CHEMICAL COMPATIBILITY OF SIC COMPOSITE STRUCTURES WITH FUSION-REACTOR HELIUM COOLANT AT HIGH-TEMPERATURES

The thermodynamic stability of SiC/SiC composite structures proposed f or fusion applications is presented in this paper. Minimization of the free energy for reacting species in the temperature range 773-1273 K is achieved by utilizing the NASA-Lewis Chemical Equilibrium Thermodyn amics Code (CET). The chemical stability of the matrix (SiC), as well as several fiber coatings (BN and graphite) are studied. Helium coolan t is assumed to contain O2 and water moisture impurities in the range 100-1000 ppm. The work is applied to recent Magnetic and Inertial Conf inement Conceptual designs. The present study indicates that the upper useful temperature limit for SiC/SiC composites, from the standpoint of high-temperature corrosion, will be in the neighborhood of 1273 K. Up to this temperature, corrosion of SiC is shown to be negligible. Th e main mechanism of weight loss will be by evaporation to the plasma s ide. The presence of a protective SiO2 condensed phase is discussed, a nd is shown to result in further reduction of high-temperature corrosi on. The thermodynamic stability of C and BN is shown to be very poor u nder typical fusion reactor conditions. Further development of chemica lly stable interface materials is required.