Eutectic Salt Mixture Research Articles

Effect of hydrothermal carbonization and eutectic salt mixture (KCl/LiCl) on the pyrolysis of Kraft lignin as revealed by thermal analysis coupled to advanced high-resolution mass spectrometry

The production of graphite requires high temperatures, and fossil petroleum, coal, or nutshells are frequently used as a carbon source. As a replacement, Kraft lignin, a by-product of the pulp and paper industry, is a promising carbon source with a prefigured aromatic network. Biomass-based feedstocks with improved characteristics can be obtained by hydrothermal carbonization, but the chemical nature of this process is not fully understood yet. Moreover, adding a eutectic salt mixture (LiCl/KCl) to the pyrolysis of Kraft lignin and HTC lignin can improve the graphitization at lower temperatures. In this study, thermal analysis with online mass spectrometric detection of the evolved gas mixture was applied to explore the influence of the eutectic salt mixture on the char conversion process. Aside from classical pyrolysis gas chromatography mass spectrometry, thermogravimetry coupled with soft photoionization mass spectrometry allowed to identify phenol, hydrogen sulfide, dimethyl sulfide, and various larger lignin fragments. These larger dimeric/trimeric methoxyphenol derivatives were successfully validated by means of high-resolution mass spectrometry equipped with soft atmospheric pressure chemical ionization. The investigations indicated the catalytic influence of the salt mixture on the production process of the biochars, achieving partially graphitization already at relatively low temperatures (700 °C). On the morphology, Raman spectroscopy and electron microscopy revealed the evolution of the carbon structures and revealed that the materials have typical features for amorphous carbon.

A Novel LiNO3-Based Eutectic Salt Mixture for Industrial Heat Treatment

Abstract A potassium nitrate–lithium nitrate–sodium nitrate (KNO3-LiNO3-NaNO3) eutectic mixture having a low melting point has been proposed as an alternative high-temperature quench medium. Inconel and steel probes were used to compare the quench performance of a conventional sodium nitrite (NaNO2) eutectic mixture and the proposed alternative medium at different bath temperatures. For the Inconel probe, the heat extraction rate was higher in the eutectic LiNO3 mixture maintained at 150°C. At elevated bath temperatures of 200°C, 250°C, and 300°C, the heat extraction rate was higher in the eutectic NaNO2 mixture. AISI 52100 steel probes quenched in eutectic LiNO3 quench medium at 150°C and 200°C showed higher hardness. At bath temperatures of 250°C and 300°C, the hardness of AISI 4140 steel probes quenched in both media was comparable. Wettability studies on Inconel and steel surfaces revealed the occurrence of nonuniform dilation of a LiNO3 eutectic mixture droplet. On the steel surface, the phenomenon occurred at lower temperatures, which resulted in an extended boiling stage and increased hardness.

C-276 nickel alloy corrosion in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions

Thermal energy storage (TES) is a key component for the practical application of concentrating solar thermal power. Recently, a push for higher temperatures has increased interest in molten salts with melting points above 550 °C as phase change thermal energy storage media. Corrosion of a nickel superalloy (C-276) with a eutectic salt mixture (59.5 %wt Na 2 CO 3 /40.5 %wt NaCl) at 650 °C was analysed under both isothermal conditions and with thermal cycle between 600 and 650 °C. Corrosion rates of 400–500 μm/year under isothermal conditions and 200–300 μm/year under thermal cycling were determined by measuring thickness loss, and oxide growth was primarily responsible for the metal loss. Thermal cycling reduced corrosion thickness, however it remained intact with no cracking or buckling. Post-test analysis of the salt found that molybdenum, manganese and tungsten were most likely to be removed from the alloy. Isothermal conditions did not cause chromium to be removed from the alloy by the salt. However, chromium was present in the salt after thermal cycling, due to its removal from the oxide layer. Whilst the corrosion was above the maximum acceptable corrosion rate for a TES system, this study shows that oxide layers formed on alloy C-276 have protective capabilities in this salt, gaining maximum thickness after about 400 h and preventing the salt from attacking the metal directly. Longer testing with more samples is require to determine if the corrosion of this alloy is halted due to the oxide layer growth on the metal surface. • Alloy C-276's compatibility with Na 2 CO 3 /NaCl molten salt at 650 °C was assessed. • Tests were undertaken under both isothermal and thermal cycling conditions. • Cross sections were analysed using microscopy for corrosion thickness and structure. • Oxide growth was the main corrosion mechanism, forming 40–60 μm thick scale layers. • Analysis of the salt found Mo, Mn and W dissolved into the salt from the alloy.

Electrochemical Corrosion Studies in Molten Chloride Salts

This study investigates corrosion of Fe–(Cr)–Ni model alloys in eutectic molten chloride salt mixtures that broadly simulate the heat transfer circuits of molten salt cooled nuclear reactors. The primary focus is impurity-driven corrosion and selective dissolution of electrochemically reactive element(s) (dealloying), which are known degradation modes for Fe– and Ni-based alloys in molten salts. This study demonstrates that a Mg rod performs close to a true Mg∣Mg2+ reference electrode system in a MgCl2 containing mixture. Mg dynamic reference electrodes validated the performance of the ordinary Mg reference electrode. The residual moisture content in molten salts is measured by cathodic polarization of Pt, which showed that convection of molten salts, after step-wise heating, through a bundle of Mg ribbons dropped the moisture content by 30-fold. The effect of impurities in the molten salt mixture, moisture and NiCl2, on dealloying was also investigated at low homologous temperature. It was found that the universal parting limit for dealloying is decreased to a value of approximately 32 at% Fe when Ni from the parent alloy is close to equilibrium with Ni2+ ions added to the eutectic molten salts. Results support the percolation/surface diffusion concept for dealloying in molten salts at low homologous temperature.

Molten Salt‐Shielded Synthesis (MS3) of MXenes in Air

MXenes are two‐dimensional transition metal carbides and/or nitrides with unique physiochemical properties and have attracted extensive interest in numerous fields. However, current MXene synthesis methods are limited by hazardous synthesis conditions, high production costs, or difficulty in large‐scale production. Therefore, a general, safe, cost‐effective, and scalable synthesis method for MXenes is crucial. Here, we report the fast synthesis of MXenes in the open air using a molten salt‐shielded synthesis (MS3) method, which uses Lewis‐acid salts as etchants and a low‐melting‐point eutectic salt mixture as the reaction medium and shield to prevent MXene oxidation at high temperatures. Carbide and nitride MXenes, including Ti3C2Tx, Ti2CTx, Ti3CNTx, and Ti4N3Tx, were successfully synthesized using the MS3 method. We also present the flexibility of the MS3 method by scaling the etching process to large batches of 20 and 60 g of Ti3AlC2 MAX precursor in one pot. When used as negative electrodes, the prepared MS3‐MXenes delivered excellent electrochemical properties for high‐rate Li‐ion storage.

Ionothermal Synthesis of Covalent Triazine Frameworks in a NaCl-KCl-ZnCl2 Eutectic Salt for the Hydrogen Evolution Reaction.

Covalent triazine-based frameworks (CTFs) are typically produced by the salt-melt polycondensation of aromatic nitriles in the presence of ZnCl2 . In this reaction, molten ZnCl2 salt acts as both a solvent and Lewis acid catalyst. However, when cyclotrimerization takes place at temperatures above 300 °C, undesired carbonization occurs. In this study, an ionothermal synthesis method for CTF-based photocatalysts was developed using a ternary NaCl-KCl-ZnCl2 eutectic salt (ES) mixture with a melting point of approximately 200 °C. This temperature is lower than the melting point of pure ZnCl2 (318 °C), thus providing milder salt-melt conditions. These conditions facilitated the polycondensation process, while avoiding carbonization of the polymeric backbone. The resulting CTF-ES200 exhibited enhanced optical and electronic properties, and displayed remarkable photocatalytic performance in the hydrogen evolution reaction.

Ionothermal Synthesis of Covalent Triazine Frameworks in a NaCl‐KCl‐ZnCl2Eutectic Salt for the Hydrogen Evolution Reaction

AbstractCovalent triazine‐based frameworks (CTFs) are typically produced by the salt‐melt polycondensation of aromatic nitriles in the presence of ZnCl2. In this reaction, molten ZnCl2salt acts as both a solvent and Lewis acid catalyst. However, when cyclotrimerization takes place at temperatures above 300 °C, undesired carbonization occurs. In this study, an ionothermal synthesis method for CTF‐based photocatalysts was developed using a ternary NaCl‐KCl‐ZnCl2eutectic salt (ES) mixture with a melting point of approximately 200 °C. This temperature is lower than the melting point of pure ZnCl2(318 °C), thus providing milder salt‐melt conditions. These conditions facilitated the polycondensation process, while avoiding carbonization of the polymeric backbone. The resulting CTF‐ES200exhibited enhanced optical and electronic properties, and displayed remarkable photocatalytic performance in the hydrogen evolution reaction.

A high temperature cell for investigating interfacial structure on the molecular scale in molten salt/alloy systems.

In this work, we describe the designand development of an in situ neutron reflectometry cell for high temperature investigations of structural changes occurring at the interface between inorganic salts, in their molten state up to 800°C, and corrosion resistant alloys or other surfaces. In the cell, a molten salt is confined by an annular ring of single crystal sapphire constrained between the sample substrate and a sapphire plate using two gold O-rings, enclosing a liquid salt volume of 20 ml, along with a dynamic cell volume to accommodate expansion of the liquid with heating. As a test case for the cell, we report on an in situ neutron reflectometry measurement of the interface between a eutectic salt mixture of MgCl2-KCl (32:68 molar ratio) and a single crystal sapphire substrate at 450°C, resulting in the formation of a 60 Å layer having a scattering length density of 1.72 × 10-6 Å-2. While the origin of this layer is uncertain, it is likely to have resulted from the salt reacting with an existing impurity layer on the sapphire substrate.

Effect of excess Mg to control corrosion in molten MgCl2 and KCl eutectic salt mixture

Structural alloys may experience corrosion when exposed to molten chloride salts due to selective dissolution of active alloying elements. One way to prevent this is to make the molten salt reducing. For the KCl + MgCl2 eutectic salt mixture, pure Mg can be added to achieve this. However, Mg can form intermetallic compounds with nickel at high temperatures, which may cause alloy embrittlement. This study shows that an optimum level of excess Mg could be added to the molten salt which will prevent corrosion of alloys like 316 H, while not forming any detectable Ni-Mg intermetallic phases on Ni-rich alloy surfaces.

Calculation of the Liquidus in Eutectic Salt Mixtures LiF–NaF, LiF–KF, NaF–KF, NaF–CsF, and KF–CsF Using a Thermodynamic Perturbation Theory

Calculation of the Liquidus in Eutectic Salt Mixtures LiF–NaF, LiF–KF, NaF–KF, NaF–CsF, and KF–CsF Using a Thermodynamic Perturbation Theory

Perspectives on selected alloys in contact with eutectic melts for thermal storage: Nitrates, carbonates and chlorides

Increasing the operating temperature of molten salt-based concentrated solar power plants is of paramount importance to enable next-generation gas turbines and an overall increase in power conversion efficiency. The issue is how to mitigate the degradation of necessary metallic components in highly corrosive salt environments. In this study, three eutectic salt mixture candidates, nitrates, carbonates, and chlorides, are brought into contact with stainless steel (316H or 304L) and the FeCrAl alloy Kanthal® APMT. The post-exposure analysis is discussed in terms of the overall performance of each alloy as concerns mass change, scale growth, internal attack, and leaching. Significant reduction of corrosion is realised through the ability of Kanthal® APMT to form aluminium oxide species at the surface in contact with alkali nitrates and carbonates. On the other hand, aluminium is leached most efficiently in contact with chlorides, which causes a deeper attack on Kanthal® APMT than alloy 304L.The overall conclusion is that only by employing a holistic perspective on all individual measurements can a long-term performance estimation be formulated.

Molten Salt Templated Synthesis of Covalent Isocyanurate Frameworks with Tunable Morphology and High CO2 Uptake Capacity.

The use of reactive molten salts, i.e., ZnCl2, as a soft template and a catalyst has been actively investigated in the preparation of covalent triazine frameworks (CTFs). Although the soft templating effect of the salt melt is more prominent at low temperatures, close to the melting point of ZnCl2, leading to the formation of abundant micropores, a significant mesopore formation is observed that is due to the partial carbonization and other side reactions at higher temperatures (>400 °C). Evidently, high-temperature synthesis of CTFs in various eutectic salt mixtures of ZnCl2 with alkali metal chloride salts also leads to mesopore formation. We reasoned that using the isocyanate moieties instead of cyano groups in the monomer, 1,4-phenylene isocyanate, could enable efficient interactions between carbonyl moieties and alkali metal ions to realize efficient salt templating to form covalent isocyanurate frameworks (CICFs). In this direction, the trimerization of 1,4-phenylene diisocyanate was carried out under ionothermal conditions at different reaction temperatures using ZnCl2 (CICF) and the eutectic salt mixture of KCl/NaCl/ZnCl2 (CICF-KCl/NaCl) as the reactive solvents. We observed notable differences in the morphologies of the two polymers, whereas CICF showed irregular-shaped micrometer-sized particles, the CICF-KCl/NaCl exhibited a filmlike morphology. Moreover, favorable ion-dipole interactions between alkali metal cations and oxygen atoms of the monomer facilitated two-dimensional growth and the formation of a purely microporous framework in the case of CICF-KCl/NaCl along with a near theoretical retention of the nitrogen content at 500 °C. The CICF-KCl/NaCl showed a BET surface area of 590 m2 g-1 along with a CO2 uptake capacity of 5.9 mmol g-1 at 273 K and 1.1 bar because of its high microporosity and nitrogen content. On the contrary, in the absence of alkali metal ions, CICF showed high mesopore content and a moderate CO2 uptake capacity. This study underscores the importance of the strength of the interactions between the salts and the monomer in the ionothermal synthesis to control the morphology, porosity, and gas uptake properties of the porous organic polymers.

Corrosion, rheology, and thermal ageing behaviour of the eutectic salt-based graphene hybrid nanofluid for high-temperature TES applications

The eutectic salt-based nanofluids have recently been explored as a Thermal Energy Storage (TES) material for high-temperature applications due to their improved thermal properties. However, the investigation of such fluids in corrosion, rheology, and thermal ageing is minimal. This study focused on the effect of newly developed hybrid nanofluids based on GO/TiO2 nanoparticles and the eutectic salt mixture on thermophysical properties for high-temperature applications. The results showed that the SS316 immersed in hybrid nanofluid formed a protective layer to prevent corrosion compared to the eutectic salt mixture after 7 days. The surface morphologies and cross-sections were studied using SEM (Scanning Electron Microscope) and EDX (Energy-dispersive X-ray spectroscopy). The addition of hybrid nanoparticles into the eutectic salt mixture exhibits minimal effect on the viscosity that varies about −0.51–11.05% from 250–400 °C range at a shear rate of 250/s. Moreover, the rheological study displayed a strong correlation (R2 of 0.99) with the experimental data. Comparative studies showed that the viscosity values of both eutectic salt mixture and hybrid nanofluid showed good agreement with the literature. Finally, the thermal ageing behaviour of eutectic salt mixture and hybrid nanofluids was investigated for 15 days, and the samples were collected on the 1st, 7th, and 15th day. The average specific heat capacity of hybrid nanofluids and eutectic salt was reduced by 8.76% and 28.94%, respectively, after 15 days, indicating poor thermal stability of eutectic salt compare to hybrid nanofluids.

Parametric study of thermodynamic and cost performance of thermal energy storage coupled with nuclear power

Thermal energy storage for nuclear power can increase the flexibility of low carbon baseload power plants and facilitate greater use of renewable energy sources. The thermodynamic performance and cost of approaches to integrate thermal energy storage with a 1050 MW nuclear power plant are compared in a parametric study over practical ranges of charge/discharge durations, peaking power and round-trip efficiency of the storage. Conceptual designs for sensible and latent heat storage modules are presented. The results quantify for the first time how different options for thermal energy storage affect technical performance of a nuclear power plant and are interpreted to identify the most efficient options and operating conditions. The three configurations are distinguished by charge and discharge operation. Configuration I charges the storage via high-pressure steam supply and discharges steam to the low-pressure turbine. Configuration II charges via high–pressure steam and discharges preheated condensate to the steam generator. Configuration IIII charges via low-pressure steam and discharges steam to a secondary Rankine cycle. The diurnal energy production ratio, or capacity factor, versus peaking power, and storage material cost are the metrics used to compare configurations. Configuration III, which does not require changes to the primary cycle turbines and thus has no detrimental impact on the efficiency of baseload operation, has the highest energy production ratio of 0.99. Energy production ratio increases as charging duration and discharge power are reduced. Configurations I and III can provide peaking power more than 1.5 times the baseload plant. Configuration II is limited to a peaking power of less than 1.1 times that of baseload and is cost effective in this range, albeit with lower energy production ratio than configuration III. Sensible heat storage in a rock bed is more economical than latent heat storage due to the relatively high cost of eutectic salt mixtures with appropriate melt temperatures. Keywords: thermal storage, nuclear, thermodynamic, sensible heat, latent heat

Heat capacity and viscosity of ternary carbonate nanofluids

SummaryRecent studies have shown that eutectic salt mixtures show remarkable enhancement in heat capacity after nanoparticles are dispersed at small concentrations. The exact mechanism behind these heat capacity enhancements is still inconclusive. However, recent studies proposed that the enhancement could be associated with nucleation and grain growth of salt dendritic structures. To investigate the hypothesis, we synthesized several samples of ternary carbonate salt mixture doped with 1% alumina nanoparticles and thermally cycled them at various heating rates and cooled them back to the solid‐state during the synthesis procedure. It can affect the nucleation/grain growth of salt dendritic nanostructures and, as a result, there can be different heat capacity enhancements. A differential scanning calorimetry was employed to characterize the heat capacity values of the systems. It was observed that the heat capacity enhancements decreased with increases in the heating rates. The highest heat capacity enhancement was observed at the lowest heating rate (ie, 2°C/min). A transmission electron microscope was employed to confirm the effect of heating cycling rates on the formation of dendritic structures. Moreover, pH variation method was used to study the effect of the dendritic structures on the heat capacity of the mixture. Furthermore, a rheometer was employed to characterize the rheological behavior. It was observed that nanofluid samples showed shear‐thinning behavior, whereas shear thinning was not observed in pure and nanofluid prepared with the pH variation method.

High-temperature molten salts optimisation using mixture design for energy storage application

In this paper, the d-optimal mixture design method was used to determine the formulation of molten salts and optimise the following salt components: NaNO3 (A), KNO3 (B), LiNO3 (C), CsNO3 (D), Ca(NO3)2 (E). The one-step method was used to synthesise 22 different combinations of a high-temperature molten salt mixture by varying molar ratios. The screening of high-temperature molten salt was based on the melting point, and the optimum eutectic salt mixture obtained from the Design of Experiment (DOE) shows the lowest melting point of 61.4 °C. Based on the experiments designed by Design of Experiment (DOE) software V9, a quadratic model was generated as a function of different salt components. Differential scanning calorimeter (DSC) was used to analyse the melting point of the eutectic salt mixture. Moreover, the optimum values fitted by the model was justified by using Analysis of Variance (ANOVA). The deviation between the experimental and predicted value was negligible (-0.56 %), which is in close relation with the DOE model.

Rheological behaviour of eutectic nanofluids containing a low fraction of GO/TiO2 hybrid nanoparticles

In this study, rheological properties of hybrid eutectic nanofluids were investigated. The eutectic salt mixture was prepared using nitrate salts in varying proportions, and eutectic nanofluid was prepared by dispersing low fractions of graphene oxide/titanium oxide (GO/TiO2) hybrid nanoparticles in the eutectic salt mixture. The material characterisation was performed using environmental scanning electron microscopy. Viscosity was measured by varying shear rates and temperatures from 0.001 to 1000 s−1 and 70–190 °C, respectively. Results show the effect of GO/TiO2 concentration (0.01, 0.05, and 0.1 wt%) reduced the viscosity by − 2.29%~22.63% at a higher shear rate (1000 s−1) and temperature (190 °C). Moreover, the eutectic nanofluid displayed strong Newtonian behaviour and was directly related to temperature and nanoparticle concentration. Herschel Bulkley model displayed the best fit and predicted all data points accurately even at higher temperatures with the coefficient of determination, R2 of 0.9987 (0.01 wt%), 0.9999 (0.05 wt%), 0.9999 (0.1 wt%) with standard deviation of 0.02 (0.01 wt%), 0.0018 (0.05 wt%), and 0.0027 (0.1 wt%), respectively. Viscoelastic studies of eutectic nanofluid at different wt% showed a slight thixotropic behaviour at 0.1 wt%, indicating the presence of nanostructures that is primarily responsible for the Newtonian behaviour of eutectic nanofluids. The proposed correlation predicts R between 0.873 and 0.878 while P-values < 0.05 at 190 °C, which is in good agreement with the statistical data. The variation in viscosity with temperature is better studied with a second-order polynomial function and the behaviour is explained with the proposed equations.

Synthesis of Zn doped g-C3N4 in KCl-ZnCl2 molten salts: The temperature window for promoting the photocatalytic activity

The Zn doped g-C3N4 is successfully synthesized in eutectic KCl-ZnCl2 salt mixture with low melting temperature for the first time. More importantly, the significant improvement in the photocatalytic activity can only be achieved by locating the melting point of the salt mixture within the temperature window between dicyandiamide and melamine oligomer formation steps in the polycondensation process. The prepared g-C3N4 using dicyandiamide as the precursor (C3N4-D) shows ~9.3 times larger specific surface area (SSA), ~9.8 times larger photocurrent density and ~1.56 times longer photo-excited carrier lifetime than the bulk g-C3N4 synthesized via the conventional thermal polymerization method in air (C3N4-M-Air). In photocatalytic methyl orange (MO) degradation, the pseudo-first reaction rate constant of C3N4-D is ~4.15 times higher than the C3N4-M-Air control group. The significantly promoted MO photodegradation performance is attributed to the remarkably enlarged SSA, enhanced MO adsorption, the positively shifted valence band maximum (VBM) and suppressed electron-hole recombination due to Zn dopants.

Ionothermal Synthesis of Imide-Linked Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are an extensively studied class of porous materials, which distinguish themselves from other porous polymers in their crystallinity and high degree of modularity, enabling a wide range of applications. COFs are most commonly synthesized solvothermally, which is often a time‐consuming process and restricted to well‐soluble precursor molecules. Synthesis of polyimide‐linked COFs (PI‐COFs) is further complicated by the poor reversibility of the ring‐closing reaction under solvothermal conditions. Herein, we report the ionothermal synthesis of crystalline and porous PI‐COFs in zinc chloride and eutectic salt mixtures. This synthesis does not require soluble precursors and the reaction time is significantly reduced as compared to standard solvothermal synthesis methods. In addition to applying the synthesis to previously reported imide COFs, a new perylene‐based COF was also synthesized, which could not be obtained by the classical solvothermal route. In situ high‐temperature XRPD analysis hints to the formation of precursor–salt adducts as crystalline intermediates, which then react with each other to form the COF.

Ionothermal Synthesis of Imide‐Linked Covalent Organic Frameworks

AbstractCovalent organic frameworks (COFs) are an extensively studied class of porous materials, which distinguish themselves from other porous polymers in their crystallinity and high degree of modularity, enabling a wide range of applications. COFs are most commonly synthesized solvothermally, which is often a time‐consuming process and restricted to well‐soluble precursor molecules. Synthesis of polyimide‐linked COFs (PI‐COFs) is further complicated by the poor reversibility of the ring‐closing reaction under solvothermal conditions. Herein, we report the ionothermal synthesis of crystalline and porous PI‐COFs in zinc chloride and eutectic salt mixtures. This synthesis does not require soluble precursors and the reaction time is significantly reduced as compared to standard solvothermal synthesis methods. In addition to applying the synthesis to previously reported imide COFs, a new perylene‐based COF was also synthesized, which could not be obtained by the classical solvothermal route. In situ high‐temperature XRPD analysis hints to the formation of precursor–salt adducts as crystalline intermediates, which then react with each other to form the COF.