Sol-Gel Synthesis of Amorphous Calcium Phosphate and Sintering into Microporous Hydroxyapatite Bioceramics

A new route for preparing hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) bioceramic has been described. An amorphous, nanosized, and carbonate-containing calcium phosphate powder that had a Ca:P ratio of 1.67 was synthesized from calcium diethoxide and phosphoric acid in ethanol via a sol-gel method. The powder was pressed at 98 MPa into green specimens and then heated to a temperature range of 500°-1300°C. At 600°C, the powder crystallized to a carbonated hydroxyapatite and a trace of ß-tricalcium phosphate before converting to hydroxyapatite at 900°C. The thermal crystallization was associated with grain growth, shrinkage, and active surface diffusion. The activation energy of grain growth was 37 ± 2 kJ/mol. After sintering at 1100°C, the decomposition of carbonated hydroxyapatite generated a microporous ceramic with an average pore size of 0.2 µm and an open porosity of 15.5%. This microporous bioceramic can be used as a bone filler.