Initial stages of silicate growth on and oxidation of graphite: A model for composites
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Initial stages of silicate growth on and oxidation of graphite: A model for composites Dawn A. Bonnell, Darrin Jiron, and Timothy Flinn Department of Materials Science and Engineering, The University of Pennsylvania, Philadelphia, Pennsylvania 19104 (Received 12 October 1995; accepted 15 June 1997)
The initial stages of silicate growth on graphite are characterized with atomic force microscopy. The morphological development indicates that decomposition of tetra ethyloxysilane at low pressure produces films of 3 nm clusters located at undercoordinated carbon sites. Clusters eventually cover the surface, at which point a second layer grows. In higher pressure deposition multiple layers of clusters grow simultaneously. A comparison of the oxidation behavior of surfaces with defects completely and incompletely terminated with SiOx shows that edge recession is the primary oxidation mechanism and that the site specificity of SiOx is effective in inhibiting oxidation.
I. INTRODUCTION
The structure and properties of carbon in the form of graphite have been vigorously studied for years in the field of fast conduction in intercalated compounds and in the field of catalysis. Recently, carbon chemistry has become essential to the development of advanced materials such as carbon-carbon composites, diamond and diamond-like films, graphite-epoxy, and metal matrix composites. Particularly challenging has been the development of carbonaceous materials for high temperature structural applications. A class of carbon based materials (carbon vapor deposited-carbon fiber composites) has been produced which exhibits better strength retention than superalloys or ceramics at temperatures above 1000 ±C.1 The obvious difficulty with these materials is their low chemical stability in oxidizing environments at temperatures as low as 500 ±C. Oxidation resistance is the limiting factor in applications of carbon composites. Strategies for providing oxidation resistance include coating with carbides (SiC, TiC), oxides (SiO2 , B2 O3 ), or nitrides (Si3 N4 , BN).2–6 The potential and demonstrated applications of carbon composites have motivated studies of high temperature oxidation and of reactions with coating materials. Much work involving oxidation of different forms of carbon, including graphite, fibers, and composites, centered on the kinetics of carbon oxidation as opposed to specific reaction mechanisms. Oxidation has been characterized with temperature programmed desorption, x-ray photoelectron spectroscopy,7,8 Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy,9 scanning electron microscopy, x-ray diffraction, and more recently scanning tunneling microscopy and spectroscopy (STM).10–13 It is now well understood that the basal plane of graphite is very difficult to J. Mater. Res., Vol. 13, No. 1, Jan 1998
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oxidize with molecular oxygen, while edges and defects oxidize more readily.14,15 Edge sites of the basal plane
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