Advanced Metallic Interconnect Development

Advanced Metallic Advanced Metallic Interconnect DevelopmentInterconnect Development

Z. Gary Yang, Gordon Xia, Prabhakar Singh, Jeff Stevenson

SECA Annual Workshop and Core Technology Program Peer Review

Boston, May 11-13, 2004

2

Interconnect DevelopmentInterconnect DevelopmentInterconnect Development

ApproachesApproaches:Evaluation of conventional and newly developed alloys (chemical, electrical, mechanical properties, cost).Investigation and understanding of degradations in bulk alloy interconnects and at their interfaces under SOFC operating conditions. Materials development

Surface modificationBulk modification or alloy developmentCathode/interconnect interfaces

Objectives:Objectives:Develop cost-effective, optimized materials and coatings for intermediate temperature SOFC interconnect and interconnect/electrode interface applications.Identify and understand degradation processes in interconnects and at their interfaces.

3

Focus Areas & ProgressFocus Areas & ProgressFocus Areas & Progress

Study of Ni-based alloys.Investigation of oxidation behavior of candidate alloys under SOFC operating conditionsDevelopment of cathode-side functional interfaces

4

Focus Areas & ProgressFocus Areas & ProgressFocus Areas & Progress

Study of Ni-based alloysInvestigation of oxidation Investigation of oxidation Investigation of oxidation behavior of candidate behavior of candidate behavior of candidate alloys under SOFC alloys under SOFC alloys under SOFC operating conditionsoperating conditionsoperating conditionsDevelopment of cathodeDevelopment of cathodeDevelopment of cathode---side functional interfaces side functional interfaces side functional interfaces

5

Ferritic Stainless Steels: Status and IssuesIn-situ X-Ray Diffraction Analysis

28 30 32 34 36 38 40 42 44 46 48

2θ

M: Fe-Cr substrate

C: Cr2O3

S: (Mn,Cr)3O4 spinel

300 h

2 h8 h

32 h

100 h

S (220)

S (311)

S (321)

S (400)

C (104)

C (110)

C (113) M (111)

M (111)

800oC100 h

air

800oC300 h

air

Scale volatility;

Long term oxidation resistance under SOFC operating conditions;

Life time scale electrical properties;

Mechanical/thermomechanicalstability.

Cr

Al Mn

(Al,Ti)xOy

800oC 1,200 h

air

Crofer22 APU

6

Study and Evaluation of Ni-Based Alloys

Study and Evaluation of Ni-Based Alloys

Why Ni-based Alloys?Excellent oxidation resistance, super high temperature strength, and good manufacturability.Formation of NiO top scale as potential Cr stopping layer.CTE can be modified through alloying.Scale can be potentially engineered for improved electrical conductivity.

QuestionsCan the required combination of properties be found in a single alloy composition?Cost?

----0.0150.3----14.02.00.40.50.105.0b3.0b22.0BalHaynes230

0.02La--0.015b0.25----1.0b15.00.40.50.02b2.0b3.0b16.0BalHastelloy S

------0.70----16.00.08b1.0b0.01b2.0b3.0b16.0BalHastelloy C-4

0.91.118.012.0BalLTES700

0.5b Cu--0.006b0.5b------25.00.8b0.8b0.03b2.5b2.0b8.0BalHaynes242

OthersVBAlTiCbWMoSiMnC Co Fe Cr Ni

Nominal composition, wt%Alloysa

Haynes242, C, S, and 230 were developed by Haynes International; LTES700 by Mitsubishi Heavy Industries.

7

Low CTE Ni-Based AlloysLow CTE Ni-Based Alloys

15.225-800oC

Haynes 230

13.3-14.420-540-760oC

Hastelloy S

13.3-14.420-540-760oC

Hastelloy C-4

13.6RT-760oC

LTES700

12.2-13.920-540-760oC

Haynes242

12.2RT-760oC

Crofer22 APU

TEC×10-6.K-1

(from manufacturers)Alloys

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600 700 800

Tempearture oC

δL/L

Haynes 230Hastelloy SHaynes 242

Haynes230: 14.9 (25-800oC)Hastelloy S: 14.8 (25-800oC)Haynes242: 13.1 (25-800oC)

TixMoxWxAlxCoxNbTaxCrx

1222

222

1063.11024.81095.71084.11098.1)95.1(1075.31028.787.13

−−−−

−−−

−−−−

++++=α

Yamamoto, et al.

From in-house measurement

Traditional Ni-based alloys have a CTE of 15.0~19.0 µm/m.K-1

(RT~800oC). A relatively low CTE of 13.0~14.5 µm/m.K-1 (RT~800ºC) can be achieved via alloying. Mo, W, Ti and Al reduce CTE of Ni-based alloys; while Cr, Ta+Nb and Co increase it;Cr concentration has to be relatively low in these alloys.

8

Scale Structure and CompositionScale Structure and Composition

Hastelloy SHaynes242

NiO

Haynes230

30 35 40 45 50 55

2Η

Inte

nsity

(a.u

.)

Haynes242Hastelloy SHaynes230

2θ

M: alloy substrateG-γ’ precipitatesS: M3O4 (spinel)C: Cr2O3

N: NiOM

C CCGG

SS N

G G

N

M

After oxidation at 800oC for 300 hours in moist AIR.

Crack

9

Scale Structure and CompositionScale Structure and Composition

After oxidation at 800oC for 300 hours in moist HYDROGEN.

Haynes230Hastelloy SHaynes242

30 35 40 45 50 55

2

Inte

nsity

(a.u

.)

Haynes230Hastelloy SHaynes240M: alloy substrate

G-γ’ precipitatesS: M3O4 (spinel)C: Cr2O3

C CCGS G

M

M

G G

2θ

10

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

0.00 50.00 100.00 150.00 200.00 250.00 300.00

Time (h)

ASR

(ohm

-cm

2 )

Haynes230

Hastelloy-S

haynes242

Power failure

Scale ASRScale ASRsample

P

P

v

v

I

I

alumina

Pt paste

The measurement was carried out at 800oC in moist air.

11

SummarySummary

CTE of Ni-based alloys can be adjusted to a relatively low value via lowering Cr% and adding metal elements such as W, Mo, etc.The decreased Cr% may however raises concerns over the oxidation resistance of an alloy in cathode environment; The heavy alloying also creates nonlinearity in the CTE curve.A scale with a NiO outer-layer can be formed on low Cr% Ni-alloys in cathode-side environment, but its suitability as an electrically conductive protective layer is questionable.

The newly developed FSS demonstrates reduced scale volatility, good CTE matching, reduced scaled resistance, and improved surface compatibility with sealing glasses.There is however a need for further improvement in long term scale chemical, electrical, and mechanical stability (for temperatures >700oC).

Ni-Based AlloysNi-Based Alloys

Ferritic Stainless SteelsFerritic Stainless Steels

12

Focus Areas & ProgressFocus Areas & ProgressFocus Areas & Progress

Study of NiStudy of NiStudy of Ni---based alloysbased alloysbased alloysInvestigation of oxidation behavior of candidate alloys under SOFC operating conditionsDevelopment of cathodeDevelopment of cathodeDevelopment of cathode---side functional interfaces side functional interfaces side functional interfaces

13

Oxidation Behavior of Alloys under Interconnect Dual Exposures

Variables: Alloy compositionIsothermal vs. cyclingMoisture

Air Air

T/Cxxxx

x

xxxx

x

Seal

FuelAirAir

T/C

Stainless steel

{NiBS FeSSHaynes 230-22%CrHastelloy S-17%CrHaynes 242-8%Cr

{E-brite-27%CrCrofer22-22%CrAISI430-17%Cr

Motivation:Oxidation study has been a common area of

interest, but typically under single atmosphere exposure.

Dual exposures are commonly found in SOFC stacks and BOP, as well as other systems.

Understanding helps develop robust materials.

Materials studied:

14

Anomalous Oxidation of FSS under Interconnect Dual Exposures: A Summary

Fe

Cr

Mn

Airside of Crofer22 APU

Mn

Cr

Fe

Isothermal: 800oC, 300h

Thermal cycling: 800oC, 3x100h

The DUAL exposures lead to an anomalous oxidation behavior of ferritic stainless steels under the SOFC interconnect dual exposure conditions;

The anomalous oxidation behavior appears to be caused by hydrogen diffusion from the fuel side to the airside of alloy interconnects.

For 430 with 17% Cr, dual exposures enhanced the iron transport in the scale on the airside, leading to hematite formation and localized attack; Fro Crofer22 (22% Cr), Fe enrichment was found in the spinel layer after isothermal oxidation; thermal cycling resulted in the hematite nodule formation and localized attack;For ferritic stainless steels with enough chromium, e.g. E-brite (27% Cr), the accelerated iron transport and iron oxide formation are inhibited, though differences in scale microstructure and morphology are observed

15

Crofer22 APU: Effects of Moisture

Grown on the coupon in moist (3%H2O) air only and on the airside of the coupon that was ISOTHERMALLYISOTHERMALLY heat-treated at 800oC, 300 hours.

Fe2O3-rich nodules

Airside of dual exposures

0

500

1000

1500

2000

2500

3000

3500

4000

20 30 40 50 60 70

2θ

Inte

nsity

(cou

nts)

M: Fe-Cr substrateC: Cr2O3

S: (Mn,Cr,Fe)3O4 spinelO: Fe2O3 hematite

C

C

C

C

C

C

C

CC

CC

C

C

C C O

O

O

SO O

OO O

S

S

S

S

SS

S

S

S

S S

S

S

M

MM

M

Air only

Airside of dual

Presence of moisture accelerated the anomalous oxidation.

Fe

Cr

Mn

16

Haynes230: Oxidation Behavior

Air exposure at both sides

Airside of dual exposures

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

30 35 40 45 50 55 60

2θ

Inte

nsity

(cou

nts)

Both sides to dry airAirside of dual exposures

M: alloy substrateG-γ’ precipitatesS: M3O4 (spinel)C: Cr2O3

N: NiO

M

M M

M

C

C

C

C C

CC

C

G

G GS

S

S

S

N

N

GG

G GS

S

S

SG

Grown on the coupon in air only air only (ambient air) and on the airsideairside of the coupon that was isothermallyisothermally heat-treated at 800oC, 300 hours.

17

0

100

200

300

400

500

600

700

800

30 35 40 45 50 55

2θ

Rel

ativ

e In

tens

ity C

C

C

C

C

C

CSG

SG

G

G

G

N

M M

N

G

GC

S

SG

M-metal substrateG-γ’ precipitationC-Cr2O3S-(Mn,Ni,Cr)3O4N-NiO

Hastelloy S: Oxidation Behavior

Air exposure at both sides

Airside of dual exposures

Grown on the coupon in air onlyair only and on the airsideairside of the coupon that was isothermallyisothermallyheat-treated at 800oC, 300 hours.

Cr

Ni

18

0

500

1000

1500

2000

2500

30 35 40 45 50 55

2θ

Inte

nsity

(a.u

.)

To air at both sides

Air side of dual exposures

M-metal substrateG-γ’ precipitationC-Cr2O3S-(Mn,Ni,Cr)3O4N-NiO

C

C

C

C

CSG

G

G

N

M

M

N

G

C

S

G

G

S

SS

SG

Haynes242: Oxidation BehaviorAir exposure at both sides

Airside of dual exposures

Grown on the coupon in air onlyair only and on the airsideairside of the coupon that was isothermallyisothermallyheat-treated at 800oC, 300 hours.

19

SummarySummarySummary

For ferriticferritic stainless steelsstainless steels with relatively low chromium levels (22% or less), dual exposure enhances the iron transport in scale on the airside, leading to hematite formation and localized attack. The presence of moisture enhances the anomalous oxidation, leading to localized attack.For NiNi--based alloysbased alloys, dual atmosphere

exposure tends to reduce NiO formation, and to facilitate the formation of a uniform chromia/spinel dominated scale.

20

Focus Areas & ProgressFocus Areas & ProgressFocus Areas & Progress

Study of NiStudy of NiStudy of Ni---based alloysbased alloysbased alloysInvestigation of oxidation Investigation of oxidation Investigation of oxidation behavior of candidate behavior of candidate behavior of candidate alloys under SOFC alloys under SOFC alloys under SOFC operating conditionsoperating conditionsoperating conditionsDevelopment of cathode-side functional interfaces

21

CathodeCathode--Side Functional InterfacesSide Functional Interfaces

Chromia forming alloy interconnects

Chromia forming alloy interconnects

Protection layer Cr2O3

Before assembly During operation

Cathode

Contact layerContact layer

Cathode

Protection layer Functional interfaces}

Protection layer acts as a mass barrier to mitigate or prevent Cr migration via both gas transport and solid state reactions, as well as to decrease electrical contact resistance. The subsequently grown chromia sub-scale serves as cation and anion transport barrier, protecting the alloy interconnect.

Contact layer promotes contact between cathodes and interconnects, and helps minimize interfacial resistance and power loss.

22

0

0.005

0.01

0.015

0.02

0.025

0 50 100 150 200 250 300

Time(hours)

ASR

(Ohm

.cm

2 )

ITO coating LSF coatingLSCr coatings Crofer22 APU

Both bare and coated samples were pre-oxidized in air at 800oC for 100h before carrying out tests in air at 800oC.

FeCr

MnO

La0.8Sr0.2FeO3

Fe

Cr

Al

MnO La0.8Sr0.2CrO3

ProvskiteProvskite Coatings as Coatings as Protection LayersProtection Layers

The provskite coatings decrease electrical resistance and mitigate or prevent Cr migration;

The growth rate of the chromia beneath the coatings and the eventual scale depends on the ionic conductivity of coatings.

Long term stability needs to be further studied.

23

Thermal Grown Thermal Grown SpinelSpinelProtection LayerProtection Layer

Concept

Chromia forming alloy interconnects

Chromia forming alloy interconnects

Ms3O4 spinel

Ms3O4 spinelCr2O3

As prepared

During operation

The protection layer is intended to be thermally grown.

Solution coating, PVD, CVD or EC plating of spinelformation metals.

Growth of a thin spinel layer via reactions during a heat treatment in an optimized environment

Formation of a spinel-chromiafunctional scale on interconnects during subsequent oxidation or SOFC operations.

Approach

24

Current focus is on thermally grown spinels which contain no Cr and/or are more stable than (Cr,Mn)3O4.

MnCO3+Co3O4

Slurry coating

Heat treating in 2.75H2+Ar at 950ºC for 24

hours.

Oxidation in oxidizing

environment 0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

28 33 38 43 48 53 58

2θ

Inte

nsity

(a.u

.)

Mn0.1Co0.9Mn0.3Co0.7Mn0.5Co0.5Mn0.7Co0.3

S: Spinel Ms3O4

Co: Cobalt

M: Crofer22 APU

SS

S S S

M

CoCo

Growth of (Mn,Co)3O4 on Crofer22 APU

25

Interfacial ASR of Crofer22 APU Grown with Spinel Protection Layers

The (Mn,Co)3O4 spinel protection layer on Crofer22 APU minimizes the interfacial resistance when (La0.8Sr0.2)Co0.5Mn0.5O3 used as a electrical contact.

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0.0 50.0 100.0 150.0 200.0 250.0Time (h)

Crofer22 APU+Mn1.5Co1.5O4, after reducingBare Crofer22 APUCrofer22 APU+Mn1.5Co1.5O4, after reducing and air heating

)( reactionscontactsscaleASR erconnectcathode ,,int/ Φ=

LSF LSF

LSF cathode

LSF cathodeLSCM contact

LSCM contact

Crofer22 APU

Crofer22 APU

6.5 PSI

V

V

I

I

6.5 PSI

500mA.cm-2

500mA.cm-2

AS

R (m

ohm

.cm

2 )

Pre-heat treated at 800oC for 100h

Pre-heat treated at 950oC for 24h

Bare Crofer22 APU

26

SummarySummarySummary

Continuous, thin spinel protection layer can be thermally grown on chromia forming alloys during optimized pre-heat treating; the spinel protection layer is intended to help minimize volatilization of Cr vapor species and the interfacial electrical resistance.Preliminary work on Co/Mn spinel layers indicates low interfacial electrical resistance.Mitigation of Cr volatility to be verified experimentally.

27

Future Work: Study oxidation behavior under dual exposures

Investigate and develop cathode-side functional interfaces

Develop and investigate cladded composite-structure interconnects

Mechanistic understanding: Interaction and transport of H/H+ at the metal/oxide interface and in the oxide scale; their effects on defect structure, transport properties, scale growth.

Study effects of dual exposure on scale electrical conductivity.Oxidation behavior of alloys under the reforming gas/air dual exposures.

Spinel protection layers: Continue to screen and search for spinels that compatible to candidate alloys and more thermochemically stable than (Mn,Cr)3O4; optimize processing and materials composition.

Electrical contact layers: Continue to study the interactions between conductive oxides and candidate alloys; investigate the interfacial ASR and optimize the composition for a minimized interfacial resistance.

Continue to the proof of concept investigation.

Study interdiffusion and predict life via modeling.

Optimize structure and compositions.

28

AcknowledgementsAcknowledgementsAcknowledgements

The authors wish to thank Wayne Surdoval, Lane Wilson, and Don Collins (NETL) for their helpful discussions regarding this work. This work was funded by the U.S. Department of Energy’s Solid-State Energy Conversion Alliance (SECA) Core Technology Program.