SrZrO₃-modulated O–T polymorphic phase boundary and improved electromechanical strain in KNNS-BNZ lead-free ceramics

Lead‑free potassium‑sodium niobate‑based piezoceramics have attracted extensive attention owing to their excellent piezoelectric performance and environment‑friendly characteristics. The effects of SrZrO 3 (SZ) doping content on the phase structure, microstructure, dielectric, ferroelectric, strain and piezoelectric properties of (0.96- x )(K 0.48 Na 0.52 ) 0.96 Li 0.04 Nb 0.96 Sb 0.04 O 3 -0.04Bi 0.5 Na 0.5 ZrO 3 - x SrZrO 3 (KNNS-BNZ- x SZ) ceramics were systematically investigated. X‑ray diffraction confirmed pure perovskite phase in all samples, with SZ doping inducing an orthorhombic‑tetragonal (O‑T) phase transition. SEM observations revealed gradual grain‑size reduction upon increasing SZ content, whereas excessive doping degraded ceramic densification. However, excessive SZ doping compromised ceramic densification and led to the deterioration of electrical performance.. Dielectric‑temperature measurements showed a downward shift in Curie temperature after SZ modification. Benefiting from O‑T phase coexistence, the x = 0.015 ceramic exhibited optimal overall electrical properties: piezoelectric coefficient d 33 =303 pC/N, planar electromechanical coupling factor k p =0.39, mechanical quality factor Q m =65, room‑temperature relative permittivity of 2183, Curie temperature T C =235 ℃, remnant polarization P r =14.7 µC/cm 2 , and a large unipolar strain of 0.12%. This work provides a useful experimental reference for performance optimization of KNNS‑BNZ-based lead‑free piezoceramics.