HIROTA KEN
Faculty of Science and Engineering Department of Molecular Chemistry and Biochemistry
Professor
Last Updated :2020/08/09

Researcher Profile and Settings

Research Areas

  • Nanotechnology/Materials/Inorganic compounds/materials chemistry

Education

  • Osaka University, 1975
  • Osaka University, Graduate School, Division of Engineering Science, 1975
  • Osaka University, School of Engineering Science Direct Affiliates, 1973
  • Osaka University, Faculty of Engineering Science, 1973

Academic Degrees

  • Doctor of Engineering, Osaka University
  • Master of Engineering, Osaka University

Published Papers

  • Effect of oxygen potential on Co solubility limit in La?Co co-substituted magnetoplumbite-type strontium ferrite
    T. Waki, S. Okazaki, Y. Tabata, M. Kato, K. Hirota, H. Nakamura
    ? 2018 La?Co co-substituted magnetoplumbite-type (M-type) strontium ferrite is typically used as a base material for high-performance hard ferrite magnets. Generally, the unquenched orbital moment of Co2+, substituted for Fe3+, is thought to enhance magnetic anisotropy and then coercivity. La3+is substituted for Sr2+to compensate for the charge unbalance associated with Co2+substitution. Even if the synthesis starts with equal amounts of Co and La, the Co contents of the resulting samples are generally lower than the La contents because of the partial reduction of Fe3+to Fe2+. We studied how the oxygen potential suppresses the formation of Fe2+and expands the Co solubility range in this system. Polycrystalline samples, synthesized under oxygen partial pressures of 0.2, 1.0, and 387 atm, were characterized by powder X-ray diffraction, wavelength-dispersive X-ray analysis, and magnetization measurements. We reveal that the application of higher oxygen pressure dramatically expands the Co solubility range, resulting in the enhancement in magnetic anisotropy. We successfully synthesized Sr-free LaFe11CoO19with high magnetic anisotropy using the hot isostatic pressure (HIP) technique., Materials Research Bulletin, 10487 - 91, 01 Aug. 2018
  • Strongly-correlated electron behavior and metal-insulator crossover in Ru-based oxides with pyrochlore-type structure
    Masaki Kato, Shin ya Yamato, Riko Kajita, Ken Hirota
    ? 2018 The Society of Materials Science, Japan. The pyrochlore ruthenates Pb2−xRExRu2O7−δ(RE = Y, Nd, Eu, Sm, Lu) have been successfully synthesized by the solid-state reaction. The solid-solution compounds of Pb2−xRExRu2O7−δshow a crossover from metallic to insulating with increasing x. Owing to a localized magnetic moment on Ru site, these compounds with x > 0.2 also indicate the spin-glass like behaviors at low temperature (T < TSG). As a result of electric resistivity measurements, the crossover was observed around x = 0.4 which is closely related to the spin-glass like behavior. In the metallic region, values of electric resistivity obey T2dependencies which indicate the strong correlation between electrons in these compounds. It is concluded that the crossover from metallic to insulating is based on the strongly-correlated electron system., Zairyo/Journal of the Society of Materials Science, Japan, 67545 - 550, 01 May 2018
  • Fabrication of dense composite ceramics utilizing pulsed electric current and pressure
    Ken Hirota, Kota Kasahara, Kota Kasahara, Masaki Kato
    ? 2018 The Society of Materials Science, Japan. Aluminum oxide (Al2O3)-based composite ceramics consisting of vapor-grown carbon nanofiber (CNF) and titanium nitride (TiN) with the compositions of (Al2O3/CNF)/TiN=(100-y)[(100-x)/x] /y vol%, (x=0, 1, 3, 5, 10, y=0, 1, 3, 5, 10), have been fabricated utilizing both pulsed electric current and pressure to meet the demand for electric discharge processing for engineering ceramics. Mixtures of fine Al2O3powder (particle size Ps: 0.1×10-6m), CNF (150 ×10-9m in diameter and ~6 ×10-6m in length), and nanoparticle TiN (Ps: 20×10-9m) were sintered at 1623 K (1350°C) under 50 MPa for 6.0×103s (10 min) in Ar to produce electrically conductive aluminum oxide ceramics (alumina) with high mechanical properties at the same time. The relative densities of Al2O3/CNF/TiN composites decreased from 99.9% to 96.1% as the content of TiN increased to 10 vol%, and their electrical resistivity ρ was much reduced from ~1013(insulator) to less than ~1.0×10-2Ω?m by the addition of a small amount of 3~5vol% CNF and TiN simultaneously. High bending strength σband high fracture toughness KIChave been achieved in the compositions of Al2O3/CNF/TiN=95[(97/3)]/5 vol% (x=3, y=5) and 95[(95/5)]/5 vol% (x=5, y=5), respectively. The former revealed the following values: σb: 511 MPa, Vickers hardness Hv: 20.4 GPa,KIC: 6.08 MPa?m1/2, and ρ: 6.5×10-3Ω?m. The latter showed σb: 562 MPa, Hv: 19.9 GPa, KIC: 6.08 MPa?m1/2, and ρ: 2.0×10-4Ω?m. These data suggested that these composites could be candidates for electric discharge processing for engineering ceramics., Zairyo/Journal of the Society of Materials Science, Japan, 67539 - 544, 01 May 2018
  • Preparation of ZnO powders with strong antibacterial activity under dark conditions
    Thi Minh, Phuong Nguyen, Shoko Hirota, Yusei Suzuki, Masaki Kato, Ken Hirota, Hideki Taguchi, Hirohisa Yamada, Kazuhiko Tsukagoshi
    © 2018 Journal of the Japan Society of Powder and Powder Metallurgy. All rights reserved. Fine ZnO powders have been prepared from plate-like ZnO particles by hydrothermal-treatment in 3 mol/L Zn(NO3)2aqueous solution at 170°C for 7 h (443 K for 25.2 × 103s), followed by the heating at 400~700°C for 1 h (673~973 K for 3.6 × 103s) in air. They show strong antimicrobial activity in the sunshade; those disinfect many kinds of bacteria even MRSA. The origin of antibacterial activity might be explained in terms of the generation of most powerful reactive oxygen species (ROS) of hydroxyl radical (OH·). As the intensity of chemiluminescence (CL) reflects the amount of ROS, the antibacterial activity has been estimated by measuring the CL emitted from the surface of ZnO in the luminol solution. Both electron spin resonance (ESR) spectroscopy and CL detection were used for identification of ROS. Furthermore, in order to increase the CL values, the ZnO powders have been ball-milled (BM) under the various conditions. It is cleared that the suitable BM conditions, such as BM time and ball size, can improve CL values greatly in comparison with those of ZnO without BM. This effect might be brought by both increasing surface area of ZnO powder and the lattice strain from “Debye effect”., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 65316 - 324, 01 Jan. 2018
  • Metal-insulator crossover in Pb-Ru based oxides with pyrochlore-type structure
    Masaki Kato, Kazuya Iwamoto, Yuki Iwakura, Ken Hirota, Angel Arevalo-Lopez, John Paul Attfield
    ? 2018 Journal of the Japan Society of Powder and Powder Metallurgy. All Rights Reserved. Synthesis and physical properties of the Pb-Ru based pyrochlores Pb2−xMxRu2O7−δ(M = Eu and Ca) have been studied. For Pb-Eu solid solution Pb2−xEuxRu2O7−δ, single phases were successfully synthesized by the solid-state reaction. Magnetic susceptibility measurements indicated the magnetic transition due to spin-glass freezing in the samples with x ? 0.8. From electric resistivity measurements, a crossover from metallic to semiconducting behaviors was observed in the range of 0.6 ? x ? 0.8. In the metallic region, normalized electric resistivity was found to be almost proportional to T2, indicating the strongly-correlated electron behaviors. The crossover between insulating and metallic phases can be caused by the strong electron-electron couplings between the itinerant 4d electrons. For Pb-Ca solid solution Pb2−xCaxRu2O7−δ, single phases were obtained for x ? 0.6 samples by heat treatments in air. Pb2−xCaxRu2O7−δsamples with 0.8 ? x ? 1.6 were successfully synthesized under high oxygen pressures. Lattice parameter of Pb2−xCaxRu2O7−δcompounds was found to decrease with Ca content x, resulting in the positive chemical pressure. As a result of electric resistivity measurements, the samples with 0.0 ? x ? 1.0 showed the metallic behaviors., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 65249 - 254, 01 Jan. 2018
  • Fabrication of B4C/TiB2composite ceramics using pulsed electric current pressure sintering
    Ken Hirota, Tomoki Taniguchi, Naoki Goto, Masaki Kato, Hideki Taguchi
    Dense B4C/TiB2composite ceramics have been fabricated from the amorphous B and C, and fine TiC powders (25 nm in size Ps), using pulsed electric current pressure sintering (PECPS) at 2173 K under 50 MPa for 6.0 × 102s in a vacuum. During PECPS the solid-state reaction between B and C was induced around 1373 K and almost at the same time TiB2phase was gradually formed, followed by a continuous formation of B4C matrix including TiB2particles. On the other hand as the reference, two kinds of B4C/TiB2composites from the B/C mixtures and TiB2powders (1.86 μm and 58 nm in Ps) were also fabricated as the same manner. The former composites using TiC showed high relative density ? 98.9% up to 11 vol% TiB2and improved mechanical properties, such as three-point bending strength σb of 650 MPa, Vickers hardness Hvof 33.5 GPa and fracture toughness KIC around 5.45 MPa?m1/2at the composition of B4C/TiB2= 93/7 vol%, however, the latter composites revealed lower relative densities of 96.5% at 7 vol%TiB2composition and deteriorated mechanical behaviors as Psof TiB2increased., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 64538 - 546, 01 Oct. 2017
  • Preparation of B-site Mn-doped Spinel Fe(Al1-xMnx)2O4(0 ? x ? 0.9) ferrite powders using a modified citric acid route
    Ken Hirota, Mei Okamoto, Yoshiaki Funahashi, Dao Hoang Anh, Masaki Kato, Hideki Taguchi
    Preparation of spinel Mn-doped Fe(Al1-xMnx)2O4(0 ? x ? 0.9) ferrite powders, in which B-site Al3+were replaced partially by Mn3+, has been tried via a modified citric acid route. Fe and Al nitrate salts and fine MnO2 powder were used as starting materials. Homogeneous mixed solutions containing Fe3+, Al3+ions and MnO2 were added to the mixed solution of citrate acid (CA) and ethylene glycol (EG) at room temperature with stirring. After boiling and continuous drying, thus obtained precursors were heated in air to burn out the organic substances and calcined at 1173 K (900°C) for 7.2 × 103s (2 h) under nitrogen atmosphere with a small amount of hydrogen to control the valances of Fe and Mn in the ferrite powders. Single spinel B-site Mn-doped Fe(Al1-xMnx)2O4powders with the compositions of x = 0.0, 0.2, and 0.6 - 0.9 have been obtained. With increasing x value, the magnetic property of spinel powder changed from paramagnetism (x = 0) to ferrimagnetism (x = 0.2, 0.6 - 0.9) at room temperature. Neutron-diffraction analysis measured at room temperature proved that some of Mn ions occupied B-site and on the contrary Fe did A-site. Magnetic properties also supported this configuration determined by XRD and NRD., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 64383 - 391, 01 Jul. 2017
  • Fabrication and thermoelectric properties of Perovskite like la0.3(Ca1-xax)0.55Cu3Ti4O12(A = Sr, Mg) ceramics
    Mikoto Okumura, Masaki Kato, Ken Hirota, Hideki Taguchi
    ? 2015, Journal of the Japan Society of Powder and Powder Metallurgy. All rights reserved. The powders of La0.3(Ca1-xAx)0.55Cu3Ti4O12(A = Sr, 0 ? x < 0.8, A = Mg, 0 ? x ? 0.1) with cubic Perovskite like structure were synthesized at 1323?K in air via a polymerized complex route. Lattice constants a of the dense (? 97?%) ceramics, fabricated in air using a hot-pressing (1273?K/3.60 × 103? s/50?MPa), increased and decreased with increasing x for the ceramics (A = Sr) and (A = Mg), respectively, On the other hand, the electrical conductivities σedecreased and increased with increasing x and temperature, respectively. Their negative Seebeck coefficients S had the maximum values of ca. -560?μV?K-1for the ceramics (A = Sr at x = 0.8: Sr0.8) and -515 μV?K-1for Mg0.02, showing the behavior that the absolute S values decreased and increased slightly with increasing temperature and x, respectively. Their power factor S2σehad the maximum values at Sr0.2 and Mg0.05. Thermal conductivities λ for the ceramics (x = 0.0, Sr0.2 and Mg0.1) were measured from 300 to 1073?K in air; lower λ were attained for Mg0.1 in comparison with others. Their dimensionless figure of merit ZT reached ~4.0 × 10-3at 1073?K. These results indicated that La0.3(Ca0.8Mg0.05)0.55Cu3Ti4O12could be a candidate of the host materials of the new hybrid-type thermoelectric element., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 62185 - 193, 01 Apr. 2015
  • Fabrication of perovskite-type Ba(Sn1-xTax)O-3 ceramics and their power factors
    Masao Takada, Hideki Taguchi, Masaki Kato, Ken Hirota
    Perovskite-type Ba(Sn1-xTax)O-3 (0.01 <= x <= 0.06) ceramics with high relative densities (92.7-94.4 %) were fabricated using the hot isostatic pressing (HIP) method at 1273 K and 196 MPa for 4 h in an atmosphere of argon gas. The lattice parameter decreased slightly with increasing x. From the XPS measurement, the Ta5+ ion was stable in Ba(Sn1-xTax)O-3 ceramics and the broad peak of the Ta4f level was the overlap between the Ta(5+)4f(5/2) and Ta(5+)4f(7/2) levels. Ba(Sn1-xTax)O-3 ceramics were n-type semiconductors, and their electrical resistivities increased with increasing x. The increase in the electrical resistivity was explained by impurity scattering due to the presence of the Ta ions. The absolute value of the Seebeck coefficient (S) increased with increasing temperature and x. The power factor (S-2 sigma), which was calculated from electrical conductivity (r) and the Seebeck coefficient, was ca. 1.0 x 10(-5) W m(-1) K-2 at x = 0.01., JOURNAL OF MATERIALS SCIENCE, SPRINGER, 50(1)476 - 481, Jan. 2015, English, Research paper (scientific journal)
  • Al<inf>2</inf>O<inf>3</inf>-Compositional dependence of mechanical properties of ZrO<inf>2</inf> based ceramics fabricated from ZrO<inf>2</inf>(Y<inf>2</inf>O<inf>3</inf>)-Al<inf&g
    Ken Hirota, Kenta Yamamoto, Koki Sasai, Masaki Kato, Hideki Taguchi, Hideo Kimura, Masayuki Takai, Masao Terada
    ? 2015, Journal of the Japan Society of Powder and Powder Metallurgy. All rights reserved. ZrO&lt;inf&gt;2&lt;/inf&gt; composite ceramics containing 10~30 mol% Al&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt; and 1.5 mol% Y&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt;, i.e., 90~70 mol% ZrO&lt;inf&gt;2&lt;/inf&gt;(1.5 mol% Y&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt;)-10~30 mol% Al&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt; ceramics, have been fabricated by sintering cubic ZrO&lt;inf&gt;2&lt;/inf&gt; solid solution (ss) nano-powders prepared via the neutralization co-precipitation method. The sintering was performed using a pulsed electric-current pressure sintering (PECPS) at 1598 K for 10 min under 50~60 MPa in Ar. Dense tetragonal- ZrO&lt;inf&gt;2&lt;/inf&gt; (ss) ceramics containing α-Al&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt; were composed of 277-130 nm grains. FE-SEM observation revealed that their microstructures, that is the grain sizes of both t-ZrO&lt;inf&gt;2&lt;/inf&gt; (ss) and α-Al&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt;, and the distribution of α-Al&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt; particles, were much influenced by the content of α-Al&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt;. The best mechanical properties of extreme high bending strength (σ&lt;inf&gt;b&lt;/inf&gt; ? 1 GPa) and high fracture toughness (K&lt;inf&gt;IC&lt;/inf&gt; ? 15 MPa?m&lt;sup&gt;1/2&lt;/sup&gt;) simultaneously were obtained at the composition of 25 mol% Al&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt; with a high relative density 99.9%, as the same composition reported for the HIP sintered ZrO&lt;inf&gt;2&lt;/inf&gt;(2~3 mol% Y&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt;)-Al&lt;inf&gt;2&lt;/inf&gt;O&lt;inf&gt;3&lt;/inf&gt; composite ceramics., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 62134 - 143, 01 Jan. 2015
  • Pulsed electric-current pressure sintering of spinel type (Fe, Mn)Al2O4ferrites and their physical property evaluation
    Masahiro Ueda, Masaki Kato, Ken Hirota
    Single-phase spinel type FeAl2O4and Mn-doped solid solutions of (Fe1-xMnx)Al2O4(0 &lt; x ? 1.0) powders have been prepared by solid state reaction at 900 °C for 6 h in 1.0% H2-bal.N2using reactive fine powders of α-Fe2O3(particle size of 0.1 μmφ), γ-Al2O3(about 10 μmφ) and MnO (about 0.15 μmφ). Afrer the powder characterization using XRD, they were densified using pulsed electiric-curuent pressure sintereing (PECPS) at 1050 °C for 10 min under 50 MPa in Ar atmossphere. Thus fabricated dense ceramics, having single-phase spinel strucure with the relative desity ? 97.0 %, were evaluated by XRD, SEM, and a SQUID magnetometer. They showed antiferromagnetic properties, except for x = 1.0, i.e., MnAl2O4(paramagnetic), and at the compositions of x: 0.4 ? 0.6, Mn-doped solid solutions ceramics revealed moderate high magnetic susceptibility., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 61171 - 178, 01 Apr. 2014
  • Fabrication of novel ZrO2(Y2O3)-Al2O3ceramics having high strength and toughness by pulsed electric-current pressure sintering (PECPS) of sol-gel derived solid solution powders
    Ken Hirota, Kengo Shibaya, Masaki Kato, Hideki Taguchi
    ZrO2based ceramics containing 25 mol% Al2O3and 0.90?1.125 mol% Y2O3, i.e., Zr02(1.2?1.5 mol%Y2O3)-25mol%Al2O3have been fabricated at 1523 to 1623 K (1250? 1350°C) for 10 min under 60 MPa in Ar by pulsed electric-current pressure sintering (PECPS) of sol-gel derived cubic ZrO2solid solution (ss) powders. Dense tetragonal-ZrO2(t-ZrO2) phase composite ceramics (?99.5%) sintered at 1623 K (1350°C), being composed of ? ?φ200 nm grains, revealed high bending strength σb ?1.5 GPa and high fracture toughness KIc?15.5 MPam1/2simultaneously. Precise investigation has been performed on the relationship between their microstructures and mechanical properties, the former of which depend on the content of Y2O3and calcining temperatures. SEM/TEM observations cleared that these improved mechanical properties might be originated from homogeneous distribution of α-Al2O3particles around the dense t-ZrO2grain matrix; the precipitation of α-Al2O3could be achieved by adopting the (ss) powders and PECPS. The Y2O3content in fine ZrO2grains has much effect of controlling the stress-induced transformation toughening of tetragonal to monoclinic ZrO2., Ceramic Transactions, 2493 - 13, 01 Jan. 2014
  • Study on fabrication of novel monolithic and hybrid materials by combined use of newly synthesized ceramics powders and various kinds of sintering methods
    Ken Hirota
    Bronze-type TiO2(B) single-crystal fibers with several microns in length and 300 ~ 400 nm in diameter, were synthesized from anatase TiO 2 by the supercritical fluid hydrothermal treatment at c.a. 375~425 °C under 23~37 MPa in 10 mol ? L-1 NaOH aq. followed by ion-exchange of Na to H in dilute HC1 aq. and post-heating at 400 °C for 5 h in air. Thus prepared TiO2(B) fibers were characterized by XRD analysis, SEM/TEM observations, ED analysis, DTA/TG and BET surface area measurements. Electrical conductivity of the fibers was also evaluated using &quot;Nano-probe&quot; measurement system. Compared to the conventional TiO 2(B) fibers prepared from the normal hydrothermal treatment at c.a. 185 °C for 8 h, the present TiO2(B) showed higher thermal stability; TiO2(B) phase was stable even after heating at 800 °C for 1 h in air. And they show an extreme high electrical conductivities of 1.2 ~1.3 S ? m-1; they were higher 1011 and 1.5 times than those of anatase TiO2 and conventional TiO2(B), respectively., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 613 - 10, 01 Jan. 2014
  • Fabrication of high strength and toughness ceramics using pulsed electric-current pressure sintering of ZrO2(Y2O3)-Al2O3solid solution powders prepared by the neutralization co-precipitation method
    Kenta Yamamoto, Masaki Kato, Ken Hirota, Hideki Taguchi, Hideo Kimura, Taichi Kunisada, Yuta Kageyama, Hiroaki Morita
    ZrO2composite ceramics containing 25mol%Al2O3and a small amount of Y2O3, i.e., 75mol%ZrO2(0.5?2.0 mol%Y2O3: 0.5?2.0Y)-25mol%Al2O3ceramics, have been fabricated by sintering cubic ZrO2solid solution (ss) nano-powder prepared via the neutralization coprecipitation method. Their sintering was performed using a pulsed electric-current pressure sintering (PECPS) at 1300? 1350°C for 10min under 60MPa in Ar. Dense tetragonal-Zr02(ss) ceramics containing α-Al2O3were composed of 135-150 nm grains with a high relative density ? 99.9%. They showed extreme high bending strength (ωb? 1 GPa) and high fracture toughness (KiC? 15 MPa. m1/2) simultaneously. FE-SEM observation revealed that these mechanical properties were originated from homogeneous distribution of α-Al203particles around the t-ZrO2matrix; the precipitation of α-Al2O3could be achieved by adopting both the (ss) powders containing α-Al2O3and PECPS., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 60428 - 435, 01 Dec. 2013
  • The relationship between the crystal structure and electrical properties of K2NiF4-type (Ca2-xNdx)MnO4
    Hideki Taguchi, Hiroyasu Kido, Ken Hirota
    K2NiF4-type (Ca2-xNdx)MnO4synthesized using a polymerized complex route crystallized in a tetragonal structure with space group I41/acd in the range of 0 ? x ? 0.2 and crystallized in an orthorhombic structure with space group Fmmm in the range of 0.3 ? x ? 0.4. The Mn-O(2) distance of the a- and b-axis directions increased with values of x. The samples were n-type semiconductors and exhibited hopping conduction in a small-polaron model at temperature greater than 393 K. At room temperature, the carrier concentration increased slightly, whereas mobility decreased by 20% with increasing x values. The absolute value of the Seebeck coefficient monotonously increased with temperature and abruptly decreased with increasing values of x at a given temperature. These results indicated that electrical conductivity was strongly affected by the antagonism of the electron transfer in the Mn4+-O-Mn3+path and the overlap between the Mnt2gand Opπorbitals. ? 2013 Elsevier Masson SAS. All rights reserved., Solid State Sciences, 2522 - 27, 11 Sep. 2013
  • Synthesis and characterization of highly crystallized TiO2(B) nanofibers via the hydrothermal treatment in supercritical fluid
    Noritsugu Taniguchi, Masaki Kato, Ken Hirota
    Single-crystal TiO2(B) nanofibers were synthesized by the hydrothermal treatment in supercritical fluid at c.a. 375 ?425 °C under 23?37 MPa using 10mol.L-1 NaOH aq. followed by ion-exchange of Na to H in dilute HCI aq. and post-heating at 400 °C in air. Thus prepared TiO2(B) nanofibers were characterized by XRD analysis, SEM/TEM observations, DTA/TG and BET surface area measurements. Electrical conductivity of the nanofibers and thermal conductivity of the nanofiber-compacts were also evaluated using &quot;Nano-probe&quot; and an Xenon flash measurement systems, respectively. Compared to the conventional TiO2(B) fibers prepared from the normal hydrothermal treatment at c.a.185 °C for 8 h, the present TiO2(B) with several microns in length and 300?400 nm in diameter, revealed higher thermal stability; TiO2(B) phase was stable even after heating at 800 °C for 5 h in air, and an extreme high electrical conductivity of 1.2?1.3 S.m-1 in addition to high thermal conductivity; the former and the latter were higher 1011 and 1.5 times than those of anatase TiO2 and normal TiO2(B)., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 59326 - 332, 01 Jun. 2012
  • Fabrication of high-strength & -fracture toughness ZrO 2/25mol%Al 2O 3 composite ceramics using high-pressure (1GPa) sintering
    Ken Hirota, Hideyuki Sato, Katsuya Takaoka, Masaki Kato, Hideki Taguchi
    ZrO 2 ceramics containing 25mol% Al 2O 3 have been fabricated by sintering cubic or tetragonal ZrO 2 (t -ZrO 2) solid solution (ss) nanoparticles prepared via a sol-gel process with a piston-cylinder type high-pressure apparatus at 850?950°C for 10 to 60min under 1GPa. Dense t -ZrO 2(ss)/Al 2O 3 composite ceramics with a small amount of monoclinic ZrO 2 (m- ZrO 2) sintered at 900°C for 30min using the calcined powder prepared at 1000°C for 1 h in air were composed of around 55nm grains with a high relative density of 99.9%. They showed extreme high bending strength (σ b : 1125MPa) and high fracture toughness (K IC : 15.8MPa?m 1/2) simultaneously. Their mechanical properties were investigated from viewpoints of the microstructure of ceramics by high-resolution TEM observation and XRD analysis. It was clear that high mechanical properties were much related with high relative density, high ratio of t -ZrO 2 phase to m- ZrO 2, homogeneous distribution of α Al 2O 3 particles in the nanometer-size grain matrix of t -ZrO 2 ; these might be achieved by adopting both the the sol-gel technique for preparation of solid solution powders containing Al2O 3 and high-pressure sintering at low temperatures. ? 2012 The Society of Materials Science, Japan., Zairyo/Journal of the Society of Materials Science, Japan, 61419 - 425, 01 May 2012
  • Synthesis and magnetic properties of Cu oxides with low-dimensional structure
    M. Kato, Y. Nogi, S. Hongo, K. Hirota
    We report the synthesis and magnetic properties of Cu oxides with low-dimensional structure. For CuSb2-xTixO6compounds with tri-rutile type structure, magnetic susceptibility χ showed that a characteristic temperature, J/kB, where a broad maximum of χ appears due to the short-range antiferromagnetic order, decreases with increase of x. Thus, it is considered that a hole carrier doped by Ti substitution results in the decrease of J/kB. Furthermore, it was found that the antiferromagnetic long-range order at the transition temperature (TN), around 8.5 K for x=0.0, still remains in the substituted compounds for x=0.3, which indicates the one-dimensional interaction along the diagonal direction is dominant. CuNb2-xTixO6compounds have zig-zag chain structure in the edge-sharing CuO6octahedra. Temperature dependences of χ were found to be well analyzed by the alternating 1DHAF model. Hole doping into this material is realaized by substituting Ti4+ions for Sb5+ions., Journal of Physics: Conference Series, 344, 01 Jan. 2012
  • Magnetic susceptibility of η-carbide-type molybdenum and tungsten carbides and nitrides
    T. Waki, S. Terazawa, Y. Umemoto, Y. Tabata, Y. Murase, M. Kato, K. Hirota, H. Nakamura
    We synthesized a series of molybdenum and tungsten η-carbide-type compounds and measured their magnetic susceptibility χ. Although the enhancement of χ was observed in several compounds, apparent magnetic order was found only in Fe3W3N (ferromagnetic at 110 K). The broad peak in χ of Fe6W6C is possibly due to antiferromagnetic order. Itinerant electron metamagnetic transitions were observed in Fe3W3C, Fe6W6C, Fe6W6N and Co6W6C. The magnetic enhancement depends on the 3d element in the order Fe &gt; Co &gt; Ni corresponding to DOS at theFermi energy in η-12 systems, whereas no clear trend has been observed in η-6 systems., Journal of Physics: Conference Series, 344, 01 Jan. 2012
  • Fabrication of ZrO 2 solid solution ceramics containing Al 2O 3 having high bending strength (σ b ? 1 GPa) and high fracture toughness (K IC ? 20 MPa ? m 1/2) simultaneously by pulsed elect
    Kengo Shibaya, Hideyuki Sato, Hiroyuki Matsuda, Katsuya Takaoka, Masaki Kato, Ken Hirota, Hideki Taguchi
    ZrO 2 solid solution (ss) ceramics containing 25mol%Al 2O 3 and 1.125mol% Y 2O 3, i.e., 75mol%ZrO 2(1.5mol% Y 2O 3:1.5Y)-25mol%Al 2O 3 ceramics, have been fabricated by sintering cubic ZrO 2 (ss) nanoparticles prepared via a sol-gel process with a pulsed electric-current pressure sintering (PECPS) at 1100°?1350°C for 10 min under 60 MPa in Ar. Dense tetragonal-ZrO 2 (ss) ceramics with a small amount of α-Al 2O 3, sintered at 1300° to 1350°C, were composed of 0.2-0.3 μm grains with a high relative density of ? 97.5%. They showed extreme high bending strength (σ b ? 1 GPa) and high fracture toughness (K IC ? 20 MPa ? m 1/2) simultaneously. High-resolution TEM observation revealed that these mechanical properties were originated from homogeneous distribution of α-Al 2O 3 particles in the nanometer-grain matrix of t-ZrO 2; the precipitation of α- Al 2O 3 could be achieved by adopting the solid solution powders containing α-Al 2O 3 and PECPS., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 58727 - 732, 01 Dec. 2011
  • Fabrication of TiN particle-dispersed A1 2O 3 composites utilizing high N 2-pressure SHS
    Ken Hirota, Hajime Yagura, Katsuya Takaoka, Masaki Kato
    Fabrication of fine TiN particle-dispersed dense Al 2O 3 composites with the compositions of Al 2O 3/TiN=100/0?90/10 vol% has been conducted from Al 2O 3/(Ti, TiN 0.3) mixed powder compacts by capsule-free hot isostatic pressing (HIP) utilizing high-pressure N 2 SHS. Fine Ti powders (φ ? 0.3 μm) with TiN 0.3 phase were prepared by thermal decomposition of planetary ball-milled fine TiH 2 powders at 400°C (673 K) for 1 h in a vacuum, followed by heating in N 2 at 200°C (473 K) for 2 h. The Al 2O 3 powder compacts (relative densities of 57.2-57.8%) with homogeneously dispersed (Ti, TiN 0.3) particles were prepared. The mixed powder compacts were hot isostatically pressed (HIPed) under the conditions of 1350°C (1623 K) at 7 MPa N 2 for 1 h, followed by the heating at the same temperature for 2 h under 196 MPa-N 2. At the first stage of heating [1350°C (1623K)/7MPa/1h], solid/gas reaction of SHS between (Ti, TiN 0.3) and N 2 was introduced to form TiN and densification of the Al 2O 3 powder compacts up to the relative density of 92-93% with closed pores was performed. And at the sequent second stage [1350°C (1623K)/196MPa/2h], densification of the most of pre-sintered composites consisting of Al 2O 3 and TiN reached higher relative densities than 98.5%. Dispersion of TiN particles (φ ? 0.30 μum) in the composites suppressed the grain growth of Al 2O 3 during HIP-sintering. Mechanical properties, such as bending strength (σ b), Vickers hardness (H v), fracture toughness (K IC), and electrical resistivity (ρ) of the composites were evaluated as a function of TiN content; the maximum values of σ b=640 MPa, H v=19.5 GPa, and K IC=4.5 MPa ? m 1/2 were obtained in the Al 2O 3/TiN=97/3?95/5 vol% composites. Among the composites, the lowest ρ value of 2.6× 10 3 Ω ? m was attained at Al 2O 3/TiN=90/10 vol% composite. ? 2011 al-Farabi Kazakh National University., Eurasian Chemico-Technological Journal, 13115 - 124, 01 Dec. 2011
  • HIP synthesis of eta-carbide-type nitrides Fe3W3N and Fe6W6N and their magnetic properties
    T. Waki, S. Terazawa, Y. Tabata, Y. Murase, M. Kato, K. Hirota, S. Ikeda, H. Kobayashi, K. Sato, K. Kindo, H. Nakamura
    We have succeeded in synthesising iron-tungsten nitrides using the hot isostatic pressing (HIP) method and have measured their magnetic properties. Two eta-carbide-type iron-tungsten nitrides with lattice constants a = 11.043(1) and 10.937(2) angstrom were synthesised directly from metal elements under high-pressure nitrogen gas. Their compositions are expected to be Fe3W3N and Fe6W6N in analogy with other eta-carbide-type compounds. Fe3W3N is a ferromagnet with a Curie temperature T-C = 110 K and a saturation moment PS = 0.78 mu(B)/Fe, whereas Fe6W6N is an antiferromagnet with a Neel temperature T-N = 75 K and shows a metamagnetic transition at around 25 T. (C) 2011 Elsevier B.V. All rights reserved., JOURNAL OF ALLOYS AND COMPOUNDS, ELSEVIER SCIENCE SA, 509(39)9451 - 9455, Sep. 2011, English, Research paper (scientific journal)
  • Simultaneous synthesis and sintering of Al2O3/TiN composites using capsule-free N2-HIPing from Al2O 3 and fine Ti mixed powder compacts
    Hajime Yagura, Katsuya Takaoka, Masaki Kato, Ken Hirota, Toshinori Fujii
    Synthesis of dense composite materials with the compositions of Al 2O3/TiN= 100/0 ? 90/10 vol% has been attempted directly from Al2O3/Ti mixed raw powder compacts using capsule-free N2 hot isostatic pressing (HIP). Fine Ti powders (0.3 μm in diameter) with a small amount of TiN0.3 phase were prepared by thermal decomposition of fine TiH2 powders at 400 °C for 3.6 × 103 s in a vacuum, followed by thermal treatment in N 2. Then, Al2O3/Ti mixed powder compacts with homogeneously dispersed Ti particles were fabricated and then HIP-sintered. During the first stage of HIPing [1350°C/6MPa/3.6× 103S], solid/gas reaction between Ti and N2, in which both were contained in the powder compacts, was introduced to form TiN. Then, after the successive second stage of HIPing [1350°C/196MPa/7.2× 103s], the most of sintered composites consisting of A12O3 and TiN phases reached to a higher relative density than 98.5 % with closed pores, nevertheless a capsule-free HIPing. Dispersion of TiN particles (0.30?0.35μm) just formed suppressed the grain growth of Al2O3 during sintering. Mechanical properties, such as bending strength (σb) , Vickers hardness (Hv), and fracture toughness (K1c) have been evaluated as a function of TiN content., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 58214 - 219, 01 Apr. 2011
  • Fabrication of Mg ferrite/permalloy magnetic nano-composites
    Motoki Obata, Masaki Kato, Ken Hirota
    Dense {Mg(Fe 1-x, Mn x) 2O 4/53Fe-47Ni permalloy (0 ? 0.4)} magnetic nano-composites consisting of spinel and alloy phases have been fabricated from N 2-atomized metal particles and fine-powder mixtures of MgOZa-Fe 2O 3/ MnO using a combination of pulsed electric current pressure sintering (PECPS) and hot isostatic pressing (HIP). Fe-Ni permalloy particles were coated with the mixed metal oxide powders homogeneously by passing these mixtures through a narrow gap (1 mm) under high shear compression in Ar. Mixed powder compacts of permalloy with the fine metal oxides were densified into the green bodies having relative densities of 83-85 % under isostatic high pressure of 1 GPa at room temperature. Then green powder compacts were pre-sintered at 823 K for 3 min under a uniaxial pressure of 100 MPa in Ar using a PECPS. The pre-sintered bodies composed of spinel ferrite and permalloy revealed the relative densities of 94-96%; the mixed metal oxides changed into magnesium spinel ferrites Mg(Fe1-XMnJ2O4 during a short heating time. Then hot isostatic pressing (HIP: (1123K/6 h/200 MPa-Ar)) was adopted to sinter the pre-sintered bodies into a near full density without an encapsulation, however, a sintering atmosphere control was performed using a ferrite muffle in order to maintain the both phase. Sintered nanocomposites with the relative density more than 99.5 % have a saturation magnetization density &gt; 1T and moderate permeability values of 30-35 at 10 MHz, which values have been expected from a material design., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 57672 - 677, 01 Oct. 2010
  • Synthesis of Al2O3/Mo2N composite materials using capsule-free nitrogen HIPing and their evaluation
    Katsuya Takaoka, Yasushi Murase, Masaki Kato, Ken Hirota
    Synthesis of dense composite materials with the compositions of Al 2O3/Mo2N = 100/0 ? 40/60 vol% has been attempted directly from Al2O3/Mo mixed raw powder compacts using capsule-free N2 hot isostatic pressing (HIP). During HIPing [1500°C/(16?20-200MPa)/1 h], solid/gas reaction between Mo and N2 was introduced to form Mo2N. The most of sintered composites consisting of only Al2O3 and Mo2N phases reached to a higher relative density than 98.0% with closed pores nevertheless capsule-free HIPing. Dispersion Of Mo2N particles just formed suppressed the grain growth of Al2O3 during sintering. Mechanical properties, such as bending strength (σb). Vickers hardness (Hv), fracture toughness (KIC), and other properties have been evaluated as a function of their compositions. The best mechanical values of σb (?573 MPa), Hv (?20.3 GPa) and K IC (?5.00MPa?m1/2) were attained at the composition of Al2O3/Mo2N = 90/10 vol%, due to a high density (98.6%) and small grain size of Al2O3 matrix (Gs ? 4.70μm). Further addition of Mo2N reduced the sinterability of matrix grains, resulting in low densities of around 90% at the 40/60 vol% composition., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 57112 - 117, 01 Feb. 2010
  • Synthesis of new antibacterial ZnO powders by hydrothermal treatment and their characterization
    Sho Matsuoka, Masaki Kato, Ken Hirota, Kazuhiko Tsukagoshi
    Homogeneous ZnO powders with antibacterial activity have been synthesized by hydrothermal treatment at 160 ? 190 °C under 1.01 ?1.09 MPa for 10 ? 50 h using Zn(NO 3) 2?6H 2O, hexamethylenetetramine ((CH 2) 6N 4) and ethanol. Their particle morphology and specific surface areas have been characterized in relation with the synthesis conditions. A luminol chemiluminescence (CL) measurement has been applied to evaluate their antibacterial activity under dark conditions. Experimental results revealed that the ZnO nanorod powders (a length? 500nm and a diameter&lt;100nm), prepared under the conditions of 160 *C/20h/1.09 MPa, showed the highest photon counts of 3.75x 10 5, which value was ten times higher than that (3.59 x 10 4 counts) of conventional ZnO powders commercially available., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 57106 - 111, 01 Feb. 2010
  • Crossover from Metallic to Semiconducting in Pb2-xLnxRu(2)O(7-delta) (Ln = Eu, Sm) Compounds with Pyrochlore Structure
    Masaki Kato, Tetsuya Imamaura, Miho Fukatsu, Ken Hirota
    We study the synthesis and physical properties of Pb(2-x)Ln(x)Ru(2)O(7-delta) (Ln = Sm, Eu) compounds with pyrochlore structure. The lead-lanthanoid pyrochlores Pb(2-x)Ln(x)Ru(2)O(7-delta) (Ln = Sm, Eu) have been successfully synthesized by the solid-state reaction. All samples were characterized by X-ray diffraction, magnetic susceptibility, and electrical resistivity measurements. Magnetic susceptibility measurements showed that the spin-glass transition temperature, T-SG, decreases with Ln content x and goes to zero at x = 0.2 and 0.6 for Sm and Eu, respectively. From electric resistivity measurements, a crossover from metallic to semiconducting behavior was observed in the range of 0.2 &lt;= x &lt;= 0.4 and 0.4 &lt;= x &lt;= 0.8 in Pb2-xSmxRu2O7-delta and Pb2-xEuxRu2O7-delta, respectively. These boundaries between metallic and insulating phases mostly correspond to values of x with T-SG = 0. These metal-insulator transitions can be caused by the strongly correlated couplings between the itinerant 4d electrons which are possibly attributed to the magnetic frustration., MULTISCALE, MULTIFUNCTIONAL AND FUNCTIONALLY GRADED MATERIALS, TRANS TECH PUBLICATIONS LTD, 631-632483 - 488, 2010, English, Research paper (international conference proceedings)
  • Simultaneous synthesis and densification of alpha-Zr(N)/ZrB2 composites by self-propagating high-temperature combustion under high nitrogen pressure
    Shingo Nakane, Takaya Endo, Ken Hirota
    Simultaneous synthesis and densification of alpha-Zr(N)/ZrB2 composites from a 85 mol% Zr/15 mol% B mixed-powder compacts have been achieved by self-propagating high-temperature under a nitrogen pressure of 10 MPa. Composites consist of fine and short rodlike ZrB2 grains (0.1 mu m(phi)-0.5 mu m(1)) dispersed into alpha-Zr(N) matrix (3 mu m). Dense composite materials (96.5% of theoretical) exhibit excellent mechanical properties, in which their bending strength and H, are 560 MPa and 6.5 GPa, respectively. This bending strength is much superior to those (205 and 480 MPa) of dense equi-axial alpha-Zr(N) (10 mu m) and dense ZrB2 (6 mu m). Fine and rodlike ZrB2 grains greatly enhanced their mechanical properties. (C) 2009 Elsevier Ltd and Techna Group S.r.l. All fights reserved., CERAMICS INTERNATIONAL, ELSEVIER SCI LTD, 35(6)2145 - 2149, Aug. 2009, English, Research paper (scientific journal)
  • Fabrication of Mg-Fe-O ferrite/magnetic metal bulk composites by high-pressure sintering
    Kengo Fujimura, Masaki Kato, Ken Hirota
    Fabrication of MgFe 2O 4/Fe-Ni alloy bulk composites, in which Fe-Ni grains ( ? 5 μm in diameter) are fully surrounded with thin MgFe 2O 4layer ( ? 0.3μm in thick), has been tried by adopting high-pressure sintering for two powders: &quot;Composite Powder I&quot; and &quot;Composite Powder II&quot;. The former was prepared as follows; fine MgO particle-coated Fe-Ni powders were heat-treated under various oxygen partial pressures at 600° and 700°C. The latter was fine MgO+α-Fe 2O 3 mixed powder particles-coated Fe-Ni powders. For the composite powders, two sintering processes were tried; i.e., pulsed-electric current pressure sintering PECPS (800°C/30MPa/3 min) and HIP (950°- 1250°C/196MPa/2h). Phase identification by XRD and SEM observations revealed that dense microstructure consisting of two phases (spinel MgFe 2O 4 and Fe-Ni alloy) was attained from the combination of composite powders II, PECPS and a re-oxidation process. However, other process resulted in formation of W?steit phase of Mg 1-xFe xO and FeO in addition to Fe-Ni due to highly reductive atmosphere during the sintering. Thus obtained composite materials consisting two phases showed good magnetic properties at room temperature as expected from their compositions and microstructures: saturation magnetization density B s of 1.20 T, and permeability μ? 15 - 30 at 1 MHz, and a Curie point above 460 °C., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 55629 - 636, 01 Sep. 2008
  • Simultaneously synthesis and sintering of carbon nanofiber dispersed B 4C nanocomposites by pulsed electric-current pressure sintering and their mechanical properties
    Shingo Nakane, Yoshihiro Nakayama, Masaki Kato, Ken Hirota, Toshiyuki Nishimura, Kiyoshi Mizuuchi, Yoshiaki Morisada
    Carbon nanofiber (CNF)-dispersed B4C nanocomposites containing 2.5vol%Al2O3 as a sintering aid were fabricated directly from mixtures of amorphous B and C, CNF, and A12O3 powders by pulsed electric-current pressure sintering (PECPS) at 1800°C for 10min under 30 MPa in a vacuum. Dispersion of CNF into the B4C matrix up to ?15 vol% introduced the enhancement of densification. A15 vol%CNF/B 4C nanocomposite revealed excellent mechanical properties: a bending strength (σb) of ?710MPa, a Vickers hardness (H v) of ?36GPa, and a fracture toughness (KIC) of ?7.6 MPa ? m1/2, in comparison with those (σb~530MPa, Hv ?29 GPa, and K IC?4.3 MPa ? m1/2) of monolithic B 4C. Interfaces between CNF and B4C matrix were investigated using high-resolution TEM and Eels. Thin Al-B-C-O layer with a several nm order thickness was observed along with CNF, and Eels showed that the densification and mechanical properties of CNF/B4C nanocomposites might be related to this thin binding layer., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 55582 - 587, 01 Aug. 2008
  • Preparation of Ruddlesden-Popper type Can+1(Mn1-xvx)nO3n+1(n=2) powders and thermoelectric conversion properties of their sintered compacts
    Masashi Yoshinaga, Ken Hirota, Masaki Kato, Hideki Taguchi, Hiroyasu Kido
    Ruddlesden-Popper type compounds, being presented as (ABO3)n-AO or An+1BnO3n+1(n = 1, 2, 3. . . ∞), have a layer structure where perovskite layer ABO3and rock salt type layer AO are alternately in place; Can+1(Mn1-xVx)nO3n+1(0≦x≦ 1) belongs also to this compound. As CaO layer shows low thermal conductivity whereas CaMnO3layer reveals high electrical conductivity and high Seebeck coefficient, therefore, Can+1(Mn1-xVx)nO3n+1seems to become a promising module as a thermoelectric conversion material. However, there was no report concerning about the thermoelectric conversion characteristic of this compound. In this study we have investigated electrical conductivity, Seebeck coefficient, and thermal conductivity of Can+1(Mn1-xVx)nO3n+1and discussed about the possibility of the application as thermoelectric conversion elements from dimensionless performance index ZT., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 55497 - 501, 01 Jul. 2008
  • Metal-insulator transition in Pb(2-x)Ln(x)Ru(2)O(7-delta) (Ln = sm, eu) compounds
    M. Kato, Y. Tani, T. Imamura, K. Hirota, K. Yoshimura
    We have investigated the physical properties of Pb(2-x)Ln(x)Ru(2)O(7-delta) (Ln = Sm, Eu) compounds with pyrochlore crystal structure. The solid-solution compounds of Pb(2-x)Ln(x)Ru(2)O(7-delta) show from metallic (x = 0) to insulating (x = 2) behaviors. These compounds with x &gt; 1.0 also indicate the spin-glass like behaviors at low temperature (T &lt; T-SG) which are attributed to the geometrical frustration on pyrochlore lattice. The transition temperature TSG decreases monotonously with decreasing x and goes to zero at x = 0.6. The sample with 0.4 &lt;= x &lt;= 0.8 shows the metal-insulator (MI) transition at 10-30K where the sign of temperature derivative of electric resistivity changes. (C) 2007 Elsevier B.V. All rights reserved., PHYSICA B-CONDENSED MATTER, ELSEVIER SCIENCE BV, 403(5-9)1315 - 1317, Apr. 2008, English, Research paper (scientific journal)
  • Simultaneous synthesis and consolidation of W-added ZrB2 by pulsed electric current pressure sintering and their mechanical properties
    Ken Hirota, Takaya Endo, Masaki Kato, Shingo Nakane, Toshiyuki Nishimura, Yoshiaki Morisada, Kiyoshi Mizuuchi
    Dense zirconium boride (ZrB2)-based materials with and without tungsten (W) have been fabricated directly from mixtures of constituent elemental powders by pulsed electric current pressure sintering (PECPS) at 1800°C for 10 min under 30 MPa in a vacuum. Formation processes of monolithic, W-doped ZrB2 solid solutions (Zr1-xW x)B2 (0〈x?0.12), and composites consisting of ZrB2(Si) and WB2 were investigated. Their mechanical properties of Vickers hardness (Hv), fracture toughness (K IC), and bending strength (σb) at room temperature were evaluated. Solid solution and composite materials gave higher Hv (?20.7 GPa), KIC (?4.4 MPam1/2), and σb (?600 and 690 MPa for the former and the latter, respectively) than those (14.1 GPa, 3.21 MPam1/2, and ?500 MPa) of the monolithic ZrB2 fabricated under the same conditions. Furthermore, the latter two materials exhibited excellent high-temperature σb values (-550-600 MPa) up to 1600°C in N2, in comparison with that (-320 MPa) of monolithic ZrB2 materials., Materials Science Forum, 561-565527 - 530, 01 Dec. 2007
  • Fabrication of carbon nanofiber (CNF)/SiAlON composites by pulsed electric-current pressure sintering and their mechanical and electrical properties
    Ken Hirota, Tadateru Shibata, Masaki Kato, Toshiyuki Nishimura
    Carbon nanofibers (CNFs: ?150 nm φ and 4?5 μm 1) added SiAlON (Si 4Al 2O 2N 6) composites were fabricated directly from mixtures of Si 3N 4, Al 2O 3, AlN, and CNF by pulsed electric-current pressure sintering (PECPS) at 1700°C for 10 min under 30 MPa in a vacuum. Dispersion of CNFs up to ?5vol% into SiAlON matrix introduced the enhancement of densification and diminution of matrix grains, which resulted in improvement of mechanical properties, such as bending strength σ b (450 → 600 MPa) and fracture toughness K IC (4.1 → 4.5 MPa?m 1/2); however, Vickers hardness H v decreased monotonously from 17.7 to 12.2 GPa with increasing CNF content. In addition, homogeneous CNFs dispersion brought a drastic change in electric conductivity; an insulative SiAlON changed into a conductive material with improved mechanical properties by a small amount of CNFs (?3vol%). High-temperature σ b in N 2 was measured up to 1600°C; the monolithic ceramics gave higher σ b than that of 5vol%CNF/SiAlON composites. Copyright ? 2007 MS&amp;T&#039;07?., Materials Science and Technology Conference and Exhibition, MS and T'07 - "Exploring Structure, Processing, and Applications Across Multiple Materials Systems", 5309 - 320, 01 Dec. 2007
  • Fabrication and Characterization of Carbon Nanofiber (CNF)-dispersed SiAlON ceramics via Spark Plasma Sintering (SPS)
    Tadateru Shibata, Masaki Kato, Ken Hirota, Toshiyuki Nishimura, Kiyoshi Mizuuchi, Yoshiaki Morisada
    Carbon nanofibers (CNFs: ? 150nm# and 4 ? 5 μm 1) added SiAlON (Si4Al2O2N 6) composites were fabricated directly from mixtures of Si 3N4, Al2O3, AlN, and CNF by spark plasma sintering (SPS) at 1700°C for 10 min under 30 MPa in a vacuum. Dispersion of CNFs up to ? 5 vol% into SiAlON matrix introduced the enhancement of densification and diminution of matrix grains, resulted in improvement of mechanical properties, such as bending strength σb, (450 → 600 MPa) and fracture toughness KIC (4.1→4.5 MPa?m1/2); however, Vickers hardness H v decreased monotonously from 17.7 to 12.2 GPa with increasing CNF content. In addition, homogeneous CNFs dispersion brought a drastic change in electric conductivity; insulative SiAlON ceramics changed into conductive materials with improved mechanical properties by a small amount of CNFs with more than ? 3 vol%., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 54606 - 611, 01 Aug. 2007
  • Simultaneous synthesis and consolidation of W-added ZrB2 by Spark Plasma Sintering (SPS) and their mechanical properties
    Takaya Endo, Shingo Nakane, Masaki Kato, Ken Hirota, Toshiyuki Nishimura, Kiyoshi Mizuuchi, Yoshiaki Morisada
    Dense zirconium boride (ZrB2)-based materials with and without tungsten (W) have been fabricated directly from mixtures of constituent elemental powders (Zr, B and W) by spark plasma sintering (SPS) at 1800°C for 10 min under 30 MPa in a vacuum. Formation processes of monolithic, W-doped ZrB2 solid solutions ZrB2 (ss) [(Zr1-xW x)B2 (0 &lt;×? 0.12)], and composites consisting of ZrB2 (ss) and WB2 were investigated. Their mechanical properties of bending strength (σb), Vickers hardness (H v) and fracture toughness (KIC) at room temperature were evaluated. Solid solution and composite materials gave higher Hv (? 20 GPa), KIC (? 4.4MPam1/2), and σb (? 600 and 690 MPa for the former and the latter, respectively) than those (14.1 GPa, 3.21 MPam1/2, and ?500 MPa) of the monolithic ZrB2 ceramics fabricated under the same conditions., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 54601 - 605, 01 Aug. 2007
  • Effect of the Mn3+ ion on electrical and magnetic properties of orthorhombic perovskite-type Ca(Mn1-xTix)O3-delta
    Katsuyuki Nakade, Ken Hirota, Masaki Kato, Hideki Taguchi
    Orthorhombic perovskite-type Ca(Mn1-xTix)O3-delta (0 &lt;= x &lt;= 0.7) was synthesized at 1173 K for 12 h in a flow of oxygen from a precursor gel prepared using citric acid and ethylene glycol. The Mn3+ ion was generated by substituting a Ti4+ ion in CaMnO3. The average particle size was 100-300 nm and did not depend on x. The lattice constants and the (Mn, Ti)-O distance increased linearly with increasing x. The variation in global instability index (GII) indicated that the instability of the structure increases monotonically with increasing x. Ca(Mn1-xTix)O3-delta was an n-type semiconductor that had its minimum values of electrical resistivity (rho) and activation energy (E-a) at x = 0.1.Ca(Mn1-xTix)O3-delta(x = 0 and 0.1) exhibited a weak ferromagnetic behavior. The variation in mu(eff) indicated that the spin state of the Mn3+ ion changes from low to high at x = 0.1, then reverts to low in the range of 0.2 &lt;= x &lt;= 0.7. The variations in rho and E-a are explained by the number of electrons according to the change in the spin state of the Mn3+ ion. (C) 2006 Elsevier Ltd. All rights reserved., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 42(6)1069 - 1076, Jun. 2007, English, Research paper (scientific journal)
  • Fabrication of meta-stable monoclinic zirconia ceramics with a high density using a piston-cylinder type high-pressure apparatus
    Katsuya Takaoka, Ken Hirota, Masaki Kato, Osamu Yamaguchi, Satoshi Ohtaka
    Single phase m-ZrO 2 ceramics (? 13mm φ and 3-5mm t) have been fabricated using a piston-cylinder type high-pressure apparatus under 1GPa at 1023-1173K, in which a newly developed high-pressure cell has been utilized to introduce pseudo-isostatic pressure. Starting material, meta-stable m-ZrO 2 powder prepared via a sol-gel method, was characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), and transmission electron microscopy (TEM). Dense ceramics sintered at 1123K, free from macro-cracks, showed a high relative density of ? 99.5% and homogeneous microstructures consisting of fine grains (- 1μm). In-situ XRD analysis under high pressures and temperatures revealed that m- to t-ZrO 2 transition took place around 1123K under 1GPa. Mechanical properties of m-ZrO 2 ceramics have been evaluated; the maximum values of bending strength (σ b = 520MPa), Vickers hardness (H v = 9.8GPa), and fracture toughness (K IC = 3.9MPa?m 1/2) were achieved at the samples fabricated at 1123K., Zairyo/Journal of the Society of Materials Science, Japan, 55258 - 263, 01 Mar. 2006
  • Fabrication and corrosion resistance of hot-dip galvanized steel plates coated with silica-sol prepared from tetraethoxysilane
    Tetsuya Komeda, Koshi Takada, Yoshihiko Takano, Kazuki Sakai, Shigekazu Tanaka, Atsushi Kawada, Ken Hirota, Osamu Yamaguchi
    Silica-sol solution was prepared using tetraethoxysilane as a starting material. A silica-gel film was formed on both silica glass substrates and hot-dip galvanized steel plates by single-step dip coating and drying at room temperature. X-ray diffraction (XRD) analysis was performed on the films formed on the silica glasses and the steel plates; it revealed that the film formed on the former consisted of H 2Si 2O 5 and both Zn 2SiO 4 and ZnNO 3 (OH) ? H 2O were detected in the film on the latter. From these results, it was cleared that the as-prepared silica-sol itself was H 2Si 2O 5, however the silica-sol was changed to Zn 2SiO 4 when coated on the hot-dip galvanized steel plates resulting from a reaction with Zn on the surface of plates. The thickness of tripe-layered film formed by three-times dipping was estimated to be about 1.3μm by direct SEM observation. Using a salt spray test and a stack test, corrosion resistance of the hot-dip galvanized steel plates coated with the silica-sol was examined, in comparison with a chromate-coated and a naked hot-dip galvanized steel plates. The silica-sol coated plates exhibited superior corrosion resistance to both former and latter samples in each test. This might be explained in terms of the formation of Zn 2SiO 4 and ZnNO 3 (OH) ? H 2O. The coating materials, insoluble in water, can protect the steel plates from corrosive oxygen and electrolyte ions etc., Zairyo/Journal of the Society of Materials Science, Japan, 5578 - 82, 01 Jan. 2006
  • Effects of Mn3+ ions on the electrical and magnetic properties of Ca(Mn1-xZrx)O3-delta (0 <= x <= 0.07)
    H Taguchi, K Hirota, S Nishihara, S Morimoto, K Takaoka, M Yoshinaka, O Yamaguchi
    Perovskite-type Ca(Mn1-xZrx)O3-delta (0 &lt;= x &lt;= 0.07) has an orthorhombic GdFeO3-type structure with space group Pnma. According to the Rietveld method, the (Mn, Zr)-O distance is independent of x and the (Mn, Zr)-O-(Mn, Zr) angles decrease very slightly with x. The electrical measurement indicates that Ca(Mn1-xZrx)O3-delta is an n-type semiconductor and exhibits hopping conductivity in a small-polaron model. The magnetic measurement indicates that Ca(Mn1-xZrx)O3-delta is antiferromagnetic and that the spin state of the Mn3+ ion is low. The activation energy for hopping (E-h) has a minimum value (approximate to 0.043 eV) at x = 0.03 due to acceleration of the electron transfer through the Mn(III)-O-Mn(IV) path and the inhibition of the electron transfer by Zr4+ ions. (c) 2005 Elsevier B.V. All rights reserved., PHYSICA B-CONDENSED MATTER, ELSEVIER SCIENCE BV, 367(1-4)188 - 194, Oct. 2005, English, Research paper (scientific journal)
  • Simultaneous synthesis and sintering of alpha-Ti1-xAlx(N) (0 <= x <= 0.08) by a self-propagating high-temperature combustion method
    K Hirota, H Hayashi, M Yoshinaka, O Yamaguchi
    Simultaneous synthesis and sintering of hexagonal alpha-Ti1-xAlx(N) (0 less than or equal to x less than or equal to 0.08) solid solutions, which contain a small amount of nitrogen, have been performed by a self-propagating high-temperature combustion method under a nitrogen pressure of 4 MPa. Dense materials (similar to99% of theoretical) prepared directly from a mixture of elemental (Ti and Al) powders reveal homogeneous microstructure composed of fine grains (12-16 mum). alpha-Ti1-xAlx(N) (x = 0.02; Ti0.98Al0.02N0.26) exhibits a three-point bending strength or, of 390 MPa, a Vickers hardness H-v of 9.24 GPa, and a fracture toughness K-IC of 4.89 MPa(.)m(1/2); their mechanical properties are much improved by doping Al into alpha-Ti(N), in comparison with those (sigma(b) = 245 MPa, H-v = 9.02 GPa, and K-IC = 3.77 MPa(.)m(1/2)) of alpha-Ti(N) fabricated under the same conditions., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 87(11)2042 - 2046, Nov. 2004, English, Research paper (scientific journal)
  • Sol-gel-derived VxTi1-xO2 films and their photocatalytic activities under visible light irradiation
    K Iketani, RD Sun, M Toki, K Hirota, O Yamaguchi
    VchiTi1-chiO2 films with various V contents (chi = 0-0.1) were prepared on pyrex glass substrates by a sol-gel method. X-ray diffraction (XRD) measurements revealed that all the films crystallized in anatase after heat-treatment at 500degreesC for 1 h, and no V-derived peak was detected. V doping resulted in a slight shift of the anatase peak position to higher 20 value due to the substitution of part of the Ti sites by V. X-ray photoelectron spectroscopic (XPS) measurements showed that vanadium presented in the film mainly in the valence state of V4+ when X was below 0.075. The absorption spectrum of the VchiTi1-chiO2 film shifted to longer wavelengths when X increased. Thus-formed VchiTi1-chiO2 films were proved to be photocatalytically active not only under ultraviolet (UV) light but also under visible light irradiation, which was confirmed by the decomposition of methylene blue and gaseous acetaldehyde, as well as by the visible light induced change of water contact angle in the film surface. Highest activity was obtained in the film with a V content of 0.01. (C) 2003 Elsevier B.V. All rights reserved., MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, ELSEVIER SCIENCE SA, 108(3)187 - 193, May 2004, English, Research paper (scientific journal)
  • Morphology Change of Undoped and Sulfate-Ion-Doped Yttria Powders during Firing
    Ikegami Takayasu, Ikegami Takayasu, Ji Guang Li, Ji Guang Li, Isao Sakaguchi, Isao Sakaguchi, Ken Hirota, Ken Hirota
    Morphologic changes that occurred during firing in undoped and sulfate-ion-doped yttria powders were examined in the present study. Clear scanning electron microscopy (SEM) images of uncoated insulators were achieved and charging of electrons was avoided by observing small samples, throughout which most of the electrons of the incident beam penetrated. SEM observation and firing of the samples were repeated several times. Searching the observed areas or particles started at low magnification, with the aid of photographs taken earlier. The sulfate-ion dopant inhibited volume diffusion and/or grain-boundary diffusion, and then particle growth of the sulfate-ion-doped yttria proceeded by surface diffusion or evaporation-condensation along with pore growth, which re sulted in collapse of the agglomerates of primary particles. Although most of the other particles exhibited slight pore growth along with particle growth at temperatures as low as 800°C, a hardening of the agglomerated particles, because of pore elimination by volume diffusion and/or grain-boundary diffusion, occurred at temperatures &gt;850°C., Journal of the American Ceramic Society, 87517 - 519, 01 Jan. 2004
  • Simultaneous synthesis and sintering of chromium carbide (Cr7C3) powder by spark plasma sintering method
    Hiroyuki Motono, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Chromium carbide (Cr7C3) ceramics have been synthesized and sintered simultaneously from a mixture of Cr (? 2 μmφ) and C (? 5 μmφ) powders by spark plasma sintering method. Dense ceramics were obtained and are composed of single-phase Cr-C3with a small amount of Cr2O3, which originates from a trace amount of oxygen adsorbed on as-received starting powders. Cr7C3ceramics sintered at 1350°C have 99.5% relative density and fine structure with 2.5 μm grain size. They exhibit excellent mechanical properties: bending strength σb540 MPa, Vicker&#039;s hardness Hv16.9 GPa, and fracture toughness KIC5.3 MPa?m1/2., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 50372 - 376, 01 Jan. 2003
  • Hot isostatic pressing of CrB prepared by self-propagating high-temperature synthesis
    Yuji Hiroki, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    The synthesis of single-phase CrB was succeeded by self-propagating high-temperature synthesis (SHS) in an argon atmosphere from an elemental mixed powders (Cr: ? 2 μm and B: 0.2 μm) preheated at 300° to 600°C in a reduced pressure. The dense monolithic CrB ceramics (?98% of theoretical) were obtained by hot-isostatically pressing CrB powder for 2 h under 196 MPa at 1200° to 1500°C. They revealed excellent mechanical properties: Vicker&#039;s hardness Hv? 23.3 GPa, bending strength σb? 690 MPa and fracture toughness KIC? 6.6 MPa?m1/2., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 50367 - 371, 01 Jan. 2003
  • Crystal structure of zirconia prepared with alumina by coprecipitation
    Shinichi Kikkawa, Shinichi Kikkawa, Akio Kijima, Ken Hirota, Osamu Yamamoto
    Zirconia was prepared by firing the coprecipltate from ZrOCl 2 and AlCl 3 mixed aqueous solution with ammonia. When fired above 600°C, the products were fine crystalline tetragonal zirconia of crystallite size &lt; 10 nm. In previous studies, the tetragonal phase had been assumed to be a (Zr 1-x4+Al x3+)O 2-x/2 solid solution, where x ? 0.25. However, X-ray diffraction pattern simulation and Al K-edge XANES spectroscopy confirmed the present product to be a mixture of t-ZrO 2 fine powder with a small amount of δ-Al 2O 3 of very low crystallinlty, even below the expected compositional range of x ? 0.25 in the (Zr 1-x4+Al x3+)O 2-x/2 solid solution., Journal of the American Ceramic Society, 85721 - 23, 01 Mar. 2002
  • Fabrication of bulk chromium nitrides using self-propagating high-temperature synthesis and hot isostatic pressing
    Yoshihiro Shoji, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Cr2N, CrN and their mixtures with desired fractions were prepared by self-propagating high-temperature synthesis (SHS) under controlled nitrogen pressure and then followed by hot isostatic pressing at 1300 °C and 196 MPa under argon gas. The combustion temperature increased with increasing nitrogen pressure. Single-phase Cr2N and CrN were formed at 1040° under 0.18 MPa N2and 1730 °C under 2 MPa N2, respectively. The mechanical properties of chromium nitride ceramics with relative density of 99.2% were examined; Vickers hardness Hv(CrN: 11.2 GPa and Cr2N: 14.5 GPa) linearly increased with increasing fraction of Cr2N, whereas fracture toughness KIC(?4.7 MPa?m1/2) and bending strength σ (?355 MPa) were constant regardless of the fraction of Cr2N/CrN., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 49323 - 326, 01 Jan. 2002
  • Characterization and sintering of LaAlO3powder prepared by the hydrazine method
    Ken Hirota, Kazuya Iketani, Masaru Yoshinaka, Osamu Yamaguchi
    Lanthanum aluminum oxide LaAlO3is formed at low temperatures (?870°C) from an amorphous material prepared by the hydrazine method. Powder characterization has been conducted using XRD, DTA/TG, TEM, and BET method; LaAlO3powders heated at 900°-1500°C revealed that they consisted of submicrometer-sized translucent particles. They were sintered at 1800°C for 2 h in air, dense ceramics with a 96.4% of theoretical have been obtained from the powder calcined at 1200°C. They exhibit the microstructure composed of well-developed grains (?140 μm) with intergranular and intragranular pores of ?13 and ? 7 μmm, respectively. Electric property has been evaluated; their dielectric constant at room temperature is 36.5 ± 1., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 49576 - 580, 01 Jan. 2002
  • Formation and electrical conductivity of air-sinterable Nd(Cr1 - xMgx)O3
    Yoshiyuki Kunifusa, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    In Mg2+-substituted NdCrO3, single-phase perovskite compounds Nd(Cr1 - xMgx)O3, where x=0 to 0.25, have been formed at low temperatures by decomposition of Nd(Cr1 - xMgx)O4, which crystallize from amorphous materials prepared by the hydrazine method. Nd(Cr1 - xMgx)O3powders consisting of submicrometer-size particles are highly sinterable; dense materials can be fabricated by sintering for 4 h at 1800 °C under atmospheric pressure. The relative densities, grain sizes, and electrical conductivities increase with increased Mg2+content. Nd(Cr0.75Mg0.25)O3materials show an excellent electrical conductivity of 1.4 × 103S m-1at 1000 °C. ? 2002 Published by Elsevier Science B.V., Solid State Ionics, 149107 - 113, 01 Jan. 2002
  • Characterization and sintering of Y3Al5O12(YAG) powder prepared by the hydrazine method
    Koki Fujii, Masaru Yoshinaka, Ken Hirota, Takayasu Ikegami, Osamu Yamaguchi
    A YAlO3(hexagonal) solid solution with a = 0.3678 nm and c = 1.046 nm crystallizes at 880° to 935°C from an amorphous material prepared in the molar ratio Y3+/Al3+= 3: 5 by a solution technique using hydrazine. The hexagonal YAlO3phase transforms to Y3Al5On12(YAG) at 1005° to 1075 °C. The YAG powder calcined at 1200°C can be densified to transparency in a vacuum for 2 h at 1600°C without additives., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 49486 - 490, 01 Jan. 2002
  • Formation, powder characterization and sintering of YCrO3 prepared by a sol-gel technique using hydrazine
    Tokumatsu Tachiwaki, Yoshiyuki Kunifusa, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Orthorhombic YCrO3is formed at low temperatures (750-830°C) by decomposition of YCrO4which crystallizes between 490 and 530°C from an amorphous material prepared by a sol-gel technique using hydrazine. Powder characteristics have been examined. Specific surface area, crystallite size and particle size are strongly dependent on heating temperature. YCrO3powders, which consist of aggregates, indicate necklace-like morphology. Ceramics with 94.2% of theoretical density can be fabricated by sintering for 4 h at 1800°C in air. They show an electrical conductivity of 61 S/m at 1000°C. ? 2001 Elsevier Science Ltd., International Journal of Inorganic Materials, 3107 - 111, 28 Apr. 2001
  • Fabrication of Super-Sendust/Mg-ferrite composite materials by spark plasma sintering and their magnetic and electrical properties
    K. Hirota, M. Yoshinaka, O. Yamaguchi
    Dense composite materials (98.6% of theoretical) consisting of Super-Sendust (86.5Fe-6Si-4Al-3.5Niwt%) and MgFe2O4(75/25 vol%), which have been fabricated by spark plasma sintering for 5 min at 850°C and 130 MPa, exhibit a high saturation magnetization density Bsof 1.2 T and a high electrical resistivity ρ of 1 × 10-2Ω ? m. Permeabilities μ&gt; 1000 at 1kHz are obtained. Fine ferrite grains (?0.6 μm) are located at the boundaries of the large spherical grains of Super-Sendust (? 30 μm)., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 48174 - 176, 01 Jan. 2001
  • Toughening and strengthening of NiAl with Al2O3 by the addition of ZrO2(3Y)
    Atsushi Kitaoka, Ken Hirota, Masaru Yoshinaka, Yoshinari Miyamoto, Osamu Yamaguchi
    NiAl/10-mol%-ZrO2(3Y) composites of almost full density have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The former intermetallic compound, which contains a trace amount of Al2O3, has been prepared via self-propagating high-temperature synthesis. The composite microstructures are such that tetragonal ZrO2 (approx. 0.2 μm) and Al2O3 (approx. 0.5 μm) particles are located at the grain boundaries of the NiAl (approx. 46 μm) matrix. Improved mechanical properties are obtained: the fracture toughness and bending strength are 8.8 MPa?m1/2 and 1045 MPa, respectively, and high strength (&gt;800 MPa) can be retained up to 800°C., Journal of the American Ceramic Society, 831311 - 1313, 01 Jan. 2000
  • Microstructure and mechanical properties of hot isostatically pressed Cr2O3/5 vol% Mo composites
    Ken Hirota, Yoshihiko Takano, Masaru Yoshinaka, Osamu Yamaguchi
    Dense Cr2O3/5 vol% Mo composites (99.3% of theoretical) consisting of fine Cr2O3matrix grains (approximately 0.35 μm) have been fabricated by hot isostatic pressing (1200 °C/2 h/196 MPa) using a mixture of sol-gel-derived Cr2O3and commercially available Mo powders. Nanoscale Mo particles are dispersed either in the boundaries of grains or in the matrix, while submicrometer-sized particles are located in the grain boundaries. They are a strengthening and toughening agent. The strength and toughness of the composites are 620 MPa and 4.6 MPa?m1/2, respectively., Materials Research Bulletin, 352217 - 2223, 01 Jan. 2000
  • New composite material with high saturation magnetization density and high electrical resistivity
    Ken Hirota, Yoshihiko Takano, Masaru Yoshinaka, Osamu Yamaguchi
    Dense composite materials (98.6% of theoretical) consisting of Super Sendust (86.5Fe-6Si-4Al-3.5Ni wt%) and MgFe2O4(75/25 vol%), which were fabricated by spark plasma sintering for 5 min at 850 °C and 130 MPa, exhibit a high saturation magnetization density Bsof 1.2 T and high electrical resistivity ρ of 1×10-2Ω?m. Permeabilities μ&gt;1000 at 1 kHz were obtained. Fine ferrite grains (approximately 0.6 μm) were located at the boundaries of the large spherical grains of Super Sendust (approximately 30 μm)., Materials Research Bulletin, 351137 - 1141, 01 Jan. 2000
  • Processing and mechanical behavior of CrN/ZrO2(2Y) composites
    Yoshihiko Takano, Masahiro Hachiya, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    CrN powder consisting of granular particles of approx. 3 μm has been prepared by self-propagating high-temperature synthesis under a nitrogen pressure of 12 MPa using Cr metal. Dense pure CrN ceramics and CrN/ZrO2(2Y) composites in the CrN-rich region have been fabricated by hot isostatic pressing for 2 h at 1300°C and 196 MPa. The former ceramics have a fracture toughness (KIC) of 3.3 MPa?m 1/2 and a bending strength (σb) of 400 MPa. In the latter materials almost all of the ZrO2(2Y) grains (0.36-0.41 μm) are located in the grain boundaries of CrN (approx. 4.6 μm). The values of KIC (6.1 MPa ? m 1/2 ) and σb (1070 MPa) are obtained in the composites containing 50 vol% ZrO2(2Y)., Journal of the American Ceramic Society, 83448 - 450, 01 Jan. 2000
  • Fabrication of structural materials by the combination of self-propagating high-temperature synthesis (SHS) and high-pressure technology
    K. Hirota, S. Nakane, M. Yoshinaka, O. Yamaguchi
    Self-propagating high-temperature synthesis (SHS) is currently developed for the production of intermetallic compounds and ceramic powders. SHS has some features, i.e., a very short synthetic time, low energy consumption, high productivity, and furthermore synthesizing novel materials which cannot be prepared by solid state reaction and powder metallurgy. This paper reviews recent research trends and progress related to the structural materials fabricated by the combination of SHS and high-pressure technology. Especially, the characteristics of dense sintered composite materials by spark plasma sintering (SPS) are described, focusing on their microstructures and mechanical properties., Review of High Pressure Science and Technology/Koatsuryoku No Kagaku To Gijutsu, 10319 - 325, 01 Jan. 2000
  • Giant magnetism in Fe metal/AlN multilayered thin film prepared by rf-sputter deposition
    Shinichi Kikkawa, M. Fujiki, S. Enomoto, K. Hirota, O. Yamaguchi
    Magnetic measurements revealed a possibility for a giant magnetization of rf-sputter deposited Fe/AlN multilayers. The [Fe(178.2 nm)/AlN(169.4 nm)]3 showed the largest enhancement in about 40% of the saturation magnetization among the multilayers deposited at an applied rf-power of 30 W. The magnetization reached a value of 233 emu/g on [Fe(500 nm)/AlN(500 nm)]1 prepared at the applied rf-power of 100W. The enhanced magnetization was assumed to be due to a formation of the iron nitrides at the multilayered interface. It strongly depends on the nitrogen content in the iron nitrides. The nitrogen was supplied from the AlN layer and was also released by spontaneous substrate heating during the sputter deposition process., Materials Science Forum, 325111 - 116, 01 Jan. 2000
  • Hot isostatic pressing of reactive SnO 2 powder
    Masaru Yoshinaka, Ken Hirota, Ken Hirota, Masayuki Ito, Hiroshi Takano, Osamu Yamaguchi, Osamu Yamaguchi
    Tin(IV) oxide (SnO 2) crystallizes at room temperature by adding hydrazine monohydrate ((NH 2) 2?H 2O) to a hydrochloric acid solution of tin, followed by washing and drying. Well-densified SnO 2 ceramics (99.8% of theoretical) with an average grain size of 0.9 μm have been fabricated by hot isostatic pressing for 2 h at 900°C and 196 MPa. Their Vickers hardness and bending strength are 14.4 GPa and 200 MPa, respectively. They exhibit an electrical conductivity of 2 × 10 -3-9 × 10 -3 S?cm -1 at room temperature., Journal of the American Ceramic Society, 82216 - 218, 01 Dec. 1999
  • Simultaneous synthesis and densification of TiB/α-Ti(N) composite material by self-propagating combustion under nitrogen pressure
    S. Nakane, O. Yamada, Y. Miyamoto, M. Yoshinaka, K. Hirota, O. Yamaguchi
    Simultaneous synthesis and densification of TiB/α-Ti(N) composites from Ti (70 mol%)/B (30 mol%) powder compacts are achieved by self-propagating combustion under a nitrogen pressure of 5 MPa. Dense composite materials (97.5% of theoretical) exhibit excellent mechanical properties, in which their fracture toughness and bending strength are 7.8 MPa m1/2and 640 MPa, respectively. Rodlike TiB grains greatly enhance the mechanical properties., Solid State Communications, 110(110)447 - 50, 29 Apr. 1999
  • Electric and magnetic properties of low-temperature sintering Mn-Zn ferrite by addition of lithium borosilicate glass
    Tatsuji Aoyama, Sinsuke Enomoto, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    An attempt was made to improve the electric and magnetic properties of low-temperature sintering Mn-Zn ferrites by adding lithium borosilicate glass. Five specimens, denoted A through E, were used in this study. Phases were identified by X-ray diffraction (XRD) analysis. The results indicate that both the magnetic and electric properties of the Mn-Zn ferrites are significantly improved by the addition of lithium borosilicate glass., Journal of Materials Science Letters, 18(18)497 - 99, 01 Mar. 1999
  • Simultaneous synthesis and sintering of α-Ti(N) by self-propagating high-temperature combustion under nitrogen pressure
    O. Yamada, M. Hachiya, S. Nakane, M. Yoshinaka, K. Hirota, O. Yamaguchi
    Dense α-Ti(N) (TiNx(0&lt; × ? 0.3), hexagonal) materials were fabricated from titanium powder compacts under high nitrogen pressure. The resulting materials were examined from a viewpoint of nitrogen contents. Using X-ray diffraction (XRD), the phases of the products were classified into six groups. The results suggest that higher-order titanium nitrides were obtained with increasing nitrogen pressure and decreasing compacting pressure. In the products formed in each region, only α-Ti(N) was produced as a dense material., Journal of Materials Science Letters, 18(18)363 - 65, 01 Mar. 1999
  • Low temperature preparation of transparent conducting material in ZnO-In2O3system by homogeneous aqueous solution method
    Shinichi Kikkawa, Hiroshi Kanasaki, Masao Takahashi, Kalyan Adhikary, Ken Hirota, Osamu Yamaguchi
    Formation of both (Zn1-xInx)O1+x/2solid solution and (ZnO)mIn2O3compound was investigated by firing the precipitates through mixed aqueous solution of Zn and In nitrates. The solid solution range was enlarged to 15% by its gel formation adding citric acid. The homologous (ZnO)mIn2O3compound (m&gt;12) with very long interlayer distance precipitated around 650°C by heating the homogeneous solid solution. It gradually decomposed to the compounds with m&lt;12 and finally crystallized as a compound with the corresponding starting m-composition. Its thin film was prepared by dip coating method using diethylene glycol as a solvent. The film fired at 600°C was transparent in a wavelength range longer than 400nm. Its electrical resistivity gradually decreased to ca. 0.01 Ωcm with the amount of doped In., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 46189 - 193, 01 Jan. 1999
  • One-step sintering of thermoelectric conversion units in the W/TiB2/SiGe and W/MoSi2/SiGe systems
    J. S. Lin, Y. Miyamoto, K. Shibata, K. Hirota, O. Yamaguchi
    Dense thermoelectric conversion units in the systems of the W/TiB2/SiGe and W/MoSi2/SiGe were fabricated in one step by using glass encapsulation HIP. In order to relax the thermal residual stress and prohibit the chemical reaction between the W and SiGe, the electrode was designed as a graded structure through TiB2 and MoSi2 interlayers. The electrical resistivity, and their thermal stability were evaluated. The results showed that the SiGe kept the same electrical resistivity as its initial value (about 1.5 approx. 2.0×10-3Ω? cm). In the zone of electrode, the electrical resistivity decreased to about 10-4Ω? cm. The measurements of electrical resistivity suggested that the combinations of the W/TiB2/SiGe for p-type, W/MoSi2/SiGe for n-type thermoelectric conversion units were suitable for obtaining optimum and thermally stable electrical resistivity profiles., Materials Science Forum, 308-311760 - 765, 01 Jan. 1999
  • Fabrication and mechanical properties of titanium boride ceramics
    Shingo Nakane, Yoshihiko Takano, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Dense TiB ceramics (99.6% of theoretical) with a grain size of approx. 5 μm have been fabricated by reaction hot pressing of TiB2 and titanium for 2 h at 1900°C and 28.5 MPa. The TiB ceramics exhibit a fracture toughness (KIC) of 4.5 MPa?m 1/2 and a bending strength (σb) of 360 MPa. Electrical resistivity (ρ) is 3.4 × 10-7 Ω?m at room temperature., Journal of the American Ceramic Society, 821627 - 1628, 01 Jan. 1999
  • Simultaneous Synthesis and Densification of α-Zr (n) by Self Propagating Combustion under Nitrogen Pressure
    Osamu Yamada, Yoshinari Miyamoto, Ken Hirota, Shingo Nakane, Masaru Yoshinaka, Osamu Yamaguchi
    Simultaneous synthesis and densification of α-Zr (N) (hexagonal) from zirconium powder compacts are achieved by self-propagating combustion under high nitrogen pressures. Nitrogen contents increase from 12.2 to 23.5 at% with decreasing compacting pressure and increasing nitrogen pressure. The material with the highest density (99% of theoretical) exhibits a Vickers hard ness (Hv) of ~6.2 GPa and a bending strength (σb) of ~205 MPa. Electrical resistivity (ρ) is determined to be ~2.0×10-6Ω?m at room temperature. ? 1999, The Society of Materials Engineering for Resources of Japan. All rights reserved., International Journal of the Society of Materials Engineering for Resources, 7(1)188 - 94, 01 Jan. 1999
  • Microstructure and mechanical properties of ZrO2(2Y)-toughened Al2O3ceramics fabricated by spark plasma sintering
    Yoshihiko Takano, Tomohito Ozawa, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Spark plasma sintering (SPS) has been performed for 5 min at 1500 °C and 30 MPa using sub-micrometer-sized Al2O3/ZrO2(2Y) composite powders in the Al2O3-rich region. Dense ZrO2-toughened Al2O3(ZTA) ceramics show excellent mechanical strength; the strength of 1620 MPa is achieved in the ZTA with 50 mol% ZrO2. The grain size of Al2O3in ZTA decreases from 1.5 to 0.6 μm with increased ZrO2content. Almost all the ZrO2grains (approximately 0.3 μm) are located in the boundaries of the Al2O3grains. Mechanical properties are discussed, with an emphasis on the relation between t-/m-ZrO2ratios and microstructures of ZTA., Journal of Materials Synthesis and Processing, 7107 - 111, 01 Jan. 1999
  • Microstructures and magnetic and electric properties of low-temperature sintering Mn-Zn ferrites without and with addition of lithium borosilicate glass
    Ken Hirota, Tatsuji Aoyama, Sinsuke Enomoto, Masaru Yoshinaka, Osamu Yamaguchi
    Highly sinterable Mn-Zn ferrite powder consisting of submicrometer-size particles (approx. 0.18 μm) with structural distortion has been prepared by grinding a coprecipitated powder with a media agitation mill. Dense pure Mn-Zn ferrites (&gt;98% of theoretical) can be fabricated by heating at 900°C in a nitrogen atmosphere. They show magnetic and electric properties in which their permeability μ and electric resistivity ρ are 520-635 and 0.8-1.6 Ω m, respectively. These properties have been further improved by adding small amounts of lithium borosilicate glass; especially, high values of μ = 1015 and ρ = 474 Ω m are achieved in ferrites (900°C/12 h) with 0.1 wt% glass addition. The microstructures consist of large grains (approx. 35 μm) with high resistive layers at grain boundaries., Journal of Magnetism and Magnetic Materials, 205283 - 289, 01 Jan. 1999
  • Fabrication and mechanical properties of Al2O3-BN/TiC/Ni FGMs
    Koji Higuchi, Junshan Lin, Yoshinari Miyamoto, Ken Hirota, Osamu Yamaguchi, Kazunori Umeda, Akihiro Tanaka, Masaaki Yamamoto, Ryohei Tanaka
    Symmetrically graded materials of the Al2O3-BN/TiC/Ni were fabricated by SHS/HIP. High residual compressive stresses were produced in the outer layer due to the thermal expansion mismatch between the outer Al2O3-BN ceramic and the inner TiC/Ni cermet layer. The dispersion of BN in the Al2O3layer increased the residual stress to higher levels than that induced in the Al2O3/TiC/Ni system. The BN has the layer structure with cleavage faces. The tribological properties of the outer Al2O3-BN layer were investigated at room temperature and 500 °C in air. The friction coefficient of the outer layer was lower than that of monolithic Al2O3, especially at 500 °C. The residual compressive stress was found to improve wear resistance of the surface layer., Zairyo/Journal of the Society of Materials Science, Japan, 48541 - 545, 01 Jan. 1999
  • Novel synthesis of mullite powder with high surface area
    Masayuki Imose, Yoshihiko Takano, Masaru Yoshinaka, Ken Hirota, Ken Hirota, Osamu Yamaguchi, Osamu Yamaguchi
    Boehmite (AlO(OH)) solid-solution gel, which yields stoichiometric mullite (3Al 2O 3?2SiO 2) at high temperatures, has been prepared by the hydrazine method. The formation process leading to 3Al 2O 3?2SiO 2 is discussed. The as-prepared powder and powders heated below 1200°C consist of very fine particles showing needlelike morphology, whereas the particles of mullite powder show thin prismatic morphology. The mullite powder after heating at 1300°C has a high surface area (87 m 2/g)., Journal of the American Ceramic Society, 81(6)1537 - 40, 01 Jun. 1998
  • Fabrication of calcite (CaCO3) ceramics with high density
    Kazuto Honma, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Journal of Materials Science Letters, 17745 - 746, 01 Jan. 1998
  • Hot Isostatic Pressing of Submicron La2Zr2O7 Powder Prepared by the Hydrazine Method
    Y. Matsumura, M. Yoshinaka, K. Hirota, O. Yamaguchi
    In the ZrO2-La2O3 system, metastable c-ZrO2 solid solutions(ss) containing up to ?27 mol% La2O3 crystallize at low temperatures from amorphous materials prepared by the hydrazine method The formation of La2Zr2O7 proceeds via two reaction processes; (i) decomposition of c-ZrO2(ss) (c-ZrO2(ss) → La2Zr2O7 + t-ZrO2) above 1100°C and (ii) a solid-state reaction of t-ZrO2 and La2O3 at 1200° to 1300°C. Submicron La2Zr2O7 powder is produced at 1300°C. Dense La2Zr2O7 ceramics (99.5% of theoretical) with an average grain size of 10 have been fabricated by hot isostatic pressing (1500°C/2h/196MPa). Their fracture toughness and bending strength are 1.9 MPa m1/2 and 172 MPa, respectively. They exhibit an electrical conductivity of 1.5x10−1 S m−1 at l000°C. ? 1998, The Japan Society of High Pressure Science and Technology. All rights reserved., Review of High Pressure Science and Technology/Koatsuryoku No Kagaku To Gijutsu, 71063 - 1065, 01 Jan. 1998
  • Fabrication and mechanical properties of continuously graded WSi2-ZrO2(2Y) materials using wet-molding
    K. Arata, M. Yoshinaka, K. Hirota, O. Yamaguchi
    Continuously graded WSi2-ZrO2(2Y) materials with high density (99.2% of theoretical) have been fabricated by uniaxial wet-molding, followed by hot pressing (1000°C/1h/30MPa) and hot isostatic pressing (1500°C/2h/196MPa). A mixed solution of glycerin and ethanol with the viscosity of ?6 mPa?s has been used as a dispersion medium in the molding process. The sintered materials show a linear composition profile with a transition from WSi2/ ZrO2(2Y) ?5/25 to ?5/95 mol%. Vickers hardness, Hv, for which the value is ?11.2 GPa, is almost independent of the compositional change. Fracture toughness, KC, increases from 5.6 to 10.3 MPa?m1/2with increasing ZrO2(2Y) Composition. ? 1998 Elsevier Science Ltd., Materials Research Bulletin, 33627 - 633, 01 Jan. 1998
  • Hot Isostatic Pressing of SnO2Prepared by the Hydrazine Method
    Noriyuki Maeda, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Reactive SnO2powders have been prepared by the hydrazine method. Their individual particles tend toward a hexagonal morphology with increasing temperature. Well-densified SnO2ceramics (99.8% of theoretical) with an average grain size of 0.9μm have been fabricated by hot isostatic pressing for 2 h at 900°C and 196 MPa. Their Vickers hardness and bending strength are ?14.4 GPa and ?200 MPa, respectively. They exhibit an electrical conductivity of ?5 × 10-3S?cm-1at room temperature., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 451157 - 1161, 01 Jan. 1998
  • Preparation Conditions of rf-Sputter Deposited Fe/AlN Multilayered Thin Films and Their Properties
    Shinichi Kikkawa, Shinsuke Enomoto, Masao Takahashi, Ken Hirota, Osamu Yamaguchi
    Formation of iron nitride was observed on Fe metal film rf-sputter deposited on polycrystalline AlN substrate as already reported on Fe metal/amorphous AlN multilayer. It was not found on the AlN thin film deposited on Fe metal substrate. These results suggest that Fe reacts with even crystallized AlN at their interface but that the iron nitride was not formed by nitrogen plasma during a reaction sputter deposition of AlN thin film. Multilayered Fe/AlN thin films were also prepared by rf-sputter deposition changing applied rf-power at 30W and 100W. Giant magnetization component was observed at the multilayered interface because of the iron nitride formed by a solid state reaction between Fe and AlN. Small amount of nitrogen in AlN was assumed to be supplied to α-Fe forming a gradient of nitrogen content at the interface., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 45253 - 256, 01 Jan. 1998
  • Hot Isostatic Pressing of Y2O3-Doped La2Zr2O7Prepared by the Hydrazine Method
    Yoshihiko Takano, Yoshinori Matsumura, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Pyrochlore La2Zr2O7containing 8 mol% Y2O3crystallizes at 825° to 885°C from an amorphous material prepared by the hydrazine method. The lattice parameter is a =1.0751 nm; this value is smaller than that (a =1.0793 nm) of pure La2Zr2O7. Dense Y2O3-doped La2Zr2O7ceramics (99.7% of theoretical) with an average grain size of 3.8 μm have been fabricated by hot isostatic pressing for 2 h at 1500°C and 196 MPa. They exhibit electrical conductivity of 1.6 S?m-1at 1000°C. Activation energies are 0.82 and 0.58 eV for initial (400°-540°C) and final (540°-1000°C) stages, respectively., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 45586 - 589, 01 Jan. 1998
  • Formation and sintering of LaCrO3prepared by the hydrazine method
    Kouichi Azegami, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Sinterable LaCrO3powder consisting of submicron particles (?0.2 μm) has been obtained at low temperatures (?850°C) by decomposition of LaCrO4, which crystallizes at 495°C, from an amorphous material prepared by the hydrazine method. Powder characterization is examined. Dense LaCrO3ceramics (93.9% of theoretical) with small grains (?2.1 μm) can be fabricated by sintering for 2 h at 1600°C without any control of oxygen pressure. They show a high electrical conductivity of 108 S?m-1at 1000°C. ? 1998 Elsevier Science Ltd., Materials Research Bulletin, 33341 - 348, 01 Jan. 1998
  • Formation and characterization of stoichiometric mullite (3Al(2)O(3)center dot 2SiO(2)) prepared by the hydrazine method
    M Imose, M Yoshinaka, K Hirota, O Yamaguchi
    MATERIALS SCIENCE RESEARCH INTERNATIONAL, SOC MATERIALS SCIENCE, JAPAN, 3(4)258 - 259, Dec. 1997, English, Research paper (scientific journal)
  • Fabrication of grain-oriented MoO3ceramics using wet-moulding process
    K. Honma, K. Hirota, O. Yamaguchi, J. Asai, Y. Makiyama
    Journal of Materials Science Letters, 161195 - 1197, 01 Dec. 1997
  • Characterization and Sintering of Alkoxy-Derived Vanadium Pentaoxide
    Kazuto Honma, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi, Junji Asai, Yukio Makiyama
    V2O5gels corresponding to the formula V2O5?nH2O with n=2.07, 0.5, and 0.2 have been prepared by the hydrolysis of VO (OC2H5) 3, followed by washing and drying. After dehydration, V2O5crystallizes at 310° to 400°C. V2O5powders with striplike particles are produced after heating at 630°C. Well-densified V2O5ceramics (97.7% of theoretical) have been fabricated by the combined use of hot pressing (630°C/2h/30MPa) and hot isostatic pressing (630°C/1h/196 MPa). The texture is of a plate structure, the grain being ? 30 μm long and ? 6 μm wide. Electrical conductivities have been measured in the temperature range of 25° to 600°C. Activation energies are determined to be 0.09 and 0.18 e V for initial and final stages, respectively. ? 1997, The Society of Materials Engineering for Resources of Japan. All rights reserved., International Journal of the Society of Materials Engineering for Resources, 5(1)9 - 14, 01 Jan. 1997
  • Formation and sintering of La2Zr2O7by the hydrazine method
    Yoshinori Matsumura, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    In the ZrO2-La2O3system, metastable c-ZrO2solid solutions (ss) containing up to ? 27 mol% La2O3crystallize at low temperatures from amorphous materials prepared by the hydrazine method. The lattice parameter a increases linearly from 0.5152 to 0.5350 nm. The formation of pyrochlore La2Zr2O7proceeds via two reaction processes: (1) decomposition of C-ZrO2(ss) (c-ZrO2(ss) → La2Zr2O7+ t-ZrO2) and (2) solid-state reaction of t-ZrO2and La2O3. Dense La2Zr2O7ceramics (99.5% of theoretical) with an average grain size of 10 μm have been fabricated by hot isostatic pressing for 2 h at 1500°C and 196 MPa. Their fracture toughness and bending strength are 1.9MPa m1/2and 172 MPa, respectively. They exhibit a high electrical conductivity of 1.5 × 10-1S m-1at 1000°C. ? 1997 Elsevier Science Ltd., Solid State Communications, 104341 - 345, 01 Jan. 1997
  • Formation and characterization of stoichiometric mullite (3Al2O3? 2SiO2) prepared by the hydrazine method
    Masayuki Imose, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Zairyo/Journal of the Society of Materials Science, Japan, 46258 - 259, 01 Jan. 1997
  • Electrical and mechanical properties of ZrO2(2Y)/TiN composites and laminates made from these materials
    Kenro Shibata, Rikiya Sato, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    High density composites with the compositions of ZrO2(2Y):TiN = 40:60 and 70:30 mol % have been fabricated by hot isostatic pressing for 2 h at 1500°C and 196MPa. The electrical resistivities (p) of the two composites are very different; showing metallic behaviour in the first case and insulating behaviour in the latter case. These properties are highly dependent on the sample texture. Laminated materials with compositions of ZrO2(2Y)/TiN = (40:60)/(70:30)/ (40:60) mol % have been prepared by hot isostatic pressing. The electrical resistivities in the perpendicular and parallel to the interface directions have been determined to be ρ⊥ ? 1 × 109and ρ‖, ? 1 × 10-6Ω m, respectively. A residual stress of as much as ? 150 MPa is induced in the interfaces. The fracture toughness is greatly affected by the residual stress., Journal of Materials Science, 32583 - 587, 01 Jan. 1997
  • Formation and sintering of 8 mol % Y2O3-substituted La2Zr2O7by the hydrazine method
    Akihiro Ota, Yoshinori Matsumura, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Journal of Materials Science Letters, 17199 - 201, 01 Jan. 1997
  • Fabrication and mechanical properties of Cr2O3solid solution ceramics in the system Cr2O3-Al2O3
    K. Shibata, M. Yoshinaka, K. Hirota, O. Yamaguchi
    In the system Cr2O3-Al2O3, Cr2O3solid solution (ss) powders containing up to 25 mol% Al2O3have been prepared by the hydrazine method. Dense Cr2O3(ss) ceramics (99-99.2% of theoretical) have been fabricated by HIPing (1500°C/2h/196MPa) of the compacts sealed under reduced pressure. Their mechanical data are discussed, in comparison with those of Al2O3(ss) ceramics in this system. Microhardness (23.4 GPa) and fracture toughness (3.9 MPa?m1/2) are not dependent on the compositions of Cr2O3(ss) ceramics. The former value is higher than that of much improved Al2O3(Al2O3(ss)) ceramics. Bending strength increases linearly with increasing Al2O3content; the maximum value of 380 MPa is achieved in the ceramics containing 25 mol% Al2O3. Copyright ? 1997 Elsevier Science Ltd., Materials Research Bulletin, 32627 - 632, 01 Jan. 1997
  • Fabrication of functionally graded MoSi2/ZrO2 (2 mol% Y2O3) materials by wet-molding process
    Kenji Arata, Nobutaka Takeuchi, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Functionally graded MoSi2/ZrO2(2Y) materials with high density (97.5% of theoretical) have been fabricated by uniaxial wet-molding, followed by hot pressing (1000°C/1h/30MPa) and hot isostatic pressing (1400°C/2h/ 196MPa). Their composition profiles are much influenced by the viscosity of the mixed solutions of glycerin and ethanol used as dispersion media; a linear compositional gradient from MoSi2/ZrO2(2Y)?70/30 to ?20/80 mol% is obtained from the solution (50/50 vol%) with the viscosity of 20 mPa ? 7s. Vickers hardness Hv and fracture toughness KIc increase from 9.7 to 12.4 GPa and 5.1 to 12.5 MPa ? m1/2, respectively, with increasing ZrO2(2Y) composition., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 44565 - 569, 01 Jan. 1997
  • Fabrication of grain-oriented Cr2O3ceramics
    Tetsuya Komeda, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    CrO(OH) powders with thin hexagonal plate particles have been prepared by the hydrothermal treatment (380°C/2 h/15.2 MPa) of sol-gel-derived amorphous Cr2O3powders. Densification has been performed by hot-pressing (1350°C/2 h/30 MPa), followed by pressureless sintering (1600°C/ 2 h) in carbon powders. Grain-oriented Cr2O3ceramics (&gt;98% of theoretical) are fabricated, in which the 〈0 0 1〉 axis is parallel to the hot pressing direction. They exhibit an anisotropy in electrical conductivity; the values measured at 700°C in the perpendicular and parallel directions for the 〈0 0 1〉 axis are 9.36 and 1.09 S m-1, respectively. ? 1997 Elsevier Science Ltd. All rights reserved., Solid State Communications, 101855 - 859, 01 Jan. 1997
  • Electrical and mechanical properties of TiC/ZrO2(2Y) composites and their three-layer laminated materials
    Yoshiyuki Muraoka, Akihiro Ota, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Well-densified composites with compositions of TiC/ZrO2(2Y) = 50/50 and 20/80 mol% were fabricated by hot isostatic pressing for 2 h at 1500°C under 196 MPa. Electric resistivities (ρ) between both composites change drastically; the former and latter composites correspond to the metallic and insulating materials, respectively. They are strongly dependent on the textures. Three-layer laminated materials with compositions of TiC/ZrO2(2Y) = (50/50)/(20/80)/(50/50) mol% were also prepared by the same technique. Overall electric resistivities in the perpendicular and parallel directions for the interfaces are determined to be ρ⊥ ? 1 × 1010 and ρ‖ ? 1 × 10-5 Ω ?m, respectively. Residual stress of as much as ? 320 MPa is induced in the interfaces. The fracture toughness (KIC) is much affected by the residual stress. ? 1996 Plenum Publishing Corporation., Journal of Materials Synthesis and Processing, 4397 - 403, 01 Nov. 1996
  • Characterization and low-temperature sintering of reactive Mn-Zn ferrite powder
    Tatsuji Aoyama, Ken Hirota, Ken Hirota, Osamu Yamaguchi, Osamu Yamaguchi, Yasuyuki Aono, Nobuya Matsutani
    Highly sinterable Mn-Zn ferrite powder consisting of submicrometer-size particles with structural distortion has been prepared by grinding a coprecipitated powder with a media agitation mill. Dense pure Mn-Zn ferrites (&gt;98% of theoretical) with an average grain size of 1.9 μm can be fabricated by heating at 900°C in a stream of N2. They show high permeability of 820 at 10 kHz through 1 MHz and an electric resistivity of 50 Ω?cm at 1 kHz., Journal of the American Ceramic Society, 79(10)2792 - 94, 01 Oct. 1996
  • Fabrication and mechanical properties of Y2O3- and CaO-doped tetragonal ZrO2 polycrystal
    Akihiro Ota, Rikiya Sato, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Metastable t-ZrO2 solid solutions with small amounts of Y2O3 and CaO crystallize at low temperatures from amorphous materials prepared by the hydrazine method. Well-densified TZP ceramics (&gt;99.2% of theoretical) have been fabricated by HIPing for 2 h at 1400°C and 196 MPa. The texture is of a homogeneous structure consisting of fine grains. Mechanical properties have been examined, in relation to t-/m-ZrO2 ratios and microstructures. An excellent fracture toughness (10.2 MPa ? m1/2) and a high bending strength (1080 MPa) are achieved in TZP ceramics. ? 1996 Plenum Publishing Corporation., Journal of Materials Synthesis and Processing, 4121 - 126, 01 Mar. 1996
  • Hot isostatic pressing of TiB2-ZrO2(2 mol% Y2O3) composite powders
    Yoshiyuki Muraoka, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Dense sintered composites of TiB2and ZrO2(2 mol% Y2O3) have been fabricated by hot isostatic pressing for 2 h at 1500°C under 196 MPa. The ZrO2particles in the composites consist of m-≫ t-ZrO2. There is no reaction between TiB2and ZrO2. Microstructures and mechanical properties are examined, in connection with increased ZrO2 content. The fracture toughness and bending strength of the composites with 30 mol% ZrO2content are 11.2 MPa?m1/2and 680 MPa, respectively., Materials Research Bulletin, 31787 - 792, 01 Jan. 1996
  • Fabrication of composite in the system ZrO2(2Y)/WSi2part 2 - Electrical and mechanical properties of laminated materials with three layers
    Yoshiyuki Muraoka, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Well-densified composites with the compositions of ZrO2(2Y)/WSi2=60/40 and 80/20 mol% have been fabricated by hot isostatic pressing for 2 h at 1500°C and 196 MPa. Electric resistivities (p) between both composites change drastically; the former and latter composites correspond to the metallic and insulating materials, respectively. They are strongly dependent on the textures. Laminated materials with the compositions of ZrO2(2Y)/WSi2=(60/40)/(80/20)/(60/40) mol% have been able to be prepared by the same technique. Overall electric resistivities in the perpendicular and parallel directions for the interfaces are determined to be ρ⊥?1×1011and ρ//?1×10-4Ω?cm, respectively. The residual stress as much as ?200 MPa is induced in the interfaces. The fracture toughness (KIC) is much affected by the residual stress., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 43426 - 432, 01 Jan. 1996
  • Electrical and mechanical properties of ZrO2(2 mol% Y2O3)/WSi2composites and their laminated materials
    Yoshiyuki Muraoka, Satoshi Kawahara, Ken Hirota, Osamu Yamaguchi
    Well-densified composites with the compositions ZrO2(2Y)/WSi2= 60/40 and 80/20 mol% have been fabricated by hot isostatic pressing for 2 h at 1500 °C and 196 MPa. Electric resistivities (ρ) of the two composites are very different; the former and latter composites correspond to metallic and insulating materials, respectively. The properties are strongly dependent on the textures. Laminated materials with the ZrO2(2Y)/WSi2= (60/40)/(80/20)/(60/40) mol% have been prepared by the same technique. Overall electric resistivities in directions perpendicular and parallel to the interfaces have been determined to be ρ⊥?1×1011Ω?cm and ρ‖?1×10-4Ω?cm, respectively. Residual stress as much as ?150 MPa is induced in the interfaces. The fracture toughness (KIc) is greatly affected by the residual stress., Materials Research Bulletin, 31397 - 404, 01 Jan. 1996
  • Microstructure, Mechanical Properteis, and Ionic Conductivity of ZrO2-based Ceramics Fabricated by Hot Isostic Pressing
    Osamu Yamaguchi, Ken Hirota, Suguru Inamura, Hiroki Miyamoto
    Special attention has been given to ZrO2-based ceramics, such as partially stabihzed ZrO2 (PSZ), tetragonal ZrO2 polycrystal (TZP), and ZrO2-toughened Al2O3 (ZTA), to improve the mechanical properties of ceramic materials. Hot isostatic pressing (HIP) is applied to fabricate dense ZrO2-based ceramics from solid solution powders developed by sol-gel techniques. This paper reviews recent research trends and progresses related to the solid solution and composite ceramics focussing on the microstructiuies, strength and fracture toughness, and ionic conductivity. ? 1996, The Japan Society of High Pressure Science and Technology. All rights reserved., Review of High Pressure Science and Technology/Koatsuryoku No Kagaku To Gijutsu, 5257 - 264, 01 Jan. 1996
  • Fabrication of composite in the system ZrO2(2Y)/WSi2Part I: Microstructures and mechanical properties of the composites in the WSi2rich region
    Yoshiyuki Muraoka, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Dense sintered composites of ZrO2(2 mol% Y2O3) and WSi2have been fabricated by hot isostatic pressing for 2 h at 1500°C under 196 MPa. They contain a small amount of W5Si3; during sintering, WSi2decomposes into W5Si3and amorphous Si. The ZrO2particles in the composites consist of only t-ZrO2. Microstructures and mechanical properties are examined, in connection with ZrO2content. The fracture toughness and bending strength of the composite with 40 mol% ZrO2addition are 7.1 MPa?m1/2and 1010 MPa, respectively., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 43267 - 271, 01 Jan. 1996
  • Electrical and mechanical properties of ZrO2(2 mol% Y2O3)/MoSi2composites and their laminated materials
    Yoshiyuki Muraoka, Fumihito Yoshikawa, Ken Hirota, Osamu Yamaguchi, Junji Asai, Yukio Makiyama
    Well-densified composites with the compositions ZrO2(2Y)/MoSi2=60/40 and 80/20 mol% have been fabricated by hot isostatic pressing for 2 h at 1400 °C and 196 MPa. Electric resistivities (p) of the two composites are very different; the former and latter composites correspond to metallic and insulating materials, respectively. The properties are strongly dependent on the textures. Laminated materials with the compositions ZrO2(2Y)/MoSi2=(60/40)/(80/20)/(60/40) mol% have been prepared by the same technique. Overall electric resistivities in directions perpendicular and parallel to the interfaces have been determined to be ρ⊥?1×1011Ω?cm and ρ‖?1×10-4Ω?cm, respectively. Residual stress as much as ?150 MPa is induced in the interfaces. The fracture toughness (KIc) is greatly affected by the residual stress., Materials Research Bulletin, 31405 - 411, 01 Jan. 1996
  • Formation of continuous series of solid solutions from powders prepared by hydrazine method: The system Cr2O3-Al2O3
    M. Watanabe, T. Hirayama, M. Yoshinaka, K. Hirota, O. Yamaguchi
    Chromia (Cr2O3)/alumina (Al2O3) solid solution (ss) powders have been prepared by the hydrazine method. As-prepared powders are classified into three groups: (i) amorphous below 25 mol% Al2O3, (ii) mixtures of this and AlO(OH) gel(ss) between 25 and 80 mol% Al2O3, and (iii) AlO(OH) gel(ss) above 80 mol% Al2O3. Cr2O3solid solutions crystallize at low temperatures from amorphous materials. The formation process leading to continuous series of solid solutions is examined. Cr2O3(ss) powders prepared at a composition of Cr2O3/Al2O3= 80/20 mol% have been characterized for particle size and shape. Individual particles tend toward a hexagonal morphology above 900°C., Materials Research Bulletin, 31861 - 868, 01 Jan. 1996
  • Preparation and hot isostatic pressing of ZrO2solid solution powders in the system ZrO2-Al2O3
    Osamu Yamaguchi, Ken Hirota, Suguru Inamura
    Special attention has been given to partially stabilized ZrO2(PSZ), tetragonal ZrO2polycrystal (TZP), and ZrO2-toughened Al2O3(ZTA) to improve the fracture toughness (KIC) and bending strength (σb) of ceramics. In a recent study, it was shown that in the system ZrO2-Al2O3, ZrO2solid solutions (ss) are formed metastably up to ?40 mol% Al2O3from amorphous materials prepared by the alkoxy-method. Isostatically hot-pressed ZrO2(ss) ceramics exhibit very high KIC(23 MPa ? m1/2) and plastic deformation at room temperature, although their σbis weak (?700 MPa). The texture consists of a homogeneous microstructure with a grain size of ?50nm. To improve σb, two kinds of ceramics have been fabricated by the addition of a small amount of Y2O3: (1) perfect ZrO2(ss) ceramics and (2) composite ceramics of ZrO2(ss) and α-Al2O3. Composites with a homogeneous dispersed α-Al2O3result in a remarkable increase of σb(1445 MPa). The ratio of ZrO2(ss) and α-Al2O3can be controlled by the amount of Y2O3addition and sintering temperature. A new method for preparing solid solutions from chlorides using hydrazine monohydrate has been developed in some systems. This paper describes the mechanical properties of PSZ, TZP and ZTA ceramics fabricated using ZrO2(ss) powders prepared by some sol-gel techniques., Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 43899 - 906, 01 Jan. 1996
  • Solid solutions of metastable tetragonal ZrO2and Ce3ZrO8in the system ZrO2-CeO2
    Akio Kawabata, Shingo Hirano, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    In the system ZrO2-CeO2, metastable t-ZrO2solid solutions containing up to 30 mol% CeO2crystallize at temperatures of 385-430°C from amorphous materials prepared by the hydrazine method. Crystalline Ce3ZrO8solid solutions are formed in as-prepared powders between 30-75 mol % CeO2. The variation of the lattice parameters of both solid solutions is determined as a function of CeO2content. The value of the lattice parameter of pure Ce3ZrO8(cubic) is a = 0.5342 nm. Detailed characterization of the Ce3ZrO8powder has been performed. Crystallite size and particle size are strongly dependent on the heating temperature. Specific surface areas do not drop below 40 m2g-1until the heating temperature is above 1000°C. ? 1996 Chapman &amp; Hall., Journal of Materials Science, 314945 - 4949, 01 Jan. 1996
  • Formation and Characterization of Ce3ZrO8Prepared by the Hydrazine Method
    Shingo Hirano, Akio Kawabata, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    A compound denoted as (Ce0.75Zr0.25)O2(Ce, ZrO8) is formed near room temperature from cerium and zirconium nitrates using hydrazine monohydrate. It has a cubic unit cell with a= 0.5342 nm. Characterization of powders heated to various temperatures at 10°C/min demonstrates that the specific surface area does not decrease below 20 mVg until &gt;1000°C. Copyright ? 1995, Wiley Blackwell. All rights reserved, Journal of the American Ceramic Society, 781414 - 1416, 01 Jan. 1995
  • Synthesis of LaCrO3 by the hydrazine method
    T. Kikkawa, M. Yoshinaka, K. Hirota, O. Yamaguchi
    Journal of Materials Science Letters, 141071 - 1073, 01 Jan. 1995
  • Formation of PbZrO3 by Transformation of Cubic ZrO2 Solid Solution
    Tatsuji Aoyama, Naoki Kurata, Ken Hirota, Osamu Yamaguchi
    Metastable ZrO2 solid soutions containing up to 50 mol% PbO are formed at low temperatures from amorphous materials prepared by the hydrazine method: the tetragonal phase up to 30 mol% PbO and the cubic phase between 30 and 20 mol% PbO. The compound PbZrO3 is formed at 615° to 690°C by transformation of c‐ZrO2 solid solution. Copyright ? 1995, Wiley Blackwell. All rights reserved, Journal of the American Ceramic Society, 783163 - 3164, 01 Jan. 1995
  • Mechanical properties of hot isostatically pressed zirconia (2 mol% yttria)-reinforced molybdenum disilicide composite
    Toshihiro Yamada, Ken Hirota, Osamu Yamaguchi, Junji Asai, Yukio Makiyama
    Dense sintered composites of ZrO2(2 mol% Y2O3) and MoSi2have been fabricated by hot isostatic pressing for 2 h at 1400 °C under 196 MPa. The ZrO2particles in the composites consist of only t-ZrO2. There is no reaction between ZrO2and MoSi2. Microstructures and mechanical properties are examined, in connection with increased ZrO2content. The fracture toughness and bending strength of the composite with 40 mol% ZrO2addition are 6.18 MPa ? m1 2and 1034 MPa, respectively. ? 1995., Materials Research Bulletin, 30(7)851 - 57, 01 Jan. 1995
  • Microstructure and mechanical properties of hot isostatically pressed zirconia (2 mol% yttria)-reinforced tungsten disilicide composites
    Kotaro Yamaguchi, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Dense sintered composites of ZrO2(2 mol% Y2O3) and WSi2have been fabricated by hot isostatic pressing for 2 h at 1500 °C under 196 MPa. They contain a small amount of W5Si3; during sintering, WSi2decomposes into W5Si3and amorphous Si. The ZrO2particles in the composites consist of only t-ZrO2. Microstructures and mechanical properties are examined, in connection with ZrO2content. The fracture toughness and bending strength of the composite with 40 mol% ZrO2addition are 7.1 MPa ? m1 2and 1011 MPa, respectively. ? 1995., Materials Research Bulletin, 30953 - 958, 01 Jan. 1995
  • Hot Isostatic Pressing and Characterization of Sol‐Gel‐Derived Chromium(III) Oxide
    Akio Kawabata, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi
    Chromium (III) oxide (Cr2O3) crystallizes at low temperatures from an amorphous material prepared by adding hydrazine monohydrate to an aqueous solution of Cr(NO3)3‐9H2O. Individual particles of Cr2O3tend toward a hexagonal morphology above 800°C. Well‐densified Cr2O3pellets (98.8% of theoretical density) have been fabricated by hot isostatic pressing for 2 h at 1100°C and 196 MPa. Their fracture toughness is 4.4 MPa.m1/2. The sample annealed in air for 12 h at 1300°C exhibits a high electrical conductivity of 3.6 Ω‐1.m‐1at 700°C. Copyright ? 1995, Wiley Blackwell. All rights reserved, Journal of the American Ceramic Society, 78(8)2271 - 73, 01 Jan. 1995
  • Electrical and mechanical properties of three-layer laminated materials composed of ZrO2(2y)/MoSi2 composites
    Yoshiyuki Muraoka, Fumihito Yoshikawa, Ken Hirota, Osamu Yamaguchi
    Dense sintered materials (&lt;99% of theoretical) consisting of three-layer structures with the compositions of ZrO2(2Y)/MOSi2=(60/40:A)/(80/20:B)/(60/40:A) mol% have been able to be fabricated by hot isostatic pressing at 1400°C for 2 h under 196 MPa. The materials show an anisotropy in overall electric resistivities measured in the perpendicular and parallel directions for the laminated interfaces: the former and latter resistivities correspond to insulating (ρ⊥?1×1011Ω?cm) and metallic (ρ″?1×10-4 Ω?cm) materials, respectively. The fracture toughness (KIc) is much affected by the residual stress estimated to be as much as ?150 MPa at the interfaces. The residual stress acts as compressive and tensile stresses in the layers A and B, respectively. ? 1995, Japan Society of Powder and Powder Metallurgy. All rights reserved., Journal of the Japan Society of Powder and Powder Metallurgy, 421468 - 1472, 01 Jan. 1995
  • Formation and Sintering of Zirconia (3 mol% Yttria)?Toughened Alumina Powders Prepared by the Hydrazine Method
    Koji Yamakata, Hideki Kume, Ken Hirota, Suguru Inamura, Osamu Yamaguchi, Hiroki Miyamoto
    Alumina/3 mol% yttria-doped zirconia composite powders have been prepared by the hydrazine method. As-prepared powders are AIO(OH) gel solid solutions and the mixtures of this and amorphous ZrO2below and above 10 mol% ZrOz, respectively. Formation process leading to α- AI203-f-ZrO2composite powders is examined. Hot isostatic pressing has been performed for 1 h at 1500°C under 196 MPa using the composite powders of α-AI2O3and f-ZrO2obtained by heating for 1 h at 1350°C. The resulting dense ZrO2-toughened Al2O3(ZTA) ceramics with 30 mol% ZrO2show excellent mechanical strength (1100 MPa). The grain sizes of α-AI2O3and intergranular ZrO2in ZTA decrease with increased ZrO2addition. Intragranular ZrO2grains are 0.3 regardless of the starting composition. Mechanical properties are discussed in connection with relative densities and microstructures. ? 1994, Japan Society of Powder and Powder Metallurgy. All rights reserved., Journal of the Japan Society of Powder and Powder Metallurgy, 41(9)1054 - 60, 01 Feb. 1994
  • Metastable zirconia phases prepared from zirconium alkoxide and yttrium acetylacetonate Part 2: Hot isostatic pressing of tetragonal zirconia solid solution powders
    M. Shimazaki, K. Hirota, O. Yamaguchi, S. Inamura, H. Miyamoto, N. Shiokawa, K. Tsuji
    Metastable ZrO2(tetragonal) solid solution powders with 2 to 5 mol% Y2O3prepared from zirconium isopropoxide and yttrium acetylacetonate have been isostatically hotpressed for 1 h at 1230°C under 196 MPa. Dense sintered materials with an average grain size of 0.3 μm are fabricated from powders containing 2 and 2.5 mol% Y2O3. High fracture toughness (13.5 MPam1 2) and excellent bending strength (1320 MPa) are achieved in these materials. The materials, especially with 5 mol% Y2O3addition, exhibit high ionic conductivity at high temperatures. The conductivity at 800°C is evaluated to be 1.8×10-2S/cm. ? 1994., Materials Research Bulletin, 29(3)277 - 85, 01 Jan. 1994
  • Hot isostatic pressing of tetragonal ZrO2 solid-solution powders prepared from acetylacetonates in the system ZrO2-Y2O3-Al2O3
    H. Watanabe, K. Hirota, O. Yamaguchi, S. Inamura, H. Miyamoto, N. Shiokawa, K. Tsuji
    In compositions having ZrO2/Y2O3=(74.25-71.25)/(0.75-3.75) (mol% ratio) with 25 mol% Al2O3, metastable t-ZrO2 solid solutions crystallize at ? 780° to ? 860°C from amorphous materials prepared by the simultaneous hydrolysis of zirconium, yttrium and aluminium acetylacetonates. Hot isostatic pressing has been performed for 1 h at 1130 and 1230°C under 196 MPa using their powders. Two kinds of material are fabricated: (i) perfect ZrO2 solid-solution ceramics and (ii) composites of ZrO2 solid solution and α-Al2O3. Their mechanical properties are examined, in connection with microstructures and t/m ZrO2 ratios. Composites with a homogeneous dispersed α-Al2O3 derived from solid-solution ceramics result in a remarkable increase of strength. ? 1994 Chapman &amp; Hall., Journal of Materials Science, 293719 - 3723, 01 Jan. 1994
  • Formation of Zirconia Solid Solutions Containing Alumina Prepared by New Preparation Method
    Koji Ishida, Ken Hirota, Osamu Yamaguchi, Hideki Kume, Suguru Inamura, Hiroki Miyamoto
    In the system ZrO2?Al2O3, a new method for preparing ZrO2 solid solutions from ZrCl4 and AlCl3 using hydrazine monohydrate is investigated. c‐ZrO2 solid solutions containing up to ?40 mol% Al2O3 crystallize at low temperatures from amorphous materials. The formation mechanism is discussed from IR spectral data. The values of the lattice parameter α increase linearly from 0.5072 to 0.5105 nm with increasing Al2O3 content. At higher temperatures, transformation of the solid solutions proceeds as follows: c(SS) →t(ss) →t(ss) +α‐Al2O3→m+α‐Al2O3. m‐ZrO2?α‐Al2O3 composite ceramics are fabricated by hot isostatic pressing for 2 h at 1250°C and 196 MPa. Microstructures and mechanical properties are examined, in connection with increasing Al2O3 content. Copyright ? 1994, Wiley Blackwell. All rights reserved, Journal of the American Ceramic Society, 77(5)1391 - 95, 01 Jan. 1994
  • Formation and hot isostatic pressing of ZrO2 solid solution in the system ZrO2-Al2O3
    S. Inamura, H. Miyamoto, Y. Imaida, M. Takagawa, K. Hirota, O. Yamaguchi
    In the system of ZrO2-Al2O3, cubic ZrO2 solid solutions containing up to 40 mol% Al2O3 crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminium alkoxides. At higher temperatures, they transform into tetragonal solid solutions. Metastable ZrO2 solid solution powders containing 25 mol% Al2O3 have been sintered at 1000-1150 °C under 196 M Pausing the hot isostatic pressing technique. The solid solution ceramics consisting of homogeneous microstructure with an average grain size of ? 50 nm exhibited a very high fracture toughness of 23 MN m -1.5. They have been characterized by X-ray diffraction and electron probe surface analyses. ? 1994 Chapman &amp; Hall., Journal of Materials Science, 294913 - 4917, 01 Jan. 1994
  • Formation of Alumina/Zirconia (3 mol% Yttria) Composite Powders Prepared by the Hydrazine Methods
    K?ji Yamakata, Ken Hirota, Osamu Yamaguchi, Hideki Kume, Suguru Inamura, Hiroki Miyamoto
    Alumina/3 mol% yttria‐doped zirconia composite powders have been prepared by the hydrazine method. As‐prepared powders are AlO(OH) gel solid solutions and the mixtures of this and amorphous ZrO2 below and above 10 mol% ZrO2, respectively. The formation process leading to α?Al2O3?t‐ZrO2 composite powders is examined. Copyright ? 1994, Wiley Blackwell. All rights reserved, Journal of the American Ceramic Society, 772207 - 2208, 01 Jan. 1994
  • Sintering of Alumina-Zirconia Composite Powders Prepared from Aluminum Sulfate and Zirconium Isopropoxide
    Yousuke Matsumoto, Ken Hirota, Osamu Yamaguchi, Hideki Kume, Suguru Inamura, Hiroki Miyamoto
    Amorphous Al2O3-ZrO2 composite powders with 5?30mol% ZrO2 have been prepared by adding aqueous ammonia to the mixed solution of aqueous aluminum sulfate and zirconium alkoxide containing 2-propanol. Simultaneous crystallization of γ-Al2O3 and t-ZrO2 occurs at 870°?980°C. The γ-Al2O3 transforms to α-Al2O3 at 1160°?1220°C. Hot isostatic pressing has been performed for 1h at 1400°C under 196MPa using α-Al2O3-t-ZrO2 composite powders. Dense ZrO2-toughened Al2O3 (ZTA) ceramics with homogeneous-dispersed ZrO2, particles shows excellent mechanical properties. The toughening mechanism is discussed. The microstructures and t/m ratios are examined, with emphasis on the relation between strength and fracture toughness. ? 1994, The Society of Materials Science, Japan. All rights reserved., Journal of the Society of Materials Science, Japan, 431042 - 1046, 01 Jan. 1994
  • Formation and Sintering of Yttria‐Doped Tetragonal Zirconia with 50 mol% Alumina Prepared by the Hydrazine Method
    Shinya Kimoto, Ken Hirota, Osamu Yamaguchi, Hideki Kume, Suguru Inamura, Hiroki Miyamoto
    Al2O3/Y2O3‐doped ZrO2 composite powders with 50 mol% Al2O3 are prepared by the hydrazine method. As‐prepared powders are mixtures of AlO(OH) gel and amorphous ZrO2 solid solutions containing Y2O3 and Al2O3. The formation process leading to α‐Al2O3‐t‐ZrO2 composite powders is examined. Hot isostatic pressing is performed for 2 h at 1400°C under 196 MPa using θ‐Al2O3‐t‐ZrO2 composite powders. The resulting dense, sintered α‐Al2O3‐t‐ZrO2 composites show excellent mechanical strength. Copyright ? 1994, Wiley Blackwell. All rights reserved, Journal of the American Ceramic Society, 77(6)1694 - 96, 01 Jan. 1994
  • Sintering and characterization of yttria-stabilized cubic zirconia with alumina derived from solid solution
    Michikazu Fukuya, Ken Hirota, Osamu Yamaguchi, Hideki Kume, Suguru Inamura, Hiroki Miyamoto, Nobuaki Shiokawa, Ryoichi Shikata
    In the composition of ZrO2(8 mol% Y2O3) Al2O3 = (100-85) (0-15) mol%, metastable c-ZrO2 solid solutions crystallize at low temperatures from amorphous materials prepared by the hydrazine method. Powders have been characterized. Crystallite size and specific surface area are strongly dependent on heating temperature. Hot isostatic pressing has been performed for 1 h at 1600°C under 196 MPa. Yttria-stabilized c-ZrO2 materials with homogeneous-dispersed Al2O3 derived from the decomposition of solid solutions give &gt;99% of theoretical densities. Materials with small amounts of Al2O3 show good strength and excellent high ionic conductivity at high temperatures. ? 1994., Materials Research Bulletin, 29(6)619 - 28, 01 Jan. 1994
  • Metastable zirconia phases prepared from zirconium alkoxide and yttrium acetylacetonate part 1: Formation and characterization of metastable zirconia solid solutions
    M. Shimazaki, K. Hirota, O. Yamaguchi, S. Fujii, M. Tamamaki, S. Inamura, M. Miyamoto, S. Kume
    In the ZrO2-Y2O3system, metastable ZrO2solid solutions containing up to ?46 mol% Y2O3crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium isopropoxide and yttrium acetylacetonate: t- and c-ZrO2phases below and above 8 mol% Y2O3, respectively. The variation of the lattice parameters of both phases is determined as a function of Y2O3content. The value of the lattice parameter of pure ZrO2(tetragonal) is a=0.5085 nm and c=0.5170 nm. Powder characterization has been examined using c-ZrO2solid solution powders prepared in the composition of ZrO2Y2O3= 80 20 mol%. Crystallite size, particle size and particle morphology are strongly dependent on heating temperature. Surface areas of powders do not drop below 10 m2/g until the heating temperature is above 900°C. ? 1993., Materials Research Bulletin, 28(28)877 - 884, 01 Jan. 1993
  • Mechanical Properties of HIP Sintered Bodies of Sendust Metal Powders
    Yutaka Imaida, Mitsuteru Takagawa, Tsuyoshi Kirimura, Ken Hirota, Osamu Yamaguchi, Atsushi Kakitsuji, Hiroki Miyamoto
    Sendust metal (Fe -9.56wt%Si -5.47wt%Al) powders have been isostatically hot pressed and the mechanical properties of sintered bodies with various porosities examined at room temperature. Vickers hardness decreases linearly with increasing porosity. The porosity dependence of bending and compressive strengths is expressed by the equation σ=σoexp(-np); σo=540 MPa for the former and σo=2800 MPa for the latter are determined, respectively, whereas the same value of n=0.22 is estimated from both strength-porosity data. Young&#039;s modulus is 300 GPa for sintered bodies with nearly theoretical density. ? 1993, Japan Society of Powder and Powder Metallurgy. All rights reserved., Journal of the Japan Society of Powder and Powder Metallurgy, 40359 - 362, 01 Jan. 1993
  • Formation and Sintering of Alkoxy-derived Al2O3-TiO2 Composite Powders
    Yousuke Matsumoto, Ken Hirota, Osamu Yamaguchi, Souichi Fujii, Masahiro Tamamaki
    In the Al2O3-TiO2 system, solid solutions of AIO(OH) gel are formed between 5 and 20 mol% TiO2 by the simultaneous hydrolysis of aluminium and titanium alkoxides, followed by washing and drying. After dehydration γ-Al2O3 solid solutions crystallize slowly from amorphous materials. At higher temperatures, rutile (TiO2) is formed directly with the transformation of γ-Al2O3 phases into corundum. Corundum-rutile composite ceramics with dispersed fine pores generated from the decomposition of Al2TiO5 have been fabricated as follows:(1) hot pressing for 2 h at 1350°C under 35 MPa using γ-Al2O3 solid solutions, (2) heating for 1 h at 1500°C to achieve the complete formation of Al2TiO5, and (3) annealing for 7 h at 1000°C to decompose of Al2TiO5 into corundum and rutile. Their mechanical properties have been examined. The bending strength yields the maximum value of 365 MPa with 15 mol% TiO2 content whereas the fracture toughness increases from 4.7 to 6.2 MPam1/2 with increasing TiO2 content. ? 1993, Japan Society of Powder and Powder Metallurgy. All rights reserved., Journal of the Japan Society of Powder and Powder Metallurgy, 40993 - 997, 01 Jan. 1993
  • Formation and sintering of corundum-rutile composite powders prepared from alkoxides
    Y. Matsumoto, K. Hirota, O. Yamaguchi, S. Fujii, M. Tamamaki
    In the Al2O3-TiO2system, solid solutions of AlO(OH) gel are formed between 5 and 20 mol% TiO2by the simultaneous hydrolysis of aluminium and titanium alkoxides, followed by washing and drying. After dehydration γ-Al2O3solid solutions crystallize slowly from amorphous materials. At higher temperatures, rutile is formed directly with the transformation of γ-Al2O3phases into corundum. Corundum-rutile composite ceramics with dispersed fine pores generated from the decomposition of Al2TiO5have been fabricated as follows: (1) hot pressing for 2 h at 1350°C under 35 MPa using γ-Al2O3solid solutions, (2) heating for 1 h at 1500°C to achieve the complete formation of Al2TiO5, and (3) annealing for 7 h at 1100°C to decompose of Al2TiO5into corundum and rutile. Their mechanical properties have been examined. The bending strength yields the maximum value of 365 MPa with 15 mol% TiO2content whereas the fracture toughness increases from 4.7 to 6.2 MPam1/2with increasing TiO2content. ? 1993., Materials Research Bulletin, 28(28)305 - 312, 01 Jan. 1993
  • Formation of Alkoxy-derived Yttrium Aluminates in the Al2O3-rich Region
    Ken Hirota, Osamu Yamaguchi, Suguru Inamura, Hiroki Miyamoto
    Hexagonal YAIO3 crystallizes at low temperatures from an amorphous material prepared by the simultaneous hydrolysis of yttrium and aluminum alkoxides. It transforms to the cubic phase with a garnet structure as an intermediate product at elevated temperatures. The formation process of YAlO3 is described. Solid solutions of hexagonal YAlO3 crystallize between 50 and 62.5 mol% Al2O3. Yttrium aluminum garnet, Y3Al5O12 (YAG), is formed by the transformation of the hexagonal YAlO3 solid solution. ? 1993, Japan Society of Powder and Powder Metallurgy. All rights reserved., Journal of the Japan Society of Powder and Powder Metallurgy, 40789 - 793, 01 Jan. 1993
  • High fracture toughness of ZrO2solid-solution ceramics with nanometre grain size in the system ZrO2-Al2O3
    S. Inamura, M. Miyamoto, Y. Imaida, M. Takagawa, K. Hirota, O. Yamaguchi
    Journal of Materials Science Letters, 121368 - 1370, 01 Jan. 1993
  • Mechanical Properties of Hot Isostatically Pressed Zirconia‐Toughened Alumina Ceramics Prepared from Coprecipitated Powders
    Yosuke Matsumoto, Ken Hirota, Osamu Yamaguchi, Suguru Inamura, Hiroki Miyamoto, Nobuaki Shiokawa, Kazuhiro Tsuji
    Amorphous Al2O3?ZrO2 composite powders with 5?30 mol% ZrO2 have been prepared by adding aqueous ammonia to the mixed solution of aqueous aluminum sulfate and zirconium alkoxide containing 2‐propanol. Simultaneous crystallization of γ‐Al2O3 and t‐ZrO2 occurs at 870°?980°C. The γ‐Al2O3 transforms to α‐Al2O3 at 1160°?1220°C. Hot isostatic pressing has been performed for 1 h at 1400°C under 196 MPa using α‐Al2O3?t‐ZrO2 composite powders. Dense ZrO2‐toughened Al2O3 (ZTA) ceramics with homogeneous‐dispersed ZrO2 particles show excellent mechanical properties. The toughening mechanism is discussed. The microstructures and t/m ratios of ZTA are examined, with emphasis on the relation between strength and fracture toughness. Copyright ? 1993, Wiley Blackwell. All rights reserved, Journal of the American Ceramic Society, 76(10)2677 - 80, 01 Jan. 1993
  • Formation, characterization and sintering of alkoxy-derived bismuth vanadate
    Ken Hirota, Goro Komatsu, Masamichi Yamashita, Hideaki Takemura, Osamu Yamaguchi
    Monoclinic BiVO4is formed by the hydrolysis of bismuth and vanadyl double alkoxide, followed by washing and drying. Its average particle size is ?45 nm. The chemical structure is described by the formula Bi(VO4). Sintered pellets with nearly theoretical density are prepared by hot pressing for 2 h at 750°C and 29.4 MPa. The electrical conductivity of the pellets has been measured at 150° to 550°C. Data are interpreted in terms of ionic conductivity due to oxygen ion vacancies. ? 1992., Materials Research Bulletin, 27823 - 830, 01 Jan. 1992
  • Formation of alkoxy-derived Bi4Ge3O12
    K. Hirota, H. Oohata, O. Yamaguchi
    Journal of Materials Science Letters, 11802 - 804, 01 Jan. 1992
  • Hot isostatic pressing of sendust metal powders
    Y. Imaida, M. Takagawa, T. Kirimura, K. Hirota, O. Yamaguchi, A. Kakitsuji, H. Miyamoto
    Sendust metal (Fe/1b9.56wt%Si/1b5.47wt%Al) powders have been isostatically hot pressed and the mechanical properties of sintered bodies with various porosities examined at room temperature. Vickers hardness decreases linearly with increasing porosity. The porosity dependence of bending and compressive strengths is expressed by the equation σ=σoexp(-np); σo=540 MPa for the σo=2800 MPa for the latter are determined, respectively, whereas the same value of n=0.22 is estimated from both strength-porosity data. Young&#039;s modulus is 300 GPa for sintered bodies with nearly theoretical density. ? 1992., Materials Research Bulletin, 27(27)661 - 68, 01 Jan. 1992
  • Characterization and sintering of reactive MgO
    Ken Hirota, Noritoshi Okabayashi, Kenji Toyoda, Osamu Yamaguchi
    Powder characteristics and sinterability of reactive MgO prepared by calcination of alkoxy-derived Mg(OH)2have been studied. Specific surface areas of powders calcined at low temperatures are very high, and these do not drop below 20 m2/g until the temperature is above 1000°C. Crystallite size, particle size, and particle morphology are dependent on calcining temperature. Optimum calcining temperature for preparing high sintered pellets is 800°C. The highest sintered densities (&gt;99.5 %) are obtained by heating at 1350°C. ? 1992., Materials Research Bulletin, 27(27)319 - 26, 01 Jan. 1992
  • Formation and characterization of alkoxy-derived Bi4Ti3O12
    Osamu Yamaguchi, Noboru Maruyama, Ken Hirota
    Tetragonal Bi4Ti3O12 crystallizes metastably at low temperatures from an amorphous material prepared by the simultaneous hydrolysis of bismuth and titanium alkoxides. It can be preserved at room temperature by quenching and has a unit cell with a = 0.546 nm and c = 3.281 nm. The structure contains octahedral TiO6 groups. Definite signs of particle growth and a marked increase in crystallite growth are seen in powders produced above 900°C and 800°C, respectively. Heating at 1000°C causes the formation of plate-like particles. The specific surface areas of powders heated at low temperatures are very high., British ceramic. Transactions and journal, 90111 - 113, 01 Jan. 1991
  • Formation and characterization of alkoxy-derived Zn2GeO4
    O. Yamaguchi, J. Hidaka, K. Hirota
    Journal of Materials Science Letters, 101471 - 1474, 01 Jan. 1991
  • Fabrication and Mechanical Properties of Yttria-doped Zirconia Ceramics Containing Alumina
    Hiroshi Watanabe, Hideki Kume, Ken Hirota, Suguru Inamura, Osamu Yamaguchi, Hiroki Miyamoto
    Metastable t-ZrO2 solid solutions with the composition of ZrO2/Y2O3/Al2O3=(74.25-71.25)/(0.75-3.75)/25 mol% crystallize at 780°C-860°C from amorphous materials prepared by the simultaneous hydrolysis of zirconium, yttrium and aluminum acetylacetonates. Two kinds of materials are fabricated by hot isostatic pressing for 1 hat 1130°Cand 1230°C under 196 MPa: (i) perfect ZrO2 solid solution ceramics and (ii) composites of ZrO2 solid solution and α-Al2O3. Mechanical properties are examined in connection with the microstructures and t/m ratios. ? 1994, Japan Society of Powder and Powder Metallurgy. All rights reserved., Journal of the Japan Society of Powder and Powder Metallurgy, 41790 - 794, 01 Feb. 1984
  • Crystallisation of hydroxyapatite in phosphorylated poly(vinyl alcohol) as a synthetic route to tough mechanical hybrid materials
    Akane Kusakabe, Ken Hirota, Tadashi Mizutani
    Partially phosphorylated poly(vinyl alcohol) was prepared by treating poly(vinyl alcohol) with 100% phosphoric acid, and 5,10 and 20% of the hydroxyl groups were converted to phosphoric acid ester. Addition of Ca2+ to an aqueous solution of phosphorylated poly(vinyl alcohol) gave a transparent gel. Five cycles of altemate soaking of the gel in aqueous CaCl2 and aqueous (NH4)(2)HPO4 were carried out to crystallise hydroxyapatite (HAP) in the phosphorylated poly(vinyl alcohol) matrix. The X-ray diffraction peaks of HAP formed in 5% phosphorylated PVA were sharp, while those of HAP formed in 20% phosphorylated PVA were broad. The contents of inorganic phase in the hybrid powder were increased from 58 to 76 wt% as the fraction of phosphate groups in the gel was decreased from 20% to 5%. The hybrid powder was first subjected to uniaxial pressing, followed by cold iso-static pressing (CIP) and warm isostatic pressing (WIP) at 120 degrees C at pressures of 300-980 MPa, to obtain the specimens for three-point bending test. These hybrid specimens showed bending strengths of 15-53 MPa. The hybrid compacts prepared from 10% phosphorylated poly(vinyl alcohol) showed the smallest Young's modulus, the largest displacement at break, and the largest fracture energy, showing that it has the highest toughness among the hybrid materials prepared from poly(vinyl alcohol) with varying degrees of phosphorylation. (C) 2016 Elsevier B.V. All rights reserved., MATERIALS SCIENCE & ENGINEERING C-MATERIALS FOR BIOLOGICAL APPLICATIONS, ELSEVIER SCIENCE BV, 70487 - 493, Jan. 2017, English, Research paper (scientific journal)
  • The crystal structure and electrical properties of K2NiF4-type (Ca2-xSmx)MnO4
    Hideki Taguchi, Hiroyasu Kido, Masaki Kato, Ken Hirota
    K2NiF4-type (Ca2-xSmx)MnO4 (0.02 &lt;= x &lt;= 03) was synthesized using a polymerized complex route. The crystal structure changed from tetragonal to orthorhombic at x= 0.3. A variation in the lattice parameters was explained by the Mn-O(2) distance, the Mn-O(2)-Mn angle, and the (Ca,Sm)-(Ca,Sm) distance. The samples were n-type semiconductors, and the minimum values of electrical resistivity (rho) and an energy gap (E-g) at x=0.1 were accounted by electron transfer through an Mn-O(2) -Mn path and overlap between the Mnt(2g) and Op(pi) orbitals. In the sample (x= 0.3), the log rho-1000/T curve consisted of two different lines that intersected at ca. 325 K, and a peak was observed at ca. 315 K in the magnetic susceptibility-temperature curve. These results indicated the presence of a charge-ordering (CO) transition at 315-325K. (C) 2015 Elsevier Ltd. All rights reserved., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 64318 - 322, Apr. 2015, English, Research paper (scientific journal)
  • Fabrication of dense B4C/CNF composites having extraordinary high strength and toughness at elevated temperatures
    Ken Hirota, Mitsuhiro Shima, Xiaolei Chen, Naoki Goto, Masaki Kato, Toshiyuki Nishimura
    Fully dense B4C/CNF composites have been fabricated from the mixture of amorphous fine powders of boron and carbon, and carbon nano-fiber (CNF) using pulsed electric-current pressure sintering (PECPS) at 2173 K (1900 degrees C) for 10 min under 30 MPa in a vacuum. They showed a moderate mechanical properties at room temperature: bending strength sigma(b) more than 450 MPa, Vickers hardness H-v of 29-36 GPa, and fracture toughness K-Ic ranging from 4.1 to 5.7 MPa m(1/2). However, they revealed extraordinary high strength more than 600 MPa at 1673-1873 K (1400-1600 degrees C) in an inert gas atmosphere, especially, sigma(b) of 830 MPa at 1773 K (1500 degrees C) (12.5 vol% CNF added composite) and sigma(b) of 800 MPa at 1873 K (1600 degrees C) (10.0 vol% CNF added composite). In addition, these B4C/CNF composites also showed much improved high-temperature fracture toughness by a factor of around 20, in comparison with monolithic B-4 degrees C ceramics at 1773 K (1500 degrees C). These extraordinary both high strength and toughness might be explained in terms of the suppression of grain growth by CNF at elevated temperatures, an inverse strength temperature-dependence for carbides, and anchor effect induced by CNF. (C) 2015 Elsevier B.V. All rights reserved., MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, ELSEVIER SCIENCE SA, 62841 - 49, Mar. 2015, English, Research paper (scientific journal)
  • Synthesis of murdochite-type Ni6MnO8 with variable specific surface areas and the application in methane oxidation
    Hideki Taguchi, Shohei Tahara, Mikoto Okumura, Ken Hirota
    To synthesize a murdochite-type Ni6MnO8 with variable specific surface areas, an oxalate precursor was calcined at 350-500 degrees C in flowing argon, and the calcined sample was heated to 600 degrees C in air. The lattice constant of the Ni6MnO8 did not depend on the calcination temperature of the precursor, while the specific surface area decreased from 8.4 m(2)/g to 2.6 m(2)/g when increasing the calcination temperature of the precursor. The methane (CH4) oxidation data indicated that the temperature corresponding to the 50% conversion (T-50%) of Ni6MnO8 calcined at 350 degrees C or 400 degrees C was lower than that of all other Ni6MnO8 specimens. However, the intrinsic conversion of Ni6MnO8 calcined at 350 degrees C, which is defined as conversion per specific surface area, was half of that of all other Ni6MnO8 specimens. The degree of crystallinity and catalytic performance of the Ni6MnO8 calcined at 400 degrees C were high. (C) 2014 Elsevier Inc. All rights reserved., JOURNAL OF SOLID STATE CHEMISTRY, ACADEMIC PRESS INC ELSEVIER SCIENCE, 215300 - 304, Jul. 2014, English, Research paper (scientific journal)
  • Fabrication of novel ZrO2(Y2O3)-Al2O3 ceramics having high strength and toughness utilising pulsed electric current pressure sintering (PECPS)
    K. Hirota, K. Shibaya, H. Matsuda, M. Kato, H. Taguchi
    Pulsed electric current pressure sintering (PECPS) makes it possible to produce novel monolithic and composite materials with well controlled microstructure or nanostructure, which has not been realised before. ZrO2 based ceramics containing 25 mol.-% Al2O3 and 0.90-1.125 mol.-% Y2O3, i.e. ZrO2(1.2-1.5 mol.-% Y2O3)-25 mol.-% Al2O3, revealing high bending strength sigma(b) &gt;= 1 GPa and high fracture toughness K-IC &gt;= 15.5 MPa m(1/2) simultaneously, have been fabricated by PECPS of cubic ZrO2 solid solution (ss) powders containing both Al2O3 and Y2O3 prepared via a sol-gel process. Tetragonal ZrO2 (t-ZrO2) phase composite ceramics sintered at 1573 to 1623 K (1300-1350 degrees C) for 10 min under 60 MPa in Ar were composed of similar to phi 200 nm grains with high relative density &gt;= 97.5%. Transmission electron microscopy observation cleared that these mechanical properties might originate from homogeneous distribution of alpha-Al2O3 particles in the dense t-ZrO2 matrix; the precipitation of a-Al2O3 could be achieved by adopting the ss powders and PECPS., ADVANCES IN APPLIED CERAMICS, MANEY PUBLISHING, 113(2)73 - 79, Feb. 2014, English, Research paper (scientific journal)
  • Hot isostatic press sintering and dielectric properties of SrTaO2N ceramics
    Y-R. Zhang, Y. Masubuchi, T. Motohashi, S. Kikkawa, K. Hirota
    SrTaO2N prepared by a citrate route was capsule hot isostatic press sintered at 196 MPa for 3 h at 1200, 1400 and 1600 degrees C. Densification over 80% was attained for all the sintered bodies with partial reduction, which was observed as a change of color from brown to dark gray. Exaggerated grain growth was observed with some amount of open pore for the product sintered at 1400 degrees C. The sample was successfully returned to the brown insulator color after annealing in ammonia flow at 1000 degrees C. The resultant SrTaO2N ceramics exhibited a considerably high relative dielectric permittivity of 7.8 x 10(3) at 10(2) Hz. (C) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved., CERAMICS INTERNATIONAL, ELSEVIER SCI LTD, 39(3)3377 - 3380, Apr. 2013, English, Research paper (scientific journal)
  • Fabrication of Full-Density Mg-Ferrite/Fe-Ni Permalloy Nanocomposites with a High-Saturation Magnetization Density of 1 T
    Ken Hirota, Motoki Obatal, Masaki Kato, Hideki Taguchi
    Dense nanocomposites consisting of spinel Mn-doped Mg-ferrite and permalloy, (Mg[Fe1 - xMnx]2O4(0 = x = 0.4)/53Fe47Ni), have been fabricated from powder mixtures of MgO/a-Fe2O3/MnO and N2-atomized metal particles utilizing high-pressure cold isostatic press (CIP), pulsed electric current pressure sintering (PECPS), and capsule-free hot isostatic pressing (HIP). Metal particles were coated with the mixed oxide powders homogeneously under high shear compression in Ar. The coated powders were densified into relative densities of 8385% by high pressure CIPing (1 GPa). They were then presintered at 823 K for 3 min under uniaxial pressure of 100 MPa in Ar using a PECPS. The pre-sintered bodies had relative densities of 9395% composed of spinel ferrite and permalloy; the mixed metal oxides were changed into magnesium ferrites by heating briefly at low temperature. Then, HIP (1123 K/6 h/196 MPa-Ar) was adopted to densify the pre-sintered bodies into a nearly full density; furthermore, a sintering atmosphere control was performed using ferrite muffles to maintain both phases. Sintered materials consisting of granular metal particles (f 8.5 mu m) isolated and surrounded by a thin ferrite discontinuous layer (0.21.3 mu m) with a relative density =99.5% revealed both a saturation magnetization density =1 T and moderate permeability values of 3035 at 1 MHz., INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, WILEY-BLACKWELL, 9(6)1085 - 1097, Nov. 2012, English, Research paper (scientific journal)
  • Effect of the A-site cation on methane oxidation of perovskite-type (La1-xMx)CoO3 (M=Ca, Sr, and Ba)
    Hideki Taguchi, Keiko Matsu-ura, Masao Takada, Ken Hirota
    Perovskite-type (La1-xMx)CoO3 (M=Ca, Sr, and Ba) synthesized at 700 degrees C in air using the polymerizable complex method had a rhombohedral perovskite-type structure in the range of x &lt;= 0.06. The oxygen content of the samples was 2.98-2.99, and the specific surface area was 3.5-8.3 m(2)/g regardless of the M ion or x. The Rietveld method indicated that the Co-O distance was also constant regardless of the M ion or x, and that the Co-O-Co angle between two CoO6 octahedra increased with an increase in the ionic radius of the M ion. The temperature corresponding to the 50% conversion (T-1/2) of CH4 oxidation was almost constant for the samples with x=0.04, while the T-1/2 of the samples with x=0.06 was linearly lowered with increases in the ionic radius of the M ion. (C) 2012 Elsevier Inc. All rights reserved., JOURNAL OF SOLID STATE CHEMISTRY, ACADEMIC PRESS INC ELSEVIER SCIENCE, 190157 - 161, Jun. 2012, English, Research paper (scientific journal)
  • Fabrication of (Ca1-xLax)MnO3 Ceramics with a High Relative Density and their Power Factor
    Hideki Taguchi, Toshihisa Kugi, Masaki Kato, Ken Hirota
    Perovskite-type (Ca1-xLax)MnO3 powders were synthesized using a Pechini-type polymerized complex route, and (Ca1-x La-x)MnO3 ceramics with a high relative density (85.6%-98.5%) were fabricated using a planetary mill and cold-isostatic pressing (CIP) in order to improve the electrical and thermoelectric properties. The electrical conductivity (Sigma) was independent of both x and the temperature, and the relationship between log Sigma T vs. 1000/T was linear. The Seebeck coefficient (S) was negative, and the absolute value of the Seebeck coefficient increased with temperature and decreased with increases of x. The highest value of the power factor (S2 Sigma) was 3-4 x 10-4 W/(K2 center dot m) for the (Ca0.96La0.04)MnO3 ceramic., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, WILEY-BLACKWELL, 93(10)3009 - 3011, Oct. 2010, English, Research paper (scientific journal)
  • Fabrication of (Ca1-xLax)MnO3 Ceramics with a High Relative Density and their Power Factor
    Hideki Taguchi, Toshihisa Kugi, Masaki Kato, Ken Hirota
    Perovskite-type (Ca1-xLax)MnO3 powders were synthesized using a Pechini-type polymerized complex route, and (Ca1-x La-x)MnO3 ceramics with a high relative density (85.6%-98.5%) were fabricated using a planetary mill and cold-isostatic pressing (CIP) in order to improve the electrical and thermoelectric properties. The electrical conductivity (Sigma) was independent of both x and the temperature, and the relationship between log Sigma T vs. 1000/T was linear. The Seebeck coefficient (S) was negative, and the absolute value of the Seebeck coefficient increased with temperature and decreased with increases of x. The highest value of the power factor (S2 Sigma) was 3-4 x 10-4 W/(K2 center dot m) for the (Ca0.96La0.04)MnO3 ceramic., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, WILEY-BLACKWELL, 93(10)3009 - 3011, Oct. 2010, English, Research paper (scientific journal)
  • Preparation of zinc oxide ceramics with a sustainable antibacterial activity under dark conditions
    Ken Hirota, Maiko Sugimoto, Masaki Kato, Kazuhiko Tsukagoshi, Tooru Tanigawa, Hiroshi Sugimoto
    Fabrication of ZnO ceramics with a sustainable antibacterial activity even in the dark has been conducted. Fine ZnO powders were hydrothermally treated in 0.5-3 mol ml(-1) Zn(NO(3))(2) aqueous solutions at 110-180 degrees C for 3-20 h. After an uniaxial pressing of the ZnO powders thus prepared, they were sintered at 400-600 degrees C for I h in air. Sustainability in antibacterial activity was evaluated using a colony count method with Escherichia coli bacteria on nutrient agar medium (36 degrees C/24 h) in a Na-P-buffer solution. The best data was attained for the ZnO ceramics prepared from the following conditions: a 3 mol ml(-1) zinc nitrate solution for the hydrothermal treatment at 120 degrees C for 7 h and sintering in air with a step-by-step pattern (470 degrees C/1 h-485 degrees C/1 h-500 degrees C/1 h). ESR and chemical photoluminescence analyses have cleared that radical oxygen of super-oxide ((center dot)O(2)(-)) originated from the surface of ZnO might exhibit an antibacterial activity even under the dark condition. (C) 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved., CERAMICS INTERNATIONAL, ELSEVIER SCI LTD, 36(2)497 - 506, Mar. 2010, English, Research paper (scientific journal)
  • Crossover from Metallic to Semiconducting in Pb2-xLnxRu(2)O(7-delta) (Ln = Eu, Sm) Compounds with Pyrochlore Structure
    Masaki Kato, Tetsuya Imamaura, Miho Fukatsu, Ken Hirota
    We study the synthesis and physical properties of Pb(2-x)Ln(x)Ru(2)O(7-delta) (Ln = Sm, Eu) compounds with pyrochlore structure. The lead-lanthanoid pyrochlores Pb(2-x)Ln(x)Ru(2)O(7-delta) (Ln = Sm, Eu) have been successfully synthesized by the solid-state reaction. All samples were characterized by X-ray diffraction, magnetic susceptibility, and electrical resistivity measurements. Magnetic susceptibility measurements showed that the spin-glass transition temperature, T-SG, decreases with Ln content x and goes to zero at x = 0.2 and 0.6 for Sm and Eu, respectively. From electric resistivity measurements, a crossover from metallic to semiconducting behavior was observed in the range of 0.2 &lt;= x &lt;= 0.4 and 0.4 &lt;= x &lt;= 0.8 in Pb2-xSmxRu2O7-delta and Pb2-xEuxRu2O7-delta, respectively. These boundaries between metallic and insulating phases mostly correspond to values of x with T-SG = 0. These metal-insulator transitions can be caused by the strongly correlated couplings between the itinerant 4d electrons which are possibly attributed to the magnetic frustration., MULTISCALE, MULTIFUNCTIONAL AND FUNCTIONALLY GRADED MATERIALS, TRANS TECH PUBLICATIONS LTD, 631-632483 - 488, 2010, English, Research paper (international conference proceedings)
  • Crystal structure and methane oxidation on perovskite-type (La1-x Nd (x) )CoO3 synthesized using citric acid
    Hideki Taguchi, Sho Matsuoka, Masaki Kato, Ken Hirota
    Perovskite-type (La1-x Nd (x) )CoO3 was synthesized using citric acid at 700 A degrees C. The Rietveld method indicated that the crystal structure changed from a rhombohedral to an orthorhombic system at x = 0.4. The Co-O distance of the rhombohedral structure connected continuously with the average Co-O(2) distance of the orthorhombic structure, and the Co-O-Co angle of the rhombohedral structure and the Co-O(2)-Co angle of the orthorhombic structure were continuous. The average particle size of the samples was approximately 55 nm. CH4 oxidation started above 300 A degrees C, and the temperature corresponding to the 50% conversion (T (1/2)) of CH4 increased linearly with increases in x. It is considered that the amount of adsorbed oxygen decreased in response to the steric hindrance, and that T (1/2) increased as a result., JOURNAL OF MATERIALS SCIENCE, SPRINGER, 44(21)5732 - 5736, Nov. 2009, English, Research paper (scientific journal)
  • Simultaneous synthesis and densification of alpha-Zr(N)/ZrB2 composites by self-propagating high-temperature combustion under high nitrogen pressure
    Shingo Nakane, Takaya Endo, Ken Hirota
    Simultaneous synthesis and densification of alpha-Zr(N)/ZrB2 composites from a 85 mol% Zr/15 mol% B mixed-powder compacts have been achieved by self-propagating high-temperature under a nitrogen pressure of 10 MPa. Composites consist of fine and short rodlike ZrB2 grains (0.1 mu m(phi)-0.5 mu m(1)) dispersed into alpha-Zr(N) matrix (3 mu m). Dense composite materials (96.5% of theoretical) exhibit excellent mechanical properties, in which their bending strength and H, are 560 MPa and 6.5 GPa, respectively. This bending strength is much superior to those (205 and 480 MPa) of dense equi-axial alpha-Zr(N) (10 mu m) and dense ZrB2 (6 mu m). Fine and rodlike ZrB2 grains greatly enhanced their mechanical properties. (C) 2009 Elsevier Ltd and Techna Group S.r.l. All fights reserved., CERAMICS INTERNATIONAL, ELSEVIER SCI LTD, 35(6)2145 - 2149, Aug. 2009, English, Research paper (scientific journal)
  • Hopping conductivity of distorted K2NiF4-type (Ca1+xNd1-x)CrO4
    Hideki Taguchi, Kazuhiro Hirata, Hiroyasu Kido, Yasuo Takeda, Masaki Kato, Ken Hirota
    Orthorhombic distorted K2NiF4-type (Ca1+xNd1-x)CrO4 (0.00 &lt;= x &lt;= 0.15) was synthesized using a standard ceramic technique. The cell parameters (a and c) decreased, whereas the cell parameter (b) increased with the increase in x. The variation in the global instability index (GII) indicated that the crystal stability of (Ca1+xNd1-x)CrO4 was not influenced by the Cr4+ ion content. At all temperatures, the electrical conductivity (sigma) of (Ca1+xNd1-x)CrO4 increased with the increase in x. (Ca1+xNd1-x)CrO4 was a p-type semiconductor and exhibited hopping conductivity in a small-polaron model in the temperature range of 290 K &lt;= T &lt;= 713 K. The Cr4+ ion acts as an acceptor, and the electron transfer through the Cr3+-O-Cr4+ path becomes active as a result of the Cr4+ ion content and the Cr-O(1) distance. (C) 2009 Elsevier Masson SAS. All rights reserved., SOLID STATE SCIENCES, ELSEVIER SCIENCE BV, 11(7)1222 - 1225, Jul. 2009, English, Research paper (scientific journal)
  • TiO2(B) as a promising high potential negative electrode for large-size lithium-ion batteries
    Minoru Inaba, Yasuyuki Oba, Fumiharu Niina, Yosuke Murota, Yasuyuki Ogino, Akimasa Tasaka, Ken Hirota
    Needle-like TiO2(B) powder was obtained from K2Ti4O9 precursor by ion exchange to protons, followed by dehydration. The charge and discharge characteristics of the TiO2(B) powder were investigated as a high potential negative electrode in lithium-ion batteries. It had a high discharge capacity of 200-250 mAh g(-1) at around 1.6V vs. Li/Li+, which was comparable with that of TiO2(B) nanowires and nanotubes prepared via a hydrothermal reaction in alkaline solution. It showed very good cycleability, and gave a discharge capacity of 170 mAh g(-1) even in the 650th cycle. It also had a high rate capability, and gave a discharge capacity of 106 mAh g(-1) even at 10 degrees C. In most of ethylene carbonate-based solutions, the TiO2(B) powder exhibited good charge and discharge characteristics. However, it showed a poor compatibility with LiBF4, propylene carbonate, and gamma-butyrolactone. The TiO2(B) powder showed good cycle performance in the presence of a non-flammable additive, trimethyl phosphate, up to 20vol.%, and a high tolerance to water up to 1000 ppm. It was also found that inexpensive Al foil can be used as a current collector of the TiO2(B) powder instead of Cu foil without sacrificing the performance. (c) 2008 Elsevier B.V. All rights reserved., JOURNAL OF POWER SOURCES, ELSEVIER SCIENCE BV, 189(1)580 - 584, Apr. 2009, English, Research paper (scientific journal)
  • Behavior of Mn3+ ions in four-layered hexagonal (Sr1-x-yLaxBay)MnO3
    Hideki Taguchi, Kazuhiro Hirata, Masaki Kato, Ken Hirota
    Stoichiometric four-layered hexagonal (4H) (Sr1-x-yBaxLay)MnO3 was synthesized using a standard ceramic technique. Rietveld analysis at room temperature indicated that the Mn-O(1) distance increased and the Mn-O(2) distance decreased with the increase in x. The samples were n-type semiconductors and exhibited hopping conductivity in a small-polaron model below 533 K. The Mn3+ ion acted as a donor and the electron transfer became active through the Mn3+-O-Mn4+ path. The samples were antiferromagnetic and the Neel temperature (T-N) was constant regardless of y when x was fixed to 0.3, whereas T-N shifted to a high temperature when y was fixed to 0.02. The face-sharing Mn3+-O(2)-Mn4+ interaction strengthened as the Mn-O(2) distance decreased, and TN shifted to a high temperature as a result. (c) 2008 Elsevier Masson SAS. All rights reserved., SOLID STATE SCIENCES, ELSEVIER SCIENCE BV, 11(2)491 - 495, Feb. 2009, English, Research paper (scientific journal)
  • The Study on Carbon Nanofiber (CNF)-Dispersed B4C Composites
    Ken Hirota, Yoshihiro Nakayama, Masaki Kato, Shingo Nakane, Toshiyuki Nishimura
    Carbon nanofiber (CNF)-dispersed B4C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800 degrees C/10 min/30 MPa). A 15 vol% CNF/B4C composite with similar to 99% of dense homogeneous microstructures (similar to 0.40 mu m grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (Sigma(b)) of similar to 710 MPa, a Vickers hardness (H-v) of similar to 36 GPa, a fracture toughness (K-IC) of similar to 7.9 MPa m(1/2), and high-temperature Sigma(b) of 590 MPa at 1600 degrees C in N-2. Interfaces between the CNF and the B4C matrix were investigated using high-resolution transmission electron microscopy, EDS, and electron energy-loss spectroscopy., INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, WILEY-BLACKWELL, 6(5)607 - 616, 2009, English, Research paper (scientific journal)
  • The Study on Carbon Nanofiber (CNF)-Dispersed B4C Composites
    Ken Hirota, Yoshihiro Nakayama, Masaki Kato, Shingo Nakane, Toshiyuki Nishimura
    Carbon nanofiber (CNF)-dispersed B4C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800 degrees C/10 min/30 MPa). A 15 vol% CNF/B4C composite with similar to 99% of dense homogeneous microstructures (similar to 0.40 mu m grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (Sigma(b)) of similar to 710 MPa, a Vickers hardness (H-v) of similar to 36 GPa, a fracture toughness (K-IC) of similar to 7.9 MPa m(1/2), and high-temperature Sigma(b) of 590 MPa at 1600 degrees C in N-2. Interfaces between the CNF and the B4C matrix were investigated using high-resolution transmission electron microscopy, EDS, and electron energy-loss spectroscopy., INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, WILEY-BLACKWELL, 6(5)607 - 616, 2009, English, Research paper (scientific journal)
  • CO oxidation on perovskite-type LaCoO3 synthesized using ethylene glycol and citric acid
    Hideki Taguchi, Satoshi Yamasaki, Atsushi Itadani, Masashi Yosinaga, Ken Hirota
    In order to synthesize perovskite-type LaCoO3 with good surface crystallinity, the gel prepared by adding both ethylene glycol (EG) and citric acid (CA) to the aqueous solution of La(NO3)3 center dot 6H(2)O and Co(NO3)(2) center dot 6H(2)O was fired at 600 degrees C in air for 3 h. The transmission electron microscopy (TEM) observation indicated that the particles of LaCoO3 tended to have a uniform shape at EG/CA = 4. Although, the specific surface area of LaCoO3 synthesized using both EG and CA was slightly smaller than that of LaCoO3 synthesized using only CA, the catalytic activity of CO oxidation became higher by adding EG. (C) 2008 Elsevier B.V. All rights reserved., CATALYSIS COMMUNICATIONS, ELSEVIER SCIENCE BV, 9(9)1913 - 1915, May 2008, English, Research paper (scientific journal)
  • Thermal expansion and mechanical properties of phenolic resin/ZrW2O8 composites
    Jun-ichi Tani, Hajime Kimura, Ken Hirota, Hiroyasu Kido
    Phenolic resin/ZrW2O8 composites were successfully fabricated and their coefficient of thermal expansion (CTE) as well as mechanical properties was investigated. The CTE of the composites decreases from 46 X 10(-6) to 14 X 10(-6) K-1 when the ZrW2O8 volume fraction increases from 0 to 52 vol %. The CTE of the composites is analyzed by some theoretical models; Schapery's upper bound provides the best estimate of the reduction in CTE. The Barcol hardness of the composites increases with an increase in the ZrW2O8 volume fraction. The bending strength of the composites with 19-25 vol % of ZrW2O8 fillers shows a maximum value of 130 MPa, which is 45% larger than that of phenolic resin without fillers. (c) 2007 Wiley Periodicals, Inc., JOURNAL OF APPLIED POLYMER SCIENCE, JOHN WILEY & SONS INC, 106(5)3343 - 3347, Dec. 2007, English, Research paper (scientific journal)
  • Fabrication of carbon nanofiber(CNF)-dispersed Al2O3 composites by pulsed electric-current pressure sintering and their mechanical and electrical properties
    Ken Hirota, Yuichi Takaura, Masaki Kato, Yoshinari Miyamoto
    Dense Al2O3-based composites (&gt;= 99.0% of theoretical) dispersed with carbon nanofibers (CNFs) were fabricated using the pulsed electric-current pressure sintering (PECPS) for 5 min at 1300 degrees C and 30 MPa in a vacuum. The dispersion of CNFs into the matrix depended much on the particle size of the starting Al2O3 powders. Mechanical properties of the composites were evaluated in relation with their microstructures; high values of three-point bending strength sigma(b) (similar to 800 MPa) and fracture toughness K-IC (similar to 5 MPa.m(1/2)) were attained at the composition of CNF/Al2O3 = 5:95 vol%, which sigma(b) and KIC values were similar to 25% and similar to 5%, respectively, higher than those of monolithic Al2O3. This might be due to the small Al2O3 grains (1.6 mu m) of dense sintered compacts compared with that (4.4 mu m) for the pure Al2O3 ceramics, resulting from the suppression of grain growth during sintering induced by uniformly dispersed CNFs in the matrix. Electrical resistivity of CNF/Al2O3 composites decreased rapidly from &gt;10(15) to similar to 2.1 x 10(-2) Omega m (5vol%CNF addition), suggesting the machinability of Al2O3-based composites by electrical discharge machining., JOURNAL OF MATERIALS SCIENCE, SPRINGER, 42(13)4792 - 4800, Jul. 2007, English, Research paper (scientific journal)
  • Fabrication of carbon nanofiber(CNF)-dispersed Al2O3 composites by pulsed electric-current pressure sintering and their mechanical and electrical properties
    Ken Hirota, Yuichi Takaura, Masaki Kato, Yoshinari Miyamoto
    Dense Al2O3-based composites (&gt;= 99.0% of theoretical) dispersed with carbon nanofibers (CNFs) were fabricated using the pulsed electric-current pressure sintering (PECPS) for 5 min at 1300 degrees C and 30 MPa in a vacuum. The dispersion of CNFs into the matrix depended much on the particle size of the starting Al2O3 powders. Mechanical properties of the composites were evaluated in relation with their microstructures; high values of three-point bending strength sigma(b) (similar to 800 MPa) and fracture toughness K-IC (similar to 5 MPa.m(1/2)) were attained at the composition of CNF/Al2O3 = 5:95 vol%, which sigma(b) and KIC values were similar to 25% and similar to 5%, respectively, higher than those of monolithic Al2O3. This might be due to the small Al2O3 grains (1.6 mu m) of dense sintered compacts compared with that (4.4 mu m) for the pure Al2O3 ceramics, resulting from the suppression of grain growth during sintering induced by uniformly dispersed CNFs in the matrix. Electrical resistivity of CNF/Al2O3 composites decreased rapidly from &gt;10(15) to similar to 2.1 x 10(-2) Omega m (5vol%CNF addition), suggesting the machinability of Al2O3-based composites by electrical discharge machining., JOURNAL OF MATERIALS SCIENCE, SPRINGER, 42(13)4792 - 4800, Jul. 2007, English, Research paper (scientific journal)
  • Mechanical properties of simultaneously synthesized and consolidated carbon nanofiber (CNF)-dispersed SiC composites by pulsed electric-current pressure sintering
    Ken Hirota, Hiroaki Hara, Masaki Kato
    Carbon nanofiber (CNF) dispersed beta-SiC composites with the addition of 0.2 mass% boron and 2.0 mass% carbon as sintering aids have been synthesized and consolidated simultaneously from mixtures of Si, amorphous C and B powders and CNF by pulsed electric-current pressure sintering (PECPS). Synthesis and consolidation process, which were observed from their expansion and shrinkage curves during PECPS, have been examined using X-ray diffraction and scanning electron microscopy for the powder compacts. CNF/SiC composites sintered at 1800 degrees C for 10 min under 40 MPa in a vacuum have similar to 96.0% of theoretical density and homogeneous structures consisting of similar to 4.0 mu m grains. A 10 vol% CNF/SiC composite exhibited excellent mechanical properties: a bending strength of similar to 720 MPa, a Vickers hardness of similar to 26.0 GPa, and a fracture toughness of similar to 5.5 MPa m(1/2). High-temperature bending strength of similar to 890 MPa at 1200 degrees C in air was attained with the same nanocomposites. (c) 2006 Elsevier B.V. All rights reserved., MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, ELSEVIER SCIENCE SA, 458(1-2)216 - 225, Jun. 2007, English, Research paper (scientific journal)
  • Synthesis and characterization of K2NiF4-type CaLnCoO(4) (Ln = Sm and Gd)
    Hideki Taguchi, Katsuyuki Nakade, Ken Hirota
    K2NiF4-type CaLnCoO(4) (Ln = Sm and Gd) has been synthesized at 1173 or 1223 K in air using citric acid (CA) and ethylene glycol (EG). CaLnCoO(4) (Ln = Sm and Gd) has an orthorhombic structure with the space group Bmab. The average particle sizes are approximately 300 nm for CaSmCoO4 and approximately 170 nm for CaGdCoO4, respectively. The global instability index (GII) indicates that the crystal structure of CaGdCoO4 is more stable than that of CaSmCoO4, CaLnCoO(4) (Ln = Sm and Gd) is a p-type semiconductor and shows paramagnetic behavior above 5 K. The 1/chi-T curve of CaSmCoO4 deviates from the Curie-Weiss law, whereas the 1/chi-T curve of CaGdCoO4 follows the Curie-Weiss law in the temperature range of 5 &lt;= T &lt;= 300 K. From the values of the observed effective magnetic moment (mu(eff)) of CaLnCoO(4) (Ln = Sm and Gd), it is considered that the spin state of the Co3+ ion is low. (c) 2006 Elsevier Ltd. All rights reserved., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 42(4)649 - 656, Apr. 2007, English, Research paper (scientific journal)
  • Fabrication of Al-added TiN materials by the combination of double self-propagating high-temperature synthesis and pulsed electric-current pressure sintering
    Shingo Nakane, Sho Hikawa, Takaya Endo, Masaki Kato, Ken Hirota, Hiroya Miyauchi, Tadashi Hioki
    Crystalline phases of Al-added TiN, denoted as (Ti1-xAlx)N-y (0 &lt;= x &lt;= 0.10, 0.8 &lt; y &lt; 1.0), prepared from a mixture of Ti and Al powders by self-propagating high-temperature synthesis (SHS) in a nitrogen atmosphere, have been investigated. By repeating SHS twice, in the region of 0.0 &lt; x &lt;= 0.02 cubic (Ti1-xAlx)N-y solid solutions, and in the region of 0.02 &lt; x &lt;= 0.10 composites consisting of (Ti1-xAlx)N-y and hexagonal Ti2AlN were formed. After powder characterization, they were consolidated to dense materials (&gt; 97% of theoretical) by pulsed electric-current pressure sintering. With increasing Al addition, the optimum sintering temperatures were lowered, followed by reduction of grain size. Their mechanical properties, that is, three-point bending strength sigma(b), Vickers hardness H-v, and fracture toughness K-IC were evaluated as a function of Al content., INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, BLACKWELL PUBLISHING, 4(2)175 - 183, 2007, English, Research paper (scientific journal)
  • Simultaneous synthesis and consolidation of W-added ZrB2 by pulsed electric current pressure sintering and their mechanical properties
    Ken Hirota, Takaya Endo, Masaki Kato, Shingo Nakane, Toshiyuki Nishimura, Yoshiaki Morisada, Kiyoshi Mizuuchi
    Dense zirconium boride (ZrB2)-based materials with and without tungsten (W) have been fabricated directly from mixtures of constituent elemental powders by pulsed electric current pressure sintering (PECPS) at 1800 degrees C for 10 min under 30 MPa in a vacuum. Formation processes of monolithic, W-doped ZrB2 solid solutions (Zr1-xWx)B-2 (0&lt;x &lt;= 0.12), and composites consisting of ZrB2(ss) and WB2 were investigated. Their mechanical properties of Vickers hardness (H-v), fracture toughness (K-IC), and bending strength (sigma(b)) at room temperature were evaluated. Solid solution and composite materials gave higher H-v (similar to 20.7 GPa), K-IC (similar to 4.4 MPam(1/2)), and sigma(b) (similar to 600 and 690 MPa for the former and the latter, respectively) than those (14.1 GPa, 3.21 MPam(1/2), and similar to 500 MPa) of the monolithic ZrB2 fabricated under the same conditions. Furthermore, the latter two materials exhibited excellent high-temperature sigma(b) values (similar to 550-600 MPa) up to 1600 degrees C in N-2, in comparison with that (similar to 320 MPa) of monolithic ZrB2 materials., PRICM 6: SIXTH PACIFIC RIM INTERNATIONAL CONFERENCE ON ADVANCED MATERIALS AND PROCESSING, PTS 1-3, TRANS TECH PUBLICATIONS LTD, 561-565527 - +, 2007, English, Research paper (international conference proceedings)
  • Hole doping effects on spin-gapped Na2Cu2TeO6 via topochemical Na deficiency
    Kumiko Morimoto, Yutaka Itoh, Kazuyoshi Yoshimura, Masaki Kato, Ken Hirota
    We report the magnetic susceptibility and NMR studies of a spin-gapped layered compound Na2Cu2TeO6 (the spin gap Delta similar to 250 K), the hole doping effect on the Cu2TeO6 plane via a topochemical Na deficiency by soft chemical treatment, and the static spin vacancy effect by nonmagnetic impurity Zn substitution for Cu. A finite Knight shift at the Te-125 site was observed for pure Na2Cu2TeO6. The negative hyperfine coupling constant (125)A(tr) is an evidence for the existence of a superexchange pathway of the Cu-O-Te-O-Cu bond. It turned out that both the Na deficiency and Zn impurities induce a Curie-type magnetism in the uniform spin susceptibility in an external magnetic field of 1 T, but only the Zn impurities enhance the low-temperature Na-23 nuclear spin-lattice relaxation rate whereas the Na deficiency suppresses it. A spin glass behavior was observed for the Na-deficient samples but not for the Zn-substituted samples. The dynamics of the unpaired moments of the doped holes are different from that of the spin vacancy in the spin-gapped Cu2TeO6 planes., JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, PHYSICAL SOC JAPAN, 75(8), Aug. 2006, English, Research paper (scientific journal)
  • Simultaneous synthesis and consolidation of chromium carbides (Cr3C2, Cr7C3 and Cr23C6) by pulsed electric-current pressure sintering
    K Hirota, K Mitani, M Yoshinaka, O Yamaguchi
    Chromium carbides (Cr3C2,Cr7C3 and Cr23C6) have been synthesized and consolidated simultaneously front mixtures of Cr and amorphous carbon powders by pulsed electric-current pressure sintering (PECPS). Dense ceramics thus obtained were composed of chromium carbides with a small amount of Cr,03, which originates from a trace amount of oxygen adsorbed on the as-received starting Cr powder. Synthesis and consolidation processes, which were observed from their shrinkage curves during PECPS, have been examined using X-ray diffraction (XRD) and scanning electron microscopy for the powder compacts. Cr(3)C(2)ceramics sintered at 1300 C for 10 min tinder 30 MPa have a 98.9% of theoretical density and fine structures with a 3.6 mu m grain size. They exhibit excellent mechanical properties: a bending strength sigma(b) of 690 MPa, a Vickers hardness H(1)of 18.9 GPa and a fracture toughness K-10 of 7.1 Mpa m(1/2). (c) 2005 Elsevier B.V. All rights reserved., MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, ELSEVIER SCIENCE SA, 399(1-2)154 - 160, Jun. 2005, English, Research paper (scientific journal)
  • Synthesis of perovskite-type (La1-xCax)FeO3 (0 <= x <= 0.2) at low temperature
    H Taguchi, Y Masunaga, K Hirota, O Yamaguchi
    Gel formation was realized by adding citric acid to a solution of La(NO3)(3)center dot 5H(2)O, Ca(NO3)(2)center dot 4H(2)O, and Fe(NO3)(2)center dot 9H(2)O. Perovskite-type (La1-xCax)FeO3 (0 &lt;= x &lt;= 0.2) was synthesized by firing the gel at 500 degrees C in air for 1 h. The crystallite size (D-1 2 1) decreased with increasing x, while the specific surface area was 6.8-9.4 m(2)/g and independent of x. The XPS measurement of the (La1-xCax)FeO3 surface indicated that the Ca2+ ion content increased with increasing x, while the Fe ion content was independent of x. Catalytic activity for CO oxidation increased with increasing x. (c) 2005 Elsevier Ltd. All rights reserved., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 40(5)773 - 780, May 2005, English, Research paper (scientific journal)
  • Formation and sintering of (Y1-xCex)MnO3 (0 <= x <= 0.06) via a citric acid process
    K Hirota, K Fujita, M Yoshinaka, O Yamaguchi
    The formation and sintering of (Y1-xCex)MnO3solid solution ceramics, where 0?x?0.06, via a citric acid process was investigated. The effect of Ce addition to YMnO3also studied. The x-ray diffraction (XRD) analysis, differential thermal analysis (DTA) and transmission electron microscopy (TEM) were used for the study. It was observed that the dc electrical conductivity of the solid solution increased and the activation energy decreased with increased Ce content at high temperature., JOURNAL OF MATERIALS SCIENCE, SPRINGER, 40(3)801 - 804, Feb. 2005, English, Research paper (scientific journal)
  • Low-temperature synthesis of perovskite-type (Nd1-xCax)MnO3 (0.0 <= x <= 0.5) via polymerized complex route
    H Taguchi, M Io, M Yoshinaka, K Hirota, O Yamaguchi
    To synthesize perovskite-type (Nd1-xCax)MnO3 (0.0 less than or equal to x less than or equal to 0.5) at relatively low temperatures as 1273 K, a homogeneous gel has been prepared by adding an equimolar amount of citric acid and ethylene glycol to the mixed aqueous solutions of Nd(NO3)(3).5H(2)O, Ca(NO3)(2).4H(2)O and Mn(NO3)(2).6H(2)O. By heating the gel, we could reduce the synthetic temperature of Ca-doped NdMnO3. The oxygen content of the sample was slightly higher than 3.00 and independent of x. The crystallite and average particle sizes revealed maximum values, ca. 27.8 and. 56.0 nm, respectively, at x=0.1. The sample was an n-type semiconductor; the electrical conductivity (sigma) and Seebeck coefficient (alpha) as a function of temperature (T) were examined. Since the log sigmaT- 1000/T and the alpha - Tcurves varied linearly, it is obvious that the sample exhibits the hopping conduction on the small polaron model. (C) 2004 Elsevier B.V. All rights reserved., SOLID STATE IONICS, ELSEVIER SCIENCE BV, 172(1-4)611 - 615, Aug. 2004, English, Research paper (scientific journal)
  • Formation, sintering, and electrical conductivity of perovskite Sm(Cr1-xMgx)O-3 (0 <= x <= 0.23) prepared by the hydrazine method
    K Hirota, M Io, H Hatta, M Yoshinaka, O Yamaguchi
    The presence of SmCrO4 is experimentally established. In Mg2+-substituted SmCrO3, single-phase perovskite Sm(Cr1-xMgx)O-3, where x=0-0.23, are formed at similar to830degreesC by decomposition of Sm(Cr1-xMgx)O-4 which crystallizes at 530-570degreesC from amorphous materials prepared by the hydrazine method. Sm(Cr1-xMgx)O-3 solid solution powders consisting of submicrometer-size particles are sinterable; dense materials can be fabricated by sintering for 2h at 1700degreesC in air. The relative densities, grain sizes, and electrical conductivities increase with increased Mg2+ content. Sm(Cr0.77Mg0.23)O-3 materials exhibit an excellent direct current electrical conductivity of 2.2 x 10(3) S m(-1) at 1000degreesC. (C) 2003 Elsevier Science (USA). All rights reserved., JOURNAL OF SOLID STATE CHEMISTRY, ACADEMIC PRESS INC ELSEVIER SCIENCE, 174(1)80 - 86, Aug. 2003, English, Research paper (scientific journal)
  • Formation, sintering, and electrical conductivity of perovskite Sm(Cr1-xMgx)O-3 (0 <= x <= 0.23) prepared by the hydrazine method
    K Hirota, M Io, H Hatta, M Yoshinaka, O Yamaguchi
    The presence of SmCrO4 is experimentally established. In Mg2+-substituted SmCrO3, single-phase perovskite Sm(Cr1-xMgx)O-3, where x=0-0.23, are formed at similar to830degreesC by decomposition of Sm(Cr1-xMgx)O-4 which crystallizes at 530-570degreesC from amorphous materials prepared by the hydrazine method. Sm(Cr1-xMgx)O-3 solid solution powders consisting of submicrometer-size particles are sinterable; dense materials can be fabricated by sintering for 2h at 1700degreesC in air. The relative densities, grain sizes, and electrical conductivities increase with increased Mg2+ content. Sm(Cr0.77Mg0.23)O-3 materials exhibit an excellent direct current electrical conductivity of 2.2 x 10(3) S m(-1) at 1000degreesC. (C) 2003 Elsevier Science (USA). All rights reserved., JOURNAL OF SOLID STATE CHEMISTRY, ACADEMIC PRESS INC ELSEVIER SCIENCE, 174(1)80 - 86, Aug. 2003, English, Research paper (scientific journal)
  • Formation, densification, and electrical conductivity of air-sinterable (Sm1-xCax) CrO3 prepared through citric acid route
    K Hirota, H Hatta, M Io, M Yoshinaka, O Yamaguchi
    In Ca2+-substituted SmCrO3, single-phase perovskite (Sm1-xCax) CrO3, where x = 0 to 0.27, have been formed at low temperatures by a citric acid processing. (Sm1-xCax) CrO3 powders consisting of submicrometer-size particles are sinterable; dense materials can be fabricated by sintering for 2 h at 1700 degreesC under atmospheric pressure. The relative densities, grain sizes, and electrical conductivities increase with increased Ca2+ content. (Sm0.73Ca0.27) CrO3 materials show an excellent electrical conductivity of 2.6 x 10(3) S . m(-1) at 1000 degreesC. (C) 2003 Kluwer Academic Publishers., JOURNAL OF MATERIALS SCIENCE, SPRINGER, 38(16)3431 - 3435, Aug. 2003, English, Research paper (scientific journal)
  • Sol-gel-derived TiO2/poly(dimethylsiloxane) hybrid films and their photocatalytic activities
    K Iketani, RD Sun, M Toki, K Hirota, O Yamaguchi
    TiO2/Poly(dimethylsiloxane) (PDMS) hybrid films with various ratios were prepared on poly(methyl methacrylate) (PMMA) substrates by a sol-gel method. The films were highly transparent (transmittance &gt; 90%), and amorphous TiO2 in the films crystallized in anatase upon treatment at 100 degreesC for 1 h at an atmosphere with relative humidity higher than 60%. They showed good photocatalytic activity for the decomposition of methylene blue and acetaldehyde, though the reactivity exhibited a slightly decreasing trend with the increase of PDMS content. Photodegradation of the PMMA substrate was greatly suppressed by the hybridization, which demonstrates the possibility to apply the films to organic substrates as photocatalysts. (C) 2002 Elsevier Science Ltd. All rights reserved., JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, PERGAMON-ELSEVIER SCIENCE LTD, 64(3)507 - 513, Mar. 2003, English, Research paper (scientific journal)
  • Fabrication of new cemented carbide containing diamond coated with nanometer-sized SiC particles
    Y Miyamoto, T Kashiwagi, K Hirota, O Yamaguchi, H Moriguchi, K Tsuduki, A Ikegaya
    A simple process for depositing a coating of silicon carbide (SiC) crystallites similar to10 mn in size onto diamond particles has been developed. SiO powders react with diamond in a vacuum at 1350degreesC to form a uniform beta-SiC polycrystalline layer similar to60 nm thick. The SiC coating improves the oxidation resistance of the diamond. A cemented carbide material containing 20-vol%-SiC-coated diamond particles was sintered to a relative density of 99.5% by pulsed-electric-current sintering. A Vickers hardness and indentation fracture toughness of 15 GPa and 16.3 MPa(.)m(1/2), respectively, were obtained. This toughness is two times higher than that of cemented carbide containing no particles. The higher toughness is attributed to deflection and blockage of crack propagation by the diamond particles., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 86(1)73 - 76, Jan. 2003, English, Research paper (scientific journal)
  • Soft solution preparation methods in a ZrO2-Al2O3 binary system
    S Kikkawa, A Kijima, K Hirota, O Yamaguchi
    Two types of soft solution preparation methods were investigated for the production of zirconia in the presence of alumina or yttria from mixed aqueous solution of zirconyl chloride and aluminum or yttrium chloride. Tetragonal zirconia powder was obtained below 1000 degreesC by firing the mixed solution gel, which was prepared by adding citric acid. The particle size distribution was much narrower than the product obtained by a conventional coprecipitation method. For the second method, microgravity was applied for 4.5 s to the freezing of the mixed chloride aqueous solution. The precipitates were freeze-dried and then fired to 1000 degreesC in TG-DTA. The dried precipitates showed good crystallinity and homogenous mixing of the metal salts when the freezing of aqueous solution was started under microgravity. The main crystalline phase after firing was tetragonal ZrO2 due to a homogenous doping of Y2O3 and a homogenous dispersion of Al2O3, respectively. (C) 2002 Elsevier Science B.V. All rights reserved., SOLID STATE IONICS, ELSEVIER SCIENCE BV, 151(1-4)359 - 364, Nov. 2002, English, Research paper (scientific journal)
  • Formation, sintering, and electrical conductivity of (Nd1-xCax)CrO3 (0 <= x <= 0.25) using citric acid as a gelling agent
    K Hirota, Y Kunifusa, M Yoshinaka, O Yamaguchi
    In Ca2+-substituted NdCrO3, single-phase perovskite compounds (Nd1-xCax)CrO3, where x = 0-0.25, have been formed by a citric acid processing. (Nd1-xCax)CrO3 powders consisting of submicrometer-size particles are sinterable; dense materials can be fabricated by sintering for 2 h at 1700degreesC under atmospheric pressure. The relative densities, grain sizes, and electrical conductivities increase with increased Ca2+ content. (Nd0.75Ca0.25)CrO3 materials show an excellent electrical conductivity of 1.9 x 10 S m(-1) at 1000degreesC. (C) 2002 Elsevier Science Ltd. All rights reserved., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 37(14)2335 - 2344, Nov. 2002, English, Research paper (scientific journal)
  • Fabrication and mechanical properties of monolithic MoSi2 by spark plasma sintering
    H Shimizu, M Yoshinaka, K Hirota, O Yamaguchi
    Fine MoSi2 powders containing a small amount of Mo5Si3 have been prepared by self-propagating high-temperature synthesis (SHS), followed by spark plasma sintering (SPS) for 10 min at 1200-1500degreesC and 30 MPa. Dense MoSi2 materials, in which the grain size is similar to7.5 mum, have been fabricated at 1300degreesC. They exhibit excellent mechanical properties Vicker's hardness H-v (10.6 GPa), fracture toughness K-IC (4.5 MPa m(1/2)), and bending strength sigma(b) (560 MPa). The strength of 325 MPa can be retained up to 1000degreesC. (C) 2002 Elsevier Science Ltd. All rights reserved., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 37(9)1557 - 1563, Aug. 2002, English, Research paper (scientific journal)
  • Fabrication and mechanical properties of almost fully-densified Cr2O3 ceramics
    K Hirota, H Motono, M Yoshinaka, O Yamaguchi
    A piston-cylinder-type high-pressure apparatus with a sample cell of larger sizes was designed to fabricate Cr2O3ceramics. Almost fully-densified Cr2O3ceramics were fabricated by sintering for 0.5 h at 1100°C and 1 GPa., JOURNAL OF MATERIALS SCIENCE LETTERS, KLUWER ACADEMIC PUBL, 21(11)853 - 854, Jun. 2002, English, Research paper (scientific journal)
  • Mechanical properties of CoAl materials with the combined additions of ZrO2(3Y) and Al2O3
    Y Takano, M Yoshinaka, K Hirota, O Yamaguchi
    Intermetallic CoAl powder has been prepared via self-propagating high-temperature synthesis (SHS). Dense CoAl materials (99.6% of theoretical) with the combined additions of ZrO2(3Y) and Al2O3 have been fabricated via spark plasma sintering (SPS) for 10 min at 1300 degreesC and 30 MPa. The microstructures are such that tetragonal ZrO2 (0.3 mum) and Al2O3 (0.5 mum) particles are located at the grain boundaries of the CoAl (8.5 mum) matrix. Improved mechanical properties are obtained; especially the fracture toughness and the bending strength of the materials with ZrO2(3Y)/Al2O3 = 16/4 mol% are 3.87 MPa(.)m(1/2) and 1080 MPa, respectively, and high strength (&gt; 600 MPa) can be retained up to 1000 degreesC., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 84(10)2445 - 2447, Oct. 2001, English, Research paper (scientific journal)
  • Formation, densification, and electrical conductivity of air-sinterable Y(Cr1-xMgx)O-3 prepared by the hydrazine method
    T Tachiwaki, Y Kunifusa, M Yoshinaka, K Hirota, O Yamaguchi
    In Mg2+-substituted YCrO3, single-phase perovskite compounds Y(Cr1-xMgx)O-3, where x = 0-0.1, have been formed at low temperatures (similar to 850 degreesC) by decomposition of Y(Cr1-xMgx)O-4 which crystallizes at similar to 600 degreesC from amorphous materials prepared by the hydrazine method. Y(Cr1-xMgx)O-3 powders consisting of submicrometer-size particles are sinterable; dense ceramics can be fabricated by sintering for 4 h at 1800 degreesC under atmospheric pressure. The relative densities, grain sizes, and electrical conductivity increase with increased Mg2+-content. Y(Cr0.9Mg0.1)O-3 ceramics show a good electrical conductivity at elevated temperatures. (C) 2001 Elsevier Science B.V. All rights reserved., MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, ELSEVIER SCIENCE SA, 86(3)255 - 259, Oct. 2001, English, Research paper (scientific journal)
  • Hot isostatic pressing of chromium nitrides (Cr2N and CrN) prepared by self-propagating high-temperature synthesis
    K Hirota, Y Takano, M Yoshinaka, O Yamaguchi
    Cr2N, CrN, and their mixtures (with desired fractions) have been prepared via self-propagating high-temperature synthesis (SHS) under a controlled nitrogen pressure, followed by hot isostatic pressing at 1300 degreesC and 196 MPa under an atmosphere of argon gas. The combustion temperature increased as the nitrogen pressure increased. Single-phase CrN was formed at 1040 degreesC under a pressure of 0.18 MPa, and single-phase CrN was formed at 1730 degreesC under a pressure of 2 MPa. The mechanical properties of the dense nitride ceramics (99.2% of the theoretical density) have been examined; the Vickers hardness (11.2 GPa. for CrN and 14.5 GPa for Cr2N) increased linearly as the fraction of Cr2N increased, whereas the fracture toughness (similar to4.7 MPa(.)m(1/2)) and bending strength (similar to 355 MPa) are constant, regardless of the fraction of Cr2N/CrN., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 84(9)2120 - 2122, Sep. 2001, English, Research paper (scientific journal)
  • Microstructure and mechanical behavior of Cr2O3/10 vol% W composites fabricated by spark plasma sintering
    Y Takano, K Fujii, M Yoshinaka, K Hirota, O Yamaguchi
    JOURNAL OF MATERIALS SCIENCE LETTERS, KLUWER ACADEMIC PUBL, 20(1)73 - 75, Jan. 2001, English, Research paper (scientific journal)
  • Microstructure and mechanical behavior of Cr2O3/10 vol% W composites fabricated by spark plasma sintering
    Y Takano, K Fujii, M Yoshinaka, K Hirota, O Yamaguchi
    Spark plasma sintering was used to fabricate the Cr2O3refractory material. The metal particulate dispersions, observed in ceramic materials, show a high possibility for improving their mechanical properties. Scanning electron microscopy and transmission electron microscopy were used for making microstructural observations of Cr2O3and WO3refractory materials., JOURNAL OF MATERIALS SCIENCE LETTERS, KLUWER ACADEMIC PUBL, 20(1)73 - 75, Jan. 2001, English, Research paper (scientific journal)
  • Novel synthesis Of Y3Al5O12 (YAG) leading to transparent ceramics
    T Tachiwaki, M Yoshinaka, K Hirota, T Ikegami, O Yamaguchi
    A YAlO3 (hexagonal) solid solution with a = 0.3678 nm and c = 1.046 nm crystallizes at 880-935 degreesC from an amorphous material prepared in the mole ratio Y3+/Al3+ = 3 : 5 by a sol-gel technique using hydrazine. The hexagonal YAlO3 phase transforms to Y3Al5O12 (YAG) at 1005-1075 degreesC. The YAG powder calcined at 1200 degreesC can be densified to transparency under vacuum for 2 h at 1600 degreesC without additives. (C) 2001 Elsevier Science Ltd. All rights reserved., SOLID STATE COMMUNICATIONS, PERGAMON-ELSEVIER SCIENCE LTD, 119(10-11)603 - 606, 2001, English, Research paper (scientific journal)
  • Low-temperature synthesis and sintering of LaAlO3
    K Iketani, M Yoshinaka, K Hirota, O Yamaguchi
    A powder preparation method using hydrazine monohydrate was used to obtain reactive LaAlO3powder. LaAlO3crystallized at low temperatures (800-870°C) from an amorphous material. The LaAlO3powder observed by transmission electron microscopy consisted of aggregates of translucent particles of ?50 nm. The dielectric constant of the ceramics was also determined., JOURNAL OF MATERIALS SCIENCE LETTERS, KLUWER ACADEMIC PUBL, 20(22)2045 - 2046, 2001, English, Research paper (scientific journal)
  • Formation, sinterability, and characteristics of La(Cr1-xMgx)O-3 prepared by a sol-gel technique using hydrazine
    K Azegami, M Yoshinaka, K Hirota, O Yamaguchi
    In Mg2+-substituted LaCrO3, single-phase perovskite compounds La(Cr1-xMgx)O-3, where x = 0 to 0.28, have been formed at low temperatures (similar to 800 degrees C) by decomposition of La(Cr1-xMgx)O-4 which crystallizes at similar to 550 degrees C from amorphous materials prepared by the hydrazine method. La(Cr1-xMgx)O-3 powders consisting of submicrometer-size are highly sinterable; dense materials can be fabricated by sintering for 2 h at 1600 degrees C under atmospheric pressure. The relative densities, grain sizes, electrical conductivities, and thermal expansion coefficients of the materials increase with increased Mg2+ content. La(Cr0.75Mg0.25)O-3 materials show an excellent electrical conductivity at elevated temperatures. Their thermal expansion coefficient is in good agreement with that of cubic ZrO2(10Y) as an electrolyte. (C) 2000 The Electrochemical Society. S0013-4651(99)11-049-8. All rights reserved., JOURNAL OF THE ELECTROCHEMICAL SOCIETY, ELECTROCHEMICAL SOC INC, 147(7)2830 - 2834, Jul. 2000, English, Research paper (scientific journal)
  • Formation, sintering and electrical conductivity of sol-gel-derived LaCl0.75Mg0.25O3
    K Azegami, M Yoshinaka, K Hirota, O Yamaguchi
    A sinterable LaCr0.75Mg0.25O3 powder consisting of submicrometer-size particles (&lt;0.2 mu m) has been obtained at low temperatures (similar to 850 degrees C) by decomposition of LaCr0.75Mg0.25O4 which crystallizes at similar to 530 degrees C from an amorphous material prepared by a sol-gel technique. Dense ceramics (98.5% of theoretical) can be fabricated by sintering for 2 h at 1600 degrees C without any control of oxygen pressure. They show an excellent electrical conductivity of 3.17 x 10(3) S m(-1) at 1000 degrees C. Their thermal expansion coefficient is 10.1 x 10(-6)/degrees C. (C) 1999 Elsevier Science Ltd. All rights reserved., SOLID STATE COMMUNICATIONS, PERGAMON-ELSEVIER SCIENCE LTD, 112(5)281 - 284, 1999, English, Research paper (scientific journal)
  • Fabrication, microstructure, and mechanical properties of Cr2O3/ZrO2(2.5Y) composite ceramics in the Cr2O3-rich region
    Y Takano, T Komeda, M Yoshinaka, K Hirota, O Yamaguchi
    Intimate mixtures of Cr2O3ZrO2(2.5Y) in the Cr2O3-rich region are produced at low temperatures from amorphous materials prepared by the hydrazine method. Spark plasma sintering (SPS) has been performed for 10 min at 1300 degrees C and 30 MPa. Composite ceramics with homogeneously dispersed fine ZrO2 (0.2 mu m) give 99.8% of theoretical densities. Their mechanical properties are examined in connection with increased ZrO2 content. A high fracture toughness of 9.3 MPa.m(1/2) and an excellent bending strength of 1290 MPa are achieved in the composite ceramics containing 50 mol% ZrO2., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 81(9)2497 - 2500, Sep. 1998, English, Research paper (scientific journal)
  • Formation, powder characterization and sintering of MgCr2O4 by the hydrazine method
    N Yoshida, Y Takano, M Yoshinaka, K Hirota, O Yamaguchi
    Picrochromite (MgCr2O4) crystallizes at 480 degrees to 530 degrees C from an amorphous material prepared by the hydrazine method. The MgCr2O4 powders were characterized for particle size and surface area. Individual particles tend toward a hexagonal morphology above 1000 degrees C. Dense MgCr2O4 ceramics (99.5% of theoretical) with an average grain size of 2 mu m have been fabricated by spark plasma sintering for 5 min at 1400 degrees C and 30 MPa. Their fracture toughness and bending strength are 3.7 MPa.m(1/2) and 310 MPa, respectively., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 81(8)2213 - 2215, Aug. 1998, English, Research paper (scientific journal)
  • Fabrication of calcite (CaCO3) ceramics with high density
    K Honma, M Yoshinaka, K Hirota, O Yamaguchi
    JOURNAL OF MATERIALS SCIENCE LETTERS, KLUWER ACADEMIC PUBL, 17(9)745 - 746, May 1998, English, Research paper (scientific journal)
  • Fabrication and mechanical properties of continuously graded WSi2-ZrO2(2Y) materials using wet-molding
    K Arata, M Yoshinaka, K Hirota, O Yamaguchi
    Continuously graded WSi2-ZrO2(2Y) materials with high density (99.2% of theoretical) have been fabricated by uniaxial wet-molding, followed by hot pressing (1000 degrees C/1h/30MPa) and hot isostatic pressing (1500 degrees C/2h/196MPa). A mixed solution of glycerin and ethanol with the viscosity of approximate to 6 mPa.s has been used as a dispersion medium in the molding process. The sintered materials show a linear composition profile with a transition from WSi2/ZrO2(2Y) approximate to 75/25 to approximate to 5/95 mol%. Vickers hardness, Hv, for which the value is approximate to 11.2 GPa, is almost independent of the compositional change. Fracture toughness, K-C, increases from 5.6 to 10.3 MPa.m(1/2) with increasing ZrO2(2Y)composition. (C) 1998 Elsevier Science Ltd., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 33(4)627 - 633, Apr. 1998, English, Research paper (scientific journal)
  • Low-temperature sintering of mullite/yttria-doped zirconia composites in the mullite-rich region
    M Imose, A Ohta, Y Takano, M Yoshinaka, K Hirota, O Yamaguchi
    Mullite/Y-TZP composites in the mullite-rich region, using powders of mullite prepared via the hydrazine method and a commercially available zirconia compound (ZrO2(2Y)), have been fabricated by sintering for 3 h at 1450 degrees C in air. The grain sizes of mullite and ZrO2(2Y) are changed with increased ZrO2(ZY) content; the former decreases from 0.25 mu m to 0.13 mu m, and the latter increases from 0.21 mu m to 0.35 mu m. High strength (780 MPa) and fracture toughness (5.4 MPa.m(1/2)) are obtained in the 50/50 (vol%) mullite/ZrO2(2Y) composite with 99.6% of theoretical density., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 81(4)1050 - 1052, Apr. 1998, English, Research paper (scientific journal)
  • Formation and sintering of LaCrO3 prepared by the hydrazine method
    K Azegami, M Yoshinaka, K Hirota, O Yamaguchi
    Sinterable LaCrO3 powder consisting of submicron particles (approximate to 0.2 mu m) has been obtained at low temperatures (approximate to 850 degrees C) by decomposition of LaCrO4, which crystallizes at 495 degrees C, from an amorphous material prepared by the hydrazine method. Powder characterization is examined. Dense LaCrO3 ceramics (93.9% of theoretical) with small grains (approximate to 2.1 mu m) can be fabricated by sintering for 2 h at 1600 degrees C without any control of oxygen pressure. They show a high electrical conductivity of 108 S.m(-1) at 1000 degrees C. (C) 1998 Elsevier Science Ltd., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 33(2)341 - 348, Feb. 1998, English, Research paper (scientific journal)
  • Formation and sintering of 8 mol % Y2O3-substituted La2Zr2O7 by the hydrazine method
    A Ota, Y Matsumura, M Yoshinaka, K Hirota, O Yamaguchi
    JOURNAL OF MATERIALS SCIENCE LETTERS, KLUWER ACADEMIC PUBL, 17(3)199 - 201, Feb. 1998, English, Research paper (scientific journal)
  • Formation and characterization of stoichiometric mullite (3Al(2)O(3)center dot 2SiO(2)) prepared by the hydrazine method
    M Imose, M Yoshinaka, K Hirota, O Yamaguchi
    MATERIALS SCIENCE RESEARCH INTERNATIONAL, SOC MATERIALS SCIENCE, JAPAN, 3(4)258 - 259, Dec. 1997, English, Research paper (scientific journal)
  • Characterization and sintering of reactive cerium(IV) oxide powders prepared by the hydrazine method
    S Nakane, T Tachi, M Yoshinaka, K Hirota, O Yamaguchi
    Nanocrystalline cerium(IV) oxide (CeO2) powders have been prepared by adding hydrazine monohydrate to an aqueous solution of hydrous cerium nitrate (Ce(NO3)(3) . 6H(2)O), followed by mashing and drying. The lattice parameter of the as-prepared powder is a = 0.5415 nm. The powder characteristics and sinterability of reactive CeO2 have been studied. The surface areas of powders that have been heated at low temperatures are high, and these surface areas do not decrease to 10 m(2)/g until the temperature is &gt;1200 degrees C. Crystallite size and particle size are strongly dependent on the heating temperature. Optimum sintered densities are obtained by calcining in the temperature range of 700 degrees-800 degrees C. Ceramics with almost-full density can be fabricated at a temperature as low as 1150 degrees C., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 80(12)3221 - 3224, Dec. 1997, English, Research paper (scientific journal)
  • Formation and sintering of La2Zr2O7 by the hydrazine method
    Y Matsumura, M Yoshinaka, K Hirota, O Yamaguchi
    In the ZrO2-La2O3 system, metastable c-ZrO2 solid solutions (ss) containing up to approximate to 27 mol% La2O3 crystallize at low temperatures from amorphous materials prepared by the hydrazine method. The lattice parameter a increases linearly from 0.5152 to 0.5350 nm. The formation of pyrochlore La2Zr2O7 proceeds via two reaction processes: (1) decomposition of c-ZrO2(ss) (c-ZrO2(ss) --&gt; La2Zr2O7 + t-ZrO2) and (2) solid-state reaction of t-ZrO2 and La2O(3). Dense La2Zr2O7 ceramics (99.5% of theoretical) with an average grain size of 10 mu m have been fabricated by hot isostatic pressing for 2 h at 1500 degrees C and 196 MPa. Their fracture toughness and bending strength are 1.9 MPa m(1/2) and 172 MPa, respectively. They exhibit a high electrical conductivity of 1.5 X 10(-1) S m(-1) at 1000 degrees C. (C) 1997 Elsevier Science Ltd., SOLID STATE COMMUNICATIONS, PERGAMON-ELSEVIER SCIENCE LTD, 104(6)341 - 345, Nov. 1997, English, Research paper (scientific journal)
  • Formation and sintering of TiO2 (anatase) solid solution in the system TiO2-SiO2
    M Yoshinaka, K Hirota, O Yamaguchi
    In the TiO2-SiO2 system, anatase solid solutions (ss) containing up to approximate to 15 mol% SiO2 are formed in the as-prepared state by the hydrazine method. The lattice parameters a and c decrease linearly from 0.3785 to 0.3776 nm and from 0.9514 to 0.9494 nm, respectively, with increased SiO, content, At high temperatures, the solid solutions by transformation decompose into rutile and amorphous SiO2, The anatase(ss) powders have been characterized for particle size and surface area, They consist of very fine particles (7-25 nm). Surface areas at low temperatures are very high and do not drop below 60 m(2)/g at 1000 degrees C. Nanostructured anatase(ss) ceramics, with greater than or equal to 99.5% of theoretical density and an average grain size of 72 nm, have been fabricated by hot isostatic pressing for 1 h at 850 degrees C and 196 MPa. Their mechanical and electrical properties have been examined., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 80(10)2749 - 2753, Oct. 1997, English, Research paper (scientific journal)
  • Fabrication and mechanical properties of continuously graded MoSi2-ZrO2(2Y) materials using wet-molding
    K Arata, N Takeuchi, M Yoshinaka, K Hirota, O Yamaguchi
    Continuously graded MoSi2-ZrO2(2Y) materials with high density (97.5% of theoretical) have been fabricated by uniaxial wet-molding followed by hot pressing (1000 degrees C/1 h/30 MPa) and hot isostatic pressing (1400 degrees C/2 h/196 MPa). Their composition profiles are greatly influenced by the viscosity of mixed solutions of glycerin and ethanol used as a dispersion medium; a linear compositional gradient from MoSi2/ZrO2(2Y) approximate to 70/30 to approximate to 20/80 mol% is obtained from the solution (50/50 vol%) with a viscosity of approximate to 20 mPa . s. Vickers hardness (H-v) and fracture toughness (K-IC) increase from 9.7 to 12.4 GPa and from 5.1 to 12.5 MPa . m(1/2), respectively, with increasing ZrO2(2Y) composition., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, WILEY-BLACKWELL, 80(8)2168 - 2170, Aug. 1997, English, Research paper (scientific journal)
  • Characterization and sintering of MoO3 prepared by thermal decomposition of hexaammonium heptamolybdate tetrahydrate
    K Honma, K Hirota, O Yamaguchi, J Asai, Y Makiyama
    MoO3 powders with plate particles have been prepared by heating hexaammonium heptamolybdate tetrahydrate ((NH4)(6)Mo7O24 . 4H(2)O) for 1 h at 650 degrees C. Densification has been performed by hot pressing for 2 h at 630 degrees C and 30 MPa. Grain-oriented MoO3 ceramics (&gt;98% of theoretical) are fabricated, in which the [010] axis is parallel to the hot pressing direction. Their electrical conductivities have been measured in the temperature range of 100 degrees to 600 degrees C. Sintered MoO3 is a ceramic semiconductor. Activation energies are determined to be 0.49 and 0.76 eV for initial and final stages, respectively., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 32(1)1 - 7, Jan. 1997, English, Research paper (scientific journal)
  • Fabrication, mechanical properties, and electrical conductivity of Co3O4 ceramics
    S Sakamoto, M Yoshinaka, K Hirota, O Yamaguchi
    Well-densified Co3O4 ceramics (98.3% of theoretical) have been fabricated by the combined use of hot pressing (800 degrees C/1 h/30 MPa) and hot isostatic pressing (880 degrees C/2 h/196 MPa). Their Vickers hardness and fracture toughness are 10.3 GPa and 4.2 MPa . m(1/2), respectively. They exhibit a high electrical conductivity of 3.35 x 10(1) S . cm(-1) at 800 degrees C., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 80(1)267 - 268, Jan. 1997, English, Research paper (scientific journal)
  • Characterization and sintering of MoO3 prepared by thermal decomposition of hexaammonium heptamolybdate tetrahydrate
    K Honma, K Hirota, O Yamaguchi, J Asai, Y Makiyama
    MoO3 powders with plate particles have been prepared by heating hexaammonium heptamolybdate tetrahydrate ((NH4)(6)Mo7O24 . 4H(2)O) for 1 h at 650 degrees C. Densification has been performed by hot pressing for 2 h at 630 degrees C and 30 MPa. Grain-oriented MoO3 ceramics (&gt;98% of theoretical) are fabricated, in which the [010] axis is parallel to the hot pressing direction. Their electrical conductivities have been measured in the temperature range of 100 degrees to 600 degrees C. Sintered MoO3 is a ceramic semiconductor. Activation energies are determined to be 0.49 and 0.76 eV for initial and final stages, respectively., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 32(1)1 - 7, Jan. 1997, English, Research paper (scientific journal)
  • Improvement of magnetic and electric properties of low-temperature sintering Mn-Zn ferrite by adding B2O3
    Y Aono, N Matsutani, T Aoyama, K Hirota, O Yamaguchi
    MATERIALS SCIENCE RESEARCH INTERNATIONAL, SOC MATERIALS SCIENCE, JAPAN, 2(4)275 - 276, Dec. 1996, English, Research paper (scientific journal)
  • Fabrication and mechanical properties of TiN/ZrO2(2Y) composites by hot isostatic pressing
    K Okuda, K Shibata, M Yoshinaka, K Hirota, O Yamaguchi
    JOURNAL OF MATERIALS SCIENCE LETTERS, CHAPMAN HALL LTD, 15(18)1615 - 1617, Sep. 1996, English, Research paper (scientific journal)
  • Hot pressing of Cr2O3 powder with thin hexagonal plate particles
    T Komeda, Y Fukumoto, M Yoshinaka, K Hirota, O Yamaguchi
    Chromium oxide (Cr2O3) powders with thin hexagonal plate particles have been obtained at 1200 degrees C to 1400 degrees C from an amorphous material prepared by adding hydrazine monohydrate to the aqueous solution of chromium nitrate. Tablets molded by wet-uniaxial-pressing were hot-pressed for 2 h at 1500 degrees C and 30 MPa. The dense sintered Cr2O3 ceramics (&gt;98% of theoretical) obtained had crystal-orientation, in which a &lt; 001 &gt; axis was parallel to the pressing direction. They exhibited an anisotropy in electrical conductivity, s; the values measured at 700 degrees C in the perpendicular and parallel directions for the &lt; 001 &gt; axis were 5.5 and 0.58 S . m(-1), respectively. Vickers hardness, HV,and fracture toughness, KIC, were 24.1 GPa and 4.6 MPa . m(1/2), respectively., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 31(8)965 - 971, Aug. 1996, English, Research paper (scientific journal)
  • Fabrication, microstructure and electrical conductivity of V2O5 ceramics
    K Honma, M Yoshinaka, K Hirota, O Yamaguchi, J Asai, Y Makiyama
    V2O5 gels corresponding to the formula V2O5 . nH(2)O with n = 2.07, 0.5, and 0.2 have been prepared by the hydrolysis of VO(OC2H5)(3), followed by washing and drying. After dehydration, V2O5 crystallizes at 310 degrees to 400 degrees C. V2O5 powders with strip-like particles are produced after heating at 630 degrees C. Well-densified V2O5 ceramics (97.7% of theoretical) have been fabricated by the combined use of hot pressing (630 degrees C/2h/30MPa) and hot isostatic pressing (630 degrees C/1h/196MPa). The texture is of a plate structure, the grain being approximate to 30 mu m long and approximate to 6 mu m wide. Electrical conductivities have been measured in the temperature range of 25 degrees to 600 degrees C. Activation energies are determined to be 0.09 and 0.18 eV for initial and final stages, respectively., MATERIALS RESEARCH BULLETIN, PERGAMON-ELSEVIER SCIENCE LTD, 31(5)531 - 537, May 1996, English, Research paper (scientific journal)
  • Formation, characterization, and hot isostatic pressing of Cr2O3-doped ZrO2 (0,3 mol% Y2O3) prepared by hydrazine method
    S Hirano, M Yoshinaka, K Hirota, O Yamaguchi
    In the ZrO2-Cr2O3 system, metastable t-ZrO2 solid solutions containing up to 11 mol% Cr2O3 crystallize at low temperatures from amorphous materials prepared by the hydrazine method. The lattice parameter c decreases linearly from 0.5149 to 0.5077 nm with increased Cr2O3 content, whereas the lattice parameter a is a constant value (a = 0.5077 nm) regardless of the starting composition. At higher temperatures, transformation (decomposition) of the solid solutions proceeds in the following way: t(ss) --&gt; t(ss) + m + Cr2O3 --&gt; m + Cr2O3. Above 11 mol% Cr2O3 addition, c-ZrO2 phases are formed in the presence of Cr2O3. The t-ZrO2 solid solution powders have been characterized for particle size, shape, and surface area. They consist of very fine particles (15-30 nm) showing thin plate-like morphology, Dense ZrO2(3Y)-Cr2O3 composite ceramics (approximate to 99.7% of theoretical) with an average grain size of 0.3 mu m have been fabricated by hot isostatic pressing for 2 h at 1400 degrees C and 196 MPa. Their fracture toughness increases with increased Cr2O3 content. The highest K-Ic value of 9.5 MPa-m(1/2) is achieved in the composite ceramics containing 10 mol% Cr2O3., JOURNAL OF THE AMERICAN CERAMIC SOCIETY, AMER CERAMIC SOC, 79(1)171 - 176, Jan. 1996, English, Research paper (scientific journal)
  • Formation and sintering of 75 mol% alumina 25 mol% zirconia (2-3.5 mol% yttria) composite powder prepared by the hydrazine method
    K Goto, K Hirota, O Yamaguchi, H Kume, S Inamura, H Miyamoto
    Al2O3/Y2O3-doped ZrO2 composite powders with 25 mol% ZrO2 have been prepared by the hydrazine method. As-prepared powders are the mixtures of AlO(OH) gel solid solutions and amorphous ZrO2. The formation process leading to alpha-Al2O3/t-ZrO2 composite powders is investigated. Hot isostatic pressing has been performed for 1 h at 1500 degrees C under 196 MPa. Dense ZrO2-toughened Al2O3 (ZTA) ceramics with homogeneous-dispersed ZrO2 particles show excellent mechanical properties. The toughening mechanism is discussed., JOURNAL OF MATERIALS SCIENCE, CHAPMAN HALL LTD, 31(1)204 - 208, Jan. 1996, English, Research paper (scientific journal)
  • CHARACTERIZATION AND SINTERING OF YTTRIA-DOPED ZIRCONIA POWDERS PREPARED BY HYDRAZINE METHOD
    M YOSHINAKA, K HIROTA, O YAMAGUCHI, H KUME, S INAMURA, H MIYAMOTO, N SHIOKAWA, R SHIKATA
    In the ZrO2-Y2O3 system, metastable ZrO2 solid solutions containing up to approximately 46 mol.-%Y2O3 crystallise at low temperatures from amorphous materials prepared by the hydrazine method into t- and c-ZrO2 phases below and above 8 mol.-%Y2O3, respectively. The variation of the lattice parameters of both phases was determined as a function of Y2O3 addition and the formation mechanism of the solid solutions is discussed. Powders calcined at various temperatures from a starting powder consisting of t-ZrO2 were characterised in terms of crystallite size and surface area. Sintered compacts were prepared from powders containing 2-5 mol.%Y2O3 by hot isostatic pressing for 2 h at 1400-degrees-C under 196 MPa. Dense compacts of high fracture toughness (11.4 MN m-3/2) and bend strength (1313 MN m-2) with an average grain size of 0.3 mum were obtained from powders with 2-3 mol.-%Y2O3. Compacts with 2.5 and 3.0 mol.-%Y2O3 exhibited high ionic conductivity at high temperatures., BRITISH CERAMIC TRANSACTIONS, INST MATERIALS, 93(6)234 - 238, 1994, English, Research paper (scientific journal)
  • FORMATION, TRANSFORMATION AND SINTERING OF ZRO2-MGO COMPOSITIONS PREPARED FROM ALKOXIDES
    J SAIKI, K HIROTA, O YAMAGUCHI, S INAMURA, H MIYAMOTO, N SHIOKAWA, K TSUJI
    In the ZrO2-MgO system, c-ZrO2 solid solutions containing between 5 and almost-equal-to 23 mol.-% MgO were found to crystallise at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and magnesium alkoxides. The values of the lattice parameter a decreased linearly from 0.5091 to 0.5062 nm with increasing MgO content. At higher temperatures, transformation of the solid solutions proceeded in the following way: c --&gt; c + t + MgO --&gt; t + MgO --&gt; t + m + MgO --&gt; m + MgO. Pure ZrO2 crystallised in the tetragonal form at 415-440-degrees-C. Dense ZrO2(m)-MgO composite ceramics fabricated by hot pressing for 2 h at 1350-degrees-C and 35 MPa showed increasing bend strength with increasing MgO content. A composite containing 5 mol.-% MgO gave a fracture toughness of 4.5 MPa m1/2. (C) 1993 The Institute of Materials., BRITISH CERAMIC TRANSACTIONS, MANEY PUBLISHING, 92(4)161 - 164, 1993, English, Research paper (scientific journal)
  • FORMATION, TRANSFORMATION AND SINTERING OF ZRO2-MGO COMPOSITIONS PREPARED FROM ALKOXIDES
    J SAIKI, K HIROTA, O YAMAGUCHI, S INAMURA, H MIYAMOTO, N SHIOKAWA, K TSUJI
    In the ZrO2-MgO system, c-ZrO2 solid solutions containing between 5 and almost-equal-to 23 mol.-% MgO were found to crystallise at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and magnesium alkoxides. The values of the lattice parameter a decreased linearly from 0.5091 to 0.5062 nm with increasing MgO content. At higher temperatures, transformation of the solid solutions proceeded in the following way: c --&gt; c + t + MgO --&gt; t + MgO --&gt; t + m + MgO --&gt; m + MgO. Pure ZrO2 crystallised in the tetragonal form at 415-440-degrees-C. Dense ZrO2(m)-MgO composite ceramics fabricated by hot pressing for 2 h at 1350-degrees-C and 35 MPa showed increasing bend strength with increasing MgO content. A composite containing 5 mol.-% MgO gave a fracture toughness of 4.5 MPa m1/2. (C) 1993 The Institute of Materials., BRITISH CERAMIC TRANSACTIONS, MANEY PUBLISHING, 92(4)161 - 164, 1993, English, Research paper (scientific journal)
  • FORMATION OF ALKOXY-DERIVED YTTRIUM ALUMINUM-OXIDES
    O YAMAGUCHI, K TAKEOKA, K HIROTA, H TAKANO, A HAYASHIDA
    Monoclinic Y4Al2O9 and hexagonal YAlO3 crystallize at low temperatures from amorphous materials prepared by the hydrolysis of yttrium and aluminium double alkoxides. Hexagonal YAlO3 transforms to the cubic phase with a garnet structure as an intermediate product at elevated temperatures. The formation process of YAlO3 is described. Solid solutions of hexagonal YAlO3 crystallize between 50 and 62.5 mol% Al2O3. Yttrium aluminium garnet Y3Al5O12(YAG) is formed by transformation of the solid solution., JOURNAL OF MATERIALS SCIENCE, CHAPMAN HALL LTD, 27(5)1261 - 1264, Mar. 1992, English, Research paper (scientific journal)
  • FORMATION OF ALKOXY-DERIVED BIVO4
    K HIROTA, G KOMATSU, H TAKEMURA, O YAMAGUCHI
    Monoclinic BiVO4 has been formed by the hydrolysis of bismuth and vanadyl double alkoxide, followed by washing and drying. The chemical formula is Bi(VO4)., CERAMICS INTERNATIONAL, ELSEVIER SCI LTD, 18(4)285 - 287, 1992, English
  • NEW MODIFICATION OF 3ZNO-NB2O5
    O YAMAGUCHI, N MARUYAMA, K HIROTA
    JOURNAL OF MATERIALS SCIENCE LETTERS, CHAPMAN HALL LTD, 10(8)445 - 447, Apr. 1991, English, Research paper (scientific journal)

Books etc

  • Low Temperature Sintering of Mn-Zn Ferrites
    Sintering '87, 1987
  • High-B and-μ′ Mn-Zn Ferrite with Improved Mechanical Strength
    Adv. Ceram., 1986

Conference Activities & Talks

  • Control of cobalt content in La-Co cosubstituted magnetoplumbite-type strontium ferrite (invited)
    Hiroyuki Nakamura, Shusuke Okazaki, Takeshi Waki, Yoshikazu Tabata, Masaki Kato, Ken Hirota
    JSPM International Conference on Powder and Powder Metallurgy, 08 Nov. 2017, English, Oral presentation (general)
  • Metal-insulator crossover in Pb-Ru based oxides with pyrochlore-type structure (invited)
    Masaki Kato, Kazuya Iwamoto, Yuki Iwakura, Ken Hirota, Angel Arevalo-Lopez, John Paul Attfield
    JSPM International Conference on Powder and Powder Metallurgy, 08 Nov. 2017, English, Oral presentation (general)
  • Preparation of ZnO Powders with Strong Antibacterial Activity under Dark Conditions
    Thi Minh Phuong NGUYEN, Shoko HIROTA, Yusei SUZUKI, Masaki KATO, Ken HIROTA, Hideki TAGUCHI, Hirohisa YAMADA, Kazuhiko TSUKAGOSHI
    JSPMIC2017, Nov. 2017, English, Oral presentation (general)
  • Fabrication of c-BN/[ZrO2(Y2O3)-Al2O3] composite ceramics using pulsed electric-current pressure sintering
    Ken HIROTA, Dao Hoang Anh, Masaki KATO
    Nov. 2017, English, Poster presentation
  • Fabrication of c‐BN/[ZrO2(Y2O3)‐Al2O3] Composite Ceramics Using Pulsed Electric‐Current Press Sintering
    Hoang Anh DAO, Masaki KATO, Ken HIROTA
    01 Jun. 2017, English, Oral presentation (general)
  • Simultaneous synthesis and densification of new-type carbon nano-fibers (CNF) dispersed B4C/CNF composites by pulsed electric current pressure sintering (PECPS) and their mechanical properties
    Hironobu Hirahara, Masaki Kato, Ken Hirota, Hideki Taguchi, Toshiyuki Nishimura
    Materials Science & Technology 2016,2016.10, Oct. 2016, English, Poster presentation
  • Mechanical properties of dense ZrO2-Al2O3 composites fabricated using various sintering methods
    Xiaoteng Ge, Masaki Kato, Ken Hirota, Hideki Taguchi, Hideo Kimura
    Materials Science & Technology 2016,2016.10, Oct. 2016, English, Oral presentation (general)
  • Metal-insulator crossover in Pb2-xRExRu2O7-δ (RE: rare earth) compounds with pyrochlore-type structure
    Masaki Kato, Yusuke Sakamoto, Shinya Yamato, Riko Kajita, Ken Hirota, Angel M. Arevalo-Lopez, Edinburgh Univ, J. Paul Attfiel, Edinburgh Un
    St Andrews Advanced Materials Conference, Jan. 2016, English, Oral presentation (general)
  • High-temperature properties of B4C/CNF composites fabricated using Pulsed Electric Current Pressure Sintering
    Ken Hirota, Naoki Goto, Xiaolei Chen, Masaki Kato, Toshiyuki Nishimura (National, Institute of Materials Science
    Dec. 2015, English, Oral presentation (invited, special)
  • Thermoelectric properties of dense B4C/CNF composites fabricated using a pulsed electric-current pressure sintering
    Xiaolei Chen, Naoki Goto, Mitsuhiro Shima, Masaki Kato, Ken Hirota, Toshiyuki Nishimura (National, Institute of Materials Science
    APMA2015, Nov. 2015, English, Oral presentation (general)
  • Simultaneous synthesis and sintering of dense B4C/CNF composites using a pulsed electric-current pressure sintering and their mechanical properties at elevated temperatures
    Naoki Goto, Mitsuhiro Shima, Xiaolei Chen, Masaki Kato, Ken Hirota, Toshiyuki Nishimura (National, Institute of Materials Science
    MS&T15, Oct. 2015, English, Oral presentation (general)
  • Pulsed Electric-Current Pressure Sintering of ZrO2(Y2O3)-Al2O3 Solid Solution Powders Prepared by the Neutralization Co-precipitation Method
    Ken HIROTA, Kenta YAMAMOTO, Koki SASAI, Masaki KATO, Hideki TAGUCHI, Hideo KIMURA, Daiichi Kigenso, Kagaku Kogyo, Co, Masayuki TAKAI, Daiichi Kigenso, Kagaku Kogyo, Co, Masao TERADA, Daiichi Kigenso, Kagaku Kogyo C
    PACRIM11, Sep. 2015, English, Oral presentation (general)
  • Fabrication of dense Al2O3/CNF/TiN composites using pulsed-electric pressure sintering
    Ken Hirota, Kota Kasahara, Masaki Kato
    Dec. 2014, English, Oral presentation (invited, special)
  • Extraordinary High-temperature Strength of B4C/CNF Composites Fabricated Using Pulsed Electric-Current Pressure Sintering (PECPS)
    Ken Hirota, Mitsuhiro Shima, Xiaolei Chen, Naoki Goto, Masaki Kato, Toshiyuki Nishimura (National, Institute of Materials Science
    The 15th IUMRS- international conference, Aug. 2014, English, Oral presentation (general)
  • Fabrication of dense Al2O3/TiN/CNF composites using both HIP and pulsed electric-current pressure sintering (PECPS)
    Ken Hirota, Kota Kasahara, Takahiro Ishiguro, Hajime Yagura(Metal Technology, Co. Ltd, Masaki Kato
    11th Int'l Conf.on Hot Isostatic Pressing, Jun. 2014, English, Oral presentation (general)
  • Pulsed Electric-Current Pressure Sintering of ZrO2(Y2O3)-Al2O3 Solid Solution Powders Prepared by the Neutralization Co-precipitation Method
    Ken HIROTA, Kenta YAMAMOTO, Masaki KATO
    The 30th Japan-Korea International Seminar on Ceramics, Nov. 2013, English, Oral presentation (general)
  • Fabrication of novel ZrO2(Y2O3)-Al2O3 ceramics having high strength and toughness utilizing pulsed-electric current pressure sintering (PECPS)
    Ken Hirota, Kengo Shibaya, Hiroyuki Matsuda, Masaki Kato, Hideki Taguchi
    STAC-6, 27 Jun. 2012, English, Oral presentation (general)
  • Fabrication of galvanized steel having antibacterial activity
    Ken Hirota, Hiroaki Sakai, Masaki Kato
    InterGalva 2012, 12 Jun. 2012, English, Oral presentation (general)
  • Fabrication of Dense Magnetic-metal/oxide Composites for Induction Heating (IH) Applications Using N2-atomized Iron BasedParticles and MgO powder
    HIROTA Ken
    PM2012 YOKOHAMA, 2012
  • Fabrication of novel ZrO2(Y2O3)-Al2O3 composite ceramics having high strength and toughness utilizing pulsed-electric current pressure sintering (PECPS)
    HIROTA Ken
    STAC-6, 2012
  • Fabrication of galvanized steel having antibacterial activity
    HIROTA Ken
    InterGalva2012, 2012
  • Synthesis and magnetic properties of Cu oxides with low-dimensional structure
    Masaki Kato, Yusaku Nogi, Satoko Hongo, Ken Hirota
    7th Joint Symposium between Doshisha University and Chonnam National University, Aug. 2010, English, Oral presentation (general)
  • Crossover from metallic to semiconducting behaviors in Pb2-xLnxRu2O7-δ (Ln = Eu, Lu, Y) compounds with pyrochlore structure
    Syuji Harada, Masaki Kato, Miho Fukatsu, Ken Hirota
    7th Joint Symposium between Doshisha University and Chonnam National University, Aug. 2010, English, Oral presentation (general)
  • Physical Properties of Elementally Substituted Ruthenates with Pyrochlore-type Structure
    Masaki Kato, Syuji Harada, Miho Fukatsu, Ken Hirota
    International Conference on Strongly Correlated Electron Systems (SCES2010), Jul. 2010, English, Oral presentation (general)
  • Simultaneous Synthesis and Sintering of Al2O3/Mo2N Composites Using Capsule-free Nitrogen Hot Isostatic Pressing and their Characterization
    KEN Hirota, KATSUYA Takaoka, YASUSHI Murase, MASAKI Kato
    CIMTEC2010, Jun. 2010, English, Oral presentation (general)
  • Simultaneous Synthesis and Sintering of Al2O3/Mo2N Composites Using Capsule-free Nitrogen Hot Isostatic Pressing and their Characterization
    HIROTA Ken
    CIMTEC2010, 2010
  • Unconventional Superconductivity and Strongly Correlated Electron Behaviors in Layered Transition Metal Oxides
    HIROTA Ken
    6th Joint Symposium between Chonnam National University and Doshisha University, 2008
  • Crossover from Metallic to Semiconducting in Pb2-xLnxRu2O7-d (Ln = Sm, Eu) Compounds with Pyrochlore Structure
    HIROTA Ken
    The 10th International Symposium on Multiscale, Multifunctional and Functionally Graded Materials, 2008
  • Mechanical Properties of Simultaneously Synthesized and Consolidated Carbon Nanofiber (CNF)-Dispersed B4C Composites by Pulsed Electric-Current Pressure Sintering
    HIROTA Ken
    6th International Conference on Inorganic Materials, 2008
  • Simultaneous Synthesis and Consolidation of W-Added ZrB2by Pulsed Electric Current Pressure Sintering and Their Mechanical Properties
    HIROTA Ken
    The Sixth Pacific Rim International Conference on Advanced Materials and Processing, 2007, ICC Jeju Island, Korea
  • Simultaneous synthesis and consolidation of carbon nanofiber (CNF)-dispersed composites by spark plasma sintering (SPS)
    HIROTA Ken
    7th Pacific Rim Conference on Ceramic and Glass Technology, Shanghai International Convention Center, 2007, Shanghai, China.
  • Fabrication of carbon nanofiber (CNF)/SiAlON composites by pulsed electric-current pressure sintering and their mechanical and electrical properties
    HIROTA Ken
    MS&T '07 (Materials Science and Technology '07), 2007, Detroit

Works

  • Process for Producing Single-Crystal Ceramics U. S. P. 4900393
    1990
  • Magnetic Head Having a Laminated Structure U. S. P.4901179
    1990
  • Polycrystlline Ferrite and a Magnetic Head Using the Same U. S. P.4521323
    1984

Research Grants & Projects

  • Fabrication and Evaluation of Advanced Ceramics

Research Seeds

  • Simultaneous synthesis and consolidation of carbon nanofiber(CNF)-dispersed composites by pulsed-electric current pressure sintering(PECPS)
    Research Fields: Chemicals, Nanotechnology and Materials
    Affiliation/Name: Ken Hirota

    Patent Information: 特願 2007-239028(外国出願有:アメリカ)
  • Fabrication of novel ZrO2-Al2O3 composite ceramics having high strength and toughness
    Research Fields: Chemicals, Nanotechnology and Materials
    Affiliation/Name: Ken Hirota

    Patent Information: 特願 2013-513978
  • Preparation of zinc oxide ceramics with a sustainable antibacterial activity under dark conditions
    Research Fields: Life Sciences, Medical and Pharmaceutical Sciences, Chemicals, Nanotechnology and Materials
    Affiliation/Name: Ken Hirota

    Patent Information: 特願 2007-010176