Direct current magnetron sputtered W–AlN cermet solar absorber films

W–AlN cermet selective surfaces have been investigated using a special direct current (dc) magnetron sputtering technology. During the deposition of the W–AlN cermet layer, both Al and W targets were run simultaneously in a gas mixture of argon and nitrogen. The total sputtering gas pressure was 1 Pa and the reactive nitrogen gas partial pressure was chosen at 2.5×10−2 Pa, which is beyond the transition point from metal phase to dielectric phase in the deposited material. The ceramic AlN component was deposited by dc reactive sputtering and the metallic W component by dc sputtering due to the excellent nitriding resistance of metallic tungsten. By substrate movement a multi-sublayer system, consisting of alternating W and AlN sublayers, was deposited. This deposited multi-sublayer system can be considered as a macro-homogeneous W–AlN cermet layer with metal volume fraction determined by controlling the thickness of the sublayers. W–AlN cermet selective surfaces with a double cermet layer film structure were deposited. The typical film structure from surface to bottom substrate is an AlN anti-reflection layer on a double W–AlN cermet layer on an Al metal infrared reflection layer. A solar absorptance of 0.92–0.94 with emittance of 0.03–0.05 at room temperature and 0.08–0.10 at 350°C has been achieved for such W–AlN cermet selective surfaces. The W–AlN cermet has excellent thermal stability in vacuum at high operating temperature.