Revealing the influence of Nb-doping on the crystal structure and electromechanical properties of (K, Bi)(Mg, Ti, Nb)O₃ ceramics

Nb-modified lead-free ceramics (K_0.48Bi_0.52)(Mg_0.02Ti_0.98−xNb _x)O₃, (KBT-BMTNbx with x = 0.00 − 0.05) were synthesized by a conventional solid-state reaction route followed by furnace cooling. The effects of Nb-doping on the structural properties and electrical properties of KBT-BMTNbx ceramics have been investigated. The X-ray diffraction pattern indicates a mixed tetragonal and cubic phase for the pure KBT-BMTNbx ceramics. Therefore, a large piezoelectric actuator coefficient d ₃₃^* ≈ 700 pm/V, piezoelectric sensor coefficient (d ₃₃ ≈ 133 pC/N) along with remnant polarization (P _r ≈ 17.5 µC/cm²), maximum electromechanical strain ≈ 0.35% and maximum temperature (T _m ≈ 336 ºC) were obtained for KBT-BMTNbx. However, with Nb-doping, a compositionally driven phase transformation occurred from mixed rhombohedral and tetragonal phases to cubic phase. Because of the excess Nb-doping in the KBT-BMT ceramics, the grain size suddenly decreased, as a result, the long-range ferroelectric phase was converted into a short-range relaxor phase. Hence, a low dielectric loss tanδ ≈ 0.02 was achieved at x = 0.02 composition. This superior dielectric performance is correlated to the crystal structure morphotropic phase boundary, optimum grain size (≈ 2 μm), maximum lattice distortion, and soft-ferroelectric effect induced by the donor doping. The main aim of recent research is to investigate P _r, d ₃₃, d ₃₃^*, S _max, and reduced tanδ for practical applications in the real world.