Roughened, Graded-Index, Cermet Photothermal Absorbers With Very High Absorptivities

Mo/A1203, composite coatings are shown to be useful in the fabrication of stable, effi-cient, selective solar absorbers. One such absorber deposited at 400°C showed no decrease in its solar absorptivity after 33 days in 500°C air. It was found that a higher substrate temperature during deposition can induce a surprisingly large surface roughness in these co-evaporated Mo/Al203 composites. This effect, when combined with grading the cermet composition, was used to produce solar absorptivities greater than 0.99 with a calculated thermal emissivity of 0.20 at 500°C, but it also resulted in a decrease in thermal stability in air. Regardless of the degree of surface roughness, all absorbers were stable in vacuum, to 750°C.