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One of the major challenges in Concentrating Solar Power (CSP) implies an increase of the working temperature of the solar receiver. In particular, current central tower systems operate at maximum temperatures of 550 ºC mainly due to the severe degradation that the state of the art absorber paints (i.e. Pyromark®) suffer at higher temperatures. In previous works [1,2] aluminum titanium oxynitrides AlyTi1-y(OxN1-x) were shown to be excellent candidate materials for solar selective coatings (SSC). These results confirmed that the designed SSCs based on materials withstand breakdown at 600 ºC in air after 900 hours of thermal cycling. In this paper we discuss the high temperature (up to 700ºC) stability in air of a solar absorber coating based on AlyTi1-y(OxN1-x) deposited by cathodic vacuum arc (CVA) at higher working pressure (P = 2.1 Pa) than those discussed in [1] and [2]. The composition, morphology and microstructure of the films were characterized by ion beam analysis, scanning and transmission electron microscopy and X-ray diffraction. The optical properties were determined by ellipsometry and spectrophotometry (UV-Vis-NIR, FTIR). The microstructural and morphological characterization shows the formation of a solid solution of AlTiN crystalline nanoparticles embedded in an amorphous Al2(O, N)3 matrix. This particular microstructure results in a coating with a high absorption coefficient within the whole wavelength range of interest (0,3 to 25 um) as modeled by spectroscopic ellipsometry. Hence, this single layer absorber shows a solar absorptance, α, of 92% and an emissivity, εRT, of 70%. The addition of an antireflective Al2O3 layer and post deposition thermal treatments improved the optical properties of the absorber to better values (α=96% and εRT=60%) than those of Pyromark®. The thermal stability in air of the absorber was firstly analyzed by cyclic heating tests, showing no degradation after 300h of cycles in air at 700ºC. Subsequently, the samples were tested in a solar furnace at 650 °C and 800 ºC for 12 hours at environmental conditions. Therefore, this absorber coating can be a feasible alternative to absorber paints for next generation of concentrated solar power plants operating at high temperature. [1] I. Heras et al., Sol. Energy Mat. Solar Cells, 176, 81-92 (2018) [2] R. Escobar-Galindo et al., Sol. Energy Mat. Solar Cells, 185, 183-191 (2018)

The durability of two solar-selective aluminium titanium oxynitride multilayer coatings was studied under conditions simulating realistic operation of central receiver power plants. The coatings were deposited by cathodic vacuum arc applying an optimized design concept for complete solar-selective coating (SSC) stacks. Compositional, structural and optical characterization of initial and final stacks was performed by scanning electron microscopy, elastic recoil detection, UV-Vis-NIR-IR spectrophotometry and X-Ray diffraction. The design concept of the solar selective coatings was validated by an excellent agreement between simulated and initial experimental stacking order, composition and optical properties. Both SSC stacks were stable in single stage tests of 12 hours at 650°C. At 800°C, they underwent a structural transformation by full oxidation and they lost their solar selectivity. During cyclic durability tests, multilayer 1, comprised of TiN, Al0.64Ti0.36N and an Al1.37Ti0.54O top layer, fulfilled the performance criterion (PC) ≤ 5% for 300 symmetric, 3 hours long cycles at 600°C in air. Multilayer 2, which was constituted of four AlyTi1-y(OxN1-x) layers, met the performance criterion for 250 cycles (750 hours), but was more sensitive to these harsh conditions. With regard to the degradation mechanisms, the coarser microstructure of multilayer 1 is more resistant against oxidation than multilayer 2 with its graded oxygen content. These results confirm that the designed SSCs based on AlyTi1-y(OxN1-x) materials withstand breakdown at 600ºC in air. Therefore, they can be an exciting candidate material for concentrated solar power applications at high temperature.

Aluminium titanium oxynitrides were studied as candidate materials for high temperature absorbers in solar selective coatings due to their excellent stability and their tuneable optical behaviour. A set of individual AlyTi1-y(OxN1-x) layers with different oxygen content was prepared by cathodic vacuum arc (CVA) deposition. The composition, morphology, phase structure and microstructure of the films were characterized by elastic recoil detection (ERD), scanning and transmission electron microscopy and X-ray diffraction. An fcc phase structure is found in a broad compositional range of AlyTi1-y(OxN1-x). Simultaneously, sample microstructure and morphology undergo systematic changes from a columnar growth to the development of a heterogeneous structure with spherical nanoparticle inclusions when the oxygen concentration is increased. The optical properties were determined by spectroscopic ellipsometry and UV–Vis–NIR and FTIR spectrophotometry. A comprehensive analysis of the film properties allowed an accurate modelling of the optical constants of the AlyTi1-y(OxN1-x) in the whole wavelength range of solar interest (from 190 nm to 25 µm). It points to a transition from metallic to dielectric behaviour with increasing oxygen content. Consequently, it is demonstrated that the optical properties of these AlyTi1-y(OxN1-x) deposited films can be controlled in a wide range from metallic to dielectric character by adjusting the oxygen concentration, opening a huge range of possibilities for the design of solar selective coatings (SSC) based on this material. Complete SSC, including a TiN layer as IR reflector, were designed by applying optical simulations, obtaining excellent optical selective properties of α=94.0% and εRT = 4.8%.

Aluminium titanium oxynitrides were studied as candidate materials for high temperature absorbers in solar-selective coatings (SSC) due to their excellent stability and their tuneable optical behaviour. A set of individual AlyTi1-y(OxN1-x) layers with different oxygen content was prepared by cathodic vacuum arc (CVA) deposition. A comprehensive analysis of the sample microstructure and morphology allowed an accurate modelling of the optical constants in the whole wavelength range of solar interest (190 nm - 25 μm). The optical properties of these films can be controlled from metallic to dielectric character by adjusting the oxygen content. Complete multilayer SSC, including a TiN layer as IR reflector, were designed by applying optical simulations, obtaining excellent optical selective properties (α = 94.0% and εRT = 4.8%). The design concepts were validated by an excellent agreement between simulated and experimental stacking order, composition and optical properties. The durability of two multilayers was studied under conditions simulating realistic operation of central receiver power plants. Both SSC stacks were stable in single stage tests of 12 h at 650°C in air. During cyclic tests, the coarser microstructure of multilayer 1, was found to be more resistant against oxidation than multilayer 2 constituted of four oxynitride layers with a graded oxygen content. Multilayer 1 fulfilled the performance criterion of PC ≤ 5% for 300 symmetric, 3 h long cycles at 600°C in air confirming that the designed SSCs are exciting candidate material for concentrated solar power applications at high temperature. Financial support by the EU, grant No. 645725, project FRIENDS22, is gratefully acknowledged.

The development of solar-selective CSP receiver coatings with high-temperature and environmental stability requires new concepts of design and in operando monitoring. Solar receiver tubes are a key component of solar thermal power plants. The increase of their operation temperature from today’s maximum of 550°C to about 800°C could increase the CSP efficiency by approximately 15 to 20% and improve the competiveness of this technology compared to other ones of carbon-free electricity generation. Potential alternatives to fast degrading state-of-the-art pigment paint receiver tube coatings are based on refractive metal carbides, nitrides, and oxides because of their high thermal stability and oxidation resistance. New types of solar-selective coatings were studied in situ at temperatures of up to 830°C by Rutherford backscattering spectrometry, Raman spectroscopy, and spectroscopic ellipsometry within a cluster tool. High-temperature stability in high-vacuum is demonstrated for carbon-metal- and oxynitride-absorber-based multilayers as well as for a solar-selective transmitter based on a transparent conductive oxide. Financial support by the EU, grant No. 645725, project FRIENDS2, and the HGF via the W3 program (S.G.) is gratefully acknowledged.

Aluminium titanium oxynitrides were studied as candidate materials for high temperature solar-selective coatings due to their excellent stability and their tuneable optical behaviour. A set of individual AlyTi1-y(OxN1-x) layers with different oxygen content was prepared by cathodic vacuum arc (CVA) deposition. The composition, morphology, phase structure and microstructure of the films were characterized by elastic recoil detection (ERD), scanning and transmission electron microscopy and X-ray diffraction.