Microstructure, Electrical Properties of Mn―doped Sr2―xCaxNaNb5O15 (x=0.05―0.35) Lead Free Ceramics

Lead―free piezoelectric ceramics Sr2―xCaxNaNb5O15+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 Qm=1455, planer electromechanical coupling factors Kp=13.4%, thickness electromechanical coupling factors Kt=36.5%, dielectric constant Er=2123, loss tangent tand=0.038, remanent polarization Pr=4.76uC/cm2, spontaneous polarization Ps=9.36uC/cm2, coercive field Ec=12.68kV/cm, and curie temperature Tc=260 . Therefore, the SCNN―M ceramics are a good candidate material for lead―free piezoelectric ceramics.