Single Crystals Move More With High Voltage

May 13, 1997

University Park, Pa -- High voltage causes a family of crystals known as relaxor ferroelectrics to deform 10 times more than any other material currently known, according to a Penn State materials scientist.

"No one has seen this large a movement before," says Dr. Thomas R. Shrout, senior scientist and professor of materials, Penn State's Materials Research Laboratory.

Materials that produce electrical voltage when mechanically stressed and change shape under applied voltage are piezoelectric. Ultrasound equipment, acoustic transducers, devices to position specimens under electron microscopes and switches commonly employ piezoelectric materials.

Most common piezoelectric materials are polycrystalline like PZT -- lead zirconate titanate. Relaxor ferroelectrics like PZN-PT -- lead zirconate niobate lead titanate, however, can be grown as single crystals. PZT-type materials are generally limited in deformation to about .17 percent, making a one centimeter piece 1.0017 centimeters long when a voltage is applied.

Shrout and Seung-Eek Park, post-doctoral researcher at Penn State's Materials Research Laboratory, were trying to improve the piezoelectric performance of medical ultrasound devices, by growing large single crystals.

"We grew some good quality crystals and tested them at low voltages," says Shrout. "Then someone suggested we test the crystals for high voltage properties."

The researchers found that the material deformed 1.7 percent. A one centimeter crystal elongates to 1.017 centimeters, 10 times more deformation than other materials. With materials that deform 10 times more, devices can either use crystals that are 10 times smaller with the same result, or not alter the size of the piezoelectric material and achieve 10 times the effect.

To be piezoelectric, materials must be asymmetrical. The applied voltage shifts some of the atoms in the crystal lattice, elongating it. PZN-PT is an asymmetrical rhombohedral crystal, and this shape may be the key to its large movement.

"The effects of a voltage on a crystal usually occur in the diagonal ëpolar' direction," says Shrout. "With PZN-PT, the direction of most movement, is along the face. This is very unusual."

The researchers do not know the mechanism behind the large movement or why these crystals react differently to high voltage. They also do not know if this effect is a property of all rhombohedral materials or specific to relaxors.

"Until we did the experiment, no one had modeled or predicted this large movement in this type of material -- relaxor ferroelectrics," says Shrout. "We don't yet understand exactly how the crystal is shifting shape or how the effect takes place."

While the researchers are searching for other materials that behave in the same way, they have applied for a patent on acoustic actuators and will be explaining their work at a Piezoelectric Crystals Planning Workshop May 14-16 in Washington, D.C. sponsored by the Office of Naval Research. ONR is also the funding agency for this work.
EDITORS: Dr. Shrout may be reached at (814) 865-1645 or TShrout@alpha.mrl.psu

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