Fabrication and characterization of high temperature ceramics for novel solar absorbers

The best candidate materials for the novel solar absorber should possess a combination of properties including appropriate absorbance, good electrical conductivity and resistance to oxidation in air. Porous SiC and dense SiC were rejected for several reasons. Porous SiC has a proper absorbance but no electrical conductivity and poor mechanical strength. Dense SiC has modest absorbance and poor electrical conductivity. The multiphase composites, SiC-MoSi2, AlN-SiC-MoSi2 have a better combination of properties, that can be tailored through compositional rearrangement of the constituent phases. Moreover, they have a superior oxidation resistance thanks to the development of a silica-based protective layer. HfC-MoSi2 have a good absorbance, but too high reflectance and poor resistance to oxidation. Results of the optical properties measurements suggest to reduce the amount of secondary phases such as MoSi2, basically for two reasons: -as MoSi2 behaves like a sintering aid, reduction of its volumetric content hinders complete elimination of porosity during sintering. Residual porosity seems to increase the material's absorbance. -MoSi2 can increase the material's reflectance, especially for matrices such as SiC and AlN. On this basis, the activity has concentrated on the design of optimized compositions with a lower content of MoSi2. On selected compositions, long oxidation tests have been performed in order to verify the capability of the material to withstand 1100°C for 100 hours.