Nucleation at cordierite glass surfaces : kinetic aspects

The surface nucleation of high-quartz solid solution crystals at fractured surfaces of glasses of the stoichiometric cordierite composition (2MgO. 2Al 2 O 3 . 5SiO 2 ) was studied by optical microscopy. Particular attention was focused on the nucleation kinetics. A constant nucleation density, N ≃ 10 -4 μm -2 , was found not to be significantly influenced by the time and the temperature of nucleation treatment. Even a very fast heating of samples employing heating rates up to 1200 K/min does not lower N substantially. However, for small average crystal diameters (<20 μm) a distribution of crystal size in the same order of magnitude is detectable indicating a simultaneous appearance of both measurable nucleation rates and growth velocities. It can be concluded that the surface nucleation of μ-cordierite occurs during the thermal treatment from a limited number of preferred nucleation sites ; these sites are used up rapidly enough to cause a strong saturation effect of nucleation, but slow enough to cause a crystal size distribution at the same time. The surface nucleation rate, I s , was calculated from the observed distribution of crystal sizes. Is progressively increases with rising temperature similar to the crystal growth velocity indicating a broad temperature range of essential nucleation activity. The latter must be regarded as the main obstacle to measure or to control surface nucleation density by means of two-step nucleation and growth treatments and must therefore be claimed to be mainly responsible for the observed constancy of N.