Thermoelectrics
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Benedikt Klobes
JCNS-2 & PGI-4, Forschungszentrum Jülich, Germany
19th September 2014 | Hercules Specialized Course 17
What are
thermoelectrics?
Why using
synchroton X-rays
and neutrons?
R. Simon wondering at ID18.
1. Basics of thermoelectricity (TE)
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2. Dynamical aspects of thermoelectric materials
3. Synchrotron X-rays for TE research
4. Neutrons for TE research
examples
Basics of Thermoelectrics
based on the Seebeck effect
G. Snyder et al., Nat. Mater. 7 (2008) 105.
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V  ST
hot
cold
reversed: Peltier effect
 can be used for thermoelectric
cooling
P  S T
Kelvin relation
Basics of Thermoelectrics
www.aist.go.jp
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wikipedia
actual usage of thermoelectric
generators (TEG):
1. space missions/probes
2. remote locations
3. gadgets (e.g. charging …)
there is no actual large-scale
application of TEG
Basics of Thermoelectrics – the figure-of-merit
I 2R
QH  T  STH I 
2
2
2
S

T
RL
2
W  I RL 
( R  RL )2
QH
hot junction @ TH
I
n-type
p-type
thermoelectric efficiency: T  (TC  TH ) / 2
I
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W
T

 ... 
QH
TH
cold junctions @ TC
QC
RL
R
1  ZT  1
1  ZT  TC / TH
some assumptions and redefinitions
ZT 
S 2

T
!!!!!
Basics of Thermoelectrics – the figure-of-merit
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ZT
www.nistep.go.jp
 > 0.1  ZT > 1 ,
but also depending on the
temperature range
Carnot efficiency for a 800 K
to 300 K „machine“ ~ 0.63
real-world applications require
 > 0.1 (economically …)
temperature (K)
Basics of Thermoelectrics – routes for research
S 2
S 2
ZT 

T

 elec   lat
the electronic transport part  maximizing the power factor S2
2
8 2kb

S
m *T 3
2
3eh
9n 2
2
ne2

m*
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but carrier concentration and
thermal transport, i.e. , are
coupled via the
Wiedemann-Franz-law
 elec  LT
A. Shakouri, Ann. Rev. Mat. Sci. 41 (2011) 399.
Basics of Thermoelectrics – routes for research
S 2
S 2
ZT 

T

 elec   lat
 elec  LT
the thermal transport part  reducing the lattice part of thermal conductivity
without impeding electronic transport properties
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 lat 
electron crystal phonon glass
max
 C ()v
s
0
G
( )l ( )d
reduce phonon mean free path l
reduce phonon group velocity vG
reduce phonon heat capacity Cs
Basics of Thermoelectrics – routes for research
 lat 
max
 C ()v
s
G
( )l ( )d
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0
possible strategies:
• introduce scattering centers: defects, rattling atoms and many more …
• use low density materials
• push the acoustic phonon branches down using complex cells …
• make use of highly anisotropic properties (layered materials …)
•…
 in any case, X-rays and neutrons are mandatory for a microscopic
understanding of the vibrational properties of thermoelectrics
Lattice dynamics in simple PbTe
PbTe is the gold standard of TE:
• applied since mid of the 1950s
• high ZT values
• great flexibility concerning doping
• possibility to tune microstructure
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special Ti
alloys
glass
K. Biswas et al., Nature 489 (2012) 414.
Lattice dynamics in simple PbTe
and in related SnTe and GeTe …
<u2>
vS
EA
Eint
P. Bauer Pereira et al., Phys. Status Solidi B 250 (2013) 1300.
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using nuclear inelastic scattering
in order to obtain the element
specific densities of phonon states,
here via the 119Sn & 125Te
resonances
fLM
<F>
Svib
ID18 @ ESRF
Fourier-Log
decomposition
P. Bauer Pereira et al., Phys. Status Solidi B 250 (2013) 1300.
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Lattice dynamics in simple PbTe
Lattice dynamics in simple PbTe
g(E)
M
lim

E 0 E 2
2 2  3vS3
PbTe
SnTe
GeTe
vS = 1850(80) m/s
vS = 1800(80) m/s
vS = 1900(70) m/s
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 2  g(E) 
 D  3 /  k B  2 dE 
 0 E

PbTe
SnTe
GeTe
QD = 170(2) m/s
QD = 160(5) m/s
QD = 180(5) m/s
GeTe slightly „harder“ than PbTe
and SnTe …
Lattice dynamics in simple PbTe
in some sense just numbers, but test for acoustic mismatch hypothesis
from PbTe to AgPb18SbTe20 :
PbTe matrix plus
coherent precipitates rich in Ag & Sb
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reduction of lattice thermal conductivity
due to „impedance“ mismatch between
matrix and precipitates?
PbTe
SnTe
GeTe
vS = 1850(80) m/s
vS = 1800(80) m/s
vS = 1900(70) m/s
PbTe
SnTe
GeTe
QD = 170(2) m/s
QD = 160(5) m/s
QD = 180(5) m/s
125Te
& 121Sb NIS on AgPb18SbTe20
speak with Atefeh
during coffee break
Nanocrystallinity and lattice dynamics – Si
high-energy
ball milling
spark plasma
sintering
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IN6 @ ILL
T. Claudio et al., J. Mater. Sci. 48 (2013) 2863.
www2.cpfs.mpg.de
How to achieve nanocrystalline, but bulk compounds?
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T. Claudio et al., J. Mater. Sci. 48 (2013) 2863.
Nanocrystallinity and lattice dynamics – Si
• nanocrystallinity has a strong impact on thermal conductivity
• peak at around 6 meV indicative for amorphous SiO2
confirmed by
PGAA, Raman
• broad feature 80 – 160 meV : H impurities in Si
• Deybe level changes drastically  different acoustic group velocities
from 6000 m/s to ~ 3400 m/s
Limits of  – Bi2Te3 based thermoelectrics
Besides PbTe, Bi2Te3 is the other gold standard of thermoelectrics
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grainsizes:
5 mm (as-cast)
vs.
25 nm (nano)
FOCUS @ SINQ
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Limits of  – Bi2Te3 based thermoelectrics
• boundary scattering not sufficient
• other mechanisms due to synthesis?
point defects, dislocations …
 strain and mass fluctuations
Rattling atoms in thermoelectrics
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Introducing additional atoms in voids of some structures
 significant decrease of thermal conductivity
skutterudite, e.g. In0.2Co4Sb12
clathrate, e.g. Sr8Ga16Ge30
possible reasons:
mass density fluctuations, lower speed of sound, rattling behavior …
Rattling of In in In0.2Co4Sb12
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In0.2Co4Sb12
Rattling of In in In0.2Co4Sb12
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Atomic dynamics also present in „simple“ neutron diffraction, i.e. in the
atomic displacement parameters (ADP)
POWGEN @ SNS
• huge difference between guest- and host ADP
• excessive specific heat at low-T  Einstein
Lattice dynamics of rattling atoms
besides Einstein-like ADP and specific heat, „confined“ and low energetic
phonons are found e.g. using NIS  support for „rattling“ notion
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ID18 @ ESRF
Xe clathrate hydrate
G. J. Long et al., Phys. Rev. B 71 (2005) 140302.
B. Klobes et al., EPL 103 (2013) 36001.
Do rattling atoms really rattle (independently)?
large ADP and Einstein-mode like behavior  independent rattlers?
resonant scattering mechanism?
Maybe: avoided crossing between acoustical and optical branches?
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RITA-II @ SINQ
Ba8Ga16Ge30
M. Christensen et al., Nat. Mater. 7 (2008) 811.
Do rattling atoms really rattle (independently)?
large ADP and Einstein-mode like behavior  independent rattlers?
resonant scattering mechanism?
Maybe: avoided crossing between acoustical and optical branches?
M. Christensen et al., Nat. Mater. 7 (2008) 811.
H. Euchner et al., Phys. Rev. B 86 (2012) 224303.
Ba8Ni3.5Ge42.1□0.4
T2 @ LLB
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Ba8Ga16Ge30
Do rattling atoms really rattle (independently)?
well, not that independently … and probably not solely rattling …
comparative study using
La and Ce filled Fe4Sb12
IN4,IN6 @ LLB
G. Nolas et al., Phys. Rev. B 58 (1998) 164.
calculation
assuming
coupling
M. Koza et al., Nat. Mater. 7 (2008) 805.
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measurement
LaxCo4Sn2Sb10
first decrease, then increase in x
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nanoengineering:
• quantum dots
• nanowires
• superlattices
•…
specific design
of thermal and
electronic transport
properties
M. Beekman et al., Semicond. Sci. Technol. 29 (2004) to be published
Artificial structures – misfit layered compounds
NIS by [(SnSe)1.04]n[MoSe2]m
k
3-ID-B @ APS
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NIS probes
phonons with
polarization
along k
Thermoelectrics – other challenges
Zhao et al., Nature 508 (2014) 373.
G. Snyder et al., Nat. Mater. 7 (2008) 105.
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in the present discussion, electronic transport properties were completely
omitted
• band engineering: narrow gaps,
2
2
S
S
sharp slope of electronic DOS
ZT 

T
• exploit crystal anisotropy

 elec   lat
• control synthesis of complex alloys
• scalability !!!
• module related issues  contacts
• replacement of toxic elements
Thermoelectrics, neutrons and X-rays …
Improvement of Thermoelectrics  Dynamical Properties
Lattice dynamics  X-ray and Neutron Scattering
PbTe based Systems
Nanocrystalline Compounds
Notion of Rattling in Skutterudites
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Artifically Engineered Systems
Other Challenges for Thermoelectric Research
The people behind the science …
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R. Hermann
Group leader
B. Klobes
PostDoc
V. Potapkin
PostDoc
A. Mahmoud
PostDoc
M. Herlitschke
PhD student
R. Simon
PhD student
A. Jafari
PhD student
10 former members:
4 PhD, 1 Diploma, 3 B.Sc. thesis
P. Alexeev
PhD student
M. Mebonia
PhD student
F. Deng
M.Sc. student