Traveling solvent crystal growth of anisotropic Zintl thermoelectrics.

第一部分：非技术概述热电材料是用来将热能转化为电能的。它们的应用范围从工业废热回收到遥感，再到太空探索。高效热电材料需要同时表现出高电导率、大的塞贝克系数和低的热导率。这两种特性的结合非常难以实现。具有高度各向异性晶体结构的材料提供了一种潜在的提高热电效率的策略，因为它们的物理性质在沿不同的晶体方向测量时显示出不同的值。由固态和材料化学计划资助的这项研究的目标是开发一种强大且适用的方法来生长具有复杂、各向异性晶体结构的化合物的单晶。表征这些大晶体有助于我们理解材料的原子结构与各向异性的热和电子行为之间的基本联系，为提高热电效率提供了途径。这个跨学科和协作的项目利用了NSF支持的PARADIM材料创新平台的设施。有了这项由固态化学和材料化学资助的奖项，研究生和本科生将接受固态合成和材料物理方面的培训；首席研究人员还将这笔赠款的成果用于推广活动，如“向女孩介绍工程”日、“斯巴达夫人工程夏令营”和当地高中的课堂活动。第2部分：技术概述本奖项的研究是基于这样一种理解，即热电材料设计中最根本的冲突之一--同时需要高电子迁移率和费米能级附近的高状态密度--可以通过利用各向异性电子传输来规避。Zint1金属间化合物相具有广泛的结构多样性和优异的高温热电性能，是研究热各向异性和电子输运各向异性的重要研究对象。虽然理论研究预测在某些锌相中沿着共价键合的高电导率方向有显著的热电效率提高，但由于难以生长适合于输运测量的单晶，缺乏实验证实。有了固态材料化学计划的这笔赠款，首席研究员的目标是通过将流动溶剂浮动区(TSFZ)晶体生长技术应用于生长大尺寸的锌单晶，来弥合理论和实验之间的差距。TSFZ技术是一种结合了助熔剂生长和定向凝固的无坩埚方法，特别适合熔化转变不一致的耐火化合物。这是助熔剂生长的自然延伸-历史上成功的锌晶体生长方法-但它允许生长更大的晶体，适合于运输测量。锌锑化合物，即使在多晶态下也表现出很高的热电效率，是这项工作的重点。通过对生长晶体的表征，结合第一性原理的研究，探索了多阴离子的尺寸和输运之间的联系，重点是电子和声子速度和散射率的各向异性。该项目通过直接验证理论预测，将闭环方法应用于材料设计和选择，从而更全面地了解在广泛应用中使用的复杂半导体中的各向异性输运行为。与此同时，这项研究的结果也被用于推广活动，如“让女孩接触工程学”日、“斯巴达夫人工程学夏令营”以及当地高中的课堂活动。

Part 1: Non-Technical SummaryThermoelectric materials are used to convert heat into electricity. They have applications ranging from industrial waste-heat recovery, to remote sensing, to space exploration. High efficiency thermoelectric materials need to simultaneously exhibit high electrical conductivity, a large Seebeck coefficient and low thermal conductivity. This combination of properties is exceptionally difficult to achieve. Materials with highly anisotropic crystal structures offer a potential strategy to increase the thermoelectric efficiency, because their physical properties display different values when measured along different crystallographic directions. The goal of the research funded by this award from the Solid State and Materials Chemistry program is to develop a powerful and adaptable method for single crystal growth of compounds with complex, anisotropic crystal structures. Characterizing these large crystals advances our understanding of the fundamental connection between the atomic structure of materials and the anisotropic thermal and electronic behavior, providing routes to enhanced thermoelectric efficiency. This interdisciplinary and collaborative project utilizes facilities at the NSF-supported PARADIM Materials Innovation Platform. With this Solid State and Materials Chemistry funded award graduate and undergraduate students are trained in solid state synthesis and physics of materials; the principal investigator also leverages results from this grant for outreach activities such as the 'Introduce a Girl to Engineering' day, the 'Lady Spartans Engineering Summer Camp' and in-class activities at local high schools. Part 2: Technical SummaryThe research for this award is based on the understanding that one of the most fundamental conflicts in the design of thermoelectric materials - the need for simultaneous high electronic mobility and a high density of states near the Fermi level - can be circumvented by exploiting anisotropic electronic transport. Zintl intermetallic phases, with their vast structural variety and excellent high-temperature thermoelectric performance, stand out as an intriguing subject area for the study of thermal and electronic transport anisotropy. Although theoretical studies predict significant thermoelectric efficiency gains along the covalently-bonded, high-conductivity directions in some Zintl phases, experimental confirmation is lacking due to the difficulty of growing single crystals suitable for transport measurements. With this grant from the Solid State Materials Chemistry program the principal investigator aims to bridge this gap between theory and experiment by adapting the traveling-solvent floating-zone (TSFZ) crystal growth technique to the growth of large Zintl single crystals. The TSFZ technique is a crucible-less method that combines flux growth and directional solidification and is particularly well-suited for refractory compounds with incongruent melting transitions. It is a natural extension of flux growth - a historically successful method for the growth of Zintl crystals - but it allows for the growth of larger crystals suitable for transport measurements. Zintl antimonides, which exhibit high thermoelectric efficiency even in polycrystalline form, are the focus of this work. Characterization of the grown crystals combined with first principles investigations are used to explore the link between polyanion dimensionality and transport, with an emphasis on the anisotropy of electron and phonon velocities and scattering rates. This project applies a closed-loop approach to material design and selection by directly validating theoretical predictions, thus leading to a more complete picture of anisotropic transport behavior in complex semiconductors used in a broad range of applications. At the same time results from this research are leveraged for outreach activities such as the 'Introduce a Girl to Engineering' day, the 'Lady Spartans Engineering Summer Camp' and in-class activities at local high schools.