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Posted: January 29, 2007
Nanotechnology creates switchable mirror
(Nanowerk News) Researchers in the Energy Control Thin Film Group of the Materials Research Institute for Sustainable Development, a part of the National Institute of Advance Industrial Science and Technology (AIST) have developed a new nanotechnology 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 x 70 cm and it shows switchable behavior. 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.
By using a apparatus with triple magnetron sputtering guns, the researchers sputtered magnesium and titanium metals simultaneously onto a glass plate to give a thin layer of magnesium-titanium alloy 40-nm thick. Over this, they 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.