Influenceof partial substitution of Co by Pb on the microstructure and thermoelectricproperties of Na_xCo_1-yPb_yO₂

A. Mrotzek¹, D. Muchilo¹, D.Platzek¹, E. Müller¹, J. Plewa², H.Altenburg²

¹German Aerospace Center(DLR), Institute of Materials Research, D-51170 Cologne, Germany

²Univ.of Applied Sciences, Department of Chemical Engineering, D-48565 Steinfurt,Germany

Recently, thereis strong interest in developing novel thermoelectric materials for high temperatureenergy conversion. In this context the highly anisotropic oxide Na_xCoO₂ exhibitspromising thermoelectric properties: thermopower up to 150 µV/K,electrical conductivity of 300 S/cm in combination with low thermalconductivity. Four different phases of the non-stoichiometric oxide are known: a-Na_xCoO₂ (0.9 £ x £ 1), aa'-Na_xCoO₂ (x = 0.75), b-Na_xCoO₂ (0.55 £ x £ 0.6) and g-Na_xCoO₂ (0.55 £ x £ 0.74) depending oncomposition and reaction temperature, where g-Na_xCoO₂ exhibitspromising thermoelectric properties. The step-wise phase transition of g-Na_xCoO₂ to a-Na_xCoO₂ via a'-Na_xCoO₂ in Aratmosphere and also the influence of the crystal structure on thethermoelectric properties have been reported before. While a- and a'-Na_xCoO₂ possess higherthermopower the electrical conductivity is much lower than the one of g-Na_xCoO₂. Toinvestigate the effect of the crystal structure on the thermoelectricproperties of Na_xCoO₂ further the current study was expanded onthe oxides Na_xCo_1-yPb_yO₂ (y = 0.01, 0.02, 0.05, 0.1, 0.2,0.5). An improved crystallinity of the Pb substituted samples was observed aswell as an enhancement of the thermopower. A similar transformation of g-Na_xCo_1-yPb_yO₂to a-Na_xCo_1-yPb_yO₂via a'-Na_xCoO₂ was detectedunder Ar atmosphere at lower temperature than reported for g-Na_xCoO₂. The effect ofpartial substitution of Co by Pb on the microstructure, thermophysical behaviorand thermoelectric properties of Na_xCoO₂ will be discussed either.

This work is supported by the DeutscheForschungsgemeinschaft, No. KA 664/10-1