Influence of partial substitution of Co by Pb on the microstructure and thermoelectric properties of NaxCo1-yPbyO2

The highly anisotropic oxide Na_xCoO₂ exhibits promising thermoelectric properties: thermopower up to 150 μV/K, electrical conductivity of 300 S/cm in combination with low thermal conductivity. Four different phases of the nonstoichiometric oxide are known: α-Na_xCoO₂ (0.9≤x≤1), α´-Na_xCoO₂ (x= 0.75), β-Na_xCoO₂ (0.55≤ x ≤ 0.6) and γ-Na_xCoO₂ (0.55≤ x ≤0.74) depending on composition and reaction temperature, but only γ-Na_xCoO₂ was studied in detail. The stepwise phase transition of γ-Na_xCoO₂ to α-Na_xCoO₂ via α´-Na_xCoO₂ in Ar atmosphere and also the influence of the crystal structure on the thermoelectric properties have been reported before. While α- and α´-Na_xCoO₂ possess higher thermopower the electrical conductivity is much lower than the one of g-Na_xCoO₂. In order to investigate the influence of doping on the thermoelectric properties of Na_xCoO₂ we expanded our studies on the alloyed oxides Na_xCo_1-yPb_yO₂ (y = 0.01, 0.02, 0.05). An improved crystallinity of the Pb substituted samples and an overall enhancement of the thermoelectric properties were observed. The composition Na_xCo_0.95Pb_0.05O₂ was studied in detail. The effect of partial substitution of Co by Pb on the microstructure, thermophysical behavior and thermoelectric properties of Na_xCoO₂ will be discussed here.