New findings are shedding light on longstanding debates over the behavior of ferroelectric materials when those materials are exposed to electric fields. The findings stem from the use of a novel technique that allows researchers to observe the real-time behavior of domain walls in ferroelectric materials as they are “poled” and “depoled.”

Ferroelectric materials are used in a wide range of technologies, from sensors to actuators, and their electrical properties are critical to their utility. It’s well established that you can bring the various domains in a ferroelectric material into alignment by applying an electric field – either direct current (DC) or alternating current (AC). This is called ‘poling.’ However, there has been significant debate about what exactly is taking place during the poling process.

“We’re now able to observe what is happening in real time, which gives us deeper insights into the mechanisms at play – which will inform our ability to engineer materials in order to produce the electrical characteristics we’re looking for,” says Jun Liu, co-corresponding author of two papers on the work and an associate professor of mechanical and aerospace engineering at North Carolina State University.

“Prior to this work, you could measure a material’s polarization state, then apply an electric field, then measure the polarization state again,” says Kara Peters, co-corresponding author of the papers and Distinguished Professor of Mechanical and Aerospace Engineering at NC State. “So you could see the before and after but not understand what’s driving the transition.”

Conventional techniques for measuring a material’s polarization state require capturing multiple images of the material sample and altering the optical polarization of the light source with each image. Researchers can then measure changes in the material’s electrical polarization by analyzing the resulting data.

“We’ve developed a technique that allows us to capture polarization changes in ferroelectric material in a single image,” says Peters. “We do this by splitting a beam of white light into multiple wavelengths of light, each of which has a different optical polarization. We can then capture all the relevant data in one shot. The limiting factor here is the frame rate of your camera, so you can capture up to thousands of frames per second – though we only captured 100 frames per second for this paper. That’s all we needed.”

“This technique allowed us to see how the boundaries between domains in the material – the domain walls – move and change in response to the introduction of an electric field,” says Liu.

There have been a number of debates among researchers over what is actually happening during these poling processes. One such debate centered on whether applying an AC field led to a rapidly alternating polarization of the material while the field was being applied.

“This was previously impossible to resolve at a high frame rate using a single image, and thus we could not see what was happening while the field was actually being applied – we could only see before and after or at a much lower frame rate than the rate of change actually happens,” says Xiaoning Jiang, co-corresponding author of the papers and the Dean F. Duncan Distinguished Professor of Mechanical and Aerospace Engineering at NC State. “And, yes, the polarization does alternate when an AC field is being applied. We’ve been able to shed light on several of these debates.”

“The new technique offers unprecedented insights into how AC poling, DC poling, and electrical depoling influence the polarization of the material,” says Liu. “Can we use this to develop better protocols that allow us to better engineer ferroelectric materials? That’s a question we’ll be exploring in future work.”

A journal article on the behavior of domain walls when exposed to an electric field is published open access in the journal Advanced Science. Co-lead authors of that paper are Ziqi Wang and Anastasia Timofeeva, both former Ph.D. students at NC State; and Zhengze Xu, a current Ph.D. student. A third co-corresponding author is Xiaoning Jiang, the Dean F. Duncan Distinguished Professor of Mechanical and Aerospace Engineering at NC State. The paper was co-authored by Hossam Elnaggar and Yusen Pei, Ph.D. students at NC State; Reece Henry, a postdoctoral researcher at NC State; Sipan Liu, a former postdoc at NC State; Brendan O’Connor, professor of mechanical and aerospace engineering at NC State; Eunkyoung Shim, an associate professor of textile engineering at NC State; and Franky So, the Walter and Ida Freeman Distinguished Professor of Materials Science and Engineering at NC State.

A journal article focused specifically on the novel method for capturing images of ferroelectic domain walls was published in Review of Scientific Instruments. First author of this paper is Timofeeva. Co-corresponding authors are Peters, Jiang and Liu. Co-authors are Wang, Xu, Liu, Pei, Elnaggar, So and Shim.

This work was done with support from the National Science Foundation under grants DMR2011978 and DMR2309184; the Air Force Office of Scientific Research under grant FA95502310311; the Office of Naval Research under grants N000142112058, N000142312001 and N000142412101; and NC State’s Nonwovens Institute under project number 21255SB.