A. S. Tolkacheva, M. А. Konopel’ko. 

ABSTRACT

Research and technology studies on Molten Carbonate Fuel Cells (MCFC) are being directed at improving their performance in mode of greenhouse gas conversion for chemical energy storage. The changes in gas composition feeding MCFC demand new insulating materials to be found. In the current work, the equilibrium solubility of magnesia ceramics in air in contact with Li₂CO₃–Na₂CO₃ and Li₂CO₃–K₂CO₃ eutectic mixtures was measured at 600°С. The study shows that magnesia is completely stable in the tested melts for at least more than 270 h. Its solubility was found to increase in Li–K carbonate eutectic. Conclusions about the material stability are based on results of inductively coupled plasma atomic absorption spectroscopy of melts and scanning electron microscopy combined with X-ray diffraction employed to ceramics testing. Magnesia is recommended as an insulating material in electrolysis cells containing Li₂CO₃–Na₂CO₃ and Li₂CO₃–K₂CO₃ carbonate eutectics for the conversion of the greenhouse gas in chemical energy storage devices operating in air.

KEYWORDS

MgO, insulators, fuel cells, microstructure, eutectic, carbonate.

DOI: 10.31857/S0235010623010097

EDN: HEDRVF

V. E. Krotov, Yu. P. Zaikov. 

ABSTRACT

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.

KEYWORDS

 intermetallic compounds, formation mechanism; electrolysis; co-reduction of ions, molten salt, electrochemical reactions

DOI: 10.31857/S023501062301005X

EDN: HIPZOH

E. A. Karfidov, Yu. P. Zaikov, E. V. Nikitina, K. E. Seliverstov. 

ABSTRACT

Experiments were carried out to determine the corrosion rate of stainless steel AISI 316 in a fluoride melt with different concentrations of O^2– (by adding lithium oxide to the melt in the concentration range from 0 to 5 wt %). The corrosion rate decreases by an order of magnitude at an oxygen anion concentration in the melt from 0.2 to 0.4 wt %, which may indicate the detection of the phenomenon of high-temperature passivation of the material due to the modification of the composition of the fluoride melt and a decrease in its corrosion activity. In addition, the type of intergranular and pitting corrosion typical of stainless steels in fluoride melts, which is the most dangerous from the point of view of the structural reactor material, changes to continuous when lithium oxide is added due to the “healing” of individual corrosion centers with excess oxygen-containing compounds. The formation of a protective layer of the spinel type with a thickness of 1 μm was established.

KEYWORDS

corrosion, candidate materials for LSR, alkali metal halide melt, high-temperature passivation, spinel-type oxides

DOI: 10.31857/S0235010622040065

EDN: DHSZUO

A. A. Yuryev, M. M. Tsymbalist, M. A. Samoylova, A. A. Khokhryakov. 

ABSTRACT

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.

KEYWORDS

alkali-borate melt, ab initio molecular dynamics, short-range order structure, three- and four-coordinated boron atoms, superstructural units, bridging and non-bridging oxygen atoms.

DOI: 10.31857/S0235010623010115

EDN: HCLEND

I. V. Evdokimov, E. V. Sterkhov, V. A. Bykov, K. Yu. Shunyaev, L. D. Son. 

ABSTRACT

Eutectic alloys, due to their low melting point, are promising materials for the production of metal glasses. Unlike crystalline alloys, amorphous alloys have no long-range order, which leads to an increase in hardness, mechanical strength, corrosion resistance and magnetic permeability. The main criterion for the application of amorphous materials in practice, under which the formation of a single-phase metallic glass during quenching, is the critical diameter. Therefore, an urgent task in this area of research is to increase the critical diameter of current amorphous alloys. One of the methods of improving the glass-forming ability of alloys is their dilution with more refractory metals. In this paper, we present amorphous metallic glasses of (Co₄₁Fe₇Cr₁₅Mo₁₄C₁₅B₆)_100-xR_x alloys with dilution with rare earth metals (R = Gd, Ho, La, Nd, Y, Yb; x = 0; 2). The critical diameter, phase and elemental composition of the alloys depending on the quenching rate were determined by X-ray diffraction and scanning electron microscopy. It is shown that the addition of 2 at % Gd, Ho and Y significantly increases the glass-forming ability of the Co-Fe-Cr-Mo-C-B alloy. By spinning method, when quenched on an aluminum disk rotating at a linear speed of 11.5 m/ s, tapes with a thickness of 19 to 73 microns were obtained. Rare inclusions with a high content of rare earth elements up to 35 at % were found on the surface of the tapes, which can act as crystallization centers.

KEYWORDS

Metal glasses, amorphous alloys, rare earth elements, glass-forming ability, critical diameter

DOI: 10.31857/S0235010623010036

EDN: HIDAUH

A. Yu. Nikolaev, A. V. Suzdaltsev, Yu. P. Zaikov.

ABSTRACT

Due to its abundance in nature as well as its properties, silicon is one of the most demanded materials in various industry areas. Currently, metallurgical silicon is obtained by carbothermic reduction of quartz. In order to obtain solar grade silicon, the last should be treated by hydrochlorination and multiple chlorination. This brief review presents an analysis of alternative methods for obtaining silicon by electrolysis of molten salts. The factors that determine the choice of the composition of molten salts, typical silicon deposits obtained by electrolysis of molten salts are shown. An assessment of the results and prospects for further use of electrodeposited silicon in lithium-ion power sources and representative test results on the use of electrolytic silicon for solar energy conversion devices were presented. The problems that need to be solved for the practical implementation of methods for the electrolytic production of silicon samples suitable for new devices and materials for energy conversion and storage are noted.

KEYWORDS

silicon, electrodeposition, thin solid films, fibers, molten salts, lithium-ion power sources, photoconverters

DOI: 10.31857/S0235010623010073

EDN: HEOLXM

С О N T E N T S

D. O. Chukhvantsev, N. I. Shurov, E. V. Nikitina, E. S. Filatov. 

ABSTRACT

The possibility of synthesizing an ultrafine powder of gadolinium hexaboride from a chloride-oxide melt by electrochemical deposition of components on an inert cathode has been experimentally proven. Previously, a similar synthesis of borides was carried out in a chloride-fluoride melt, and anhydrous lanthanide fluorides, which are produced industrially by fluorination of oxides, were used as a source of lanthanum ions.

Technological advantages of our melt are: exclusion of the intermediate redistribution of oxide fluorination; the possibility of electrolysis in air without an inert atmosphere; the use of the simplest design of the electrolyzer and the minimum set of cheap equipment. All this will significantly reduce the cost of the synthesized borides.

All reagents were preliminarily prepared and certified by chemical analysis and X-ray diffraction analysis and were stored in an inert box until the experiment. The optimal parameters of the boride synthesis process were preliminarily determined: temperature and concentrations of B₂O₃ and Gd₂O₃ oxides; cathodic current densities in the production of gadolinium borides. The experiment was carried out in an air atmosphere. The simplest electrolyzer design consisted of: an inert molybdenum wire cathode, a graphite anode, and an alundum crucible. First, a cleaning electrolysis was carried out at a current density of 0.3 A/cm² for 15 minutes to remove residual traces of moisture. The main electrolysis was carried out at a current density of 0.4 A/cm² for 4 hours, with the replacement of the working electrode with an interval of one hour. The resulting precipitate was washed by decantation in distilled water with acidified hydrochloric acid, then the solid phase was separated by vacuum filtration and dried.

The phase composition and morphology of the obtained borides were identified by XRD, and the sizes of crystals and their shapes were determined by SEM. It is shown that during electrolysis, a mixture of two phases GdB₆ (cubic system, Pm-3m) and GdB₄ (tetragonal system, P4/mbm) is formed on the cathode. Chemical reactions of dissolution of boron and gadolinium oxides in calcium chloride due to interoxide interaction, as well as electrochemical gross reactions of formation of borides on the cathode are proposed.

KEYWORDS

salt melt, calcium chloride, boron oxide, gadolinium hexaboride, borides.

DOI: 10.31857/S0235010622060032

Yu. V. Stulov.

ABSTRACT

The main problems appearing in the quantum-chemical modeling of complex lanthanide (Ln) ions are considered. The principal difficulty is the presence of an open 4f shell in the Ln ions, which leads to a large number of quasi-revealed electronic states that cannot be correctly described by single-configuration methods. Various approaches to the quantum chemical modeling of systems containing Ln complexes are analyzed. We considered the basis sets in which 4f electrons are taken into account explicitly (4f-in-valence) and the bases in which 4f electrons are taken into account in the effective core potential (the core potential is determined by a number of electrons located near the nucleus and not involved in the formation of chemical bonds, 4f-in-core). It is shown that quantum-chemical calculations by the Hartree-Fock methods and the density functional theory can lead both to the incorrect distribution of the electron density of the complex, which is typical for the 4f-in-valence bases, and to the incorrect description of the coordination number of the complex ion when using the 4f-in-core basis sets. The incorrect reproduction of the structure of the complexes is apparently related to the inability to take into account the interaction of the 4f orbitals with the ligand field. Nevertheless, we proposed an approach that consists in using the 4f-in-core basis for the samarium atom together with locking the structure of the complex. This makes it possible to obtain a correct simulation of the composition of the first coordination sphere of the complexes and the undistorted electronic structure of the complex particle. A good agreement between the results of quantum-chemical modeling and experimental data is observed. It is shown that the use of 4f-in-core bases also leads to a significant reduction in the amount of machine time required, which makes it possible to use these bases in routine calculations. This approach can be extended for other Ln-containing model systems as well.

KEYWORDS

Quantum-chemical calculations, Hartree-Fock method, Density Functional Theory, Lanthanide complexes, Samarium, 4f-in-core.

DOI: 10.31857/S0235010622060123

A. M. Potapov, V. A. Kesikopoulos, A. E. Dedyukhin, Yu. P. Zaikov. 

ABSTRACT

The equilibrium composition of (LiCl-KCl)_eut. + 1 mol % UCl₃ system during its titration with lead chloride at the temperatures of 773, 873 and 973 K was calculated by thermodynamic modeling methods. The calculations were carried out taking into account the activity coefficients of lead and uranium chlorides in the molten LiCl–KCl eutectic. At 773 K, the activity coefficients g(PbCl₂) = 0.42; g(UCl₃) = 5.73·10^-3 и g(UCl₄) = 7.54·10^-5 were used. As the temperature rises, the activity coefficients increase. Thus, at 973 K the activity coefficients are already g(PbCl₂) = 0.49; g(UCl₃) = 0.020 and g(UCl₄) = 3.04·10^-4. It was found that in the LiCl-KCl melt, lead chloride only partially oxidizes UCl₃ to UCl₄. This is due to the fact that the difference between the formal standard potentials  and  is only about 0.2 V. Therefore, PbCl₂ is a weak oxidizing agent in this case. The difference between  и  potentials increases with increasing temperature. Therefore, UCl₄ equilibrium fraction increases with increasing temperature and also with excess of PbCl₂. For example, at 773 K and a threefold excess of PbCl₂ against the stoichiometry, the average uranium valence is n = 3.27. And at 973 K and the same excess of PbCl₂, the average valency of uranium increases to n = 3.36.

Also, the process of reduction of UCl₄ to UCl₃ by metallic uranium in the molten LiCl-KCl eutectic was modeled taking into account the activity coefficients of the components. In the considered interval, 773–973 K, the reduction has no thermodynamic difficulties and proceeds completely.

The HSC-9.9 software package was used for thermodynamic calculations.

KEYWORDS

Thermodynamic modeling, Uranium, Uranium chlorides, Lead chloride, “soft” chlorination.

DOI: 10.31857/S0235010622060111