Study identifies high-performance alternative to conventional ferroelectrics

Lighting a gas grill, taking an ultrasound, using an ultrasonic toothbrush, these actions involve the use of materials that can translate an electrical voltage into a change in shape and vice versa.

Known as piezoelectricity, the ability to exchange mechanical stress and electrical charge can be widely exploited in capacitors, actuators, transducers and sensors such as accelerometers and gyroscopes for next-generation electronics. However, integrating these materials into miniaturized systems has been difficult due to the tendency of electromechanically active materials to — at the submicrometer scale, when thicknesses are only a few millionths of an inch — “squeeze” from the material to which they are attached. , which significantly reduces their performance.

Rice University researchers and collaborators at the University of California, Berkeley, have discovered that a class of active electromechanical materials called antiferroelectrics may hold the key to overcoming performance limitations due to strain in miniature electromechanical systems.

A new study published in Materials of Nature reports that a model antiferroelectric system, lead zirconate (PbZrO₃), produces an electromechanical response that can be up to five times that of conventional piezoelectric materials even in films that are only 100 nanometers (or 4 millionths of an inch) thick.

“We’ve been using piezoelectric materials for decades,” said Rice materials scientist Lane Martin, who is the corresponding author on the study. “Recently there has been a strong motivation to further integrate these materials into new types of devices that are very small—as you would want to do for, say, a microchip that goes inside your phone or computer . The problem is that these materials are usually less usable at these small scales.”

According to current industry standards, a material is considered to have very good electromechanical performance if it can undergo a 1% change in shape – or strain – in response to an electric field. For an object measuring 100 inches in length, for example, getting 1 inch longer or shorter represents 1% strain.

“From a materials science perspective, this is a significant answer, since most hard materials can only change by a fraction of a percent,” said Martin, the Robert A. Welch Professor of Materials Science and Nanoengineering and director of the Rice Advanced. Institute of Materials.

When conventional piezoelectric materials are scaled down to systems smaller than one micrometer (1000 nanometers), their performance generally deteriorates significantly due to substrate interference, which dampens their ability to change shape in response to an electric field or , conversely, in generate tension in response to a change in shape.

According to Martin, if electromechanical performance were rated on a scale of 1-10—where 1 is the lowest performance and 10 is the industry standard of 1% strain—then straining would typically be expected to reduce the electromechanical response of conventional piezoelectrics from 10 to a range of 1- 4.

“To understand how compression affects movement, first picture being in a middle seat on an airplane with no one on either side of you—you’ll be free to adjust your position if you’re uncomfortable, hot, etc. ” Martin said. “Now imagine the same scenario, except now you’re sitting between two big offensive linemen from the Rice football team. You’ll be ‘locked’ between them so you can’t really adjust your position in response to a stimulus .”

The researchers wanted to understand how very thin films of antiferroelectrics—a class of materials that remained understudied until recently due to a lack of access to “model” versions of the materials and their complex structure and properties—changed their shape in response of tension. and whether they were also sensitive to coercion.

First, they grew thin films of the model antiferroelectric material PbZrO₃ with very careful control of the thickness, quality and orientation of the material. Next, they performed a series of electrical and electromechanical measurements to quantify the thin films’ responses to applied electrical voltage.

“We found that the response was significantly larger in thin films of the antiferroelectric material than that achieved in similar geometries of traditional materials,” said Hao Pan, a postdoctoral researcher in Martin’s research group and lead author of the study.

Measuring shape change on such small scales was no easy task. In fact, optimizing the measurement setup required so much work that the researchers documented the process in a separate publication.

“With the perfect measurement setup, we can get a resolution of two picometers—that’s about one-thousandth of a nanometer,” Pan said. “But just showing that a shape change happened doesn’t mean we understand what’s going on, so we had to explain it. This was one of the first studies to reveal the mechanisms behind this high performance.”

With the support of their collaborators at the Massachusetts Institute of Technology, the researchers used a state-of-the-art transmission electron microscope to observe the shape change of the nanoscale material with atomic resolution in real time.

“In other words, we watched the electromechanical actuation as it was happening, so we could see the mechanism for the large shape changes,” Martin said. “What we discovered was that there is a voltage-induced change in the crystal structure of the material, which is like the basic building block or the single type of Lego block from which the material is built. In this case, that Lego block gets stretched reversibly with applied electrical voltage, giving us a large electromechanical response.”

Amazingly, the researchers found that not only does crimping not interfere with the material’s performance, it actually enhances it. Together with collaborators at Lawrence Berkeley National Laboratory and Dartmouth College, they recreated the material computationally in order to get a different view of how strain affects actuation under applied electrical voltage.

“Our results are the culmination of years of work on related materials, including the development of new techniques to probe them,” Martin said. “By understanding how to make these thin materials work better, we hope to enable the development of smaller and more powerful electromechanical devices or microelectromechanical systems (MEMS)—and even nanoelectromechanical systems (NEMS)—that use less energy and can do things we never thought of before.”