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

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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.