Study Of Mid-High Temperature Solar Selective Absorbing Coatings Preparation By Plasma Spraying And Sol-Gel

The solar absorbing coatings with high selective efficient in mid -high temperature have been widely used on the surface of power heatabsorber and trough heatabsorbe.The using environment of solar selective absorbing coatings are decided by service ability and intrinsic property. The high selective efficient absorbing coatings in mid-high temperature usually take the well absorptivity/emissivity, thermal stability and atmosphere corrosion resistance. The traditional low temperature selective absorbing coatings were produced by electroplating, electrolessplating, brushplating and sol-gel. The coatings which were produce by those methods taking the bad thermal stability were limited in the low using environment. The mid-high temperature solar selective absorbing coatings were generally produced by magnetron sputtering and radio trequency sputtering, but those methods were limited on large scale industrialized production because of the high production cost, the high require of workpiece structure and the complex production technology. The plasma spraying technology have been widely used in the area of thermal barrier coating,abradable seal coating and corrosion-resistant coating because of its good performance of bonding force and thermal stability and corrosion resistance. But it is rarely reported to produce solar selective absorbing coatings by plasma spraying method.In this paper, we will adopt plasma spraying method to prepare solar selective absorbing coatings.The sol-gel method has been widely used in large scale industrialized production on many areas because of the simple production method and low production coast. In this paper,we will take the idea of anti reflection coating reported by many papers of solar selective absorbing coatings,The selective transmission film is produced on the plasma spraying cermet coating by the method of sol-gel processing and dip-coating technique,which could reduce emissivity and increase absorptivity.In this paper, We take the Al2O3,Cr,WC,Co as the main raw material and produce the two serials of Cr- Al2O3 and Co- WC cermet powders by spray drying method. The powders have the well fluidity and spraying performance after drying and screening. We take the plasma spraying technology to produce two series cermet on the shining stainless steel substrate and do the research of heat treatment and polishing. We take the SnCl4·5H2O pure water and NH3·H2O as the main material to prepare SnO2 sol-gel and take the dipcoating and the heat treatment method to produce the SnO2 permselective membrane.The microstructure and absorptivity,emissivity of plasma spraying absorbing coatings and composite coatings were tested and analysed, The main research content and conclusion are as:(1) The powder,plasma spraying coatings and composite coatings were XRD tested and analysed. The result revealed:the agglomerated powders were pure Cr-Al2O3 and Co-WC cermet, The phase of plasma spraying coatings did not change in contrast with powders and we prepared SnO2 permselective membrane on the plasma spraying coatings.(2) The microstructure of plasma spraying coatings and composite coatings were SEM tested and analysed. The result revealed:the plasma spraying cermet coatings were laminar structure,the surface of coatings were hillock structure with pore phase and the flakiness SnO2 permselective membrane was formed on the surface of plasma spraying coating(3) The relationship of roughness of the surface with absorptivity and emissivity were analysed,the research result revealed:absorptivity and emissivity were reduced with decreasing of roughness of the surface,but the absorptivity /emissivity were increased. The roughness of the surface was related with accidented hillock structure of surface of plasma spraying coatings(4) the relationship of absorptivity and emissivity with the content of metal were analysed,the research result revealed:the absorptivity was increased and emissivity was decreased with the content of metal decreasing. The absorptivity and emissivity related with energy gap of metal and irradiation of ceramics(5) The relationship of absorptivity and emissivity with heat treatment of plasma spraying coatings were analysed,the research result revealed:the 700-900℃heat treatment could decrease emissivity and absorptivity of coatings. The heat treatment could homogenize the surface of coating and the grain of coatings grown. (6) The relationship of absorptivity and emissivity with the SnO2 film were analysed,the research result revealed:the SnO2 film could decrease emissivity greatly,The SnO2 film took effect on permselective and filling pore of plasma spraying coatings...