Anodically Produced Multiple Oxide Films for Increasing the Reflectance of Evaporated Aluminum*

Abstract

The paper describes a method for preparing reflectance-increasing film pairs of Al2O3 and TiO2 on evaporated aluminum mirrors. With one film pair of Al2O3 and TiO2, each film effectively one-quarter wavelength thick, the visible reflectance of aluminum can be increased by 5 percent. To produce such a multilayer coating, TiO2 is evaporated onto freshly deposited aluminum until the aluminum reflectance decreases to a minimum at λ=620 mµ. The direct evaporation of TiO2 results in a slightly reduced, strongly absorbing film material (TiOx). By anodic treatment in an electrolyte, such as 3 percent ammonium tartrate, the absorbing TiOx on aluminum can be oxidized to nonabsorbing TiO2, and part of the aluminum can be converted to Al2O3 underneath the TiO2. Seventy-five volts must be applied to produce quarter-wavelength thick films of Al2O3 and TiO2 at λ=530 mµ. Al2O3–TiO2 coatings furnish better protection than the most frequently used MgF2–ZnS film pairs. Two film pairs of Al2O3 and TiO2 can also be applied in one anodic process.