Electric Properties And Piezoelectric Mechnasm Of Li⁺ And Al³⁺ Co-doped ABO₃ Ferroelectric Ceramics

Doping is a significant method to obtain large piezoelectric effect in ABO₃ perovskite structure piezoelectric ceramics.The phase instability induced by doping is usually the intrinsic source of large piezoelectric effect,such as Morphotropic phase boundary?MPB?and Polymorphic phase transition?PPT?,Whether there are other intrinsic mechanisms for obtaining large piezoelectric effects in the ABO₃ system have been widely concerned.(Li⁺-Al³⁺)co-doped Ba Ti O₃ and Pb?Zr Ti?O₃ ceramics were synthesized by conventional sintering method.The phase structure and microstructure of the ceramics were characterized.The dielectric,ferroelectric and piezoelectric properties are carried out.The reason for enhanced piezoelectricity related to this doping is discussed.The present study provides a promising route to the large piezoelectric effect in the ABO₃,and reveals the mechansm for the enhanced piezoelectricity.Oxygen vacancy,as a kind of the defects will appear in the synthesis process of ABO₃ structural oxides.The electrical properties of Ba Ti O₃ containing oxygen vacancies are more like conductor,which suggests that the defects have great influence on the electrical properties of Ba Ti O₃.In the ABO₃ system,the donor and acceptor co-doping can guarantee the local electricity of the lattice,and inhibit the formation of oxygen vacancy defects.The X-ray diffraction?XRD?results show that Li⁺ and Al³⁺ ions are inclined to occupy the neighboring A-sites?along the [001] pc direction?in the lattice.The DFT analysis proves the rationality and feasibility of this occupation.In the process of investigating domain structure,the abnormal domain structure which is resulted from the electricfield distortion around ionic pairs is found.The discovery of the domain also confirms the ordering distribution of the ionic pair.Besides,the orientation of ionic pair is achieved under the action of temperature-and electric-field.The bias?about 2.1 k V/m?is observed in the BLAT?1 mol%?ceramics including oriented ionic pairs.BLAT?1 mol%?exhibits largely thermo-stable piezoelectric constant?d33?300 p C/N?and huge mechanical quality factor?Qm>2000?.The depolarization temperature is close to the Curie temperature.In order to verify the universality of this doping in ABO 3 systems,the ferroelectric and piezoelectric properties of PZT ceramics with various symmetries?T,M or R?doped by Li⁺ and Al³⁺ were systematically studied.Similar XRD results obtained in doped PZT ceramics suggest that the distribution of Li⁺-Al³⁺ in PZT is as same as that in BLAT.A contricted hysteresis loop is found in the doped PZT ceramics,which is derived from the effect of the dipole moment generated by the ionic pair on the spontaneous polarization inside the domain.This constricted hysteresis loop induced by ionic pairs does not exhibit relaxation characteristics.This doping can largely affect the tetragonal crystal symmetry and the corresponding piezoelectric properties.For the rhombohedral lattice,doping has little effect on their structure and properties.The composition range for the optimum properties in doped PZT has the potential to be expanded?0.46?Ti?0.51?.The dependence of dielectric properties and in-situ XRD results indicate that the T-C phase transition is completed in a wide temperature range,namely,diffuse.Based on the fact that lattice distorstion is related to the doping stress,the effect of stress on the free energy profiles of BT,PZT?40/60?and PZT?60/40?is analysized in the framework of the Landau-Ginzburg-Devonshire?LGD?theory.The results show that the energy barrier between the ferroelectric and paraelectric phases in the BT and PZT?40/60?will decrease with the compressive stress.The energy barrier of PZT?60/40?is insensitive to this kind of stress.The above analysis explains the reason why the dopant improves the piezoelectric properties of T-phase BT and PZT ceramics.A piezoelectricity enhancement mechanism related to the stress-driven flattening of a free-energy profile is proposed.Finally,the sensitive stress direction for the ABO₃ with the different phase structure can be calculated.This study provides theoretical support for obtaining the large piezoelectric effect through this doping route in other ABO₃ systems.