Thermal Conductivity Reduction of SiGe Nanocomposites
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S7.2.1
Thermal Conductivity Reduction of SiGe Nanocomposites T. Harris1, H. Lee1, D. Z. Wang2, J. Y. Huang2, Z. F. Ren2, B. Klotz3, R. Dowding3, M. S. Dresselhaus4, and G. Chen1 1 Department of Mechanical Engineering, Massachusetts Institute of Technology (MIT) Cambridge, MA 02139-4307, U.S.A. 2 Department of Physics, Boston College, Chestnut Hill, MA 02476, USA 3 Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5069, USA 4 Department of Physics and Department of Electrical Engineering and Computer Science, MIT ABSTRACT High figure of merit has been reported in superlattice structures in recent years mainly due to a large reduction in thermal conductivity, while maintaining or even enhancing the power factor. These findings suggest the possibility of using nanocomposites to reduce a material’s thermal conductivity and thereby increasing the thermoelectric figure of merit. The current work reports on the thermal conductivity of various Si-Ge nanocomposites synthesized in a simple and straightforward process that allows one to exploit nanoscale physics while rapidly producing macroscale devices. Composite synthesis consisted of combining Si nanoparticles approximately 70 nm in diameter with micron-sized Ge particles in various atomic ratios via hotpressing to form macroscale samples. Room-temperature thermal conductivity was measured for each of the nanocomposite samples and compared with the values of SiGe bulk alloys. In this preliminary work, we observed a significant reduction in bulk thermal conductivity in such materials. INTRODUCTION The efficiency and energy density of a thermoelectric device is determined by the dimensionless thermoelectric figure of merit, ZT,
ZT =
S 2σ T, k
(1)
where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and k is the thermal conductivity of the material. Recent results of high-ZT superlattice (SL) structures [1, 2] show that for comparable power factors (S2σ) between the SL structure and its bulk counterpart, a substantial reduction in k has been observed. This result is elucidated in Table 1 and leads one to believe that by substantially reducing the thermal conductivity of a material, while maintaining the material’s power factor, one can achieve a large ZT enhancement.
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Power Factor, S2σ [µW/cm-K2] Conductivity, k [W/mK]
PbSeTe/PbTe Nanostructure 32 0.6
[1] Bulk 28 2.5
Bi2Te3/Sb2Te3 Nanostructure 40 0.5
[2] Bulk 35 2
Table 1. Power factor and thermal conductivity for high-ZT structures [1] and [2]. Bulk values were estimated from [3]. The observed power factors for both instances are comparable, but the conductivities between the nanostructures and the bulk show a substantial difference.
Recent theoretical work [4] regarding the thermal conductivity for both in-plane and cross-plane thermal conductivity of SL thin films shows that phonon coherence is not necessary to achieve a reduction in thermal conductivity. The key ingredient for achieving a “low” thermal conductivity lies in the reduction of the
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