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Rosa Robert
Synthesis, structure and
Supervisor: Prof. Dr. Armin Reller [Solid state chemistry]
Date of oral examination: 12/21/2007
169 pages, english
This thesis focuses on the study of different cobaltate phases suitable for thermoelectric applications at high temperature. Complex cobalt oxide phases are synthesized by %93Chimie douce%94 and classical methods. These potential thermoelectric materials are characterized with respect to their crystal structure, microstructure, composition, and thermal stability by EXAFS, XRPD, EM/ED, TGA, XRF, HGE and DSC. The Seebeck coefficient, thermal conductivity and electrical resistivity of polycrystalline cobaltates with perovskite-type and layered-cobaltite structure are evaluated in a wide temperature range. The electrical transport of epitaxial La(Co,Ni)O3 thin films grown by pulsed laser deposition are compared with its bulk counterpart. The perovskite structure possesses a very high degree of compositional flexibility being able to tolerate a wide variety of cations on both the A- and B-site and allowing the fine tuning of physical properties. In this experimental study, the influence of A- and B-site substitution in the LaCoO3 system is reported. In the studied B-site substituted LaCoO3 phases, the charge carrier concentration plays an important role in the enhancement of the electrical transport. Furthermore, p- and n-type phases can be derived by creating aliovalent oxidation states of Co. Ti-substituted cobaltates show low lattice thermal conductivity which can be attributed to an increase of the lattice disorder in these phases. It is shown that the A-site substitution in LnCoO3 with smaller rare-earth elements leads to higher Seebeck coefficient values as a result of the stabilization of the Co3+ ions in the low spin state. The comparison of the electrical transport of La(Co,Ni)O3 thin films with the polycrystalline phase reveals that the thin films display better thermoelectric performance at high temperatures. The large Seebeck coefficient exhibited by both perovskite-type and layered cobaltite phases is analysed using the Heikes formula. It can be concluded that the thermopower in cobaltate phases is governed by the spin and orbital degeneracy of the electronic states of the Co ions. The results evidence the good potential of La(Co,Ti)O3 and Dy(Co,Ni)O3 phases for applications as high temperature thermoelectric materials.