Microstructure and spectral selectivity of Mo–Al 2O 3 solar selective absorbing coatings after annealing

Abstract

Mo, Al 2O 3 single layer, Mo–Al 2O 3 granular cermet layer, Mo/Al 2O 3 tandem and an optimized Mo–Al 2O 3 multilayer coating with a double cermet layer configuration were deposited on stainless steel substrates by magnetron sputtering technique. The samples were annealed in vacuum at different temperatures ranging from 350 °C to 1000 °C for 2–5 h to evaluate their thermal stability. The spectral absorbance and thermal emissivity for the multi-layer selective coatings in the region of 1.3–25 μm were 0.91–0.93 and 0.19–0.27, respectively, depending on heat treatment temperature. Increasing annealing temperature has more obvious influence on the rise of emissivity than the drop of spectral absorbance. The microstructure, surface morphology, composition distribution and diffusion for various films before and after high temperature aging were investigated employing scanning electronic microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Fe 2O 3, as a diffusion barrier between Mo layer and SS substrate, effectively holds back the presence of Mo 2C and Fe 2MoC complex phases. Al 2O 3 is a stable phase even at 1000 °C for 3 h. However, defects such as widened boundaries, cracks and holes, which could appear in Al 2O 3 layer when heated at higher temperature than 650 °C, will provide the paths of Mo diffusion. Two diffusion modes of Mo, including inner self-congregation of Mo inside the cermet layer and Mo infiltration through the Al 2O 3 layer in Mo/Al 2O 3 tandem at 800 °C for 5 h, were observed after annealing.