CAREER: Understanding Kirkendall Pore Formation and Evolution: Correlating Compositional, Geometrical, and Thermal Influences

第一部分：非技术概述当你早上醒来闻到培根的气味或看到食品色素在水中扩散时，你是在欣赏扩散。一般说来，扩散是指物体从集中度较高的区域移动到集中度较低的区域。尽管在室温下扩散并不明显，但在我们周围的固体中也发生了扩散-只是速度比我们在空气中闻到的东西或看到水中的东西扩散时慢得多。由于某些类型的原子比其他类型的原子扩散速度更快，它们会留下空位，称为空位。如果一种材料中有足够多的空位彼此接近，它们就会合并并形成气孔。通常，科学家试图避免材料中的孔洞，因为它会降低材料的机械、电或热性能，特别是当气孔出现在表面附近时。有趣的是，20世纪40年代的S发现，固体中的扩散可以被仔细地控制，从而影响材料中孔洞的位置。这一发现被称为“柯肯德尔效应”，它可以被用来有意地调整材料中气孔的位置，使它们变得有益。例如，中空结构在从电池到生物医学植入物的各种应用中可以有几个优势。该项目支持基础研究，以确定成分、几何形状和温度如何影响空位的移动和这些“Kirkendall”气孔的演化。通过了解成分、几何形状和温度的影响，我们可以设计出更好的材料，如果对材料有害，则消除孔隙率，如果对特定应用有利，则使用它们来创建有用的结构。该项目还包括一个重要的教育组成部分，重点是让学生接触材料科学，并让他们参与到高中、本科和研究生水平。通过与该项目相结合的招聘和外联活动，将强调扩大传统上代表性不足的群体在STEM中的参与，以帮助在未来几年使工程职业人才库多样化。第2部分：技术概述这个职业项目旨在通过柯肯德尔效应进一步了解和控制空缺引起的移徙，同时参与多种教育和外联活动，对高中、本科和研究生水平的学生产生深远影响。总的研究目标是系统地研究组成、热和几何对空位迁移和Kirkendall孔演化的影响。具体地说，这三个研究目标是：(I)评估添加铬的效果；(Ii)评估径向对称性和纵横比的作用；(Iii)确定Ni-Cr-Al-Ti系中Kirkendall气孔合并所需的温度梯度的大小。这些目标是通过实验实现的，这些实验涉及通过传统的非原位金相和原位X射线层析技术能够实现的四维微结构表征的扩散偶和扩散涂层微物体的制造和分析。这项工作的更广泛影响是，通过将本项目涉及的中心概念纳入教育和外联活动，帮助培养一支多样化的、有STEM能力的劳动力队伍，以适应不同的职业道路，让学生接触材料科学。教育和外展活动围绕以下三个教育目标：(I)为高中生和教师扩大材料营(Ii)加强“如何制作”本科生荣誉研讨会课程，以及(Iii)创建创新的扩散模块，为材料科学研究生提供翻转课程。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。

PART 1: NON-TECHNICAL SUMMARY When you wake up in the morning to the smell of bacon or watch food coloring spread in water you are appreciating diffusion. In general terms, diffusion is the movement of things from areas of higher concentration to areas of lower concentration. Even though it is not obvious at room temperature, diffusion is happening in the solids around us as well - just at much slower speeds than what we witness when we smell something in the air or see something spread in water. Because certain types of atoms diffuse at higher rates than others, they can leave behind empty spaces called vacancies. If there are enough vacancies near one another in a material, they can merge and form pores. Typically, scientists try to avoid porosity in a material because it can decrease the mechanical, electrical or thermal properties especially when pores occur near a surface. Interestingly, it was discovered in the 1940's that diffusion in solids could be carefully controlled and as a result, influence where porosity in a material is located. This discovery is called the "Kirkendall effect" and it can be used to intentionally tailor the location of pores in a material so they become beneficial. Hollow structures for example can have several advantages in applications ranging from batteries to biomedical implants. This project supports fundamental research to determine how composition, geometry, and temperature influence the movement of vacancies and the evolution of these "Kirkendall" pores. By understanding the effect of composition, geometry and temperature we can design better materials that either eliminate porosity if harmful to the material or use them to create useful structures if advantageous for a given application. This project also includes a significant educational component that focuses on exposing students to, and engaging them in, materials science across high school, undergraduate and graduate levels. Through both recruiting and outreach activities integrated with this project, broadening participation of traditionally underrepresented groups in STEM will be emphasized to help diversify the engineering careers talent pool for years to come. PART 2: TECHNICAL SUMMARY This CAREER project aims to further understanding and control of vacancy-induced migration via the Kirkendall effect while engaging in multiple education and outreach activities to profoundly impact students in high school, undergraduate study and at the graduate level. The overall research goal is to systematically investigate the compositional, thermal and geometric influences on vacancy migration and Kirkendall pore evolution. Specifically, the three research objectives are to (i) evaluate the effect of Cr additions, (ii) assess the role of radial symmetry and aspect ratio and (iii) determine the magnitude of temperature gradient required for Kirkendall pore coalescence in Ni-Cr-Al-Ti based systems. These objectives are accomplished via experiments involving the fabrication and analysis of diffusion couples and diffusion coated micro-objects via conventional ex-situ metallography and 4-D microstructural characterization enabled by in-situ X-ray tomography. The broader impacts of this work are to help produce a diverse STEM-capable workforce for various career pathways by incorporating central concepts addressed in this project into education and outreach activities that expose and engage students in materials science. The education and outreach activities center on the following three education objectives: (i) expanding a Materials Camp for high school students and teachers (ii) enhancing a “How It’s Made” hands-on undergraduate honors seminar course, and (iii) creating innovative diffusion modules to enable flipped classes for materials science graduate students.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.