Three types of poorly crystallized calcium-deficient hydroxyapatites (CDHA) with Ca/P
molar ratios 1.50, 1.58, and 1.67 were prepared from CaHPO4·2H2O and KOH. These were
sintered at 1050 °C for 4 h. Well-crystallized β-tricalcium phosphate (β-TCP), biphasic calcium
phosphate (BCP), and stoichiometric hydroxyapatite (HA) were produced, respectively. The
sintered and unsintered calcium phosphates were studied by X-ray diffraction (XRD) and
Fourier transform infrared (FTIR) spectroscopy. In addition, Ca/P molar ratios were
determined by a chemical analysis. Thermogravimetric analysis (TGA) revealed a loss of
water for the period of sintering. On the basis of the experimental data, a structure of BCP
was suggested. After applying the numerical values of ionic (OH-, Ca2+, and PO4
3-) diffusion
coefficients at 1000 °C, the solid-state transformation mechanism of CDHA into BCP was
proposed.
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