Surface Oxide Phase Research Articles

Formation of alternative surface oxide phases on GaAs by adsorption of O2 or H2O: A UPS, XPS, and SIMS study

Adsorption of O2 and H2O on GaAs exhibits a sharp contrast in behavior: O2 bonds to As sites almost exclusively while H2O predominantly bonds to Ga sites. The formation of Ga–OH bonds due to dissociative adsorption of H2O preceded by a molecular adsorption step is inferred from UPS data. Corroboration of these findings is provided by SIMS data for positive Ga and negative As ions. The role of hydrogen is discussed in the formation of stable oxides on the GaAs surface.

Asymmetric surface states: A source for surface reconstruction and structure sensitivity

Analysis of an oscillatory kinetic mechanism whose variables are surface oxide and gas phase reactant, shows that in some range of operating conditions the uniform state becomes unattainable beyond a critical system length, and an asymmetric state of surface oxide emerges. Beyond the bifurcation length the integral rate per unit length varies with the size of the system and reaches an asymptotic level for large systems. The asymmetric state is stable for most of this range, except near the bifurcation point where it might be oscillatory. The inhomogeneous profile of oxide concentration induces flux of metal from the oxidized to the reduced section and facilitates the formation of permanent surface structure. Due to the low oxide diffusivity an asymmetric state may emerge on catalyst crystallites (10–1000 Å) and thus, the proposed mechanism may account for the phenomena of surface reconstruction and structure sensitivity. Indeed, the discussion of the class of reactions where these phenomena were o

The electrochemistry of silver in koh at elevated temperatures—III. Potentiostatic study

The electrochemistry of polycrystalline silver in 1 mol kg −1 KOH at temperatures between 295 K and 478 K has been studied using potentiostatic polarization. At potentials corresponding to silver dissolution and to the early stages of Ag 2O formation, the transient current data at all the temperatures studied exhibit regions that are linearly dependent on the inverse square root of time. This dependence indicates mass transport to be a rate limiting step in these processes. Log (steady-state current) vs potential data show that at all temperatures silver undergoes an active-to-passive transition. However, the potential of the transition shifts to less positive values and the passive current increases with increasing temperature. In the region of formation of surface oxide phases, the transients exhibit maxima due to nucleation and growth phenomena. The effect of temperature on the transients is similar to the effect of overpotential; in both cases, the time taken to reach the maximum current decreases.

XPS study of the initial growth of oxide films on Inconel 600 alloy

The gas phase oxidation of Inconel 600 alloy has been studied under very mild oxidation conditions. Depth-composition profiles of thin oxide films formed on the alloy have been studied using X-ray photoelectron spectroscopy (XPS) and ion bombardment. The oxide and metal phases within the surface film are reproducibly differentiated using the XPS chemical shift. The film composition and structure were studied as a function of oxidation time, temperature, oxygen concentration and surface topography. Oxidation at 100°C results in formation of a duplex NiO-Cr 2O 3 layer. At 280°C a duplex layer also forms consisting of a NiFe 2O 4 spinel, near the surface, and Cr 2O 3 near the metal interface. Iron diffusion is found to be strongly affected by the method of preparation; chromium diffusion is not. Oxygen concentration affects the relative concentration of nickel oxide produced during oxidation. The chromium oxide phase, however, is unaffected by oxygen concentration at this temperature, suggesting that growth of this protective layer is severely limited by the low outward diffusion rate of chromium ion. The initial oxide film is more enriched in chromium at 500°C than at 300°C, and some evidence of internal oxidation is seen. Oxidation of the nickel phase occurs after a limiting thickness of Cr 2O 3 is established. At 700°C, a surface oxide phase of CrO 2 is established initially. The optimum conditions for formation of a pure chromium oxide layer on Inconel 600 are high temperature, annealed alloy surfaces and a low oxidant concentration.

The interactions of nitric oxide and oxygen with Ru(101̄0)

The interaction of nitric oxide with ruthenium (101̄0) has been studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) techniques at substrate temperatures and reactant partial pressures as high as 950° C and 10 −6 Torr, respectively. Nitric oxide adsorption is rapid at temperatures below 200°C and results in the formation of c(2 × 4) and (2 × 1) ordered LEED structures at proposed coverages of 0.25 and 0.50 monolayers of dissociated adatoms. The nitrogen adatoms desorb if the sample temperature is raised above 200°C. Prolonged exposures at temperatures greater than 400°C result in the slow development of (7 × 1), c(4 × 8) or c(2 × 6) LEED structures depending upon the temperature and the fractional surface coverage of oxygen adatoms. A model involving the formation of non-stoichiometric surface oxide phases is proposed. The data are correlated with the results of earlier studies on the nitric oxide reduction reaction.

Wetting of amalgam alloys by mercury

The degree of wetting of amalgam alloy by mercury during trituration is an important variable in determining reaction rates and the degree of amalgamation. The purpose of this study was to measure the contact angle made by mercury on various phases present in amalgam alloy in air and on commercial alloys. During amalgamation, mercury is in contact with the oxides present on the amalgam alloy as well as the metallic amalgam phase. To measure the degree of wetting of the metallic phases, silver, tin, and copper were melted in such proportions as to give specimens of silver, tin, the alpha, beta, and gamma silver-tin phases, the eutectic in the silver-copper system. These were cast in the form of cylinders, sectioned, and the surfaces prepared by sandblasting. Oxide specimens of AgO, Ag2O, SnO, and SnO2 were prepared by compaction in a mold. Four commercial amalgam alloys also were tested. Mercury drops were then placed on the surfaces and the contact angles measured at different time intervals. The initial contact angles of mercury on the alpha, beta, gamma, and eutectic phases were found to be 90degrees, 112 degrees, 145 degrees, and 138 degrees, respectively. Mercury reacted with silver and tin, resulting in a change of contact angle with time. Contact angles measured on the commerical alloys were also high, on the order of 145 degrees. The gamma phase had contact angles closest to those of commercial alloys. Surface oxide phases are most likely responsible for this poor wettablility by mercury.

Field ion microscope observations of surface rearrangement and oxide formation at clean rhenium surfaces heated in oxygen

The interaction between oxygen and clean, well-ordered rhenium surfaces has been investigated in the field ion microscope over a wide range of temperatures. The surface structure of vacuum annealed specimens was extensively modified by the presence of adsorbed oxygen. At partial monolayer coverages, surface metal atom rearrangement was inhibited, particularly at {112̄2}, but at coverages near saturation, facets were produced at {101̄0} at a lower temperature than on the clean surface, resulting in the exposure of an increased proportion of closepacked surface structures and suggesting a change in the rearrangement mechanism at high coverages. The production of facets at the major poles was temperature and coverage dependent as a result of the varying influence of adsorbed oxygen on surface free energy. A decrease in oxygen coverage was observed at 1200–1300 K on near-saturated surfaces, which may be associated with the formation of volatile ReO 3 from regions where oxygen concentration exceeds a critical value. There was little evidence of the production of a discrete surface oxide phase formed on specimens heated in the presence of gas phase oxygen, but a reduction in specimen radius as a result of volatile oxide formation was observed at pressures greater than 10 −4 Torr and temperatures above 1400 K. It is concluded that the final structure of rhenium surfaces heated in oxygen is dependent upon the rate of oxide formation, the effective oxygen coverage during oxidation, and the extent of surface rearrangement.

Effects of Si, Mn and C on metallurgical properties of wateratomized SUS 316 stainless steel powders

The main objective of the present work was synthetically to elucidate and evaluate the addition effects of Si, Mn and C on various properties of SUS 316 stainless steel powders produced by water-atomization of liquid metals. The results were summarized as follows:1) Every component mainly affected particle shapes, the amount of surface oxide and metallographical phase states of powders, where especially Si and Mn gave each other some reverse effects. Consequently, apparent density, compactibility and the like were numerically correlated to Si/Mn ratio.2) Compressibility was increased when Si: 0.6-0.9% and Mn≤0.2%, because of above-mentioned powder characteristics. On the other hand, for the higher C-content, green density less increased in the stage of plastic deformation of particles.3) When the powders were vacuum-sintered at temperature below 1200°C, the same Si-component range as2) gave a maximum sintered density, and also gave a lower activation energy for sintering than any other ranges.4) The sintering rate and pore-spherodization of compacts were accelerated with increase of Si-content. Therefore, mechanical properties, especially porosity-sensitive ductility were appreciably improved in Si-rich materials which were sintered at higher temperature.5) Corrosion resistance in boiling acid solutions was complicatedly related to every component, being overlapped by the influence of residual pore shape and oxides along grain boundary. All these sintered materials seemed stronger for boiling 65% HNO3-solution.

Adsorption of Oxygen on the (110) Plane of Tungsten

The sticking coefficients and desorption kinetics of oxygen on a (110) tungsten crystal have been investigated by a step desorption/reflection technique. Sticking coefficients follow a generalized Kisliuk model, if allowance for reflection on first impact with the surface is made. Unlike CO on (110) W, reflection of O2 is appreciable and appears to have an activation energy of 0.6 kcal, with respect to substrate temperature. Desorption follows first-order kinetics and has an activation energy of ∼ 92 kcal. The process is complex, however, as shown by the variation in pre-exponential with surface treatment. A mechanism involving the formation and subsequent decomposition of two distinct surface oxide phases is postulated and shown to fit the experimental results. On the basis of this mechanism the observed activation energy corresponds to the process oxide→mobile adsorbed O, and it is shown that this implies an upper limit of 126 kcal for the heat of adsorption of O atoms on (110) W.

Structure of Surface Oxides Formed on A 20% Cr/25% Ni/Nb-Stabilised Steel in Carbon Dioxide at High Temperatures

Abstract Electron and X-ray diffraction studies of oxide growth on a 20% Cr/25% Ni/Nb-stabilised austenitic steel in carbon dioxide at 750° and 850° are described. The results are interpreted in terms of a duplex structure comprising an inner rhombohedral Cr2O3-layer and an outer spinel oxide of the M2+(M3+)2O4 type. The influence of minor alloying elements, including rare earth metal additions, on the constitution and distribution of the surface oxide phases formed on the same basic steel is also discussed as a means of improving oxidation resistance.