The crystallization of calcium phosphate glass with the ratio [CaO]/[P₂O₅] < 1

Abstract

The phase evolution in a calcium phosphate glass with the molar ratio [CaO]/[P₂O₅] < 1, to which 6.4 mol% TiO₂ and 10 mol% Al₂O₃ were added as nucleation agents, was studied. The results indicate that the primary phase β-Ca₂P₂O₇ is formed during surface nucleation and crystallization which are dominant at T < 930°C. No presence of metastable calcium phosphates was registered during the formation of the primary phase. The growth of β-Ca₂P₂O₇ crystals occurs on the faceted crystal/glass interface with dendritic morphology at a crystal growth rate that is independent of time. The kinetics of β-Ca₂P₂O₇ crystal growth are characterised by a growth activation energy of E _a = 426 ± 15 kJ/mol. The parameters of the unit cell of the β-Ca₂P₂O₇ phase decrease with increasing temperature. The secondary TiP₂O₇ and AlPO₄ phases are formed by volume nucleation and crystallization. The temperature of the maximum nucleation rate was determined to be T _n = 690°C, and it is higher than the transformation temperature T _g. In the temperature interval T < 930°C secondary phases appear during long annealing times. At T > 930°C glass volume crystallization is dominant.