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Ferroelectric Microstructure Observation Could Lead to Safer Piezoelectric Materials

Ferroelectric Microstructure Observation Could Lead to Safer Piezoelectric Materials content piece image
An atomically resolved scanning transmission electron microscopy (STEM) image of the polar nanoregions (PNRs) embedded in the nonpolar matrix in the layered perovskite material (Ca, Sr)3Mn2O7. Bright contrast in the images can be directly interpreted as the atomic columns in the crystal. Aberration corrected STEM was employed to direct capture the arrangement of the atoms in the (a-type and b-type) polar nanoregions in the crystal and the displacement measurement at picometer precision were performed on the STEM images to extract the distortion in the structure. Credit: Alem Group/Jennifer M. McCann, MRI.
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A team of researchers have 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 demonstrate a spontaneous electric polarization when an external electric charge is applied. This causes a spontaneous electric polarization when positive and negative charges in the materials head to different poles. These materials also have piezoelectric properties, which means the material generates an electrical charge under an applied mechanical force.

This enables these materials to make electricity from energy like heat, movement, or even noise that might otherwise be wasted. Therefore, they hold potential for alternatives to carbon-based energy, such as harvesting energy from waste heat. In addition, ferroelectric materials are especially useful for data storage and memory as they can remain in one polarized state without additional power, making them attractive for energy-saving data storage and electronics. They are also widely used in beneficial applications such as switches, important medical devices like heart-rate monitors and ultrasounds, energy storage and actuators.

However, the strongest piezoelectric materials contain lead, which is a major issue given lead is toxic for humans and animals.

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

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

"The designing principle of this material is combining the motion of the material’s little oxygen octahedra,” said Leixin Miao, doctoral candidate in materials science and first author of the study in Nature Communications. “In the material, there are octahedra of oxygen atoms that can tilt and rotate. The term ‘hybrid improper ferroelectric’ means we combine the rotation and the tilting of the octahedra to produce ferroelectricity. It is considered a ‘hybrid’ because it is the combination of two motions of the octahedra generating that polarization for ferroelectricity. It is considered an ‘improper’ ferroelectric since the polarization is generated as a secondary effect.”

Reference: Miao L, Hasin KE, Moradifar P, et al. Double-Bilayer polar nanoregions and Mn antisites in (Ca, Sr)3Mn2O7. Nat Commun. 2022;13(1):4927. doi:10.1038/s41467-022-32090-w

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