№ 3 (2024)

This study is concerned with the theoretical description of a quasi-stationary process of directional crystallization of binary melts and solutions in the presence of a quasi-equilibrium two-phase region. The quasi-equilibrium process is ensured by the fact that the system supercooling is almost completely compensated by heat released during the phase transformation. Quasi-stationarity of the process determining constancy of the crystallization rate is ensured by given temperature gradients in the solid and liquid phases. The system of heat and mass transfer equations and boundary conditions to them under these assumptions is dependent on a single spatial variable in the reference frame moving with the crystallization rate relative to a laboratory coordinate system. Exact analytical solutions to the formulated problem in parametric form are obtained. The parameter of the solution is the solid phase fraction in a two-phase region. The distributions of temperature and impurity concentration in the solid, liquid and two-phase regions of the crystallizing system, the rate of solidification, and the spatial coordinate in the two-phase region depending on the solid phase fraction in it are found. An algebraic equation for the solid phase fraction at the interface between the solid material and the two-phase region is derived. Exact analytical solutions show that the impurity concentration in the two-phase layer increases as the solid phase fraction increases. Moreover, the solid phase fraction at the interface solid phase — two phase region and its thickness increase as the temperature gradient in the solid phase and the solidification rate increase. The developed theory allows us to determine analytically the permeability of the two-phase region and a characteristic interdendritic spacing in it. Analytical solutions show that the relative permeability in the two-phase region increases from a certain value at the interface with the solid phase to unity at the interface with the liquid phase. The selection theory of stable dendritic growth allows us to determine analytically a characteristic interdendritic distance in the two-phase layer that decreases as the temperature gradient in the solid phase increases. An increase of impurity in the molten phase gives a decrease in the interdendritic spacing within a two-phase region.

The object of the study is high-temperature oxide-metal melts obtained in furnaces of induction melting in a cold crucible (IMCC). The results of pilot tests in IMCC furnaces at melt temperatures of more than 2200 ^оC in air, conducted to study the distribution of components between the oxide and metal phases of a two-phase melt with limited miscibility of components, are presented. The results of physicochemical studies of materials obtained by quenching crystallization of a high-temperature melt are presented, confirming the reduction of silicon and the oxidation of iron with the redistribution of these components between the oxide and metal phases. This experimental result contradicts the well-known Ellingham diagrams and thermodynamic calculations, but a similar effect is observed experimentally in the U–O–Fe system. Thus, the IMCC method allows for the inversion of redox processes in a number of oxide-metal systems, which can be used to obtain new materials and create technologies for high-temperature extraction of target components.

With the growing demand for renewable energy sources, much of the research in the battery industry is focused on creating safe and high-capacity energy storage systems that can handle high current loads using inexpensive and readily available materials. The aluminum-ion batteries (AIB) are considered as one of the most promising systems. Such materials as aluminum metal, carbon materials and chloroaluminate ionic liquids are used as anode, cathode and electrolyte, respectively. A low-temperature chloroaluminate melt based on triethylamine hydrochloride (Et₃NHCl) is promising and inexpensive electrolytes for AIBs. This melt has the ability to reversibly precipitate/dissolve aluminum metal due to the presence of the Al₂Cl₇^– ion in it. However, the diffusion of Al₂Cl₇^– ions in the Et₃NHCl–AlCl₃ system has not been studied previously. In the presented work, the concentration dependence of the diffusion coefficients of the Al₂Cl₇^– anion was studied using chronopotentiometry in the concentration range N = 1.3–1.95 (where N is the molar ratio of aluminum chloride to organic salt). It was shown that diffusion coefficients increase with aluminum chloride content growth in the studied melt: from 1.71·10^–7 (N = 1.3) to 4.50·10^–7 cm²·s^–1 (N = 1.95). This behavior can be caused by the viscosity decrease of the melts with Al₂Cl₇^– concentration growth. Based on the obtained results it can be concluded that Et₃NHCl–AlCl₃ with N = 1.95 is the most suitable electrolyte for AIB. Moreover, it was established that the electrochemical reduction of the Al₂Cl₇^– on the surface of the aluminum electrode is complicated by the nucleation process, which has the lowest overvoltage at N = 1.95.

Molten alkali metal chlorides used in pyrotechnologies are aggressive corrosive agents. The high operating temperature of the process, the heterogeneity of the environment, and the significant corrosion activity of the molten salt necessitate both the search for stable structural materials and the development of methods for protecting the structural elements of high-temperature technological devices. Corrosion loss reduction techniques traditionally used in low temperature environments are not applicable at high temperatures.

The article examines the influence of oxygen-containing impurities (lithium oxide and hydroxide) on the corrosion behavior of metallic nickel (grade NP1) – the main component of candidate structural alloys, a thermodynamically and structurally stable material in the melt for the process of electrolytic refining of spent nuclear fuel. A method for preparing the LiCl–KCl salt electrolyte and obtaining lithium oxide by thermal decomposition of anhydrous lithium hydroxide under vacuum is described, and the concentrations of impurities in the electrolyte and the synthesized lithium oxide are determined. An installation for conducting corrosion tests in an inert atmosphere of a glove box is presented.

To assess the corrosion resistance of the material, the following were used: gravimetric analysis, X-ray diffraction analysis of the surface and cross-sectional sections, and X-ray diffraction analysis of the surface of the samples. The dependences of the corrosion rate of the material on the concentration of oxygen-containing additives Li₂O and LiOH were obtained. Based on a combination of gravimetric, X-ray microspectral and X-ray phase analysis data, it was established that metallic nickel samples demonstrate high corrosion resistance in the studied melts with the introduction of Li₂O and LiOH additives.