Collaborative Research: Experimental and Computational Studies of Solid-State Diffusion and New Phase Formation in Bimetallic Nanostructures

原子的扩散运动可导致固体的物理性质随时间的变化，范围从简单的组成变化到新结构的形成和生长。随着样本量的减少，这些变化的影响(可能是有利的，也可能是不利的)在较低的温度、较短的时间段和较小的长度尺度下变得显著。因此，对这些过程的起源和结果的基本了解在纳米结构材料中尤为重要。本项目的研究部分包括在纳米尺度上对双金属样品中扩散原子运动的实验和计算研究。对实验和计算结果的分析将提供这些纳米结构在广泛的长度和时间尺度上扩散运动的详细微观图像，并最终将有助于具有有用性能的新材料的设计、合成和加工。该项目的教育和推广部分包括开发一门新的本科生/研究生水平的X射线衍射方法的实际应用课程，为本科生和研究生提供有意义的研究机会，以及通过参加特拉华州科学奥林匹克竞赛针对初中和高中学生的推广计划。虽然多组元块体和二维(2D)薄膜系统中的固态扩散(SSD)和新相形成(NPF)已经被研究了很多年，但对零维(0D)和一维(1D)扩散对中的SSD和NPF的了解要少得多。本项目的研究部分旨在通过对零维核/壳纳米颗粒和一维多层纳米线的结构演化进行系统的实验和计算研究来解决这一问题。特别是利用光泵/x射线探针超快时间分辨x射线衍射仪和常规高温x射线粉末衍射仪对化学制备的0D扩散偶和电化学制备的1D扩散偶的结构演化进行了实验研究。这些实验研究将得到0D和1D扩散耦合中SSD和NPF的计算研究的补充。研究项目的实验部分将包括使用内部设施和阿贡国家实验室先进光子源提供的设施进行的泵浦/探头TRX射线测量。在这两种情况下，都将使用钛蓝宝石激光来泵浦样品，泵送大约50飞秒的光脉冲，然后是x射线探测脉冲。光抽运导致相同温度的快速升高，通过相对于抽运脉冲延迟到达温度，可以获得时间分辨率为10‘S-100’S的飞秒的X射线衍射图，时间周期从10‘S到100’S纳秒。传统的高温粉末X射线衍射测量(CXRD)将允许在更长的时间内进行互补的结构测量(尽管时间分辨率有限)。该研究项目的计算部分将包括通过结合密度泛函理论计算、团簇展开方法和动力学蒙特卡罗模拟来模拟0D和1D扩散对中的SSD和NPF。这类计算研究特别重要，因为它们适用于连续体模型分解的短长度尺度。这些实验和计算研究将在很短和很长的时间尺度上提供0D和1D纳米结构中SSD和NPF的详细微观图像，最终将有助于具有有用性能的新材料的设计、合成和加工。

Non-Technical Summary The diffusive motions of atoms can lead to changes in the physical properties of solids over time that range from simple composition variations to the formation and growth of new structures. The effects of these changes (which can be advantageous or disadvantageous) become significant at lower temperatures, shorter time periods, and smaller length scales as the sample size is reduced. As a result, a fundamental understanding of the origins and consequences of these processes is particularly important in nanostructured materials.The research component of this project consists of an experimental and computational study of diffusive atomic motion in bimetallic samples at the nano-scale. The analysis of the experimental and computational results will provide a detailed microscopic picture of diffusive motion in these nanostructures over a wide range of length and time scales, and ultimately will contribute to the design, synthesis, and processing of new materials with useful properties.The educational and outreach component of the project consists of the development of a new undergraduate/graduate level course in the practical application of x-ray diffraction methods, providing meaningful research opportunities for undergraduate and graduate students, and an outreach program targeted at junior and senior level high school students by participating in the Delaware Science Olympiad. Technical SummaryWhile solid-state diffusion (SSD) and new phase formation (NPF) in multicomponent bulk and two-dimensional (2D) thin film systems has been studied for many years, much less is known about SSD and NPF in zero-dimensional (0D) and one-dimensional (1D) diffusion couples. The research component of this project has been designed to address this issue through a systematic experimental and computational study of the structural evolution in 0D core/shell nanoparticles and 1D multilayered nanowires. In particular, the structural evolution of chemically prepared 0D and electrochemically prepared 1D diffusion couples will be experimentally studied using both optical pump/x-ray probe ultrafast time-resolved x-ray diffraction (TRXRD) and conventional high temperature x-ray powder diffraction (cXRD) measurement techniques. These experimental studies will be complemented by computational studies of SSD and NPF in model 0D and 1D diffusion couples.The experimental component of the research project will consist of pump/probe TRXRD measurements carried out using in-house facilities and facilities available at the Advanced Photon Source at Argonne National Laboratory. In both cases a Ti:sapphire laser will be used to pump the sample with an approximately 50 femtosecond optical pulse followed by an x-ray probe pulse. The optical pump leads to a very rapid increase in the same temperature, and by delaying the arrival of the x-ray pulse relative to the pump pulse temperature depend x-ray diffraction patterns can be acquired with a time resolution of 10's - 100's of femtoseconds over time periods from 10's to 100's of nanoseconds. Conventional high temperature powder x-ray diffraction measurements (cXRD) will allow complementary structural measurements to be carried out over much longer time periods (although with limited time resolution). The computational component of the research project will consist of modeling SSD and NPF in 0D and 1D diffusion couples by combining density functional theory calculations with cluster expansion methods and kinetic Monte Carlo simulations. Computational studies of this kind are of particular importance because they are applicable at the short length scales over which continuum models break down. These experimental and computational studies will provide a detailed microscopic picture of SSD and NPF in 0D and 1D nanostructures over both very short and very long time scales, and ultimately will contribute to the design, synthesis, and processing of new materials with useful properties.