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Porous ceramics with complex pore structure were synthesized by a freeze-dry process. Freezing-in of a water-based ceramics slurry was done while controlling the growth direction of the ice. Sublimation marks of the ice were generated by drying under reduced pressure. Porous ceramics having a complex pore structure were obtained by sintering the green body: aligned macroscopic open pores contained micropores in their internal walls. The pore structure was substantially affected by the starting slurry concentration and sintering temperature. The pore formation mechanism is discussed in relation to these effects.

Porous silicon nitride with macroscopically aligned channels was synthesized using a freeze‐drying process. Freezing of a water‐based slurry of silicon nitride was done while unidirectionally controlling the growth direction of the ice. Pores were generated subsequently by sublimation of the columnar ice during freeze‐drying. By sintering this green body, a porous silicon nitride with high porosity (over 50%) was obtained and its porosity was controllable by the slurry concentration. The porous Si3N4 had a unique microstructure, where macroscopically aligned open pores contained fibrous grains protruding from the internal walls of the Si3N4 matrix. It is hypothesized that vapor/solid phase reactions were important to the formation mechanism of the fibrous grains.

Dense ␤-Si 3 N 4 with various Y 2 O 3 /SiO 2 additive ratios were fabricated by hot pressing and subsequent annealing. The thermal conductivity of the sintered bodies increased as the Y 2 O 3 /SiO 2 ratio increased. The oxygen contents in the ␤-Si 3 N 4 crystal lattice of these samples were determined using hot-gas extraction and electron spin resonance techniques. A good correlation between the lattice oxygen content and the thermal resistivity was observed. The relationship between the microstructure, grain-boundary phase, lattice oxygen content, and thermal conductivity of ␤-Si 3 N 4 that was sintered at various Y 2 O 3 /SiO 2 additive ratios has been clarified.

Homogeneous Y‐Si‐O‐N glasses containing 15 or 20 eq% nitrogen (N) were prepared from compositions with Y/Si ratios in the vicinity of that of the lowest eutectic point on the Y2O3–SiO2 phase diagram. The liquidus on the phase diagram shifted toward lower temperatures by incorporation of N. The density, the elastic moduli, and the glass transition temperature of the Y‐Si‐O‐N glasses increased with incorporation of N. This is due to the closer packing of atoms in the glasses by the substitution of N, which is in three‐fold coordination with Si, for O which is in two‐fold coordination, and the stronger covalent nature of the Si–N bond compared with the Si–O bond. The coefficient of thermal expansion of the Y‐Si‐O‐N glasses increased with increasing Y content, because the discontinuity of the glass network developed with increasing nonbridging anions by the introduction of Y. In contrast, the glass transition temperature and the elastic moduli increased with Y content due to the high coordination of Y for O, and the relatively high cationic field strength of Y. Furthermore, the effect of cationic field strength on properties of Ln‐Si‐O‐N glasses (Ln = lanthanides or Y) is discussed.

Silicon nitride with a preferred orientation of large elongated grains was obtained by tape casting of raw powder slurry seeded with rodlike P-Si,N, particles, followed by a gas pressure sintering under 1 MPa nitrogen pressure. The large elongated grains developed from seeds lay in planes parallel to the casting direction in a two-dimensional distribution. Increased fracture toughness (11.1 MPa-m"*) and bending strength (1100 MPa) were achieved in the direction perpendicular to the grains alignment compared to specimens with a random distribution of elongated grains. Moreover, the specimens exhibited a high Weibull modulus of 46 due to the uniform distribution of large grains.

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