rajalekshmi chockalingam, vasantha r.w. amarakoon, and herbert giesche new york state college of...
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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
NYSCC Alfred University
Center for Advanced Ceramic Technology
But first: “Where on earth is Alfred ?”
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
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)
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
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)
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
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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Silica-Yttria: Schematic Examples of Microstructures
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Example
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Silica spheres coated with yttria.
Heteronucleation Example
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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
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.
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
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
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
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Center for Advanced Ceramic Technology
Sintering: Temperature Profile
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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
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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SEM Micrographs: Al2O3-0.34%Mn,Co-Gd0.2Ce0.8O1.9
MW1250C-40min
500nm500nm
CONV-1350C-5hr
NYSCC Alfred University
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
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’)
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
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
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?