In-situ high temperature XRD and TEM study of the thermal stability and sintering behavior of octacalcium phosphate

Currently, the characteristics of solid-state phase transformation corresponding to octacalcium phosphate (OCP) sintering is still not completely understood. Although it was known that Ca-deficient (Ca-def) hydroxyapatite (HA) is usually unstable at high temperature, the thermal stability and sintering behavior of OCP have been rarely reported. The objective of this study is to systematically investigate the thermal stability and sintering behavior of OCP powders by comprehensive characterization techniques including in-situ high temperature X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA) and scanning electron microscopy (SEM). In-situ XRD results showed that the “collapsed OCP”, Ca-def HA and β-tricalcium phosphate (TCP) crystal structures formed by heating OCP at 200–250 °C, 300–600 °C and 800–900 °C, respectively. Furthermore, DTA analysis revealed that the peak temperatures of the phase transitions of OCP to Ca-def HA and Ca-def HA to β-TCP were ∼170 °C and ∼720 °C, respectively. SEM examination indicated that a large number of pores were introduced during sintering OCP powders due to the release of structurally contained water at 200–250 °C. Therefore, a porous β-TCP ceramic can be directly prepared by sintering OCP powders at 900 °C. This phase transition induced foaming method may be extended for preparing porous bioceramics by sintering calcium phosphate phases as long as they contain crystalline water, which could provide guidelines for better utilizing the phase transformations of calcium phosphate to produce special bioceramics for biomedical applications. •“Collapsed octacalcium phosphate (OCP)” formed by heating OCP at 200–250 °C.•Ca-deficient (Ca-def) hydroxyapatite (HA) formed by heating OCP at 300–600 °C.•β-tricalcium phosphate (TCP) formed by heating OCP at 800–900 °C.•Peak temperatures of OCP to Ca-def HA and Ca-def HA to β-TCP were ∼170 °C and ∼720 °C.•Many pores were introduced during sintering OCP due to crystalline water release.