Formation of nano-tendril bundle (NTB) by evaporation of tungsten in neutral gases

In the fusion reactor ITER, helium (He) ions will irradiate the first wall. Under He irradiation conditions, the tungsten (W) wall may develop a foam filament structure, so-called fuzz. Recently, nano -tendril bundles (NTB) and large-scale fiberform nanostructure (LFN) were observed to develop on W surfaces under He irradiation conditions close to the one where fuzz formed. However, the mechanisms leading to NTB and LFN still need to be fully understood. This work reports the formation of NTB without He ion irradiation for the first time. The NTBs were formed by evaporation of W in He, Ar, Ar/H2O mixture or O2. The evaporations were performed on silicon substrates, at room temperature (RT) and for pressures between 20 to 100 Pa. After evaporation, the NTBs were characterized using scanning electron microscopy (SEM). This work demonstrates the importance of the evaporation conditions in the formation of NTB and proposes mechanisms to explain the formation of NTB and LFN.