US1787694A - Electron-emission material - Google Patents

Description

Patented Jan. 6, 1931 No Drawing.

UNITED STATES PATENT OFFICE JOHN WESLEY MARDEN AND HARVEY CLAYTON BENTSCHLER, OF EAST ORANGE, NEW

JERSEY, ABQIGNORS TO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENN- SYLVANIA ELECTRON-EMISSION MATERIAL This invention relates to electron-emitting cathodes and more particularly to a cathode comprising a refractory metal body having incorporated or deposited thereon, material having high electron-emitting properties.

One of the objects of this invention is to provide a material of the above nature having high electron-emission and which will maintain such emission under adverse conditions for a long commercial life.

Another. object is to provide an electronemitting material, the emission of which may be readily restored should the same be impaired or destroyed. d

Other objects and advantages will appear as the description proceeds.

The present invention is based on the discovery that vanadium when incorporated in small percentages in refractory metals, such as tungsten, molybdenum, tantalum, etc., or coated thereon imparts to the metal body a high emission comparable with that obtained with thorium under similar conditions. Vanadium, however, as an electron-emitting material, has certain advantages over thorium as will hereinafter more fully appear.

Heretofore thorium has been considered one of the most practical of electron-emitting materials. Tt is readily worked into tungsten in small percentages and has a high electron emission at a moderate temperature at which temperature the rate of evaporation is very low. In producing thoriated tungsten wire for electron emission purposes it is usual to introduce the thorium into the wire in the form of thorium oxide since it is difiicult' to prevent thorium metal from volatilizing out at the temperatures to which the material is subjected during manufacture. The thorium oxide is later reduced to thorium metal. One method largely used for obtaining this reduction of thorium oxide is to flash the filament in a hydrocarbon atmosphere such as described in application Serial No. 636,510 of Ralph E. Myers, filed May 3, 1923, entitled Evacuated devices and method of exhaust, and assigned to the Westinghouse Lamp Company.

Another process is that described in application Serial No. 620,485 of John W.

Application filed March 8, 1926. Serial No. 93,808.

Marden et al., filed February 21, 1923, en titled Activation of electron-emission mate rial, and assigned to the Westinghouse Lamp Company.

In this latter method the thorium is introduced into the filament in the form of thorium carbide. In both of the above methods it is believed that thorium carbide is ultimately formed, which after the filament is sealed in the envelope and the same exhaust ed, is decomposed by heating the filament to a high temperature. While such process produces material of hi h electron-emitting properties it is somew at involved and requires careful supervision. Difliculty has also been experienced when employing thoriated cathodes in maintaining the thorium emission for a considerable eriod of time due to oxidation of the thor um by minute quantities of oxygen liberated from the electrodes. This is particularly true in low power tubes such as the usual dry cell tubes employing a short filament having a very limited quantity of thorium metal available for electron emission. In order to prevent this oxidation of the thorium it is the usual practice to bake out the anode plates and grids in hydrogen before sealingsin in order to remove all oxygen therefrom and after completion of the tube to heatthe plate by high frequency induction current to a high temperature, i. e., around '900 to 1000 to drive out all occluded gases and vaporize the metallic clean-up agent therefrom.

In spite of these precautions, however, an oxide film persists on the surface of the plate which is subsequently decomposed by electron bombardment from the cathode during operation of the device, with the liberation of oxygen which migrates to the filament and oxidizes the thorium film thereon destroying its emission. The thorium oxide so formed cannot be reduced again and the emission of the filament is permanentlyimpaired.

It is one of the aims of the present invention to avoid the difiiculties inherent in the use of thorium activated cathodes and to provide an electron-emitting material, the emission of which may be readily developed and maintained or if impaired may be readily restored.

lVe have observed when treating vanadium at high temperature in a high vacuum that near the melting point of the vanadium it has a very low vapor pressure and does not volatilize to any extent. We have also discovered that at such temperatures vanadium has an extremely high electron emission and when incorporated into a carrier metal such as molybdenum in small percentages, that it imparts to the metal body substantially the same emission as if the body were composed entirely of vanadium.

It has been proposed heretofore to roduce a tungsten filament containing thorium, vanadium, yttrium oxide and a number of other materials. Such a product is disclosed in the patent to Laise 1,569,095. The method of producing such a wire is extremely involved and laborious. The vanadium which is added to the Wire in addition to the thorium and other materials is ineffective to increase or augment the electron emission of the thorium under the existing theories of electron emission from thoriated cathodes. It is generally accepted that the thorium forms a film on the surface of the filament,

one atom deep and that any surplus thorium rapidly vaporizes away. If the entire filament is thus covered with an atom deep layer of thorium the filament will have its maximum emission and the addition of vanadium to the filament will not increase this emission. If a portion of the surface layer of thorium is replaced by vanadium the electron emission of the filament as a whole will be decreased slightly since the emission of vanadium is slightly less than that of thorium. It is obviously impossible to determine in a filament containing both thorium and vanadium how much emission is due to the presence of thorium and how much is due to vanadium.

The present applicants have found that it 1s unnecessary to employ the involved process, such as disclosed in the Laise patent, or to include in the filament the many different materials specified by him. Electronemitting filaments havin properties substantially equal to those 0% thorium activated cathodes and in addition being free from the objections specified above may be readily produced by the usual processesof producing tungsten, molybdenum and tantalum which are highly developed and greatly simplified. In order to produce a molybdenum filament containing vanadium, for instance, it is only necessary to mix the powdered molybdenum or molybdenum oxide with a vanadium compound, such as ammonium vanadate, reduce the same in hydrogen, press the powdered material into slugs and sinter, swage and draw the same in the manner well known in the art for producing molybdenum filaments.

The proportion of vanadium required in the completed Wire is very small, excellent results having been obtained with wire containing less than .5% of vanadium metal. Preferably, sufiiciently finely divided vanadium or reducible vanadium compound such as ammonium vanadate or vanadyl sulphate or nitrate should be added to the refractory metal powder to produce, from .5 to 1% of vanadium. Larger percentages do not appreciably augment the emission and render the wire more diflicult to Work. Caution should be observed during the various heat treatments to which the material is subjected during fabrication to treat the wire at as low a temperature as is consistent with proper mechanical working of the body in order to avoid distillation of the vanadium from the metal. When employing tungsten as the base or carrier metal, heating the wire to high temperatures in hydrogen should be avoided since hydrogen has a strong tendency to embrittle the same. Such brittleness, however, may be readily removed by subjecting the wire to an anodic treatment in a caustic soda solution such as that described and claimed in a copending application of Malcolm N. Rich, Serial No. 4140, filed January 23, 1925, entitled Pliable tungsten and method of producing the same and assigned to the Westinghouse Lamp Compan When the vanadium is incorporated into tantalum filaments such Wires may be fabricated by the cold working process Well known in the art for producing tantalum wire.

As a specific example of the production of vanadium activated cathodes, vanadyl nitrate was added to finely powdered molybdenum in su'fiicient quantity to produce 625% of vanadium metal to molybdenum metal. The mixture was reduced in a stream of hydrogen in the usual manner, the temperature being maintained at about 1000 C. for a period of several hours in order to insure complete reduction. The reduced mixture was then sieved and pressed into a slug of one-quarter inch cross section. The slug was smtered and treated in hydrogen by passing the current therethrough sufiieient to heat the same to about 80% of its fusing temperature. In the case of the one-quarter inch slug a current of 920 amperes was required for obtaining the desired heat. The slug was then swaged to 36 mil size in the usual manner and drawn to final size, annealing being resorted to at the 14 mil size to remove work hardening effects and recrystallize the wire so that the further drawing could be accomplished.

While vanadium activated molybdenum has been cited by way of specific example it is to be understood that the vanadium may be readily incorporated into other metals, such as tungsten and tantalum in accordance after reduction,

with the well known methods for producing such metals. 7

We havev found that the vanadium activated filaments when enclosed in an evacuated container, have a high emission at moderate temperatures and that such emission may be maintained for a long period oftime without diminution. By way of comparison several tests were made on electron tubes employing thorium activated and vanadium ac-. tivated cathodes. In the case of the thorium activated cathode tubes it was found that the emission was quite constant at plate potentials around volts but that when the voltage was raised to volts there was a decided loss in emission, the emission dropping from approximatel 5O milliamperes to 20 milliamperes in a out 40 minutes. This is explained on the theory that at higher late voltages the electron bombardment o the plate was sufiicient to decompose the oxide film which is always present there, liberating oxygen which oxidized the thorium impairing the emission of the cathode. In the case of vanadium activated filaments under similar treatment, the emission dropped only from 48 milliamperes to 38 milliamperes, or about one-third of the loss of emission which was experienced with the thoriated cathode. This property of vanadium activated filament of resisting due to the property of vanadium oxide of decomposing below the temperature at which the filament is normally operated and is of extreme value since it permits of the use of 7 high plate current, which heretofore in the commercial production of electron devices has caused eat difliculty due to the oxidation of the t orium on initial operation of the device.

The vanadium activated filaments, moreover, were found to regain their emission much more readily than did the thorium activated cathodes due to the property of the vanadium oxide of being decomposed by heat alone. In the case of the vanadium doped filaments it is only necessary when oss of emission by oxidation is dications are that in the commercial production of tubes, after seasoning, the vanadium activated filaments will have a higher emission and longer life than thorium activated cathodes.

While the vanadium is preferably incorporated into the cathode as an alloy or mixture with the carrier metal, if desired it may be coated thereon or employed in various other manners for electron emission purposes. Obviously, various other modifications and changes may be made in the invention described without departing therefrom.

What is claimed is:

1. In an electron discharge device utilizing a thermionically active cathode and at least one cooperating electrode, a cathode comprised of a refractory metal base containing approximately 1.0 per cent of vanadium.

2. In an electron discharge device utiliz ing a thermionically active cathode and at least one cooperating electrode, a cathode comprised of molybdenum containing approximately 1.0 per cent of vanadium.

In testimony whereof, we have hereunto subscribed our names this 5th.day of March, 1926.

JOHN WESLEY MARDEN. HARVEY CLAYTON RENTSCHLER.

this emission is impaired by oxidation to elevate the temperature of the cathode slightly in excess of its normal operating temgerature in order to reduce the vanadium oxide and restore the original high emission of the filament. In the test referred to above, the plate voltageswere raised to 100 volts and urther loss in emission in both filaments experienced. The filaments were then reseasoned to restore the emission and after such treatment the vanadium activated filament gave an emission much in excess of that of the thorium activated filament, indicating clearly that the vanadium is much more resistant to the abuseto which electron devices ma be subjected in use than are thoriated cat odes and while their initial emission may be slightly less than that of thorium, the in-