US 3717503 A
To increase selectively the electron emission from a cathode, a control grid is placed above the cathode and osmium or iridium is evaporated through the openings of the grid onto the cathode. On the resulting cathode the coated regions provide a value of work function which is lower than that of the uncoated regions.
Description (OCR text may contain errors)
United States Patent [191 Beggs  METHOD OF CONSTRUCTING A VAPOR DEPOSITED Ill-POTENTIAL CATHODE  Inventor: James E. Beggs, Schenectady, NY.
 Assignee: General Electric Company  Filed: Dec. 15, 1970  Appl. No.: 98,269
Related US. Application Data  Division of Ser. No. 762,797, Sept 26, 1968.
 US. Cl. ..117/2l2, 117/210, 117/221, 313/337, 313/346, 313/349  Int. Cl, ..B44d l/18, HOlj 1/00  Field oiSearch ..117/210,2l2,217, 221; 313/349, 346, 337
 References Cited UNITED STATES PATENTS 2,600,121 6/1952 McGee et a1. ..117/106 R 1 Feb. 20, 1973 3,119,041 1/1964 Harris ..313/346 R 3,373,307 3/1968 Zalm et al... ....117/231 X 3,023,131 2/1962 Cassman...,. 117/217 X 2,779,887 1/1957 Jennings..... 117/210 X 3,154,711 10/1964 Beggs 313/337 X 3,567,989 3/1971 Koshizuka.. ....313/346 X 3,599,031 8/1971 Beggs ..3l3/346 X Primary Examiner-Alfred L. Leavitt Assistant Examiner-Kenneth P. Glynn Attorney-Paul A. Frank, John F. Ahern, Julius J. Zaskalicky, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [5 7 ABSTRACT To increase selectively the electron emission from a cathode, a control grid is placed above the cathode and osmium or iridium is evaporated through the openings of the grid onto the cathode. On the resulting cathode the coated regions provide a value of work function which is lower than that of the uncoated regions.
5 Claims, 3 Drawing Figures METHOD OF CONSTRUCTING A VAPOR DEPOSITED BI-POTENTIAL CATHODE This present application is a division of my co-pending application Ser. No. 762,797, filed Sept. 26, 1968.
This invention relates to electron discharge devices and more particularly to a method of manufacturing such a device which provides electron beam focusing. The invention herein described was made in the course of, or under a contract or sub-contract thereunder, with the Department of the Army.
Power triodes for operation at microwave frequencies must necessarily have a large control electrode to anode spacing in order to minimize interelectrode capacitance. To obtain suitable electron current flow across such a wide gap necessitates operation of the control electrode at a relatively large positive potential. Operation at such large positive potential, however, tends to cause collection of current by the control electrode during the positive portions of the voltage wave applied to the control electrode.
In my U.S. Pat. No. 3,154,711, granted Oct. 27, 1964 and assigned to the assignee of the present invention, there is disclosed an electron discharge device in which electron beam focusing is achieved by employing a cathode having a self-biasing focusing electrode of small dimensions in relatively close proximity to the cathode and conductively interconnected therewith. The cathode electrode and focusing electrode have respective low and high work functions so as to establish a negative contact potential difference on the focusing electrode with respect to the cathode, the focusing electrode being maintained clean by operating it at or near cathode temperatures to produce the contact potential difference. The use of such a bi-potential cathode has been found to be helpful for reducing to a low level the magnitude of current collected by a control electrode operating at a positive potential with respect to the cathode. Also in order to obtain a high performance electron discharge device the control grid must be formed of a mesh of very fine wires. However, when a device with such a control grid is operated with a large output current, the fine wires are rapidly heated and distorted unless the construction of the device is such that collection of current by the wires is avoided.
It is a principal object of my invention to provide a method of constructing a new and improved type of bipotential cathode.
It is another object of my invention to provide a method of constructing improved means for focusing electron beams.
In accordance 'with my present invention, a bi-potential cathode is formed with a coating of material in precise alignment with openings in a control electrode structure by evaporation of the material onto the cathode through the openings after the control electrode is mounted adjacent to it. The material evaporated on the cathode may be either osmium or iridium. On the resulting cathode the coated regions have a value of work function which is lower than that of the uncoated regions. As a result, the material deposited on the surface of the cathode not only inpre-coated with a material to assist in maintaining a clean surface when operating at elevated temperatures.
The novel features believed characteristic of my invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the attached drawing in which:
FIG. 1 is an enlarged, cross-sectional side view of a portion of an electron discharge device employing a focusing electrode constructed in accordance with the present invention;
FIG. 2 is a partial enlarged plan view of a portion of the bipotential cathode of FIG. 1; and
FIG. 3 is a cross-sectional side view of a triode electron discharge device incorporating a bi-potential cathode constructed in accordance with my present invention and showing electrode spacings exaggerated.
In the portion of the electron discharge device illustrated in FIG. 1, a cathode 1 is supported by a cylinder of foil 2 which in turn is connected to a cathode contact ring 3. Cathode 1 may be of a well-known type and comprises a porous tungsten body impregnated with a suitable electron-emissive material such as barium calcium aluminate or barium calcium tungstate. The cathode is raised to operating temperatures by means of a heater 4 bonded to the bottom surface of cathode l, heater 4 being energized through a wire 5 and a cathode contact member 3. A grid electrode 6 of mesh construction is disposed closely adjacent cathode 1. Grid 6 may comprise closely spaced, very fine wires 7 having diameters of the order of 0.0004 inch with approximately 500 turns per inch arranged in a first direction and more widely spaced, slightly heavier wires 8 arranged transverse to wires 7 to provide an orthogonal pattern of tiny openings between wires 7,8. The mesh arrangement of grid wires is brazed to a grid ring 9 which in turn is welded to a grid support ring 10 which provides an external connection for the control grid. A ceramic insulator 11 is positioned between and sealed to cathode contact ring 3 and grid support ring 10 in a well-known manner.
In forming the cathode-control grid structure, the cathode is mounted by welding support cylinder 2 which may comprise hafnium foil or any other suitable material to cathode contact ring 3, with the cathode surface positioned at a predetermined distance above the upper surface of grid support ring 10. Grid ring 9 which is of sufficient thickness to support the mesh of fine grid wires slightly above the cathodes surface is then brazed to grid support ring 10. In this manner, the grid is affixed securely in its desired final relationship with respect to the cathode prior to the formation of the bi-potential cathode surface.
In accordance with my invention, coated regions 12 are deposited on the upper surface of cathode 1 in exact alignment with the openings in control grid 6 by evaporating through the openings of grid 6 a material providing a work function which is lower than that of the surface of cathode 1. When a metal having such a high work function and comprising, for example, osmium or iridium, is deposited onto the surface of the impregnated cathode 1, it is effective to reduce the work function of the cathode by approximately 0.2 volt. Accordingly, I evaporate such a material in a conventional manner by supporting a bead or piece (not shown) of the material above the control grid and heating it. This causes the material to descend in straight lines and be deposited through the openings in the control grid so that the respective portions of the coating are in exact alignment with such openings. During such coating operation the material is also deposited on the upper surface of the control grid, i.e, the surface remote from cathode 1. The material thus deposited on the grid wires may be either allowed to remain or wiped ofi.
FIG. 2 is an enlarged partial plan view of cathode 1 after being coated with the material. In this figure numeral 13 designates the porous tungsten structure of the cathode, l4 designating the impregnating electron emissive material, and numeral 12 the rectangular regions of coating vapor deposited on the surface of cathode 1. In this structure, uncoated regions 15 of the cathode between the grid wires maintain their original higher value of work function while coated regions 12 assume the new lower value. For electron tubes where the spacing between the control grid and the cathode is very close, for example of the order of 0.001 inch or less, the 0.2 volt negative potential existing on the uncoated cathode surface beneath the grid wires provides a focusing capability sufficient to appreciably reduce current collection by the grid wires. Additionally, I have found that the electron emission from the coated regions is of the order of three or four times higher than that from the uncoated regions below the wires so that an additional advantage is gained by deposition of the surface coating on the cathode with the grid in position.
In accordance with my invention, in some instances it is desirable first to vaporize over the entire surface of the cathode, a material (not shown) which can maintain a clean surface when operating at elevated materials. Titanium, zirconium, platinum, or a non-emitting material, such as molybdenum carbide, is suitable for such coating material. Thereafter, when l evaporate osmium or iridium and deposit it through the openings in the grid structure, a much larger contact difference of a potential is achieved and even more effective focusing of the electron current results.
FIG. 3 shows a cross-sectional elevation of an electron discharge triode incorporating the foregoing fea tures of my invention. In FIG. 3, heater lead is connected to a heater contact button 16 sealed in a wellknown manner to a ceramic insulator 17, in turn sealed to the bottom portion of contact ring 3. The relatively massive anode 18 having a planar surface 19 in opposed relation to cathode l is sealed to an anode contact ring 20 which in turn is spaced from grid support ring by means of a ceramic insulator 21 sealed to metal ring 22.
In an electron discharge device constructed as described, vapor deposition of iridium or osmium on the surface of the cathode not only increases its emission, but also provides a potential to aid in focusing electrons through the openings in the control and screen grids. This is accomplished by depositing the material on the cathode with the grids already mounted in their final positions so that the material is deposited only on the cathode surface in exact alignment with the openings between the grid wires. By this procedure,
emission from areas below the grid wires is less than that from the coated areas. A difference of potential of approximately 0.2 volt between the uncoated and coated areas is present and tends to deflect electrons and form them into beams that more readily pass between the grid wires. Although the focusing potential is small, it becomes more effective as the grid is spaced more closely to the cathode. An electron discharge device embodying my invention characteristically has a power output three times that of an otherwise identical device but not including my invention. In both instances the power output is that obtainable without overheating the fine grid wires.
While the present invention has been described by reference to particular embodiments thereof, it will be understood that modifications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. The method of making a bi-potential cathode for an electron tube including a cathode member having a planar face and a control grid member constructed of closely spaced fine wires, said wires defining apertures in said control grid member, said method comprising the steps of:
permanently affixing said control grid member in a preselected closely spaced relation to said cathode member, said spaced relation being preselected to determine operating characteristics of said electron tube; and
vapor depositing onto said planar face of said cathode member through said control grid member a material providing a lower work function than that of the material of said cathode member, said vapor deposition step providing a coated pattern of said lower work function material on said planar face, said pattern corresponding exactly to and in registration with said apertures in said control grid member.
2. The method of claim 1 in which the coating material is selected from the group consisting of osmium and iridium.
3. The method of claim 1 in which a piece of iridium is supported above the cathode member and control grid member and vaporized so that the iridium vapor descends in straight lines through apertures between the wires of the grid member to form a coating on the cathode.
4. The method of claim 2 which includes depositing a material selected from the group consisting of titanium, zirconium, platinum, and molybdenum carbide on the cathode member before the coating is vapor deposited thereon.
5. The method of claim 1 which includes the step of vapor depositing on the planar face of said cathode member, prior to affixing the control grid member to the cathode member, a material selected from the group consisting of titanium, zirconium, platinum, and molybdenum carbide.