Enhancing the electrical properties of K0.5Na0.5Nb0.96Sb0.04O3-based piezoelectric ceramics via Bi0.5K0.5HfO3  doping

Lead-free piezoelectric ceramics with the composition (0.996- x )K _0.5 Na _0.5 Nb _0.96 Sb _0.04 O ₃ -0.004AlFeO₃₋ x Bi _0.5 K _0.5 HfO ₃ (abbreviated as KNNS-AF- x BKH,where x  = 0.02, 0.025, 0.03, 0.035) were synthesized via a conventional solid-state reaction method. The effects of Bi _0.5 K _0.5 HfO₃ (BKH) doping concentration on the phase structure, Raman spectra, microstructure, and electrical properties were systematically investigated. Results indicate that the ceramic with x  = 0.03, sintered at 1102°C for 4 hours, exhibits the optimal piezoelectric performance. X-ray diffraction (XRD) and Raman spectroscopy confirm that all samples crystallize in a perovskite structure at room temperature, with a coexistence of tetragonal and orthorhombic phases observed when x  > 0.025. Scanning electron microscopy (SEM) analysis reveals a dense and uniform microstructure for the x  = 0.03 composition; Temperature-dependent dielectric measurements show that both the orthorhombic-tetragonal phase transition temperature and the Curie temperature shift toward lower temperatures with increasing BKH content. For x  = 0.03, the dielectric loss tangent (tanδ) displays a non-monotonic temperature dependence, first increasing, then decreasing, before rising sharply at elevated temperatures. This composition also achieves a high remnant polarization ( P _r =40.51 µC/cm²), a notable piezoelectric coefficient ( d ₃₃=232 pC/N), a low coercive field ( E _c =18.7 kV/cm) and a planar electromechanical coupling coefficient k _p of 0.38.