Thermal reconstruction behavior of the quenched hydroxyapatite powder during reheating in air

Commercial hydroxyapatite (HAP) powders were quenched from 1500°C to room temperature and reheated at different temperatures by a program controlled SiC-heated furnace to investigate the reconstruction behavior of quenched HAP powder in air. X-ray diffractometer (XRD) and Fourier-transformed infrared (FTIR) analysis were used to examine changes in crystalline phases and functional groups of quenched HAP powders at different temperatures. Weight changes of the quenched powders during heating were recorded by thermogravimetric analysis (TGA). The XRD results showed that the quenched HAP powder was composed of two crystalline phases of tetracalcium phosphate (TTCP) and α-tricalcium phosphate (αTCP). There was no other calcium phosphate phases to be traced. When quenched HAP powders were reheated, TTCP gradually reconstructed into HAP around 500°C and yield Ca(OH)2. αTCP did not convert to HAP at a temperature lower than 700°C but the transformation of αTCP to βTCP was observed during heating. αTCP reacted with Ca(OH)2, the product of reconstruction of TTCP, to form more HAP at temperatures over 700°C. In addition, the CO32− ion was incorporated into the lattice of the reconstructed HAP around 500°C. The intensity of CO32− ion in FTIR decreased as the temperature increased and totally disappeared around 900°C. Above 1000°C, reconstructed HAP lost OH ions and transformed into oxyhydroapatite.