Recent advances on

thermoelectric glasses

A.P. Gonçalvesa, E.B. Lopesa, L.M. Ferreirab,

G. Delaizirc, O. Rouleauc, C. Godartc

aDep. Química, Instituto Tecnológico e Nuclear/CFMC-UL,

P-2686-953 Sacavém, Portugal.

bDep. Física, Instituto Tecnológico e Nuclear, P-2686-953 Sacavém,

Portugal.

cCNRS, ICMPE, CMTR, 2/8 rue Henri Dunant, 94320 Thiais, France.

OUTLINE

•INTRODUCTION : ZT, PF (a,),

•BULK : generally high PF => minimize lattice

•GLASS : lattice minima => increase PF => TE ? (2006)

•CONDUCTING GLASS : -semiconductor (SC); -new results

•SUMMARY

Thermoelectric Effect

n- type

-

p- type

+

No temperature gradient:

Uniform distribution

of the charge carriers.

With temperature gradient:

Carriers at the hot end have higher kinetic

energy

Charge carriers diffusion to the cold zone

A voltage appear:

V = apn (Th – Tc)

hot

cold

e- h

I x

0

Electrical and Thermal transport

•Two kinds: n (a<0) e p (a>0);

•Electrical current (I) direction: charge carriers move from cold to hot=>

Entropy transfer from the cold to the hot junction (opposite to normal heat flux);

L

Heat flux due to Peltier effect and

to (opposite) thermal conductivity

dx

dTATIQ pppp a

dx

dTATIQ nnnn a

Total heat flux

(extracted from cold zone)

Qf = (Qn + Qp)x=0

BUT: heat is produced by Joule effect, I2r/A (per unit length)

RITKITQ fnpf

2

2

1)( aa

Q flux

I current

thermal cond.

A section

elect. cond.

r = 1/

L

A

L

AK

n

nn

p

pp

A

L

A

LR

n

nn

p

pp r

r

Thermal conductance

Electrical resistance

and

Dissipation of an electric power (Seebeck + Joule)

W= I . [(ap-a

n).T + IR]

Cooling: coefficient of performance

IR).Δ-[( I

RI2

1KΔ)IT(

W

QC

np

2cnp

f

T

T

OPaa

aa

Qf heat extracted from cold

Electricity generation: efficiency

Qc heat flux passing through

Pu useful electric power

r charge resistance

r)(RΔT

ΔT

2

I2

1K)IT-(

IR).-I [(

Q

Pη

cn

n

c

u

p

p

aa

aa

Cooling

2/1

2/1

maxZT)(11

ZT)(1

TT

TTCOP

hc

hc

Generation

TT

TT

hc

ch

1/2

1/2

maxZT)(1

1ZT)(1

Maximizing

2

2

rr

aa

nnpp

np

Z

2

ch TTT

Z only depends

on the materials

Simplification:

Similar values of Z for n & p types at T => Z mean

2

2

rr

aa

nnpp

np

Z

r

a 2

Z

It is convenient to define a figure of merit zT for a single material

in order to compare and optimize them

zT = a2T/

(adimensional, depends only on the material)

a = Seebeck coefficient , = electrical conductivity, = thermal conductivity

Efficiency or Coefficient of

Performance maximization

zT

maximization

Maximization of a2 (power factor)

Minimization of

T

Maximization of a2

a large => only a single type of carrier;

Metals or degenerate semiconductors:

n – carrier concentration

m* - effective mass of carrier

m – carrier mobility

Maximization of a2

10

14

10

16

10

18

10

20

10

22

Semiconductors

Metals Insulators

a2

a

Carrier content concentration (cm−3)

Minimization of

= elect + lat

Wiedemann-Franz law

elect = LT

Decrease of lat

Approach Effects on phonons Materials

(examples)

Heavy atoms weakly

bounded to the structures Phonon-scattering centers

Skutterudites

Clathrates

Complex structures Increase the optical phonon modes Clathrates

Yb14MnSb11

Inclusions, impurities Increase diffusion

(affects more phonons than carriers) Composites

Solid solutions Increase mass fluctuations

(higher phonon scattering)

Half-Heusler

systems

Grain boundaries Reduce the phonons mean free path Low dimensional

systems

Phonon-glass/electron-crystal (PGEC) materials

G. Slack, in CRC Handbook of Thermoelectrics, 1995

0 200 400 600 800 1000 1200 1400

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

CuMo6Se

8

Yb14

MnSb11

NaxCoO

2

Ca3 -x

NaxCo

4O

9

Ba8Ga

18Ge

28

Borides

-FeSi2

Si0.80

Ge0.20

Pb1-x

SnxTe

1-ySe

y

CeFe3.5

Co0.5

Sb12Zn

4Sb

3Zn

4 -xCd

xSb

3

Bi2-x

SbxTe

3

Thermoelectric materials: p-type Z

T

Temperature (K)

0 200 400 600 800 1000 1200 14000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

In2-x

GexO

3

Ti0.5

(Zr0.5

Hf0.5

)0.5

NiSn0.998

Sb0.002

In0.2

Ce0.2

Co4Sb

12

In0.2

Co4Sb

12

Ba8Ga

16Ge

30

Ba0.3

Co3.95

Ni0.05

Sb12

SrPbO3

LaTe1.45

Si0.80

Ge0.20

-FeSi2

Pb1-x

SnxTe

1-ySe

y

CoS

b 3

Bi2(Sb,Te)

3

Bi2-x

SbxTe

3

Thermoelectric materials: n-type

ZT

Temperature (K)

BULK

higher zT

many chalcogenides

pnictides

Approach Effects on phonons Materials

(examples)

Heavy atoms weakly

bounded to the structures Phonon-scattering centers

Skutterudites

Clathrates

Complex structures Increase the optical phonon modes Clathrates

Yb14MnSb11

Inclusions, impurities Increase diffusion

(affects more phonons than carriers) Composites

Solid solutions Increase mass fluctuations

(higher phonon-scattering)

Half-Heusler

systems

Grain boundaries Reduce the phonons mean free path Low dimensional

systems

Approach Effects on phonons Materials

(examples)

Heavy atoms weakly

bounded to the structures Phonon-scattering centers

Skutterudites

Clathrates

Complex structures Increase the optical phonon modes Clathrates

Yb14MnSb11

Inclusions, impurities Increase diffusion

(affects more phonons than carriers) Composites

Solid solutions Increase mass fluctuations

(higher phonon-scattering)

Half-Heusler

systems

Grain boundaries Reduce the phonons mean free path Low dimensional

systems

Glasses

Glasses

Low lat

Maximization of a2

Insulating

~ 0

Metallic

a ~ 0 a(300 K) ~ 1 mV/K

Semiconducting Glasses?

Is it always?

“Glasses exhibit some of the lowest lattice thermal conductivities. In a

glass, thermal conductivity is viewed as a random walk of energy

through a lattice rather than rapid transport via phonons, and leads to

the concept of a minimum thermal conductivity, κmin. Actual glasses,

however, make poor thermoelectrics because they lack the needed

„electron-crystal‟ properties — compared with crystalline

semiconductors they have lower mobility due to increased electron

scattering and lower effective masses because of broader bands.”

G. Jeffrey Snyder and Eric S. Toberer, nature materials VOL 7 (2008) 105 - 114

Most cases is true!!!

chalcogenides

pnictides

H.J. Goldsmid,

Proc. Phys. Soc.

London 67 (4)

(1954) 360–363

Chalcogenide-

Pnictide-based glasses?

Are they Semiconducting Glasses?

Yes!

Silicon's ~1.5×1010 cm−3 at 300 K

0.001% As => 1017 extra free electrons

Chalcogenide

with a Pnictide

2C30

C3+ + C1

M. Kastner, D. Adler, and H. Fritzsche, Phys. Rev. Lett. 37,1504 (1976).

VAP’s - valence alternation pairs

~1018 1019 cm-3

•C3+ and C1

present in equal concentrations;

•pin EF near the gap center.

Y. Taniguchi, H. Yamamoto, S. Hirogome, J. Appl. Phys., 52, 261 (1981).

What about electrical conductivity?

Ge20Te80 - r(300 K) = 2.77x108 mWm**

•M.A.Popescu in: Non-Crystalline Chalcogenides, 2002 Kluwer Academic Publishers New York

**G. Perthasarathy, et al. Sol. State Com. 51, 195-197 (1984)

P20Se80 - r(300 K) = 2x109 mWm*

Sb20S30 - r(300 K) = 4x1014 mWm*

GeS2 - r(300 K) = 1x1018 mWm*

(W-1cm-1)

Ge20Te80

Tg = 428 K, Tc = 493.5 K*

r(300 K) = 2.77x108 mWm** S(300 K) = 960 mV/K

*M Abu El-Oyoun, J. Phys. D: Appl. Phys. 33, 2211 (2000)

** G. Perthasarathy, et al. Sol. State Com. 51, 195-197 (1984)

I. Kaban et al. / Journal of Non-Crystalline Solids 326&327 (2003) 120–124

K. Ramesh, S. Asokan, K.S. Sangunni, E.S.R. Gopal,

J. Phys.: Condens. Matter 8 (1996) 2755-2762

CuxGe15Te85-x up to 9% at Cu

=> r decreases

M.A.Popescu in: Non-Crystalline Chalcogenides,

2002 Kluwer Academic Publishers New York

Electrical conductivity of

chalcogenide glasses:

(i) bond strengths,

(ii) network connectivity,

(iii) density.

K. Ramesh, S. Asokan, K. S. Sangunni and E. S. R. Gopal, J. Phys.:

Condens. Matter, 1999, 11, 3897–3906

Ge–Te => 396.7 kJ mol-1

Cu–Te => 230.5 kJ mol-1

A.P.Goncalves, E.B. Lopes, O. Rouleau, C. Godart, J. Mater. Chem., 2010, 20, 1516–1521

Melt-spinning

Glass Composition r300K

(mWm)

Er(High T)

(meV)

a300K

(mV/K)

Ea(High T)

(meV)

EHopp

(meV)

a2/r

(mW/K2m)

Reference

Ge20Te80 2.8x108 470 960 - - 3.3x10-3 [13]

Cu7Ge13Te80 5. 8x106 340 505 84 256 4.4x10-2 This work

Cu7.5Ge15Te77.5 2.1x107 351 562 58 293 1.5x10-2 This work

Cu12Ge12Te76 1.2x106 298 361 - - 1.1x10-1 This work

Cu15Ge7.5Te77.5 1.6x105 244 540 122 122 1.8 This work

Cu20Ge5Te75 2.9x105 263 453 34 229 0.7 This work

Cu22.5Ge2.5Te75 6x103 164 415 46 117 29 This work

Cu27.5Ge2.5Te70 2.6x103 126 394 45 81 60 This work

A.P.Goncalves, E.B. Lopes, O. Rouleau, C. Godart, J. Mater. Chem., 2010, 20, 1516–1521

Best Glass Composition

r300K (mWm)

Ea(High T)

(meV) a300K

(mV/K) a2/r

(mWK-2m-1)

Cu27.5Ge2.5Te70 2600 126 394 60

Ge20Te80 => (300K) ~ 0.1 W/K.m

L = vr2/3CV/3M2/3NA1/3

v -velocity of sound

r-density

CV -specific heat capacity per mole at constant volume

M -molecular weight

Cu27.5Ge2.5Te70 => elect ~3x10-4 W/Km

with of Ge20Te80 => ZT(300K) ~ 0.2

A.P.Goncalves, E.B. Lopes, O. Rouleau, C. Godart, J. Mater. Chem., 2010, 20, 1516–1521

Cu27.5Ge2.5Te70 => Tg = ~380 K

M.A.Popescu in: Non-Crystalline Chalcogenides, 2002 Kluwer Academic Publishers New York

As40Te60

Tg = ~390 K r(300 K) = 1x1012 mWm

S(300 K) = ? mV/K

A. Giridhar, S. Mahadevan

B. Journal of Non-Crystalline Solids 238 (1998) 225

N. Zotov et al. / Physica B 276}278 (2000) 463}464

CuyAs45-yTe55 (0 < y < 35)

100 150 200 250 30010

2

103

104

105

106

107

108

109

Cu20

As25

Te55

Cu25

As20

Te55

Cu30

As15

Te55

Cu35

As10

Te55

Cu26

Ga2Ge

2Te

70

r (

µW

.m)

Temperature (K)

100 150 200 250 300 350200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

Cu20

As25

Te55

Cu25

As20

Te55

Cu30

As15

Te55

Cu35

As10

Te55

Cu26

Ga2Ge

2Te

70

Th

erm

op

ow

er

(µV

/K)

Temperature (K)

20 25 30 35

100

150

200

CuxAs

45-xTe

55

Cu content

Ea (

meV

)

40

60

80

100P

ow

er fa

cto

r (µW

K-2m

-1

a2/r(300K) in Cu-As-Te ~ 100µWK2/m

a2/r(300K) in Cu-Ge-Te ~ 60µWK2/m

Tg ~390 K

50 100 150 200 250

0

2

4

6

8

10

12

Tg=132°C Tx=164°C

Tg=137°C Tx=168°C

Tg=135°C Tx=173°C

Tg=136°C Tx=167°C

Cu20

As25

Te55

Cu25

As20

Te55

Cu30

As15

Te55

Cu35

As10

Te55

Heat

flow

(m

W/m

g)

Temperature (K)

a2/r(300K) in Cu-As-Te ~ 100µWK2/m

=> ZT probably 0.3

Cu15Te55As30

Cu20Te47As33

D = 0.14-1.4 mm/s

Spark Plasma Sintering treatment:

from ribbons to bulk

Ribbons

SPS

Bulk

Cu35As10Te55 XRD patterns before and after

SPS

Before SPS

After SPS

Thermal conductivity and specific heat of bulk amorphous

chalcogenides Ge20Te80-xSex (x = 0,1,2,8)

S.-N. Zhang et al. / Journal of Non-Crystalline Solids 355 (2009) 79–83

r(300 K) = 106 Wcm or above

Preparation and thermoelectric properties of

bulk in situ nanocomposites with

amorphous/nanocrystal hybrid structure

T.J. Zhu, F. Yan, X.B. Zhao, S.N. Zhang, Y. Chen, S.H. Yang, J. Phys.: Appl. Phys. 40 (2007) 6094

Summary

Increase : ZT, , S, semiconductor - PF = S2

thermal conductivity : electronic él , lattice latt

bulk (many) chalcogenides - nanostructure effects

Semiconducting glasses

encouraging results in Cu-Ge-Te glass*

As favors glass domain as well as melt spinning

New results on Cu-Te-As glass : PF twice that in Cu-Ge-Te

A.P. Gonçalves, E.B. Lopes, C. Godart, E. Alleno, O. Rouleau - Portuguese patent, 103351, (2006)

* A.P. Gonçalves, E.B. Lopes, O. Rouleau, C. Godart - J. of Mat. Chem. 20, 1516-1521 (2010)

glass easy

large size samples

in some doped CuyAs45-yTe55 => ZT

Thank you for your attention

Acknowledgements

• INTELBIOMAT - ESF;

20 30 40 50 600

500

1000

1500

Cu30

Te70

Cu15

Ge10

Te75

Cu7.5

Ge15

Te77.5

Ge20

Te80

Inte

nsity

(a.u

.)

2 (º)