Binary Molten Salt Mixtures Research Articles

Molecular dynamics analysis of thermophysical properties of Na2CO3-K2CO3 binary molten salts in temperature range of 1125–1475 K

Molten carbonates are highly promising solar thermal storage materials. In this paper, the molecular dynamics (MD) method was used to study the thermal properties of binary molten salt mixtures of Na2CO3-K2CO3 as a function of temperature and composition. Their thermal storage mechanisms were analyzed from the perspectives of microstructure and atomic diffusion. It was confirmed that temperature and various components have significant effects on the density, thermal conductivity and shear viscosity of binary molten salts. Based upon the MD data, the correlation for the temperature-thermal properties-component with the correlation coefficient of higher than 0.98 was obtained. In addition, the radial distribution function (RDF) showed that the change in thermal properties was the result of variation in distance between the ionic clusters. Finally, based on the guidance of material composition design strategy, a coefficient that could reflect the synergistic effect between the binary molten salts was proposed for the first time. The results showed that the synergy coefficient reached its highest point for the molten salt molar ratio of 58:42, and at this point, the positive synergy was most obvious. This study can provide theoretical guidance for the design and enhancement in the performance of molten carbonates as thermal storage materials.

Study on the preparation and thermal properties of binary mixed chloride salts

Nitrate phase change materials (PCMs) are the most widely used PCMs in solar thermal power generation technology. The maximum service temperature of Nitrate phase change materials is only 600°C. Therefore, to find a phase change material with large heat capacity, wide temperature range, low heat loss and low price is the focus of current research. According to different mass ratios, nine binary molten salt mixtures were prepared by mixing lithium chloride and sodium chloride. The phase change temperature and latent heat of phase transition of them were studied by differential scanning calorimeter (DSC). The experiment results showed that since the melting point of sodium chloride was high, when the content of sodium chloride in the binary mixture of lithium chloride and sodium chloride was large, a small amount of lithium chloride could not reduce the melting point of the mixture below 600°C, the mixture could not be melted. Meanwhile, when sodium chloride and lithium chloride were melted, the phase transition temperature of lithium chloride and sodium chloride remained at about 540°C and floated at ±15°C. The melting temperature and crystallization temperature of the binary mixture of 90% lithium chloride and 10% sodium chloride were quite different, and the latent heat of phase transformation was relatively high. Therefore, the binary mixed molten salt can be used in the heat transfer and storage technology of solar power generation.

Thermal energy storage characteristics of binary molten salt nanofluids: Specific heat and latent heat

This study aims to examine the thermal energy storage characteristics, in terms of specific heat and latent heat, of nanoparticle-doped binary molten salt nanofluids. Two different binary molten salt mixtures, namely carbonate salts (Li2CO3 and K2CO3) and nitrate salts (NaNO3 and KNO3), were used as base solvents. Graphite nanoparticles were dispersed into the salt mixtures. The chemical compositions (molar fractions) of the two salt components were varied from hypoeutectic to hypereutectic, including eutectic compositions, to investigate their effects on the specific heat and latent heat of both the pure salts and their nanofluids. The specific heat of the carbonate salt nanofluid was enhanced, irrespective of the chemical composition, and the enhancement was proportional to the nanoparticle concentration. However, the specific heat of the nitrate salt nanofluid was decreased by the addition of 0.1 wt% of graphite nanoparticles and increased by the addition of a larger amount of 1 wt%. The latent heat of the nanofluids was enhanced by doping nanoparticles into the salt mixtures and was affected by the chemical composition of the solvent salt mixtures. The distinct behaviors of specific heat and latent heat obtained in this study were discussed based on a compressed liquid (solid-like) layer formed near the nanoparticles. The thermal energy storage capacities of the nanofluids were estimated and compared by using their specific heat and latent heat.

Low-melting-temperature binary molten nitrate salt mixtures for solar energy storage

Thermal analysis of the binary system (Al(NO3)3)–(Cu(NO3)2) of different ratios was performed by using differential scanning calorimetry and thermogravimetric analysis. The eutectic temperature of the binary salt mixture was determined to be 65 °C. Moreover, the degradation temperature, specific heat, latent heat of fusion and thermal stability were reported, respectively. Further, this work focuses on synthesising low-temperature salt mixtures comprised of aluminium nitrate non-hydrate (Al(NO3)3) and copper nitrate hydrate (Cu(NO3)2) of varying compositions (60:40, 50:50, 55:45 and 57.5:42.5), respectively, which are optimised using Design of Expert v13. The performance of the binary salt mixture was determined from the results obtained from thermal stability measurements after the exposure of salt mixture to 120 h at a higher temperature. The results showed that the novel binary molten salt mixtures exhibit decomposition at 215 °C and the latent heat was found to increase from 17.7 to 20.73 J g−1 with the increase in aluminium nitrate composition with excellent repeatability after three sets of experiments. The average specific heat of binary molten salt mixture was observed from 2.73 to 2.13 J g−1 K−1. The optimal molar ratio of binary molten salts is found to be 60:40 (Al/Cu) from the obtained results. Field emission scanning electron microscopy and energy-dispersive X-ray showed a uniform distribution of both the salts in the mixture. The average specific heat of the optimal molar ratio was 21.8% higher compared to other molar ratios, indicating the most promising material to be used in solar applications.

Unravelling the Internal Complexities of Molten Salts

Abstract Much experimental and theoretical effort has gone into revealing the internal complexities of molten salts for the past two decades. In this paper we shall show how neutron diffraction and computer simulation techniques have helped in gaining a better understanding of these systems at the microscopic level. Firstly, a short review on the structure of molten halide systems as revealed by these techniques will be presented. Complementarity of using X-rays with neutrons and, some recent results on the structure of molten DyCl3 obtained by combining neutron and X-ray diffraction with molecular dynamic simulations will be discussed. Neutron diffraction isotopic substitution techniques have played an important role in elucidating the interatomic structure of a diversity of molten salts. Pair distribution functions (PDFs), determined for a number of 1:1 and 2 :1 halide melts, provided theorists with a critical test of their model potentials. It is now clear that for 1:1 molten systems theoretical models based on Fumi-Tosi potentials can adequately describe many of the structural features. Nevertheless, the chal-lenge is two fold: (i) to determine real interatomic potentials for 2:1 and 3 :1 molten systems capable of reproducing not only the microscopic structural details obtained at the partial PDF, gαβ (r) level, but also their macroscopic behaviour and, (ii) to characterise the structure in binary molten salt mixtures.

Adaptive filter algorithms for interference patterns of diffusion measurements

A holographic interferometer is used to measure diffusion coefficients of binary molten salt mixtures. The composition of such transparent mixtures is proportional to the refractive index. The composition changes cause local variations of the refractive index during diffusion. These variations are visualized by interference fringes, which are photographed by a CCD camera. Exact detection of the intensity extreme values of the fringes is essential to calculate diffusion coefficients. As irregular illumination and noise disturb the detection, adaptive filter algorithms are applied to reduce noise sufficiently while maintaining all relevant information.

Association in Molten Salts and Mobility Isotherms showing the Chemla Effect

Abstract In 1984 a model for additive binary molten salt mixtures had been proposed that allowed for a qualitative understanding of the occurence of the Chemla effect (equal internal mobilities at a certain mixing ratio of the salts). In that model the presence of three ions (1,2 and 3) and two neutral molecules (4 and 5) consisting of two ions is assumed, and the ratios K1 = x4 / x1x3 and K2 = X5 / X2X3 of their mole fractions are assumed to be independent of the mixing ratio of the salts. In the present paper, that model is compared with recent experimental results on the system (Li, Cs)Cl obtained in Japan. The comparison shows that the model is too simple for a quantitative description of systems showing the Chemla effect. Rather a dependence of K1 and K2 on the mixing ratio of the salts must be assumed.

An isotopic substitution study of and binary molten salt mixtures

The scattering from binary molten salt mixtures of nominally two thirds alkali halide concentration, where x = `natural', `zero-scattering' and `62' enrichments of nickel, has been measured by time-of-flight neutron diffraction on ISIS LAD. A complementary experiment was performed on molten samples, also of nominally two thirds alkali halide concentration, using isotopic substitution of chlorine. In the case of - KCl, the results indicate a large first sharp diffraction peak (FSDP) in and but not in . Reverse Monte Carlo (RMC) modelling indicates a well-ordered, almost regularly tetrahedral local structure for the ion, the preponderance of large ions as next-nearest neighbours to the metal cation and the apparent association of this with enhanced intermediate-range order (IRO) in the mixtures. The results confirm the findings of the initial composition study of this system and provide further support for the suggestion that enhanced IRO arises from ordering between tetrahedral units and alkali counter-ions. Modelling of the data suggests a strong similarity with molten even though the pure and molten salt structures are confirmed to be dissimilar.

The structure of liquid AlCl3 and structural modification in AlCl3-MCl (M=Li, Na) molten salt mixtures

The scattering from pure liquid AlCl3 and AlCl3-MCl (M=Li, Na) binary molten salt mixtures has been measured by time-of-flight neutron diffraction. Reverse Monte Carlo simulation of the AlCl3 data whilst confirming a regular tetrahedral local geometry for the Al3+ ion, challenges the accepted view of the structure as consisting (predominantly) of discrete Al2Cl6 dimers. The data for the mixtures suggest that the connectivity in the structure of pure AlCl3, formed by Al-Cl-Al bridges, is proportionately reduced upon the addition of alkali halide as isolated AlCl4- units are created. In addition, a gradual shift in the position of the first sharp diffraction peak to higher Q is observed, implying a systematic reduction in the length scale of intermediate-range order. The findings for the mixtures are generally consistent with those from previous structural studies using X-ray diffraction and also with Raman and electrochemical measurements.

Structural modification in molten metal chloride and alkali chloride mixtures

Structure factors have been measured by time-of-flight neutron diffraction for NiCl2-KCl, NiCl2-LiCl, and ZnCl2-LiCl binary molten salt mixtures. The results are consistent with those from earlier thermodynamic and spectrophotometric studies, in which the degree of structural modification was found to depend on the type of alkali cation. The NiCl2-LiCl and ZnCl2-LiCl systems appear to be almost ideal admixtures of the pure salts. However, adding KCl to NiCl2 results in enhancement of the local structure and intermediate range order. The first sharp diffraction peak at Q approximately=1 AA-1 in pure NiCl2, which is more prominent than previously reported, appears to be enhanced by the addition of KCl up to a high concentration of alkali halide. No change is apparent in NiCl2-LiCl or ZnCl2-LiCl.

Lithium ion-crown ether complexes in a molten salt

Abstract

Comparative investigations among binary molten salt mixtures PbCI2- AgCI, PbCI2- UCI, and PbCI2-KCI using an isopiestic technique

Comparative investigations among binary molten salt mixtures PbCI2- AgCI, PbCI2- UCI, and PbCI2-KCI using an isopiestic technique

NMR Relaxation of Ions in Molten Salt Mixtures: Cs—133 in the (Li-Cs)NO3 System

We have measured the spin-lattice relaxation times of Cs + ions in the molten (Li-Cs)NO 3 system. The relaxation times of Cs + increase with increase of LiNO 3 concentration, while those of Li + decrease with increasing CsNO 3 concentration. These results are explained in terms of the anion polarization model, in which we suppose that the electron clouds of anions are polarized toward the smaller cations, if the size of cations is different in binary molten salt mixtures with common anions

Nuclear magnetic resonance study of thallium-205 in binary molten salt mixtures

Mesures des deplacements chimiques de Tl + , en melanges binaires fondus de nitrates et d'halogenures, en fonction de la composition du melange et de sa temperature, afin d'etudier la structure de ces sels, et de revoir le modele de polarisation de l'anion proposee par Hafner et Nachtrieb

Surface tension of binary molten salt mixtures

Surface tension of binary molten salt mixtures

Calculation of phase diagrams of binary salt mixtures with a common anion

In order to calculate excess chemical potentials of the components in binary molten salt mixtures, a simple model is proposed which requires three physical parameters for each salt: the crystal lattice energy, the latent heat of melting and the sum of the ionic radii. The model has been used to calculate the liquidus temperature for various compositions of the binary system. Calculated and experimentally determined phase equilibria for the following systems are compared: LiF–KF, LiF–NaF, NaF–KF, LiCl–KCl, NaCl–CsCl, LiNO3–NaNO3, CaF2–MgF2, LiF–CaF2, NaF–CaF2 and NaCl–SrCl2.

Water solubility in molten-salt mixtures. A theory for selective ionic hydration

A statistical thermodynamic treatment based on the quasi-lattice model of molten salts is developed to explain the non-linear dependence of the enthalpy of transfer of water from liquid water to an infinitely dilute solution of water in a binary molten-salt mixture on the molar composition of the mixture. The derived equation is used to fit previously determined data for the enthalpy of transfer of water from liquid water to (undercooled) liquid LiNO3+ NH4NO3 mixtures in the whole composition range; a satisfactory fit is obtained for a reasonable choice of the values for the quasi-lattice coordination numbers of LiNO3 and NH4NO3. Literature data for the enthalpy of dissolution of water in molten LiNO3+ NaNO3 and LiNO3+ KNO3 mixtures are in qualitative agreement with the predictions of the present theory.

Composition Gradients in Molten Salt Binary Mixtures during Electrolysis at High Current Density

Separation of components has been measured in binary molten salt mixtures, , subjected to electrolysis between silver electrodes. The analysis of thin slices of frozen mixture contained in silica frits gives qualitatively the composition changes between anode and cathode and shows the expected enrichment of potassium ions at the cathode. Changes in concentration at the electrodes are measured electrochemically in free electrolytes and in electrolytes contained in frits. The time dependence of concentration at the electrodes is obtained both during electrolysis and during relaxation following electrolyses of different durations. The experimental results confirm the predictions from a mass transport model proposed previously for systems analogous to mixed molten salt batteries operated at high current densities.

Steady‐State Composition Profiles in Mixed Molten Salt Electrochemical Devices: I . Lithium/Sulfur Battery Analogs

During charge or discharge of batteries with a binary molten salt mixture as the electrolyte, composition gradients are produced by the electrode reactions and the differences in mobilities of the electroactive and nonelectroactive ions. The effects of current density, electrode separation, and initial composition of the electrolyte are predicted by an analytical solution of the flux equations derived with transport properties similar to those of mixtures. Numerical solution of the flux equations predicts the composition profiles in lithium sulfur battery analogs with mixtures of differing compositions. Either complete depletion of the electroactive constituent at one electrode, or precipitation of a solid phase at the electrodes, could result from the predicted composition gradients. Changes in electrolyte composition at the electrodes may also affect J‐phase formation at the sulfur electrode during discharge.

ChemInform Abstract: ELECTROCHEMICAL STUDIES OF MASS TRANSPORT IN HIGH TEMPERATURE ELECTROLYTES

AbstractDie Interdiffusionskoeffizienten in BeF2‐LiF‐Salzschmelzen mit S3, 65 bzw. 70 Mol % BeF2 werden chronopotentiometrisch bestimmt, wobei der chronopotentiometrische Übergang eine Folge der Verarmung an nichtelektroaktiven Li‐Ionen durch Elektromigration ist.