GB2140608A - Energy conversion devices using liquid sodium and beta alumina ceramic electrolyte material - Google Patents

Abstract

An energy conversion device employing beta alumina electrolyte material and sodium, for example a sodium sulphur cell, has wicking means (12) adjacent a surface of the electrolyte material (10) to maintain that surface wetted with liquid sodium, which wicking means is formed of an iron chromium aluminium alloy also containing yttrium. <IMAGE>

Description

SPECIFICATION Energy conversion devices using liquid sodium and beta alumina ceramic electrolyte material This invention relates to energy conversion devices using liquid sodium and beta alumina solid ceramic electrolyte material. Atypical example of such a device is a sodium sulphur cell. Otherforms of energy conversion devices include for exmaple sodiumsodium thermo electric generators.

Beta alumina is ionically conductive forsodium ions and, in devices ofthe kind referred to above, the sodium, in one stage of a cycle of operations, passes through the beta alumina thereby depleting the level of beta alumina against one face of the electrolyte material. For example in a sodium sulphur cell, the sodium constituting the anode, on discharge ofthe cell, passesthrough the electrolyte to form sodium polysulphides in the cathodic region. The operation is reversed on recharging the cell. In orderto obtain full discharge current evenwhen the sodium level is depleted, wicking means are provided to maintain a layer of sodium over one surface ofthe electrolyte material even when there is only a small quantity of sodium in the anodic region.Such a wicking medium must withstand the temperatures of operation and temperature cycling of the cell, it must not react chemically or electrochemically with the othercompo- nents ofthe cell and it must be a material which, despite any surface changes which may occur, is readilywetted by liquid sodium. The wicking medium is preferably a mesh or foil slightly spaced away from the surface of the electrolyte material to leave a capillary region in between the medium and the electrolyte. Very few materials will meet all these required conditions and heretofore, although stainless steel has been proposed, the preferred material which has heretofore been employed as a wicking medium in sodium sulphur cells and similar devices is iron foil.

In electrochemical cells such as sodium sulphur cells, the anodic and cathodic reactants are highly reactive chemically and sealing of the anodic and cathodic compartments is very important. Such sealing is necessary in general in all devices employing liquid sodium and beta alumina electrolyte material.

Because of the temperatures of operation and temperature cycling which occur in devices employed in liquid sodium,the sealing of a closure orclosuresto the electrolyte material is commonly effected using a glaze, employing a glass having a temperature coefficient of expansion over the appropriatetemper- ature ranges which closely matches that of the beta alumina. Even so, such a glass seal is subjected to stress changes, particularly if the beta alumina is sealed to a different material, for example alpha alumina.

In manyforms of construction, the wicking medium must be assembled with the beta alumina electrolyte material before sealing ofcomponentstothe beta alumina. Considerfor example a sodium sulphur cell having tubular electrolyte material with sodium inside the tube and a cathodic reactant around the outside of the electrolyte tu be. The wicking medium in this case would typically be a cylinder or spiral of metal foil inside the electrolyte tube, slightly spaced away from the internal surface of the electrolyte tube to leave a capillary region. Such material has to be put in place before the electrolyte tube can be closed at its open end or ends. We have found that the raising of such an assemblyto the temperature required for glazing, e.g.

900 C, if carried out in airoran oxidising atmosphere with iron foil or stainless steel as the wicking material results in contamination ofthe glass by oxidation scale from the wicking material. Owing to theformation of oxide scale from wick materials such as stainless steel and iron foil, the glazing operation has heretofore been carried out under vacuum or in a suitable inert atmosphere. Also annealing ofironfoil and other such materials reduces the compliance of the material and reduces the effectiveness as awick.

The present invention is directed to an improved construction and method of production for an energy conversion device utilising liquid sodium and beta alumina electrolyte material which permits ofthe use of glazing for sealing components to the beta alumina without necessitating that the glazing should be carried out in a vacuum or other controlled atmosphere furnace.

According to one aspect ofthe present invention, in an energy conversion device employing beta alumina electrolyte material and sodium which is liquid at the operating temperature of the device, wicking means are provided adjacent a surface ofthe electrolyte material to be exposed to sodium during operation of the cell, which wicking means is formed of an iron chromium aluminium alloy containing also yttrium.

Iron chromium aluminium alloys, under oxidising conditions, form a stable alumina coating. The inclusion of yttrium within the alloy results in this alumina coating being keyed to the matrix by preferential grain boundary oxidation. It also enhances the resistance to attack by oxides and other oxidising agents and other materials. Afurther advantage of the inclusion of yttrium is that the material remains rigid at the temperatures involved with devices ofthe nature to which the present application is directed whilst still retaining mechanical properties such as ductility.

The use ofthis materialforthewicking meansthus enables glazing of components to the beta alumina to be effected even in an oxidising atmosphere after the wicking means is in position adjacentthe electrolyte material.

The amount of yttrium required in the alloy is relativelysmall compared with the iron chromium aluminium content and may be less than 0.5% by weight,typicallybeing about0.3% byweight. A convenient material to use is that sold underthe name FecralloyA (Fecralloy is a RegisteredTrade Mark) which material typically has a composition by weight of carbon 0.03%, silicon 0.3%, chromium 15.8%, aluminium 4.8%, yttrium 0.3%, balance iron.

The material is conveniently used in the form of a foil. For use with beta alumina electrolyte material in the form of a tube, the foil may be formed into a cylinderwhichwould be insidethetubeoroutsidethe tube according as to whether the sodium is to be inside or outside the electrolyte tube. Such a cylinder may readily beformed by wrapping the foil around the tube or around a mandrel and welding the edges ofthe foil to secure them together.

In some cases apertures may beformedthrough the foil to permit sodium to pass through the foil into the capillary region between the foil and the electrolyte. In a central sodium cell for example the region insidethe foil mayform a sodium reservoir, the foil passing into the capillary region which maintains the internal surface ofthe electrolyte tube wetted with sodium irrespective ofthe level of sodium in the reservoir.

Commonly however it is sufficient, for permitting passage ofthe sodium intothe capillary region, to form a series of scallops or cut-outs along the edge of the foil at least at one end of the foil cylinder. These scallops or cut-outs should be at the bottom edge of the foil if the cell is to be used with its axis upright and it is convenienttoform them at both ends.

The invention furthermore includeswithin its scope a method of securing a component such as a closure or part-closure element, to a beta aluminatube for use in an electrochemical energy conversion device in which the tube is to contain liquid sodium, wherein a wicking medium comprising an iron chromium aluminium alloy containing also yttrium is put inside or is put around the electrolyte tube and said component is secured by glazing to the electrolyte tube, the glazing operation being carried out in air.

Thefollowing is a description of one embodiment of the invention, reference being made to the accompanying drawing which is a longitudinal section through part of a beta alumina electrolyte tube and certain further components of a sub-assemblyform- ing part of a sodium sulphurcell.

Referring to the drawing there is shown a beta alumina electrolyte tube 10 formed with a closed end 11. Located within the electrolyte tube is a wicking medium 12 made of an iron chromium aluminium alloy containing a small quantity of yttrium. Convenientlythe material has a typical composition by weight of carbon 0.03%, silica 0.3%, chromium 15.8%, aluminium 4.8%, yttrium 0.3%, balance iron. This metal alloy is in theform of a foil which has been bent around to form a cylinderwith the overlapping edges welded together. The cylinderfitsfairly closely within theelectrolytetubeto leave an annular capillary region between the internal surface ofthetubeandthe external surface ofthe wicking cylinder.If necessary the metal foil may be dimpled to maintain the required spacing. The upper and lower edges ofthe cylinder formed by the metal foil have small scallops or cutaways 13 to permit sodium to pass from the region insidethemetalfoil into the capillary region.

In the manufacture of a sodium sulphur cell, the wicking medium is positioned inside the beta alumina tube and then a partial closure element in the form of an annular ceramic member 14 of alpha alumina or of beta alumina is sealed tothe open end of the beta alumina tube 10 by a glazed seal 15. The wick may be preoxidised priorto insertion into the electrolyte, typically at 11 000C for 2 hours. The glazing is effected in air using an alumina borosilicate glass containing appropriate proportions of alkaline earth materials chosen to give a coefficient ofthermal expansion closely matching that of the beta alumina. Reference may be made to U.K. Patent Specification No. 2048237 for details of the composition of suitable glasses for this purpose.

The partial closure member 14 has a central aperture 16to be utilised forfilling the beta alumina tube with liquid sodium, the aperture then being sealed by insertion of a closure member (not shown) carrying a current collectorwhich will extend into the liquid sodium.

By the choice of the material for the wicking medium, it has become possible to effecttheglazing operation in airthereby obviating any need for using a vacuum furnace or a furnace with a controlled atmosphere of inert gas. The alloy material described above, for example, Fecralloy, forms a tenacious oxide layer and does not cause oxide contamination of the glass.

Although reference has been made more particularly in the foregoing exampleto a construction in which the wicking medium lies inside a beta alumina tube, it will be immediately apparent that similar techniques can be employedfor a wicking medium aroundthe outside ofthe beta alumina tube; for example in a central sulphurtype of tubular sodium sulphur cell, an alpha alumina collar may have to be secured, by glazing,to a beta alumina electrolytetube atthe open end ofthattubetoform part of a closureforthe annular sodium compartment between the outer surface ofthe electrolyte tube and a surrounding cylindrical housing.Awicking medium may also equallywell be employed in a flat plate cell,the wicking medium in this case being a flat plate, typically a disc, which would lieadjacenttotheflat plate constituting the electrolyte material. In this case glazing may be employed to seal the electrolyte disc into a housing.

Claims (10)

1. An energy conversion device employing beta alumina electrolyte material and sodium which is liquid atthe operating temperature of the device, wherein wicking means are provided adjacent a surface of the electrolyte material to be exposed to sodium during operation of the cell, which wicking means is formed of an iron chromium aluminium alloy containing also yttrium.

2. An energy conversion device as claimed in claim 1 whereintheamountofyttrium in the alloy is less than 0.5% by weight ofthe alloy material.

3. An energy conversion device as claimed in claim 1 wherein the wicking means has substantially a composition by weight of carbon 0.03%, silicon 0.3%, chromium 15.8%, aluminium 4.8%,yttrium Q.3%, balance iron.

4. An energy conversion device as claimed in any ofthe preceding claims wherein the wicking means comprises a foil.

5. An energy conversion device as claimed in any ofthe preceding claims and having beta alumina electrolyte material in the form ofatubewherein the wicking means comprises foil formed into a cylinder.

6. An energy conversion device as claimed in either claim 4 or cla i m 5 and having aperturesthrough the foil to permit sodium to pass through the foil into the capillary region between the foil and the electrolyte.

7. An energy conversion device as claimed in claim wherein, for permitting passage ofthe sodium into the capillary region, a series of scallops or cut-outs are provided along the edge of the foil at least at one end of the foil cylinder.

8. A method of securing a component to a beta alumina tube for use in an electrochemical energy conversion device in which the tube is to contain liquid sodium, wherein a wicking medium comprising an iron chromium aluminium alloy containing also yttrium is put inside or is put around the electrolyte tube and said component is secured by glazing to the electrolyte tube, the glazing operation being carried out in air.

9. An energy conversion device having a betaalumina electrolyte element secured to a further component by glazing and having a wicking means substantially as hereinbefore described with reference to the accompanying drawing.

10. A method of securing a componentto a beta-alumina tube containing a wicking means substantially as hereinbeforedescribed.