Mechanochemical Synthesis of SiO 4 4– ‐Substituted Hydroxyapatite, Part III – Thermal Stability

Thermal stability of mechanochemically synthesized silicon-substituted hydroxyapatite containing 0.6, 0.8, 1.0, and 1.2 mol of silicon per mol of apatite unit cell has been studied for the first time. The powders of the silicon-substituted hydroxyapatite were annealed within the temperature range 700–1300 °C. It was found that the substituted hydroxyapatites remain single-phase after annealing at temperatures of up to 900 °C for all concentrations of added silicon. The second phase – silicocarnotite Ca5(PO4)2SiO4 – appears in the materials containing 1.0 and 1.2 mol of added silicon that have been annealed at 1000 °C. In the powder containing 0.8 mol of added silicon, silicocarnotite was detected after annealing at 1100 °C. The formation of silicocarnotite is likely to occur in the nanoparticles of the silicon-substituted hydroxyapatite with silicon concentrations much higher than the average value. It was found that the silicon-substituted hydroxyapatite containing 0.6 mol of silicon per mol of apatite unit cell has the highest thermal stability and remains single-phase up to a temperature of 1200 °C, similarly to pure hydroxyapatite obtained by the same synthesis method.