Showing papers on "Tungsten published in 1974"

Reaction mechanism in chemisorption kinetics: nitrogen on the {100} plane of tungsten

Abstract: Kinetic models are developed for the formation of a chemisorbed overlayer on a homotattic surface with fourfold symmetry, taking into account the existence of short range order in the overlayer. It is shown that where the pairwise lateral interaction energy between nearest neighbour adatoms is large (repulsive or attractive), and the overlayer is consequently highly ordered, dissociative adsorption follows a pseudo first-order rate mechanism. Accurate sticking probabilities as a function of surface stoichiometry are reported for the interaction of nitrogen with the {100} plane of tungsten, obtained by a molecular beam technique, over a wide range of both surface and gas temperatures. The results are shown to be in quantitative agreement with the kinetic model, allowing for initial trapping into a mobile physisorbed state, and consistency is established with structural (low-energy electron diffraction) studies.

Sputtering of chemisorbed gas (nitrogen on tungsten) by low-energy ions

Abstract: Flash filament techniques and mass spectrometry have been used to measure sputtering yields for nitrogen chemisorbed on tungsten and bombarded with noble‐gas ions in the energy range up to 500 eV. The experimental results show that primarily nitrogen atoms rather than molecules are sputtered. Despite a high binding energy (∼6.7 eV/atom), we find high sputtering yields and low threshold energies. The results are found to be in reasonable agreement with simple theoretical estimates. It is sugguested that, in the investigated energy range, adsorbed nitrogen atoms are sputtered primarily as a consequence of direct knock‐on collisions with impinging and/or reflected noble‐gas ions. Estimates are also given for the yield of nitrogen atoms knocked off by sputtered tungsten atoms. This latter process is expected to dominate at much higher energies.

Electronic Structure and Catalytic Behavior of Tungsten Carbide

Abstract: Tungsten carbide has been shown to be an effective catalyst for a number of reactions that are readily catalyzed by platinum, but not at all by tungsten, and it was speculated that this behavior is due to changes in the electron distribution when carbon is added to tungsten. A test of this hypothesis, made by measuring the valence band x-ray photoelectron spectrum of tungsten carbide and comparing it with the spectra of tungsten and platinum, shows that, near the Fermi level, the electronic density of states of tungsten carbide more nearly resembles that of platinum than that of tungsten.

Surface Electronic Properties of Tungsten, Tungsten Carbide, and Platinum

Abstract: The local electronic structures of the surface regions of tungsten, tungsten carbide, and platinum have been compared. Contrary to the hypothesis that the platinum-like catalytic activity of tungsten carbide results from the contribution of carbon valence electrons to the 5d band of tungsten, the width of the unfilled portion of the d band increases on going from tungsten to tungsten carbide.

Structural studies by high‐resolution electron microscopy: tetragonal tungsten bronze‐type structures in the system Nb2O5–WO3

Abstract: The contrast in many-beam lattice images from very thin crystals of 4Nb2O5. 9WO3 is shown to be directly related to its known structure. On the basis of this correlation, the structure of 2Nb2O5.7WO3 is derived from the observed image contrast; it is an ordered intergrowth of the ReO3 and tetragonal tungsten bronze structural types. The structures of typical fault boundaries and disordered intergrowths are described, and those of the reported compounds 6Nb2O3. 11WO3 and 3Nb2O5.8WO3 are discussed.

Tungsten metallization for LSI applications

Abstract: It is shown that MOS–LSI devices can be satisfactorily metallized with tungsten thin films using the rf-diode sputtering technique. The apparatus used consisted of a roots-blower and a Vacion pumped belljar system equipped with an 8-in. diam CVD W-cathode and a W-substrate table (anode) to which a dc bias could be applied. The control substrates were oxidized silicon wafers in which the back-oxide was removed to facilitate proper application of the bias. The effect of sputtering power (200–400 W) and of substrate bias (−200−+250 V) was studied on the deposition rate, substrate temperature, and various properties of the film such as electrical resistivity, stress, impurity concentration, constitution, and microstructure. The present process of W-film depostition has been found to offer several advantages. Low-power and low-voltage operation results in high quality MOS-compatible W-films. Substrate heating during the sputter deposition is kept to a minimum, thereby eliminating any undesirable metallurgical ...

Process for making linings for friction operated apparatus

Abstract: A friction lining according to the invention is formed of bounded solidified previously molten particles formed on its support by plasma deposition; the base planes 0001 of a major proportion of the crytals in said lining are oriented in directions substantially parallel to the friction surfaces; the materials of which said linings are formed contain metals, metallic oxides or carbides having high melting points; they include cobalt, magnesium, neodymium, titanium, tungsten under its hexagonal form, yttrium, nickel under its hexagonal form, MoC, Mo 2 C, NbC, Nb 2 C, Ta 2 C, WC, W 2 C, V 2 C, Cr 2 O 3 , TiO 2 ; they may contain under an alloyed form proportions of non hexagonal materials, including molybdenum, chromium, aluminum, copper, iron, the non hexagonal nickel, niobium, B 4 C, TaC, TiC, Cr 3 C 2 , VC, ZrC, ZrO 2 , Al 2 O 3 , which do not affect the hexagonal structure of the alloys obtained.

The preparation and characterization of beta-tungsten, a metastable tungsten phase

Abstract: A metastable phase of tungsten (β-W) is obtained by hydrogen reduction of WO3 at 500°C. Five different samples of WO3 have been used to investigate the effects of oxide surface area and impurity content on the reduction process. The initial oxide samples and the reduction product have been characterized by scanning electron micrographs, and measurements of the oxygen content of β-W have been performed. It is concluded that β-W is not a suboxide as previously reported by Hagg and Schonberg. The effect of other elements on the β-W to α-@#@ W transformation temperature has been investigated.

An Evaluation of the Resistance to Electrolysis of Metals for Use in Biostimulation Microprobes

Abstract: Electrolysis measurements have been made on electrodes of platinum, stainless steel, tungsten, iridium, palladium, rhodium, rhenium, gold, tantalum, titanium, zirconium, and some conducting oxides of the tungsten bronze family. Electrodes were immersed in physiologic saline solution and subjected to biphasic rectangular current pulses of various current densities at 50 pps, 24 hours per day for periods up to nine months. Weight loss was recorded at intervals during this time. We conclude that iridium,rhodium, platinum and palladium are extremely resistant to electrolysis under these conditions and electrolysis under 5? thick, made of such materials should have lifetimes on the order of decades. Gold is somewhat poorer but probably acceptable; all the other materials tested, including tungsten and stainless steel, are unacceptable as chronic microelectrode materials.

Chemically Vapor Deposited Tungsten for Semiconductor Metallizations

Abstract: The properties of CVD tungsten films, produced by hydrogen reduction of WF/sub 6/ or WCl/sub 6/, were investigated to determine the suitability of the films for use as silicon integrated circuit metallizations. Both low pressure and atmospheric pressure flow systems were investigated. The tungsten films show excellent adhesion to silicon substrates, have resistivities between 6 and 15 mu ohm-cm depending on thickness, and are easily etched into submicron patterns. In addition, the contact resistance to silicon and MOS properties of tungsten films from both deposition reactions were measured. (8 figures) (auth)

Shock compressibility of porous tungsten, molybdenum, copper, and aluminum in the low pressure domain

Abstract: Results are presented of an experimental study of the behavior of four porous metals, tungsten, molybdenum, copper, and aluminum with an initial density corresponding to ≈30 and 50% of the density of the continuous (solid) material under loading by shocks with intensity between 10–800 kbar. The behavior of the shock compression curves in the p-ϱ plane shows that for some pressure p* the compression of the porous substance to the state of a continuous material is completed. At initial densities ≈50% of the continuous substance, the values are p*=65, 40 and 35 kbar, respectively, for W, Mo, and Cu. At lesser densities (≈30% of the density of the continuous substance), the values of p* are approximately halved as compared with those mentioned. The state of complete packing of the porous material into a monolithic solid was not determined in aluminum in the pressure range examined 6.5–200 kbar.

Study of the x‐ray production mechanism of a dense plasma focus

Abstract: The x‐ray emission from a 375‐kJ plasma focus is reported as measured on a time‐resolved basis with Ross filters and silicon diode detectors. Thick‐target electron beam spectra are calculated with a Monte Carlo electron beam transport program and fit to the experimental time‐integrated x‐ray spectra. A good fit is obtained with a 2‐MA E−2 power law electron beam, with cutoff energies at 8 and 200 keV, incident normal to a thick tungsten target. The source of the beam is judged to be electrons which are accelerated under the influence of strong electric fields. A 20‐nsec Q‐switched ruby laser pulse injected tungsten ions via laser ablation prior to the initial dense pinch, and the x‐ray emission below 4 keV was enhanced and that above 10 keV was reduced. The enhanced low‐energy x‐ray emission is in agreement with calculated radiation for collisional M‐shell excitation of highly stripped tungsten in a 1.5‐keV thermal plasma. A 900‐ μsec ruby laser pulse injected sufficient tungsten ions to degrade greatly t...

Interrelationships between process parameters, structure, and properties of CVD tungsten and tungsten–rhenium alloys

Abstract: The development of CVD techniques for fabricating free-standing tungsten and tungsten–rhenium alloy structures is reviewed. Relationships between plating parameters, kinetics, morphology, microstructure, and properties of thick polycrystalline deposits are discussed. It is emphasized that porosity may be grown into the grain boundaries when the depostition rate is controlled by gas phase diffusion, and that fully dense deposits are generally obtained when the rate is limited by a surface process. The origin and control of many of the microstructural features peculiar to CVD are also discussed.