Investigation of the Room Temperature Brittle-to-Ductile Transition of Single-Crystal Silicon at Sub-Micron Length Scale Using Accelerated Molecular Dynamics

众所周知，硅很脆，一受到冲击就会碎。由于这种脆性断裂，硅基结构不是机械加工的，而是以各种巧妙的方式制造的。然而，在超过临界温度（约600°C）时，硅就会变得具有延展性，也就是说，它可以塑性变形。最近的几个实验发现，亚微米硅结构即使在室温下也表现出塑性变形。虽然这种与尺寸相关的脆性到延性转变具有很大的潜力，可以提高硅基纳米技术的可靠性和可制造性，但我们目前对这种现象的理解仍然不完整。这项资助的目的是利用计算模型对单晶硅的大小相关的脆性到延性转变进行基本的原子水平的理解。这将通过原子模拟再现实验观测，然后分析模拟结果并构建预测多尺度模型来实现。在这项研究中获得的知识和理解将通过更好的设计和成本效益的制造工艺提高无处不在的微纳米机电系统的可靠性，这将对国民经济产生重大影响，因为全球纳米技术市场预计到2021年将达到905亿美元。这笔拨款还将分别用于辛辛那提大学的女性科学与工程暑期研究项目和合作项目，以吸引本科生参与研究。在本研究中，将进行加速分子动力学模拟，以揭示单晶硅在亚微米尺度下室温塑性变形的原子尺度机制。单缺口块和纳米线的计算模型将考虑分析各种关键因素的影响，如温度、尺寸、几何形状、加载速率、自由表面结构和氧化层。为了在几乎相同的实验条件下进行模拟，将采用一种称为超动力学的加速分子动力学模拟方法以及空间多尺度准连续体方法。这项研究的结果将有助于研究其他脆性材料，如蓝宝石和氧化锆，其可加工性也一直是一个持续的问题。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。

As is well-known, silicon is brittle so that it shatters on impact. Due to this brittle fracture, silicon-based structures are not machined, but fabricated in a variety of artful ways. Above a critical temperature though, at about 600 °C for bulk silicon, silicon becomes ductile, i.e., it can deform plastically. Several recent experiments have found that sub-micron silicon structures exhibit plastic deformation even at room temperature. While this size-dependent brittle-to-ductile transition has a strong potential to improve the reliability and manufacturability of silicon-based nanotechnology, our current understanding of the phenomenon remains incomplete. The objective of this grant is to achieve a fundamental atomic-level understanding of the size-dependent brittle-to-ductile transition of single-crystal silicon using computational modeling. This will be accomplished by reproducing the experimental observations with atomistic simulations and then analyzing the simulation results and constructing predictive multiscale models. The knowledge and understanding obtained in this research will improve the reliability of the ubiquitous micro- and nano-electro-mechanical systems through better designs as well as cost-efficient manufacturing processes, which will have significant impact on the national economy as the global nanotechnology market is estimated to reach $90.5 billion by 2021. This grant will also be used to engage undergraduates in research by leveraging the women in science and engineering summer research program and the co-op program, respectively, at the University of Cincinnati.In this study, accelerated molecular dynamics simulations will be performed to unveil the atomic-scale mechanisms responsible for the room-temperature plastic deformation of single-crystal silicon at sub-micrometer scale. Computational models of single-notched blocks and nanowires will be considered to analyze the effects of various key factors such as temperature, size, geometry, loading rate, and free surface structures and oxide layers. To carry out the simulations under near-identical experimental conditions, an accelerated molecular dynamics simulation method called hyperdynamics as well as a spatial multi-scale quasi-continuum method will be employed. The outcomes of this research will enable investigation of other brittle materials such as sapphire and zirconia whose machinability has also been an ongoing issue.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.