Stability of W-Al2O3 cermet based solar coating for receiver tube operating at high temperature

An optimized solar multilayer coating based on W-Al2O3 graded cermet layer was fabricated in our laboratories by magnetron sputtering technique. To use this coating for the receiver tube of solar parabolic trough plants it is necessary for it to have very high structural, physical and chemical stability at high temperature (580 °C) under vacuum, maintaining good performances in terms of high solar absorption and low thermal emissivity. An investigation on the stability of our coating was carried out by means of several material characterizations before and after annealing processes at high temperature. The salient characteristics emerging in this study were the partial oxidation of the tungsten nano-particles inside the cermet forming shells of WOx, the nucleation of a nano-crystalline porous alumina γ phase in the matrix of the prevailing amorphous one and the phase transition from β to α of the tungsten back-reflector. All these phenomena were responsible for the coating reflectance modification promoted by the increased temperature. However we verified that such light optical modification was not progressive but at an annealing time of 2 days a definitive adjustment of the coating optical properties was reached, attesting to a remarkable thermal stability of this solar coating under vacuum and making it very suitable to be used at high temperature.