Preparation And Properties Of CrON Based Solar Selective Absorbing Coatings

Solar selective abosrbing coatings is the core component of solar thermal conversion due to the advantages of clean and pollution-free,low cost and high efficiency,which also make the solar thermal conversion become one of the main ways for solar energy utilization.However,the solar spectrum selective absorbing coating currently faces the following problems:?1?the coating has significant elemental diffusion and oxidation at high temperatures;?2?the amorphous or nanocrystalline structure in the coating will crystallize or recrystallize at high temperatures to change the microstructure of the coating;?3?the difference in thermal expansion coefficient between the coating and the substrate material leads to delamination,cracking and spalling of the film layer;?4?the corrosion resistance of the coating needs to be improved.For the problems that the solar selective absorbing coatings faces the phenomenon of element diffusion,oxidation,cracking and shedding of the coating at high temperatures,weak corrosion resistance in the salt spray environment,as well as its physical and chemical mechanisms are difficut to reveal,we focus our attention on transition metals?Cr?and their oxides and nitrogen oxides because of their high resistance to high temperature oxidation.In addition,the band gap of such materials can be effectively controled by controlling the content of oxygen,nitrogen or other doping elements,which results in that the materials have good solar absorptance.At the same time,we will also focus on the physical and chemical mechanisms of the structural and compositional changes of the coatings caused by the diffusion and oxidation of the elements at high temperatures and in a neutral salt spray environment,providing experimental and theoretical for subsequent optimization or development of solar selective absorbing coatings that with superior thermal stability and corrosion resistance.However,the physical and chemical mechanisms of some transition metals and their oxides,nitrides,and oxynitrides are not clear in terms of element diffusion,oxidation,coating cracking,shedding,and weather resistance under salt spray conditions.In this paper,metal-chromium and its oxides and nitrides are used as light-absorption layer,and the Al/CrO_x/CrO_x N_y/SiO₂?abbreviated as CrON-based?multilayered cermet solar spectrum selective absorption coating prepared by magnetron sputtering serves as the research vehicle.Meantime the physical and chemical mechanisms for exploring the degradation of optical properties under high temperature and salt spray environment are taken as research targets.The research carried out includes the following four main aspects:?1?Design and preparation of CrO_x and CrO_x N_y absorber layers;?2?Photothermal properties of CrON-based solar selective absorber coatings;?3?Thermal stability study of CrON-based solar spectrum selective absorbing coatings;?4?Study on corrosion resistance of CrON-based solar selective absorption coatings.Using exploratory research methods combined with quantitative and qualitative analysis,the following results and conclusions have been obtained:?1?In CrO_x and CrO_x N_y films,when the oxygen content is high,the main phase of CrO_x and CrO_x N_y films is Cr₂O₃;when the oxygen content is low,the main phase is Cr,and when the oxygen content is in the middle,it consists of Cr₂O₃ and Cr.In the CrO_x N_y thin film,there is also a trace of CrO₂N₆ crystal phase.In addition,as the oxygen decreases,the refractive index and extinction coefficient of the CrO_x and CrO_x N_y films decrease gradually,exhibiting typical dielectric properties.?2?The optical absorption of the prepared Al/CrO_x/CrO_x N_y/SiO₂ cermet coating is 0.947,and the thermal emissivity is 0.05?100°C?.The CrO_x layer is the main light absorbing layer;the distrubution of the obtained surface grains size of the coating are uniform.No obvious defects are observed except for a small amount of crystallites.There are a large number of amorphous structures in the coating.There are also crystal phases with different sizes and distributions,such as Cr₂O₃,Cr,CrO₂N₆ and so on.At the same time,there are different degrees of lattice distortion in these crystalline phases.?3?According to the EN ISO 9806-2013 standards,Al/CrO_x/CrO_x N_y/SiO₂ cermet coating has good thermal stability,can be used for the solar energy spectrum absorber layer flat plate solar collector;while after accelerated aging at elevated temperature in the experiment,different degrees of element diffusion and oxidation occurred in the as-deposited and annealed interfaces of the coating.The oxidation products are mainly Cr₂O₃ and Al₂O₃;in addition,the defects of the coating itself,such as microcracks,holes,dislocations,and stacking faults,will greatly promote the diffusion and oxidation of the elements under high temperature conditions.?4?The corrosion process of Al/CrO_x/CrO_x N_y/SiO₂ coating in salt spray environment is that the Cr,Al and other metal particles in the coating are dissolved as the anode;at the same time,chloride ions easily converte the insoluble materials such as Al₂O₃,CrO_x,and CrN_x O_y in the coating into soluble chlorides.The obtained Cr³⁺and Al³⁺react with the OH^-ions that dissolved from the oxygen to form the corresponding pro-oxide.When the Cr?OH?₃ and Al?OH?₃ are dehydrated,the corresponding corrosion products are obtained,and the corrosion products are easily accumulated on the layer surface and sublayer interface of the coatings.Morphology shows that with the increase of salt spray corrosion time,the surface roughness of the coating gradually increases,and small crystal grains and amorphous phase gradually disappear,leaving more micro-pits,micro-holes and micro-cracks.?5?After the Al/CrO_x/CrO_x N_y/SiO₂ coatings were corroded by sodium chloride salt sprays of1.0wt%,3.0wt%and 5.0wt%for 96h at 35°C,the absorpttance decreased by 0.040,0.059,and0.115,,and the emittance increased by 0.098,0.221,and 0.381,respectively.The change in light absorption of the coating during salt spray corrosion can be described by an empirical formulaY=y₀+Ae^-t/B.The change in heat emissivity can be described by an empirical formulaY=y₀+B₁ t+B₂ t²+B₃ t³.