|Scanning electron microscopy image of a synthesized porous gold sample shows the sponge-like morphology of interconnecting ligaments after selective dealloying.|
Among the most interesting accomplishments of materials science is the design and synthesis of new materials with engineered properties. Cellular materials—in particular, nanoporous metal foam—are one example. Through a process of casting, powder metallurgy, and sputter deposition, cellular materials with large pores (200 μm to 2 mm) can be produced. However, creating nanoporous metal foam with pores less than 100 nm requires unconventional techniques such as selective dealloying to achieve homogeneous pores and pore size distribution.
Current research on nanoporous foam processing has focused on gold/silver alloys, and the challenges are to produce nanoporous foam of lower density from a range of metals and to understand the relationship of pore evolution and mechanical properties. Testing and characterizing the foam require new techniques as well as the development of foam scaling laws, which include factors such as nanoporosity and nanocrystallinity.
Lawrence Livermore’s long-term strategic plan aims for the development and understanding of nanoscale materials science and technologies to achieve programmatic missions. Processing and characterization of nanoporous structures (pore size 10–50 nm) are necessary for this research endeavor. Most of the research on nanoporous materials has focused on synthesis, with much less attention given to characterization and mechanical behavior. Our studies of the processing, characterization, and mechanical behavior of these materials will accelerate the development of technologies that use these materials. Specifically, sophisticated targets for high-energy-density experiments demand new classes of designer materials with low densities and thoroughly characterized mechanical behavior. High-energy-density science will benefit from this work because our research will provide the foundation for the fabrication of highly tailored and complex targets.
We have demonstrated for the first time that electrolytic dealloying leads to a nanoporous structure formed of nanocrystalline foam ligaments. In selective dealloying, the more electrochemically active element is dissolved, leaving behind a sponge-like morphology of interconnecting ligaments made from the less electrochemically active element (Figure 1). Using dealloying, we have prepared nanoporous gold (Au) samples by selective electrolytic dissolution of silver (Ag) from various Au/Ag alloys.
Contrary to our results, previous research on free-corrosion dealloying (electroless) indicated that the dealloying process would not change the crystal grain structure to nanocrystalline. This discrepancy could suggest that electrochemically driven dealloying produces a higher supersaturation of gold adatoms, which in turn should increase the nucleation rate. The actual evolving morphology should be strongly influenced by the mobility of vacancies and adatoms and, in particular, by the presence of nucleation sites and thus the development of a nanocrystalline structure.
Even though nanoporous metals have recently attracted considerable interest— fueled by potential sensor and actuator applications—very little is known about their mechanical properties. To elucidate the yield strength of nanocrystalline gold, we have conducted both compression and tensile studies with nanoindentation and bending tests. Based on foam scaling laws, our results suggest that the nanoporous nanocrystalline gold is a high-yield, highstrength material that approaches the intrinsic yield strength of gold.
It is also important to understand how the ligament diameters affect when the materials would fail. We have deduced that the failure of a few ligaments triggers brittle fracture of the crystal lattice network. Interestingly, the failure mechanism of the ligaments seems to change with the ligament diameters. Our microscopic characterization of fracture surfaces has shown that in nanoporous gold with a ligament diameter of ~100 nm, the ligaments fail by plastic flow and necking (elongation of filaments). On the other hand, failure by slip (atomic plane movement) was observed for ligaments with a diameter of ~1000 nm. Figure 2 shows the fracture characteristics of a heat-treated sample. The heat treatment increases the ligament diameter from ~100 nm to ~1 μm
|Figure 2. Scanning electron microscopy images show the fracture characteristics of a heattreated sample of foam with large pores (~1 μm). Cell collapse is seen in regions of compressive stress (c), and elongation of the cell structure is visible in regions of tensile stress (t). Higher magnification (inset) reveals plastic deformation of individual ligaments by slip (s).|
Prior to our discovery, the general belief in the field was that there was no recrystallization during dealloying, and therefore the nanoporous foam was composed of single-crystal ligaments. We have shown for the first time that nanoporous foam synthesized by dealloying (free-corrosion or electrochemically driven) is composed of nanocrystalline ligaments.
We also introduced a two-step dealloying/compaction process to produce nanocrystalline monolithic gold (Figure 3). The compacted nanocrystalline gold exhibits an average grain size of less than 10 nm and hardness values of up to 4.5 times higher than the values obtained from polycrystalline gold. This two-step process presents an alternative route to producing monolithic nanocrystalline metals.
J. Biener, et al., “Nanoporous Au—a High Yield Strength Material,” J. Appl. Phys. 97, 024301 (2005).
A.M. Hodge, et al., “Monolithic Nanocrystalline Au Fabricated by the Compaction of Nanoscale Foam,” J. Mater. Res. 20, 554 (2005).