铁酸铋（BiFeO₃，BF）是一种具有钙钛矿结构的多铁性材料，表现出高居里温度（T_C = 850 ℃）、高自发极化强度（P_s = 100 μC/cm²）和良好的应变性能，受到广泛研究。本课题以铁酸铋为基体，针对其高漏导和大介电损耗问题，选择位于准同型相界（Morphotropic Phase Boundary，MPB）附近具有较大铁电压电活性的BiFeO₃-BaTiO₃（BF-BT）体系进行改性研究，分别在储能和应变特性上达到性能改善的目的。主要研究内容如下：

为降低BF-BT陶瓷的剩余极化强度，实现优异的储能性能，采用传统固相法制备0.7((1-x)BiFeO₃-xSr_0.7Bi_0.2TiO₃)-0.3BaTiO₃（BF-BT-SBT）三元无铅弛豫铁电陶瓷，研究了SBT（Sr_0.7Bi_0.2TiO₃）掺杂对BF-BT陶瓷的阻抗、介电、铁电和应变特性的影响。SBT的取代诱导从三方相（R3c）到平均赝立方（P_c）结构的相变，并在高掺杂陶瓷组分产生了第二相。同时，电阻率、电均匀性和带隙显著增加，这与抑制氧空位的形成有关。此外，铁电长程有序被破坏，促进了局部缺陷场和极性纳米微区（PNRs）的形成。高掺杂组分中的结构异质性导致了弛豫行为的增强和对外部电场具有几乎无滞后的极化响应。在低电场下分别获得了具有良好温度稳定性的介电响应和优异的介电储能特性。这些结果表明，SBT的掺杂是调节铁酸铋基弛豫铁电陶瓷介电性能的有效途径。

为进一步增强BF-BT-SBT体系的的介电弛豫特性，在BF-BT-0.2SBT陶瓷中引入高价Nb⁵⁺取代Ti⁴⁺，通过掺杂不同质量比的Nb₂O₅调控BF-BT基陶瓷的缺陷机制，制备(0.56BiFeO₃-0.14Sr_0.7Bi_0.2TiO₃)-0.3BaTiO₃-xwt%Nb₂O₅（BS-xNb）陶瓷。结果表明，Nb掺杂诱导了P_c钙钛矿结构，随着掺杂量的增加，在富Nb组分中产生了第二相。微量Nb⁵⁺掺杂导致离子电导率和介电损耗降低，提高了电阻率，这归因于氧空位和相关电子的形成受到抑制。而引入过量的Nb₂O₅会产生更多的价电子缺陷，导致漏电流增大。此外，Nb₂O₅掺杂的加入增强了无序性，铁电遍历长程有序被破坏，促进了PNRs的形成，导致纤细的电滞回线。该项工作证明了对Ti⁴⁺进行低水平的Nb⁵⁺掺杂可以补偿陶瓷在烧结过程中氧空位的生成并抑制离子电导率，从而改善铁酸铋基陶瓷的介电行为。

由于BF本身存在的高漏导和大损耗问题仍需对BF-BT陶瓷的应变性作进一步改善。对MPB附近的0.67BF-0.33BT体系在其A位掺杂稀土元素Sm和Nd可以抑制氧空位，降低漏导；同时引入Bi(Mg_1/2Ti_1/2)O₃（BMT）与氧空位结合形成缺陷偶极子诱导非对称应变（S-E）曲线，从而在低电场下获得大的应变。采用固相法制备(0.67-x)Bi_0.99Sm_0.01FeO₃-0.33BaTiO₃-xBi(Mg_1/2Ti_1/2)O₃ (BSF-BT-BMT)和(0.67-y)Bi_0.99Nd_0.01FeO₃-0.33BaTiO₃-yBi(Mg_1/2Ti_1/2)O₃ (BNF-BT-BMT)无铅陶瓷。结果表明，两种陶瓷均为P_c结构，没有第二相。复阻抗谱揭示了晶界响应和双电离氧空位的主导作用。随着BMT的增加，弛豫性得到提高，BSF-BT-BMT陶瓷在低电场下获得了大的非对称应变，在高温下，应变达到最大，同时实现了超高逆压电系数。结果表明，共存相与缺陷工程相结合，可以显著增强应变。这项工作将促进无铅铁电陶瓷大应变的进一步研究。

Bismuth ferrite (BiFeO₃, BF) is a multiferroic material with a perovskite structure, exhibiting high Curie temperature (T_c = 850 ℃) and high spontaneous polarization intensity (P_s=100 mC/cm² and excellent piezoelectric performance have been widely studied in various fields such as biomedical, aerospace, and ocean exploration. This study focuses on the modification of BiFeO₃-BaTiO₃ (BF-BT) system located near the morphotropic phase boundary (MPB), using bismuth ferrite as the matrix to address the issues of high leakage conductivity and large dielectric loss caused by the volatilization of Bi₂O₃ and the valence change of Fe. The aim is to improve its dielectric, energy storage, and strain characteristics. The main research content is as follows:

To reduce the residual polarization strength of BF-BT ceramics and achieve excellent energy storage performance, 0.7((1-x)BiFeO₃-xSr_0.7Bi_0.2TiO₃)-0.3BaTiO₃ (BF-BT-SBT) ternary lead-free relaxor ferroelectric ceramics were prepared by a conventional solid-phase method, investigating the effect of Sr_0.7Bi_0.2TiO₃ (SBT) on the impedance, dielectric, ferroelectric, and strain properties. The substitution of SBT induces a phase transition from an R3c to an average pseudo-cubic structure and forms a secondary phase at high dopants. Simultaneously, the resistivity, electrically homogeneous, and band gaps are increased significantly, related to the inhibition of oxygen vacancy formation. Besides, the ferroelectric long-range order is disrupted, which promotes the formation of local defect fields and polar nanoregions (PNRs). The structural heterogeneity for the higher doped components leads to an increased relaxation behavior with an almost hysteresis-free polarization response to external electric fields. Excellent temperature-insensitive dielectric response and promising dielectric energy storage properties in low electric fields are achieved, respectively. These results indicate that the SBT modification is an effective route in regulating the dielectric performance of bismuth ferrite based relaxors.

To further enhance the dielectric relaxation properties of the BF-BT-SBT system, high valence Nb⁵⁺was introduced to replace Ti⁴⁺in BF-BT-0.2SBT ceramics. The defect mechanism of BF-BT based ceramics was controlled by doping Nb₂O₅ with different mass ratios. (0.56BiFeO₃-0.4Sr_0.7Bi_0.2TiO₃)-0.3BaTiO₃-xwt%Nb₂O₅ (BS-xNb) ceramics was prepared. The results indicate that Nb doping induced a P_c phase structure, and with the increase of doping amount, a second phase was generated in the Nb rich component. The doping of trace amounts of Nb⁵⁺ leads to a decrease in ion conductivity and dielectric loss, and increases the resistivity, which is attributed to the inhibition of the formation of oxygen vacancies and related electrons. While, the introduction of excessive Nb₂O₅ will generate more valence electron defects, leading to charge leakage. In addition, the doping can enhance disorder, disrupt the ferroelectric ergodic long-range ordering, and promote the formation of PNRs, leading to slender P-E loops. This work demonstrates that low-level Nb⁵⁺ doping of Ti⁴⁺can compensate for the generation of oxygen vacancies during sintering and suppress ion conductivity, thereby restoring the dielectric behavior of bismuth ferrite-based ceramics.

Due to the high leakage conductivity and large losses inherent in BF, further improvement of the strain properties of BF-BT ceramics is still needed. The 0.67BF-0.33BT system near MPB was doped with rare earth elements Sm and Nd at A-site can inhibit oxygen vacancies and reduce leakage conductivity. The introduction of Bi(Mg_1/2Ti_1/2)O₃ (BMT) can combine with oxygen vacancies to form defect dipoles induced an asymmetric S-E curves, resulting in large strain response under low electric field. The (0.67-x)Bi_0.99Sm_0.01FeO₃-0.33BaTiO₃-xBi(Mg_1/2Ti_1/2)O₃ (BSF-BT-BMT) and (0.67-y)-Bi_0.99Nd_0.01FeO₃-0.33BaTiO₃-yBi(Mg_1/2Ti_1/2)O₃ (BNF-BT-BMT) lead-free ceramics were prepared. Both ceramics have a P_c structure and no second phase. The complex impedance spectrum reveals the dominant role of grain boundary response and double ionized oxygen vacancies. With increasing BMT, the relaxation property is enhanced. A large asymmetric strain was obtained in BSF-BT-BMT ceramics under low electric field. The super high strain and signal inverse piezoelectric coefficient simultaneously obtained under high temperatures. All these indicate that the combination of coexisting phases and defect engineering can significantly enhance strain. This work will promote further research on lead-free ferroelectric ceramics with high strain.

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