Phase transitions and electrical properties of (Na_1-xK_x)(Nb_1-ySb_y)O₃lead-free piezoelectric ceramics with a MnO₂sintering aid

Lead-free piezoelectric ceramics (Na1-xKx)(Nb1-ySby)O3+z mol% MnO2 have been prepared by a conventional solid-state sintering technique. Our results reveal that Sb5+ diffuses into the K0.5Na0.5NbO3 lattices to form a solid solution with a single-phase orthorhombic perovskite structure. The partial substitution of Sb5+ for B-site ion Nb5+ decreases the paraelectric cubic-ferroelectric tetragonal phase transition (Tc) and the ferroelectric tetragonal-ferroelectric orthorhombic phase transition (TO-F), and retains strong ferroelectricity. A small amount of MnO2 is enough to improve the densification of the ceramics. The co-effects of MnO2 doping and Sb substitution lead to significant improvements in ferroelectric and piezoelectric properties. The ceramics with x=0.45-0.525, y=0.06-0.08, and z=0.5-1 exhibit excellent ferroelectric and piezoelectric properties: d33=163-204 pC/N, kP=0.47-0.51, kt=0.46-0.52, ε=640-1053, tan δ=1.3-3.0%, Pr=18.1-22.6 μC/cm2, Ec=0.72-0.98 kV/mm, and TC=269°-314°C.