rajalekshmi chockalingam, vasantha r.w. amarakoon, and herbert giesche
DESCRIPTION
Center for Advanced Ceramic Technology. Alumina / Cerium Oxide Nano-Composite Electrolyte for Solid Oxide Fuel Cell Applications. Rajalekshmi Chockalingam, Vasantha R.W. Amarakoon, and Herbert Giesche New York State College of Ceramics at Alfred University, Alfred, NY, USA. - PowerPoint PPT PresentationTRANSCRIPT
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Rajalekshmi Chockalingam, Vasantha R.W. Amarakoon,
and Herbert Giesche
New York State College of Ceramics at Alfred University, Alfred, NY, USA
Alumina / Cerium Oxide
Nano-Composite Electrolyte
for
Solid Oxide Fuel Cell Applications
NYSCC Alfred University
Center for Advanced Ceramic Technology
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NYSCC Alfred University
Center for Advanced Ceramic Technology
But first: “Where on earth is Alfred ?”
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NYSCC Alfred University
Center for Advanced Ceramic Technology
eOHOHAnodeat 2: 22
2
22 2
2
1: OeOCathodeat
OHOHOverall 222 2
1:
Anode: Ni + YSZ
Cathode: La1−xSrxMnO3-δ
Electrolyte: 8 mol% Y2O3 stabilized ZrO2
Operated at close to 10001000°C°C.
http://en.wikipedia.org/wiki/Image:Fcell_diagram_sofc.gif
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Alternative Electrolyte Materials
NYSCC Alfred University
Center for Advanced Ceramic Technology
For Example:
Gadolinium doped Ceria
Ce0.8Gd0.2O1.9
Leading to lower operation temp.
However !!!
under reducing conditions:
Ce+IV → Ce+III electronic conduction.
Idea !!!
Electron Trapping Interfaces.
S.M. Haile, “Fuel cell materials and components,” Acta materialia, 51, 5981-6000 (2003)
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Conduction Band
Acceptor states
Fermi Level
Valence Band
ΦL
2
1
02
dqNW
W= width of the depletion layer
εo= permittivity of free space
ε’= dielectric constant
Φ= potential barrier
q= electronic charge
Nd= charge carrier density
ZnO varistor microstructure Al2O3/CeO2 composite microstructure
Semiconductor Phase
Insulating Phase
Grain Boundary
CB
VB
Fermi Level
Acceptor States
ΦL
N N P P
N
P
P N
“Electron Trapping”
NYSCC Alfred University
Center for Advanced Ceramic Technology
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So, how do we make such a microstructure?
NYSCC Alfred University
Center for Advanced Ceramic Technology
Coated Nano-particles.
Densify/sinter and retain microstructure.(microwave sintering; fast & uniform)
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Center for Advanced Ceramic Technology
Use two suspensions of particles with opposite charge.
Zeta-potential (pH)
Surfactant adsorption
Porous coating; weak adhesion forces;requires large difference in particle size
Heterocoagulation
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NYSCC Alfred University
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Center for Advanced Ceramic Technology
Alfred University
Precipitate coating material onto seed particles.
Essentially “any” precipitation reaction can be used.
As long as it is a “controlled” (slow) precipitation
Dense and uniform coating
Heteronucleation
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Center for Advanced Ceramic Technology
Silica-Yttria: Schematic Examples of Microstructures
NYSCC Alfred University
Example
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Center for Advanced Ceramic Technology
Silica spheres coated with yttria.
Heteronucleation Example
NYSCC Alfred University
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Center for Advanced Ceramic Technology
Excess silica cores remain after phase transformation and sintering.
Visualized by
etching with HF.
Heteronucleation Example cont.
NYSCC Alfred University
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Center for Advanced Ceramic Technology
Alfred University
Alumina core (seed)
Cobalt and Manganese surface layer; acceptor states at the interface
Gadolinium doped Ceria (50 to 100 nm) oxygen-ion-conductor; ‘continuous phase’
Microwave sintering to retainthe proposed microstructure
Schematic of the “new” nano-composite electrolyte.
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Center for Advanced Ceramic Technology
NYSCC Alfred University
T h erm om eter
S tirrer
C on sta n t tem p era tu re b a th
A l2 O 3 -M n -C o -S O L
Al(OC4H9)3 H2O (75°C)
Hydrolysis under vigorous stirring for 30 min
Peptization with HNO3 & Aging at 95°C for 5 days
Al2O3 SOLMn (NO3)2 6H2O + H2O
Co (NO3)2 6H2O
+ H2O
NH4OH
+ H2O
Mn, Co coated Al2O3 Sol
Stirr at 90°C for 4 hrs & Age 24 hrs
Synthesis of Mn, Co doped Al2O3 Sol
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Center for Advanced Ceramic Technology
NYSCC Alfred University
Coating of Gd doped CeO2 on Al2O3 Sol
T h erm om eter
S tirrer
C on sta n t tem p era tu re b a th
A l2 O 3 -M n -C o -2 0 % G d d o p e d C e O 2
Gd(NO3)3 6H 2O
+H2O
Aging at RT for 24 hours
Dry and heat Treatment
Mn, Co coated
Al2O3 Sol
Forming and sintering
Gd0.2Ce0.8O1.9coated on Mn, Co coated Al2O3 Sol
NH4OH + H2O
Vigorous stirring at 93°C for 6 hours
Ce(NO3)3.6H2O
+H2O
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Microwave Sintering Set up
Figure (A) 2.45 GHz MW Furnace and Figure (B) Sample set up with alumina insulation box and Thermocouple.
A B
NYSCC Alfred University
Center for Advanced Ceramic Technology
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Sintering: Temperature Profile
NYSCC Alfred University
Center for Advanced Ceramic Technology
0
200
400
600
800
1000
1200
1400
1600
0 250 500 750 1000 1250 1500 1750Time in minutes
Tem
per
atu
re°C
Conventional Microw ave
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XRD Results for Gd0.2Ce0.8O1.9 -0.34%Mn-0.34%Co-Al2O3
NYSCC Alfred University
Center for Advanced Ceramic Technology
0
500
1000
1500
2000
2500
20 30 40 50 60 70
2 theta
Inte
nsi
ty
900°C - 6h
900°C - 4h
900°C - 2h
700°C - 2h
500°C - 2h
300°C - 2h
room temp.
♥ matches Gd0.2Ce0.8O1.9
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Center for Advanced Ceramic Technology
SEM Micrographs: Al2O3-0.34%Mn,Co-Gd0.2Ce0.8O1.9
MW1250C-40min
500nm500nm
CONV-1350C-5hr
NYSCC Alfred University
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NYSCC Alfred University
Center for Advanced Ceramic Technology
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
1200 1250 1300 1350 1400 1450 1500
Temperature (°C)
De
nsi
ty (
g/c
m3 )
GdC(MW)Coat-B(MW)Coat-C(MW)GdC (CON)Coat-B (CON)Coat-C (CON
Microwave sintering Conventional sintering
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NYSCC Alfred University
Center for Advanced Ceramic Technology
-1
1
3
5
7
9
11
0.7 0.8 0.9 1 1.1 1.2 1.3 1.4
1000/T (K)
ln(s
T)
(S
cm
-1 K
)
GdC CON
Coat-B CON
Coat-C CON
Coat-B MW
Coat-C MW
Impedance Spectroscopy in Air (‘Ionic Conductivity’)
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NYSCC Alfred University
Center for Advanced Ceramic Technology
-5
-4
-3
-2
-1
0
1
2
3
4
-30 -25 -20 -15 -10 -5 0
Log Po2
Lo
g (s ) (
S/c
m)
GdC CON
Coat-B CON
Coat-C CON
600°C
Electrical Conductivity as a Function of Oxygen Partial Pressure
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NYSCC Alfred University
Center for Advanced Ceramic Technology
-5
-4
-3
-2
-1
0
1
2
3
4
-30 -25 -20 -15 -10 -5 0
Log Po2
Lo
g (
s) (
S/c
m)
GdC CON
Coat-B CON
Coat-C CON
800°C
Electrical Conductivity as a Function of Oxygen Partial Pressure
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Center for Advanced Ceramic Technology
• Coated powders lead to unique microstructure.
• Microwave sintering is substantially faster.
• Submicron grain size can be retained
• Increased hardness.
• Electron trapping states at the alumina-ceria interface reduce electronic conductivity.
• Al2O3 inclusions have no major effect on ionic-conductivity.
Conclusions
NYSCC Alfred University
• Better control of coating - thickness and uniformity.
• Test of other material combinations.
• Measure oxygen conductivity ‘directly’ (transference number).
• Test in a ‘real’ device !!!
What’s next?