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ComparativeStudyoftheLeakCharacteristicsofTwoCeramic/GlassCompositeSealsforSolidOxideFuelCells

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Comparative Study of the LeakCharacteristics of Two Ceramic/GlassComposite Seals for Solid Oxide FuelCellsB. Dev1, M. E. Walter1,*1 Department of Mechanical and Aerospace Engineering, The Ohio State University, Scott Laboratory, 201 West 19th Avenue,

OH 43210, USA

Received May 28, 2014; accepted October 27, 2014, published online November 25, 2014

Abstract

Solid oxide fuel cells (SOFCs) have the potential to play a sig-nificant role in a future clean energy economy. However,SOFCs still face major obstacles before they can be commer-cialized, with efficient sealing being among the most promi-nent. The present research focuses on the comparative studyof microstructure, crystal phase evolution, and leak rates, fortwo ceramic/glass seals used in an SOFC. The leak test appa-ratus is a controlled facility designed to incorporate differentmechanical loading, stack configurations, and thermal cycles.Simultaneous leak testing with an acoustic emission (AE)sensor was also used to identify any micro-damage in seals.A two-level factorial design was applied to the first sealing

composition to identify the main and the interactive factorsfor leak rates. MINITAB� was also used to determine a linearregression-based leak rate model. The second seal formula-tion employed a more stable glass which led to reduced leakrates. Additional factors in a two-level factorial design wereinvestigated for the second seal formulation. Based on multi-ple experiments with different stack components, it wasdetermined that the number of interfaces is most critical forleak rate, showing that even in the presence of thermalcycling, leakage is an interfacial dominated phenomenon.

Keywords: Ceramic/Glass Composite Seals for SOFCs,Ceramic/Glass Ratio, Crystallization, Factorial Design, LeakRate Measurements, Micro-voids

1 Introduction

SOFCs are a growing technology for highly efficient, envir-onmental friendly power generation [1–3]. The majority ofSOFC developers currently target systems that operate at tem-peratures of 700-850 �C. The higher operating temperaturesallow catalysis of fuels without special, expensive materials.The main components of SOFCs are thin, multi-layer ceramicoxide electrolytes, porous cermet composite electrodes, oxide-coated stainless steel interconnects, and high temperatureseals. These materials were traditionally selected for their elec-trochemical properties, cost, and ease of manufacturing [2].Presently, many researchers have also been pursuing electro-lyte-supported planar technology for ease of manufacturing,lower operating temperatures, fuel flexibility, simplified inter-connects, and higher power densities. Furthermore, electro-lyte-supported geometries avoid unwanted issues associatedwith oxidation-reduction cycling, mechanical integrity, andseal complexity [2–5]. However, sealing remains a significantissue. In fact, many claim that seals are the most significant

barrier to large-scale commercialization of the entire SOFCtechnology [6–8].

SOFC seals must perform three essential functions: separatefuel and oxygen streams, electrically insulate various cell com-ponents, and provide mechanical support. In particular, soft-ening is good for absorbing mechanical loads, but softening isalso associated with unwanted crystallization in glass ceramicseals. In addition, seals need to have long-term stability at ele-vated temperatures, in harsh environments, and should notcause degradation or be degraded by contacting interconnects,electrodes, electrolytes, and/or separators. Unfortunatelysatisfying the necessary functionality requires contradictoryproperties, and the balance is difficult to achieve.

To date most SOFC development has focused on glass orglass–ceramic seals also popularly known as bonding seals.The primary advantages of bonding seals are superior chemi-cal stability under reactive atmospheres and not needing an

–[*] Corresponding author, [email protected]

FUEL CELLS 15, 2015, No. 1, 115–130 ª 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 115

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DOI: 10.1002/fuce.201400095

Dev, Walter: Comparative Study of the Leak Characteristics of Two Ceramic/Glass Composite Seals

external load frame for effective sealing [5–9]. The main disad-vantages of bonding seals are brittleness at low temperatures,which results in susceptibility to coefficient of thermal expan-sion (CTE) mis-match [9–12] and softening and crystallizationof the glass phases, which compromises long term durability[8, 9]. Compressive seals are another class of seals. Compressiveseals are advantageous as they allow appreciable degrees of rel-ative motion between the mating surfaces without opening leakpaths [8–12]. The primary disadvantage of the compressive seal-ing method is the lack of suitable materials that are compliant inthe aggressive SOFC environment [9, 11]. Furthermore, com-pressive seals require complicated infrastructure for providingmechanical loading [11]. Past work has studied the leakagecharacteristics of different seals [11–22]. In particular, Chou et al.and Simner and Stevenson [11] characterized the leak rates formica and hybrid mica based seals. In addition, R.N. Singh [5],Sang et al. [19] and Le et al. [21] had also characterized differentcategories of seals for SOFC applications.

The present research deals with a novel compressive seal-ing concept that utilizes a ceramic/glass composite. Thisapproach differs from the ‘‘pure’’ glass approach that startswith a single glass system that later partially crystallizes into aglass-ceramic. While not fully dense, the microstructure doesresult in tortuous leakage paths and allows micro-damagewhich prevents catastrophic failure. There are a large numberof variables associated with the ceramic/glass composite sys-tem as well as the application of this system to sealing applica-tions. The current work explores the leakage characteristics oftwo different composite seals by comparing the leak rateresults under multiple cycles, the microstructures, and the for-mation of crystalline phases. Within this study, the number ofthermal cycles, surface smoothness, types of interfaces and theapplied compressive load are also varied. Factorial designapproaches are employed to understand the main and theinteraction factors among the various variables that affectedleak rates in seals.

2 Material Systems

The first composite seal (Seal 1) has a ceramic to glass ratioof 60:40, and is processed by either roll compaction or tapecasting of nominally 14 mm glass and 0.5 mm ceramic (alumina)powders mixed with a proprietary binder system developedat Ragan Technologies Inc (RTI). Binder burn-out and consoli-dation of the glass powder occurs during a heat treatment at800 �C for 4 hours. The glass powder used in the compositeseal is V-1716 from Viox, which has Pacific Northwest Nation-al Laboratory’s G-18 composition [25–28]. The second compos-ite seal (Seal 2) has a ceramic to glass ratio of 80:20. It is alsoprocessed by either roll compaction or tape casting, but thistime nominally 4 mm glass from Schott and 0.5 mm ceramic(alumina) powders were mixed again with RTI’s binder sys-tem. The main constituents of Schott based glass are MgO,BaO, Al2O3, and SiO2. The glass and alumina phases for boththe compositions are shown in the SEM backscatter image pro-vided in Figures 1a and 1b.

To replicate an actual stack, the other components used inthe leak tests were electrolytes, shims, and interconnects. Theseals, shims, and electrolytes were each donut-shaped cou-pons having an inner and outer diameters of 25.6 and50.8 mm, respectively. Pre-tested seal coupons were 0.25 mmthick. Shim and electrolyte coupons were each 1 mm thick andwere made from 18SR stainless steel and Scandia-stabilizedZirconia, respectively. 18SR steel is composed of the followingelements with the percentages being weight percentages: Car-bon (0.015%), Manganese (0.30%), Chromium (17.30%), Nickel(0.25%), Titanium (0.25%), Aluminium (1.7%), and Iron (bal-ance)1. Interconnects function as physical separators and elec-trical connector in a stack and will be described in the nextsection. Shims are used as spacers to provide gaps betweenparts in order to support and ensure a better fit in a stack.The following configurations: interconnect-seal-interconnect(IC-S-IC), interconnect-seal-shim-seal-interconnect (IC-S-SH-S-IC), and interconnect-seal-shim-seal-electrolyte-seal-shim-seal-interconnect (IC-S-SH-S-E-S-SH-S-IC) were tested.

3 Leak Test Experiments

3.1 Experimental Apparatus

Figure 2 shows a schematic diagram of the facility designedto experimentally characterize the leak rates. The apparatushas the potential to vary stack configurations under differentcompressive and thermal loadings. To mimic the conditions inan actual SOFC stack the two manifolds were made of Crofer22 APU, a commonly used SOFC interconnect material. Themanifolds were reused for all runs. For the first compositeseal, the manifolds were initially polished with 600 and then1000 grit sandpapers to ensure repeatable surfaces before eachtest. However, for the second composite seal, the manifoldswere initially polished with the same 600 and 1000 grit sand-papers, followed by 2000 grit sand paper and 7 mm diamondmedia polish (3000 grit equivalent).

Thermal cycling was performed with an integrated clam-shell furnace. Previous research had determined an appropri-ate curing cycle to be 2 �C/min heating, followed by a 4 hourhold at 800 �C and finished with 5 �C/min cooling to room

–1 http://www.nks.com/18_sr_stainless_steel.html

Fig. 1 (a) Backscattered image of 0.25 mm green seal 1 (b) Backscat-tered image of 0.25 mm green seal 2.

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temperature [25]. The stack components were subjected to 1,5, and 10 thermal cycles. For all leak rate experiments, the sys-tem was air pressurized to 4.48 kPa by controlling the pressureregulator and monitoring the pressure with an OMEGAPX309-005G5V transducer. The compressive loading of theseal constructs was generated with the Test Resources 800Lload frame. Each configuration was initially cycled withoutthe mechanical loading. The compressive load and the airpressure were applied simultaneously during the dwell peri-ods of the 1st, 5th, and 10th thermal cycle in each configurationwhile performing the leak test.

To investigate if the propagation of micro-damage in cycledseals could affect the leakage characteristics in a stack, an AEsensor was attached to the leak test apparatus as shown in Fig-ure 2. The AE sensor is a piezo material that detects transientsurface waves generated from cracking and other deformationphenomenon that release strain energy. One end of a chromel-alumel wave guide was coupled to the bottom manifold whilethe other end was connected to a polished surface on whichthe AE sensor was placed. The AE signal was pre-amplifiedby 34 dB and then sent to a Vallen System AMSY-4 AE system.A signal threshold was set to 40 dB to eliminate unwanted

noise from the load frame, the test set up, and the surround-ings. AE recording began once the system was pressurized.

3.2 Leak Rate Calculation

Experimental leak rate is based on the assumption that airbehaves like an ideal gas as the system was pressurized to a rela-tively low value of only 4.48 kPa [26]. Thus, it was appropriateto apply a linearized form of the ideal gas law to fit the loss inpressure. The leak rate was then calculated from the known sys-tem volumes and the rate of pressure decays [6, 21]. The systemvolume was divided into two regions, a cold and a hot region.The hot region volume varied depending on the configurationbeing tested. Moles of air lost from the entire system were deter-mined by applying the ideal gas law in each region separately asfollows:

ntot ¼ ncold þ nhot ¼DP � Vcold

R � Tcoldþ DP�Vhot

R � Thot(1)

where ntot; DP, Vcold;Tcold;Vhot;Thot, and R represent totalnumber of moles in the system, pressure decay, volume andtemperature in the cold and hot regions, and the universal gasconstant, respectively.

Fig. 2 Schematic of the leak test set up equipped with an AE sensor.

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After determining the numbers of moles lost, the mol: literratio was then used to determine the total volume lost acrossDP. Assuming ideal gas behaviour for air, 1 mole of gas fills avolume of 22400 cm3 [6, 26]. References [6, 21] have indicatedthat the calculated leak rates are under ambient conditions.Hence, the present leakage formulation encompasses the stan-dard ambient conditions. Thus, the volume of air lost from thesystem in cm3 can be expressed by Eq. (2)

Vtot ¼ ntot � 22400 (2)

The leak rate can then expressed as Vtot divided by theelapsed time. As give in previous literature, the leak rate wasfurther normalized by the following approximation for seallength, [4–6, 21, 27]:

Seal Length ¼ p � Do �Di

ln Do=Dið Þ

� �(3)

where Do and Di are the outer and the inner diameters, respec-tively.

4 Results and Discussion

4.1 Leak Rates for Seal 1

Rates of pressure decays were compared across differentcycled components for each stack configuration and for multi-ple cycles and are presented in Figure 3. Using the procedureoutlined in the previous section, the leak rates were calculated

for each configuration and are presented in Figure 4. It is ob-served that the leak rates vary with multiple thermal cyclesand number of interfaces. Past research had indicated thatthere was evolution of crystalline phases with multiple ther-mal cycling [25]. Thus it is believed that increasing leak rateswith increasing thermal cycling was due to crystallization.The increase in leak rates with additional interfaces was dueto the increase in number of potential leakage paths.

To investigate if the type of interface had a significant effecton leak rate, IC-S-E-S-IC and IC-S-SH-S-IC configurationswere compared. In addition to the usual variability due to thenumber of thermal cycles, Figure 5 shows that the leak ratesfor the seal-electrolyte interface were higher. Finally, leak testswere conducted on IC-S-IC configurations cycled once withdifferent compressive loadings. The resulting leak rates arepresented in Figure 6. It is observed that with increase in com-pressive loading, the leak rate decreases. This is due to thereduction of interfacial gaps between the mating surfaces withincreased loading. From these studies with different config-urations and loadings, it became apparent that a more sys-tematic approach to determine the main factors was needed.

4.2 Two Level Factorial Design for Seal 1

Factorial design allows study of the effect of each factor onthe response variable, as well as the effects of interactionsbetween factors on the response variable. Table 1 presents thedifferent factors along with the corresponding response for thefirst composite seal. The number of interfaces varied with each

Fig. 3 Decay of pressure differentials with time for different stack configurations using Seal 1.

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Fig. 4 Variations in leak rates for different stack configurations (Seal 1).

Fig. 5 Comparison of leak rates for IC-S-IC configuration (Seal 1) cycled once under different compressive loading.

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stack configuration and was thus considered as one of the fac-tors in the factorial design. In addition, mechanical loadingand number of thermal cycles were considered as the othertwo factors. The low and high values were 1 and 10 for ther-mal cycles, 34.5 and 138 kPa for mechanical loading, and 2and 8 for number of interfaces.

A two level factorial design model was adopted for devel-oping the predictive leak rate equation based on regressionanalysis [28, 29]. Factors that had confidence levels of 95% orp-values less than 0.05 were considered significant and werethus incorporated into the analysis [28–30]. Figure 7 presentsthe Pareto chart representation for different factors. It is ob-

Table 1 Two Level Factorial Design applied to identify the fundamental factors effecting leakage in Seal 1.

Runs Number of Interfaces Grit Size Mechanical loading in kPa Thermal Cycling Leak Rate in SCCM/CM

1 (IC-SEAL-IC)2 1000 34.5 1 0.035

2 (IC-SEAL-IC)2 1000 34.5 5 0.036

3 (IC-SEAL-IC)2 1000 34.5 10 0.037

4 (IC-S-SH-S-IC)4 1000 34.5 1 0.069

5 (IC-S-SH-S-IC)4 1000 34.5 5 0.071

6 (IC-S-SH-S-IC)4 1000 34.5 10 0.072

7 (IC-S-EL-S-IC)4 1000 34.5 1 0.071

8 (IC-S-EL-S-IC)4 1000 34.5 5 0.074

9 (IC-S-EL-S-IC)4 1000 34.5 10 0.076

10 (IC-S-SH-S-EL-S-SH-S-IC)8 1000 34.5 1 0.135

11 (IC-S-SH-S-EL-S-SH-S-IC)8 1000 34.5 5 0.144

12 (IC-S-SH-S-EL-S-SH-S-IC)8 1000 34.5 10 0.148

13 (IC-SEAL-IC)2 1000 69 1 0.032

14 (IC-SEAL-IC)2 1000 103.5 1 0.031

15 (IC-SEAL-IC)2 1000 138 1 0.029

Fig. 6 Comparison of leak rates across IC-S-SH-S-IC and IC-S-E-S-IC configurations (Seal 1) subjected to multiple thermal cycles.

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served that the number of interfaces for a stack configurationhas the greatest influence on leak rates. It is also interesting tosee that the interaction between the applied thermal cyclesand number of interfaces affected the leak rates in seals.However, the effect of interactions between factors on theresponse variable was insignificant compared to the effect ofnumber of interfaces. The predictive leak rate response equa-tion based on the linear regression analysis is given as follows:

Leak Rate ¼ 0:004716þ 0:0166375 · N � 4:7308 · 10�4 · M

� 2:57761 · 10�4 · T þ 0:0002 · N · T (4)

where N represents the numbers of interfaces, M representsthe mechanical loading/pressure in kPa, and T is the numberof thermal cycles. The predictive leak rate model was com-pared against the experimental leak rates for the IC-S-SH-S-ICconfiguration. IC-S-SH-S-IC was chosen for validating themodel as it has 4 interfaces, which was directly not includedin the two level factorial design. Figure 8 presents the compar-ison between the experimental and predictive leak rates forthe IC-S-SH-S-IC configuration. It is observed that the regres-sion model closely agrees with the experimental leak rates,with a maximum error of only 1.4%.

4.3 Leak Rates for Seal 2

With the number of interfaces dominating leak response, itis also expected that the leak rates would be affected by sand-ing/polishing of the manifolds. Thus, grit sizes were consid-ered as an additional factor in studying the leakage character-istics for Seal 2. Prior to performing leak tests on Seal 2, themanifolds were initially polished with the same 600 and 1000grit sandpapers, followed by 2000 grit sand paper and finallya 7 mm diamond media polish (3000 grit equivalent). It wasalso important to check the repeatability of the leak ratesexperiments. Thus, for each of the grit sizes considered above,three runs were performed. For each test the IC-S-IC config-uration was subjected to 1 thermal cycle under a mechanicalloading of 34.5 kPa and the results are shown in Figure 9. It isobserved that the leak rates varied with grit sizes, withsmoother surfaces performing better. The variability in the

Fig. 7 Pareto chart representation of various factors for studying leakrates for Seal 1.

Fig. 8 Experimental and predicted leak rates for the IC-S-SH-S-IC stack configuration for Seal 1.

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leakage measurement for the 7 mm diamond media polish isnegligible, indicating that the data is repeatable. However,since there were multiple runs it is also possible and insightfulto examine propagation of errors.

From Eq. (1), it is observed that leak rate is mainly a func-tion of pressure differentials DP, hot and cold temperaturesThot and Tcold, and hot and cold volumes Vhot and Vcold. For thepresent analysis, Tcold is assumed to be 25 �C for the durationof the test. Based on the uncertainties associated with the digitalcallipers (accuracy –0:05 mmÞ, the uncertainty in volume mea-surements are Vhot ¼ 140:5–0:15 cm3 and Vcold ¼ 3:5–0:05 cm3

respectively. The uncertainty associated with the temperaturemeasurements in the hot volume is based on the accuracy ofthe Watlow F4 Series controller, and from manufacturer speci-fications, the accuracy is taken to be Thot ¼ 800–2�C. Finally,the uncertainty in pressure differential measurement is basedon the accuracy of the PX309-005G5V and is determined to beDP ¼ 4:48–0:17 kPa. Based on previously published method-ology [31], the uncertainly in leakage measurement can beapproximated as follows:

sLeak rate

Leak rate

� �2¼ sDP

DP

� �2þ

sVhot

Vhot

� �2

þsVhot

Vhot

� �2

þsThot

Thot

� �2

(5)

where s is the uncertainty associated with each measurementsystem. Referring to the results from 3000 grit size for IC-S-ICconfiguration as highlighted in Figure 9, the uncertainty inleak rate is calculated to be:

sLeak rate ¼ 9:04 · 10�4sccm=cm

The error associated with the leakage calculations for 3000grit size is therefore 0.022–9:0 · 10�4sccm=cm.

Figure 9 also presents comparison between the leak ratesfor the two composite seals. Although there is only 1000 Gritdata for Seal 1, it is observed that Seal 2 has far better sealingperformance compared to Seal 1. The influence of mechanicalloading was again studied with the Seal 2 using the IC-S-ICconfiguration cycled 1, 5, and 10 times, and the results areshown in Figure 10. For the same configuration and for onethermal cycle it was found that leak rate for Seal 1 haddecreased by 20.7% for increased loading from 34.5 to 138kPa. On the other hand, for Seal 2, the total decrease in leakrate for the same conditions was 15.7%.

The effects of multiple thermal cycling on IC-S-IC configura-tion under multiple loading were also investigated for Seal 2.Figure 10 shows the variations of leak rates with multiple ther-mal cycling and loading for IC-S-IC configuration. It is ob-served that the thermal cycling affects sealing performance.However, there are no striking variations in leak rates across 5and 10 thermal cycles for each mechanical loading. This indi-cates that the glass is relatively stable in Seal 2.

From the analysis on the Seal 1, it was observed that thenumbers of interfaces for an SOFC stack has the greatest influ-ence on leak rates. Thus, it was important to study the effect ofadditional interfaces on leak rates for Seal 2. Figure 11 presentsthe comparison of leak rates for IC-S-IC and IC-S-SH-S-IC con-figurations for Seal 2. It is observed that an increase in thenumber of interfaces again results in significant increase inleak rates. The effect of increased mechanical loading and ther-

Fig. 9 Comparison of leak rates for IC-S-IC configurations with manifolds polished with different grit sizes for Seal 2. Three runs were performed foreach configuration. The leak rate for Seal 1 with 1000 Grit polish is also shown.

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Fig. 10 Variations in leak rates for IC-S-IC configuration (Seal 2) subjected to different numbers of thermal cycles and different loadings.

Fig. 11 Comparison between the leak rates for IC-S-IC and IC-S-SH-S-IC configurations (Seal 2) for different numbers of thermal cycles and differentloads.

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mal cycling on leak rates for IC-S-SH-S-IC configuration aresimilar to that of IC-S-IC configuration.

4.4 Two level factorial designs for Seal 2

Based on the leakage data for the second composite seal, atwo level factorial design was applied to study the effect ofdifferent factors on the response. Table 2 presents the variousfactors with the corresponding leak rates. The regressionmodel was developed based on the influences from number ofinterfaces, thermal cycles, mechanical loading, and the gritsizes used for polishing the manifolds. The low and high val-ues in the factorial design were 1 and 10 for thermal cycles,34.5 and 138 kPa for mechanical loading, 2 and 8 for numberof interfaces and 1000 and 3000 for grit sizes. Based on MINI-TAB’s two level factorial design, the predictive leak rate equa-tion is as follows:

Leak Rate ¼ 0:00767þ 0:01083 · N � 2:0845 � 10�6 · G

� 3:2296 · 10�4 · M� 1:5908 · 10�4 · T

þ 3:78884 · 10�5 · N · Mþ 0:0001647 · N · T

þ 3:2973 · 10�5 · M · T

� 1:1814 · 10�5 · N · M · T (6)

where N is the numbers of interfaces, M is the mechanicalloading/pressure in kPa, T is the number of thermal cycles,and G is the grit size. Figure 12 presents the Pareto Chartrepresentation of different factors. It is again observed thatnumbers of interfaces in a stack has the greatest influence onleak rates. The interactions between different factors havenominal effect on leak rates compared to the main factors.

The experimental leak rates for different configurationscycled five times were compared against the regression modelas it was not directly included in the factorial design. Figure 13presents the comparative study between the predicted and theexperimental leak rates. It is observed that the regressionmodel is able to capture the experimental trend. The predic-tive leak rate equation for the second composite seal hadslightly underestimated the experimental value with a maxi-mum error of 2.0%.

4.5 Study of micrographs and crystalline phases of cycled seals

Based on the experimental leak tests conducted on both thesealing compositions, it is observed that the second compositeseal has a better sealing efficiency compared to the first. It was

Table 2 Two Level Factorial Design applied to identify the fundamental factors effecting leak rates in Seal 2.

Runs Number of Interfaces Grit Size Mechanical loading inkPa

Thermal Cycling Leak Rate in SCCM/CM

1 2 1000 34.5 1 0.026

2 2 2000 34.5 1 0.025

3 2 3000 34.5 1 0.022

4 2 3000 69 1 0.020

5 2 3000 138 1 0.019

6 2 3000 34.5 5 0.024

7 2 3000 69 5 0.022

8 2 3000 138 5 0.020

9 2 3000 34.5 10 0.024

10 2 3000 69 10 0.023

11 2 3000 138 10 0.022

12 4 3000 34.5 1 0.045

13 4 3000 69 1 0.043

14 4 3000 138 1 0.042

15 4 3000 34.5 5 0.047

16 4 3000 69 5 0.044

Fig. 12 Pareto Chart representation of various factors for studying theleak rates in Seal 2.

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thus essential to connect the macroscopic leakage properties tothe microstructural features in each composition. Hence, themicrostructures and crystalline phases of each cycled seals forboth the compositions were further investigated using thescanning electron microscopy (SEM) and X-ray diffraction(XRD) techniques.

Post-test cross section of each cycled seal was gold coatedand imaged with an FEI Quanta 200 scanning electron micro-scope. Micro-voids in the SEM images were manually identi-fied. MATLAB� was then used to threshold the images anddetermine void percentages, thus providing an estimate of theevolution of micro-voids with multiple thermal cycles. Fig-ure 14 presents the cross-sectional SEM micrographs, their cor-responding processed images, and the percentages of micro-voids for both the composite seals. It is observed that in eachcomposition the percentages of micro-voids evolved with ther-mal cycles. However, the percentage of micro-voids is higherin the first composite seal for each thermal cycle compared tothat in the second composition. Past studies have indicatedthat micro-voids could evolve due to CTE differences betweenthe crystalline and amorphous glassy phases [28, 30]. Micro-voids could also develop in ceramic/glass composite seals ifthe crystallization temperature of the glass is below the sinter-ing temperature of the composite [33].

To verify the evolution of crystalline phases in both thecomposite seals cycled multiple times, X-Ray Diffraction(XRD) techniques were employed. Scans for 5<2q< 60� took

approximately 4 hours and 30 minutes. The parent elementspresent in G-18 [23] and Schott glasses were used as inputs foranalyzing the XRD data. Figures 15a and 15b present the XRDscan patterns and crystalline phases of seals cycled 1, 5, and 10times. It is observed that the dominant peaks in the first com-posite seal are corundum (Al2O3), barium calcium silicate(Ba1.5Ca0.5SiO4), barium aluminum silicate (BaAl2Si2O8), bar-ium aluminum oxide (Al2Ba5O8), and barium silicate (BaSiO3).For the second composite seal, it is seen that the dominantpeaks are all corundum (Al2O3). There are few barium alumi-num silicates (BaAl2Si2O8) and quartz (SiO2) peaks formed inthe seals cycled five and ten times. This shows that the secondcomposite seals do not crystallize under multiple thermalcycles. It is also seen in Figure 15a that for the first compositeseal, the percentages of crystalline phases increase withincreasing number of cycles. Figure 15b shows that for the sec-ond composite seal the intensities of Al2O3 peaks haveincreased with multiple thermal cycles. It is believed that themotion of viscous glassy phases flowing into the ceramic pow-der by capillary action would cause Al2O3 particles to segre-gate on the surfaces and hence the intensities of Al2O3 peaksare increasing in the second composite.

The evolution of micro-voids with multiple cycles in thefirst composite seal was consistent with the increasingamounts of crystallized glass. Thus it is again inferred thatcrystallization plays the dominant role in development ofmicro-voids. Crystallization of glass decreases the percentage

Fig. 13 Experimental and predicted leak rates for Seal 2 with different configurations cycled five times.

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of viscous glassy phase that is essential for efficient sealing atthe operating temperature [32, 33]. This could explain thepoorer sealing performance in the first composite seal as com-pared to the second.

4.6 AE response during leak test experiments

AE sensor did not register any hits during the dwell periodof the 1st, 5th, and the 10th thermal cycle at 800 �C and also thestack pressure had leaked out completely before the coolingcycle had initiated. Thus, leakage is not expected to be the con-sequence of any kind of bulk failure of the cycled seal. Fig-ure 16 presents the thermal cooling rate along with the syn-chronized AE hits for the IC-S-IC configurations cycledmultiple times under a compressive pressure of 34.5 kPa. It isobserved that the AE responses varied with the sealing com-position and number of thermal cycles. The first compositioncycled once more AE hits compared to the second. However,there are more AE hits in the second composite seal cycled fiveand ten times.

The AE energies in decibels for seal 1 and 2 cycled for 1, 5and 10 times are individually compared in Figure 17(a)–17(f).It is observed that there are high AE energy peaks in Seal 1. It

is believed that the occurrence of these high energy peakscould be due to propagation of micro damage and evolutionof micro voids through crystalline phases. In addition it isbelieved that progressive phase transformations of residualglassy phases into crystalline phases with multiple thermalcycling could have contributed to these high energy peaks[23, 33]. Progressive crystallization of amorphous glassyphases in the first composite seal also results in degradation ofinterfacial properties [23, 34, 35]. This could presumably resultin interfacial delamination across IC-S interfaces and thusresult in high AE energy peaks. The magnitudes of energypeaks in the second composite seals are lower compared tothe first. The second composite has high percentages of lowdensity alumina powders and hence has greater compliance.The microstructure and compliance of the second compositeseal could result in more hits with lower AE energy in each hit.

5 Summary

The present research conducted a comparative study onleak rates of a 60:40 ceramic/glass seal as well as an 80:20ceramic/glass seal with a different glass composition. For each

Fig. 14 Cross sectional SEM and processed images of a) Seal 1 cured at 800oC for 1 cycle b) Seal 1 cured at 800oC for 5 cycles c) Seal 1 cured at800 �C for 10 cycles d) Seal 2 cured at 800 �C for 1 cycle d) Seal 2 cured at 800 �C for 5 cycles f) Seal 2 cured at 800 �C for 10 cycles.

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seal, leak tests were performed for different stack configura-tions subjected to multiple mechanical loading and thermalcycles. A two level factorial design was applied to identify themain and the interactive factors affecting the leak rates in boththe composite seals. MINITAB was used to determine a regres-sion-based predictive leak rate model. The predictive leak rate

model was able to capture the experimental data fairly accu-rately with a maximum error of 1.43 and 2.42% for the firstand the second compositions, respectively. It was also ob-served that leak rates depended on the sealing composition.The second composite seal had a superior sealing efficiencycompared to the first.

Fig. 15 X-Ray diffraction patterns and crystalline phases from a) Seal 1 b) Seal 2 cured at 800 �C for 1 cycle, 5 cycles, and 10 cycles.

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Microstructural studies showed that the first compositehad high percentage of micro-voids that had evolved withmultiple thermal cycles. X-ray Diffractometery indicated thatthere was evolution of high percentages of crystalline phasesin the first composite compared to the second. The percen-tages of crystalline phases in the first composite had increasedwith multiple thermal cycling. It was observed that the inten-sities of Al2O3 peaks in the second composite seal hadincreased with progressive thermal cycling from 5 to 10 cycles.AE hits had varied with the sealing composition and thermalcycles. AE energy peaks in the second composite seals werelower compared to the first.

Acknowledgements

This work was supported by the Ohio Department ofDevelopment’s Third Frontier Fuel Cell Program. The authorswould also like to thank the staff of NexTech Materials Ltd. formany helpful discussions concerning SOFCs and ceramic/glass composite seals.

List of Symbols

Latin Letters

ntot Total number of molesDP Pressure DifferentialVcold Volume of Cold regionVHot Volume of Hot regionTcold Temperature of Cold region: 25 �CTHot Temperature of Hot region: 800 �CVtot Total Volume of the system�R Specific Universal Gas Constant: 0.287 KJ kg–1K–1

R Universal Gas Constant: 8.314 JK–1mol–1

1 psi = 0.0068 MPaICs InterconnectsSH ShimsE ElectrolytesS SealsM Mechanical loading/pressure in kPaT Number of thermal cyclesN Number of InterfacesG Grit Sizes

Fig. 16 Variations in AE hits for the two sealing composition comprising of IC-S-IC configuration cycled multiple times.

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