Crystallization of Na2O-doped colloidal gel-derived silica

Crystallization kinetics of the colloidal gel-derived silica has been investigated when the powder compacts are pressureless-sintered at 700–900°C in air. α-Cristobalite is the only crystalline phase in the sintered mixture as detected by X-ray diffractometry, whose quantity is used to establish the crystallization kinetic curves. The Avrami exponents of n=0.8–2.3 thus derived indicate that the crystallization has taken place by predominantly thermal nucleation of the diffusion-controlled nature. Both lamellar and spherulitic growth of the α-cristobalite crystals are observed by scanning electron microscopy in sintered samples containing Na2O. The growth morphology alters from predominantly chunky crystals (n=2.3) to spherulitic grains (n=0.8) as the Na2O-content of the initial colloidal sols is increased beyond a critical level. The activation enthalpy values of ΔH=216–288 kJ mol−1 suggest also that oxygen transport by a mechanism involving limited exchange of network and interstitials is the rate-determining mechanism for the crystallization of cristobalite.