Qi Zhanga,
Hsiwen Yangb,
Fanrong Zengc,
Shaorong Wangc,
Dian Tanga and
Teng Zhang*a
aCollege of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian 350108, China. E-mail: teng_zhang@fzu.edu.cn; Fax: +86 591 22866537; Tel: +86 591 22866540
bDepartment of Materials Science and Engineering, National United University, Miao-Li 36003, Taiwan, China
cCAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), 1295 Dingxi Road, Shanghai 200050, China
First published on 27th April 2015
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.
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.
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 |
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).
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
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 |
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.
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.
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 (Ca0.612Sr0.388)ZrO3 (ICDD card no. 89-8016) are the main phases in the glass–ceramics. The standard diffraction pattern of (Ca0.612Sr0.338)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 (Ca0.612Sr0.338)ZrO3 phase match the measured patterns very well.
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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 (Ca0.612Sr0.388)ZrO3 (2θ = 30.9°) increases with increasing ZrO2 content. This implies that the crystallization of (Ca0.612Sr0.388)ZrO3 in glass–ceramics can be facilitated by ZrO2 dopant.
Although this trend does not hold for glass#4ZrO2 (4 mol% ZrO2), the onset crystallization temperature (Tx) of the sealing glasses increases from 869 ± 3 °C for glass#2ZrO2 to 901 ± 3 °C for glass#8ZrO2. The temperature window between the Td (or TMS, the maximum shrinkage temperature) and Tx 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 (logσ 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 (Ca0.612Sr0.388)ZrO3 and CaZrO3, one can deduce that the increase in conductivity of glass–ceramics relates to the increasing (Ca0.612Sr0.388)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
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Fig. 3 The temperature dependence of conductivity (log![]() |
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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 (Ca0.612Sr0.338)ZrO3 phase in present work.
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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. |
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 |
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.
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