Annamareddy Ajay, Li Yuhui, Yu Lian, Voyles Paul M, Morgan Dane
Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
School of Pharmacy, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
J Chem Phys. 2021 Mar 14;154(10):104502. doi: 10.1063/5.0039078.
The enhancement of surface diffusion (D) over the bulk (D) in metallic glasses (MGs) is well documented and likely to strongly influence the properties of glasses grown by vapor deposition. Here, we use classical molecular dynamics (MD) simulations to identify different factors influencing the enhancement of surface diffusion in MGs. MGs have a simple atomic structure and belong to the category of moderately fragile glasses that undergo pronounced slowdown of bulk dynamics with cooling close to the glass transition temperature (T). We observe that D exhibits a much more moderate slowdown compared to D when approaching T, and D/D at T varies by two orders of magnitude among the MGs investigated. We demonstrate that both the surface energy and the fraction of missing bonds for surface atoms show good correlation to D/D, implying that the loss of nearest neighbors at the surface directly translates into higher mobility, unlike the behavior of network-bonded and hydrogen-bonded organic glasses. Fragility, a measure of the slowdown of bulk dynamics close to T, also correlates to D/D, with more fragile systems having larger surface enhancement of mobility. The deviations observed in the fragility-D/D relationship are shown to be correlated to the extent of segregation or depletion of the mobile element at the surface. Finally, we explore the relationship between the diffusion pre-exponential factor (D) and the activation energy (Q) and compare it to a ln(D)-Q correlation previously established for bulk glasses, demonstrating similar correlations from MD as in the experiments and that the surface and bulk have very similar ln(D)-Q correlations.
金属玻璃（MGs）中表面扩散系数（D）相较于体扩散系数（D）的增强现象已有充分记录，且可能对气相沉积生长的玻璃的性能产生强烈影响。在此，我们使用经典分子动力学（MD）模拟来确定影响MGs中表面扩散增强的不同因素。MGs具有简单的原子结构，属于中等脆性玻璃类别，随着冷却接近玻璃化转变温度（T），其体动力学显著减慢。我们观察到，当接近T时，D的减慢程度比D温和得多，在所研究的MGs中，T时的D/D相差两个数量级。我们证明，表面能和表面原子的缺失键分数与D/D都有良好的相关性，这意味着表面最近邻原子的缺失直接转化为更高的迁移率，这与网络键合和氢键有机玻璃的行为不同。脆性是衡量接近T时体动力学减慢程度的指标，它也与D/D相关，脆性越大的系统表面迁移率增强越大。脆性与D/D关系中观察到的偏差与表面可移动元素的偏析或耗尽程度相关。最后，我们探讨了扩散预指数因子（D）与活化能（Q）之间的关系，并将其与先前为体玻璃建立的ln（D）-Q相关性进行比较，结果表明MD模拟得到的相关性与实验相似，且表面和体具有非常相似的ln（D）-Q相关性。
