GB1586073A

GB1586073A - Metal-to-ceramic seal

Info

Publication number: GB1586073A
Application number: GB22211/78A
Authority: GB
Original assignee: Chloride Silent Power Ltd
Current assignee: Chloride Silent Power Ltd
Priority date: 1978-05-24
Filing date: 1978-05-24
Publication date: 1981-03-18

Description

(54) IMPROVEMENTS IN OR RELATING TO METAL-TO-CERAMIC SEAL (71) We, CHLORIDE SILENT POWER LIMITED, a British Company, of 52 Grosvenor Gardens, London, SW1W OUA, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be prepared, to be particularly described in and by the following statement: This invention relates to ceramic seals and is concerned more particularly with the sealing of an annular metal member onto a cylindrical ceramic element.

Such seals may be used for example in electrochemical cells of the kind having a solid ceramic electrolyte. Such cells, of which sodium sulphur cells are a typical example, commonly have to operate at elevated temperatures where the electrode materials are liquid.

The seals necessary to seal these electrode materials within the cell have therefore not only to withstand the highly reactive electrode materials at these temperatures but are also subjected to temperature cycling. In the case for example of a sodium sulphur cell, the electrolyte may be a beta-alumina tube closed at one end with the sodium on one face of the tube, preferably the outer face, and sulphur/sodium polysulphides on the other face. The cell has to be sealed to prevent escape or mixing of these materials. A number of proposals have been made for various types of sealing arrangement.

Compared with most metals, ceramic materials are generally weak, particularly in tensile strength and it is necessary, in any seal for such a cell, to ensure that the ceramic material is not overstressed.

In a sodium sulphur cell, the beta-alumina electrolyte tube may have an alpha-alumina tubular extension at its open end so that the end portion is not ionically conductive. The alpha-alumina and beta-alumina have similar coefficients of thermal expansion and it is wellknown that the alpha-alumina extension can be secured to the beta-alumina with a glass seal, in this case, the closure of the cell has to be effected by sealing to the alpha-alumina tube.

This is essentially the same problem of forming a metal ceramic seal on the end of a ceramic tube as arises if the seal is made directly to the beta-alumina.

In our earlier Application No. 20959/76 (Serial No. 1574804), it is proposed to secure annular metal elements to a cylindrical ceramic member by the steps of putting elements or coatings of'relatively soft material, e.g. aluminium, over inner and outer shaped surfaces of the ceramic member at or near an end thereof, putting inner and outer annular metal elements having shaped surfaces over the respective soft metal covered inner and outer surfaces of the ceramic, the shaping of said shaped surfaces of the metal elements being such that the internal diameter of the outside element decreases in the direction towards the end of the cylindrical member and the- external diameter of the inner element increases in this direction over the same portion of the member, the inner and outer annular members being of a metal which is hard compared with the aforementioned soft metal and having coefficients of thermal expansion such that the coefficient of expansion of the outer annular member is greater than that of the ceramic material and that of the inner annular member is less than that of the ceramic material, heating the assembly and, when the assembly is hot, forcing the annular members axially into tight engagement with the ceramic material whereby the ceramic material is tightly secured on cooling between the two annular members. With this construction, by a suitable choice of the coefficients of thermal expansion of the materials, there need not be any hoop stress in the ceramic material. The choice of the coefficients of thermal expansion, set out above, ensures that the annular members are forced into tight engagement with the ceramic material on cooling subsequent to the pressing operation. The shaped surfaces are conveniently conical surfaces. By this technique, it is possible to obtain a tight seal onto the ceramic material without undue stress of that material.

According to the present invention, there is provided a method of sealing an annular metal element or elements to a cylindrical ceramic element, one of the elements to be sealed together having an external cylindrical surfaces shaped so that the diameter of that surface varies along the axis and the other element having an inside cylindrical surface shaped so that its inside diameter varies along the axis whereby the shaped surfaces can be nested to gether, the two elements being pressed together in an axial direction with an interlayer of a deformable metal between said shaped surfaces, the pressure and the relative movement being such as to cause deformation of at least part of the interlayer around the whole of the annular region between the elements with the formation of cold pressure bonds giving hermetic seals between the interlayer material and both the inner and outer elements. Typically the surfaces would be shaped so that a relative movement of between 1 and 10 millimetres in the axial direction causes a reduction in thickness of at least 50% of at least a part of the interlayer extending around the whole of the annular region by deformation of this interlayer.

By applying pressure in this way without heating, it has been found possible to effect a hermetic seal in a much shorter time than is required for the technique in which heating of the assembly is required before applying the pressure. The cold bonding technique, in the case of a sodium sulphur cell, moreover enables the cell to be sealed with the sulphur electrode in place. A further advantage of this technique utilising cold pressure bonding is that it becomes possible to use an annular metal element which, if it is outside the ceramic, has a lower coefficient of thermal expansion than the ceramic and, if it is inside the ceramic, has a higher coefficient of thermal expansion than the ceramic so that, on heating the assembly, the compressive stress on the bond is increased.

With such an arrangement in which the compressive stress is increased on heating, it is preferable to have the ceramic material between inner and outer annular elements, each of which is sealed to the ceramic material.

The soft metal interlayer may be formed of aluminium or aluminium alloy. Using a cold pressure deformation bonding technique as described above, it is convenient to employ a high purity aluminium, which is a very soft material. In this case, aninterlayer element of aluminium for putting between the surfaces of the ceramic and annular metal elements may conveniently be formed by a pressing opera Uon. It is alternatively possible however to provide the alumiunium as a coating on one or both of the elements which are to be sealed with the aluminium interlayer.

The shaped surfaces of the two elements are conveniently conical surfaces which are correspondingly tapered. Preferably at least one annular groove is formed on the shaped surfaces of one of the elements, conveniently the ceramic element. The soft metal can deform into such a groove and this groove thus helps to avoid the development of interfacial pressure except where it is needed. In the region of the groove the soft metal deforming into the groove relieves interfacial pressure which may be substantially zero in this region. Such a groove or grooves therefore enables a non-linear pressure distribution to be developed across the tapered surface and hence enables higher local pressures to be developed for a given axial loading.

In some cases the groove may be formed at the end of the ceramic material; in other words the surface of the material has a step with a shoulder facing towards the end of the ceramic tube.

If a deformation bond is formed as described above with cold pressure bonding between aluminium and the ceramic material and between aluminium and a metal annular element, it is preferable to heat treat the assembly prior to use to improve bonding. This relieves any local strain.

As described in the specification of the aforementioned Application No. 20959/76 (Serial No. 1574804) inner and outer tapered surfaces may be formed on a ceramic tube and separate annular metal elements may be secured thereto. In an electrochemical cell for example, it may be required to seal separately the regions lying inside and outside a ceramic electrolyte tube. In a sodium sulphur cell, conveniently the sodium is outside the electrolyte tube and the sulphur/polysulphide cathodic reactant is on the inside of the electrolyte tube and, in this embodiment, it is convenient to make the outer annular metal element of a low expansion metal e.g. mild steel or a nickel-iron alloy such as Nilo K (Nilo is a Registered Trade Mark) and the inner metal annular element of a steel with a coating of a nickel-chromium material and to use aluminium as the soft metal. These inner and outer metal elements are sealed to the ceramic by a cold pressing operation as described above. During the sealing operation, the ceramic is subjected to compressive forces between the two tapered surfaces but any hoop stress is dependent only on the difference of these inwardly and outwardly directed radial srresses.

The invention includes within its scope an assembly comprising a cylindrical ceramic member having shaped inner and outer surfaces at or near one end of the member, these surfaces being such that the outer diameter decreases towards the end of the member and the inner diameter increases towards that end of the member, and inner and outer annular elements of a hard metal having complementary tapered surfaces bonded to the shaped surfaces of the ceramic member with interlayers of a soft metal between the ceramic and each of the inner and outer elements by cold pressure bonding as described above. In particular the invention includes within its scope an assembly as described and for use in a sodium sulphur cell wherein the ceramic member is a tube or cylinder of beta-alumina or alpha-alumina and wherein the inner annular metal element is a corrosion-resistant alloy such as Inconel 600 (Inconel is a Registered Trade Mark) or mild steel protected by a coating of a corrosion resistant alloy and the outer metal annular element is mild steel or a low expansion alloy such as Nilo K. (Nilo is a Registered Trade Mark).

Instead of having tapers on the inside and outside surfaces of the ceramic, it may be preferred in some cases to have tapers on opposite ends of a ceramic annular element so that the metal elements may be secured thereto by pressing them on to the ceramic element from opposite ends thereof.

Thus, according to another aspect of the present invention, a method of sealing annular metal elements to the open end of a ceramic electrolyte tube for an electrochemical cell comprises the steps of providing a rigid annular element of a ceramic material having a coefficient of expansion substantially the same as that of the electrolyte material, forming upper and lower shaped surfaces on said annular ceramic element at or near the ends thereof, the shaping being arranged so that the outside diameter decreases in the direction towards the respective end of the ceramic annular element, putting conical metal elements or coatings over the shaped surfaces, such metal elements or coatings being of a relatively soft metal compared with said annular metal elements, putting said inner and outer annular metal elements over the respective soft metal covered upper and lower shaped surfaces of the ceramic element, and then forcing the annular metal elements axially together to deform the soft metal the pressure and relative movement being such as to cause deformation of at least part of soft metal between the shaped surfaces around the whole of each annular region between the ceramic and the annular metal elements with the formation of cold pressure bonds giving hermetic seals, the deformation causing a reduction in thickness of at least 50% of at least a part of the soft metal interlayer, the annular ceramic element being sealed to the electrolyte tube either before or after the metal elements have been assembled to the annular ceramic element.

For a sodium sulphur cell, which typically operates at a temperature of 350"C to 4000C, the outer annular metal element is preferably of a material having a coefficient of thermal expansion slightly less than that of the ceramic material and the inner annular metal element has a coefficient of expansion slightly greater than the ceramic material so that the seal is tightened when the cell is brought up to the operating temperature.

The shaped surfaces of the ceramic material may be tapered surfaces, for example with straight tapers, and preferably an annular groove is formed in each tapered surface of the ceramic. The annular metal elements have conical surfaces and these may have a straight taper corresponding to the taper on the ceramic surfaces.

The ceramic annular element may be secured, for example with glass, to the end of the electrolyte tube either before putting on the metal elements or after forming the assembly with the metal elements.

It will be seen that with the above-described construction, the hoop stress is taken by the annular ceramic element which can be made quite thick in the radial direction and hence capable of withstanding the stresses with might arise.

By the above-described technique, it becomes readily possible, in a simple operation which can be automated, to secure the metal elements onto the rigid ceramic element. The metal elements can conveniently both be made of the same material and this facilitates the sealing operation because the similarly shaped surfaces with the same maximum and minimum diameters may be used and it is readily possible to obtain the same thermal conditions for effecting seals to both of the tapered surfaces since these are both external surfaces.

The choice of materials to be used will, in general, depend on the materials in the cell.

In an electrochemical cell having a liquid alkali metal and a liquid cathodic reactant such as for example suiphur/polysuiphides, the various materials would have to be chemically inert with respect to the cell materials at the operating temperature of the cell. For a sodium sulphur cell, conveniently the ceramic annular element is formed of alpha-alumina. The soft metal interlayer may be formed of aluminium or aluminium alloy.

Very conveniently the aluminium interlayer between the hard metal and the ceramic is extended beyond the seal region, for example to form an internal protective coating protecting the outer hard metal members from contact with the anodic and cathodic reactants in the cell.

One of the two hard metal annular members may be formed integrally with an outer casing for the cell. This member, which may be formed for example of steel, would be secured on the shaped surface at that end of the ceramic element which is joined to the electrolyte tube, the casing extending from the annular portion around the outside of the electroyte tube which has the anodic and cathodic reactants on opposite faces. Preferably in a sodium sulphur cell, the sodium forming the anode is arranged in the region outside the electrolyte tube and the sulphur/polysulphides forming the cathodic reactant is put inside the electrolyte tube.

In the following description of two embodiments of the invention, reference will be made to the accompanying drawings in which: Figure 1 is a diagrammatic axial section through part of a ceramic cylinder with metal rings secured thereto for use in a sodium sulphur cell; and Figure 2 is a diagrammatic axial section through part of another construction of cer amic cylinder with metal rings secured thereto for use in a sodium sulphur cell.

Referring to Figure 1, there is shown part of a cylindrical ceramic element 10 which, in this particular embodiment is an alpha-alumina ring suitable for securing onto the open end of a beta-alumina ceramic tube (not shown) constituting a solid electrolyte in a sodium sulphur cell. At one end of the element 10, its inner and outer faces are tapered as shown at 11 and 12 respectively with straight tapers such that the outer diameter decreases and the inner diameter increases towards the end of the member 10. In this particular example, the two tapers have the same slope but this is not essential. Secured on these tapered faces are on outer ring 13 and an inner ring 14. For a sodium- sulphur cell in which the sodium is on the outside of the ceramic electrolyte and the cathodic reactant (sulphur/polysulphides) inside the electrolyte, the outer ring 13 may be formed of mild steel or Nilo K and the inner ring 14 may be formed of steel plated, e.g. ion plated, with a protective coating, e.g. of a nickel chromium alloy. These rings have tapered surfaces complementary to the tapers on the ceramic material. A thin layer of alumnium is provided between each of the tapered surfaces. In this particular embodiment the aluminium is made in the form of two cones 15, 16, one of which is put inside the inner taper and the other is put over the outer taper. In a typical case, this aluminium, after final assembly of the ring, might be 0.2 mm thick but, for clarity in the drawings, its thickness is exaggerated. A high purity, i.e. a soft aluminium is used and these cones 15, 16 are conveniently formed as pressings.

To make the assembly, the inner and outer metal rings 14, 13 are pushed onto the ceramic material 10 and an axial load is applied onto the metal elements with respect to the ceramic material to force the metal elements tightly onto the ceramic giving a deformation bond.

The loading is applied with the assembly at room temperature. For this cold pressure bond, there must be relative movement of the surfaces, which removes any oxide layer. Conveniently the movement is between 1 and 10 mm in the axial direction and is such as to reduce the thickness of the aluminium by at least 50% over at least a part of the aluminium layer extending completely around the ceramic. However, the relative movement depends on the angle of taper. This pressure produces a tight bond giving a hermetic seal between the components. The seal formed in this way has been found to be helium-tight but it is preferred to anneal the assembly causing diffusion to occur between the bonded layers so increasing the strength of the bond. In one embodiment, the coefficient of linear expansion of the ceramic material, over the temperature operating range of a sodium sulphur cell, typically from room temperature up to 350 or 4000C, is slightly greater than the coefficients of linear expansion of the Nilo K (Nilo is a Registered Trade Mark) outer ring 13 and less than the coefficient of expansion of the plated steel inner ring 14. The radial compressive forces on the ceramic material due to these differing expansions when the cell is brough up to its operating temperature will be in opposite directions. The hoop stress in the ceramic material depends only on the difference between these radial compressive forces and is therefore necessarily less than would be the case if a single ring were shrunk onto the outside or the inside of the ceramic.

Local increase in the compressive forces during the bonding operation are obtained by shaping the ceramic material, for example by forming circumferential grooves 20, 21. It will be seen that.the radial compressive load is distributed over a smaller area hence resulting in greater pressure in the localised regions but that the hoop stress will remain unchanged if the same materials and overall dimensions are used.

The soft metal flows into the groove and, because of this, the interfacial pressure will be very substantially relieved in the region immediately adjacent the groove where the soft metal can readily flow away into the groove. The pressure area is thus in the regions away from the groove. Such a groove therefore permits of selective choice of regions of higher pressure.

After the deformation bond has been formed, as described above, the assembly is annealed to relieve any remaining local stress.

In the above-described arrangements, a separate aluminium element has been put between the ceramic tapered surface and the annular metal element 13 or 14 to form the soft deformable metal. In some cases, it may be preferred to put aluminium coatings on the ceramic tapered surface and/or on the cooperating tapered surface of the metal element.

Preferably the coating is put on both surfaces.

Ion-plating may be used for forming these coatings.

In designing the annular metal elements 13 and 14, their thickness, and the yield points and moduli of elasticity should be taken into account to ensure that they will deform before the ceramic material is over-stressed.

Figure 2 of the drawings illustrates a construction in which two annular metal elements are secured on external surfaces of a ceramic member for use in a sodium sulphur cell. Referring to Figure 2, there is shown part of a cylindrical ceramic electrolyte tube 30 which, in this particular embodiment, is formed of beta-alumina and which serves to separate an anodic region from a cathodic region. This electrolyte tube 30 is closed at one end (the lower end in the drawing). In this embodiment the anodic region 31 containing liquid sodium is on the outside of the electrolyte tube 30.

Inside this tube is the cathodic region containing, in the known way, a porous matrix of electronically-conductive material, e.g. carbon or graphite fibre 32 impregnated with sulphur/ polysulphides and with a central cathode current collector. This current collector comprises an outer tube 33 of electrically-conductive material which is impermeable and chemically and electrocemically inert to the cathodic reactant, and within the tube 33 is a conductive metal core 34 with a deformable interface material 35 between the core 34 and the tube 33. The cell has an outer steel housing 36 which extends around the outside of the anodic region 31.

An annular element 40 which is formed of ceramic material having a coefficient of thermal expansion similar to that of the electrolyte tube 30 and which is an electronic insulator and also an ionic insulator is secured to the upper end of the electrolyte tube. Conveniently this member 40 is formed of alpha-alumina and is bonded by glass 41 to the end of the electrolyte tube.

The member 40 has a radial thickness substantially greater than that of the electrolyte tube 30 so as to be capable of withstanding substantially higher radial stresses than the electrolyte tube. This member 40 is shaped on its external surface to have two tapered regions 43 and 44.

The lower region 43 tapers from a maximum diameter halfway up the height of the element 40 to a minimum diameter at the lower extremity. The tapered surface 44 extends over the upper half of the element 40 and is tapered in the opposite sense. The two tapers are of substantially the same angle and are linear tapers.

Annular grooves 45, 46 are formed around the element 40 in respective tapered surfaces 43, 44. The outer housing 36, at its upper end is formed of Nilo K (Nilo is a Registered Trade Mark) and is flared outwardly as shown at 47 to have a shape corresponding to the taper on the surface 43. A top closure member 49 has a downwardly-extending flange portion 50 at an angle conforming to the slope of the tapered surface 44. An interlayer 51 of a deformable metal such as aluminium or aluminium alloy is provided between the flared portion 47 of the housing and the surface 43 and a similar interlayer 52 is provided between the sloping flange portion 50 and the tapered surface 44. Preferably the interlayers 51, 52 are formed of a high purity aluminium.

Deformation seals are made between the outer housing 36 and the element 40 and between the top closure 49 and the element 40 by loading the closure member 49 in the direction of the axis of the assembly against a reactive load on the housing 36. This loading is effected at room temperature. The surfaces are shaped so that relative movement of between 1 and 10 mm occurs between the ceramic and metal elements at each of the bonds. The loading causes the interlayers 51, 52 to be extensively deformed during relative movement between the housing 36 and the closure 49 with a reduction in thickness of the aluminium of at least 50% over at least a part of the seal region extending completely around the ceramic tube.

This extensive deformation breaks up the oxide surface layer on the material or the inerlayers and exposes the pure metal surface. Deformation can also scrub oxide layers from the inner surfaces of the housing and the top closure member where they extend over the tapered surfaces. The freshly exposed metal of the interlayers makes contact with the metal housing 36 and the top closure member 49 and also to the alpha-alumina element 40 so that all three are sealed together.

The grooves 45,46 reduce the area of contact of the tapered surface and thus increase the compressive stress on the interlayer and thereby cause increased deformation. These grooves provide a region into which material from the deformed interlayers is able to flow. This reduces the interfacial pressure in the immediate region of the grooves and hence causes, for any given axial loading of the assembly, increased interfacial pressure in other parts remote from the groove. It thereby increases the amount of deformation that is able to occur at a given stress level. The provision of such a groove or grooves enables a non-linear pressure distribution to be obtained with the maximum stress occurring in chosen areas.

The interlayers 51, 52 of soft deformable metal might be coatings bonded to one or both of the tapered surfaces but, as illustrated in the drawing, are preferably separate elements which conveniently extend beyond the region of the seal to perform further functions. The lower interlayer 51 extends down inside the housing 36 to form a protective barrier 54. The anodic region 31 between this barrier 54 and the electrolyte tube 30 is formed as a capillary region or contains a sodium electrode capillary feeder means so that the outer surface of the electrolyte tube 30 can be kept wet with liquid sodium from a sodium reservoir. Such an interlayer member 51, 54 may conveniently be formed as deep drawn tube of aluminium or aluminium alloy with a hemispherical closed lower end. Preferably high purity aluminium is used since this very soft material facilitates the formation of the required cold pressure seals.

Suitable small holes or fissures may be formed in this layer at appropriate locations to allow a restricted feed of sodium metal from the sodium reservoir. In a cell having a cylindrical casing, the sodium is conveniently stored in a reservoir (not shown) beyond the closed end of the ceramic electrolyte tube 30. In a cell with a square section housing, the sodium may be stored between the outer surface of the electrolyte tube and the inner surface of the outer casing. In such a cell, the housing 36 may be of substantially square cross section over the greater part of its length with a short transformation region from the square cross section to a round cross section at the upper end where the housing is formed with a flared portion 47 of conical form which is sealed to the alpha alumina element 40.

The upper interlayer 52 extends across the top of the alpha-alumina element 40 across the top of the cell to the current collector as a protective lining 55 inside the closure member to give protection against corrosion by the cathodic reactants during operation of the cell. This interlayer 52, 55 is conveniently also used as an interlayer for a deformation seal 56 between the closure member and the outer tube 33 of the current collector; this current collector is provided with an internal closure 57 through which electrical connection between the current collector and external terminal is effected.

The cold pressure bonding technique described above enables the cell to be sealed with the sulphur electrode in place.

In the embodiments described above, two seals are made simultaneously by a cold pressing technique. These seals could be made in separate operations. In this case, one of the seals may be made by cold pressing as described above and the above may include a heating stage as described in the specification of Application No. 20959/76 (Serial No. 1574804). For example in a sodium sulphur cell of the central sulphur type, i.e. having the sulphur electrode within an electrolyte tube, the outer seal might be made using heating as described in Application No. 20959/76 (Serial No. 1574804) and the inner seal could be made by cold pressing as described above, preferably under vacuum after insertion of the sulphur electrode. In this instance, both the inner tapered ring and the outer tapered ring should have coefficients of expansion slightly greater than that of the ceramic (e.g. an alpha alumina ring on the end of the electrolyte tube) to which the metal rings are to be sealed. The seals and the alpha alumina would be under compressive stress at the operating temperature of the cell.

In the specification of Application No.

18870/77 (Serial No. 1586072) out of which the present application is partially divided, there is claimed a method of sealing annular metal members to

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

alumina element 40.

The upper interlayer 52 extends across the top of the alpha-alumina element 40 across the top of the cell to the current collector as a protective lining 55 inside the closure member to give protection against corrosion by the cathodic reactants during operation of the cell. This interlayer 52, 55 is conveniently also used as an interlayer for a deformation seal 56 between the closure member and the outer tube 33 of the current collector; this current collector is provided with an internal closure 57 through which electrical connection between the current collector and external terminal is effected.

The cold pressure bonding technique described above enables the cell to be sealed with the sulphur electrode in place.

In the embodiments described above, two seals are made simultaneously by a cold pressing technique. These seals could be made in separate operations. In this case, one of the seals may be made by cold pressing as described above and the above may include a heating stage as described in the specification of Application No. 20959/76 (Serial No. 1574804). For example in a sodium sulphur cell of the central sulphur type, i.e. having the sulphur electrode within an electrolyte tube, the outer seal might be made using heating as described in Application No. 20959/76 (Serial No. 1574804) and the inner seal could be made by cold pressing as described above, preferably under vacuum after insertion of the sulphur electrode. In this instance, both the inner tapered ring and the outer tapered ring should have coefficients of expansion slightly greater than that of the ceramic (e.g. an alpha alumina ring on the end of the electrolyte tube) to which the metal rings are to be sealed. The seals and the alpha alumina would be under compressive stress at the operating temperature of the cell.

In the specification of Application No.

18870/77 (Serial No. 1586072) out of which the present application is partially divided, there is claimed a method of sealing annular metal members to the open end of a ceramic electrolyte tube for an electrochemical cell comprising the steps of providing a rigid annular element of a ceramic material having a coefficient of expansion substantially the same as that of the electrolyte material, forming upper and lower shaped surfaces on said annular ceramic element at or near the ends thereof, the shaping being arranged so that the outside diameter decreases in the direction towards the respective end of the ceramic annular element, putting conical metal elements or coatings over the shaped surfaces, such metal elements or coatings being of relatively soft metal compared with said annular metal members, putting said annular metal members over the respective soft metal covered upper and lower shaped surfaces of the ceramic element, and the forcing the annular metal members axially together to deform the soft metal so that said metal members are secured on the ceramic element, said ceramic annular element being secured to the electrolyte tube either before putting on the metal elements or after forming the assembly with the metal elements.

In the last-mentioned specification, there is also claimed an electronichemical cell having a ceramic electrolyte tube closed at one end and separating anodic and cathodic reactants, wherein an annular element of ceramic material is sealed with glass to the open end of the electrolyte tube, said annular element having shaped external surfaces such that, over these surfaces, the diameter of the element decreases from a maximum diameter intermediate the ends of the element to minimum diameters at or near the respective ends of the element, soft metal being provided over said shaped surfaces and outer metal elements being provided around said shaped surfaces and secured thereto by the soft metal, one of said metal elements forming part of a housing enclosing the electrolyte tube and the other of the metal elements forming a top closure extending at least partly across the top of said annular ceramic element.

WHAT WE CLAIM IS: 1. A method of sealing an annular metal element or elements to a cylindrical ceramic element, one of the elements to be sealed together having an external cylindrical surface shaped so that the diameter of that surface varies along the axis and the other element having an inside cylindrical surface shaped so that its inside diameter varies along the axis whereby the shaped surfaces can be nested together, the two elements being pressed together in an axial direction with an interlayer of a deformable metal between said shaped surfaces, the pressure and the relative movement being such as to cause deformation of at least part of the interlayer around the whole of the annular region between the elements with the formation of cold pressure bonds giving hermetic seals between the interlayer material and both the inner and outer elements.
2. A method as claimed in Claim 1 wherein said surfaces are shaped so that a relative movement of between 1 and 10 millimetres in the axial direction causes a reduction in thickness of at least 50% of at least a part of the interlayer extending around the whole of the annular region by deformation of this interlayer.
3. A method as claimed in either Claim 1 or Claim 2 wherein the annular metal element is outside the ceramic element and has a lower coefficient of thermal expansion than that of the ceramic element.
4. A method as claimed in either Claim 1 or Claim 2 wherein the metal element is inside the ceramic element and has a higher coefficient of thermal expansion than that of the ceramic element.
5. A method as claimed in either Claim 3 or Claim 4 wherein the ceramic material is arranged between the inner and outer annular

elements, each of which is sealed to the ceramic material.
6. A method as claimed in any of the previous claims when the deformable metal interlayer is formed of aluminium or aluminium alloy.
7. A method as claimed in any of the preceding claims wherein said interlayer comprises an interlayer element of aluminium formed by a pressing operation and put between the surfaces of the ceramic and annular metal elements.
8. A method as claimed in any of Claims 1 to 6 wherein said interlayer comprises an aluminium coating on one or both of the elements.
9. A method as claimed in any of the preceding claims wherein said shape surfaces of the two elements comprise correspondingly tapered conical surfaces.
10. A method as claimed in any of the preceding claims wherein at least one annular groove is formed on the shaped surfaces of one of the elements.
11. A method as claimed in Claim 10 wherein said annular groove is formed on the ceramic element.
12. A method as claimed in any of Claims 1 to 9 wherein the ceramic material has a step with a shoulder facing towards the end of the cylindrical ceramic element.
13. A method as claimed in any of the preceding claims wherein said cylindrical element comprises a ceramic tube with inner and outer tapered surfaces and wherein two separate annular metal elements are secured thereto.
14. A method as claimed in Claim 13 and for sealing a sodium sulphur cell in which the sodium is outside a ceramic electrolyte tube and the sulphur/polysulphide cathodic reactant is on the inside of the electrolyte tube and wherein said outer annular metal element is formed of a low expansion metal and wherein the inner annular element is formed of a steel with a coating of a nickel-chromium material and wherein the soft metal is aluminium.
15. An assembly comprising a cylindrical ceramic member having shaped inner and outer surfaces at or near one end of the member, these surfaces being such that the outer diameter towards the end of the member and the inner diameter increases towards the end of the member, and inner and outer annular elements of a hard metal having complementary tapered surfaces bonded to the shaped surfaces of the ceramic member with interlayers of a soft metal between the ceramic and each of the inner and outer elements by cold pressure bonding as claimed in any of the preceding claims.
16. An assembly as claimed in Claim 15 and for use in a sodium sulphur cell wherein the ceramic member is a tube or cylinder of betaalumina or alpha-alumina and wherein the inner annular metal member is a corrosion-resistant alloy or mild steel protected by a coating of a corrosion-resistant alloy and wherein the outer metal annular element is mild steel or a low expansion alloy.
17. A method of sealing annular metal elements to the open end of a ceramic electrolyte tube for an electrochemical cell comprising the steps of providing a rigid annular element of a ceramic material having a coefficient of expansion substantially the same as that of the electrolyte material, forming upper and lower shaped surfaces on said annular ceramic element at or near the ends thereof, the shaping being arranged so that the outside diameter decreases in the direction towards the respective end of the ceramic annular element, putting conical metal elements or coatings over the shaped surfaces, such metal elements or coatings being of a relatively soft metal compared with said annular metal elements putting said annular metal elements over the respective soft metal covered upper and lower shaped surfaces of the ceramic element, and then forcing the annular metal elements axially together to deform the soft metal the pressure and relative movement being such as to cause deformation of at least part of soft metal between the shaped surfaces around the whole of each annular region between the ceramic and the annular metal elements with the formation of cold pressure bonds giving hermetic seals, the deformation causing a reduction in thickness of at least 50% of at least a part of the soft metal interlayer, the annular ceramic element being sealed to the electrolyte tube either before or after the metal elements have been assembled to the annular ceramic element.
18. A method as claimed in Claim 17 wherein the shaped surfaces of the ceramic material are tapered surfaces with an annular groove formed in each tapered surface of the ceramic.
19. A method of sealing an annular metal element or elements to a cylindrical ceramic element substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.
20. A sodium sulphur cell having an end closure sealed by the method of any of the preceding claims.