Functionally graded porous ceramics with dense surface layer produced by freeze-casting

Graded and porous Al₂O₃ ceramics with dense surface layer were fabricated by camphene-based freeze-casting process. The Al ₂O₃/camphene/dispersant slurries were prepared by ball-milling at 60 °C for 24 h, before pouring into silicone rubber die. The warm slurry was cast into a mold at 25 °C, where the top surface of the cast body was exposed to air to allow for the controlled evaporation of molten camphene, and the bottom was attached to a copper plate before being placed on the top of a metal plate immersed in a water bath which can be cooled by ice-water (°C) and liquid nitrogen (-196 °C). This processing method can produce a graded porosity and pore structure distribution. A typical four pore structure, that is surface dense layer, transition layer, aligned pore distribution region and inner porous region is formed between the top surface and bottom surface. This technique is considered potentially useful in fabricating novel porous ceramics with special structure.