ИСТИНА

Bioceramics as bone grafts have to demonstrate special biological properties, such as gradual dissolution in liquid media of organism (resorption) and replacement by newly formed bone. Bone mineral has a composition very similar to Ca10(PO4)6(OH)2 (hydroxyapatite-HA), but, additionally, contains several ionic substitutions. Since the resorption rate of HA is too low to induce massive bone growth, biphasic calcium phosphate ceramics, combinations of -Ca3(PO4)2 (tricalcium phosphate) and HA, are commonly used as bioactive implants. Another way to improve resorptivity of such materials is chemical modification of HA or Ca3(PO4)2, e.g. by Ca/M (=alkaline metal) substitution leading to formation of CaMPO4 compounds with rhenanite structure. The main objective of this study was to create materials for human bone surgery with better resorption properties compared to HA. We have chosen double phosphates of calcium and alkaline metals Ca(3-x)М2x(PO4)2 (x=0÷1, М=Na, K) with crystal structure of β-Ca3(PO4)2 (х<0.15) and β-CaNaPO4 (x=1) as the components of ceramics and multiphase composites. The phase diagrams of the systems Ca3(PO4)2-CaNaPO4 and Ca3(PO4)2-CaKPO4 have been studied by different techniques of thermal analysis – DTA (STA 409 PC Luxx, Netzsch, and Pyris Diamond, Perkin Elmer), thermal expansion measurements (dilatometer DIL 402 C, Netzsch), high temperature X-ray diffraction (D/Max-2500, Rigaku, up to 1370°C). Phase diagram of Ca3(PO4)2-CaNaPO4 was verified according to that one after Ando. Phase «A» - a solid solution near the composition Ca5Na2(PO4)4, was considered as a superlattice of α-CaNaPO4 with doubled a lattice parameter and tripled c. It was determined that single-phase CaNaPO4 cannot be kept under cooling down at any rate, even by quenching, due to fast α-CaNaPO4 to β-CaNaPO4 transformation. This transformation accompanied by significant volume change can negatively affects the ceramics causing formation of cracks. Also, kinetics of α↔β transformation in pure Ca3(PO4)2 was explored, and critical cooling rate to produce single-phase α-Ca3(PO4)2 was determined. “Dangerous” temperature region in sense of the rapid transformation was found to be located near 800-900C. Phase diagram of Ca3(PO4)2-CaKPO4 system has been constructed for the first time. The main feature of it compared to the sodium system is an area of solid solution, called us by «B», at the vicinity of the composition with x=0.33 considered as a superstructure of α-CaKPO4. The solid solution «B» is transformed to apatite-like phase called us by «X» under cooling.