Physico-Chemical nanomaterials science

Synthesis of Molybdenum and Tungsten Silicides Nanopowders from Ionic Melts
Tretiakov A.V.1, Gab A.I.1, Uskova N.M.2, Malyshev V.V.1,2
1 University “Ukraine”. 23 Lvivska St., Kyiv-03115, Ukraine. 
E-mail: a.tretiakov@meta.ua
2 Vernadsky Institute of General and Inorganic Chemistry of the NAS of Ukraine. 32/34 Palladina Ave., Kyiv-03142, Ukraine.

The aim of the present study is to implement high-temperature electrochemical synthesis of nanopowders of silicides of chromium, molybdenum, and tungsten during electrolysis of chromium (molybdenum, tungsten) and silicon containing ionic melts.
Temperature of electrochemical synthesis of molybdenum and tungsten silicides is 850-900 ºC. Gain in energy of interaction compared with known synthesis processes is provided by interaction of deposited at cathode fine powders of Mo (W) and silicon at atomic level.
Essential role in electrochemical synthesis of Mo and W silicides is played by temperature and current density values. Lowering the temperature below 850 ºC does not ensure completeness of interaction of Mo (W) and Si, and temperature rise above 950 ºC reduces stability of metal-salt “pear” causing silicides not to be formed. With optimal melt composition, pure products were obtained with current density 0.5-1.2 A/cm2 for MoSi2 and 0.5-1.5 A/cm2 for WSi2. With ik < 0.5 A/cm2, obtained product is contaminated with Mo or W. Within the current density range 0.5-1.5 A/cm2, current density increase raises Mo and W silicides dispersity.
During electrochemical synthesis of silicides of Mo and W, as well as during direct interaction of elements, main process is silicon diffusion through metal layer. In such a case, metal is weakly involved in diffusion, and higher silicides are formed in such a system from lower ones. Therefore, obtained products composition is influenced significantly by process duration. Thus, with melt composition, wt.%: NaCl - 49; Na3AlF6 - 49.0; Na2MoO4 - 1.0; SiO2 - 1.0, at 900 ºC, with current density 1.0 A/cm2, depending on electrolysis duration, various products are formed including different molybdenum silicides. Optimal electrolysis duration is 45-50 min.
Similar dependence of phase composition on electrolysis duration is observed in NaCl-Na3AlF6-Na2WO4-SiO2 system.