The dissertation is dedicated to the study of physical properties and characteristics of calcium phosphate materials with impurities in the temperature interval from room temperature to 1400°C, which covers the production of calcium phosphate powders and the transformation of dense and porous ceramics based on the calcium phosphates at high temperatures. For the first time, calcium phosphate coatings were obtained by the deposition in the aqueous solutions on a new type of metal substrates of the MAX phase Ti3AlC2.
The first chapter describes the crystal structures of calcium phosphates, the stoichiometry of these compounds, and their phase equilibrium conditions. The origin of the impurities in calcium phosphate materials were discussed. The mechanisms of sintering of ceramics, the methods of production of dense and porous ceramics were reported in details in this chapter. In the part of the section devoted to the electrical properties of calcium phosphate materials, the mechanisms of the transfer of electrical charges are considered. This section also discusses the methods of sputtering of calcium phosphate coatings on the metal substrates.
The second chapter of the thesis is devoted to the synthesis methods of calcium phosphate materials and the study of physical properties and characteristics of calcium phosphate materials. The equipment and methods used to measure the physical characteristics were described in detail.
The third chapter of the thesis represents the results of sputtering of calcium phosphate coatings on the substrates from the MAX phase of Ti3AlC2 by the deposition in the aqueous solutions. The study was conducted separately for hydroxyapatite (Ca/Р=1.67) and other calcium phosphates of variable chemical composition (Ca/Р≠1.67).
The fourth chapter examines the electrical characteristics of calcium phosphate materials with impurities depending on the annealing temperature in the temperature interval from room temperatutre to 1200°C. It has been demonstrated for the first time, that the impurities in hydroxyapatite samples affect the behavior of the temperature dependence of electrical resistance at DC. Such impurities have been identified.
The fifth chapter of the thesis is devoted to the research of the sintering of calcium phosphate materials. It was established, that the sintering of calcium phosphates take place in accordance with the mechanisms of surface diffusion and grain boundaries.
The main scientific results obtained in the work are as follows.
1. Calcium-phosphate coatings on Ti3AlC2 substrates were obtained by the sol-gel method. It has been shown, that the phase composition of the coatings depends on the dipping time of the samples in the calcium phosphate solution.
2. It has been shown, that the optimal conditions for the formation of the calcium-phosphate coating on Ti3AlC2 substrates are dipping time in the solution of calcium phosphates for 24 hours. The coatings have a porous structure, which should promote the formation of a tight contact of metal implants with the bone.
3. It has been shown, that the changes in the electrical capacity, electrical resistance, and dielectric constant of the samples depending on the annealing temperature are related to the decomposition and interaction of impurities: desorption of water from the surface of calcium phosphate nanoparticles, as well as from the crystal lattice, decomposition of HPO42- and interaction of the obtained products with СО32-, and also occur as a result of the release of СО2 from the crystal lattice of hydroxyapatite.
4. It has been shown, that the electrical resistance of the samples at DC significantly depends on the presence of impurities in the calcium phosphate samples, which lead to the appearance of minima on the corresponding dependences. The main carriers of electric charge in the samples are OH-, as well as impurity ions (NH4+, СО32-, P2O74-), which are formed at the stage of the synthesis of calcium phosphates.
5. The activation energy of ion transfer processes in hydroxyapatite samples with the impurities at the high temperatures was determined.
6. For the first time, it has been shown, that the dependences of linear and volume shrinkage, density and Vickers microhardness have a nonmonotonic behavior, which is associated with the presence of the impurities both in the form of individual phases and in the form of ions located in the crystal lattice of hydroxyapatite.
7. It has been shown, that the sintering of hydroxyapatite samples with impurities occurs by the mechanism of lattice diffusion from the surface and grain boundary diffusion.
8. The activation energies of the growth of hydroxyapatite crystallites with impurities during the sintering of the samples were calculated. The values of the activation energy of hydroxyapatite samples are in good agreement with the data given in the literature for the growth of calcium phosphate particles.
