Sol-Gel Protective Coatings For Black Chrome Solar Selective Films

Abstract Electro-deposited black chrome solar selective films degrade rapidly when heated to tem­ peratures above 300°C in air. The application of sol-gel protective coatings to the black chrome films has been investigated as a possible means to improve the oxidation resistance of black chrome at high temperatures. The sol-gel coating process consists of applying an alcoholic solution containing polymeric glass precursors. After the coating is fired for about one-half hour at a moderate temperature (~450°C), a glass layer is obtained. Because of the wide range of sol-gel processing parameters, initial studies have concentrated on determining the effect of the following variables on the thermal stability of sol-gel coated black chrome: (1) sol-gel composition; (2) firing temperature; (3) firing atmosphere; (4) sol-gel coating thickness; (5) pre-aging of the black chrome films. Of the compositions studied, only SiO2/B2C>3 and SiC>2 /B2O3/Al2C>3/Na2O/BaO resulted in improved thermal stability. Of the other processing parameters, the firing temperature had the most significant effect on the resulting black chrome thermal stability, with 500°C firing being optimum. For the best combination of process variables studied to date, the solar absorptance of a sol-gel coated sample decreased from 0.97 to 0.95 after 100 hrs at 400°C, while, for an uncoated control sample, the solar absorptance decreased to 0.89. For most of the sol-gel coated films, continued aging at 400°C resulted in continued protection of the black chrome film.*IntroductionElectrodeposited black chrome solar selective films are currently the primary candidate for use in parabolic trough solar concentrators1 that operate near 300°C. Recent studies2 have shown that the thermal stability of the black chrome films that are electroplated from Harshaw Chemical Co. Chromonyx bath ^ is a strong function of both the bath chemistry and contaminants in the bath. Degradation of films deposited onto nickel substrates results in a rapid decrease in the solar absorptance by ~10%. Investigation of changes in the film microstructure and composition with thermal aging have shown that metallic chromium (Cr) within the film oxidizes to form C^O^. Therefore if the rate of oxidation of the metallic chromium could be reduced through the use of a protective coating, the useful lifetime of the black chrome film may be extended or the maximum permissible operating temperature might be increased.Suitable protective coatings would include inorganic oxides that are stable at high tem­ perature, such as SiO2» TiO2, MgO, A12O3, etc. There are two areas of concern in using such a protective coating. First, the coating itself may adversely affect the optical properties of the black chrome film and thereby reduce the collector operating efficiency. This can occur either because of a reduction in the solar absorptance (probably caused by increasing the reflectance at the coating/black chrome interface) or because of an increase in the emittance (probably caused by increased infrared absorption in the coating). Thus the thick­ ness of the protective coating may have to be carefully controlled. The second area of concern involves the cost and ease of application of the coating to the solar collector receiver surface. In the case of Sandia's parabolic collector, the receiver is a ~3.6 m long tube, approximately 33 mm in diameter.A coating technique that allows various inorganic oxides to be deposited on large surfaces at relatively low cost is the sol-gel process. In this process, glasslike macromolecules are formed in solution at room temperature by chemical polymerization.^ Before the solution transforms to a stiff, amorphous gel, it can be diluted and applied to a surface by dipping, spraying or other thin film coating techniques. After drying, the porous gel coating is heated to a moderate temperature (less than the softening point) where it converts to a dense, transparent glass layer.*This work was supported by the Division of Solar Energy, U. S. Department of Energy (DOE),under contract DE-AC04-76-DP00789. **A U.S. DOE facility.