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.