Microscopic mechanisms of grain size effects on domain switching in sodium potassium niobate ceramics

Lead-free potassium sodium niobate [(K0.5Na0.5)NbO3, KNN]-based piezoceramics have emerged as promising alternatives to lead-based counterparts. Although grain-size effects in KNN ceramics have been widely investigated, most prior studies relied on doping strategies, introducing additional variables that complicate interpretation. The intrinsic microscopic mechanisms of their grain size effects remain inadequately understood. In this work, the influence of grain size on domain structures and ferroelectric properties was systematically investigated in pure KNN ceramics with controlled uniform grain sizes (∼0.5, ∼3, and ∼9 μm). Comprehensive characterization combining piezoresponse force microscopy and macroscopic ferroelectric measurements reveals that although saturated polarization is similar across different grain sizes, polarization switching responses to applied electric fields vary substantially. Small grains predominantly exhibit simplified 180° domain configurations resulting from elevated grain-boundary-induced residual stresses, leading to higher coercive fields and reduced domain growth dynamics (growth rate, ∼169 nm2 V−1). Conversely, large grains feature diverse non-180° domains, which facilitate polarization switching at lower electric fields with an enhanced domain growth of ∼270 000 nm2 V−1. These results demonstrate that different grain boundary densities critically affect internal stress distributions and domain structures, thereby determining domain switching kinetics and macroscopic electromechanical performances. This study provides essential insights into the microscopic mechanisms underlying grain size effects in lead-free piezoelectric ceramics.