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

通过适当的受体掺杂，可以显著提高钛酸铋钠压电陶瓷的离子电导率。镁掺杂后，离子电导率达到固体氧化物燃料电池（SOFC）中常用的良好固体电解质的电导率。导致这种高导电性的缺陷化学性质目前尚不清楚。了解缺陷的化学性质将有助于评估NBT是否可以作为氧离子导体应用的新材料。在本项目中，将研究掺杂NBT受体的缺陷化学和导电机制。使用系统的方法结合电导率和扩散实验与计算机辅助建模，应确定电导率特性的起源。特别是高离子电导率将是该项目的主要重点，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确定电导率机制和贡献缺陷。利用TOF - sims进行的18o示踪剂扩散实验对于研究氧的输运及其对整体电导率的贡献具有重要意义。除了多晶材料外，还将检查单晶，以更好地区分体积和晶界的贡献。本征缺陷（主要是O-和bi -空位）的形成焓和迁移焓将在密度泛函理论的基础上借助电子结构计算来确定。计算了这些缺陷与掺杂剂的关联，以预测o迁移的活化能。与实验获得的信息一起，将揭示NBT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.