| Solar selective absorber film is a functional material that could realize the high-efficient photo-thermal conversion, regard as a kind of new energy materials. It could help mankind to utilize the solar energy in a highly efficient way, so that there is an urgent need for the research of solar selective absorbers due to today’s increasingly serious energy crisis. Nano metal ceramic composite film is currently the most widely used solar selective absorber film, mainly owin to its advantages on the spectrum performance, film quality, substrate adhesion, etc, but its poor thermal stability has been widely criticised, hindering its application on high temperature photo-thermal fileds. On the other hand, in terms of the preparation of solar selective absorber film, magnetron sputtering method, as the dominated preparation method, has been facing the issues including complex preparation process and high cost, thus it is also need to overcomed by developing new technologies.Aiming at the above problems in the field of solar selective absorber films, this thesis has successful prepared a C-Cu-TiO2 composite porous film which could be applied as an solar selective absorber film, by using the sol-gel method, combined with the ligand exchange, sol network crosslinking, induced phase seperation, Marangoni effect and vacuum annealing process. In this ternay composite film, TiO2 performs as particle-like skeleton material, coverd by carbon material, while Cu particles are embedded in the base material to form nano composite reinforced structure. Furthermore, hierarchical porous structure is formed on the surface of film, improving the optical selectivity of this ternay composite film. This thesis has focused on the sol precursor wet chemical process, annealing process and micro-nano structure tuning, explored the relationships between film structure and its optical selectivity performance and thermal stability. Finally, theoretical model has been established, helping us fully understand the mechanisms and the insighs into the optical selectivity of this ternay composite porous film.The results show that C-Cu-TiO2 composite porous film has good solar absorptivity, low emissivity and excellent thermal stability, qualified to be used in high temperature photo-thermal applications, and more importantly, its one-step preparation process is simple, low cost, controllable, repeatable. The details of research content and results are as follows:1) C-TiO2 composite film has been prepared by sol-gel method, using tetrabutyl titanate, acetyl acetone, salicylic acid and ethanol as the precursor. In this composite structure, TiO2 performs as a particle-like sketlon structure, of which the particle size is in the range from 20 nm to 55 nm, rutile phase, and the C material totally covers around TiO2 particles, mainly made up with nano-grapihte crystallines, of which the carbon sp2 clusters size is in the range of 1.1 nm-1.4 nm. In the precursor sol network, the hydrolysis and polymerization of tetrabutyl titanate result in the formation of Ti-O-Ti network; acetyl acetone could control the degree of its hydrolysis and polymerization process by passivation complexation, avoiding the cracks caused by a oversized Ti-O-Ti network; in addition, since SA anions have two functional groups with negative effective charges, the Ti-O-Ti network would be easily surrounded by ionized SA anions through electrostatic attraction extremely, forming an intimate interface.2) Annealing temperature and time of C-TiO2 composite film would impact on its film structure and photo-electric properties. When the annealing temperature ranges from 600℃ to 1000℃, the TiO2 particle size of composite film of decreases, film thickness decreases, the film resistivity drops rapidly for four orders of magnitude, the optical absorption properties of composite film gradually decrease. As the nnnealing time ranges from 1 h to 10 h, TiO2 particle size changs little, film thickness decreases before reaching 2h’s annealing process, the film resistivity gradually rises, the optical absorption properties of composite film also gradually decrease.3) By tuning the content of acetyl acetone in the initial sol, we are able to control the polymerization degree and viscosity of sol network, so as to adjust the film thickness of C-TiO2 composite film in a wide range, while keeping the composition ratio of C-TiO2 unchanged. By tuning the content of acetyl acetone in the initial sol, we are able to control the composition ratio of C-TiO2 in a wide range, while keeping the absolute content of TiO2 consistent.4) As the carbon content C-TiO2 composite film gets higher, th solar absorptivity increases slightly with the, but the emissivity is deteriorated. In terms of the C-TiO2 composite film with the best optical selectivity performance, the volume ratio of TiO2 to carbon is about 3:7, and its film thickness is about 400 nm. The results of accelerated aging test show that the C-TiO2 composite film has excellent thermal stability performance, fully meeting the aging criterion of solar selective absorber film.5) Adding CuCl2 into the precursor sol of C-TiO2 composite film, then processing the sol as film by spin coating method at room temperature, the surface structure of the film would rearrange itself under the driving force of surface tension induced by the Marangoni effect, resulting in uniform circular pore structure. In addition, the porous film presents hierarchical structure, i.e. some pores are micrometer grade but some others are only one tenth of the former pores in size. Annealing the composite film with hierarchical porous structure at 600℃ in vacuum, the porous structure of the gel film preserved. In this C-Cu-TiO2 composite porous film, C-TiO2 composite serves as base material, and some Cu particles or rich Cu oxides (Cu2O) reduced by carbon are embedded in the base material, whose size is in the range of 20nm-80nm.6) By tuning the content of CuCl2 in the initial sol, the micro-nano structure of C-Cu-TiO2 composite porous film could be tailored. Since CuCl2 is both the Marangoni effect trigger agents and Cu particle precursor material, the increase of content of CuCl2 facilitate the development of porous morphology and the increase of Cu particles. When the CuCl2 content varies from 0 to 0.6 (the molar ratio to Ti in the initial sol), the smooth surface of composite film transforms into a hierarchical porous morphology gradually, the solar absorptivity and emissivity increase slightly with the content of CuCl2. As the CuCl2 content reaches to 0.8 and 1.0, the average size of pores increases to micrometer grade, and the average depth of pores increases from 30%(CuCl2 content is 0.6) to more than 80%, almost throughout the entire film thickness. The porous structure of the film has developed too thoroughly, so that some regions of substrate has been directly exposed to the ambient, deteriorating both absorptivity and emissivity of C-Cu-TiO2 composite porous film. Therefore, the optimized C-Cu-TiO2 composite porous film is determined to the one with 0.6 CuCl2 content.7) Since there are chemical-active Cu particles embedded in the with C-Cu-TiO2 composite porous film, the thermal stability of ternary composite film has slightly deteriorated compared with C-TiO2 composite film. However, thermal stability of C-Cu-TiO2 composite porous film still fulfill the requirement of a qualified solar selective absrobers.8) Through the calculation of theoretical models, both nano-metal composite structure and light-trapping structure could enhance the optical performance of thin film, and nano metal metal composite perform better when the film thickness is lower. More importantly, combining two kinds of enhanced mechanisms, the optical performance of film would be improved much higher. The calculation results provide an important theoretical support for the preparation of complementary structure of C-Cu-TiO2 composite porous film, i.e. with both nano-metal composite structure and light-trapping structure. |
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