Drastic modification of low temperature thermoelectric properties of Na-doped Bi 2 Sr 2 Co 2 O y ceramics prepared via l
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Drastic modification of low temperature thermoelectric properties of Na‑doped Bi2Sr2Co2Oy ceramics prepared via laser floating zone technique G. Çetin1 · B. Özçelik1 · M. Gürsul1 · M. A. Madre2 · A. Sotelo2 · S. Adachi3 · Y. Takano3 Received: 27 May 2020 / Accepted: 28 July 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract In this study, B i2Sr2−xNaxCo2Oy (x = 0.0, 0.05, 0.075, 0.10, and 0.15) ceramic powders have been fabricated via the classical ceramic route, followed by a texturing process through the laser floating zone technique. XRD patterns show the thermoelectric phase as the major one. In addition, Na-substitution reduces the amount of secondary phases, when compared to the pure sample. SEM observations point out that grain orientation is significantly improved when Na-content is increased. Na-substitution reduces electrical resistivity from 35 (in pure samples) to 19.6 mΩ cm (in Na = 0.05 ones) at around room temperature, while Seebeck coefficient is, approximately, twice measured in Na-free. On the other hand, thermal conductivity is slightly lower in undoped samples (0.83 W/K m), when compared to the Na-substituted ones (1.10–1.40 W/K m) at room temperature, due to their lower electrical conductivity. Finally, ZT values are higher when the Na-content is increased, reaching 0.022 at around 400 K.
1 Introduction Research on alternative energy sources is quite popular due to challenges like the decrease of fossil-fuel-based energy sources, increase in energy demand and global warming. Thermoelectric materials have an important place in this kind of research to enable the reuse of waste heat as electrical energy. These materials allow producing electric energy from a temperature gradient when they are integrated into thermoelectric generators formed by p-n legs. The conversion efficiency of these materials can be obtained from the dimensionless figure of merit, ZT, defined as TS2/ρκ, where T is the absolute temperature, S Seebeck coefficient, ρ electrical resistivity, and κ thermal conductivity [1]. Bi2Te3, PbTe and C oSe3 intermetallics are commonly used as thermoelectric materials in commercial modules. However, this * G. Çetin [email protected] 1
Department of Physics, Faculty of Sciences and Letters, Çukurova University, 01330 Adana, Turkey
2
ICMA (CSIC-Universidad de Zaragoza), C/María de Luna 3, 50018 Zaragoza, Spain
3
National Institute for Materials Science, 1‑2‑1 Sengen, Tsukuba, Ibaraki 305‑0047, Japan
compounds show some important drawbacks such as their low abundance in the earth’s crust [2], and their degradation and/or liberation of heavy elements at high temperatures under air [3]. Such problems have caused the emergence of Co-based oxides, which are abundant in earth’s crust, stable at high temperatures, and much less toxic. The first discovered member of this CoO-based family, N axCoO2, exhibits large thermoelectric power, breaking the general belief that oxides had poor thermoelectric properties [4]. This work led to the discovery
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