Study On The Preparation Of High-temperature Solar Selective Coatings Based On Duplex Ceramics Of ZrB₂-Al₂O₃

As one of the chief ways in which solar energy is exploited,the key material basis of solar thermal conversion is solar absorber film,and a solar absorber film with excellent performance can increase the efficiency and lifetime of power generation systems.The results of the study show that increasing the operating temperature of solar thermal power can increase the efficiency of power generation and reduce the cost of electricity,but the high-temperature working conditions put more demanding requirements on the high-temperature resistance of solar absorber film.Therefore,it is very important and urgent to explore and develop new types of high-temperature resistant solar absorber films.In this thesis,ZrB₂,a refractory metal boride,was selected as the photon absorber layer,and W IR reflector layer,ZrB₂ absorber layer,and Al₂O₃ reflector layer were sequentially deposited on 321 mirrors stainless steel substrate by magnetron sputtering coating technology to form a ZrB₂-Al₂O₃ duplex ceramic composite solar absorber film.With the help of XPS,XRD,FESEM,AFM,four-probe resistivity test system,UV-VIS-NIR spectrophotometer,and Fourier infrared transform instrument,the effects of sputtering power,working air pressure,substrate temperature,and other coating processes on the microstructure,electrical properties,and optical properties of the ZrB₂ film were investigated,and the structural changes of the ZrB₂-Al₂O₃ biceramic composite solar absorber film were analyzed.Finally,the thermal stability of the ZrB₂-Al₂O₃ biceramic composite solar absorber film in a vacuum environment was studied.The findings reveal that the deposition rate of ZrB₂ films is linearly increasing with the sputtering power,while the deposition rate tends to increase and then decrease as the working air pressure increases.At the sputtering power of 100 W and working air pressure of 0.4 Pa,the structure of ZrB₂ film is completely amorphous.With increasing sputtering power and working pressure,the film gradually transitions from an amorphous to a crystalline structure at a power of 300 W and a working pressure of 0.8 Pa,the film is completely in a nanocrystalline state.The cross-sectional morphology results show that as the sputtering power increases from 100W to 400 W,the ZrB₂ film gradually shows a looser columnar structure from dense without obvious features.The results of XPS analysis showed that all prepared ZrB₂ films showed a B-rich state,i.e.,the B/Zr atomic ratio was slightly higher than its stoichiometric ratio of 2.The resistivity test results show that the resistivity of nanocrystalline ZrB₂ films was lower than that of amorphous ZrB₂ films.The resistivity of both nanocrystalline ZrB₂ films and amorphous ZrB₂ films decreased with increasing temperature in the temperature range of 77-600 K.Moreover,the resistivity decreased slowly below 500 K,and decreased sharply above 500 K.The results of the optical performance study show that the deposition of W film with thickness over 120 nm on the stainless steel substrate can significantly improve the reflectivity of the stainless steel substrate in the near-infrared band and play a role in suppressing thermal radiation,thus reducing the energy loss due to thermal radiation and improving the photothermal conversion efficiency of the solar absorber film.The ZrB₂ film superimposed on the W film has a good absorption effect on the incident sunlight,but as the thickness of the ZrB₂ film increases above 40 nm,the absorption capacity of the ZrB₂ film gradually tends to saturate,and the absorption rate value can reach 67.1%.Further re-deposition of Al₂O₃ film on the surface of ZrB₂ film as anti-reflective layer can significantly weaken the reflection of incident sunlight by the composite coating,and at the same time,the solar absorption of the composite coating increases to 89.7%due to the interference phase extinction between the absorbing ZrB₂ film and the antireflective Al₂O₃ film;when the absorbing layer is increased to a double layer of ZrB₂,the solar absorption of the composite coating can be further increased to 92.1%.After the sample was subjected to vacuum heat treatment at 550?×96 h,the absorptance value was slightly reduced to 91.7%,and the emission ratio increased to 0.126 at550?,but the durability evaluation factor PC=0.0055,which is much smaller than the threshold value of PC?0.05,indicating that the ZrB₂-Al₂O₃ double-layer coating prepared in this study has a higher absorption rate than that of the composite coating.ZrB₂-Al₂O₃ biceramic composite absorber film prepared in this study can meet the requirement of a long-time continuous thermal power generation use under a high-temperature vacuum environment at550°C.