A. V. Hamza

Monolithic nanocrystalline Au fabricated by the compaction of nanoscale foam

We describe a two-step dealloying/compaction process to produce nanocrystalline Au. First, nanocrystalline/nanoporous Au foam is synthesized by electrochemically-driven dealloying. The resulting Au foams exhibit porosities of 60 and 70% with pore sizes of {approx} 40 and 100 nm, respectively, and a typical grain size of <50 nm. Second, the nanoporous foams are fully compacted to produce nanocrystalline monolithic Au. The compacted Au was characterized by TEM and X-ray diffraction and tested by depth-sensing nanoindentation. The compacted nanocrystalline Au exhibits an average grain size of <50 nm and hardness values ranging from 1.4 to 2.0 GPa, which are up to 4.5 times higher than the hardness values obtained from polycrystalline Au.

Negative strain rate sensitivity in ultrahigh-strength nanocrystalline tantalum

Nanocrystalline tantalum prepared through direct current magnetron sputtering exhibits a negative strain rate sensitivity behavior under nanoindentation; i.e., the hardness value of nanocrystalline tantalum decreases with increasing indentation loading rates. High resolution transmission electron microscopy reveals a beta- to alpha-phase transformation underneath the indents, suggesting that the pseudohexagonal to body-centered cubic structural transformation in nanocrystalline tantalum is likely pressure induced. Evidence of shear banding was documented along the edge of indents, consistent with the negative strain rate sensitivity observed.

Orientation-dependent hardness and strain rate sensitivity in nanotwin copper

We observe a strong twin-orientation dependent hardness and strain rate sensitivity (m) in nanotwin copper. A highest m value of 0.059 ± 0.004 and an activation volume (V) of ∼10b3 are measured when deformation is predominately vertical to twin boundaries (TBs), whereas a much smaller m and larger V are observed for the direction parallel to TBs. Dislocation density is found to have a stronger impact on m and V in nanotwin materials, compared to that in coarse-grained materials.

Synthesis of bi-modal nanoporous Cu, CuO and Cu2O monoliths with tailored porosity

Nanoporous structures are exceptionally useful in catalytic, sensing and mechanical applications. However, precise control over the structure and composition of the nanoporous material is critical for the material to behave as desired. We report here a new bottom-up synthesis technique termed filter-casting for the creation of large scale (>1 cm) nanoporous structures which provide this precise control. Cu, CuO and Cu2O and bi-modal macro/nanoporous Cu structures were created with this technique to demonstrate the range of materials and structures which can be formed into nanoporous monoliths. Homogeneous nanoporous monoliths are synthesized using nanoparticles, and bi-modal or higher-order porosities are achieved using a sacrificial polystyrene template. The higher-order pore size is determined by the polystyrene particle diameter, and the nanopore size is set by the diameter of the nanoparticles. Surface areas as high as 34 m2 g−1, and relative densities between 12 and 58%, have been achieved. Filter-casting is a powerful new method for directly synthesizing large nanoporous monoliths with predetermined composition, pore size and pore structure.

Incipient plasticity in metallic glass modulated nanolaminates

The plastic deformation in copper-zirconium nanocrystalline-amorphous nanolaminates is investigated by means of stress-relaxation experiments at a range of initial stress levels. Progressive multistep relaxation cycles reveal that the onset of plastic deformation occurs at a much lower stress level in nanocrystalline-amorphous nanolaminate than in crystalline-crystalline nanolaminates or other nanostructured materials. The derived activation volumes and strain rate sensitivities imply interfacial dislocation mechanisms, consistent with the observations from postmorterm transmission electron microscopy. This indicates that the crystalline-amorphous interfaces may be the preferred source for dislocation nucleation and/or emission.

Single crystal growth and formation of defects in deuterium-tritium layers for inertial confinement nuclear fusion

We identify vapor-etched grain boundary grooves on the solid-vapor interface as the main source of surface roughness in the deuterium-tritium (D–T) fuel layers, which are solidified and then cooled. Current inertial confinement fusion target designs impose stringent limits to the cross-sectional area and total volume of these grooves. Formation of these grain boundaries occurs over time scales of hours as the dislocation network anneals and is inevitable in a plastically deformed material. Therefore, either cooling on a much shorter time scale or a technique that requires no cooling after solidification should be used to minimize the roughness.

Light-ion-irradiation-induced thermal spikes in nanoporous silica

Improving mechanical properties of low-density nanoporous solids has been a long standing challenge. Here, we study how alpha particle bombardment and thermal annealing can be used to improve mechanical properties of nanoporous silica aerogels analysed by depth-sensing nanoindentation. Data suggest that light-ion irradiation creates non-melting thermal spikes in unconstrained nanoligaments of the aerogel, resulting in improved ligament connectivity.