Absorber Coatings Research Articles

Optical simulation, characterization and thermal stability of Cr2O3/Cr/Cr2O3multilayer solar selective absorber coatings

Cr2O3/Cr/Cr2O3 multilayered selective solar absorber coating was deposited onto stainless steel substrates using electron beam vacuum evaporation at room temperature. The optical constants of the individual layers were obtained from reflectance measurements fitted by CODE software using the suitable dielectric function model, and showed that both Cr2O3 layers were dielectric in nature, while the Cr layer was semi-transparent due to its low thickness. Then, the experimental reflectance spectrum of the SS/Cr2O3/Cr/Cr2O3solar absorber was successfully simulated using CODE software. It was found that this coating exhibited a good spectral selectivity of 0.82/0.21 at 300 °C. The effect of annealing on the structural, morphological, mechanical and optical properties of the SS/Cr2O3/Cr/Cr2O3 solar absorber was investigated. SEM revealed an increase in Cr2O3 crystal size after annealing at 400 °C for 24 h in air, this leads to a surface smoothing illustrated by a decrease in roughness. The structural investigations of the annealed multilayered coatings showed a decrease in intensity of the stainless steel XRD peak linked to the increase in crystallinity of Cr2O3. Nanoindentation measurements showed that after annealing the samples were more resistant to mechanical solicitations as compared to as-deposited samples. Optical analysis showed that the sample annealed at 400 °C for 24 h exhibited higher absorptivity α = 0.89 and lower emissivity ε at 300 °C = 0.15than the as-deposited sample due to material structuring with temperature, which is promising for mid-to high temperature concentrated solar power applications in air. Coating optimization in CODE also indicated that α could be further improved to 0.916 without increasing emissivity.

Sputtered SiC coatings for radiative cooling and light absorption

The outstanding thermomechanical and broadband spectral absorptance of silicon carbide (SiC) coatings makes it a promising material for high-performance optoelectronics devices and photothermal applications. We propose sputtered SiC coatings to develop: (i) broadband infrared emitters for passive radiative cooling and (ii) a spectrally selective ultrathin nanocomposite absorber. For radiative cooling, a proof of concept device has been fabricated by topping the as-sputtered 2.5-μm-thick SiC films with a single pair of distributed Bragg’s reflector (DBR). Owing to the infrared surface polariton resonances of SiC, broadband infrared emissivity of 0.8 is achieved in atmospheric transparency window (8 to 14 μm). The higher refractive index contrast (Δn) in DBR is achieved by depositing high refractive index dense Si film on top of low refractive index nanoporous SiO2, which was fabricated by oblique angle deposition approach. A single pair of DBR on top of SiC considerably reduces the absorption of the overall solar spectrum by 18%. In addition, impact of various materials and number of pairs of DBRs on radiative cooling are also numerically investigated. We also designed and fabricated SiC-based spectrally selective solar absorber coatings. An ultrathin SiC-based nanocomposite absorber with sandwich-like configuration consisting of SiC/Ag-SiC/SiC on reflective substrate has exhibited an outstanding absorptance of 91.24% in the wavelength range of 280 to 2000 nm. The surface plasmon polaritons of metal inclusion within the SiC matrix of an ultrathin nanocomposite (40 nm) layer has contributed to broadband light absorption, whereas the top most layer of SiC (40 nm) acts as antireflection layer. Conclusively, the versatility of sputtered SiC coatings has proven to be promising for radiative cooling and spectrally selective solar absorbers.

Plasmonic Nanocomposites Based on Silver Nanocube-Polymer Blends Displaying Nearly Perfect Absorption in the UV Region.

Plasmonic nanocomposites based on well-dispersed silver nanocubes in poly(vinylpyrrolidone) are presented that are solution-processed into layers of varying volume fractions of nanocubes. We show that the high-energy modes of the nanocubes are almost insensitive to plasmonic coupling within the nanocube assemblies, leading to a linear increase in light absorption in the UV region with the nanocube densities. Concerning the main dipolar resonance mode at 450 nm, it is strongly affected by the formation of these assemblies, leading to an increased absorption in the UV region as well as a large absorption band in the visible region. Simulations of the optical response of the nanocube assemblies as a function of nanocube spacing and electric field polarization reveal that optical features in the visible region are due to intercube couplings at short intercube distances and parallel electric field orientation. In contrast, the additional plasmonic band in the UV region has its origin in residual dipolar oscillations of the nanocubes in combination with weak dipolar coupling for both parallel and transversal field polarizations. The combination of these effects leads to an enlarged absorption band in the UV region with nearly perfect light absorption of 98.8% at a high silver volume fraction of 8% that is accompanied by a very weak specular reflection of only 0.28%. Although such perfect absorption is usually observed only when nanocubes are assembled on a gold surface, nearly perfect absorption herein is achieved on a large palette of substrates including glass, plastic, and cheap metals such as aluminum, making it a promising approach for solution-processed robust and cheap quasi-perfect absorption coatings.

Transition metal based CuxNiyCoz-x-yO4 spinel composite solar selective absorber coatings for concentrated solar thermal applications

Solar selective absorber coatings, combining of nanostructured, ternary transition metal spinel structure and composite oxides were prepared by a series of transition metal (Co, Ni and Cu) based nitrate salts. A single-layered structure of crystalline phase spinel-type oxide AB2O4 in the framework of simple wet chemical route and dip coating method was prepared in the crystalline structure of CuxNiyCoz-x-yO4. The X-ray diffraction profile of the oxide film was found to coincide with that of crystalline [Cu2+0.31Co3+0.16Co2+0.53]A[Cu2+0.17Ni3+0.16Co3+1.67]BO4 in the ICDD database. Precursor salts were heated at a particular temperature to form hydroxy nitrate solution followed by suitable solvent with different binders to form adherent sol for dip coating technique. By optimizing sol concentration, withdrawal speed and annealing temperature, uniform absorber layer with solar absorptance of 0.91 and emittance of 0.14 were achieved in a single layer coating with a composite oxide formation. The optical, structural and morphological properties of the optimized coatings of single layer CuxNiyCoz-x-yO4 spinel and composite oxide have been characterized by UV–Vis–NIR spectrophotometer, emissometer, X-ray diffraction, field emission - scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy.

Effect of surface modified SiO2 powders on microwave absorbing properties of flaky FeSiAl coatings

Flake FeSiAl (FFSA) powders were used as absorbent in absorption coatings with a thickness of 1 mm. Two types of SiO2 powders (DNS-2, RNS-A) were added to the coatings in order to improve the impedance matching and the absorption properties. The electromagnetic parameters of FFSA and two types of SiO2 powders were measured by transmission/reflection technology, and the reflection loss of coatings was measured by the arch method in the frequency range of 2–18 GHz. The effect of FFSA content on the microwave absorbing properties was analyzed. And as the content increases, the effective absorption bandwidth (RL ≤ − 10 dB) firstly increases and then decreases and the absorbing peak shifts to lower frequency ranges. Then the absorbing properties of the FFSA/SiO2 coatings were investigated. The results show that the microwave absorption performance was improved significantly when the appropriate amount of SiO2 powder was added to the coating. The minimum reflection loss decreased by 10 dB (DNS-2) and 24.3 dB (RNS-A), respectively. The effective absorption bandwidth (RL ≤ − 10 dB) increased by 2 GHz and 1.8 GHz. The interface polarization and SiO2-channel models were built to analyse the enhanced microwave absorption mechanism of the coatings.

Shifting of the absorption edge in TiB2/TiB(N)/Si3N4 solar selective coating for enhanced photothermal conversion

The concentrated solar power systems for solar energy applications demand the development of solar absorber coatings with high spectral selectivity. This is possible by careful compositional selection of absorber thin films with an aim to shift the optical absorption edge towards higher wavelength side. In this context, the present work aims at developing such a coating, based on TiB2/TiB(N)/Si3N4 by tailoring magnetron sputtering parameters. This multilayer exhibits a high absorptance of 0.964 and a moderately low emittance of 0.18 at 82 °C. The attainment of excellent spectral selectivity has been explained in terms of systematic variation or gradient in refractive index and extinction co-efficient throughout the multilayer stack.

Multi-layer solar selective absorber coatings based on W/WSiAlNx /WSiAlOyNx/SiAlOx for high temperature applications

A simulated and an experimental design of multilayer solar selective absorber coatings for high temperature applications is presented in this study. The optical tandem is composed of four layers deposited by magnetron sputtering on stainless steel substrates at room temperature. The first is a back-reflector tungsten layer, that is followed by two absorption layers based on WSiAlNx/ WSiAlOyNx structure for phase interference. The final layer is an antireflection layer of SiAlOx. The design was conducted with the help of SCOUT software creating a multilayer model based on transmittance (T) and reflectance (R) spectra of individual thin layers deposited on glass substrates. The final design shows simultaneously high solar absorptance α = 96.0% and low emissivity ε = 10.5% (calculated at 400 °C) together with high thermal stability at 450 °C, in air, and 600 °C in vacuum for 400 h and 300 h, respectively.

High-Temperature Tolerance in Multi-Scale Cermet Solar-Selective Absorbing Coatings Prepared by Laser Cladding.

In order to achieve cermet-based solar absorber coatings with long-term thermal stability at high temperatures, a novel single-layer, multi-scale TiC-Ni/Mo cermet coating was first prepared using laser cladding technology in atmosphere. The results show that the optical properties of the cermet coatings using laser cladding were much better than the preplaced coating. In addition, the thermal stability of the optical properties for the laser cladding coating were excellent after annealing at 650 °C for 200 h. The solar absorptance and thermal emittance of multi-scale cermet coating were 85% and 4.7% at 650 °C. The results show that multi-scale cermet materials are more suitable for solar-selective absorbing coating. In addition, laser cladding is a new technology that can be used for the preparation of spectrally-selective coatings.

Effect of multilayered CoO-CoAl2O4 films on improving solar absorptance of Co-WC solar selective absorbing coatings

The multilayered CoO-CoAl2O4 solar selective films were prepared on Co-WC coatings with different mole ratio of Co and Al by sol-gel process. The thin film has high and low refractive indices, respectively. The inner layer can improve surface condition of Co-WC coating as a high absorbing film and buffer lay, annealed at 650 °C in vacuum. The outer layer was annealed at the same condition. The absorptance (α) of the multilayer structure coating increased from 0.873 to 0.932. The properties of the sols were characterized by thermogravimetry-differential scanning calorimetry (TG-DSC). The phase compositions and structure of samples annealed at 650 °C were analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructure and elements distribution of thin films were investigated by field emission scanning electron microscopy (FE-SEM) and energy dispersive spectrometer (EDS), respectively. The absorptance of coatings indirectly indicate the binding force between the Co-WC substrate, durability, long-time stability and corrosion resistance are fine. The results illustrated that defects on surface of the Co-WC coating gradually decrease with the deposition of sol films. And the good element gradients in multilayer structures and the structure of thin films might be the reasons for its improvement in absorptance.

Effects of environmental and operational variability on the spectrally selective properties of W/WAlN/WAlON/Al2O3-based solar absorber coating

The stability of solar selective absorber coatings in hostile environments (e.g., humid condition, corrosive medium, longer exposure at elevated temperature) needs to be critically assessed to ensure durability of such coatings. In the present work, magnetron sputtered W/WAlN/WAlON/Al2O3-based absorber coating with a high absorptance (0.958) and low emittance (82 °C) has been tested in humid and corrosive environments. The selectivity (absorptance/emittance) of the coating did not change after keeping it in 95% humidity at 37 °C for 400 h. Corrosion study in 3.5% NaCl solution reveals that this novel multilayer coating has a better corrosion resistance than that of uncoated stainless steel (SS) substrate. The nanoindentation test on the coating indicates that it has a hardness of 9.6 ± 0.5 GPa. The performance of the coating did not degrade after heat treatment at 350 °C in air for 1000 h. Additionally, the activation energy for degradation has been determined to predict the stability of the coating at high temperature. Further, the normal spectral emissivity of the tandem solar absorber was measured in the full angular range from 200 to 500 °C. The results of emissivity measurements by varying observation angles are analysed using numerical integration to obtain the total hemispherical emissivity. In summary, W/WAlN/WAlON/Al2O3 stack is an attractive candidate absorber coating for photo-thermal conversion systems.

Microstructure and Optical Properties of Spray-Pyrolysed FeAlOx Cermet as a Spectrally Selective Solar Absorber Coating

Microstructure and Optical Properties of Spray-Pyrolysed FeAlO<i>_x</i> Cermet as a Spectrally Selective Solar Absorber Coating

A Path to High-Efficiency Optical Coupling for HIRMES

The high-resolution mid-infrared spectrometer (HIRMES) under development for Stratospheric Observatory for Infrared Astronomy is an instrument operating in the 25–122 μm spectral range with a spectral resolution R = Δλ/λ ~ 100,000 and has two absorber-coupled transition edge sensor bolometric detector focal planes. We have developed novel NbTiN low-stress absorber coatings which have the required optical impedance across the HIRMES operating band. The low intrinsic stress of these coatings allow for a peak-to-valley corrugation amplitude < 5 μm of the 450 nm thick, 1.4 mm × 1.7 mm detector pixels. Furthermore, these coatings have a superconducting transition temperature ~ 10 K, which allows them to simultaneously serve as an absorber in the desired signal band and a rejection filter at long wavelengths. This attribute makes them especially attractive for ultrasensitive absorber-coupled bolometric detector applications, because it helps in controlling the optical loading from out-of-band radiation. We also discuss a novel method for integrating a wedged-reflective absorber termination to the detector array.

Hafnium carbide based solar absorber coatings with high spectral selectivity

Single layer coatings of hafnium carbide were deposited on stainless steel, copper and silicon substrates by means of reactive sputtering. The deposition process was optimized for maximum solar absorptance by varying the process parameters. In the case of coated stainless steel substrates, we obtained a maximum solar absorptance of 0.925 along with a thermal emittance of 0.13. For coated copper substrates we obtained a maximum solar absorptance of 0.948 along with a thermal emittance of 0.03. Reflectance spectra of SS and Cu substrates coated at optimized parameters were measured in the wavelength range 250 nm to 20 µm. Detailed x-ray photoelectron spectroscopy (XPS) studies were carried out to investigate the chemical composition of the coatings. XPS analysis of coated samples showed that because of the tendency of HfC to oxidize, the coating had a surface consisting mostly of HfO2 but the extent of oxidation reduced with depth and partially oxidized HfC was present on the subsurface layers of the coatings. Along with the absorbing nature of HfC and appropriate coating thickness, the high solar absorptance of the coating can be partially attributed to the presence of HfO2 near the surface which acts similar to an antireflection layer. Assuming a two-layer model of the coating, an upper HfO2 layer and a HfC layer underneath it, reflectance spectra of the coated samples were simulated using SCOUT software and compared to the experimentally obtained spectra. For all the samples, best fit values of coating thicknesses were in the range 120–130 nm.

Reduction of short wavelength reflectance of multi-wall carbon nanotubes through ultraviolet laser irradiation

Multi-wall carbon nanotube coatings are used as broadband, low-reflectance absorbers for bolometric applications and for stray light control. They are also used as high emittance blackbody radiators. Irradiation of single wall carbon nanotubes with ultraviolet (UV) laser light has been shown to remove amorphous carbon debris, but there have been few investigations of the interaction of UV light with the more complex physics of multi-wall carbon nanotubes. We present measurements of reflectance and surface morphology before and after exposure of multi-wall carbon nanotube coatings to 248 nm UV laser light. We show that UV exposure reduces the reflectivity at wavelengths below 600 nm and present modeling of the thermal cycling the UV exposure causes at the surface of the carbon nanotubes. This effect can be used to flatten the spectral shape of the reflectivity curve of carbon nanotube absorber coatings used for broadband applications. Finally, we find that the effect of UV exposure depends on the nanotube growth process.

A study of solar thermal absorber stack based on CrAlSiNx/CrAlSiNxOy structure by ion beams

Renewable energies are foreseen as a major energy resource for next generations. Among several energy sources and technologies available, Concentrated Solar Power (CSP) technology has a great potential, but it needs to be optimised, in particular to reduce the costs, with an increase of the operating temperature and long term stability. This goal can be achieved by tailoring the composition and multilayer structure of films. In this work we present and discuss the results obtained from solar absorber coatings based on nitride/oxynitride structures. A four-layer film structure, W/CrAlSiNx(HA)/CrAlSiNxOy(LA)/SiAlOx, was deposited on stainless steel substrates using magnetron sputtering deposition method. Simulations were performed to establish the best spectral properties of the multilayer stacks with optical constants of single layers and film thickness. The elemental analysis was performed using Rutherford Backscattering Spectrometry (RBS) and Time of flight Elastic Recoil Detection Analysis (TOF-ERDA). To assess the thermal stability of the coatings the samples were thermal annealed at 400 °C, in air, and at 600 °C, in vacuum. The results obtained by RBS and TOF-ERDA reveal good oxidation resistance and thermal stability. Also, the optical measurements confirm the potential of these materials for the use in CSP technology.

Comparative analysis of different surfaces for integrated solar heating and radiative cooling: A numerical study

The spectral selectivity of solar selective absorbing coatings enhances coating performance in diurnal heating collection but also limits the potential application of these materials in nocturnal radiative cooling. A radiative cooling surface shows poor solar heating performance due to the same reason. The present study proposed a novel surface that combines solar heating and radiative cooling (SH-RC) considering the spectral selectivity of photo-thermic conversion and radiative cooling. A hypothetical SH-RC surface was also proposed. This hypothetical surface had an absorptivity of 0.92 in the solar radiation band, emissivity of 0.70 in the “atmospheric window” band, and absorptivity (emissivity) of 0.05 in other bands. The thermal performance of this spectrally selective SH-RC surface (SH-RCs surface) was numerically investigated by comparing it with three surfaces, namely, solar selective absorbing coating surface (SH surface), spectrally selective radiative cooling surface (RC surface), and spectrally non-selective black surface (SH-RCblack surface). Results indicated that the SH-RCs surface is most suitable for achieving integrated SH and RC. In a typical summer day, the heat gains of the SH, RC, SH-RCblack, and SH-RCs surfaces are 17.14, 0, 15.57, and 13.22 MJ/m2, respectively. The cooling energy gains of the four surfaces are 0, 1.02, 0.95, and 1.01 MJ/m2, respectively.

Delineating the role of surface characteristics on the solar selectivity of colored chromium oxide coating on 304 stainless steel substrate

Solar selective colored chromium oxide coatings were prepared on the SS304 substrate through the electrochemical deposition route to provide more options and flexibility in aesthetically design solar absorbers with acceptable energy performance. The variation of colors in the coatings was achieved by varying the coating thickness as a function of deposition time. Color changes in the coatings were also evident in the samples that were annealed under vacuum. Electron microscopic images show that nanometer sized pores are present on the coating surfaces. The pore-density, pore-size distribution and surface roughness exhibit significant changes with variations in deposition time and annealing temperature. Initially, the selectivity factor of 6.08 was recorded in the as prepared sample coated for 30 min, which improved to 7.36 on annealing the samples under vacuum at 700 °C, however, it drops to 4.22 when annealed at 900 °C. A similar trend was also observed for samples that are coated for 60 min. The results indicate that the pore-density, pore-size distribution and surface roughness play a dominant role in governing the solar selectivity of the coatings. Nanoindentation tests were performed on the coated samples and the results show that mechanical properties of the solar absorber coatings degrades with the increase in coating thickness but improve significantly with increase in annealing temperature.

Copper-oxide spinel absorber coatings for high-temperature concentrated solar power systems

Concentrated Solar Power (CSP), a promising renewable energy technology, involves methods to concentrate the sun's energy onto receiver systems that generate steam, activate turbines, and consequently generate electrical power. To ensure CSP technologies remain cost-competitive, absorber coatings on CSP receiver systems require performance enhancements for increasing solar-thermal energy conversion efficiency. In this work, black metal-oxide nanoparticles comprising copper-cobalt oxides (CuxCo3−xO4) and copper-manganese oxides (CuxMn3−xO4) are synthesized for solar absorptive potential by hydrothermal syntheses – selected for low-cost, energy-efficient fabrication capable for bulk manufacturability. The material is deposited onto high-temperature, durable Inconel substrates by a flexible spray-coating method, and characterization is performed by Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray Spectroscopy (EDS), and X-Ray Powder Diffraction (XRD) analyses, as well as measurements to gauge thermal performance. High temperature stability of a model solar receiver surface using these synthesized materials are assessed by comparing spectral reflectance and a figure-of-merit efficiency metric before and after high temperature exposure beyond 1000 h. To extend spectrally-selective absorbance capability, the coating surfaces are geometrically-textured using sacrificial polymer beads that are jointly implemented in the spray-coating process. This study ultimately showcases materials produced with high figure-of-merit conversion efficiency, demonstrating solar absorber coatings capable of interfacing with next-generation CSP receiver systems.

Air-based sputtering deposition of TiNxOy films for solar selective absorber coatings applications

Titanium oxynitride (TiNxOy) thin films with tunable electrical and optical properties were produced by sputtering using air as a reactive gas at a high base pressure of 1.3 × 10−2 Pa. Pure N2 and O2 with a fixed ratio of 79/21 was also used as a reference reactive gas (denoted as S-air) at a low base pressure of 6.7 × 10−4 Pa to investigate the influence of trace components in air on the characteristics of the films. By merely changing either the air/Ar or the S-air/Ar flow ratio within suitable processing windows, crystal phases, microstructures, electrical resistivities, and optical properties could be tailored. In practice, the TiNxOy thin films produced by such air-based sputtering could turn from conductive, semiconductive, to dielectric and would correspondingly change from opaque, translucent, to transparent. For solar selective absorber (SSA) coatings evaluations, bilayer and gradient TiNxOy-based SSA structures were tested, respectively. The gradient TiNxOy SSA coatings exhibited the best performance with solar absorptance of 86.3% and thermal emittance of 11.3%. This facile air-based sputtering technique with such a gradient coating design has great potential for solar selective absorber coatings applications.

Development of Coatings for Radar Absorbing Materials at X-band

The present review gives a brief account on some of the technical features of radar absorbing materials (RAMs). The paper has been presented with a concentrated approach towards the material aspects for achieving enhanced radar absorption characteristics for its application as a promising candidate in stealth technology and electromagnetic interference (EMI) minimization problems. The effect of metal particles doping/dispersion in the ferrites and dielectrics has been discussed for obtaining tunable radar absorbing characteristics. A short theoretical overview on the development of absorber materials, implementation of genetic algorithm (GA) in multi-layering and frequency selective surfaces (FSSs) based multi-layer has also been presented for the development of radar absorbing coatings for achieving better absorption augmented with broadband features in order to counter the radar detection systems.