Structure And Properties Of PT-based Relaxor Ferroelectric Material

Based on the relation of structure and properties of PbTiO3 (PT) relaxor ferroelectric, the microstructure, piezoelectric, dielectric and ferroelectric properties of Pb(Sci/2Nbi/2)O3-Pb(Mgi/3Nb2/3)O3-PbTiO3 (PSN-PMN-PT) and Pb(Mgi/3Nb2/3)03-PbTiO3 (PMN-PT) relaxor ferroelectric were systematically studied in this thesis.A series of PSN-PMN-PT ternary ferroelectric ceramics with different compositions at MPB were prepared by a modified two-step columbite precursor method. The influences of sintering temperature and PSN additives on the microstructure, density and electrical properties of ceramics were investigated. With the increasing of sintering temperature, the perovskite structure with the coexistence of rhombohedral and tetragonal is unchanged. The grain size increases, while the density firstly increases then decreases. When the sintering temperature is 1140℃, the density reaches the maximum (relatively density 98.26%). The SEM micrographs indicated that the ceramics exhibit the minimum porosity, uniform grain size and dense structure at 1140℃. Thus, the best sintering temperature located at 1140℃. With increasing content of PSN, the phase structure and grain size of PSN-PMN-PT ceramics have few changes. With increasing mole fraction of PSN from 5% to 45%, the Curie temperature Tc is increased from 154.9℃ to 235.6℃, while the piezoelectric, dielectric and ferroelectric properties is decreased. The optimal properties were found for 25PSN-38PMN-37PT with d33～478 pC/N, kp ～65.1%, εmax～8338, tan δ～0.03, Pr～19.2 μC/cm2, Ec～9.5 kV/cm and Tc～192.2℃, respectively.Based on the observation of ferroelectric domains for PMN-PT ferroelectric single crystal, we found many small domains with different polarization orientations and various sizes in poled PMN-PT. These small domains with different orientations may explain the high piezoelectric constant (d33～1500 pC/N) and piezoelectric inhomogeneity of PMN-PT ferro-electric single crystal. However, the domains is almost invisible in unpoled crystal except a little small domain. With increasing temperature to 89℃, where is rhombohedral-tetragonal phase transition temperature, the ferroelectric domains grow quickly along a specific direction in unpoled crystal. During the cooling process, it forms many small domains along different orientations, which could be attributed to the interaction of polar nanoregions in the ferroelectric domains. Polar nanoregions could explain the relaxation behavior of PMN-PT ferroelectric.In contrast to the PMN-PT single crystal, ternary PSN-PMN-PT ferroelectric single crystals exhibit excellent dielectric and ferroelectric properties:the Curie temperature Tc～170 ℃, dielectric constant εrt～4714 at room temperature, remnant polarization Pr～ 37.11 μC/cm2, coercive Ec～4.74 kV/cm. The irregular ferroelectric domains evolve into the flat strip until extinction with increasing temperature. The phase transition of ferroelectric-paraelectric occurs in a wide temperature range (160～180℃), which suggests the relaxation behavior in agreement with the previous investigation of dielectric.