On the Mechanics of Ripplocations: A New Defect in Layered Solids

最近在分层材料方面的发现为裁剪更好的材料开辟了新的领域。分层材料在我们的环境中无处不在；它们包括石墨、云母和二维固体（具有原子级厚度）等。人们早就知道，要使固体变形，必须存在微观缺陷。直到最近，这些缺陷被认为是称为位错的线缺陷，这是一种众所周知的和研究过的材料缺陷。然而，2016年的研究表明，关于层状材料的变形，传统观念是不正确或不完整的。一种潜在的新的微机制被称为涟漪定位，最好描述为原子尺度的涟漪，被认为是控制机制。在宏观尺度上，涟漪的位置就像地毯上的涟漪。目前，人们对这些缺陷，它们的机制或行为，以及如何利用它们来创造改进的材料知之甚少。本研究通过在两个长度尺度上的综合建模和实验来研究波纹定位的机制。总体目标是将力学、材料科学和科学计算领域整合在一起，建立一个设计更先进材料的平台，并为与层材料相关的技术部门（如地质技术和微电子技术）提供基础知识基础。此外，作为研究的一部分，将通过暑期项目向高中生进行推广，将聘请来自代表性不足群体的本科生和研究生进行研究，并将在课程中增加互动学习模块，以培养下一代多样化的科学家和工程师。涟漪定位和错位行为之间存在着根本的差异，并激励着当前的项目。根据位错理论，当层状固体材料的平面被边对边加载和卸载时，如果是弹性材料，它们要么会发生可逆变形，并在不耗散任何能量的情况下恢复到原来的形状，要么会永久凹陷。涟漪定位行为解释了第三种观察到的选择，即材料返回到原始形态，同时消耗大量能量。这项工作的目标是在多个层面上理解波纹定位力学，从原子，通过分子模型和理论，到宏观层状固体将被压痕的地方。分子建模的结果，以捕捉波纹的基本物理将被用来创建一个新的波纹动力学模型。该模型的压痕预测，包括强度和微观结构变形，将直接与纳米尺度的原子模型和微尺度的实验压痕研究的结果进行比较和验证。目的是证明波纹定位机制是尺度不变的，并提供一个基于力学的预测框架，可以扩展到无数其他分层材料，以实现改进的设计策略。

Recent discoveries in layered materials have opened new dimensions for tailoring better materials. Layered materials are ubiquitous in our surroundings; they encompass graphite, mica, and two-dimensional solids (with an atomic-scale thickness), among many others. It has long been understood that for solids to deform, microscopic defects must be present. Up to quite recently these defects were presumed to be line defects called dislocations, a well-known and studied material defect. In 2016, however, research suggested that conventional wisdom was incorrect, or incomplete, regarding the deformation of layered materials. A potentially new micro-mechanism called a ripplocation, best described as an atomic scale ripple, was suggested as the governing mechanism. At the macroscale, a ripplocation would be a carpet ripple. Currently, little is known about these defects, their mechanics or behavior, and how to leverage them to create improved materials. This research investigates the mechanics of ripplocation through an integrated suite of modeling and experimentation at two length scales. The overall goal integrates fields of mechanics, materials science, and scientific computing to build a platform to engineer more advanced materials as well as provide fundamental knowledgebase to the technology sectors where layers materials are relevant, such as geo-technology and microelectronics. Also, as part of the research, outreach to high school students will be made through summer programs, undergraduate and graduate students from underrepresented groups will be hired to perform research, and interactive learning modules will be added to the curriculum to educate the next generation of diverse scientists and engineers.Fundamental differences between ripplocation and dislocation behavior exist and motivate the current project. According to dislocation theory, when the planes of layered solid materials are loaded and unloaded edge-on they will either reversibly deform, and return to their original form without dissipating any energy if it's an elastic material or they will remain permanently indented. Ripplocation behavior explains the third observed option, which is the material returning to its original form, while dissipating considerable amounts of energy. The goal of this work is to understand ripplocation mechanics at multiple levels, from the atomic, through molecular modeling and theory, to the macroscopic where layered solids will be indented. Results from molecular modeling to capture the fundamental physics of ripplocations will be leveraged to create a new ripplocation dynamics model. Indentation predictions, including both strength and microstructural deformation, from the model will be directly compared to and verified with results from atomistic modeling at the nanoscale and experimental indentation studies at the microscale. The goal is to show that ripplocation mechanics are scale invariant and provide a mechanics-based predictive framework that can be extended to a myriad of other layered materials to enable improved design strategies.