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43324-G10
Characterizing the Nanostructure and Mechanical Behavior of Nanoporous Noble Metals
Nanoporous noble metals possess chemical properties and high surface-to-volume ratios making them promising candidates for applications such as actuators, sensors, and catalysts. Exhibiting an interconnected, porous structure with ligament widths as low as 3-4 nm, these materials also facilitate investigation of the effects of nanoscale geometric confinement on the mechanical properties of metals. In the second year of this grant, we have optimized the conditions for fabricating thin film nanoporous gold (np-Au) and palladium (np-Pd), and have subjected them to stress measurements and in situ nanoindentation.
While initially problematic (thin film np-Au initially flaked off the substrate), a reliable method for producing thin films with mechanical integrity has been developed, yielding crack-free np-Au films that exhibit strong adhesion to Si substrates. An example is shown in the figure, where an as-dealloyed np-Au film exhibits a consistently open nanoporous structure, with no crack-like defects at grain boundaries.
Stress-temperature behavior of films with various thicknesses was measured during thermal cycling. Surprisingly, thinner films (75 nm) carried higher stresses than thicker films (300 nm), even though the ligament width (30 nm) was much smaller than film thickness. Typically, the smallest geometrical parameter determines the strength of small-scale metals. Using scaling laws for porous solids, equivalent strengths of 2.0 GPa or greater are calculated, suggesting that the strength of np-Au may approach the theoretical strength level.
This project funded by ACS PRF has had a strong and positive impact on the PI's research group. One graduate student is involved full-time with this project, which forms the basis of his dissertation, and he presented his results at the TMS Annual Meeting in February 2007. Overall, the project has resulted in six conference presentations, including an invited talk at the “Workshop on In Situ Methods in Nanomechanics”, held at LBNL in August 2007. This latter presentation discussed the collaborative work performed by the PI and colleagues at the National Center for Electron Microscopy (NCEM).
A series of snapshots taken from an in situ recording of nanoindentation in the JEOL 3010 TEM at NCEM is shown below. In this and other in situ indentation experiments, individual np-Au ligaments deform in a ductile manner, with dislocation activity apparent in the ligaments (examples indicated by white arrow in image b). In contrast, np-Au samples produced from alloy films with lower Au content exhibited mixed ductile and brittle behavior: although individual ligaments were ductile, deep indentation caused the compacted np-Au to cluster into regions, ≈100-200 nm in size, which broke apart from each other and slid past other clusters.
Preliminary work on the fabrication of np-Pd has already produced thin films with high porosity and excellent adhesion after dealloying. The films exhibit small pores with diameters on the order of 5 nm and do not exhibit grain boundary cracking. The PI has also performed hydrogen sensing experiments with np-Pd thin films, and their increased surface-to-volume ratio appears to improve their response time in comparison to blanket Pd thin films. During exposure to a flowing H2-N2 gas mixture, the np-Pd film quickly entered a compressive stress state, and each time the amount of H2 in the stream was changed, a new steady state stress was reached within 5 min. The blanket Pd film, on the other hand, did not reach a plateau stress within 10 min of changes in H2.
In summary, the second year of this project has seen advances in both processing and testing of nanoporous noble metals. Crack-free thin films of np-Au and np-Pd are now readily produced, and have been subjected to thermal cycling, in situ nanoindentation, and environmental testing. For comparison, nanoporous iridium will be fabricated next, and is expected to form even finer pores, in addition to the brittle behavior exhibited by bulk iridium. Comparing this new material to np-Au and np-Pd will result in a more complete understanding of the structure and mechanical properties of nanoporous noble metals.
