Method for preparing alkaline beta alumina parts

Method for manufacturing alkaline beta alumina parts, consisting in heating a mixture of alumina and of sodium carbonate, in crushing the powder obtained, in shaping the parts, in sintering them at 1650.degree. C, those parts being arranged in a sintering enclosure suitable for creating, in the vicinity of the parts, an atmosphere which is rich in sodium. The invention is implemented in the embodiment of an electrolyte for sodium-sulphur cells.

The present invention has as its object a method for preparing alkaline 
beta alumina parts and more particularly sodium beta alumina parts. 
It is known that great applications can be found at present for alkaline 
beta alumina and more particularly beta sodium alumina as a solid 
electrolyte in electrochemical cells such as sodium sulphur cells. 
A known method for preparing beta sodium alumina having a formula of 
xA1.sub.2 0.sub.3, Na.sub.2 0 consists, for example, in effecting, at a 
temperature comprised between 1850.degree. and 2000.degree. C, the melting 
of a mixture of alpha or gamma alumina and of sodium carbonate in such 
proportions that x be comprised between 5 and 11. The alumina thus 
obtained is broken up and crushed until the required granulometry is 
obtained. 
For that purpose, the block of alumina obtained is crushed during 24 hours, 
approximately by means of steel balls in a stainless steel jar. 
A part of the iron which is contingently inserted during the crushing 
operation is removed by magnetic sorting, the remainder being removed by 
corroding with hydrochloric acid. 
After washing, rinsing, drying, beta alumina powder whose grains can have 
dimensions in the order of a few microns is obtained. 
Beta alumina powder is then shaped with a view to its use as an 
electrolyte. 
In the case where it is required to form a tube, it is an advantage to 
proceed as follows: 
In a first phase, the powder is deposited on a mandrel, for example by 
electrophoresis or spraying with a gun, after putting into suspension in 
an organic solvent contingently containing a binding agent. Then, an 
isostatic compression of the deposit thus formed is effected. 
After stripping, sintering in air is effected, for example in a gas 
furnace, at a temperature in the order of 1800.degree. to 1900.degree. C 
for about 4 hours. 
The rise in temperature up to that sintering temperature is rapid. It can 
be effected in 3 hours. Rapid cooling is also effected. 
In this way, it is possible to obtain tubes having a thickness comprised 
between 200 and 600 microns and whose walls are very fluid-tight, the said 
tubes being suitable for use as solid electrolytes. 
Nevertheless, such methods have a certain number of disadvantages. One of 
these disadvantages results from the fact that the crushing times are long 
and that it is necessary to effect a purifying of the product obtained as 
stated hereinabove. 
Another disadvantage results from the fact that subsequent to the use of 
high sintering temperatures, there occurs a great loss of sodium in the 
beta sodium alumina parts, this leading, more particularly, to a certain 
dispersion of the properties between different batches of sintered parts. 
The present invention makes it possible to overcome those major 
disadvantages and it has as its object a method enabling the easy 
preparing of the alkaline beta alumina and more particularly of the beta 
sodium alumina having compositions and properties which are very 
reproducible, such a method being simple to implement at a moderate cost 
price. 
The invention therefore has as its object a method for preparing alkaline 
beta alumina parts, in which the following successive phases are carried 
out: 
A. A close mixture of alumina powder and alkaline carbonate powder, more 
particularly sodium carbonate powder is formed in respective quantities 
such that a predetermined ratio of A1203/Na20 be obtained; 
B. The said close mixture is heated in an open crucible; 
C. The said mixture is allowed to cool freely; 
D. The powder thus obtained is crushed; 
E. The shaping of the said parts is effected; 
F. The sintering of these latter is effected; 
A method in which the said sintering is effected by arranging the said 
parts in a sintering enclosure, suitable for creating, during the 
operation and in the immediate vicinity of the parts, an atmosphere which 
is rich in sodium, the said enclosure being heated to a temperature 
comprised between 1600.degree.0 and 1700.degree., maintained for 30 
minutes to 4 hours, then being cooled freely down to ambient temperature, 
that method being characterized in that the said sintering enclosure 
comprises a body suitable for receiving the said parts stopped up at its 
ends by means of a top plate and a bottom plate, each of the said plates 
comprising a hollowed out part filled with the said alkaline carbonate, 
the said bottom plate resting on a bottom, the said top plate being 
stopped up by means of a cover. 
According to one method of embodiment, the said body has a shape which is 
preferably tubular, the said parts being arranged inside the body on 
chocks made of the said alkaline beta alumina, the hollowed out part of 
the said top plate being arranged at the upper part of the latter with 
respect to the said cover, the hollowed out part of the said bottom plate 
also being arranged at the top part of the latter with respect to the said 
body. 
According to another embodiment, the said body comprises alveoli opening 
out at its ends and accomodating the said parts arranged bearing against 
the said bottom plate, the hollowed out part of the said top plate being 
arranged at the upper part of the latter with respect to the said cover, 
the hollowed out part of the said bottom plate being arranged at the lower 
part of the latter with respect to the said bottom and communicating with 
each of the said alveoli by means of a channel opening out into the inside 
part of each of the parts. 
In these two embodiments, the sintering assembly is entirely made of the 
said alkaline beta alumina. 
According to yet another embodiment, the said sintering enclosure is made 
of a refractory material preferably chosen from the group formed by alpha 
alumina, carborundum (silicon carbide) zirconia and it comprises a body 
suitable for accomodating the said parts, stopped up at its ends by means 
of a top plate and a bottom plate, each of the said plates comprising a 
hollowed out part filled with the said alkaline carbonate, the said bottom 
plate resting on the bottom, the said top plate being stopped up by means 
of a cover, the said body having a preferably tubular shape, the said 
parts being arranged inside the body on chocks made of alkaline beta 
alumina, the hollowd out part of the said top plate being arranged at the 
upper part of the latter, facing the said cover, the hollowed out part of 
the said bottom plate also being arranged at the upper part of the latter, 
facing the said body, the said hollowed out part of the top plate 
communicating with the inside of the said body by means of channels formed 
in the said top plate, the said channels opening out into the said 
hollowed out part of the top plate by means of parts in relief, the 
alkaline carbonate being spread round the said parts in relief.

Firstly, powders of alpha or gamma alumina and of sodium carbonate are 
closely mixed together in respective quantities such that beta alumina 
xA1.sub.2 0.sub.3, Na.sub.2 0, with x comprised between 5 and 11, is 
obtained. 
Such a mixture is arranged in an open crucible in such a way that the 
reaction takes place in an open atmosphere. 
The said crucible is inserted in a furnace and brought to a temperature 
comprised between 1150.degree. and 1300.degree., for example 1200.degree., 
maintained for 1 to 5 hours, for example. 
At the end of the heating time, the crucible is allowed to cool freely. The 
beta sodium alumina thus obtained in the form of powder is then crushed 
for about 30 minutes and passed through a sieve. 
The shaping of the parts, for example, of tubes, is then effected, this 
being done by electrophoresis and isostatic compression of the deposit 
obtained. 
According to the invention, the sintering is effected as follows: 
With reference to FIGS. 1 and 2, the tubes 1 of beta sodium alumina shaped 
as previously described are inserted in a sintering enclosure which is 
made entirely of beta sodium alumina. That enclosure comprises, in a first 
embodiment, a tubular body 2, stopped up at its ends by a top plate 3 and 
a bottom plate 4. The top plate 3 comprises a circular hollowed out part 5 
at its upper part, filled with sodium carbonate powder or grains 6. 
Likewise, the bottom plate 4 also comprises, at its upper part, a hollowed 
out part 7 filled in a like manner with sodium carbonate powder or grains 
6. The assembly is stopped up by a cover 9 and rests on a bottom 10. The 
said tubes 1 rest on chocks 11 made of beta sodium alumina. 
With reference to FIGS. 3 and 4, the tubes 1 of beta sodium alumina shaped 
as previously described are inserted in a sintering enclosure also made of 
beta sodium alumina, which, according to a second embodiment, comprises a 
body 22 which is, for example, cylindrical, perforated with alveoli 23 
which are, for example, cylindrical, the said body being stopped up at its 
ends by means of a top plate 24 and a bottom plate 25, on which the tubes 
1 rest. 
The top plate 24 comprises a circular hollowed out part 26 at its upper 
part, filled with sodium carbonate powder or grains 27. Likewise, the 
bottom plate 25 comprises, at its lower part, a hollowed out part 28 
filled in a like way with sodium carbonate powder or grains 27. Moreover, 
each of the alveoli 23 communicates with the hollowed out part 28 of the 
plate 25 by means of a channel 29 opening out inside the part 1. The 
assembly is stopped up by a cover 30 and rests on a bottom 31. 
Whatever the mode of embodiment chosen, the enclosure is inserted in a 
furnace where the sintering of the parts 1 is effected. For that purpose, 
the temperature is raised to 1650.degree. C and maintained for 11/2 hours, 
such operating conditions varying, contingently, between 30 minutes and 4 
hours inasmuch as concerns time and between 1600.degree. and 1700.degree. 
inasmuch as concerns temperature. The speed of the rise in temperature is 
in the order of 3 hours. Then the furnace is allowed to cool freely. 
With reference to FIGS. 5 and 7, the tubes 1 of beta sodium alumina which 
are prepared and shaped as previously described are inserted in a 
sintering enclosure made entirely, in the third embodiment, either of 
alpha alumina or of carborundum, or of zirconia, or of any other 
refractory material. That enclosure comprises a tubular body 42, stopped 
up at its ends by a top plate 43 and a bottom plate 44. The top plate 43 
comprises a circular hollowed out part 45 at its upper part in which 
sodium carbonate powder or grains 46 are inserted. Likewise, the bottom 
plate 44 comprises, also, at its upper part, a hollowed out part 47 filled 
with sodium carbonate powder or grains 46. The assembly is stopped up by a 
cover 49 and rests on a bottom 50. The said tubes 1 rest on chocks 51 made 
of beta sodium alumina. 
According to that embodiment, the hollowed out part 45 of the top plate 43 
communicates with the inside of the body 42 by means of channels such as 
52. The channels 52 open out, moreover, into the hollowed out part 45 
through the parts in relief 53 intended for preventing the sodium 
carbonate 46 spread in the hollowed out part 45 round those parts in 
relief from falling in the body 42. 
The enclosure is therefore inserted in a furnace where the sintering of the 
parts 1 is effected. For that purpose, as in the preceding cases, the 
temperature is raised to 1650.degree. C and maintained for 11/2 hours, 
such operating conditions contingently varying, moreover, between 30 
minutes and 4 hours, inasmuch as concerns time and between 1600.degree. 
and 1700.degree. C inasmuch as concerns temperature. The rising time of 
the temperature is in the order of 3 hours. Then the furnace is allowed to 
cool freely. 
Whatever the embodiment chosen may be, the advantages of such a method are 
as follows: 
In the first instance, the implementing of sources or tanks of sodium 
carbonate in the sintering enclosures makes it possible to create, in the 
vicinity of the parts 1 to be sintered, an atmosphere which is rich in 
sodium, suitable for compensating all contingent losses of the parts in 
that element. 
The parts obtained in this way have a composition and properties which are 
constant and very reproducible, even from one sintering batch to another. 
Moreover, the use of such a relatively low sintering temperature ensures, 
on the one hand, great saving of energy and, on the other hand, a 
remarkable service life of the sintering enclosures, which can therefore 
be used for a great number of operations. 
It must be understood that the invention is in no way limited to the 
embodiments described and illustrated, but on the contrary, it covers all 
the variations therein.