New Electrochromic Mirror Systems

Variable reflectance coatings (switchable mirrors) have significant advantages over traditional absorbing devices for radiant energy control in a variety of architectural and aerospace applications due to their large dynamic ranges in both transmission and reflection in the visible and near infrared regimes. Although electrochromic and gasochromic metal hydride films have been the primary focus of recent research in this field, other systems merit consideration. Two of these, based on electrochemical conversion of copper to copper oxides and of pnicogens to lithium pnictides are discussed here. Three distinct states are available in the copper system: the highly reflecting metal, the transparent Cu(I) oxide, and the black, highly absorbing Cu(II) oxide. Metallic thin films of elemental antimony and bismuth are reversibly converted to transparent, semiconducting lithium pnictides by cathodic polarization in a non-aqueous lithium electrolyte. Like the metal hydrides, these systems provide substantial modulation of near infrared transmission and reflection, but have somewhat lower visible reflectance in their mirror states.