Nanocomposites and nanomaterials

Electrochemical behavior and galvanic treatment of boron carbide in ionic melts
Malyshev V.V.1, Gab A.I.1, Shakhnin D.B.1, Bogovik K.I.1
1 Institute of Engineering & Technology, Open International University of Human Development “Ukraine”, 23 Lvivska St., Kyiv-03115, Ukraine. 
E-mail: viktor.malyshev.igic@gmail.com

The thermodynamic analysis was carried out of possible reactions between boron carbide and oxygen-containing compounds of the Group IV and VI elements usually used to modify acid-base properties of melts in electrodeposition and synthesis of refractory metal carbides and borides.
The oxidation of boron carbide to metaborate or boron oxide was found to be the energetically most preferable processes. The compounds involving carbon or a Group VI element are expected to reduce to give the element in the free state or its intermediate-valence oxide. At 900-1200 K, boron carbide should actively interact almost with all the considered compounds. The redox processes occur at the semiconductor-melt interface in the presence of "mild" oxidizers with establishment of the electrochemical equilibrium and space separation of charges. In contact with "strong" oxidizers, B4C undergoes noticeable corrosion.
The electroreduction of oxyanions MoO42- (WO42-) was studied in the NaCl- KCl melt containing Ca2+ cations, at the platinum-pin, platinum-gauze and boron carbide electrodes. Comparison of the limiting currents at the empty and filled platinum containers allowed us to estimate the surface area of B4C grains working as the cathode. From the difference in currents at the empty and filled gauze-container electrodes, the partial currents due to reduction of MoO42- at the B4C grain surface were calculated.
Chemical deposition of molybdenum, tungsten, and their carbides were performed onto B4C grains from the halide-oxide, NaCl-KCl-Na2MO4-CaCl2, and oxide melts Na2WO4-MO3, Na2WO4-MO3-Li2CO3 at different temperatures, cathodic current densities and duration of the process. The chemical deposition of molybdenum and tungsten was carried out from the melts NaCl-KCl-3 mol.% Na2MO4-5 mol.% CaCl2 and Na2WO4-5 mol.% MO3. The electrochemical deposition of molybdenum carbide was conducted in the melt Na2WO4-5 mol.% MoO3-10 mol.% Li2CO3 at the current density of 10 to 200 A/m2 and the temperature of 1073 to 1173 K (powder deposits are formed at < 1073 K). The degree of metallization of B4C grains was found to depend on the cathodic current density and duration of the electrochemical or chemical deposition.