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Research paper : Development of switchable mirror glass (K. Yoshimura et al.)−263−Synthesiology - English edition Vol.5 No.4 (2013) 2 Issues in achieving the practical use of the switchable mirrorWhen the research for the switchable mirror was started at AIST in 2001, the following issues were identified as points that must be solved, and researches were started: (1) the improvement of optical property, (2) the improvement of durability, and (3) the development of an electrochromic switchable mirror.2.1 Improvement of optical propertyFigure 1(a) shows the basic structure of the switchable mirror composed of the magnesium alloy thin film as the switching layer, that is coated with a thin catalyst layer. In an as-deposited state after film deposition, the magnesium alloy layer is in a mirror state since it is metallic, and when it is exposed to an atmosphere containing diluted hydrogen, the switching layer becomes hydrogenated by the action of palladium catalysts, and the layer turns into an insulator and turns transparent. Next, when this is exposed to an atmosphere containing oxygen, the hydrogen in the hydride reacts with oxygen by the action of palladium catalysts and gets pulled out as H2O, and the film returns to a metallic state and becomes a mirror. The switching mirror state where the mirror and transparent states switch back and forth was first found in the thin films of rare earth metals such as yttrium and lanthanum, and several new materials were found later (Table 1). The alloy of magnesium and transition metal was found in the United States in 2001, and this was considered to be a better material to be applied to large glass, because it was less expensive and was thought to have higher durability compared to the rare earth metals. AIST focused on this material and started research for the first time in Japan. However, when this material was first discovered, the transmittance of visible light was about 20 % in its transparent state, and the color was dark brown that was not suitable for practical use. AIST started research on increasing the visible light transmittance in the transparent state of the switching mirror, and found that the visible light transmittance in the transparent state can be increased to about 50 % by using the Mg rich Mg-Ni alloy thin film, as shown in Fig. 1(b).[13] This was a big step forward to practical use.However, when the Mg-Ni alloy thin film is used as the switching mirror layer, the sample is slightly brown in the transparent state although it has excellent switching property. This color is not considered preferable for use in the windows of buildings and vehicles. Therefore, we conducted research for improving the color neutrality in the transparent state, and found that almost coloreless transparency can be obtained in cases of hydrogenation by using the Mg-Ti alloy[14] or Mg-Nb alloy[15] as the switching layer. However, although the Mg-Ti alloy thin film or Mg-Nb alloy thin film is colorless in the transparent state, the visible light transmittance is inferior compared to the Mg-Ni alloy thin film. Our research group recently found that we could obtain colorlessness during hydrogenation and increase the visible light transmittance by using the thin films of Mg-Ca,[11] Mg-Ba, and Mg-Sr[12] alloys. Figure 2 shows the optical transmittance spectra in the transparent state of each material. The materials of magnesium and alkaline-earth metal can be called the fourth generation materials made originally by AIST, unlike the Fig. 1 (a) Basic structure of the switchable mirror, and (b) the magnesium-nickel (Pd/Mg6Ni) switchable mirror thin film with excellent optical property developed at AISTMagnesium/alkaline earth metal alloyMg-Ca[11],Mg-Ba[12],Mg-Sr[12]AIST 2009Fourth generationMagnesium/transition metal alloyMg-Ni[8],Mg-Ti[9],Mg-Co[10],etc.Lawrence Berkeley Laboratory 2001Third generationRare earth/magnesium alloyGd-Mg[5],Sm-Mg[6],Y-Mg[7],etc.Phillips 1997Second generationRare earth metalY[4],La[4],etc.Vrije Universiteit, Amsterdam 1996First generationTransparent stateMirror state(b) Pd~4 nm(a)Mg-Ni~40 nmGlass substrateTable 1. Types of switchable mirror thin film materialsFig. 2 Comparison of the transmission spectra in the transparent state of the magnesium-nickel (Mg6Ni) switchable mirror, magnesium-titanium (Mg0.88Ti0.12) switchable mirror, and magnesium-calcium (Mg0.94Ca0.06) switchable mirrorTransmittance (%)Wavelength (nm)25002000150010005000102030405060Mg6NiMg0.94Ca0.06Mg0.88Ti0.12

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