Polar‐State Regulation and Processing‐Assisted Breakdown Strengthening in BNT‐Based Ceramics for High Electrostrain

High‐field electrostrain in lead‐free ferroelectrics is limited by the intrinsic trade‐off among polarization strength, strain reversibility, and dielectric breakdown strength. Here, (1− x )[0.7(Bi 0.5 Na 0.5 )TiO 3 –0.3BiFeO 3 ]– x SrTiO 3 ceramics were designed to regulate polar‐state evolution through a composition–processing co‐design strategy. x‐Ray diffraction confirms a perovskite structure for all compositions, while Raman spectra show gradual changes in A‐site vibration, B‐O bonding, and BO 6 octahedral modes after SrTiO 3 addition. Dielectric measurements show that the dielectric maximum temperature decreases from 402°C to 371°C as x increases, indicating a weakening of long‐range ferroelectric order. Among the investigated compositions, the x = 0.04 composition achieves an optimal balance between polarization magnitude and reversibility, enabling efficient field‐induced strain generation. Piezoresponse force microscopy directly reveals nanoscale switchable polar heterogeneity, providing microscopic evidence for a spatially distributed energy landscape governing the electromechanical response. Viscous polymer processing further enhances breakdown strength by improving thickness uniformity and suppressing processing‐induced defects, enabling operation up to 300 kV/cm. As a result, a large reversible electrostrain of 0.57% is achieved, corresponding to a 138% enhancement compared with bulk ceramics. This work demonstrates that integrates polar‐state regulation with processing‐enabled electric‐field extension for high‐performance lead‐free actuator materials.