Thin ceramics make strong piezoelectrics

Piezoelectrics are materials that deform in response to an electric field. They are important in all sorts of industrial and consumer devices, typically acting as sensors, fine actuators and sound generators. Generally speaking, the bigger the piezoelectric response, the better.

Some of the biggest responses are found in ferroelectrics – that is, materials with a permanent electric polarization – in single crystal form, but these take a lot of time and energy to synthesise. On the other hand, ceramics can easily be processed on a large scale. These are brittle materials composed of many crystal grains, but usually they have less than two-fifths of the piezoelectric response of their counterparts made of single-crystal ferroelectrics.

Groups led by Rajeev Ranjan at the Indian Institute of Science in Bangalore, and John Daniels at the University of New South Wales in Sydney, have been investigating whether the piezoelectric response in ceramics might be limited by the “clamping” of individual crystal grains by their neighbours. If so, then a thinner ceramic ought to perform better, as a bigger portion of its grains would be at the material’s surfaces, where there are fewer neighbouring grains constraining it and therefore, theoretically, more freedom. “What we didn’t know is how deep from the surface the grains can still feel relatively free,” says Ranjan.

To answer that question, they had to come to the ID15A beamline at the ESRF. Here, armed with discs 0.2 to 1 mm thick of a range of ceramic compositions, they could perform high-energy X-ray diffraction to chart the movement of grains at variable depth as they applied an electric field and measured the subsequent deformation, in situ. Observing these dynamics at micron resolution relied on the quality of the focusing optics and the mechanical sample positioning. “High energy x-rays with a planar beam geometry of about 1 x 100 µm2 were necessary to have the depth resolution and sufficient grain sampling. The source and optics of ID15A allowed this unique combination,” says Daniels.

To the researchers’ surprise, the crystal grains were relatively free even 0.1 mm below the surface.  This explained why the discs that were just 0.2 mm thick deformed by as much as 1% in that axis – a threefold improvement on 0.7 mm discs, and comparable to deformations found in expensive single-crystal ferroelectrics. “This is a revelation in the piezoceramic community,” says Ranjan.

Building on these findings Ranjan, Daniels and colleagues could address another two fundamental questions. First, they managed to clear up some confusion in previous measurements of piezoceramics, showing that apparently high readings do not always reflect true, symmetrical deformations, but instead a bending effect. Second, the researchers found that thin piezoceramics  bend on application of an electric field due to the removal of oxygen atoms from the crystal grains during the processing of the ceramics at high temperatures. Bending piezoceramics could find applications as simple cantilevers, they say.

There is still more work to be done, however. For instance, the researchers need to figure out how far these effects can be pushed. But they are excited to keep going. “I’m looking forward to more experiments at the ESRF with my colleagues Stefano and John,” says Ranjan.

Reference:

Das Adhikary,et al.  Nature 637, 333–338 (2025). https://doi.org/10.1038/s41586-024-08292-1

Text by Jon Cartwright

Top image: Piezoelectric ceramics bending under electric field. Credits: Rajeev Ranjan