Showing papers on "Tungsten published in 1991"

Multilayer coated abrasive element for bonding to a backing

Abstract: To prevent thermal stress from damaging a PCD or diamond, multilayer metal coatings for bonding polycrystalline diamond compacts and diamond crystals to a matrix comprise a first metal layer of a refractory metal, such as tungsten, a compliant metal layer of copper, and an outer metal layer of a refractory metal such as tungsten. Metallic bonding layers of a metal, such as nickel, are placed between the tungsten and copper layers for improved bonding. The method of manufacturing multilayer metal coatings comprises applying the inner metal layer by chemical vapor deposition, applying the first bonding layer metal by electrolytic deposition, applying the compliant layer metal by electrolytic deposition, applying the second bonding layer by electrolytic deposition and applying the outer layer by chemical vapor deposition. A superabrasive tool element comprises a coated diamond product bonded either to a matrix comprising tungsten carbide or iron powder or to a cemented tungsten carbide support.

Metastable phase transformations induced by ball-milling in the CuW system

Abstract: For the first time, based on X-ray diffraction, scanning electron microscopy/energy dispersion X-ray analyses, differential thermal analysis and differential scanning calorimetry experiments, the crystal to non-equilibrium phase transition induced by ball-milling has been evidenced in the CuW system. This system exhibits a total immiscibility in both solid and liquid states. Starting from elemental copper and tungsten powders, a partial solubility of copper into the cubic tungsten lattice as well as of tungsten into the f.c.c. copper lattice is induced by ball-milling. Such an enhancement of the solubility is revealed by an effect on the lattice parameter of both Cu(W) and W(Cu) crystalline solid solutions over the whole investigated composition range, i.e. Cu 5 W 95 to Cu 95 W 5 (wt.%). Such an experimental investigation of the crystal to amorphous phase transition induced by ball-milling in a system which exhibits a positive heat of mixing either in the solid or in the liquid states supports our previous results leading to the conclusion that the mechanisms of the phase transitions induced by ball-milling are different from those of the so-called classical solid-state amorphization.

X-ray absorption spectroscopy study of the titania- and alumina-supported tungsten oxide system

Abstract: Tungsten oxide supported on titania is an important material for the selective catalytic reduction of nitrogen oxides NO{sub x}. A structure analysis by means of X-ray absorption spectroscopy (XANES and EXAFS) is reported for materials prepared by spreading of WO{sub 3} in physical mixtures with the support oxide as well as by impregnation from aqueous solution. A comparison is also made with alumina-supported materials. Analysis of the XANES region at the W L{sub 1} and W L{sub 3} edges indicated that tungsten is hexavalent and anchored to the surface as WO{sub 5} and WO{sub 4} units, the relative proportion of which increases with loading. When water is absorbed, pseudooctahedrally coordinated species are formed in both cases. The analysis of the EXAFS provides additional support for the existence of these structures, which contain oxo groups W{double bond}O and W-O-W bridges. A tentative structure model is proposed, in which islands of surface tungstate species are formed by branched chains of WO{sub 5} units. The chains are assumed to be terminated by WO{sub 4} units, the WO{sub 5}/WO{sub 4} ratio thus increasing with chain length or island size, which obviously increases with loading.

Role of platinum in aluminide coatings

Abstract: An investigation has been carried out to determine the role of platinum in aluminide coatings on selected nickel-based superalloys. Various microscopy, spectroscopy and diffraction techniques were used to characterize the microstructure. It is found that platinum improves the protective nature of the coating by (1) increasing its diffusional stability, (2) eliminating chromium-rich precipitates from the outer coating layer and (3) preventing refractory transition elements such as molybdenum, vanadium and tungsten from diffusing into the outer coating layer. However, the oxidation behaviour of the coating varies from one alloy substrate to another depending upon its chemical composition and, in particular, the rare earth elements.

The origin of stress in sputter‐deposited tungsten films for x‐ray masks

Abstract: The mechanism for the cause of stress in a sputter‐deposited tungsten (W) film has been clarified. The tensile stress of the film was calculated using the interatomic forces acting on the grain boundary. The average distance of the grain boundary gaps was determined from the measured film density assuming the film had homogeneous size rectangular grains. The calculated and measured stress values were in good agreement in the high working gas pressure region. The difference between these values in the low working gas pressure region has been able to be explained by the compressive stress due to the peening effect of Ar. The low stress in the high pressure region was obtained by large opened grain boundaries which produced low film density. A low film density causes a low x‐ray stopping power. The film deposited in the low pressure region is suitable as an x‐ray absorber because of its high film density.

Electrothermal vaporization using a tungsten furnace for the determination of rare-earth elements by inductively coupled plasma mass spectrometry

Abstract: In electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) using a tungsten furnace, the effect of vaporizing parameters on signals and the performance of the ETV technique in the measurement of rare-earth elements (REEs) have been investigated. The precision for La was 1-2% at absolute amounts above 1 pg. The detection limits for 14 REEs were 0.0001-0.0006 ng mL −1 , 1 order of magnitude better than those obtained by NEB-ICP-MS

Metal/metal‐oxide interfaces: A surface science approach to the study of adhesion

Abstract: Metal‐oxide/metal interfaces play an important role, for example, in the joining of an oxide ceramic to a metal for sealing applications. In order to probe the chemical and physical properties of such an interface, we have performed Auger electron spectroscopic (AES) and temperature programed desorption (TPD) experiments on a model system composed of very thin films of Cr, Fe, Ni, or Cu evaporated onto a very thin thermally grown oxide on a W single crystal. Monolayer films of Fe and Cr were found (by AES) to completely wet the oxide surface upon deposition, and were stable up to temperatures at which the films desorbed (≊1300 K). In contrast, monolayer Ni and Cu films formed three‐dimensional islands exposing the oxidized W surface either upon annealing (Ni) or even upon room‐temperature deposition (Cu). The relative interfacial interaction between the overlayer metal and the oxide, as assessed by TPD, increases in the series Cu

The Performance of Electrochromic Tungsten Trioxide Films Doped with Cobalt or Nickel

Abstract: Tungsten trioxide is being developed as a electrochromic material for smart windows. However, the relatively high electrical resistivity and overpotential for hydrogen evolution and oxidation affect their response time and electrical power consumption. Previous work on Pt/WO{sub 3} indicated that admixing of Pt onto WO{sub 3} significantly reduced the overpotential for hydrogen oxidation and evolution reactions since the formation of hydrogen tungsten bronze results in the spill-over of the hydrogen reaction to the tungsten bronze surface. This paper reports work on the preparation of code-posited Ni/WO{sub 3} and Co/WO{sub 3} films which give significantly faster response times and consumes less power than traditional WO{sub 3} films.

Sol–gel synthesis of WO3 thin films

Abstract: Versatile molecular precursors for the sol–gel synthesis of tungsten oxide thin films can be obtained via the reaction of tungsten oxychloride WOCl4 with alcohols. Oligomeric species [WOCl4 –x(OR)x]n are formed. Their molecular structure is analysed by infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy (1H, 13C, 183W), X-ray absorption spectroscopy (EXAFS) and small-angle X-ray scattering (SAXS). Hydrolysis of these precursors leads to the formation of tungsten oxide colloidal solutions. They can be easily deposited by dipcoating. Thin films ca. 3000 A thick are obtained. Their morphology depends on the nature of the alcohol. Homogeneous films are obtained with bulky alkyl groups such as PriOH. These amorphous tungsten oxide layers exhibit electrochromic properties and could be used for display devices or smart windows. They can also be easily transformed, at room temperature, into crystalline hydrates WO3·nH2O (n= 1 or 2) when left in a humid atmosphere.

Etchback process for tungsten utilizing a NF3/AR chemistry

Abstract: A process for etching a tungsten layer formed on a semiconductor substrate is described. The etch is carried out in a parallel plate plasma reactor. The etchant gases include nitrogen trifluoride (NF 3 ) and argon (Ar). The use of NF 3 in a tungsten etching process reduces the build-up of polymers or sulfur residues on the electrode as occurs with processes utilizing sulfur or carbon fluorides as etchant gases. The process has a sufficiently high etch rate for volume production. The NF 3 -Ar etch process can be used to etchback a blanket layer of deposited tungsten to form tungsten via plugs in contact areas of the device. In the via plug process, reduced micro-loading effect, that is, the tendency of some plugs to be etched away before the complete etching of the blanket layer, has been achieved. The etching of tunsten with NF 3 -Ar process can be preformed in one or more steps in process utilizing several etching steps. Additionally, a tungsten etch incorporating one or more NF 3 -Ar steps and one or more steps utilizing etchants such as SF 6 , Cl 2 , O 2 , CF 4 , CBrF 3 , CF 3 Cl, CF 2 Cl 2 or similar etchants can be used to optimize etch rate and uniformity while obtaining the benefit of reduced residue build-up.

Solution softening mechanism of iridium and rhenium in tungsten at room temperature

Abstract: With the highest melting temperature among metals, tungsten possesses a combination of properties desired for space nuclear power applications. One of the major deterrents to the application of tungsten lies in its low-temperature brittleness. The poor ductility of tungsten at room temperatures results in great difficulty in fabrication. Various approaches, including purification and alloying, have been employed to ductilize tungsten at room temperature. The fact that rhenium additions increase the ductility of tungsten was first reported by Geach and Hughes 1 in 1956, and following study confirmed that rhenium added to tungsten not only improves the room temperature fabricability, but also increases the high-temperature strength. The alloy system studied most in the past was tungsten-rhenium binary system, and the mechanical test conducted was limited to the bending test which was far from enough to examine the solution softening mechanism of rhenium in tungsten. The effect of rhenium on the room temperature mechanical properties of tungsten-rhenium-thoria ternary system has not been researched. Besides, based on its similarity to rhenium in electron structure, iridium (Ir) appears to be another potential softening element in tungsten. But so far, there are no mechanical data available for tungsten-iridium system, either at room temperature or at high temperatures. In the present study, a comprehensive investigation has been performed on the room-temperature hardness, tensile properties, microstructure and fracture behavior of various tungsten-iridium and tungsten-rhenium-thoria alloys. The objective of this study was to examine the solid-solution softening mechanism of iridium in unalloyed tungsten and that of rhenium in thoriated tungsten at room temperature. The effect of alloying elements on the fracture behavior of tungsten-iridium and tungsten-rhenium-thoria alloys was also examined.

Method for fabricating storage node capacitor having tungsten and etched tin storage node capacitor plate

Abstract: A dynamic random access memory (DRAM) storage cell having a storage contact capacitor comprising a tungsten and TiN storage node capacitor plate and the method for fabricating the same. At least a portion of the storage node capacitor plate is formed vertically in the DRAM. The TiN is controllably etched to increase the area of the storage node capacitor plate. An upper poly layer functions as the cell plate and is insulated from the storage node capacitor plate by a dielectric layer.

Method for forming low resistance and low defect density tungsten contacts to silicon semiconductor wafer

Abstract: An improved process is described for forming planar tungsten-filled contacts to a silicon substrate in contact openings through an insulating layer which provides for the formation of titanium silicide in and on the silicon surface at the bottom of the contact openings to provide low resistance silicide interconnections between the silicon substrate and the tungsten. A titanium nitride layer is formed over the titanium silicide and on the surfaces of the insulation layer, including the top surface of the insulation layer and the sidewall surfaces of the contact openings through the insulating layer. This titanium nitride layer provides a nucleation layer which permits a good bond to form from the tungsten through the titanium nitride and titanium silicide in the contact openings to the silicon substrate; and from the tungsten through the titanium nitride layer to the insulator material such as silicon dioxide (SO2), resulting in the formation of low resistance and low defect density contacts.

Raman Study on Sol-Gel Derived Tungsten Oxides from Tungsten Ethoxide

Abstract: Tungsten oxide gels were prepared by a sol-gel process from a tungsten ethoxide/ethanol/water solution (sol). Tungstic hydrates were observed in high-viscosity sol by Raman spectroscopy. As the sol is reduced to the gel, Raman bands ascribed to amorphous tungsten oxide appeared. The result of the Raman measurement shows that the structure of the gel changes progressively during heat treatment and that the amorphous tungsten oxide becomes a crystalline solid at 410?C.

Optical properties of dilute hydrogen tungsten bronze thin films

Abstract: Transmission and reflection spectra in the range of 0.5–4.1 eV have been obtained for HxWO3 polycrystalline thin films of 5‐nm mean grain size. The composition range x=0–0.16 has been investigated. The measurements were refined to give the absorption coefficient and other optical parameters versus photon energy. The data have been interpreted in terms of the interband transition model due to Green and Travlos [Philos. Mag. B 51, 501 (1985)]. It is noted that main gap changes because of changes in crystal structure, and there is a sensitivity of the main gap value to grain size which may have its origins in quantum confinement effects.

Sputtering of Tungsten by Carbon

Abstract: The sputtering of tungsten by carbon is investigated experimentally and by computer simulation. The erosion and deposition is studied as a function of the fluence of the incident carbon at 1 and 6 keV and for different angles of incidence. Both, experimental data and computer simulation show a transition from net carbon deposition to net erosion as the angle of incidence exeeds values of 40°.

A Mathematical Model for Chemical Vapor Deposition Processes Influenced by Surface Reaction Kinetics: Application to Low‐Pressure Deposition of Tungsten

Abstract: This paper reports on a mode for the simultaneous reaction kinetics and transport processes in chemical vapor deposition (CVD) reactors extended to treat deposition of materials that have a broad range of surface characteristics, e.g. sites with multiple dangling bonds and adsorbates with multiple bonding configurations. The model uses the nature of the surface to determine the elementary processes that can take place during growth. Rate constants for these processes are calculated from first principles using statistical thermodynamics, transition state theory, and bond dissociation enthalpies. In this way, deposition rates are determined without either assuming the reaction mechanism or arbitrarily choosing any kinetic parameter values. The utility of the approach is illustrated by modeling low-pressure CVD of tungsten from tungsten hexafluoride and hydrogen. The treatment considers 14 species and eight reactions in the gas together with 21 species and 65 processes at the surface.

Stability of tungsten/carbon and tungsten/silicon multilayer x‐ray mirrors under thermal annealing and x‐radiation exposure

Abstract: The effect of thermal annealing and irradiation in an intense white synchrotron x‐ray beam on the x‐ray reflectance of tungsten/carbon and tungsten/silicon multilayers is reported. Thermal annealing at 400 °C for two hours produces larger effects than irradiation of cooled multilayers in the white beam of a 20‐pole hard x‐ray wiggler with 0.94‐T peak field on the storage ring DORIS operating at 5.42 GeV and electron currents of 20–36 mA for 40 h. Thermal annealing caused the period and first order reflectance of a W/Si sample to decrease, in contrast to a W/C sample whose period and reflectance increased on annealing. Of five actively cooled samples irradiated, one W/C sample showed significant change in reflectance. Preannealing of this multilayer stabilized it to radiation‐induced changes. Irradiation effects also depend on multilayer period and constituent materials. Implications of these results for models describing multilayer reflectance and for multilayer applications in the new generation of synchrotron radiation sources are discussed.

Mechanical alloying of high melting point intermetallics

Abstract: We report on the synthesis of high melting point metal-metalloid intermetallics using mechanical alloying. For all of the over 50 binary systems containing iron, titanium, nickel, vanadium, tungsten, silicon, boron and carbon studied by us the intermetallic phases from via one of the four reaction channels: (1) direct formation of an intermetallic; (2) amorphous phase formation followed by annealing; (3) metastable nanostructure formation followed by annealing; or (4) fine mixture formation followed by annealing. In all of these cases annealing is performed at temperatures well below the melting point. These reaction paths are documented with X-ray diffractometry results.

Study on mechanism of selective chemical vapor deposition of tungsten using in situ infrared spectroscopy and Auger electron spectroscopy

Abstract: Selective chemical vapor deposition (CVD) of tungsten (W) using tungsten hexafluoride (WF6) and monosilane (SiH4) is investigated by in situ infrared spectroscopy and Auger electron spectroscopy. The infrared spectra show that trifluorosilane (SiHF3) is the main by‐product species, and that silicon‐tetrafluoride (SiF4) is less than 20%–25% of SiHF3 in partial pressure. The main chemical reaction involved in selective W CVD can be expressed as WF6+2SiH4→W+2SiHF3+3H2. Based on our experimental results, a new mechanism of selective W CVD, which notes hydrogen dissociation having a central role in this process, is proposed. It disproves the widely accepted model, which is based on the assumption that SiF4 is the major reaction product.