In situ observation of hydride nucleation and selective growth in magnesium thin-films with environmental transmission electron microscopy
In situ observation of hydride nucleation and selective growth in magnesium thin-films with environmental transmission electron microscopy
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DOI：
10.1016/j.ijhydene.2019.10.057
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发表时间：
2019-12-06
影响因子：
7.2
通讯作者：
Pundt, Astrid
中科院分区：
文献类型：
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作者：
Hamm, Magnus;Bongers, Marian David;Pundt, Astrid
对溶质诱导相变的理解在催化、记忆切换或储能等各种研究领域中都是至关重要的。我们研究了模型镁-氢(Mg-H)体系的溶质诱导相变，它在相变过程中提供了较高的晶格膨胀。结合电子能量损失谱(EELS)和各种成像技术，在环境电子显微镜(ETEM)中原位分析了氢化镁的析出和生长过程。研究发现，镁氢化物的形成是通过形成被小角度晶界分隔开的纳米晶来进行的。这种微观结构的变化在ETEM中很容易检测到，并使研究氢化物相的生长成为可能。EELS结果证实了纳米晶组织与氢氧化镁相的直接匹配。我们将这种微观结构的变化归因于转变过程中在基质和镁氢化物之间产生的大应变和应力。在薄膜中观察到了半球形和指状的镁氢化合物区域。将ETEM研究与有限元模拟相结合，对板层的局部应力分布进行了模拟，结果表明局部应力对板层的生长行为有影响。有限元模拟表明，局部应力分布取决于氢化区的形状。这包括氢化物成核后和氢化物生长过程中产生的应力。氢在MgH_2中沿Pd/MgH_2界面和高角度晶界扩散较快，在小角度晶界扩散较慢，而在晶界扩散较慢。本文强调了氢化物生长与其自产生的局部应力场之间的相互依赖关系。我们认为这些结果并不局限于Mg-H体系，而是具有更一般的性质。(C)2019氢能出版有限责任公司。爱思唯尔有限公司出版。保留所有权利。
The comprehension of solute-induced phase transformations is crucial in a variety of research fields such as catalysis, memory switching or energy storage. We study solute induced phase transformations on the model magnesium-hydrogen (Mg-H) system which provides high lattice expansion during the phase transformations. In situ precipitation and growth of MgH2 is analyzed in an environmental transmission electron microscope (ETEM), combining electron energy loss spectroscopy (EELS) and various imaging techniques. It is found that the Mg-hydride (MgH2) formation proceeds through the formation of nanocrystals that are separated by low-angle grain boundaries. This change in the microstructure is easily detectable in the ETEM and allows studying the growth of the hydride phase. The EELS results confirm the direct match between the nanocrystalline microstructure and the MgH2 phase. We attribute this microstructural change to large strains and stresses between the matrix and the MgH2 created during the transformation process. Half-spherical as well as finger-like regions of MgH2 is observed in the film. Combing the ETEM studies with finite element method simulations on the local stress distribution in the lamella suggests an influence of local stresses on the growth behavior. As the FEM simulations reveal, the local stress distribution depends on the shape of the hydrided region. This includes stresses that occur after the hydride nucleation and during the growth of the hydride. Hydrogen diffusion is suggested to be fast along the Pd/MgH2 interphase as well as along the high angle grain boundaries and less fast in low angle grain boundaries in MgH2, in contrast to the very slow diffusion in the MgH2 grains. This paper highlights the interdependency of the hydride growth and its self-created local stress fields in an in situ ETEM study. We consider these results not to be limited to the Mg-H system, but being of more general nature. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.