Preparation And Properties Of Co-Cr₃C₂-WC High Temperature Cermet Spectral Selective Absorbing Coating

Concentrating Solar Power technology(CSP)is one of the most widely ways used for solar energy utilization.As the key component of the heat collection system in CSP,the operating temperature and optical properties of the solar selective absorbing coating directly affect the energy conversion efficiency of the system.The efficiency of solar thermal power generation will become higher as the solar absorbing coating can withstand higher temperature.The focus and difficulty of future research on solar selective absorbing coatings is how to maintain stable optical performance at high temperatures.In this paper,the SS/Co-Cr₃C₂-WC/Al₂O₃ solar selective absorbing coating was prepared using high temperature stable cermet materials and a combination of high velocity oxygen-fuel(HVOF)spraying technology and sol-gel methods.The optical properties and high-temperature thermal stability of the coating were systematically studied.In this study,five groups of Co-Cr₃C₂-WC agglomerated powders with different proportions were prepared by spray granulation.The powders were sieved and calcined before used for spraying.The phase,micro morphology and process performance(fluidity and bulk density)of the agglomerated powder were tested and analyzed.The results showed that the agglomerated powder consisted of Co skeleton,Cr₃C₂ and WC particles uniformly adhere to the powder surface,and had a good spherical shape.The powder with a particle size ranging from 40 to 60?m was selected through sieving.The powder had excellent spraying properties after a calcination process at 400?with protective atmosphere,which was conducive to the preparation of high-quality coatings.Co-Cr₃C₂-WC cermet absorbing layers were prepared by HVOF spraying.First,the influence of coating structure,morphology and properties at different Cr₃C₂/WC ratios was studied by analyzing the coating phase,surface morphology,element distribution,and optical properties,the ratio with optimal properties was obtained.Then the content of metal Co was reduced to further reduce the emittance of the absorbing layer and improve the optical performance of the absorbing layer.The results show that when the Co content is 80 wt.%,The absorptance of the coating decreases with the decrease of the Cr₃C₂/WC value,and the emittance decreases first and then increases.When the Cr₃C₂/WC is 1:1,the coating has the highest absorptance/emittance of 0.828/0.47.Then fixing the composite ratio of the ceramic phase,when the Co content is increased to 90%,the emittance of the90Co-5Cr₃C₂-5WC absorbing layer is reduced to 0.308,the absorptance is 0.805,and the quality factor is increased to 2.61.Considering the intrinsic contribution of each component to the optical performance of the coating and the influence of the state of the coating surface on the emittance,the ratio with the highest optical performance of absorbing coating is 90Co-5Cr₃C₂-5WC.Al₂O₃ anti-reflection layer was prepared on the surface of the A0 absorbing layer with the best optical properties by a sol-gel spin coating method.After spin-coating and heat treatments,the absorptance of the C0 coating was greatly increased,and the emittance was also reduced.The final absorptance of the C0coating was 0.903 and the emittance was 0.278.The optical performance of the coating was stable after heat treatment at 600?for 100 h in the atmospheric environment,which met the stability requirements at high temperature.Al₂O₃anti-reflection coatings increase the absorptance of composite coatings by increasing the transmission of short-wavelength light and sealing effect,which improves the surface state of coatings and improves the high-temperature service performance of coatings.It is expected to be the selective absorbing coating material in medium and high temperature solar heat collection systems.