ne-

(LS-BrPVDexCFclinh ptermediate spinel layers were formed spontaneously in all thin lms, withositions of the perovskite lms. With the exception of the BSCF lms, the

spinel layers was highly effective in blocking Cr d

operachallenThe Crless ste

the Cr content provides good oxidation resista

include Crofer22, E-Brite, and 430 stainless steel. Some newly use of commercial-scale coating methods for depositing chromium

Solid State Ionics 181 (2010) 12941302

Contents lists available at ScienceDirect

Solid Stat

w.edeveloped alloys and several promising oxide dispersion strength-ened (ODS) alloys are also being considered.

The role of Cr poisoning in the degradation of the electrochemicalperformance of SOFCs has been well documented [79]. To minimizethe effect of Cr contamination, engineers and component designershave employed protective surface coatings on the interconnectmaterials to act as barriers against outward diffusion of Cr. In recentyears, a variety of coating systems have been investigated, including:perovskite oxides such as (La,Sr)MnO3, (La,Sr)CoO3, (La,Sr)CrO3 ortheir combinations[4,1116]; spinels such as (Co,Mn)3O4 and (Mn,

barrier layers. In this work we have adopted a commercial scalephysical vapor deposition (PVD) approach to deposit perovskite andmetallic Co coatings on commercial stainless steel substrates, atthicknesses of 13 m. The deposited lms are characterized withregard to their microstructure and phase content before and afterthermal treatments in order to dene the roles of the chemicalconstituents in mitigating Cr diffusion from the stainless steel.

2. Experimental methodsCr)3O4 [1720]; metallic coatings such as Co, Ncomplex oxide coatings[22].

A variety of techniques have been empcoatings including vacuumplasma spraying[1[13], electrodeposition [2326] slurry dip coa

Corresponding author. Tel.: +1 (607)871 2438; faxE-mail address: misture@alfred.edu (R. Lacey).

0167-2738/$ see front matter 2010 Elsevier B.V. Adoi:10.1016/j.ssi.2010.07.007nce at high temperaturecial alloys of this kind

be oxidized to produce high-density oxides.The large-scale production of coated interconnects will require the[10]. The most commonly studied commer1. Introduction

Materials developments enablingtemperatures have introduced a new(Cr) poisoning of the cathode [19].temperature SOFCs is the high-Cr stain 2010 Elsevier B.V. All rights reserved.

tion of SOFCs at lowerge related to chromiumsource in intermediateel interconnect wherein

ing[4,12,14,27], and ltered arc deposition[22]. A very recent reviewarticle highlights differences among the performance of variouscoating materials and processes[31], citing electrodeposition asperhaps the most effective method reported to date. The mostcommonly used methods allow deposition of oxide lms, butelectrodeposition yields dense metallic lms which can subsequentlyi and Cu [21]; and other

loyed to deposit these5], wet powder sprayingting, magnetron sputter-

Thin lms weelectron-gun PVand residual gasSpecimens werecoating on oneincluding La0.3SrCoO3- (LSC82),(BSCF), LaNi0.5F

: +1 (607)871 2354.

ll rights reserved.iffusion from the interconnect materials.

combination of a perovskite or Co coating and the spontaneously-formed, chemically graded intermediateChromium blocking lmsubstrate interface, inconcurrent shifts in compEvaluation of Co and perovskite Cr-blocki

Robert Lacey a, Abhijit Pramanick a, Jae Chun Lee b, JaRobert Naum c, Scott T. Misture a,a Kazuo Inamori School of Engineering, Alfred University, Alfred, NY 14802, USAb Materials Sci. & Eng., Myongji University, Kyunggi-do, Republic of Koreac Applied Coatings, Inc. Rochester, NY, USA

a b s t r a c ta r t i c l e i n f o

Article history:Received 18 February 2010Received in revised form 6 July 2010Accepted 16 July 2010

Keywords:SOFCInterconnect

The viability of perovskitemigration from Crofer22, Ephysical vapor deposition (LSC, LSCF, LNF and Co lmsmaterials. Although the BSformation upon thermal cyinvestigated using XPS dept

j ourna l homepage: wwg thin lms on SOFC interconnects

Il Jung a, Bo Jiang a, Doreen D. Edwards a,

C, LSCF, LNF and BSCF) and metallic (Co) thin lm coatings to reduce Crite and SS430 interconnect materials has been examined. Production-scale) systems were used to obtain uniform lms with thickness near 1 m. Thehibited good adhesion, thermal stability and chemistry similar to the targetlms exhibited good initial adhesion, subsequent reactions caused blisterg. Chromium migration upon extended thermal treatments of 168 h wasroling through the lms. As a result of interdiffusion of elements across the

e Ionics

l sev ie r.com/ locate /ss ire deposited on commercial alloys using commercialD coaters, equipped with crystal thickness monitorsanalyzers, by Applied Coatings, Inc., Rochester, NY.coated then cut to size, yielding specimens with aface only. Eight coating materials were tested,

0.7CoO3- (LSC37), La0.6Sr0.4CoO3- (LSC64), La0.8Sr0.2-La0.6Sr0.4Co0.2Fe0.8O3- (LSCF), Ba0.5Sr0.5Co0.2Fe0.8O3-e0.5O3- (LNF55), LaNi0.6Fe0.4O3- (LNF64), and Co

diffraction using a Co K sourcewas used to study the evolution of thecrystalline phases, up to a maximum temperature of 1000 C

bulk crystalline LSC at 500 C[29]. However, both crystalline andamorphous LSCF display similar total conductivity at 500 C [30].

The amorphous LNF, LSC, and LSCF lms exhibited thermallyactivated conductivity with either one or two activation energies. TheLSC37 and LSC64 exhibited a single activation energy (~0.40.5 eV)whereas LSC82 exhibited two activation energies (0.2 eV below200 C and 1.2 eV above). LNF and LSCF behaved similarly, withactivation energies of 0.16 or 0.17 eV below 200 C and 0.55 and 0.74above, respectively. The structural origin of the change in activationenergy near 200 C remains unclear, and additional work is underwayto establish the interfacial resistance of the crystalline coatings after

Fig. 1. Cross-sectional view of lms of composition LSC82 on (a) glass, (b) sapphire (off-axis view), and (c) Crofer22 substrates.

1295R. Lacey et al. / Solid State Ionics 181 (2010) 12941302with heating and cooling rates of 30 C/min in a custom diffractionfurnace [28]. The diffraction data were collected over a 2-theta rangeof 10 to 85. The crystalline phases were identied using the full scale(10902) diffraction patterns using a commercial software packageJADE.

Post-situ analysis was also performed for lms heat treated undertwo conditions: 1000 C for 3 h or 1000 C for 3 h followed by 168 h(1 week) at 850 C. X-ray diffraction, SEM, and x-ray photoelectronspectroscopy (XPS) depth proling were used to characterize thelms after heat treatment. XPS was performed using monochromaticAl K radiation in a Phi Quantera SXM instrument. The spectra werecollected from a region 100 m in diameter. It was noted that the XPSdepth proles do not provide high accuracy depth analysis. This is theresult of inherent roughness of the various metal substrates anddifferences in chemical composition and density of the depositedlms. Nevertheless, the depth proles provide sufcient informationto understand the general trends in diffusion of the different elementsacross the lmsubstrate interface.

To verify the expected equilibrium phase assemblage in the thinlms, bulk reactions of the perovskite phases with Cr2O3 and Crofer22in powder formwere investigated. Powdermixtures at a ratio of 50:50by weight were annealed at 850 C in air for 168 h and thencharacterized using XRD.

The in-plane electrical conductivity of oxide lms (LSC37, LSC64,LSC82, LSCF, LNF55, and LNF64 deposited on glass substrates wasmeasured from 50 C to 500 C using impedance spectroscopy with Ptelectrodes. Samples were heated at a rate of 10 C/min, and heldisothermally at 50 C increments for 20 min prior to taking theimpedance measurements. Impedance spectra were collected byscanning from 10 MHz to 1 Hz with a 1 V excitation signal using aSolatron 1260 impedance analyzer.

3. Results and discussion

3.1. Microstructure and electrical behavior of as-deposited lms

The PVD process produced uniform, homogeneous lms ofsubmicron thicknesses, which was conrmed from SEM micrographsof the coatings as illustrated in Fig. 1 for a thin lm coating of LSC82.Coatings deposited on glass or sapphire were dense, uniform andsmooth. Coatings deposited on the steel substrates, which had arougher surface, followed the contour of the substrate.

The oxide lms were amorphous and remained so upon heating to~600 C as determined by XRD, shown later. The electrical conduc-tivity of the amorphous lms at 50 C ranged from 2104 S/cm forLSC37 to 3 S/cm to LNF55. At 500 C the conductivities ranged from4 S/cm for LSC37 to 102 S/cm for LSCF. The conductivity of themetal. During each coating run, the thin lms were deposited on vedifferent substrates, including E-Brite, Crofer22, SS430, sapphire andborosilicate glass substrates. All coatings were deposited withnominal thickness of ~1000 nm, except the Co coatings which weredeposited at nominal thickness of 3000 nm. Prior to deposition, thesubstrates were sequentially wiped with alcohol, ultrasonicated in amild soap solution, rinsed with hot deionized water (16 Mresistance), and again wiped with alcohol. During deposition, thesubstrates were held on counter-rotational tooling and positioned inthe coating chamber to control deposition uniformity and maximizecontrol of coating chemistry.

The microstructures of the lms were examined using eldemission gun (FEG) scanning electron microscopy (SEM). Crosssections of the lms and substrates were examined using semi-quantitative energy dispersive x-ray spectroscopy (EDS). In-situ x-rayamorphous LSC lm is some 2.5 orders of magnitude lower than for long term annealing.

3.2. Short term crystallization and phase evolution

Fig. 2(a) shows a cross section and composition gradient of anLSC82 thin lm on a Crofer22 substrate after heat treatment at1000 C for 3 h. The thickness of the lm is ~1 m, as can be estimatedfrom the contrast in the cross-sectional micrograph and thedistribution of the chemical elements shown in the EDS line scan.The particle size is in the range of ~100 nm, as can be observed from

the top-view of the microstructure shown in Fig. 2(b). The evolutionof the different phases during heat treatment is shown in the in-situdiffraction data in Fig. 2(c). At room temperature, the lms areamorphous, and the perovskite phase (La,Sr)CoO3 starts to crystallizeat ~600 C. The XRD patterns are typical of a random powder,demonstrating that the amorphous lms crystallize without anypreferred crystallite orientation. A spinel phase begins to form at~800 C, which is most likely of the form (Cr,Co,Mn)3O4, though the

canh; (

1296 R. Lacey et al. / Solid State Ionics 181 (2010) 12941302Fig. 2. (a) Cross-sectional view of LSC82 lm on Crofer22 substrate, along with line EDS s(B) after crystallization at 1000 C, (C) after isothermal heat treatment at 850 C for 168

spinel, M2O3 where M is transition metal, and (La,Sr)CoO3- in the deposited lm on Crofacross the lmsubstrate interface; (b) microstructure of LSC82 lm: (A) as-deposited,c) in situ X-ray diffraction showing evolution of crystalline reaction products SrCrO4,

er22 substrate.

Fig. 3. (a) Cross-sectional view of LNF64 lm on Ebrite substrate, along with line EDSscan across the lmsubstrate interface; (b) in situ X-ray diffraction pattern of LNF55lm on Crofer22 substrate, showing evolution of crystalline reaction products spinel,M2O3 where M is transition metal, and La(Ni,Fe)O3- in the deposited lm on Crofer22substrate.

1297R. Lacey et al. / Solid State Ionics 181 (2010) 12941302exact composition of the spinel phase cannot be determined usingonly diffraction data. In addition to the primary phases of (La,Sr)CoO3and spinel phase, the formation of an appreciable fraction of SrCrO4was also observed, beginning at ~750 C.

Phase evolution for the LSC37 and LSC64 lms was similar toLSC82, with the rst crystallization of the perovskite phase alsooccurring at ~600 C. For LSCF, formation of (La,Sr)(Co,Fe)O3, a spinelphase, and SrCrO4 from the amorphous lm was observed upon heattreatment, again similar to the LSC lms.

The results for LSC, in a general sense, agree with earlier results ofFujita et al. [11] who also used a PVD process but found only the spinelphase as a reaction product after heating to 750 C. The moreaggressive heat treatment used in the current work shows thatSrCrO4 also forms given ample time at elevated temperatures.

The LNF64 and LNF55 thin lms were similar in microstructure tothe LSC thin lms, regardless of the interconnect alloy used as asubstrate. Fig. 3(a) shows a cross section of an LNF64 lm after heattreatment at 1000 C for 3 h on E-Brite and the composition gradientof the elements present. The contrast in the cross-sectional micro-graph and the distribution of the thin lm composition elements inthe EDS plot suggest that the lm thickness is~1 m. Fig. 3(b) showsthe in-situ diffraction patterns for LNF55 during thermal treatment ofthe thin lm on Crofer22 to 1000 C. Similar to the phase formationsequence in LSC lms, perovskite La(Ni,Fe)O3 begins to form at~600 C and a spinel phase begins to form at ~800 C. In addition,minor diffraction peaks begin to appear from the formation of M2O3(M=Cr, Fe, Ni, Mn, Co, etc.) at ~800 C. The EDS line scans show someCr enrichment at the metal-oxide interface (also clearly detectedusing XPS as discussed later) which is likely due to the formation ofCr2O3.

Fig. 4(a) shows the microstructure of the BSCF thin lm after heattreating at 1000 C for 3 h. The blisters formed on the surface are likelythe result of thermal mismatch between the reaction products formedand the substrate. The in-situ diffraction patterns at varioustemperatures are shown in Fig. 4(b). In this case, the large relativecontent of Ba and/or Sr results in crystallization of major fractions of(Ba,Sr)CrO4, in addition to the expected spinel phase. It is likely thatthe large quantity of (Ba,Sr)CrO4, with its large CTE mismatch, isresponsible for the blistering.

LSC, LSCF, and BSCF all react to form chromate phases of the form(Ba,Sr)CrO4 because of the divalent A-site cations, but LNF does not.To verify the expected equilibrium phase assemblage in the thin lmswith longer heat treatments, bulk reactions of the perovskite phaseswith Cr2O3 and Crofer22 in powder form were investigated. Theresults for mixtures with LSC37, LSC64, LSC82, LSCF6428, BSCF5582and LNF46 are summarized in Table 1. With the exception of BSCF, allof the reaction mixtures produced the same phases observed in thethin lms. In the case of BSCF, several additional phases were noted inthe bulk samples that were not detected in the thin lm. Twoadditional observations can be made from the data in Table 1. First,the phase assemblage for reaction with Crofer22 and Cr2O3 is nearlyidentical in all cases (ignoring any Fe in solid solution in the variousphases). Second, the LNF is nearly inert to Cr, forming only traceamounts of spinel and NiO.

Examination of the Co thin lm indicates that the microstructureand phase evolution sequences differ appreciably from the perovs-kites. A cross section of the lm after heat treatment at 1000 C for 3 his shown in Fig. 5(a). The thickness of the lm on average was ~3 m,as estimated from the contrast in the cross-sectional micrograph andthe distribution of Co in the EDS plot. The heat treated lm showed adendritic morphology with subsequent transformation to equiaxedgrains as shown in Fig. 5(b). The evolution of the crystalline phases inthe lm upon heat treatment was determined from the in-situdiffraction patterns, as shown in Fig. 5(c). The as-deposited lmcontained some crystalline Co, and upon heating in air oxidation

occurs to form rst CoO, beginning at ~500 C and then Co3O4. On

1298 R. Lacey et al. / Solid State Ionics 181 (2010) 12941302further heating, Co3O4 reacts with the Crofer22 to form a spinel-typephase and excess CoO, which is chemically reasonable from a massbalance analysis.

3.3. Long-term stability and chemical interdiffusion

LSC82, LNF55 and Co lms on Crofer22, E-Brite, and SS430substrates were studied in detail using XPS depth proling. The

Fig. 4. (a) Microstructure of crystallized BSCF lm showing formation of blisters on the lwhereas the inset in the picture on the left shows crystalline nature of the lms. (b) In situ X-spinel, (Ba,Sr)MO3 and M2O3 where M is transition metal in the deposited lm on Crofer

Table 1Thin lm and bulk reaction products of different coating compositions with Cr2O3 andCrofer22.

LSC LSCF BSCF LNF

Cr2O3 (La,Sr)CoO3- (La,Sr)FeO3- BaCrO4 La(Ni,Fe)O3-SrCrO4 SrCrO4 Sr3Cr2O8 Trace spinelSpinel Spinel Spinel Trace NiO

Ba12Co11O33(Sr,Ba)2M2O5

Crofer22 (La,Sr)CoO3- (La,Sr)FeO3- BaCrO4 La(Ni,Fe)O3-SrCrO4 SrCrO4 Sr3Cr2O8 SpinelSpinel Spinel Spinel Trace NiO

Ba12Co11O33(Sr,Ba)2M2O5inherent rough surface nish of the substrates (see for example Fig. 1(c)) prevents accurate determinations of the sputtered depth, but theproles nonetheless show trends in chemical interdiffusion.

Fig. 6 shows the XPS depth proles for several samples, whichhighlights similarities in the Cr diffusion behavior:

Cr enrichment at the oxidemetal interface, with oxygen present The Cr diffusion prole from the metal into the oxide is well-dened, and reaches a concentration of zero at a depth of 350400 nm below the surface of the oxide (~1100 nm for the thicker Colm)

Small Cr concentrations are noted on the surface of each specimen,whichwe attribute to vapor phase transport of Cr from the uncoatedregions of the specimens rather than from Cr diffusion through thecoatings.

Fig. 6(a) shows the concentration depth prole for the LSC82 thinlm on Crofer 22 substrate after heat treating at 1000 C for 3 h and850 C for 168 h. Outward diffusion of Fe, Mn and Cr from the Crofer22 substrate toward the lm is clear, even after the short periods ofexposure to high temperature environments. Concurrent inwarddiffusion of Co from the lm toward the substrate is also evident.

The composition proles for the LSC82 lms treated at 168 and 3 hare remarkably similar, with no signicant amount of Cr noted at ornear the surface and indistinguishable Cr concentration proles. The

m surface (marked by dotted circles). A close-up of the blisters is shown on the right,ray diffraction pattern showing evolution of crystalline reaction products (Ba,Sr)CrO4,22 substrate.

Fig. 5. (a) Cross-sectional view of Co lm on Crofer22 substrate, along with line EDS scan across the lmsubstrate interface; (b) microstructure of Co lm: after crystallization at1000 C (A), and after isothermal heat treatment at 850 C for 168 h (B); (c) in situ X-ray diffraction pattern showing evolution of crystalline reaction products Co3O4, spinel, andCoO in the deposited lm on Crofer22 substrate.

1299R. Lacey et al. / Solid State Ionics 181 (2010) 12941302

primary differences between the depth proles for long and shortheat treatments are the increased Mn content below the lm and thenearly complete diffusion of Co from the lm toward the substrate. An

analysis of the La proles suggests that the initial reaction of the lmwith Crofer22 results in a largely stable phase assemblage that trapsthe Cr below the initial thin lm boundary.

, (d

1300 R. Lacey et al. / Solid State Ionics 181 (2010) 12941302Fig. 6. XPS proles for (a)LSC 82 on Crofer22, (b)LNF55 on Crofer22, (c)LNF64 on EBrite

treated at 1000 C for 3 h, the plots marked B are for samples heat treated at 850 C for 168)LNF64 on 430SS, and (e)Co lms on Crofer22. The plots marked A are for sample heat

h subsequent to treatment at 1000 C for 3 h.

1301R. Lacey et al. / Solid State Ionics 181 (2010) 12941302Qualitative XRD analyses of the LSC82 lms before and after the168 h heat treatments show that identical phases are present. Fromthis result, it is clear that the diffusion of a fraction of the Co towardthe Crofer22 substrate, in concert with outward diffusion of Mn, doesnot require decomposition of the perovskite coating. Instead, thechemical composition of the initial LSC82 lm changes with time,

for the lms before and after the 168 h heat treatments arequalitatively identical, with the formation of a perovskite phase, a

Fig. 6 (continued).spinel phase, and either M2O3 or SrCrO4. Evidence that the diffusion ofthe various cations simply changes the compositions of the varioussolid solutions of the perovskite, spinel, and M2O3 phases can beconrmed with an XRD measurement that probes the entire depth ofthe lm. Again, the XRD results show substantial mass transportthrough the depth without changing the phases present.

The XPS depth proles of LNF lms on E-Brite and SS430 substratesafter heating for 3 h at 1000 C are qualitatively similar, as shown inFig. 6(c) and (d), respectively. This result suggests that minor changesin the alloy composition have little effect on cation diffusion and thatchemical differences can be accommodated by shifting solid solutioncompositions without altering the phase assemblage. Fig. 6(e) showsthe concentration prole through the Co thin lm on Crofer 22substrate. The Co lm is observed to be highly effective in preventingdiffusion of Cr to the surface of the lm, in agreement with recentwork by Stanislowski et al. [21] who studied lms of ~10 m thicknessprepared by sputtering. The effectiveness of the PVD Co coating isclearly demonstrated, with only 3 m resulting in a spinel phasecapped by CoO/ Co3O4 oxides.

4. Conclusions

The PVD process produced LSC, LSCF, LNF and Co lms on Crofer22,SS430 and E-Brite that are each highly effective in blocking Crdiffusion from the steel substrate. BSCF lms were reactive andformed blisters upon thermal cycling in air, so were not consideredfurther. All of the perovskite lms formed a similar phase assemblageafter heat treatment, consisting of the target perovskite, a spinelintermediate layer, and minor fractions of M2O3 or SrCrO4 phases,depending on the initial chemistry. Reactions of the as-depositedlms with the substrate occurred rapidly, with little or no differencein the chemical diffusion, microstructure, or phase assemblage foundbetween samples heat treated for 3 vs. 168 h.

The chemical depth proles clearly demonstrate that diffusion of theperovskite B-site cations (Ni, Fe, Co) and the transition metals from thesteel (Fe, Cr, Mn) occurs rapidly at high temperature to spontaneouslyform chemically graded lms with stable spinel and perovskite phasesthat inhibit outward diffusion of Cr. The wide range of solid solutionaccommodated by the spinel, perovskite, and M2O3 phases allows thisextensive chemical interdiffusionwithout changing the phases present.

In the case of the Co metal coating, heat treatment results inincorporating some Mn in place of Co, while concurrently shiftingtoward lower Sr content as a result of the partial reaction to formSrCrO4. Indeed, it is also possible that the SrCrO4 layer mayincorporate some Mn, as hexavalent Mn and Cr have very similarionic radii in tetrahedral coordination (0.255 vs. 0.250 ). Heating thesamples for 168 h does not result in any notable changes inmicrostructure, as shown in Fig. 2(b). The microstructures are similarto the as-deposited lms, even maintaining the nano-scale grain sizeafter one week at 850 C.

A comparison of the concentration depth proles for the LNF55thin lm before and after the long heat treatment is shown in Fig. 6(b). The diffusion behavior is similar to that found for the LSC82 thinlm, with outward diffusion of Fe, Mn and Cr toward the lm from theCrofer 22 substrate, and inward diffusion of Ni from the lm towardthe substrate. A large increase in Mn concentration is detected nearthe surface of the lm. Similar to LSC82, the LNF55 thin lm is shownto be effective in mitigating diffusion of Cr from the substrate to thesample surface.

For both LSC82 and LNF55, the La prole exhibited an exceedinglysharp concentration boundary between La on the surface and theunderlying oxides, even after 168 h at 850 C. X-ray diffraction dataoxidation and reaction to form a compositionally graded spinel layer

and a CoO/ Co3O4 surface. This lm is also highly effective in blockingCr diffusion.

Overall, the results suggest that these PVD lms are effective formitigating diffusion of Cr from SOFC metal interconnects and thatrenements in lm chemistry and processing might improve theirperformance even further.

Acknowledgments

Financial support for this work was provided by the New YorkState Foundation for Science, Technology and Innovation, NYSTAR,under contract C030093, and Applied Coatings, Inc.

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1302 R. Lacey et al. / Solid State Ionics 181 (2010) 12941302

Evaluation of Co and perovskite Cr-blocking thin films on SOFC interconnectsIntroductionExperimental methodsResults and discussionMicrostructure and electrical behavior of as-deposited filmsShort term crystallization and phase evolutionLong-term stability and chemical interdiffusion

ConclusionsAcknowledgmentsReferences