Influenceof partial substitution of Co by Pb on the microstructure and thermoelectricproperties of NaxCo1-yPbyO2


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 NaxCoO2 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-NaxCoO2 (0.9 £ x £ 1), aa'-NaxCoO2 (x = 0.75), b-NaxCoO2 (0.55 £ x £ 0.6) and g-NaxCoO2 (0.55 £ x £ 0.74) depending oncomposition and reaction temperature, where g-NaxCoO2 exhibitspromising thermoelectric properties. The step-wise phase transition of g-NaxCoO2 to  a-NaxCoO2 via a'-NaxCoO2 in Aratmosphere and also the influence of the crystal structure on thethermoelectric properties have been reported before. While a- and a'-NaxCoO2 possess higherthermopower the electrical conductivity is much lower than the one of g-NaxCoO2. Toinvestigate the effect of the crystal structure on the thermoelectricproperties of NaxCoO2 further the current study was expanded onthe oxides NaxCo1-yPbyO2 (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-NaxCo1-yPbyO2to a-NaxCo1-yPbyO2via a'-NaxCoO2 was detectedunder Ar atmosphere at lower temperature than reported for g-NaxCoO2. The effect ofpartial substitution of Co by Pb on the microstructure, thermophysical behaviorand thermoelectric properties of NaxCoO2 will be discussed either.

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