Corrosion of a dense, co-continuous SiC/Si composite in CO2 and synthetic air at 750 °C

Abstract

The heat-to-electricity conversion efficiency of concentrated solar power plants may be enhanced, for lower cost electricity generation, by using high-temperature (750 °C) supercritical CO2 as a working fluid to drive the turbines in these plants. Unfortunately, such operation has been inhibited by the reduction in thermomechanical performance at ≥550 °C of metal alloys used, or considered for use, in compact heat exchangers for heat transfer to supercritical CO2. Co-continuous, melt-infiltration-processed SiC/Si composites possess an attractive combination of high-temperature mechanical and thermal properties for use in such compact heat exchangers. However, the corrosion behavior of melt-infiltration-processed, equivolume SiC/Si composites in CO2 at 750 °C has not been reported. In this work, the oxidation kinetics of SiC/Si composites, fabricated by Si liquid infiltration into porous SiC preforms, have been examined in CO2 and in air at 750 °C for up to 700 h. Continuous, thin, slow-growing external amorphous SiO2 scales formed on the SiC/Si composites during oxidation in both gases, without detectable oxide formation at internal (subsurface) SiC/Si interfaces. Corrosion-induced SiC formation in CO2, and Si3N4 formation in air, were also not detected below the SiO2 scales. The weight gains were small, with a significantly lower rate of weight gain detected in CO2 than in air. SiO2 scales formed on SiC in both gases were thinner than the scales formed on Si. The SiO2 scales formed on Si in air were notably thicker than the scales formed on Si in CO2, whereas SiO2 scales formed on SiC in air and CO2 were of similar thickness.