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No 1 (2025)
Articles
Quasi-equilibrium and nonequilibrium explosive crystallization of InBi and In2Bi compounds
Abstract
The process of quasi-equilibrium and nonequilibrium explosive crystallizations of chemical compounds InBi and In2Bi, as well as their components bismuth and indium, has been studied using cyclic thermal analysis (CTA) and differential thermal analysis (DTA). The experiments were carried out under the same conditions. It has been established that the chemical compound In2Bi behaves like indium during crystallization, i.e., regardless of the preliminary overheating and the time of isothermal exposure of the melt to four hours, it crystallizes quasi-equilibriously with a slight pre-crystallization overcooling of 1.5-2 K. And the chemical compound InBi behaves like bismuth during crystallization. The temperature of critical overheating of the melt has been found, upon cooling from which crystallization has a quasi-equilibrium character (PK), and upon cooling from temperatures above, crystallization has an explosive character from the supercooled state, i.e., the dependence of melt overheating on overcooling is abrupt. The experimental results are interpreted from the point of view of the cluster-coagulation model of melt crystallization.
3-9
Random ensembles of particles with pentagonal symmetry: densification and properties
Abstract
The paper studies the densities and statistical-geometric characteristics of random packings of regular pentagons on a plane. The initial ensemble was generated by random sequential adsorption (RSA). A packing densification algorithm is proposed, which is a modification of the Lubachevsky-Stillinger (LS) method. The final ensemble was obtained by stepwise increasing the linear dimensions of 2-D particles at a fixed density of the square “box”. It is shown that the packing density of the final ensemble for this algorithm is practically independent of the density of the initial ensemble (with a total number of particles of about 104 or more). The maximum packing density of the starting ensemble of regular pentagons obtained by the RSA method was 0.54306 ± 0.00220, which is in good agreement with the literature value of 0.54132. The highest (final) density achieved after compaction of the starting ensemble was 0.8381 ± 0.0020 for pentagons. This value is close to the value found by a similar algorithm for packing hard disks (0.84-0.86). The correlation functions of hard disks and pentagons demonstrate a number of common patterns. At the same time, the “crystallization” of the ensemble of hard disks at relatively high densities close to the maximum achieved is expressed more sharply. At the same time, the “peaks” of the correlation function for pentagons (compared to disks) are expected to have a smaller height and a larger width, a more complex structure. Ensembles of non-convex particles with pentagonal symmetry (such as five-pointed stars) demonstrate significantly lower packing densities and do not compact to partial “crystallization”. A relatively simple algorithm for compacting “starting” random packings of polygons, applied in the work, allows “compacting” two-dimensional ensembles of any polygons (without self-intersections). However, partial ordering and sufficiently high densities (corresponding to the beginning of “crystallization” of the ensemble) are achieved when using it only for convex polygonal particles.
10-23
Specific electrical conductivity of molten (LiCl–KCl)eut – HfCl4 mixtures
Abstract
Electrical conductivity is one of the most important properties that are required for the proper organization of electrolytic processes occurring in molten salts, in particular, during the production and refining of metallic hafnium and its separation from zirconium. In this work, we have measured for the first time the electrical conductivity of molten HfCl4 mixtures with a low-melting solvent (LiCl-KCl)eut, which makes it possible to lower significantly (by hundreds of degrees) the temperature of technological processes. The liquidus line of this pseudobinary system has been also constructed for the first time at the HfCl4 concentrations up to 30 mol. %. A specially designed capillary quartz cell with a constant in the range of 95.2–91.9 cm–1 and high-purity chlorides were used to measure the electrical conductivity. The resistances of the molten mixtures in the HfCl4 concentration ranges of 0–30 mol.% and temperatures of 780–1063 K were recorded using an AC bridge P-5058 at a frequency of 10 kHz, and the melt temperature was measured with a Pt/Pt–Rh thermocouple. It was found that the values of electrical conductivity of the molten (LiCl–KCl)eut.-HfCl4 mixtures increase as the temperature increases from 0.86 to 2.08 S/cm. This occurs as a result of the increased ion mobility (simple and complex) and decreased melt viscosity. With an increase in the HfCl4 concentration, the electrical conductivity decreases. In the same direction, the concentration of relatively low-mobility complex groups HfCl62– containing 6 chlorine anions tightly bound to the tetra-charged metal, increases in the melts. The concentration of the main current carriers, Li+, K+ and especially the mobile Cl– anions, decreases more and more, which leads to a decrease in the electrical conductivity of the melt. In the molten (LiCl–KCl)eut.–ZrCl4 mixtures that we studied earlier, the electrical conductivity decreases less as the tetrachloride concentration increases, which indicates a lower strength of the ZrCl62– complexes compared to HfCl62–.
24-34
Emissivity of elements of the scandium subgroup
Abstract
The results of an experimental study of the normal integral emissivity of metals of the scandium subgroup: scandium, yttrium and lanthanum in a wide range of solid–liquid temperatures, including the phase transition, are presented. The study is due to the lack of data on the normal integral emissivity of metals in periodicals and reference publications. The interest in the metals under study is also related to their unique physico-chemical properties, which make them promising for use in high-temperature systems. The authors of the article interpret the presented data as obtained for the first time, and they are preliminary in nature, requiring clarification. The measurement method is radiation, the method of heating samples is resistive. The experimental error is ± 3–5%. Measurements of the solid phase of metals were carried out in a vacuum, the liquid phase was studied in an atmosphere of specially prepared gas – argon. Graphical illustrations and numerical values of the results are presented. The obtained complex data on the normal integral emissivity within each of the phases of the metal state are monotonously increasing. This behavior of the normal integral emissivity is associated with structural changes in crystal lattices due to an increase in temperature. In the region of the solid–liquid phase transition, a jump in the normal integral emissivity was detected for each of the metals studied. In this case, the jump is associated with a sharp increase in free electrons during the restructuring of the metal structure due to melting, and the magnitude of the jump as a percentage of the solid phase is individual for each metal. All the results of the study have been analyzed and discussed. Numerical simulation based on classical electromagnetic theory using the Foot approximation is carried out, the results of which are compared with experimental values. It is concluded that the theoretical calculation of the emissivity qualitatively, but not quantitatively, makes it possible to describe the behavior of the emissivity of metals, provided that the values of the specific electrical resistance of metals in a given temperature range are known.
35-45
Immiscibility of ionic melts: a simple model with charge differences
Abstract
This work is devoted to the analysis of the immiscibility mechanism and the peculiarities of its manifestation in the case of mixtures of classical electrolytes. This mechanism should be deduced from the differences in the potential energy of the ions constituting the components of the mixture with respect to their surroundings. Since electrostatic interactions are shielded at a large distance from the central ion in any electrolytes, it is important for the considered mechanism what contribution to the concentration dependence of the chemical potential of a component is given by one or another sort of ions. In this paper we consider a simplified model of a binary solution in which the interaction of cations and anions in each of the ionic liquids is approximated by the model of charged hard spheres, i.e., they are considered as primitive electrolytes. Since the problem of liquid-phase immiscibility cannot be considered without taking into account the finite sizes of ions, it is necessary, firstly, to choose at least the full version of the Debye-Hückel model, and, secondly, to take into account the direct contribution of excluded volume effects or hard-sphere repulsion, for which a van der Waals-type model can be used. As a result, the reasoning about the concentration dependence of the density in a liquid-phase system and the equation of state that allows us to find it become key for describing the features of the miscibility gap. The theoretical analysis of the immiscibility problem can be carried out by considering that a cation and an anion belonging to one of the components of a binary mixture possess the same value of ionic radius and equal but opposite charge, while differing in their values for the other component of the solution. Thus, a binary restricted primitive model (RPM) is formulated to consider the effects of charge differences on the miscibility gap. In the present work, analytical expressions describing the position of the critical point in the asymptotic limit of small charge differences are derived in detail. It is shown that the critical temperature is proportional to the fourth degree of the charge mismatch, and the shift of the critical composition from equimolar occurs towards the component with smaller charge values. The latter result seems to be quite general, describing the preference in solubility of salts, which have larger charge values, in ionic melts with smaller charges on cations and anions.
46-61
Electroreduction of nickel (II) chloride, nickel (II) fluoride and tungsten (VI) oxide mixtures in a heat activated battery
Abstract
The paper presents the results of studies of the discharge characteristics of elements of a heat activated battery (HAB) containing mixtures of NiCl2-NiF2-WO3 as a positive electrode. It is shown that the addition of tungsten oxide to a mixture of lithium halides makes it possible to increase the removable current density and discharge voltage. It has been established that the reason for the increase in these electrical characteristics is the presence of tungstate compounds (nickel tungstate, lithium tungstate) in the reduction products, which are formed during the operation of the studied HAB elements in a stationary mode. These compounds have a sufficiently high conductivity, which makes it possible to reduce the passivation of the positive electrode and reduce the internal resistance of the HAB element. The optimal composition of the cathode mixture for the studied discharge conditions of HAB elements has been determined. The maximum capacity of the discharge plateau is 0.4 A×h×g–1, the voltage of the discharge plateau varies from 2.40 to 1.65 V, depending on the density of the discharge current. The reduction products of NiCl2–NiF2–WO3 cathode mixtures were studied using XRD, SEM and STA methods. It has been established that nickel halides, which are part of the studied cathode mixtures, are reduced to metal and lithium halides by a two-electron mechanism, according to the electrochemical reaction: NiX2+Li++2e–→Ni+2LiX, where X is Cl, F. The reduced nickel forms a metallic dendritic sponge, the density of which, under equal discharge conditions, is determined by the ratio of the components in the initial cathode mixture. The pores of the dendritic sponge are partially filled with a salt fraction based on lithium halides. The reduction of tungsten oxide to metal has an intermediate stage of formation in tungstate compounds occurring during the stationary operation of the HAB element. In the reduction products of cathode mixtures with a content of up to 5 wt.% of tungsten oxides, the formation of LiCl–Li2O solid solutions is observed. At higher concentrations of the oxide component in the cathode mixtures, zones containing pure lithium oxide are formed in the salt fraction of the reduction products. Tungsten is deposited on the surface of nickel dendrites, forming areas in the form of dotted inclusions. The STA curves of the salt fraction formed during electrochemical reactions have a single thermal effect corresponding to the temperatures of co–melting of a solid solution of LiCl–Li2O and a triple mixture of lithium halides LiF–LiCl–LiBr.
62-72
Modernization of the design of a measuring cell for determining the thermal diffusivity of salt melts using the laser flash method
Abstract
In this work, a well-known cell for measuring the diffusivity of fluoride salt melts by the laser flash method has been modernized. Alkali metal halide melts, such as the eutectic mixture FLiNaK (46.5 mol% LiF – 11.5 mol% NaF – 42 mol% KF), are considered promising materials for use in nuclear power engineering, particularly in molten salt reactors (MSRs), where they act as coolants and actinide fission media. This makes the study of their thermophysical properties extremely important for the design of reactor cores and heat transfer systems. However, as data from the literature show, measurements of the thermal diffusivity of FLiNaK melts are accompanied by significant discrepancies associated with the influence of unaccounted for heat transfer factors and errors in experimental techniques. The laser flash method is one of the preferred methods for studying the thermal diffusivity of salt melts at high temperatures due to its ability to account for convective and radiative heat transfer. However, this method using a known cell leads to overestimated values of thermal diffusivity due to dissipated heat flow. In order to modernize this cell, a numerical model was built in COMSOL Multiphysics, which allowed us to study the influence of materials (Ni, BN, Au) and cell geometry on the heat transfer processes. Data analysis allowed to obtain an optimized cell design that minimized the fraction of heat flow lost, reduced the time to reach the temperature peak, eliminated the need for calibration measurements and extended the temperature range of measurements. Experimental validation of the improved cell was carried out using Netzsch LFA 467 HT HyperFlash equipment. The data obtained confirmed the possibility of more accurate measurement of FLiNaK diffusivity in the temperature range of 550-800°C. Particularly, using of the modernized measuring cell improves the reproducibility of the results and reduces the data scatter, reducing measurement error from 33,8 to 2,6%. These widens the prospects for further studies of high temperature melts, contributing to the development of new generation MSR technologies.
73-84
On the anniversary of V.V. Zakharov
85-86