In‐line production system for sputter deposition of graded index solar absorbing films

An apparatus for producing a selective solar‐absorbing coating on tubes is described. The coating has absorptance α = 0.88±0.02 and room‐temperature emittance e = 0.035±0.005. When applied to a glass substrate, it consists of layers of stainless steel, copper, and a composite metal–carbon layer. The coating is applied by translation of the substrate through two hollow‐cathode magnetron sputter‐coating chambers. The metal–carbon layer is applied by reactive sputtering in the presence of acetylene and has a graded refractive index. Coatings have been incorporated into evacuated tubular collectors and their properties are discussed.