BLACK COBALT CADMIUM ALLOY SOLAR SELECTIVE COATING

A systematic method is described here for the development of electrodeposited selective black coatings. Electroplating bath compositions hitherto reported were empirical. But we propose here a systematic method using the Hull cell to optimize the composition of the electroplating bath. An additive had been identified and incorporated into the system to improve the quality of the black coating. The effect of the additive (1,2,3-benzotriazole) on the coating parameters are studied. A new complexing agent (ammonium acetate) other than thiocyanate usually reported for the development of selective black coatings is used in our system. The present system produced a black coating in the current density range of 6-8 A/dm2. Optimized coatings possessed a solar absorptance (alpha) of 0.96 and thermal emittance (epsilon) of 0.12. Metallic content of the coating was analysed by atomic absorption spectroscopy. Microstructure of the coating studied by scanning electron microscopy showed good grain refinement of the coating. Tape tests and thermal stability tests proved good adhesion of the coating. Salt spray analysis implied a high corrosion resistance of the coating. A stagnation temperature of 132.4-degrees-C was recorded for this coating for an average insolation of 810 W/m2. Optimized bath compositions and operating conditions of the system are reported.