Structure And Properties Of AlCrON-based Solar Selective Absorbing Coatings With High-temperature Tolerance

Solar selective absorbing coatings have been considered as the key components for the solar thermal applications.The metal-dielectric coatings consist of extremely fine metal particles embedding in dielectric matrices are identified as promising materials for high-temperature spectral selective absorption coating applications due to their outstanding thermal stability and sound integrated optical properties.Moreover,the optical properties of metal-dielectric coatings can be tuned by controlling the size,content and distribution of the metal particles.However,during long time aging under high temperatures,the metal particles are prone to agglomerating,coarsening,oxidizing and diffusing across different layers,which would result in the composition and microstructure changes.Even worse,the metal-dielectric coatings would experience irreversible degradations in optical properties.Thus,developing solar selective absorbing coatings with high absorptance(?)in the visible and near-infrared regions and low emittance(?)in the infrared region(2.5-25 ?m)as well as excellent thermal stability is of great importance to improve the solar-thermal conversion efficiency.To enhance the thermal stability and optical properties of ceramics coatings simultaneously,we propose to replace the metal particles with metal nitrides that are extremely stability.The binding energy of metal nitrides can be tuned by doping to achieve desired optical properties.In the present study,metal-chromium nitrides and nitride oxides were selected as the absorbing layers,while Cr and AlCrO were selected as the infrared reflection layer and anti-reflection layer.The AlCrON-based multilayer solar selective absorbing coatings were prepared by cathodic arc ion plating technology.Then,the amorphization processing was employed to form a nanocrystallineamorphous heterostructure.Systematic investigations were carried out to test the optical properties and microstructure evolution of the AlCrON-based multilayer solar selective absorbing coatings during the aging tests.The research contents are as follows:(1)design and fabrication of the AlCrON cermet-based multilayer solar selective absorbing coatings;(2)the optical properties and selective absorbing mechanism;(3)the amorphization and thermal stability during high temperature aging in air;(4)the evolutions of phase,morphology and composition distribution in the deposited and aged coatings.The main results and conclusions of the studies are as follows:(1)Double-absorbing layer AlCrON-based solar selective absorbing coating with a high solar absorptance of 0.90-0.93 and a relatively low emittance of 0.11-0.15 are obtained by the cathodic arc ion plating technique.The microstructure analysis indicates that the as-deposited Al Cr N and AlCrON layers,acting as the main absorbers,are consisted of the Al N,Cr2 N and Al N,Cr N,Cr2 N nanocrystallites embedding in amorphous matrix,respectively while the AlCrO layer,acting as the anti-reflection layer,is consisted of mainly amorphous.After aging at 500 °C for 2 h in air,a solar selectivity of 0.94/0.12 was achieved.Based on the microstructural analysis,the enhancement in absorptance of the multilayer AlCrON-based coating during the initial stage of thermal aging could be attributed to the decomposition of Cr2 N nanocrystallites and the recrystallization of the Al Cr N and AlCrON layers.While the enhanced emittance after aging can be ascribed to the decomposition of the macro-droplets and the decrease of surface roughness.(2)After short time aging at 500 °C in air,a nanocrystalline-amorphous heterostructure was achieved due to the amorphization during the initial stage of aging.Moreover,the nanosized nitrides particles are separated by amorphous.The absorptance and emittance of the double-absorbing layer AlCrON-based solar selective absorbing coating slightly increased after aged at 500 °C for 1000 h in air.However,the coatings show an outstanding thermal stability since the PC value stabilized around0.016,which is far smaller than the critical value for failure(PC=0.05).The improvement in absorptance is ascribed to the formation of nanosized chromium nitrides particles in the absorbing layers,which can increase the reflecting and scattering of light and thus promote the absorption.The enhancement in thermal stability is attributed to occurrence of amorphous matrices.During high temperature aging,the main change for amorphous is structural relaxation.In this case,the amorphous matrices can retard the element diffusion and agglomeration in the nanoparticles.Moreover,the aluminum atoms in the nanoparticles tend to agglomerate at the surface to reduce the Gibbs energy.These aluminum atoms will be oxidized in air and thus form a layer of Al2O3 covered the nanoparticles,which can prevent the agglomeration and coarsening of the nanoparticles.(3)To reduce the emittance of the AlCrON cermet-based multilayer solar selective absorbing coatings,a novel Cr/Al Cr N/AlCrON/Al Cr N/AlCrON/AlCrO multilayer coating was deposited on stainless steel substrates using cathodic arc ion plating.In this tandem absorber coating,Cr,Al Cr N,AlCrON and AlCrO act as the reflector layer,semi-absorbing layer,the main absorbing layer and the antireflection layer,respectively.An average absorptance of 0.90 and emittance of 0.15 are achieved in this multilayer coating fabricated with optimized parameters.More importantly,the coating exhibits an outstanding thermal stability with a selectivity of 0.94/0.11 even after aged at 500 °C for 1000 h in air.The microstructure analysis indicate that the multilayer coating consists of aperiodic Al Cr N and AlCrON layers in addition to the Cr and AlCrO layers.After the long-term aging,small amounts of Al N,Cr N and Cr2 N nanocrystallites are observed to be homogeneously embedded in the Al Cr N and AlCrON amorphous matrices.The nanoparticles in the Al Cr N 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 embedded in the amorphous AlCrO antireflection layer,which can effectively 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 dielectric amorphous matrices and the sluggish atomic diffusion in the nanoparticles,which could effectively slow down the inward diffusion of oxygen and avoid agglomeration of nanoparticles.