When we talk about ‘materials’, we generally imagine solids, although liquids also satisfy the dictionary definition of ‘matter’. In fact, molten or fused salts have a long and distinguished history in the preparation and modification of solids, and have helped realize unique engineering systems by solving important problems.
For the uninitiated, let me state some things about the nature and properties of molten/fused salts. In the past, they were considered to comprise a fused inorganic compound or, more usually, a solution of several compounds that could be characterized in terms of an oxidation potential and an acid-base parameter (equivalent to the pH of aqueous solutions). Salts of binary, ternary, and higher-order systems have (generally known) phase diagrams and thermodynamic descriptions similar to metallic systems. More recently, organic-based fused salts, which generally also provide liquids at a lower temperature range, have been included as fused salts under the label ‘ionic liquids’. Both types of molten salts usually exhibit exclusive ionic conduction with small alkali cations as carriers and, thus, are suited to support electrochemical (EC) processes for applications such as open-circuit sensors and, especially, EC processes driven by applied voltages. The electrolytic nature of fused salts also allows the use of transient EC studies to identify the dominant oxidation and reduction reactions at the electrodes. Depending on several factors, these solutions can also act as good solvents for oxides and other compounds.
Perhaps the most famous application of a molten salt is the use of fused cryolite (Na3AlF6) as both the solvent for alumina and the electrolyte for electrowinning liquid Al in the 120 year-old Hall-Herault cell. The almost identical ionic radii of fluoride and oxide ions permit the dissolution of alumina upon the formation of soluble oxyfluoride complex anions, a favorable stereochemistry for which an improvement or replacement has not been found. Another example is the use of fused fluorides or chlorides for the electrodeposition of refractory metals, Mg, B, U, rare-earth metals, etc. Thermodynamically, electrowinning is the only route available for the production of metals from very stable compounds, although a reasonably stable compound will react with the liquid metal of a more reactive metal. A special effort is underway to reduce the cost of Ti by developing novel electrodeposition processes to replace the Kroll process. Fused chloride pyrochemical processes are being developed for reprocessing and recycling spent nuclear fuels. At the high end of the temperature scale, fused oxides (slags) are used in the refinement of specialty steels in an electro-slag-refining process. These EC processes are all subject to the inherent slowness imposed by Faraday's Law and a high electrical cost.
Because of their relatively high thermal conductivity and reasonably low viscosity, fused nitrate salts are used for heat transfer or cooling in many systems, including solar power generation stations and in the heat treatment of steels. The molten carbonate fuel cell, which generates electrical energy via the EC oxidation of a fuel, is on the verge of being commercialized. In this case, a binary alkali carbonate salt is really a thick paste held in a thin porous oxide separator.
On the negative side, highly corrosive fused alkali sulfate films are deposited onto metallic parts from combustion product gases in coal-burning power plants and gas turbines. Of course, fused chlorides and fluorides are also corrosive to hardware if an oxidant is present. One strange deleterious aspect of fused salts is the enormously high volume expansion (about 25%) upon melting of the solid; the associated stresses in constrained containers can burst the hardware when freezing-melting cycles are involved. Environmental restrictions on the use and disposal of inorganic salts has led to an emphasis on recycling and the substitution of ionic liquids, which can offer a greener technology.
If the applications and problems associated with fused salts seem worthy of continued research and development – and I think they are – then the thermodynamics, chemistry, electrochemistry, spectroscopy, and modeling that constitute the basis for fused salt technology must continue to find some emphasis in college curricula and research funding agencies. I think that a good chemistry background is sufficient for scientists to shift to consider fused salt systems. There is some commonality between the concepts of aqueous solutions and fused salt systems, but one needs to make some adaptation for studies that require elevated temperatures and a different sort of solution chemistry.