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Electrochromism

Another more well known form of chromism, after thermochromism, is electrochromism, or the process whereby a material changes color after the application (or removal) of an electric field. The term was first recorded in 1961, though research on the phenomena has been recorded as early as the early 1800s. Metal oxides, such as tungsten trioxide, some conjugated or aromatic polymers, and organic materials known as viologens are most of interest as electrochromic materials. The largest application of electrochromism can be found in smart glass, smart windows, and optical displays, which can change the transparency of the material as directed and block or transmit specific wavelengths of light.

Sources/Further Reading: (Image 1—2015 article) (Image 2—2019 book chapter) (Image 3—2021 article) (Image 4—2019 article) (Wikipedia) (Smart Glass World) (Georgia Tech)

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Gradation-tint smart window

NIMS, Waseda University and Tama Art University developed together smart glass capable of producing various shades on its surface. Unlike the conventional types, the newly developed tinting smart glass allows users to easily change the shaded area of a window. For example, a user would be able to change the shaded area of a window in accordance with the elevation of the sun. The technology may be applicable to various types of windows, including those of automobiles and buildings, enabling them to offer both shade and clear visibility simultaneously.

Electrochromic glass capable of blocking sunlight in response to electricity has been used in airplane windows in recent years. However, because these windows become shaded uniformly across their entire surface, once they are darkened, passengers are unable to enjoy any exterior views.

The joint research group recently developed a tinting smart glass that can produce gradational shade using electrochromic polymers (metallo-supramolecular polymers) studied by the group. The darkened part of the glass surface can be changed simply by applying a low voltage (3 V) for different durations. The desired shade can be maintained even after the voltage is reduced to turn off the current. In addition, the shading and transparency can be reversed by reversing the electric current, allowing users to easily change the darkened area of the glass surface as desired.

Spray-on electric rainbows: Making safer electrochromic inks

An innovation with water allows electrochromic conjugated polymer films to be applied more safely

Anyone who has a rear-view mirror that automatically dims blue in reaction to annoying high-beam headlights glaring from behind has seen an electrochromic film in action.

Now, chemists at the Georgia Institute of Technology have developed a new method to more safely and, by extension, easily produce these shear films, which change their color with the help of a tiny electric current. This could make them available to many industries that have not been able to feasibly use them before.

In manufacturing, electrochromic films are often coated onto other materials, such as the surface of a mirror, as inks. They are usually based in solvents that are flammable and have toxic fumes, making them unsuitable for many work settings that rely on printing and spraying machinery to apply colors.

Georgia Tech researchers have developed electrochromic film inks that are water-based, making them safer for diffuse application in settings where the kinds of safety precautions and protective equipment that are standard in handling volatile organic chemicals would be impractical.

Self-powered system makes smart windows smarter

Smart windows equipped with controllable glazing can augment lighting, cooling and heating systems by varying their tint, saving up to 40 percent in an average building's energy costs.

These smart windows require power for operation, so they are relatively complicated to install in existing buildings. But by applying a new solar cell technology, researchers at Princeton University have developed a different type of smart window: a self-powered version that promises to be inexpensive and easy to apply to existing windows. This system features solar cells that selectively absorb near-ultraviolet (near-UV) light, so the new windows are completely self-powered.

"Sunlight is a mixture of electromagnetic radiation made up of near-UV rays, visible light, and infrared energy, or heat," said Yueh-Lin (Lynn) Loo, director of the Andlinger Center for Energy and the Environment, and the Theodora D. '78 and William H. Walton III '74 Professor in Engineering. "We wanted the smart window to dynamically control the amount of natural light and heat that can come inside, saving on energy cost and making the space more comfortable."

The smart window controls the transmission of visible light and infrared heat into the building, while the new type of solar cell uses near-UV light to power the system.

Nanophotonics team creates low-voltage, multicolor, electrochromic glass

Rice University's latest nanophotonics research could expand the color palette for companies in the fast-growing market for glass windows that change color at the flick of an electric switch.

In a new paper in the American Chemical Society journal ACS Nano, researchers from the laboratory of Rice plasmonics pioneer Naomi Halas report using a readily available, inexpensive hydrocarbon molecule called perylene to create glass that can turn two different colors at low voltages.

"When we put charges on the molecules or remove charges from them, they go from clear to a vivid color," said Halas, director of the Laboratory for Nanophotonics (LANP), lead scientist on the new study and the director of Rice's Smalley-Curl Institute. "We sandwiched these molecules between glass, and we're able to make something that looks like a window, but the window changes to different types of color depending on how we apply a very low voltage."

Controlling the way cracks form and spread to make a coating for electrochromic materials

Cracks in a material typically compromise its strength and integrity, so research focus has traditionally been on preventing their occurrence and spread. An A*STAR team has now taken a different approach, prompting and directing the propagation of cracks on thin films to make highly-ordered patterned coatings for electrochromic materials.

The transmission of light by electrochromic materials alters in response to brief bursts of electrical charge. They have optical uses ranging from the windows in Boeing 787-9 Dreamliners which change color at the touch of a button, to privacy glass around hotel bathrooms which switch between clear and opaque, to auto-dimming rear view car mirrors.

To feasibly expand the potential uses for these materials, scientists must reduce the amount of electrical power needed to modulate their optical property changes, explains team leader Sing Yang Chiam from the A*STAR Institute of Materials Research and Engineering. To achieve this, "devices will require a greater surface area of contact for enhanced interaction", he says. "If you use nanoparticles for a large surface area, scattering makes for poor optical properties. Using a film with controlled cracks allows us to increase the surface area for better electrical efficiency, without sacrifice of the optical properties."

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New flexible material can make any window 'smart'

Researchers in the Cockrell School of Engineering at The University of Texas at Austin have invented a new flexible smart window material that, when incorporated into windows, sunroofs, or even curved glass surfaces, will have the ability to control both heat and light from the sun. Their article about the new material will be published in the September issue of Nature Materials.

Delia Milliron, an associate professor in the McKetta Department of Chemical Engineering, and her team's advancement is a new low-temperature process for coating the new smart material on plastic, which makes it easier and cheaper to apply than conventional coatings made directly on the glass itself. The team demonstrated a flexible electrochromic device, which means a small electric charge (about 4 volts) can lighten or darken the material and control the transmission of heat-producing, near-infrared radiation. Such smart windows are aimed at saving on cooling and heating bills for homes and businesses.

The research team is an international collaboration, including scientists at the European Synchrotron Radiation Facility and CNRS in France, and Ikerbasque in Spain. Researchers at UT Austin's College of Natural Sciences provided key theoretical work.

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