| NSF Org: |
CMMI Division of Civil, Mechanical, and Manufacturing Innovation |
| Recipient: |
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| Initial Amendment Date: | August 22, 2018 |
| Latest Amendment Date: | August 22, 2018 |
| Award Number: | 1762463 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Siddiq Qidwai
sqidwai@nsf.gov (703)292-2211 CMMI Division of Civil, Mechanical, and Manufacturing Innovation ENG Directorate for Engineering |
| Start Date: | September 1, 2018 |
| End Date: | August 31, 2022 (Estimated) |
| Total Intended Award Amount: | $274,503.00 |
| Total Awarded Amount to Date: | $274,503.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
926 DALNEY ST NW ATLANTA GA US 30318-6395 (404)894-4819 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
225 North Avenue, NW Atlanta GA US 30332-0420 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Mechanics of Materials and Str |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.041 |
ABSTRACT
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.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Silicon (Si) is the most commonly used material in the modern electronic and micro/nano electro-mechanical systems. Bulk Si is brittle at room temperature and can become ductile at elevated temperature. However, understanding of the brittle-to-ductile transition in Si, especially, at the nanoscale remains elusive. The outcomes of this research project, obtained through collaboration with Professor Yong Zhu at North Carolina State University, include quantitative characterization and mechanistic insights into brittle to ductile transition in Si nanostructures. We conducted in situ temperature-controlled nanomechanical testing inside a transmission electron microscope (TEM) and tested 78 Si nanowires under tension in the temperature range of room temperature and 600 K. We obtained the size- and temperature-dependent mechanical properties of Si nanowires and further constructed a brittle-to-ductile-transition map in the parameter space of temperature and nanowire diameter. We performed molecular dynamics simulations of Si nanowire deformation as well as atomistic calculations of the energy barriers that control the stress-assisted, thermally activated processes of dislocation nucleation using the free-end nudged elastic band method. A key research outcome includes the novel mechanism of surface nucleation of 1/2⟨110⟩{001} dislocations in Si nanostructures at elevated temperatures, which contrasts with the prevalent activities of the conventional type of 1/2⟨110⟩{111} dislocations in bulk Si at room temperature. As a result, the 1/2⟨110⟩{001} dislocations can become highly active with increasing temperature and thus play a critical role in the formation of deformation bands, leading to a transition from brittle fracture to dislocation-mediated failure in Si nanowires at elevated temperatures. This finding provides mechanistic insights, namely, promoting 1/2⟨110⟩{001} dislocations through tuning the sample size, composition and temperature, for enhancing the ductility of covalently bonded nanostructures such as Si, Ge, C and their alloys, which are critical for micro/nano-electrical-mechanical system applications in the future. Opportunities for learning through research have been provided to the graduate student supported by this project at Georgia Tech. Results from this project have been integrated into the graduate courses taught at Georgia Tech.
Last Modified: 07/12/2023
Modified by: Ting Zhu
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