Synthesis of Calcium Hexaboride by Electrolysis of Molten Salt
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INTRODUCTION
HEXABORIDES of alkaline earth and rare earth metals have been at the center of numerous experimental and theoretical studies for more than four decades. This interest is due to the great variety and unique combination of their properties. Calcium hexaborides show low density (about 2.5 g/cm3), high hardness (25 to 30 GPa), a high melting point (2200 °C), good electrical conductivity at high temperatures and chemical stability.[1] In addition, calcium hexaboride has such useful properties as low work function, stable resistivity and a low coefficient of linear expansion across a relatively wide temperature range. Calcium hexaboride is known to be the most effective deoxidizer in the production of pure copper and silver. It is also a good semi-finished product for the production of abrasive tools and structural ceramics. The discovery of an unusual type of ferromagnetism in hexaboride CaB6 and SrB6 about 20 years ago caused a new surge of interest in them. The relatively high Curie temperature (600 K) was a surprise discovery because these compounds do not have partially occupied d or f electronic states. Extensive
YACOV CHERNOV, NIKOLAY SHUROV and NIKOLAI TKACHEV are with the Institute of High-Temperature Electrochemistry UB RAS, Yekaterinburg 620137, Russian Federation. EUGENE FILATOV is with the Institute of HighTemperature Electrochemistry UB RAS and also with Ural Federal University, Yekaterinburg 620002, Russian Federation. Contact email: e.fi[email protected] VALERY SMOLENSKI is with the Institute of High-Temperature Electrochemistry UB RAS and also with the College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China. Manuscript submitted September 26, 2018 Article published online June 19, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B
theoretical and experimental studies aimed at explaining the observed magnetic properties of the band structure of CaB6 are still being carried out.[2–6] Known methods for preparing hexaborides of alkaline earth metals include the direct reaction of metals and boron at high temperature and pressure, reduction of CaO and B2O3 by carbon (carbothermia) or metals, gas-phase reduction methods and mechanochemical synthesis.[7] There are also several works on obtaining powders of CaB6 by electrolysis of molten salts: Uchida,[8] Wang and Zhai,[9] Angappan et al.[10] In work[9] CaB6 was obtained by electrodeposition in molten CaCl2-NaCl. The difference is that the working temperature is lower (600 °C and 700 °C), while the solubility of calcium borate in CaCl2-NaCl was lower than in CaCl2 (the present paper). In the work[10] hexaboride calcium was synthesized at a temperature of 900 °C as a result of electrolysis of the molten salt. In this work, a CaO-B2O3-LiF melt was used in the synthesis in an argon atmosphere. In contrast to this work, we carried out electrolysis in an atmosphere of air. Furthermore, corrosive active alkali metal fluorides as, for example, in papers[8–12] were not used by us. The closest to our work is t
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