Impermeable SiC Composites

Next generation nuclear plants will operate with helium as the working fluid and operate most efficiently at temperatures approaching 1000°C. Steel alloys used in reactor construction lose their strength at temperatures above 850°C, and SiC/SiC composite systems generally have helium permeability that is too large for duct components. This project will develop a carbon-fiber-reinforced silicon carbide matrix composite system that retains its strength above 1000°C and has helium permeability less than 10-9m2/s. Nanophase materials used in the matrix will minimize porosity without sacrificing strength or high temperature performance. In Phase I, the permeation of gas through a silicon-carbide-based ceramic composite was reduced by more than two orders of magnitude by adding a combination of oxides to the composite matrix. The oxides reacted during processing with the composite matrix to fill pores and gaps, thereby reducing the ability of both air and helium to flow through the matrix. It was shown that the permeability can be reduced to 10-9m2/s, comparable to the best silicon carbide composite materials available. In Phase II, the processing of the matrix additives will be improved to further reduce gas permeability through the carbon-fiber-reinforced silicon carbide composites. In addition, it will be shown that complex assemblies can be fabricated using these materials, with the strength and gas permeability of joints comparable to the composite system. Commercial Applications and other Benefits as described by the awardee: The successful development of a family of impermeable high-temperature, oxidation-resistant refractory composites would enable the full-scale commercial development of next generation nuclear plants. These high efficiency systems would produce both electricity and hydrogen to enable the U.S. to reduce its dependence on fossil fuels while minimally impacting the environment. The development of high temperature composite systems that are impermeable to gas flow also should facilitate the incorporation of high temperature ceramic composites into next generation gas turbines