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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—heat, movement, or even noise—that might otherwise be wasted. Therefore, it holds 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 it can remain in one polarized state without additional power, making it attractive for energy-saving data storage and electronics. These materials are also widely used in beneficial applications such as switches, medical devices, 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,” said Nasim Alem, associate professor of materials science and engineering and the study’s corresponding author. “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, materials science and engineering, 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.”
There is also a unique characteristic of CMO’s microstructure that is something of a mystery to researchers.
“At room temperature, there are some polar and nonpolar phases coexisting at room temperature in the crystal,” Miao said. “And those coexisting phases are believed to be correlated with negative thermal expansion behavior. It is well-known that normally, a material expands when heated, but this one shrinks. That is interesting, but we know very little about the structure, like how the polar and nonpolar 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 double bilayer polar nanoregions in the CMO crystals. To our knowledge, it is the first time that such microstructure was directly imaged in the layered perovskite materials.”
Before, it was never 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.
"We monitored the material, what’s going on during the phase transition, and were able to probe atom by atom at what type of bonding we have, what type of structural distortions we have in the material, and how that may change as a function of temperature,” Alem said. “And this is very much explaining some of the observations that people have had with this material. For example, when they get the thermal expansion coefficient, no one has really known where this comes from. Basically, this was going down into the atomic level and understanding the underlying atomic-scale physics, chemistry, and also the phase transition's dynamics, how it's changing.”
This in turn would enable the development of lead-free, powerful piezoelectric materials.
“Scientists have been trying to find new paths to discover lead-free ferroelectric materials for many beneficial applications,” Miao said. “The existence of the polar nanoregions is considered to benefit the piezoelectric properties, and now we showed that via defect engineering, we may be able to design new strong piezoelectric crystals that would ultimately replace all lead containing materials for ultrasonic or actuator applications.”
The characterization work that revealed these never-before-seen processes in the material was carried out at the Materials Research Institute (MRI) in the Millennium Science Complex. This included multiple transmission electron microscopes (TEM) experiments that enabled the never-before-seen to be seen.
Another benefit of the study was free software developed by the research team, EASY-STEM, that enables easier TEM image data processing. This could potentially shorten the time needed to advance scientific research and move it to practical application.
“The software has a graphical user interface that allows users to input with mouse clicks, so they do not need to be an expert in coding but still can generate amazing analysis,” Miao said.
Other authors of the study from Penn State include Parivash Moradifar, doctoral candidate (at the time), and Ke Wang, staff scientist with MRI.
The study was supported by the National Science Foundation.
A new computational model developed by Penn State scientists represent an advance in the phase-field model – a tool for modeling how the internal structures of materials evolve at the mesoscale, which refers to the size of objects and phenomena occurring between the atomic scale and those observable by the human eye Credit: Provided by Tiannan Yang . All Rights Reserved.
New model that examines materials at mesoscale may be bridge to next-gen devices
A newly developed model may serve as a bridge between quantum mechanical calculations at the atomic scale and devices that could enable next-generation quantum technologies, according to a team of Penn State researchers.
“We establish a new computational model for understanding the dynamics of simultaneous structural and electronic processes in functional and quantum materials, discovering their mesoscale fundamental physics and predicting their functionalities,” said Tiannan Yang, an assistant research professor in materials science and engineering.
The findings, published in the journal NPJ Computational Materials, represent an advance in the phase-field model—a tool for modeling how the internal structures of materials evolve at the mesoscale, which refers to the size of objects and phenomena occurring between the atomic scale and those observable by the human eye, such as crystal grains, magnetic domains, junctions, and nanoscale materials and devices, the scientists said. Predicting and controlling material behaviors at this spatial scale is critical to translating quantum phenomena into functional devices and systems.
“In terms of the phase field model, this is a really important, even transformational event,” said Long Qing Chen, Donald W. Hamer Professor of Materials Science and Engineering. “We now have a phase field model that can simultaneously describe the dynamics of structural and electronic processes. This can be applied to many different problems in functional and quantum materials.”
Understanding how the atoms and electrons inside materials will respond to external stimuli, for example heat, force, electric field, or light, is essential for predicting the material properties and ultimately harnessing the materials’ functionalities, the scientists said.
The phase field method, co-developed by Chen, has emerged over the past several decades as a powerful tool to model microstructure and physical properties at the mesoscale. But the method did not take into account the dynamical interactions between electrons and the crystal lattice, an effect that becomes particularly significant in fast processes excited by strong stimuli.
“Once you hit a material with some stimulus, it is going through a lot of processes,” said Chen, who also has appointments in mathematics and engineering science and mechanics. “And a lot of times those are simultaneous electronic and structural processes. Now, we have a way to describe these together.”
The new model allows scientists to examine the dynamics of these processes—or changes that happen over very short timescales, from picoseconds to nanoseconds—like when researchers shine short pulses of lasers onto a material to alter its electronic properties.
“A lot of properties depend on frequency,” Chen said. “When you apply a field, whether mechanical, electrical, or light at different frequencies, the material will respond differently. This model now lets us look at the frequency dependence of these responses and see how the structure has actually evolved inside the material and how that connects to the properties.”
The findings offer a theoretical framework for understanding and predicting the coupled electron and structural dynamics of excited-state materials and lay the foundation for further mesoscale models for a wide variety of functional and quantum materials, the scientists said.
Quantum materials is a broad term that refers to materials with collective properties governed by quantum behavior, such as special magnetic and electronic ordering phenomena that could lead to revolutionary, next-generation technologies, like quantum computing.
The underlying physics of the phenomena inherent to quantum materials, such as strongly interacting electrons, topologically driven spin, charge, and orbital and lattice textures, will be captured by the computational phase-field method to help researchers and engineers harness the materials’ specific properties, the scientists said.
This research stems from a $2.75 million grant awarded to a Penn State-led team to improve the phase field method. The effort is part of a larger DOE project that aims to integrate computational simulation, software development and experimental validation to accelerate the development and utilization of quantum and functional materials.
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Penn State has named Corning Inc., one of the world's leading innovators in materials science, as its 2022 Corporate Partner of the Year. The annual award celebrates corporate partners that have demonstrated exceptional commitment in the promotion and support of Penn State, have excellent track records of philanthropy and research, and actively engage Penn State students and alumni in the workplace and the classroom.
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Lawrence Hancock, Ph.D.
1983, 1988g
Chief Executive Officer, Akita Innovations LLC
Lawrence "Larry" Hancock is a Founder and the Chief Executive Officer of Akita Innovations LLC. Hancock has over 30 years of experience in research, development, and commercialization of emerging materials technologies. He has a diverse background in applied polymer science, including materials chemistry and physical properties as well as engineered materials such composites, membranes, textiles, and sensory materials. Previously, Hancock led the Materials and Photonics group of FLIR Systems, CBRNE Detection Division.
Hancock holds BS, MS and Ph.D. degrees in Polymer Science from The Pennsylvania State University and completed post-doctoral studies at the Massachusetts Institute of Technology.
He delivered the Department of Materials Science and Engineering Richard E. Tressler Lecture in 2020 and served as a member of the Graduates of Earth and Mineral Sciences (GEMS) board from 2014-19.
Larry is married to Beth Hancock (’84 BS, ’87 MS Polymer Science Penn State) and they have two children.