Multi-role of Sodium Doping in BiCuSeO on HighThermoelectric Performance

In this work, Na-doped BiCuSeO thermoelectric materials have been prepared and the effect of Na doping on their microstructure and thermoelectric properties has been studied. When the doping content is less than 6%, all Na+ can dissolve into the matrix and substitute for Bi3+ sites and play a role of acceptor; when the content is above 6%, the substitution saturates and excessive doping results in the formation of Na2CO3 and Na2SeO3 secondary phases. On the one hand, the doping of Na+ for Bi3+ can significantly improve the electrical properties due to the significant increase of carrier concentration. Furthermore, the phonon and total thermal conductivity also decrease with Na doping because of the dual phonon scattering by the point defects and secondary phases both resulting from Na+ doping. As a result, the thermoelectric performance is enhanced, and a maximum ZT value of 0.97, which is approximately triple that of the undoped BiCuSeO, is achieved at 873 K for the Bi0.92Na0.08CuSeO sample.