Kinetics of wollastonite nucleation in CaO•SiO2 glass

The nucleation and growth of wollastonite crystals have been investigated in stoichiometric CaO . SiO2 glass. In the temperature range of 1043 to 1073 K, the nucleation rate was evaluated from crystallite densities, determined by optical microscopy on samples subjected to double-stage heat treatment. Turnbull's method was used to analyze the nucleation rates in terms of various theoretical models: the classical theory (CNT), a semiempirical density functional approach (SDFA) based on the model of Bagdassarian and Oxtoby, and the phenomenological diffuse interface theory (DIT). When either the viscosity or the transient time of nucleation is used in determining the rate constants, a strongly temperature dependent interfacial free energy is obtained that approaches a linear behavior at high temperatures. Accordingly, Turnbull's plot is curved; however, the high-temperature asymptotic behavior is well described by both the SDFA and the DIT. The DIT analysis indicates a homogeneous process, as opposed to the heterogeneous mechanism suggested by the SDFA. The melting point values of the dimensionless interfacial free energy (Turnbull's alpha) deduced from the slope of Turnbull's plot are 0.79-0.85 (DIT) and >1.06-1.14 (SDFA). The somewhat too high values in the latter case originate from-a broad (similar to 200 Angstrom) interfacial region predicted by the SDFA that exceeds considerably the interface width from computer simulations. Attempts to remove this problem by adjusting the parameters of the SDFA failed, suggesting that a more accurate free energy functional needs to be introduced into the model. Possible origins of the nonlinear Turnbull plot are discussed. (C) 1998 American Institute of Physics.