Light-controlling glass, its manufacturing method, and vanadium dioxide thin film with porous moth-eye structure that constitutes the light-controlling glass

Abstract

To provide a light-control glass having a vanadium dioxide thin film arranged on a transparent substrate, which light-control glass has a high light transmittance in the visible region when a light control rate (rate of change in light transmittance when the phase transition temperature is exceeded) in the infrared region is large, and also to provide a vanadium dioxide thin film constituting the light-control glass.SOLUTION: A light-control glass is provided, comprising a transparent substrate and a vanadium dioxide thin film, wherein the vanadium dioxide thin film has a moth-eye structure whose surface is constituted of protrusions with a period of 100 nm to 800 nm and is porous consisting of fine particles, with a particle area ratio of 30% to 60% as measured by AFM (atomic force microscopy), has a refractive index that results in a transmittance of 40% to 80% at a wavelength of 400 nm to 800 nm for the light-control glass at temperatures lower than or equal to its phase transition temperature, and has a control ratio (change in light transmittance) of 10% or more at a wavelength of 1000 nm to 1600 nm. A production method of the light-control glass, and a vanadium dioxide thin film constituting the light-control glass are also provided.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a thermochromic light-control glass having a vanadium dioxide thin film with a porous moth-eye structure formed on a transparent substrate. The present invention further relates to a vanadium dioxide thin film with a porous moth-eye structure that constitutes the light-control glass, and a method for producing the same. [Background technology]

[0002] The use of thermochromic light-controlling glass has been attracting attention because it can efficiently utilize thermal energy and contribute to energy conservation, etc. Known examples of light-controlling glass include electrochromic and gasochromic glass, which are used in the windows of buildings such as houses and office buildings, and in vehicles such as automobiles, but they have the drawback of being complicated in structure and requiring additional equipment for light control, resulting in high costs. Additionally, to save energy, low-emissivity glass, which transmits visible light but reflects infrared rays (part of solar radiation and radiant heat), and heat-reflecting glass, which blocks solar heat, are also used. However, these do not have the function of automatically controlling the amount of light and blocking solar radiation and heat radiation according to changes in the season and environmental temperature. Therefore, they are not thermochromic light-controlling glass.

[0003] Vanadium dioxide (VO2) undergoes a crystalline phase transition at around 68°C, resulting in significant changes in its optical and electrical properties. As the temperature rises, its infrared transmittance decreases significantly, while its reflectance increases. In other words, it is known to have thermochromic properties, automatically controlling and blocking solar radiation and thermal radiation without electrical driving force (and therefore without additional equipment) (see, for example, Non-Patent Document 1). For this reason, it has attracted attention as a smart window material, whose optical properties, such as transmittance and reflectance, change reversibly with temperature. For example, Patent Document 1 describes the use of a vanadium dioxide thin film with a moth-eye structure as a smart window material. Thus, vanadium dioxide thin films transmit infrared rays well at a specific temperature, i.e., at temperatures below the crystalline phase transition temperature of vanadium dioxide (hereinafter sometimes simply referred to as the "transition temperature" or "phase transition temperature"), and when the transition temperature is exceeded, the infrared transmittance decreases significantly, providing automatic light-control and heat-blocking properties. Therefore, various studies are being conducted on their application to light-control glass. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Chinese Patent Publication No. 111139432 [Non-patent literature]

[0005] [Non-Patent Document 1] CAO Xun et al., Nanoporous Thermochromic VO2 (M) Thin Films: Langmuir Vol.30 No.6 Pages.1710-1715 (2014.02.18) Summary of the Invention [Problem to be solved by the invention]

[0006] However, in conventional switchable glass using vanadium dioxide thin films, once the transition temperature is exceeded, infrared transmittance drops significantly, but heat radiation from the glass surface increases. Furthermore, high light transmittance in the visible range is desirable for switchable glass, but conventional switchable glass using vanadium dioxide thin films does not have sufficient transmittance (light transmittance) in the visible range, and measures such as the application of an anti-reflection coating were required to ensure high transmittance. Therefore, there is a demand for vanadium dioxide thin films that exhibit a significant decrease in infrared transmittance as the temperature rises (i.e., a high dimming rate in the infrared range) and high light transmittance in the visible range, as well as dimming glass using such thin films.

[0007] An object of the present invention is to provide a light-controlling glass having a vanadium dioxide thin film formed on a transparent substrate, which has a high light-controlling rate in the infrared range (the rate at which infrared transmittance decreases when the transition temperature is exceeded) and a high light transmittance in the visible range, and a vanadium dioxide thin film that constitutes the light-controlling glass and imparts thermochromic properties. Here, the visible range refers to a wavelength range of 400 nm to 800 nm, and the infrared range refers to a wavelength range of over 800 nm, and in particular, the infrared range refers to the near-infrared range of wavelengths of 1000 nm to 1600 nm. Another object of the present invention is to provide a method for producing the vanadium dioxide thin film that constitutes the light control glass.

[0008] Furthermore, the above-mentioned light-controlling glass is also desired to have the property of low thermal radiation from its surface in order to increase the effect of storing heat without releasing it outdoors (outdoors) in winter when applied to buildings, etc., and to reduce the impact of exhaust heat on global warming, etc. An object of the present invention is to provide the above-mentioned light-controlling glass having a vanadium dioxide thin film formed on a transparent substrate, which has a high light control rate in the infrared range, high light transmittance in the visible range, and low thermal radiation from its surface.

[0009] The temperatures (ambient temperatures) at which window glass, etc. in buildings and mobile vehicles are used are usually lower than the transition temperature of vanadium dioxide, which is around 68°C. Therefore, it is desirable for light-controlling glass used in these applications to have a lower transition temperature in order to adapt to environmental temperatures. An object of the present invention is to provide light-controlling glass having a vanadium dioxide thin film formed on a transparent substrate, which has properties such as a high light control rate in the infrared range and high light transmittance in the visible range, and whose transition temperature is lowered so as to adapt to environmental temperatures. [Means for solving the problem]

[0010] As a result of extensive investigations to solve the above problems, the present inventors have found that The inventors have found that by forming a vanadium dioxide thin film on a transparent substrate with a moth-eye structure on the surface consisting of minute protrusions and by making the film porous with a predetermined porosity, and by adjusting the refractive index so that it varies within a predetermined range in the thickness direction, it is possible to obtain high light transmittance in the visible range and a large dimming rate in the infrared range. They have also found that the use of this vanadium dioxide thin film makes it possible to obtain light-controllable glass with high light transmittance in the visible range and a large dimming rate in the infrared range.

[0011] The inventors have further found that in the light control glass having the vanadium dioxide thin film, by providing a buffer layer made of a wide-gap oxide semiconductor between the vanadium dioxide thin film and the transparent substrate, it is possible to reduce thermal radiation from the surface while maintaining a large light control rate in the infrared range and high transmittance in the visible range.

[0012] It is known that the transition temperature can be lowered by adding impurities to a vanadium dioxide thin film (doping). However, the present inventors discovered that by using an impurity selected from the group consisting of Nb, Ta, Mo, and W and controlling the amount of impurity added using the MOD method, it is possible to control the transition temperature of the vanadium dioxide thin film, and that it is also possible to set a transition temperature that is compatible with the environmental temperatures in which window glass, etc., in buildings and mobile vehicles are used.

[0013] The present inventors have further found that if a vanadium dioxide thin film is formed on a transparent substrate by the metal organic decomposition (MOD) method under specific conditions of rapid heating to the firing temperature and rapid cooling, a vanadium dioxide thin film with a moth-eye structure having the specified porosity and light control glass having the above-mentioned properties can be produced. That is, the above-mentioned object of the present invention is achieved by the invention having the following configuration.

[0014] The first aspect of the present invention is light-controllable glass having a transparent substrate and a vanadium dioxide thin film formed on the transparent substrate or on one or more transparent layers formed on the transparent substrate, wherein the vanadium dioxide thin film has a moth-eye structure on its surface composed of protrusions with a period of 100 nm to 800 nm, is porous composed of fine particles, and has a particle area ratio of 30% to 60% as measured by AFM (atomic force microscope), and at a temperature below its phase transition temperature, the light-controllable glass has a refractive index that gives a transmittance of 40% to 80% at wavelengths of 400 nm to 800 nm, and a light-control rate (change in transmittance) of 10% or more at wavelengths of 1000 nm to 1600 nm. A nanoscale porous moth-eye structure is a thin film structure whose surface is a moth-eye structure made up of protrusions with a period of 100 nm to 800 nm, and which is porous made up of fine particles, with a particle area ratio measured by AFM (atomic force microscope) of 30% to 60%.

[0015] A second aspect of the present invention is the above-mentioned first light-controlling glass, characterized in that a buffer layer made of a wide-gap oxide semiconductor is provided between the transparent substrate and the vanadium dioxide thin film.

[0016] A third aspect of the present invention is the light-controlling glass according to the first or second aspect of the present invention, characterized in that the vanadium dioxide thin film contains one or more cations selected from the group consisting of Nb, Ta, Mo, and W as a dopant in an amount of 1.0 mol % or more relative to vanadium, and the light-controlling glass has a phase transition temperature of 65°C or lower. Here, the phase transition temperature refers to the temperature at which the infrared transmittance of the light-controlling glass changes critically, and specifically refers to the temperature between the upper and lower limits of the center of the phase transition at which the transmittance changes significantly. More specifically, it refers to Tc shown in Figure 5.

[0017] The fourth aspect of the present invention is a vanadium dioxide thin film constituting the light control glass according to any one of the first to third aspects of the present invention. That is, the vanadium dioxide thin film is characterized in that its surface has a moth-eye structure composed of protrusions at a period of 100 nm to 800 nm, is porous composed of fine particles, has a particle area ratio of 30% to 60% as measured by AFM (atomic force microscope), and has a refractive index of 1.5 to 2.0 at temperatures below its phase transition temperature and decreases at temperatures above the phase transition temperature.

[0018] The fifth aspect of the present invention is a method for producing a vanadium dioxide thin film, which comprises applying a solution containing a vanadium organic compound to a uniform thickness on a transparent substrate or on a transparent layer provided on the transparent substrate, repeating pre-baking by heating at 300°C to 400°C once or a plurality of times, and then heating to a temperature higher than 550°C but lower than 600°C for main baking, wherein the temperature is increased to the main baking temperature at a rate of 30°C / min or more and the temperature is decreased from the main baking temperature at a rate of 25°C / min or more. [Effects of the Invention]

[0019] According to the first aspect of the present invention, there is provided a light control glass that has high light transmittance in the visible range and a high light control rate in the infrared range. According to the second aspect of the present invention, there is provided a light control glass that has a large light control rate in the infrared range and a high light transmittance in the visible range, while reducing heat radiation from the surface. According to the third aspect of the present invention, there is provided a light control glass having a phase transition temperature lowered to 65° C. or lower while maintaining a high light control rate in the infrared region and a high light transmittance in the visible region. According to a fourth aspect of the present invention, there is provided a vanadium dioxide thin film having a nanoscale porous moth-eye structure that constitutes the light control glass of the first aspect of the present invention. According to a fifth aspect of the present invention, there is provided a method for producing the light control glass according to any one of the first to third aspects of the present invention. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a schematic cross-sectional view of a light-control glass according to the present invention. [Figure 2] 1 is a schematic cross-sectional view showing a nanoscale porous moth-eye structure according to an example of this embodiment, and a graph showing the change in refractive index in the thickness direction. [Figure 3] FIG. 10 is a flow chart showing each step of the fifth production method of the present invention. [Figure 4] 1 is a graph showing the relationship between transmittance and wavelength for the light control glass of Examples 1 to 3. [Figure 5] 1 is a graph showing the relationship between the transmittance and temperature of the light control glass of Examples 5, 6, and 7. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments for carrying out the present invention will be described in more detail, but the scope of the present invention is not limited to the following embodiments.

[0022] The first aspect of the present invention is light-controllable glass having a transparent substrate and a vanadium dioxide thin film formed on the transparent substrate or on one or more transparent layers formed on the transparent substrate, wherein the vanadium dioxide thin film has a moth-eye structure on its surface composed of protrusions with a period of 100 nm to 800 nm, is porous composed of fine particles, and has a particle area ratio of 30% to 60% as measured by AFM (atomic force microscope).At a temperature below its phase transition temperature, the light-controllable glass has a refractive index that gives the light-controllable glass a transmittance of 40% to 80% in the wavelength range of 400 nm to 800 nm, and a light-control rate (change in transmittance) of 10% or more in the wavelength range of 1000 nm to 1600 nm.

[0023] The light-controlling glass of this embodiment has a transparent substrate and a vanadium dioxide thin film formed thereon. Figure 1(a) is a schematic cross-sectional view of the light-controlling glass made of a transparent substrate and a vanadium dioxide thin film. Another transparent layer, for example, a buffer layer made of a wide-gap oxide semiconductor (described later) may be provided between the transparent substrate and the vanadium dioxide thin film. Figure 1(b) is a schematic cross-sectional view of a light-control glass comprising a transparent substrate, a vanadium dioxide thin film, and a buffer layer.

[0024] Examples of transparent substrates include transparent bodies that are used as substrates for light-controlling glass, such as glass, sapphire, and quartz. The thickness of the substrate is appropriately selected in consideration of strength and other factors depending on the application of the light-controlling glass, but generally, a substrate with a thickness of 10 mm or less is used.

[0025] The vanadium dioxide thin film is characterized by a nanoscale porous moth-eye structure, i.e., a moth-eye structure whose surface is composed of minute protrusions and is porous containing minute pores. Figure 2(a) is a schematic cross-sectional view showing an example of the nanoscale porous moth-eye structure of this embodiment. The pitch of the projections of the protrusion-like structure (and the size of the projections) does not necessarily have to be uniform, but is generally 100 nm to 800 nm. Preferably, 50% or more of the pitch of the projections is within the above range, more preferably, 80% or more is within the above range, and even more preferably, 80% or more is within the range of 200 nm to 400 nm.

[0026] The vanadium dioxide thin film used in this embodiment may have a film-like portion connected to the transparent substrate side of the protrusion-like structures on the surface, but this film-like portion is also porous, containing fine voids (air layers). The presence of the moth-eye structure and the voids, i.e., the presence of air in the film, reduces the refractive index of the film to be lower than that of vanadium dioxide alone. The transmittance of the light control glass of this embodiment is affected by the relationship between the refractive index of the vanadium dioxide thin film and the refractive index of the transparent layer or transparent substrate that it contacts, but the vanadium dioxide thin film constituting the first aspect of the present invention is characterized by having a refractive index that, at a temperature below its phase transition temperature, gives the light control glass a transmittance of 40% to 80%, preferably 60% to 80%, at wavelengths of 400 nm to 800 nm. This refractive index can be obtained by adjusting the period of the protrusions of the moth-eye structure within the above-mentioned range and adjusting the particle area ratio measured by AFM (atomic force microscope) within the range of 30% to 60%. The particle area ratio measured by AFM (atomic force microscope) is a value obtained by binarizing an image obtained by measurement in contact mode based on the average particle size according to the particle size, and then averaging the results.

[0027] Even in conventional light-controlling glass, the reflectance at the surface, which affects the transmittance, can be reduced by reducing the average refractive index of the thin film provided on the surface to approximately the refractive index of the substrate (preferably to approximately the square root value). The average refractive index of the vanadium dioxide thin film can be reduced by controlling its crystal grain density, but the refractive index of vanadium dioxide bulk cannot be sufficiently reduced to this level. Therefore, in this embodiment, the refractive index is reduced by the protrusion structure or porous structure due to the moth-eye structure (i.e., the presence of air in the film).

[0028] The change in refractive index in the thickness direction depends on the air occupancy rate in the thickness direction and the period of the protrusion array structure. However, it is preferable that the vanadium dioxide thin film constituting the light-controllable glass of this embodiment has an adjusted air occupancy rate in the thickness direction and protrusion array structure, so that the refractive index on the transparent substrate side is close to the refractive index of the transparent substrate or transparent layer with which the vanadium dioxide thin film is in contact, and that the refractive index increases from the surface side toward the transparent substrate side.

[0029] In the preferred embodiment, the tip of the protrusion in the moth-eye structure contains almost no vanadium dioxide and is essentially an air layer, so the refractive index is approximately 1.0, just like the air layer. However, the refractive index increases toward the base of the protrusion (the transparent substrate side) because the ratio of vanadium dioxide to the air layer increases. When there is no other transparent layer between the vanadium dioxide thin film and the transparent substrate, the refractive index on the transparent substrate side is preferably the same as or close to the refractive index of the transparent substrate. When there is another transparent layer (e.g., a buffer layer) between the vanadium dioxide thin film and the transparent substrate, the refractive index on the transparent substrate side is preferably the same as or close to the refractive index of the transparent layer.

[0030] Figure 2(b) shows the change in refractive index in the thickness direction of the vanadium dioxide thin film in the example of Figure 2(a). As shown in the figure, the refractive index n on the surface side of the vanadium dioxide thin film is 1, and the refractive index increases (not necessarily uniformly) toward the transparent substrate, until the side in contact with the transparent substrate has a refractive index nearly equal to that of the transparent substrate.

[0031] By forming the vanadium dioxide thin film into the nanoscale porous moth-eye structure and adjusting the refractive index to the above configuration, reflection on the film surface can be efficiently suppressed, and the light transmittance of the light control glass, particularly in the visible range, can be increased to 40% to 80%. As a result, a light control glass with high light transmittance can be obtained without the need for an anti-reflection film.

[0032] The thickness of the vanadium dioxide thin film is preferably in the range of 100 nm to 300 nm. Within this range, the high light transmittance and a high dimming rate (change in transmittance) of 10% or more in the near-infrared wavelength range of 1000 nm to 1600 nm can be easily obtained. In other words, an excellent dimming rate can be obtained.

[0033] In a nanoscale porous moth-eye structure, the structural period must be sufficiently short compared to the wavelength of light in order for scattered light or diffracted waves to not be generated. By satisfying this condition, namely, by having a moth-eye structure in which the period between protrusions is 100 nm to 800 nm, and which is porous consisting of fine particles and voids between the particles, with a particle area ratio of 30% to 60% as measured by AFM (atomic force microscope), and by having a refractive index such that the transmittance of the light control glass at wavelengths of 400 nm to 800 nm is 40% to 80%, preferably 60% to 80%, at temperatures below its phase transition temperature, a low refractive index film that is independent of the angle of incidence can be realized, and reflection can be kept low over a wide wavelength range and a large angle of incidence. The nanoscale porous moth-eye structure having the above characteristics can be produced by the fifth production method of the present invention.

[0034] The second aspect of the present invention is a light-controlling glass according to the first aspect, characterized in that a buffer layer made of a wide-gap oxide semiconductor is provided between the transparent substrate and the vanadium dioxide thin film. By providing a buffer layer made of a wide-gap oxide semiconductor, it is possible to reduce thermal radiation from the surface while maintaining a high light control rate in the infrared range and high light transmittance in the visible range. A schematic cross-sectional view of the light-controlling glass of this embodiment is shown in Figure 1(b).

[0035] A wide-gap oxide semiconductor is one having a band gap of 3.0 eV or more. The wide-gap oxide semiconductor used in the present invention can be formed into a transparent layer by film formation. Examples of such wide-gap oxide semiconductors include HfO2, HfZrO2, ZnO, WO3, and Ga2O3.

[0036] The buffer layer made of the wide-gap oxide semiconductor can be formed, for example, by applying a liquid containing the wide-gap oxide semiconductor to a predetermined thickness by a general-purpose coating method, such as spin coating, on a transparent substrate such as sapphire or glass, or on a transparent layer formed on the transparent substrate, followed by baking. It can also be formed by a method similar to the metal-organic decomposition (MOD) method used in the fifth aspect of the present invention. The thickness of the buffer layer made of the wide-gap oxide semiconductor is preferably in the range of 30 nm to 100 nm.

[0037] By forming a vanadium dioxide thin film on top of the buffer layer formed as described above, a light control glass having a heat radiation suppressing function can be produced. In addition to the buffer layer, another transparent layer may be provided between the vanadium dioxide thin film and the transparent substrate.

[0038] A third aspect of the present invention is the light-controlling glass according to the first or second aspect of the present invention, characterized in that the vanadium dioxide thin film contains one or more cations selected from the group consisting of Nb, Ta, Mo, and W as a dopant in an amount of 1 mol % or more relative to vanadium, and the phase transition temperature is 65°C or lower.

[0039] It is known that adding cationic impurities (dopants) such as Nb and Mo to vanadium dioxide lowers the crystal phase transition temperature (WO 2011 / 118700 A1). The present inventors discovered that the fifth manufacturing method of the present invention allows for more stable and uniform addition of impurities to lower the phase transition temperature compared to other manufacturing methods for vanadium dioxide thin films. Specifically, the fifth manufacturing method of the present invention allows for easy control of the addition of cations selected from Nb, Ta, Mo, and W to a vanadium organic compound solution in an amount of 1 mol% or more relative to vanadium, thereby lowering the phase transition temperature and making it compatible with ambient temperatures (e.g., 65°C or less). As a result, a third light-controllable glass of the present invention has been obtained, which contains one or more cations selected from the group consisting of Nb, Ta, Mo, and W in an amount of 1 mol% or more relative to vanadium and has a phase transition temperature of 65°C or less.

[0040] The fourth aspect of the present invention is a vanadium dioxide thin film that constitutes the light control glass of the first to third aspects of the present invention, and is a film that contains vanadium dioxide as a main component and has a nanoscale porous moth-eye structure. The film may contain vanadium oxides other than vanadium dioxide, but preferably 70% or more of the vanadium in the film forms vanadium dioxide. This vanadium dioxide thin film is produced by the method for producing light control glass of the fifth aspect of the present invention.

[0041] The fifth aspect of the present invention is a method for producing a vanadium dioxide thin film, which comprises applying a solution containing a vanadium organic compound to a uniform thickness on a transparent substrate or on a transparent layer provided on the transparent substrate, repeating pre-baking by heating at 300°C to 400°C once or a plurality of times, and then heating to a temperature higher than 550°C but lower than 600°C for main baking, wherein the temperature is increased to the main baking temperature at a rate of 30°C / min or more and the temperature is decreased from the main baking temperature at a rate of 25°C / min or more.

[0042] This manufacturing method can produce the light control glass of the first to third aspects of the present invention, and also forms the vanadium dioxide thin film having the nanoscale porous moth-eye structure of the fourth aspect of the present invention.

[0043] Other known methods for producing light-control glass that uses a vanadium dioxide thin film as a component include sputtering, PLD, and hydrothermal synthesis. However, these methods cannot produce light-control glass with a vanadium dioxide thin film with a nanoscale porous moth-eye structure. Furthermore, sputtering and PLD have the problem of prone to segregation and non-uniformity in the doping of impurities when lowering the transition temperature. Furthermore, while hydrothermal synthesis has been reported to produce microparticles that can be physically mixed, it does not achieve a lower transition temperature.

[0044] In the fifth manufacturing method of the present invention, a vanadium dioxide thin film with a moth-eye structure is produced by the metalorganic decomposition (MOD) method. The production of a vanadium dioxide thin film with a moth-eye structure by the metalorganic decomposition (MOD) method is disclosed in Japanese Patent Laid-Open No. 2020-142961 and WO2011 / 118700A1. However, these disclosed methods cannot form a vanadium dioxide thin film with a nanoscale porous moth-eye structure. Therefore, high light transmittance in the visible range and excellent dimming efficiency in the infrared range cannot be obtained. The fifth manufacturing method of the present invention is characterized in that the main firing performed in the metal organic decomposition (MOD) method is performed at a temperature higher than 550°C and lower than 600°C, and the temperature is increased to the main firing temperature and decreased from the main firing temperature at rates of 30°C / min or more and 25°C / min or more, respectively. Due to these characteristics, a vanadium dioxide thin film with a nanoscale porous moth-eye structure is formed, and switchable glass is obtained that exhibits high light transmittance in the visible range and a high dimming rate in the infrared range.

[0045] Fig. 3 is a flow diagram showing each step of the fifth production method of the present invention (the specific conditions shown in Fig. 3 are preferred conditions). In this method, first, a liquid containing a vanadium organic compound (MOD solution) is applied onto a transparent substrate made of glass, sapphire, or the like, or onto a transparent layer (e.g., a buffer layer made of a wide-gap oxide semiconductor) formed on the transparent substrate ((1) in Figure 3). The MOD solution and application conditions can be the same as those for applying vanadium organic compounds by the conventional MOD method.

[0046] The coating method is not particularly limited as long as it can uniformly coat the MOD solution to a thickness of about 100 nm to 300 nm, but spin coating is the most suitable method for uniformly coating the above thickness. When the concentration of the solution is within the above range, the rotation speed in spin coating is preferably 2000 rpm to 7000 rpm, and more preferably 4000 rpm to 6000 rpm. After the MOD solution is applied, the applied film is usually heated (pre-baked) to dry and remove the solvent. The above steps from coating to pre-baking may be repeated about twice.

[0047] After the above steps from application to pre-baking, the transparent substrate coated with the MOD solution is preferably heated and baked at 300° C. to 350° C. for 15 to 20 minutes (pre-baking: (2) in FIG. 3). The calcination is preferably carried out in an oxygen-free inert atmosphere, for example, an N2 atmosphere. The rate of temperature increase from room temperature to the calcination temperature and the rate of temperature decrease from the calcination temperature to room temperature are preferably 30° C. / min or more and 25° C. / min or more, respectively. By setting the temperature increase rate and temperature decrease rate to the above rates or higher, a vanadium dioxide thin film having a nanoscale porous moth-eye structure is more easily obtained.

[0048] The process from coating to pre-baking ((1) in FIG. 3) and the process of pre-baking ((2) in FIG. 3) may be repeated two or more times. For example, if a thin film of the desired thickness cannot be obtained within the above range of spin coating rotation speed, the process may be repeated two or more times to obtain the desired thickness.

[0049] After the steps from coating to pre-baking ((1) in FIG. 3) and the pre-baking ((2) in FIG. 3) are performed once or repeated two or more times, the substrate is heated to 550°C to 600°C for 15 to 25 minutes and baked (main baking: (3) in FIG. 3). In the main baking, the temperature rise rate from room temperature to the main baking temperature is 30°C / min or more, and the temperature drop rate from the main baking temperature to room temperature is 25°C / min or more. These characteristics enable the formation of a vanadium dioxide thin film with a nanoscale porous moth-eye structure on the substrate. If the temperature rise rate and temperature drop rate are lower than the above rates, a vanadium dioxide thin film with a nanoscale porous moth-eye structure cannot be obtained.

[0050] (Features of the light-controlling glass of the present invention Nos. 1-3) Conventional light-controlling glass using vanadium dioxide, etc., exhibits a characteristic that while infrared transmittance decreases significantly with increasing temperature, thermal radiation from the surface increases, necessitating the application of anti-reflection coatings or other treatments. The first invention utilizes a porous moth-eye structure, while the second invention further incorporates a buffer layer made of a wide-gap oxide semiconductor to reduce thermal radiation from the surface while maintaining transmittance in the visible range. Furthermore, the third invention, in addition to the above features, lowers the transition temperature by adding a specified amount or more of specific impurities, thereby achieving thermal radiation suppression and adaptability to environmental temperatures. Compared to vanadium dioxide thin films of 50 nm or less obtained by conventional methods such as sputtering, the fourth invention's vanadium dioxide thin film exhibits half the reflectance in the visible to near-infrared range, and is expected to achieve transmittance of 50% or more in the visible range. [Example]

[0051] The present invention will be described below with reference to examples, but the scope of the present invention is not limited to these examples. In the following Examples 1 to 4, an organometallic decomposition solution (V-02, manufactured by Kojundo Chemical Laboratory Co., Ltd.) prepared by dissolving a vanadyl carboxylate solution in butyl acetate so that the oxide concentration was 2.0% by mass was used as the MOD solution. As the transparent substrate, a (0001) cut C-plane sapphire substrate (10×10×0.3 mm) was used.

[0052] Example 1 Light-control glass was fabricated by depositing a vanadium dioxide thin film on a transparent substrate according to the flow shown in Figure 3. The specific fabrication process and conditions are as follows: (Coating process) As a pretreatment, the transparent substrate was washed with an organic solvent. 0.1 mm of the MOD solution was spin-coated onto the transparent substrate at a rotation speed of 6000 rpm for 30 seconds. The substrate was then pre-baked at 120°C for 2 minutes, and the solvent was dried and removed. The above coating and pre-baking steps were repeated.

[0053] (Pre-firing) After the coating step, the transparent substrate coated with the MOD solution was heated in an inert atmosphere (1 atm) using a water-cooled infrared lamp heating device (MILA-3000 manufactured by ULVAC-RIKO) at a heating rate of 30°C / min until the substrate temperature reached 300°C, and then the substrate temperature was maintained at 300°C to 350°C for 15 minutes for pre-baking, and then cooled to room temperature at a heating rate of 30°C / min.

[0054] (Final firing) After the pre-firing, the transparent substrate coated with the MOD solution was heated in an inert atmosphere (1 atm) using a water-cooled infrared lamp heating device (MILA-3000 manufactured by ULVAC-RIKO) at a temperature increase rate of 30°C / min until the substrate temperature reached 580°C, and then the substrate temperature was maintained at 580°C to 600°C for 15 minutes for main firing, and then the substrate was cooled to room temperature at a temperature decrease rate of 30°C / min to obtain a light-control glass having a 100 nm-thick vanadium dioxide thin film formed on a sapphire substrate.

[0055] Example 2 A light-controllable glass having a vanadium dioxide thin film with a thickness of 200 nm formed on a sapphire substrate was obtained in the same manner as in Example 1, except that the coating step and pre-baking were repeated twice and then main baking was performed.

[0056] Example 3 A light-controllable glass having a vanadium dioxide thin film with a thickness of 300 nm formed on a sapphire substrate was obtained in the same manner as in Example 1, except that the coating step and pre-baking were repeated three times and then main baking was performed.

[0057] Example 4 A HZO thin film was prepared on a transparent substrate, which had been pretreated by washing with an organic solvent, using the chemically dissolved in solution (CSD) method to a thickness of 30 to 100 nm, and a buffer layer was then formed. Light-controlling glass was obtained in the same manner as in Examples 1 to 3, except that the substrate on which the obtained buffer layer was formed was used instead of a transparent substrate, and a vanadium dioxide thin film having a thickness of 100 nm, 200 nm, or 300 nm was formed on the substrate.

[0058] Example 5 Light-controlling glass was obtained in the same manner as in Examples 1 to 3, except that a (1102)-cut R-plane sapphire substrate (10 × 10 × 0.5 mm) was used as the transparent substrate, and a vanadium dioxide thin film having a thickness of 100 nm, 200 nm, or 300 nm was formed on the sapphire substrate.

[0059] Example 6 Nb2O5 was added to the MOD solution in an amount that would give 1 mol % Nb relative to the vanadium in the solution, and dissolved to obtain a Nb-added MOD solution. Light-controlling glass having a vanadium dioxide thin film with a thickness of 100 nm, 200 nm or 300 nm formed on a transparent substrate was obtained in the same manner as in Example 5, except that this Nb-added MOD solution was used instead of the MOD solution.

[0060] Example 7 Light-controlling glass was obtained in the same manner as in Example 6, except that the amount of Nb added was changed from 1 mol % to 2 mol %. A vanadium dioxide thin film having a thickness of 100 nm, 200 nm, or 300 nm was formed on a transparent substrate.

[0061] (Evaluation of crystallinity) X-ray diffraction (crystallinity evaluation by XRD) was performed on the light-control glasses obtained in Examples 1, 4, and 7 using an X-ray diffractometer (Rigaku Corporation: SmartLab). It was confirmed that the thin films formed on the substrates in all cases were vanadium dioxide thin films in which 70% or more of the vanadium was vanadium dioxide.

[0062] (Evaluation of surface morphology) When the vanadium dioxide thin films of the light-controlling glasses obtained in Examples 1 to 3 were observed using a confocal laser scanning microscope (LSM, Olympus OLS3000), it was confirmed that all of them had a nanoscale porous moth-eye structure with a cross section as shown in Figure 2(a), and that the pitch spacing of the protrusion array was approximately 200 nm.

[0063] (Transmittance change evaluation 1) The light-controlling glasses having a vanadium dioxide thin film thickness of 100 nm obtained in Examples 1, 4, and 7 were measured for transmittance in the visible to near-infrared region using a visible / infrared spectrophotometer (Shimadzu Corporation, UV3600Plus) and a heated glass stage (ST Japan) with a temperature accuracy of 0.1°C or less. The measurement results confirmed that the light-controlling glass obtained in Example 4, which has a buffer layer, has a higher transmittance than the light-controlling glass obtained in Examples 1 and 7, which do not have a buffer layer. These results also show that the presence of a buffer layer reduces thermal radiation.

[0064] (Evaluation of transmittance change 2) The relationship between light transmittance and wavelength in the visible to infrared range (wavelengths of 400 nm to 2000 nm) was measured for the light control glasses obtained in Examples 1 to 3 at 30° C. and 80° C. using a visible / infrared spectrophotometer (Shimadzu Corporation: UV3600Plus). The results are shown in Figure 4 (in Figure 4, (a) shows the results for Example 1, (b) for Example 2, and (c) for Example 3).

[0065] As is clear from FIG. 4, all of the light control glasses obtained in Examples 1 to 3 have high transmittance in the visible range and large light control rates in the infrared range. In particular, Example 1, which was pre-baked once (two layers), had a relatively high transmittance in the visible range, with an average transmittance in the visible range of approximately 70%, but the dimming rate (change in transmittance in the near-infrared range after phase transition) was relatively small. On the other hand, Example 3, which was pre-baked three times (six layers), had a high dimming rate in the infrared range, but a relatively low transmittance in the visible range. In order to obtain a high dimming rate while maintaining a high transmittance in the visible range, it seems appropriate to pre-baked twice (four layers). The difference depending on the number of pre-bakings is presumed to be due to differences in the thickness of the vanadium dioxide thin film formed.

[0066] (Evaluation of transmittance change 3) For the light control glasses obtained in Examples 5, 6, and 7, the light transmittance at a wavelength of 1600 nm was measured every 5°C while the temperature was raised at a rate of 5°C / minute, and the change in light transmittance with respect to temperature (transmittance change) was measured. The measurements were performed using a visible / infrared spectrophotometer (Shimadzu Corporation: UV3600Plus) and a heated glass stage (ST Japan) with a temperature accuracy of 0.1°C or less. The results are shown in Figure 5.

[0067] As shown in Figure 5, in all cases, the infrared transmittance changed critically at a certain temperature, and the phase transition temperature (Tc in Figure 5), which is the temperature at which the critical change occurred, was 73°C (Figure 5(a)) for the light-controlling glass obtained in Example 5, to which no Nb was added, 64°C (Figure 5(b)) for the light-controlling glass obtained in Example 6, to which 1 mol% Nb was added, and 53°C (Figure 5(c)) for the light-controlling glass obtained in Example 7, to which 2 mol% Nb was added. These results show that Nb can be added by the MOD method, and that by controlling the amount of Nb added, light-controlling glass with a phase transition temperature that is suited to the ambient temperature can be obtained.

[0068] (Other experimental examples) When the transmittance before the phase transition (near room temperature) of the light-controlling glass obtained in Example 1 was compared with that of a light-controlling glass (comparison product) that was the same as the light-controlling glass obtained in Example 1 except that the vanadium dioxide thin film had a uniform thin film rather than a nanoscale porous moth-eye structure, the light-controlling glass obtained in Example 1 had a transmittance that was 20% or more higher. The refractive index of the vanadium dioxide thin film of the comparison product was higher than that of the substrate below the phase transition temperature, resulting in a low transmittance before the phase transition, but in Example 1 (the first example of the present invention), the transmittance was improved by the moth-eye structure.

Claims

1. A light-controllable glass having a transparent substrate and a vanadium dioxide thin film provided on the transparent substrate or on one or more transparent layers provided on the transparent substrate, wherein the vanadium dioxide thin film has a moth-eye structure on its surface composed of protrusions at a period of 100 nm to 800 nm, and is porous composed of fine particles, with a particle area ratio measured by AFM (atomic force microscope) of 30% to 60%; and at a temperature equal to or lower than its phase transition temperature, the light-controllable glass has a refractive index such that the transmittance of the light-controllable glass at wavelengths of 600 nm to 800 nm is 40% to 80%, and the light-control rate (change in transmittance) at a wavelength of 1600 nm is 10% or more.

2. 2. The light-controlling glass according to claim 1, wherein a buffer layer made of a wide-gap oxide semiconductor is provided between the transparent substrate and the vanadium dioxide thin film.

3. 3. The light-controlling glass according to claim 1, wherein the vanadium dioxide thin film contains one or more cations selected from the group consisting of Nb, Ta, Mo, and W as a dopant in an amount of 1 mol % or more relative to vanadium, and has a phase transition temperature of 65° C. or lower.

4. A vanadium dioxide thin film characterized in that its surface has a moth-eye structure composed of protrusions with a period of 100 nm to 800 nm, is porous composed of fine particles, and has a particle area ratio of 30% to 60% as measured by AFM (atomic force microscope), and exhibits a refractive index that is lower at temperatures higher than its phase transition temperature than at temperatures below the phase transition temperature.

5. A method for producing a vanadium dioxide thin film, comprising applying a solution containing a vanadium organic compound to a uniform thickness on a transparent substrate or on a transparent layer provided on the transparent substrate, repeating pre-baking by heating at 300°C to 400°C once or a plurality of times, and then heating to a temperature higher than 550°C but lower than 600°C for main baking, wherein the temperature is increased to the main baking temperature at a rate of 30°C / min or more and the temperature is decreased from the main baking temperature at a rate of 25°C / min or more.

Images