Vol.5 No.4 2013
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Research paper : Development of switchable mirror glass (K. Yoshimura et al.)−264−Synthesiology - English edition Vol.5 No.4 (2013) conventional switchable mirror thin film category. Their practical use is expected.At what level the variation range of the transmittance and reflectivity is set depends on the use. For example, for use in the windshield of a car, visible light transmittance of 70 % or above in the transparent state is necessary, and this condition has not been cleared at present. To raise the transmittance to this level, it is necessary to optically design the whole multilayer thin film including the palladium and protective film layers, not just the switchable layer composed of magnesium alloy thin film. We are currently engaging in this development.2.2 Improvement of durabilityFor the switchable mirror thin film material, a product with fairly practical performance has been achieved in terms of its optical property. However, the greatest problem of this material is the rapid deterioration due to repeated switching. For example, Fig. 3(a) shows the variation in the optical transmittance at wavelength of 670 nm when switching is repeated using hydrogen gas diluted to 4 % using argon, for the Pd/Mg4Ni thin film. It can be seen that the optical variation range gradually decreases with repeated switching, and the deterioration progresses rapidly particularly after 140 times.[16] In the practical use of the switchable mirror, the improvement of durability is the most important issue, and we have made various attempts to improve this.Deterioration occurs due to a combination of several factors, and one factor is that the Mg in the Mg-Ni thin film migrates to the surface through the Pd layer, due to the repetition of hydrogenation and dehydrogenation. We found that the migration of Mg can be controlled by inserting a metal thin film of Ti or others as a buffer layer between the Pd and Mg-Ni layers.[17] As seen in Fig. 3(b), the deterioration does not occur up to about 400 times when this buffer layer is inserted. However, rapid deterioration occurs in this sample also. This is due to the change in volume due to the hydrogenation and dehydrogenation, and this damages the Pd layer. To prevent this, a certain type of protective film is useful. For example, when the Teflon (PTFE) protective film was vapor-deposited on the surface of the sample to which the Ti buffer layer is inserted, the switching could be repeated up to about 1,500 times (Fig. 3(c)).[18] Yet for practical use, durability of about 10,000 times is necessary, and we are continuing the research for further improvement of durability.2.3 Realization of the electrochromic switchable mirrorTo switch the switchable mirror by a gaschromic method, it is necessary to use the double layer (pair) glass. Since this may be difficult depending on the use, electric switching may become necessary in some cases. For switching by an electrochromic method, we initially studied a device using an alkaline water solution. However in a device that uses a solution, the magnesium dissolves when the positive charge is applied to the switchable thin film side and short-circuiting was the only way to return from the transparent to the mirror state, and this was not practical. Therefore we developed an all-solid-state electrochromic switchable mirror that did not use the solution state.[19]Figure 4 shows the structure of the all-solid-state switchable mirror device that is being fabricated presently.[20] It is a multilayer structure in which the transparent conductive film (ITO), tungsten oxide thin film, tantalum oxide thin film, Al thin film, Pd thin film, and Mg-Ni alloy thin film are stacked. The films are fabricated by the magnetron sputtering method. The tungsten oxide thin film is a layer for storing the hydrogen ion, tantalum oxide thin film is the electrolyte layer, Pd thin film is the layer for promoting the passing of hydrogen, and the Mg-Ni alloy thin film is the switchable layer that switches from the mirror to the transparent state. The Al thin film is the buffering layer that prevents the Pd from dispersing into the tantalum oxide layer through repeated switching. The state of (c)(b)(a)PTFE/Pd/Ti/Mg4NiPd/Ti/Mg4NiPd/Mg4NiTransmittance (%)01020304016001400120010008006004002000Transmittance (%)01020304016001400120010008006004002000Transmittance (%)01020304016001400120010008006004002000Number repeatedTa2O5 WO3 ITO Ta2O5 HXWO3 V Pd Al Al Pd ITO V -5 V +5 V Mg alloyGlassGlassHydrideSwitchable mirror layerCatalysts layerBuffer layerSolid electrolyte layerIon storage layerTransparent conducting layerFig. 3 Comparison of switching durability(a) Switchable mirror thin film with magnesium alloy layer and palladium layer only, (b) switchable mirror thin film with inserted buffer layer, and (c) switchable mirror thin film with buffer layer and protective film coating (Teflon layer).Fig. 4 Photograph and structure of the all-solid-state electrochromic switchable mirror device

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