Showing papers on "Oxide published in 1989"

A superconducting copper oxide compound with electrons as the charge carriers

Abstract: Since the discovery of high-temperature superconductivity in cop-per oxide compounds1, there has been much effort to understand what is required for a compound to have a high transition tem-perature (Tc). Up to now, all of the families of high-Tc copper oxide superconductors have contained two-dimensional sheets of Cu–O pyramids or octahedra, and the carriers of the superconduct-ing current have been electron vacancies, or 'holes'. By contrast, we report here the discovery of a family of superconducting copper oxides in which the carriers are electrons. The new superconductors are Ce4+-doped compounds, with the formula Ln2–xCexCuO4–y, where Ln stands for the lanthanides Pr, Nd or Sm. The compounds have the Nd2CuO4 (T′-phase) structure2, which is composed of sheets of Cu–O squares (see Fig. la). This structure has no apical oxygen atoms, in contrast to the T-phase structure with Cu–O octahedra (Fig. 1b), as observed in La2–xSrxCuO4 (refs 1, 2), and the T*-phase structure with Cu–O pyramids (Fig. 1c), as in Nd2–x–z Cex Srz CuO4 (ref. 3 and Y. T. et al., manuscript in preparation).

The formation of controlled-porosity membranes from anodically oxidized aluminium

Abstract: Synthetic membranes are used in a number of diverse applications, such as filtration1,2, bioreactors2,3, tissue culture4, analytical devices including sensors2,5, and as supports for active materials1,5. Narrow pore-size distribution, high pore density and thinness are often important attributes. The anodic oxidation of aluminium6 can produce porous films possessing these features; the anodizing voltage controls the pore size and pore density, whereas the thick-ness is determined by the amount of charge transferred. A major problem with this technique, however, is that the films remain attached to the aluminium, with the pore base closed by an oxide barrier layer. Here we overcome this problem by progressively reducing the anodizing voltage, thereby causing perforation of the barrier layer and separation of the film as a porous membrane.

Transition Metal Oxides: Surface Chemistry and Catalysis

Abstract: 1. Introduction. 2. Bulk and Surface Structure of Transition Metal Oxide. 3. Physical and Electronic Properties. 4. Surface Coordinative Unsaturation. 5. Surface Acidity. 6. Reduction of Oxides. 7. Oxygen on Oxides. 8. Preparation of Oxides. 9. Metathesis and Isomerization. 10. Decomposition, Hydrogenation and Related Reactions. 11. Selective Oxidation Reactions I. 12. Selective Oxidation Reactions II. 13. Catalytic Reaction Between Hydrogen and Carbon Oxides. 14. Photo-Assisted Surface Processes. Index.

A 1/f noise technique to extract the oxide trap density near the conduction band edge of silicon

Abstract: The use of 1/f noise measurements in n-channel MOSFETs to extract the oxide trap density in space and energy near and above the conduction band edge of silicon is investigated. The conventional carrier number fluctuation model of 1/f noise that attributes 1/f noise to the trapping and detrapping of inversion layer carriers by oxide traps is reviewed. It is shown that oxide band bending in devices with a nonuniform oxide trap distribution leads to a gate voltage dependence in the magnitude and exponent gamma (V/sub gs/) of the 1/f/sup gamma / noise spectrum. An extension of the 1/f noise number fluctuation model that includes both carrier number fluctuations and correlated mobility fluctuations is then studied. Correlated mobility fluctuations are attributed to the coulombic scattering of inversion layer carriers by the fluctuating trapped charge. It is shown that the correlated fluctuation model predicts a gate voltage dependence in the magnitude and exponent gamma of the 1/f/sup gamma / noise spectrum even for a uniform oxide trap distribution. By analyzing the 1/f noise magnitude and exponent data in n-channel MOSFETs having various oxide thicknesses, both models are used to extract the oxide trap density over a wide range of space and energy. >

Solid-state vanadium-51 NMR structural studies on supported vanadium(V) oxide catalysts: vanadium oxide surface layers on alumina and titania supports

Abstract: Solid-state wide-line, magic-angle spinning (MAS) and pulse excitation (nutation) NMR techniques are applied in a study of local environments in two-dimensional vanadium(V) oxide surface layers on titania and alumina supports. Two main surface vanadium oxide species with different bonding environments are detected, which are, on the basis of their anisotropic chemical shift and nuclear electric quadrupolar coupling properties, assigned to 4- and 6-coordinate V-O environments. In all materials, the relative amount of 6-coordinate surface species increases monotonically as a function of the vanadium oxide surface coverage. However, the results indicate a marked dependence of the surface vanadium oxide structure on the metal oxide support material. Vanadium(V) oxide on TiO{sub 2} (anatase) substrates displays the highest tendency to be 6-coordinated down to very low surface coverages. Hydrous species (OH and H{sub 2}O) participate in this environment, and upon dehydration in vacuo, a new tetrahedral species is formed. These results illustrate the suitability of {sup 51}V NMR as a unique quantitative spectroscopic tool in the structural analysis of vanadium(V) oxide catalytic materials.

Oxidation studies of SiGe

Abstract: We have studied the kinetics and mechanism of oxidation of SiGe alloys deposited epitaxially onto Si substrates by low‐temperature chemical vapor deposition. Ge is shown to enhance oxidation rates by a factor of about 3 in the linear regime, and to be completely rejected from the oxide so that it piles up at the SiO2/SiGe interface. We demonstrate that Ge plays a purely catalytic role, i.e., it enhances the reaction rate while remaining unchanged itself. Electrical properties of the oxides formed under these conditions are presented, as well as microstructures of the oxide/substrate, Ge‐enriched/SiGe substrate, and SiGe/Si substrate interfaces, and x‐ray photoemission studies of the early stages of oxidation. Possible mechanisms are discussed and compared with oxidation of pure silicon.

The reactive element effect in high-temperature corrosion

Abstract: When small quantities (typically 1 wt.% or less) of a number of reactive elements are added to high-temperature alloys containing chromium or aluminum, a number of beneficial effects result. This is called the “reactive element effect” (REE). Some or all aspects of the REE can be developed when the reactive element is added as an alloy addition, when it is present as a dispersion of oxide particles in the alloy, when it is ion implanted into the surface, or when a coating of the reactive element oxide is applied to the surface of the alloy. In this paper, the experimental evidence is briefly summarized, and different models that have been proposed to account for one or more aspects of the REE are assessed in terms of this evidence.

The effect of fluorine in silicon dioxide gate dielectrics

Abstract: The effect of post-oxide-growth fluorine incorporation in gate dielectrics is reported. Fluorine was introduced through ion implantation into polysilicon and diffused into the gate oxide, as indicated by SIMS measurements. No great decrease in the breakdown field was observed, although a decrease in charge-to-breakdown was seen. Interface characteristics also improved with medium to high doses of fluoride. High doses were found to grow additional oxide. NMOS FETs showed increased immunity to hot-electron-induced stress. These results are explained by a model wherein fluorine bonds to silicon, and the displaced oxygen grows the additional oxide. >

Mixed-cation oxide powders via polymeric precursors

Abstract: Revue des developpements recents dans le domaine de la preparation des poudres ceramiques d'oxydes mixtes a partir de polymeres precurseurs

Reaction of water with hydrofluoric acid treated silicon(111) and (100) surfaces

Abstract: Si wafers with (100) or (111) oriented surfaces were treated in hydrofluoric acid (40% HF, 1 min) and then water rinsed for different times from 10 s to more than 50 h. Oxygen uptake and oxide formation were investigated by x‐ray photoelectron spectroscopy and high‐resolution electron energy‐loss spectroscopy. The initial state after the HF dip is characterized by a coverage with Si–hydride and small amounts of oxygen and fluorine. The interaction with the liquid phase of water was investigated up to the monolayer range. It shows distinct features: The first step is a rapid exchange of Si–F with H2 O to form Si–OH groups followed by a slow nucleophilic attack of OH− on surface Si–H to produce Si–OH. Growth law is logarithmic and extends to 3–5 h of water contact. The surface Si–OH act as nuclei for the attack of water on the polarized Si–SiOH backbonds. Interior Si–H and Si–OH groups develop. Further attack of OH− on interior Si–H yields Si–OH. Condensation of Si–OH forms Si–O–Si bridges and SiO2−x nuclei appear. Strain and altered surface topography lead to a changed rate of the logarithmic oxide growth. The oxide formation is accompanied by a slight corrosive attack of H2 O, leading to roughening of the surface.

Electrical and physical properties of ultrathin reoxidized nitrided oxides prepared by rapid thermal processing

Abstract: A systematic study of the correlations between electrical and physical properties of nanometer-range reoxidized nitrided oxide films is reported. Rapid thermal processing was applied to the full fabrication process 7.7-nm-thick oxides nitrided at various conditions were reoxidized at 900-1150 degrees C for 15-600 s. Nitridation- and reoxidation-condition dependences of charge-trapping properties, i.e. the flat-band voltage shift and the increase of midgap interface state density induced by a high-field stress, were studied. The hydrogen concentration in the film and the nitrogen concentration near the Si-SiO/sub 2/ interface were measured by secondary ion mass-spectroscopy and Auger electron spectroscopy respectively. It was shown that striking improvement of the charge-trapping properties by rapid reoxidation is achieved by the reducing of hydrogen concentration while keeping the nitrogen concentration near the interface unchanged. >

Transient oxidation of Single-Crystal β-NiAl

Abstract: The transient oxidation of β-NiAl in air at 800 °C and 1100 °C has been studied using electron microscopy. The oxide scale consists predominatly of metastable Al2O3 phases. θ-Al2O3 is the major oxide phase within 10.0 hr of oxidation at 800 °C and 0.1 hr at 1100 °C. The scales form epitaxially on (001)β and (012)β specimens throughout the transient stage, whereas the degree of preferred oxide orientation decreases with oxidation time on (011)β and (111)β specimens. The orientation relationships reflect the small mismatch between parallel close-packed directions in the metal and in the cation sublattice of the oxides. The correlation of distinctive oxide surface morphologies with internal structural defects indicates the strong tendency of the Al2O3 scale to growvia short-circuit diffusion paths.

Oxygen chemisorption and laser Raman spectroscopy of unsupported and silica-supported vanadium oxide catalysts

Abstract: An oxygen chemisorption method has been developed for measuring the active surface area of supported and unsupported V{sub 2}O{sub 5} following reduction in hydrogen. It is shown that to achieve complete reduction of the vanadia surface without reducing the bulk, reduction must be carried out at 640 K. Oxygen uptakes of unsupported samples reduced at close to this temperature yield an oxygen atom site density of 3.2 {times} 10{sup 18} m{sup {minus}2}, a value near that expected for a monolayer. The same oxygen chemisorption technique is applied to silica-supported V{sub 2}O{sub 5}. Laser Raman spectroscopy confirms that, near 640 K, oxygen chemisorbs primarily at the surface of the dispersed vanadia but does not exchange with the bulk of the oxide. For very low weight loadings, a limiting stoichiometry of one adsorbed oxygen atom per vanadium atom is obtained. This stoichiometry is used to calculate dispersions ranging from 92% to 50% for supported V{sub 2}O{sub 5} samples of 0.3-9.8% weight loading.

Differences in growth mechanisms of oxide scales formed on ODS and conventional wrought alloys

Abstract: The oxidation behavior in air of two ODS alloys, MA 754, a chromia former, and MA 956 an alumina-scale former, has been compared with that of conventional wrought model alloys with similar compositions. The main effects on scale properties of both oxide types due to oxide dispersions were found to be improved adherence, decreased growth rates, and enhanced selective oxidation. In addition to metallography, X-ray diffraction, energy dispersive X-ray analysis of the scales, and studies of scale morphology, the detailed growth mechanisms of the oxide layers were studied using an18O tracer technique. The results show that the oxides on the conventional alloys grow by both metal and oxygen transport, and that the addition of oxide dispersions suppresses the outward scale growth. This change in growth mechanism is a possible explanation for the observed improved scale adherence, decreased growth, and enhanced selective oxidation in the yttria-containing alloys.

Deposition of silicon-containing films using organosilicon compounds and nitrogen trifluoride

Abstract: In a low-pressure reactor, the addition of nitrogen trifluoride to a gaseous organosilicon compound such as tetraethoxysilane (TEOS) or tetramethylcyclotetroxysilane (TMCTS) results in surprisingly enhanced silicon dioxide deposition rates. The oxide deposited using this process also has the capability of filling features having aspects ratios up to at least 1.0, and may exhibit low mobile ion concentrations. The process is also applicable for depositing other silicon-containing films such as polysilicon and silicon nitride.

Superconductivity near liquid nitrogen temperature in the PbSrRCaCuO system (R=Y or rare earth)

Abstract: Superconductivity close to the boiling point of liquid nitrogen has been discovered in the P-Sr-R-Ca-Cu-O system where R is Y or rare earth. The superconducting compound has been identified as Pb 2 Sr 2 (R,Ca) 1 Cu 3 O 8+ y and the structure of the R=Y compound has been solved by single-crystal X-ray diffraction methods. The structure consists of double CuO 2 sheets interleaved by (Ca,R), a unit which is common to most of the high- T c copper oxide superconductors. A new structural feature found in this system is a double PbO layer separated by a sheet of copper atoms. A non-superconducting oxide of approximate composition (Y,Ca)Sr 2 (Cu,Pb) 3 O 7− y formed as a second phase in some preparations has been also characterized; its structure is related YBa 2 Cu 3 O 7 .

Control Factor of Native Oxide Growth on Silicon in Air or in Ultrapure Water

Abstract: Native silicon (Si) oxide growth on Si (100) wafers in air and in ultrapure water at room temperature requires coexistence of water and oxygen in the air and ultrapure water ambients. The growth rate data on n‐, n+‐, and p+‐Si (100) in air indicate layer‐by‐layer growth of an oxide. The growth rate on n‐Si (100) in ultrapure water may be governed by a parabolic law. For native oxide growth in ultrapure water, the number of Si atoms dissolved in ultrapure water is over one order of magnitude larger than the number of Si atoms contained in the grown native oxide film. The structural difference between the native oxide film in air and in ultrapure water is also discussed.

Role of reactive elements in alloy protection

Abstract: Reactive element additions may promote the formation of protective oxide scales on high temperature alloys, and improve long term oxidation resistance by limiting the rate of scale growth and inhibiting scale failure processes. Various modifications to transport properties and mechanical behaviour have been proposed to explain the observed benefits. These mechanisms are reviewed with reference to available experimental evidence. Key steps in enhancing oxidation resistance are probably the provision of reactive element oxide particles for incorporation into a scale and consequent segregation of reactive element ions to high diffusivity pathways and interfaces. Such behaviour is consistent with known variations in protection resulting from changes in reactive element type, concentration, and initial distribution.MST/1055

TEM studies of oxidized NiAl and Ni3Al cross sections

Abstract: Cross sections of oxide scale/(Ni-Al) intermetallics were prepared by a new method and studied using primarily transmission electron microscopy (TEM) The cross sections were prepared by encasing an oxidized metal specimen sandwich in a low-melting-temperature zinc alloy Observations of oxidized zirconium-doped β-NiAl cross sections revealed crystallographic voids beneath an adherent Al2O3 scale The oxide-metal interface was incoherent, but a high dislocation density in the metal near the interface suggested that a large tensile stress was induced by the attached oxide scale A duplex Al2O3-NiAl2O4 scale formed on zirconium-doped and zirconium/boron-doped γ′-Ni3Al alloys Additional results are presented involving oxidation mechanisms and oxide-metal interface structures