Synthesis and thermal dehydroxylation kinetic of anhydrous calcium phosphate monetite CaHPO4

Abstract

In this study, we have synthesized and performed the thermal decomposition kinetics of anhydrous calcium phosphate monetite CaHPO4 under air atmosphere by TG/DTG and DTA techniques using non-isothermal experiments. The prepared and calcined products at 500 °C were characterized by powder X-ray diffraction, infrared spectroscopy, transmission electron microscopy and X-ray microanalysis. The degradation of CaHPO4 was observed between [425–490 °C] and leads to the formation of calcium pyrophosphate by condensation of orthophosphate groups. The kinetic parameter results achieved by the isoconversional methods combined to Malek’s procedure showed that the thermal dehydroxylation is a single-step process which related to crystal nucleation and growth mechanism, according to JMA (n) model, with n = 1.37 and an activation energy of Friedman E α = 299.4 ± 2.1 kJ mol−1. The thermodynamic functions (ΔS*, ΔH* and ΔG*) of the thermal dehydroxylation step were calculated using the activated complex theory. The obtained values showed that the reaction is directly related to the introduction of heat and is non-spontaneous process.