Solar Selective Coatings: Industrial State-of-the-Art
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Solar selective absorber materials have been the focus of much research and development over the last 60 years as the functionality of solar thermal collection devices relies considerably on these coatings and their operation and deposition specifications are complex. This article reviews the solar selective absorber coatings which have been patented and covers a wide variety of materials and techniques including metal/ceramic composites (cermets), semiconductors, paint coatings, electroplating and vapour deposition processes. The cermet class of materials has emerged as the most utilised with the recent selective coatings based on nanoscale particulates. As the operating temperatures in solar devices increases so demands on reliability, thermal stability and durability continue to grow. Furthermore, there is pressure to reduce costs and the advances in nanoscale technologies and thin film deposition seem to offer the greatest potential for new selective coatings.Keywords:
Cermet
Deposition
Three kinds of Cr_2O_3 ceramic coating structure,i.e.,two layer,three layer and five layer ceramic coatings were prepared by plasma spraying.Effect of coating structure on the coatings microstructure,bond strength,the thermal schock and wear resistances were investigated.The results show that the multi-layer ceramic coatings can increase the bond strength,and the resistance to thermal shock and wear.Among the three kinds of multi-layer coating,the five-layer coating is the best one.The microstructure observation and the micro-hardness measurement show that the five layer coating has the characteristics of gradient variation,which means that the gradient ceramic coating could be prepared by plasma spraying through gradient change of the composition from the substrate to the surface layer.
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Thermal spray coating process is a surface modification technique in which a coating material like cermets, metallic, ceramic and some other materials in form powder are feed into a torch or a gun, the powder inserted into torch will be melted by high temperature developed by torch. Coating thickness can achieve by applying multiple layer of melted coated material. This paper aims at the study of mechanical characterization of thermal spray single layer and multi-layer coatings. Coatings on SS 316L is followed by the wear test .It has been found that the wear rate of base metal i.e. SS 316L is more than single layer and multi layer coatings. The multilayer has shown the maximum resistance to the wear rate.
Cermet
Torch
Characterization
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Plating (geology)
Deposition
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There is an increasing interest in the last years for materials with nanometric grain size because of the enhanced properties that could be achieved when reducing to the nanometric scale. Three coatings using conventional, nanostructured and bimodal (mixture of conventional and nano) WC-Co powders were obtained through High Velocity Oxygen-Fuel (HVOF) technique. The powders were sprayed under different spraying conditions in order to improve trybological properties for nanostructured powders. The relationship between spraying conditions and decomposition has been studied using in-flight measurements and XRD techniques. The nanostructured coating showed more decomposition than the other ones during spraying process, but this decomposition was reduced through the new conditions. Nanostructured coating showed more hardness using producer conditions, but the bimodal coating showed better abrasive and friction wear resistance. Nanostructured and bimodal coatings also provided an enhanced corrosion protection to the substrate when compared with the conventional one. An important improvement in the abrasive wear resistance for the nanostructured coating was obtained with the new conditions, due to the reduction in the decomposition.
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Herein, heat transfer from the coating to the substrate during the thermal spraying process is simplified as one-dimensional heat conduction and a formula to express the temperature distribution in the substrate is provided. To achieve this, the spray process was divided into two stages, namely deposition (coating sprayed onto the substrate) and post-deposition (cooling of coating and substrate to atmospheric temperature). The coating was achieved through a layer-by-layer deposition method. Residual stresses in the system (including both the coating and substrate) following deposition of each layer were calculated, as well as those induced by post-deposition. Finally, the proposed formulae were implemented in a real-case example to illustrate the effect of heat transfer with regards to torch velocity on residual stresses. The simulative results were shown to have a better agreement with experimental results at low rather than at high torch velocities. The residual stresses in the coating surface decreased with the increase in heat transfer time. When the heat transfer time exceeded a certain value, a sharp decline in residual stresses was observed.
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Nichrome
Cermet
Titanium carbide
Decarburization
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Thermal spray coating process is a surface modification technique in which a coating material like cermets, metallic, ceramic and some other materials in form powder are feed into a torch or a gun, the powder inserted into torch will be melted by high temperature developed by torch. Coating thickness can achieve by applying multiple layer of melted coated material. This paper aims at the study of micro structural characterization of thermal spray single layer and multi-layer coatings. Coatings on the substrate were followed by Scanning electron microscopy and X-ray diffraction to know the different phases present in the coated as well uncoated SS 316L. By seeing SEM result it’s found that single layer coating is not done properly. As compared to SEM result of single layer coated AISI 316 L multilayer SEM results is more accurate, there is no crack on the coating surface and there is much less porosity in the multilayer coated sample. AISI 316L contain Nickel (Ni) and Chromium (Cr) as major phase.
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Cermet
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This paper describes the various coatings used for absorber tube of Parabolic Trough Collector (PTC) and their properties. Solar thermal selective absorber coatings are currently characterized by their solar absorptance and their thermal emittance. Mostly coating is by electroplating, paint coatings and deposited cermet. This paper mainly focus on properties of Black chrome, Black Nickel , Black cobalt, Black-colored CuFeMnO4 spinel powder, Thickness-sensitive spectrally selective (TSSS) paint coating, spray-coated graphitic films and there manufacturing process. The coatings need to be stable in air in case the vacuum is breached. Current coatings do not have the stability and performance desired for moving to higher operating temperatures.
Parabolic trough
Cermet
Chrome plating
Selective surface
Absorptance
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Thermal spray coating process is a surface modification technique in which a coating material like cermet’s, metallic, ceramic and some other materials in form powder are feed into a torch or a gun, the powder inserted into torch will be melted by high temperature developed by torch. Coating thickness can achieve by applying multiple layer of melted coated material. This paper aims at the study of microstructural characterisation of thermal spray single layer and multi-layer coatings. Coatings on the substrate were followed by Scanning electron microscopy to know the different phases present in the coated as well uncoated SS 316L. By seeing SEM result it’s found that single layer coating is not done properly. As compared to SEM result of single layer coated AISI 316 L multilayer SEM results is more accurate, there is no crack on the coating surface and there is much less porosity in the multilayer coated sample.
Cermet
Torch
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A novel alumina-assisted treatment process was proposed to manufacture fully densified and perfectly spherical WC-Co particles for thermal spraying applications. In as-sprayed coating, the decarburization phenomenon common in previous studies was almost entirely eliminated, and a unique binder structure with nanocrystalline and amorphous Co co-existing was obtained. Owing to the strengthening effects of nanocrystalline Co and WC/Co interfacial bonding, the wear resistance of the new coating was 4 times higher than that of the conventional coating prepared with a porous feedstock. Instead of plastic deformation, interface fracture and micro-cutting that mainly occurred in conventional WC-based coatings, fracture of WC grains and subsequent oxidation dominated wear failure of the new WC-Co coating. The present work opens up new opportunities for the fabrication of dense ceramic-metal composite particles and wear-resistance enhancement of cermet coatings.
Decarburization
Cermet
Nanocrystalline material
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