Piezoelectric materials deform when subjected to an electric field, a property that makes them essential for applications ranging from medical ultrasound imaging to missile actuators. While some synthetic single-crystal piezoelectrics exhibit large longitudinal electrostrain (over 1%), these materials are rare and expensive to produce. For most commercial uses, cost-effective polycrystalline piezoelectric ceramics are preferred, though they typically achieve lower strain values of 0.2% to 0.4%.
"The maximum electrostrain reported in polycrystalline lead-free piezoelectrics is 0.7%," said Gobinda Das Adhikary, the study's first author and a former PhD student at IISc's Department of Materials Engineering (MatE). "Our goal was to surpass this limit."
Piezoelectric ceramics are composed of grains, each containing regions of aligned polarization called domains. These domains collectively switch orientation in response to an electric field, causing the material to deform. Grains near the surface deform more easily due to fewer constraints, while those deeper inside are restricted by surrounding grains, leading to lower overall deformation. In standard piezoceramic discs-10 mm in diameter and 1 mm thick-most grains exhibit minimal deformation, resulting in limited strain, explained Rajeev Ranjan, Professor at MatE and the study's corresponding author.
The research team experimented with altering the dimensions of lead zirconate titanate (PZT), a well-known piezoceramic. They observed that reducing the thickness of a circular PZT disc from 0.7 mm to 0.2 mm increased its electrostrain from 0.3% to 1%. "At 0.2 mm, the proximity to the surface allows domains to switch more freely," Ranjan explained. "Stacking five 0.2 mm discs in place of a single 1 mm disc can significantly boost strain performance."
A critical focus of the study was addressing the environmental and health concerns associated with lead-based piezoelectrics. The researchers found that some reports of high strain values in lead-free piezoceramics were likely misinterpreted. Adhikary recounted his initial excitement upon measuring a strain value of 1.5% in a lead-free material, only to discover that the apparent deformation was due to bending rather than true longitudinal strain.
The team identified oxygen vacancies, positively charged defects formed during high-temperature manufacturing, as a key factor in these anomalies. These vacancies migrate under an electric field, restricting domain switching on one side of the material and causing bending. "Reducing oxygen vacancies can unlock higher longitudinal strain values in lead-free piezoceramics," Ranjan noted. In fact, the team recently achieved an electrostrain of approximately 2.5% in a lead-free piezoceramic by minimizing such defects, a result they plan to publish soon.
The study underscores the importance of revisiting manufacturing and testing methods for piezoceramics. It also calls for further research into the mechanisms driving electrostrain at reduced thicknesses. "Uncovering these mechanisms is crucial for advancing our understanding of piezoceramics," Adhikary added.
Research Report:Longitudinal strain enhancement and bending deformations in piezoceramics
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