Transcript Nano-Crystalline Super
Nanocrystalline Super-Ionic Conductors for Solid Oxide Fuel Cells
Daniel Strickland (Seattle University)
University of California – Irvine Material Science and Engineering
Mentor: Professor Martha L. Mecartney
Graduate Student: Sungrok Bang Collaborator: Jeremy Roth
Support from NSF REU program UCI IM-SURE
Introduction to SOFC
• • •
Basic fuel cell operation Cathode Reaction
O
2 4
e
2
O
2
Anode Reactions
H
2 2
H
2
e
4
H
2
O
2 2
H
2
O
Taken from fuelcellworks.com
Daniel Strickland IM-SURE July 27, 2005
Electrolyte Material Challenges
•
Operating Temperature
•
Design Challenges
–
Current materials require high operating T > 800 ºC
–
Sacrifice long-term stability and encourage material degradation
–
Similar thermal expansion coefficients
–
High chemical compatibility
K. Sundmacher, L.K. Rihko-Struckmann and V. Galvita, Solid electrolyte membrane reactors: Status and trends, Catalysis Today, Volume 104, Issues 2-4, 30 June 2005, Pages 185-199.
Electrolyte Material Challenges
•
Implementation Challenges
–
Operational costs are significantly increased
–
Potential applications are limited
Ionic conductance
•
SOFC operating temp can be reduced by increasing ionic conductance
•
Two ways to increase:
–
Increase ionic conductivity
–
Decrease ion travel distance
Increasing Ionic Conductivity
• •
Doped zirconia used as electrolyte material (Scandium and Yttrium used) Zirconia grain structure:
Increasing Ionic Conductivity
•
Traditional theory:
–
High ionic conductivity through grain interior
–
Low ionic conductivity through grain boundaries
•
Increase grain size to increase overall conductivity
Decreasing Ion Travel Distance
•
Ion travel distance reduced by decreasing electrolyte thickness
•
Thin film fabrication techniques employed to create electrolytes of sub-micron thickness
How to improve overall conductance?
•
Nanocrystalline grain microstructure required for sub-micron thicknessess 2 :
–
Prevent pinholes
–
Must be gas-tight
•
It appears as if ionic conductivity must be sacrificed to decrease ion travel distance
2. B.P. Gorman, V. Petrovsky, H.U. Anderson, and T. Petrovsky (2004), “Optical Characterization of Ceramic Thin Films: Applications in Low-Temperature Solid Oxide Fuel Cell Materials Research,”
Journal of Materials Research
,
19
, 573-578.
A potential solution
•
Possible grain boundary conductivity improvements at nano-scale!
•
Other factors may begin to dominate:
–
Decreased impurity concentration 3
3. H.L. Tuller (2000), “Ionic Conduction in Nanocrystalline Materials,”
Solid State Ionics
,
131
, 143-157.
Goal of Research
•
Fabricate yittria stabilized and scandia stabilized zirconia nanocrystalline thin films
•
Characterize microstructure and ionic conductivity Atomic Force Microscope image of YSZ thin film
C.D. Baertsch et al,
Journal of Materials Research
,
19
, 2604-2615 (2004)
Daniel Strickland IM-SURE July 27, 2005
Fabrication Process
Zirconium propoxide Zr(OC 3 H 7 ) 4 Isopropanol (dilutant) Yttrium isopropoxide Scandium isopropoxide Multiple 0.05-0.25 M Solution Add 70% Nitric 30% H 2 O (hydrolysis) Spin-coat (silicon wafer) Dry T = 130º C Pyrolyze T = 420º C Crystallize T = 520ºC DSC/TGA (Optimize Heating Regime) SEM X-Ray Diffraction Impedance Spectroscopy
Finding optimized condition
•
Parameters involved:
–
Solution viscosity
–
Spin speed and time
–
Heating regime
Viscosity
•
Three factors influence viscosity:
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Reaction rate: Hydrolysis
•
Process where H 2 O breaks organics off of propoxides
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Reaction Time
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Solution concentration
Reaction time and concentration
•
Viscosity was assumed constant for initial 48 hours
•
Viscosity linearly dependant of sol gel concentration
•
Concentration varied from .05M to .30M to find optimized condition
Sol-gel concentration
0.05 M 0.10 M 0.15 M 0.30 M
Heating regime
Nano-Cracks Delamination
Heating regime
4Y-4Sc DSC/TGA
100% 95% 90% 85% 80% 75% 70% 65% 60% 55% 50% 0 100 200 300 400 500 600
Optimized Fabrication Conditions
• • • • •
.05 M solution .9:1 water to propoxide molar ratio Spin coating at 2000 rpm, for 30 sec Heat treatment between each coat:
–
3 ºC/min to 130 ºC
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Hold 30 min
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2 ºC/min to 520 ºC
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Hold 60 min Coat up to 8 layers
Optimized thin Films
Optimized thin Films
X-Ray Diffraction Studies
• Confirm crystalline zirconia thin film • Calculate grain size • Calculate lattice parameters
X-Ray Diffraction Studies
•
How XRD works:
– Incident X-Rays in phase – Phase shift function of plane spacing and incident angle: phase shift 2
d
sin – Phase shift = multiple of wavelength, beams react constructively – Detected X-ray intensity peaks
Taken from Callister
XRD: Confirm Crystalline Zirconia 8 Y XRD
30.125
50.2
8 Y Bulk 30.425
34.95
59.675
62.7
73.725
8 Y Thin Film 8 Y Powder 30.2
35.15
34.98
20 25 30 35 40 50.725
60.125
62 74.075
50.26
45 50
2-Theta
55 59.76
62.66
60 65 70 73.86
75 80
XRD: Calculate grain size
•
Used integral breadth formula:
( 2 tan 2 ) 2
K
L
tan 2 sin 16
e
2 •
Some interesting trends:
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Dopants influenced grain size
–
Heating to 700 C did not induce grain growth
8YSZ 4YSZ 4Y-4Sc 8ScSZ 4Sc 500 C 17 nm 18 nm 20 nm 21 nm 22 nm 700 C 17 nm
XRD: Lattice parameters
• • Each peak corresponds to a plane of atoms Crystal structure unit cube length can be calculated:
a
d hkl
h
2
k
2
l
2 Ǻ Thin Film 4YSZ 5.091
8YSZ 5.096
4ScSZ 5.055
8ScSZ 5.054
4Y-4Sc 5.075
Sol-Gel Powder 5.113
5.128
5.086
5.081
5.101
Impedance Spectroscopy (IS)
• • •
IS needs to be performed to quantify ionic conductivity Substrate conditions:
– –
Not an ionic conductor Not and electronic conductor
– –
Smooth surface Mechanically strong Need silver paint for electrodes
Conclusions
• • • •
We can fabricate high quality, 1 μ thin films
– –
Crack free Highly dense Correlation found between dopants and grain size Lattice parameter for thin film is smaller than that of powder or bulk material Thin films are ready for impedance spectroscopy
Acknowledgements
Mentor: Prof. Martha L. Mecartney Graduate Students: Sungrok Bang Tiandan Chen Collaboration: Jeremy Roth IM-SURE Program: Said Shokair University of California – Irvine National Science Foundation Daniel Strickland IM-SURE July 27, 2005