Real Time Neutron Diffraction Studies of apatite glass ceramics

Apatite-mullite glass-ceramics have been developed based on SiO2-Al2O3-P2O5-CaOCaF2 glasses. The glasses crystallise to form fluorapatite (FAP) and mullite with an appropriate heat treatment. The crystallisation mechanism has been thought to occur via a prior amorphous phase separation. The aim of this study was to carry out real time neutron diffraction of apatite-mullite glass-ceramics in order to develop an understanding of the crystal growth and crystal dissolution phenomena in the temperature regime between 550 and 1200oC. The results show that during the initial stages of fluorapatite crystallisation pronounced line broadening was observed indicating crystallisation on a nanoscale. Mullite starts to crystallise once there is insufficient charge balancing cations in the glass to maintain aluminium in a four fold coordination state. As a result of this work it is suggested that the mechanism of crystal growth of FAP is thought to involve the dissolution of smaller FAP crystals during the crystal growth temperature followed by re-crystallisation of FAP on the remaining larger crystallites during cooling.