Electrodeposition of tungsten from Li2WO4-Na 2WO4-K2WO4 based melts

Abstract

We examined a eutectic Li2WO4-Na2WO 4-K2WO4 melt and several its derivative melts at 873 K to develop a stable bath which enables electrodeposition of high quality tungsten. Galvanostatic electrolysis at 25 mA cm-2 in the eutectic Li2WO4-Na2WO4-K 2WO4 melt gave a powdery deposit consisting of both tungsten and tungsten oxide. A dendritic deposit of tungsten was obtained from a Li2WO4-Na2WO4-K2WO 4-LiCl-NaCl-KCl melt, which has a lower viscosity, in the same electrolysis condition. The dendrite formation was completely suppressed by vibrating the electrode during the electrodeposition. However, the surface was covered with large and angular crystal grains. By addition of KF to the melt, the size of the crystal grains became much smaller and smooth surface was obtained. The XRD analysis and cross-sectional SIM observation revealed that the addition of KF suppressed the preferential growth of α-W crystal and gave the mixture of α-W and β-W resulting in smaller crystal grain size.