Ken Hirota

Formation, characterization and sintering of alkoxy-derived bismuth vanadate

Abstract Monoclinic BiVO 4 is 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(VO 4 ). 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.

Formation of alkoxy-derived yttrium aluminium oxides

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.

Formation and sintering of La2Zr2O7 by the hydrazine method

Abstract 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 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)→La2Zr2O7 + t-ZrO2) and (2) solid-state reaction of t-ZrO2 and La2O3. Dense La2Zr2O7 ceramics (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 1.9 MPa m 1 2 and 172 MPa, respectively. They exhibit a high electrical conductivity of 1.5 × 10−1 S m−1 at 1000°C.

Formation and sintering of LaCrO3 prepared by the hydrazine method

Abstract Sinterable LaCrO 3 powder consisting of submicron particles (≈0.2 μm) has been obtained at low temperatures (≈850°C) by decomposition of LaCrO 4 , which crystallizes at 495°C, from an amorphous material prepared by the hydrazine method. Powder characterization is examined. Dense LaCrO 3 ceramics (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 −1 at 1000°C.

Formation, powder characterization and sintering of YCrO3 prepared by a sol–gel technique using hydrazine

Abstract Orthorhombic YCrO 3 is formed at low temperatures (750–830°C) by decomposition of YCrO 4 which 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. YCrO 3 powders, 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.

Hot isostatic pressing of TiB2-ZrO2(2 mol% Y2O3) composite powders

Abstract Dense sintered composites of TiB 2 and ZrO 2 (2 mol% Y 2 O 3 ) have been fabricated by hot isostatic pressing for 2 h at 1500 °C under 196 MPa. The ZrO 2 particles in the composites consist of m - ⪢ t - ZrO 2 . There is no reaction between TiB 2 and ZrO 2 . Microstructures and mechanical properties are examined, in connection with increased ZrO2 content. The fracture toughness and bending strength of the composites with 30 mol% ZrO 2 content are 11.2 MPa. 1 2 and 680 MPa, respectively.

Formation and hot isostatic pressing of ZrO2 solid solution in the system ZrO2-Al2O3

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.

Fabrication, microstructure and electrical conductivity of V2O5 ceramics

V{sub 2}O{sub 5} gels corresponding to the formula V{sub 2}O{sub 5} {center_dot} nH{sub 2}O with n = 2.07, 0.5, and 0.2 have been prepared by the hydrolysis of VO(OC{sub 2}H{sub 5}){sub 3}, followed by washing and drying. After dehydration, V{sub 2}O{sub 5} crystallizes at 310--400 C. V{sub 2}O{sub 5} powders with strip-like particles are produced after heating at 630 C. Well-densified V{sub 2}O{sub 5} ceramics (97.7% of theoretical) have been fabricated by the combined use of hot pressing (630C/2h/30MPa) and hot isostatic pressing (630 C/1h/196MPa). The texture is of a plate structure, the grain being {approx}30 {micro}m long and {approx}6 {micro}m wide. Electrical conductivities have been measured in the temperature range of 25--600 C. Activation energies are determined to be 0.09 and 0.18 eV for initial and final stages, respectively.

Sintering and characterization of yttria-stabilized cubic zirconia with alumina derived from solid solution

In the composition of ZrO[sub 2](8 mol% Y[sub 2]O[sub 3])/Al[sub 2]O[sub 3] = (100--85)/(0--15) mol%, metastable c-ZrO[sub 2] 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 1,600 C under 196 MPa. Yttria-stabilized c-ZrO[sub 2] materials with homogeneous-dispersed Al[sub 2]O[sub 3] derived from the decomposition of solid solutions give >99% of theoretical densities. Materials with small amounts of Al[sub 2]O[sub 3] show good strength and excellent high ionic conductivity at high temperatures.

A New Composite Material with High Saturation Magnetization Density and High Electrical Resistivity

Abstract Dense composite materials (98.6% of theoretical) consisting of Super Sendust (86.5Fe–6Si–4Al–3.5Ni wt%) and MgFe 2 O 4 (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 B s of 1.2 T and high electrical resistivity ρ of 1 × 10 −2 Ω·m. Permeabilities μ >1000 at 1 kHz were obtained. Fine ferrite grains (∼0.6 μm) were located at the boundaries of the large spherical grains of Super Sendust (∼30 μm).