Influence of the oxygen content on thermoelectric properties of Ca3−xBixCo4O9+δ system

Oxygen contents and thermoelectric properties of hot-pressed polycrystalline Ca3−xBixCo4O9+δ (0.00 < x < 0.20) samples were investigated both in Ar and air atmospheres in the temperature range 300 K < T < 870 K. Ar-annealed samples showed reduction in electrical conductivity and an elevation in Seebeck coefficient at T > 650 K with increasing x, whereas no systematic behaviour was found for the air-annealed samples. The figure-of-merit ZT at 870 K was improved from 0.12 (x = 0.00, both air and Ar) to 0.19 (x = 0.20, air) and 0.15 (x = 0.20, Ar). The results mark the significance of the oxygen content when assessing the thermoelectric properties of oxide compounds.

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► Bi substitution in Ca3−xBixCo4O9+δ system reduces both electrical and heat conductivity while increasing the Seebeck.
► ZT values at 870 K in x = 0.20 samples are 0.19 after air-annealing and 0.15 after Ar-annealing.
► Ar-annealed (reduced δ) samples show a systematic trend with x.
► No systematic trends in thermoelectric properties with x are observed in air-annealed Ca3−xBixCo4O9+δ (0.00 < x < 0.20).
► Oxygen content has a high significance in assessing thermoelectric properties of oxide compounds.