Structural Characterization of Oxide layers on Aluminum Formed by Exposure to Hyperthermal Atomic Oxygen
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Structural Characterization of Oxide layers on Aluminum Formed by Exposure to Hyperthermal Atomic Oxygen Long Li1, Liang Wang1, Timothy K. Minton2, Judith C. Yang1 1 Materials Science and Engineering Department University of Pittsburgh, Pittsburgh, PA.USA 2 Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT. USA ABSTRACT Single crystal Al (100) was exposed to 5 eV atomic oxygen beam. The sample was maintained at a temperature of 220oC and the total atomic oxygen fluence was 8×1019 atom·cm-2. We have characterized the resulting oxide and interface structures by cross-sectional (scanning) transmission electron microscopy ((S)TEM) and scanning electron microscope(SEM). Our TEM results show that an amorphous aluminum oxide layer with ~6 nm thickness formed on the aluminum crystal, and a rough alumina/Al(100) interface forms. For a systematic study of the evolution of the oxide, a unique Physical Sciences, Inc. Pitt FASTTM AO laser detonation atomic oxygen source in a UHV chamber is employed. The system is equipped with a Maxtek RQCMTM system, a research quartz crystal microbalance (QCM) with a dual-sensor head, to dynamically measure the mass change of an aluminum film coated on the sensor crystal during exposure to atomic oxygen. The Al film initially experiences mass loss, and then parabolic mass gain. To observe the structural evolution of the oxide, a very thin Al (100) single crystal was exposed inside the AO source, characterized by SEM and TEM. The surface morphology changed from flat to rough after 5.5 minutes of exposure. This surface roughening could be related to the initial mass loss measured by QCM.
INTRODUCTION Space vehicles residing in Low Earth Orbit (LEO), ranging from 200 km to 700 km above the earth, experience a harsh oxidizing environment, where atomic oxygen (AO) is the primary corrosive species [1, 2]. The atomic oxygen is generated by the dissociation of molecular oxygen by intense UV sunlight, and has a large flux of approximately 1012 to 1014 atoms·cm-2s-1. The relative kinetic energy of the hyperthermal atomic oxygen is ~5 eV since space vehicles travel at about 8 km/s in the low earth orbit [3-5]. We focused on the morphological changes on a model metal material, Al, when exposed to hyperthermal atomic oxygen. Aluminum and its alloys are used as structural components on space vehicles, and as a coating material to enhance reflectivity and also as protective coating for polymers since the passivation of aluminum after exposing to oxygen [6, 7]. Aluminum usually
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forms only about 2 nanometer amorphous alumina layer when exposed to air. Previous investigations compared the oxidation of aluminum exposed to different species of oxygen [8], and the electrostatic charging of an Al2O3 film by electron bombardment produces a greatly enhanced rate of Al(111) oxidation by O2 at 90 K [9]. Ozone produced oxide film on aluminum enhances the passivation properties, and the TEM measurements revealed that the O3-grown oxide is a slightly denser and less porous
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