FUNDAMENTAL STUDIES OF THE DURABILITY OF MATERIALS FOR INTERCONNECTS IN SOLID

OXIDE FUEL CELLSAGREEMENT NO. DE-FC26-02NT41578

(Start Date September 30, 2002)

J. Hammer, S. Laney, F. Pettit, & G. MeierDepartment of Materials Science & Engineering

University of Pittsburgh

N. Dhanaraj & J. BeuthDepartment of Mechanical Engineering

Carnegie Mellon University

SECA Annual Workshop and Core Technology Program Peer Review

May 13, 2004

PROJECT STRUCTURENETL

Dr. Lane Wilson Technical MonitorDr. Chris Johnson NETL Fellowship Mentor

University of PittsburghProf. Frederick. S. Pettit Co-principal InvestigatorProf. Gerald. H. Meier Co-principal InvestigatorMs. Julie Hammer Graduate StudentMr. Scot Laney NETL Partnership FellowMs. Mary Birch Senior Project Student

Carnegie Mellon UniversityProf. Jack L. Beuth Co-principal InvestigatorMs. Nandhini Dhanaraj Graduate Student

CompletedMs. Kelly Coyne B.S. Engineering Phys.Ms. Carrie Davis B. S. Materials ScienceMr. Wesley Jackson B. S. Materials Science

PROGRAM FOCUSTASK I: Mechanism-Based Evaluation Procedures

(Chromia-Forming Alloys)

• Characterization of Exposed Fuel Cell Interfaces• Growth Rates of Chromia Scales on Cr and Ferritic

Alloys• Adhesion of Chromia Scales• Oxide Evaporation• Complex Atmosphere Testing

Note: An important theme which cuts across Tasks I and II is the establishment of accelerated testing protocols.

PROGRAM FOCUS;TASK II: FUNDAMENTAL ASPECTS OF

THERMOMECHANICAL BEHAVIOR• XRD Stress Measurements (Chromia Films)• Indentation Testing of Interface Adhesion• Indentation Test Fracture Mechanics Analysis

Key Issues: What leads to spallation: scale thickening, stress changes or changes at the interface?Can we quickly evaluate alloy systems without testing to the time for spontaneous spallation?

Debonding Oxide Scale

Substrate

Indenter

CompressiveStress

a = Contact RadiusR = Debond Radius

aR

Plastic Zone

E-BRITE SAG 364 hrs 900 °C

PROGRAM FOCUSTASK III: Alternative Material Choices

This Task involves theoretical analysis of possible alternative metallic interconnect schemes including:

• Control of Growth Rate and Conductivity of Simple Oxides (e. g. CoO, NiO)

• Ni and dispersion-strengthened Ni• Low CTE Alloys Based on Fe-Ni (Invar)• Bi-layer Alloys

The most promising systems will be evaluated experimentally with regard to durability and oxide conductivity

TASK I: RESULTSOxidation of Ferritic Alloys

Alloys• E-BRITE (26 Cr-1 Mo)• AL 453 (22 Cr + Ce/La)• Crofer (22 Cr + La)• ZMG232 (22 Cr + La/Zr)

Exposure Conditions• T = 700°C, 900°C• One-Hour Cycles• Atmospheres

- Dry Air (SCG)- Air + 0.1 atm H2O- Ar/H2/H2O (SAG)

(pO2 = 10 –20 atm at 700°C and 10 –17

atm at 900°C)

TASK I: RESULTSDiagram of Apparatus

Previous Results

• Oxidation in wet air produced the most severe degradation at 900˚C (accelerated chromia growth on Crofer and AL453 and increased spallation from E-brite).

• ASR correlated with oxide thickness.• Thin specimens deform under oxidation-induced

stresses.

TASK I: RESULTSDry Air Exposures – 700oC

Time vs. Mass Change / Area (700oC, dry air)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 200 400 600 800 1000 1200 1400 1600 1800 2000Exposure (Cycles)

Mas

s C

hang

e / A

rea

(mg/

cm2 )

Crofer - 1

Crofer - 2

E-brite - 1

E-brite - 2

AL453 - 1

AL453 - 2

ZMG232 - 1

ZMG232 - 2

TASK I: RESULTSSimulated Anode Gas (Ar-4%H2, H2O) Exposures – 700oC

Time vs. Mass Change / Area for Crofer, E-brite, AL453, & Ni (700oC, Ar/H2/H20)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Exposure (cycles)

Mas

s C

hang

e / A

rea

(mg/

cm2 )

Crofer - 1Crofer - 2E-brite - 1E-brite - 2E-brite - 3AL453 - 1AL453 - 2Ni

TASK I: RESULTSWet Air (0.1 atm H2O) Exposures - 700oC

Time vs. Mass Change / Area (700oC, wet air)

-0.10

0.00

0.10

0.20

0.30

0.40

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Exposure (Cycles)

Mas

s C

hang

e / A

rea

(mg/

cm2 )

Crofer - 1

Crofer - 2

E-brite - 1

E-brite - 2

AL453 - 1

AL453 - 2

ZMG232 - 1

ZMG232 - 2

TASK I: RESULTSMicrostructural and Phase Identification

Crofer 700oC

Dry Air2000 cycles

Wet Air1017 cycles

SAG2000 cycles

Internal Al2O3Internal Al2O3

Internal Al2O3

Cr2O3

Cr2O3

Cr2O3

MnCr2O4

MnCr2O4

MnCr2O4

Si rich oxide Si rich oxide

Si rich oxide

TASK I: RESULTSMicrostructural and Phase Identification

AL453 700oC

Dry Air2000 cycles

Wet Air1017 cycles

SAG2000 cycles

Internal Al2O3

Internal Al2O3

Internal Al2O3

Metal Layer Cr2O3 Cr2O3

Cr2O3Cr2O3 with ~7% FeCr2O3 with ~11.5% Fe

TASK I: RESULTSMicrostructural and Phase Identification

ZMG232 700oC

Dry Air1019 cycles

Wet Air1017 cycles

Internal Al2O3

Internal Al2O3

Cr2O3

Cr2O3MnCr2O4 MnCr2O4

Si rich oxideSi rich oxide

TASK I: RESULTSMicrostructural and Phase Identification

E-brite 900oC

Dry Air 2000 cycles

Wet Air 2005 cycles

SAG 2000 cycles

Cr2O3

MnCr2O4

Si rich oxides

Cr2O3

Cr2O3

Si rich oxides

Internal Al2O3

Internal Al2O3

Internal Al2O3

TASK I: RESULTSMicrostructural and Phase Identification

E-brite 700oC

Dry Air2000 cycles

Wet Air1017 cycles

SAG2000 cycles

Sigma Phase

Sigma Phase

Sigma Phase

Internal Al2O3

Internal Al2O3

Internal Al2O3

Cr2O3 Cr2O3

Cr2O3

TASK I: RESULTSSigma Phase in E-brite at 700oC

Dry Air2000 cycles

SAG2000 cycles

Wet Air1017 cycles

Wet Air1017 cycles

Cracks

Sigma Phase (37% Cr, 63% Fe)

Sigma Phase (36.6% Cr, 63.4% Fe)

Sigma Phase (35.9% Cr, 64.1% Fe)

Sigma Phase (36.6% Cr, 63.4% Fe)

Chromia Evaporation

moleKJGoK /891100 −≈∆

510632

−= xa OCr

Crofer

MnCr2O4

)(2)()( 3223

32 gCrOgOsOCr =+43

2

21

32

21

3 OOcrCrO paKp =

)()()( 4232 sOMnCrsOCrsMnO =+

132=OCra atmxpCrO

111043

−=

71032

−=OCra atmxpCrO14101

3

−=

Chromia Saturation

MnO Saturation

LaCrO3 (Activity data from Hilpert et al)

atmxpCrO13103

3

−=

Pressure of CrO3

-18

-16

-14

-12

-10

-8

-6

0.7 0.75 0.8 0.85 0.9 0.95 1 1.051/T*103 (K-1)

log

p CrO

3 (at

m)

log p(Cr) log p(Mn)(A) log p(La) log p(Mn)(B)

Cr2O3

LaCrO3

MnCr2O4

Partial pressures of CrO3 in Equilibrium with

Cr2O3, MnO-saturated MnCr2O4, and LaCrO3

Note: similar reductions would be achieved in the pressure of CrO2(OH)2

Crofer oxidized in contact with LaSrMnO4(cathode) for 88hrs at 900oC in air + 0.1atm

H2O

Crofer (side in contact with cathode)

Crofer (side opposite cathode)

MnCr2O4with ~1.5% Sr and ~2.6% La

MnCr2O4

Cr2O3

Cr2O3 with ~2% Sr, ~4% La, and ~4.7% Mn

~74.9% Fe ~20% Cr ~2.2% La ~2% Mn ~0.9% Sr

After exposure, the cathode contained ~0.9% Cr and ~1.8% Al

La0.8Sr0.2CrO3 Coated E-Brite (~5µm thick)

La0.8Sr0.2CrO3*

Cr2O3

SiO2

Substrate

*Coating is porous due to a phase transformation during devitrification

La0.8Sr0.2CrO3 (amorphous)

Substrate

SubstrateSiO2

Cr2O3

100 hrs Isothermal, 900oC in Dry Air

Coated side

Uncoated side

As Coated

La0.8Sr0.2FeO3 Coated E-Brite (~5µm thick)

10 20 30 40 50 60 70 80°2Theta

0

100

400

900

1600

2500

3600

counts/s

10 20 30 40 50 60 70 80°2Theta

0

100

400

900

1600

2500

3600

counts/s

As Coated Surface

SubstrateLa0.8Sr0.2FeO3

SubstrateLa0.8Sr0.2FeO3

Cracks

800oC 2 hrs in Ar-H2 (pO2~10-8)

La0.8Sr0.2FeO3 Coated E-Brite (~5µm thick)

La0.8Sr0.2CrO3 Coated Al 29-4C (~5µm thick)

Chromite coating cracked as well after same exposure conditions

La0.8Sr0.2FeO3

La0.8Sr0.2FeO3

Substrate Substrate Cr2O3/SiO2

Cross sections show the coating to be much more dense, but also confirms the cracks seen from the surface

Task I Summary

• Oxidation morphologies were similar at 700 and 900ºC.• MnCr2O4 is more stable than other transition metal

chromates.• Measurable interaction between Crofer and cathode

material.• Sigma-phase was observed to form in the higher Cr

content alloys at 700˚C.• Chromia growth reduced under chromite coating.• Chromite and ferrite coatings cracked during

devitrification.

TASK I: FUTURE WORKWork Planned for Next Twelve Months

• Continue Conductivity Measurements on Scales• Continue Study of Effect of Contact with Anode

and Cathode Materials• Experiments to Decrease Chromia Growth Rate

(Reactive Elements, Elimination of Grain Boundaries in Chromia)

• Study the kinetics of sigma-phase formation.• Investigate Effects of Simultaneous Exposure to

Cathode and Anode Gases• Continue Study of Effects of Coatings (Chromite)

on Chromia Growth and Evaporation

TASK II SUMMARY• Indentation Has Been Used to Induce Spallation in Vapor- and

SAG-Exposed E-BRITE and in Coated Specimens• Initial Observations and Fracture Calculations are Consistent

with Observations• Ability to Predict Spallation Behavior at Early Times is Key as

Testing Temperatures are Reduced

• Extend Modeling of Indentation of E-BRITE to Other Substrate Systems

• Incorporate Oxide Thickness and XRD Stress Measurements into Models: Identify Mechanisms Leading to Spallation

• Indentation Tests on E-BRITE for Longer Exposures at 900° C in Wet Air and Simulated Anode Gas

• Indentation Tests on Specimens Exposed at 700° C • Study of Adherence of Exposed Coated Specimens

TASK II: FUTURE WORKWork Planned for Next Twelve Months

TASK III: RESULTSComparison of oxide thickness for NiO and Cr2O3 -700˚C

Ni Wet Air1017 cycles

NiO

Crofer Wet Air1017 cycles

Internal Al2O3

Cr2O3

MnCr2O4

Si rich oxide

E-brite Wet Air1017 cycles

Sigma Phase

Internal Al2O3

Cr2O3

AL453 Wet Air1017 cycles

Internal Al2O3

Cr2O3

Estimated ASR is approximately the same for Ni and Crofer after oxidation

Task III: TYPICAL RESULTSCo-8 wt% Cu - 900ºC Dry Air - 28 hours

Pt markers observed above scale/alloy interface

Platinum markers

CoO

Alloy

Scale

Copper Concentrations 125 hr exposure

Copper to Cobaltratio Position

12.51 ±1.06 5 µm from gas/scale interface7.01 ±0.95 midway through scale2.55 ±1.05 5 µm above scale/alloy interface

Copper Concentration Position6.38% ±0.83% 5 µm below scale/alloy interface8.40% ±1.17% midway through alloy

Note significant “uphill diffusion” of Cu in CoO.

Parabolic Rate Constants at 900ºC

y = 5.434E-09x

y = 5.429E-09xy = 9.95E-09x

y = 4.79E-09x

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000

T ime Seco nds

(∆M ass/ area) 2

(g/ cm 2 ) 228hours

125 hours

Gesmundo1atmcobalt 1atm

Summary: Cu was successfully doped into CoO but growth rate was not decreased.

TASK III: PRELIMINARY RESULTSAlternative Material Choices

•This Task involves a theoretical evaluation of alternate metallic materials which have properties superior to the ferritic alloys.

•CoO scales have been successfully doped with Cu from an alloy but growth rate was not decreased

TASK II: FUTURE WORKWork Planned for Next Twelve Months

•Work on doping of CoO and NiO by alloying will continue focussing on growth rate and conductivity.

•The most promising materials will be fabricated and tested.

SUMMARY AND CONCLUSIONS

The aim of this project is to evaluate the chemical and thermomechanical stability of ferritic alloys in the fuel cell environment.

The understanding gained will be used to attempt to optimize the properties of the ferritic alloys.

A parallel study is evaluating the potential use of alternate metallic materials as interconnects.

COLLABORATION

Collaboration, advice, and support from the following organizations is gratefully acknowledged:

• PNNL (P. Singh, G. Yang)

• NETL Morgantown (C. Johnson, L. Wilson, G. Richards)

We would welcome the opportunity to collaborate with other branches of the National Laboratories and Industry.

TASK I: RESULTSWet Air (0.1 atm H2O) Exposures (for Ni) - 700oC

Time vs. Mass Change / Area for Ni (700oC, wet air)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 100 200 300 400 500 600 700 800 900 1000 1100Exposure (Cycles)

Mas

s C

hang

e / A

rea

(mg/

cm2 )

Side 1 Side 2

TASK I: RESULTSDual Atmosphere Conditions

800oC, 3 cycles, 100 hours per cycle

Internal Al2O3Internal Al2O3

Si rich oxideSi rich oxide

Cr2O3 MnCr2O4 MnCr2O4Cr2O3

Crofer Stress Measurement Using the 220 Lattice Plane (900oC, Dry Air, 100 Cycles)

y = -0.0107x + 1.2458

1.234

1.236

1.238

1.240

1.242

1.244

1.246

1.248

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

sine squared psi

d-sp

acin

g

Stress = -2.19 ± 0.03 GPa

TYPICAL RESULTSTASK II: THERMOMECHANICAL BEHAVIOR

Stress Measurements with 220 Lattice Plane – 900oC, Dry Air

TASK II: THERMOMECHANICAL BEHAVIORUSE OF TINTING TO VIEW SAG SPECIMEN DEBONDS

• Two Indents after 464 Hours at 900°C• 60kg Indent, 10min at 700°C (tint), 150kg Indent (Mechanics Analysis

Shows Increased Load Only Increases the Size Scale of the Damage) • Tinting allows Clear Visualization of Debond Size at 60kg• Visual Extent of Debonding Roughly Equals Actual Extent of Debonding

SAG 264 HrsSAG 464 Hrs/150KgSAG 464 Hrs/150Kg

60 Kg Indent

60 Kg Debond

150 Kg Indent

60 Kg Debond(Tint)

150 Kg Debond

TASK II: THERMOMECHANICAL BEHAVIORSAG SPECIMEN INDENT FRACTURE MODELING

Large Substrate

Larg

e Su

bstra

te

Conical Indenter r

z 0

50

100

150

200

2 2.5 3 3.5 4

R/a

Gc

(J/m

2 )

t = 0.25umt = 0.5umt = 1umt = 2umt = 4um

• Finite Element Model of the Indent Problem: Substrate Strains Transferred to the Chromia Scale

• Fracture Mechanics Formulas Estimate Gc vs. Normalized Debond Radius (Residual Stress of -2.22 GPa in Chromia Scale)

• R/a = 2.5 and toxide = 2µm Yields: Gc = 34 J/m2