• Norrman, Kion
• Hira, Uzma
• Pryds, Nini
• Christensen, Dennis Valbjørn
• Sher, Falak
• Han, Li

Abstract

The detailed crystal structures and high temperature thermoelectric properties of polycrystalline Ca_{3−2<em>x</em>}Na_2<em>x</em>Co_{4−<em>x</em>}W_<em>x</em>O₉ (0 ≤ <em>x</em>≤ 0.075) samples have been investigated. Powder X-ray diffraction datashow that all samples are phase pure, with no detectable traces ofimpurity. The diffraction peaks shift to lower angle values withincrease in doping (<em>x</em>), 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 (<em>ρ</em>), Seebeck coefficient (<em>S</em>) and thermal conductivity (<em>κ</em>) 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 <em>x</em> = 0.025 sample at 1000 K. The corresponding thermoelectric figure of merit (<em>zT</em>) for <em>x</em> = 0.025 sample is calculated to be 0.21 at 1000 K, which is ∼2.3 times higher than <em>zT</em>value of the undoped sample. These results suggest that Na and W dualdoping is a promising approach for improving thermoelectric propertiesof Ca₃Co₄O₉ system.

Topics

• impedance spectroscopy
• resistivity
• phase
• x-ray photoelectron spectroscopy
• powder X-ray diffraction
• thermal conductivity