Balabanov A. Processes of synthesis and properties of IR-transparent ceramics in the MgO-Y2O3 system

The dissertation is dedicated to determining the features of obtaining IR-transparent ceramics in the MgO-Y2O3 system, as well as the influence of structural-morphological characteristics of nanopowders on the optical and functional properties of ceramics. The main physico-technological parameters of the synthesis of IR-transparent ceramics in the MgO-Y2O3 system are determined. A comprehensive approach to the synthesis of yttrium oxide as an IR-transparent material for windows of technological devices is implemented at all stages of the ceramic production route, as well as the conditions for the synthesis of 50:50 vol.% MgO-Y2O3 nanocomposites by spark plasma sintering. The first chapter describes the main features of methods for obtaining transparent media for modern infrared (IR) optics in the MgO-Y2O3 system. The second chapter provides a detailed description of experimental methods for obtaining powders, manufacturing samples of IR-transparent ceramics and methods of their characterization are described. The third chapter investigates the sintering activity of Y2O3 nanopowders as a function of their structural-morphological characteristics. It is determined that commercial Y2O3 powders form three-dimensional agglomerates of complex hierarchy with sizes up to 3-5 microns, which are formed from finer primary particles with sizes of 25-60 nm. It is established that the effective densification of Alpha-nano powders is due to the high dispersion of the powder mixture, a low degree of agglomeration, and a small size of primary particles. The regularities of the formation of substitution solid solutions Y2O3:La3+ are investigated. It is shown that complete dissolution of hexagonal La2O3 in cubic Y2O3 occurs at temperatures below 1500°C with the participation of intermediate phases enriched with lanthanum. Phase transformations accompanied by an increase in specific volume, as well as the hygroscopicity of lanthanum oxide, lead to the cracking of highly alloyed Y2O3 ceramics:La3+ during sintering. The fourth chapter investigates the features of the mesostructure of Y2O3 powder compacts, the influence of sintering-promoting additives, and the conditions for the formation of Y2O3 ceramics. It is shown that the pre-sintering of Y2O3 compacts is advisable at the maximum temperature at which sintering does not occur with the approach of particle centers, and the increase in the size of pores does not exceed ≈30%. Compacts fired at T=800°C are characterized by an optimal mesostructure in terms of densification efficiency during subsequent vacuum sintering (T=72% at λ=1100 nm). It is shown that a complex doping of La3++Zr4+, promoting sintering, effectively suppresses the mobility of grain boundaries and simultaneously activates the diffusion transport along the branched system of intergranular boundaries by forming defects in the cationic sublattice. Through the optimization of the temperature-time route of vacuum sintering, conditions for the synthesis of IR-transparent Y2O3 ceramics with high transparency in the visible and IR ranges (83% at 5 μm) are established. The fifth chapter investigates the physico-chemical conditions for the formation of amorphous precursors and nanopowders in the MgO-Y2O3 system. Using the method of self-propagating glycine-nitrate synthesis with an excess of glycine and nitric acid, composite MgO-Y2O3 nanopowders in a ratio of 1:1 by volume were synthesized. It is established that non-equilibrium conditions of precursor synthesis contribute to the formation of a product with coral-like geometry. It is shown that at temperatures above 800°C, the fibers break down into individual isolated quasi-spherical particles suitable for further sintering within the ceramic technology framework. The sixth chapter is dedicated to studying the influence of sintering temperature and Ho3+ ion doping on the processes of diffusion mass transfer and physical properties of MgO-Y2O3 nanocomposite ceramics. Conditions for the synthesis of 50:50 vol.% MgO-Y2O3 nanocomposites by spark plasma sintering of nanopowders, allowing the achievement of optical transparency of ceramics in the IR wavelength range, are implemented. It is shown that linear optical transmittance of the composite T≈68% at λ=5000 nm can be realized under the conditions of forming a homogeneous two-phase structure, spatial restriction of recrystallization processes of components, and preservation of grain size at the level of 200-250 nm, where the contribution of light scattering at interphase boundaries is minimal. It is established that doping with Ho3+ ions activates diffusion mass transfer during the formation of MgO-Y2O3:Ho3+ ceramics (3-12 at.%) through the redistribution of electronic density near the doping ions. This leads to an increase in the optical transmittance coefficient of MgO-Y2O3:Ho3+ ceramics (3 at.%) from 68 to 75% compared to the undoped material.