Structural behavior and thermoelectric properties of the brownmillerite system Ca2(ZnxFe2−x)O5 |
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Authors: | Emily Asenath-Smith Scott T Misture Doreen D Edwards |
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Institution: | Alfred University, Kazuo Inamori School of Engineering, Materials Science and Engineering, 2 Pine Street, Alfred, NY 14802, USA |
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Abstract: | The Ca2(ZnxFe2−x)O5 series was synthesized and characterized to determine the influence of zinc dopant on the brownmillerite structure for thermoelectric applications. All single-phase compounds exhibited Pnma symmetry at room temperature up to the solubility limit at x=0.10. High-temperature X-ray powder diffraction was used to show that the nature of the Pnma-Imma(0 0 γ)s00 transition in Ca2Fe2O5 is modified by the presence of zinc. While the Zn-free composition transitions to an incommensurate phase, the Zn-containing phases transition instead to a commensurate phase, Imma(0 0 γ)s00 with γ=1/2. Both the Néel temperature and the onset temperature of the Pnma-Imma(0 0 γ)s00 phase transition decreased with increasing zinc concentration. Rietveld analysis of the in situ diffraction pattern for the x=0 sample at 1300 °C demonstrates that the structure contains statistically disordered chain orientations as described by space group Imma. Thermoelectric properties were analyzed in air from 100 to 800 °C. The positive Seebeck coefficient revealed hole-type conduction for all compositions. Doped samples exhibited electrical conductivities up to 3.4 S/cm and thermal conductivity of 1.5 W/mK. Transport analysis revealed thermally activated mobility consistent with polaron conduction behavior for all compositions. |
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Keywords: | Structure modulation Browmillerite Seebeck coefficient Thermal properties Electrical conductivity High-temperature X-ray diffraction |
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