Study Of Doping Effects On The Properties Of NBT Based Lead Free Ceramics

Piezoelectric ceramics is an advanced functional material based on its mutual transfer between mechanical energy and electric energy. It is applied to modern electric field widely. However, current worked piezoelectric materials are mainly lead-based. The development and study of lead-free piezoelectric materials have been more and more urgent because of the environment problem. In this paper, 0.92Na0.5Bi0.5TiO3-0.08BaTiO3(NBT-BT) lead-free piezoelectric ceramics modified with different amounts of La2O3 and MnCO3 were prepared by the conventional mixed oxide method. Microstructures were studied according x-ray diffraction, scanning electron microscope, transmission electron microscope and Raman-scattering spectroscopy etc. Dielectric, piezoelectric, ferroelectric and mechanical properties were measured by digital electron bridge, d33 meter, impedance analyzer, high resistance system, ferroelectric parameter meter and stress measurement system etc. According, doping effects and mechanisms of La₂O₃ and MnCO₃ were analyzed and discussed. Results of Mn doping implied that samples located at rhombohedral-tetragonal morphotropic phase boundary (MPB). Small MnCO3 doping resulted in indistinct transition from rhombohedral to tetragonal. Mn ions diffused into B-site of the lattice in the value of +2/+3 and resulted in ''hard doping'' effects and ''particle size'' effect. The super electric properties have been acquired at 0.34wt% MnCO₃ addition: εr =1596, d33 =145 pC/N, kp =0.162, Qm =191.3, tanδ=1.10%, with obvious increase of diffuse degree and well temperature stability. Results of La doping implied that La2O3 doping resulted in transition from rhombohedral to tetragonal at MPB to a certain degree and clear increase of diffuse degree and Curie temperature. Especially, in the range of 0.22O₃ addition, the dielectric properties almost didn't change before and after poling, while piezoelectric decreased irregularly. Aiming at this dielectric and piezoelectric abnormality, more researches was open out. Results implied that La addition induced relaxor-antiferroelectric phase transition at room temperature, while La3+ prior substituted Ba2+ most likely. This is most important for the rules of designing of new NBT based lead free materials in the future.