Crystallization and properties of CaO-P2O2-B2O3 glasses

Glasses were prepared with compositions (50–0.5 x) CaO.(50–0.5 x) P₂O₅ · x B₂O₃ with B₂O₃ contents (x) from 0 to 45 mol%. The glass transformation temperature (T _g), dilatational softening temperature (T _D) and Vickers hardness (H _V) initially increased with x, but showed maxima at about x=20 for T _g and T _D and at about x=35 for H _V. The thermal expansion coefficient decreased with x, levelling off at about 35 mol% B₂O₃. The maximum tendency to crystallize occurred at around 25 mol% B₂O₃. Volume nucleation (and hence glass-ceramic formation) and surface nucleation were obtained for x between 15 and 25 mol%. The first phase to appear was BPO₄, which was probably homogeneously nucleated. Subsequently the 4CaO · P₂O₅ phase was heterogeneously nucleated on the BPO₄. For 10 x 35 only surface nucleation was observed. The kinetics of nucleation were investigated in the 20 mol% B₂O₃ glass. The changes in properties and crystallisation behaviour with B₂O₃ content were related to short-range structural information. Infrared spectra and literature data indicated a threedimensional network of B-O-B and B-O-P linkages in the glasses.