An extension to isothermal processes of the theoretical method developed (TMD) to analyze the glass-crystal transformation kinetics by differential scanning calorimetry. Application to the crystallization of the Sb_(0.16)As_(0.22)Se_(0.62) glassy alloy

摘要

It is well known that in the study of the glass-crystal transformation kinetics, sometimes is necessary that both the nucleation frequency and the crystal growth rate depend on time as a power law. To explain the probable physical nature of this time dependence, the theoretical method developed (TMD) for non-isothermal processes, which we have published recently, considers oriented nucleation and oriented growth processes, which are non-linear. In the present article we extend the quoted TMD to isothermal glass-crystal transformations. From this view point, and considering the impingement effect, a procedure has been developed to obtain an evolution equation with the time, t, for the actual volume fraction transformed, x, under isothermal regime. In order to calculate the kinetic parameters corresponding to isothermal processes, we follow the already quoted TMD adequately extended to isothermal regime. Thus, by means of an adequate computer program, from the isothermal experimental data, t and x, obtained by differential scanning calorimetry for each fixed temperature, T, it is possible to choose the corresponding impingement factor, δ_i, more suitable. Next, by using the corresponding straight regression lines ln t vs. ln[f(x,δ_i)], the kinetic exponent, n, and the reaction rate constant, K_A, are evaluated from the slope and intercept, respectively. Besides, considering that the quoted constant has a temperature dependence of Arrhenius type, the slope and intercept of the straight regression line lnK_A vs. 1/T give the values of the kinetic parameters: activation energy and frequency factor, respectively. The quoted extension of TMD and the Johnson-Mehl-Avrami (JMA) model have been applied to the isothermal crystallization kinetics of Sb_(0.15)As_(0.22)Se_(0.62) glassy semiconductor, since, given the generality of the above-mentioned TMD, the JMA model is a particular case of the already quoted extension of TMD. It is important to indicate that the experimental curve of the actual volume fraction transformed vs. time shows a better agreement with the theoretical curve of the extension of the TMD than with the corresponding curve of the JMA model, confirming the reliability of the quoted extension in order to analyze the isothermal transformation kinetics of the above-mentioned glassy semiconductor.