Experimental investigations on a novel thermoelectric material (Na_xCoO₂)

Abstract

Thermoelectric (TE) devices can be used reversely as a heat pump for refrigeration (heat management) and mostly oxide materials are used in these devices. Metal oxide-based TE materials possess several advantages compared to metal chalcogenides. These materials can be employed, since they are environment-friendly and cost-effective. TE performance can be easily tuned and this can be achieved through composition and structure. In the present work, Na_0.77CoO₂/nano Co₃O₄ composites are chosen. Although Na_xCoO₂ possesses superior TE performance to other oxide-based materials, its application has been limited due to its instability in air. It decomposes into the insulating Co(OH)₂ and/or CoCO₃ by absorbing moisture and/or CO₂ from the ambient environment. Upon heating, both Co(OH)₂ and CoCO₃ decompose further into the insulating Co₃O₄ and CoO. Therefore, it is important to explore the chemical stability of Na_xCoO₂. This study analyses the reasons for the chemical instability of Na_xCoO₂ without impairing its TE properties. Na_0.77 CoO₂ was prepared by the solid-state synthesis method and the single phase was observed in XRD. Again XRD was taken after 15 days, to analyse the stability of the prepared sample. The peaks corresponding to Na₀.₇₇CoO₂ disappeared. The reasons for this instability were analysed and possible solutions are suggested.

KEYWORDS:

• Thermoelectric materials
• metal oxides
• Seebeck effect
• figure of merit
• nanocomposites

Acknowledgements

The authors thank Dr R. Gopalan, Scientist ‘G’ and Associate Director and Dr D. Siva Prahasam, Scientist-E, Centre for Automotive Energy Materials, ARCI, IIT Madras Research Park, Chennai, for providing the laboratory facilities to carry out the research work.

Disclosure statement

No potential conflict of interest was reported by the authors.