Aminah Rumjahn Chemical Engineering and Material Science University of California, Davis

1. 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

2. Outline • Intro to Solid Oxide Fuel Cells (SOFCs) • Motivation for Work and Goals • Background of Apatite • Experimental procedure • Data • Results • Conclusion/Future Work

3. SOFCs www.eos.polito.it/h_fuel_ing2.htm

4. 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

5. 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

6. 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

7. 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

8. 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)

9. 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

10. Results: XRD STANDARD CRYOMILLED Water Contamination!! Secondary phase: La2SiO5

11. 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

12. Results: SEM Standard Sol Gel Cryomilled Sol Gel 

13. Results: SEM STANDARD

14. Results: SEM CRYOMILLED

15. 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!!

16. 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

17. 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