Oxidation of an ultra high temperature ceramic : zirconium carbide

This project reports on the study of the oxidation of ZrC – 20vol% MoSi2 in the temperature range 1800 – 2400 K in two oxidizing atmospheres. One is air in order to partially reproduce the operating conditions of a high-temperature receiver for concentrated solar radiation. Such receivers are used in solar tower power plants, this technology being likely to grow in scale in the future due to environmental concerns. The other is helium with a low oxygen partial pressure in order to study incidental and accidental conditions of a Gas-Cooled Fast Reactor (GFR), a Generation IV nuclear reactor. A thermodynamical calculation showed the existence of a limit temperature above which the solid oxide turns into gaseous species. This temperature was influenced by the oxygen partial pressure and the kind of oxidizing atmosphere. During oxidation experiments carried out in a 6 kW solar furnace in PROMES laboratory, France, reliable results were obtained in air. From 2000 K, bubbles emerged from the surface of the samples, grew and burst more and more rapidly with increasing temperature. The formation of these bubbles was accompanied by an increase in oxidation damage for the material. It was assumed that these bubbles formed because of the surface melting of the samples in addition to an increased release of CO. However ZrC was found to undergo less oxidation damage than SiC under the same conditions, as demonstrated in this study. Two different ZrC surface states were studied, but no major differences were observed in term of oxidation behavior. In impure helium, the experiments were not sufficiently reliable to draw conclusions about the possibility of using this material for a GFR. Nevertheless, the observation of a very thin, perhaps protective, glassy layer at 1800K might lead to interesting results in the future. Results of thermodynamical calculation and characterization of the oxide layer suggest that the addition of such a high amount of MoSi2 was detrimental to the oxidation behavior above 1800 K because of its dissociation and its role in melting of the surface in air. A numerical analysis of the reactor was performed with Ansys-Fluent® in order to model the solar furnace in operation. This analysis gave interesting results about temperature, flows and chemical species showing that the reactor is well designed for this kind of experiments, and the development of a flame-like flow as reported by the analysis was observed in reality.Validerat; 20101217 (root