Study On The Doping Behavior Of Thermoelectric Material NaCo₂O₄

NaCo2O4 is a ceramic oxide of great thermoelectric property. In this work, we mainly focus on the study of doping effects on the thermoelectric properties of NaCo2O4. The doping includes substituting Co with Cu and substituting Na with Ag in NaCo2O4. The best doping amount and mechanisms of effects of doping on the materials were studied by doping different amount of Cu or Ag. All the samples were prepared by conventional solid state reaction method. Structural characterization and measurements on thermoelectric properties were done for all the samples. Experimental methods include XRD, SEM, and electric resistivity at high and low temperatures, and thermal conductivity.From the Cu doped NaCo2O4 samples, it can be noticed that the structure of the material changes when the doping ratio is large. When x≤0.35, Na (Co1-xCux)2O4 still has the structure of NaCo2O4, while x=0.40 the main diffraction peaks shift apparently, this may owing to the limit of solid solution with Cu. It shows the change of lattice constants. The samples of x=0.15 and x=0.20 have smaller thermal conductivity than the non-substituted one. Seebeck coefficient of doped samples was considerately improved comparing with undoped sample. The one with x=0.15 is of the biggest Seebeck coefficient. In all, the thermoelectric performances of doped samples are improved compared with the undoped when the doping ratio x of Cu for Co is 0.05 and 0.15.For the Ag doped NaCo2O4, the electrical resistivity increases with the increasing temperature, showing a metallic-like behavior. The undoped sample has the largest electrical resistivity at lower temperatures, while the sample of x=0.05 has the lowest value. The reason is that Ag doping changes the concentration of carriers. At high temperatures, the sample of x=0.05 has the lowest resistivity. The thermal conductivity of doped samples is smaller than the undoped one, while theκNaCo2O4 is extremely low, the low thermal conductivity is due to the random occupancy of. Na ions. The sample of x=0.05 has the highest Seebeck coefficient. The effect of doping on the thermoelectric properties was evaluated from the parameters above. The sample of x=0.05 has the best ZT value among all the Ag doped samples, suggesting a satisfied doping ratio with considerately improved thermoelectric properties.