US1663561A - Electron-discharge device - Google Patents

Description

Patented Mar. 27, 1928.

UNITED STATES PATENT OFFICE.

GEORGE DEAN O'NEILL, OF BLOOMFIELD, NEW JERSEY, ASSIGNOR TO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

ELECTRON-DISCHARGE DEVICE.

No Drawing.

This invention relates to an electron discharge device employing an anode or plate electrode and more particularly to the removal of deleterious gases and other impurities from such a device.

The invention is especially concerned with electron discharge devices designed for operation with high plate voltages in which the plate electrode. attains a relatively high temperature during operation and in which the electron-emitting cathode has its emission readily impaired by gases either residual in the device after exhaustion or liberated from the electrodes. The invention is not limited. however, to such devices, but may be employed in creating high vacuums in evacuated containers of whatever type or in the purification of the gaseous atmosphere in devices employing monatomic gases.

In discharge devices employing cathodes containing thorium or thorium compounds. for instance, difficulty has been experienced in maintaining the thorium in an active condition. due to the minute quantity of this material contained in the cathode and the facility with which it combines with the gases. particularly oxygen. given oil from the other electrodes or metal parts of the device under electron bombardment from the cathode.

It is common practice in the manufacture of discharge devices employing thoriated cathodes, to .bake the envelope to high temperature during exhaust and to heat the plate and other electrodes to a high temperature as by a high frequency induction current or electron bombardment to drive oif the gases and vapors occluded therein. It is also usual to bake the electrodes in hydrogen prior to sealing them in the envelope in order to eliminate all traces of oxygen therefrom.

However, in spite of the precautions. a. small amount of oxygen or metallic oxides may be retained in the electrodes at the completion of the hydrogen treatment and upon exposure of the electrodes tothe atmosphere, after this treatment and before the electrodes can be sealed into the envelope and the same exhausted, additional gas may be absorbed Application filed March 3, 1927. Serial No. 172,569.

by the electrode and a thin film of oxide formed on the surface thereof, sufficient to impair or destroy the electron emission from the cathode.

In the construction of low power tubes, that is, tubes in which the plate electrode does not become appreciably heated during operation, it has been proposed to eliminate the deleterious eflect of the oxide and oxygen contained in or on the electrode by forming the electrode of an alloy or mixture of nickel and a readily vaporizable clean-up metal, such as zinc. as described and claimed in copending application of Lederer et al., Serial No. 85,016, filed January 30, 1926, and entitled Electrodes for electron discharge devices. However. this electrode, because of its low melting point, is not capable of being used in power tubes. the plates of which become highly heated during operation.

Among the residual gases most difiicult to eliminate from the evacuated container after the completion of the pumping process, are carbon dioxide, carbon monoxide, hydrogen and water vapor.

While certain of these gases are cleaned up to some extent by the usual getters heretofore employed, such as magnesium. they are not entirely removed thereby and (lith culty has been experienced. due to their presence in the tube, causing the same to have a short life and the cathode to exhibit a rapidly falling emission. This short life is apparently due to the positive ion bombardment of the filament by the ionized residual gas. causing the thorium to be knocked bodily from the surface thereof.

One of the objects of the present invention is to provide a plate electrode for an electron discharge device operable at high voltage which will be self cleaning and which will not liberate oxygen or other gases capable of destroying the emission from the cathode.

Another object is to provide a method of producing a refractory metalelectrode which will contain or have associated therewith, an ingredient capable of precluding the libera-v tion of deleterious gases therefrom.

A further object is to provide a clean-up agent for residual gases in an electron dis charge device which becomes highly activated at relatively high temperatures.

A still further object is to provide a method and means for introducing aluminum into an evacuated device as a clean-up material in a convenient manner and by which it may be rendered highly active for the elimination of undesired gases.

Other objects and advantages will appear as the description proceeds.

In accordance with the present invention I employ as the plate electrode of a vacuum discharge device a refractory metal, such as molybdenum. capable of withstanding the high temperatures to which the electrode may be subjected during operation. In this electrode I incorporate as an alloy therewith, a small percentage of a reducing metal such as aluminum or chromium. The function of the aluminum or chromium is threefold; first to protect the electrode from gas contamination prior to mounting and exhaustion of the device: secondly, to act as a cleansing agent to purify the electrode during or after exhaust and thirdly, to clean up the residual gases in the envelope and ob tain a high vacuum therein. If desired, the clean-up metal may be incorporated directly into the molybdenum before or during fabrication thereof into sheet form. However, I prefer to produce the alloy of aluminum and molybdenum, for instance, after the molybdenum has been fabricated into sheet form and either before or after the molybdenum sheet has been formed into the electrode.

A method which I have found satisfactory for effecting this alloy is as follows: Previously, formed molybdenum plates were packed in a mixture of powdered aluminum and powdered oxide in the proportions of 40% of the former and 60% of the latter in a refractory metal box and heated to about 1000 C. in an electric or gas furnace in a hydrogen atmosphere for about ten minutes. The percentages of aluminum and aluminum oxide, of course, may be varied within a wide ran the essential feature being that sufficient a uminum oxide or other refractory metal oxide, such as magnesium oxide or chromium oxide be present in sufficient quantity to prevent the aluminum from sintering together during the, calorizing treatment. After this treatment, the plates are removed from the powdered material and scrubbed lightly in water to remove the loose material adhering to the surface. The plates may then be rinsed in dilute hydrochloric acid (10% solution) for a brief period of from 10 to 20 seconds to dissolve off the excess aluminum and aluminum oxide from the surface of the plate. Care should be taken, however, not to subject the lates to the action of the hydrochloric acid for a suflicient period to dissolve away a substantial portion of the plate material. The plate should then be Washed in water, then in alcohol and dried. They may then be mounted in the usual manner without further treatment and the mount incorporated into an envelope and the same exhausted. During the latter part of the exhaust treatment the plate is heated, as by high frequency induction current, to a temperature sufliciently high to release a portion of the aluminum to clean up residual gases and to degasify the electrode. \Vhilc a good clean-up action may be obtained by heating the plates to a temperature of from 1200 to 1300 C., I prefer to heat the plate above l750 in order to volatilize any aluminum oxide which may be formed thereon.

During the baking of the molybdenum plates and the aluminum-aluminum oxide mixture, a portion of the aluminum is absorbed into the molybdenum and becomes alloyed therewith. The electrodes, during this process, moreover, are thoroughly degasified and left in a pure condition. The aluminum on the surface of the plate and the aluminum-molybdenum alloy adjacent, the surface, serves to protect the plate from further gas contamination prior to sealing-in and exhausting of the electrode and the heat treatment to which the plate is subjected after sealing-in serves to insure the complete removal of all gases therefrom. This clean-up action is facilitated by the diffusion of the aluminum, which has been absorbed into the electrode, to the surface thereof, during which it comes in contact with and cleans up all traces of absorbed and occluded gases contained in the electrode. Due to its high aflinity for oxygen it serves at the same time to decompose any metallic oxides which may be present.

I have found that when aluminum is alloyed with a refractory metal, such as molybdenum, that it is much more effective as a clean-up of residual gases than is aluminum when employed in the unalloyed state. This is due to the much higher temperature to which the aluminum-molybdenum alloy may be heated before vaporization of the aluminum therefrom than the unalloyed aluminum may be heated before it vaporizes. Due to the high temperature at which the aluminum is liberated, from the alloy, it serves to reduce such gases as carbon dioxide, carbon monoxide, water vapor, etc., and thus readily cleans up the same. It also readily combines with hydrogen, oxygen, nitrogen, or other gases present.

It is to be understood, of course, that the invention is not limited to the particular alloy of molybdenum and aluminum but other refractory metals, such as tun ten, tantalum, etc., may be similarly a oyed either with aluminum or with other clean-up a ents, such as chromium. Moreover, the a loy may be produced in various manners and I do not desire to be restricted to the exact details set forth in the above description.

\Vhat is claimed is:

1. A clean-up material for common gases comprising a refractory metal having incorporatcd therein, a quantity of aluminum.

2. Aclean-up material for common gases consisting of an alloy of molybdenum and aluminum.

3. An electrode for a vacuum discharge device composed of a refractory metal having a clean-up metal alloyed therewith.

4. An electrode for an electron discharge device of the type employing a cathode, the thermionic activity of which is readily destroyed by oxygen consisting of a refractory metal having alloyed therewith, a reducing agent which forms solid compounds with the oxygen and oxides contained in the electrode.

5. An electrode for a vacuum electric device composed of an alloy of a refractory metal and aluminum.

6. An electrode for a vacuum electric device composed of an alloy of molybdenum and a clean-up metal.

7. An electrode for an electron discharge device composed of molybdenum having the surface portion thereof alloyed with aluminum.

S. An electrode for an electron discharge device composed of a refractory metal coated with aluminum.

9. An electron discharge device having a cathode, the emission of which is readily impaired by oxidation or positive ion bombardment and a cooperating electrode composed of a refractory metal having a cleanup metal incorporated therein.

In testimony whereof, I have hereunto subscribed my name this 1st day of March, 1927.

GEORGE DEAN ONEILL.

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