Showing papers on "Tungsten published in 1971"
TL;DR: The binding states, adsorbate densities, and desorption kinetics of hydrogen on the (110), (111), and (100) crystal planes of tungsten are examined in this paper.
Abstract: The binding states, adsorbate densities, and desorption kinetics of hydrogen on the (110), (111), and (100) crystal planes of tungsten are examined. There are two atomic states on the (110) plane and four atomic states on the (111) plane. Saturation hydrogen densities are approximately equal in the major states on both planes and correspond to one hydrogen atom for each surface tungsten atom on the (110) plane and approximately four hydrogen atoms for each surface tungsten atom on the (111) plane. While the desorption activation energies of the most tightly bound states on all three planes are nearly identical, there are weakly bound (Ed = 14 and 22 kcal mole−1) atomic states on the (111) plane which are entirely lacking on the other planes. These results show that there are considerable variations in hydrogen adsorption on these planes. Arguments are advanced that the surface atom configurations are nearly those of the bulk crystal, and hydrogen surface structures are proposed to account for the observed...
198 citations
TL;DR: In this article, the hydrogenation of cyclohexene and benzene on nickel-tungsten sulfide and tungsten disulfide catalysts is compared. And the role of nickel in multiplying the number of active tungststen sites is evaluated.
170 citations
TL;DR: In this paper, the LEED structures resulting from hydrogen adsorption on (100)W are studied as functions of hydrogen coverage and surface temperature for temperatures above 78 °K, and it is suggested that this structure on the clean surface may be due to structural rearrangement of tungsten or an adsorbed state of hydrogen which goes into solution upon heating.
143 citations
TL;DR: In this article, it was shown that the active centers in these catalysts are tungsten ions, probably in the trivalent state, and that the hydrogenation activity for benzene is proportional to the intensity of this ESR absorption.
113 citations
01 Jan 1971-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: The mechanisms and kinetics of bubble formation that occur during annealing of tungsten doped with small amounts of K, Al, and Si compounds has been investigated by electron microscopy as mentioned in this paper.
Abstract: The mechanisms and kinetics of bubble formation that occur during annealing of tungsten doped with small amounts of K, Al, and Si compounds has been investigated by electron microscopy. Bubbles are present in the sintered ingot due to volatilization of the dope during the sintering treatment. As the ingot is worked down into wire, the bubbles present in the ingot become elongated. On annealing these elongated bubbles undergo shape changes which depend on the amount of working. Elongated bubbles with a length-to-width ratio of less than ten spheroidized while those with a length-to-width ratio greater than twenty are unstable and break up into a row of bubbles. If working has been sufficient to close up the elongated bubbles,e.g., in ribbon, 0.125 mm thick rolled from 0.9 mm diam wire, bubble formation results from the diffusion of vacancies to the volatile dopant particles.
97 citations
89 citations
84 citations
TL;DR: The binding energies of transition-metal atoms of the $5d$ series adsorbed on a transition metal are calculated in the tight-binding approximation as discussed by the authors, and the general features of the variation of the binding energy with the number of adatom $5D$ electrons are similar for all the substrates.
Abstract: The binding energies of transition-metal atoms of the $5d$ series adsorbed on a $5d$ transition metal are calculated in the tight-binding approximation. The general features of the variation of the binding energy with the number of adatom $5d$ electrons are similar for all the substrates. It shows a parabolic behavior with a maximum of the order of the cohesive energy for the substrate, and a subsequent sharper decrease as the number of adatom $5d$ electrons is increased. The maximum is always located close to tungsten, as in the case of a tungsten substrate, where it is between tungsten and rhenium.
83 citations
TL;DR: In this article, the relative field evaporation rates of tungsten kink site atoms at (110) plane edges and the absolute field eva-cation rates of individual Tungsten adatoms on ( 110) planes have been measured covering nine orders of magnitude.
Abstract: The relative field evaporation rates of tungsten kink site atoms at (110) plane edges and the absolute field evaporation rates of individual tungsten adatoms on tungsten (110) planes have been measured covering nine orders of magnitude. From the experimental data, the effective polarizabilities of the tungsten kink site atoms and the adatoms are found to be 4.6 ± 0.6 and 6.8 ± 1.0 A3, respectively. A theoretical consideration is given to explain why the effective polarizability of metal surface atoms depends on the atom‐to‐surface mirror plane distance, and therefore the atomic sites. The calculation also reveals a first power field dependent energy level shift, which is dependent upon the atom‐to‐surface plane distance. It is further shown that the classical image potential no longer holds at a distance smaller than 1 A from the metal surface.
79 citations
69 citations
TL;DR: In this article, the work function dependence on adsorption of alkali metals on Ta/110/ and W/100/ crystals under ultrahigh vacuum was investigated and the results showed that the dependence on adorption was independent of work function.
TL;DR: In this paper, the formation of a corrosive oxide layer of apparently unlimited thickness during the interaction of oxygen with a polycrystalline tungsten filament at temperatures between 500 and 1000°K has been investigated by line-of-sight flash desorption mass spectrometry.
Abstract: The kinetics of formation of a corrosive oxide layer of apparently unlimited thickness during the interaction of oxygen with a polycrystalline tungsten filament at temperatures between 500 and 1000°K has been investigated by line‐of‐sight flash desorption mass spectrometry. The major desorption product from this multilayer oxide film is WO2, which desorbs over the temperature range 1000–1300°K. The rate of formation of the oxide film was shown to be directly proportional to the oxygen gas phase pressure at pressures ≲ 10−4 torr. A study of the temperature dependence of the oxidation rate revealed that the formation of the WO2‐producing oxide layer proceeds through two distinct steps; these occur subsequent to the first three stages in the interaction of oxygen with W characterized in Paper I. In the final state of oxidation, the rate is shown to be independent of the thickness of the oxide film. The results are in qualitative accord with the theoretical model of Mott. Desorption of the oxide film is a zer...
IBM1
TL;DR: In this paper, exposure of polycrystalline tungsten surface to atomic nitrogen or bombardment by N2+ is shown to enhance chemisorption, and a somewhat greater saturation coverage is obtained by exposure to neutral atomic nitrogen.
TL;DR: In this article, the authors examined the condensation kinetics of hydrogen on the (110, (111), and (100) planes of tungsten as functions of crystal temperature, coverage, and population of the various binding states.
Abstract: The condensation kinetics of hydrogen on the (110), (111), and (100) planes of tungsten are examined as functions of crystal temperature, coverage, and population of the various binding states. Measurements were carried out simultaneously on all three crystals in the same ultrahigh‐vacuum system in order to accurately compare sticking coefficients and amounts adsorbed. There are large variations in the initial sticking coefficients s0; at 78°K these vary between 0.26 on the (111) plane to 0.07 on the (110) plane. This implies that the electronic transition involved in bond formation depends sensitively on the geometric and electronic structures of the substrates. Also s0 is observed to be almost independent of substrate temperature for all planes, showing that phonon assisted processes do not determine the energy transfer. The coverage dependence of s shows considerable variation between states. In all cases a weakly adsorbed precursor state is involved, but the kinetics of conversion to the tightly bound...
TL;DR: In this article, the anodic dissolution of tungsten was studied in solutions and solutions at 25°C and the results showed that the metal was oxidized to the +6 state.
Abstract: The anodic dissolution of tungsten was studied in solutions and solutions at 25°C. Faradaic efficiency studies in basic solutions showed the metal to be oxidized to the +6 state. In acid solutions, a thick protective film of yellow was formed. Polarization curves possessed linear Tafel regions at potentials slightly more positive than the rest potentials. An anodic dissolution mechanism is proposed which involves a surface film of that is further oxidized to and dissolves by hydrolyzation.
TL;DR: In this article, an abrupt transition was observed between a condition of "passivation", in which a thin film of SiO2 formed at low temperatures, and a steady-state "combustion" condition at high temperatures.
Abstract: The reaction of oxygen at low pressures with silicon layers on tungsten ribbons was studied. An abrupt transition was observed between a condition of “passivation,” in which a thin film of SiO2 formed at low temperatures, and a steady-state “combustion” condition at high temperatures. The latter state is characterized by the formation of volatile SiO. The boundary between these two states has been defined in terms of the pressure-temperature relation. Oxygen consumption in the “combustion” state is represented by first-order reaction kinetics with an activation energy of 13 ± 1 kcal/mole. The stability of the two states has been defined by a thermodynamic analysis of the SiO2 layer stability. The oxygen consumption dependence on temperature has been described by a kinetic model which involves a consideration of the various elementary steps in the reaction.
TL;DR: The adsorption of CH4 by the (100) plane of tungsten has been studied using flash desorption and work function methods in this paper, where it was found that CH4 adsorbs to appreciable coverage as an undissociated molecular species at ∼ 110 K. This observed binding energy is greater than expected for dispersion forces alone, and added contributions to the binding energy are discussed.
TL;DR: In this article, secondary electron emission coefficients for thermal energy He(21S), He(23S), Ne(3P0.2) and Ar(3p0.4) metastable atoms incident on a gold surface have been measured using an improved gas cell absorption method.
Abstract: For pt. I see ibid., vol.4, 1683 (1971). Secondary electron emission coefficients for thermal energy He(21S), He(23S), Ne(3P0.2) and Ar(3P0.2) metastable atoms incident on a gold surface have been measured using an improved gas cell absorption method. The coefficients for the same metastable species incident on chemically cleaned stainless steel and copper surfaces and on atomically clean cadmium and tungsten surfaces have been determined by calibration against the gold surface. The results indicate that rather larger coefficients than those used hitherto should be used for gold surfaces. The implications of the results presented for the measurement of metastable fluxes are examined.
TL;DR: In this article, the electron temperature of a quiescent plasma in a metal discharge chamber, where the walls serve as the anode, can be raised by introducing a second anode consisting of 0.0025 cm−diam tungsten wires held at potentials as high as 100 V with respect to the walls.
Abstract: The electron temperature of a quiescent plasma in a metal discharge chamber, where the walls serve as the anode, can be raised by introducing a second anode consisting of 0.0025‐cm‐diam tungsten wires held at potentials as high as 100 V with respect to the walls. Hot electrons orbit the wires and return to the plasma. Cold electrons are absorbed. Smooth temperature variation, without introduction of noise, has been achieved between 0.5 and 4.0 eV.
TL;DR: In this article, single crystals of nickel have been grown on tungsten field emitter tips by vapour deposition in UHV, using substrate temperatures in the range 425-700° K, and nickel fluxes of the order of 0.1-10 atom layers per minute.
TL;DR: The superconducting critical temperatures of a total of 18 samples of α-Mo2C, β-Mo 2C, η-MoC1−x and α-W2C have been determined resistively.
Abstract: The superconducting critical temperatures of a total of 18 samples of α-Mo2C, β-Mo2C, η-MoC1−x and α-W2C have been determined resistively. The critical temperatures lie within the ranges 6°–7.3°, 5.1°–7.2°, 7.4°–8.9° and 2.95°–3.05°K, respectively. The critical temperature for β-Mo2C is much higher than found previously, and similar to that for α-Mo2C.
TL;DR: In this article, the results of a series of experiments demonstrated that self-interstitial atoms (SIAs) were immobile at an irradiation temperature of 15°K, and that these same SIAs...
Abstract: In Part I (see previous paper) the results of a series of experiments demonstrated that self-interstitial atoms (SIAs) were immobile at an irradiation temperature of 15°K, and that these same SIAs ...
TL;DR: The decomposition of ammonium paratungstate 5(NH4)2O.5H2O in hydrogen has been studied thermogravimetrically and the formation of a hexagonal ammonium tungsten bronze phase was established as mentioned in this paper.
Abstract: The decomposition of ammonium paratungstate 5(NH4)2O.12WO3.5H2O in hydrogen has been studied thermogravimetrically and the formation of a hexagonal ammonium tungsten bronze phase, (NH4)xWO3(x∼0.25) established. Powder X-ray diffractometer studies, and measurements of electrical conductivity, magnetic susceptibility and reflectance spectra show that this phase is both iso-structural and iso-electronic with corresponding tungsten bronzes formed by potassium and rubidium.
TL;DR: In this article, an X-radiographic technique was developed to obtain profiles of molten ceramic menisci and pendant drops sealed in molybdenum or tungsten capsules.
Abstract: An X-radiographic technique was developed to obtain profiles of molten ceramic menisci and pendant drops sealed in molybdenum or tungsten capsules. The surface tension for Al2O3 was calculated from the shape of the drops or menisci. Molybdenum capsules lowered the surface tension value. The density was 3.01 g/cm8 at the melting point.
TL;DR: In this paper, the electrical and piezoresistive properties of vacuum deposited films of copper, tungsten, platinum and gold were investigated in the thickness range from 100 to 1000 A.
Abstract: The electrical and piezoresistive properties of vacuum deposited films of copper, tungsten, platinum and gold were investigated in the thickness range from 100 to 1000 A. Both the conductivity and the longitudinal strain coefficient of resistance exhibit a size dependence, that was interpreted by applying the Sondheimer theory of charge transport in thin films on the assumption that the reflection of electrons by the film surfaces is partially specular. The proportion of electrons elastically scattered has been deduced from considerations based on the anomalous skin effect. The calculations are in reasonable agreement with the observations; the discrepancies can be attributed to the chemical contamination of the samples and to departures of the surface structure from the idealized flat configuration assumed in the theoretical treatment.
TL;DR: In this article, a method is described to calculate the temperature distribution along the tip of a conical tip of tungsten and some other metals at temperatures between 1000 and 2900 °K.
TL;DR: In this paper, the multilayer adsorption and growth of cadmium on clean polycrystalline tungsten ribbons was investigated with a molecular beam and mass-spectrometric technique.
Abstract: The multilayer adsorption and growth of cadmium on clean polycrystalline tungsten ribbons (with strong preferred orientation) has been investigated with a molecular‐beam and mass‐spectrometric technique. The adsorption and desorption kinetics have been studied as a function of temperature, supersaturation, and coverage. Five distinct adsorption states have been found between 100 and 600°C. Growth of bulk cadmium occurs only after multilayer adsorption is complete. No measurable supersaturation or critical nucleation event is observed for the growth of cadmium. However, even at high supersaturation and heavy coverage, the condensation coefficient is still smaller than unity. The kinetic and structural data are explained by the evolution of faceted, crystallographic, and epitaxial cadmium islands on the cadmium multilayer adsorbate.