US2403033A - Gaseous electrical space discharge rectifying device - Google Patents

Description

July 2, 1946. P; w. STUTSMAN GASEOUS ELECTRICAL SPACE DISCHARGE RECTIFYING DEVICE Filed March 51, 1942' \NV'ENTOR. PAUL W; STUTSMAN BY AT'TY.

Patented July 2, 1946 GASEOUS ELECTRICAL SPACE DISCHARGE RECTIFYING DEVICE Paul W. Stutsman, Waltham, Mass., assignor, by

mesne assignments, to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application March 31, 1942, Serial No. 436,939

6 Claims.

This invention relates to gaseous electrical space discharge rectifying devices and particularly to the cathodes thereof. This invention has special applicability to cathodes of the type which are adapted to be heated to the temperature of electronic emission by the discharge occurring in such a device.

In certain devices of the character described, a. glow discharge is produced which thereafter changes into an arc. During the period of the glow discharge, particularly during the abnormal glow period, a comparatively high cathode drop exists. At the same time a substantial amount of positive ions has been produced due to ionization of the gas. Because of the high drop the positive ions rush towards the cathode and bombard it with destructive effect. The life of the cathode is thus considerably shortened. It therefore is desirable that the transition from a glow discharge to an arc occur as quickly as possible. Furthermore, after the normal glow has spread on the cathode, the abnormal glow which follows and the transition to the arc make greater demands on the circuit with which said device is associated, and in certain cases the circuit is unable to meet these demands. For the above reasons and for other reasons which will be apparent to those versed in the art it is desirable that the transition from the glow discharge to an arc occur as easily and as rapidly as possible. Therefore, it is a further object of the present invention to provide means for enabling an easy and rapid transition from a glow discharge to an arc in such devices.

Another object of this invention is the provision of a gaseous electrical space discharge rectifying device which has a comparatively long operative life.

Other and further objects and advantages will become apparent, and the foregoing will be best understood, from the following description of an exemplification thereof, reference being had to the drawing in which the figure is a perspective view of a gaseous rectifying tube employing my invention.

In the drawing the tube illustrated consists of a glass envelope l adapted to contain a suitable gaseous atmosphere and having a, reentrant stem 2 with a press 3 at the upper end thereof, said press carrying a cathode 4 and a plurality of anodes 5. The anodes 5 may be made of graphite and are preferably surrounded by conducting shields 6. A getter assembly I may also be arranged on the press 3. Suitable lead-in conductors 9 and an exhaust stem l0 may be provided as 88 will be apparent to those versed in the art.

The cathode t is preferably made in the form ends of the coil being connected to standards 8. The cathode is preferably made of a, solid core portion on which a fine wire is wound. For example, the cathode may consist of a tungsten core of approximately .005" on which a fine tungsten wire is spirally wound. The fine tungsten wire preferably Weighs 4 mg. per 200 mm. and has a diameter of approximately .00146". Said fine tungsten wire is wound with approximately 460 turns per inch on the core, which is relatively close, since about 680 turns per inch would completely cover said core. This assembly consisting of the fine wire wound around the heavier tungsten core is then wound in the form of the spiral as illustrated, preferably having about thirteen turns, the exterior diameter of the spiral being approximately Thus a comparatively large surface of cathode is provided.

The cathode is preferably coated with a suitable electron emissive material comprised of a compound or a mixture of compounds of alkali and/or alkaline earth metals. The term alkaline metals as used hereinafter in the, claims is intended to include both alkali and/or alkaline earth metals.

In tubes of the character described after a starting voltage is applied thereto a glow discharge is produced. This glow discharge, after a certain short interval, changes to an are. For the reasons hereinabove pointed out, it is preferable that this transition occur as soon as possible and with the minimum consumption of electrical energy. I have found that by providing thorium oxide on the cathode this transition occurs sooner and more readily. An additional advantage obtained from the use of thorium oxide is that due to its radioactivity it will maintain some ionization in aseous tubes when said tubes are not in operation. This enables initiation of the operation of such tubes at lower starting voltages.

In applying the thorium oxide to the cathode the thorium oxide may be made mixed with the electron-emissive materials that are to form the cathode coating. The proportion of thorium oxide to the electron-emissive material may be varied within considerable limits. I have found, however, that the use of too much thorium oxide will cause the cathode to deteriorate more rapidly and will increase the likelihood of arc-back, while too little will produce substantially less than the optimum advantages that may be obtained. I have found that when the thorium oxide contained in the mixture has a weight approximatey of the order of 4% to 8% of weight of the electron-emissive material therein, good of a spiral coil, the

s 3 results will be obtained. Above said 8% in a specified embodiment.- I found a tendency towards arc-back, especially when 16% was used. When 18% was used, I also found that the operative life of said tubes was substantially less than the optimum obtained.

A specific example of a coating of a cathode which I have found gives good results is as follows. A mixture is made of 400 grams barium monoxide, 32 grams thorium oxide, grams of barium hydroxide and A of 1 cc. of dimethyl phthalate and is dispersed in 140 cc. of refined white mineral oil, such as "Nuiol" or the like. The dimethyl phthalate serves to reduce the viscosity of said mixture and to thereby produce 'a composition which is suillciently workable or fluid to provide a uniform coating on the body portion of the coil. In order to obtain a more homogeneous mixture. it is preferred that the barium oxide and thorium oxide particles used in said mixture be small enough to pass through a wire mesh having about 200-300 openings per square inch.

Another satisfactory mixture employed only half the quantity of thorium oxide. In this latter mixture 16 grams instead of the 32 grams was substituted in the above formula. This homogeneous mixture is applied to the cathode coil in the manner described in my copending application for Emissive filament and method of making," flied January 3, 1942, Serial Number 425,546, now Patent No. 2,362,510, issued November 14. 1944. After the coil has been immersed in this mixture and has become coated. it is removed and heated in air by passing a current therethrough. In order to permit the ready removal of the coating mixture from the ends of the coil so that these ends may be clean for welding to standards 0, it is preferred that the current does not pass through said ends so that the ends will be cool and the coating mixture will not bind thereto. As the coil is heated it is manipulated so as to expand and contract the convolutions thereof. The coil is gradually heated to a temperature of the order of 800 C. after which it is immersed in a protective rinse consisting of a relatively light oil,

such as kerosene or No. 2 fuel oil.

The protective rinse is effective not only to prevent atmospheric hydration of the coated coil. but also to dissolve such residual coating composition as may adhere to the end portions of the core. The coated coil may remain in the rinse until the assemblage with the coil standards at which time it is withdrawn and its ends are welded or otherwise secured to standards I.

The assemblage is then subjected to a further heat treatment in air at a temperature of the order of 800 to 900 C., which treatment is eflective to drive all the kerosene, and this operation may be carried out in an enclosed chamber under reduced pressure to avoid flashing of the kerosene and possible inJury to the coated core. Following this step the assemblage is then set into the tube and subjected to a further heat treatment at a temperature of the order of 1100' 0., this treatment being effective to fuse the barium monoxide particles, burn off any residues and,

reduce any barium compounds incidentally formed during the previous heat treatments.

In this last heating treatment the temperature is gradually raised towards that indicated and during the latter part of the raising of this temperature the air is gradually pumped out of the tube through the exhaust stem l8. which is conescapes nccted to a suitable exhaustpump. After this heat treatment and after substantial evacuation of the air from said tube. the getter is while the evacuating pump continues to operate. After the gases in said tube have been substantially exhausted, a suitable gas, such as for example a monatomic gas, like argon, is preferably introduced into said tube at a pressure of the order of a few mm. of mercury. The exhaust stem I8 is then sealed.

While I have described the specificjdetails of a particular embodiment of my invention it is. of course, to be understood that modifications may be made without departing therefrom. For example, other types of anodes and other shapes of cathodes might be employed. While I have shown two anodes in this embodiment, it is, of course, obvious that only one or more than two anodes might be employed. Furthermore, while I have described a cathode which is adapted to be heated by the energy of the discharge, my invention may be employed with cathodes which are heated by means external to the discharge, such as for example a current passed through said cathode. Numerous other modifications will be apparent to those versed in the art and therefore it is desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A gaseous electrical space discharge rectifying device of the type in which a glow discharge changes to an arc comprising an envelope, an ionizabie gas in said envelope for supporting a discharge, an anode and a cathode in said envelope between which the discharge' is adapted to occur, said cathode having a coating of electron emissive material, said coating including thorium oxide, said thorium oxide having a weight of the order of 4% to 8% of the weight of the electron emissive material.

2. A gaseous electrical space discharge rectifying device comprising an envelope containing gas adapted to be ionized to support the discharge, an anode and a cathode in said envelope between which the discharge isadapted to occur, said cathode having a coating comprised of alkaline metals and thorium oxide, said thorium oxide having a weight of the order of 4% to 8% of the weight of the alkaline metals.

3. In the method of-forming a coating for cathodes adapted to be used in gaseous electrical space discharge rectifying device the step comprising mixing an electron emissive material with thorium oxide in the proportion of 4 to 8% of thorium oxide by weight to the weight of the electron emissive material.

4. In the method of forming a coating for cathodes adapted to be used in gaseous electrical space discharge rectifying devices, the step com mixing an alkaline metal compound with thorium oxid in the proportion by weight of approximately 4% to 8% of thorium oxide to the alkaline metal compound.

PAUL W. STUTSMAN.

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