No 6 (2023)

The eutectic mixture LiF–NaF–KF (FLiNaK) is promising as a carrier salt for molten-salt reactors. Cerium fluoride CeF₃ can be considered as a simulant of plutonium fluoride PuF₃. In this work, the temperature dependence of the electrical conductivity of the molten ternary eutectic LiF–KF–NaF containing from 0 to 25 mol % cerium fluoride in the temperature range 480–777°C. FliNaK–CeF₃ mixtures containing up to 25 mol. % of cerium fluoride were prepared directly in the experimental cell, dropping the required amount of CeF₃ into the melt and keeping mixtures of a given composition for 12 h at temperatures exceeding the liquidus temperatures of the studied compositions. The electrolyte resistance was determined from impedance hodographs, which were taken using the Z-1500J impedance meter. A two-electrode cell with Pt electrodes was used. Analysis of the experimental values of the electrical conductivity of double and triple fluoride melts KF–LiF available in the literature; KF–NaF; LiF–NaF–KF shows that our data on the electrical conductivity of the LiF–NaF–KF (FLiNaK) eutectic mixture are consistent with the general trend in electrical conductivity with increasing KF concentration in the melt. A significant decrease in the specific electrical conductivity of the molten system and a slight increase in the activation energy of electrical conductivity with an increase in the concentration of cerium fluoride are shown. The temperature dependences of the electrical conductivity of the molten system were approximated by second-order polynomials. The formation of the \({\text{CeF}}_{6}^{{3 - }}\) complex ion in the melt was confirmed by spectral studies. The Raman spectra of molten FLiNaK and FliNaK–CeF₃ mixtures containing 8 and 15 mol. % of cerium fluoride were recorded. With an increase in the concentration of CeF₃ in the melt, the amount of \({\text{CeF}}_{6}^{{3 - }}\) complex ions increases, which leads to an increase in the intensity of the observed vibrational bands.

Porous iron is considered one of the most promising biodegradable or resorbable materials in traumatology. To date, only a few works on the preparation of nanoporous iron by dealloying or selective dissolution of ferroalloys are known. Among the promising candidates for precursor alloys, ferrotitanium is of great interest. The difference of iron and titanium release potentials in chloride melts can be about 700 mV. However, two intermetallides are present in the phase diagram: Fe₂Ti and FeTi with melting temperatures of 1427 and 1317°C, respectively. A sample of the composition Fe_0.6Ti_0.4 was melted with the congruent Fe₂Ti compound as its base. Dealloying was carried out in the molten chloride eutectic LiCl–KCl at two temperatures of 400 and 600°C in the potentiostatic mode at the following potentials relative to the platinum quasi-electrode –0.23 and +0.1 V, respectively. The potentials were chosen to provide sufficiently large currents at the beginning of the electrode process, but to avoid mechanical cracking of the sample. Bi-continuous iron nanostructure with characteristic pore sizes of the order of 100 nm at dealloying slightly below its recrystallization start temperature (400°C) was obtained. The residual titanium content found by EDS spectroscopy did not exceed 5 at % by surface. At increasing, the process temperature up to 600°C the expected enlargement of pores and ligaments was observed, and extended areas of larger voids appeared – of the order of several microns, arising, apparently, due to more intensive destruction of micro-inclusions of the FeTi phase in the precursor alloy. The XRD of the samples after dealloying unequivocally testifies to the formation of the cermet iron phase on the surface of the samples. The weak XRD reflections of impurities of the defective wustite phase (FeO) are noted, as there is a partial oxidation of the active nanoporous iron surface already after the sample removal and washing in the air at room temperature. The conclusion is made that it is fundamentally possible to obtain nanoporous iron with a bicontinuous pore structure and ligaments (≈100 nm) from ferrotitanium in molten chloride eutectics by electrochemical dealloying at temperatures slightly below the beginning of iron recrystallization (400°C).

The paper presents an ab initio study of neodymium containing clusters modeling the structure of corresponding molten salts. The relevance of such study is dictated by development of new methodologies and technologies for processing electronic and magnetic wastes, which are a valuable source of rare earth metals. In turn, quantum chemical calculations provide a powerful tool for investigation of structural features of model systems mimicking high temperature molten salts. In the present study, the simulations are performed within the Hartree–Fock and density functional theory approaches using the Firefly 8.20 software package. We propose a methodology for calculation of interaction energies in ternary systems including the neodymium complex, the outer-sphere cation shell, and the rest of the cluster. The interaction energies between the neodymium complex and other parts of a system are calculated. The dependence of interaction energies on the number of outer-sphere cations is investigated and the most stable “neodymium complex + outer-sphere shell” structures are determined. The calculated data are compared to direct spectroscopic investigations available in literature. The obtained interatomic Nd–X (X – F, Cl) distances coincide with experimentally deduced values. The computed Raman spectra for the 18MCl + M₃NdCl₆ (M – Na, K, Rb, Cs) model systems demonstrate a good agreement between calculated and experimentally observed positions of the most intense peak. Therefore, the chosen systems provide a reliable minimalistic model for quantum chemical investigations of molten salts structure.

Effective viscosity (viscoelasticity) of cesium and boron oxides melts was measured at temperatures 900–1600 K and concentrations 0 ≤ x ≤16 mol % Cs₂O by vibration viscosimetry. It was shown that vibration leads to non-Newtonian flow of melts. This means that not only configuration activation energy, the switching energy of bridging oxygen bonds but also the elastic energy of structural units of melts associated with activation energy of viscous flow. Using parameters under conditions of Newtonian and non-Newtonian flow of melts, shear viscosity η', elastic modulus G' and stored viscosity η'' were calculated. It was shown that cesium boron melts in conditions of high shear rates can be considered as liquids with viscous and elastic properties. Glass transition temperature (T_g, K) was measured by DSC, its dependence of content of cesium oxide was plotted and explained.

The study of the influence of the content of cerium oxide and the basicity of the slag on the viscosity and temperature of the onset of crystallization of the CaO–SiO₂–Ce₂O₃ system containing 15% Al₂O₃ and 8% MgO was carried out using the simplex-lattice method of experiment planning, which makes it possible to obtain mathematical models describing the dependence of the property on the composition as a continuous function. Using the experimental data, we built mathematical models that describe the relationship between the temperature of a given viscosity and the composition of the oxide system. Then, by combining the obtained composition-temperature diagrams of a given viscosity on the isothermal section of the composition-viscosity diagram, a set of viscosity isolines was obtained. Generalization of the results of mathematical modeling and graphical display on the isothermal section of the composition-viscosity diagram made it possible to obtain new data on the viscosity of the CaO–SiO₂–Ce₂O₃ oxide system containing 15% Al₂O₃ and 8% MgO, in the range of basicity 2–5 and the content of 0–15% Ce₂O₃. Experimental data show that the slags of the studied oxide system, which do not contain cerium oxide, are characterized by an increased crystallization temperature and viscosity in the studied range of basicity. The presence of cerium oxide in the slags of the studied oxide system provides a rather low viscosity and crystallization start temperature in the temperature range of 1500–1550°C. An increase in the content of cerium oxide in slags with a basicity of 2–3 from 1 to 15% is accompanied by a decrease in the crystallization onset temperature from 1490 to 1410°C. Increasing the basicity to 5.0 leads to an increase in the temperature of the onset of crystallization to 1520°C. At a temperature of 1500°C, the viscosity of slags with a basicity of 2.0–3.0, containing 7–15% Ce₂O₃, varies within 0.2–0.3 Pa · s. An increase in slag basicity to 3.0–5.0 at a fixed Ce₂O₃ content of 7–15% is accompanied by an increase in slag viscosity up to 1.0 Pa · s and reaches 2.0 Pa · s with a decrease in Ce₂O₃ to 1–6%. An increase in temperature to 1550°C and a fixed basicity of 3–5 is accompanied by a significant decrease in viscosity, which does not exceed 0.35 Pa · s at a Ce₂O₃ content of 1–15%.

The electrochemical behavior of lanthanum ions on a nickel electrode has been studied using various electrochemical methods such as cyclic voltammetry, chronopotentiometry, open circuit chronopotentiometry (on-off curves), and square wave voltammetry in an equimolar melt of potassium and sodium chlorides at 973 K. The cyclic voltammetry curves has several reduction waves on the cathodic branch and corresponding oxidation waves on the anodic branch. The first wave A is located in the potential region –(0.0–0.1) V, where the reduction of Ni²⁺ ions takes place. The second wave B is in the region of potentials –(1.72–1.77) V, on it occurs electroreduction of ions \({\text{LaCl}}_{6}^{{3 - }}\) on nickel electrode with certain depolarization with formation of intermetallide of lanthanum with nickel La_xNi_y. The appearance of the third wave C in the potential region –(2.09–2.13) V, we associate with the electroreduction of chloride complexes \({\text{LaCl}}_{6}^{{3 - }}\) on intermetallide La_xNi_y with the formation of metallic lanthanum. On the basis of the obtained data it is shown that during the electroreduction of lanthanum chloride complexes in KCl–NaCl melt at T = 973K the nickel electrode interacts with the released lanthanum, causing a significant depolarization of the process of electroreduction of the chloride complex, also formation of intermetallide with Ni electrode occurs.