Effects of Added Clay on the Properties of Freeze-Casted Composites of Silica Nanoparticles

摘要

An experimental study was performed to investigate the effects of added clay particles on the structure and properties of porous, freeze-dried composites produced from silica nanoparticles. The sol-gel transition of the silica suspension arrests the sedimentation of the dense clay platelets to produce a gel, containing large clay particles dispersed throughout. No chemical additives other than sodium chloride were needed to cause the gelation. Dry composites were obtained by subjecting the gelled suspensions to rapid freezing in one of the following ways: submersion in liquid nitrogen or liquid propane, or exposure to low-temperature refrigeration. Frozen samples were then sublimated under vacuum to remove the water. Scanning electron microscope images of the dried composites showed a porous material whose pore sizes decrease with an increasing rate of freezing. The dependence on freeze rate suggests that the porous structure is the result of ice formation. Addition of the clay particles significantly increases the rate of gel formation. Furthermore, the strength of the clayanoparticle composites, as measured by normal compression tests, was substantially greater than that of a similar material made with only silica nanoparticles. Finally, BET surface area analysis shows a decreasing surface area with increasing volume fraction of clay at a constant volume fraction of silica.