Collaborative Research: Brittle-to-Ductile Transition and Strength of Silicon Nanowires at Elevated Temperatures

硅是现代电子和微纳米机电系统中最常用的材料。脆性断裂是开发可靠的硅纳米结构用于微纳米器件的一个严重障碍。初步证据表明，在高温下，硅纳米线的脆性到延展性行为转变是可能的，这为更可靠的应用带来了希望。这项研究将促进对高温下这种转变的变形机制的基本理解。这一发现将为高温下强韧性硅纳米结构的设计提供力学基础，从而促进国家健康、繁荣和福利。此外，该项目将通过开发高温纳米尺度研究的新实验和建模方法，促进纳米工程的进展。为了产生更广泛的影响，研究中的适当课程将被整合到亚特兰大一所拥有大量少数民族学生的高中的课程模块中，以及北卡罗莱纳州立大学的本科课程中。此外，将招收本科生进行高级研究。目前，人们对纳米级硅（Si）在高温下的热力学行为缺乏基本的认识和理解。本项目的目的是量化温度、应变率和样品尺寸对Si纳米线强度和脆性到延性转变（BDT）的影响，借助透射电子显微镜（TEM）和原子模型的新型原位热力学实验。为了了解BDT及其相关的强度控制变形机制，研究涉及三个紧密耦合的推力：(i)测量和计算不同温度、应变速率和样品尺寸下Si纳米线的屈服/断裂强度，并基于Weibull统计分析数据；（ii）获得Si纳米线的活化参数（包括活化能和活化体积）与温度、应变速率、样品尺寸、表面和内部结构的关系，并对位错和断裂机制进行原位TEM表征；（iii）进行分子动力学和原子反应途径建模，通过将建模结果与原位测量和TEM表征相结合，阐明控制强度和BDT的限速位错机制。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。

Silicon is the most commonly used material in the modern electronic and micro/nano electro-mechanical systems. Brittle fracture is a serious roadblock to the development of reliable silicon nanostructures for practical use in micro/nano devices. Initial evidence suggests that at elevated temperatures, brittle to ductile behavior transition is possible in silicon nanowires, which presents hope for more reliable applications. This research will advance the fundamental understanding of the deformation mechanisms underlying such transition at elevated temperatures. The findings will provide the mechanical basis for the design of strong and ductile silicon nanostructures at elevated temperatures, thus advancing national health, prosperity, and welfare. In addition, the project will promote the progress of nanoengineering by developing novel experimental and modeling methods for nanoscale research at elevated temperatures. For broader impact, appropriate lessons from research will be integrated into a course module for an Atlanta high school with a large minority student body as well as in an undergraduate course at North Carolina State University. Moreover, undergraduate students will be recruited to perform advanced research.There is currently a critical lack of fundamental knowledge and understanding of the thermomechanical behavior of nanoscale silicon (Si) at elevated temperatures. The objective of this project is to quantify the temperature, strain rate, and sample size effects on the strength and brittle-to-ductile transition (BDT) in Si nanowires, with the help of novel in-situ thermomechanical experimentation in transmission electron microscopy (TEM) and atomistic modeling. To understand BDT and associated strength-controlling deformation mechanisms, the research involves three tightly coupled thrusts: (i) to measure and calculate the yield/fracture strengths of Si nanowires at different temperatures, strain rates and sample sizes and analyze the data based on the Weibull statistics; (ii) to obtain activation parameters (including activation energy and activation volume) of Si nanowires as functions of temperature, strain rate, sample size, surface and internal structures, and to perform in-situ TEM characterization of dislocation and fracture mechanisms; (iii) to conduct the molecular dynamics and atomistic reaction pathway modeling to elucidate the rate-limiting dislocation mechanisms that control the strength and BDT by coupling modeling results with in-situ measurements and TEM characterization.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.