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.

The mechanical properties of Al 2 O 3 -based porous ceramics fabricated from pure Al 2 O 3 powder and the mixtures with Al(OH) 3 were investigated. The fracture strength of the porous Al 2 O 3 specimens sintered from the mixture was substantially higher than that of the pure Al 2 O 3 sintered specimens because of strong grain bonding that resulted from the fine Al 2 O 3 grains produced by the decomposition of Al(OH) 3 . However, the elastic modulus of the porous Al 2 O 3 specimens did not increase with the incorporation of Al(OH) 3 , so that the strain to failure of the porous Al 2 O 3 ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH) 3 particles. Fracture toughness also increased with the addition of Al(OH) 3 in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture-mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.

Al 2 O 3 -based porous ceramics with high surface areas were fabricated by adding Al(OH) 3 to the starting powder, followed by pressureless sintering at temperatures >1100°C. Three types of starting powders were used in the present study: pure (OH) 3 . The addition of Al(OH) 3 considerably increased the surface area of the porous Al 2 O 3 , and the addition of ZrO 2 further increased the surface area; a surface area as high as 36.39 m 2 /g was obtained, and the high surface area was retained at higher temperature. The pore size distribution of the specimens with high surface area was bimodal, with one peak at ϳ100 nm and the other, which contributed most of the surface area, at ϳ10 nm. X-ray analysis showed that in the sample with the fine pores and high surface area, there was -Al 2 O 3 phase produced by the decomposition of Al(OH) 3 , presumably because the phase transformation of -Al 2 O 3 to ␣-Al 2 O 3 was incomplete after low-temperature sintering. Moreover, the porous Al 2 O 3 with high surface area retained superior mechanical properties, attributed to the good sinterability of the fine ␣-Al 2 O 3 powder used in the present study. The sintered specimens could be large and designed to any shape, because pressureless sintering was used for fabrication. The present approach provides a new way of fabricating porous Al 2 O 3 ceramics that could be widely used as catalyst supports in industry, especially for high-temperature catalysis.

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