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产生革命性的影响，如传感、催化、等离子体和电子应用，但仍然具有挑战性。最近，将大量原料压在具有纳米孔的模具上的热机械成型技术已经显示出能够制造直径和长度可控的大型单晶纳米线阵列的能力，从而对上述许多应用具有广泛的意义。然而，这些纳米线在热机械成型过程中是如何形成的尚不清楚，这严重限制了该技术的广泛适用性。本项目旨在从根本上了解原子尺度的成型过程，利用原位透射电子显微镜（TEM）直接实时观察金属体系在热机械成型过程中纳米线的形成过程。这些新知识将通过提供加工过程中的基础科学来帮助改进热机械成型技术，以便更好地选择所需的加工条件来控制这些制造的纳米线的尺寸和长宽比，并适用于更广泛的可被热机械成型的材料类别，并允许更容易地生产大量纳米级材料。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就业机会做好准备。现场TEM电影将与K-12学生和公众分享，以教育和吸引公众参与纳米科学和制造。纳米尺度的热机械成型，将大量的原料压在具有纳米尺度通道的模具上，温度只有熔化温度的一小部分，最近已经证明了生产有序相单晶纳米线的大阵列的能力。本项目旨在通过原子尺度的透射电子显微镜（TEM）来了解不同金属间化合物和固溶体系在非原位和原位下的热机械成型扩散过程。再结晶过程，单晶纳米线从多晶原料中挤出的机制，将被详细研究，以解释由吉布斯自由能的降低严格调节的扩散动力学，这对金属间化合物或固溶体的化学计量相当敏感。原位透射电镜提供了扩散机制的实时信息，也可以证明排除了常见的位错滑移和晶界运动。总之，该项目将提供金属间化合物和固溶体纳米级固体扩散过程的原子尺度信息，这是目前尚未开发的。这种固体在受限通道中扩散的直接可视化也将有助于更好地理解纳米尺度上的蠕变行为。这一新知识将有助于发展热机械成型作为一种更方便和定制的纳米材料生产方法，用于广泛的技术应用。为了推广，在该项目下获得的原位TEM电影将被用作视觉工具，教育K-12学生和公众关于原子运动的基本方面及其与纳米制造的相关性。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.