Some swell materials give up their secret

Piezoelectric materials change their size in response to electricity and produce an electric signal in response to pressure. Some of these substances, which play roles in sonar equipment, ultrasonic scanners, ink-jet printer heads, and other devices, are much more responsive than others. Now scientists may have identified the specific atomic arrangement, or phase, that underlies the behavior of the best piezoelectric substances, a find that could lead to even better materials.

Three-peak pattern of X-ray intensity reflects an atomic arrangement in PZN-PT that may explain this piezoelectric material’s superior ability to expand. Red is highest intensity, blue lowest. Noheda et al./Brookhaven

Researchers say the newfound phase may explain the superior performance of the most widely used piezoelectric material, a lead alloy called PZT. A similar phase appears in two related alloys that outperform PZT but haven’t yet been commercialized.

The phase showed up under bright illumination from high-energy X rays at the National Synchrotron Light Source, an electron accelerator at Brookhaven National Laboratory in Upton, N.Y. Called a monoclinic phase, it appears to allow a piezoelectric material to expand in an unusual manner when zapped by an electric field, scientists say. In the newly recognized phase, the material enlarges along two perpendicular directions. In contrast, less-expansive piezoelectric materials, such as quartz, can swell in only one direction.

“We are learning now that one circumstance implies the other. Each time you have . . . this huge piezoelectric response, you have this monoclinic phase,” says Brookhaven’s Beatriz Noheda.

She described the evidence for the new structure this week at the March meeting of the American Physical Society in Seattle. Noheda collaborated on the research with her Brookhaven colleagues Gen Shirane and David E. Cox, as well as with scientists at Pennsylvania State University in State College and at Simon Fraser University in Burnaby, British Columbia.

Not until a few weeks ago was it apparent that all three of the top piezoelectric materials harbor a monoclinic phase, Noheda says. That’s when she and her coworkers determined that certain compositions of the most recently discovered of the lead alloys, known as PMN-PT, also has the telltale arrangement at room temperature.

A year ago, Noheda and her coworkers stunned their fellow piezoelectric researchers with the first sighting of the surprising phase in the already heavily studied PZT. “PZT has been around a long time. Everyone was very surprised there was anything new to it,” says theorist David Vanderbilt of Rutgers University in Piscataway, N.J.

In the wake of that experiment, he and other theorists refined their models of PZT. Now their revised models and calculations agree with the Noheda team’s empirical findings. In the past year, Noheda and her colleagues have also identified a monoclinic phase in PZN-PT, which they will report in an upcoming issue of Physical Review Letters. “They’ve done beautiful, careful work that really maps out where these phases are,” says Vanderbilt.

Discovered in the 1950s, PZT is named for lead (represented by the P in its chemical symbol, Pb), zirconium, and titanium. PZN-PT combines lead with zinc, niobium, and lead-titanate, whereas PMN-PT has a similar makeup but swaps magnesium for zinc. All the alloys also contain oxygen.

In 1997, Penn State researchers showed that PZN-PT and PMN-PT have piezoelectric effects about 10 times greater than PZT’s. Unlike pieces of PZT, which form from many small crystals, the more complex alloys can grow as single crystals, enhancing their piezoelectric responses.

The finding of a correlation between phase and piezoelectric prowess could bolster future efforts to custom-design still more effective piezoelectric materials, comments Ronald E. Cohen of the Carnegie Institution of Washington (D.C.). “In order to make something better, you have to know how it works,” he notes. The Navy has just established a consortium of theorists, including Cohen, to pursue the goal of designer piezoelectric materials, he says.

More Stories from Science News on Physics