Center of Electron Microscopy and State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University , Hangzhou, China , 310027.
Department of Materials Science and Engineering, Johns Hopkins University , Baltimore, Maryland 21218, United States.
Nano Lett. 2016 Feb 10;16(2):1156-60. doi: 10.1021/acs.nanolett.5b04439. Epub 2016 Jan 28.
Sublimation is an important endothermic phase transition in which the atoms break away from their neighbors in the crystal lattice and are removed into the gas phase. Such debonding process may be significantly influenced by dislocations, the crystal defect that changes the bonding environment of local atoms. By performing systematic defects characterization and in situ transmission electron microscopy (TEM) tests on a core--shell MgO-Mg system, which enables us to "modulate" the internal dislocation density, we investigated the role of dislocations on materials' sublimation with particular focus on the sublimation kinetics and mechanism. It was observed that the sublimation rate increases significantly with dislocation density. As the density of screw dislocations is high, the intersection of screw dislocation spirals creates a large number of monatomic ledges, resulting in a "liquid-like" motion of solid-gas interface, which significantly deviates from the theoretically predicted sublimation plane. Our calculation based on density functional theory demonstrated that the remarkable change of sublimation rate with dislocation density is due to the dramatic reduction in binding energy of the monatomic ledges. This study provides direct observation to improve our understanding on this fundamental phase transition as well as to shed light on tuning materials' sublimation by "engineering" dislocation density in applications.
升华是一种重要的吸热相转变，其中原子从晶格中的相邻原子中脱离出来并进入气相。这种脱附和过程可能会受到位错的显著影响，位错是改变局部原子键合环境的晶体缺陷。通过对具有可调节内部位错密度的核壳型 MgO-Mg 体系进行系统的缺陷特征描述和原位透射电子显微镜（TEM）测试，我们研究了位错对材料升华的作用，特别关注升华动力学和机制。结果表明，升华速率随位错密度的增加而显著提高。由于螺位错密度较高，螺位错螺旋的交叉会产生大量的单原子台阶，导致固-气界面呈现出“类液体”的运动状态，这与理论上预测的升华面明显偏离。我们基于密度泛函理论的计算表明，升华速率随位错密度的显著变化归因于单原子台阶结合能的急剧降低。本研究提供了直接的观察结果，有助于提高我们对这一基本相转变的理解，并为通过“工程化”位错密度来调控材料升华提供了启示。
