Influence of partial substitution of Co by Pb on the microstructure and thermoelectric properties of Na/sub x/Co/sub 1-y/Pb/sub y/O/sub 2/

The highly anisotropic oxide Na/sub x/CoO/sub 2/ exhibits promising thermoelectric properties: thermopower up to 150 /spl mu/V/K, electrical conductivity of 300 S/cm in combination with low thermal conductivity. Four different phases of the nonstoichiometric oxide are known: /spl alpha/-Na/sub x/CoO/sub 2/ (0.9/spl les/x/spl les/1), /spl alpha/'-Na/sub x/CoO/sub 2/ (x= 0.75), /spl beta/-Na/sub x/CoO/sub 2/ (0.55/spl les/ x /spl les/ 0.6) and /spl gamma/-Na/sub x/CoO/sub 2/ (0.55/spl les/ x /spl les/0.74) depending on composition and reaction temperature, but only /spl gamma/-Na/sub x/CoO/sub 2/ was studied in detail. The stepwise phase transition of /spl gamma/-Na/sub x/CoO/sub 2/ to /spl alpha/-Na/sub x/CoO/sub 2/ via /spl alpha/'-Na/sub x/CoO/sub 2/ in Ar atmosphere and also the influence of the crystal structure on the thermoelectric properties have been reported before. While /spl alpha/- and /spl alpha/'-Na/sub x/CoO/sub 2/ possess higher thermopower the electrical conductivity is much lower than the one of g-Na/sub x/CoO/sub 2/. In order to investigate the influence of doping on the thermoelectric properties of Na/sub x/CoO/sub 2/ we expanded our studies on the alloyed oxides Na/sub x/Co/sub 1-y/Pb/sub y/O/sub 2/ (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/sub x/Co/sub 0.95/Pb/sub 0.05/O/sub 2/ was studied in detail. The effect of partial substitution of Co by Pb on the microstructure, thermophysical behavior and thermoelectric properties of Na/sub x/CoO/sub 2/ will be discussed here.