With the dog days of summer upon us, things are heating up, especially cars and buildings with windows to soak up the sun's rays. Help may soon be available in the form of a smart film that can block heat--but not light--from the sun. Recent findings indicate that the window coating can reduce room heating by as much as 50 percent.

Troy D. Manning, now at the University of Liverpool, and Ivan P. Parkin of University College London developed the novel coating using vanadium dioxide and a small amount of the metal tungsten. The film allows light with wavelengths in the visible spectrum and infrared light, at low temperatures, to pass through it. When the temperature exceeds 29 degrees Celsius, however, the material reflects infrared radiation. While the heat reflective properties of vanadium dioxide are well recognized, the stumbling block has been the switching temperature, Parkin notes. It's not much good if the material starts to reflect infrared light at 70 degrees Celsius. At the switching temperature, the material changes from being semi-conducting (and therefore absorbing infrared light) to acting more like a metal (and thus reflecting infrared).

The thin films can be laid down on windows as they are being produced and wouldn't require additional infrastructure. Unfortunately, the covering currently has a somewhat unattractive yellowy brown color, which the researchers hope to be able to remove with future refinements. The next step in getting the coating to market is to investigate how durable it is, Parkin says. Ideally, because it's laid down at the point of manufacture you want it to last for the life time of the window, but looking around you see many windows that date from the Victorian era, so we need the coating to last for over 100 years. The scientists report their results in the August 10 issue of the Journal of Materials Chemistry.