Electrolytic preparation of CaB6 by molten salt technique

Abstract

Calcium hexaboride (CaB 6 ) crystals with high melting point (2,235°C) have been conveniently synthesized at low temperature (900°C) from molten salt electrolysis. The synthesis was carried out using CaO-B 2 O 3 -LiF melt under argon atmosphere. Electrochemical experiments were carried out in an inconel reactor to having a high purity graphite crucible, which served as an electrolyte holding vessel as well as the anode. An electropolished molybdenum rod was employed as the cathode. The electrolysis was performed at 900°C under argon atmosphere at current densities ranging from 0.5 to 1.5 A/cm 2 at 1:6 M ratios of calcium and boron content. After the electrolysis, the cathode product was removed and cleaned using dilute HCl solution followed by triple distilled water. Characterization of the crystalline product by TG/DTA, XRD, CHNS, EDAX, XPS, EPR, and SEM were reported. From the studies, it has been observed that CaB 6 can be synthesized at all current densities and the products have some impurities.

Electrochemical Synthesis of Magnesium Hexaboride by Molten Salt Technique

“…The mechanism of hexaboride formation was proposed by many authors [ 12 , 14 – 17 , 37 – 47 ]. According to Li and Jin, the negative charged boron atoms and the positive charged alkaline earth metal atoms form complexes of M 2+ metal cation and B 6 2− dianion due to electrostatic attraction.…”

“…The whole assembly was placed in an inconel reactor, which was kept in an electrical heating furnace with thermocouple. The experimental setup for the electrosynthesis of magnesium hexaboride is described elsewhere [ 14 – 17 ]. Then the salts were melted slowly under a continuous flow of argon gas.…”

“…Then the salts were melted slowly under a continuous flow of argon gas. The melt was equilibrated at 900°C for one hour before proceeding electrolysis [ 15 – 17 ]. The bath was preelectrolyzed at 2.0 V for one hour to remove the impurities and moisture prior to electrolysis.…”

Collective synthesis of the CaB₆‒TiB₂ heterogeneous powders

“…В литературе описывается достаточно широкий спектр различных методов синтеза CaB 6 (в том числе в нанокристаллическом состоянии): электролиз в расплавах солей [10,11], синтез сжиганием [12,13], механосинтез [14] и др. Доступность исходных компонентов и низкое содержание примесей в конечном продукте являются преимуществами методов синтеза, связанных с твердофазным высокотемпературным карботермическим восстановлением CaCO 3 с использованием различных источников бора (элементарного B, B 2 O 3 , H 3 BO 3 , B 4 C) [15][16][17][18][19][20][21][22].…”

Low-temperature synthesis of calcium hexaboride nanoparticles via magnesiothermic reduction in molten salt

“…The techniques/methodologies used to date for the CaB 6 powder synthesis include mainly, direct elemental reaction, 12) borothermal or borocarbide reduction, 13)19) mechanochemical treatment or combustion assisted metallothermic reduction, 3), 20) solution-based processing, 21)23) molten salt electrolysis, 10),24), 25) floating zone method, 26) and aluminum flux method. 4) Unfortunately, these techniques suffer from at least one of the following drawbacks: use of expensive and/or hazardous raw materials/ precursors (e.g., elemental boron, calcium and NaBH 4 ), requirement of specialty equipment/reaction vessels and high processing temperature/pressure, and/or long processing time, heavy agglomeration and poor purity of final products, and high overall production cost.…”