Measurements and analysis of oxygen bubble distributions in LiCl–KCl molten salt

Transparent system experimental studies have been performed to provide measurement and analysis of oxygen bubble distributions at different sparging rates in LiCl–KCl molten salt at 500 °C using a high-speed digital camera and an oxygen sensor. The results reveal that bubble sizes and rise velocities increased with an increase in oxygen sparging rate. The bubbles observed were ellipsoidal in shape, and an equivalent diameter based on the ellipsoid volume was calculated, ranging from 0.00263 m to 0.00407 m. Results also show that the bubble equivalent diameters are normally distributed. A Fanning friction factor correlation was used to predict a bubble's rise velocity. The oxygen mass transfer coefficients were calculated using the oxygenation model; these values were on the order of 10⁻⁴ m/s and followed a decreasing trend corresponding to an increasing bubble size and sparging rate. The diffusivities were calculated based on two different approaches—one based on physics of the bubbles and the other on systematic properties. The diffusivity values calculated from bubble physics are 1.65 × 10⁻⁹ m²/s to 8.40 × 10⁻⁹ m²/s, which are within the range suggested by literature for gases in liquids of a similar viscosity.     

The formation mechanism of nanocrystalline TiC from KCl–LiCl molten salt medium

In this research, an attempt was made to artificially synthesis bulk nanocrystalline TiC using molten salt bath medium. To do so, titanium powder and carbon black (CB) were mixed as precursors with prepared KCl–LiCl eutectic composition and the effect of the synthesis temperature and dwelling time were studied on the final powder. The X-ray diffraction (XRD) patterns revealed that the increase of the temperature and dwelling time has a considerable effect on the purity and crystallinity of the final TiC. Based on results attained from simultaneous thermal analysis (STA) and microscopic micrographs (TEM and SEM), the mechanism of TiC synthesis was determined as “template growth”. The XRD and STA analyses results showed that by increasing the temperature up to 950 °C, some intermediate phases such as LiTiO₂ and TiCl_x (x = 2 and 3) were formed. The results of Rietveld refinement showed that with increasing the temperature from 815 to 950 °C, the crystallite size of the final product decreased from 75 to 30 nm and the lattice parameter of TiC powder increased from 4.298 to 4.324 Å (The lattice parameter of TiC is 4.327 Å). The Energy Dispersive X-ray Spectroscopy (EDS) declared that the nanocrystalline TiC synthesized at 950 °C was with maximum accordance with the composition of the stoichiometric compound.     

Electrochemical behaviour of LaCl3 at tungsten and aluminium cathodes in LiCl–KCl eutectic melt

The electrochemical behaviour of lanthanum was studied at inert tungsten electrode and reactive aluminium electrode in LiCl–KCl eutectic melt in the temperature range 698–798 K using transient electrochemical techniques. Reduction of La(III) to La(0) at the tungsten electrode takes place in a single step. The reduction shows quasi-reversible behaviour for polarization rates, 25 ≤ ν ≤ 150 mV s^?1 and is predominantly controlled by charge transfer of La(III) ions for scan rates higher than 75 mV s^?1. The heterogenous rate constant of the process was estimated from impedance spectroscopy and from the semi-integrals of the cyclic voltammograms. The redox potential of the La(III)/La couple at the Al electrode was observed to be more positive than that at the inert electrode. This potential shift is due to the lowering of the activity of La in the metal phase caused by the formation of the intermetallic compound Al₁₁La₃. Thermodynamic properties such as Gibbs energy of formation of Al₁₁La₃, excess Gibbs energy and the activity coefficient of La in Al were calculated from the open circuit potential measurement.     

Molten salt corrosion of high density graphite and partially stabilized zirconia coated high density graphite in molten LiCl–KCl salt

Corrosion studies were performed on uncoated high density graphite and plasma sprayed partially stabilized zirconia (PSZ) coated high density graphite with NiCrAlY bond coat in molten LiCl–KCl eutectic salt at 600 °C for periods of 250 h, 1000 h and 2000 h under inert argon atmosphere. High density graphite showed weight loss while PSZ coated high density graphite showed weight gain. There is no significant attack and degradation of top PSZ coating in molten salt, however microcracks were observed at the bond coat-substrate interface after 2000 h of exposure. PSZ coated high density graphite exhibited excellent corrosion resistance in molten LiCl–KCl salt due to chemical stability and absence of phase transformation as confirmed from scanning electron microscopy, X-ray diffraction and laser Raman studies, however adhesion of the coating has to be improved.     

KCl–LiCl molten salt synthesis of LaOCl/CeO2-g-C3N4 with excellent photocatalytic-adsorbed removal performance for organic dye pollutant

A ternary photocatalyst of LaOCl/CeO₂-g-C₃N₄ was designed and firstly synthesized by a facile KCl–LiCl molten salt method. The 60 %LaOCl/40 %CeO₂-g-C₃N₄ with a 6: 4 weight ratio of LaOCl to CeO₂ has an optimal degradation ratio (nearly 100%) for methylene blue (MB) with an impressive cyclic stability. Both of adsorption and photodegradation contribute to the high MB removal ratio, and kinetic study results show that the rate constant of photocatalytic degradation route is 0.0174 min^?1, which is less than that of adsorption process (2.4406 min^?1). The excellent adsorption performance of 60 %LaOCl/40 %CeO₂-g-C₃N₄ is attributed to its negative zeta potential (?35.40 mV) and larger average pore size. The photochemical characterizations suggest that the 60 %LaOCl/40 %CeO₂-g-C₃N₄ form a dual Z-scheme heterojunction, which enhances its separation efficiency of photogenerated e^-/h⁺ with an outstanding redox ability, and further promotes the photocatalytic activity.     

Corrosion protection of steel in molten Li2CO3–K2CO3 and Na2CO3–K2CO3 mixtures in a hydrogen-containing atmosphere

The electrochemical behaviour of TiN-, TiN–AlN-, Cr- and CrN-coated 316L stainless steel in molten Li₂CO₃–K₂CO₃ and Na₂CO₃–K₂CO₃ melts in a reducing gaseous atmosphere (10% H₂–90% N₂) was studied using voltammetry and scanning electron microscopy combined with energy-dispersed X-ray analysis in the temperature range of 600–730 C. To facilitate the identification of the electrochemical reactions the voltammetric behaviour of stainless steel, titanium, nickel and gold was also investigated. Voltammetric characteristics obtained at AlN–TiN coated electrodes showed no anodic reactions at potentials more negative than that of CO^2– ₃ oxidation. Cr- and CrN-coated electrodes demonstrated a suppressed anodic dissolution after the first steady state voltammetric cycle. The voltammograms obtained for the other electrodes studied displayed the corresponding anodic metal-dissolution waves. TiN, AlN, Cr and CrN coatings seem to be the most promising as corrosion-resistant materials for the anodic compartments of molten carbonate fuel cells.     

Electrodeposition of cohesive carbon films on aluminum in a LiCl–KCl–K2CO3 melt

Electrodeposition of carbon on an aluminum electrode was studied in a LiCl–KCl–K₂CO₃ melt. A cyclic voltammogram for an aluminum electrode indicates that the cathodic current is due to the reduction of CO₃^2– ions. Carbon films on aluminum substrates were obtained by potentiostatic electrolysis, and the cohesiveness of the films depended on the potential. SEM observations showed that the morphology of the deposited carbon film depends on the electrolytic conditions. Raman spectroscopy, XPS and XAES measurement showed that the film consisted of carbon in the sp² state.     

A study on the recovery of actinide elements from molten LiCl–KCl eutectic salt by an electrochemical separation

Pyroprocessing is a prominent way for the recovery of the long-lived elements from the spent nuclear fuel. Electrorefining is a key technology for pyroprocessing and generally composed of two recovery steps—deposition of uranium onto a solid cathode and the recovery of TRU (TRansUranic) elements. In this study, it was investigated on electrochemical separation of actinides to develop an actinide recovery system in a molten LiCl–KCl eutectic salt. In the electrorefining experiment, uranium was successfully separated from cerium. The effects of the anode material and the surface area were investigated during the electrolysis experiments for a more efficient electrorefining system. Anode potential decreased with an increasing anode surface area, however, an anode effect was observed in case of a complicated anode structure for high surface area. Glassy carbon was found to be the best anode material among the molybdenum, graphite, glassy carbon, and oxide materials. It was found that the solid cathode with a perforated ceramic container could be one of the candidates for a salt clean-up process to remove residual actinide elements in the salt after the recovery step.     

Electrode reaction of the Np3+/Np couple in LiCl–KCl eutectic melts

The electrochemical behaviour of the Np³⁺/Np couple in the LiCl–KCl eutectic salt was investigated by electromotive force measurements, cyclic voltammetry and chronopotentiometry in the temperature region between 723 and 823 K. The standard redox potential of the Np³⁺/Np couple vs Ag/AgCl (1.00 wt %) was measured and given by the equation, E _Np ³⁺ _/Np ^° = –2.0298 + 0.000706 T, where E is in V and T in K. The electrode reaction of the Np³⁺/Np couple was almost reversible under the conditions studied. The diffusion coefficient of Np³⁺, D _Np ³⁺, in the LiCl–KCl eutectic melts between 723 and 823 K was represented by the equation, D _Np ³⁺ = 2.22 × 10^–6 – 6.88 × 10^–9 T + 5.60 × 10^–12 T ² cm² s^–1. The adsorption and desorption peaks of Np at the Mo working electrode caused by underpotential deposition were also observed in the cyclic voltammograms, and the work function of Np was evaluated as 3.04 eV by peak analysis of the cyclic voltammograms.     

Microstructural and microtextural investigations of boron nitride deposited from BCl3–NH3–H2 gas mixtures

The structural, morphological and textural characteristics of BN coatings processed by CVD from (BCl₃, NH₃, H₂) gas mixtures, at low pressure (P=1.3 kPa) and low temperature (T=800 °C), with different Q_NH3/Q_BCl3 gas flow rate ratios, have been investigated. Whereas the as-processed coatings are amorphous, a high degree of crystallisation can be achieved after a post-deposition heat treatment. The sole post-elaboration heat treatment does not allow the improvement of the crystallisation degree of the boron nitride. The presence of a small amount of oxygen, resulting from a simple exposure of the coating to a controlled atmosphere (temperature, moisture rate), is also necessary. For given temperature and pressure, a wide range of microstructures of the heat-treated BN coatings, from isotropic to anisotropic, can be observed by varying the Q_NH3/Q_BCl3 ratio.     

Dissolution behavior of silica in molten CaO–SiO2–Fe2O3–MgO–MnO slag

The modification of basic oxygen furnace (BOF) slag by adding silica can improve the properties of BOF slag for applications in the cement industry. The rapid dissolution of silica is essential to hot slag modification. In this work, the dissolution behavior of silica in the molten CaO–SiO₂–Fe₂O₃–MgO–MnO system as synthetic BOF slag was investigated by using the traditional rotating cylinder technique. Effects of rotation speed, temperature, immersion time, and slag basicity on the silica dissolution were studied. Scanning electron microscopy equipped with energy dispersive spectrometer (SEM-EDS) and FactSage simulations were employed to reveal the dissolution mechanism. It was found that the dissolution of the silica rod was affected by both the thermodynamic driving force and the slag viscosity. The silica dissolution rate in molten CaO–SiO₂–Fe₂O₃–MgO–MnO slag increased with increasing the rotation speed and temperature, but first increased and then decreased when decreasing the slag basicity from 2.5 to 1.5. A linear correlation between the logarithm of the dissolution rate and the logarithm of cylinder periphery velocity with a slope of 0.44 was observed, indicating the mass transfer within the boundary layer as the dissolution rate determining step. A direct dissolution way was found during the dissolution of silica in molten CaO–SiO₂–Fe₂O₃–MgO–MnO slag.     

Study on the preparation of Mg–Li–Mn alloys by electrochemical codeposition from LiCl–KCl–MgCl<Subscript>2</Subscript>–MnCl<Subscript>2</Subscript> molten salt

This study presents a novel electrochemical study on the codeposition of Mg, Li, and Mn on a molybdenum electrode in LiCl–KCl–MgCl₂–MnCl₂ melts at 893 K to form different phases Mg–Li–Mn alloys. Transient electrochemical techniques such as cyclic voltammetry, chronopotentiometry, and chronoamperometry have been used in order to investigate the codeposition behavior of Mg, Li, and Mn ions. The results obtained show that the potential of Li metal deposition, after the addition of MgCl₂ and MnCl₂, is more positive than the one of Li metal deposition before the addition. The codeposition of Mg, Li, and Mn occurs at current densities lower than −1.43 A cm⁻² in LiCl–KCl–MgCl₂ (8 wt%) melts containing 2 wt% MnCl₂. The onset potential for the codeposition of Mg, Li, and Mn is −2.100 V. α, α + β, and β phases Mg–Li–Mn alloys with different lithium and manganese contents were obtained via galvanostatic electrolysis from LiCl–KCl melts with different concentrations of MgCl₂ and MnCl₂. The microstructures of typical α and β phases of Mg–Li–Mn alloys were characterized by X-ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy (SEM). The analysis of energy dispersive spectrometry (EDS) and EPMA area analysis showed that the elements of Mg and Mn distribute homogeneously in the Mg–Li–Mn alloys. The results of inductively coupled plasma analysis determined that the chemical compositions of Mg–Li–Mn alloys correspond with the phase structures of XRD patterns, and lithium and manganese contents of Mg–Li–Mn alloys depend on the concentrations of MgCl₂ and MnCl₂.     

Study and modeling of the liquid–solid equilibrium of the KCl–KNO3–HCl–H2O system at 283.15 K

In this work, the liquid–solid equilibrium of the KCl–KNO₃–HCl–H₂O system at 283.15 K was investigated. To calculate the solubility product constants of KCl and KNO₃ in HCl solution, the Pitzer equation was used to calculate the activity coefficient of the ionic species in aqueous solution. In addition, the thermodynamic model with the Pitzer equation was constructed to regress the temperature coefficient parameters for HCl and HNO₃. The calculation and prediction of the KCl–KNO₃–HCl–H₂O system using the results of the regression confirmed that the regression was successful and the Pitzer equation was suitable. For the reaction KCl(s) + HNO₃ → KNO₃(s) + HCl , the equilibrium constant K can be obtained from the solubility product constants of KCl and KNO₃. Eutectic points for the KCl–KNO₃–HCl–H₂O system were also predicted. These equilibrium data could be expressed by the triangular phase diagram. The process of the reaction to produce KNO₃ could be explained through this triangular phase.     

Calculation of flammability limits of silane–oxygen and silane–air mixtures

The upper and lower flammability limits of silane–oxygen and silane–air mixtures at pressures of 0.05–1.1 atm and temperatures of 350–640 K are found on the basis of Westbrook’s detailed chemical kinetics. It is demonstrated that the death of OH radicals has a minor effect on these limits (their stability) within the framework of the Arrhenius kinetic model. The effect of the silane–air mixture composition on the flammability limits is found. It is shown that the behavior of the ultimate temperature of ignition is nonmonotonic as the fraction of silane increases.     

Influence of mechanical activation of reactive mixtures on the microstructure and properties of SHS-ceramics MoSi2–HfB2–MoB

In this work, the influence of modifications of SHS-process on the microstructure and performance characteristics of composite ceramics MoSi₂-HfB₂-MoB with two-level structure was studied. Partial texturing of MoSi₂ grains in samples obtained by force SHS pressing technology was revealed. The effect of preliminary mechanical activation on the macrokinetic parameters of combustion and on the microstructure of the synthesized ceramics was studied. A significant grinding of the synthesized ceramics grain and an increasing of physical-mechanical properties are achieved by increasing the velocity and lowering the combustion temperature of the activated mixtures. The sample obtained by hot pressing of SHS powder from MA reaction mixture showed the most optimal combination of hardness (19.5 GPa), porosity (0.4%) and oxidation resistance (1.82∙10^-6 mg/(cm²∙s)).     

Solid-state formation of lithium ferrites from mechanicallyactivated Li2CO3–Fe2O3 mixtures

The formation of lithium ferrites (LiFe₅O₈ and LiFeO₂) from mechanically activated mixtures of Li₂CO₃–Fe₂O₃ has been studied using thermal analysis (TGA, DSC), evolved gas analysis (TG/FT-IR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and particle size analysis. It is shown that mechanical activation of the precursors considerably enhances the reactivity of the solid system analysed and makes it possible to obtain reaction products with a much lower expense of thermal energy. In particular, lithium ferrites can be obtained at temperatures at least 160 °C lower than those necessary in the absence of mechanical activation. Moreover, both the microstructure and the allotropic ratio of the products, as well as the reaction path, are affected by mechanical activation.     

Role of B2O3 in formation of mullite from kaolinite and α-Al2O3 mixtures

Abstract

The microstructure, phase constitution, and physical properties of mullite bodies prepared from α-Al₂O₃- kaolin mixtures with added B₂O₃ were investigated. Densification was found to be enhanced with small additions of B₂O₃. The results indicate that 0.5 wt-% B₂O₃ increases the content and growth rate of the mullite. It was found to be the optimum addition with respect to densification and resulting properties.     

Effect of applied voltage on wetting and corrosion of corundum refractory by CaO–SiO2–MgO molten slag

The wetting and corrosion behavior of the corundum substrate anode by CaO–SiO₂–MgO molten slag was investigated via the joint application of the sessile drop method with applied voltage and SEM-EDS technique. The slag drop exhibited a good wettability on the corundum substrate. The apparent contact angle at zero voltage slightly exceeded that at a 1 V applied voltage but was lower than those at 1.5 V and 2 V ones. Low applied voltage of 1 V had little effect on the corundum substrate's direct dissolution corrosion processes; high ones of not less than 1.5 V caused the electrode reaction to occur. The stirring effect of O₂ bubbles from the anode reaction aggravated the substrate's direct dissolution and physical stripping. It was found that the applied voltage could inhibit the slag penetration, and the apparent contact angle had no obvious relation with the direct dissolution thickness and penetration depth. A thin but almost continuous MgO?Al₂O₃ (MA) layer could form at the slag/substrate interface at the applied voltage of 1.5 V. These results indicate the positive effect of applied voltage on the distribution of interfacial products and the molten slag penetration in reducing the corrosion of corundum anode under certain conditions. However, when the applied voltage was too high, the vigorous electrode reaction could aggravate the direct dissolution and physical stripping of the corundum anode, and damage the continuation of the formed interface product layer with a high melting point.     

Dy3+ doped LiCl–CaO–Bi2O3–B2O3 glasses for WLED applications

Dy³⁺ doped calcium bismuth borate glasses were synthesized in the composition range of xLiCl-(30 − x)CaO-20Bi₂O₃-50B₂O₃ + 1 mol% Dy₂O₃ (x = 0, 2, 5, 7, 10 and 15 mol%, LC0, LC2, LC5, LC7, LC10 and LC15 respectively) using conventional melt-quench technique. Broad XRD profiles confirmed non-crystalline nature of synthesized compositions. The compositional dependencies of structural changes (using FTIR spectra), thermal behavior (using DSC thermographs) and optical band gap (using UV–Vis–NIR spectra) were discussed. Photoluminescence (PL) excitation spectra recorded at 577 nm yielded six different excitation peaks belonging to Dy³⁺ ions. The PL emission spectra recorded at 451 nm were analyzed to extract different light emission parameters viz. Y/B ratio, color coordinates, correlated color temperature (CCT) following CIE 1931 chromaticity diagram. The emission colors were found to lie in white light region and lies very close to standard white light emission. The CCT of sample LC10 (5335 K) is closest to CCT of standard white light (5615 K) which depicted the optimized concentration of LiCl for application of these glasses in WLED application.