WO2014049119A1

WO2014049119A1 - Method of producing a joined product

Info

Publication number: WO2014049119A1
Application number: PCT/EP2013/070187
Authority: WO
Inventors: Wolff-Ragnar Kiebach; Peter Vang Hendriksen
Original assignee: Danmarks Tekniske Universitet
Priority date: 2012-09-28
Filing date: 2013-09-27
Publication date: 2014-04-03

Abstract

The present invention provides a method of producing a joined product by reactive joining a ceramic member to a further member using a joining material selected among glass and glass ceramic.

Description

Method of producing a joined product

The present invention relates to a method of producing a joined product by reactive joining a ceramic member to a further member with a glass or glass-ceramic composition. Furthermore, the present invention relates to a joined product of a ceramic member and a further member, which is obtainable by said method. Likewise, the invention concerns the use of a glass or glass- ceramic composition for producing such a joined product.

Background

Glass or glass-ceramic compositions are commonly employed as adhesives or sealants for joining ceramic materials with other parts, such as other ceramics, metals, or metal alloys. Common applications of such joined products are composite materials that are built of tubular ceramic oxygen membranes bonded to steel pipes, wherein the glass or glass-ceramic compositions act as adhesives or sealing materials.

In particular, US 2011/0209618 Al discloses a ceramic product comprising an oxygen separation membrane made of an oxide ceramic with a perovskite structure, at least one ceramic connecting member bonded to the oxygen separation membrane, and a bond part between the oxygen membrane and the connecting member, wherein the bond part is formed by adhesive bonding via a glass-ceramic layer comprising leucite crystals.

US 5,725,218 shows a fritted sealant material for adhesive sealing a ceramic tube of Sr, Fe, Co oxides to an oxidation resistant nickel-based metal alloy, wherein said sealant comprises a compound of Sr oxide and boric oxide and Sr, Fe, Coo.sOx present in the range of about 30 to about 70 percent by weight of the fritted sealant material.

Jinhua et al. Journal of Rare Earths, vol.25, August 2007, 434-438 discloses a method of producing a joined product. When heated Ga of the LSGM diffuses into the sealant, while other components of LSGM and the components in glass-ceramics do no diffuse with each other.

Commonly-known products joined by adhesive bonding often show an inadequate adhesion of the joined members to each other. This is especially the case for oxygen membranes bonded to steel pipes due to the fact that the operating temperatures of these devices are in the range of 500 to 1000°C, preferably 600 to 950°C, and taking into account the relatively high differences of the coefficients of thermal expansion (CTE) of these members. This in turn may lead to an insufficient sealing and reduced life time of the device.

In view of this, a need exists for providing an improved method for joining ceramic members such as oxygen membranes to other members that may have a different CTE, and which provides a joined member with an improved adhesion and sealing.

One commonly known other method of bonding materials to each other is diffusion bonding, such as described e.g. in US 3,517,432. Said document teaches a method of diffusion bonding of ceramics by placing a metal selected from the class consisting of aluminum, titanium, and vanadium between the two ceramic members, heating in an oxidizing or inert atmosphere until the metal melts and forms a non-conducting ceramic phase with the ceramic members, and then cooling to room temperature. This diffusion bonding method only employs the metal, metal oxides, or metal hydrides as sealing adhesive materials. Furthermore, diffusion bonding usually requires high pressures that must be applied during the bonding process.

Furthermore, also diffusion bonding is not sufficient for producing joined products of ceramics and other materials with regard to sufficient adhesion and sealing, especially at high operating temperatures and when joining materials with different CTE's.

Object of the present invention

The object of the present invention is the provision of a method for joining a ceramic element to a further member, wherein the method provides a joined product with a high sealing ability and adhesion of the joined members, even at high operating temperatures of the joined device, such as in a range of 500 to 1000°C, preferably 600 to 950°C, and even if the joined members show different coefficients of thermal expansion. Further objects will be apparent from the below description.

Summary

This object is achieved by a method in accordance with claim 1. Preferred embodiments of the method are specified in claims 2 to 13. The present invention also relates to a joined product in accordance with claim 14 and a use in accordance with claim 15.

Detailed description Method steps and definitions:

The present invention, in order to solve one or more problems of the prior art, provides a method of producing a joined product by reactive joining a ceramic member to a further member, wherein the method comprises a first step of providing a joining layer between the ceramic member and the further member in order to form an intermediate product, wherein the joining layer comprises a glass or glass-ceramic composition, or a precursor thereof, and a second step of joining the ceramic member to the further member by heating the intermediate to a specific temperature.

This specific temperature is lower than the melting temperature of the ceramic member.

Furthermore, the specific temperature is higher than the glass transition temperature of the glass composition or higher than the melting temperature of the glass ceramic composition. The heating in the second step forms a reaction layer between the ceramic member and the joining layer by inter-diffusion, and subsequent chemical reaction of components of the ceramic member and the joining layer. Contrary to the process of the prior art, where, by means of pressure and diffusion, an intimate contact is provided between members to be joined, the reactive joining process of the present invention achieves improvement in bonding by ensuring that the material of the joining layer melts, or is present above its glass transition temperature. This leads to the fact that the interface of the ceramic member to be joined is at least partially dissolved by the molten joining layer material and/or reacts therewith. This process in turn generates a new reaction layer with a different chemical composition, which is a result from the at least partial dissolution/reaction of the ceramic member and the subsequent reaction with the melted joining layer, so that an improved bonding is achieved. In this context, suitable for the present invention that the specific temperature to which the intermediate product is heated is also lower than the sintering temperature of the ceramic member, in order to ensure that the properties of the ceramic member (such as a membrane) are not altered. As will be shown and explained in detail in the context of the examples in accordance with the present invention, this reactive joining provides an improved joining layer resulting in improved overall product properties.

First step:

In the first step, the joining layer is provided between the ceramic member and the further member in order to form an intermediate product, wherein the joining layer comprises a glass or glass-ceramic composition, or a precursor thereof. Thus, the intermediate product forms a sandwich structure, wherein the joining layer is located between the ceramic member and the further member. The term "sandwich structure" is to be understood in that the area or void space, which is located between the ceramic member and the further member (i.e. independently from its shape), is at least partly filled with the material for the joining layer comprising the glass or glass-ceramic composition.

The term "glass or a glass-ceramic composition, or a precursor thereof encompasses two alternative embodiments, namely a glass composition or a glass-ceramic composition. Said compositions may be provided in the first step in the form of a precursor of the glass or glass- ceramic composition, wherein the term "precursor" is to be understood as any starting

composition that may be subsequently transformed (for example during the heating step) to a glass composition or a glass-ceramic composition. The term "glass composition" refers to an amorphous solid material. The term "glass-ceramic composition" refers to a semi-crystalline material showing crystalline areas in a glass matrix. The glass-ceramic composition may be produced from the glass composition by controlled crystallization, i.e. the glass composition and the glass-ceramic composition may show the same chemical composition may only differ in their degree of crystallinity. The controlled crystallization may be induced by the heating in the second step.

Suitable methods for producing the glass or glass-ceramic composition are commonly known. In particular, the compositions may be prepared by mixing the oxides of the components and/or any suitable precursor substances of the components, heating to a temperature of higher than the melting temperature and cooling the mixture, for example by quenching with water. This results in an amorphous starting glass, which may be subsequently pulverized by a milling process to obtain a pulverized glass composition. The glass composition may then be reheated to induce a controlled crystallization in order to form a glass-ceramic composition.

The glass/glass-ceramic composition can be provided to the ceramic member and the further member in any conventional manner. One embodiment is the application of the composition in the form of a paste which may be obtained by mixing the glass/glass-ceramic composition with a suitable additive such as water, organic solvents, or an organic binder. However, the void spaces between the ceramic member and the further member may also be filled by a powder of the glass/glass-ceramic composition in order to form the joining layer. Typical application examples are screen printing, tape casting and other processes known to the skilled person. The application conditions are not crucial as far as they produce an intermediate product. Typically, the application is performed at room temperature. If pastes or slurries are employed, the process may comprise an additional drying step in a conventional manner, in order to remove the

solvent/water.

Second step:

The second step of joining the ceramic member to the further member is achieved by heating the intermediate product obtained in the first step to a specific temperature, which may be designated as joining temperature.

It is essential for the inventive method that said temperature is lower than the melting temperature of the ceramic member. Furthermore, said temperature must be higher than the glass transition temperature of the glass, if the joining layer comprises a glass composition or a precursor thereof. If the joining layer comprises a glass-ceramic or a precursor thereof, the temperature must be higher than the melting temperature of the glass-ceramic. The glass temperature (Tg) and the melting temperature (Tm) refer to the onset temperatures as determined via differential scanning calorimetry (DSC). In embodiments, the joining temperature in the second step is 10 °C or higher, preferably 20 °C or higher, more preferably 50 °C or higher, than the Tg or the Tm.

Embodiments comprise joining temperatures of 800 °C or higher, preferably 900 °C or higher, more preferably 1100 °C or higher. The desired joining temperature may be reached by increasing the temperature in specified heating rates, which are typically selected so that thermal stress for the intermediate product is minimized. Suitable heating rates are 0.5 to 5 °C/min. In accordance with the present invention these heating rates may however be lower or higher. The desired final joining temperature is then maintained for a suitable duration, in order to allow the desired reactive joining process to occur. Typical final heating times (i.e. duration of maintaining the intermediate product at the final heating temperature) may range from several minutes to several hours, and embodiments are in the range of from 10 to 100 minutes, preferably 20 to 80 minutes.

Such a heat treatment of the intermediate product ensures at least a partial dissolution of the ceramic member by the molten glass/glass-ceramic composition (or the precursor thereof), comprised in the joining layer. This results in an effective inter-diffusion of components of the ceramic member and the glass or glass-ceramic composition, which in turn induces a chemical reaction at their interface. The nature of this chemical reaction is not limited as far as in results in the formation of a reaction layer between the ceramic member and the joining layer.

The second step of joining may be carried out by pressurizing the ceramic member and the further member; however, it is a specific advantage of the present invention that the method can be carried out at atmospheric pressure, i.e. without any pressurizing of the intermediate product. The heating may be performed in an oxidizing or in an inert atmosphere, depending on circumstances.

This method ensures the provision of a joined product, which shows a high adhesion of the ceramic member and the further member. This can be explained by the at least partial dissolution of the ceramic layer by the molten the glass/glass-ceramic layer, which ensures an effective inter- diffusion and which induces a chemical reaction of components of both parts that in turn leads to the formation of a reaction layer. In consequence, the joined product shows a mechanically strong bonding of the members and thus provides an improved sealing of the components. This is even the case at high operating temperatures, such as 800 to 1000 °C, and even if the joined members show different coefficients of thermal expansion. The inventive method of reactive joining particularly ensures a stronger bonding/sealing, when compared to methods employing only a physical boning, such as adhesive or diffusion bonding.

The method optionally comprises a third step of subsequent controlled cooling, which

particularly results in the formation of crystalline areas in the ceramic member and/or the joining layer. In embodiments, the cooling rates are 10 to 200 °C/h, preferably 25 to 100 °C/h, more preferably 50 °C/h.

The heating temperature employed in the second step of joining is preferably lower than the sintering temperature of the ceramic member. This ensures that the composition of the ceramic member in the resulting joined product is not altered.

The inventive method is usually applied for joining of a ceramic member to a further member, wherein both members show similar coefficients of thermal expansion (CTE). In particular, the coefficient of thermal expansion (CTE) of the ceramic member may be in the range of 5 to 25 x 10^"6/°C, preferably 8 to 17 x 10^"6/°C. The CTE of the further member may be from 5 to 25 x 10^" ⁶/°C, preferably 8 to 17 x 10^"6/°C. However, the inventive method also enables the joining of a ceramic member to a further member, which shows differences in the coefficient of thermal expansion (CTE). In embodiments, the CTE of the ceramic member and the CTE of the further member differs in 1 x 10^~6/°C or less, preferably in 5 x 10^"6/°C or less, more preferably in 8 x 10^" ⁶/°C or less.

In any case, the CTE of the glass layer or glass-ceramic layer is preferably adjusted in that its value is between the CTE of the ceramic member and the CTE of the further member. In embodiments, the CTE of the glass layer or glass-ceramic layer may be from 6 to 16 x 10^"6/°C, preferably 8 to 14 x 10^"6/°C, more preferably 11 to 13 x 10^"6/°C. The CTE is determined by dilatometry measurement.

Ceramic member:

The ceramic member may be constituted of any ceramic material. In preferred embodiments, the ceramic member is an oxide ceramic that shows a perovskite structure. Preferably the perovskite structure is selected from CaTii__xFe_x0₃ (0<x<l, preferably 0.5<x<l, more preferably

0.75<x<0.95), Bai__xSr_xCoi__yFe_y0₃ (0<x<l, preferably 0.1<x<0.9, more preferably 0.25<x<0.75; and 0<y<l, preferably 0.1<y<0.9, more preferably 0.25<y<0.75), Lai__xSr_xCo0₃ (0<x<l, preferably 0.1<x<0.9, more preferably 0.25<x<0.75), Lai__xSr_xCoi__yFe_y0₃ (0<x<l, preferably 0.1<x<0.9, more preferably 0.20<x<0.65 and 0<y<l, preferably 0.4<y<0.95, more preferably 0.6<y<0.9). In Examples, the oxide ceramic member is selected from CaTi₀.₉Fe₀.iO₃ (CTF), Bao.₅Sro.₅Coo.₈Feo.₂O₅ (BSCF), Lao.₅Sr₀.₅Co0₃ (LSC) and Lao.₆Sro.₄Coo.₂Feo.sO₅ (LSCF). These oxide ceramics with perovskite structures can act as oxygen ion conductors, i.e. show O^2" conductivity, i.e. may be used as oxygen separation membranes, which are capable of transmitting oxygen ions continuously from one side of the membrane to the other. In addition, they can be used as oxygen separation materials for selectively transmitting to one side of a membrane oxygen from an oxygen-containing gas (such as air) supplied to the other side, particularly at high operating temperatures of 500 to 1000 °C.

Alternatively, the ceramic member may comprise a metal oxide, preferably MgO. Such metal oxides and in particular MgO can be used as s support for membrane structures. The metal oxide is present in porous form and serves as carrier and support for functional materials, such as oxygen conductors and/or electron conductors. In particular the present invention covers ceramic members comprising such a support in combination with an oxygen membrane material, such as the perovskite materials mentined above.

The ceramic member may have any shape, such as planar shape or a tubular shape. In a particularly preferred embodiment, the ceramic member is a tubular oxygen membrane mode from an oxide ceramic showing a perovskite structure. Alternatively, the ceramic member may be a tubular membrane with a porous MgO support in combination with an oxygen membrane.

Further member:

The further member can be made of any material. In embodiments the further member is selected from the group comprising ceramics, metals, metal-alloys, cermets, or semiconductor materials. Preferably, the further member is a metal, such as steel, preferably Kanthal APM steel. The further member may have any shape, such as planar shape or a pipe shape.

In a particularly preferred embodiment, the further member is a steel member, such as a steel pipe or steel adapter used for providing a connection from a ceramic member to a steel member (for example, if both members have a tubular shape).

Glass or glass-ceramic composition:

In principle, any glass or glass-ceramic composition, or any precursor thereof, may be employed in the inventive method. In other words, the chemical composition of the glass or glass-ceramic composition is not crucial as far as it is comprises components that can undergo a chemical reaction with diffusing components of the ceramic member.

In embodiments, the glass or a glass-ceramic composition comprises 5-70 mol% CaO, 5-45 mol% ZnO, 5-50 mol% B₂0₃ 1-85 mol% Si0₂. Any ranges for the composition given in the present invention are based on the total glass composition.

Further preferred embodiments of the glass or glass-ceramic composition and their effects are explained in the following. All these embodiments are presented within the context of the present invention, i.e. all these embodiments may be combined as they describe aspects of the glass or glass-ceramic composition.

CaO: The content of CaO in the glass or glass-ceramic composition may be 5-70 mol%. In embodiments, the glass or glass-ceramic composition comprises CaO in 25-60 mol%, preferably 35-55 mol%, and more preferably 45-50 mol%. More specifically, CaO preferably is the major component of the glass composition, i.e. may be present in 50 mol% or more. This amount of CaO ensures a coefficient of thermal expansion (CTE) that matches with the other components of joined product.

ZnO:

The glass or glass-ceramic composition may comprise 5-45 mol% ZnO. In embodiments, the ZnO content is 10-35 mol%, preferably 12.5-30 mol% and more preferably 17.5-25 mol%. ZnO acts as a nucleating agent in the glass or glass-ceramic composition. The presence of ZnO is this range ensures a sufficiently high and fast nucleation, which, on the other hand, leads to small crystallite sizes and a fine microstructure. ZnO also imparts a high stability against deformation under stress resulting in improved mechanical properties. The presence of ZnO in the

composition in furthermore enables, when the composition in accordance with the present invention is used in contact with a steel surface, the formation of a thin layer of ZnO near the interface with the steel surface, which protects the metal from corrosion or reaction with other components of the composition. This is an additional benefit of the composition in accordance with the present invention.

B₂0₃:

The glass or glass-ceramic composition also may comprise 5-50 mol% B₂0₃. In embodiments, the composition can comprise 10-45 mol%, preferably 15-30 mol% and more preferably 17.5-25 mol% B₂0₃. B₂0₃ acts as a glass former, i.e. decreases the viscosity and amount of crystallization of the glass or glass-ceramic composition. This content of B₂0₃ ensures a sufficiently low viscosity at the desired joining temperature.

Si0₂:

The glass or glass-ceramic composition may further comprise 1-85 mol% Si0₂. In embodiments, the glass composition comprises 2.5-45 mol%, preferably 5-30 mol% and more preferably 7.5-15 mol% Si0₂. This range ensures a sufficient glass forming capability of the composition, while keeping in preferred embodiments the Si0₂ content relatively low. Other components:

The glass or glass-ceramic composition is in preferred embodiments also substantially free of any of the elements Ba, Na and Sr, which means that each element of the group comprising Ba, Na and Sr is present in the composition in an amount of 1 mol% or less. Preferably, the composition also comprises any other alkali elements in an amount of 1 mol% or less. In further preferred embodiments, the content of any of these elements is 10 mmol% or less, more preferably 0.1 mmol% or less. This low Ba, Na and Sr content and optionally low content of other alkali metals suppresses the formation of chromates which would lead to failure of the sealing properties of the layer due to brittleness and thermal mismatch during operation (if in contact with a Cr containing surface, such as a steel support). In view of this, the life time, mechanical stability and durability of the joined product can be enhanced.

In further embodiments, the composition comprises no MnO, which means in embodiments that the content of MnO is 5 mol% or less, preferably 2.5 mol% and more preferably 1 mol% or less. Since MnO acts as a crystallization agent in a glass or glass-ceramic composition, a low content of MnO as indentified above enables a sufficiently low viscosity and appropriate amount of crystallization of the composition.

In embodiments, the composition comprises 35 mol% or less, preferably 20 mol% or less, more preferably 10 mol% or less MgO; and/or 35 mol% or less, preferably 20 mol% or less, more preferably 10 mol% or less AI₂O₃.

In further embodiments, the glass or glass-ceramic composition comprises 30 mol% or less, preferably 20 mol% or less, more preferably 10 mol% or less Na₂0; and/or 30 mol% or less, preferably 20 mol% or less, more preferably 10 mol% or less MgO.

The composition also may contain one or more other oxides, such as oxides selected from the group, comprising La₂0₃, Y₂0₃, PbO, Cr₂0₃, V₂0₅, NiO, CuO, Ti0₂, Zr0₂, As₂0₃, Sb₂0₃, A1₂0₃ and Fe₂0₃. These compounds may be used as additives to adjust the properties of glass composition, such as the coefficient of thermal expansion, the melting temperature, the glass transition temperature, the softening temperature, the viscosity, the elastic modulus, the surface tension, the adhesion, the crystallization behavior, the corrosion resistance and the diffusion properties of the glass composition. The effects of these elements in glass or glass-ceramic compositions are known in the prior art and will not be explained in detail. The inventive glass composition may contain these additives in usual amounts, such as in e.g. up to 5 mol%.

However, a preferred embodiment of the glass composition does not comprise any of these compounds, which means in embodiments that the content of the sum of these components is 5 mol% or less, preferably 2.5 mol% and more preferably 1 mol% or less.

Preferred embodiments of the glass or glass-ceramic-composition:

A preferred glass or glass-ceramic composition comprises 25-60 mol% CaO, 10-35 mol% ZnO, 10-40 mol% B₂0₃ and 2.5-45 mol% Si0₂. A more preferred glass composition comprises 35-50 mol% CaO, 12.5-30 mol% ZnO, 15-30 mol% B₂0₃ and 5-30 mol% Si0₂. A particularly preferred glass composition comprises 45-55 mol% CaO, 17.5-25 mol% ZnO, 17.5-25 mol% B₂0₃ and 7.5- 15 mol% Si0₂. These compositions particularly ensure a high adhesion to the members of the joined product, due to the formation of a reaction layer. Furthermore, these embodiments show a thermal coefficient of expansion that fits with the other members of the product, and thus ensures a sufficient life time, a high durability and a high chemical and mechanical stability of the joined product.

In a further preferred aspect of the present invention, the glass or glass-ceramic composition consists of the components CaO, ZnO, B₂0₃ and Si0₂, which may be present in any combination of the ranges as defined herein. The term "consists of means that the glass composition is substantially free of other components than CaO, ZnO, B₂0₃ and Si0₂. The term "substantially free" means in embodiments that the sum of any other components in the glass composition is 3 mol% or less, preferably 1 mol% or less, more preferably 10 mmol% or less, most preferably 0.1 mmol% or less.

In a further preferred embodiment, the sum of CaO, ZnO and B₂0₃ is 60 mol% or more, preferably 70 mol% or more and more preferably 80 mol% or more. This embodiment facilitates a particularly high adhesion, high CTE, excellent life time and high durability.

Physical properties and microstructure:

The glass-ceramic composition may show in embodiments a melting temperature T_m of 1200°C or less, preferably 1100°C or less, more preferably 1000°C or less. In further preferred embodiments, the glass transition temperature T_g of the glass composition is 1000°C or less, preferably 800°C or less, more preferably 600°C or less. The glass transition temperature and the melting temperature correspond to the onset temperatures as determined by differential scanning calorimetry (DSC).

This low melting temperature and/or glass transition temperature facilitates that the composition shows the ability of self-healing. The term "self-healing" is defined as the possibility to cure or close cracks or other leaks in the structure during operation of the joined product. This effect can be achieved, when the operating temperature of the joined product is higher than the melting temperature of the joining layer and/or the reaction layer. In this case, the joining layer and/or the reaction layer at least partially (re)melts and, thus, can refill any cracks and gaps that may have occurred in the sealing structure. Since the present composition may show a favorably low T_m, the composition can exhibit a high self-healing ability due to the increased fluidity that closes leaks or gaps.

In a further preferred embodiment, the glass-ceramic composition shows a predominantly crystalline structure, which preferably means that the composition comprises crystalline areas in 50 % or more, more preferably in 60 % or more, most preferably 75 % or more. The amount of crystalline areas is determined vie scanning electron microscopy (SEM) by visual inspection. Such a high content of crystallinity is achieved by the specific chemical constitution of the glass composition and/or by appropriately adjusting the sealing process

The glass-ceramic composition in accordance with the above embodiments also shows a fast crystallization behavior (i.e. preferably within 10 hours, more preferably within 5 hours, most preferably within 1 hour) and the final stable structure is quickly reached already after the joining step. This fast crystallization also results a microstructure with a high mechanical stability and durability, whereas slow crystallization and ageing will not change the microstructure significantly over time.

The crystalline areas in the glass-ceramic composition may be formed by one single crystalline phase. However, the present invention preferably encompasses glass-ceramic compositions with more than one crystalline phase, such as two or more than two crystalline phases. This ensures a particularly high mechanical stability and durability, while remaining a favorable high CTE.

Reaction layer: The reaction layer is formed in situ by a chemical reaction at the interface between the ceramic member and the glass or glass-ceramic composition constituting the joining layer during the second step due to the dissolution and inter-diffusion of components from the ceramic member and the glass or glass-ceramic composition. The chemical nature of this reaction layer is not limited but is generally may constitute one or more chemically distinct phases, which are formed as a result of the inter-diffused components of the ceramic layer and the glass or glass-ceramic composition.

The thickness of the reaction layer is not crucial. Preferred thicknesses are in the range of 1 to 10.000 μιη, preferably 10 to 5.000 μιη, more preferably 100 to 1.000 μιη.

The reaction layer may form a separate and chemically distinct layer, i.e. may show an interface to the ceramic member and the joining layer. However, it is preferred that the reaction layer forms a gradient of at least one physical or chemical parameter, which is different in the ceramic member and the glass or glass-ceramic composition, in a way that the difference in said parameter is gradually equalized. It is particularly preferred that said parameter is selected from the group, comprising the coefficient of thermal expansion (CTE) and the concentration of at least one component of the ceramic member and/or the glass or glass-ceramic composition. In other words, the reaction layer preferably shows no distinct interface towards the ceramic member and the joining layer, but rather forms a continuous gradient between the ceramic member and the joining layer. Such an embodiment particularly ensures a strong bonding of the ceramic member and the joining layer and thus provides an improved sealing.

A reaction layer formed in accordance with the present invention is illustrated in the examples and exemplified in figure 8. This figure shows that in this illustrative embodiment the gradient generated by the formation of the reaction layer concerns specific components of the joining material and the ceramic member. Namely, it can be seen that zinc and silicon, which are predominantly contained in the joining layer material, show a gradient from high values to low values in the direction from the joining layer material to the ceramic member. Titanium, on the other hand, being predominantly contained in the ceramic material shows a gradient in the other direction. Components which are present in both materials, such as calcium, do not show a corresponding gradient, but their respective concentrations remain relatively stable over the entire width of the reaction layer. Figure 8, in this context, furthermore shows that a reliable distinction between the ceramic member and the joining layer material and the reaction layer/reaction zone can be made by appropriate analysis, i.e. in the present example EDX analysis.

A reaction layer as illustrated in figure 8 is the gist and core of the present invention since it represents a novel interlayer between the material of the joining layer and the material of the ceramic member, which differs from the bonding layer obtained for example by the prior art diffusion process, where the molten material of the (metal based) joining layer diffuses into the member to be joined so that no novel phases are generated (as is the case in the present invention) but merely a finely dispersed system of the two distinct materials (metal component of the joining layer and component of the joined member) is given.

Joined product and use

The present invention also concerns a joined product of a ceramic member and further member, which is obtainable by the method as identified above. The invention encompasses joined products according to any combination of preferred embodiments identified above or in connection with the method, which likewise apply to the joined product.

In particular, the joined product may be an oxygen membrane element comprising a tubular oxygen membrane of an oxide ceramic showing a perovskite structure and a steel pipe. Potential applications of such oxygen membrane elements are oxygen purification means in place of low- temperature separation or PSA (Pressure Swing Adsorption). Alternatively, such oxygen separation membrane elements can also be used in the field of fuel cells, or in GTL (Gas to Liquid) technology in which a synthetic liquid fuel (methanol or the like) is produced by using oxygen ions supplied from one side of the membrane to the other to oxidize a hydrocarbon (methane gas or the like) supplied to the other side.

The oxygen membrane elements are used in high temperature-high pressure applications to produce oxygen, wherein the final joined product operates at temperatures of between 500 and 1000°C. On the one side of the membrane, a high pressure is applied, which creates an oxygen gradient. Alternatively, the driving force for oxygen transport across the membrane may be created by generating a vacuum on one side versus an atmospheric or small overpressure on the other side. The oxygen membranes, or more exactly Oxygen Transport Membranes (OTMs), are dense ceramic membranes which exhibit mixed conductivity of oxygen ions and electrons at elevated temperatures. Alternatively, OTMs are dense ceramic membranes on a porous support. OTM technology offers promising alternatives to oxygen production by cryogenic distillation for many applications like gasification of coal and biomass for power production, oxy-fuel combustion processes, partial oxidation for syngas production, etc. These membranes are of particular interest for integration in high temperature systems where the hot non-permeate gas can be used for power generation from gas turbines.

Tubular OTMs based on perovskite materials, such as CaTio.9Feo.1O3, are typical examples for this type of application, due to high stability over a large range of oxygen partial pressures at high temperature and in the presence of C0₂, a moderate thermal expansion coefficient of 12-13 x 10^" ⁶/°C and due to their reduced costs, when compared than other membrane materials employing rare elements. The tubular oxygen membranes may be produced by known methods, which e.g. involve extrusion processes.

Alternatively, the joined product may form a solid oxide fuel cell (SOFC) or a solid oxide electro lyser cell (SOEC) These SOFC's or SOEC's comprise an anode layer and a cathode layer and an ion conducting electrolyte interposed between these layers, which can be sealed by the joining method in accordance with the present invention. In other words, the anode layer and the cathode layer of the SOFC/SOEC can constitute the joined members in accordance with the present invention. In particular, the SOFC or SOEC includes commonly known set-ups comprising e.g. a porous strontium-doped lanthanum manganite (LSM) electrode, a dense yttria- stabilized zirconia (YSZ) electrode and a porous nickel-zirconium cermet (NZC) fuel electrode. These cells may be arranged in series of stacks, i.e. the SOFC/SOEC comprises a plurality of planar conductive sheets of thin layers of an anode and cathode and a layer of a ceramic ion conducting electrode disposed between the anode and the cathode, which is arranged as a stack. The operating temperature of the SOFC/SOEC is usually in the range of 700°C to 1000°C and in embodiments 650°C or lower, preferably 600 °C or lower.

The SOFC operates by electrochemically reacting fuel gas with an oxidant gas to produce DC output voltage. The SOEC acts in a reverse way by electrochemically generating a fuel gas under a DC input voltage by consumption of a gas such as CO, C0₂ or H₂0 or a combination thereof. Suitable fuel gases constitute CO, H₂0 or mixtures thereof. The joining method in accordance with the present invention is suitable to join/seal any area of a SOFC/SOEC cell or stack, i.e. the place/area and type of substrate is not specifically limited. The joining/sealing in particular ensures the separation of the fuel and the (oxidant) gas. Sealed areas are in preferably the edges of the cells or stacks, which is particularly suitable for the case of planar cell designs that are arranged in a stack. The method in accordance with the present invention is also suitable for joining/sealing further parts of the SOEC/SOFC, such as adjacent sheets. Furthermore, the method is suitable for sealing the external manifolds of the stack or for sealing the gas flow channels in an internally manifold of a SOFC/SOEC stack. However, the method in accordance with the present invention may also be used at other areas in the fuel cell that is operated at a high temperature.

The present invention also concerns the use of a glass or glass-ceramic composition for producing a joined product as defined above. In particular, the preferred embodiments of the glass or glass- ceramic composition as identified above ensure the formation of a reaction layer, i.e. a reactive joining of the ceramic member and the further member and, thus, enable an improved bonding and sealing of the joined product.

Figures:

The invention is further illustrated by the below examples and under reference to the following figures:

Figure la: DSC measurement of Glass 1 Figure lb: DSC measurement of Glass 2

Figure 2a: Linear thermal expansion curve obtained by dilatometry measurement of Glass 1 Figure 2b: Linear thermal expansion curve obtained by dilatometry measurement of Glass 2 Figure 3a: SEM micrograph of Crofer 22 APU steel sealed to YSZ using Glass 1 Figure 3b: SEM micrograph of Crofer 22 APU steel sealed to YSZ using Glass 2 Figure 4: Temperature resolved XRD spectrum of Glass 2

Figure 5 : Experimental setup of CTF tubular membrane in Kanthal sample holder before sealing Figure 6: SEM micrograph of the cross section of the setup after sealing at 1150 C.

Figure 7: SEM micrograph of the cross section of the reaction zone between the CTF and the glass.

Figure 8: EDX analysis of the reaction zone

Figure 9: SEM micrographs of the cross section of comparative example (upper picture and) inventive example (lower picture).

Examples

1. Preparation of glass compositions:

Two glass compositions in accordance with preferred embodiments of the present invention are prepared. Glass composition 1 (Glass 1) is prepared by mixing 48 mol% CaO, 19 mol% ZnO, 21 mol% B₂0₃ and 12 mol% Si0₂. Glass composition 2 (Glass 2) is prepared by mixing 50 mol% CaO, 20 mol% ZnO, 20 mol% B₂0₃ and 10 mol% Si0₂. As set out above, the glass compositions show a fast crystallization behavior, i.e. also can form a glass-ceramic composition under appropriate conditions.

The glass compositions are synthesized in the following manner: All reactants are mixed and transferred to a Pt crucible. The mixture is heated up to 1200 °C with a heating rate of 200 °C/h and kept at this temperature for 2 hours. Thereafter, the liquid glass melt is quenched by pouring the melt into water in order to obtain an amorphous starting glass. The chemical composition of the starting glass is identical to the mixture of the reactants. Subsequently, the glass composition is produced by milling the start glass in a ball-mill to obtain a powder with a particle size d₅o of less than 22 μιη.

2. Characterization of the glass compositions:

The thermal behavior of the glass compositions is evaluated by DSC measurement in a temperature range from 30 to 1050 °C in a Pt crucible, by employing 50 mg of the glass. The measurement is performed under argon (flow rate 40 ml/min) with a heating rate of 10 °C/min. Figures la and lb show DSC curves of Glass 1 and Glass 2, respectively, revealing glass transition temperatures of 565°C/594°C (Tg onset). Crystallization of Glass 1 and Glass 2 starts at 660°C and 700°C, respectively. Glass 1 and Glass 2 show melting points (Tm onset) of approximately 990°C.

The coefficient of thermal expansion (CTE) is obtained by dilatometry measurement, which is performed on sintered glass bars in argon (flow rate 50 ml/min) with a heating rate of 3 °C/min in a temperature range from 25 °C. Figure 2a shows dilatometry measurement results of three samples of Glass 1 (amorphous state, glass-ceramic state and partly crystallized state) and reveals CTE values of 1 l ^ lO^K^"1, 1 l ^ lO^K ¹ and 12.0* lO^K ¹, respectively. Figure 2b shows a dilatometry measurement result of a sample of Glass 2 (glass-ceramic state) and reveals a CTE value of 12.0* lO^K^"1.

The adhesion behavior of Glass 1 and Glass 2 on steel is measured by the following method: The glass composition is applied in the form of powder. YSZ (Y₂0₃-Zr0₂) with 8 mol% Y₂0₃ is produced by tape casting and sintering. The YSZ electrolyte has a thickness of 200 μιη after sintering. Crofer22APU (W.-Nr. 1.4760, ThysenKrupp VDM, Werdohl, Germany) with a thickness of 230 μιη is used as ferritic steel. All materials are cut into 2 cm x 2 cm pieces and joining is conducted by placing the glass powder of Glass 1 and Glass 2, respectively, between the YSZ and the steel. To ensure contact during sealing a load of 4 kg is applied. The assemblies are heated in air with 100 °C/h to the final sealing temperature of 800 °C (Glass 2) and 925 °C (Glass 1), respectively. After being held for 20 min at these temperatures, the samples are cooled down at a cooling rate of 100 °C/h to room temperature.

These samples were analyzed by SEM/EDX in the following manner: The samples including the glass sealing are vacuum embedded in Struers epoxy resin (epofix), ground using SiC paper, polished using 6,3 μιη and 1 μιη diamond paste, and are then carbon coated to eliminate surface charging. Images are taken on a Zeiss Supra 35 scanning electron microscope (SEM) equipped with a field emission gun and an EDS detector in backscattered mode with an acceleration voltage of 15 kV.

Figures 3a and 3b show SEM micrographs of the samples employing Glass 1 and Glass 2, respectively. These micrographs illustrate that the glass compositions shows good adhesion and wetting to the steel surface due to the fact that no cracks, voids or delamination is found. The crystallization behavior of the Glass 2 is analyzed by X-ray diffraction analysis (XRD) by recording a temperature resolved XRD spectrum from 30 °C up to 900 °C. XRD spectra in a 2 theta range from 10 to 60° are taken in air with an interval of 5 °C and a heating rate of 60 °C/min between the measurements. Figure 4 shows a temperature resolved XRD spectrum of Glass 2 and reveals the formation of Ca₂ZnSi₂07 (hardystonite), CaZnSi₂0₆, ZnO and Ca₂B₂0₅ crystals at different temperatures. The crystalline areas have an average diameter of crystalline domains of 500-800 nm. The average diameter is visually detected by measuring the average diameter of crystalline domains.

3. Production a joined product of a ceramic oxide membrane and a steel pipe

A joined product comprising a tubular CaTio.8Feo.2O3 (CTF) oxygen membrane as ceramic member and a Kanthal APU steel sample as further member is produced as follows. The CTF tube is inserted into a steel sample holder and the voids are filled with Glass 1, i.e. a pulverized glass composition having the composition 48 mol% CaO, 21 mol% B₂0₃, 19 mol% ZnO and 12 mol% Si0₂. Subsequently, the intermediate product is heated to 1150 °C for 20 min. The experimental setup is shown in Figure 3. At this temperature, the glass melts and reacts with CTF tube under the formation of a reaction layer. Thereafter, the joined product is cooled to room temperature at a cooling rate of 100 °C/h.

4. Characterization of the joined product of a ceramic oxide membrane and a steel pipe:

The joined product is analyzed by SEM/EDX in the following manner: The sample is vacuum embedded in Struers epoxy resin (epofix), ground using SiC paper, polished using 6,3 μιη and 1 μιη diamond paste, and are then carbon coated to eliminate surface charging. Images are taken on a Zeiss Supra 35 scanning electron microscope (SEM) equipped with a field emission gun and an EDS detector in backscattered mode with an acceleration voltage of 15 kV.

Figure 6 shows an SEM micrograph of the cross section of the setup, wherein the dotted line marks illustrate the originally dimension of the CTF tube and indicates the area of the reaction layer, whereby a sharp interface is not visible. The area of the reaction zone between the glass and the membrane tube is lager than 100 μιη. The presence of a reaction layer is confirmed by detailed SEM/EDX analysis as shown in figure 7, which illustrates a large inter-diffusion of elements from the glass into the CTF tube as well as from CTF tube into the glass. Figure 8 also shows a EDX analysis of the reaction zone. The circles designated as 1 to 6 mark the area where EDX analysis were performed. A gradient for Zn and Si, which are part of the glass ceramic, is found (decreasing from the left to the right end) as well as for Ti, which is part of the CTF tube (increasing from left to right).

A summary of the EDX analysis is given in the below table:

Comparative Example

As comparative example, an analogous experiment is performed but with using conventional C- glass instead of Glass 1 in order to show the result of conventional adhesive bonding. C-glass is constituted of 64% Si0₂, 4.1 % Al₂0₃*Fe₂0₃, 13.4 % CaO, 3.3 % MgO, 9,6 % Na₂0*K₂0, 4.7 % B₂03, and 0.9 % BaO. The C-glass shows a softening point of 750 °C, and a CTE of

6.3* 10^~6/°C.

Figure 9 shows a SEM micrograph illustrating that conventional adhesive bonding shows a sharp interface between the tubular oxygen membrane and the steel sample (upper picture), proving that no reactive joining has taken place. In contrast thereto, the lower picture of Figure 9 shows a joined product according to the present invention.

Claims

1. Method of producing a joined product by reactive joining a ceramic member to a further

member, wherein the method comprises: a first step of providing a joining layer between the ceramic member and the further member in order to form an intermediate product, wherein the joining layer comprises a glass or glass- ceramic composition, or a precursor thereof, a second step of joining the ceramic member to the further member by heating the

intermediate product to a specific temperature, wherein the specific temperature is lower than the melting temperature of the ceramic member, and wherein the specific temperature is higher than the glass transition temperature of the glass composition or higher than the melting temperature of the glass-ceramic composition, and wherein the heating forms a reaction layer between the ceramic member and the joining layer by inter-diffusion and chemical reaction of components of the ceramic member and the joining layer.

2. Method according to claim 1, wherein the specific temperature is lower than the sintering temperature of the ceramic member.

3. Method according to of claims 1 or 2, wherein the thermal coefficient of expansion (TCF) of the ceramic member is in the range of 5 to 25 x 10^"6/°C, preferably 8 to 17 x 10^"6/°C; and/or wherein the TCE of the further member is from 5 to 25 x 10^~6/°C, preferably 8 to 17 x 10^~6/°C; and/or wherein the CTE of the glass layer or glass-ceramic layer is from 6 to 16 x 10^~6/°C, preferably 8 to 14 x 10^"6/°C, more preferably 11 to 13 x 10^"6/°C, wherein the CTE is determined by dilatometry measurement.

4. Method according to any of claims 1 to 3, wherein the ceramic member is an oxide ceramic that shows a perovskite structure, which is preferably selected from CaTii__xFe_x03 (0<x<l, preferably 0.5<χ<1, more preferably 0.75<x<0.95), Bai__xSr_xCoi__yFe_y0₃ (0<x<l, preferably 0.1<x<0.9, more preferably 0.25<x<0.75; and 0<y<l, preferably 0.1<y<0.9, more preferably 0.25<y<0.75), Lai__xSr_xCo0₃ (0<x<l, preferably 0.1<x<0.9, more preferably 0.25<x<0.75), Lai__xSr_xCoi__yFe_y0₃ (0<x<l, preferably 0.1<x<0.9, more preferably 0.20<x<0.65 and 0<y<l, preferably 0.4<y<0.95, more preferably 0.6<y<0.9), and which is more preferably selected from CaTi₀.₉Fe₀.iO₃ (CTF), Bao.₅Sro.₅Coo.sFeo.2O3 (BSCF), Lao.₅Sr₀.₅Co0₃ (LSC) and

La₀.₆Sr₀.₄Coo.₂Feo.₈0₃ (LSCF), and/or wherein the ceramic member comprises a non- perovskite metal oxide, preferably MgO.

5. Method according to any of claims 1 to 3, wherein the further member is made of a material selected from the group comprising ceramics, metals, metal-alloys, cermets, or semiconductor materials, preferably metals such as steel.

6. Method according to any of claims 1 to 4, wherein the joined product is a tubular oxygen membrane joined to a steel pipe.

7. Method according to any of claims 1 to 8, wherein the glass or a glass-ceramic layer is formed of a glass composition, comprising 5-70 mol% CaO, 5-45 mol% ZnO, 5-50 mol% B₂O₃ 1-85 mol% S1O₂, based on the total glass composition.

8. Method according to claim 9, wherein the glass composition comprises 25-60 mol%,

preferably 35-55 mol%, more preferably 45-50 mol% CaO; and/or 10-35 mol%, preferably 12.5-30 mol%, more preferably 17.5-25 mol% ZnO; and/or 10-40 mol%, preferably 15-30 mol%, more preferably 17.5-25 mol% B₂O₃; and/or 2.5-45 mol%, preferably 5-30 mol%, more preferably 7.5-15 mol% S1O₂.

9. Method according to claim 9 or 10, wherein the glass composition comprises 35 mol% or less, preferably 20 mol% or less, more preferably 10 mol% or less MgO; and/or 35 mol% or less, preferably 20 mol% or less, more preferably 10 mol% or less AI₂O₃,

10. Method according to any of claims 9 to 11, wherein the glass composition comprises 30

mol% or less, preferably 20 mol% or less, more preferably 10 mol% or less Na₂0; and/or 30 mol% or less, preferably 20 mol% or less, more preferably 10 mol% or less MgO.

11. Method according to any of claims 9 to 12, wherein the glass composition comprises one or more compounds of the group, comprising La₂0₃, Y₂0₃, PbO, Cr₂0₃, V₂0₅, NiO, CuO, Ti0₂, Zr0₂, As₂0₃, Sb₂0₃, Fe₂0₃, SrO, and BaO, preferably in a total amount of 5 mol% or less.

12. Method according to any of claims 1 to 5, wherein reaction layer forms a gradient of at least one physical or chemical parameter, which is different in the ceramic member and the glass or glass-ceramic composition, in a way that the difference in said parameter is gradually equalized.

13. Method according to any of claims 1 to 7, wherein said parameter is selected from the group, comprising the thermal coefficient of expansion (CTE) and the concentration of at least one component of the ceramic member and/or the glass or glass-ceramic composition.

14. Joined product of a ceramic member and a further member, which is obtainable by a method according to any of claims 1 to 13.

15. Use of a glass or a glass-ceramic composition for producing a joined product in accordance with claim 14.