Investigation of Phase Evolution During the Thermochemical Synthesis of Tricalcium Phosphate

Abstract

The formation of tricalcium phosphate (TCP) from dicalcium phosphate and calcium carbonate was investigated using differential thermal analysis (DTA), thermogravimetric analysis (TGA), and powder X-ray diffraction (XRD). DTA showed three distinct thermal events attributed to dehydration of dicalcium phosphate dihydrate (brushite, DCPD) to dicalcium phosphate anhydrous (monetite, DCPA), the formation of beta-calcium pyrophosphate (β-Ca2P2O7), and the calcination of calcium carbonate. TGA showed three weight losses corresponding to the three thermal events, respectively. XRD analysis showed that β-TCP formed, beginning at about 900°C, by the reaction of β-Ca2P2O7 with CaO and β-TCP changed to α-TCP above 1200°C. Further examination of the formation of TCP from calcium hydroxide (Ca(OH)2) and DCPA showed that β-TCP would form only after the decomposition of Ca(OH)2 to CaO and from the reaction of CaO with β-Ca2P2O7 at a fairly low temperature of 800°C. In addition, by naturally cooling α-TCP, formed at 1300°C within the heating furnace, to room temperature, it was difficult to obtain a pure phase of β-TCP. The proposed mechanism of the reaction to form TCP may include the dehydration of brushite to monetite, dehydration of monetite to beta-calcium pyrophosphate, decomposition of calcium carbonate, the formation of β-TCP, and phase transition.