Luo, Junhang
(2011)
In-Situ TEM Study on the Mechanical Behavior of Nanowires.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
One-dimensional nanomaterials have qualitatively different mechanical behavior comparing with their bulk form owing to their small length scale and huge surface area1. To predict the mechanical properties and deformation behaviors of material in nanometer length scale and disclose the deformation mechanisms of them, plenty of computational simulations have been conducted. However, due to the sample mounting difficulty and their quite small volume, it is very tough to perform high-quality mechanical testing and validate the predictions from computational simulations.
Using the unique Nanofactory probing systems, in-situ mechanical tests combined with observations by transmission electron microscopy (TEM) with atomic resolution have been performed successfully on one-dimensional nanomaterials such as silver (Ag) nanowries, silica (SiO2) nanowires, nanoscale Al90Fe5Ce5 metallic glass and sodium chloride (NaCl) nanowires. 19.3% strain was achieved in the bicystalline Ag nanowires. Stacking faults formed on the (111) plane and interestingly, the stacking fault (local hexagonal close-packed (hcp) structure) was not induced by partial dislocations movement, but by the Frank loops formation and expansion.
SiO2 glass at room temperature is usually brittle due to fracture instability. However, showered by electron beam, silica nanowires with big diameters (>100 nm) can flow superplastically more than 670%. But once beam is blanked more than 2 minutes, the mechanical response can recover back to brittle failure if silica nanowire’s diameter is large than 20 nm. However, unrecovered beam damage will trigger the brittle to ductile transition if silica nanowire’s diameter is less than 20 nm.
Al90Fe5Ce5 metallic glass with size less than 20 nm can be super plastic deformed with elongation ~200%. Necking occurred without shear bands in the nanoscale sample with an area reduction nearly 100%. Surprisingly, it is first time to see atomic chain formation in metallic glasses. Fast diffusion of surface atom and no chance to form shear band is thought to attributed to such extraordinary ductility.
The mechanical test on common salt shows very interesting results. NaCl nanowires can be formed by touching sharp probe with NaCl surface in the transmission electron microscope and deform superplastically. The nanowires can be stretched to 280% and be very flexible under compression(can be bent over 90°) under the electron beam irradiation. During the elongation process, there were no dislocations observed due to the fast diffusion.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
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| Date: |
30 June 2011 |
| Date Type: |
Completion |
| Defense Date: |
14 December 2009 |
| Approval Date: |
30 June 2011 |
| Submission Date: |
16 March 2010 |
| Access Restriction: |
5 year -- Restrict access to University of Pittsburgh for a period of 5 years. |
| Number of Pages: |
143 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Mechanical Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
In-situ; mechanical testing; deformation; nanowires |
| Other ID: |
http://etd.library.pitt.edu/ETD/available/etd-03162010-105547/, etd-03162010-105547 |
| Date Deposited: |
10 Nov 2011 19:32 |
| Last Modified: |
15 Nov 2016 13:37 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/6514 |
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