Optical properties of anodic aluminum oxide films on Al1050 alloys

In this study the reflectance and nano-structure of anodized aluminum oxide (AAO) films that formed on Al1050 alloys in a 15% w/w sulphuric acid solution were examined. It was found that AAO films that formed under a high bath temperature and/or low anodic current density had a high pore density (count dm^{- 2}). The reflectance of AAO films formed at different anodizing times is a function of their thickness. The surface reflectance spectra showed interference when the AAO films exceeded a critical thickness (~ 100 nm). The reflectance curves oscillated and were affected by the pore density. We calculated the refractive index (n) and extinction coefficient (k) of the films based on the measured film thickness and reflectance. We can see from the experimental results that reflectance decreased with increasing thickness, while increasing pore density led to an increased refractive index but decreased extinction coefficient.