US2206509A - Radio tube manufacture - Google Patents

Description

y 2, 1940- a. A LEDERER I 7 6,509

RADIO TUBE MANUFACTURE.

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July 2, 1940. E. A. LEDERER 2,206,509

/ RADIO TUBE MANUFACTURE Filed May 27, 1939 I 2 Sheets-Sheet 2 INVENTOR. ERNEST A. LEDERER BY WQ'M/ A TTORNE Y.

Patented July 2, 1940 PATENT OFFICE RADIO TUBE MANUFACTURE Ernest A. Lederer, Essex Fells, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application May 27, 1939, Serial No. 276,127

8 Claims.

My invention relates to electron discharge devices, particularly to the manufacture of cathodes for such devices.

The usual cathode is made by coating a cathode wire or metal cylinder with a stable compound of active metal, such as barium and strontium carbonates, and by mounting the coated cathode with its cooperating electrodes in an envelope where the compound is decomposed by heat and if) gaseous products of decomposition are pumped out. The remaining oxides of the active metals must then be heated at rather high temperatures to electronically activate the cathode. Although long experimentation and factory practice has established optimum exhausting schedules, temperatures and compositions, many cathodes fail to activate, causing considerable waste of labor and tube materials.

An object of my invention is improved methods of making cathodes and of insuring good electron emission from the cathodes in the electron discharge devices in which they are mounted.

A more specific object of my invention is to make cathodes and to completely activate them before they are placed with other electrodes in an electron discharge device.

According to my invention a cathode coated, for example, with barium strontium carbonates is placed in a chamber which is evacuated to a vacuum good enough to permit activation of oxide coated cathodes. The cathode is then heated to decompose the compound, and activated until it gives good electronic emission. The degree of activation may then be conveniently determined by applying a high voltage between the cathode and an adjacent electrode in circuit with a meter to indicate the space current flowing through the cathode coating. If the cathode fails to properly activate and show good electron emission, it may be discarded and the cost saved of tube parts that would, according to the old method of manufacture, have been assembled with the defective cathode and placed in a tube envelope. A number of cathodes may be simultaneously processed in this way, a large batch ofcoated cathodes being heated in an exhausted chamber, with circuit connections for individually testing each cathode for emissivity after a predetermined heating and exhausting schedule. The cathodes which fail to show the desired electron emissivity may thus be easily detected and separated from those cathodes which properly activate.

After the cathode is well activated, and while it is still in the well evacuated chamber, and in order to preserve the sensitive coatings of the ac- (Cl. 2502'l.5)

tivated cathode, I then according to my invention, overcoat each cathode with an air impervious overcoat of metal, the overcoating being conveniently applied by vapor condensation or by cathodic sputtering from an adjacent filament I or source from which the metal for the overcoating may be easily vaporized, as suggested in the United States patent to Hunter 1,897,480 of February 14, 1933. Magnesium has been found to be one of the more desirable metals for over- [1.) coating activated cathodes.

After the overcoat has been applied the chamber is opened and the overcoated cathodes removed, those cathodes which by test prove to have low emissivity being discarded and the re- 1 mainder placed in the hands of operators who mount them in the conventional way in electrode assemblies, which are mounted in envelopes, ex-' hausted and sealed. After evacuation of the envelope of the electron discharge device only enough heat need be applied to the cathode to evaporate the overcoating and to expose the activated oxides. The metal selected for overcoating the activated surface of the cathode should not harm the activated cathode and should have a 35 higher vapor pressure than the activated materials of the cathode so that the overcoat may be removed by heat without destroying the activation. Cathodes processed according to my invention are gas free when placed in the tubes in go which they are to be used and do not evolve the large amounts of gas characteristic of the cathodes placed in tubes with conventional coating compounds.

The characteristic features of my invention are u defined in the accompanying claims, and apparatus for carrying out my improved process is described in the following specification and shown in the accompanying drawings in which Figures 1 and 2 show diagrammatically apparatus for actio vating and testing indirectly heated cathodes according to my invention, Figure 3 shows an electron discharge device including a cathode prepared according to my invention, and Figure 4 shows a tube for activating and overcoating a 5 cathode.

The apparatus shown here is for illustrative purposes only and it will be obvious to those skilled in the art that while a cathode sleeve of the indirectly heated type is described, filamen- 50 tary cathodes may be made according to my invention. A cathode sleeve I is sprayed with a stable compound of active metals and is mounted in a chamber or oven 2 that may be evacuated, and is supported in an upright position between 5 two filaments I which are composed of or coated with the metal to be used for overcoating the sleeve. 'Ihe cathode is heated by its indirect heater connected to power source 4 to slowly I decompose the coating on the sleeve as the air is pumped from the oven. After decomposition of the coating the cathode temperature is raised to reduce the oxides and liberate some ofthe active metals in the coating, the heating being I. continued until the coating is well activated.

The temperature of the cathode is then reduced to its rated operating temperature and the cathode is tested for electron emissivity by connecting it through a high voltage source 5 to the anode ll electrode 6 opposite the sleeve. The emissivity of the cathode is measured in the current meter 1 in circuit with the cathode and its degree of activation is readily determined.

If the materials of the sleeve and its oxide coating properly respond to the heat treatment to produce a well activated cathode, an air impervious protective overcoat of metal may then be applied to the activated surface of the sleeve by vaporizing. metal from the filaments 3 arranged parallel to opposite sides of the sleeve. Current passed through the filament from power source 8 may raise the metal of the filament to vaporization temperature and evaporation of the filament continued until the desired thickness of Q overcoating on the sleeve is obtained. The anode electrodes 6 in the apparatus illustrated may conveniently be curved metal sheets placed around the filaments to direct the evaporated overcoating metal toward the cathode sleeves. My novel method of making cathodes may be easily adapted for factory use with an oven in which a batch of cathodes may be treated, activated, tested and overcoated. As diagrammatically represented in Figure 2 the cathodes may be arranged in a row, each cathode being positioned between two filaments 3 from which the overcoating metal may be vaporized. After the oven is exhausted and the coating on the cathode reduced by heat from power source 4, the elec- Q tron emissivity of each cathode is individually tested by connecting it in circuit with the high voltage source 5 and the current meter I connected through push button switches to the anode electrodes 8. Those cathodes which fail to activate and show the desired electron emissivity may thus be easily detected and separated from those cathodes which properly activate. Current is then passed though filaments 3 from power source I to evaporate metal which when I condensed upon the activated surfaces of the cathodes renders the cathodes impervious to air so that they may be removed from the oven and handled without deterioration of the activated coatings. Each cathode proven by test to have Q good electron emissivity may then be assembled in the usual way in an electrode assembly for a radio tube as shown in Figure 3, and sealed into the exhausted envelope of the tube. The cathode may then be heated to evaporate the pro- Q tective overcoating and to expose the activated surface of the cathode. By discarding those cathodes which fail to properly activate, considerable labor and materials in electrode assemblies are saved. I. The active metal of the cathode may conveniently be applied to the cathode core by condensing vaporized active metal on the core. As shown in Figure 4, for example, the cathode l is mounted inside a coiled filament from which the active metal, such as barium, may be vaporized by heat obtained from the A. C. source ll. After the desired thi, ness of active metal is obtained, and the core is sufliciently electronically activated, as by heating, it is overcoated by a protective film vaporized from filaments 3, connected to power source ll. Battery I2 may be employed to heat the cathode l and apply the testing voltage between the cathode and coil I.

Various combinations of core metals, active coatings and overcoating materials may be used 10 within the scope of my invention. Good results have been obtained by coating a tungsten cathode with pure barium condensed on the tungsten from an adjacent filament of tantal overcoated with barium oxide or barium dioxide. The layer of pure barium on the tungsten is then easily overcoated with an air impervious layer of zinc-cadmium alloy, good results having been obtained with an alloy of equal parts by weight of zinc and cadmium, the alloy preferably being 10 carried on a core of nickel. This cathode when transferred to its final envelope can be heated to a temperature of about 300 to remove the overcoat of zinc-cadmium and expose the active barium surface of the cathode. The core metal may, in addition to the tungsten, be tantalum, molybdenum, nickel and nickel alloys. The active surface of the cathode may be obtained in the conventional manner by applying a bariumstrontium oxide coating to the core, activating $0 and overcoating with the zinc-cadmium alloy. Alternatively the active surface may comprise magnesium oxide impregnated with barium derived preferably from a barium azlde source, or the active coating may be convenientlyobtained from barium-berylliate compound painted or sprayed on the cathode core and heated to partially decompose the compound and liberate the barium. As an alternative for the zinc-cadmium overcoat pure magnesium may be used which is particularly good for barium applied to the core by the vapor process. The magnesium however must be vaporized from the activated surface at a temperature of about 500 C. In case an oxide coating is chosen it is first heated at a tempera- 48 ture to decompose the compound and is then given a hot shot of about 1200 C. for one minute, after which it must be aged for several minutes to fully electronically activate the oxides. These cathodes are then operated at a temperature of about 800 C. Activated surfaces of the cathode when applied in the oxide form are usually rough, requiring a thicker overcoat to protect the active metal. It is preferable, therefore, to smooth the oxide layer by drawing through a die or rolling before it is placed in the oven whereit is activated and overcoated. Oxide coatings applied cataphoretically are in general smoother than oxides applied by spraying or dipping. Overcoats for oxide coated cathodes should for good air imperviousness be of the order of .001" in thickness whereas active surfaces applied by the vapor process may be overcoated with metal layers as thin as .00025". 6 Either the magnesium or zinc-cadmium overcoat vaporizes and condenses on the coolest part of the bulb and it is my belief that neither overcoating metal enters into any chemical reaction with the core metal or with the active metal of the cathode surface.

Cathodes prepared according to my invention are insured of good electron emission in the electron discharge devices in which they are mounted and the waste of labor and materials by u the assembly of defective cathodes in such devices is obviated.

What I claim as new is:

1. The process of manufacturing an electron discharge device containing a cathode with a refractory core comprising treating said core in vacuum to obtain electron emission from the surface of said core, covering said surface in said vacuum with an air impervious layer of metal, removing the cathode from said vacuum and mounting the cathode with cooperating electrodes of said cathode in a vacuum, evaporating a metal.

neutral to the activated surface of the cathode and condensing said metal on said surface in the vacuum, then mounting the cathode with cooperating electrodes in an envelope, exhausting the envelope and removing said metal to expose said activated surface.

4. The process of manufacturing cathodes for electron discharge devices comprising coating the cathode core in a vacuum with a layer of electronically active metal, overcoating said active metal layer with an air impervious overcoat, transferring the'overcoated activated cathode to an envelope, exhausting said envelope and vaporizing the overcoat to expose said active metal.

5. The process of manufacturing cathodes for electron discharge devices comprising mounting the core of said cathode in an evacuated chamber, vaporizing in said chamber an electronically active metal and condensing the metal on said core, then vaporizing a second metal in said chamber with a vaporization temperature lower than the vaporization temperature-of said active metal and condensing said second metal on the layer of active metal, transferring the overcoated cathode to an envelope in which it is to be used,

exhausting the envelope and heating the cathode to a temperature between the vaporization temperatures of the two mentioned metals.

6. The process of manufacturing a cathode for an electron discharge device comprising mounting in an exhausted chamber a cathode core of a refractory metal selected from the group consisting of tungsten, tantalum, molybdenum and nickel, vaporizing barium in said chamber and condensing the barium on said core, then vaporizing a zinc-cadmium alloy in the chamber and condensing the alloy on the barium coated core to form a protective overcoat, transferring the overcoated core to an envelope, exhausting the envelope and heating the core to vaporize the alloy of said overcoat.

7. The process of manufacturing an electron discharge device containing an electron emisslve cathode with a refractory core coated with an active metal comprising coating the core with a stable compound of said metal, heating in a vacuum to decompose the compound and electronically activate the coated core, covering the activated cathode in vacuum with an air impervious layer of metal, removing the cathode from said vacuum and mounting the cathode with cooperating electrodes in an envelope, exhausting said envelope and heating the cathode to vaporize the metal covering and expose said active metal.

8. The process of manufacturing an electron discharge device containing an electron emissive cathode with a refractory metal core coated with an active metal comprising coating the core with a. stable oxygen compound of said metal, heating the coated core in a vacuum .to decompose the compound, reducesome of-the compound to the pure metal and electronically activate the coated .core, vaporizing a metal chemically inert to said active metal in said chamber, and condensing the second mentioned metal on the coated'core, the depth of the condensed layer of the second mentioned metal being thick enough to be impervious to air, removing the cathode from said vacuum and mounting the cathode with cooperating electrodes in an envelope, exhausting said envelope and heating the cathode to vaporize the second mentioned metal and expose said active metal.

ERNEST A. LEDERER.

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