NaCl KCl Research Articles

Study on diffusion kinetics of chromium and nickel electrochemical co-deposition in a NaCl–KCl–NaF–Cr2O3–NiO molten salt

Abstract The process of preparing surface composite by molten salt co-deposition is the result of the mass transfer of active particles in molten salt, electrochemical reduction, and solid diffusion. In this study, we prepared Cr–Ni alloy/low-carbon steel surface composites in NaCl, KCl, NaF, Cr2O3, and NiO melt salt system successfully, and analyzed the entire diffusion dynamics process, aiming to find out the limiting links and provide ideas for further improving the preparation efficiency. The results show that chromium and nickel ions are simultaneously reduced on the cathode surface through two and one steps, respectively. And an alloy layer with Fe content of 64.52 wt%, Ni content of 28.96 wt%, and Cr content of 6.52 wt% is formed on the surface of low-carbon steel substrate. The average diffusion coefficients of chromium and nickel atoms in the surface composites are 1.16 × 10−14 and 1.44 × 10−14 m2·s−1. The mass transfer process in molten salt is the limiting link in the whole preparation process.

Electrodeposition of Niobium Coatings on the Cryogyroscope Rotor

The discharge process of Nb(IV) complexes to Nb was studied by cyclic voltammetry. A galvanostatic electrolysis with the cathodic current density of 1.5·10−2 A cm−2 in the NaCl–KCl–NaF–K2NbF7 melt in contact with Nb anode was used for deposition of niobium coatings on the cryogyroscope rotor at temperature 1023 K, and time of process from 8 to 12 h. The coatings were continuous and smooth due to the special design of the cathode. The rotation speed of the stirrer was set to 35 rpm and did not change in all experiments. Such conditions made it possible to obtain coatings with the thickness up to 150 μm. The main characteristics of the niobium coatings such as purity, roughness, nonsphericity, and superconductivity were investigated.

Electrochemical synthesis of a hydrogen storage material based on lanthanum and nickel intermetallics in the equimolar KCl–NaCl melt

Co-reduction of lanthanum and nickel ions on inert tungsten and active nickel electrodes were studied using the methods of cyclic chronovoltammetry, chronopotentiometry and open circuit chronopotentiometry in equimolar KCl–NaCl at 973 K. When both lanthanum and nickel ions are present in equimolar KCl–NaCl, the voltammetry dependences demonstrate the nickel ions reduction wave in the region of potentials –(0.00–0.1) V and lanthanum ions reduction wave in the region of potentials –(1.85–1.9) V relative to a silver chloride reference electrode. There are two more reduction waves on the opposite sides of voltammograms in the regions of potentials –(1.5–1.6) V and –(1.75–1.8) V. These waves are associated with the reduction of lanthanum ions and their depolarization on metallic nickel preliminarily deposited on the tungsten electrode, which results in the formation of intermetallic LaxNiy phases. The (E–t) dependences of open circuit chronopotentiometry demonstrate a delay plateau, which corresponds to the time required for dissolution of separate phases of intermetallic compounds. LaNi2 and LaNi5 intermetallic phases were electrochemically synthesized at a definite concentration of lanthanum and nickel chlorides in equimolar KCl–NaCl. The synthesized lanthanum and nickel intermetallic compounds were characterized by the X-ray diffraction analysis, scanning electron microscopy and photon correlation spectroscopy.

Synthesis of CaCu3Ti4O12 Micron-cubes and rods via high proportion molten salt method

CaCu3Ti4O12 (CCTO) particles were produced from a CuO–CaCO3–TiO2 peroxo-hydroxide precursor material in NaCl–KCl and Na2SO4–K2SO4 salt mixtures via the molten-salt synthesis method at different salt-to-precursor mass ratios. Regular-shaped CCTO particles of cubes, rods, and polyhedrons can be obtained at large salt-to-precursor mass ratios of above 50:1. With the extension of sintering time, the particle shape is more regular and the size is larger. Long micro rods with a length of about 53 μm can be obtained at a mass ratio of 125:1 and a long sintering time of 72 h in sulfate salts. The formation mechanisms are also discussed and the results suggest that a large salt-to-precursor mass ratio may provide a sufficient number of Na − K ions to sufficiently modify the particle shape and form regular-shaped cubes and rod-like particles. At the same time, CCTO ceramics synthesized by Na2SO4–K2SO4 molten-salt method show good dielectric properties, with a dielectric constant higher than 104 and a loss factor less than 0.45 in the range of 20 Hz to 1 MHz.

Binary molten salt in situ synthesis of sandwich‐structure hybrids of hollow β‐Mo2C nanotubes and N‐doped carbon nanosheets for hydrogen evolution reaction

AbstractFocused exploration of earth‐abundant and cost‐efficient non‐noble metal electrocatalysts with superior hydrogen evolution reaction (HER) performance is very important for large‐scale and efficient electrolysis of water. Herein, a sandwich composite structure (designed as MS‐Mo2C@NCNS) of β‐Mo2C hollow nanotubes (HNT) and N‐doped carbon nanosheets (NCNS) is designed and prepared using a binary NaCl–KCl molten salt (MS) strategy for HER. The temperature‐dominant Kirkendall formation mechanism is tentatively proposed for such a three‐dimensional hierarchical framework. Due to its attractive structure and componential synergism, MS‐Mo2C@NCNS exposes more effective active sites, confers robust structural stability, and shows significant electrocatalytic activity/stability in HER, with a current density of 10 mA cm−2 and an overpotential of only 98 mV in 1 M KOH. Density functional theory calculations point to the synergistic effect of Mo2C HNT and NCNS, leading to enhanced electronic transport and suitable adsorption free energies of H* (ΔGH*) on the surface of electroactive Mo2C. More significantly, the MS‐assisted synthetic methodology here provides an enormous perspective for the commercial development of highly active non‐noble metal electrocatalysts toward efficient hydrogen evolution.

Synthesis of Ti3SiC2 MAX phase powder through molten salt method

AbstractTitanium silicon carbide (Ti3SiC2) MAX phase powder was synthesized from elemental reactants using the molten salt synthesis (MSS) method. Optimum experimental parameters were also investigated to determine the purity and synthesis pathway of the Ti3SiC2 MAX phase. The results showed that Ti3SiC2 was not synthesized using carbon black as the carbon source in the starting materials because of the high quantity of TiC formed along with the TiSi2 silicide phase. However, Ti3SiC2 was successfully synthesized in a relatively high purity (93%) at 1200°C for 2 h using graphite as the source of carbon because of the formation of TiC and Ti5Si3 intermediate phases. The Ti5Si3 silicide phase was found to play a crucial role in the formation of the Ti3SiC2 MAX phase using the MSS method. Moreover, applying a pressure of 150 MPa to the prepared samples and using the eutectic mixture of NaCl–KCl (molar ratio: 1:1) instead of NaCl also resulted in the higher formation of the Ti3SiC2 MAX phase. The formation mechanism of Ti3SiC2 was determined to be the reaction among Ti5Si3, TiC, and residual carbon through the template‐growth and dissolution–precipitation mechanisms that occurred at different stages of the synthesis process.

Effect of Microchannel Diameter on Electroosmotic Flow Hysteresis

Electroosmotic flow (EOF) commonly involves inhomogeneous fluids in practical applications. EOF hysteresis, which is defined as direction-dependent flow behavior, has been extensively investigated for dissimilar solution pair systems. Hitherto, there is no investigation being conducted to examine the effect of microchannel diameter on the hysteresis phenomenon. In this investigation, current monitoring experiments and finite element numerical simulations were performed to examine the intensification of the hysteretic behavior with reduction in the microchannel diameter. Three solution pairs were selected for the study, namely KCl–NaCl (dissimilar ionic species with similar concentration), NaCl and KCl (similar ionic species but different concentrations) solution pairs, with microchannels of 5 μm and 100 μm internal diameters. EOF hysteresis augmentation for reduced channel diameter (i.e., 5 μm microchannel) is due to the coupling effect of the resultant wider/tighter interfacial width and the minority pH-governing ion-driven hysteresis, which was earlier discovered to be the origin of EOF hysteresis. This investigation provides an appropriate understanding of the channel dimensional effect on EOF behavior involving multiple fluids, and the outcomes can potentially be implemented on chemical and biological microfluidic systems with adjustable throughput.

Electrodriven Chemical Looping Ammonia Synthesis Mediated by Lithium Imide

The chemical looping process for ammonia synthesis (CLAS) has advantages in avoiding competitive adsorptions of nitrogen and hydrogen and optimizing the thermodynamics and kinetics of a reaction step by step. Nitrogen carriers with favorable thermodynamic and kinetic properties are critical for such a process, especially under mild conditions. Recent studies have shown that alkali- and alkaline-earth-metal imides are attractive nitrogen carriers; however, the equilibrium concentration of NH3 in the hydrogenation of imides to ammonia and hydrides is low at atmospheric pressure. Moreover, the reaction rate of nitrogen fixation on the hydrides is slow. Here, we show that an electrodriven CLAS (ECLAS) mediated by a lithium imide is feasible in a LiCl–NaCl–KCl eutectic electrolytic cell with a nickel-foam electrode. We demonstrate that the reactions of lithium imide hydrogenation and lithium hydride nitridation are both promoted by the input of electric energy. The average ammonia production rate over such an electrodriven CLAS is ca. 8 times higher than that of a thermal-driven CLAS.

Effect of Chlorine Salts on TiC Coating Morphology and Bonding Strength of Diamond Surface in Molten Salt Reaction

To obtain a high-quality TiC coating on the diamond (D) surface and improve the interfacial properties of metal matrix composites, the optimized molten salt method is used to prepare TiC coatings on the D surface. The effects of NaCl, KCl, and NaCl–KCl molten salts on the uniformity, surface morphology, composition, and bonding ability of TiC coatings are investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction, and cold–hot cycling experiments. The results show that the boiling and pickling processes after the molten salt reaction can effectively remove the residual chloride salts and Ti powders to obtain clean TiC-coated D. The TiC particles of the coating surfaces prepared by NaCl and KCl molten salts are finer and more uniform, which is due to their higher melting points and shorter real reaction time compared with NaCl–KCl mixed salts. The TiC coating prepared by KCl has the best bonding ability and can withstand 50 times as many cold–hot cycles due to its uniform fine deposition particles and appropriate thickness. And, during cold–hot cycles, it can be found that the TiC coating cracking and shedding occur primarily at the edges and corners of the D with internal thermal stress concentration.

Molybdenum electrodeposition in NaCl–KCl–MoCl3 melt using pulse electrolysis

Electrodeposition of molybdenum in the NaCl–KCl–MoCl3 (10 wt% of Мо) melt was studied by the pulsed electrolysis at 780 °C. The compact Mo deposits were obtained. The electrolyte deposits were studied using scanning electron microscopy and X-ray diffraction analysis. It was found that the solid Mo cathode deposits of the primarily (110) orientation may be obtained by the pulse electrolysis at the cathode current (ic) ranging from 2 up to 6 А/cm2, average cathode current density (im) ranging from 0.286 up to 0.875 А/cm2 and ratio of the relaxation time to the pulse time (Tr/Tc) ranging from 6 up to 18. The influence of the electrodeposition conditions on the current efficiency at the pulse electrolysis was studied. The pulse electrolysis parameters allowing the electroreduction with 100% current efficiency are determined. An increase in the cathode pulse time (Tc) was found to result in a decrease in the current efficiency of the im and Tr/Tc constants. An increase of the cathode current density (ic) as well as a decrease of the cathode pulse time (Tc) lead to a decrease of the crystal dimensions. The optimization of the im and Tr/Tc parameters increases the electrodeposition rate by factor 5 comparing to the galvanostatic electrolysis.

Formation of a nanoporous layer on the surface of silver and copper by dealloying of thermal diffusion coatings Me5Zn8 (Me=Ag, Cu) in a deep eutectic solvent

ABSTRACT Selective anodic dissolution of pre-galvanized surfaces was used to obtain nano-porous layers of approximately 10 microns thick on copper and silver specimens. To achieve single-phase homogeneous thermal diffusion layers of Cu5Zn8 and Ag5Zn8, we used a zinc plating process carried out in molten eutectic (KCl–NaCl–ZnCl2) at Т = 370°С. Dealloying of these coatings in a deep eutectic solvent (choline chloride/urea + 0.1 mol/l ZnCl2) at T = 133°C produced a typical bi-continuous structure of pores and ligaments (100 nm) on the surface of the silver and the copper.

Joint Electroreduction of La3+ and Ni2+ Ions in the Equimolar KCl–NaCl Melt at 973 K

Joint Electroreduction of La3+ and Ni2+ Ions in the Equimolar KCl–NaCl Melt at 973 K

Sintering Properties of Tetragonal Zirconia Nanopowder Preparation of the NaCl + KCl Binary System by the Sol–Gel–Flux Method

Tetragonal zirconia nanopowders (3Y-TZP NPs) stabilized by 3 mol % Y2O3 were synthesized by a novel sol–gel–flux method. The effects of calcination temperature, Y2O3 content, and molten salt system (NaCl + KCl binary system and NaCl unary system) on the phase composition and morphology of nanopowders (NPs) were investigated. Meanwhile, the sintering properties of the prepared 3Y-TZP NPs were studied. The 3Y-TZP NPs and sintered ceramics were characterized by thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) analysis, and X-ray fluorescence (XRF) spectroscopy. The results show that the optimum calcination temperature for obtaining dispersed ZrO2 NPs is reduced from 800 to 660 °C when the NaCl unary system is substituted by the NaCl + KCl binary system. 3Y-TZP NPs with a single tetragonal phase are obtained in the NaCl + KCl binary system when Y2O3 content in the raw material is 5 mol %. The average particle size of the obtained ZrO2 NPs is reduced from 47.25 nm in the unary system to 26.18 nm in the binary system because of the lower calcining temperature. Furthermore, the 3Y-TZP NPs obtained in the binary system have narrower particle size distribution and better sintering performance than ZrO2 NPs obtained in the unary system. After sintering for 4 h at 1200 °C, the former generated 3Y-TZP ceramics with higher density and finer grain size. This work produced narrow-distributed well-dispersed 3Y-TZP NPs with quasi-spherical morphology, which are good candidates for the applications, including oxygen sensors and thermal barrier coatings.

Insights into the local structure evolution and thermophysical properties of NaCl–KCl–MgCl2–LaCl3 melt driven by machine learning

The local structure evolution and thermophysical properties of the NaCl–KCl–MgCl2–LaCl3 melt were thoroughly understood, which facilitates the advancement and innovation of molten salt electrolytic production for Mg–La alloys.

CAPACITANCE AND IMPEDANCE OF IRIDIUM ELECTRODE IN MOLTEN ALKALI CHLORIDES

Сapacitance of the iridium electrode was studied by the electrochemical impedance spectroscopy with variation of the main physical and chemical parameters: electrical potential, temperature, radius of the alkali cation. The influence of the signal frequency used in AC electrochemical methods on the capacitance value and the shape of the curve was also checked. Capacitance of the iridium electrode was obtained in molten sodium, potassium and cesium chlorides in the temperature range 1093–1123 K and the frequency range of the AC signal 3 · 100–3 · 104 Hz in the entire accessible range of electrical polarization. The obtained capacitance curves have two main minima with a maximum between them. One of these minima (cathodic one) was identified as the classical potential of minimum capacitance. A decrease in the signal frequency and the temperature of experiment, as well as an increase in the cation radius in the NaCl–KCl–CsCl order, leads to the appearance of one or two additional minima in the potential region between the main minima. The depth of these intermediate minima increases and their potential shifts in the positive direction with an increase in the radius of the alkali metal cation of the salt electrolyte. The calculated values of the capacitance of the electrical double layer and the adsorption capacitance were obtained by the method of equivalent electrical circuits. One of the additional minima obtained by direct measurement of the dependence of the electrode capacitance on the potential at a high AC frequency corresponds to the calculated capacitance of the double layer. The other additional minimum obtained at a low AC frequency corresponds to the calculated adsorption capacitance.

Corrosion behaviors and protective layer stabilization mechanism of 316 stainless steel in NaCl–KCl–NaF reciprocal salts at high temperatures

Corrosion tests of 316 stainless steel (316SS) in NaCl–KCl–NaF reciprocal salts were conducted at different temperatures. Results showed that the reciprocal salts had perfect compatibility with 316SS even without special treatment, and a stable Cr–O enriched layer was first observed. The stabilization mechanism of chromium oxides in molten salts was explored, where the low oxoacidity was conducive to from the stable chromium oxides. These findings can provide a new direction for the design and corrosion control of molten salts.

Structural diversity in salt excreting glands and salinity tolerance in Oryza coarctata, Sporobolus anglicus and Urochondra setulosa

Unlike the bicellular glands characteristic of all known excreting grasses, unique single-celled salt glands were discovered in the only salt tolerant species of the genus Oryza, Oryza coarctata. Salt tolerance has evolved frequently in a large number of grass lineages with distinct difference in mechanisms. Mechanisms of salt tolerance were studied in three species of grasses characterized by salt excretion: C3 wild rice species Oryza coarctata, and C4 species Sporobolus anglicus and Urochondra setulosa. The leaf anatomy and ultrastructure of salt glands, pattern of salt excretion, gas exchange, accumulation of key photosynthetic enzymes, leaf water content and osmolality, and levels of some osmolytes, were compared when grown without salt, with 200mM NaCl versus 200mM KCl. Under salt treatments, there was little effect on the capacity for CO2 assimilation, while stomatal conductance decreased with a reduction in water loss by transpiration and an increase in water use efficiency. All three species accumulate compatible solutes but with drastic differences in osmolyte composition. Having high capacity for salt excretion, they have distinct structural differences in the salt excreting machinery. S. anglicus and U. setulosa have bicellular glands while O. coarctata has unique single-celled salt glands with a partitioning membrane system that are responsible for salt excretion rather than multiple hairs as previously suggested. The features of physiological responses and salt excretion indicate similar mechanisms are involved in providing tolerance and excretion of Na+ and K+.

Fabrication plate-like BaBi4Ti4O15 single-crystalline particles by the molten salt synthesis method

Plate-like single-crystalline BaBi4Ti4[Formula: see text] particles were synthesized by the molten salt synthesis (MSS) method. The effects of sintering temperature, holding time, and NaCl–KCl molten salt content on the phase structure and morphology of plate-like BaBi4-Ti4[Formula: see text] particles were investigated. The results show that plate-like BaBi4Ti4[Formula: see text] particles can be synthesized when the sintering temperature is above 800[Formula: see text]C. The size of particles increases with increasing sintering temperature and molten salt content. Largely anisotropic plate-like BaBi4Ti4[Formula: see text] particles with diameter [Formula: see text]10[Formula: see text]m and thickness of [Formula: see text]0.3 [Formula: see text]m can be obtained under the optimum process parameters. The crystal structure of BaBi4Ti4[Formula: see text] was determined as A21am by TEM, which should be attributed to the Bi[Formula: see text] and Ba[Formula: see text] diffusing into [TiO6] octahedrons.

Evolution of micro-pores in Ni–Cr alloys via molten salt dealloying

Porous materials with high specific surface area, high porosity, and high electrical conductivity are promising materials for functional applications, including catalysis, sensing, and energy storage. Molten salt dealloying was recently demonstrated in microwires as an alternative method to fabricate porous structures. The method takes advantage of the selective dissolution process introduced by impurities often observed in molten salt corrosion. This work further investigates molten salt dealloying in bulk Ni–20Cr alloy in both KCl–MgCl2 and KCl–NaCl salts at 700 ℃, using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction (XRD), as well as synchrotron X-ray nano-tomography. Micro-sized pores with irregular shapes and sizes ranging from sub-micron to several microns and ligaments formed during the process, while the molten salt dealloying was found to progress several microns into the bulk materials within 1–16 h, a relatively short reaction time, enhancing the practicality of using the method for synthesis. The ligament size increased from ~ 0.7 μm to ~ 1.3 μm in KCl–MgCl2 from 1 to 16 h due to coarsening, while remaining ~ 0.4 μm in KCl–NaCl during 16 h of exposure. The XRD analysis shows that the corrosion occurred primarily near the surface of the bulk sample, and Cr2O3 was identified as a corrosion product when the reaction was conducted in an air environment (controlled amount sealed in capillaries); thus surface oxides are likely to slow the morphological coarsening rate by hindering the surface diffusion in the dealloyed structure. 3D-connected pores and grain boundary corrosion were visualized by synchrotron X-ray nano-tomography. This study provides insights into the morphological and chemical evolution of molten salt dealloying in bulk materials, with a connection to molten salt corrosion concerns in the design of next-generation nuclear and solar energy power plants.

Micromorphology and texture of niobium coating electrodeposited in NaCl—KCl—CsCl molten salt system

A low-toxicity and environment-friendly NaCl—KCl—CsCl—K2NbF7 system was used to prepare Nb coatings on Mo substrates. The effects of temperature, current density and electrodeposition time on the micromorphologies and textures of the electrodeposited Nb coatings were studied. The results showed that Nb coatings obtained at 30—70 mA/cm2 in the temperature range of 700—750 °C were continuous and compact, with a hardness range of 2.16—2.45 GPa. As the columnar crystals grew with time, the preferential growth orientations of the Nb coatings changed from 〈200〉 to 〈211〉 and then became disordered. With increasing polarization, the morphologies of the Nb coatings changed from hexagonal star-like surface to conical or pyramid-like surface.