Crystallization of Na2O-doped colloidal gel-derived silica

Crystallization kinetics of the colloidal gel-derived silica has been inves tigated when the powder compacts are pressureless-sintered at 700-900 degre es C in air. alpha-Cristobalite is the only crystalline phase in the sinter ed mixture as detected by X-ray diffractometry, whose quantity is used to e stablish the crystallization kinetic curves. The Avrami exponents of n = 0. 8-2.3 thus derived indicate that the crystallization has taken place by pre dominantly thermal nucleation of the diffusion-controlled nature. Both lame llar and spherulitic growth of the alpha-cristobalite crystals are observed by scanning electron microscopy in sintered samples containing Na2O. The g rowth morphology alters from predominantly chunky crystals (n = 2.3) to sph erulitic grains (n = 0.8) as the Na2O-content of the initial colloidal sols is increased beyond a critical level. The activation enthalpy values of De lta H = 216-288 kJ mol(-1) suggest also that oxygen transport by a mechanis m involving limited exchange of network and interstitials is the rate-deter mining mechanism for the crystallization of cristobalite. (C) 2000 Elsevier Science S.A. All rights reserved.