Induced crystallization of glass-forming melts : Part 1. Heterogeneous nucleation. Effect of noble metal microcrystals on the crystallization of calcium metaphosphate glasses

Abstract

The possibilities of initiating crystallization in glass-forming melts are analyzed, using experimental evidence, obtained with several model glass-forming systems. In the present Part 1 of the investigation the process of heterogeneous nucleation, catalyzed by insoluble crystallization cores is studied, using a new theoretieal thermodynamie model. The nucleation activity coefficient, Φ, of the aubstrates is described by the adhesion energy, β, at the interface crystallization core/overgrowing crystal. Lattice disregistry is accounted for as an additional correlation factor, influencing the thermodynamie work of adhesion. The crystallization of calcium metaphosphate glass-forming melts, initiated by noble metal microerystals (Ag, Au, Pd, Pt, Rh, Ru, Ir, Os), is studied by differential thermal analysis, optical and scanning electron microseopy. Ir microerystals show highest nucleation activity promoting intensive bulk crystallization in the Ca(PO3)2 systems investigated. The activity of other crystallization cores decreases in the following sequence: Ru > Rh > Os > Pt > Pd > Au > Ag. The above outlined thermodynamic theory of nucleation activity is used to correlate experimental data and to calculate activity coefficients, Φ, in dependence on the properties of the substrates employed. Under tangential stress, applied by pressure in extrusion experiments, the metal microcrystals in the glass samples are stratified in concentric rings with particle density increasing towards the sample surface. Thus a new method of formation of pre-oriented glass-ceramic materials is indicated. In Part 2 of the investigation the effect of soluble oxide additives on nucleation and crystallization in glass-forming melts is considered