Friday, March 26, 2010

Invisible Microblinds for Smart Windows Control Heat and Light Transmission-Easily Manufactured by Optical Lithography

National Research Council of Canada (Ottawa, ON, CA) scientists have developed a microblind system for smart windows that can be used let light or heat through a window or keep them out. For optimal energy efficiency, smart windows using the blinds would allow the transmission of IR in the winter so it contributes to heating the building, and block it in the summer to help the building stay cool. The amount of visible light could also be tuned to optimize the comfort of ambient conditions to users and minimize the amount of general lighting needed. The microblinds could even be used to make window displays.

According to inventors Boris Lamontagne  and Christophe Py, writing in U.S. Patent 7,684,105, the system has an array of overhanging stressed microblinds, each having an anchor portion attached a substrate and a mobile portion. The microblinds are responsive to electrostatic forces to mutate between a deployed configuration wherein the mobile portion obscure the glass substrate and a curled configuration wherein the mobile portion exposes the substrate. A transparent conductive layer permits the application of an electric field to the microblinds to control the open and shut modes.

Microblinds for windows would modify-their visible and/or infrared transmission. This allows better comfort for the occupant as well as major energy savings in terms of heating and cooling costs.  The microblinds also overcome the limitations of current technology for smart windows, which is mainly based on the use of electrochromic layers, has severe limitations in terms of lifetime, speed, maximum IR transmission and visual aspect. 

Smart windows using the microblinds would be capable of varying the transmission characteristics depending on the solar radiation, the outdoor weather conditions and the different requirements of the users (for instance, high ambient light for a meeting, low ambient light for the projection of a film).

The figure is a micrograph image of curled microblinds on a 500 micron scale as developed by National Research Council of Canada scientists

The miroblinds provides a means to make smart windows by fabricating microblinds directly on a large scale substrate, such as a glass window pane or illuminated panel, or on an additional substrate that could be added to existing windows or panels. These microblinds are fabricated using a thin layer or thin layers under controlled stress. 

The microblinds are activated by electrostatic forces and can therefore be actuated either by an automated monitoring system (ambient light and/or temperature) or directly by the user. In the case of windows, they are intended to be applied to buildings, or in the case of an illuminated panel they could be applied to billboards.

The term window as used herein thus means a transparent pane of the type that would typically be found in a building, or vehicle, such as an airplane, space vehicle, or automobile, for example, or that is designed to be looked through or at by a human occupant. Typically, such a window would have dimensions in the order of 1 meter square, although it could be smaller. The microblinds can also be formed on a supporting layer to be applied to such a window, or also they could be applied to an illuminated panel, such as found on a billboard.        

Patterning of the microblinds can be accomplished by standard optical lithography. However, owing to the large dimensions involved, some methods are particularly advantageous: micro-templating using very large rollers with a mold, laser patterning or a combination of those methods or others can be used. The optical lithography technique is being developed very actively by the nano-industry. It involves a soft material to imprint the microblind geometry. The second technique is available for very large area patterning, especially for the display industry.

The dimensions of microblinds need not have a geometric progression: grey levels could be obtained with any distribution of dimensions, including having all blinds of the same size, as long as some of the blinds can be actuated independently from some others.

A particular advantage of having microblinds of different dimensions is to overcome the possibly deleterious effect of diffraction of light through periodic structures, for example the Moire effect. The Moire effect is observed when superimposing a repetitive design, such as a grid, on the same or a different design in order to produce a pattern distinct from its components. If such a periodic structure were to be observed through a smart window whose microblind array is normally invisible to the user, that periodic structure may appear to the viewer different than if it was not viewed through the window. That undesirable effect is overcome by arranging the microblinds array in a non-periodic structure.

Partial opening of the window pane, or grey levels, can be obtained by using different geometry-size of microblinds and having means of actuating them independently.  The blinds can also be configured to provide grey level with additional refinement, and allow the window pane to be used as a display.  

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