The research objective of this award is to experimentally study, via nanomechanical tests, the effect of internal geometry on the mechanical properties of nanoporous metal foams. The ultimate goal is to provide understanding of the extent to which the mechanical properties can be enhanced via intelligent material design of nanoporous foams. Nanoporous metal foams consist of an interconnected network of nanosized struts and show great promise in a variety of applications including batteries, fuel cells, sensors, etc. due to their large surface to volume ratios. Due to their unique synthesis technique by dealloying that results in distinct geometrical structure with nanosized features, nanoporous metal foam properties cannot be simply inferred from the properties of bulk metal foams or other nanostructured materials. Successful completion of the proposed research will establish a deeper understanding of the structure-property relation of these novel materials.
The majority of the experimental work will be performed by graduate students who will gain invaluable experience in nanotechnology research. Through Research Experience for Undergraduates (REU) program the PI will attract undergraduate students to work on parts of this project. The PI will also work with local high-school teachers through the Georgia Intern-Fellowships for Teachers Program (GIFT) to introduce results from the proposed work into demos/experiments that introduce nanotechnology and energy concepts to pre-college students.
Nanoporous (NP) metals have attracted attention because they possess very high surface-to-volume ratios (good for catalysis or as electrodes for electrochemical energy storage devices for example) as well as extremely good mechanical properties (strength, conductivity, etc). It is this combination of properties as well as the fact that one can obtain a well integrated, interconnected structure that distinguishes them from other high surface systems. At the same time, there is an overlap of effects at the nanoscale that pose challenges in the analysis of these complex structures. Existing investigations have extended scaling laws that have been derived for porous materials at larger scales while simultaneously underestimating the solid mass fraction. This investigation synthesized two unique NP metal systems (nanograined NP platinum and nanotwinned, nanograined NP Copper), measured the relative density (solid mass fraction) and examined NP metal mechanical behavior through nanoindentation. This type of analysis enables one to examine the effect of the intricate structure on the overall mechanical properties of NP metals. It was found that the geometrical structure has much more pronounced role on affecting the NP metal foam mechanical behavior. In the case of NP metal stiffness, the presence of mass agglomeration can enhance stiffness by more than one order of magnitude. Yet, there are still effects that are due to the fact that the solid material is at the nanoscale. The hierarchical NP metals that have been synthesized have mechanical behavior significantly enhanced from that of bulk metals. The hierarchical strut synthesis points to a yet unexplored parameter in the optimization of mechanical properties of NP metals. This analysis is the first of its kind to demonstrate how profoundly the geometry can influence properties. This study also points to an unexplored regime of NP metals with strut diameters <15 nm that can have much different mechanical behavior due to a coupling of the surface and internal structure. Two publications are already the result of this study and two more are in preparation. This project funded the efforts of two Ph.D. graduate students (one a female PhD student) and contributed to their high skill training. During the Summer 2013, the PI leveraged funds from an NSF RET program and worked closely with a high school teacher from the Atlanta area. The teacher performed research on the NP Cu and is a co-author to an article in press at Applied Physics Letters. At the same time, the teacher worked closely with one of the PhD graduate students to build an open source 3D printer, the first of its kind to be given to Cross Keys High School in Atlanta. This helps the school build an Invention Studio around the printer to help high school students learn about concepts of engineering.