Glass-to-crystal transformation

Until now we have considered the ideal structures of crystals only when each atom or ion is on a regular site in the crystal. Real crystals contain a variety of imperfections or defects. In crystalline ceramics and glasses, the structure and chemistry of the material will be determined by the kinetics of defect movement. For example, the kinetics of the glass-to-crystal transformation are slow if the temperature is low (typically less that 1000°C) because the transformation occurs by atoms moving—in ceranucs, this usually occurs by point defects moving. If point defects move too slowly, the structure with the lowest energy (the equilibrium structure) may never actually be achieved. How fast they move is determined by their structure. 

As opposed to the liquid-crystal transformation, the liquid-glass transformation is not a phase transition and therefore it can not be characterized by a certain transition temperature. Nevertheless, the term "the vitrification temperature , Tv, is widely used. It has the following physical meaning. As opposed to crystallization, vitrification occurs when the temperature changes continuously, i.e. over some temperature interval, rather than jump-wise. Inside this interval, the sample behaves as a liquid relative to some of the processes occurring in it, and as a solid relative to other processes occurring in it. The character of this behaviour is determined by the ratio between the characteristic time of the process, t, and the characteristic relaxation time of the matrix, x = t//G, where tj is the macroscopic viscosity and G is the matrix elasticity module. If t x, then the matrix should be considered as a solid relative to the process, and if t > x it should be considered as a liquid. The relation tjx = 1 can be considered as the condition of the matrix transition from the liquid to the solid (vitreous) state, and the temperature Tv at which this condition is realized as the temperature of vitrification. Evidently, Tv determined by such means will be somewhat different for the processes with different characteristic times t. However, due to the rapid (exponential) dependence of the viscosity rj on T, the dependence of Tw on t (i.e. on the kind of process) will be comparatively weak (logarith-... 

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are the most widely used thermal analysis techniques. Both techniques have the same objective to examine thermal events in a sample by heating or cooling without mass exchange with its surroundings. The thermal events examined by DTA and DSC include solid phase transformation, glass transition, crystallization and melting. Differential emphasizes that analysis is based on differences between sample material and a reference material in which the examined thermal events do not occur. 

To mention a second very simple example, U is certainly greater than A in the transformation of quartz glass into crystallized quartz. In accord with Richards rule, the specific heat of quartz glass at low temperatures is therefore notably greater than that of quartz. At higher temperatures, very definitely even at T — 400°, the converse is, however, the case, and at T = 6oo° the difference is fairly considerable (cf. paper 47, p. 430). The fact that quartz glass has a lower specific heat than quartz above room temperatures has also been confirmed recently by the careful experiments of K. Schulz. At about room temperature the specific heats of the two modifications are equal, so that here, according to Richards, there must be equality between A and U, which is of course out of the question. 

Chapter IX, page 99.—An interesting case of a condensed system, namely, the affinity of the transformation of amorphous silica (quartz glass) into crystallized quartz, has been worked out by R. Wietzel (" Zeitsch. anorg. Chem., 116, p. 71, 1921) in my old laboratory. Since, however, the specific heat of quartz glass falls off so slowly that it was not possible to get anywhere near the region of the T3-Law, the experimental investigation of this case is not yet complete. This is one of the cases where it is very desirable that accurate measurement of specific heat should be continued down to helium temperatures. This is the more important in that no direct experimental test of the application of the Heat Theorem to amorphous substances has yet been made with satisfactory reliability. The transformation from quartz into cristobalite could, however, be followed with adequate accuracy from the standpoint of the Heat Theorem. 

Fig. 51. Traces of measurements on Gd0 64Co0 36 made using a differential scanning calorimeter operated at two different heating rates. The positions of the glass transition temperature Tg and the crystallization temperature 7X are indicated for the higher heating rate. The inset shows the level scheme for the amorphous-to-crystalhne transformation. The experimental values for the activation enthalpy AE = Eac — Ea is 3. 29 eV or 317 kJ mol 1. The value for the crystallization enthalpy A Hc, = Ec — EA is 3.8...

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