Goncharenko A. Crystallization and structural transformations in calcium phosphates with metastable and ion-substituted phases

Object of the study: nanodisperse amorphous, nanocrystalline and submicrocrystalline samples of calcium phosphates with a ratio of 1.0 ≤ Ca / P <1.67. Objective: establishing the physical regularities and mechanism of the formation of the initial composition and structure, the crystallization of equilibrium and metastable phases, the thermal phase transition and the peculiarities of compaction of calcium phosphates with a ratio 1.0 ≤ Ca/P < 1.67, as well as the optimal conditions for obtaining perspective biomaterials based on them. Methods: complementary methods of solid state physics: X-ray diffraction, scanning electron microscopy with X-ray microanalysis, thermogravimetric and differential thermal analyzes, IR spectrometry, mass spectrometry? atomic absorption spectroscopy and colirometry. Theoretical and practical results, scientific novelty: the evolution of the composition, structure and morphology of amorphous nanopowders with a starting ratio Ca/P = 1 in the temperature range 20–1000 ºС was studied; it was found that the formed phases were nanocrystalline, due to which the powders showed a high sintering ability; compaction processes in them began at temperatures that are 150–200 ºС lower than typical for sintering of calcium phosphate compacts; for the first time the mechanisms and crystallization characteristics of metastable α/-calcium pyrophosphate (α/-CРР), as well as the phase transition of α/-CРР → β-CРР and grain growth in these phases were determined; based on the values of activation energies and Avrami exponent, the mechanisms of transformations were defined as mainly surface diffusion at medium temperatures and combined surface-volume diffusion at high temperatures; the formation of high-temperature phases at medium temperatures is associated with the Ostwald's rule of stages, and at the same time two phases α/-CРР and β-CРР from one precursor - with the formation of two types of clusters before crystallization; based on the obtained scientific results, the optimal conditions for obtaining promising biomaterials from calcium phosphates with metastable phases of CРР and tricalcium phosphate, as well as gradient ion-substituted ceramics based on calcium-deficient hydroxyapatite were determined. Practical significance: The results of the performed research supplement the knowledge about the crystallization processes of amorphous formations, the nature of crystallized phases and thermal phase transitions in calcium phosphates with metastable and ion-substituted phases. The proposed approach in the microwave synthesis of gradient sodium-substituted CP can be used in the development of the novel technology for obtaining of related materials. Scope of application: in educational and methodical work in the training of specialists in the specialization "Biomaterials Physics and Medical Materials Science", as well as in the preparation of textbooks, manuals and other educational literature in these disciplines.