US1760454A - Manufacture of electron-emitting devices and the like - Google Patents

Description

May 27, 1930.- c, T. ULREY 1,760,454

MANUFAG TURE of ELECTRON EMITTING DEVICES AND vTHE LIKE Filed July 24, 1922 iNvENToR CLAYTON 7- UL/ EY ATTORNEY Patented May 27, I930 UNITED STATES. PATENT OFFICE CLAYTON TRI'DLE ULEEY, 0E EAsa' ORANGE, NEw JERSEY, AssI GNoE gro WESTING- HOUSE LAMP oomrANY, A CORPORATION OF PENNSYLVANIA MANUFACTURE OF ELECTRON-EMITTING DEVICES AND THE LIKE Application-filed July 24,

This invention relates to the manufacture of electron-emitting devices and the like and more particularly to the use of thorium 'to sensitize and activate electron-emitting cathodes and as a clean-up agent.

An object of my invention is the prepara tion of a refractory filament coated with thorium by vaporization from a mass of thorium.

Another object of my invention is the manufacture of radio bulbs, X-ray tubes or the like, each having a late or target consisting of thorium, in who e or in part, whereby the electron-emitting element or filament may be sensitized and activated therefrom initially and whereby such activation may be repeated as often as may be necessary or desirable.

A further object of my invention is the production of electron-emitting devices, each with a late therein composed, at least partly, o? thorium whereby the same may be utilized for coating the electron-emitting filament with thorium by vaporization or sputtering therefrom, under the influence of heat or electronic bombardment, to increase its electron-emissivity and, at the same time, acting as a clean-up agent for improving the vacuum in the bulb.-

A still further object of my invention is the manufacture of radio bulbs, X-ray tubes and other electron-emitting devices, in which the hot-cathode is composed principally of a filament of tungsten or the like and is adapted to operate at a much lower temperature than pure uncoated tungsten and stlll develop the requisite electron-emission, be-

' cause of the application thereon, by distillation from a component part of the device, of a coating of material of large electronemissivity. A

Other objects and advantages will be apparent upon reading the following description.

In electron-emitting devices such as radio bulbs, X-ray tubes and the-like, it is des rable to have the hot-cathode or electron-em tting element thereof composed of a mater al having a high thermionic electronic-emissivity. Experiments have shown that, of

1922. Serial No. 577,021.

materials suitable for this purpose, thorium has a high value but it also has "a lower melting point than tungsten and molybdenum andthere is considerable dificulty in producing it in filamentary form. It oxidizes readily when so produced and hence is not well adapted for electron-emitting material in the pure, metallic form ifnot combined or consolidated with other materials. Because of these properties of thorium, various attempts have been made to use it consolidated with, or attached to, other more refractory metals, such as tungsten.

In Patent #1244216, October 23, 1917, Langmuir describes a method of heat-treat- .ing thoriated tungsten filament to develop a film of thorium thereon. The process described in the patent is a delicate one and, apparently, has to be closely followed to obtain the desired result for it is stated on page 2, lines 105 to 109, of said patent that, if a trace of oxygen is admitted to the bulb, the electron-emission immediately falls as low as, or even lower than, the low value observed when a cathode of pure tungsten is slightly oxidized.

In Patent #1244217, October 23, 1917,

Langmuir describes a method of developing 1 a surface coating of thorium. on a thoriated tungsten filament by reduction of some of the thoria in the filament, which is more or less difficult topractice, and the results obtained are not as good as that from pure thorium.

According to my invention, I have avoided the defects of the prior art by mounting an electron-emitting cathode adjacent to a plate or target consistin atleast in part, of thorium. The oath e, preferably in filamentary form, may comprise a refractory metal, such as tungsten, and the same is sensitized or activated, that is, the electron-emissivity thereof is increased to that of thorium by depositing thereon a film or coatingof thorium from the plate or target, when used for an X-ray tube, by heating said plate or the like to incandescence to vaporize or sputter therefrom the requisite amount of thorium to deposit the .same on the filamentary cathode.

The process is preferably carried out by energizing the cathode to an electron-emit ting temperature from a suitable source of electricity and then impressing a hi h potential, between said cat ode and t e plate, thereby heating the plate to in'candescence by electron-bombardment. Such treatment causes partial vaporization of the thorium of the plate which will exert a clean-up action on residual gases, if any, in the bulb, and a fihn of metallic thorium will condense and will be deposited on the surface of the heated filamentary cathode, under the directing influence of the potential between the cathode and the anode or plate.

My invention will better-be understood by referring-to the aecompanyin drawing, in which, an electron tube or ra io device, as one embodiment of my invention, is illustrated in side elevation, partly in section, and diagramatically shown connected for sensitization or activation thereof.

The radio device 1 may comprise an evacuated sealed envelope or bulb 2, a filamentary cathode or electron-emitting element 3, a grid 4: and an anode or plate 5. Extending throughthe press 8 are leading-in wires 6 and 7 for the energization of the filament 3. Leading-in wires 9 and 11 also extend through the press 8 to make electrical connection to the grid 4e and the plate 5, respectively.

The plate 5 may be composed entirely of metallic thorium or may comprise a plate of tungsten or molybdenum painted with powdered thorium metal in a suitable binder, but it preferably consists merely ofa plate of tungsten or molybdenum with a strip or clip of thorium metal attached thereto, as represented at 12.

The thorium may be prepared in powdered form by the calcium-reduction process or by the process described and claimed in the application of John W. Marden, Serial Number 432,324, filed December 21, 1920, Preparation of rare metals, and assigned to the Westinghouse Lamp Company; If a plate of clip of thorium is to be used, the metal may be sintered and fused into a coherent mass by the method described and claimed in the application of John W. Marden et 9.1., Serial Number 432,325, filed December 21, 1920, Sintering and fusing of metal powders into coherent metals, and assigned to the Westinghouse Lamp Company. The furnace used for such sintering and fusing may be that described and claimed in the application of Harvey C. Rentschler, Serial Number 430,118, filed December 13, 1920, Furnaces, and assigned to the Westinghouse Lamp Company.

The grid 4 may be a coil of tungsten or molybdenum wire. The filament 3 may be composed of refractory metal such as tungsten, molybdenum or the like.

The bulb 2 may be exhausted in the usual current through the same.

manner after which the filament 3 is activated for the purpose of increasing its electron-emissivity to that of pure thorium by the following method.- The filament 3 is energized from a battery 13 or other suitable source of electricity by closing the switch 14, whereby it becomes heated to an electron-emitting temperature. A high potential may then be established between the filament 3 and the plate 5 by closing the switch 15 to ener ize the primary 16 of the adjustable transfbrmer 17 from a suitable source of be heated inductively by surrounding the de-' vice by a coil and passing high-frequency This will make the plate 5 a secondary of the coil and heat the plate by means of the current induced therein.

The degree of heating of the plate may be regulated by adjusting the impressed potential from the transformer 17 or by regulating the temperature of the energized filament 3 by adjusting the rheostat 21 in circuit therewith. The activation of the filament 3 by vaporization or sputtering from the plate 5 or the strip of thorium 12 attached thereto may be determined by noting the amount of the thermionic current registered on the milliammeter 22 with the otential of the secondary 19 reduced until t e plate is cold, the filament still being energized. After the requisite increase has occurred in the current registered by the milliammeter 22, for a certain potential between the filament and the plate, the filament 3 will then be sufiiciently activated, because of the film or coating of thorium deposited thereon from the plate 5 or strip of thorium 12 attached thereto.

The device 1 may then be utilized for the purpose desired, as, for instance, radio work.

The filament 3 will be found to have the same to increase the electron-emissivity thereof, it ing the vaporized thorium on to said cawill be found that such treatment will eflithode.

In testimony whereof,; 1 have hereunto subciently clean-up residual gases in the bulb because of the efiicient clean-up action of highly heated or vaporized thorium. This clean-up action may be repeated in the same scribed my name this2 1st' day of July, 1922.

CLAYTON TRIDLE ULREY.

; way in which the activation of the filament is repeated. I

It will be apparent that, according to my improved process for activating an electronemitting filament, the film of thorium deposited thereon will tend to adhere thereto 1n a very efiicient manner, because the filament is heated during the processof distillation. For this reason, the surface coating of thorium is not only firmly attached to the filament but may even form a superficial alloy therewith. The distillation of the thorium by this method is also more efiicient because of the directive influence of the potential difference between the filament and the plate, during the I distillation of the thorium to the filament,

that is, the charged particles of thorium distilled from the plate will be drawn to the filament and condensed thereon rather than be distilled uniformly to all adjacent parts of the device.

' On account of providing a relatively large mass of thorium in the electron device, the activation of the filament may be repeated a large number of times and, for this reason,

the eifieiency of the device for electron-emission may be maintained for an indefinite period. For this same reason, the thorium may be used simultaneously as a clean-up agent because there is a surplus provided beyond that needed for activating the filament.

Although I have described what I consider the preferred means of practicing my inven-.

tion and have shown a preferred embodiment thereof, it is to be understood that the same are merely illustrative and that my invention is limited only by the spirit and scope of the appended claims.

What is claimed is:

1. The method of activating a refractory incandescible cathode for electron emission purposes, comprising enclosing the same in an evacuated envelope with another electrode consisting at least in art of thorium metal and heating the said electrode by electron bombardment to a temperature sufficiently high to partially vaporize said thorium and sputtering said vaporized thorium from said electrode to the cathode.

2. The method of activating a refractory incandescible cathode for electron emission purposes comprising encloslng the same in an

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