Power factor enhancement induced by Bi and Mn co-substitution in NaxCoO2 thermoelectric materials

Polycrystalline Na0.81−xBixCo1−yMnyO2 (0 ≤ x ≤ 0.15, 0 ≤ y ≤ 0.05) thermoelectric materials have been synthesized by solid-state reaction method using Bi and Mn as the property-manipulated substitution elements. The thermoelectric ceramics were characterized by X-ray diffraction (XRD) for phase composition, field emission scanning electron microscopy (FE-SEM) for grain morphology and cross section of sintered samples, and inductive-coupled plasma atomic emission spectroscopy (ICP-AES) for stoichiometric composition. The bulk density and average grain size of the system were also examined. The effects of partial substitution of Bi for Na and/or Mn for Co on the thermoelectric properties of Na0.81CoO2 have been systematically investigated. The power factor was substantially improved by Bi and Mn co-substitution in Na0.81CoO2, and the power factor value of Na0.66Bi0.15Co0.99Mn0.01O2 was about three times as large as that of Na0.81CoO2 in the whole measured temperature range. It is evident that the Bi and Mn co-substitution is effective for optimizing thermoelectric properties of Na0.81CoO2.