Showing papers on "Tungsten published in 2012"

Design of Stable Nanocrystalline Alloys

Abstract: Nanostructured metals are generally unstable; their grains grow rapidly even at low temperatures, rendering them difficult to process and often unsuitable for usage. Alloying has been found to improve stability, but only in a few empirically discovered systems. We have developed a theoretical framework with which stable nanostructured alloys can be designed. A nanostructure stability map based on a thermodynamic model is applied to design stable nanostructured tungsten alloys. We identify a candidate alloy, W-Ti, and demonstrate substantially enhanced stability for the high-temperature, long-duration conditions amenable to powder-route production of bulk nanostructured tungsten. This nanostructured alloy adopts a heterogeneous chemical distribution that is anticipated by the present theoretical framework but unexpected on the basis of conventional bulk thermodynamics.

A new class of electrocatalysts for hydrogen production from water electrolysis: metal monolayers supported on low-cost transition metal carbides.

Abstract: This work explores the opportunity to substantially reduce the cost of hydrogen evolution reaction (HER) catalysts by supporting monolayer (ML) amounts of precious metals on transition metal carbide substrates. The metal component includes platinum (Pt), palladium (Pd), and gold (Au); the low-cost carbide substrate includes tungsten carbides (WC and W2C) and molybdenum carbide (Mo2C). As a platform for these studies, single-phase carbide thin films with well-characterized surfaces have been synthesized, allowing for a direct comparison of the intrinsic HER activity of bare and Pt-modified carbide surfaces. It is found that WC and W2C are both excellent cathode support materials for ML Pt, exhibiting HER activities that are comparable to bulk Pt while displaying stable HER activity during chronopotentiometric HER measurements. The findings of excellent stability and HER activity of the ML Pt–WC and Pt–W2C surfaces may be explained by the similar bulk electronic properties of tungsten carbides to Pt, as is ...

Synthesis of Surface-Functionalized WS2 Nanosheets and Performance as Li-Ion Battery Anodes

Abstract: Separation of bulk tungsten disulfide (or WS2) into few-layer two-dimensional (2-D) crystals is of interest because of their high surface area for certain chemical processes and size-dependent optical and electronic characteristics. Herein, we demonstrate a process that involves the physical separation of weakly bonded WS2 layers by use of a strong acid treatment (chlorosulfonic acid) at 2 mg/mL, followed by quenching in deionized (DI) water. X-ray photoelectron spectroscopy of the superacid-treated WS2 suggests the formation of W–O type bonds, signifying oxidation of tungsten and reduction of the sulfur phase. Thermogravimetric analysis showed a three-phase weight-loss pattern, suggesting acid functionalization of WS2 surfaces. We also studied the electrochemical behavior of an acid-treated WS2 anode in a lithium half-cell configuration that showed a three-step charge–discharge behavior, characteristic of a conversion reaction. The electrochemical capacity was 118 mAh/g after 50 cycles.

4.17 – Tungsten as a Plasma-Facing Material

Abstract: As the most promising plasma-facing material for actual and future nuclear fusion devices, tungsten has to face and withstand a broad variety of severe operational conditions. These comprise high transient thermal loads, neutron irradiation-induced material degradation and transmutation, hydrogen and helium attack at the plasma-facing surface, and thermal fatigue under steady state heat fluxes as part of a plasma-facing component. The characterization and understanding of the material’s behavior under these conditions is essential for finding a grade of tungsten or tungsten alloy compatible with such an extreme environment.

NO2-sensing properties of porous WO3 gas sensor based on anodized sputtered tungsten thin film

Abstract: In this paper, a novel porous WO3 sensor was prepared by anodic oxidation of DC magnetron sputtered metallic tungsten (W) film deposited on alumina substrate. The structural and morphological properties of these films are investigated using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). Coral-like porous crystalline WO3 film with a grain size of about 9.3 nm was obtained after annealing of the anodized W film. The porous WO3 sensor achieved its maximum response value to NO2 at a low operating temperature of 150 °C. In comparison to sputtered WO3 sensor, the porous WO3 sensor showed markedly higher responses, much better response–recovery characteristics and lower optimal operating temperature to different concentration of NO2 gas due to its high specific surface area and small grain size.

Modern trends in tungsten alloys electrodeposition with iron group metals

Abstract: Theoretical and applied studies of tungsten alloys with iron group metals (Me-W) are being carried out worldwide, in the light of their versatile applications. The aim of this paper is to provide an overview of the works on electrodeposition of tungsten alloys with iron group metals, their properties and applications. There are 221 papers reviewing on the following theoretical and practical topics: chemistry of electrolytes used for electrodeposition, codeposition mechanisms, and properties of electrodeposited tungsten alloys. In addition, the formation of W(VI) and iron group metal (Me) complexes (polytungstates and complexes of Me(II) and W(VI)) with citrates and OH− is analysed based on the published data and the calculated distribution of species as a function of pH (ranged from 1 to 10) is provided for solutions with/without citrates. The adduced data are correlated with the compositions of electrodeposited alloys. Various codeposition models of tungsten with iron group metals described in the literature are critically discussed as well. The peculiarities of the structure of tungsten alloys and their thermal stability, mechanical, tribological, and magnetic properties, corrosion performance, their applications in hydrogen electrocatalysis, template-assisted deposition into recesses (aimed to obtain micro- and nanostructures) are also reviewed and mapped.

Fracture Toughness of Polycrystalline Tungsten Alloys

Abstract: Tungsten and tungsten alloys show the typical change in fracture behavior frombrittle at low temperatures to ductile at high temperatures. In order to improve theunderstanding of the effect of microstructure the fracture toughness of pure tungsten,potassium doped tungsten, tungsten with 1wt% La2O3 and tungsten rheniumalloys were investigated by means of 3-point bending -, double cantilever beam -and compact tension specimens. All these materials show the expected increase infracture toughness with increasing temperature. The experiments demonstrate thatthe grain size, texture, chemical composition, grain boundary segregation and dislocationdensity seem to have a large effect on fracture toughness below the DBTT.These influences can be seen in the fracture behavior and morphology, where twokinds of fracture occur: on one hand the transgranular and on the other hand the intergranularfracture. Therefor techniques like electron backscatter diffraction, augerelectron spectroscopy and x-ray line profile analysis were used to improve the understandingof the parameters influencing fracture toughness.

Synthesis, Crystal Structure, and Elastic Properties of Novel Tungsten Nitrides

Abstract: Among transition metal nitrides, tungsten nitrides possess unique and/or superior chemical, mechanical, and thermal properties. Preparation of these nitrides, however, is challenging because the incorporation of nitrogen into tungsten lattice is thermodynamically unfavorable at atmospheric pressure. To date, most materials in the W–N system are in the form of thin films produced by nonequilibrium processes and are often poorly crystallized, which severely limits their use in diverse technological applications. Here we report synthesis of tungsten nitrides through new approaches involving solid-state ion exchange and nitrogen degassing under pressure. We unveil a number of novel nitrides including hexagonal and rhombohedral W2N3. The final products are phase-pure and well-crystallized in bulk forms. For hexagonal W2N3, hexagonal WN, and cubic W3N4, they exhibit elastic properties rivaling or even exceeding cubic-BN. All four nitrides are prepared at a moderate pressure of 5 GPa, the lowest among high-press...

Comparative study of IVB–VIB transition metal compound electrocatalysts for the hydrogen evolution reaction

Abstract: A systematic and comparative investigation of 18 different IVB–VIB transition metal carbides, nitrides, sulfides, silicides and borides as well as of the three main group element ceramics AlN, B4C and h-BN for their electrocatalytic performance towards the hydrogen evolution reaction (HER) is presented. We exploited the voltammetry of immobilised particles as a method that allows a rapid electrocatalyst screening and characterisation. The performances of the electrocatalysts – by means of the overpotential of the hydrogen evolution reaction in 100 mM H2SO4 – were compared with platinum and nickel as benchmarks. Although none of the catalyst materials investigated reached the performance of platinum with respect to overpotential and stability, the electrocatalytic properties of several compounds, i.e. tungsten and molybdenum carbide but also of tungsten boride calls for continuative work.

Inkjet printing of sol-gel synthesized hydrated tungsten oxide nanoparticles for flexible electrochromic devices.

Abstract: Tungsten oxide nanoparticles were synthesized via a sol-gel route using metallic tungsten as precursor, and were printed on a flexible electrode using inkjet printing in order to build solid-state electrochromic cells. Several spectroscopic techniques were used to characterize and compare tungsten oxide particles obtained from different origins. FTIR, Raman and X-ray diffraction spectroscopic measurements showed that the sol-gel synthesis described here produces nanoparticles mainly in an amorphous state with hexagonal crystalline domains and allowed the analysis of the hydration extent of those nanoparticles. The size was measured combining dynamic light scattering, sedimentation, and microscopic techniques (AFM), showing a consistent size of about 200 nm. The tungsten oxide nanoparticles were used to produce an ink formulation for application in inkjet printing. Solid-state electrochromic devices were assembled at room temperature, without sintering the tungsten oxide printed films, showing excellent contrast between on/off states. Electrochemical characterization of those films is described using cyclic voltammetry. The devices were then tested through spectroelectrochemistry by Visible/NIR absorption spectroscopy (400-2200 nm range), showing a dual spectroscopic response depending on the applied voltage. This phenomenon is attributed to the presence of two different crystalline states in accordance with results obtained from the spectroscopic characterization of the nanoparticles. The electrochromic cells had a good cycling stability showing high reversibility and a cyclability up to more than 50,000 cycles with a degradation of 25%.

Tungsten nano-tendril growth in the Alcator C-Mod divertor

Abstract: Growth of tungsten nano-tendrils (?fuzz?) has been observed for the first time in the divertor region of a high-power density tokamak experiment. After 14 consecutive helium L-mode discharges in Alcator C-Mod, the tip of a tungsten Langmuir probe at the outer strike point was fully covered with a layer of nano-tendrils. The thickness of the individual nano-tendrils (50?100?nm) and the depth of the layer (600???150?nm) are consistent with observations from experiments on linear plasma devices. The observation of tungsten fuzz in a tokamak may have important implications for material erosion, dust formation, divertor lifetime and tokamak operations in next-step devices.

Nanostructuring of molybdenum and tungsten surfaces by low-energy helium ions

Abstract: The formation of metallic nanostructures by exposure of molybdenum and tungsten surfaces to high fluxes of low energy helium ions is studied as a function of the ion energy, plasma exposure time, and surface temperature. Helium plasma exposure leads to the formation of nanoscopic filaments on the surface of both metals. The size of the helium-induced nanostructure increases with increasing surface temperature while the thickness of the modified layer increases with time. In addition, the growth rate of the nanostructured layer also depends on the surface temperature. The size of the nanostructure appears linked with the size of the near-surface voids induced by the low energy ions. The results presented here thus demonstrate that surface processing by low-energy helium ions provides an efficient route for the formation of porous metallic nanostructures.

Activation of plasmons and polarons in solar control cesium tungsten bronze and reduced tungsten oxide nanoparticles

Abstract: Dispersions of reduced tungsten oxide and tungsten bronze nanoparticles are known to show a remarkable absorption of near-infrared (NIR) light applicable to solar control filters for automotive and architectural windows. Origin of the NIR absorption has been investigated by analyzing dielectric constants of CsxWO3 (x = 0.15, 0.25, and 0.33) and WO2.72, and using Mie scattering theory. The optical analysis and Mie scattering theory analysis indicate that a localized surface plasmon resonance and polarons of localized electrons contribute alongside to the observed NIR absorption at different wavelengths.

Synthesis of Nanosize Tungsten Oxide and Its Evaluation as an Electrocatalyst Support for Oxygen Reduction in Acid Media

Abstract: WO3 was investigated as a possible support material for monolayer Pt ORR electrocatalysts in acid electrolyte. It was found that as-prepared Pt/WO3 showed enhanced activity for the ORR and superior electron transfer ability compared to conventional Pt/C and bulk polycrystalline Pt. However, similar to other tungsten derived supports, most notably WC, the Pt/WO3 electrocatalyst showed significant performance degradation that was tied directly to the formation of water-soluble hydrogen tungsten bronze (HxWO3) on the support surface that was identified by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). This information, combined with previous work by our group on WC supports, are also combined to propose a series degradation pathway for WC and WO3 supports in acid media, which is relevant for proton exchange membrane (PEM) fuel cells.

Synthesis of mesoporous nitrogen–tungsten co-doped TiO2 photocatalysts with high visible light activity

Abstract: Mesoporous N,W co-doped TiO 2 photocatalysts that contained various percentages of atomic tungsten dopant levels were synthesized by a facile solution combustion method which utilized urea as a nitrogen source and sodium tungstate as a tungsten source. The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), N 2 physisorption, UV–vis absorbance spectroscopy, and X-ray photoelectron spectroscopy (XPS). The results reveal that the synthesized N and W co-doped TiO 2 nanomaterials have high surface areas and mesoporous structures. In addition, the co-doping significantly narrows the band gap (∼2.7 eV) that is responsible for the high visible light response of these samples in comparison to that of pure anatase TiO 2 (∼3.2 eV). The photocatalytic activity of the prepared samples was evaluated on the basis of the photodegradation rate of Rhodamine B under visible light ( λ > 420). It was found that the mesoporous N,W co-doped TiO 2 nanomaterials fabricated in this study exhibited high visible light activity. This significant improvement in photocatalytic activity may be attributed to the synergistic effect of the red shift in absorption combined with a high surface area.

Toward inexpensive superhard materials: tungsten tetraboride-based solid solutions.

Abstract: To enhance the hardness of tungsten tetraboride (WB4), a notable lower cost member of the late transition-metal borides, we have synthesized and characterized solid solutions of this material with tantalum (Ta), manganese (Mn), and chromium (Cr). Various concentrations of these transition-metal elements, ranging from 0.0 to 50.0 at. %, on a metals basis, were made. Arc melting was used to synthesize these refractory compounds from the pure elements. Elemental and phase purity of the samples were examined using energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), and microindentation was utilized to measure the Vickers hardness under applied loads of 0.49–4.9 N. XRD results indicate that the solubility limit is below 10 at. % for Cr and below 20 at. % for Mn, while Ta is soluble in WB4 above 20 at. %. Optimized Vickers hardness values of 52.8 ± 2.2, 53.7 ± 1.8, and 53.5 ± 1.9 GPa were achieved, under an applied load of 0.49 N, when ∼2.0, 4.0, and 10.0 at. % Ta, Mn, and Cr were added to W...

Complete structural model for lanthanum tungstate: a chemically stable high temperature proton conductor by means of intrinsic defects

Abstract: This is the first paper reporting that lanthanum tungstate, earlier believed to be La6WO12, is in fact La28−xW4+xO54+δ , where tungsten dissolves in lanthanum sites to form a stable solid-state electrolyte, exhibiting proton conduction by hydration at intermediate temperatures.

Atomic layer deposition of tungsten materials

Abstract: Embodiments of the invention provide a method for depositing tungsten-containing materials. In one embodiment, a method includes forming a tungsten nucleation layer over an underlayer disposed on the substrate while sequentially providing a tungsten precursor and a reducing gas into a process chamber during an atomic layer deposition (ALD) process and depositing a tungsten bulk layer over the tungsten nucleation layer, wherein the reducing gas contains hydrogen gas and a hydride compound (e.g., diborane) and has a hydrogen/hydride flow rate ratio of about 500:1 or greater. In some examples, the method includes flowing the hydrogen gas into the process chamber at a flow rate within a range from about 1 slm to about 20 slm and flowing a mixture of the hydride compound and a carrier gas into the process chamber at a flow rate within a range from about 50 sccm to about 500 sccm.

Saturation of deuterium retention in self-damaged tungsten exposed to high-flux plasmas

Abstract: Polycrystalline, annealed tungsten targets were bombarded with 12.3 MeV W4+ ions to various damage levels. Deuterium was implanted by high-flux plasmas in Pilot-PSI (>1024 m−2 s−1) at a surface temperature below 525 K. Deuterium retention has been studied by nuclear reaction analysis and by thermal desorption spectroscopy. We found that deuterium retention is strongly enhanced by the tungsten bombardment and that saturation occurs at a W4+ fluence of about 3 × 1017 m−2. The maximum deuterium concentration in the damaged region was measured to be 1.4 at.%. This is in accordance with other experiments that were carried out at much lower fluxes. We therefore conclude that the saturation behaviour and the maximum retention are not affected by the high fluxes used in our experiments.A simple geometric model is presented that assumes that the saturation solely originates in the tungsten irradiation and that explains it in terms of overlapping saturated volumes. The saturated volume per incident MeV ion amounts to 3 × 104 nm3. From our results, we are able to obtain an approximate value for the average occupation number of the vacancies.