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压电陶瓷钛酸铋钠（NBT）的离子电导率。通过Mg掺杂，离子电导率达到通常对于固体氧化物燃料电池（SOFC）中使用的良好固体电解质获得的值。导致这种高电导率的缺陷化学性质迄今尚不清楚。关于缺陷化学的知识将允许评估NBT是否可以是用于氧离子导体应用的新材料。在本计画中，将研究受主掺杂NBT的缺陷化学与导电机制。应采用将电导率和扩散实验与计算机辅助建模相结合的系统方法，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测量将使我们能够识别导电机制和贡献缺陷。飞行时间-（TOF）-二次离子质谱（西姆斯）研究的18 O示踪剂扩散实验将是非常重要的氧气的运输及其对整体电导率的贡献的调查。除了多晶材料，还将检查单晶，以更好地区分体和晶界的贡献。本征缺陷（主要是O-和Bi-空位）的形成和迁移的焓将通过基于密度泛函理论的电子结构计算来确定。这些缺陷与掺杂剂的关联计算O-迁移的活化能预测。结合实验获得的信息，应揭示NBT的缺陷化学和缺陷动力学的一致图片。

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.