Elsevier

Journal of Power Sources

Volume 196, Issue 18, 15 September 2011, Pages 7578-7584
Journal of Power Sources

Barium silicates as high thermal expansion seals for solid oxide fuel cells studied by high-temperature X-ray diffraction (HT-XRD)

https://doi.org/10.1016/j.jpowsour.2011.04.035Get rights and content

Abstract

Gas-tight seals between metals and ceramics in solid-oxide fuel cells can be fabricated from glasses which enable the crystallization of phases with high thermal expansion coefficients (mostly barium silicates). This article mainly reports on high-temperature X-ray diffraction studies on these silicates. It is shown that all barium silicates exhibit thermal expansion coefficients in the range from 10.5 to 15.4 × 10−6 K−1 (100–800 °C). The expansions are strongly dependent on the respective crystallographic axis. The ortho- and metasilicates exhibit the largest thermal expansion coefficients. The coefficient of thermal expansion of a sealing glass is attributed to the thermal expansion of the crystalline phases and the residual glassy phase. The phase formation should carefully be controlled also with respect to aging. Crystalline phases with high coefficients of thermal expansion, such as the barium silicates, are advantageous as components in such sealing glasses.

Highlights

► We examined the thermal expansion of various crystalline barium silicates. ► Thermal expansion was studied by dilatometry and high-temperature X-ray diffraction. ► High-temperature X-ray diffraction is advantageous to determine thermal expansion. ► Barium orthosilicate and barium metasilicate exhibit the highest thermal expansion.

Introduction

For gas-tight and electrically insulating seals, glass or materials derived hereof are preferred for numerous applications [1]. In a first step, a glass is melted, then it is crushed and powdered and subsequently brought in between the components to be joined. During high temperature treatment, the glass powder sinters and – in some cases – subsequently crystallizes. The crystallization behavior is controlled by the chemical composition, especially by the addition of nucleating agents or nucleation inhibitors, and furthermore by the temperature time schedule.

To enable the utilization as a sealing glass for solid-oxide fuel cells (SOFCs) or high temperature reactors, these materials have to meet various thermal, mechanical and chemical properties [1]. In particular, they have to withstand high temperatures as well as thermal cycling. For example for rigidly bonded sealing glasses, the softening temperature should be higher than the operating temperature and the thermal expansion coefficients (CTEs) of the different components of the cell and of the seal glass should match within a value of ±1 × 10−6 K−1. Various materials have been used as components in SOFCs. For example stabilized zirconia is used as electrolyte, the attributed coefficient of thermal expansion (30–800 °C) is in the range from 10.3 to 10.6 × 10−6 K−1 [2], depending on the dopants used for the stabilization of zirconia. As cathode material various perovskites are used, for example Nd1−xSrxMnOδ, which has a CTE (30–700 °C) in the range from 11 to 14 × 10−6 K−1 for x = 0.15  0.5 [3]. The common anode material is a cermet of nickel and yttrium-stabilized zirconia which has a CTE (30–1000 °C) of 12.6 × 10−6 K−1 [4]. Ferritic steels often serve as interconnect in SOFCs; a typical value of the CTE (at 800 °C) of a steel with 20–25% chromium is 12.3 × 10−6 K−1 [5].

Glasses with high CTE often possess a fairly low glass transition temperature. For example Flügel et al. [6] reported a glass with the mol% composition of 35 SiO2, 10 B2O3, 5 Al2O3, 37 BaO and 13 SrO, which has a CTE (200–500 °C) of 13.6 × 10−6 K−1 and glass transition and softening temperatures of 585 and 640 °C, respectively. Hence, especially if the seal has to withstand high temperatures (>700 °C) and at the same time, high thermal expansion coefficients (CTE > 10 × 10−6 K−1) are required, the use of crystallizing glasses is especially advantageous.

In principle also so called composite seals might be used. Here, a glass is mixed with a crystalline component with high thermal expansion coefficient, such as metals or metal fluorides. The composite seal has a CTE in between that of the glass and that of the added crystalline phase. For example alumina has been reported in the literature as suitable filler for a composite seal together with a soda-lime alumino borosilicate glass matrix. This leads to increased mechanical toughness, thermal shock resistance and viscosity which simultaneously decreases the thermal expansion [7]. MgO has been reported to be suitable as filler in a soda aluminosilicate glass if higher thermal expansion coefficients are required [8]. In principle, also powdered metals should be suitable as fillers in composite seals.

Another possibility is the use of a compressive load on the sealing material for accommodation of differential thermal expansion [9]. For example mica and Al2O3 are materials used in such compressive seals. Sang et al. [10], [11], [12] described methods to determine and to predict the leak rate of such sealing materials. Compressive seals often show a good performance but one problem is the higher technical afford to apply an external load [1].

An interesting field are alkaline earth silicates. Here, the thermal expansion coefficients are fairly high and the respective phases can either be used as fillers or, however, the crystalline phases can directly be crystallized from suitable glass compositions. A crystal phase with a high CTE as a component of a sealing glass can rise the CTE of the seal, which should lie in between the CTEs of its constituents (all crystalline phases and possibly a residual glassy phase). Although the most glass-ceramics reported to be suitable for sealing fuel cells are based on alkaline earth silicates, most of the thermal expansion coefficients of the respective crystalline phases up to now have not been reported in the literature.

Previous work has been done by Vasil’ev [13] already in the 50s. He determined the dilatometric expansion coefficients of several alkaline earth orthosilicates, including barium orthosilicate. In the early 70s Oelschlegel [14], [15] examined the barium silicates BaSi2O5, Ba5Si8O21 and Ba2Si3O8 and determined CTE using high-temperature X-ray diffraction. He also took into account the occurrence of low-temperature (LT) and high-temperature (HT) modifications. Weil et al. [16] published dilatometric measurements of the CTE of BaSi2O5, Ba2Si3O8 and BaSiO3. However, these samples were not prepared from stoichiometric mixtures of raw materials, but from a glass with the composition (wt%): 56.4 BaO, 22.1 SiO2, 5.4 Al2O3, 8.8 CaO and 7.3 B2O3 by devitrification. The reported CTE for a certain phase, however, is indeed the CTE of glass-ceramics with the desired phase as the main component, and hence also other phases are present.

This article reports on the thermal expansion behavior of barium silicates studied by both high temperature X-ray diffraction and dilatometry. Those barium silicates were studied which according to the phase diagram reported by Huntelaar and Cordfunke [17] are stable in the desired temperature range from room temperature to 1000 °C.

Section snippets

Materials and methods

The barium silicates were prepared from reagent grade raw materials. Mixtures of SiO2 (quartz powder C, SCHOTT) and BaCO3 (chemical pure, REACHIM) with the respective stoichiometry were ball milled and annealed at 1500–1550 °C for 1–3 h. After cooling, the samples were ball milled again. This procedure was repeated at least twice. The formed phases were studied by powder X-ray diffraction (XRD), using a SIEMENS D5000 diffractometer with Cu Kα radiation. The XRD-patterns were analyzed using

Results

Fig. 1 shows (as an example) a set of diffractograms measured by HT-XRD of barium orthosilicate Ba2SiO4. All detected XRD lines are attributable to crystalline Ba2SiO4. With increasing temperature, the lines were shifted to smaller Bragg angles. Also an increase in intensity for some reflections was observed. The other studied barium silicates showed a fairly similar behavior.

Fig. 2 shows the XRD-pattern of barium metasilicate BaSiO3 as an example of the Rietveld calculations performed. The

Discussion

Concerning the XRD-measurements, all barium silicates were measured both on standard sample holder and on heating stage. The deviations of the cell parameters measured at room temperature (see Table 1) were less than 0.70%. The deviations of the room temperature measurements from those given in the literature were less than 0.50%.

In the investigated temperature range, all studied barium silicates show a positive thermal expansion in all their cell parameters with increasing temperatures.

All

Conclusions

All barium silicates exhibit high CTEs > 10 × 10−6 K−1, the highest thermal expansion (according to the HT-XRD measurements) show the orthorhombic barium silicates BaSi2O5, BaSiO3 and Ba2SiO4. Especially advantageous as components of crystallizing sealing glasses for high-temperature applications are Ba2SiO4 and BaSiO3 because they have not only a high mean CTE of 15.4 × 10−6 K−1, but also large CTEs (12.7–19.1 × 10−6 K−1) in all crystallographic directions. If powdered sealing glasses are first sintered

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