Synthesis, Characterization, and Thermoelectric Properties of Electrospun Boron-Doped Barium-Stabilized Bismuth-Cobalt O
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THERMOELECTRIC nanocomposites are promising materials for the conversion of waste heat in to electricity in a cost-effective and environmentally benign manner. Along with the large thermopower effect of NaxCoO2, the layered cobalt oxides were the subject of intense research. However, the discovery of the substitution between sodium and bismuth has increased the thermoelectric properties (ZT factor), and other layered cobalt oxides such as bismuth-doped cobalt oxides have also drawn great interest.[1,2] The ZT factor is a measure of the potential of a particular material for an efficient thermoelectric device.[3] The ZT factor has a dimensionless ratio and depends on the electrical conductivity, the thermal conductivity, and the Seebeck coefficient. A high-performance thermoelectric material requires a high Seebeck coefficient and electrical conductivity; and low thermal conductivity.[4] The Seebeck coefficient d is named as ‘‘thermopower,’’ too. All materials have a specific Seebeck coefficient. It depends on the electrical EMRE C¸INAR, M.Sc. Student, and AHMET AKDEMIR, Professor, are with the Department of Mechanical Engineering, Selcuk University, Konya, Turkey. SERHAT KOC¸YIG˘IT and ARDA AYTIMUR, M.Sc. Students, are with the Department of Advanced Technologies, Gazi University, Ankara, Turkey. Contact e-mail: _ [email protected]. IBRAHIM USLU, Professor, is with the Department of Chemical Education, Gazi University, Ankara, Turkey. Manuscript submitted February 26, 2014. Article published online May 23, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
potential difference (DV) and the temperature difference (DT); the formula is (d = DV/DT).[3] Nanostructuring polymer composites are effective in enhancing the ZT factor through phonon scattering at the nanograin boundaries.[5] The electrical conductivity may be increased through the use of the quantum dots to enhance the high electron density because the electron mobility was greater in the nanocrystalline structures.[6] Conversely, the thermal conductivity may be decreased through the phonon-blocking effects through the use of the nanostructures. The electrospinning technique is a simple and straightforward process that can be used to produce nanofibers and nanograins—after calcination and sintering—for the thermoelectric composite materials. According to the literature, it possible to generate the ceramic nanocrystals with the enhanced ZT factor using the electrospinning technique.[7,8] Another important advantage for the electrospun thermoelectric ceramic nanofibers is that the synthesis process is highly efficient and scalable.[9] This work reports a novel technique for the synthesis of high surface area of the bismuth-cobalt oxide-based nanofibers and nanocrystalline ceramic powders, use of the polymeric precursor, polyvinyl alcohol (PVA) sol-gel based electrospinning technique, and the consolidation into bulk ceramics by calcining and sintering in air atmosphere. Barium was added to the bismuth-cobalt oxide structure as the stabilized material. The Ba2+ ion has a significantly
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