Thermoelectric figure of merit and thermal conductivity of type-l clathrate alloy nanowires
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Research Letter
Thermoelectric figure of merit and thermal conductivity of type-I clathrate alloy nanowires Prabhjot Kaur, Institute of Nano Science and Technology, Habitat Center, Phase-X, Mohali, Punjab – 160062, India Georg K. H. Madsen, Institute of Materials Chemistry, TU Wien, A-1060 Vienna, Austria Chandan Bera, Institute of Nano Science and Technology, Habitat Center, Phase-X, Mohali, Punjab – 160062, India Address all correspondence to Chandan Bera at [email protected] (Received 1 November 2018; accepted 17 December 2018)
Abstract Clathrates based on Si and Ge have very low lattice thermal conductivity (∼1 W/m-K). This value can potentially be further reduced by alloying and nano-structuring. In this work, the thermal conductivity of Si and Ge clathrates alloy have been investigated using model based on the relaxation time approximation. By including alloy scattering, we find that the lattice thermal conductivity of Ba8Cu6Si40 is reduced by 50% from 1.64 to 0.80 W/m-K in Ba8Cu6Si40(1−x)Ge40x alloy. Further ∼90% reduction of the thermal conductivity is possible for nanowire clathrate alloys. The ultra-low thermal conductivity in the nanowire will be very suitable for the thermoelectric application.
Thermal conductivity is one of the dominant parameters for efficiency optimization in a vast range of devices. It plays an important role in energy conversion, transfer and storage applications. Understanding of thermal transport properties in complex nanomaterials can lead to the design of many modern devices such as thermoelectric,[1,2] optoelectronic,[3,4] photovoltaics[3,5] and storage devices.[6] The ability to optimize the thermoelectric performance of materials by minimizing its lattice thermal conductivity opens for the fabrication of a much larger category of efficient thermoelectric materials. There is a close relation between the complexity of the crystal structure and the thermal conductivity. Thermal conductivity is directly related to the phonon velocities in the system, which in turn can be strongly anisotropic depending on the crystal structure.[7] The anharmonic scattering rates also depend intricately on the crystal structure and constituent atoms.[8] Besides the intrinsic properties, alloying, substitution, and nano-structuring have been found to be very effective in controlling the thermal conductivity of large range of materials. For thermoelectric application, a semi-conducting material with low (lattice) thermal conductivity and high electrical conductivity is a long-standing quest. Clathrates are one type of compounds which show such behavior and have been reported as potential efficient thermoelectric materials due to their very low thermal conductivity.[9–12] Clathrates are made up of a guest-host assembly with a complex crystal structure (Fig. 1), and the anharmonicity of the atoms depends on the chemical bonding between the different atoms. The vibrations of the guest atoms depend on their position in the clathrate cage[13] as off-center and on-center vibrations of the guest atoms will b
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