Stable glass-ceramic sealants for solid oxide fuel cells: Influence of Bi2O3 doping

https://doi.org/10.1016/j.ijhydene.2010.04.106Get rights and content

Abstract

Diopside (CaMgSi2O6) based glass-ceramics in the system SrO–CaO–MgO–Al2O3–B2O3–La2O3–Bi2O3–SiO2 have been synthesized for sealing applications in solid oxide fuel cells (SOFC). The parent glass composition in the primary crystallization field of diopside has been doped with different amounts of Bi2O3 (1, 3, 5 wt.%). The sintering behavior by hot-stage microscopy (HSM) reveals that all the investigated glass compositions exhibit a two-stage shrinkage behavior. The crystallization kinetics of the glasses has been studied by differential thermal analysis (DTA) while X-ray diffraction adjoined with Rietveld-R.I.R. analysis have been employed to quantify the amount of crystalline and amorphous phases in the glass-ceramics. Diopside and augite crystallized as the primary crystalline phases in all the glass-ceramics. The coefficient of thermal expansion (CTE) of the investigated glass-ceramics varied between (9.06–10.14) × 10−6 K−1 after heat treatment at SOFC operating temperature for a duration varying between 1 h and 200 h. Further, low electrical conductivity, good joining behavior and negligible reactivity with metallic interconnects (Crofer22 APU and Sanergy HT) in air indicate that the investigated glass-ceramics are suitable candidates for further experimentation as sealants in SOFC.

Introduction

Glass-ceramics (GCs) combine the generally superior properties of crystalline ceramics with the ease of processing of glass. Major attributes of GCs include more refractory behavior and superior mechanical properties, relative to glasses as well as ceramics. Undoubtedly, one of the major qualities, however, is an ability to tailor their thermal expansion characteristics. This makes GCs ideal candidates where compatible thermal expansions are necessary. Most recently, there has been a dramatic revival of interest in both glass- and GC- to metal seals [1], particularly, for new applications including SOFC [2], [3] and high temperature sensors [4], [5].

In planar design of SOFC, which involves stacking of tens of repeating unit cells (anode/electrolyte/cathode) separated by metallic interconnect plates, seal is required to prevent fuel leakage and air mixing at high temperature (800–1000 °C) along with to seal the electrolyte against the metallic body of the device, in order to create a hermetic rugged and stable stack. Any leakage of fuel into the air (or air into the fuel) will lead to direct combustion of fuel and may cause local overheating (hot spots) and sometimes may burst. Therefore, the seals must be stable in a wide range of oxygen partial pressure (air and fuel) and be chemically compatible with other fuel cell components, while minimizing thermal stresses during high temperature operation which creates a major challenge in the development of planar SOFCs [2], [3].

As mentioned above, glasses and GCs are ideal candidates for the job of sealing in SOFCs due to the flexible and compliant nature of glass at temperatures above glass transition, which leads to decrease in mechanical stresses caused by the difference in CTE between the sealing material and SOFC component(s). Moreover, controlled crystallization of glass seal leads to an increase in the mechanical strength and electrical resistivity of the GC, while tailoring the CTE of the final product with respect to the crystalline phases formed.

Majority of the glass/GC sealants developed so far are either BaO-based [6] or Na2O-based [7], [8] aluminosilicates. However, due to the stringent requirements most sealants are not practicable because of the drawbacks concerning either thermal expansion mismatch or due to reactions with SOFC components [9], [10]. Further, significant content of BaO may also promote interaction with water vapor, leading to slow sealant degradation under SOFC operating conditions. A remedy to these problems lies in the development of BaO- and Na2O-free GC sealant which exhibits good CTE matching and low/negligible reactivity with other SOFC components, in particular with metallic interconnect.

An attempt in this direction has been made by various research groups. Ley et al. [11] studied the glass and GC system of SrO–Al2O3–La2O3–SiO2–B2O3. The CTE values of the as-made materials were in the range of (8–13) × 10−6 K−1, while the long term stability was not reported. Recently, Brochu et al. [12] compared the performance of the BaO- and SrO-based borate glass-composites for sealing materials in SOFCs and reported the formation of low CTE crystalline phase, BaZrO3, on interaction with 8YSZ (ZrO2 stabilized by 8 mol% Y2O3), for BaO-containing glass-composites. However, in case of SrO-based glass-composites, formation of strontium zirconates was observed, which has CTE similar to 8YSZ. Mahapatra et al. [13], [14] studied the structure and thermophysical properties and devitrification behavior of the glasses in the system (25−X)SrO–20La2O3–(7 + X)Al2O3–40B2O3–8SiO2 (mol.%) (X = 0–10). Similarly, Kumar et al. [15] studied the influence of substituting La2O3, Y2O3 and Al2O3 on thermal and physical properties of a glass with composition (mol.%) 30SrO–40SiO2–20B2O3–10A2O3 (A: La, Y, Al) and studied their chemical interaction with bismuth vanadate based electrolyte material. However, high boron seals proposed in earlier studies [11], [12], [13], [14], [15] are apt to eventually corrode under humidified hydrogen environments (common in fuel cell operation) over time. Glasses with B2O3 as the only glass former have shown up to 20% weight loss in the humidified H2 environment and extensive interactions with cell component materials both in air and wet fuel gas [16].

Therefore, in the light of above mentioned perspective, a SrO-based aluminosilicate GC composition has been formulated in the primary crystallization field of diopside (CaMgSi2O6) via substitution scheme 0.2Ca2+ + 0.1Mg2+ ↔ 0.3Sr2+ and 0.1 (Ca2+ + Si4+) ↔ 0.1(La3+ + Al3+) in pure CaMgSi2O6 system, thus resulting in a theoretical composition Sr0.3Ca0.7Mg0.9Al0.1La0.1Si1.9O6. Further, the influence of Bi2O3 addition (1, 3 and 5 wt. %) on the sintering and crystallization behavior, flow properties of glasses along with CTE, electrical properties and chemical interaction of resultant GCs with metallic interconnects has been investigated. The deliberate addition of Bi2O3 has been made due to its low melting point (817 °C) which might be helpful in tailoring the flow properties of sealants. Also, Bi2O3 is a major component of bismuth vanadate based electrolyte materials for SOFC [15]. However, the amount of Bi2O3 has been kept ≤5 wt.% (<1 mol%) because if present in higher concentration it might exhibit reducing behavior (Bi3+ → Bi0) in hydrogen rich environment on the anode side of SOFC [17]. It is noteworthy that even though minor amounts of PbO in GC sealant leads to rapid and massive internal oxidation and iron oxide formation on the metallic interconnect surface at the air side of SOFC stack [18]; no such results pertaining to Bi2O3 addition have been reported to the best of our knowledge. Furthermore, 1 wt.% NiO and 2 wt.% B2O3 have been added to all the investigated glass compositions in order to improve adhesion behavior of GCs to metal and decrease the viscosity and glass transition temperature (Tg), respectively. Table 1 presents the compositions of all the investigated glasses.

Section snippets

Synthesis of glasses

Homogeneous mixtures of batches (∼100 g) in accordance with glass compositions presented in Table 1 were prepared by ball milling of powders of SiO2 (purity >99.5%), CaCO3 (>99.5%), Al2O3 (Sigma Aldrich, ≥98%), H3BO3 (Merck, 99.8%), MgCO3 (BDH chemicals, UK, >99%), SrCO3 (Sigma Aldrich, 99+%), La2O3 (Sigma Aldrich, 99.9%), Bi2O3 (Sigma Aldrich, 99.9%) and NiO (Sigma Aldrich, 99%) and calcination at 900 °C for 1 h. The glass batch was melted in Pt crucibles at 1550 °C for 1 h, in air. Glasses in

Density and dilatometry

The density of glasses increased with increase in Bi2O3 content (Fig. 1) due to its highest density (8.9 g cm−3) in comparison to other constituents of glasses. The molar volume of glasses was observed to follow a similar trend as it increased with increasing Bi2O3 concentration in glasses (Fig. 1). These results are in good agreement with the results of Ahlawat et al. [26] and may be explained on the basis of the fact that Bi2O3 is an unconventional glass network former and increasing Bi2O3/SiO

Conclusions

Glass-ceramic sealants free from BaO and Na2O have been designed and investigated in the crystallization field of diopside (CaMgSi2O6). Further, the influence of Bi2O3 (1–5 wt. %) addition on the flow properties, sintering and crystallization behavior along with electrical conductivity and long term thermal stability of sealants has been investigated. All the glasses exhibit two-stage shrinkage behavior resulting in well sintered glass-ceramics with diopside based crystalline phases. The amount

Acknowledgements

Ashutosh Goel is thankful to FCT-Portugal for research grant (SFRH/BPD/65901/2009). The support of CICECO is also acknowledged.

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