Molten salts electrical conductivity

In the following text the general term electrical conductivity means the property of a molten salt to conduct the electric current, and when necessary the specific and equivalent conductivities will be specified. 

Electrolysis Chemical change produced by passage of an electric current through a substance. Electrolyte Water solution or molten salt that conducts electricity. 

Bismuthides. Many intermetaUic compounds of bismuth with alkafl metals and alkaline earth metals have the expected formulas M Bi and M Bi, respectively. These compounds ate not saltlike but have high coordination numbers, interatomic distances similar to those found in metals, and metallic electrical conductivities. They dissolve to some extent in molten salts (eg, NaCl—Nal) to form solutions that have been interpreted from cryoscopic data as containing some Bi . Both the alkafl and alkaline earth metals form another series of alloylike bismuth compounds that become superconducting at low temperatures (Table 1). The MBi compounds are particularly noteworthy as having extremely short bond distances between the alkafl metal atoms. 

The mobilities of ions in molten salts, as reflected in their electrical conductivities, are an order of magnitude larger than Arose in Are conesponding solids. A typical value for diffusion coefficient of cations in molten salts is about 5 X lO cm s which is about one hundred times higher Aran in the solid near the melting point. The diffusion coefficients of cation and anion appear to be about the same in Are alkali halides, wiAr the cation being about 30% higher tlrair Are anion in the carbonates and nitrates. 

Electrochemically, the system metal/molten salt is somewhat similar to the system metal/aqueous solution, although there are important differences, arising largely from differences in temperature and in electrical conductivity. Most fused salts are predominantly ionic, but contain a proportion of molecular constituents, while pure water is predominantly molecular, containing very low activities of hydrogen and hydroxyl ions. Since the aqueous system has been extensively studied, it may be instructive to point out some analogues in fused-salt systems. 

Electrolyte a substance, liquid or solid, which conducts electrical current by movement of ions (not of electrons). In corrosion science, an electrolyte is usually a liquid solution of salts dissolved in a solvent, or a molten salt. The term also applies to polymers and ceramics which are ionically conductive. 

When an ionic solid like sodium chloride is melted, the molten salt conducts electric current. The conductivity is like that of an aqueous salt solution Na+ and Cl- ions are present. The extremely high melting temperature (808°C) shows that a large amount of energy is needed to tfear apart the regular NaCl crystalline arrangement to free the ions so they can move. 

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. 

Good electrical conductance is one of the characteristics of many though not all molten salts. This characteristic has often been employed industrially. Various models have been proposed for the mechanism of electrical conductance. Electrolytic conductivity is related to the structure, although structure and thermodynamic properties are not the main subjects of this chapter. Electrolytic conductivities of various metal chlorides at the melting points are given in Table 4 together with some other related properties. "... 

It is not as easy as it appears to know whieh are the eleetrieally conducting species in molten salts. It seems to be nearly impossible to determine the electrically conducting species by experiment alone. ... 

Data on high-temperature melts are still limited. Conventional methods are difficult to apply because of the high values of thermal conductivity. Other difficulties in measuring molten salts are their corrosiveness, high electrical conductivities, and the necessity of careful preparation. Special care should be taken to exclude convection errors, which are usually the most serious source of errors, even at room temperature. 

Gustafsson et al. measured the thermal conductivity and thermal diffusivity of molten NaNOs KNO3. The approximate dimension of the foil used for NaNO, was platinum measuring 0.010 X 40 X 86 mm The foil was heated by a constant electric current and the measurements were completed within 10 s. Errors due to radiation were considered to be negligible. The accuracy was claimed to be +2.6% for thermal conductivity and 3% for thermal diffusivity, but the effect of current leak from the metallic foil to the molten salts was neglected. 

G. J. Janz, J. Phys. Chem. Ref Data 9 (1980) 791 Molten Salts Data as Reference Standards for Density, Surface Tension, viscosity and Electrical Conductance KN03 and NaCl, American Chemical Society-American Institute of Physics-National Bureau of Standards, Washington, DC, 1980. 

The composition of the electrolyte is quite important in controlling the electrolytic deposition of the pertinent metal, the chemical interaction of the deposit with the electrolyte, and the electrical conductivity of the electrolyte. In the case of molten salts, the solvent cations and the solvent anions influence the electrodeposition process through the formation of complexes. The stability of these complexes determines the extent of the reversibility of the overall electroreduction process and, hence, the type of the deposit formed. By selecting a suitable mixture of solvent cations to produce a chemically stable solution with strong solute cation-anion interactions, it is possible to optimize the stability of the complexes so as to obtain the best deposition kinetics. In the case of refractory and reactive metals, the presence of a reasonably stable complex is necessary in order to yield a coherent deposition rather than a dendritic type of deposition. 

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. 

A dense and electronically insulating layer of LiA102 is not suitable for providing corrosion resistance to the cell current collectors because these components must remain electrically conductive. The typical materials used for this application are 316 stainless steel and chromium plated stainless steels. However, materials with better corrosion resistance are required for longterm operation of MCFCs. Research is continuing to understand the corrosion processes of chromium in molten carbonate salts under both fuel gas and oxidizing gas environments (23,25) and to identify improved alloys (29) for MCFCs. Stainless steels such as Type 310 and 446 have demonstrated better corrosion resistance than Type 316 in corrosion tests (29). 

So from electrical data, it is possible to get information on partial thermodynamic functions of the salt and then develop thermodynamic models for quantitative interpretation of the conductivity variation with composition. These models are not very different from those already developed for molten salt mixtures or metallic alloys. 

According to H. W. Foote and L. H. Levy, soln. of the alkali chlorates in other molten salts gave f.p. indicating that the mol. wt. under those conditions are normal. A. Rosenheim and 0. Liebknecht48 found the mol. wt. of acid by the f.p. method depend on the concentration, and that the acid in dil. soln. is a monobasic acid, and in cone. soln. the acid is polymerized and exists as a dibasic acid, H2l206. E. Gro-schuff and P. Walden also showed that the electrical conductivity of iodic acid soln. corresponds with a monobasic acid, as is also the case with chloric and bromic acids. The monobasic acid, therefore, can be represented by A. Kekule s type of formula with iodine univalent or C. W. Blomstrand s type of formula, with iodine quinquevalent and iodic anhydride, i.e. iodine pentoxide will be represented ... 

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