Fiber-form nanostructured tungsten formation by helium arc discharge plasma irradiation under a gas pressure of 5 kPa

Abstract

Fiber-form nanostructured tungsten (W) formation was successfully demonstrated using a helium (He) arc discharge plasma generated at a gas pressure of 5 kPa. The W substrate was exposed to the He arc discharge plasma at a surface temperature of about 1000 °C. A bias voltage, Vb, was applied between the W substrate and the vacuum chamber to control the incident He ion energy, Ei. Scanning electron microscope (SEM) observations showed that the W surface was slightly roughened, but no fiber-form nanostructure was formed at a Vb of −22 V. On the other hand, fiber-form nanostructure growth was observed at a Vb of −112 V. Furthermore, cross-sectional SEM observations revealed the presence of He bubbles inside the W fibers and the sub-surface layer of the W substrate. As collisions between ions and neutrals in the sheath cannot be ignored at 5 kPa, a theoretical evaluation of Ei distribution was performed using the collisional sheath model. Furthermore, an experimental evaluation of Ei using the sputtering yield of carbon substrates by He arc discharge plasma irradiation was carried out, and then Ei was obtained to be 40–50 eV for Vb = −112 V. Therefore, the distribution of Ei at Vb = −112 V is considered to contain more than 20 eV, which is necessary for the growth of fiber-form nanostructured W found in conventional low-gas pressure plasma irradiation experiments.