Study Of Compositional Design,Electromechanical Properties And Mechanisms Of Bismuth-based Perovskite Relaxor Ferroelectric Ceramics

The electric field induced strain property in the bismuth-based perovskite systems has attracted much attention in recent years owing to the achievement of a large electric field induced strain in the (Na0.5Bi0.5)TiO3 (BNT)-based and other bismuth-based perovskite systems. However, there still exist some problems in bismuth-based perovskite systems. For example, much of the work was done based on the BNT-BaTi03/(K0.5Bi0.5)TiO3 (BNT-BT/KBT) binary systems or the related ternary systems based on BNT-BT/KBT, but little work was focusd on other BNT-based systems or other bismuth-based perovskite systems. The large strain obtained in these systems is usually induced by a very large electric field and always shows a strong composition and temperature sensitivity as well as a large strain hysteresis. The origin of the electric field induced large strain in bismuth-based perovskite systems was still unclear and needed more detailed work.Therefore, a comparative study of different bismuth-based perovskite systems and corresponding composition optimization design were carried out in this work and the electric field induced strain in the bismuth-based perovskite systems was further improved. The relations between the composition, structure and electrical properties, the origin of the electric field induced strain and the origin of the improved strain property in bismuth-based perovskite systems were especially investigated with different analysis methods. The main contents were outlined as below:(1) In chapter 2, the compositional dependent evolution of the phase structure and electrical properties were investigated in a new BNT-based ternary system of (Na0.5Bi0.5)TiO3-PbTiO3-xBi(Mg0.5Ti0.5)O3 (BNT-PT-xBMT) with the substitution of BMT for BNT. It was found that with increasing the BMT content, the phase structure gradually changed from a coexistence of rhombohedral and tetragonal phases (x<0.18) to a pseudocubic phase (x>0.18). During this process, a ferroelectric-nonergodic relaxor-ergodic relaxor transformation was observed and a large strain of 0.45% at 7 kV/mm was obtained for samples with x=0.20-0.22. For samples with x<0.12, two dielectric anomalies with different characteristics were observed from the dielectric measurement and were believed to be related with the ferroelectric to relaxor phase transition of rhombohedral and tetragonal phase respectively. Moreover, from the measurement of the synchrotron X-ray diffraction, it was found that the application of an electric field induced an obvious domain texture for both rhombohedral phase and tetragonal phase for samples with x<0.18. With increasing the temperature, the detexture process was firstly observed for the rhombohedral phase and the stability of this domain texture is more stable for the tetragonal phase for samples with x<0.12. On the other hand, with further increase the BMT content, the stability of this domain texture for the tetragonal phase decreased and lose its domain texture at lower temperature.(2) In chapter 3, a new ternary system of Bi(Mgo.5Tio.5)03-PbTi03-(Nao.5Bio.5)Ti03 (BMT-PT-BNT) was constructed by putting two systems BMT-PT and BNT together in terms of their respective characteristics and the strain property was successfully improved. It was found that large electric field induced strains could be obtained in different compositions with the coexistence of nonergodic and ergodic relaxors and the optimum strain value was 0.41-0.43% at 7 kV/mm. From the measurement of the in-situ electric field synchrotron X-ray diffraction, the origin of the electric field induced large strain was believed to be related with the electric field induced relaxor to ferroelectric transition. Moreover, a frequency-insensitive strain was observed in the 0.45BMT-PT-0.32BNT composition which was ascribed to a fast response of both the polar nanoregions and the domain walls to the external electric field. On the other hand, from an investigation of the compositions which lie closely to the phase boundary and have a coexistence of pseudocubic and tetragonal phases, it was found that the electrical property of these compositions was decided not only and simply by the phase coexistence but also influenced by the properties of each phases as well as the relative amount of the pseudocubic and tetragonal phases.(3) In chapter 4, composition and temperature insensitive large strains were reported in a new ternary system, Bi(Mgo.5Ti0.5)03-PbTi03-Pb(Mg1/3Nb2/3)03 (BMT-PT-PMN), for the first time. Meanwhile, an obvious decrease of the strain hysteresis was also observed in this system. A composition insensitive large strain of ～0.40% at 7 kV/mm was obtained in a wide composition range of 0.2<x<0.5 in the BMT-0.3PT-xPMN system and a temperature insensitive large strain of ～0.30% at 5 kV/mm varying less than 10% from room temperature up to 160℃ was obtained in the BMT-0.3PT-0.20PMN composition. Moreover, a large strain of 0.42%at 7 kV/mm with a small strain hysteresis of ～23% was observed in the BMT-0.3PT-0.45PMN composition. All of these would make this system to be a very promising ceramic material for actuator applications. The origin of the electric field induced strain and the improved strain properties were investigated by using synchrotron X-ray diffraction combined with other testing methods. It was found that the electric field induced large strain was believed to be related with the electric field induced relaxor to ferroelectric transition while the appearance of an additional intermediate phase during this process could obviously reduce the strain hysteresis. The achievement of the temperature insensitive strain was mainly ascribed to be related with the special domain structure for BMT-rich compositions. This special domain structure gave rise to a relatively large difference between the ferroelectric-relaxor transition temperature and the freezing temperature, resulting in a coexistence of nonergodic and ergodic relaxors in a wide temperature range. In addition, the temperature stability of phase structures and the origin of the electric field induced strains under different electric fields also have an obvious influence on the temperature dependence of the strain property. Meanwhile, such a special domain structure also resulted in a coexistence of nonergodic and ergodic relaxors in a wide temperature range, resulting in the composition insensitive strain. This may provide a new method for compositional design to improve the strain property in the bismuth-based perovskite systems.(4) In chapter 5, an obvious decrease of the electric field needed to induce the large strain and an obvious enhancement of the normalized strain d33* were achieved in a new ternary system, BiFeO3-PbTiO3-Pb(Mg1/3Nb2/3)O3 (BMT-PT-PMN), for the first time. Meanwhile, an obvious decrease of the strain hysteresis was also observed in this system. An ultrahigh electrostrain of ～0.22%-0.55% was obtained in the field range between 2.5 kV/mm and 7 kV/mm in the x=0.35 composition while the optimum normalized strain d33*of～1100 pm/V was obtained at 3.5 kV/mm. A relatively large strain of ～0.33% at 5 kV/mm with a low strain hysteresis of ～25% was obtained in the x=0.68 composition. Both of these two aspects would be very favorable for the practical application of this system. The origin of this large strain was especially studied from the measurement of the in-situ electric field synchrotron X-ray diffraction on different compositions with increasing and decreasing the electric field. It was found that a collective effects of the electric field induced orientation and growth of the polar nanoregions, the switching of rhombohedral domains and the rhombohedral-tetragonal phase transition would be responsible for this large electrostrain. Most importantly and uniquely, the switching of the rhombohedral domains, which was ascribed to be related with a large lattice distortion of the rhombohedral phase in this system and gives an additional contribution to the electric field induced strain, was believed to be responsible for the enhancement of the electric field induced strain in this system.(5) In chapter 6, the reasons why the electric field induced strain was generated and why the strain was improved were further explored from a systematic analysis of the relations between the phase structure and strain property of all the studied systems in this work. First, the origin of the electric field induced relaxor to ferroelectric transition was found to be an orientation and growth of the polar nanoregions under the external electric field as well as the following switching of ferroelectric domains and ferroelectric phase transition for the first time. In addition, the field dependent evolution of the domain structure in bismuth-based systems was also summarized as shown in the schematic diagram. Second, the strain hysteresis in the bismuth based perovskite relaxor ferroelectrics was mainly ascribed to be related with a delayed phase transition process with loading and unloading the electric field and the strain hysteresis was successfully reduced by optimizing the compositional design. The origin of the delayed phase transition process was also investigated from the viewpoint of the microstructures and it was ascribed to be related with the movement of the chemical ordered regions with increasing the electric field and their concentration at the boundary of the ferroelectric domains.