Traveling solvent crystal growth of anisotropic Zintl thermoelectrics.

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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.