Selective Solar Absorber Coatings Research Articles

The investigation of thermal stability of Al/NbMoN/NbMoON/SiO2 solar selective absorbing coating

The thermal stability of the Al/NbMoN/NbMoON/SiO2 solar selective absorbing coatings is investigated to determine its potential applications in concentrated solar power (CSP). The coatings are deposited on stainless steel (SS) substrate by using magnetron reactive sputtering method, which exhibit high solar absorptance (αs = 0.948) and low thermal emittance (ε = 0.11, at 400°C). The coatings are aged (in vacuum and air) at different temperatures and time durations. The aging effect is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and Vis.-IR spectroscopy. There is no significant change in surface morphology and spectral selectivity (performance criterion (PC) value is 0.021) of the coating after aging at 500°C for 240h in vacuum. The coatings also keep high spectral selectivity (PC = 0.034) after aging at 450°C for 200h in air. Unfortunately, the value of PC increases to 0.161 when the coating is aged at the temperature of 500°C for 200h in air, which is attributed to diffusion and oxidation in the absorbing layers and degradation of the dense surface morphology and structure, as evidenced by XRD and SEM analysis. These results indicate that the Al/NbMoN/NbMoON/SiO2 coatings is stable up to 500°C in vacuum and below 450°C in air, which means enough thermal stability for potential applications in parabolic trough CSP.

Optical characterization of TiAlNx/TiAlNy/Al2O3 tandem solar selective absorber coatings

Solar selective absorber coatings, consisting in a TiAlNx / TiAlNy tandem absorber with low (metallic-like) and high (semiconductor-like) nitrogen content, and an Al2O3 antireflective coating, were deposited on stainless steel. A full optical characterization was carried out on these coatings. Reflectance spectra were measured by spectrophotometry in the UV to mid-IR range (0.25−25µm), allowing for the calculation of total solar absorptance and thermal emittance. An absorptance of 0.93 and an emittance of 0.22 at 550°C were obtained for the best coating. The temperature dependence of reflectance was investigated in the infrared region and results demonstrated the validity of the classical room temperature measurement approximation to calculate thermal emittance at high temperature for these coatings. The total hemispherical emittance of the coatings was calculated from hemispherical directional reflectance spectra measured at different angles, showing that the near normal hemispherical emittance is a good approximation of the total hemispherical emittance. The total solar absorptance was also directly measured under natural solar irradiation using a unique solar facility.

High-temperature tolerance in WTi-Al2O3 cermet-based solar selective absorbing coatings with low thermal emissivity

Cermet-based solar selective absorbing coatings are widely used, however, the long-term thermal instability and pretty high infrared emissivity at high temperatures (>550°C) are still challenging issues to be addressed, which essentially lies in suppressing the growing up and agglomeration behaviors of metal nanoparticles (NPs) and maintaining the interface integrity in the multi-layer stacked structure. Herein, we develop and explore WTi-Al2O3 cermet-based absorbing coatings, demonstrating a solar absorptance of ~93% and a very low thermal emissivity of 10.3% @500°C even after annealing at 600°C for 840h in vacuum. It is revealed that the surface segregation of solute Ti atoms from the parent alloyed NPs and their partial oxidation to form protective layer restrain outward diffusion of W element, agglomeration of NPs, and interface structure degradation, in favor of enhancing the thermal tolerance of the coatings. These results suggest that the WTi-Al2O3 based absorbing coating is a good candidate for high-temperature solar thermal conversion.

Optical and structural analysis of solar selective absorbing coatings based on AlSiOx:W cermets

It is reported in this work the development and study of the optical and structural properties of a solar selective absorber cermet based on AlSiOx:W. A four-layer composite film structure, W/AlSiOx:W(HA)/AlSiOx:W(LA)/AlSiOx, was deposited on stainless steel substrates using the magnetron sputtering deposition method. Numerical calculations were performed to simulate the spectral properties of multilayer stacks with varying metal volume fraction cermets and film thickness. The chemical analysis was performed using X-ray photoelectron spectroscopy and the results show that in the high metal volume fraction cermet layer, AlSiOx:W(HA), about one third of W atoms are in the W0 oxidation state, another third in the Wx+ oxidation state and the last third in the W4+, W5+ and W6+ oxidation states. The X-ray diffractograms of AlSiOx:W layers show a broad peak indicating that both, W and AlSiOx, are amorphous. These results indicate that this film structure has a good spectral selective property that is suitable for solar thermal applications, with the coatings exhibiting a solar absorptance of 94–95.5% and emissivities of 8–9% (at 100°C) and 10–14% (at 400°C). The samples were subjected to a thermal annealing at 450°C, in air, and 580°C, in vacuum and showed very good oxidation resistance and thermal stability. Morphological characterizations were carried out using scanning electron microscopy and atomic force microscopy. Rutherford Backscattering experiments were also performed to analyze the tungsten depth profile.

Improved Solar Absorptance of WC/Co Solar Selective Absorbing Coating with Multimodal WC Particles

One type of multimodal (comprised of nanometer and sub-micrometer WC particles) and two types of conventional (comprised of nanometer and sub-micrometer WC particles, respectively) WC/Co powders were deposited on AISI 304L stainless steel substrates by using high velocity oxygen fuel spraying. The use of multimodal WC particles was indicated to have a beneficial effect on the solar absorptance (α) of the WC/Co coatings. The α of the multimodal WC coating reached 0.87, which was much higher than what can be achieved by either fine (0.82) or coarse powders (0.80) alone. By microstructural analysis, the enhancement in solar absorptance of the multimodal WC/Co coating was ascribed to the layer of distributed WC particles. During the thermal spraying, the nanostructured WC particles underwent rapid melting for the large specific surface area while the aggregated powders were heated, but not necessarily melted. The molten nano-WC would fill the available pores between the softened and heated aggregates, providing a layered distribution of WC particles for the spray-deposited coating. In this condition, the light-trapping in the multimodal coating will be enhanced due to the efficient light reflection among the multimodal WC particles, which contributes to the enhancement of solar absorptance.

Durability of different selective solar absorber coatings in environments with different corrosivity

The degradation and durability of different selective solar absorbers surfaces in outdoor exposure testing (OET) sites with different atmospheric corrosivity (maritime/industrial and urban atmospheres) were investigated for the optimization of accelerated aging tests to the qualification in terms of durability of selective solar absorber coatings in different environments.Two Outdoor Exposure Testing (OET) sites were constructed, the climate was characterized and the atmospheric corrosivity categories were determined by corrosion rate measurements of standard specimens (carbon steel, zinc, copper and aluminium) during 12 months, according to ISO 9223, ISO 9225 and ISO 9226:2012. Two Physical Vapour Deposition (PVD) coatings (commercially available) and three paint coatings (commercially available) were also exposed in the two OET sites with evaluation of degradation after 1, 2, 4, 6, 9 and 12 months of exposure in terms of optical properties (solar absorptance and thermal emittance), morphology and chemical evaluation (SEM/EDS and XRD). The selective solar absorber surfaces submitted to OET sites for one year were ranked according to their performance in terms of optical properties and corrosion over time.The results obtained show the relevance of outdoor exposure testing sites, namely in places with high corrosivity as in marine and/or industrial areas as a reliable way to verify the corrosion resistance of new materials and products and the evolution of optical properties degradation of absorber surfaces in the presence of high concentration of contaminants.

Enhanced optical properties of TiN-based spectrally selective solar absorbers deposited at a high substrate temperature

In this study, the spectrally selective solar absorber coatings of TiN/Al2O3 are deposited onto stainless steels (SS) at a high substrate temperature of 300°C, which exhibits a high absorptance of 0.92 and a low emittance of 0.11 at 82°C. The structure, optical properties and thermal stability have been characterized by ultra-high resolution scanning electron microscopy, UV–vis–NIR spectrophotometry, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The study's results indicate that the substrate temperature (300°C) has an important effect on the solar absorptance and emittance. The SS/TiN/Al2O3 coating exhibits good thermal stability in a vacuum at 500°C for 5h. The coating also exhibits good thermal stability in air at 200°C for 2h. Raman analysis confirms that the oxidation of the SS/TiN/Al2O3 coating in air causes the composition to approach the stable oxide TiO2.

ALUMINA MATRIX WITH CONTROLLED MORPHOLOGY FOR COLORED SPECTRALLY SELECTIVE COATINGS

This study presents the results obtained in tailoring thin films of alumina, deposited onto aluminium substrates by spray pyrolysis, at 250.C, using AlCl3 solutions (0.15M) in a water:ethanol (1:1) solvent mixture. As morphology controlling agents, acetylacetone and maleic acid based copolymers are added in the precursors solutions. The precursors systems are analyzed by FTIR spectroscopy and the surface morphology of alumina thin films is investigated by atomic force microscopy. The addition of hydrophilic copolymer in precursors solution favors the development of regular open structures, while fractured large open structures are obtained when hydrophobic copolymer is used. The addition of acetylacetone in spraying solution leads to irregular large open structures. Combined additive systems (acetylacetone and hydrophylic/hydrophobic copolymers) have a complex influence and can be a tool in tailoring the morphology of the alumina thin films, as suitable matrix for further infiltration of red (Fe2O3) and gray-green (CuxS) pigments and development of colored solar selective absorber coatings with promising optical performances. Thus, spectral selectivities (S) of 3.4 - 4.07 are obtained for red absorber coatings, while for gray-green absorber coatings spectral selectivities are slightly lower (S = 2.35 - 2.7).

Microstructure and Optical Properties of SS/Mo/Al2O3 Spectrally Selective Solar Absorber Coating

Surface-textured Mo thin film is fabricated by magnetron sputtering through the adjustment of deposition parameters, which exhibits a high absorptance of 0.80 and a low emittance of 0.09. The single-layer Mo deposited on stainless steel (SS) is characterized by x-ray diffraction (XRD), ultra-high resolution scanning electron microscope, atomic force microscope and optical measurement. The controlled surface roughness combined with larger aspect ratio contributes much to the high absorptance and low emittance. Based on the SS/Mo coating, a spectrally selective coating (SS/Mo/Al2O3) is designed and fabricated. The coating shows an amorphous structure and exhibits an absorptance of 0.90 and an emittance of 0.08. Tauc-Lorentz and Drude free-electron models are used to modeling the optical properties of Al2O3 and Mo layers by phase-modulated spectroscopic ellipsometry.

Thermal stability test and ageing mechanisms study of different solar selective absorbing coatings

Long term thermal stability and temperature induced ageing mechanisms of different solar absorber coatings are investigated in this study. These coatings are chromium, chromium nitride and chromium oxynitride based coatings on aluminium substrate. The coatings were annealed at 278°C for 600h during the first round test. The second round test was performed at 308°C for 342h. An additional test at 400°C for 200h is designed for the sake of obtaining more degradation information. Thermal stability, microstructure, composition distribution, element diffusion and oxidation were studied using UV/VIS/NIR spectrophotometer, FTIR spectrometer, scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and Auger electron spectroscopy (AES) respectively. The result shows that the main ageing mechanism is diffusion and oxidation of Cr, Al, N and O. And the microstructure defects, such as enlarged grain boundaries, micro cracks and holes, act as a facilitating role during the ageing process because they provide fast diffusion paths for short-circuit diffusion.

Controllable fabrication and characterization of porous C/TiO2 nanocomposite films as solar selective absorber

Abstract Porous C-TiO 2 nanocomposite films prepared by photopolymerization-induced phase-separation method (PIPS) exhibited three dimensional interconnected pores structure. Scanning electron microscopy (SEM), atomic force microscope (AFM), thermo-gravimetric and differential thermal analysis (TG-DTA), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, UV–Vis-NIR spectrophotometer are employed to characterize the structure and optical properties. The results illustrated that the photomonomer pentaerythritol triarylate (PETA) directly affects the relative molecular weight of polymer formed by photopolymerization to regulate the morphology and residual carbon of C-TiO 2 nanocomposite films, and ultimately affects the solar selective absorption properties. The single layer of as-prepared porous C-TiO 2 nanocomposite films exhibits high solar absorptance (α = 0.766–0.863) with low thermal emittance (e = 0.06–0.12), yielding an optimized photothermal conversion efficiency η = α-e of (0.706–0.762) corresponding to a wide redundancy PETA concentration 0.3–0.9 (molar ratio to Ti precursor). The proportion of residual carbon contributed by PVP, PETA and poly-PETA, and other substances of the sol is 40.2%, 56.2% and 3.6%, respectively, which might be regulated by a suitable annealing process. Based on these results, porous C-TiO 2 nanocomposite films are proved to be a novel candidate material for mid-temperature solar selective absorber coatings.

Thermal stability of WAlN/WAlON/Al2O3-based solar selective absorber coating

The solar absorptance property of W/WAlN/WAlON/Al2O3-based coatings, deposited by DC/RF magnetron sputtering on stainless steel substrate was studied by measuring the reflectance spectra in the wavelength range of 250 - 2500 nm. The effect of thermal annealing on the optical properties of the solar selective absorber coatings was investigated. Annealing the coatings at 450°C for 150 hrs in air did not show any significant change in the spectral properties of the absorber coating indicating the excellent thermal stability of the coating. The W layer acts as infrared reflective layer and diffusion barrier on stainless steel substrate. The top Al2O3 layer serves as dense shield to protect the under layers from oxidation in air. In summary, the present study indicates the potential application of W/WAlN/WAlON/Al2O3-based selective coatings in high temperature photo thermal conversion systems.

Functional multi-layer solar spectral selective absorbing coatings of AlCrSiN/AlCrSiON/AlCrO for high temperature applications

A novel AlCrSiN/AlCrSiON/AlCrO tandem multi-layer nanocomposite coating has been prepared by cathodic arc ion plating for solar selective absorbing. The composition, structure, and surface morphology of the coating were characterized using SEM, EDS, XPS, TEM, and AFM. The optical performances of the coating were measured by spectrophotometer, while the thermal stability of the coating was evaluated by Raman spectroscopy. The coating exhibited a relatively high absorptance of 0.958 and a relatively low emittance of 0.14, as well as an outstanding thermal stability with a selectivity of 0.924/0.16 even after annealing at 600°C for 600h in air, which properties rendered this multi-layer nanocomposite coating a potential material for photo-thermal conversion at high temperatures.

Interfacial engineering of solution-processed Ni nanochain-SiOx (x < 2) cermets towards thermodynamically stable, anti-oxidation solar selective absorbers

Cermet solar thermal selective absorber coatings are an important component of high-efficiency concentrated solar power (CSP) receivers. The oxidation of the metal nanoparticles in cermet solar absorbers is a great challenge for vacuum-free operation. Recently, we have demonstrated that oxidation is kinetically retarded in solution processed, high-optical-performance Ni nanochain-SiOx cermet system compared to conventional Ni-Al2O3 system when annealed in air at 450–600 °C for several hours. However, for long-term, high-temperature applications in CSP systems, thermodynamically stable antioxidation behavior is highly desirable, which requires new mechanisms beyond kinetically reducing the oxidation rate. Towards this goal, in this paper, we demonstrate that pre-operation annealing of Ni nanochain-SiOx cermets at 900 °C in N2 forms the thermodynamically stable orthorhombic phase of NiSi at the Ni/SiOx interfaces, leading to self-terminated oxidation at 550 °C in air due to this interfacial engineering. In contrast, pre-operation annealing at a lower temperature of 750 °C in N2 (as conducted in our previous work) cannot achieve interfacial NiSi formation directly, and further annealing in air at 450–600 °C for >4 h only leads to the formation of the less stable (metastable) hexagonal phase of NiSi. Therefore, the high-temperature pre-operation annealing is critical to form the desirable orthorhombic phase of NiSi at Ni/SiOx interfaces towards thermodynamically stable antioxidation behavior. Remarkably, with this improved interfacial engineering, the oxidation of 80-nm-diameter Ni nanochain-SiOx saturates after annealing at 550 °C in air for 12 h. Additional annealing at 550 °C in air for as long as 20 h (i.e., 32 h air annealing at >550 °C in total) has almost no further impact on the structural or optical properties of the coatings, the latter being very sensitive to any interfacial changes due to the localized surface plasmon resonances of the metal nanostructures. This phenomenon holds true for Ni nanoparticle diameter down to 40 nm in Ni-SiOx system, where the optical response remains stable for 53 h at 550 °C in air. The oxidation vs. time curve also shows saturation behavior deviating from the kinetic Deal-Grove oxidation model. These results strongly suggest a promising approach to thermodynamically stable, anti-oxidation Ni/SiOx cermet absorbers via interfacial engineering.

Effects of substrates, film thickness and temperature on thermal emittance of Mo/substrate deposited by magnetron sputtering

The thermal emittance of the Mo film, as an IR-reflector in solar selective absorbing coatings, is the most important property. The effects of the substrate material, the substrate surface roughness, the film thickness and the temperature on the thermal emittance of the Mo/substrate have been investigated. A series of Mo films with increasing film thickness were deposited on two types of substrate materials (glass and stainless steel). A saturated Mo thickness of 50 nm is found to produce the lowest thermal emittance. The thermal emittance of the Mo film is reduced by decreasing the substrate surface roughness. The emittance of the optimal Mo film remains 0.05 from 25 °C to 400 °C, which can meet the optical requirements for the IR-reflector.

Spectrally selective CuS solar absorber coatings on stainless steel and aluminum

CuS and ZrO2/CuS thin films have been deposited at 200 °C in ambient atmosphere by spray pyrolysis onto stainless steel S304 and aluminum AA1050 sheets in order to be used as prospective solar absorbers for thermal energy harvesting. Zirconia has been deposited as a top layer working as an antireflection coating increasing the solar absorption and protecting the underlying copper sulfide layer, which works as the solar light absorbing material. Solar selectivity of the samples was evaluated measuring the total hemispherical reflectivity in the ultraviolet‐visible infrared range and calculating solar absorptance and thermal emittance. The solar selectivity was optimized by tuning the film thickness of the coating stack. X‐ray diffraction, SEM and X‐ray photoelectron spectroscopy analyses have been used to proof crystal phase, coverage, film thickness, and surface contamination of optimized samples, revealing that the copper sulfide film is CuS covellite and the optimized film thicknesses are in the range of 50 nm for the ZrO2 top layer and 140 nm for the CuS layer. Copyright © 2016 John Wiley & Sons, Ltd.

Investigation on low thermal emittance of Al films deposited by magnetron sputtering

A series of Al films with different thicknesses were deposited on polished stainless steel by direct current (DC) magnetron sputtering as a metal IR-reflector layer in solar selective absorbing coating (SSAC). The effects of the film thickness and the temperature on the thermal emittance of the Al films are studied. An optimal thickness 78nm of the Al film for the lowest total thermal emittance is obtained. The thermal emittance of the optimal Al film keeps close to 0.02 from 25°C to 400°C, which are low enough to satisfy the optical requirements in SSAC. The optical constants of the Al film are deduced by fitting the reflectance and transmission spectra using SCOUT software.

Optical properties and thermal stability of colored solar selective absorbing coatings with double-layer antireflection coatings

Multilayer solar selective absorbing coatings of Cu/Ti/SiO2/Ti/TiO2/SiO2 were designed first and then deposited on quartz substrates using the magnetron sputtering method. The thicknesses of the individual layers were optimized to achieve high solar absorptance, while the colors were designed with the TFCalc optical design software. The colors of the coatings including purple, blue, blue green, yellow green and orange yellow were prepared based on the designed parameter. The colored solar selective absorbing coatings exhibit solar absorptance higher than 0.95 and thermal emissivity lower than 0.09 at 80°C. Furthermore, the thermal stability of the Cu/Ti/SiO2/Ti/TiO2/SiO2 coatings was studied by annealing at high temperature in vacuum and in air for 2h. The X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and spectrophotometry were used to characterize properties of the coatings. The results showed that the multilayer solar selective absorbing coatings were thermally stable at 450°C in vacuum and 300°C in air for 2h, which may provide an instruction to design and fabricate high performance colored solar selective absorbing coatings.

Optimization and preparation of Pt–Al2O3 double cermet as selective solar absorber coatings

To prepare high performance Pt–Al2O3 double cermet selective solar absorber coatings, a computer simulation programme has been used to calculate the appropriate complex refractive index of the Pt–Al2O3 double cermet structure. Based on the reflectance, the factors which affect the spectral selectivity of the solar absorber surfaces such as the nature of the IR reflecting metal, and the volume fraction as well as the thickness were optimized. A series of Pt–Al2O3 cermet films with different values of volume fraction and thickness were prepared onto glass and stainless steel substrates. The reflectance spectra of all the single films were in a good agreement with the calculated spectra. The measured optical constants as well as those deduced from the fitting were used as a database to optimize the selective solar absorber coating. Based on the optimized parameters, a Pt–Al2O3 double cermet with a layer stratification consisting of Mo ∼100 nm/Pt–Al2O3 ∼ 60 nm (0.39 HMVF)/Pt–Al2O3∼ 40 nm (0.19 LMVF)/Al2O3∼ 80 nm was prepared using a sputtering system at room temperature. The reflectance of the optimized Pt–Al2O3 double selective solar absorber coatings was also consistent with the theoretical calculation, the corresponding absorptance and thermal emittance were excellent: α ∼0.97 and ε ∼0.05, respectively. Considering the literature by enlarge, such experimental results overcome almost all Al2O3 based selective solar absorber coatings realized so far.

Influence of temperature-induced copper diffusion on degradation of selective chromium oxy-nitride solar absorber coatings

Temperature-induced copper diffusion process and its influences on optical degradation and long-term stability of solar absorber coatings on copper substrates were investigated at intermediate temperatures of 248–500°C. The studied absorbers were sputtered chromium oxy-nitride absorbers having tin oxide anti-reflection coatings. The absorbers were aged by means of thermal accelerated ageing studies and short-period heat treatments up to 500°C for two hours.Ageing mechanisms and degradation of the absorbers were analysed before and after the ageing studies by optical measurements (solar absorptance with a UV/Vis/NIR spectrophotometer and thermal emittance by FTIR spectrophotometry), microstructural analysis using a field-emission scanning electron microscope (FESEM) equipped with an energy dispersive X-ray spectrometer (EDS) and a transmission electron microscope (TEM) with an EDS, composition by time-of-flight elastic recoil detection analysis (TOF-ERDA) and an X-ray photoelectron spectroscope (XPS), and adhesion by tensile test. The relation between optical degradation and diffusion mechanisms was studied using optical modelling and simulation. The results clearly revealed the mechanism of outward copper diffusion: diffusion of copper substrate atoms into the coating and through the coating to the surface, formation of copper oxide islands on the surface of the coating, and formation of voids in the substrate surface. The relation between the diffusion mechanisms and increase in thermal emittance of the absorber surface was demonstrated.