CAREER: A roadmap to atomically ordered complex materials via control of entropic mixing

非技术摘要原子排序晶体材料的能力是先进技术的核心。在20世纪80年代，发展了两元素材料中99%的原子有序，其中两种元素在其原子位置上几乎完全交替。这使得高频晶体管成为可能，从而导致了手机革命和高效太阳能电池板技术。理论预测吹捧了复杂(3+元素)材料的革命性新材料特性，这将使新器件在信息技术、太阳能电池、照明、微波通信、热电和电力电子中得到广泛应用。然而，实现这些性质所需的99%的原子有序仍然难以实现。这个项目的目标是系统地了解复杂材料中的基本有序化机制。这项研究将计算理论和实验结果结合起来，创建了一套标准，可以用来设计足够高的原子有序度(99%)的材料，以实现其内在性质。这项研究直接整合了教育活动，以影响STEM领域中代表性不足的少数民族、妇女和服务不足的农村社区，并确保本科生教育包括研究经验。这一基于导师的战略侧重于在阿拉巴马州和密西西比州农村地区代表性不足的社区提升科学、技术、工程和数学(STEM)教育工作者。K-12教育工作者可以接触到阿拉巴马州动态科学计划的大学教员和专家来计划课程和实验室课程，并通过阿拉巴马州数学、科学和技术暑期学院接受培训，以使他们的农村学区有资格获得计划/设备资金。本科生暑期研究人员来自当地历史悠久的黑人学院和大学、少数族裔服务机构和美国物理学会的本科生物理女性会议。本科生还将在每个学年担任研究助理。技术摘要在凝聚态物质和材料领域，复杂材料中不完美的原子有序是一个普遍存在的问题：在复合材料能够广泛应用之前，需要一个能够解释熵混合无序的模型。第一性原理计算可以准确地预测有序体系的本征(结构、电子、磁/磁动力学)材料性质，但需要近乎完美(99%)的原子有序才能体现这些性质。目前，在热能和生长能存在的情况下，确定系统是否能形成所需的原子有序化是一项复杂且计算昂贵的任务。这项研究的中心假设是，可以通过结合现有的冶金度量来预测原子的高有序性，这些度量表明了熵混合的水平。为了验证这一假设，对一系列三元素L21有序的Heusler合金进行了有序性和性质的计算预测，之所以选择这些合金，是因为几十年来在实现原子无序导致的高自旋极化预测方面遇到了挫折。尽管应用程序通常喜欢高度有序的系统，但选择具有一系列有序级别的材料是为了完善稳健的定量模型，并为材料设计提供路线图。每个材料系统的薄膜都是由首席研究员发明的溅射束外延方法以低能量生长的，因此每个系统的原子有序性和材料性质可以与外部贡献最小的预测进行比较。这些结果在理论和实验之间形成了一个反馈回路，以建立和完善当使用三个或更多元素时的原子有序性的定量模型。复杂合金中原子有序度的定量预测模型广泛适用于材料系统受到熵混合困扰的许多其他领域。该项目由电子和光子材料计划以及旨在促进竞争性研究的既定计划联合资助。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 AbstractThe ability to atomically order crystalline materials is central to advancing technology. In the 1980s, 99% atomic ordering in two-element materials was developed, wherein two elements alternate nearly perfectly in their atomic site occupations. This enabled high-frequency transistors, which led to the cell phone revolution and high-efficiency solar panel technologies. Theoretical predictions tout revolutionary new material properties in complex (3+ elements) materials that will make possible new devices with broad application in information technology, solar cells, lighting, microwave communications, thermoelectrics, and power electronics. However, achieving the 99% atomic ordering required to realize those properties has remained elusive. The goal of this project is to systematically gain an understanding of the fundamental ordering mechanisms in complex materials. This research integrates computational theory and experimental results to create a set of criteria that can be used to design materials of sufficiently high atomic ordering (99%) to realize their intrinsic properties. This research directly integrates educational activities to impact underrepresented minorities, women, and underserved rural communities in STEM fields, and ensure that undergraduate education includes research experience. This mentor-based strategy focuses on elevating science, technology, engineering and mathematics (STEM) educators in underrepresented communities in rural Alabama and Mississippi. K-12 educators gain access to university faculty and specialists at the Alabama Science in Motion program to plan classes and laboratory sessions, and through the Alabama Math, Science and Technology Summer Institute receive training to qualify their rural school district for program/equipment funding. Undergraduate summer researchers are recruited from local Historically Black Colleges and Universities, minority-serving institutions and the American Physical Society’s Conferences for Undergraduate Women in Physics. Undergraduate students will also work as research assistants during each school year.Technical AbstractImperfect atomic ordering in complex materials is a pervasive issue in the condensed matter and materials communities: A model that accounts for entropic mixing disorder is required before complex materials can be applied widely. First principles calculations can accurately predict the intrinsic (structural, electronics, magnetic/magnetodynamic) material properties of an ordered system, but near-perfect (99%) atomic ordering is needed to manifest those properties. At present, it is a complex and computationally expensive task to determine if the system can form with the required atomic ordering in the presence of thermal and growth energies. The central hypothesis of this research is that high atomic ordering can be predicted by incorporating existing metallurgical metrics that indicate the level of entropic mixing. To test the hypothesis, computational predictions of ordering and properties are produced for a range of three-element L21-ordered Heusler alloys, chosen specifically due to decades-long frustration in realizing predictions of high spin polarization due to atomic disorder. Although applications often prefer highly ordered systems, materials with a range of ordering levels are selected to refine a robust quantitative model and provide a roadmap for material design. Thin films of each material system are grown with low energetics by the Sputter Beam Epitaxy method invented by the principal investigator, such that atomic ordering and material properties of each system can be compared to predictions with minimal extrinsic contribution. The results form a feedback loop between theory and experiment to establish and refine a quantitative model of atomic ordering when three or more elements are used. A quantitative predictive model for atomic ordering in complex alloys broadly translates to the many other fields whose material systems are plagued by entropic mixing.This project is jointly funded by the Electronic and Photonic Materials Program and the Established Program to Stimulate Competitive Research.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.