The Effect of Annealing under Non-vacuum on the Optical Properties of TiAlN Non-vacuum Solar Selective Absorbing Coating Prepared by Cathodic Arc Evaporation

TiAIN solar selective absorbing coatings which were deposited on 304L stainless steel using cathodic arc evaporation method were annealed under non-vacuum at different temperatures with different times. The optical properties （absorptance and emittance） of the coatings were measured by a spectrophotometer. It was found that, after being annealed for 2 hours at different temperatures, the absorptance of the coatings reached the highest value of 0.92 at 700 ℃ while the emittance got the lowest value of 0.38 at 800 ℃. When the coatings were annealed at 600 ℃ for 24 hours, the optical properties changed to 0.92/0.44 （absorptance/ emittance）. By measuring the structure, morphology, elements and surface roughness of the coatings, it was found that both the elemental composition and the surface roughness of the coatings changed as a result of annealing, and these changes caused the change of the optical properties of the coatings.

Thickness Dependence of Structural and Optical Properties of Chromium Thin Films as an Infrared Reflector for Solar-thermal Conversion Applications

The thermal emittance of Cr film, as an IR reflector, was investigated for the use in SSAC. The Cr thin films with different thicknesses were deposited on silicon wafers, optical quartz and stainless steel substrates by cathodic arc ion plating technology as a metallic IR reflector layer in SSAC. The thickness of Cr thin films was optimized to achieve the minimum thermal emittance. The effects of structural, microstructural, optical, surface and cross-sectional morphological properties of Cr thin films were investigated on the emittance. An optimal thickness about 450 nm of the Cr thin film for the lowest total thermal emittance of 0.05 was obtained. The experimental results suggested that the Cr metallic thin film with optimal thickness could be used as an effective infrared reflector for the development of SSAC structure.

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/SiO_(2) 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℃ for 1500 h in vacuum.At aging temperature of 500℃ for 1000 h or 600℃ for 300 h in vacuum,the slight inter-diffusion between Mo layer and stainless steel(SS)substrate occurs.At higher aging temperature of 700℃ 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℃for 300 h in air.A heating-cooling cycling(HCC)treatment evidences the good thermal stability of this coating working in cold environment(60℃).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.

A review of high-temperature selective absorbing coatings for solar thermal applications

Solar selective absorbing coatings directly harvest solar energy in the form of heat.The higher temperatures are required to drive higher power-cycle efficiencies in favor of lower costs of energy.According to different dielectrics,high temperature coatings can mainly be divided to double cermet solar selective coatings,transition metal nitride multilayer coatings and transition metal oxide multilayer coatings.This paper assesses the photothermal conversion efficiency and thermal stability,and discusses the challenges and strategies of improving both thermal and optical properties.Double cermet layers can stabilize nanocrystalline structures by alloying,while transition metal nitride/oxide layers generally choose the reliable materials with superior mechanical properties and thermal stability.The purpose of this review is to get the optimized systems,and propose further research directions at higher temperature,such as all-ceramic absorbing coatings.

Optical Design and Preparation of High Performance Mo/NiAlN/NiAlON/SiO_2 Solar Selective Absorbing Coating

A new solar selective absorbing coating of Mo/NiAlN/NiAlON/SiO_2 multilayer was prepared on stainless steel(SS) substrates by magnetron sputtering for solar thermal power applications. The optical constants and thickness of individual layers were simulated using the Scout software based on the experimentally measured reflectance and transmittance spectra of individual layers. The coating of Mo/NiAlN/NiAlON/SiO_2 with an ideal solar absorptance(α) of 0.945 in the solar spectrum range was designed via the optical constants of each layer in the Scout software. The Mo/NiAlN/NiAlON/SiO_2 coating was deposited via the optimized layer thickness. A good spectral selectivity with absorptance(α=0.936) and emittance(ε=0.09, T=80 ℃) was obtained. This method, which incorporates the optical simulation with the related experiments, provides a convenient approach to obtain the ideal solar selective absorbing coatings.

Preparation of solar selective absorbing CuO coating for medium temperature application

A new method of preparing CuO solar selective absorbing coating for medium temperature is presented.After pretreatment,brass was overlaid with CuO by chemical plating.The effects of reactant concentration,reaction tem-perature and reaction time on the absorptivity of CuO coating were investigated.The optimized condition of preparing CuO coating was obtained.The CuO coating was analyzed with X-ray photoelectron spectroscopy(XPS)and scanning electron microscopy(SEM).In order to prolong the period of use,the CuO coating was protected by TiO2.The experi-ment shows that the TiO2/CuO coating is more heat-resistant,acid-resistant,and wear resistant than CuO coating,without losing absorptivity markedly.The TiO2 coating can reduce emissivity and protect the CuO coating.

A novel multilayer high temperature solar absorber coating based on high-entropy alloy NbMoTaW:Optical properties, thermal stability and corrosion properties

With the development of new materials and technology,high entropy alloy(HEA)nitride films have attracted much attention of researchers due to their excellent optical properties and mechanical properties.Herein,a novel SS/NbMoTaWN(HEAN)/NbMoTaWON(HEAON)/SiO_(2) coatings are prepared,which shows a high spectral selectivity of a/ε=0.944/0.12.The preparation and optimization of the coating are studied by combining experiments with ellipsometric program and CODE software.High temperature thermal stability test is performed in depth,which proves that the coating could bear 400℃ in air for 2 h,and 600℃for 2 h in vacuum.Long-term thermal stability researches indicate that the SSACs still keep good optical properties(a=0.902,ε=0.106)even after annealing at 600℃ for 100 h.The failure mechanism is analyzed by XRD and Raman spectra.In addition,neutral salt spray test is performed to investigate the anti-corrosion ability,which indicates the coating has a good optical performance after soaking in 3.5 wt%NaCl solution for 30 days.Obviously,this work provides a new strategy to construct solar absorber coatings based on NbMoTaW high entropy alloy.

Greatly enhanced solar absorption via high entropy ceramic AlCrTaTiZrN based solar selective absorber coatings

High entropy alloys(HEAs),which is at the expense of high cost compared to traditional alloy,should not be confined to the mechanical properties,but should be employed to devise a novel combination with unique functional and mechanical performances.In this work,high entropy alloy nitride(HEAN)is utilized as a novel double absorption layer to improve solar absorption in the high temperature solar selective absorbing coatings(SSACs).Our primary motivation is to lower thermal emittance(ε)and enhance solar absorptance(a).In order to realize this goal,coating design(CODE)software is employed to design and optimize the proposed HEAN based SSACs using appropriate dielectric function model.The ultimate as-deposited coating shows good optical properties with a high a value of 0.965 and a lowεvalue of 0.086(at 82C).The estimate of thermal stability tests indicates that HEAN based SSACs has the ability to resist instability in high working temperature,which keeps good optical properties(a¼0.925,ε¼0.070)after annealing at 600C for 10 h.