Effect of Temperature on Crystal Structure of W Films Electrodeposited from Molten CsF–CsCl–WO3

Abstract

The electrodeposition of W was studied in detail using CsF–CsCl–WO3. Prior to electrodeposition, the WO3 solubility was confirmed to be 1.0 mol% at 773 K and increase with temperature. To investigate the effect of temperature on the crystal structure, electrodeposition was conducted at 6–25 mA cm−2 between 773 and 923 K with a unified charge density of 90 C cm−2. X-ray diffraction analysis confirmed that the crystal structures of the electrodeposited W films were β-W at 773 and 823 K, a mixed phase (α-W and β-W) at 873 K, and α-W at 923 K. The shape of the crystal grains varied with temperature: grains of β-W obtained at 773 and 823 K were spherical, while those of α-W obtained at 923 K were angular. Scanning electron microscopy observations showed that W films with smoother surfaces were obtained at lower current densities at all temperatures. In particular, a dense and smooth W film (surface roughness: 0.66 μm, thickness: 10 μm) was obtained at 6 mA cm−2 and 773 K. When the charge density was increased to 210 C cm−2 at 6 mA cm−2 and 773 K, a W film with a smooth surface and thickness of 30 μm was obtained.