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The compositional, optical and structural stability of transparent conductive oxide SnO2:Ta (1.25 at.% Ta) thin films at 650 °C and 800 °C in air was studied under isothermal conditions. After the high-temperature treatment, the element composition and the optical spectra of the material were unchanged. The X-ray diffraction confirmed the conservation of a single rutile-type phase. Two strong Raman lines located out of the SnO2 phonon range indicated point defects in the material, which were identified as Sn vacancies and O interstitials by theoretical calculations. These point defects were partially healed out during the high-temperature treatment, without affecting the transmittance and reflectance of the material. Our study demonstrates an exceptional high in-air stability of Ta-doped SnO2 and encourages it for applications in fields, where transparent conductive oxides with high-temperature and oxidation stability are required. These are, e.g., selective transmitters for concentrated solar power or electrodes for dye-sensitized solar cells and dynamic random-access memories.

Tetrahedral, hydrogen-free amorphous carbon (ta-C) is characterized by the highest sp3-carbon content and the highest hardness of HIT > 40 GPa of the group of diamond-like carbon (DLC) coatings [1]. The formation of sp3-C bonds in this material requires impinging atoms or ions with energies of around 100 eV, provided by cathodic vacuum arc or laser arc deposition [2]. Plasma filter techniques implemented in the recent years enable the production of ta-C coatings with significantly improved surface quality. In this study, friction and wear properties of ta-C coatings deposited by filtered Laser arc are studied as a function of the environment: in humid and dry air, in nitrogen as well as in vacuum from 1 mbar to 10-7 mbar. Low friction with coefficients of friction (COF) < 0.1 is found for the friction pair ta-C/ ta-C at normal pressure, nearly independently on the relative humidity. Likewise, the wear rates are not dependent on whether dry or humid conditions are established. In vacuum, the COF and wear rates increased by a factor of approx. ten and three, respectively. Raman studies reveal a complex structure evolution of ta-C in the wear track and on the contact area of the counter body with decreasing pressure. In the wear tracks at least two types of carbon are found. One of them shows an almost unchanged G line position and an unmeasurable ID/IG ratio as the initial coating, while the second one has a by 20 cm-1 lowered G line position and an ID/IG ratio of approx. 0.4. This structure has similar Raman signatures as the counter body contact area, indicating the formation of an identical tribolayer on both friction partners. Laterally and depth resolved atomic insight in the transfer layer formation is obtained by micro-beam Rutherford backscattering spectrometry for ta-C coatings in contact with steel, brass, alumina, and silicon carbide. Financial support by the DFG, grant No. 4157267

In situ measurements using RBS, ERD, and SE are less explored in characterizing solar absorber materials at high temperatures [1, 2]. In the present work, we report the W/WAlSiN/SiON/SiOO2 based spectrally selective solar absorber coating with excellent optical, compositional and structural properties at high temperatures [3, 4]. We have carried out in situ Rutherford backscattering spectrometry, elastic recoil detection and spectroscopic ellipsometry measurements at three different temperatures at 450°C, 650°C, and 800°C. An optical model describing perfectly the reflectance and ellipsometric data was developed. Further, the microstructural properties of the solar absorber coating are evaluated using cross-sectional transmission electron microscopy before and after annealing. Our data obtained before and after the heating experiments demonstrate excellent compositional, optical and structural stability of the coatings under the applied conditions. Furthermore, in situ ellipsometry showed the conservation of the optical properties of the W/WAlSiN/SiON/SiOO2 based spectrally selective solar absorber up to 800 °C, which is crucial for high-temperature applications. [1] Ramón Escobar Galindo, Matthias Krause, K. Niranjan and Harish Barshilia, in Sustainable Material Solutions for Solar Energy Technologies (ed. Mariana Fraga, Delaina Amos, Savas Sonmezoglu, Velumani Subramaniam, Elsevier, 2021). [2] Lungwitz, F. et al. Transparent conductive tantalum doped tin oxide as selectively solar-transmitting coating for high temperature solar thermal applications, Solar Energy Mater. Solar Cells 196, 84-93, doi:10.1016/j.solmat.2019.03.012 (2019). [3] K. Niranjan, A. Soum-Glaude, A. Carling-Plaza, S. Bysakh, S. John, H.C. Barshilia, Extremely high temperature stable nanometric scale multilayer spectrally selective absorber coating: Emissivity measurements at elevated temperatures and a comprehensive study on ageing mechanism, Solar Energy Mater. Sol. Cells

A series of CuCr₂O₄ thin films has been successfully synthesized via a facile and cost-effective reactive Direct Current Magnetron Sputtering technique at room temperature. These coatings were deposited to evaluate their suitability as absorber material for the next generation of concentrated solar power plants [1]. The composition of the films was controlled using as key parameters the power ratio between Cu and Cr targets as well as the oxygen flux. The films deposited without intentional substrate heating were initially amorphous and needed to be annealed at 800 °C for one hour to obtain a spinel-like crystal structure [2]. RBS was used to characterize the composition of the as-deposited and annealed coatings. The structural properties were investigated by Raman spectroscopy and XRD. Structural characterization allows us to evidence that slight deviation in stoichiometry promotes the formation of secondary phases in the films. In this concern, we use in-situ Raman spectroscopy and spectroscopic ellipsometry to characterize the structural evolution of the films as a function of temperature in a controlled oxygen atmosphere. The study evidences the evolution from amorphous to fully crystallized material. Additionally, the influence of the film roughness in the optical performance of the coatings with appropriate composition was explored to enhance the optical properties of the film. References: 1) Ramón Escobar Galindo, Matthias Krause, K. Niranjan and Harish Barshilia, in Sustainable Material Solutions for Solar Energy Technologies (ed. Mariana Fraga, Delaina Amos, Savas Sonmezoglu, Velumani Subramaniam, Elsevier, 2021). 2) Matthias Krause, Johanna Sonnenberg, Frans Munnik, Jörg Grenzer, René Hübner, Aurelio Garcia-Valenzuela, Sibylle Gemming in Materialia 18 (2021) 101156.

Tetrahedral, hydrogen-free amorphous carbon (ta-C) is characterized by the highest sp3-carbon content and the highest hardness of HIT > 40 GPa of the group of diamond-like carbon (DLC) coatings [1]. The formation of sp3-C bonds in this material requires impinging atoms or ions with energies of around 100 eV, provided by cathodic vacuum arc or laser arc deposition [2]. Plasma filter techniques implemented in the recent years enable the production of ta-C coatings with significantly improved surface quality. In this study, friction and wear properties of ta-C coatings deposited by filtered Laser arc are studied as a function of the environment: in humid and dry air, in nitrogen as well as in vacuum from 1 mbar to 10-7 mbar. Low friction with coefficients of friction (COF) < 0.1 is found for the friction pair ta-C/ ta-C at normal pressure, nearly independently on the relative humidity. Likewise, the wear rates are not dependent on whether dry or humid conditions are established. In vacuum, the COF and wear rates increased by a factor of approx. ten and three, respectively. Raman studies reveal a complex structure evolution of ta-C in the wear track and on the contact area of the counter body with decreasing pressure. In the wear tracks at least two types of carbon are found. One of them shows an almost unchanged G line position and an unmeasurable ID/IG ratio as the initial coating, while the second one has a by 20 cm-1 lowered G line position and an ID/IG ratio of approx. 0.4. This structure has similar Raman signatures as the counter body contact area, indicating the formation of an identical tribolayer on both friction partners. Laterally and depth resolved atomic insight in the transfer layer formation is obtained by micro-beam Rutherford backscattering spectrometry for ta-C coatings in contact with steel, brass, alumina, and silicon carbide. Financial support by the DFG, grant No. 4157267

In situ measurements using RBS, ERD, and SE are less explored in characterizing solar absorber materials at high temperatures [1, 2]. In the present work, we report the W/WAlSiN/SiON/SiOO2 based spectrally selective solar absorber coating with excellent optical, compositional and structural properties at high temperatures [3, 4]. We have carried out in situ Rutherford backscattering spectrometry, elastic recoil detection and spectroscopic ellipsometry measurements at three different temperatures at 450°C, 650°C, and 800°C. An optical model describing perfectly the reflectance and ellipsometric data was developed. Further, the microstructural properties of the solar absorber coating are evaluated using cross-sectional transmission electron microscopy before and after annealing. Our data obtained before and after the heating experiments demonstrate excellent compositional, optical and structural stability of the coatings under the applied conditions. Furthermore, in situ ellipsometry showed the conservation of the optical properties of the W/WAlSiN/SiON/SiOO2 based spectrally selective solar absorber up to 800 °C, which is crucial for high-temperature applications. [1] Ramón Escobar Galindo, Matthias Krause, K. Niranjan and Harish Barshilia, in Sustainable Material Solutions for Solar Energy Technologies (ed. Mariana Fraga, Delaina Amos, Savas Sonmezoglu, Velumani Subramaniam, Elsevier, 2021). [2] Lungwitz, F. et al. Transparent conductive tantalum doped tin oxide as selectively solar-transmitting coating for high temperature solar thermal applications, Solar Energy Mater. Solar Cells 196, 84-93, doi:10.1016/j.solmat.2019.03.012 (2019). [3] K. Niranjan, A. Soum-Glaude, A. Carling-Plaza, S. Bysakh, S. John, H.C. Barshilia, Extremely high temperature stable nanometric scale multilayer spectrally selective absorber coating: Emissivity measurements at elevated temperatures and a comprehensive study on ageing mechanism, Solar Energy Mater. Sol. Cells

A series of CuCr₂O₄ thin films has been successfully synthesized via a facile and cost-effective reactive Direct Current Magnetron Sputtering technique at room temperature. These coatings were deposited to evaluate their suitability as absorber material for the next generation of concentrated solar power plants [1]. The composition of the films was controlled using as key parameters the power ratio between Cu and Cr targets as well as the oxygen flux. The films deposited without intentional substrate heating were initially amorphous and needed to be annealed at 800 °C for one hour to obtain a spinel-like crystal structure [2]. RBS was used to characterize the composition of the as-deposited and annealed coatings. The structural properties were investigated by Raman spectroscopy and XRD. Structural characterization allows us to evidence that slight deviation in stoichiometry promotes the formation of secondary phases in the films. In this concern, we use in-situ Raman spectroscopy and spectroscopic ellipsometry to characterize the structural evolution of the films as a function of temperature in a controlled oxygen atmosphere. The study evidences the evolution from amorphous to fully crystallized material. Additionally, the influence of the film roughness in the optical performance of the coatings with appropriate composition was explored to enhance the optical properties of the film. References: 1) Ramón Escobar Galindo, Matthias Krause, K. Niranjan and Harish Barshilia, in Sustainable Material Solutions for Solar Energy Technologies (ed. Mariana Fraga, Delaina Amos, Savas Sonmezoglu, Velumani Subramaniam, Elsevier, 2021). 2) Matthias Krause, Johanna Sonnenberg, Frans Munnik, Jörg Grenzer, René Hübner, Aurelio Garcia-Valenzuela, Sibylle Gemming in Materialia 18 (2021) 101156.

CuCr2O4 thin films grown by physical vapour deposition were studied in order to evaluate their potential as absorber material for the next generation of concentrated solar power plants. A series of Cu-Cr-O thin films was deposited by reactive ion beam sputtering. The Cr/Cu ratio in the sputter target is demonstrated as the most important parameter to achieve the intended film stoichiometry. In-air annealing at 800 °C leads to structural transformations of the as-deposited films and results in phase compositions according to those expected from the ternary Cu-Cr-O phase diagram. Tetragonal CuCr2O4 with 98.6 at.% phase purity regarding the solid film constituents is obtained for the appropriate Cr/Cu ratio in the sputter target. CuCr2O4 thin films absorb light in the entire solar spectral range from 300 nm to 2500 nm. Their energy gap is found to be < 0.5 eV, and their solar absorptance is estimated to be (0.85 +/- 0.03). The dense microstructure with good thermal conductivity, full adhesion to the substrate, and a relatively low surface roughness are discussed as technological advantages of CuCr2O4 thin films grown by physical vapour deposition.

An alternative concept to achieve solar selectivity for solar thermal materials and applications consists in the use of spectrally selective transmitter coatings.[1] These are characterized by a high transmittance in the solar range and a high reflectance in the thermal range of the electromagnetic spectrum. Suitable materials for selective transmitters are dielectric/metal/dielectric multilayers and transparent conductive oxides (TCOs).[2] The concept has a series of advantages compared to multilayer- or cermet-based solar-selective coatings (SSCs) like the easiness of manufacturing, the possibility to use standard materials as transmitter (e.g., indium tin oxide (ITO)) and absorber (e.g. Pyromark or black chrome), and the adaptability to specific requirements with respect to receiver temperature and solar concentration factor. After a conceptual introduction, an analysis of solar plant parameters, i.e., operation temperature and solar concentration, for which this concept provides a better solar efficiency than state-of-the-art bare black body absorber, will be given.[3] We will then review the recent developments in the field, which include an excellent high-temperature in-air stability of such type of solar coatings.[4] In the second part of the talk, we will report own results toward a new TCO on black body absorber coating. Vacuum and in-air stability of the TCO SnO2:Ta at 800 °C and its structural properties before and after heat exposure are demonstrated. As potential absorber, the formation, structure, and optical properties of dense, PVD-grown CuCr2O4 thin films are studied. They are obtained in high purity from as-deposited samples by a simple in-air annealing step at 800 °C and absorb light in the whole solar range from 300 nm to 2500 nm. [1] C.E. Kennedy, Review of Mid- to High-Temperature Solar Selective Absorber Materials, NREL Technical Reports, NREL - National Renewable Energy Laboratory, Golden, Colorado, USA, 2002.