Molten salts density

Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN. The specific gravity of iridium is only very slightly lower than osmium, which is generally credited as the heaviest known element. Calculations of the densities of iridium and osmium from the space lattices give values of 22.65 and 22.61 g/cm 3, respectively. These values may be more reliable than actual physical measurements. At present, therefore, we know that either iridium or osmium is the densest known element, but the data do not yet allow selection between the two. 

The electrorefining of many metals can be carried out using molten salt electrolytes, but these processes are usually expensive and have found Httie commercial use in spite of possible technical advantages. The only appHcation on an industrial scale is the electrorefining of aluminum by the three-layer process. The density of the molten salt electrolyte is adjusted so that a pure molten aluminum cathode floats on the electrolyte, which in turn floats on the impure anode consisting of a molten copper—aluminum alloy. The process is used to manufacture high purity aluminum. 

X-ray diffraction has been used for the study both of simple molten salts and of binary mixtures thereof, as well as for liquid crystalline materials. The scattering process is similar to that described above for neutron diffraction, with the exception that the scattering of the photons arises from the electron density and not the nuclei. The X-ray scattering factor therefore increases with atomic number and the scattering pattern is dominated by the heavy atoms in the sample. Unlike in neutron diffraction, hydrogen (for example) scatters very wealdy and its position cannot be determined with any great accuracy. 

The measurement of correlation times in molten salts and ionic liquids has recently been reviewed [11] (for more recent references refer to Carper et al. [12]). We have measured the spin-lattice relaxation rates l/Tj and nuclear Overhauser factors p in temperature ranges in and outside the extreme narrowing region for the neat ionic liquid [BMIM][PFg], in order to observe the temperature dependence of the spectral density. Subsequently, the models for the description of the reorientation-al dynamics introduced in the theoretical section (Section 4.5.3) were fitted to the experimental relaxation data. The nuclei of the aliphatic chains can be assumed to relax only through the dipolar mechanism. This is in contrast to the aromatic nuclei, which can also relax to some extent through the chemical-shift anisotropy mechanism. The latter mechanism has to be taken into account to fit the models to the experimental relaxation data (cf [1] or [3] for more details). Preliminary results are shown in Figures 4.5-1 and 4.5-2, together with the curves for the fitted functions. 

Molten salt investigation methods can be divided into two classes thermodynamic and kinetic. In some cases, the analysis of melting diagrams and isotherms of physical-chemical properties such as density, surface tension, viscosity and electroconductivity enables the determination of the ionic composition of the melt. Direct investigation of the complex structure is performed using spectral methods [294]. 

Table 5.3. Exchange current densities in molten salt electrolyte systems.

The popular and well-studied primitive model is a degenerate case of the SPM with = 0, shown schematically in Figure (c). The restricted primitive model (RPM) refers to the case when the ions are of equal diameter. This model can realistically represent the packing of a molten salt in which no solvent is present. For an aqueous electrolyte, the primitive model does not treat the solvent molecules exphcitly and the number density of the electrolyte is umealistically low. For modeling nano-surface interactions, short-range interactions are important and the primitive model is expected not to give adequate account of confinement effects. For its simphcity, however, many theories [18-22] and simulation studies [23-25] have been made based on the primitive model for the bulk electrolyte. Ap-phcations to electrolyte interfaces have also been widely reported [26-30]. 

The electrolysis apparatus operates well above the melting point of aluminum (660 °C), and liquid aluminum has a higher density than the molten salt mixture, so pure liquid metal settles to the bottom of the reactor. The pure metal is drained through a plug and cast into ingots. 

The National Institute of Standards and Technology (NIST) molten salts database has been designed to provide engineers and scientists with rapid access to critically evaluated data for inorganic salts in the molten state. Properties include density, viscosity, electrical conductance, and surface tension. Properties for approximately 320 single salts and 4000 multicomponent systems are included, the latter being primarily binary. Data have been abstracted from the literature over the period 1890-1990. The primary data sources are the National Bureau of Standards-National... 

The molten salt standard program was initiated at Rensselaer Polytechnic Institute (RPI) in 1973 because of difficulties being encountered with accuracy estimates in the NBS-NSRDS molten salt series. The density, surface tension, electrical conductivity, and viscosity of KNO3 and NaCl were measured by seven laboratories over the world using samples distributed by RPI. The data from these round-robin measurements of raw properties were submitted to RPI for evaluation. Their recommendations are summarized in Table 2. 

Some 30 years ago, transport properties of molten salts were reviewed by Janz and Reeves, who described classical experimental techniques for measuring density, electrical conductance, viscosity, transport number, and self-diffusion coefficient. 

G. J. Janz, F. W. Dampier, andP. K. Lorenz, Molten Salts, Vol.l Eleetrieal Conductance, Density, and Viseosity Data, U.S. Dept of Commerce, National Bureau of Standards, NBS-NSRDS-15, Washington, DC, 1968. 

G. J. Janz, G. Gardener, U. Krebs, andR. P. T. Tomkins, J. Phys. Chem. Ref Data i (1974) 115 Molten Salts, Vol. 4, Part 1 Fluorides Eleetrieal Conduetance, Density, viseosity and Surfaee Tension Data, American Chemical Society-American Institute of Physics-National Bureau of Standards, Washington, DC, 1974. 

The electrolyte is made by in situ chlorination of vanadium to vanadium dichloride in a molten salt bath. Higher valent chlorides are difficult to retain in the bath and thus are not preferred. The molten bath, which is formed by sodium chloride or an equimolar mixture of potassium chloride-sodium chloride or of potassium chloride-lithium chloride or of sodium chloride-calcium chloride, is contained in a graphite crucible. The crucible also serves as an anode. Electrolysis is conducted at a temperature about 50 °C above the melting point of the salt bath, using an iron or a molybdenum cathode and a cathode current density of 25 to 75 A dnT2. The overall electrochemical deposition reaction involves the formation and the discharge of the divalent ionic species, V2+ ... 

Janz, G. J., Thermodynamic and Transport Properties for Molten Salts Correlation Equations for Critically Evaluated Density, Surface Tension, Electrical Conductance, and Viscosity Data. 1988, New York American Institute of Physics. 

The methodology most practiced is referred to here as codeposition, where a single solution contains precursors for all the elements being deposited and is reduced at a fixed potential or current density. The earliest report appears to be that by Gobrecht et al., which was published in 1963 [45]. Two anodes were used in the study, one of Se and one of Cd (or Ag), to form selenite and cadmium ions, respectively. CdSe was then formed by co-reduction of both species at the cathode. Reports of the formation of GaP in 1968 [46] and ZnSe in 1975 [47] via codeposition were subsequently published, and both involved molten salt electrolysis. 

Lithium Iron Sulfide (High Temperature). High-temperature molten salt Li—Al/LiCl— KCl/FeS - cells are known for their high energy density and superior safety. At one point they were being actively pursued for electric vehicle and pulse-power applications. Historically, boron nitride (BN) cloth or felt has been used as the separator in flooded-electrolyte cells, while MgO pressed-powder plaques have been used in starved-electrolyte cells. 

During the electrorefining of uranium metal in a molten salt eutectic, a low current density favors the formation of large single crystals. Up to 5-cm single crystals of uranium metal have resulted from the large-scale (100 kg of U) electrorefining of uranium metal in molten LiCl/KCl eutectic (17). 

E. J. Casey (Defense Research Establishment, Ottawa) reviewed the selection of anodes and electrolytes for high-energy density storage batteries. The present state of development of batteries by using light metal anodes in nonaqueous, molten salt and solid electrolytes was reviewed, and suggestions were made on the feasibility of novel systems. 

In considering the selection of anodes for high energy density (HED) storage (or secondary) batteries (SB), we note that there are some 19 metals whose free-energy density (TED) of reaction with oxidants such as O2, Cl2, and F2 are higher than those of Zn with the same oxidants. Most of these metals react violently with water. The remainder are passivated by water. Therefore other electrolytes must be considered for these metals, based on non-aqueous, molten salt, or solid-state ionic conductors. Much experimental work has been carried out during the last two decades on primary and secondary batteries based on anhydrous electrolytes, aimed at utilization of the active metals. 

Ionic liquids are a class of solvents and they are the subject of keen research interest in chemistry (Freemantle, 1998). Hydrophobic ionic liquids with low melting points (from -30°C to ambient temperature) have been synthesized and investigated, based on 1,3-dialkyl imidazolium cations and hydrophobic anions. Other imidazolium molten salts with hydrophilic anions and thus water-soluble are also of interest. NMR and elemental analysis have characterized the molten salts. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. The influence of the alkyl substituents in 1,2, 3, and 4(5)-positions on the imidazolium cation on these properties has been scrutinized. Viscosities as low as 35 cP (for l-ethyl-3-methylimi-dazolium bis((trifluoromethyl)sulfonyl)amide (bis(triflyl)amide) and trifluoroacetate) and conductivities as high as 9.6 mS/cm were obtained. Photophysical probe studies were carried out to establish more precisely the solvent properties of l-ethyl-3-methyl-imidazolium bis((trifluoromethyl)sulfonyl)amide. The hydrophobic molten salts are promising solvents for electrochemical, photovoltaic, and synthetic applications (Bon-hote et al., 1996). 

V at 40°. This AV. /pCl" value for the (100) orientation n-GaAs is approximately one-half that obtained with (111) n-GaAs crystals (2), indicating that the crystal surface atom density and type can be a significant factor in the interactions between substrate and electrolyte. Flat-band potential values for (100) and (111) n-GaAs/molten salt interphases and for the (111) n-GaAs/aqueous electrolyte interphase are compared in Table I. 

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