Development of the CaO–SrO–ZrO₂–B₂O₃–SiO₂ sealing glasses for solid oxide fuel cell applications: structure–property correlation

Abstract

In this study, the structure–property correlation in the CaO–SrO–ZrO₂–B₂O₃–SiO₂ sealing system is systematically investigated. In particular, the effect of ZrO₂ on the structure and chemical compatibility of the glasses is clearly demonstrated. The increases in the glass transition temperature and softening temperature of the glasses indicate that the glass structure is condensed by the ZrO₂ dopant, which is further confirmed by the ¹¹B and ²⁹Si NMR spectra. Furthermore, the addition of ZrO₂ reduces the crystallization tendency of the glasses. The fraction of Cr⁶⁺ in the glass/Cr₂O₃ reaction couple decreases significantly with increasing ZrO₂ content at 700 °C because of the condensed glass structure, whereas the fraction of Cr⁶⁺ increases with increasing ZrO₂ content at 750 and 800 °C. This behavior is related to the increase in residual glass content in the glass–ceramics. Good bondage can be observed at the interfaces between glasses and Crofer 22APU after being held at 800 °C for 500 hours. These results demonstrate the suitability of the Zr-containing glass–ceramics as sealing materials for SOFC applications.

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1. Introduction

Solid oxide fuel cells (SOFCs) are attractive electrochemical devices that convert chemical energy into electricity with great efficiency.^1–4 As a key component of SOFCs, the sealing glass has to meet certain requirements for its electrical resistivity,^5–7 thermal stability,^8–14 and chemical compatibility.^15–17

Interactions between glass–ceramic sealants and ferritic interconnects at the SOFC operational temperatures have been found to result in the formation of interfacial phases potentially detrimental to the performance of the SOFC stacks.^18–21 Alkali ions or residual water in some glass compositions significantly increase the vaporization of chromium from the interconnects.^18,22 The transport of Cr-vapors, as CrO₃ or CrO₂(OH)₂, can lead to the formation of alkaline earth chromates on glass surfaces well-removed from the glass–metal interface.²² The high coefficients of thermal expansion (CTEs) of these chromates, e.g., BaCrO₄ and SrCrO₄ (∼18 to 20 × 10⁻⁶ K⁻¹),²³ contributes to the physical separation of the sealing glass (CTE ∼ 12.5 × 10⁻⁶ K⁻¹) and the interconnect (∼13 × 10⁻⁶ K⁻¹) because of the large CTE differences, and leads to significant losses in bond strength between SOFC glasses and interconnect materials.²⁴

It has been reported that the extent and nature of the reactions between glass sealants and different types of alloys depend on the alloy compositions and the exposure conditions.^19,22 For chromia–forming alloys, the edges of the seals where oxygen or air is accessible typically exhibit BaCrO₄ formation. For a more oxidation resistant alloy, e.g., a Ni-based super alloy, the extent of the formation of BaCrO₄ can be limited. The addition of aluminum to the interconnect alloy can suppress the formation of interfacial chromates and so lead to stronger glass–metal seals.²⁵ Likewise, the addition of protective coatings to the interconnects has been used to suppress chromate formation,²⁶ but often at the cost of conductivity. Compared with the great efforts made to modify the interconnects,^27–30 few works have been done to improve the chemical compatibility of sealing glass because of the difficulty of quantitative investigation of the glass–metal interaction in the complex SOFC stack.^31–33

On the other hand, the design of compliant seals has attracted increasing attention in recent years. Ideally, the glass should remain viscous at high temperature, i.e., 700–800 °C, to release the thermal stress generated by any mismatch in the coefficient of thermal expansion (CTE) during thermal cycling. The glass melt should also be able to flow to heal any cracks in the seals that form during operation, while still maintaining the necessary mechanical strength to support the substrates. The key of this design is to have sufficient amount of residual glass under the operational condition of SOFC. However, our recent work has revealed that alkaline earth ions (Sr²⁺) can react with a Cr-containing interconnect to form a detrimental chromate phase, e.g., SrCrO₄, much more easily in the glass matrix than they can in crystalline phases.³⁴ Therefore, chemical compatibility is of great importance for fabricating compliant seals.

It is known that ZrO₂ generally reduces the crystallization tendency of glasses and thus constitutes a representative model system to investigate the chemical compatibility of compliant seals. In this paper, ZrO₂ was added gradually (from 2 to 8 mol%) into the SrO–CaO–SiO₂–B₂O₃ sealing glass system (hereafter, these samples are called glass#2ZrO₂ and glass#8ZrO₂, respectively). Attention was focused on the following questions: (1) how does ZrO₂ affect the structures of glasses and glass–ceramics? (2) How does ZrO₂ affect the thermal and electrical properties of glasses? (3) How does ZrO₂ affect the interfacial reaction between sealing glasses and Cr-containing interconnects? In particular, the reaction temperature was chosen to be 700, 750, or 800 °C to clarify the structural dependence of the interfacial reaction.

2. Experimental

2.1 Preparation of glasses and glass–ceramics

Glasses from the CaO–SrO–B₂O₃–SiO₂–ZrO₂ system were melted in a Pt-crucible in air at 1500 °C for one hour. The nominal compositions of the glasses (mol%) are shown in Table 1. Some of the melt was poured into a stainless steel mold to obtain cylindrical glass specimens (25 mm length and 6 mm diameter), and the rest of the melt was quenched on a steel plate to form the glass. Glass powders were then crushed and sieved to a particle size of 45 to 53 μm. Pellets (10 mm diameter and 2 mm thickness) were obtained after uniaxial pressing of glass powders and were held at 700 and 800 °C for up to 500 hours in air (referred to as ‘glass–ceramics’).

Table 1 Batch composition (in mol%) of sealing glasses

Sample ID	SrO	CaO	B2O3	SiO2	ZrO2
glass#2ZrO2	25.48	25.48	7.84	39.20	2.00
glass#4ZrO2	24.96	24.96	7.68	38.40	4.00
glass#6ZrO2	24.44	24.44	7.52	37.60	6.00
glass#8ZrO2	23.92	23.92	7.36	36.80	8.00

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In recent years, there is a need to reduce the operational temperature from 900–1000 °C for traditional SOFC into the so-called intermediate temperature (IT) range of 500–750 °C, in order to reduce the corrosion rate of SOFC components.³⁵ Thus, most of sealing glasses are designed to be operated at the temperature range of 700–750 °C, including the ZrO₂-containing glasses in present work.

It is known that long-term stability (static state) and thermal-cycling stability (dynamic state) are critical for sealing glasses. Many works on the sealing materials have been performed in static state in literature.^{20,21,36–38} Therefore, the test scenarios in preset work are chosen to be at 800 °C for 500 hours to accelerate the evolution of glass–ceramics as well as the glass–metal interface, which provides useful information on the long stability of sealing glasses. This will need a much longer time if testing at a low temperature (e.g., 700 °C).

2.2 Characterization of glass structure and crystalline structure of glass–ceramics

²⁹Si and ¹¹B magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded for all quenched glasses. The spectra were acquired on a Bruker DSX-400MHZ spectrometer operating at a B₀ field of 9.4 T with ²⁹Si and ¹¹B Larmor frequencies of 79.5 and 128.4 MHz, respectively. The detailed procedure for the NMR measurement has been reported elsewhere.³⁹

In addition, the crystalline phases in the glass–ceramic powders were identified by X-ray diffraction (XDS 2000, Scintag, Inc.). The crystalline content (wt%) in the glass–ceramics that were held at 800 °C for 2 hours was then calculated by the RIQAS software (Release 4.0.0.8, Materials Data, Inc., CA). The detailed procedure for the calculation has been described elsewhere.^40,41

2.3 Characterization of thermal and electrical properties of glasses

The onset crystallization temperature (T_x) of the glass powders was determined by performing differential scanning calorimetry (SDTQ600, TA, Inc.) at a heating rate of 10 °C min⁻¹. The thermal characteristics of the quenched glasses, including the CTE (between 200 and 600 °C), glass transition temperature (T_g), and softening temperature (T_d), were determined by operating a dilatometer (DIL402C, NETZSCH, Inc.) at 10 °C min⁻¹ in air. Glass–ceramics held at 800 °C for 500 hours were also subjected to dilatometric measurements for comparison. The thermal properties of the quenched glasses and glass–ceramics are summarized in Table 2. The density of the quenched glasses was measured using the Archimedes method in water.

Table 2 Thermal properties of sealing glasses and glass–ceramics^a

Sample ID	Glass#2ZrO2	Glass#4ZrO2	Glass#6ZrO2	Glass#8ZrO2
a Tg: glass transition temperature. Td: softening temperature. Tx: onset crystallization temperature. CTE: coefficient of thermal expansion.
Thermal parameters
Tg (°C)	685 ± 3	705 ± 3	713 ± 3	726 ± 3
Td (°C)	748 ± 3	752 ± 3	779 ± 3	787 ± 3
Tx (°C)	869 ± 3	909 ± 3	894 ± 3	901 ± 3
CTE (×10^−6 K^−1, 200–600 °C)
Glasses	12.6 ± 0.1	13.1 ± 0.1	12.1 ± 0.1	12.4 ± 0.1
800 °C for 500 hours	12.2 ± 0.1	12.2 ± 0.1	11.1 ± 0.1	10.5 ± 0.1
Crystalline content (weight%)
800 °C for 2 hours	71 ± 5	67 ± 5	65 ± 5	60 ± 5

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The electrical conductivity of the quenched glasses and glass–ceramics was measured in air from 650 to 750 °C using a high resistance meter (4339B, Agilent, Inc.). The measurement was performed with Pt current collectors and Pt wire, at a heating rate of 5 °C min⁻¹.

2.4 Characterization of interfacial reaction

Based on the previous experience in our lab, the adhesion test, such as pin-pull test, is hard to distinguish the adhesion strength between glass and metallic interconnect. First of all, the broken point often exists in the fragile glass instead of interface. In addition, the measured strength will be very close within experimental uncertainty if the reaction zone of different specimens has a similar thickness. Therefore, we propose a more reliable and sensitive approach for evaluating the interfacial reaction between sealing glass and Cr-containing interconnect, which is the main point of present work.

A ∼20 mg mixture of glass and 10 wt% Cr₂O₃ powders was reacted in an alumina boat in air for different times at 700, 750, or 800 °C. After the reaction, the mixture was dissolved into ∼200 ml of room-temperature deionized water, and the UV-VIS absorption spectra were recorded using an Optima 2000 DV (Perkin Elmer, Inc.). The concentration of Cr⁶⁺ in each solution was calculated by fitting the corresponding absorbance from the calibration curve derived from the K₂CrO₄ solutions. Three samples were made under identical conditions and were analyzed to determine the experimental uncertainty. The detailed procedure for this measurement has also been discussed elsewhere.^34,42,43

In addition, the glasses were bonded to Crofer 22APU substrates and the interfacial reactions were characterized. The coated samples were held in air at 800 °C for 500 hours. The glass–metal sealing couples were polished using SiC paper and an alumina suspension, as discussed elsewhere.⁴² The polished samples were analyzed using field emission scanning electron microscopy (Supra-55, Zeiss, Inc.) and X-ray energy dispersive analysis (EDS, X-Max, OXFORD instruments, Inc.). Sealing glasses were coated on only one side of the Crofer in present work, as reported by many colleagues in literature.^{20,21,38,44,45} This fixture may not be a true representation of the interfacial chromate formation.

3. Results and discussion

3.1 Glass structure and crystalline structure of glass–ceramics

Fig. 1a shows the ¹¹B MAS NMR spectra of the glasses. The peak centers at −20 ppm can be assigned to BO₄ (four-fold coordinated boron species), whereas the peaks positioned at −9 and −14 ppm correspond to BO₃ (three-fold coordinated boron species).⁴⁶ The ratio of BO₃ to BO₄ increases with increasing ZrO₂ content, which reveals that a BO₄ → BO₃ transition occurs in the glasses upon the addition of ZrO₂.

Fig. 1 MAS NMR spectra of glasses for (a) ¹¹B and (b) ²⁹Si.

In addition, a single broad resonance peak positioned at approximately −80 ppm, corresponding mainly to Q² and Q³ units, can be observed in the ²⁹Si MAS NMR spectra of the glasses, as shown in Fig. 1b. The chemical shift of ²⁹Si from −79 ppm for glass#2ZrO₂ to −78 ppm for glass#8ZrO₂ indicates an increasing densification degree of [SiO₄] tetrahedral with increasing ZrO₂ content.³⁹ Therefore, the addition of ZrO₂ strengthens the glass structure in present work, consistent with the previous results in literature.⁴⁷

Fig. 2 shows the XRD patterns of glass–ceramics held at different temperatures. All species remain in an amorphous state after being held at 700 °C for 5 hours, as shown in Fig. 2a. Shown in Fig. 2b and c are the XRD patterns of glass–ceramics held at 800 °C for 2 and 500 hours, respectively. The patterns have been normalized by the intensity of the strongest peak (2θ = 31.2°). Sr₂SiO₄ (ICDD card no. 76-1494), CaSiO₃ (ICDD card no. 43-1460), CaB₄O₇ (ICDD card no. 83-2025) and (Ca_0.612Sr_0.388)ZrO₃ (ICDD card no. 89-8016) are the main phases in the glass–ceramics. The standard diffraction pattern of (Ca_0.612Sr_0.338)ZrO₃ (89-8016) has also been included at the bottom of Fig.2c for comparison. It is clear that most of the diffraction peaks corresponding for (Ca_0.612Sr_0.338)ZrO₃ phase match the measured patterns very well.

Fig. 2 XRD patterns of glass–ceramics held at (a) 700 °C for 5 hours, (b) 800 °C for 2 hours and (c) 800 °C for 500 hours. (d) The patterns of (c) plotted in the 2θ range of 20–35°.

To clarify the change in the diffraction peaks, the XRD patterns in Fig.2c have also been plotted in the 2θ range of 20–35°, as shown in Fig.2d. It is clear that the diffraction intensity of (400) plane in CaSiO₃ (2θ = 23.1°) increases when the ZrO₂ content increases from 2 to 8 mol%, indicating the increasing CaSiO₃ content in glass–ceramics with the addition of ZrO₂. It is also worth noting that the intensity of (004) plane in (Ca_0.612Sr_0.388)ZrO₃ (2θ = 30.9°) increases with increasing ZrO₂ content. This implies that the crystallization of (Ca_0.612Sr_0.388)ZrO₃ in glass–ceramics can be facilitated by ZrO₂ dopant.

3.2 Thermal and electrical properties of glasses and glass–ceramics

The thermal properties of sealing glasses and glass–ceramics are summarized in Table 2. The glass transition temperature (T_g) increases from 685 ± 3 to 726 ± 3 °C when the ZrO₂ content increases from 2 to 8 mol%. In addition, the softening temperature (T_d) increases from 748 ± 3 °C for glass#2ZrO₂ to 787 ± 3 °C for glass#8ZrO₂. It has been found that increases in the glass transition temperature and softening temperature often indicate condensation of the glass network.⁴⁸ Considering the increases in the glass transition temperature and the softening temperature due to the ZrO₂ dopant observed in present work, it can be concluded that the Si–O bonding plays a predominant role in the overall glass structure and therefore results in condensation of the glass network.

Although this trend does not hold for glass#4ZrO₂ (4 mol% ZrO₂), the onset crystallization temperature (T_x) of the sealing glasses increases from 869 ± 3 °C for glass#2ZrO₂ to 901 ± 3 °C for glass#8ZrO₂. The temperature window between the T_d (or T_MS, the maximum shrinkage temperature) and T_x of sealing glasses, referred to as ΔT, is often considered an important criterion for evaluating their sealing behavior.³⁹ These ΔT values of the glasses with ZrO₂ dopants in the present work were high (from 114 to 157 °C), indicating that these samples exhibit better sealing behavior than ZrO₂-free glass (823 − 739 = 84 °C).⁴⁹

Moreover, the crystalline content of the glass–ceramics held at 800 °C for 2 hours decreases from 71 ± 5 wt% for glass#2ZrO₂ to 60 ± 5 wt% for glass#8ZrO₂. The increasing residual glass content in the glass–ceramics indicates that the addition of ZrO₂ reduces the tendency of the glasses to crystallize, due to the condensed glass network (in Fig. 1 and Table 2). It is known that the addition of ZrO₂ can strengthen the glass network of borosilicate glasses and promote the formation of Q² structural units, which is consistent with the experimental results in present work.^39,50,51 The strengthened glass network reduces the crystallization tendency of glasses and results in the decrease in crystalline content of glass–ceramics held at 800 °C for 2 hours (in Table 2).

It has also been reported that ZrO₂ conduced a-quartz-like structure in borosilicate glasses, which thus promoted a-quartz crystallization upon subsequent heat-treatment.⁵² Therefore, one can deduce that ZrO₂ favors the formation of Q² structural units in glass network (in Fig. 1b) and consequently facilitates the crystallization of Q²-type species, i.e., CaSiO₃,⁵³ after long-term heat-treatment in present work (in Fig. 2c and d).

The CTEs of the quenched glass#2ZrO₂, glass#4ZrO₂, glass#6ZrO₂, and glass#8ZrO₂ samples are 12.6, 13.1, 12.1, and 12.4 × 10⁻⁶ K⁻¹, respectively. The change in the CTE of the quenched glasses seems to be unrelated to the ZrO₂ content and might be affected by technical factors such as the formation of pores in the glass cylinders during the pouring process. In contrast, it is clear that the CTE of the glass–ceramics held at 800 °C for 500 hours, decreases from 12.2 × 10⁻⁶ K⁻¹ for glass#2ZrO₂ to 10.5 × 10⁻⁶ K⁻¹ for glass#8ZrO₂. The decrease in CTE can be related to the increasing CaSiO₃ content in glass–ceramics (in Fig.2c), because of its low CTE (9.4 × 10⁻⁶ K⁻¹).⁵⁴ It is also worth noting that the CTEs of the glasses and glass–ceramics are almost within the design range for SOFC sealing applications (e.g., 11–13 × 10⁻⁶ K⁻¹).¹

Fig.3 shows the temperature dependence of conductivity (log σ versus 1000 T⁻¹) for glasses and glass–ceramics, measured in air from 650 to 750 °C. The conductivity for glass#2ZrO₂ and glass#8ZrO₂ is chosen for simplicity. It is clear that the conductivity (σ) of all species increases with increasing temperature, indicating the predominant role of ionic conductivity in present work. In addition, the conductivity of quenched glasses at each temperature decreases with ZrO₂ dopant. For example, the conductivity of glasses at 750 °C decreases from 8.5 × 10⁻⁷ S cm⁻¹ for glass#2ZrO₂ to 5.6 × 10⁻⁷ S cm⁻¹ for glass#8ZrO₂. The decrease in conductivity of glasses with increasing ZrO₂ content further confirms the densification of glass network by ZrO₂ dopant in present work. It is worth noting that the conductivity of glass–ceramics held at 800 °C for 500 hours increases with ZrO₂ dopant. For example, the conductivity of glass–ceramics at 750 °C increases from 2.0 × 10⁻⁸ S cm⁻¹ for glass#2ZrO₂ to 2.4 × 10⁻⁷ S cm⁻¹ for glass#8ZrO₂. It has been reported that the conductivity of CaZrO₃ at 750 °C is about 5.6 × 10⁻⁵ S cm⁻¹.⁵⁵ Considering the similar perovskite structure of (Ca_0.612Sr_0.388)ZrO₃ and CaZrO₃, one can deduce that the increase in conductivity of glass–ceramics relates to the increasing (Ca_0.612Sr_0.388)ZrO₃ content with ZrO₂ dopant (in Fig.2d). On the other hand, we have also analyzed the residual glass in glass–ceramics by EDS. The results show that no zirconia can be detected in residual glass, which excludes the effect of ZrO₂ on the glass matrix in glass–ceramics. In addition, the ratio of (Ca + Sr) to Si is close to 1.0 for both glass#2ZrO₂ and glass#8ZrO₂, indicating the similar residual glass in glass–ceramics in present work. Therefore, the increase in conductivity of glass–ceramics can be mainly attributed to the increasing (Ca, Sr)ZrO₃ phase. It is also worth noting that the conductivity of glasses and glass–ceramics at 750 °C ranges from 2.0 × 10⁻⁸ S cm⁻¹ to 8.5 × 10⁻⁷ S cm⁻¹, meeting the insulating requirement of sealing glasses (<10⁻⁴ S cm⁻¹) for SOFCs application.⁵⁶

Fig. 3 The temperature dependence of conductivity (log σ versus 1000 T⁻¹) for glasses and glass–ceramics, measured in air from 650 to 750 °C.

3.3 Interfacial reaction

Fig. 4a shows the fraction of Cr⁶⁺ in the reaction couples between Cr₂O₃ and the glass powders after they have reacted in air at 700 °C for different times. It is clear that the fraction of Cr⁶⁺ in the reaction couple decreases with increasing ZrO₂ content. Considering the amorphous nature of all species (in Fig. 2a), the improved chemical compatibility of the glasses at 700 °C can be attributed to the condensed glass structure (in Fig. 1 and Table 2).

Fig. 4 Fraction of Cr⁶⁺ in the reaction couples between Cr₂O₃ and glass powders after they have reacted in air for different times at (a) 700 °C, (b) 750 °C, and (c) 800 °C.

The fraction of Cr⁶⁺ in the reaction couples at 750 and 800 °C increases with increasing ZrO₂ content, as shown in Fig. 4b and c. Our previous work showed that the chemical compatibility of a CaO–SrO–B₂O₃–SiO₂ sealing system can be improved by controlled crystallization of a Sr-containing phase. Thermodynamic calculation has also revealed that the Sr²⁺ ions in the crystalline phase (e.g., Sr₂SiO₄) are more stable than their analogues in the glass matrix.^43,49,57,58 Therefore, a greater amount of residual glass weakens the chemical compatibility of the sealing glasses at 750 and 800 °C.

Fig. 5 shows SEM micrographs of sealing couples held at 800 °C for 500 hours and EDS line scans across the respective interfaces. Representative micrographs of glass#2ZrO₂ and glass#8ZrO₂ are chosen for comparison, as shown in Fig. 5a and b. The compositions of the selected spots in the glass–ceramics determined by EDS are summarized in Table 3. The results have been normalized for O content. Considerable amount of Sr and Ca can be detected in the Zr-enrichment zones (point#2 in Fig. 5a, point#4 and point#5 in Fig.5b), as shown in Table 3. Therefore, one can deduce the presence of phases containing Ca, Sr and Zr according to EDS results. Based on XRD and EDS results (in Fig. 2 and 5), one can confirm the existence of the exact (Ca_0.612Sr_0.338)ZrO₃ phase in present work.

Fig. 5 Micrographs of glass–metal interfaces held at 800 °C for 500 hours and EDS elemental line scans across the respective interfaces for (a) glass#2ZrO₂ and (b) glass#8ZrO₂.

Table 3 EDS results for selected spots in Fig. 5 (in at.%). The results have been normalized for O content

Spot	1	2	3	4	5
Sr	27	40	25	38	21
Ca	22	10	25	10	26
Si	51	3	50	0	40
Zr	0	47	0	52	14

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In addition, good bonding can be observed at the interfaces between the Crofer 22APU and ZrO₂-containing glasses. Moreover, the EDS line scan reveals that the width of the reaction zones (enriched in Sr and Cr) at the glass–metal interfaces is ∼2 μm, which is much thinner than the previous results under similar condition (5–30 μm) in literature.³⁶ This indicates that the addition of ZrO₂ significantly improves the chemical compatibility of sealing glasses in present work.

4. Conclusions

The relationship between the structures and properties of ZrO₂-containing sealing glasses has been systematically investigated in present work. The addition of ZrO₂ strengthens the glass network, and thus, reduces the tendency of the glasses to crystallize. The CTE and electrical conductivity of glasses also decrease with increasing ZrO₂ content due to the condensed glass network. The CTE of glass–ceramics decreases with the addition of ZrO₂ because of the facilitated crystallization of a low CTE phase (CaSiO₃); whereas, the electrical conductivity of glass–ceramics increases with ZrO₂ dopant due to the accelerated crystallization of (Ca_0.612Sr_0.388)ZrO₃. The condensed glass network also reduces the interfacial reaction, whereas the greater amount of residual glass in glass–ceramics with ZrO₂ dopants favors the reaction. Moreover, a reaction zone of ∼2 μm thick can be observed at the interface between ZrO₂-containing sealing glasses and Crofer 22APU held at 800 °C for 500 hours. These findings on the structure–property correlation of sealing glass will be useful for the design of stable sealing glasses for SOFC applications.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (no. 51102045), Program for New Century Excellent Talents in Fujian Province University (no. JA12013), and the Undergraduate Innovative Experiment Program of Fujian Province (no. 201313470020). They would also like to thank Zhenhuan Zheng for assistance with SEM/EDS, Qingming Huang for assistance with X-ray diffraction, Fen Lin for assistance with DSC analysis, and Yunlong Yu for assistance with dilatometric measurement.

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