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Posted on: 11/17/2009 12:00:00 AM... A lead-free alternative to current piezoelectric materials has been identified by researchers with the Lawrence Berkeley National Laboratory and the University of California, Berkeley.

Piezoelectric materials, as a result of their unique crystal structures, are able to couple mechanical and electrical properties. The electromechanical properties of piezoelectric materials have made them valuable components in a broad range of devices, including sensors, actuators, and transducers. They are especially valuable for medical ultrasounds and for non-destructive testing of roads and bridges.

The most widely used piezoelectric materials today are lead-based perovskite compounds, especially lead zirconate titanate (PZT). These perovskites display superior piezoelectric properties in areas where the phase of their crystal structure abruptly changes. Such areas, known as morphotropic phase boundaries, are produced via complex chemical alloying of a PZT-type perovskite’s metal and oxide constituents.

The Berkeley research team has discovered how to make this technology less toxic by using bismuth ferrite, a compound with a perovskite crystal structure which has crystal planes of oxygen and bismuth atoms alternating with planes of oxygen and iron atoms. These planes can move relative to one other when the proper strain is applied. The scientists determined that the piezoelectric effect in bismuth ferrite becomes significantly enhanced in response to the application of epitaxial strain-compression in the direction of its crystal planes.

“We have demonstrated that epitaxial strain can be used to create large piezoelectric responses in thin films of bismuth ferrite,” says Ramamoorthy Ramesh, lead researcher and TMS member. “The piezoelectric effect is reversible when the strain is relaxed.”

By alternating between squeezing and relaxing, the material can be shuttled back and forth between phases.

The team’s research is described in the paper, “A Strain-Driven Morphotropic Phase Boundary in BiFeO₃,” published November 13 in Science.

Ramesh and his group are now testing their strain-driven piezoelectric technique on other perovskites. They are also exploring the use of epitaxial strain to generate other phenomena, such as magnetoresistance.

For More Information:

• Access Science Paper

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