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Thermal cycling and ageing of a glass-ceramic sealant for planar SOFCs
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 1 8 9 5 e1 1 9 0 3
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Thermal cycling and ageing of a glass-ceramic sealant for planar SOFCs F. Smeacetto a,*, A. Chrysanthou b, M. Salvo a, T. Moskalewicz c, F. D’Herin Bytner d, L.C. Ajitdoss a, M. Ferraris a a
Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy School of Aerospace, Automotive and Design Engineering, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK c Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Krako´w, Poland d Saint-Gobain Isover CRIR, France b
article info
abstract
Article history:
Thermal cycling and thermal ageing tests were performed on Crofer22APU/glass-ceramic/
Received 14 January 2011
Anode-Supported-Electrolyte (ASE) joined samples in air at the SOFC operating tempera-
Received in revised form
ture of 800 C. The Crofer22APU had been polished and preoxidised at 900 C for 2 h. The
31 March 2011
diffusion behaviour at the two interfaces was examined and revealed slight diffusion of
Accepted 10 April 2011
chromium and manganese from Crofer22APU into the glass-ceramic. No interactions,
Available online 18 July 2011
failure or crack formation were observed at the Crofer22APU/glass-ceramic interface and between the glass-ceramic and YSZ.
Keywords:
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Glass-ceramic sealant
reserved.
Solid oxide fuel cell Trasmission electron microscopy Microstructure Crofer22APU
1.
Introduction
Solid Oxide Fuel Cells (SOFCs) are highly efficient energy conversion devices which produce electricity by the electrochemical reaction between a fuel and an oxidant. The repeating unit of a planar SOFC cell is formed by series of anodeeelectrolyteecathode and interconnects. The interconnect links the anode of one cell to the cathode of the neighbouring cell [1]. Depending on the stack design, a sealant is needed to join components (metallic and ceramic) and form gastight seals to separate both the oxidant and the fuel chambers. The development of sealants for solid oxide fuel
cells is a significant challenge because they must meet very restrictive requirements; they must withstand the severe environment of the SOFC, be resistant to oxidative and reducing environments and be thermo-chemically compatible with the materials to which are in contact. The problem becomes even more challenging as there is also a requirement for thermal cycle stability for planar stacks in which different SOFC components with dissimilar thermo-mechanical properties are sealed together. The sealants must survive for several hundreds of thermal cycles during SOFC operations. Any crack that forms within the sealant or poor adhesion at the interfacial regions can cause leakage that leads to lower
* Corresponding author. Tel.: þ39 011564706; fax: þ39 0115644699. E-mail address: [email protected] (F. Smeacetto). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.083
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cell performance and efficiency. Three general design approaches are currently being used for the sealant in the SOFC planar configuration: rigid bonded sealants, compressive sealants, and compliant-bonded sealants. Glasses and glass-ceramics are widely used to prepare metal-ceramic joints and in principle meet most of the requirements of an ideal SOFC sealant. Moreover, their characteristic temperatures, thermal and thermo-mechanical properties can be tailored by varying the composition. Many glass-forming systems have been considered as SOFC sealants, including those based on phosphates, borates, and silicates [2e19]. Glass-ceramics, which are prepared by crystallization of glasses, are considered advantageous for SOFC sealant applications because the resulting glass-ceramic is typically stronger than the parent glass. Moreover, it is possible to tailor the properties of the resulting glass-ceramic sealant by controlling the kinetics of crystallization and the crystalline phases that are formed. Finally, glass-ceramic sealants have good hermeticity and are usually thermally and environmentally stable. Barium aluminosilicate sealants have shown high reactivity with the metallic interconnect at temperatures between 800 C and 900 C by forming a weak interfacial layer composed of barium chromate (BaCrO4) and monocelsian (BaAl2Si2O8), while borate glasses are not sufficiently stable in a humidified fuel gas environment [20,21]. The difference in the coefficient of thermal expansion (CTE) between the sealant and anode, the electrolyte and the cathode together with the inherent brittleness of glasses, may cause cracks to develop in seals during thermal cycling or thermal shock. This can cause leakage that would lower the cell performance and efficiency. In order to remain hermetic, a sealant must exhibit strong bonding both with the electrolyte and the interconnect. In addition, the coefficient of thermal expansion of the interconnect and of the sealant must closely match each other and interfacial reactions must not lead to products with thermal expansion markedly different from those of the bulk phases, otherwise high residual stresses will be generated within the interfacial reaction zone giving rise to cracks. The authors have recently developed a new sealant based on a sodium-calcium-aluminosilicate glass-ceramic [22]which has a coefficient of thermal expansion of 10.7 106 C1, a value that is compatible with YSZ and Crofer22APU substrates, in order to obtain a very good thermo-mechanical compatibility. The study that is presented here is focused mainly on the morphological/microstructural behaviour of the glass-ceramic sealant in contact with Crofer22APU. Prior to the joining process the Crofer22APU, had undergone a preoxidation treatment at 900 C for 2 h. This was different from our previously reported investigation [23] where the Crofer22APU had been preoxidised at 950 C for 2 h and the subsequent thermal treatments showed little or no diffusion of chromium and manganese into the glass-ceramic sealant. The current study aimed to investigate whether the reduction in the pre-oxidation temperature would result in any changes in the subsequent thermal behaviour. In addition, samples underwent two different thermal cycles and were examined for interfacial cracks. A total of three types of thermal cyclic and thermal ageing tests were employed in air for 500 h.
2.
Experimental
The heat resistant metal alloy used for this study was Crofer22APU (Cr 20e24, C 0.03, Mn 0.30e0.80, Si 0.50, Al 0.50 max, Fe balance, wt.%) manufactured by Tyssen Krupp, Germany and supplied by HT Ceramix, Switzerland). Crofer22APU, was polished using SiC paper to 4000 grit and then preoxidised at 900 C for 2 h in air. The anode-supported-electrolyte (ASE) (electrolyte: 8%mol yttria stabilised cubic zirconia, YSZ; anode; NiO-YSZ) half cell were purchased by H. C. StarckGmbH (Germany). The Crofer22APU and ASE samples to be joined were cut to obtain a final joined sample of 6 6 2 mm3. The melting procedure, thermal and thermo-mechanical characterization of the sealant are described elsewhere [22,23]. The sealant composition ranged between 53 and 58 mol% SiO2, 16e18 mol% Al2O3, 24e26 mol% CaO and 9e12 mol% Na2O. The glass transition temperature, Tg, and the softening temperature, were measured to be 670 C and 740 C respectively, by differential thermal analysis (DTA) and dilatometer, while two crystallization temperatures were detected at 830 C and 940 C respectively. The joints were prepared by placing the Crofer22APU plates on the yttria-stabilised zirconia (YSZ) surface of the anode-supported-electrolyte with a slurry of glass powder dispersed in ethanol (solid content of 40 %wt.) sandwiched inbetween. The sealed samples were then heat-treated in a tubular oven in argon at a temperature above the glass softening point without applying any load. Reproducible results, in terms of joint thickness and homogeneity were obtained. The joining thermal treatment was carried out by heating from room temperature to 900 Cat a heating rate of 5 C/min and a dwelling time of 30 min at 900 C. The cooling rate was 5 C/min. Thermal cycling tests were performed in a muffle furnace with static air from room temperature to 800 C for different times for a total period of 500 h. Two kinds of thermal cycling tests were conducted on the joined samples: Thermal Cycling 1: six samples had a cooling cycle of 10 C/min from 800 C to room temperature every 72 h for a total of 500 h giving a total of 7 heatingecooling cycles.
Fig. 1 e Crofer22APU/glass-ceramic/ASE SEM cross-section after thermal cycling 2.
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Fig. 2 e SEM cross-section magnification of the interface between Crofer22APU and the glass-ceramic sealant for a sample that followed thermal cycling 1.
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Thermal Cycling 2: six other joined samples were extracted and then reinserted into the furnace at 800 Cevery 24 h for a total of 500 h. The latter of the two thermal cyclic treatments included 21 heatingecooling cycles and was thus more severe than the former cyclic treatment. Thermal ageing tests were conducted on six joined samples at 800 C in static air for a total period of 500 h (heating rate 13 C/min, cooling rate 1 C/min). Cross-sections of all joined samples were characterized by scanning electron microscopy (SEM) (FEI Inspect, Philips 525 M and JEOL 5700) after polishing. EDS (SW9100 EDAX) analysis was carried out in order to detect any elemental diffusion into or away from the sealant and to examine for any chemical interactions between Crofer22APU and the glass-ceramic sealant. One of the samples for SEM examination was submitted to etching with 4%vol HF for 40 s, in order to evidence the microstructure. The microstructure and chemical composition at the Crofer22APU/glass-ceramic interface were also examined by transmission electron microscopy (TEM)using JEM-2010 ARP on cross-section thin foils prepared by the Precision Ion Polishing System (PIPS; Gatan). Phases were identified using selected area electron diffraction (SAED)
Fig. 3 e Elemental EDAX mapping at the interface between Crofer22APU and the glass-ceramic sealant for a sample with thermal cycling 1.
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and energy dispersive X-ray spectroscopy (STEM-EDS). STEM images were acquired with a resolution of 256 224 pixels. The electron beam size was about 3 nm.
3.
Results and discussion
3.1. Crofer22APU/glass-ceramic sealant/ASE joined samples after thermal cycling 1 and 2 SEM examination of these twelve samples showed that the interfaces Crofer22APU/glass-ceramic/YSZ (comparable with the as fabricated microstructure of the joint, before tests, that can be found in reference 23) remained joined together without any evidence of cracking or delamination, suggesting good thermo-mechanical compatibility between the glassceramic sealant and both Crofer22APU and YSZ. A typical
example of this is shown in Fig. 1 after thermal cycling 2which represents the more severe of the two undertaken thermal cycles. Qualitative observations indicate very good adhesion between the glass-ceramic sealant and the two substrates after thermal cycling treatments. Fig. 2 shows a magnification of the interface between Crofer22APU and the glass-ceramic sealant (in order to evidence features in the glass-ceramic) for a sample that followed thermal cycling 1. Elemental EDAX mapping for the same sample is presented in Fig. 3. It can be observed that aluminium within the Crofer22APU alloy diffused, suffered internal oxidation and formed discontinuous pockets of Al2O3 just below the Crofer22APU protective oxide layer (Fig. 2). This observation is in agreement with the observation reported by Horita et al. [24] where pockets of aluminium oxide were detected in the metallic interconnect (air at 800 C) suggesting significant oxidation in the inner parts of the alloy. In the
Fig. 4 e SEM cross-section of Crofer22APU/glass-ceramic sealant interface and EDS investigation on 4%vol HF etched samples (thermal cycles 1 from room temperature to 800 C for 500 h).
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Fig. 5 e SEM cross-section of YSZ/glass-ceramic sealant interface after thermal cycles from room temperature to 800 C for 500 h 1 according to thermal cycle 2).
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regions where there was enrichment and oxidation of aluminium, there was also evidence of small pores developing. This appeared to be the result of aluminium diffusion followed by internal oxidation at the grain boundaries. This did not seem to be a problem after 500 h of thermal cyclic treatment, as there was no evidence of cracks at the interface between the protective oxide layer and the stainless steel. There was also no evidence at all of any aluminium diffusing into the protective oxide layer. Slight diffusion of chromium and manganese from Crofer22APU into the glass-ceramic sealant was also observed. At the same time, sodium and calcium from the glass-ceramic sealant were observed to diffuse away from the Crofer22APU/glass-ceramic interface. Further EDS investigation is presented in Fig. 4 for a sample that was etched with 4% vol HF and shows the development of three distinct microstructural zones within the glass-ceramic sealant near the interface with Crofer22APU. The EDS spectra indicated that zone 1 which is adjacent to the Crofer22APU and about 10 mm thick, is rich in Si, Cr and O with some Al, Ca and Mn also being present. Zone 2 contains Si, Al, Ca, O and only a trace of Na, while zone 3 contains the original phases of the glass-ceramic.
Fig. 6 e Crofer22APU/glass-ceramic sealant SEM cross-section after thermal ageing at 800 C for 500 h and the relative EDS mapping.
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Fig. 5 shows a SEM cross-section of YSZ/glass-ceramic sealant interface after thermal cycles from room temperature to 800 C for 500 h(according to thermal cycle 2). No cracks are observed at the interface and no elements diffusion was detected into the glass-ceramic.
3.2. Crofer22APU/glass-ceramic sealant/ASE joined samples after thermal ageing tests As in the case of the samples submitted to thermal cyclic treatment, all the Crofer22APU/glass-ceramic sealant/ASE joined samples after the thermal ageing tests maintained their integrity and showed no evidence of fracture or delamination. Fig. 6 shows a Crofer22APU/glass-ceramic sealant SEM cross-section after thermal ageing at 800 C for 500 h and the relative EDS mapping. The results appear to be the same as with the thermally cycled samples as described in the previous section with slight diffusion of both chromium and
manganese into the glass-ceramic sealant having taken place. Therefore the further characterization (TEM and SAED)of theCrofer22APU/glass-ceramic sealant interface was concentrated on thermally aged joined samples. The Crofer22APU/ glass-ceramic interface was also examined by using brightfield TEM and the results are shown in Fig. 7. This part of the investigation focused on the interactions within about 2 mm of the Crofer22APU/glass-ceramic interface. In some areas adjacent to the interface, an amorphous zone of up to 500 nm thickness was detected (Fig. 7, point 1) which contained Si (26.6 at.%) and O (61.7 at.%) suggesting the formation of amorphous SiO2. A small amount of some of the other elements such as Al (7.1 at.%), Cr (2.3 at.%), Mn (1.8 at.%), Fe (0.3 at.%) and Ca (0.2 at.%) which appeared to be dissolved in the amorphous SiO2were also detected in this zone. Fig. 7 also confirms the presence of the CrMn2O4 protective oxide layer (point 2, Fig. 7) on the surface of Crofer22APU. Just below the protective oxide layer, pockets of Al2O3 particles were
Fig. 7 e The interface of the Crofer22APU/glass-ceramic and SAED patterns taken from areas marked on figure as 1e6 and their identification, TEM BF cross-section thin foil.
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detected (point 3, Fig. 7). This adds further support to the SEM observations in Fig. 2 where pockets of Al2O3 were detected just below the protective oxide layer. Point 4 represents the aFe solid solution. As chromium and manganese diffuse into the glass-ceramic, they react with SiO2 to form Mn3Cr2Si3O12 (face-centred orthorhombic) which was detected by TEM at point 5 in Fig. 7. The observation of Mn3Cr2Si3O12is rather surprising as this phase has previously been produced in laboratory conditions only by the combined application of high pressures and temperatures. However, the presence of Mn, Cr, Si and O in areas close to the Crofer22APU/glassceramic interface in STEM-EDS (Fig. 8) elemental maps also confirmed the possibility of the presence of the Mn3Cr2Si3O12 phase. After an exhaustive search, the only known phase that fits the TEM data is Mn3Cr2Si3O12. The authors have no reason to doubt the validity of the TEM data, but as it is not clear why this phase forms, further work is planned in the near future to address this. Further into the glass-ceramic, Ca2Al2SiO7 was detected at point 6 in Fig. 7. There was no evidence of any sodium at all in the areas close to the Crofer22APU/glassceramic interface. The SEM observations indicate that sodium diffuses away from the Crofer22APU/glass-ceramic interface at least 10 mm into the glass-ceramic. This observation is rather interesting because sodium is always diffusing away from chromium. As a consequence there is no evidence of any reaction between sodium and chromium oxides which are known to react together to form the volatile sodium chromate [25], Na2CrO4. Further support for the absence of any Na2CrO4 formation was also provided by the fact that there was no porosity within the diffusion layer (i.e. no evidence for any evaporation). In previous work by Ogasawara et al. [26] who used a glassy sealant, the formation of sodium chromate was reported in some areas. The chromate is volatile and loss of chromium hindered the formation of a stable protective oxide. Clearly this is not the case in the present work as the behaviour of the glass-ceramic sealant was such that it prevented reaction between the sodium and chromium oxides. Furthermore, there was no direct contact between sodium and chromium oxides as they were well-separated by a distinct layer (zone 2 in Fig. 4) making any adverse reaction to produce sodium chromate unlikely. Therefore the slight diffusion of chromium and manganese into the glass-ceramic produced no detrimental effects. Within this region (zone 2), only a small trace of sodium was detected. Chromates are rather volatile and volatilization would be quite obvious resulting in big voids and failure of the Crofer22APU-glass-ceramic bond. Furthermore, SEM examination (picture not reported here) around the three phase boundary (Crofer22APU-glass-ceramic sealant-air, i.e. outer seal edge) revealed no presence of Na2CrO4 as well as no anomalous corrosion of the Crofer22APU interconnect. Comparison of the Gibbs Free energy of formation data under standard conditions for NaAlSiO4 (4,163,500 þ 248.5T kJ/mol) and Na2CrO4 (1,334,000 þ 176.6T kJ/mol) as compiled by Kubaschewski et al. [27] indicates that at 800 C the NaAlSiO4 phase is more stable and that formation of the chromate would be unlikely. The absence of any interfacial reaction between the Crofer22APU and sodium glass-ceramic sealant (where amorphous residual phase is very low [22],) is likely due to
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the fact that sodium is mainly present in one of the crystalline phases, limiting alkali fast diffusion process through the glassy phase. Concerning other glass-ceramic sealants Ba or Sr silicate glass containing and non-containing alkalis,
Fig. 8 e STEM-EDS (Fig. 8) elemental maps at Crofer22APU/ glass-ceramic interface.
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Fig. 9 e SAED patterns taken from area in the glass-ceramic at 15e20 microns from Crofer22APU/glass-ceramic interface; TEM BF cross-section thin foil.
in [6] the authors discussed the compatibility between Crofer22APU (as-received and aluminized) and potassium oxide containing glasses: the addition of K2O in the parent glass sealant led to the detachment of sealant from Crofer22APU, but due to alkaline-earth chromates formation; microstructure characterization showed extensive chromate formation for the as-received Crofer22APU and complete loss of the protective alumina layer for the aluminized Crofer22APU. Further analysis into the glass-ceramic using bright-field TEM in Fig. 9 also showed evidence of small pockets of CaSiO3 (point 1, Fig. 9) and NaAlSiO4 (point 2, Fig. 9) as well as an amorphous phase (point 3, Fig. 9). The results of the thermal treatments have led to phase changes at the interface between the Crofer22APU and the glass-ceramic up to 5 microns from the interface. Both chromium and manganese from the pre-oxidation layer obtained by heating Crofer22APU at 900 C for 2 h in air had diffused up to 5 mm into the glassceramic to form Mn3Cr2Si3O12. In a recent study, Smedskjaer and Yue [28] investigated the behaviour of Na2OeCaOeSiO2 system in oxidising and reducing conditions. Sodium and calcium were reported to diffuse outwards under oxidising conditions. In diffusion studies of the Na2OeCaOeSiO2, system Natrup et al. [29] observed that the diffusion of Naþ ions was faster than that of Ca2þ ions. The results of these previous studies have similarity to the present investigation which has shown the
formation of a very thin (about 200 nm) SiO2-rich layer at the interface between Crofer22APU and the glass-ceramic and the diffusion of sodium and calcium away from the interface. Furthermore, the faster diffusion of the Naþ ion in comparison to the Ca2þ ion enabled sodium to diffuse further away leaving, behind it, the Ca2Al2SiO7 zone. The diffusion profiles and the microstructural observations for the two types of cyclic tests and for the thermal ageing tests were very similar. The reason for carrying out the various types of tests was to investigate whether the two types of thermal cycles would lead to cracking and failure at the interface between the glass-ceramic sealant and the Crofer22APU. As reported above, good adhesion between the two was maintained through the various thermal tests. The results of the present investigation differ from our previously published study [30] where no Cr interfacial diffusion was observed at the Crofer22APU/glass-ceramic interface. The importance of the condition of the preoxidised protective layer was demonstrated in another of our earlier investigations [30]. The reason for the difference in the behaviour lies in the type of pre-oxidation treatment that was used for Crofer22APU. In our earlier study, the material was preoxidised at 950 C for 2 h instead of 900 C as for the current study. The temperature and time of pre-treatment in the current study were not high enough to sufficiently stabilise and to obtain an homogeneous and coherent preoxidised layer.
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4.
Conclusions
The observations of the current study have led to the following conclusions: 1. Crofer22APU/glass-ceramic sealant/ASE joined samples submitted to thermal cycles and thermal ageing at 800 C for 500 h in air showed no evidence of interfacial failure and cracks; slight migration of both Cr and Mn into the glassceramic was observed. 2. A very thin layer measuring 0.5 mm was detected at the Crofer22APU/glass-ceramic interface after thermal ageing for 500 h at 800 C, while Cr and Mn were observed to interact with SiO2 to form Mn3Cr2Si3O12. At the same time both sodium and calcium were observed to diffuse inwards away for the Crofer22APU/glass-ceramic interface. As a result there was no direct contact between sodium and chromium oxides making any reaction to produce the volatile sodium chromate unlikely. 3. No interfacial migration was observed at the interface between the glass-ceramic and YSZ. 4. The temperature and time of Croferr22APU pre-treatment should be high enough to obtain an homogeneous and coherent preoxidised layer.
references
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Thermal cycling and ageing of a glass-ceramic sealant for planar SOFCs F. Smeacetto a,*, A. Chrysanthou b, M. Salvo a, T. Moskalewicz c, F. D’Herin Bytner d, L.C. Ajitdoss a, M. Ferraris a a
Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy School of Aerospace, Automotive and Design Engineering, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK c Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Krako´w, Poland d Saint-Gobain Isover CRIR, France b
article info
abstract
Article history:
Thermal cycling and thermal ageing tests were performed on Crofer22APU/glass-ceramic/
Received 14 January 2011
Anode-Supported-Electrolyte (ASE) joined samples in air at the SOFC operating tempera-
Received in revised form
ture of 800 C. The Crofer22APU had been polished and preoxidised at 900 C for 2 h. The
31 March 2011
diffusion behaviour at the two interfaces was examined and revealed slight diffusion of
Accepted 10 April 2011
chromium and manganese from Crofer22APU into the glass-ceramic. No interactions,
Available online 18 July 2011
failure or crack formation were observed at the Crofer22APU/glass-ceramic interface and between the glass-ceramic and YSZ.
Keywords:
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Glass-ceramic sealant
reserved.
Solid oxide fuel cell Trasmission electron microscopy Microstructure Crofer22APU
1.
Introduction
Solid Oxide Fuel Cells (SOFCs) are highly efficient energy conversion devices which produce electricity by the electrochemical reaction between a fuel and an oxidant. The repeating unit of a planar SOFC cell is formed by series of anodeeelectrolyteecathode and interconnects. The interconnect links the anode of one cell to the cathode of the neighbouring cell [1]. Depending on the stack design, a sealant is needed to join components (metallic and ceramic) and form gastight seals to separate both the oxidant and the fuel chambers. The development of sealants for solid oxide fuel
cells is a significant challenge because they must meet very restrictive requirements; they must withstand the severe environment of the SOFC, be resistant to oxidative and reducing environments and be thermo-chemically compatible with the materials to which are in contact. The problem becomes even more challenging as there is also a requirement for thermal cycle stability for planar stacks in which different SOFC components with dissimilar thermo-mechanical properties are sealed together. The sealants must survive for several hundreds of thermal cycles during SOFC operations. Any crack that forms within the sealant or poor adhesion at the interfacial regions can cause leakage that leads to lower
* Corresponding author. Tel.: þ39 011564706; fax: þ39 0115644699. E-mail address: [email protected] (F. Smeacetto). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.083
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cell performance and efficiency. Three general design approaches are currently being used for the sealant in the SOFC planar configuration: rigid bonded sealants, compressive sealants, and compliant-bonded sealants. Glasses and glass-ceramics are widely used to prepare metal-ceramic joints and in principle meet most of the requirements of an ideal SOFC sealant. Moreover, their characteristic temperatures, thermal and thermo-mechanical properties can be tailored by varying the composition. Many glass-forming systems have been considered as SOFC sealants, including those based on phosphates, borates, and silicates [2e19]. Glass-ceramics, which are prepared by crystallization of glasses, are considered advantageous for SOFC sealant applications because the resulting glass-ceramic is typically stronger than the parent glass. Moreover, it is possible to tailor the properties of the resulting glass-ceramic sealant by controlling the kinetics of crystallization and the crystalline phases that are formed. Finally, glass-ceramic sealants have good hermeticity and are usually thermally and environmentally stable. Barium aluminosilicate sealants have shown high reactivity with the metallic interconnect at temperatures between 800 C and 900 C by forming a weak interfacial layer composed of barium chromate (BaCrO4) and monocelsian (BaAl2Si2O8), while borate glasses are not sufficiently stable in a humidified fuel gas environment [20,21]. The difference in the coefficient of thermal expansion (CTE) between the sealant and anode, the electrolyte and the cathode together with the inherent brittleness of glasses, may cause cracks to develop in seals during thermal cycling or thermal shock. This can cause leakage that would lower the cell performance and efficiency. In order to remain hermetic, a sealant must exhibit strong bonding both with the electrolyte and the interconnect. In addition, the coefficient of thermal expansion of the interconnect and of the sealant must closely match each other and interfacial reactions must not lead to products with thermal expansion markedly different from those of the bulk phases, otherwise high residual stresses will be generated within the interfacial reaction zone giving rise to cracks. The authors have recently developed a new sealant based on a sodium-calcium-aluminosilicate glass-ceramic [22]which has a coefficient of thermal expansion of 10.7 106 C1, a value that is compatible with YSZ and Crofer22APU substrates, in order to obtain a very good thermo-mechanical compatibility. The study that is presented here is focused mainly on the morphological/microstructural behaviour of the glass-ceramic sealant in contact with Crofer22APU. Prior to the joining process the Crofer22APU, had undergone a preoxidation treatment at 900 C for 2 h. This was different from our previously reported investigation [23] where the Crofer22APU had been preoxidised at 950 C for 2 h and the subsequent thermal treatments showed little or no diffusion of chromium and manganese into the glass-ceramic sealant. The current study aimed to investigate whether the reduction in the pre-oxidation temperature would result in any changes in the subsequent thermal behaviour. In addition, samples underwent two different thermal cycles and were examined for interfacial cracks. A total of three types of thermal cyclic and thermal ageing tests were employed in air for 500 h.
2.
Experimental
The heat resistant metal alloy used for this study was Crofer22APU (Cr 20e24, C 0.03, Mn 0.30e0.80, Si 0.50, Al 0.50 max, Fe balance, wt.%) manufactured by Tyssen Krupp, Germany and supplied by HT Ceramix, Switzerland). Crofer22APU, was polished using SiC paper to 4000 grit and then preoxidised at 900 C for 2 h in air. The anode-supported-electrolyte (ASE) (electrolyte: 8%mol yttria stabilised cubic zirconia, YSZ; anode; NiO-YSZ) half cell were purchased by H. C. StarckGmbH (Germany). The Crofer22APU and ASE samples to be joined were cut to obtain a final joined sample of 6 6 2 mm3. The melting procedure, thermal and thermo-mechanical characterization of the sealant are described elsewhere [22,23]. The sealant composition ranged between 53 and 58 mol% SiO2, 16e18 mol% Al2O3, 24e26 mol% CaO and 9e12 mol% Na2O. The glass transition temperature, Tg, and the softening temperature, were measured to be 670 C and 740 C respectively, by differential thermal analysis (DTA) and dilatometer, while two crystallization temperatures were detected at 830 C and 940 C respectively. The joints were prepared by placing the Crofer22APU plates on the yttria-stabilised zirconia (YSZ) surface of the anode-supported-electrolyte with a slurry of glass powder dispersed in ethanol (solid content of 40 %wt.) sandwiched inbetween. The sealed samples were then heat-treated in a tubular oven in argon at a temperature above the glass softening point without applying any load. Reproducible results, in terms of joint thickness and homogeneity were obtained. The joining thermal treatment was carried out by heating from room temperature to 900 Cat a heating rate of 5 C/min and a dwelling time of 30 min at 900 C. The cooling rate was 5 C/min. Thermal cycling tests were performed in a muffle furnace with static air from room temperature to 800 C for different times for a total period of 500 h. Two kinds of thermal cycling tests were conducted on the joined samples: Thermal Cycling 1: six samples had a cooling cycle of 10 C/min from 800 C to room temperature every 72 h for a total of 500 h giving a total of 7 heatingecooling cycles.
Fig. 1 e Crofer22APU/glass-ceramic/ASE SEM cross-section after thermal cycling 2.
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Fig. 2 e SEM cross-section magnification of the interface between Crofer22APU and the glass-ceramic sealant for a sample that followed thermal cycling 1.
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Thermal Cycling 2: six other joined samples were extracted and then reinserted into the furnace at 800 Cevery 24 h for a total of 500 h. The latter of the two thermal cyclic treatments included 21 heatingecooling cycles and was thus more severe than the former cyclic treatment. Thermal ageing tests were conducted on six joined samples at 800 C in static air for a total period of 500 h (heating rate 13 C/min, cooling rate 1 C/min). Cross-sections of all joined samples were characterized by scanning electron microscopy (SEM) (FEI Inspect, Philips 525 M and JEOL 5700) after polishing. EDS (SW9100 EDAX) analysis was carried out in order to detect any elemental diffusion into or away from the sealant and to examine for any chemical interactions between Crofer22APU and the glass-ceramic sealant. One of the samples for SEM examination was submitted to etching with 4%vol HF for 40 s, in order to evidence the microstructure. The microstructure and chemical composition at the Crofer22APU/glass-ceramic interface were also examined by transmission electron microscopy (TEM)using JEM-2010 ARP on cross-section thin foils prepared by the Precision Ion Polishing System (PIPS; Gatan). Phases were identified using selected area electron diffraction (SAED)
Fig. 3 e Elemental EDAX mapping at the interface between Crofer22APU and the glass-ceramic sealant for a sample with thermal cycling 1.
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and energy dispersive X-ray spectroscopy (STEM-EDS). STEM images were acquired with a resolution of 256 224 pixels. The electron beam size was about 3 nm.
3.
Results and discussion
3.1. Crofer22APU/glass-ceramic sealant/ASE joined samples after thermal cycling 1 and 2 SEM examination of these twelve samples showed that the interfaces Crofer22APU/glass-ceramic/YSZ (comparable with the as fabricated microstructure of the joint, before tests, that can be found in reference 23) remained joined together without any evidence of cracking or delamination, suggesting good thermo-mechanical compatibility between the glassceramic sealant and both Crofer22APU and YSZ. A typical
example of this is shown in Fig. 1 after thermal cycling 2which represents the more severe of the two undertaken thermal cycles. Qualitative observations indicate very good adhesion between the glass-ceramic sealant and the two substrates after thermal cycling treatments. Fig. 2 shows a magnification of the interface between Crofer22APU and the glass-ceramic sealant (in order to evidence features in the glass-ceramic) for a sample that followed thermal cycling 1. Elemental EDAX mapping for the same sample is presented in Fig. 3. It can be observed that aluminium within the Crofer22APU alloy diffused, suffered internal oxidation and formed discontinuous pockets of Al2O3 just below the Crofer22APU protective oxide layer (Fig. 2). This observation is in agreement with the observation reported by Horita et al. [24] where pockets of aluminium oxide were detected in the metallic interconnect (air at 800 C) suggesting significant oxidation in the inner parts of the alloy. In the
Fig. 4 e SEM cross-section of Crofer22APU/glass-ceramic sealant interface and EDS investigation on 4%vol HF etched samples (thermal cycles 1 from room temperature to 800 C for 500 h).
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Fig. 5 e SEM cross-section of YSZ/glass-ceramic sealant interface after thermal cycles from room temperature to 800 C for 500 h 1 according to thermal cycle 2).
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regions where there was enrichment and oxidation of aluminium, there was also evidence of small pores developing. This appeared to be the result of aluminium diffusion followed by internal oxidation at the grain boundaries. This did not seem to be a problem after 500 h of thermal cyclic treatment, as there was no evidence of cracks at the interface between the protective oxide layer and the stainless steel. There was also no evidence at all of any aluminium diffusing into the protective oxide layer. Slight diffusion of chromium and manganese from Crofer22APU into the glass-ceramic sealant was also observed. At the same time, sodium and calcium from the glass-ceramic sealant were observed to diffuse away from the Crofer22APU/glass-ceramic interface. Further EDS investigation is presented in Fig. 4 for a sample that was etched with 4% vol HF and shows the development of three distinct microstructural zones within the glass-ceramic sealant near the interface with Crofer22APU. The EDS spectra indicated that zone 1 which is adjacent to the Crofer22APU and about 10 mm thick, is rich in Si, Cr and O with some Al, Ca and Mn also being present. Zone 2 contains Si, Al, Ca, O and only a trace of Na, while zone 3 contains the original phases of the glass-ceramic.
Fig. 6 e Crofer22APU/glass-ceramic sealant SEM cross-section after thermal ageing at 800 C for 500 h and the relative EDS mapping.
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Fig. 5 shows a SEM cross-section of YSZ/glass-ceramic sealant interface after thermal cycles from room temperature to 800 C for 500 h(according to thermal cycle 2). No cracks are observed at the interface and no elements diffusion was detected into the glass-ceramic.
3.2. Crofer22APU/glass-ceramic sealant/ASE joined samples after thermal ageing tests As in the case of the samples submitted to thermal cyclic treatment, all the Crofer22APU/glass-ceramic sealant/ASE joined samples after the thermal ageing tests maintained their integrity and showed no evidence of fracture or delamination. Fig. 6 shows a Crofer22APU/glass-ceramic sealant SEM cross-section after thermal ageing at 800 C for 500 h and the relative EDS mapping. The results appear to be the same as with the thermally cycled samples as described in the previous section with slight diffusion of both chromium and
manganese into the glass-ceramic sealant having taken place. Therefore the further characterization (TEM and SAED)of theCrofer22APU/glass-ceramic sealant interface was concentrated on thermally aged joined samples. The Crofer22APU/ glass-ceramic interface was also examined by using brightfield TEM and the results are shown in Fig. 7. This part of the investigation focused on the interactions within about 2 mm of the Crofer22APU/glass-ceramic interface. In some areas adjacent to the interface, an amorphous zone of up to 500 nm thickness was detected (Fig. 7, point 1) which contained Si (26.6 at.%) and O (61.7 at.%) suggesting the formation of amorphous SiO2. A small amount of some of the other elements such as Al (7.1 at.%), Cr (2.3 at.%), Mn (1.8 at.%), Fe (0.3 at.%) and Ca (0.2 at.%) which appeared to be dissolved in the amorphous SiO2were also detected in this zone. Fig. 7 also confirms the presence of the CrMn2O4 protective oxide layer (point 2, Fig. 7) on the surface of Crofer22APU. Just below the protective oxide layer, pockets of Al2O3 particles were
Fig. 7 e The interface of the Crofer22APU/glass-ceramic and SAED patterns taken from areas marked on figure as 1e6 and their identification, TEM BF cross-section thin foil.
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detected (point 3, Fig. 7). This adds further support to the SEM observations in Fig. 2 where pockets of Al2O3 were detected just below the protective oxide layer. Point 4 represents the aFe solid solution. As chromium and manganese diffuse into the glass-ceramic, they react with SiO2 to form Mn3Cr2Si3O12 (face-centred orthorhombic) which was detected by TEM at point 5 in Fig. 7. The observation of Mn3Cr2Si3O12is rather surprising as this phase has previously been produced in laboratory conditions only by the combined application of high pressures and temperatures. However, the presence of Mn, Cr, Si and O in areas close to the Crofer22APU/glassceramic interface in STEM-EDS (Fig. 8) elemental maps also confirmed the possibility of the presence of the Mn3Cr2Si3O12 phase. After an exhaustive search, the only known phase that fits the TEM data is Mn3Cr2Si3O12. The authors have no reason to doubt the validity of the TEM data, but as it is not clear why this phase forms, further work is planned in the near future to address this. Further into the glass-ceramic, Ca2Al2SiO7 was detected at point 6 in Fig. 7. There was no evidence of any sodium at all in the areas close to the Crofer22APU/glassceramic interface. The SEM observations indicate that sodium diffuses away from the Crofer22APU/glass-ceramic interface at least 10 mm into the glass-ceramic. This observation is rather interesting because sodium is always diffusing away from chromium. As a consequence there is no evidence of any reaction between sodium and chromium oxides which are known to react together to form the volatile sodium chromate [25], Na2CrO4. Further support for the absence of any Na2CrO4 formation was also provided by the fact that there was no porosity within the diffusion layer (i.e. no evidence for any evaporation). In previous work by Ogasawara et al. [26] who used a glassy sealant, the formation of sodium chromate was reported in some areas. The chromate is volatile and loss of chromium hindered the formation of a stable protective oxide. Clearly this is not the case in the present work as the behaviour of the glass-ceramic sealant was such that it prevented reaction between the sodium and chromium oxides. Furthermore, there was no direct contact between sodium and chromium oxides as they were well-separated by a distinct layer (zone 2 in Fig. 4) making any adverse reaction to produce sodium chromate unlikely. Therefore the slight diffusion of chromium and manganese into the glass-ceramic produced no detrimental effects. Within this region (zone 2), only a small trace of sodium was detected. Chromates are rather volatile and volatilization would be quite obvious resulting in big voids and failure of the Crofer22APU-glass-ceramic bond. Furthermore, SEM examination (picture not reported here) around the three phase boundary (Crofer22APU-glass-ceramic sealant-air, i.e. outer seal edge) revealed no presence of Na2CrO4 as well as no anomalous corrosion of the Crofer22APU interconnect. Comparison of the Gibbs Free energy of formation data under standard conditions for NaAlSiO4 (4,163,500 þ 248.5T kJ/mol) and Na2CrO4 (1,334,000 þ 176.6T kJ/mol) as compiled by Kubaschewski et al. [27] indicates that at 800 C the NaAlSiO4 phase is more stable and that formation of the chromate would be unlikely. The absence of any interfacial reaction between the Crofer22APU and sodium glass-ceramic sealant (where amorphous residual phase is very low [22],) is likely due to
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the fact that sodium is mainly present in one of the crystalline phases, limiting alkali fast diffusion process through the glassy phase. Concerning other glass-ceramic sealants Ba or Sr silicate glass containing and non-containing alkalis,
Fig. 8 e STEM-EDS (Fig. 8) elemental maps at Crofer22APU/ glass-ceramic interface.
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Fig. 9 e SAED patterns taken from area in the glass-ceramic at 15e20 microns from Crofer22APU/glass-ceramic interface; TEM BF cross-section thin foil.
in [6] the authors discussed the compatibility between Crofer22APU (as-received and aluminized) and potassium oxide containing glasses: the addition of K2O in the parent glass sealant led to the detachment of sealant from Crofer22APU, but due to alkaline-earth chromates formation; microstructure characterization showed extensive chromate formation for the as-received Crofer22APU and complete loss of the protective alumina layer for the aluminized Crofer22APU. Further analysis into the glass-ceramic using bright-field TEM in Fig. 9 also showed evidence of small pockets of CaSiO3 (point 1, Fig. 9) and NaAlSiO4 (point 2, Fig. 9) as well as an amorphous phase (point 3, Fig. 9). The results of the thermal treatments have led to phase changes at the interface between the Crofer22APU and the glass-ceramic up to 5 microns from the interface. Both chromium and manganese from the pre-oxidation layer obtained by heating Crofer22APU at 900 C for 2 h in air had diffused up to 5 mm into the glassceramic to form Mn3Cr2Si3O12. In a recent study, Smedskjaer and Yue [28] investigated the behaviour of Na2OeCaOeSiO2 system in oxidising and reducing conditions. Sodium and calcium were reported to diffuse outwards under oxidising conditions. In diffusion studies of the Na2OeCaOeSiO2, system Natrup et al. [29] observed that the diffusion of Naþ ions was faster than that of Ca2þ ions. The results of these previous studies have similarity to the present investigation which has shown the
formation of a very thin (about 200 nm) SiO2-rich layer at the interface between Crofer22APU and the glass-ceramic and the diffusion of sodium and calcium away from the interface. Furthermore, the faster diffusion of the Naþ ion in comparison to the Ca2þ ion enabled sodium to diffuse further away leaving, behind it, the Ca2Al2SiO7 zone. The diffusion profiles and the microstructural observations for the two types of cyclic tests and for the thermal ageing tests were very similar. The reason for carrying out the various types of tests was to investigate whether the two types of thermal cycles would lead to cracking and failure at the interface between the glass-ceramic sealant and the Crofer22APU. As reported above, good adhesion between the two was maintained through the various thermal tests. The results of the present investigation differ from our previously published study [30] where no Cr interfacial diffusion was observed at the Crofer22APU/glass-ceramic interface. The importance of the condition of the preoxidised protective layer was demonstrated in another of our earlier investigations [30]. The reason for the difference in the behaviour lies in the type of pre-oxidation treatment that was used for Crofer22APU. In our earlier study, the material was preoxidised at 950 C for 2 h instead of 900 C as for the current study. The temperature and time of pre-treatment in the current study were not high enough to sufficiently stabilise and to obtain an homogeneous and coherent preoxidised layer.
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4.
Conclusions
The observations of the current study have led to the following conclusions: 1. Crofer22APU/glass-ceramic sealant/ASE joined samples submitted to thermal cycles and thermal ageing at 800 C for 500 h in air showed no evidence of interfacial failure and cracks; slight migration of both Cr and Mn into the glassceramic was observed. 2. A very thin layer measuring 0.5 mm was detected at the Crofer22APU/glass-ceramic interface after thermal ageing for 500 h at 800 C, while Cr and Mn were observed to interact with SiO2 to form Mn3Cr2Si3O12. At the same time both sodium and calcium were observed to diffuse inwards away for the Crofer22APU/glass-ceramic interface. As a result there was no direct contact between sodium and chromium oxides making any reaction to produce the volatile sodium chromate unlikely. 3. No interfacial migration was observed at the interface between the glass-ceramic and YSZ. 4. The temperature and time of Croferr22APU pre-treatment should be high enough to obtain an homogeneous and coherent preoxidised layer.
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