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%.

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.

The design of efficient and stable solar selective coatings for Concentrating Solar Power (CSP) central receivers requires a comprehensive knowledge about the incorporated materials. In this work solar selective coatings were grown by filtered cathodic vacuum arc (FCVA) deposition. The complete stacks consist of an infrared reflection layer, an absorber layer of C:ZrC nanocomposites and an antireflection layer. The Carbon-transition metal nanocomposites were studied as absorber materials because they show appropriate optical properties, i.e. high absorption in the solar region and low thermal emittance. Furthermore metal carbides are thermally and mechanically stabile in air at high temperatures. In order to optimize the absorber layer, the metal content was controlled by adjusting the pulse ratio between the two arc sources. The elemental composition of the absorber layers was determined by Ion Beam Analysis. X-Ray diffraction (XRD) measurements show the formation of metal carbides when the metal content is high enough. The optical properties of the deposited coatings were characterized by spectroscopic ellipsometry (SE). The reflectance spectra of the complete selective coating were simulated with the optical software CODE. Bruggeman effective medium approximation (EMA) was employed to average the dielectric functions of the two components which compose the nanocomposite in the absorber layer. Good agreement was found between simulated and measured reflectance spectra of the solar selective multilayer.

Solar selective coatings based on carbon transition metal carbide nanocomposite absorber layers were designed. Pulsed filtered cathodic arc was used for depositing amorphous carbon: metal carbide (a-C:MeC, Me = V, Mo) thin films. Composition and structure of the samples were characterized by ion beam analysis, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The optical properties were determined by ellipsometry and spectrophotometry. Three effective medium approximations (EMA), namely Maxwell-Garnett, Bruggeman, and Bergman, were applied to simulate the optical behaviour of the nanocomposite thin films. Excellent agreement was achieved between simulated and measured reflectance spectra in the entire wavelength range by using the Bergman approach, where in-depth knowledge of the nanocomposite thin film microstructure is included. The reflectance is shown to be a function of the metal carbide volume fraction and its degree of percolation, but not dependent on whether the nanocomposite microstructure is homogeneous or a self-organized multilayer. Solar selective coatings based on an optimized a-C:MeC absorber layer were designed exhibiting a maximum solar absorptance of 96% and a low thermal emittance of ~5 and 15% at 25 and 600ºC, respectively. The results of this study can be considered as predictive design tool for nanomaterial-based optical coatings in general.

Dans les centrales solaires à concentration, le flux solaire est concentré sur des récepteurs pour chauffer un fluide de transfert jusqu’à 600°C. Dans le but d’améliorer leurs propriétés optiques, ces récepteurs peuvent être recouverts par des revêtements multicouches à sélectivité spectrale. Ces travaux portent sur le développement de structures optiquement sélectives innovantes, absorbantes dans le domaine du rayonnement visible et proche infrarouge (faible réflectivité) et peu émissives (forte réflectivité) pour le rayonnement infrarouge. Nous avons développé des empilements multicouches associant un métal réfractaire avec une céramique ce qui permet d’améliorer l’absorption dans le visible ainsi que la stabilité thermique. Les couches sont déposées par plasma combinant la PACVD et la PVD. L’étude de faisabilité du procédé transférable à l’industrie, son développement et son optimisation via la conception d’un réacteur innovant, le dépôt et la caractérisation des couches, leur association dans des empilements optiquement sélectifs performants et l’étude de leur vieillissement ont été réalisés. / In concentrated solar power (CSP) plants, solar flux is concentrated on receivers to heat a transfer fluid up to 600°C. In order to improve their optical properties, these receivers can be covered by multilayered spectrally selective coatings. This work is devoted to designing and developing innovative spectrally selective structures showing strong absorption (low reflectivity) in the visible and near infrared range and low emissivity (high reflectivity) in the infrared range. We developed such stacks associating a refractory metal with high IR reflectivity and a ceramic which improves absorption in the visible range and thermal stability. The coatings were synthesized by plasma techniques, combining PACVD and PVD. Pre-industrial process feasibility, its development and optimization through the design of an innovative reactor, layer deposition and characterization, their combination in efficient optically selective stacks and the study of their aging have been achieved.