Real-time nucleation and crystallisation studies of a fluorapatite glass-ceramics using small-angle neutron scattering and neutron diffraction

Real-time small-angle neutron scattering (SANS) and neutron diffraction (ND) studies have been performed on a calcium fluorapatite (Ca-5(PO4)(3)F) (FAP) glass-ceramic composition. The cast glass exhibited scattering at low q and a peak in I(q) at higher q. The scattering at low q is thought to arise from a larger-scale nucleated structure, while the peak in I(q) is thought to arise from a finer spinodally decomposed structure. High temperature viscoelastic measurements show two reductions in the storage modulus (E') and two peaks in the damping factor (tan delta) consistent with a glass that has undergone amorphous phase separation (APS) during the casting process. On heating to 780 degrees C, the scattering at low q increased in intensity, while the peak in I(q) increased in intensity and moved to lower q, consistent with the coarsening of the finer scale phase separated structure. During isothermal experiments, the scattering at low q increased in intensity and the peak in I(q) moved to lower q, corresponding to a final spacing of about 35 nm. After about 30 min at 740 degrees C, and 12 min at 750 degrees C the coarsening process effectively stopped and is inhibited by the glass transition temperature (T-g) of the second glass phase. ND showed the glass to crystallize on heating to FAP and then mullite (2SiO(2).3Al(2)O(3)). At high temperatures, both the FAP and mullite crystal phases partially re-dissolved, but were found to re-crystallize rapidly on subsequent cooling. The results indicate that the proposed crystal growth hold is actually a crystal dissolution hold, with re-crystallization occurring rapidly on cooling. The results indicate that it is important to control not only the heat-treatment cycle but also the cooling cycle.