| Ferroelectrics are typical functional materials, which can convert mechanical energy into electrical energy and vice versa, thus they are ideal for many electromechanical devices. Ferroelectric single crystals (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) exhibit ultrahigh electromechanical properties, so they have attracted much attention recently. Because the macroscopic properties of materials are determined by their microscopic structures, the macroscopic properties of PZN-PT single crystals are strongly related to their domain structures. A small difference in domain structure may cause big change on macroscopic electromechanical properties. So the properties of single crystals measured by different groups exhibit obvious variation, which cause serious confusion on the practical application of this material. For now, there are few reports on the relationship between the macroscopic electromechanical performance and domain structures of PZN-PT single crystals. Moreover, the domain walls are ignored in many theoretical calculations, which lead to large inconsistence between the theoretical results and experimental values. This will greatly affects the designs of electromechanical devices. To solve these problems, the relationship between the macroscopic electromechanical properties and domain structures of PZN-PT single crystals have been studied, and the nature of domain walls and their effect on the electromechanical properties of crystals were discussed using theoretical and experimental methods.Domain engineering and poling at high temperature techniques were used to control the domain structure and improve their macroscopic electromechanical properties of ferroelectric single crystals PZN-6%PT with composition away from the morphotropic phase boundary. The domain structure effect on the macroscopic electromechanical properties of [001]c oriented PZN-6%PT crystals has been studied. The results show the electromechanical property could be improved when the domain sizes are reduced. When the size of 71°domains is about 8μm, very high piezoelectric constant d33 3450 pC/N have been produced in PZN-6%PT single crystals, which is 42.7% higher than the corresponding reported values. In addition, the layered twin domains and a lot of 71°neutral domain walls were found in PZN-6% PT single crystals with excellent piezoelectric properties, rather than the charged ones usually assumed in the literature.The relationship between electromechanical property and the domain structure of ferroelectric single crystals PZN-9%PT with composition near the morphotropic phase boundary was investigated as a function of temperature and poling electric field. The results indicate that, the ferroelectric domain structure can be controlled and the macroscopic electromechanical properties can be improved by adjusting the poling field at room temperature. For [001]c oriented PZN-9%PT single crystals, an optimal poling electric field is 1110 V/mm at room temperature. After poling, engineered domain configuration are formed, and the superior longitudinal electromechanical coupling factors being 0.90, longitudinal piezoelectric coefficient being 1670 pC/N can be obtained in PZN-9%PT single crystals. When the poling electric field is above the critical value, the piezoelectric coefficients of PZN-9%PT single crystal begin to decrease. When the poling electric field is up to 1200 V/mm, PZN-9%PT single crystals is in single domain state, resulting in the longitudinal piezoelectric coefficient decreased to 50% less than that in engineered domain configuration. These results are significant for understanding the characteristics of PZN-9%PT single crystals, finding suitable work environment, and improving the application dependability of this material.Based on periodic medium theory, a piezoelectric domain wall model was presented. Since domain wall is a transition zone between the twin domains, the property of domain wall is associated with that of adjacent twin domains. Piezoelectric domain wall model takes the piezoelectric properties of domain wall into account, and it solves the contradictions between conductive domain wall model and the insulation nature of piezoelectric material. Piezoelectric domain wall model reasonably explain the calculated results and some important experimental phenomena.Weighting factors were introduced in the periodic medium theory, which provides a new method to describe the properties of domain walls and the property difference between the domain wall and adjacent twin domains in the crystals with engineered domain configuration. The laminate theory was improved, and the effective method to study the domain size effect was found, which extended the application scope of laminate theory. The ferroelectric domain size dependences of the macroscopic electromechanical properties and the domain wall properties in normal ferroelectric single crystals BaTiO3 and new ferroelectric single crystals PZN-6%PT have been studied by piezoelectric domain wall model and laminate theory. The results indicated the 90°charged domain wall in BaTiO3 single crystals and 71°neutral domain walls in PZN-6% single crystals have significant effects on the macroscopic electromechanical properties of ferroelectric single crystals. It can be used to improve the piezoelectric properties of PZN-6% and BaTiO3 single crystals by reducing ferroelectric domain size and increasing the volume ratio between domain walls and domains in the unit cell, which is consistent with the experimental results. Both 90°charged domain walls in BaTiO3 crystals an-d 71°neutral domain walls in PZN-6% crystals have piezoelectric properties, and 71°neutral domain walls have a stronger anisotropy.In summary, the results of this work present theoretical evidences and effective methods to optimize the electromechanical performances of PZN-PT single crystals, and provide the necessary experimental support for the application of PZN-PT single crystals in the practicable electromechanical devices.
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