Unique ferroelectric microstructure visible for the first time

A team of researchers has observed and reported for the first time the unique microstructure of a novel ferroelectric material, enabling the development of lead-free piezoelectric materials for electronics, sensors and energy storage that are safer for human use. This work was led by the Alem Group at Penn State and in collaboration with research teams at Rutgers University and the University of California, Merced.

Ferroelectrics are a class of materials that exhibit spontaneous electrical polarization when an external electrical charge is applied. This causes spontaneous electrical polarization when positive and negative charges in the materials lead to opposite poles. These materials also have piezoelectric properties, meaning that the material generates an electrical charge under an applied mechanical force.

This allows these materials to generate electricity from energy such as heat, motion, or even noise that might otherwise be wasted. Therefore, they hold potential for alternatives to carbon-based energy, such as B. the production of energy from waste heat. Additionally, ferroelectric materials are particularly useful for data storage and storage because they can remain in a polarized state without additional power supply, making them attractive for low-power data storage and electronics. They are also widely used in useful applications such as switches, essential medical devices such as heart rate monitors and ultrasound, energy storage, and actuators.

However, the strongest piezoelectric materials contain lead, which is a major problem as lead is toxic to humans and animals.

“We would like to design a piezoelectric material that doesn’t have the disadvantages of current materials,” said Nasim Alem, associate professor of materials science and engineering at Penn State and corresponding author of the study. “And right now, in all of these materials, lead is a major disadvantage because the lead is dangerous. We hope our study can lead to a suitable candidate for a better piezoelectric system.”

To develop a route to such a lead-free material with strong piezoelectric properties, the research team worked with calcium manganate, Ca₃Mn₂O₇ (CMO). CMO is a novel hybrid unsuitable ferroelectric material with some interesting properties.

“The design principle of this material is to combine the motion of the material’s small oxygen octahedra,” said Leixin Miao, a PhD student in materials science and first author of the study in nature communication. “In the material there are octahedrons of oxygen atoms that can tilt and rotate. The term ‘hybrid mismatched ferroelectric’ means that we combine the rotation and tilting of the octahedron to create ferroelectricity. It is considered a ‘hybrid’ because it is the combination of two movements of the octahedron that produce this polarization for ferroelectricity. It is considered an “unsuitable” ferroelectric because polarization is created as a secondary effect.”

There is also a unique feature of CMO’s microstructure that has puzzled researchers.

“At room temperature, there are some polar and non-polar phases that coexist in the crystal at room temperature,” Miao said. “And these coexisting phases are thought to correlate with negative thermal expansion behavior. It is known that when a material is heated, it normally expands, but this one shrinks. This is interesting, but we know very little about the structure, e.g. B. how the polar and non-polar phases coexist.”

To better understand this, the researchers used atomic-scale transmission electron microscopy.

“We used electron microscopy because with electron microscopy we can use atomic-scale probes to see the exact atomic arrangement in the structure,” Miao said. “And it was very surprising to observe the polar bilayer nanoregions in the CMO crystals. To our knowledge, it is the first time such a microstructure has been directly imaged in the layered perovskite materials.”

According to the researchers, what happens to a material that undergoes such a ferroelectric phase transition has never been observed before. But with electron microscopy, they were able to monitor the material and what was happening during the phase transition.

“We’ve been monitoring the material, what’s going on during the phase transition, and we’ve been able to study, atom by atom, what kind of bonding we have, what kind of structural distortions we have in the material, and how that can change as a function of temperature,” Alem said, “And that explains very well some of the observations that people have made with this material. For example, when they get the coefficient of thermal expansion, nobody really knew where that came from. Basically, it went down to the atomic level and that Understanding the underlying physics, chemistry and also the dynamics of the phase transition at the atomic level as it changes.”

This in turn would enable the development of lead-free, high-performance piezoelectric materials.

“Scientists have been trying to find new ways to discover lead-free ferroelectric materials for many useful applications,” Miao said. “The existence of the polar nanoregions is thought to be beneficial for the piezoelectric properties, and now we have shown that through defect engineering we might be able to design new strong piezoelectric crystals that will eventually use any lead-containing materials for ultrasonic or actuator applications would replace.”

The characterization work that revealed these never-before-seen processes in the material was carried out at the Materials Research Institute facilities at the Millennium Science Complex. This included experiments with several transmission electron microscopes (TEM), which made it possible to see things never seen before.

Another benefit of the study was the free software developed by the EASY-STEM research team, which allows for easier processing of TEM image data. This could potentially reduce the time required to advance scientific research and translate it into practical application.

“The software has a graphical user interface that allows users to input with mouse clicks, so users don’t need to be an expert in coding but can still create amazing analysis,” Miao said.