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Interfacial reaction behavior and bonding mechanism between liquid Sn and ZrO2 ceramic exposed in ultrasonic waves
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Interfacial reaction behavior and bonding mechanism between liquid Sn and ZrO2 ceramic exposed in ultrasonic waves ⁎
Dan Luoa, Yong Xiaoa, , Ling Wangb, Li Liua, Xian Zenga, Mingyu Lic a b c
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China China National Electric Apparatus Research Institute Co, Ltd Guangzhou 510300, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, China
A R T I C L E I N F O
A BS T RAC T
Keywords: ZrO2 ceramic Pure Sn Ultrasound-assisted dipping ZrSnO4
Ultrasound-assisted dipping of ZrO2 ceramics into molten Sn solder was performed to realize the lowtemperature joining of ZrO2 ceramics in this study. Scanning electron microscopy with energy dispersive spectrometer, X-ray diffraction and X-ray photoelectron spectroscopy were employed to study the effects of ultrasonic vibration on the microstructure of Sn/ZrO2 interface, and to elucidate the joining mechanism between Sn coating layer and ZrO2 ceramic. Results showed that, after ultrasonically dipping in molten Sn for 1200 s, a pure Sn solder layer with a thickness of approximately 8–9 µm was coated on the ZrO2 surface. The Sn coating layer exhibited excellent metallurgic bonding with ZrO2 ceramic. A nano-sized ZrSnO4 ternary phase, which was beneficial to the smooth transition of the lattice from Sn solder to ZrO2 ceramic, was formed at the Sn/ZrO2 interface. The formation of ZrSnO4 interlayer was ascribed to the acoustic cavitation induced hightemperature reaction of Sn, O and ZrO2 at the molten Sn/ZrO2 ceramic interface. The tested average shear strength of ZrO2/Sn/ZrO2 joints was approximately 32 MPa, and the shearing failure mainly took place within the Sn solder layer.
1. Introduction As an engineering ceramic, ZrO2 ceramic has been largely used in solid oxide fuel cells (SOFCs) [1–6], dental applications [7,8], optical instruments and electronic devices [3,9] due to its superiority in mechanical, optical and high temperature physical and chemical stabilities. However, it is difficult to fabricate some complex and large ZrO2 ceramic components, which impedes their wide application. To solve these problems, some brazing methods, such as solid-state diffusion bonding and active brazing, were always performed in joining ZrO2 and ZrO2 or ZrO2 and other materials [10–12]. Active brazing was an attractive technique in joining ZrO2 ceramics because of its convenience, cost-effectiveness and high-efficiency [12–15]. However, the brazing temperature during using active filler metals was usually higher than 700 °C. Large residual stress was easy to accumulate at the interface of filler metal layer and ceramic substrate because of the significant mismatch of thermal expansion coefficients between ceramics and metal alloys. Furthermore, the high brazing temperature may destroy the metal counterparts with low melting point. Low temperature soldering is necessary to achieve low residual stress and high accuracy connection between ceramic and ceramic or
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ceramic and metal, especially in applications of some optical instruments and electronic devices with high precision requirement and served in low temperature environment. Unfortunately, the wettability between low melting solders and ZrO2 ceramic is poor. Hence, several pre-metallization methods, such as physical vapor deposition and electroless or chemical plating [7,16], were usually performed to fabricate an easy-wetted metallization layer on the faying surface of ZrO2 substrate. A key problem facing the use of these traditional metallization methods was that the plating process was time consuming and the bonding strength between the metallization layer and ZrO2 substrate was poor. Therefore, an improved method to realize the rapid fabrication of high strength metallization layer on ZrO2 ceramic surface is of importance. Ultrasound-assisted fluxless soldering or brazing process has been largely reported in previous studies [17–20]. The ultrasound-induced cavitation effects have been simply considered as a driving force to promote the wetting of liquid solders on metal substrates by breaking the oxide film on the surface of molten solder and solid substrate [17,18]. Actually, it has been reported that the implosion of cavitation bubble induced by ultrasonic waves inducted in liquid medium could generate localized high-temperature and high-pressure conditions (i.e.
Corresponding author. E-mail address: [email protected] (Y. Xiao).
http://dx.doi.org/10.1016/j.ceramint.2017.03.042 Received 23 January 2017; Received in revised form 6 March 2017; Accepted 6 March 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Luo, D., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.03.042
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“hot spots”) near the bubble, which may produce a non-equilibrium reaction environment at low temperature and dramatically affect the physical-chemical interaction behavior at the solid-liquid interface [21,22]. Chen et al. [19] reported that, a strong SiC/SiC joint with a maximum shear strength of 148.1 MPa was obtained by ultrasonically brazing at 420 °C with a ZnAlMg filler metal. The bonding mechanism between the ZnAlMg filler metal layer and SiC substrate was ascribed to the increased mass transfer from SiO2 to ZnAlMg caused by acoustic cavitation induced erosion effects. Cui et al. [20] found that a nanocrystalline α-alumina layer was built at molten Sn-Zn-Al alloy and sapphire interface with the assistance of ultrasonic waves at a processing temperature of 230 °C, which resulted in a smooth transition of the lattice from sapphire to solder metal. These results imply that it is feasible to fabricate a metallization layer on the ZrO2 substrate surface with the assistance of ultrasonic waves. However, less previous studies have been found on the ultrasound-assisted coating of Sn solder on ZrO2 ceramic surface. Thus, in this study, we attempt to fabricate a pure Sn coating layer on the ZrO2 substrate surface with an ultrasonic dipping process. Emphasis is focused on studying the ultrasonic effects correlated microstructure evolution of Sn/ZrO2 interface. Moreover, the bonding mechanism between pure Sn coating layer and ZrO2 substrate is discussed. Such a study is expected to be helpful in developing a high bonding strength Sn metallization layer on the ZrO2 ceramic surface.
approximately 300 °C and was kept constant at this temperature for 60 s, the ultrasonic vibration with a frequency of 20 kHz and a power of 850 W was imposed on the Ti alloy plate for 1200 s. After that, the ZrO2 ceramic cylinder was taken out of the molten Sn bath and cooled in air. Finally, a Sn metallization layer was coated on the ZrO2 substrate surface. All the ultrasonic dipping process was operated in air condition. The Sn-coated ZrO2 substrates were grinded perpendicularly to the Sn coating layer and polished according to standard metallographic technique. To observe the surface morphology of ZrO2 substrate after ultrasonic dipping treatment, some as-coated ZrO2 substrates were deep etched in 20% nitric acid solution for 1 h, then cleaned in 1% hydrochloric acid alcohol solution to remove the Sn coating layer completely. The morphologies of Sn/ZrO2 interface and deep-etched ZrO2 substrate surface were observed by scanning electron microscopy (SEM, FEI Quanta FEG 250) equipped with an energy dispersive spectrometer (EDS). Line scanning analysis across the Sn/ZrO2 interface was performed with an operation voltage of 20 kV and a current of 10 nA, the scanning step was approximately 0.0637 µm. The phase constitutions of the deep-etched ZrO2 substrate surface were characterized by X-ray diffraction (XRD, D/MAX-RB, Cu Kα) with a scanning step of 0.02°. Moreover, X-ray photoelectron spectroscopy (XPS, ESCALAB 250) measurement was performed to analyze the element constitutions and valance states on the deep-etched ZrO2 surface. Carbon C 1 s line at 285.0 eV was taken as a reference for calibration. To investigate the bonding properties between Sn coating layer and ZrO2 ceramic, two Sn-coated ZrO2 cylinders were soldered together by remelting the Sn coating layer in the Sn-bath. Shear strength tests of ZrO2/Sn/ZrO2 joints were performed in an electron tension testing machine (Instron-5569) with a strain rate of 1 mm/min. Five specimens were prepared to obtain an average shear strength value. During the shear strength test, the as-soldered joints were fixed in a specially designed fixture, the schematic diagram of which can be found in our previous studies [23].
2. Experimental Sintering yttria-stabilized ZrO2 ceramic (purity > 97%, Shenzhen Hard Precision Ceramic Co., Ltd) with a cylinder shape and a dimension of Ø5×5 mm was used as base material. Pure Sn ingots (purity > 99.5%, Yik Shing Tat Industrial Co., Ltd) were used as the coating material. Prior to the ultrasonic dipping process, the faying surface of ZrO2 cylinder was grinded and polished to a mirror finish then ultrasonically cleaned in anhydrous ethanol. Fig. 1 shows the schematic diagram of the ultrasonic dipping process. A T4-Ti alloy plate (20×50×4 mm) with a groove (15 mm in diameter, 3 mm in depth) in the center was used as the Sn bath. It should be noted that, a TiN barrier layer was fabricated on the surface of Ti alloy plate by gas nitriding treatment, so that the erosion of Ti alloy plate during the ultrasonic dipping process could be mitigated. A stainless steel platform was applied to support and confine the Ti alloy plate, and a resistance-heating unit was set at the bottom of this platform. The heating temperature was monitored by inserting a Ktype thermal couple into the Sn bath. An ultrasonic vibration horn was performed perpendicularly on one side of the Ti alloy plate, which could apply a vertical vibration on the Ti alloy plate and induct the ultrasonic waves through the Ti alloy plate into the molten Sn bath. During the ultrasonic dipping process, several Sn ingots (approximately 2.5 g in weight) were put into the Sn bath and heated to liquid state, then a ZrO2 cylinder was placed perpendicularly into the molten Sn bath. When the temperature of molten Sn was increased to
3. Results and discussion Fig. 2 shows the cross-section image of Sn-coated ZrO2 substrate obtained by ultrasonically dipping for 1200 s. An uniform Sn solder layer with a measured thickness of approximately 8–9 µm is created on the ZrO2 surface, as shown in Fig. 2a. The Sn coating layer exhibits excellent bonding with the ZrO2 ceramic, no cracks or pores are found in the Sn coating layer and at the Sn/ZrO2 interface. Fig. 2b shows the magnified image of “A” area marked in Fig. 2a. A discontinuous interface between Sn coating layer and ZrO2 substrate is presented in this image. The ZrO2 substrate is “eroded” by molten Sn solder with the assistance of ultrasonic waves, and the erosion pits on the ZrO2 surface are filled with Sn solder. Some ZrO2 particles can be found in the Sn coating layer located near the Sn/ZrO2 interface, which may be stripped from the ZrO2 substrate. The concentration profiles of Zr and Sn elements across the Sn/ZrO2 interface are shown in Fig. 3. The BC line in Fig. 2a indicates the position analyzed by EDS line scanning. It can be seen that there exists a relatively thin transition zone at the Sn/ZrO2 interface. The line scanning result of Sn element in Fig. 3 reveals that the Sn content declines gently from the Sn coating layer to the ZrO2 substrate. Zr element concentration profile shows that this element also penetrates into transition zone and its content decreases from the transition zone and becomes zero in the Sn coating layer. These results imply that the mutual diffusion of Sn and Zr elements between Sn coating layer and ZrO2 substrate has happened after the ultrasonic dipping treatment, indicating that there formed a metallurgic bonding between Sn metallization layer and ZrO2 ceramic. In order to observe the surface morphology of Sn-coated ZrO2 substrate much more clearly, the Sn coating layer was removed by deep etching treatment and the surface of ZrO2 substrate was exposed. Fig. 4a shows the surface microstructure of the deep-etched ZrO2
Fig. 1. Schematic diagram of ultrasonic dipping process.
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Fig. 2. SEM images of (a) the cross-section of Sn-coated ZrO2 ceramic obtained by ultrasonically dipping in molten Sn for 1200 s, and (b) the magnified “A” area marked in Fig. 2a.
black line in this figure exhibits the XRD pattern of ZrO2 ceramic without ultrasonic treatment. No additional peaks of other phases can be found except the ZrO2 peaks from the measurement. The red line in Fig. 5a displays the XRD pattern of deep-etched ZrO2 ceramic obtained by ultrasonically dipping for 1200 s. It can be seen that the peaks of ZrO2 ceramic along with those of ZrSnO4 ternary phase are indexed in the profile. The peaks of ZrSnO4 ternary phase appear at 31.4° and 34.4° (PDF No. 48–0889), exhibiting an orthorhombic crystal structure according to previous studies [24]. Thus, it can be deduced that the nano-sized particles formed on the surface of ZrO2 substrate obtained by ultrasonically dipping for 1200 s were mainly composed of metastable ZrSnO4 phase. Interestingly, compared with ZrO2 ceramic without ultrasonic dipping treatment, the XRD diffraction peaks of ZrO2 substrate obtained by ultrasonically dipping in molten Sn for 1200 s shift a little to the right side, as shown in Fig. 5b, but maintain a tetragonal crystal structure. Since the XRD characterization has a penetration depth of several micrometers on ZrO2 ceramic surface [25], it can be anticipated that some trace phases may not be detected in this test. The sampling depth of XRS is much less than that of XRD, thus XRS experiments are much more sensitive to surface species. To better understand the elemental compositions of ZrO2 substrate surface, XPS measurements were carried out and the results are shown in Fig. 6. Fig. 6 shows the XPS characterization results of deep-etched ZrO2 ceramic obtained by ultrasonically dipping in molten Sn for 1200 s. The C 1 s spectrum reveals only one peak at 284.7 eV, as shown in Fig. 6a, which corresponds to C-H species and mainly comes from the surface carbon contamination. The broad O 1 s line with a distinct shoulder, as shown in Fig. 6b, is fitted to a combination of components located at 530.2 eV and 532.2 eV, which can be ascribed to O2- ions in lattice and O-H bonds from absorbed hydroxyl groups, respectively. The existence of multi peaks may associated with the presence of surface oxygen defects [26]. The peaks at 182.4 eV and 184.8 eV in the Zr 3d spectrum, as shown in Fig. 6c, are ascribed to spin-orbit splitting of the Zr 3d, Zr 3d5/2 and Zr 3d3/2, respectively. These peaks demonstrate that the valance state of Zr atoms presents as +4 according to previous studies [27]. Fig. 6d shows the curve-fitted Sn 3d XPS spectra. It can be seen that Sn 3d5/2 and Sn 3d3/2 were measured at 486.6 eV and 495.2 eV, respectively, which indicates that the valence state of Sn is +4. A shoulder appears on the right side of Sn 3d3/2, which may be associated with satellites or signal noise. The results above demonstrate that after deep-etching in 20% nitric acid solution for 1 h and cleaning with alcohol, Sn element with a valance state of +4 still exists on the ZrO2 ceramic surface. Furthermore, the valance states of O, Zr and Sn convincingly suggest that the nano-sized particles formed on the Sn/ ZrO2 interface are composed of ZrSnO4 ternary phase. Actually, it was amazed and confused to find that there formed a
Fig. 3. EDS line scans of the Sn/ZrO2 interface of BC line marked in Fig. 2a.
substrate obtained by ultrasonic dipping for 1200 s. The surface of deep-etched ZrO2 substrate exhibits an uneven morphology, some corrosion pits and particles can be found on the deep-etched ZrO2 substrate surface. The EDS analysis results of the region marked by “1” in Fig. 4a demonstrate that these bright particles are composed of 63.68 at% O, 14.86 at% Zr and 21.46 at% Sn elements, respectively. The magnified image of “A” region marked in Fig. 4a is shown in Fig. 4b. It can be seen that some nano-sized particles are presented on the ZrO2 surface. The cross-section image of deep-etched ZrO2 substrate obtained by ultrasonically dipping for 1200 s is shown in Fig. 4c. Obviously, a reaction layer with a thickness range from approximately 0.5 to 2.5 µm can be found on the ZrO2 surface. Fig. 4d shows the surface morphology of deep-etched ZrO2 substrate without ultrasonic dipping treatment. Comparing to the ZrO2 ceramic obtained by ultrasonically dipping in molten Sn for 1200 s, the ZrO2 substrate without ultrasonic dipping treatment exhibits a relatively smooth surface. The results above demonstrate that a Sn metallization layer with an even thickness can be successfully coated on the ZrO2 ceramic surface with the ultrasonic dipping method. The metallurgic bonding between Sn coating layer and ZrO2 substrate may be associated with the nanosized particles formed at the Sn/ZrO2 interface. To make clear the phase composition of these newly formed nano-particles shown in Fig. 4b, XRD analysis was performed and the results are displayed in Fig. 5. Fig. 5a shows XRD patterns of deep-etched ZrO2 ceramics. The 3
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Fig. 4. SEM images of (a) the surface morphology of deep-etched ZrO2 substrate obtained by ultrasonically dipping in molten Sn for 1200 s, (b) magnified “A” region marked in Fig. 3a, (c) the cross-section of deep-etched ZrO2 substrate ultrasonically dipped for 1200 s, and (d) the surface morphology of ZrO2 ceramic without ultrasonic dipping treatment.
by-product of nanocomposite ZrO2-SnO2 thin film obtained by sol-gel co-deposition processing and subsequently annealing at temperatures higher than 500 °C for approximately 1 h. In their studies, the formation of ZrSnO4 bimetallic oxide in the film was ascribed to the maximum solution of Sn4+ ions in ZrO2 lattice obtained upon crystallization. Furthermore, Kim et al. [30] and Gaillard-Allemand et al. [31]
ZrSnO4 ternary phase at the molten Sn/ZrO2 ceramic interface in this study, since the ZrSnO4 phase was usually synthesized via chemiedouce followed by calcining at high temperature for several hours in previous studies [28,31]. A number of literatures have been published on the fabrication of ZrSnO4 ternary phase [9,24,28–31]. For instance, Anitha et al. [9] and Joy et al. [28] found that the ZrSnO4 phase was a
Fig. 5. (a) XRD profiles of deep-etched ZrO2 ceramic obtained by ultrasonically dipping for 1200 s (red line) and ultrasonically dipping for 0 s (black line), (b) the magnified region marked in Fig. 5a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
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Fig. 6. Curve-fitted (a) C 1 s, (b) O 1 s, (c) Zr 3d and (d) Sn 3d XPS core-level spectra of deep-etched ZrO2 ceramic obtained by ultrasonically dipping in molten Sn for 1200 s.
corrosion pits on the surface of ZrO2 substrate after ultrasonic dipping for 1200 s, as shown in Fig. 2b. Furthermore, acoustic cavitation induced micro-jets and shock waves can generate locally extreme high temperature in liquid medium, approximately 1600 °C surrounding the cavitation bubble [18]. In this study, the ultrasonic waves were inducted in molten Sn solder, whose boiling temperature is 2602 °C. The molten Sn can store and deliver the ultrahigh energy generated by bubble collapse without evaporation unlike aqueous solution. Thus, the acoustic cavitation can provide a special high temperature environment at the Sn/ZrO2 interface for the formation of ZrSnO4 phase. As cavitations are more likely to appear at the liquid/solid interface, the oxidation in this place may be more significant than that elsewhere [21]. The O element dissolved in the molten Sn may diffuse continually to the Sn/ZrO2 interface with the help of ultrasonic-induced streaming and acoustic cavitation effects. Ultimately, the high temperature reaction environment produced by cavitation effects may promote the reaction of Sn, O and ZrO2 at the molten Sn/ZrO2 interface, which results in the formation of ZrSnO4 ternary phase on the ZrO2 ceramic surface. The reaction can be expressed as: Sn+2 O+ZrO2→ZrSnO4. The changes in the microstructure of Sn/ZrO2 interface induced by exposure to ultrasonic waves will surely have an influence on the bonding strength of Sn coating layer and ZrO2 substrate. To verify the reliability between Sn coating layer and ZrO2 substrate, ZrO2/Sn/ZrO2 joints were made and shear strength tests were performed. Results show that the average shear strength of ZrO2/Sn/ZrO2 joints was 32 MPa, and the joints were mainly failed in the Sn solder layer.
found that, when a mixed powder of ZrO2 and SnO2 was calcined in a temperature range of 1230–1750 °C, the immiscibility gap existed between ZrO2 and SnO2 could result in two limited solid solution of (Zr1−xSnx)O2 and (Sn1−yZry)O2, and the solubility increased slowly with increasing calcining temperature. These studies demonstrate that both the high reaction temperature and the existence of SnO2 and ZrO2 are necessary to the formation of ZrSnO4 phase. In this study, the ZrSnO4 phase was formed on the surface of ZrO2 ceramic by ultrasonically dipping ZrO2 in molten Sn for only 1200 s; moreover, the ultrasonic dipping temperature was only 300 °C, which was much lower than the reacting temperature mentioned in previous studies. Then, how did the ZrSnO4 ternary phase form at the molten Sn/ZrO2 ceramic interface during the ultrasonic dipping process? Obviously, the formation of ZrSnO4 ternary phase is associated with the ultrasonic waves. During the ultrasonic dipping process, high intensity ultrasonic waves were inducted into the molten Sn solder through the Ti alloy plate. This may induce the creation of acoustic cavitation effects, which is the formation, growth, and implosive collapse of vacuum cavities. The cavities collapse near the ZrO2 surface can induce the formation of shock waves and liquid mirco-jets, which may impinge the ZrO2 surface and create locally extreme high pressure on it. When the distance between cavitation bubble and solid surface is approximately 30–300 µm, the local pressure induced by mirco-jets and shockwaves can reach to approximately 4 GPa [21,22]. Such a high pressure can break the oxide film on the surface of molten Sn and strip the sintered ZrO2 ceramic. That is why there formed extensive
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Obviously, the coarsening of ZrO2 surface and the formation of ZrSnO4 interlayer at the Sn/ZrO2 interface may most probably improve the bonding properties of Sn coating layer and ZrO2 ceramic. 4. Conclusions In this study, ultrasound-assisted dipping process was performed to fabricate a Sn metallization layer on the ZrO2 ceramic surface and thus realize the low temperature soldering of ZrO2 ceramic. The microstructure of Sn/ZrO2 interface and the bonding mechanism between Sn coating layer and ZrO2 ceramic were studied. A pure Sn metallization layer with a thickness of approximately 8–9 µm was successfully coated on the ZrO2 ceramic surface by ultrasonically dipping the ZrO2 ceramic into molten Sn for 1200 s. The ZrO2 ceramic exhibited a coarsening surface and the pits located on the ZrO2 surface were completely filled with Sn solder. Furthermore, there formed a ZrSnO4 bimetal oxide interlayer at the Sn/ZrO2 interface, which resulted in an excellent metallurgic bonding between Sn coating layer and ZrO2 ceramic. The formation of ZrSnO4 ternary phase was attributed to the acoustic cavitation induced high temperature reaction of Sn, O and ZrO2 at the molten Sn/ZrO2 interface. The tested shear strength of ZrO2/Sn/ZrO2 joints was 32 MPa, and the joints were mainly failed in the pure Sn solder layer. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 51605357), Zhongshan Science and Technology Plan Project (No. 2016A1013), and the Fundamental Research Funds for the Central Universities (2015IVA003). References [1] J. Cao, X.G. Song, C. Li, L.Y. Zhao, J.C. Feng, Brazing ZrO2 ceramic to Ti-6Al-4V alloy using NiCrSiB amorphous filler foil: interfacial microstructure and joint properties, Mater. Charact. 81 (2013) 85–91. [2] M.O. Curi, H.C. Ferraz, J.G.M. Furtado, A.R. Secchi, Dispersant effects on YSZ electrolyte characteristics for solid oxide fuel cells, Ceram. Int. 41 (2015) 6141–6148. [3] A.H. Gorji, A. Simchi, A.H. Kokabi, Development of composite silver/nickel nanopastes for low temperature joining of yttria-stabilized zirconia to stainless steels, Ceram. Int. 41 (2015) 1815–1822. [4] S.Y. Zhen, W. Sun, G.Z. Tang, D. Rooney, K. Sun, X.X. Ma, Fabrication and evaluation of NiO/Y2O3-stabilized-ZrO2 hollow fibers for anode-supported microtubular solid oxide fuel cells, Ceram. Int. 42 (2016) 8559–8564. [5] J. Yang, A.F. Molouk, T. Okanishi, H. Muroyama, T. Matsui, K. Eguchi, A stability study of Ni/yttria-stabilized zirconia anode for direct ammonia solid oxide fuel cells, ACS Appl. Mater. Interfaces 7 (2015) 28701–28707. [6] C.L. Chao, C.L. Chu, Y.K. Fuh, R.Q. Hsu, S. Lee, Y.N. Cheng, Interfacial characterization of nickel-yttria-stabilized zirconia cermet anode/interconnect joints with Ag-Pd-Ga active filler for use in solid-oxide fuel cells, Int. J. Hydrog. Energy 40 (2015) 1523–1533. [7] C. Hubsch, P. Dellinger, H.J. Maier, F. Stemme, M. Bruns, M. Stiesch, L. Borchers, Protection of yttria-stabilized zirconia for dental applications by oxidic PVD coating, Acta Biomater. 11 (2015) 488–493.
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Interfacial reaction behavior and bonding mechanism between liquid Sn and ZrO2 ceramic exposed in ultrasonic waves ⁎
Dan Luoa, Yong Xiaoa, , Ling Wangb, Li Liua, Xian Zenga, Mingyu Lic a b c
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China China National Electric Apparatus Research Institute Co, Ltd Guangzhou 510300, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, China
A R T I C L E I N F O
A BS T RAC T
Keywords: ZrO2 ceramic Pure Sn Ultrasound-assisted dipping ZrSnO4
Ultrasound-assisted dipping of ZrO2 ceramics into molten Sn solder was performed to realize the lowtemperature joining of ZrO2 ceramics in this study. Scanning electron microscopy with energy dispersive spectrometer, X-ray diffraction and X-ray photoelectron spectroscopy were employed to study the effects of ultrasonic vibration on the microstructure of Sn/ZrO2 interface, and to elucidate the joining mechanism between Sn coating layer and ZrO2 ceramic. Results showed that, after ultrasonically dipping in molten Sn for 1200 s, a pure Sn solder layer with a thickness of approximately 8–9 µm was coated on the ZrO2 surface. The Sn coating layer exhibited excellent metallurgic bonding with ZrO2 ceramic. A nano-sized ZrSnO4 ternary phase, which was beneficial to the smooth transition of the lattice from Sn solder to ZrO2 ceramic, was formed at the Sn/ZrO2 interface. The formation of ZrSnO4 interlayer was ascribed to the acoustic cavitation induced hightemperature reaction of Sn, O and ZrO2 at the molten Sn/ZrO2 ceramic interface. The tested average shear strength of ZrO2/Sn/ZrO2 joints was approximately 32 MPa, and the shearing failure mainly took place within the Sn solder layer.
1. Introduction As an engineering ceramic, ZrO2 ceramic has been largely used in solid oxide fuel cells (SOFCs) [1–6], dental applications [7,8], optical instruments and electronic devices [3,9] due to its superiority in mechanical, optical and high temperature physical and chemical stabilities. However, it is difficult to fabricate some complex and large ZrO2 ceramic components, which impedes their wide application. To solve these problems, some brazing methods, such as solid-state diffusion bonding and active brazing, were always performed in joining ZrO2 and ZrO2 or ZrO2 and other materials [10–12]. Active brazing was an attractive technique in joining ZrO2 ceramics because of its convenience, cost-effectiveness and high-efficiency [12–15]. However, the brazing temperature during using active filler metals was usually higher than 700 °C. Large residual stress was easy to accumulate at the interface of filler metal layer and ceramic substrate because of the significant mismatch of thermal expansion coefficients between ceramics and metal alloys. Furthermore, the high brazing temperature may destroy the metal counterparts with low melting point. Low temperature soldering is necessary to achieve low residual stress and high accuracy connection between ceramic and ceramic or
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ceramic and metal, especially in applications of some optical instruments and electronic devices with high precision requirement and served in low temperature environment. Unfortunately, the wettability between low melting solders and ZrO2 ceramic is poor. Hence, several pre-metallization methods, such as physical vapor deposition and electroless or chemical plating [7,16], were usually performed to fabricate an easy-wetted metallization layer on the faying surface of ZrO2 substrate. A key problem facing the use of these traditional metallization methods was that the plating process was time consuming and the bonding strength between the metallization layer and ZrO2 substrate was poor. Therefore, an improved method to realize the rapid fabrication of high strength metallization layer on ZrO2 ceramic surface is of importance. Ultrasound-assisted fluxless soldering or brazing process has been largely reported in previous studies [17–20]. The ultrasound-induced cavitation effects have been simply considered as a driving force to promote the wetting of liquid solders on metal substrates by breaking the oxide film on the surface of molten solder and solid substrate [17,18]. Actually, it has been reported that the implosion of cavitation bubble induced by ultrasonic waves inducted in liquid medium could generate localized high-temperature and high-pressure conditions (i.e.
Corresponding author. E-mail address: [email protected] (Y. Xiao).
http://dx.doi.org/10.1016/j.ceramint.2017.03.042 Received 23 January 2017; Received in revised form 6 March 2017; Accepted 6 March 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Luo, D., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.03.042
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“hot spots”) near the bubble, which may produce a non-equilibrium reaction environment at low temperature and dramatically affect the physical-chemical interaction behavior at the solid-liquid interface [21,22]. Chen et al. [19] reported that, a strong SiC/SiC joint with a maximum shear strength of 148.1 MPa was obtained by ultrasonically brazing at 420 °C with a ZnAlMg filler metal. The bonding mechanism between the ZnAlMg filler metal layer and SiC substrate was ascribed to the increased mass transfer from SiO2 to ZnAlMg caused by acoustic cavitation induced erosion effects. Cui et al. [20] found that a nanocrystalline α-alumina layer was built at molten Sn-Zn-Al alloy and sapphire interface with the assistance of ultrasonic waves at a processing temperature of 230 °C, which resulted in a smooth transition of the lattice from sapphire to solder metal. These results imply that it is feasible to fabricate a metallization layer on the ZrO2 substrate surface with the assistance of ultrasonic waves. However, less previous studies have been found on the ultrasound-assisted coating of Sn solder on ZrO2 ceramic surface. Thus, in this study, we attempt to fabricate a pure Sn coating layer on the ZrO2 substrate surface with an ultrasonic dipping process. Emphasis is focused on studying the ultrasonic effects correlated microstructure evolution of Sn/ZrO2 interface. Moreover, the bonding mechanism between pure Sn coating layer and ZrO2 substrate is discussed. Such a study is expected to be helpful in developing a high bonding strength Sn metallization layer on the ZrO2 ceramic surface.
approximately 300 °C and was kept constant at this temperature for 60 s, the ultrasonic vibration with a frequency of 20 kHz and a power of 850 W was imposed on the Ti alloy plate for 1200 s. After that, the ZrO2 ceramic cylinder was taken out of the molten Sn bath and cooled in air. Finally, a Sn metallization layer was coated on the ZrO2 substrate surface. All the ultrasonic dipping process was operated in air condition. The Sn-coated ZrO2 substrates were grinded perpendicularly to the Sn coating layer and polished according to standard metallographic technique. To observe the surface morphology of ZrO2 substrate after ultrasonic dipping treatment, some as-coated ZrO2 substrates were deep etched in 20% nitric acid solution for 1 h, then cleaned in 1% hydrochloric acid alcohol solution to remove the Sn coating layer completely. The morphologies of Sn/ZrO2 interface and deep-etched ZrO2 substrate surface were observed by scanning electron microscopy (SEM, FEI Quanta FEG 250) equipped with an energy dispersive spectrometer (EDS). Line scanning analysis across the Sn/ZrO2 interface was performed with an operation voltage of 20 kV and a current of 10 nA, the scanning step was approximately 0.0637 µm. The phase constitutions of the deep-etched ZrO2 substrate surface were characterized by X-ray diffraction (XRD, D/MAX-RB, Cu Kα) with a scanning step of 0.02°. Moreover, X-ray photoelectron spectroscopy (XPS, ESCALAB 250) measurement was performed to analyze the element constitutions and valance states on the deep-etched ZrO2 surface. Carbon C 1 s line at 285.0 eV was taken as a reference for calibration. To investigate the bonding properties between Sn coating layer and ZrO2 ceramic, two Sn-coated ZrO2 cylinders were soldered together by remelting the Sn coating layer in the Sn-bath. Shear strength tests of ZrO2/Sn/ZrO2 joints were performed in an electron tension testing machine (Instron-5569) with a strain rate of 1 mm/min. Five specimens were prepared to obtain an average shear strength value. During the shear strength test, the as-soldered joints were fixed in a specially designed fixture, the schematic diagram of which can be found in our previous studies [23].
2. Experimental Sintering yttria-stabilized ZrO2 ceramic (purity > 97%, Shenzhen Hard Precision Ceramic Co., Ltd) with a cylinder shape and a dimension of Ø5×5 mm was used as base material. Pure Sn ingots (purity > 99.5%, Yik Shing Tat Industrial Co., Ltd) were used as the coating material. Prior to the ultrasonic dipping process, the faying surface of ZrO2 cylinder was grinded and polished to a mirror finish then ultrasonically cleaned in anhydrous ethanol. Fig. 1 shows the schematic diagram of the ultrasonic dipping process. A T4-Ti alloy plate (20×50×4 mm) with a groove (15 mm in diameter, 3 mm in depth) in the center was used as the Sn bath. It should be noted that, a TiN barrier layer was fabricated on the surface of Ti alloy plate by gas nitriding treatment, so that the erosion of Ti alloy plate during the ultrasonic dipping process could be mitigated. A stainless steel platform was applied to support and confine the Ti alloy plate, and a resistance-heating unit was set at the bottom of this platform. The heating temperature was monitored by inserting a Ktype thermal couple into the Sn bath. An ultrasonic vibration horn was performed perpendicularly on one side of the Ti alloy plate, which could apply a vertical vibration on the Ti alloy plate and induct the ultrasonic waves through the Ti alloy plate into the molten Sn bath. During the ultrasonic dipping process, several Sn ingots (approximately 2.5 g in weight) were put into the Sn bath and heated to liquid state, then a ZrO2 cylinder was placed perpendicularly into the molten Sn bath. When the temperature of molten Sn was increased to
3. Results and discussion Fig. 2 shows the cross-section image of Sn-coated ZrO2 substrate obtained by ultrasonically dipping for 1200 s. An uniform Sn solder layer with a measured thickness of approximately 8–9 µm is created on the ZrO2 surface, as shown in Fig. 2a. The Sn coating layer exhibits excellent bonding with the ZrO2 ceramic, no cracks or pores are found in the Sn coating layer and at the Sn/ZrO2 interface. Fig. 2b shows the magnified image of “A” area marked in Fig. 2a. A discontinuous interface between Sn coating layer and ZrO2 substrate is presented in this image. The ZrO2 substrate is “eroded” by molten Sn solder with the assistance of ultrasonic waves, and the erosion pits on the ZrO2 surface are filled with Sn solder. Some ZrO2 particles can be found in the Sn coating layer located near the Sn/ZrO2 interface, which may be stripped from the ZrO2 substrate. The concentration profiles of Zr and Sn elements across the Sn/ZrO2 interface are shown in Fig. 3. The BC line in Fig. 2a indicates the position analyzed by EDS line scanning. It can be seen that there exists a relatively thin transition zone at the Sn/ZrO2 interface. The line scanning result of Sn element in Fig. 3 reveals that the Sn content declines gently from the Sn coating layer to the ZrO2 substrate. Zr element concentration profile shows that this element also penetrates into transition zone and its content decreases from the transition zone and becomes zero in the Sn coating layer. These results imply that the mutual diffusion of Sn and Zr elements between Sn coating layer and ZrO2 substrate has happened after the ultrasonic dipping treatment, indicating that there formed a metallurgic bonding between Sn metallization layer and ZrO2 ceramic. In order to observe the surface morphology of Sn-coated ZrO2 substrate much more clearly, the Sn coating layer was removed by deep etching treatment and the surface of ZrO2 substrate was exposed. Fig. 4a shows the surface microstructure of the deep-etched ZrO2
Fig. 1. Schematic diagram of ultrasonic dipping process.
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Fig. 2. SEM images of (a) the cross-section of Sn-coated ZrO2 ceramic obtained by ultrasonically dipping in molten Sn for 1200 s, and (b) the magnified “A” area marked in Fig. 2a.
black line in this figure exhibits the XRD pattern of ZrO2 ceramic without ultrasonic treatment. No additional peaks of other phases can be found except the ZrO2 peaks from the measurement. The red line in Fig. 5a displays the XRD pattern of deep-etched ZrO2 ceramic obtained by ultrasonically dipping for 1200 s. It can be seen that the peaks of ZrO2 ceramic along with those of ZrSnO4 ternary phase are indexed in the profile. The peaks of ZrSnO4 ternary phase appear at 31.4° and 34.4° (PDF No. 48–0889), exhibiting an orthorhombic crystal structure according to previous studies [24]. Thus, it can be deduced that the nano-sized particles formed on the surface of ZrO2 substrate obtained by ultrasonically dipping for 1200 s were mainly composed of metastable ZrSnO4 phase. Interestingly, compared with ZrO2 ceramic without ultrasonic dipping treatment, the XRD diffraction peaks of ZrO2 substrate obtained by ultrasonically dipping in molten Sn for 1200 s shift a little to the right side, as shown in Fig. 5b, but maintain a tetragonal crystal structure. Since the XRD characterization has a penetration depth of several micrometers on ZrO2 ceramic surface [25], it can be anticipated that some trace phases may not be detected in this test. The sampling depth of XRS is much less than that of XRD, thus XRS experiments are much more sensitive to surface species. To better understand the elemental compositions of ZrO2 substrate surface, XPS measurements were carried out and the results are shown in Fig. 6. Fig. 6 shows the XPS characterization results of deep-etched ZrO2 ceramic obtained by ultrasonically dipping in molten Sn for 1200 s. The C 1 s spectrum reveals only one peak at 284.7 eV, as shown in Fig. 6a, which corresponds to C-H species and mainly comes from the surface carbon contamination. The broad O 1 s line with a distinct shoulder, as shown in Fig. 6b, is fitted to a combination of components located at 530.2 eV and 532.2 eV, which can be ascribed to O2- ions in lattice and O-H bonds from absorbed hydroxyl groups, respectively. The existence of multi peaks may associated with the presence of surface oxygen defects [26]. The peaks at 182.4 eV and 184.8 eV in the Zr 3d spectrum, as shown in Fig. 6c, are ascribed to spin-orbit splitting of the Zr 3d, Zr 3d5/2 and Zr 3d3/2, respectively. These peaks demonstrate that the valance state of Zr atoms presents as +4 according to previous studies [27]. Fig. 6d shows the curve-fitted Sn 3d XPS spectra. It can be seen that Sn 3d5/2 and Sn 3d3/2 were measured at 486.6 eV and 495.2 eV, respectively, which indicates that the valence state of Sn is +4. A shoulder appears on the right side of Sn 3d3/2, which may be associated with satellites or signal noise. The results above demonstrate that after deep-etching in 20% nitric acid solution for 1 h and cleaning with alcohol, Sn element with a valance state of +4 still exists on the ZrO2 ceramic surface. Furthermore, the valance states of O, Zr and Sn convincingly suggest that the nano-sized particles formed on the Sn/ ZrO2 interface are composed of ZrSnO4 ternary phase. Actually, it was amazed and confused to find that there formed a
Fig. 3. EDS line scans of the Sn/ZrO2 interface of BC line marked in Fig. 2a.
substrate obtained by ultrasonic dipping for 1200 s. The surface of deep-etched ZrO2 substrate exhibits an uneven morphology, some corrosion pits and particles can be found on the deep-etched ZrO2 substrate surface. The EDS analysis results of the region marked by “1” in Fig. 4a demonstrate that these bright particles are composed of 63.68 at% O, 14.86 at% Zr and 21.46 at% Sn elements, respectively. The magnified image of “A” region marked in Fig. 4a is shown in Fig. 4b. It can be seen that some nano-sized particles are presented on the ZrO2 surface. The cross-section image of deep-etched ZrO2 substrate obtained by ultrasonically dipping for 1200 s is shown in Fig. 4c. Obviously, a reaction layer with a thickness range from approximately 0.5 to 2.5 µm can be found on the ZrO2 surface. Fig. 4d shows the surface morphology of deep-etched ZrO2 substrate without ultrasonic dipping treatment. Comparing to the ZrO2 ceramic obtained by ultrasonically dipping in molten Sn for 1200 s, the ZrO2 substrate without ultrasonic dipping treatment exhibits a relatively smooth surface. The results above demonstrate that a Sn metallization layer with an even thickness can be successfully coated on the ZrO2 ceramic surface with the ultrasonic dipping method. The metallurgic bonding between Sn coating layer and ZrO2 substrate may be associated with the nanosized particles formed at the Sn/ZrO2 interface. To make clear the phase composition of these newly formed nano-particles shown in Fig. 4b, XRD analysis was performed and the results are displayed in Fig. 5. Fig. 5a shows XRD patterns of deep-etched ZrO2 ceramics. The 3
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Fig. 4. SEM images of (a) the surface morphology of deep-etched ZrO2 substrate obtained by ultrasonically dipping in molten Sn for 1200 s, (b) magnified “A” region marked in Fig. 3a, (c) the cross-section of deep-etched ZrO2 substrate ultrasonically dipped for 1200 s, and (d) the surface morphology of ZrO2 ceramic without ultrasonic dipping treatment.
by-product of nanocomposite ZrO2-SnO2 thin film obtained by sol-gel co-deposition processing and subsequently annealing at temperatures higher than 500 °C for approximately 1 h. In their studies, the formation of ZrSnO4 bimetallic oxide in the film was ascribed to the maximum solution of Sn4+ ions in ZrO2 lattice obtained upon crystallization. Furthermore, Kim et al. [30] and Gaillard-Allemand et al. [31]
ZrSnO4 ternary phase at the molten Sn/ZrO2 ceramic interface in this study, since the ZrSnO4 phase was usually synthesized via chemiedouce followed by calcining at high temperature for several hours in previous studies [28,31]. A number of literatures have been published on the fabrication of ZrSnO4 ternary phase [9,24,28–31]. For instance, Anitha et al. [9] and Joy et al. [28] found that the ZrSnO4 phase was a
Fig. 5. (a) XRD profiles of deep-etched ZrO2 ceramic obtained by ultrasonically dipping for 1200 s (red line) and ultrasonically dipping for 0 s (black line), (b) the magnified region marked in Fig. 5a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
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Fig. 6. Curve-fitted (a) C 1 s, (b) O 1 s, (c) Zr 3d and (d) Sn 3d XPS core-level spectra of deep-etched ZrO2 ceramic obtained by ultrasonically dipping in molten Sn for 1200 s.
corrosion pits on the surface of ZrO2 substrate after ultrasonic dipping for 1200 s, as shown in Fig. 2b. Furthermore, acoustic cavitation induced micro-jets and shock waves can generate locally extreme high temperature in liquid medium, approximately 1600 °C surrounding the cavitation bubble [18]. In this study, the ultrasonic waves were inducted in molten Sn solder, whose boiling temperature is 2602 °C. The molten Sn can store and deliver the ultrahigh energy generated by bubble collapse without evaporation unlike aqueous solution. Thus, the acoustic cavitation can provide a special high temperature environment at the Sn/ZrO2 interface for the formation of ZrSnO4 phase. As cavitations are more likely to appear at the liquid/solid interface, the oxidation in this place may be more significant than that elsewhere [21]. The O element dissolved in the molten Sn may diffuse continually to the Sn/ZrO2 interface with the help of ultrasonic-induced streaming and acoustic cavitation effects. Ultimately, the high temperature reaction environment produced by cavitation effects may promote the reaction of Sn, O and ZrO2 at the molten Sn/ZrO2 interface, which results in the formation of ZrSnO4 ternary phase on the ZrO2 ceramic surface. The reaction can be expressed as: Sn+2 O+ZrO2→ZrSnO4. The changes in the microstructure of Sn/ZrO2 interface induced by exposure to ultrasonic waves will surely have an influence on the bonding strength of Sn coating layer and ZrO2 substrate. To verify the reliability between Sn coating layer and ZrO2 substrate, ZrO2/Sn/ZrO2 joints were made and shear strength tests were performed. Results show that the average shear strength of ZrO2/Sn/ZrO2 joints was 32 MPa, and the joints were mainly failed in the Sn solder layer.
found that, when a mixed powder of ZrO2 and SnO2 was calcined in a temperature range of 1230–1750 °C, the immiscibility gap existed between ZrO2 and SnO2 could result in two limited solid solution of (Zr1−xSnx)O2 and (Sn1−yZry)O2, and the solubility increased slowly with increasing calcining temperature. These studies demonstrate that both the high reaction temperature and the existence of SnO2 and ZrO2 are necessary to the formation of ZrSnO4 phase. In this study, the ZrSnO4 phase was formed on the surface of ZrO2 ceramic by ultrasonically dipping ZrO2 in molten Sn for only 1200 s; moreover, the ultrasonic dipping temperature was only 300 °C, which was much lower than the reacting temperature mentioned in previous studies. Then, how did the ZrSnO4 ternary phase form at the molten Sn/ZrO2 ceramic interface during the ultrasonic dipping process? Obviously, the formation of ZrSnO4 ternary phase is associated with the ultrasonic waves. During the ultrasonic dipping process, high intensity ultrasonic waves were inducted into the molten Sn solder through the Ti alloy plate. This may induce the creation of acoustic cavitation effects, which is the formation, growth, and implosive collapse of vacuum cavities. The cavities collapse near the ZrO2 surface can induce the formation of shock waves and liquid mirco-jets, which may impinge the ZrO2 surface and create locally extreme high pressure on it. When the distance between cavitation bubble and solid surface is approximately 30–300 µm, the local pressure induced by mirco-jets and shockwaves can reach to approximately 4 GPa [21,22]. Such a high pressure can break the oxide film on the surface of molten Sn and strip the sintered ZrO2 ceramic. That is why there formed extensive
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Obviously, the coarsening of ZrO2 surface and the formation of ZrSnO4 interlayer at the Sn/ZrO2 interface may most probably improve the bonding properties of Sn coating layer and ZrO2 ceramic. 4. Conclusions In this study, ultrasound-assisted dipping process was performed to fabricate a Sn metallization layer on the ZrO2 ceramic surface and thus realize the low temperature soldering of ZrO2 ceramic. The microstructure of Sn/ZrO2 interface and the bonding mechanism between Sn coating layer and ZrO2 ceramic were studied. A pure Sn metallization layer with a thickness of approximately 8–9 µm was successfully coated on the ZrO2 ceramic surface by ultrasonically dipping the ZrO2 ceramic into molten Sn for 1200 s. The ZrO2 ceramic exhibited a coarsening surface and the pits located on the ZrO2 surface were completely filled with Sn solder. Furthermore, there formed a ZrSnO4 bimetal oxide interlayer at the Sn/ZrO2 interface, which resulted in an excellent metallurgic bonding between Sn coating layer and ZrO2 ceramic. The formation of ZrSnO4 ternary phase was attributed to the acoustic cavitation induced high temperature reaction of Sn, O and ZrO2 at the molten Sn/ZrO2 interface. The tested shear strength of ZrO2/Sn/ZrO2 joints was 32 MPa, and the joints were mainly failed in the pure Sn solder layer. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 51605357), Zhongshan Science and Technology Plan Project (No. 2016A1013), and the Fundamental Research Funds for the Central Universities (2015IVA003). References [1] J. Cao, X.G. Song, C. Li, L.Y. Zhao, J.C. Feng, Brazing ZrO2 ceramic to Ti-6Al-4V alloy using NiCrSiB amorphous filler foil: interfacial microstructure and joint properties, Mater. Charact. 81 (2013) 85–91. [2] M.O. Curi, H.C. Ferraz, J.G.M. Furtado, A.R. Secchi, Dispersant effects on YSZ electrolyte characteristics for solid oxide fuel cells, Ceram. Int. 41 (2015) 6141–6148. [3] A.H. Gorji, A. Simchi, A.H. Kokabi, Development of composite silver/nickel nanopastes for low temperature joining of yttria-stabilized zirconia to stainless steels, Ceram. Int. 41 (2015) 1815–1822. [4] S.Y. Zhen, W. Sun, G.Z. Tang, D. Rooney, K. Sun, X.X. Ma, Fabrication and evaluation of NiO/Y2O3-stabilized-ZrO2 hollow fibers for anode-supported microtubular solid oxide fuel cells, Ceram. Int. 42 (2016) 8559–8564. [5] J. Yang, A.F. Molouk, T. Okanishi, H. Muroyama, T. Matsui, K. Eguchi, A stability study of Ni/yttria-stabilized zirconia anode for direct ammonia solid oxide fuel cells, ACS Appl. Mater. Interfaces 7 (2015) 28701–28707. [6] C.L. Chao, C.L. Chu, Y.K. Fuh, R.Q. Hsu, S. Lee, Y.N. Cheng, Interfacial characterization of nickel-yttria-stabilized zirconia cermet anode/interconnect joints with Ag-Pd-Ga active filler for use in solid-oxide fuel cells, Int. J. Hydrog. Energy 40 (2015) 1523–1533. [7] C. Hubsch, P. Dellinger, H.J. Maier, F. Stemme, M. Bruns, M. Stiesch, L. Borchers, Protection of yttria-stabilized zirconia for dental applications by oxidic PVD coating, Acta Biomater. 11 (2015) 488–493.
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