“Ultrahigh” strain in Pb-free piezoceramics: Electroding effects

Abstract

Recently, reports showing ultra-high electrostrain (>1%) have appeared in K0.5Na0.5NbO3-based Pb-free piezoceramics. One view considers it to be an intrinsic longitudinal strain of the piezoceramic [Fu et al., Science 378, 1125 (2022)]. Another view considers the large strain displayed in conventional measurement systems as a manifestation of the bending of the piezoceramics [Adhikary et al., Nature 637, 333 (2025)]. While this debate continues, we demonstrate here a profound effect of the commonly used silver electrode paints on the overall electrostrain behavior of the piezoceramic discs in the small thickness regime (<0.4 mm). We performed a comparative study by applying the two most commonly used silver electrodes, namely: (i) air-dried room-temperature silver paint (RTSP) and (ii) fired-on high-temperature silver paint (HTSP). We performed measurements on (K0.5Na0.5)NbO3-based and Na0.5Bi0.5TiO3-based Pb-free piezoceramics. Invariably, we found that when RTSP is used, the bipolar strain field (S–E) loops measured with the conventional setup are much more asymmetric (with large strain displayed in one half cycle) than when HTSP is used. We show evidence of elemental mixing at the HTSP-piezoceramic interface and argue that the relatively strong bonding of the HTSP with the piezoceramic increasingly clamps the surface grains and limits the overall electro-deformation. Our findings are important for the piezoceramic community in highlighting that the way we do electroding of the piezoceramics profoundly affects the electrostrain behavior of thin piezoceramic discs.