UNIVERSITY PARK, Pa. — The ability of piezoelectric materials to convert mechanical energy into electrical energy and vice versa makes them useful for various applications from robotics to communication to sensors. A new design strategy for creating ultrahigh-performing piezoelectric ceramics opens the door to even more beneficial uses for these materials, according to a team of researchers from Penn State and Michigan Technological University.  

“For a long time, piezoelectric polycrystalline ceramics have shown limited piezoelectric response in comparison to single crystals,” said Shashank Priya, associate vice president for research and professor of materials science and engineering at Penn State and co-author of the study published in the journal Advanced Science. “There are many mechanisms that limit the magnitude of piezoelectricity in polycrystalline ceramic materials. In this paper, we demonstrate a novel mechanism that allows us to enhance the magnitude of the piezoelectric coefficient several times higher than is normally expected for a ceramic.” 

The piezoelectric coefficient, which describes the level of a material's piezoelectric response, is measured in picocoulombs per Newton.  

“We achieved close to 2,000 picocoulombs per Newton, which is a significant advance, because in polycrystalline ceramics, this magnitude has always been limited to around 1,000 picocoulombs per Newton,” Priya said. "2,000 was considered an unreachable target in the ceramics community, so achieving that number is very dramatic.” 

The path to discovering the new mechanism began with a question: What factors control the magnitude of piezoelectric constant? The piezoelectric constant is the charge generated by a unit of applied force, picocoulomb per Newton, which in turn is dependent on effects occurring at atomic to mesoscale. 

“We wondered what are some basic effects, almost at the atomic scale, of the fundamental parameters that limit or control the response?” Priya said. “Using the multiscale model developed at Michigan Tech, which is a combination of different modeling techniques to bridge the length scale, we carried out a very detailed investigation on two phenomena.” 

One was chemical heterogeneity, which describes how atoms of different elements in a material are distributed at the nanoscale. This is important because the different atomic positions and the sites that they occupy are critical to piezoelectric response.  The second is anisotropy, the influence of crystallographic orientation. This is important because piezoelectric properties in a material are higher along a certain crystallographic direction.  

“Imagine the material is like a cube — a cube has different axes, a face diagonal, and a body diagonal, and so piezoelectric response changes across all these different directions,” Yu U. Wang, professor in materials science and engineering, Michigan Technical University, said. “And so, we show that by aligning all the grains in a ceramic material along certain crystallographic axes, we can get a very high piezoelectric response. We created a very high amount of local heterogeneity and a very high grain orientation in the ceramic material, and the combination of these two basic controlling parameters led to high piezoelectric response in ceramics.” 

The researchers discovered if you add a small amount of the rare earth element europium to the ceramic, the europium will occupy the corner of the cubic lattice. This creates the chemical heterogeneity in the material that is necessary for a high piezoelectric response. The researchers were able to further amplify the response by getting 99% of the crystal grains oriented.  

The combination of these two effects has not been explored before, according to Yongke Yan, associate research professor in materials science and engineering and lead author in this study. 

"I think this mechanism that we were able to identify not only leads to enhancement but leads to dramatic enhancement, and pushes it close to ideal value, which is much higher than what many people would expect,” Yan said.  

To collect the necessary data to prove their concept, Priya and his team worked with Dabin Lin, formerly a visiting scholar with Penn State's Materials Research Institute (MRI) and currently a lecturer in photoelectrical engineering at Xi’an Technological University in China, and Ke Wang, MRI staff scientist in MRI’s Materials Characterization Lab. This included gathering transmission electron microscope data by scanning the ceramic materials, which they combined with energy-dispersive X-ray spectroscopy (EDS) techniques. EDS can determine what chemical elements are present and enables researchers to “see” at the single atom level that the europium is present in the ceramic in a way that gives it the heterogeneity necessary for high piezoelectric response.  

These findings have the potential to lead to improved and even novel piezoelectric materials, with a variety of new actuator and transducer applications. This could mean better robotics, sensors, transformers, ultrasonic motors and medical technologies. In addition, since the ultrahigh piezoelectric ceramics in the study can be processed using traditional multilayer manufacturing processes, the materials would be cost-effective and scalable. 

“People benefit from electronics, and they are present in so many things, such as robots, microscopes, transportation systems, any personal device with a screen such as a phone, medical devices such as body imaging or scanning tools, and even things used in space exploration like robots that might operate outside a spacecraft,” Priya said. “All of these things can be improved with ultrahigh piezoelectric ceramics.” 

Along with Priya, Ke, Yang, Yan and Lin, other co-authors of the study from Penn State include Li-Feng Zhu, visiting scholar, Haoyang Leng, doctoral candidate materials science and engineering; Xiaotian Li, assistant research professor in materials science and engineering; and Hairui Liu, postdoctoral researcher in materials science and engineering. Liwei Geng, instructor in materials science and engineering at Michigan Technological University, was also a co-author.

Support for the study was provided by the Department of Defense’s Defense Advanced Research Projects Agency and the National Science Foundation.