Effects of noble metal addition on microstructure and thermoelectric properties of NaxCo2O4

The synthesis of NaxCo2O4/Ag and NaxCo2O4/Au composites was tried by mechanical milling and subsequent sintering. Ag and Au particles were added to the NaxCo2O4 powder prior to the mechanical milling. The microstructure and thermoelectric properties of the NaxCo2O4/Au composite were compared to those of the NaxCo2O4/Ag composite and the NaxCo2O4 single phase, and the effects of the Ag and Au addition on the thermoelectric performance of NaxCo2O4 were discussed. Au particles around 2 μm or smaller in size, which were significantly smaller than Ag particles around 10 μm in size, were dispersed in the NaxCo2O4 matrix. The Seebeck coefficient and the electrical resistivity of NaxCo2O4 were slightly enhanced and significantly reduced by these noble metals addition, resulting in the large power factor of these composites. On the other hand, the NaxCo2O4/Au composite showed the electrical resistivity larger than that of the NaxCo2O4/Ag composite. Ag and Au addition markedly increased the thermal conductivity, and the dimensionless figure of merit of NaxCo2O4 could not be improved by these noble metals addition.