Cooled by an electric pulse
Electric fields leave unusual ceramic cold.
A material has been made that turns cold at the push of a button, a feature that could be harnessed by novel cooling systems for computers.
Alex Mischenko of the University of Cambridge, UK, and his colleagues say that their material may open the way to practical applications of 'electrical refrigeration', in which electric fields are used to keep things cool.
This principle, called the electrocaloric effect, has been known since the 1960s, but it had seemed too weak an effect to be of much use. The Cambridge team has now found a ceramic material that shows what they call a giant electrocaloric effect; it's more than 100 times larger that seen previously.
The material is actually a variant of a well-known one, called lead zirconate titanate, or PZT. This hard, crystalline solid is piezoelectric: squeezing it creates an electric field inside it. This makes PZT useful for inter-converting sound and electrical energy, as is done in some microphone and ultrasound technologies.
Mischenko and colleagues found that they could turn PZT into a promising electrocaloric material simply by making it from about 20 times more zirconium than titanium, they report in Science11.
The electrocaloric effect isn't very well understood, the researchers say. But Mischenko's colleague Neil Mathur at Cambridge explains that basically what is needed is a material that undergoes an abrupt change of its crystal structure, a so-called phase change, at a particular temperature.
In PZT the temperature of this phase change can be altered by an electric field. This means that, close to the phase change, an electric field can pull the atoms in the crystal lattice into a low-energy state, allowing them to "suck energy" into the lattice and thereby to produce cooling.
The electrocaloric effect is very closely related to the better-known pyroelectric effect, in which warming a material creates an electric field. PZT has this property too, which is why the researchers chose to investigate it. "Whatever is good for the pyroelectric effect is good for the electrocaloric effect," says Mathur.
They found that by applying 25 volts to a film of zirconium-rich PZT a third of a micrometre thick, they could reduce its temperature by 12 oC. This effect could be used to make a heat pump for refrigeration, Mathur says.
The idea would be to repeatedly cycle the material through the temperature drop, each time connecting it up to the system that is to be cooled and then disconnecting it again once it had cooled the system down. On each cycle, the temperature of the system would drop in a stepwise fashion.
The PZT material isn't going to be very useful in itself, Mathur admits, because the strongest electrocaloric effect happens only at relatively high temperatures: around 220 oC. He and his team are now looking for alternatives that work well closer to room temperature.
Mathur points out that a related effect, in which cooling is induced by magnetism, has been used already by several companies to make prototype refrigeration units. But these require impractically strong magnetic fields. Could electrical triggering of the cooling be more practical? "We can't say it will and we can't say it won't," Mathur confesses.
- Mischenko A.S., et al. Science, 311 . 1270 - 1271 (2006).
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