Oxide thermoelectrics prepared by laser melting: effects of processing atmosphere

Abstract

Thermoelectric materials can convert waste heat into electrical energy, thus contributing to the sustainable energy technologies. Traditional thermoelectric materials, despite their good performance, suffer from two main problems, the toxicity/scarcity of the elements used and their stability in harsher work conditions like high temperatures or non-inert atmospheres. Thermoelectric oxides appear to be a promising alternative due to natural abundance of the constituents and high thermal stability [1]. This work focus on the processing of these materials using the Laser Floating Zone (LFZ) technique, with particular emphasis given to laser processing under various redox atmospheres, allowing unique opportunities for tuning the structural, microstructural and thermoelectric properties [2], including growth of fully dense fibres, formation of metastable phases and/or promoting different oxidation states by adjusting the growth conditions. Here we report the processing of model manganite- and titanate-based materials including donorsubstituted Ca(Pr)MnO3 and Ti(Ta)O2 systems. The results suggest successful incorporation of the dopants in the structures of the base material. Electrical conductivity studies and microstructural characterization of the Ca(Pr)MnO3 samples indicate the formation of core-shell structures with different resistivities. These core-shell structures are not always desirable and may negatively affect the transport properties, as observed when compared to the Ti(Ta)O2 system. This work shows how these structures can be tuned or eliminated by a posterior thermal treatment. XRD/SEM/EDS studies demonstrate some guidelines for tuning the phase composition and microstructure by adjusting the growth rate under different redox conditions. We report high power factor values of 303 μWm-1K-2 at 1120 K for the Ca(Pr)MnO3 system [3] and 317 μWm-1K-2 for the Ti(Ta)O2 system. The obtained guidelines suggest that LFZ is a suitable technique for processing thermoelectric oxides, if optimized control over growth parameters and reequilibration conditions is imposed.