Porous tungsten nanostructure formation using a helium arc discharge plasma under sub-atmospheric pressure

Porous tungsten (W) nanostructure formation was performed for the first time using a helium (He) arc discharge plasma under sub-atmospheric pressure of 80 kPa. Scanning electron microscope observations showed that micron-sized bubble and hole structures were formed in the W substrate at a surface temperature of 1700 degrees C, indicating He penetration into the W substrate. At the surface temperature of 800 degrees C, surface blackening of the W substrate due to the He plasma irradiation for two hours was found. The unique surface morphology has a W nanofiber network with a thickness of similar to 10 mu m which consists of assembly of W nanoparticles with a diameter of 20-30 nm. It was suggested that an interaction between W vapor evaporated from the W electrodes and the surrounding He plasma plays a key role for physical processes of the nanostructured W formation identified in this study. It could be formed due to a process of cluster-cluster aggregation of W nanoparticles produced by nucleation and condensation from the W vapor phase. The W nanoparticle network deposition is considered to be a new process for the porous W nanostructure formation as an alternative to standard fuzz formation associated by bubble formation on the surface of base W substrate.