High-temperature thermoelectric properties of Na- and W-Doped Ca₃Co₄O₉ system

Abstract

The detailed crystal structures and high temperature thermoelectric properties of polycrystalline Ca_3-2<i>x</i> Na_2<i>x</i> Co_4-<i>x</i> W _<i>x</i> O₉ (0 ≤ <i>x</i> ≤ 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 (<i>x</i>), which is consistent with larger ionic radii of Na⁺ and W⁶⁺ ions. X-ray photoelectron spectroscopy data reveal that a mixture of Co²⁺, Co³⁺ and Co⁴⁺ valence states are present in all samples. It has been observed that electrical resistivity (<i>ρ</i>), Seebeck coefficient (<i>S</i>) and thermal conductivity (<i>κ</i>) are all improved with dual doping of Na and W in Ca₃Co₄O₉ system. A maximum power factor (PF) of 2.71 × 10^-4 W m^-1 K^-2 has been obtained for <i>x</i> = 0.025 sample at 1000 K. The corresponding thermoelectric figure of merit (<i>zT</i>) for <i>x</i> = 0.025 sample is calculated to be 0.21 at 1000 K, which is ∼2.3 times higher than <i>zT</i> value of the undoped sample. These results suggest that Na and W dual doping is a promising approach for improving thermoelectric properties of Ca₃Co₄O₉ system.