报告题目：The Effect of Microstructure in Understanding the Electronic Properties of Complex Materials
报告人：Prof. G. Jeffrey Snyder(Northwestern University,Materials Science and Engineering)
The influence of micro/nanostructure on electrical conductivity is a topic of great scientific interest and of particular technological importance to oxide electronics and thermoelectrics. Many doped oxides and organic conductors are thought to be hopping conductors due to their rapidly increasing electrical conductivity with temperature. This brings into question the usefulness of DFT band structures in predicting and explaining electronic properties as electronic bands should give rise to metallic like conduction in a heavily doped system, and the physics of strong electron correlations are often invoked. In clearly metallic systems the presence of ionized impurity scattering is often assumed when an increasing conductivity with temperature is observed.
In our work on complex thermoelectric materials we have found that grain boundaries are often responsible for these deviations from metallic behavior in heavily doped systems. In many oxides such as the perovskite SrTiO3, the grain boundaries can have such a dramatic effect as to change the material from a metal to an insulator. By analyzing single crystal or large grain material and combining results from Seebeck and Hall effect measurements which are much less influenced by grain boundary resistance we can separate the effect of the grains and the grain boundaries. We find for example that bulk degenerately doped SrTiO3 has electronic transport different from a traditional metal due to its unique electronic structure that makes a strongly warped Fermi surface that gives the density of states a 2D rather than 3D character. This leads to a T2 dependence of the resistivity without invoking strong correlation physics.
In the new high-efficiency Mg3Sb2-based thermoelectric material we find that grain boundaries dramatically increase the resistance at low temperatures. At the atomic level it is found that this is due to the presence of Mg vacancies in a <10nm thick region at the grain boundary that repels and/or traps charge carriers. but, once the role and mechanism of grain boundaries is recognized they can be engineered to greatly improve thermoelectric performance. in srtio3 wrapping the grain boundaries with graphene largely recovers single crystal type behavior while maintain the low thermal conductivity of a polycrystalline material. in mg3sb2-based thermoelectric materials growing grains and engineering them to be less resistive results in such high thermoelectric performance as to rival the state of the art thermoelectric material bi2te3.
G. Jeffrey Snyder obtained his B.S. degree in physics, chemistry and mathematics at Cornell University (1991) focusing on solid state chemistry which he continued during a two year stay at the Max Planck Institut FKF (Festk?rperrperforschung) in Stuttgart, Germany. He received his Ph.D. in applied physics from Stanford University (1997) where he studied magnetic and magneto-electrical transport properties of metallic perovskites as a Hertz Fellow. He was a Senior Member of the Technical Staff in the thermoelectrics group at NASA’s Jet Propulsion Laboratory for 9 years (1997-2006) and as a Faculty Associate in materials science at the California Institute of Technology (Caltech) 2006-2014 where he focused on thermoelectric materials and devices. His interests include the discovery of new Zintl phase thermoelectric materials and nanostructured thermoelectric composites using bulk processing, band structure engineering and thermoelectric performance optimization. Dr. Snyder has published over 400 articles, book chapters and patents. He served as treasurer of the international thermoelectric society and vice president of the international thermoelectric academy.
Dr. Snyder is one of the world’s most prominent and highly cited (Thomson Reuters 2016-8) scientists particularly in the rapidly growing field of thermoelectrics. His 2008 review article in Nature Materials, is used internationally to instruct many new students, and introduce the essentials of thermoelectricity to a multi-disciplinary audience. It is the most cited article in thermoelectrics in 2013.