No 1 (2023)

The electrochemical behavior of scandium in halide melts is of interest both from the point of view of developing new electrochemical methods for producing scandium and its materials, and from the point of view of simulating electrochemical behavior of fission products during pyrochemical processing of spent nuclear fuel in molten salts. Using the methods of cyclic voltammetry, square-wave voltammetry and chronopotentiometry, the regularities of electrical reduction of scandium ions depending on the electrolysis parameters of the LiF–CaF₂–ScF₃ melt at a temperature of 800°C were studied. It is shown that the electrical reduction of scandium in the melt under study occurs at potentials more negative than –0.45 V relative to the potential of the aluminum electrode, while the electrodeposition of scandium on the electrode contributes to the electrical reduction of lithium cations with depolarization. When analyzing the obtained polarization dependences, it was noted that the process of electroreduction of scandium proceeds in one 3-electrode stage, while it is not electrochemically reversible. It has been suggested that the cause of irreversibility is the stage of formation of a new phase. As a result of electrochemical measurements, it was concluded that, due to the wide “electrochemical window”, the LiF–CaF₂ melt can be used both for the electrochemical synthesis of scandium and studying regularities of the selective electroreduction or co-electroreduction of minor actinides and lanthanides.

The aim of this work was to develop a computational-theoretical method for a detailed study of the geometry and statistical characteristics of local structural groups of complex liquids such as alkaline borate systems, tending to form a bulk boron-oxygen network. The technique was worked out a melt 30Na₂O–70B₂O₃ as an example at T = 1273 K. Ab initio molecular dynamics was used, implemented in the VASP program code for a supercell consisting of 250 atoms. The ion coordinates obtained at each step were used to obtain statistically significant information about the detailed structure of the melt. Using the original program developed for this purpose, we determined the partial radial distribution functions of the of atoms and analyzed all the closest coordinations found in the model around each type of ions, also the types and number of stable groups, bond lengths and angles in them. In addition, the tetrahedrality criterion for units BO₄ and ВB₄ were defined. Almost regular triangles (~80% of boron atoms) and tetrahedra (~19% of boron atoms) with a boron ion in the center and oxygen ions at the vertices proved to be the basic structural units. These simple structures form a boron-oxygen network connected by common (bridging) oxygen atoms. This network includes almost all boron atoms. Superstructural units, namely combinations of three or more basic structures have been found. For example, two triangles and one tetrahedron are forming rings of six alternating boron and oxygen atoms. Besides, the existence of rings that are formed from four basic structural units were discovered, but they in contrast to six-atom rings, are not planar formations. The proposed technique allows to obtain almost any details on the structural features of systems of this type, in particular, to answer the important question about the number of bridging and non-bridging oxygen atoms. It turned out that there are approximately 86% of bridging oxygens in studied system. The approach used considers correctly covalent and ionic bonds in liquid systems based on network-forming oxides and modifier-oxides. That will make possible to study the change in local structural characteristics and its dependence on concentration and temperature explaining the behavior of various physico-chemical properties.

Two mechanisms of electrolytic synthesis of intermetallic (IMs) during the simultaneous reduction (co-reduction) of their ions at the cathode in salt melts are known. And both are wrong. One of them is in contradiction with the experimental data. He violates also the thermodynamics law. Another does not represent a co-reduction process, since the ions of both metals must be simultaneously reduced at the cathode and not just one some of them. The present work does not contain new experimental data, she is purely theoretical. The mechanism of the co-reduction process is proposed and thermodynamically substantiated. It is shown for the first time that the first IM crystals appear not on the surface of an electropositive metal. They appear on the surface of a binary homogeneous solid solution consisting of IM components and which is formed at the initial moment of electrolysis. It has been shown and reconfirmed thermodynamically that electronegative metal ions are reduced with depolarization For the first time, the electrochemical equations for the crystallization of a phase of constant composition during long-term electrolysis, as well electrochemical equations as for crystallization of other intermetallic phases on the surface of the previous ones are presented. The presence of several IM phases in the cathode deposits obtained during long-term electrolysis under galvanostatic conditions is explained for the first time. The IMs electrocrystallization mechanism is examined for the cases of galvanostatic and potentiostatic electrolysis modes, as well as for cyclic and square wave voltammetry. It is also applicable to the co-reduction process of crystallization of metal compounds with non-metals.

Сapacitance of the iridium electrode was studied by the electrochemical impedance spectroscopy with variation of the main physical and chemical parameters: electrical potential, temperature, radius of the alkali cation. The influence of the signal frequency used in AC electrochemical methods on the capacitance value and the shape of the curve was also checked. Capacitance of the iridium electrode was obtained in molten sodium, potassium and cesium chlorides in the temperature range 1093–1123 K and the frequency range of the AC signal 3 · 10⁰–3 · 10⁴ Hz in the entire accessible range of electrical polarization. The obtained capacitance curves have two main minima with a maximum between them. One of these minima (cathodic one) was identified as the classical potential of minimum capacitance. A decrease in the signal frequency and the temperature of experiment, as well as an increase in the cation radius in the NaCl–KCl–CsCl order, leads to the appearance of one or two additional minima in the potential region between the main minima. The depth of these intermediate minima increases and their potential shifts in the positive direction with an increase in the radius of the alkali metal cation of the salt electrolyte. The calculated values of the capacitance of the electrical double layer and the adsorption capacitance were obtained by the method of equivalent electrical circuits. One of the additional minima obtained by direct measurement of the dependence of the electrode capacitance on the potential at a high AC frequency corresponds to the calculated capacitance of the double layer. The other additional minimum obtained at a low AC frequency corresponds to the calculated adsorption capacitance.

Information about the density and electrical conductivity of salt melts is of interest both for assessing the possibility of their use for electrolytic obtaining and refining of beryllium and other technological processes, and analyzing the possible interaction of components. Data on the density of molten saline systems containing fluoride and alkaline metals chloride are obtained by hydrostatic weighing. The balloon and the thread of the suspension were made of platinum. Berylia oxide used the material and cover of thermocouple. In system BeF₂–MeCl (Me = Li, Na, K, Cs) и BeF₂–(Li–K)_eut–Cl investigated from 9 to 14 molten saline mixtures containing from 0 to 100% beryllium fluoride with an increase in temperature by 100–200 K from the melting point of the mixture with an average step of 10 K. Due to the behavior of the individual fluoride of beryllium when heated above the melting temperature (high viscosity and intense evaporation), the density of molten salt was measured by maximum pressure in the gas bubble. Simultaneously with the density of the capillary method, the electrical conductivity of these melts was measured. Material of the measuring cell—beryllium oxide, measuring electrodes—platinum rods with a diameter of 1 mm. The permanent cells were determined and regularly controlled by melting of high-purple potassium chloride. All operations for the preparation of saline mixtures, the selection of samples for chemical analysis and the measurement of properties were carried out in an isolated atmosphere of a dry and additionally cleaned argon. The measurement results are presented on the graphs and in the form of the first and second-order polynomas, reflecting the dependence of density and electrical conductivity on temperature for various compounds of saline mixtures. The values of the simultaneously measured density and electrical conductivity values were used to calculate the molar volume and molar electrical conductivity of electrolytes. The isotherms of the molar volume are almost linear in nature, which indicates the weak interaction of the components of the melt. The isotherms of molar electrical conductivity have a characteristic outrage in the area of compositions containing about 30 mol % beryllium fluoride, which may be associated with the formation of complex compounds in the liquid phase.