In situ TEM mechanical molding of intermetallic nanowires

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Non-technical SummaryLarge-scale manufacturing of nanostructures with controlled shapes and high sample quality will be transformative for many applications, such as sensing, catalysis, plasmonic and electronic applications, yet still challenging to achieve. Recently, thermomechanical molding, in which bulk feedstocks are pressed onto a mold with nano-sized pores, has shown the ability to fabricate large arrays of single-crystalline nanowires with well-controlled diameters and lengths, thus having broad implications for the many applications listed above. However, how these nanowires are formed during thermomechanical molding is not well understood, severely limiting the broad applicability of this technique. This project aims to fundamentally understand the molding process at the atomic scale by the use of in situ transmission electron microscopy (TEM) to directly observe in real-time the formation of nanowires of metallic systems during thermomechanical molding. Such new knowledge will help to improve the thermomechanical molding technique by providing the underlying science during processing in order to better select the processing conditions needed to control the dimensions and aspect ratios of these manufactured nanowires and be applicable to a broader class of materials that can be thermomechanically molded, and allow for more facile production of large quantities of nanoscale materials. Additionally, the project will provide research opportunities to undergraduate students to perform research in nanoscale metallic systems, thus preparing them for career opportunities in advanced nanomanufacturing. The in situ TEM movies will be shared with K-12 students and the general public in order to educate and engage the public in nano- science and manufacturing.Technical SummaryNanoscale thermomechanical molding, in which bulk feedstocks are pressed onto a mold with nanoscale channels at a fraction of the melting temperature, has recently demonstrated the capability to produce large arrays of single-crystalline nanowires of ordered phases. This project aims to understand the diffusion process of the thermomechanical molding by atomic scale structure characterization using transmission electron microscopy (TEM), both ex situ and in situ, with various intermetallic and solid-solution systems. The process of recrystallization, the mechanism in which a single-crystalline nanowire is extruded out from a polycrystalline bulk feedstock, will be examined in detail to explain diffusion dynamics that are tightly regulated by lowering of the Gibbs free energy, which depends quite sensitively to the stoichiometry of the intermetallics or solid solution. In situ TEM provides real-time information on the diffusion mechanism that could also demonstrate the exclusion of common dislocation slips and grain boundary movements. In summary, the project will provide atomic scale information on nanoscale solid diffusion processes of intermetallics and solid solutions, which is currently largely unexplored. Such direct visualization of solid diffusion in confined channels will also help to establish a better understanding of creep behaviors at the nanoscale. This new knowledge will help develop thermomechanically molding as a more facile and tailored production method of nanoscale materials for a broad range of technical applications. For outreach, the in situ TEM movies obtained under the project will be used as visual tools to educate K-12 students and general public about the fundamental aspects of atomic motions and their relevance to nanomanufacturing.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.