الفهرس 
ContentsOnly 14 pages are availabe for public view
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Abstract
In the last decades, many drastic efforts have been undertaken to attain solar selective absorber coatings with high thermal stability and performance for better solar energy capture. Nanomaterials that are attached at the back-side, front-side, or inside of laminated absorber coatings play a determinative novel role, thus improving the thermal performance of solar harnessing systems. To cite a general example, a deficiency of stationary non-concentrating solar power technologies, as one of the potentials to eliminate traditional fossil fuels, is the lack of high performance absorbing materials at high temperatures. Such enhancement is an obviously requirement for tracked concentrating solar collectors as well. In this thesis, Nanometer polymeric composites were prepared as selective coatings for absorbing solar thermal energy. These composites are silicone rubber embedded with nanoparticles of Co3O4, Fe3O4, and Graphene at different concentrations (0, 0.05, 0.25, and 1 wt. %). The characterization of the samples was studied using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), infrared spectroscopy (FT-IR), and electron microscopy (SEM). Thermal stability analysis was conducted using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The optical properties were studied using spectrophotometers UV-VIS-NIR. The XRD results showed diffraction peaks at peak angle ϴ≈22 for Co3O4/SiR, Fe3O4/SiR, and G/SiR, which did not change with nanoparticle concentrations. The surface morphology of the samples showed no aggregation and no cracks or pores between the nanoparticles and the polymer. The FT-IR spectrum reveals characteristic peaks in the silicone rubber matrix, including stretching vibrations from Si-H bonds, stretching vibrations from C-H bonds, bending vibrations from Si-O bonds, and Si-Si vibrations. The thermal analysis showed that the silicone rubber is thermally stable up to 350°C, with higher thermal stability due to the presence of nanoparticles. The composites decompose to Co2O3 after 450°C, while Fe3O4 and Graphene decompose at 800 and 890°C, respectively. Differential scanning calorimetry (DSC) showed no endothermic or exothermic thermal peak, and no phase shift occurred in temperature from room temperature to 300°C. The specific heat of the prepared composites decreased with increasing nanoparticle concentration, with Fe3O4 having the lowest values. The thermal conductivity of the prepared samples was measured using the Steady State Method (Lee Method) It was found that by increasing the concentration of nanoparticles in silicone rubber matrix, the thermal conductivity increased and G/SiR has the highest values of thermal conductivity due to the properties of graphene. The UV-Vis-NIR spectrophotometer revealed excellent average solar thermal absorption α = 86.99% for Co3O4/SiR composites and α = 91.87% for Fe3O4/SiR composites. The contact angle of water on SiR / Fe3O4 film was found to be approximately 123o, indicating hydrophobicity and dust-preventing properties. The photothermal conversion efficiency of the solar selective coating was studied using a solar simulator on a tube filled with second distilled water coated with a Fe3O4/SiR film. The coated tube absorbs heat at a better rate than the uncoated tube, resulting in a 30% higher internal temperature and preservation of heat.