High-temperature thermoelectric properties of Na- and W-Doped Ca 3 Co 4 O 9 system.

The detailed crystal structures and high temperature thermoelectric properties of polycrystalline Ca 3-2 x Na 2 x Co 4- x W x O 9 (0 ≤ x ≤ 0.075) samples have been investigated. Powder X-ray diffraction data show that all samples are phase pure, with no detectable traces of impurity. The diffraction peaks shift to lower angle values with increase in doping ( x ), which is consistent with larger ionic radii of Na + and W 6+ ions. X-ray photoelectron spectroscopy data reveal that a mixture of Co 2+ , Co 3+ and Co 4+ valence states are present in all samples. It has been observed that electrical resistivity ( ρ ), Seebeck coefficient ( S ) and thermal conductivity ( κ ) are all improved with dual doping of Na and W in Ca 3 Co 4 O 9 system. A maximum power factor (PF) of 2.71 × 10 -4 W m -1 K -2 has been obtained for x = 0.025 sample at 1000 K. The corresponding thermoelectric figure of merit ( zT ) for x = 0.025 sample is calculated to be 0.21 at 1000 K, which is ∼2.3 times higher than zT value of the undoped sample. These results suggest that Na and W dual doping is a promising approach for improving thermoelectric properties of Ca 3 Co 4 O 9 system.