A team of Penn State researchers have observed and reported the unique microstructure of a new ferroelectric material for the first time, enabling the development of lead-free piezoelectric materials for electronics, sensors and energy storage that are safer for human use. The work was led by Penn State’s Alem Group and in collaboration with research teams from Rutgers University and the University of California, Merced. The article reporting the results was published in Nature Communications.
Ferroelectrics are a class of materials which demonstrate spontaneous electrical polarization when an external electrical charge is applied. This causes spontaneous electrical polarization when the positive and negative charges of materials point to different poles. These materials also have piezoelectric properties, which means that the material generates an electrical charge under an applied mechanical force.
The piezoelectric property allows these materials to generate electricity from energy like heat, motion, or even noise that might otherwise be wasted. Therefore, they have potential for alternatives to carbon-based energy, such as energy recovery from waste heat. Additionally, ferroelectric materials are particularly useful for data storage and memory because they can remain in a biased state without additional power, making them attractive for data storage and power-efficient electronics. They are also widely used in beneficial applications such as switches, important medical devices such as heart rate and ultrasound monitors, energy storage and actuators.
But the toughest piezoelectric materials contain lead, which is a major problem given that lead is toxic to humans and animals.
Nasim Alem, associate professor of materials science and engineering at Penn State and corresponding author of the study, said, “We would like to design a piezoelectric material that does not have the drawbacks of current materials. And right now the lead in all of these materials is a big minus because lead is dangerous. We hope that 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 new improper hybrid ferroelectric material with interesting properties.
Leixin Miao, PhD student in materials science and first author of the study, explained: “The design principle of this material is to combine the movement of the small oxygen octahedra of the material. Within the material there are octahedrons of oxygen atoms that can tilt and rotate. The term “hybrid improper ferroelectric” means that we combine the rotation and tilting of the octahedra to produce ferroelectricity. It is considered a “hybrid” because it is the combination of two movements of the octahedra generating this polarization for ferroelectricity. It is considered a “dirty” ferroelectric since polarization is generated as a side effect.
There is also a unique feature of CMO’s microstructure that is a mystery to researchers.
“At room temperature, some polar and non-polar phases coexist at room temperature in the crystal,” Miao said. “And these coexisting phases are thought to be correlated with negative thermal expansion behavior. It is well known that normally a material expands when heated, but this material shrinks. It’s interesting, but we know very little about the structure, like how polar and non-polar phases coexist.
To better understand this, the researchers used atomic-scale transmission electron microscopy.
“Why we used electron microscopy is 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 double-bilayer nanoregions in the CMO crystals. To our knowledge, this is the first time that such a microstructure has been directly imaged in layered perovskite materials.
Previously, it had never been observed what happens to a material that goes through such a ferroelectric phase transition, according to the researchers. But with electron microscopy, they could monitor the material and what was happening during the phase transition.
Alem noted, “We monitored the material, what happens during the phase transition, and were able to probe atom by atom what kind of bonding we have, what kind of structural distortions we have in the material and how that might change. as a function of temperature. And that goes a long way to explaining some of the observations that people have made with this material. For example, when they get the coefficient of thermal expansion, no one really knows where it came from. Basically it went down to the atomic level and included the underlying atomic scale physics, chemistry and also the dynamics of the phase transition, how it changes.
This then allowed the development of powerful, lead-free piezoelectric materials.
“Scientists have been trying to find new ways to discover lead-free ferroelectric materials for many beneficial applications,” Miao said. “The existence of polar nanoregions is believed to be beneficial for piezoelectric properties, and we have now shown that, through defect engineering, we may be able to design powerful new piezoelectric crystals that would eventually replace all materials containing lead for ultrasonic or actuator applications.”
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The characterization work that revealed these novel processes in the material was carried out at the facilities of the Materials Research Institute at the Millennium Science Complex. This included multiple transmission electron microscope (TEM) experiments that made it possible to see the never-before-seen.
There is a long list of authors to credit, too long for a blog post. However, it should be mentioned that the list can be viewed on both the press release page and the study summary page.
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Lead, although very useful and extremely convenient, is actually quite a toxic material when ingested. It’s gentle enough to soak into the skin and work its way into your mouth and nose – something you really don’t want to happen.
The hope is that products and devices that simply don’t exist today could come to the consumer market as new properties of materials are more widely explored.
For now, existing better and safer products seem to be available fairly quickly if the cost of calcium manganate in manufacturing is low enough in terms of cost and processing. There will likely be more materials to come in the future as well.
By Brian Westenhaus via New Energy and Fuel
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