FIELD OF THE INVENTION 0001 The present invention relates to a switchable mirror glass using a magnesium thin film or a magnesium-nickel alloy thin film (hereinafter collectively, magnesium-containing thin film), and, more specifically, the present invention relates to a novel switchable mirror material for use in a switchable glass capable of automatically controlling sunlight incident from a windowpane without a sunblind or curtain, a switchable mirror glass produced using the same material, and a method of controlling the same switchable mirror glass. 0002 The present invention is a valuable material technology for controlling the light transmittance of a window in buildings or vehicles.

BACKGROUND OF THE INVENTION 0003 Generally, a great amount of heat comes in and out through windows (openings) in a building. For example, about 48% of heat from a heating system for winter flows out through windows, and about 71% of heat comes in an air-cooled room through windows in summer time. If the sunlight/heat transfer through windows is adequately controlled, an enormous amount of energy savings can be effectively achieved. 0004 A switchable glass has been developed for the purpose of controlling the sunlight/heat transfer through windows. 0005 There have been proposed various types of systems for controlling such a switchable glass. Among them, 1) a material having a light transmittance property reversibly changeable in response to current/voltage applied thereto is referred to as electrochromic material, 2) a material having a light transmittance property changeable in response to the temperature thereof is referred to as thermochromic material, and 3) a material having a light transmittance property changeable by controlling the state of atmospheric gas is referred to as gasochromic material. Particularly, an electrochromic switchable glass using a tungsten trioxide thin film as a switchable layer has been extensively researched toward the practical use thereof, and some related products have already been placed on the market. 0006 In principle, all of the conventional electrochromic switchable glasses are designed to control absorption level of incident light by the switchable layer such as a tungsten trioxide thin film. Thus, the switchable layer is inevitably heated as it absorbs the light, and the resulting heat will be reradiated toward a room, resulting in a deteriorated energy saving effect. To solve this problem, reflection level instead of absorption level of the incident light should be controlled. That is, it is desired to provide a material that can be reversibly changed between mirror and transparent states. 0007 While it was quite difficult to discover such a material having a property reversibly changeable between mirror and transparent states, a research group in Holland discovered in 1996 that hydrides of rare earth metals, such as, yttrium or lanthanum, are reversibly changeable between mirror and transparent states in response to hydrogen. This material was named a switchable mirror (J. N. Huiberts, R. Griessen, J. H. Rector. J. R. Wijngaarden, J. P. Dekker, D. G. de Groot, N. J. Koeman, Nature 380 (1996) 231). While the rare earth hydrides have an excellent switchable mirror property of a large variation of light transmittance, the limited availability of rare earth materials and their hydrides, and high cost make it disadvantageous to use the switchable mirror as a coating of windows. 0008 In 2001, a research group in the USA discovered a magnesium-nickel alloy Mg2Ni as a new switchable mirror material (T. J. Richardson, J. L. Slack, R. D. Armitage, R. Kostecki, B. Farangis, and M. D. Rubin, Appl. Phys. Lett. 78 (2001) 3047). Mg2Ni is expected to be a more suitable material for windowpane coatings because it is made of magnesium and nickel, which are more readily available and cost less than rare earth materials. Mg2Ni has a high reflectance in its mirror state, but a low visible light transmittance (20% from literature) in its transparent state. Thus, this material can be practically used only after the visible light transmittance in the transparent state is increased up to an adequate level. 0009 Some materials are known to have the switchable mirror properties, such as hydrides of rare earth metal such as yttrium or lanthanum, rare earth-magnesium alloy hydrides, rare earth-gallium alloy hydrides, and magnesium-nickel alloy hydrides. Among them, the magnesium-nickel alloy hydrides are suitable as windowpane coatings in view of availability and cost. However, none of the previously reported magnesium-nickel alloys could be practically used due to their low light transmittance in their hydrogenated state. While the electrochromic switchable glass using a tungsten trioxide thin film has been extensively researched toward the practical use thereof, and some related products have already been placed on the market, as described above, the thin film has a complicated multilayer thin-film coating structure. This leads to extremely high cost which is a negative factor against promoting its use as a switchable window. Thus, there is still a strong need for new materials for a switchable glass with excellent properties and suitable for practical use.

SUMMARY OF THE INVENTION 0010 Under the above circumstances, after extensive research on switchable properties of various compositions of magnesium and magnesium-nickel alloy thin films prepared through a sputtering method, the inventor found that (1) a magnesium thin film containing no nickel exhibits a specifically excellent switchable mirror property when it has a thickness less than a given value, (2) a film transparentized through hydrogenation exhibits a thermochromic property allowing the light transmittance of the film to be changed in response to its temperature, and (3) a new composition MgNix (0.1<x<0.3) has a significantly higher light transmittance in the transparent state than Mg2Ni. Based on these findings, the present invention was completed. 0011 One object of the present invention is to provide a new type of switchable mirror material comprising a magnesium-containing thin film that is low in cost, and a catalyst layer containing, for example, a slight amount of palladium. 0012 Another object of the present invention is to provide a switchable mirror glass window structure using the above switchable mirror material. 0013 Still another object of the present invention is to provide a new switchable mirror material having a simple structure of two coating layers capable of facilitating cost reduction. 0014 In order to achieve the above objects, according to a first aspect of the present invention, there is provided a switchable mirror material comprising (1) a magnesium thin film having a thickness of 40 nm or less, and preferably 10 to 20 nm, or a magnesium-nickel alloy thin film having an alloy composition represented by MgNix wherein 0.1<x<0.3, preferably, 0.16<x<0.2, (2) a transparent catalyst layer formed on a surface of the thin film, and optionally (3) a transparent protective layer formed on the catalyst layer. The switchable mirror material has a chromic property which allows the thin film to be switched from a mirror state into a transparent state by means of hydrogenation of the thin film, and to be switched from the transparent state into the mirror state by means of dehydrogenation of the thin film in a temperature range of about 20 C. to 100 C. 0015 In the switchable mirror material in the first aspect of the present invention, the catalyst layer may be a palladium or platinum layer having a thickness in the range of 0.5 to 10 nm. The protective layer may be made of a hydrogen-permeable and water-impermeable material. 0016 According to a second aspect of the present invention, there is provided a switchable mirror member comprising a transparent substrate, and a switchable layer including the switchable mirror material set forth in the first aspect of the present invention, wherein the switchable layer is formed on a surface of the transparent substrate. 0017 In the switchable mirror member set forth in the second aspect of the present invention, the transparent substrate may be a transparent sheet made of acrylic or plastic material. 0018 According to a third aspect of the present invention, there is provided a switchable mirror glass comprising a glass substrate, and a switchable layer including the switchable mirror material set forth in the first aspect of the present invention, wherein the switchable layer is formed on the glass substrate. 0019 According to a fourth aspect of the present invention, there is provided a switchable mirror glass window which consists of double or triple glass plates. In this case, at least one of the glass plates is composed of the switchable mirror glass as set forth in the third aspect of the present invention with the switchable layer facing in the inward direction of the insulating glass structure. 0020 In the switchable mirror glass window set forth in the fourth aspect of the present invention, the enclosed space between the glass plates is selectively supplied with either one of (1) hydrogen gas and (2) heated air or oxygen gas, from an atmospheric-state controller. 0021 According to a fifth aspect of the present invention, there is provided a method of switching the states of a switchable mirror glass with a switchable layer including the switchable mirror material set forth in the first aspect of the present invention, by selectively causing hydrogenation and dehydrogenation in the switchable layer, in such a manner that the level of hydrogenation is selectively adjusted through at least one of a gasochromic process using hydrogen gas and an electrochromic process using electrolyte containing proton, and the level of dehydrogenation is selectively adjusted through at least one of a gasochromic process using air or oxygen gas, an electrochromic process using electrolyte and a thermochromic process based on heating. 0022 According to a sixth aspect of the present invention, there is provided a method of switching the states of a switchable mirror member or glass with a switchable layer including the switchable mirror material set forth in the first aspect of the present invention, wherein the transition temperature of the magnesium thin film is set at any temperature in a given range in advance to allow the magnesium thin film to be automatically switched from the transparent state to the mirror state at the preset temperature in response to rise in atmospheric temperature. 0023 In the method set forth in the sixth aspect of the present invention, the transition temperature may be set at any temperature in the range of about 20 C. to 100 C. 0024 The transition temperature may be set depending on the thickness of the catalyst layer. 0025 According to a seventh aspect of the present invention, there is provided a method of switching the states of a switchable mirror member or glass with a switchable layer including the switchable mirror material set forth in the first aspect of the present invention, wherein speed of dehydrogenation in the switchable layer is adjusted by controlling the temperature of air or oxygen gas to be supplied around the switchable layer. In this case, the level of dehydrogenation in the switchable layer to be varied according to the temperature of the air or oxygen gas may further be adjusted by controlling the thickness of the catalyst layer. 0026 According to a eighth aspect of the present invention, there is provided an article comprising a switchable layer including the switchable mirror material set forth in the first aspect of the present invention, and either one of a switchable mirror glass and a switchable mirror member having a transparent substrate. The switchable layer is attached onto the surface of the switchable mirror glass or the transparent substrate. 0027 The switchable mirror material having a magnesium-containing thin film or the switchable mirror glass using the switchable mirror material of the present invention has the following significant effects. 0028 (1) The switchable mirror material of the present invention exhibits an excellent switchable mirror property far superior to the previously reported magnesium-nickel alloy Mg2Ni. 0029 (2) The switchable layer having the magnesium-containing thin film can be advantageously applied onto the inward-facing surface of the switchable mirror glass used as at least one of the glass plates in the switchable mirror glass window having an insulating glass structure, for example a double glazing structure, to allow the states of the magnesium-containing thin film to be switched by introducing hydrogen gas and others into the enclosed space between the glass plates. 0030 (3) The switchable mirror material can be prepared by using a low-cost metal such as magnesium and nickel, and a slight amount of catalytic material such as palladium coated on the thin film. Thus, the switchable mirror material is significantly advantageous in cost benefit terms. 0031 (4) The switchable mirror glass can be effectively used as various window glasses for not only buildings but also vehicles including automobiles. 0032 (5) The switchable mirror glass can be widely used in not only windows but also various kinds of articles, such as, shielding for privacy protection, ornamental articles or toys utilizing the feature of switching between mirror and transparent states, to add the switchable mirror function thereto.

BRIEF DESCRIPTION OF THE DRAWINGS 0033 FIG. 1 is a schematic diagram showing a sputtering apparatus used for preparing a magnesium and magnesium-nickel alloy thin film of a switchable mirror material of the present invention. 0034 FIG. 2 is a schematic diagram showing an apparatus for evaluating the switchable property of the prepared magnesium-containing thin film. 0035 FIG. 3 shows the variance of light transmittance at a wavelength of 670 nm for samples each different in thickness of a magnesium thin film (hydrogen gas of 1 atm was introduced at the time T50 s). 0036 FIG. 4 shows optical transmittance spectra for a sample of a switchable mirror material comprising a magnesium thin film. 0037

FIGS. 5

a and 5b show the respective states of a switchable mirror material comprising a magnesium thin film, wherein FIG. 5a shows a metallic state, and FIG. 5b shows a transparent state. 0038 FIG. 6 shows light transmittances of samples each having a different composition ratio of Ni to Mg, at a wavelength of 670 nm after 30 minutes from exposure of the samples to hydrogen gas. 0039

FIGS. 7

a and 7b show optical spectra of a switchable mirror material comprising a magnesium-nickel alloy thin film, wherein FIG. 7a is an optical transmittance spectrum, and FIG. 7b is an optical reflectance spectrum. 0040 FIG. 8 shows the variation of light transmittance of a sample having a composition of MgNi0.2 at different temperatures, wherein hydrogen gas of 1 atm was introduced at the time T5 s, and then air of 1 atm was introduced at the time T100 s. 0041

FIGS. 9

a, 9b and 9c are schematic block diagrams showing three types of switchable mirror glass windows of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 0042 The present invention will now be described in more detail. 0043 A switchable mirror material of the present invention comprises (1) a magnesium thin film having a thickness of 40 nm or less, or a magnesium-nickel alloy thin film having a specific composition represented by MgNix (0.1<x<0.3), and (2) a catalyst layer formed on the surface of the magnesium or magnesium-nickel alloy thin film. The catalyst layer may be made of palladium, platinum or any other suitable material having an effect equivalent to that of palladium or platinum. 0044 The magnesium-containing thin film having an excellent switchable property may be prepared by, for example, a sputtering method, a vacuum deposition method, an electron-beam deposition method, a chemical vapor deposition (CVD) method or an electroplating method. Other techniques will be apparent to one skilled in the art. 0045 The catalyst layer may be formed by, for example, coating palladium or platinum on the surface of the magnesium-containing thin film in a thickness range of 0.5 nm to 10 nm. Again, other materials and techniques will be apparent to one skilled in the art. The catalyst layer may also be prepared by, for example, a sputtering method, a vacuum deposition method, an electron-beam deposition method, a chemical vapor deposition (CVD) method or an electroplating method. Preferably, each of the magnesium-containing thin film and the catalyst layer is formed using a magnetron sputtering apparatus. A switchable layer including the switchable mirror material is formed on the surface of a transparent substrate to provide a switchable mirror member, or on the surface of a glass substrate to provide a switchable mirror glass. The transparent substrate may be a transparent sheet made of acrylic material, plastic material, or any other suitable material having properties equivalent to those to of acrylic or plastic material. 0046 Optionally, a protective layer may be formed on the surface of the switchable layer. The protective layer may be made of a hydrogen-permeable and water-impermeable material. The protective layer may be formed by any suitable method including, for example, by forming the protective layer as a sheet and attaching the sheet on the surface of the switchable layer, or by applying a protective material over the surface of the switchable layer. Other techniques for forming the protective layer will be apparent to one skilled in the art. 0047 The protective layer may be formed as a sheet or thin film made of polyethylene, tantalum oxide (Ta2O5), zirconium oxide (ZrO2), or any other suitable material having properties equivalent to those of polyethylene, tantalum oxide or zirconium oxide. The protective layer can provide enhanced durability in the switchable layer. 0048 The switchable layer is formed on the inward-facing surface of the switchable mirror glass used as at least one of the glass plates in a window having a double or triple glazing structure, to provide a switchable mirror glass window. In this case, the states of the magnesium-containing thin film of the switchable layer in the switchable mirror glass window are switched by selectively supplying hydrogen gas and heated air or oxygen gas into at least one enclosed space defined between the glass plates of the switchable mirror glass window from an atmospheric-state controller to selectively adjust the level of hydrogenation and dehydrogenation in the magnesium-containing thin film. The atmospheric-state controller may be constructed as a modular unit detachably mounted onto an appropriate position of the switchable mirror glass window. 0049 To switch the states of the magnesium-containing thin film, the level of hydrogenation is selectively adjusted through at least one of a gasochromic process using hydrogen gas and an electrochromic process using electrolyte containing proton, and the level of the dehydrogenation is selectively adjusted through at least one of a gasochromic process using air or oxygen gas, an electrochromic process using electrolyte and a thermochromic process based on heating. 0050 A transition temperature at which the magnesium thin film is switched reversibly from a transparent state to a mirror state may be set at any temperature in the range of about 20 C. to 100 C. in advance to allow the magnesium thin film to be automatically switched from the transparent state to the mirror state at the preset temperature in response to a rise in atmospheric temperature. In this case, the transition temperature may be set depending on the thickness of the catalyst layer formed on the magnesium thin film. That is, the thickness of the catalyst layer can be adjusted to set the transition temperature at any temperature, for example, in the range of about 20 C. to 80 C. 0051 A transition speed at which the magnesium-nickel alloy thin film is switched from a transparent state to a mirror state has a temperature dependency. Based on this property, the state of the magnesium-nickel alloy thin film can be automatically switched from the transparent state to the mirror state by increasing the temperature of the atmosphere around the switchable layer. The temperature dependency of the switchable layer can be adjusted by controlling the thickness of the catalyst layer formed on the surface of the magnesium-nickel alloy thin film. 0052 The switchable layer may be formed on both the inward-facing surfaces of a pair of glass plates defining an enclosed space of a window having a double insulating glass structure, to reduce the light transmittance of the window in the mirror state and to increase the range of reflectivity. The switchable mirror glass may be used as an intermediate glass plate of a window having a triple glazing structure to provide enhanced thermal insulating effect. 0053 In the switchable mirror glass, the variable range of the light transmittance is reduced as the thickness of the magnesium thin film as described in an aforementioned embodiment is reduced. For example, when the thickness of the magnesium thin film is 50 nm or more, the light transmittance of the switchable mirror glass is not changed at all even if it is exposed to hydrogen. When the thickness of the magnesium thin film is reduced less than 16 nm, the light transmittance in the transparent state increases. Under this condition, while the light transmittance in the mirror state also increases, more than 50% of the light transmittance can be assured. Thus, the thickness of the magnesium thin film is desirably set in the range of about 5 nm to about 40 nm. 0054 The magnesium thin film is initially in the mirror state of providing metallic luster and high reflectivity in the switchable mirror glass. Through hydrogenation, the magnesium thin film is switched into the transparent state, and its optical spectrum is drastically changed. While there was a report on the optical change of magnesium thin films for exposure to hydrogen at a high temperature of 100 C. or more to switch into the transparent state, it is the first time that such a drastic change has been observed at about 20 C. The switchable mirror glass of the present invention can achieve more than 60% of visible-light transmittance. Thus, the transmittance value of the switchable mirror material of the present invention fulfills the rough standard for practically used window glasses, i.e., a light transmittance of 60% or more. 0055 The switchable property of the switchable mirror glass comprising magnesium-nickel alloy thin film largely depends on a composition ratio of nickel to magnesium in the magnesium-nickel alloy thin film. The switchable mirror glass exhibits an excellent specific/selective switchable property when the magnesium-nickel alloy thin film has a composition ratio of nickel to magnesium is in the range of 0.1:1 to 0.3:1. 0056 When the hydrogenated magnesium-nickel alloy Mg2Ni thin film is exposed to air after discharging the hydrogen gas, the thin film is oxidized at the same speed as that of hydrogenation therein, and returned to the mirror state. 0057 In contrast, in the magnesium thin film of the present invention, even if the film having a very thin thickness is hydrogenated and then exposed to air, the thin film will be maintained in the transparent state, and its light transmittance will have almost no variation. However, when this film is heated up to 60 C., the light transmittance is reduced to return the film to the original mirror state. Such phenomenon that the state is changed in response to temperature is referred to as thermochromic phenomenon. That is, the combination of the magnesium thin film and the catalyst layer, such as palladium, coated on the magnesium thin film provides a new material having both the gasochromic and thermochromic phenomenon. 0058 On the other hand, in the magnesium-nickel alloy thin film of the present invention, the speed of dehydrogenation under air atmosphere largely depends on the temperature of the air atmosphere, and the dehydrogenation speed increases as the temperature increases. Such phenomenon that transmittance is varied according to temperature is also referred to as thermochromic phenomenon. Thus, the above property allows the switchable mirror material of the present invention to be thermochromcally used. Such a switchable mirror material has not been found until the present invention. 0059 These properties are effectively exerted particularly when the magnesium-containing thin film is used in windshields or another window glass of automobiles. That is, when an automobile is parked in hot summer, the thin film is automatically switched from the transparent state to the mirror state to reflect sunlight and prevent the temperature of the cabin space from increasing. The mirror state can be returned to the transparent state at any time by supplying hydrogen around the thin film. In addition, the temperature allowing the thin film to be returned to the mirror state can be varied by changing the thickness of the catalyst layer. Specifically, the thin film is switched to the mirror state at a lower transition temperature as the thickness of the catalyst layer increases. However, the increased thickness of the catalyst layer causes degradation of the light transmittance in the transparent state. 0060 When the switchable mirror material of the present invention is used as a switchable mirror glass, it may be used as at least one of glass plates of a window having a double glazing structure. Further, the magnesium-containing thin film having a thin catalyst layer such as palladium coated thereon is positioned to face in the inward direction of the window, and an enclosed space is sealingly defined in the window. Then, a pair of holes are formed in any of the glass plates. One of the holes is connected to a discharge pipe having a cap, and the other hole is connected to a unit (atmospheric-state controller) for electrolyzing water to generate hydrogen and supplying hydrogen into the enclosed space, and for electrolyzing air or water to generate oxygen and supplying oxygen into the enclosed space. The magnesium-containing thin film can be switched to the transparent state at any time by supplying hydrogen from the atmospheric-state controller into the enclosed space. 0061 If the atmospheric temperature is not changed, this state will be maintained. If the atmospheric temperature increases, the magnesium thin film will be automatically switched into the mirror state to reflect light and prevent light from transmitting therethrough. The film in the mirror state is returned to the transparent state at any time by supplying hydrogen into the enclosed space. The discharge pipe can be closed with the cap so as to hold the supplied hydrogen in the enclosed space to prevent the film from being returned to the mirror state. A hot air can be selectively supplied from the atmospheric-state controller into the enclosed space to switch from the transparent state to the mirror state. 0062 When the atmosphere around the magnesium-nickel alloy thin film is changed to air or oxygen atmosphere containing no hydrogen, the thin film is dehydrogenated and gradually switched from the transparent state to the mirror state. The dehydrogenation speed increases as the temperature of the atmosphere increases. For example, under an atmospheric temperature of 40 C., the thin film is switched to the mirror state at the same speed as that in hydrogenation. Under an atmospheric temperature of 20 C., the thin film is gradually switched to the mirror state at a speed 10 times slower than that in hydrogenation. The mirror state can be switched to the transparent state by supplying oxygen gas or heated air from the atmospheric-state controller. 0063 As described above, the present invention is directed to the switchable mirror material including the magnesium-containing thin film excellent in switchable mirror property, and the switchable mirror glass using the switchable mirror material. The switchable mirror material of the present invention having a thin palladium or platinum catalyst layer exhibits an excellent switchable mirror property far superior to the previously reported magnesium-nickel alloy Mg2Ni. Thus, the switchable mirror material would open the way for putting switchable mirrors in practical use. The windows in houses are widely regarded as one of important matters, and a double or triple insulating glass structure is gradually taking the dominant position for windows in new houses. The switchable layer including the switchable mirror material of the present invention can be advantageously applied onto the inward-facing surface of the switchable mirror glass used as at least one of glass plates in the switchable mirror glass window having a double or triple glazing glass structure to allow the states of the magnesium-containing thin film to be switched by introducing hydrogen gas and others into the enclosed space between the glass plates. 0064 While the electrochromic switchable glass closest to practical use among switchable glasses has an excellent performance as described above, it is required to coat at least five layers with high quality, which leads to unacceptably increased cost. In contrast, the switchable mirror material of the present invention can be prepared by using a low-cost magnesium-containing thin film and a slight amount of catalytic material such as palladium coated on the thin film. Thus, the switchable mirror material is significantly advantageous in cost benefit terms. Hydrogen for switching the states can be handled more simply with enhanced safety, for example, by adjusting the distance between the glass plated in a double insulating glass structure. Under the prospect of increasing demand for a highly functional glass, it is noteworthy that the present invention provides a new switchable mirror material for switchable mirror glasses. 0065 As discussed above, the switchable mirror glass can be effectively used as various window glasses for not only buildings but also vehicles including automobiles. In particular, when an automobile is parked in summer, the cabin space will be heated up to a high temperature of several dozen degrees. If the switchable mirror glass of the present invention is employed in the automobile, the magnesium-containing thin film of the switchable mirror glass is automatically switched from the transparent state to the mirror state to prevent the temperature of the cabin space from increasing. The mirror state can be conveniently returned to the transparent state at any time. 0066 The switchable mirror glass can be widely used in not only windows but also various kinds of articles such as shielding for privacy protection, ornamental articles or toys utilizing the feature of switching between mirror and transparent states, to add the switchable mirror function thereto. The term article herein means the complete range of articles equipped with the switchable mirror glass. 0067 While the present invention will now be described in conjunction with specific Examples, it is not limited thereto.

EXAMPLE 1 0068 A magnesium thin film was formed by a triple magnetron sputtering apparatus as shown in FIG. 1. Metal magnesium and metal palladium are targets of two of three sputtering guns. A glass plate having a thickness of 1 mm was used as a substrate. After surface cleaning, the glass plate was placed in a vacuum chamber, and then the vacuum chamber was evacuated. In a sputtering film-forming process, a magnesium thin film was first formed by using the magnesium target. The film was sputtered under an argon gas pressure of 1.2 Pa, at Power 30 W through a DC sputtering method. Then, a palladium thin film was deposited under the same vacuum condition at Power 6W.

EXAMPLE 2 0069 The film obtained from Example 1 was attached to an apparatus as shown in FIG. 2 to evaluate the optical properties of the film. The sample is attached to a small chamber to allow the magnesium thin film to face toward the inside of the chamber, and hydrogen gas at 1 atm was introduced into the chamber. While maintaining this state, the variation of the light transmittance of the sample was determined by a combination measuring system of a semiconductor laser of 670 nm wavelength and a silicon photodiode. 0070 FIG. 3 shows the variation of light transmittance caused by hydrogenation under the conditions that the thickness of the palladium layer was kept at a constant value of 4 nm, and the thickness of the magnesium layer was increased. It was verified that the variation of light transmittance reduced as the thickness of the magnesium layer increased, and no variation of light transmittance was caused by the exposure of the magnesium layer to hydrogen when the thickness of the magnesium layer increased up to 50 nm or more. 0071 The solid line in FIG. 3 shows the variation of light transmittance at 670 nm wavelength for a sample with the magnesium layer having a thickness of 16 nm, and the palladium layer having a thickness of 4 nm. The film had a metallic luster and almost no light transmittance just after film formation. Then, the gas in the chamber was once discharged and hydrogen was introduced into the chamber. After about 20 seconds, the light transmittance increased up to about 45%, and the film is switched into the transparent state. While the light transmittance in the transparent state increased as the thickness of the magnesium layer was reduced down to less than 16 nm, the transmittance in the mirror state also increased. When the thickness of the magnesium layer was further reduced down to 5 nm or less, the light transmittance increased up to greater than 50% even in the transparent state. Thus, a suitable thickness of the magnesium layer for use in a switchable mirror material is in the range of about 5 nm to about 40 nm.

EXAMPLE 3 0072 FIG. 4 shows an optical spectrum in the metallic state just after deposition for samples prepared such that the magnesium layer had a thickness of 25 nm, and the palladium layer had a thickness of 1.2 nm, and an optical spectrum after the sample was exposed to hydrogen atmosphere, and switched into the transparent state. FIG. 5 is a picture showing the state switching at that time. While this sample was initially in the mirror state having a metallic luster, it was hydrogenated and switched into the transparent state. As seen in FIG. 4, the transmittance spectrum is drastically changed in response to the state switching. 0073 As seen in FIG. 5(a), this sample was in the mirror state providing a high reflectivity in the mirror state. Upon exposure to hydrogen atmosphere, the sample was switched into the transparent state allowing the backside of the sample to be seen therethrough. While the conventional magnesium-nickel alloy Mg2Ni thin film has a dark brown even if it is hydrogenated, the material of the present invention was achromatic A visible-light transmittance calculated from the light transmittance spectrum in the transparent state was about 63%. Thus, it was proved that the light transmittance of the material of the present invention fulfilled the rough standard for practically used window glasses, i.e., a light transmittance of 60% or more. A thin polyethylene sheet was attached to the above sample, and a test was performed in the same manner. As a result, it was verified that the durability of the sample was significantly improved.

EXAMPLE 4 0074 A magnesium-nickel alloy thin film was formed by a triple magnetron sputtering apparatus as shown in FIG. 1. Metal magnesium, metal nickel and metal palladium serving as targets are set up to three sputtering guns, respectively. A glass plate having a thickness of 1 mm was used as a substrate. After rinse, the glass plate was placed in a vacuum chamber, and then the vacuum chamber was evacuated. In a sputtering film-forming process, a magnesium-nickel alloy thin film was first formed by using the magnesium and nickel targets. The film was sputtered under an argon gas pressure of 0.8 Pa, at Power 30 W for the magnesium target and Power 11 W for the nickel target through a DC sputtering method. Then, a palladium thin film was deposited under the same vacuum condition at Power 6W.

EXAMPLE 5 0075 The film obtained in Example 4 was attached to an apparatus as shown in FIG. 2 to evaluate the optical properties of the film. The sample is attached to a small chamber to allow the magnesium-nickel alloy thin film to face toward the inside of the chamber. The chamber was evacuated, and then hydrogen gas of 1 atm was introduced into the chamber. While maintaining this state, the variation of the light transmittance of the sample was determined by a combination measuring system of a semiconductor laser of 670 nm wavelength and a silicon photodiode. Just after the deposition process, the magnesium-nickel alloy thin film has a metallic luster and high reflectivity. When this thin film is exposed to hydrogen gas, it is hydrogenated to increase the light transmittance of the thin film. It was verified that the speed of hydrogenation largely depends on the composition of the magnesium-nickel alloy. Thus, various magnesium-nickel alloy thin films having different compositions were prepared by variably adjusting the respective powers to be applied to the magnesium and nickel targets during the film formation process through sputtering method, and the respective switchable properties of the prepared thin films were evaluated. 0076 FIG. 6 shows light transmittance of the prepared magnesium-nickel alloy thin films having different compositions of the magnesium-nickel alloy (MgNix), at a wavelength of 670 nm, wherein the thin films were exposure to hydrogen gas of 1 atm for 30 minutes. Each of the magnesium-nickel alloy thin films had a thickness of 40 nm, and a thin palladium layer of 4 nm thickness was coated thereon. 0077 As shown in FIG. 6, the visible-light transmittance in the transparent state is changed to have a maximum value at the composition ratio x of about 0.16. Further, an excellent switchable property is specifically exhibited in the x range of 0.1 to 0.3, particularly 0.16 to 0.2.

EXAMPLE 6 0078

FIG. 7

a shows an optical spectrum of a magnesium-nickel alloy thin film having a composition MgNi0.2 or the ratio of magnesium to nickel of 5:1 (the thickness of the magnesium-nickel alloy thin film: 40 nm, the thickness of the palladium layer: 4 nm) in the mirror state just after deposition. FIG. 7b shows an optical spectrum of the magnesium-nickel alloy thin film in the transparent state caused by exposing the thin film to hydrogen atmosphere. These figures show that the sample initially in the mirror state with metallic luster, low light transmittance and high reflectivity is switched to the transparent with largely increased light transmittance and reduced reflectivity through hydrogenation. For reference, an optical spectrum of a magnesium-nickel alloy thin film having a composition MgNi0.5 (Mg2Ni) in the transparent state is also shown in FIG. 7a. As is apparent from FIG. 7a, the light transmittance of MgNi0.2 has a variance about 3 times larger than that of MgNi0.5.

EXAMPLE 7 0079 FIG. 8 shows the temperature dependence of dehydrogenation of a magnesium-nickel alloy thin film having a composition MgNi0.2 or the ratio of magnesium to nickel of 5:1 (the thickness of the magnesium-nickel alloy thin film: 40 nm, the thickness of the palladium layer: 4 nm) when the thin film was exposed to hydrogen gas of 1 atm and to air of 1 atm. When the thin film was exposed to the hydrogen gas, it was hydrogenated and quickly switched to the transparent state within about 20 seconds. When the thin film was exposed to air, it was dehydrogenated and switched to the mirror state. Under an atmospheric temperature of 20 C., the speed in the dehydrogenation was slower than that in the hydrogenation, and it takes about 500 seconds. 0080 As shown in FIG. 8, when the atmospheric temperature is set at 40 C., the speed in the hydrogenation does not change, but the speed in the dehydrogenation increases. From this data, it was verified that the transition speed from the transparent state to the mirror state can be controlled by adjusting the atmospheric temperature. 0081 A deposited magnesium-nickel alloy thin film has metallic luster and high reflectivity in the mirror state. Upon exposure to hydrogen atmosphere, the sample is switched to the transparent state allowing the backside of the sample to be seen therethrough. While the conventional magnesium-nickel alloy Mg2Ni thin film has a dark brown color even if it is hydrogenated, the material of the present invention has a light yellow color. A visible-light transmittance calculated from the light transmittance spectrum in the transparent state in FIG. 7 is about 50%, which is a significantly improved value as compared to 20% in the conventional Mg2Ni.

EXAMPLE 8 0082 The magnesium-containing thin film having a thin palladium coating were positioned to face in the inward direction of a window in a double insulating glass structure, and an enclosed space was sealingly defined in the window. Then, a pair of holes was formed in the window. One of the holes was connected to a discharge pipe having a cap, and the other hole was connected to a unit (atmospheric-state controller) for electrolyzing water to generate hydrogen and supplying hydrogen into the enclosed space, and for electrolyzing air or water to generate oxygen and supplying oxygen into the enclosed space. In this way, a switchable mirror glass window was constructed. This switchable mirror glass window is shown in FIG. 9a. The magnesium-containing thin film could be switched to the transparent state at any time by supplying hydrogen from the atmospheric-state controller into the enclosed space. 0083 When the atmospheric temperature was not changed, this state of the magnesium thin film was maintained. When the atmospheric temperature was increased, the magnesium thin film was automatically switched into the mirror state to reflect light and prevent light from transmitting therethrough. The film in the mirror state could be returned to the transparent state at any time by supplying hydrogen into the enclosed space. It was verified that the discharge pipe could be closed with the cap so as to hold the supplied hydrogen in the enclosed space to prevent the film from being returned to the metallic state. It was also verified that hot air could be selectively supplied from the atmospheric-state controller into the enclosed space to switch from the transparent state to the mirror state. 0084 The transparent state of the magnesium-nickel alloy thin film could be maintained by filling hydrogen gas in the enclosed space. When air or oxygen gas was supplied to the enclosed space, the magnesium-nickel alloy thin film was gradually switched to the metallic state automatically to reflect light and prevent light from transmitting therethrough. When heated air or heated oxygen gas was supplied to the enclosed space, the thin film was quickly returned to the metallic state. 0085 As shown in FIG. 9b, another switchable mirror glass window having a reduced light transmittance and an increased reflectivity range in the mirror state was constructed by forming the switchable layer on both the inward-facing surfaces of a pair of glass plates defining an enclosed space of a window having a double insulating glass structure. 0086 As shown in FIG. 9c, still another switchable mirror glass window having an excellent heat insulating performance was constructed by using the switchable mirror glass as an intermediate glass plate of a window having a triple insulating glass structure. These windows have the same switchable property as that of the aforementioned switchable mirror glass window. 0087 Although the invention has been described with respect to specific embodiments, the details are not to be construed as limitations, for it will become apparent that various embodiments, changes and modifications may be resorted to without departing from the spirit and scope thereof, and it is understood that such equivalent embodiments are intended to be included within the scope of this invention.