Reaction Sintering of Bioceramic Based on Substituted Calcium Phosphates CaMPO4 (M = K, Na)

Abstract

The synthesis of double phosphates based on calcium and alkali metals from different two-phase mixtures was studied. The phase relationships in the CaO–K2O–P2O5 and CaO–Na2O–P2O5 system were clarified, especially the phase transition temperature in potassium rhenanite CaKPO4. It was shown that some compounds that are mentioned in the literature, in particular, CaK2P2O7, CaNa2P2O7, CaK4(PO4)2, and CaNa4(PO4)2, cannot be obtained at 1000–1200°C at normal pressure. The densification and recrystallization were investigated during the reaction sintering of selected two-phase mixtures. Reaction sintering of a mixture of Ca3(PO4)2 and CaK4(PO4)2 gives CaKРO4 ceramic with a density of 78% (linear shrinkage of the sample is 5%); when sintering a mixture of K2CO3 and Ca2P2O7, the density of ceramic was 62% with a linear shrinkage of 15%. The reaction sintering of sodium-containing mixtures made it possible to obtain dense ceramics (density above 90%, linear shrinkage more than 6.5%) from a mixture of CaNa4(PO4)2 and Ca3(PO4)2. The experimental features of the reaction sintering of calcium and potassium/sodium double phosphates are discussed from the point of view of volume change during the course of the reaction. Another aspect that determines the density and microstructure of ceramics is the phase transformation of α → β in alkali metal rhenanites CаMPO4.