Abstract
Many new high efficiency thermoelectric materials have nanoscale grains or are multi-phase composites at the nanometer scale.  These include materials such as PbTe alloys, and Zn4Sb3, which can contain small ~10nm nanoparticles that can be difficult to control.  Typically the goal is to reduce the lattice thermal conductivity of the matrix by scattering phonons at the interfaces of the particles.  In principle it is not the small size of the particles but the spacing between particles that matter small nanoparticles widely spaced may not be effective while large nanoparticles with high surface area can be quite effective at scattering phonons. These microstructures can be engineered using well understood metallurgical principles for the formation and control of microstructure [1]
The typical strategy to engineer grain boundary scattering is simply to reduce the grain size. However a growing body of experimental and computational evidence shows that not all grain boundaries are the same at scattering phonons [2]. Dramatic improvements in zT has been observed in Bi2Te3 and PbSe based materials with dislocations. Dislocation strain appears to be effective at scattering phonons without scattering electrons. 2-D materials such as graphene, wrapped around individual grains alter the grain boundary structure or complexion. Such altered grain boundary complexions in CoSb3 skutterudites for example have decreased thermal conductivity without harming electron mobility and lead to improved thermoelectric generators [3].
References
[1] Nicholas A. Heinz, Teruyuki Ikeda, Yanzhong Pei and G. Jeffrey Snyder "Applying quantitative microstructure control in advanced functional composites" Advanced Functional Materials 24, 2135 (2014)
[2] Hyun-Sik Kim, Stephen D. Kang, Yinglu Tang, Riley Hanus, G. J. Snyder “Dislocation strain as the mechanism of phonon scattering at grain boundaries” Materials Horizons  3, 234 (2016)
[3] Peng-an Zong, Riley Hanus, Maxwell Dylla, Yunshan Tang, Jingcheng Liao, Qihao Zhang, G Jeffrey Snyder, Lidong Chen “Skutterudite with Graphene-modified Grain-boundary Complexion Enhances zT Enabling High-efficiency Thermoelectric Device” Energy Environ. Sci. 10, 183 (2017)

Biography
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 (Festkörperrperforschung) 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) 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.