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

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

doi:10.1039/c8ra01691g

Uzma Hira, Dennis Valbjørn Christensen + Show 4 more

Open Access

https://doi.org/10.1039/c8ra01691g

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Abstract

The detailed crystal structures and high temperature thermoelectric properties of polycrystalline Ca3−2xNa2xCo4−xWxO9 (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 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 (ρ), Seebeck coefficient (S) and thermal conductivity (κ) 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 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 Ca3Co4O9 system.

Highlights

Thermoelectric (TE) power generation from waste heat is considered as a promising renewable energy technology.[1,2] TE devices convert thermal energy into electricity via the Seebeck effect, and electrical power into solid state refrigeration via the Peltier effect.[3]
There are some research studies on simultaneous substitution of two different metal cations at Ca- and Co-sites in C-349 system with signi cant improvement in TE properties with zT values of $0.20–0.25.40–42 there are no reports published on dual doping of Na and W metals in Ca3Co4O9 cobaltite as yet
Powder X-ray diffraction data revealed that Na+ and W6+ ions enter into the Ca2CoO3 and CoO2 layers of Ca3Co4O9 system respectively

Summary

Introduction

Thermoelectric (TE) power generation from waste heat is considered as a promising renewable energy technology.[1,2] TE devices convert thermal energy into electricity via the Seebeck effect, and electrical power into solid state refrigeration via the Peltier effect.[3]. It was reported that doping of Na ions at Ca-sites resulted in decrease of electrical resistivity and as a consequence increase of thermoelectric power factor to $5.5 Â 10À4 W mÀ1 KÀ2 at 1000 K, though thermal conductivity (k) of these samples was still too high (4.0 W mÀ1 KÀ1), impeded the further improvement of zT values.[25] On the other hand, high valence 4d and 5d transition metal-doped C-349 samples exhibited much smaller thermal conductivity with reasonably good zT values.[39] There are some research studies on simultaneous substitution of two different metal cations at Ca- and Co-sites in C-349 system with signi cant improvement in TE properties with zT values of $0.20–0.25.40–42 there are no reports published on dual doping of Na and W metals in Ca3Co4O9 cobaltite as yet This prompted us to prepare a series of Ca3À2xNa2xCo4ÀxWxO9 (0 # x # 0.075) oxides by the conventional solid-state reaction method, and investigate their structural and hightemperature thermoelectric properties.

Experimental

Result and discussion

Thermoelectric properties

Conclusion