Fabrication of porous silica with controllable and tunable porosity via freeze casting

Porous ceramics offer unique properties that can bring advances to many application areas. The freeze-casting process has a strong potential for fabricating porous ceramics; however, the effects of process parameters on part porosity must be well understood for scalable manufacturing via freeze casting. This paper presents an experimental analysis of the freeze-casting process that correlates the freeze-casting parameters with pore characteristics. A full-factorial design of experiments is conducted on a unidirectional freeze-casting testbed using silica as the ceramic material and camphene as the solvent. The effects of solid loading, particle size, cooling temperature, and the distance from the cooling surface on porosity characteristics are evaluated. The fabricated samples are cross-sectioned vertically and horizontally and imaged using scanning electron microscopy. Image processing is used to obtain the porosity characteristics of areal porosity, pore size, pore shape, and pore orientation. The capability to steer the pore orientation is also demonstrated through bidirectional freezing experiments supported by a finite-element model. As a result, a quantitative understanding of the effects of freeze-casting process parameters on porosity characteristics is gained for the silica-camphene system. These results and the presented approach can be used for reproducible manufacture of porous ceramics with controlled porosity.