Nano-microstructural control of phonon engineering for thermoelectric energy harvesting
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Introduction Manipulating thermal-transport properties is important for various applications ranging from integrated circuits and optoelectronics to thermoelectric energy conversion.1,2 Thermoelectric generators, capable of realizing direct energy conversion from heat into electricity, are solid-state devices with broad future promise for low-grade waste heat harvesting. The conversion efficiency is dominated by the material’s dimensionless thermoelectric figure of merit:
(
)
ZT = S 2σ κ T ,
(1)
where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity (including contributions from lattice, κlat , and electrons, κel), and absolute temperature, respectively. For more than a half century, a ZT of approximately 1 has been generally considered as the benchmark for pursuing high ZT. Stimulated by the concept of “quantum confinement” coupled with the introduction of advanced synthesis techniques,3,4 a series of record-high ZT values have been realized for materials and structures ranging from quantum dots
(zero-dimensional structures) to bulk nanostructured materials (three-dimensional [3D] structures) over the past two decades.5–7 These critical advances are mainly attributed to reduced κlat, since κlat is relatively independent of the electronic properties.8 Generally, heat conduction in a semiconductor is mostly dominated by lattice vibrations, which can be approximately understood using kinetic theory:
1 3
κ lat = Cvvl ,
(2)
where Cv is the specific heat of the semiconductor, ν is the phonon group velocity, and l is the phonon mean free path (MFP). Heat transport in thermoelectric materials results from the cumulative contributions of phonons with a broad range of MFPs, and different MFPs contribute different amounts to κlat for a specific material.9 Taking bulk crystalline silicon as a representative example, its MFP spectrum spans more than five orders of magnitude from 1 nm to 100 μm at room temperature.10 Thus, designing multiscale microstructures encompassing point defects, linear defects, interfacial defects, and volume defects could scatter phonons across a wide range of length scales, maximally suppress κlat, and thereby increase ZT.11–14
Zihang Liu, Department of Physics, University of Houston, USA; [email protected] Jun Mao, Department of Mechanical Engineering, University of Houston, USA; [email protected] Te-Huan Liu, Department of Mechanical Engineering, Massachusetts Institute of Technology, USA; [email protected] Gang Chen, Department of Mechanical Engineering, Massachusetts Institute of Technology, USA; [email protected] Zhifeng Ren, Department of Physics, University of Houston, USA; [email protected] doi:10.1557/mrs.2018.7
• VOLUME © 2018 Materials Research Society MRS 43 • of MARCH 2018 • www.mrs.org/bulletin Downloaded from https://www.cambridge.org/core. University of New England, on 10 Mar 2018 at 01:50:48, subject to theBULLETIN Cambridge Core terms use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2018.7
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