A cylindrical magnetron sputtering system for depositing metal aluminium nitride cermet solar coatings onto batches of tubes

A cylindrical direct current (dc) magnetron sputtering system of commercial scale for coating glass tubes with metal-aluminium nitride (M-AlN) cermet solar selective surfaces is described. The stainless steel-aluminium nitride (SS-AlN) cermet material has been investigated as a solar absorbing layer. During the deposition of a SS-AlN cermet layer, both an Al metal target and stainless steel target are sputtered simultaneously in a 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 onto the surrounding tubes by dc reactive sputtering. The excellent nitriding resistance of stainless steel leads to a nearly pure SS sublayer being deposited onto the tubes. Tube planetary rotation results in a deposited multi-layered system consisting of alternating SS and AlN sublayers. This multi-sublayer system can be considered as a macro-homogeneous cermet layer with the metal volume fraction determined by controlling the thicknesses of SS and AlN sublayers. Varying the SS target current achieves different SS volume fractions in the cermet layers. Following this procedure the SS-AlN cermet solar selective coatings with a double cermet layer structure were deposited onto 32 borosilicate glass tubes of 37 mm outside diameter and 1.2 m long. A solar absorptance of 0.93-0.95 and emittance of 0.04-0.05 at room temperature has been achieved. (C) 1998 American Vacuum Society.Materials Science, Coatings & FilmsPhysics, AppliedSCI(E)10ARTICLE2628-6321