US2160583A - Pool-type discharge device - Google Patents

Description

May 30, 1939'.

J. M. CAGE POOL-TYPE DISCHARGE DEVICE Filed March 13, 1937 SEM/-- obwucrwa MA TERI/4L Inventor:

e .y 2 m a r C o M n m h m o Patented May 30, 1939 UNITED STATES PATENT OFFICE POOL-TYPE DISCHARGE DEVICE York Application March 13, 1937, Serial No. 130,760

8 Claims.

This invention relates to pool-type discharge devices, and more particularly to improvements in starting electrodes for such devices. The present application is a continuation in part of my application, Serial No. 48,864, filed November 8, 1935.

It has been found that very accurate and sensitive starting of a tube having a pool-type cathode may be obtained by the use of a semi-conducting immersion electrode having a tip or active portion projecting below the surface of the cathode pool. This effect is believed to be due in part to the nature of the surface contact existing between the starter material and the cathode pool and, in part, to the production of a sufficiently high voltage gradient along the body of the starting electrode to cause field emission of electrons from the surface of the pool. This type of starting and the theoretical considerations which govern its operation are discussed, for example, by J. Slepian and L. A. Ludwig at page 693 of vol. 52 of the Transactions of the A. I. E. E. for June, 1933.

It is an object of the present invention to provide improved immersion electrodes suitable for producing the type of arc initiation described in the foregoing. More specifically, it is an object to provide electrodes which require a low starting current, are reliable in operation and which do not change their characteristics even after a long period of use.

According to a preferred embodiment of the invention these objects are attained by forming the body or core of the starting electrode from a chemically stable semi-conductor, preferably recrystallized silicon carbide, having inherent electrical characteristics well adapted for such use and covering the active surface of the electrode pended claims. My invention itself, together with further objects and advantages thereof, will best be understood by reference to the following description taken in connection with the accom- Da ying drawing, in which Fig. 1 illustrates a discharge device capable of utilizing my improved electrodes, Fig. 2 is a view, partly in section, of an immersion electrode formed in accordance with the invention and associated with a suitable mounting structure and Fig. 3 shows the electrode of Fig. 2 partly broken away.

In pool-type discharge devices such as the three electrode tube I illustrated'in Fig. l, the anode 2 and the pool-type cathode 3 (suitably of mercury) are adapted to be connected in series with an external load circuit and a source of operating potential. The latter (not shown on the drawin typically comprises an alternator or other energy source for generating a voltage which varies sinusoidally from positive to negative at the rate of many cycles per second. For many purposes, as for instance, in welding circuits it is desired to have the tube rendered conductive only during a limited portion of the positive half cycle of voltage. This is accomplished by the use of an immersion starting electrode 4 of semi-conducting material (i. e., a material having a specific resistivity between about and 100,000 times that of mercury) positioned so that its tip is in permanent contact with the cathode pool 3. The elecrode 4 is subjected to a starting potential so regulated as to permit effective starting current to flow only at exactly predetermined times with respect to the variations of the main electrode potential.

For satisfactory operation, it is necessary that th starting electrode have electrical and surface characteristics which favor the particular type of starting under consideration. In this connection, it has been proposed to use recrystallized silicon carbide, such material being obtained, for example, by the following procedure:A quantity of crystallized silicon carbide is reduced to a workable form by pulverizing the crystals, mixing the pulverized product with a cellulosic binder such as a starch paste and extruding the mixture through a suitable die orifice. The extruded material is fired in silicon vapor at a temperature of about 2000, thus converting the cellulosic binder to silicon carbide.

Recrystallized silicon carbide prepared in this manner is chemically stable even at temperatures comparable to those encountered in the presence of an arc discharge and is highly refractory. Also, while it is exceptionally strong mechanically, it is capable of being machined and shaped with facility. Electrically, it is well adapted for use as an immersion starting electrode in that its resistivity inherently falls within the range indicated above as being suitable for such use (100 to 100,000 times that of mercury).

In spite of the factors described in the foregoing, starters consisting entirely of silicon carbide present the disadvantage of having a relatively short life. I have found that this difliculty is primarily due to the fact that silicon carbide, when used as a starting electrode in connection with a cathode material such as mercury, tends to become progressively wet by such material to an extent which seriously interferes with the operation of the electrode. When the wetting has proceeded beyond a given point, the electrical resistance of the starting electrode may become so diminished that no starting action whatever can be obtained.

In accordance with my invention, this difficulty is overcome by providing the active surface of a recrystallized silicon carbide electrode with a superficial non-insulating or partially conducting covering of a substance which is resistant to wetting by the cathode material. Such a covering may take the form of a coating applied to the exterior surface of the active portion ofthe electrode or alternatively may be provided, by an impregnant which coats the surfaces of the discrete particles or crystals which, form the structure of the electrode body.

A covering of the first type may be provided, for example, by applyingto the active electrode tip acoating comprising a carbide of boron, tungsten, or molybdenum preferably mixed with a refractory diluent such as clay. These materials are all resistant to Wetting by mercury and considerably increase the useful life of a, silicon carbide starter. Of the named substances I prefer to use boron carbide because its surface characteristics permit very low starting currents to be employed and because it resists wetting to a somewhat greater extent than is true with the other materials named.

In Fig. 2, I have exemplified a finished electrode constructed as indicated in the foregoing. The electrode is illustrated as comprising a substantially cylindrical core member 5 of silicon carbide terminating in a tapered tip portion bounded by a concave conical surface of revolution. The base of the cylinder is rigidly mounted in conducting relation with a'metallic coupling member 6 which in turn is secured in good electrical contact with a lead-in conductor 1. After the silicon carbide core is mounted in the manner illustrated, the tip which is to be immersed in the cathode pool is dipped into an aqueous suspension of powdered boron carbide mixed with from to'40 per cent of a non-conducting diluent or binder, for example, clay. Upon drying the electrode surface will retain enough of the suspended material to form thereon a coating, designated in the drawing by the numeral 8, which possesses surface characteristics suitable for producing arc initiation of the character hereinbefore set forth and which is resistant to wetting by the cathode pool material. The adherence of the coating, which consists essentially of boron carbide thoroughly dispersed. in the clay binder, may be assured by firing in a good vacuum at a temperature of about 1000 C. The coating 8 is shown in its relation to the core in Fig. 3 which illustratesthe starting electrode partly broken away.

Boron carbide itself when used as a starter material is difiicult to shape and appears to be incapable of withstanding ordinary manufacturing operations without appreciable chemical decomposition. Furthermore, the products of its decomposition are such as tend to produce faulty operation of the starter and of the discharge device with which it is associated However, when it is applied as indicated, that is, as a thin coating covering only the active portion of a preformed core member which is itself chemically stable, these factors are of no signif cancej Composite startersof'th'e type described above have" been used for many hundred hours without showing appreciable wetting or deterioration.

Instead of using an external coating of the character referred to, in certain cases even better results may be obtained by impregnating or treating the active or tip portion of the starter body with a substance adapted to render the individual silicon carbide particles incapable of being wet with mercury. The mechanism by which this is accomplished is that the individual crystals of silicon carbide are coated with a protective layer of the treating substance.

For this use, one may employ various noninsulating substances which are not wet by mercury. However, because of the relatively great chemical stability of the resulting protective layer, I prefer to utilize substances having the characteristics of products obtained by the decomposition of certain ferrous compounds, for example, of iron nitrate or iron chloride. A particular impregnating procedure which I have found to be highly satisfactory in this connection includes immersing the active portion of a silicon carbide electrode in an aqueous solution of an iron salt and subsequently subjecting the same to the action of heat. A solution of iron nitrate is particularly suitable for this purpose in that it may be first dried by heating suificiently to eliminate the water content and then melted at about 60 C. so as to become fully spread over the surfaces of the carbide crystals. As a final step the nitrate may be decomposed by heating in a hydrogen atmosphere or in vacuum. The resultant protective layer is believed to comprise a chemically stable mixture of iron and iron oxide (F6304) these substances being sufliciently conductive not to impair the normal operation of the electrode. They are also resistant to wetting by mercury to an extent which assures the absence of any change in the characteristics of the electrode as a Whole even after a very long period of use.

Since the treating process described in the foregoing protects the crystal surfaces throughout the whole body of the treated portion of the electrode, chipping or disintegration of the exterior surface of the electrode is not harmful. The new surfaces exposed by such chipping are nevertheless fully coated and protected against wetting by the mercury. Furthermore, an electrode construction of the type described may be prepared easily and cheaply with the assurance of consistent results.

While I have described my invention in connection with a particular structural embodiment, it should be understood that numerous modifications may be made without departing from the invention. I therefore aim in the appended claims to cover all such equivalent modifications as come within the true spirit and scope of the foregoing disclosure.

I claim:

1. An electrical discharge device including an 3 saidstarting electrode comprising a core consist- 15 ing of a chemically stable semi-conductor having a specific resistivity between about 100 and about 100,000 times that of mercury and being coated on said immersed tip portion with a mixture of boron carbide and a non-conducting diluent.

3. An electrical discharge device including an anode, a mercury pool cathode and a starting electrode having a tip portion which is in contact with the cathode during normal operation of the device, said starting electrode comprising a core of silicon carbide coated on said tip portion with a mixture of boron carbide and a non-conducting diluent.

4. An immersion starting electrode for a pooltype discharge device comprising a substantially cylindrical core of silicon carbide, said core having a tapered tip portion which is covered superficially and throughout its structure with a substance having substantially the characteristics of the products of decomposition of iron nitrate.

5. An immersion starting electrode for a pooltype discharge device comprising a substantially cylindrical core of silicon carbide, said core having a tapered tip portion coated externally with a material containing boron carbide.

6. The method of producing a durable silicon carbide immersion starting electrode for pooltype discharge devices which comprises treating the active portion of the electrode with iron nitrate and thereafter heating said portion sufficiently to cause decomposition of the iron nitrate.

'7. A discharge device including an anode, a pool type cathode and an immersion starting electrode of the type which, when energized, operates by virtue of its high resistivity to produce electron emission from the cathode, wherein the starting electrode comprises a core formed of a chemically stable semi-conducting material which tends to be wet by the cathode material, said core having a portion which is immersed in the cathode material during normal operation of the device, and means for maintaining the core material out of direct contact with the cathode material, said means comprising a non-insulating substance which is resistant to wetting by the cathode material and which is applied as a superficial coating to the immersed portion of the core.

8. A discharge device including an anode, a mercury cathode and an immersion starting electrode of the type which when energized operates by virtue of its high resistivity to produce electron emission from the cathode, wherein the starting electrode comprises a core of silicon carbide having a portion which is immersed in the mercury cathode during normal operation of the device, and means for maintaining the core material out of direct contact with the mercury, said means comprising a non-insulating substance which is resistant to wetting by mercury and which is applied as a superficial coating to the immersed portion of the core.

JOHN M. CAGE.

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