A Plastic That Chills

Materials that change temperature in response to electric fields could keep computers--and kitchen fridges--cool.

Thin films of a new polymer developed at Penn State change temperature in response to changing electric fields. The Penn State researchers, who reported the new material in Science last week, say that it could lead to new technologies for cooling computer chips and to environmentally friendly refrigerators.

Cool spool: Films of a specially designed polymer, just 0.4 to 2.0 micrometers thick, can get colder or hotter by 12 °C when an electric field is removed or applied across them. Credit: Qiming Zhang, Penn State

Changing the electric field rearranges the polymer's atoms, changing its temperature; this is called the electrocaloric effect. In a cooling device, a voltage would be applied to the material, which would then be brought in contact with whatever it's intended to cool. The material would heat up, passing its energy to a heat sink or releasing it into the atmosphere. Reducing the electric field would bring the polymer back to a low temperature so that it could be reused.

In a 2006 paper in Science, Cambridge University researchers led by materials scientist Neil Mathur described ceramic materials that also exhibited the electrocaloric effect, but only at temperatures of about 220 °C. The operating temperature of a computer chip is significantly lower--usually somewhere around 85 °C--and a kitchen refrigerator would have to operate at lower temperatures still. The Penn State polymer shows the same 12-degree swing that the ceramics did, but it works at a relatively low 55 °C.

The polymer also absorbs heat better. "In a cooling device, besides temperature change, you also need to know how much heat it can absorb from places you need to cool," says Qiming Zhang, an electrical-engineering professor at Penn State, who led the new work. The polymer, Zhang says, can absorb seven times as much heat as the ceramic.

Zhang attributes these qualities to the more flexible arrangement of atoms in polymers. "In a ceramic, atoms are more rigid, so it's harder to move them," he says. "Atoms can be moved in polymers much more easily using an electric field, so the electrocaloric effect in polymer is much better than ceramics."

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The material's properties make it an attractive candidate for laptop cooling applications, says Intel engineer Rajiv Mongia, who studies refrigeration technologies. Computer manufacturers are looking for less bulky alternatives to the heat sinks and noisy fans currently used in laptops and desktop computers. The ideal technology would be small enough to be integrated into a computer chip.

Until now, says Mongia, exploring the electrocaloric effect for chip cooling had not made sense. The first ceramic materials didn't exhibit large enough temperature changes--chip cooling requires reductions of at least 10 °C--and the more recent ceramics don't work at low enough temperatures. They also contain lead, a hazardous material that is hard to dispose of safely. The polymers do not have those drawbacks. "The fact that they've been able to develop a polymer-type material that can be used in a relatively thin film is worth a second look," Mongia says. "Also, it's working in a temperature range that is of interest to us."