Phase transitions and structural changes of nanostructured chlorapatite under thermal treatment

The influence of thermal treatment on phase transformations and structural features of mechanosynthesized chlorapatite nanopowders was investigated. Results showed that the phase composition of the product was influenced by the ball to powder weight ratio. During annealing in the range of 900–1300 °C for 1 h, decomposition of chlorapatite to tricalcium phosphate, tetracalcium phosphate, and hydrochloric acid occurred. The rate of decomposition increased gradually with increasing the annealing temperature to 1300 °C. According to the obtained data, during milling process the lattice strain increased significantly to about 1.08%, while the crystalline size declined sharply to around 28.89 nm. After thermal treatment, the crystallite size of chlorapatite went up notably due to a dramatic decrease in residual elastic strain. The crystallinity degree of the 5 h milled sample was about 78.14%. This value reached around 92.86% after annealing at 1300 °C. Scanning electron microscope images indicated that the milled sample had cluster-like structure which was consisted of fine particles with a mean size of about 95 nm. During heating in the range of 900–1100 °C, the coalescence of nanoparticles with spheroidal and polygonal morphologies occurred. After annealing at 1300 °C the microstructure showed a coarse structure characterized by the presence of grains with an average size of around 3 μm.