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제126회 대한화학회 학술발표회 및 총회 Phase Boundary Mapping Thermoelectric Semiconductors

2020년 9월 18일 11시 02분 20초
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Inorganic Chemistry - Recent Research Trends in Soli-State Chemistry and Metal Complex Chemistry
저자 및
G Jeffrey Snyder
Materials Science, Northwestern University, United States
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승인 1건
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)