Composition and temperature dependence of structure and piezoelectricity in (1−x)(K1−yNay)NbO3-x(Bi1/2Na1/2)ZrO3 lead-free ceramics

Lead-free piezoceramics with the composition (1-x)(K1-yNay)NbO3-x(Bi1/2Na1/2)ZrO3 (KNyN-xBNZ) were prepared using a conventional solid state route. X ray diffraction, Raman spectroscopy and dielectric measurements as a function of temperature indicated the coexistence of rhombohedral (R) and tetragonal (T) phase, typical of a morphotropic phase boundary (MPB) as the BNZ concentration increased and by adjusting the K/Na ratio. High remnant polarization (Pr = 24 μC/cm2), piezoelectric coefficient (d33 = 320 pC/N), effective piezocoefficient (d33* = 420 pm/V), coupling coefficient (kp = 48%) and high strain (S = 0.168%) were obtained at room temperature but significant deterioration of Pr, d33* and kp were observed by increasing from room temperature to 160 °C (17.5 μC/cm2, 338 pm/V and 32%, respectively) associated with a transition to a purely T phase. Despite these compositions showing promise for room temperature applications, the deterioration in properties as a function of increasing temperature poses challenges for device design and remains to be resolved.