WO2010099634A1

WO2010099634A1 - Sealing technology

Info

Publication number: WO2010099634A1
Application number: PCT/CN2009/000240
Authority: WO
Inventors: Jinsong Zhang; Chuanyong Hao; Zhenglin Li; Xianfeng Mao
Original assignee: Institute Of Metal Research, Chinese Academy Of Sciences; Bp Alternative Energy International Limited
Priority date: 2009-03-06
Filing date: 2009-03-06
Publication date: 2010-09-10

Abstract

An oxide composition comprising Ag and Cu, characterised by the alloy additionally comprising an element X, where X is Al and/or Pd, and its use as a seal for joining or sealing a porous ceramic with a ceramic of lower porosity.

Description

SEALING TECHNOLOGY

This invention relates to seals between metal and ceramic surfaces, more specifically to seals that can be used in hermetically joining a ceramic material used in selective separation, or as a support for a selective separation membrane, with a metallic surface.

The use of membrane technology is known for selectively separating certain components from mixtures of components. For example, US 5,554,286 describes the separation of water from ethanol using a zeolite membrane, WO 2008/074181 describes the separation of oxygen from air using ceramic membranes, and WO 2005/065806 describes a palladium membrane for selectively separating hydrogen from a hydrogen- containing gaseous mixture.

The composition of the membrane depends on its application. For example, in US 5,554,296 the porous properties of the zeolite are responsible for the separation, the pores being large enough to allow transport of water through the porous structure, while being too small to allow ingress of alcohol molecules. In WO 2008/074181 non-porous ceramic membranes separate oxygen by allowing the transport of oxide ions through the ceramic structure, while simultaneously allowing transfer of electrons to enable charge neutrality across the membrane to be maintained. In WO 2005/065806, a non-porous palladium film deposited on a ceramic support acts as a selective hydrogen-permeable membrane. In the practical application of a selectively permeable membrane, the membrane typically needs to be attached to a reactor or separation vessel, which is usually of metallic construction, for example iron, steel or stainless steel. As membranes are often made from, or are supported on a ceramic material, a means of securely attaching the ceramic membrane material or support to the metallic vessel is required. Furthermore, the attachment must be made hermetically so as to prevent leakage of undesired components which would reduce the selectivity of the separation process for which the membrane is used.

WO 2007/054462 describes a process for producing a high temperature ceramic to metal seal, comprising preparing and densifying a pre-form made from 10-90% of a glass mixture and 10-90% of at least one ceramic, which is placed between the metal and ceramic surfaces to be joined and heated to provide the seal. A problem with glass to metal seals, however, is that bonding between the glass and metallic surfaces tends not to be strong.

Another method of sealing ceramics to metals is to use a metal or alloy which, through processes such as brazing, can provide an effective join to the metal, and also to the ceramic. In order to be effective, the alloy seal must have good so-called "wettability" properties, in that it can at least partially interact or react with the ceramic surface.

GB 1,151,473 describes a method of sealing a ceramic with a refractory metal in which a non-porous layer of molybdenum and/or tungsten is deposited on a ceramic surface and brazed to provide a bond between the refractory metal surface and the coated ceramic surface. Ruthenium-molybdenum and rhodium molybdenum alloys are stated to have good wettability characteristics.

There remains a need for an alternative seal and method of sealing ceramic and metallic surfaces.

According to a first aspect of the present invention, there is provided an alloy composition comprising Ag and Cu, characterised by the alloy additionally comprising an element X, where X is Al and/or Pd.

Copper is typically present in the alloy in an amount of from 10 to 50 wt%, preferably 20 to 40 wt%, for example 25 to 30 wt%.

When present in the alloy, the Al content is typically in the range of from 0.1 to 20wt%, for example 0.5 to 10wt%, such as 2 to 8 wt%.

When present in the alloy, the Pd content is typically in the range of from 0.1 to 20wt%, for example 0.5 to 10wt%_s such as 2 to 8 wt%.

Other elements, typically silver, make up the balance, silver preferably comprising 80wt% or more of the balance. In a preferred embodiment, Titanium is also present in the alloy, preferably in an amount of from 0.1 to 20 wt%, for example 1 to 10wt%, such as 2 to 8 wt%.

In a preferred embodiment, the alloy consists of Ag, Cu, Ti and X, where X is Al and/or Pd, and comprises for example from 25 to 30 wt% Cu, from 2 to 8 wt% Al, from 2 to 8wt% Pd, from 2 to 8 wt% Ti, the balance being silver. In a further embodiment, the alloy comprises 5wt% Ti, 27wt% Cu, 5wt% Al and 63wt% Ag.

The alloy can be placed between a ceramic and a metallic surface which, after melting and setting, provides a hermetic seal that effectively prevents fluids permeating between the metal and ceramic surfaces. By a hermetic seal is meant that the seal is able to prevent greater than 99.50%, preferably greater than 99.90%, and even more preferably greater than 99.99% of a fluid or mixture of fluids permeating through the seal under operating conditions. It has been found that the alloy of the present invention has good wettability properties for the ceramic, and is able to interact or react effectively -with the ceramic material to provide a strong adherence thereto, and is also able to adhere well to a metallic surface, which thus enables an effective seal between a metallic and ceramic surface to be achieved. The seal can be formed by using techniques such as brazing, preferably under inert atmosphere, such as a nitrogen and/or argon atmosphere or under vacuum. In one embodiment a reducing atmosphere is used, such as a hydrogen-containing atmosphere, in which the atmosphere can be pure hydrogen, or optionally hydrogen diluted by an inert gas, such as nitrogen or argon, for example as a mixture of 20% by volume or less in inert gas. In a further embodiment, additives or fillers (often referred to as "flux") are used to help reduce any oxidation, a typical example being borax.

Another advantage of the alloy of the present invention is that it can act as an effective seal between a metal and ceramic, and can remain effective at high temperatures. It is also chemically resistant, for example to oxidation. This is important where the seal is in contact with oxidising environments, for example when in contact with oxygen- containing gases or high temperature steam.

The alloy of the present invention can be used to prepare a seal between two surfaces, where the sealed surfaces are to be used in high temperature applications. The alloy and alloy seal are stable, for example, under sustained temperatures of up to 700⁰C, for example up to 65O⁰C. The alloy can be prepared by melting together the constituent elements in the desired ratio, and then cooling the resulting melt. Cooling can be passive, for example by allowing the melt to cool gradually to ambient temperatures by removing the source of heat, or removing the melt from the heat source. Alternatively, a rapid quenching method can be used, for example by contacting the melt with a cooled surface, such as a water-cooled copper surface as described in US 4,678,720. Melting is usually conducted under an inert atmosphere, for example under an argon atmosphere, to prevent oxidation. In one embodiment, inductive melting is used to melt the metals forming the alloy. There are a number of ways in which an alloy seal can be made between a metal surface and ceramic surface which are to be joined and sealed. In one embodiment, a brazing technique is used. Brazing is a joining process in which an alloy is heated to a melting temperature, typically 45O⁰C or more, which enables melted alloy to penetrate the gap between two surfaces. On cooling, the alloy solidifies, sealing the gap. The high temperatures typically involved in brazing often cause alloying to take place between the alloy seal and the metallic surface which is being joined or sealed, which contributes to the high strength and stability of the seal.

The alloy to be used as the sealing material will have a lower melting point than the surfaces to be joined. The alloy compositions can be used to provide a seal where at least one of the surfaces is a metallic surface with a greater melting point than the alloy seal, for example iron, steel, and stainless steel. The melting point of the alloy is typically in the range of from 45O⁰C to 1400⁰C, for example in the range of from 600 to 1100⁰C or 850 to 1000⁰C. The alloy is particularly suitable for providing a seal between a metallic surface and a ceramic surface. The ceramic can be porous or non-porous. As an example, alumina can be prepared in a porous form, with surface areas of 100 m g^" or more, for example in the range of from 100 to 300m²g⁴ • Such forms of alumina generally have a bulk density in the range of from 0.6 to 1.4 g/mL at 2O⁰C. Alumina can also exist in a higher density and lower porosity form. Such high density, low porosity alumina can be prepared by sintering porous, lower density alumina at temperatures in the range of from 1500 to 2000⁰C₅ for example in the range of from 1650 to 175O⁰C. The resulting sintered alumina has a density typically in the range of from 3.0 to 4.1 g/mL at 2O⁰C, for example in the range of 3.5 to 4.0 g/mL at 2O⁰C. Surface areas of such materials tend to be less than 100m² g^"1, for example less than 50m²g^"1, or 20m²g^"1.

It is preferable for the ceramic to be a low porosity ceramic. The surface of high porosity ceramics can be quite loosely bound, and hence can become detached from the bulk relatively easily. Thus, although a seal may be strong, the ceramic surface may be prone to cracking or flaking, thus reducing the effectiveness of the seal. The structure can weaken further when exposed to cycling of high and low temperatures. With lower porosity ceramics, the surface tends to be more strongly bound to the bulk of the material, which makes a seal stronger and less prone to degradation during temperature cycling. The alloy of the present invention is particularly suitable for providing a seal between a metallic surface, and the surface of a low porosity ceramic material, for example a ceramic material having a surface area of less than 100

preferably less than 50 m²g^"1, more preferably less than 20 m²g^"1. The linear thermal expansion coefficient of the alloy is typically in the range of from greater than 5 xlO^"6 to less than 20 x 10^'6 m m^'1 K^"1 at 2O⁰C, and preferably in the range of from greater than 7 x 10^"6 to less than 20 x 10^"6 m m^"1 K^"1 at 2O⁰C, for example in the range of from 13 x 10^"6 to 19 x 10-⁶ m m ¹ K⁴ at 2O⁰C.

The alloy can be used as a seal between a metal and a ceramic surface. The metal surface can itself be an alloy, for example steel, stainless steel or other iron-containing alloys, for example iron-chromium alloys. Examples of stainless steels include those comprising one or more of Cr Ni and Ti (in addition to Fe), such as lCrl8Ni9Ti steel comprising 18wt% Cr, 19wt% Ni, <0.8wt% Ti₅ < 0.12wt%C, the balance being iron. This material mainly adopts the austenite structure. Preferably, the metal surface is chosen so as to minimise the differences in the linear thermal expansion coefficient and the ceramic, while maintaining its integrity to perform its own function, for example as a support for attaching the ceramic to a reactor or other vessel. In one embodiment, the surface is a ICr 13 martensite steel (comprising < 0.15wt%C, 18wt% Cr, the balance being iron, and being mainly of martensite structure), which has a relatively low linear thermal expansion coefficient compared to stainless steels with the austenite structure (12 x 10^"6 m m^"1 K^'1 at 2O⁰C), while maintaining physical and chemical resistance at high temperatures. In a further embodiment, the metal can be 4J33, which has a composition of 33wt% Ni, 15wt%Co, and the balance being Fe. Its linear expansion coefficient is 6 x 10^"6 to 8 x 10^"6 m m^"1 K^"1, which is even lower than that for lCrl3. In a further embodiment of the invention, the ceramic is sealed to a low expansion metal, for example 4J33 steel, which in turn is attached to a higher expansivity metal, for example a lower cost steel that is used in or forms part of a reactor or other vessel. This acts to minimise further the differences in thermal expansion coefficients between metal and ceramic surfaces, thus further reducing stresses to which the alloy seal of the present invention is exposed during temperature cycling. It also means that the whole reactor or vessel to which the ceramic needs to be joined does not need to be fabricated from low expansivity metals or alloys, which are generally significantly more costly than conventional construction metals, such as iron, steel and stainless steel.

In another aspect of the present invention, an alloy seal between a metal and ceramic surface is prepared by creating the alloy in situ during the sealing process. This has been found to be a suitable method for sealing processes which employ melting of the alloy, such as brazing, particularly where one or more of the metals can have a tendency to separate out from the rest of the alloy as a separate phase, thus creating inhomogeneity in the alloy composition. Such an effect has been observed in AgCuTi alloys, as noted for example in US 4,678,720, which describes the production of a AgCuTi foil by a rapid quenching method, to produce an alloy comprising an even distribution of fine titanium- containing phases. However, there remains a need for an alternative method of producing an alloy which can be used to form seals, for example between metal and ceramic surfaces.

Thus, according to a second aspect of the present invention, there is provided a process for producing an alloy seal between a first material and a second material, which alloy comprises preliminary alloy metals and a further alloy metal, which process comprises applying to the gap between surfaces of the first and second materials the further alloy metal, and a preliminary alloy comprising the preliminary alloy metals, and heating the further metal, the preliminary alloy and the first and second materials to a temperature below the melting temperature of the first and second materials, and above the melting temperature of the preliminary alloy.

In the process of this second aspect of the present invention, an alloy seal, comprising preliminary alloy metals and a further alloy metal (optionally more than one further alloy metal), can be prepared in situ during seal formation, for example during brazing, in which the preliminary and further alloy metals are separately applied to the gap between the surfaces of the two materials to be joined or sealed. The preliminary alloy metals are preferably themselves applied in the form of an alloy, herein a "preliminary alloy", such that the melting temperature of the preliminary alloy is lower than that of the two materials to be sealed. The seal can be formed by heating to a temperature above the melting point of the preliminary alloy, but below the melting temperature of the materials forming the first and second surfaces, for example through brazing at temperatures of 45O⁰C or more, and typically 1400⁰C or below, for example 1200⁰C or below. The method of the present invention is useful where one or more of the further alloy metals has a melting point above that of one of the materials to be joined or sealed. For example, in cases where one of the materials is iron, steel, stainless steel or some other alloy of iron, then titanium can be a further alloy metal (melting point of titanium is 1668⁰C, whereas that of iron is 1538⁰C. Steels generally have lower melting points than iron, depending on the specific grade).

The first and the second material can be the same or different. The sealing method is particularly suitable for providing a seal between a ceramic material and a metallic material, the ceramic preferably being a low-porosity ceramic, for example having a surface area of less than 100 m²g^'1, preferably less than 50 m²g^"1, more preferably less than 20 m²g^"1. The ceramic surface can be, for example, a low-porosity, high density refractory oxide, such as sintered alumina as described above. The metal can be iron or an alloy thereof, for example steel or a grade of stainless steel.

The further alloy metal can be applied to one of the surfaces to be joined or sealed. In one embodiment, this can be achieved by sputtering the further alloy metal thereon. Alternatively, a foil of the further alloy metal can be used, which can be pressed against one of the surfaces or placed between the two surfaces to be sealed. The layer should be thick enough to ensure sufficient metal is available to form the alloy, while being thin enough to ensure that the desired alloy can be created across the thickness of the layer. The thickness of the further alloy metal is typically 20μm or less, and 1 μm or more, for example in the range of from 1 to 20 μm or preferably 2 to 10 μm.

An advantage of the method of the present is that it is a relatively simple process to form an alloy comprising a high melting temperature metal, and also enables an effective alloy seal to be prepared in situ, which reduces the complexity of the sealing process. The brazing time is dependent on the individual alloy, but is typically in the range of from 1 minute to 16 hours, for example in the range of from 1 minute to 1 hour.

In one embodiment, the alloy is an alloy as described above in relation to the first aspect of the present invention, and comprises Ti, Ag, Cu and Al. Ti is the further alloy metal. Ti foil (as the further alloy metal) is placed in the gap between the two surfaces to be sealed, for example a ceramic surface, such as a low porosity, high density alumina as herein described, and a metal surface, such as iron, steel or a stainless steel. The preliminary alloy, comprising Ag, Cu and Al as the preliminary alloy metals, is also applied to the gap between the two surfaces, and heating is carried out to enable the seal to be formed, typically through brazing. During the heating process, the preliminary alloy melts, enters the gap between the two surfaces (for example through capillary action) either on one side of the further alloy metal foil or both sides. During the heating or brazing process, the titanium (as the further alloy metal) and the preliminary alloy combine to produce the desired alloy incorporating the Ti during the heating or brazing process.

In an alternative embodiment of the invention, the further alloy metal is coated onto one or both of the surfaces to be joined, for example by sputtering. The preliminary alloy can be applied between the gap, and heated as previously described. In a further embodiment of the invention, there can be more than one further alloy metal. For example, an alloy can be prepared by applying one further alloy metal layer on one of the surfaces to be joined, and a different further alloy metal layer on the other surface, before applying the preliminary alloy before heating or brazing.

As already mentioned above, there are certain disadvantages of providing a direct seal between a metal and a porous ceramic surface using an alloy composition as a seal.

In the preparation of membranes, for example those used for the purposes of selective separations, it is often advantageous for the membrane material itself to be very thin, not least because a thinner membrane generally proffers improved separation kinetics. As thin membranes are often fragile and not capable of being self-supported, they are often deposited onto a porous support, for example a porous ceramic support such as alpha- alumina. This provides the mechanical strength required for the membrane to maintain its integrity, while the porosity allows components to flow to and from the membrane. Thus, where a ceramic membrane or ceramic membrane support is to be connected to a metallic vessel, there remains a need for a metal to ceramic connection of improved stability, in particular improved stability to high temperatures and to thermal cycling.

According to a third aspect of the present invention, there is provided an apparatus comprising a metal, a first ceramic which is porous and a second ceramic with lower porosity than the first ceramic, in which the second ceramic is sealed to the first ceramic by a first seal, and the metal is sealed to the second ceramic by a second seal. In one embodiment, the first seal is an oxide seal, and the second seal is an alloy seal. Preferably, the seals are hermetic. When an alloy is used to provide a seal between two surfaces, the process of creating the seal typically involves melting the alloy and allowing it to re-solidify. In this process, where a porous ceramic is one of the surfaces, the molten alloy at least partially reacts with the ceramic surface, which enables the join or seal to be made. In addition, during heating or brazing, the alloy itself partially alloys with the metallic surface to which the ceramic surface is to be joined, which also provides a strong bond or seal.

However, as already described above, it has been found that the bond between the alloy and a porous ceramic surface can be quite unstable when subjected to thermal cycling, which is exacerbated where there are imperfections in the ceramic surface. In addition, as metals and ceramics often have very different thermal expansion coefficients, there is still a large amount of stress on the alloy joint or seal when it is exposed to high temperature conditions, or conditions of thermal cycling. Furthermore, where the ceramic material comprises compressed particles of ceramic, typically associated with low density ceramics, such as porous alumina membrane support tubes, the mechanical integrity of the ceramic material itself can be quite low, which also can suffer damage when thermal stresses are applied.

It has now been found that, when sealing or joining a porous first ceramic material with a metal surface, the above problems can be overcome by sealing the surface of the first ceramic material and the surface of the metal with a second ceramic which has lower porosity, and often higher density, than the first ceramic.

The seal between the first and second ceramic materials can be achieved using an oxide seal, such as a mixed oxide seal. The oxide is often in the form of a glass, preferably one in which there is an even or homogeneous distribution of the different elements of the oxide. Oxides can provide strong seals between two ceramic surfaces, and are effective in preventing passage of fluid compositions (i.e. are hermetic), and also are more resistant to degradation from thermal effects.

The low porosity ceramic (second ceramic) preferably has a thermal expansion coefficient similar to that of the high porosity ceramic (first ceramic). This is advantageous in that thermally-induced stresses between the joined and/or sealed ceramic surfaces are reduced or minimised. Typically, the low porosity ceramic surface is capable of withstanding thermal expansion differences between it and a metallic surface or support to which it may be attached. Another advantage of sealing the first ceramic and metal to a lower porosity ceramic is that porosity ceramics tend to have greater mechanically integrity than higher porosity ceramics, and are more resistant to degradation, for example when subjected to thermal cycling. It has also been found (as described above) that an alloy seal can adhere strongly and provide an effective hermetic seal between the low porosity ceramic surfaces and metallic surfaces.

The first and second ceramic can be selected from alumina, silica, zirconia, hafnia, titania, magnesia, any lanthanide oxide, or combinations of two or more thereof.

The surface area of the first ceramic is typically 100m g^" or more, for example in the range of from 100 to 1000m²g^"1, such as 100 to 300m²g^"1. The second ceramic generally has a surface area of less than

for example less than 50m²g^"1, such as less than 20m²g^"1. The first and second ceramic can be of the same or different compositions. For example, the first ceramic can be porous alumina, with a surface area of 100m²g^"1 or more, for example in the range of from 100 to 300m²g^"1, with a typical density in the range of from 0.6 to 1.4 g/mL (at 2O⁰C), and the second ceramic can be a sintered alumina, with a surface area of less than 100 m g , for example less than 50 m g^' , such as less than 20 m²g^"1, and having a typical density in the range of from 3.0 to 4.1 g/mL, for example 3.5 to 4.0 g/mL (at 2O⁰C).

The second seal, i.e. the seal between the second ceramic and the metal, is preferably an alloy seal produced, for example, by a brazing method. Advantageously, the alloy is an alloy as herein described in relation to the first aspect of the present invention.

The first seal, i.e. between the first ceramic and the second ceramic, is preferably in the form of an oxide. In one embodiment, the oxide is in the form of a glass, in which there is little or no long-range crystalline structure, but wherein the elements in the oxide are evenly distributed.

The seal is created by applying oxide sealing material, for example as glass granules, powder or beads, in the gap between the first and second ceramic surfaces, and heating to high temperature, for example temperatures in the range of from 800 to 2000⁰C. Preferably, the temperature should be lower than the sintering temperature of the first ceramic, for example a temperature of 1500⁰C or lower and/or 1000⁰C or more, such as in the range of from 1000 to 1500⁰C or 1400 to 1500⁰C. Producing the seal does not require an inert atmosphere, although one can be used if desired. As the formation of the oxide (first) seal is usually conducted at higher temperatures than those used for producing the second (alloy) seal, it is usually advantageous to prepare the first seal before the second seal, to prevent excessive temperatures affecting the stability and integrity of the second seal. Where alloy seals are used to provide the second seal, i.e. to provide a join or seal between the metal surface and the second ceramic, the metal is preferably chosen from those that melt at higher temperatures than the alloy seal material. The melting point of the metal is preferably greater than about 1200⁰C. Examples include alloys of titanium, alloys of nickel, iron, steel, stainless steel and other alloys of iron. The alloy seal can be an alloy as herein described in relation to the first aspect of the present invention.

In one embodiment, the first ceramic supports a membrane that is used for selectively separating one or more compounds from a fluid mixture. The membrane can be a zeolite, for example for example zeolite 3 A or 4 A as used in removing water from organic liquids, such as a mixture of water with one or more alcohols such as C₁ to C₄ alcohols. The membrane can be palladium, optionally additionally comprising silver and/or copper, which can be useful in the selective separation of hydrogen from a gaseous mixture, for example a mixture additionally comprising one or more of CO, CO₂, nitrogen, water, oxygen and one or more hydrocarbons such as one or more C₁ to C₆ hydrocarbons. The membrane can be a selective oxygen separation membrane, having a composition comprising electronic and oxide conducting properties, for example an oxide with both electronic and oxide conducting properties, or a composite of different oxides having separate oxide conducting and electronic conducting properties, and which is capable of separating oxygen from a mixture of gases, for example those additionally comprising hydrogen, nitrogen, water, CO, CO₂ and one or more hydrocarbons, such as one or more C₁ to C₆ hydrocarbons. Examples of compositions that can be used in the selective separation of oxygen from a mixture of gases or other fluids comprising oxygen are described in WO 2008/074181.

The apparatus described herein can be part of a larger apparatus, for example a reactor or other vessel. Thus, the metal that is sealed or joined with the second ceramic can be part of a metallic vessel. Alternatively, the metal can be joined, for example through brazing or welding, to a metallic vessel wall. The vessel can be a separation vessel, for example for allowing selective separations using a selectively permeable membrane, the membrane being on the first ceramic. In another embodiment, the vessel is a combined reactor and separation vessel, in which a chemical reaction takes place in the reactor, and one or more products are separated by the selectively permeable membrane. As an example, the membrane may be a palladium membrane (or a Ag and/or Cu-modified palladium membrane) deposited on the first ceramic, e.g. porous alumina, which enables hydrogen produced in a reaction to permeate across the membrane. The reaction can be catalysed, the reactor on one side of the membrane having a catalyst. In one embodiment, the reaction is the steam reforming, autothermal reforming or partial oxidation of a hydrocarbon. The first seal typically has a thermal expansion coefficient that is similar to those of the first and second ceramics in order to reduce stresses to the seal during thermal cycling. Typically, the first seal has a linear thermal expansion coefficient in the range of from 6xlO^~6 to 10x 10^"6 m m^"1 K^"1. Typically, the first seal comprises one or more OfAl₂O₃, CaO and SiO₂. According to a fourth aspect of the present invention, there is provided an oxide composition comprising, in addition to oxygen, Al, Ca and Si, characterised by the composition further comprising one or more of Ba, Zr and Ga.

Glass seals made using the above composition are effective for joining and/or sealing two ceramic surfaces together. The compositions are typically produced by mixing together separate oxides, and then heating to high temperature, typically at temperatures in the range of from 800 to 2000⁰C. Preferably, the temperature should be lower than the sintering temperature of the first ceramic, for example a temperature of 1500⁰C or lower, while ensuring that the oxides are in a molten state, such as at temperatures of 1000⁰C or more. Temperatures in the range of from 1000 to 1500⁰C are preferred, for example in the range of from 1400 to 1500⁰C.

The composition is an oxide, and typically comprises a homogeneous distribution of the non-oxygen elements. The presence of one or more of Ba, Ga and Zr in the Al, Ca and Si mixed oxide enables control over the density and linear thermal expansion coefficient of the glass to be achieved, without negatively affecting the stablility, chemical resistance and effectiveness when used as a seal between a first and a second ceramic surfaces.

Typically, when used as a (first) seal between a first and second ceramic surface, the composition of the seal composition, expressed as the individual constituent oxides, is in the range of from 30 to 60wt% BaO, ZrO₂ and/or Ga₂O₃ (combined total where more than one of these are present), in the range of from 1 to 15wt% CaO, in the range of from 1 to 10wt% Al₂O₃, and in the range of from 20 to 40wt% SiO₂. Preferably, the oxide composition comprises BaO, and in one embodiment the oxide composition comprises BaO and ZrO₂. The BaO content of the oxide can in one embodiment be in the range of 30 to 40 wt%. The ZrO₂ content of the oxide can in one embodiment be up to 15wt%, for example in the range greater than O to 15wt%, such as 1 to 15wt%.

The oxide composition can be prepared by heating together the different oxide components to form a melt, and quenching the melt to form a glass comprising a mixture of the constituent oxides.

During preparation of the oxide composition, the heated oxide mixture or melt is preferably rapidly cooled or quenched. This has the advantage of preventing the different oxide components from separating out as separate oxide phases, which helps improve homogeneity of the composition and reduces the chances of defects and loss of sealing effectiveness. Rapid quenching can be achieved by contact with cold water, for example by submerging in water, or by spraying water onto the heated mixed oxides.

To form the seal, the mixed oxide glass composition can be applied (typically in powdered or bead form) to the gap between the first and second surfaces, and heating to above the melting temperature of the mixed oxide glass, for example in the range of from 800 to 2000⁰C₅ for example a temperature of 1500⁰C or less, and/or 1000⁰C or more, for example in the range of from 1000 to 1500⁰C, such as a temperature in the range of from 1400 to 1500⁰C.

In one embodiment of the invention, an apparatus comprising a metal surface sealed to a porous ceramic surface can be used to support a selectively permeable membrane. The selectively permeable membrane is able to allow the selective permeation of a component in a fluid mixture comprising that component. Thus, in one embodiment, the membrane is palladium, or palladium modified with Cu and/or Ag. This is able to separate hydrogen from a fluid mixture comprising hydrogen, such as a mixture of hydrogen with one or more of carbon monoxide, carbon dioxide, oxygen and water. The art-skilled person is aware of how to prepare a palladium membrane supported on a porous ceramic. An example of one such method is described in WO 2005/065806, in which a porous ceramic support, such as alumina, is treated with a solution of a palladium salt, such as aqueous Pd(NHs)₂Cl₂, in the presence of a reducing agents such as hyrdazine.

The art-skilled person is also aware of how to operate a supported palladium membrane for separating hydrogen from a hydrogen-containing mixture. Examples of such processes are described in WO 2007/129024 and WO 2007/031713.

Another type of membrane that can be supported onto the porous first ceramic includes a zeolite membrane for separating water from organic liquids. In one embodiment the zeolite is one with the LTA structure (according to the database of zeolite structures published by the International Zeolite Association), such as silicalite or zeolite- A, e.g. Zeolite 3 A or 4A (which comprise potassium and sodium cations respectively to balance the negative charge of the zeolite framework). The zeolite membrane can be produced and used to dewater an organic liquid, typical examples being an alcohol, an ester or a mixture of two or more alcohols and/or esters.

A method of making a supported zeolite membrane, and its use in alcohol dewatering, is exemplified in EP-A- 1 980 314, which describes the treatment of a porous support pre- treated with silica and optionally also zeolite-A crystals as crystal nucleation centres, with an aqueous zeolite-A synthesis gel. Examples of organic liquids which, when additionally comprising water, can be dried or dewatered using zeolite membranes, for example zeolites with LTA structure include one or more C₁ to C₄ alcohols, for example ethanol, isopropanol, or a mixture of C₂ to C₄ alcohols. The method of separating water from an organic liquid using a selectively permeable membrane is often termed pervaporation.

Typically the membrane, the membrane support (i.e. the first, porous ceramic), and the metal to which the first ceramic is sealed (for example via a low porosity ceramic as herein described), are attached to a reactor or vessel wall. The vessel comprising the selectively permeable membrane can be considered to have two zones; a first zone on one side of the membrane, and a second zone on the other side of the membrane. Herein, the first zone relates to the zone having the mixture of fluids comprising the component to be selectively separated, and the second zone relates to the zone into which the selectively separated component enters after permeating the membrane. Thus, for example, where the mixture of fluids is a mixture of one or more alcohols with water, the wet alcohol mixture (in gaseous or liquid form) is fed to the first zone of the separation vessel, where water selectively permeates through the membrane into the second zone, leaving an alcohol-rich mixture that can be removed from the first zone.

A similar principle applies in the example where the mixture of fluids is a gaseous mixture comprising hydrogen, such as when in combination with one or more of water, oxygen, carbon monoxide, carbon dioxide and one or more hydrocarbons, for example a gaseous mixture produced from the steam reforming, autothermal reforming or partial oxidation of a hydrocarbon, such as methane or natural gas. The gaseous mixture is fed to the first zone of the separation vessel, hydrogen permeates the selective permeable membrane (typically a Pd membrane, or Pd modified with Ag and/or Cu) into the second zone, leaving a hydrogen deficient mixture in the first zone.

Although they can be conducted batch- wise, membrane separation processes are typically continuous processes, where a mixture of fluids is constantly fed to the first zone of a separation vessel and a mixture deficient in the separated component (or components where applicable) is removed from the first zone, while the component to be separated permeates the membrane into the second zone of the vessel, from which it is removed. To assist removal of the permeated component from the second zone of the separation vessel, a vacuum can be used, or a sweep or flush gas to flush the permeated component therefrom.

In a further embodiment, the component to be separated is generated in situ in the first zone of the separation vessel. For example, hydrogen can be generated by the steam reforming, partial oxidation and/or autothermal reforming of one or more hydrocarbons, such as hydrocarbons selected from one or more C₁ to C₄ hydrocarbons, typically in the presence of a catalyst. Thus the separation vessel can be a combined reactor/separation vessel, in which the first zone of the reactor is adapted to allow reactants to be fed thereto, to enable a desired component to be produced therein through a chemical reaction, ' optionally in the presence of a catalyst, and whereby the desired component permeates into the second zone through the membrane, thus separating it from the other reactants and products.

WO 2007/129024 and WO 2007/031713 both describe process in which hydrogen is produced in a combined reactor/separation vessel, in which the first zone of the reactor comprises a catalyst for producing hydrogen from a hydrogen-containing compound, wherein the hydrogen permeates a Pd membrane or a Ag/Cu-modified Pd membrane within the reactor into a second zone, while a hydrogen-deficient product stream is removed from the first zone. Examples of reactions include a water gas shift reaction, where water and carbon monoxide react to produce hydrogen and carbon dioxide, or the steam reforming, autothermal reforming or partial oxidation of a hydrocarbon feedstock, such as methane or natural gas, to produce hydrogen and one or more of carbon monoxide and carbon dioxide.

Use of combined reaction and membrane separation is advantageous, particularly in a continuous process, because removal of the desired component continually alters the reaction equilibrium, and enables greater yields of the desired component to be produced. An additional benefit is that product is separated from the remaining reactants and any other products, making purification significantly easier.

There now follows non-limiting examples illustrating the invention, with reference to the Figures in which:

Figure 1 is a perspective view of a porous ceramic tube joined to a metal pipe via a ring of a lower porosity ceramic ring. Figure 2 is a view the same apparatus as shown in Figure 1 after an approximately

45° anti-clockwise rotation about axis A-A'.

Figure 3 is an exploded view of the apparatus shown in Figures 1 and 2. Figure 4 is a more detailed schematic view of a low porosity ceramic ring, as shown in Figures 1, 2 and 3. Figure 5 is a more detailed schematic view of a low porosity ceramic plug, as shown in Figures 1, 2 and 3.

Figure 6 is a cutaway view of an alloy seal prepared from a metal foil and a preliminary alloy.

Figure 7 is a longitudinal section through the apparatus illustrated in Figure 6. Figure 8 illustrates an alternative way of sealing a metal with a ceramic, compared to that illustrated in Figures 6 and 7.

Figure 9 is a longitudinal section through a separation vessel comprising a membrane supported on a ceramic tube/metal tube apparatus as shown in Figures 1, 2 and 3.

Figure 10 is a longitudinal section through a combined reaction and separation vessel incorporating a membrane supported on a ceramic tube/metal tube apparatus as shown in Figures 1, 2 and 3. Figure 11 is an apparatus having a metal to ceramic seal, where the metal is joined to a different metal with different thermal expansion characteristics.

Figure 12 is a photograph demonstrating wettability characteristics of an alloy not according to the present invention on a ceramic surface. Figure 13 is a photograph demonstrating wettability characteristics of an alloy according to the present invention on a ceramic surface.

Figures 1, 2 and 3, show how a metal can be sealed to a first ceramic by means of an intermediate object, in this case a small tube or ring, made of a second ceramic, the first ceramic being porous, and the second ceramic having a lower porosity than the first ceramic. Metal tube, 1, and a tube made from the first ceramic, 2, are both sealed to a ring of the second ceramic, 3. A passage is formed between the interior, 4, of the metal tube 1, the interior, 9, of the first ceramic tube 2 and the interior, 10, of the ring of second ceramic 3. The tube of first ceramic, 2, is sealed at its other end by a plug, 5, which is also made of a low porosity ceramic, typically made from the same ceramic as the ring of second ceramic, 3. In one embodiment, the metal tube is 4J33 stainless steel, the low porosity second ceramic is sintered alumina, and the porous first ceramic is porous gamma alumina. The metal tube, 1, is joined and sealed to the ring of second ceramic, 3, with alloy, 6. The first ceramic tube, 2, is sealed to the second ceramic ring, 3, by a glass seal, 7. The first ceramic tube, 2, is also sealed to the plug of second ceramic, 5, using a glass seal, 8. Although in this embodiment, one end of the apparatus is closed, by plug 5, in an alternative embodiment both ends can have hollow interiors, for example where plug 5 more closely resembles the ring of second ceramic 3, which can also optionally be attached to a metal tube, typically one that is similar or identical to metal tube 1.

Figures 4 and 5 illustrate respectively in further detail the ceramic ring 3 and ceramic plug 5, each of which has a lip (11 and 12 respectively for ring 3 and plug 5) and a protrusion (13 and 14 respectively for ring 3 and plug 5). In each case, the protrusion (13 and 14) extends into the interior, 9, of the tube of first ceramic, 2, maintaining contact with the inner wall of the tube of first ceramic when sealed. The lips (11 and 12) contact the top and bottom edges of the tube of first ceramic when sealed. In this Figure, the ring of second ceramic, 3, is shown after 180° rotation about axis B-B', as identified in Figure 3. Oxide seal is typically applied to the portion of the lip (11 and 12) that makes contact with the edges of the porous ceramic tube 2, and/or the portion of the protrusions (13 and 14) that contact the inner surface of the porous ceramic tube 2.

Figures 6 and 7 show how an alloy seal can be made using a metal foil (i.e. a foil of the further alloy metal) and a preliminary alloy. The metal foil of further alloy metal, 15, (for example titanium) is inserted into the gap between metal tube, 1, and the internal surface of the ring of second ceramic, 3. As an alternative to using a metal foil, the further alloy metal can be sputtered onto the internal surface of ceramic tube, 3, or the outer surface of the metal tube, 1, to produce a metallic layer thereon. The preliminary alloy, 16, comprising the preliminary alloy metals, is then placed at the gap between the second ceramic tube, 3, and the metal tube, 1. The desired alloy forming the seal material is then produced by heating, for example by brazing at a temperature of 45O⁰C or more. In one embodiment, the alloy seal is prepared using 5wt% titanium foil, and 95wt% of a preliminary alloy comprising Ag, Cu and Al. In a further embodiment, the preliminary alloy comprises Cu at 27wt%, Al at 5wt%, the balance being Ag. Figure 8 shows an alternative method of sealing a metal to a ceramic surface. A layer of further alloy metal, 15, (for example titanium) is formed on a portion of the outer surface of ceramic ring, 3, by sputtering. A ring of preliminary alloy, 16, (an example being a Ag, Cu and Al alloy) is placed on top of the further alloy metal surface, and one end of the metal tube placed on top of the further alloy metal ring. The formation of the desired alloy seal can be effected by brazing.

Figure 9 shows an apparatus that can be used for a separation process using a selectively permeable membrane supported on a porous first ceramic as described herein.

The separation vessel is equipped with metal tubes, 1 and V, integrated into the separation vessel walls. The metal tubes and the vessel can be made from the same metal, or alternatively the metal tubes can be of a different metal or alloy to the vessel. Having the reactor/vessel wall made from a different metal is advantageous where a metal with low expansion coefficient needs to be joined to the ceramic tube, in order to more closely match the expansion coefficient of the ceramic (ceramics tend to have lower thermal expansion coefficients than metals) for reducing stress and degradation of the seal during thermal cycling. Low expansion metals tend to be rare or expensive, and hence construction of a complete reactor or vessel from such a low expansivity metal can be avoided by connecting the metal tube, 1, to a reactor or vessel, 20, made of more readily available material, such as iron, or alloys of iron such as steel and stainless steel. Techniques such as welding or brazing can be used to provide a strong, stable and impermeable seal between the different metals.

The metal tubes are each joined and sealed with rings, 3 and 3' of a low porosity second ceramic, for example as described above. The low porosity tubes of second ceramic are in turned joined and sealed with a high porosity tube of first ceramic, 2, as described above in relation to Figure 3. The surface of the first ceramic tube is coated with a membrane, 22. For separating hydrogen from a hydrogen-containing fluid mixture, the membrane is typically palladium, or palladium additionally comprising Cu and/or Ag. The membrane extends over the entire surface of the porous tube of first ceramic to prevent any fluids other than hydrogen permeating through to the other side of the membrane. Although this Figure shows the membrane on the outer surface of the porous ceramic tube 2, it is also possible to coat the internal surface of the porous first ceramic either instead of, or in addition to, the outer surface. In use, a fluid mixture enters the first zone of separation vessel 20, through inlet 21.

One of the components in the fluid mixture permeates the membrane, 22, and enters the internal portion of the porous ceramic tube, 9. The unpermeated portion of the fluid mixture, deficient in the selectively permeated component, leaves the separation vessel via outlet, 23. Thus, in an example where the membrane is a palladium membrane, and the fluid mixture is a mixture of hydrogen, carbon monoxide and carbon dioxide, hydrogen permeates the membrane, and the hydrogen-deficient gas mixture comprising predominantly carbon monoxide and carbon dioxide is removed via the outlet 23.

The selectively separated component, (for example hydrogen), permeates the membrane, 22, and enters the interior 9 of the porous tube of first ceramic 2, where it can diffuse out of the reactor through the interior, 4 or 4' of either of the metal tubes 1 or I⁵, via the interior, 10 or 10', of the low porosity second ceramic tubes, 3 or 3'. In an alternative embodiment, a sweep gas can be used to flush the permeated fluid from the interior of the porous ceramic tube, 2. Thus, in one embodiment, a flow of sweep gas, such as nitrogen, argon or steam, can be fed through metal tube 1, and a mixed fluid comprising the sweep gas and the permeated component leaves through metal tube 1 ' .

In this Figure, the second zone of the separation vessel can be defined as the portion of the vessel accessible only to the selectively permeated component, and which comprises the interior of the metal tubes, 4 and 4', the interior of the low porosity tubes of second ceramic, 10 and 10', and the interior 9 of the porous tube of first ceramic 2. The first zone comprises the portion of the vessel on the other side of the membrane 22 and within the walls of vessel 20. Although only a single supported membrane is shown in Figure 8, the reactor can comprise two or more supported membranes. Having more than one membrane allows increased surface area for permeation, which improves the efficiency of the separation process.

Figure 10 is a modification of Figure 9, in that the first zone comprises a catalyst, 24, wherein one or more reactants are fed into the first zone of the combined reactor/separation vessel, 20, through inlet, 21, where they contact catalyst 24 and undergo reaction to produce the separable component, which permeates through membrane 22 and into the second zone of the vessel. Unreacted reactants and other products are removed through outlet 23. An example of such a reaction is the steam reforming, partial oxidation or autothermal reforming of a hydrocarbon, such as methane, in the presence of steam and/or oxygen to produce a gaseous mixture comprising hydrogen and one or more oxides of carbon, in the presence of a selective hydrogen-permeable membrane, for example a palladium membrane or Pd-Cu or Pd-Ag alloy membrane. When hydrogen is produced, it permeates the membrane into the second zone of the reactor/separation vessel. This drives the reaction in the first zone towards further conversion of the hydrocarbon to produce more hydrogen, which improves reactant conversion, and also produces a pure supply of hydrogen, which can be removed from the second zone of the reactor/separation vessel. An example of such a process is described in WO 2007/031713.

In Figure 11 the low porosity ceramic ring, 3, is attached to metal tube, 1, using an alloy seal, 6, as shown above in Figures 1, 2 and 3. The metal tube, 1, is in turn attached to a second metal tube, 25, which forms part of a reactor or separation vessel, 20, and which is typically of the same material as the reactor or separation vessel. The join (for example weld) between the metal tube, 1, and a tube of second metal, 25, which is integrated into vessel 20 is shown at 26. This is an alternative to one of the embodiments described in relation to Figures 9 and 10, and provides another means by which the use of an expensive or rare metal or alloy to fabricate an entire reactor or vessel can be avoided. Figures 12 and 13 illustrate how an alloy comprising silver as described herein exhibits improved wettability characteristics on a ceramic surface than a corresponding Ag-free alloy. Figure 12 shows a copper based Cu-2Al-3Si-2Ti alloy (i.e. 2wt% Al, 3wt% Si, 2wt% Ti and the balance Cu), 30, on a low density, high porosity alumina surface, 31, after treatment under vacuum at 1100⁰C. The footprint made by the alloy on the surface is small, and does not spread across the ceramic surface, demonstrating a weak interaction with the surface. This alloy is not in accordance with the first aspect of the present invention, as it does not comprise silver.

Figure 13 shows an Ag-27Cu-5Al-5Ti alloy (i.e. 27wt% Cu 5wt% Al, 5wt% Ti, and the balance Ag), 32, on a low density, high porosity alumina surface, 31 , after treatment under vacuum at 96O⁰C. The footprint made by the alloy is larger than the corresponding alloy in Figure 12, the alloy spreading out across a wider area compared to the Ag-free alloy, demonstrating greater interaction with the surface.

Ag-25Cu-10Ti alloy (i.e. 25wt% Cu, 10wt% Ti, the balance being Ag), which is a common alloy used in brazing, is not able to withstand temperatures of greater than about 45O⁰C, and hence is not suitable for high temperature application. Addition of Al and/or Pd to the alloy composition improves temperature stability and resistance to oxidation.

Experimental Procedure A homogeneous oxide mixture comprising 40-45% BaO, 5-10% Al₂O₃, 10-15% CaO, and 30-40% SiO₂ powders was prepared by ball-milling. The mixture was then melted at 1450-1480⁰C for 1 to 2 hours in an alumina crucible, followed by rapid water quenching. The obtained glass was ground into fine powders of about 5-15 μm in diameter by ball milling. To improve the high temperature stability, ZrO₂ powder at up to 15wt% content was homogeneously mixed into the above powdered glass seal.

A bimetallic tube was manufactured through butt welding a 1 CrI 8Ni9Ti 304 stainless steel rod (comprising lwt% Cr, 18wt% Ni, 9wt% Ti) to a 4J33 (cover) alloy rod, followed by machining to produce a tube shape.

The 4J33 end of the bimetallic tube was brazed to a low porosity, dense alumina ring using Ag 25-30Cu 2-8Al 2-8Pd 2-8Ti alloy (comprising 25-30wt% Cu, 2-8wt% Al, 2- 8wt% Pd and 2-8wt% Ti, the balance being silver). At first, a Ag 25-30Cu 2-8Al 2-8Pd preliminary alloy (comprising 25-30wt% Cu, 2-8wt% Al, 2-8 wt% Pd, the balance being Ag) was fabricated by inductive melting method under an argon atmosphere. The obtained preliminary alloy was rolled into a thin plate before use. The titanium was incorporated into the alloy by magnetron-controlled sputtering of a Ti film of about 2-10 μm thick on the end face of the dense, low porosity alumina ring, followed by brazing. The seal the low porosity alumina plug and ring (with an alumina density 3.5g/cm³) to the two open ends of the high porosity ceramic tube was effected as follows. About 0,6 to 0.8 g of a glass sealing powder was mixed with alcohol to form a sticky paste; then a uniform seal paste layer was put around the two end surfaces of the porous ceramic tube. The low porosity alumina plug and ring were inserted into the two ends of the high porosity ceramic tube, such that the lips were held in contact with the pasted end surfaces of the ceramic tube. The assembly was placed vertically into the air furnace and heated to a temperature of 115O⁰C at a rate of 10°C/min, and held at that temperature for 15 min. The joint was then allowed to cool to room temperature.

The brazing of bimetallic tube to the low porosity alumina ring was achieved using a vertical vacuum furnace which capable of reaching temperatures of up to 125O⁰C. First, a Ti film of about 2-10 μm thick was sputtered onto the end face of the dense alumina ring; a Ag-Cu-Al-Pd alloy ring (0.15-0.2 g) was inserted into the interface between the 4J33 end of the bimetallic tube and the dense alumina transition ring. The whole assembly, vertically disposed, was put into a vacuum furnace, and brazed at to a temperature of 96O⁰C at a rate of 10°C/min under vacuum, the being about 5 x 10^"3 Pa. The j oint was then allowed to cool to room temperature after brazing.

Claims

1. An oxide composition comprising, in addition to oxygen, Al, Ca and Si, characterised by the composition further comprising one or more of Ba, Zr and Ga.

2. An oxide composition as claimed in claim 1, in which the Ca content (expressed as CaO) is in the range of from 10 to 15 wt%.

3. An oxide composition as claimed in claim 1 or claim 2, in which the Al content (expressed as Al₂O₃) is in the range of from 10 to 15 wt%.

4. An oxide composition as claimed in any one of claims 1 to 3, in which the Si content (expressed as SiO₂) is in the range of from 30 to 40wt%.

5. An oxide composition as claimed in any one of claims 1 to 4, in which the combined content of Ba, Zr and Ga (expressed as BaO, ZrO₂ and Ga₂O₃ respectively) is in the range of from 40 to 45 wt%.

6. An oxide composition as claimed in any one of claims 1 to 5 which, in addition to oxygen, consists of Al, Ca, Si, and one or more of Ba, Zr and Ga.

7. An oxide composition as claimed in any one of claims 1 to 6, in which the thermal expansion coefficient is in the range of from 6^χlO^"6 to 1OxIO^"6 m m^'1 K^"1.

8. An oxide composition as claimed in any one of claims 1 to 7, the composition being a glass.

9. An apparatus comprising a first surface, a second surface and a seal, in which the seal comprises a composition as claimed in any one of claims 1 to 8.

10. An apparatus comprising a porous ceramic, a low porosity ceramic, and a seal, the porous ceramic having higher surface area than the low porosity ceramic, and the seal being in contact with the surface of the porous ceramic and the surface of the low porosity ceramic, characterised by the seal comprising a composition as claimed in any one of claims 1 to 8.

11. An apparatus as claimed in claim 10, in which the low porosity ceramic and/or the porous ceramic is an alumina.

12. An apparatus as claimed in claim 10 or claim 11, in which the surface area of the low porosity ceramic is less than 100 m² g^"1 and/or has a density in the range of from 3 to 4.1 g ml/¹ at 2O⁰C.

13. An apparatus as claimed in any one of claims 10 to 12, in which the porous ceramic has a surface area of 100 m² g^"1 or more, and/or has a density in the range of from 0.6 to 1.4 g mL^"1 at 2O⁰C.

14. An apparatus as claimed in any one of claims 10 to 13, in which at least one surface of the porous or low porosity ceramic, preferably the porous ceramic, comprises a selectively permeable membrane.

15. An apparatus as claimed in claim 14, in which the selectively permeable membrane is selectively permeable towards water, oxygen or hydrogen.

16. An apparatus as claimed in claim 14 or claim 15, in which the selectively permeable membrane is selected from a zeolite with LTA structure, palladium or an alloy of palladium with silver and/or copper, an oxide with both electronic and oxide conducting properties, or a composite of different oxides having separate oxide conducting and electronic conducting properties.

17. An apparatus as claimed in any one of claims 10 to 16, in which the first and/or second ceramic is joined or sealed to a metal by an alloy seal.

18. An apparatus as claimed in any one of claims 10 to 18, in which the metal is connected to a second metal having a higher linear thermal expansion coefficient.

19. An apparatus as claimed in any one of claims 10 to 18, in which the metal or second metal is connected to or is part of a reactor or other vessel.

20. An apparatus as claimed in claim 19, in which the reactor or vessel is made from iron, steel or stainless steel.

21. An apparatus as claimed in claim 20, in which the first ceramic comprises a selectively permeable membrane, which reactor or vessel comprises a first zone, a second zone, an inlet to the first zone, and outlet from the first zone, an outlet from the second zone and optionally an inlet to the second zone, which first zone is defined as the portion of the vessel on one side of the selectively permeable membrane in which a fluid mixture comprising a component to be selectively separated can be produced or fed, and the second zone is defined as the portion of the vessel on the other side of the selectively permeable membrane, into which the selectively separated component permeates .

22. A process for preparing a seal between a porous ceramic and a low porosity ceramic, which process comprises applying an oxide composition between the surface of the porous ceramic and low porosity ceramic and heating to a temperature above the melting temperature of the oxide composition, which oxide composition is a composition as claimed in any one of claims 1 to 8.

23. A process for separating a component from a fluid mixture comprising the component, which process comprises contacting the fluid mixture with an apparatus as claimed in claim 14, which apparatus comprises a membrane that is selectively permeable to the component, wherein the component permeates the membrane.

24. A process as claimed in claim 23, in which the component is hydrogen, and the membrane is palladium or an alloy of palladium with silver and/or copper.