In general, a window (opening) of a house or building is a location for the transfer in and out of a lot of heat. For example, a rate of heat loss flowing from a window in a case of heating in winter is approximately 48% and a rate of heat inflowing from a window in a case of cooling in summer is even approximately 71%. Therefore, it is possible to obtain an effect of enormous energy saving by appropriately controlling light or heat through a window.
A light control glass has been developed for such a purpose and has a function of controlling an inflow and/or outflow of light and/or heat.
There are some kinds of methods for conducting a light control of such a light control glass. Among those, 1) a material with a light transmittance that is reversibly changed by applying an electric current or an electric voltage thereto is referred to as an electrochromic material, 2) a material with a light transmittance that is changed depending on a temperature is referred to as a thermochromic material, and further, 3) a material with a light transmittance that is changed by a control of an atmospheric gas is referred to as a gasochromic material. Among these, a study of an electrochromic light control glass that uses a tungsten oxide thin film for a light control layer is most advanced, so that a stage of practical application has generally been attained at present and a marketed product has also been provided.
However, an electrochromic light control glass wherein a tungsten thin film is used for this light control layer is such that a principle thereof is that light is absorbed by the light control layer to conduct a light control. Therefore, there is a problem in that the energy saving effect is degraded because a light control layer absorbs light so as to be heated and it is also re-radiated into the room interior. In order to eliminate this, a light control is not conducted by absorbing light and it is necessary to conduct a light control by reflecting light. That is, a material (reflection-type light control element) has been desired that has a characteristic in such a manner that a state thereof is reversibly changed between a transparent state and a reflection state.
For a material that has such a characteristic, it has been found and reported in recent years that a state is reversibly changed between a transparent state and a reflection state due to a hydrogenation and a dehydrogenation of a rare earth metal such as a yttrium or a lanthanum (see, for example, U.S. Pat. No. 5,635,729).
Otherwise, an alloy of a rare earth metal such as a gadolinium and a magnesium (see, for example, U.S. Pat. No. 5,905,590), an alloy of a magnesium and a transition metal (for example, see U.S. Pat. No. 6,647,166), and an alloy of an alkaline-earth metal such as a calcium and a magnesium (for example, see Japanese Patent Application Publication No. 2010-066747) have already been known as a material that has a reflection-type light control characteristic (light control mirror characteristic).
However, a switching repetition durability of the above-mentioned reflection-type light control element between a transparent state and a reflection state is low. Accordingly, a method that interposes a buffer layer between a layer that has a reflection-type light control characteristic and a catalyst layer and further forms a hydrogen-permeable and water-repellent protection layer on a surface of the catalyst layer has been conducted in order to improve durability. However, there is a problem in that a switching repetition durability of about 1600 times is not provided even if a buffer layer or a protection layer is provided (for example, see Bao, et al., Solar Energy Materials & Solar Cells, Vol. 93, 1642 (2009)).