High performance aperiodic metal-dielectric multilayer stacks for solar energy thermal conversion

Abstract

A novel spectrally selective Cr/AlCrN/AlCrNO/AlCrN/AlCrNO/AlCrO multilayer coating was deposited on stainless steel substrates using cathodic arc ion plating. In this tandem absorber coating, Cr, AlCrN, AlCrNO and AlCrO act as the reflector layer, semi-absorbing layer, the main absorbing layer and the antireflection layer, respectively. The spectral properties of this multilayer coating during long-term annealing in air have been investigated with the aim of evaluating the thermal stability of this aperiodic metal-dielectric multilayer stacks. An average absorptance of 0.90 and emittance of 0.15 are achieved in this multilayer coatings fabricated with optimized parameters. More importantly, the coating exhibits an outstanding thermal stability with a selectivity of 0.94/0.11 even after annealed at 500 °C for 1000 h in air. The microstructure analysis indicate that the multilayer coating consists of aperiodic AlCrN and AlCrON layers in addition to the Cr and AlCrO layers. After the long-term annealing, small amounts of AlN, CrN and Cr2N nanocrystallites are observed to be homogeneously embedded in the AlCrN and AlCrON amorphous matrices. The nanoparticles in the AlCrN and AlCrON layers can effectively scatter the incident light into a broadband wavelength range, increasing the optical path length in the absorbing layers and thus resulting in a pronounced enhancement in the absorptivity. A handful of Cr2O3 and Al2O3 nanograins are observed to embedd in the amorphous AlCrO antireflection layer, which can efficitively reflect the solar infrared radiation and the thermal emittance from the substrate and thus result in pretty low infrared emissivity. The good thermal stability is attributed to the excellent thermal stability of the cermet-based amorphous matrices and the sluggish atomic diffusion in the nanoparticles, which could effectively slow down the inward diffusion of oxygen and avoid agglomeration of naonparticles. These results suggest that the AlCrON-based selective absorbing coating is a good candidate for photo-thermal conversion at high temperatures.