Showing papers on "Oxide published in 1996"

Thermodynamic stability of binary oxides in contact With silicon

Abstract: Using tabulated thermodynamic data, a comprehensive investigation of the thermo-dynamic stability of binary oxides in contact with silicon at 1000 K was conducted. Reactions between silicon and each binary oxide at 1000 K, including those involving ternary phases, were considered. Sufficient data exist to conclude that all binary oxides except the following are thermodynamically unstable in contact with silicon at 1000 K: Li2O, most of the alkaline earth oxides (BeO, MgO, CaO, and SrO), the column IIIB oxides (Sc2O3, Y2O3, and Re2O3, where Re is a rare earth), ThO2, UO2, ZrO2, HfO2, and Al2O3. Of these remaining oxides, sufficient data exist to conclude that BeO, MgO, and ZrO2 are thermodynamically stable in contact with silicon at 1000 K. Our results are consistent with reported investigations of silicon/binary oxide interfaces and identify candidate materials for future investigations.

Electronic oxide polarizability and optical basicity of simple oxides. I

Abstract: The average electronic oxide polarizability α02− of numerous single component oxides has been calculated on the basis of two different properties: linear refractive index n0 and energy gap Eg, which have demonstrated remarkable correlation. The optical basicity Λ of the oxides has been estimated on the basis of average electronic oxide polarizability calculated from the refractive index Λ(n0) and the energy gap Λ(Eg). A good agreement has been observed between the optical basicity data obtained using independent initial quantities. The simple oxides have been separated into three groups according to the values of their oxide polarizability. The α02− values (above 3 A) obtained for PbO, Sb2O3, and Bi2O3 have been attributed to the high cation polarizability and the presence of a lone pair in the valence shell of the cation.

Microbial Reduction of Crystalline Iron(III) Oxides: Influence of Oxide Surface Area and Potential for Cell Growth

Abstract: Quantitative aspects of microbial crystalline iron(III) oxide reduction were examined using a dissimilatory iron(III) oxide-reducing bacterium (Shewanella alga strain BrY). The initial rate and lon...

Control of Electron Transfer Kinetics at Glassy Carbon Electrodes by Specific Surface Modification

Abstract: Various well-established and novel surface modification procedures were used on glassy carbon (GC) electrodes to yield surfaces with low oxide content or which lack specific oxide functional groups. In addition, monolayers of several different adsorbates were formed on GC surfaces before electrochemical evaluation. Both the nonspecific monolayer adsorbates and reagents which chemisorb to specific functional groups were observed on the surface with Raman and photoelectron spectroscopy. The various GC surfaces were then evaluated for their electron transfer reactivity with nine redox systems in aqueous electrolyte, including Ru(NH3)62+/3+, Fe(CN6)3-/4-, ascorbic acid, and Feaq3+/2+. The nine systems were categorized according to their kinetic sensitivity to surface modification. Several, including Ru(NH3)62+/3+, are insensitive to surface modifications and are considered outer sphere. Feaq3+/2+, Vaq2+/3+, and Euaq2+/3+ are catalyzed by surface carbonyl groups and are very sensitive to the removal of surface...

Chromium Vapor Species over Solid Oxide Fuel Cell Interconnect Materials and Their Potential for Degradation Processes

Abstract: Alloys protected from corrosion by a chromia scale and LaCrO{sub 3}-based perovskites are used as materials for the interconnect of solid oxide fuel cells (SOFCs). The chromium vaporization of these materials was studied by thermochemical modeling. Partial pressures of the vaporizing species were determined for different O{sub 2} and H{sub 2}O concentrations in the oxidizing gas. CrO{sub 2}(OH){sub 2}(g) and CrO{sub 3}(g) are the most abundant species in air with and without humidity, respectively. The potential of the Cr-containing vapor species for the degradation of the electrical properties of an SOFC was analyzed by thermodynamic computations. The electrochemical reduction of the Cr-containing vapor species at the cathode/electrolyte/gas phase boundary can lead to polarization losses.

Metal oxides as a hole-injecting layer for an organic electroluminescent device

Abstract: We demonstrate that by using thin films of metal oxides, such as vanadium oxide (VOx), molybdenum oxide (MoOx) and ruthenium oxide (RuOx), as a hole-injecting layer for an organic electroluminescent (EL) device consisting of N,N'-diphenyl-N,N-bis(3-methylphenyl1)1,1'-biphenyl-4,4' diamine (TPD) and tris-(8-quinolinolato) aluminium (Alq), the EL device performance can be significantly improved. The `operating voltage' of the device is reduced with respect to a device with a well known indium - tin-oxide (ITO) electrode for hole injection. We consider that the improvement of the operating voltage is attributable to the lower energy barrier for hole injection at the metal oxide/TPD interface.

Magnetoresistance effect element

Abstract: There are provided a magnetoresistance effect element, a magnetic head, a magnetic head assembly and a magnetic recording system, which have high sensitivity and high reliability. The magnetoresistance effect element has two ferromagnetic layers, a non-magnetic layer provided between the ferromagnetic layers, and a layer containing an oxide or nitride as a principal component, wherein the layer containing the oxide or nitride as the principal component contains a magnetic transition metal element which does not bond to oxygen and nitrogen and which is at least one of Co, Fe and Ni.

Oxygen diffusion barrier properties of transparent oxide coatings on polymeric substrates

Abstract: Gas diffusion properties of transparent thin film coatings have been under investigation by a number of workers for application in food and medical packaging. Work on the oxygen permeation properties of high barrier thin film coatings is reviewed, with emphasis on the coating thickness dependence of the oxygen permeation rate. Comparison of the coating thickness dependence of the oxygen permeation properties of evaporated, sputtered and plasma-deposited transparent oxide and organic coatings suggests that the reduction in permeation due to the coatings is limited by transport through coating defects (e.g. pinholes, grain boundaries or microcracks). The usefulness of such measurements as a probe of coating microstructure is assessed by reviewing structural and permeation investigations of metallized coatings on polymers.

Electrochemical Deposition of Copper(I) Oxide Films

Abstract: Films of copper(I) oxide can be electrodeposited by reduction of copper(II) lactate in alkaline solution. Rietveld analysis of electrochemically grown films reveals pure copper(I) oxide with no copper(II) oxide or copper metal present in the films and a lattice parameter of a = 0.4266 nm. The cathodic deposition current is limited by a Schottky-like barrier that forms between the Cu2O and the deposition solution. A barrier height of 0.6 eV was determined from the exponential dependence of the deposition current on the solution temperature. At a solution pH of 9 the orientation of the film is [100], while at a solution pH of 12 the orientation changes to [111]. Atomic force images of the [100] oriented films have crystals shaped as four-sided pyramids, while the [111] films have triangular crystals. The grain size for films grown at 65 °C ranges from 2 to 5 μm. A refractive index of 2.6 was measured from the transmission spectrum for wavelengths between 1350 and 2800 nm. The p-type semiconductor has a dire...

Mobility of Surface Species on Oxides. 1. Isotopic Exchange of 18O2 with 16O of SiO2, Al2O3, ZrO2, MgO, CeO2, and CeO2-Al2O3. Activation by Noble Metals. Correlation with Oxide Basicity†

Abstract: Temperature-programmed isotopic exchange of 18O2 with 16O of several oxides was carried out in the 200−900 °C temperature range. The oxides can be ranked according to their maximal rates of exchange obtained at the following temperatures: CeO2, 410 °C ≫ CeO2-Al2O3, 480 °C ≈ MgO, 490 °C > ZrO2, 530 °C ≫ γ-Al2O3, 620 °C ≫ SiO2, 850 °C. Except on CeO2 and on CeO2-Al2O3 a simple exchange yielding initially 18O16O can be observed. With ceria containing oxides, the reaction occurs in part via a multiple exchange mechanism yielding initially 16O2 which is indicative of the presence of binuclear species (O2, O2-, or O22-) at the ceria surface. Chlorine-free rhodium catalysts supported on these oxides were prepared with metal dispersions between 32 and 89%. The presence of rhodium accelerates considerably the oxygen exchange with the support: the maximal rates of the exchange can be observed at much lower temperatures, by about 200−300 °C with respect to the bare oxides. This is attributed to a spillover of oxyg...

Bulk Synthesis of Inorganic Fullerene-like MS2 (M = Mo, W) from the Respective Trioxides and the Reaction Mechanism

Abstract: Recently, milligram quantities of MoS2 fullerene-like nanotubes and negative curvature polyhedra (generically called inorganic fullerene-like material, IF), were reproducibly obtained by a gas phase reaction from an oxide precursor (Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222. Srolovitz, D. J.; Safran, S. A.; Homyonfer, M.; Tenne, R. Phys. Rev. Lett. 1995, 74, 1778). The present work focuses on the mechanism of the synthesis of IF-MS2 (M = W, Mo). The IF material is obtained from oxide particles smaller than ca. 0.2 μm, while larger oxide particles result in 2H-MS2 platelets. The key step in the reaction mechanism is the formation of a closed layer of MS2, which isolates the nanoparticle from its surroundings and prevents its fusion into larger particles. Subsequently, the oxide core of the nanoparticle is progressively converted into a sulfide nanoparticle with an empty core (IF). Taking advantage of this process, we report here a routine for the fabrication of macrosco...

On the partial oxidation of propane and propylene on mixed metal oxide catalysts

Abstract: The present review analyses the literature data reported on the partial oxidation of propane to organic compounds (acrolein, acrylic acid and acrylonitrile) over mixed metal oxides, mainly magnesium vanadates, vanadia bismuth molybdates and vanadia antimony. The data were compared to those reported on the partial oxidation of propylene over bismuth molybdate and antimony—tin multicomponent oxides and over cuprous simple oxide. For both reactions, we analyzed the involved reaction mechanisms, intermediate species, active phases and active sites. The role of water produced during the reaction and that of the Bronsted acid sites were shown to be important in the determination of the selectivity of the expected products. The main conclusion of our study is that a good mix of acid-base and redox properties of the oxide surface should permit a controlled orientation of the reaction towards selective products.

Synthesis and Characterization of Model Polyalkane−Poly(ethylene oxide) Block Copolymers

Abstract: We describe the preparation of a new set of amphiphilic block copolymers with well-defined molecular weights and block volume fractions. The synthesis of a variety of polyalkane−poly(ethylene oxide) block copolymers was accomplished by a new polymerization−hydrogenation sequence. Initially, anionic polymerization of either butadiene or isoprene was performed followed by end capping with ethylene oxide. The resulting hydroxyl-terminated polydienes were catalytically hydrogenated to give the corresponding hydroxyl-terminated polyalkanes. These polymeric alcohols were then titrated with potassium naphthalenide to yield the analogous potassium alkoxides. This type of macroinitiator was employed in the polymerization of ethylene oxide. Seventeen polyalkane−poly(ethylene oxide) block copolymers were prepared in near quantitative yields with molecular weights ranging from (1.4 to 8.7) × 103 and poly(ethylene oxide) volume fractions ranging from 0.29 to 0.73. These polymers are model materials for block copolymer...

Soft breakdown of ultra-thin gate oxide layers

Abstract: The dielectric breakdown of ultra-thin 3 nm and 4 nm SiO/sub 2/ layers used as a gate dielectric in poly-Si gate capacitors is investigated. The ultra-thin gate oxide reliability was determined using tunnel current injection stressing measurements. A soft breakdown mechanism is demonstrated for these ultra-thin gate oxide layers. The soft breakdown phenomenon corresponds with an anomalous increase of the stress induced leakage current and the occurrence of fluctuations in the current. The soft breakdown phenomenon is explained by the decrease of the applied power during the stressing for thinner oxides so that thermal effects are avoided during the breakdown of the ultra-thin oxide capacitor. It is proposed that multiple tunnelling via generated electron traps in the ultra-thin gate oxide layer is the physical mechanism of the electron transport after soft breakdown. The statistical distributions of the charge to dielectric breakdown and to soft breakdown for a constant current stress of the ultra-thin oxides are compared. It is shown that for accurate ultra-thin gate oxide reliability measurements it is necessary to take the soft breakdown phenomenon into account.

Chapter 10 Dense ceramic membranes for oxygen separation

Abstract: Publisher Summary This chapter reviews the recent developments in the area of mixed ionic-electronic conducting membranes for oxygen separation, in which the membrane material is made dense—that is, free of cracks and connected-through porosity, being susceptible only for oxygen ionic and electronic transport. Emphasis is on the defect chemistry, mass transport, and the associated surface exchange kinetics. The basic elements of mixed ionic and electronic transport through dense ceramic membranes are focused. The chapter discusses mixed-conducting acceptor-doped perovskite and perovskite-related oxides and gives examples to illustrate the fundamental factors determining the oxygen fluxes through dense ceramic membranes. A key factor in the possible application of oxygen ion conducting ceramics is that, for use as solid electrolyte in fuel cells, batteries, oxygen pumps or sensors, their electronic transport number should be as low as possible. Stimulated by the search for candidate materials for electrodes in solid oxide fuel cells (SOFC) and oxygen separation membranes, researchers have explored the possibility of introducing electronic conductivity in oxygen-ion conducting fluorite-type matrices by doping with multi-valent dopants.

Measurement of the stress in oxide scales formed by oxidation of alumina-forming alloys

Abstract: The alumina scales on a variety of high-temperature alloys are found to fluoresce when illuminated with light having a frequency greater than 18,000 cm−1. The fluorescence exhibits two narrow lines characteristic of chromium-doped alpha-aluminum oxide. The frequency shift of the two lines from the room-temperature, stress-free values of 14,402 cm−1 (1.786 eV) and 14432 cm−1 (1.789 eV) provides a noncontact measure of the stress in the alumina scales using the piezospectroscopic effect. In addition, the broadening of the lines is a measure of the stress gradient in the scale. The physical basis for the fluorescence technique is described together with its implementation for highspatial-resolution (∼2 μm) measurements. As illustration, room-temperature measurements of the residual stress in scales formed at 1100°C on single-crystal NiAl, polycrystalline Ni3Al, two Fe−Ni−Cr−Al alloys, and two Ni−Al base superalloys are presented.

A structural model for phosphate glasses

Abstract: The structural component of the network-former oxide in phosphate glasses has a decreasing number of links between the PO4 tetrahedra. However, in addition to the common depolymerization process, further structural principles are in effect. In the present paper, it is proposed that all of the terminal oxygen atoms, including those doubly bonded within the PO4 branching groups, tend to coordinate a network-modifier cation. In a first range, that leads to an effect which can be called a re-polymerization based on modifier oxide additions. Due to the surfeit of terminal oxygen atoms in phosphate glasses with small metal oxide content, the modifier cations, Me, occupy places with high MeO coordination numbers surrounded by terminal oxygen atoms on MeOP bridges. The system tends to stabilize at the point at which all of the terminal oxygen atoms occupy MeOP positions. For even higher concentrations of metal oxide, the coordination number of modifying cations around terminal oxygens exceeds unity, and a modified random network ensues. The model is used for the explanation of the behaviour of packing densities and MeO coordination numbers. The phenomenon of drastic changes of glass properties at about 20 mol% metal oxide content has been related to a network reconstruction based on the change in the type of linkage between the metal cations and the PO4 middle groups.

Synthesis of oxide thin films and overlayers by atomic layer epitaxy for advanced applications

Abstract: Atomic layer epitaxy (ALE) as a self-limiting thin film growth process results in several practical advantages including accurate and simple thickness control, large area and large batch capability, good conformality and reproducibility. straightforward doping and scale-up, possibility to produce sharp and tailored interfaces as well as capability to prepare multilayer structures in a continuous process. In this paper the use of ALE in deposition of numerous conducting and insulating oxide thin films will be reviewed. The versatile ALE chemistry allows the use of different types of metal-containing precursors from simple inorganic compounds to organometallics. Commonly used oxygen sources include water, oxygen, ozone, hydrogen peroxide and alcohols. The recent advances in oxide ALE processes, i.e. deposition of ternary and quaternary oxides, controlled doping of transparent conductors, preparation of multilayer structures as well as conformai coating of porous silicon will also be discussed in respect to their practical applications.

Electrolyte Optimization for Cathodic Growth of Zinc Oxide Films

Abstract: Zinc oxide is of considerable interest to the optical and electronic industries, because of its electrical, optical, and acoustic characteristics. ZnO films can be prepared by several techniques, such as radio frequency (RF) magnetron sputtering, chemical vapor deposition, and molecular beam epitaxy. Preparation of oxide films by electrodeposition from aqueous solutions has several potential advantages over the other techniques. However, the formation of oxide films through electrochemical reactions have been demonstrated only on thallic oxide by Switzer and zirconium oxide by Gal-Or. In this work, the authors have prepared transparent ZnO films with optical bandgap energy of 3.3 eV by electrodeposition from an aqueous, 0.1 mol/liter zinc nitrate electrolyte. The deposition technology is still being developed. This paper reports the effects of the electrolyte concentration on the electrodeposition and properties of the ZnO films.