Equivalent conductivity, molten salts

The most direct effect of defects on tire properties of a material usually derive from altered ionic conductivity and diffusion properties. So-called superionic conductors materials which have an ionic conductivity comparable to that of molten salts. This h conductivity is due to the presence of defects, which can be introduced thermally or the presence of impurities. Diffusion affects important processes such as corrosion z catalysis. The specific heat capacity is also affected near the melting temperature the h capacity of a defective material is higher than for the equivalent ideal crystal. This refle the fact that the creation of defects is enthalpically unfavourable but is more than comp sated for by the increase in entropy, so leading to an overall decrease in the free energy... 

As in the case of solutions, the specific conductance, K, the equivalent conductance, a, and the molar conductance, am, are also distinguished for molten electrolytes. These are defined in the same manner as done for the case of solutions of electrolytes. It may, however, be pointed out that molten salts generally have much higher conductivities than equivalent aqueous systems. 

Linear sweep voltammetry, capacitance-voltage and automated admittance measurements have been applied to characterize the n-GaAs/room temperature molten salt interphase. Semiconductor crystal orientation is shown to be an important factor in the manner in which chemical interactions with the electrolyte can influence the surface potentials. For example, the flat-band shift for (100) orientation was (2.3RT/F)V per pCl" unit compared to 2(2.3RT/F)V per pCl" for (111) orientation. The manner in which these interactions may be used to optimize cell performance is discussed. The equivalent parallel conductance method has been used to identify the circuit elements for the non-illum-inated semi conductor/electrolyte interphase. The utility of this... 

The electrical conductivity of molten salts can be expressed in two ways equivalent conductivity A (ohm-1 cm2 cquiv ) and specific conductivity k (ohm-1 cm-1), and between these terms there is the relation... 

Equivalent conductivities (and ionic mobilities) of the melts are similar to that of aqueous solutions. Very high specific conductivities are typical for molten salts, as seen in Table 1 [49], The reason for this is the fact that molten salts are very concentrated solutions (for example, the concentration of molten LiF is about 65 molar the concentration of molten KC1 is about 20 molar, etc.). The electrical conductivities of various molten salts cannot be compared at constant temperature because of their different melting points. Therefore, in Table 1 the values of conductivities were selected at 50° above the melting point of each salt. 

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. 

Viscosity is an internal force of friction which acts oppositely to the flowing fluid. Walden s rule is also applicable to molten salts. Shabanov [54] applied that rule for the limiting equivalent electrical conductivities (see Figure 7) ... 

The Nernst-Einstein relation shows the dependence between the self-diffusion coefficient Dt and the equivalent conductivity A of molten salts ... 

There are numerous data in the literature [59] which demonstrate that for molten halides the equivalent conductivity calculated by means of the Nernst-Einstein relation is significantly higher than the directly measured conductivity value. This is due to the fact that the structural entities of molten salts make unequal contributions to diffusion and electrical conductivity. 

The electrical conductivity of molten salts has been used to elucidate the structure of the salts. Generally the variation of that property was measured as a function of composition and temperature. The isotherm of the ideal equivalent conductivity of a binary system is additive, and it can be calculated by the relationship... 

It is noticed that the law of additive equivalent electrical conductivity does not apply for simple molten salt mixtures, like the NaCl-KCl system. All other properties indicate this system to be a very simple mixture. However, the isotherm of equivalent electrical conductivity shows a maximum negative deviation from additivity at the 50-50 composition, and that indicates the presence of ionic interactions. Generally, if the deviations are not large, they can be due to the presence of ionic interactions. Large deviations indicate the presence of ionic complexes. 

Markov et al. [60,61] proposed an equation for the equivalent electrical conductivity of simple binary molten salt mixtures. In binary systems (MjX + M2X or MXj + MX2) there is the possibility of the following ionic arrangements MjX — MjX M2X — M2X MjX — M2X. The probabilities of forming the combinations MjX - MjX M2X - M2X and MjX - M2X are proportional to X, x2 and 2xxx2, respectively, where Xt and x2 are the molar fractions of the two salts. For monovalent molten salts, the equivalent electrical conductivity of a mixture of these salts, Am, can be written as... 

The above argument brings out an important point about the limitations of the Nernst-Einstein equation. It does not matter whether the diffusion coefficient and the equivalent conductivity vary with concentration to introduce deviations into the Nernst-Einstein equation, D and A must have different concentration dependencies. The concentration dependence of the diffusion coefficient has been shown to be due to nonideality (f 1), i.e., due to ion-ion interactions, and it will be shown later that the concentration dependence of the equivalent conductivity is also due to ion-ion interactions. It is not the existence of interactions perse that underlies deviations from the Nernst-Einstein equation otherwise, molten salts and ionic crystals, in which there are strong interionic forces, would show far more than the observed few percent deviation of experimental data from values calculated by the Nernst-Einstein equation. The essential point is that the interactions must affect the diffusion coefficient and the equivalent conductivity by different mechanism and thus to different extents. How this comes about for diffusion and conduction in solution will be seen later. 

Molten Salt Valence of Cation Equivalent Conductivity... 

Calculate the transport numbers of the cation and the anion in molten CsCl at 943 K. The experimental equivalent conductivity of the fused salt is 67.7 ohms cm equiv. The observed diffusion coefficients of Cs" and Cl ions in molten CsCl are 3.5 x 10 - and 3.8 x 10" cm s", respectively. (Contractor)... 

The equivalent conductivity of molten salts depends upon the cationic radius. Plot the equivalent conductivities of molten salts of monovalent cations against the corresponding cationic radii. Comment on this linear dependence. (Contractor)... 

Theoretical interpretation of the concentration dependence of equivalent conductivity for simple binary mixtures was first presented by Markov and Shumina (1956). It should be emphasized that this theory, even when considering simple structural aspects, represents rather a method of interpretation of the experimental data than a genuine picture of the structure of the melt. In molten salts generally only ions and not molecules are present, hence the conception of Markov and Shumina (1956) is to be considered also from this aspect. Their theory is based on the assumption that the electrical conductivity of a mixture of molten salts varies with temperature like pure components. In this respect, general character of the electrical conductivity dependence on composition, indicating the interaction of components in an ideal solution, could be expected. 

In a mixture of univalent salts of the type AX-BX, the following interactions should be present AA, BB, AXB, and BXA. The last two interactions are equal, thus they can be written as 2AXB. Considering that the probability of the interactions mentioned is proportional to their molar fractions, Markov and Shumina derived a relation for the composition dependence of the equivalent conductivity in a mixture of molten salts in the form... 

This is the simplest NEMD scheme. Formally, the algorithm is equivalent to a method to simulate electrical conductivity of a 1-1 molten salt (say alkali halide melt). Each particle is assigned a charge (-Fl or —1) which couples to the external field in the same way as real charges couple to the (constant) electrical field. The charges are called color charges because they are visible only to the external field, the interparticle potentials being unaffected by them. The derivation starts from the color Hamiltonian... 

The conductance increases with the temperature, so that the minus sign before the activation energy for the conductance should be noted, contrary to the positive values of numerator of the exponent for the viscosity. The equivalent conductance of the molten salt, which is the product of the specific conductance with the molar volume of the molten salt, also follows an Arrhenius-type expression ... 

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