Impact of electric fields on microstructure evolution in functional ceramics

电场烧结是材料科学研究的一个新兴领域。它导致了闪速烧结的发现，这已被证明是粉末致密化的通用技术。然而，由于许多过程在场辅助烧结过程中是活跃的，电场的具体影响是难以隔离的。在优先计划的第一期，我们研究了钛酸锶（STO）在电场中微观结构演变过程中微观结构梯度的来源。我们将重点放在闪速烧结、场辅助晶粒生长和缺陷形成能量和点缺陷迁移势垒的原子模拟上。通过分析籽粒多晶在不同温度、加热时间和电场强度下的微观结构演变，研究了籽粒在STO中的生长。我们确定了由氧空位的电迁移引起的空间电荷特征的变化，作为微观结构梯度形成的可能机制。介绍了负极氧空位的重新分布导致静电势和移动晶界（GB）固有阻力的减小，从而局部加速晶粒生长的模型。这些发现得到了TEM观测和空间电荷层连续水平热力学模型的支持。采用经典原子计算建立了晶界点缺陷形成和迁移的计算模拟框架。结果导致详细了解微观结构演变对缺陷化学的依赖，甚至超出了场辅助烧结和晶粒生长。这些发现有助于更好地理解电场对物质的操纵。在此基础上，第二阶段的重点是通过引入掺杂剂、改变气氛和施加交流电场来改变钛酸锶和其他钙钛矿材料的缺陷化学和缺陷迁移动力学。理论部分将侧重于建立一个更有效的工作流程来预测相关缺陷物种的宏观扩散常数，并实现电场效应的直接（迁移和空位形成能）和间接（场驱动的晶界构型变化）。

Sintering in an electric field is a new and promising field of materials science. It resulted in the discovery of flash sintering, which has proved to be a versatile technique for powder densification. However, as many processes are active during field assisted sintering, the specific influence of the electric field is difficult to isolate. In the first period of the priority programme, we investigated the origin of microstructural gradients during microstructure evolution in electric fields for strontium titanate (STO). We placed the emphasis on flash sintering, field assisted grain growth, and atomistic simulations of defect formation energies and migration barriers of point defects. Grain growth in STO was studied by analyzing the microstructure evolution of seeded polycrystals at different temperatures, heating times and electric field strengths. We identified changes in space charge characteristics, induced by electromigration of oxygen vacancies, as the probable mechanism for the formation of microstructural gradients. A model was introduced that suggests redistribution of oxygen vacancies at the negative electrode causes a decrease of the electrostatic potential and the intrinsic drag on a moving grain boundary (GB), thereby locally accelerating grain growth. These findings are supported by TEM observations and continuum-level thermodynamic modelling of space charge layers. Classical atomistic calculations were used to establish a computational simulation framework for the formation and migration of point defects at grain boundaries.The results have led to detailed insights into the dependence of microstructure evolution on defect chemistry even beyond field assisted sintering and grain growth. The findings contribute to a better understanding of the manipulation of mater by electric fields.On the basis of the knowledge acquired, the focus of the second period, experimentally, will be on altering the defect chemistry and defect migration kinetics of strontium titanate and other perovskite materials by introducing dopants, changing atmospheres and applying AC electric fields. The theoretical part will focus on building a more efficient workflow to predict macroscopic diffusion constants for the relevant defect species based on master equations and to implement the electric field effects directly (migration and vacancy formation energies) and indirectly (field driven configurational changes of the grain boundaries).