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Sol Gel Approach: Sol Gel Approach: Lanthanum Silicates as a Lanthanum Silicates as a Replacement for Yttria Replacement for Yttria
Stabilized Zirconia (YSZ) in Stabilized Zirconia (YSZ) in Solid Oxide Fuel Cell Solid Oxide Fuel Cell (SOFC) Electrolytes(SOFC) Electrolytes
Aminah RumjahnAminah RumjahnChemical Engineering and Material ScienceChemical Engineering and Material Science
University of California, DavisUniversity of California, Davis
University of California, IrvineChemical Engineering and Material Science PI: Martha MecartneyGraduate student: Mai Ng
OutlineOutline
• Intro to Solid Oxide Fuel Cells (SOFCs)Intro to Solid Oxide Fuel Cells (SOFCs)• Motivation for Work and GoalsMotivation for Work and Goals• Background of ApatiteBackground of Apatite• Experimental procedureExperimental procedure• DataData• ResultsResults• Conclusion/Future WorkConclusion/Future Work
MotivationMotivation
• Yttria Stabilized Zirconia (YSZ) is Yttria Stabilized Zirconia (YSZ) is traditional material used for electrolytetraditional material used for electrolyte
Main disadvantage: High operating tempMain disadvantage: High operating temp
Diffusion equation: D = DDiffusion equation: D = D00exp(-Q/RT)exp(-Q/RT)• New materials must be consideredNew materials must be considered• Good electrolyte: stable, lower operating Good electrolyte: stable, lower operating
temp, high oxygen ion conductivitytemp, high oxygen ion conductivity• Apatite = lower operating temp and high Apatite = lower operating temp and high
oxygen ion conductivity oxygen ion conductivity
Grain Size Must Be Grain Size Must Be SMALL!SMALL!
• Studies have shown some nanocrystalline Studies have shown some nanocrystalline ceramics have high ionic conductivityceramics have high ionic conductivity
• Ionic conductivity governed by grain Ionic conductivity governed by grain boundariesboundaries
• Ions in oxy-apatite travel faster in interstitial Ions in oxy-apatite travel faster in interstitial regionsregions
• Hypothesis is more grain boundaries = Hypothesis is more grain boundaries = higher conductivityhigher conductivity
Small grains Small grains More grain More grain boundariesboundaries = More efficient = More efficient electrolyte materialelectrolyte material
ApatiteApatite
Silicate-based apatite with SiO4 tetrahedra (yellow). The calculated pathway for oxygen diffusion is shown.*M S Islam, University of Bath
• A type of mineral
•Structure Hexagonal monoclinic
•Rare earth oxy-apatites:
La, Ce, Gd, Sm
• Specifically, apatite-type lanthanum silicates exhibit highest ionic conductivity
About Sol GelAbout Sol Gel• Solution chemistry based Solution chemistry based • Inorganic metal salts/metal organic compounds Inorganic metal salts/metal organic compounds
(metal alkoxides)(metal alkoxides)• Hydrolysis and polymerization forms liquid SOLHydrolysis and polymerization forms liquid SOL• Condensation forms solid GELCondensation forms solid GEL• Heat treatment Heat treatment crystalline ceramics crystalline ceramics• AdvantagesAdvantages
• composition highly controllablecomposition highly controllable• low temperatureslow temperatures• homogenous mixinghomogenous mixing• more freedom for applications -- coat = thin filmsmore freedom for applications -- coat = thin films
SOL GEL
ProcedureProcedure• Dissolve lanthanum nitrate hexahydrate in Dissolve lanthanum nitrate hexahydrate in
ethanol and acetic acid ethanol and acetic acid • Add tetraethylorthosilicate (TEOS)Add tetraethylorthosilicate (TEOS) SolSol• Dry overnightDry overnight GelGel• Heat treatments: Decompose at 600°C for Heat treatments: Decompose at 600°C for
4hrs and Calcine at 1000°C for 2hrs4hrs and Calcine at 1000°C for 2hrs
Solid oxy-apatite (LaSolid oxy-apatite (La9.339.33SiSi66OO2626))
CryomillingCryomilling
• No previous studies on No previous studies on cryomilling of ceramicscryomilling of ceramics
• Success with Success with cryomilling of metalscryomilling of metals
Reduced grain size Reduced grain size of Al to ~26nm*of Al to ~26nm*
• Cryogenic = very low Cryogenic = very low temps temps liqN liqN22(-200°C)(-200°C)
*F.Zhou, D.Witkin, S.R. Nutt, E.J. Lavernia, Mater. Sci. Eng. A375-377 (2004) 917-921
Results: XRDResults: XRD
XRD for Standard Sol Gel La9.33(SiO4)6O2
-2000
0
2000
4000
6000
8000
10000
12000
14000
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
2-Theta Scale
Lin
(Co
un
ts)
XRD for Cryomilled Sol Gel La9.33(SiO4)6O2
-1000
0
1000
2000
3000
4000
5000
6000
7000
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
2-Theta Scale
Lin
(C
ou
nts
)
STANDARD CRYOMILLED
Water
Contamination!!
Secondary phase: La2SiO5
Scherrer EquationScherrer Equation t = t = 0.90.9λλ
Bcos (Bcos (θθBB) )
t = crystallite sizet = crystallite size
λλ = = wavelength of Cu filament (wavelength of Cu filament (1.541.54Å)Å)
B = width of peak at ½IB = width of peak at ½Imaxmax
θθBB = angle of peak = angle of peak
NOT APPLICABLE FOR SIZES > 200nmNOT APPLICABLE FOR SIZES > 200nm
Crystallite size of Crystallite size of standard standard sample:sample:
~21nm~21nm
Crystallite size of Crystallite size of cryomilled cryomilled sample:sample:
~14nm~14nm
Results: SEMResults: SEM
Standard Sol Gel
Cryomilled Sol Gel
Results: SEMResults: SEMSTANDARD
Results: SEMResults: SEMCRYOMILLED
ConclusionsConclusions
• Fabricated apatite-type LaFabricated apatite-type La9.339.33SiSi66OO26 26
through sol gel routethrough sol gel route• Scherrer formula gives similar Scherrer formula gives similar
crystallite sizescrystallite sizes• SEM shows cryomilled powders are SEM shows cryomilled powders are
less agglomeratedless agglomerated
Uncertain of the effects of Uncertain of the effects of cryomilling!!cryomilling!!
Future WorkFuture Work
• Fabrication of sintered pellets to conduct Fabrication of sintered pellets to conduct impedance spectroscopy (IS) to determine impedance spectroscopy (IS) to determine ionic conductivity of Laionic conductivity of La9.339.33SiSi66OO26 26
• More Cryomilling!!!More Cryomilling!!!
Devise a better collection method toDevise a better collection method to
avoid water contaminationavoid water contamination
Vary the milling timeVary the milling time
Characterization testsCharacterization tests
Density measurementsDensity measurements
•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
AcknowledgementsAcknowledgements