Improvement of Thermoelectric Properties through Reduction of Thermal Conductivity by Nanoparticle Addition and Stoichio
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MRS Advances © 2017 Materials Research Society DOI: 10.1557/adv.2017.507
Improvement of Thermoelectric Properties through Reduction of Thermal Conductivity by Nanoparticle Addition and Stoichiometric Change to Mg2Si William T. Yorgason1, Arden N. Barnes1, and Nick Roberts1 Mechanical and Aerospace Engineering, Utah State University, Logan, Utah 84341, U.S.A.
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ABSTRACT Thermoelectric materials have been of interest for several decades due to their ability to recapture waste heat of various systems and convert it to useful electricity. One method used to improve the thermoelectric efficiency of a material is to reduce the lattice thermal conductivity (kp) while not affecting the other properties. In order to reduce the kp of the material, this paper introduces silicon (Si) nanoparticles (NPs) in Mg2Si to manipulate phonon scattering and mean free path. A series of simulations is performed with the metal silicide thermoelectric material MgxSix. The objective of this work is two-fold: 1) to determine the optimal Si nanoparticle (NP) concentration and 2) to determine the optimal MgxSix stoichiometry for minimizing the kp of the system. It should be noted, however, that the assumed reduction in thermal conductivity is only a result of reduced phonon transport and that minimal impact is made on the transport of electrons. Interestingly, the uniform off-stoichiometry (49.55 atomic percent (a/o) Si) sample of MgxSix resulted in a reduction of kp of 84.62 %, while the Si NP sample, with matching a/o Si, resulted in a reduction of kp of 78.82 %. INTRODUCTION Addition of Si NPs to Magnesium Silicide (Mg2Si), a well-known thermoelectric, may increase this material’s efficiency in waste-heat recovery, as shown in similar work [1]. The figure of merit (ZT), which is used to determine the efficiency of a thermoelectric material to convert heat into electric power, is a dimensionless parameter described by equation 1 [2]: ܼܶ ൌ ܵ ଶ
ఙ்
(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 thermoelectric material. Equation 1 shows that ZT can be increased by lowering the k of the material. k is defined by equation 2 [2]: ݇ ൌ ݇ ݇
(2)
where kp is the lattice contribution, and ke the electron contribution, to the k of the material. Usually, the positive correlation between k and σ, which occurs due to the positive correlation between ke and σ, presents a challenge in achieving a high ZT value in thermoelectric materials. However, at 300 K, the ke of Mg2Si is very small relative to k, being 0.2 Wm-1 K-1 or less [3], while k has been measured at 7.8 Wm-1K-1 [4]. Therefore, k should depend much more on changes in kp than changes in ke. Further, the Si NPs (32.562 Å in diameter) should cause much more phonon scattering than electron scattering, due to the size difference in the mean free paths of phonons and electrons [5]. Therefore, the presence of Si NPs should not cause a significant change in σ or ke. In addition, adding Si NPs to
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