Oxide Structures formed on Silver Single Crystals due to Hyperthermal Atomic Oxygen Exposure
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Oxide Structures formed on Silver Single Crystals due to Hyperthermal Atomic Oxygen Exposure Long Li and Judith C. Yang Materials Science and Engineering Department, University of Pittsburgh, Pittsburgh, PA 15261, USA. ABSTRACT Silver (100) and (111) single crystals were exposed to a unique laser detonation atomic oxygen source, which produces a high flux of 5 eV atomic oxygen, for seven hours at 220°C. The resultant oxide and oxide-metal interfaces were characterized by optical, scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM). The oxide scale was more than 10-micron thick and very weakly attached to the Ag substrate. The silver oxides were complex and surprising, differ in their thickness and the oxide phases due to the orientation of the Ag single crystals. The cross-section TEM studies revealed complex microstructures with many defects, such as micro-twins, porosity and irregular shaped grains. INTRODUCTION Satellites, spacecrafts and sensors residing in Low Earth Orbit (LEO) experience a harsh oxidizing surroundings, where atomic oxygen (AO) is the major 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 1015 atoms.cm-2s-1. The relative kinetic energy of the hyperthermal atomic oxygen is ~5 eV since space vehicles travel at ~8 km.s-1 in the low earth orbit [3-5]. We focused on the morphological changes on a model metal material, Ag, when exposed to hyperthermal atomic oxygen. Silver is used in a wide variety of applications in LEO, such as electron conductive material [6] as well as a detector or sensor material for calibrating atomic oxygen exposure [3-5]. Hence, strong interests in the ground-based simulation of oxidation of silver exist [3-7]. Silver forms only a few tens of nanometer oxide when exposed to air at ambient temperature and atmosphere [8], but it strongly reacts with atomic oxygen even at low oxygen pressures and ambient temperatures because of the high reactivity of the atomic oxygen [3-6,9]. The oxide and oxide/metal microstructures should provide insights into the oxidation mechanisms of silver by atomic oxygen. Yet, the microstructural changes of Ag exposed to atomic oxygen have not been studied methodically. In this paper, we present our results of Ag (100) and Ag (111) exposed to hyperthermal atomic oxygen created by laser detonation [10]. We investigated systematically the possibility of non-passivation of silver and the oxide microstructure by means of x-ray diffraction (XRD) spectroscopy, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
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EXPERIMENTAL Ag (100) and Ag (111) single crystals with a native oxide layer were exposed simultaneously o 15 under same conditions at 220 C by atomic oxygen with the flux of 3x10 atoms.cm-2s-1 for 7 hours, where the 5 eV atomic oxygen are created by laser detonatio
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