Reversible Strain in Porous Metals Charged in Electrolytes
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Reversible Strain in Porous Metals Charged in Electrolytes Dominik Kramer1, Raghavan Nadar Viswanath1, Smrutiranjan Parida1 and Jörg Weissmüller1,2 1 Institut für Nanotechnologie, Forschungszentrum Karlsruhe, Karlsruhe, Germany 2 Technische Physik, Universität des Saarlandes, Saarbrücken, Germany ABSTRACT Nanoporous metal samples with millimetre size were prepared either by compacting nanocrystalline powders or by dealloying, the dissolution of the less noble metal(s) of an alloy. The samples were immersed in an electrolyte, and their length was measured as a function of the applied potential in-situ in a dilatometer. The results obtained for nanocrystalline platinum, nanoporous gold and for gold platinum alloys show that the length varies in dependence of the surface charge. The strain amplitude of nanocrystalline platinum was 0.15%, and even larger strains have been measured using an Au-Pt alloy. This strain is comparable to commercial piezoceramics, but it is achieved using smaller voltages. The strain measured for nanoporous gold prepared by dealloying was smaller than that mainly due to the larger structure size (20 nm structure size compared to 6 nm Pt crystallite size), but in the case of gold, it was possible to prepare stable composite structures of a metal foil and of the nanoporous gold. If such a bimetallic foil is charged, it is found to bend. Due to the mechanical amplification of the contraction or expansion of the nanoporous part of the foil, it was possible to observe the effect of electric charges on the surface stress of metals directly with the naked eye for the first time. These results demonstrate that nanoporous metals might be useful for actuator applications and for the study of surface strain effects. Furthermore, they are the first realization of a general concept that suggests that most of the properties of conducting nanomaterials can be tuned by controlling the surface charge. INTRODUCTION Because of the high surface-to-volume ratio of nanomaterials, changes at the surface can contribute significantly to the materials overall properties. In the case of conducting substances, the surface charge density can be readily varied by applying a bias voltage relative to a counter electrode. If the nanostructures are charged in an electrolyte, considerable surface charge density variations can be obtained on the whole material, even if it has an arbitrary shape, e.g. a porous structure. Many properties depend on the charge density and therefore, it has been suggested that it should be possible to tune electron-density dependent structural and physical properties of conducting nanomaterials with their high surface-to-volume-ratio using an external voltage [1,2]. Examples of such tuneable properties are the lattice constant and the dimensions of porous samples with nanosized metal structures, which vary in response to changes in the surface stress of the porous bodies. The results summarized here are experimental evidence that the strain in a metal can indeed be reversibly changed by chargi
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