Solid reaction mechanism of CaHPO4•2H2O + CaCO3 with and without yttria

The effect of yttria on the solid reaction mechanism of a CaHPO4 center dot 2H(2)O + CaCO3 system at different temperatures was experimentally studied. The samples with and without yttria were subjected to thermogravimetric/differential scanning calorimetry measurement. The samples were heat treated at the temperatures corresponding to the peaks on the DSC spectra, and the resulted phase compositions were identified by X-ray diffraction. The transformation mechanism was deduced by comparing the phases obtained at different temperatures. The results show that the transformations at below 1073 K are not affected by yttria, but all those at above 1073 K are completely altered. The formation temperature of hydroxyapatite decreases by 134 K, and the decomposition temperature increases by 38 K. The polymorphous transformation of Ca-3(PO4)(2) from beta phase to alpha phase increases by 47 K. The thermodynamic properties of the transformations at above 1073 K are also modified by the addition of yttria; that is, the endothermal peaks are substituted by exothermal peaks.