Crystal Nucleation and Growth in Lithium Diborate Glass by Thermal Analysis

Abstract

The kinetics of nucleation and crystal growth in a Li2B4O7 glass were investigated using differential thermal analysis (DTA) and differential scanning calorimetry (DSC). This melt forms a glass even when it is cooled at a very slow rate (2 K/min). The temperature range of nucleation was determined by comparing the heights of the exothermic DTA crystallization peaks for samples that were annealed at different temperatures for a fixed time (10 min). Nucleation for this glass occurred in the temperature region of glass transition, where the nucleation rate had a maximum value at ∼770 K. The real detectable enthalpy change that was due to crystallization occurred at temperatures well above the temperature range for nucleation; hence, the temperature regions for nucleation and crystal growth were assumed to be well separated. The glass sample with an almost‐saturated number of nuclei, which was prepared by annealing at a temperature of 770 K for 10 min while the melt was cooled, was subjected to DSC measurements at various heating rates (φh). The kinetics of crystal growth were analyzed via the conventional isoconversion method. For the sample without any annealing treatment, the number of nuclei for growth varied, depending on the thermal histories within the temperature region for nucleation, which was characterized by the linear cooling rate (φc) of the melt and the linear heating rate φh of the glass. A kinetic approach that considered the dependence of the number of nuclei on φc and φh was applied to the crystal growth in the nonannealed glass. The three‐dimensional growth of the preexisting nuclei with apparent activation energies (E) of 315.6 ± 4.6 kJ/mol for the annealed glasses and 300.6 ± 9.5 kJ/mol for the nonannealed glasses was determined. This agreement in the values of E indicates that the change in the number of nuclei, depending on φc and φh, can be treated successfully by the present kinetic approach for crystal growth.