Papers

AVS2001 Session AS-ThM: Oxides and Oxidation

Thursday, November 1, 2001 9:00 AM in Room 134

Thursday Morning

Start	Invited?	Item
9:00 AM		AS-ThM-3 Atomic Oxygen Irradiation of Polymers: Ground Simulation of LEO Environment G. Lempert, R. Intrater, I. Gouzman, E. Grossman, Y. Lifshitz (Soreq NRC, Israel); A. Hoffman (Technion Haifa, Israel) External surfaces of spacecraft in low earth orbit (LEO) are exposed to the various constituents of the space environment, including atomic oxygen (AO) and solar UV radiation, which often compromise the material properties. Material degradation by LEO atomic oxygen is simulated in a variety of experimental facilities, each with their respective limitations. RF oxygen plasma is a widely used facility for screening materials for LEO application. In addition to the neutral oxygen atoms, the plasma generates electrons and ions as well as vacuum ultraviolet (VUV) radiation, which may also contribute, either directly or synergistically, to degradation of materials. In the present study, direct and synergistic degradation effects on polymers were investigated by exposing the polymers to various combinations of the different species and the VUV emanating from an RF oxygen plasma. The different exposure conditions, including AO alone, AO+VUV, AO+e^-+O⁺+VUV, AO+e^-+VUV, AO+O⁺+VUV, were separated downstream beyond the RF reactor exit. The samples were characterized by mass loss measurements, changes in surface composition, by XPS, and ATR-FTIR spectroscopy, and changes in surface morphology by AFM. The polymer samples investigated were, Kapton, fluorocarbons such as Teflon FEP, Tefzel and Tedlar, having different C/F ratios, and polyethylene. Fluorine containing polymers were confirmed to be more resistant to AO alone than hydrocarbons. However, fluorocarbon polymers, sensitive to VUV radiation, showed synergistic degradation effects and higher erosion yield under AO+VUV irradiations. The effect may be interpreted by VUV radiation induced scissioning, thereby facilitating the material erosion under AO attack. The results presented demonstrate the synergistic effects on the degradation of different polymer materials and elucidate the limitations of the validity of the common RF plasma for screening materials for LEO applications.
9:20 AM		AS-ThM-4 Surface Oxidation on Pure Liquid InSn Alloy M. Jenko (Institute of Metals and Technology, Slovenia) Fluxless vacuum soldering is widely used for hermetical encapsulation of image intensifier tubes and recently for miniature channel photomultiplier. Extremely clean surfaces are required to obtain good wetting at soldering process of this sophisticated sealing technique. An ultra thin oxide film, covering the liquid solder, can often cause leakage of fluxless soldering seals. The model study of pure indium oxidation in the temperature range from 20 to 400°C in a vacuum at 5x10^-5 mbar oxygen pressure was performed using in situ AES measurements. The formation of In₂O₃ in the temperature range from 20 to 360°C was established. At temperatures higher than 360°C isothermal dissociation of ultra thin In₂O₃ film on liquid In surface was found. We also studied the initial phases of surface oxidation on pure liquid InSn alloy (20at.% In, 80at.% Sn). The surface of liquid solder was exposed to pure oxygen at 250°C. On oxidized InSn surface (1,5 x 10 ⁵L) only In₂O₃ was found. Thermodynamic calculations showed that a mixture of SnO, SnO and In₂O₃ was formed at these conditions. The mixed oxides appeared to be thermodynamic unstable near the alloy-oxide interface. The only stable oxide formed on the surface of liquid InSn alloy was found to be pure In₂O₃ at the temperatures of 250 ° and below 360°C. At higher temperatures (higher than 360°C) isothermal dissociation of ultra thin In₂O₃ on liquid InSn surface following by the process In₂O₃ (s)+ 4 In(l)= 3 In₂O (g) was established too.
9:40 AM		AS-ThM-5 Oxidation of Scandium Layers on Tungsten A. Shih, J.E. Yater, C. Hor, R. Abrams (Naval Research Laboratory) Scandate cathodes have demonstrated an emission density as high as 400A/cm² at 1300 K. In order to develop a reliable fabrication technique for scandate cathodes, a thorough understanding of the surface chemistry involving Sc, Ba, O and W is essential. An initial approach is to investigate the interaction among the elements in subsets separately. In this study, Sc on W is studied by AES and TPD. The study reveals three states of Sc adsorption on W that originate from the first-, second- and multi ple-layer. It is desirable to incorporate metallic Sc in the fabrication because its high mobility is conducive to coating uniformity and to Sc replenishment. However, can metallic Sc survive the cathode activation process, which takes a few hours at 1500 K? The desorption-rate equation for the multiple-layer state has been determined as a function of the substrate temperature. At 1500 K, the evaporation rate is 40 µm/hour. Oxidation increases the binding energy of Sc. At 1500 K the evaporation rate of oxidized Sc is less than 0.04 layers/hour. Consequently Sc in the scandate coating should be in an oxidized form. We studied the oxidation process of Sc films and found that at room temperature oxygen has a very limited diffusion length in Sc films. In a 13-layer film after an exposure of 2000 L, 7 layers of Sc remains unoxidized. However, an elevated substrate temperature accelerates the oxidation process. For a 13-layer film, 3 layers and 1.5 layers of Sc remain un-oxidized after an oxygen exposure 48 L at substrate temperatures of 520K and 720K, respectively. This underlies the need of a thermal treatment of a scandate cathode in an oxidizing environment before activation.
10:00 AM		AS-ThM-6 Mechanism of the Activation of Mg₂Ni for Hydrogen Up-Take by Surface Treatment With Liquid Water and Water Vapor J.K. Lomness, L.A. Giannuzzi, M.D. Hampton (University of Central Florida) Magnesium nickel alloy (Mg₂Ni) has been considered an excellent hydrogen storage medium because it has a high hydrogen capacity, forms a very stable hydride, is inexpensive, and it presents no environmental hazards. One of the major problems associated with the use of magnesium nickel alloy for hydrogen storage is its activation for initial hydrogen uptake. Traditionally, high temperatures and high pressures are needed for initial activation of this alloy. It has been discovered that treatment of Mg₂Ni with liquid water and water vapor activates the alloy for hydrogen uptake. The purpose of this paper is to present results obtained from the surface modification of Mg₂Ni with liquid water and water vapor. The data obtained suggest the presence of a hydroxide on the surface. It is believed that this is the first report of the presence of hydroxides on the surface of an active hydrogen storage alloy. Data obtained from x-ray photoelectron spectroscopy and transmission electron microscopy will be presented.
10:20 AM		AS-ThM-7 The Interaction of Organic Molecules with Oxidised Metal Surfaces: Studies by High Resolution XPS and ToF-SIMS J.F. Watts (University of Surrey, UK) The interaction of organic molecules with oxide surfaces is important in many areas of endeavor. At the University of Surrey we have been concerned with this phenomenon for more than two decades, and special emphasis has been applied in the arena of adhesion and adhesive bonding. Our work in this area has been concerned, predominantly, with adsorption from the liquid phase, and it will be shown that XPS and SIMS are ideally suited to the study of adsorption characteristics of such systems. This paper will review fundamental studies that have been undertaken with a view to providing guidelines that can be used in studies of organics on oxide surfaces in other spheres of interest. There are two basic parameters that must be addressed if we are to start to develop a model of the manner of interaction of an organic molecule with an oxide surface. The first is the capacity of the solid surface for the adsorbing molecule, the second is the nature of the bond that is formed between adsorbate and substrate. If the concern is of the behavior of very small amounts of adsorbing species, (up to a few monolayers, for example), there is also the concern regarding the conformation of the molecules and the degree of order in the adsorbed layer. In more complex situations the interest will be in the phenomenon of competitive adsorption where the adsorbing solution contains not one, but many potential adsorbates. Adsorption characteristics of substrates are readily deduced by the construction of adsorption isotherms based on XPS or ToF-SIMS data. In essence, once kinetic equilibrium has been found for a particular system, a series of measurements are made on the adsorption of organic molecules, from solution of various concentrations. The uptake curve, in the case of chemisorption, takes the form of a classical adsorption isotherm, and may be treated by procedures that reflect adsorption in accord with Langmuir, Temkin or other model as appropriate.
11:00 AM		AS-ThM-9 Characterization of Rhenium Oxide Using ESCA B. Panda (National Aeronautics and Space Administration) High melting point and inherent ductility (toughness) over a wide range of temperature has made Rhenium an engineering material of choice for several thrust chambers in propulsion systems. Although the material remains tough at high temperatures, it still can transform to several oxides. As many as eight different oxides have been reported in literature.^1,2 When characterized using ESCA (Electron Spectroscopy for Chemical Analysis) these oxides show large shifts in the Re 4f line positions. While this unique property could be used as a tool for oxide characterization, literature indicates that only a few of these oxides have been characterized.³ Current work focuses on characterizing oxides of Rhenium using ESCA. Spectral line (Re 4f) shifts have been measured for various oxides and the results have been compared with the line positions of real-time oxidation products from space hardware. ¹ ¹ Druce,J. G. F. in "Rhenium", Cambridge, At The University Press, 1948,pp.29-35 ² Lebedev, K. B. in "The Chemistry of Rhenium", Translated by Ronson, L. and Woolf, A. A., Butterworths, London, 1962, pp. 13 -17. ³ Broclawik, E., Haber, J., and Ungier, L. in "Electronic Structure of Rhenium Oxides" J. of Physics and Chemistry of Solids, Vol. 42, 1981, pp. 203 -208.