Microstructure and thermoelectric properties of NaxCo2O4/Ag composite synthesized by the polymerized complex method

The synthesis of a thermoelectric NaxCo2O4/Ag composite was attempted by the polymerized complex (PC) process using AgNO3 as an Ag source and subsequent sintering at 1153 K for 72 ks. The effects of the PC process and Ag addition to NaxCo2O4 on microstructure and thermoelectric properties of the NaxCo2O4/Ag composite were investigated. Ag was hardly substituted for Na and Co sites, and the sintered sample was composed of the NaxCo2O4 and Ag phases. The electrical resistivity of the composite was smaller than that of the NaxCo2O4 single phase and the Seebeck coefficient was slightly enhanced by Ag addition, resulting in the significantly large power factor. However, most of precipitated Ag particles in the NaxCo2O4 matrix were coarse, 5–8 μm in size, and the thermal conductivity of the composite was high as compared to the NaxCo2O4 single phase. From these results, the dimensionless figure of merit of the composite was almost the same as that of the NaxCo2O4 single phase.