Adhesion and structural stability enhancement for Ag layers deposited on steel in selective solar coatings technology

Improvement of structural and morphological stability of Ag layers has to be pursued for effective employment in solar coating technology for “parabolic trough” thermal plants. Starting from the consideration that stainless steel surface of a receiver tube does not behave as an excellent substrate for fabricating appropriate Ag layers in terms of adhesion strength and thermal stability, both substrate pre-treatments and adaptive layer deposition have been taken into account. Beneficial effect of substrate ionic etching pre-treatment on adhesion coupled with utilization of “α phase” tungsten film as adaptive layer gave evidence for large improvement both on adhesion strength and on morphological stability of Ag layers subjected to thermal cycles at 400 °C under vacuum. Using Ag as infrared reflector and Wα as adaptive layer, an optimized cermet-based selective coating suitable for receiver tube operating at 400 °C has been fabricated. This coating, subjected to annealing processes at 400 °C under vacuum up to 29 days, showed excellent thermal and optical stability.