Keeping cool with oxide films
Mar 3, 2006
Thin oxide films made from the mineral perovskite could lead to a new type of refrigeration, according to new work from scientists in the UK. Alex Mischenko of the University of Cambridge and colleagues have found that the films, which cool when an applied electric field is removed from them, are 100 times more effective than any similar material known to date. This phenomenon, known as the electrocaloric effect, has been largely ignored until now because it was too small to be useful for commercial applications (Science 311 1270).
Traditionally, most household fridges used environmentally damaging chemicals, such as chlorofluorocarbons, which damage the ozone layer. Although these chemicals have been phased out from domestic appliances, the search is still on for alternative cooling methods that do not use hazardous substances or are more efficient.
Such alternatives include magnetic refrigeration systems and the use of electrocaloric materials, which change temperature when an applied electric field is removed from them. These materials generated much interest in the 1960s and 1970s but were not exploited commercially because the electrocaloric effects were too small – just 2.5° for the best material at applied fields of 750 volts.
Mischenko and co-workers now report on a "giant electrocaloric effect" in perovskite PZT, which is an insulating oxide containing lead, zirconium and titanium. By measuring the polarization of thin films of the PZT at different temperatures coupled with some basic thermodynamic equations, the scientists calculated that the material cools down by as much as 12° in a field of just 25 V. This means that it is about a 100 times better at cooling surrounding matter than other known substances. Unfortunately, the effect is strongest at 222°C, which is well above room temperature.
Although practical applications are still some years away, electrocaloric thin films could be used to cool down electronic components like computer chips, and in biotechnology systems such as reactors and sensors and microelectromechanical and infrared imaging systems. The technology might also find use in automotive and aerospace applications, air conditioning and even domestic refrigeration.
The team now plans to introduce some dopants into the PZT to bring down the working temperature to near room temperature. It will also look for new lead-free electrocaloric materials.
About the author
Belle Dumé is science writer at PhysicsWeb