Fabrication And Compositonal Tailoring Of Bismuth Sodium Titanate Based Lead-Free Piezoelectric Ceramics

Sodium bismuth titanate Bi_0.5Na_0.5TiO₃?BNT?, which has strong ferroelectric properties at room temperature, is a kind of A-site complex ions perovskite-type ferroelectric with relatively high Curie temperature 320 °C, large remanent polarization 38 ?C/cm2 and good development prospects. In this paper, BNT-based ceramics were prepared by the conventional solid-state sintering method, and the effect of various additives on the microstructure and electrical properties of the ceramics have been investigated. The dielectric relaxation and phase transition behavior of the ceramics with different compositions have been examined with respect to temperature and measuring frequency.Firstly, the perovskite oxides Bi_0.5Na0.5Ti1-xMnx O3-??BNTM10000x, x = 0, 0.25%, 0.5%, 0.75%, 1%, 1.5%, 2%? were prepared via the conventional solid state reaction method. The crystal structure of BNT and BNTM25 ceramics were refined by the Rietveld method with powder X-ray diffraction at room temperature. Small amount of oxygen vacancy, significant increased tetragonality and grain size effect had made the electromechanical and electrical properties of Bi_0.5Na0.5Ti1-xMnxO3-? ceramics optimized at the MnO2 addition of x = 0.25%. The excessive doping amount led to the concentration of oxygen vacancies and ferroelectric-to-relaxor phase transition, which was thought to deteriorate the electrical properties. The BNTM25 ceramics was found to have a high remnant polarization?Pr? of 48.5 mC/cm2, an electromechanical coupling factor?kp? of 0.18, an electrostrictive strain?S33? of 0.24% and a piezoelectric coefficient?d33? of 105 pC/N. In addition, almost no profound fatigue was observed after switching over 106 cycles at room temperature, indicating a potential application on transduceras as lead-free piezoelectric materials.Secondly, in order to probe the effects of different doping types on the BNT-based ceramics, the perovskite oxide Bi_0.5Na0.5MnxTi1-xO3-?, Bi_0.5Na0.5NbxTi1-xO3, Bi_0.5Na0.5?Mn0.5Nb0.5?xTi1-xO3 and Bi_0.5Na_0.5TiO₃ ceramics?abbreviated as BNT-Mn, BNT-Nb, BNT-MnNb, BNT, respectively, x = 0.25%? were prepared. The crysyal structures and electrical properties of Nb-donor, Mn-acceptor and?Mn0.5Nb0.5? substitution at B site were investigated. The?Mn0.5Nb0.5? substitution gave rise to large defect-dipole clusters containing highly localized electrons and the inhibited of reduction of Ti4+ to Ti3+, which should be responsible for the increase of Tc and Td. The ferroelectric properties and field-induced strains were both improved by Mn-acceptor and?Mn0.5Nb0.5? substitution at B site. The fatigue-resistant properties of BNT-Nb ceramics were comparable to BNT ceramics, BNT-Mn ceramics were found to have significantly improved fatigue-resistant properties, while almost no profound fatigue was observed in BNT-MnNb ceramics after switching over 106 cycles at room temperature.Finally, the perovskite oxide?1- x?Bi_0.5?Na0.9K0.1?0.5TiO3 – xSrTi0.8Zr0.2O3?SZT1000x, x = 0, 0.2%, 0.4%, 0.6%, 0.8%, 1%? were prepared via the conventional solid state reaction method. The room-temperature ferroelectric P–E loops coordinate with polarization current density J–E curves exhaustively illustrated the changes of ferroelectric domains and polar nanoregions?PNRs? under different driving field. The origin of the large strain is due to the presence of a nonpolar phase at a zero field that can easily transform into a long-range ferroelectric phase upon the application of an electric field and brings the system back to its unpoled state once the applied electric field is removed. The composition and electric field dependent strain behavior of this system were investigated to develop a lead-free piezoelectric material with a large strain response at a lower electric field. A large strain of 0.44%?Smax/Emax=744 pm/V? at an applied field of 50 kV/cm was obtained at the composition of 0.6%mol SZT. Notably, the electric field required to deliver large strains was reduced to 40kV/cm while the Smax/Emax reached up to 717 pm/V, indicating that the developed material is highly promising for actuator applications.