Formation of Nanosized Metallic Ag Grains by Oxidation of Ag Single Crystals with Hyperthermal Atomic Oxygen
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Formation of Nanosized Metallic Ag Grains by Oxidation of Ag Single Crystals with Hyperthermal Atomic Oxygen Long Li and Judith C. Yang Materials Science and Engineering Department, University of Pittsburgh, Pittsburgh, PA 15261, USA. ABSTRACT Silver single crystals (Ag(100), Ag(111)) were exposed to 5eV hyperthermal atomic oxygen, created by a laser detonation of molecular oxygen at a substrate temperature of 220oC for 7 hours. Oxide scales of more than ten microns formed on both Ag (100) and Ag (111) substrates. The microstructural investigation of the oxide layers by high resolution transmission electron microscopy (HRTEM) revealed that the “oxide”scales are predominately composed of nanosized polycrystalline silver grains (5-100nm) as well as a small amount of nanosized silver oxides. The results were remarkably different than the O2 oxidation. The HRTEM investigation suggested that the grains of polycrystalline silver were first carved off from the substrate into “oxide”scale by lattice expansion and decohesion, which are driven by atomic oxygen diffusion in Ag lattice, occupation of oxygen atoms at the interstitial sites of Ag lattice, and partially internal oxidation. The grains in the scale were also subject to continuing oxidations with the atomic oxygen-secondary poly-crystallization, and changed to smaller grains. The preferred oxidation fronts in silver lattice is along the {111} planes. INTRODUCTION Satellites and space vehicles residing in the Low Earth Orbit (LEO) 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 1015 atoms cm-2 s-1. The relative kinetic energy of the hyperthermal atomic oxygen is ~5 eV since space vehicles travel at ~8 km s-1 in LEO [3-5]. We focused on the morphological changes of 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], a sensor material for calibrating atomic oxygen exposure [3-5]. In catalysis studies, silver acts as an important heterogeneous catalyst and is the subject of many experimental and theoretical investigations [7-9]. Recent investigation revealed silver a potential material for optical data storage, based on a process of photo-redox reaction of silver [10]. Most of the studies are related to the mechanism of silver oxidation, i.e. silver reacts with different oxygen species, yet silver does not react much with O2 at ambient or moderate temperature (below 360oC) [7, 11,12]. Sliver strongly reacts with atomic oxygen even at low oxygen pressures and ambient temperatures because of the non-passivation of silver and the high reactivity of the atomic oxygen [3-6,]. Our previous work [13,14] revealed that tens microns of thick oxide scales formed on Ag
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(100) and Ag (111) created by exposure to atomic oxygen at 220oC with complex compounds of
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