Gas Chromatographic Study of Decomposition of Hydroxyapatite in the Presence of Dispersed Zirconia at Elevated Temperature

Abstract

Release of water from hydroxyapatite (HAp), partially stabilized zirconia (PSZ) and mixture of HAp and PSZ powders at elevated temperature has been measured by gas chromatography. These powders were heated at a constant heating rate of 2°C/min up to 1400°C in helium and the released gas sampled every 15min. At temperatures ranging from 160 to 190°C PSZ released weakly absorbed water and the released water diminished with increasing temperature. Both HAp powder and the mixed HAp/PSZ powder showed four water release peaks during heating in the whole temperature range. The first peak in the temperature range from 160 to 190°C was from the weakly absorbed water as in the PSZ powder. The second peak, which appeared between 200 and 1000°C for HAp and 200 and 600°C for the mixed powder, was attributed to the strongly bound water. The third one, which was found only in mixed HAp/PSZ powders, was of dehydroxylation of HAp which caused transformation into oxyhydroxyapatite (OHA). In the case of HAp powder, no peak was observed for dehydroxylation of HAp by gas chromatography, but infrared spectra showed the dehydroxylation by the existence of OH- absorption and it has been confirmed that it occurred above 1000°C. The last peak indicates the decomposition of HAp into α-TCP which occurred above 1150 and 1000°C for the HAp and for the mixed powder, respectively. After this decomposition, cubic zirconia and CaZrO3 were also detected in the mixed powder by XRD. The PSZ mixed in HAp enhanced the decomposition of HAp, as well as evolution of water, and had an effect of lowering the temperatures of HAp decomposition and releasing the strongly bound water on HAp.