Showing papers on "Oxide published in 1990"

Growth of native oxide on a silicon surface

Abstract: The control factors controlling the growth of native silicon oxide on silicon (Si) surfaces have been identified. The coexistence of oxygen and water or moisture is required for growth of native oxide both in air and in ultrapure water at room temperature. Layer‐by‐layer growth of native oxide films occurs on Si surfaces exposed to air. Growth of native oxides on n‐Si in ultrapure water is described by a parabolic law, while the native oxide film thickness on n +‐Si in ultrapure water saturates at 10 A. The native oxide growth on n‐Si in ultrapure water is continuously accompanied by a dissolution of Si into the water and degrades the atomic flatness at the oxide‐Si interface, producing a rough oxide surface. A dissolution of Si into the water has not been observed for the Si wafer having surface covered by the native oxide grown in air. Native oxides grown in air and in ultrapure de‐ionized water have been demonstrated experimentally to exhibit remarkable differences such as contact angles of ultrapure waterdrops and chemical binding energy. These chemical bond structures for native oxide filmsgrown in air and in ultrapure water are also discussed.

Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides

Abstract: Nature of oxide surface centres characterization of the state of surface cations on oxide catalysts by IR study of the absorbed probe molecules CO and NO complexation of alkenes on oxide surfaces application of infrared spectroscopy to mechanistic studies of heterogeneous catalytic reactions

Dependence of catalytic rates on catalyst work function

Abstract: THE catalytic activity and selectivity of metals can be altered dramatically and reversibly by supplying or removing oxide anions, O2–, at the metal catalyst surface by interfacing the catalyst with an O2– conductor such as zirconia1–6. This effect, termed non-faradaic electrochemical modification of catalytic activity (NEMCA), leads to a steady-state catalytic reaction rate increase up to 3 × 105 times higher than the rate of supply or removal of O2– at the catalyst surface1,3,4. Here we report that β"-Al2O3, which is a Na+ conductor, can also induce the NEMCA effect. Furthermore we show that the origin of the NEMCA effect lies in the controlled variation of catalyst work function on polarization of the metal-solid-electrolyte interface, and demonstrate that the rates of metal-catalysed reactions depend exponentially on the average catalyst work function. Thus the influence of catalyst electronic structure, rather than surface geometry, is here the key factor in catalytic activity.

Oxidation of metastable single‐phase polycrystalline Ti0.5Al0.5N films: Kinetics and mechanisms

Abstract: Metastable single‐phase, NaCl‐structure, polycrystalline Ti0.5Al0.5N alloy films have been shown to exhibit much better high‐temperature (750–900 °C) oxidation resistance than polycrystalline TiN films grown under similar conditions. The Ti0.5Al0.5N alloys, ≂3 μm thick, were deposited at temperatures between 400 and 500 °C on stainless‐steel substrates by dc magnetron sputter deposition in mixed Ar+N2 discharges with an applied negative substrate bias Vs of either 0 or 150 V. Oxidation in pure O2 initially occurred at a rate that varied parabolically with time. The oxide overlayers consisted of two partially crystalline sublayers, the upper one Al‐rich and the lower one Ti‐rich, with no measurable N concentrations in either. Inert‐marker transport experiments showed that oxidation proceeded by the simultaneous outward diffusion of Al to the oxide/vapor interface and inward diffusion of O to the oxide/nitride interface. The oxidation rate constant K increased with oxidation temperature Tox at a rate much h...

An ESCA method for determining the oxide thickness on aluminum alloys

Abstract: A simple method for estimating the oxide thickness on aluminum alloys using ESCA was investigated. The method is based on a uniform overlayer model and requires only a single ESCA measurement per sample. Expressions are shown which relate the experimental intensity ration of the oxidic and metallic Al 2p ESCA peaks to the oxide thickness, for both MG and Al x-ray sources. The method does not require ion etching or separate calibration procedures. Several practical examples that illustrate the usefulness of this method are presented. The method can also be applied to other thin film (i.e. <∼ 100 A) metal oxide/metal systems. The maximum oxide thickness that can be measured using the method is limited to approximately three times the inelastic mean free path of the appropriate photoelectrons within the material of interest.

Partial oxidation of methane to synthesis gas

Abstract: Partial oxidation of methane to synthesis gas has been carried out over a number of transition metal catalysts under a range of conditions. It is found that the metals Ni, Ru, Rh, Pd, Ir and Pt, either supported on alumina or present in mixed metal oxide precursors, will bring the system to equilibrium. The yield of CO and H2 improves with increasing temperature in the range 650–1050 K, and decreases with increasing pressure between 1 and 20 atm. An excellent yield (∼92%) is obtained with a 4∶2∶1 N2∶CH4∶O2 ratio at 1050 K and atmospheric pressure, with a space velocity of 4×104 hour−1.

Theory of Steady‐State Passive Films

Abstract: A theory is developed for the steady-state properties of passive films that form on metals and alloys in aqueous environments. This theory is based on the point defect model developed earlier, and predicts that they steady-state thickness of the barrier film and the log of the steady-state current will vary linearly with applied voltage. These relationships may be used to estimate empirical parameters that describe the dependencies of the potential drop across the barrier film/environment interface on the applied voltage and pH and to estimate kinetic parameters for dissolution of the film. If dissolution at the film-solution interface occurs very slowly, the primary passive film is envisaged to consist of a rigid oxide sublattice that transmit cations from the metal to a gel-like, precipitated upper layer. If dissolution at the barrier film/environment interface occurs rapidly, then a steady-state thickness is achieved by a balance between the rate of dissolution of the film at the film-solution interface and the rate of growth of the film into the underlying metal phase. Due to the outward movement of oxygen vacancies (i.e., inward movement of oxygen ions) through the barrier layer. The model is found to account for many experimental data that have beenmore » published in the literature on the quasi steady-state properties of passive films.« less

Reflection high-energy electron diffraction oscillations during epitaxial growth of high-temperature superconducting oxides

Abstract: Strong intensity oscillations have been found in reflection high-energy electron diffraction during epitaxial growth of ${\mathrm{BaTiO}}_{3}$, ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ with perovskite-type structures. The oscillation period corresponds to the height of the minimum unit of each oxide that satisfies the chemical composition and electrical neutrality. The data provide a basic understanding of the epitaxial growth process of ionic oxides.

The role of noble metals in the chemistry of solid-state gas sensors

Abstract: Surface reactions occurring on semiconductor- and pellistor-type sensors are related to the generation of a sensor signal. The role of chemisorbed and lattice oxygen on oxides and noble metals is demonstrated by examples. Reactions including spillover of hydrogen from the metal to the oxide are discussed. Finally the chemical aspects in the detection of hydrogen by three types of palladium-based sensors are compared.

Electrical and optical properties of amorphous indium oxide

Abstract: The authors present a detailed analysis of the electrical and optical properties of amorphous transparent conducting thin films of indium oxide prepared by ion beam sputtering with a wide range of carrier concentrations. They show that the resistivity is dominated by ionised impurity scattering despite the amorphous structure of the films. The weak effect of the structural disorder is confirmed by studies of the interband absorption and is explained by a consideration of the relative length scales of the structural disorder and the Fermi wavelength.

X‐ray photoelectron spectroscopic studies of thin film oxides of cobalt and molybdenum

Abstract: Surface oxidation reactions of cobalt, molybdenum and mixed cobalt–molybdenum metals have been investigated using x-ray photoelectron spectroscopy (XPS). The oxide stoichiometries have been determined from XPS intensity measurements. Such quantification has been important in identifying oxide compositions with characteristic XPS spectra. A number of discrete binary molybdenum oxides have been characterized after reactions at 200°C and pressures ranging from 1 atm to 10−6 Pa. At the lowest pressure, the Mo 3d spectra and O/Mo ratios suggest the formation of a molybdenum–oxygen structure with a stoichiometry near unity. By contrast, at higher pressures, oxides that are predominantly MoO2 and MoO3 appear, but other intermediate oxide structures are also identified. Well-defined binary oxides of cobalt can be prepared from cobalt metal after heating in oxygen at different pressures. New binary oxides of cobalt and molybdenum have been generated by the reaction on MoO3 or Co3O4 substrates. Oxidation of a thin film of molybdenum on a Co3O4 substrate has been shown to produce a nearly stoichiometric surface layer of CoMoO4. The procedure could be useful for the production of other ternary oxides of possible interest for use as XPS reference materials.

Electrical characteristics of ultrathin oxynitride gate dielectric prepared by rapid thermal oxidation of Si in N2O

Abstract: This letter presents a unique process to grow high quality ultrathin (∼60 A) gate dielectrics using N2O (nitrous oxide) gas Compared with conventional rapid thermally grown oxide in the O2, the new oxynitride dielectrics show very large charge‐to‐breakdown (at +50 mA/cm2, 850 C/cm2 for oxynitride compared to 95 C/cm2 for the control thermal oxide) and less charge trapping under constant current stress Significantly reduced interface state generation was also observed under constant current stress and x‐ray radiation A secondary‐ion mass spectroscopy depth profile indicates a nitrogen‐rich layer at the Si/SiO2 interface, which can explain the improved integrity of oxynitride dielectric

Displacement reactions during mechanical alloying

Abstract: The occurrence of simple solid-state displacement reactions during mechanical alloying has been investigated. The reduction of cupric oxide to pure copper metal by a variety of metallic reducing agents was studied, and the powders were examined by X-ray diffractometry and electron microscopy. When milled with a liquid process control agent, the reaction progressed gradually with time, whereas an unstable combustion reaction occurred when no such control agent was employed. A minimum adiabatic temperature of 1300 K is necessary for combustion to occur in these systems. The reaction enthalpy is an important factor in determining the precombustion period. The as-milled powders consisted of finely divided, nanometer-sized crystallites with an extremely high defect density. It is proposed that the increased reactivity of the system arises through the unique conditions prevailing during mechanical alloying.

Limits to adherence of oxide scales

Abstract: Strains can cause oxide scales to fracture, often leading to loss of protectiveness and creation of spalling debris. Fracture mechanics is used to identify a number of criteria under which uniform scales may be expected to fail as a result of rapidly applied strains. The failure mode most frequently found occurs when the strain ɛ builds up in the scale until the strain energy density per unit area exceeds the fracture surface energy γ of the oxide. This produces spalling when ɛ>(2·8 γ/hE)1/2, where h is the scale thickness and E is the Young's modulus of the oxide. Two further regimes are identified in thin scales. First, as the external strain is applied to the oxide via the metal substrate, it is obvious that no further stress can be applied to the oxide if the substrate has itself been stressed beyond yield. This condition gives rise to a second regime, that of extended oxide adherence on substrates under external tensile strains in which the oxide cracks and forms a series of islands, but rema...

Isolated redox centers within microporous environments. 2. Vanadium-containing aluminophosphate molecular sieve five

Abstract: Cobalt-containing aluminophosphate five (CoAPO-5) was synthesized and shown to contain tetrahedral Co{sup 2+} residing within framework atomic positions and also as extraframework cations. The isolated framework Co{sup 2+} atoms can be oxidized by O{sub 2} at 500{degree}C to Co{sup 3+} which is stabilized in tetrahedral coordination by the oxide lattice. The framework Co{sup 3+} is easily reduced to Co{sup 2+} by a variety of reducing agents.

A surface‐electrochemical basis for the direct logarithmic growth law for initial stages of extension of anodic oxide films formed at noble metals

Abstract: Measurements on anodic surface oxidation of noble metals as a function of time and electrode potential show that the initial extension and subsequent thickening of such oxide films is directly logarithmic in time. A striking feature of this behavior is that the direct logarithmic extension law already applies to increase of coverage of Pt or Au electrodes with time well below the limit of formation of one monolayer of OH or O species on the metal surface. A direct logarithmic law of oxide film growth also applies to post‐monolayer growth involving early stages of quasi‐three‐dimensional film formation. Eventually, as the oxide film thickens, the Mott–Cabrera ‘‘high‐field’’ growth mechanism can apply. However, below the monolayer level of oxide film formation, electrochemisorption of two‐dimensional (2D) structures of OH or O arises so that the Mott–Cabrera mechanism cannot be applicable to that situation. It is shown that the kinetic relation for direct electrodeposition of OH or O species onto available ...

Gas turbine blade

Abstract: PURPOSE:To aim at the coexistance of tenacity and heat resistance contrary to each other by forming the base material of a blade with a particular plurality of oxide group ceramics and also forming a hollow portion inside the base material of the blade and also a cover layer made of a particular material on the outer surface thereof. CONSTITUTION:The base material 1 of a blade forming the profile of the gas turbine blade is formed by using Si3N4 or Al2O3-ZnO2 as oxide group ceramics. A cover layer 2 is formed on the outer surface of the base material of the blade by using SiC. A hollow portion 3 is formed inside the base material of the blade 1 and cooling air is blown in the hollow portion 3 to make up for heat resistance of the base material of the blade. Ceramics having different natures are thereby used properly and joined together to satisfy characteristics of tenacity and heat resistance contrary to each other at the same time. Conse quently, the utility and the wide use of the gas turbine blade made of ceramics can be improved.

Metal Oxide Cathode Materials for Electrochemical Energy Storage: A Review

Abstract: Due to their rather low molecular weight and their favorable electrochemical and solid‐state properties, first row transition metal oxides seem to be specially attractive as cathode materials in electrochemical energy storage systems. Therefore, we undertook a detailed overview, covering electrochemical, conductivity, ion diffusivity, spectroscopic, and other physico‐chemical data on metal oxides in relation to their behavior in batteries. Metal oxide‐based primary batteries have achieved a high technological level and yield energy densities of up to 300 Wh kg−1 or 880 Wh l−1. Oxide‐based secondary batteries, on the other hand, typically yield less than 100 Wh kg−1. Based on the present review, V, Cr, Mn, and Co oxides seem to be the most promising solid‐state cathode materials for future high performance secondary batteries.

Partial Melt Growth Process of Bi2Sr2Ca1Cu2Ox Textured Tapes on Silver

Abstract: Bi2Sr2Ca1Cu2Ox textured tapes with an excellent Jc were synthesized by doctor-blade casting and a melt process Green tape was laid on a silver foil and oxide/silver composite tape was heat treated A highly textured microstructure was formed by slow cooling from 890°C to 870°C with the oxide in a partially melted state The highest Tc and Jc (77 K, 0 T), 89 K and 13×104 A/cm2, respectively, were obtained by quenching from 870°C However, Jc at 42 K was higher for the sample slowly cooled to room temperature and reached 14×105 A/cm2 at 25 T These results indicate the potential for the practical application of Bi-based oxide superconductors at both 77 K and 42 K