Effect of the polymerized complex process on doping limit of thermoelectric NaxCo1-yMyO2 (M = Mn, Ni)

The thermoelectric NaxCoO2 ceramics partially substituted by Mn or Ni at the Co site were synthesized by a polymerized complex (PC) process and subsequent pressureless sintering. The effects of the PC process and the partial substitution on the NaxCoO2 phase formation and their thermoelectric properties were investigated. For NaxCo1-yMnyO2, the sintered samples were composed of the single phase of y-NaxCoO2 without any second phases up to y = 0.10. On the other hand, the X-ray diffraction analysis showed that small peaks from the CoNiO2 phase were clearly detected in the pattern of the NaxCo0.90Ni0.10O2. However, the Ni content in the y phase matrix of the NaxCo1-yNiyO2 synthesized by the PC process was about 1.9 times greater than that of the ceramic sample prepared by the conventional solid state reaction method, indicating that the PC process is effective for expanding the doping level. Both the Seebeck coefficient and the electrical resistivity were increased by the Mn and Ni substitutions over the entire temperature range, and the dimensionless figure of merit was improved by these substitutions, especially by the Ni substitution.