Molten Salt Process Research Articles

Effectiveness evaluation of molten salt processing and ultrasonic cavitation techniques during the production of aluminium based hybrid nanocomposites - An experimental investigation

The aim of this research work is to evaluate the effectiveness of two processing methods and treatments on the mechanical properties Al 7075 based hybrid nanocomposite. In this study, Al 7075 based hybrid nanocomposites were fabricated through ultrasonic assisted cavitation and molten salt processing methods, which were further subjected to T6 treatment and deep cryogenic treatment. The microstructural observation was carried out on hybrid nanocomposites using optical and scanning electron microscopy. A hybrid nanocomposite produced by molten salt processing with ultrasonic assistance and optimized mechanical stirring was found to possess superior properties in terms of tensile strength, percentage elongation and hardness over all other samples. Out of these treatments, T6 was found to better enhance the mechanical properties of aluminium alloy and hybrid nanocomposite than deep cryogenic treatment.

Molten-salt synthesis of g-C3N4-Cu2O heterojunctions with highly enhanced photocatalytic performance

A novel heterojunction composite photocatalyst, g-C3N4-Cu2O, was fabricated via a high-temperature solid-phase molten salt method, hydrothermal method, and high-temperature calcination method. The structures and properties of as-synthesized samples were characterized using a range of techniques, such as X-ray photoelectron spectroscopy, scanning electron microscopy, UV–vis diffuse reflectance spectra and the Brunauer Emmet Teller (BET) theory. Their photocatalytic activity was evaluated based on the degradation of methyl orange (MO) under visible light irradiation. Based on the results obtained via TEM, XPS, EPR, and the other techniques, we verified that a heterojunction had formed. The g-C3N4-Cu2O heterojunction had the largest specific surface area, which provided plentiful activated sites for photocatalytic reaction. Moreover, g-C3N4-Cu2O showed the highest photocurrent effect and the minimum charge-transfer resistance. Furthermore, the g-C3N4-Cu2O heterojunction exhibited the highest MO photodegradation rate. After a series of radical trapping experiments and EPR analysis, we demonstrated that the holes and O2− radicals could be the main active species involved in MO photodegradation. The molten-salt process can improve the BET surface area to form abundant heterojunction interfaces, which serve as channels for photogenerated carrier separation and thereby enhance its light utilization and quantum efficiency.

In situ nano-sized spinel Li4Ti5O12 powder fabricated by a one-step roasting process in molten salts

In this work, for the first time, a one-step roasting process to fabricate nano-sized spinel Li4Ti5O12 powder (LTO-Ti) of which electronic conductivity was enhanced by doping unreacted titanium powder in the molten salts environment growing in situ directly from Ti powder is described. The crystal structure and morphology of LTO-Ti were examined with X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). When used as anodes for LIBs, the as synthesized nano-sized spinel Li4Ti5O12 powder (LTO-Ti) manifest an excellent rate capability and a significantly enhanced cycling performance. At charge/discharge current of 0.5C, 1C, 2C, 5C, 10C and 15C, the discharge capacity of the LTO-Ti electrode is 170, 162, 149, 143, 131, and 110 mAhg−1 respectively. After 200 cycles at current of 2C, its capacity retention was 96.7% with almost no capacity fading, exhibiting high capacity, superior rate property and outstanding cycling stability.

(Invited) Graphene Nanostructure By Molten Salt Process for Supercapacitor Applications

Graphene has a two-dimensional honeycomb structure consisting of a single layer of carbon atoms with a sp2-bonding configuration and has recently attracted significant attention owing to its unique properties, which include an exceptionally high specific surface area and high electrical and thermal conductivities as well as mechanical and chemical stability. However, graphene sheets exhibit an extreme tendency to agglomerate or restack. This leads to a surface that is electrochemically less effective, low electrical conductivity, and an interlayer spacing that hinders access to electrolyte ions, thus limiting the use of the sheets in supercapacitor applications. Introducing spacer materials such as metal oxide, carbonaceous between the graphene sheets is a viable strategy for preventing the sheets from agglomerating. In this study, we report the scalable fabrication of a high-quality hierarchical interconnected carbon nanostructure, combining two-dimensional graphene nanosheets and one-dimensional graphene nanoscrolls from graphite by a simple high temperature molten salt method. The electrode using the prepared material exhibited high reversible specific capacitance, excellent rate capability, and good cyclability because of their unique interconnected networks of carbon nanostructures with high electrochemically active surface area and high electrical conductivity. More details on the synthetic procedure, electrochemical and structural properties will be presented at the meeting.

Efficient photodegradation of methylthioninium chloride dye in aqueous using barium tungstate nanoparticles

BaWO4 nanoparticles were successfully used as the photocatalysts in the degradation of methylthioninium chloride (MTC) dye at different pH levels of aqueous solution. Pure phase of barium tungstate (BaWO4) nanoparticles was synthesized by modified molten salt process at 500 °C for 6 h. Structural and morphological characterizations of BaWO4 nanoparticles (average particle size of ~40 nm) were studied in details using powder x-ray diffraction (XRD), FTIR, Raman, energy-dispersive, electron microscopic, and x-ray photoelectron spectroscopy (XPS) techniques. Direct band gap energy of BaWO4 nanoparticles was found to be ~3.06 eV from the UV–visible absorption spectroscopy followed by Tauc’s model. Photocatalytic properties of the nanoparticles were also investigated systematically for the degradation of MTC dye solution in various mediums. BaWO4 nanoparticles claim the significant enhancement of the photocatalytic degradation of aqueous MTC dye to non-hazardous inorganic constitutes under alkaline, neutral, and acidic mediums.

Production of metals and oxygen from coal fly ash by aluminothothermic and electrochemical reduction process

In-situ utilization of lunar resources for the production of metals and oxygen is essential for lunar exploration. The main emphasis of this article is on a candidate approach that combined aluminothermic reduction and electrolysis process in cryolite molten salts to produce both Al-based alloy and oxygen from lunar soil simulants. The influences of reductant amount and reaction time on the aluminothermic reduction of coal fly ash in cryolite melts at 1233 K (960 °C) were studied. The results obtained by XRF, XRD, SEM and EDS analysis show that the products mainly contain Al, Si, Fe, and little Ti, while the main phases of the products are Al and Si. With the increase of reaction time, the content of Si in alloy products increases from 4.43 wt% to 7.47 wt%. In addition, the remaining alumina-rich cryolite melts was electrolyzed at 1233 K (960 °C) using 56 wt%Fe-44 wt%Ni alloy inert anode. The cathode product obtained by galvanostatic electrolysis for 4 h was characterized, and the results showed that the phase composition of the cathode product was pure Al, while the highest aluminum content in the cathode product is 99.431 wt%. Besides, the outlet gas at the anode during electrolysis is pure oxygen according to the analysis of gas chromatography.

Microstructural and Radioluminescence Characteristics of Nd3+ Doped Columbite-Type SrNb2O6 Phosphor.

Undoped and different concentration Nd3+ doped SrNb2O6 powders with columbite structure were synthesized by molten salt process using a mixture of strontium nitrate and niobium (V) oxide and NaCl-KCl salt mixture as a flux under relatively low calcining temperature. X-ray diffraction analysis results indicated that SrNb2O6 phases found to be orthorhombic columbite single phase for undoped, 0.5 and 3mol% Nd3+ doping concentrations. Phase composition of the powders was examined by SEM-EDS analyses. Radioluminescence properties of Nd3+ doped samples from UV to near-IR spectral region were studied. The emissions increased with the doping concentration of up to 3mol%, and then decreased due to concentration quenching effect. There is a sharp emission peak around 880nm associated with 4F5/2→4I9/2 transition in the Nd3+ ion between 300 and 1100nm. The broad emission band intensity was observed from 400 to 650nm where the peak intensities increased by increasing Nd3+ doping concentration. All the measurements were taken under the room temperature.

In situ nano-sized ZrC/ZrSi composite powder fabricated by a one-pot electrochemical process in molten salts

Nano-sized ZrC/ZrSi composites are in situ synthesized from ZrSiO4 and carbon, through a one-pot electrochemical process at relatively lower temperature.

Solid‐state immobilisation of titanium dioxide nanoparticles onto nanoclay

The impregnation of nanoclay with TiO2 was carried out by new and fast method. Nanoclay was submitted only to a single step molten salt process. Scanning electron microscopy showed that TiO2 nanoparticles precipitated over the nanoclay surface after treatment. Structural characterisation of nanocomposite by X-ray diffraction displayed the change of basal peak after even 1 min process at both temperatures. UV-visible spectroscopy showed the presence of TiO2. However, the band gap was not affected by process temperature and time. XRF graphs revealed that by increasing temperature and time of the molten salt process, the amount of loaded titanium dioxide on the nanoclays was enhanced. Through this quick process, an inexpensive nanocomposite with properties comparable with those from more elaborate processes is produced.

Thermal Energy Storage in Molten Salts: Overview of Novel Concepts and the DLR Test Facility TESIS

The paper gives an overview of various high temperature thermal energy storage concepts such as thermocline [3], floating barrier [4] or embedded heat exchanger [7] that have been developed in recent years. In this context, a description of functionality, a summary of the technical specification and the state of development of each concept is given. The latter includes the next steps and open questions, respectively. Additionally, further research topics, such as the stability of material selections are presented. In order to answer many of the open questions, a new molten salt test facility called “Test facility for thermal energy storage in molten salts (TESIS)” is under construction at DLR. This facility has two separate sub-systems, which can be operated independently from each other. One system is designed to investigate new single tank storage concepts focusing on the thermocline-filler concept. The other system is a flexible test section for molten salt process technology and component tests, such as valves, solar receiver tubes and instrumentation. Both sub-systems can operate at temperatures up to 560°C, using nitrate salt as the storage and heat transfer media. Experimental results on the compatibility of filler materials and molten salt are presented. In addition, the current construction and development status of the facility will be reported.

Microwave-assisted molten-salt rapid synthesis of isotype triazine-/heptazine based g-C3N4 heterojunctions with highly enhanced photocatalytic hydrogen evolution performance

Rapid synthesis and construction of graphitic carbon nitride (g-C3N4) based heterojunctions, cost-effective metal-free photocatalysts for hydrogen evolution reaction (HER) under solar irradiation, is of highly practical significance. This work reports a one-pot microwave-assisted molten-salt (mw-ms) process to rapidly synthesize isotype triazine-/heptazine based g-C3N4 heterojunctions with highly enhanced photocatalytic HER performance using melamine as the single-source precursor. The typical sample (mw-ms-g-C3N4) was obtained by thermally polymerizing melamine molecules at 550°C for 30min in the media of eutectic KCl/LiCl salts under microwave irradiation in air. The analyses of phases, chemical compositions and microstructures indicate that the mw-ms-g-C3N4 sample consists of an isotype triazine-/heptazine based g-C3N4 heterojunction, taking on a plate-like morphology with a specific surface area (SBET) of 25.7m2g−1. Comparatively, the g-C3N4 sample synthesized via an electric-resistance molten-salt (er-ms) process at 550°C for 240min is composed of a triazine-based g-C3N4 phase with a SBET of 58.1m2g−1, whereas the samples obtained by electric-resistance heating (er, at 550°C for 240min) and microwave heating (mw, at 550°C for 30min) processes consist of a heptazine-based g-C3N4 phase. The mw-ms-g-C3N4 sample shows a photocatalytic HER rate of 1480μmolg−1h−1, which is 5 times that (300μmolg−1h−1) of the er-ms-g-C3N4 sample, 15 times that (95μmolg−1h−1) of the er-g-C3N4 sample and 23 times that (63μmolg−1h−1) of the mw-g-C3N4 sample, under the similar visible-light (λ≥420nm) irradiation. The typical apparent quantum yield of the mw-ms-g-C3N4 sample at 420nm is up to 10.7%. The UV–vis DR spectra suggest that both the triazine-based g-C3N4 and heptazine-based g-C3N4 phases have a similar bandgap of ∼2.66eV, whereas the Mott-Schottky analysis indicates that the triazine-based g-C3N4 phase has a more positive flat conductive potential (−0.90V) than the triazine-based g-C3N4 phase (−1.22V). Due to the suitable alignment of their energy bandgap structures, the isotype triazine-/heptazine based g-C3N4 hybrids in the mw-ms-g-C3N4 sample form a type II heterojunction of semiconductor/semiconductor, which provides a convenient carrier transfer path and leads to more efficient separation of photo-generated electron-hole pairs than the other samples. The synergistic effects of microwave heating and molten-salt liquid polycondensation provide a robust platform for rapid and large-scale construction of isotype g-C3N4/g-C3N4 heterojunctions as metal-free high-performance HER photocatalysts using a simple single-source precursor.

Hydrogen Diffusion and Trapping in Fluoride Salt Melts and Graphite Electrodes

Molten salt processes were developed decades ago as a promising technology for material processing and industrial process heat applications. This technology is still considered applicable for thermal energy storage and power production. Liquid salts can indeed be considered legitimate candidates for several heat transfer applications, thanks to favorable physical properties such as high boiling point, low vapor pressures, large specific heat, and large thermal conductivity. In particular, fluoride-based molten salts are considered as a coolant for several nuclear reactor advanced design owing to favorable neutronic cross sections and reduced corrosion of structural materials. FHR, Fluoride-salt cooled High-temperature Reactors, use flibe salt (2LiF-BeF2) to cool a graphite pebble-bed-fueled core. A detailed knowledge of the chemistry of fluoride melts is important for an efficient corrosion control and reliable FHR reactor design. The corrosion mechanisms in a molten salt system are different from aqueous systems; molten salt chemistry prevents the formation of a thermodynamically stable passivation layer on structural metals, therefore the corrosion control has to be achieved through the control of the redox potential of the fluoride salt. Solubilities and diffusivities of metals, rare earths, fission products and tritium in the fluoride melt and within graphite framework, are of direct interest for the FHR design community. Ionic coordination in the liquid and gas phase also needs to be fundamentally understood in order to address the above mentioned problems. This paper will provide a review of the electrochemical techniques and spectroscopy methods used in the past to study fluoride salt systems. Furthermore, state of the art of techniques used in other high temperature molten salt environments and low temperature ionic liquids will be reviewed. Tritium management is the main design challenge for the FHR reactors. Tritium is produced through transmutation of Lithium-6 in a neutron environment and might be released into the atmosphere through the heat exchanger walls. However, in FHR reactors, the graphite pebble bed and graphite reflector may be the ultimate sink for tritium. Goal of the ongoing research at UW Madison is to define tritium diffusion and absorption into graphite, using hydrogen as a surrogate. This paper will address theoretical electrochemical techniques used in aqueous system to study absorption of hydrogen into metals (Potentiostatic Double-Step [1]) and also experimental techniques (Electrochemical Impedance Spectroscopy [2]) applicable to the same issue. These techniques will be applied initially to graphite/water systems and later to the graphite/flibe system. The paper will present preliminary results and report the challenges pertaining to experimental setup with fluoride salt systems, due to the high melting temperatures, the highly corrosive nature of the salt, in addition tothe toxicity of Beryllium, main component of flibe salt. REFERENCES: [1] B. Pound, G. Wright, et al., ‘A potentiostatic double-step method for measuring hydrogen atom diffusion and trapping in metal electrodes—II. Experimental’, Acta Metall., vol. 35, no. I, pp. 263–270, (1987). [2] D. D. Macdonald and M.C.H. McKubre. “Impedance Measurements in Electrochemical Systems”. Chapter 2 in Modern Aspects of Electrochemistry. 14, 61(1982).

Development of Carbon Anodes for Use in Electrolytic Reduction of Uranium(IV) Oxide

Recycling of used nuclear fuel using pyrochemical methods involves a series of molten salt processes performed at elevated temperatures and results in the separation of actinides from fission products. The first step in the pyrochemical treatment of light water reactor (LWR) oxide fuel is electrolytic reduction. This electrochemical operation converts used fuel oxides to metal before electrochemical separations can be performed by electrorefining. Conversion of oxides to metal is commonly performed in molten LiCl-Li2O at 650℃. Electrolytic reduction of UO2, the primary component of used nuclear fuel, has been successfully demonstrated at the kilogram-scale using platinum anodes1-2, but progression toward engineering scale presents different anode material requirements. Carbon, specifically graphite, is commonly used for ore refining in commercial molten salt processes because it is cost effective, readily available, and easily fabricated to meet scale-up requirements. Use of carbon anodes in LiCl-Li2O results in different salt chemistries, with respect to using platinum or other oxygen evolving electrode materials. In particular, the evolved gaseous byproduct of carbon anodes is CO and CO2, which can react with Li2O to form Li2CO3. If the carbonate concentration is allowed to increase, the desired reaction, namely the transport of oxide from the cathode to the anode via Li2O, is replaced by the undesirable transport of carbon from the anode to the cathode via Li2CO3. This report will discuss use of carbon as an anode material for the electrolytic reduction of UO2 to uranium metal in molten LiCl-Li2O at 650℃. A discussion of the salt chemistry, minimizing Li2CO3 formation, method developments, cell operation, and reduction efficiency will be presented.

Highly efficient supercapacitor electrode with two-dimensional tungsten disulfide and reduced graphene oxide hybrid nanosheets

Two-dimensional (2D) nanostructures with their high surface area and large in-plane conductivity have been regarded as promising materials for supercapacitors (SCs). Tungsten disulfide (WS2) is highly suitable for charge accumulation with its abundant active sites in the interspacing between the 2D structures and the intraspacing of each atomic layer, as well as on the tungsten centers with the charges generated by the Faradaic reactions. This study proposes the preparation of well-constructed WS2/reduced graphene oxide (RGO) nanosheets using a simple molten salt process as the electroactive material for SCs, which presents a high specific capacitance (CF) of 2508.07 F g−1 at the scan rate of 1 mV s−1, because of the synergic effect of WS2 with its large charge-accumulating sites on the 2D planes and RGO with its highly enhanced conductivity and improved connections in the WS2 networks. The excellent cycling stability of 98.6% retention after 5000 cycles charge/discharge process and the Coulombic efficiency close to 100% for the entire measurement are also achieved for the WS2/RGO-based SC electrode. The results suggest the potential for the combination of the 2D metal sulfide and carbon materials as the charge storage material to solve the energy problems and attain a sustainable society.

Highly adsorptive mesoporous carbon from biomass using molten-salt route

A general, simple, eco-friendly, and scalable route of obtaining highly adsorptive activated carbon from biomass using ZnCl2 molten-salt process is reported. Upon using three biomasses (pine wood chips, banana peels, and pine leaves) as the model carbon precursors, the molten-salt process yields different porous structures of carbons with a specific surface area up to 1624.8 m2 g−1. The properties of the three obtained activated carbons were characterized by SEM, TEM, XRD, FT-IR, and BET. The results show that all of them are amorphous with oxygen-containing functional groups on the surface. Besides, they exhibit remarkable adsorption capacity for removal of methylene blue of 425, 225, and 300 mg g−1, respectively. Since various biomasses can be transformed into activated carbons, molten-salt route is proved general and feasible, which offers a simple and environmentally sound way for transforming low-cost and abundant biomass to highly adsorptive and valuable activated carbon. This method also opens up a new avenue toward the application of biomass in various fields.

Development of Electrochemical Methods for High Temperature Molten Fluoride Salt Systems

Molten salt processes were developed several decades ago as a promising technology for material processing and industrial process heat applications. This technology is still seen today as extremely suitable for thermal energy storage and power production. Liquid salts can indeed be considered legitimate candidates for several heat transfer applications thanks to favorable physical properties such as high boiling point, low vapor pressures, large specific heat, and large thermal conductivity. In December 2015 a “Molten Salt Electrochemistry Short Course” was organized at University of Wisconsin Madison. The course was taught by Professor Digby MacDonald, a renowned expert in electrochemistry techniques for corrosion studies in aqueous environments. The course concentrated on the fundamentals of electrochemistry, with specific references to processes such as corrosion, electro-deposition, passivity and passivity breakdown, stress corrosion cracking, and corrosion fatigue. The course also included a discussion of the specialized techniques employed to study hydrogen in metals and carbon (graphite). This paper will discuss the applicability of each of these electrochemistry techniques to molten fluoride salts and will provide an overview of the current status of these techniques in fluoride ionic liquids. The definition of a thermodynamically stable reference electrode (RE) has emerged as one of the critical points required to rigorously apply electrochemistry analytical techniques to fluoride salt systems. Therefore, this paper will review the fluoride molten salt RE designs developed in the 1960s and 1970s and in recent years. Additionally this paper will report designs and testing of the fluoride salt REs under development at University of Wisconsin -Madison.

Synthesize CCTO Using Different Mixing Media

In recent years, there has been an increasing interest on high dielectric constant that have significant applications in electronic devices. Dielectric materials have many technological applications such as capacitors, resonators and filters. High dielectric ceramic capacitors based perovskite oxides are necessary for modern electronic devices and are found to be suitable for a wide range of applications. Subramanian et al. discovered the high dielectric constant of CaCu3Ti4O12 (CCTO) ~ 10,000 at room temperature. CCTO has the cubic perovskite crystal structure and high dielectric constant of ~ 104 up to 105 at radio frequency and good temperature stability over a wide temperature range [1,2]. These properties were desired for various microelectronic applications. With the high dielectric constant, the material can store more charge and the values make CCTO an attractive material for ultra-high energy density capacitors. However, this properties can be accomplished if single phase of CCTO is formed. Many research have been done recently on the synthesis of the cubic perovskite CCTO and many techniques are working such as sol-gel route [3], combustion techniques [4], molten salt process [5] and etc., but this technique is difficult and complex process during sample preparation.

Chemically synthesized boron carbon oxynitride as a new cold cathode material

Synthesis of boron carbon oxynitride (BCNO) nanosheets at different temperature from amorphous to crystalline regime has been reported. The synthesis was done by a simple molten salt process using sodium borohydride and urea as precursors.Transmission electron microscopic study confirms the formation of sheet-like structure of the as-synthesized material. The performances of the as-synthesized BCNO nanosheets as cold cathode materials have been studied for the first time in the high vacuum electron field emission set up. It has been seen that the material gives considerable field emission current with turn on field as low as 2.95V/μm with good stability and thus a new cold cathode material can be postulated.

Particle Size and Crystal Phase Dependent Photoluminescence of La2Zr2O7:Eu3+ Nanoparticles

Herein, La2Zr2O7:5% Eu3+ nanoparticles (NPs) with different sizes have been synthesized for the first time through a modified facile molten salt process using a single‐source complex precursor of La(OH)3·ZrO(OH)2:Eu(OH)3·nH2O. It was found that the concentration of the added ammonia to co‐precipitate the corresponding metallic ions to form the precursor can influence the final particle size of the fluorite La2Zr2O7:5%Eu3+ NPs. Furthermore, the crystal phase of the La2Zr2O7:5%Eu3+ NPs was transferred from fluorite to pyrochlore after thermal treatment at 1000°C. The relationship between photoluminescence (PL), quantum yield (QY), particles size and crystal phase has been further investigated through fluorescence decay, site symmetry, and Judd–Ofelt (J–O) analysis. Specifically, PLQY and lifetime increase with increasing particle size of the fluorite La2Zr2O7:5%Eu3+ NPs. Additionally, crystal phase transfer from fluorite to pyrochlore resulted in large PLQY decrease and moderate lifetime increase in the La2Zr2O7:5%Eu3+ NPs.

Preparation of TiC/SiC composites from Ti-enriched slag by an electrochemical process in molten salts

TiC/SiC composites were successfully prepared from Ti-enriched slag via an electro-deoxidization process in molten CaCl2–NaCl eutectic melt at 700°C in argon. Electrolysis was conducted at an applied voltage of 3.2V using a graphite anode and a composite cathode composed of graphite powder and Ti-enriched slag. According to the results, a possible reaction pathway was suggested: firstly the complex Ti-compounds were decomposed into TiO2-based simple oxides; then a mass of CaTiO3 and a spot of CaSiO3 formed accompanied by the reduction of titanium dioxide to titanium sub-oxide such as TiO, Ti2O; finally the graphite powder involved in the formation of SiC, TiC1−xOx, which were further converted into SiC and TiC by prolonging electrolysis.