Two-Step Sintering and Sr(Fe0.5Nb0.5)O3 Addition Enhancing Electrical Properties of KNN-Based Lead-Free Piezoelectric Ceramics

Lead-free (1−x)(K0.49Na0.51)(Nb0.95Sb0.05)O3-xSr(Fe0.5Nb0.5)O3 [(1−x)KNNbS-xSrFNb, x = 0.01–0.07] ceramics with 0.5 wt% CuO were prepared by conventional sintering (CS) and two-step sintering (TSS). Increasing the SrFNb content drives the structure from orthorhombic–tetragonal (O-T, x ≤ 0.03) coexistence to rhombohedral–tetragonal (R-T, x ≥ 0.05) coexistence, and shifts the phase boundary toward room temperature, while Fe3+/Nb5+ B-site disorder induces polar nanoregions, diffuse dielectric behavior (dispersion index γ > 1.7) and a drop of Tm (temperature of dielectric constant maximum) from 295 °C to below room temperature. Increasing SrFNb significantly reduces the average grain size from 0.6 μm to 0.3 μm, while TSS decouples densification from grain growth, raising the maximum relative density from 92.13% to 94.51% at x = 0.05 with only a slight reduction in average grain size from 0.35 μm to 0.32 μm. These changes directly tailor the electrical properties: the near-room-temperature O-T boundary at x = 0.03 gives the best soft-type response (piezoelectric constant d33 = 133 pC/N, electromechanical coupling coefficient Kp = 0.285, remnant polarization Pr = 22.72 μC/cm2), whereas acceptor-oxygen-vacancy dipoles at x = 0.01 pin domain walls, yielding the highest mechanical quality factor Qm = 329 with low tan δ, and TSS further improves fatigue resistance. Combining SrFNb-driven structural evolution with TSS microstructural control thus offers an effective route to tailor KNN-based lead-free piezoelectrics.