State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, China.
Hebei Provincial Key Lab of Optoelectronic Information Materials, College of Physics Science and Technology, Hebei University, Baoding, 071002, China.
Nanoscale. 2019 Dec 28;11(48):23346-23356. doi: 10.1039/c9nr07265a. Epub 2019 Dec 3.
Understanding the oxidation process of active metals plays a crucial role in improving their mechanical/oxidation properties. Using in situ environmental transmission electron microscopy and density-functional theory, we firstly clarify the oxidation process of single-crystal Mg at the atomic scale by using a new double-hole technique. A unique incipient interval-layered oxidation mechanism of single-crystal Mg has been confirmed, in which O atoms intercalate through the clean (21[combining macron]1[combining macron]0) surface into the alternate-layered tetrahedral sites, forming a metastable HCP-type MgO structure. Upon the increased incorporation of oxygen at the neighboring interstitial sites, the HCP-type Mg-O tetrahedron structure sharply transforms into the FCC-type MgO oxide In addition, a typical anisotropic growth mechanism of oxides has been identified, wherein it involves two routes: the epitaxial growth of the MgO layer and the inward migration of the MgO/Mg interface. The whole oxidation rate of single-crystal Mg is mostly determined by the inward migration rate of the MgO/Mg interface, which is about six times higher than that of the epitaxial growth rate of the MgO layer along the same orientation planes. Moreover, the inward migration rate of the (020)‖(011[combining macron]0) interface is about twice as large as that of the (200)‖(0002) interface. This continuous oxide growth is mainly related to the defects in the MgO layer, which builds effective channels for the diffusion of O and Mg atoms. The in situ double-hole observations together with theoretical calculations provide a potential trajectory to probe the oxidation fundamentals of other active metals.
理解活性金属的氧化过程对于改善其机械/氧化性能至关重要。本工作利用原位环境透射电子显微镜和密度泛函理论，首次采用双孔技术在原子尺度上阐明了单晶 Mg 的氧化过程。证实了单晶 Mg 独特的初始层状氧化机制，其中 O 原子通过清洁（21[combining macron]1[combining macron]0）表面插入到交替层状四面体位置，形成亚稳 HCP 型 MgO 结构。随着氧在相邻间隙位置的不断掺入，HCP 型 Mg-O 四面体结构迅速转变为 FCC 型 MgO 氧化物。此外，还确定了氧化物典型的各向异性生长机制，其中涉及两种途径：MgO 层的外延生长和 MgO/Mg 界面的向内迁移。单晶 Mg 的整体氧化速率主要由 MgO/Mg 界面的向内迁移速率决定，该速率约比沿相同取向面的 MgO 层的外延生长速率快 6 倍。此外，（020）‖（011[combining macron]0）界面的向内迁移速率约为（200）‖（0002）界面的两倍。这种连续的氧化生长主要与 MgO 层中的缺陷有关，这些缺陷为 O 和 Mg 原子的扩散建立了有效的通道。原位双孔观察与理论计算相结合，为研究其他活性金属的氧化基础提供了一个潜在的轨迹。
