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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