Surface and volume nucleation and growth in TiO2±cordierite glasses

Systematic measurements and analyses of surface nucleation and growth of l-cordierite on diamond-polished surfaces and on fractured surfaces of cordierite glasses containing 0.3, 1.2, 3.4, 6.2 and 8.1 wt% TiO2 were carried out. All the glasses exhibit surface nucleation, however, the glass having 8.1 wt% TiO2 also exhibits volume nucleation. The maximum surface nucleation rate is located at a temperature considerably higher than the maximum volume nucleation rate. Both surface and volume crystallization occur by heterogeneous nucleation. The average nucleus/substrate wetting angles, h, were estimated from surface and volume nucleation measurements. The widely di€erent values of h indicate that the active catalyzing sites are di€erent for surface and volume crystallization. It is likely that Al2TiO5 crystals induce volume crystallization of l-cordierite, while surface crystallization is a€ected by TiO2, defects and relicts of polishing power. Nucleation on fractured surfaces was induced mainly by solid particles, most likely by broken glass particles. The crystal growth velocity of l-cordierite crystals is well ®tted by the 2D-surface nucleation growth model. Ó 1999 Published by Elsevier Science B.V. All rights reserved.