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

Abstract

The Ca-deficient apatite (CDHA) was prepared from the precursors of (CH 3COO) 2Ca· xH 2O, Ca(NO 3) 2·4H 2O and H 3PO 4, (NH 4)H 2PO 4 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 (NH 4)H 2PO 4 as precursor was higher than that with H 3PO 4. 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.