Thermal decomposition and reconstruction 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 degrees C and cooled to room temperature by a program controlled SiC heated furnace. X-ray diffractometer (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 would gradually release its OH- ions and transform into OHAP in the temperature of 1000-1360 degrees C. Above 1360 degrees C, the OHAP would decompose into TTCP and alpha TCP phase. The OH- stretching bands of HAP could be traced by FTIR even at the temperature of 1350 degrees C which was the eve of HAP decomposition. It reflected that the HAP did not dehydrate completely before decomposed. We speculated that some of OAP might be formed during dehydration and there were a great amount of OHAP still left in the system even up to the temperature of decomposition. In the temperature of 1400-1500 degrees C, there were no significant different in XRD patterns, only TTCP and alpha TCP crystalline phase were observed. When the HAP gradually cooled down from 1500 degrees C, a part of TTCP and alpha TCP would directly reconstruct into oxyapatite (OAP) around 1350 degrees C. OAP existed in the temperature of 1350 to 1300 degrees C during cooling. When the temperature down to 1290 degrees C, a part of TTCP and alpha TCP reconstructed into OHAP by rehydration reaction and OAP were rehydrated into OHAP as well. At 1100 degrees C, the rest of TTCP and alpha TCP would reconstitute into HAP. As the temperature decreased, the OHAP were gradually rehydrated and reconstituted into HAP.