Investigation of the sintering pressure and thermal conductivity anisotropy of melt-spun spark-plasma-sintered (Bi,Sb) 2
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Jian He, Song Zhu, and Tim Holgate Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634-0978
Shanyu Wang, Xinfeng Tang,a) and Qingjie Zhang State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
Terry M. Trittb),c) Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634-0978 (Received 24 January 2011; accepted 3 May 2011)
A combined melt-spinning and spark-plasma-sintering (SPS) procedure has proven to be effective in preparing high-performance (Bi,Sb)2Te3 thermoelectric (TE) nanocomposites via creating and optimizing their resulting multiscale microstructures. (Bi,Sb)2Te3 possesses a highly anisotropic crystal structure; therefore, it is important to investigate any potential correlation between the SPS conditions, the as-formed microstructures, and the resulting TE properties. In this work, we investigate the correlation between the SPS pressure, the microstructure texture, and the anisotropy of the total thermal conductivity in these melt-spun spark-plasma-sintered (Bi,Sb)2Te3 compounds. The thermal conductivity has been measured in directions that are both perpendicular and parallel to the pressing (or force) direction by rearranging the sample geometry as described in the text. The results show that the anisotropy of thermal conductivity is ;0, 2–3, 6–7, and 13–15% for the samples sintered at pressures of 20, 30, 45, and 60 MPa, respectively. These results are consistent with an increasing degree of orientation observed by x-ray diffraction and electron microscopy. I. INTRODUCTION
The heavily doped Bi2Te3 alloys are one of the most important commercial thermoelectric (TE) materials that exist to date.1,2 Bi2Te3 compounds possess a rhombohedral symmetry with the space group R3m and they crystallize in a layered structure. The layered structure of Bi2Te3 is composed of five monatomic hexagonal networks alternatingly stacked in the sequence Te(1)-Bi-Te(2)-Bi-Te(1).3,4 The bonds between the quintet layers are van der Waals type, whereas the Bi-Te(2) and Bi-Te(1) bonds within the layer are mainly covalent, however, with a small fraction of ionic bonding.5 Such an anisotropic crystal structure and chemical bonds lead to strong anisotropy in the electrical resistivity (q) and the thermal conductivity (j) Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] c) This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/ DOI: 10.1557/jmr.2011.170 J. Mater. Res., Vol. 26, No. 15, Aug 14, 2011
but much less for the Seebeck coefficient (a). These are the material’s parameters that govern the dimensionless figure of merit, ZT, of a TE material (ZT 5 a2T/qj, in which T is absolute temperature).6–8 Anisotropy in the TE properties of commercial zonemelted p-type (Bi,S
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