Decreased thermal conductivity in Bi 2 Sr 2 Co 2 O x bulk materials prepared by partial melting
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Feridoon Azough and Robert Freer Material Science Center, School of Materials, University of Manchester, Grosvenor Street, Manchester M13 9PL, UK (Received 6 October 2015; accepted 21 March 2016)
Bi2Sr2Co2Ox (BSC-222) bulk materials have been prepared in air by single hot pressing or partial melting followed by hot pressing. Thermal transport properties of as-prepared samples were compared with BSC-222 samples prepared by single partial melting. Samples prepared through hot pressing have a high bulk density and improved preferential grain orientation. Bulk density and preferential grain orientation are significantly better in samples processed by partial melting followed by hot pressing. In samples prepared by partial melting, the presence of sub-micrometer-sized secondary phases Bi0.75Sr0.25Ox, Sr-based oxide, and CoO, is observed. The lattice contribution to the total thermal conductivity decreases significantly in partially melted BSC-222 samples. By using a classical phonon transport model, this result demonstrates that the decrease in lattice thermal conductivity in partially melted BSC-222 samples may be linked not only to porosity but also to the presence of defects, induced by partial melting process, which may play an important role in phonon scattering.
I. INTRODUCTION
Among the different oxide compounds investigated as thermoelectric materials (TEs),1–3 cobaltite Bi2Sr2Co2Ox (BSC-222) is a promising p-type material for high temperature thermoelectric applications in air. The efficiency of TE conversion increases with the dimensionless figure of merit, ZT, which is defined as ZT 5 S2Tr/j, where S is the Seebeck coefficient, T is the absolute temperature, and r and j are the electrical and thermal conductivity, respectively. BSC-222 single crystals exhibit a promising ZT value of 1.1 at 700 °C in air.4 The BSC-222 phase exhibits a particular layered structure where CoO2 layers alternate with Bi0.87SrO2 block layers along the c-axis with a misfit structure.5 Linked to this layered structure, phonon scattering at the interface between layers is expected to impact thermal transport properties beneficially. As predicted, the ZT value for BSC-222 polycrystalline samples is lower than the BSC222 single crystal. To the best of our knowledge, a ZT value of 0.19 in air at 700 °C is the best value reported thus far for BSC-222 polycrystalline materials.6 Recently, by using partial melting processing, the ZT value has been improved to 0.27 at 700 °C in air.7 Green pellets are Contributing Editor: Terry M. Tritt a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.142 1296
J. Mater. Res., Vol. 31, No. 9, May 14, 2016
http://journals.cambridge.org
Downloaded: 12 May 2016
heated to a state known as “partial melting”. In this state, BSC-222 phase being an incongruent system is decomposed into a liquid (Bi-rich) and a solid (Bi-poor) phase. By slow cooling to room temperature, the BSC-222 grains are reformed through a peritectic reaction between the solid and liquid phases.8 In
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