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Study of sealants for SOFC
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Study of sealants for SOFC Xuan-Vien Nguyen a,*, Chang-Tsair Chang b, Guo-Bin Jung a, Shih-Hung Chan a, Win-Tai Lee c, Shu-Wei Chang c, I-Cheng Kao c a
Department of Mechanical Engineering & Fuel Cell Center, Yuan Ze University, Taoyuan 320, Taiwan Taoyuan Aerotropolis Company, Taoyuan 320, Taiwan c Taiwan Power Company, New Taipei City, Taiwan b
article info
abstract
Article history:
The development of suitable sealants for high-temperature solid oxide fuel cells (SOFC) is a
Received 10 March 2016
major challenge, as such sealants must withstand harsh conditions. They must be inert at
Received in revised form
the high temperatures used in SOFC operation (i.e., resistant to environments composed of
19 July 2016
oxidative and reducing gases), as well as thermo-chemically and thermo-mechanically
Accepted 19 July 2016
compatible with the materials with which they are in contact. This paper presents a
Available online xxx
post-experimental analysis of variously sealantsdmica paper, flexible mica paper, and thermiculite 866dused in high-temperature SOFC operation. The sealants are exposed to
Keywords:
air or hydrogen at 600 Ce1000 C for 100 h. The goal of this work is to investigate the
Solid oxide fuel cell
thermal expansion properties (thickness expansion, coefficients of thermal expansion CTE
Thermiculite 866
and porosity), mechanical stability, and leakage during midterm operation. After the
Mica
sealants fired at 1000 C, their relative thicknesses increased to around 0.98, 1.01, and
Coefficients of thermal expansion
0.568 mm, respectively. The coefficients of thermal expansion CTEs (600e1000 C) for mica
CTE
paper and flexible mica paper were calculated to be from 8.0 104 K1 to 10.0 104 K1,
Mechanical stability
the CTEs of thermiculite 866 were around 1.0 104 K1. The relative porosity of thermi-
Porosity
culite 866, as determined through the density method, changed from 15.4% to 28.7% for temperatures from 600 C to 1000 C, respectively. Scanning electron microscopy is used to investigate the structure of thermiculite 866. It is tested for leakage using hydrogen from 500 to 3000 cc min1 at 25 C and 800 C. The leakage rates at 3000 cc min1 are 4.06% and 8.4% at 25 C and 800 C, respectively, showing that thermiculite 866 is a suitable sealant for SOFC applications. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, anode-supported planar solid oxide fuel cells (SOFC) have received much attention because of their satisfactory power density at intermediate temperatures. The main advantages of power production using SOFCs are their high conversion efficiency, the absence of combustion, and
the variety of fuels (including those derived from renewable sources) that may be utilized with them. They also have a simple manufacturing process and yield a high production rate. However, one of the major technological challenges with planar SOFCs is the development of suitable sealants for separating the fuel and oxidant [1e5]. Reliable sealants must be amenable to high-temperature operations (from 600 C to 1000 C) and must withstand the
* Corresponding author. Tel.: þ886 975 470 278; Fax: þ886 3 455 5574. E-mail addresses: [email protected], [email protected] (X.-V. Nguyen). http://dx.doi.org/10.1016/j.ijhydene.2016.07.156 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Nguyen X-V, et al., Study of sealants for SOFC, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.07.156
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severe environments of SOFCs (i.e., oxidative and reducing environments). Furthermore, the sealants need to have longterm performance stability (>40,000 h) at the operating temperature and need to endure thermal cycles thermochemically and thermo-mechanically during routine operation [6e8]. Seals in SOFC stacks have to satisfy a variety of requirements [9]. The sealants should also sufficiently requite the mechanical and thermal mismatch of the stack components, while not effecting or deteriorating the functional layers of the SOFC. Furthermore, electrical insulation is required to avoid short circuits when stacking cells and interconnects. The problem becomes more challenging when thermal cycle stability is also required for planar stacks in which dissimilar SOFC components are sealed together. The sealants have to survive hundreds to several thousands of thermal cycles during life service. To date, several different approaches for SOFC seals have been investigated, including rigid glass, glass ceramic, and fiber-reinforced glass seals [10e13], compressive mica seals [14,15], and metallic seals such as active brazing alloys or silver wire [16,17]. Planar-type SOFCs are generally regarded as superior to their counterparts [18]. However, although they offer a new option for future green-power generation and utilization, such SOFCs have been difficult to commercialize because of their insufficient long-term durability, caused primarily by cracking of the seals among cell components during thermal cycling, which creates passageways for fuel and oxidizing gases. An ideal sealing material needs to meet several stringent requirements, notably low leakage, mechanical integrity, as well as chemical, electrical, and thermal compatibility with the fuel and the cell components. It also must be robust enough to withstand numerous thermal cycles and reliable enough to sustain long-term operation at high temperatures. One of the important factors for selection of sealants is the coefficient of thermal expansion (CTE). The matching of CTE of the sealants with other cell components, i.e., YSZ electrolyte and interconnect material, is compulsory for minimizing the thermal stresses [19]. Currently, glass ceramics are the sealing materials most commonly used. They offer gas tightness and the potential for adapting the coefficients of thermal expansion (CTE) to other stack components by controlling the crystallizing phase content. However, properties of glasses or glasseceramics, such as coefficient of thermal expansion (CTE), viscosity and porosity, often change over time. During long term operation these changes can create additional thermo-mechanical stresses leading to seal failure [20]. Based on the requirements of sealants, some previous studies focus on four important successive steps: (i) investigation and selection of potential sealants (with matching CTE) and approximation of the composition, (ii) processing techniques employed, (iii) investigation of the quantitative impacts of each constituent on the sealing behavior and (iv) optimization of chemical processes and technique (controlling crystallization kinetics by surface engineering, chemistry and long-term stability) [21]. The quality of the sealants must be high, since even small leaks in these seals can affect the cell voltage and thus can decrease performance [22]. As the thermal expansion of the sealants increase with heating and cooling rates, the development of sealants is particularly important for achieving rapid startup times [23], which is a
major challenge with SOFCs. The goal of this work, therefore, is to investigate the thermal-expansion properties (thickness expansion, coefficient of thermal expansion (CTE) and porosity), mechanical stability, microstructural properties, and leakage of mica paper, flexible mica paper, and thermiculite 866 sealants during intermediate temperature operation.
Experimental Sealant design and production Three sealants were used in this study, namely, mica paper (thickness: 500 mm, Ming Wei Corp., Taiwan), flexible mica paper (thickness: 500 mm, FCM Fuel Cell Materials Co. Ltd., U.S.A.), and thermiculite 866 (thickness: 500 mm; Flexitallic Co. Ltd., UK). The mica paper, flexible mica paper, and thermiculite 886 were designed and fabricated for application as sealants in planar SOFC stacks. Photographs of the sealants and the interconnector for the SOFC stacks are shown in Fig. 1. The design of the SOFC sealant is very important for ensuring proper fuel and oxidant supply through the gas flow paths. The stacks must not leak and gas pressure must not build up, as both of these factors can affect the performance and efficiency of SOFCs during power generation. In this study, an SOFC sealant design was developed.
Thermal expansion tests Thickness expansion test The original thicknesses of the mica paper, flexible mica paper, and thermiculite 866 were 0.5 mm each. The sealants were heated up to targeted temperature 600 C, 700 C, 800 C, 900 C, and 1000 C in a heating box. The heating rate is 2.0e3.33 C min1 and then kept at targeted temperature for 100 h. An electronic measurement apparatus was used to investigate the expansion of the fired sealants.
Coefficient of thermal expansion (CTE) calculation After firing, the thickness of fired sealants was measured. Coefficient of thermal expansion (CTE) of fired sealants was calculated by using Equation (1) [24]. CTE ¼
1 DL L DT
(1)
where L is the original thickness of sealant samples, DL is the change in thickness of fired sealants, and DT is the change in temperatures.
Porosity determination After firing of the sealants, the Archimedes method was used to investigate the porosity of the fired sealants [25]. The bulk density (rb) of the fired sealants was calculated by using Equation (2): rb ¼
rs Wfa Wfa Wfs
(2)
where rs is the density of the solvent (boiling water). Samples of the fired sealants were first weighed on a pan, the weight
Please cite this article in press as: Nguyen X-V, et al., Study of sealants for SOFC, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.07.156
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Fig. 1 e Photographs of the SOFC stack sealants: a) mica paper, b) flexible mica paper, c) thermiculite 866, and d) the interconnector.
(Wfa) was then recorded. The samples were then immersed in boiling water and placed on the weighing pan submerged in the solvent. The weight in kilograms (Wfs) of the immersed sample was then recorded. The porosity of the fired sealants was then determined from the absolute and bulk densities of the fibers through Equation (3): P¼
r 1 b 100 ra
hydrogen flow rates at the inlet equal to the test at room temperature. As shown in Fig. 3, the flow meter was connected to the outlet of fuel. It shows variety of changes in flow rate of the hydrogen outlet.
Results and discussion (3)
where P is the percentage porosity (%), and ra is the absolute density of the original sealant.
Mechanical stability test After firing, the sealants were tested under a capillary flow porometer machine (CFP-1500AEXDH, Porous Materials, Inc., U.S.A.). As shown in Fig. 2, the fired sealants were put inside a compressing chamber and then compressed at 0.3 MPa by using compressed air.
Microstructural characterization The morphologies of the fired sealants were studied under a scanning electron microscope (JSM-5600, Jeol Co., Japan) equipped with an energy-dispersive spectroscope.
Leakage test Fig. 3 depicts the experimental setup system for leakage test. The sealants were assembled in SOFC device and then the SOFC unit was placed in heating box. Firstly, leakage test was carried out with hydrogen at room temperature (25 C). The sealants were tested for leaks at different hydrogen flow rates at the inlet (500e3000 cc min1). After that, the temperature of heating box was then set at 800 C for 5 h with heating rate 2.7 C min1. The leakage test was investigated at 800 C with
Thermal expansion The sealants were tested at 600 C, 700 C, 800 C, 900 C, and 1000 C for 100 h in a heating box. As shown in Fig. 4, the thicknesses of the fired sealants e mica paper, flexible mica paper, and thermiculite 866 are 1.01, 0.98, and 0.568 mm, respectively. The thickness changes of mica paper, flexible mica paper, and thermiculite 866 are 0.51, 0.48, and 0.068 mm, respectively. These changes suggest that the variation in the porosity of thermiculite 866 under high temperature condition is slightly compared to that of mica paper and flexible mica paper. Thus, thermiculite 866 is a better candidate in terms of decreasing the leakage rate compared to sealants e mica paper and flexible mica paper. Fig. 5 shows that the porosity of the flexible mica paper increased very rapidly, from 21.8% at 600 C to 52.7% at 1000 C. The porosity of the mica paper increased from 25.3% at 600 C to 58.9% at 1000 C, higher than that of the flexible mica paper. Meanwhile, the porosity of thermiculite 866 changed from 15.4% at 600 C to 28.7% at 1000 C, gentle compared to sealants e mica paper and flexible mica paper. As shown in Fig. 6, the coefficient of thermal expansion (CTE) of fired sealant samples were calculated as a relative values. The relative (600e1000 C) CTEs for mica paper were found to be 8.7 104 K1, 9.2 104 K1, 9.7 104 K1, 10.3 104 K1, 9.85 104 K1for 600, 700, 800, 900, and 1000 C, respectively. The relative (600e1000 C) CTEs for flexible mica paper were found to be 9.74 104 K1,
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Fig. 2 e Schematic illustration of experimental set-up for mechanical stability test.
Fig. 3 e Schematic illustration of experimental set-up for leakage test. 8.9 104 K1, 10.29 104 K1, 10.97 104 K1, 10.46 104 K1 for 600, 700, 800, 900 C, and 1000 C, respectively. The relative (600e1000 C) CTEs for thermiculite 866 were found to be 1.04 104 K1, 1.15 104 K1, 1.13 104 K1, 1.07 104 K1, and 1.39 104 K1 for 600 C, 700 C, 800 C, 900 C, and 1000 C, respectively. It can be found that the CTEs of mica paper and flexible mica paper (8.0 104 K1e10.0 104 K1) are much higher than that of thermiculite 866 (1.0 104 K1).
Mechanical stability
Fig. 4 e Changes in the thicknesses of the sealants with temperatures.
Fig. 7 shows structural comparisons of the sealing materials before and after firing. After firing, the mica paper and flexible mica paper samples could be easily detached by a pressing machine and could be easily pulverized by applying a light force using two fingers, and the structures of both materials
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Fig. 5 e Changes in the porosities of the sealants with temperature.
Fig. 6 e The coefficient of thermal expansion (CTE) of the sealants at different temperatures.
could be easily broken. However, the fired sample of thermiculite 866 was not broken. Thus, the structures of mica paper and flexible mica paper are softer than that of thermiculite 866 and the porosities after firing of the former two are higher than that of the latter. Thus, thermiculite 866 is a better candidate in terms of higher mechanical stability compared to sealants e mica paper and flexible mica paper.
Microstructural characterization Fig. 8 shows the structures of the sealing materials before firing. Mica paper and flexible mica paper appear to be thick and blocky, consisting of many stacked layers. These materials depend on the inclusion of an elastomer in the material body to bind them together and to ensure a good seal at ambient temperature. Addition of excess elastomer, however, leads to poor creep characteristics. At high temperatures, the binder burns off, resulting in a leakage rate higher than that at room temperature. In contrast, the structure of thermiculite 866 resembles that of muscle (Fig. 8(c)). This structure indicates why the sealing is robust; the highly aligned sheets of chemically exfoliated vermiculite interspersed with steatite
5
Fig. 7 e The sealing materials before and after firing: (a) mica paper, (b) flexible mica paper, and (c) thermiculite 866.
create a very circuitous path along which gas molecules have to travel in order to escape [26]. This structure is thus suitable for reducing the leakage rate. Fig. 9 shows the structures of mica paper, flexible mica paper, and thermiculite 866 after firing. As shown in the figure, the structures of fired mica paper and flexible mica paper changed at high temperature. The blocky layers in their structures expanded (Fig. 9(a) and (b)) because these materials depend on an elastomer in the material body that binds them together. At high temperatures, the binder burned off, creating an expanded structure. As gaps appeared in the material body at high temperature, the thicknesses of the mica paper and flexible mica paper increased. This finding is consistent to big change in thickness and porosity for sealants e mica paper and flexible mica paper. In contrast, the structure of thermiculite 866 did not expand at high temperature (Fig. 9(c)) because the materials lacked organic content to burn off. This finding is related to its small change in thickness and porosity. Therefore for sealants e mica paper and flexible mica paper, the changes in thickness and porosity were more significant due to burning of the elastomer, which is included in the material body, at high temperatures.
Leakage test Fig. 10 depicts the rate of leakage through the materials at room temperature (25 C) and at 800 C with different hydrogen flow rates at the inlet (from 500 to 3000 cc min1 compressive stress). The standard cubic centimeters per minute per unit leak length of seal is also calculated, the length of seal is 4 14 cm ¼ 56 cm. The figure shows that the room-temperature leakage rates for mica paper are 1.34% at 500 cc min1 (1.19 101 sccm cm1), 5.75% at 2000 cc min1 (2.05 sccm cm1), and 9.78% at 3000 cc min1 (5.24 sccm cm1); for flexible mica paper are 1.14% at 500 cc min1 (1.01 101 sccm cm1), 5.06% at 2000 cc min1 (1.8 sccm cm1), and 9.22% at 3000 cc min1 (4.94 sccm cm1); and for thermiculite 866 are 0.51% at 500 cc min1 (0.45 101 sccm cm1), 2.52% at 2000 cc min1
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Fig. 8 e Microstructures of the sealants before firing.
Fig. 9 e Microstructure of the sealants after firing.
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Fig. 10 e Rate of leakage through the sealants: a) mica paper, b) flexible mica paper, and c) thermiculite 866.
(0.9 sccm cm1), and 4.06% at 3000 cc min1 (2.175 sccm cm1). The leakage rates at 800 C of mica paper are 3.8% at 500 cc min1 (3.34 101 sccm cm1), 12.6% at 2000 cc min1 (4.5 sccm cm1), and 20.6% at 3000 cc min1 (11.03 sccm cm1); for flexible mica paper are 3.2% at 500 cc min1 (2.85 101 sccm cm1), 10.2% at 2000 cc min1 (3.64 sccm cm1), and 17.4% at 3000 cc min1 (9.32 sccm cm1); and 1.62% at 500 cc min1 (1.44 101 sccm cm1), 4.7% at 2000 cc min1 (1.67 sccm cm1), and 8.4% at 3000 cc min1 (4.5 sccm cm1) for thermiculite 866. The leakage rates for all sealants increase with increasing hydrogen flow rate and temperature. Also, the leakage rates for mica and flexible mica are similar and almost twice higher than that of thermiculite 866. These results are compared with those of previous studies. The best results were obtained using muscovite, single-crystal mica, phlogopite mica, and 125 mm silver layers. Using the paper form of muscovite and phlogopite mica, the leak rates were still far superior (~1 101 sccm cm1), mica without the compliant silver layer (about 6e9 sccm cm1) [15].
Muscovite and phlogopite micas have been evaluated as SOFC seals at 800 C. Cleaved natural mica sheets (with no binder) indicated far superior sealing characteristics with leak rates lower than 0.1 sccm cm1 at 800 C [14]. It can be seen that the leakage rate of sealants with organic binder is much higher than sealants without organic binder. Therefore, the organic binder sealants were usually combined with other sealing materials to use in high operating temperature SOFC.
Conclusion This paper investigated the thermochemical and thermomechanical compatibility of mica paper, flexible mica paper, and thermiculite 866 at high temperature. In general, mica paper and flexible mica paper have highly attractive features for sealing purposes, namely, their structural deformability and gas tightness at both room temperature and high temperature. However, their seriously limited thermochemical
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and thermomechanical stabilities still need to be addressed. In contrast, thermiculite 866 contains no organic content that burns off. This lack is related to the small change in its thickness and porosity. Furthermore, its lack of organic content, which could contaminate and thus cause degradation of solid oxide fuel cells (SOFC), ensures no increase in porosity or additional leakage during high-temperature operation. Furthermore, various sealing patterns using thermiculite 866 are easy to design and to cut; patterns with simple shapes can be cut with a knife or with scissors. These features imply that thermiculite 866 is a suitable sealant for SOFC applications.
[11]
[12]
[13]
[14] [15]
Acknowledgements [16]
The authors gratefully thank the National Science Council of Taiwan under contracts NSC 99-2221-E-155-063 and NSC 1003113-E-155-001 and the Taiwan Power Company for their financial support.
[17] [18]
references
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Study of sealants for SOFC Xuan-Vien Nguyen a,*, Chang-Tsair Chang b, Guo-Bin Jung a, Shih-Hung Chan a, Win-Tai Lee c, Shu-Wei Chang c, I-Cheng Kao c a
Department of Mechanical Engineering & Fuel Cell Center, Yuan Ze University, Taoyuan 320, Taiwan Taoyuan Aerotropolis Company, Taoyuan 320, Taiwan c Taiwan Power Company, New Taipei City, Taiwan b
article info
abstract
Article history:
The development of suitable sealants for high-temperature solid oxide fuel cells (SOFC) is a
Received 10 March 2016
major challenge, as such sealants must withstand harsh conditions. They must be inert at
Received in revised form
the high temperatures used in SOFC operation (i.e., resistant to environments composed of
19 July 2016
oxidative and reducing gases), as well as thermo-chemically and thermo-mechanically
Accepted 19 July 2016
compatible with the materials with which they are in contact. This paper presents a
Available online xxx
post-experimental analysis of variously sealantsdmica paper, flexible mica paper, and thermiculite 866dused in high-temperature SOFC operation. The sealants are exposed to
Keywords:
air or hydrogen at 600 Ce1000 C for 100 h. The goal of this work is to investigate the
Solid oxide fuel cell
thermal expansion properties (thickness expansion, coefficients of thermal expansion CTE
Thermiculite 866
and porosity), mechanical stability, and leakage during midterm operation. After the
Mica
sealants fired at 1000 C, their relative thicknesses increased to around 0.98, 1.01, and
Coefficients of thermal expansion
0.568 mm, respectively. The coefficients of thermal expansion CTEs (600e1000 C) for mica
CTE
paper and flexible mica paper were calculated to be from 8.0 104 K1 to 10.0 104 K1,
Mechanical stability
the CTEs of thermiculite 866 were around 1.0 104 K1. The relative porosity of thermi-
Porosity
culite 866, as determined through the density method, changed from 15.4% to 28.7% for temperatures from 600 C to 1000 C, respectively. Scanning electron microscopy is used to investigate the structure of thermiculite 866. It is tested for leakage using hydrogen from 500 to 3000 cc min1 at 25 C and 800 C. The leakage rates at 3000 cc min1 are 4.06% and 8.4% at 25 C and 800 C, respectively, showing that thermiculite 866 is a suitable sealant for SOFC applications. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, anode-supported planar solid oxide fuel cells (SOFC) have received much attention because of their satisfactory power density at intermediate temperatures. The main advantages of power production using SOFCs are their high conversion efficiency, the absence of combustion, and
the variety of fuels (including those derived from renewable sources) that may be utilized with them. They also have a simple manufacturing process and yield a high production rate. However, one of the major technological challenges with planar SOFCs is the development of suitable sealants for separating the fuel and oxidant [1e5]. Reliable sealants must be amenable to high-temperature operations (from 600 C to 1000 C) and must withstand the
* Corresponding author. Tel.: þ886 975 470 278; Fax: þ886 3 455 5574. E-mail addresses: [email protected], [email protected] (X.-V. Nguyen). http://dx.doi.org/10.1016/j.ijhydene.2016.07.156 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Nguyen X-V, et al., Study of sealants for SOFC, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.07.156
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severe environments of SOFCs (i.e., oxidative and reducing environments). Furthermore, the sealants need to have longterm performance stability (>40,000 h) at the operating temperature and need to endure thermal cycles thermochemically and thermo-mechanically during routine operation [6e8]. Seals in SOFC stacks have to satisfy a variety of requirements [9]. The sealants should also sufficiently requite the mechanical and thermal mismatch of the stack components, while not effecting or deteriorating the functional layers of the SOFC. Furthermore, electrical insulation is required to avoid short circuits when stacking cells and interconnects. The problem becomes more challenging when thermal cycle stability is also required for planar stacks in which dissimilar SOFC components are sealed together. The sealants have to survive hundreds to several thousands of thermal cycles during life service. To date, several different approaches for SOFC seals have been investigated, including rigid glass, glass ceramic, and fiber-reinforced glass seals [10e13], compressive mica seals [14,15], and metallic seals such as active brazing alloys or silver wire [16,17]. Planar-type SOFCs are generally regarded as superior to their counterparts [18]. However, although they offer a new option for future green-power generation and utilization, such SOFCs have been difficult to commercialize because of their insufficient long-term durability, caused primarily by cracking of the seals among cell components during thermal cycling, which creates passageways for fuel and oxidizing gases. An ideal sealing material needs to meet several stringent requirements, notably low leakage, mechanical integrity, as well as chemical, electrical, and thermal compatibility with the fuel and the cell components. It also must be robust enough to withstand numerous thermal cycles and reliable enough to sustain long-term operation at high temperatures. One of the important factors for selection of sealants is the coefficient of thermal expansion (CTE). The matching of CTE of the sealants with other cell components, i.e., YSZ electrolyte and interconnect material, is compulsory for minimizing the thermal stresses [19]. Currently, glass ceramics are the sealing materials most commonly used. They offer gas tightness and the potential for adapting the coefficients of thermal expansion (CTE) to other stack components by controlling the crystallizing phase content. However, properties of glasses or glasseceramics, such as coefficient of thermal expansion (CTE), viscosity and porosity, often change over time. During long term operation these changes can create additional thermo-mechanical stresses leading to seal failure [20]. Based on the requirements of sealants, some previous studies focus on four important successive steps: (i) investigation and selection of potential sealants (with matching CTE) and approximation of the composition, (ii) processing techniques employed, (iii) investigation of the quantitative impacts of each constituent on the sealing behavior and (iv) optimization of chemical processes and technique (controlling crystallization kinetics by surface engineering, chemistry and long-term stability) [21]. The quality of the sealants must be high, since even small leaks in these seals can affect the cell voltage and thus can decrease performance [22]. As the thermal expansion of the sealants increase with heating and cooling rates, the development of sealants is particularly important for achieving rapid startup times [23], which is a
major challenge with SOFCs. The goal of this work, therefore, is to investigate the thermal-expansion properties (thickness expansion, coefficient of thermal expansion (CTE) and porosity), mechanical stability, microstructural properties, and leakage of mica paper, flexible mica paper, and thermiculite 866 sealants during intermediate temperature operation.
Experimental Sealant design and production Three sealants were used in this study, namely, mica paper (thickness: 500 mm, Ming Wei Corp., Taiwan), flexible mica paper (thickness: 500 mm, FCM Fuel Cell Materials Co. Ltd., U.S.A.), and thermiculite 866 (thickness: 500 mm; Flexitallic Co. Ltd., UK). The mica paper, flexible mica paper, and thermiculite 886 were designed and fabricated for application as sealants in planar SOFC stacks. Photographs of the sealants and the interconnector for the SOFC stacks are shown in Fig. 1. The design of the SOFC sealant is very important for ensuring proper fuel and oxidant supply through the gas flow paths. The stacks must not leak and gas pressure must not build up, as both of these factors can affect the performance and efficiency of SOFCs during power generation. In this study, an SOFC sealant design was developed.
Thermal expansion tests Thickness expansion test The original thicknesses of the mica paper, flexible mica paper, and thermiculite 866 were 0.5 mm each. The sealants were heated up to targeted temperature 600 C, 700 C, 800 C, 900 C, and 1000 C in a heating box. The heating rate is 2.0e3.33 C min1 and then kept at targeted temperature for 100 h. An electronic measurement apparatus was used to investigate the expansion of the fired sealants.
Coefficient of thermal expansion (CTE) calculation After firing, the thickness of fired sealants was measured. Coefficient of thermal expansion (CTE) of fired sealants was calculated by using Equation (1) [24]. CTE ¼
1 DL L DT
(1)
where L is the original thickness of sealant samples, DL is the change in thickness of fired sealants, and DT is the change in temperatures.
Porosity determination After firing of the sealants, the Archimedes method was used to investigate the porosity of the fired sealants [25]. The bulk density (rb) of the fired sealants was calculated by using Equation (2): rb ¼
rs Wfa Wfa Wfs
(2)
where rs is the density of the solvent (boiling water). Samples of the fired sealants were first weighed on a pan, the weight
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Fig. 1 e Photographs of the SOFC stack sealants: a) mica paper, b) flexible mica paper, c) thermiculite 866, and d) the interconnector.
(Wfa) was then recorded. The samples were then immersed in boiling water and placed on the weighing pan submerged in the solvent. The weight in kilograms (Wfs) of the immersed sample was then recorded. The porosity of the fired sealants was then determined from the absolute and bulk densities of the fibers through Equation (3): P¼
r 1 b 100 ra
hydrogen flow rates at the inlet equal to the test at room temperature. As shown in Fig. 3, the flow meter was connected to the outlet of fuel. It shows variety of changes in flow rate of the hydrogen outlet.
Results and discussion (3)
where P is the percentage porosity (%), and ra is the absolute density of the original sealant.
Mechanical stability test After firing, the sealants were tested under a capillary flow porometer machine (CFP-1500AEXDH, Porous Materials, Inc., U.S.A.). As shown in Fig. 2, the fired sealants were put inside a compressing chamber and then compressed at 0.3 MPa by using compressed air.
Microstructural characterization The morphologies of the fired sealants were studied under a scanning electron microscope (JSM-5600, Jeol Co., Japan) equipped with an energy-dispersive spectroscope.
Leakage test Fig. 3 depicts the experimental setup system for leakage test. The sealants were assembled in SOFC device and then the SOFC unit was placed in heating box. Firstly, leakage test was carried out with hydrogen at room temperature (25 C). The sealants were tested for leaks at different hydrogen flow rates at the inlet (500e3000 cc min1). After that, the temperature of heating box was then set at 800 C for 5 h with heating rate 2.7 C min1. The leakage test was investigated at 800 C with
Thermal expansion The sealants were tested at 600 C, 700 C, 800 C, 900 C, and 1000 C for 100 h in a heating box. As shown in Fig. 4, the thicknesses of the fired sealants e mica paper, flexible mica paper, and thermiculite 866 are 1.01, 0.98, and 0.568 mm, respectively. The thickness changes of mica paper, flexible mica paper, and thermiculite 866 are 0.51, 0.48, and 0.068 mm, respectively. These changes suggest that the variation in the porosity of thermiculite 866 under high temperature condition is slightly compared to that of mica paper and flexible mica paper. Thus, thermiculite 866 is a better candidate in terms of decreasing the leakage rate compared to sealants e mica paper and flexible mica paper. Fig. 5 shows that the porosity of the flexible mica paper increased very rapidly, from 21.8% at 600 C to 52.7% at 1000 C. The porosity of the mica paper increased from 25.3% at 600 C to 58.9% at 1000 C, higher than that of the flexible mica paper. Meanwhile, the porosity of thermiculite 866 changed from 15.4% at 600 C to 28.7% at 1000 C, gentle compared to sealants e mica paper and flexible mica paper. As shown in Fig. 6, the coefficient of thermal expansion (CTE) of fired sealant samples were calculated as a relative values. The relative (600e1000 C) CTEs for mica paper were found to be 8.7 104 K1, 9.2 104 K1, 9.7 104 K1, 10.3 104 K1, 9.85 104 K1for 600, 700, 800, 900, and 1000 C, respectively. The relative (600e1000 C) CTEs for flexible mica paper were found to be 9.74 104 K1,
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Fig. 2 e Schematic illustration of experimental set-up for mechanical stability test.
Fig. 3 e Schematic illustration of experimental set-up for leakage test. 8.9 104 K1, 10.29 104 K1, 10.97 104 K1, 10.46 104 K1 for 600, 700, 800, 900 C, and 1000 C, respectively. The relative (600e1000 C) CTEs for thermiculite 866 were found to be 1.04 104 K1, 1.15 104 K1, 1.13 104 K1, 1.07 104 K1, and 1.39 104 K1 for 600 C, 700 C, 800 C, 900 C, and 1000 C, respectively. It can be found that the CTEs of mica paper and flexible mica paper (8.0 104 K1e10.0 104 K1) are much higher than that of thermiculite 866 (1.0 104 K1).
Mechanical stability
Fig. 4 e Changes in the thicknesses of the sealants with temperatures.
Fig. 7 shows structural comparisons of the sealing materials before and after firing. After firing, the mica paper and flexible mica paper samples could be easily detached by a pressing machine and could be easily pulverized by applying a light force using two fingers, and the structures of both materials
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Fig. 5 e Changes in the porosities of the sealants with temperature.
Fig. 6 e The coefficient of thermal expansion (CTE) of the sealants at different temperatures.
could be easily broken. However, the fired sample of thermiculite 866 was not broken. Thus, the structures of mica paper and flexible mica paper are softer than that of thermiculite 866 and the porosities after firing of the former two are higher than that of the latter. Thus, thermiculite 866 is a better candidate in terms of higher mechanical stability compared to sealants e mica paper and flexible mica paper.
Microstructural characterization Fig. 8 shows the structures of the sealing materials before firing. Mica paper and flexible mica paper appear to be thick and blocky, consisting of many stacked layers. These materials depend on the inclusion of an elastomer in the material body to bind them together and to ensure a good seal at ambient temperature. Addition of excess elastomer, however, leads to poor creep characteristics. At high temperatures, the binder burns off, resulting in a leakage rate higher than that at room temperature. In contrast, the structure of thermiculite 866 resembles that of muscle (Fig. 8(c)). This structure indicates why the sealing is robust; the highly aligned sheets of chemically exfoliated vermiculite interspersed with steatite
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Fig. 7 e The sealing materials before and after firing: (a) mica paper, (b) flexible mica paper, and (c) thermiculite 866.
create a very circuitous path along which gas molecules have to travel in order to escape [26]. This structure is thus suitable for reducing the leakage rate. Fig. 9 shows the structures of mica paper, flexible mica paper, and thermiculite 866 after firing. As shown in the figure, the structures of fired mica paper and flexible mica paper changed at high temperature. The blocky layers in their structures expanded (Fig. 9(a) and (b)) because these materials depend on an elastomer in the material body that binds them together. At high temperatures, the binder burned off, creating an expanded structure. As gaps appeared in the material body at high temperature, the thicknesses of the mica paper and flexible mica paper increased. This finding is consistent to big change in thickness and porosity for sealants e mica paper and flexible mica paper. In contrast, the structure of thermiculite 866 did not expand at high temperature (Fig. 9(c)) because the materials lacked organic content to burn off. This finding is related to its small change in thickness and porosity. Therefore for sealants e mica paper and flexible mica paper, the changes in thickness and porosity were more significant due to burning of the elastomer, which is included in the material body, at high temperatures.
Leakage test Fig. 10 depicts the rate of leakage through the materials at room temperature (25 C) and at 800 C with different hydrogen flow rates at the inlet (from 500 to 3000 cc min1 compressive stress). The standard cubic centimeters per minute per unit leak length of seal is also calculated, the length of seal is 4 14 cm ¼ 56 cm. The figure shows that the room-temperature leakage rates for mica paper are 1.34% at 500 cc min1 (1.19 101 sccm cm1), 5.75% at 2000 cc min1 (2.05 sccm cm1), and 9.78% at 3000 cc min1 (5.24 sccm cm1); for flexible mica paper are 1.14% at 500 cc min1 (1.01 101 sccm cm1), 5.06% at 2000 cc min1 (1.8 sccm cm1), and 9.22% at 3000 cc min1 (4.94 sccm cm1); and for thermiculite 866 are 0.51% at 500 cc min1 (0.45 101 sccm cm1), 2.52% at 2000 cc min1
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Fig. 8 e Microstructures of the sealants before firing.
Fig. 9 e Microstructure of the sealants after firing.
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Fig. 10 e Rate of leakage through the sealants: a) mica paper, b) flexible mica paper, and c) thermiculite 866.
(0.9 sccm cm1), and 4.06% at 3000 cc min1 (2.175 sccm cm1). The leakage rates at 800 C of mica paper are 3.8% at 500 cc min1 (3.34 101 sccm cm1), 12.6% at 2000 cc min1 (4.5 sccm cm1), and 20.6% at 3000 cc min1 (11.03 sccm cm1); for flexible mica paper are 3.2% at 500 cc min1 (2.85 101 sccm cm1), 10.2% at 2000 cc min1 (3.64 sccm cm1), and 17.4% at 3000 cc min1 (9.32 sccm cm1); and 1.62% at 500 cc min1 (1.44 101 sccm cm1), 4.7% at 2000 cc min1 (1.67 sccm cm1), and 8.4% at 3000 cc min1 (4.5 sccm cm1) for thermiculite 866. The leakage rates for all sealants increase with increasing hydrogen flow rate and temperature. Also, the leakage rates for mica and flexible mica are similar and almost twice higher than that of thermiculite 866. These results are compared with those of previous studies. The best results were obtained using muscovite, single-crystal mica, phlogopite mica, and 125 mm silver layers. Using the paper form of muscovite and phlogopite mica, the leak rates were still far superior (~1 101 sccm cm1), mica without the compliant silver layer (about 6e9 sccm cm1) [15].
Muscovite and phlogopite micas have been evaluated as SOFC seals at 800 C. Cleaved natural mica sheets (with no binder) indicated far superior sealing characteristics with leak rates lower than 0.1 sccm cm1 at 800 C [14]. It can be seen that the leakage rate of sealants with organic binder is much higher than sealants without organic binder. Therefore, the organic binder sealants were usually combined with other sealing materials to use in high operating temperature SOFC.
Conclusion This paper investigated the thermochemical and thermomechanical compatibility of mica paper, flexible mica paper, and thermiculite 866 at high temperature. In general, mica paper and flexible mica paper have highly attractive features for sealing purposes, namely, their structural deformability and gas tightness at both room temperature and high temperature. However, their seriously limited thermochemical
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and thermomechanical stabilities still need to be addressed. In contrast, thermiculite 866 contains no organic content that burns off. This lack is related to the small change in its thickness and porosity. Furthermore, its lack of organic content, which could contaminate and thus cause degradation of solid oxide fuel cells (SOFC), ensures no increase in porosity or additional leakage during high-temperature operation. Furthermore, various sealing patterns using thermiculite 866 are easy to design and to cut; patterns with simple shapes can be cut with a knife or with scissors. These features imply that thermiculite 866 is a suitable sealant for SOFC applications.
[11]
[12]
[13]
[14] [15]
Acknowledgements [16]
The authors gratefully thank the National Science Council of Taiwan under contracts NSC 99-2221-E-155-063 and NSC 1003113-E-155-001 and the Taiwan Power Company for their financial support.
[17] [18]
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