Defect chemistry and conductivity mechanisms in acceptor doped sodium-bismuth titanate (NBT)

适当的受主掺杂可以显著提高无铅钛酸铋钠(NBT)压电陶瓷的离子导电性。掺镁后，离子电导率达到了固体氧化物燃料电池(SOFC)中常用的优良固体电解质的电导率。到目前为止，导致这种高电导率的缺陷化学性质尚不清楚。对缺陷化学的了解将使人们能够评估NBT是否可以成为氧离子导体应用的新材料。在本项目中，我们将研究受主掺杂NBT的缺陷化学和导电机制。采用将电导率和扩散实验与计算机辅助模拟相结合的系统方法，应确定电导率特性的来源。特别是高离子电导率将是该项目的主要焦点，因为它具有很高的科学和技术重要性。温度和氧分压相关的阻抗谱和介电常数的测量将使我们能够识别导电机制和造成缺陷的原因。由飞行时间(TOF)-SIMS研究的18O示踪剂扩散实验对于研究氧的传输及其对整体电导率的贡献将是非常重要的。除了多晶材料外，还将研究单晶，以更好地区分块状和晶界的贡献。本征缺陷(主要是O空位和铋空位)的形成和迁移热将在密度泛函理论的基础上通过电子结构计算确定。计算了这些缺陷与掺杂的缔合关系，以预测O迁移的活化能。结合实验获得的信息，将揭示NBT的缺陷化学和缺陷动力学的一致图像。

The ionic conductivity of the lead-free piezoelectric ceramic sodium-bismuth titanate (NBT) can be significantly enhanced by appropriate acceptor doping. With Mg-doping the ionic conductivity reaches values which are usually obtained for good solid electrolytes used in solid oxide fuel cells (SOFC). The defect chemical properties resulting in this high conductivity are not known so far. Knowledge about the defect chemistry would allow the evaluation whether NBT could be a new material for oxygen ion conductor applications. In this project, the defect chemistry and conductivity mechanisms of acceptor doped NBT will be investigated. Using a systematic approach combining conductivity and diffusion experiments with computer assisted modelling, the origin of the conductivity properties shall be identified. Particularly the high ionic conductivity will be the main focus of the project because it is of high scientific and technological importance.Temperature and oxygen partial pressure dependent impedance spectroscopy and permittivity measurements will allow us to identify the conductivity mechanisms and the contributing defects. 18O-tracer diffusion experiments investigated by Time-of-flight-(TOF)-SIMS will be of high importance for the investigation of the transport of oxygen and its contribution to the overall conductivity. Apart from polycrystalline material single crystals will also be examined to better distinguish between bulk and grain boundary contribution. Enthalpy of formation and migration of intrinsic defects (mainly O- and Bi-vacancies) will be determined with the help of electronic structure calculations on the basis of density functional theory. The association of these defects with dopants are calculated to predict activation energies for O-migration. Together with the experimentally obtained information a consistent picture of the defect chemistry and defect kinetics of NBT shall be revealed.