A study on the dimensionality of the crystal growth and the exponents of the power laws of the time-dependence for the nucleation frequency and for the crystal growth rate: Application to the non-isothermal crystallization of the Ag_(0.16)As_(0.42)Se_(0.42) glassy alloy

摘要

A procedure has been developed to establish the thermal process type, to determine the dimensionality of the crystal growth and to evaluate the exponents of the power laws of the time-dependence both for the nucleation frequency and for the crystal growth rate in glass-crystal transformations under non-isothermal regime. To carry out the quoted procedure, it is established a linear relation between the logarithms of the heating rate and of the temperature corresponding to the maximum crystallization rate, which allows to obtain other linear relation between the logarithm of the quoted temperature and the reciprocal of this temperature. From the slope of the last relation, once the thermal process type has been determined by means of the comparison of the kinetic exponent values for the as-quenched and reheated samples, the dimensionality of the crystal growth is deduced. By comparing the theoretical values of the activation energy for one-, two- and three-dimensional crystal growth, obtained from the quoted slope, with the experimental value for this quantity, it is chosen the theoretical value nearest to the experimental one. Thus, the dimensionality of the crystal growth is determined. From the dimensionality and of the kinetic exponent values for the as-quenched and reheated samples, the exponents of the above-mentioned power laws have been evaluated. The procedure developed has been applied to the crystallization kinetics of the Ag_(0.16)As_(0.42)Se_(0.42) glassy semiconductor, which presents two exothermic peaks. For the first one, the thermal process is a glass-crystal transformation with "site saturation" and two-dimensional crystal growth; whereas, for the last one, it is a process of continuous nucleation and three-dimensional crystal growth.