Year of Publication

2008

Degree Name

Doctor of Philosophy (PhD)

Document Type

Dissertation

College

Engineering

Department

Materials Science and Engineering

First Advisor

Dr. T. John Balk

Abstract

The structure of nanoporous gold (np-Au) provides a very limited volume for deformation to occur, and thus offers an opportunity to study the role of defects such as dislocations in nanoscale metal volumes. A practical goal is to understand mechanical properties of np-Au so that it can be can produced in stable form, for use in applications that require some mechanical integrity. Bulk and thin film np-Au have been fabricated and studied here.

Bulk np-Au was prepared by electrochemically dealloying Au-Ag alloys with 25 and 30 at.% Au. In the lower Au content material, cracks always formed during dealloying. When Au content increased to 30 at.% and a two-step electrochemical dealloying method was used (first using diluted electrolyte and then concentrated acid), bulk np-Au with no volume change and minimal cracking was successfully fabricated. Thermal and mechanical behavior of np-Au was studied by heat treatment and microindentation. During annealing in air, Ostwald ripening governed ligament coarsening, while annealing of ligaments in vacuum was more likely a sintering process.

Nanoporous Au thin films were produced by dealloying sputtered Au-Ag alloy films. Residual stresses in np-Au films were measured with wafer curvature. Similar to bulk materials, np-Au thin films made from 25 at.% Au alloy films exhibited extensive cracking during dealloying, whereas films from 30 at.% Au precursor alloys were completely crack-free. 25 at.% Au np-Au films carried almost no stress because of extensive cracking, whereas stress in 30 at.% Au np-Au films was up to ~230 MPa. Ligament coarsening followed a t1/8 time dependence for stress-free films, versus t1/4 in films under stress. It was proposed that bulk diffusion was responsible for formation of larger pits at grain centers during the incipient stages of dealloying.

In situ nanoindentation experiments inside the transmission electron microscope were performed to investigate deformation of np-Au films and dislocation motion within ligaments. Dislocations were generated easily and moved along ligament axes, after which they interacted with other dislocations in the nodes of the porous network. It was found that slower displacement rates caused load drops to occur at shorter distance intervals and longer time intervals.

Included in

Metallurgy Commons

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