Effect Of The Microstructure And Orientation Of Grains On The Performances Of Perovskite Ferroelectric Ceramics

To date ferroelectric materials have been widely applied in enormous fields,such as transducers,actuators,sensors and so forth.In light of this,the ferroelectric materials are generally regarded as being related to an important class of smart materials supporting science and techonology development.Among the most popular ferroelectric materials,those based on Pb Zr_1-xTi_xO₃?PZT?structures show superior performances in workable devices.From fundamental understanding of physical features in PZT based systems,some further investigations are still under consideration by a wide scientific community.For instance,the soft-hard doping processes,the effect of defect dipoles remain essential concerns in material sciences area.On the other hand,in view of the environmental-friendly requirement,more attention have been paid by researchers to lead-free ferroelectric materials.Bi Fe O₃-Ba Ti O₃?BF-BT?systems have attracted a great deal of interest as promising candidate for lead-free ferroelectric materials.In addition to the doping method,the synthesis of single crystals also can be relevant to enhance significantly the ferroelectric properties.But this route is limited for the development because of the high cost and difficulties of synthesizing large-size single crystals.In this case,the realization of textured ferroelectric materials offers an alternative approach to achieve high-performance ferroelectric systems.The mechanism is based on the grains of textured samples being forced to align along a defined direction,as similar as that of single crystals.Thus,the texturing approach is an effective method to enhance the ferroelectric performance of materials.Based on the research background above,PZT-based and BF-BT-based ceramics were investigated systematically in this dissertation.The first contribution was dealing with the analysis of microstructures,crystalline phases and electric properties in PZT systems.It was revealed the involvement of softening-hardening features and the relation between defect dipoles and electric properties in acceptor-doped materials.In a second part,the interplay of microstructures and electric properties was studied in BF-BT systems with ion doping,providing the relevant analysis of the optimized performance such as the enhanced piezoelectric properties of lead-free systems.In the third contribution,experimental investigations and deep analysis were devoted to the effects of the microstructures,and oriented grains by the reactive templated grain growth method on ferroelectric performances in textured BF-BT ceramics.The main achieved results are hereafter highlighted:?1?The correlation between the microstructures and the involved properties were investigated in soft Pb_0.94Sr_0.05La_0.01(Zr_0.53Ti_0.47)O₃?PSLZT?ceramics fabricated by the conventional solid-state reaction.The variation of Fe³⁺doping rate modulated ferroelectric characteristics and the electromechanical behavior in PSL?ZT?_1-x-Fe_xsystems.When x=0,lead vacancies caused by the substitution of Pb²⁺by La³⁺in the sample,were prone to enhance the crystal distortion.This sample showed apparent soft features,with the remnant polarization?P_r?of 35.7?C/cm²,the piezoelectric constant(d₃₃)of 412 p C/N and the strain of 0.22%?25 k V/cm?respectively.When x=0.005,the partial lead vacancies of the sample were neutralized by the positive charges originating from the substitution of Ti⁴⁺and Zr⁴⁺by Fe³⁺.This sample?x=0.005?also exhibited the soft feature with electromechanical quality factor Q_m of 71.When x=0.010 and 0.015,the Q_m of two samples enhanced tremendously to 210 and 500,respectively,which indicated that oxygen vacancies caused by Fe³⁺substitution pinned domain switching and led to the hard features in the two samples.The result also revealed the effect of defect dipoles on ferroelectric performances in the aged hard-doped sample?x=0.015?.It was found that domain switching of the unpoled hard-doped sample was pinned under the external electric field.When the sample was poled,the polarization of defect dipoles P_D was aligned along the spontaneous polarization P_s.Aligned P_D weakened the pinning effect of domain switching under the parallel electric field,promoting the ferroelectric properties of the sample.?2?Bi Fe_1-xAl_xO₃-Ba Ti O₃?BFA-BT?ceramics prepared by solid-state reaction method were systematically investigated with a particular focus on their microstructures and electric properties,in order to optimize ferroelectric performances.It was found that the distortion 60°-of the rhombohedral phase?R3c?and(8/(6of the tetragonal phase?P4mm?reached their maximum values in doping content of x=0.030.The large distortion of lattices enhanced the displacement of samples under the electric field,resulting in the high ferroelectric properties for BFA-BT samples.The highest saturated polarization and remnant polarization achieved in this system were36.8?C/cm² and 31.5?C/cm²,respectively.In addition,⁵⁷Fe Mo?ssbauer spectrum indicated that the structural disorder at the B site was increased with enhancing Al³⁺doping,while dielectric properties also disclosed intensified features of diffuse phase transition and the relaxation behavior.With x?0.045,the samples exhibited two distinct dielectric relaxtion responses relevant to effects of grains and grain boundaries,which were explained according to the“Brick-wall”model.?3?<001>oriented Bi Fe Ga O₃-Ba Ti O₃?BFG-BT?textured ceramics were synthesized via the reactive templated grain growth method.The effective piezoelectric coefficient d₃₃*of the textured sample reached 685 pm/V at 180?.XRD investigations indicated the stable ferroelectric structure of textured samples.Because it was not found the phase transition from ferroelectric phase?P4mm and R3c?to the paraelectric phase?cubic Pm3m?when temperatures increased to 300?.The high fraction of the rhombohedral phase and high-density nanodomains were exhibited in textured samples.The large portion of the rhombohedral phase in textured samples contributed to the large displacement under the electric field.While the low energy barrier of domain switching induced by nanodomains also reduced the coercive field,resulting in the easy switching of domains in the external electric field.Moreover,the<001>oriented grains of textured samples lowered the angle between the external electric field and the spontaneous polarization,and meanwhile weakened the stress in neighboring grains,leading to the easy switching of domains in the external electric field.These optimized microstructures enhanced ferroelectric performances in textured samples.