Structure And Properties Of Perovskite-type Ferroelectric Solid Solutions Near The Morphotropic Phase Boundary

Perovskite-type ferroelectric solid solutions near the morphotropic phase boundary (MPB) have attracted much attention owing to their special structure and properties. This dissertation mainly focuses on such ferroelectric systems aiming at the improvement of their electrical properties.The linear combination rule of compositions near the MPB in ternary ceramic systems has been presented, and its generality was further proved through a series of designed experiments. According to this rule, compositions with super electrical properties can be conveniently exploited in ternary systems.A series of compositions in the PNN–PZN–PT system were selected according to the linear combination rule. Ceramics with such compositions were fabricated using the columbite precursor method, and their phase structure and properties were investigated in detail. The results demonstrated that there is a wide linear MPB region in such ternary system. Since ceramics with compositions in the Zn-rich region have partial contents of pyrochlore phases, compositions with better properties are in the Ni-rich region. Ceramics with the composition of 0.56PNN–0.14PZN–0.30PT has high peak dielectric constant (24500) and piezoelectric coefficient (730pC/N). In the same way the wide linear MPB region was also demonstrated in the PMN–PNN–PT system. The peak dielectric constant and the piezoelectric coefficient of the 0.53PMN–0.13PNN–0.34PT ceramics reach 32000 and 700pC/N, respectively.The linear combination rule was further applied in the quaternary system. A series of PMN–PNN–PZN–PT compositions were obtained through the linear combination of ternary compositions with super properties in PNN–PZN–PT and PMN–PNN–PT systems. The results demonstrated that the selected quaternary compositions exhibit superior electrical properties than those in ternary systems. The peak dielectric constant and the piezoelectric coefficient of some quaternary compositions reach as high as 36000 ? 37000 and 860 ? 900pC/N, respectively, indicating that the introduction of more components in the ferroelectric system mayhelp to improve the electrical properties of materials, and the linear combination rule is also applicable for multiple-component systems.Electrical properties of the PMN–PNN–PT ceramics sintered in different atmosphere were compared. It was found that the loss of lead at high temperature severely degraded the properties.The growth mechanism of lead-based relaxor ferroelectric single crystals were proposed based on the observation of the morphology of freely-grown crystalline grains in ceramics. This mechanism can help explain the phenomena shown in our experiments and reported in many literatures, according to which the growth arts of single crystals can be improved.