A SODIUM ELECTRODE ENERGY CONVERSION DEVICE AND A METHOD OF CLOSING THE CASE OF A SODIUM ELECTRODE ENERGY CONVERSION DEVICE

Abstract: A sodium electrode energy conversion device includes a case closed by a closure member. The case and the closure member are sealed together at interfacing surfaces thereof. At least one of the case and the closure member is formed from a composite material comprising a metal substrate and a layer of a metal more ductile than the substrate on at least the interfacing surface of the substrate. The case and the closure member are joined together by a roll seam joint.

Description: The present invention relates to a sodium electrode energy conversion device and, in particular, to a method of closing the case of such a device. Examples of sodium electrode energy conversion devices include sodium sulphur cells, sodium metal chloride cells and sodium-sodium cells. In conventional lead acid batteries, a liquid electrolyte - dilute sulphuric acid - separates two solid electrodes. In contrast, a sodium/sulphur cell uses a solid electrolyte - generally beta alumina - which separates two liquid electrodes, namely liquid sulphur and liquid sodium electrodes. Such a sodium/sulphur cell is shown in Figure l of the drawings which is a perspective view of the cell with an area of the case shown removed to reveal its construction. As shown, the cell comprises a case, for example of steel with an internal anti-corrosion coating of aluminium or aluminium alloy, in the form of a right circular cylinder. The case contains a solid electrolyte cup 2 of beta alumina, the cup 2 containing a sodium electrode 3. A space between the case 1 and the cup 2 contains a sulphur electrode 4. For use, the cell is maintained at a temperature of between 300°C and 400°C such that the sodium and sulphur electrodes 3, 4 are in liquid form. The open end of the cup 2 is closed by an insulating disc 5 of alpha alumina. The case 1, itself, is closed by an annular sealing closure disc 6 of steel, again with an internal anti-corrosion coating of aluminium or aluminium alloy. The case 1 serves as a terminal for the sulphur electrode 4. The sodium electrode 3 contains an elongate metal current collector 8 which extends axially of the case 1 out through the disc 5 where it is connected to a centre terminal disc 7 mounted on the disc 5. The necessary connections are made by welding. As sulphur is essentially non-conducting, a means of making an electrical connection between the case 1 and the cup 2 has to be provided. This is generally achieved by forming the sulphur electrode 4 as a carbon fibre mat impregnated with sulphur. It will be appreciated that with such a cell, the sodium and sulphur electrodes 3, 4 can have their locations reversed. With such a cell it is necessary for the alpha alumina disc 5 to seal the open end of the beta alumina cup 2 and this is generally effected by a glazing technique. It is also necessary for the disc 6 and the terminal disc 7 to be secured to the alpha alumina disc 5 to form seals. It is also necessary for the disc 6 to be secured to the case 1, and this is generally effected using electron beam, laser or TIG fusion welding techniques. However, a difficulty which arises when using the aforementioned techniques is that the heat required can damage the corrosion resistant coating provided on the case l and on the closure disc 6, or can create a heat affected zone in the weld region, either of which can result in a more rapid than usual localised degradation in the cell due to corrosive attack by the cell electrode materials. Generally, it has been found difficult to provide a sufficiently hermetic bond between the case and the closure member of prior art sodium/sulphur cells. It is an object of the present invention to provide an improved sodium electrode energy conversion device and a method of making such a cell. According to a first aspect of the present invention, there is provided a sodium electrode energy conversion device including a case closed by a closure member, the case and the closure member being sealed together at interfacing surfaces thereof, at least one of the case and the closure member being formed from a composite material comprising a metal substrate and a layer of a metal more ductile than the substrate on at least the interfacing surface of the substrate, wherein the case and the closure member are joined together by a roll seam joint. The inventors have surprisingly found that a satisfactory seal for a sodium electrode energy conversion device can be produced by a roll seam joint between the case and closure member if at least one of the case and the closure member is made of a composite material as defined. The roll seam joint was found to be sufficiently hermetic throughout the temperature ranges experienced by a sodium electrode energy conversion device, such as a sodium/sulpur cell, in operation, from ambient temperature to 350°C (the nominal operating temperature of the cell) despite the mis-match of thermal expansion of the deformable metal and the substrate at their interfaces. According to a second aspect of the present invention there is provided a method of closing a case of a sodium electrode energy conversion device, by a closure member, the case and the closure member being sealed together at interfacing surfaces thereof, at least one f the case and the closure member being formed of composite material comprising a metal substrate and a layer of a metal more ductile than the substrate on at least an interfacing surface of the metal substrate, the method including the step of rolling together adjacent parts of the closure member and the case whereby a roll seam joint is produced. As outlined previously, the inventors have surprisingly found that a satisfactory seal can be produced between the case and the closure member of a sodium electrode energy conversion device, such as a sodium/sulphur cell, using a roll seam joint. The method of the present invention has the advantage that the localised elevated temperatures of the prior art methods are not required and so no heat damage of the corrosion resistant coating or heat affected zone in the seal region is produced. Accordingly, the prior art problem of a more rapid than usual localised degradation in the cell due to corrosive attack of the case or closure member by the cell electrode materials is alleviated. Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a prior art sodium/sulphur cell as described previously; Figure 2 is a flow chart illustrating schematically a preferred embodiment of the method of the present invention; Figures 3 and 4 show first and second embodiments of a closure member for use in the present invention; Figure 5 shows a case of a sodium/sulphur cell for use with the present invention; Figure 6 shows an approximation of the stages of the rolling operation, for forming a roll seam joint; Figure 7 is a view of a part of a sodium/sulphur cell provided in accordance with the present invention showing the roll seam joint; and Figure 8 is a drawing taken from a photograph of a cross section of a roll seam joint in a sodium/sulphur cell provided in accordance with the present invention. A preferred embodiment of the method of the present invention is illustrated schematically in Figure 2. As indicated, the first step is to form the closure member and the case from a composite material consisting of a steel substrate on at least one surface of which is mechanically fixed a layer of a ductile metal, such as aluminium or an aluminium alloy, which is more ductile than the substrate. In one practical embodiment, the composite material comprises a deep drawing quality 250 microns thick low carbon steel substrate with an aluminium coating of thickness 60 microns on one surface and an aluminium coating of thickness of 25 microns on the other surface. In another practical embodiment, the composite material comprises a low carbon steel substrate of thickness 250 microns with an aluminium coating of thickness 300 microns on one surface and an aluminium coating of thickness 60 microns on the other surface. The surface with the thicker aluminium coating provides the inner surfaces of the case and closure member which need to be corrosion resistant. Figure 3 is a cross sectional view of one embodiment of a closure member 10 for use with the present invention. As described previously, the inner surface 12, which needs to be corrosion resistant, has an aluminium coating of thickness 60 microns whereas the outer surface 14 has an aluminium coating of thickness 25 microns. The closure member 10 includes an outwardly extending annular flange 16 which is shown in enlarged detail in Figure 3A. As shown in Figure 3A, an annular lip 18 is provided at the end of the flange 16. Typically, for a closure member of external diameter "D" of about 54 mm, the flange 16 has a radial dimension "B" of about 5 mm. Figure 4 shows a second embodiment 20 of a closure member for use with the present invention. As with the embodiment of Figure 3, the inner surface 22 of the closure member, which needs to be corrosion resistant, has an aluminium coating of thickness 60 microns, whereas the outer surface 24 has an aluminium coating of thickness 25 microns. The closure member 20 includes an outwardly extending annular flange 26, shown in enlarged detail in Figure 4A, but no lip is provided at its edge. Typically, a closure member 20 of external diameter "D" of about 58 mm has a flange 26 of radial dimension "B" of about 7 mm. Figure 5 is a schematic representation of a case 30 for a sodium/sulphur cell provided in accordance with an embodiment of the present invention. As described previously, the case 30 is formed of a steel substrate of thickness 0.25 mm having an aluminium coating of thickness 60 microns on its inner surface 32 and an aluminium coating of thickness 20 microns on its outer surface 34. The thickness of the composite material is indicated by "t". The case 30 is provided with an outwardly extending annular flange 36. Typically, for a case of external diameter "d" of about 50 mm, the flange 36 has a radial dimension "A" of about 4 mm. The case 30 may also be formed with a kink 38 at its open end, as indicated in Figure 6. The closure members 10, 20 and the case 30 are formed from the composite material by methods known to those skilled in the art. Advantageously, as indicated in Figure 2, the case and the closure member are then heat-treated by being heated to an elevated temperature, maintained at that temperature for a period of time and then slowly cooled to ambient temperature. A typical thermal profile of the regime is as follows: ramp to 375°C at 7°C per minute; hold at 375°C for one hour; cool to ambient temperature at 7 C per hour or greater. It is believed that this heat-treatment either stress relieves the steel to prevent spring back after the roll seam joint has been produced, or anneals the aluminium to aid in the deformation of the aluminium during formation of the roll joint. The inventors have found that heat-treatment of at least one of the closure member and cell case prior to rolling has a dramatic effect on the hermeticity of the roll seam joints produced. It is to be noted that this step of heat-treatment could be either an additional step in the method of manufacturing a sodium/sulphur cell or could take place due to another manufacturing step, such as diffusion bonding of the closure member to an alpha alumina disc. After heat treatment of the case and closure member, the sodium/sulphur cell is then assembled and located in the roll forming apparatus, as indicated in Figure 2. The configuration of the case and closure member at this stage is shown in Figure 6A. Roll forming machines are known to those skilled in the art and so will not be described in detail. After location, the cell assembly is raised on to a central mandrel. As the cell assembly comes into contact with the mandrel, a clutch is operated, causing a first set of rollers to be closed on to the cell assembly for the first rolling operation. During the rolling operation, the cell assembly is stationary and the first set of rollers are rotating as they approach the flanges 16 or 26, 36. An approximation of the configuration of the cell assembly after the first rolling operation is shown in Figure 6b. Once the first rolling operation has been completed, the first set of rollers are returned to the "open" position and a second set of rollers are operated for the second rolling operation. The configuration of the cell assembly after the second rolling operation is shown in Figure 6C. Finally, the second set of rollers are returned to the rest position and the cell assembly is released from the central mandrel. Figure 7 shows, in cross section, a part of a sodium/sulphur cell 40 provided in accordance with one embodiment of the present invention. A sodium electrode 42 and a sulphur electrode 44 are separated by a beta alumina solid electrolyte cup 46 which contains the liquid sodium. The electrolyte cup 46 is closed by an insulating disc 48 of alpha alumina which is sealed to the cup 46 by a glassy seal 50. The sulphur electrode 44 is contained within the case 52 of the cell. A closure member 54 is sealed to the case 52 by a roll seam joint 56, as outlined previously, to close the case 52. To accommodate the roll seam joint 56, the case 52 is kinked inwardly at 58 so that the overall diameter of the sodium/sulphur cell is no greater than the maximum diameter of the case 52. That part of the roll seam joint 56 which can be accessed by the sulphur 44 is shown in greater detail in Figure 7a. The aluminium coatings of thickness 60 microns are indicated by the letter "a" and the aluminium coating of thickness 25 microns by the letter "b". From Figure 7, it can be seen that little potential exists for damaging the internal protective aluminium coatings 52a, 54a of the cell at the roll seam joint as those parts of the coatings 52A, 54A which can be accessed by the sulphur 44 are not much affected by the formation of the roll seam joint. It has been found that the amount of material that is rolled into the roll seam joint 56 affects the hermeticity of the joint. If B - t > 2A - t will tend to be hermetic. If B - t = 2A - t will be variable. If B - t < 2A - t will tend to leak. Those skilled in the art will appreciate that the hermeticity required for any joint or seal in a structure will depend on the position of the joint or seal in that structure. In equations (1) , (2) and (3) , the values of B, t and A are as defined in Figures 3, 4 and 5 of the accompanying drawings, namely: B = radial dimension of flange of closure member; t is the thickness of the composite material, ie. the thickness of the case or of the closure member; A is the radial dimension of the flange on the case. If there is a deficiency of material at C, the roll seam joint may leak. Sodium/sulphur cells including a roll seam joint between the closure member and the case, manufactured in accordance with the present invention as described previously, were subject to 50 thermal cycles up to 350°C as follows: heat up to 350°C at 3°C per minute; maintain at 35o°C for three hours; cool down to 20°C at 3°C per minute. The roll seam joints were hermetic on manufacture and remained hermetic after the above thermal cycling. In order to maximise the hermeticity of the roll seam joint, a certain amount of relative movement and formation between the aluminium coatings of the case and closure member should be introduced. Accordingly, the closure member is formed such that the flange of the closure member will fold upon itself during the rolling operation. Figure 8 shows a roll seam joint 60, so produced, between a case 62 and a closure member 64. For clarity, the ends of the steel substrate of the composite material from which the case and closure member have been manufactured have been referenced, respectively 62S and 64S. The aluminium in the roll seam joint is referenced by the numeral 66. It can be seen from Figure 8 that a certain amount of relative movement and deformation, under pressure, of the aluminium coatings of the case and the closure member has occurred during the rolling operation, resulting in the formation of a cold weld. Accordingly, the flange (as described previously) of the closure member is formed with a sufficiently large radial dimension, or may include a lip as described with reference to Figure 3, to provide sufficient of the composite material to cause the relative movement between the aluminium coatings of the case and closure member. It would therefore appear preferable that both the inner and outer surfaces of both the case and closure member should be provided with a deformable aluminium coating. It is however envisaged that a sufficiently hermetic roll seam joint can be produced in which only one surface of each of the case and closure member was provided with a deformable metal layer. Table 1 shows the results of an investigation into the effect of different material combinations at the roll seam interface on the hermeticity of the joint produced by rolling. Each of three different material combinations was investigated by the construction of case/closure member assemblies. In order to ensure that the hermeticity of the rolled joint was not affected by the component rolling sequence, one cell from each group of material combinations was rolled in turn. After rolling, each assembly was leak tested. After leak testing, all of the assemblies were subjected to a thermal cycle up to 400°C with a ramp rate of 15°C/min with a one hour hold. After the thermal cycle, the assemblies were again leak tested. Table 1 Sample No. Leak Rate After Rolling Leak Rate After (Torr Litre Per Second) Thermal Cycling (Torr Litre Per Second 5 -4 3xlθ "3 3X10 6 NDL NDL 7 NDL NDL 8 -9 1X10 -9 1X10 9 NDL NDL 10 -9 5x10 -9 8X10 11 -4 7X10 -6 4X10 12 -5 6X10 -8 2X10 3 4x10 * -7 2X10 4 -4 5x10 -7 1X10 5 -4 2X10 -8 6x10 (NDL indicates no leak was detected) The combinations of materials used for the case and the closure member for each of the samples is indicated in Table 2 below. Table 2 Sample Nos. Cell Case Top Cap Description 1 to 5 Composite Steel Al/Fe/Al composite Material pressed cell case with internal aluminium coating of 25 microns and external aluminium coating of 60 microns on a mild steel substrate of 250 microns. Cell case annealed for one hour at 400°C prior to rolling. Pressed steel top cap with no aluminium coating on either surface. to 10 Composite Composite Al/Fe/Al composite Material Material pressed cell case with internal aluminium coating of 25 microns and external aluminium coating of 60 microns on a mild steel substrate of 250 microns. Table 2 fCont/d. Sample Nos. Cell Case Top Cap Description 6 to 10 Pressed Al/Fe/Al (Cont/d) composite top cap with internal aluminium coating of 25 microns and external aluminium coating of 60 microns on a mild steel substrate of 250 microns. Both components annealed for one hour at 400°C prior to rolling. 1 to 15 Steel Composite Mild steel spun cell Material case of thickness of 300 microns (250 microns prior to spinning) . Spun Al/Fe/Al composite top cap with internal aluminium coating of 25 microns and external aluminium coating of 60 microns on a mild steel substrate of 250 microns. Top cap annealed at 400°C for one hour prior to rolling. From Table 1, it can be seen that the group of assemblies (samples 6 to 10) in which both the cell case and the top case were made of a composite material had roll seam joints of a hermeticity greater than the hermeticity of the assemblies in which only one of the components was made of a composite material. Accordingly, for roll seam joints in which the hermeticity of the seal is critical, a seal in which both components are made of the composite material is preferred. The samples in which only one of the cell case and top cap was made of the composite material (samples 1 to 5 and 11 to 15) had a more variable hermeticity than the group in which both components were made of composite material. However, it was found that the hermeticity of the seals was, in fact, improved by the thermal cycling. Roll seam joints in which only one of the components is made of the composite material may have sufficient hermeticity, particularly after thermal cycling, for certain applications. In roll seam joints in which both the components are made of the composite material, the aluminium coatings provide some degree of corrosion resistance for both of the components. The corrosion resistance of the roll seam joint may be improved by increasing the thickness of the aluminium coating. In roll seam joints where only one of the components is made of the composite material, the material of the other component must be chosen with the environment of the roll seam joint in mind. For example, constructions having an interface of steel and aluminium are susceptible to attack by sodium. Therefore, a roll seam joint which is adjacent to sodium may have one ■ component made of the composite material and the other component made of a material other than mild steel, such as Inconel. For cells in which a roll seam joint is at an outer edge of one end, the outer electrode is adjacent the roll seam joint and is a sodium electrode if the cell is a sodium-sodium cell, a central sulphur cell or a central metal chloride cell. A significant advantage of the present invention is that the roll seam joints for the sodium electrode energy conversion devices can be produced using standard machines and pressures to provide practical embodiments. An attempt was made to produce an assembly with a roll seam joint between two components of mild steel without any coating of a metal more ductile than the steel. However, any seal produced between the two components of mild steel was so poor that it was not possible to reduce the pressure in the assembly to a level at which the assembly could be leak tested. This suggests that the hermeticity of any seal produced was at least two orders of magnitude worst than the hermeticity of the assemblies of Tables 1 and 2. It is envisaged that a roll seam joint having at least some degree of hermeticity can be produced between two components if one of the components is made of a composite material having a layer of aluminium, or other ductile material, of at least 5 microns. A coating of this thickness should include sufficient aluminium to fill in all the surface irregularities. Such a thickness of aluminium should also mean that relative movement of adjoining surfaces of aluminium can occur during formation of the roll seam joint. This relative movement at the surfaces of the aluminium disrupts the aluminium oxide from the surfaces, producing clean surfaces of aluminium between which a diffusion bond can be formed, in effect, cold welding at ambient temperature. In roll seam joints where both components are made of the composite material, relative movement will occur between the interfacing surfaces of the aluminium coatings on the two components. In roll seam joints where only one of the components is made of the composite material, relative movement of aluminium between interfacing surfaces will occur at a position where the component of composite material is rolled back on itself. To provide a bulk of aluminium to enable relative movement at the surfaces of the aluminium, the aluminium coatings preferably have a thickness of at least 10 microns, advantageously at least 25 microns. Thicker coatings of aluminium may be used to improve the corrosion resistance of the components in addition to providing aluminium for the roll seam joint. Modifications to the embodiments described, within the scope of the present invention, will be apparent to those skilled in the art. In particular, it is envisaged that a sodium/sulphur cell in accordance with the present invention may be manufactured from a composite type material which was formed of separate layers of aluminium and steel.

Claims: CLAIMS 1. A sodium electrode energy conversion device including a case closed by a closure member, the case and the closure member being sealed together at interfacing surfaces thereof, at least one of the case and the closure member being formed from a composite material comprising a metal substrate and a layer of a metal more ductile than the substrate on at least the interfacing surface of the substrate, wherein the case and the closure member are joined together by a roll seam joint. 2. A device according to Claim 1 wherein the composite material has a layer of a ductile metal on each surface. 3. A device according to Claims 1 or 2 wherein said layer of a ductile metal is mechanically fixed to said metal substrate. 4. A device according to any one of the preceding Claims wherein the closure member is formed to include an outwardly extending annular flange. 5. A device according to any one of the preceding claims wherein an annular lip is provided on the annular flange. 6. A device according to any one of the preceding claims, an edge of the case defining an opening which is closed by the closure member. 7. A device according to Claim 6 wherein said edge of the case is at an end of the case. 8. A device according to Claims 6 or 7 wherein said edge of the case includes an outwardly extending annular flange. 9. A device according to Claim 8 dependent on Claims 4 or 5 wherein B - t > 2A - t + π t/2 (4) where: A = radial dimension of outwardly extending annular flange of the case; B = radial dimension of outwardly extending annular flange of the closure member; t = thickness of composite material. 10. A device according to any one of the preceding Claims wherein the case is kinked inwardly below the position of said roll seam joint such that the overall diameter of the cell is no more than the basic diameter of the case. 11. A device according to any one of the preceding Claims wherein said ductile metal is aluminium or an aluminium alloy. 12. A device according to any one of the preceding claims wherein the layer of ductile metal has a thickness of at least 5 microns. 13. A device according to Claim 12 wherein the layer of ductile metal has a thickness of at least 10 microns. 14. A device according to Claim 13 wherein the layer of ductile metal has a thickness of at least 25 microns. 15. A device according to any one of the preceding Claims where said metal substrate is formed of steel. 16. A device according to any one of Claims 12 to 15 wherein only one of the case and the closure member is formed from the composite material, the other of the case and the closure member being formed from a material other than mild steel. 17. A device according to Claim 17 wherein said material other than mild steel is resistant to sodium attack. 18. A device according to Claim 17 wherein the material other than mild steel is selected from a group including Inconel and Fecralloy. 19. A device according to any one of Claims 16 to 18 wherein the sodium electrode is adjacent the roll seam joint. 20. A device according to any one of Claims 1 to 15 wherein both the case and the closure member are made of the composite material. 21. A device according to any one of the preceding claims wherein the device is a sodium sulphur cell. 22. A method of closing a case of a sodium electrode energy conversion device, by a closure member, the case and the closure member being sealed together at interfacing surfaces thereof, at least one of the case and the closure member being formed of a composite material comprising a metal substrate and a layer of a metal more ductile than the substrate on at least an interfacing surface of the metal substrate, the method including the step of rolling together adjacent parts of the closure member and the case whereby a roll seam joint is produced. 23. A method according to Claim 22 wherein the composite material has a layer of a ductile metal on each surface. 24. A method according to any one of Claims 22 or 23 wherein said layer of a ductile metal is mechanically fixed to said metal substrate. 25. A method according to any one of Claims 22 to 24 further comprising the step of forming an outwardly extending annular flange on the closure member wherein said step of rolling together the closure member and the case comprises the step of rolling together said outwardly extending annular flange of the closure member and the adjacent part of the case. 26. A method according to Claim 25 further comprising the step of forming an annular lip on the annular flange of the closure member. 27. A method according to any one of Claims 22 to 26 further comprising the step of forming said adjacent part of the case as an outwardly extending annular flange. 28. A method according to Claims 25 or 26 comprising the step of forming said adjacent part of the case as an outwardly extending annular flange wherein said step of rolling adjacent parts of the closure member and the case comprises the step of rolling together said outwardly extending annular flanges of the closure member and the case. 29. A method according to Claim 28 wherein B - t > 2A - t + π t/2 (4) where: A = radial dimension of outwardly extending annular flange of the case; B = radial dimension of outwardly extending annular flange of the closure member; t = thickness of composite material. 30. A method according to any one of Claims 22 to 29 comprising the step of closing an end of the case with the closure member. 31. A method according to any one of Claims 22 to 30 further comprising the step of forming the case to be kinked inwardly below the position of said roll seam joint such that the overall diameter of the cell is no more than the basic diameter of the case. 32. A method according to any one of Claims 22 to 31 further comprising the step of thermally treating the closure member and the case after said step of rolling. 33. A method according to any one of Claims 22 to 32 further comprising the step of thermally treating the composite material prior to said step of rolling. 34. A method according Claim 33 comprising the step of thermally treating at least one of the closure member and the case prior to said step of rolling. 35. A method according to any one of Claims 22 to 34 comprising the step of forming only one of the closure member and the case from the composite material. 36. A method according to any one of Claims 22 to 34 comprising the step of forming said adjacent parts of both the closure member and the case from the composite material. 37. A method according to Claim 36 comprising the step of thermally treating both of the closure member and the case prior to said step of rolling. 38. A method according to any one of Claims 22 to 37 for closing the case of a sodium sulphur cell with a closure member. 39. A sodium electrode energy conversion device substantially as hereinbefore described. 40. A method of closing the case of a sodium electrode energy conversion device substantially as hereinbefore described.

Inventors: GEE PAUL, MACLACHLAN STUART, O'NEIL-BELL CHRISTOPHER, GEE, PAUL, MACLACHLAN, STUART, O'NEIL-BELL, CHRISTOPHER
Applicants: CHLORIDE SILENT POWER LTD, CHLORIDE SILENT POWER LIMITED
Application year: 1991
Main classification: H01M10
IPC codes: H01M2, H01M10