June 7, 2000 CHAMPAIGN, Ill. -- As active materials become increasingly smaller for the next generation of smart materials systems, the need to understand and predict material response becomes critical. At the University of Illinois, an experimental investigation into how the properties and responses of smart materials -- such as piezoelectric ceramics -- change as a function of size has yielded a few surprises.
"Both the piezoelectric properties and the dielectric constants of smart materials cast as thin films were found to be strongly dependent on thickness," said Nancy Sottos, a professor of theoretical and applied mechanics. "As the films became thinner, the desired responses became smaller."
Piezoelectric ceramics are commonly used in pressure sensors, microphones and accelerometers. Deposited as thin films, the material can serve as tiny sensors and actuators in microelectromechanical (MEMS) devices, as elements in ultrasonic motors and as switching capacitors for integrated circuitry. While thin films have much better mechanical properties than the bulk ceramics -- for example, films are far less brittle - other physical and electrical properties may change in undesirable ways.
"The properties of piezoelectric films are critical to the quality and the reliability of MEMS devices," Sottos said. "To optimize the performance of thin-film structures, we must first understand the factors that influence those properties."
For their experiments, Sottos and graduate research assistant Lei Lian obtained a number of lead-zirconate-titanate thin films that ranged in thickness from 0.5 to 2.0 microns (a micron is one millionth of a meter). To record the films' tiny displacements (on the order of trillionths of a meter), Sottos and Lian developed a high-resolution, laser Doppler heterodyne interferometric technique.
The measurement scheme is based on the Doppler shift. First, the beam from an argon laser strikes a 40 MHz acousto-optic modulator, which produces two beams and sends them along different arms of the interferometer. One beam then bounces off the sample while the other beam serves as a reference. When the two beams are recombined, the researchers can very accurately extract the displacement signal from the Doppler shift riding on top of the 40 MHz carrier.
"It's clear from our experiments that as the films become thinner and thinner, there is an undesirable decrease in both piezoelectric response and dielectric constant," said Sottos, who published the results in the April 15 issue of the Journal of Applied Physics. "Fortunately, however, it may be possible to avoid these effects by controlling the residual stress in the material."
Significant stresses build up in piezoelectric thin-film structures during the fabrication process, Sottos said. "Changes in the residual stress state might be one major cause for the change in properties with film thickness that we observed. By applying a mechanical stress -- to relieve some of the residual stress -- the response of the film can be greatly enhanced."
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