Charge Transport Analysis Using the Seebeck Coefficient-Conductivity Relation

Citation

Kang, Stephen Dongmin (2018) Charge Transport Analysis Using the Seebeck Coefficient-Conductivity Relation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ZAQP-MN67. https://resolver.caltech.edu/CaltechTHESIS:05292018-140812451

Abstract

Charge transport properties like electrical conductivity or the Seebeck coefficient are defined phenomenologically from near-equilibrium thermodynamics, while the analysis or modeling of them often involves a physical model based on mechanistic principles. In other words, physical models connect microscopic and physical parameters to phenomenological and experimental properties. One of the challenges is that the complexity of solid state requires many physical parameters, whereas the measurable properties which help to determine those parameters are limited. The interrelations of measured properties are very important to overcome this challenge, but this aspect is not well recognized in conventional analysis themes. In this thesis, the concept of using a phenomenological transport function is devised to help combine a collection of measurements into an intermediate level of phenomenology, relevant for Fermion transport but not dependent on a particular physical model. This phenomenological transport function can be determined by examining the electrical conductivity, the Seebeck coefficient, and potentially the Lorenz number. Because the phenomenological transport function combines information from a set of multiple measurable properties, a direct comparison to the transport function of a physical model serves as a strong test for the model.

Particular usefulness comes from extracting transport functions from the Seebeck coefficient-conductivity relation, especially in doped semiconductors. This approach is applied to contrast CeO_2-x and n-type SrTiO₃ as narrow and dispersive transport function materials, each consistent with polaron and band conduction, respectively. In band conductors such as SrTiO₃ and Mg₃Sb₂, the approach is used to test and refute previous claims about the scattering mechanism and find consistency with deformation potential scattering in both cases. In conducting polymers, which do not resemble any other type of conventional conductors, the Seebeck-conductivity relation reveals a qualitative disagreement with the commonly cited Mott's models. For the case of Cu₂Se, a peculiar band conductor which shows anomalies in the Hall measurement of the high temperature phase and also in other transport properties at the phase transition, the transport function approach is applied as a workaround for modeling. On the practical side, for thermoelectric applications, the transport function approach is used to characterize material quality factors for both majority carrier conduction and bipolar conduction. Finally, experimental efforts for improving the accuracy and applicability of Seebeck measurements is discussed.

Thesis Files