Smart glass

Smart glass or switchable glass, also called smart windows or switchable windows in its application to windows or skylights, refers to electrically switchable glass or glazing which changes light transmission properties when voltage is applied.

Certain types of smart glass can allow users to control the amount of light and heat passing through: with the press of a button, it changes from transparent to translucent, partially blocking light while maintaining a clear view of what lies behind the window. Another type of smart glass can provide privacy at the turn of a switch.

Smart glass technologies include electrochromic devices, suspended particle devices, Micro-Blinds and liquid crystal devices.

The use of smart glass can save costs for heating, air-conditioning and lighting and avoid the cost of installing and maintaining motorized light screens or blinds or curtains. When opaque, liquid crystal or electrochromic smart glass blocks most UV, thereby reducing fabric fading; for SPD-type smart glass, this is achieved when used in conjunction with low-e low emissivity coatings.

Critical aspects of smart glass include installation costs, the use of electricity, durability, as well as functional features such as the speed of control, possibilities for dimming, and the degree of transparency of the glass.

Electrically switchable smart glass

Electrochromic devices

Main article: electrochromism

Electrochromic devices change light transmission properties in response to voltage and thus allow to control the amount of light and heat passing through. In electrochromic windows, the electrochromic material changes its opacity: it changes between a colored, translucent state (usually blue) and a transparent state. A burst of electricity is required for changing its opacity, but once the change has been effectuated, no electricity is needed for maintaining the particular shade which has been reached. Darkening occurs from the edges, moving inward, and is a slow process, ranging from many seconds to several minutes depending on window size. Electrochromic glass provides visibility even in the darkened state and thus preserves visible contact with the outside environment. It has been used in small-scale applications such as rearview mirrors. Electrochromic technology also finds use in indoor applications, for example, for protection of objects under the glass of museum display cases and picture frame glass from the damaging effects of the UV and visible wavelengths of artificial light.

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 (such as ITO or PEDOT:PSS). Each printed layer has a specific set of functions. A working electrode consists of a positive porous semiconductor (say Titanium Dioxide, TiO2) with adsorbed chromogens (different chromogens for different colors). 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 (and maintain overall device charge neutrality). 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 a electrical potential to the transparent conducting substrate relative to the conductive carbon layer. This causes a reduction of viologen molecules (coloration) 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 (around 1 Volt) 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.

Suspended particle devices

In suspended particle devices (SPDs), a thin film laminate of rod-like particles suspended in a fluid is placed between two glass or plastic layers, or attached to one layer. When no voltage is applied, the suspended particles are arranged in random orientations and tend to absorb light, so that the glass panel looks dark (or opaque), blue or, in more recent developments, grey or black colour. When voltage is applied, the suspended particles align and let light pass. SPDs can be dimmed, and allow instant control of the amount of light and heat passing through. A small but constant electrical current is required for keeping the SPD smart window in its transparent stage.

Polymer dispersed liquid crystal devices

In polymer dispersed liquid crystal devices (PDLCs), 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, as discovered by Al Coat Ltd. and SmartGlass International, when tints and special inner layers are used. It is also possible to create fire-rated and anti X-Ray versions for use in special applications. In addition, Al Coat has demonstrated that patterns can be formed in the transparent electrodes or in the polymer, enabling fabrication of display devices and decorative windows. Most of the devices offered today operate in on or off states only, even though the technology to provide for variable levels of transparency is easily applied. This technology has been used in interior and exterior settings for privacy control (for example conference rooms, intensive-care areas, bathroom/shower doors) and as a temporary projection screen. It has been marketed under the name of "Polyvision privacy glass". New generation nano-technology switchable films with improved transparency are offered by Polytronix, Inc. and Scienstry, with working voltages lowered to the 12 to 48 VAC range; the lower driving voltage could extend life expectancy to some extent.

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 so 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 conductive oxide (TCO) 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 (milliseconds), UV durability, customized appearance and transmission. The micro-blinds are under development at the National Research Council (Canada). One of the novelties is their simple and cost-efficient fabrication scheme.

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.

• Different types of glazing can show a variety of chromic phenomena, that is, based on photochemical effects the glazing changes its light transmission properties in response to an environmental signal such as light (photochromism), temperature (thermochromism), or voltage (electrochromism).
• Liquid crystals, when they are in a thermotropic state, can change light transmission properties in response to temperature.
• Recent advancements in electrochromic materials have led to the discovery that transition metal hydride electrochromics that create a reflective face instead of an absorbent. These materials have the same idea but go about the problem in a different way by switching between a transparent state when they are off to a reflective state when a voltage is applied . Switchable mirrors were originally developed by Tom Richardson and Jonathon Slack of Berkeley Lab’s Environmental Energy Technologies Division. They used rare earth metals and created the first metal-hydride switchable mirrors. Low emittance coatings reject unwanted thermal heat due to solar infrared21. These mirrors have become common place in cars’ rearview mirrors in order to block the glare of following vehicles. An optically absorbing electrochromic color reduces the reflection intensity. These mirrors must be fully transformed to a reflective state as muted reflection must persist in the darkened state. Originally a metal, they are converted into a transparent hydride by injecting hydrogen in a gas or liquid phase. It then switches to a reflective state.
• Various metals have been looked into. Thin Ni-Mg films have low visible transmittance and are reflective. When they are exposed to H2 gas or reduced by an alkaline electrolyte, they become transparent. This transition is attributed to the formation of nickel magnesium hydride, Mg2NiH4. Films were created by cosputtering from separate targets of Ni and Mg to facilitate variations in composition. Single-target d.c. magnetron sputtering could be used eventually which would be relatively simple compared to deposition of electrochromic oxides, making them more affordable. Lawrence Berkeley National Laboratory determined that new transition metals were cheaper and less reactive but contained the same qualities further reducing the cost .

• Tungsten doped Vanadium dioxide VO₂ coating reflects infrared light when the temperature rises over 29 degrees Celsius, to block out sunlight transmission through windows at high ambient temperatures.

These types of glazing 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.

The topic of smart windows in a wider sense includes also self-cleaning glass and the automatic opening or closing of windows for ventilation purposes, for example according to a timer or in response to a rain sensor.

Examples of use

Smart Glass using one of the aforementioned technologies has been seen in a number of high profile applications. SmartGlass International have completed large scale installations such as the Guinness Storehouse in Dublin where over 800,000 people per year can see LC SmartGlass being used in interactive displays and privacy windows. LC SmartGlass was used to launch the Nissan Micra CC in London using a four-sided glass box made up of 150 switchable glass panels which switched in sequence to create a striking outdoor display. The main use for LC SmartGlass is in internal partitions where many companies now enjoy the ability to switch screens and doors from clear to private.

SmartGlass International has manufactured and installed a number of SPD smart glass panels into rooflights for high end domestic projects. These SPD SmartGlass windows ranged from 400 by 400 mm to 1000 by 2000 mm and allowed clients to regulate the amount of light entering master bedroom suites.

Another example of use is the installation of PDLC-based smart glass provided by iGlass in Australia, in the EDGE, a glass cube which protrudes out from the 88th floor skydeck of the world's highest residential tower, Eureka Towers, located in Melbourne. The cube can hold 13 people. When it extends out of the building by 3 metres, the glass is made transparent, giving the cube's occupants views of Melbourne from a height of 275 metres. The same type of smart glass has also been proposed for use in hospital settings to controllably provide patients with privacy as needed.

PDLC technology provided by KEEN Projection Media Ltd. was used in a display to unveil the Nissan GTR at the Canadian International Auto Show in Toronto.

In the media, the updated set for the Seven Network's Sunrise program features a Smart Glass background that uses liquid crystal switchable glass (AGP UMU Glass) supplied by Architectural Glass Projects. The technology is especially suited to this purpose, as the set was originally open to a public place, meaning that people could do obscene things behind the presenters. The new set with Smart Glass allows the street scene to be visible at times, or replaced with either opaque or transparent blue colouring, masking the view.

The new Boeing 787 Dreamliner features electrochromic windows which replace 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. The Ferrari 575 M Superamerica had an electrochromic roof as standard, and the Maybach has a PDLC roof as option. Some Polyvision Privacy Glass has been applied in the Maybach 62 car for privacy protection purposes.

A Hongkong office uses 130 square meters of Polyvision Privacy Glass, which is available in sizes up to 1500 x 3200 mm.

ICE-3 high speed trains use electrochromatic glass panels between the passenger compartment and the driver's cabin. The standard mode is clear/lucent and can be switched by the driver to frosted/translucent mainly to keep passengers off "unwanted sights" for example in case of (human) obstacles.

An interesting project of soundproof PDLC smart glass was completed in Moscow by Shtiever Ltd. The installation features smart glass panels made of Polyvision Privacy film and special grade acoustic PVB interlayer. This combination of materials made it possible to create a unique soundproof glass barrier within the private apartment.

Soundproof Smart Glass On Русский: Звукоизолирующее смарт стекло - включено

Soundproof Smart Glass Off Русский: Звукоизолирующее смарт стекло - выключено

Switchable LED Glass

From the technology of PDLC glass, Polytron Technologies Inc. has developed new products in the PDLC glass family using LEDs embedded into two interlayers of laminated glass together with PDLC film. This creates a unique product called Switchable LED Glass. This is brand new technology of PDLC glass which combines energy efficient LEDs to create a material for uses such as office partitions, business showcases, or special countertops.

ICE 3 train with view in driver's cab

Glass panel switched to "frosted" mode

External links

• Electronic Smart Glasses at GlassOnWeb
• Lower Voltage Switchable Glass, at Polytronix
• Chromogenics, in: Windows and Daylighting at Lawrence Berkeley National Laboratory
• Smart glass blocks infrared when heat is on, NewScientist.com news service
• PDLC switchable windows, Liquid Crystal Institute at Kent State University
• Switchable Glass: A possible medium for Evolvable Hardware, NASA conference on Adaptive Hardware Systems, IEEE CS Press, pp 81–87, 2006.
• Switchable Glazing Windows Change the light transmittance, transparency, or shading of windows at toolbase.org
• SPD Control System Corporation Intelligent Control Systems for SPD Smart Glass
• A new generation of switchable windows, at Scienstry