Structure and Electrical Conductivity of Mn
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Transcript Structure and Electrical Conductivity of Mn
Structure and Electrical
Conductivity of Mn-based Spinels
Used as SOFC Interconnect coating
Supervisor: Dr. A. Petric
Yadi Wang
Jan. 25.2013
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OUTLINE
Literature Review
Objectives
Preliminary Results
Conclusions & Future Work
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Literature Review
-----SOFC
Reactions:
Cathode
O2 + 4e- = 2O 2Anode
H2 +O2- =H2O+2eCO +O2-= CO2 +2eCH4 + 4O2- =2H2O+CO2+8e-
Environmental friendly
High Efficiency
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Literature Review
-----Interconnect
Interconnect is a critical part of the fuel cell
electrical connection between cells
gas separation within the cell stack
Requirements
High electrical conductivity
Chemical stability in both fuel and air
Thermal expansion match to other cell components
High mechanical strength
Chemical stability with regard to other cell components
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Literature Review
-----Interconnect
Hard to fabricate,
high cost
Ceramic interconnect
1990s
Al---alumina
Metallic interconnect
Si---silica
Cr---chromia
Ferritic stainless
steel 16%<Cr<18%
Chromium-based
alloy Cr ~95%
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High
resistance
Literature Review
-----Metallic Interconnect
Main disadvantages of metallic interconnects
--Growth of Cr2O3 scale causes high resistance in the cell stack
--Volatile Cr (VI) causes chromium poisoning of the cell
2Cr2O3 + 3O2 + 4H2O ⟺ 4H2CrO4 (g)
2Cr2O3 + 3O2 ⟺ 4CrO3 (g)
2Cr2O3 + 3O2 + 4H2O ⟺ 4CrO2(OH)2 (g)
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Literature Review
-----Metallic Interconnect
A protective coating is needed on the cathode side to
isolate Cr from air :
--maintain electrical conductivity
--prevent chromium (VI) formation
Candidates to use for coating
--Perovskite oxides (high cost, difficult to apply coating)
--Spinel oxides
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Literature Review
-----Spinel
Many famous rubies
on the crown turned
out to be spinels
“Black Prince's ruby”
A magnificent
170-carat red spinel
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Literature Review
-----Spinel
Has formula of AB2O4
Cubic closed-packed oxygen
lattice with 32 O2- in one cell
A and B cations occupy
interstitial sites
1/8 of tetrahedral and 1/2 of
octahedral sites are occupied
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Literature Review
-----Spinel
Spinels can form solutions with many different cations
A element: Mg, Ni, Mn, Zn, Cu, Co
B element: Al, Cr, Mn, Fe, Co
Multivalent elements needed for conductivity
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Literature Review
-----Spinel
Table 1. Properties of common spinels (800oC except where noted)
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Petric, A., Ling, H., J. Am. Ceram. Soc., 90[5] 1515-1520 (2007).
Literature Review
-----Spinel Conductivity
Conduction occurs by hopping of charge on octahedral sites
----Thus presence of different valence states among octahedral
cations is essential to conduction. (Al-based spinels have low
conductivity.)
----Spinels containing manganese have high conductivity because of
multiple valence states of Mn
Conduction is temperature activated
σ=σ0 exp(-EF /kT)
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Literature Review
-----Mn-based Spinel
Mn valence state
Tetrahedral: Mn2+
Octahedral : Mn2+ , Mn3+ , Mn4+ (2Mn3+ Mn4+ +Mn2+ )
Conductivity of Mn-based spinel is attributed to
hopping between Mn3+/Mn4+ on octahedral sites.
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Literature Review
-----Mn-based Spinel
Co-Mn spinel
MnCo2O4---has conductivity of 60 S/cm
Promising coating material for SOFC metallic interconnect
CuMn2O4 has conductivity around 200 S/cm, volatile
Cu doped Co-Mn spinel
Increase conductivity
Decrease cost
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Objectives
Measure electrical conductivity of Co-Mn-Cu spinel
Electroplate spinel coating for stainless steel 430 and
chromium alloy interconnects
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Co - Mn - O2
p(O2) = 0.21 atm
2000
Aukrust & Muan (air)
Liquid
1800
1600
Monoxide
1400
T(C)
1200
1000
Spinel
800
Mn2O3 + Sp
600
Mn2O3 + CoMnO3
400
CoMnO3 + Sp
MnO2 + CoMnO3
200
MnO2 + Sp
0
0
.2
.4
.6
.8
1
mole Co/(Co+Mn)
Petric, A., Ling, H., J. Am. Ceram. Soc., 90[5] 1515-1520 (2007).
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Co-Cu-Mn Spinel Conductivity
Measured
conductivity
of 13
compositions
within the
cubic region
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Co-Cu-Mn Spinel Conductivity
Blue region
identifies
the region
of optimum
properties
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Coating methods
Coating methods:
Slurry coating, screen-printing, physical vapor deposition,
electroplating, plasma spray
Electroplating + oxidation
--cost-effective
--low processing temperature (room temperature)
--uniform films on complex substrate
--easy to control thickness
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Electroplating
Reactions:
Cathode: Cu2++2e-= Cu
Co2++2e-=Co
Mn2++2e-=Mn
Anode: 2H2O= O2+ 4H++4eUse Faraday’s law:
m=(Q*M)/(F*Z) =
to control electroplating time
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Preliminary Results
-----C00.5Cu0.5Mn2O4 Coating
On stainless steel 430, 24h
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Preliminary Results
-----C00.5Cu0.5Mn2O4 Coating
On Stainless steel 430, 150h
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Preliminary Results
-----C00.5Cu0.5Mn2O4 Coating
On chromium-iron plate, 24h
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Conclusions & Future Work
Cu doped Co-Mn spinels have ideal electrical conductivity
Co0.5Cu0.5Mn2O4 coating successfully stops Cr migration
Improve the quality of spinel coating.
Apply different compositions
Characterize the coating
Apply to large interconnect plate
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Acknowledgement
Dr. A. Petric
Dr. Y. Mozharivskyj
Dr. P. Wei, Dr. S. Joshi, Xavier Michaud
Chris Butcher, Doug Culley, Xiaogang Li, Wenhe Gong
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