Method Of Making A Directly-heated Cathode

Abstract

A directly-heated cathode having a very short warm-up time comprising a tubular sheet of refractory metal having an array of apertures therein defined by a grid of intersecting filaments. The filament grid may be integral with spaced end sections of the sheet, the end sections serving as heat-dam and top-cap sections of the cathode. Also included is a method of making such a cathode comprising etching the apertures in a refractory metal sheet.

Parent Case Text

This is a division, of application Ser. No. 81,449, filed 10-16-70 now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates to a novel directly-heated cathode having a very short warm-up time and to a method of making the directly-heated cathode.

Directly-heated cathodes are employed in electron-beam tubes, such as the RCA type 8462 power tetrode, when short warm-up times are required. A typical directly-heated cathode comprises a hollow cylinder made from a woven mesh or screen of fine wire. As is known in the art, the finer the wire of the mesh, the shorter is the warm-up time of the cathode. However, a mesh cylinder made from very-fine wire has little structural strength or rigidity and is difficult to make.

A simplified method of making directly-heated cathodes having short warm-up times and improved structural properties is described in U.S. Pat. No. 3,404,442, issued to S. E. Aungst et al. The method comprises forming a thin-wire mesh, i.e., a filament section, between a pair of supports mounted in spaced relation on a mandrel made of a disposable material. The ends of the mesh are bonded to the supports, and the wire strands of the mesh are bonded to one another at their cross-over points. Typically, the bondings are accomplished by plating the parts with nickel and then firing the assembly to diffuse the nickel.

Cathodes made by this method, however, have several disadvantages. Thin wire meshes present structural problems, so that the cathode warm-up times are longer than would be obtained if thinner wires could be used. The various metal bonds are sources of cathode failures. Also, the choice of mesh-wire metals is limited by the need for nickel plating; e.g., tungsten is not used. The method itself has several disadvantages. Very accurate control is required of the wire-winding, nickel-plating, and final-firing procedures. Also, the filament, heat-dam, and top-cap sections of the cathodes must be separately made.

SUMMARY OF THE INVENTION

The novel directly-heated cathode comprises a tubular sheet of refractory metal having an array of apertures therein defined by a grid of intersecting filaments formed from a single piece of refractory metal. A source of electrical current is connected to the ends of the sheet to heat the filaments. Preferably, the sheet includes end sections integral (i.e., made from the same piece) with the grid in a central section therebetween. By being formed from a single piece of refractory metal, the filaments may be made very thin without suffering the structural problems of prior cathodes. The prior bonds at the filament cross-over points are eliminated as sources of cathode failure. Also, by having end sections integral with the filament grid, the prior bonds between the filament and heat dam sections and between the filament and top-cap sections can be eliminated as sources of cathode failure. Thus, the novel cathode has a shorter warm-up time and longer operating lifetime than have prior directly-heated cathodes.

The method of making the novel directly-heated cathode comprises etching an array of apertures in a sheet of refractory metal to produce a grid of intersecting filaments, shaping the sheet into a tube, and sealing together the adjacent sides of the tube. Means are then attached for connecting a source of current to the upper and lower ends of the tube. Preferably, the apertures are etched in a central section integral with spaced end sections of the sheet. By etching the apertures in the sheet instead of winding thin wire to produce a filament section, the prior wire-winding, metal-bonding, and final-firing procedures are eliminated. The choice of filament material is no longer limited by the need for nickel plating, so that tungsten and other refractory metals can be used. Also, by etching the apertures in the central section of the sheet, the filament section may be integral with the heat-dam and top-cap sections of the cathode. Thus, with respect to prior cathodes, directly-heated cathodes made by this method are simpler and less costly to construct, and have shorter warm-up times and longer operating lifetimes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view, in axial section, of a cathode assembly comprising the novel directly-heated cathode;

FIG. 2 is a side view, in axial section, of a sub-assembly of the cathode assembly of FIG. 1;

FIG. 3 is an unfolded view of a cylindrical member of the sub-assembly of FIG. 2;

FIG. 4 is an enlarged view of a portion of the member of FIG. 3;

FIG. 5 is a sectional view through line 5--5 of the sub-assembly of FIG. 2; and

FIG. 6 is a side view, in axial section, of another sub-assembly of the cathode assembly of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is an example of the novel directly-heated cathode. FIG. 1 shows a novel directly-heated cathode assembly 21 suitable for use in an electron beam tube such as an RCA type 8462 power tetrode. The cathode assembly 21 comprises two sub-assemblies: a filament assembly 23 and a support assembly 25.

The filament assembly 23, shown separately in FIG. 2, comprises a cylindrical sheet 27 made of tungsten metal and having a thickness of the order of 0.001 inch. The cylindrical sheet 27 comprises a central section 29 integral (i.e., made from the same piece) with and located between a lower end section 31 and an upper end section 33. The central section 29, the lower end section 31, and the upper end section 33 serve as filament, heat-dam, and top-cap sections, respectively, for the filament assembly 23.

For further describing the cylindrical sheet 27, reference is made to FIGS. 3 and 4. FIG. 3, which is an unfolded view of the cylindrical sheet 27, shows a rectangular sheet 35 comprising a central section 37 integral with and located between a lower end section 39 and an upper end section 41. The central section 37, lower end section 39, and upper end section 41 of the rectangular sheet 35 correspond to the central section 29, lower end section 31, and upper end section 33, respectively, of the cylindrical sheet 27.

The central section 37 has a plurality of smaller apertures 43 therein, shown in detail in FIG. 4. The smaller apertures 43 are substantially rhombic in shape, the rhombuses being uniformly separated by a plurality, i.e., grid, of intersecting filaments 45. The rhombuses have rounded corners to increase the effective areas of intersection of the filaments 45. In normal operation, the filaments 45 have currents flowing along their lengths. Since these currents add at the intersections of the filaments 45, the effective areas of intersection are increased to maintain a substantially uniform current density throughout the central, i.e., filament, section 29 of the filament assembly 23. Typically, the width of each filament 41 is of the order of 0.001 inch and the effective width of each intersection of the filaments 41 is of the order of 0.002 inch.

The lower end section 39 of the rectangular sheet 35 has a plurality of larger apertures 47 therein. The larger apertures 47, which are substantially rectangular in shape, serve to decrease the area for heat conduction in the lower end, i.e., heat dam, section 31 (whereby heat is contained in the central, i.e., filament, section 29) of the filament assembly 23.

Within the filament assembly 23, the cylindrical sheet 27 is supported at its lower end section 31 by a metal support cylinder 49 secured to the inner circumference thereof. As shown in FIG. 2, the support cylinder 49 has a step-like outer portion or flange upon which the cylindrical sheet 27 rests. The cylindrical sheet 27 is in turn locked to the support cylinder 49 by a metal locking ring 51, which fits around the outer circumference of the lower end section 31 and also rests upon the outer flange of the support cylinder 49. The cylindrical sheet 27 is supported at its upper end section 33 by a metal filament cap 53 secured to the inner circumference thereof. As shown in FIGS. 2 and 5, the filament cap 53 is a disk-shaped member comprising a plurality of spokes 55, the purpose of which is discussed below. The cylindrical sheet 27 is further supported by a metal alignment ring 57 secured to the inner circumference of the lower end section 31, along an area adjacent the central section 29, thereof.

As recited above, the cathode assembly 21 comprises the filament assembly 23 and a support assembly 25. The support assembly 25, shown separately in FIG. 6, comprises a metal support tube 59, the lower end of which is secured to a terminal assembly 61. The terminal assembly 61 includes a metal first ring 63 secured to the support tube 59, a metal cup-shaped member 65, and an insulating second ring 67 connected to and electrically separating the first ring 63 and the member 45.

The filament assembly 23 and the support assembly 25 are combined, as shown in FIG. 1, such that the support cylinder 49 and the filament cap 53 (of the former) are secured to the cup-shaped member 65 and the upper end of the support tube 59 (of the latter), respectively. To compensate for any expansion of the cylindrical sheet 27 relative to the support tube 59, due to differential heating, the filament cap 53 comprises the plurality of spokes 55 cited above. The spokes 55 serve as flexible mechanical couplers, the absence of which could cause the central section 29 of the cylindrical sheet 27 to bow in relation to the support assembly 25 or otherwise cause the cathode assembly 21 to fail.

For operating the cathode, a source of electrical current (not shown) is connected to the ends of the cylindrical sheet 27 by means of a first electrical lead 69, attached to the lower end of the support tube 59, and a second electrical lead 71, attached to the support cylinder 49. The lower end of the support tube 59 is electrically connected to the upper end section 33 of the cylindrical sheet 27 by means of the filament cap 53 secured thereto, and the support cylinder 49 is electrically connected to the lower end section 31 of the cylindrical sheet 27.

Cathodes as described above have had warm-up times as much as 30-to-50 percent shorter than the warm-up times of prior (nickel-plated) directly-heated cathodes. Also, the structural properties (e.g. the shock and vibration characteristics) and, therefore, the operating lifetimes have been improved over the prior art.

The following is an example of the novel method of making the directly-heated cathode described above. As shown in FIG. 3, a rectangular sheet 35, made of tungsten metal and having a thickness of the order of 0.001 inch, comprises a central section 37 integral with and located between a lower end section 39 and an upper end section 41. A plurality of smaller apertures 43 and a plurality of larger apertures 47 are photo-etched in the central section 37 and the lower end section 39, respectively. The smaller apertures 43 are produced in a grid-like array as shown in FIG. 4 and described above.

Various techniques for photo-etching a pattern of apertures in a metal sheet are known, particularly in the cathode ray tube art. For example, to produce the apertured rectangular sheet 35 shown in FIG. 3, the entire (tungsten) sheet is first coated with a very thin layer of photosensitive material or photoresist, such as KMER or KTFR, both of which are commercially available from Eastman Kodak Company. The photoresist is then exposed to arc-lamp or ultra-violet light rays by contact printing through a negative replica of the pattern of apertures ultimately to be contained in the sheet. The sheet is next washed with a suitable solvent, such as trichloroethylene, so that the unexposed photoresist is dissolved and washed away while the exposed photoresist remains intact. The sheet is then subjected to a chemical etchant, e.g., one having a 1:1 volume ratio of hydrofluoric acid-to-hot nitric acid, which attacks the resist-free or uncovered areas, but not the covered areas, of the sheet.

The photo-etched rectangular sheet 35 is shaped into a cylindrical sheet 27 by first placing, i.e., curving, the rectangular sheet 35 into an annular space defined by two concentric ceramic cylinders (not shown). The ceramic cylinders are then inserted within a larger-diameter molybdenum cylinder (not shown). The cylinder-shaping assembly comprising the molybdenum cylinder, the two ceramic cylinders, and the curved sheet 35 is then heated at about 1,000.degree. C, which temperature is sufficient to fix the shape of the curved sheet 35. This heating, in combination with the pressure resulting from the greater expansion of each of the inner ceramic cylinders relative to the outer molybdenum cylinder, causes the sheet 35 to take the permanent shape of a cylinder. The adjacent longitudinal edges or sides of the cylindrically-shaped sheet 35 typically have overlapping or abutting portions (not shown), which are then sealed, by spot welding or brazing, to complete the cylindrical sheet 27.

As shown in FIG. 2 and described above, a filament assembly 23 comprises the cylindrical sheet 27 secured to each of a metal support cylinder 49, a metal locking ring 51, a metal filament cap 53, and a metal alignment ring 57. Employing a known metal-brazing technique, the metal parts are first plated with a 0.0002 to 0.0003-inch layer of copper and then a 0.0002 to 0.0003-inch layer of gold. Then the complete filament assembly 23 is heated at the flow temperature of the copper-gold combination, about 950.degree. C., to secure the parts.

A separate support assembly 25 is constructed as shown in FIG. 6. As described above, the support assembly 25 comprises a metal support tube 59, the lower end of which is secured to a terminal assembly 61. The terminal assembly 61 comprises a metal first ring 63, a metal cup-shaped member 65, and an insulating second ring 67 therebetween. Preferably, the parts of the support assembly 25 are secured by brazing.

The support assembly 25 is inserted within the filament assembly 23 such that the cup-shaped member 65 and the upper end of the support tube 59 (of the former) contact the support cylinder 49 and the filament cap 53 (of the latter), respectively. The cup-shaped member 65 is then rf-brazed to the support cylinder 49, and the support tube 59 is rf-brazed to the filament cap 53. A first electrical lead 69 is attached to the lower end of the support tube 59, and a second electrical lead 71 is attached to the support cylinder 49. The resulting structure is a completed directly-heated cathode assembly as shown in FIG. 1.

Cathodes made by the method described above haave exhibited the previously-cited improved characteristics. In addition, they have been simpler and less costly to construct than have been prior directly-heated cathodes, since the wire-winding, metal-bonding, and final-firing steps have been eliminated.

GENERAL CONSIDERATIONS

There are various embodiments of the invention other than those described above. For example, the filament assembly comprises a "tubular" sheet which may have a cylindrical, conical, or other shape which is open at both ends. The sheet may be made of a "refractory" metal (i.e., a material that is difficult to melt) other than tungsten, such as molybdenum, nickel, rhenium, or an alloy thereof. The sheet may comprise only the filament section of the filament assembly the heat-dam and top-cap sections of which must be secured separately to the sheet. Also, the outer refractory-metal surface of the filament section may be coated with a lower-work-function material to effect election emission at a lower temperature; typically in the art, carbonate coatings are so employed.

The geometry of the filament section of the sheet, including the width of the intersecting filaments thereof and the shape and size of the apertures defined by these filaments, may be a function of the grid-to-cathode spacing in the tube. In an RCA type 8462 power tetrode, where the grid-to-cathode spacing is typically about 0.008 inch, the filament section described above appears (to the grid) as a solid cylindrical emitting surface. For other grid-to-cathode spacings, the width of the intersecting filaments may be other than 0.001 inch. The smaller apertures in the filament section may be other than rhombic in shape; for example, they may be substantially rectangular or circular.

The larger apertures in the heat-dam section of the sheet may be variously dimensioned and have other than a rectangular shape; also, they may not be required. The supporting means for the tubular sheet may comprise members other than the filament cap, support cylinder, locking ring, and alignment ring described above; the latter may even be eliminated. Also, the support assembly may have a structure other than that shown in FIG. 6.

As indicated above, the filament, heat-dam, and top-cap sections of the filament assembly may be made separately and then secured as required. The smaller and larger apertures may be etched in the filament and heat-dam sections, respectively, using techniques and procedures other than those described above. Also, the etching step may follow, rather than precede, the tube-shaping step. That is, the flat rectangular sheet may first be formed into a cylinder and then suitably etched to produce the apertured cylindrical sheet. Various metal-shaping techniques may be employed in the tube-shaping step. For example, the cylinder may be a section of seamless tubing.

Claims

What is claimed is:

1. A method of making a directly-heated cathode having integral filament and heat-dam sections comprising:

a. etching an array of apertures in a central section of a sheet of refractory metal to produce a grid of intersecting filaments, thereby forming said central section into said filament section;

b. etching a plurality of openings in a lower end section of said sheet integral with said central section, thereby forming said lower end section into said heat-dam section;

c. shaping said sheet into a tube; and

d. attaching means for connecting a source of current to the upper and lower ends of said tube.

2. The method of claim 1, wherein said apertures are photo-etched in said sheet.