Microstructure, electrical properties of Mn-doped Sr_2-xCa_xNaNb₅O₁₅ (x=0.05-0.35) lead free ceramics

Lead-free piezoelectric ceramics Sr_2-xCa_xNaNb₅O₁₅ +ywt%MnO₂ (x=0.05-0.35, y=0, 0.3, 0.5) (SCNN-M) were fabricated by conventional solid-state processes, and their microstructure, phase transformation, dielectric, piezoelectric and ferroelectric properties were investigated systematically. Our results reveal that Ca²⁺ ions have entered into the Sr₂NaNb₅O₁₅ lattices to form a solid solution and the doping of MnO₂ is effective in promoting the densification of the ceramics. With the MnO₂ doping, all the ceramics can be well sintered at a relative low temperature and exhibit a dense and pure tungsten-bronze structure. The addition of Ca²⁺ has no remarkable effect on the microstructure and MnO₂ caused a significant promoted grain growth. Compared with the pure SCNN ceramics, the dielectric and piezoelectric properties of SCNN-M ceramics were greatly enhanced. At room temperature, the SCNN-M ceramics with x=0.5 and y=0.5 exhibits the optimum properties: piezoelectric coefficient d₃₃=190pC/N, mechanical quality factor Q_m=1455, planer electromechanical coupling factors K_p=13.4%, thickness electromechanical coupling factors K_t=36.5%, dielectric constant ε_r=2123, loss tangent tanδ=0.038, remanent polarization P_r=4.76μC/cm², spontaneous polarization P_s=9.36μC/cm², coercive field E_c=12.68kV/cm, and curie temperature T_c=260°C. Therefore, the SCNN-M ceramics are a good candidate material for lead-free piezoelectric ceramics.