Kinetic and thermodynamic characteristics of glassy

The main difference between a glass and its liquid is not structural but kinetic and depends on a microscopic quantity called the structural relaxation time t. This time is the mean life time for the movement of a structural unit over a distance equivalent to its size. Such a structural unit may consist of several SiO units in the case of a silicate glass. The 

Currently, the dependence of t on temperature is deduced from viscosity-temperature measurements. At T T, the temperature dependence of T obeys an Arrhenius law, but this dependence is much more complex at T T. In the latter case it is referred to an empirical Vogel-Tamman-Fulcher (VTF) law (Vogel, 1921 Tamman and Hesse, 1926 Fulcher, 1925). 

The glass transition temperature is thus closely related to kinetic parameters and to the duration of the experiment conducted on the material. Thus, the glass transition temperature is an increasing function of the quenching rate. In practice a variation of about 10-20 K for T may be observed for the same glass (Menetrier, Hojjaji, Estournes and Levasseur, 1991). Note also that for a well defined compound which may be obtained in the form of a glass T and Tq are linked by an empirical 

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