Supercooled glasses

In the foregoing article we have applied the methods of statistical mechanics to a determination of the entropy of crystals and supercooled glasses, and have reached the following conclusions. 

First-order solid-state amorphization occurs due to an entropy catastrophe [39] causing melting of superheated graphite and decompressed diamond below Pg when the entropy of the ordered crystal would exceed the entropy of the disordered liquid. This condition is resolved with the occurrence of a kinetic transition to a (supercooled) glass whereby the exact kinetic conditions during carbon transformation will be critically Pg-depen-dent [39]. It is important to consider the crystal to liquid transition and the effect of a superheated crystal whereof the ultimate stability is determined by the equality of crystal and liquid entropies [40]. When this condition is met, a solid below its Pg will melt to an amorphous solid, particularly... 

Above Tg, the supercooled glass-forming hquid exhibits decreasing viscosity with increasing temperature. On the one hand the glass-forming hquid of silica exhibits a relatively small and constant temperature coefficient of viscosity, e.g. decreasing from Pas... 

To continue the a measiuement beyond the softening point requires that the supercooled glass-forming liquid be enclosed in containment such that volume change can be measured. But a above Mg is not usually relevant to the performance of a glass material in normal service since above Mg the glass melt cannot support sustained stress and undergoes viscous flow and stress relaxation. 

The viscosity of the supercooled glass melt varies over many orders of magnitude, so it is common to express t on a log scale or by powers of 10 such that 7 is approximately 10 Pa s and the viscosity at the melting temperature can be of the order of < 1 Pa s. With such a large range of viscosities possible, no one measurement technique is sufficient for the whole range. Measurement methods can be broadly classified as high- or low-viscosity methods. 

The free enthalpy G (see also (2.2)) for nucleation of a supercooled glass melt consists of two terms, the volume and the interface term and Go (2.11) ... 

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