New cermet solar coatings for solar thermal electricity applications

On the basis of successful research into new metal-aluminum nitride (M-AlN) cermet solar selective coatings, a commercial-scale cylindrical direct current (dc) magnetron sputter coater has been developed for the deposition of these cermet solar coatings onto batches of tubes. The coater vacuum chamber has a diameter of 0.8 m and is 1.8 m in height. 32 glass tubes may be accommodated in the chamber. Stainless steel-aluminum nitride (SS-AlN) cermet material has been investigated as a solar absorbing layer. Three cylindrical post cathodes, consisting of stainless steel, aluminum and copper tubes, are installed in the chamber. During the deposition of an SS-AlN cermet layer, both the Al and SS targets are sputtered simultaneously in the gas mixture of argon and nitrogen. The nitrogen partial pressure is set sufficiently high at the aluminum target side to ensure that a nearly pure AlN ceramic sublayer is deposited by dc reactive sputtering. The planetary rotation of the 32 tubes results in a multi-layered system consisting of alternating SS and AlN sublayers. This multi-sublayer system can be considered as a macro-homogeneous cermet layer. Varying SS target current achieves different SS volume fractions in the cermet layers. A solar absorptance of 0.94-0.95 and emittance of 0.04-0.05 at room temperature has been achieved for the SS-AlN cermet solar coatings. These solar tubes are stable at 330-400 ??C. Using this commercial-scale coater, it should be possible to produce solar tubes stable at even higher temperatures (450-550 ??C), based on a high melting point transition metal-aluminum nitride cermet.EI1-3109-1146