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Thermoelectrics

Benedikt Klobes

JCNS-2 & PGI-4, Forschungszentrum Jülich, Germany

19th September 2014 | Hercules Specialized Course 17

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What arethermoelectrics?

Why using synchroton X-rays

and neutrons?

R. Simon wondering at ID18.

1. Basics of thermoelectricity (TE)

2. Dynamical aspects of thermoelectric materials

3. Synchrotron X-rays for TE research

4. Neutrons for TE research examples

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Basics of Thermoelectrics

G.

Sn

yde

r e

t a

l., N

at.

Ma

ter.

7 (

20

08

) 1

05

.

based on the Seebeck effect

TSV

hot cold

reversed: Peltier effect can be used for thermoelectriccooling

TSP Kelvin relation

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Basics of Thermoelectricsw

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.ais

t.g

o.jp

wik

ipe

dia

actual usage of thermoelectric

generators (TEG): 1. space missions/probes 2. remote locations 3. gadgets (e.g. charging …)

there is no actual large-scaleapplication of TEG

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Basics of Thermoelectrics – the figure-of-merit

p-type n-type

hot junction @ TH

cold junctions @ TC

RL

I I

QH

QC

2

2RIISTTQ HH

2

222

)( L

LL RR

RTSRIW

R

thermoelectric efficiency:

HCHH TTZT

ZT

T

T

Q

W

/1

11...

some assumptions and redefinitions

2/)( HC TTT

TS

ZT2

!!!!!

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Basics of Thermoelectrics – the figure-of-merit

temperature (K)

ZT

Carnot efficiency for a 800 Kto 300 K „machine“ ~ 0.63

real-world applications require h > 0.1 (economically …)

h > 0.1 ZT > 1 , but also depending on the temperature range

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w.n

iste

p.g

o.jp

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Basics of Thermoelectrics – routes for research

TSS

ZTlatelec

22

the electronic transport part maximizing the power factor S2s

but carrier concentration and thermal transport, i.e. k, are coupled via theWiedemann-Franz-law

A. Shakouri, Ann. Rev. Mat. Sci. 41 (2011) 399.

LTelec

32

2

2

22

9*

3

8

nTm

eh

kS b

*

2

m

ne

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Basics of Thermoelectrics – routes for research

TSS

ZTlatelec

22

the thermal transport part reducing the lattice part of thermal conductivitywithout impeding electronic transport properties

electron crystal phonon glass

dlvC Gslat )()()(max

0

reduce phonon mean free path l

reduce phonon group velocity vG

reduce phonon heat capacity Cs

LTelec

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Basics of Thermoelectrics – routes for research

dlvC Gslat )()()(max

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 microscopicunderstanding of the vibrational properties of thermoelectrics

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Lattice dynamics in simple PbTe

K. Biswas et al., Nature 489 (2012) 414.

PbTe is the gold standard of TE:• applied since mid of the 1950s• high ZT values• great flexibility concerning doping• possibility to tune microstructure

glass

special Ti alloys

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Lattice dynamics in simple PbTe

P. B

auer

Per

eira

et a

l., P

hys.

Sta

tus

Sol

idi B

250

(20

13)

1300

.

ID18 @ ESRF

and in related SnTe and GeTe …

using nuclear inelastic scatteringin order to obtain the elementspecific densities of phonon states,here via the 119Sn & 125Te resonances

vS

EA fLM

<u2>

Eint <F> Svib

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Lattice dynamics in simple PbTe

P. B

auer

Per

eira

et a

l., P

hys.

Sta

tus

Sol

idi B

250

(20

13)

1300

.

Fourier-Logdecomposition

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Lattice dynamics in simple PbTe

33220 2

)(lim

SE v

M

E

Eg

PbTe vS = 1850(80) m/sSnTe vS = 1800(80) m/sGeTe vS = 1900(70) m/s

dEE

EgkBD

02

2 )(/3

PbTe QD = 170(2) m/sSnTe QD = 160(5) m/sGeTe QD = 180(5) m/s

GeTe slightly „harder“ than PbTe and SnTe …

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Lattice dynamics in simple PbTe

PbTe vS = 1850(80) m/sSnTe vS = 1800(80) m/sGeTe vS = 1900(70) m/s

PbTe QD = 170(2) m/sSnTe QD = 160(5) m/sGeTe QD = 180(5) m/s

in some sense just numbers, but test for acoustic mismatch hypothesis

from PbTe to AgPb18SbTe20 :PbTe matrix pluscoherent precipitates rich in Ag & Sb

reduction of lattice thermal conductivitydue to „impedance“ mismatch betweenmatrix and precipitates?

125Te & 121Sb NIS on AgPb18SbTe20

speak with Atefehduring coffee break

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Nanocrystallinity and lattice dynamics – Si

How to achieve nanocrystalline, but bulk compounds?

high-energyball milling

spark plasmasintering

T. Claudio et al., J. Mater. Sci. 48 (2013) 2863.

IN6 @ ILL

ww

w2.

cpfs

.mpg

.de

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Nanocrystallinity and lattice dynamics – Si

T. C

laud

io e

t al.,

J. M

ater

. Sci

. 48

(201

3) 2

863.

• nanocrystallinity has a strong impact on thermal conductivity

• peak at around 6 meV indicative for amorphous SiO2

• broad feature 80 – 160 meV : H impurities in Si

• Deybe level changes drastically different acoustic group velocities

confirmed byPGAA, Raman

from 6000 m/s to ~ 3400 m/s

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Limits of k – Bi2Te3 based thermoelectrics

FOCUS @ SINQ

Besides PbTe, Bi2Te3 is the other gold standard of thermoelectrics

grainsizes:5 mm (as-cast)

vs.25 nm (nano)

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Limits of k – Bi2Te3 based thermoelectrics

• boundary scattering not sufficient• other mechanisms due to synthesis?

point defects, dislocations … strain and mass fluctuations

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Rattling atoms in thermoelectrics

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 …

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Rattling of In in In0.2Co4Sb12

In0.2Co4Sb12

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Rattling of In in In0.2Co4Sb12

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

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Lattice dynamics of rattling atoms

G. J. Long et al., Phys. Rev. B 71 (2005) 140302. B. Klobes et al., EPL 103 (2013) 36001.

ID1

8 @

ES

RF

besides Einstein-like ADP and specific heat, „confined“ and low energeticphonons are found e.g. using NIS support for „rattling“ notion

Xe clathrate hydrate

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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?

Ba8Ga16Ge30

RIT

A-I

I @ S

INQ

M. Christensen et al., Nat. Mater. 7 (2008) 811.

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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?

Ba8Ga16Ge30

M. Christensen et al., Nat. Mater. 7 (2008) 811.

Ba8Ni3.5Ge42.1□0.4

T2

@ L

LB

H. E

uchn

er e

t al.,

Phy

s. R

ev. B

86

(201

2) 2

2430

3.

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Do rattling atoms really rattle (independently)?

well, not that independently … and probably not solely rattling …

M. K

oza

et a

l., N

at. M

ater

. 7 (

2008

) 80

5.

IN4,IN6 @ LLB

comparative study usingLa and Ce filled Fe4Sb12

G. N

olas

et a

l., P

hys.

Rev

. B 5

8 (1

998)

164

.

LaxCo4Sn2Sb10

measurement calculationassumingcoupling

first decrease, then increase in x

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Artificial structures – misfit layered compounds

M. B

eekm

an e

t al.,

Sem

icon

d. S

ci. T

echn

ol. 2

9 (2

004)

to b

e pu

blis

hed

nanoengineering:• quantum dots• nanowires• superlattices • …

specific designof thermal and electronic transportproperties

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NIS by [(SnSe)1.04]n[MoSe2]m

3-I

D-B

@ A

PS

k

NIS probesphonons with polarizationalong k

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Thermoelectrics – other challenges

TSS

ZTlatelec

22

in the present discussion, electronic transport properties were completely omitted

• band engineering: narrow gaps, sharp slope of electronic DOS• exploit crystal anisotropy• control synthesis of complex alloys• scalability !!!• module related issues contacts• replacement of toxic elements

G.

Sn

yde

r e

t a

l., N

at.

Ma

ter.

7 (

20

08

) 1

05

.

Zh

ao

et

al.,

Na

ture

50

8 (

20

14

) 3

73

.

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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

Artifically Engineered Systems

Other Challenges for Thermoelectric Research

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R. HermannGroup leader

B. KlobesPostDoc

V. PotapkinPostDoc

A. MahmoudPostDoc

M. HerlitschkePhD student

R. SimonPhD student

A. JafariPhD student

P. AlexeevPhD student

M. MeboniaPhD student

F. DengM.Sc. student

10 former members:4 PhD, 1 Diploma, 3 B.Sc. thesis

The people behind the science …