Amount Of Tungsten Research Articles

Engineered chemistry of Cu–W composites sintered by field-assisted sintering technology for heat sink applications

High performance microelectronics require superior thermal management systems. Heat sink plates that dissipate the heat generated by the microelectronic components are of critical importance for this purpose. The two primary concerns with heat sink materials are the coefficient of thermal expansion (CTE), which should be low to match that of silicon (4 × 10−6/°C), and the thermal conductivity, which should be as high as possible to move the heat efficiently. This study created copper–tungsten composite materials using the field assisted sintering technology (FAST) process. The amount of tungsten used in the composites was varied from 0 to 70 wt% to study the effects of tungsten as well as to show the ability of the composite’s properties to be tailored based on the percentage of tungsten in the composite. It is shown that increasing pressure has a positive effect on final density while heating rate has no effect. High densities of >97.5% were achieved in all of the composites. As predicted, the CTE decreases as the percentage of tungsten increases. Thermal conductivities are also reported for each sample.

Effect of infiltration parameters on composition of W–ZrC composites produced by displacive compensation of porosity (DCP) method

In this study, the effect of infiltration parameters on composition of W–ZrC composites produced by displacive compensation of porosity (DCP) method was investigated. For this purpose, a porous tungsten carbide preform was prepared by cold isostatic pressing (CIP) of tungsten carbide powder and polyvinyl alcohol (PVA). The porous preform was debinded for 4 h at 400 °C and sintered for 4 h at 1400 °C. The sintered preform was then infiltrated by molten Zr 2Cu at 1300 °C and 1200 °C for 1, 3, 5 and 7 h. Scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS) and X-ray diffraction (XRD) were used to study the cross section of infiltrated specimens. The results indicated that the amount of tungsten and zirconium carbide phases increased by increasing the infiltration temperature, whereas the effect of infiltration time on composition of W–ZrC composite was negligible.

Catalytic adsorptive stripping voltammetry determination of ultra trace amount of tungsten using factorial design for optimization

A highly sensitive procedure is presented for the determination of ultra-trace concentration of tungsten by catalytic adsorptive stripping voltammetry. The method is based on adsorptive accumulation of the tungsten-pyrocatechol violet complex onto a hanging mercury drop electrode, followed by reduction of the adsorbed species by voltammetric scan using differential pulse modulation. The reduction current is enhanced catalytically by chlorate. The influence of variables was completely studied by factorial design analysis. Optimum analytical conditions for the determination of tungsten were established. Tungsten can be determined in the range 0.06–12.0 ng/mL with a limit of detection of 0.02 ng/mL. The influence of potentially interfering ions on the determination of tungsten was studied. The procedure was applied to the determination of tungsten in one sandwich polyoxometalate and some synthetic samples similar to alloy compounds with satisfactory results.

Study of soft X-ray emission from Z-pinches with a complex atomic composition

Results are presented from experimental studies of Z-pinches produced by implosion of aluminum and tungsten cylindrical wire arrays in the Angara-5-1 facility. The electron temperature T e and density n e of the high-temperature pinch plasma have been determined by analyzing line emission from multicharged ions. For the same mass and radius of the array and the same number of wires in it, the intensity of line emission of H- and He-like Al ions from an imploded Al + W wire array containing even a small amount of tungsten (7 wt %) is one order of magnitude lower than that from an Al array. As the W content increases, the total soft X-ray (SXR) yield increases, while the duration of the SXR pulse decreases. For the 30% W content in the array, the power and duration of the SXR pulse are nearly the same as those recorded during the implosion of a W array with the same linear mass and radius and the same number of wires. Results are also presented from experiments with nested wire arrays in which the outer and inner shells were made of Al and W wires, respectively. It is found that, in this case, the effect of tungsten on the line emission of aluminum is much weaker than that in experiments with arrays in which tungsten and aluminum wires were placed in the same shell, even if the mass of the inner (tungsten) shell was larger than that of the outer (aluminum) one. At the same time, the inner W shell plays a significant role in the implosion dynamics of a nested wire array, reducing the duration of the SXR pulse and increasing the SXR power.

Determination of tin in marine sediments by slurry sampling atomic absorption spectrometry with electrothermal atomization using a permanent modifier

Slurry sampling atomic absorption spectrometry with electrothermal atomization was used for the determination of tin in marine sediments. A mixture of 4 μg of iridium and 40 μg of tungsten and 12 μg of niobium alone were used as permanent modifiers. Suspensions were prepared in 7% nitric acid. The influence of amounts of tungsten on iridium removal from graphite tubes modified by iridium and the mechanism of atomization of tin in slurries were studied in detail. The effect of the amount of added iridium, tungsten and niobium on the analytical signal, and of the soil matrix on the background signal is described. In order to prevent interferences by sulfur compounds, barium chloride was added as a matrix modifier. Calibration was performed directly using aqueous standards. The analysis of certified reference materials confirmed the reliability of the approach. The precision and accuracy of the determination of tin in sediments by this method is acceptable.

Hydrogenation of p-Chloronitrobenzene on Tungsten-Modified NiCoB Catalyst

NiCoB has been reported to be a good catalyst for the hydrogenation of p-chloronitrobenzene (p-CNB). However, it aggregates easily. In this study, a series of tungsten-modified NiCoB catalysts with various tungsten contents were synthesized by the chemical reduction method using NaBH4 as the reducing agent. The products were characterized by X-ray diffraction (XRD), N2 sorption, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The catalysts were tested for the liquid-phase hydrogenation of p-CNB. Ni, Co, W, and B were present in both elemental and oxidized states. The addition of tungsten could influence the surface composition of the catalyst. The electronic structures of Ni, Co, W, and B were also changed with changing content of tungsten. XPS showed that the elemental states of Ni, Co, W, and B formed an amorphous nanoalloy. The interactions of these species affected their electron densities and further influenced their catalytic activities. The tungsten oxide located among the NiCoB particles could act as a spacer that separates the NiCoB particles from their neighbors and inhibits aggregation. Compared to the unmodified NiCoB catalyst, the catalytic activity of tungsten-modified NiCoB exhibited an obvious increase, along with a slight decrease in the selectivity of p-chloroaniline (p-CAN). The differences in the atom radius and electronegativity of tungsten from those of the other elements in the alloy resulted in the formation of active sites for the catalytic reaction. The concentration of boron on the catalyst surface decreased with increasing tungsten concentration, which was responsible for the slight decrease in the selectivity of p-CAN. Considering the electronic effects, the structural effects, and the amount of B3+ (Lewis acid), good catalytic performance in the hydrogenation of p-CNB was achieved by modifying the NiCoB catalysts with the proper amount of tungsten. Tungsten species not only acted as a spacer to prevent NiCoB particles from aggregating, but also donated partial electrons to Ni and Co. An overdose of tungsten would cover the surface of Ni and result in low activity.

Nanostructured Co–W coatings produced by electrodeposition to replace hard Cr on aerospace components

Co–W alloys with 12–26 at.-%W (25–45 wt-%W) can be produced by electrodeposition from complexed aqueous plating baths. The deposits can be in the amorphous or nanostructured conditions, depending on the plating bath operations. Most recent attention has been focused on the nanocrystalline conditions which have hardness values in excess of 850 kg mm−2, which makes them competitive with hard engineering chromium. This may allow the coatings to replace hard Cr which is produced from hexavalent chromium baths having associated health and safety problems. Co–W coatings, 2–120 μm thick, can be deposited with a low incidence of cracking, with good corrosion and wear properties (dry and lubricated) when deposited on a variety of steel substrates. The underlying reasons, i.e. the amount of tungsten in solution, the density of grain boundaries and the reduction in bonding between coating and counterface under dry sliding conditions, which account for these enhanced properties are discussed. Potential applications are in many sectors of engineering including aerospace, hydraulics and automobile.

Pharmacokinetics of radiolabeled tungsten (188W) in male Sprague-Dawley rats following acute sodium tungstate inhalation

Aerosol cloud formation may occur when certain tungsten munitions strike hard targets, placing military personnel at increased risk of exposure. Although the pharmacokinetics of various forms of tungsten have been studied in animals following intravenous and oral administration, tungsten disposition following inhalation remains incompletely characterized. The objective of this study was to evaluate the pharmacokinetics of inhaled tungstate (WO4) in rats. Male, 16-wk-old, CD rats (n = 7 rats/time point) underwent a single, 90-min, nose-only exposure to an aerosol (mass median aerodynamic diameter [MMAD] 1.50 μm ) containing 256 mg W/m3 as radiolabeled sodium tungstate (Na2188WO4). 188W tissue concentrations were determined at 0, 1, 3, 7, and 21 days postexposure by gamma spectrometry. The thyroid and urine had the highest 188W levels postexposure, and urinary excretion was the primary route of 188W elimination. The pharmacokinetics of tungsten in most tissues was best described with a two-compartment pharmacokinetic model with initial phase half-lives of approximately 4 to 6 h and a longer terminal phase with half-lives of approximately 6 to 67 days. The kidney, adrenal, spleen, femur, lymph nodes, and brain continued to accumulate small amounts of tungsten as reflected by tissue:blood activity ratios that increased throughout the 21-day period. At day 21 all tissues except the thyroid, urine, lung, femur, and spleen had only trace levels of 188W. Data from this study can be used for development and refinement of pharmacokinetic models for tungsten inhalation exposure in environmental and occupational settings.

Dust investigations at ASDEX Upgrade

Investigations on dust in the full tungsten divertor tokamak, ASDEX Upgrade, are summarized. Newly designed collectors are used to distinguish plasma-produced dust from other kinds of debris. Whereas submicron particles are often tungsten spheres, larger ones are agglomerates from tungsten in a boron/carbon matrix. Extrapolation of the measurement yields 80 mg of carbon and 1200 mg of tungsten in the dust, including 510 mg of tungsten spheres. In the case of its mobilization, the amount of tungsten observed may constitute a significant impurity source. Droplets produced by arcs seem to be the production mechanism for tungsten spheres. The strong light emission of dust particles penetrating the scrape off layer (SOL) is rarely observed by a fast camera during normal discharges, but is very frequently observed after disruptions. As the particles follow straight lines, we infer only a weak interaction with the SOL plasma. This is also confirmed by the observation of fragile agglomerates.

Vanadium loaded carbon-based monoliths for the on-board No reduction: Influence of vanadia and tungsten loadings

Carbon-coated catalysts doped with tungsten and vanadia oxides with different V and W loadings have been prepared by the ionic exchange method and characterized. The surface, structure and composition have been investigated by XPS, Raman, N 2 sorption at 77 K, TPD-NH 3 and reactivity tests for the SCR of NO with NH 3 at low temperatures. Under reaction conditions, NO conversions were found to go through a maximum with vanadia surface coverage at approximately half a monolayer. The observed decrease in the SCR activity at higher vanadia loadings can be attributed to either a loss of dispersion or loss of textural properties. Maximum NO conversion is ascribed to the higher Brönsted proton acidity (V 4+) of the centres that decreases with increasing vanadia loadings up to 3 wt% loading due to the increase of V 4+/V 5+ ratio. Large amounts of tungsten (5%, w/w) upon or before addition of vanadia do not provide an enhancement of activity. The results indicate that W addition increases surface acidity leading to stronger Brönsted or even Lewis acid centre creation.

Acute sodium tungstate inhalation is associated with minimal olfactory transport of tungsten ( 188W) to the rat brain

Olfactory transport of represents an important mechanism for direct delivery of certain metals to the central nervous system (CNS). The objective of this study was to determine whether inhaled tungsten (W) undergoes olfactory uptake and transport to the rat brain. Male, 16-week-old, Sprague–Dawley rats underwent a single, 90-min, nose-only exposure to a Na 2 188WO 4 aerosol (256 mg W/m 3). Rats had the right nostril plugged to prevent nasal deposition of 188W on the occluded side. The left and right sides of the nose and brain, including the olfactory pathway and striatum, were sampled at 0, 1, 3, 7, and 21 days post-exposure. Gamma spectrometry ( n = 7 rats/time point) was used to compare the levels of 188W found on the left and right sides of the nose and brain and blood to determine the contribution of olfactory uptake to brain 188W levels. Respiratory and olfactory epithelial samples from the side with the occluded nostril had significantly lower end-of-exposure 188W levels confirming the occlusion procedure. Olfactory bulb, olfactory tract/tubercle, striatum, cerebellum, rest of brain 188W levels paralleled blood 188W concentrations at approximately 2–3% of measured blood levels. Brain 188W concentrations were highest immediately following exposure, and returned to near background concentrations within 3 days. A statistically significant difference in olfactory bulb 188W concentration was seen at 3 days post-exposure. At this time, 188W concentrations in the olfactory bulb from the side ipsilateral to the unoccluded nostril were approximately 4-fold higher than those seen in the contralateral olfactory bulb. Our data suggest that the concentration of 188W in the olfactory bulb remained low throughout the experiment, i.e., approximately 1–3% of the amount of tungsten seen in the olfactory epithelium suggesting that olfactory transport plays a minimal role in delivering tungsten to the rat brain.

Tribological behaviour of electrodeposited cobalt–tungsten coatings: dependence on current parameters

The tribological behaviour of cobalt–tungsten alloy coatings electrodeposited under direct or pulse current mode from a cobalt sulphate–sodium tungstate bath was investigated. At a low average current density of 0˙5 A dm–2 the coatings contain a low amount of tungsten that leads to a milky appearance of the coatings. These coatings have a high roughness, a low hardness, and exhibit a high coefficient of friction and a high wear in sliding tests against corundum. In contrast, cobalt–tungsten coatings deposited at a higher average current density of 1 A dm–2 also have a milky appearance but exhibit tribological properties comparable to the ones of electrodeposited hard chromium and physical vapour deposited TiN.

Strengthening electroless Co-based barrier layers by minor refractory-metal doping

An electrochemical seeding process without involving sensitization-activation and displacement-activation is presented to grow catalytic particles of sizes only ~ 4 nm, subsequently initializing the deposition of Co–B and Co–W–B barrier layers which have controlled amounts of solutes, Co 92B 8 and Co 90W 3B 7, on thermally oxidized SiO 2 layer on Si using electroless plating. Analyzing the Si/SiO 2/barrier layer samples before and after thermal annealing (450 °C/1 h) using depth-profiling secondary ion mass spectroscopy, transmission electron microscopy, and scanning electron microscopy reveals that the Co 92B 8 barrier layer is degraded by the mutual reactions between the barrier’s components, Co and B, and the outgoing Si, ultimately forming Co 2Si and Co 2B. However, doping a minor amount of tungsten not only strengthens the thermo-chemical and microstructure stability of the barrier layers, but also significantly reduces the self-diffusion of the matrix cobalt, and hence can prevent the intermixing of Co and Si. The strengthening behavior of the barrier layer by the doping of tungsten solute atoms will be clarified based on elemental depth-profiling and microstructure analyses.

Preparation and Characterization of Bismuth Tungstate Polycrystalline Flake-Ball Particles for Photocatalytic Reactions

Micrometer-sized spherical polycrystalline particles of bismuth tungstate (Bi2WO6) of a hierarchical “flake-ball” shape were prepared by a facile hydrothermal reaction without using any surfactants and polymers as structure-directing agents. The flake-ball particles were assemblies of polycrystalline flakes composed of rectangular platelets with a lateral size of a few hundred nanometers and thickness of 20−35 nm. An excess amount of a tungstate precursor (10%) and an acidic condition (pH 1.2) during the hydrothermal reaction were required to obtain a high yield of uniform particles with the flake-ball architecture. The mechanism by which the flake-ball particles are formed is discussed. The photocatalytic activities under ultraviolet light irradiation were investigated by using oxygen liberation from water, oxidative decomposition of acetic acid in an aqueous solution, and oxidative decomposition of gaseous acetaldehyde. The photocatalytic activity level of the flake-ball particles was higher than the photocatalytic activity levels of other Bi2WO6 samples prepared by conventional solid-state and hydrothermal reactions using a stoichiometric amount of a tungstate precursor. It was revealed that the 10% excess amount of tungsten plays a key role in the high level of photocatalytic activity of flake-ball particles.

Illuminating Tungsten’s Life Cycle in the United States: 1975−2000

Tungsten's unique properties, such as its high density and melting point, are manifest in a variety of technological applications, and attention has recently centered upon the health and environmental effects of tungsten. Utilizing material flow analysis, the amounts of tungsten produced, fabricated and manufactured, entering use, and entering waste management in the United States from 1975-2000 were determined, with the inclusion of net trade, and with attention paid to the way that tungsten partitions into in-use reservoirs and the quantities of tungsten that enter the environment as a result of anthropogenic processes. The results from two approaches are presented: the first and typical, approach for historical material flow analysis studies, whereby tungsten use in products over time was approximated using end-use sectors used by the United States Bureau of Mines and the United States Geological Survey (the "end-use sector model") and a second, more detailed approach whereby the historical pattern of end-use products was determined (the "finished product model"). These models represent the first life cycle of tungsten that takes into account both finished product trade and varying residence times for products entering use. For the cumulative time period of 1975-2000, approximately 60% of the supply to fabrication and manufacturing processes in the United States consisted of imports. The tungsten embodied in products used in applications other than transportation is discarded (i.e., enters the waste stream, which encompasses both material that is eventually landfilled and that is eventually recycled) in the same year it enters service. Of its items entering end-of-life for the cumulative time period, 50-70% of tungsten is estimated to be landfilled. In both models, the largest flows of tungsten to the environment were estimated to originate from post-consumer discards. Approximately 8-17 times as much tungsten entered landfills than was estimated to be generated as waste from fabrication and manufacturing processes; for tailings, these ratios are even greater. Tungsten is an example of a critical mineral resource where the majority of the United States' supply comes from imports, it has vital uses in manufacturing, and it has a high rate of loss at end-of-life.

From tungsten hexacarbonyl adsorption on TiO 2(1 1 0) surface to supported tungsten oxide phases

Synchrotron-based photoemission spectroscopies were used to study the adsorption of tungsten hexacarbonyl on (1 1 0) TiO 2 surfaces: experiments using W4f and Ti2p intensities variations show that, at 140 K, the hexacarbonyl growth proceeds via a layer-by-layer mode. Moreover, it was evidenced using both core levels and valence band experiments that, after back to room temperature, W(CO) 6 desorbs without significant decomposition. However, low energy (500 eV) ion (Ar +) irradiation can allow partial decomposition of tungsten hexacarbonyl molecules leading to sub-carbonyl tungsten molecules. The bonding of sub-carbonyl species to the TiO 2 surface was then stronger than the one of hexacarbonyl: these chemisorbed species do not desorb when going back to room temperature allowing a higher amount of metal to remain on the surface. Subsequent annealing gets rid of remaining carbonyl groups. This phenomenon leaves tungsten atoms at the TiO 2 surface. The annealing organizes these W atoms in the 400–600 K temperature range leading to WO x phases, which are almost free of carbon. The oxidation degree of these phases is strongly related to tungsten amount remaining at the surface.

Synthesis of tungsten–vanadium mixed oxides for ethanol partial oxidation

Vanadium–tungsten mixed oxides with various compositions were synthesized from a mixture of sodium metavanadate and sodium tungstate solution by sol–gel method. For comparison, pure tungsten oxide and pure vanadium oxide were synthesized. X-ray diffraction analyzes and Raman spectroscopy were employed to examine structure after calcination at 600 °C. In the cases of mixed oxides, new peaks were observed from X-ray diffraction analysis, which were not found in either pure tungsten oxide or pure vanadium oxide. X-ray photoelectron spectroscopy was used to determine oxidation states of vanadium and tungsten. The sample compositions were determined by energy dispersive spectroscopy (EDS) and correlated remarkably well to the V:W ratios in solution. Catalytic activities for partial oxidation of ethanol to acetaldehyde were investigated in a fixed-bed reactor and it was found that the catalytic activity was strongly dependent on composition. When small amount of tungsten was added into vanadium oxide, catalytic activity was improved, compared with pure vanadium oxide. Among synthesized samples, 5% WO 3–95% V 2O 5 mixed oxide showed highest catalytic activity.

Direct Synthesis and Structural Characteristics of Ordered SBA-15 Mesoporous Silica Containing Tungsten Oxides and Tungsten Carbides

A series Of WO3-SBA-15 materials with different Si/W ratios have been hydrothermally synthesized using tetraethyl orthosilicate (TEOS) as silica precursor, ammonium paratungstate as tungsten precursor, and EO20PO70EO20 (P] 23) as structure-directing reagent. After temperature-programmed carburization (TPC) in flowing CH4/H-2 (20/80 v/v mixture), the materials were converted to the corresponding WxC-SBA- 15 materials. The structure of the oxide and carbide materials has been characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), nitrogen adsorption -desorption measurements, Si-29 magic-angle spinning (MAS) NMR spectroscopy, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and thermogravimetric and differential scanning calorimetric analysis (TG-DSC) measurements. The results show that after hydrothermal synthesis using different amounts of tungsten and subsequent carburization, the materials retain the mesopore structure of SBA- 15. When Si/W = 30-15, the majority of the tungsten is dispersed in the channels of SBA-15 with the remainder being incorporated into the framework of SBA-15 with the formation of Si-O-W bonds. The tungsten carbide exists as a single W2C phase after carburization. At higher tungsten content (Si/W = 7.5), the amount of tungsten in the framework of SBA-15 increases with the formation of both Si-O-W bonds and W-O-W bonds. The tungsten carbide formed after carburization exists as a mixture Of W2C and WC phases. A model for the distribution of tungsten in SBA-15 is proposed involving three different tungsten species: alpha-W inside SBA-15 channels, beta-W embedded in the internal surfaces of the SBA- 15 channels, and gamma-W inside the framework of SBA- 15. After temperature-programmed carburization, alpha-W sites are transformed into W2C, whereas beta-W sites afford WC; in contrast, gamma-W sites show little change after carburization.

Electrodeposition of CoWP film: III. Effect of pH and temperature

Polarisation behaviour and nucleation mechanism of induced co-deposition of cobalt, tungsten and phosphorus from citrate electrolytes containing cobalt sulphate, sodium tungstate and sodium hypophosphite on copper disc electrode at various electrolyte pH values were studied using cyclic voltammetry and chronoamperometry, respectively. It was found that the onset potentials for reduction and oxidation gradually shifted towards more cathodic direction with increase in pH. Co-deposition of cobalt, tungsten and phosphorus occurred under diffusion control and followed instantaneous type of nucleation mechanism. The highest current efficiency (>70%) was found when the CoWP films were deposited from neutral or slightly acidic solution. Larger variation of film composition, cobalt from 48 to 74 at.%, tungsten from 13 to 38 at.% and phosphorus from 8 to 33 at.%, could be made by changing the pH of the electrolyte between 3 and 8. The films deposited from very acidic solutions contained larger amounts of tungsten and phosphorus and were amorphous; while the films deposited from near neutral solutions were polycrystalline and contained hexagonal cobalt. The surfaces of the amorphous films were smoother than those of the polycrystalline films. Needle-like dendritic crystallites were obtained when the films were deposited from basic solution or at elevated temperature.

Preparation and Characterization of Tungsten Carbide Confined in the Channels of SBA-15 Mesoporous Silica

A series of WO3/SBA-15 materials with different Si/W ratios have been prepared by impregnating the host material SBA-15 with aqueous ammonium paratungstate solutions. After temperature-programmed carburization (TPC) in flowing CH4/H2 (20/80 v/v mixture), the materials are converted to the corresponding W2C/SBA-15 species. Both the oxide and carbide materials are characterized using X-ray diffraction, nitrogen adsorption-desorption, 29Si NMR spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and TEM measurements. The XRD results show that after impregnation with different amounts of tungsten and subsequent carburization, the materials retain the mesopore structure of SBA-15. The nitrogen adsorption-desorption results indicate that a thin layer of W2C covers the internal walls of SBA-15. Quantitative 29Si single-pulse excitation MAS experiments and FTIR spectroscopy show that the incorporation of W2C in the channels of SBA-15 is correlated with the formation of Si-O-W bonds. Some Si-O-W bonds are transformed into Si-O-H bonds after carburization. The TEM results show that the thickness of the W2C thin layer is 1.7-1.9 nm in W2C/SBA-15. A model involving a discrete W2C thin layer in the channels of SBA-15 is proposed on the basis of the NMR data. The calculated thickness of the discrete W2C thin layer is consistent with value given by HRTEM.