Thin-Film Fellow

"A piezoelectric material," Susan Trolier-McKinstry explains, "is one that has a coupling between electrical and mechanical energies. If I take such a material and apply an electrical field to it, I can make it change shape. If I squeeze it—apply mechanical stress—I can generate an electrical field.

"We use these materials all the time without knowing it," adds Trolier-McKinstry, professor of ceramic science and engineering at Penn State. "Medical ultrasound is a good example." An ultrasound probe, she explains, contains piezoelectric crystals, which vibrate, i.e., change shape, when an electric current is applied. These vibrations produce sound waves that travel outward into the body until they reach the boundaries between fluid, soft tissue, and bone. Some of the sound waves are bounced back to the probe, where the crystals then emit an electrical current.

Most bulk piezoelectric devices, however, operate at fairly high voltages—often more than 60 volts. Says Trolier-McKinstry, "It would be really nice to make an actuator that you could drive with really low voltage, like that produced by a silicon chip."

She recently received major support to do just that. In 2008 Trolier-McKinstry was selected as one of six distinguished researchers from U.S. universities to form the inaugural class of the Department of Defense's new National Security Science and Engineering Faculty Fellows Program. The program provides long-term funding to scientists and engineers to pursue basic research of crucial importance to next-generation DOD technologies. The six Fellows receive grants of up to $3 million each over a five-year period.

Trolier-McKinstry's proposal to the DOD contained three parts. Her first object is to improve the piezoelectric response of the thin-film materials that are crucial to increasingly miniaturized devices. "I want to be able to create a big change of shape with just a little bit of voltage," she says.

Next, Trolier-McKinstry is looking at ways to create usable structure on the surfaces of thin films without ruining them. "The materials we use are rather complicated in their chemistry," she explains. "They usually contain at least four different types of atoms, and inevitable these don't all etch at the same rate. So it's often easy to induce damage when you etch into these materials. Instead, we're trying to print the patterns we want directly, using a stamp. We're the first group that has ever done this."

Her third goal, she says, is to be more precise in applying the high temperatures necessary for crystallization of thin films. "We start out with a film that's amorphous, like glass," she says. "And it's not piezoelectric until we crystallize it." That usually requires 600 to 700 degrees Celsius, however—a heat much too intense for integrated circuits and other potential components to withstand. "So instead of heating up the whole film and everything that it sits on, we're using laser adsorption to target just the film."

The materials she's working to perfect have lots of potential applications, both defense-related and not. "DOD is very interested in switches for high-speed communications," she says. "For radar applications, they need to be able to switch parts of the circuit in and out. This is hard to miniaturize.

"The same generic technology is useful in lots of sensors for condition-based maintenance," she continues. "These allow you to tell when a bridge or a big industrial tool is in need of repair—before damage is being done."

Trolier-McKinstry also envisions a thin-film version of an ultrasound system. "Our long-term goal would be to make something small enough that you could swallow it. So you could maybe replace the colonoscopy with a pill that just passes through."

As director of the W.M. Keck Smart Materials Integration Laboratory and the Center of Excellence in Piezoelectric Materials and Devices at Penn State, she says the DOD fellowship "gives me a tremendous amount of flexibility to address the key science and engineering challenges in piezoelectrics for microelectromechanical systems. This kind of sustained funding allows us to explore deeper, fundamental problems.

"What we'd really like to do is move beyond the incremental and make big improvements in the functionality of these materials."

Susan Trolier-McKinstry, Ph.D., is professor of Ceramic Science and Engineering and Director of the W. M. Keck Smart Materials Integration Laboratory in the College of Earth and Mineral Sciences, STMcKinstry@psu.edu.

Last Updated June 01, 2010