Molten Salt Composition Research Articles

Intermediate Temperature Sodium Batteries Enabled By an Inorganic Molten Catholyte

As the national electrical energy grid continues to expand and evolve, there is an increasing need for safe, cost-effective and reliable large scale batteries. Current large scale battery technologies continue to suffer from limited cyclability, high cost, and potentially hazardous runaway reaction behaviors. Here, we describe a promising alternative approach to intermediate temperature molten sodium-based batteries, enabled by advanced solid state ceramic electrolytes, such as NaSICON, that offer high Na+ conductivity at temperatures well below 200 °C. Integrating molten sodium anodes, ceramic solid state electrolytes, and fully molten AlCl3-based molten salt catholytes, we create high performance, all-inorganic batteries. In particular, we explore an AlCl3-NaI molten salt catholyte, and we show that these batteries operate efficiently below 200 °C. We describe the relationships between the molten salt composition and electrochemical properties of the catholyte, revealing how these promising, intermediate temperature technologies boast coulombic efficiencies near 100% and energy efficiencies >80%. Moreover, we validate the safety of this material set using accelerated rate calorimetry to demonstrate that the system exhibits neither the runaway exothermic reactions nor hazardous pressurized gas generation that plague other large-scale battery systems. Finally, we discuss the potential benefits to long term battery reliability afforded by low-to-intermediate temperature battery operation. The encouraging performance of this emergent intermediate temperature molten salt battery shows a path to providing the reliable, safe, and cost-effective energy storage solutions demanded by our rapidly growing national electrical energy grid. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Phase Equilibria Studies of the LiCl-KCl-UCl3 System

Regions of the LiCl-KCl-UCl3 phase diagram used to represent the molten salt compositions generated during the electrorefining of used nuclear fuel were evaluated by studying the LiCl-UCl3 and KCl-UCl3 binary systems and several ternary mixtures. Phase transition temperatures of several binary and ternary mixtures made with LiCl, KCl, and UCl3 were measured by using differential scanning calorimetry. Inductively coupled plasma-atomic emission spectroscopy was used to measure the gross compositions of the salt mixtures and X-ray diffraction (XRD) was used to identify the phases formed after they were thermally cycled and had cooled to room temperature. The LiCl-UCl3 system has a eutectic transition at 763 ± 2 K for a mixture with 25 mol % UCl3. The KCl-UCl3 system has two eutectic transitions, one at 827 ± 3 K and another at 805 ± 4 K for mixtures with 19 mol % UCl3 and 57 mol % UCl3, respectively, and the congruently melting compound K2UCl5 was identified to have formed by XRD. The LiCl-UCl3 and KCl-UCl3 binary phase diagrams were developed and combined with the LiCl-KCl phase diagram to produce a portion of the LiCl-KCl-UCl3 phase diagram. The LiCl-KCl-UCl3 system includes two ternary eutectics, one occurring at 681 ± 6 K for the mixture with 33 mol % UCl3, 42.0 mol % LiCl, and 25 mol % KCl, and the other at 619 ± 1 K for the mixture with 8 mol % UCl3, 50.0 mol % LiCl, and 42 mol % KCl. The evaluation of these phase diagrams provides an improved understanding of the LiCl-KCl-UCl3 systems generated during electrorefining.

Preparation of ultra-small molecule-like Ag nano-clusters in silicate glass based on ion-exchange process: Energy transfer investigation from molecule-like Ag nano-clusters to Eu3+ ions

Ultra-small sliver nano-clusters are a novel kind of materials with excellent photoluminescence properties, which have potential application in the fields of integrated optical storage, biological optics sensor and solid state lighting. In this work, ultra-small molecule-like Ag nano-clusters were prepared in the silicate glass by the ion-exchange process. The influence of ion-exchange temperature, ion-exchange time and ratio of molten salt composition on the formation of molecule-like Ag nano-clusters was investigated systematically in the silicate glass by the transmission electron microscopy, optical absorption and photoluminescence spectroscopy. The results demonstrated that the stabilized molecule-like Ag nano-clusters with the size of 1–3 nm were formed by controlling the ion-exchange parameters. In addition, the influence of Eu3+ doping on the formation of molecule-like Ag nano-clusters was observed in the silicate glass, promoting the growth of molecule-like Ag nano-clusters. The luminescence enhancement of Eu3+ was obtained due to efficient energy transfer from molecule-like Ag nano-clusters to Eu3+.

Electrochemistry of the NaI-AlCl3 Molten Salt System for Use as Catholyte in Sodium Metal Batteries

Molten salt electrolytes show promise as safe, effective elements of emerging low to intermediate temperature molten sodium batteries. Here we investigate the NaI-AlCl3 molten salt system for its electrochemical and physical properties at 150 and 180°C, temperatures recently used to demonstrate a new NaI battery using this molten salt system. Molten salt compositions ranging from 20–75% NaI were prepared and electrochemically interrogated with carbon fiber ultramicroelectrodes utilizing cyclic voltammetry, chronoamperometry, and differential pulse voltammetry. Results indicate that at very high or very low NaI concentrations, secondary phases present hinder diffusion of the redox-active species, potentially impacting the current density of the system. Furthermore, a concentration-independent chronoamperometric analysis technique was leveraged to determine effective diffusion coefficients of active I− in the melt phase. Collectively, the physical characterization and electrochemical properties of the tested salts indicate that the catholyte composition can significantly affect the physical state, current density, ionic diffusion, and voltage window of these promising NaI-AlCl3 molten salt battery catholytes.

Kinetics of CO2 Absorption by Calcium Looping in Molten Halide Salts

Carbon Capture in Molten Salts (CCMS) is a high temperature method for extracting CO2 from a variety of flue gases related to power generation and carbon-intensive industries. The chemical principles are similar to those in calcium looping in the solid state; a carbonation reaction of CaO with CO2 to form CaCO3 followed by regeneration of CO2 through the reverse reaction. In CCMS, the active substances (CaO/CaCO3) are dissolved or partly dissolved in molten salts, allowing fast reaction kinetics, high CO2 sorption capacities, and avoiding solids attrition issues. In our previous studies, the focus has been on the total CO2 sorption capacity and demonstration of cyclic absorption and desorption. Experiments have been performed with up to 20 wt% CaO in molten CaCl2 and eutectic CaF2/CaCl2. It has been demonstrated that up to 85% of the CaO reacts during absorption, and ∼100% of the CaCO3 is decomposed during desorption. No degradation of the sorbent has been observed after 12 cycles. In the present study, the focus is turned to the reaction kinetics between CO2 and CaO. The raw data from previous experiments are analyzed to obtain the sorption capacity (g CO2 / 100g sorbent) as a function of time, and the linear region of the capacity is further used to evaluate the reaction kinetics. The effect of absorption temperature, molten salt composition, CaO content and cyclic CO2 capture is studied. The results show that CaF2/CaCl2 is more favorable for CCMS than pure CaCl2; the kinetically controlled regime lasts longer and the total sorption capacity is higher. For both of the salt mixtures, the sorption capacities are stable during cyclic CO2 capture, without any deterioration of the reaction kinetics.

Molten salt synthesis of morphology controlled α-alumina platelets

Abstract Controlling the morphology of plate-like α-Al 2 O 3 powders is essential since Al 2 O 3 platelets of different sizes and shapes are needed in numerous applications. In this work, non-aggregated α-Al 2 O 3 single-crystal platelets with diameters ranging from ~1 µm to more than 20 µm and thicknesses from ~0.1 µm to 1.3 µm were produced by molten salt synthesis. The effects of alumina precursor and molten salt compositions on phase formation and morphology development of Al 2 O 3 particles were investigated. Al 2 (SO 4 ) 3 precursor reacts with K 2 SO 4 salt to form K 3 Al(SO 4 ) 3 liquid phase at ~625 °C. The α-Al 2 O 3 seeds generated in-situ from the decomposition of K 3 Al(SO 4 ) 3 can serve as low energy nucleation sites, and thus accelerate complete transformation to α-Al 2 O 3 at much lower temperatures, yielding platelets of ≥20 µm diameter. Conversely, γ-Al 2 O 3 precursor is stable until higher temperatures, resulting in the formation of small hexagonal α-Al 2 O 3 platelets (1–2 µm in diameter) in the K 2 SO 4 molten salt. In addition to alumina precursors, the salt species also strongly affect the morphology of Al 2 O 3 particles. Compared with chloride salts (NaCl and KCl), sulfate salts (K 2 SO 4 and Na 2 SO 4 ) create favorable conditions for growth of higher- aspect-ratio α-Al 2 O 3 hexagonal platelets.

Molten salt composition and composites for improved latent heat thermal energy storage

The growing concern over nonrenewable fuel sources, coupled with the continued increase of global energy demand has incentivised research into numerous new and pre-existing renewable energy sources. Concentrated solar thermal (CST) takes advantage of the high heat capacity of molten salts to provide an alternative solar solution to photovoltaic cells that allows reduced downtime through heat storage for use during suboptimal conditions. This meta-study examines the effectiveness of various eutectic Molten Salt compositions and materials as thermo-physical augmenters, with a focus on improved thermal conductivity when composited with molten salts as a method of enhancing the efficiency of concentrated solar thermal storage technology. The study is based on literature retrieved from scientific databases to investigate information available about enhancing LHTES technology. Research into carbon composites such as Expanded Graphite (EG) exhibits promising results revealing thermal conductivity increases as high as 40% in eutectic salt materials, however inconsistencies in measurements and materials used reveal a need for a greater analysis. Although molten salt thermal storage systems are not optimal in their current state, indication was found that composite storage mediums could potentially solidify their spot as a viable renewable energy source.

Alkali Carbonate Molten Salt Coated Calcium Oxide with Highly Improved Carbon Dioxide Capture Capacity

AbstractCO2 reduction is crucial if the effects of this gas on global warming are to be alleviated. We report for the first time an alkali carbonate molten salt promoted CaO‐based CO2 sorbent with CO2 capture performance superior to that of neat CaO. The influences of chemical composition, loading, and melting temperature of the (Li–Na–K)2CO3 molten salts and of the calcination and adsorption temperatures on CO2 capture were evaluated systematically. The microstructural and morphological evolution of the samples during CO2 adsorption was studied by X‐ray diffraction, scanning electron microscopy, and Fourier‐transform infrared spectroscopy analyses. The (Li–K)2CO3 molten salt coating was found not only to promote CO2 uptake but also to facilitate CO2 desorption from CaO. In particular, at low temperatures of 500 and 600 °C, the CO2 capture capacity increased significantly from 1.19 and 3.26 mmol g−1 to 6.93 and 10.38 mmol g−1, respectively. The melting point of the molten salts was also a crucial factor in the improvement of CO2 uptake. Kinetic studies based on fractal‐like models indicated that the rate coefficients for (Li–K)2CO3/CaO were approximately 3.3 to 3.8 times larger than those for neat CaO. The coating of alkali carbonate molten salts is believed to prevent the formation of a rigid CaCO3 layer on the surface of the CaO particles and to provide continuous delivery of CO32− to promote CO2 capture. During the CO2 adsorption/desorption cycling tests, (Li–K)2CO3/CaO resulted in a stable and reversible CO2 uptake of 6.0–6.3 mmol g−1, which is much higher than that of neat CaO (2.0 mmol g−1).

Effect of Polyhydric Alcohol Addition on Electrodeposition of Fe-Al Alloy in AlCl3-NaCl-KCl-FeCl2 Molten Salts

Introduction The thermoelectric material can directly convert the temperature difference into the electrical energy and is a candidate for clean energy source. Iron-aluminum binary alloy is composed of inexpensive common metals and a fundamental system of the Heusler-type iron-based thermoelectric materials, having a characteristic that the sign of Seebeck coefficient changes based on its composition. The usage of thermoelectric materials made of films has advantage to produce lightweight and high-capacity thermoelectric conversion devices, because the devices usually consist of pairs of p- and n-type materials. Thus, the electrodeposition method, which uses simple apparatus and is low cost film forming method, has been investigated to prepare thermoelectric materials in film. In the previous study, the electrodeposition of Fe-Al binary alloy in AlCl3-NaCl-KCl-FeCl2 quaternary molten salts was reported. Fe-Al alloy films having various compositions were obtained by varying the composition of molten salts, particularly the mole ratio of AlCl3 and FeCl2. In this study, the effects of addition of polyhydric alcohol such as ethylene glycol or glycerin in AlCl3-NaCl-KCl-FeCl2 molten salts were investigated to understand the effect on the composition of Fe-Al electrodeposit. Experimental The basic composition of molten salts used in this study was 64.0 mol%AlCl3-26.0 mol%NaCl-10.0 mol%KCl in the vicinity of AlCl3-NaCl-KCl ternary eutectic composition. For the electrodeposition of Fe-Al alloy, FeCl2 was used as Fe ion source, and the mole ratios of AlCl3 and FeCl2 in the molten salts were 40 to 1 - 100 to 1. Ethylene glycol (EG) or glycerin (G) was added in 0 - 5.96 mol% in the molten salts. Electrolysis was conducted under constant current density of 100 Am-2 for 2.5×105 Cm-2 at a temperature of 423 K using a copper cathode and an iron anode. The preparations of molten salts and electrolyses were carried out in an Ar-gas-filled glove box. After rinsing with acetone and water, and drying, the electrodeposits were analyzed by using X-ray diffraction and an electron probe micro analyzer. Results The composition of electrodeposit obtained in the 63.59 mol% AlCl3-25.83 mol% NaCl-9.94 mol% KCl-0.64 mol% FeCl2 molten salts without additive, where the mole ratio of AlCl3 and FeCl2 were 100 to 1, was 99.4 mol% Fe-0.6 mol% Al. On the other hand, that of electrodeposits obtained in the 4.84 mol%EG-added and 4.84 mol%G-added molten salts were 31.20 mol% Fe-68.80 mol% Al and 94.59 mol% Fe-5.41 mol% Al, respectively. It was found that Al content in the electrodeposit was increased by EG or G addition to AlCl3-NaCl-KCl-FeCl2 molten salts, particularly the addition of EG was very effective. Then the relationship between the amount of EG addition and the composition of Fe-Al alloy electrodeposit obtained by the galvanostatic electrolysis at 100 Am-2 was investigated. Al content in the electrodeposit increased with the amount of EG addition in the molten salts, and reached the limit at 4.84 mol% EG. The molten salts got gelatinous at 5.96 mol%EG, and were solidified by the addition of EG more than 7.09 mol%. Thus it was found that the addition of 4.84 mol%EG was effective for increasing Al content in the Fe-Al alloy electrodeposit. The electrodeposition of Fe-Al alloy was investigated in AlCl3-NaCl-KCl-FeCl2-EG molten salts, where the mole ratio of AlCl3 and FeCl2 are 40 to 1 - 100 to 1 and EG concentration was 4.84 mol%. Al content in the electrodeposit increased as the FeCl2 concentration in the molten salts decreased. Particularly, Al content increased significantly, when the FeCl2 concentrations in the molten salts were lower than 1.2 mol%. Within the limit of experiments, Fe-Al alloy films containing 2.6 - 68.8 mol% Al were obtained by varying the mole ratio of AlCl3 and FeCl2 (FeCl2 concentration) in molten salts. In the molten salts without EG addition, the Al contents in the electrodeposits were less than 1.0 mol% and the FeCl2 concentration was 0.6 mol% or lower where Al content in the electrodeposit increased significantly. It was clarified that EG addition enabled the control of the composition for Fe-Al alloy obtained in the AlCl3-NaCl-KCl-FeCl2 molten salts in which the co-electrodeposition of Al was difficult. From the cyclic voltammograms measured in the molten salts with and without EG, both cathodic currents derived from the reductions of iron and aluminum ionic species were decreased. Since the electrodeposition potential of Fe is much nobler that that of Al, it was considered that the decrease of Fe electrodeposit due to the EG addition led the relative increase of Al in the electrodeposit.

Electrolytic Refining of Rare Earth Element from Neodymium Magnet Using Molten Salt

Introduction Neodymium magnets contain large amounts of neodymium, praseodymium, and dysprosium. Dysprosium, which is more expensive than neodymium and praseodymium, is added to neodymium magnets to increase their heat resistance. Neodymium magnets are used in numerous consumer electronics and electric- and hybrid-powered vehicles. Large amounts of neodymium are wasted each year when products that contain these magnets are discarded. Processes for the recovery of rare earth elements from neodymium magnets have been developed. Rare earth oxides are recovered from these processes, and these oxides are subsequently reduced to rare earth metals using molten salt electrolysis. We have reported a recovery process for rare earth elements from neodymium magnets using molten salt electrolysis, where these elements were recovered as alloys. In this study, we focus on the electrical behavior of rare earth elements in neodymium magnets during molten salt electrolysis. We use anodic polarization experiments to determine the optimal electric leaching conditions and observe the leaching behavior of rare earth elements. Materials and Methods The electrolysis potential was controlled using a potentiostat. The reactor was made of Pyrex glass and purged with Ar gas. A eutectic salt mixture of 59-mol% LiCl and 41-mol% KCl (melting point: 626 K), which melted at 723 K, was used in the electrolysis bath. The cathode electrode was a glassy carbon rod. The anode electrode was a neodymium magnet, a neodymium rod, a dysprosium rod, or an iron wire. The reference electrode was Ag/AgCl (0.1 N) in a eutectic composition of LiCl–KCl; this electrode was placed in a mullite tube. The electrolysis bath was maintained at 473 K for 24 h under a vacuum to eliminate water. Results and Discussion The scan rate of anodic polarization was 5 mV s−1. The oxidation current of neodymium and dysprosium is generated at approximately −2.2 V, and oxidation potential is the lowest among the elements in the neodymium magnet. The iron oxidation potential was the highest at −0.7 V. Iron is the main component of the neodymium magnet, and the leaching of iron must be avoided in the recovery process. The use of potentiostatic electrolysis enables the selective leaching of rare earth elements from the neodymium magnet, as reported in our previous research. Rare earth elements were leached from the neodymium magnets using potentiostatic electrolysis. The neodymium magnets were used as the anode, and a carbon rod served as the cathode. The electrolysis potential was −1.0 V, and the quantity of electricity was approximately 1200 C. the residual magnet and molten salt compositions were similar for all three neodymium magnets used in the experiments. The residual iron content increased, and the concentrations of the rare earth elements decreased. The rare earth elements were leached from the neodymium magnet into the molten salt, and the total rare earth content in the molten salt was greater than 99.0 mass%. Conclusion Rare earth elements were leached from neodymium magnets using electrolysis in a molten eutectic mixture of LiCl and KCl. The oxidation potential of all neodymium magnet was −1.0 V. The oxidation potential of dysprosium was similar to those for neodymium and praseodymium. The content of rare earth elements in the leaching component was greater than 99.0 mass%. Acknowledgement This work was supported by Environment Research and Technology Development Fund of the Ministry of the Environment, Japan 3K143005.

Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry

The electrochemical transformation of CO2 into more valuable chemicals such as CO offers great interest in the energetic field. In this paper, the feasibility of this valorisation process is investigated by cyclic voltammetry in different binary molten carbonates: Li2CO3Na2CO3 (52:48 mol. %), Li2CO3K2CO3 (62:38 mol. %) and Na2CO3K2CO3 (56:44 mol. %) and in a ternary eutectic: Li2CO3Na2CO3K2CO3 (43.5:31.5:25 mol. %). The eutectic nature, the oxoacidity, temperature and scanning conditions on the reduction and oxidation peaks is thoroughly analysed. The reduction peak related to CO2 reduction into CO is diffusion-controlled and slow quasi-reversible; in specific experimental conditions, it is split into two peaks of similar intensity. Re-oxidation peaks are most probably due to dissolved and adsorbed CO. Based on our experimental investigation under varied conditions such as molten salt composition, oxoacidity, temperature of the system and thermodynamic predictions regarding carbon presence, two mechanistic schemes are proposed, involving gaseous, dissolved and adsorbed species.

Preparation and characterization of Zn2TiO4 micro/nano crystals by molten salt method

The high pure single phase Zn2TiO4 was synthesized by a molten salt method. The influences of the molten salt system and the calcinations temperature on the phase composition and crystalline morphology were studied. The results show that high pure single phase Zn2TiO4 with plate-like crystalline morphology and well-developed faceted surfaces were synthesized at 800–900 °C in NaCl–ZnCl2 salt system when 0–20 % of zinc-bearing raw material is replaced by ZnCl2 salt, which is distinctly beneficial to simplify the composition of molten salt. Compared to the conventional solid phase reaction process, the formation temperature of single phase Zn2TiO4 is lower and the crystal growth is more complete.

One-Step Extraction of Antimony in Low Temperature from Stibnite Concentrate Using Iron Oxide as Sulfur-Fixing Agent

A new process for one-step extraction of antimony in low temperature from stibnite concentrate by reductive sulfur-fixation smelting in sodium molten salt, using iron oxide as sulfur-fixing agent, was presented. The influences of molten salt addition and composition, ferric oxide dosage, smelting temperature and duration on extraction efficiency of antimony were investigated in details, respectively. The optimum conditions were determined as follows: 1.0 time stoichiometric requirement (α) of mixed sodium salt (αsalt = 1.0), WNaCl:Wsalt = 40%, αFe2O3 = 1.0, Wcoke:Wstibnite = 40%, where W represents weight, smelting at 850 °C (1123 K) for 60 min. Under the optimum conditions, the direct recovery rate of antimony can reach 91.48%, and crude antimony with a purity of 96.00% has been achieved. 95.31% of sulfur is fixed in form of FeS in the presence of iron oxide. Meanwhile, precious metals contained in stibnite concentrate are enriched and recovered comprehensively in crude antimony. In comparison to traditional antimony pyrometallurgical process, the smelting temperature of present process is reduced from 1150–1200 °C (1423–1473 K) to 850–900 °C (1123–1173 K). Sulfur obtained in stibnite is fixed in FeS which avoids SO2 emission owing to the sulfur-fixing agent. Sodium salt can be regenerated and recycled in smelting system when the molten slag is operated to filter solid residue. The solid residue is subjected to mineral dressing operation to obtain iron sulfide concentrate which can be sold directly or roasted to regenerate into iron oxide.

Novel methodology to prepare homogenous ternary molten salts for concentrated solar power applications and their thermo-physical characterization

Homogeneous ternary molten salt mixtures have been synthesized using novel and robust preparation method. In a typical preparation method, the ternary molten salt compositions contain two salts in common (KNO3 and LiNO3) and one additional hydrated salt. Thermo-physical properties of various synthesized ternary molten salt mixtures were evaluated by using simultaneous TGA-DSC, Dynamic Shear Rheometer and Thermal conductivity analyzer. Thus prepared ternary molten salt mixtures exhibited improved thermo-physical properties in the form of low melting point, high thermal stability, high specific heat capacity, high thermal conductivity and low viscosity, which are essential for the high temperature heat transfer fluids in concentrated solar power (CSP) applications.

Molten salt CO2 capture and electro-transformation (MSCC-ET) into capacitive carbon at medium temperature: effect of the electrolyte composition.

Electrochemical transformation of CO2 into functional materials or fuels (i.e., carbon, CO) in high temperature molten salts has been demonstrated as a promising way of carbon capture, utilisation and storage (CCUS) in recent years. In a view of continuous operation, the electrolysis process should match very well with the CO2 absorption kinetics. At the same time, in consideration of the energy efficiency, a molten salt electrochemical cell running at lower temperature is more beneficial to a process powered by the fluctuating renewable electricity from solar/wind farms. Ternary carbonates (Li : Na : K = 43.5 : 31.5 : 25.0) and binary chlorides (Li : K = 58.5 : 41.5), two typical kinds of eutectic melt with low melting points and a wide electrochemical potential window, could be the ideal supporting electrolyte for the molten salt CO2 capture and electro-transformation (MSCC-ET) process. In this work, the CO2 absorption behaviour in Li2O/CaO containing carbonates and chlorides were investigated on a home-made gas absorption testing system. The electrode processes as well as the morphology and properties of carbon obtained in different salts are compared to each other. It was found that the composition of molten salts significantly affects the absorption of CO2, electrode processes and performance of the product. Furthermore, the relationship between the absorption and electro-transformation kinetics are discussed based on the findings.

Thermal Property of Eutectic Molten Salts by Using Thermogravimetric Analyzer (TGA)

The thermal property of new composition of eutectic molten salt was investigated to obtain low melting point and better stability at temperature of 500°C as heat transfer fluid in solar thermal energy system. The NaCl used was purified from seawater. The eutectic molten salts were prepared in ten different weight ratios and experiments were carried out using nitrogen as inert gas with heating of 10°C/min to the temperature from 25°C to 500°C. Experimental results indicated that all mixtures exhibited low melting point (<163°C) and high stability. The thermal degradation of LiNO3, NaNO3, KNO3 and NaCl exhibit in the DTG profiles respectively. From the present study it can be concluded that major weight loss of the system is due to the dissociation of lithium nitrate to lithium oxides.

Corrosion Behavior of Eutectic Molten Salt solution on Stainless Steel 316L

The effect of eutectic molten salt on the corrosion behavior of a stainless steel 316L was investigated. To be used as thermal energy storage fluid, low melting point is one of the utmost important thermal properties amongst other. Eutectic salt was defined as a combination of two or more salt at different composition producing lower melting point than of it purest form. Four types of salts, sodium chloride (NaCl), potassium nitrate (KNO3), lithium nitrate (LiNO3), and sodium nitrate (NaNO3) were used to form fifteen mixtures of different composition to be tested on SS316L. The composition based on weight ratio was generated using response surface methodology (RSM). Eutectic composition of molten salt used as thermal energy storage fluid can possibly affect the corrosion occurrences due to the existence of Cl− ion and thereby influence the lifespan of the 316L stainless steel. This study indicates that the compositional different of eutectic mixture significantly affected the lifespan of the 316L stainless steel against corrosion. Two types of approaches are used in describing the corrosion behavior, which are resistance potential, Rp and corrosion rate. After test of 24hours in total immersion condition, 316L stainless samples have developed a scale of corrosion products made up of multiple oxides. Highest proportion of NaCl salt inhibits higher corrosion rate in eutectic molten salt as observed in resistance potential value and corrosion rate. Morphology of the stainless steel SS316L was further investigated under scanning electron microscope (SEM).

Preparation of Al-La Master Alloy by Thermite Reaction in NaF-NaCl-KCl Molten Salt

A NaF-NaCl-KCl ternary system containing La2O3 was investigated for the preparation of Al-La master alloy by the thermite reaction method. The solubility of La2O3 in NaF-NaCl-KCl molten salt was determined by the method of isothermal solution saturation. Inductively coupled plasma-optical emission spectroscopy and x-ray diffraction (XRD) analyses were used to consider the content of La2O3 in molten salt and the supernatant composition of molten salt after dissolution of La2O3, respectively. The results showed that the content of NaF had a positive influence on the solubility of La2O3 in NaF-NaCl-KCl molten salts, and the solubility of La2O3 could reach 8.71 wt.% in molten salts of 50 wt.%NaF-50 wt.% (44 wt.%NaCl + 56 wt.%KCl). The XRD pattern of cooling molten salt indicated the formation of LaOF in molten salt, which was probably obtained by the reaction between NaF and La2O3. The kinetic study showed that the thermite reaction was in accord with a first-order reaction model. The main influence factors on La content in the Al-La master alloy product, including molten salt composition, amount of Al, concentration of La2O3, stirring, reduction time and temperature, were investigated by single-factor experimentation. The content of La in the Al-La master alloy could be reached to 10.1 wt.%.

다성분 염화물계 진공 증류의 잔류 액체 거동 계산

파이로 공정은 사용후핵연료 관리 이슈 해결과 유용자원 재활용 제고의 목적으로 개발되고 있다. 파이로 공정 중 전해환원 공정은 LiCl을 전해질로 사용하여 산화물을 금속으로 전환시키는 공정으로 금속 전환체에 잔류염이 포함되므로 후속 공정이 요구된다. 진공 증류 공정은 다양한 용융염계에서 적용되어 왔으며 금속 전환체에서도 활용될 수 있다. 전해환원 금속 전환체 잔류염은 LiCl과 알카리 및 알카리토 금속 염화물을 포함한다. 본 연구에서는 이들 염화물들의 증기압을 추산하여 진공 증류 공정에서 잔류 액체의 조성변화를 계산하였다. 증류된 기체가 일정하게 제거되는 조건에서 물질수지와 기-액 평형식을 결합한 모델을 개발하였으며 증기압을 이용하여 무차원 시간에 대한 액체 조성 변화를 계산하였다. 공정 조건 변화 모사를 위해 온도와 용융염 조성을 변화시켜 거동을 비교하였다. 잔류염의 증류는 주성분인 LiCl에 의해 지배되었으며 LiCl 보다 증기압이 높은 CsCl은 쉽게 제거될 것이 예상되었다. 증기압이 유사한 RbCl은 LiCl과 일정한 조성이 유지되었다. 반면 증기압이 낮은 <TEX>$SrCl_2$</TEX>와 <TEX>$BaCl_2$</TEX>는 시간에 따라 농축되며 초기 조성이 높은 경우 증류 과정에서 석출될 가능성이 있는 것으로 예상되었다. Pyroprocessing has been developed for the purpose of resolving the current spent nuclear fuel management issue and enhancing the recycle of valuable resources. An electrolytic reduction of the pyroprocessing is a process to reduce oxides into metals using LiCl as an electrolyte and requires a post-treatment process due to the inclusion of residual salt in porous metal products. A vacuum distillation has been adopted for various molten salt systems and could be applied to the post-treatment process of the electrolytic reduction. The residual salt in the metal products includes LiCl, alkali chlorides, and alkaline earth chlorides. In this paper, vapor pressures of chlorides have been estimated and the composition changes on the residual liquid during the vacuum distillation process have been calculated. A model combining a material balance and vapor-liquid equilibrium relations has been proposed under a constant vapor discharging flow rate and liquid composition changes have been calculated using the vapor pressures with respect to a dimensionless time. The behaviors have been compared with temperature and molten salt composition changes to simulate the process condition variation. The distillation of the residual salt has been dominated by LiCl which is the main component of the salt and CsCl of which vapor pressure is higher than that of LiCl would be readily removed. RbCl exhibits similar vapor pressure with LiCl and maintains its composition. However, <TEX>$SrCl_2$</TEX> and <TEX>$BaCl_2$</TEX> of which vapor pressures are much lower than that of LiCl are concentrated with time and expected to be possibly precipitated during the distillation when the initial compositions are increased.

A New Electrodeposition Process of Crystalline Silicon Utilizing Water-Soluble KF–KCl Molten Salt

As a new production process of silicon solar cells, electrodeposition in high temperature molten salts is worth investigating. One of the most studied molten salt systems for electrodeposition of silicon is LiF-KF or LiF-NaF-KF containing K2SiF6. Many important works regarding this molten salt were undertaken in 1970s and 1980s [1-4]. Cohen reported that both epitaxial and polycrystalline continuous films of compact, coherent and well-adherent silicon were obtained from LiF-KF containing K2SiF6 at 1023 K [2]. One of the problems in the previous works is the removal of the adhered salt on the deposited silicon. This is due to the very low solubility of LiF and NaF to water. Another problem of the previous studies, when aiming at the solar grade purity, is the availability of high purity and low cost silicon ion sources. Conventionally, either K2SiF6 or anode dissolution of Si rod was utilized. However, it is difficult to prepare K2SiF6and Si rod having solar grade purity with low cost.From this background, we propose a new electrodeposition process of crystalline silicon, as shown in Fig. 1. Here, fluoride–chloride molten salt is used as an electrolyte and SiCl4 as a silicon ion source. Among many kinds of fluoride–chloride molten salts, we especially focus on the eutectic KF–KCl (45:55 mol%) molten salt. Since both KF and KCl are highly soluble to water, the adhered salt can be easily removed by water washing. Moreover, high purity SiCl4, which is produced with low cost, can be used as an ion source. When gaseous SiCl4is introduced into the molten KF–KCl, the following reaction proceeds:SiCl4 + 6F− → SiF6 2− + 4Cl− (1)Thus, after the introduction of SiCl4 gas, the resultant molten salt is same as the molten KF–KCl containing K2SiF6. Another advantage of the use of fluoride–chloride molten salt is the capability of Cl2 gas evolution on the carbon anode, which realizes the fixed composition of molten salt as well as the reuse of Cl2 for the production of SiCl4.Firstly, electrochemical reduction of Si(IV) ion on a silver electrode was investigated by cyclic voltammetry in a molten KF–KCl–K2SiF6 system at 923 K. Then, the optimum conditions were explored for electrodepositing adherent, compact and smooth silicon layer. Galvanostatic electrolysis was conducted for silver electrodes with various current densities and K2SiF6 concentrations. As typical results, Fig. 2 shows cross-sectional SEM images of the samples electrodeposited at (a) 38.8, (b) 155 and (c) 310 mA cm-2 in 2.0 mol% K2SiF6. A compact and smooth Si layer was obtained at an intermediate current density of 155 mA cm-2. At a lower current density of 38.8 mA cm-2, detachment of Si layer from the Ag substrate occurred during the water washing. At a higher current density of 310 mA cm-2, the morphology changed into nodular.AcknowledgementsThis study was financially supported in part by the CREST program by Japan Science and Technology Agency (JST).