Temperature-dependent surface porosity of Nb2O5 under high-flux, low-energy He+ ion irradiation

Abstract

The present study reports on high-flux, low-energy He + ion irradiation as a novel method of enhancing the surface porosity and surface area of naturally oxidized niobium (Nb). Our study shows that ion-irradiation-induced Nb surface micro-and nano-structures are highly tunable by varying the target temperature during ion bombardment. Mirrorpolished Nb samples were irradiated with 100 eV He + ions at a flux of 1.2×10 21 ions m-2 s-1 to a total fluence of 4.3×10 24 ions m-2 with simultaneous sample annealing in the temperature range of 773-1223 K to demonstrate the influence of sample temperature on the resulting Nb surface morphology. This surface morphology was primarily characterized using field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Below 923 K, Nb surfaces form nano-scale tendrils and exhibit significant increases in surface porosity. Above 923 K, homogeneously populated nano-pores with an average diameter of ~60 nm are observed in addition to a smaller population of sub-micron sized pores (up to ~230 nm in diameter). Our analysis shows a significant reduction in surface pore number density and surface porosity with increasing sample temperature. High-resolution ex-situ X-ray photoelectron spectroscopy (XPS) shows Nb 2 O 5 phase in all of the ion-irradiated samples. To further demonstrate the length scales in which radiation-induced surface roughening occurs,

One‐Step Plasma Synthesis of Nb2O5 Nanofibers and their Enhanced Photocatalytic activity

“…Also, the difference in the ion flux by~20 % (2:4 À 3:1 Â 10 22 m À2 ) had a minor effect, considering the fact the irradiationtime was changed by a factor of three or greater. As was discussed previously, [48] the surface was easily oxidized naturally at room temperature. We also observed the cross section of FN layers both on 3 h and 10 h irradiated Nb samples; the thickness of the FN layer was almost the same on both samples.…”

Effect of high-flux, low-energy He+ ion irradiation on Ta as a plasma-facing material

“…Ex situ XPS has previously been reported by us1920 to be a valuable technique not only for determining surface chemistry and oxidation states of samples, but also for indirectly measuring surface roughness post-irradiation. Since a roughened surface has a larger effective surface area, and larger surface area provides increased number of sites available for oxidation, one would expect a very rough surface to oxidize more efficiently when exposed to air.…”

“…However, there are only a few preliminary studies investigating the response of these alternative refractory metals to high fluxes of He and H isotopes. It has been shown that under these similar He + ion irradiation conditions, significant nanostructuring is also observed in most other refractory metals such as molybdenum (Mo)1819, niobium (Nb)20, and vanadium (V)21. However, similar preliminary temperature-dependent studies in Ta22 show that Ta does not form the same high aspect ratio surface structures as observed in W, Mo, Nb, and V; rather, Ta only forms shallow surface pores when irradiated with high-flux, low-energy He + ion irradiation.…”

Tuning surface porosity on vanadium surface by low energy He+ ion irradiation

“…All irradiations were performed in ultra-high vacuum (UHV) chamber with a base pressure of 5.0  10 -8 Torr; however, the working pressure during ion irradiation was 1.0  10 -4 Torr. The selected ion energy (100 eV), ion-flux (7.2  10 20 ions m -2 s -1 ), ion-fluence (2.6  10 24 ions m -2 ), and range of target temperature (823-1173 K) were based on our previous studies involving He + -induced surface modifications in refractory metals [40,41,43,44]. For precise temperature measurement, we used thermocouples rather than radiation thermometers; a thermal-feedback mechanism in the sample heater were corrected for the ion- Ocean Optics was used for signal detection.…”

“…This study is also significant in the understanding of microstructure evolution and fuzz formation on high-Z refractory metals for fusion applications. As with W [8,9,[36][37][38][39], Mo [40][41][42], Ta [43], and Nb [44], the He + ion-irradiated V surface could also have a weakness against plasma and/or ion heat flux. Therefore, such V nano-and submicron-structures may not be favorable for protecting the V surface from possible melting and erosion.…”