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Abstract
The encountering problems of energy shortage and environmental pollution lead to the attention toward the limitless amount of clean and renewable energy received from the sun in the form of solar radiation. For efficient photothermal conversion the solar absorber surfaces must have the combined properties of having high absorptance (?) and low infrared emittance (?) at the operating temperatures. This is achieved through black selective coatings.
In this work electrodeposition technique was used to produce two types of selective black nickel coatings; the first sulfur–bearing coating from solutions containing Ni-sulfate, Zn-sulfate and thiocyanate and the second phosphorus–bearing coating from a hypophosphite bath.
The objective of this research is to obtain new selective black nickel coatings with improved properties against environmental degradation. Evaluation of optical properties of both sulfur- and phosphorus-bearing coatings was presented. A selectivity (S) value of 15.5 and 7 was achieved for sulfur and phosphorus bearing coatings, respectively. In spite of the high selectivity of sulfur–bearing coating, its endurance at elevated temperatures and highly humid atmospheres is rather poor due to the oxidation of metallic species.
The black coloration of sulfur–bearing coating was attributed to the deposition of Ni and Zn sulfides (NiS2, ZnS) in conjunction with free Ni, free Zn and intermetallic Ni-Zn (Ni3Zn22). On the other hand, the black coloration of the coatings bearing phosphorus was due to the formation of NiP2 phase with free Ni and Ni–Zn compound (Ni3Zn22). In either case, the structure of the as- deposited coating is amorphous or microcrystalline in nature which is transformed to crystalline state after heat treatment at 250o C for one hour.
The new P-bearing coating showed good thermal stability at 250o C and excellent endurance in severe conditions of humidity and high temperatures.