The effect of annealing temperature on selective solar absorptance of Ni-Al coating prepared by flame spray technique

Abstract

Ni-Al coating has been considered a promising candidate as a solar absorber material in solar thermal applications operating at mid-to high-temperatures. In a previous study by the current authors, a Ni-Al composite layer was applied as a coating onto a stainless steel 316L pipe using the flame spray technique. The solar absorptance (αsol) achieved, however, was relatively low compared to other commercial solar absorber coatings. To enhance the αsol of the Ni-Al, the annealing process was performed on this coating. The results clearly revealed that the metal oxides on the surface of the Ni-Al coating were NiO and Al2O3. As the annealing temperature increased, the reflectance (R) slightly decreased, while the αsol significantly increased. This was caused by the layer of metal oxides covering the surface of the Ni-Al coating functioning as an antireflection (AR) layer. The αsol of the Ni-Al coating increased approximately 13% after annealing at 800°C, compared to the unannealed Ni-Al coating. It is therefore clear that the annealed Ni-Al coating enhances the αsol due to the antireflection effect between the metal oxide layer and the underlying Ni-Al composite coating.