Fiberform Nanostructures Research Articles

Fabrication of photocatalytically active vanadium oxide nanostructures via plasma route

Plasma irradiation was used to create nanostructured vanadium oxide with potential commercial and industrial applications. Morphology changes were induced at the nano- and micro-meter scale, accompanied by the growth of helium nanobubbles. Micrometer-sized pillars, cube-shaped nanostructures, and fuzzy fiberform nanostructures were grown on the surface; the necessary conditions in terms of the incident ion energy and the surface temperature for those morphology changes were revealed. Hydrogen production experiments using a photocatalytic reaction with aqueous methanol solution were conducted on the fabricated samples. Enhanced H2 production was confirmed with the plasma irradiated nanostructured sample that had been oxidized in air atmosphere. Photocatalytically inactive vanadium oxide exhibited a high photocatalytic activity after nanostructurization of the surface by helium plasma irradiation.

Fuzzy nanostructure growth on precious metals by He plasma irradiation

Growth of helium bubbles near the surface of metals leads to various morphology changes when sufficient helium ions are continuously implanted on the surface. In this study, low energy (< 100 eV) helium plasma irradiations to various noble metals were conducted, and consequent surface morphology changes were analyzed. It was found that fiberform nanostructures were formed by the helium plasma irradiation on rhodium and ruthenium thin films, which were deposited by magnetron sputtering, and on platinum and iridium wires. Growth of fiberform structures were not identified on gold, silver, and palladium samples exposed to helium plasmas even though the irradiations were conducted at various surface temperatures, though pinholes were observed on palladium surface. We discussed the relation between the morphology changes and material properties.

Thermal radiative characteristics of nanostructured tungsten at high-temperature

Fiber-form nanostructures grown on tungsten (W) surfaces with helium (He) plasma irradiation exhibit an excellent thermal radiative property at low sample temperatures up to 1200 °C. To maintain this radiative property at higher temperatures, the recovery to undefected surface and annealing effect of nanostructured W surfaces need to be investigated while focusing on the dopant effect. TFGR-W-1.1%TiC/H (toughing, fine-grained recrystallized tungsten with TiC dispersoids) was experimentally found to have the best characteristics among the tested doped W samples. However, it was noted and discussed that a further improvement at higher temperatures needs an additional surface treatment. Possibilities of maintaining complicated structures with nano-scale lengths are discussed in terms of dopant pinning effect and self-diffusion on tungsten surface.

Initial growth phase of W-fuzz formation in ultra-long pulse helium discharge in LHD

In order to confirm the formation of a tungsten fiberform nanostructure (W-fuzz) by helium plasma exposure in the large-sized plasma confinement device, the bulk tungsten with the size of 80 × 20 × 1.5 mm3 was inserted into the divertor leg position in the Large Helical Device (LHD). Then, it was exposed to the divertor plasma during the ultra-long pulse helium discharges with 10,190s in total. The width of the divertor plasma, incident ion energy and total fluence were expected to be ∼2cm, 100–200eV and ∼5 ×1025 He/m2 (strike position), respectively. The surface temperature of the tungsten specimen was monitored by IR camera. The typical surface temperature of the divertor strike point was estimated to be around 1900∼2300K. After the exposure, an initial growth phase of tungsten fiberform nanostructure (W-fuzz) was able to be identified on the tungsten surface. The finest initial growth phase of the W-fuzz structure was able to be identified on the central region of the divertor strike point, where retention amount of helium was estimated to be ∼8 ×1021 He/m2. This study is the first simultaneous evaluation of the W-fuzz growth and quantification of the helium retention in the large-sized plasma confinement device.

Towards understanding the differences in irradiation effects of He, Ne and Ar plasma by investigating the physical origin of their clustering in tungsten

While inert gas seeding to improve energy confinement has been successfully applied in many tokamak experiments, questions remain as to the irradiation effects of inert gases on tungsten. In this paper, we have systematically investigated the clustering behaviors of the inert gas atoms He, Ne and Ar in plasma-facing tungsten using first-principles calculations. Small interstitial clusters, Hem, Nem, and Arm, can form due to the attraction between the atoms and tend to expand along the (1 1 0) planes. The inert gas clusters induce strong lattice distortions and so it is energetically favorable for a self-interstitial atom to be emitted from the clusters when the numbers of atoms are above six, three, three for Hem, Nem, and Arm respectively. The clustering behaviors can be well explained by the intrinsic repulsive interaction between the inert gas atoms and the attractive interaction coming from the reduced valence-electron density by interstitial inert gas atoms. Compared to He, the much greater attraction between the Ne/Ar atoms and the lower trigger condition of ‘self-trapping process’ for Ne/Ar clusters provide a reasonable explanation for the difference of irradiation effects on tungsten between He and Ne/Ar plasmas, i.e. Ne/Ar plasmas cannot result in the formation of holes/bubbles and fiber-form nanostructures on tungsten surface under the same irradiation conditions as He plasma.

Increase in the work function of W/WO3 by helium plasma irradiation

Helium plasma irradiation to tungsten leads to formation of helium clusters, helium bubbles, and fiberform nanostructures near the surface. In this study, tungsten samples exposed to helium plasmas are analyzed by X-ray photoelectron spectroscopy in detail. The W 4f spectra and valence band spectra show chemical composition and electronic state of the surface. It was found that significant differences were not identified on the electronic state between the helium plasma irradiated and pristine samples when the degree of oxidization was the same level. From a low kinetic energy cut-off of X-ray photoelectron spectrum, the work function of the samples was measured. It was found that helium plasma irradiation increased the work function by approximately 0.5 eV when WO3/W was higher than unity. The possible mechanism to arise the difference in the work function is discussed.

Experimental identification for physical mechanism of fiber-form nanostructure growth on metal surfaces with helium plasma irradiation

The initial stage of fiber-form nanostructure growth on metal surface with helium plasma irradiation is illustrated, taking recent research knowledge using a flux gradient technique, and including loop-like nano-scale structure as precursors. The growth mechanism of fibers is discussed in terms of the shear modulus of various materials that is influenced by the helium content as well as the surface temperature, and the mobility of helium atoms, clusters and/or nano-bubbles in the bulk, loops and fibers. This model may explain the reason why tantalum does not provide fiber-form nanostructure although the loop-like structure was identified. The model also suggests the mechanism of an existence of two kinds of nanostructure of titanium depending on surface temperature. Industrial applications of such nanostructures are suggested in the properties and the possibilities of its growth on other basic materials.

Radiative cooling properties of He-defected tungsten with fiber-form nanostructured surface

The radiative cooling capability of tungsten with a fiber-form nanostructure is evaluated precisely using an electron-beam power system. It is found that the total emissivity of black nanostructured tungsten is very close to 1.0, meaning an almost complete black body. The discrepancies in the determination of the total emissivity between the electron-beam method and a fixed biasing procedure are discussed in terms of the power transmission through the plasma sheath, focusing on the ion energy reflection and cooling associated with electron emission.

Growth of multifractal tungsten nanostructure by He bubble induced directional swelling

Helium (He) plasma irradiation to tungsten (W) leads to morphology changes in nanometer scale by the formation and growth of He bubbles. Initially pinholes and protrusions are formed on the surface followed by the formation of nanostructures. In this study, based on experimental observation, the growth process of the fiberform nanostructures are revisited and the swelling process of the structure is discussed. The novel nanostructures are analyzed from the viewpoint of fractality. It is found that the number of the initially formed pinholes and its sizes have a fractal relation, indicating that the size and number of bubbles formed near the surface have fractality. The fractal dimension is estimated from the brightness variation of a transmission electron microscope (TEM) micrograph and gas adsorption property. Moreover, it is revealed from TEM image analysis that the nanostructure has multifractal feature, probably because of the fractality identified between the number and the size of bubbles near the surface.

In situ observation of structural change of nanostructured tungsten during annealing

Deformation of fiberform nanostructure and the dynamic behavior of helium (He) bubbles in fuzz tungsten (W) during annealing have been investigated by means of in situ cross-section observation using transmission electron microscopy and He desorption rate observation using thermal desorption spectroscopy (TDS). Thermal recovery of the nanostructure, such as shrinkage and coalescence of fine structure, annihilation of He bubbles, and large desorption of He gas, occurred around 1073–1173K. The activation energy of He was estimated from a TDS peak that appeared around 300–400K by using the Kissinger–Akahira–Sunose model-free-kinetics method. In addition, the TDS results of fiberform nanostructured tungsten were compared with those of tungsten samples irradiated with a high-energy He ion beam.

Initial Stage of Fiber-Form Nanostructure Growth on Refractory Metal Surfaces with Helium Plasma Irradiation

The initial stage of fiber-form nanostructure growth on tungsten and molybdenum surfaces with helium plasma irradiation is investigated by a technique that allows time evolution of nanostructure growth to its spatial variation. The pitting of the original base surface resulting from successive hole formation is clearly demonstrated. The surface fine-grained miniatuarization proceeds with increasing hole area. Then, loop-like nano-scale structures appear with some coherency, which are thought to be precursors of fiber-form tendrils. Loop raptures and branching out are considered as possible candidates of the initial stage of nano-fiber growth.

Temperature Range for Fiber-Form Nanostructure Growth on Molybdenum Surfaces due to Helium Plasma Irradiation

Temperature Range for Fiber-Form Nanostructure Growth on Molybdenum Surfaces due to Helium Plasma Irradiation

Power Transmission Factor for Tungsten Target w/wo Fiber-Form Nanostructure in He Plasmas with Hot Electron Component Using Compact Plasma Device AIT-PID

After giving the specifications of compact plasma device for the study on plasma-wall interactions, AIT-PID, focusing on the electron energy distribution of helium plasmas, the experimental and analytical results for the determination of power transmission factor through the sheath formed on the helium defected tungsten with fiber-form nanostructure on its surface are presented.

Comparison of Damages on Tungsten Surface Exposed to Noble Gas Plasmas

Tungsten was exposed to pure Ar or Ne plasmas over 1550 K at several incident ion energies. Even under the irradiation condition that the tungsten nanostructure is formed by He plasma irradiation, holes/bubbles and fiberform nanostructures were not formed on the surface by exposure to Ar or Ne plasmas. In addition, the results from energy dispersive X-ray spectroscopy supported the facts that Ar and Ne did not remain in the sample. We will discuss the reason for the differences in the damage to the tungsten surface exposed to noble gas plasmas.

Tritium retention in nanostructured tungsten with large effective surface area

Abstract Tungsten specimens with fiberform nanostructure (Nano-W) were prepared by exposure to helium plasma in the divertor plasma simulator NAGDIS-II with different amounts of helium fluence. For comparison, tungsten specimens with smooth surface (Polished-W) were also prepared. Surface area of Nano-W was measured by using Brunauer, Emmet and Teller (B.E.T.) method. Tritium retention of Nano-W and Polished-W was investigated by an imaging plate (IP) and β-ray induced X-ray spectrometry (BIXS) technique exposure to mixture gas of deuterium and tritium. It was found that surface area of Nano-W was significantly larger than that of Polished-W and increased in proportion to the amount of helium irradiation. On the other hand, tritium retention showed a saturation trend when the helium fluence was higher than 5.0 × 1025 m−2.

Recovery of Tungsten Surface with Fiber-Form Nanostructure by Plasmas Exposures

One of the serious concerns for tungsten materials in fusion devices is the radiation defects caused by helium plasma irradiation since helium is a fusion product. The fiber-formed nanostructure is thought to have a possible weakness against the plasma heat flux on the plasma-facing component and also may destroy the reflectivity of optical mirrors. In this paper an interesting method for the recovery of such tungsten surfaces is shown. The recovery process depends on the grade and manufacturing process of tungsten materials.

Development of Nanostructured Black Metal by Self-Growing Helium Bubbles for Optical Application

When tungsten is exposed to a helium plasma under specific conditions, fiberform nanostructures grow from the surface, and optical properties significantly alter. It is shown that the optical absorptivity increases significantly by the helium irradiation from visible to near-infrared wavelength range. The optical absorptivity became almost 100% from visible to near infrared wavelength range after the nanostructure was formed on the surface. The result indicates that the emissivity of radiation in visible and near infrared wavelength range also increases significantly by the helium irradiation. The emissivity around 1550 nm is 4.4 times higher than the initial value, while it is twice at the visible wavelength range. The helium fluence dependences of the specular and total optical reflectances and the morphology change observed by transmission electron microscope (TEM) are revealed.

Helium bubble formation on tungsten in dependence of fabrication method

In order to investigate the morphology change of tungsten due to helium plasma exposure, tungsten (W) materials fabricated in different processes, ITER grade W, ultra-fine grained W (W–0.8TiC), and powder metallurgy W after helium plasma exposure were observed by scanning electron microscope. In all the three cases, pin-holes are observed on the surface; moreover, the non-uniformity of the formation of helium bubbles and fiberform nanostructure was observed on ITER grade W. From the detailed analysis of the crystal orientation with the help of focused ion beam method (FIB) indicates that the initial growth of the nanobubbles may be slow on the (110) face of tungsten.

Recovery of Tungsten Surface with Fiber-Form Nanostructure by the Argon Plasma Irradiation at a High Surface Temperature

One of the serious concerns for tungsten materials in fusion devices is the radiation defects caused by helium plasma irradiation, while the helium is one of fusion products. Fiber-formed nanostructure is worried to have a possible weakness against the plasma heat flux and may destroy the reflectivity as an optical mirror. In this communication an interesting method for a recovery of such a tungsten surface is shown.

Investigation on the effect of temperature excursion on the helium defects of tungsten surface by using compact plasma device

The effects of temperature excursion on the helium defects of tungsten surface have been investigated by using compact plasma device AIT-PID (Aichi Institute of Technology – Plasma Irradiation Device). An initial stage of bubble formation has been identified with an order of smaller (sub-micron) bubbles and holes than those in the past in which the micron size is the standard magnitude. The radiation cooling has been detected when a blacking of tungsten surface coming from nanostructure formation is proceeding due to an increase in the emissivity. The temperature increase to the domain (∼1600K) in bubble/hole formation from that in nanostructure formation has been found to bring a constriction in diameter and a reduction in length of fiber-form nanostructure.