Development of novel glass-based composite seals for planar intermediate temperature solid oxide fuel cells
Highlights
► Novel glass/ceramic composite seals exhibited excellent gas tight and chemical compatibility. ► The leakage rates of the seals were lower than 0.01 sccm cm-1 within 25 thermal cycles. ► The seals can maintain well interface compatibility and adhesion after test. ► The feasibility of the seals applied on SOFC was confirmed by single cell test.
Introduction
The planar intermediate temperature solid oxide fuel cell (SOFC) has been highly regarded as a promising power conversion system in recent years [1]. However, one of the major challenges for SOFC is to develop suitable sealing materials to prevent gas leakage and ensure electrical insulation in the SOFC stack [2], [3], [4]. In order to meet sealing requirements in SOFC operation condition, the seals should also fulfill the requirements like thermo-mechanical, chemical stability and matching coefficient of thermal expansion (CTE) with adjacent components [5], [6], [7].
So far, seal materials are generally divided into rigid seals and compressive seals [4]. Glass-based materials, as typical rigid seals, could tune their composition to optimize the required physical properties such as glass transition temperature (Tg), glass soft temperature (Ts) and the CTE. Because borosilicate glass seal can modify composition to obtain suitable Tg and Ts, it can keep a “soft” state under certain compressive load at SOFC operation temperature which is very important to the sealing and contact of stack [8], [9], [10]. Although the glass seals have disadvantages in assembling stack due to the property of brittleness in room temperature, they can provide well adherence with the adjoining components, flexible deformation and reliable sealing ability above Tg [8], [9], [11], [12]. Mica and mica-based composite seals, as a kind of compressive seals, can satisfy the sealing requirements under suitable compressive stress. In special, hybrid phlogopite mica-based compressive seals can even undergone 1000 thermal cycles or more [13]. But the K element from mica seals tends to react with the electrode materials of cell, and then degrade cell performance [13], [14], [15], [16]. Deformable metallic seals, such as Au and Ag, are limited in application because of their high electronic conductivity. They must be applied together with insulating materials to prevent electrical conduction [4], [7]. Considering all these seals, glass-based composite seals seems to be the promising materials as seals in SOFC application due to their high geometric stability and gas tightness.
In this paper, a novel glass-based composite seals was developed whose leakage rates has been investigated under different operation temperature, gas input pressure and thermal circles. With certain amount of Al2O3 powders added into glass matrix, the strength of the composite seals was improved by avoiding excessive flow of melting glass in low viscosity. So the reliability and thermal cycle stability of glass-based composite seals were enhanced under SOFC operation condition. To examine further sealing design and effect of the glass-based composite materials, single cell test has been performed, followed by phase identification and microstructure characterization.
Section snippets
Seal preparation
The mixed fine powder of Al2O3 and glass were employed for seal fabrication through tape casting. The glass powder, named as HGS, with a nominal composition (in molar percentage) 8.3%B2O3-12.1%BaO-21.8%MgO-12.1% ZnO-45.7%SiO2, was provided by New Huaguang, Ltd Company. All the original reagents were of chemical grade. 30% Al2O3 powders were subsequently added in glass matrix. The phase structure of mixed powder has been identified as shown in Fig. 1, which exhibited sharp Al2O3 crystalline
Leakage rates of glass-based composite seals
Fig. 4 shows the leakage rates of HGS-1 seals measured at the inlet pressure of 1, 2 and 3 psi under compressive stresses from 10 psi to 40 psi at 700 °C (a), 750 °C (b) and 800 °C (c) in air, respectively. Generally, the leakage rates were all lower than 0.01 sccm cm−1 and their gas tightness were more excellent than Al2O3-based seals in our previous studies [5]. The leakage rates increased with the inlet pressure varied from 1 psi to 2 psi and basically remained stable when the inlet pressure
Conclusion
Novel glass-based composite seal was developed through tape casting technique whose performances such as leakage rates, compatibility have been evaluated. The Al2O3 enhanced glass seals was reasonably good candidates for SOFC sealing materials. The leakage rates and thermal cycled leakage rates were all lower than 0.01 sccm cm−1 under compressive stress of 20 psi at 750 °C. The compressive stress was suitable and enough to ensure gas tightness of HGS-1 seals. The interfaces of
Acknowledgments
This research was financially supported by the Hubei Science Foundation of China under contract 2008CDA004, the “863” high-tech project under contract 2011AA050702. SEM and XRD analysis were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology.
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