Thermal stability of stoichiometric strontium-incorporated hydroxyapatite

Several Stoichiometric strontium-incorporated hydroxyapatite powders with different content of strontium ion in place of calcium ion have been investigated on their thermal stabilities, including phase structure, chemical composition and crystal size and morphology at different temperatures. Results show that there exists a competition between phase decomposition and crystal growth of apatite. With the firing temperature climbing to 1300°C, no change happens to phase component of pure HAP, but the crystal apparently grows from 20 nm-40 nm at 600°C to -20 μm at 1300°C. However, after some Sr²⁺ incorporating into hydroxyapatite crystal by replacing equivalent Ca²⁺, the larger ion radius and the inhibiting action on growth of apatite of Sr²⁺ make the decomposition of Sr-incorporated hydroxyapatite (Sr-HAP) prior to its crystal growth. Thus, Sr-HAP starts to decompose into tricalcium phosphate at 900°C, which becomes serious at a higher firing temperature and more incorporation of Sr²⁺. Although the crystal further grows up when increasing the firing temperature, the range of growth is much lower than that of pure HAP. At 1300°C, Sr-HAP also maintains a similar equiaxial crystal (440 nm-730 nm × 220 nm-370 nm).