Upload
ronna
View
62
Download
2
Tags:
Embed Size (px)
DESCRIPTION
Thermoelectricity of Semiconductors. Jungyun Kim December 2, 2008. Outline. Discovery of the thermoelectricity – Seebeck coefficient Operation of thermoelectric devices Architectural and materials enhancement Large impact of shrinking to nanoscale. Seebeck Effect. - PowerPoint PPT Presentation
Citation preview
Thermoelectricity of Semiconductors
Jungyun KimDecember 2, 2008
Outline
Discovery of the thermoelectricity – Seebeck coefficient
Operation of thermoelectric devices
Architectural and materials enhancement
Large impact of shrinking to nanoscale
Thermoelectricity - known in physics as the "Seebeck Effect"
• In 1821, Thomas Seebeck, a German physicist, twisted two wires of different metals together and heated one end.
• Discovered a small current flow and so demonstrated that heat could be converted to electricity.
www.worldofenergy.com.au/07_timeline_world_1812_1827.html
Seebeck Effect
www.dkimages.com/discover/DKIMAGES/Discover/Home/Science/Physics-and-Chemistry/Electricity-and-Magnetism/General/General-18.html
chem.ch.huji.ac.il/history/seebeck.html
Seebeck Effect
Metal rod
Electron mobility Phonon motion
Photon
Phonon motion
Electron mobility
Electrons in the hot region are more energetic and therefore have greater velocities than those in the cold regiondT
dVS
Seebeck Coefficient
Heat transfer through electrons and phonons (lattice vibrations)
Al Al
Thermoelectric Operation
Rowe D.M., Thermoelectrics Handbook, 2006. Snyder et al. Nature 7, 105-114, (2008).
e-
h+
• Electron/hole pairs created at the hot end absorbs heat.
• Pairs recombine and reject heat at the cold end.
• The net voltage appears across the bottom of the thermoelectric legs.
TSzT2
Figure of Merit – Conflicting Properties
S - Seebeck Coefficient3/2
*2
22
338
n
TmehkS B
n – carrier concentration m* - effective mass of carrier μ – carrier mobility
σ - Electron Conductivity
ne
1
Figure of Merit - zT
κ - Thermal Conductivity
LTneTLκκκκ
e
le
=>2Sz
Effect of Carrier Concentration
Snyder et al. Nature 7, 105-114, (2008).
TSzT2
Figure of Merit – Conflicting Properties
S - Seebeck Coefficient3/2
*2
22
338
n
TmehkS B
n – carrier concentration m* - effective mass of carrier μ – carrier mobility
σ - Electron Conductivity
ne
1
Figure of Merit - zT
κ - Thermal Conductivity
LTneTLκκκκ
e
le
=>2Sz
Effect of Temperature
Snyder et al. Nature 7, 105-114, (2008).
TSzT2
Figure of Merit – Conflicting Properties
S - Seebeck Coefficient3/2
*2
22
338
n
TmehkS B
n – carrier concentration m* - effective mass of carrier μ – carrier mobility
σ - Electron Conductivity
ne
1
Figure of Merit - zT
κ - Thermal Conductivity
LTneTLκκκκ
e
le
=>2Sz
Bell. Science, 321 (2008)
DiSalvo, Science, 285 (1999)
• Best micro-scale materials operate at ZT = 1
(10% of Carnot efficiency)
• To run at 30% efficiency (home refrigeration) need a ZT=4.
Architectural Enhancement
Functionally graded and segmented thermoelements
High-performance multisegmented thermoelectric
Rowe D.M., Thermoelectrics Handbook, 2006.
Materials Enhancement
Fleurial, J.-P. et al. Int. Conf. Thermoelectrics, (2001).
Snyder et al. Nature 7, 105-114, (2008).
Void spaces in CoSb2 are filled by alloying and doping decreasing thermal conductivity.
Complex crystal structures that yield lowlattice thermal conductivity.
Zn4Sb3 (left), highly disordered Zn sublattice with filled interstitial sites, and complexity of Yb14MnSb11 (right) unit cell
Calculated dependence of zT for Bi2Te3 structure material
Macro to Nano – Thermal conductivity
Hicks, L.D. and Dresselhaus, M.S. Effect of quantum-well structures on the thermoelectric figure of merit. Physical Review B, 47, 12727-12731 (1993).
Venkatasubramanian R. et al. Thin-film thermoelectric devices with high room-temperature figures of merit. Nature 413, 597-602 (2001).
Recent Developments – Si Nanowires
Hochbaum, A.I. et al. Enhanced thermoelectric performance of rough silicon nanowires. Nature 45, 10 163-167 (2008).
SEM image of a Pt-bonded EE Si nanowire. Scale bar 2um.
• Near both ends are resistive heating and sensing coils to create a temperature gradient.
• To measure conductivity, I-V curves were recorded by a source meter
• Seebeck voltage (∆Vs) was measured by multimeter with a corresponding temperature difference ∆T
Single nanowire power factor (red) and calculated zT (blue)for 52nm nanowire. Uncertainty in measurements 21% for power factor and 31% for zT.
Thermoelectric enhancement through introduction ofnanostructures at different length scales1. Diameter2. Surface roughness3. Point defects
Motivation and Applications• Approximately 90% of world’s power is generated by heat engines that
use fossil fuels combustion– Operates at 30-40% of the Carnot efficiency – Serves as a heat source of potentially 15 terawatts lost to the
environment• Thermoelectrics could potentially generate electricity from waste heat• Thermoelectrics could be used as solid state Peltier coolers
www.chinatraderonline.com
www.solarsolutions.ca
Rowe D.M., Thermoelectrics Handbook, 2006.
http://www.phys.psu.edu/nuggets/?year=2004
Summary
• Enhanced scattering of phonons– Increased surface area to volume– Greater surface roughness– Inclusion of dopants and point defects
• Macro to Nano– Greater decrease in thermal conductivity than
electron conductivity from decrease in diameter (3D → 2D → 1D)
• Current research– Development of Si nanowire thermoelectric
properties– Advancement in nanowire processing of well-
known thermoelectric materials
Macro to Nano - Electron ConductivityElectron scattering from surface imperfections and grain boundaries and interfaces
Quantum confinement: external conduction and valence band move in opposite directions to open up band-gap
Bulk
90 nm
65 nm
www.itrs.net/reports.html Dresselhaus, M.S. Physical Review B 61, 7 (2000).