| Relaxor ferroelectric materials owned large electrostrains and high energy-storage properties, which have been widely applied in actuators, sensors, energy-storage capacitors, and so on. Perovskite-type Bi-based ferroelectrics have strong ferroelectrieity and multi-component ferroelectric materials are effective in improving the strain and energy-storage properties. In this dissertation, we selected three kinds of Bi-based ferroelectric ceramics of such as BiFeO3 (BF), Bi(Mg1/2Ti1/2)O3 (BMT) and Bi(Ni1/2Ti1/2)O3 (BNiT), through optimizing the compositional design to explore the relationship between composition, structure and property, and emphatically investigated the electrostrain and energy-staorge properties. The main contents were outlined below:(1) Based on the good ferroelectric characteristic of the BF-PbTiO3 (PT), a new ternary system of BF-BaTiO3-xBaZrO3 (BF-BT-xBZ) was constructed by relaxor ferroelectric Ba(Zr0.25Ti0.75)O3 (BZT) composition substitution BF. The results show that with increasing the BZT content, the phase structure gradually changed from a rhombohedral phase to a coexistence of rhombohedral and tetragonal phases and then into a pseudocubic phase, at the same time, a normal-relaxor ferroelectric phase transformation was detected. When x=0.19, piezoelectric properties reached optimal performance (d33～390 pC/N) and large electric-field induced strain (d33*～390 pC/N). When the content of BZT reached to x=0.27, a relatively larger strain S～0.36% (d33*～900 pm/V) can be induced under the external electric field of 4 kV/mm, while appeared with the degradation of piezoelectric properties. Hence we adopted in-situ high-resolution synchrotron X-ray diffraction measurement and other characteristic methods to explore the cause of this phenomenon.(2) Based on lead-free ceramics has attracted much attention in recent years, hence a novel BF-BT-xBaZrO3 (BZ) lead-free relaxor ferroelectric ceramic was designed and constructed. Measurements of temperature-dependent dielectric permittivity and the polarization/strain hysteresis loops demonstrated an obvious evolution of dielectric relaxor behavior at room temperature from nonergodic to ergodic states. A significantly enhanced electrostrain of ～0.37% at 7 kV/mm with a relatively small hysteresis of ～39 % and a low-frequency sensitivity was found at x=0.04, showing large potential for actuator applications.(3) The relaxor ferroelectric Pb(Ni1/3Nb2/3)O3 (PNN) compositional substitution was found to induce a diffuse-relaxor-weak relaxor ferroelectric phase transition in a new BMT-PT-xPNN ternary system, leading to the evolution of nonergodic-ergodic-nonergodic relaxor states at RT from the BMT-rich side to the PNN-rich side. Two strain maxima of～0.42%(d33*～600 pm/V) and ～0.29%(d33*～420 pm/V) at 7 kV/mm were obtained in the x=0.2 and x=0.65 samples, respectively. It is worthy of note that the x=0.2 composition owned a temperature-insensitive but highly hysteretic high-field strain. By comparison, the x=0.65 composition exhibited a weakly hysteretic strain with a relatively small threshold electric field. This phenomenon was ascribed to larger sizes and less dynamics of polar nanoregions in BMT-rich compositions than those in PNN-rich compositions.(4) A pseudo-cubic to tetragonal phase transformation was found to be accompanied by a relaxor-normal ferroelectric phase transition in new perovskite-type BNiT-PT-xPb(Mg1/3Nb2/3)O3 ceramics. A large-strain platform of ～0.35% was generated under 6kV/mm (dynamic d33*=583 pm/V) within 0.15≤x≤0.4. The temperature-dependent measurement demonstrated that enhanced electrostrains appeared not only near a freezing temperature Tf, but also near an ergodic to ferroelectric phase transition temperature Tfr, both of which are ascribed to a reversible field induced ergodic to ferroelectric phase transition. An interesting finding would be that more similar free energies between ergodic and ferroelectric phases near Tfr have brought about smaller driving fields and strain hysteresis but lower maximum strains.(5) In this communication, the relaxor, ferroelectric and energy-storage properties of a new lead-free ferroelectric BF-BT-xBa(Mg1/3Nb2/3)O3 (BMN) was reported by a microwave dielectric material (BMN) composition substitution. It is found that the substitution of BMN for BF was found to induce a typical dielectric relaxor behavior, during which a relatively high Pmax of 38 μC/cm2 and a low Pr of 5.7 μC/cm2 were obtained under 12.5 kV/mm in the x=0.06 sample, leading to an energy-storage density of W～1.56 J/cm3 and energy-storage efficiency of η～75%. The achievement of these characteristics was basically attributed to an electric field induced reversible ergodic to ferroelectric phase transition. Moreover, a good temperature stability of the energy storage was obtained in the x=0.06 sample from 25 to 190℃, which should largely benefit from both the diffuse phase transformation process and the ergodic-nonergodic phase coexistence in a wide temperature range.(6) A new ternary system of BF-BT-xLa(Mg1/2Ti1/2)O3 (LMT) was constructed by substituting LMT for BF. It is found that the substitution of LMT for BF was found to more easily induce relaxor behaviour in this system. Excellent energy-storage properties of W～1.56 J/cm3 and η-75% were achieved in x=0.06 sample. These properties obviously show overall advantage compared to other ceramics, making this ceramics become promising lead-free energy-storage material. Our work may provide a new method for designing high energy-storage properties in lead-free relaxor ceramics. |
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