Oxide thermoelectrics prepared by laser melting: effects of processing atmosphere

Abstract

Traditional thermoelectric materials suffer from two main problems, the toxicity/scarcity of the elements used and their stability at high temperatures or in non-inert atmospheres. Thermoelectric oxides appear to be a promising alternative to traditional materials due to natural abundance of the constituents and high thermal stability. This work focuses on the processing of these materials using the Laser Floating Zone (LFZ) technique. Particular emphasis is given to LFZ processing under various redox atmospheres, allowing unique opportunities for tuning the structural, microstructural and thermoelectric properties. This technique allows the growth of fully dense fibres, as well as the 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. 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 and 317 μWm-1K -2 for the Ti(Ta)O2 system. The obtained results suggest that LFZ is a suitable technique for processing thermoelectric oxides if optimized control over growth parameters and re-equilibration conditions is imposed.