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

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

NONTECHNICAL SUMMARY This 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 SUMMARY This 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.