In a new study published in Nature, researchers at the Indian Institute of Science (IISc) and collaborators show that the efficacy of a commonly-used piezoelectric ceramic material can be dramatically increased just by reducing its thickness and by preventing atomic defects inadvertently formed during manufacturing.
Piezoelectric materials deform (stretch or contract) when an electric field is applied, making them suitable for wide-ranging applications, from ultrasound imaging in hospitals to actuators in guided missiles. Single crystals of some synthetic piezoelectrics show large longitudinal electrostrain (>1%), a value that indicates how much the material deforms in the direction of the field. But such materials are rare, and mass-producing single crystals is costly. They are therefore used only for niche applications. For most commercial applications, the more economical polycrystalline piezoelectric ceramics are used. However, these show much lower longitudinal strain (0.2-0.4%).
“The maximum electrostrain reported in polycrystalline lead-free piezoelectrics is 0.7%,” says Gobinda Das Adhikary, first author and former PhD student at the Department of Materials Engineering (MatE), IISc. “Our intention was to increase the strain beyond this.”
Every grain in a piezoceramic contains spontaneously polarised regions called domains that switch their orientation towards the electric field in tandem, causing the material to deform as a whole. Grains near the surface of the material are better at deforming because they are freer, whereas deep inside, they are more tightly bound by other grains from all sides and find it difficult to deform. In commonly-used piezoceramic discs (typically 10 mm in diameter and 1 mm thick) most grains deform very less, resulting in an overall low longitudinal strain, explains Rajeev Ranjan, Professor at MatE and corresponding author.
While tweaking the size and shape of a widely-known piezoceramic called PZT (lead zirconate titanate), Ranjan’s team discovered that when the thickness of a circular PZT disc was reduced from 0.7 mm to 0.2 mm, its electrostrain jumped from 0.3% to 1%.
Intrigued, they collaborated with scientists at the European Synchrotron Radiation Facility (ESRF) to carry out X-ray diffraction experiments. They found that reducing the thickness made the grains show significantly increased switching of the polarised domains. “At 0.2 mm, the domains feel that the surface is closer and switch better,” Ranjan explains. “This means that by replacing a 1 mm ceramic disc that has 0.3% strain with five 0.2 mm discs stacked on top of each other, you can get a much higher strain.”
Most commercial advanced piezoelectrics contain lead, which is cancerous and harmful for the environment. The team discovered that values of >1% electrostrain reported in some lead-free piezoceramics by other groups may actually be off the mark. Nearly three years ago, Adhikary was testing a lead-free piezoceramic called sodium bismuth titanate when he suddenly measured a strain value of 1.5%. “I was on cloud nine,” he recalls. But his excitement was short-lived. It turned out that the material was not stretching or contracting along the thickness direction, but bending, a completely different phenomenon. When they tested several other materials with reportedly high electrostrain values, all of them turned out to be measures of bending and not longitudinal deformation.
The team also found a possible cause for the bending. Piezoceramics are generally heated to 1,100-1,300°C during manufacture, which can create positively-charged defects called oxygen vacancies. Under an electric field, these vacancies move towards the negatively-charged electrode. “Domains closer to the negative side get pinned by these vacancies and are unable to switch, whereas domains on the positive side are able to switch more,” Ranjan says. This imbalance causes the material to bend, which might be mistaken for longitudinal deformation during routine measurement.
“In all lead-free piezoceramics, if we can reduce oxygen vacancies, we can increase the longitudinal strain to 1% or higher, as we demonstrated in PZT,” he adds. His team recently discovered that reducing oxygen vacancies in a lead-free piezoceramic boosted its electrostrain to ~2.5% (results yet to be published). The new insights into bending in piezoceramics from the group can also be useful for designing monolith cantilevers.
The findings emphasise the need to revisit how piezoceramics are made and tested, and point to how little is known about what drives electrostrain below a certain thickness, the researchers say. “We need to uncover new mechanisms to explain such anomalous behaviour,” Adhikary adds.
