­New Piezoelectric Material Doesn’t Suffer Clamping

Piezoelectric materials are widely used in technological products. These materials can transmit electrical energy to mechanical stress and vice versa. The phenomenon is used in everyday devices, such as lighters and ultrasound machines and in components such as actuators, transducers and sensors. However, until now at least, piezoelectricity loses some of its effect when the application decreases in size, which is becoming a problem as electronic components are miniaturized even further. The problem with these materials is that at the submicrometer scale, the electromechanically active materials get ‘clamped’ down to the materials that they are attached to, decreasing their performance.

This problem is being tackled by researchers from Rice University and their colleagues at the University of California, Berkeley. The scientists have identified a new class of electromechanically active materials called antiferroelectrics that could overcome the performance limitations of clamping to create miniaturized electromechanical devices. The antiferroelectric material that they studied, lead zirconate (PbZrO3), can output an electromechanical response that is up to five times greater than conventional piezoelectric materials, even in films that are only 100 nanometers thick.

Traditionally, materials are thought to have very good electromechanical performance if they can undergo a 1% change in shape in response to an electric field. “This is a significant response, since most hard materials can only change by a fraction of a percent,” said Lane Martin, the Robert A. Welch Professor, professor of materials science and nanoengineering and director of the Rice Advanced Materials Institute.

When conventional piezoelectric materials get below 1,000 nanometers in size, their performance deteriorates due to the interference of the substrate, which dampens their ability to change shape in response to electric field or to generate voltage in response to a change in shape. The researchers wished to understand how very thin films of antiferroelectrics changed their shape in response to voltage and whether they were susceptible to clamping. Antiferroelectrics have remained understudied until recently due to a lack of access to “model” versions of the materials and their complex structure and properties.

The researchers started by growing thin films of the model antiferroelectric material PbZrO 3 with very careful control of the material thickness, quality and orientation. Next, they performed an array of electrical and electromechanical measurements to quantify the responses of the thin films to applied electric voltage. They found that the response was larger in the thin films of antiferroelectric material than similar geometries of traditional materials. Measuring shape change at such small scales was not easy to achieve. To perform the measurements, the team worked with MIT, using a state-of-the-art transmission electron microscope to observe the nanoscale material shapeshift with atomic resolution in real time.

“We watched the electromechanical actuation as it was happening, so we could see the mechanism for the large shape changes,” Martin said. “What we found was that there is an electric voltage-induced change in the crystal structure of the material, which is like the fundamental building unit or single type of Lego block from which the material is built. In this case, that Lego block gets reversibly stretched with applied electric voltage, giving us a big electromechanical response.”

From the tests, the researchers discovered that performance was actually enhanced in antiferroelectric materials. Together with collaborators at Lawrence Berkeley National Laboratory and Dartmouth College, they recreated the material computationally in order to get another view of how the clamping affects the actuation under applied electric voltage.

Martin said. “By figuring out how to make these thin materials work better, we’re hoping to enable the development of smaller and more powerful electromechanical devices or microelectromechanical systems (MEMS) ⎯ and even nanoelectromechanical systems (NEMS).”

The team’s work was published in Nature Materials

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