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Interfacial reaction and microstructural evolution of Ag-Cu braze on BaCo0.7Fe0.2Nb0.1O3-δ at high temperature in air
Ceramics International 43 (2017) 810–819
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Interfacial reaction and microstructural evolution of Ag-Cu braze on BaCo0.7Fe0.2Nb0.1O3-δ at high temperature in air
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Yuwen Zhang , Lili Zhang, Wei Guo, Ting Liu, Chengzhang Wu, Weizhong Ding, Xionggang Lu State Key Laboratory of Advanced Special Steel & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Ag-Cu braze Mixed ionic-electronic conducting oxides ceramics Reactive air brazing Wetting Interfacial reaction Ridge evolution
Based reactive air brazing technique, interfacial reaction and microstructure formation of Ag-6.6 mol% Cu wetting on BaCo0.7Fe0.2Nb0.1O3-δ(BCFNO) membrane in air were investigated. The interfacial microstructural analysis revealed that at high temperature Ag-Cu alloy transformed into Ag-Cu-O braze in air and the liquid braze penetrated into BCFNO matrix along the grain boundaries. Cu-O in the liquid braze reacted with BCFNO to form CoCuO2 and Ba2Cu3O5+x confirmed by XRD. Ridge formed quickly at the triple line as a result of the migration of the interfacial reaction products and in response to the vertical component of force from surface tension. The rapid interfacial reaction and ridging at the spreading front actually stalled the spreading of the liquid braze, rather than a continuous layer of smoother, barrier-free reaction phase forming and driving accompanied spreading. Low macroscopic contact angles between the braze and BCFNO substrate were observed due to the evolution of the ridge composed of the interfacial reaction products near the tripe line.
1. Introduction Mixed ionic/electronic conducting oxygen transport membranes (MIEC-OTM) have been used for oxygen separation, catalytic membrane reaction and oxyfuel combustion processes [1–6]. For an industrial application, the gas-tight joining of the membranes to metal components in MIEC-OTM systems is an engineering challenge because of high thermal expansion coefficient, low chemical stability and strength of the membranes. Metal brazing has been attracting more and more attentions for sealing MIEC-OTM modules to metal interconnect components. Due to the good ductility, oxidation resistance and compatibility with the MIECOTM modules and metal components, Ag-based alloys have been widely used as metallic sealing fillers [7]. To avoid the stability degradation of the MIEC-OTM during the brazing at a reducing atmosphere or high vacuum and the deterioration of the joint during the high temperature operation at an oxidizing atmosphere, reactive air brazing (RAB) was developed by Weil et al. [8]. RAB is a method that forms a metallic joint directly in air without the use of fluxes and other special atmosphere. The wetting of (La0.6Sr0.4)(Co0.2Fe0.8)O3-δ (LSCFO) by Ag-Cu alloys with RAB was investigated by Weil group [7,9]. They found that the wettability of the Ag-CuO brazes on LSCFO substrates increased with CuO content up to 34% while a nonwetting behavior was observed for pure silver. The addition of CuO to silver enhanced wetting on LSCFO substrates through
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a non-reactive mechanism. The wettability between Ag-CuO alloys and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCFO) membrane in air was improved as the CuO content increased and a reaction product layer formed along the wetting interface [10–13]. A continuous Pb(Mg0.33Nb0.67)O3(PMN) /silver interface was observed without an interfacial reaction product and the wetting was controlled by a physical wetting mechanism [14]. The wetting of Ag-Cu alloys on the zirconia and alumina ceramics was also improved due to the presence of Cu oxides and the new reaction products along the joining interface [15–17]. Most characterizations on the microstructures were focused on the interfacial cross section for the center of the solidified Ag-Cu sessile drops on the substrates. Near the triple line of the sessile drop on the substrates, the reaction and microstructure development directly affects the spreading and wetting behaviors. Ridging occurred at the triple line for higher CuO contents (2 mol% and 8 mol%) sessile drop on BSCFO in air and no ridge was observed for solidified Ag-1 mol% CuO [13]. However, there was no ridge found in wetting test with Ag4 mol% CuO and a pronounced ridge at the triple point formed for Ag16 mol% CuO on BSCFO [12]. The mechanism on the ridge formation and its effect have not been reported in these studies. An understanding of the wetting, spreading, interfacial reaction and ridging behaviors during high-temperature brazing in air is critical. To the best of our knowledge, the relative study has not been published, especially for Ag-Cu braze alloys wetting on MIEC-OTMs with RAB.
Corresponding author. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2016.10.012 Received 20 June 2016; Received in revised form 30 September 2016; Accepted 2 October 2016 Available online 13 October 2016 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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samples were characterized separately.
Our group has used BaCo0.7Fe0.2Nb0.1O3-δ (BCFNO) reactors to produce hydrogen by partial oxidation of methane in coke oven gas (COG) and investigated the fabrication and performance of the membrane reactors in lab [18–20]. To develop a commercial process, the gas-tight BCFNO membrane tubes must be brazed to metal supports using Ag-based alloys to construct high temperature modules. In this paper, the interfacial reactions between Ag-Cu braze and BCFNO in air were investigated systematically. The microstructural evolution of the joining interfaces at the center and near the triple line of the sessile drops was studied. The formation of the triple line ridge during high-temperature brazing in air was given special attention.
3.1.1. The center of the wetting drops The micrographs in Fig. 2 display the cross-section microstructure of the center of three solidified wetting sample quenched immediately after heating to 970 °C. The corresponding EDS analysis taken on the different denoted in Fig. 2 was given in Table 1. It can be seen from Fig. 2a that there were three zones for the cross-section of Ag-6.6 mol% Cu/BCFNO wetting sample: Ag-based filler (Ⅰ), interfacial reaction zone (Ⅱ) and the bulk of BCFNO substrate (Ⅲ). Below the braze drop, an interfacial reaction layer about 100 µm thick existed in the substrate. For the zone of Ag-based filler, the braze matrix was pure silver (position 1 in Fig. 2b and Table 1). Copper oxide precipitates were found within the pure silver matrix (position 2 in Fig. 2b). The EDX data collected on these oxide precipitates indicated that their composition was a nearly 1:1 ratio of CuO and Cu2O (Table 1, as position 2 in Fig. 2b). The result was consistent with that reported by Weil [9]. With temperature decreasing, CuO partially dissociated due to the decrease of oxygen activity in silver. Silver displays no solubility for copper oxide in the solid state and CuO dissolved in the liquid silver melt precipitated within the bulk filler metal. The similar microstructure of the silver bulk interspersed copper oxide precipitates was also observed in RAB reported in references [7–17]. As shown in Fig. 2c, the interfacial reaction zone consisted of two layers with one thin layer being in contact with the filler metal (denoted by A), and the other layer bordering the bulk of BCFNO substrate (denoted by B). It can be seen from Fig. 2c that, in the layer A, there were two phases: dark phase and grey phase. The grey phase was encircled by the dark phase and the island structure was formed. EDX analyses performed at different places showed that the dark phase (as position 3 in Fig. 2c) was composed of Co, Cu and O (as position 3 in Table 1). The grey phase (as position 4 in Fig. 2c) consisted of Ba, Co, Fe, Cu and O (as position 4 in Table 1). The thickness of the reaction layer A was approximately 4 µm. For the layer B in the interfacial reaction zone, there were about four phases: dark phase, grey phase, high-grey phase and white phase (Fig. 2c and d). EDX analyses revealed that the dark phase was composed of Ba-Co-Cu-O complex phases (as position 6 in Fig. 2d and Table 1) along the BCFNO grain boundaries (as position 5 in Table 1). There were some Co-riched oxides (as position 7 in Table 1) which corresponded to the high-grey phase in the bulk BCFNO. The silver appeared to infiltrate into the underlying substrate (as position 8 in Fig. 2d and Table 1, white phase). After heating and holding at 970 °C for 10 min, three regions for the cross-section of the quenched wetting sample was also observed (Fig. 3a). The central interface microstructure was found to be similar in appearance to that of the wetting sample holding at 970 °C for 0 min. The copper oxide was found within the filler matrix (as denoted by a white arrow in Fig. 3b). The interfacial reaction layer amounted to 120 µm thick. Two layers with different microstructures were also observed in the interfacial reaction zone (Fig. 3c and d). The corre-
2. Experimental The preparation procedures of the BCFNO powder and the membrane disks have been given elsewhere [18]. Prior to wetting tests, the wetting surface of each BCFNO substrate was ground to 10 µm finish and cleaned ultrasonically using acetone and ethanol. Base on our previous study, Ag-6.6 mol% Cu was chosen as brazing alloy used in present experiments. The Ag-Cu braze alloys were prepared by melting pure Ag and Cu in a vacuum induction furnace. And the braze pellets with 3 mm in diameter by 5 mm high were made from the prepared AgCu alloys. The sessile drop experiments were conducted in a tube atmosphere furnace by placing a braze pellet on the polished surface of a substrate. The samples were heated to 970 °C at 10 °C/min in air and hold at 970 °C for 0 min, 10 min and 20 min, respectively. Then each sample was cooled to room temperature in the furnace. After cooling to room temperature, the samples were removed from the furnace. The microstructure and the interface components analysis were conducted on the polished cross-sectioned samples by SEM (JSM-6700F) and EDX (Oxford INCA EDX). The phase and crystal structures of the interfacial reaction zones were analyzed with an X-ray diffractometer (Rigaku DLLMAX-2200). 3. Results and discussion 3.1. Microstructural analysis The different wetting samples after droplets solidification are presented in Fig. 1. Fig. 1a illustrates the state of a sessile drop of Ag-6.6 mol% Cu quenched immediately after heating to 970 °C. Fig. 1b illustrates the state after 10 min holding at 970 °C; Fig. 1c corresponds to the state after 20 min holding at 970 °C. As the holding time for liquid braze at high temperature increased, the liquid phase gradually flowed out the braze droplet and a halo was observed around each solidified braze droplet on BCFNO substrate. The polished crosssection of the interfaces of wetting samples were prepared to determine the microstructure and reactions between the braze alloy and the substrate. The center and the region near the triple line of the wetting
Halo
Halo
Halo
BCFNO
BCFNO
BCFNO
Ag6.6Cu drop Ag6.6Cu drop
Ag6.6Cu drop 3mm
3mm
Fig. 1. Images of the top view of wetting samples holding at 970 °C for (a) 0 min, (b) 10 min and (c) 20 min.
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Fig. 2. Cross-section SEM micrographs of the center interface between Ag-6.6 mol% Cu drop and BCFNO substrate holding at 970 °C for 0 min.
well with the previous EDS analysis. For easy compare, XRD pattern of fresh BCFNO membrane was also included in Fig. 5. For joining of MIEC-OTM ceramics with RAB, wetting of Ag-Cu alloys on LSCFO and BSCFO in air had been investigated. No metathetic reaction between the braze and LSCFO substrate was found, but the apparent compositional substitution (Cu2+↔Co2+) occurred between copper oxide and LSCFO [7,9]. The copper-cobalt-oxygen rich product was observed along with BSCFO and silver. The interface contained Co3O4, CoO and CuO phases identified by XRD pattern [13]. They proposed that a selective reaction of the copper oxide in the sessile drop with cobalt in the BSCFO substrate. In present study, CoCuO2and Ba2Cu3O5+x oxide compounds were found in the interfacial reaction zone. And Cobalt-riched oxides precipitated along the BCFNO grain boundaries. In the interfacial reaction zone, the island structure that the grey phase was encircled by the dark phase was formed. Combining the XRD with EDS analysis, the dark phases should be composed of CoCuO2 and Cu oxides and the grey phase should contain Ba2Cu3O5+x oxide. This indicated that liquid Cu-O phase penetrated along the BCFNO grain boundary and reacted with the Cobalt-riched oxide separated from BCFNO to form the dark phases. At the same time, the compositional substitution (Cu↔Co) occurred between Cu-O phase and BCFNO. The grey phase formed. Based on the previous characterization, the thickness of the different interfacial reaction layers as a function of the dwelling time at 970 °C was aggregated in Fig. 6. With increasing the dwelling time, the total thickness of the reaction layer increased lineally. And the thickness of the island-like reaction layer correlated directly with dwelling time, with an abrupt change in slope observed for dwelling time of 10 min. The thickness of the island-like reaction layer increased slightly. This indicated that the diffusion of Cu-O from the liquid braze
Table 1 The composition of different points in Fig. 2 (mol%). Position Ba Co Fe Cu O Ag
1
2
60 40 100
3
4
5
6
7
10.4 – 55.3 34.3 –
34 25.4 5.9 1.8 32.8 –
37.1 23.5 7.8
4.1 6.9
24.4 37.2 6.2
31.6
55.6 33.5
8
32.3 100
sponding upper layer A with an island structure increased to around 11 µm thick. The interface containing similar three regions as denoted in Fig. 4a were also found after the wetting sample held at 970 °C for 20 min and immediately quenched to room temperature. The copper oxide was also found within the filler matrix (as denoted by the white arrow in Fig. 4b). More filler materials have infiltrated into the BCFNO bulk and reacted with each other (Fig. 4c and d). Thickness of the whole interfacial reaction layer increased to about 180 µm and the islandlike reaction layer was approximately 13 µm thick for this case. Based on the previous analysis in wetting BCFNO by Ag-6.6 mol% Cu in air, the composition of the reaction products were composed of Ba-Cu-O and Co-Cu-O oxides according to the BaO-CuO and CoO-CuO phase diagrams [21,22]. Micro XRD was used to precisely identify the new phases formed in the reaction zone. Fig. 5 presents the XRD patterns obtained at the interfacial reaction zones between BCFNO substrates and Ag-6.6% Cu braze. It was found that the new phases were composed mainly of Ba2Cu3O5+x, CoCuO2 and CuO in the reaction interfaces of the wetting samples (Fig. 5b, c and d). This also correlated 812
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Fig. 3. Cross-section SEM micrographs of the center interface between Ag-6.6 mol% Cu drop and BCFNO substrate holding at 970 °C for 10 min.
and d respectively. As shown in Fig. 7b, the island structure that the grey phase was encircled by the dark phase was also observed in most part of the ridge (zone Ⅲ). The microstructure was similar to that in the upper layer A at the center interface (Fig. 2c). The corresponding EDS analyses revealed that the dark phase was rich in Co, Cu and O and the grey phase was composed of Ba, Co, Fe, Cu and O. Those results indicated that the ridge region and the upper layer A in the center interfacial reaction zone were integrated and continuous (as denoted by the white arrow in Fig. 7d). Between the ridge and the bulk BCFNO, there was a diffusion reaction layer (zone Ⅳ), which was integrated with the low layer B in the center interfacial reaction zone (Fig. 2c). A mixture of island structure (denoted A in Fig. 7c) and diffusion reaction structure (denoted B in Fig. 7c) was observed in the ridge near the triple line. Fig. 8a and c present the cross-section SEM micrographs of the interface near the triple line of the solidified wetting samples after heating and holding at 970 °C for 10 min and 20 min respectively. Two similar ridges were observed around the solidified braze droplets on the BCFNO substrates. The high magnification cross-sections of the triple line ridges were illustrated in Fig. 8b and d respectively. As the holding time for liquid braze at high temperature increased, the microstructure became uniform and the island structure was observed for two sessile droplet samples. And the ridges extended to the interfacial center below the braze alloy drops. The reaction products contained similar Co-Cu-O and Ba-Cu-O rich phases according to EDS analyses. As denoted by short arrows in Fig. 8a and c, the fronts of the ridges on the BCFNO substrate and the interfacial reaction in the BCFNO substrate were misaligned. In the initial stage, the ridge spread rapidly compared with the growth of the interfacial reaction layer in the BCFNO bulk (as denoted by short arrows). With increasing the reaction time, a part of
might be a controlling step. Under present experimental condition, a maximum thickness of the reaction layer amounted to about 180 µm. A ~200 µm thick layer with micro-structure changes was observed in the Ba0.5Sr0.5Co0.8Fe0.2O3–δ ceramic wetting by Ag-4 mol% Cu in air [12]. For a commercial process, the thickness of the dense ceramic membranes integrated into a pilot module is usually about one thousand microns. Excessive reaction between the ceramic membranes and the braze alloy can have a deleterious effect on not only the membranes stability but also the joint strength. A thin, continuous reaction product layer is desirable and the optimum thickness of the reaction product layer varies for different ceramic/braze alloy systems. Further study is needed to control the reaction kinetics and obtain an optimum interfacial microstructure. 3.1.2. The region near the triple line of the wetting drops Fig. 7 shows the cross-section SEM micrographs of the interface at the triple line of the solidified wetting sample after heating to 970 °C and immediately quenching. A ridge was observed around the solidified braze droplet on the BCFNO substrate (Fig. 7a). The ridge corresponded to the halo observed in Fig. 1a. The typical microstructure of the cross-section essentially consisted of four distinct zones marked as Ⅰ-Ⅳ, in Fig. 7a. In the center of the braze droplet (zone Ⅰ), copper oxide precipitates were found within the pure silver matrix. The micrograph was same with that of the silver matrix shown in Fig. 2b. Around the zone Ⅰ, there was a lean Cu-oxide area (zone Ⅱ) near the interface within the braze drop. Most of Cu-oxide in zone Ⅱ diffused into the interface and reacted with the BCFNO substrate to form the interfacial reaction layers, zone Ⅲ (the ridge) and zone Ⅳ. In order to precisely indentify the microstructure of the triple line ridge, the high magnification cross-sections of different local regions were presented in Fig. 7b, c 813
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Fig. 4. Cross-section SEM micrographs of the center interface between Ag-6.6 mol% Cu drop and BCFNO substrate holding at 970 °C for 20 min.
Fig. 6. The thickness of reaction layer as a function of dwelling time at 970 °C.
interfaces near the triple line of the solidified wetting samples. When the braze alloy began melting, a low microscopic contact angle was observed near the triple line (Fig. 9a). Surprisingly, a microscopic contact angle exhibited obtuse after the braze melt completely (Fig. 9b). Though no ridge at the triple line was found after the droplet solidification for this case, an obvious interfacial reaction layer had formed under the braze droplet. With the temperature increasing and the interfacial reaction proceeding, a ridge began formed near the triple line (Fig. 9c and d). Because there was no enough liquid reaction products in the ridge at high temperature, an incontinuous ridge (denoted by the discrete lines in Fig. 9c) was observed due to the shrinkage during the droplet solidification. An obvious ridge was
Fig. 5. Micro XRD patterns for the interfacial reaction zones of different samples after wetting experiments.
the liquid phase in the ridge infiltrated into the BCFNO bulk and the ridge volume around the braze alloy began to decrease. As the liquid consumed and the reaction layer in the BCFNO bulk extended, the ridge front might leave behind the reaction layer due to the shrinkage during the solidification. To clarify the ridge formation at the triple line, another series of sessile drop experiments were conducted in air. After heating to a certain temperature, each wetting sample was cooled quickly in furnace. Fig. 9 presents the cross-section SEM micrographs of the
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Fig. 7. Cross-section SEM micrographs of the interface near the triple line between BCFNO substrate and Ag-6.6 mol% Cu: (a) the whole micrographs of the interface; (b) high magnification cross-section of the ridge encircling the braze; (c) high magnification cross-section of the ridge near the substrate; (d) the ridge extending to the interfacial center.
penetration and reaction layer. The present investigations clearly indicated that the wetting of BCFNO by Ag-Cu alloys in air was achieved through the formation of an interfacial reaction zone and a chemical wetting mechanism. As shown in Fig. 10, for reactive air brazing with Ag-Cu-O, the reactive spreading, wetting and ridge formation can be deduced as follows: (1) Formation of liquid Ag-Cu-O: With the temperature increasing, the braze melts in contact with BCFNO substrate in air. Exposure of the Ag-Cu melt to oxygen in air changed the originally binary metallic alloy system into a ternary Ag-Cu-O system via the following reaction (1) (StageⅠin Fig. 10). The molten brazing filler decomposed into two demixed liquid phases, an Ag-rich L2 liquid and an L1 liquid rich Cu and O.
formed at the triple point when the wetting sample was heated to 965 °C. The triple line ridges encircled the circular section of the solidified sessile drops. 3.2. Interfacial reaction mechanism and ridge formation Based on the experimental results, it is evident that interfacial reactions occurred between BCFNO and Ag-6.6%mol Cu during the wetting process in air. Firstly, at high temperature, exposure of the AgCu melt to oxygen changed the originally binary metallic alloy system into a ternary Ag-Cu-O metallic/nonmetallic thermodynamic system. This was confirmed by the results that only copper oxide no metal copper was observed in the solidified braze droplets (Fig. 2b, Fig. 3b and Fig. 4b). According to Ag-CuO phase diagram [9], for the Ag-6.6 mol% Cu used in present experiment, when the Ag-Cu-O braze was heated to 970 °C, two liquid phases formed, one which was rich in silver (L2), and the other rich in copper (L1). The general phase formation sequence was (L1+L2→L2+CuO→CuO+fcc-Ag) during brazing temperature decreasing. The silver-rich liquid was the major phase. Because the two liquid phases were immiscible, it is expected that they will segregate. The copper-rich liquid preferentially migrated to wet the BCFNO surfaces because of its higher CuO content and therefore lower the interfacial energy with the oxide substrate. The capillary forces also might drive the liquid to infiltrate the adjacent BCFNO porosity. The penetration of the copper-rich liquid occurred along the grain boundary regions of BCFNO. The molten copper oxide in copper-rich liquid reacted with BCFNO substrate to form the interfacial compounds such as Co-Cu-O and Ba-Cu-O. The remained silver was found in the
Ag-Cu + O2(in air) → Ag-Cu-O (L1+L2)
(1)
(2) Initial spreading: At the stage Ⅱ in Fig. 10, the capillary forces drive a liquid Ag-Cu-O braze towards a shape of constant curvature and the contact angle towards the θ value (Fig. 11a) given by the traditional wetting equation [23]. In Fig. 11, γlv and γsv are the liquid (Ag-Cu-O braze) and solid surface energies, and γsl is the solid (BCFNO substrate)/liquid (Ag-Cu-O braze) interfacial energy. At the same time, Cu-O in liquid braze diffuses and adsorbs at the solid-liquid interface. With high Cu content, the liquid braze displayed the lowest interfacial energy with the oxides ceramic substrate and the L1 liquid rich Cu and O preferentially wet the ceramic substrate to the complete exclusion of the silver-rich L2 liquid [9]. (3) Interaction between Cu-O and BCFNO: With the diffusion and migration of liquid Cu and O-rich phase to the brazing interface, the penetration of copper oxide occurred along the grain boundary of 815
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Fig. 8. Cross-section SEM of the interfaces at the triple line of the wetting samples after holding at 970 °C for 10 min (a, b) and 20 min (c, d).
φs, φl and φv were the equilibrium dihedral angles in the solid (interfacial reaction layer between the molten braze and BCFNO substrate), liquid (molten Ag-Cu-O alloy) and vapor (air) phases, respectively. Under such conditions, a small ridge developed at the triple junction as a result of the migration of the interfacial reaction products near the triple line to satisfy the wetting equation for ridge formation. During the initial spreading, it is proposed that a small enough ridge can be carried by the triple junction under certain condition, resulting in a variable macroscopic angle while simultaneously satisfying the two independent relations for ridge formation locally [25]. As a consequence, the motion of the triple line can be inhibited by dragging from the ridge. When the reaction front spread ahead of the liquid braze, a triple line ridge became big enough to essentially arrested the liquid braze spreading. Subsequently, the reaction product will form rather uniformly under the braze. Due to the limitation by the ridge drag and the surface tension, the microstructure of the braze near the triple line changed from the initial contacting status on BCFNO substrate to a spheroidicity as shown in Fig. 11a and b respectively. Based on the previous discussion, ridging at the triple line indicated that the diffusion rate and the reaction kinetics at the wetting interface of Ag-6.6 mol% Cu/BCFNO were relative rapid. The initial isolated nucleation and the formed ridge at the triple line can stall the spreading of the liquid braze. The liquid front remained attached to the ridge and decelerated. If the arrested macroscopic angle is large enough, the front can be disloged from this position and move ahead by some driving force. The front of the liquid braze will move ahead until it is virtually re-arrested by a growing ridge. The occurrence of such arrest and jump-off may depend on heterogeneities that trigger ridge formation
BCFNO. At the same time, the following reactions occurred. The reaction products nucleated, grew at the interface and finally covered the interface (Stage Ⅲ in Fig. 10). Co oxide phase separation: BaCo0.7−xFe0.2Nb0.1O3-δ-y+Co-oxide
BaCo0.7Fe0.2Nb0.1O3-δ→ (2)
Reaction or solid solution: Cu-O+Co-oxide→CoCuO2
(3)
Exchange reaction: Cu2+↔Co2+
(4)
The overall reaction was: BCFNO+Ag-Cu-O→Ba2Cu3O5+x+CoCuO2+ Ag (5) (4) Formation of the ridge: For molten metals or oxides on ceramics, exposure temperatures are typically ≥(0.2–0.5)Tm (Tm being the substrate melting point), which will allow some local diffusion or solution of the ceramics [24]. During the preparation of the membrane samples, we found the melting temperature of BCFNO substrate was about 1250 °C. The wetting temperature of 970 °C in present study was > (0.2–0.5)Tm (Tm≈1250 °C). So, the local diffusion of BCFNO substrate into the liquid reaction products or molten Ag-Cu alloy can occur (as shown in Figs. 7 and 8). With the interfacial reactions occurring, the initial triple line ridge formed near the spreading front (Stage Ⅳ in Fig. 10). The ridge evolved and propagated until certain equilibrium was attained (Stage Ⅴ and Ⅵ in Fig. 10). As shown in Fig. 11b, the microscopic 2D (two-dimensional, 2D, horizontally and vertically) equilibrium was established by the migration of the interfacial reaction products in the region close to the triple line according to the wo independent relations for ridge formation [25]. In Fig. 11(b), 816
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Fig. 9. Cross-section SEM of the interfaces near the triple line of the wetting samples after heating to (a) 950 °C, (b) 955 °C, (c) 960 °C and (d) 965 °C.
Fig. 10. Schematic presentation of the reactive spreading model for the wetting system of Ag-Cu/BCFNO in air: adsorption, compound formation and ridging [with vertical magnification].
stochastically in limited composition ranges [25]. However, a similar behavior had not been observed for Ag-Cu-O liquid on BCFNO substrate. With the interfacial reactions proceeding, the liquid Ag-Cu-
O braze spread slowly very much. There was not an additional thermodynamic driving force big enough to overcome the pinning effect of the ridge resulting in achieving an equilibrium contact angle. 817
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Fig. 11. Sketch shows expected shapes of the triple zone before the ridge formation (a), after the ridge formation (b) and a hypothetical status (c).
References
The decreasing of the macroscopic contact angle was due to the evolution of the triple line ridge not the liquid braze spreading. The rapid interfacial reaction and ridging at the spreading front actually stalled the liquid braze spreading, rather than a continuous layer of smoother, barrier-free reaction phase forming and driving rapidly and accompanied spreading (Fig. 11(c)). The special contacting status between the Ag-Cu-O braze and BCFNO membrane provides important boundary conditions for modeling the mechanical behavior of the joining modules.
[1] J. Sunarso, S. Baumann, J.M. Serra, W.A. Meulenberg, S. Liu, Y.S. Lin, J.C. Diniz da Costa, Mixed ionic–electronic conducting (MIEC) ceramic-based membranes for oxygen separation, J. Membr. Sci. 320 (2008) 13–41. [2] P.-M. Geffroy, J. Fouletier, N. Richet, T. Chartier, Rational selection of MIEC materials in energy production processes, Chem. Eng. Sci. 87 (2013) 408–433. [3] Y.Y. Wei, W.S. Yang, J.ürgen Caro, H.H. Wang, Dense ceramic oxygen permeable membranes and catalytic membrane reactors, Chem. Eng. J. 220 (2013) 185–203. [4] X.L. Dong, W.Q. Jin, Mixed conducting ceramic membranes for high efficiency power generation with CO2 capture, Curr. Opin. Chem. Eng. 1 (2012) 163–170. [5] S. Engels, T. Markus, M. Modigell, L. Singheiser, Oxygen permeation and stability investigations on MIEC membrane materials under operating conditions for power plant processes, J. Membr. Sci. 370 (2011) 58–69. [6] S.M. Hashim, A.R. Mohamed, S. Bhatia, Current status of ceramic-based membranes for oxygen separation from air, Adv. Colloid Interface Sci. 160 (2010) 88–100. [7] J.S. Hary, J.Y. Kim, K.S. Weil, Joining mixed conducting oxides using an air-fired electrically conductive braze, J. Electrochem. Soc. 151 (2004) J43–J49. [8] K.S.Weil, D.M.Paxton,in: H.-T. Lin, M. Singh, (Eds.) Proceedings of the 26th Annual Conference on Composites, Advanced Ceramics, Materials, and Structures: A, American Ceramic Society, Westerville, OH, 2002. [9] K.S. Weil, J.Y. Kim, J.S. Hardy, Interfacial analysis of (La0.6Sr0.4)(Co0.2Fe00.8)O3-δ substrates wetted by Ag-CuO, J. Mater. Sci. 40 (2005) 2341–2348. [10] S. Dabbarh, E. Pfaff, A. Ziombra, A. Bezold, Bezold, Brazing of MIEC ceramics to high temperature metals, in: design, development, and applications of engineering ceramics and composites, Ceram. Trans. 215 (2010) 213–223. [11] K. Bobzin, T. Schlafer, N. Kopp, Thermochemistry of brazing ceramics and metals in air, Int. J. Mater. Res. 102 (2011) 972–976. [12] A. Kaletsch, J. Hummes, E.M. Pfaff, C. Broeckmann, Joining oxygen transport membranes by reactive air brazing, in: R. Gourley, C. Walker (Eds.) Proceedings of the 5th International Brazing and Soldering Conference, ASM International, Materials Park, 2012, pp. 437–443 [13] V.V. Joshi, A. Meier, J. Darsell, K.S. Weil, M. Bowden, Trends in wetting behavior for Ag-CuO braze alloys on Ba0.5Sr0.5Co0.8Fe0.2O3-δ at elevated temperatures in air, J. Mater. Sci. 48 (2013) 7153–7161. [14] K.M. Erskine, Brazing perovskite ceramics with siliver/copper oxide braze alloys, J. Mater. Sci. 37 (2002) 1705–1709. [15] K.S. Weil, C.A. Coyle, J.S. Hardy, J.Y. Kim, G.G. Xia, Alternative planar SOFC sealing concepts, Fuel Cells Bull. 5 (2004) 11–16. [16] J.Y. Kim, J.S. Hardy, K.S. Weil, Novel metal-ceramic joining for planar SOFCs, J. Electrochem. Soc. 152 (2005) 52–58. [17] J.Y. Kim, J.S. Hardy, K.S. Weil, Effects of CuO content on the wetting behavior and mechanical properties of a Ag-CuO braze for ceramic joining, J. Am. Ceram. Soc. 88 (2005) 2521–2527. [18] Y.W. Zhang, Q. Li, P.J. Shen, Y. Liu, Z.B. Yang, W.Z. Ding, X.G. Lu, Hydrogen amplification of coke oven gas by reforming of methane in a ceramic membrane reactor, Int. J. Hydrog. Energy 33 (2008) 3311–3319. [19] Y.W. Zhang, J. Liu, W.Z. Ding, X.G. Lu, Performance of an oxygen-permeable membrane reactor for partial oxidation of methane in coke oven gas to syngas, Fuel
4. Conclusions In current study, the high-temperature wetting and interfacial reaction of Ag-6.6 mol% Cu alloy on BCFNO membrane in air was investigated. The cross-sectional microstructure of the center and the triple line of the solidified sessile drops were characterized by SEMEDS and XRD. Two distinctive interfacial reaction layers were formed between the Ag-6.6 mol% Cu braze and the substrates. The new phases in reaction layers were composed of CoCuO2, Ba2Cu3O5+x, Co-riched oxide and Cu oxides. With increasing the time for the liquid braze, the thickness of the reaction layers increased. Rapid interfacial reaction caused the ridging at the triple line. Due to the pinning of the triple line ridge, the front spreading of the liquid braze was limited. Low macroscopic contact angles were observed due to the evolution of the ridge composed of the interfacial reaction products near the tripe line, rather than the liquid braze spreading. In wetting of the Ag-6.6 mol% Cu/BCFNO system in air, ridging led to a special contacting status between the solidified braze and BCFNO membrane at the triple line.
Acknowledgments The authors would like to thank Yuliang Chu for SEM analysis work. This work was supported by the National Natural Science Foundation of China (Grant No. 51174133, 51274139 and 51225401). 818
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oxide/copper oxide in air, J. Inorg. Nucl. Chem. 30 (1968) 747–753. [23] A.W. Adamson, Physical Chemistry of Surface, 4th edn, John Wiley & Sons, New York, 1982. [24] E. Saiz, R.M. Cannon, A.P. Tomsia, Reactive spreading: adsorption, ridging and compound formation, Acta Mater. 48 (2000) 4449–4462. [25] E. Saiz, A.P. Tomsia, R.M. Cannon, Ridge effects on wetting and spreading of liquids and solids, Acta Mater. 46 (1998) 2349–2361.
90 (2011) 324–330. [20] Y.W. Zhang, H.W. Cheng, J. Liu, W.Z. Ding, Performance of a tubular oxygenpermeable membrane reactor for partial oxidation of CH4 in coke oven gas to syngas, J. Nat. Gas. Chem. 19 (2010) 280–283. [21] T.B. Lindermer, E.D. Specht, The BaO-Cu-CuO system Solid-liquid equilibria and thermodynamics of BaCuO2 and BaCu2O2, Physica C: Supercond. 255 (1995) 81–94. [22] F.C.M. Driessens, G.F. Rieck, H.N. Coehen, Phase equilibria in the system cobalt
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Interfacial reaction and microstructural evolution of Ag-Cu braze on BaCo0.7Fe0.2Nb0.1O3-δ at high temperature in air
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Yuwen Zhang , Lili Zhang, Wei Guo, Ting Liu, Chengzhang Wu, Weizhong Ding, Xionggang Lu State Key Laboratory of Advanced Special Steel & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Ag-Cu braze Mixed ionic-electronic conducting oxides ceramics Reactive air brazing Wetting Interfacial reaction Ridge evolution
Based reactive air brazing technique, interfacial reaction and microstructure formation of Ag-6.6 mol% Cu wetting on BaCo0.7Fe0.2Nb0.1O3-δ(BCFNO) membrane in air were investigated. The interfacial microstructural analysis revealed that at high temperature Ag-Cu alloy transformed into Ag-Cu-O braze in air and the liquid braze penetrated into BCFNO matrix along the grain boundaries. Cu-O in the liquid braze reacted with BCFNO to form CoCuO2 and Ba2Cu3O5+x confirmed by XRD. Ridge formed quickly at the triple line as a result of the migration of the interfacial reaction products and in response to the vertical component of force from surface tension. The rapid interfacial reaction and ridging at the spreading front actually stalled the spreading of the liquid braze, rather than a continuous layer of smoother, barrier-free reaction phase forming and driving accompanied spreading. Low macroscopic contact angles between the braze and BCFNO substrate were observed due to the evolution of the ridge composed of the interfacial reaction products near the tripe line.
1. Introduction Mixed ionic/electronic conducting oxygen transport membranes (MIEC-OTM) have been used for oxygen separation, catalytic membrane reaction and oxyfuel combustion processes [1–6]. For an industrial application, the gas-tight joining of the membranes to metal components in MIEC-OTM systems is an engineering challenge because of high thermal expansion coefficient, low chemical stability and strength of the membranes. Metal brazing has been attracting more and more attentions for sealing MIEC-OTM modules to metal interconnect components. Due to the good ductility, oxidation resistance and compatibility with the MIECOTM modules and metal components, Ag-based alloys have been widely used as metallic sealing fillers [7]. To avoid the stability degradation of the MIEC-OTM during the brazing at a reducing atmosphere or high vacuum and the deterioration of the joint during the high temperature operation at an oxidizing atmosphere, reactive air brazing (RAB) was developed by Weil et al. [8]. RAB is a method that forms a metallic joint directly in air without the use of fluxes and other special atmosphere. The wetting of (La0.6Sr0.4)(Co0.2Fe0.8)O3-δ (LSCFO) by Ag-Cu alloys with RAB was investigated by Weil group [7,9]. They found that the wettability of the Ag-CuO brazes on LSCFO substrates increased with CuO content up to 34% while a nonwetting behavior was observed for pure silver. The addition of CuO to silver enhanced wetting on LSCFO substrates through
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a non-reactive mechanism. The wettability between Ag-CuO alloys and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCFO) membrane in air was improved as the CuO content increased and a reaction product layer formed along the wetting interface [10–13]. A continuous Pb(Mg0.33Nb0.67)O3(PMN) /silver interface was observed without an interfacial reaction product and the wetting was controlled by a physical wetting mechanism [14]. The wetting of Ag-Cu alloys on the zirconia and alumina ceramics was also improved due to the presence of Cu oxides and the new reaction products along the joining interface [15–17]. Most characterizations on the microstructures were focused on the interfacial cross section for the center of the solidified Ag-Cu sessile drops on the substrates. Near the triple line of the sessile drop on the substrates, the reaction and microstructure development directly affects the spreading and wetting behaviors. Ridging occurred at the triple line for higher CuO contents (2 mol% and 8 mol%) sessile drop on BSCFO in air and no ridge was observed for solidified Ag-1 mol% CuO [13]. However, there was no ridge found in wetting test with Ag4 mol% CuO and a pronounced ridge at the triple point formed for Ag16 mol% CuO on BSCFO [12]. The mechanism on the ridge formation and its effect have not been reported in these studies. An understanding of the wetting, spreading, interfacial reaction and ridging behaviors during high-temperature brazing in air is critical. To the best of our knowledge, the relative study has not been published, especially for Ag-Cu braze alloys wetting on MIEC-OTMs with RAB.
Corresponding author. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2016.10.012 Received 20 June 2016; Received in revised form 30 September 2016; Accepted 2 October 2016 Available online 13 October 2016 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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samples were characterized separately.
Our group has used BaCo0.7Fe0.2Nb0.1O3-δ (BCFNO) reactors to produce hydrogen by partial oxidation of methane in coke oven gas (COG) and investigated the fabrication and performance of the membrane reactors in lab [18–20]. To develop a commercial process, the gas-tight BCFNO membrane tubes must be brazed to metal supports using Ag-based alloys to construct high temperature modules. In this paper, the interfacial reactions between Ag-Cu braze and BCFNO in air were investigated systematically. The microstructural evolution of the joining interfaces at the center and near the triple line of the sessile drops was studied. The formation of the triple line ridge during high-temperature brazing in air was given special attention.
3.1.1. The center of the wetting drops The micrographs in Fig. 2 display the cross-section microstructure of the center of three solidified wetting sample quenched immediately after heating to 970 °C. The corresponding EDS analysis taken on the different denoted in Fig. 2 was given in Table 1. It can be seen from Fig. 2a that there were three zones for the cross-section of Ag-6.6 mol% Cu/BCFNO wetting sample: Ag-based filler (Ⅰ), interfacial reaction zone (Ⅱ) and the bulk of BCFNO substrate (Ⅲ). Below the braze drop, an interfacial reaction layer about 100 µm thick existed in the substrate. For the zone of Ag-based filler, the braze matrix was pure silver (position 1 in Fig. 2b and Table 1). Copper oxide precipitates were found within the pure silver matrix (position 2 in Fig. 2b). The EDX data collected on these oxide precipitates indicated that their composition was a nearly 1:1 ratio of CuO and Cu2O (Table 1, as position 2 in Fig. 2b). The result was consistent with that reported by Weil [9]. With temperature decreasing, CuO partially dissociated due to the decrease of oxygen activity in silver. Silver displays no solubility for copper oxide in the solid state and CuO dissolved in the liquid silver melt precipitated within the bulk filler metal. The similar microstructure of the silver bulk interspersed copper oxide precipitates was also observed in RAB reported in references [7–17]. As shown in Fig. 2c, the interfacial reaction zone consisted of two layers with one thin layer being in contact with the filler metal (denoted by A), and the other layer bordering the bulk of BCFNO substrate (denoted by B). It can be seen from Fig. 2c that, in the layer A, there were two phases: dark phase and grey phase. The grey phase was encircled by the dark phase and the island structure was formed. EDX analyses performed at different places showed that the dark phase (as position 3 in Fig. 2c) was composed of Co, Cu and O (as position 3 in Table 1). The grey phase (as position 4 in Fig. 2c) consisted of Ba, Co, Fe, Cu and O (as position 4 in Table 1). The thickness of the reaction layer A was approximately 4 µm. For the layer B in the interfacial reaction zone, there were about four phases: dark phase, grey phase, high-grey phase and white phase (Fig. 2c and d). EDX analyses revealed that the dark phase was composed of Ba-Co-Cu-O complex phases (as position 6 in Fig. 2d and Table 1) along the BCFNO grain boundaries (as position 5 in Table 1). There were some Co-riched oxides (as position 7 in Table 1) which corresponded to the high-grey phase in the bulk BCFNO. The silver appeared to infiltrate into the underlying substrate (as position 8 in Fig. 2d and Table 1, white phase). After heating and holding at 970 °C for 10 min, three regions for the cross-section of the quenched wetting sample was also observed (Fig. 3a). The central interface microstructure was found to be similar in appearance to that of the wetting sample holding at 970 °C for 0 min. The copper oxide was found within the filler matrix (as denoted by a white arrow in Fig. 3b). The interfacial reaction layer amounted to 120 µm thick. Two layers with different microstructures were also observed in the interfacial reaction zone (Fig. 3c and d). The corre-
2. Experimental The preparation procedures of the BCFNO powder and the membrane disks have been given elsewhere [18]. Prior to wetting tests, the wetting surface of each BCFNO substrate was ground to 10 µm finish and cleaned ultrasonically using acetone and ethanol. Base on our previous study, Ag-6.6 mol% Cu was chosen as brazing alloy used in present experiments. The Ag-Cu braze alloys were prepared by melting pure Ag and Cu in a vacuum induction furnace. And the braze pellets with 3 mm in diameter by 5 mm high were made from the prepared AgCu alloys. The sessile drop experiments were conducted in a tube atmosphere furnace by placing a braze pellet on the polished surface of a substrate. The samples were heated to 970 °C at 10 °C/min in air and hold at 970 °C for 0 min, 10 min and 20 min, respectively. Then each sample was cooled to room temperature in the furnace. After cooling to room temperature, the samples were removed from the furnace. The microstructure and the interface components analysis were conducted on the polished cross-sectioned samples by SEM (JSM-6700F) and EDX (Oxford INCA EDX). The phase and crystal structures of the interfacial reaction zones were analyzed with an X-ray diffractometer (Rigaku DLLMAX-2200). 3. Results and discussion 3.1. Microstructural analysis The different wetting samples after droplets solidification are presented in Fig. 1. Fig. 1a illustrates the state of a sessile drop of Ag-6.6 mol% Cu quenched immediately after heating to 970 °C. Fig. 1b illustrates the state after 10 min holding at 970 °C; Fig. 1c corresponds to the state after 20 min holding at 970 °C. As the holding time for liquid braze at high temperature increased, the liquid phase gradually flowed out the braze droplet and a halo was observed around each solidified braze droplet on BCFNO substrate. The polished crosssection of the interfaces of wetting samples were prepared to determine the microstructure and reactions between the braze alloy and the substrate. The center and the region near the triple line of the wetting
Halo
Halo
Halo
BCFNO
BCFNO
BCFNO
Ag6.6Cu drop Ag6.6Cu drop
Ag6.6Cu drop 3mm
3mm
Fig. 1. Images of the top view of wetting samples holding at 970 °C for (a) 0 min, (b) 10 min and (c) 20 min.
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Fig. 2. Cross-section SEM micrographs of the center interface between Ag-6.6 mol% Cu drop and BCFNO substrate holding at 970 °C for 0 min.
well with the previous EDS analysis. For easy compare, XRD pattern of fresh BCFNO membrane was also included in Fig. 5. For joining of MIEC-OTM ceramics with RAB, wetting of Ag-Cu alloys on LSCFO and BSCFO in air had been investigated. No metathetic reaction between the braze and LSCFO substrate was found, but the apparent compositional substitution (Cu2+↔Co2+) occurred between copper oxide and LSCFO [7,9]. The copper-cobalt-oxygen rich product was observed along with BSCFO and silver. The interface contained Co3O4, CoO and CuO phases identified by XRD pattern [13]. They proposed that a selective reaction of the copper oxide in the sessile drop with cobalt in the BSCFO substrate. In present study, CoCuO2and Ba2Cu3O5+x oxide compounds were found in the interfacial reaction zone. And Cobalt-riched oxides precipitated along the BCFNO grain boundaries. In the interfacial reaction zone, the island structure that the grey phase was encircled by the dark phase was formed. Combining the XRD with EDS analysis, the dark phases should be composed of CoCuO2 and Cu oxides and the grey phase should contain Ba2Cu3O5+x oxide. This indicated that liquid Cu-O phase penetrated along the BCFNO grain boundary and reacted with the Cobalt-riched oxide separated from BCFNO to form the dark phases. At the same time, the compositional substitution (Cu↔Co) occurred between Cu-O phase and BCFNO. The grey phase formed. Based on the previous characterization, the thickness of the different interfacial reaction layers as a function of the dwelling time at 970 °C was aggregated in Fig. 6. With increasing the dwelling time, the total thickness of the reaction layer increased lineally. And the thickness of the island-like reaction layer correlated directly with dwelling time, with an abrupt change in slope observed for dwelling time of 10 min. The thickness of the island-like reaction layer increased slightly. This indicated that the diffusion of Cu-O from the liquid braze
Table 1 The composition of different points in Fig. 2 (mol%). Position Ba Co Fe Cu O Ag
1
2
60 40 100
3
4
5
6
7
10.4 – 55.3 34.3 –
34 25.4 5.9 1.8 32.8 –
37.1 23.5 7.8
4.1 6.9
24.4 37.2 6.2
31.6
55.6 33.5
8
32.3 100
sponding upper layer A with an island structure increased to around 11 µm thick. The interface containing similar three regions as denoted in Fig. 4a were also found after the wetting sample held at 970 °C for 20 min and immediately quenched to room temperature. The copper oxide was also found within the filler matrix (as denoted by the white arrow in Fig. 4b). More filler materials have infiltrated into the BCFNO bulk and reacted with each other (Fig. 4c and d). Thickness of the whole interfacial reaction layer increased to about 180 µm and the islandlike reaction layer was approximately 13 µm thick for this case. Based on the previous analysis in wetting BCFNO by Ag-6.6 mol% Cu in air, the composition of the reaction products were composed of Ba-Cu-O and Co-Cu-O oxides according to the BaO-CuO and CoO-CuO phase diagrams [21,22]. Micro XRD was used to precisely identify the new phases formed in the reaction zone. Fig. 5 presents the XRD patterns obtained at the interfacial reaction zones between BCFNO substrates and Ag-6.6% Cu braze. It was found that the new phases were composed mainly of Ba2Cu3O5+x, CoCuO2 and CuO in the reaction interfaces of the wetting samples (Fig. 5b, c and d). This also correlated 812
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Fig. 3. Cross-section SEM micrographs of the center interface between Ag-6.6 mol% Cu drop and BCFNO substrate holding at 970 °C for 10 min.
and d respectively. As shown in Fig. 7b, the island structure that the grey phase was encircled by the dark phase was also observed in most part of the ridge (zone Ⅲ). The microstructure was similar to that in the upper layer A at the center interface (Fig. 2c). The corresponding EDS analyses revealed that the dark phase was rich in Co, Cu and O and the grey phase was composed of Ba, Co, Fe, Cu and O. Those results indicated that the ridge region and the upper layer A in the center interfacial reaction zone were integrated and continuous (as denoted by the white arrow in Fig. 7d). Between the ridge and the bulk BCFNO, there was a diffusion reaction layer (zone Ⅳ), which was integrated with the low layer B in the center interfacial reaction zone (Fig. 2c). A mixture of island structure (denoted A in Fig. 7c) and diffusion reaction structure (denoted B in Fig. 7c) was observed in the ridge near the triple line. Fig. 8a and c present the cross-section SEM micrographs of the interface near the triple line of the solidified wetting samples after heating and holding at 970 °C for 10 min and 20 min respectively. Two similar ridges were observed around the solidified braze droplets on the BCFNO substrates. The high magnification cross-sections of the triple line ridges were illustrated in Fig. 8b and d respectively. As the holding time for liquid braze at high temperature increased, the microstructure became uniform and the island structure was observed for two sessile droplet samples. And the ridges extended to the interfacial center below the braze alloy drops. The reaction products contained similar Co-Cu-O and Ba-Cu-O rich phases according to EDS analyses. As denoted by short arrows in Fig. 8a and c, the fronts of the ridges on the BCFNO substrate and the interfacial reaction in the BCFNO substrate were misaligned. In the initial stage, the ridge spread rapidly compared with the growth of the interfacial reaction layer in the BCFNO bulk (as denoted by short arrows). With increasing the reaction time, a part of
might be a controlling step. Under present experimental condition, a maximum thickness of the reaction layer amounted to about 180 µm. A ~200 µm thick layer with micro-structure changes was observed in the Ba0.5Sr0.5Co0.8Fe0.2O3–δ ceramic wetting by Ag-4 mol% Cu in air [12]. For a commercial process, the thickness of the dense ceramic membranes integrated into a pilot module is usually about one thousand microns. Excessive reaction between the ceramic membranes and the braze alloy can have a deleterious effect on not only the membranes stability but also the joint strength. A thin, continuous reaction product layer is desirable and the optimum thickness of the reaction product layer varies for different ceramic/braze alloy systems. Further study is needed to control the reaction kinetics and obtain an optimum interfacial microstructure. 3.1.2. The region near the triple line of the wetting drops Fig. 7 shows the cross-section SEM micrographs of the interface at the triple line of the solidified wetting sample after heating to 970 °C and immediately quenching. A ridge was observed around the solidified braze droplet on the BCFNO substrate (Fig. 7a). The ridge corresponded to the halo observed in Fig. 1a. The typical microstructure of the cross-section essentially consisted of four distinct zones marked as Ⅰ-Ⅳ, in Fig. 7a. In the center of the braze droplet (zone Ⅰ), copper oxide precipitates were found within the pure silver matrix. The micrograph was same with that of the silver matrix shown in Fig. 2b. Around the zone Ⅰ, there was a lean Cu-oxide area (zone Ⅱ) near the interface within the braze drop. Most of Cu-oxide in zone Ⅱ diffused into the interface and reacted with the BCFNO substrate to form the interfacial reaction layers, zone Ⅲ (the ridge) and zone Ⅳ. In order to precisely indentify the microstructure of the triple line ridge, the high magnification cross-sections of different local regions were presented in Fig. 7b, c 813
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Fig. 4. Cross-section SEM micrographs of the center interface between Ag-6.6 mol% Cu drop and BCFNO substrate holding at 970 °C for 20 min.
Fig. 6. The thickness of reaction layer as a function of dwelling time at 970 °C.
interfaces near the triple line of the solidified wetting samples. When the braze alloy began melting, a low microscopic contact angle was observed near the triple line (Fig. 9a). Surprisingly, a microscopic contact angle exhibited obtuse after the braze melt completely (Fig. 9b). Though no ridge at the triple line was found after the droplet solidification for this case, an obvious interfacial reaction layer had formed under the braze droplet. With the temperature increasing and the interfacial reaction proceeding, a ridge began formed near the triple line (Fig. 9c and d). Because there was no enough liquid reaction products in the ridge at high temperature, an incontinuous ridge (denoted by the discrete lines in Fig. 9c) was observed due to the shrinkage during the droplet solidification. An obvious ridge was
Fig. 5. Micro XRD patterns for the interfacial reaction zones of different samples after wetting experiments.
the liquid phase in the ridge infiltrated into the BCFNO bulk and the ridge volume around the braze alloy began to decrease. As the liquid consumed and the reaction layer in the BCFNO bulk extended, the ridge front might leave behind the reaction layer due to the shrinkage during the solidification. To clarify the ridge formation at the triple line, another series of sessile drop experiments were conducted in air. After heating to a certain temperature, each wetting sample was cooled quickly in furnace. Fig. 9 presents the cross-section SEM micrographs of the
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Fig. 7. Cross-section SEM micrographs of the interface near the triple line between BCFNO substrate and Ag-6.6 mol% Cu: (a) the whole micrographs of the interface; (b) high magnification cross-section of the ridge encircling the braze; (c) high magnification cross-section of the ridge near the substrate; (d) the ridge extending to the interfacial center.
penetration and reaction layer. The present investigations clearly indicated that the wetting of BCFNO by Ag-Cu alloys in air was achieved through the formation of an interfacial reaction zone and a chemical wetting mechanism. As shown in Fig. 10, for reactive air brazing with Ag-Cu-O, the reactive spreading, wetting and ridge formation can be deduced as follows: (1) Formation of liquid Ag-Cu-O: With the temperature increasing, the braze melts in contact with BCFNO substrate in air. Exposure of the Ag-Cu melt to oxygen in air changed the originally binary metallic alloy system into a ternary Ag-Cu-O system via the following reaction (1) (StageⅠin Fig. 10). The molten brazing filler decomposed into two demixed liquid phases, an Ag-rich L2 liquid and an L1 liquid rich Cu and O.
formed at the triple point when the wetting sample was heated to 965 °C. The triple line ridges encircled the circular section of the solidified sessile drops. 3.2. Interfacial reaction mechanism and ridge formation Based on the experimental results, it is evident that interfacial reactions occurred between BCFNO and Ag-6.6%mol Cu during the wetting process in air. Firstly, at high temperature, exposure of the AgCu melt to oxygen changed the originally binary metallic alloy system into a ternary Ag-Cu-O metallic/nonmetallic thermodynamic system. This was confirmed by the results that only copper oxide no metal copper was observed in the solidified braze droplets (Fig. 2b, Fig. 3b and Fig. 4b). According to Ag-CuO phase diagram [9], for the Ag-6.6 mol% Cu used in present experiment, when the Ag-Cu-O braze was heated to 970 °C, two liquid phases formed, one which was rich in silver (L2), and the other rich in copper (L1). The general phase formation sequence was (L1+L2→L2+CuO→CuO+fcc-Ag) during brazing temperature decreasing. The silver-rich liquid was the major phase. Because the two liquid phases were immiscible, it is expected that they will segregate. The copper-rich liquid preferentially migrated to wet the BCFNO surfaces because of its higher CuO content and therefore lower the interfacial energy with the oxide substrate. The capillary forces also might drive the liquid to infiltrate the adjacent BCFNO porosity. The penetration of the copper-rich liquid occurred along the grain boundary regions of BCFNO. The molten copper oxide in copper-rich liquid reacted with BCFNO substrate to form the interfacial compounds such as Co-Cu-O and Ba-Cu-O. The remained silver was found in the
Ag-Cu + O2(in air) → Ag-Cu-O (L1+L2)
(1)
(2) Initial spreading: At the stage Ⅱ in Fig. 10, the capillary forces drive a liquid Ag-Cu-O braze towards a shape of constant curvature and the contact angle towards the θ value (Fig. 11a) given by the traditional wetting equation [23]. In Fig. 11, γlv and γsv are the liquid (Ag-Cu-O braze) and solid surface energies, and γsl is the solid (BCFNO substrate)/liquid (Ag-Cu-O braze) interfacial energy. At the same time, Cu-O in liquid braze diffuses and adsorbs at the solid-liquid interface. With high Cu content, the liquid braze displayed the lowest interfacial energy with the oxides ceramic substrate and the L1 liquid rich Cu and O preferentially wet the ceramic substrate to the complete exclusion of the silver-rich L2 liquid [9]. (3) Interaction between Cu-O and BCFNO: With the diffusion and migration of liquid Cu and O-rich phase to the brazing interface, the penetration of copper oxide occurred along the grain boundary of 815
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Fig. 8. Cross-section SEM of the interfaces at the triple line of the wetting samples after holding at 970 °C for 10 min (a, b) and 20 min (c, d).
φs, φl and φv were the equilibrium dihedral angles in the solid (interfacial reaction layer between the molten braze and BCFNO substrate), liquid (molten Ag-Cu-O alloy) and vapor (air) phases, respectively. Under such conditions, a small ridge developed at the triple junction as a result of the migration of the interfacial reaction products near the triple line to satisfy the wetting equation for ridge formation. During the initial spreading, it is proposed that a small enough ridge can be carried by the triple junction under certain condition, resulting in a variable macroscopic angle while simultaneously satisfying the two independent relations for ridge formation locally [25]. As a consequence, the motion of the triple line can be inhibited by dragging from the ridge. When the reaction front spread ahead of the liquid braze, a triple line ridge became big enough to essentially arrested the liquid braze spreading. Subsequently, the reaction product will form rather uniformly under the braze. Due to the limitation by the ridge drag and the surface tension, the microstructure of the braze near the triple line changed from the initial contacting status on BCFNO substrate to a spheroidicity as shown in Fig. 11a and b respectively. Based on the previous discussion, ridging at the triple line indicated that the diffusion rate and the reaction kinetics at the wetting interface of Ag-6.6 mol% Cu/BCFNO were relative rapid. The initial isolated nucleation and the formed ridge at the triple line can stall the spreading of the liquid braze. The liquid front remained attached to the ridge and decelerated. If the arrested macroscopic angle is large enough, the front can be disloged from this position and move ahead by some driving force. The front of the liquid braze will move ahead until it is virtually re-arrested by a growing ridge. The occurrence of such arrest and jump-off may depend on heterogeneities that trigger ridge formation
BCFNO. At the same time, the following reactions occurred. The reaction products nucleated, grew at the interface and finally covered the interface (Stage Ⅲ in Fig. 10). Co oxide phase separation: BaCo0.7−xFe0.2Nb0.1O3-δ-y+Co-oxide
BaCo0.7Fe0.2Nb0.1O3-δ→ (2)
Reaction or solid solution: Cu-O+Co-oxide→CoCuO2
(3)
Exchange reaction: Cu2+↔Co2+
(4)
The overall reaction was: BCFNO+Ag-Cu-O→Ba2Cu3O5+x+CoCuO2+ Ag (5) (4) Formation of the ridge: For molten metals or oxides on ceramics, exposure temperatures are typically ≥(0.2–0.5)Tm (Tm being the substrate melting point), which will allow some local diffusion or solution of the ceramics [24]. During the preparation of the membrane samples, we found the melting temperature of BCFNO substrate was about 1250 °C. The wetting temperature of 970 °C in present study was > (0.2–0.5)Tm (Tm≈1250 °C). So, the local diffusion of BCFNO substrate into the liquid reaction products or molten Ag-Cu alloy can occur (as shown in Figs. 7 and 8). With the interfacial reactions occurring, the initial triple line ridge formed near the spreading front (Stage Ⅳ in Fig. 10). The ridge evolved and propagated until certain equilibrium was attained (Stage Ⅴ and Ⅵ in Fig. 10). As shown in Fig. 11b, the microscopic 2D (two-dimensional, 2D, horizontally and vertically) equilibrium was established by the migration of the interfacial reaction products in the region close to the triple line according to the wo independent relations for ridge formation [25]. In Fig. 11(b), 816
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Fig. 9. Cross-section SEM of the interfaces near the triple line of the wetting samples after heating to (a) 950 °C, (b) 955 °C, (c) 960 °C and (d) 965 °C.
Fig. 10. Schematic presentation of the reactive spreading model for the wetting system of Ag-Cu/BCFNO in air: adsorption, compound formation and ridging [with vertical magnification].
stochastically in limited composition ranges [25]. However, a similar behavior had not been observed for Ag-Cu-O liquid on BCFNO substrate. With the interfacial reactions proceeding, the liquid Ag-Cu-
O braze spread slowly very much. There was not an additional thermodynamic driving force big enough to overcome the pinning effect of the ridge resulting in achieving an equilibrium contact angle. 817
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Fig. 11. Sketch shows expected shapes of the triple zone before the ridge formation (a), after the ridge formation (b) and a hypothetical status (c).
References
The decreasing of the macroscopic contact angle was due to the evolution of the triple line ridge not the liquid braze spreading. The rapid interfacial reaction and ridging at the spreading front actually stalled the liquid braze spreading, rather than a continuous layer of smoother, barrier-free reaction phase forming and driving rapidly and accompanied spreading (Fig. 11(c)). The special contacting status between the Ag-Cu-O braze and BCFNO membrane provides important boundary conditions for modeling the mechanical behavior of the joining modules.
[1] J. Sunarso, S. Baumann, J.M. Serra, W.A. Meulenberg, S. Liu, Y.S. Lin, J.C. Diniz da Costa, Mixed ionic–electronic conducting (MIEC) ceramic-based membranes for oxygen separation, J. Membr. Sci. 320 (2008) 13–41. [2] P.-M. Geffroy, J. Fouletier, N. Richet, T. Chartier, Rational selection of MIEC materials in energy production processes, Chem. Eng. Sci. 87 (2013) 408–433. [3] Y.Y. Wei, W.S. Yang, J.ürgen Caro, H.H. Wang, Dense ceramic oxygen permeable membranes and catalytic membrane reactors, Chem. Eng. J. 220 (2013) 185–203. [4] X.L. Dong, W.Q. Jin, Mixed conducting ceramic membranes for high efficiency power generation with CO2 capture, Curr. Opin. Chem. Eng. 1 (2012) 163–170. [5] S. Engels, T. Markus, M. Modigell, L. Singheiser, Oxygen permeation and stability investigations on MIEC membrane materials under operating conditions for power plant processes, J. Membr. Sci. 370 (2011) 58–69. [6] S.M. Hashim, A.R. Mohamed, S. Bhatia, Current status of ceramic-based membranes for oxygen separation from air, Adv. Colloid Interface Sci. 160 (2010) 88–100. [7] J.S. Hary, J.Y. Kim, K.S. Weil, Joining mixed conducting oxides using an air-fired electrically conductive braze, J. Electrochem. Soc. 151 (2004) J43–J49. [8] K.S.Weil, D.M.Paxton,in: H.-T. Lin, M. Singh, (Eds.) Proceedings of the 26th Annual Conference on Composites, Advanced Ceramics, Materials, and Structures: A, American Ceramic Society, Westerville, OH, 2002. [9] K.S. Weil, J.Y. Kim, J.S. Hardy, Interfacial analysis of (La0.6Sr0.4)(Co0.2Fe00.8)O3-δ substrates wetted by Ag-CuO, J. Mater. Sci. 40 (2005) 2341–2348. [10] S. Dabbarh, E. Pfaff, A. Ziombra, A. Bezold, Bezold, Brazing of MIEC ceramics to high temperature metals, in: design, development, and applications of engineering ceramics and composites, Ceram. Trans. 215 (2010) 213–223. [11] K. Bobzin, T. Schlafer, N. Kopp, Thermochemistry of brazing ceramics and metals in air, Int. J. Mater. Res. 102 (2011) 972–976. [12] A. Kaletsch, J. Hummes, E.M. Pfaff, C. Broeckmann, Joining oxygen transport membranes by reactive air brazing, in: R. Gourley, C. Walker (Eds.) Proceedings of the 5th International Brazing and Soldering Conference, ASM International, Materials Park, 2012, pp. 437–443 [13] V.V. Joshi, A. Meier, J. Darsell, K.S. Weil, M. Bowden, Trends in wetting behavior for Ag-CuO braze alloys on Ba0.5Sr0.5Co0.8Fe0.2O3-δ at elevated temperatures in air, J. Mater. Sci. 48 (2013) 7153–7161. [14] K.M. Erskine, Brazing perovskite ceramics with siliver/copper oxide braze alloys, J. Mater. Sci. 37 (2002) 1705–1709. [15] K.S. Weil, C.A. Coyle, J.S. Hardy, J.Y. Kim, G.G. Xia, Alternative planar SOFC sealing concepts, Fuel Cells Bull. 5 (2004) 11–16. [16] J.Y. Kim, J.S. Hardy, K.S. Weil, Novel metal-ceramic joining for planar SOFCs, J. Electrochem. Soc. 152 (2005) 52–58. [17] J.Y. Kim, J.S. Hardy, K.S. Weil, Effects of CuO content on the wetting behavior and mechanical properties of a Ag-CuO braze for ceramic joining, J. Am. Ceram. Soc. 88 (2005) 2521–2527. [18] Y.W. Zhang, Q. Li, P.J. Shen, Y. Liu, Z.B. Yang, W.Z. Ding, X.G. Lu, Hydrogen amplification of coke oven gas by reforming of methane in a ceramic membrane reactor, Int. J. Hydrog. Energy 33 (2008) 3311–3319. [19] Y.W. Zhang, J. Liu, W.Z. Ding, X.G. Lu, Performance of an oxygen-permeable membrane reactor for partial oxidation of methane in coke oven gas to syngas, Fuel
4. Conclusions In current study, the high-temperature wetting and interfacial reaction of Ag-6.6 mol% Cu alloy on BCFNO membrane in air was investigated. The cross-sectional microstructure of the center and the triple line of the solidified sessile drops were characterized by SEMEDS and XRD. Two distinctive interfacial reaction layers were formed between the Ag-6.6 mol% Cu braze and the substrates. The new phases in reaction layers were composed of CoCuO2, Ba2Cu3O5+x, Co-riched oxide and Cu oxides. With increasing the time for the liquid braze, the thickness of the reaction layers increased. Rapid interfacial reaction caused the ridging at the triple line. Due to the pinning of the triple line ridge, the front spreading of the liquid braze was limited. Low macroscopic contact angles were observed due to the evolution of the ridge composed of the interfacial reaction products near the tripe line, rather than the liquid braze spreading. In wetting of the Ag-6.6 mol% Cu/BCFNO system in air, ridging led to a special contacting status between the solidified braze and BCFNO membrane at the triple line.
Acknowledgments The authors would like to thank Yuliang Chu for SEM analysis work. This work was supported by the National Natural Science Foundation of China (Grant No. 51174133, 51274139 and 51225401). 818
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oxide/copper oxide in air, J. Inorg. Nucl. Chem. 30 (1968) 747–753. [23] A.W. Adamson, Physical Chemistry of Surface, 4th edn, John Wiley & Sons, New York, 1982. [24] E. Saiz, R.M. Cannon, A.P. Tomsia, Reactive spreading: adsorption, ridging and compound formation, Acta Mater. 48 (2000) 4449–4462. [25] E. Saiz, A.P. Tomsia, R.M. Cannon, Ridge effects on wetting and spreading of liquids and solids, Acta Mater. 46 (1998) 2349–2361.
90 (2011) 324–330. [20] Y.W. Zhang, H.W. Cheng, J. Liu, W.Z. Ding, Performance of a tubular oxygenpermeable membrane reactor for partial oxidation of CH4 in coke oven gas to syngas, J. Nat. Gas. Chem. 19 (2010) 280–283. [21] T.B. Lindermer, E.D. Specht, The BaO-Cu-CuO system Solid-liquid equilibria and thermodynamics of BaCuO2 and BaCu2O2, Physica C: Supercond. 255 (1995) 81–94. [22] F.C.M. Driessens, G.F. Rieck, H.N. Coehen, Phase equilibria in the system cobalt
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