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Electrochromism

Electrochromism

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Electrochromism is the phenomenon displayed by some materials of reversibly
Reversible reaction
A reversible reaction is a chemical reaction that results in an equilibrium mixture of reactants and products. For a reaction involving two reactants and two products this can be expressed symbolically as...

 changing color when a burst of charge
Charge (physics)
In physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...

 is applied. Various types of materials and structures can be used to construct electrochromic devices, depending on the specific applications.

One good example of an electrochromic material is polyaniline
Polyaniline
Polyaniline is a conducting polymer of the semi-flexible rod polymer family. Although the compound itself was discovered over 150 years ago, only since the early 1980s has polyaniline captured the intense attention of the scientific community. This is due to the rediscovery of its high electrical...

 which can be formed either by the electrochemical or chemical oxidation of aniline
Aniline
Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...

. If an electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

 is immersed in hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

 which contains a small concentration of aniline, then a film of polyaniline can be grown on the electrode. Depending on the oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

, polyaniline can either be pale yellow or dark green/black. Other electrochromic materials that have found technological application include the viologen
Viologen
Viologens are toxic bipyridinium derivatives of 4,4'-bipyridyl. The name is because this class of compounds is easily reduced to the radical mono cation, which is intensely blue coloured....

s and polyoxotungstates. Other electrochromic materials include tungsten oxide (WO3), which is the main chemical used in the production of electrochromic window
Window
A window is a transparent or translucent opening in a wall or door that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material like float glass. Windows are held in place by frames, which...

s or smart glass.

Polymer-based solutions have recently been developed by John Reynolds and colleagues at the University of Florida
University of Florida
The University of Florida is an American public land-grant, sea-grant, and space-grant research university located on a campus in Gainesville, Florida. The university traces its historical origins to 1853, and has operated continuously on its present Gainesville campus since September 1906...

. These promise to provide flexible and cheap electrochromics in a variety of colours, going all the way up to black.

As the color change is persistent and energy need only be applied to effect a change, electrochromic materials are used to control the amount of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 and heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 allowed to pass through windows ("smart windows"), and has also been applied in the automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 industry
Industry
Industry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...

 to automatically tint
Window film
There are many types of window tint available in the market for a wide variety of uses from solar heat reduction to UV protection, privacy to safety and security, decorative applications to heat retention.- Primary properties :...

 rear-view mirror
Rear-view mirror
A rear-view mirror is a mirror in automobiles and other vehicles, designed to allow the driver to see rearward through the vehicle's backlight ....

s in various lighting
Lighting
Lighting or illumination is the deliberate application of light to achieve some practical or aesthetic effect. Lighting includes the use of both artificial light sources such as lamps and light fixtures, as well as natural illumination by capturing daylight...

 conditions. Viologen
Viologen
Viologens are toxic bipyridinium derivatives of 4,4'-bipyridyl. The name is because this class of compounds is easily reduced to the radical mono cation, which is intensely blue coloured....

 is used in conjunction with titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...

 (TiO2) in the creation of small digital displays. It is hoped that these will replace liquid crystal display
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....

s as the viologen, which is typically dark blue, has a high contrast compared to the bright white of the titania, thereby providing the display high visibility.

ICE 3
ICE 3
ICE 3 is a family of high-speed EMUs of Deutsche Bahn. It includes classes 403 and 406, which are known as ICE 3 and ICE 3M respectively...

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

See also

  • e-paper
  • C.G. Granqvist
    Claes-Göran Granqvist
    Claes-Göran Granqvist was born in Helsingborg, Sweden, in 1946. He is one of the leading figures of Swedish and international science in various fields including nanomaterials; green nanotechnology; materials for solar energy utilization and energy efficiency , electrochromic materials...

    , Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995, reprinted 2002, approx. 650 pages.

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