Crystal nucleation in silicate glasses: the temperature and size dependence of crystal/liquid surface energy

The most basic assumption of the classical nucleation theory (CNT) is to treat nucleus/liquid surface energy, sigma, as a macroscopic property having a value equal to that of a planar interface, sigma(infinity). Therefore, when the CNT is used to analyze experimental data, the size dependence of surface energy is often neglected. To date, there has been no reliable method to measure the surface energy of the nucleus/liquid interface except by fitting nucleation rate data to the theory. In this case, one obtains the surface energy of critical size nuclei as a function of temperature. However, the fitted sigma(T) dependence arises from two different factors: the temperature dependence of sigma for a planar interface and its size dependence. This paper focuses on the temperature dependence of the macroscopic value of surface energy, decoupling it from the size dependent part. Tolman's equation was used to eliminate the size dependence of surface energy from published nucleation data for two stoichiometric silicate glasses (Li2O . 2SiO(2) and Na2O . 2CaO . 3SiO(2)). It is shown that the Tolman parameter may be chosen so that surface tension decreases with temperature; d sigma(infinity)/dT < 0. The value of d sigma(infinity)/dT obtained in this way is close to theoretical predictions. (C) 2000 Elsevier Science B.V. All rights reserved.