Sol Gel Approach: Lanthanum
Silicates as a Replacement for
Yttria Stabilized Zirconia (YSZ)
in Solid Oxide Fuel Cell (SOFC)
Electrolytes
Aminah Rumjahn
Chemical Engineering and Material Science
University of California, Davis
University of California, Irvine
Chemical Engineering and Material Science
PI: Martha Mecartney
Graduate student: Mai Ng
Outline
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Intro to Solid Oxide Fuel Cells (SOFCs)
Motivation for Work and Goals
Background of Apatite
Experimental procedure
Data
Results
Conclusion/Future Work
SOFCs
www.eos.polito.it/h_fuel_ing2.htm
Motivation
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Yttria Stabilized Zirconia (YSZ) is traditional
material used for electrolyte
Main disadvantage: High operating temp
Diffusion equation: D = D0exp(-Q/RT)
New materials must be considered
Good electrolyte: stable, lower operating temp,
high oxygen ion conductivity
Apatite = lower operating temp and high oxygen
ion conductivity
Grain Size Must Be SMALL!
• Studies have shown some nanocrystalline
ceramics have high ionic conductivity
• Ionic conductivity governed by grain boundaries
• Ions in oxy-apatite travel faster in interstitial
regions
• Hypothesis is more grain boundaries = higher
conductivity
Small grains  More
grain boundaries
= More efficient
electrolyte material
Apatite
• A type of mineral
•Structure
 Hexagonal monoclinic
•Rare earth oxy-apatites:
La, Ce, Gd, Sm
• Specifically, apatite-type
lanthanum silicates exhibit
highest ionic conductivity
Silicate-based apatite with SiO4 tetrahedra (yellow).
The calculated pathway for oxygen diffusion is shown.
*M S Islam, University of Bath
About Sol Gel
• Solution chemistry based
• Inorganic metal salts/metal organic compounds (metal
alkoxides)
• Hydrolysis and polymerization forms liquid SOL
• Condensation forms solid GEL
• Heat treatment  crystalline ceramics
• Advantages
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composition highly controllable
low temperatures
homogenous mixing
more freedom for applications -- coat = thin films
SOL
GEL
Procedure
• Dissolve lanthanum nitrate hexahydrate in
ethanol and acetic acid
• Add tetraethylorthosilicate (TEOS)
 Sol
• Dry overnight
Gel
• Heat treatments: Decompose at 600°C for 4hrs
and Calcine at 1000°C for 2hrs
Solid oxy-apatite (La9.33Si6O26)
Cryomilling
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No previous studies
on cryomilling of
ceramics
Success with
cryomilling of metals
Reduced grain size
of Al to ~26nm*
Cryogenic = very low
temps  liqN2(-200°C)
*F.Zhou, D.Witkin, S.R. Nutt, E.J. Lavernia, Mater. Sci. Eng. A375-377 (2004) 917-921
Results: XRD
STANDARD
CRYOMILLED
XRD for Cryomilled Sol Gel La9.33(SiO4)6O2
14000
7000
12000
6000
10000
5000
8000
4000
Lin (Counts)
Lin (Counts)
XRD for Standard Sol Gel La 9.33(SiO4)6O2
6000
4000
3000
2000
2000
1000
0
0
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
18
34
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
-1000
-2000
2-Theta Scale
2-Theta Scale
Secondary phase: La2SiO5
34
Scherrer Equation
t =
0.9λ
Bcos (θB)
t = crystallite size
λ = wavelength of Cu filament (1.54Å)
B = width of peak at ½Imax
θB = angle of peak
NOT APPLICABLE FOR SIZES > 200nm
Crystallite size of
standard sample:
Crystallite size of
cryomilled sample:
~21nm
~14nm
Results: SEM
Standard Sol Gel
Cryomilled Sol Gel 
Results: SEM
STANDARD
Results: SEM
CRYOMILLED
Conclusions
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Fabricated apatite-type La9.33Si6O26
through sol gel route
Scherrer formula gives similar crystallite
sizes
SEM shows cryomilled powders are less
agglomerated
Uncertain of the effects of cryomilling!!
Future Work
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Fabrication of sintered pellets to conduct
impedance spectroscopy (IS) to determine ionic
conductivity of La9.33Si6O26
More Cryomilling!!!
Devise a better collection method to
avoid water contamination
Vary the milling time
Characterization tests
Density measurements
Acknowledgements
•Professor Martha Mecartney and Graduate
student Mai Ng for their enthusiasm,
guidance and support
•Mecartney and Mumm groups
•UC Irvine and the UROP team for the
IMSURE program
• NSF for financial support
•Zeiss Center of Excellence for microscopy
support