Stainless-steel–AlN cermet selective surfaces deposited by direct current magnetron sputtering technology

Abstract

Stainless-steel–AlN cermet selective surfaces were deposited by a direct-current (DC) magnetron sputtering technology. The aluminum nitride ceramic and stainless-steel (SS) components in the cermet were deposited by simultaneously running both SS and Al metal targets in a gas mixture of argon and nitrogen. Nitrogen partial pressure was set sufficiently high to ensure that a nearly pure AlN ceramic sublayer was deposited by DC reactive sputtering. Because of the excellent nitriding resistance of stainless steel and the very low nitrogen partial pressure, a nearly pure SS sublayer was deposited by DC sputtering. By substrate rotation, a multi-sublayer system, consisting of alternating SS and AlN sublayers, was deposited. By controlling the thickness of SS and AlN sublayers this multi-sublayer system can be considered as a macrohomogeneous cermet layer. Following this procedure, the SS–AlN cermet selective surfaces with a double cermet layer structure were deposited. The deposited films have the following structure: a low metal volume fraction cermet layer is placed on a high metal volume fraction cermet layer which in turn is placed on an aluminum metal infrared reflection layer. The top surface layer consists of an aluminum nitride anti-reflection layer. A solar absorptance of 0.93–0.96 and normal emittance of 0.03–0.04 at room temperature have been achieved for the SS–AlN cermet selective surfaces. After vacuum baking at 500°C for 1 h there is only a slight change in optical reflectance in the wavelength range of 0.3–2.0 μm for the SS–AlN cermet selective surfaces. However, the values of solar absorptance and emittance change insignificantly, around 0.005.