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

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