Synthesis and properties of materials based on the system SiO<sub>2</sub>-P<sub>2</sub>O<sub>5</sub>-CaO-MgO
In this work, a calcium-phosphate system was obtained by sol-gel method from solutions based on ethyl alcohol, tetraethoxysilane, phosphoric acid, calcium nitrate, and magnesium nitrate (concentration of reagents was 0.4 M). To investigate the physico-chemical properties, four solutions with a varying content of calcium and magnesium were chosen. The composition of Mg5 was 58-12-25-5; of Mg10, 58-1220-10; of Mg15, 58-12-15-15; and of Mg20, 58-12-10-20 mas. %. On the first day, for solutions Mg5, Mg10, Mg15, and Mg20, depending on the composition of the solution, the viscosity was 1.72 to 1.82 mm<sup>2</sup>/s. On day 2, a significant decrease in viscosity value was observed in solutions of Mg5, Mg10, Mg15, and on day 3 a white finely dispersed precipitation was precipitated. The Mg20 solution was stable for 13 days. On day 14, a precipitate formed in the Mg20 solution. Thus, with increasing magnesium content in the system, the stability of solutions increased. Precipitation in solutions with a magnesium content of 5, 10, and 15 mas. % was because in the solutions, in addition to the processes of hydrolysis and polycondensa-tion, there was an electrostatic interaction between dissolved ions and solvent molecules, which can lead to both an increase and a decrease in the value of viscosity. Since the viscosity of solutions decreased and the hydrolysis and polycondensation reactions occurred almost instantaneously, the viscosity decreased due to the electrostatic interaction of dissolved ions and solvent molecules. Using IR spectroscopy and thermal analysis, the stages of the formation of oxide systems were established. Removal of physically and chemically bound water occurred at up to 200°C. At 200-600°C, alcohol and the products of the thermooxida-tive destruction of ethoxy groups were burned, and the decomposition of nitrates occurred. At > 600°C, transitions of amorphous of compounds to crystalline ones were observed. Using X-ray powder diffraction analysis, quartz SiO2 and vitlokite Ca2.589Mg0.411 (PO4)2 were identified in the Mg5 sample. In the Mg10, Mg15, Mg20 samples, fi-cristobalite SiO2 and stentifieldite Mg3Ca3(PO4)4 were detected. Scanning electron microscopy showed that the surface of powders had an inhomogeneous porous structure. Optimum modes of heat treatment of the samples were established: drying at 60°C for 30 min and annealing at 600-800°C for 60 min.