Microstructure of SiO2-Al2O3-CaO-P2O5-K2O-F- glass ceramics.: 1.: Needlelike versus isometric morphology of apatite crystals

In SiO2-Al2O3-CaO-P2O5-K2O-F- glasses, either spherical or needlelike fluorapatite crystals san be formed depending on the heat-treatment schedule. If the annealing is performed at 800 degreesC or low heating rates are applied, isometric apatite crystallizes whereas the heat treatment at 1200 degreesC in conjunction with high heating rates results in apatite needles. On the basis of the combination of P-31 MAS NMR spectrometry, transmission-electron microscopic imaging, and energy-dispersive X-ray spectrometry, a crystallization model capable of explaining the morphology change is derived. Phase-separation processes are shown to have a major impact on the composition of the residual glass hosting apatite crystals and the structure and composition of the glass matrix in turn determine the fluorapatite morphology observed. At 800 degreesC, spherical crystals are formed since the two different growth modes parallel and perpendicular to the crystallographic c axis of apatite (provoking anisometric growth at high temperature) are outweighed by hampered diffusion.