Showing papers on "Oxide published in 1976"

THE SPECIFIC ADSORPTION OF DIVALENT Cd, Co, Cu, Pb, AND Zn ON GOETHITE

Abstract: Summary The specific adsorption of divalent Cd, Co, Cu, Pb, and Zn on goethite is measured as a function of pH. For each mole of heavy metal adsorbed approximately two moles of H+ ions are displaced from the interface. Using these results the heavy metal adsorption data are expressed as functions of the solution concentrations of both H+ and metal ions, and the interfacial reaction is described by the equation, 2SH+M2+= S2M+2H+. The adsorption data are consistent with an electrochemical model of the simultaneous adsorption of H+ ions and divalent metal ions on to the oxide. The intrinsic affinities of the metal ions for the oxide surface increase in the order, Cd < Co < Zn < Pb < Cu. However, besides the affinity of the metal ion for the surface, the adsorption curves are considered to be influenced by surface charge, the adsorption density of the metal ions and their size. The analysis of the data in terms of H+ and M2+ ion adsorption is considered to be complementary to the hydrolysis model for heavy metal adsorption.

The influence of yttrium additions on the oxide-scale adhesion to an iron-chromium-aluminum alloy

Abstract: The oxidation behavior of an Fe-27%Cr-4%Al alloy and similar alloys containing 0.023% and 0.82% Y in 1 atm oxygen at 1200°C has been examined. The oxide formed on the yttrium-free alloy develops a highly convoluted configuration, apparently resulting from lateral growth of the oxide. The latter leads to oxide detachment from the alloy at temperature and extensive spalling during cooling. It is postulated that lateral growth results from the formation of oxide within the existing oxide layer by reaction between oxygen diffusing inward down the oxide grain boundaries and aluminum diffusing outward through the bulk oxide. Additions of yttrium to the alloy apparently prevent the formation of oxide within the oxide layer, the oxide-forming reaction occurring as the alloy-oxide interface. Thus lateral growth is prevented and spalling during cooling does not occur. Secondary advantages conferred by the addition of 0.82% Y to the alloy are the prevention of void formation at the alloy-oxide interface, the avoidance of alloy grain growth during oxidation, and the creation of an oxide “keying” or “pegging” effect.

Physical Properties of SnO2 Materials I . Preparation and Defect Structure

Abstract: Stannic oxide in its pure form is an n‐type wide‐bandgap semiconductor. Its electrical conduction results from the existence of point defects (native and foreign atoms) which act as donors or acceptors. Some unique properties of make the material useful for many applications. Therefore, increasing attention is being paid to studies on this oxide, especially on the methods of preparation, and its electrical and optical properties. The purpose of this work is to provide a general up‐to‐date review of the investigations carried out and to help identify important areas for further studies. This part is concerned with the preparation and defect structure of single crystals, sintered polycrystalline samples, and thin films. Parts II and III2 are reviews of the electrical and optical properties of materials.

Physical Properties of SnO2 Materials II . Electrical Properties

Abstract: Stannic oxide in its pure form is an n‐type wide‐bandgap semiconductor. Its electrical conduction results from the existence of point defects (native and foreign atoms) which act as donors or acceptors. Some unique properties of make the material useful for many applications. Therefore, increasing attention is being paid to studies on this oxide, especially on the methods of preparation, and its electrical and optical properties. The purpose of this series is to provide a general up‐to‐date review of the investigations carried out and to help identify important areas for further studies. The first part was concerned with the preparation and defect structure of single crystals, sintered polycrystalline samples, and thin films. In this part we review the electrical properties of these materials. The electrical properties of single crystals are well understood and the properties of sintered powder are similar to those of single crystals. However, those of films have not been explained satisfactorily up to the present time. They depend strongly on the manner of preparation and subsequent heat‐treatments in various ambients. In the third part, the optical properties of will be reviewed.

Studies on the mechanism of atom formation in graphite furnace atomic absorption spectrometry

Abstract: The mechanism of atom formation for a number of elements in a Perkin-Elmer heated graphite atomizer, HGA 2100, has been studied using a combined thermodynamic and kinetic approach. Assuming that an analyte surface-gas phase equilibrium exists within the furnace and the production of observable atoms is characterized by a unimolecular rate constant, a plot of the logarithm of the absorbance as a function of the inverse of the absolute temperature yields a straight line from which the activation energy, E/sub a/, of the limiting step in the atomization pathway can be obtained. These E/sub a/ values reveal that three major atomization mechanisms are operative: thermal dissociation of the analyte oxide or halide, and carbon reduction of the oxide followed by atomization of the free metal.

Thermal barrier coating system

Abstract: This invention relates to a high temperature oxidation resistant thermal barrier coating system for a nickel-, cobalt-, or iron-base alloy substrate. An inner metal bond coating contacts the substrate, and a thermal barrier coating covers the bond coating. NiCrAlR, FeCrAlR, and CoCrAlR alloys have been satisfactory as bond coating compositions where R=Y or Yb. These alloys contain, by weight, 24.9-36.7% chromium, 5.4-18.5% aluminum, and 0.05 to 1.55% yttrium or 0.05 to 0.53% ytterbium. The coatings containing ytterbium are preferred over those containing yttrium. An outer thermal barrier coating of partially stabilized zirconium oxide (zirconia) which is between 6% and 8%, by weight, of yttrium oxide (yttria) covers the bond coating. Partial stabilization provides a material with superior durability. Partially stabilized zirconia consists of mixtures of cubic, tetragonal, and monoclinic phases.

The oxidation of alloys

Abstract: A review of principles governing alloy oxidation is presented together with the means by which practical oxidation-resistance alloys have been developed. The reactions involved are complex and involve acquiring data from many fields. Wagner developed the basic diffusional models for the formation of oxide scales, but these need to be expanded using detailed physical and metallurgical studies of the alloy phases, oxides and their defects, as well as the mode of scale formation and the structural relationships between alloy and oxide scale. At this time the engineering solutions have outstripped fundamental understanding, but an attempt is made to show how the development of oxidation theory has contributed to this effort by a term now referred to as 'informed empiricism'.

Physical properties of the electrical barriers in varistors

Abstract: The nonlinear relationship between current and voltage in the zinc oxide varistor arises from electrical barriers at the grain boundaries in the polycrystalline ceramic. At very low fields, thermionic emission accounts for the majority of the current, with Schottky emission becoming more important as the field increases and assists in lowering the barrier. Tunneling through the barrier dominates in the presence of high fields, giving the highly nonlinear behavior. The bismuth (oxide) which segregates at the grain boundries occurs in two forms: an adsorbed layer present on the interfaces between grains, and a discrete phase which is continuous along grain edges. The latter is in the form of triangular prisms which exhibit a dihedral angle of about 60° as they approach a ZnO–ZnO interface, and thus this phase does not completely wet the ZnO grains. The adsorbed layer can account for the electrical barrier. Its chemical thickness is less than 50 A as determined by Auger electron analysis. A depletion layer i...

Thermodynamics and kinetics of oxidation of hot-pressed silicon nitride

Abstract: The “passive” oxidation behaviour of silicon nitride hot-pressed with 1 wt% MgO has been studied in dry oxygen in the temperature range 1000 to 1400‡ C. The oxidation follows the classical parabolic behaviour with an apparent activation energy of 375 kJ mol−1. Except for minor amounts of a glass and cristobalite, the oxide film consists predominantly of MgSiO3 in which various impurity elements, e.g. Ca, Fe, Al, etc., concentrate. The outward diffusion of Mg2+ and impurity cations from the grainboundary glass phase through the oxide film appears to be oxidation rate controlling.

Development, Growth and Adhesion of Al2O3 on Platinum-Aluminum Alloys.

Abstract: The development, growth, and adhesion of α-Al2O3 scales on platinum-aluminum alloys containing between 0.5 and 6 wt.% aluminum have been studied at temperatures in the interval between 1000 and 1450° C. The morphologies and microstructures of the α-Al2O3 scales were found to be influenced by the temperature, oxygen pressure, and the microstructures of the alloys. The oxidation rates of the alloys appeared to be controlled by transport of oxygen along grain boundaries in the α-Al2O3 scales. The α-Al2O3 scales adhered to the platinum-aluminum substrates even after extensive periods of cyclic oxidation. The good adhesion of the α-Al2O3 may result from mechanical keying of the oxide to the alloys due to the development of irregular oxide-alloy interfaces.

Origin of Interface States and Oxide Charges Generated by Ionizing Radiation

Abstract: The randomly located trivalent silicon atoms are shown to account for the thermally generated interface states at the SiO2-Si interface. The interface state density is greatly reduced in water containing ambients at low temperatures (450°C) by forming trivalent silicon hydroxide bonds. Interface states are regenerated when the ?Si-OH bonds are broken by ionizing radiation and the OH ions are drifted away. In the bulk of the oxide film, the trivalent silicon and the interstitial oxygen donor centers are shown to be responsible for the heat and radiation generated positive space charge build-up (oxide charge) in thermally grown silicon oxide.

High‐field transport in SiO2 on silicon induced by corona charging of the unmetallized surface

Abstract: A study has been made of high‐field electronic transport in thermally grown SiO2 in which the injection and transport of carriers were induced by charging the exposed surface of the insulator with ions extracted from a corona discharge. This technique totally avoids destructive breakdown through weak spots in the insulator. Auxiliary developments included a comparison technique for measuring the steady‐state potential difference across the unmetallized insulator and a p‐n junction method for determining the sign of the principal charge carrier in the oxide. Using a corona discharge in dry air, breakdown fields of approximately 6.5 and 13.5 MV/cm were obtained for charging with positive and negative ions, respectively. Electrons were identified as the current carriers for both polarities of applied field. Measurements of the discharge of the samples, made with a Kelvin probe, yield results that are consistent with Fowler‐Nordheim tunneling of electrons from the silicon into the oxide, with an electron effective mass in the oxide m*ox=0.48m. Positive charge accumulation was observed in the oxide after charging with negative corona to fields above 11 MV/cm.

Efficient photovoltaic heterojunctions of indium tin oxides on silicon

Abstract: Heterojunction diodes of indium tin oxide films sputtered on to p-silicon using ion-beam techniques display significant photovoltaic effects when exposed to sunlight. Galvanomagnetic and optical measurements confirm that the oxide films are highly degenerate transparent semiconductors. At a tin oxide concentration of 10%, an open-circuit voltage of 0.51 V was observed along with a short-circuit current of 32 mA/sq cm, a fill factor of 0.70, and a conversion efficiency of 12%. As the concentration was raised to 70%, the voltage remained steady, the current fell to 27 mA/sq cm, and the fill factor fell to 0.60

Oxidation kinetics of hot-pressed silicon carbide

Abstract: The oxidation of silicon carbide, hot-pressed with ∼ 4 wt % Al2O3, in 1 atm dry oxygen follows classical parabolic behaviour with an activation energy of 481 kJ mol−1 in the temperature range 1200 to 1400° C. The oxide film consists predominantly of cristobalite and a glassy phase in which additive (Al) and various impurity elements (Fe, Na, K, etc) concentrate. The desorption of CO(g) from the SiC/SiO2 interface appears to be oxidation rate controlling.

Melting behaviour of low molecular weight poly (ethylene-oxide) fractions: 2. Folded chain crystals

Abstract: SummaryMelting temperatureTm and enthalpy of fusion have been measured, by DSC, for folded chain crystals of low molecular weight poly(ethylene-oxide) fractions ranging from 3000 to 10000. These crystals are formed by molecules folded a small integer number,n, of times and show unusual thermal stability on heating. The rates of chain unfolding during isothermal crystal growth and subsequent heating were measured and a reliable stability criterion could be defined for folded chain polymer crystals.Extending the theoretical treatment ofFlory andVrij to folded chain crystals and usingTm data, a reasonable estimate was derived for the respective surface free energy contributions of chain ends and chain folds. The results suggest considerable hydrogen bonding between OH end groups, with a bonding energy of 3.1 Kcal/mole, when the crystal surface contains only chain ends. Hydrogen bonding is essentially destroyed by chain folding. Further analysis leads to an estimate of the contour length of cilia, associated with chain ends and to that of chain folds containing, on average, 2.8 and 3.5 monomer units respectively. Chain folds must thus be sharp involving adjacent re-entry. Finally, an analytical expression is derived showing the separate dependence ofTm on chain length andn, parameters which determine the crystal thicknessL. Critical comparison of this relationship to that commonly used for determining surface free energies from linearTm vs 1/L plots shows that the latter only applies accurately to chains of infinite length and to crystals of thickness larger than a critical valueL*.RésuméLa températureTm et l'enthalpie de fusion ont été mesurées sur des cristaux à chaines repliées de fractions de polyoxyéthyléne, de masse moléculaire variant entre 3000 et 10000. Ces cristaux sont constitués par des molécules repliées un nombren entier de fois et ils ont une stabilité thermique inhabituelle. La vitesse de dépliement des chaines a été mesurée lots de la croissance isotherme des cristaux et du chauffage consécutif et l'on a pu définir un critère de stabilité pour des cristaux à chaines repliées.En étendant la théorie deFlory etVrij aux cristaux à chaines repliées et en utilisant les valeurs deTm on a pu estimer les contributions respectives des bouts de chaine et des repliements à l'énergie libre superficielle. Les résultats suggèrent un couplage important, par liaison hydrogène entre les groupes terminaux OH lorsque la surface des cristaux est constituée entièrement de bouts de chaine, l'énergie de couplage étant de 3,1 Kcal/mole. Ce couplage disparait pratiquement par le rephement des chaines. L'analyse des résultats permet aussi d'estimer la longueur des bouts de chaines et celle des repliements qui émergent du réseau cristallin: ils sont formés, respectivement de 2,8 et 3,5 unités de monomère, en moyenne. Les repliements sont serrés et relient des positions adjacentes du réseau. On déduit finalement une expression deTm en fonction de la longueur des chaines et den, paramètres qui déterminent l'épaisseurL du cristal. La comparaison critique de cette relation avec celle généralement utilisée pour déterminer l'énergie libre superficielle, impliquant une variation linéaire deTm avec 1/L, montre que cette dernière ne s'applique en toute rigueur qu'aux chaines de longueur infinie et aux cristaux d'épaisseur supérieure à une valeur critiqueL*.

Electrochemical Semipermeability and the Electrode Microsystem in Solid Oxide Electrolyte Cells

Abstract: New evidence is put forward within the framework of a recently introduced model. The electrodes are viewed as measuring the oxygen chemical potential in a "microsystem" which exchanges oxygen at a finite rate with the surrounding gas being analyzed. In high temperature oxygen gauges, the oxygen flux resulting from the electrochemical oxygen semipermeability of the electrolyte can have two effects other than reducing the theoretical emf by the factor which appears to induce error of second order only. The oxygen semipermeability can obviously modify the oxygen content of the gas analyzed and also can disturb the equilibrium between the electrode microsystem and the analyzed gas. A special experimental setup was assembled to study both phenomena. The results showed that the second is generally far from being negligible and may lead to noticeable measurement error. A new arrangement with a zirconia point electrode is proposed to eliminate this source of error in oxygen gauges working at very high temperatures. The new arrangement was utilized to measure accurately the oxygen semipermeability of a [9 mole per cent (m/o)] electrolyte at temperatures up to 1650°C. The semipermeability was found to be proportional to, where and are the equivalent oxygen pressures in the layers of oxygen adsorbed on both bases of the pellet. The activation energy of the electronic conductivity equaled 2.02 eV.

Surface Chemistry of Oxides

Abstract: The chemistry of the surface of oxides depends to a large extent on the quantity and type of both physically and chemically (OH groups) absorbed water. Infrared spectroscopy coupled with a variety of chemical exchange processes has provided useful information on the nature and stability of the adsorbed species, and on the interaction of the surface hydroxyl groups with adsorbates at the solid/vapour and solid/liquid interfaces. Electrochemical techniques are used to study the oxide/aqueous solution interface to provide surface charge data and establish zero points of charge, both of which are related to the character of the hydroxyl group and its environment. This paper briefly reviews the application of these methods to the oxides of titanium, silicon and aluminium, in both the pure and mixed forms, and to the study of the deposition of one oxide on the surface of another.

Foaming and silt suspending agent

Abstract: A foaming and silt suspending agent is disclosed containing at least one alkyltrimethylammonium chloride and an amine oxide selected from the group consisting of bis-(2-hydroxyethyl) cocoamine oxide, dimethylhexadecylamine oxide, and dimethyl-hydrogenated tallowamine oxide. The agent is particularly useful in a variety of aqueous based fluids of the type used in treating subterranean formations, including fluids containing other cationic additives such as certain acid inhibitors and clay stabilization agents which tend to impair the effectiveness of prior art agents.

Hydrogen sensitivity of palladium--thin-oxide--silicon Schottky barriers

Abstract: It is shown that current transport through Schottky barriers formed by Pd on n-type silicon with a thin thermally grown oxide is sensitive to hydrogen in the ambient. It is shown that a transition from minority- to majority-carrier dominated current occurs with increasing hydrogen pressure.