Study identifies high-performance alternative to conventional ferroelectrics

Lighting a gas grill, using ultrasound, using an ultrasonic toothbrush: these actions involve the use of materials capable of translating 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, at the submicrometer scale, when the thickness is only a few millionths of an inch, to be “clamped together.” by the material to which they are attached. , which significantly reduces their performance.

Researchers at Rice University and collaborators at the University of California, Berkeley have discovered that a class of electromechanically active materials called antiferroelectrics could be the key to overcoming performance limitations due to clamping in miniaturized electromechanical systems.

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

“We’ve been using piezoelectric materials for decades,” said Lane Martin, a materials scientist at Rice who is the study’s corresponding author. “Recently, there has been a strong push to further integrate these materials into new types of very small devices, as you would want to do, for example, for a microchip placed inside your phone or computer The problem is that these materials are generally just 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 deformation) in response to an electric field. For an object measuring 100 inches in length, for example, getting 1 inch more or less represents 1% stress.

“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, professor of materials science and nanoengineering and director of Rice Advanced. Materials Institute.

When conventional piezoelectric materials are scaled down to sub-micrometer (1,000 nanometer) sized systems, their performance typically deteriorates significantly due to substrate interference, dampening their ability to change shape in response to a electric field or, conversely, to generate a voltage in response to a change in shape.

According to Martin, if electromechanical performance were rated on a scale of 1 to 10 (where 1 is the lowest performance and 10 is the industry standard of 1% strain), then tightening should generally lower the electromechanical response of piezoelectrics. conventional from 10 to 1%. range 1-4.

“To understand the impact of tightness on movement, first imagine that you are sitting in a middle seat on an airplane with no one on either side of you: you would be free to adjust your position if you are uncomfortable. ‘comfortable, overheat, etc.,’ Martin said. “Now imagine the same scenario, except that you are now sitting between two huge offensive linemen from the Rice football team. You would be ‘squeezed’ between them in such a way that you really couldn’t adjust your position. significantly in response to a stimulus.”

The researchers wanted to understand how very thin films of antiferroelectrics, a class of materials that remained little studied until recently due to lack of access to “model” versions of the materials and their complex structure and properties, changed. shape in response to tension. and whether they were equally likely to squeeze.

First, they developed thin films of the model antiferroelectric material PbZrO.₃ 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 responses of the thin films to applied electrical voltage.

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

Measuring shape change on such small scales was not an easy task. In fact, optimizing the measurement setup required so much work that the researchers documented the process in a separate publication.

“With an advanced measurement setup, we can achieve a resolution of two picometers, or about a thousandth of a nanometer,” Pan said. “But just showing that a shape change occurred doesn’t mean we understand what’s happening, so we had to explain it. This was one of the first studies to reveal the mechanisms behind it. origin of this high performance.”

With support from 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 material at the nanoscale with real-time atomic resolution.

“In other words, we observed the electromechanical actuation as it happened, so we could see the mechanism responsible for the large shape changes,” Martin said. “What we found is that there is an electrical voltage-induced change in the crystal structure of the material, which is like the fundamental building unit or the single type of Lego block from which the material is constructed. In this case, this Lego block is reversibly stretched with an applied electrical voltage, giving us a large electromechanical response.

Surprisingly, the researchers found that clamping not only does not interfere with the material’s performance, it actually improves it. Working with collaborators at Lawrence Berkeley National Laboratory and Dartmouth College, they recreated the material computationally to get another view of how tightening affects actuation under applied electrical voltage.

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