Department of Chemistry, Stanford University , Stanford, California 94305, United States.
Advanced Photon Source, Argonne National Laboratory , Argonne, Illinois 60439, United States.
ACS Nano. 2017 Nov 28;11(11):10945-10954. doi: 10.1021/acsnano.7b04735. Epub 2017 Oct 23.
Defects such as dislocations and grain boundaries often control the properties of polycrystalline materials. In nanocrystalline materials, investigating this structure-function relationship while preserving the sample remains challenging because of the short length scales and buried interfaces involved. Here we use Bragg coherent diffractive imaging to investigate the role of structural inhomogeneity on the hydriding phase transformation dynamics of individual Pd grains in polycrystalline films in three-dimensional detail. In contrast to previous reports on single- and polycrystalline nanoparticles, we observe no evidence of a hydrogen-rich surface layer and consequently no size dependence in the hydriding phase transformation pressure over a 125-325 nm size range. We do observe interesting grain boundary dynamics, including reversible rotations of grain lattices while the material remains in the hydrogen-poor phase. The mobility of the grain boundaries, combined with the lack of a hydrogen-rich surface layer, suggests that the grain boundaries are acting as fast diffusion sites for the hydrogen atoms. Such hydrogen-enhanced plasticity in the hydrogen-poor phase provides insight into the switch from the size-dependent behavior of single-crystal nanoparticles to the lower transformation pressures of polycrystalline materials and may play a role in hydrogen embrittlement.
位错和晶界等缺陷通常控制着多晶材料的性能。在纳米晶材料中，由于涉及的短长度尺度和埋入的界面，在保留样品的情况下研究这种结构-功能关系仍然具有挑战性。在这里，我们使用布拉格相干衍射成象技术，以三维细节研究结构不均匀性对多晶薄膜中单个 Pd 晶粒的氢化相转变动力学的作用。与之前关于单晶和多晶纳米粒子的报告相反，我们没有观察到富含氢的表面层的证据，因此在 125-325nm 的尺寸范围内，氢化相转变压力没有尺寸依赖性。我们确实观察到有趣的晶界动力学，包括在材料仍处于贫氢相时晶格的可逆旋转。晶界的迁移率，加上缺乏富含氢的表面层，表明晶界是氢原子的快速扩散位点。在贫氢相中增强的氢塑性为从单晶纳米粒子的尺寸依赖性行为转变为多晶材料的较低转变压力提供了深入的了解，并且可能在氢致脆化中起作用。
