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Dual-atmosphere tolerance of Ag–CuO-based air braze
International Journal of Hydrogen Energy 32 (2007) 3655 – 3663 www.elsevier.com/locate/ijhydene
Dual-atmosphere tolerance of Ag–CuO-based air braze Jin Yong Kim, John S. Hardy, Scott Weil ∗ Pacific Northwest National Laboratory, Richland, WA 99352, USA Available online 12 October 2006
Abstract Recently, a new braze filler metal based on the silver–copper oxide system was developed for use in sealing high-temperature, solid-state electrochemical devices such as solid oxide fuel cells. One of the concerns regarding the viability of this joining technique is the long-term stability of silver-based alloys under a high-temperature, dual oxidizing/reducing gas environment. This paper reports on an initial series of exposure experiments that were conducted to characterize the effects of (1) filler metal composition, (2) brazing temperature, and (3) exposure time on the microstructural stability of Ag–CuO-brazed Al2 O3 /Al2 O3 joints under a prototypic operating environment for an intermediate temperature solid oxide fuel cell stack. In general joints exposed simultaneously to air on one side and hydrogen on the other for short periods of time at 800 ◦ C (100 h) showed no signs of degradation with respect to hermeticity or joint microstructure. Samples exposed for longer periods of time (1000 h) displayed some internal porosity, which extends approximately halfway across the joint and is not interconnected. Little effect of the filler metal’s composition on its tolerance to dual-atmosphere exposure was observed. However, brazing temperature was found to have a measurable effect. Higher brazing temperature leads to a more extensive formation of an interfacial reaction phase, copper aluminate, which tends to tie up some of the free CuO in the filler metal and minimize the formation of porosity in the air-brazed joints during long-term, dual-atmosphere exposure. The effect is due to the greater chemical stability of the copper aluminate relative to copper oxide. Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. Keywords: Ag–CuO braze; Dual-atmosphere; Microstructure
1. Introduction One of the key components in the design of highperformance intermediate temperature planar solid oxide fuel cell (IT-pSOFC) stacks is the seal between the metal and ceramic cell components (i.e. a “cell-to-frame seal”) in the stack. A significant engineering challenge in fabricating these devices is how to make this seal hermetic, rugged, and stable during continuous high-temperature use over its operational lifetime, which is anticipated to be on the order of 10,000 h or longer depending on the application. At present, there are only a few methods that are used in sealing pSOFC stacks, none of which fully meets the design requirements of stack builders. The most popular technique is glass joining. While this method of sealing is relatively simple and cost-effective [1–3], it yields a final seal that is brittle and therefore susceptible to failure by thermal stresses induced during rapid stack heating and ∗ Corresponding author. Tel.: +1 509 376 6796; fax: +1 509 375 2186.
E-mail address: [email protected] (K.S. Weil).
cooling or due to thermal expansion mismatches between the sealing glass and the joining substrates [3]. Compressive seals employ deformable materials that do not bond to the pSOFC components but instead serve as gaskets [4–6]. Thus, sealing is achieved when the entire stack is compressively loaded. Because the sealing material conforms to the adjacent surfaces and is under constant compression during use, it forms a dynamic seal. That is, the sealing surfaces can slide past one another without a disruption in hermeticity and the individual stack components are free to expand and contract during thermal cycling with no need to consider CTE matching. This offers stack designers greater freedom in utilizing alloys other than ferritic stainless steels for the metal components. The gaskets are readily produced and easy to apply. Additionally, they offer the potential for mid-term stack repair by releasing the compressive load, disassembling the stack, and replacing the damaged cell or separator components. However, in order to employ compressive seals in a pSOFC stack, a load frame is required to maintain the desired level of compression on the stack over the entire period of operation and the stack
0360-3199/$ - see front matter Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. doi:10.1016/j.ijhydene.2006.08.054
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components must be capable of withstanding the sealing load. The load frame introduces several complexities in stack design, including: oxidation of the frame material, load relaxation due to creep, and increased weight and thermal mass (and therefore reduced specific power and thermal response of the overall system). In addition to increased system cost, these factors seriously limit the use of compressive seals in mobile applications. A third option recently developed for sealing hightemperature electrochemical devices is air brazing [7]. Originally developed as a means of sealing tubular hydrogen and oxygen separation membranes to the respective metallic gas manifold structure in a coal gasification system [8], the technique is currently being examined for its potential use in sealing planar SOFCs [9]. Unlike traditional ceramic-to-metal brazing (active metal brazing), which must be carried out in vacuum or reducing gas environment, air brazing takes place directly in air without need of surface reactive fluxing agents or inert cover gases. One material system that has shown particular promise for forming air braze filler metals 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 room temperature strengths on the order of 60%–80% of the base ceramic [7,9,10]. Like glass sealing, Ag–CuO filler metals can be used to braze together components directly in air, forming a joint that is inherently oxidation resistant. When the filler metal composition and braze temperature are optimized, the air-brazed joint is ductile and can be heated and cooled at rapid rate from room temperature to upwards of 750 ◦ C through numerous cycles without failure [11]. However, there is a concern with using silver-based alloys for pSOFC applications. Silver is known to undergo a form of high-temperature embrittlement, which occurs when it is simultaneously exposed to oxygen (or air) and hydrogen [12,13]. 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 were the first to observe that under certain conditions water vapor bubbles will form due to reaction between the two diffused species primarily along the grain boundaries of silver [12]. In the present investigation, a series of dual-atmosphere exposure experiments were conducted to determine if this effect takes place in the Ag–CuO braze alloys and if so, identify whether the mechanism of microstructural evolution is similar to that of high-purity silver. 2. Experimental Alumina cap and tube specimens of the type shown in Fig. 1 were employed in exposure testing of the Ag–CuO seal. Although in the SOFC stack it is envisioned that the air braze seal will be between YSZ and ferritic stainless steel, given extensive prior filler metal testing with alumina [10], this material was selected as a surrogate substrate to isolate the primary effects of dual-atmosphere exposure on the filler metal. The dual-atmosphere exposure condition was established by heating the braze-sealed end of the pipe in air while circulating wet H2 gas within the pipe. The cold end of the pipe that extends
H2 in H2 out Alumina tube Air
Furnace
Ag-CuO braze ring Alumina cap Fig. 1. A schematic of the dual-atmosphere testing apparatus.
outside of the muffle furnace shown in Fig. 1 was sealed using Swagelok䉸 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 external leaks. High-purity (99.7% Al2 O3 , ∼99% dense; Alfa Aesar) alumina tubes measuring 25 mm in outer diameter ×18 in long with a 2 mm wall thickness were used in the study. The alumina discs employed as caps were also high-purity (99.7% Al2 O3 , 98% dense; Alfa Aesar) and measured 25 mm in diameter by 4 mm thick. The faying surfaces of both the tubes and caps were cleaned with acetone and air dried prior to brazing. Two air braze filler metal compositions were employed in air brazing: 2 and 8 mol% CuO in Ag, referred to as CA02 and CA08, respectively. These were formulated by ball-milling the appropriate amounts of copper powder (99%, Alfa Aesar) and silver powder (99.9%, Alfa Aesar) and isostatically cold pressing the mixtures into discs measuring approximately 25 mm in diameter by 2 mm high. The discs were sintered in air at 920 ◦ C for 2 h, during which the copper fully oxidized to CuO [10]. After sintering, the discs were rolled into foils measuring ∼50 m thick, then cut into 25 mm O.D. rings each with a wall thickness of 2 mm. Joining was conducted by placing a braze foil ring between an alumina disc and the faying surface of an alumina tube. The weight of the tube (∼300 g) on the braze foil and disc served as a dead load, ensuring good contact during brazing. The assembly was heated in air at 2 ◦ C/ min to a final braze temperature of either 980 or 1100 ◦ C and held at temperature for 30 min before furnace cooling to room temperature. Two different brazing temperatures were employed to determine the role that temperature has on the extent of interfacial reaction between the filler metal and the alumina substrates and whether the presence of a reaction zone affects the dual-atmosphere tolerance of the resulting joint. After brazing, each sealed tube was exposed to a dual-atmosphere environment at 800 ◦ C for 100 or 1000 h. Ultra high-purity hydrogen (UHP-H2 ) was bubbled through 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 the duration of the test. Three variables were examined in this set of experiments: (1) braze filler composition, 2 mol% CuO in Ag (CA02) vs. 8 mol% CuO in Ag (CA08); (2) brazing temperature,
J.Y. Kim et al. / International Journal of Hydrogen Energy 32 (2007) 3655 – 3663
980 vs. 1100 ◦ C; and (3) period of exposure at 800 ◦ C, 100 vs. 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 quartered to allow examination of several cross-sections during microstructural analysis. For comparison, as-brazed alumina discs (20 mm diameter, 3 mm thick) joined at each brazing condition (i.e. filler metal composition and braze temperature) were also cross-sectioned and examined. Microstructural analysis was performed using a scanning electron microscope (SEM, JEOL JSM-5900LV) equipped with an Oxford energy dispersive X-ray spectrometry (EDS) system,
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which employs a windowless detector for quantitative detection of both light and heavy elements. 3. Results and discussion 3.1. Microstructure of the as-brazed joints Shown in Figs. 2 and 3 are cross-sectional SEM micrographs of specimens in the four different as-brazed conditions. Low magnification images collected from alumina joints brazed with a 2 mol% CuO filler metal, shown in Figs. 2(a) and (c) after brazing at 980 and 1100 ◦ C, respectively, display the presence
Fig. 2. Cross-sectional SEM micrographs of the alumina joints brazed with CA02 (2 mol% CuO): (a) and (b) at 980 ◦ C (a: X40, b: X1000), (c) and (d) at 1100 ◦ C (c: X40, d: X500).
Fig. 3. Cross-sectional SEM micrographs of the alumina joints brazed with CA08 (8 mol% CuO): (a) and (b) at 980 ◦ C (a: X40, b: X1000), (c) and (d) at 1100 ◦ C (c: X40, d: X500).
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of large air pockets. These defects form during joining presumably due to insufficient wetting between this braze filler and alumina [14]. The joint brazed at 980 ◦ C contains a small amount of copper oxide both in the braze filler and at the braze/substrate interface, as seen in Fig. 2(b). SEM examination of specimens joined at 1100 ◦ C reveals the presence of copper aluminate, CuAl2 O4 , in addition to copper oxide observed in Fig. 2(d). Thus, the higher brazing temperature facilitates reaction between the copper oxide in the braze filler and the alumina substrate. Air pockets were also found in joints brazed with an 8 mol% CuO filler metal, as seen in Figs. 3(a) and (c). In general, however, the size of these flaws was substantially smaller than those observed in the CA02 joints, likely because the increased amount of CuO leads to enhanced wetting behavior [7,9,14]. At higher magnification, shown in Fig. 3(b), it is apparent that the CA08 sample brazed at 980 ◦ C contains significantly more copper oxide than the comparable CA02 joint. Also it is noted that a small amount of copper aluminate is present in this specimen, implying that the higher CuO content appears to promote the formation of copper aluminate. This is likely due to the liquid phase separation that occurs in the CA08 composition at the brazing temperature, forming two liquids one rich in silver (L1) and the other in CuO (L2) [15]. When this occurs, it has been observed that the L2 liquid will preferentially wet the alumina substrate [7,10,14]. It is speculated that the high activity of the CuO in the wetting L2 phase promotes the reaction that forms CuAl2 O4 . This reaction is even more extensive when the joint is formed at 1100 ◦ C, as seen in Fig. 3(d). 3.2. Effects of exposure time and filler metal composition Shown in Figs. 4 and 5 are the microstructural results obtained on specimens that were brazed at 980 ◦ C using the
CA02 filler metal and then exposed at 800 ◦ C to the dual wet H2 /ambient air environment for 100 and 1000 h, respectively. Results from EDS analyses that correspond to the numbered regions in these two figures are provided in Table 1. Displayed in Fig. 4(a) is a low magnification cross-sectional image of the CA02 sample after 100 h of exposure. The width of the brazed joint is ∼2.12 mm and as was observed in the comparable as-brazed condition (Fig. 2(a)), there are large air pockets in the filler metal region of the joint. The microstructure on the air-exposed side of the sample is shown at higher magnification in Fig. 2(b). EDS measurements indicate that this region consists of copper oxide particles in a pure silver matrix, similar to the as-brazed specimen shown in Fig. 2(b). A region further inside of the joint, located 0.51 mm from the airexposed side or 1.61 mm from the H2 -exposed side, exhibits essentially the same microstructure, indicating that it was not influenced by dissolved hydrogen. However, in a region closer to the hydrogen-exposed side of the joint, shown in Fig. 4(d) at 0.67 mm from the H2 side, the lack of copper oxide precipitates is notable. EDS analysis of the filler metal (Spot “3”) indicates that it is composed of ∼1.9 at% copper in silver. This result suggests that during the 100 h of exposure, hydrogen diffuses into this side of the joint up to this point and reduces the original copper oxide precipitates to metallic copper, which subsequently alloys with the surrounding silver forming a solid solution. Even though approximately half of the joint experiences hydrogen-induced reduction of this type, there is no indication of hydrogen embrittlement or the formation of porosity within the silver of the type observed by Klueh and Mullins [12]. The cross-sectional micrographs shown in Fig. 5 display the microstructure of the CA02 specimen that was exposed for 1000 h. In this case, the width of the joint is ∼2.26 mm, with several air pockets present in the low magnification image of Fig. 5(a). As seen in Fig. 5(b), the air-exposed side of the
Fig. 4. A capsule sealed with CA02 (2 mol% CuO) at 980 ◦ C, exposed under dual atmospheres at 800 ◦ C for 100 h: (a) low magnification, (b) air side, (c) 0.51 mm from air, and (d) 1.45 mm.
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Fig. 5. A capsule sealed with CA02 (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.
Table 1 Results of EDS quantitative analysis conducted on the samples sealed with CA02 (2 mol% CuO) at 980 ◦ C (Fig. 4: exposed for 100 h and Fig. 5: exposed for 1000 h) Elementa
Ag L Cu K a All
Fig. 4
Fig. 5
1
2
3
1
2
3
4
100.00 —
100 —
98.13 1.87
100 —
99.15 0.85
97.92 2.08
98.13 1.87
compositions listed are in at%.
joint exhibits a microstructure comparable to that of the asbrazed joint. However on the right-hand side of this image, some porosity is present and no copper oxide precipitates are observed. EDS 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, as listed in Table 1. This result suggests that over the 1000 h exposure period, hydrogen was able to diffuse nearly through the entire width of the joint. At a region located 0.79 mm from the air-exposed side of the joint, porosity is also present and appears to occur along the grain boundaries of the resulting silver–copper alloy, as shown in Fig. 5(c). EDS analysis indicates that the filler metal (Spot “3”) contains 2.08 at% of copper in silver. Again, reduction of copper oxide precipitates originally in this region appears to have occurred due to hydrogen diffusion across the joint. At a region closer to the hydrogen-exposed side of the joint (1.10 mm from the air side, as seen in Fig. 5(d)), the microstructure displays only the copper–silver filler metal but no evidence of any porosity or CuO precipitates. That is, in CA02 joints brazed at 980 ◦ C and exposure tested for 1000 h internal pore formation is localized toward the air-exposed side of the joint side, whereas after only 100 h of exposure no pore formation
takes place. It was also noted in the 1000 h CA02 exposure specimen that porosity is found less than a halfway across 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. Shown in the micrographs of Fig. 6 is the microstructure of the CA08 joining specimen brazed at 980 ◦ C after exposure at 800 ◦ C for 100 h. Similar to what was noted in the comparison between the as-brazed specimens, the low magnification image in Fig. 6(a) reveals the presence of a couple of air pockets in the joint, but they are far smaller in size than in the comparable CA02 specimens. The air-exposed side of the joint shown in Fig. 6(b) displays a typical as-brazed microstructure consisting of copper oxide precipitates in a pure silver matrix, as determined by the EDS analysis reported in Table 2. A micrograph of the region located 0.27 mm from the air-exposed side, Fig. 6(c), appears to capture the filler metal in transition with respect to reduction. In the left-hand side of the micrograph, copper oxide precipitates are clearly present in the silver matrix, whereas on the right-hand side no precipitates are present and the silver matrix contains 9.91 at% copper, suggesting that hydrogen reduction has taken place. It is particularly interesting to note that while this evidence suggests both dissolved hydrogen and oxygen are present in this region, there are no obvious signs of pore formation in the filler metal. Although some porosity is found at the braze/substrate interface, it is attributable to the reduction of interfacial CuO precipitates as reported in a previous study [14]. At a region closer to the hydrogen-exposed side of the joint, shown in Fig. 6(d) (1.10 mm from the air side), no evidence of CuO could be found and large-scale porosity was observed. EDS analysis indicates that the filler metal contains 10.95 at% copper in silver (Spot “4”) demonstrating that this part of the joint underwent hydrogen-induced reduction. It is speculated
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Fig. 6. A capsule sealed with CA08 (8 mol% CuO) at 980 ◦ C, exposed under dual atmospheres at 800 ◦ C for 100 h: (a) low magnification, (b) air side, (c) 0.27 mm from air, and (d) 1.10 mm from air.
Table 2 Results of EDS quantitative analysis conducted on the samples sealed with CA08 (8 mol% CuO) at 980 ◦ C (Fig. 6: exposed for 100 h and Fig. 7: exposed for 1000 h) Elementa
OK Al K Ag L Cu K a All
Fig. 6
Fig. 7
1
2
3
4
1
2
3
4
5
6
7
— — 100.0 —
37.19 58.13 — 4.68
— — 90.09 9.91
— — 89.05 10.95
— — 100.0 —
29.21 51.81 3.05 15.93
— — 97.83 2.17
— — 94.30 5.70
— — 91.97 8.03
29.48 54.90 12.49 3.12
— — 92.34 7.66
compositions listed are in at%.
that the porosity observed is due to solely to the reduction of the copper oxide, rather than to the hydrogen embrittlement mechanism based on the following reasons. First, the rounded morphology and the location of the porosity are similar to those of the copper oxide precipitates that originally resided in the as-brazed sample (see Fig. 3(b) for comparison). Previously it has been found that porosity attributable to hydrogen embrittlement exhibits an elongated shape because pore formation takes place along the grain boundaries of the filler metal [12,13]. Second, as mentioned above no evidence of hydrogen embrittlement was observed in the comparable 100 h CA02 exposure specimen. Thus, the porosity found in the CA08 sample of Fig. 6 is presumed to have formed via reduction of large copper oxide precipitates. Micrographs of the CA08 specimen brazed at 980 ◦ C and subsequently exposure tested for 1000 h are shown in Figs. 7(a)–(f). Again the air-exposed side of the specimen exhibits a microstructure similar to that of the comparable as-brazed joint, although the presence of the CuAl2 O4 product is a bit more extensive as seen in Fig. 7(b). At a region located 0.66 mm from the air side of the joint, Fig. 7(c), unreduced copper oxide can be observed in the filler metal matrix.
However, the matrix appears to contain a small amount of metallic copper (Spot “3”, reported in Table 2), which suggests that after 1000 h of exposure the concentration of hydrogen at this point in the joint is high enough to begin inducing a small amount of CuO reduction. In addition, a small amount of sub-micron size porosity is observed in this micrograph, although the mechanism responsible for its presence it is not obvious. Also detected were precipitates of copper aluminate within the bulk of the filler metal (indicated by Spot “2” in the micrograph). However, EDS analysis indicates that ratio of copper to aluminum in this phase is 1:3.3, which differs significantly from that measured previously and attributable to CuAl2 O4 (Cu : Al = 1 : 2). The result suggests that the copper in the original CuAl2 O4 precipitates partially reduces in the hydrogen-rich environment of the exposed filler metal, leading to a lower copper content in the aluminate phase. Porosity attributable to hydrogen embrittlement was observed in a region closer to the hydrogen-exposed side of the joint, ∼0.92 mm from the air side as shown in Fig. 7(d). As discussed previously, it can be difficult to identify the origin of porosity in the high-CuO-containing exposure specimens since it can be caused by either hydrogen embrittlement or by the
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Fig. 7. A capsule sealed with CA08 (8 mol% CuO) at 980 ◦ C, exposed under dual atmospheres at 800 ◦ C for 1000 h: (a) low magnification, (b) air side, (c) 0.66 mm from air, (d) 0.92 mm from air, (e) 1.19 mm from air, and (f) H2 side.
reduction of large copper oxide particles initially in the filler metal. The key distinguishing features are the morphology and location of the porosity. Pores generated by the reduction of copper oxide should initially assume approximately the same rounded, equiaxed morphology as the original precipitates, whereas pores created by the hydrogen embrittlement mechanism develop as a result of reaction between dissolved oxygen and hydrogen along the paths of fastest transport, i.e. the grain boundaries of the filler metal. Because of this, their morphology is more elongated. Therefore, the pores observed within the bulk filler metal of the joint in Fig. 7(d) appear to be generally attributable to hydrogen embrittlement because of their elongated shape, while the smaller more equiaxed pores along the braze/substrate interface are characterized as those originating from the reduction of copper oxide. This analysis is prone to some error because it assumes that the prior CuO particles are equiaxed, which those observed in Fig. 7(c), for example, generally are. But where the precipitates or pores have agglomerated, this type of interpretation is less clear-cut. Further toward the hydrogen side of the joint, ∼1.19 mm from the air side, as shown in Fig. 7(e), the morphologies of the pores are nearly all equiaxed, suggesting that they formed via CuO reduction. Additionally, the microstructure adjacent to the hydrogen-exposed portion of the joint exhibits almost
no porosity. Thus, only a small length of this joint displays porosity directly attributable to hydrogen embrittlement. In general, it can be concluded that the CA08 samples exhibit a trend in dual-atmosphere behavior similar to that of the CA02 joining samples: exposure for 1000 h leads to some hydrogen embrittlement, while exposure for 100 h shows no evidence of this. That is, other than the observation of CuO → Cuinduced porosity in the high-CuO-containing specimens, there is no significant effect of filler metal composition on the dualatmosphere tolerance of the air-brazed joints out to 1000 h of exposure. 3.3. Effects of brazing temperature Shown in Figs. 8 and 9 are the microstructure of joining specimens brazed at 1100 ◦ C using the CA02 and CA08 filler metals, respectively, after exposure testing at 800 ◦ C for 1000 h. Results from EDS analysis collected at spots labeled in both figures are listed in Table 3. The CA02 specimen shown in Fig. 8 displays microstructures along the joint from air to the hydrogen-exposed side that are quite similar as those observed in the comparable specimen-brazed at 980 ◦ C, which is shown in Fig. 5. The elongated pores generated by the hydrogen embrittlement mechanism are found solely on the air side of the joint, as seen in Fig. 8(b) and (c). However again, more than
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Fig. 8. A capsule sealed with CA02 (2 mol% CuO) at 1100 ◦ C, exposed under dual atmospheres at 800 ◦ C for 1000 h: (a) low magnification, (b) air side, (c) 0.32 mm from air, and (d) 1.12 mm from air.
Fig. 9. A capsule sealed with CA08 (8 mol% CuO) at 1100 ◦ C, exposed under dual atmospheres at 800 ◦ C for 1000 h: (a) low magnification, (b) air side, (c) 0.46 mm from air, (d) 0.94 mm from air, (e) 1.23 mm from air, and (f) H2 side.
half of the width of the joint is free of porosity even though all the copper oxide appears to have been reduced and subsequently alloyed into the silver-based matrix.
The micrographs of the CA08 specimen in Fig. 9 are also similar to those of the comparable CA08 joint prepared by brazing at lower temperature, which is shown in Fig. 7. Pore
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Table 3 Results of EDS quantitative analysis conducted on the samples sealed with 1100 ◦ C after dual-atmosphere exposure for 1000 h (Fig. 8: CA02, and Fig. 9: CA08) Elementa
OK Al K Ag L Cu K a All
Fig. 8
Fig. 9
1
2
3
1
2
3
4
5
6
— — 98.73 1.27
— — 98.69 1.31
— — 95.18 4.82
— — 100.0 —
33.84 53.70 — 12.46
— — 93.03 6.97
34.34 53.52 2.31 9.82
— — 90.68 9.32
— — 94.12 5.88
compositions listed are in at%.
formation that could clearly be attributed to hydrogen embrittlement was limited to a ∼0.4 mm wide region near the middle of specimen, shown in Fig. 9(d). However, there is at least one difference between the joining samples shown in Figs. 7 and 9. Compared to the specimen brazed at 980 ◦ C, substantially more copper aluminate is found in the sample joined at 1100 ◦ C. In general, this phase appears to have undergone partial reduction, except on the air side, as evidenced by a measurable decrease in copper content. Furthermore, a comparison of Figs. 7(d) and 9(d) demonstrates that the porosity found in the specimen brazed at 1100 ◦ C is substantially smaller in size. The correlation suggests that the copper aluminate may buffer or suppress the formation of large pores in the high-CuO-containing filler metals, probably by tying up some of the copper in this more reduction-resistant oxide and preventing the formation of large CuO precipitates in the resulting joint. This observation might be a key in forming an air braze filler metal composition that is more resistant to internal pore formation under high-temperature, dual-atmosphere exposure. 4. Summary and conclusions The high-temperature tolerance of silver–copper oxide air braze filler metals was examined under dual oxidizing/reducing atmospheres at 800 ◦ C. Regardless of the filler metal composition or brazing temperature employed, none of the specimens that were tested for 100 h displayed any signs of hydrogen embrittlement of the type reported by Klueh and Mullins and by Singh et al. in high-purity silver. However, specimens exposed for a longer period of time, 1000 h, did exhibit evidence of this type of internal porosity, the extent of which was limited to less than half the width of the joint. The type of porosity assumed two different forms, depending on the mechanism responsible for its formation. That which was generated by hydrogen embrittlement displayed an elongated morphology, while pores formed due to CuO precipitate reduction (observed only in the high-CuO-containing specimens) were more equiaxed in shape. CuO reduction was accompanied by subsequent diffusion of the resulting metallic copper into the surrounding silver matrix. Even though differences in filler metal composition result in braze joints of significantly different microstructure, the apparent influence of filler metal composition on the dual-atmosphere tolerance of the joint was found to be marginal. On the other hand, brazing temperature appears to play a role in possibly
mitigating the extent of internal pore formation during exposure due to increased generation of a copper aluminate reaction product. This phase ties up some of the copper from the filler metal and leads to finer size CuO precipitates in the as-brazed microstructure. Because the copper aluminate is more resistant to reduction than CuO, one mechanism of pore formation in the air-brazed joints is effectively suppressed. While the issue of hydrogen embrittlement remains a concern with respect to the long-term stability of silver-based brazes under dual oxidizing/reducing environment, its development in the Ag–CuO filler metals is much slower than was previously reported in high-purity silver, both due to the longer diffusion path in the braze joints and the buffering effects that CuO reduction appears to afford. Acknowledgments The authors would like to thank Jorrod Crum for his assistance in sample preparation for SEM analysis work. This work was supported by the U.S. Department of Energy, Solidstate Energy Conversion Alliance (SECA) Core Technology Program. References [1] 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. [2] Ley K, Krumpelt M, Kumar R, Meiser J, Bloom I. J Mater Res 1996;11:1489. [3] Yang ZG, Xia GG, Meinhardt KD, Weil KS, Stevenson JW. J Mater Eng Perf 2004;13:327. [4] Simner SP, Stevenson JW. J Power Sources 2001;102:310. [5] Chou YS, Stevenson JW. J Power Sources 2002;112:376–83. [6] Chou YS, Stevenson JW. J Power Sources 2003;115:274–8. [7] Weil KS, Kim JY, Hardy JS. Electrochem Solid-State Lett 2005;8:A133. [8] Weil KS, Hardy JS, Rice JP, Kim JY. Fuel 2006;85:156. [9] Kim JY, Hardy JS, Weil KS. J Electrochem Soc 2005;152:J52. [10] Kim JY, Hardy JS, Weil KS. J Am Ceram Soc 2005;88:2521. [11] Weil KS, Coyle CA, Darsell JT, Xia GG, Hardy JS. J Power Sources 2005;152C:97. [12] Kleuh RL, Mullins WW. Trans Metall Soc AIME 1968;242:237. [13] Singh P, Yang ZG, Viswanathan V, Stevenson JW. J Mater Eng Perform 2004;13:287. [14] Kim JY, Hardy JS, Weil KS. J Mat Res 2006;21:1434. [15] Shao ZB, Liu KR, Liu LQ, Liu HK, Dou S. J Am Ceram Soc 1993;76:2663.
Dual-atmosphere tolerance of Ag–CuO-based air braze Jin Yong Kim, John S. Hardy, Scott Weil ∗ Pacific Northwest National Laboratory, Richland, WA 99352, USA Available online 12 October 2006
Abstract Recently, a new braze filler metal based on the silver–copper oxide system was developed for use in sealing high-temperature, solid-state electrochemical devices such as solid oxide fuel cells. One of the concerns regarding the viability of this joining technique is the long-term stability of silver-based alloys under a high-temperature, dual oxidizing/reducing gas environment. This paper reports on an initial series of exposure experiments that were conducted to characterize the effects of (1) filler metal composition, (2) brazing temperature, and (3) exposure time on the microstructural stability of Ag–CuO-brazed Al2 O3 /Al2 O3 joints under a prototypic operating environment for an intermediate temperature solid oxide fuel cell stack. In general joints exposed simultaneously to air on one side and hydrogen on the other for short periods of time at 800 ◦ C (100 h) showed no signs of degradation with respect to hermeticity or joint microstructure. Samples exposed for longer periods of time (1000 h) displayed some internal porosity, which extends approximately halfway across the joint and is not interconnected. Little effect of the filler metal’s composition on its tolerance to dual-atmosphere exposure was observed. However, brazing temperature was found to have a measurable effect. Higher brazing temperature leads to a more extensive formation of an interfacial reaction phase, copper aluminate, which tends to tie up some of the free CuO in the filler metal and minimize the formation of porosity in the air-brazed joints during long-term, dual-atmosphere exposure. The effect is due to the greater chemical stability of the copper aluminate relative to copper oxide. Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. Keywords: Ag–CuO braze; Dual-atmosphere; Microstructure
1. Introduction One of the key components in the design of highperformance intermediate temperature planar solid oxide fuel cell (IT-pSOFC) stacks is the seal between the metal and ceramic cell components (i.e. a “cell-to-frame seal”) in the stack. A significant engineering challenge in fabricating these devices is how to make this seal hermetic, rugged, and stable during continuous high-temperature use over its operational lifetime, which is anticipated to be on the order of 10,000 h or longer depending on the application. At present, there are only a few methods that are used in sealing pSOFC stacks, none of which fully meets the design requirements of stack builders. The most popular technique is glass joining. While this method of sealing is relatively simple and cost-effective [1–3], it yields a final seal that is brittle and therefore susceptible to failure by thermal stresses induced during rapid stack heating and ∗ Corresponding author. Tel.: +1 509 376 6796; fax: +1 509 375 2186.
E-mail address: [email protected] (K.S. Weil).
cooling or due to thermal expansion mismatches between the sealing glass and the joining substrates [3]. Compressive seals employ deformable materials that do not bond to the pSOFC components but instead serve as gaskets [4–6]. Thus, sealing is achieved when the entire stack is compressively loaded. Because the sealing material conforms to the adjacent surfaces and is under constant compression during use, it forms a dynamic seal. That is, the sealing surfaces can slide past one another without a disruption in hermeticity and the individual stack components are free to expand and contract during thermal cycling with no need to consider CTE matching. This offers stack designers greater freedom in utilizing alloys other than ferritic stainless steels for the metal components. The gaskets are readily produced and easy to apply. Additionally, they offer the potential for mid-term stack repair by releasing the compressive load, disassembling the stack, and replacing the damaged cell or separator components. However, in order to employ compressive seals in a pSOFC stack, a load frame is required to maintain the desired level of compression on the stack over the entire period of operation and the stack
0360-3199/$ - see front matter Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. doi:10.1016/j.ijhydene.2006.08.054
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components must be capable of withstanding the sealing load. The load frame introduces several complexities in stack design, including: oxidation of the frame material, load relaxation due to creep, and increased weight and thermal mass (and therefore reduced specific power and thermal response of the overall system). In addition to increased system cost, these factors seriously limit the use of compressive seals in mobile applications. A third option recently developed for sealing hightemperature electrochemical devices is air brazing [7]. Originally developed as a means of sealing tubular hydrogen and oxygen separation membranes to the respective metallic gas manifold structure in a coal gasification system [8], the technique is currently being examined for its potential use in sealing planar SOFCs [9]. Unlike traditional ceramic-to-metal brazing (active metal brazing), which must be carried out in vacuum or reducing gas environment, air brazing takes place directly in air without need of surface reactive fluxing agents or inert cover gases. One material system that has shown particular promise for forming air braze filler metals 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 room temperature strengths on the order of 60%–80% of the base ceramic [7,9,10]. Like glass sealing, Ag–CuO filler metals can be used to braze together components directly in air, forming a joint that is inherently oxidation resistant. When the filler metal composition and braze temperature are optimized, the air-brazed joint is ductile and can be heated and cooled at rapid rate from room temperature to upwards of 750 ◦ C through numerous cycles without failure [11]. However, there is a concern with using silver-based alloys for pSOFC applications. Silver is known to undergo a form of high-temperature embrittlement, which occurs when it is simultaneously exposed to oxygen (or air) and hydrogen [12,13]. 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 were the first to observe that under certain conditions water vapor bubbles will form due to reaction between the two diffused species primarily along the grain boundaries of silver [12]. In the present investigation, a series of dual-atmosphere exposure experiments were conducted to determine if this effect takes place in the Ag–CuO braze alloys and if so, identify whether the mechanism of microstructural evolution is similar to that of high-purity silver. 2. Experimental Alumina cap and tube specimens of the type shown in Fig. 1 were employed in exposure testing of the Ag–CuO seal. Although in the SOFC stack it is envisioned that the air braze seal will be between YSZ and ferritic stainless steel, given extensive prior filler metal testing with alumina [10], this material was selected as a surrogate substrate to isolate the primary effects of dual-atmosphere exposure on the filler metal. The dual-atmosphere exposure condition was established by heating the braze-sealed end of the pipe in air while circulating wet H2 gas within the pipe. The cold end of the pipe that extends
H2 in H2 out Alumina tube Air
Furnace
Ag-CuO braze ring Alumina cap Fig. 1. A schematic of the dual-atmosphere testing apparatus.
outside of the muffle furnace shown in Fig. 1 was sealed using Swagelok䉸 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 external leaks. High-purity (99.7% Al2 O3 , ∼99% dense; Alfa Aesar) alumina tubes measuring 25 mm in outer diameter ×18 in long with a 2 mm wall thickness were used in the study. The alumina discs employed as caps were also high-purity (99.7% Al2 O3 , 98% dense; Alfa Aesar) and measured 25 mm in diameter by 4 mm thick. The faying surfaces of both the tubes and caps were cleaned with acetone and air dried prior to brazing. Two air braze filler metal compositions were employed in air brazing: 2 and 8 mol% CuO in Ag, referred to as CA02 and CA08, respectively. These were formulated by ball-milling the appropriate amounts of copper powder (99%, Alfa Aesar) and silver powder (99.9%, Alfa Aesar) and isostatically cold pressing the mixtures into discs measuring approximately 25 mm in diameter by 2 mm high. The discs were sintered in air at 920 ◦ C for 2 h, during which the copper fully oxidized to CuO [10]. After sintering, the discs were rolled into foils measuring ∼50 m thick, then cut into 25 mm O.D. rings each with a wall thickness of 2 mm. Joining was conducted by placing a braze foil ring between an alumina disc and the faying surface of an alumina tube. The weight of the tube (∼300 g) on the braze foil and disc served as a dead load, ensuring good contact during brazing. The assembly was heated in air at 2 ◦ C/ min to a final braze temperature of either 980 or 1100 ◦ C and held at temperature for 30 min before furnace cooling to room temperature. Two different brazing temperatures were employed to determine the role that temperature has on the extent of interfacial reaction between the filler metal and the alumina substrates and whether the presence of a reaction zone affects the dual-atmosphere tolerance of the resulting joint. After brazing, each sealed tube was exposed to a dual-atmosphere environment at 800 ◦ C for 100 or 1000 h. Ultra high-purity hydrogen (UHP-H2 ) was bubbled through 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 the duration of the test. Three variables were examined in this set of experiments: (1) braze filler composition, 2 mol% CuO in Ag (CA02) vs. 8 mol% CuO in Ag (CA08); (2) brazing temperature,
J.Y. Kim et al. / International Journal of Hydrogen Energy 32 (2007) 3655 – 3663
980 vs. 1100 ◦ C; and (3) period of exposure at 800 ◦ C, 100 vs. 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 quartered to allow examination of several cross-sections during microstructural analysis. For comparison, as-brazed alumina discs (20 mm diameter, 3 mm thick) joined at each brazing condition (i.e. filler metal composition and braze temperature) were also cross-sectioned and examined. Microstructural analysis was performed using a scanning electron microscope (SEM, JEOL JSM-5900LV) equipped with an Oxford energy dispersive X-ray spectrometry (EDS) system,
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which employs a windowless detector for quantitative detection of both light and heavy elements. 3. Results and discussion 3.1. Microstructure of the as-brazed joints Shown in Figs. 2 and 3 are cross-sectional SEM micrographs of specimens in the four different as-brazed conditions. Low magnification images collected from alumina joints brazed with a 2 mol% CuO filler metal, shown in Figs. 2(a) and (c) after brazing at 980 and 1100 ◦ C, respectively, display the presence
Fig. 2. Cross-sectional SEM micrographs of the alumina joints brazed with CA02 (2 mol% CuO): (a) and (b) at 980 ◦ C (a: X40, b: X1000), (c) and (d) at 1100 ◦ C (c: X40, d: X500).
Fig. 3. Cross-sectional SEM micrographs of the alumina joints brazed with CA08 (8 mol% CuO): (a) and (b) at 980 ◦ C (a: X40, b: X1000), (c) and (d) at 1100 ◦ C (c: X40, d: X500).
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of large air pockets. These defects form during joining presumably due to insufficient wetting between this braze filler and alumina [14]. The joint brazed at 980 ◦ C contains a small amount of copper oxide both in the braze filler and at the braze/substrate interface, as seen in Fig. 2(b). SEM examination of specimens joined at 1100 ◦ C reveals the presence of copper aluminate, CuAl2 O4 , in addition to copper oxide observed in Fig. 2(d). Thus, the higher brazing temperature facilitates reaction between the copper oxide in the braze filler and the alumina substrate. Air pockets were also found in joints brazed with an 8 mol% CuO filler metal, as seen in Figs. 3(a) and (c). In general, however, the size of these flaws was substantially smaller than those observed in the CA02 joints, likely because the increased amount of CuO leads to enhanced wetting behavior [7,9,14]. At higher magnification, shown in Fig. 3(b), it is apparent that the CA08 sample brazed at 980 ◦ C contains significantly more copper oxide than the comparable CA02 joint. Also it is noted that a small amount of copper aluminate is present in this specimen, implying that the higher CuO content appears to promote the formation of copper aluminate. This is likely due to the liquid phase separation that occurs in the CA08 composition at the brazing temperature, forming two liquids one rich in silver (L1) and the other in CuO (L2) [15]. When this occurs, it has been observed that the L2 liquid will preferentially wet the alumina substrate [7,10,14]. It is speculated that the high activity of the CuO in the wetting L2 phase promotes the reaction that forms CuAl2 O4 . This reaction is even more extensive when the joint is formed at 1100 ◦ C, as seen in Fig. 3(d). 3.2. Effects of exposure time and filler metal composition Shown in Figs. 4 and 5 are the microstructural results obtained on specimens that were brazed at 980 ◦ C using the
CA02 filler metal and then exposed at 800 ◦ C to the dual wet H2 /ambient air environment for 100 and 1000 h, respectively. Results from EDS analyses that correspond to the numbered regions in these two figures are provided in Table 1. Displayed in Fig. 4(a) is a low magnification cross-sectional image of the CA02 sample after 100 h of exposure. The width of the brazed joint is ∼2.12 mm and as was observed in the comparable as-brazed condition (Fig. 2(a)), there are large air pockets in the filler metal region of the joint. The microstructure on the air-exposed side of the sample is shown at higher magnification in Fig. 2(b). EDS measurements indicate that this region consists of copper oxide particles in a pure silver matrix, similar to the as-brazed specimen shown in Fig. 2(b). A region further inside of the joint, located 0.51 mm from the airexposed side or 1.61 mm from the H2 -exposed side, exhibits essentially the same microstructure, indicating that it was not influenced by dissolved hydrogen. However, in a region closer to the hydrogen-exposed side of the joint, shown in Fig. 4(d) at 0.67 mm from the H2 side, the lack of copper oxide precipitates is notable. EDS analysis of the filler metal (Spot “3”) indicates that it is composed of ∼1.9 at% copper in silver. This result suggests that during the 100 h of exposure, hydrogen diffuses into this side of the joint up to this point and reduces the original copper oxide precipitates to metallic copper, which subsequently alloys with the surrounding silver forming a solid solution. Even though approximately half of the joint experiences hydrogen-induced reduction of this type, there is no indication of hydrogen embrittlement or the formation of porosity within the silver of the type observed by Klueh and Mullins [12]. The cross-sectional micrographs shown in Fig. 5 display the microstructure of the CA02 specimen that was exposed for 1000 h. In this case, the width of the joint is ∼2.26 mm, with several air pockets present in the low magnification image of Fig. 5(a). As seen in Fig. 5(b), the air-exposed side of the
Fig. 4. A capsule sealed with CA02 (2 mol% CuO) at 980 ◦ C, exposed under dual atmospheres at 800 ◦ C for 100 h: (a) low magnification, (b) air side, (c) 0.51 mm from air, and (d) 1.45 mm.
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Fig. 5. A capsule sealed with CA02 (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.
Table 1 Results of EDS quantitative analysis conducted on the samples sealed with CA02 (2 mol% CuO) at 980 ◦ C (Fig. 4: exposed for 100 h and Fig. 5: exposed for 1000 h) Elementa
Ag L Cu K a All
Fig. 4
Fig. 5
1
2
3
1
2
3
4
100.00 —
100 —
98.13 1.87
100 —
99.15 0.85
97.92 2.08
98.13 1.87
compositions listed are in at%.
joint exhibits a microstructure comparable to that of the asbrazed joint. However on the right-hand side of this image, some porosity is present and no copper oxide precipitates are observed. EDS 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, as listed in Table 1. This result suggests that over the 1000 h exposure period, hydrogen was able to diffuse nearly through the entire width of the joint. At a region located 0.79 mm from the air-exposed side of the joint, porosity is also present and appears to occur along the grain boundaries of the resulting silver–copper alloy, as shown in Fig. 5(c). EDS analysis indicates that the filler metal (Spot “3”) contains 2.08 at% of copper in silver. Again, reduction of copper oxide precipitates originally in this region appears to have occurred due to hydrogen diffusion across the joint. At a region closer to the hydrogen-exposed side of the joint (1.10 mm from the air side, as seen in Fig. 5(d)), the microstructure displays only the copper–silver filler metal but no evidence of any porosity or CuO precipitates. That is, in CA02 joints brazed at 980 ◦ C and exposure tested for 1000 h internal pore formation is localized toward the air-exposed side of the joint side, whereas after only 100 h of exposure no pore formation
takes place. It was also noted in the 1000 h CA02 exposure specimen that porosity is found less than a halfway across 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. Shown in the micrographs of Fig. 6 is the microstructure of the CA08 joining specimen brazed at 980 ◦ C after exposure at 800 ◦ C for 100 h. Similar to what was noted in the comparison between the as-brazed specimens, the low magnification image in Fig. 6(a) reveals the presence of a couple of air pockets in the joint, but they are far smaller in size than in the comparable CA02 specimens. The air-exposed side of the joint shown in Fig. 6(b) displays a typical as-brazed microstructure consisting of copper oxide precipitates in a pure silver matrix, as determined by the EDS analysis reported in Table 2. A micrograph of the region located 0.27 mm from the air-exposed side, Fig. 6(c), appears to capture the filler metal in transition with respect to reduction. In the left-hand side of the micrograph, copper oxide precipitates are clearly present in the silver matrix, whereas on the right-hand side no precipitates are present and the silver matrix contains 9.91 at% copper, suggesting that hydrogen reduction has taken place. It is particularly interesting to note that while this evidence suggests both dissolved hydrogen and oxygen are present in this region, there are no obvious signs of pore formation in the filler metal. Although some porosity is found at the braze/substrate interface, it is attributable to the reduction of interfacial CuO precipitates as reported in a previous study [14]. At a region closer to the hydrogen-exposed side of the joint, shown in Fig. 6(d) (1.10 mm from the air side), no evidence of CuO could be found and large-scale porosity was observed. EDS analysis indicates that the filler metal contains 10.95 at% copper in silver (Spot “4”) demonstrating that this part of the joint underwent hydrogen-induced reduction. It is speculated
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Fig. 6. A capsule sealed with CA08 (8 mol% CuO) at 980 ◦ C, exposed under dual atmospheres at 800 ◦ C for 100 h: (a) low magnification, (b) air side, (c) 0.27 mm from air, and (d) 1.10 mm from air.
Table 2 Results of EDS quantitative analysis conducted on the samples sealed with CA08 (8 mol% CuO) at 980 ◦ C (Fig. 6: exposed for 100 h and Fig. 7: exposed for 1000 h) Elementa
OK Al K Ag L Cu K a All
Fig. 6
Fig. 7
1
2
3
4
1
2
3
4
5
6
7
— — 100.0 —
37.19 58.13 — 4.68
— — 90.09 9.91
— — 89.05 10.95
— — 100.0 —
29.21 51.81 3.05 15.93
— — 97.83 2.17
— — 94.30 5.70
— — 91.97 8.03
29.48 54.90 12.49 3.12
— — 92.34 7.66
compositions listed are in at%.
that the porosity observed is due to solely to the reduction of the copper oxide, rather than to the hydrogen embrittlement mechanism based on the following reasons. First, the rounded morphology and the location of the porosity are similar to those of the copper oxide precipitates that originally resided in the as-brazed sample (see Fig. 3(b) for comparison). Previously it has been found that porosity attributable to hydrogen embrittlement exhibits an elongated shape because pore formation takes place along the grain boundaries of the filler metal [12,13]. Second, as mentioned above no evidence of hydrogen embrittlement was observed in the comparable 100 h CA02 exposure specimen. Thus, the porosity found in the CA08 sample of Fig. 6 is presumed to have formed via reduction of large copper oxide precipitates. Micrographs of the CA08 specimen brazed at 980 ◦ C and subsequently exposure tested for 1000 h are shown in Figs. 7(a)–(f). Again the air-exposed side of the specimen exhibits a microstructure similar to that of the comparable as-brazed joint, although the presence of the CuAl2 O4 product is a bit more extensive as seen in Fig. 7(b). At a region located 0.66 mm from the air side of the joint, Fig. 7(c), unreduced copper oxide can be observed in the filler metal matrix.
However, the matrix appears to contain a small amount of metallic copper (Spot “3”, reported in Table 2), which suggests that after 1000 h of exposure the concentration of hydrogen at this point in the joint is high enough to begin inducing a small amount of CuO reduction. In addition, a small amount of sub-micron size porosity is observed in this micrograph, although the mechanism responsible for its presence it is not obvious. Also detected were precipitates of copper aluminate within the bulk of the filler metal (indicated by Spot “2” in the micrograph). However, EDS analysis indicates that ratio of copper to aluminum in this phase is 1:3.3, which differs significantly from that measured previously and attributable to CuAl2 O4 (Cu : Al = 1 : 2). The result suggests that the copper in the original CuAl2 O4 precipitates partially reduces in the hydrogen-rich environment of the exposed filler metal, leading to a lower copper content in the aluminate phase. Porosity attributable to hydrogen embrittlement was observed in a region closer to the hydrogen-exposed side of the joint, ∼0.92 mm from the air side as shown in Fig. 7(d). As discussed previously, it can be difficult to identify the origin of porosity in the high-CuO-containing exposure specimens since it can be caused by either hydrogen embrittlement or by the
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Fig. 7. A capsule sealed with CA08 (8 mol% CuO) at 980 ◦ C, exposed under dual atmospheres at 800 ◦ C for 1000 h: (a) low magnification, (b) air side, (c) 0.66 mm from air, (d) 0.92 mm from air, (e) 1.19 mm from air, and (f) H2 side.
reduction of large copper oxide particles initially in the filler metal. The key distinguishing features are the morphology and location of the porosity. Pores generated by the reduction of copper oxide should initially assume approximately the same rounded, equiaxed morphology as the original precipitates, whereas pores created by the hydrogen embrittlement mechanism develop as a result of reaction between dissolved oxygen and hydrogen along the paths of fastest transport, i.e. the grain boundaries of the filler metal. Because of this, their morphology is more elongated. Therefore, the pores observed within the bulk filler metal of the joint in Fig. 7(d) appear to be generally attributable to hydrogen embrittlement because of their elongated shape, while the smaller more equiaxed pores along the braze/substrate interface are characterized as those originating from the reduction of copper oxide. This analysis is prone to some error because it assumes that the prior CuO particles are equiaxed, which those observed in Fig. 7(c), for example, generally are. But where the precipitates or pores have agglomerated, this type of interpretation is less clear-cut. Further toward the hydrogen side of the joint, ∼1.19 mm from the air side, as shown in Fig. 7(e), the morphologies of the pores are nearly all equiaxed, suggesting that they formed via CuO reduction. Additionally, the microstructure adjacent to the hydrogen-exposed portion of the joint exhibits almost
no porosity. Thus, only a small length of this joint displays porosity directly attributable to hydrogen embrittlement. In general, it can be concluded that the CA08 samples exhibit a trend in dual-atmosphere behavior similar to that of the CA02 joining samples: exposure for 1000 h leads to some hydrogen embrittlement, while exposure for 100 h shows no evidence of this. That is, other than the observation of CuO → Cuinduced porosity in the high-CuO-containing specimens, there is no significant effect of filler metal composition on the dualatmosphere tolerance of the air-brazed joints out to 1000 h of exposure. 3.3. Effects of brazing temperature Shown in Figs. 8 and 9 are the microstructure of joining specimens brazed at 1100 ◦ C using the CA02 and CA08 filler metals, respectively, after exposure testing at 800 ◦ C for 1000 h. Results from EDS analysis collected at spots labeled in both figures are listed in Table 3. The CA02 specimen shown in Fig. 8 displays microstructures along the joint from air to the hydrogen-exposed side that are quite similar as those observed in the comparable specimen-brazed at 980 ◦ C, which is shown in Fig. 5. The elongated pores generated by the hydrogen embrittlement mechanism are found solely on the air side of the joint, as seen in Fig. 8(b) and (c). However again, more than
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Fig. 8. A capsule sealed with CA02 (2 mol% CuO) at 1100 ◦ C, exposed under dual atmospheres at 800 ◦ C for 1000 h: (a) low magnification, (b) air side, (c) 0.32 mm from air, and (d) 1.12 mm from air.
Fig. 9. A capsule sealed with CA08 (8 mol% CuO) at 1100 ◦ C, exposed under dual atmospheres at 800 ◦ C for 1000 h: (a) low magnification, (b) air side, (c) 0.46 mm from air, (d) 0.94 mm from air, (e) 1.23 mm from air, and (f) H2 side.
half of the width of the joint is free of porosity even though all the copper oxide appears to have been reduced and subsequently alloyed into the silver-based matrix.
The micrographs of the CA08 specimen in Fig. 9 are also similar to those of the comparable CA08 joint prepared by brazing at lower temperature, which is shown in Fig. 7. Pore
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Table 3 Results of EDS quantitative analysis conducted on the samples sealed with 1100 ◦ C after dual-atmosphere exposure for 1000 h (Fig. 8: CA02, and Fig. 9: CA08) Elementa
OK Al K Ag L Cu K a All
Fig. 8
Fig. 9
1
2
3
1
2
3
4
5
6
— — 98.73 1.27
— — 98.69 1.31
— — 95.18 4.82
— — 100.0 —
33.84 53.70 — 12.46
— — 93.03 6.97
34.34 53.52 2.31 9.82
— — 90.68 9.32
— — 94.12 5.88
compositions listed are in at%.
formation that could clearly be attributed to hydrogen embrittlement was limited to a ∼0.4 mm wide region near the middle of specimen, shown in Fig. 9(d). However, there is at least one difference between the joining samples shown in Figs. 7 and 9. Compared to the specimen brazed at 980 ◦ C, substantially more copper aluminate is found in the sample joined at 1100 ◦ C. In general, this phase appears to have undergone partial reduction, except on the air side, as evidenced by a measurable decrease in copper content. Furthermore, a comparison of Figs. 7(d) and 9(d) demonstrates that the porosity found in the specimen brazed at 1100 ◦ C is substantially smaller in size. The correlation suggests that the copper aluminate may buffer or suppress the formation of large pores in the high-CuO-containing filler metals, probably by tying up some of the copper in this more reduction-resistant oxide and preventing the formation of large CuO precipitates in the resulting joint. This observation might be a key in forming an air braze filler metal composition that is more resistant to internal pore formation under high-temperature, dual-atmosphere exposure. 4. Summary and conclusions The high-temperature tolerance of silver–copper oxide air braze filler metals was examined under dual oxidizing/reducing atmospheres at 800 ◦ C. Regardless of the filler metal composition or brazing temperature employed, none of the specimens that were tested for 100 h displayed any signs of hydrogen embrittlement of the type reported by Klueh and Mullins and by Singh et al. in high-purity silver. However, specimens exposed for a longer period of time, 1000 h, did exhibit evidence of this type of internal porosity, the extent of which was limited to less than half the width of the joint. The type of porosity assumed two different forms, depending on the mechanism responsible for its formation. That which was generated by hydrogen embrittlement displayed an elongated morphology, while pores formed due to CuO precipitate reduction (observed only in the high-CuO-containing specimens) were more equiaxed in shape. CuO reduction was accompanied by subsequent diffusion of the resulting metallic copper into the surrounding silver matrix. Even though differences in filler metal composition result in braze joints of significantly different microstructure, the apparent influence of filler metal composition on the dual-atmosphere tolerance of the joint was found to be marginal. On the other hand, brazing temperature appears to play a role in possibly
mitigating the extent of internal pore formation during exposure due to increased generation of a copper aluminate reaction product. This phase ties up some of the copper from the filler metal and leads to finer size CuO precipitates in the as-brazed microstructure. Because the copper aluminate is more resistant to reduction than CuO, one mechanism of pore formation in the air-brazed joints is effectively suppressed. While the issue of hydrogen embrittlement remains a concern with respect to the long-term stability of silver-based brazes under dual oxidizing/reducing environment, its development in the Ag–CuO filler metals is much slower than was previously reported in high-purity silver, both due to the longer diffusion path in the braze joints and the buffering effects that CuO reduction appears to afford. Acknowledgments The authors would like to thank Jorrod Crum for his assistance in sample preparation for SEM analysis work. This work was supported by the U.S. Department of Energy, Solidstate Energy Conversion Alliance (SECA) Core Technology Program. References [1] 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. [2] Ley K, Krumpelt M, Kumar R, Meiser J, Bloom I. J Mater Res 1996;11:1489. [3] Yang ZG, Xia GG, Meinhardt KD, Weil KS, Stevenson JW. J Mater Eng Perf 2004;13:327. [4] Simner SP, Stevenson JW. J Power Sources 2001;102:310. [5] Chou YS, Stevenson JW. J Power Sources 2002;112:376–83. [6] Chou YS, Stevenson JW. J Power Sources 2003;115:274–8. [7] Weil KS, Kim JY, Hardy JS. Electrochem Solid-State Lett 2005;8:A133. [8] Weil KS, Hardy JS, Rice JP, Kim JY. Fuel 2006;85:156. [9] Kim JY, Hardy JS, Weil KS. J Electrochem Soc 2005;152:J52. [10] Kim JY, Hardy JS, Weil KS. J Am Ceram Soc 2005;88:2521. [11] Weil KS, Coyle CA, Darsell JT, Xia GG, Hardy JS. J Power Sources 2005;152C:97. [12] Kleuh RL, Mullins WW. Trans Metall Soc AIME 1968;242:237. [13] Singh P, Yang ZG, Viswanathan V, Stevenson JW. J Mater Eng Perform 2004;13:287. [14] Kim JY, Hardy JS, Weil KS. J Mat Res 2006;21:1434. [15] Shao ZB, Liu KR, Liu LQ, Liu HK, Dou S. J Am Ceram Soc 1993;76:2663.