Announcements

KABAM-CHEMISTRY JOINT SEMINAR - G. Jeffrey Snyder

Author: COLLEGE OF SCIENCES
Time: 14:30
Location: SCI 103

KOÇ UNIVERSITY KABAM-CHEMISTRY JOINT SEMINAR
********************************************************

Speaker          : G. Jeffrey Snyder, Northwestern University

Title                : Engineering Dopant Defects and Zintl Phases  

Date                : February 25, 2020, Tuesday
Time               : 14:30  
Cookie & Tea: 14:15 SCI 103
Place               : SCI 103 

 

Abstract         :    

We often understand the physical properties of Zintl Phases by considering the perfect crystalline material that is defect free. Yet this perfect, stoichiometric (valence balanced) crystal is an intrinsic semiconductor with equal number of electrons and holes. To make a n-type or p-type semiconductor we typically use point defects to introduce a slight valence imbalance that leads to excess electrons or holes. Often intrinsic defects such as vacancies, interstials or antisite defects, provide the necessary carriers to make the material a good thermoelectric (e.g. Zn4Sb3, Bi2Te3-Sb2Te3, YbxCoSb3, etc.). Most materials, however, require extrinsic dopants to be a good thermoelectric and intrinsic defects only make it more complicated. Sometimes intrinsic defects are so prevalent they are killer defects that prevent any dopant from making the material n-type or p-type. By understanding that atomic chemical potentials influence defect energy and also define regions in phase diagrams we can use phase boundary mapping to explore all the possible thermodynamic defect states of a material to avoid killer defects. The most dramatic demonstration of this is the discovery of high zT n-type Mg3Sb2 which only occurs in Mg-rich Mg3Sb2 where Mg-vacancies are suppressed.

Point defects can also make gradual but profound changes to the band structure compared to the defect free compound. This includes increasing band gap for higher temperature application, reducing conductivity mass for higher mobility or band convergence for dramatic increase in density of states (Pb(Se,Te), Mg2(Si,Sn), Bi2Te3-Sb2Te3,). In principle all of these defects can be better controlled by engineering chemical potentials through phase boundaries. Even the Ni content MNiSn (M = Ti, Zr, Hf) Half-Heusler thermoelectrics can be sufficiently altered to make substantial differences in electronic properties. The excess Ni produces impurity states in the band gap that changes the effective band gap and leads to additional electron and phonon scattering.

 

References

[1] Ohno, Snyder et al, Joule 1, 141 (2018)

[2] Gregory S. Pomrehn, Alex Zevalkink, W. G. Zeier, A. van de Walle and G. J. Snyder “Defect controlled electronic properties in AZn2Sb2 Zintl phases (A=Ca, Sr, Eu, Yb)”, Angewandte Chemistry, 126, 3490 (2014)

[3] Yinglu Tang, S-W Chen, G. J Snyder “Temperature Dependent Solubility of Yb in Yb-CoSb3 Skutterudite and its Effect on Preparation, Optimization and Lifetime of Thermoelectrics“ Journal of Materiomics 1, 75 (2015)

[4] Alex Zevalkink, Gregory S. Pomrehn, Samantha Johnson, Jessica Swallow, Zachary M. Gibbs, and G. Jeffrey Snyder “Influence of the triel elements (M = Al, Ga, In) on the transport properties of Ca5M2Sb6 Zintl compounds” Chem. Mater 24, 2091 (2012)

[5] Gregory S. Pomrehn, Alex Zevalkink, Wolfgang G. Zeier, Axel van de Walle and G. Jeffrey Snyder “Defect controlled electronic properties in AZn2Sb2 Zintl phases (A=Ca, Sr, Eu, Yb)”, Angewandte Chemistry, 126, 3490 (2014)

 

 

Bio                 :

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 (Festkörperforschung) 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 (Clarivate Analytics highly cited researcher each year since 2016 to 2019) 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 and is the most cited article in thermoelectrics.