Electrodeposition of tungsten from ZnCl 2–NaCl–KCl–KF–WO 3 melt and investigation on tungsten species in the melt

Abstract

The electrodeposition of tungsten in ZnCl 2–NaCl–KCl–KF–WO 3 melt at 250 °C was further studied to obtain a thicker deposit. In the ordinary electrolysis at 0.08 V vs. Zn(II)/Zn, the current density decreased from 1.2 mA cm −2 to 0.3 mA cm −2 in 6 h. A thickness of the obtained tungsten layer was 2.1 μm and the estimated current efficiency was 93%. A supernatant salt and a bottom salt were sampled after 6 h from the melting and were analyzed by ICP-AES and XRD. It was found that the soluble tungsten species slowly changes to insoluble ones in the melt. The soluble species was suggested to be WO 3F − anion. One of the insoluble species was confirmed to be ZnWO 4 and the other one was suggested to be K 2WO 2F 4. Electrodeposition was carried out under the same condition as above except for the intermittent addition of WO 3 every 2 h. The current density was kept at the initial value and the thickness was 4.2 μm. The intermittent addition of WO 3 was confirmed to be effective to obtain a thicker tungsten film.