Showing papers on "Oxide published in 1988"

A New High-TcOxide Superconductor without a Rare Earth Element

Abstract: We have discovered a new high-Tc oxide superconductor of the Bi-Sr-Ca-Cu-O system without any rare earth element. The oxide BiSrCaCu2Ox has Tc of about 105 K, higher than that of YBa2Cu3O7 by more than 10 K. In this oxide, the coexistence of Sr and Ca is necessary to obtain high Tc.

Bacterial Manganese Reduction and Growth with Manganese Oxide as the Sole Electron Acceptor

Abstract: Microbes that couple growth to the reduction of manganese could play an important role in the biogeochemistry of certain anaerobic environments. Such a bacterium, Alteromonas putrefaciens MR-1, couples its growth to the reduction of manganese oxides only under anaerobic conditions. The characteristics of this reduction are consistent with a biological, and not an indirect chemical, reduction of manganese, which suggest that this bacterium uses manganic oxide as a terminal electron acceptor. It can also utilize a large number of other compounds as terminal electron acceptors; this versatility could provide a distinct advantage in environments where electron-acceptor concentrations may vary.

Metal oxides as heterogeneous catalysts for oxygen evolution under photochemical conditions

Abstract: Metal oxides, in the form of dispersed powders, have been tested as potential catalysts for the four-electron oxidation of water to O2 under photochemical conditions. The most efficient catalysts were found to be IrO2, Co3O4, RuO2, NiCo2O4, Rh2O3 and Mn2O3 and, in particular, high activity was observed with IrO2. Comparison of the oxide structure with its observed rate of O2 generation under standard conditions has allowed formulation of a few general requisites for an effective catalyst. Samples of iridium oxide deposited onto the surface of a second (inert) oxide were tested for their O2-evolving capability. The efficiency of the system depended markedly upon the nature of the support. Materials that favour formation of small deposits of iridium oxide (e.g. ZnO, MgO, TiO2) are the best supports, whilst O2 production is almost completely inhibited with acidic supports. Many metal oxides can be prepared in the form of hydrates of variable composition. These materials also function as O2-evolving catalysts, the efficiency of the process depending upon any thermal pretreatment. This finding is explained in terms of changes in structure and composition of the oxide that occur upon heating.

Two-dimensional thermal oxidation of silicon. II. Modeling stress effects in wet oxides

Abstract: For pt.I see ibid., vol.ED-34, p.1008-17 (May 1987). The authors propose that the stress from two-dimensional oxide deformation affects the kinetic parameter in the Deal-Grove model (1965). In particular, the viscous stress associated with the nonuniform deformation of the oxide is identified as the fundamental force of retardation. In this model, the stress normal to the Si-SiO/sub 2/ interface reduces the surface reaction rate in both convex and concave surfaces, whereas the stress in the bulk of the oxide (compressive for concave and tensile for convex surfaces) is responsible for the thinner oxides on the concave structures. The model is described by a simplified mathematical formulation made possible by the symmetry in cylindrical structures. Comparisons with experimental data, possible applications, and limitations of the model are also discussed. >

Modeling and characterization of gate oxide reliability

Abstract: A technique of predicting the lifetime of an oxide to different voltages, different oxide areas, and different temperatures is presented. Using the defect density model in which defects are modeled as effective oxide thinning, many reliability parameters such as yield, failure rate, and screen time/screen yield can be predicted. This modeling procedure is applicable to both wafer-level and long-term reliability tests. Process improvements including defect gettering and alternative dielectrics such as chemical-vapor-deposited oxides are evaluated in the format of defect density as a function of effective oxide thinning. >

Oxygen ion conductivity of the ceria-samarium oxide system with fluorite structure

Abstract: Ionic conduction of oxygen in the ceria-samarium oxide system was investigated as a function of temperature, partial pressure of oxygen and the oxide composition, together with its crystal structure, density and defect structure. The ionic conductivity of (CeO2)1−x(SmO1.5)x was the highest in ZrO2-, ThO2- and CeO2-based oxide systems. The system CeO2-SmO1.5 consisted of the solid solution with a fluorite structure atx<50 at.%. The ionic transference number was nearly unity between 600 and 900°C. With an increase in Sm2O3 content, the ionic conductivity gradually decreased due to a decrease in mobility of oxygen ions. The samarium oxide-doped ceria was less reducible than pure and alkaline earth oxide-doped ceria.

Kinetics of CO oxidation on single-crystal Pd, Pt, and Ir

Abstract: The activity of Pt(100), Pd(110), Ir(111), and Ir(110) single-crystal catalysts for CO oxidation has been studied as a function of temperature and partial pressure of O/sub 2/ and CO in a high-pressure reactor-ultra-high-vacuum surface analysis apparatus over the temperature range 425-725 K and pressure range 0.1-600 Torr. The specific rates and the partial pressure dependencies determined for the single crystals are in excellent agreement with results obtained previously for high surface area supported catalysts, demonstrating the structure insensitivity of this reaction. The single-crystal catalysts exhibit simple Arrhenius behavior over most of the temperature range studied, and the observed activation energies lie between 22 and 33 kcal/mol, close to the desorption energy of CO from these surfaces. These results are consistent with the generally accepted model in which the surface is primarily covered with CO and the reaction rate is controlled by the desorption of CO. Deviation from Arrhenius behavior below 500 K for Pt is interpreted as a change in the reaction mechanism. Under highly oxidizing conditions surfaces of both Pd and Ir show negative-order dependence on O/sub 2/ partial pressure, indicating the presence of a strongly bound oxygen species. The oxygen species was similar to surface oxide formed bymore » deliberate oxidation and could be detected as CO/sub 2/ desorbing at high temperatures in postreaction temperature-programmed desorption. Oxide formed by oxidation of the Pd and Ir samples prior to high-pressure reaction was only stable at 475 K on Pd(110) in an 11:1 O/sub 2/:CO mixture and to 500 K on Ir(111) in an 80:1 O/sub 2/CO:CO mixture. Deliberate oxidation resulted in a rate decrease but did not affect the activation energy significantly, indicating that the oxide served merely as a simple site blocker.« less

Superconducting oxide films with high transition temperature prepared from metal trifluoroacetate precursors

Abstract: Superconducting thin films of Y‐Ba‐Cu oxide have been prepared on yttria‐stabilized zirconia substrates using metal trifluoroacetate spin‐on precursors. The films exhibit an extremely sharp resistive transition with zero resistance at temperatures as high as 94 K. The superconducting phase is formed by a three‐step process: (a) decomposition of the spun‐on trifluoroacetate film to the fluorides, (b) conversion of the fluorides to oxides by reacting with water vapor, and (c) annealing followed by slow cooling in oxygen. The properties of the films depend on the amount of conversion of the fluorides by reaction with water. Films which show the presence of some unreacted barium fluoride have strong c‐axis normal preferred orientation, with a sharp resistive transition. When all the barium fluoride is converted, the film is more randomly oriented and exhibits a broader transition to zero resistance.

Crystal structure of the 80 K superconductor YBa2Cu4O8

Abstract: Recently a new Y–Ba–Cu–O superconducting oxide has been prepared as a distinct phase in thin films1–3. The new phase was first seen as a defect structure in bulk samples of YBa2Cu3O7 (refs 4–7) and found to contain an additional copper layer7. The proposed defect structure has a c-axis spacing of ∼27 A and a cation ratio of YBa2Cu4. Y–Ba–Cu–O films containing a majority phase with a 27 A unit cell have been proposed as evidence of a distinct phase with the same structure as the defects (ref. 1 and K. Char et al., unpublished results). Films of the new phase superconduct with a transition temperature of 80 K (K. Char et al., unpublished results and refs 2 and 3). We show by X-ray crystallography of a film containing 85% of the new phase that the stoichiometry can be written as YBa2Cu4O8. The new structure differs from YBa2Cu3O7 in that the single, linear Cu–O chain parallel to the b-axis has been replaced by a double Cu–O chain with edge-sharing, square-planar oxygen coordination.

High-field-induced degradation in ultra-thin SiO/sub 2/ films

Abstract: Very thin thermal oxides are shown to exhibit a failure mode that is undetected by conventional breakdown tests. This failure mode appears in the form of excessive leakage current at low field and is induced by high-field stresses. The stress-induced oxide leakage is permanent and stable with time and thermal annealing. It becomes the dominant failure mode of thin oxides because it always precedes destructive breakdown. Experimental results and theoretical calculations show that the leakage current is not caused by positive charge generation and accumulation in the oxide. It is proposed that the oxide leakage originates from localized defect-related weak spots where the insulator has experienced significant deterioration from electrical stress. The leakage conduction mechanism appears to be thermally assisted tunneling through the locally reduced injection barrier, and the model seems to be consistent with both I-V measurements at temperatures from 77 K to 250 degrees C and theoretical calculations. >

Surface and near-surface chemistry of oxide materials

Abstract: 1. The Theory of Ceramic Surfaces (A.M. Stoneham, P.W. Tasker). 2. Electronic and Geometric Structure of Defects on Oxides and their Role in Chemisorption (V.E. Henrich). 3. Selected Experimental Methods in the Characterization of Oxide Surfaces (H. Hirschwald). 4. Surface Reactivity of Oxide Materials in Oxidation-Reduction Environment (T. Seyama). 5. Metal Oxide Overlayers and Oxygen Induced Chemical Reactivity Studied by Photoelectron Spectroscopy (M.W. Roberts). 6. Surface Segregation in Metal Oxides (P. Wynblatt, R.C. McCune). 7. Work Function of Oxide Ceramic Materials (J. Nowotny, M. Sloma). 8. Photoeffects on Metal Oxide Powders. (J. Cunningham). 9. The Role of the Surface on Bulk Physical Properties of Glasses (A.A. Kruger). 10. Segregation in Oxide Surfaces Solid Electrolytes and Mixed Conductors (A.J. Burggraaf, A.J.A. Winnubst). 11. Grain Size and Grain Boundary Effects in Passive Electronic Components (D. Hennings). 12. Reactions at Phase Boundaries during Metal/Ceramic Bonding (H.J. de Bruin). 13. Oxide Surfaces in Solution (R.L. Segall et al.). 14. Metals on Oxides: Formation, Characterization and Properties (L.C. Dufour, M. Perderau). 15. Electronic Structure and Transport Properties of Interfaces in Metal Oxides (M.H. Sukkar, H.L. Tuller). 16. High T c Oxide Superconductors. Possible Effect of Interfaces (J. Nowotny, M. Rekas, D.D. Sarma, W. Weppner). Subject Index. Formula Index.

Stress induced leakage current limiting to scale down EEPROM tunnel oxide thickness

Abstract: The effects of thinning the FLOTEX EEPROM tunnel oxide on its reliability are investigated using capacitors and cell structures with oxide thickness ranging from 47 to 100 AA. A low-electric-field oxide leakage current is induced by charge injection stressing, and it increases with decreasing oxide thickness. Its conduction mechanism is found to be different from that caused by positive charge accumulation in that it has the opposite thickness dependence. A corresponding increase of charge loss in a write/erase (W/E)-cycled EEPROM cell is observed with decreasing oxide thickness in a room-temperature retention test. When oxide thickness is decreased, the maximum number of W/E cycles to tunnel-oxide breakdown decreases with the decrease in charge to breakdown of the negatively biased gate. For scaling down the EEPROM tunnel oxide, the most serious limiting factor is oxide leakage current induced by W/E cycling stress, resulting in data-retention degradation. >

The mechanism of operation of tin(iv) oxide carbon monoxide sensors

Abstract: The electrical properties of tin(iv) oxide render it an excellent material for the detection of very low levels of carbon monoxide, and several devices based on sintered pellets of SnO2 are commercially available1,2. Investigations of the operation of such sensors have generally been restricted to studies of the electrical behaviour of the oxide and little attention has been paid to the relationship between the surface adsorption phenomena and electrical changes in the bulk oxide. These studies3 demonstrated the consistency of a model in which conductance is effectively controlled by the population of negatively charged oxygen adsorbates. The chemical reactions occurring at the oxide surface, although unsubstantiated, have been assumed to involve CO adsorption, desorption of CO2 (which produces an increase in conductance) and replenishment of the resulting surface oxygen vacancy by adsorption of molecular oxygen4. Here we describe experiments in which the nature of the surface species adsorbed into tin(iv) oxide from atmospheres containing low levels of carbon monoxide is monitored using transmission infrared spectroscopy while simultaneously measuring the electrical changes produced in the bulk oxide. The data demonstrate the effect of heat treatment on the formation of adsorbate species and the mutual interdependence of adsorbate species and bulk oxide electrical conductance. This relationship is important both for sensor operation and heterogeneous catalysis.

Precursors for sol-gel preparations

Abstract: Precursors used for the synthesis of oxide systems by the sol-gel method are reviewed and their role in the various stages of the process is discussed. Emphasis is given to alkoxide precursors and to their physical and chemical properties. In particular, the following topics are discussed: degree of oligomerization, volatility, viscosity, reactions with alcohol, molecular association between alkoxides, hydrolysis, stabilization against hydrolysis. Some information about preparation methods, commercial products and availability is also given. Among non-alkoxide precursors, nitrates, carboxylates, acetylacetonates, chlorides and other inorganics are described from the point of view of their use in sol-gel preparations. They are compared, when possible, with the corresponding alkoxides.

High oxide ion conductivity in Ca 12 Al 14 O 33

Abstract: Oxide ion conducting solids have much potential for use in fuel cells, sensors, oxygen pumps and gauges. Existing oxide ion conductors are essentially confined to a small group of ceramic oxides with structures related to fluorite, the most important examples being ZrO2- and Bi2O3-related materials. There is a clear need for new oxide ion conductors with improved properties. We report measurements of the conductivity of Ca12Al14O33, which indicate it to be an oxide ion conductor with bulk conductivity only 8–10 times less than that of yttria-stabilized zirconia. The structure of Ca12Al14O33 is unrelated to the fluorite structure. The reasons for the high conductivity are not well understood but may be associated with the fact that the structure contains a mixture of anions, a three-dimensional aluminate framework and essentially free oxide ions.

Kinetics of carbon monoxide oxidation by oxygen or nitric oxide on rhodium(111) and rhodium(100) single crystals

Abstract: The oxidation of CO by O/sub 2/ or NO over Rh(111) and Rh(100) single crystals has been studied in a high-pressure reaction-high-vacuum surface analysis apparatus. Steady-state catalytic activity as a function of temperature and partial pressures of CO, O/sub 2/, and NO has been measured. The CO-O/sub 2/ reaction was found to be insensitive to the structure of the surface as evidenced by the identical rates, activation energies, and partial pressure dependencies measured on the two-single-crystal surfaces. Both surfaces deactivated at high O/sub 2/ partial pressures to the formation of a near-surface oxide (probably Rh/sub 2/O/sub 3/) which is catalytically inactive for this reaction. The deactivation occurred at a slightly lower O/sub 2/ partial pressure on Rh(100), likely due to the relative ease of oxygen diffusion through this surface leading to more rapid bulk oxidation. Unlike the CO-O/sub 2/ reaction, CO oxidation by NO was very sensitive to the geometric structure of the surface. This behavior was evident in all kinetic parameters measured on the two surfaces. The kinetic data obtained on Rh(111) agree quantitatively with a surface chemistry model previously developed for this reaction. The model predicts that the rate is limited by the formation of N/sub 2/ frommore » the recombination of adsorbed N atoms. and that the surface, under steady-state reaction conditions, is covered largely with adsorbed N atoms and smaller amounts of adsorbed NO. On Rh(100) the data are consistent with a reaction rate limited by the formation of N/sub 2/ from the surface reaction of adsorbed NO and N atoms.« less

Tin dioxide gas sensors. Part 2.—The role of surface additives

Abstract: It has been shown earlier (J. F. McAleer, P. T. Moseley, J. O. W. Norris and D. E. Williams, J. Chem. Soc., Faraday Trans. 1, 1987, 83, 1323) that the gas response of porous pellets of tin dioxide depends crucially on electronic surface states involving adsorbed oxygen, and on the rates of combustion reactions involving the gases to be detected. It is expected, therefore, that the use of surface additives capable of either pinning the Fermi level of the tin dioxide or altering the rate of combustion (depending on choice of material and temperature) would profoundly affect the gas sensor response of the material. This is found to be the case. Studies of the influence of surface additions of precious metals and of metal oxide particles on the gas response behaviour of tin dioxide are described. The use of precious metals at temperatures near to ambient imposes an oxygen-independent Schottky barrier on the tin dioxide surface and results in a distinct low-temperature response to carbon monoxide, probably by adsorption on the precious metal modifying the surface potential (and hence the Schottky barrier). This particular effect is expected only when the precious metal particles distributed over the oxide surface are extremely small (1–10 nm): the effect disappears if the catalyst particles are aggregated by heating. The distribution of particles of a foreign oxide (here Ag2O or ZnO) on the surface of the tin dioxide also appears to pin the surface-energy levels to those of the additives, and tin dioxide treated in this way exhibits gas-sensing properties that are modified to resemble those of the additive oxide in bulk. At higher temperatures, especially in the presence of a precious-metal catalyst, the gas is completely oxidised within a thin outer shell of the specimen and consequently the response to the gas of the measured resistance of the pellet disappears. Indeed, a response to the reaction products (H2O and CO2) is obtained.

Reversibility of trapped hole annealing

Abstract: Annealing under negative bias of metal-oxide-semiconductor field-effect transistors (MOSFETs) previously irradiated and annealed under positive bias is studied as a function of oxide field and time. Using three different sets of samples ranging from hard to soft, it is possible to observe a considerable reversal of the oxide trapped charge component in the two harder sets and essentially none in the soft set. These results are interpreted in terms of electrons tunneling back and forth from the Si substrate to electron traps associated with the simple oxygen vacancy (an E' center) in the oxide. Some of these compensating electrons do reform the Si-Si bonds broken by the trapped holes. The results are consistent with a large body of previous work. In addition, the data suggest that the energy level of the hole trap is below the valence band edge of Si. >

Manufacture of semiconductor device

Abstract: PURPOSE:To prevent the leakage current of the base-emitter junction by providing processes for selectively removing a nitride film, diffusing an impurity of a first conductivity type, and forming a shallow second region of the first conductivity type widely than a deep emitter region of the first conductivity type. CONSTITUTION:In an N-type semiconductor region 1 which is the collector of a bi-polar transistor, a P-type base region 2 is formed, an oxide film 5 is provided, the oxide film is removed in the portion in which a shallow second region 4 of a first conductivity type is to be formed, and a nitride film 6 having a film thickness of about 1-2mu is grown. Thereafter, the nitride film is etched by means of anisotropic dry etching to form such a structure that the nitride film 6 remains inside the oxide film 5, an N-type impurity is driven in by ion implantation, and a heat treatment is performed, forming a deep emitter region 3. Then, the nitride film 6 is selectively removed by not phosphoric acid or the like, and the N-type impurity is fiffused at a low temperature, forming the shallow second region of the first conductivity type. With this, crystal defects due to the ion implantation are isolated from the emitter-base junction of a bi-polar transistor, and thus the leakage current of the emitter-base junction can be prevented.

An XPS Investigation into the Passivity of an Amorphous Ni‐20P Alloy

Abstract: Anodic polarization experiments in and electrolytes show that amorphous Ni‐20P alloy passivates at potentials between −0.3 and about 0.2V (SCE) and dissolves transpassively above 0.2V (SCE). In , in which crystalline Ni pits readily, Ni‐20P resists pitting and passivates as effectively as in . Analysis by x‐ray photoelectron spectroscopy of Ni‐20P surfaces after polarization indicates an enrichment of elemental P on the alloy surface, the presence of hypophosphite anions and the absence of oxidized nickel species. Thus, the Ni‐20P alloy does not passivate by development of a three‐dimensional oxide film, such as the film that forms on nickel. A chemical passivation process is proposed that is controlled by the formation and surface retention of hypophosphite anion, which acts as a barrier between the alloy and the electrolyte.

The influence of yttrium on oxide scale growth and adherence

Abstract: Alloys and coatings for high-temperature service are designed to form selectively chromia scales, alumina scales, or, to a limited extent, silica scales upon exposure to the environment. For such oxide scales to be protective, they should be both slow growing and adherent. It turns out that the addition of yttrium to such alloys can often impart both characteristics to the oxide scale. However, the actual operating mechanisms continue to be a matter of controversy among researchers in the area of oxidation. In the present study, the growth and adherence of alumina and chromia scales on alloys containing yttrium, either as an oxide dispersion or as an intermetallic phase, have been investigated in conjunction with detailed oxide scale characterization using the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS). The results of the study are used for critical assessment of the proposed mechanisms, especially the more recent ones, and to suggest some new mechanisms for adherence.