- Development of a switchable mirror that turns into a color-neutral transparent state -
Kazuki Yoshimura, Group Leader of the Energy Control Thin Film Group of the Materials Research Institute for Sustainable Development (Director: Mamoru Nakamura), a part of the National Institute of Advance Industrial Science and Technology (President: Hiroyuki Yoshikawa) (hereinafter referred to as AIST) has, in collaboration with Shanhu Bao (Post-Doctoral Research Scientist at AIST), developed a new thin film material for a switchable mirror that can be switched between reflective and transparent states.
Previous research works have focused on the use of thin films made of magnesium-nickel alloy that behave as switchable mirrors: these, however, all have a yellow tinge in their transparent state. This is an obstacle to the practical application of such products, because such a yellow color renders the materials unsuitable for use in windows for buildings or automobiles.
The newly developed switchable thin film is made of an alloy of magnesium and titanium: this was successfully produced as a uniform layer on sheets of glass with the size of 60 × 70 cm and we confirmed that it showed switchable behavior (Figures 1 and 2). The switchable mirror consists of two layers of glass enclosing a cavity and coated on their interior surfaces with the alloy film. A gas that contains hydrogen at a low concentration of about 1% or oxygen at a concentration of about 20% is introduced to the cavity to activate the switching behavior.
Fig 1. Mirror State
Fig 2. Transparent State
A switchable glass is a glass that has an adjustable degree of transparency or color, and the use of such glass in windows for buildings or automobiles could allow considerable reductions to be made in energy consumption.
Various kinds of switchable glasses have been developed, and electrochromic glasses, the light transmissivity of which can be controlled by changing the color of the glass in response to an electrical signal, have already been commercialized. These, however, are not free from problems in terms of energy-saving efficiency. The temperature of the glass increases as a result of absorption of light by the thin colored film that modulates light transmission. This increase in temperature causes infrared radiation to be re-radiated inside the room, decreasing the energy-saving efficiency of the glass.
To overcome this defect, a reflective material that modulates light by reflection rather than by absorption is desirable. In 1996, a research group in the Netherlands developed thin films based on yttrium and lanthanum with a thin layer of palladium: these could be switched between transparent and reflective states by means of hydrogenation and dehydrogenation reactions. These materials cannot, however, be easily used in industrial applications, such large windows, because these elements are rare and expensive.
A research group at the Lawrence Berkeley National Laboratory of the U.S.A. developed a thin-film switchable mirror made of a magnesium-nickel alloy. This has a dark brown color, even in its transparent state, and the materials have poor optical characteristics. There was therefore a need for the development of cheap switchable materials with a high transparency.
Since 2002, the Energy Control Thin Film Group has been developing a thin film for use in a switchable mirror, and it has developed materials made of magnesium-nickel alloy that have excellent optical characteristics when used as thin films for switchable mirrors.
However, we were unable to make these materials color-neutral in their transparent state, because the yellowish tinge could not be completely removed by any means. We therefore explored the possibility of creating a thin film from materials other than magnesium-nickel alloy, and we tried using a magnesium-titanium alloy instead. By using magnesium-titanium alloy, we reduced the degree of tinting considerably, and we developed a thin film that is almost color-neutral in its transparent state.
By using a apparatus with triple magnetron sputtering guns, we sputtered magnesium and titanium metals simultaneously onto a glass plate to give a thin layer of magnesium-titanium alloy 40-nm thick. Over this, we applied a very thin layer of palladium (about 4-nm thick) by vacuum sputtering to create a switchable thin film.
The thin film forms a reflecting mirror when it is applied to the glass, but it 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. The change between states is very impressive.
The switchable windows that are actually used have a pair-glass structure with the thin-film applied as a coating on the inner sides of the two panes. 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. We created a trial switchable mirror with size of 60 × 70 cm and we tested its switching behavior. The thin film showed excellent switching characteristics, and this is the first full-size sample of switchable mirror glass ever to be produced.
We are developing the technology to increase the durability of the switchable mirror control by reducing 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, we 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. We will further develop the new thin film so that we can conserve energy simply by applying it to windows.