Yasusei Yamada (Senior Researcher) and others of Energy Control Thin Film Group (Leader: Kazuki Yoshimura), the Materials Research Institute for Sustainable Development (Director: Mamoru Nakamura) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Ryoji Chubachi), have developed a new switchable mirror with a visible transmittance of over 70% in the transparent state.

Window glass using switchable mirrors that can change their optical properties between a reflective state and a transparent state has a high cooling-load reduction effect in summer, compared to transparent double-glazed window glass. It is anticipated that such window glass will be commercialized early through the development of switchable mirrors for practical use that can withstand repeated switching between the reflective and transparent states and safe switching methods.

The visible transmittance of the developed switchable mirror in a transparent state has been improved to over 70 % by coating the surface of a highly-durable switchable mirror using a magnesium-yttrium-based alloy with an appropriate antireflective layer. This satisfies the legal requirements for light transmission in automobile windshields under the safety standards for road transport vehicles and has demonstrated the possibility of adapting the developed switchable mirror for window glass for thermal control of automobile interiors.

Developed switchable mirror with a 70% visible transmittance in the transparent state seen from the transparent substrate side (Left: Mirror state; Right: Transparent state)

Window glass is a necessity to bring in light and to secure visibility in buildings and automobiles, but at the same time, it acts as a large opening for heat. This being the case, if a glass that can adjust (control) light transmission from the outside (switchable glass) can be used, a large energy-saving effect can be expected. Various kinds of switchable glass have been developed so far. Although switchable glass that adjusts light transmittance electrically (electro-chromic switchable glass) is commercially available, the cost must be further reduced to gain popularity.

Furthermore, conventional electro-chromic switchable glass controls light by turning a thin film into dark blue to absorb the light, causing the temperature of the thin film to rise. This had a drawback of impairing the cooling load reduction effect since the heat is re-radiated into the room from the thin film. On the other hand, if light can be reflected rather than absorbed to control transmittance, it would be possible to shield sunlight more efficiently. Thus, commercialization of switchable glass (switchable mirror) that allows switching between a transparent state and a reflective state is expected.

AIST began R&D for thin-film materials for switchable mirrors in 2002 and has produced full-sized switchable mirror window glass using a magnesium-nickel alloy and installed it in a building to measure the cooling load. The results indicated a cooling load reduction effect of over 30 % compared to normal transparent double-glazed window glass.

However, the durability of this type of switchable mirror under repeated switching between the reflective state and the transparent state was insufficient for application as window glass. Therefore, a switchable mirror using a magnesium-yttrium-based alloy with the switching durability to hold up for at least 10,000 cycles was developed (AIST press release on September 20, 2012). However, the transmittance of this switchable mirror in the transparent state was approximately 55% at most and when this was used as a window facing a southerly direction, it was found that the increase in heating load in winter would exceed the decrease in cooling load in summer. Therefore, a target was set to develop a switchable mirror with a visible transmittance of over 70 % in the transparent state, in consideration of its adoption for automobile windshields.

A part of this research has been supported by a grant under the Industrial Technology Research Grant Program of FY2008, “Fabrication of switchable mirrors with optimized optical properties for energy efficient windows and evaluation of the energy saving potential of such windows in different buildings and climates” of the New Energy and industrial Technology Development Organization.

An antireflective coating was used to maximize visible transmittance in a transparent state. The most appropriate refractive index and film thickness of dielectric materials for an antireflective coating were estimated through simulations and it was found that visible transmittance will reach 74 % in the case of a refractive index of 2.1 and a film thickness of 60 nm. Low cost and versatile titanium oxide was selected as the material with a refractive index of 2.1, and was coated on the surface of a switchable mirror using a magnesium-yttrium-based alloy.

The transmittance spectra of this switchable mirror in both a transparent state and a reflective state are shown in Fig. 1. From the spectra, visible transmittance T_vis and solar transmittance T_sol in the transparent state are estimated to be 71.3 % and 60.9 % respectively, indicating a visible transmittance of over 70 %, close to the simulation results. T_vis and T_sol in the reflective state were estimated to be 5.6 % and 5.5 % respectively, showing a very wide range of light-control. The developed switchable mirror has a visible transmittance of over 70 % and will lessen the annual air-conditioning load not only by the reduction in cooling load in summer but taking into account the heating load as well.

In the reflective state, the mirror shows a deep navy blue color from the titanium oxide coated side (Fig. 2, Left) and a silver-colored mirror from the transparent substrate side (Fig. 2, Right). When the antireflective coating is not applied, the external appearance in the reflective state is a silver-colored mirror seen from either side (Fig. 3). It can be seen that the effects of titanium oxide have reduced the reflection in the reflective state. When the switchable mirror with antireflective coating is exposed to an atmosphere containing hydrogen but without oxygen, it will switch to the transparent state and conversely, when exposed to an atmosphere containing oxygen but without hydrogen, it will return to the reflective state (Fig. 1).

Furthermore, in cases in which a switchable mirror is used in buildings, when the outside is dark but the room is well lit, such as at night, conventional switchable mirrors turned the entire surface of the window into a full-length mirror, causing a severe sense of discomfort (Fig. 3, Left). With the developed switchable mirror, the reflection in the reflective state seen from the interior of the room has diminished drastically, making it possible to suppress the discomfort.

In addition, the developed switchable mirror will satisfy the legal requirement for automobile windshields to have a visible transmittance of over 70 %. Electric vehicles, which are expected to gain popularity in the future, do not have engines that are a large heat source and require energy to heat the cabin in winter. The use of solar radiation will greatly influence fuel efficiency. The cooling load when a car is parked in an outdoor parking will also affect the fuel efficiency.

Switchable mirrors that can switch their optical characteristics will be able to respond to both winter and summer seasons and their commercialization is expected to contribute greatly to the improvement of the fuel economy of automobiles.

Figure 1 : Transmittance spectra of the developed switchable mirror

Figure 2 : Appearance of the developed switchable mirror in the reflective state (Left: from the antireflective coating side; Right: from the transparent substrate side)

Appearance of a conventional switchable mirror without an antireflective coating in the reflective state (Left: from the switchable film side; Right: from the transparent substrate side)

To use switchable mirror as window glass, evaluation of weather resistance against solar radiation is essential. Exposure tests will be conducted, and R&D will be promoted to commercialize switchable mirror glass windows in the near future for use in office buildings and automobiles to drastically reduce air-conditioning load.