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Sol Gel Approach: Lanthanum Silicates as a Replacement for Yttria Stabilized Zirconia (YSZ) in Solid Oxide Fuel Cell (SOFC) Electrolytes PowerPoint PPT Presentation


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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

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Sol Gel Approach: Lanthanum Silicates as a Replacement for Yttria Stabilized Zirconia (YSZ) in Solid Oxide Fuel Cell (SOFC) Electrolytes

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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


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Outline

  • Intro to Solid Oxide Fuel Cells (SOFCs)

  • Motivation for Work and Goals

  • Background of Apatite

  • Experimental procedure

  • Data

  • Results

  • Conclusion/Future Work


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SOFCs

www.eos.polito.it/h_fuel_ing2.htm


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Motivation

  • 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


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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


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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


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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

    • composition highly controllable

    • low temperatures

    • homogenous mixing

    • more freedom for applications -- coat = thin films

SOL

GEL


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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)


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Cryomilling

  • 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


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Results: XRD

STANDARD

CRYOMILLED

Water Contamination!!

Secondary phase: La2SiO5


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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:

~21nm

Crystallite size of cryomilled sample:

~14nm


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Results: SEM

Standard Sol Gel

Cryomilled Sol Gel 


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Results: SEM

STANDARD


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Results: SEM

CRYOMILLED


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Conclusions

  • 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!!


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Future Work

  • 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


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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


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