Effect of the Polymerized Complex Process on Doping Limit of Thermoelectric Na_xCo_1−yM_yO₂ (M=Mn, Ni)

Abstract

The thermoelectric Na_xCoO₂ 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 Na_xCoO₂ phase formation and their thermoelectric properties were investigated. For Na_xCo_1−yMn_yO₂, the sintered samples were composed of the single phase of γ-Na_xCoO₂ 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 Na_xCo_0.90Ni_0.10O₂. However, the Ni content in the γ phase matrix of the Na_xCo_1−yNi_yO₂ 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.