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Smart glass
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== Electrically switchable smart glass == The following table shows an overview of the different electrically switchable smart glass technologies: {| class="wikitable sortable" !Technology !State with electricity !State without electricity !Comment |- |Electrochromic devices |Electric pulses are used for changing the light transmission |Maintains previous state |Transition times and light transmissivity vary by manufacturer. |- |Polymer-dispersed liquid-crystal devices |Transparent |Opaque |Clear or opaque states only. Primarily used for privacy control for interior settings. |- |Suspended-particle devices |Transparent |Partly opaque |Enables control of light transmissivity. |- |Micro-blinds |Opaque |Transparent |Switches state quickly, handles wear from UV radiation well |- |} ===Electrochromic devices=== [[Electrochromic devices]] change light transmission properties in response to voltage and thus allow control over the amount of light and heat passing through.<ref name="Nature">{{cite journal |last1=Xu |first1=Ting |last2=Walter |first2=Erich C. |last3=Agrawal |first3=Amit |last4=Bohn |first4=Christopher |last5=Velmurugan |first5=Jeyavel |last6=Zhu |first6=Wenqi |last7=Lezec |first7=J. |last8=Talin |first8=A.Alec |title=High-contrast and fast electrochromic switching enabled by plasmonics |journal=Nature Communications |volume=7 |pages=10479 |date=27 January 2016 |pmc=4737852 |pmid=26814453 |doi=10.1038/ncomms10479 |bibcode=2016NatCo...710479X }}</ref> In electrochromic windows, the material changes its [[opacity (optics)|opacity]]. A burst of electricity is required for changing its opacity, but the material maintains its shade with little to no additional electrical signals.<ref name="Mortimer">{{cite news |last1=Mortimer |first1=Roger J. |title=Switching Colors with Electricity |url=https://www.americanscientist.org/article/switching-colors-with-electricity |access-date=15 July 2022 |work=American Scientist |date=6 February 2017 |language=en}}</ref> Old 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 20β30 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 maintains visibility in its darkened state and thus preserves visual contact with the outside environment. Recent advances in electrochromic materials pertaining to [[Transition metal|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 [[Nanocrystalline material|nanocrystalline]] 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 [[Indium tin oxide|ITO]] or [[PEDOT:PSS]]). Each printed layer has a specific set of functions. A working electrode consists of a positive porous semiconductor such as titanium dioxide, with adsorbed [[chromogen]]s. These chromogens change color via reduction or oxidation. A [[Passivation (chemistry)|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 electrically separating the working electrode from the counter [[electrode]]. The counter electrode provides a high capacitance to counterbalance the charges inserted/extracted on the SEG electrode (and maintain charge neutrality in the overall device). Carbon is an example of a 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, often 30 micrometers. 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 (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 [[Viologen|viologens]]. This can be explained by the small over- potentials needed to drive the electrochemical reduction of the surface adsorbed viologens/chromogens. Most types of smart film require voltage (e.g. 110VAC) to operate, and therefore such types of smart films must be enclosed within glass, acrylic or polycarbonate laminates to provide electrical safety to users.{{ciation needed|date=August 2021}} ===Polymer-dispersed liquid-crystal devices=== In polymer-dispersed [[liquid crystal|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, when tints and special inner layers are used. === Suspended-particle devices === In suspended-particle devices (SPDs), a thin film laminate of rod-like [[Nanoscopic scale|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. === Micro-blinds === [[File:Microblind.JPG|thumb|[[Scanning electron microscope]] (SEM) image of 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]] (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 technology of micro-blinds was developed at the [[National Research Council (Canada)]].
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