DC reactive sputtered stainless steel-carbon selective absorbing surfaces for evacuated solar energy collectors

This paper reports on the results of a technical study involving a large cylindrical magnetron sputtering system designed and produced in China, in which selective surface coatings can be applied to thirty absorber tubes at once. The experiments have been carried out using copper, 1Cr18Ni9Ti stainless steel, and acetylene gas. After dc reactive and non-reactive sputtering, stainless steel-carbon/copper selective surfaces are deposited onto glass tubes. Absorptance of the coating is α2=0.92-0.94 and normal emittance εn=0.07-0.08. Some of the evacuated collectors have been exposed outdoors for one year, and their steady temperature went up to 222°C without deterioration of the coating. Some collectors have been electrically heated for 8770 hours at 300°C, then for another 1000 hours at 350°C, but the coating did not show any change.