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

非技术总结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，固体冰融化成液态水。然而，冰块不会一下子融化；相反，它们会在表面融化。因此，科学家们通过了解固体和液体之间表面的性质，即固体/液体界面，以及在那里发生的过程来研究熔化过程。固体/液体界面的特征取决于分子结构，分子非常小，速度非常快，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，以受控的方式减缓分子运动，以便团队能够提取关于固体和液体中的界面属性和过程的信息。这一结果将帮助科学家更好地了解熔化和冻结，这在金属铸造和3D打印中非常重要。此外，研究小组将研究固体/固体界面和固体/蒸汽界面，固体/固体界面存在于从电池到钢铁的各种材料中，固体/蒸汽界面影响电子材料的生产。这类计算机模拟对于支持科学和技术进步越来越重要。为了帮助材料科学工作者掌握这些新方法，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提供给所有感兴趣的科学家。技术总结该奖项支持关于可能处于不同状态的材料界面的理论和计算研究和教育。界面是一种平面缺陷，它出现在状态、相、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的交叉处。由于界面代表电子、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，如果接口是移动的，它将移动以最小化系统总自由能。当界面移动时，它与其他界面、内部和外部场以及几何边界条件相互作用，不断改变其配置。随着其局部环境的演变，界面结构、形状和限速运动机制也可能发生变化。这种集体相互作用确保了在真实材料中，界面很少达到亚稳平衡构型。由于界面调节材料的热、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构型的界面，而不能应用于材料加工过程中发生的移动的、不断演变的界面。这项工作的目的是开发一种新的方法来获得处于平衡状态和远离平衡状态的界面的真实自由能和绝对迁移率，以便于物理发现，加深对机理和结果的理解，并与介观材料过程相联系。提出了一种计算有限温度界面自由能和迁移率的新方法。被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更多已知的驱动力，产生一组可以求解界面自由能和迁移率的方程。一个驱动力是合成的，因此施加在系统上；另一个驱动力(S)可能包括曲率、化学、应力、磁、缺陷或其他贡献。由于界面不需要处于平衡构型，因此可以获得弯曲和/或移动边界的性质。DFB-MD方法可以推广到其他的系统几何结构、驱动力和过程。通过定义适当的有序参数并基于该参数施加已知的过剩能量，许多类型的界面--可能包括其他缺陷--的运动可能被改变。这种影响移动界面运动的能力有可能为许多涉及复杂过程的公开问题提供洞察力，包括位错运动、晶体生长和粗化、沉淀、晶体生长和空位形成。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.