Transformation mechanism of different chemically precipitated apatitic precursors into β-tricalcium phosphate upon calcination

The Ca-deficient apatite (CDHA) was prepared from the precursors of (CH₃COO)₂Ca·xH₂O, Ca(NO₃)₂·4H₂O and H₃PO₄, (NH₄)H₂PO₄ to investigate the transformation mechanism of β-tricalcium phosphate (β-TCP). X-ray diffraction analysis shows that the development of β-TCP is not via direct reaction between Ca and P for all the different combinations between Ca and P precursors. The activation energy of β-TCP formation with (NH₄)H₂PO₄ as precursor was higher than that with H₃PO₄. Following the Johnson-Mehl-Avrami equation, the reaction kinetics of β-TCP phase formation is found one-dimension growth with interface-controlled and diffusion controlled growth depending on the annealing temperature. There exists a transition between 750°C and 825°C, and the transition rate from interface-controlled to diffusion-controlled growth is precursor-dependent.