Effects of Preparation Conditions in Aqueous Solution on Properties of Hydroxyapatites

Hydroxyapatite powders (HAP: Ca₁₀(PO₄)₆(OH)₂) were prepared from calcium nitrate and di-ammonium hydrogen phosphate at various temperatures in aqueous solution in the absence and presence of carbon dioxide. In the absence of carbon dioxide, the crystallinities as well as the calcium to phosphate ratio (Ca/P ratio) of HAP powders increased with an increase in the preparation temperature. Calcium ion deficiency caused low Ca/P ratios of HAP powders. Such HAP powders decomposed thermally to α-tricalcium phosphate (α-TCP : Ca₃(PO₄)₂) at 800 °C in 3 hours whereas HAP powders with sufficient calcium were stable at 1200 °C for 3 hours. In the presence of carbon dioxide as well the crystallinities of HAP powders increased with an increase in the preparation temperature, but carbonate-bearing HAPs were obtained instead of pure HAP. In addition, calcium carbonate was obtained along with carbonate-bearing HAP prepared below 60 °C.