bicuseo-layered oxychalcogenides with high zt values, from...
TRANSCRIPT
BiCuSeO-layered oxychalcogenides with high ZT values, from basic science to applications
David Berardan, L-D. Zhao, C. Barreteau, L. Pan, J. Li, N. Dragoe
ICMMO – Univ. Paris-Sud, Orsay, France
a brief history
Timeline:• 1990’s: R3+CuChO as potential high-Tc superconductor
(but no superconductivity…)
• early 2000’s: RCuChO as transparent conducting oxide (p type)
• late 2000’s: from RCuChO to RFePnO
• 2006: superconductivity in LaFePO (low temp)
• 2008: superconductivity in LaFeAsO1-xFx (Tc = 26K)
55K with Sm a few months later
→ 2008: promising TE properties around 100K in doped LaFeAsO
→ 2010: we moved to the oxychalcogenides
13-14 oct. 2014
ZrCuSiAs structure type
layered structured, P 4/n m m space group
Cu lies in a CuCh4 distorted environment
Bi lies in distorted square antiprism
edge sharing tetrahedra
many analogous and sister compounds
13-14 oct. 2014
Electronic structure
[email protected] Hiramatsu et al. Chem Mat 23 326 (2008)
BiCuChO as compared to RECuChO
→ strongly reduced band gap
O
Ch
Cu
: Bi
[R3+2O
2-2]
2+ insulating layer
[Cu+2Se2-
2]2- conducting layer
« natural quantum-well »?
2D transport behavior?
13-14 oct. 2014
electronic structure
BiCuChO: semiconductor, band gap between 0.4 eV (Ch = Te) and 1.1 eV (Ch = S)
• “large” band-gap• multivalley• multi hole band
equal contribution from Se and Cu to the DOS near EF
small Bi 6p contribution
but
no DOS « peak » at EF
Chem Mat 24 3168 (2012) 13-14 oct. 2014
electronic structure
[Bi2O2 ]2+
[Cu2Se2 ]2-
[Bi2O2 ]2+
[Cu2Se2 ]2-
moderately covalent inter-layer bonding
covalent Cu-Se intra-layer bonding →→→→ intermetallic layer
(LCAO-B3LYP): Bi1.46Cu0.24Se-0.66O-1.06→ far from ionic model
ionic Bi-O intra-layer bonding →→→→ oxide layer
Mulliken overlap population:Cu-Se = 0.11Cu-Cu = 0.05Bi-Se = 0.04 (underestimated by the calculation)
Chem Mat 24 3168 (2012) 13-14 oct. 2014
transport properties
carriers concentration optimization?
APL 97 092118 (2010), JACS 133 20112 (2011)
0 50 100 150 200 250 3000
50
100
150
200
0.00
0.05
0.10
0.15
0.20
0.25
S²σ
(m
W.m
-1.K
-2)
ρ (m
Ω.c
m)
Temperature (K)
“large” electrical resistivity
→→→→ low power factor despite large S
but intrinsically very thermal conductivity
Degenerate semiconductor ≠ Eg > 0,8 eV ??
→ Cu vacancies formation:
2 BiCu1-xSeO = (Bi2O2)2+ + (Cu2-2xSe2)
(2+2x)- + 2xh+
λlat ≈ 0.5 W.m-1.K-1 at 850K !
13-14 oct. 2014
electronic transport
A2+ doping on the Bi site
2(Bi1-xMxCuSeO) = (Bi2(1-x)M2xO2)2(1-x)+ + (Cu2Se2)
2 + 2xh+
simple carriers counting:
Energ. & Environ. Sci. 7 2900 (2014) 13-14 oct. 2014
electronic transport
0.0 0.1 0.2 0.31
10
100
0
5
10
15
20
25
30
ρ (
mΩ
.cm
)
x0.0 0.1 0.2 0.3
0
100
200
300
400
S (
µV
.K-1)
x
0.0 0.1 0.2 0.30
1x10-4
2x10-4
3x10-4
4x10-4
5x10-4
S2 σ (
W.m
-1.K
-2)
x0.0 0.1 0.2 0.3
1018
1019
1020
1021
[p]
(cm
-3)
x
µ (
cm2 .V
-1.s
-1)
Bi1-xSrxCuSeO
similar results with Ba2+, Ca2+, Pb2+
• Optimum [p] around 1021 .cm-3
• Low mobility
• Accoustic phonons scattering
Chem Mat 24 3168 (2012)
A2+ doping on the Bi site
13-14 oct. 2014
S²σ and λ
moderate ρ (low µ) + large S
moderate power factor S²/ρ
BUT
Very low thermal conductivity
unknown origin at the moment:
probably linked to the crystal structure
+ presence of Bi atoms
APL 97 092118 (2010)
(similar with other 2+ elements)
13-14 oct. 2014
promising ZT values
Bi1-xSrxCuSeO
very promising ZT values
large possible improvement:
• λlat ≈ 50% λtot
• only « moderate » power factor
(low µ)
APL 97 092118 (2010)
what about reduced grains size ?
13-14 oct. 2014
smaller grains size, lower λ
mechanical grinding before SPS (Ba doping)
0.3-0.4 µm instead of 3-10 µm
~ 20-25% decrease of λλλλlat
S²σσσσ almost unaffected
(intrisically low µµµµ)
stable upon time at 650ºC
Ener & Environ Sci 5 8543 (2012) 13-14 oct. 2014
Large ZT values
[email protected] Ener & Environ Sci 5 8543 (2012)
x µm
0.x µm
• stable upon time
• average ZT=0.8 between 350ºC and 650ºC
ZT at 600-650ºC
low doping efficiency
Pb2+ (not shown): low solubility limit
13-14 oct. 2014
Platelet grains → texturation?
Bi0.875Ba0.125CuSeO
Ener & Environ Sci 6 2916 (2013)
(performed in a « classical » hot-press system using various molds sizes)
13-14 oct. 2014
Platelet grains → texturation?
Bi0.875Ba0.125CuSeO
001
010
110
EBSD microstructures of textured: Band contrast images, grain
size distribution histograms and Z-Euler images
Pressing
direction
• no pref. orientation for as-synthesized sample
• Lotgering factor L=0.82 after texturation
(platelet grains)
Ener & Environ Sci 6 2916 (2013) 13-14 oct. 2014
Transport properties anisotropy
[email protected] Ener & Environ Sci 6 2916 (2013)
anisotropy of σ aniso
tropy o
f λ
13-14 oct. 2014
And improved ZT values
0 T 1 T 2 T 3 T0 T 1 T 2 T 3 T0 T 1 T 2 T 3 T
• very low anisotropy for as-synthesized sample
• 25% ZT improvement for the textured sample
best ZT ever for a polycristalline lead-free p-type materials in this T range
(average ZT = 1.1 between 350ºC and 650ºC)
Ener & Environ Sci 6 2916 (2013) 13-14 oct. 2014
ZT is not enough…
• microstructure stable upon time at least after one week at 650ºC
• no ionic conductivity → can be used in modules with dc current
→ 2010: ZT ~ 0.8 at 600ºC in Bi1.xSrxCuSeO ([p] optimization)
→ 2012: ZT ~ 1.1 at 650ºC in Bi1-xBaxCuSeO (~ 300 nm grains)
→ 2013: ZT ~ 1.4 at 650ºC in Bi1-xBaxCuSeO (texturation)
but what about the applications??
…but
• sealed silica tubes are not nice for industrial production…
• what about the long term stability?
13-14 oct. 2014
Towards applications:the synthesis
Weighedraw powders
Mixed and grounded the powders
Silica tube filled with Ar
300×12hCalcined powders
700×8hsynthesized
powders
SPS
700×7min
thermal conductivity Seebeck coefficientelectrical resistivity
BAG BAG
H2O, O2 < 0.5 ppm H2O, O2 < 0.5 ppm
“LnCuChO” powder
density ~ 97%
« intermetallic-like »synthesis↓
Not industrially-friendly
13-14 oct. 2014
Other synthesis route:mechanical alloying?
first attempts using agate balls and vials
→ large SiO2 contamination + grinding of the media + no BiCuSeO phase
solution: use of tempered steel balls and vials
20 30 40 50 60 70
5h 6h 6.5h 7.5h
1h 2h 3h 4h
I (a
.u.)
2θ (°)
rotation speed: 400 rpm
• 1 hour: nanocrystalline precursors
• 2 hours: minor amounts of BiCuSeO
• 2-6.5 hours: BiCuSeO ↑ , precursors ↓
• 7.5 hours: single phase BiCuSeO
(simulations+Rietveld:
Bi2O3 can be detected down to 0.5 atm%)
Precursors: Bi2O3, Bi, Cu & Se powders
JSSC 203 187 (2013) 13-14 oct. 2014
Mechanical alloying
optimized conditions:• mpowder/mballs = 1/7• rotation speed: 600 rpm• between 3-7h milling time
Single phase (XRD, DSC, SEM)
• easily densified using Spark Plasma Sintering
• after densification, platelet grains, ~ 50nm cross-plane, ~ 300 nm in-plane• « mechanical behaviour » (handling) similar to conventional synthesis
JSSC 203 187 (2013) 13-14 oct. 2014
Mechanical alloying
20 30 40 50 60 70
0.00 0.05 0.10 0.158.92
8.96
9.00
9.04
experimental calculated difference Bragg positions
I (
a.u
.)
2θ (°)
c (Å
)
Ba Fraction
the best ZT values of BiCuSeO based materials → heavily doped samples
• single phase Bi1-xBaxCuSeO samples
• no contamination by the grinding media
• good TE properties of densified samples
(S²σ ~ 450 µW.m-1.K-2 at 300K, similar to conventional synthesis)
DopedDoped--BiCuSeO synthesized: under air, at room temperature, within a feBiCuSeO synthesized: under air, at room temperature, within a few hours w hours
JSSC 203 187 (2013) 13-14 oct. 2014
Stability?
oxide layer
intermetallic layer
oxide layer
BiCuChO:
Oxide or intermetallic?
Which stability under air?
under inert atmosphere?
Se → volatilization?
13-14 oct. 2014
Stability under Ar?
0 200 400 600 800 1000
heat
flux
(a
.u.)
Temperature (°C)
DSC, argon heating cooling
~ 750°C
exo
endo0 200 400 600 800 1000
-6
-4
-2
0
2
TG
A (
%)
Temperature (°C)
TGA, argon, powder, 5K/min
• decomposition at ~750ºC• volatilization starts around 700ºC• no significant volatilization up to 650ºC• no reduction/oxygen loss (from TE properties)
BiCuSeO
13-14 oct. 2014
0 3 6 9 12 15 18 21-0.004
-0.003
-0.002
-0.001
0.000
BiCuSeO power
0.255 m2.g-1
650°C
Wei
gh
t lo
ss (
mg
.cm-2
)
days
-1.64 10-4 mg.cm-2.day-1
Submitted to JSSC
Stability under air?
BiCuSeO powder
• mass gain onset at moderate temperature• quick volatilization above 500ºC• no mass gain after 1week at 175ºC, but significant after 24h at 250ºC• linked to the oxidation of the powder
Submitted to JSSC 13-14 oct. 2014
Stability under air?
Grazing incidence XRD
BiCuSeO: stable under argon at 600BiCuSeO: stable under argon at 600--650650ººC under Ar, but not under air > 175C under Ar, but not under air > 175ººC C
13-14 oct. 2014Submitted to JSSC
conclusion
• ZTpeak = 1.4 at 650ºC
• ZTaverage = 1.1 between 350-650ºC
• Possible synthesis at 300K under air & raw materials price << PbTe
best Pb-free polycristalline p-
type thermoelectric
+
=
very promising for waste heat recovery applications…
… but still much work
…to applications
• Not stable under air → coating?
• Which n-type material?
• Contacts (thermal exp. coef. ~ 2.10-5 K-1)?
• Mechanical properties?
• (…)
from basic science…
• Origin of the very low λlat
• Engineering of the band structure for ↑ ZT:
• Resonant doping?
• Band convergence (multiband system)?
• (…)
13-14 oct. 2014
on course collaborations??
acknowledgments
thanks to:thanks to:
• my coworkers:
• Pr. Nita Dragoe & Dr. Emilie Amzallag• L.D. Zhao & L. Pan (postdocs)• C. Barreteau & J. Li (Ph’D students)• V. Pelé (Ms Student)
• my funding agencies:
• RTRA « Physics triangle », under « STP » contract
• French National Research Agency, under « OTher » contract• Univ. Paris-Sud International Relations Department
and coworkers from Harbin Institute of Technology & Beihang Univ. Beijing, China
13-14 oct. 2014
acknowledgments
Thanks for your attentionThanks for your attentionfurther reading :
• Appl. Phys. Lett. 97, 09118 (2010) (Sr doping, ZT = 0.8)
• JACS 133, 20112 (2011) (Cu vacancies, ZT = 0.8)
• Chem. Mat. 24, 3168 (2012) (Sr doping, physics)
• Ener. & Environ. Sci. 5, 8543 (2012) (Ba doping, reduced grains size, ZT = 1.1)
• Ener. & Environ. Sci. 6, 2916 (2013) (texturation, ZT = 1.4)
• J. Solid State Chem. 203, 187 (2013) (produced by mechanical alloying under air)
• Semiconductor Science & Technology 29, 064001 (2014) (review)
13-14 oct. 2014