Windows that switch between transmissive and reflective states could dramatically improve the energy efficiency of buildings, to say nothing of increasing the comfort level of their inhabitants. Rare earth and rare earth-magnesium alloy films have the ability to convert between transparent and mirror states when they take up hydrogen. Researchers at the Lawrence Berkeley National Laboratory are investigating the effect in mirrors based on a transition metal-magnesium alloy. A film under investigation at Lawrence Berkeley National Laboratory may make switchable mirrors commonplace. Unlike existing solutions that use costly rare earths, the films are based on a nickel-magnesium alloy. Courtesy of Thomas J. Richardson. Using nickel-magnesium films deposited onto glass substrates with or without transparent coatings, Thomas J. Richardson and his colleagues have demonstrated that the mirrors change reflectivity as a function of hydrogen uptake. The films as deposited are reflective, but they become transparent when they are exposed to a dry gas stream containing 4 percent hydrogen mixed with a noble gas. They recover the reflective mirror state upon exposure to ambient air. Electrochemical switching is also possible. When the researchers immersed the film in a potassium hydroxide electrolyte, they could induce electrochemical hydrogen loading by applying voltage between the conductively coated glass substrate and the electrolyte. Coating the nickel-magnesium film with palladium enhanced the hydrogen-uptake efficiency, decreasing the time required to make the transition between the two states. Although the film exhibited some hysteresis, it demonstrated repeatable switching from fully reflective to semitransparent states. The researchers postulated that the changes in reflectance were due to structural changes driven by chemical changes. The initial Mg2Ni film is metallic, but they believed that it converts to two insulators, Mg2NiH4 and MgH2, as hydrogen is introduced. Infrared and Raman spectroscopy confirmed this explanation. Richardson expects that windows based on the film will see applications first in satellite shielding, airplane cockpit windows, and helmet shields for astronauts and pilots, where cost is a secondary concern. He believes that the work also will yield consumer applications. The study demonstrates that switchable mirrors do not require rare earths. Besides their superior reflectance and transparency, the films should be less expensive to manufacture. They may also be more stable against oxidation. Imagination is the limit According to Richardson, the only limit to the technology is our imagination. "I believe that switchable mirrors will become part of everyday life," he said, "from automobile windows to motorcycle helmets to sunglasses, from display and information technology to energy conservation, control and production." The researchers described the mirrors in the May 14 issue of Applied Physics Letters.