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

The heat-to-electricity conversion efficiency of concentrated solar power plants may be enhanced, for lower cost electricity generation, by using high-temperature (750 degrees 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 degrees 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 degrees 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 degrees 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. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

In this study, the oxidation behavior of SiC/Si composites at 750 degrees Celsius in CO2 and air was examined, revealing the formation of thin SiO2 scales in both gases without any detectable oxide formation at the SiC/Si interfaces. The weight gains were minimal, with a significantly lower rate of weight gain observed in CO2 compared to air. The SiO2 scales formed on SiC were thinner than those formed on Si, and the SiO2 scales on Si in air were thicker than in CO2, while the SiO2 scales on SiC were of similar thickness in both air and CO2.