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See Inside November 2008

Cool Polymers: Toward the Microwave Oven Version of the Refrigerator

Getting a bigger chill out of polymers that respond to electric fields
ice cubes



TOM GRILL / Corbis

Whether they sit in your kitchen or inside your personal computer, refrigerators and other cooling devices are typically bulky, often noisy and frequently power-hungry. A team at Pennsylvania State University recently found that certain plastics cool off a significant amount—12 degrees Celsius—when an applied electric field is removed. Should the technique become feasible, the resulting solid-state coolers could efficiently and quietly eliminate heat from, say, integrated-circuit boards, enabling smaller, faster computers.

Engineers have long known of so-called electrocaloric substances that drop in temperature when an external electric field is withdrawn, but the amount of chilling either was too small at practical temperatures or occurred at too high a temperature to be useful. Effective chip cooling, for instance, requires reductions of at least 10 degrees C from typical operating temperatures—about 85 degrees C, says G. Dan Hutcheson, chief executive officer at VLSI Research, a microelectronics industry market research firm in Santa Clara, Calif. Computers usually require heat sinks, radiators, fans, heat pipes or even fluid-based heat pumps to extract the surplus degrees.

If successful, the new technology should be compact and at least 10 times more energy-efficient than conventional cooling techniques, according to Penn State electrical engineer Qiming Zhang, who led the team. The group found that a micron-thick film of a polyvinylidene fluo­ride co-polymer—polyvinylidene fluo­ride trifluoroethylene—heats up a dozen degrees C when zapped with 120 volts at ambient temperatures as low as 55 degrees C. Such a rise constitutes an order of magnitude improvement over other electrocaloric materials (mostly ceramics) at that temperature range.

Zhang, who in the past worked on plastic “artificial muscles” that alter shape under electric fields, says that years ago he “started thinking about melting ice into water, which is one of the most effective ways to cool objects.” That effect is based on a phase change in which an ordered system (solid ice) transforms in­to a disordered one (liquid water). In time, the scientists identified several promising polymers in which an applied voltage caused the atoms or molecules to align, thus creating greater order.

The electrocaloric materials, Zhang reports, consist of long molecular chains with a positive electric charge on one end and negative on the other. These dipolar chains, which can move around freely, are normally oriented randomly. But “when you apply an electric field, the dipoles tend to spin around until they align with the field,” he says. Thermodynamically speaking, this molecular ordering lowers the system’s entropy, so the system compensates by heating up as a consequence of energy conservation. When the field is disengaged, the chains randomize and the polymer cools off. The rigid microstructures of electrocaloric ceramics, in contrast, “can move only a little bit,” Zhang notes, which accounts for their weak temperature response. The polymers can also absorb seven times as much heat as the ceramics.

In an ideal solid-state refrigerator, a chilling cycle starts when contact breaks between the polymer and the object that is being cooled, thermally isolating the polymer. An applied electric field causes the temperature of the polymer to rise. It is then placed into momentary thermal contact with a heat sink, which absorbs any heat and entropy that the polymer has. The polymer is next isolated from the heat sink; the electric field is then lowered, which reduces the temperature of the polymer and enables it to cool the target object once again.

A workable system could in particular prove a boon for the computer industry. Silicon chips run hotter than is desirable for optimal performance, comments Benson Inkley, a senior power/thermal engineer at Intel in Hillsboro, Ore. Cooling with electrocaloric plastics offers intriguing possibilities, Inkley states: “Imagine coating an entire circuit board with a layer of polymer, in effect, forming a cooling blanket.”

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