CDS&E: A New Approach for Determining the Free Energy and Absolute Mobility of Flat, Curved, and Moving Interfaces

该奖项支持可能处于不同状态的材料界面的理论和计算研究和教育。在自然界中，物质经常自发地改变它们的状态。例如，在温暖的日子里，固体冰融化成为液态水。然而，冰块不会一下子全部融化；相反，它们在表面融化。因此，科学家通过了解固体和液体之间的表面（称为固体/液体界面）的特性以及在那里发生的过程来研究熔化过程。固/液界面的特性取决于分子结构，分子非常小，非常快，非常难以控制。在这个项目中，研究团队将开发一种新的计算机模拟方法，以一种可控的方式减缓分子运动，以便团队可以提取有关固体和液体中的界面特性和过程的信息。研究结果将帮助科学家更好地理解熔化和冻结，这在金属铸造和3D打印中很重要。此外，研究小组还将研究从电池到钢铁等各种材料中的固体/固体界面，以及影响电子材料生产的固体/蒸汽界面。这种类型的计算机模拟对于支持科学技术进步越来越重要。为了用这些新方法培养材料科学工作者，该项目将帮助培养计算材料科学原理方面的学士、硕士和博士生。此外，为了最大限度地发挥这项工作的影响，这个项目的方法和结果将对所有感兴趣的科学家开放。该奖项支持材料界面的理论和计算研究和教育，这些界面可能处于不同的状态。界面是发生在状态、相位、晶体取向、磁自旋、原子有序或任何其他结构参数不同的材料交叉处的平面缺陷。因为界面代表了电子、磁性或原子结构的破坏，它们为系统贡献了正的自由能。因此，如果一个界面是移动的，它将移动，使系统的总自由能最小。当一个接口移动时，它与其他接口、内部和外部场以及几何边界条件相互作用，不断改变其结构。随着其局部环境的变化，界面结构、形状和限速运动机制也会发生变化。这种集体相互作用确保了在实际材料中，界面很少达到亚稳态平衡构型。由于界面调节着材料的热、电、机械、光学、化学和功能特性，材料科学家研究它们的热力学和动力学。然而，几乎所有的方法都局限于处于亚稳态平衡配置的界面，而不能应用于材料加工过程中发生的移动、进化界面。这项工作的目标是开发一种新的方法来获得平衡和远离平衡的界面的真正自由能和绝对迁移率，以实现物理发现，提供对机制和结果的更深入理解，并与中尺度材料过程联系起来。提出了一种计算有限温度界面自由能和迁移率的新方法。称为驱动力平衡分子动力学（DFB-MD）方法，它依赖于平衡两个或多个已知的驱动力，产生一个可以求解界面自由能和迁移率的方程组。一种驱动力是合成的，因此强加于系统；其他因素可能包括曲率、化学、应力、磁性、缺陷或其他因素。由于界面不需要处于平衡状态，因此可以获得弯曲和/或移动边界的性质。然后，这些材料的性质可用于在更大的长度和时间尺度上建立材料模型或解释实验观察结果。DFB-MD方法可以推广到其他系统几何、驱动力和过程。通过定义一个适当的顺序参数，并在此参数的基础上施加已知的过剩能量，可以改变许多类型的界面（可能包括其他缺陷）的运动。这种影响移动界面运动的能力有可能为涉及复杂过程的许多开放问题提供见解，包括位错运动、晶粒生长和粗化、沉淀、晶体生长和空位形成。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NONTECHNICAL SUMMARYThis award supports theoretical and computational research and education on interfaces in materials which may be in different states. In the natural world, materials often spontaneously change their state. For example, on a warm day, solid ice melts to become liquid water. However, ice cubes don't melt all at once; instead, they melt at the surface. Scientists study the process of melting, therefore, by understanding the properties of the surface between the solid and liquid, termed the solid/liquid interface, and the processes that take place there.The characteristics of the solid/liquid interface depend on the molecular structure, and molecules are very small, very fast, and very difficult to control. In this project, the research team will develop a new computer simulation method that slows down the molecular motions in a controlled manner so that the team can extract information about the interfacial properties and processes in solids and liquids. The results will help scientists better understand melting and freezing, which is important in metal casting and in 3D printing. In addition, the research team will study solid/solid interfaces, which occur in diverse materials from batteries to steel, and solid/vapor interfaces, which influence the production of electronic materials.Computer simulations of this type are increasingly important to support scientific and technological advancement. To prepare the materials science workforce in these new methods, this project will help train bachelors, masters, and doctoral students in the principles of computational materials science. Furthermore, to maximize the impact of this work, the methods and results of this project will be available to all interested scientists.TECHNICAL SUMMARYThis award supports theoretical and computational research and education on interfaces in materials which may be in different states. An interface is a planar defect that occurs at the intersection of materials that differ in state, phase, crystal orientation, magnetic spin, atomic ordering, or any other structural parameter. Because interfaces represent a disruption in electronic, magnetic, or atomic structure, they contribute a positive free energy to the system. Thus, if an interface is mobile, it will move so as to minimize the total system free energy. When an interface moves, it interacts with other interfaces, with internal and external fields, and with geometric boundary conditions, continuously altering its configuration. As its local environment evolves, the interface structure, shape, and rate-limiting motion mechanism may change as well. Such collective interactions ensure that in real materials, interfaces rarely attain metastable equilibrium configurations. Because interfaces mediate the thermal, electrical, mechanical, optical, chemical, and functional properties of materials, materials scientists study their thermodynamics and kinetics. However, nearly all methods are limited to interfaces that are in metastable equilibrium configurations and cannot be applied to the mobile, evolving interfaces that occur during material processing. The goal of this work is to develop a new approach for obtaining the true free energy and absolute mobility of interfaces at and away from equilibrium in order to enable physical discovery, provide deeper understanding of mechanisms and outcomes, and link to mesoscale material processes.A new method for calculating finite temperature interfacial free energy and mobility is proposed. Termed driving force balanced molecular dynamics (DFB-MD) method, it relies on balancing two or more known driving forces, yielding a system of equations that can be solved for interface free energy and mobility. One driving force is synthetic, thus imposed upon the system; the other(s) may include curvature, chemical, stress, magnetic, defect, or other contributions. Because the interface need not be in an equilibrium configuration, the properties of curved and/or moving boundaries can be obtained. These materials properties may then be used to inform materials models at larger length and time scales or to interpret experimental observations.The DFB-MD approach may be generalized to other system geometries, driving forces, and processes. By defining an appropriate order parameter and applying a known excess energy based on that parameter, the motion of many types of interfaces - potentially including other defects - may be altered. This ability to influence the motion of a moving interface has the potential to offer insight into a number of open problems involving complex processes, including dislocation motion, grain growth and coarsening, precipitation, crystal growth, and vacancy formation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.