Apatite crystal-growth mechanism in SiO2-Al2O3-CaO-P2O5-K2O-F- glass-ceramics

Depending on the heat-treatment schedule in a SiO2-Al2O3-CaO-P2O5-K2O-F gla ss, apatite of either needle-like or isometric shape is shown to crystallis e. The combined application of P-31 MAS NMR spectrometry and transmission e lectron microscopy in conjunction with energy-dispersive X-ray spectrometry proves the degree of dissolution of liquid-liquid phase separation droplet s present in the casted glass to have a major influence on the morphology o f apatite crystals. Since at 800 degreesC, a smaller number of droplets bec ome dissolved as compared with a treatment at 1200 degreesC (instead they a re converted into apatite), the supersaturation of the glass responsible fo r the formation of apatite is lower at 800 degreesC. Together with kinetic factors affecting the growth of apatite crystals (viscosity, diffusion), th e different Ca/P-ratio is responsible for the two apatite morphologies obse rved.