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 DT A, XRD and SEM. The crystallization temperature was found to vary from 940 degrees C to 1040 degrees C, and two phases appeared in the crystallized sa mples: Ca12Al14O33 (major phase) and CaAl2O4 (minor phase). Two kinds of cr ystal structures, dendritic and spherulitic, were observed by optical micro scopy. Heating rate and particle size effects on crystal growth were studie d by DTA. The average activation energies of crystal growth in two differen t particle sizes were determined to be 739 +/- 4 kJ/mol by the Kissinger me thod and 694 +/- 4 kJ/mol by the Augis-Bennett method. The average Avrami c onstant for two different sized particles was determined to be 1.7 +/- 0.2 by the Ozawa equation. The DTA peak heights decreased with increasing parti cle size, which shows that the crystallization mechanism includes surface c rystallization. A nucleation-rate type curve was obtained by investigating the relation between nucleation temperature and DTA peak height, which sugg ests that there are two independent nucleation rate maxima. Two, distinct c rystallization peaks were observed in monolithic (non-powdered) samples by DTA, and were further studied by in situ XRD and SEM. It was determined tha t the two peaks correspond to surface and internal crystallization, respect ively. (C) 1999 Elsevier Science B.V. All rights reserved.