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Shape-Changing Crystals Get Shiftier

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Science  28 Mar 1997:
Vol. 275, Issue 5308, pp. 1878
DOI: 10.1126/science.275.5308.1878

A talented family of materials has gained some even more gifted members. So-called piezoelectric crystals have the unique ability to swell or shrink when zapped with electricity, as well as give off a jolt of juice themselves when compressed or pulled apart. Engineers have exploited this trait for decades to convert mechanical energy to electricity and back again in applications ranging from phonograph needles to telephone speakers.

Crystal growth.

A weak field displaces atoms toward the corners of the unit cells, but a stronger field rearranges the lattice.

Source: T. Shrout and S.-E. Park

Now, a pair of researchers from Pennsylvania State University has bred new piezoelectric wunderkinds, some of which display an effect 10 times greater than that of current family members. A paper by the researchers, materials scientists Thomas Shrout and Seung-Eek Park, is scheduled to appear this spring in the inaugural issue of the journal Materials Research Innovations, but early word of the new work is already turning a few heads. “It's an exciting breakthrough,” says Eric Cross, another piezoelectric materials expert at Penn State, who is not affiliated with the project. “Improvements by a factor of 10 are not easy to come by in a field that's 50 years old and considered mature.” If the materials are commercialized, as Cross and others believe they will be, they could usher in a new generation of piezoelectric devices that would improve everything from the resolution of ultrasound machines to the range of sonar listening devices.

Piezoelectric materials owe their abilities largely to the asymmetrical arrangement of positively and negatively charged atoms in their crystal structure. The positive and negative charges balance out in each of the crystal's unit cells—its basic repeating units—but the positive charges, for instance, may be weighted toward the top of each cell. An electric field can displace the charges even farther, which distorts the overall shape of the unit cell and of the crystal as a whole. The process can also run in reverse: Squeezing or stretching the material shifts the charges relative to each other, redistributing electric charge around the surface of the crystal, which can produce a small electric current.

The usual showcase for these properties is a cheap ceramic material called PZT, containing millions of crystalline grains in different orientations. PZT, which is composed primarily of lead, zirconium, titanium, and oxygen, can deform by as much as 0.17% in a strong applied field. To boost this shape-shifting ability, researchers have tried to grow single crystals of PZT, in which all the unit cells would line up in the same direction. Their contributions to the piezoelectric effect would also line up, enhancing it. But because PZT's components tend to separate during processing, the ceramic is extremely difficult to grow as a single crystal, says Shrout.

To coax the material into forming single crystals, Shrout and Park tried varying its composition. They settled on a couple of different mixtures, such as a combination of lead, zinc, and niobium spiked with varying amounts of lead-titanate (PT). The researchers found that a small admixture of PT—less than 9%—yielded materials that not only grew into single crystals, but also ended up with piezoelectric abilities that are enhanced more than they expected.

Just why that is, “we still don't know for sure,” says Shrout. But he and Park believe that at least part of the enhancement is due to the fact that an electric field applied to the new materials does more than just shift a few atoms around in the unit cell, as in PZT: “We think it causes the whole crystalline lattice structure to change from one form to another,” says Shrout. The changed crystal structure, in turn, frees individual atoms to respond more strongly to the field, increasing the overall distortion of the material. Likewise, a mechanical distortion probably produces a similar lattice shift, enabling the material to generate more current than standard PZT.

Whatever the reason for the effect, it's likely to be very useful, says Robert Newnham, another piezoelectricity expert at Penn State. The new crystals will undoubtedly cost more than ceramics like PZT, says Park, because growing single crystals is a slow and painstaking process. But he adds that he and Shrout are working on ways to speed it up. If they succeed, the new piezoelectric wunderkinds could grow up to live expansive lives indeed.

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