Freeze casting of porous zirconia through sol-gel

Abstract

Shape memory materials experience diffusionless phase transitions through martensitic transformations where the material returns to its original phase after heat is applied (shape memory), or where it returns on its own (superelasticity). In ceramics, this transformation is optimized at high surface area-to-vol. ratios, where cracking due to the transitions is reduced. Freeze casting is a technique used to make porous materials which have high surface area and low d. Sol-gel processing allows for good control over the fabrication of ceramic powders, and consequently, the microstructure. We propose that by combining sol-gel and directional freeze casting, a homogeneous and directional zirconia porous structure can be achieved. By controlling the chem. compns. of acetylacetone and water (Zr/AcAc/H_2O 1/0.1/7.5 molar ratio), and thus, balancing the processes of hydrolysis and polycondensation, a suspension was produced which was made into a porous foam by freeze casting. Freezing conditions at -30°C yielded ceramic foams with minimal cracking, micropores, and 1-2 m wall thickness. Ceramics sintered at 1500°C were found to have a higher grain d. and larger grain sizes than those sintered at 1200°C and 1400°C. The mech. properties were studied by compressing the sintered ceramics, and their correlation with the microstructure were explored.

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