Metal-molten salt solutions

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). 

Causo, M.S., Ciccotti, G., Montemayor, D., Bonella, S., Coker, D.F. An adiabatic linearized path integral approach for quantum time correlation functions electronic transport in metal-molten salt solutions. J. Phys. Chem. B 109 6855... 

Electronic Transport in a Metal-Molten Salt Solution ... 

In this section we present some applications of the LAND-map approach for computing time correlation functions and time dependent quantum expectation values for realistic model condensed phase systems. These representative applications demonstrate how the methodology can be implemented in general and provide challenging tests of the approach. The first test application is the spin-boson model where exact results are known from numerical path integral calculations [59-62]. The second system we study is a fully atomistic model for excess electronic transport in metal - molten salt solutions. Here the potentials are sufficiently reliable that findings from our calculations can be compared with experimental results. 

The behavior of an excess electron in dilute metal-molten salt solutions has been the subject of many experimental and theoretical studies [69-72]. The details of the model we employ are exactly the same as the early calculations of Selloni and coworkers [71,72]. Specifically, our simulations have been performed on a periodically repeated system of 32 K+ cations, 31 CP anions, and 1 electron. The mass density was set to p = 1.52 x 10 kg/m. The temperature we use here is T = 1800 K. 

E. S. Fois, A. Selloni, and M. Parrinello (1989) Approach to metallic behavior in metal-molten-salt solutions. Phys. Rev. 39, p. 4812... 

High density of excess electrons. Concentrated metal ammonia, metal-molten salt solutions and liquid metals exhibit a transition from a localized to the metallic state. 

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. 

Metal-ammonia solutions are to be compared with molten salt solutions of the type KI-K, which show a two-phase region (Bredig 1964, Warren 1985), and with caesium vapour. This was emphasized particularly by Krumhansl (1965). 

What is the nature of the transition to the metallic state in a liquid containing a high density of excess electrons When the concentration of excess electrons in a liquid is gradually increased, a transition from behavior characteristic of a localized state to that characteristic of a delocalized state is observed—e.g., in concentrated metal-ammonia solutions and in metal-molten salt mixtures. 

Bredig [67] considered two categories of metal-molten salt mixtures metallic and nonmetallic solutions. In metallic solutions the metal dissolves without interacting strongly with the melt. Metal ions and partially free electrons are formed. The electrical conductivity of these mixtures increases strongly due to the presence of very mobile partially free electrons. Therefore an electronic conductivity appears in these melts. In nonmetallic solutions the metal reacts with the melt under the formation of subvalent ions or subvalent compounds. The electrical conductivity of these mixtures depends only to a small extent on the concentration of dissolved metal. The variation of properties of the metal-molten salt mixtures shows a continuous change from the metallic solutions to the nonmetallic if the temperature is sufficiently high. 

No Bi2Xi compound is known, but it has long been known that when metallic bismuth is dissolved in molten BiCl3 a black solid of approximate composition BiCl can be obtained. This solid is Bi24Cl28, and it has an elaborate constitution, consisting of four BiCll", one Bi2Clg, and two Bi + ions, the structures of which are depicted in Fig. 10-5. The electronic structure of the Bi + ion, a metal atom cluster, is best understood in terms of delocalized molecular orbitals. Other low-valent species present in various molten salt solutions are Bi+, Bi3+, Bi +, and Bi +. The last, in Bi8(AlCl4)2, has a square antiprismatic structure. 

The most obvious and frequently used route to sub-valent species in molten halide solution is the reduction of a normal-valent halide with its parent metal, i.e. sym-proportionation reactions such as (2). This methodology is natural, since it disfavors disproportionation of the sub-valent compound to the zero-valent state according to (1). In addition, the methodology minimizes the number of components in the system. Post-transition metal-molten salt systems from which solid, sub-valent compounds have been isolated through symproportionation reactions are summarized in Table 1. 

In Fig. 9.13, the heat treatments are necessary to improve the efficiency of the sulphation step. The latter can be engineered in several alternative types of plant. Alternatives are available for the subsequent steps to pure oxide, but usually based upon precipitation and crystallization, as is the one shown in Fig. 9.13. The precipitation of beryllium hydroxide by boiling an alkaline solution of sodium beryllate, is a particularly valuable purification step, and is also used in Fig. 9.14. Chlorination of oxide mixed with carbon is a standard type of operation as used for the preparation of chloride intermediates of other metals. Molten salt electrolysis is one of the two alternative commercial routes to pure beryllium metal, the other being shown in Fig. 9.14. 

In conclusion, it appears that few metal-molten salt systems behave in the ideally polarizable sense generally associated with the mercury/aqueous solution interface at 298 K. Possible exceptions include some noble liquid metal/melt systems such as mercury/molten nitrates and lead/molten halides at low temperatures (<773 K), but only when the molten electrolyte is extensively purified. Otherwise, systems need to be analyzed as complex impedances, using ac or pulse techniques, to determine whether the minimum interfacial capacitance is affected by extensive factors, leading to parallel pseudocapacitances and Faradaic components. The range of potentials and measuring frequencies for which the interface approaches ideally polarizable behavior also needs to be established. It now seems clear that the multilayer ionic model of charge distribution at the metal/melt interface is more pertinent to molten media than the familiar double layer associated with aqueous solutions. However, the quantitative theories derived for the former model will have to be revised if it is confirmed that the interfacial capacitance is, indeed, independent of temperature in the ideally polarizable region. 

A number of unique difficulties pertain to oxidation states of metal ions encountered in molten salt solutions. For example, for first-row transition metals, the highest oxidation state prevailing is often +3, as in the case of Fe and Cr. Frequently, for chlorides in particular, the +3 state compounds are volatile at suitable operating temperatures and, hence, their solutions are thermally unstable.Other problems encountered include rapidly dispropor-tionating states, the formation of oxyhalides, and precipitation of complexes by reaction with the melt. While redox reactions per se involve very fast charge transfer steps, these may occur at the extremes of the range of electrochemical stability, thus leading to concomitant solvent melt decomposition. Nevertheless, suitable processes such as Fe /Fe on vitreous carbon in chloride melts can be employed to determine the effective electrochemical areas of electrodes where diffusion coefficients are accurately known. ... 

Lithium Iodide. Lithium iodide [10377-51 -2/, Lil, is the most difficult lithium halide to prepare and has few appHcations. Aqueous solutions of the salt can be prepared by carehil neutralization of hydroiodic acid with lithium carbonate or lithium hydroxide. Concentration of the aqueous solution leads successively to the trihydrate [7790-22-9] dihydrate [17023-25-5] and monohydrate [17023-24 ] which melt congmendy at 75, 79, and 130°C, respectively. The anhydrous salt can be obtained by carehil removal of water under vacuum, but because of the strong tendency to oxidize and eliminate iodine which occurs on heating the salt ia air, it is often prepared from reactions of lithium metal or lithium hydride with iodine ia organic solvents. The salt is extremely soluble ia water (62.6 wt % at 25°C) (59) and the solutions have extremely low vapor pressures (60). Lithium iodide is used as an electrolyte ia selected lithium battery appHcations, where it is formed in situ from reaction of lithium metal with iodine. It can also be a component of low melting molten salts and as a catalyst ia aldol condensations. 

In atomization, a stream of molten metal is stmck with air or water jets. The particles formed are collected, sieved, and aimealed. This is the most common commercial method in use for all powders. Reduction of iron oxides or other compounds in soHd or gaseous media gives sponge iron or hydrogen-reduced mill scale. Decomposition of Hquid or gaseous metal carbonyls (qv) (iron or nickel) yields a fine powder (see Nickel and nickel alloys). Electrolytic deposition from molten salts or solutions either gives powder direcdy, or an adherent mass that has to be mechanically comminuted. 

Electrolysis. Electrowinning of zirconium has long been considered as an alternative to the KroU process, and at one time zirconium was produced electrolyticaHy in a prototype production cell (70). Electrolysis of an aH-chloride molten-salt system is inefficient because of the stabiUty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of in solution, decreasing the concentration of lower valence zirconium ions, and results in much higher current efficiencies. The chloride—electrolyte systems and electrolysis approaches are reviewed in References 71 and 72. The recovery of zirconium metal by electrolysis of aqueous solutions in not thermodynamically feasible, although efforts in this direction persist. 

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