Glasses supercooled melt

This is the process of the crystallization of one or more of the constituents of glass. Generally a glass is thermodynamically unstable with respect to these crystals, but at ordinary temperatures the crystallization rate is quite negligible. Crystallization may occur when the glass is worked at high temperature. The crystals which appear in a supercooled melt are not necessarily those of the stable solid phase at the temperatiure concerned for example, cristobalite can appear at temperatures for which tridymite is the stable crystalline... 

Physical properties of solid polyphosphate glasses and their melts are also in accord with the conclusions drawn from chemical studies. The X-ray diffraction pattern shows the polyphosphate anions to consist of long chains of P04 tetrahedra (32) and the same conclusion is reached by studying the double refraction of fibers formed by rapidly drawing supercooled melts of Graham s salt (101). 

The viscosity of molten glass and of supercooled melts was found by Fulcher S and by Tammann and Hesse to be given by ... 

The supercooled melt and the glass are in the metastable states, which are well separated from the corresponding crystalline states by large barriers. We may therefore assume that equilibrium thermodynamics can be applied to such metastable states. Hence the free energy variation of a glass is also represented by equation (2.04) and we may therefore write,... 

Fig. 7.10 FT-IR spectra of (R,5)-proxyphylline and (R,5 )-diprophylline polymorphs and the noncrystalline state SCM supercooled melt, Am glass [51, 107],...

Fig. 7.12 Polarized-Iight photomicrographs of the different forms of (/f,S>proxyphylline (Mod. I, II, and in, SCM supercooled melt) in a melt-film preparation between a glass sUde and a cover slip as well as the coresponding Raman spectra, recorded with an NIR-FT-IR microscope directly from the film preparations [107].

At the glass transition temperature T of the system the configurational part of entropy vanishes. It is assumed that the transition of the supercooled melt to the glass is a type of second-order transition to obtain Eq. (78), where B is a temperature-independent energy term of transport, R is the gas constant, and A is a temperature-independent quantity, depending on the composition of the solution. 

Consistent with the above discussions, the ability to form glasses depends on the experimental conditions. For instance, while it is possible to supercool a melt for most materials, many materials tend to crystallize near the melting temperature, and will form supercooled melts only if relatively high cooling rates are employed. This is because the conversion of supercooled melt to more stable crystalline forms occurs as a function of relaxation times, in which shorter relaxation times make crystallization... 

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