Cathode for an electric discharge tube

Abstract

A cathode having a short heating time and a long lifetime for an electric discharge tube is provided. The cathode comprises a metal (particularly nickel) support base coated with a layer of potentially electron-emissive material, which support base has a thickness ranging between 20 and 150 .mu.m, while the metal crystallites have a size which does not permit of any further crystallite growth or recrystallization. Particularly, the crystallites of the support base have a size which corresponds to the thickness of the support base. The cathode is obtained by a method in which the recrystallization thermal treatment is effective to prevent additions in the metal of the support base from forming oxides to a depth which is further than 1 micrometer from the surface.

Description

FIELD OF THE INVENTION

The invention relates to a cathode for an electric discharge tube, comprising a metal support base coated with a layer of potentially electron-emissive material.

BACKGROUND OF THE INVENTION

In the manufacture of cathodes for electron tubes a basic composition is usually formed to a desired configuration and then coated with a layer of alkaline earth carbonates in order to form a cathode or filament. Subsequently the cathode or the filament is placed in an electron tube structure and heat is directly or indirectly applied to the cathode so as to reduce the carbonates to oxides and free metal and thereby activate the cathode. Subsequently heat is applied to the cathode during operation of the tube in order to realize emission of electrons during a period (i.e. lifetime) and to an extent which is dependent on a large number of factors. A relatively thick support base has appeared to be favourable, for example for a long lifetime. A drawback of a relatively thick support base is, however, that the cathode has a long heating time, which is undesirable in many applications.

SUMMARY OF THE INVENTION

The invention has for its object the provision of a cathode having a short heating time and yet a long lifetime.

According to the invention a cathode of the type described in the opening paragraph i.e., for an electric discharge tube comprising a metal support base coated with a layer of potentially electron-emissive material, is therefore characterized in that the support base has a thickness ranging between 20 and 150 .mu.m, the metal crystallites in the base having a size which does not permit any further crystallite growth or recrystallization.

The invention is based on the recognition that the temperature conditions which prevail in an electron tube during operation may cause grain growth or recrystallization of the grains of the support base, which grain growth or recrystallization in its turn causes the electron-emissive coating to scale or come off in the case of a relatively thin support base. This is a factor which detrimentally influences the lifetime of the cathode. The lifetime of a cathode having a relatively thin support base and hence a short heating time can be improved considerably by ensuring that the metal crystallites have a size which no longer permits grain growth or recrystallization.

Generally grain growth or recrystallization is not possible if the metal crystallites have a size which corresponds to the thickness of the support base. An embodiment of the cathode according to the invention is therefore characterized in that the crystallites of the support base have a size which corresponds to the thickness of the support base.

During operation the cathode according to the invention can be heated directly or indirectly (by means of heat generated by a separate heating body, for example a filament). In the latter case it is advantageous for the stability of the thin support base if the heating body remains free from contact with the support base during operation of the cathode. Otherwise the heating body may detrimentally influence the stability of this base, particularly in the case where it is continuously switched on and off during operation.

The favourable effect on the cathode lifetime caused by crystallites which cannot exhibit any further crystal growth could thereby be at least partially destroyed.

The heating body is preferably placed at a distance ranging between 20 and 300 .mu.m from the support base. If the distance is smaller than 20 .mu.m, the heating base and the support body may still come into contact with each other during use of the cathode due to thermal expansion of the heating body. If the distance is larger than 300 .mu.m, the support body is less efficiently heated by the heating body.

In the manufacture of a support base for a cathode it is common practice to combine specific additives (such as Mg, Si and Al) and a base material (such as nickel, nickel alloys such as nickel-lanthanum and tungsten) by means of a melting process so as to obtain a cathode support base material. This material is hot-rolled, then cold-rolled to a strip having a desired thickness and subsequently formed to a cathode support base configuration. The crystals the support base can be given the desired size which does not permit of any further grain growth by giving, according to a further aspect of the invention, the support base a suitable recrystallization thermal treatment prior to the formation of the cathode.

The invention is also based on the recognition that the decrease of the electron emission during the lifetime of the cathode results, inter alia, from the reduction of the quantity of emission activators in the support body, notably in the surface of the support body, due to diffusion and oxidation of the activators. These activators are constituted by the additions which are present in the support body which mainly comprises nickel. The activators diffuse during use of the cathode to the surface of the support body where they activate the electron emission.

Particularly in thin supports, which in total comprise a smaller quantity of additions, hence activators, it is thus important that these activators are not rendered partly or totally "inactive" for by the thermal treatment which is performed to obtain a maximum size of the crystals. A further aspect of the invention is therefore characterized in that the recrystallization thermal treatment is performed under conditions which prevent additions in the metal of the support base from forming oxides to a depth which is further than 1 micrometer from the surface, and preferably not further than 0.5 micrometer.

If the support body is heated in a dry hydrogen atmosphere at a temperature between 850.degree. and 1100.degree. C., optionally preceded by a thermal treatment in an oxygen-containing atmosphere at a temperature ranging between 300.degree. and 450.degree. C., it not only appears that the nickel in the support body recrystallizes to a sufficient extent but also that only a very small quantity of activators becomes inactive. As a result the cathode has a sufficiently constant emission of electrons during its lifetime. Moreover, the cathode appears to be improved in a number of zero-hour emission properties such as an increase of the saturation current, because the free activator elements are present near the surface of the support body.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described in greater detail by way of example with reference to the accompanying drawing in which

FIG. 1 is a diagrammatic longitudinal section view of an indirectly heated cathode assembly

FIG. 2 is a plan view of the assembly of FIG. 1, and

FIG. 3 is a longitudinal section view of another cathode assembly having an alternative support base structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The cathode assembly 1 of FIG. 1 has a cylindrical nickel-chromium cathode support shaft 2, which is provided with a support base or support body 3. The support body 3 mainly consists of nickel and may comprise free activator elements such as, for example Cr, Mg, Al, W, Ta, Si, Ti, Co, Mn and Zr. The cathode shaft 2 accommodates a heating body in the form of a helical filament 4 which may consist of a helically wound metal core having an electrically insulating aluminium oxide coating. A layer of potentially electron-emissive material 7, which is several dozen micrometers thick and which may be provided, for example by means of spraying, is present on the support body 3.

When manufacturing such a cathode the support body 3 is secured to the cathode shaft 2 during a process step. According to the invention, the support body is subjected to a thermal treatment before it is secured to the cathode shaft, i.e., the support body initially is heated in air for 10 to 20 minutes at a temperature of between 300.degree. C. and 450.degree. C., in order to oxidize of organic compounds. Subsequently the support body is heated in a dry hydrogen atmosphere (dew point -60.degree. C.) for 10 to 20 minutes at a temperature of between 850.degree. C. and 1100.degree. C. As a result of this latter heating step the nickel crystals grow to their maximum size in the support body so that problems of bonding the emissive layer to the support body are prevented from occurring at a later stage, for example, when activating the cathode in the tube at which temperatures up to 1000.degree. C. may occur. After the above-described treatment the support body has a glossy appearance.

The cathode shaft may be bright or it may be provided with a thermally black radiating layer, for example by a separate thermal treatment so as to obtain such a thermally black radiating layer on the inner side and on the outer side of the cathode shaft. An example of a suitable thermal treatment of a cathode shaft consisting of a chromium-nickel alloy is to heat the cathode shaft in a dry hydrogen atmosphere at a temperature of approximately 950.degree. C. at which contaminants on the surface are removed. Subsequently the cathode shaft is heated in air at a temperature of approximately 700.degree. C., to form chromium oxide and nickel oxide crystals on the surface. By subsequently heating the cathode shaft in a humid hydrogen atmosphere (dew point 14.degree. C.) at 1050.degree. C., the nickel oxide which has formed on the support body is reduced to nickel, while the chromium oxide is not reduced. Since the humid hydrogen atmosphere has an oxidizing effect on chromium, the chromium oxide film on the shaft will become thicker during this thermal treatment ultimately forming a stable thermally black radiating layer.

After the possible thermal treatments of the support body 3 and the cathode shaft 2 they are secured to each other, for example, by means of welding.

During a subsequent process step a layer of potentially electron-emissive material is provided on the support body.

It has been found that the reduction of electron emission of the layer which always occurs during the lifetime of the cathode may be kept very small (in a given case no more than 8% as against a reduction of more than 25% in conventional cathodes) when the support body is subjected to the previously mentioned thermal treatment so as to give the metal crystals a maximum size. Moreover, a number of zero-hour emission properties of the cathode also appears to be improved.

The cathode shaft 2 with the support base 3 of the cathode 1 of FIG. 1 is suspended in an opening of a housing 6 by means of three suspension means 8a, 8b and 8c (see FIG. 2). The filament 4 is connected to current supply leads 5a and 5b.

FIG. 3 shows an alternative construction in which the shaft and the support base consist of one piece 13. The emissive layer 7 and the filament 4 are the same as in FIG. 1.

In both cases it is advantageous for the lifetime of the cathode when the filament 4 cannot come into contact with the thin (20-150 .mu.m thick) support base 3 or 13. The filament 4 is preferably placed in the cathode shaft 2 in such a way that the distance d (FIG. 1) between the support body 3 and the filament 4 ranges between 20 .mu.m and 300 .mu.m. Dependent on the permissible lower cathode temperature, the distance d is preferably between 50 and 200 .mu.m.

A cathode according to the invention not only has a substantially constant electron emission during its lifetime but it can also be operated at a lower temperature due to its increased zero-hour emission.

Claims

1. A method of manufacturing an oxide cathode in which a layer of potentially electron-emissive material is provided on a metal support base, characterized in that the support base has a thickness of about 20 to 150 .mu.m, the method comprising:

subjecting the support base to a recrystallization thermal treatment comprising heating the support base in a dry hydrogen atmosphere at a temperature ranging between 850.degree. and 1100.degree. C. under conditions that cause the metal crystallites in said support base to grow to a maximum size which does not permit further crystallite growth or recrystallization; and after said recrystallization thermal treatment, providing said layer of potentially electron-emissive material on said support base.

2. A method as claimed in claim 1, characterized in that the recrystallization thermal treatment is effective to prevent additions in the metal of the support base from forming oxides to a depth which is further than 1 micrometer from the surface.

3. A method of manufacturing an oxide cathode in which a layer of potentially electron-emissive material is provided on a metal support base, the method comprising:

initially subjecting said support base to a thermal treatment in an oxygen-containing atmosphere at a temperature within the range of 300.degree. to 450.degree. C.;

then subjecting said support base to a recrystallization thermal treatment of heating the support base in a dry hydrogen atmosphere at a temperature ranging between 850.degree. C. and 1100.degree. C. under conditions that cause the metal crystallites contained in said support body to grow to a maximum size which does not permit further crystallite growth or recrystallization; and

after said recrystallization thermal treatment, providing said layer of potentially electron-emissive material on said support base.

4. A method as claimed in claim 3 wherein the support base comprises nickel.

5. A method of manufacturing an oxide cathode comprising a metal support body coated with a layer of potentially electron-emissive material, the support body having a thickness between 20 and 150 .mu.m which method comprises the steps of:

(a) providing a cathode shaft;

(b) providing a support body;

(c) subjecting the support body to a thermal treatment wherein the support body is heated to a temperature of about 300.degree. C. to about 450.degree. C.; and

(d) subsequently subjecting the support body to a recrystallization thermal treatment wherein the support body is heated to a temperature of about 850.degree. to about 1100.degree. C. in a dry hydrogen atmosphere to cause metal crystallites contained in said support body to grow to a maximum size which does not permit further crystallite growth or recrystallization.

6. A method as claimed in claim 5 wherein step (c) is conducted in an oxygen-containing atmosphere.

7. A method as claimed in claim 5 wherein step (d) is conducted in a dry hydrogen atmosphere, the dew point of such atmosphere being about -60.degree. C.

8. A method as claimed in claim 5 wherein the crystallites of the support body have a size which corresponds to the thickness of the support base.

9. A method as claimed in claim 5 wherein the support body comprises nickel.

10. A method as claimed in claim 5 wherein a layer of potentially electron-emissive material is provided on the support body subsequent to step (d).

11. A method as claimed in claim 5 comprising the additional steps of:

(e) securing the cathode shaft and the support body to each other; and

(f) providing a layer of potentially electron-emissive material on the support body.