Electric Foam: Scientists uncover basis of material oddball

Some crystals change their size when exposed to an electric current and also generate electric signals when squeezed. These so-called piezoelectric materials, which are usually ceramic, appear widely in ink-jet printer heads, microphones, and some other electronic products (SN: 3/17/01, p. 167: Some swell materials give up their secret).

PORE PERFORMANCE. High voltage separates electric charges in pores (bottom image) to make plastic foam suitable for sensors and other devices. Bauer

Specially treated polypropylene foam, a mainstay of the packaging industry, can mimic the defining behavior of traditional piezoelectric crystals. Now, researchers have shown that the resemblance extends to other desirable properties.

Finnish researchers 15 years ago discovered that the lightweight foam acquires piezoelectric properties after it’s zapped with several thousand volts. Compared to ceramic piezoelectrics, the foam is soft, flexible, and relatively inexpensive. It has already been incorporated into a few products, including keypads and musical-instrument pickups. Because it can cover large areas and conform to irregular shapes, the foam opened new technological prospects, says Siegfried Bauer of Johannes Kepler University in Linz, Austria.

Investigating how the foam becomes a piezoelectric impersonator, Bauer and his colleagues showed a year ago that air in a pore breaks down into electrons and positively charged ions that cling to opposite walls. Like lightning, visible flashes of light accompany those “microstorms,” Bauer says.

Additional research had shown that, like many piezoelectric materials, the foam is ferroelectric. Such a substance harbors an electric field that can be flipped by a voltage.

At a symposium on ultrasound research this October, Bauer and his coworkers plan to present evidence that the similarity between the foam and conventional piezoelectrics extends even further. Both materials can respond to an electric current by simultaneously expanding in some areas and contracting in others. To implement this differential response, scientists take advantage of materials’ ferroelectric nature and flip internal fields in some regions but not others.

This permits such capabilities as sophisticated ultrasound focusing and hidden piezoelectric bar codes, Bauer says.

The foam loses its piezoelectric quality above 55C, a temperature sometimes reached in a car’s glove box, notes materials scientist Tom Rosenmayer of W.L. Gore in Munich.

If it can be made to retain piezoelectric properties at higher temperatures, the foam could be a “breakthrough” discovery in electrically active plastics, comments Michael R. Wertheimer of the École Polytechnique in Montreal.


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