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Enhanced Thermoelectric Performance at the Superionic Phase Transitions of Mixed Ion-Electron Conducting Materials

Citation

Brown, David Ross (2015) Enhanced Thermoelectric Performance at the Superionic Phase Transitions of Mixed Ion-Electron Conducting Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9B56GNM. http://resolver.caltech.edu/CaltechTHESIS:08112014-121653693

Abstract

The quality of a thermoelectric material is judged by the size of its temperature de- pendent thermoeletric-figure-of-merit (zT ). Superionic materials, particularly Zn4Sb3 and Cu2Se, are of current interest for the high zT and low thermal conductivity of their disordered, superionic phase. In this work it is reported that the super-ionic materials Ag2Se, Cu2Se and Cu1.97Ag0.03Se show enhanced zT in their ordered, normal ion-conducting phases. The zT of Ag2Se is increased by 30% in its ordered phase as compared to its disordered phase, as measured just below and above its first order phase transition. The zT ’s of Cu2Se and Cu1.97Ag0.03Se both increase by more than 100% over a 30 K temperatures range just below their super-ionic phase transitions. The peak zT of Cu2Se is 0.7 at 406 K and of Cu1.97Ag0.03Se is 1.0 at 400 K. In all three materials these enhancements are due to anomalous increases in their Seebeck coefficients, beyond that predicted by carrier concentration measurements and band structure modeling. As the Seebeck coefficient is the entropy transported per carrier, this suggests that there is an additional quantity of entropy co-transported with charge carriers. Such co-transport has been previously observed via co-transport of vibrational entropy in bipolaron conductors and spin-state entropy in NaxCo2O4. The correlation of the temperature profile of the increases in each material with the nature of their phase transitions indicates that the entropy is associated with the thermodynamcis of ion-ordering. This suggests a new mechanism by which high thermoelectric performance may be understood and engineered.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Thermoelectrics, Entropy, Seebeck, Superionic, Metrology, Thermodynamics
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Applied Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Snyder, G. Jeffrey
Group:Caltech Thermoelectrics Group, Resnick Sustainability Institute
Thesis Committee:
  • Atwater, Harry Albert (chair)
  • Snyder, G. Jeffrey
  • Johnson, William Lewis
  • Minnich, Austin J.
Defense Date:17 July 2014
Non-Caltech Author Email:dbrownte (AT) gmail.com
Funders:
Funding AgencyGrant Number
Air Force Office of Science Research (AFOSR)FA9550-12-1-002
Resnick Sustainability InstituteUNSPECIFIED
Record Number:CaltechTHESIS:08112014-121653693
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:08112014-121653693
DOI:10.7907/Z9B56GNM
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8631
Collection:CaltechTHESIS
Deposited By: David Brown
Deposited On:02 Nov 2015 17:26
Last Modified:03 Feb 2016 22:58

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