Patent Description:
SOC can either operate as a solid oxide fuel cell (SOFC) where they convert chemical energy into electrical energy by oxidizing a fuel or as a solid oxide electrolysis cell (SOEC) where they convert electrical energy into chemical energy by electrolysis of water (and/or carbon dioxide and/or hydrocarbons in a co-electrolysis process). SOC are typically operated in a temperature range of <NUM> - <NUM>, whereas reactants and products of the chemical conversion process are present in gaseous state.

SOC comprise at least three essential ceramic components: two porous electrodes (i.e., a cathode and an anode) separated by a thin and dense electrolyte, together forming the membrane electrode assembly (MEA).

For practical applications, cells are connected in a series forming a stack, which requires additional components such as interconnects, frames, and sealants in order to enable the cells to perform in a series connection and to direct flow of reactant and product gases. The most common stack geometry is planar, wherein cells are placed one on top of each other, connected through the interconnects, most commonly made from ferritic stainless steel. Interconnects and frame elements are designed to comprise channels for directing the flow of reactant and product gases to and from the electrolyte, respectively, and separating anodic and cathodic compartments of a cell to prevent reactants and products from mixing.

The parts in contact with the ceramic electrodes provide for electrical connection between the cathode of a first cell and the anode of a second cell stacked on top of the first. Metallic interconnects may exhibit a protective metal oxide layer on their surface, preventing them from high-temperature corrosion, such as Chromium oxide.

Sealants used for joining the abovementioned components must provide for electrical insulation and gas tightness. To that end, sealants must have a good adhesion to adjoining elements and ensure stable bonds with ceramic and metallic components and avoid interactions leading to local changes of the sealant properties, causing cracks or delamination.

The selection of a suitable glass-ceramic composition as sealant material for SOC is driven by its coefficient of thermal expansion (CTE) and its transition temperature. The CTE must be compatible with that of the adjoining components to keep mechanical stresses resulting from different CTEs at an acceptable level and reduce risk of material failure and consequential leakage. Further, the formation of undesirable phases having a much lower or higher CTE than the remaining material must be avoided. The transition temperature must be close to the operating temperature of the stack to allow for the sealant to soften and provide for good adhesion and sufficient rigidity even at high temperatures.

Cells and stack components are subject to a variety of degradation mechanisms due exposure to reactants and high temperatures which may lead to undesirable phases being formed at the sealant/substrate interface which can lead to the local deterioration of sealant properties. Furthermore, some glass-ceramics undergo progressive crystallization at SOC operating temperatures, leading to changes in the residual glass composition.

In SOFC/SOEC applications, glass-ceramic based sealants are most commonly used due to their high thermal and chemical stability, high electrical resistivity, high flexibility in composition and low production cost. Such glass-ceramic sealants are typically developed by using different glass formers, glass modifiers and additives in an optimum concentration to obtain suitable thermal, electrical and thermo-mechanical properties.

Various studies have been carried out regarding the development of reliable glass-ceramic sealants. However, most of the glass-ceramics reported in literature show certain limitations, i.e., formation of undesirable crystalline phases with CTE considerably different from other SOFC/SOEC components, presence of high porosity, chemical reactivity with neighboring components, etc..

<CIT> discloses a glass ceramic sealant for solid oxide fuel cells. The sealing material in weight percent (wt%) is comprised of (i) <NUM> - <NUM> wt% glass frit and (ii) <NUM> - <NUM> wt% zirconia or leucite addition, wherein the glass frit comprises SiO<NUM> to <NUM> - <NUM> mol%, Li<NUM>O to <NUM> - <NUM> mol%, Na<NUM>O to <NUM> - <NUM> mol%, K<NUM>O to <NUM> - <NUM> mol%, MgO to <NUM> - <NUM> mol%, CaO to <NUM> - <NUM> mol%, Al<NUM>O<NUM> to <NUM> - <NUM> mol%, B<NUM>O<NUM> to <NUM> - <NUM> mol%, SrO to <NUM> - <NUM> mol% and alkali metal oxide to <<NUM> mol%. The glass-ceramics compositions may contain a relatively high concentration (up to <NUM> mol%) of B<NUM>O<NUM> which is not suitable for high temperature SOFC/SOEC applications. A higher concentration of B<NUM>O<NUM> results in its evaporation at high temperatures and therefore can adversely affect the sealant's properties. In literature, the evaporation of B<NUM>O<NUM> is often reported as main source of increased porosity in glass-ceramic based sealants. The patent also discloses the glass-ceramics to comprise up to <NUM> mol% of Al<NUM>O<NUM> which increases the possibility of formation of undesirable low-CTE phases such as SrAl<NUM>Si<NUM>O<NUM> or BaAl<NUM>Si<NUM>O<NUM> in case of SrO and BaO as main modifiers, respectively. The presence of low-CTE phases may introduce thermal stress leading to crack formation or delamination in particular during applied thermal cycles during operation. Furthermore, the patent discloses the glass-ceramics to comprise a total amount of alkali metal oxides of up to <NUM> mol%. Alkali metal oxides have a small ionic radius and can reduce the electrical resistivity of the glass, which is not desirable for SOFC/SOEC applications. Moreover, alkali metal oxides tend to be more chemically reactive, intruding the risk of undesired phases being formed at the sealant/substrate interface. Issues related to electrical resistivity and chemical reactivity become more critical for SOEC applications where applied voltages are higher than in typical SOFC applications. All the sealant compositions discussed in the patent have a much lower SrO content (≤<NUM> mol%) as compared to the compositions according to the present invention. The higher SrO-content is required to form high-CTE Sr-containing crystalline phases such as SrSiO<NUM> and/or Sr<NUM>SiO<NUM>, which is important for the performance of the sealant according to the present invention, as the formation of a Sr-based crystalline phase minimizes the concentration of SrO in a residual glassy phase and consequently reduces the possibility of high-CTE SrCrO<NUM> formation in the presence of Chromium at the interconnects surface.

<CIT> discloses the following composition in mol%: M1 (BaO, SrO, CaO, and/or MgO) to <NUM> - <NUM> mol%, M2 (Al<NUM>O<NUM>) to <NUM> - <NUM> mol%, M3 (SiO<NUM> and B<NUM>O<NUM>) to <NUM> - <NUM> mol%. The glass-ceramic composition may have a high concentration of Al<NUM>O<NUM> (up to <NUM> mol%), thus there is a high possibility for formation of low-CTE phases such as SrAl<NUM>Si<NUM>O<NUM> or BaAl<NUM>Si<NUM>O<NUM>. Further, the composition comprises BaO, wherein the composition according to the present invention is preferably BaO-free thus to prevent the formation of a high-CTE BaCrO<NUM>-phase in the presence of Chromium. Further, the composition according to the patent comprises less than <NUM> mol% of SrO, wherein the composition to the present invention comprises a much higher mol-percentage of SrO at <NUM> - <NUM> mol% with the advantages discussed in the previous paragraph. Further, the patent discloses compositions comprising up to <NUM> mol% of SiO<NUM>. Such high concentration of SiO<NUM> can lead to the formation cristobalite (SiO<NUM>) phase which is not desirable in SOFC/SOEC applications especially under thermal cyclic conditions due to the fact that cristobalite phase undergoes volume change around <NUM> and can lead to crack initiation. Alternatively, the composition may comprise an extremely high B<NUM>O<NUM> content of up to <NUM> mol%, which is undesirable for high temperature applications because such concentrations can cause high porosity in the sealant.

Both patents discussed above do not provide information about the possible crystalline phases in glass-ceramics. Due to the presence of various components in a glass, it is difficult to control the type and relative qualities of formed crystalline phases. The formation of undesirable primary phase with very high or low coefficient of thermal expansion can disturb the composition of residual glass and thus can either minimize the chances of the formation of desired crystalline phase or its relative concentration. It can affect the overall CTE of the glass-ceramic.

In the publication <NPL> -different glass systems have been discussed which are of the following composition in mol%: SiO<NUM> to <NUM> - <NUM> mol%, CaO to <NUM> - <NUM> mol%, SrO to <NUM> - <NUM> mol%, Al<NUM>O<NUM> to <NUM> - <NUM> mol%, B<NUM>O<NUM> to <NUM> - <NUM> mol%. According to the reported results, the glass-ceramics had been investigated up to <NUM> hours at <NUM>, while no information is provided about the glass-ceramics behavior at higher temperatures typically above <NUM>. Some of the reported glass formed undesirable high CTE SrCrO<NUM> phase even at <NUM>, which can increase dramatically at high temperatures, such as in the range typicall for SOC operation. The glasses studied have a relatively low SiO<NUM> contents (max. up to <NUM> mol%), which can reduce the thermal stability of glass-ceramics at higher temperatures (typically above <NUM>). Further, the glasses comprise a lower SrO content and a much higher CaO content than composite according to the present invention.

In the publication <NPL> which has been published by the inventors in <NUM>, three glass-ceramic compositions were described, which have been designed and characterized as sealant materials for solid oxide electrolysis cells (SOEC), having operating temperature of <NUM>. The crystallization and the sintering behavior of the glasses are investigated by using differential thermal analysis (DTA) and heating stage microscopy (HSM) respectively. The glasses show glass transition temperatures of <NUM> - <NUM>, while the coefficients of thermal expansion (CTE) of <NUM> - <NUM> × <NUM>-<NUM> K-<NUM> (<NUM> - <NUM>) are measured for the glass-ceramics, matching with the CTEs of the other cell components. The compatibility between the glass-ceramic sealants, the 3YSZ electrolyte and the Crofer22APU interconnect is examined by means of SEM and EDS, in the as-joined condition and after <NUM> hours at <NUM> in air. Compositional changes in the glass-ceramic sealants are reviewed and discussed with respect to the formed crystalline phases before and after the aging treatment at <NUM>. All the compositions subject of the paper exhibited undesirable phases, limiting the sealant's durability under SOFC/SOEC conditions, which is a common problem faced in SiO<NUM>-SrO based glass systems. As the formation of low-CTE phases, such as SrAl<NUM>Si<NUM>O<NUM>, or the formation of a phases undergoing undesired volume expansion, such as the cristobalite phase, causes stress within the glass-ceramic sealant and consequently can lead to crack initiation, the results of the study were unsatisfactory.

Therefore, the object of the present invention is to present a glass ceramic sealing composition which is thermally and chemically stable for high temperature and long-term operations under SOFC/SOEC conditions and hence suitable for being used as a sealant in SOC applications.

A secondary object of the present invention is to present a glass ceramic sealing composition which is free from low-CTE phases, and comprises SrSiO<NUM> and/or Sr<NUM>SiO<NUM> as main crystalline phases, with CTEs in the desired range of <NUM> - <NUM> × <NUM>-<NUM> K-<NUM>.

Another object of this invention is to minimize the formation of a cristobalite phase (SiO<NUM>).

The object is solved by the combination of features according to the claim <NUM>.

The glass ceramic sealing-composition is characterized in that the composition is selected from the following components in mol%:.

wherein the total concentration of CaO and MgO and SrO is <NUM> - <NUM> mol% and wherein the total concentration of SiO<NUM> and B<NUM>O<NUM> is <NUM> - <NUM> mol %.

According to the invention the composition has SrSiO3 and/or Sr2SiO4 as main crystalline phases in addition to a residual glassy phase after crystallisation at <NUM>-<NUM>. Such glass ceramic compositions are free from low CTE phases, and exhibit CTEs in the desired range of <NUM> - <NUM> × <NUM>-<NUM>-<NUM>.

According to the invention a small amount of B<NUM>O<NUM> with a minimum of <NUM> mol%, is added to improve the wettability of the glass ceramic sealing composition for better adhesion. However, with higher B<NUM>O<NUM> concentration, the possible evaporation of boron can impart adverse effect. Therefore, the maximum concentration of B<NUM>O<NUM> in the presented glass-ceramic is below <NUM> mol%. Further, according to the invention, the composition is alkali-metal-oxides-free so to eliminate the issues related to low electrical resistivity of alkali metal oxides respectively.

The glass ceramic sealing composition according to the present invention has a crystalline phase providing a desirable coefficient of thermal expansion for application as sealant material for SOC stacks and to ensure strong adhesion to other components. The formation of crystalline phases with undesirable coefficient of thermal expansion has been avoided by using suitable ratio between glass formers and modifiers. The sealant is thermally and chemically stable for high temperature and long-term operations under conditions typical for SOC stacks. The developed glass-ceramic sealing composition can be used to join ceramics-to-ceramics, ceramics-to-metals and metals-to-metals.

The described glass ceramic sealing composition is a Sr-SiO<NUM> based glass. In contrast to various state of art glass ceramics of the same family (Sr- SiO<NUM>), the formation of high CTE SrCrO<NUM> in the presence of Chromium oxide at the sealants interface has been minimized, while the formation of the low CTE SrAl<NUM>Si<NUM>O<NUM> phase has been eliminated. These undesirable phases have either very high or very low coefficient of thermal expansions as compared with other SOC components, thus can lead to thermal stresses and eventual loss of adhesion / consequential leakage. Additionally, the glass ceramic sealing composition is chemically stable and does not undergo undesirable phase transitions even after ageing.

In one preferred embodiment the composition has <NUM> - <NUM> mol% of SrO.

The presence of Al<NUM>O<NUM> mainly delays the crystallization process in glass ceramics and stabilizes the amorphous phase. However, higher Al<NUM>O<NUM> contents can increase the possibility of formation of low CTE SrAl<NUM>Si<NUM>O<NUM> phase in SiO2-SrO based glass systems. For this purpose, the Al<NUM>O<NUM> contents were restricted to maximum of <NUM> mol %.

The composition may contain Y<NUM>O<NUM>, La<NUM>O<NUM>, CeO<NUM>, or Ga<NUM>O<NUM> of up to <NUM> mol% or a combination thereof of up to <NUM> mol%. It has been found that rare earth metal oxides i.e. La<NUM>O<NUM>, Y<NUM>O<NUM> etc. and Ga<NUM>O<NUM> improve the CTE and wettability of glass ceramic. Therefore, the glass ceramic sealing composition may preferably contain a small concentration of one of these oxides or their combination.

According to a preferred embodiment, the composition is BaO-free and/or ZnO-free so to eliminate the issues related to the possible formation of undesirable high CTE BaCrO<NUM> phase.

BaO and SrO containing glass ceramics for SOC applications mainly suffer by the formation of BaCrO<NUM> or SrCrO<NUM> respectively. Both of these phases (BaCrO<NUM> or SrCrO<NUM>) have a high CTE and adversely effects the sealing properties of glass-ceramic sealants. The glass ceramic sealing composition according to the present invention in one preferred embodiment is free of BaO, therefore, the formation of BaCrO<NUM> has been eliminated. On the other hand, the formation of SrCrO<NUM> has been minimized to a maximum up to <NUM>% of the material, by optimizing the SrO/SiO<NUM> - ratio.

In one embodiment the glass ceramic sealing composition has a coefficient of thermal expansion of <NUM> - <NUM> x <NUM><NUM> K-<NUM> and/or a coefficient of thermal expansion of <NUM> - <NUM> x <NUM>-<NUM> K-<NUM> after crystallization at <NUM> - <NUM>. The glass ceramic according to the invention therefore has an excellent compatibility with metallic interconnect and ceramic electrolyte materials commonly used in SOC applications.

The composition has a SiO<NUM> -content of <NUM> - <NUM> mol% or specifically in the range of <NUM> - <NUM> mol%.

The composition has a SrO -content of <NUM> - <NUM> mol% or specifically below <NUM> mol%.

Furthermore, the composition specifically can have a SiO<NUM>/SrO ratio of <NUM> - <NUM>. For the glass ceramic composition according to the invention, the SiO<NUM>/SrO ratio is preferably kept in the range of <NUM> - <NUM> in order to have desirable Sr-Silicate based phase and to avoid cristobalite phase, while maintaining the overall CTE of glass-ceramic in the range of <NUM>-<NUM> x <NUM>-<NUM> K-<NUM>. By using a SiO<NUM>/SrO ratio of <NUM>-<NUM>, the cristobalite phase content has minimized to <NUM>-<NUM> weight%.

Undesirable phases were avoided by optimizing the SrO content and SiO<NUM>/SrO - ratio of the composition.

The composition can have crystalline phases comprising of at least <NUM>% of volume content of overall glass-ceramic.

In one embodiment the composition comprises SiO<NUM> and B<NUM>O<NUM> in the range of <NUM> - <NUM> mol% in total, more preferably <NUM> - <NUM> mol% in total.

The composition can contain CaO up to <NUM> mol% and MgO below <NUM> mol%.

Furthermore, the composition may also have an overall concentration of CaO and MgO and SrO up to maximum of <NUM> mol%.

Moreover the composition can comprise of Y<NUM>O<NUM> and/or La<NUM>O<NUM> and/or CeO<NUM> and/or Ga<NUM>O<NUM> or a combination thereof in the range of <NUM> - <NUM> mol%, more preferably in the range of <NUM> - <NUM> mol%.

The application of the glass ceramic sealing composition according to one of the preceding embodiments can be used for bonding of at least two SOFC or SOEC components.

The bonding can be established between ceramic-to-metal and/or ceramic-to-ceramic and/or metal-to-metal, such that the glass ceramic sealing compositionserves as a sealant and/or bonding agent in SOFC or SOEC.

In the following, the invention is further explained, whereby this is not necessarily restrictive but possibly restrictive.

As with BaO containing glass ceramic systems which allow for the formation of BaCrO<NUM>, SrO containing glass ceramic systems can form a high CTE SrCrO<NUM> phase, however, the formation of SrCrO<NUM> seems to be kinetically slower than that BaCrO<NUM>, probably due to a lower Gibbs free energy of the BaCrO<NUM> formation. Additionally, the volume content of the residual glassy phase and the corresponding Ba/Sr contents, can affect the formation of chromates (BaCrO<NUM> and SrCrO<NUM>). High volume content of residual glass or a high concentration of Ba/Sr in the residual glass enhances the possibility of respective chromates formation. The glass ceramic composition according to the invention comprises > <NUM> vol% of crystalline phases, thus minimized the possibility of formation of SrCrO<NUM>.

To act as an effective sealant, the glass ceramic composition should have a CTE in a suitable range and closely matching with joining components. In SOFC/SOEC, the glass ceramic sealant is mainly used to join steel interconnects and zirconia-based electrolytes having CTEs in the range of <NUM> - <NUM> x <NUM>-<NUM> K-<NUM>. The CTE of the crystallized glass ceramic mainly depends on the crystalline phases formed in it. The formed crystalline phases should therefore also have an optimum CTE in the range of <NUM> - <NUM> x <NUM>-<NUM> K-<NUM> to obtain stable joints. The glass ceramic sealant composition has CTE in the range of <NUM> - <NUM> x <NUM>-<NUM> K-<NUM> and more specifically a CTE of <NUM> - <NUM> x <NUM>-<NUM> K-<NUM>, and is thus suitable for SOFC/SOEC applications.

The addition of SrO improves the CTE and wettability of glass ceramics. Besides that, increasing the concentration of SrO in SiO<NUM> based glass ceramic systems, can lead to the formation of SrSiO<NUM> or Sr<NUM>SiO<NUM> phases having CTEs of <NUM> - <NUM> x <NUM>-<NUM> K-<NUM>. However, on the other hand, an excess of SrO can also lead to the formation of a high CTE SrCrO<NUM> phase due to chemical reaction between Cr contaminants originating from stainless steel alloys used for SOFC/SOEC interconnects and SrO from the glass ceramic. Therefore, it is necessary to have an optimum concentration of SrO.

Due to the presence of >50vol% of crystalline content, the glass ceramic sealant is thermally stable at high working temperatures of SOFC/SOEC. Moreover, the glass ceramic sealant has a high electrical resistivity (><NUM><NUM> Ω. cm) in the temperature range of <NUM> - <NUM>, and is thus suitable for SOFC/SOEC applications.

Claim 1:
Glass ceramic sealing composition, characterized in that the composition is selected from the following components in mol%:

<TAB>

wherein the total concentration of CaO and MgO and SrO is <NUM> - <NUM> mol% and wherein the total concentration of SiO<NUM> and B<NUM>O<NUM> is <NUM> - <NUM> mol %, wherein the composition is alkali-metal-oxide-free and the composition comprises SrSiO<NUM> and/or Sr<NUM>SiO<NUM> as main crystalline phase(s) in addition to residual glassy phase after crystallization at <NUM> - <NUM>.