新型钙钛矿结构钛酸铋钠氧离子导体的电导调控与机理研究

Oxide-ion conductors are of considerable importance for electrochemical applications, such as solid oxide fuel cells (SOFCs), sensors and active catalyst supports. Bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT) is one of the most promising lead-free piezoelectrics. However, it opens up a new direction to design oxide ion conductors in perovskite oxides since their high oxygen-ion conductivity at medium temperatures was recently reported in non-stoichiometry BNT system. As a new family of oxide-ion conductors, it is still lacked of details on the structural characteristics and phase transition, and raises a problem in controlling the nature of defects species in BNT that will significantly influence their oxide ionic conductive properties. Meanwhile, it is probed to be problematic for the chemical stability in reducing atmospheres and cycling properties. Thus, great effects on the properties promotion and structural analysis are necessary. In this project, BNT-based materials with ionic conductive grain are chosen as the object of study. The oxide ionic conductivity and chemical stability are investigated by optimizing the phase structure, defect structure and microstructure. The contribution of electronic and ionic conductivities is needed to be clarified and special attention is paid on the interface effect to realize the controllability of the charge in micro-region. The Rietveld structural refinement method is used to quantify the displacement of the cation and the tilting of the oxygen octahedral as the irreversible phase transformation nature of these BNT under strong electrical field. The conductive mechanism is revealed by modifying and calculating the defect form and the path of oxygen ionic diffusion. These will provide fundamental principle and technology supports for optimizing the properties and expanding the application.

氧离子导体在固体氧化物燃料电池、传感器及活性催化剂载体等电化学应用中起着非常重要的作用。钛酸铋钠（Bi0.5Na0.5TiO3，BNT）基材料是最有前途的无铅压电材料之一，而近期研究表明非化学计量比BNT具有优异的氧离子导电特性，开辟了钙钛矿氧化物中氧离子导体设计的新方向。但是作为氧离子导体家族新成员，其结构特征与相转变不够明确，材料设计时缺陷的形成类型难以控制并伴随不同导电机制，同时它在还原气氛下的化学稳定性和循环特性也有待研究。本项目拟以具有离子导电晶粒BNT基材料为研究对象，通过相结构、缺陷结构及显微结构协同调控，提高材料的电导率和稳定性，明确电子电导和离子电导贡献，分析和优化材料的界面效应，实现电荷微区可控。同时利用电场下相变的不可逆性，结合结构精修量化钙钛矿中离子位移和八面体旋转，调控、计算缺陷存在形式和氧离子跃迁路径，揭示导电机理，为进一步性能优化和扩展应用提供技术和理论支撑。

氧离子导体在固体氧化物燃料电池、传感器及活性催化剂载体等电化学应用中起着非常重要的作用，其中高氧离子电导率是其最重要的技术指标之一。本课题以钛酸铋钠基材料为对象，研究了不同组成与其电学性能的关系，获得了具有高氧离子导电的材料体系；通过缺陷设计，获得了高介电性能的介质材料。讨论了相结构、缺陷结构及显微结构对材料电学性能的影响机制，为新型钛酸铋钠基氧离子导体的性能设计及扩展应用提供了重要的指导意义：(a)设计了一系列非化学计量比阳离子空位/受主掺杂的钛酸铋钠材料体系，研究了其组成、交流阻抗响应和导电性能与之间的关系。系统地研究了不同掺杂量稀土元素(La、Nd、Sm)等价取代Bi0.5Na0.5TiO3（BNT）和具有氧空位BNT基陶瓷，研究了温度演变过程中，其组成、晶体结构、缺陷结构及导电性能的关系，获得了高晶粒导电BNT材料体系，在600℃时，(Bi0.445Nd0.05Ca0.04)Na0.5TiO2.965具有最大的晶粒导电率，达12.7mS/cm，明显优于文献中报道。(b)基于准同型相界，设计了不同阳离子空位的BNT基材料体系，空位的加入引入了一个随机局域极化场，获得了延迟的无滞后电滞回线，在低场强下(100kV/cm)获得了高储能密度(Wrec=1.5J/cm3)和效率(91%)的BNT基陶瓷体系，且具有良好的温度稳定性。同场强下，储能效率和密度远高于其他报道，在储能应用中的具有极大的优势。另外，通过设计遍历弛豫态下Bi离子缺失，BNT基陶瓷在很宽温度范围内(＞320℃)具有大的介电常数ε′(~3000)和极小的损耗 (<0.003)，且具有优异的储能和电致伸缩性能。总之，项目研制出一系列性能优异的BNT基陶瓷，为制备环境友好的氧离子导体应用和介电、铁电及压电应用提供了科学依据和技术支撑。