Glass forming molten salts

A number of molten salt systems [e.g., the simple ionic system Ca(N03)2-KN0j], have the property of being able to be supercooled, i.e., to remain liquid at temperatures below the melting point down to a final temperature. This is called the glass transition temperature, and at this temperature the salts form what is called a glass. This glass is only apparently solid. It is a highly disordered substance in which a liquid structure 

Comparison of Calculated Isothermal Compressibilities with the Experimental Values (Hole Theory) 

Mathematical treatment of molten salts that supercool was first carried out by Cohen and Turnbull. The principal idea of the hole theory—that diffusion involves ions that wait for a void to turn up before jumping into it—is maintained. However, Cohen and Turnbull introduced into their model a property called thefree volume, Vf. What is meant by this free volume It is the amount of space in addition to that, Vq, filled by matter in a closely packed liquid. Cohen and Turnbull proposed that the free volume is linearly related to temperature 

To express the probability that the free volume occasionally opens up to form a hole, Cohen and Turnbull first defined a factor y, which allows for the partial filling of the expanded free volume to the size of a hole. It can vary between 1/2 y 1. A value of y = 1 means that the holes are empty, and a value of y = 1/2 that they are half filled. 

The authors rejected the normal thermal probability term, involving They 

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Cohen and Tumbull s model, compared with that for non-glass forming molten salts, 645 theory, for glass forming molten salts, 644 College Station, Texas, and centers for electrochemistry, 26... 

Glass forming, molten salts, 642 Glucose, and diabetics, 22 Gouy, originator of the ion-atmosphere model, 292... 

K.L. Ngai, in R. Rickert, A. Blumen (Eds.), Effects of Disorder in Relaxational Processes, Transport and Relaxation in Glass Forming Molten Salts, Springer-Verlag, Berlin 1994, p. 89. 

Table 4.1 Properties of binary glass-forming molten salts... 

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

The last part of ionic electrochemistry, ionics, is about pure electrolytes. A few decades back this electrochemistry would have been all about high-temperature liquids (liquid common salt at 850 °C was the role model). However, this has changed, and the temperatures for eliminating the solvent have deaeased considerably. Some molten salts are now room temperature liquids. At the other end of the temperature scale are the molten silicates, where large polyanions predominate. These are important not only in the steel industry, where molten silicate mixtures form blast furnace slags, but also in the corresponding frozen liquids, the glasses. 

As a consequence of impossibility to arrange particles in the liquid phase in one solution only, the formation of two liquids, which are not miscible, may occur. Such a situation can be frequently observed in glass-forming silicate melts in the region of high concentrations of SiOa. Partial miscibility in the liquid phase is quite uncommon in binaries of molten salts. Partial miscibility is more frequent in reciprocal salt systems and probably as a rule, in the high metal concentrations of the metal-metal halide systems. 

A special quartz cell for XAFS measurements of molten salts was developed by Okamoto et al. (2002). The cell having a sand-glass form is shown in Figure 10.1. The solid sample is placed in the upper container and the cell is heated. When the sample melts, the fused sample runs down through the narrow measurement part of the cell, where the... 

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