Nanocomposite Bi(Sb)Te(Se) Materials by Cryogenic Mechanical Alloying and Optimized High Pressure Hot-pressing
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Nanocomposite Bi(Sb)Te(Se) Materials by Cryogenic Mechanical Alloying and Optimized High Pressure Hot-pressing Tsung-ta E. Chan1, Rama Venkatasubramanian2, James M. LeBeau1, Peter Thomas2, Judy Stuart2 and Carl C. Koch1 1
Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27606, U.S.A. 2
RTI International, Research Triangle Park, NC 27709, U.S.A.
ABSTRACT Nanocomposite Bi2Te3 based alloys are attractive for their potentially high thermoelectric figure-of-merit (ZT) around room temperature. The nano-scale structural features embedded in the matrix provide more scattering of phonons and can thus reduce the lattice thermal conductivity. To further take advantage of such nanocomposite structures, we focus on the development of nanocrystalline Bi(Sb)Te(Se) powders by high energy cryogenic mechanical alloying followed by an optimized hot pressing process. This approach is shown to successfully produce Bi(Sb)Te(Se) alloy powders with grain size averaging about 9 nm for n-type BiTe(Se) and about 16 nm for p-type Bi(Sb)Te respectively. This cryogenic process offers much less milling time and prevents thermally activated contamination or imperfections from being introduced during the milling process. The nanocrystalline powders are then compacted at optimized pressures and temperatures to achieve full density compactions and preserve the grain sizes effectively. The resulting nano-bulk materials have optimal Seebeck coefficients and are expected to have improved ZT. Thermoelectric properties and microstructure studies by X-ray diffraction and transmission electron microscopy will also be presented and discussed. INTRODUCTION Limited efficiency, represented by the figure of merit (ZT), of the thermoelectric (TE) materials has been the main challenge for the TE devices to be competitive in “green” power generation. Since the concept of utilizing nanostructure to scatter phonons has been proposed [1], thin film TE materials have achieved peak ZT greater than 3 by the reductions in the thermal conductivity from their superlattice structures [2, 3]. Similar phonon scattering mechanism has been theoretically established for other geometry [4] and then experimentally shown feasible in the bulk BiSbTe and SiGeP materials [5, 6]. Such bulk “nanocomposite” structure, i.e., nanoscale particles/grains embedded in the matrix materials, is typically produced by the bottom up method: start with nanocrystalline powders followed by a consolidation process [5, 6]. In order to compensate the grain growth during the consolidation, it is desirable to have as small grains in the powders as possible. Cryogenic milling, an alternative method of the more common room temperature ball milling, is able to achieve nanocrystalline copper and zinc powders in a shorter amount of milling time [7, 8]. Combined with optimized hot pressing, this time and cost efficient approach is expected to create a nanocomposite structure with high density of interfaces to serve as phonon scattering sites and therefore lead to im
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