Showing papers on "Oxide published in 1970"

The morphology and mechanism of formation of porous anodic films on aluminium

Abstract: The morphology of porous anodic oxide films formed on aluminium in phosphoric acid electrolytes at constant current density or voltage, and under changing electrical or electrolytic conditions, has been studied quantitatively by electron microscopy. Replicas from film sections and from both film interfaces have been prepared, as well as transmission micrographs of thin films, produced under accurately defined conditions. During formation at constant current density, pore initiation occurs by the merging of locally thickening oxide regions, which seem related to the substructure of the substrate, and the consequent concentration of current into the residual thin areas. The pores grow in diameter and change in number until the steady-state morphology is established. The film barrier layer thickness has been measured directly for the first time. The steady-state barrier-layer thickness, cell diameter and pore diameter are all observed to be directly proportional to the formation voltage. It becomes evident that the barrier-layer thickness, decided largely by an equilibrium established between oxide formation in the barrier-layer and field-assisted dissolution (probably thermally enhanced) at the pore bases, determines the cell and pore sizes by a simple geometrical mechanism. Anion incorporation into the film and its hydrogen-bonded structure play secondary roles to these factors in determining the actual film morphology, although not its subsequent properties. A consequence of the mechanism is that, at constant current density, relatively non-aggressive electrolytes give thicker barrier layers, larger cells and larger pores next to the barrier layer than aggressive media, although subsequent pore widening at the outer surface of the film by simple chemical dissolution is more severe in aggressive electrolytes.

Low-temperature oxidation

Abstract: Low-temperature oxidation is a reaction, occurring at or below room temperature, between a solid and a gas. It usually involves the combination of oxygen with metals, and it has the greatest commercial impact in the presence of moisture, as in corrosion. Cabrera and Mott put forward a theory of low-temperature oxidation, based on the assumption that cation migration occurs under the influence of a potential built up across the growing oxide film. Recent experimental results require that this theory be expanded to explain recent observations such as anion migration during oxide growth and the transition from the initial chemisorbed monolayer to a bulk, threedimensional oxide. The additional ideas put forward in the present paper may be summarized as follows. Low-temperature oxidation is controlled by the nature of the oxide; whether it is a network former or a modifier. A period of fast, linear oxidation is followed by a slow logarithmic reaction whose rate, in turn, can increase if the oxide film crystallizes to form grain boundaries. The initial fast oxidation is a continuation of the chemisorption process. Place exchange (anions and cations interchanging positions) occurs when the energy due to the image force of an oxygen ion is greater than the bond energy holding the ion in place. A stable film forms when this bond energy is greater than the image force energy. The oxygen ions formed on the oxide surface then set up a potential across the film. This potential provides the driving force for continued reaction. Oxide growth during this later stage is a slow, logarithmic process. A barrier to ion transport exists at the gas-oxide interface in the case of anion migration and at the metal-oxide interface in the case of cation migration. In both cases, the field built up across the oxide lowers the barrier sufficiently so that ion migration can occur. Network modifiers allow cation migration. The reaction rate is sensitive to crystallographic orientation of the metal, but not to oxygen pressure. A constant voltage is maintained across the film, so that the Cabrera-Mott theory explains the logarithmic kinetics. Network-forming oxides allow onion migration. The number of anions, and hence, the rate of reaction, is sensitive to oxygen pressure, but not crystallographic orientation of the metal substrate. Since the potential is a result of the mobile anions, the film tends to grow under constant field. The logarithmic kinetics then must be explained by an increasing activation energy for ion transport, as proposed by Eley and Wilkinson. The logarithmic growth rate can be increased by the presence of water vapor if the water introduces “dangling” bonds into an oxide network structure. Crystallization of the oxide film also increases its rate of growth and results in the formation of oxide islands.

Crystalline zeolite zsm-11

Abstract: A CRYSTALLINE ZEOLITE, DESIGNATED ZSM-11, HAVING THE COMPOSITION AS FOLLOWS: 0.9$0.3M2O:W2O3-20 TO 90 YO2:ZH2O WHEREIN M IS A CATION, N IS TTHE VALENCE OF SAID CATION, W IS ALUMINUM OR GALLIUM, Y IS SILICON OR GERMANIUM AND Z IS FROM 6 TO 12, SAID ZEOLITE CHARACTERIZED BY SPECIFIED X-RAY DIFFRACTION VALUES, SAID ZEOLITE IS PREPARED BY DIGESTING A REACTION MIXTURE COMPRISING (R4X)2O, SODIUM OXIDE, AN OXIDE OF ALUMINUM OR GALLIUM, AN OXIDE OF SILICON OR GERMANIUM, AND WATER, R4X BEING A CATION OF A QUATERNARY COMPOUND. ORGANIC COMPOUND CONVERSION IS CARRIED OUT IN THE PRESENCE OF CATALYTICALLY-ACTIVE FORMS OF SAID ZEOLITE.

The role of oxide microstructure and growth stresses in the high-temperature scaling of nickel

Abstract: The oxidation of nickel near 1000°C is accompanied by the generation of stresses parallel with the metal-oxide interface and of magnitude (~1500 psi) sufficient to elongate nickel rod, increase sheet area, and sharpen the angle of bend of ells and helices. A primary cause of this stress is identified with the formation of layers of new nickel oxide upon boundaries of columnar grains where nickel, diffusing through the oxide crystals, meets oxygen, diffusing along grain boundaries. Classical parabolic growth of the scale gives way to a slower quasilinear rate when the major site of new oxide formation is abruptly shifted to a system of grain boundaries lying close to the metal surface and created by recrystallization of the oxide under the influence of stress and high temperature. Another source of stress in the scale arises from the constantly changing area of the metal-oxide interface when oxidation is occurring upon curved metal surfaces. This stress reinforces that generated by deposition of material at oxide grain boundaries when the surface is convex and opposes it when the surface is concave.

Na 2 SO 4 -induced accelerated oxidation (hot corrosion) of nickel

Abstract: The Na2SO4-induced accelerated oxidation of nickel has been studied at 1000°C. It has been found that low oxygen activities in the Na2SO4, which are produced by the formation of NiO, cause the sulfur activity of the Na2SO4 to be increased. Nickel and sulfur from the Na2SO4 combine to form nickel sulfide and the oxide ion activity of the Na2SO4 is increased. The accelerated oxidation of nickel occurs because oxide ions react with NiO to form a nonprotective oxide scale. The accelerated oxidation of nickel is not self-sustaining since oxide ions are not produced when conditions in the Na2SO4 are no longer favorable for the formation of nickel sulfide.

High-temperature oxidation of alloys

Abstract: Some recent developments in the understanding of the oxidation of alloys at elevated temperatures are reviewed, with special reference to binary and ternary alloys upon which many commercial materials are based. Following an initial classification of alloy systems, certain basic principles and their limitations are considered, including factors determining whether an alloy displays surface scaling only, internal oxidation only, or both phenomena; the mode of distribution of cations in an oxide solid solution growing on an alloy; and doping and the Wagner-Hauffe rules. Previous alloy oxidation theory has concerned itself principally with steady-state oxidation but, in practice, behavior of the oxidizing sample in approaching the steady state, both upon initial exposure to the environment and after scale spalling induced isothermally or during thermal cycling, is often as important. Such behavior is illustrated in terms of scale establishment and with respect to compositional changes in the subjacent alloy, the importance of certain alloy and oxide properties being emphasized. After further brief consideration of steady-state scaling, the causes and consequences of breakaway oxidation are presented. The role of major and minor concentrations of third elements on scaling behavior is discussed. The paper concludes with brief reference to situations not considered in detail and with aspects requiring further study.

The single-crystal Raman spectra of nearly opaque materials. Iron(III) oxide and chromium(III) oxide

Abstract: A technique for studying orientation effects on nearly opaque single crystals with laser Raman spectroscopy is described in detail. The instrument used in these studies is a Spex monochromator. The scattering equations derived are applied to a study of iron(III) oxide and chromium(III) oxide.

The transient oxidation of alloys

Abstract: The pre-steady-state oxidation of a wide range of binary alloys of practical importance, in 1 atm oxygen at 600° C, is discussed in terms of the main determining parameters, namely the free energies of formation and growth rates of the component and complex oxides, the bulk alloy composition, the alloy interdiffusion coefficient, the oxygen solubility and diffusivity in the alloy, and effects such as epitaxy. Schematic diagrams are used to illustrate the morphology and structure of the films, as revealed by electron microscopy and diffraction. After tabulation of the sparse, and often apparently unreliable, fundamental parameters, a comparison is made within each of the following groups of alloys: (1) Fe-Cr, Ni-Cr, and Co-Cr alloys, in which the less noble metal is the same and the noble metal is varied, there being composition ranges in which noble metal oxide and less noble metal oxide respectively predominate but in which the oxides are partially miscible or react. (2) Ni-Al, Ni-Cr, Ni-Si, Ni-Mn, and Ni-Co alloys, in which the noble metal is the same, the less noble metals have a wide range of affinities for oxygen and oxidation rates, and the oxide phases produced include solid solutions, largely immiscible simple oxides, and complex oxides. (3) Cu-Ni, Cu-Zn, and Cu-Al alloys, in which the noble metal is the same and the less noble metal is varied, the oxides being largely immiscible.

Phonon and Electronic Raman Spectra of Cubic Rare-Earth Oxides and Isomorphous Yttrium Oxide*

Abstract: Laser-excited Raman spectra have been observed in single crystals of Y2O3, Y2O3(Eu3+), Er2O3, and YB2O3 at 10, 80, and 300 K. In Y2O3(Eu3+) the symmetries of the excited levels have been determined. The optically active phonons can be separated into two groups; in one group (above 300 cm−1) the internal vibrations of (R.E.-O6) octahedrons are preponderant, whereas in the other group (below 200 cm−1) the translational motions of these octahedrons and the R.E. ions are dominant. In Y2O3(Eu3+) electronic transitions (2F0 → 2F2) of the Eu ions in C3i-site symmetry have been observed and the symmetries of the crystal-field-split levels have been determined. Other effects, which may also be attributed to electronic Raman transitions, have been observed in Er2O3 and Yb2O3.

Electrode having platinum metal oxide coating thereon,and method of use thereof

Abstract: AN ELECTRODE FOR USE IN AN ELECTROLYTIC PROCESS. THE ELECTRODE HAS A CORE OF A FILM-FORMING METAL AND A LAYER HAVING AT LEAST THE OUTSIDE PORTION OF THE THICKNESS THEREOF OF AN ELECTTOLYTE RESISTANT AND ELECTROLYSIS PRODUCT RESISTANT MATERIAL, WHICH OUTSIDE PORTION HAS A THICKNESS OF AT LEAST ABOUT 0.054 MICRON. THE LAYER IS ON AT LEAST PART OF THE SURFACE OF SAID CORE. THE MATERIAL OF THE PORTION CONSISTS ESSENTIALLY OF AT LEAST ONE OXIDE WHICH IS AN OXIDE OF AT LEAST ONE PLATINUM METAL TAKEN FROM THE GROUP CONSISTING OF PLATINUM, IRIDIUM, RHODIUM, PALLADIUM, RUTHENIUM, AND OSMIUM. THE ELECTRODE IS ESPECIALLY USEFUL IN ELECTROLYSIS OF BRINE TO OBTAIN CHLORINE.

A Comparison of the Oxidation of Fe&bCr, Ni ? Cr and Co ? Cr alloys in oxygen and water vapour

Abstract: The rates and modes of oxidation of a wide range of pure Fe-Cr, Ni-Cr and Co-Cr alloys in 1 atm oxygen, and in 0.1 atm water vapour/0.9 atm argon contaminated with traces of air, at 1000°C are compared, using data originating largely from a single laboratory. The main features of the mechanisms of oxidation for specific groups of alloys are considered in the light of existing knowledge on the alloy and oxide Properties determining these mechanisms. Vergleich der Oxydation von Fe-Cr, Ni-Cr und Co-Cr in Sauerstoff und Wasserdampf Die Geschwindigkeit und der Verlauf der Oxydation zahlreicher Legierungen aus Fe-Cr, Ni-Cr und Co-Cr in Sauerstoff (1 at) und Wasserdampf (0,1 at) + Argon (0,9 at) + Spuren von Lu# als Verunreinigung bei 1000°C werden unter Verwendung von weitgehend aus einem einzigen Laboratorium stammenden Daten verglichen. Die kennzeichnenden Eigenschaften der Oxydationsmechanismen fur spezifische Legierungsgruppen werden im Lichte der verfugbaren Kenntnisse uber die Legierungs- und Oxid- eigenschaften betrachtet, welche die Oxydationsmechanismen bestimmen.

Differential thermal analysis study of various oxalates in oxygen and nitrogen

Abstract: Differential thermal analysis (DTA) of some 25 oxalates are reported in atmospheres of oxygen and nitrogen. It is shown that in some cases in nitrogen the metal is formed, in other cases the oxide of lowest valency state is produced, while a third group give the same oxide product in nitrogen and oxygen. The production in oxygen of the oxide for the first group, and of a higher oxide in the second group is a phenomenon which in the first case involves a further reaction of the metal with the oxygen atmosphere. Another group of oxalates produce characteristic DTA curves due to the formation of carbonate as the initial decomposition product. The dehydrations of these oxalates are marked by endothermic peaks.

The ionic double layer at the interface

Abstract: An experimental study of the adsorption of potential-determining ions at the zincite (ZnO) aqueous solution interface is reported. Changes in solubility of the oxide with pH are shown to limit adsorption measurements to the pH range 8–10. The pzc of ZnO, as determined by the intersection of adsorption curves at low ionic strength, is shown to depend on the method of preparation and on the nature of the supporting electrolyte.

Nucleation and Growth of Iron Oxides in Olivines, (Mg,Fe)2SiO4

Abstract: Iron-rich olivines have been oxidized in air in the laboratory and the mechanism of their breakdown has been elucidated using X-ray diffraction and electron microscopy. Low-temperature oxidation (500–800 °C) produces well-oriented hematite- and magnetite-like precipitates together with amorphous silica. The reaction is a cellular one in which thin needles of oxide about 50–100 A apart grow into the matrix separated by regions of amorphous silica. Nucleation of spherical colonies of the iron oxide and silica occurs on dislocations. Although the hematite or magnetite always shows the same topotactic relationship with the matrix, the direction in which the needle-like precipitates grow is determined by the orientation of the nucleating dislocation. The small size and highly distorted nature of these precipitates accounts for the diffuseness of their X-ray reflections. Oxidation at 1000 °C produces undistorted equiaxed grains of the oxides about 0·2 μm in size. They are surrounded by silica, which produces a disordered electron diffraction pattern. As the temperature is raised, the silica achieves more structural order and the oxide grains increase in size.

Oxygen Reduction on Oxide‐Free Platinum in 85%Orthophosphoric Acid: Temperature and Impurity Dependence

Abstract: Oxygen electrode kinetics in 85% orthophosphoric acid have been studied as a function of temperature and impurity level in the electrolyte. It is shown that the mechanism of reaction is the same as that in N perchloric acid, and that rate constants and Tafel slopes are dependent on impurity adsorption. Hydrogen peroxide is not a significant reaction product, at least at high temperatures. The activation energy of the reaction at the reversible potential was determined to be

A study of the stresses generated in zirconia films during the oxidation of zirconium alloys

Abstract: The deformation kinetics of zirconium and Zircaloy-2 strips, resulting from oxidation at 500°C in dry oxygen, were measured as a function of time. The stress distribution in the oxidized specimens was analyzed using a stress model based on the theory of elasticity and was correlated with the oxidation kinetics. The magnitude of the stress generated during oxidation differed significantly between zirconium and Zircaloy-2; while a simple linear relationship appeared to exist between the stress and the oxide thickness on zirconium, the stresses in oxides on Zircaloy-2 increased very rapidly and discontinuously to a maximum at a film thickness near the transition in the kinetic curve. In order to characterize the stress distribution in the oxide film, vacuum annealing experiments were performed on preoxidized specimens. These indicated that the stress relaxation which occurs during vacuum annealing resulted from dissolution in the metal of the highly stressed inner layers of the oxide film. Although these measurements have provided valuable information on the stresses generated during the oxidation of zirconium and Zircaloy-2, they do not require a causal relationship between the stresses and any oxidation process (e.g., transition).

Effects of Material and Processing Parameters on the Dielectric Strength of Thermally Grown SiO2 Films

Abstract: The influence of several material and processing parameters on the dielectric trength of thermally grown films on silicon has been assessed. This was accomplished by statistically analyzing the breakdown characteristics of a large number of MOS capacitor structures, which had been fabricated in various ways. Although the results of this investigation are only qualitative, they clearly demonstrate that the effective breakdown strength of these films is strongly dependent on: purity, structural perfection and thickness; the presence of a passivating phosphosilicate glass layer; the presence and reactivity of the metal electrode; and, the duration of the post‐ metallization heat treatment. The morphology of certain micron‐size defects which develop in the MOS structure during annealing is described in some detail, since these faults are apparently responsible for oxide shorting in some instances.

Transport Numbers and the Structure of Porous Anodic Films on Aluminum

Abstract: Experiments are described in which aluminum samples, covered with a porous oxide layer by anodization in oxalic acid, are subsequently re‐anodized in boric acid ("forming"). The experimental techniques employed combine standard voltage and current measurements with gravimetric and optical methods for film thickness determination and measurements of the rate of film dissolution. The results show that for a given anodizing electrolyte the porosity depends only on the anodizing voltage and that it decreases with increasing voltage. The growth of the porous films is determined solely by the formation of oxide at the oxide/metal interface. This can be used to define a transport number for the positive ions during anodization, which was found to be . During forming in boric acid of a sample already covered with a porous oxide layer some of the oxide is formed in the pores and some at the metal/oxide interface. For this process a transport number was obtained. The average pore diameter, cell diameter, and barrier layer thickness vary linearly with the anodizing voltage, the cell wall thickness being somewhat thinner than the barrier layer.