Study of Freeze-Cast Porous Silica Nanoparticle-Based Composites

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Virginia Tech

Porous silica-based nanocomposites are promising ceramics, as they exhibit high specific surface area, highly porous network, and a surface that can be easily functionalized. This dissertation describes the results of a study on the formation and properties of porous silica nanoparticle-based composites, using techniques of freeze casting and sintering. Kaolinite platelets and silica nanorods were added into the nanoparticle system, and their effects on modifying the porous microstructures and physical properties were investigated.

During freeze casting, homogeneous microstructures with highly interconnected porosity are fabricated. Kaolinite addition results in large and more interconnected pores, while added silica nanorods cause a pore morphology evolution from circular to elongated spherical pores with increasing aspect ratio. The specific surface areas (area/mass) of the particles are conserved during freeze casting and values for the resulting composites can be accurately predicted using the area and mass of the components assuming conservation of area. Both kaolinite platelets and silica nanorods effectively improved the strength of the freeze cast green composites as they distribute any applied stress over a larger portion of the sample.

Upon sintering, added kaolinite is found to modify the sintering behavior of the silica nanoparticles and a transitioning interfacial phase is identified when sintering temperature is above 1250 °C. This new phase contributes to the further enhancement of strength and this strengthening effect depends on composition and initial solids loading. After sintering at 1250 °C for 1 h, a ceramic containing 10 vol% kaolinite and 8 vol% silica has a maximum strength while maintaining a ~69% porosity. The kaolinite-silica composites with lower solids loading exhibit faster sintering (e.g. larger shrinkage, more extensive thickening of the pore walls), which, in turn, results in a rapid increase in mechanical strength.

Based on the understanding of the composite properties and the underlying principles, a novel method for creating nanocomposites with precisely controllable specific surface area is developed. With repeated nanoparticle suspension infiltration, freeze drying, and sintering, the specific surface area can be varied from less than one to well over 100 m2/g, demonstrating potential application as liquid membranes.

Silica, Kaolinite, Freeze casting, Porous microstructure, Strength