Modification And Characterization Of BiFeO₃ -PbTiO₃ Multiferroic Solid Solutions

The nomenclature"multiferroics"has been coined for in which two or all three ferroic orders, ferroelectrics (antiferroelectrics), ferromagnetics (anti-ferromagnetic) and ferroelastic coexist in the same phase. These kinds of materials not only provide an additional degree of freedom in design of actuators, transducers, and next generation memory devices, but also boost the study of basic physics. Bismuth ferrite and Lead Titanate (BiFeO₃-PbTiO₃) solid solution is multiferroic materials with Morphotropic phase boundary (MPB). In vicinity of MPB, TN and Tc of BiFeO₃-PbTiO₃ ceramics are 650℃and 290℃, respectively. Therefore, it has potential applications in high-performance electronic devices and high-temperature piezoelectric transducers. However, the high conductivity, large coercive field and weak ferromagnetism of BiFeO₃-PbTiO₃ ceramics prevent its practical applications. In this paper, the modified BiFeO₃-PbTiO₃ ceramics were prepared by solid-state reaction processing, and the influence of chemical modifications on the structure, dielectric, ferroelectric, piezoelectric and magnetic properties of BiFeO₃-PbTiO₃ solid solutions were investigated. Furthermore, the diffused phase transition and conductivity mechanisms of modified BiFeO₃-PbTiO₃ ceramic were studied. Finally, two new methods were exploited to prepare modified BiFeO₃-PbTiO₃ ceramics.Lanthanum (La) modified (1-y)(Bi_1-xLa_x)FeO₃-yPbTiO₃ (BLF-PT) ceramics shows the MPB characteristics. The values of dielectric constant, piezoelectric constant and planar electromechanical factor of BLF-PT ceramics reach maximum at MPB composition. The mainly contribution to the overall piezoelectric constant of the BLF-PT ceramics is from the motion of domain walls. With the increase of La concentration, the MPB of BLF-PT is shifted to the PbTiO₃ content. The La substitution is also found to decrease the coercive filed and improve the resistivity, resulting in significant enhancement of dielectric and piezoelectric properties. Furthermore, for 0.57(Bi_1-xLa_x)FeO₃-0.43 PbTiO₃ ceramics, the substitution of La at A site for Bi can promote the formation of rhombohedral structure and grain growth, enhance dielectric, ferroelectric and piezoelectric properties. Moreover, La modification also decreases the phase transition temperature and increase the degree of diffused phase transition. The electric properties of 0.57BLF-0.43PT reach the maximum at x = 0.2.The substitution of Ti for Fe can also promote the formation of tetragonal structure and decrease the c/a ratio. It is noted that the dielectric constant and loss of 0.6BLFT?0.4PT significantly decreased by Ti doping in the range of 100³50℃. 0.6BLFT?0.4PT ceramics is an NTC material and its overall resistivity is mainly from the contribution of grain, and the oxygen vacancies are the dominant charge defects at high temperature. Ti doping increases the overall resistivity.The remnant magnetization of (1-y)(Bi_1-xLa_x)FeO₃-yPbTiO₃ ceramics reached the maximum at the MPB composition of x = 0.2 and y = 0.43, which is 0.11 emu/g. The remnant magnetic polarization can be further improved by 5 at% Gallium (Ga) modification at Fe-site. At antiresonate frequency, the variation ratio of dielectric constant under 0.7 T magnetic fields for La and Ga co-modified BiFeO₃-PbTiO₃ ceramics is 7 %, indicating that Magneto-capacitance effect exists in the La and Ga co-modified BiFeO₃-PbTiO₃ ceramics. The BiFeO₃-PbTiO₃ ceramics modified by 20 at% La with MPB composition are insulating multiferroic materials, which show strong ferroelectricity and enhanced magnetization.The introduction of strong magnetic field (10 Tesla) in the sintering process increases the c/a ratio slightly,enlarges the grain size of 0.57BLF-0.43PT ceramics significantly. As a results, 0.57BLF-0.43PT ceramics exhibit the enhanced dielectric constant,remnant polarization and high temperature resistivity. The 0.6BLF-0.4PT powders with a single phase and the grain size of 40⁵0 nm is obtained under 600℃for 2h by sol-gel technique. The ceramics prepared using nano-powders under 1060℃show rhombohedral structure with grain size of 1-2μm. The 0.6BLF-0.4PT ceramics prepared using nano-powders show enhanced dielectric, ferroelectric and piezoelectric properties than those of ceramics with the same composition prepared by traditional solid-state reaction process.