It has been known for some time that the presence of alkali metal oxides in liquid alkali metals is undesirable, or even intolerable, when the liquid metal is to be used as an electrical conductor or as a heat transfer medium--particularly in atomic reactor systems. Further, it has now been discovered that if sodium containing any substantial amount of sodium oxide(s) is used in a hollow-fiber type sodium/sulfur battery cell, the useful life of the cell will be curtailed by corrosion (by the sodium oxide) at the inner surfaces of the fibers--which serve as the electrolyte/separator membrane in the cell.
Similar corrosion would be expected to occur in alkali metal/chalcogen battery cells in general when the alkali metal used is contaminated with alkali metal oxides. However, the geometry and restricted cross section of hollow fibers, which are typically hair-like, are such that the problem of corrosion is particularly acute in Na/S cells utilizing such fibers. (Cells of the latter type operate at temperatures of about 300.degree.-400.degree. C. and are described in U.S. Pat. Nos. 3,476,062; 3,672,995; 3,791,868 and others.) Experience with such cells strongly suggests that fiber failures which occur when the cell is put back on charge after prolonged, "deep" discharge are caused by the structural changes resulting from corrosion and accompanied by the (relatively abrupt) development of a high resistance in the latter stages of the discharge period.
The solubility of sodium oxide in sodium varies from several parts per million, at the melting point of sodium, up to several hundred parts per million at elevated temperatures (300.degree.-400.degree. C., for example). For most applications, an oxide content of more than a couple of parts per million is considered "substantial" and must be reduced in some manner.
In a well known "gettering" procedure, the sodium oxide is reduced with an active metal, such as zirconium, which is insoluble in sodium. The sodium is repeatedly passed over a gauze or foil of the zirconium-type metal at elevated temperatures (600.degree.-800.degree. C., for example). This method is effective but requires long residence times and is relatively inefficient for treatment of small batches of sodium.
An ancillary problem is to be able to monitor the oxygen/oxide content in alkali metals which are employed under conditions where oxygen uptake may occur. For example, when liquid sodium is circulated as a coolant in atomic reactor loops, it is essential to monitor oxygen content, even though the sodium is charged in an oxygen-free condition to a purged system which is subsequently sealed. U.S. Pat. No. 3,309,233 discloses the use of a solid electrolyte, electrochemical cell, in which the electrolyte is composed of yttria, (Y.sub.2 O.sub.3), for monitoring the dissolved oxygen content of sodium at temperatures within the range of 400.degree.-500.degree. C. This is done by measuring the e.m.f. developed between half-cells on opposite sides of the yttria membrane. One half-cell contains Na/Na.sub.2 O saturated with oxygen and the other half-cell contains the sodium being monitored, which is continuously circulated through the cell as a bleed stream from the main sodium body. The oxygen content of the monitored sodium is calculated from the developed e.m.f. by appropriate relationships. According to the patent, this method is generally not suitable for use at temperatures outside of the 400.degree.-500.degree. C. range.
At the present stage of development of alkali metal/chalcogen batteries, the alkali metals (sodium, principally) are required in much smaller amounts than are appropriate for really efficient (continuous) gettering treatments. A need for a faster, more convenient and efficient method of removing oxygen (oxides) from relatively small bodies of alkali metals is apparent.
It is well known that alkali metal oxides can be reduced, with reactive metals, such as the alkaline earth metals and certain rare earth metals, which are soluble in the alkali metals. It is also known that the oxides of the latter metals produced by the reaction are essentially insoluble in molten alkali metals. However, it does not appear that removal of dissolved alkali-metal oxides from molten alkali metals by means of this reaction has seriously been considered. This may be at least partly due to the fact that the amount of the reducible oxide present in a given body of the alkali metal must first be determined if introduction of more than a stoichiometric amount of the reductant (and soluble) metal is to be ensured. (This is not a problem in the preceding "gettering" procedure.)