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Liquid alkali metals have several physical properties which favor their use in a number of important applications. For example, their large liquidus temperature range and their excellent heat transfer properties are important for use as heat transfer media. They are used in large nuclear reactors in which hundreds of tons of sodium are circulating, and in small parts of engines for cooling of valves. Since these metals are among the most electropositive elements, several of them (Li, Na) can be used in high specific capacity and high energy density batteries at moderately elevated temperatures. The compatibility of metallic constructional materials which are used to contain the liquid metals is strongly influenced by nonmetals present in the liquids. The physical properties of the liquid metals are also influenced by dissolved substances. Several nonmetals dissolved in alkali metals are able to form ternary compounds with components of the constructional materials. Thus, corrosion and compatibility studies have been accompanied by extensive chemical work related to the solutions of non-metallic substances in liquid alkali metals. All available solubility data of nonmetallic elements and some of their compounds in the five liquid alkali metal solvents (Li, Na, K, Rb, and Cs) are collected and compiled. Original publications with reliable data and information on the methods used to generate them are reported in individual Compilations. When numerical data are not given in a publication, the data are often read out from figures and converted into numerical data by the compilers. The precision of this procedure is indicated in the Compilations under Estimated Error. Evaluated solubility data are tabulated at the end of the Critical Evaluations: if there is agreement of at least two independent studies within the experimental error, the solubility values are assigned to the “recommended” category. Values are assigned as “tentative,” if only one reliable result was reported, or if the mean value of two or more reliable studies was outside the error limits. In the tabulation, three, two, or one significant figures are assigned for respective precisions that are better than ±1% and ±10% and worse than ±10%. If necessary, the solubilities are recalculated into mol %. The completeness of this investigation of the literature has been confirmed and extended by studying several reviews dealing with the solution chemistry of substances in the alkali metals. Solubility data are sometimes measured under parameters, which are not standard conditions of such measurements. Frequently measurements are performed under constrained pressure. The solubility of noble gases or other gases, which do not form compounds with the alkali metals, depends on the gas pressures. This dependency is documented in the data sheets. Schematic phase diagrams are presented in systems for which they assist the understanding of the data and the conclusions. They are based on the most recent state of knowledge and generally presented in the Critical Evaluations. Some solubility diagrams are shown in form of a log solubility versus reciprocal temperature function. These figures illustrate the larger scatter of data for systems in which interfering reactions cause unstable behavior of solutions. While several solutes are well defined substances, other systems need still additional studies to define the equilibrium solid state compound. One should realize that estimations of the stoichiometry and thermal stability of ternary compounds are experimentally difficult, and their results are often uncertain.
A ruLe, based on the work describing the factors governing the alloying behavior of metaLs [1], allows the prediction of the miscibility or immiscibility of aLkali or aLkaline earth metaLs with other metaLs in the Liquid state. GeneraLLy, metallic eLements are unlikeLy to dissoLve each other if their atomic diameters differ by more than 15% and if there is a high eLectrochemi-caL tendency to favor the formation of intermetaLLic compounds. An eLement is aLso more likeLy to dissoLve in one of higher vaLency rather than the converse [2].The atomic radii of the aLkali metaLs range from 0.156 to 0.27 nm and those of the aLkaline earth metaLs from 0.113 to 0.217 nm. The radius of U in the metallic state is 0.138 nm. Miscibility of the eLements in the Liquid state shouLd, therefore, be restricted to incompLete miscibility of U with the Light aLkaLi and aLkaline earth metaLs. The Low vaLency of the group I a and II a eLements compared to U dictate that they are more likeLy to dissoLve in the moLten heavy metaL [2].AdditionaLLy, a maximum eLectrochemicaL factor for soLubility, in terms of Pauling's eLectronegativity concept, was formuLated. According to this factor, the difference in eLectronegativity between the two eLements under consideration shouLd be Less than 0.4 eV to permit miscibility [3]. One shouLd expect onLy a Limited miscibility of the aLkali and aLkaline earth metaLs with U owing to their eLectrochemicaL factors. The miscibility in the Liquid state is determined by the Hildebrand soLubility parameter, (), which is defined as () = (dEylV)1/2, where dEv is the heat of vaporization and V is the atomic voLume [4]. According to Hildebrand's ruLe, two non-poLar Liquids shouLd be compLeteLy miscibLe if V ()A -()B)2 < 2 RT. However, this ruLe negLects the influence of the eLectronegativity, X. Therefore, Mott introduced the eLectronegativity difference (XA -XB) and n, the number of bonds, which the two eLements can form. The expression for the compLete miscibility in the modified form is V ()A -()B)2 -23060 n (XA -XB)2 < 2 RT. This ruLe was verified for a Large number of aLkali metaL-heavy metaL systems [5].The temperature coefficients of the soLubility in binary metallic systems are dependent on an atomic size effect [6]. According to Mott's ruLe, the ratios of the atomic radii influence the temperature gradients of the soLubility. Experiments showed that the soLubilities, which can be expressed by equations of the generaL form Log N = B -A/T, are influenced by the atomic size factor in both constants, Band A. High vaLues of A and B are found in systems with rB/rA beLow 0.8 and around 1.4 [7]. The vaLues of rB/rA caLcuLated for the aLkali and aLkaline earth metaLs and U atomic radii aLso predict very poor miscibility of the metaLs in their Liquid state. Gmelin Handbook U Suppl. Vol. B 2 H. U. Borgstedt et al., Alloys of Uranium © Springer-Verlag Berlin Heidelberg 1989 Gmelin Handbook U Suppl. Vol. B 2 It is obvious from gettering reactions of Liquid Na with solid U that the alkali metal does not dissolve in U in m...
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