The interest in thermoelectric oxides as candidate materials for high temperature waste heat recovery has generated vital scientific activity during the last years. However, while it is well established in several other fields of materials science that the electronic structure of oxides at high temperatures can be significantly modified by the formation of oxygen vacancies, the precise control of the sample and its surrounding atmosphere is rather seldomly seen in scientific publications on high temperature thermoelectric oxides. Therefore, during this thesis, I have investigated the influence of the oxygen content on the properties of two of the most prominent thermoelectric oxides.
A variation of the oxygen content of a material at high temperatures can be achieved by a variation of the surrounding atmosphere and the subsequent in- and out-diffusion of oxygen ions until the new thermodynamic equilibrium state is reached. Oxygen vacancies can usually be described as effectively charged point defects and the precise control of their concentration provides a means to change the charge carrier concentration of a material in situ.
Therefore, one goal of this thesis is to establish that the high temperature thermoelectric characterisation of oxides should preferably be done under controlled atmospheric conditions. In fact, part of the scatter observed in published results on nominally identical samples can be explained by (unintentionally) different oxygen content, due to different measurement atmospheres or sample kinetics. Moreover, the results from this thesis aim to contribute to the fundamental understanding of the charge transport processes in the studied and related materials.
The thesis comprises the design and characterisation of an appropriate system to measure the electrical transport properties of the materials under investigation (Manuscript 3). Manuscript 1 and 4 study one of the most prominent thermoelectric oxides: Misfit calcium cobalt oxide (Ca2CoO3-δ)q(CoO2) (CCO), which shows one of the best reproducible p-type thermoelectric performances among all oxides.
In Manuscript 1, we established a defect chemical model of this material. Due to its misfit structure, it is inherently mixed-valent, so that a modified defect notation was chosen. The dependency of both electrical conductivity and Seebeck coefficient on the oxygen content was measured in a wide range of temperature and oxygen partial pressure. It was concluded that – at high temperatures – charge carriers should be described as itinerant in this material. We further showed that the often used Heikes formula cannot be used for a quantitative analysis of the Seebeck coeffcient in CCO. Instead, we suggested a modified Mott formula with a significant contribution from the energy dependent mobility to describe the Seebeck coefficient in CCO.
In Manuscript 4, a combined experimental and theoretical study of the oxygen nonstoichiometry in CCO is presented. Based on DFT-calculations and experimental Raman-spectroscopy, it is shown that oxygen is preferentially removed from an atomic position within the central layer of the Ca2CoO3-subsystem. The computational results further indicated that the electronic properties are sensitive to small variations in the crystal structure. The thermodynamics of oxidation were investigated by three different techniques (TG, TG-DSC, and DFT) and differences were discussed.
In Manuscript 2, the high temperature charge transport in CaMnO3-δ (CMO) was investigated. CMO is – when doped with small amounts of niobium – among the most promising n-type oxides with a figure of merit reaching 0.3 at high temperatures. When forming oxygen vacancies – thereby increasing the electron concentration of the material – we observed an unusual simultaneous decrease of both conductivity and the absolute value of the Seebeck coefficient. These findings were analysed as an indication of strongly interacting small polarons as the charge carrier in this material. We generalised this result to develop a simple model for the powerfactor and concluded that mutual Coulomb repulsion limits the thermoelectric performance of these materials.
List of papers. Papers 1, 3 and 4 are removed from the thesis due to publisher restrictions.
Paper 1 – Electronic Transport Properties of [Ca2CoO3-δ]q[CoO2] M. Schrade, H. Fjeld, T.G. Finstad, T. Norby J. Phys. Chem. C, 118, (2014), 2908-2918 doi:10.1021/jp409581n
Paper 2 – High Temperature Transport Properties of Thermoelectric CaMnO3-δ – Indication of Strongly Interacting Small Polarons M. Schrade, R. Kabir, S. Li, T.Norby, T.G. Finstad J. Appl. Phys., 115, (2014), 103705 doi:10.1063/1.4868321 copyright 2014 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Paper 3 – Versatile Apparatus for Thermoelectric Characterisation of Oxides at High Temperatures M. Schrade, H. Fjeld, T. Norby, T.G. Finstad submitted to Review of Scientific Instruments
Paper 4 – Oxygen Nonstoichiometry in (Ca2CoO3)0.62(CoO2) - A Combined Experimental and Computational Study M. Schrade, S. Casolo, P. Graham, C. Ulrich, S. Li, O.-M. Løvvik, T.G. Finstad, T. Norby J. Phys. Chem. C, Article ASAP, Publication Date (Web): July 28, 2014. doi:10.1021/jp5048437