Showing papers on "Tungsten published in 2008"

Synthesis of Mo and W carbide and nitride nanoparticles via a simple "urea glass" route.

Abstract: A simple, inexpensive, and versatile route for the synthesis of metal nitrides and carbides (such as Mo2N, Mo2C, W2N and WC) nanoparticles was set up. For the first time, metal carbides were obtained using urea as carbon-source. MoCl5 and WCl4 are in a first step contacted with alcohols and an appropriate amount of urea to form a polymer-like, glassy phase, which acts as the starting product for further conversions. Just by heating this phase it was possible to prepare either molybdenum and tungsten nitrides or carbides simply by changing the metal precursor/urea molar ratio. In this procedure, urea plays a double role as a nitrogen/carbon source and stabilizing agent (necessary for the nanoparticle dispersion). Molybdenum and tungsten nitride and carbides synthesized are almost pure and highly crystalline. Sizes estimated by WAXS range around 20 and 4 nm in diameter for Mo and W nitrides or carbides, respectively. The specific surface area was found between 10 and 80 m2/g, depending on the metal and the initial ratio of metal precursor to urea.

Plumbing carbon nanotubes

Abstract: Since their discovery, the possibility of connecting carbon nanotubes together like water pipes has been an intriguing prospect for these hollow nanostructures. The serial joining of carbon nanotubes in a controlled manner offers a promising approach for the bottom-up engineering of nanotube structures—from simply increasing their aspect ratio to making integrated carbon nanotube devices. To date, however, there have been few reports of the joining of two different carbon nanotubes1,2,3. Here we demonstrate that a Joule heating process, and associated electro-migration effects, can be used to connect two carbon nanotubes that have the same (or similar) diameters. More generally, with the assistance of a tungsten metal particle, this technique can be used to seamlessly join any two carbon nanotubes—regardless of their diameters—to form new nanotube structures.

Ion-driven deuterium retention in tungsten

Abstract: The ion-driven retention of deuterium in polycrystalline tungsten (PCW) is studied experimentally and theoretically as a function of temperature, incident ion energy, and ion fluence. Deuterium retention was investigated by thermodesorption spectroscopy and ion beam analysis. The peculiarities of deuterium behavior in PCW such as (i) ion-induced defect formation at low-energy implantation and (ii) higher D retention of low-energy ions (60–200eV) compared to high-energy ions (3keV) at high fluences are considered. The effect of intrinsic defects (dislocations, vacancies, grain boundaries) and ion-induced defects (vacancies, dislocations, deuterium clusters) is discussed for polycrystalline tungsten.

WO3 gas sensors prepared by thermal oxidization of tungsten

Abstract: Tungsten trioxide (WO3) was grown by direct thermal evaporation of metal tungsten foils in an oxygen atmosphere. The samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-rays (EDX) analysis and Raman spectroscopy. The synthesized products consisted of crystalline aggregates having a mean size of about 0.8 μm. Raman spectroscopy evidenced typical vibrational peaks of the monoclinic WO3 phase. The samples were investigated as resistive gas sensors towards NO2, NO and NH3 gases. The influence of the working temperature on the sensor response was also studied. The sensors were sensitive enough to detect NO2 and less sensitive to NO and NH3. The relative variation of resistance as a function of gas concentration was found to be reversible and to follow a power-law relationship. The response and recovery times were determined according to a simple adsorption kinetic model.

Tungsten black absorber for solar light with wide angular operation range

Abstract: Using three-dimensional finite-difference time-domain simulations, we designed a structured tungsten slab with subwavelength periodicity that displays near-complete absorptivity throughout the entire solar spectrum over a wide angular range. The structure consists of a square lattice array of pyramids. The parameters of the pyramids are chosen to provide the optimal impedance matching between free space and tungsten, while the period is chosen to be subwavelength in order to generate absorption over an ultrabroad band of wavelengths.

Pressure-induced incompressibility of ReC and effect of metallic bonding on its hardness

Abstract: The compressible behaviors of the selected 5d transition metal carbides MC (M=W,Re,Os,Ir) with hexagonal tungsten carbide-type structure were studied by first-principles calculations. Results indicate that the incompressibility of ReC exceeds that of diamond under higher pressure. The calculated method for hardness of crystals with partial metallic bonding is suggested and the calculated results indicate that hexagonal ReC crystal possesses excellent mechanical properties.

A molecular dynamics study of the phase transition in bcc metal nanoparticles

Abstract: The phase transition between liquid and solid phases in body-centered cubic (bcc) metal nanoparticles of iron, chromium, molybdenum, and tungsten with size ranging from 2000 to 31,250 atoms was investigated using a molecular dynamics simulation. The nucleation from an undercooled liquid droplet was observed during cooling in all nanoparticles considered. It was found that a nucleus was generated near one side of the particle and solidification spread toward the other side the during nucleation process. On the other hand, the surface melting and subsequent inward melting of the solid core of the nanoparticles were observed during heating. The depression of the melting point was proportional to the inverse of the particle radius due to the Gibbs-Thomson effect. On the other hand, the depression of the nucleation temperature during cooling was not monotonic with respect to the particle radius since the nucleation from an undercooled liquid depends on the event probability of an embryo or a nucleus.

Tracer Investigation of Pore Formation in Anodic Titania

Abstract: Using a sputtering-deposited titanium substrate, incorporating six equally spaced nanolayers of Ti-W alloy, the volume and composition changes accompanying the formation of porous anodic films on titanium in 0.5 wt % NH 4 F in glycerol are investigated. The findings reveal amorphous films with nanotubes of TiO 2 , containing fluoride ions and possibly glycerol derivatives. Tungsten and titanium species are lost to the electrolyte at differing rates during anodizing, leading to an enrichment of tungsten in the film relative to the composition of the substrate. The spacing of tungsten-containing bands in the film is ∼2.3 that of the original alloy layers during growth of the major pores. The generation of the nanotubes can be explained either by field-assisted flow of film material within the barrier layer to the pore walls, with cation and anion transport numbers of anodic titania in the barrier layer region similar to those of barrier films and with field-assisted ejection of Ti 4+ ions to the electrolyte, or by field-assisted dissolution, but with a reduction in cation transport number.

Femtosecond laser-induced periodic surface structure formation on tungsten

Abstract: In this paper, we demonstrate the generation of periodic surface structures on a technologically important material, tungsten, at both 400 and 800 nm, despite that the table values of dielectric constants for tungsten at these two wavelengths suggest the absence of surface plasmons, a wave necessary for forming periodic structures on metals. Furthermore, we find that the structure periods formed on tungsten are significantly less than the laser wavelengths. We believe that the dielectric constants of tungsten change significantly due to intense laser pulse heating and surface structuring and roughening at nanometer scales, permitting surface plasmon excitation and periodic structure formation.

Tungsten and nitrogen co-doped TiO2 nano-powders with strong visible light response

Abstract: A two-step method, combining with sol-gel and mechanical alloying (MA) method, was used to fabricate the tungsten and nitrogen co-doped TiO2 nano-powders ((W, N) co-doped TiO2 NPs). The (W, N) co-doped TiO2 NPs showed strong absorbance in visible range, as long as 650 nm. Enhanced photocatalytic activities under visible light irradiation were also observed from the results of photodegradation experiments and chemical oxygen demand (COD) analysis. Physical, chemical, and optical properties of the samples were investigated. Possible reasons for the enhanced photocatalytic activities were analyzed based on the experimental results. Oxygen vacancies detected by electron spin response (ESR) spectra, acting as trapping agencies for electrons (e(-)) to produce active oxygen species (O-center dot(2-)), were proved to be the main cause for the improved photocatalytic performances. (c) 2008 Published by Elsevier B.V.

Heterogeneous deformation and spall of an extruded tungsten alloy : plate impact experiments and crystal plasticity modeling

Abstract: The role of microstructure in the dynamic deformation and fracture of a dual phase, polycrystalline tungsten alloy under high-rate impact loading is investigated via experiments and modeling. The material studied consists of pure tungsten crystals embedded in a ductile binder alloy comprised of tungsten, nickel, and iron. The tungsten crystals are elongated in a preferred direction of extrusion during processing. Plate impact tests were conducted on samples oriented either perpendicular or parallel to the extrusion direction. Spatially resolved interferometric data from these tests were used to extract wave propagation behavior and spall strength dependent upon position in the sample microstructure. Finite element simulations of impact and spall in digitally reproduced microstructural geometries were conducted in parallel with the experiments. Finite deformation crystal plasticity theory describes the behavior of the pure tungsten and binder phases, and a stress- and temperature-based cohesive zone model captures fracture at grain and phase boundaries in the microstructure. In results from both experiments and modeling, the grain orientations affect the free-surface velocity profile and spall behavior. Some aspects of distributions of free-surface velocity and spall strength among different microstructure configurations are qualitatively similar between experimental and numerical results, while others are not as a result of differing scales of resolution and modeling assumptions. Following a comparison of experimental and numerical results for different microstructures, intergranular fracture is identified as an important mechanism underlying the spall event.

Analysis of the side gap resulting from micro electrochemical machining with a tungsten wire and ultrashort voltage pulses

Abstract: Micro wire electrochemical machining (ECM) using a O 10 µm tungsten wire as the tool electrode is presented. Since electrochemical machining does not wear out the tool, it can be easily applied to the fabrication of micro grooves by moving the tool electrode along a programmed toolpath. To minimize the side gap, ultrashort voltage pulses were applied between the tool electrode and the workpiece. Changes in the side gap according to the applied pulse voltage, pulse on-time and pulse period were investigated, and the optimal pulse condition for stable machining was obtained. By using this method, micro features such as micro grooves and gears were fabricated into stainless steel plates.

Nanosize hexagonal tungsten oxide for gas sensing applications

Abstract: Tungsten oxides and tungsten oxide hydrates are among the most used materials in electro-, photo- and gaso-chromic applications. Lately, tungsten oxides have been commonly applied as sensing layers for hazardous gas detection as well. In this work, a soft chemical nanocrystalline processing route has been demonstrated for the preparation of hexagonal tungsten oxides. The acidic precipitation was followed by hydrothermal and heat treatments at low temperatures. The morphology of parent phases, such as amorphous WO 3 ·2H 2 O, orthorhombic WO 3 ·1/3H 2 O, and resulting oxides with open structured nanosized hexagonal platelets of h-WO 3 particles have been studied by scanning electron microscopy (SEM), by conventional transmission electron microscopy (TEM) and by high resolution transmission electron microscopy (HRTEM). Structural and electrochemical performance of thin films have been determined by atomic force microscopy and cyclic voltammetry. The ion insertion properties of tungsten oxide hydrate and tungsten oxide films show a clear dependence on the presence of structural water and on the close packed structure. Sensing properties of the prepared tungsten oxides have been tested with respect to ammonia gas.

Redox behavior of molybdenum and tungsten in phosphate glasses.

Abstract: In this work, vitreous samples were prepared in the binary system (100 - x)NaPO3-xMO3 with M = Mo and W and x varying from 10 to 60. The transmittance properties in the UV, visible, and near-infrared were monitored as a function of MO3 concentration. In both cases, an increase in the amount of transition metal results in an intense and broad absorption band in the visible and near-infrared attributed to metal reduction under synthesis conditions. It was shown that this large absorption can be partially or totally removed using specific oxidizing agents or by improving synthesis parameters such as melting temperature or cooling rate of the melt. In addition, structural investigations by Raman and X-ray absorption spectroscopy suggest that reduction only occurs when the metal cation is in octahedral geometry and that the transmittance improvement is not related with any structural changes. These results were explained in terms of thermodynamic equilibrium of redox species in the melt and allowed to obtain for the first time transparent and chemically stable glasses containing high concentrations of MO3 with transition metals in octahedral geometry inside the glass network.

Methods for growing low-resistivity tungsten for high aspect ratio and small features

Abstract: The present invention addresses this need by providing methods for depositing low resistivity tungsten films in small features and features having high aspect ratios. The methods involve depositing very thin tungsten nucleation layers by pulsed nucleation layer (PNL) processes and then using chemical vapor deposition (CVD) to deposit a tungsten layer to fill the feature. Depositing the tungsten nucleation layer involves exposing the substrate to alternating pulses of a boron-containing reducing agent and a tungsten-containing precursor without using any hydrogen gas, e.g., as a carrier or background gas. Using this process, a conformal tungsten nucleation layer can be deposited to a thickness as small as about 10 Angstroms. The feature may then be wholly or partially filled with tungsten by a hydrogen reduction chemical vapor deposition process. Resistivities of about 14 μΩ-cm for a 500 Angstrom film may be obtained.

One-pot synthesis of twist-like helix tungsten–nitrogen-codoped titania photocatalysts with highly improved visible light activity in the abatement of phenol

Abstract: The twist-like helix W,N-codoped TiO2 photocatalysts were prepared by a simple one-pot synthesis route to hydrolysis of titania tetrachloride using ammonium tungstate as tungsten and nitrogen sources. The morphology and microstructure characteristics of W,N-codoped titania photocatalysts with different amount of tungsten doping were characterized by means of BET, TEM, SEM, XPS, UV–vis DRS, PLS and XRD. The probable mechanism of codoping effect is proposed. It is presumed that cooperation of nitrogen and tungsten ions leads to produce new states and narrow the band gap between the valence band and conduction band effectively, which will greatly improve the photocatalytic activity in the visible light region. On the other hand, the tungsten ions with changing valences in the W,N-TiO2 samples are considered to act as trapping sites, which will effectively decrease the recombination rate of photo-induced electrons and holes and then increase the photo-oxidation efficiency of the catalysts. The metal and nonmetal codoped 1%-W,N-TiO2 sample shows the best photocatalytic activity, which is much superior to P25 under both visible and ultraviolet light irradiation. The superior activity of W,N-TiO2 photocatalysts can also be ascribed to the special twist-like helix structure with regular holes on the wall, high surface area, large pore volume and well-crystallized anatase phase.

Influence of Tungsten Precursors on the Structure and Catalytic Properties of WO3/SBA-15 in the Selective Oxidation of Cyclopentene to Glutaraldehyde

Abstract: Mesoporous WO3/SBA-15 was synthesized by the conventional incipient wetness impregnation method from various tungsten precursors. The influence of different tungsten precursors on the structure of WO3/SBA-15, as well as their catalytic performance for the selective oxidation of cyclopentene (CPE) to glutaraldehyde (GA), was investigated. The dispersion and nature of the tungsten species were systematically characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV−visible diffuse reflectance spectra (DRS), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (NH3-TPD), and temperature-programmed reduction (H2-TPR). Among the different tungsten sources, the tungstenic complex sources produced from the reaction of tungstic acid with oxalic acid showed the highest dispersion of WO3 species, the strongest surface Bronsted and Lewis acid properties, and the strongest interaction with the SBA-15 support. As a consequ...

Synthesis, Characterization, and Rietveld Refinement of Tungsten-Framework-Doped Porous Manganese Oxide (K-OMS-2) Material

Abstract: Tungsten was successfully doped at 1 and 2 mol % tungsten into the K-OMS-2 framework. Sodium tungstate and tungsten pentabromide were used in a reflux synthesis preparation. The data collected from the characterization methods collectively affirm the substitution of tungsten into the K-OMS-2 framework. Conductivity measurements showed an increase in the resistivity. Conversion of benzyl alcohol to benzaldehyde had a conversion of 25 and 15% for the sodium tungstate and tungsten pentabromide, respectively, while retaining 100% selectivity. Properties such as the resistivity, thermal stability, and crystallinity of the material were altered depending on the amount and type of starting reactants used.