A Plastic That Chills

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

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

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.

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."

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."

But chip-cooling devices will take a while to arrive. It now takes 120 volts to get the polymer to change its atomic arrangement, and that figure would need to be much lower if the material is to be used in laptops. "Ideally, you want it to work at voltages common within the realm of a notebook, in the tens of volts or less," Mongia says. The researchers will also need to engineer a working device containing the thin films.

Electrocaloric materials could make fridges greener. Current household fridges use a vapor-compression cycle, in which a refrigerant is converted back and forth between liquid and vapor to absorb heat from the insulated compartment. The need for mechanical compression lowers the fridge's efficiency. "Vapor-cooled fridges are 30 to 40 percent efficient," Mathur says. But because electrocaloric materials have no moving parts, they could lead to cooling devices that are more energy efficient than current fridges. What's more, current hydrofluorocarbon refrigerants contribute to global warming.

Refrigerators that use electrocaloric materials would have an advantage over the magnetic cooling systems that some companies and research groups are developing. Electric fields large enough to produce substantial temperature changes in electrocaloric materials are much easier and cheaper to produce than the magnetic fields used in experimental refrigeration systems, which require large superconducting magnets or expensive permanent magnets. However, refrigerators need temperature spans of 40 °C, which is a tall order for electrocaloric materials right now, Mathur says. "The main sticking point in terms of the technology is that we have thin films, and you can't cool very much with a thin film."

Zhang and his colleagues are now trying to design better electrocaloric polymers. They plan to study polymers made from liquid crystals, which are used in flat-panel displays. Liquid crystals contain rod-shaped molecules that will align with an electric field and revert to their original arrangement when the field is removed. Zhang says that this property could be exploited to make materials that absorb and release large amounts of heat in response to electric fields.