Masaru Yoshinaka

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.

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.

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).

Fabrication and mechanical properties of Cr2O3 solid solution ceramics in the system Cr2O3-Al2O3

In the system Cr{sub 2}O{sub 3}-Al{sub 2}O{sub 3}, Cr{sub 2}O{sub 3} solid solution (ss) powders containing up to 25 mol% A/l{sub 2}O{sub 3} have been prepared by the hydrazine method. Dense Cr{sub 2}O{sub 3}(ss) ceramics (99--99.2% of theoretical) have been fabricated by HIPing (1,500 C/2h/196MPa) of the compacts sealed under reduced pressure. Their mechanical data are discussed, in comparison with those of Al{sub 2}O{sub 3}(ss) ceramics in this system. Microhardness (23.4 GPa) and fracture toughness (3.9 MPa{center_dot}m{sup 1/2}) are not dependent on the compositions of Cr{sub 2}O{sub 3}(ss) ceramics. The former value is higher than that of much improved Al{sub 2}O{sub 3} content; the maximum value of 380 MPa is achieved in the ceramics containing 25 mol% Al{sub 2}O{sub 3}.

Formation of continuous series of solid solutions from powders prepared by hydrazine method: The system Cr2O3-Al2O3

Chromia (Cr2O3)/alumina (A12O3) solid solution (ss) powders have been prepared by the hydrazine method. As-prepared powders are classified into three groups: (i) amorphous below 25 mol% A12O3, (ii) mixtures of this and AIO (OH) gel(ss) between 25 and 80 mol% A12O3, and (iii) AIO(OH) gel(ss) above 80 mol% A12O3. Cr2O3 solid 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 Cr2O3Al2O3 = 8020 mol% have been characterized for particle size and shape. Individual particles tend toward a hexagonal morphology above 900 °C.

Solid solutions of metastable tetragonal ZrO2 and Ce3ZrO8 in the system ZrO2-CeO2

In the system ZrO2-CeO2, metastable t-ZrO2 solid solutions containing up to 30 mol% CeO2 crystallize at temperatures of 385–430 °C from amorphous materials prepared by the hydrazine method. Crystalline Ce3ZrO8 solid 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 CeO2 content. The value of the lattice parameter of pure Ce3ZrO8 (cubic) is a = 0.5342 nm. Detailed characterization of the Ce3ZrO8 powder has been performed. Crystallite size and particle size are strongly dependent on the heating temperature. Specific surface areas do not drop below 40 m2g−1 until the heating temperature is above 1000°C.

Formation and electrical conductivity of air-sinterable Nd(Cr1-xMgx)O3

Abstract 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)O3 powders 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)O3 materials show an excellent electrical conductivity of 1.4×103 S m−1 at 1000 °C.