Thermal decomposition and reconstitution of hydroxyapatite in air atmosphere

In this paper, the decomposition and reconstruction behavior of hydroxyapatite (HAP) during heating and cooling in air atmosphere were studied. The commercial HAP were chosen and gradually heated to 1500°C and cooled to room temperature by a program controlled SiC heated furnace. X-ray diffraction (XRD) and Fourier-transformed infrared (FTIR) analysis were used to investigate the change of crystalline phases and functional groups of HAP at different temperatures. Weight change of samples was recorded by thermogravimetric analysis (TGA) during heating and cooling. The results revealed that HAP gradually releases its OH− ions and transforms into OHAP in the temperature of 1000–1360°C. Above 1360°C, the OHAP would decompose into TTCP and αTCP phase. The OH− stretching bands of HAP could be traced by FTIR even at the temperature of 1350°C which indicates HAP decomposition. HAP does not dehydrate completely before decomposition. We speculated that some oxyapatite (OAP) might be formed during dehydration with a great amount of OHAP still left in the system even up to the temperature of decomposition. In the temperature range of 1400–1500°C, there was no significant difference in XRD patterns, only TTCP and αTCP crystalline phases were observed. When the HAP gradually cools from 1500°C, a part of TTCP and αTCP would directly reconstruct into OAP around 1350°C. OAP existed in the temperature range of 1350–1300°C during cooling. When the temperature decreased to 1290°C, a part of TTCP and αTCP reconstructed into OHAP by rehydration reaction and OAP were rehydrated into OHAP as well. At 1100°C, the rest of TTCP and αTCP reconstitutes into HAP. As the temperature decreases, the OHAP is gradually rehydrated and reconstituted into HAP.