Showing papers on "Tungsten published in 1997"

A simplified unified theory of arcs and their electrodes

Abstract: A recently developed unified theory of arcs and their electrodes, with cathodes which are thermionic emitters, has been simplified so that there is a reduction in computation times by approximately a factor of 100. Electrode and arc regions are treated together and points at the surface of the electrodes are treated in a special way to account for electrode effects; no assumptions are made concerning the current density at the cathode surface. The theory is used to make predictions of arc and electrode temperatures and arc voltages for arcs in argon as a function of current in the range 50 - 400 A. The maximum temperatures of the arc and the cathode and also the current - voltage characteristics are in reasonable agreement with experimental results for cathodes with a included angle. For a cathode with a included angle, we predict a maximum in the temperature several millimetres from the tip of the electrode, in approximate agreement with experiment. Temperatures of the cathode tip are predicted to be much higher for tungsten cathodes than for thoriated tungsten cathodes and are in reasonable agreement with experimental results. Tungsten electrodes are hotter than thoriated tungsten electrodes, partly due to increased ion heating, but largely due to greater heat conduction from the arc to the electrode due to the arc plasma covering a greater area of the electrode surface for tungsten.

Thermal Expansion of Periclase (MgO) and Tungsten (W) to Melting Temperatures

Abstract: In situ x-ray data on molar volumes of periclase and tungsten have been collected over the temperature range from 300 K to melting. We determine the temperature by combining the technique of spectroradiometry and electrical resistance wire heating. The thermal expansion (α) of periclase between 300 and 3100 K is given by α=2.6025 10−5+1.3535 10−8 T+6.5687 10−3 T−1−1.8281 T−2.

Thermal expansion of periclase (MgO) and tungsten (W) to melting temperatures Sample: T = 298 K

Abstract: Abstract In situ x-ray data on molar volumes of periclase and tungsten have been collected over the temperature range from 300 K to melting. We determine the temperature by combining the technique of spectroradiometry and electrical resistance wire heating. The thermal expansion (α) of periclase between 300 and 3100 K is given by α=2.6025 10−5+1.3535 10−8 T+6.5687 10−3 T−1−1.8281 T−2.For tungsten, we have (300 to 3600 K) α=7.862 10−6+6.392 10−9 T.The data at 298 K for periclase is: molar volume 11.246 (0.031) cm3, α=3.15 (0.07) 10−5 K−1, and for tungsten: molar volume 9.55 cm3, α=9.77 (10.08) 10−6 K−1.

Mechanism of WO 3 Electrodeposition from Peroxy‐Tungstate Solution

Abstract: Electrodeposition of amorphous electrochromic tungsten trioxide was studied. Good quality films were deposited cathodically onto tin‐doped indium oxide (ITO) from a water/isopropanol solution containing dissolved tungsten(VI) species. Three reducible species are present in this solution: unbound , polytungstate ions, and peroxy‐tungstate ions. The effects of the content, which is related to the age of the deposition solution, and of mass transport on the deposition current and deposition efficiency were determined and used to elucidate the mechanism of electrodeposition. Reduction of unbound and polytungstate did not result in film growth. This part of the current was under mixed diffusion‐kinetic control. The remainder of the current was due to reduction of peroxy‐tungstate and was kinetically limited. This process led to deposition. The current efficiency, defined as the number of electrons consumed by reduction of peroxy‐tungstate per W atom deposited, was about 1.6, in good agreement with the proposed deposition reaction.

Method for anisotropically etching tungsten using SF6, CHF3, and N2

Abstract: A method for etching a tungsten containing layer 25 on a substrate 10 substantially anisotropically, with good etching selectivity, and without forming excessive passivating deposits on the etched features. In the method, the substrate 10 is placed in a plasma zone 55, and process gas comprising SF 6 , CHF 3 , and N 2 , is introduced into the plasma zone. A plasma is formed from the process gas to anisotropically etch the tungsten containing layer 22. Preferably, the plasma is formed using combined inductive and capacitive plasma operated at a predefined inductive:capacitive power ratio.

Theoretical tensile stress in tungsten single crystals by full-potential first-principles calculations

Abstract: Fully self-consistent ab initio electronic structure calculation of theoretical tensile strength is performed for the first time using full-potential LAPW method. As a specific example, tensile strength of single-crystalline tungsten loaded uniaxially along the (001) and (111) directions is analyzed. Although tungsten is elastically nearly isotropic ( C 44 ≈ C′), theoretical tensile strength exhibits a marked anisotropy ( σ 001 th = 0.289 Mbar, σ 111 th = 0.401 Mbar). This anisotropy is explained in terms of structural energy differences between bcc, fcc and simple cubic structures which occur on the calculated deformation paths. Theoretical results compare favorably with experimental value of 0.247 ± 0.036 Mbar obtained for tungsten whiskers grown along the (110) direction.

Si nanowire growth with ultrahigh vacuum scanning tunneling microscopy

Abstract: Using a scanning tunneling microscope (STM), Si nanowires were grown by applying a voltage at a constant current between a Si substrate and a gold STM tip. Silicon atoms were deposited onto a gold tip by field evaporation. The field evaporation rate of silicon atoms was activated by heating the substrate. Silicon nanowire was grown on the gold tip at a substrate temperature of 700 °C. Nanowires could not be grown on a clean tungsten tip when using a gold-free Si substrate. The presence of gold atoms is important for the growth of silicon. Apparently, gold atoms deposited on the silicon substrate by field evaporation reduce the activation energy of field evaporation by attacking Si–Si bonds.

Infrared and X-ray studies of hydrogen intercalation in different tungsten trioxides and tungsten trioxide hydrates

Abstract: Hydrogen intercalation via spillover reaction in various tungsten trioxides leads to the formation of blue hydrogen bronzes. These reversible reactions induce changes in the W-O bond system while maintaining the W-O skeleton. The effect of the intercalation process on the host crystalline structure has been studied with respect to the ν(O-W-O) stretching vibration changes and lattice parameter variations by means of infrared and X-ray diffraction measurements. Among the main results, the intercalation process is shown to be strongly influenced by the structural type of the host compound as well as its amorphous versus crystalline nature. For instance, for the ReO3 type oxides (monoclinic and cubic WO3) and hexagonal WO3, ν(O-W-O) shifts to higher frequency are assigned to a shortening effect of W-O bonds. A W-O bond system arrangement is also measured for the crystallized and amorphous hydrates WO3 · H2O, but no detectable changes could be found in the pyrochlore WO3 and in the hydrate WO3·1/3 H2O.

Physico-chemical properties of WO3/TiO2 systems employed for 4-nitrophenol photodegradation in aqueous medium

Abstract: A series of polycrystalline WOx/TiO2 samples were prepared by means of a conventional impregnation method. The samples were characterized by means of X-ray diffraction, Vis-UV diffuse reflectance and Raman spectroscopies, nitrogen adsorption at 77 K for determining specific surface areas and surface texture, scanning electron microscopy, and FT-IR monitoring of pyridine adsorption for measuring the surface acidity. Catalytic activity of the samples has been assessed by carrying out as a ``probe'' reaction the photodegradation of 4- nitrophenol in aqueous medium. The results obtained indicate that incorporation of tungsten on titania leads to formation of different surface species, depending on the tungsten loading. Tungsta microcrystals were detected by X-ray diffraction when the nominal molar W/Ti ratio reached a value of 8.0%. FT-IR investigation indicated that the presence of tungsten induces formation of Bronsted and Lewis surface acid sites. The photoactivity results confirm the beneficial effect of tungsten in TiO2 for 4-nitrophenol photodegradation in aqueous medium. The reaction rates are higher than those reported in literature for another set of samples and maximum photoactivity was achieved for a sample containing 1.96 moles of W per 100 moles of Ti.

Tungsten Oxides as Active Supports for Highly Dispersed Platinum Microcenters: Electrocatalytic Reactivity Toward Reduction of Hydrogen Peroxide and Oxygen

Abstract: The electrochemical deposition of spatially dispersed platinum centers within a tungsten oxide film onto the surface of a carbon substrate is described. The approach explores cathodic fabrication of metallic Pt microparticles via anodic dissolution of a Pt counterelectrode. Nonstoichiometric tungsten(VI,V) oxides are simultaneously electrodeposited with Pt particles during reductive potential cycles. We describe electrocatalytic properties of the resulting composite films toward reduction of dioxygen. We also demonstrate that a Pt-free tungsten oxide film shows strong electrocatalytic activity for reduction of hydrogen peroxide. The method permits preparation of films containing platinum microcenters at low loadings (ca. 5 μg cm 2 ) within the reactive oxide matrix. We postulate bifunctional activity of the system in terms of promoting catalytic reduction of oxygen (by the traces of platinum) and reduction of any hydrogen peroxide intermediate [by the W(VI,V) oxide matrix].

Tungsten silicide deposition process

Abstract: A method of forming a gate metallization in a semiconductor integrated circuit by forming a polycrystalline silicon layer over a gate dielectric layer and then converting the polycrystalline silicon layer into tungsten or tungsten silicide by exposing the polycrystalline silicon to tungsten hexafluoride gas. The method enables the formation of polycrystalline silicon and tungsten or tungsten silicide in the same process cycle in the same reactor or in two similarly configured reactors or in two similarly configured clustered reactors.

Fabrication and mechanical properties of fine-tungsten-dispersed alumina-based composites

Abstract: The mechanical properties and microstructure of fine-tungsten-dispersed alumina-based composites, which were fabricated by hot pressing a mixture of fine α-Al2O3 and W powders, have been investigated Small W particles of approximately 140 nm average size were located within the Al2O3 matrix grains The mechanical properties were influenced by the metal content and sintering conditions When the appropriate W content and sintering condition were selected (typically 5–10 vol% W and sintered at 1400°C), the fracture strength was enhanced compared with that of monolithic Al2O3 The metal content dependence of Young's modulus and the Vickers hardness did not obey the rule of mixtures This may be attributed to the presence of localized residual stress caused by the incorporation of fine W dispersion into Al2O3 On the other hand, high-temperature (1600°C) sintering caused degradation in the properties of the composites due to the grain growth and chemical reaction of W dispersion, which was revealed by X-ray photoelectron spectroscopy analysis The relations between fabrication condition and mechanical properties are discussed

Influence of the tungsten oxide precursor on WOxZrO2 and Pt/WOxZrO2 properties

Abstract: The influence of the tungsten precursor and of the preparation conditions on the properties of platinum free or platinum containing WOx promoted zirconia has been studied. Catalysts were characterized by hydrogen chemisorption at room temperature, TPR, XRD and Laser Raman spectroscopy and during the n-hexane reaction at 200°C, 6 kg cm−2, and a hydrogen/n-hexane molar ratio 7. For all catalysts, the same WOx species were observed after calcination: crystalline WO3 and an amorphous WOx species were detected by Laser Raman. The best catalytic activity and stability were observed on catalysts in which platinum was added after calcination, independently of the WOx precursor used in the preparation.

Carbon nanotubes are coherent electron sources

Abstract: The observation of electron interference effects provides direct evidence of the coherence of the electrons emitted from a carbon nanotube. To demonstrate this, the low-energy electron point source microscope has been used to mount an individual carbon nanotube onto a tungsten tip. In subsequent experiments, the electrons emitted from the nanotube were used to generate holograms. Comparison with a standard tungsten atomic point source emitter establishes a high degree of coherence for a nanotube emitter.

Effect of phosphorus concentration and method of preparation on the structure of the oxide form of phosphorus-nickel-tungsten/alumina hydrotreating catalysts

Abstract: Two series of phosphorus containing NiW/Al2O3 catalysts were prepared by different preparation methods and varying phosphorus content from 0 to 7.6 wt.% P2O5. The influence of the phosphorus concentration and the preparation procedure on the structure and the dispersion of the compounds formed in the oxide form of the catalysts was studied. Diffuse reflectance spectroscopy (DRS), infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) were used for the characterization of the catalysts. It was demonstrated that introduction of phosphorus in NiW/Al2O3 catalysts impedes the formation of NiAl2O4 and increases the amount of Ni2+(Oh) ions in the oxide form of the samples. This effect of phosphorus is better expressed when phosphorus is first introduced to alumina, followed by co-impregnation with nickel and tungsten. Surface AlPO4 is formed in the phosphorus containing samples irrespective of the preparation procedure. Phosphorus is distributed as a monolayer up to concentrations of 1.3 P at/nm2 in both series of catalysts. It was also found that the increased phosphorus content in the samples leads to an increased degree of polymerization of the tungsten species via WOW bonds. The presence of phosphorus changes the dispersion of the active components. Evaluation of the average particle size of tungsten species shows that it increases from 10Afor the phosphorus free sample up to 20–25Afor the samples with high phosphorus content. The relationship between the structure of the phosphorus promoted NiW/Al2O3 catalysts and their hydrodesulfurization activity is discussed.

Synthesis and structural evolution of tungsten carbide prepared by ball milling

Abstract: Tungsten carbide has been synthesized directly by ball-milling tungsten powder and activated carbon in vacuum. The structural development of the WC phase with milling times up to 310 h has been followed using X-ray, neutron diffraction and scanning electron microscopy. Subsequent annealing (at 1000 °C for 1 and 20 h) of material milled for 90 h or longer, results in samples comprising almost entirely crystalline WC. The production of WC itself during milling results in enhanced iron contamination from the steel mill and balls on extended milling which were monitored by energy-dispersive X-ray and Mossbauer spectroscopies.

Spectroscopic measurements of tungsten erosion in the ASDEX Upgrade divertor

Abstract: Tungsten erosion in the ASDEX Upgrade divertor has been measured by observing the WI emission at 400.9 nm. The sputtering yield is in the region of atoms/ion for typical divertor plasma conditions. By comparison of the measured sputtering yield with modelled sputtering yields one can conclude that tungsten erosion is dominated by plasma impurities. Adding impurities without cooling the boundary plasma increases the sputtering yield due to the additional impact energy gained by the particle in the sheath potential caused by its higher net charge. Cooling the boundary plasma by adding more impurities or increasing the plasma density leads to a strong decrease in the tungsten erosion. By comparison of the tungsten influx with the measured net erosion the prompt redeposition was determined. The redeposited fraction of tungsten increases significantly with a higher divertor electron density as expected.

Electrochemical Measurements during the Chemical Mechanical Polishing of Tungsten Thin Films

Abstract: A polishing tool and a potentiostat were used to simultaneously polish and measure the direct current (dc) open-circuit potential and anodic polarization behavior of chemical vapor deposited tungsten films in the presence of various oxidants. Of the different oxidants tested at pH 1.5 or pH 4.4, (NH 4 ) 6 Mo 7 O 24 formed the most protective passive layer on tungsten. Even in the presence of the most aggressive oxidant, Fe(NO 3 ) 3 , the dissolution rates of chemical vapor deposited tungsten were approximately 3 nm/min during abrasion, which is a very small fraction of typical removal rates reported for chemical mechanical polishing of tungsten. This indicates that electrochemical oxidation followed by abrasive removal of the oxidation product and dissolution may not be the primary mechanism for tungsten removal. Atomic force microscopy scans of polis ed tungsten films indicate that corrosion assisted fracture may be an important removal mechanism for tungsten during chemical mechanical polishing.

Titanium carbonitride coated stratified substrate and cutting inserts made from the same

Abstract: A process for forming a titanium carbonitride coated stratified cobalt enriched cemented carbide cutting insert, said cemented carbide is comprised of more than 70 percent by weight tungsten carbide and has a matrix binder comprising cobalt with a cobalt enriched stratified surface. The process comprises chemically vapor depositing a layer of titanium carbonitride on said cemented carbide substrate by heating gaseous reactants comprising titanium chloride, methane, hydrogen, and nitrogen with an optimum methane/nitrogen ratio, under suitable conditions to form a titanium carbonitride coated insert having an eta phase in the cemented carbide substrate adjacent said titanium carbonitride coating. The eta phase comprises chemical compounds consisting essentially of cobalt, tungsten and carbon, and the titanium carbonitride surface is contacted with a carburizing gas for a sufficient time and at a sufficient temperature to convert substantially all of said eta phase to elemental cobalt and tungsten carbide.

Observations of X-Ray Spectra from Highly Charged Tungsten Ions in Tokamak Plasmas

Abstract: X-ray spectra in the range 7 - 10 A from highly charged tungsten (Z = 74) ions were observed in the tokamak ASDEX Upgrade. Lines emitted from to were identified by comparison with ab initio calculations and with the expected ionization equilibrium charge-state distribution. Most of the features in the observed spectra coincide with predicted lines. A spectral line of As I-like could be identified as the predominant one for plasmas with 2.5 - 3 keV central electron temperature. The calculations predict clearly separated spectral lines for Kr I-like W at about 18.4 A and strong lines between 5.6 and 6.0 A for all charge states under consideration that can be used for monitoring W densities in the soft x-ray region. Tungsten concentrations in the range of were extracted from the emissivities of spectral lines from and .

Chemical Vapor Deposition of Tungsten Carbide, Molybdenum Carbide Nitride, and Molybdenum Nitride Films

Abstract: Tungsten carbides, β-WC 1-x and WC, molybdenum carbide nitride, Mo(C, N), and molybdenum nitride, MoN, films were prepared by chemical vapor deposition using WF 6 and MoF 6 with (CH)N, NH 3 , and H. The reaction of WF 6 with (CH 3 ) 3 N and H yielded WC films at 700-800°C and β-WC I-X films at 400-600°C. This reaction did not give tungsten carbide nitride, probably because of the higher structural instability of the tungsten nitride than the carbide. The interaction of MoF 6 with (CH 3 ) 3 N and H z , however, gave a molybdenum carbide nitride film at 700°C though the crystallinity was lower than that of cubic molybdenum nitride prepared at the same temperature. Cubic molybdenum nitride MoN films were obtained at 500-700°C from the reactions of MoF 6 with NH 3 and H z using Ar as a carrier gas. The powder product obtained at 800°C by the same reaction was composed of cubic and hexagonal MoN phases, while that prepared at 850°C was hexagonal MoN and metallic molybdenum. Molybdenum nitrides contained nitrogen atoms trapped in the grain boundary in addition to those occupying lattice points. Vickers hardness values of the MoN films prepared at 600-750°C were in the range of 1300 to 1400 kgf mm 2 .