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

硅是现代电子和微纳机电系统中最常用的材料。脆性断裂是发展实用于微纳米器件的可靠硅纳米结构的严重障碍。初步证据表明，在高温下，硅纳米线的脆性行为可以转变为韧性行为，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背后的变形机制的基本理解。这一发现将为高温下设计强韧的硅纳米结构提供力学基础，从而促进国家的健康、繁荣和福利。此外，该项目还将通过为高温下的纳米尺度研究开发新的实验和建模方法来促进纳米工程的进步。为了产生更广泛的影响，适当的研究课程将被整合到亚特兰大一所拥有大量少数族裔学生的高中的课程模块中，以及北卡罗来纳州立大学的本科课程中。此外，本科生将被招募进行高级研究。目前，对纳米硅(Si)在高温下的热机械行为严重缺乏基本知识和理解。本项目的目的是通过新的透射电子显微镜原位热机械实验和原子模拟来量化温度、应变速率和样品尺寸对硅纳米线的强度和脆塑性转变(BDT)的影响。为了理解BDT和相关的强度控制变形机制，本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硅纳米线的屈服/断裂强度，并基于威布尔统计分析数据；(Ii)获得硅纳米线的激活参数(包括激活能和激活体积)随温度、应变速率、样品尺寸、表面和内部结构的函数，并进行位错和断裂机制的原位电子显微镜表征；(Iii)进行分子动力学和原子反应路径模拟，通过将模拟结果与现场测量和透射电子显微镜表征相结合，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。

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.