Jun 16 2017
A research team from the University of Pennsylvania have gained new knowledge on the smart materials used in ultrasound technology. When they developed a detailed model to explain the working of these materials, they discovered evident similarities with the behavior of water.
Penn Scientists explore the materials that make medical ultrasound and SONAR devices work better than ever. Their theory (left) provides unprecedented agreement with experimental X-ray diffuse scattering data (right), showing that their material model is revealing the behavior of relaxor ferroelectric materials. Credit: University of Pennsylvania
The research has been reported in the journal Nature and has been headed by Andrew M. Rappe, the Blanchard Professor of Chemistry in the School of Arts & Sciences and a Professor of Materials Science and Engineering in the School of Engineering and Applied Science, and Hiroyuki Takenaka, Postdoc in the Department of Chemistry. Ilya Grinberg, Penn Research Specialist, and Shi Liu, Alumnus, were the Co-Authors of the research.
The Researchers focused on throwing light on the way materials interact with, utilize and transform energy into disparate forms. As part of this research, they analyzed the behavior of a smart material, namely piezoelectricity, wherein mechanical energy is interchanged with electrical energy.
In piezoelectricity, when an electric field is applied to a material, the dipoles within it are reoriented. This plays a significant role in the material’s behavior.
You can imagine that there’s a cage of oxygen atoms and there’s a positive ion in the middle. If it sits in the middle of the cage then there’s no dipole, but if it moves off-center then there’s a dipole. The rearrangement of those dipoles is what leads to these smart material properties.
Andrew M. Rappe, the Blanchard Professor of Chemistry in the School of Arts & Sciences and a Professor of Materials Science and Engineering in the School of Engineering and Applied Science
When the positive ions move away from the center, the ion cages around them get shrunk or elongated in a coordinated manner, resulting in a change in shape of the material.
When voltage is supplied to ultrasound devices, the shape of the material changes, or the material vibrates. These vibrations penetrate the human body and get echoed. Piezoelectric materials are even employed in sonar to enable instruments to obtain under-water images.
In the recent past, Researchers have found out a set of materials that they consider will give greater piezoelectric performance than materials discovered earlier. According to Rappe, at a basic level, Researchers could not perceive the reason behind the exceptional behavior of these materials.
“If you don’t know why it works, how could you possibly reverse engineer it and get to the next level?” asked Rappe.
Researchers mostly use theory as well as modeling to analyze smart materials. They form a concept of operation of a system according to their understanding and can illustrate the behavior of an actual material by solving certain equations.
One thing that we often do is solve the equations of quantum mechanics because quantum mechanics is known to be an accurate model for how electrons behave. The electrons are the glue that holds the nuclei together. If you know how they’re behaving, then you know what determines when bonds break and form and so forth.
Andrew M. Rappe, the Blanchard Professor of Chemistry in the School of Arts & Sciences and a Professor of Materials Science and Engineering in the School of Engineering and Applied Science
According to Rappe, one exciting development is the ability to go beyond what Researchers can afford quantum mechanically and build mechanical models and give them a more approximate way of dealing with the bonds in a solid while also allowing them to model finite temperature, larger amounts of material and for longer periods of time.
“This allows us to observe behaviors that take a long time to happen or only happen deep inside a material, and this gives us unique perspectives on complicated behaviors,” stated Rappe.
Although previous experiments have investigated this material and certain theoretical models have unearthed specific properties of the material, at present, the Penn research team have developed a complete model, to date, related to the behavior of the material.
Earlier Researchers believed that at higher temperatures it was “every dipole for himself,” rendering it simple for them to react to external stimuli (e.g. electric fields).
When the material cools down, the dipoles assemble in groups known as polar nanoregions. When the regions become large, they become languid, making it highly difficult for them to react.
The new paper published by the Researchers reports that while the dipoles are free to float at higher temperatures, when the temperature is reduced, the dipoles clump together and form these polar nanoregions, wherein the regions actually do not grow larger; on the contrary, but they are more thoroughly aligned.
This results in the formation of domain walls inside the material that separates patches of different alignment. These domain walls between dipolar regions are reason behind the enhanced piezoelectric characteristics of the material.
This is similar to the behavior in water where the dipoles become highly correlated as the temperature decreases. However, here, the correlation does not remain intact at larger distances.
“They’re never perfectly aligned,” stated Rappe. “Nearby water dipoles may get more and more aligned, but because of hydrogen bonding there’s some intrinsic size beyond which it doesn’t grow.”
Piezoelectric materials are significant materials used in actuators, transducers and sensors used in various industries. Restricted knowledge on the behavior of these materials has made the development of higher quality materials very slow. This paper offers an innovative and in-depth knowledge of the way in which they function and unearths the similarities between their behavior and the behavior of water.
A thorough knowledge of the reasons behind the behavior of these materials can open the door for new materials design, resulting in the development of higher quality piezoelectrics that may transform smart material applications.
It’s exciting to be able to build up a model from individual electrons up to millions of atoms at finite temperature and observe complex properties. It’s exciting that observing those complex properties gives us new productive directions where we can enhance materials that will more efficiently convert energy for useful devices to help people.
Andrew M. Rappe, the Blanchard Professor of Chemistry in the School of Arts & Sciences and a Professor of Materials Science and Engineering in the School of Engineering and Applied Science
The Office of Naval Research supported the study under Grant N00014- 12-1-1033. The U.S. Department of Defense offered computational support through a Challenge Grant from the High Performance Computing Modernization Office.