Yasuo Fukushige

Colloidal processing and mechanical properties of silicon carbide with alumina

Submicrometer-sized SiC coated with SiO 2 of 0.4–1.8 wt.% and α–Al 2 O 3 powder of median size 0.2 μ m were mixed in aqueous solutions in the pH range 3.0–10.0. The SiC/Al 2 O 3 (4.3–6.9 wt. %) powders were consolidated by filtration through gypsum molds and hot-pressed at 1600°–2040 °C under a pressure of 39 MPa. These compacts were densified to near the theoretical density at 1700°–1800 °C. The sintering mechanisms are discussed based on the analysis of shrinkage curves of SiC/Al 2 O 3 compacts during hot-pressing. The equiaxed SiC grains grew with low aspect ratios below 1800 °C and changed to plate-like grains at 1900 °C. The fracture toughness of SiC as a function of average grain size reached a maximum of 5 Mpa · m 0.5 at 2.5 μ m grains of low aspect ratios of 1–2. The flexural strengths at room temperature were 230–430 MPa in the SiC above 98% of the theoretical density and showed a similar grain size dependence.

Colloidal processing of silicon carbide

Abstract Rheological properties of silicon carbide aqueous suspensions and sintering behavior of consolidated powder compacts were studied on two kinds of submicrometer-sized powders (powder A: median size 0.1 μm, 0.38 wt% SiO 2 , powder B: median size 0.7 μm, 1.75 wt% SiO 2 ). Highly concentrated fluid suspensions (>50 vol%) and high-density green compacts (> 65% TD (theoretical densi were prepared from electrostatically stabilized powder B in the solutions at pH = 10. Addition of fine γ-alumina particles (≈ 20 nm) as a sintering additive (0–5 wt%) lowered both the green densities of powders A and B to about 50% TD. The density of SiC with alumina hot-pressed at 2100–2150°C was higher for powder B than for powder A.

Electrical properties of carbon fiber/shirasu glass composite

Abstract A kind of aluminosilicate glass, shirasu glass, with a chemical composition (mass%) of 75.4 SiO2, 14.6 Al2O3, 0.2 TiO2, 1.7 Fe2O3, 0.2 MgO, 1.5 CaO, 3.0 Na2O and 3.6 K2O was pressureless-sintered to full density in air at 1000 °C. The dense glass showed specific resistivity of 1440 Ωm at 350 °C and activation energy of 71.7 kJ/ mol at 200–800 °C. These values were comparable to those for compositions of 25–35 mol% R2O in the R2OSiO2 glass (R = Na, K) and associated with the enhanced migration of Na and K ions resulting from the substitution of Al for Si in shirasu glass and the relatively large open glass structure. A carbon fiber (40 vol%)/shirasu glass (60 vol%) composite was formed by filtration of an aqueous suspension and hot-pressed to full density at 1000 °C under a pressure of 10 MPa in a N2 atmosphere. The specific resistivity of the composite was lower in parallel (0.04 Ωm at 350 °C) than it was when perpendicular (14.6 Ωm at 350 °C) to the direction of hot pressing. The activation energy when perpendicular to the direction of hot-pressing (68.5 kJ/mol) was comparable to that of shirasu glass. Little temperature dependence of the conductivity was measured in parallel to the direction of hot-pressing. The above results for the composite were discussed using the mixed conduction mechanisms of ions and electrons, which were affected by the orientation of the carbon fibers.