Scientists Solve Long-standing Challenge for Piezoelectric Materials

Piezoelectric materials have many applications, including sonar technologies and devices that generate and detect ultrasonic waves. However, for these devices to effectively generate sonar or ultrasonic waves, the material must be “polarized.”

This is because the piezoelectric materials used in sonar and ultrasonic applications are mostly ferroelectric. And like all ferroelectric materials, they exhibit a phenomenon called spontaneous polarization. This means that they contain pairs of positively and negatively charged ions called dipoles. When a ferroelectric material is polarized, it means that all of its dipoles have been pulled into alignment with an external electric field. In other words, all of the dipoles are oriented in the same direction, which makes their piezoelectric properties more apparent.

“If these dipoles are not aligned, it is difficult to generate directed ultrasonic waves with the amplitude needed to be practical,” says Xiaoning Jiang, corresponding author of a paper on the subject and Dean F. Duncan Distinguished Professor of Mechanical and Aerospace Engineering at North Carolina State University.

“The polarity preservation of piezoelectric-ferroelectric materials poses some significant challenges because the dipoles can start to lose their alignment when exposed to high temperatures or high pressure,” Jiang says.

“It’s also a manufacturing issue, because it limits what other materials and processes can be used to manufacture ultrasonic devices,” Jiang says. “And because the elevated temperatures aren’t nearly as high—alignment issues can be seen at 70 degrees Celsius—even shipping or storing these technologies can sometimes adversely affect the polarity and performance of the devices.

“Moreover, long-term use of some technologies can result in the device itself generating heat, which risks depolarizing the piezoelectric-ferroelectric material.”

And once the dipoles in the material get out of alignment, getting them back into proper alignment isn’t easy. The piezoelectric-ferroelectric material must be removed from the device and exposed to high temperatures—300 degrees Celsius or more—to completely depolarize the material before “repolarizing” and pulling the dipoles back into proper alignment.

“It’s important to reuse these piezoelectric-ferroelectric materials because they tend to be expensive—you don’t want to just throw them away,” Jiang says. “But often the material is salvaged and the rest of the ultrasonic device is thrown away.

“We’ve developed a technique that lets us depolate and repolate piezoelectric-ferroelectric materials at room temperature. This means we can return the dipoles to their correct alignment without removing the material from the device—and we can do this multiple times, as needed.”

To understand the new technique, you need to understand that there are two ways to pull the dipoles in a piezoelectric-ferroelectric material into alignment. The most commonly used technique involves applying a direct current (DC) electric field to the material, which pulls all the dipoles in the same direction.

“This method works well for creating alignment, but it is practically impossible to remove the polarization of the material using only a constant field,” Jiang said.

The second technique involves applying an alternating current (AC) electric field to the material, causing the dipoles to oscillate in response to the field waves until the field is removed, at which point the dipoles lock into place, aligning themselves in the appropriate orientation.

“We found that we could also depolate the material with an AC field, even at room temperature. If the material was originally polarized with a DC field, we could remove a significant portion of the polarization with an AC field—but not all of it,” Jiang said. “However, if the material was originally polarized with an AC field, we found that we could also completely depolate the material with an AC field.”

This discovery has at least two important implications for ultrasound technology.

“If we can polarize piezoelectric-ferroelectric materials at room temperature, that means we can change other materials and manufacturing processes we use to make ultrasonic devices to optimize their performance,” Jiang says. “We are no longer limited to materials and processes that won’t affect polarization in piezoelectric-ferroelectric elements because we can polarize the material with an AC field after the device is assembled.”

“Furthermore, it means we can easily swap out materials in existing devices, hopefully ensuring a long lifetime of peak performance for these technologies.”