Power factor enhancement induced by Bi and Mn co-substitution in Na_xCoO₂ thermoelectric materials

Polycrystalline Na_0.81-xBi_xCo_1-yMn_yO₂ (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 Na_0.81CoO₂ have been systematically investigated. The power factor was substantially improved by Bi and Mn co-substitution in Na_0.81CoO₂, and the power factor value of Na_0.66Bi_0.15Co_0.99Mn_0.01O₂ was about three times as large as that of Na_0.81CoO₂ in the whole measured temperature range. It is evident that the Bi and Mn co-substitution is effective for optimizing thermoelectric properties of Na_0.81CoO₂.