US1752747A - Electron-discharge device and getter therefor - Google Patents

Description

Patented Apr. 1, 1930 UNITED 's'ra manner enron nnnnnna, on msr omen, NEW JERSEY, ASSIGNOR 'ro wns'rme- HOUSE m1 COMPANY, A conromrron or rmmsnvanm nLEc'rnon-mscnmen neuron am) encr'rnn. *rnnnnroa Re Drawing.

This invention relates to an electron discharge device employing an anode or plate electrode and an electron-emitting cathode, the active electron-emitting ingredient of which has its emitting properties destroyed or impaired by oxidation. I

In electron discharge devices employing cathodes activated by thorium, for instance, difficulty has been experienced heretofore in maintaining the thorium in an active condition due to the'small quantity. of this material contained in the cathode and the ease with which it combines with the gases, particularly oxygen given off by the other electrodes or parts of the device under electron bombardment from the cathode.

It is the common practice in the manufacture of discharge devices employing thoriated cathodes to bake out the envelope at a high temperature during exhaust and to heat the plate and other electrodes to a high temperature as by high frequency induction currents to drive off the gases and vapors occluded therein. The electrodes are also baked out in hydrogen prior to being sealed in the envelope in order to eliminate all traces of oxygen therefrom.

However, a small amount of oxygen or metallic oxides may remain in the electrodes at the completion of the hydrogen treatment and upon exposure of the electrodes to the atmosphere after this treatment and before they can be sealed into the envelope, a thin film of oxide forms on the surface thereof so that in spite of these precautions sufiicient oxygen or oxide is present in the electrodes to impair or destroy the emission from the cathode. This back oxidation of the cathode by oxides liberated from the other electrodes is particularly troublesome in tubes employing filaments having a low power consumption such as dry cell tubes because of the limited size of the filament used in such tubes and the minute quantity of thorium contained therein.

At the time the plates are heated by high frequency induction currents to drive out the occluded gases, it is usual practlce to vaporize a metallic clean-up agent such as misch metal or magnesium from the plate to eliminate the Application filed March 10, 1926. Serial No. 93,792.

residual gases contained in the tube. The temperature to which the electrodes may be heated during this process is about l000 C. since above this temperature trouble is experienced due to warping and distortion of the electrode assembly and'overheating of the glass portions of the device. This temperature, however, is not sufiiciently high todecompose the oxides of the plate material such, for example, as nickel oxide and as a consequence, the electrodes at the completion of the high frequency treatment are still cov- 'ered with a film of metallic oxide. This oxide is readily decomposed by electron bombardment, particularly at high plate voltages and when a discharge is passed through the device, the oxygen may be liberated and migrate to the cathode resulting in oxidizing the thorium film thereon and destroying or impairing the emission from the cathode.

The cathodes are heat treated in hydrogen during manufacture and absorb considerable quantities of this gas. After the electrodes are assembled in the device and the envelope evacuated, the cathode is seasoned by operat- 1ng it under predetermined conditions for a definite length of time to develop its electron emission. During this initial burning of the filament the hydrogen is driven therefrom and on coming in contact with the other electrodes, assists in decomposing the oxide film thereon with the formation of water vapor. This water vapor finds it way back to the cathode where it oxidizes the thorium liberating the hydrogen which repeats the cycle until eventually it becomes ionized and is cleaned up by the getter material employed. One of the objects of this invention is to prevent back oxidation of the cathode by oxygen liberatedfrom the other electrodes.

Another object is to provide agetter material for an electron discharge device which will decompose the oxide film on the electrodes and prevent oxidation of the electron emitting material.

Another object is to provide a getter material for use in electron discharge devices employing cathodes, the emission of which is impaired or destroyed by oxygen which will-reduce or prevent loss of emission from such oxidation.

Other objects and advantages will hereinafter appear.

It has been proposed heretofore, to employ phosphorus getters in electron discharge devices but such use has been limited to red phosphorus for the purpose of cleaning up residual gases and vapors. Briefly, the method emplo ed consists in' painting the red phosphorus dlrectly onto the plate or grid of the device and vaporizing the same by heating the anode as by high frequency induction currents. The results obtained when using red phosphorus in this manner are erratic and the tubes produced have non-uniform emissivity. Red phosphorus is produced chiefl for the manufacture of matches and firewor s and is diflicult to obtain in a pure condition. It usually contains from 5 to 10% of impurities, such as hydrocarbons, silica, iron oxide, etc., which are liberated when the phosphorus is vaporized and exert a decided detrimental efiect on the electron emission from the filament.

Red phosphorus, moreover, even if obtained in a pure condition, is very unstable in the atmosphere, rapidly oxidizing into phosphorus pentoxide which in turn readily absorbs moisture forming phosphoric acid. It has been found impossible in the commercial production of electron discharge devices, such as radio tubes, to prevent the formation of phosphoric acid to an appreciable extent before the phosphorus can be introduced into the device. The phosphoric acid on being heated to liberate the phosphorus, also liberates water vapor in .the envelope, causing oxidation of the active cathode material, resulting in loss of emission from the cathode and thus defeating the purpose for which the phosphorus was employed.

In accordance with the present invention the difliculties incident to the use of red phosphorus for clean-up purposes have been overcome by employing the phosphorus in such manner that the impurities therein are cleaned up before they have an opportunity of impairing the emission from the cathode or in employing the phosphorus in such form that it is free from impurities. I have discovered that the oxide film which is always resent on the surface of cold electrodes may be decomposed by the action of yellow phosphorus vapor thereon and that when this In is destroyed prior to the operation of the cathode, that high electron emission is obtained for a long commercial life. In carrying out my invention I preferably introduce the phosphorus into the envelope in the form of a compound stable in t 1e atmosphere but which is readily reducible within the envelope to liberate pure yellow phosphorus. A compound may be used which is decomposible by heat alone or if desired, a reducing agent such as aluminum or magnesium may be employed to effect the reduction.

In accordance with one embodiment of my invention commercial red phosphorus may be introduced into the device intimately mixed with a reducing agent in such manner that when the phosphorus is vaporized, the lmpurities contained therein are cleaned up by the reducing agent, thus precluding the possibility of their impairing the emission from the filament.

One compound of phosphorus which I have found very suitable for producing pure yellow phosphorus within the envelope, is calcium metaphosphate mixed with silica and a reducing agent, such as powdered aluminum or magnesium or both. Silica is added to the getter material to prevent the formation of orthophosphate and the liberation of free calcium or sodium in the en.- velope which afiect the grid bias by deposition thereon. The reaction takes place according to the following equation:

Instead of calcium metaphosphate, sodium metaphosphate or any other suitable meta phosphate may be used. The constituents of the getter should be mixed in their molecular proportions with approximately 50% excess of the reducing agent. The quantity of the getter applied to the plate should be restricted to as small an amount as is compatible with thorough reduction of the oxide film on the plate. If too much phosphorus is employed, the tube will be gassy since yellow phosphorus has a very appreciable vapor pressure at operating temperature. If too much phosphorus vapor is present when the discharge is passed through the device, positlve bombardment of the filament will occur resulting in temporary loss in emission. In such case, the gas condition can be cured and the electron emission restored by seasoning the device for a suflicient length of time. This is due to the conversion of yellow phosphorus to red phosphorus when the vapor of the same is ionized and diffusion of a new supply of thorium to the surface.

In order to control uniformly the quantity of getter applied to the anode, the materials in finely divided form may be mixed with a suitable binder, such as a solution of nitrocellulose in amyl acetate and a definite quantity thereof painted or otherwise applied to the plate. One method which I have found effective for determining the amount of getter employed is by forming an eyelet of wire which when dipped in the solution will retain a definite quantity thereof which may be deposited on the plate.

Another suitable phosphorus compound which I have employed for producin pure yellow phosphorus in the envelope, is orylphosphate. This compound is preferable for some purposes over the metaphosphates since boron even if set free by the reduction, does not deleteriously effect the operation of the device as does sodium or calcium and therefore does not require the use of compounds such as silica for preventing the liberation of the boron. Boron is a solid element which does not change either the grid-bias or produce a gassy condition within the tube. Preferably, aluminum or magnesium reducing material is used with the borylphosphate to effect a liberation of the phosphorus. Aluminum is preferred to magnesium since the reduction is not as violent and can be more easily regulated. A getter which has proven very satisfactory utilizing this compound may be made up as follows: i

Grams PBO4 10 Powdered Al 7 Powdered Mg 2 These materials may be mixed with suflicient binder such as a solution of nitrocellulose in amyl acetate to give them the proper consistency for applying to the plates.

A still further compound which I have also employed with success is copperphosphide. Copper phosphide dissociates in vacuumon being heated to 300 0., the phosphorus vaporizing entirely at 700. This compound decomposes by heat alone, and therefore need not be mixed with any reducing agent. It is preferably applied to the plate in powdered condition with a binder, since the copper hosphide is extremely brittle and cannot lie welded to the plates in solid form. In addition to the copper phosphide, another metallic clean-up agent such as magnesium should be employed to absorb the residual gases in the tubes.

In the manufacture of low power electron devices such as the so-called UV-199 tube, I have found that 5 milligrams of magnesium and 2% milligrams of copper phosphide are sufficient to produce the desired effect.

The compound which I prefer to use is phosphorus dinitride. This compound decomposes into its components in vacuum at about 800 C. At the instant the decomposition takes place, suificient phosphorus is liberated to clean-up the oxide film from the plate and at the same time the nitrogen which is liberated sweeps out all remnants of gases which might have been left in the tube, such as water vapor, hydrocarbon, residual air, etc., and replaces them by very pure nitrogen which, not being detrimental to the thorium metal, can be cleaned up electrically, that is, by ionization and absorption on seasoning by metallic clean-up agents, such as misch metal. The magnesium may be mixed in finely divided form with the phosphorus dinitride or applied to the plate separately. This compound is particularly good since it can be prepared with a high degree of purity and it is absolutely indifferent to most reagents at room temperature.

While a number of compounds have been specified, it is to be understood that others may be employed within the scope of this invention. In choosing a suitable compound the following properties of the compound should be given consideration. It should be stable in air and obtainable in a pure condition; it should not contain any ingredients, such as fluorides which deleteriously effect the emission of the filament or the operation of the device by producing a gassy condition therein or by creating an unfavorable grid bias; it should develop sufiicient phosphorus to reduce the oxide on the plate and the generation of the phosphorus should be suficiently slow to prevent sputtering or explosive reaction of the getter.

I have further discovered that red phosphorus may be employed when intimately mixed with powdered magnesium or aluminum and applied to the plate so as to be vaporized therefrom. The red phosphorus on eing heated decomposes into yellow phosphorus and the magnesium or alummum which is in intimate association therewith cleans up the liberated impurities, such as hydrocarbons, water vapor, etc., before they have an opportunity of escaping from the getter material. A suitable composition for such a getter is as follows:

Red phosphorus 15 grams Powdered Mg grams Powdered Al 15 grams Binder 110 c. c.

The binder used may be a nitrocellulose solution in amyl acetate. The aluminum was employed in the mixture in order to obtain a better consistency for paintingthe getter on the plate.

It will be noted in all of the compounds specified that upon heating the anode preferably while still on the pumps, pure yellow phosphorous is evolved free from impurities which might deleteriously effect the life or emission of the filament. This phosphorus readily decomposes the oxide film on the anode or grid cleaning up the oxygen content thereof. This effectively prevents the liber ation of oxygen from the plate during subsequent operation of the device and precludes back oxidation of the cathode. In addition to the phosphorus, a metallic clean-up agent, such as magnesium or aluminum or both, is vaporized and cleans up the residual gases and vapors.

It is obvious, of course, that man changes may be made in the invention described herein without departing therefrom.

What is claimed is:

1. A getter for an evacuated container comprising a mixture of a phosphorus compound and a reducing agent which vaporizes' upon heating the mixture to clear n5) residual gases.

2. A getter for an evacuate container comprising a mixture of a phosphorus compound and magnesium.

3. A getter for an evacuated container comprising a mixture of a phosphorus compound, aluminum and magnesium.

4. A getter for an evacuated container comprising a mixture of a phosphorus compound, a finely divided metallic clean-up material and a binder.

5, The method of introducing yellow phosphorus in an evacuated container which comprises introducing into said container prior to evacuation, a mixture of a stable phos- P l1c clean-up agent, which reduces with respect to the phosphorus compound and reacts with respect to the residual gases within said container, evacuating said container and thereafter heating the phosphorus compound and admixed clean-up agent to efi'ect reduction of the phosphorus compound, substantial clean-up of residual gases contained within the evacuated container and a liberation of yellow phosphorus.

6. The method of introducing phosphorus in an evacuated container which comprises introducing into said container prior to evacuating the same, a mixture of phosphorus dinitride and a finely divided metallic clean-up agent which reduces with respect to the phosphorus dinitride and reacts with respect to residual gases within said evacuated container, evacuating the container, and thereafter heating the phosphorus compound and admixed clean-up agent to effect reduction of the phosphorus dinitride, substantial cleanup of residual gases contained within the I evacuated containerand a liberation of yellow phosphorus.

7. The combination in an electron discharge device of an evacuated envelope, a thermionic cathode, the activating constituent of which is destroyed by oxygen, at least one cooperating anode containing electronically dissociable oxygen compounds, and a getter material substantially comprised of a phosphorus compound admixed with a metallic clean-up agent which when heated is reducing with respect to the phosphorus compound to liberate substantially pure phos horus.

In testimony whereo scribed my name this 8th day of March, 1926'.

ERNEST ANTON LEDERER.

horus compound and a finely divided metal- ,I have hereunto. sub- Q