Absorber Coatings Research Articles

Temperature-induced degradation of bilayer MoOx-based solar absorbers under air annealing conditions

Thermal stabilities of solar selective absorber coatings in operating environment are crucial to evaluate the durability, which are directly related to the optical properties. Degradation induced by temperature is an important guarantee for assessing long service life. Here, the sputtered bilayer MoOx cermet-based solar selective absorber coatings with an absorptance of 0.91 and an emittance of 0.06 are investigated under air annealing conditions. Thermal aging test shows no degradation of the MoOx/SiO2 absorber with a selectivity of 0.904/0.06 when annealing up to 250 °C for 200 h. In contrast, the MoOx/MoOx absorber shows significantly poor thermal tolerance at 200 °C in aerobic environment. As extending the temperatures and durations, the solar absorber is severely degraded owing to the diffusion and oxidation of Mo atoms induced by temperature, which is also responsible for the optical and structural deterioration of the absorber. These results highlight that the as-obtained absorber coatings have good possibility for solar thermal harvesting application.

Cu(Mn0.748Ni0.252)2O4/SiO2 Nanoparticle Layers for Wide-Angle Spectral Selectivity and High Thermal Stability

Spectrally selective absorber coatings (SSAC) have attracted tremendous research attention owing to its high conversion efficiency of solar energy into heat with minimal emittance. In this regard, ...

Thermal annealing characteristics of solar selective absorber coatings based on nano-multilayered MoOx films

Designing double metal-dielectric (cermet) solar selective absorber coatings (SSACs) often requires complex co-sputtering techniques with multiple targets. This inevitably limits the simple and low-cost industrial fabrication. Here, we develop novel nano-multilayered MoOx-based SSACs by simple and stable one-step reactive magnetron sputtering process using single molybdenum target. The proposed multilayer SSACs exhibit good solar absorptance of 0.93 and low thermal emittance of 0.06. Owing to the temperature-induced oxygen diffusion and oxidation phenomenon the as-sputtered SSACs have a poor thermal tolerance under air atmosphere, and after annealing at 200 °C for 150 h, the resulting absorptance is diminished from 0.93 to 0.90. However, the optical performance of the annealed SSAC is relatively stable in high-vacuum environment, even after annealing at 450 °C for 200 h, it still displays an ideal spectral selectivity of 0.92/0.07. With above properties, the resulting MoOx-based SSAC is a promising absorber for enduring thermal harvesting in solar vacuum collectors.

Improvement of thermal stability of ZrSiON based solar selective absorbing coating

Solar selective absorbing coatings (SSACs) are required to have not only excellent optical property but also outstanding thermal stability for high temperature applications. The optical properties of Mo/ZrSiN/ZrSiON/SiO2 SSAC had been optimized successfully before. Herein, we are focusing on the evaluation and mechanism of thermal stability of this multilayer coating for its potential applications in concentrated solar power (CSP) systems. Fortunately, the coating exhibits excellent thermal stability after aging at 400 °C for 1500 h in vacuum. At aging temperature of 500 °C for 1000 h or 600 °C for 300 h in vacuum, the slight inter-diffusion between Mo layer and stainless steel (SS) substrate occurs. At higher aging temperature of 700 °C for 100 h in vacuum, the serious inter-diffusion between Mo layer and SS substrate leads to invalidation of the coating, which has been evidenced by Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD) technologies. Additionally, this coating also has an outstanding thermal stability after aging at 400 °C for 300 h in air. A heating-cooling cycling (HCC) treatment evidences the good thermal stability of this coating working in cold environment (−60 °C). The results reveal that this coating can be a promising candidate not only for CSP system in high temperatures but also for usage in cold environment.

Electroless deposited black nickel-phosphorous solar absorber coatings on carbon steel: Effect of plating bath pH

One of the challenges of solar energy collection is energy conversion efficiency, particularly the absorbent surface capability to convert solar energy to thermal energy. In this work, nickel-phosphorous (Ni-P) coating was deposited onto a carbon steel substrates using the electroless plating process containing nickel sulfate as the nickel source and sodium hypophosphite as the reducing agent. This study focuses on the study of plating bath pH effect on the light absorption properties of the black surface after blackening process and at the same time maintain the characteristic of protective coating to protect the substrate from corrosion. The electroless nickel plating was conducted at various plating bath pH (4.5, 5.0, and 6.0) for 3 h at a constant bath temperature and composition. Electroless black Ni-P surfaces were obtained through an etching process of Ni-P coating with 9 mol/L nitric acid solution. The surface morphology of Ni-P coating and black Ni-P coating were observed using scanning electron microscopy (SEM). The chemical composition of Ni-P coating deposited at various pH was measured using energy dispersive X-ray (EDX) equipped on the SEM. The SEM results show that black Ni-P coating for pH 4.5 sample has the highest porosity which is 47.1% of the surface area with highest phosphorus content shown by EDX analysis. The cross-sectional analysis showed that the depth of the etching effect occurred only up to the 33% of the coating thickness. The remaining 67% of coating thickness still uniformly attached on the substrate without the presence of any defects or flacking after the etching process. The visible near-infrared spectrophotometer, (VIS-Nir) showed that the black Ni-P coating of pH 4.5 sample had the highest absorption capability which is 92% of light absorption due to its high surface roughness.

ADVANCEMENT OF SOLAR SELECTIVE DLC COATING USING CAPVD FOR SOLAR THERMAL APPLICATIONS

Metal nitride multilayer films display a unique combination of exceptional properties with respect to optical absorption, thermal emission, corrosion resistance, adhesion between coating and surface and high temperature withstand. Most considerable aspects of nitride coatings were economical, environmentally friendly and easy to develop. Similar to nitride thin films, to achieve a considerable absorption (α) -0.92 and low emission (Є) -0.08 along with chemical and radiation stable solar selective coatings, Diamond Like Carbon (DLC) thin films exhibit the desirable properties for Concentrated Solar thermal Power(CSP)applications. The main advantages of DLC films were high hardness, chemical and radiation stability and good control over the optical properties. To achieve above-mentioned properties, optimization of each layer of the DLC coating has needed. The main aim of this research is optimization of Cr-base layer using Cr-Target current 175A to get 125 nm thicknesses, optimise the AlSiN absorber layer by controlling the AlSi- target current 175A to maintain 35nm thickness. The sequence of the DLC coating layers was selected based on their relative thickness, which was optimize to get good solar selectivity (α/Є). Individual layers of the DLC solar coatings have unique properties to get overall required high absorbance and low emission along with chemical and radiation stability. These solar selective multi-layers (Cr/DLC/AlSiN) have deposited by using available Cathodic targets (Cr , AlSi & Ti) in Cathodic Arc Physical Vapor Deposition (CAPVD) and optimized parameters were mainly depend on the target currents to control over the thickness of the each layer, base pressure 1*10-5 mbar and deposition temperature 400°C. The DLC multilayer solar selective coatings were characterized using Ultraviolet Visible Near infrared (UV- Vis- NIR) spectrophotometer, Scanning Electronic Microscopy (SEM), Transmission Electron Microscopy (TEM) and Raman spectroscopy etc. Scratch test and corrosion tests have conducted for these absorber coatings testing.

Thermal Performance Enhancement in Flat Plate Solar Collector Solar Water Heater: A Review

Various studies to improve the thermal performance of flat plate solar collector (FPSC) solar water heater have been conducted, and more are currently in progress. This study aims to review existing methods on thermal performance enhancement for FPSC and discuss on heat-transfer enhancement using vibration and its potential application for FPSC. Ten methods for improving thermal performance are identified, which include applications of nanofluids, absorber coatings, phase change materials (PCM), thermal performance enhancers, FPSC design modifications, polymer materials, heat loss reduction, mini and micro channel and heat-transfer enhancement using vibration. An examination of heat-transfer enhancement using vibration in low frequency ranges for an evacuated-tube solar collector (ETSC) solar water heater system showed that it can potentially achieve heat-transfer enhancement of up to 78%. Nevertheless, there is still a lack of research on the applications of heat-transfer enhancement using vibration on FPSC to date.

A 117-W 1.66-Times Diffraction Limited Continuous-Wave Nd:YVO4 Zigzag Slab Laser with Multilayer Amplified-Spontaneous-Emission Absorbing Coatings**Supported by the National Key R&D of China (Grant No. 2016YFB0402103), the National Natural Science Foundation of China (Grant No. 61875208), and the Knowledge Innovation Program of Chinese Academy of Sciences (Grant No. GJJSTD20180004).

We report a continuous-wave end-pumped Nd:YVO4 zigzag slab laser with multilayer amplified spontaneous emission (ASE) absorbing coatings. The coatings are deposited on the slab faces. A five-layer structure consists of SiO2-Ti-SiO2-Ti-Au, and the thicknesses are 2520 nm, 10 nm, 160 nm, 24 nm and 200 nm, respectively. The designed coatings show good performance for the ASE control in the experimental tests. A stable-unstable hybrid laser oscillator along orthogonal directions in the slab aperture is further configured, achieving the 117W output at a pump of 328 W. The beam quality factors M2 in the unstable direction and stable direction are 1.57 and 1.66, respectively.

Femtosecond laser damage resistance of beam dump materials for high-peak power laser systems

Ultrahigh peak power laser systems require highly absorptive and high-damage threshold beam dumps, which allow safe and reliable termination of the beam path in vacuum. We present promising optical materials for high-power laser beam dump designs. In order to evaluate the optimal beam dump performance of the various materials, we characterized the optical properties of these materials and measured their damage thresholds. The tested beam dump materials include highly absorptive (>99.8 % ) Vantablack coatings and absorptive glasses with different optical densities. The laser-induced damage threshold (LIDT) tests were performed using 800-nm ultrafast laser beams with a pulse duration of 40 fs at a 1-kHz repetition rate. The LIDT test methods, such as S-on-1 and R-on-1, were used for determining the damage threshold. The Vantablack coatings showed a moderate ultrafast damage threshold while being highly absorptive and applicable on metallic substrates. The absorptive glasses with high optical density have higher damage resistance and can be used for low-repetition rate applications with the highest intensities. By comparing the optical properties and LIDT results, we discuss the suitability of these beam dump materials for ultrafast high-power laser systems.

Achievements in mid and high-temperature selective absorber coatings by physical vapor deposition (PVD) for solar thermal Application-A review

Abstract Solar energy can be harnessed using different evolving technologies like photovoltaic (PV), artificial photosynthesis, solar heating, solar thermal power plant, etc., whose progress depends on the materials in the collector. Most solar collectors use spectrally selective absorber coating with high absorptance and low emittance in the solar and at the infrared spectrum range respectively. Several efforts have been put since 1990s by different authors towards synthesizing highly efficient and thermally stable solar absorber coating for mid to high temperature applications using different deposition methods. It has been established that, the photothermal conversion efficiency of solar thermal power plant depends on the selective absorber coating with high thermal stability and excellent optical properties. This paper reviews the different types of solar absorber coatings and the current state of the art of the existing mid and high temperature absorber coatings with an emphasis on the sputter deposited coatings. A detail survey on the commercially available mid and high temperature absorber coatings with high absorptance and low emittance have been carried out. The aim of this review article is to know what has been achieved so far on the performance and thermal stability of absorber coatings especially in an air ambient. Also, the scope of performance improvement has been highlighted in this paper.

Multifunctional Thin Film Optically Graded Flexible Absorber

Flexible near perfect optical absorbers have many applications in the field of photovoltaic, energy harvesting and defence technologies. Even though flexible absorber coatings can be easily applied on the curved surfaces with complex geometry, the experimental realization of such absorbers having high absorption capacity in the broadband range is a challenging task. Here, we report the design and fabrication of a polydimethylsiloxane (PDMS) based optically graded thin (total thickness - 750 μm) assembly which shows near perfect absorption (95%) in the wavelength range of 300 to 2000 nm. The observed high absorbing capacity may be attributed to the presence of multiscale feature size in optically graded assembly which leads to efficient light trapping and multiple scattering of incident beam. The multilayer assembly comprises layers of PDMS reinforced with iron, zinc oxide (ZnO) and carbon nanotubes (CNTs). In addition to this, detailed fracture mechanism of fabricated absorber is studied. Moreover, contact angle study of fabricated thin film structure reveals that it is not only a near perfect absorber but also hydrophobic in nature. The ease of fabrication process combined with the excellent properties shown by the multilayer flexible assembly makes it an attractive option for industrial applications.

Effect of surface roughness on the solar photothermal conversion efficiency of spray-coated CuCo2O4 films

Mixed transition metal oxide films are emerging as efficient and inexpensive potential alternatives to multilayer cermet spectrally selective coatings. However, to replace the current standards involving a complex metal–dielectric structure, oxides must be optimized in terms of their electronic structure and mainly their film morphology. In the present work, a simple ultrasonic nebulized spray pyrolysis technique is used to deposit CuCo2O4 films for solar absorber coatings. Their photothermal efficiencies are studied for solar thermal energy harvesting for different film thicknesses obtained by varying the deposition time. The film surface attributes are studied using atomic force microscopy and scanning electron microscopy. The films deposited for 5 and 10 min show relatively high visible absorptance (∼0.79) and relatively low thermal emittance (∼0.1) and thus are promising candidates for spectrally selective coatings. Meanwhile, increasing the deposition time (>10 min) increases the thickness, thereby increasing the solar absorptance. However, this results in an uncontrolled increase in the surface roughness, which affects the spectral selectivity adversely, leading to the films having higher thermal emittance of between 0.1 and ∼0.25. Analysis of the specular reflection contribution shows that this deterioration is governed predominantly by interference effects due to surface attributes. This study is important for the technological applications of spectrally selective coatings and makes a significant quantitative contribution to emphasize the importance of surface morphology in optics.

Optical design, thermal shock resistance and failure mechanism of a novel multilayer spectrally selective absorber coating based on HfB2 and ZrB2

Exploring the inherent spectral selectivity of ultra-high temperature ceramics is of great significance to prepare solar absorber coatings with a simple structure and low cost. The present work shows a further improvement towards the design and deposition of the multilayer spectrally selective absorber coatings based on HfB2 and ZrB2. This novel HfB2-ZrB2/ZrB2/Al2O3 solar absorber coating exhibits a high spectral selectivity (α/ε) of 0.93/0.07. Considering the application environment, the thermal shock resistance experiments are carried out at −70 °C, room temperature and 400 °C in air to test the applicability. After 5 cycling tests, the optical properties of the coating do not degrade. In addition, the detailed failure mechanism at high temperature of this coating is also investigated.

Nonequilibrium carbon black suspensions used in synthesis of polymer composite material

Nowadays polymer matrix-based composite material with various carbon fillers are widely used to protect radioequipment from different interference, to improve characteristics of radar absorbing coatings. Current synthesis processes are sophisticated and rather costly. The challenge is to develop new methods of producing composite materials by efficient knowledge intensive technologies to reduce the cost of products. The paper studies possibility of producing composite material on the basis of elastic polyurethane foam with carbon fillers using polyurethane impregnation in nonequilibrium black carbon suspension. Suspension composition: running water and carbon-bearing powders of nanometer range: fullerene black carbon, Taunite (multi-layer carbon nano tube material), technical carbon T900, wood soot. Nonequilibrium suspension was produced by treatment in hydrodynamic generator of rotor type (cavitation mode). Angular rotation speed of the rotor ω = 10000 rpm.

Ultra-thin and high temperature NiCrAlY alloy metamaterial enhanced radar absorbing coating

Abstract In this article, we present an enhanced metamaterial radar absorbing coating (MRAC) which possesses good electromagnetic absorption performance and ultra-thin thickness at high temperatures. The absorber is composed of radar EM absorbing coating and a layer of metamaterial. The metamaterial layer on the coating can enhance the absorption in two aspects. First, due to the coupling between the high permittivity coating and metamaterial layer, the impedance can match a smaller thickness better than a coating alone. Meanwhile, intense EM fields appear the surface of metamaterial pattern, which can also promote the absorption. The periodic pattern is made of special alloy which provides high temperature resistance quality. It is worth noting that the processing merely requires traditional plasma spraying, which ensures the practicability and feasibility. A 0.96 mm thick sample can achieve EM reflection loss below -5 dB at 800 °C over the whole X band. The experimental results are also consistent with the simulation, which successfully verifies the feasibility and proposes a competitive design for radar absorbing coatings (RAC).

A comparative experimental investigation on thermal performance for two types of vacuum tube solar air collectors based on flat micro-heat pipe arrays (FMHPA)

Two types of solar air collectors (SACs), namely, a conventional-tube collector (C-TC) and a transparent-tube collector (T-TC), are designed on the basis of flat micro-heat pipe arrays (FMHPA) and are compared. FMHPA is used as the core heat transfer element for both collectors. Absorption coatings are located at different places. It is located at, namely, on the inner wall of vacuum tube and on the FMHPA evaporation section in the C-TC and the T-TC. A large volumetric flow rate of air contributes to high efficiency. Results of the comparative experiment show that even if the thermal efficiency of the C-TC (77.6%) is smaller than that of the T-TC (85.0%), the useful energy of the C-TC (641 W) is larger than that of the T-TC (497 W). Moreover, the COP values of the C-TC and T-TC are 11.4 and 8.9, respectively. The unit initial investments are 0.15 and 0.21 $/W. Hence, considering the heating capability and price advantage, the C-TC is a better collector form than the T-TC in the same collector gross area. The collector pressure drop does not exceed 19.1 Pa. These results can serve as a reference for the selection and design of SACs for heating applications and crop drying.

Thermal Stability of Chromium-Iron Oxidation Mixture Cermet-Based Solar Selective Absorbing Coatings.

The solar selective absorber coating (SSAC) are at the core of the efficient solar-thermal system. In this paper, for the first time, the Chromium-iron oxidation mixture cermet was successfully prepared on the surface of ultra-pure ferritic stainless steel by chemical coloring as SSAC. The coating surface has an optical trap structure, and the chromium-iron oxidation mixture cermet is used as an absorption layer to realize solar-thermal conversion. The solar absorptance (AM1.5) of the coating reached 93.66, and the thermal emittance was less than 13. After thermal shock tests at 25/300 °C done 32 times (accumulated 812.8 h), the Performance Criterion (PC) of the coating was 0.01375 < 0.05, showing outstanding thermal stability.

Experimental and Modelling the Combined Measurements Technique for Microwave Absorbing Coatings Testing with Finite Difference Time Domain Method

Because of the structure property of radar absorbing coatings, using of the existing methods for testing these materials is quite difficult. In this paper, a combined-measurement technique was proposed for broadband characterization of microwave absorbing coatings using flanged rectangular waveguide sensor. The technique combines both frequency sweep reflections measurement and two-layer method to provide two different test conditions of the needed reflection coefficients necessary to extract both complex permittivity and complex permeability over a given frequency range. The first set of reflection coefficients is obtained by directly placing the sensor alongside the coating material and performing broadband measurement using frequency sweep over the selected frequency points of frequency range. The same procedure is applied on a combination of known material followed by the test material to measure the second set of the respective reflection coefficients. Finite Difference Time Domain (FDTD) method was employed to numerically calculate the sensor reflection coefficient and analyse the influence of the sensor geometry and coating material thickness on measurement accuracy. The measurement results of both complex permittivity and permeability for various samples of microwave absorbing coatings were agreed well with reference data. Simple interpolation approximation was introduced in their extraction process from multiple reflection coefficient data since they are frequency dependent parameters . The proposed technique speeds up and simplifies coating materials testing, while it decreases the error due to repeatability of measurement.

Selective properties of high-temperature stable spinel absorber coatings for concentrated solar thermal application

In concentrated solar thermal (CST) system, receiver tube is one of the key important elements in the photothermal conversion process. The high photothermal efficiency of the receiver tube greatly depends upon the coating type, angular selectiveness of the coating, and radiative, conductive and convective losses. Apart from the efficiency, cost-effectiveness of the components is the major hurdle for the CST system to make it a viable technology. In this regard, we have implemented wet-chemical based spinel absorber coatings in a tandem layer approach to make the coating more selective in terms of high absorptance (95%), low emissivity (13%) and wide angular selectiveness (0 to 60°). The coatings are tested for thermal stability and corrosion resistance to check their stability in open-air atmosphere condition. Further, the photothermal conversion efficiencies of the developed coatings are calculated at different temperatures ranging from 300 to 500 °C by considering the actual thermal emissivity values of the absorber coating at that particular temperature.

Structural, Optical and Electrical Properties of Electron Beam Evaporated Tioxny Films as Selective Solar Absorber Coatings

Titanium oxinitride (TiOx Ny ) solar absorber coatings were deposited at different oxygen partial pressures onto Cu, Si and glass substrates using electron beam evaporation technique. XRD diffraction patterns evidenced (111), (200) and (220) orientation of TiNx phase. The preferred orientation of the films changed with oxygen partial pressure. XPS revealed the intensity of both Ti 2P3/2 and Ti 2P1/2 increases as a function of oxygen flow, and also shifted towards higher binding energy, indicating more oxidized state of Ti species than that of TiO2 due to incorporation of nitrogen atoms. Formation of uniformly distributed spherical like particles and an increase in surface roughness of the TiOx Ny films were observed as a function of oxygen partial pressure as depicted from SEM and AFM, respectively. Ellipsometric and resistivity measurements showed a shift from metallic to semiconductor behaviour of the TiOx Ny films as oxygen flow changed. A solar absorptance value of 0.94 in the solar spectrum region and a low thermal emittance value of 0.05 were achieved for the TiOx Ny solar absorber coatings prepared at the oxygen partial pressure of 7.5x10-5 Torr due to both interference and intrinsic absorption. This study confirmed that a single layer of TiOx Ny film can be a good candidate as selective solar absorber.