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.