Structure and energetics of silica in the sol-gel to ceramic transitions

Sol-gels of colloidal silica were characterized with photoacoustic Fourier-transform infrared spectroscopy, differential scanning calorimetry, thermogravimetry, drop solution calorimetry and X-ray diffraction. Infrared spectroscopy showed that water and hydrogen-bonded silanol were removed from the gels with increasing temperature. The intensity of the isolated SiOH stretching band increased as temperatures increased to 800°C, then decreased as the temperature was increased further and densification occurred. Sintering was facilitated by an oxygen atmosphere. The colloidal silica gel densified at 1150°C in O₂, but densification was negligible in an N₂ atmosphere. When polyvinyl alcohol was used as a binding additive to the gel, the resulting gel densified and crystallized to cristobalite between 1000 and 1150°C in both oxygen and nitrogen. The crystallization was found to result from alkali impurities in the PVA. Photoacoustic spectra of the gels showed that the PVA adsorbed to the silica surfaces through the surface hydroxyl groups, helping to tether particles together.