dc.contributor.advisor Clarke, David R. dc.contributor.author Sparks, Taylor David dc.date.accessioned 2012-08-10T19:10:45Z dc.date.issued 2012-08-10 dc.date.submitted 2012 dc.identifier.citation Sparks, Taylor David. 2012. Oxide Thermoelectrics: The Role of Crystal Structure on Thermopower in Strongly Correlated Spinels. Doctoral dissertation, Harvard University. en_US dc.identifier.other http://dissertations.umi.com/gsas.harvard:10243 en dc.identifier.uri http://nrs.harvard.edu/urn-3:HUL.InstRepos:9396420 dc.description.abstract This dissertation reports on the synthesis, structural and thermal characterization and electrical and thermal transport properties of a variety of strongly correlated spinels. General structure property relationships for electrical and thermal transport are discussed. However, the relationship between thermopower and features of the crystal structure such as spin, crystal field, anti-site disorder, and structural distortions are explored in depth. The experimental findings are reported in the context of improving existing oxide thermoelectric materials, screening for new materials or using thermopower as a unique characterization tool to determine the cation distribution in spinels. The need for improved n-type oxide thermoelectric materials has led researchers to consider mixed valence $(+3/+4)$ manganese oxides. Contrary to previous findings we report herein that the $LiMn_2O_4$ compound reaches the relatively large n-type thermopower of $-73 \mu V/K$ which is three times larger than the value observed in other manganese oxides, $-25 \mu V/K$. The cause of this increase in thermopower is shown to be the absence of a Jahn-Teller distortion on the $Mn^{3+}$ ions in $LiMn_2O_4$. By avoiding this structural distortion the orbital degeneracy is doubled and the Koshibae et al.’s modified Heikes formula predicts a thermopower of $-79 \mu V/K$ in good agreement with the experiment. Altering the $Mn^{3+/4+}$ ratio via aliovalent doping did not affect the thermopower and is a second evidence of universal charge transport first reported by Kobayashi et al. The role of anti-site disorder was further examined in $Fe_xMn_{1-x}NiCrO_4$ x=0, ½, ¾, 1 spinels but the effect on thermopower was inconclusive due to the presence of impurity phases. Next, the thermopower as a function of temperature in $Co_3O_4$ was investigated as a means whereby the Wu and Mason’s 30 year old model for using thermopower to calculate cation distribution in spinels could be revisited. We report evidence that Wu and Mason’s original model using the standard Heikes formula and considering octahedral sites alone leads to a stoichiometrically inconsistent result at high temperatures. Alternate models are evaluated considering Koshibae et al.’s modified Heikes formula and accounting for tetrahedral site contributions. Furthermore, the effect of a possible spin state transition is considered. en_US dc.description.sponsorship Engineering and Applied Sciences en_US dc.language.iso en_US en_US dash.license LAA dc.subject Jahn-Teller distortion en_US dc.subject condensed matter physics en_US dc.subject materials science en_US dc.subject alternative energy en_US dc.subject neutron diffraction en_US dc.subject oxide thermoelectric en_US dc.subject spinel en_US dc.subject strongly correlated systems en_US dc.subject thermopower en_US dc.title Oxide Thermoelectrics: The Role of Crystal Structure on Thermopower in Strongly Correlated Spinels en_US dc.type Thesis or Dissertation en_US dc.date.available 2012-08-10T19:10:45Z thesis.degree.date 2012 en_US thesis.degree.discipline Applied Physics en_US thesis.degree.grantor Harvard University en_US thesis.degree.level doctoral en_US thesis.degree.name Ph.D. en_US
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