Morphotropic Phase Transition And Piezoelectric Properties Of The Ba TiO₃-BaZrO₃-CaTiO₃ Ternary Ferroelectric Ceramics

Pseudo-binary Ba Zr0.2Ti0.8O3-x Ba0.7Ca0.3Ti O3(x BC0.3T) is a new lead-free piezoelectric ceramic system,which has attracted much attentions for the excellent piezoelectric constant that is good as lead contain material. Study the origin of its large piezoelectric constant would be helpful for the design of lead free piezoelectric materials. In fact, the pseudo-binary x BC0.3T system is a section of the ternary system based on Ba Ti O3, Ba Zr O3 and Ca Ti O3(BT–BZ–CT). And it is still unclear that the phase transition and its huge piezoelectric responses mechanism of BT–BZ–CT ternary system. In this thesis, based on the studies on the x BC0.3T system, the relationship between the dielectric, ferroelectric and piezoelectric properties of BT-BZ-CT and its phase transitions around MPB have been extensitively studied, especially for the mechanism of huge piezoelectric responses around MPB.The characteristic temperature has been got through the study of dielectric constant versus temperature curve of the x BC0.3T system. Furthermore, one transition phase region has been found, which has been ignored before, at the boundary between R and T phase region. Combining the result of XRD, the phase composition of the transition phase region is confirmed to be a mixed phase. On the basis of this, the phase diagram of the x BC0.3T system has been revised.In this study, it has been found that in the x BC0.3T system the phase compositions of the powder and bulk samples with the same composition around MPB are different because of the different state of stress. This phenomena suggests that the phase compositions of the samples around MPB is very sensitive to stress. The result also suggests that the relationship between intrinsic phase transition and huge piezoelectric effect need to be studied using stress-free powder samples.The phase compositions and phase transitions of BT-BZ-CT ternary ceramics with other compositions have been studied according to the x BC0.3T system phase diagram, and the whole phase diagram of BT-BZ-CT ternary system has been obtained. On the basis of the phase diagram, the MPB is existed in other peseudo-binary systems, which is similar to x BC0.3T.x BC0.3T /PI composite films have been prepared via adding the stress-free x BC0.3T ceramic powders into the PI matrix. The intrinsic phase transition behavior of x BC0.3T ceramic powders under stress has been studied, through uploading stress on the films. The relationship between intrinsic phase transition and huge piezoelectric effect has been illustrated. The result suggests that the origin of huge piezoelectric effect of x BC0.3T around MPB is the co-effect of phase transition and polarization rotation.Porous 0.5BC0.3T ceramic with different porosity have been prepared by using glucose as pore-forming agency. The influence of pores on the relaxation of residual stress for porous 0.5BC0.3T ceramics was studied using XRD, and the influence of stress relaxation on the phase structure has been clarified. The dispersion coefficients of the samples have been studied by using the revised Curie-Weiss’ law. The stress relaxation effect on the dispersion coefficients has been studied. The stress induced diffuse phase transition has been found in this system. And the results also show that the dispersion coefficients could be designable by changing stress conditions in the materials.The ferroelectric and piezoelectric properties of BT-BZ-CT ternary system have been studied, the results show that this system has excellent ferroelectric and piezoelectric properties. Huge piezoelectric response is existed in the samples around MPB. The energy barriers of the phase transitions of the samples have been calculated using Landau- Devonshire theory. The influence of energy barrier of the phase transition on huge piezoelectric effect of this system has been studied. The result suggests that the origin of huge piezoelectric effect around MPB is the same as x BC0.3T system, which is the co-effect of phase transition and polarization rotation. At the same time, the effect of phase transition and polarization rotation on the hunge piezoelectric effect have been found, that is when energy barrier is lower and both phase transition and polarization rotation are easier, the dominate factor of the peizoelectric property of this material is the change of polarization due to the phase transition and polarization rotation. When the energy barrier is higher, the decisive factor of the peizoelectric property of this material is the height of the energy barrier, namely the difficulty of the phase transition and polarization rotation. The result illustrates the two mechanisms of the huge piezoelectric property and the details of their contributions to the piezoelectric responses of this system. The result provides a reference for the design of huge peizoelectirc material.