Smart glass


Smart glass or switchable glass is a glass or glazing whose light transmission properties are altered when voltage, light, or heat is applied. In general, the glass changes from transparent to translucent and vice versa, changing from letting light pass through to blocking some wavelengths of light and vice versa.
Smart glass technologies include electrochromic, photochromic, thermochromic, [|suspended-particle], [|micro-blind], and [|polymer-dispersed liquid-crystal] devices.
When installed in the envelope of buildings, smart glass creates climate adaptive building shells.

Electrically switchable smart glass

Suspended-particle devices

In suspended-particle devices, a thin film laminate of rod-like nano-scale particles is suspended in a liquid and placed between two pieces of glass or plastic, or attached to one layer. When no voltage is applied, the suspended particles are randomly organized, thus blocking and absorbing light. When voltage is applied, the suspended particles align and let light pass. Varying the voltage of the film varies the orientation of the suspended particles, thereby regulating the tint of the glazing and the amount of light transmitted. SPDs can be manually or automatically "tuned" to precisely control the amount of light, glare and heat passing through.

Electrochromic devices

change light transmission properties in response to voltage and thus allow control over the amount of light and heat passing through. In electrochromic windows, the electrochromic material changes its opacity. A burst of electricity is required for changing its opacity, but once the change has been effected, no electricity is needed for maintaining the particular shade which has been reached.
First generation electrochromic technologies tend to have a yellow cast in their clear states and blue hues in their tinted states. Darkening occurs from the edges, moving inward, and is a slow process, ranging from many seconds to several minutes depending on window size. Newer electrochromic technologies eliminate the yellow cast in the clear state and tinting to more neutral shades of gray, tinting evenly rather than from the outside in, and accelerate the tinting speeds to less than three minutes, regardless of the size of the glass. Electrochromic glass provides visibility even in the darkened state and thus preserves visible contact with the outside environment.
Recent advances in electrochromic materials pertaining to transition-metal hydride electrochromics have led to the development of reflective hydrides, which become reflective rather than absorbing, and thus switch states between transparent and mirror-like.
Recent advancements in modified porous nano-crystalline films have enabled the creation of electrochromic display. The single substrate display structure consists of several stacked porous layers printed on top of each other on a substrate modified with a transparent conductor. Each printed layer has a specific set of functions. A working electrode consists of a positive porous semiconductor such as Titanium Dioxide, with adsorbed chromogens. These chromogens change color by reduction or oxidation. A passivator is used as the negative of the image to improve electrical performance. The insulator layer serves the purpose of increasing the contrast ratio and separating the working electrode electrically from the counter electrode. The counter electrode provides a high capacitance to counterbalances the charge inserted/extracted on the SEG electrode. Carbon is an example of charge reservoir film. A conducting carbon layer is typically used as the conductive back contact for the counter electrode. In the last printing step, the porous monolith structure is overprinted with a liquid or polymer-gel electrolyte, dried, and then may be incorporated into various encapsulation or enclosures, depending on the application requirements. Displays are very thin, typically 30 micrometer, or about 1/3 of a human hair. The device can be switched on by applying an electrical potential to the transparent conducting substrate relative to the conductive carbon layer. This causes a reduction of viologen molecules to occur inside the working electrode. By reversing the applied potential or providing a discharge path, the device bleaches. A unique feature of the electrochromic monolith is the relatively low voltage needed to color or bleach the viologens. This can be explained by the small over- potentials needed to drive the electrochemical reduction of the surface adsorbed viologens/chromogens.

Polymer-dispersed liquid-crystal devices

In polymer-dispersed liquid-crystal devices, liquid crystals are dissolved or dispersed into a liquid polymer followed by solidification or curing of the polymer. During the change of the polymer from a liquid to solid, the liquid crystals become incompatible with the solid polymer and form droplets throughout the solid polymer. The curing conditions affect the size of the droplets that in turn affect the final operating properties of the "smart window". Typically, the liquid mix of polymer and liquid crystals is placed between two layers of glass or plastic that include a thin layer of a transparent, conductive material followed by curing of the polymer, thereby forming the basic sandwich structure of the smart window. This structure is in effect a capacitor.
Electrodes from a power supply are attached to the transparent electrodes. With no applied voltage, the liquid crystals are randomly arranged in the droplets, resulting in scattering of light as it passes through the smart window assembly. This results in the translucent, "milky white" appearance. When a voltage is applied to the electrodes, the electric field formed between the two transparent electrodes on the glass causes the liquid crystals to align, allowing light to pass through the droplets with very little scattering and resulting in a transparent state. The degree of transparency can be controlled by the applied voltage. This is possible because at lower voltages, only a few of the liquid crystals align completely in the electric field, so only a small portion of the light passes through while most of the light is scattered. As the voltage is increased, fewer liquid crystals remain out of alignment, resulting in less light being scattered. It is also possible to control the amount of light and heat passing through, when tints and special inner layers are used.

Micro-blinds

Micro-blinds control the amount of light passing through in response to applied voltage. The micro-blinds are composed of rolled thin metal blinds on glass. They are very small and thus practically invisible to the eye. The metal layer is deposited by magnetron sputtering and patterned by laser or lithography process. The glass substrate includes a thin layer of a transparent conducting oxide layer. A thin insulator is deposited between the rolled metal layer and the TCO layer for electrical disconnection. With no applied voltage, the micro-blinds are rolled and let light pass through. When there is a potential difference between the rolled metal layer and the transparent conductive layer, the electric field formed between the two electrodes causes the rolled micro-blinds to stretch out and thus block light. The micro-blinds have several advantages including switching speed, UV durability, customized appearance and transmission. The micro-blinds is developed at the National Research Council.

Related areas of technology

The expression smart glass can be interpreted in a wider sense to include also glazings that change light transmission properties in response to an environmental signal such as light or temperature.
These types of glazings cannot be controlled manually. In contrast, all electrically switched smart windows can be made to automatically adapt their light transmission properties in response to temperature or brightness by integration with a thermometer or photosensor, respectively.

Examples of use

in Melbourne has a glass cube which projects out from the building with visitors inside, suspended almost above the ground. When one enters, the glass is opaque as the cube moves out over the edge of the building. Once fully extended over the edge, the glass becomes clear.
The Boeing 787 Dreamliner features electrochromic windows which replaced the pull down window shades on existing aircraft.
NASA is looking into using electrochromics to manage the thermal environment experienced by the newly developed Orion and Altair space vehicles.
Smart glass has been used in some small-production cars including the Ferrari 575 M Superamerica.
ICE 3 high speed trains use electrochromatic glass panels between the passenger compartment and the driver's cabin.
The elevators in the Washington Monument use smart glass in order for passengers to view the commemorative stones inside the monument.
The city's restroom in Amsterdam's Museumplein square features smart glass for ease of determining the occupancy status of an empty stall when the door is shut, and then for privacy when occupied.
Bombardier Transportation has intelligent on-blur windows in the Bombardier Innovia APM 100 operating on Singapore's Bukit Panjang LRT Line, to prevent passengers from peering into apartments while the train is moving and is planning to offer windows using smart glass technology in its Flexity 2 light rail vehicles.
Chinese phone manufacturer OnePlus demonstrated a phone whose rear cameras are placed behind a pane of electrochromic glass.

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