A team of researchers has observed and reported for the first time the unique microstructure of a new 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 a scientist 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 spontaneously exhibit electrical polarization when an external electric charge is applied. This leads to spontaneous electrical polarization when the positive and negative charges in the materials point to different electrodes. These materials also have piezoelectric properties, which means that the material generates an electrical charge under an applied mechanical force.
This enables these materials to produce electricity from energy such as heat, motion or even noise that would otherwise be wasted. Therefore, they have potential for carbon-based energy alternatives, such as harvesting energy from waste heat. In addition, ferroelectric materials are particularly useful for data and memory storage as they can remain in a single polarized state without additional energy, making them attractive for energy-efficient data storage and electronics. They are also widely used in useful applications such as switches and important medical devices such as heart rate monitors, ultrasound, energy storage and actuators.
However, the strongest piezoelectric materials contain lead, which is a major issue since lead is toxic to humans and animals.
“We would like to design a piezoelectric material that does not have the defects of existing materials,” said Nassim Alem, associate professor of materials science and engineering at Penn State and corresponding author of the study. “Currently, lead in all of these materials is a major drawback because lead is dangerous. We hope our study will lead to a suitable candidate for a better piezoelectric system.”
To develop a pathway for such lead-free materials with strong piezoelectric properties, the research team worked with Ca3Mn2O7 (CMO) calcium manganese. CMO is a new hybrid material unsuitable for photovoltaic iron electricity with some interesting properties.
said Lixin Miao, a doctoral candidate in materials science and first author of the study at Nature Communications. “In the material, there are eight faces of oxygen atoms that can tilt and rotate. The term ‘electroelectric improper hybrid’ means that we combine rotation and tilt octahedron to produce electrical energy. It is considered ‘hybrid’ because it is the combination of two octahedral motions that generate this polarization. of ferroelectricity. It is considered ‘improper’ ferroelectricity because polarization is produced as a secondary effect.”
There is also a unique property of CMO’s micro-structure which is a mystery to researchers.
“At room temperature, some polar and nonpolar phases coexist at room temperature in the crystal,” Miao said. “These coexisting phases are thought to be related to the behavior of negative thermal expansion. It is well known that a material typically expands when heated, but this phase contracts. This is interesting, but we know very little about the structure, such as how the polar and non-polar phases coexist.”
To better understand this, the researchers used atomic-scale transmission electron microscopy.
“The reason we use the electron microscope is that with the electron microscope, we can use atomic sensors to see the exact atomic arrangement in the structure,” Miao said. “And it was very surprising to observe bilayer polar nano-regions in CMO crystals. To our knowledge, this is the first time such a microstructure has been directly imaged in layered perovskite materials.”
Before, it had never been observed what happens to a material undergoing such an electrical phase transition, according to the researchers. But using an electron microscope, they could observe the material and what was going on during its phase transition.
“We’ve been watching the material, what happens during the phase transition, and we’ve been able to probe an atom by seed at what kind of bonding we have, what kind of structural distortions there are in the material, and how that might change as a function of that,” said the scientist. “And that largely explains some of the observations he got. people with this article. For example, when they got the coefficient of thermal expansion, no one really knew where this came from. Basically, this was going down to the atomic level and understanding basic physics and chemistry at the atomic level as well as phase transition dynamics, and how it changes. “
This, in turn, will enable the development of strong, lead-free piezoelectric materials.
“Scientists are trying to find new ways to discover lead-free ferroelectric materials for many useful applications,” Miao said. “The presence of polar nanoregions is beneficial for piezoelectric properties, and now we have shown that by engineering the defects, we may be able to design new strong piezoelectric crystals that will eventually replace all lead-containing materials for ultrasonic or actuator applications.”
Characterization work that revealed these unprecedented processes in matter was performed at the facilities of the Institute for Materials Research at the Millennium Science Park. This included experiments with multiple transmission electron microscopes (TEM) that made it possible to see the unprecedented.
Another benefit of the study is the free software developed by the research team, EASY-STEM, which allows easier processing of TEM image data. This can shorten the time needed to advance scientific research and transfer it to practical application.
“The software has a graphical user interface that allows users to input with mouse clicks, so people don’t need to be coding experts but can still create amazing analysis,” Miao said.
Along with Miu and Alem, other study authors from Penn State included Parivash Moradifar, a doctoral candidate at the time, and Ke Wang, a staff scientist with MRI. The authors from UCSD include Kishwar-e Hasin, a graduate student in computational and simulation materials science, and Elizabeth A. Nowadnik, associate professor of materials science and engineering. Additional authors from Oak Ridge National Laboratory include Dibangshu Mukherjee, Associate Research and Development Scientist, and Rutgers University’s Sang Wook Cheung, Distinguished Professor, Henry Rutgers Professor, Board Professor and Director of the Center for Synthesis of Quantum Materials.
The study was supported by the National Science Foundation.