Thin Film Hybrid Ceramic-Polymeric Low Cost Solar Absorber
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Thin Film Hybrid Ceramic-Polymeric Low Cost Solar Absorber Edgar A. Chávez-Urbiola and Juan F. Pérez-Robles CINVESTAV-Querétaro, C.P. 76230 Querétaro Qro., México
ABSTRACT Different hybrid Ceramic-Polymeric coatings were prepared, from a suspension consisting of a mixture of tetraethyl orthosilicate (TEOS) as a silicon dioxide precursor, polyvinyl acetate (PVA) and colorant to obtain sol-gel SiO2-PVA thin films. The films were prepared using Sol-Gel technology, applied by dip-coating technique. In order to determine the optimal formulation, different samples varying the proportion of PVA were prepared and evaluated. Both optical and mechanical properties were tested, finding an optimal value of 30 percent of PVA for the mechanical properties, and a value of 50 percent for the optimal optical properties. In both cases, the coatings made can be considered as a reasonable alternative for use as a Solar Absorber for Low-Mid range temperature, with a smaller thickness than the comparable commercial coating but with similar performance and lower cost. INTRODUCTION In the last decades, the use of solar energy has shown a remarkable increase due to the necessity of alternative power sources, as a result of the scarcity of fossil fuels and their constantly increasing costs [1]. The use of solar absorbers to convert sunlight to thermal power is one of the more affordable and economical way to use solar radiation [2]. The solar absorbers in systems like solar water heaters have been used for decades and now form a solid part of the market of renewable energy devices. It is also important to mention that there are many different techniques, configurations and materials that are suitable for the fabrication of solar absorbers [3]. The most common materials used in the mid-temperature range (T9H >9H 5B 0B F2 20 5H 4H 5B 1B F3 30 9H 9H 5B 5B F4 40 9H 3H 4B 1B F5 50 9H 9H 4B 2B
The adherence was more affected by degradation process than the hardness. The Figure 1 shows the images for the different films after the adherence test in two columns. Left side of each of the two columns presents the sample before degradation and after degradation in the right side.
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Figure 1. Different samples after the adherence test, undegraded and degraded. What is expected is that a lower content of PVA leads to a more ceramic film, thus more fragile and also harder, but it is not clear in Table I nor Figure 1. Figure 2 shows different micro photographs, revealing that there exist cracks on the surface for some of the formulations that are the cause of loosing of the mechanical properties, which agrees with the results of the Table I.
Figure 2. Micro photographs of surface cracking of the different coating formulations (1000X). Also the changes in optical absorbance were analyzed before and after degradation process. In Figure 3 the obtained data is shown. Before degradation (Left), F5 and REF are overlapped, but after it the reference material absorbs less than the REF one. F3 shows little decrease in absorbance
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