Impact of electric fields on grain growth in strontium titanate

在过去的几年里，人们花了大量的精力来研究电场辅助烧结(快速闪速烧结)，它可以显著地加速烧结过程。这一过程中发生的基本机制仍在讨论中。探索现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。高电流会导致焦耳发热和温度急剧升高，并在几秒钟内发生收缩。观察电场中的晶粒长大不仅可以简化实验，而且可以更清楚地了解活化机理，而不是试图了解电场辅助烧结的全部复杂性。我们开展了一系列初步实验，使用定制的模型几何观察了钛酸锶在电场中的晶粒长大行为。绝缘氧化铝板被用来防止任何电流流过样品，从而防止焦耳加热。我们认为电场对晶体生长的影响与样品中的缺陷(尤其是氧和锶空位)的再分布有关。带正电的缺陷向负极迁移，反之亦然。我们的方法跨越了原子论理论(计算晶界缺陷结构和势能)、晶界非化学计量的电子显微镜观察、热力学和动力学(缺陷迁移动力学、缺陷分布和空间电荷理论的计算)和宏观晶体生长实验。我们基于特殊的晶界(双晶)和一般的晶界(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).