ME 381R Lecture 20

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ME 381R Lecture 20 Nanostructured
Thermoelectric Materials
Dr. Li Shi Department of Mechanical Engineering
The University of Texas at Austin Austin, TX
78712 www.me.utexas.edu/lishi lishi_at_mail.utexas.
edu
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Thermoelectrics
250C

• Thermocouple

250C
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Thermoelectric Cooling Performance
Venkatasubramanian et al. Nature 413, 597
Cold

Nanostructured thermoelectric materials
2.5-25nm
Bi2Te3/Sb2Te3 Superlattices
Harman et al., Science 297, 2229
Hot
Quantum dot superlattices

• Coefficient of Performance (COP?Q/IV)

2
CFC unit
1
COP
Bi2Te3
0
0
1
2
3
4
5
ZT

• ZT Figure of Merit

Seebeck coefficient
Electrical conductivity
Thermal conductivity
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Thin Film Superlattice Thermoelectric Materials

• Thin film superlattice

• Approaches to improve Z ? S2s/k
• --Frequent phonon-boundary scattering low k
• --High density of states near EF high S2s in QWs

Quantum well (smaller Eg)
Barrier (larger Eg)
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Electronic Density of States in 3D

• Each state can hold 2 electrons
• of opposite spin(Paulis principle)
• Number of states with wavevectoreltk

2D projection of 3D k space
ky
dk
k
kx
2p/L

• Number of states with energyltE

Density of States
Number of k-states available between energy E and
EdE
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Electronic Density of States in 2D

• Each state can hold 2 electrons
• of opposite spin(Paulis principle)
• Number of states with wavevectoreltk

2D k space (kz 0)
ky
dk
k
kx
2p/L

• Number of states with energyltE

Density of States
Number of k-states available between energy E and
EdE
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Electronic Density of States in 1 D
1D k space (ky kz 0)
k

• Each state can hold 2 electrons
• of opposite spin(Paulis principle)
• Number of states with wavevectoreltk

• Number of states with energy lt E

Density of States
Number of k-states available between energy E and
EdE
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Electronic Density of States
Ref Chen and Shakouri, J. Heat Transfer 124, p.
242 (2002)
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Low-Dimensional Thermoelectric Materials

• Thin Film Superlattices of
• Bi2Te3,Si/Ge, GaAs/AlAs

Barrier
Quantum well
Ec
E
Ev
x
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Potential Z Enhancement in Low-Dimensional
Materials

• Increased Density of States near the Fermi Level
  
• high S2s (power factor)
• Increased phonon-boundary scattering low k

? high Z S2s/k
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Thin Film Superlattices for TE CoolingVenkatasubr
amanian et al, Nature 413, P. 597 (2001)
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Z Enhancement in Nanowires
Experiment
Theory
Nanowire
Prof. Dresselhaus, MIT Phys. Rev. B. 62, 4610
Heremans et at, Phys. Rev. Lett. 88, 216801
Challenge Epitaxial growth of TE nanowires with
a precise doping and size control
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Imbedded Nanostructures in Bulk Materials

• Nanodot Superlattice

Data from A. Majumdar et al.

• Bulk materials with embedded nanodots

Images from Elisabeth Müller Paul Scherrer
Institut Wueren-lingen und Villigen, Switzerland
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Phonon Scattering with Imbedded Nanostructures
Spectral distribution of phonon energy (eb)
group velocity (v) _at_ 300 K
Long-wavelength or low-frequency phonons are
scattered by imbedded nanostructures!
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Challenges and Opportunities

• Designing interfaces for low thermal conductance
  at high temperatures
• Fabrication of thermoelectric coolers using
  low-thermal conductivity, high-ZT nanowire
  materials
• Large-scale manufacturing of bulk materials with
  imbedded nanostructures to suppress the thermal
  conductivity