Nanoporous Metals by Alloy Corrosion: Formation and Mechanical Properties
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by Alloy Corrosion: Formation and Mechanical Properties
Jörg Weissmüller, Roger C. Newman, Hai-Jun Jin, Andrea M. Hodge, and Jeffrey W. Kysar Abstract Nanoporous metals prepared by the corrosion of an alloy can take the form of monolithic, millimeter-sized bodies containing approximately 1015 nanoscale ligaments per cubic millimeter. The ligament size can reach down to the very limits of stability of nanoscale objects. The processes by which nanoporous metals are formed have continued to be fascinating, even though their study in relation to surface treatment, metal refinement, and failure mechanisms can be traced back to ancient times. In fact, the prospect of using alloy corrosion as a means of making nanomaterials for fundamental studies and functional applications has led to a revived interest in the process. The quite distinct mechanical properties of nanoporous metals are one of the focus points of this interest, as relevant studies probe the deformation behavior of crystals at the lower end of the size scale. Furthermore, the coupling of bulk stress and strain to the forces acting along the surface of nanoporous metals provide unique opportunities for controlling the mechanical behavior through external variables such as the electrical or chemical potentials.
Introduction Condensed matter may exhibit porosity in a variety of expressions.1 Some substances (e.g., zeolites) will spontaneously form crystalline structures with regular arrays of interstices. Others, such as bone, are formed by biological growth or by more or less random agglomeration of objects, either controlled by the kinetics of aggregation, as in granular matter, or by minimization of their energy (e.g., capillary energy), as in foams. There are many other processes that lead to the formation of porous matter, and expressions of porosity are ubiquitous in many branches of science, including hard and soft condensed matter sciences, materials sciences, and life sciences.
The class of porous materials that are of interest in the present article are formed by the combined processes of (1) the selective removal of atoms of one species from a solid solution—forming random porosity at an atomic scale, and (2) the rearrangement—at the scale of a few nanometers—of the resulting atomic-scale mixture of interstitials and atoms of the remaining species into two phases, solid and void, that are separated by an interface. Typically, the microstructural length scales associated with the two phases, pores and solid “ligaments,” are both on the order of few nanometers. Our interest in these objects relates directly to the formation mechanism: it is a particularly
MRS BULLETIN • VOLUME 34 • AUGUST 2009 • www.mrs.org/bulletin
well-defined example of the nonequilibrium, driven processes that are also relevant to current issues in fields such as materials behavior under irradiation, mechanical alloying, spinodal decomposition, and self-organization.2 The structure sizes of interest—nanometers—are large enough to suggest a description of these “nanoporous mate
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