Joint strength of a solid oxide fuel cell glass–ceramic sealant with metallic interconnect in a reducing environment

Journal of Power Sources 280 (2015) 272e288

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Joint strength of a solid oxide fuel cell glasseceramic sealant with metallic interconnect in a reducing environment Chih-Kuang Lin a, *, Yu-An Liu a, Si-Han Wu b, Chien-Kuo Liu b, Ruey-Yi Lee b a b

Department of Mechanical Engineering, National Central University, Jhong-Li 32001, Taiwan Physics Division, Institute of Nuclear Energy Research, Lung-Tan 32546, Taiwan

h i g h l i g h t s
A setup is built to assess sealant/interconnect joint strength in humidified H2.
Effects of reducing environment and thermal aging on joint strength are studied.
Thermal aging in wet H2 enhances joint strength at 25  C but degrades it at 800  C.
Unaged joints show a comparable strength when tested in air or humidified H2.
Thermal aging in air or wet H2 degrades high-temperature joint strength comparably.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 November 2014 Received in revised form 4 January 2015 Accepted 18 January 2015 Available online 21 January 2015

Effects of reducing environment and thermal aging on the joint strength of a BaOeB2O3eAl2O3eSiO2 glasseceramic sealant (GC-9) with a ferritic-stainless-steel interconnect (Crofer 22 H) for planar solid oxide fuel cells are investigated. A technique is developed for conducting mechanical tests at room temperature and 800  C in H2-7 vol% H2O under shear and tensile loadings. Given an aged condition and loading mode, the joint strength at 800  C is lower than that at room temperature in the given humidified hydrogen atmosphere. A thermal aging at 800  C in H2-7 vol% H2O for 100 h or 1000 h enhances both shear and tensile joint strengths at room temperature but degrades them at 800  C in the same reducing environment. Non-aged specimens show a comparable joint strength and fracture mode when tested in humidified hydrogen and in air under a given loading mode and testing temperature. The shear strength at 800  C for joint specimens after a 1000-h thermal aging at 800  C in air or humidified hydrogen is reduced by a similar extent of 19%, compared to the counterpart of non-aged joint specimens tested in the same oxidizing or reducing environment. © 2015 Elsevier B.V. All rights reserved.

Keywords: Planar solid oxide fuel cell Glasseceramic sealant Metallic interconnect Joint strength Reducing environment Thermal aging

1. Introduction Planar and tubular cells are the two major configuration designs being developed for solid oxide fuel cell (SOFC). Planar SOFCs (pSOFCs) have attracted more attention as they are easier to fabricate, operate at a lower temperature, and offer a higher power density over tubular ones. Thanks to a thinner electrolyte, the operation temperature for anode-supported pSOFCs can be lowered to less than 800  C and the ohmic loss is also reduced, compared to the electrolyte-supported configuration. Repeated units of ceramic anodeeelectrolyteecathode assembly and metallic

* Corresponding author. E-mail address: [email protected] (C.-K. Lin). http://dx.doi.org/10.1016/j.jpowsour.2015.01.126 0378-7753/© 2015 Elsevier B.V. All rights reserved.

components are usually integrated into a multi-cell stack to generate a high voltage and power for pSOFCs. Metallic interconnects are often employed to separate fuel and oxidant gasses and to connect individual cells structurally and electrically in series in order to obtain sufficient power density. Hermetic sealing is required to maintain the electrochemical performance of a pSOFC system by bonding adjacent components and avoiding a mixing of fuel and oxidant. Joining glasseceramic sealants to metallic interconnects is often seen in application of a rigid type of sealing technique to pSOFC stacks [1]. Typical locations of sealants applied in a pSOFC stack include: (a) cell to metal frame; (b) metal frame to metal interconnect; (c) frame/interconnect pair to electrically insulating spacer; (d) stack to base manifold plate [2]. Seals at locations (b) and (d) are regarded as a joint of glasseceramic sealant and metallic interconnect.

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Fig. 2. Typical forceedisplacement curves of non-aged joint specimens tested in H2-7 vol% H2O: (a) shear loading; (b) tensile loading. Fig. 1. Schematic of specimen enclosed by a gas-tight tube in mechanical test.

SOFC sealants must have good adherence, chemical stability, and chemical compatibility with adjacent components in both oxidizing and reducing environments. The high-temperature operation, however, gives rise to significant thermal stresses due to mismatch of coefficient of thermal expansion (CTE) between components and temperature gradients in pSOFCs [3,4]. Therefore, sealants for pSOFCs must also have good mechanical integrity and proper thermal expansion match with adjacent components at operating temperature. Thermal stresses generated at a joint between glasseceramic sealant and metallic interconnect in pSOFC stacks may cause failure of sealing with excessive deformation and/ or debonding, leading to gas leakage. Accordingly, undesired chemical interaction and CTE mismatch between glasseceramic sealant and metallic interconnect could generate damages in pSOFC stacks and degrade the electrochemical performance [5e7]. It is thus necessary to investigate the chemical compatibility as well as mechanical integrity of such a joint in both oxidizing and reducing environments for assessing the reliability and durability of a pSOFC stack. Many studies, e.g. Refs. [7e18], have investigated the bonding and chemical interaction of various glasseceramic sealants with Crcontaining ferritic stainless steels (for interconnect) under simulated SOFC environments. Most of those studies [10e18] were

carried out in a single exposure atmosphere, either air (or moist air) representing the cathode-side environment, or a reducing atmosphere simulating the anode-side environment. A few studies [7e9] were conducted in dual-atmospheres that closely simulate exposure environments of the joint during SOFC operation to characterize the interactions between glasseceramic sealants and metallic interconnects. Menzler et al. [11] characterize the interactions of two interconnect steels, Crofer 22 APU and JS-3, with a BaOeCaOeAl2O3eSiO2 (BCAS) glasseceramic sealant in three different environments (air, humidified air, and humidified hydrogen) at 800  C for durations from 1 h to 500 h. Their results show that air or humidified air exerts no significant, detrimental effect on the glasseceramic/steel joint, in terms of adhesion, cracking, and inter-diffusion [11]. In contrast, interaction in the humidified hydrogen atmosphere leads to an internal oxidation in the steel [11]. The electrical resistance of steel/glasseceramic/steel sandwich joint shows no degradation when both sides are exposed to air or humidified hydrogen, as reported by Haanappel et al. [8,9]. On the other hand, the insulation capacity of the joint is obviously affected by presence of a dual environment (one side exposed to air and the other side exposed to humidified hydrogen), as conductive iron-rich oxides are formed and result in “short-circuiting” between the two metallic sheets [8,9]. As described above, the interaction between glasseceramic sealant and metallic interconnect in an oxidizing environment

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Fig. 4. Failure patterns of non-aged shear specimens tested in H2-7 vol% H2O at (a) room temperature and (b) 800  C.

Fig. 3. Joint strength of non-aged specimens tested in H2-7 vol% H2O: (a) shear loading; (b) tensile loading.

differs from that in a reducing environment, which could lead to a different chemical and/or electrical behavior. Such a variation of interaction with environment between sealant and interconnect may also influence the mechanical integrity of their joints. Therefore, a systematic investigation of mechanical properties of joints between glasseceramic sealant and metallic interconnect at operation temperature in both oxidizing and reducing environments is essential for development of a reliable pSOFC stack. For those steels and glasseceramics developed for pSOFCs, effects of environment on their bonding characteristics, such as electrical performance, chemical properties, and long-term stabilities, have been widely investigated [7e18]. Although many studies have investigated the mechanical properties and fracture resistance of the joint of glasseceramic sealant/metallic interconnect, they are mainly conducted in an oxidizing environment (mostly air) [19e30]. However, there is still lack of study on the effect of reducing environment on joint strength of glasseceramic sealant/metallic interconnect, in particular at high temperature, due to a challenge in developing a proper mechanical test technique. Such a subject is important to advance development of SOFC technologies. As part of a series of studies [28e37] on the mechanical properties of glasseceramic sealants and metallic interconnects for pSOFCs, the aim of this study is to investigate the effect of reducing environment on the

mechanical strength of a joint between a BaOeB2O3eAl2O3eSiO2 glasseceramic sealant (GC-9) and a ferritic-stainless-steel interconnect (Crofer 22 H) for pSOFC applications. A counterpart study in an oxidizing environment has been conducted and reported in Ref. [28] of which some results are compared with the results in this study to characterize the environmental effect on the joint strength. Two loading modes, tensile and shear forces, are applied to characterizing the mechanical properties of the joint at room temperature and 800  C in a humidified hydrogen atmosphere. In addition, some samples are also tested after thermal aging at 800  C in the same humidified hydrogen atmosphere to simulate an SOFC working environment. Fractography and microstructural analyses are conducted with scanning electron microscopy (SEM) and correlated with the mechanical testing results. It is hoped that results of the present study and previous work [3,4,28e37] can provide useful information for assessing the structural integrity of pSOFC stacks in both oxidizing and reducing environments. 2. Experimental procedures 2.1. Materials and specimens To simulate a joint of glasseceramic sealant/metallic interconnect subjected to thermal stresses at room temperature and operating temperature, two types of sandwich joint specimen (metal/ sealant/metal) are designed and applied to determining the tensile and shear strength of the joint. Details of the specimen geometry and dimensions are given in a previous study [28]. Metallic coupons of the joint specimen are made of a newly developed commercial ferritic stainless steel, Crofer 22 H (ThyssenKrupp VDM GmbH, Germany), which is a heat-resistant alloy developed for pSOFC interconnects. Chemical composition of the Crofer 22 H alloy in wt% includes 22.93 Cr, 1.94 W, 0.51 Nb, 0.43 Mn, 0.21 Si, 0.08 La, 0.07 Ti, 0.02 Cu, 0.02 Al, 0.014 P, 0.007 C, < 0.002 S, and balance of Fe. Mechanical properties at 25  Ce800  C and high-temperature creep behavior of Crofer 22 H have been characterized in previous studies [36,37]. The as-received steel plates were cut into slices in dimensions of 95 mm (length) x 25 mm (width) x 2.5 mm (thickness). A pin hole was drilled in each steel slice for applying

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pin loading. It is effective to minimize bending and twisting effects during a test by means of pin loading. For shear test specimens, one edge of each steel slice was milled from the original thickness of 2.5 mm to 1 mm on an area of 8 mm  25 mm to be spread with the glasseceramic sealant. The glasseceramic sealant, designated as GC-9, used to join the two metallic coupons is a novel BaOeB2O3eAl2O3eSiO2 glass which has recently been developed at the Institute of Nuclear Energy Research (INER, Taiwan) for applications in pSOFC. Chemical composition of the patented GC-9 glasseceramic in mol% includes 34 BaO, 9.5 B2O3, 4.5 Al2O3, 34 SiO2, 12 CaO, 5 La2O3, and 1 ZrO2 [38]. The GC-9 glasseceramic sealant shows good thermal properties, chemical compatibility and stability, and hermetic properties which are appropriate for application in pSOFCs [39e44]. Mechanical properties of the GC-9 glasseceramic at 25  Ce800  C and high-temperature creep behavior have been investigated previously [31e34]. The GC-9 glass was made by mixing the constituent oxide powders followed by melting at 1550  C for 10 h. After melting, it was poured into a mold preheated to 680  C to produce GC-9 glass ingots. The GC-9 glass ingots were then annealed at 680  C for 8 h and cooled down to room temperature. GC-9 glass powders were made by crushing the as-cast glass ingots and sieving with 325-mesh sieves. The average size of the glass powders is 45 mm. Slurries were then made by adding into the GC-9 powders the desired amounts of solvent (alcohol), binder (ethyl celluloid), and plasticizer (polyethylene glycol). A GC-9 glass slurry was spread on the joining region of each asmachined steel slice to make a half-specimen. The apparent joining area is 25 mm  2.5 mm and 25 mm  6 mm for each tensile and shear specimen, respectively. The glass slurry was made of a mixture of GC-9 glass powders dispersed in ethanol. Each halfspecimen was then put in a furnace at 70  C to dry the slurry. A joint specimen was assembled by placing a half-specimen onto another to form a Crofer 22 H/GC-9/Crofer 22 H sandwich joint through appropriate heat treatments. In the assembling process, the joint specimen was firstly held at 500  C for 1 h and heated to 900  C followed by a hold time of 4 h. The heating rate at each heating step is 5  C min1. After the joining process was completed, the apparent thickness of glasseceramic sealant is of 0.5 mm in the shear specimen and of 0.44 mm in the tensile specimen. For investigating environmental effects on the joining quality, some specimens were thermally aged in a reducing atmosphere of humidified hydrogen (H2-7 vol% H2O) at 800  C for 100 h or 1000 h. The thermal aging treatment was carried out in an environmentcontrolled furnace attached with a quartz tube filled with desired

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gas. The reducing atmosphere was generated using helium as a carrier gas to pass through water at 90  C and then to mix with pure hydrogen. During the heating step, helium was used to purge the quartz tube continuously. After reaching 800  C, humidified hydrogen gas (H2-7 vol% H2O) began to flow into the quartz tube for thermal aging treatment. To avoid water condensation, gas pipelines were wrapped with a heating tape to maintain at a certain, warm temperature. 2.2. Mechanical testing To characterize mechanical properties of the joint at room temperature and 800  C, mechanical tests were conducted under uni-axial loading on a commercial closed-loop servo-hydraulic test machine attached with a furnace. A stainless steel tube (AISI 310), which is designed in house and fabricated by a local machine shop, is attached to the specimen for mechanical test in a humidified hydrogen environment. The tube encloses the specimen in a gastight condition, as shown in Fig. 1. During the mechanical test, hydrogen gas with 7 vol% H2O flows in and out of the attached tube to keep the entire tube filled with humidified hydrogen. All of the mechanical tests at both room temperature and 800  C are conducted in the given humidified hydrogen atmosphere using such an attachment. For mechanical tests at 800  C, a compact furnace is used to heat the central portion of the specimen and attachment, as shown in Fig. 1. For high-temperature tests, specimens were heated to 800  C with a rate of 5  C min1 and then held for 15 min to reach thermal equilibrium before applying the mechanical loads. During the heating step, the attached stainless tube was continuously flushed with helium gas. When the temperature reached 800  C, the helium gas was replaced by humidified hydrogen gas (H2-7 vol% H2O). Mechanical tests were conducted by means of displacement control with a stroke rate of 0.5 mm min1. For each condition, 5e7 specimens were repeatedly tested and the average strength was determined. 2.3. Microstructural analysis Fracture surface of each specimen was examined after mechanical testing with an optical microscope to determine the true joining area for calculating the nominal tensile or shear stress. Fractography analysis was also conducted on the broken specimens using SEM. Some specimens were cut along the longitudinal direction and then finely polished for observing the cross sections to investigate the characteristics of interfaces in the joint. The

Fig. 5. High-magnification SEM micrographs of two selected areas in the upper fracture surface of Fig. 4(a): (a) at the center (metal substrate); (b) at the periphery (BaCrO4 chromate layer).

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Fig. 6. EDS analysis results of the fracture surfaces in Fig. 5: (a) metal substrate (Fig. 5(a)); (b) BaCrO4 chromate layer (Fig. 5(b)).

interfacial morphology between the glasseceramic sealant and metallic interconnect was then examined using SEM. An energy dispersive spectrometer (EDS) module was used to analyze elemental distribution on the fracture surfaces as well as in the cross-sectional oxide layers between glasseceramic sealant and metallic interconnect. Through these analyses, fracture mode of each given testing condition was then characterized. 3. Results and discussion 3.1. Non-aged joint of glasseceramic sealant and metallic interconnect Fig. 7. Failure patterns of non-aged tensile specimens tested in H2-7 vol% H2O at (a) room temperature and (b) 800  C.

After sintering, non-aged joint specimens were tested under

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tensile or shear loading in H2-7 vol% H2O at room temperature and 800  C. Typical forceedisplacement curves under shear loading are shown in Fig. 2(a). The forceedisplacement curve at room temperature exhibits a typical linear brittle fracture pattern. However, the forceedisplacement curve at 800  C exhibits a non-linear failure mode. Note the initial non-linear behavior in each forceedisplacement curve is due to some compliance of the pin-loading rigs. As 800  C is higher than the softening temperature (745  C) of GC-9 glasseceramic [31], failure under shear loading at 800  C changes from a brittle mode to a ductile one. The shear strength of the non-aged joint specimens tested at room temperature and 800  C is shown in Fig. 3(a). In Fig. 3 and the following similar figures, the height of a column indicates the average strength value and the attached error bar represents the standard deviation. For shear loading, the joint strength is of 6.8 ± 1.4 MPa at room temperature and decreases to 4.2 ± 0.3 MPa at 800  C, due to a hightemperature softening mechanism. Formation of adhesive oxide layers is the main mechanism of interfacial joining between glasseceramic sealants and metallic interconnects. The high-temperature joining mechanism of GC-9 glasseceramic with Crofer 22 APU and Crofer 22 H alloys involves formation of two major oxide layers, namely a Cr2O3 chromia layer on the surface of Crofer 22 steel and a BaCrO4 chromate layer on the surface of GC-9 [28,39]. A (Cr, Mn)3O4 spinel layer is then formed between these two oxide layers [28,39]. Fig. 4 shows typical failure patterns of the non-aged shear specimens tested in H2-7 vol% H2O at room temperature and 800  C in the present study. Note that fracture surfaces on both halves of each broken specimen are presented in Fig. 4 with a mirror symmetry. As shown in Fig. 4(a), the glasseceramic layer is peeled from the metal substrate on the upper fracture surface and the metal substrate on the lower fracture surface is adhered with the peeled glasseceramic and relevant oxide layers. An inner region of metallic lust with machining marks is present in the upper fracture surface of Fig. 4(a), and there is a counterpart dark region in the lower fracture surface. The dark region is a Cr2O3 chromia layer which is peeled from the upper metal substrate. Note that BaCrO4 chromate also appears at the periphery of the upper fracture surface shown in Fig. 4(a). However, these features are not found in Fig. 4(b) for high-temperature shear specimens. A dark green (in the web version) inner region with dispersive white spots is observed on the upper fracture surface, as shown in Fig. 4(b). The major composition of this inner region is identified to be the GC-9 glasseceramic. BaCrO4 chromate also appears at the periphery of the upper fracture surface shown in Fig. 4(b). Identification of each layer on the fracture surfaces shown in Fig. 4 and the following figures is confirmed through SEM/EDS analysis and evidences of microstructural observation are given as follows. High-magnification SEM micrographs of two selected areas in the upper fracture surface of Fig. 4(a), one from the center and the other from the periphery, are shown in Fig. 5(a) and (b), respectively. As high intensities of Cr, Fe, and O are detected in Fig. 5(a) through EDS analysis, the inner region is confirmed to be the Crofer 22 H substrate. In a similar way, high intensities of Cr, Ba, and O are detected in Fig. 5(b), indicating that the peripheral region is indeed a BaCrO4 chromate layer. Similar features of SEM micrograph for the metal substrate and BaCrO4 chromate are also observed in the counterparts tested in air [28]. The EDS results of Fig. 5(a) and (b) are presented in Fig. 6(a) and (b), respectively, as an example of EDS evidence. Accordingly, for the non-aged shear specimens tested at room temperature, delamination occurs primarily at the interfaces between the metal substrate and chromia and occasionally between the glasseceramic substrate and chromate. For the non-aged shear specimens tested at 800  C, fracture occurs mostly within the glasseceramic layer and occasionally at

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the interface between the glasseceramic substrate and chromate. Representative forceedisplacement curves for non-aged tensile specimens tested at room temperature and 800  C in H2-7 vol% H2O are shown in Fig. 2(b). The tensile forceedisplacement curves at room temperature and 800  C both exhibit a linear brittle fracture pattern, which is different from the shear loading trend. The expected high-temperature viscous behavior of the GC-9 glasseceramic sealant apparently does not influence the tensile forceedisplacement response at 800  C. A similar phenomenon is also observed in the tensile tests performed in air [28]. Fig. 3(b) shows tensile strength of the non-aged joint specimens tested at room temperature and 800  C in H2-7 vol% H2O. For tensile loading, the joint strength at room temperature is of 31.3 ± 4.4 MPa and drops to 11.9 ± 4.7 MPa at 800  C. Fracture patterns of non-aged tensile specimens tested at room temperature and 800  C are not as diverse as those of shear specimens. As shown in Fig. 7, both upper and lower fracture surfaces of the room-temperature and high-temperature tensile specimens are covered with a glasseceramic layer in white color. Highmagnification SEM micrographs of two selected white spots in Fig. 7(a), one from the upper fracture surface and the other from the lower fracture surface, are shown in Fig. 8(a) and (b), respectively. Needle-shape crystalline phases (alpha-Ba (Al2Si2O8)) are observed in both Fig. 8(a) and (b), indicating the fracture surface is indeed covered with a glasseceramic layer. Note that alpha-Ba(Al2Si2O8) is the main crystalline phase in GC-9 glasseceramic [33]. These fractography observations indicate that fracture indeed occurs within the glasseceramic layer for tensile specimens tested at room temperature in H2-7 vol% H2O. High-magnification SEM micrographs are also taken for the white regions in Fig. 7(b) and show similar features to those observed in Fig. 8. Therefore, fracture is also identified to take place within the glasseceramic layer for tensile specimens tested at 800  C in H2-7 vol% H2O. Some non-aged tensile specimens were cut along the longitudinal direction to observe the cross sections in a lateral view (thickness side) and to characterize interfacial features in the joint. SEM micrographs of the interface between GC-9 and Crofer 22 H are shown in Fig. 9 with a back-scattered electron (BSE) mode. In Fig. 9(a), vertical direction is the direction of applied force and horizontal direction is the direction of specimen thickness. During the joining process, specimens are subjected to a small compressive loading from the holding fixtures such that part of glasseceramic sealant is squeezed out of edge of the joining area (Fig. 9(a)). As shown in Fig. 9(a), the upper zone is Crofer 22 H alloy, while the lower region is the glasseceramic sealant. In Fig. 9(b), a light gray region between Crofer 22 H and GC-9 is observed. Note that the light gray region is not formed uniformly. It is thicker at the edge of the joining area, which is expected to be exposed to more oxygen during the assembling process in air, compared to that formed at the inner region of the joint. Through EDS analysis, element distributions in these zones are confirmed and shown in Fig. 10. A high intensity of Cr and Fe is found in the region of Crofer 22 H, as expected. As shown in Fig. 10(b)e(e), a high intensity of Cr, Ba, and O, respectively, is found in the light gray region, indicating it is a BaCrO4 chromate layer. As described above, the main mechanism of interfacial joining between glasseceramic and metallic interconnect is formation of two oxide layers, a Cr2O3 chromia layer on the surface of Crofer 22 steel and a BaCrO4 chromate layer on the surface of GC-9 [28,39]. As shown in Fig. 9(c), a Cr2O3 chromia layer is seen on the metal side and has a thickness about 0.2 mm. The BaCrO4 chromate layer present in Fig. 9(b) is hardly visible in the SEM micrographs at the inner central region of the non-aged specimen. It is implied that BaCrO4 chromate layer is too thin to be observed due to a less amount of oxygen available in the central region. Similar observations of a chromate layer at the outer sealing

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Fig. 8. High-magnification SEM micrographs of two selected white spots in Fig. 7(a): (a) on the upper fracture surface; (b) on the lower fracture surface.

edges are also reported in the study of Chou et al. [21].

3.2. 100 h-aged joint of glasseceramic sealant and metallic interconnect After a 100-h thermal aging treatment, the aged specimens were then tested under tensile or shear loading at room temperature and 800  C in H2-7 vol% H2O. As shown in Fig. 11(a), the forceedisplacement curves of 100 h-aged shear specimens exhibit a brittle fracture pattern and a non-linear failure mode at room temperature and 800  C, respectively. Such patterns are similar to

those of non-aged shear specimens (Fig. 2(a)) except that fracture displacement of the 100 h-aged shear specimens at 800  C is smaller than that at room temperature. For 100 h-aged tensile specimens, the forceedisplacement curves at room temperature and 800  C both exhibit a brittle fracture mode (Fig. 11(b)). These patterns are similar to those of non-aged tensile specimens (Fig. 2(b)) except that the 100-h aged tensile specimen has a greater fracture displacement at room temperature but a smaller fracture displacement at 800  C than the non-aged one. Compared to the non-aged condition, a 100-h thermal aging treatment in H2-7 vol% H2O slightly enlarges the fracture displacement at room

Fig. 9. SEM micrographs (BSE mode) of a cross section of an interface between the GC-9 and Crofer 22 H in a non-aged joint specimen: (a) low magnification view; (b) high magnification of the outlined region in (a); (c) higher magnification of the outlined region in (b).

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Fig. 10. EDS mapping of elements on the cross section of an interface between the GC-9 and Crofer 22 H shown in Fig. 9(b): (a) observed region; (b) Cr; (c) Ba; (d) Fe; (e) O.

temperature but significantly shortens the fracture displacement at 800  C for both loading modes. Such a phenomenon is also observed for the 1000 h-aged specimens tested at room temperature and 800  C in H2-7 vol% H2O. Water in glass has a great influence on a variety of properties for glass. For instance, water increases the rate of structural relaxation and crystallization of glass [45e47]. In addition, water reduces the viscosity of glass [45e47]. Accordingly, fracture displacement of the aged specimens tested at 800  C is lower than that of the non-aged specimens, as the aged specimens are heat treated in humidified hydrogen at 800  C for 100 h or 1000 h. Shear strength of the 100 h-aged joint specimens tested at room temperature and 800  C in H2-7 vol% H2O is shown in Fig. 12(a). For 100 h-aged shear specimens, the joint strength decreases from 8.2 ± 1.4 MPa at room temperature to 2.3 ± 0.3 MPa at 800  C due to a high-temperature softening mechanism. Typical failure patterns of the 100 h-aged shear specimens are shown in Fig. 13. As shown in each of Fig. 13(a) and (b), the glasseceramic layer is peeled from the Crofer 22 H substrate in the upper fracture surface and the lower fracture surface is adhered with a GC-9 glasseceramic/

BaCrO4 chromate layer. As the failure patterns at room temperature and 800  C are identified to be similar for the 100 h-aged shear specimens, only detailed microstructural observations of roomtemperature specimens are given below as an example. A slightly bright region is observed on the upper fracture surface in Fig. 13(a). A high-magnification SEM micrograph of an area selected from the central region in the upper fracture surface of Fig. 13(a) is shown in Fig. 14(a). As Cr, Ba, Fe, and O elements are detected in Fig. 14(a) through EDS analysis, the region observed is indeed a chromia layer mixed with some residual BaCrO4 chromate and GC-9 glasseceramic. A high-magnification SEM micrograph of a central region in the lower fracture surface of Fig. 13(a) is shown in Fig. 14(b). Aggregated particles and few needle-shape crystals are observed in Fig. 14(b). As aggregated particles are buried in the residual glass of GC-9 glasseceramic, it is difficult to identify their phase. Needleshape crystals are the alpha-Ba(Al2Si2O8) crystalline phase. By means of EDS analysis, high intensities of Si, Ba, and O elements are detected in Fig. 14(b), indicating that this region is a GC-9 glasseceramic layer. Based on the fractography features described above, fracture does not take place along a single interface, such as

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Fig. 11. Typical forceedisplacement curves of 100 h-aged joint specimens tested in H27 vol% H2O: (a) shear loading; (b) tensile loading.

that of chromia/metal substrate or chromate/glasseceramic substrate, but through cracking in the oxide layers and glasseceramic substrate. Therefore, for 100 h-aged shear specimens tested at room temperature in H2-7 vol% H2O, fracture occurs in the mixed layer of glasseceramic/chromate/chromia. Such a failure pattern is also observed for 100 h-aged shear specimens tested at 800  C in H2-7 vol% H2O. Fig. 12(b) shows tensile strength of the 100 h-aged joint specimens tested at room temperature and 800  C in H2-7 vol% H2O. The joint strength under tensile loading at room temperature is of 46.6 ± 5.0 MPa and reduces significantly to 3.2 ± 0.3 MPa at 800  C. Again, this is due to a high-temperature softening effect. Failure patterns of the 100 h-aged tensile specimens tested at room temperature and 800  C in H2-7 vol% H2O are shown in Fig. 15. A white region is visible at the right portion of the upper and lower micrographs in Fig. 15(a). Microstructure of the GC-9 glasseceramic is observed in these white regions. High-magnification SEM micrographs of two selected areas in Fig. 15(a), one from the central gray region of the upper fracture surface and the other from the central green (in the web version) region of the lower fracture surface, are shown in Fig. 16(a) and (b), respectively. EDS analysis results indicate the regions observed in Fig. 16(a) and (b) are metal substrate and chromia, respectively. Therefore, for 100 h-aged tensile specimens tested at room temperature, fracture occurs at the interfaces

Fig. 12. Joint strength of 100 h-aged specimens tested in H2-7 vol% H2O: (a) shear loading; (b) tensile loading.

Fig. 13. Failure patterns of 100 h-aged shear specimens tested in H2-7 vol% H2O at (a) room temperature and (b) 800  C.

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Fig. 14. High-magnification SEM micrographs of two selected areas in Fig. 13(a): (a) a central region of the upper fracture surface; (b) a central region of the lower fracture surface.

between the metal substrate and chromia and within the glasseceramic layer. Microstructures in the upper and lower fracture surfaces of Fig. 15(b) for the 100 h-aged tensile specimens tested at 800  C are similar to those of the 100 h-aged shear specimens tested at room temperature and 800  C (Fig. 13). EDS analysis results are also similar to those for 100 h-aged shear specimens indicating that fracture occurs in the mixed layer of glasseceramic/ chromate/chromia for 100 h-aged tensile specimens tested at 800  C in H2-7 vol% H2O. Fig. 17 shows the cross-sectional SEM micrographs (BSE mode) of the interface of Crofer 22 H/GC-9 in a 100 h-aged tensile specimen. A non-uniform chromate layer at the interface is observed in Fig. 17(a). Again, BaCrO4 chromate is too thin to be observed at the interior region of the joint. In comparison of the cross-sectional micrographs between non-aged and 100 h-aged specimens (Figs. 9 and 17), an internal Cr-oxidation zone is found on the metal side in the aged specimen (Fig. 17(b)). As shown in Fig. 17(c), thickness of the Cr2O3 chromia layer on the metal side is about 0.2 mm. Below the chromia layer in Fig. 17(c), needle-shape crystals are buried in the amorphous glass phase, indicating that it is glasseceramic. Note that internal oxidation takes place mostly at the triple-phase boundary of metal/glasseceramic/humidified hydrogen. Similar observations of internal oxidation at the triplephase boundary are also reported in the study of Menzler et al. [11]. The internal metal oxidation may be attributed to an interaction of hydrogen/water vapor with the double oxide layer to form hydroxides or other volatile species [11]. Additionally, not only aluminum and silicon in the interconnect steel but also elements in the glasseceramic could enhance the internal oxidation [8,11]. In general, air promotes chromate formation and humidified

Fig. 15. Failure patterns of 100 h-aged tensile specimens tested in H2-7 vol% H2O at (a) room temperature and (b) 800  C.

hydrogen promotes iron- and chromium-containing phase formation at the interface [8]. Note that the internal oxidation features observed in Fig. 17 are not found in the counterpart specimens thermally aged in air [28]. 3.3. 1000 h-aged joint of glasseceramic sealant and metallic interconnect The 1000 h-aged specimens were also tested under tensile or shear loading at room temperature and 800  C in H2-7 vol% H2O. Trends of the forceedisplacement relationship in the 1000 h-aged specimens are similar to those of 100 h-aged specimens (Fig. 11). Fig. 18(a) shows shear strength of the 1000 h-aged joint specimens tested in H2-7 vol% H2O. The shear strength drops from 8.4 ± 1.2 MPa at room temperature to 3.4 ± 0.3 MPa at 800  C, again, due to a high-temperature softening mechanism. Typical failure patterns of 1000 h-aged shear specimens tested at room temperature and 800  C are shown in Fig. 19. Needle-shape crystalline phases, like those present in Fig. 8, are observed in highmagnification SEM micrographs of the white regions on the upper and lower fracture surfaces in each of Fig. 19(a) and (b). High intensities of Si, Ba, and O are detected, through EDS analysis, on the fracture surfaces in both Fig. 19(a) and (b). Therefore, fracture occurs within the GC-9 glasseceramic layer for 1000 h-aged shear specimens tested at room temperature and 800  C in H2-7 vol% H2O. Tensile strength of the 1000 h-aged joint specimens tested at room temperature and 800  C in H2-7 vol% H2O is presented in Fig. 18(b). For a tensile loading mode, the joint strength at room temperature is of 46.0 ± 9.4 MPa and drops significantly to 5.8 ± 2.2 MPa at 800  C because of a high-temperature softening mechanism. Fig. 20 show typical failure patterns of the 1000 h-aged tensile specimens tested at room temperature and 800  C. Glasseceramic features (white regions) are clearly observed on the upper and lower fractures in each of Fig. 20(a) and (b). Similar to 1000 h-aged shear specimens, both SEM and EDS analysis results indicate that fracture occurs within the GC-9 glasseceramic layer for 1000 h-aged tensile specimens tested at room temperature and 800  C in H2-7 vol% H2O. Cross-sectional views of microstructure at the interface of Crofer 22 H/GC-9 in a 1000 h-aged tensile specimen are shown in Fig. 21. Again, a non-uniform chromate layer and an internal Cr-oxidation zone of steel at the interface are observed, as shown in Fig. 21(a). However, after a 1000-h thermal aging treatment in H2-7 vol% H2O, thickness of the chromia layer becomes non-uniform at interface (Fig. 21(b)). At some areas, it is thicker than that in the 100 h-aged

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Fig. 16. High-magnification SEM micrographs of two selected areas in Fig. 15(a): (a) a central gray region of the upper fracture surface; (b) a central green region of the lower fracture surface.

specimens, while it becomes thinner at other areas. It is presumably due to a chromia depletion mechanism that provides chromium vapor species for interfacial diffusion [13]. 3.4. Effects of thermal aging in reducing environment on joint strength Tables 1 and 2 summarize the average strength, standard deviation, and fracture site for the given joint specimens subject to shear and tensile loading, respectively, in H2-7 vol% H2O. As described above, four types of fracture site are generally observed

in the joint specimens. Fracture taking place in GC-9 glasseceramic layer is classified as Type A, while that at the interface between GC9 substrate and BaCrO4 layer is classified as Type B. Type C denotes fracture at the interface between Crofer 22 H substrate and Cr2O3 layer and Type D represents fracture in the mixed layer of glasseceramic/chromate/chromia. If fracture involves two sites, it is marked with two types together, as listed in Tables 1 and 2. As shown in Table 1, for variously aged shear specimens tested at room temperature, the joint strength is low if fracture involves an interfacial delamination. For shear specimens tested at 800  C, fracture occurring within the glasseceramic layer accompanies a

Fig. 17. SEM micrographs (BSE mode) of a cross section of an interface between the GC-9 and Crofer 22 H in a 100 h-aged joint specimen: (a) low magnification view; (b) high magnification of Region 1 in (a); (c) high magnification of Region 2 in (a).

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Fig. 19. Failure patterns of 1000 h-aged shear specimens tested in H2-7 vol% H2O at (a) room temperature and (b) 800  C.

Fig. 18. Joint strength of 1000 h-aged specimens tested in H2-7 vol% H2O: (a) shear loading; (b) tensile loading.

greater joint strength. As shown in Table 2, fracture takes place mostly within the glasseceramic layer for specimens tested under tensile loading except for the 100 h-aged specimens tested at 800  C. The 100 h-aged joint specimens tested at 800  C have the lowest tensile strength and a D type of fracture site, indicating a low tensile joint strength for fracture involving cracking in the mixed layer of glasseceramic/chromate/chromia. A similar phenomenon was found for metal interconnect/glasseceramic sealant/metallic interconnect joints tested in air [20,28]. For joint specimens tested in H2-7 vol% H2O after a 1000-h aging treatment in the same humidified hydrogen atmosphere, fracture all occurs within the glasseceramic layer, regardless of loading mode and testing temperature (Tables 1 and 2). It indicates that the interfacial bonding strength between various layers is enhanced and greater than the strength of bulk glasseceramic sealant after a long-term thermal aging in a reducing environment. Apparently, GC-9 glasseceramic layer is the weakest layer to resist both shear and tensile loading for joint specimens thermally aged for 1000 h in H2-7 vol% H2O followed by mechanical testing in the same reducing environment. Shear strength of variously aged joint specimens tested in H2-7 vol% H2O is plotted in Fig. 22(a) for comparison. In general, a thermal aging treatment in the given reducing environment

enhances the shear strength at room temperature but deteriorates it at 800  C. Compared to the average shear strength of non-aged specimens, an extent of 21%e24% of increase is observed for aged specimens tested at room temperature and 19%e45% of reduction is seen at 800  C. Fig. 22(b) shows a comparison of tensile strength of the joint specimens in variously aged conditions. The tensile strength shows a similar trend to that of the shear strength. The average tensile strength for aged specimens increases by 47%e49% at room temperature and decreases by 51%e73% at 800  C in comparison with the non-aged ones. As shown in Fig. 22, 100 haged specimens have a comparable room-temperature shear and tensile strength with that of 1000 h-aged specimens, while their high-temperature shear and tensile strength is much lower than the counterpart of 1000 h-aged specimens. In other words, a 100-h thermal aging treatment in H2-7 vol% H2O degrades the joint strength at 800  C to a greater extent than does a 1000-h one. However, in the previous counterpart study for a given oxidizing environment [28], the extent of reduction in high-temperature

Fig. 20. Failure patterns of 1000 h-aged tensile specimens tested in H2-7 vol% H2O at (a) room temperature and (b) 800  C.

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Fig. 21. SEM micrographs (BSE mode) of a cross section of an interface between the GC-9 and Crofer 22 H in a 1000 h-aged joint specimen: (a) low magnification view; (b) high magnification view.

shear strength by a thermal aging of 250 h, 500 h, or 1000 h in air is of no significant difference (17%e19%). Whether a 100-h thermal aging treatment in air would generate a greater detrimental effect on the high-temperature joint strength in air is unknown, as no such tests were conducted in Ref. [28] for comparison. Some possible mechanisms to explain the thermal aging effect are given as follows. Firstly, it might be related to changes in defect morphology and/or size due to thermal aging treatment [21]. As the sealing glass is devitrified into a polycrystalline multi-phase mixture, changes in the flaw shape and size may occur in the thermal aging treatment, leading to a reduction in size and/or blunting of the critical defect for failure during mechanical test [21]. Secondly, it might be associated with a relief of residual stresses during thermal aging. Thirdly, devitrification may reduce the residual glass content in the sealant and increase the ceramic crystalline content. A greater extent of crystallization in the aged specimens is expected to generate a higher strength in the glasseceramic substrate. Representative SEM micrographs (BSE mode) for GC-9 glasseceramic layer in variously aged joint specimens are shown in Fig. 23. Gray needles, which are the primary crystalline alpha-Ba(Al2eSi2O8) phase, are clearly observed in the given three aged conditions (Fig. 23). The extent of alpha-Ba(Al2Si2O8) phase for each aged condition is estimated by calculating the average area proportion of the gray needles in fifteen SEM micrographs like that present in Fig. 23. As a result, the area proportion of the crystalline alpha-Ba(Al2Si2O8) phase is approximately 23%, 32%, and 32%, in

Table 1 Average shear strength, standard deviation, and fracture site of variously aged joint specimens tested in H2-7 vol% H2O. Test temperature ( C)

Aged condition

Average shear strength (MPa)

Standard deviation (MPa)

Fracture sitea

25 25

Non-aged 100 haged 1000 haged Non-aged 100 haged 1000 haged

6.8 8.2

1.4 1.4

BþC D

8.4

1.2

A

4.2 2.3

0.3 0.3

AþB D

3.4

0.3

A

25 800 800 800

a A: in glasseceramic sealant layer; B: at the interface between glasseceramic substrate and BaCrO4 layer; C: at the interface between metal substrate and Cr2O3 layer; D: in the mixed layer of glasseceramic/chromate/chromia.

the non-aged, 100 h-aged, and 1000 h-aged GC-9 layer, respectively. The three mechanisms described above might contribute to an enhanced joint strength at room temperature for aged specimens over non-aged ones. Micro-voids (the darkest area) are also found in Fig. 23 as a result of burning of binder and/or plasticizer during sintering process. They might even be formed during the cooling step due to CTE mismatch between crystalline phases and residual glass in the GC-9 glasseceramic. It is indicated in the study of Liu et al. [48] that micro-voids are formed in the aged samples due to CTE difference between ceramic and glass phases during a cooling process after thermal aging. In Fig. 23, the extent (area proportion) of microvoids in GC-9 layer is also calculated for each aged condition. The porosity ratio for the non-aged, 100 h-aged, and 1000 h-aged GC-9 is thus estimated as 15%, 22%, and 22%, respectively. It might explain why the joint strength at 800  C for aged specimens is inferior to that for non-aged specimens, as most of the hightemperature fracture sites (A and D) involve the bulk glasseceramic sealant. Presumably, this fourth mechanism together with the other three described above competes with each other to play a major role in determining the joint strength of variously aged specimens under the given thermal aging and mechanical testing conditions. Chou et al. [21] developed a (SrO,CaO)eY2O3eB2O3eSiO2 glass (YSO75) for SOFC applications and performed mechanical testing at room temperature on the joint of YSO75 glass with a Cr-containing ferritic steel (Crofer 22 APU). They found a similar trend to that of the current work, namely increasing joint strength at room temperature after thermal aging in a wet (~30% H2O) dilute hydrogen

Table 2 Average tensile strength, standard deviation, and fracture site of variously aged joint specimens tested in H2-7 vol% H2O. Test temperature ( C)

Aged condition

Average tensile strength (MPa)

Standard deviation (MPa)

Fracture site*

25 25

Non-aged 100 haged 1000 haged Non-aged 100 haged 1000 haged

31.3 46.6

4.4 5.0

A AþC

46.0

9.4

A

11.9 3.2

4.7 0.3

A D

5.8

2.2

A

25 800 800 800

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Fig. 22. Comparison of joint strength for variously aged specimens tested in H2-7 vol% H2O: (a) shear loading; (b) tensile loading.

(2.7% H2/Ar) atmosphere at 850  C. Note that neither hightemperature test nor shear loading test was conducted in that study [21]. The room-temperature tensile strength of as-sealed YSO75/Crofer 22 APU joints is 6.3 ± 0.9 MPa which slightly increases to 7.0 ± 1.6 MPa after thermal aging (850  C/250 h) in the wet reducing environment [21]. In the present study, the tensile strength at room temperature for the as-sealed GC-9/Crofer 22 H joints is 31.3 ± 4.4 MPa when tested in H2-7 vol% H2O. After thermal aging in the given humidified hydrogen atmosphere at 800  C for 100 h or 1000 h, the tensile strength at room temperature for the GC-9/Crofer 22 H joints increases to 46.6 ± 5.0 MPa or 46.0 ± 9.4 MPa, respectively. Apparently, the given GC-9/Crofer 22 H joint has a greater joint strength than the YSO75/Crofer 22 APU joint indicating the newly developed GC-9 glasseceramic is also appropriate for SOFC applications. 3.5. Overall comparison of joint strength in reducing and oxidizing environments Fig. 24 shows comparisons of shear and tensile strength at room temperature and 800  C for non-aged joint specimens tested in various environments. For comparison, previously obtained data in

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air [28] are also included in Fig. 24. For non-aged joint specimens tested in air, the shear strength is of 6.6 ± 1.6 MPa at room temperature and decreases to 4.7 ± 0.3 MPa at 800  C, while the tensile strength decreases from 23.0 ± 3.3 MPa at room temperature to 12.7 ± 3.3 MPa at 800  C [28]. As shown in Fig. 24, joint strength of the non-aged specimens tested in air is comparable with the counterpart in H2-7 vol% H2O except for tensile strength at room temperature. Given a loading mode and testing temperature, fractography observations in Ref. [28] and those in the present study indicate that fracture modes for the non-aged specimens are similar between the given two environments. As duration of each mechanical test, including 15 min of thermal equilibrium for a high-temperature one, is generally less than 20 min, no significant difference of environmental effect on the joint strength is found between the given oxidizing and reducing environments. Fig. 25 shows a comparison of high-temperature shear strength of the non-aged and 1000 h-aged joint specimens which are thermally aged and tested at 800  C in H2-7 vol% H2O or air. Data in air from previous work [28] are also included in Fig. 25 for comparison. The shear strength at 800  C in air is of 4.7 ± 0.3 MPa and 3.8 ± 0.5 MPa, respectively, for non-aged and 1000 h-aged joint specimens [28]. For both non-aged and 1000-h aged conditions, the average high-temperature shear strength in air is slightly higher than that in humidified hydrogen (Fig. 25). However, the difference is within the scattering range such that shear strength at 800  C for the 1000-h aged joint specimens may be seen as comparable in both given reducing and oxidizing environments. This is consistent with the fractography observations, as fracture of the 1000-h aged shear specimens takes place primarily within the glasseceramic layer in both air and H2-7 vol% H2O. Note that a direct comparison of the high-temperature tensile strength of 1000 h-aged joint specimens in air and H2-7 vol% H2O is not accessible, as no such tests in air were conducted in Ref. [28]. As shown in Fig. 25, thermal aging treatments in the given reducing and oxidizing environments have similar detrimental effects on the shear strength at 800  C. Compared to the hightemperature shear strength of non-aged specimens, 19% of reduction in strength is observed for the joint specimens thermally aged at 800  C in both air and H2-7 vol% H2O for 1000 h. Compared with the non-aged condition, a 1000-h thermal aging in air greatly enlarges the thickness of Cr2O3 chromia layer and BaCrO4 chromate layer at the metal/glasseceramic interface [28]. However, the thickness of BaCrO4 chromate layer after a 1000-h thermal aging in H2-7 vol% H2O does not change significantly (Fig. 21), but internal Cr-oxidation of steel takes place at the interface of Crofer 22 H/GC9. Apparently, these differences in the interfacial microstructure of the joint specimens which are thermally aged in either environment do not play a major role in determining the high-temperature shear strength. For a comparably detrimental effect described above, the mechanism responsible for reduction of hightemperature shear strength after a 1000-h thermal aging in the oxidizing atmosphere is likely similar to that in the reducing atmosphere. As mentioned in the previous section, micro-voids are formed in the aged samples during a cooling process after thermal aging because of CTE mismatch between ceramic and glass phases such that fracture takes place in the glasseceramic layer. Such a mechanism is presumably responsible for failure of the 1000 haged joint specimens in both given reducing and oxidizing environments. Note that fracture occurs primarily within the glasseceramic layer for 1000 h-aged shear specimens tested at 800  C in either given environment. In the study of Chou et al. [21], the room-temperature tensile strength of YSO75/Crofer 22 APU joints degrades substantially from 6.3 ± 0.9 MPa to 0.5 ± 0.3 MPa after aging in air (850  C/500 h), but slightly increases to 7.0 ± 1.6 MPa after thermal aging (850  C/

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Fig. 23. SEM micrographs (BSE mode) of GC-9 glasseceramic layer in variously aged joint specimens: (a) non-aged; (b) 100 h-aged; (c) 1000 h-aged.

250 h) in a wet hydrogen atmosphere. In the present study, a thermal aging treatment in the given humidified hydrogen environment also enhances the mechanical strength of GC-9/Crofer 22 H joints at room temperature, but degrades it at 800  C. As described above, the high-temperature shear strength at 800  C is degraded to a similar extent (19%) after a 1000-h thermal aging in both air and H2-7 vol% H2O for the given GC-9/Crofer 22 H joints. As no high-temperature test was conducted in the study of Chou et al. [21], it is unknown how the high-temperature mechanical strength of their YSO75/Crofer 22 APU joints is affected after thermal aging in both oxidizing and reducing environments. Based on the data available for comparison, the GC-9/Crofer 22 H joint given in the present study shows a greater value of mechanical strength than the YSO75/Crofer 22 APU joint in the study of Chou et al. [21]. To the authors' best knowledge, no common threshold values of mechanical strength for the pSOFC stack joint have been reported in the literature or specified by the SOFC industry. It is simply because it is related to the configuration design of pSOFC stack and the materials chosen for the components. A smaller mismatch of CTE between components and a lower temperature gradient would reduce the critical thermal stress in a pSOFC stack as well as the relevant mechanical strength required [3,4]. On the other hand, a greater mechanical joint strength would tolerate a greater magnitude of thermal stress on the interface of a sealant/interconnect joint and improve the structural integrity and performance of a pSOFC stack. The interfacial properties of the sealant/interconnect joint have not been taken into account in previous thermal stress analyses for pSOFC stacks as a highly challenging simulation and modeling technique needs to be developed. More research is needed to quantitatively address the interfacial, mechanical

properties of the sealant/interconnect joint into thermal stress analysis so as to assure the robustness of a pSOFC stack. A greater mechanical joint strength on the interface between the sealant and interconnect in both oxidizing and reducing environments presumably indicates a better bonding behavior and gastight sealing for a given sealant. As a result, a better electrochemical performance and a lower degradation rate are expected for application of such a sealant in SOFCs. The GC-9 glasseceramic sealant is developed specifically for application in pSOFCs. In addition to the quantitative, mechanical properties reported in the current and prior work [28e34], GC-9 has been proved in many aspects for appropriate application in pSOFCs [38e44]. For reducing thermal stresses during thermal cycles of SOFC operation, the CTE of GC-9 has been shown to match very well with those of ceramic electrolyteeelectrode assembly and metallic interconnect [38,44]. Liu et al. [42,44] have studied the hermetic properties of the GC-9 glasseceramic sealant joined with a metallic interconnect (Crofer 22 APU) under isothermal aging at 800  C as well as thermal cycling (room temperature-800  C). The average leakage rate of the Crofer 22 APU/GC-9/Crofer 22 APU sandwich joint is of 2.25  105 mbar l s1 cm1 at 800  C for more than 1000 h and of 5.58  105 mbar l s1 cm1 for 50 thermal cycles between room temperature and 800  C [42,44]. Furthermore, Liu et al. [43,44] have also investigated the interfacial compatibility and stability of the GC-9 sealant with adjacent components of pSOFC. Their results indicate that the GC-9 sealant is chemically stable and has good adhesion characteristics when joined with a metallic interconnect (Crofer 22 APU) and a ceramic electrolyteeelectrode assembly (ASC1) during a thermal aging at 800  C for 1000 h [43,44]. As described above, the main purpose of this study is to

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quantitatively measure the mechanical strength of the interface between the given sealant and interconnect steel in an SOFC reducing environment containing wet hydrogen. It is not able to conduct an integrated mechanicaleelectrochemical test to simultaneously measure the mechanical strength of the sealant/interconnect joint and the currentevoltage characteristics (or power density) during operation of a pSOFC stack. In general, electrochemical performance of a sealing technology is usually assessed by testing a full pSOFC stack of unit cell or multiple cells. An electrochemical test has been conducted on a 3-cell pSOFC stack by the coauthors at INER using GC-9 as the sealant. The currentevoltage characteristics indicate that an average degradation rate of 0.45% per 1000 h in electrochemical performance is found after operation of the 3-cell pSOFC stack for 6300 h. It indicates that the GC-9 glasseceramic sealant provides an excellent gas-tight sealing for the given pSOFC stack in both oxidizing and reducing environments and proves its applicability in pSOFCs. One advantage of SOFC over other fuel cells is its fuel flexibility. In the present study, the effect of a humidified hydrogen atmosphere on the mechanical strength of the sealant/interconnect joint is characterized. Effect of carbon monoxide on the joint strength and long-term electrochemical performance is not considered and is beyond the scope of the current work. The co-authors at INER

have been conducting an experiment trying to use CH4 as the fuel for a prototypical pSOFC stack. After reforming, the content of CO is around 10e15% in the fuel. Preliminary results of that pilot study show that no damage/leakage in the sealing or degradation in electrochemical performance is found after a steady operation for 1000 h using GC-9 as the sealant. It indicates that GC-9 also shows a good chemical stability in a reducing environment containing CO. However, a more systematic study is needed to investigate the effect of CO on the mechanical strength of the sealant/interconnect joint, which could be considered as a future work in this series of studies.

Fig. 24. Comparison of joint strength of non-aged specimens tested in H2-7 vol% H2O and air: (a) shear loading; (b) tensile loading. (Data of air are taken from Ref. [28].)

Fig. 25. Comparison of shear strength at 800  C for variously aged specimens tested in H2-7 vol% H2O and air. (Data of air are taken from Ref. [28].).

4. Conclusions Mechanical strength of the interface between the given GC-9 glasseceramic sealant and Crofer 22 H interconnect steel is quantitatively measured in a humidified hydrogen reducing environment. Effects of thermal aging on the joint strength are characterized and compared in both reducing and oxidizing environments. Results of mechanical testing and fractography analysis are summarized as follows: (1) A mechanical testing technique for determining both the shear and tensile joint strength between SOFC glasseceramic sealant and metallic interconnect at room temperature and high temperature in a reducing environment is developed. Through this technique, the high-temperature strength of such a joint in a reducing atmosphere is able to be quantitatively determined, which has not yet been reported in the literature. (2) Both tensile and shear strengths of Crofer 22 H/GC-9/Crofer 22 H sandwich joint in H2-7 vol% H2O increase at room temperature but decrease at 800  C after a thermal aging at 800  C in the same reducing environment. A 100-h thermal aging increases the shear and tensile strengths at room temperature by 21% and 49%, respectively, but deteriorates them at 800  C by 45% and 73%, respectively. After a thermal aging treatment of 1000 h, the shear and tensile strengths are enhanced respectively by 24% and 47% at room temperature, while they are degraded respectively by 19% and 51% at 800  C.

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(3) For joint specimens tested in H2-7 vol% H2O, fracture occurring within the glasseceramic layer accompanies a greater shear strength, while fracture for a lower shear strength involves interfacial delamination or cracking in the mixed layer of glasseceramic/chromate/chromia. For the counterparts under tensile loading, a greater joint strength corresponds to fracture occurring only in the glasseceramic layer, while a lower joint strength is accompanied by cracking in the mixed layer of glasseceramic/chromate/chromia. (4) Given a loading mode and testing temperature, the joint strength and fracture mode for non-aged specimens are generally comparable when tested either in H2-7 vol% H2O or in air. (5) A 1000-h thermal aging at 800  C in either of the given reducing and oxidizing environments generates comparably detrimental effects on the shear strength at 800  C. A reduction of high-temperature shear strength by 19% is observed for joint specimens after a 1000-h thermal aging in either atmosphere, compared to the counterpart of non-aged joint specimens. (6) The GC-9/Crofer 22 H joint given in this study shows a greater mechanical strength compared with other SOFC glasseceramic sealant/interconnect steel joints reported in the literature. Accordingly, it is expected for a pSOFC stack, which uses GC-9 and Crofer 22 H as the sealant and interconnect, respectively, to tolerate a greater magnitude of thermal stress on the joint interface and to have a better structural robustness. Results of the present work and previous studies on the newly developed GC-9 glasseceramic sealant indicate that the GC-9 sealant has good thermal properties, chemical compatibility and stability, hermetic properties, and mechanical properties appropriately for applications in pSOFC. In this regard, the joint strength data in a reducing environment presented in this study and those previously obtained in an oxidizing atmosphere can serve as a basis to support development, modeling, and optimization of pSOFC stack designs when using GC-9 as the sealant. Acknowledgments This work was supported by the Ministry of Science and Technology (Taiwan) under Contract No. MOST 103-2221-E-008-014MY2 and by the Institute of Nuclear Energy Research (Taiwan) under Contract No. 103-2001-INER-025. References [1] J.W. Fergus, J. Power Sources 147 (2005) 46e57. [2] P.A. Lessing, J. Mater. Sci. 42 (2007) 3465e3476. [3] C.-K. Lin, T.-T. Chen, Y.-P. Chyou, L.-K. Chiang, J. Power Sources 164 (2007) 238e251. [4] C.-K. Lin, L.-H. Huang, L.-K. Chiang, Y.-P. Chyou, J. Power Sources 192 (2009) 515e524. [5] Y. Zhao, J. Malzbender, J. Power Sources 239 (2013) 500e504. [6] A. Nakajo, J. Kuebler, A. Faes, U.F. Vogt, H.J. Schindler, L.-K. Chiang, S. Modena, J. van Herle, T. Hocker, Ceram. Int. 38 (2012) 3907e3927.

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