US 2842706 A
Description (OCR text may contain errors)
July s, 1958 FIG/ D. DQBISCHEK ETAL COLD CATHODE. VACUUM TUBE Filed March 1, 1956 IN VEN TORS, DIE TIP/CH DOB/SCHEK, HAROLD JACOBS.
A TTOR/VE Y United States Patent M COLD CATHODE VACUUM TUBE Dietrich Dobischek, Asbury Park, and Harold Jacobs, West Long Branch, N. J., assignors to the United States of America as represented by the Secretary of the Application March 1, 1956, Serial No. 568,941 8 Claims. (Cl. 315-11 (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.
The invention relates to electron discharge devices, and more particularly to improvements in cold cathodes for such devices which are capable of self-sustained field emission. One type of such device is disclosed and claimed in the copending application of Dietrich Dobischek et al., Ser. No. 408,053, filed February 3, 1954, now U. S. Patent No. 2,802,127, which is hereby incorporated by reference as part of the present application. The cathode coating described in said copending application requires the use of a temporary source of electrons, spark discharge, or ultraviolet radiation to initiate a production of secondary emission in the cathode. After secondary emission is initiated, the electric field which develops across the cathode coating maintains the emission.
The principal object of the present invention is to provide a cathode coating in which secondary emission can be more easily initiated.
In accordance with the present invention there is first deposited a layer of porous magnesium oxide upon a metal base in the manner set forth in said copending application. Thereafter, a cesium getter is flashed to deposit a relatively small amount of cesium on the magnesium oxide layer. It has been found that the trace of cesium permits the cathode coating to be excited into secondary emission by a small amount of visible or infrared light which is temporarily flashed on the coating, after which self-sustained field emission occurs.
For a more detailed description of the invention together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, in which:
Figs. 1 and 2 are schematic diagrams of electron discharge tubes incorporating the invention; and
Fig. 3 is a perspective view of an experimental electron discharge tube incorporating the invention.
Referring now to Fig. 1, the reference numeral indicates an evacuated envelope of glass or other material transparent to visible or infrared light. Mounted within said envelope is a cathode 12 comprising a metal base which may be of nickel, copper, platinum, zirconium or other suitable material. Upon this base is deposited a coating 14 which is capable of secondary emission. Said coating comprises a layer of magnesium oxide upon the surface of which is deposited a trace of cesium. Spaced from cathode 12 is a collector electrode 16 of nickel or other suitable metal. The collector electrode is positively biased relative to the cathode 12 by a suitable source of potential 18 connected in series with a limiting resistor 20.
' Outside of envelope 10 there may be positioned a source of visible or infrared light 22. If desired, the source of light can be placed within envelope 10.
In operation, the light 22 is energized. When the light K 2,842,706. Patented July 8, 1958 strikes the cathode coating, emission of electrons is initiated therefrom. Thereafter, even after the light source 18 is extinguished, the electric field applied between electrodes 12 and 16 maintains the emission. It has been found that if the light is maintained in operation, variations in light intensity will cause a variation in the space current. To interrupt the self-sustained emission the source of potential 18 is disconnected from the tube electrodes by a suitable switch means.
With tubes of this type, currents as high as milliamperes per cm. can be obtained. As the current increases the coating exhibits a bluish fluorescence, Whereby the tube can be used for display purposes.
If the magnesium oxide coating is porous and consists of coarse granular particles, the tube has a high noise content and can be used as an excellent source of noise signals, or wherever the noise content is not objectionable. To reduce the noise content, a more uniform coating of magnesium oxide must be provided. It has also been found that less noise will be produced if zircorium is used for the metal base of cathode 12.
By incorporating more electrodes in the tube, it can be used for the same purposes as ordinary electron tubes. Fig. 2 shows such a tube comprising an envelope 10', a cathode 12' and a grid electrode 16 which serve the same purpose as elements 10, 12 and 16 in Fig. 1. Electrode 16' is positively biased relative to cathode 12 by means of a source of potential 18. A current limiting resistor 20 is placed in series with electrode 16'. In addition the tube may have one or more control grids 26 and an anode 28. Electrode 16' functions as an accelerating grid to draw electrons toward the anode 28. By varying the potential on control grid 26 the space current between anode 28 and cathode 12' can be varied. Electrodes 12' and 16 are the equivalent of the cathode of an ordinary tube, while the remaining electrodes act in the same way as in any other tube. The electrodes may be of any conventional shape or form. They may be planar or cylindrical and concentric with each other.
In Fig. 3, there is shown a structural embodiment of anexperimental tube built in accordance with the invention. It comprises an envelope 10 of glass provided with means (not shown) for exhausting gas therefrom. Mounted within the tube is a cathode 12" having a cylindrical metal base upon which is first deposited a coating of magnesium oxide. Upon this layer there is then superimposed a coating comprising a relatively small amount of cesium in a manner hereinafter described. On one side of the cylinder 12", and spaced a short distance therefrom, is a semicylindrical collector grid electrode 16''. On the opposite side of said cylinder, and spaced therefrom as shown, is a nickel receptacle 32 containing magnesium foil. Mounted near a metal cup 32 is a disclike electrode 34 of nickel. Above receptacle 32 is situated a cesium getter 36. Cylinder 12 is mounted so that it can be rotated on its axis by means of a tab 38 of magnetic material fastened thereto and adapted to be actuated by an external magnet (not shown), whereby tab 38 and cathode 12" to which it is fastened can be rotated any desired amount. A source of D. C. potential 18" is connected to the various electrodes, as shown.
Envelope 10" is now filled with oxygen to a pressure of 20-40 mm. of mercury. Element 34 is then made several hundred volts more positive than magnesium receptacle 32 so that a glow discharge is produced therebetween. A current limiting resistor may be connected in the circuit of the element 34 to prevent excess current from being drawn. The glow discharge results in an ion bombardment which causes the receptacle 32 to heat Very rapidly, thereby igniting the magnesium therein.
3 The magnesium burns violently and deposits on cylinder 12" as a fine, white stalactite-like magnesium oxide film, constituting the porous undercoating of the composite emissive layer.
After the magnesium oxide layer is so formed, the applied positive potential is disconnected from element 34, and the cathode is induction heated to about 700 800 C. in the oxygen for about a minute to complete the oxidation. The oxygen is now pumped out. The cesium getter 26 is then flashed in the usual manner and a fine cesium film is deposited on the magnesium oxide layer. The cesium film constitutes the outer coat of the emissive layer. Grid 16" is then made positive by about 150-250 volts with respect to cathode 12", and said cathode is rotated by application of an external magnetic field to tab 38 so that the coated area of the cathode 12 faces grid 16". The cathode is now momentarily illuminated by a source of light (not shown) to initiate electron emission, after which the emission becomes selfsustained because the potential of grid 16 is positive relative to the cathode. The usual barium getter (not shown) is now flashed, although this can be done before the cesium getter is flashed and the tube is sealed at a pressure of about 10- mm. of mercury. Finally, all the potentials are removed from the tube, and the tube is then baked for approximately a half hour at a temperature of 50-90 C. Thetube is now ready for operation.
The amount of cesium should be relatively minute and may amount to about one part cesium to a hundred parts magnesium oxide. It can be as low as one part cesium to 100,000 parts magnesium oxide. Too much cesium will render the coating too conductive. Since it is often difiicult to provide a cesium getter of such minute proportions, any excess amount of cesium can be removed by oxidation. To this end oxygen at a pressure of about 5 mm. of mercury can be reintroduced into the tube for about a minute to oxidize the excess amount of cesium.
While there has been described what is at present considered a preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.
7 What is claimed is:
l. A cold cathode comprising a metal base and a coating thereon, said coating comprising magnesium oxide and cesium, the amount of said magnesium oxide being at least 100 times greater than the amount of cesium.
2. A cold cathode as set forth in claim 1, wherein said magnesium oxide is highly porous.
3. A cold cathode as set forth in claim 2, wherein said coating is irregular.
4. A cold cathode as set forth in claim 2, wherein said coating is uniform.
5. A cold cathode as set forth in claim 4, wherein the base metal to which said coating is applied is zirconium.
6. A field emission electron discharge device comprising a cold cathode, a collector electrode, and means for positively biasing said collector electrode relative to said cathode to produce field emission from said cathode after emission has been initiated, said cathode comprising a metallic surface having thereon a coating of a mixture of porous magnesium oxide and a trace of cesium, the amount of said magnesium oxide being at least times greater than the amount of cesium so as to render said coating emissive upon irradiation with light.
7. A field emission electron discharge device comprising a cold cathode, a collector electrode, and means for positively biasing said collector electrode relative to said cathode to produce field emission from said cathode after emission has been initiated, said cathode comprising a metallic surface having thereon a coating comprising a layer of porous magnesium oxide and cesium deposited upon said coating, the amount of said cesium being small relative to the amount of magnesium oxide and being insuificient to materially reduce the insulating properties of said coating, said amount of cesium being 1 part to from 100 to 100,000 parts magnesium oxide so as to render said coating emissive upon irradiation with light having a wavelength at least as great as that of visible light.
8. A field emission electron discharge device comprising a cold cathode, an accelerating grid, a control grid and a collector electrode, and means for positively biasing said accelerating grid and collector electrode relative to said cathode to produce field emission from said cathode after emission has been initiated, said cathode comprising a metallic surface having thereon a coating comprising a layer of magnesium oxide and cesium deposited upon said layer, the amount of said cesium being small relative to the amount of magnesium oxide and being insuflicient to materially reduce the insulating properties of said coating, the amount of said magnesium oxide being at least 100 times greater than the amount of cesium so as to render said coating emissive upon irradiation With visible light.
References Cited in the file of this patent UNITED STATES PATENTS 2,024,762 Geflcken et a1. Dec. 17, 1935 2,568,449 Hansen Sept. 18, 1951 2,676,282 Polkosky Apr. 20, 1954 OTHER REFERENCES Bruining: Physics and Applications of Secondary Electron Emission, McGraw-Hill Book Co.. New York. 1954, pages 59 to 63.