Windows That Block Heat Only On Hot Days: New Research Brings Us Closer

BUFFALO, N.Y. -- New materials science research from the University at Buffalo could hasten the creation of "smart" windows that reflect heat from the sun on hot summer days but let in the heat in colder weather.

The findings concern a unique class of synthetic chemical compounds that are transparent to infrared light at lower temperatures, but undergo a phase transition to begin reflecting infrared when they heat up past a certain point.

An article detailing some of these discoveries appears today (April 7) on the cover of the Journal of Physical Chemistry Letters. Additional papers have appeared online or in print in CrystEngComm, the Journal of Materials Chemistry and Physical Review B. The research is described in a video at http://pubs.acs.org/page/jpclcd/banerjee-video.html

In the papers, UB researchers report that they have managed to manipulate the trigger temperature for vanadium oxide, one such material. The advance is a crucial step toward making the compound useful for applications such as coatings for energy-saving windows.

By preparing vanadium oxide as a nanomaterial instead of in bulk, the scientists managed to lower the compound's trigger point from 153 degrees Fahrenheit to 90. Doping vanadium oxide nanowires with tungsten brought the temperature down further, to 7 degrees Fahrenheit. Molybdenum doping had a similar but smaller effect.

Researchers also found that they were able to induce a phase transition using an electric current instead of heat.

UB chemist Sarbajit Banerjee led the studies, collaborating with Sambandamurthy Ganapathy, an assistant professor of physics, to head the Physical Review B research on the use of the electric current.

"Definitely, we are closer than we've ever been to being able to incorporate these materials into window coatings and other systems that sense infrared light," said Banerjee, an assistant professor. "What we found is an example of how much of a difference finite size can make. You have a material like vanadium oxide, where the phase transition temperature is too high for it to be useful, and you produce it as a nanomaterial and you can then use it right away."

Banerjee and Ganapathy previously led research projects demonstrating that, in nanoscale form, two additional synthetic compounds -- copper vanadate and potassium vanadate -- exhibit phase transitions akin to those in vanadium oxide.

Banerjee's work has caught the attention of the National Renewable Energy Laboratory, which has contacted him to discuss developing window coatings that could improve the energy efficiency of buildings with heating or air conditioning systems. The technology could be particularly useful in places like Phoenix and Las Vegas that experience extreme summer temperatures.

Besides smart windows, vanadium oxide could also be useful in products including computer chips, night-vision instruments and missile guidance systems, Banerjee said.

Two major awards are funding Banerjee's research on the material: A Cottrell Scholar Award from the Research Corporation for Science Advancement, announced this year, and a National Science Foundation CAREER award, the foundation's most prestigious award for junior investigators.

Both honors recognize Banerjee's accomplishments in teaching as well as in research. He has mentored numerous graduate students, including Luisa Whittaker, Christopher J. Patridge and Jesus M. Velazquez, who appear as first authors on some of Banerjee's papers on vanadium oxide. Tai-lung Wu, supervised by Ganapathy, is the first author of the Physical Review B paper.

The University at Buffalo is a premier research-intensive public university, a flagship institution in the State University of New York system and its largest and most comprehensive campus. UB's more than 28,000 students pursue their academic interests through more than 300 undergraduate, graduate and professional degree programs. Founded in 1846, the University at Buffalo is a member of the Association of American Universities.