CA1287875C

CA1287875C - Manufacture of electrochemical cells

Info

Publication number: CA1287875C
Application number: CA000535119A
Authority: CA
Inventors: Peter Raymond Smith; Walter George Bugden
Original assignee: Lilliwyte SA
Current assignee: Lilliwyte SA
Priority date: 1986-04-22
Filing date: 1987-04-21
Publication date: 1991-08-20
Anticipated expiration: 2008-08-20
Also published as: ZA872684B; GB2190236B; AU591406B2; SE8701637D0; IT8720199A0; GB8709392D0; DE8705788U1; AU7158787A; GB2190236A; JPS62264551A; IT1203944B; FR2598559A1; SE8701637L; DE3713380C2; DE3713380A1; GB8609771D0; FR2598559B1; US4772293A

Abstract

ABSTRACT

The invention provides a method of manufacturing an electrochemical cell housing wherein a beta-alumina tube has an open end attached to a casing via an alpha-alumina ring. In the method the alpha-alumina ring is thermocompression bonded to a metal ring by hot isostatic pressing, the alpha-alumina ring is then glass welded to the open end of the tube, and the metal ring is metal welded to the casing. The invention also provides a cell housing wherein a beta-alumina tube is located in spaced relationship within a metal casing. The tube is glass welded to an alpha-alumina ring which is thermocompression bonded on a curved radially facing surface thereof to a metal ring, the metal ring being metal welded to the casing.

Description

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~AN~F~CT~R~ OF EL~CTROC~ENICa~ CELIS

THIS INVENTION relates to an electrochemical cell housing. More particularly, the invention relates to a method of manu~acturing an electrochsmical cell housing.
According to the invention in the manufacture of an electrochemical cell housing comprising a b~ta-alumina tube :~ located within an outer metal casing wherein the tube has an open end and is attached to the casing via an annular alpha-alumina ring at said open end, the alpha-alumina ring being hollow-cylindrical in shape and having a pair of flat end faces, a cylindrical radially inner curved surface and ~ cylindrical outer curved sur~ace, by a method which ; includes the step of thermocompression bonding the alpha-~ alumina ring to at least one metal ring, and of thereafter : attaching the alpha-alumina ring to the open end o~ the beta-alumina tube by glass welding and attaching at least one said metal ring by metal welding to the casing or to a metal closure ~or the tube, the improvement whereby the thermocompression bonding is effected by hot i60static pressing by means of a fluid under pre~ure, the pressure being exerted in a radial direction and th~ metal ring ; being bonded to one o~ the cylindrical curved sur~aces of the alpha~alumina ring.

The term 'glass welding' as used herein, is also known in the art as glass sealing or glassing.

The hot isostatic pressing of each metal ring to the alpha-alumina ring will be at a tamperature and pressure su~icient to cause the thermocompression bonding. The metal employed for the meta} rings will naturally be compatible in the intended cell environment with the intended active cell substanc~s~ ~or example, when the cell is intended to have, as active anode or cathode substances, . --:

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7~5 substances such as the alkali metals or alkaline earth metals, chalcogens (eg sulphur or selenium) and electrolytes such as alkali metal or alkaline earth metal halides or haloaluminates, the metal of the rings may comprise nickel or a nickel-based or nickel-containing alloy, or a ferrous alloy, such as Inconel, Nilo ~ or FecralloyTM Possible thermal shock arising from differential thermal expansion between the alpha-alumina and the metal in question should also be borne in mind in selecting the metal to be used, and the aforesaid metals are believed to be suitable from the point of view of avoiding~thermal sock.

For thermocompression bonding such metal~ to alpha~
alumina with a reasonably short heating regime or cycle time, temperatures in excess o~ 1000C are typically required, with the alpha alumina and metal being held together by ~he hot isostatic pressing with considerable ~orce. The isostatic pressing step of the present invention may thus take place at a temperature in the range 1000 -1400C, preferably 1050 - 1250C and typically 1100C, the alpha-alumina and metal being pressed together by pressures in the order o~ 50 - 200 mPa, preferably 10 -50 mPa and typically 25 mPa, for cycle times o~ the order o~ 15 -120 minutes, preferably 30 - 80 minutPs, and typically 60 minutes, ~or the aforesaid nickel~ or iron-containirlg metals. For example, ~or nickel, a temperature of 1050C and a pressure of 50 mPa, applled by way of hot isostatic pressing is suitable for a cycle time o~ 60 minutes. Heating rates ~rom ambient up t~ the maximum temperature o~ up to 1000C/hr or more may be employed, such heating rates conveniently being in the range of 500 -700C/hr, typically 600C/hr. Similar cooling rates may be employed.

Two metal rings may be attached to the alpha~alumina ring by the hot isostatic pres~ing to form a collar . . , :' ~
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assembly, to which the beta-alumina tube i~ then attached by glass welding, which is also known as glassing.
Typically beta-alumina tubes are closed at one end, so that such collar will usually be attached only to one end thereof in the manufacture of the cell housing.

The rim or periphery of the open end of the beta-alumina tube may be welded by means of glass into an axially outwardly facing groove, provided for this purpose on the alpha-alumina ring, the groove extending lo circumferentially along an axially facing side of the ring and coaxi~al with the ring. Furthermore two concentric rings of metal may be attached to the alpha-alumina ring prior to the glass welding, concentric with the alpha-alumina ring, the metal rings being pre~erably attached respectively to the inner and outer curved cylindrical surfaces o~ the alpha-alumina ring and being in the form of truncated cylinders which may project from the alpha-alumina ring in the axial direction opposite to the axial direction in which the annular groove i~ the alpha-alumina ring ~aces.
The cell housing can then be completed by attachiny a metal closure, eg in the form of a circular or annular disc, to the inner metal ring to close off the tube, and by attaching a metal closure, which casing may also be in the form of an annular disc, to the outer metal ring, to clo~e of~ an annular opening in the cell housing defined between the beta-alumina tube and an outer metal casing, which may be in the form of a canister, in which the tube is located, the casing being attached to the outer periphery of said closure. Attachment of the closures to the metal rings will be by welding, conveniently tungsten inert gas welding, said closure~ and casing being, for example, nickel, nickel alloys, steel or the like In a particular embodiment o~ the invention the method may accordingly include simultaneously thermocompression bonding two matal rings to the alpha-alumina ring by said : ' ' ~
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: hot isostatic pressing in a radial direction, one to the radially inner cylindrical curved sur~ace of the alpha-alumina ring and one to the radially outer cylindrical curved surface thereof, the method including metal welding the radially inner metal ring to a metal closure to close off said open end of the tube and metal welding the radially outer metal ring to the casing, and the method furthar including forming a circumferentially extending axially facing groove in the alpha-alumlna ring between the metal rings, locating the open end of the beta-alumina tube in said groove, and glass welding said open end in position in said ~roove. As mantioned above, the outer metal ring can be attached to the casing indirectly, by welding an annular metal closure between the outer ring and the casing or canister, or said outer ring can be welded directly to the casing or canister.

The method may include, as a preliminary step, the formation of the collar assembly comprising the alpha-alumina ring with the two metal ring~ concentrically ! 20 attached thereto as described above, and a plurality of such assemblies may be formed simultaneously. According to this aspect o~ the invention a plurality of alpha-alumina tubes are simultaneously eaah thermocompression bonded to two metal rings formed prior to the glas weldiny khereof to beta-alumina tubes, by locating an alpha-alumina ring concentrically between a pair of metal tllbes, thermocompression bonding the metal tubes simultaneously to the alpha-alumina ring by hot isostatia pres~ing to form a composite assembly, and then slicing the composite assembly into a plurality of annular slices, each of which slices comprises an alpha-alumina ring thermocompression bonded to two metal rings. In this case, the alpha-alumina ring may be a composite tube, b~ing formed by stacking ~ plurality of alpha-alumina tubes end-to-end, the slicing being into 3S ~lices which each com~rise a pair o~ alpha-alumina ~ubes located b~twaen a pair of me~al rings, one of the alpha-.' : ' ,, ;, , . . , ~ .

7~37~

alumina tubes being removed and discarded be~ore the slice is attached to the bet~-alumina tube and casing. In okher words, a plurality of alpha-alumina tubes may be pre~ormed individually, being then assembled together t~ form a compo ite or segmented tube which is sandwiched concentrically between two tubes o~ metal from which the metal rings are to be sliced.

The annular space betwe2n the metal tubes which is occupied by the alpha~alumina ring may be evacuated of gas prior to the thermocompression bonding, opposite ends of the annu~ar space occupied by the alpha-alumina ring being closed off by welding annular closures to the ends of the metal tubes to seal said annular space under vacuum prior to said thermocompression bonding.

Th~ method may in this case include the step of loadin~ a getter material into the interior of said annular space prior to the sealing, the getter material acting to resist a pressure build-up in said annular space during the thermocompression bonding by gettering at least some o~
such gases as are evolved in said interior during the hot isostatic pressing.

Thu~, the annular open ends o~ the annular space between the pipes may be closed o~'f by annular closure discs ~uitably welded thereto, eg by tungsten inert gas : 25 welding, the ~inal weld being by electron beam walding under vacuum, after which the as~embly as a whole will be subjected to the hot isostatic pressing. Electron beam welding is pre~erred ~or the final weld as the interior of the assembly should be evacuated of gas before the hot isostatic pressing, an~ electron beam welding can be effected under vacuum. The initial welds however may take place under inert gas.

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g ~7~75 Instead, all the welds may be ef~ecked by kungst~n inert gas welding, one of the metal tubes or one of the closures being provided with a bleed opening or passage, via which the assembly may be evacuated before the hot isostatic pressing, the bleed opening or passage being suitably sealed off prior to the hot isostatic pressing.

Providing slices as described above which each contain two alpha-alumina tubes, permits upon removal of one of the~e rinys, a collar to be obtained wherein the metal rings project axially to one side of the remaining alpha alumina ring in the collar, to ~acilitate sub~equent metal w~lding of these metal rings to the beta-alumina tube closure and to the casing. Removal of the one alpha-alumina ring may be by machining or grinding, which can also be employed to provide the groove in the remaining beta-alumina ring for receiving ~he open end of the beta-alumina tube. Instead, however, both alpha-alumina tubes may be left in the collar, and welding of the metal rings to the beta-alumina tube closure and to the casing may be effected alongside one of these alpha-alumina tubes, any damage caused to this alpha-alumina ring by the welding being pr~vented from extendiny or propagating into the other alpha-alumina ring which will remain whole and undamaged and suitable for sealing -the end of the beta-alumilla tubeO

Naturally, if desired, the radially lnner and outer curved sur~aces of the alpha alumina tubes from which the composite tube is formed may be gr~und and/or polished to a desired degree of smoothness prior to the hot isostatic pressing, to promote good thermocompression bonding, adhesion and sealing of the metal tubes to the alpha-alumina ring. ~his grindin~ and polishing may be effeGted by means o~ a suitable abrasive paper and/or diamond paste.

The methvd may further i.nclude the step of providing, - on each metal surface which is to be thermocompression ~, ~ ,~

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bonded to alpha-alumina by the hot isostatic pressiny, a continuous coating of a different metal. In a particular case the metal sur~ace may be a nickel surface, the coaking being at most 2 microns thick and the dif~erent metal being a member of the group comprising platinum, gold and copper.
Th~ dif~erent metal may be applied by any suitable method, eg electrolysis, vapour phase deposition or sputtering.

Instead, th~ method may include the step of forming on each metal surface which is to be thermocompression bonded to alpha-alumina by the hot isostatic pressing, a layer of oxide of~the metal less than 1 micron thick. Forming the oxide layer may be by heating the mstal at an elevated temperature in an oxidizing atmosphere. The heating may be at a temperature of at least 250C, in air. In this case also, the metal may be nickel.

The oxidizing will usually be at a temperature above 250C and, naturally, below the melting point of the metal.
Preferably this temperature is about 300 - 500C. The period for which the metal is held at the elevated temperature in the oxidizing atmosphere is inversely related to the temperature, being no longer when the t mperature is lower and vice versa. This period can vary from a few minutes or less at temperatures clvse to the melting point of the metal, and can extend typically up to about 2 hours or more for temperatures o~ about 250C.

As is the case with the hot isostatic pressing, where longer cycle times are typically employed ~or lower hot isostatic pressing temperatures and pressures, than are employed for higher pressing temperatures and pressures~
and higher isostatic pressing pressures are employed at lower pressing temperatures than at higher pressing temperatures, the bes~, most convenient or most economic combination o~ parameters to be u~ed for ~ormation of the .

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layer of oxide should be determined by routine experimentation, within the range~ specified above.

The purpose o~ the metal coating or oxide layer is ko improve the thermocompression bonding, thereby increasing the bond strength and gas-tightness thereo~. For thermocompression bonding nickel to alpha-alumina, good results have been obtain~d ~or heating nickel in air at 3609C for 1 hour, being ~etter than the results obtained when nickel i5 heated for eg 15 minutes at 900C in air.
In these cases, when the hot isostatic pressing took place at 50 mPa at 1150C for 30 minutes, bond strengths were obtained for the samples oxidized at 360C of about 32 mPa, compared with about 17,5 mPa for those oxidized in air at gOOC.

The method of the invention accordingly provides for the manufacturing of an electrochemical cell housing which comprises a beta~alumina tube located within a metal casing and defining a space therebetween, the interior of the tube and the space between the casing and tube respectively providing electrode compartments, the tube hav.ing an open end glass welded to an alpha-alumina ring and the alpha-alumina ring having at least one annular metal ring thermocompression bonded to a curved radially directed surface thereof, the metal ring bei~g metal welded to the ca~ing or to a metal closure whioh close~ the tube.

There may be two metal rings thermocompression bonded to the alpha-alumina, namely a radially inner metal ring bonded to the radially inwardly directed curved sur~ace of the alpha-alumina ring an~ a radially outer metal ring bonded to the radially ou~wardly directed curved surface of the alpha~alumina ring, the radially inner metal ring being metal welded to ~ metal closur which closes the tube and the radially outer metal ring being metal welded to the casing.

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-':." : , ~ , . ' - ~ i s The in~ention will now be described, by way of example, with r~erence to the accompanying diagrammatic drawings, in which:

Fi~ure 1 shows a schematic sectional side elevation of a cell housing made in accordance with the method of the present in~-ention;

Figure 2 shows a schematic sectional side elevation of a composite assembly of nickel tubes and an alpha-alumina ring formed from alpha-alumina tubes prior to hot isostatic pressing;
and Figures 3 to 5 show similar views of a collar assembly made from the assembly of Figure 2, in successive stages of manufacture.

In Figure 1 of the drawings, reerence numeral 10 generally designates a cell housing manufactured L~ accordance with the method of the present i~vention. The housing .is suitable, for example, for an electrochemical cell which has molten sodium as its acti~e anode ma~erial, a transition metal , . , . .. , . . .. , _ .. ..
chloride such as FeCl or NiC12 in the form of a porous matrix as 2 . .. ... .
its active cathode material, and a molten salt liquid ..................
electrolyte comprising sodium aluminium chloride, the active anode material on the one hand, and the molten salt electrolyte and active cathode material on the other h~uld, being provided on opposite sides of a beta-alumina separator which acts as a solid electrolyte.

The housing 10 co~prises an outer cylindrical casing 12 in th~ form of a canister, eg of nic~el or preferably steel and, conce~trically located ~herein, a s-al~na tube 14, closed at one end at 16 and open at its o~her end at 18. The periphery .: . .
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of the open end 18 of the tube 14 is provided Wit}l a collar asselllbly, generally designated ~0. The tube 14 forms the solid electrolyte of the eventual cell.

The casing 12 has a cylindrical side wall 22 welded to a circul~r floor 24, the closed end 16 of the tube ]4 being located adjacent but spaced from the floor 24.

The collar 20 comprises a circular ring or truncated cylinder 26 o-f alpha-alumina, the axially inner end face of which has a circumferentially extending groove therein at 28, within which the periphery of the open end 18 of the tube 14 is located and is welded, in fluid-tight fashion, by means of glass.
Two concentric truncated cylinders of nickel, designated 30 and 32, are thermocompression bonded in fluid-tight fashion respectively to the outer and inner curved surfaces of the ring 26. The open end 18 of the tube 14 is closed off by an annular closure disc 3~ of nickel or stainless steel, welded to the ring 32 a~ 36 by tungsten inert gas welding; and the end of the casing 12 remote from the iloor 24 is closed off by means of an annular closure disc 40 of nickel or stainless steel, welded to the casing at 42 and welded to the ring 30 at 44 by tungsten inert gas welding. A stainless steel rod current collector 46 is shaw projecting into the tube 14 via the disc 34, to which i~ is similarly welded as at 48, and a stainless steel rod current collector 50 is shown welded to the ~xially outer surface of the disc 40 at 52. This arrangement is suitable for a cell in which the anode material is located inside the tube 14, the cathode material and molten salt elec~rolyte being located in the annular space between the tube 14 ~ld casing 12~

Turning to Figure 2, reference numeral 54 generally designates an assembly for the mass production of collar assemblies 20 (Figure 1) in accordance with ~he method of the - : . ::: , .
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inveiltion. The assembly 54 comprises a plura]ity of alpha-alwnina tubes 56, 58 which have been formed from alpha-alumina cmd, a~ter such grinding or polishing as is required for -their outer and inner curved surfaces, have been stacked in end-to-end abutment concentrically between two nickel pipes 60, 62, fitting between said pipès Witll a close sliding or fTiction fit. It will be noted that the rings 58 are somewhat longer (see B in Figure 2) in the axial direction than the rings 56 ~see A in Figure 2) and that a pair of rings 56 is located between successive rings 58. The rings at the end of the stackJ
designated 64, are half the axial length of the rings 58.

To complete the manufacture of the assembly 54, annular closure discs 66 are welded to the pipes 60, 62 at 68, 70~ 72 and 74, to close off the annular space between the pipes 60 and 62, within which the rings 56, 58 and 64 are located. Three of these welds, eg 68, 70 and 72 are tungsten inert gas welds which are formed first, after which the annular space between the pipes 60, 62 is evacuated, eg by locating the assembly in a vacuum chamber7 wherein the final wcld 74 is made by electron beam welding, so that the assembly 54 is closed with a vacuum therein. Some titanium or tantalum, eg in the form of granules or foil ~not shown) may be provided in the interior of the asseTnbly 54 for the purpose of gettering gases such as oxygen given off by the hot isostatic pressing described hereunder.

The assembly 54, or a plurality of such assemblies simultaneously, is/are then subjected to hot isostatic pressing at a temperature of 1050~C for 60 minutes under a fluid pressure of 50 MPa, to thermocompression bond the pipes 60, 62 respectively to the outer and inner curved surfaces of rings 56, 58, 64. After cooling, the assembly 54 is then sliced or cut into rings, at the~positions shown by the arrows 76, 78, the cuts at 76 being between two abutting alpha-alun~ina tubes 56, and the , : ' ' -.

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cuts at 78 being midway, lengthwise, along each of the alpha-~lumina tubes 58. Before or after this cutting the closure ~iscs 66 Call be removed; or the cut en~ portions having the discs 66 can be discarded~

This cutting at 76, 78 produces a plurality of annular collar assembly blanks, one of which is shown at 80 in Figure 3.
The blank 80 comprises a ring 56 of alpha-alumina (designated also 26 as it will form the ring 26 o-f the collar assembly 20 of Fi~ure 1), a ring 82 of alpha-alumina which is half of one of the rings 58 of Figure 2, and two rings 60, 62 of nickel (also designated 30 9 32 as they will form the rings 30, 32 of the assembly 20 of Figure 1).

The ring 82 is then machined out of the blcmk 80 to provide a part-~rocessed bl~lk as shown at 84 in Figure 4, in which the same numerals refer to the same parts as in Figure 3;
and diamond grindillg is them employed to form the groove at 28 ~Figure 5) for receiving the periphery of the open end 18 of the tube 14 (Fi~ure 1). l~le finished collar assembly is shown in Figure 5 where the parts are designated by the numerals used in Figure 1.

With reference also to Figure 1, the tube 14 is then glass welded at its open end 18 into the groove at 28 in the ring 26, and, after sodium is charged into the tu~e 14, the disc 40 (having the current collector 46 pre welded thereto at 48) is titanium ~lert gas welded to the nickel ring 32 at 36. The tube 14 is then located concentrically within the casing 12, molten salt electrolyte is charged into the annular space therebetween together with porous active cathode material, and the cell housing is completed by tungsten inert gas welding the disc 40 to the nickel ring 30 at 44 and to the casing 12 at 42.

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~ nployillg hot isostatic pre~sing to thermocompression bond the nickel rings 3~, 32 to the alpha-alumina ring 26 has a number of material ancl ~mexpected advantages. t\n important adv~ltage is that machilling and preparation o-f the rings, particularly the alpha-alumina ring, can be kept to a minimum.
This arises from the fact that isostatic pressing, as opposed for example to ~liaxial pressing or die pressing, exerts its pressure in all directions, so that close tolerances and a close surface-to-surface fit between ~he nickel rings and the alpha-alumina ring, with the rings preferably seating flat and in continuous sur-face-to-surface contact with each other is less important. Relatively poor fits OT con~act between the rings to be thermocompression bonded can in principle be tolerated, the isostatic pressing automatically bringing the materials to be thermocompression bonded into contact with each other, and spreading them out and bending them into contact, if necessary, before the thermocompression bonding actually takes place. A high degree of surface finish~ and finishing and machining of the components prior to the thermocompression bonding can thus be substantially reduced, i~` not eliminated. This is of major importance in keeping costs to a minim~

A -further material aclvantage of the invenkion is that the employment of hot isostatic pressing permits the nickel rings to be attached to the curved cylindrical inner and outer surfaces of the alpha-alumina ring. This is believed to be impossible, or at best extremely difficult, wikh die pressing or uniaxial pressing. Attaching the nickel rings to the curved inner and outer surfaces of the alpha-alumina ring allows the assembly of said three rings to be kept to a minimum in radial thickness, but, at the same time) relatively large curved surfaces, made large by their extending in the axial direction, can be employed for the thermocompression bonding, thus ensuring bonding over a large area, with the attendant advankages of mechanical strength, ';

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durabiiity and -fluid-tightness. This perrnits a cell to be made with an electrode coml~artment outside (or inside) the beta-alwnina tube of extremely narrow radial dimensions~ as the alpha-al~nina ring need stand proud of the beta-alumina ring in the radially outward (or inward) direction by a spacing which is not larger than about half the width of the axially facing end face of the alpha-alumina ring9 which in turn need ~e no wider than required for proper welding to the beta-alwnina tube. In short,' the alpha-alumina ring can be made of narrow radial dimensions, with the attendant advantages, ie narrow electrode cornpartments as described above, requiring reduced amounts o~
electrode or electrolyte material to fill them sufficiently to wet the beta-alurnina tuhe ully.

In this regard it should be no~ed that good results have been obtained with 40 mm nominal diameter beta-alumina tubes, but less successful results have been obtained with 54 mm nominal diameter beta-alumina tubes. It is believed, however, that with better quality control the difficulties encoun*ered with larger tubes will be overcome and, in any event as mentioned above, the invention has particular advantages when applied to narrow beta-alurni~na tubes.

A further material advantage of the present inventio~
is that a large i-ndustrial scale isostatic pressing device or apparatus can be used, simultaneously to prepare large numbers of ring assemblies, according to the method described above. Cycle times are kept to a minimum, and to match ~hese cycle times by iaxial or die pressing, large nwnbe~s of dies, with the attendant extremely high cost, would be required.

Furtherrnore~ with the particular geometry shown in Figure 1 of the drawings, the nickel rings can project in the axial direction from the alpha-alumina ring, to provide , ~ : , , ~ , .
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r~l~tiv~ly large sur:Eace areas for welding to the casing (via disc 40) and to the circular closure disc 34, thus promoting the easy formation of strong fluid-tight welds.

Also, if desired, it should be noted that the nickel tubes or pipes 60, OE2 can have their suraces which are to abut the alpha-alumina tubes 56, 58, 64 treated to improve the thermocompression bond strengths therebetween. Thus these tube surfaces can have an oxide layer formed thereon, eg by heating the tubes in air at 360C for 1 hour, or can be provided with eg a gold surface 1 - 2 microns thick by for example vapour phase deposition or sputtering~ Purthermore, such surface ~reatment can act to improve the ~luid tightness of the thennocompression bonds obtained. As regards the oxide layer, tests have shown that it need not be thick to improve the bond strength of nickel to alpha-alumina, and layer thicknesses which are not detectable by a weight increase on a four-figure chemical balance have been found to be effective.
.
Finally, it should be noted that the method of the invention can be applied by thermocompression bonding a metal rin~ to the outer curved surface of an alpha-alumina ring, followed by cutting an annular circumferentially extending slot in the metal ring, thereby dividing it into two axially spaced metal rings bonded to said curved surface and separated by sai.d slot. These rings can then be welded, in the fashion of rings 30 and 32 in Figure 1, to the tube closure 34 and casing 12, to form the housing.
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Claims

1. In the manufacture of an electrochemical cell housing comprising a beta-alumina tube located within an outer metal casing wherein the tube has an open end and is attached to the casing via an annular alpha-alumina ring at said open end, the alpha-alumina ring being hollow-cylindrical in shape and having a pair of flat end faces, a cylindrical radially inner curved surface and cylindrical outer curved surface, by a method which includes the step of thermocompression bonding the alpha-alumina ring to at least one metal ring, and of thereafter attaching the alpha-alumina ring to the open end of the beta-alumina tube by glass welding and attaching at least one said metal ring by metal welding to the casing or to a metal closure for the tube, the improvement whereby the thermocompression bonding is effected by hot isostatic pressing by means of a fluid under pressure, the pressure being exerted in a radial direction and the metal ring being bonded to one of the cylindrical curved surfaces of the alpha-alumina ring.

2. A method as claimed in claim 1, which includes simultaneously thermocompression bonding two metal rings to the alpha-alumina ring by said hot isostatic pressing in a radial direction, one to the radially inner cylindrical curved surface of the alpha-alumina ring and one to the radially outer cylindrical curved surface thereof, the method including metal welding the radially inner metal ring to a metal closure to close off said open end of the tube and metal welding the radially outer metal ring to the casing, and the method further including forming a circumferentially extending axially facing groove in the alpha-alumina ring between the metal rings, locating the open end of the beta-alumina tube in said groove, and glass welding said open end in position in said groove.

3. A method as claimed in claim 1, in which a plurality of alpha-alumina tubes are simultaneously each thermocompression bonded to two metal rings formed prior to the glass welding thereof to beta-alumina tubes, by locating an alpha-alumina ring concentrically between a pair of metal tubes, thermocompression bonding the metal tubes simultaneously to the alpha-alumina ring by hot isostatic pressing to form a composite assembly, and then slicing the composite assembly into a plurality of annular slices, each of which slices comprises an alpha-alumina ring thermocompression bonded to two metal rings.

4. A method as claimed in claim 3, in which the alpha-alumina ring is a composite tube and is formed by stacking a plurality of alpha-alumina tubes end-to-end, and in which the slicing is into slices which each comprise a pair of alpha-alumina tubes located between a pair of metal rings, one of the alpha-alumina tubes being removed and discarded before the slice is attached to the beta-alumina tube and casing.

5. A method as claimed in claim 3, in which the annular space between the metal tubes which is occupied by the alpha-alumina ring is evacuated of gas prior to the thermocompression bonding, opposite ends of the annular space occupied by the alpha-alumina ring being closed off by welding annular closures to the ends of the metal tubes to seal said annular space under a vacuum prior to said thermocompression bonding.

6. A method as claimed in claim 5, which includes the step of loading a getter material into the interior of said annular space prior to the sealing, the getter material acting to resist a pressure build-up in said annular space during the thermocompression bonding by gettering at least some of such gases as are evolved in said interior during the hot isostatic pressing.

7. A method as claimed in claim 1, which includes the step of providing, on each metal surface which is to be thermocompression bonded to alpha-alumina by the hot isostatic pressing, a continuous coating of a different metal.

8. A method as claimed in claim 7, in which the metal surface is a nickel surface, the coating is at most 2 microns thick and the different metal is a member of the group comprising platinum, gold and copper.

9. A method as claimed in claim 1, which includes the step of forming on each metal surface which is to be thermocompression bonded to alpha-alumina by the hot isostatic pressing, a layer of oxide of the metal less then 1 micron thick.

10. A method as claimed in claim 9, in which forming the oxide layer is by heating the metal at an elevated temperature in an oxidizing atmosphere.

11. A method as claimed in claim 10, in which the heating is at a temperature of at least 250°C and is in air.

12. A method as claimed in claim 9, in which the metal is nickel.