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In prior works, a novel method for continuously obtaining Al-Zr master alloys from oxide raw materials by electrolysis of a low-melting electrolyte based on the KF-NaF-AlF3-ZrO2 system at 800 °C to 820 °C was proposed. In the present work, the method for preparing the Al-Zr master alloy proceeded in an enlarged 100 A electrolyzer and the obtained master alloy was applied for grain refinement and improvement of its aluminum alloy properties. The modifying ability of the master alloy was studied, drawing on the example of the Al-Si-Fe alloy. Different amounts of the Al-Zr master alloy with a content of 10 wt pct zirconium were added to the Al-Si-Fe alloy at 900 °C. The effect of the zirconium content in the aluminum alloy and its cooling rate on its structure and properties was revealed. It was found that the addition of zirconium to the alloy refined the grain by 4 to 5 times without changing its shape or structure. A study of the joint effect of alloying and rate cooling indicated that a grain size reduction of up to 5 mkm can be achieved by these procedures.

Viscosity and phase transitions were studied in the (KF-KCl)eut.-(10 mol. %)K2SiF6 system. Synchronous thermal analysis (STA) was used to study the phase transformations in the system in the temperature range of 298–1113 K. The (KF-KCl)eut.-(10 mol. %)K2SiF6 melt was found to be homogeneous at 903–993 K. The viscosity of the system under study was investigated by the rotational viscometer FRS 1600. The results may be described by linear function in coordinates ln(η) – 10 000/T at 898–984 K when viscosity changes from 2.8 to 2.1 mPa s. The melt's nonhomogeneity at 984–1055 K is manifested by the change of the temperature dependence slope. The correlation of viscosity and STA data allows determining the decomposition mechanism of K2SiF6 in the melt under study. The decomposition results in the formation of KF and SiF4. The viscosity of the investigated system was measured during the crystallization process at 876–898 K. The melt at the temperatures below 898 K was found to be in the form of suspension, which is illustrated by the deviation of the linear dependence in ln(η) – 10 000/T coordinates. At 876–898 K, the viscosity rises from 2.8 mPa s to 85.1 Pa s as temperature decreases.

The regularities of silicon and aluminum co-deposition on glassy carbon from KF?KCl (2:1)?75 mol % KI?0.15 mol % K2SiF6?(up to 0.15 mol %) AlF3 melts at 998 K were studied by cyclic voltammetry, chronoamperometry, scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The cyclic voltammograms demonstrated the presence of only one cathodic peak (or nucleation loop at a low reverse potential) and the corresponding anodic peak. The cathodic peak shifted in the cathodic direction with decreasing concentration of aluminum ions in the melt or with increasing scan rate. The Scharifker?Hills model was used to analyze potentiostatic current density transients and estimate the values of the apparent diffusion coefficient and the number density of nuclei. The morphology and elemental analysis of the samples obtained during potentiostatic and galvanostatic deposition for 30?60 s were studied. Continuous thin silicon films doped with aluminum were obtained under galvanostatic conditions.

A KCl–PbCl2 (50.0–50.0 mol %) molten system was used as an electrolyte for secondary lead refining. As a result, pure lead and an anode product containing 52.81 mol % Sb, 33.66 mol % Bi, 7.54 mol %...

The liquidus and solidus temperatures and electrical conductivity of the KF–KCl–KI–K2SiF6 systems were measured as a function of the concentration of KI and K2SiF6. The quasi-binary diagram ([66 mo...

The liquidus temperatures and electrical conductivity of the (CsCl-NaCl-KCl)eut-ReCl4 melts, feasible for Re production, were studied by thermal analysis and impedance spectroscopy. A part of the quasi-binary phase diagram for [CsCl-KCl-NaCl]eut-ReCl4 in the ReCl4 concertation range of 0–7.61 mol % was compiled on the basis of obtained thermal analysis data. ReCl4 is most likely to dissolve in molten eutectic CsCl-KCl-NaCl with the formation of the ReCl62– ions. Compound Cs2ReCl6 was identified in the frozen samples by XRD analysis. The electrical conductivity of molten mixture (CsCl-KCl-NaCl)eut.-ReCl4, measured in the cell with the coaxial glassy carbon electrodes, was found to decrease with increasing ReCl4 content.

Among the available protection systems for steel, the use of coatings is the most popular and economical method. One can protect the steel electrode from aggressive media with an aluminum coating. A thin Al film on an Fe substrate has been studied by the molecular dynamics method at a heating temperature from 300 K to 1500 K. A significant horizontal displacement of individual Al atoms on the edges of the film is observed during the simulation. The film begins to “spread” slightly near the edges. This “spreading” creates the conditions for the beginning of diffusion of iron atoms into aluminum. Some Al atoms were found to penetrate the Fe matrix at a temperature of 873 K. The total energy curve of the system shows both the melting transition in aluminum and phase transition from the body-centered cubic lattice to the face-centered cubic one at 1173 K. The binding energy for the Al atom in the lattice of the Fe crystal is smaller than that for Fe atoms. The calculated diffusion coefficients for Al and Fe have a significantly slower growth with a temperature in the range of 673 K ≤ T ≤1500 K. To describe the diffusion in a crystal using the molecular dynamics model, a temperature-dependent correction to the activation energy is calculated. The temperature dependence of the diffusion coefficient of aluminum atoms in an iron crystal can be represented as an Arrhenius expression with a temperature-dependent energy barrier for diffusion. © 2019, Institute for Metals Superplasticity Problems of Russian Academy of Sciences. All rights reserved.

The aluminum electrolysis process in the conventional cryolite-alumina electrolyte with cryolite ratio of 2.7 was carried out at an initial temperature of 970 °C and the anode current density of 0.5 A/cm2 in a 15A lab-scale cell in order to study the formation of the side ledge during electrolysis and the alumina distribution between electrolyte and side ledge. The alumina contained 35.97% α-phase and 64.03% γ-phase with the particles size in the range of 10-120 μm. The cryolite ratio and the alumina concentration were determined in molten electrolyte during electrolysis and in frozen bath after electrolysis. The side ledge in the electrolysis cell was formed only by the 13th hour of electrolysis. With a slight temperature decrease a significant increase in the side ledge thickness was observed. The basic components of the side ledge obtained by the XRD phase analysis were Na3AlF6, Na5Al3F14, Al2O3, and NaF.5CaF2.AlF3. As in the industrial cell, the increased alumina concentration in the side ledge formed on the cell walls and at the ledge-electrolyte-aluminum three-phase boundary during aluminum electrolysis in the lab cell was found (FTP No 05.604.21.0239, IN RFMEFI60419X0239).

Texturing silicon wafers is one way to increase the performance of solar cells. This work is the first to report on the surface modification of Si wafers by processing in polytungstate melts. Scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction analysis (XRD), the Brunauer–Emmett–Teller (BET) method, and photoelectrochemical measurements were used to elucidate the effect of texturing conditions in the Na2WO4—K2WO4 (1:1) melt containing 35 or 50 mol% WO3 at 973 K in air. As a result of cathodic treatment in the melt containing 50 mol% WO3 at the potential of –0.92 V (vs Pt) for 15 s, upright pyramids were formed on the Si surface. In addition, inverted pyramids appeared at the OTB/Si contact points. The photocurrent density of these samples was several times higher than that for the initial Si wafer or the Si wafer etched in 5 M NaOH solution at 353 K for 20 min. Mechanisms for the formation of upright and inverted pyramids were proposed. Unusual eight-faceted pyramids were formed on the Si surface during cathodic treatment in the melt containing 35 mol% WO3 at –1.19 V for 15 s, but the photocurrent density of such samples was low.

Received: 01.08.2019. Accepted: 11.10.2019. Published: 15.10.2019. The heat of fusion of eutectic mixtures of sodium cryolite with alumina and calcium fluoride was measured using differential scanning calorimetry. Melting temperatures were found to be in good agreement with literature data. The molar heat of fusion of cryolite salts and eutectic mixtures was found to be directly dependent on melting temperature. The temperature dependence coefficient is the same as that of alkali halides. The work was supported by RFBR. Grants No 18‑03‑00785 А.

The regularities of the formation and dissolution of independent new-phase nuclei on an indifferent electrode during a cyclic potential sweep were analyzed. Equations for cyclic voltammograms and nuclei sizes were derived for cases of simultaneous or progressive nucleation with diffusion-controlled growth or kinetically (charge transfer) controlled growth. Results of numerical simulation obtained at various values of the scan rate were presented. It was found that current extrema can be proportional to the scan rate raised to the power of –1/2, –3/2, –2, –3, depending on the nucleation type and the limiting growth stage. Criteria for identification of phase formation mechanism were verified by analyzing the experimental cyclic voltammograms for the electrodeposition/electrodissolution of silver nuclei on iridium in the KNO3-NaNO3-AgNO3 melt.

Neutron transmutation doping (NTD) was applied to produce a stable isotope 31P admixture in thin silicon films electrodeposited on glassy carbon (GC) from the KF-KCl (2:1) – 75 mol% KI – 0.5 mol% K2SiF6 melt. The irradiation of samples with 5 and 15 m silicon film thickness was carried out under the neutron flux density of 1.46x1012 cm–2s–1 for 168 h at 318 K in the IVV-2M research reactor (RR). The irradiated samples were studied by methods of gamma-ray and mass spectrometry, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). Impurities the activities of which reach levels above the minimum significant ones (MSA) were detected and identified. The presence of 31P atoms (1.11014 cm–3) in thin silicon films after the NTD process was confirmed. The specific electrical resistance of the irradiated samples was evaluated. Arrangements were defined to optimize the NTD process for obtaining acceptable results, including a higher 31P concentration and lower exposure time to achieve the minimum significant specific activity (MSSA).

The heat capacities of molten salts are very important for their practical use. Experimental investigation of this property is challenging because of the high temperatures involved and the corrosive nature of these materials. It is preferable to combine experimental investigations with empirical relationships, which allows for the evaluation of the heat capacity of molten salt mixtures. The isobaric molar heat capacities of all molten alkali and alkaline-earth halides were found to be constant for each group of salts. The value depends on the number of atoms in the salt, and the molar heat capacity per atom is constant for all molten halide salts with the exception of the lithium halides. The molar heat capacities of molten halides do not change when the anions are changed.

Recent developments in the field of low-temperature aluminum electrolysis utilizing oxygen-evolving inert anodes prove the efficiency of potassium cryolite-based electrolytes. Such electrolytes give an opportunity to perform electrolysis at temperature below 800 °C. The potassium cryolite-based electrolytes can be used as prospective media for other electrochemical and chemical technological processes including aluminum alloys production. However, the introduction to the electrolytic bath the metal-containing additives along with alumina can result in electrolysis violation. It is a consequence of significant change in electrolyte properties with varied composition. In present work the KBF4impact on the physical-chemical properties of the potassium cryolite-based melts was studied. Liquidus temperature, electrical conductivity, density, alumina solubility in the KF-AlF3 and KF-NaF(15.9 mol%)-AlF3 molten mixtures containing KBF4 were determined. Cryolite ratio (CR), the ratio between molar concentrations of the alkali metal fluoride and aluminum fluoride (CR=NMF/NAlF3), was varied from 1.3 to 1.5. It should be mentioned that the cryolite ratio for the mixed KF-NaF-AlF3 cryolite melts was calculated as CR=(NKF+NNaF)/NAlF3. Elemental composition of the molten mixtures being studied was controlled by ICP analysis and X-ray fluorescence spectroscopy on XRF-1800 (Shimadzu). The liquidus temperature of the cryolite melts was measured by the specially developed thermal analysis technique. The data obtained in the KF-AlF3-KBF4 and KF-NaF(15.9 mol%)-AlF3-KBF4 systems showed that the liquidus temperature rises with the KBF4 content. It should be noted that the NaF, LiF, and CaF2, potential additives to the electrolyte for low-temperature electrolysis, being added to the KF-AlF3 melts with low CR, impact the liquidus temperature the same manner as KBF4. The alumina additions to the potassium cryolite-based melts decrease the liquidus temperature as it is shown in Fig.1. The quasi-binary “cryolite mixture”– alumina phase diagrams are simple eutectics. The KBF4 presence in the KF-NaF-AlF3-Al2O3 electrolytes shifts the eutectic point to the right and increases the alumina solubility in these melts. The eutectic points were found equal to 759 and 761°C at the Al2O3 content 4 and 5 mol%, respectively, in the [KF-NaF(15.9mol%)-AlF3+KBF4 (3mol%)]-Al2O3 and [KF-NaF(15.9 mol%)-AlF3+KBF4(5 mol%)]-Al2O3 mixtures with CR=1.3. The Al2O3addition of 6 mol% to the both electrolytes results in significant rise of the liquidus. The bend point on the cooling curves corresponding to the temperature of primary crystallization was not detected because the beginning temperature of the cooling process in these tests was below the liquidus. The presence of not dissolved deposit at the bottom of crucible at 850 °C was also confirmed by visual observation. The original technique to study the electrical conductivity of aggressive fluoride and oxide-fluoride melts was developed. It consists in combination of two types cells: capillary (BN) and with parallel electrodes (Mo). The resistivity of electrolyte was measured by means of impedancemeter (Zahner electrik IM6E) in a frequency range from 100 Hz to 1 MHz using a signal with amplitude 5 mV/s. The electrical conductivity of the potassium and potassium-sodium cryolites was measured in the capillary-type cell whereas the electrical conductivity of electrolytes with KBF4 and Al2O3additions was studied in the cell with parallel electrodes. The temperature dependence of a cell constant was taken into account that made the electrical conductivity measurements more accurate in a wide temperature range. The density of cryolite melts was measured using the Archimedean method. A platinum sphere suspended with platinum wire and attached to an electronic balance unit (Mettler AT20) was immersed into the electrolyte located in the glassy carbon crucible. The measurements were performed in the temperature range 700-800 °C. The temperature dependence of the Pt sphere volume was considered and determined in the calibration test with FLiNaK (eutectic). The density of the KF-AlF3-KBF4 melts decreases with increasing the KBF4 content. So, the addition of the KBF4 3 mol% reduces the density of the molten mixture on about 5%.

This paper describes the interaction between calcium and molten CaCl2 and the solubility of calcium in this melt, depending on the calcium content in the copper-calcium alloy that comes in contact with the molten CaCl2. The negative influence of the dissolved calcium on the current efficiency was verified. The negative effects of moisture and CaO impurities on the calcium current efficiency were demonstrated. The dependence of the current efficiency and the purity of the metal obtained by the electrolysis conditions were studied in a laboratory electrolyzer (20 to 80 A).

The electrolyzers used to extract calcium from copper-calcium alloys at 0.5 to 3.0 kA were designed, manufactured, and utilized to study the process parameters more precisely. The pilot electrolysis device used to produce calcium from copper-calcium alloys at 12 to 15 kA was developed. The calcium production using the bipolar electrolyzer was proved to be possible and was experimentally tested.

Recent developments in the field of low-temperature electrolytes and electrolysis are considered. Although the investigations have not found fundamental problems in low-temperature aluminum electrolysis development as an industrial process, challenges remain in cell design, construction, and operation of cells with these emerging technologies.

The interrelationship between electrical conductivity, molar volume and enthalpy of mixing was studied for molten chlorides and their mixtures. The dependence of electrical conductivity and activation energy on the molar volume is different for various groups of salts. The dependence of specific conductivity on molar volume obtained for molten alkali chlorides was found to be similar to other chloride salts. The specific conductivity of binary mixtures that lack strong chemical interactions between the components can also be described by the proposed empirical equation. The enthalpy of mixing should be taken into consideration for these chemical interactions.

The effect of calcium fluoride on liquidus temperatures of the cryolite melts with a low cryolite ratio (CR) was studied. The systems KF-NaF-AlF3 and KF-LiF-AlF3 with CRs of 1.3, 1.5, and 1.7 have been investigated. The liquidus curves of systems containing CaF2 are different and depend on the K/(K + Na) and K/(K + Li) ratios. In potassium cryolite with CRs of 1.3 and 1.5, the calcium fluoride solubility is low and increases with NaF (LiF) concentration.

Electrical conductivity of fluoride-chloride melts is discussed. The available experimental data for binary and ternary systems are analyzed and presented as a molar volume function.

The electrical conductivity of low temperature electrolytes for aluminum electrolysis has been measured. The potassium cryolite was the basic melt. Additions of LiF and NaF increased the conductivity of (KF +AlF3) molten mixtures but Al2O3 decreased it.

Salt mixtures based on sodium or/and potassium fluorides were tested as electrolyte for oxygen evolving anodes. Electrolyte compositions used in experiments were suggested on the basis of suitable alumina solubility and dissolution rate in the previous papers. Platinum, cermets and Ni-Cu-Fe based metal alloys were tested as inert anodes. Preferable inert anode materials with low polarization, suitable high current density range and low corrosion rate for low-temperature electrolyte systems were proposed.

Experimental investigation of their properties such as density, electrical and thermal conductivity, etc., takes much time. In order to reduce this time it is possible to combine direct measurements with empirical evaluation using data available for molten salts. Empirical equations for electrical and thermal conductivity have been obtained for basic molten electrolytes of industrial electrolysis.

The electrical conductivity of fluoride–chloride electrolytes for solar silicon electrolysis was investigated using impedance spectroscopy. Electrolytes containing potassium fluoride, potassium chloride, cesium chloride, potassium hexafluorosilicate, and silicon oxide were studied. The influence of the electrolyte composition on the electrical conductivity was examined.

The Al-Sc alloys were obtained in the KF-NaF-AlF3 electrolyte containing Sc2O3 and Al2O3 at 800 oC by chemical and electrochemical techniques. Electrolysis was performed in a 20 A lab-scale cell with graphite anode and aluminum cathode. An impact of the cathode current density (0-1 A∙cm-2), the Sc2O3 content (1, 2, 4, and 8 wt%) in the KF-NaF-AlF3 electrolyte, the rate of the molten aluminum agitation (0, 100 rotates/min), and the synthesis duration (30-210 min) on the Sc content in the Al-Sc alloy was studied. The Al-Sc alloys with the Sc content of 0.30-1.80 wt%, depending on the synthesis conditions, and with the uniform Sc distribution throughout the aluminum matrix were produced. The peculiarities of the electrolytic and electroless fabrication of the Al-Sc alloys in the KF-NaF-AlF3-Al2O3-Sc2O3 electrolytes at 800 oC in comparison with the processes occurred in the NaF-AlF3-Al2O3-Sc2O3 melts at 980 oC are discussed.

The Al-Sc alloys are typically obtained by aluminothermy method. However, the high alloy’s price is defined by the cost of a scandium-containing substance (such as ScF3 or Sc, etc.) prepared from its raw material (Sc2O3) for the reaction with aluminum. Expenses for a multistage scandium extraction process are also substantial. Another way for producing the Al-Sc alloys can be the electrochemical co-reduction of Al and Sc from their oxides dissolved in a cryolite (NaF-AlF3) melt. To develop the electrolysis process and to determine its operating parameters it is necessary to know some basic physical-chemical properties of the NaF-AlF3-Sc2O3-Al2O3 molten system, such as liquidus temperature, Sc2O3 solubility in cryolite including a cross impact of both Sc2O3 and Al2O3 oxides on their solubility and electrical conductivity of molten mixture. A thermo analysis (TA) was used for liquidus temperature measurements. An oxide solubility was determined by the TA along with isothermic saturation method. An electrical conductivity was measured in the capillary (BN) cell and in the cell with parallel Mo-electrodes using an impedance technique. The obtained results testified that the Sc2O3 behaved similar to the Al2O3. Its additions to the molten cryolite decreased the liquidus temperature and the electrical conductivity. The Sc2O3 solubility in the NaF-AlF3 melt with molar ratio of NaF/AlF3=2.3 was found to be about 10 wt% at 980 ᵒC. The Al2O3 solubility in the scandium-containing cryolite decreased with the Sc2O3 concentration rise. The total solubility of both oxides, in controversial to known data, was determined to be slightly higher than the solubility of each separate oxide in the NaF-AlF3 melt.

The ability to dissolve alumina makes the sodium cryolite melt applicable for the primary aluminum production despite the high liquidus temperature of the melt. Numerous cryolite based compositions are used by aluminum smelters nowadays. The majority of industrial baths contain various salt additions intended to improve physical and chemical properties of the melt, as well as the overall process parameters. Nevertheless, the question of an appropriate medium for the inert anode application remains relevant in the aluminum industry because of high corrosion activity of fluoride melts at temperatures around 950 ºC. Lowering the operating temperature by means of reducing the liquidus temperature seems an advantageous approach to extend the implementation of the technology with the oxygen evolving inert anode. Recently KF-NaF-AlF3 melts with low cryolite ratio have been recognized a prospective medium for the new environmentally benign technology for the primary aluminum production [1,2]. The change in the chemical composition of the melt affects noticeably its physical and chemical properties. The electrolyte density is one of the properties dramatically affecting the technological parameters of the electrolytic cell. The density of conventional electrolytes for the aluminum production has been well studied [3,4] however, the need to scrutinize the physical and chemical properties of low melting electrolytes is clearly warranted. Yan with coauthors [5] addressed the KF-NaF-AlF3 melt with a different NaF content at (([KF] + [NaF])/[AlF3]) 1.3 and 1.41 but data presented concerning the influence of alumina and calcium fluoride are scarce. The aim of the present work was to study the density and molar volume of the KF-NaF-AlF3 system at CR=1.3, 1.5, with Al2O3 and CaF2 additions by the Archimedean method in the temperature range 920–1150 K. In the present work the cryolite ratio was defined as a ratio between molar concentrations of the alkali metal fluoride and aluminum fluoride (CR=NMF/NAlF3) – for the mixed cryolite melts (KF-NaF-AlF3) was calculated as CR=(NKF+NNaF)/NAlF3. Analysis of the data obtained for the density of low-melting cryolite mixtures illustrates the existence of some general regularities and specific features of the melts under study. Over a broad interval of the SCR, the molar volume of the melt rises upon the replacement of sodium fluoride with the potassium one (Fig. 1). The molar volume was shown to decrease with the CR rise. The calcium fluoride additives cause the density increase and molar volume depression (Fig. 2). The alumina additions up to 6 mol.% did not show to significantly change either the density or the molar volume of the melts. References Redkin, A.; Apisarov, A.; Dedyukhin, A.; Kovrov, V.; Zaikov, Yu.; Tkacheva O.; and Hryn J. Recent Developments in Low-Temperature Electrolysis of Aluminum. ECS Trans. 2013, 50 (11), 205-213. Tkacheva, O. J. Hryn, J. Spangenberger, B. Davis, T. Alcorn Operating parameters of aluminum electrolysis in KF-AlF3 based electrolytes. Light Metals. – 2012. - P. 675-680 Fellner, P.; Haarberg, G.M.; Hives, J.; Kvande, H.; Sterten A.;, Thonstad, J. Aluminium Electrolysis. 3 rd Edition – Aluminium; Verlag: Dusseldorf, 2001. Fernandez, R.; Ostvold, T. Surface Tension and Density of molten Fluorides and Fluoride Mixtures Containing Cryolite. Acta Chemica Scandinavica 1989, 43, 151-159. Yan, H.; Yang, J.; Li, W. Surface Tension and Density in the KF-NaF-AlF3-Based Electrolyte. J. Chem. Eng. Data 2011, 56, 4147-4151.

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