Design, properties and degradation mechanisms of Pt-Al2O3 multilayer coating for high temperature solar thermal applications

• Solar thermal power needs new thermoresistant materials.
• Pt-Al2O3 multilayer coatings were developed to meet this need.
• Different IR reflector materials were used and aging tests performed.
• Pt IR reflector showed the best results after aging at 650 °C.

Thin film composite materials, especially alumina-platinum (Al2O3-Pt) multilayer coatings, are promising materials for Concentrated Solar Power (CSP) applications, because of their good resistance to heat and oxidation. In this paper, we will present our work on the design and realization of such coatings. Our absorbers are composed of a substrate (Si, stainless steel and Inconel), a metallic infrared (IR) reflector and an alternation of thin Al2O3 and Pt layers. The absorber structure was optimized by optical simulations and then we used magnetron sputtering to deposit these coatings on different substrates. Then we made optical characterization, transmission electron microscopy (TEM) and chemical characterization to study these coating as deposited and after thermal aging at 650 °C in air. By using different kinds of IR reflector (molybdenum (Mo) reflector, Pt reflector or no reflector) we demonstrate that the choice of this layer is of great importance for the stability of the whole absorber. We show that Mo reflector is not suitable for applications at high temperature in air. Best results were obtained with a 7 layer stack, comprising a Pt reflector. A solar absorption of α = 0.93 and a thermal emissivity of ε = 0.43 (calculated for a temperature of 650 °C) were achieved after aging at a constant temperature of 650 °C in air during 100 h.