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

SOFCsSOFCs

www.eos.polito.it/h_fuel_ing2.htm

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