Molten Salt Medium Research Articles

Investigating microstructural evolution during the electroreduction of UO2 to U in LiCl-KCl eutectic using focused ion beam tomography

Reprocessing of spent nuclear fuels using molten salt media is an attractive alternative to liquid-liquid extraction techniques. Pyroelectrochemical processing utilizes direct, selective, electrochemical reduction of uranium dioxide, followed by selective electroplating of a uranium metal. Thermodynamic prediction of the electrochemical reduction of UO2 to U in LiCl-KCl eutectic has shown to be a function of the oxide ion activity. The pO2− of the salt may be affected by the microstructure of the UO2 electrode. A uranium dioxide filled “micro-bucket” electrode has been partially electroreduced to uranium metal in molten lithium chloride-potassium chloride eutectic. This partial electroreduction resulted in two distinct microstructures: a dense UO2 and a porous U metal structure were characterised by energy dispersive X-ray spectroscopy. Focused ion beam tomography was performed on five regions of this electrode which revealed an overall porosity ranging from 17.36% at the outer edge to 3.91% towards the centre, commensurate with the expected extent of reaction in each location. The pore connectivity was also seen to reduce from 88.32% to 17.86% in the same regions and the tortuosity through the sample was modelled along the axis of propagation of the electroreduction, which was seen to increase from a value of 4.42 to a value of infinity (disconnected pores). These microstructural characteristics could impede the transport of O2− ions resulting in a change in the local pO2− which could result in the inability to perform the electroreduction.

Ni-catalyzed synthesis of hexagonal plate-like alpha silicon nitride from nitridation of Si powder in molten salt media

The catalytic effects of Ni during the nitridation of Si from a molten salt and the morphologies of the nitridation products were investigated. The phase composition, particle size, and morphological structure of the α-Si3N4 powder were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and a Brunauer–Emmett–Teller (BET) specific-surface-area analysis. The results showed that Ni significantly accelerated Si nitridation and the formation of the hexagonal plate-like α-Si3N4 phase by the formation of Ni–Si in the eutectic salt at 1050–1350°C. Further, the nitridation rate of the Si powder was increased to 93% after 5h at 1250°C with 2-wt% Ni used as the catalyst. The specific surface area of the α-Si3N4 synthesized powder was 5.70–10m2/g, exceeding that of 1.70m2/g of the Si powder.

The synthesis, structure and properties of a new lithium-rich manganese(ii) phosphate Li5CsMn(P2O7)2: a congruently melting compound with a 'lithium hamburger' structure.

The crystals of the title compound have been grown in molten salt media. Single-crystal X-ray diffraction experiment revealed that the compound crystallizes in an orthorhombic space group Pbcn (no. 60) with cell parameters: a = 5.1023(10) Å, b = 19.717(4) Å, c = 12.557(3) Å, and Z = 4. The structure of Li5CsMn(P2O7)2 consists of one-dimensional manganese pyrophosphate chains [MnP4O14]∞ interleaved with Cs(+) and Li(+) cations. A pseudo-two-dimensional layer [CsMnP4O14]∞ parallel to the ab plane is made up of [MnP4O14]∞ chains and Cs-O polyhedra through sharing the oxygen atoms. The three crystallographically distinct Li atoms are all located in the interlayered space to form a 'lithium hamburger' structure. Magnetic studies demonstrated that the compound shows paramagnetic behavior. DSC and XRD investigations revealed that the compound melts congruently. AC impedance studies show that the conductivity is 1.3 × 10(-5) S cm(-1) at 573 K and the activation energy is 0.42 eV. In addition, Li ion diffusional pathways were obtained using the bond valence map.

Catalyst-assisted synthesis of α-Si3N4 in molten salt

Silicon nitride (Si3N4) powder was synthesised under a nitrogen gas flow (150mL/min) using molten salt media at relatively low temperatures (950–1350°C) to investigate the effects of temperature and cobalt (Co) content on its phase assembly and micro-morphology. Analysis of purified α-Si3N4 obtained using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed that the fabrication of α-Si3N4 from silicon by molten salt media is possible at temperatures as low as 1050°C. Furthermore, Co played an important role in accelerating Si nitridation through the formation of a Co–Si liquid phase, with overall conversion increasing from 5% to 60% with the addition of 0.5wt% Co at 1150°C. At temperatures below 1350°C, silicon nitride exists mainly as flaky nanoparticles, but at higher temperatures changes to α-Si3N4 nanorods that are 200–500nm in diameter and 500–1000nm in length. The growth of these nanorods and nanoparticles was governed by the vapour–liquid–solid and vapour–solid mechanisms.

Effect of formation of “long range” secondary dendritic nanostructures in molten salt nanofluids on the values of specific heat capacity

Several studies in recent literature have demonstrated the enhancement of specific heat capacity (Cp) of molten-salts on doping with minute concentration of nanoparticles, especially when the synthesis conditions enabled the formation of stable colloidal suspensions (which are also known as “molten-salt nanofluids”). In this study we present additional evidence in support of theory proposed earlier in the literature that stable colloidal suspensions of nanoparticles in a molten-salt medium induces the preferential surface adsorption of the constitutive chemical species in the salt mixture which in turn leads to the nucleation of solid phase of the molten salt (with perhaps a different chemical composition than in the bulk phase) on the nanoparticle surface. The surface adsorbed salt species leads to the nucleation and growth of a semi-solid layer of dendritic shaped phase (dendritic shaped secondary “long range” nanostructures). Incidentally, such nanostructures were not observed in electron microscopy images for samples of pure molten-salt mixtures subjected to control experiments (i.e., without nanoparticles). Hence, this study conclusively demonstrates that the existence of these nanostructures is primarily responsible for the enhancement of specific heat capacity. In this study, three different types of nanoparticles are dispersed in the same molten-salt mixture (“base fluid”) and the experimentally measured values of specific heat capacity enhancements obtained in this study are correlated to the formation of dendritic nanostructures that are observed in the images obtained from the electron microscopy of the molten-salt nanomaterial samples.

Thermodynamic Stability Assessment of MIEC Cathode Materials for Solid Oxide Fuel Cells

Mixed ionic-electronic conductor (MIEC) cathode materials such as La(1-x)SrxCo0.2Fe0.8O3-δ (LSCF,where x=0.2, 0.3, 0.4), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF 5582), Sr2Fe1.5Mo0.5O6-δ (SFMO), La2NiO4+δ (LNO), GdBaCo2O6-δ (GBCO), and PrBaCo2O6-δ (PBCO) exhibit excellent oxygen transport properties as well as high electronic conductivity at operational temperatures of SOFCs. Their promising electrochemical performance make these materials excellent candidates for potential application as SOFC cathodes. However, their long-term thermodynamic stability in the operating environment of SOFCs, including unexpected and uncommon situations such as CO2-leakage from the anodic side is of concern and should be thoroughly investigated prior to deploying them in cells and stacks. This study investigates the thermodynamic stability of promising SOFC cathodes under operating conditions, in molten salt media, and through galvanic cell measurements. The cathode materials of interest are La(1-x)SrxCo0.2Fe0.8O3-δ (LSCF, where x=0.2, 0.3, 0.4), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF 5582), Sr2Fe1.5Mo0.5O6-δ (SFMO), La2NiO4+δ (LNO), GdBaCo2O6-δ (GBCO), and PrBaCo2O6-δ (PBCO). The stability of these materials is studied in air and at various CO2 partial pressures.

Molten salt assisted synthesis of 3C–SiC nanowire and its photoluminescence properties

Single crystalline silicon carbide nanowires (SiC NWs) were prepared through the carbothermal reduction route using silica fume and phenolic resin in a molten salt (NaCl and NaF) medium at 1450°C. The presence of molten salts was found to have a noticeable effect on the complete conversion of SiO2 to 3C–SiC with the temperature increasing from 1300°C to 1450°C, and NaF/NaCl ratios and molten salts/reactant ratios also played vital roles in the growth of SiC NWs. The as-achieved SiC NWs were 100–150nm in diameter and 10μm in length, which grew along the direction perpendicular to the (111) plane and contained stacking faults (SFs). The as-synthesized SiC NWs had a strong emission peak (422nm) located in the violet–blue spectral range, suggesting potential applications in optoelectronic devices. All these discoveries suggest a cheap and feasible route for large scale growth of high quality SiC NWs at low temperature.

Thermodynamic investigations of oxyfluoride of thorium and uranium

The standard molar Gibbs energy of formation of ThOF2(s) and UO2F2(s) has been determined using an e.m.f. technique. For this purpose, separate fluoride cell has been constructed using CaF2(s) as the solid electrolyte. From the measured e.m.f. values and required Gibbs energy data available in the literature, ΔfGom(T) for these oxyfluorides has been calculated. The enthalpy of formation of ThOF2(s) and UO2F2(s) at 298.15 K has been calculated from the experimentally measured Gibbs energy data using the second and the third law methods. To determine the stability domains of ThOF2(s) and UO2F2(s), the phase diagram and chemical potential diagrams of Th–F–O and U–F–O systems were calculated by the CALPHAD method and FactSage software. These calculations can be used to predict the oxygen partial pressures and the temperature domains in which thorium and uranium oxyfluorides might be formed in the molten salt medium.

Thermodynamic Stability Assessment of MIEC Cathode Materials for Solid Oxide Fuel Cells

Promising electrochemical performance of mixed ionic-electronic conductors (MIEC) such as La(1-x)SrxCo0.2Fe0.8O3-δ (LSCF,where x=0.2, 0.3, 0.4), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF 5582), GdBaCo2O6-δ (GBCO), and PrBaCo2O6-δ (PBCO) make these materials potentially excellent candidates for SOFC applications. However, in order to consider these materials for cathode application in SOFC stacks, their long-term thermodynamic stability in the operating environment of SOFCs, including unexpected and uncommon situations such as CO2-leakage from the anodic side should be thoroughly considered. This study investigates the thermodynamic stability of promising SOFC cathodes under operating conditions, in molten salt media, and through galvanic cell experiments. The cathode materials of interest are La(1-x)SrxCo0.2Fe0.8O3-δ (LSCF, where x=0.2, 0.3, 0.4), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF 5582), GdBaCo2O6-δ (GBCO), and PrBaCo2O6-δ (PBCO). The stability of these materials is studied in air and at various CO2 partial pressures.

Preparation and characterisation of porous biomorphic SiC/C ceramic from molten salt

This paper presents a new technique for synthesizing porous silicon carbide (SiC) ceramics with a biomorphic microstructure through the molten salt reaction of a carbon template/silicon composite under a static argon atmosphere. The effects of varying the sintering temperature (1200 to 1400°C) and corresponding holding time on the pore size distribution of the obtained ceramics is investigated, and the biomorphic cellular morphology of the residual carbon template in the porous β-SiC ceramic is analysed for the presence of α-SiC. The results reveal that the formation of SiC starts at about 1250°C, at which point SiC exists as nanowires in the cellular pores of the carbon template. This unique hierarchical morphology is determined to be influenced by the molten salt media.

An investigation on ZnWO4 nano-powder synthesis using LiNO3–KNO3 medium at low temperature

A state of art molten salt method has been introduced to synthesise ZnWO4 nano-powders, employing ZnCl2 and Na2WO4·2H2O as raw materials. The newly introduced eutectic composition of LiNO3-KNO3 with the melting point temperature of 125°C was employed as salt media. It has been observed that the displacement reaction ZnCl2 + Na2WO4·2H2O → ZnWO4+2NaCl+2H2O was conducted in molten salt media during the synthesis. Characterisation of as-synthesised powders was performed using X-ray diffraction (XRD), Fourier transformation infrared (FTIR) spectroscopy, Brunner–Emmett–Teller (BET), transition electron microscopy (TEM), and photoluminescence (PL). The results showed that temperature had a crucial effect on the crystallite size of synthesised nano-powders. Furthermore, TEM micrographs demonstrated that the synthesised nanoparticles at 350°C were almost angularly shaped with grain size less than 80 nm. In addition, the average activation energy of crystallites growth during synthesis was found to be about 2.47 kJ mol− 1; proposing that the crystallite growth during the synthesis procedure occurred by means of an interfacial reaction.

On the covalency of U(III)–Cl, U(IV)–Cl bonding in a LiCl–KCl eutectic melt at 450 °C: Spectroscopic evidences from their 5f–6d and 5f–5f electronic transitions

UV–VIS spectra of uranium ions provided information not only on the electronic state of U(III)–Cl and U(IV)–Cl species, but also on the metal–ligand bonding properties. The 5f–6d transition lines in the electronic absorption spectra of U(III) and U(IV) provided key information on the covalent bonding properties of U(III)–Cl and U(IV)–Cl complexes. The position and intensities of 5f–6d and 5f–f5 transitions varied depending on the oxidation state and bonding properties. The 5f–6d transitions of U(III) ion occur at a lower energy of as low as the 400–600nm range with decreased intensity. However, U(IV) ions occur at the much higher ~250nm range. The 5f–6d/5f–5f intensity ratio indicated the degree of covalency. The U(III)–Cl species exhibited stronger covalent bonding than the U(IV)–Cl. This means that U(III)–Cl bonding is stronger than U(IV)–Cl species. Compared with uranium ions, Eu(II) ion showed no measurable covalent bonding under the same conditions, which means that Eu(II)–Cl bonding is predominantly ionic in nature. The bond strength (covalency) was evaluated in the order of U(III)–Cl>U(IV)–Cl≫Eu(II)–Cl. The reduction rate of U(IV) is ~4 times faster than that of the oxidation of U(III), indicating that U(III)–Cl is more stable than U(IV)–Cl species. The UV–VIS spectral analysis method may also be effective in investigating the bonding properties of neptunium and plutonium ions with chloride ligands in molten salt media.

Corrosion Behavior of Structural Materials in LiCl-KCl Molten Salt by Thermogravimetric Study

Spent metallic fuel from future fast-breeder reactors would be processed by non-aqueous pyrochemical reprocessing. Different unit operations of pyrochemical reprocessing demands various structural materials that are compatible with molten salt medium at high temperatures. In the present study, the corrosion behavior of the candidate materials stainless steel (SS) 410, alloy 600 and 9Cr-1Mo (9% chromium and 1% molybdenum) steel was investigated in molten lithium chloride–potassium chloride (LiCl-KCl) salt by thermogravimetric studies under inert and reactive atmospheres. The test specimens, along with molten salt, were isothermally heated at 500 and 600°C for 24 h in both atmospheres. Alloy 600 showed marginally higher weight gain than 9Cr-1Mo and SS410 under both atmospheres at 600°C. Higher weight gain was observed under reactive atmosphere than in inert atmosphere at 600°C. All of the samples investigated under both atmospheres at 500°C showed insignificant weight loss. The surface morphology and energy...

High nitrogen-content carbon nanosheets formed using the Schiff-base reaction in a molten salt medium as efficient anode materials for lithium-ion batteries

A series of porous carbon nanosheet materials with a high nitrogen content have been prepared using melamine and terephthalaldehyde as carbon precursors through the Schiff-base reaction in a molten salt medium.

Preparing high surface area porous carbon from biomass by carbonization in a molten salt medium

Peanut shells were transformed into porous carbon with a high surface area through a simple ZnCl2-molten salt synthesis process.

Molten salt medium synthesis of wormlike platinum silver nanotubes without any organic surfactant or solvent for methanol and formic acid oxidation.

In the current research, the PtxAgy (x/y = 86/14, 79/21, 52/48, 21/79, 11/89) nanoparticles (NPs) are synthesized in the KNO3-LiNO3 molten salts without using any organic surfactant or solvent. The SEM results suggest that when the content of Ag is higher than 48%, the wormlike PtxAgy nanotubes (NTs) can be synthesized. The diameter of the PtxAgyNTs shows a slow decrease with the increase of Ag content. The TEM and HRTEM results indicate that the growth of hollow PtxAgy NTs undergoes an oriented attachment process and a Kirkendall effect approach. The results of cyclic voltammetry (CV) measurement indicate that the Pt52Ag48 catalyst presents a remarkable enhancement for methanol electrooxidation, while the Pt86Ag14 catalyst prefers electrochemically oxidizing formic acid compared with that of the commercially available Pt black.

Electrochemical properties of Ce(iii) in an equimolar mixture of LiCl–KCl and NaCl–KCl molten salts

Cyclic voltammetry of Ce(iii) in LiCl–KCl molten salt medium and the morphology of Ce metal deposited by electrochemical deposition.

Hydrogenolysis of lignin in ZnCl2 and KCl as an inorganic molten salt medium

Abstract Lignin can be converted into monomeric products with the aid of molten salt media. Molten zinc chloride (ZnCl2)/potassium chloride (KCl) mixtures are suitable for this purpose. The application of an eutectic mixture with low melting points leads to similar main products as are obtained by pyrolysis. The hydrogenolysis of an organosolv lignin in molten salts of ZnCl2/KCl was investigated as a function of reaction temperature, residence time, and lignin concentration, and the composition of liquid products and monophenols was analyzed by gas chromatography-mass spectrometry (GC-MS). The yields can be optimized by the proper selection of the reaction temperature. A longer residence time and higher lignin concentrations lead to increased formation of solid residues and gaseous products. The liquid products mainly consist of substituted phenols derived from lignins. Polymeric products are the result of condensation reactions (i.e., the formation of new C-C linkages in the course of secondary reactions).

Crystallization of Y3Al5O12:Ce3+ glass microspheres prepared by flame-spraying synthesis

Ce3+-doped Y3Al5O12 microspheres and microcubes were prepared by controlled crystallization of flame-sprayed glass microspheres. The morphology and microstructure of the Y3Al5O12:Ce3+ phosphor particles were affected by the heat-treatment temperature and the crystallization environment. Powder sample heat-treated in air was composed of randomly aggregated grains and their size can be tuned from nanometer scale to micrometer size by adjusting the heat-treatment temperature. Cubic Y3Al5O12:Ce3+ particles were obtained when the glass microspheres were heat-treated in NaCl–NaF molten salt medium. The formation mechanisms for the Y3Al5O12:Ce3+ microspheres and microcubes were analyzed. The photoluminescence spectra of the prepared Y3Al5O12:Ce3+ microspheres and microcubes were investigated.

Combined molten salt–Ni/Al2O3 as synergistic medium for high-quality syngas production

Proposed synergistic use of a combined medium of molten salt and Ni/Al2O3 (MS-Ni) was investigated for its enhancement of cellulose pyrolysis for high-quality syngas production. Clean renewable solar energy is to be stored at a high temperature in molten salt (MS) and provides the heat of pyrolysis. The MS-Ni medium could increase H2 yield by 3-folds while CO yield slightly increased by 15%, compared to the case of only MS medium. The peak rate of H2 production nearly quadrupled while the peak rate of CO production increased 2.5times at about 150 and 80K lower temperatures, respectively. The ratio of selectivity of syngas to undesired CH4 was nearly doubled. Arrhenius rate expressions for pseudo-first-order pyrolytic reaction are derived from the experimental data to give activation energies of 206 and 128kJmol−1 for the MS and the MS-Ni mediums, respectively. The experimental results clearly validated the role of MS-Ni as a synergistic medium for high-quality syngas production from cellulosic biomass pyrolysis.