Effects of Solids Loading on Sintering and Properties of Freeze-Cast Kaolinite–Silica Porous Composites

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Author to whom correspondence should be addressed. e-mail: klu@vt.edu

Abstract

Kaolinite–silica nanocomposites with a green porosity ranging from 75% to 87% were prepared using a freeze-casting technique. The initial solids loading values (kaolinite platelets plus silica nanospheres) greatly influence the sintering behavior as well as the phase and strength of the resulting porous composites. The composites with lower solids loading exhibit faster sintering (e.g., larger shrinkage, more extensive thickening of the pore walls) when sintered at 1250°C, which in turn, results in a rapid increase in flexural strength. All the composites maintain a high porosity (above 50%) after sintering at 1250°C for 72 h, whereas the flexural strength of the composites increases from roughly 0.2 MPa for the green samples to 13.3, 7.5, and 6.5 MPa for 12, 18, and 24 vol% solids samples, respectively, after sintering. It is believed that solids loading affects kaolinite–silica packing during the sol-to-gel transition as a minimum amount of silica nanoparticles is required to build the gel network. This particle packing difference influences the amount of kaolinite–silica interfacial contact, which in turn affects the strength. The strength increase through solids loading change is a combined effect of changes in the porous structure during sintering plus the development of a new phase at the silica–kaolinite interface.

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