Physico-Chemical nanomaterials science

High Temperature Electrochemical Synthesis of Tungsten Carbide Nanopowders from Ionic Melts
Yasko O.M.1, Gab A.I.1, Malyshev V.V.1
1 Institute for Engineering & Technology, University “Ukraine”. Lvivska St. 23, Kyiv-03115, Ukraine. 
E-mail: o.yasko@meta.ua

We studied the simultaneous electroreduction of CO32- and WO42- in oxide (Na2WO4) melts. It is difficult to detect the reduction wave of lithium tungstate on the background of Na2WO4. Adding lithium tungstate up to 20 mol % gave rise to no waves but caused a positive shift of the voltammetric curve relative to that of pure tungstate. A wave is detectable only at lithium tungstate concentrations above 20 mol %. The reduction wave of lithium tungstate is observed at its concentration 40 mol % and a potential of -1.8 to -1.9 V. Adding lithium carbonate to the Na2WO4-Li2WO4 melt initially causes an increase in the height of the lithium tungstate reduction wave. The reduction wave of the carbonate ion, followed by lithium tungstate electroreduction, appears at a lithium carbonate concentration of 10 mol %. At the lithium carbonate concentration of 15 mol %, the tungstate and carbonate3 reduction waves merge in a single broad electroreduction wave. 
Tungsten carbide was obtained from the electrolyte composed of 10-20 mol % Li2CO3, 30-45 mol % Na2WO4, and Li2WO4 at T= 1073-1173 K and i = 1.0-1.7 A/cm2. Varying the concentrations of the melt components modifies the product composition. Melts containing less than 10 mol % Li2CO3 yield a WC + W2C + W mixture with component proportions depending on the carbonate concentration. At Li2CO3 concentrations above 20 mol %, the product is contaminated by free carbon. Pure tungsten carbide forms from melts containing 5-45 mol % lithium tungstate. At the other Li2WO4 concentrations, the desired product is contaminated by tungsten oxide.
The product composition depends strongly on the synthetic temperature. A WC + W2C mixture forms below 1073 K, and a W2C or a W2C + W mixture results at 1023 K and below. Raising the temperature above 1173 K is inexpedient because of the thermal instability of the carbonate ion and the increasing volatility of the melt components. The cathodic current density is governed by the carbonate concentration in the melt. Striving for finer powders, we conducted the electrosynthesis at the highest possible current densities. In optimal melts, i varied between 1.0 and 1.7 A/cm2, yielding tungsten carbide powders with a specific surface area of 10-20m2/g.