Catalyzed nucleation under microgravity conditions:: results with a model glass

Abstract

In the present paper results are reported on the influence of microgravity on the heterogeneous crystallization in a glass-forming melt (Graham's glass (NaPO 3) x ) in which nucleation is induced by Pd microcrystals. In its crystallization behavior the system resembles a model of the catalyzed nucleation used in glass-ceramics synthesis. Moreover, heterogeneous nucleation guarantees maximal reproducibility in experimentation and conditions, at which the absence of convection plays a decisive role in crystallization. The microgravity experimentation was performed on the MIR Cosmic Station and simultaneously repeated at the MUSC Laboratory in Cologne in an arrangement guaranteeing a DTA-like comparison between cosmic and terrestrial crystallization kinetics. Both series of samples crystallized were additionally examined at terrestrial conditions using different techniques. The thermodynamic, rheologic and crystallization characteristics of the NaPO 3 glass are known from previous terrestrial experiments. From the devitrification kinetics of NaPO 3 glassy samples in heating run experiments it is found that microgravity considerably enhances the induced nucleation of the glass-forming melt in the vicinity of glass transition temperature T g . This effect is determined mainly by an increased impingement rate z of the ambient-phase building units caused by an enhanced rise of the sample temperature. At microgravity conditions convection and heat transfer are reduced and this causes a steep temperature rise, viscosity decrease and thus, increased z values. In crystallization experiments at low undercoolings, where nucleation barrier and not attachment rate z is decisive, no difference is observed between terrestrial and microgravity samples.