In situ TEM mechanical molding of intermetallic nanowires

具有可控形状和高样品质量的纳米结构的大规模制造将对许多应用具有变革性，如传感、催化、等离子体和电子应用，但仍具有挑战性。最近，热机械成型--将大量原料压制在具有纳米孔径的模具上--已经显示出制造直径和长度可控的大阵列单晶纳米线的能力，因此对上面列出的许多应用具有广泛的意义。然而，这些纳米线是如何在热机械成型过程中形成的还不是很清楚，这严重限制了这项技术的广泛应用。本项目旨在通过使用原位透射电子显微镜(TEM)直接实时观察热机械成型过程中金属体系纳米线的形成，从原子尺度上了解成型过程。这些新知识将有助于改进热机械成型技术，在加工过程中提供基础科学，以便更好地选择控制这些已制造纳米线的尺寸和长宽比所需的加工条件，并适用于更广泛类别的可以热机械成型的材料，并允许更容易地生产大量纳米材料。此外，该项目将为本科生提供研究纳米级金属系统的机会，从而为他们在先进纳米制造领域的职业机会做好准备。为了教育和吸引公众参与纳米科学和制造，将与K-12学生和普通公众分享现场的TEM电影。技术摘要纳米级热机械模塑，其中大量原料被压在具有纳米级通道的模具上，熔化温度的一小部分，最近展示了生产大阵列有序相单晶纳米线的能力。本项目旨在通过使用各种金属间化合物和固溶体体系的非原位和原位的透射电子显微镜(TEM)的原子级结构表征来了解热机械成型的扩散过程。再结晶过程，即单晶纳米线从多晶块状原料中挤出的机制，将被详细研究，以解释通过降低吉布斯自由能而严格控制的扩散动力学，吉布斯自由能对金属间化合物或固溶体的化学计量比非常敏感。原位透射电子显微镜提供了扩散机制的实时信息，也可以证明排除常见的位错滑移和晶界运动。总而言之，该项目将提供关于金属间化合物和固溶体的纳米级固体扩散过程的原子尺度信息，这一过程目前在很大程度上是未被探索的。这种对受限通道中固体扩散的直接可视化也将有助于在纳米尺度上更好地理解蠕变行为。这一新知识将有助于开发热机械成型作为一种更简单和量身定做的纳米材料生产方法，用于广泛的技术应用。在外展方面，在这个项目下获得的现场瞬变电磁电影将被用作可视化工具，教育K-12学生和普通公众关于原子运动的基本方面及其与纳米制造的相关性。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.