GRADED METAL CARBIDE SOLAR SELECTIVE SURFACES COATED ONTO GLASS TUBES BY A MAGNETRON SPUTTERING SYSTEM

A solar selective absorbing surface consisting of mixed iron, chromium, and nickel carbides on copper has been deposited onto 1. 5 m long, 22 mm o. d. glass tubes by reactive and nonreactive dc sputtering. Grading the properties of the metal carbide from metal rich at the copper substrate to dielectric at the surface leads to solar absorptances of approximately 93% and emittances of approximately 4% at 300 K (both properties averaged along the length of a tube). Aging experiments indicate that absorptances deteriorate slowly at 300 degree C in vacuum due to diffusion in the graded film. Emittances at elevated temperature for copper and for the selective surfaces deposited onto tubes have been determined using a calorimetric technique.