On the topological, morphological, and microstructural characterization of nanoporous metals
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Introduction While the adage “form follows function” may serve as a fundamental architectural principle, the materials science community knows better. This paradigm of the Bauhaus design cannot be embraced for nanoporous metals produced by dealloying. Without a critical understanding of the nuanced structural characteristics of these unique materials, their optimization and exploitation for multifunctional applications cannot be adequately achieved. This is perhaps in contrast to other open-cellular structures, such as those fabricated using additive manufacturing techniques. Unlike those structures where structural elements are chosen and manipulated to achieve a targeted stiffness, specific weight, or strength, the resultant structure of dealloyed materials is dictated by their inherent dealloying and coarsening mechanisms. In this article, we review the state of the art in the structural characterization of nanoporous metals fabricated by dealloying processes, focusing largely on nanoporous gold (NPG), and explore the significance of various aspects of structure—that of structural geometry and of microstructure. The former can be considered from both morphological and topological points of view, while the latter explores the internal crystallography and defect structure. We do not explicitly explore the mechanisms of structural evolution, a topic
better explored by McCue et al. in this issue.1 We do, however, address the implications from structural characterization on revealing active deformation mechanisms. Although the structure–property relationships important to nanoporous metals span a broad spectrum of complex and interdependent functionalities (see the Introductory article in this issue),2 we use mechanical behavior as a framework to exemplify the significance of certain structural aspects. For more details on the mechanical behavior of nanoporous metals, we refer the reader to the Jin et al. article in this issue.3
Key challenges for structural characterization In order to establish adequate characteristics of structural geometry and microstructure of nanoporous metals, one must first understand the challenges unique to this material. The structural length scales of interest range from the crystalline defect structure at the smallest scales up to the structural geometry scale (e.g., the ligaments and pores). In NPG, these range from nanometers up to microns. Moreover, the concept of a representative volume element (RVE) establishes the smallest volume of material needed for investigation. An RVE is the smallest volume over which a measurement can be made that will yield a value representative of the whole;4 it is difficult to experimentally identify rigorously. It is possible
Erica T. Lilleodden, Helmholtz-Zentrum Geesthacht; and Hamburg University of Technology, Germany; [email protected] Peter W. Voorhees, Northwestern University, USA; [email protected] doi:10.1557/mrs.2017.303
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• VOLUME 43 • JANUARY 2018 • www.mrs.org/bulletin Materials Research Society Downloaded MRS fromBULLE
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