Characterization and Mechanical Behavior of Nanoporous Gold

Metallic foams with pore sizes from 250 lm to 2 mm have been a subject of research for over two decades due to their high surface area, which allows for a variety of applications, such as thermal and sound insulation. Recently, processing of metallic foams at the nanoscale (pores sizes less than 100 nm) has opened the door to new and interesting applications, such as sensors and actuators. In general, processing of nanoporous metal foams has been focused on selective dealloying techniques which produce materials with an open sponge-like structure of interconnecting ligaments and a typical pore size distribution on the nanometer length scale. Selective dealloying is defined as the selective dissolution of one or more components from a metallic alloy. Typically, the less noble components are removed, and the more noble components remain behind. This process requires a significant difference in the reversible metal/metal ion potential of the metals in the alloy. The morphology of dealloyed structures is of key importance in many engineering applications. While any alloy which meets the electrochemical criteria may be dealloyed, ideal bicontinuous porous structures are obtained from binary alloys with complete single phase solid solubility across all compositions. An ideal structure can be described as a uniform interpenetrating solid-void composite, with a narrow void/ligament size distribution. The most well known system which meets this criterion is Ag/Au. Au is relatively inert in electrolytes which can dissolve Ag, therefore the dissolution current is solely due to Ag dissolution. Currently, research of nanoporous metals has been focused on synthesis. However, in order to further study possible nanoporous foam applications, their mechanical behavior needs to be addressed. Few studies have been focused on macro-foam behavior, such as Li et al. who reported a ductile-brittle transition in nanoporous Au, which seemed to be controlled by the microstructural length scale of the material. Biener et al. reported on the fracture behavior of nanoporous Au as a function of the length scale. Recently, we studied the mechanical properties of nanoporous-Au under compressive stress by depth-sensing nanoindentation, and determined a yield strength of 145 (± 11) MPa and a Young’s modulus of 11.1 (± 0.9) GPa. A striking result of this study is that the experimentally determined value of the yield strength is almost one order of magnitude higher than the R ES EA R C H N EW S