Transcript Слайд 1 - University of Groningen

Thermoelectric energy
Oleksandr Shpak
Outline
1. Overview






1.1 Why Thermoelectricity?
1.2 Possible Applications
1.3 How Does Thermoelectricity Work?
1.4 Efficiency
1.5 Historical Overview
2. Nano-scale Materials Technology




2.1 Quantum-Dot Superlattice (QDSL)
2.2 Silicon Nanowires
2.3 Nanocomposites
3. Summary

2
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
1.1 Why Thermoelectricity?
With thermoelectric energy conversion heat is converted directly into electricity
using a class of materials known as thermoelectric materials.




Solid-state reliability,
predictability and stability
Does not use any moving
parts
Vibration/noise free
Chlorofluorocarbon-free, no
environmentally harmful
fluids.
A. J. Minnich, M. S. Dresselhaus, Z. F. Ren and G. Chen, Energy
Environ. Sci., 2009, 2, 466–479
3
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
1.2 Applications
Vehicle waste heat recovery to improve fuel
economy.
Industrial waste heat recovery (incinerators, cement,
steel mills, and so on).
Site-specific and on-demand cooling in electronic
TE Climate Control Technologies.
Radioisotope heat-powered thermoelectric
generators in space probes sent beyond Mars
4
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
1.3 How thermoelectricity works?
F.J. DiSalvo, Science 1999, 285, 703
5

Conversion of temperature differences
directly into electricity (Seebeck effect )

Diffusion of mobile carriers from the
hot side to the cold side

Electrochemical potential will form in
response to a temperature gradient
(Seebeck voltage)

Thermoelectric characteristic of the
material is the amount of voltage
generated per unit temperature
gradient (Seebeck coefficient )

Can also be used as solid-state
refrigerators or heat pumps by driving
a current in a circuit with two dissimilar
materials (Peltier effect)
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
1.4 Efficiency

Modern devices consist of many legs of
alternating n-type and p-type materials,
allowing a current to flow through each leg
sequentially while heat flows through each
leg in parallel

Today thermoelectric devices are not in
common use because of low efficiency
and engineering considerations

We desire materials with high electrical
conductivity, high Seebeck coefficient,
and low thermal conductivity
G.J Snyder and E.S. Toberer, Nature Materials 2008, 7, 105
6
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
1.4 Efficiency
Power conversion efficiency is
critically dependent on the
material Figure of Merit (ZT)
S - Seebeck coefficient
σ - electrical conductivity
k - thermal conductivity
T - absolute temperature at which the
properties are measured
www.eere.energy.gov/vehiclesandfuels/pdfs/deer_2002/session2/
2002_deer_ venkatasubramanian1.pdf
7
Optimizing all the parameters
together turns out to be very difficult
because the properties are
interdependent.
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
1.5 Historical Overview
http://epa.gov/ncer/publications/workshop/9-15-2003/pdf/Rama.pdf
During the period 1960–1990 the (Bi1–xSbx)2(Se1–yTey)3 alloy family remained the best
commercial material with ZT≈ 1.
An alternative approach is to create nanostructured materials. By using the same
materials but in a nanostructured form, it is possible to modify thermoelectric properties in
ways that are not possible with bulk materials, which can lead to an enhancement in ZT.
8
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
2.1 Quantum-Dot Superlattice (QDSL)
Schematic drawing of a QDSL
Two strategies: the use of quantumconfinement phenomena to enhance S (to
control S and σ somewhat independently), and
the use of numerous interfaces to scatter
phonons more effectively than electrons
With a quantum well width below 4 nm S
could be increased relative to bulk.
T. C. Harman, P. J. Taylor, M. P. Walsh, B. E. LaForge, Science
2002, 297, 2229.
9
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
2.1 Quantum-Dot Superlattice (QDSL)
Now it is possible to grow superlattices of
such sandwich structures over thousands
of periods to produce a QDSL of
composition PbTe/PbSe0.98Te0.02. Using Bi
as an n-type dopant for this QDSL, a value
of ZT~1.6 was achieved at 300K.
These materials are not practical for
large-scale commercial use because
they are fabricated by atomic layer
deposition processes (molecular beam
epitaxy etc.), making them slow and
expensive to fabricate.
T. C. Harman, M. P. Walsh, B. E. LaForge, G. W. Turner, J. Electron.
Mater. 2005, 34, L19.
10
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
2.2 Silicon Nanowires
Si is the most abundant and widely used
semiconductor, with a large industrial
infrastructure for low-cost and high-yield
processing.
Arrays of Si nanowires were synthesized by an
aqueous electroless etching (EE) method.
The nanowires varied from 20 to 300 nm in
diameter with an average diameter of
approximately 100 nm.
l.I. Hochbaum, R. Chen, R.D. Delgado, W. Liang, E.C. Garnett,
M. Najarian, A. Majumdar, and P. Yang, Nature 2008, 451, 163
11
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
2.2 Silicon Nanowires
Large difference in mean free path lengths
between electrons and phonons at room
temperature: 110 nm for electrons in highly
doped samples and 300 nm for phonons.
Reduction in thermal conductivity without
significantly affecting S.
It is possible to achieve ZT = 0.6 at room
temperature in rough Si nanowires of 50 nm
diameter. With optimized doping, diameter
reduction and roughness control, the ZT is
likely to rise even higher.
A.I. Boukai, Y. Bunimovich, J. Tahir-Kheli, J.-K. Yu, W.A. Goddard III
and J.R. Heath,,Nature 2008, 451, 168
12
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
2.3 Nanocomposites
Reduced lattice thermal conductivity does
not require an atomically perfect interface
or an exact geometry.
Ball milling and hot pressing can be used to
create nanograined materials.
Nanocomposites retain the high density of
interfaces but do not have a special
geometry or structure, significantly
simplifying the fabrication process and
allowing the material to be produced in
large quantities.
A.J. Minnich, M.S. Dresselhaus, Z.F. Ren and G. Chen, Energy
Environ. Sci. 2009, 2, 466–479
13
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
3. Summary
Even with the current efficiencies of thermoelectric devices billions of dollars
could be saved each year if thermoelectric generators were used on the exhaust
of vehicles.
Modeling and experimental efforts help to understand the fundamental physics of
phonon transport which is the key to further reducing the thermal conductivity.
Further increases in ZT should be possible.
14
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009
Thank you for you attention
15
"Energy & Nano" - Top Master in Nanoscience Symposium
17 June 2009