EP0013201A1 - Directly heated cathode and high frequency electron tube comprising such a cathode - Google Patents

Directly heated cathode and high frequency electron tube comprising such a cathode Download PDF

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EP0013201A1
EP0013201A1 EP79400941A EP79400941A EP0013201A1 EP 0013201 A1 EP0013201 A1 EP 0013201A1 EP 79400941 A EP79400941 A EP 79400941A EP 79400941 A EP79400941 A EP 79400941A EP 0013201 A1 EP0013201 A1 EP 0013201A1
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cathode
cathode according
layer
thermoemissive
intermediate layer
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EP0013201B1 (en
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Guy Clerc
Arvind Shroff
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Thales SA
Thomson CSF Scpi
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Thomson CSF Scpi
Thomson CSF SA
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J23/00—Details of transit-time tubes of the types covered by group H01J25/00
    • H01J23/02—Electrodes; Magnetic control means; Screens
    • H01J23/04—Cathodes
    • H01J23/05—Cathodes having a cylindrical emissive surface, e.g. cathodes for magnetrons
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/13—Solid thermionic cathodes
    • H01J1/15—Cathodes heated directly by an electric current

Definitions

  • 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.
  • the subject of the present invention is a cathode which makes it possible to avoid thermomechanical problems inside the tube while ensuring good thermoelectronic emissivity.
  • it comprises a support in pyrolytic graphite and a thermoemissive material based on lanthanum hexaboride, the support and the material thermoemissive being separated by a layer forming a diffusion barrier between these two elements.
  • the layer 2 of emissive material is made necessary by the choice of graphite for the support 1; indeed, graphite is a poor thermoelectronic emitter, the work of output of an electron being of the order of 4.7 eV.
  • a good emitting material 2 is then placed on its surface, such as a boron compound of lanthanides, for example lanthanum hexaboride (LaB 6 ), 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 1300 ° to 1600 Q , while that of a tungsten or thoriated tungsten cathode, often used materials, is around 1900 ° -2000 ° C.
  • 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 BaB 6 , formation of a boron carbide and release 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.
  • a layer 3 intended to isolate the carbon atoms from the atoms of lanthanum hexaboride.
  • the intermediate layer 3 can be made of a stable carbide, tantalum (TaC) or hafnium (HfC) for example.
  • 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 ⁇ m.
  • 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.

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Abstract

L'invention a pour objet une cathode à émission thermoélectronique à chauffage direct. Elle comporte un support (1) en graphite pyrolytique et un matériau thermoémissif (2) à base d'hexaborure de lanthane, ces deux éléments étant séparés par une couche (3) formant barrière de diffusion, constituée par un carbure de tantale ou de hafnium, un métal de la famille du platine, ou un composé de bore. Elle est utilisable notamment dans les tubes haute fréquence du type triode, tétrode ou pentode.The invention relates to a thermoelectronic emission cathode with direct heating. It comprises a support (1) in pyrolytic graphite and a thermoemissive material (2) based on lanthanum hexaboride, these two elements being separated by a layer (3) forming a diffusion barrier, constituted by a tantalum or hafnium carbide , a platinum family metal, or a boron compound. It can be used in particular in high frequency tubes of the triode, tetrode or pentode type.

Description

La présente invention a pour objet une cathode pour tube électronique haute fréquence, et plus particulièrement une cathode à émission thermoélectronique à chauffage direct. Elle a également pour objet un tube électronique comportant une telle cathode.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.

Dans les tubes électroniques haute fréquence du type triode, tétrode ou pentode qui comportent une cathode, une anode et une, deux ou trois grilles, il est avantageux de réaliser les grilles en graphite pyrolytique, matériau connu pour ses qualités mécaniques et thermiques. Toutefois, dans ces mêmes tubes, les cathodes sont généralement réalisées en fils de tungstène thorié pour des raisons d'émissivité thermoélectronique. Il se pose alors, en fonctionnement, des problèmes mécaniques du fait de la différence de comportement thermique de ces matériaux. Ces problèmes ne sont résolus qu'imparfaitement par des montages mécaniques coûteux ou des conditions d'utilisation des tubes par ailleurs contraignantes, telles que l'allumage permanent des cathodes par exemple.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.

La présente invention a pour objet une cathode permettant d'éviter les problèmes thermo- mécaniques à l'intérieur du tube tout en assurant une bonne émissivité thermoélectronique. Elle comporte à cet effet un support en graphite pyrolytique et un matériau thermoémissif à base d'hexaborure de lanthane, le support et le matériau thermoémissif étant séparés par une couche formant barrière de diffusion entre ces deux éléments.The subject of the present invention is a cathode which makes it possible to avoid thermomechanical problems inside the tube while ensuring good thermoelectronic emissivity. To this end, it comprises a support in pyrolytic graphite and a thermoemissive material based on lanthanum hexaboride, the support and the material thermoemissive being separated by a layer forming a diffusion barrier between these two elements.

D'autres objets, caractéristiques et résultats de l'invention ressortiront de la description suivante et des dessins annexés, ou :

  • - 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, or:
  • - Figure 1 shows, seen in section, an embodiment of the cathode according to the invention;
  • - Figure 2 shows an alternative embodiment of the cathode shown in Figure 1.

Sur ces différentes figures, les mêmes références se rapportent aux mêmes éléments.In these different figures, the same references relate to the same elements.

Sur la figure 1,.on a donc représenté un premier mode de réalisation de la cathode selon l'invention, dans lequel elle comporte trois éléments :

  • - 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, therefore, a first embodiment of the cathode according to the invention has been 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.

En ce qui concerne le support 1, le graphite pyrolytique est préféré à d'autres matériaux pour deux raisons principales :

  • - 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 (1000° à 20009 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 cathode by current flow in the support 1, up to high temperatures (1000 ° to 20009 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 making the cathode and the grids leads to better geometric definition of the internal structure of the tube.

La couche 2 de matériau émissif est rendue nécessaire par le choix du graphite pour le support 1 ; en effet, le graphite est un mauvais émetteur thermoélectronique, le travail de sortie d'un électron étant de l'ordre de 4,7 eV. On dispose alors à sa surface un matériau 2 bon émetteur, tel qu'un composé boré des lanthanides, par exemple de l'hexaborure de lanthane (LaB6), ou un mélange d'hexaborure de lanthane et d'un autre matériau permettant de diminuer encore le travail de sortie, tel qu'un autre lanthanide. L'avantage des composés de ce type est qu'ils sont bons émetteurs à des températures plus faibles que d'autres matériaux émissifs connus ; la température d'utilisation d'une cathode en hexaborure de lanthane peut être de l'ordre de 1300° à 1600Q, alors que celle d'une cathode en tungstène ou tungstène thorié, matériaux souvent utilisés, se situe vers 1900°-2000° C.The layer 2 of emissive material is made necessary by the choice of graphite for the support 1; indeed, graphite is a poor thermoelectronic emitter, the work of output of an electron being of the order of 4.7 eV. A good emitting material 2 is then placed on its surface, such as a boron compound of lanthanides, for example lanthanum hexaboride (LaB 6 ), 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 1300 ° to 1600 Q , while that of a tungsten or thoriated tungsten cathode, often used materials, is around 1900 ° -2000 ° C.

Toutefois, un inconvénient de tels matériaux pour réaliser la couche émissive 2 est leur grande activité chimique vis-à-vis du graphite, à chaud. Il se produit alors, par exemple dans le cas du BaB6, formation d'un carbure de bore et libération de lanthane, qui a une tension de vapeur élevée comparée à celle de l'hexaborure de lanthane, selon la réaction suivante :

Figure imgb0001
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 BaB 6 , formation of a boron carbide and release of lanthanum, which has a high vapor pressure compared to that of lanthanum hexaboride, according to the following reaction:
Figure imgb0001
 which leads to the destruction of the cathode.

Pour éviter ce phénomène, on dispose entre les éléments 1 et 2 une couche 3 destinée à isoler les atomes de carbone des atomes de l'hexaborure de lanthane.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.

Deux solutions sont possibles pour interdire la réaction précédente :

  • - 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 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 diborures de ces corps sont stables et l'occupation des sites intersticiels 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 of a transition metal from columns IV B (titanium, zirconium or hafnium) or VB (niobium or tantalum for example) of the periodic classification of elements. The diborides of these bodies are stable and the occupation of the interstitial sites of the metal by boron atoms prohibits the diffusion of the boron atoms belonging to the emissive layer 2.

Dans une variante de réalisation, lorsqu'il est nécessaire non plus d'interdire la réaction chimique rappelée ci-dessus, mais de la retarder, dans le cas par exemple où la durée de vie du tube est limitée par ailleurs, la couche intermédiaire 3 peut être constituée d'un carbure stable, de tantale (TaC) ou de hafnium (HfC) par exemple.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 can be made of a stable carbide, tantalum (TaC) or hafnium (HfC) for example.

En ce qui concerne la réalisation technologique de la cathode selon l'invention, on utilise donc un support 1 en graphite pyrolytique, usiné par tous moyens connus pour constituer un cylindre creux, de structure maillée ou non maillée, dont la conductivité est maximale parallèlement à l'axe du cylindre ; l'épaisseur de ce support est, à titre d'exemple, comprise entre 0,2 et 1 mm. Ce support est alimenté par des tigelles d'amenée de courant qui sont également en graphite.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.

La couche intermédiaire 3 est déposée sur le support 1 par évaporation, pulvérisation cathodique, électrolyse ou par phase vapeur ; elle a une épaisseur qui est de préférence comprise entre 5 et 20 µm.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 μm.

La couche émissive 2 est déposée sur la couche 3 au pinceau, au pistolet, par cataphorèse, par pulvérisation cathodique, par évaporation sous vide ou par dépôt ionique ; elle a une épaisseur qui est de préférence comprise entre 0,04 et 0,1 mm.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.

La figure 2 représente une variante de réalisation technologique de la cathode selon l'invention.FIG. 2 represents an alternative technological embodiment of the cathode according to the invention.

Sur cette figure, on retrouve la couche 1 en graphite pyrolytique sur laquelle est déposée la couche intermédiaire 3 telle que décrite ci-dessus. Mais dans le cas de la figure 2, on ajoute de la poudre 4 d'un métal de la famille du platine (iridium ou rhénium de préférence) frittée à la surface de la couche 3, afin d'améliorer l'adhérence de la couche émissive 2 d'hexaborure de lanthane sur la couche intermédiaire 3.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 à chauffage direct, caractérisée par le fait qu'elle comporte un support (1) en graphite pyrolytiqus et une couche d'un matériau thermoémissif (2) comportant de l'hexaborure de lanthane, le support (1) et le matériau thermoémissif (2) étant séparés par une couche intermédiaire (3) formant barrière de diffusion pour les atomes constituant le support (1) et le matériau thermoémissif (2).1. Direct heating cathode, characterized in that it comprises a support (1) in pyrolytic graphite and a layer of a thermoemissive material (2) comprising lanthanum hexaboride, the support (1) and the material thermoemissive (2) being separated by an intermediate layer (3) forming diffusion barrier for the atoms constituting the support (1) and the thermoemissive material (2). 2. Cathode selon la revendication 1, caractérisée par le fait que la couche intermédiaire (3) est constituée par un matériau non réactif chimiquement au carbone et au bore.2. Cathode according to claim 1, characterized in that the intermediate layer (3) consists of a material which is not chemically reactive with carbon and boron. 3. Cathode selon la revendication 2, caractérisée par le fait que ce matériau non réactif est constitué par l'un des métaux suivants : platine, osmium, rhénium ou iridium.3. Cathode according to claim 2, characterized in that this non-reactive material consists of one of the following metals: platinum, osmium, rhenium or iridium. 4. Cathode selon la revendication 1, caractérisée par le fait que la couche intermédiaire (3) est constituée par un composé de bore et d'un des métaux des colonnes IV B et V B de la classification périodique des éléments.4. Cathode according to claim 1, characterized in that the intermediate layer (3) consists of a boron compound and one of the metals of columns IV B and V B of the periodic table of the elements. 5. Cathode selon la revendication 4, caractérisée par le fait que ce composé est un diborure de l'un des métaux suivants : titane, ziconium, hafnium, niobium, tantale.5. Cathode according to claim 4, characterized in that this compound is a diboride of one of the following metals: titanium, ziconium, hafnium, niobium, tantalum. 6. Cathode selon la revendication 1, caractérisée par le fait que la couche intermédiaire (3) est constituée par un carbure stable.6. Cathode according to claim 1, characterized in that the intermediate layer (3) consists of a stable carbide. 7. Cathode selon la revendication 6, carac-1 térisée par le fait que ce carbure est un carbure de tantale ou de hafnium.7. Cathode according to claim 6, carac-1 terized in the fact that this carbide is a tantalum or hafnium carbide. 8. Cathode selon l'une des revendications précédentes, caractérisée par le fait que le matériau thermoémissif (2) est constitué d'hexaborure de lanthane.8. Cathode according to one of the preceding claims, characterized in that the thermoemissive material (2) consists of lanthanum hexaboride. 9. Cathode selon l'une des revendications 1 à 7, caractérisée par le fait que le matériau thermoémissif (2) est constitué par un mélange d'hexaborure de lanthane et d'un autre lanthanide.9. Cathode according to one of claims 1 to 7, characterized in that the thermoemissive material (2) consists of a mixture of lanthanum hexaboride and another lanthanide. 10. Cathode selon l'une des revendications précédentes, caractérisée par le fait qu'elle comporte de plus une couche (4) d'une poudre d'un métal non réactif au carbone ou au bore, frittée sur la surface de la couche intermédiaire (3) et sur laquelle est déposée la couche de matériau thermoémissif (2).10. Cathode according to one of the preceding claims, characterized in that it further comprises a layer (4) of a powder of a metal not reactive to carbon or boron, sintered on the surface of the intermediate layer (3) and on which is deposited the layer of thermoemissive material (2). 11. Cathode selon la revendication 10, caractérisée par le fait que la couche (4) d'une poudre d'un métal non réactif est constituée de poudre d'iridium ou de rhénium.11. Cathode according to claim 10, characterized in that the layer (4) of a powder of a non-reactive metal consists of iridium or rhenium powder. 12. Tube électronique haute fréquence, comportant une cathode, une anode et au moins une grille, caractérisé par le fait que ladite cathode est constituée selon l'une des revendications précédentes.12. High frequency electronic tube, comprising a cathode, an anode and at least one grid, characterized in that said cathode is constituted according to one of the preceding claims.
EP79400941A 1978-12-27 1979-11-30 Directly heated cathode and high frequency electron tube comprising such a cathode Expired EP0013201B1 (en)

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FR7836487A FR2445605A1 (en) 1978-12-27 1978-12-27 DIRECT HEATING CATHODE AND HIGH FREQUENCY ELECTRONIC TUBE COMPRISING SUCH A CATHODE
FR7836487 1978-12-27

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EP0013201B1 (en) 1982-05-19
FR2445605A1 (en) 1980-07-25
US4429250A (en) 1984-01-31
DE2962924D1 (en) 1982-07-08
FR2445605B1 (en) 1981-06-12

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