Carbon deposition from electronically-excited gas-graphite oxidation reactions in closed systems

Abstract

An investigation was made of microstructural changes produced in nuclear graphites by electronically-excited gas-oxidation reactions under static conditions. The principal change in the static system which was not found with flowing gases is carbon deposition. The carbon deposits, which appear not to contain suboxides, are of two types, (1) clusters, and (2) tree-like formations. The clusters are random arrays of small crystallites and appear to form in the gas phase and settle out on nearby surfaces. In contrast, the tree-like structures grow on the substrate and consist of larger crystallites oriented with the graphite basal planes at a high angle to the surface. Both carbon gasification and deposition were found in this study. However, deposition occurred only at 900 μ initial oxygen pressure, the highest pressure investigated. Gasification results from oxidation by excited oxygen and carbon dioxide. Deposition processes involve the excited reaction of carbon monoxide to produce carbon clusters directly in the gas phase and carbon growth on the heated substrate from carbon suboxide pyrolysis. Apparently, gasification and deposition can occur simultaneously with the net result being determined by the relative rates of the two processes.