Bonded Heater, Cathode, Control Electrode Structure

Abstract

A unitary heater, cathode, and control electrode structure for an electron discharge device is formed by coating a disk of a porous refractory metal with an inorganic insulating layer, overcoating the insulating layer with a film of refractory metal, forming a grid pattern in the film on one side of the disk, forming openings in the film and insulating layer corresponding to the pattern, and impregnating the disk with thermionic emissive material.

Description

This invention relates to electrodes for electron discharge devices, and particularly, to a method of manufacturing a unitary heater, cathode, and control grid structure for an electron discharge device which can be mounted in a tube envelope for completion of the construction of the device. The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Army.

The three-electrode electron discharge device, or triode, basically consists of a heater, a cathode, a control grid, and an anode operating in a vacuum environment. Of these electrodes, the anode is a relatively rugged structure while the cathode and its associated heater and control electrode are the chief components determining the degree of performance of the device. The life of the device is determined by successful operation of these components. Because the control grid is usually closely spaced to the cathode, and must be maintained in a fixed position relative to the cathode during operation of the device, a major portion of the cost of constructing the device consists in manufacturing and assembling the heater, cathode, and control grid in the evacuated envelope. If these elements can be preassembled into a rugged and reliable structure, construction of the electron tube, or electron discharge device, can be simplified, manufacturing costs reduced, and operation made more dependable.

It is a principal object of my invention to provide a new and improved unitary heater, cathode, and control grid structure for an electron discharge device.

It is another object of my invention to provide a thermionic cathode with a finely detailed grid bonded to one side and an efficient heater bonded to the other side.

Another object of my invention is to provide new and improved methods of forming a unitary heater, cathode, control grid structure for an electron discharge device.

In accordance with the present invention, a unitary heater, cathode, control grid structure for an electron discharge device is formed by coating a disk of a porous refractory metal with an inorganic insulating layer, coating the insulating layer with a film of refractory metal and forming a grid pattern in the film on one side of the disk. Thereafter, openings are formed in the film and the insulating layer corresponding to the grid pattern and the disk is impregnated with thermionic emissive material. When a heater is attached to the other side of the disk, a rugged unitary structure is available which can easily be assembled in a tube envelope in opposition to the anode structure to form a completed electron discharge device.

While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the details of the preferred process and embodiment of the invention may be more readily ascertained from the following detailed description when read in conjunction with the accompanying drawings in which:

FIG. 1 illustrates schematically the assembly stages of a unitary control electrode, cathode structure;

FIG. 2 illustrates the successive assembly stages of a unitary heater, cathode, control electrode structure;

FIG. 3 illustrates the successive assembly stages of a modification of the unitary heater, cathode, control electrode structure of FIG. 2; and

FIG. 4 is a vertical cross section of an electron discharge device employing the unitary heater, cathode, control electrode structure of FIG. 2.

In the group of sketches which form FIG. 1 and show from left to right the successive stages used to manufacture a bonded control electrode, cathode structure, a porous refractory metal disk 1 is provided with two outer parallel surfaces 2, 3 and a groove 4 which extends around the periphery of the disk. Porous disk 1 is coated on its upper surface with an inorganic insulating layer 5 upon which is deposited a thin film 6 of a refractory metal. Porous disk 1 may be formed of any suitable refractory metal such as tungsten or molybdenum. The material of the inorganic insulating layer 5 preferably is a material having a low dielectric constant and a high dielectric strength such as boron nitride or alumina and may be deposited on the upper surface of the disk from a chemical vapor. The film 6 of a refractory metal which overcoats the insulating layer 5 may be deposited on the layer 5 by sputtering or chemical vapor deposition and may comprise either tungsten or a tungsten alloy such as tungsten carbide, molybdenum or molybdenum carbide, or zirconium.

After the composite structure comprising disk 1 with layers 5 and 6 has been formed, an etched grid is placed in contact with the upper surface of layer 6 and a blast of abrasive particles such as aluminum oxide powder is employed to erode openings 7 in the layers 5 and 6. Preferably, openings 7 are sufficiently deep that a portion of the porous disk 1, such as portions 8, are removed to form a concave surface that can aid in focusing electron current through the openings 7 in the control grid.

To obtain thermionic emission from the cathode, it is necessary to fill the pores of the porous disk 1 with a suitable mixture of oxides. For this purpose, a mixture of barium, calcium, and aluminum oxides is coated on the surface 3, the composite structure is inverted, and inductively heated to a temperature of approximately 1,650.degree. C. whence the oxides form a liquid and penetrate the pores of the disk 1. After this assembly operation has been completed, a metal clad heater 9 is brazed to the bottom surface 3 of the assembly.

In the form of the invention illustrated in FIG. 2, the porous refractory metal disk 1 is provided with a concave rim 10 which corresponds to the groove 4 of the disk of FIG. 1 and which is used, as is explained later, for positioning and supporting the assembly. Two or more lateral holes 11 are formed in the rim of the disk and extend from one edge almost to the opposite edge. Plugs 12 are used to close the openings of these holes. The entire disk is then coated with a thin layer (0.0001 inch to 0.001 inch thick) of an insulating layer 5 of an inorganic material having a low dielectric constant. Thereafter, the top and bottom surfaces of the insulating layer are covered with an overcoating film of refractory metal having a thickness of the order of 0.5 to 10 microns. Layer 6 of this film of refractory material on the upper surface of the composite structure as seen in FIG. 2 extends over the edges to provide a portion 13 to function as a contact region for the control electrode to be formed in the upper layer. Thereafter, both upper layer 6 of the film of refractory metal and the lower layer 14 are coated with a photoresist. A control electrode or grid pattern is developed on layer 6 and a heater pattern on layer 14. The exposed metal is then removed by etching to form a grid in upper layer 6 and a heater in lower layer 14.

In constructing the unitary heater, cathode, control grid assembly, I prefer to form porous disk 1 of tungsten, insulating layer 5 of boron nitride, and films 6 of tungsten although the other materials of the previously mentioned groups of material may be used. After a grid has been formed in layer 6 by the photoetching process, the grid pattern is carburized to provide a surface capable of operating hot with a low level of grid emission. After the carburizing step, a mask (not shown) is placed over the heater side of the disk. This mask is provided with an annular opening 15. Thereafter, the two surfaces are subjected to a blast of aluminum particles to erode the boron nitride layer 5 from the openings in the grid pattern and also opening 15 in the heater mask.

After the eroding step, plugs 12 are removed from openings 11 and the assembly is fired in a vacuum at a high temperature of the order of 1,200.degree. C.--1,800.degree. C. to thoroughly free it from gases. A mixture of cathode oxides is then inserted in holes 11 and the assembly is heated to a temperature where the oxides form a liquid that impregnates the pores of the tungsten disk. Contact to the cathode is made through opening 15 in the insulating film 5.

In the modification of my unitary heater, cathode, control grid assembly illustrated in FIG. 3, a plurality of conductive tubes 20 are inserted in disk 1 prior to the step of coating the disk with inorganic insulating layer 5. In all other respects, the assembly of FIG. 3 is formed as explained in connection with the description of FIG. 2. After the step of eroding the areas in films 6 and 5 to form openings 7, a mixture of cathode oxides such as barium, calcium and aluminum oxides is introduced into the porous disk 1 through tubes 20. Tubes 20 likewise form terminals for the cathode obviating the need for opening 15.

FIG. 4 illustrates my unitary heater, cathode, and control grid structure as embodied in an electron discharge device of a type described and claimed in my concurrently filed application Ser. No. 39,284 (RD-3478) and assigned to the assignee of this present invention. This device 30 comprises three concentric cylinders 31, 32, and 33. Cylinders 31 and 32 are formed of a suitable metal, such as titanium, while cylinder 33 is formed of an insulating material such as a spinel. The cylinders are supported on an insulating base 34 of a suitable material such as forsterite, cylinders 31 and 32 having at their lower ends outwardly extending flanges which are insulated from each other by a layer 35 of forsterite. Cylinder 33 provides a support for an anode structure 36 while cylinder 32 provides a support for a unitary heater, cathode, control grid structure 37 of my invention. A garter spring 38, comprising a resilient conductive helix, snaps into the concave rim of structure 37 and rests upon a shoulder 39 at the upper end of cylinder 32. In this manner, garter spring 38 contacts both the grid contact region 13 of assembly 37 and shoulder 39 so that the outwardly extending flange of cylinder 32 forms an externally accessible contact for the control grid. Contact to cathode disk 1 is made by means of a plurality of conductors 40 which extend from cylinder 31 and are inclined to cathode disk to make a resilient contact with the disk. Contact to heater 14 is made through a pair of leads 41, 42 which pass through openings provided in base 34.

One of the advantages of my invention is that it permits preassembling the unitary heater, cathode, grid structure as a rugged and reliable structure and facilitates positioning of that composite structure in an electron discharge device. By etching film 6, very fine and exact control grid details can be formed. Furthermore, by carburizing the control grid, it is maintained at a low level of grid emission. The continuous film of insulation beneath the entire heater permits it to be operated at a large potential difference with respect to the cathode. By providing a layer 5 which is not only an insulating layer but also a good thermal conductor, heat is transferred directly from the control grid to the cathode so that the grid runs at the same temperature as the cathode.

While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects and I, therefore, intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

Claims

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A unitary heater, cathode, and control grid structure for an electron discharge device which comprises a circular disk of porous refractory metal having two spaced parallel outer surfaces and a peripheral edge, an inorganic insulating layer covering the surfaces of said disk, and a film of refractory metal overlying substantially all of said insulating layer, the film on one surface having a gridlike configuration, the film on the other surface having a configuration of a heating coil, said disk containing thermionic emissive material, and the insulating layer on said one surface having openings extending into the porous disk corresponding to the openings in the gridlike configuration of said film, whereby when the film on the other surface is heated, electrons are directed through said openings in the insulating layer and the film on said one surface.

2. The structure of claim 1 in which the disk of refractory metal comprises tungsten and the inorganic insulating layer comprises a material selected from the group consisting of boron nitride and alumina.

3. The structure of claim 1 in which the refractory metal comprises tungsten and the inorganic insulating layer comprises boron nitride.

4. The structure of claim 1 in which the film of overcoating refractory metal comprises a material selected from the group consisting of tungsten, tungsten carbide, molybdenum, molybdenum carbide, and zirconium.

5. The structure of claim 2 in which the film of overcoating refractory metal comprises tungsten.