BASKET ELECTRODE SHAPING

Abstract: The invention is a method of manufacture of a cylindrical electrode such as a thermionic cathode (10) comprising an array (13) of helical filaments (12). The method comprises the step of placing the array (13), attached to its support electrodes (14, 16) which are fixedly mounted to a common subassembly support (18, 20, 21, 22, 24), inside a close-fitting refractory dielectric cylinder (26) and heating the filament assembly (13) by passing current through the filaments. The filament assembly (13) expands to contact the form (26). On further heating it is plastically deformed to form a perfect cylindrical shape. After cooling and shrinking, it is removed from the form (26).

Description: The invention pertains to basket structures formed of wires, such as used as thermionic cathodes for grid-controlled, high powered electron tubes. Many grid-controlled electron tubes have used thermionic cathodes formed by a cylindrical array of helical filaments, generally of thoriated tungsten. One set of said filaments had a certain helical pitch and the other set an opposite helical pitch. The cylindrical array is formed by spacing the filaments around a cylindrical metallic mandrel which may be a spot-welding electrode. At the spots where filaments of opposite pitch cross, they typically are spot-welded together. In the prior art the filaments were joined together to form a basket-like cylindrical cage which was set into its cylindrical shape by firing at a high temperature with a cylindrical form inside the cage. Each end of each filament is then welded to a cylindrical metallic ring which serves as a common current-connecting element for all the filaments in parallel. A major problem with the prior art procedure was that after the initial shaping, the step of spot welding the filaments to their support electrodes introduced strains in the metals. After assembly of the complete electrode structure, the filaments were carburized by heating to a high temperature in an atmosphere of carbon-containing gas such as a volatile hydrocarbon. During the carburization these strains were relieved by deformation of the parts. At this point it was not practical to reshape the parts to their desired true cylindrical outline because the carburized tungsten is very brittle. US-A-3465400 discloses a method of forming an electrode structure using two cylindrical rings with a helical wire between them. With an inner mandrel and an outer clamp, the whole structure is placed in a furnace and heated in an atmosphere of wet hydrogen to fuse together all wire-to-wire and wire-to-ring contacts, after which the assembly is cooled. The invention provides a method of shaping an electrode structure as set out in Claim 1. An example of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic section through the axis of a thermionic cathode structure preparatory to formation showing ancillary equipment; and Figure 2 is a schematic section through the axis of the cold shape of the cathode after formation. In the preferred embodiment, a plurality of wires (wire filaments) as of thoriated tungsten are spaced around a cylindrical mandrel, which may be pieces of a single long continuous wire, are wound helically about the mandrel. One set has a first helical pitch and a second set has the opposite helical pitch. Where wires of opposite pitch cross, they are spotwelded together to form a unitary basket structure (basket; basket array) as an embodiment of a cylindrical array of a plurality of helical wire filaments. This basket structure may receive a first shape-setting step by being placed around a refractory cylindrical jig with a second hollow cylindrical jig outside the structure. The basket, constrained by the jig, is then fired, as in a hydrogen atmosphere or vacuum, at a high temperature to anneal the wires to be strain-free in their constrained shape. In the next step the basket array is mounted on a pair of metallic cylinders used as support electrodes as shown in Figure 1. The basket 13 of filaments (wire filaments) 12 is mounted with the ends of its filaments projecting over the cylindrical ends of an upper metallic cylinder used as support electrode 14 and a lower metallic cylinder used as support electrode 16. The ends of filaments 12 are then spotwelded to the metallic cylinders used as support electrodes. Lower metallic cylinder used as support electrode 16 has a flat flange 18 which is brazed between ceramic insulating rings 20 and 21 which are part of the vacuum envelope of the tube. Upper metallic cylinder used as support electrode 14 is mounted on an axial metallic rod 24, as of molybdenum, which in turn is brazed to ceramic ring 20 via a flat ring portion 22. The assembly of the metallic cylinders used as support electrodes 14, 16 with their support structure 18, 20, 21, 22, 24 is of course completed before the array of filaments 12 is attached to it. As described above, the process of affixing the basket array to the metallic cylinders used as support electrodes 14, 16 may produce strains in the parts. On subsequent heating, such as in carburizing thoriated tungsten filaments or in actual operation as a thermionic cathode, these strains can cause mechanical distortions which spoil the cylindrical shape. To avoid the problem,after mounting the basket filaments 12 to metallic cylinders used as support electrodes 14 and 16, a refractory dielectric form 26 is placed outside the basket array. Form 26 is typically a high-alumina ceramic. It may be of any desired external shape, but it has a hollow cylindrical bore 28 with a diameter which fits closely over the outer surfaces of basket structure 13. The assembly with form 26 in place is placed in an inert atmosphere, preferably a vacuum. Filaments 12 are then heated by passing current between metallic cylinders used as support electrodes 14, 16 via external leads 18 and 24. During this heating, basket structure 13 expands until its outer wires 12 touch form 26. Form 26 remains relatively cool and has a low thermal expansivity so it does not expand very much. On further heating, basket structure 13 is constrained from further expansion, so the wires 12 are plastically deformed to form a perfect fit in the cylindrical form recess 28. As the assembly is cooled, basket 13 shrinks away from form 26, leaving it with a slight hour-glass shape. Figure 2 illustrates the hour-glass shape of the cathode when cooled to room temperature as contrasted to the cylindrical hot envelope 30. This shape is advantageous because on subsequent heating during operation it can again expand into a perfect cylindrical outline as indicated by the dashed envelope cylinder 30. After the shaping operation, form 26 is removed and filaments 12, if made of thoriated tungsten, are carburized on their surfaces by heating basket structure 13 in a reducing or inert atmosphere containing carbonaceous material such as a volatile hydrocarbon which decomposes at the filament temperature, producing carbon which forms a carbide layer on the filament surfaces. The electrode structure is then ready for assembly into an electronic vacuum tube such as a grid-controlled triode or tetrode. The above example is intended to be illustrative of a preferred embodiment of the invention. Other embodiments may become obvious to those skilled in the art. For example, the precisely shaped basket cylinder may be used as a grid in tubes of cylindrical geometry.

Claims: A method of shaping an electrode structure formed by a cylindrical array of a plurality of helical wire filaments, said method comprising: using a first and a second common metallic cylinders (14, 16) being mechanically fixed to a can support means (24) and mutually electrically insulated, forming said cylindrical array (13) of helical wire filaments (12), each wire filament being bonded at a first end to said first common metallic cylinder (14) and at a second end to said second common metallic cylinder (16), placing around said cylindrical array a form (26) of temperature resistant dielectric material having a cylindrical opening sized to fit outside said array, placing said array and form in an inert environment, heating said wire filaments in said inert environment by passing electric current between said metallic cylinders used as support electrodes, so that at least some of said wire filaments expand to be constrained by the surface of said opening, and cooling said electrode structure, such that said wire filament array shrinks away from said form, and removing said form. The method of Claim 1 wherein said cylindrical array comprises wire filaments of opposite pitch and said forming of said cylindrical array comprises bonding said wire filaments together at their crossovers. The method of Claim 1 wherein said inert environment is a vacuum. The method of Claim 1 wherein said wire filaments are tungsten wires. The method of Claim 4 wherein said tungsten is thoriated. The method of Claim 1 further including the subsequent step of heating said wire filaments in said array in a carbonaceous atmosphere to carburize their surfaces. The method of Claim 6 wherein said carburization follows said removal from said form.

Inventors: JACKSON, SEMLER D., TORNOE, ROGER NORMAN
Applicants: VARIAN ASSOCIATES, INC.
Application year: 1987
Main classification: H01J9
IPC codes: H01J9