For the past three years, Delia Milliron and her team at the Cockrell School of Engineering at the University of Texas at Austin have been working on electrochromic smart materials that look like ordinary glass, yet can selectively keep out light or heat. Now the UT Austin team have come up with a new, flexible, smart-window material that uses a low-temperature fabrication process that makes it easier and cheaper to use than previous iterations.

Electrochromic windows are a bit like those automatic sunglasses that darken when you step out in the sun. The difference is that electrochromic materials can be made to block light or heat at the flip of a switch, which makes them ideal for use in windows, awnings, and other surfaces to control both incoming light and for heating or cooling a room.

Previously, the UT Austin team created such materials using a high temperature process that embeds compounds (such as tin oxide or niobium oxide) into glass, where they form a new nanostructure. The material is amorphous, which is to say it's more like a supercooled liquid than a solid, and when an electric current of about 4 volts is applied, the surface of the glass becomes opaque to either visible light, heat-carrying infrared rays, or both.

The problem is that embedding the compounds into the glass and altering its structure requires high temperatures, which is very inefficient, and the result is brittle and inflexible. Now the UT Austin team, in conjunction with the European Synchrotron Radiation Facility, France's CNRS and Spain's Ikerbasque, has come up with a more efficient low-temperature process to embed niobium oxide into a plastic film.

Linear structural model of chemically condensed niobium oxide(Credit: Cockrell School of Engineering)

In addition to its ability to be applied to curved glass surfaces as a coating, the team says that the new polymer film darkens faster and uses less power by means of a local arrangement of the atoms in a linear structure instead of an amorphous state, which allows ions to pass more freely.

Aside from its immediate applications, the film provides new insights into how such materials are formed and how they behave. Because of this, it may one day be possible to form new components, such as more efficient supercapacitors. In the meantime, UT Austin will continue to develop the low-temperature material to improve its performance in comparison with the high-temperature versions.

The research is scheduled to be published in Nature Materials.

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