Electromechanical and microstructural study of (1-x) Bi_0.5 (Na_0.40 K_0.10) TiO₃-x (Ba_0.70 Sr_0.30) TiO₃ lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1-x) BNKT-xBST (x = 0 ~ 0.150) were studied, and the effects of BST content on the electromechanical properties and crystal structure were examined. X-ray diffraction patterns confirmed a single perovskite phase, and the tetragonality was found to increase with increasing BST ratio. The ceramics exhibited good performance with the following properties: piezoelectric coefficient d₃₃ = 223 pC/N, coupling factor k_p = 38 %, electromechanical quality factor Q_m = 120, and tanδ = 3 % at 1 kHz. Significantly enhanced temperature stability of the ferroelectric properties was found in the range from room temperature to 125 °C. Further, the ceramics displayed a high piezoelectric constant d^*₃₃ calculated as S_max/E_max = 300 pm/V. Remnant polarization (P_r) was found to increase with the increasing temperature up to 125 °C; however, a significant increase was observed at 50 °C. Moreover, the P_r at room temperature was increased with increasing BST content up to (x = 0.075) along with a significant decrease in coercive field.