SEM of nanoporous gold structure. Credit: Helmholtz Association of German Research Centers.
Several recent, unrelated posts have been about materials with nanoscale porosity (diamond aerogels, chalcogenide PCMM). Here is another one, but perhaps this one will prove to be not so unrelated.
In the June 3 issue of Science, authors Jin and Weissmüller describe a nanostructured hybrid gold-perchloric acid electrolyte composite in which it was possible to alter the yield strength, flow stress and ductility of the composite quickly and repeatably (“A Material with Electrically Tunable Strength and Flow Stress,” doi: 10.1126/science.1202190)
A nanoporous gold skeleton with contiguous, interpenetrating porosity was made by dissolving silver out of a gold-silver alloy, and the porosity was infiltrated with perchloric acid (HClO4) electrolyte. An electrochemical potential was applied to the sample and mechanical properties were measured
The gold responded to the voltage in three regimes. Between 0-0.5 V, the surface accumulated a negative charge similar to a capacitor. Between 0.50-1.2 V, a layer of ClO4– anions adsorbed from the electrolyte to compensate for the positive charge in the surface. Finally, between 1.20-1.50 V, a monolayer oxide film formed.
The largest changes in mechanical properties were found between the anion adsorption region at about 1.0 V and the oxidation region at about 1.5 V where the measured flow stress increased by up to a factor of 2. Smaller increases of flow stress were observed when the applied voltage was pulsed from the 1.0 V adsorption region to the 0.1 V capacitive charge region, and flow stress was smallest in the 0.5 V region where charge approached zero.
In the same issue of Science (“Potential Solutions for Creating Responsive Materials,” doi: 10.1126/science.1206856), Sierdazki offered some comment about the work of Jin and Weissmüller, suggesting that the flow stress increased in the oxidation regime because of dislocation pinning by the oxide layer, or that the oxide layer may be creating a stiff layer that prevents dislocations from escaping from the surface. Other researchers have also observed correlations between mechanical properties and sample size effects that are related to a dearth of bulk dislocations in high surface-to-volume materials.
Sierdazki notes that this work may contribute to a better understanding of stress-corrosion cracking mechanisms in metal alloys, for example stainless steels, by showing how selective dissolution of an alloying element leaves behind a nanoporous structure that can form an oxide layer under the right conditions. The work also could explain the fatigue damage observed in rechargeable batteries associated with volume changes as the metal ions intercalate in and out of the electrode.
Previously, we reported on the first-time synthesis of diamond aerogel, and on the use of mesoporous silica as a scaffold for growth of chalcogenide phase-change memory materials. These nanoporous materials are effectively “bulk” surface materials, and these research programs are expanding the types of materials that can be synthesized and demonstrating ways of engineering the surfaces and properties of “monolithic” samples. If the properties of the nanoporous materials can be controlled or made into responsive surfaces as the Jin and Weissmüller work suggests, then it should be possible to design new materials and new configurations of devices that exploit surface properties like sensors, coatings, energy collectors, medical devices, lab-on-a-chip devices, etc.