Russian Metallurgy (Metally)最新文献

Abstract—The solid-phase synthesis of indium–gallium–zinc oxide (IGZO) with the formula InGaZnO₄ is studied. The synthesis is performed from the In₂O₃, Ga₂O₃, and ZnO oxides at a temperature of 1400°C for 24 h in an air atmosphere and is followed by rapid air cooling. The single-phase state of the resulting ceramic pellet is confirmed by X-ray diffraction, infrared Fourier transmission spectroscopy, and Raman spectroscopy. The dimensional characteristics, the morphology of the synthesized sample, and the distribution of elements are estimated using scanning electron microscopy and energy dispersive X-ray spectroscopy. The unit cell parameters are determined using the Rietveld full-profile analysis.

The equilibrium potentials of platinum in a KCl–PbCl₂—PtCl₂ melt at various temperatures and platinum dichloride contents are measured by the emf method. The empirical equations of the isotherms and polytherms of the equilibrium potentials of platinum are derived. The conditional standard potentials of platinum in a molten mixture of potassium and lead are calculated. The changes in the Gibbs energy upon platinum dichloride formation from elements in the melt under study are determined. The separation factors of the metals from the alloys are estimated. The obtained results indicate prospects of the separation processes of lead and noble metals in chloride melts.

Abstract—The evolution of the morphological perturbations that form at the solid–liquid interface in the presence of a convective–conductive heat transfer mechanism is studied. A mathematical model is formulated for convective–conductive heat transfer; it takes into account small sinusoidal deviations of the interface shape from plane geometry. An analytical solution describing the undisturbed ground state of a solidifying material has been found. The evolution of small sinusoidal morphological interface perturbations can cause an unstable solidification mode, which was studied using a linear analysis of morphological stability. Equations describing the evolution of small morphological interface and temperature perturbations are derived by expanding boundary conditions into Taylor series on the surface between liquid and solid phases. An analysis of the equations derived for interface and temperature perturbations allowed us to determine a dispersion relation, which is the relation between the perturbation frequency and wavenumber. At a zero perturbation frequency, the dispersion relation determines a neutral stability curve. Model calculations are performed for two systems (metal, magma) in the presence of a liquid phase flow along a solidified material. The calculations performed show that the perturbation frequency changes its sign with a wavenumber. This confirms the existence of two different solidification modes with a stable flat interfacial boundary of molten and solidified materials and an unstable wavelike interface. The maximum morphological perturbation frequency, which determines the most dangerous rapidly growing perturbations, has been found. The developed theory has a limiting transition to the previously constructed theory of a purely convective heat transfer mechanism in the liquid phase of a system.

Abstract—Molybdenum and molybdenum-containing alloys are used as structural materials in various high-temperature processes, including the operation of molten-salt reactors and pyrochemical technologies of processing spent nuclear fuel from fast neutron reactors. Tellurium is a fission product, which actively interacts with structural materials. The aim of this work is to determine the thermodynamic characteristics of tellurium–molybdenum alloys. For this purpose, the potential differences between molybdenum and molybdenum–tellurium alloys of various compositions are determined. The potentials are compared to the Mo–Te phase diagram, and the thermodynamic characteristics of the formed compounds are calculated. The activity of molybdenum in the alloys and the change in the Gibbs free energy of formation of the Mo₃Te₄ and α‑MoTe₂ intermetallics at 500 and 600°C are determined.

Abstract—Spent nuclear fuel (SNF) processing is strongly difficult because of high radioactivity of its actinides including the most active americium and curium. Primary experiments on their extraction using nonradioactive imitators are preferable, but the choice of them is impeded by a necessity to obey a broad range of physicochemical properties with imitated elements. In this work, the choice is substantiated and the most appropriate imitators are proposed on the basis of literature data to study the separation of americium and curium in chloride melts. Europium and gadolinium can serve as imitators for chemical processes in the molten LiCl–KCl eutectic, but a high difference of potentials of americium and curium does not allow their use for studying the electrochemical kinetics. The arbitrary potentials and electroreduction kinetics of a series of metals (Mg, Sc, Sr, Ba, Pr, Nd, Ce, Eu, and Gd) in the chloride melts at various temperatures are analyzed. A high convergence of the reduction potentials of magnesium and scandium with those of the imitated elements assumes their application as electrochemical imitators for studying separately americium and curium (without their simultaneous presence in the solution). In spite of the resembling reduction potentials, the combined use of the Mg/Sc pair for the study of this process seems doubtful, since the electrochemical kinetics of these elements differs strongly from the kinetics of the target actinides. The Nd/Ce pair can serve as a more suitable pair for the combined study of the electrochemical kinetics of americium and curium due to the high affinity of both chemical and electrochemical properties to the imitated elements. A comparison of the electrochemical kinetics of cerium and curium reduction in the chloride melts shows that both elements exist in the melt in the oxidation state 3+ and are reduced in one step. In turn, both neodymium and americium are reduced in two steps, and the transition from the oxidation state 3+ to 2+ begins for both elements in a potential range of –1.5 V versus the silver chloride reference electrode.

The interaction between uranium trichloride and molten LiCl–Li₂O in the eutectic LiCl–KCl melt at 773, 823, and 873 K is investigated using potentiometry in order to check the completion of UCl₃ precipitation reaction. Measuring the equilibrium U³⁺/U couple potential is found to allow the precipitation of UCl₃ to be checked in real time. Quantitative precipitation is observed at a UCl₃ : Li₂O molar ratio of 1 : 2; in this case, the uranium electrode potential shifts to the negative region by more than 300 mV relatively to equilibrium. Using the obtained experimental data, the apparent standard potentials of the U³⁺/U couple are determined, and the activity coefficients of UCl₃ in the eutectic LiCl–KCl melt are calculated. The activity coefficients of UCl₃ at 773, 823, and 873 K are 5.4 × 10^–4, 9.1 × 10^–4, and 4.8 × 10^–3, respectively.

Abstract—One of the methods for high-temperature treatment (HTT) of nitride spent nuclear fuel (SNF) of fast neutron reactors is its direct oxidation. At this stage of processing, it is desirable to remove radioactive cesium, which is in the form of uranates in an oxidizing atmosphere, from SNF. The behavior of the Cs₂U₂O₇ cesium diuranate upon heating to 1000°C is investigated by of synchronous thermal analysis, isothermal holding, and vacuum sublimation. Samples were analyzed by X-ray diffraction, microstructural, and chemical analyses. The data of thermal analysis and isothermal holding at 150, 400, 700, and 900°C indicate that the decomposition of cesium diuranate to form monouranate and volatile products begins at temperatures of 150–200°C, the complete conversion of diuranate into monouranate ends by 700°C, and complex uranium oxides form. The sample mass loss upon holding at 700°C is ~8%. At 900°C, monouranate partially decomposes into diuranates, complex polymer uranates, and volatile products. The total mass loss is ~14%. During high-temperature vacuum sublimation at 950°C in a vacuum of 2 × 10^–2 mmHg, cesium diuranate decomposes into monouranate, uranium(IV) oxide, polyuranates, and volatile products; the mass loss does not exceed 20%; and the main sublimate component is metallic cesium (>99%). Thus, cesium is incompletely removed during direct oxidation of nitride SNF, and the resulting cesium uranates would significantly complicate HTT.

The mechanism of reactions of zirconium oxide with the melts based on alkaline metal fluorides is studied. The solubilities and dissolution rates of ZrO₂ in KF–AlF₃ and NaF–AlF₃ melts under natural convection conditions are measured. According to the results obtained, the limiting content of ZrO₂ in the KF–AlF₃ ([KF]/[AlF₃] = 1.3) melt at 750°C is 0.69 ± 0.10 mol % and that in the NaF–AlF₃ ([NaF]/[AlF₃] = 1.3) melt at 800°C is 0.54 ± 0.04 mol %. The dissolution rate of weighed samples of ZrO₂ in amounts of 0.5, 1.0, and 1.5 wt % in the KF–AlF₃ melt at 750°C and in the NaF–AlF₃ melt at 800°C is 0.022 and 0.017 mol/min, respectively. The liquidus temperatures of the melts under study are measured at different contents of ZrO₂ and Al₂O₃, molar ratios, and cationic compositions of the mixture. The addition of both zirconium and aluminum oxides and the partial replacement of KF by NaF with the retention of the ([KF] + [NaF])/[AlF₃] molar ratio are found to result in an increase in the liquidus temperatures of the melts under study. The study of the thermal effects of the melts by differential scanning calorimetry (DSC) shows that regardless of the [KF]/[AlF₃] ratio the thermal effect onset in the KF–AlF₃ melts occurs at 542°C and that in the KF–NaF–AlF₃ system takes place at 597°C, which is related to the melting of the KAlF₄ and K₂NaAl₃F₁₂ phases, respectively. An insignificant (1.3%) mass loss is observed on heating the KF–AlF₃ mixture to 900°C, which is assigned, according to the mass spectroscopy data, to the partial evaporation of KAlF₄. According to the X-ray diffraction (XRD) data on the molten oxide–fluoride mixtures under study, the reaction of zirconium oxide with the fluoride melts affords fluorozirconates of the corresponding alkaline metals and more complicated complex compounds.

Abstract—The electrochemical reduction of U(III) ions to the metal in molten LiCl–KCl eutectic in a temperature range of 673–793 K on cadmium and gallium electrodes under an inert gas atmosphere is studied by cyclic voltammetry, square-wave voltammetry, and zero current potentiometry. Reagents containing no traces of moisture, oxygen, and products of their interaction are used in the experiments. All main procedures are carried out in a dry glove box under a purified argon atmosphere. As shown by cyclic and square-wave voltammetry, the cathodic reduction of uranium(III) ions on the active cadmium and gallium electrodes in the “electrochemical window” under study occurs with a depolarization of 0.2–0.4 V and depends on the cathode material. The potential shift to the electropositive range is found to be related to the formation of intermetallic compounds of uranium with the material of the active electrodes. The potentiostatic electrolysis of the melt at the potentials of the current peaks observed on the cyclic voltammograms using the active electrodes is conducted to identify the compositions of the cathodic deposits. The X-ray diffraction (XRD) data show that the intermetallic compound UCd₁₁ is formed on the cadmium electrode and UGa₃ and UGa₂ are formed on the gallium electrode. SEM image and EDS analysis of the sample surface confirm the presence of fine fragments of the U–Cd and U–Ga intermetallic compounds. The conditions for the formation of alloys of a specified composition are determined by potentiostatic electrolysis. The equilibrium potentials of the U–Cd and U–Ga alloys are measured by zero current potentiometry, and the temperature dependences of the apparent standard potentials of the alloys are calculated. The electrochemical extraction of uranium from the LiCl–KCl–UCl₃ melt is studied. The determined degree of extraction of uranium from the electrolyte on the active liquid electrodes at various electrolysis times exceeds 97%.

The possibilities of conducting a direct mass spectral determination of Li, Be, Mg, Sr, Zr, Mo, Ru, Rh, Pd, Ag, Cs, Ba, La, Ce, and Nd in the products of processing spent nuclear fuel based on metallic uranium or its oxides are discussed. The optimum method of sample preparation is selected to ensure a quantitative transition of all elements into a working solution. The influence of the uranium content on the analytical signal intensity and the detection limits of the elements to be detected is estimated. The presence of 1 g/L uranium in a solution is found to decrease the analytical signal intensity by 50 times and worsens the detection limits of individual analytes by two orders of magnitude compared to these indicators in uranium-free solutions. The possibility of using internal standards to correct matrix effects is shown.