Nucleation and crystallization in calcium aluminate glasses

The crystallization and nucleation mechanisms of a eutectic composition (46.5 wt% CaO, 53.5 wt% Al2O3) calcium aluminate glass were investigated by DTA, XRD and SEM. The crystallization temperature was found to vary from 940°C to 1040°C, and two phases appeared in the crystallized samples: Ca12Al14O33 (major phase) and CaAl2O4 (minor phase). Two kinds of crystal structures, dendritic and spherulitic, were observed by optical microscopy. Heating rate and particle size effects on crystal growth were studied by DTA. The average activation energies of crystal growth in two different particle sizes were determined to be 739 ± 4 kJ/mol by the Kissinger method and 694 ± 4 kJ/mol by the Augis–Bennett method. The average Avrami constant for two different sized particles was determined to be 1.7 ± 0.2 by the Ozawa equation. The DTA peak heights decreased with increasing particle size, which shows that the crystallization mechanism includes surface crystallization. A nucleation-rate type curve was obtained by investigating the relation between nucleation temperature and DTA peak height, which suggests that there are two independent nucleation rate maxima. Two, distinct crystallization peaks were observed in monolithic (non-powdered) samples by DTA, and were further studied by in situ XRD and SEM. It was determined that the two peaks correspond to surface and internal crystallization, respectively.