Molten salt route

Sanchez S, Lucas C, Picard GS, Bermejo MR, CastriUejo Y (2000) Molten salt route for ZnSe high-temperature electrosynthesis. Thin Solid Films 361-362 107-112... 

Cationic deltahedral Pn clusters are known for Pn=Bi. [Bis], [Big], and [Bi9], which have been structurally characterized with complex counterions such as AlCLt, AsFg, and HfCle , can be obtained either by molten salt routes or by using so-called super acidic systems [205-209]. According to Wade s formalism, [Bis] " " and [Big] correspond to 12 ske closo and 22 ske arachno structures, respectively, and the latter adopts the shape of a square antiprism. [Bi9] " which is expected to form a 22 ske nido-cluster in analogy to Fig. 2d, rather adopts a distorted tricapped trigonal prismatic topology (Fig. 2c). 

In nonreactive molten salts, on the other hand, flux components are not incorporated into the product phase. Here, the molten salt acts more in the classical sense as a reagent to promote the reaction at a lower temperature than would be required by the ceramic, or direct, route (Section 5.2). This is accomplished by two attributes of molten salts an acid-base equilibrium that enables the general dissolution-recrystallization of metal oxides and a highly electropositive (oxidizing) environment that stabilizes the highest oxidation state of many transition metals (Gopalakrishnan, 1995), which can lead to mixed valency. A plethora of complex transition metal oxides have been synthesized in nonreactive molten alkali metal hydroxides, carbonates, and hypochlorites. Examples of such molten salt routes to mixed transition metal oxides include (Rao and Raveau, 1998) ... 

A major drawback connected with the symproportionation reaction is that the number of redox reactions that can take place is limited. A more recent development, which shows obvious similarities to the traditional molten salt route, but whieh avoids the limitations of symproportionations, is the solid-state technique developed by Beck in which a volatile, high-valent transition-metal chloride acts both as halide acceptor and as oxidizing agent. The synthesis of Tes " by oxidation of tellurium with WCl6 according to (4) is representative. ... 

The use of AsFs as the oxidizing agent for the elements in cluster synthesis has several advantages over the molten salt route and the syntheses in neat superacids the reactions may be performed in inert solvents (most often liquid SO2) and the syntheses may be performed at ambient or low temperature rather than at elevated temperatures. In addition, the reaction by-products and the solvents are volatile and easily separated from the reaction mixture. Furthermore, the AsF ion is thermodynamically more stable than the A1CU and GaCU ions (Table 2), which are the protagonists of the molten salt scene. 

J. Trojan-Piegza, E. Zych, Preparation of nanocrystalline Lu203 Eu phosphor via a molten salts route, J. Alloys and Compounds, 380, 118-22 (2004). 

Compared with the conventional method, the ceramic prepared from the molten-salt route showed better dielectric properties. Tawichai et al. [77] reported BaFeo.5Nbo 03 (BFN) ceramics that were obtained at 700 C. Because the ceramics prepared by the molten-salt method had a higher density (6.12 g/cm ) than the ceramics prepared by the traditional sintering method (4.60 g/cm ), a very high dielectric constant with low loss tangent was observed. 

Apart from the most electropositive metals, most other metals extracted through molten salt routes are recovered as solids these include many important refractory and other transition metals, the lanthanides, and some actinides. Particularly interesting problems arise in the electrowinning of the refractory metals. Attempts to deposit these metals in a coherent, massive form of theoretical density usually meet with a number of difficulties. Deposits may be dendritic, for example, if electrodeposition proceeds under mass transfer control, or they may be powdery and nonadherent if secondary reactions, such as alkali metal deposition, followed by backreaction with the solute, occurs. Moreover, powdery deposits may also arise if low oxidation states, formed as intermediates during the reduction process, disproportionate in the metal-melt interphase. Charge-transfer-controlled electrodeposition or coupled chemical steps appear to be a prerequisite for obtaining dense, coherent, and adherent deposits. Such deposits have been obtained... 

Much research and development has been carried out on possible electrowinning routes for these metals however, except for tantalum,which is used in relatively large quantities for making capacitors, no other process has been operated on a commercial scale. This contrasts sharply with the extraction of liquid metals via molten salt routes, mentioned above. The reasons for this are complex but include innate conservatism and historical factors, as well as more obvious considerations such as capital investment and the high cost of preparing suitable feedstocks. One inherent factor stands out, however, and that is the form of the deposit if this is dendritic and/or powdery, as frequently happens, then postelectrolytic separation of the metal from entrained melt can be costly, time consuming, and lead to contamination of the product, especially with oxides. Nevertheless, some quantities of mischmetal as well as certain actinides are recovered in this way, in spite of their pyrophoric nature. 

Molten salt route is one relevant way for producing these metals extractive metallurgy can provide the oxides (Nb20s, Ta20s) or the fluoride compounds (K2NbF7 K2TaF7). 

The book contains 61 chapters written by authors all recognized as specialists actively working in fused salts chemistry, electrochemistry and solid state chemistry. Our purpose was to offer new aspects of Molten Salts Chemistry and Technology to readers from academia and industry. It should be useful for generating new ideas showing the interest of the molten salt route. 

Over the past few years, a large number of experimental approaches have been successfully used as routes to synthesize nanorods or nanowires based on titania, such as combining sol-gel processing with electrophoretic deposition,152 spin-on process,153 sol-gel template method,154-157 metalorganic chemical vapor deposition,158-159 anodic oxidative hydrolysis,160 sonochemical synthesis,161 inverse microemulsion method,162 molten salt-assisted and pyrolysis routes163 and hydrothermal synthesis.163-171 We will discuss more in detail the latter preparation, because the advantage of this technique is that nanorods can be obtained in relatively large amounts. 

Excision reactions are sometimes accompanied by redox chemistry. For example, dissolution of the 2D solid Na4Zr6BeCli6 in acetonitrile in the presence of an alkylammonium chloride salt results in simultaneous reduction of the cluster cores (144). Here, the oxidation product remains unidentified, but is presumably the solvent itself. As a means of preventing such redox activity, Hughbanks (6) developed the use of some room temperature molten salts as excision media, specifically with application to centered zirconium-halide cluster phases. A number of these solids have been shown to dissolve in l-ethyl-2-methylimidazolium chloride-aluminum chloride ionic liquids, providing an efficient route to molecular clusters with a full compliments of terminal chloride ligands. Such molten salts are also well suited for electrochemical studies. 

In this chapter we discuss preparative routes for inorganic materials in three basic types of systems involving the presence of a distinct solid-liquid interface those in which the liquid and solid phases are of the same chemical identity (solidification and vitrification processes), those in which the liquid and solid phases are not of the same chemical identity (crystallization, precipitation), and the special case in which the liquid phase is a pure ionic liquid or molten salt. Ionic liquids can serve as the solvent as well as a templating agent, and the liquid components may or may not become incorporated into the final solid product. We also discuss two areas where the distinct solid-liquid interface becomes somewhat blurred namely, sol-gel and solvothermal processes. 

Both reactive and nonreactive molten salts can be used in nontopochemical routes. An example of a nontopochemical route to inorganic materials utilizing reactive molten salts is when a metallic element is reduced in a low-melting alkali metal polychalcogenide (hiQn, where Q = O, S, Se, Te) to form a ternary metal chalcogenide. Potassium bismuth sulfide (KBi3Ss) has been prepared in... 

Separation of the tetrahalides can be achieved from their solutions in molten salts (K2ZrF6, NaCl, SnCh) or under pressure (10-70 atm). However, distillation of the adducts of the tetrachlorides with POCI3 is a more convenient route. Borohydrides and aUcoxides of Zr and Hf are more volatile, but their complicated syntheses and high air-sensitivity hmit their usefulness. 

The reaction between boron(III) oxide (or boric acid, borax, kernite or colemanite) and phosgene in a molten salt (AlClj/NaCI) has been patented as a commercial route to boron(III) chloride [148] ... 

In our natural environment, metals are most stable in an oxidized state, e.g., Fe203 or AI2O3. As a consequence, one step of metal ore refining is the reduction of the metal oxide to its zero oxidation state. An electrochemical reduction process, where the electrolysis medium is a molten salt, is preferred for very electropositive metals (e.g., aluminum, sodium, lithium, and magnesium) and for metal refining where the chemical route suffers from environmental problems. 

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. 

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