The tin impurity in Bi0.5Sb1.5Te3 alloys

Christopher M. JaworskiDepartment of Mechanical Engineering

The Ohio State University

Ellen Heian & Sharp HoangZT Plus

Joseph P. HeremansDepartment of Mechanical Engineering and Department of Physics

The Ohio State University

Supported by DOE-EERE as part of “High-Efficiency Zonal Thermoelcetric HVAC System for Automotive Applications”, Ford Program, Clay Maranville, Lead PI

2

Resonant levels increase thermopower

g(E)

E

FEEB

B

EE

EEnEgTk

ekS

=

∂

∂⋅+⋅⋅⋅=

)()(

1)()(

3

2 µµ

πMott relation for degenerate statistics

EF

Mechanism 1Mahan and Sofo, PNAS 93 7436, 1996

Mechanism 2: Resonant scattering A. Blandin & J. Friedel, Le Journal de Physique et le Radium 20 160, 1959In PbTe: Yu. Ravich, CRC Handbook on Thermoelectrics, D. M. Rowe, Ed. 1995In Bi2Te3: M. K. Zhitinskaya, S. A. Nemov and T. E. Svechnikova, Phys. Solid State 40 1297, 1998• Works great at cryogenic temperatures • Will NOT give high zT at operating temperatures where acoustic/optic phonon scattering dominates

Which dominates? Can be proven experimentally by measuring Nernst effect

Starting Point: binary Bi2Te3 with Sn

• Lowest Sn% -> compensated

• Middle Sn% have increased Seebeck over Ge and Pb doped

• Highest Sn% lose ‘resonance’

3

Sn Level1x1019 1x10203x1019 3x1020

p (1019 cm-3)

0

100

200

300

50

150

250

S (µ

V/K

)

PbGePbTl

UVB λ=1

UVB λ=1/2

3 101 30

Tin

Jaworski, C.M. et al., Phys Rev B 80 233201 (2009)

300K

Shubinov-de Haas oscillations -> Hall vs EF

1: Kulbachinskii, Physical Review B, 1994, Vol. 50, 23, 16921.2: Kohler, H.: Physica Status Solidi (b), 1976, Vol. 74.

Tin Content Oscillation Frequency

Fermi Surface Area

Bi2-xSnxTe3 [Δ(1/B)]-1 T (m-2)

x=0.0025 12.7 1.21E+17

x=0.0075 11.4 1.09E+17

x=0.015 13.5 1.29E+17

4

Slope of line = effective mass

Carrier density

from Hall

Fermi Level from SdH

50 100 150 200 250T (K)

0

0.5

1

1.5

λ (s

catte

ring

para

met

er)

50 100 150 200 250T (K)

0

0.4

0.8

1.2

Effe

ctiv

e M

ass

m

*/m

e

50 100 150 200 250T (K)

0

5

10

15

20

25Fe

rmi L

evel

(meV

)

Bi1.9975Sn.0025Te3Bi1.995Sn.005Te3Bi1.985Sn.015Te3

4-parameter fit to transport properties

Not resonant scattering, need λ~41

Effective mass upper valence band

5

1. physica status solidi(RRL) -, Vol. 1, No. 6. (2007), pp. 256-258.

FEEB

B

EE

EEnEgTk

ekS

=

∂

∂⋅+⋅⋅⋅=

)()(

1)()(

3

2 µµ

π

It’s this | Not that

Ionized Impurity

Optical phononλττ E0=

0

40

80

120

160

200

S (µ

V/K

)

.08%Sn

.18%Sn

.30%Sn

.30%Sn +1%Te

.30%Sn +3%Te

0 100 200 300 400 500T (K)

1x10-6

1x10-5

1x10-4

Res

istiv

ity (Ω

m)

0 100 200 300 400 500T (K)

0 100 200 300 400 500T (K)

0

10

20

30

40

Pow

er F

acto

r (µW

/ cm

K2 )

0 100 200 300 400 500T (K)

1E+19

1E+20

Car

rier D

ensi

ty (c

m-3)

(Bi.25Sb.75)2-x SnxTe3+δSingle Crystal

Carrier concentration too high

7

Powder Metallurgy: Measurement

All properties measured in same direction along pressing

thermal conductivity = specific heat * density * flash diffusivity

•Thermopower•electrical resistivity•low temperature thermal conductivity•Hall Coefficient•Nernst CoefficientNo issue with anisotropy error in zT

8

Cold Press + Sintered Bi10Sb30Te60 +SnTex

20 30 40 50 60 702θ (degrees)

Inte

nsity

(Arb

itrar

y U

nits

)

Bi10Sb30Te60SnTe

0.0% SnTe

0.05% SnTe

0.18% SnTe

0.25% SnTe

x= 0.05%, 0.10%, 0.18%, 0.25%Inorg. Mater., vol: 22 pg: 515, (1986)Inorg. Chem., vol: 36 pg: 260, (1997)

Idea: increase S by maximizing ionized impurity scattering

Add both tin and iodine to alloys

9

Seebeck, in general: dEEf

TkEEE

qkS

B

FB ∫∞

∂∂−

=0

)()(1 σσ

( )

++=n

Tkmh

qkS

BdB

*3 2

2

ln25 πλ

1. Heremans J.P. et al., Phys. Rev. B 14, 5156 (1976)

Non-degenerate statistics: The Pisarenko relation

Works in bismuth1, but will it increase power factor in BiSbTe?-Tradeoff between increased thermopower and decreased mobility

λττ E0=

10

100 200 300 400T (K)

0.1

1

10ρ

(mΩ

cm

)

12 x 1019 cm-3

6 x 1019 cm-3

3 x 1019 cm-3

0 x 1019 cm-3

100 200 300 400T (K)

1x1019

1x1020

p (c

m-3)

100 200 300 400T (K)

10

100

1000

mob

ility

(cm

2 / V

s)

SPS: Bi.5Sb1.5Te3 + Tin + Iodine

Iodineatoms

4.5 x 1019 cm-3

Sn atoms +

100 200 300 400T (K)

0

5

10

15

20

25

PF

(µW

/cm

K2 )

100 200 300 400T (K)

0

50

100

150

200

250

S (µ

V/K

)

12 x 1019 cm-3

6 x 1019 cm-3

3 x 1019 cm-3

0 x 1019 cm-3

11

SPS Bi.5Sb1.5Te3 + Tin + Iodine

No iodine

Iodine

Iodine atoms

12

SPS Bi.5Sb1.5Te3 + Tin + Iodine

100 200 300 400T (K)

0

0.2

0.4

0.6

0.8

1

1.2

zT

100 200 300 400 500 600T (K)

0

0.4

0.8

1.2

1.6

κ (W

/mK

)

12 x 1019 cm-3

6 x 1019 cm-3

3 x 1019 cm-3

0 x 1019 cm-3

Iodineatoms

4.5 x 1019 cm-3

Sn atoms +

Summation

• Dilute amounts of Sn lead to high zT

• Influence of porosity needs to be further investigated

• Ionized impurity scattering decreases mobility more than increases thermopower– Bad for power factor

13