Preparation And Study On Co-WC-M₂O₃ Composite Cermet Solar Selective Absorbing Coating

Photo-thermal conversion technology is one of the most important ways for human to use solar energy.Solar selective absorbing coating is a key component in concentrating solar power?CSP?systems,and its performance will directly affect the efficiency of energy conversion.Solar selective absorbing coating,especially those applied in mid-and high-temperature,the coating should possess two characteristics,high solar selectivity and good thermal stability.With the research on solar selective absorbing coating,it is an important trend to prepare the coating that have excellent optical properties,high thermal stability,good weather resistance and can large area production at the same time.In this paper,taking into consideration account preparation cost and service environment of coating,high velocity oxy-fuel spraying process?HVOF?were used to prepare Co-WC based absorbing coating and sol-gel method were used to prepare sealing and anti-reflection coating.Multi-layer composite structure were applied to improve optical properties of the composite coating.In this paper,the Co-WC based cermet composite powders were prepared by combined spray drying granulation process and calcination method with atmosphere protection.The micro-structure of the composite powder before and after calcination process were explored.Results shows that the agglomerated powders has not been oxidized and carbonized after calcined at 1000?for 2h with argon gas protection.Moreover,the micro-morphology of composite powder after calcination was based on spherical frame formed by Co particles,and the WC and M₂O₃ particles are filled in pores or attached to surface.Calcination treatment was helpful for the compaction of powders.Furthermore,the physical properties,phase structure and morphologies of the six kinds of sprayed powders were systematically investigated.Co-WC-M₂O₃ cermet composite coating were prepared by HVOF process.First,the effect of the addition content of Al₂O₃ on properties of Co-WC-Al₂O₃ coating were explored.The phase structure,the surface and cross-section morphologies and optical properties of Co-WC-Al₂O₃ coating were systematically investigated.Results shows that the Co-WC-Al₂O₃ coating exhibit an excellent solar selectivity when the addition content of Al₂O₃ was 10wt.%,the absorptance/emittance was 0.84/0.363.Moreover,three addition materials?Al₂O₃,Cr₂O₃,Mn₂O₃?were individually added to Co-WC based coating to improve optical properties of composite coating,and then the behavioral changes of coating were discussed.The results indicates that the influence were different on solar selectivity for each of three addition materials.The absorptance of composite coating were increased when Al₂O₃ or Cr₂O₃ particles were added to Co-WC materials,while the emittance of composite coating were also increased when Cr₂O₃ or Mn₂O₃ particles were added to Co-WC materials.In a whole,Al₂O₃ was the best addition materials for Co-WC-M₂O₃ coating in this three materials.A3 coating with good solar selectivity were selected as absorbing bottom layer,and Al₂O₃ sealing coating and SiO₂ anti-reflection coating were prepared by sol-gel method in A3 coating surface.Results shows that the optical properties of composite coating have an greatly increase after the sol coating were added,the absorptance/emittance was 0.90/0.308.The cracks and spallings phenomenon has not been found of the composite coating after 100 times thermal shock experiment under the temperature of 600?,and the coating exhibited good thermal shock resistance ability.