Thermally induced crystallization of mechanosynthesized chlorapatite–titania composite nanopowders

The thermally induced crystallization of mechanosynthesized chlorapatite–titania composite nanopowder was investigated. Firstly, the composite nanopowder was produced after 5 h of milling. The mechanosynthesized powder was annealed in the range 900–1300 °C for 1 h. The 5 h milled sample showed the crystallite size and lattice strain of about 23±1 nm and 0.0107±0.00054, respectively. During the subsequent annealing, crystallization of the milled sample at 900 °C and severe decomposition of nanocomposite at 1300 °C were detected. Accordingly, the fraction of crystalline phase reached a maximum around 96±4% at 900 °C and then declined to 80±4% at 1300 °C. The lattice strain decreased drastically to about 0.0002±0.00001 at 1300 °C, while the crystallite size increased significantly to around 277±14 nm. Based on the obtained data, the unit cell volume of CAp went down during the thermal treatment due to the ion exchange reaction between chlorapatite and titania. According to the electron microscopic observations, the morphological features of composite nanopowders were influenced strongly by the annealing temperature. The 5 h milled sample was composed of spheroidal particles with an average size of about 35 and 190 nm before and after annealing at 900 °C, respectively. At 1100 °C, the coalescence of nanospheres was dominant. Finally, a porous composite structure comprised of coarse grains with an average size of about 1 μm along with finer grains with a mean size of around 100 nm was formed at 1300 °C.