Influence of defect chemistry on ferroelectric properties of KNN

无铅铁电陶瓷(K0.5Na0.5)NbO_3(KNN)因其高的压电系数d_(33)、优良的应变性能、高的居里温度T_c和机械品质因数Q_m而脱颖而出。这主要归因于较高的漏电流和较低的密度。高泄漏电流通常是由本征和本征的点缺陷引起的。假设点缺陷热力学关于铅基陶瓷的知识可以直接转移到无铅材料上。然而，情况并非如此。例如，在(Na1/2Bi1/2)TiO3中，Li等人发现受主掺杂导致了极高的氧离子电导率而不是铁电硬化。对于KNN来说，掺杂策略对铁电性能的影响通常要小得多，因为从含铅材料的经验中可以预料到这一点。有趣的是，最近也对铅基陶瓷进行了缺陷化学研究，以阐明老化和降解机制。因此，从一开始就确定无铅陶瓷的缺陷化学性质，以合理地确定是否能够满足高可靠性标准，是非常重要的。基于电导实验和量子力学模型的实验研究相结合，将描述不同掺杂对KNN的影响。此外，将考虑微结构和相变的影响，以将这些与缺陷化学的变化分开。铁电性质将被确定为电场高度和作用时间的函数。通过这种方式，可以解释由于缺陷物种迁移而导致的可能的降解和老化现象。

The lead-free ferroeletric ceramic (K0.5Na0.5)NbO3 (KNN) stands out because of its high piezoelectric coefficient d_33, excellent strain properties, high Curie Temperature T_C and mechanical quality factor Q_m. Despite the optimized and promising properties, KNN is rarely found in applications so far. This can be mainly attributed to the comparably high leakage currents and low density. The high leakage currents often result from extrinsic and intrinsic point defects. It was assumed that the knowledge from point defect thermodynamics on lead-based ceramics can be directly transferred to the lead-free material. This is, however, not the case. For example, in case of (Na1/2Bi1/2)TiO3 extremely high oxygen ionic conductivity instead of ferroelectric hardening was found for acceptor doping by Li et al. For KNN, doping strategies usually have a far lower impact on ferroelectric properties as it could be expected from the experience with lead-containing material. Interestingly, defect chemical investigations have recently also been conducted on lead-based ceramics to elucidate aging and degradation mechanisms. Thus it is of high importance to determine the defect chemical properties of lead-free ceramics right from the beginning to rationalize whether high reliability criteria can be fulfilled. A combination of experimental investigations based on conductivity experiments and quantum mechanical modelling will lead to a description of the influence of different dopands on KNN. Furthermore, the effect of microstructure and phase transitions will be taken into account to separate these from changes in defect chemistry. The ferroelectric properties will be determined as a function of height and application duration of electrical field. This way possible degradation and aging phenomena due to migrating defect species can be elucidated.