In this thesis, nanostructured solar selective coatings used in solar thermal collectors are explored. The nanostructured stainless steel-aluminum nitride (SS-AlN) solar selective coating, a typical solar absorber, has been studied in terms of surface morphology, optical characterization, heat treatment and calculation optimization. Additionally, the potential of using the Fe3O4 nanoparticles coating as a solar absorber has been investigated. A green chemical method of co-precipitation is employed to prepare the Fe3O4 nanoparticles. The coating of Fe3O4 nanoparticles is formed on the copper substrate using dip-coating process. The effects of the thickness, substrate roughness and heat treatment on the performance of the Fe3O4 nanoparticles coating have been then analyzed.
The nanostructured SS-AlN solar selective coatings were deposited by sputtering on the glass substrates. The detailed structure of the sample as confirmed by the TEM images is as follows: AlN (73nm)/ SS-AlN (LMVF) (20nm)/ SS-AlN (HMVF) (30nm)/ AlN (8nm)/ Cu. The absorptance denoted by a and the emittance denoted by ε are 0.9033 and 0.1317, respectively, as measured by spectrometers. After being annealed below 500°C for 1 hour, the SS-AlN solar selective coating has a higher absorptance and a lower emittance compared to the unannealed sample. Especially when annealed between 200°C and 300°C，its absorptance increases while the emittance decreases, as a result a is 0.9378 and ε is 0.11. However, an annealing process above 500°C was found to cause considerable damage on the SS-AlN samples. By using Matlab’s simulation tool, the structure of SS-AlN solar selective coating is optimized as: SiO2 (40nm)/AlN (30nm)/SS-AlN (f2=0.25, d2=30nm)/SS-
AlN (f3=0.33, d3=50nm)/AlN (8nm)/Cu. We found that a double antireflective layer helps to improve the absorptance.
The Fe3O4 nanoparticles prepared by co-precipitation are needle-like with an average size estimated around 20 nanometers. With the number of dip times increasing, the thickness of Fe3O4 nanoparticles coating is enhanced which, however, leads to an unfavorable detach of the film. In terms of the effects of substrate roughness, we found that: 1) the Fe3O4 nanoparticles coating bonded better with rough copper substrate; 2) the coating on a polished copper substrate has higher reflectance in infrared region than that on a rough copper substrate, which contributes to reduce thermal emittance. The thermal emittance of the Fe3O4 nanoparticles coating reduces after the heat treatment at 100°C, 200°C, and 300°C. We found that the optical properties of coating have been significantly improved when annealed at 100°C, i.e. a=0.926, ε=0.378. Annealed above 200°C, however, the absorptance of the coating decreases
