Research identifies a powerful alternative to conventional ferroelectric energy

Lighting a gas barbecue, making 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.

The ability to switch between mechanical voltage and electrical charge is known as piezoelectricity and 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 become – at the sub-micrometer scale, when the thickness is only a few millionths of an inch – ‘trapped’ by the material to which they are attached . , which significantly reduces their performance.

Researchers and collaborators from Rice University at the University of California, Berkeley have discovered that a class of electromechanically active materials called antiferroelectric materials 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 as small as 100 nanometers (or 4 millionths of an inch) thick.

“We’ve been using piezoelectric materials for decades,” said rice materials scientist Lane Martin, the study’s corresponding author. “Lately 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 fits into your phone or computer. The problem is that these materials are usually just less useful on this small scale.”

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 elongation) in response to an electric field. For example, for an object 100 inches long, becoming 1 inch longer or shorter means 1% strain.

“From a materials science perspective, this is a significant answer, as 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 the Rice Advanced Materials Institute.

When conventional piezoelectric materials are miniaturized into systems smaller than a micrometer (1,000 nanometers), their performance typically degrades significantly due to substrate interference, which reduces their ability to change shape in response to an electric field or, conversely, is reduced. generate tension in response to a change in shape.

According to Martin, if electromechanical performance were rated on a scale of 1 to 10 (with 1 being the lowest performance and 10 being the industry standard of 1% strain), clamping would generally be expected to reduce the electromechanical response of conventional piezoelectric devices. reduce from 10 to 10. the range 1-4.

“To understand how clenching affects movement, first imagine you’re sitting in the middle seat of an airplane, with no one on either side of you – you’re free to adjust your position if you feel uncomfortable, overheated , etc.,” said Martin. “Now imagine the same scenario, except now you’re sitting between two huge offensive linemen from the Rice football team. You’d be so ‘sandwiched’ between them that you really can’t meaningfully adjust your position in response to a stimulus.”

The researchers wanted to understand how very thin films of antiferroelectric materials – a class of materials that until recently remained understudied due to a lack of access to ‘model versions’ of the materials and because of their complex structure and properties – changed their shape in response to stress . and whether they were also sensitive to clamping.

First they grew thin films of the model antiferroelectric material PbZrO₃ with very careful control of material thickness, quality and orientation. They then 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 greater in the thin films of antiferroelectric material than what is achieved in comparable 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 a small scale was not an easy task. Optimizing the measurement setup required so much work that the researchers documented the process in a separate publication.

“With the sophisticated measurement setup, we can achieve a resolution of two picometers, which is about one-thousandth of a nanometer,” Pan said. “But just showing that a shape change has occurred 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 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 nanoscale materials in real time with atomic resolution.

“In other words, we looked at the electromechanical activation as it happened so we could see the mechanism for the large shape changes,” Martin said. “What we discovered was that there is an electrical stress-induced change in the crystal structure of the material, which resembles the basic building unit or the single type of Lego block that makes up the material. In this case, that Lego block gets reversibly stretched with applied electrical voltage, which gives us a great electromechanical response.”

Surprisingly, the researchers found that clamping not only does not disrupt material performance, but actually improves it. Together with collaborators at Lawrence Berkeley National Laboratory and Dartmouth College, they simulated the material computationally to get another view of how the clamping 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 investigate them,” Martin said. “By figuring out 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 consume and be able to do things that we never thought possible before.”