The National Institute of Advanced Industrial Science and Technology (AIST) newly developed a novel multifunctional window glass capable of an automatic solar irradiation control in response to environment temperature, demonstrating some top-level performances in the world including a high visible transmission of 40-60% which has been the most challenging problem for such window. 

There have been window glasses in market for energy saving, of which the so called “heat reflecting glass” or “low-E glass” shows energy efficiency by either shading extra solar irradiation in summer, or with heat insulation in winter. Such windows, however, demonstrate only fixed optical properties without response to the seasonal requirements. Thermochromic (TC) material with changeable optical properties upon temperature, has been studied for automatic solar control window. However, a conventional TC window has been far away from commercialization due to the low visible transmission, poor heat insulation, and a very limited range of solar controllability.

The AIST has recently carried out a research project called “R&D of a heat mirror with active response to environmental temperature” entrusted from the New Energy and Industrial Technology Development Organization (NEDO), in addition to its long-time research experience on energy efficient windows including the TC ones. A unique multilayered structure was created based on the light interference theory of laminated optical system, leading to the highest performance in the world with visible transmission of 40-60% and solar controllability twice as much to the conventional ones. With an establishment of manufacturing process in addition to its potential multifunction, the window is highly promising to commercialization as a next generation window for energy saving, environmental protection and comfort.

Typical window structure and performance are demonstrated in the figure. The window shades 60-70% of the solar irradiation in summer, but receives almost twice as much the solar infrared (IR) in winter, switching automatically around 30^oC in response to the changes in environment temperature. The window can be made multifunctional with, e.g., highly heat insulation, photocatalytically self-cleaning, and even more by a careful modification of the structure.

A large amount of heat exchange, e.g., about 50% in winter and 70% in summer, takes place through the window, leading to a substantial energy loss for heating/cooling of house or building. Therefore, energy efficient windows are developed in response to the social requirements against the problem of globe warming. Pair-glassed windows have been used in house and building construction for higher insulation, and windows with heat reflecting or low-E coatings are developed for more energy efficiency. However, the above windows show only a fixed optical performance without any response to seasonally requirements. Thermochromic (TC) materials have been studied for automatic solar control. However, the conventional TC window is far away from commercialization due to the low visible transmission, poor heat insulation, and a very limited range of solar controllability.

align="left">The AIST has been dedicated to the development of the most advanced energy efficient window materials. Lately, particular efforts have been focused on a NEDO project “R&D of a heat mirror with active response to environmental temperature”. Although the use of a thermochromic material, switching optically by a semiconductor-to-metal phase transition, seems similar to the conventional works, a novel and unique multilayered structure with the thermochromic layer sandwiched by one or more functional layers was introduced for the first time, leading to substantial increases in visible transmission and solar controllability. The newly developed energy efficient window is able to perform multifunctionally with automatic solar control, total ultraviolet blocking, highly heat insulation, and photocatalytically self-cleaning. Further with the establishment of manufacturing process and possible multifunction in addition, the novel window is highly promising to commercialization as a next generation window for energy saving, environmental protection and comfort.

A thermochromic material of VO₂ crystal was used for solar irradiation control. The switching temperature was precisely set at around 30^oC by element doping.

At first, optical design was applied to the simplest case with only a VO₂ layer on glass. The most suitable layer thickness was predicted for the first time, leading to the highest solar controllability under little loss of visible transmission for the single film case. The optical calculation was confirmed by sample prepared by sputtering.

The property limitation of the single VO₂ layer system was drastically solved by a novel and unique multilayer structure, in which the VO₂ solar control layer is sandwiched by several functional layers, leading to the highest increase of visible transmission and solar controllability in the world for the VO₂-based thermochromic windows. The window shades as much as 60-70% of the solar irradiation in summer, but receives almost twice as much the solar IR in winter, switching automatically around 30^oC in response to the changes in environment temperature. Furthermore, the window can be made multifunctional with totally UV blocking, highly heat insulation, photocatalytically self-cleaning, and even more by a careful design of the structure. The establishment of the preparation process by sputtering for the multilayered structure opens the way for the window to commercialization.