玻璃形成液体中Stokes-Einstein关系式的高温异常失效及动力学慢化研究

Stokes-Einstein　(SE)　relation describes the relationship between self-diffusivity and viscosity, and thus plays an important role in studying the slowing down dynamics of glass-forming liquids. The abnormal breakdown of SE relation at temperatures above liquidus temperature contradicts our normal knowledge about the validity of SE relation and the dynamics of normal liquid, and it is thus a real challenge for experiment and theory. With a combination of molecular dynamics simulation and synchrotron radiation experiment, the project aims to elucidate the mechanism and to find a resonable criteria to predicate the abnormal breakdown of SE relation. The project includes: (1) investigate the intrinsic characteristics of materials on the validity of SE relation, and propose an empirical model for the prediction of the system with the abnormal breakdown of SE relation;(2)investigate the correlation motion of atom and the slowing down of dynamics of high temperature normal liquid from the analysis of atomic diffusion and relaxation process;(3)investigate the correlations among dynamics,structure and the validity of SE relation, and elucidate the mechanism of the abnormal breakdown of SE relation; (4) on the basis of the mechanism study, we try to propose a quantitative criteria to predict the onset temperature for the abnormal breakdown of SE relation. The combination of electrostatic levitation and synchrotron EXAFS technique is applied to determine the liquid structure, and the experimental results are used to confirm the validity of simulation. This project is important for the right application of SE relation and the elucidation of the microscopic details of glass transition in glass-forming liquids. Moreover, the project can give important information for the further improvement of the mode coupling theory.

Stokes-Einstein（SE）关系式描述了液体自扩散系数和粘度的关联，对于玻璃形成液体动力学慢化研究具有重要意义。SE关系式在液相线温度以上异常失效的现象与人们的传统认知相矛盾，给实验和理论带来了严峻挑战。本项目采用分子动力学模拟和实验相结合的方法，系统研究了SE关系式高温异常失效的物理机制及定量预测方法。对SE关系式异常失效体系的内禀属性进行研究，提出预测异常失效体系的经验模型；对原子的扩散和弛豫过程进行研究，澄清高温液体原子的耦合运动、动力学慢化和空间不均匀性；探讨液体动力学、微观结构与SE关系式失效的耦合关系，阐明SE关系式异常失效的物理机制；探索异常失效温度的定量预测方法并对其合理性进行验证。利用静电悬浮结合同步辐射EXAFS研究液体结构，对模拟结果的合理性进行验证。本项目对SE关系式的合理使用及玻璃态转变机理研究具有重要意义，并为模式耦合理论的进一步完善提供重要信息。

Stokes-Einstein (SE) 关系式描述了自扩散系数和粘度之间的关联。通常情况下，SE关系式在温度低于模式耦合理论(MCT)临界温度Tc时失效。但最新研究发现，对很多液体而言，SE关系式在液相线温度以上已经发生异常失效。项目以揭示SE关系式的高温异常失效机制并定量预测失效温度为目标，以分子动力学模拟和实验相结合的方式，对液态单质Al、二元合金Fe-Al、Ni-Nb和三元合金Al-Co-Ni的输运性质和SE关系式的失效情况进行了系统研究。研究表明，在液态Al、Fe-Al和Al-Co-Ni中SE关系式在液相线温度以上发生高温异常失效，而在Ni-Nb合金中未发现该现象。对熔体结构、动力学空间不均匀性、动力学慢化现象、结构与动力学的耦合关系等进行了深入研究，揭示了SE关系式熔点以上异常失效的物理机制是原子动力学空间不均匀性的快速增加。基于组元动力学脱耦，提出了SE关系式异常失效温度的D1/D2（即：两组元自扩散系数比值）判据，并对判据的合理性及适用范围进行了检验。研究发现，强的化学短程有序(CSRO)对组元动力学脱耦以及SE关系式的高温异常失效具有重要影响。基于动力学慢化，提出了动力学参数的超-Arrhenius行为判据；基于原子团簇分析，提出了局域五次对称 (Local Five-Fold Symmetry) 这一预测SE关系式异常失效更为普适的方法。在上海同步辐射光源(SSRF)通过高能同步辐射X射线衍射技术对Al72Ni16Co12熔体结构进行了测定，并利用实验结果对经典分子动力学模拟结果的合理性进行了验证。本项目对SE关系式的合理使用及玻璃态转变机理研究具有重要意义，并为模式耦合理论的进一步完善提供重要信息。