Formation, Structure and Properties of Freeze-Cast Kaolinite-Silica Nanocomposites

Abstract

This study is focused on understanding the effects of kaolinite content and freezing rate on the microstructure, surface area, and flexural strength of freeze‐cast kaolinite–silica nanoparticle composites. Scanning electron microscopy reveals that the bulk of the composites contain interconnected pores and that the size of the pores increases with kaolinite concentration. Lowering the freezing rate from 2.0 to 0.05 K/min produces much larger pores on the outer surface of the sample and only minor effects on the bulk morphology. BET measurements indicate that the specific surface area (area/mass) of the composites is controlled primarily by the relative amounts of kaolinite and silica in the sample and not by the freezing rate. Equibiaxial flexural strength tests show that the strength of the composites is substantially higher than freeze‐cast samples produced using either kaolinite only or silica only, indicating that the two components work cooperatively. The strength is slightly higher for composites produced at the lower freezing rate. Adsorption tests indicate that during gel formation, the silica nanoparticles strongly adsorb to the much larger kaolinite platelets, which could be the primary cause for the increased strength of the composites.

Effects of added kaolinite on the strength and porosity of freeze-cast kaolinite–silica nanocomposites

“…The structure, strength, porosity, and X-ray diffraction patterns of green samples and samples sintered up to 1250°C are measured and discussed. In combination with our previous findings on the properties of these kaolinite-silica composites [9,23], we propose several different mechanisms for the observed changes in strength.…”

“…For the green samples, the maximum strength occurs at 14 vol% kaolinite and the strength is approximately 8 times higher than that of the pure silica. As explained in an earlier paper, our hypothesis is that the silica nanoparticles bond the structure together, while the large kaolinite platelets distribute any applied stress over a larger area [23]. There is thus a clear synergistic effect of having both materials present.…”

“…The pore walls consist of kaolinite platelets and silica nanoparticles bonded together (Fig. 1a, inset) and this process has been extensively discussed in our previous paper [23]. The subsequent sintering process causes diffusion between the kaolinite and silica materials and results in changes in this microstructure.…”

“…Typical values of the Poisson's ratio for the composite ranged between 0.21 and 0.30. Details of this equibiaxial flexural strength test and the instrument's schematic illustration were described in a previous paper [23].…”

“…In this study, well-dispersed suspensions were first prepared by sequentially adding certain amounts of Ludox, de-ionized water, NaOH, and kaolinite into a processing vial. After mixing by a vortexer (Fisher Scientific Inc., Bohemia, NY), NaCl was introduced into the suspension to generate a sol-togel transition process, which was found to improve the interaction of two species and the homogeneity of the porous structure of the resulting composites [23]. The concentrations of NaCl in all suspensions were maintained at 0.5 M. To promote the gelation, NaOH was added to the suspensions before the introduction of salt to adjust the suspension pH to *7.8.…”