Control of pore structure during freeze casting of porous SiC ceramics by different freezing modes

The frozen moulds, including homogeneous, unidirectional and bidirectional freezing, were designed using materials with different thermal conductivities, and the temperature variations of the moulds and samples during the freezing process were simulated by finite element analysis. Highly porous SiC ceramics with significant differences in pore structure were fabricated by using the SiC/water slurries prepared via uniform or oriented freeze casting with various freezing modes, and porosity and compressive strength of the as-fabricated ceramics were investigated. The results showed that the pore structure of ceramics prepared by homogeneous freezing was relatively intricate and inconsistent, and had a higher compressive strength. In contrast, the pore structure of ceramics fabricated using bidirectional freezing mode was more ordered and higher porosity was observed. Moreover, porous ceramics prepared by unidirectional freezing mode exhibited a typical gradient structure with increased pore size from tens of micrometers in the bottom to hundreds of micrometers in the top.