Kinetics of Nonisothermal Crystallization of Bi2Sr2CaCu2Ox Glasses with Different Copper Valence States

Abstract

The kinetics of the nonisothermal crystallization process in Bi2Sr2CaCu2Ox glasses with different copper valance states, R(Cu+) = Cu+/(Cu++ Cu2+) = 0.79–0.99, were examined by using differential scanning calorimetry. Four different kinetic equations were used for the data analyses. It was found that the values of activation energy for crystal growth, Ea, decreased with increasing Cu+ content. The value of Ea, which was estimated from the modified Ozawa equation, for the glass with R(Cu+) = 0.79 was 483 kJ·mol‐1 and for the glass with R(Cu+) = 0.99 was 353 kJ·mol‐1. In the former glass, the number of crystal nuclei was almost independent of the heating rate. But, in the latter glass, the number of crystal nuclei varied significantly with the heating rate. The crystallization mechanism in the glasses was closely related to the thermal stability and viscosity of glasses: the glass with R(Cu+) = 0.99 showing a high thermal stability and a low activation energy for viscous flow had a small value of Ea. The plot of In (Tp2/α) against 1/Tp, where Tp and α are crystallization peak temperature and heating rate, respectively, gave a reasonable value of Ea for the glass with R(Cu+) = 0.79 but was unsuitable for the evaluation of Ea for crystal growth of the glass with R(Cu+) = 0.99.