Kh. B. Kushkhov, M. K. Vindizheva, R. A. Mukozheva, F. A. Kisheva. 

ABSTRACT

Using the methods of cyclic chronovoltammetry, chronopotentiometry, and open-circuit chronopotentiometry (turn-on and turn-off curves), the joint electroreduction of lanthanum and nickel ions on a tungsten electrode in an equimolar melt of potassium and sodium chlorides at a temperature of 973 K was studied. It was investigating that on an inert tungsten electrode the joint electroreduction is released at potentials of – 2.1…–2.2 V relative to the silver chloride reference electrode. The mechanism of the process in one three-electron stage which controlled by diffusion transfer stage at polarization rates less than 0.1 V/s. At higher polarization rates, the charge transfer stage is slower.

With a shared content of lanthanum and nickel ions in an equimolar KCl-NaCl melt at 973 K, the current-voltage dependences show a wave of nickel reduction at potentials of 0.0…–0.1 V, lanthanum ions at potentials of – 1.85…–1.9 V. In addition to these waves, there are two recovery waves on the current-voltage dependence at potentials of –1.5…–1.6 V and –1.75…–1.8 V. The appearance of the last two waves is associated with the reduction of lanthanum ions with depolarization on pre-deposited metallic nickel on a tungsten electrode, with further formation of intermetallic phases of lanthanum and nickel La_xNi_y (various compositions):

At a wave at potentials of –1.85…–1.9 V, electroreduction of LaCl₆^3- (chloride complexes on the La_xNi_y intermetallic compound takes place with the formation of intermetallic phases with a higher content of lanthanum due to the interaction of the La_xNi_y intermetallic compound with metallic lanthanum: .

The (E–t) dependences of open circuit chronopotentiometry (on and off curves) revealed potential delay plateaus corresponding to the dissolution of individual phases of lanthanum and nickel intermetallic compounds.

Based on the results of the analysis of the cyclic current-voltage dependences of the molten system KCl-NaCl–LaCl₃–NiCl₂, (E–t) dependences obtained by the direct chronopotentiometric method and open circuit chronopotentiometry, it can be concluded that the electrochemical synthesis of lanthanum- nickel intermetallides can be realized only in the kinetic mode, namely, electroreduction of LaCl₆^3- ions on pre-deposited metallic nickel on a tungsten electrode with the formation of La_xNi_y intermetallic phases as a result of diffusion reaction.

KEYWORDS

lanthanum chloride, nickel chloride, joint electroreduction, cyclic chronovoltammetry, chronopotentiometry, open circuit chronopotentiometry, chloride melt.

DOI: 10.31857/S0235010622060081

A. V. Krylosov, I. B. Polovov, O. I. Rebrin. 

ABSTRACT

Beryllium-containing halide melts can be used for electrolytic obtaining and refining of beryllium, in processing processes for the spent nuclear fuel. Information about the melting temperatures of salt mixtures is important for the organization of technological processes with their use.

The complexity of the study of salt mixtures containing beryllium fluoride is associated with a number of factors, among which salt hygroscopicity, corrosion activity in relation to structural materials and the toxicity of salt vapor. In this regard, all preparatory operations were carried out in printed boxing, in the atmosphere of Argon. Differential-termic analysis (DTA) of samples is also performed in the atmosphere of the dry and clean argon. In the work, they used crucibles from platinum and oxide beryllium. These materials showed better resistance to corrosion in the studied molten saline media.

The fusibility diagrams of the fluoride of beryllium with alkaline metals (Li, Na, K, Cs), built by the results of differential-termic and X-ray phase analysis, are given. The coordinates of eutectic points are determined.

In the interval of changes in the concentration of beryllium fluoride from individual alkaline metal chloride to the corresponding eutectic, the measurement results are represented in the form of approximation polynomas of concentration dependence of the melting temperature of systems BeF₂–MeCl (Me = Li, Na, K, Cs).

The fusibility of the salt mixtures of eutectic of lithium and potassium chloride with fluoride of beryllium has been studied. The most fusible of the studied mixtures contains 35 mol % beryllium fluoride and melts at a temperature of 331^°С

The absence of triple connections in the studied systems is shown. To identify phases, an X-ray-dissection analysis was used, which showed the presence of alkaline metals in the solid phase of the corresponding chlorides of alkaline metals. Beryllium fluoride was determined by the characteristic peak of absorption on infrared spectra. The work uses the original methodology for distillation of beryllium fluoride obtained by the decomposition of ammonium fluoride. In view of the tendency of beryllium fluoride to glass formation, information about the melting temperatures of melting in this component of salt mixtures is of an evaluative character.

KEYWORDS

fusibility diagrams, beryllium fluoride, fluoride-chloride systems, molten salt mixtures, differential-thermal analysis, halide melts

DOI: 10.31857/S0235010622060147

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

ABSTRACT

Even though alkali metal fluorides are among the most aggressive salts in terms of degradation of structural material used in reactor engineering, these salt melts have huge advantages due to high thermal conductivity, low viscosity, high boiling points, the highest heat capacity per unit volume and insensitivity to radiation.

In most high-temperature technologies using molten salts, alloys containing high concentrations of chromium, silicon or aluminum are used, since these elements easily form passive oxide films. However, in molten halide salts, these oxides either do not form or are unstable due to the very low oxygen activity in the molten salts. In this regard, experiments were carried out to determine the corrosion rate of the Hastelloy C2000 alloy with different concentrations of O^2- (by adding lithium oxide to the melt in the concentration range from 0 to 5 wt %) in a fluoride melt. The formation of a protective oxygen-containing layer with a thickness of 10 microns has been established. The corrosion rate decreases by an order of magnitude at the concentration of oxygen anions in the melt from 0.14 to 0.28 wt %, which may indicate the detection of the phenomenon of high-temperature passivation of the material due to modification of the composition of the fluoride melt and reduction of its corrosion activity. In addition, for the Hastelloy C2000 alloy, the depth of corrosion damage in fluoride melts decreases when lithium oxide is introduced into the melt due to the formation of oxygen-containing compounds of sufficient thickness on the surface.

KEYWORDS

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

DOI: 10.31857/S0235010622060093

A. V. Isakov, M. V. Laptev, A. O. Khudorozhkova, A. S. Shmygalev, A. P. Apisarov. 

ABSTRACT

A promising task in the energy sector is the development of high-capacity lithium-ion batteries. The use of silicon fibers as an anode will further increase the capacity of LIB. However it becomes necessary to obtain silicon without silicon dioxide on the surface because of since the greatly reduce the capacity of the LIB anode material. In this paper, we propose a method for purifying silicon nanofibers obtained by electrolysis of molten salts from an oxide film in a reaction with ammonium fluoride using a vacuum setup developed by the authors. A technique has been developed for preparing the charge for purification and further analysis. Silicon nanofibers are obtained by electrolysis of a KF-KCl-K₂SiF₆-SiO₂ melt. The chemical composition was studied using X-ray microanalysis combined with electron microscopy and mass spectroscopy. The structure and morphology of silicon deposits was studied by electron microscopy and X-ray diffraction. The fundamental possibility of removing the oxide film from the surface of silicon nanofibers is shown. Si-NH₄F mixtures with molar ratios of 1/1, 1/0.123 and 1/0.49 were used for purification. The composition of the Si-NH₄F mixture and the temperature at which the purification process is most complete are determined. The thermodynamic probabilities of the occurrence of chemical reactions in the interaction of silicon with ammonium fluoride are discussed. In the course of the study, the mechanism of interaction of ammonium fluoride with the surface oxide layer of silicon nanofibers was established. The interaction of the silicon oxide film with ammonium fluoride under experimental conditions proceeds with the formation of an ammonium hexafluorosilicate phase and easily volatile components, ammonia and water vapor. Mass ratios of silicon and oxygen in purified fibers are determined. It has been established that an increase in this ratio is an indicator of the improvement in the purification of the silicon surface from oxygen. It has been established that the treatment of the cathode silicon deposit by the proposed method using high-temperature vacuum distillation is an effective method for cleaning the silicon surface from oxide-containing compounds formed on its surface.

KEYWORDS

molten salts, potassium hexafluorosilicate, silicon nanofibers, electrolysis, high-temperature evacuation.

DOI: 10.31857/S0235010622060056

V. G. Kolyasnikov, A. M. Potapov, V. Yu. Shishkin, A. E. Dedyukhin, Yu. P. Zaikov.

ABSTRACT

The molten eutectic mixture LiCl-KCl is a promising medium for different high-temperature electrochemical processes of refining, separation, electrocrystallization, etc. It has a low melting point, low corrosion activity and a wide electrochemical window. The sequence of oxidation and electrodeposition of metals in electrolytes is determined by their electrochemical potentials.

The object of research is CdCl₂ solutions in the molten LiCl-KCl eutectic. Cadmium chloride is studied as a promising agent for chlorination in molten salts.

The purpose of the present work is to evaluate the thermodynamic parameters of the CdCl₂ interaction with the LiCl-KCl melt. The equilibrium electrode potentials of cadmium in the molten LiCl-KCl eutectic were experimentally measured with a cadmium chloride content of 0.25 to 20 mol % in the temperature range from 450 to 550°C, using EMF method. The values ​​of conventional standard electrode potentials of cadmium were calculated and the temperature dependence of the isobaric-isothermal potential was determined. The increase in CdCl₂ concentration and temperature rise leads to the shift of potentials to a more positive region.

Empirical equations of electrode potentials isotherms were obtained. The valency of cadmium cations calculated form the isotherms slope was fond equal to two.

The conditional standard electrode potentials of cadmium in the LiCl-KCl melt were calculated, temperature dependence was determined.

The values ​​of the enthalpy and entropy of the CdCl₂ formation reaction in the LiCl-KCl melt in the temperature range of 450–550°C are calculated: ∆H = -382.63 kJ/mol; ∆S = -119.66 J/mol∙K.

The Gibbs energy of the CdCl₂ formation reaction in the LiCl-KCl melt increases with temperature rise.

KEYWORDS

Equilibrium potentials, conditional standard potential, EMF method, cadmium chloride

DOI: 10.31857/S023501062206007X

D. A. Vetrova, S. A. Kuznetsov. 

ABSTRACT

The redox reaction Ti(IV) + e^– ↔ Ti(III) in CsCl-CsF(10 wt %)-K₂TiF₆-based melts with additions of alkaline earth metal cations (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) was studied by cyclic voltammetry method. The quasi-reversibility region of titanium complexes for recharge process has been determined and the standard rate constants of charge transfer for the redox couple Ti(IV)/Ti(III) in CsCl-CsF(10 wt %)-K₂TiF₆-based melts with additives of alkaline earth metal cations were calculated. The standard rate constants of charge transfer do not depend on the polarization rate and increase with temperature increasing. It is shown that the standard rate constants of charge transfer increase with increasing mole ratio of alkaline earth metal cations to titanium up to a certain Me²⁺/Ti(IV) ratio and then decrease with further increasing of Me²⁺ cations concentration in the melt. It was found that the rate constants of charge transfer increase with increasing ionic potential of the addition, reaching a maximum value for complexes with outer-sphere cations of magnesium, and this dependence is rectilinear. The activation energies of the charge transfer process in melts with additions of alkaline earth metal cations were calculated. The values obtained are significantly lower than the activation energies of the initial system CsCl-CsF(10 wt %)-K₂TiF₆. It is shown that the standard rate constants of charge transfer decrease and the activation energy values increase in transition from (NaCl-KCl)_equimol. based melt to KCl-KF and CsCl-CsF melts both without addition and with addition of alkaline earth metal cations.

KEYWORDS

melt, cyclic voltammetry, titanium complexes, redox couple, quasi-reversible process, standard rate constants of charge transfer.

DOI: 10.31857/S0235010622060135

М. А. Kobelev, D. О. Zakiryanov, N. К. Tkachev. 

ABSTRACT

Calculations of partial structural factors of a molten lithium fluoride at a temperature of 1200 K are carried out by three methods: ab initio and classical molecular dynamics, as well as within the model of charged hard spheres in the mean-spherical approximation. It is shown that despite the general topology of the calculated partial structure factors for all three methods, the key feature of the computer simulation results is the absence of a shoulder on the right side of the first peak of the cation-cation structure factor. It is suggested that the observed difference can be related to the Buckingham softness of the repulsive part of the pair potential in the case of the classical model in molecular dynamics calculations. Calculations using by ab initio molecular dynamics also confirm this conclusion. The results of calculating the density-density structural factor by computer simulation methods demonstrate the presence of two maxima lying at k ≈ 2.4 Å^-1 and k ≈ 4.4 Å^-1, respectively. At the same time, within the model of charged hard spheres, there is one maximum at k ≈ 3.5 Å^-1 and a small shoulder on the left, the position of which coincides with the first maximum obtained by computer simulation methods. The observed differences can be related to the fact that the calculated curve of the partial cation-anion structure factor within the framework of the model of charged hard spheres has a higher amplitude in the vicinity of the wave vector ≈ 3.5 Å^-1.

KEYWORDS

structural factors, molten LiF, molecular dynamics, density functional theory

DOI: 10.31857/S0235010622060068

С О N T E N T S

A.V. Kaibichev, I.A. Kaibichev, E.V. Ignateva. 

ABSTRACT

The gases removal from Fe-C (C content 0.8 and 2.0 wt %, Fe – the balance) melt was determined by reducing the melt weight. Weight loss could occur with the removal of iron, carbon, hydrogen, nitrogen and oxygen. The constancy of the Fe-C melt (2% wt.) weight at the end of the experiment in an electric field and without it excluded iron from the number of elements to be removed. The removal of the elements (C, H₂, N₂, and O₂) is calculated by the mass of monatomic ions taking into account the influence of experimental conditions. The weight of a monatomic ion of an element is determined by the amount of electricity required for its transfer, the proportion of ions in electricity and the element atomic weight. The ions proportion in the transmitted electricity is determined from the ratio of the ion velocity to the sum of the electron velocity. The required amount of electricity for the transfer of an element monatomic ion is calculated in relation to the product of the fraction of the element ions in the past electricity that flowed and its atomic weight to the sum of their values of all participants in the transfer. The weight of the monatomic ions of the removed elements when multiplied by the determined number of atoms is equal to the melt weight change. The removed gases (C, H₂, N₂, and O₂) were found by the determined number of their atoms from the equality of masses during the experiment and in short periods of experiment conducting. The weight of gases monatomic ions during the experiment is greater than the weight of such ions in short periods.

During the experiment hydrogen was removed in the form of Н₃⁺ ions (0.0658•10^-3 g) and nitrogen – N₃⁺ (7.2396•10^-3 g) from the Fe-C melt (0.8 wt %). The deviation of ions mass from the weight change was + 0.07%.

Hydrogen was removed from the Fe-C melt (2 wt %) in the form of Н₁₀⁺ ions (0.180•10^-3 g), nitrogen – N₂⁺ (4.0208•10^-3 g), oxygen О₅⁺ (13.04268•10^-3 g). The deviation of their mass from the weight change was 0.03%. Polyatomic ions were removed from the melt gases. Carbon was not removed, there was no carbon in the surface layer.

KEY WORDS: melt, hydrogen, carbon, nitrogen, oxygen, coulomb, volt, ions, atomicity, mass.

DOI: 10.31857/S0235010622040053

D.A. Rozhentsev, N.K. Tkachev. 

ABSTRACT

Nano-porous metals obtained by dealloying in the percolation mode have the structure of a network of ligaments with pore sizes that can vary from several nanometers to several microns. Nano-porous iron and some ferroalloys containing manganese could be extremely promising materials as bio-degradable implants for regenerating bone tissue after fractures and other injuries. Molten salts, as electrolytes, are capable of accelerating percolation dealloying due to elevated temperatures, as well as higher current densities can be applicable in this case. Electrochemical dealloying of ferromanganese of various compositions (30, 50, 70 Mn at %) in molten mixtures of alkali metal chlorides has been carried out. A complex combination of temperature and electrochemical processes in the percolation mode, which leads not only to the formation of bi-continuous structures, but also to structures of the dendritic type. The evolution of the structure of the synthesized nano- and ultraporous iron depending on temperature from a dendrite-like and streamline-oriented morphology at 300℃ to a typical bi-continuous nanoporous structure with a pore size of about 100 nm at 400℃ was observed. Activation of the effects of coagulation and coarsening of ligaments and a loss of orientation along the field line were noted. However, under these conditions, a bi-continuous structure (100 nm) sufficiently uniform in size is formed. Further increasing the temperature results in another porous material with a 3D pore hierarchy at (500℃), which is distinguished by the distribution of pores of different sizes from 100 nm to 1 μm. An observation was made about the formation of a porous structure of an enlarged, micron scale at 600℃ with «ginger-root-like» ligaments. Dividing the process of dealloying at 500℃ and simple annealing at 600℃ into two independent steps leads to a similar result, but the structure has less coherence of the metal particles.

KEYWORDS: selective anodic dissolution, dealloying, ferroalloy, chloride melt, annealing, nanoporous iron

DOI: 10.31857/S0235010622050061