Materials synthesis in the liquid phase, or wet-chemical synthesis, utilizes a solution medium in which the target materials are generated from a series of chemical and physical transformations. Although this route is central in organic chemistry, for materials synthesis the low operational temperature range of the solvent (usually below 200 °C, in extreme 350 °C) is a serious restriction. Here, salt melt synthesis (SMS) which employs a molten inorganic salt as the medium emerges as an important complementary route to conventional liquid phase synthesis. Depending on the nature of the salt, the operational temperature ranges from near 100 °C to over 1000 °C, thus allowing the access to a broad range of inorganic crystalline materials and carbons. The recent progress in SMS of inorganic materials, including oxide ceramic powders, semiconductors and carbon nanostructures, is reviewed here. We will introduce in general the range of accessible materials by SMS from oxides to non-oxides, and discuss in detail based on selected examples the mechanisms of structural evolution and the influence of synthetic conditions for certain materials. In the later sections we also present the recent developments in SMS for the synthesis of organic solids: covalent frameworks and polymeric semiconductors. Throughout this review, special emphasis is placed on materials with nanostructures generated by SMS, and the possible modulation of materials structures at the nanoscale in the salt melt. The review is finalized with the summary of the current achievements and problems, and suggestions for potential future directions in SMS.

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Salt melt synthesis of ceramics, semiconductors and carbon nanostructures

Electrochemical reduction of solid Ta2O5 to Ta metal in molten CaCl2 at 1123 K has been studied by cyclic voltammetry and potentiostatic electrolysis, together with spectroscopic, electron microscopic, and elemental analyses. The initial step in the electro-reduction process is the electrochemically driven oxide−Ca2+ interaction, leading to the formation of two or more stable calcium-containing compounds which are likely Ca0.5Ta2O5 and CaTa2O5. These compounds are then reduced to TaO and/or δ-TaO before full metallization. At potentials more negative than −0.9 V vs Ag/AgCl, fine tantalum powder (aggregates of nodular ∼200 nm particles) can be prepared by direct electrolysis of thin and porous pellets of the Ta2O5 powder (aggregates of nodular particles of ∼300 nm in size). The current efficiency and energy consumption are satisfactory, for example, achieving ∼78% and ∼2.4 kWh/kg-Ta at −1.0 V, respectively. The oxygen concentration in the produced Ta powder can be reduced to as low as 1200 ppm by electroly...

Thin pellets : Fast electrochemical preparation of capacitor tantalum powders

Electrochemical reduction behavior of U3O8 powder in a LiCl molten salt

https://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/57796/1/CO2OtakeREMTRev2.pdf

CO2 gas decomposition to carbon by electro-reduction in molten salts

Mechanistic insight of electrochemical reduction of Ta2O5 to tantalum in a eutectic CaCl2–NaCl molten salt

Tantalum and niobium powder preparation from their oxides by calciothermic reduction in the molten CaCl2

Formation of broccoli-like morphology of tantalum powder

Dielectric properties of tantalum powder with broccoli-like morphology

https://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/57797/1/REMT_Ueda_Ver9Ryo2.pdf

Formation of niobium powder by electrolysis in molten salt

https://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/193298/1/j.jpcs.2014.11.014.pdf

Masahiko Baba

Papers

Tantalum and niobium powder preparation from their oxides by calciothermic reduction in the molten CaCl2

Formation of broccoli-like morphology of tantalum powder

Dielectric properties of tantalum powder with broccoli-like morphology

Formation of niobium powder by electrolysis in molten salt

Niobium powder synthesized by calciothermic reduction of niobium hydroxide for use in capacitors