Thermal decomposition and reconstitution of hydroxyapatite in air atmosphere

In this paper, the decomposition and reconstruction behavior of hydroxyapat ite (HAP) during healing and cooling in air atmosphere were studied. The co mmercial HAP were chosen and gradually heated to 1500 degrees C and cooled to room temperature by a program controlled SIC heated furnace. X-ray diffr action (XRD) and Fourier-transformed infrared (FTIR) analysis were used to investigate the change of crystalline phases and functional groups of HAP a t different temperatures. Weight change of samples was recorded by thermogr avimetric analysis (TGA) during heating and cooling. The results revealed t hat HAP gradually releases its OH- ions and transforms into OHAP in the tem perature of 1000-1360 degrees C. Above 1360 degrees C, the OHAP would decom pose into TTCP and alpha TCP phase. The OH- stretching bands of HAP could b e traced by FTIR even at the temperature of 1350 degrees 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 temper ature of decomposition. In the temperature range of 1400-1500 degrees C, th ere was no significant difference in XRD patterns, only TTCP and alpha TCP crystalline phases were observed. When the HAP gradually cools from 1500 de grees C, a part of TTCP and alpha TCP would directly reconstruct into OAF a round 1350 degrees C. OAF existed in the temperature range of 1350-1300 deg rees C during cooling. When the temperature decreased to 1290 degrees C, a part of TTCP and alpha TCP reconstructed into OHAP by rehydration reaction and OAF were rehydrated into OHAP as well. At 1100 degrees C, the rest of T TCP and alpha TCP reconstitutes into HAP. As the temperature decreases, the OHAP is gradually rehydrated and reconstituted into HAP. (C) 1999 Elsevier Science Ltd. All rights reserved.