Absorber Coatings Research Articles

Development of a low environmental impact, porous solar absorber coating utilizing binary/ternary solvent blends for CSP systems

Concentrated solar power utilizes a field of mirrors to redirect solar rays onto a central receiver to generate thermal energy through heat transfer media and a Rankine steam cycle. To effectively transfer heat to the heat transfer material, the receiver has to efficiently convert/absorb the incoming solar flux without losing energy to radiation. Receivers are coated with a solar absorber coating evaluated with a figure of merit which weighs the energy absorbed by the sample against the total incident energy. The structure of the painted coating plays a large part in the long-term stability and optical properties of the solar absorber coatings. We investigated the effects of different solvents on the micro-structure of black oxide coated paint tiles and evaluated the stability of the paint colloid using the Gibbs free energy of mixing. We also investigated the use of low environmental impact solvents as potential alternates to standard solvents to create low-stress films. The results show that paint blends thinned by blends of dimethyl carbonate and tertbutylbenzene have low-stress surface morphology with pore-like structures due to the favorable Gibbs free energy value of the colloid and reduced evaporation rate of the primary solvents. These coatings also exhibited strong optical performance with figure of merit and solar absorbance values of 91.60% and 96.86%, making them ideal coatings for next generation concentrated solar power plants.

High-temperature air-stable solar absorbing coatings based on the cermet of MoSi2 embedded in SiO2

Solar absorbing coating is a critical component in concentrated solar power (CSP) system. How to further decrease costs and enhance thermal stability of coatings is crucial for large-scaled CSP systems. Herein, we present a cost-effective and facile strategy to prepare the MoSi2–SiO2 based coatings, which can survive at 850 °C in air for 100 h due to the formation of a SiO2 barrier in cermet that prevents the coatings from further oxidation. The absorbers demonstrate a high absorptance of ∼95%, and the total efficiency using this absorber could reach 63.5% with the conditions of a solar concentration ratio of C = 1000 at 850 °C, which exceeds most of spectrally selective coatings. These results suggest that the MoSi2–SiO2 based absorbing coating would be a potential candidate for a large-scale deployment in high-temperature solar thermal conversion systems.

Tantalum carbide based spectrally selective coatings for solar thermal absorber applications

Tantalum carbide (TaC) based tandem coatings for selective absorption of solar radiation were fabricated by magnetron sputtering technique. Tandem structure of Ti/TaC/Al2O3 coating was deposited on a stainless steel substrate in which Ti acts as back reflection layer, TaC acts as main absorber layer and Al2O3 acts as an antireflection layer. The composition and thicknesses of the individual component layers have been optimized by adjusting the sputtering process parameters. Reactively sputtered TaC coating deposited at 85 W of sputtering power and 8 sccm of C2H2 flow rate showed an absorptance of 0.845 and emissivity of 0.14. After depositing approximately 50 nm thick Al2O3 on Ti/TaC, the absorptance increased to 0.957 and emissivity increased to 0.15. The refractive indices and extinction coefficients of each layer were used to simulate the reflectance spectra of the deposited tandem stack using SCOUT simulation software. For individual layer thicknesses of Ti, TaC and Al2O3 of 15, 55 and 50 nm, respectively a good match was obtained with the experimental and simulated reflectance spectra. The layer thicknesses were consistent with the thickness data of the tandem stack obtained from cross-sectional field emission scanning electron microscope image. The thermal stability of the tandem stack was examined in vacuum at different temperatures for a short duration. The results showed that the tandem stack is stable up to 500 °C for 2 h.

Reverse Design of Ultrasonic Absorptive Coating for the Stabilization of Mack Modes

The effects of ultrasonic absorptive coating (UAC) admittance on the first and second modes in a high-speed boundary layer are analyzed using linear stability theory. It is shown that the growth rate of the first mode is increased as UAC admittance phase varies from to , whereas the second mode is amplified when tends to and damped if . The frequency range of the first mode is broadened when is in the vicinity of , and the second mode shifts to low frequencies when . Moreover, the stabilization or destabilization effects on Mack modes, frequency band broadening, and frequency shifting are promoted by large UAC admittance magnitudes. Based on the requirements of the admittance phase and magnitude for the stabilization of Mack modes, a design strategy for UAC is proposed to stabilize the second mode in a wide frequency range with the first mode little amplified.

Impedance-Near-Zero Acoustic Metasurface for Hypersonic Boundary-Layer Flow Stabilization

The laminar-turbulent transition in hypersonic flows is associated with the acoustic-wave-like Mack second mode. Understanding the physics and suppression of this mode is not only of basic scientific interest, but is also essential to future high-speed transportation. This study reveals that, counterintuitively, an almost nondissipative boundary with near-zero surface acoustic impedance can suppress this mode, and performs even better than conventional dissipation-based ultrasonic absorptive coatings. A practical realization, based on an acoustic metasurface, is furthermore presented, pointing toward full control of the hypersonic boundary-layer transition. Full speed ahead!

Synthesized of octahedral cupper chromite spinel for spectrally selective absorber (SSA) coatings

In this study, cupper chromite Spinel (CuCr2O4) pigment was synthesized via two-step combustion–calcination procedure. Mixtures of citric acid and glycine were used as fuel and nitrate ions were used as oxidizers. The samples were synthesized at different pHs. The synthesized sample was characterized by x-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and energy-dispersive X-ray spectroscopy (EDX). The X-ray diffraction (XRD) patterns of the samples revealed that the single-phase CuCr2O4 spinel was formed. The SEM results showed that the spinel particles were synthesized in octahedral morphology. Furthermore, pH and amount of fuel have remarkable effect on size of particles. Increasing the pH leads to increasing the size of particles. Also, the size of particles was significantly increased by doubling the amount of fuel. In the next step, the synthesized pigments were used as spectrally selective absorber in solvent-based acrylic coatings. The functional groups of the coatings were identified by the attenuated total reflectance (ATR) analysis. The solar absorptance (αs) and the thermal emittance (εT) of the coatings were determined from the corresponding diffuse reflectance spectra. The optimum selective sample were calculated αs = 0.9 and εt = 0.3.

An experimental study on the effect of salt spray testing on the optical properties of solar selective absorber coatings produced with different manufacturing technologies

Solar selective absorber coating (SSAC) is one of the key components of a solar collector, with its optical properties having a significant impact on the collector’s thermal performance. The key parameters characterizing the optical properties of an SSAC are the solar absorptance (absorptance of solar radiation) and the thermal emittance (emittance for long-wave radiation). However, some of high-performing SSACs suffer from some drawbacks, such as lower durability, lower resistance to corrosion and abrasion, which is particularly harmful for SSACs, as, for example, chlorides in the atmosphere have become a main contributor to corrosion in coastal areas with the increasing trend of global warming. In this paper, salt spray tests have been conducted on the SSACs manufactured by three common manufacturing technologies, i.e., the anode oxidation (AO) technology, the vacuum magnetron sputtering (VMS) technology, and the black chromium plating (BCP) technology, over the testing durations of 12 h, 24 h, 36 h, and 48 h, respectively, to examine the effect of the salt spray testing on the optical properties of SSACs manufactured by different manufacturing technologies. The salt spray testing is an accelerated aging testing method for evaluating the SSAC’s resistance to corrosion when it is under an extended exposure to a saline, or salted, spray (fog). The experimental results show that, in general, the SSACs manufactured by the BCP technology have excellent resistance to salt spray (i.e., to corrosion) and those manufactured by the AO technology have only reasonable resistance to corrosion, whereas the SSACs manufactured by the VMS technology have very poor resistance to corrosion. The results also demonstrate that there are noticeable differences in the optical properties of the SSAC samples even manufactured by the same technology but by different manufacturers, with some having significant differences. The causes for the differences have been further examined through the inspection of the physical appearance of the selected SSAC samples and the experimentally measured distributions of the monochromatic reflectance of solar radiation of the samples over the solar spectrum before and after the salt spray testing over different durations.

Optical properties and thermal stability of Cu spinel oxide nanoparticle solar absorber coatings

Optimal coatings on receivers for concentrated solar power plants (CSP) not only need to have high solar absorptance, but should also possess superior stability in air at elevated temperatures. For next-generation CSP plants, the surface temperature of the receivers is expected to exceed 750 °C. In this work, we systematically studied optical properties and long term thermal stability of solar absorbing coatings (SACs) made from various Cu(II) containing spinel oxide nanoparticles, including CuCr2O4, Cu0·5Cr1·1Mn1·4O4, CuFeMnO4, and compared these properties to those of the state-of-the-art Pyromark 2500 coating. The solar absorptance of each sample was measured after isothermal annealing at 800 °C in air for durations of 100, 300, 1000, and 2000 h. We found that porous Cu0·5Cr1·1Mn1·4O4 had the highest solar absorptance at 97.1% before the thermal annealing and remained the highest throughout thermal testing, remained at 97.2% after 2000 h, whereas Pyromark 2500 exhibited considerable degradation in solar absorptance, from 96.4% to 94.6%. We analyzed the chemical composition, microstructures, and particle sizes of all the samples before and after the thermal annealing and discussed their implications on optical properties and thermal stability.

High-temperature stable refractory nanoneedles with over 99% solar absorptance

Solar absorber coatings have widely been investigated for solar-thermal technologies including concentrated solar power and solar thermochemistry. While various nanostructures such as nanowires and nanotubes have been commonly used for high solar absorptance owing to their potent light trapping effect, the high temperature stability of these nanostructures has yet to be established due to either coarsening of nanostructures or oxidation of certain materials in air (e.g., Si and C). In this work, we developed a nanostructured solar absorber from a family of high-temperature refractory spinel oxides, Co3O4 and CuCo2O4, with ultra-high solar absorptance over 99%. Once coated with a thin layer of HfO2 or SiO2 through atomic layer deposition, the Co3O4 and CuCo2O4 nanoneedles preserve their high aspect ratio and sharp tips, allowing the solar absorbers to maintain the superior absorptivity and excellent thermal stability at an elevated temperature for an extended period (650 °C and 800 °C for 100 h for passivated Co3O4 and Cu1Co2O4, respectively). These results suggest that solar absorbers made from refractory spinel oxide nanoneedles can be used for high-temperature solar thermal applications with ultrahigh absorptance.

Improvement of surface photostability of bamboo scrimber by application of organic UV absorber coatings

Improving the durability of biological materials is essential for expanding their application areas. In this study, an organic ultraviolet (UV) shielding layer (coating) composed of a UV-curable paint, ethyl acetate, anhydrous alcohol, and an organic UV absorber is constructed on the surface of outdoor bamboo scrimber (OBS). The behaviour of OBS with or without this coating was investigated under accelerated weathering. The physical properties photostability of the samples before and during exposure were evaluated by characterizing the colour and glossiness. Contact angle (CA) measurements and surface free energy (SFE) calculations were performed to investigate the wettability of the coatings. Attenuated total reflectance Fourier transform infrared spectroscopy was used to characterize the changes in the functional groups of the coatings during weathering. The experimental results have shown that the colour, glossiness, CA, SFE, and functional groups photostability of the sample with the UV shielding coating increased significantly compared to those of the control sample. The coating with 5% organic UV absorber was considered as the best UV shielding layer in this study, as it successfully protected the OBS and reduced the surface photodegradation of the substrate and the coating itself.

Structure, thermal stability and chromaticity investigation of TiB2 based high temperature solar selective absorbing coatings

Transition metal titanium diboride (TiB2) is a promising material for the solar selective absorbing coatings due to its favorable spectral selectivity and thermal stability. In this research, spectrally selective TiB2/Al2O3 tandem absorber is deposited on stainless steel (SS), which exhibits a high solar absorptance (αs = 0.93) and a low thermal emissivity (ε = 0.11). In this tandem absorber, the TiB2 acts as the main absorber layer and the Al2O3 acts as an antireflection layer. The tandem absorber coating is systematically characterized by ultra-high resolution scanning electron microscope (SEM), UV–vis-NIR spectrophotometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Micro-Raman techniques. The thermal stability in vacuum of the SS/TiB2/Al2O3 tandem absorber coatings is investigated in the temperature range of 400–800 °C for 2 h and 100 h, respectively. SEM and Raman spectra indicate that the change of surface morphology and chemical composition for the coating result in the degradation of optical performance. With the different annealed temperature, the SS/TiB2/Al2O3 tandem absorber coatings exhibit different color, which is further investigated using chromaticity diagram.

Crowding effects of nanoparticles on energy absorption in solar absorption coatings

Metallic nanoparticles with large photo-thermal conversion capabilities have significant implications for solar absorption coatings. In this work, we develop a calculation model to analyze crowding effects on energy absorption, including effects of nanoparticle radius, size distribution, interparticle spacing, and spatial distributions in a plane. There is an optimal nanoparticle radius that maximizes energy absorption, which is different for different media, for example, 45 nm for air and 30 nm for water. For regular distributions of nanoparticles in a plane, a hexagonal lattice with an interparticle distance of about 315 nm is optimal for solar-thermal conversion. We expect these results to be useful for designing solar absorption coatings with high photo-thermal conversion efficiencies.

Structure, thermal stability and optical simulation of ZrB2 based spectrally selective solar absorber coatings

In the present work, a solar selective coating of ZrB2/Al2O3 was deposited onto stainless steel (SS) substrates using a magnetron sputtering system. The coating exhibited an good selective absorbing properties with an absorptance of 0.92 and an emittance of 0.11. To investigate the thermal stability of the coating, the samples were annealed at 500 °C and 600 °C for 100 h, respectively. Experimental results indicated that the coating was thermally stable at 500 °C. The structure and optical properties of the coating is also characterized using ultra-high resolution scanning electron microscope, UV–vis–NIR spectrophotometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS). The reflectance spectra of the SS/ZrB2/Al2O3 coating was successfully simulated by CODE software.

Accelerated aging of absorber coatings for CSP receivers under real high solar flux – Evolution of their optical properties

The use of durable high solar absorptance receivers is a key element in a CSP plant project. In this article, different receiver materials are studied: four alloy substrates (T91, T22, VM12, Inconel 617) combined with four new absorber coatings, operable in solar towers with molten salts or steam as heat transfer fluids, and a classic Pyromark® paint considered as a reference. In order to test the durability of the coatings, 200 solar accelerated aging cycles were applied on the samples, using a concentrated solar facility (named SAAF). The cycles were defined so as to apply realistic high solar flux and temperature on the front side of the samples, and with high cooling and heating rates reproducing the fast variation of solar irradiation due to cloudy weather and subsequent thermal shocks. The optical characteristics of the coatings were measured at the beginning and at regular intervals during the aging procedure. Different behaviors of the coatings were observed depending on the substrate, before any aging cycle. After this first aging campaign, some evolutions were observed on the solar absorptance or thermal emittance, depending on the substrate and the coating. Nevertheless, the degradations noticed are not significant enough to conclude about the durability of the coatings.

Thermal stability and Weatherability of Single-Layer Micron Scale Cermets Solar Selective Absorbing Coatings by Laser Cladding

The thermal stability of solar absorber coatings at high temperature is the most important challenge for solar energy photo-thermal utilization. A new one layer micrometer-size TiC/TiN-based cermet coating was designed and fabricated with laser cladding method in air. The result indicated the laser cladding could be used to obtain a TiC/TiN-Ni/Mo cermet coating with the desired optical properties. An absorptance (α) of ∼84% and a thermal emissivity (ɛ) of ∼5% at 300 K were calculated. Furthermore, the thermal stability of the coating was outstanding after heat-treatment. The absorptance and emissivity of the TiC/TiN-based cermet coating were respectively 82% and 4.1% at 650°C. The results indicated that TiC/TiN-Ni/Mo cermets are suitable for spectrally selective materials. Furthermore, the cermet coating also possesses excellent weatherability based on the test of the corrosion resistance. The results indicated that TiC/TiN-Ni/Mo cermets are suitable for spectrally selective materials. Moreover, the laser cladding was found to be an improved and novel preparation technology in the field of solar selective absorbing coatings.

石墨烯超材料微结构材质的制备及其在吸波涂层改进设计上的应用

石墨烯超材料微结构材质的制备及其在吸波涂层改进设计上的应用

Effective Complex Permittivity Determination and Microwave Absorption Properties of a Granular Dielectric Composite Material

The Transmission / Reflection (T/R) method for effective complex permittivity determination is employed to characterize granular dielectric composite material over a wideband frequency. This characterization is a starting point for designing a Radar Absorbent Material (RAM) by optimizing the thickness of the composite layer and inclusion concentrations. A series of 2D-simulations of granular composite material (Graphite / Teflon PTFE), in the dilute limit approximation, are performed; and the maximum electromagnetic absorption (i.e., minimum reflection) is reached for different frequency bands. An optimized composite sample has shown a maximum reflection loss of -31.2 dB (>99% power absorption) at 13.7 GHz with a bandwidth (full frequency width at half of the maximum response) of 2.6 GHz in a sample thickness of 4.3 mm and filler concentration of 1.38%. Materials can find applications in suppression of electromagnetic interference and microwave absorbing coatings.

Structure, optical simulation and thermal stability of the HfB2-based high-temperature solar selective absorbing coatings.

Transition metal borides are a kind of potential materials for high-temperature solar thermal applications. In this work, a novel SS/HfB2/Al2O3 tandem absorber was prepared, which exhibited high solar spectrum selectivity (α/ε) of 0.920/0.109. The optical constants of the coating were obtained using spectroscopic ellipsometry, and the dispersion model of the HfB2 layer was modeled with the Tauc–Lorentz dispersion formula. In addition, the reflectance spectrum simulated by the CODE software corroborated well with the experimental results. The thermal stability test indicated that the HfB2/Al2O3 solar absorber coating was thermally stable in vacuum at 600 °C for 2 h. When extending the annealing time to 100 h, the coating could maintain high spectral selectivity after aging at 500 °C irrespective of whether in air or vacuum. All these results indicate that the coating has good solar selectivity and is a promising candidate for high-temperature solar thermal applications.

Greatly enhanced anticorrosion of Al–AlNxOynanocermet films with self-passivated Al nanoparticles for enduring solar-thermal energy harvesting

Unprecedented anticorrosive Al–AlNxOyselective solar absorption coatings (SSACs) are fabricated on an industrial scaleviaa self-developed air-to-air continuous sputtering technique.

宽频段太赫兹辐射计高吸收率涂层的特性

宽频段太赫兹辐射计高吸收率涂层的特性