Sarbajit Banerjee

Is there a way for a window to reflect heat in the summer and let it through in the winter?
A window that changed in response to the heat might behave in just that way. Sarbajit B­anerjee, a materials chemist at the University at Buffalo in New York, is applying his work on a compound called vanadium oxide to coat glass with a material that makes this possible.

Banerjee had been studying vanadium oxide because he was interested in the physics of phase transition—for example, the way water freezes as the temperature drops. When the temperature reaches 153 °F, this compound’s crystalline structure changes from one that’s transparent to infrared light—that is, radiated heat—to one that reflects the light.

Using nanofabrication techniques to change the microscopic structure of the crystalline material, Banerjee found a way to lower the temperature at which that change occurs. When the material is formed as long, thin nanowires, it undergoes the transition at a mere 90 °F. A researcher at a window company suggested that this version had good characteristics for a switchable window coating.

Banerjee was able to bring that temperature down even further by mixing tungsten into the material. And perhaps most promising of all, he found that he could trigger the transition at a range of temperatures by sending an electric current through the material—holding out the promise of changing a room’s temperature with the flip of a switch, and without racking up an energy bill.

Banerjee is now in the process of licensing his heat-­blocking window coating to a U.S. building-materials company; he predicts that it will cost just 50 cents per square foot. He also has a partnership with Tata Steel, a global manufacturer headquartered in Mumbai, India, and they are looking at how to use the material to deflect heat from the corrugated-steel roofs that commonly turn houses stifling in India and other parts of the developing world.

—Katherine Bourzac