Microstructure, dielectric and piezoelectric properties of La-modified Bi_0.5(Na_0.84K_0.16)_0.5TiO₃ lead-free ceramics

Lead-free ceramics (Bi_1−xLa_x)_0.5(Na_0.84K_0.16)_0.5TiO₃ were prepared by a conventional ceramic technique and the effects of La doping and sintering temperature on the microstructure, ferroelectric and piezoelectric properties of the ceramics were studied. All the ceramics possess a pure perovskite structure and La³⁺ diffuses into the Bi_0.5(Na_0.84K_0.16)_0.5TiO₃ lattices to form a solid solution with a rhombohedral symmetry. The addition of La leads to the significant change in the grain morphology and size for the (Bi_1−xLa_x)_0.5(Na_0.84K_0.16)_0.5TiO₃ and a number of rod grains with the length of 10–50 μm and the diameter of 1–2 μm are observed in the ceramic with x = 0.04 sintered at 1,140 °C for 2 h. However, as sintering temperature increases to 1,160 °C, the rod grains disappears and the uniform and rectangular grains are observed in the ceramics with x = 0.04. As x increases from 0 to 0.06, the coercive field E _c of the ceramics decreases from 4.33 to 2.81 kV/mm and the remanent polarization P _r of the ceramics retains the high values of 25.9–27.7 μm/cm². The depolarization temperature T _d decreases from 154 to 50 °C with x increasing from 0 to 0.10. All the ceramics exhibit the diffusive phase transition at high temperature (280–320 °C). The ceramic with x = 0.04 sintered at 1,150 °C for 2 h exhibit the optimum piezoelectric properties, giving d ₃₃ = 165 pC/N and k _p = 32.9 %. The optimum sintering temperature is 1,150 °C at which the improved piezoelectric properties (d ₃₃ = 165 pC/N and k _p = 32.9 %) are obtained. At the high La³⁺ level (x = 0.10 and 0.12), the ceramics exhibit weak ferroelectricity (P _r = 13.0–21.2 μm/cm²) and thus possess poor piezoelectricity (d ₃₃ = 17–27 pC/N).