NEW CERMET SOLAR COATINGS FOR SOLAR THERMAL ELECTRICITY APPLICATIONS

On the basis of successful research into new metal-aluminium nitride ( M-AlN) cermet solar selective coatings, a commercial-scale cylindrical direct current (dc) magnetron sputter coater has been developed for t he deposition of these cermet solar coatings onto batches of tubes. Th e 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-a luminium nitride (SS-AIN) cermet material has been investigated as a s olar absorbing layer. Three cylindrical post cathodes, consisting of s tainless steel, aluminium and copper tubes, are installed in the chamb er. During the deposition of an SS-AIN cermet layer, both the Al and S S targets are sputtered simultaneously in the gas mixture of argon and nitrogen. The nitrogen partial pressure is set sufficiently high at t he aluminium target side to ensure that a nearly pure AlN ceramic subl ayer is deposited by de reactive sputtering. The planetary rotation of the 32 tubes results in a multi-layered system consisting of alternat ing SS and AlN sublayers. This multi-sublayer system can be considered as a macro-homogeneous cermet layer. Varying SS target current achiev es different SS volume fractions in the cermet layers. A solar absorpt ance of 0.94-0.95 and emittance of 0.04-0.05 at room temperature has b een achieved for the SS-AlN cermet solar coatings. These solar tubes a re stable at 330-400 degrees C. Using this commercial-scale coater, it should be possible to produce solar tubes stable at even higher tempe ratures (450-550 degrees C), based on a high melting point transition metal-aluminium nitride cermet. (C) 1998 Elsevier Science Ltd. All rig hts reserved.