Showing papers on "Tungsten published in 2007"

Multiscale modeling of crowdion and vacancy defects in body-centered-cubic transition metals

Abstract: We investigate the structure and mobility of single self-interstitial atom and vacancy defects in body-centered-cubic transition metals forming groups 5B (vanadium, niobium, and tantalum) and 6B (chromium, molybdenum, and tungsten) of the Periodic Table. Density-functional calculations show that in all these metals the axially symmetric self-interstitial atom configuration has the lowest formation energy. In chromium, the difference between the energies of the and the self-interstitial configurations is very small, making the two structures almost degenerate. Local densities of states for the atoms forming the core of crowdion configurations exhibit systematic widening of the "local" d band and an upward shift of the antibonding peak. Using the information provided by electronic structure calculations, we derive a family of Finnis-Sinclair-type interatomic potentials for vanadium, niobium, tantalum, molybdenum, and tungsten. Using these potentials, we investigate the thermally activated migration of self-interstitial atom defects in tungsten. We rationalize the results of simulations using analytical solutions of the multistring Frenkel-Kontorova model describing nonlinear elastic interactions between a defect and phonon excitations. We find that the discreteness of the crystal lattice plays a dominant part in the picture of mobility of defects. We are also able to explain the origin of the non-Arrhenius diffusion of crowdions and to show that at elevated temperatures the diffusion coefficient varies linearly as a function of absolute temperature.

Calculation of x-ray spectra emerging from an x-ray tube. Part I. electron penetration characteristics in x-ray targets.

Abstract: The penetration characteristics of electron beams into x-ray targets are investigated for incident electron kinetic energies in the range 50-150 keV. The frequency densities of electrons penetrating to a depth x in a target, with a fraction of initial kinetic energy, u, are calculated using Monte Carlo methods for beam energies of 50, 80, 100, 120 and 150 keV in a tungsten target. The frequency densities for 100 keV electrons in Al, Mo and Re targets are also calculated. A mixture of simple modeling with equations and interpolation from data is used to generalize the calculations in tungsten. Where possible, parameters derived from the Monte Carlo data are compared to experimental measurements. Previous electron transport approximations in the semiempirical models of other authors are discussed and related to this work. In particular, the crudity of the use of the Thomson-Whiddington law to describe electron penetration and energy loss is highlighted. The results presented here may be used towards calculating the target self-attenuation correction for bremsstrahlung photons emitted within a tungsten target.

Oxide materials with low thermal conductivity

Abstract: A heuristic approach to identifying candidate materials with low, temperature-independent thermal conductivity above room temperature is described. On the basis of this approach, a number of compounds with thermal conductivities lower than that of 8 mol% yttria-stabilized zirconia and fused silica have been found. Three compounds, in particular, the Zr 3 Y 4 O 12 delta phase, the tungsten bronzes, and the La 2 Mo 2 O 9 phase, exhibit potential for low thermal conductivity applications. As each can exhibit extensive substitutional solid solution with other, high atomic mass ions, there is the prospect that many more compounds with low thermal conductivity will be discovered.

Controlled Growth and Characterization of Tungsten Oxide Nanowires Using Thermal Evaporation of WO3 Powder

Abstract: Tungsten oxide (WO3) nanowires were prepared on a tungsten (W) substrate by thermal evaporation of WO3 powder at elevated temperature in a tube furnace. The morphology, structure, composition, and chemical state of the prepared nanowires were characterized by SEM, EDX, TEM, XRD, Raman spectroscopic, and XPS measurements. The nanowires grown using WO3 powder were found to have uniform morphology with a high density and a crystalline structure consistent with monoclinic WO3. The field-emission measurements showed that the prepared nanowires have a turn-on field of 4.8 V/μm. The role of WO3 powder in the growth of high-density nanowires has been discussed by comparing the above results with those of nanowires grown without using WO3 powder and also on a different substrate. Also, the effects of growth temperature on the chemical binding states and product morphology of the nanowires were investigated.

Near Infrared Absorption of Tungsten Oxide Nanoparticle Dispersions

Abstract: Homogeneous dispersions of reduced tungsten oxide and tungsten bronze nanoparticles with ternary additives Na, Tl, Rb, and Cs have been prepared in the wet process and examined for optical properties. The dispersions of reduced tungsten oxide and tungsten bronze nanoparticles are found to show a remarkable absorption of near infrared light while retaining a high transmittance of visible light. This property is highly suitable for solar control filters in automotive and architectural windows.

Sub-ms laser pulse irradiation on tungsten target damaged by exposure to helium plasma

Abstract: The effects of a transient heat load on tungsten damaged by helium plasma irradiation have been investigated using a ruby laser with long pulse duration in the divertor simulator NAGDIS-II (Takamura et al 2002 Plasma Sources Sci. Technol. 11 A42). The pulse width of the ruby laser was ∼0.6 ms, which is close to that of the expected heat load accompanied by type-I edge localized modes (ELMs) in ITER operation. Helium holes/bubbles, which were formed in the surface region of powder metallurgy tungsten due to the exposure to the helium plasma, disappeared after the laser pulse irradiation to the tungsten surface with sufficient pulse energy. The results indicated that the transient heat loads similar to those expected by ELMs will mitigate damages such as bubbles and holes produced by helium irradiation. When a vacuum plasma sprayed tungsten coating on graphite was exposed to the helium plasma, the surface was covered with arborescent nanostructured tungsten containing many helium bubbles inside the structure. Melting traces were found on the surface after the laser pulses irradiated the surface even though the pulse energy was lower than that for melting bulk tungsten. A numerical temperature calculation of the sample suggested that the effective thermal conductivity near the surface dramatically decreased by several orders of magnitude due to the formation of nanostructured tungsten.

High strength alloys

Abstract: High strength metal alloys are described. At least one composition of a metal alloy includes chromium, nickel, copper, manganese, silicon, niobium, tungsten and iron. A heater system may include a canister at least partially made from material containing at least one of the metal alloys. A system for heating a subterranean formation may include a tublar that is at least partially made from a material containing at least one of the metal alloys.

Strain-rate dependence of the brittle-to-ductile transition temperature in tungsten

Abstract: We have investigated the strain-rate dependence of the brittle-to-ductile transition (BDT) temperature in pre-cracked tungsten single-crystals and polycrystals. There is an unambiguous Arrhenius relationship over four decades of strain rate, giving an activation energy for the process controlling the BDT of 1.05 eV. This is equal to the activation energy for double-kink formation on screw dislocations, suggesting that their motion controls the brittle–ductile transition.

Corrosion behaviour of electrodeposited nanocrystalline Ni–W and Ni–Fe–W alloys

Abstract: The present work deals with evaluation of corrosion behaviour of electrodeposited nanocrystalline Ni–W and Ni–Fe–W alloys. Corrosion behaviour of the coatings deposited on steel substrates was studied using polarization and electrochemical impedance spectroscopy techniques in 3.5% NaCl solution while their passivation behaviour was studied in 1N sulphuric acid solution. The corrosion resistance of Ni–W alloys increased with tungsten content up to 7.54 at.% and then decreased. In case of Ni–Fe–W alloys it increased with tungsten content up to 9.20 at.% and then decreased. The ternary alloy coatings exhibited poor corrosion resistance compared to binary alloy coatings due to preferential dissolution of iron from the matrix. Regardless of composition all the alloys exhibited passivation behaviour over a wide range of potentials due to the formation of tungsten rich film on the surface.

Tungsten Monocarbide as Potential Replacement of Platinum for Methanol Electrooxidation

Abstract: Polycrystalline tungsten monocarbide (WC) surfaces are synthesized and evaluated as a potential replacement of platinum (Pt) electrocatalysts. A combined approach utilizing ultrahigh vacuum (UHV) and electrochemical techniques demonstrates that WC possesses the following prerequisites for alternative electrocatalysts: high activity toward methanol decomposition, improved resistance to poisoning by CO, and promising electrochemical stability.

Tungsten as first wall material in fusion devices

Abstract: The observation in JET of co-deposition of tritium with carbon has led to a broad discussion on the replacement of graphite by a high-Z material for the first wall coverage. Moreover, due to the high erosion rate, carbon plasma facing components (PFCs) appear to be unacceptable for a commercial fusion reactor. Research in this area has subsequently gained increased attention. This paper describes the status of investigations on the use of tungsten as a first wall material. It discusses on the physical side the plasma wall interaction, the transport of tungsten in the plasma boundary and in the core. As an intermediate step on the technological side, graphite is often coated with tungsten layers. For highly loaded surfaces in a fusion reactor finally bulk tungsten components will have to be developed.

Doped and un-doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride: the effects of thickness and crystallographic orientation on thermochromic properties

Abstract: The atmospheric pressure chemical vapour deposition reaction of vanadyl acetylacetonate and tungsten hexachloride with oxygen led to the production of thin films of tungsten doped monoclinic vanadium dioxide on glass substrates. Scanning electron microscopy and X-ray diffraction indicated that the films had different morphologies and crystallinities depending on the deposition conditions used. Transmission and reflectance measurements showed a significant change in properties in the near infra-red either side of the metal to semiconductor transition. Variable temperature transmission studies show that the metal to semiconductor transition was lowered by tungsten doping. The effect of film thickness was studied with un-doped and doped films. It was found that film thickness limited the intensity of light passing through the film and the extent of the thermochromic transition but was found not to influence the hysteresis width or temperature of transition. Different film growth conditions led to a range of film morphologies which profoundly affected the resulting optical properties of the films. It was found that film morphology and preferred crystallographic orientation had a marked influence on the width and switching temperature of the thermochromic transition.

Hydrogen isotope retention in plasma-facing materials: review of recent experimental results

Abstract: Recent data on deuterium retention in carbon fibre composites and tungsten both irradiated with D ions and exposed to D plasmas are presented. Deuterium depth profiles measured up to depths of 7–14 μm allow understanding of the mechanism which is responsible for the hydrogen isotope trapping in these materials. In the CFC materials the amount of retained deuterium increases with the ion fluence at all irradiation temperatures in the range from 323 to 723 K. No saturation is reached as observed in pyrolytic graphite. Depth profiles show that saturation occurs only within a near surface layer corresponding to the ion range. The increase in total retention at near-room temperature is accompanied by an increasing of the long profile tail extending beyond 14 μm with the D concentration of about 10−1 at.% at a depth of 10 μm for fluences above 1024 D m−2. The depth at which deuterium is retained in tungsten (W) can be divided into three zones: (i) the near-surface layer (up to a depth of 0.2–0.5 μm depending on ion energy), (ii) the sub-surface layer (from ~0.5 to ~2 μm), and (iii) the bulk (>5 μm). Low-energy D ion irradiation modifies the W structure to depths of up to about 5 μm, both for W single crystals and polycrystalline W. The high D concentration (0.1–0.3 at.%) at depths of 1–3 μm relates to accumulation of D2 molecules in vacancy clusters and voids. These defects are supposed to be generated due to plastic deformation of the W surface caused by deuterium supersaturation within the near-surface layer.

Synthesis and Electrical Properties of Tungsten-Doped Vanadium Dioxide Nanopowders by Thermolysis

Abstract: Tungsten-doped vanadium dioxide (VO2) nanopowders were synthesized by thermolysis of (NH4)5[(VO)6(CO3)4(OH)9]·10H2O at low temperature with, to the best of our knowledge, active white powdery tungstic acid (WPTA) used as a substitutional dopant for the first time. The change in electrical resistance due to the semiconductor−metal transition was measured from −5 to 150 °C by the four-probe method. Differential scanning calorimetry and the resistance−temperature curve of the nanopowders indicated that the phase transition temperature of VO2 powders was 67.15 °C, but for W-doped VO2, the temperature was reduced to 26.46 °C. The results indicated that WPTA was found to be exceptionally effective as a dopant for reducing the transition temperature.

Development of High Current-Density Cathodes With Scandia-Doped Tungsten Powders

Abstract: The development of high current-density cathodes employing scandia-doped tungsten powders is reviewed in this paper. A matrix with a submicrometer microstructure characterized by uniformly distributed nanometer particles of scandia is believed to play a dominant role in the improved emission capability of these cathodes. Space-charge-limited current densities of over 30 A/cm2 at 850 degCb have been repeatedly obtained for many runs of cathodes fabricated from the different batches of scandia-doped tungsten powders. A lifetime of over 10000 h at 950-degCb 2-A/cm2 dc loading in a test diode has been achieved. Periodic high current-density pulse testing was also carried out during the test. The performance for both the dc and pulsed current densities remained stable. When tested at Stanford Linear Accelerator Center in a cathode life test vehicle with a Pierce gun configuration, the cathode operated for 500 h at 1170 degC b, with a pulsed loading of 100 A/cm2 and with less than 5% degradation in current density. The outstanding performance of these cathodes is attributed to a surface multilayer of Ba-Sc-O of about 100-nm thickness that uniformly covers the W grains with nanometer-size particles distributed on the growth steps. The layer is formed after proper activation by diffusion of free or ionic Sc together with Ba and O from the interior of the cathode to its surface. This highly mobile, free, or ionic Sc is liberated from constituents produced during impregnation and activation by reactions between the matrix materials and impregnants

Effects of Transmutation Elements on Neutron Irradiation Hardening of Tungsten

Abstract: Tungsten (W) is a candidate material for Plasma facing materials of fusion reactors. During fusion reactor operation, not only irradiation damages but also transmutation elements such as rhenium (Re) and osmium (Os) are produced by neutron irradiation. As a result, the original pure tungsten changes to W-Re or W-Re-Os alloys. Thus, the mechanical and physical properties are expected to change. The aim of this study is to investigate the effects of transmutation elements on neutron irradiation hardening and microstructure changes of tungsten. To simulate the effects of transmutation elements, tungsten base model alloys were used in this study. The examined compositions of the alloys were selected from the calculated changes in solid solution area of W-Re-Os alloy. Neutron irradiation was performed in fast test reactor JOYO in JAEA. The irradiation damages and temperature ranges were 0.17–1.54 dpa and 400–750 � C respectively. After the irradiation, Vickers hardness test and TEM observation were performed. There were clear differences between Re and Os in effects on irradiation hardening. In the case of W-Re alloys, when damages were less than 0.40 dpa, the irradiation hardenings were nearly equal to those of pure tungsten independent of Re addition. But when the damage was 1.54 dpa, the irradiation hardenings increased lineally with Re content. Microstructural observations showed that precipitations mainly formed in W-Re alloys. In the case of W-Os alloys, the irradiation hardenings (� Hv) of W-3Os alloys were larger than those of pure tungsten. And the differences were about 400 independent of dpa and irradiation temperature. Effects of Re and Os on irradiation hardening based on the microstructural observations were discussed. [doi:10.2320/matertrans.MAW200722]

Chalcogenide clusters of Group 5–7 metals

Abstract: The state-of-the-art and prospects of the development of chemistry of chalcogenide clusters of Group 5–7 metals of the Periodic Table are discussed. The main structural types, methods of synthesis, the reactivity and possible applications of vanadium, niobium, tantalum, chromium, molybdenum, tungsten and rhenium chalcogenide clusters with nuclearity of three and larger are considered.

Explosion temperatures and pressures of metals and other elemental dust clouds

Abstract: The Pittsburgh Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH) conducted a study of the explosibility of various metals and other elemental dusts, with a focus on the experimental explosion temperatures. The data are useful for understanding the basics of dust cloud combustion, as well as for evaluating explosion hazards in the minerals and metals processing industries. The dusts studied included boron, carbon, magnesium, aluminum, silicon, sulfur, titanium, chromium, iron, nickel, copper, zinc, niobium, molybdenum, tin, hafnium, tantalum, tungsten, and lead. The dusts were chosen to cover a wide range of physical properties—from the more volatile materials such as magnesium, aluminum, sulfur, and zinc to the highly “refractory” elements such as carbon, niobium, molybdenum, tantalum, and tungsten. These flammability studies were conducted in a 20-L chamber, using strong pyrotechnic ignitors. A unique multiwavelength infrared pyrometer was used to measure the temperatures. For the elemental dusts studied, all ignited and burned as air-dispersed dust clouds except for nickel, copper, molybdenum, and lead. The measured maximum explosion temperatures ranged from ∼1550 K for tin and tungsten powders to ∼2800 K for aluminum, magnesium, and titanium powders. The measured temperatures are compared to the calculated, adiabatic flame temperatures.

Synthesis, Structure, and Electrochemical Characterization of Nanocrystalline Tantalum and Tungsten Nitrides

Abstract: Nanocrystalline tantalum and tungsten nitrides were synthesized by a two-step ammonolysis reaction of transition metal chlorides. The first step involves ammonolysis of tantalum and tungsten chlorides at room temperature using anhydrous chloroform as the solvent. A second step consists of heat treatment of these as-prepared powders under anhydrous NH3 atmosphere leading to the formation of tantalum and tungsten nitride nanocrystallites at relatively low temperature (600°–675°C). The ammonolysis, nitridation, structure, morphology, surface area, density, and electronic conductivity of the nitrides obtained were studied and characterized. The electrochemical responses of these nitrides were also measured by cyclic voltammetry using 1M KOH electrolyte.