Electrodeposition and characterization of nano crystalline cobalt coatings used for solar energy absorbers

Electrodeposition process is used for making a uniform and adherent covering, commonly of metal or mixes, upon a surface by acting of electrical stream. The coatings which have specific reaction to sunlight powered range are designated ^particular or selective coatings^. Expanding of the effectiveness was achieved high sunlight absorbance (α) in the obvious and close infra-red range and low emittance (ε) in the infra-red range. Also, thermal stability of the selective coating at high temperatures (300-500 ºC) should be established. Dark cobalt covering has a light absorption coefficient of 92-96% higher than dark chromium coatings with a higher warm security in task. Attributable to their high consumption obstruction, cobalt oxide mixes are utilized as securing brightening coatings for iron. Electro-deposition of nano cobalt metal on a stainless steel 304 substrate was established. Two different electrolytic bath compositions were used in this investigation. The first bath composition was [270 g/l CoSO₄, 35 g/l CoCl₂, 2.5 g/l Co(NO₃)₂ and 30 g/l H₃BO₃], and the second was as the previous without addition of Co(NO₃)₂. The role of nitrate ions as an oxidizing agent was investigated. Different current densities (20, 30 and 40 m A/cm²) were applied. The deposition times were varying from 1 min to 9 min. The optimum time of deposition was found to be 6 minutes for each used current density. Formation of cobalt oxide dark films were achieved by applying annealing thermal treatment to all test coupons after electro-deposition process by holding them inside furnace at temperature 400 degree C about 10 hours. Crystal structure transformation and morphology changes were investigated for coatings at all conditions. S E M and E D X were used for monitoring the gradual changes in the coated surface and nano structure formation. The degree of crystallinity and crystallite size changes were measured by using XRD. Surface roughness measurements were measured for all formed surfaces after each of electro-deposition process as well as annealing thermal treatment process. The optical properties such as absorption and reflectivity were investigated using UV-IR Spectrometry in order to find out the effect of applied electro-deposition conditions on the solar energy absorption of the formed surface coating. The cobalt based surface layer formed by using bath-1 conditions had higher compactness and more uniform distribution properties as compared to that which formed by using bath-2 conditions. A significant changes in the crystalline system morphology, degree of crystallinity and crystallite size, were occurred due to annealing heat treatment. The absorption coefficient value was up to 98%,at the high energy region of visible sunlight spectrum, when using bath- 1 composition (i.e. addition of cobalt nitrate) by applying the lowest value of current density (i.e. 20 m A/cm² ). As well as, absent of cobalt nitrate provide almost a stable absorption value about 96 - 97% over all visible sunlight spectrum and increased slightly near to infra-red region.