Showing papers on "Tungsten published in 2009"

Formation process of tungsten nanostructure by the exposure to helium plasma under fusion relevant plasma conditions

Abstract: Helium irradiation on tungsten changes the surface morphology dramatically by forming a nanometre-sized fibreform structure which could bring about serious problems for fusion reactors. From the experimental results in liner divertor simulators, it is revealed that the incident ion energy and surface temperature are key parameters for the formation of the structure. It is shown that the tungsten nanostructure is easily formed when the temperature is in the range 1000–2000 K, and the incident ion energy is higher than 20 eV. Furthermore, on the basis of the helium irradiation experiments performed in the divertor simulator NAGDIS-I, the initial formation process of the nanostructure is revealed. It is shown that the nanostructure formation is related to pinholes appearing on the bulk part of the material, and then, the rough structure develops to a much finer nanostructure. The nanostructure was also observed on the molybdenum surface that was exposed to the helium plasma. It increases interest in the possibility that nanostructure formation by helium irradiation is a common phenomenon that occurs on various metals.

Fabrication of high temperature oxides dispersion strengthened tungsten composites by spark plasma sintering process

Abstract: The paper describes the fabrication process of high temperature oxides, such as Y 2 O 3 , HfO 2 and La 2 O 3 , dispersed tungsten composites by spark plasma sintering. The oxide contents varied from 0 to 5 wt% and sintering was conducted for 3 min at 1700 °C. Among three kinds of oxides, Y 2 O 3 is the most efficient element to consolidate W powder. As dispersed up to 5 wt% Y 2 O 3 into the matrix, the relative density of the W composite is increased up to nearly 100% of theoretical value. In order to analyze the effect of Y 2 O 3 particles on the densification of W powders, the microstructure of W–Y 2 O 3 composite is observed using the transmission electron microscopy. By this experiment, it is found that dark phases, which had been known as Y 2 O 3 phase, are composed of W, Y and O. Therefore, during sintering, W atoms move through Y 2 O 3 phases as well as W grain boundaries, thereby W and Y 2 O 3 are soluble, and so sinterability of W is enhanced. The hardness of the composite is increased from 350 to 510 kg/mm 2 with increasing Y 2 O 3 contents since the relative density is increased and the grain size is reduced from 20 to 4 μm. However, in case of HfO 2 and La 2 O 3 , the hardness of the composites is decreased even though the grain size is reduced because of their lower relative densities.

Brighter light sources from black metal: significant increase in emission efficiency of incandescent light sources.

Abstract: By applying the femtosecond laser blackening technique directly to a tungsten incandescent lamp filament, we dramatically brighten the tungsten lamp and enhance its emission efficiency to approach 100%. A comparison study of emission and absorption for the structured metal surfaces shows that Kirchhoff's law is applicable for the black metal. Furthermore, we demonstrate that we can even obtain partially polarized light as well as control the spectral range of the optimal light emission from the laser-blackened tungsten lamp.

Methods for depositing tungsten films having low resistivity for gapfill applications

Abstract: Methods of filling gaps or recessed features on substrates are provided. According to various embodiments, the methods involve bulk deposition of tungsten to partially fill the feature followed by a removing a top portion of the deposited tungsten. In particular embodiments, the top portion is removed by exposing the substrate to activated fluorine species. By selectively removing sharp and protruding peaks of the deposited tungsten grains, the removal operation polishes the tungsten along the feature sidewall. Multiple deposition-removal cycles can be used to close the feature. The filled feature is less prone to coring during CMP.

Growth of monoclinic WO3nanowire array for highly sensitive NO2 detection

Abstract: A monoclinic tungsten trioxide nanowire array has been grown on silicon substrates using tungsten powders as source materials by thermal evaporation under specific synthesis conditions (1000 °C, 13–15 Torr, 200 sccm air flow). The morphology, chemical composition and crystal structure of the as-prepared tungsten trioxide nanowires were characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and transmission electron microscopy. The nanowires were identified as monoclinic in structure, with diameters ranging from 40 to 100 nm and lengths up to 5 µm. It was found that sufficient oxygen and air flow are the major factors to influence the nanowire array growth. The nanowire array was employed directly for gas sensor fabrication using photolithography. The gas sensing experiments revealed that the nanowire array sensors are highly sensitive to NO2 (50 ppb), making the tungsten trioxide nanowire array a competitive candidate for highly sensitive gas sensor fabrication.

High surface area tungsten carbide microspheres as effective Pt catalyst support for oxygen reduction reaction

Abstract: In the present work, the preparation of high surface area (256 m2 g−1) tungsten carbide microspheres (TCMSs) by the aid of a simple hydrothermal method is realized and the performance of the Pt electrocatalyst supported on the as-prepared TCMSs towards the oxygen reduction reaction (ORR) is investigated. The SEM micrographs indicated that both the synthesized carbon microspheres (CMSs) and TCMSs showed perfect microsphere structure and uniform size. The EDX measurements confirmed that when the C/W mass ratio is ∼2.5/1, tungsten and carbon coexist in the microspheres. Moreover, from the XRD results, it can be found that both W2C and WC are detected and W2C exists as the main phase. It was found that the Pt particles are uniformly dispersed on the supports, while the corresponding average particle size is ∼3.7, 4.1 and 4.3 nm for Pt/C, Pt/CMSs and Pt/TCMSs, respectively. It was also found that in terms of ORR onset potential and mass activity, the Pt/TCMSs catalyst exhibits superior performance to that of Pt/CMSs and Pt/C, enhancing the ORR catalytic activity by more than 200%. The above behavior could be attributed to its higher electrochemical surface area (ESA), as well as to the synergistic effect between Pt and tungsten carbides.

Subsurface morphology changes due to deuterium bombardment of tungsten

Abstract: Recrystallized polycrystalline tungsten was exposed to a deuterium plasma beam with high flux (1022 D m−2 s−1) and low energy (38 eV D−1) to fluences up to 1027 D m−2 The sample temperature was varied between 320 and 800 K The three-dimensional morphology of blister-like structures and the grain orientation were investigated by scanning electron microscopy combined with focused ion beam cross-sectioning and electron backscattering diffraction Cracks with distorted areas ( 480 K) were observed beneath the surface The surface blister-like structures and the defects underneath are correlated along crystallographic orientation of the W grains in accordance to the low-indexed slip system {110}111 The defects are mobile and accumulate under deuterium loading Samples exposed near room temperature do not form such large cavities by subsequent heating up to 1300 K Deuterium bombardment above 700 K does not lead to blister-like structures

New crystal structure and characterization of lanthanum tungstate "La6WO12" prepared by freeze-drying synthesis.

Abstract: Lanthanum tungstates with a La/W atomic ratio between 6 and 4.8 have been synthesized as polycrystalline materials using the freeze-drying wet-chemical precursor method. Our results show that a single phase material is obtained when the La/W ratio is between 5.3 and 5.7 (T = 1500 °C). Outside this compositional range, segregation of either La2O3 (La/W ≥ 5.8) or La6W2O15 (La/W ≤ 5.2) are found. We have solved the crystal structure for the composition with a La/W nominal atomic ratio of 5.6 by combining powder X-ray and powder neutron diffraction techniques. This structure substantially differs from that previously reported for Ln6WO12 (Ln = Y, Ho). The main differences between the two structure types are the crystal symmetry, the different coordination environment of the cations and the formula unit. The formula unit can be written as La6.63W1.17O13.43 (Z = 4; calculated density = 6.395 g/cm3), well in accordance with the diffraction techniques, He-pycnometry and electron probe microanalysis. These materials can be described as a face centred cubic structure with space group F3m. Lattice parameters vary between 11.173 and 11.188 A, depending on composition. Dense ceramic materials are obtained at 1400 °C, and microanalyses measurements indicate that no significant tungsten evaporation occurs compared to the nominal values. Compositions with La2O3 segregation show similar conductivity values as the single phase ones, but those containing segregation of W-rich phases show a considerable drop in conductivity with increasing content of the secondary phase.

High-temperature anodized WO3 nanoplatelet films for photosensitive devices.

Abstract: Anodization at elevated temperatures in nitric acid has been used for the production of highly porous and thick tungsten trioxide nanostructured films for photosensitive device applications. The an...

First-principles calculation of nitrogen-tungsten codoping effects on the band structure of anatase-titania

Abstract: The electronic properties and photocatalytic activity of nitrogen (N) and/or tungsten (W)-doped anatase are calculated using density functional theory. For N-doping, isolated N 2p states above the top of the valence band are responsible for experimentally observed redshifts in the optical absorption edge. For W-doping, W 5d states below the conduction band lead to band gap narrowing; the transition energy is reduced by 0.2 eV. Addition of W to the N-doped system yields significant band gap narrowing gap by 0.5 eV. This rationalizes recent experimental data which showed that N/W-doped titania exhibits higher visible-light photocatalytic efficiency than either N- or W-doping alone.

Investigation on diffusion wear during high-speed machining Ti-6Al-4V alloy with straight tungsten carbide tools

Abstract: During high-speed machining Ti-6Al-4V alloy, high-temperature at the tool–chip interface and the concentration gradient of chemical species between tool material and workpiece material support the activation of diffusion process, and therefore the crater wear forms on the rake surface of the cutting tool at a short distance from the cutting edge. In this paper, the diffusion analysis was theoretically proposed. The constituent diffusion at the contact interface between tool material and Ti-6Al-4V alloy at high-temperature environment, the crater wear on the rake surface of the tool, and the chips collected from high-speed milling Ti-6Al-4V alloy with straight tungsten carbide tools were analyzed by the scanning electron microscope with energy dispersive X-ray spectroscopy. The constituents inside the tool could diffuse into the workpiece and the diffusion layer was very thin and close to the interface. Compared with the diffusion of tungsten and carbon atoms, the pulling out and removing of the tungsten carbide (WC) particles due to cobalt diffusion dominated the crater wear mechanism on the rake surface of the cutting tool.

Anisotropic Structure Deformation in the VO 2 Metal-Insulator Transition

Abstract: X-ray absorption fine structure data of tungsten (VI)--doped vanadium dioxide in the insulating phase, and during the metal-insulator transition, are presented for the first time. Tungsten ${L}_{\mathrm{III}}$- and vanadium $K$-edge data suggest that significant expansion in the [110] and $[1\overline{1}0]$ directions occurs across the phase transition from low to high temperature. This distortion breaks the bonds between Peierls-paired vanadium ions, opening a band gap, and reveals the nature of the mechanism by which tungsten doping lowers the transition temperature and enthalpy.

Prompt ignition of a unipolar arc on helium irradiated tungsten

Abstract: A fibreform nanostructured layer is formed on a tungsten surface by helium plasma bombardment. The helium fluence was of the order of 1026 m−2, and the surface temperature and incident ion energy during helium irradiation were, respectively, 1900 K and 75 eV. By irradiating a laser pulse to the surface in the plasma, a unipolar arc, which many people have tried to verify in well-defined experiments, is promptly initiated and continued for a much longer time than the laser pulse width. The laser pulse width (~0.6 ms) and power (~5 MJ m−2) are similar to the heat load accompanied by type-I edge localized modes (ELMs) in ITER. The unipolar arc is verified from an increase in the floating potential, a moving arc spot detected by a fast camera and arcing traces on the surface. This result suggests that the nanostructure on the tungsten surface formed by the bombardment of helium, which is a fusion product, could significantly change the ignition property of arcing, and ELMs become a trigger of unipolar arcing, which would be a great impurity source in fusion devices.