Thermoelectricity in Complex Oxides
Worldwide activities in thermoelectrics have been snowballing during the past 5 years mainly initiated by a wide spread of applications ranging from cooling devices, energy harvesting from waste heat, power generation from sunlight to radioisotope thermoelectric generators in remote unmanned facilities in space. Now days most strategies to obtain novel thermoelectrics with the optimized power factor S2σ , where S is the Seebeck coefficient and σ is the electrical conductivity utilize effect of additional carrier scattering centers and modifications of the electronic structure and they are restricted to prepare bulk thermoelectric nanocomposites based on semiconductors. The additional carrier scattering will arise from nanoinclusions or granular interfaces, while the electronic structure changes will arise from localized distortions and internal interfaces.
Based on the expertise acquired in the Scientific Service Group Technology in preparing single crystal-type thin films of complex oxides three research directions are pursuit. One includes the investigation of the power factor of complex oxide thin films, e.g. cobaltites, and to explore their stability in a temperature range from ambient temperature to 1000°C. The other is the attempt to tailor the thermoelectric properties of heterostructures and superlattices by modifications of the electronic structure of the hetero-interfaces and the third one is the application of those materials as sensors based on the light-induced thermoelectric effect. Here, recent achievements include a systematic study of Na1-xCoO2 (0.5<x<0.8) thin films showing that at room temperature Na0.63CoO2 films have a responsivity of 1.24 Vcm2/mJ combined with the rise time of 12 ns, and an anisotropy of the Seebeck coefficient ΔS of 60.8 μV/K. [1-4]
To explore possible interface effects in heterostructures and superlattices, measurements of the temperature dependence of the dc resistivity as well as the thermopower in single YBCO and LCMO thin films as well as YBCO-LCMO superlattices are performed. Characterizing their charge and entropy transport properties we observe a sign change and a substantial increase of the thermoelectric power in the superlattices which can’t be explained by a simple superposition of those of the individual single layers. Different possible mechanisms such as interfacial strain, incomplete oxidation, charge transfer and oxygen deficiency at the interface can be accounted for these observations. Tentatively we ascribe the results to a combination of interface effects and a long range interaction affecting the entropy transport. 
 Thermoelectric conversion via laser-induced voltage in highly textured polycrystalline NaxCoO2 ceramic. Yan G. W., et al. J. Appl. Phys. 110, 103102 (2011).
 Epitaxial layered cobaltite NaxCoO2 thin films grown on planar and vicinal cut substrates. Yu L., et al. J. of Crystal Growth 328, 34 (2011).
 Epitaxial La0.9Ca0.1MnO3 films grown on vicinal cut substrates for the investigation of resistivity and thermoelectric anisotropy. Yu L., et al. J. of Crystal Growth 322, 41 (2011).
 Substrate-induced anisotropy of c-axis textured NaxCoO2 thin films. Yu L., et al. Progress in Solid State Chemistry 35, 545 (2007).
 Thermoelectric Properties of YBCO/LCMO Superlattices. Heinze S. et al. to be submitted to App. Phys. Lett. (2012).