A model for non-isothermal crystallization kinetics

In the framework of a previously proposed Integral Equation formalism, an equation is derived for the kinetics of non-isothermal crystallization (devitrification). It is shown that the Kissinger multiple-scan thermal analysis technique, commonly used for the evaluation of activation energies, cannot be applied to all non-isothermal transformations, but only to those where site saturation, heterogeneous nucleation takes place. The heating rate is the controlling factor of the nucleation mode in devitrification. If it is high enough, homogeneous nucleation is activated. Heterogeneous nucleation takes place when the heating rate is low. In that case, the high temperatures imposed by the thermodynamics for homogeneous nucleation are not reached before completion of crystallization. A critical heating rate is derived, that depends on the amount of pre-existing nucleation sites, on the activation energy for growth of the new phase and on a critical temperature. The new model is conform to the results obtained in devitrification experiments induced by high rate pulse heating, as compared to those obtained at lower heating rates usually practiced in DSC analyses for the same material system.