GB2261764A

GB2261764A - Method of making seal in alkali metal energy conversion devices

Info

Publication number: GB2261764A
Application number: GB9124811A
Authority: GB
Inventors: Stuart Maclachlan
Original assignee: Silent Power F R Energies GmbH; Chloride Silent Power Ltd
Current assignee: Silent Power F R Energies GmbH; Chloride Silent Power Ltd
Priority date: 1991-11-22
Filing date: 1991-11-22
Publication date: 1993-05-26
Also published as: JPH07501176A; WO1993010568A1; GB9124811D0; EP0613589A1

Abstract

in making alkali metal energy conversion device having a solid electrolyte member 11 separating respective electrode spaces, the electrolyte member being conductive to ions of the alkali metal, with seals between the electrolyte member 11 and an electronically conductive current collecting member 18 of the device formed via an insulating member 12, the insulating member 12 being joined to the electrolyte member by means of a hardened sealant 13 incorporating glass powder at an interface between the members, dry powder incorporating glass powder is moulded and compressed to form a self supporting dry mass of the powder having a shape substantially conforming to the interface between the members, the electrolyte member and the insulating member are then brought together into the joining position with the dry mass of the powder at this interface and the members are heated to melt the glass powder to join the members. <IMAGE>

Description

MAKING ALKALI METAL ENERGY CONVERSION DEVICES The present invention relates to making alkali metal energy conversion devices, particularly those having a solid electrolyte member separating respective electrode spaces, the electrolyte member being conductive to ions of the alkali metal, and an insulating member joined to the electrolyte member to enable seals to be formed via the insulating member between the electrolyte member and electronically conductive current collecting members of the device. An example of alkali metal energy conversion devices of this type is the sodium sulphur cell. However the present invention is applicable to any corresponding form of alkali metal energy conversion device in which it is necessary to join a solid electrolyte member to an insulating member within the cell.

Commonly, the solid electrolyte member is a ceramic, typically beta alumina ceramic which is conductive to ions of sodium. Then the insulating member can be made of alpha alumina. Hitherto most methods proposed for joining the solid electrolyte member to the insulating member comprise the use of a glazing method so that a glass seal is formed between the two members. It is important that the glass chosen for this purpose is resistant to chemical attack from the electrode materials employed within the cell, and is also matched as far as possible to the thermal expansion coefficients of the materials of the electrolyte member and the insulating member.

United Kingdom patent 2083943B has described the use as a material for forming the join or seal between the electrolyte member and the insulating member of a hardened mixture of glass powder and ceramic powder. The resulting seal comprises particles of ceramic powder in a glassy matrix and it has been found that the seals have superior characteristics and resistance to attack. In particular, the resultant seal material is more resistant to cracking under the operating conditions of a sodium sulphur cell for example.

In the above mentioned patent, the mixture of glass and ceramic powders is suspended in a liquid with a high viscosity, particularly glycerine. One of the electrolyte member or the insulating member is then immersed in this liquid mixture to coat the member where it is to be joined to the other member. The two members are then pressed together and the liquid of the coating material is expelled at a suitable temperature. The coating material is then "age hardened by heating to the operating temperature of the glass" so that a join having the required mechanical strength is formed.

This method of joining the two members together has disadvantages, particularly in application to mass production techniques, and associated with the need for two heating stages, first to drive off the glycerine, and then to fire the glass.

The present invention provides a method of making an alkali metal energy conversion device having a solid electrolyte member separating respective electrode spaces, the electrolyte member being conductive to ions of the alkali metal, and an insulating member joined to the electrolyte member to enable seals to be formed via the insulating member between the electrolyte member and an electronically conductive current collecting member of the device, the insulating member being joined to the electrolyte member by means of a hardened sealant incorporating glass powder at an interface between the members, wherein dry powder incorporating glass powder is moulded and compressed to form a self supporting dry mass of the powder having a shape substantially conforming to said interface, the electrolyte member and the insulating member are then brought together into the joining position with the dry mass of the powder at said interface, and the members are heated to melt the glass powder to join the members.

This process of moulding and compressing a dry powder mixture to form a self supporting dry mass avoids the use of a wet sealing compound. Further, only a single firing step is required to seal the members together once they have been offered up to each other in the sealing position. Preferably, the dry powder is moulded and compressed on to one of the electrolyte member and the insulating member so that said dry mass is formed affixed thereto. This allows the sealing material to be preloaded onto one of the members during the manufacturing process.

The member with the sealing material affixed thereto can then be handled in the usual way without risk of the material becoming contaminated, displaced, or contaminating extraneous parts of the handling apparatus of the machine.

An important additional advantage is that the use of dry powder applied directly to one of the members enables a successful seal to be formed even between electrolyte and insulating members which have not been accurately machined to fit each other.

Conveniently, the electrolyte member is tubular having an open end which is joined to a corresponding ring or disc shaped insulating member, whereupon the self supporting dry mass of the powder may be formed as a ring and may be affixed to the insulating member.

Preferably, the ring or disc shaped insulating member may include a first surface generally parallel to the plane of the member to interface with an annular end surface at the open end of the tubular electrolyte member, and a second surface generally perpendicular to the first surface to fit about or in the electrolyte member at said open end. Then conveniently the members are joined by placing the insulating member with its plane generally horizontal and said first surface directed upwards, applying said dry powder to said first surface between said second surface and a further surface generally parallel to said second surface and forming a channel therebetween, and pressing a tubular ram down into the channel to compress the dry powder to form said dry mass affixed to said first surface.

Then said further surface may be formed by a removable former located on or about the insulating member to form said channel. Alternatively, said further surface may be formed in said insulating member.

Preferably, the ram is shaped in cross section to deflect said dry mixture during compressing towards the said second surface.

In one embodiment, both said first surface and said further surface are extended by means of removable formers to deepen the channel therebetween so as to receive the uncompressed dry powder.

Advantages can be achieved when the dry powder is essentially pure glass powder. However, preferably, the dry powder comprises a dry mixture of glass powder and ceramic powder. The ceramic powder is preferably alpha alumina.

The dry mixture may comprise between 25 and 80% by weight of glass powder and between 20 and 75% by weight of ceramic powder. Preferably, the dry mixture comprises between 60 and 75% by weight of glass powder and between 25 and 40% by weight of ceramic powder.

Up to 10% by weight of a dry binder powder may be mixed with the dry powder. This dry binder is conveniently a dry form of polyethylene glycol, e.g.

having a molecular weight in the range 1500 to 200.

Especially when using polyethylene glycol as a binder, the dry powders may be wet mixed by adding a volatile liquid to dissolve the dry binder powder (for example acetone), and after wet mixing driving off the volatile liquid to leave a dry powder mixture.

Although the dry mass may be formed by applying pressure without heat, it may be convenient to heat the powder when moulded and compressed to produce at least partial sintering of the dry mass.

An example of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a cross sectional view of a sodium sulphur cell of the kind to which the present invention may be applied; Figure 2 is a cross sectional view illustrating a ram and former which may be used in association with an insulating member in this example of the present invention; Figures 3, 4 and 5 are part sectional views illustrating three initial steps of this example of the invention; and Figures 6 and 7 are sectional views illustrating two further steps of this example of the invention.

Referring to Figure 1, an alkali metal energy conversion device is illustrated, in this case taking the form of a sodium sulphur cell. In this example, the cell comprises an outer metal casing 10 containing an electrolyte member 11 formed as an open ended cup. The upper open end of the electrolyte cup 11 is closed by means of a disc shaped lid 12 which is joined to the open end of the cup 11 by means of a seal 13.

In this example, the electrolyte cup 11 is made of beta alumina and the lid 12 is made of alpha alumina. The seal 13 between the two can be formed of glass but in the example of this invention this seal is formed of a glass composite material comprising particles of ceramics in a glass matrix.

An outer sealing member 14 in the shape of an annulus is sealed at an inner edge to the upper surface of the lid 12, and at its outer edge to the upper rim of the outer container 10. This seal 14 thereby closes off an outer space 15, outside the electrolyte cup 11, which in this example contains the sulphur electrode of the cell.

An inner washer 16 is sealed on the upper surface of the cup 12 about a central aperture 17 in the lid. A current collecting pin 18, typically of metal, is located extending through the aperture 17 and is itself sealed to the washer 16, thereby closing off an inner space 19 inside electrolyte cup 11. This inner space 19 contains the sodium electrode in this example.

It can be seen, therefore, that in the arrangement shown in Figure 1, a seal is effectively made between the electrolyte cup 11 and the sodium current collecting pin 18 via the alpha alumina insulating lid 12. Similarly, a seal is made between the cup 11 and the outer container 10 which constitutes a current collector for the sulphur electrode, again via the insulating lid 12.

As mentioned above, it is known from GB2083943B to employ for the seal 13 a composite material formed of ceramic powder in a glass matrix. This example of the invention is concerned with a method of forming the seal 13from such a glass composite material.

Referring to Figure 2, there is shown in cross section an enlarged view of the insulating alpha alumina lid 12 before assembly in the final cell as illustrated in Figure 1. In Figure 2, the lid 12 is shown inverted relative to its orientation in Figure 1, and a circular former 20 is illustrated resting on the upwardly directed surface of the lid. The former 20 has a central nipple 21 which engages in the aperture 17 of the lid 12, so as to locate the former concentrically on the lid.

The lid itself is circular, having the aperture 17 at the centre, and an outer peripheral flange 22, shown upstanding in Figure 2. The flange 22 is sized to have an inner diameter which is slightly larger than the outer diameter of the end of the electrolyte cup 11, so that the flange 22 of the lid can fit over the open end of the cup 11 on assembly, as illustrated in Figure 1.

The lid 12 has a first annular surface 23, which is parallel to the plane of the disc shaped lid and located just inside the flange 22, so as to interface with the annular end surface of the electrolyte cup 11 when the two are to be joined together as shown in Figure 1. The inner cylindrical surface 24 of the flange 22 forms a second surface of the insulating cap 12 which is generally perpendicular to the first surface 23, and in the example illustrated is sized to fit about the electrolyte cup 11 at its open end.

The outer cylindrical surface 25 of the former 20 provides a further surface which is generally parallel to the surface 24 of the cap 12. The surfaces 24 and 25 form between them an annular channel 26. The diameter of the former 20 is chosen so that the width of the channel 26 is slightly greater than the thickness of the electrolyte cup 11 at its open end.

In accordance with this example of the method of the present invention, a dry mixture of ceramic and glass powders is poured or otherwise applied to the channel 26 all the way around inside the flange 22 of the cap 12.

Then, a cylindrical ram, 27 is inserted into the channel 26 and pressed downwards against the cap 12 so as to compress the dry particulate material located in the channel. If sufficient force is applied by the ram, the dry particulate material can be compressed into a self supporting mass of the material which is affixed to the surface 23 of the cap 12.

In the illustrated example, the end of the cylindrical ram 27 which engages the dry particulate material in the channel 26 is profiled as illustrated in Figure 2, to slope upwardly and outwardly from the inner to the outer cylindrical surface of the ram.

The operation of the ram 27 in association with the former 20 is better illustrated in Figures 3, 4 and 5. As the end of the ram 27 first engages the dry particulate material 28 in the channel 26, the profiled face of the ram tends to deflect the dry material outwards against the cylindrical surface 24 of the cap. Thus, as the ram is pressed firmly downwards against the cap with the desired pressure, a self supporting mass of the particulate material is formed by the compressive forces which is firmly affixed both to the annular surface 23 of the cap and to the immediately adjacent region of the cylindrical surface 24. On withdrawal of the ram 27 and removal of the former 20, the compressed dry mixture of glass and ceramic powders forms a ring 29 firmly affixed inside the flange 22 of the insulating cap, as illustrated in Figure 5.

At this stage, the cap with the ring of compressed powders can be handled and manipulated readily during further assembly processes without fear of contaminating the ring 29 of sealant, or having the material of the sealant contaminate other parts of the handling machine or components of the cell.

At the next stage of assembly, the alpha alumina electrolyte cup 11 is inverted and located with its open end inside the flange 22 of the cap, so that the annular end face 30 of the cup 11 engages the ring 29 of sealant material. The assembly is then placed in an oven to fire at a temperature above the melting point of the glass powders used in the powder mixture of the ring 29. On melting of the glass particles, the resulting liquid glass, loaded with ceramic particles, forms a gas-tight seal between the end of the electrolyte cup 11 and the insulating cap 12 all the way around inside the flange 22, producing a composite glass/ceramic seal 31 as illustrated in Figure 7.

As described, the seal formed in the process of the present invention may comprise a glass powder or preferably a mixture of glass and ceramic powders. The glass powder may comprise a glass having the following composition in mass percents of oxide: B203 25%, SiO2 46%, A1203 10%, CaO 3%, SrO 6%, BaO 6%, Na2O 4%.

Other glass compositions may alternatively be used, particularly those containing between 40 and 45% B203, from 3 to 11% SiO2, from 18 to 30% A1203, from 0.5 to 7% CaO, from 0 to 8% MgO, from 0 to 12% SrO, from 5 to 20% BaO, and from 0 to 5% Na2O.

Where the glass powder is mixed with a ceramic powder, the ceramic powder is preferably alpha alumina, for example a commercial grade powder known as "Universal Abrasives White Bauxilite Micro Powder. This powder is about 99.5% A1203 with small proportions of SiO2, Foe203, TiO2, CaO, Na2O and K2O. The particle size of the ceramic powder is preferably between 0.5 and 10 microns. The above commercially available powder has a mean size of about 3 microns with a 97% percentile of 7 microns and a 20% percentile of 1 micron.

Other ceramic powders which may be used comprise beta alumina, zirconium silicate, zirconia and magnesium oxide. These powders may be used either separately or in mixtures.

The dry mixture of glass and ceramic powders is preferably between 25 and 80% by weight glass powder and between 20 and 75% by weight ceramic powder. In a particular example, the mixture of glass and ceramic powders is 67% by weight glass and 33% by weight ceramic.

10% by weight of a dry binder powder may also be added to the mixture to improve handling of the compressed dry mass of the powder. A preferred binder powder is polyethylene glycol with molecular weights in the range 1500 to 200. This material is itself a dry powder.

In order to obtain adequate mixing of a polyethylene glycol binder powder with the other powder material, it is preferable to perform the mixing in wet state, by adding a volatile liquid, such as acetone to the powders, choosing a liquid which dissolves the binder powder. Once mixing is completed, the acetone is driven off again to leave a dry powdery mixture for applying to one of the members in making the seal.

In the process of compressing the dry powder onto one of the electrolyte and insulating members, a compressive force of llNmm 2 may be employed. This has been found adequate to achieve the desired consolidation of the powders as a self supporting dry mass. Higher compressive forces could be used provided the substrate member, i.e.

the electrolyte member or the insulating member, can withstand such higher forces. It is considered that forces below about 7.5Nmm 2 would be barely sufficient to ensure adequate compaction, although lower forces could be used with different choices of powders.

In order to melt the glass powder to form the seal, a firing temperature in the range 850 0C to 1100 0C is used depending on the glass composition employed. Firing can be conducted in air, though other considerations may dictate firing in inert atmospheres such as argon or nitrogen. It has been found that firing in atmospheres of a reducing gas tends to promote foaming and produces seals with unacceptably high levels of porosity.

Because the dry powder material is used to prepare the seal between the insulating and electrolyte members, it is possible to adjust the amount of powder material applied to the seal area as required. By comparison, glass seals are sometimes formed employing a preformed ring of glass located on one of the members to be sealed together. It will be understood that this ring of glass provides a predetermined quantity of sealing material.

This is satisfactory, when the interfacing surfaces of the two members to be sealed together are accurately machined. However if there are tolerance variations in the two members, so that varying gaps and spaces must be filled by the glass seal, then it is inappropriate to employ a fixed amount of sealant material for this purpose. Insufficient sealant material would then be available to fill large gaps.

By comparison, the use of a powder material enables adequate powder to be provided as required to ensure a good seal even between members of varying tolerances.

This is an important consideration in the manufacture of the components of an alkali metal cell, since it may be possible to dispense with the step of machining the fired ceramic components constituting the electrolyte and insulating members.

In the above example, the powder has been compressed to affix it to the insulating member. However, the powder may be moulded and compressed separately from the members to be joined, so as to form a separate self supporting dry mass of the powder. This separate dry mass, or sealing body, is then located between the members to be joined before these are heated.

Whether the dry mass is formed affixed to one of the members to be joined, or as a separate body, heat may be applied during moulding and compression of the dry powder to achieve at least partial sintering.

Claims

1. A method of making an alkali metal energy conversion device having a solid electrolyte member separating respective electrode spaces, the electrolyte member being conductive to ions of the alkali metal, and an insulating member joined to the electrolyte member to enable seals to be formed via the insulating member between~the electrolyte member and an electronically conductive current collecting member of the device, the insulating member being joined to the electrolyte member by means of a hardened sealant incorporating glass powder at an interface between the members, wherein dry powder incorporating glass powder is moulded and compressed to form a self supporting dry mass of the powder having a shape substantially conforming to said interface, the electrolyte member and the insulating member are then brought together into the joining position with the dry mass of the powder at said interface, and the members are heated to melt the glass powder to join the members.

2. A method as claimed in Claim 1 wherein said dry powder is moulded and compressed on to one of the electrolyte member and the insulating member so that said dry mass is formed affixed thereto.

3. A method as claimed in Claim 1 or Claim 2 wherein said dry powder is heated when moulded and compressed to produce at least partial sintering of the dry mass.

4. A method as claimed in any preceding claim wherein the dry powder comprises a dry mixture of glass powder and ceramic powder.

5. A method as claimed in Claim 4 wherein the ceramic powder is alpha alumina.

6. A method as claimed in either of Claims 4 or 5, wherein the dry mixture comprises between 25 and 80% by weight of glass powder and between 20 and 75% by weight of ceramic powder.

7. A method as claimed in Claim 6, wherein the dry mixture comprises between 60 and 75% by weight of glass powder and between 25 and 40% by weight of ceramic powder.

8. A method as claimed in any preceding claim wherein up to 10% by weight of a dry binder powder is mixed with the dry powder.

9. A method as claimed in Claim 8, wherein the dry binder powder is a dry form of polyethylene glycol.

10. A method as claimed in either of Claims 8 or 9, wherein the dry powders are wet mixed by adding a volatile liquid to dissolve the dry binder powder, and after wet mixing driving off the volatile liquid to leave a dry powder mixture.

11. A method as claimed in any preceding claim wherein the electrolyte member is tubular having an open end which is joined to a corresponding ring or disc shaped insulating member, wherein the self supporting dry mass of the powder is formed as a ring.

12. A method as claimed in Claim 11 as dependent on Claim 2 wherein the dry mass is formed affixed to the insulating member.

13. A method as claimed in Claim 12 wherein the ring or disc shaped insulating member includes a first surface generally parallel to the plane of the member to interface with an annular end surface at the open end of the tubular electrolyte member, and a second surface generally perpendicular to the first surface to fit about or in the electrolyte member at said open end, wherein the insulating member is placed with its plane generally horizontal and said first surface directed upwards, said dry powder is applied to said first surface between said second surface and a further surface generally parallel to said second surface forming a channel therebetween, and a tubular ram is pressed down into the channel to compress the dry powder to form said dry mass affixed to said first surface.

14. A method as claimed in Claim 13 wherein said further surface is formed by a removable former located on or about the insulating member to form said channel.

15. A method as claimed in Claim 13 wherein said further surface is formed in said insulating member.

16. A method as claimed in either of Claims 13 or 14, wherein the ram is shaped in cross section to deflect said dry mixture during compressing towards said second surface.

17. A method as claimed in Claim 15 wherein said first surface and said further surface are extended by means of removable formers to deepen the channel therebetween so as to receive the uncompressed dry powders.