Specific 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... 

The specific conductivities of molten salts are frequently represented, as a function of temperature by an AtTlrenius equation, but it is unlikely that the unit step in diffusion has a constant magnitude, as in the coiTesponding solids and the results for NaCl may be expressed, within experimental eiTor, by the alternative equations... 

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. 

Solutions containing a high concentration of excess electrons display a transition to the metallic state. Thus, for sodium-ammonia solutions in the concentration region 1-6 M the specific conductance increases by about three orders of magnitude, and the temperature coefficient of the conductance is very small (27). Similar behavior is exhibited by other metal-ammonia solutions (but surprisingly, not by concentrated lithium-methylamine solutions ) (10) and by metal-molten salt solutions (17). 

Molten salt electrolyte systems comprise salts in the liquid state (molten), which form ionic phases and are highly ionically conductive, thus reaching specific conductivities comparable to those of room temperature concentrated aqueous solutions (0.1 < X < 10 Q -cur1) [160], These systems can be divided into two classes ... 

Other nonaqueous systems, which were mentioned in the first chapter, such as ionically conducting polymers, molten salts and solid electrolytes, have uses that are more specific. Hence, experimental aspects that are related to polymer based systems and molten salts are mentioned in the chapters that deal with them. 

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. 

Ideal molten salts, which are high conductivity melts. By substitution of some cation with another cation the specific conductivity in this group varies according to the change in interionic distance. Therefore, all these salts (pure salts and mixtures) have similar structures. 

Using that classification, Redkin modified Biltz and Klemm s table (see Table 3). That classification was confirmed by Nakamura and Itoh [52], who found that the specific conductivity of molten alkali chlorides increases monoton-ically with decreasing cation radius, as shown in Figure 6. In the case of chlorides of alkaline earth metals there is a break at CaCl2, and the specific conductivity decreases dramatically when going to Mg2+ and Be2+. This break may be attributed to complex formation among the component ions in those molten salts [53], This means that in the molten alkali halides there are free ions and no complex formation, a fact confirmed by Raman spectroscopy [52],... 

As in the case of molten salt mixtures, the specific conductivity of the metal-molten salt mixtures can be treated as the sum of the specific conductivity of the molten salt and that of the dissolved metal ... 

Like fuel cells, batteries using molten salt electrolytes offer high performance. Molten salts have very high electrical conductivity, which permits the use of high current densities. Likewise, molten salts permit the use of highly reactive electrode materials, which cannot be used in aqueous electrolytes. For these reasons, batteries with molten salts offer very high specific energy (>100 Wh/kg). To... 

The crucial difference between molten salts and molten ice lies in the values of the specific conductivity (Table 5.7). Fused salts have about 10 times greater specific conductivity than fused ice. 

The fact that a concentration of about 1 mol% of an alkali metal in a molten salt system can cause a considerable specific conductance demands some kind of explanation. At 1 %, the electrons are about 2 nm apart, on the limit for tunneling site to site. What is the mechanism of their easy passage through the molten salt ... 

The sodium ions in the /3-type aluminas can be replaced by a host of monovalent and divalent ionic species (Ag +, Cu, Li, K, Rb, Ba ", Sr, Cd ) " , In all cases the conductivity is decreased. These cation-substituted / -type aluminas may have some specific applications as selective ion sensors however, very little is known about the preparation of these materials in the form of polycrystalline ceramics, other than by the ion exchange of sodium /3 and P" single crystals (sometimes polycrystals) in various molten salts. These homologues, while interesting, have not been developed in polycrystalline form and are not discussed further. 

On the basis of this formulation of the liquid s structure, these substances should be viewed as molten salts their specific conductance is of interest in this context see Table I). In a separate conductance study... 

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