Mechanochemical Synthesis of SiO sub(4) super(4-)-Sub stituted 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 degree C. It was found that the substituted hydroxyapatites remain single-phase after annealing at temperatures of up to 900 degree C for all concentrations of added silicon. The second phase - silicocarnotite Ca sub(5)(PO sub(4)) sub(2)SiO sub(4) - appears in the materials containing 1.0 and 1.2 mol of added silicon that have been annealed at 1000 degree C. In the powder containing 0.8 mol of added silicon, silicocarnotite was detected after annealing at 1100 degree 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 degree C, similarly to pure hydroxyapatite obtained by the same synthesis method. Partial transformation of silicon-substituted hydroxyapatite to silicocarnotite (SCar) is facilitated at higher temperatures and higher concentrations of silicon. Within the studied concentration range of added silicon, the hydroxyapatite containing 0.6 mol of silicon per mol of apatite unit cell has the highest thermal stability.