T. John Balk

Mechanical properties of bulk single crystalline nanoporous gold investigated by millimetre-scale tension and compression testing

In this work, the mechanical behaviour of millimetre-scale, bulk single crystalline, nanoporous gold at room temperature is reported for the first time. Tension and compression tests were performed with a custom-designed test system that accommodates small-scale samples. The absence of grain boundaries in the specimens allowed measurement of the inherent strength of millimetre-scale nanoporous gold in tension. The elastic modulus and strength values in tension and compression were found to be significantly lower than values measured with nanoindentation-based techniques and previously reported in the literature, but close to those reported for millimetre-scale polycrystalline samples tested using traditional compression techniques. Fracture toughness was found to be very low, in agreement with the macroscopic brittleness of nanoporous gold, but this is due to the localization of deformation to a narrow zone of ligaments, which individually exhibit significant plasticity and necking.

In Situ Indentation of Nanoporous Gold Thin Films in the Transmission Electron Microscope

The mechanical behavior of nanoporous gold was investigated during in situ nanoindentation in the transmission electron microscope. Thin films of nanoporous gold, with ligaments and pores of the order of 10‐nm diameter, offer a highly constrained geometry for deformation and thus provide an opportunity to study the role of defects such as dislocations in the plasticity of nanomaterials. Films ranging in thickness from 75 to 300 nm were indented, while the motion of dislocations and deformation of ligaments were observed in situ. Dislocations were generated and moved along ligament axes, after which they interacted with other dislocations in the nodes of the porous network. For thicker films, the load‐displacement curves exhibited load drops at regular intervals. The question of whether the spacing of these load drops was related to the collapse of pores in the nanoporous films or due to bursts of plasticity within the ligaments was investigated. Additionally, the effect of the indenter displacement rate on the mechanical response of these gold films with nanoscale porosity was investigated. Indentation rates were varied from 1.5 to 30 nm/s. There appears to be a kinetic factor related to dislocation nucleation, where slower displacement rates cause load drops to occur at shorter distance intervals and over longer time intervals. Microsc. Res. Tech., 2009.

Developing scaling relations for the yield strength of nanoporous gold

In this work, the applicability of Gibson and Ashby’s porous scaling relations to nanoporous metals is discussed, and an updated equation is proposed for relating the yield strength of nanoporous gold to the yield strength of individual gold ligaments that form the porous structure. This new relation is derived from experimental measurements obtained by small-scale tensile testing and by nanoindentation, and incorporates the average ligament diameter. Nanoindentation data, obtained experimentally by the authors as well as reported by others in the literature, are reconciled with tensile test measurements previously reported by the present authors. The values of ligament yield strength calculated with the new scaling relation are found to agree with data reported from mechanical testing of nanowires, and the scaling relation thus represents a bridge between nanowire and nanoporous metal behaviour. In addition, calculations of yield strength for nanoporous gold samples with various ligament size and relative density are consistent with the experimentally determined values.

Magnesium Alloy Precursor Thin Films for Efficient, Practical Fabrication of Nanoporous Metals

An improved approach to fabrication of nanoporous (np) metals is demonstrated for several metallic systems that were successfully created by dealloying magnesium-based precursor alloys (also containing iridium, nickel, gold, or osmium-ruthenium). A significant advantage is that magnesium alloys can be dealloyed effectively using water or, if needed, dilute acetic acid. The crystal structures of magnesium-based precursor films were significantly different from those of alloys commonly used as precursors. This approach should be generally applicable to np metal synthesis.

The mechanical response of core-shell structures for nanoporous metallic materials

Nanoporous gold (NP-Au) exhibits microscale plasticity, but macroscopically fails in a relatively brittle manner. This current study suggests that a core-shell structure can increase both ductility and strength of NP-Au. A core Au foam structure was created using conventional dealloying methods with average ligament size of 60 nm. Nickel was then electroplated on to the NP-Au with layer thicknesses ranging from 2.5 nm to 25 nm. Nanoindentation demonstrated a significant increase in the hardness of the coated Np-Au, to about five times of that of the pure Np-Au, and a decrease in creep by increasing the thickness of the coated Ni layer. Molecular dynamics simulations of Au–Ni ligaments show the same trend of strengthening behavior with increasing Ni thickness suggesting that the strengthening mechanisms of the Np-Au are comparable to those for fcc nano ligaments. The simulations demonstrate two different strengthening mechanisms with the increased activity of the twins in plated Au–Ni ligaments, which leads to more ductile behavior, as opposing to the monolithic Au ligaments where nucleation of dislocations govern the plasticity during loading.

Controlled ligament coarsening in nanoporous gold by annealing in vacuum versus nitrogen

The structural evolution of nanoporous gold during thermal treatment was studied by annealing samples in vacuum and in flowing nitrogen. As expected, ligament thickness generally increased in both environments. However, ligaments annealed at high temperature in vacuum remained relatively narrow, undergoing much less coarsening than nitrogen-annealed samples, albeit with some ligament agglomeration. When annealed in flowing nitrogen, gold ligaments coarsened significantly at temperatures above 300 °C. This discrepancy is attributed to different surface diffusion rates of gold in the two annealing environments. The current results suggest that diffusion on the surfaces of nanoporous gold ligaments proceeds more quickly in nitrogen than in vacuum.

Uniaxial compression testing of bulk nanoporous gold

Abstract The mechanical properties of bulk nanoporous gold (np–Au) with a relative density of 0.35 were investigated by compression testing of millimetre-scale specimens. Young’s modulus, Poisson’s ratio and yield strength values were determined from uniaxial, quasi-static experiments using a custom-built mechanical testing system. The cuboid-shaped specimens were fabricated following a specific, controlled process (including cutting and grinding) that guaranteed a precise and repeatable geometry. The np–Au structure was created from a silver–gold alloy by electrochemical dealloying in nitric acid. Mechanical properties obtained from compression testing are compared to values reported in the literature and to scaling relations. Values are found to agree with a recently proposed scaling relation for the yield strength of np–Au that incorporates a ligament size effect and a modified scaling exponent.

Synthesis of nanoporous nickel thin films from various precursors

Multiple approaches to fabricate nanoporous nickel (np-Ni) thin films from NiCu, NiAl, NiFe and NiMg precursors are presented. Deposition of precursor films, dealloying and characterization of the nanoporous films by electron microscopy are discussed. All precursor alloys can yield np-Ni. However, dealloying of NiMg precursor films yields the most consistent, open-porosity structure with a ligament size of 7 nm, comparable to that of Raney nickel.

Observations on cutting edge radius effects in cryogenic machining of porous tungsten

Porous tungsten is a refractory metal commonly used to manufacture dispenser cathode pellets. To produce the required geometric shape of a cathode, precision machining of the emitting surface is necessary. Because dry machining leads to excessive tool wear and smearing of surface pores, a plastic infiltrant is used to both prevent smearing and lubricate the cut. In order to develop a sustainable alternative to this plastic infiltration process, infiltrant-free cryogenic machining of porous tungsten has been identified by previous studies to be capable of producing excellent levels of surface porosity [1-3]. Cryogenic cooling during machining of porous tungsten allows for controlled brittle fracture machining, leading to relatively poor surface roughness [3]. Consequently, improving the surface quality of cryogenically machined porous tungsten surfaces is a necessary condition for the successful implementation of this technology.

Composition and work function relationship in Os–Ru–W ternary alloys

Os–Ru thin films with varying concentrations of W were sputter deposited in order to investigate their structure–property relationships. The films were analyzed with x-ray diffraction to investigate their crystal structures, and a Kelvin probe to investigate their work functions. An Os–Ru–W film with ∼30 at. % W yielded a work function maximum of approximately 5.38 eV. These results align well with other studies that found work function minima from thermionic emission data on M-type cathodes with varying amounts of W in the coatings. Furthermore, the results are consistent with other work explaining energy-level alignment and charge transfer of molecules on metal oxides. This may shed light on the mechanism behind the “anomalous effect” first reported by Zalm et al., whereby a high work function coating results in a low work function for emitting cathode surfaces. An important implication of this work is the potential for the Kelvin probe to evaluate the effectiveness of dispenser cathode coatings.