Phase transformation and sintering behavior of Ca₂P ₂O₇

The phase transformation and sintering behaviors of Ca₂P ₂O₇ with different phase composition were investigated by using X-ray powder diffraction (XRD), dilatometery and scan electron microscopy techniques in this paper. It is found that although α-Ca₂P ₂O₇ (high-temperature form) could be kept during its cooling process, it is metastable and retransformed into β-Ca ₂P₂O₇ (low-temperature form) at about 950 °C during its reheating process. This reversible phase transformation was discussed from the point view of polyhedral distortion. The sample from β-Ca₂P₂O₇ calcined at 1000 °C/2 h densifies much faster than that from α-Ca₂P₂O ₇ and α + β mixture, and bulk density as high as 98% TD can be obtained as it is sintered at 1150 °C. A dense α-Ca ₂P₂O₇ free of micro-crack could not be obtained whatever from the powder of β-Ca₂P₂O ₇ or α-Ca₂P₂O₇ or α + β mixture. Such different sintering behavior was explained in related to the reversible phase transformation between α- and β-Ca ₂P₂O₇.