Ultrahigh electrostrain >1% in lead-free piezoceramics: Role of disk dimension

Recently, a series of reports showing ultrahigh electrostrains (>1%) have appeared in several Pb-free piezoceramics. The ultrahigh electrostrain has been attributed exclusively to the defect dipoles created in these systems. We examine these claims based on another report (G. D. Adhikary and R. Ranjan, "Ultrahigh measured unipolar strain >2% in polycrystalline bulk piezoceramics: Effects of disc dimension," arxiv.org/abs/2208.07134), which demonstrated that the measured electric field driven strain increased dramatically simply by reducing the thickness of the ceramic disks. We prepared some representative Pb-free compositions reported to exhibit ultrahigh strain and performed electrostrain measurements. We found that these compositions do not show ultrahigh electrostrain if the thickness of the disks is above 0.30 mm (the disk diameters were in the range of 10-12 mm diameter). The ultrahigh strain values were obtained when the thickness was below 0.30 mm. We compare the electrostrain obtained from specimens designed to exhibit defect dipoles with those obtained from stoichiometric compositions of Na0.5Bi0.5TiO3 and K0.5Na0.5NbO3-based lead-free systems and could obtain much higher strain levels (4%-5%) in the later specimens in the small thickness regime. Our results do not favor the defect dipole theory as the exclusive factor for causing ultrahigh strain in piezoceramics. A new approach is called for to understand the phenomenon of ultrahigh electrostrain caused by the thickness reduction of piezoceramic disks.