Impact of electric fields on grain growth in strontium titanate

在过去的几年里，在研究电场辅助烧结（FAST，闪速烧结）方面花费了大量的精力，这可以显著加快烧结过程。在此过程中发生的潜在机制仍在讨论中。探索场辅助烧结的基本方面是非常具有挑战性的，因为实验很难控制。高电流引起焦耳加热，温度急剧升高，在几秒钟内发生收缩。而不是试图理解电场辅助烧结的全部复杂性，观察电场中的晶粒生长既可以简化实验，又可以更清楚地了解活性机制。我们进行了一系列的初步实验，利用定制的几何模型观察了钛酸锶在电场中的晶粒生长行为。绝缘氧化铝板被用来防止任何电流流过样品，从而防止任何焦耳加热。我们认为电场对晶粒生长的影响与样品中缺陷的重新分布有关（特别是氧和锶空位）。带正电荷的缺陷向负极迁移，反之亦然。将预期的局部缺陷浓度与已知的缺陷对晶粒生长的影响进行比较，支持这一假设。我们的方法涵盖原子理论（计算晶界缺陷结构和电势）、边界非化学计量学的电子显微镜观察、热力学和动力学（计算缺陷迁移动力学、缺陷分布和空间电荷理论）和宏观晶粒生长实验。我们基于特殊晶界（双晶）和一般晶界（种子多晶）。

During the last years, considerable effort has been expended in investigating electric-field-assisted sintering (FAST, flash sintering) that can significantly accelerate sintering processes. The underlying mechanisms occurring during the process are still under discussion. Exploring fundamental aspects of field-assisted sintering is very challenging, because experiments are hard to control. High electrical currents cause Joule heating and increase the temperature drastically and shrinkage occurs within seconds. Instead of trying to understand the full complexity of field-assisted sintering, observing grain growth in electric field can both simplify the experiments and provide clearer insights into the active mechanisms.We carried out a series of preliminary experiments, where we observed the grain growth behavior of strontium titanate in electric field using a tailored model geometry. Insulating alumina plates were used to prevent any electric current to flow through the sample and therefore any Joule heating. A gradient of the growth length appears: at the negative electrode growth is strongly acceleratedWe believe that the effect of electric fields on grain growth is related to a defect redistribution across the sample (particularly oxygen and strontium vacancies). Positively charged defects migrate towards the negative electrode and vice versa. A comparison of the expected local defect concentration to the already known impact of defects on grain growth supports this hypothesis.Our approach spans atomistic theory (calculation of defect structures and potentials at grain boundaries), electron-microscopical observation of boundary non-stoichiometry, thermodynamic and kinetics (calculation of defect migration kinetics, defect distribution and space charge theory) and macroscopic grain growth experiments. We base on special grain boundaries (bicrystals) as well as general boundaries (seeded polycrystals).