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. / Engineering and Applied Sciences

Die thermoelektrische Energiekonversion auf der Basis des Seebeck-Effekts ist eine Methode zur direkten Erzeugung elektrischer Energie aus thermischer Energie. Für die wesentlichen anwendungsrelevanten Parameter Temperaturbereich, elektrische Leistung und Herstellungskosten sind Materialauswahl und Aufbau der TEG entscheidend. In der vorliegenden Arbeit wurden erstmalig thermoelektrische Oxidkeramiken in monolithischen TEG verwendet, die auf der Grundlage des transversalen thermoelektrischen Effekts arbeiten. Die TEG wurden mit industriell skalierbaren Keramiktechnologien hergestellt, untersucht und hinsichtlich ihrer Parameter detailliert theoretisch und experimentell bewertet. Als Modellsystem für die Materialien wurde La1-xSrxCuO4 in Kombination mit Ag bzw. Ag6Pd1 verwendet. Es konnte belegt werden, dass diese monolithischen TEG im Bereich kleiner elektrischer Leistungen eine vorteilhafte Alternative zu herkömmlichen longitudinalen thermoelektrischen Generatoren sein können.