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porousau_155Reversible strain by physisorption in nanoporous gold

E. Detsi, Z. G. Chen, W. P. Vellinga, P. R. Onck, and J. T. M. De Hosson, Applied Physics Letter, 99, 083104 (2011)


struteric_250Nanoporous metallic materials are promising materials for actuation, because they are susceptible to produce large strain in combination with high stiffness and strength, in contrast to polymer actuation materials that generate large strain but with low stiffness and strength, or to piezoceramic materials which are stiff and strong but with restricted strain amplitudes.  Nanoporous materials are characterized by a bicontinuous network of interconnected solid nanostructures (ligament size about 20 nm in accompanying figure) and nanochannels (pores). (Figure) In this structural configuration, the fraction of atoms at the ligaments-pores interface is significantly high compared to those in the ligaments bulk, meaning that nanoporous materials display high surface area-to-volume ratios. A direct consequence of the high surface area-to-volume ratio for nanoporous metals is the macroscopic change in volume in response to changes in their surface stress, i.e. the stress state at the ligaments-pores interface. These changes in the surface stress are usually driven by the electric charge transfer, either during ions electrosorption at the ligaments-electrolyte interface or during chemisorption of atoms at the ligaments-gas interface. bending

In contrast we have shown that it is possible to generate and control reversible macroscopic dimensional changes in nanoporous gold specimens exposed to ambient air by varying the water vapor content in air The accompanying figure shows bending of a bilayer gold foil (left ordinate, red curve) upon variation of the relative humidity (right ordinate, blue curve). The bilayer with a total thickness of 20 µm consists of a thick nanoporous gold layer and thin solid gold layer.

Due to the absence of chemical reactions during the process and the usage of a renewable energy that arises from the liquid to vapor phase transition in water, this concept could be attractive for short-stroke environmentally of friendly actuator and sensor technologies.