Mechanical properties and dual atmosphere tolerance of Ag–Al based braze

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Mechanical properties and dual atmosphere tolerance of Ag–Al based braze Jin Yong Kim, Jung-Pyung Choi, K. Scott Weil Pacific Northwest National Laboratory, Richland, WA 99352, USA

ar t ic l e i n f o

abs tra ct

Article history:

In this paper, the effects of aluminum on the microstructure, mechanical properties, and

Received 5 June 2007

high temperature dual atmosphere tolerance of silver and silver–copper oxide filler metals

Received in revised form

were investigated. It was found that joints brazed with binary Ag–Al braze foils containing

6 December 2007

more than 2 at% Al retained a metallic form of aluminum within the metallic braze filler

Accepted 7 December 2007

matrix after brazing at 1000  C in air. The bend strengths of these joints decreased with

Available online 14 March 2008

increasing aluminum content due to the formation of interfacial aluminum oxide.

Keywords: Dual atmosphere tolerance Silver-based braze Aluminum addition

However, the existence of metallic aluminum in the braze filler matrix appeared to enhance the high-temperature dual atmosphere tolerance of the silver-based braze filler, which displayed measurably less porosity after 1000 h of exposure at 800  C in a dual reducing/oxidizing atmosphere environment than unalloyed silver. A series of binary and ternary braze pastes based on the Ag–Al(–Cu) system were also formulated as potential pSOFC (planar solid oxide fuel cell) sealants. Model alumina joints brazed with these pastes exhibited an increase in bend strength with increasing copper content. However, unlike the binary Ag–Al filler metals, the ternary compositions often retained no protective metallic aluminum after brazing. Thus, while the addition of copper improves filler metal wettability and, therefore, joint strength in the Ag–Al alloys, it appears to reduce the dual atmosphere tolerance of these filler metals. Published by Elsevier Ltd. on behalf of International Association for Hydrogen Energy.

1.

Introduction

Sealing is one of the key challenges in designing and developing durable intermediate temperature planar solid oxide fuel cell (pSOFC) stacks. To ensure good electrochemical performance, it is necessary that each seal within the stack remains hermetic and stable over the operational lifetime of the device, which is anticipated to be on the order of 10,000 h or longer. Several methods are used currently in sealing pSOFC stacks, but none of them fully meets all of the design requirements of stack builders [1]. The most popular technique is glass joining. While this sealing technique is relatively simple and cost-effective [2–4], it yields a brittle seal that is susceptible to failure by thermal stresses induced by

rapid stack heating and cooling or thermal expansion mismatches between the sealing glass and the joining substrates [4]. An alternative to glass joining is compressive sealing [5–7], in which the seal is achieved by compressing a compliant high-temperature material such as mica between two sealing surfaces using an external load frame. Since the seal conforms to both sealing surfaces under constant compression and the sealing surfaces can slide past one another, thermal expansion matching between joining substrates is not required. However, this technology suffers due to the lack of a reliable high-temperature sealing material and difficulty in designing the load frame, which must be capable of delivering moderate-to-high loads in a high-temperature, oxidizing

Corresponding author. Tel.: +1 509 376 3372.

E-mail address: [email protected] (J.Y. Kim). 0360-3199/$ - see front matter Published by Elsevier Ltd. on behalf of International Association for Hydrogen Energy. doi:10.1016/j.ijhydene.2007.12.043

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environment over the entire period of stack operation. Even the most widely used sealing material, mica, is not adequate for this application due to through-seal leakage [5]. More recently, the reactive air brazing (RAB) technique was developed for use in sealing high-temperature electrochemical devices [8–10]. Unlike traditional ceramic-to-metal brazing (active metal brazing), which must be carried out in vacuum or reducing gas environment, the air brazing process takes place directly in ambient air without need of surface reactive fluxing agents or inert cover gases. One material system that has shown a particular promise for forming air braze filler materials is Ag–CuO. Recent studies have shown that a 1.4–8 mol% addition of CuO in Ag results in a good balance of wettability and adhesion on a variety of oxide surfaces, thereby producing joints that display good room temperature strength on the order of 60–80% of the base ceramic [8,10,11]. Ag–CuO filler materials can be used to braze together ceramic and heat resistant metal components directly in air, forming a joint that is inherently oxidation resistant. Unlike glass joining, the air brazed joint is ductile and can be heated and cooled at rapid rate from room temperature to 750  C through numerous cycles without failure [12]. However, there has been some concern with using silverbased materials for pSOFC applications. Silver is known to undergo high-temperature embrittlement, which occurs when it is simultaneously exposed to dual atmospheres, oxygen (or air) and hydrogen [13,14]. Both gaseous species are relatively soluble in silver and display rapid rates of diffusion within the interstitial void space of the silver lattice. Klueh and Mullins [13] were the first to observe that under certain conditions water vapor bubbles formed by the reaction of two diffused species would create porosity primarily along the grain boundaries of silver. It was reported in our previous publication that this type of hydrogen embrittlement was observed in the Ag–CuO filler material [15], although its development was much slower than that reported in highpurity silver tubes [14]. In the present study, aluminum has been added as an alloying agent in an effort to improve the dual atmosphere tolerance of Ag-based braze filler metals. Since metallic aluminum in the braze filler can preferentially react with oxygen, it may be possible to suppress pore formation by tying up the diffused oxygen before it can react with the diffused hydrogen. In the proper concentrations, the aluminum may additionally form a protective alumina scale on the air facing surface of the joint (and possibly on the water saturated H2 side), which can serve as a diffusion barrier to further oxygen ingress. A series of aluminum-added silverbased filler metals were prepared in the form of solid foils and mixed powder pastes to investigate the effects of aluminum additions on the microstructures, mechanical properties, and high temperature dual atmosphere tolerance of a set of model brazed Al2 O3 joints.

2.

Experimental

The braze filler metals were applied in two kinds of form, as foils or pastes. Based on the binary Ag–Al phase diagram four

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Table 1 – Braze filler compositions employed in this study Filler form Foil

Paste

Sample IDa

Al (at%)

Cu (at%)

Ag (at%)

LG01F LG02Fb LG05Fb LG10F

1 2 5 10

0 0 0 0

99 98 95 90

LG05Pb LG05C2P LG05C8Pb

5 5 5

0 2 8

95 93 87

a

‘‘L’’, ‘‘G’’, and ‘‘C’’ represent aluminum, silver, and copper, respectively. Numbers on the right side of ‘‘LG’’ indicate the at% of Al and those after ‘‘C’’ imply the at% of Cu. ‘‘F’’ and ‘‘P’’ at the end of sample IDs refer to foil and paste of braze fillers, respectively. b Samples tested for dual atmosphere tolerance.

braze filler metal compositions of binary aluminum–silver foils were developed with Al contents ranging from 1 to 10 at%, as reported in Table 1, where aluminum and silver exist in a solid solution. These compositions were formulated by ball-milling the appropriate amounts of aluminum powder (99.5%, Alfa Aesar) and silver powder (99.9%, Alfa Aesar). The powder mixtures were isostatically cold pressed into discs measuring approximately 25 mm in diameter by 2 mm high, which were sintered in dry hydrogen at 920  C for 2 h. After sintering, the surface oxide layer on the sintered discs was removed using a grinding tool and the discs were rolled into foils measuring 50 mm thick. To identify the effects of copper alloying additions on the microstructure and mechanical properties of the Ag–Al alloys, three different compositions ranging from 0 to 8 at% Cu were formulated using a base mixture of Ag–5 at% Al, as reported in Table 1. Pastes were then prepared by blending the mixture of copper, aluminum, and silver powders with a polymer binder (V006, Heraeus Inc.) in a weight ratio of 85–15. To prepare specimens for bend strength testing, polycrystalline alumina plates (Al23, 99.7%, Alfa Aesar) measuring 50 mm  25 mm  4 mm were brazed along the long edge to form a 50  50  4 mm plate. Brazing was conducted by heating the sandwich component in static air at 3  C=min to 1000  C, holding at 1000  C for 1 h, and cooling to room temperature at 3  C=min. Once joined, each sample was cut into rectangular bend bar specimens, each measuring 4  3  50 mm with the joint located midway along its lengths. Fourpoint bend testing was conducted according to C1161-02c ASTM test procedure. Alumina cap and tube specimens as described in our previous publication [15] were employed in exposure testing of the Ag–Al(–CuO) seal. The dual atmosphere exposure test was conducted by heating the braze-sealed end of the tube in air while circulating wet H2 gas within the tube. The cold end of the tube that extends outside of the muffle furnace was sealed using Swageloks compression fittings to ensure that the hydrogen could be supplied to the in-board side of the seal via a central tube and exhausted via a port without any

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external leaks. High purity (Al23, 99.7% Al2 O3 , Alfa Aesar) alumina tubes of 25 mm in outer diameter and 18 in long with 2 mm wall thickness were used in the study. The alumina discs employed as caps were also high purity (Al23, 99.7% Al2 O3 , Alfa Aesar) and measured 25 mm in diameter by 4 mm thick. The faying surfaces of both the tubes and caps were cleaned with isopropanol and air dried prior to brazing. Selected compositions of the binary and ternary alloys listed in Table 1 were used in joining an alumina tube to an alumina disc by placing a braze foil ring or applying a paste in between. The weight of the tube ð300 gÞ on the braze filler and disc served as a dead load, ensuring good contact during brazing. The assembly was heated in air using the same brazing schedule reported above. After brazing, each sealed tube was exposed to a dual atmosphere environment at 800  C for 1000 h. Ultra high purity hydrogen (UHP-H2 ) was bubbled through a water prior to entering the alumina tube at the flow rate of 30 sccm. The specimens were heated at 2  C=min to 800  C and held at this temperature for 1000 h. After testing, each specimen was cut 2 mm from the braze bond line to remove the cap and adjacent joining region. The resulting piece was cut into half to observe the cross-sections of the joint in microstructural analysis. For comparison, alumina discs (20 mm diameter, 3 mm thick) were also joined for each filler metal of interest and subsequently crosssectioned to examine the microstructure of the as-brazed joints. Microstructural analysis was performed using a scanning electron microscope (SEM, JEOL JSM-5900LV) equipped with an Oxford energy dispersive X-ray spectrometer (EDX), which employs a windowless detector for quantitative detection of both light and heavy elements. To identify the mechanisms of failure in the bend bar specimens, the fracture surfaces of the failed bars were examined via SEM and EDX.

3.

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Results and discussion

3.1. Microstructure and mechanical properties of asbrazed joints Shown in Figs. 1(a)–(d) are the cross-sectional SEM micrographs of as-brazed specimens joined with four different binary Ag–Al alloy foil compositions. As seen in Fig. 1(a), the joint brazed with the 1 at% Al filler metal contains alumina particles both within the Ag matrix and along the interface between the filler metal and the alumina substrates. EDX analysis conducted on the metallic braze filler matrix at Spot ‘‘1’’ in Fig. 1(a) revealed no metallic aluminum, as reported in Table 1, implying that all the aluminum in the braze foil was consumed via oxidation during the brazing process. Alumina particulate are also found in the 2 at% Al joint (LG02F), Fig. 1(b), but a small amount of unoxidized aluminum, 1:2 at% as given in Table 2, remains alloyed in the filler metal matrix. Alumina discs joined with filler metals containing higher aluminum concentrations, for example, with 5 at% Al in Fig. 1(c) and with 10 at% Al in Fig. 1(d), appear to possess greater alumina particle content, particularly along or near the filler metal/substrate interface. The concentration of metallic aluminum in the filler metal matrix also increases, 3.65 at% for LG05F and 7.73 at% for LG10F, as reported in Table 2. Shown in Fig. 2 is a plot of the bend strength for alumina bars joined with binary filler metal foils as a function of aluminum content. Joints brazed with a pure Ag foil exhibit an average bend strength of 78 MPa. The average bend strengths of the joints decrease with increasing aluminum content in the filler metal, from 64 MPa for bars joined with a 1 at% Al containing foil (LG01F) to 1.77 MPa for those joined with a 10 at% Al containing foil (LG10F). To understand this

Fig. 1 – Cross-sectional SEM micrographs of as-brazed alumina joints brazed with Ag–Al foils: (a) LG01F, (b) LG02F, (c) LG05F, and (d) LG10F.

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Table 2 – Results of EDX quantitative analysis conducted on the braze filler matrix in Figs. 1 and 4 Elementa

‘‘1’’ Fig. 1(a) (LG01F)

‘‘2’’ Fig. 1(b) (LG02F)

‘‘3’’ Fig. 1(c) (LG05F)

‘‘4’’ Fig. 1(d) (LG10F)

‘‘5’’ Fig. 4(a) (LG05P)

‘‘6’’ Fig. 4(a) (LG05P)

‘‘7’’ Fig. 4(b) (LG05C8P)

‘‘8’’ Fig. 4(b) (LG05C8P)

0.00 100.0 –

1.22 98.78 –

3.65 96.35 –

7.73 92.27 –

0.84 99.16 –

1.26 98.74 –

0.00 100.00 0.00

0.53 99.47 0.00

Al K Ag L Cu K a

All compositions listed are in at%.

Bend Strength (MPa)

160 140 120 100 80 60 40 20 0

-2

0

2

4 6 Al content (at%)

8

10

12

Fig. 2 – Room temperature 4-point bend strength of alumina bars joined with Ag–Al foils.

trend, the fracture surfaces of broken alumina bars joined with foils containing 2 at% Al (LG02F) and 5 at% Al (LG05F) were observed under the SEM. Shown in Figs. 3(a) and (b) are the corresponding halves of an LG02F joint. Those of a failed LG05F joint can be seen in Figs. 3(c) and (d). In both cases, the primary mode of failure appears to be de-bonding along the filler metal/alumina substrate interface. Aluminum oxide particles formed along this interface are found on the both sides of the broken bars, indicating that the fracture runs through this portion of the joint and the particles are the likely source of interfacial weakening. Thus, when additional interfacial alumina is formed by increasing the amount of aluminum in the braze foil (e.g. LG10F) the interfacial bond strength decreases further, which is reflected by the reduction in joint strength. Examples of the cross-sectional microstructures of alumina joints brazed with binary and ternary pastes are shown in Figs. 4(a) and (b). In comparing joints brazed with foil, for example, LG05F in Fig. 2(c), with those joined using a paste of the same composition, LG05P in Fig. 4(a), a fewer number of larger-sized alumina particles are formed at or near the filler metal/substrate interfaces. Note also from Table 2 that less metallic aluminum was found in the paste-formed metallic braze filler matrix, 0.84–1.26 at% Al for LG05P vs. 3.65 at% for the comparable foil-formed specimen LG05F. In other words, the porous nature of the powders in the braze paste permits greater aluminum oxidation to take place during the air brazing process. This is not surprising, since the oxidation of the aluminum in the foils is eventually limited by the

diffusion of dissolved oxygen through a depleted zone established just in-board of the alumina particulate region. In the case of the ternary pastes, e.g. that shown in Fig. 4(b) containing 5 at% Al and 8 at% Cu, aluminum not only appeared to fully oxidize but also react with copper oxide to form a copper aluminate phase, CuAl2 O4 , as identified by EDX. Some unreacted copper oxide was also detected as particulate within the bulk braze filler and along the filler metal/substrate interfaces. As indicated by spot analysis, Spots ‘‘7’’ and ‘‘8’’ in Fig. 4(b), the filler metal matrix contains a very low concentration of metallic aluminum, 0–0.53 at% Al. Although the underlying mechanism is not yet clear, the addition of copper seems to enhance the oxidation of aluminum. One possibility may be that the diffusion of oxygen in the molten Ag–CuO–Al liquid may be faster than in molten Ag–Al, although additional work is required to verify this hypothesis. The bend strengths of alumina joints brazed with ternary filler metals containing 5 at% Al are plotted as a function of Cu content in Fig. 5. Interestingly, the joints brazed with the binary paste (LG05P) display a higher average bend strength (70 MPa) than those using foils of the same composition (LG05F; 10 MPa). This may be related to the fact that less interfacial Al2 O3 particulate is found in the joints brazed using a paste, as is clear by comparing Figs. 1(c) and 4(a). Bend strength increases with copper (oxide) content, a trend which has been previously reported with Ag–CuO alloys and is due to an improvement in filler metal wettability [11]. Even though the addition of copper oxide is known to increase the strength of silver-brazed joints in this fashion, the concern is that it may adversely influence the dual atmosphere tolerance of Ag–Al based air braze filler metals, given that much of the aluminum in the ternary brazing alloys was found to be oxidized.

3.2.

Dual atmosphere tolerance

Shown in Figs. 6(a)–(d) is the microstructure of a typical Ag–CuO braze filler after a dual atmosphere test at 800  C for 1000 h. This result, which was reported in an early publication [14], was obtained on a specimen brazed using a filler metal containing 2 at% of Cu (oxide). Results from local EDX analyses at the numbered points in the figure are reported in Table 3. The low magnification cross-sectional image displayed in Fig. 6(a) exhibits large air pockets in the filler material region of the joint, which is formed due to the limited wettability of Ag–2 mol% CuO liquid on the alumina

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Fig. 3 – Fracture surfaces of the two corresponding halves of fractured alumina bars joined with the Ag–Al braze filler foils: (a) and (b) LG02F, (c) and (d) LG05F.

Bend Strength (MPa)

140 120 100 80 60 40

5 mol% Al

20 0

-2

0

2 4 6 Cu content (mol%)

8

10

Fig. 5 – Room temperature 4-point bend strength of alumina bars joined with Ag–Al(–Cu) pastes.

Fig. 4 – Cross-sectional SEM micrographs of as-brazed alumina joints brazed with Ag–Al(–Cu) pastes: (a) LG05P (5 at% Al) and (b) LG05C8P (5 at% Al + 8 at% Cu).

substrate and the low viscosity of the liquid [15]. The airexposed side of the joint shown in Fig. 6(b) exhibits a microstructure similar to that of the as-brazed joint, contain-

ing copper oxide particles in the pure Ag matrix (refer to Spot ‘‘1’’ in Table 3). However, on the right-hand side of this image, some porosity is observed and little copper oxide is found. EDX analysis of Spot ‘‘2’’ indicates that the filler metal residing between the two pores in this micrograph is composed of 0.85 at% of copper in silver. It has been previously reported that no porosity forms due to the reduction of copper oxide in this composition [16]. Thus this porosity is presumably due to the dual atmosphere embrittlement mechanism, i.e. the formation of porosity resulting from the reaction between oxygen and hydrogen in silver originally described by Klueh and Mullins [13]. At a region located 0.79 mm from the air exposed side of the joint, shown in Fig. 6(c), porosity is also present and found predominantly along the grain boundaries of the silver matrix. EDX analysis indicates that the filler metal at Spot ‘‘3’’ contains 2.08 at% of copper in silver. At a region closer to the hydrogen-exposed side of the joint, the microstructure consists only of a

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Fig. 6 – A capsule sealed with Ag–CuO braze filler (2 mol% CuO) at 980  C, exposed under dual atmospheres at 800  C for 1000 h: (a) low magnification, (b) air side, (c) 0.79 mm from air, and (d) 1.10 mm from air [14].

Table 3 – Results of EDX quantitative analysis conducted on the samples sealed with 2 at% Cu containing Ag braze filler after being exposed to dual atmosphere for 1000 h (Fig. 6) [14] Elementa

‘‘1’’

‘‘2’’

‘‘3’’

‘‘4’’

Ag L Cu K

100 –

99.15 0.85

97.92 2.08

98.13 1.87

a

All compositions listed are in at%.

copper–silver filler metal, with no evidence of porosity or CuO precipitates. That is in the joints brazed with Ag–2 at% Cu, internal pore formation caused by the dual atmosphere embrittlement mechanism is localized toward the air-exposed side of the joint, even though the reduction of copper oxide in the original filler metal suggests that hydrogen has diffused nearly completely across the entire width of the sample. It is speculated that this is due to the difference in diffusion rate between oxygen and hydrogen and solubility of each species in the filler metal. Shown, respectively, in Figs. 7 and 8 are the microstructures of the specimens brazed with filler metals containing 2 and 5 at% Al that were exposed in dual atmosphere for 1000 h. Compared to specimens brazed with the Ag–CuO filler metal, the Ag–Al joints contain no large air pockets, as seen in Figs. 7(a) and 8(a). Suppression of air pocket formation is related to the increase in the viscosity of Ag–Al braze filler metals [17]. The air-exposed side of the specimen brazed with the Ag–2Al (LG02) foil shown in Fig. 7(b) exhibits a microstructure similar to that of the as-brazed joint in Fig. 2(b). Note that no metallic aluminum is detected in the filler metal, as indicated by the EDX data collected from Spot ‘‘1’’ (Fig. 7(a))

reported in Table 4, suggesting complete oxidation of the aluminum. At a region located 0.83 mm from the air-exposed side of the joint, porosity is present along the grain boundaries of the Ag matrix, as seen in Fig. 7(c). This zone of porosity extends to a region located 1.65 mm from airexposed side (Fig. 7(d)). Since the as-brazed joint exhibits no porosity, the pores found in Figs. 7(b) and (c) signify dual atmosphere embrittlement within the joint. As reported in Table 4, the metal matrix within this region (Spot ‘‘2’’ in Fig. 7(c)) contains a lower Al content (0.65 at% Al) than a region outside of this zone toward the hydrogen-exposed side (Spot ‘‘3’’ in Fig. 7(d); 1.44 at% Al). Even though the porosity forms under dual atmosphere exposure, the porosity found in the Ag–2Al filler metal is finer in size than that observed in Ag–CuO joint. Both findings suggest that the aluminum addition does reduce the formation of dual atmosphere induced porosity, possibly due to the preferential reaction between metallic aluminum in the filler metal and soluble oxygen that has diffused into the metal at the air side of the joint. Shown in Figs. 8(a)–(d) are cross-sectional images of a dual atmosphere exposed specimen formed using the Ag–5Al filler metal (LG05F). The air-exposed side contains a large porosity. EDX analysis conducted on the filler metal matrix at Spot ‘‘1’’ in Fig. 8(b) reveals no metallic aluminum, as listed in Table 4, indicating that all of the aluminum on the air-exposed side of this specimen has been completely oxidized. Thus, this area is vulnerable to dual atmosphere embrittlement when hydrogen diffuses through the braze filler. Porosity caused by dual atmosphere embrittlement is also detected in the regions located 0.56 mm, Fig. 8(c), and 0.86 mm, Fig. 8(d), from the air-exposed side of the joint. As with the Ag–2Al specimen, EDX analysis results listed in Table 4 indicate that the filler metal matrix contains a lower concentration of metallic aluminum inside the porous dual atmosphere affected zone than in a region adjacent to it, 0.77 at% at Spot ‘‘2’’ in Fig. 8(c)

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Fig. 7 – A capsule sealed with the LG02 foil (2 at% Al) at 1000  C, exposed under dual atmospheres at 800  C for 1000 h: (a) low magnification, (b) air side, (c) 0.83 mm from air, and (d) 1.65 mm from air.

Fig. 8 – A capsule sealed with the LG05 foil (5 at% Al) at 1000  C, exposed under dual atmospheres at 800  C for 1000 h: (a) low magnification, (b) air side, (c) 0.56 mm from air, and (d) 0.86 mm from air.

Table 4 – Results of EDX quantitative analysis conducted on the samples sealed with Ag–Al foils after dual atmosphere exposure for 1000 h (Fig. 7: LG02F and Fig. 8: LG05F) Elementa

Al K Ag L a

Fig. 7

Fig. 8

‘‘1’’

‘‘2’’

‘‘3’’

‘‘1’’

‘‘2’’

‘‘3’’

0.00 100.00

0.65 99.35

1.44 98.56

0.00 100.0

0.77 99.23

2.53 97.47

All compositions listed are in at%.

versus 2.53 at% Al at Spot ‘‘3’’ in Fig. 8(d). The size of porosity shown in Figs. 8(c) and (d) appears to be even smaller than that found in the Ag–2Al specimen. Additionally, the porous dual atmosphere affected zone appears to have shifted closer to the air side of the joint. The latter two observations suggest that the greater concentration of metallic aluminum available in the LG05F specimen further inhibits dual atmosphere induced pore formation, although the phenomenon is not completely eliminated. Shown in Figs. 9(a)–(e) are the results of microstructural analysis conducted on a dual atmosphere exposed specimen

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Fig. 9 – A capsule sealed with the LG05 paste (5 at% Al) at 1000  C, exposed under dual atmospheres at 800  C for 1000 h: (a) low magnification, (b) air side, (c) 0.78 mm from air, (d) 1.16 mm from air, and (e) 1.67 mm from air.

Table 5 – Results of EDX quantitative analysis conducted on the samples sealed with Ag–Al(–Cu) pastes after dual atmosphere exposure for 1000 h (Fig. 9: LG02P and Fig. 10: LG02C8P) Elementa

Al K Ag L CuK a

Fig. 9

Fig. 10

‘‘1’’

‘‘2’’

‘‘3’’

‘‘4’’

‘‘1’’

‘‘2’’

‘‘3’’

0.00 100.00 –

0.61 99.39 –

0.99 99.01 –

0.91 99.09 –

0.00 100.0 –

0.00 98.47 1.53

0.00 95.03 4.97

All compositions listed are in at%.

brazed using the Ag–5Al paste, instead of foil. The low magnification image in Fig. 9(a) shows that there are no large air pockets in the joint. As was found in the other Ag–Al specimens, no metallic aluminum was observed in the filler metal matrix on the air-exposed side of the joint, reported as Spot ‘‘1’’ in Table 5. A small amount of fine-scale porosity formed by dual atmosphere embrittlement is found in a region extending 0:78 mm (Fig. 9(c)) to 1:16 mm (Fig. 9(d)) from the air-exposed side. Again, the size and amount of porosity indicate that the addition of aluminum to the filler metal helps reduce dual atmosphere induced porosity in the joint.

The effect of copper oxide additions on the dual atmosphere tolerance of the Ag–Al based filler metals was investigated using a paste containing 5 at% Al and 8 at% Cu. Results from microstructural and compositional analyses are reported in Figs. 10(a)–(d) and Table 5, respectively. Observed on the air-exposed side of the joint are copper oxide and copper aluminate particles in a pure silver matrix (Spot ‘‘1’’ in Fig. 10(b)), both within the bulk of the joint and along the filler metal/substrate interfaces. Coarse porosity is found internally within the joint in Region ‘‘C’’ of Fig. 10(a), shown at a higher magnification in Fig. 10(c). The filler metal matrix at Spot ‘‘2’’ contains metallic copper but no metallic aluminum,

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Fig. 10 – A capsule sealed with the LG05C8 paste (5 at% Al, 8 at% Cu) at 1000  C, exposed under dual atmospheres at 800  C for 1000 h: (a) low magnification, (b) air side, (c) 0.55 mm from air, and (d) H2 side.

as indicated in Table 5. A pore-free region toward the hydrogen-exposed side of the joint is shown in Fig. 10(d). No detectable amount of metallic aluminum is observed in the filler metal at Spot ‘‘3’’ in this figure. Again, because the hydrogen-exposed side of the joint exhibits no porosity, the large-scale porosity observed in Fig. 10(c) signifies internal dual atmosphere embrittlement. This suggests that the addition of copper to Ag–Al reduces the filler metals’ dual atmosphere tolerance. That is copper degrades the active role that aluminum plays in scavenging oxygen within the filler metal and delaying its diffusion across the joint

4.

Summary and conclusions

The microstructures, mechanical properties, and high temperature dual atmosphere tolerance of various Ag–Al based air braze filler metals were investigated in this study. In the case of binary Ag–Al braze foils containing more than 2 at% Al, metallic aluminum was found to persist through the air brazing process and could be detected within filler metal. Unfortunately, the bend strengths of specimens joined using the binary Ag–Al filler metal compositions decreased with increasing aluminum content. This was due to the formation of interfacial aluminum oxide, which led to interfacial debonding between the braze filler and the alumina substrate. However, the existence of metallic aluminum in the filler metal matrix enhanced the high temperature dual atmosphere tolerance of the resulting joint. The size of the dualatmosphere induced porosity was measurably smaller and the amount of porosity significantly smaller in the aluminum alloyed filler metals than in those containing no aluminum. In addition, a set of pastes based on the binary and ternary Ag–Al(–Cu) filler metal compositions were examined as potential sealants. Alumina jointing specimens fabricated using these pastes displayed increased bend strength with

increasing copper content in the original paste formulation. However, after air brazing, the concentration of residual metallic aluminum in the final joint appeared to decrease as the amount of copper in the filler metal was raised. Subsequent exposure testing demonstrated that the porous dual atmosphere affected zone was more extensive in the Ag–Al–Cu brazed specimens than in the those brazed using Ag–Al filler metals. Thus, the conclusion from this initial series of tests is that the addition of aluminum to a silver braze filler metal degrades joint strength, but increases dual atmosphere tolerance. Alternatively, the addition of copper to Ag–Al air braze filler metals increases joint strength, but tends to degrade the dual atmosphere tolerance of the resulting joints.

Acknowledgments The authors would like to thank Douglas A. Conner for bending sample preparation. This work was supported by the U.S. Department of Energy, Solid-state Energy Conversion Alliance (SECA) Core Technology Program. R E F E R E N C E S

[1] Weil KS. JOM 2006;58:37. [2] Eichler K, Solow G, Otschik P, Schaffrath W. Degradation effects at sealing glasses for the SOFC. In: McEvoy AJ, editor. Proceedings of the fourth European solid oxide fuel cell forum. Switzerland: Oberrohrdorf; 2000. p. 899–906. [3] Ley K, Krumpelt M, Kumar R, Meiser J, Bloom I. J Mater Res 1996;11:1489. [4] Yang ZG, Xia GG, Meinhardt KD, Weil KS, Stevenson JW. J Mater Eng Perf 2004;13:327. [5] Simner SP, Stevenson JW. J Power Sources 2001;102:310. [6] Chou YS, Stevenson JW. J Power Sources 2002;112:376.

ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

[7] Chou YS, Stevenson JW. J Power Sources 2003;115:274. [8] Weil KS, Kim JY, Hardy JS. Electrochem Solid-State Lett 2005;8:A133. [9] Weil KS, Hardy JS, Rice JP, Kim JY. Fuel 2006;85:156. [10] Kim JY, Hardy JS, Weil KS. J Electrochem Soc 2005;152:J52. [11] Kim JY, Hardy JS, Weil KS. J Am Ceram Soc 2005;88:2521. [12] Weil KS, Coyle CA, Darsell JT, Xia GG, Hardy JS. J Power Sources 2005;152C:97. [13] Kleuh RL, Mullins WW. Trans Metall Soc AIME 1968;242:237.

33 (2008) 3952 – 3961

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[14] Singh P, Yang ZG, Viswanathan V, Stevenson JW. J Mater Eng Perf 2004;13:287. [15] Kim JY, Hardy JS, Weil KS. Dual atmosphere tolerance of Ag–CuO based air braze. Int J Hydrogen Energy 2007; 32(16):3655. [16] Kim JY, Hardy JS, Weil KS. J Mater Res 2006;21:1434. [17] Kim JY, Hardy JS, Weil KS. Ag–Al based air braze for high temperature electrochemical devices. Int J Hydrogen Energy 2007;32(16):3754.