- (W6I8)Cl4– A Basic Model Compound for Photophysically Active [(W6I8)L6]2–Clusters?
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The heteroleptic cluster compound (W6I8)Cl4was prepared by thermal conversion of the homoleptic clusters W6I12and W6Cl12at 700 °C to yield a bright yellow powder. The presence of the smaller chlorido ligands in apical positions of [(W6I8)Cl6]2–creates nearly spherically clusters showing thermal and chemical inertness. Photoluminescence studies revealed a strong red phosphorescence from excited spin-triplet states.
- Str?bele, Markus,Enseling, David,Jüstel, Thomas,Meyer, H.-Jürgen
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- Reduction of WO3 to W-metal by mechanochemical reaction
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A new non-thermal route for reduction of tungsten oxide (WO3) to metallic tungsten (W) by a milling operation in ammonia (NH3) gas atmosphere in the presence of lithium nitride (Li3N) is proposed in this paper. A sample of
- Kano, Junya,Kobayashi, Eiko,Tongamp, William,Miyagi, Shoko,Saito, Fumio
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- Reduction of tungsten oxides with carbon. Part 1: Thermal analyses
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The kinetics and mechanism of the reduction of WO3 with carbon (in the form of graphite and of lamp black) were studied using isothermal thermogravimetry of small sample masses (2 and the final product of the reduction was tungsten. The CO/CO2 ratio in the gaseous products had a considerable influence on the reactions occurring. The rate of the first stage of the reduction under isothermal conditions could be described by diffusion models, and is proposed to involve diffusion of CO(g) and CO2(g) through the pores of the reacting tungsten oxides. The activation energies of the graphite and lamp black systems differed significantly for this first stage of reduction (386 compared to 465 kJ mol-1). These activation energies are high for a diffusion process and may be inflated by changes in the structure of the product and the CO/CO2 equilibrium ratio as the temperature increases. The rate of the second stage of reaction can be described by a first-order rate equation, and it is proposed that the second stage of reaction is limited by the reaction of carbon with carbon dioxide, rather than by the reduction of a tungsten oxide. The measured activation energy of 438 kJ mol-1 is slightly higher than the reported values for the carbon-carbon dioxide reaction (up to 400 kJ mol-1).
- Venables, Dean S.,Brown, Michael E.
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- On the formation of defects and morphology during chemical vapor deposition of tungsten
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Face-to-face wafers were used to observe anomalous tungsten deposition in the gap-edge between wafers. In the WF6-H2 atmosphere, three regions are identified: (i) an open-deposition region (region A), (ii) a half-sealed deposition region (region B), and (iii) an etching or tunnel region (region C). In the WF6-Ar atmosphere, there are only two regions: (i) an open deposition region (region A'), and (ii) a half-sealed deposition region (region B'). The third region disappears because HF does not form in the absence of H2. Different chemical reactions are expected in different regions, dictated by the local gas composition. A half-sealed structure model proposed here is supported by thermodynamic calculations, and applied to explain encroachment, wormholes, and other well-known effects during the chemical vapor deposition of tungsten from tungsten hexafluoride.
- Wang,Cao,Wang,Zhang
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- Zirconium carbide-tungsten cermets prepared by in situ reaction sintering
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Zirconium carbide-tungsten (ZrC-W) cermets were prepared by a novel in situ reaction sintering process. Compacted stoichiometric zirconium oxide (ZrO2) and tungsten carbide (WC) powders were heated to 2100°C, which produced cermets with 35 vol% ZrC and 65 vol% W consisting of an interpenetrating-type microstructure with a relative density of ～95%. The cermets had an elastic modulus of 274 GPa, a fracture toughness of 8.3 MPa·m1/2, and a flexural strength of 402 MPa. The ZrC content could be increased by adding excess ZrC or ZrO2 and carbon to the precursors, which increased the density to >98%. The solid-state reaction between WC and ZrO2 and W-ZrC solid solution were also studied thermodynamically and experimentally.
- Zhang,Hilmas,Fahrenholtz
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- Preparation of tungsten and tungsten carbide submicron powders in a chlorine-hydrogen flame by the chemical vapor phase reaction
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A hydrogen-chlorine flame chemical vapor deposition reactor has been developed to synthesize ultrafine powders of refractory compounds (e.g. carbides and nitrides). At the laboratory scale, synthesis of tungsten and tungsten carbides (both (WC)1-x and α - W2C) gave encouraging results. The collected refractory powders do not have internal porosity, they exhibit spherical shape and have a very narrow size distribution. The size of the particles and the crystalline structure depends on the flame temperature, flow rate of the reactants and residence time of particles. In short, they depend on the flame characteristics. Thermochemical calculations were carried out to obtain the optimum conditions for different carbide powder synthesis. The flame is completely characterized and the temperature distribution within the reactor is obtained.
- Zhao, G. Y.,Revankar, V. V. S.,Hlavacek, V.
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- Structural evolution of W nano clusters with increasing cluster size
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We have recorded.the x-ray diffraction (XRD) patterns of nanometer-size W metal clusters prepared at different average cluster sizes. Nanometer-size W metal clusters were produced through a collision induced clustering mechanism of W metal atoms generated by decomposing W(CO)6 vapors. The XRD patterns clearly showed that structure changed from amorphous→face-centered-cubic (fcc)→body-centered-cubic (bcc) with increasing average cluster size. This implies that W metal clusters do not simply approach the bulk bcc structure but pass through an intermediate fcc structure before they reach the bulk structure, as predicted by Tomanek, Mukherjee, and Bennemann [Phys. Rev. B 28, 665 (1983)].
- Oh,Huh,Kim,Park,Lee
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- Carbon nanotubes produced by tungsten-based catalyst using vapor phase deposition method
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We have demonstrated that W-based catalysts can produce carbon nanotubes (CNTs) effectively. Well-aligned, high-purity CNTs were synthesized using the catalytic reaction of C2H2 and W(CO)6 mixtures. The CNTs had a multiwalled structure with a hollow inside. The graphite sheets of CNTs were highly crystalline but the outmost graphite sheets were defective.
- Lee, Cheol Jin,Lyu, Seung Chul,Kim, Hyoun-Woo,Park, Jong Wan,Jung, Hyun Min,Park, Jaiwook
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- Sc2O3-W matrix impregnated cathode with spherical grains
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Sc2O3-W matrix cathodes have been prepared by using a liquid-liquid doping method combined with high-temperature sintering. The microstructure and physical behavior of active substances of scandia-doped tungsten matrix and impregnated cathode has been studied by SEM and AES methods. The results show that the matrix has a homogeneous structure composed of W grains with spherical shape and superfine Sc2O3 particles dispersed uniformly over and among W grains. After impregnation, this Sc-type impregnated cathode has high emission capability. Space-charge-limited current density could reach 52 A/cm2 at 850 °Cb. The high emission results from a Ba-Sc-O active layer with a thickness of about 80 nm, which is formed at the cathode surface during the activation period. Both the decrease of the thickness of active surface layer and the decrease of the content of Sc at the surface could lead to the degradation of current density during operation.
- Wang, Jinshu,Li, Lili,Liu, Wei,Wang, Yanchun,Wang, Yiman,Zhou, Meiling
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- Electrodeposition of tungsten from ZnCl2-NaCl-KCl-KF-WO3 melt and investigation on tungsten species in the melt
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The electrodeposition of tungsten in ZnCl2-NaCl-KCl-KF-WO3 melt at 250 °C was further studied to obtain a thicker deposit. In the ordinary electrolysis at 0.08 V vs. Zn(II)/Zn, the current density decreased from 1.2 mA cm-2 to 0.3 mA cm-2 in 6 h. A thickness of the obtained tungsten layer was 2.1 μm and the estimated current efficiency was 93%. A supernatant salt and a bottom salt were sampled after 6 h from the melting and were analyzed by ICP-AES and XRD. It was found that the soluble tungsten species slowly changes to insoluble ones in the melt. The soluble species was suggested to be WO3F- anion. One of the insoluble species was confirmed to be ZnWO4 and the other one was suggested to be K2WO2F4. Electrodeposition was carried out under the same condition as above except for the intermittent addition of WO3 every 2 h. The current density was kept at the initial value and the thickness was 4.2 μm. The intermittent addition of WO3 was confirmed to be effective to obtain a thicker tungsten film.
- Nitta, Koji,Nohira, Toshiyuki,Hagiwara, Rika,Majima, Masatoshi,Inazawa, Shinji
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- Study on single step solid state synthesis of WC@C nanocomposite and electrochemical stability of synthesized WC@C & Pt/WC@C for alcohol oxidation (methanol/ethanol)
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WC@C nano composite was prepared by a single step solid-state reaction through in situ reduction and carburization of WO3 in the presence of Mg and activated charcoal. The XRD results and thermodynamics analysis showed that the optimization of reaction temperature facilitates the reduction as well as carburization of tungsten oxide(s) at different reaction temperature. Thermogravimetric analysis of the product was done to assess the thermal stability in air. The Raman spectroscopy was used to find out the nature (amorphous/graphitic) of carbon in the obtained phase. The N2 adsorption-desorption measurement showed a narrow pore size distribution from 3 to 4 nm with BET surface area of up to 522.5 m2/g. TEM/HRTEM images confirmed formation of the WC nano particles with spherical morphology. Electrochemical stability of pure and platinized carbide sample (Pt/WC) has been investigated using cyclic voltammetry in acidic media for alcohol (methanol and ethanol) oxidation.
- Singla, Gourav,Singh,Pandey
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- Kinetic study of tungsten atoms (a 7S3 and a 5DJ) in the presence of C2H4 and NH3 at room temperature
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The gas-phase reactivity of W (a 7S3 and a 5DJ) with C2H4 and NH3 at room temperature was investigated using a time-resolved laser induced fluorescence (LIF) spectroscopy. Tungsten atoms were produced by a 266-nm multiphoton decomposition (MPD) of W(CO)6. The reactant pressure dependence of the pseudo-first-order depletion rates of W (a 7S3) could allow an estimation of the pseudo-second-order depletion rate constant of W (a 7S3), (4.5 ± 0.5) × 10-10 cm3 molecule-1 s-1 for C2H4 and (0.73 ± 0.10) × 10-10 for NH3 at 6.0-Torr total pressure with an Ar buffer. A simulation of the transient curves based on a modification of the observed apparent decay rate constants, involving nearby a 5DJ states in the presence of C2H4 and NH3, allowed us to separately estimate the contribution of the chemical quenching (W (a 7S3) + R → product(s)) and the physical quenching (W (a 7S3) + R → W (a 5D1) + R) processes. In the case of C2H4, chemical quenching appeared to dominate over the physical quenching, while the physical quenching was the main depletion process in the case of NH3. The large reactivity of the W (a 7S3) state not only for C2H4, but also for NH3, is discussed in terms of the relativistic effects.
- Ishikawa, Yo-ichi,Matsumoto, Yoshitaka
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- Synergy between tungsten and palladium supported on titania for the catalytic total oxidation of propane
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Titania-supported palladium catalysts modified by tungsten have been tested for the total oxidation of propane. The addition of tungsten significantly enhanced the catalytic activity. Highly active catalysts were prepared containing a low loading of 0.5 wt.% palladium, and activity increased as the tungsten loading was increased up to 6 wt.%. Catalysts were characterised using a variety of techniques, including powder X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction and aberration-corrected scanning transmission electron microscopy. Highly dispersed palladium nanoparticles were present on the catalyst with and without the addition of WOx. However, the addition of WOx slightly increases the average palladium particle size, and there was some evidence for the Pd forming epitaxial islands on the support in the tungsten-doped samples. Surface analysis identified a combination of Pd0 and Pd2+ on a Pd/TiO2 catalyst, whereas all of the Pd loading was found in the form of Pd2+ with the addition of tungsten into the catalysts. At low tungsten loadings, isolated monotungstate and some polytungstate species were highly dispersed over the titania support. The concentration of polytungstate species increased as the loading was increased, and it was also promoted by the presence of palladium. The coverage of the highly dispersed tungstate species over the titania also increased as the tungsten loading increased. Some tungstate species were also found to be associated with the palladium oxide particles, and there was an enrichment of oxidised tungsten species at the peripheral interface of the palladium oxide nanoparticles and the titania. Sub-ambient temperature-programmed reduction experiments identified an increased concentration of highly reactive species on catalysts with palladium and tungsten present together, and we propose that the new WO x-decorated interface between PdOx and TiO2 particles may be responsible for the enhanced catalytic activity in the co-impregnated catalysts.
- Taylor, Marie N.,Zhou, Wu,Garcia, Tomas,Solsona, Benjamin,Carley, Albert F.,Kiely, Christopher J.,Taylor, Stuart H.
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- Study of the preparation of bulk powder tungsten carbides by temperature programmed reaction with CH4 + H2 mixtures
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The synthesis of bulk tungsten carbides by carburization of W metal or of WO3 with mixtures of CH4 in hydrogen at various pressures has been studied in temperature programmed experiments. The resulting solids have been characterized by elemental analysis, X-ray diffraction, XPS analysis, and specific surface area measurements. The carburization occurs in two distinct steps: W2C is formed in the first step taking place at about 650°C at atmospheric pressure with a 20% CH4-H2 mixture, while the formation of WC occurs only at higher temperatures. During carburization some free carbon is deposited, the importance of which is very much dependent on CH4 partial pressure and on the temperature of carburization. It has also been shown that direct carburization of WO3 by CH4-H2 does not take place, but that the carburization occurs via the reduction of WO3 to W metal. The rate of reduction of WO3 and that of carburization of W metal are very much dependent on, respectively, hydrogen partial pressure and CH4 partial pressure. The extent of reduction of WO3 into W metal required for carburization which takes place also depends on CH4 partial pressure, indicating a competition between carburization of W metal at the surface and diffusion of W metal into the bulk of the Solid.
- Leclercq,Kamal,Giraudon,Devassine,Feigenbaum,Leclercq,Frennet,Bastin,Loefberg,Decker,Dufour
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- Adsorption and desorption of Se on Si(100)2×1: Surface restoration
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This work entails a study of the adsorption of elemental Se on the reconstructed Si(100)2×1 surface. The investigation took place in an ultra high vacuum (UHV) by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and work function (WF) measurements. The adsorption of one monolayer (1 ML) of Se at room temperature (RT) causes the transition of the reconstructed Si(100)2×1 surface to its original bulk terminated Si(100)1×1 configuration, while Se adatoms form a 1×1 structure by breaking the Si-Si dimer bonds. The Si-Se bond is strong (Eb = 2.97 eV/atom), resulting in the formation of a SiSe compound. Above 1 ML, Se forms a SiSe2 compound with Eb = 2.67 eV/atom. The heating that follows causes the desorption of Se up until 1000 K, where ΘSe = 0.5 ML, and the Si(100)1×1 structure is changed back to the reconstructed Si(100)2×1 with the Se forming a 2×1 structure. The models of Se(1×1)/Si(100)1×1 and of the Se(2×1)/Si(100)2×1 structures are given.
- Papageorgopoulos,Kamaratos
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- STRUCTURE OF LPCVD TUNGSTEN FILMS FOR IC APPLICATIONS.
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Because of the widespread potential use of CVD tungsten films in integrated circuit applications, the structural properties of these films were examined over a range of deposition conditions and with different nucleating layers. When tungsten is deposited on a chromium nucleating layer on an oxidized silicon wafer, the stress is not a strong function of film thickness, and most of it is caused by the difference in the thermal coefficients of expansion of tungsten and silicon. The surface roughness increases almost linearly with increasing film thickness. The grain size also increases with increasing film thickness, with the diameters of the largest grains comparable to the film thickness. The structure is equiaxed, rather than columnar.
- Kamins,Bradbury,Cass,Laderman,Reid
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- Mechanism for selectivity loss during tungsten CVD
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We have investigated possible mechanisms for the loss of selectivity (i.e., deposition on silicon dioxide) during tungsten CVD by reduction of tungsten hexafluoride and found strong evidence that selectivity loss is initiated by desorption of tungsten sub
- Creighton
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- Recovery of tungsten from the exhaust of a tungsten chemical vapor deposition tool
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An ETC DryScrub plasma scrubber has been successfully tested for the capturing and recovery of metallic tungsten from the exhaust of a W chemical vapor deposition (CVD) tool. The scrubber operation was completely transparent to the upstream CVD process. The WF6 destruction removal efficiency of the scrubber was determined with a quadrupole mass spectrometer. The system had been tested with two different plasma frequencies: 100 and 40 kHz. With the 100 kHz frequency, the destruction efficiency of WF6 reached an initial value of 98% at a nominal dissipated power of 1200 W. However, the layer of W deposited on the scrubber electrodes contained hydrates of tungsten oxides. Moreover, the destruction efficiency of WF6 dropped to less than 70% after eight consecutively processed wafers (`memory effect'). Introducing an intermediate H2 plasma treatment ensured a continuously high efficiency, and improved significantly the purity of the deposited W layer in the scrubber. With the 40 kHz power supply, the maximum efficiency reached is more than 99% from a nominal dissipated power of 1100 W on. The purity of the deposited W layer is high (>99%). No memory effect was observed. Successful marathon runs have been performed with each tested frequency.
- Van Hoornick, Nausikaae B. H.,Van Hoeymissen, Jan A. B.,Heyns, Marc M.
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- LOW PRESSURE CHEMICAL VAPOR DEPOSITION OF TUNGSTEN ON POLYCRYSTALLINE AND SINGLE-CRYSTAL SILICON VIA THE SILICON REDUCTION.
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High purity metallic tungsten films are deposited on phosphorus-doped and undoped polycrystalline and single-crystal silicon by the silicon reduction of WF//6. Depositions are performed in a commercial LPCVD hot wall reactor at temperatures ranging from 310 degree -540 degree C. Film formation is self-limiting, meaning that after a given film thickness any further reaction between WF//6 and the underlying silicon is inhibited. Obtainable film thickness depends strongly on the doping condition of silicon and on surface preparation prior to LPCVD. Conventional wet chemical cleaning limits the maximum obtainable film thickness to approximately 400A, whereas with a low power argon plasma treatment approximately 900A thick films can be obtained reproducibly. The resistivity of these films is 18. 3 plus or minus 4. 5 mu OMEGA cm.
- Tsao,Busta
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- Influence of deposition parameters on the texture of chemical vapor deposited tungsten films by a WF6/H2/Ar gas source
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In order to investigate the relationship between the film texture and the deposition parameters including gas composition, temperatures, and substrate materials, polycrystalline tungsten films were prepared by reduction of tungsten hexafluoride with hydrogen. The experimental results show that the orientations of the textured films correlate closely with WF6 concentrations in the gas mixture and deposition temperatures. High WF6 concentrations enhance the formation of the 〈100〉 textured films, while low WF6 concentrations facilitate the formation of the 〈111〉 textured films. The 〈111〉 texture, usually observed in tungsten films deposited at high temperatures (≥400 °C) and high ratios of H2 to WF6, can also form at low deposition temperatures (370 °C) when the WF6 concentration used is low enough (0.6 vol %). The WF6 concentration for obtaining a 〈100〉 textured film increases with increasing deposition temperatures. On the other hand, the concentrations of hydrogen gas have minimal influence on the formation of the film textures. Therefore, in terms of gas composition, the WF6 concentration, not the ratio of H2 to WF6, is a compact factor which directly dominates the texture of the films. The effect of substrate on the film texture was only found in the initial growth stage. The crystals formed at the beginning of the film growth were randomly oriented. A preferential orientation became pronounced with increasing film thickness through the competitive crystal growth.
- Chang,Hon
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- Oxidation resistance of hafnium diboride ceramics with additions of silicon carbide and tungsten boride or tungsten carbide
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Dense samples of HfB2-SiC, HfB2-SiC-WC, and HfB 2-SiC-WB were prepared by field-assisted sintering. The WB and WC additives were incorporated by solid solution into the HfB2 and the HfC that formed during sintering. Oxidation of the samples was studied using isothermal furnace oxidation between 1600° and 2000°C. Sample microstructure and chemistry before and after oxidation were analyzed by scanning electron microscopy and X-ray diffraction. The addition of WC and WB did not alter oxidation kinetics of the baseline HfB2-SiC composition below 1800°C; however, at 2000°C, HfB2-SiC-WC and HfB 2-SiC-WB had oxide scales that were 30% thinner than the oxide scale of HfB2-SiC. It is believed that WC and WB promoted liquid-phase densification of the HfO2 scale, thereby reducing the path of oxygen ingress, during oxidation.
- Carney, Carmen M.,Parthasarathy, Triplicane A.,Cinibulk, Michael K.
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- EFFECTS OF CHEMICAL OXIDE ON THE DEPOSITION OF TUNGSTEN BY THE SILICON REDUCTION OF TUNGSTEN HEXAFLUORIDE.
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The effects of thin (chemical) oxide grown during the chemical cleaning of silicon wafers on the silicon reduction of tungsten hexafluoride have been investigated. Unlike tungsten deposition on samples without the chemical oxide, deposition thickness on t
- Wong,Kobayashi,Browning,Paine,Saraswat
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- Temperature-programmed and X-ray diffractometry studies of hydrogen-reduction course and products of WO3 powder: Influence of reduction parameters
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The hydrogen-reduction course and products of synthetic tungsten(VI) oxide (WO3) were examined by means of temperature-programmed reduction (TPR) and X-ray powder diffractometry (XRD) studies. A set of model tungsten compounds was procured and examined similarly for reference purposes. Results obtained could help resolving two subsequent reduction stages: (i) a low-temperature stage (3 is reduced to the tetravalent state (WO2) via formation and subsequent reduction of intermediate WO2.96, WO2.9, WO2.72 oxides; and (ii) a high-temperature stage (>1050 K) through which WO2 thus produced is reduced to the metallic state (Wo) via two intermediate oxide species (tentatively, WO and W2O-W3O). Reduction events involved in the high-temperature stage were found to be relatively more sensitive to the reduction parameters; namely, the starting oxide mass, heating temperature and rate, and gas flow rate and composition. They were also found to require lower activation energies than those required by events occurring throughout the low-temperature stage, a fact that may suspect compliance of the high-temperature reduction events to autocatalytic effects.
- Zaki,Fouad,Mansour,Muftah
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- Dissociation equilibria of tungstate ion in a molten fluoride and the effect on electrodeposition
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The dissociation equilibria of tungstate ion in a molten fluoride system have been investigated using a zirconia-air oxide ion indicator. The dissociation constant, K, of the reaction in molten LiF-KF eutectic at 973 K has been determined to be K = 5(±1) × 10-12 (mole fraction unit). With this taken into consideration, tungsten has been electrodeposited from molten LiF-KF-K2WO4 under several sets of electrolytic conditions, with addition of small amounts of Na2O. The texture of the electrodeposit was nodular or mossy, the surface becoming rougher with increasing concentration of oxide ion (up to about 0-3 × 10-3 mole fraction). When the oxide ion concentration in the molten fluoride was high (1 × 10-2mole fraction), however, the surface of the electrodeposit became uniform.
- Yabe,Ema,Ito
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- Diffusion barrier properties of tungsten nitride films grown by atomic layer deposition from bis(tert-butylimido)bis(dimethylamido)tungsten and ammonia
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The synthesis of the highly uniform, smooth and conformal coatings of tungsten nitride (WN) using atomic layer deposition (ALD) from vapors of bis(tert-butylimido)bis(dimethylamido)tungsten and ammonia was discussed. The diffusion barrier properties of th
- Becker, Jill S.,Gordon, Roy G.
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- Size control of tungsten powder synthesized by self-propagating high temperature synthesis process
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Tungsten powder was prepared by self-propagating high temperature synthesis (SHS) from a mixture of WO3 and Mg. The MgO in the product was leached with an HCl solution. The complete reduction of WO3 required a 33% excess of magnesium over the stoichiometric molar ratio Mg/WO3 of 3. The tungsten product had a purity of 99.98%, which was higher than that of the reactants. The high purity resulted because the impurities in the reactants were volatilized during the highly exothermic reaction and dissolved during leaching of the product. Size distribution and the shape of the tungsten particles produced was affected by compaction pressure on the green pellet.
- Won, Chang-Whan,Jung, Joong-Chai,Ko, Seog-Gueon,Lee, Jong-Hyeon
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- Investigations of nonstoichiometric tungsten oxide nanoparticles
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Recently, pure phases of hollow multi-walled WS2 nanotubes were prepared from a surface reaction with reduced tungsten oxide nanowhiskers. During the process, which starts with WO3-x nanoparticles and finishes with WS2 nanotubes, particular care has been devoted to the evolution of the tungsten oxide cores once the first encapsulating WS2 layer has been formed. The reduced tungsten oxide phases were previously studied by a combination of techniques, including high-resolution transmission electron microscopy and X-ray diffraction. In the present study, Raman spectroscopy combined with the previous two techniques is used to give further detail concerning the structure of the reduced oxide phases. This study sheds some further light on the reduction process of the tungsten suboxide phases and the growth mechanism of oxide nanowhiskers and subsequently WS2 nanotubes from quasi-isotropic tungsten oxide nanoparticles.
- Frey,Rothschild,Sloan,Rosentsveig,Popovitz-Biro,Tenne
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- Tungsten deposition on quartz by the reaction of WF6 and hydrogen
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The rate of deposition of tungsten on quartz was determined with the aim of clarifying the mechanism of losing the selectivity of tungsten deposition on Si/SiO2 substrates. The gaseous species above a quartz substrate proceeding during the deposition reaction were identified by the use of a mass spectrometer. From the experimental results described, it is concluded that in order to obtain good selective deposition of tungsten on Si/SiO2 substrates, the partial pressure of WF6 on substrates should be as small as possible and that of H2 should be as large as possible.
- Kato,Itsumi
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- ENTHALPY OF FORMATION OF PENTACARBONYL(TRIPHENYLPHOSPHINE) TUNGSTEN AND OF PENTACARBONYL(METHOXY(PHENYL)METHYLENE)TUNGSTEN
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Microcalorimetric studies on the sublimation, thermal decomposition and bromination of W(CO)6, and of the complexes and > have provided standard enthalpies of formation (in kJ mol-1) of the crystalline and vapour
- Al-Takhin, Ghassan,Connor, Joseph A.,Pilcher, Geoffrey,Skinner, Henry A.
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- Solid state synthesis of tungsten carbide nanorods and nanoplatelets by a single-step pyrolysis
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We report a simple and efficient single-step synthesis of tungsten carbide nanorods and nanoplatelets by direct pyrolysis of a hybrid composite material of 12-tungstophosphoric acid and hexadecyltrimethylammonium bromide in a closed Swagelok cell at 1000?°C. The product was characterized by XRD, TGA, SEM, TEM, XPS, and CV. The diameter of the nanorods is 30-50 nm, and the length varies from 200 to 500 nm. The size of the platelets is around 55 nm. The WC exhibits an interesting structural surface with kinks, steps, and terraces which is evidenced by HRTEM studies. ? 2005 American Chemical Society.
- Shanmugam, Sangaraju,Jacob, David S.,Gedanken, Aharon
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- Characterization of selective tungsten films prepared by photo-chemical vapor deposition
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Selective photo-chemical vapor deposition (CVD) of tungsten films decomposed by direct photoexcitation of WF6 have been studied. Film deposition rate increased with increasing temperature but was only slightly dependent on WF6 gas concentration. The selectivity deteriorated with increasing deposition temperature, WF6 concentration, and deposition time. Typically, in order to achieve selectivity, the flow rate of WF6 must be lower than 35 sccm and the deposition temperature must be lower than 230°C. No encroachment and self-limited thickness problems were found as in the low-pressure chemical vapor deposition method. In general, tungsten films prepared by photo-CVD were amorphous as observed by x-ray diffraction analysis. After annealing, the tungsten had a polycrystalline structure with a resistivity of 18 μΩ-cm.
- Fang,Hwang,Sun
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- KINETICS OF LPCVD TUNGSTEN DEPOSITION IN A SINGLE WAFER REACTOR.
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The rate equation for the low pressure chemical vapor deposition (LPCVD) of tungsten from tungsten hexafluoride on (100) silicon was experimentally determined in a laboratory scale single wafer vacuum reactor. The tungsten film deposition is initiated by the silicon reduction of tungsten hexafluoride. In the absence of hydrogen, the silicon reduction results in a 10-40 nm self-limiting tungsten deposit, with the thickness dependent upon the native oxide layer prior to deposition. The hydrogen reduction of tungsten hexafluoride is one-half order in hydrogen, zero order in tungsten hexafluoride, and has an activation energy of 73,000 J mol** minus **1 at temperatures from 561 to 683 K, and pressures from 0. 067-1. 3 kPa (0. 5 to 10 torr).
- McConica,Krishnamani
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- Vacuum annealing of nanocrystalline WC powders
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The effect of vacuum annealing temperature on the chemical and phase compositions, particle size, and lattice strain of nanocrystalline tungsten carbide (WC) powders with a particle size from 20 to 60 nm has been studied by X-ray diffraction and electron microscopy. The results demonstrate that vacuum annealing of WC nanopowders at tann ≤ 1400°C is accompanied by a marked decrease in carbon content and changes in phase composition due to carbon desorption from the surface of the powder as a result of the interaction of carbon with oxygen impurities. In addition, annealing leads to an increase in particle size due to coalescence of aggregated nanoparticles and reduces the lattice strain of the powder. Pleiades Publishing, Ltd., 2012.
- Kurlov,Gusev
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- Freestanding nanostructures for three-dimensional superconducting nanodevices
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Free-space nanostructures are the fundamental building blocks of three-dimensional (3D) nanodevices with multi-functionality beyond that achievable by planar devices. Here we developed a reliable technique for the site-specific post-growth geometrical manipulation of freestanding superconducting nanowires using ion-beam irradiation with nanometer-scale resolution to fabricate uniformly shaped and sized clean-surface 3D nanostructures. Such structures could integrate with conventional superconducting quantum interference devices to detect magnetic fields both parallel and normal to the substrate. Property characterizations suggest that our focused-ion-beam technique allows tailoring of freestanding superconducting loops for size and geometry, potentially for lab-on-chip experiments.
- Cui, Ajuan,Li, Wuxia,Luo, Qiang,Liu, Zhe,Gu, Changzhi
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- Nucleation on SiO2 during the selective chemical vapor deposition of tungsten by the hydrogen reduction of tungsten hexafluoride
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A horizontal hot-wall chemical vapor deposition (CVD) quartz reactor with rectangular cross section was used to study the effect of different process conditions on the nucleation of tungsten on SiO2 during selective WCVD by the H2 reduction of WF6. The experimental procedure included placing a metallic surface at the center of the reactor, and small samples of SiO2 at different positions both upstream and downstream with respect to the metallic surface. Digitized scanning electron microscopy micrographs were used to determine the particle size distributions of nuclei on the SiO2 surfaces. We found that the amount of nucleation on SiO2 decreases when smaller metallic surfaces are present and for lower temperatures and shorter process times. Although nucleation was always greatest on SiO2 samples closest to the metal sample, the effect of flow rate depended on the position of the SiO2. A statistical nearest neighbor analysis indicated a clustering of W nuclei on the SiO2. A simplified mathematical model was developed to predict concentration profiles of a gaseous intermediate generated at the metal surface during the thermal decomposition of the source gas. This intermediate has been proposed as being the reactive species that causes nucleation on SiO2 surface. Qualitative agreement between experimental and theoretical results reinforce the proposed role of the intermediate with this species being characterized by a short lifetime.
- Desatnik,Thompson
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- KINETICS AND MECHANISM OF SELECTIVE TUNGSTEN DEPOSITION BY LPCVD.
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Tungsten films have been deposited selectively on oxide-patterned silicon wafers by the H//2 reduction of WF//6 in WF//6-H//2 and WF//6-H//2-Ar gas flow systems. The deposition rate of films was investigated as a function of reactant partial pressures. The reaction orders with respect to WF//6 and H//2 are zero and one-half, respectively Under given experimental conditions, the growth rate of the selectively deposited W films was reduced by 32% when the deposition area increased by a factor of 10-100. This decrease in growth rate can be attributed to the effect of HF on the surface reaction. The selective nature of the process deteriorates with increasing deposition rate, WF//6 partial pressure, H//2 partial pressure, or deposition time.
- Pauleau,Lami
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- Electrolytes for tungsten refining
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Chloride-fluoride and phosphate-fluoride salts of alkali metals with additions of sodium tungstate and tungstic anhydride are studied as electrolyte systems for tungsten refining. The results demonstrate that the melts of these systems are suitable for el
- Pavlovskii
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- EVALUATION OF THE SELECTIVE TUNGSTEN DEPOSITION PROCESS FOR VLSI CIRCUIT APPLICATIONS.
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Tungsten films have been selectively deposited on oxide-patterned silicon wafers using two successive deposition steps, i. e. , the Si reduction of WF//6 followed by the H//2 reduction of WF//6. The deposition process was performed in a horizontal hot wall LPCVD reactor. In this reactor, the surface of vertical wafers was parallel to the axis of the quartz tube. The thickness of W films obtained for a given deposition time was found to be dependent on the 'history' (or the cleanliness) of the CVD reactor. The thickness uniformity across a wafer and from wafer to wafer was determined as a function of the wafer position in the CVD reactor. The selectivity of the process was evaluated by measuring the W nuclei density on the oxide (PSG) mask as a function of the thickness of W films.
- Lami,Pauleau
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- Nanoscale electron-beam-stimulated processing
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Electron beam stimulated deposition and etching were investigated. These processes find application in nanoscale selective processing. During etching of silicon and silicon dioxide, inelastic scattering of the electron beam with the gas was found to take
- Rack,Randolph,Deng,Fowlkes,Choi,Joy
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- Electrodeposition of metallic tungsten films in ZnCl2-NaCl-KCl-KF-WO3 melt at 250 °C
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WO3 hardly dissolved in ZnCl2-NaCl-KCl (0.60:0.20:0.20, in mole fraction) melt at 250 °C. The solubility of WO3 increased by the addition of KF to the melt, which enabled electrodeposition of metallic tungsten. A smooth and fine film of metallic tungsten was obtained on a nickel substrate by the potentiostatic electrolysis at 0.06 V versus Zn(II)/Zn. The thickness reached ca. 2.5 μm after 6-h electrolysis. The surface of the nickel contact probe pin manufactured by the conventional LIGA (Lithographie-Galvanoformung-Abformung, German abbreviation) process was successfully coated with a smooth and adhesive tungsten film by using this melt.
- Nakajima, Hironori,Nohira, Toshiyuki,Hagiwara, Rika,Nitta, Koji,Inazawa, Shinji,Okada, Kazunori
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- Reactivity of hexanes (2MP, MCP and CH) on W, W2C and WC powders. Part II. Approach to the reaction mechanisms using concepts of organometallic chemistry
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The reactivity of 2-methylpentane (2MP), methyleyelopentante (MCP) and cyclohexane (CH) on reproducible and well-characterized surfaces of W, W2C and WC has been studied. A great deal of effort was put on the characterizations by physisorption, chemisorption and photoemission spectroscopy. The results obtained with these reference materials can be used for comparison with those appearing in the literature and sometimes debated because of the inconstancy of the active states due to non-stoichiometric compositions (excess of carbon, decarburization, oxidation by oxygen impurities). In agreement with the work of Boudart et al. [F.H. Ribeiro, R.A. Dalla Betta, M. Boudart, J. Baumbgartner, E. Iglesia, J. Catal. 130 (1991) 86; F.H. Ribeiro, M. Boudart, R.A. Dalla Betta, E. Iglesia, J. Catal. 130 (1991) 498; E. Iglesia, J.E. Baumgartner, E.H. Ribeiro, M. Boudart, J. Catal. 131 (1991) 523; E. Iglesia, F.H. Ribeiro, M. Boudart, J.E. Baumgartner, Catal, Today 15 (1992) 307.] we confirmed that reforming reactions do not take place in the temperature range 80-400°C and four experimental conditions. They dehydrogenation of cyclohexane appeared only when a strong position by carbonaceous residues occurred. For 2-methylpentane and methylcyclopentane the extensive hydrogenolysis character of WC is higher than for W2C and WC surfaces are interpreted by different possible reaction intermediates deduced from concepts of organometallic chemistry.
- Hemming,Wehrer,Katrib,Maire
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- Plasma synthesis of tungsten carbide nanopowder from ammonium paratungstate
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A thermal plasma process has been applied to the synthesis of nanosized tungsten carbide powder with ammonium paratungstate (APT) as the precursor. The reduction and carburization of vaporized APT produced nanosized tungsten carbide (WC1-x) pow
- Ryu, Taegong,Sohn, Hong Yong,Hwang, Kyu Sup,Fang, Zhigang Z.
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- A reflectometric study of the reaction between Si and WF6 during W-LPCVD on Si and of the renucleation during the H2 reduction of WF6
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The formation of W through the reduction of WF6 by Si is monitored in situ using a wavelength adjustable reflectometer. The reflectance-time relation can be understood and modeled by assuming island growth and a statistical distribution of the island thickness. The model is supported by SEM and Auger observations. The effect of surface layers like native oxides or a plasma treatment on the inhomogeneous Si consumption by the reaction between Si and WF6 (gouging) and its effect on the reflectance-time relation are understood. The model is also applicable in the case of renucleation during the H2 reduction of WF6. A renucleation step consists of the deposition of SiH4 followed by the Si consumption by WF6. A renucleation step reduces the surface roughing which occurs during the H2 reduction process.
- Holleman,Hasper,Middelhoek
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- A study of formation of nanometric W by room temperature mechanosynthesis
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Nanometric particles of elemental tungsten can be produced by solid state room temperature reduction of tungsten (VI) oxide with magnesium metal. The reaction is driven by high-energy ball milling powder mixtures of WO 3 and Mg and involves a thermite route that allows for very short reaction times (as small as 8 min) under the proper milling conditions. The evolution of the reaction was followed as a function of time by X-ray diffraction, showing a gradual decrease in crystallite size of the reactant WO3, with formation of intermediate reaction products, until a near-instantaneous reaction takes place leaving only MgO and elemental W as final products. MgO was removed by leaching in diluted HCl and high purity tungsten (purity >99%) was obtained with a typical molar yield of 84%. The W powders are formed of impact welded aggregates of small particles of homogeneous dimension of about 70-100 nm, the typical crystallite size is 19 nm.
- Ricceri, Riccardo,Matteazzi, Paolo
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- Tungsten fluorides: Syntheses and electrochemical characterization in the FLINAK molten salt eutectic
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The following tungsten fluorides have been synthesized by simple addition reactions or by reduction with tungsten metal at elevated temperature: KWFT7, K2WF8, MWF6 (M = K, Na, Rb, Cs), K2WF7, M3WF8 (M = K, Na, Rb), and K3WF6. The compounds were characterized by their Raman spectra and by cyclic voltammetry in the molten FLINAK eutectic melt (46.5, 11.5, and 42.0 mol % of LiF, NaF, and KF, respectively) at 475-800 °C. X-ray crystal structures are reported for two new compounds K2WF7 and K3WF6. The crystals of K2WF7 were orthorhombic, space group Pnma (No. 62) with a = 9.800(2) A, b = 5.7360(11) A, c = 11.723(2) A, and Z = 4. Crystals of K3WF6 were cubic, space group Fm3 (No. 225) with a = b = c = 8.9160(10) A, Z = 4. Electrodeposition of tungsten metal on Pt from FLINAK, prepared by the addition of WF6 gas and metallic tungsten to the melt, is suggested to result from reduction of an equilibrium mixture of WF83- and WF63-.
- Eklund,Chambers,Mamantov,Diminnie,Barnes
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- XPS studies on surface reduction of tungsten oxide nanowire film by Ar + bombardment
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WO3 nanowire film was bombarded by Ar ion beam in the analysis chamber of an X-ray photoelectron spectroscopy (XPS) system to produce uniform tungsten cone arrays. The WO3 nanowire film itself served as an etching mask during the Ar+ bombardment. The changes of surface chemical states and electronic structures during bombardment were monitored by in situ XPS. The morphological evolution with different Ar+ bombardment time was observed by ex situ scanning electron microscopy (SEM). At the start of Ar+ bombardment partial W6+ in WO3 was reduced to W5+ immediately, subsequently to W4+ and then to Wx+ (intermediate chemical state between W4+ and W0), finally to W0. Multiple oxidation states of tungsten coexisted until finally only W0 left. SEM images showed that the nanowires were broken and then fused together to be divided into clusters with a certain orientation after long-time high-energy ion beam bombardment. The mechanism of the ion-induced reduction during bombardment and the reason of the orientated cone arrays formation were discussed respectively.
- Xie,Gong,Liu,Tao,Zhang,Chen,Meng,Chen
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- Structural Transformations of [CuEn 3]WO4 Complex Salt in the Range 100–390 K and Its Degradation to [CuEn 2](WO4)·2H2O
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The crystal structure of [CuEn3]WO4 (En is ethylenediamine) is studied in the temperature range 100–390 K. Crystallographic data at 100 K are: a = 27.6903(8) ?, c = 9.9405(3) ?, space group P3?, V = 6600.8(4) ?3, Z = 18. Copper atomic coordination is a distorted square bipyramid. Four short Cu–N distances are within 2.038(5)–2.110(6) ?; two long distances are within 2.374(8)–2.514(7) ?. The lengths of N–H…O interionic contacts lie within 1.96–2.17 ?. A temperature elevation makes the Cu–N distances equal: at 298 K they are within 2.066(2)–2.256(3) ?(a = 16.0391(8) ?, c = 9.9608(6) ?, space group P3?c1, V = 2219.1(3) ?3, Z = 6), and at 390 K they are 2.151(6) ?(a = 9.2986(10) ?, c = 10.0520(14) ?, P3?1c, V = 752.7(2) ?3, Z = 2). In the range from 100 K to 390 K the average W–O distances decrease from 1.776 ? to 1.734 ?. Hirshfeld surfaces of complex cations are analyzed. It is shown that with increasing temperature the number of interionic N–H…O contacts decreases. The [CuEn3]WO4 phase is found to be unstable and on storing in air it transforms into [CuEn2](WO4)·2H2O.
- Khranenko,Sukhikh,Komarov, V. Yu.,Gromilov
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p. 1790 - 1798
(2019/12/24)
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- Preparation of tungsten carbides by reducing and carbonizing WO2 with CO
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Tungsten carbides, which can be widely used as cutting and drilling tools, chemical catalysts and aerospace coatings, have attracted widespread attentions. The present work reported a simple method to prepare tungsten carbides by using WO2 as the raw materials and CO as the reduction and carbonization agent. It was found that at lower and moderate temperatures, all the final product was WC, while at higher temperatures, the final products were W2C or W. The reduction and carbonization mechanism was also studied theoretically and experimentally. WO2 was reduced and carburized to WC in one step at lower temperatures; WO2 was first reduced and carburized to W2C and then to WC at moderate temperatures; while at higher temperatures, WO2 was first reduced to metallic W and then carbonized to WC or W2C, but with the temperature increasing, the carbonization became difficult and required much higher CO content.
- Dang, Jie,Wu, Yijie,Lv, Zepeng,Lv, Xuewei
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p. 421 - 429
(2018/02/28)
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- Novel aluminum-graphene and aluminum-graphite metallic composite materials: Synthesis and properties
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A novel method of creating new lightweight, aluminum-metallic, composite materials under halides melt at temperatures 973-1073 K under air atmosphere is proposed. The method for synthesizing aluminum-based metallic composite materials, containing up to 2 wt. % graphene sheets uniformly distributed in a metal matrix, is entirely new, having no analogies in current science and practice. The synthesis of graphene nanosheets in a metal matrix is one-step, simultaneous process, taking place directly in molten aluminum under alkali halides melt without the necessity of a separate stage of synthesis and introduction of graphene. This has the potential to facilitate the inexpensive synthesis of aluminum-graphene composites with a high concentration of graphene. The aluminum-graphene composites formed according to this method are characterized by a high uniformity of graphene films with linear dimensions from 100 nm to 50 μm and a thickness from one to three nm in the metal bulk. No aluminum carbide forms under synthesis; the aluminum-graphene and aluminum-graphite composites are resistant to corrosion in NaCl solution. The hardness, strength and ductility of aluminum-graphene composites are at least 2-3 times higher than the initial aluminum material, proportional to the concentration of graphene.
- Yolshina,Muradymov,Korsun,Yakovlev,Smirnov
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p. 449 - 459
(2016/01/09)
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- A family of oxide-carbide-carbon and oxide-nitride-carbon nanocomposites
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This paper describes a powerful and versatile approach that combines the benefits of sol-gel processing with controlled phase separation to yield oxide-carbide-carbon or oxide-nitride-carbon nanocomposites. the Partner Organisations 2014.
- Schnepp,Hollamby,Tanaka,Matsushita,Xu,Sakka
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supporting information
p. 5364 - 5366
(2014/05/06)
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- Production of fine tungsten powder by electrolytic reduction of solid CaWO4 in molten salt
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Direct electrochemical reduction of solid CaWO4 to fine W powder in molten CaCl2-NaCl was studied by cyclic voltammetric measurement and potentiostaticconstant-voltage electrolysis. The effect of electrolysis conditions such as electrolysis time, temperature and voltage on the quality of the product (the residual amount of CaWO4) was systematically investigated. Based on the solubility tests of CaWO4 in the melt in a wide temperature range, an unprecedented strategy on electrolytic extraction of pure tungsten powder was proposed and confirmed, which involves low-temperature and short-duration electrolysis of solid CaWO4 in molten CaCl 2-NaCl eutectic salt (1023 K, 10-hour electrolysis) followed by a simple washing process in the same melt at a higher temperature (1123 K, 1-hour washing). The particle size of the as-prepared W powder is less than 200 nm. This process exhibits a high yield and increased energy efficiency for production of fine W powder from CaWO4.
- Tang, Dingding,Xiao, Wei,Yin, Huayi,Tian, Longfei,Wang, Dihua
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p. E139-E143
(2012/07/14)
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- Synthesis and characterization of new chromium, molybdenum and tungsten complexes of 2-[2-(methylaminoethyl)] pyridine
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A green chemistry route of synthesis using direct sunlight irradiation for the reactions of [M(CO)6] M = Cr, Mo or W with 2-[2- (methylaminoethyl)] pyridine (maepy) in THF. The reactions resulted in the formation of the oxo complex [Cr2(O)4(maepy)2] (1) and the tetracarbonyl complexes [Mo(CO)4(maepy)] (2) and [W(CO)4(maepy)] (3). The prepared complexes were characterized by elemental analysis, IR, NMR, mass spectrometry and magnetic measurement. The complexes (1-3) were further investigated by thermogravimetric technique (TG). The biological activity of maepy and complexes as antibacterial and antifungal reagents have been investigated.
- Ali, Saadia A.,Soliman, Ahmed A.,Marei, Amany H.,Nassar, Doaa H.
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p. 164 - 168
(2012/07/13)
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- Chemical reactions, anisotropic grain growth and sintering mechanisms of self-reinforced ZrB2-SiC doped with WC
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Transmission electron microscopy study of pressureless sintered ZrB 2-20 vol% SiC composites with WC (5-10 vol%) additions was carried out in this work. Two independent chemical reactions in which the ZrO 2 impurities were removed from ZrB2 grain surface were confirmed. Three new formed phases, (W, Zr)ssB, (Zr, W) ssC, and (W, Zr)ssSi2, apart from the plate-like ZrB2 and SiC grains, were identified in the samples. Microstructure observations strongly suggest that the liquid phases in terms of (W, Zr)ssB and (W, Zr)ssSi2, appear in this system during the sintering process below 2200°C, and the former could well wet the ZrB2 grain boundaries. The mass transport process between ZrB2 and (W, Zr)ssB was also confirmed by scanning-transmission electron microscope analysis. The above results supports that the densification at 2200°C is assisted by liquid phase and the elongation of ZrB2 platelets resulted from the Ostwald ripening during the dissolution-diffusion-precipitation process.
- Zou, Ji,Sun, Shi-Kuan,Zhang, Guo-Jun,Kan, Yan-Mei,Wang, Pei-Ling,Ohji, Tatsuki
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p. 1575 - 1583
(2011/09/15)
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- Ternary phase Zr x Cu y C z in reactively infiltrated ZrC/W composite
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A new ternary ZrxCuyCz phase is observed in the reactively infiltrated ZrC/W composite. The X-ray diffraction and transmission electron microscopy analyses reveal that the ZrxCu yCz phase has a same rocksalt structure as ZrC, but possesses sixfold long-period structure with lattice constant of 2.88 ± 0.07 nm. This finding indicates the existence of ZrxCu yCz compound, which expands the ternary carbides and nitrides permutation space.
- Zhao, Yan-Wei,Wang, Yu-Jin,Zhou, Yu,Peng, Hua-Xin,Song, Gui-Ming
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p. 3178 - 3180
(2011/11/05)
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- Tungsten-based carbides as anode for intermediate-temperature fuel cells
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Tungsten carbides were prepared from ammonium paratungstate via temperature-programmed carburization under flowing a gaseous mixture of CH 4H2 to employ as anode catalysts in fuel cells consisting of CsH2PO4SiP2O7-based composite electrolyte operative at 200C. The resulting materials were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The heat-treatment at high temperatures promoted the reduction and carburization of tungsten component. The single phase of WC was observed for the samples subjected to the carburization at and above 800°C. The single cell employing the catalyst prepared at 850C attained the best performance. The anode material containing the metallic W exhibited low stability under the power generation condition. With nickel or cobalt additives, the carburization of tungsten species was initiated at low temperatures. The samples with the additives heat-treated at high temperatures were composed of several tungsten carbides including WC. When these samples were applied as anode catalysts, the additive species lowered the cell performance. These results indicated that the WC phase was the most effective electrocatalyst for the hydrogen oxidation.
- Muroyama, Hiroki,Katsukawa, Koji,Matsui, Toshiaki,Eguchi, Koichi
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p. B1072-B1075
(2011/10/01)
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- Studies on the synthesis of a Mo-30 wt% W alloy by non-conventional approaches
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In the present study, alternate routes for the synthesis of a single phase Mo30wt%Walloy were pursued to surmount the limitations experienced in conventional technique. The process essentially consists of preparation of active Mo andWpowders by H2 reduction of the respective oxide intermediates through multiple processing steps and then converting those pure powders into Mo30wt%Walloy by mechanical alloying technique (MA) at RT under Ar atmosphere in a high-energy planetary ball mill. The structural evolution of the alloy from the milled powders at different interval of time was studied by X-ray diffraction (XRD) and phase corresponding to Mo30W alloy was confirmed. The broadening of peaks in XRD pattern was due to crystallite refinement during milling towards the formation of the designated alloy. A high rate of densification for MA powder was achieved during sintering between 900 °C and 1200 °C and density close to theoretical density was attained. The microstructure of sintered alloy exhibited uniform, polyhedral grains with average grain size of about 3μm. The morphological evolution of as-milled powder was studied by Scanning electron microscopy (SEM) which revealed the formation of nano sized crystallites with polyhedral shapes. The crystallites were initially arranged in clusters which later on got distributed uniformly with the progress in milling time. The average crystallite size of MA powder was found to be 7.3nm after 25 h of milling.
- Chakraborty,Banerjee,Sanyal, Gopal,Bhave,Paul, Bhaskar,Sharma,Suri
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p. 211 - 217
(2010/10/01)
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- Chemical synthesis of tungsten-copper nanocomposite powder
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In this study homogeneous powders of CuWO4 and WO3 was produced from ammonium paratungstate (APT) and copper nitrate. Then, the product was used to prepare nano-sized W-Cu powder. Hence, a mixture of ammonium paratungstate and copper nitrate with predetermined weight proportion was made in distilled water, while the content of the beaker was being stirred at a certain speed to reach the desired composition of W-20 wt % Cu. Mixture was heated to 80-100°C for 6 h. Also, pH range was adjusted at about 3-4. The mixture was then evaporated and dried in the air. To reach W-Cu composite powder, the precursor powders burned out at 520°C for 2 h in the air to form W-Cu oxide powder and then were ball milled and reduced in H2 atmosphere to convert it into W-Cu composite powder. The resulting powders were evaluated using scanning electron microscopy, X-ray diffraction, thermogravimetric analysis and differential thermal analysis techniques. The results showed that homogeneous powders of W-Cu with particle size of around 100 nm and a nearly spherical shape could be obtained by this process. Each particle include smaller parts with size of around 20-30 nm. Pleiades Publishing, Ltd., 2010.
- Naseri, M. Babapour,Kamali,Hadavi
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p. 167 - 173
(2010/06/18)
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- Tungsten thin-film deposition on a silicon wafer: The formation of silicides at W-Si interface
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The interphase boundary formed in the process of tungsten thin-film deposition on a silicon wafer is investigated. These films are produced via (1) a CVD technique relying on hydrogen reduction of tungsten hexafluoride, (2) the same technique supplemented
- Plyushcheva,Mikhailov,Shabel'nikov,Shapoval
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p. 140 - 144
(2009/05/30)
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- Solid metal compound, preparations and uses thereof
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The invention relates to a solid metal compound comprising (i) a solid support comprising aluminium oxide, (ii) at least one first metal compound (C1) selected from metal hydrides, organometallic compounds and organometallic hydrides, and comprising a metal (M1) selected from the lanthanides, the actinides and the metals of Groups 4 to 7 of the Periodic Table of the Elements, and (iii) at least one second metal compound (C2) comprising a metal (M2) selected from the metals of Groups 8 to 10 of said Table. The compounds (C1) and (C2) are preferably supported on, particularly grafted onto the solid support. The invention also relates to processes for preparing the solid metal compound, preferably comprising stage (1) comprising dispersing and preferably grafting (i) an organometallic precursor (Pr1) comprising the metal (M1) and (ii) a precursor (Pr2) comprising the metal (M2) onto the support, so as to produce the solid metal compound, and preferably stage (2) comprising contacting the solid metal compound thus obtained with hydrogen and/or a reducing agent. The invention also relates to the use of the solid metal compound in processes comprising hydrocarbon reactions optionally in the presence of hydrogen, and preferably involving the splitting and recombining of carbon-carbon and/or carbon-hydrogen and/or carbon-metal bonds, so as to produce final hydrocarbons different from the starting ones. The solid metal compound can be used in processes comprising alkane and/or alkene metathesis, non-oxidative methane coupling, alkene oligomerisation, methane-olysis of hydrocarbons, cross-metathesis and hydrogenolysis of hydrocarbons, e.g. saturated hydrocarbons, hydrocarbon polymers/oligomers or waxes, in the presence of hydrogen.
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- Use of ionic liquids (ILs) for the IL-anion size-dependent formation of Cr, Mo and W nanoparticles from metal carbonyl M(CO)6 precursors
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Stable chromium, molybdenum and tungsten nanoparticles are obtained reproducibly by thermal or photolytic decomposition under argon from mononuclear metal carbonyl precursors M(CO)6 (M = Cr, Mo, W) suspended in the ionic liquids BMim+BF4-, BMim +OTf- and BtMA+Tf2N- (BMim+ = n-butyl-methyl-imidazolium, BtMA+ = n-butyl-trimethyl-ammonium, Tf2N = N(O2SCF 3)2, OTf = O3SCF3) with a very small and uniform size of 1 to 1.5 nm in BMim+BF4- which increases with the molecular volume of the ionic liquid anion to ～100 nm in BtMA+Tf2N- [characterization by transmission electron microscopy (TEM), dynamic light scattering and transmission electron diffraction (TED) analysis]. The Royal Society of Chemistry.
- Redel, Engelbert,Thomann, Ralf,Janiak, Christoph
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p. 1789 - 1791
(2008/12/22)
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- Investigation of static corrosion between W Metals and TiNx Barriers in a W chemical-mechanical-polishing slurry
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In order to control the polishing qualities of tungsten (W) and titanium nitride (TiNx) films in W chemical-mechanical-polishing (WCMP) processes, the electrochemical behavior between the W and the TiNx films deposited at various N2 flow rates was examined in this study. Metrologies, including X-ray diffractrometry, Auger electron spectrometry, and scanning electron microscopy, were used to verify the physical properties of the TiNx films, while electrochemical analyses, including electrochemical impedance spectroscopy, potential dynamic curves, and potential difference measurements, were used to characterize the mechanism of galvanic corrosion between the W and the TiNx films deposited at various N2 flow rates. The results show that the N content of the TiNx films influences not only the physical properties of the TiNx films but also the chemical activity in the WCMP slurries. The equivalent circuit, including the charge-transfer resistance and the titanium-oxide resistance associated with tantalum-oxide capacitance, was built to characterize the mechanism of the galvanic corrosion between the W and the TiNx metals.
- Hung, Chi-Cheng,Wang, Ying-Lang,Lee, Wen-Hsi,Chang, Shih-Chieh
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p. H469-H473
(2009/01/31)
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- Thermolysis specifics of Tin(IV) and Tin(II) complex derivatives: Thermolysis of (Acac)2SnX2 (X = Cl, N3), (CO)5MSnCl2(thf) (M = Cr, Mo, W), (CO) 5MSn(Acac)2, (CO)5MSn
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Differential scanning calorimetry and thermogravimetry are used to study the thermolysis of following complexes: (Acac)2Sn(N3)Cl (1); (Acac)2SnCl2 (2); (CO)5MSn(Acac) 2 with M = Cr (3) or W
- Dobrokhotova,Koroteev,Novotortsev,Egorov,Nefedov
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p. 1109 - 1119
(2008/10/09)
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- Analysis of tungsten film electrodeposited from a ZnCl2-NaCl-KCl melt
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Microparts for micro-electro-mechanical systems (MEMS) are becoming increasingly important, and the Lithographie Galvanoformung Abformung (LIGA) process for producing such parts is attracting attention [1]. However, as this process requires the step of electroplating in aqueous solution, only copper, nickel and their alloys can be used because of the limit of the potential window of water. We have been attempting to apply the electroplating of refractory metals in molten salts to the LIGA process or to the surface coating of conventional LIGA microparts in order to give higher strength and heat resistance for the microparts. Herein we report the characteristics of a tungsten film electrodeposited from a ZnCl2-NaCl-KCl melt at 250 °C.
- Nitta, Koji,Inazawa, Shinji,Okada, Kazunori,Nakajima, Hironori,Nohira, Toshiyuki,Hagiwara, Rika
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- Thermo-programmed reduction study of Pt/WOx-ZrO2 materials by thermogravimetry
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The thermo-programmed reduction study of Pt/WOx-ZrO2 materials prepared with different tungsten loading were performed by thermogravimetry. The samples were synthesized by impregnation method and calcined at 600, 700 and 800°C. The characterizations of both un-calcined and calcined materials were carried out using different techniques: thermal analysis (TG and DTA), X-ray diffraction (XRD) and thermo-programmed reduction (TPR). TG and DTA analysis of un-calcined were used to determination of calcination temperatures of the samples. XRD diffractograms were useful to help us in the determination of phase presents. TPR profiles showed between three and four events at different temperatures attributed to platinum reduction and the different stages of tungsten specie reduction.
- Garrido Pedrosa, A. M.,Souza, M. J. B.,Melo, D. M. A.,Araujo, A. S.
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p. 349 - 354
(2008/10/09)
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- Thermite reactions of Al/Cu core-shell nanocomposites with WO3
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The Al/Cu core-shell nanocomposites are fabricated via a facile displacement method and characterized by scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS). In comparison with the thermite reactions of bulk Al powders with WO3, the thermite reactions of Al/Cu core-shell nanocomposites with WO3 as a function of the equivalence ratio of Al to WO3 (Φ) and reaction environment are investigated by differential scanning calorimetry (DSC) and dealt with in terms of reaction performance of thermite including onset and peak temperatures, maximum reaction rate (Qmax) and reaction heat (ΔHr) based on DSC data. The results indicate that the reactivity of Al/Cu core-shell nanocomposites with WO3 is higher than that of bulk Al powders but depends strongly on the equivalence ratio, which is optimal when the fuel in the thermites is slightly rich, i.e. Φ = 1.2. It is significant that in air, ΔHr of WO3 and core-shell Al/Cu (Φ = 1.2) is higher than that of WO3 and bulk Al by 49%. In addition, it is also observed that the solid phase reaction 2Al(s) + WO3(s) = Al2O3(s) + W(s) may be irritated prior to Al melting when heating Al/Cu-WO3 in argon, which is confirmed by DSC and X-ray diffractometry (XRD). Finally, the reasons of improving the thermal performance of thermites have been discussed.
- Wang, Yi,Jiang, Wei,Cheng, Zhipeng,Chen, Weifan,An, Chongwei,Song, Xiaolan,Li, Fengsheng
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- Pressureless densification of zirconium diboride with boron carbide additions
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Zirconium diboride (ZrB2) was densified (>98% relative density) at temperatures as low as 1850°C by pressureless sintering. Sintering was activated by removing oxide impurities (B2O3 and ZrO2) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (～150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B4C were evaluated as additives to facilitate the removal of ZrO2. Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB2 (surface area ～1 m2/g) or attritionmilled ZrB2 (surface area ～ 7.5 m2/g) with WC and/or B4C as a sintering aid. ZrB2 containing only WC could be sintered to ～95% relative density in 4 h at 2050°C under vacuum. In contrast, the addition of B4C allowed for sintering to >98% relative density in 1 h at 1850°C under vacuum.
- Zhang,Hilmas,Fahrenholtz
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p. 1544 - 1550
(2008/10/09)
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- Carbothermic reduction of cobalt and nickel tungstates
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Data are presented on the formation sequence of metallic, carbide, and intermediate oxide phases in the carbon reduction of CoWO4 and NiWO4. The surface reaction between CoWO4 particles and solid carbon yields the mixed carbide Co6W6C, while the reaction with the CO originating from carbon vaporization yields the intermetallic phase Co7W6. The initial stage of the solid-state and gas-phase reduction of NiWO4 yields a solid solution of tungsten in nickel (?10 at % W), Ni4W, and, presumably, the NiWO4-WO3 eutectic. The solid solution reacts with carbon to form Ni2W4C and with CO to form filamentary tungsten crystals. Pleiades Publishing, Inc., 2006.
- Lebukhova,Karpovich
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p. 310 - 315
(2008/10/09)
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- A comparative study of the atomic-layer-deposited tungsten thin films as nucleation layers for W-plug deposition
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The properties of three different kinds of atomic-layer-deposited (ALD) W thin films were comparatively characterized and investigated as nucleation layers for the W-plug process of 70 nm design-rule dynamic random access memory. ALD-W (A) film was deposited using alternating exposures of WF6 and SiH 4, and ALD-W (B) film was treated with B2H6 for 5 s prior to W ALD using WF6 and SiH4. Finally, ALD-W (C) film was deposited using alternating exposures of WF6 and B 2H6. All the ALD-W films showed excellent step coverage at the contact with an aspect ratio of ～14, but their resistivities were as high as 125-145 μΩ cm at the thickness of 20 nm. High resistivities of ALD-W films are discussed on the basis of impurities cooperation such as Si and B, phase (body-centered-cubic α-W or primitive cubic β-W), crystallinity (crystalline or amorphous), and grain size. It was found that ALD-W (C) film formed an amorphous phase, which was stable until 900°C annealing. This is clearly different from ALD-W (A) and ALD-W (B) with polycrystalline grains of α-W and β-W, and β-W was transformed to α-W after 800°C annealing. The formation of amorphous W resulted in the formation of large-size grains of chemical-vapor-deposited W film deposited on ALD-W (C) and the reduction in the resistivity of W-plug stack. The integration results showed that the reduced resistivity of W-plug stack with ALD-W (C) provided a significantly lower resistance at the W bit line contact. Another advantage of the integration scheme with ALD-W (C) was its stable contact resistance at the ultrahigh aspect ratio (UHAR) contact even though the step coverage of the underlayer, TiN, was poor. It was also found that the B2H6 pretreatment was effective for obtaining the low and stable contact resistance at UHAR contact.
- Kim, Soo-Hyun,Kwak, Nohjung,Kim, Jinwoong,Sohn, Hyunchul
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p. G887-G893
(2008/10/09)
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- Tungsten carbides and W-C phase diagram
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The crystal structures of the tungsten monocarbide δ-WC and the disordered lower carbide β-W2C are studied. Using magnetic susceptibility measurements, the hexagonal carbide δ-WC is shown to be stable from 300 to 1200 K. The sequence of phase t
- Kurlov,Gusev
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p. 121 - 127
(2008/10/09)
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- In situ XAS and XRD studies on the structural evolution of aMmonium paratungstate during thermal decomposition
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The bulk structural evolution during the decomposition of ammonium paratungstate (APT) under various oxidizing and reducing gases was elucidated by the complementary techniques, in situ XAS, in situ XRD, and TG/DSC, combined with mass spectrometry. In the temperature range from 300 to 650 K, the decomposition of APT proceeds nearly independently of the gas employed. At higher temperatures, an oxidizing gas results in the formation of crystalline triclinic WO3 as the majority phase at 773 K, while mildly reducing gases (propene, propene and oxygen, and helium) result in the formation of partially reduced and highly disordered tungsten bronzes. No further reduction is observed under propene or helium, and this indicates a strongly hindered oxygen mobility in the tungsten oxide lattice in the temperature range employed. The decomposition of APT under hydrogen results in the formation of WO 2 and, eventually, tungsten me tal. During the thermal treatment of APT, major changes occur at ca. 500 K where a complete structural rearrangement takes place that results in the destruction of the polyoxotungstate ion of APT and the formation of a tungsten oxide bronze. The rather low temperature for the formation of a three-dimensional lattice compared to the thermal treatment of common polyoxomolybdate precursors indicates the lower stability of the precursor and intermediates as ligands are removed. For tungsten to act as a potential structural or electronic promoter in molybdenum oxide based catalysts, tungsten needs to be incorporated in regular molybdenum oxide structures already at a very early stage of the catalyst preparation. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.
- Kirilenko, Olga,Girgsdies, Frank,Jentoft, Rolf E.,Ressler, Thorsten
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p. 2124 - 2133
(2007/10/03)
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- Electrodeposition of nano-structured nickel-21% tungsten alloy and evaluation of oxygen reduction reaction in a 1% sodium hydroxide solution
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The nano-structured nickel-21at.% tungsten alloys were electrodeposited onto the copper substrates from unstirred sulfate-citrate-chloride-bromide-sodium tungstate electrolyte at 60°C. The maximum particle sizes of the deposits, as estimated from the atomic force microscopy (AFM), and scanning electron microscopy (SEM), respectively were 125, 75, and 100nm. The Tafel plot for oxygen reduction reaction in oxygenated unstirred 1% sodium hydroxide solution showed a Tafel slope of 130mV/decade. There were minor variations in the limiting current density with a change in the particle size.
- Ma, Miaomiao,Donepudi,Sandi,Sun,Prakash
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p. 4411 - 4416
(2008/10/09)
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- Study of the reduction of tungsten trioxide doped with MCl (M=Li, Na, K)
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Thermodynamic calculations predict the formation of hydrochloric acid gas and alkali tungstates during hydrogen reduction of WO3 doped with alkali chlorides MCl (M=Li, Na, K). The formation of HCl was proved experimentally by simultaneously cou
- Schwenke,Feist,Lunk
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- Atomic layer deposition of ZrO2 on W for metal-insulator-metal capacitor application
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Atomic layer deposition of ZrO2 on W was investigated for metal-insulator-metal (MIM) capacitor application. A 13～14-A-thick interfacial layer of WOx was observed between ZrO2 and W through angle-resolved x-ray photoelectron spectroscopy. Results showed an effective dielectric constant of 22～25 was obtained for MIM capacitors with Pt top electrodes.
- Lee, Sang-Yun,Kim, Hyoungsub,McIntyre, Paul C.,Saraswat, Krishna C.,Byun, Jeong-Soo
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p. 2874 - 2876
(2008/10/08)
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- Size-distribution and emission spectroscopy of W nanoparticles generated by laser-assisted CVD for different WF6/H2/Ar mixtures
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Tungsten nanoparticles were produced by ArF excimer laser-assisted chemical vapor deposition from WF6/H2/Ar gas mixtures. Subsequent pulses excited the gas-phase particles, allowing optical emission spectroscopy to monitor the intensity of the emitted radiation and temperature of the laser-heated particles. A systematic study on size-distributions of the deposited particles, determined by electron microscopy, in connection with emission spectroscopy and rate of deposition measurements is presented, with respect to different partial pressures of the reactants. The rates of deposition of W nanoparticle films were determined by X-ray fluorescence spectroscopy. In addition, the intensity of the scattered 193 nm laser line was also monitored as the partial pressure of H2 was varied.
- Landstr?m, Lars,Lu, Jun,Heszler, Peter
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p. 11615 - 11621
(2007/10/03)
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- Mobile radiant energy sterilizer
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A portable, radiation-producing apparatus is provided that can produce highly energetic electron beam radiation and X-rays from a low voltage power source., e.g., a battery. The radiation-producing apparatus is comprised of a radiation generating device, a pulsed high voltage generator and a control system. The pulsed high voltage generator is comprised of a power source and a Tesla resonant transformer. The Tesla resonant transformer has at least one first capacitor, a primary coil, a secondary coil and at least one second capacitor. The at least one second capacitor is disposed axially within the secondary coil. The pulsed high voltage generator is connected to the radiation generating device for providing electrical energy to the radiation generating device. The control system is connected to the pulsed high voltage generator for selectively controlling the transfer of energy from the pulsed high voltage generator to the radiation generating device. The radiation-producing apparatus generates pulses of electrons and X-rays. Each pulse has a time duration of about 100 nanoseconds or less. The electrons and X-rays produced by the radiation-producing apparatus can be used to deactivate microbial contamination or irradiate various materials.
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- TA ON Mo(W)/Cu(Ag)/S clusters derived with [Cp*MS3]- kinetics of [PPh4]2[Cp*WS3 (CuBr)3]2 and [PPh4]2[Cp*MoS3 (CuBr)3]2
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Thermal analysis on two new heterometallic sulfide clusters, [PPh4]2[Cp*WS3 (CuBr)3]2 and [PPh4]2[Cp*MoS3 (CuBr)3]2 (where PPh4=tetraphenyl phosphonium, Cp=pentamethylcyclopentadienyl), was carried out using a simultaneous TG-DTA unit in an atmosphere of flowing nitrogen and at various heating rates. Supplemented using EDS method, their thermal behavior and properties, together with the composition of their intermediate product, were examined and discussed in connection with their distinctive molecular structure as a dianion, which provided some theoretically and practically significant information. Both clusters decomposed in a two-step mode, but without a stable new phase composed of Mo/W-Cu-S formed during their decomposition process as we expected. Based on TG-DTG data, four methods, i.e. Achar-Brindley-Sharp, Coats-Redfern, Kissinger and Flynn-Wall-Ozawa equation, were used to calculate the non-isothermal kinetic parameters and to determine the most probable mechanisms.
- Lu,Ding,Xu,Yao,Liu,Lang
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p. 985 - 994
(2008/10/08)
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