Scientists develop ‘switchable’ glass film

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Mirror switches from reflective to transparent

Originally published February 2007

A switchable mirror material developed by scientists at the National Institute of Advance Industrial Science and Technology could be used for windows in buildings or automobiles and could reduce the energy consumed by air conditioners by more than 30%.

Mirror state

The new material uses a thin film of magnesium-titanium alloy in a 60-cm-by-70-cm mirror window that can switch from reflective to transparent states. The switchable windows use the thin film as a coating on the inner sides of two panes of glass. Switching is achieved by introducing a gas that contains hydrogen at a low concentration of about 1% or oxygen at a concentration of about 20% between the panes.

With current commercial switchable glass, as the temperature of the glass increases from absorption of light by the thin colored film that modulates light transmission, infrared radiation is re-radiated inside the room, decreasing the energy-saving efficiency of the glass.

Scientists have been searching for a reflective material that modulates light by reflection rather than by absorption. In 1996, a research group in the Netherlands developed thin films based on yttrium and lanthanum with a thin layer of palladium which could be switched between transparent and reflective states by hydrogenation and dehydrogenation reactions. But the elements are rare and expensive. A research group at the Lawrence Berkeley National Laboratory developed a thin-film switchable mirror made of a magnesium-nickel alloy, but it has a dark brown color, even in its transparent state.

Reflective state

Since 2002 the Energy Control Thin Film Group at AIST has been developing a thin film made of magnesium-nickel alloy, but they were unable to make the materials color-neutral in their transparent state, so they explored the possibility of creating a thin film using a magnesium-titanium alloy, which resulted in a film almost color-neutral in its transparent state.

The Energy Control Thin Film Group applied magnesium and titanium metals simultaneously onto a glass plate to make a thin layer of alloy about 40-nm thick. Over that, they applied a thin layer of palladium about 4-nm thick.

The thin film forms a reflecting mirror when it is applied to the glass, but turns transparent when it is exposed to an atmosphere that contains hydrogen but no oxygen. It reverts to a reflective state when it is exposed to an atmosphere that contains oxygen but no hydrogen. Switching is achieved by introducing gas containing a low concentration of hydrogen (about 1%) or gas containing oxygen (about 20%) into the space between the panes. Small amounts of hydrogen and oxygen for use in the switching process can be readily generated by decomposition of water. The trial switchable mirror created by the Energy Control Thin Film Group showed “excellent switching characteristics,” and the group believes it is the first full-size sample of switchable mirror glass ever to be produced.

Now, the group is working on the technology to increase the durability of the switchable mirror control by reducing the deterioration as a result of repeated cycles of switching. At the same time, because the thin film can be formed on various transparent materials besides glass, they are developing various switchable mirror films coated with this material as part of the “Strategic Development of Rationalization Technology for Energy Usage” project sponsored by the New Energy and Industrial Technology Development Organization. The group hopes it can further develop the new thin film so “we conserve energy simply by applying it to windows.”

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