1 High Temperature Glass Seal High Temperature Glass Seal High Temperature Glass Seal Y-S Chou, J.W. Stevenson, X. Li, G. Yang, and P. Singh K2-44, Materials Division Pacific Northwest National Laboratory Funded under the SECA Core Technology Program through US Department of Energy’s National Energy Technology Laboratory August 7-9, 2007, San Antonio, Texas
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High Temperature Glass SealHigh Temperature Glass SealHigh Temperature Glass Seal
Y-S Chou, J.W. Stevenson, X. Li, G. Yang, and P. SinghK2-44, Materials Division
Pacific Northwest National Laboratory
Funded under the SECA Core Technology Program through US Department of Energy’s
National Energy Technology Laboratory
August 7-9, 2007, San Antonio, Texas
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OutlineOutlineOutline
Accomplishment of FY07Mechanical strength evaluation: plain crofer, aluminized, spinel-coated Electrical stability evaluation: aging under DC loading SummaryFuture work
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FY07 AccomplishmentFY07 AccomplishmentFY07 AccomplishmentCompleted seal strength evaluation of high-temperature glass. Evaluated pre-oxidation, aging, coating, and environmental effects on strength.Without coating, strength would degrade if a thick Cr2O3 oxide layer present or aged in air. No strength reduction if aged in reducing gas. Cause for strength degradation was SrCrO4formation.Alumina coating is effective in blocking Cr; however, the deposition process needs to be optimized to minimize overdose. Spinel coating showed best results with minimum strength reduction even aging in air.Tested conventional and high-temperature sealing glasses in SOFC environment and 0.7 V DC loading. Conventional glass showed severe Fe diffusion and rapid increase in conductivity (830oC/~80hr), while high-temperature glass showed excellent electrical stability over ~1200hr at 850oC.
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High-temperature sealing glass HighHigh--temperature sealing glass temperature sealing glass
To mimic long-term oxidation1200oC resulted in pores underneathOutside discrete and non-uniform (Mn,Cr) oxide, inside dense and continuousCr2O3-rich oxideT<0.5μm (800oC/2h)
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Effect of pre-oxidation on tensile strengthEffect of preEffect of pre--oxidation on tensile strengthoxidation on tensile strength
All showed brittle failure: coating, aging, oxide layer thickness, environment
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As-received
1000oC/2hPre-oxidized
Mixed fracture
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Fracture initiated mostly from edge flaws
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Effect of aging and environmentEffect of aging and environmentEffect of aging and environment
effect of aging on strength, crofer/YSO75/crofer, 1/2"x1/2"
0
1
2
3
4
5
6
7
8
9
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no aging 850C/500h air aged 850C/250h red.aged
aver
age
tens
ile s
tren
gth,
MPa
Air: 850oC/500hWet and reducing: 30%H2O, 70%(2.7H2/Ar) 850oC/250h
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Fracture surface of aged sampleFracture surface of aged sampleFracture surface of aged sample
Presence of substantial amount of SrCrO4
850oC/500h air 850oC/250h reducing gas
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Aluminized crofer showed no chromate formation
Aluminized crofer showed no Aluminized crofer showed no chromate formationchromate formation
Plain crofer Aluminized crofer
SrCrO4
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Tensile strength of aluminized croferfrom pack cementation
Tensile strength of aluminized Tensile strength of aluminized crofercroferfrom pack cementationfrom pack cementation
RT average strength, MPa
0
1
2
3
4
5
6
7
8
9
none 1000C/2h 1200C/2h
oxidation condition
aver
age
stre
ngth
, MPa
Pack cementation, followed by heat-treatment in air
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Sealing with aluminized crofer from vapor phase deposition
Sealing with aluminized Sealing with aluminized crofercrofer from vapor from vapor phase depositionphase deposition
Vapor phase deposition followed by heat-treatment at 1000oC/2h airAs-sealed coupons all fractured through glass.Glass bonded well to aluminized crofer coupons.
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Appreciable amount of Al diffusionAppreciable amount of Al diffusionAppreciable amount of Al diffusion
Fe
Cr
Al 15%10%
5%
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Issue of overdosing AlIssue of overdosing AlIssue of overdosing Al
as-received and aluminized crofer22APU
0.00E+00
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
1.40E-02
1.60E-02
0 200 400 600 800 1000
Temp (ºC)
Line
ar E
xpan
sion
Aluminizing by vapor phase deposition followed by heat-treatment at 1000-1100oC/2h air
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RT seal strength of spinel-coated crofer/YSO75 glass
RT seal strength of spinelRT seal strength of spinel--coated coated crofer/YSO75 glasscrofer/YSO75 glass
RT average tensile strength (spinel coating)
0
1
2
3
4
5
6
7
8
9
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plain as-sealed coated as-sealed
coated air aged coated red. aged
aver
age
stre
ngth
, MPa
(Mn,Co)3O4 coated crofer22APU
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Fracture surface of as-sealedMn1.5Co1.5O4-coated crofer22APU
Fracture surface ofFracture surface of asas--sealedsealedMnMn1.51.5CoCo1.51.5OO44--coated crofer22APUcoated crofer22APU
Fracture through glass, not at the interface. YSO75 not fully coverageAll 8 samples showed edge flaw
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Fracture surface of air agedMn1.5Co1.5O4-coated crofer22APU
Fracture surface ofFracture surface of air agedair agedMnMn1.51.5CoCo1.51.5OO44--coated crofer22APUcoated crofer22APU
850oC/500 h air
Suggest 5 μm coating was not enough
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Setup for resistivity measurementSetup for Setup for resistivityresistivity measurementmeasurement
air side
fuel sidehybridmica seal
alumina supportpipe
Inconel pressingfixture
crofer
crofer crofer
compressive loading
glass glass
VV
Power supply
+
+
-
-
resistor
sense
sense
out
out
air side
fuel sidehybridmica seal
alumina supportpipe
Inconel pressingfixture
crofer
crofer crofer
compressive loading
glass glass
VV
Power supply
+
+
-
-
resistor
sense
sense
out
out
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Good electrical stability for HT glass on plain crofer22APU
Good electrical stability for HT glass on Good electrical stability for HT glass on plain crofer22APUplain crofer22APU
Mechanical strength tests of M/G/M coupons showed strength degradation for uncoated crofer22APU only if aged in air or started with thick Cr2O3 surface layer.Aluminizing is effective in blocking Cr; however, overdosing would increase CTE for thin samples and leads to seal failure.(Mn,Co)3O4-coated crofer showed similar initial seal strength as plain crofer or aluminized ones. Minimal strength degradation when aged in air.HT glass showed very good electrical stability in SOFC and DC loading at 850oC up to ~1200h.Successful demonstration of 2”x2” single cell test at 800oC with candidate materials set: HT glass, spinel coating, SS441, and LSM contact paste.
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Future workFuture workFuture workMaterials set validation with single cell (2”x2”) stack testing and standardize the design:
1. Sealing glass: high-temperature, self-healing and composite.
2. Metallic interconnect: SS441 (standard or low Si), crofer22APU
3. Protective coating: (Mn,Co)3O4, aluminaShort-term (200-500h) performance test at 800-850oCShort-term thermal cycling test (800oC/24h, cool to RT)x10Collaboration with modeling workStrengthening of candidate high-temperature glass with reinforcement