EP0013201B1 - Directly heated cathode and high frequency electron tube comprising such a cathode - Google Patents

Description

The subject of the present invention is a cathode for a high frequency electronic tube, and more particularly a thermoelectronic emission cathode with direct heating. It also relates to an electronic tube comprising such a cathode.

In high frequency electronic tubes of the triode, tetrode or pentode type which comprise a cathode, an anode and one, two or three grids, it is advantageous to produce the grids in pyrolytic graphite, a material known for its mechanical and thermal qualities. However, in these same tubes, the cathodes are generally made of thoriated tungsten wires for reasons of thermoelectronic emissivity. Mechanical problems then arise due to the difference in thermal behavior of these materials. These problems are only imperfectly solved by costly mechanical assemblies or otherwise restrictive conditions of use of the tubes, such as the permanent ignition of the cathodes for example.

It has already been proposed, for example in French patent n ° 66913, to produce direct heating cathodes by superposition of two layers, one in pyrolytic graphite, the other in lanthanum hexaboride.

And, in French Patent No. 2,033,868, advantage was taken of the fact of a weak interaction existing between the graphite and the lanthanum hexaboride in order to arrange them one against the other without any intermediary.

Contrary to the assertions in this document, experiments carried out by the applicant have led it to recognize that certain risks of interaction existed between pyrolytic graphite and lanthanum hexaboride.

In the case of sintered cathodes in which the emissive material is mixed with a powder of a support material, it has been proposed, as in German patent No. 1,211,724, to avoid any contact between these two materials, to coat the particles of the metal support of a suitable metal film. And, in US Patent No. 3,436,584, rhenium has been proposed as well as the boronized or fueled metals to provide a diffusion barrier against boron.

The subject of the present invention is a direct heating cathode comprising, in known manner, a pyrolytic graphite support and a layer of a thermoemissive material comprising lanthanum hexaboride, but in which, in accordance with the invention, the support of pyrolytic graphite and the thermoemissive material are separated by an intermediate layer intended to isolate the atoms constituting the pyrolytic graphite support from the atoms constituting the thermoemissive material.

• La figure 1 représente, vu en coupe, un mode de réalisation de la cathode selon l'invention ;
• La figure 2 représente une variante de réalisation de la cathode représentée sur la figure 1.

Other objects, characteristics and results of the invention will emerge from the following description and the accompanying drawings, in which:

• FIG. 1 represents, seen in section, an embodiment of the cathode according to the invention;
• FIG. 2 represents an alternative embodiment of the cathode shown in FIG. 1.

In these different figures, the same references relate to the same elements.

• - un support 1 de préférence en graphite pyrolytique ;
• - une couche 2 d'un matériau émissif ;
• - une couche intermédiaire 3, formant barrière de diffusion entre les éléments 1 et 2.

In FIG. 1, a first embodiment of the cathode according to the invention is therefore shown, in which it comprises three elements:

• - A support 1 preferably made of pyrolytic graphite;
• - a layer 2 of an emissive material;
• an intermediate layer 3, forming a diffusion barrier between the elements 1 and 2.

• - la première tient aux qualités du graphite pyrolytique lui-même : en effet, celui-ci n'est pas isotrope et présente, dans le plan du dépôt, une assez bonne conductivité électrique et une très bonne conductivité thermique, alors que dans une direction normale au dépôt, ces conductivités sont faibles ; par ailleurs, il présente de faibles coefficients de dilatation et de bonnes propriétés mécaniques à haute température ; cela permet un chauffage direct de la cathode par circulation de courant dans le support 1, jusqu'à de hautes températures (1 000° à 2 000 °C par exemple) ;
• - la seconde tient à l'insertion de la cathode dans un tube électronique comportant une ou plusieurs grilles, elles-mêmes réalisées en graphite pyrolytique : l'utilisation d'un même matériau pour la réalisation de la cathode et des grilles conduit à une meilleure définition géométrique de la structure interne du tube.

With regard to support 1, pyrolytic graphite is preferred to other materials for two main reasons:

• - the first is due to the qualities of the pyrolytic graphite itself: in fact, it is not isotropic and has, in the deposition plane, a fairly good electrical conductivity and a very good thermal conductivity, while in one direction normal to the deposit, these conductivities are low; moreover, it has low coefficients of expansion and good mechanical properties at high temperature; this allows direct heating of the cathode by circulation of current in the support 1, up to high temperatures (1000 ° to 2000 ° C for example);
• - the second is due to the insertion of the cathode into an electronic tube comprising one or more grids, themselves made of pyrolytic graphite: the use of the same material for producing the cathode and the grids leads to better geometric definition of the internal structure of the tube.

The layer 2 of emissive material is made necessary by the choice of graphite for the support 1: in fact, graphite is a poor thermoelectronic emitter, the work of output of an electron being of the order of 4.7eV. A good emitting material 2 is then placed on its surface, such as a boron compound of lanthanides, for example lanthanum hexaboride (LaB.), Or a mixture of lanthanum hexaboride and another material making it possible to further decrease output work, such as another lanthanide. The advantage of compounds of this type is that they are good emitters at lower temperatures than other known emissive materials: the temperature of use of a lanthanum hexaboride cathode can be of the order of 1,300 ° at 1600 °, while that of a cathode made of tungsten or thoriated tungsten, often used materials, is around 1900 ° -2000 ° C.

ce qui conduit à la destruction de la cathode.However, a drawback of such materials for producing the emissive layer 2 is their high chemical activity with respect to graphite, when hot. It then occurs, for example in the case of LaS _e . formation of a boron carbide and liberated tion of lanthanum, which has a high vapor pressure compared to that of lanthanum hexaboride, according to the following reaction:

which leads to the destruction of the cathode.

To avoid this phenomenon, there is between the elements 1 and 2 a layer 3 intended to isolate the carbon atoms from the atoms of lanthanum hexaboride.

• - dans un premier mode de réalisation, on dépose une couche (3) d'un matériau pour lequel on ne connaît pas de réaction chimique avec le carbone et l'hexaborure de lanthane, tel qu'un métal de la famille du platine : platine, osmium, rhénium ou iridium ;
• - dans un deuxième mode de réalisation, la couche intermédiaire 3 est constituée par un composé de bore et d'un métal de transition des colonnes IV B (titane, zirconium ou hafnium) ou V B (niobium ou tantale par exemple) de la classification périodique des éléments. Les dibo- rures de ces corps sont stables et l'occupation des sites interstitiels du métal par des atomes de bore interdit la diffusion des atomes de bore appartenant à la couche émissive 2.

Two solutions are possible to prohibit the previous reaction:

• - In a first embodiment, a layer (3) of a material is deposited for which there is no chemical reaction with carbon and lanthanum hexaboride, such as a metal of the platinum family: platinum , osmium, rhenium or iridium;
• - in a second embodiment, the intermediate layer 3 consists of a boron compound and a transition metal from columns IV B (titanium, zirconium or hafnium) or VB (niobium or tantalum for example) of the periodic table elements. The diborides of these bodies are stable and the occupation of the interstitial sites of the metal by boron atoms prevents the diffusion of the boron atoms belonging to the emissive layer 2.

In an alternative embodiment, when it is no longer necessary to prohibit the chemical reaction mentioned above, but to delay it, in the case for example where the life of the tube is otherwise limited, the intermediate layer 3 may consist of a stable carbide, tantalum (TaC) or hafnium (HfC) for example.

With regard to the technological embodiment of the cathode according to the invention, a support 1 of pyrolytic graphite is therefore used, machined by any known means to form a hollow cylinder, of mesh or non-mesh structure, the conductivity of which is maximum parallel to the axis of the cylinder; the thickness of this support is, for example, between 0.2 and 1 mm. This support is supplied by current supply rods which are also made of graphite.

The intermediate layer 3 is deposited on the support 1 by evaporation, sputtering, electrolysis or by vapor phase; it has a thickness which is preferably between 5 and 20 wm.

The emissive layer 2 is deposited on the layer 3 with a brush, with a gun, by cataphoresis, by cathode sputtering, by evaporation under vacuum or by ionic deposition; it has a thickness which is preferably between 0.04 and 0.1 mm.

FIG. 2 represents an alternative technological embodiment of the cathode according to the invention.

In this figure, we find the layer 1 of pyrolytic graphite on which the intermediate layer 3 is deposited as described above. But in the case of FIG. 2, powder 4 of a metal of the platinum family (preferably iridium or rhenium) is added sintered on the surface of the layer 3, in order to improve the adhesion of the layer emissive 2 of lanthanum hexaboride on the intermediate layer 3.

Claims (12)

1. Cathode with direct heating, comprising a support (1) of pyrolytic graphite and a layer of a thermo-emissive material (2) comprising lanthanum hexaboride, characterized in that the support (1) and thermo-emissive material (2) are separated by an intermediate layer (3) intended to insulate the atoms forming the support (1) from the atoms forming the thermo-emissive material (2).

2. Cathode in accordance with claim 1, characterized by the fact that the intermediate layer (3) is formed by a material chemically not reacting with carbon and boron.

3. Cathode in accordance with claim 2, characterized by the fact that this non-reacting material is formed by one of the following metals : platinum, osmium, rhenium or iridium.

4. Cathode in accordance with claim 1, characterized by the fact that the intermediate layer (3) is formed by a composition of boron and one of the metals of the column IV B or V B of the periodic system of the elements.

5. Cathode in accordance with claim 4, characterized by the fact that the composition is a diboride of one of the following metals : titanium, zirconium, hafnium, niobium, tantalum.

6. Cathode in accordance with claim 1, characterized by the fact that the intermediate layer (3) is formed of a stable carbide.

7. Cathode in accordance with claim 6, characterized by the fact that the carbide is a tantalum or hafnium carbide.

8. Cathode in accordance with any of the preceding claims, characterized by the fact that the thermo-emissive material (2) is formed of lanthanum hexaboride.

9. Cathode in accordance with any of claims 1 to 7, characterized by the fact that the thermo-emissive material (2) is formed by a mixture of lanthanum hexaboride and another lanthanide.

10. Cathode in accordance with any of the preceding claims, characterized by the fact that it further comprises a layer (4) of a metal powder which is non-reacting with carbon or boron, sintered on the surface of the intermediate layer (3) and on which the layer of thermo-emissive material (2) is deposited.

11. Cathode in accordance with claim 10, characterized by the fact that the layer (4) of a non-reacting metal powder is formed of an iridium or rhenium powder.

12. Electronic high frequency tube comprising a cathode, an anode and at least one grid, characterized by the fact that said cathode is formed in accordance with one of the preceding claims.

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