Low-temperature phase transitions in β-Ca3(PO4)2 and related compounds

In order to improve the properties of hydroxyapatite (HA), HA/TiO2 composites with different TiO2 addition contents were prepared by microwave sintering. Effects of sintering temperature and TiO2 addition on phase composition, microstructure, mechanical properties and bioactivity were investigated. HA/TiO2 mixtures were prepared by the hydrolysis of tetrabutyl titanate (TBT) in HA particles suspension, which presented high specific surface areas and amorphous TiO2 phase. Compared to no phase change for pure HA, TiO2 addition promoted the HA decomposition by their mutual chemical reactions depending on the sintering temperature and TiO2 content. The sintering behavior of HA/TiO2 composites was also discussed in term of sintering temperature and TiO2 addition content. Ti atoms were uniformly dispersed in the HA/TiO2 composites with the formation of gradual Ti rich particles. TiO2 addition enhanced the sintering ability of HA powders, and HA/TiO2 composites exhibited densifications and fine grains at elevated temperature due to the nanosized raw powders and limited atomic diffusions induced by short sintering durations and byproduct suppressions. The sintering temperature can play a great effect on the microhardness and elastic modulus. New apatite was formed on the surfaces of HA/TiO2 composites after immersion in simulated body fluid, which indicates high bioactivities for composites prepared by microwave sintering.