Dealloyed nanoporous metals are made by the selective electrochemical dissolution of one component of a uniform solid solution alloy under conditions where the remaining alloy components may diffuse along the metal/electrolyte interface to re-form into a highly porous metal with pore and ligament sizes on the nanoscale. Applications for these new materials are emerging in (electro)catalysis, optical sensing, and actuation, and active research into their unusual mechanical and electronic properties are being actively investigated. Less well-studied are the kinetic processes that control the morphological and compositional evolution of nanoporous metals during the pattern forming instability of their creation, even though these kinetic processes control the ultimate shape, ligament crystal surface orientation, and the surface and bulk compositions of the final nanoporous metal. This program will make a detailed study of the fundamental kinetic processes that control the morphological evolution of nanoporous metals, that control their surface composition, that influence their processing/structure relationships, and that can be used to make new materials. The overall goal is to probe how far the morphological and compositional characteristics of nanoporous metals can be controlled.
NON-TECHNICAL SUMMARY: A challenge in the study of nanostructured materials is to make tangible quantities of materials that possess controlled structure at the near-atomic scale, and still possess the unusual and remarkable properties associated with nanometer size. Success at this kind of nanotechnology scale-up may translate to many disciplines, and help improve a range of technologies from energy to sensing to mechanical systems. This program will make an in-depth study of one class of bulk nanostructured materials, so-called nanoporous metals made by electrochemical dissolution of one or more elements from a multi-component alloy. These remarkable materials possess a contiguous network of pores whose diameters are only tens of atoms wide; they are being actively explored for a diverse range of applications from catalysis to solar energy. Understanding the fundamental physics and chemistry of how nanoporous metals form and how their microscopic shape can be changed will inform to what degree the properties of these materials can be controlled, and open up new methodologies to translate the properties of the nanoscale to the macroscale world. Along the way, pedagogical products will be developed and disseminated, including an introductory lecture course to explore the linkages between traditional metallurgy and a computer simulation code that can be used to probe nanomaterials structure evolution.
Dealloyed nanoporous metals are made by the selective electrochemical dissolution of one component of a uniform solid solution alloy under conditions where the remaining alloys components may diffuse along the metal/electrolyte interface to re-form into a highly porous metal with pore and ligament sizes on the nanoscale. Applications for these new materials are emerging in (electro)catalysis, optical sensing, actuation, structural materials and electronic materials, and active research into their unusual mechanical and electronic properties are being actively investigated. Less well-studied are the kinetic processes that control the morphological and compositional evolution of nanoporous metals during the pattern forming instability of their creation, even though these kinetic processes control the ultimate shape, ligament crystal surface orientation, and the surface and bulk compositions of the final nanoporous metal. In this study we made detailed studies of the fundamental kinetic processes that can be used to control ("tune") these characteristics. Examples of specific subjects, activities, and questions examined were: (a) morphological evolution of nanoporous metals, via experiment and computer simulations. We found that the morphology of these complex metals is best described via their topology, i.e., how many "holes" they have per unit volume. Detailed analysis of topological evolution explained why voids are sometimes seen in ligaments, and how the quick dance of atoms on the surface of nanostructured materials can guided to create core/shell and hollow particle materials; (b) we discovered that new nanoporous materials could be made out of refractory metals. These high melting point materials required dealloying at higher temperatures, and we developed new methods to make them using molten metals to dissolve out one component. Nanoporous Ta, useful for capacitors and electronic materials, was fabricated in this manner; (c) we examined "size-effects" of the development of porosity during dealloying in a simulation/experimental examination of dealloying of nanoparticles (which happens in many batteries). We elucidated the physics and chemistry that control porosity evolution as function of particle size, demostratig that porosity can't form below a particular particle size; (d) in collaboration, we used advanced transmission electron microscopy to develop techniques to find the atomic structure of nanoporous metals at unprecedented detail, literally finding the position of every atom within a representative volume of material. More broadly, examination of nanoporou metals by dealloying is a versatile framework with which to study surface science and atom motion on metals at the atomic level. And particularlly appealing, this knowledge can be translated and manifested to the scale of bulk materials, and almost immediately integrated into many important materials technologies. Many graudate and undergraduate students participated in this program, and software developed in this program to study nanoscale materials is being used internationally for research and education.
