EP0034512A2 - Heating element for indirectly heated cathodes, process for manufacturing such an element and indirectly heated cathode comprising the same - Google Patents

Abstract

This heating element consists of a filament (3) and a mixture (4), sintered between 1700 and 1800 ° C, alumina and less than 10% by weight of yttrium oxide. This mixture fills the space left free by the filament inside the cylinder (2) closed by an emissive disc (1) which constitutes the cathode.

Description

The present invention relates to a heating element for an indirectly heated cathode. It also relates to a method of manufacturing such an element and to indirectly heated cathodes comprising such an element.

Indirectly heated cathodes which are used in electronic tubes are well known in the prior art. They generally consist of an emissive disc brazed at one of the ends of a cylinder of non-emissive material which serves as a housing. Inside the cylinder is placed a filament which ensures the so-called indirect heating of the cathode.

• - celles à chauffage par filament libre dans lesquelles le filament chauffe la cathode par rayonnement ;
• - celles à chauffage par filament dit potté, dans lesquelles l'espace laissé libre par le filament à l'intérieur du cylindre est rempli par un corps bon conducteur thermique, électriquement isolant à la température de fonctionnement, dont le point de fusion est élevé et qui enfin ne réagit pas à la température de fonctionnement avec le filament et le cylindre.

There are two types of indirect heating cathodes:

• - those with free filament heating in which the filament heats the cathode by radiation;
• - those with so-called potted filament heating, in which the space left free by the filament inside the cylinder is filled by a body which is a good thermal conductor, electrically insulating at operating temperature, the melting point of which is high and which finally does not react to the operating temperature with the filament and the cylinder.

In cathodes with indirect heating by "potted" filament, the filament is less vulnerable to shocks and mechanical vibrations than in cathodes with indirect heating by free filament.

The present invention relates to cathodes with indirect heating by "potted" filament.

• - par de la poudre d'alumine frittée vers 2000°C ;
• - ou, par un mélange de poudre d'alumine et de poudre d'oxyde de calcium fritté entre 1750°C et 1800°C.

In the prior art, the "potting", that is to say the body which blocks the filament in the cylinder, consists of:

• - with alumina powder sintered at around 2000 ° C;
• - Or, by a mixture of alumina powder and sintered calcium oxide powder between 1750 ° C and 1800 ° C.

According to the present invention, the "potting" consists of a mixture of alumina and of less than 10% by weight of an oxide of one of the elements of column III A of the periodic table of the elements, this mixture being sintered between 1700 and 1800 ° C.

According to a preferred embodiment of the invention, the mixture consists of yttrium oxide and alumina of chemical composition 3 Y ₂ O ₃ . 5 Al ₂ O ₃ , plus alumina in the α phase

• - le frittage est réalisé entre 1700 et 1800°C et cette température ne peut pas fragiliser le filament en tungstène ou en tungstène rhénium, comme c'est le cas lorsqu'on doit monter à 2000°C pour fritter de la poudre d'alumine pure ;
• - le potting réalisé est solide et compact, il assure un bon contact thermique de longue durée entre le filament et le cylindre.Il assure un isolement électrique égal à celui obtenu avec un "potting " à base d'alumine uniquement.
• - l'oxyde d'yttrium'qui peut être utilisé est stable et très pur. Son coefficient de dilatation linéaire α égal à 8, 18. 10^-6 est très proche de celui des filaments couramment utilisés et identique à celui de l'alumine, ce qui permet d'obtenir un "potting" dont le coefficient de dilatation ne dépend pas des proportions d'alumine et d'oxyde.De plus, la conductibilité thermique de l'oxyde d'yttrium est identique à celle de l'alumine (λ = 0,017 cal. S^-1. cm^-1.e^-1 à 1800°C et 0,013 à 1000°C). Enfin, la température de fusion de l'oxyde d'yttrium (2410°C) est inférieure à celle de l'oxyde de calcium par exemple (2572°C).

The "potting" according to the present invention has the following advantages:

• - the sintering is carried out between 1700 and 1800 ° C and this temperature cannot weaken the tungsten or rhenium tungsten filament, as is the case when it is necessary to go up to 2000 ° C to sinter alumina powder pure;
• - the potting produced is solid and compact, it ensures good long-term thermal contact between the filament and the cylinder. It provides electrical insulation equal to that obtained with a "potting" based on alumina only.
• - the yttrium'oxide which can be used is stable and very pure. Its coefficient of linear expansion α equal to 8.18. 10 ^-6 is very close to that of commonly used filaments and identical to that of alumina, which makes it possible to obtain a "potting" whose coefficient of expansion does not depend on the proportions of alumina and oxide. In addition, the thermal conductivity of yttrium oxide is identical to that of alumina (λ = 0.017 cal. S ^{- 1.} Cm ^-1 .e ^-1 at 1800 ° C and 0.013 at 1000 ° C). Finally, the melting temperature of yttrium oxide (2410 ° C) is lower than that of calcium oxide for example (2572 ° C).

• - La figure 1, une vue en perpective d'une cathode à chauffage indirect à filament "potté" ;
• - La figure 2, un détail du diagramme de phase du mélange alumine-oxyde d'yttrium ;
• - Les figures 3a, b et c, des schémas illustrant le procédé de fabrication selon l'invention.

Other objects, characteristics and results of the invention will emerge from the following description, given by way of nonlimiting example and illustrated by the appended figures which represent:

• - Figure 1, a perspective view of an indirect heating cathode filament "potté";
• - Figure 2, a detail of the phase diagram of the alumina-yttrium oxide mixture;
• - Figures 3a, b and c, diagrams illustrating the manufacturing process according to the invention.

In the different figures, the same references designate the same elements, but, for reasons of clarity, the dimensions and proportions of the various elements are not observed.

FIG. 1 represents a perspective view of an indirect heating cathode with potted filament.

This cathode consists of an emissive disc 1 which occupies one of the ends of a cylinder 2 made of non-emissive material.

The cylinder 2 is generally made of molybdenum. It serves as a housing for the cathode and is also called the "skirt" of the cathode.

At one end of the cylinder, a porous entungsten disc 1 is brazed. Brazing takes place around 1900 ° C.

Once this brazing is completed, a filament 3 made of tungsten or tungsten is introduced into the cylinder. rhenium covered by cataphoresis with an alumina isolation layer. We carry out the "potting" 4 which blocks the filament 3 in the cylinder.

Then carried out hot, around 1750 ° C., the impregnation with barium aluminate and calcium of the porous tungsten disc 1, which makes this disc emissive.

The impregnation of the porous tungsten being done at around 1750 ° C. while the filament 3 is in the cylinder 2, there is no disadvantage in that for the preparation of the "potting", the temperature reaches 1700 or 1800 ° C as is the case for the preparation of the "potting" according to the invention.

According to the invention, the "potting" consists of a mixture of alumina and of less than 10% by weight of an oxide of one of the elements of column III A of the periodic table of the elements, this mixture being sintered between 1700 and 1800 ° C.

Column III A of the Periodic Table of the Elements has four elements: the. scandium, Sc, yttrium, Y, lanthanum, La, and finally actinium, Ac.

We will first take yttrium as an example.

FIG. 2 shows a detail of the phase diagram of the alumina-yttrium oxide mixture, which is taken from the work: "Phase diagrams for Ceramists - 1969 - supplement".

The thick line curve indicates the melting tempera- _ture of the mixture of alumina Al ₂ O _3, yttrium oxide Y ₂ 0 ₃ based on the percentages by weight of alumina and yttria.

This curve is discontinuous. For certain percentages of alumina and yttrium oxide, melting occurs at a lower temperature than for the other percentages, they are eutectic compositions.

In FIG. 2, it can be seen that for point A, which corresponds substantially to 60% of alumina and 40% of yttrium oxide, the melting temperature is 1760 ° C. Around point A, it can be seen that the melting temperature is higher than 1760 ° C. Similarly, it is remarkable when observing FIG. 2 that from a percentage greater than about 43% of aluminum, the mixture obtained in the solid state has the same chemical constitution which is 3 Y ₂ O ₃ . 5 Al ₂ O ₃ , plus alumina in the α phase.

The fact that the alumina-yttrium oxide mixture is a eutectic is interesting because it makes it possible to reduce the temperature of the sintering compared to pure alumina. Likewise, it is advantageous that the chemical composition of the solid product obtained is the same over a wide range of the respective percentages of alumina and yttrium oxide. Indeed, it is difficult when making a mixture of powders - here alumina powder and yttrium oxide powder - to ensure that the mixture is perfectly well produced and that the percentage of bodies present is constant. Consequently, it is advantageous to obtain the same chemical compound even if the mixture is not perfectly homogeneous.

We therefore wish to benefit from the advantages of the alumina-yttrium oxide mixture compared to pure alumina, while seeking to have a mixture comprising the maximum of alumina.

Indeed, if the mixture contains too much yttrium oxide, there is a risk of detachment of the "potting" from the cylinder which serves as a housing and detachment of the filament from its "potting".

Experimentally, we have determined that we get potting was born bringing together the maximum of desired advantages by sintering between 1700 and 1800 ° C. and limiting the percentage of yttrium oxide to around 10% by weight.

It is however understood that a good quality "potting" is obtained by sintering between 1700 and 1800 ° C. a mixture comprising approximately 50 to 99% of alumina and therefore approximately 1 to 50% of yttrium oxide.

In all cases, a mixture of yttrium oxide and alumina with chemical composition 3 Y ₂ O ₃ is obtained. 5 Al ₂ O ₃ , - plus, alumina in the α phase.

In Figure 2, only the interesting part of the alumina-yttrium oxide phase diagram has been shown. Indeed, on the rest of the diagram, the percentage of alumina is low (less than 40%) and the temperature of fusion and therefore the sintering temperature are too high.

The alumina which enters into the composition of the "potting" can consist of several varieties of alumina of different particle size.

Thus, for example, grains less than 10 μm in diameter and gains of 10 to 50 μm in diameter are used.

The mixture of several varieties of alumina makes it possible to find a compromise between the defects and the qualities inherent in each variety. Indeed, fine-grained alumina takes on mass easily but has a significant shrinkage. While alumina with large diameter grains is more difficult to take in mass but constitutes a porous mass without shrinking.

As has just been described in detail for yttrium, the "potting" can be carried out by sintering between 1700 and 1800 ° C. a mixture of alumina and less than 10% of a scandium oxide, lanthanum or actinium. The chemical composition of the bodies obtained will be, in the case of lanthanum oxide, La ₂ O ₃ . 11 Al ₂ O ₃ plus alumina in the α phase and in the case of scandium oxide, Sc ₂ O ₃ . Al ₂ 03, plus alumina in the α phase.

It may be pointed out that the insulation layer deposited by cataphoresis on the filament 3 may be an alumina layer as has been indicated in the description of FIG. 1, but this insulation layer may also have the same composition as the mixture which is used for potting, for example alumina and yttrium oxide.

In Figures 3a, b and c, there is shown. a method of manufacturing a heating element according to the invention.

According to this method, firstly intimately mixing, with stirring for at least 24 hours, powder of an oxide of one of the elements of column III A of the periodic table of the elements and one or more alumina powders different particle size. The alumina powder should not exceed 10% by weight of the mixture.

A binder is then added to the mixture so as to obtain a paste.

The face of the emissive disc 1 which is situated towards the inside of the cylinder 2 is coated with this paste 5. This step is shown in FIG. 3a. Then, the binder is slowly evaporated using an electric lamp, for example 100 W, or leaving to dry naturally.

The filament 3 is introduced into the cylinder 2. This step is shown in FIG. 3b.

The cylinder is then filled several times with the paste, the consistency of which can be modified by adding the binder. Whenever we have added paste to the cylinder, the binder, which may be acetone, is evaporated using the electric lamp.

Finally, sintered under hydrogen between 1700 and 1800 ° C for about half an hour so as to obtain the "potting" 4.

By evaporating the binder slowly and as a layer of paste is added to the cylinder, the formation of bubbles is prevented due to rapid evaporation of the binder from all of the paste filling the cylinder.

Claims (9)

1. Heating element for indirect heating cathode consisting of an emissive disc (1) which occupies one end of a cylinder (2) made of non-emissive material, this heating element consisting of a filament (3) located inside the cylinder and a sintered mixture of alumina and oxide, which fills the space left free by the filament inside the cylinder, characterized in that this mixture (4) is consisting of alumina and of less than 10% by weight of an oxide of one of the elements of column III A of the periodic table of the elements, this mixture being sintered between 1700 and 1800 ° C. 2. Heating element according to claim 1, characterized in that this mixture (4) consists of yttrium oxide and alumina of chemical composition, 3 Y ₂ O ₃ . Al ₂ O ₃ plus alumina in the α phase. 3. Heating element according to claim 1, characterized in that this mixture (4) consists of lanthanum oxide and alumina of chemical composition, La ₂ O ₃ . 11 Al ₂ O ₃ plus alumina in the α phase. 4. Heating element according to claim 1, characterized in that this mixture (4) consists of scandium oxide and alumina of chemical composition, Sc ₂ O ₃ . Al2 03 plus α-phase alumina 5. Heating element according to one of claims 1 to 4, characterized in that this mixture (4) comprises several varieties of alumina of different particle size. 6. Heating element according to claim 5, characterized in that one of the varieties of alumina 'has grains of less than 10 µm in diameter while the other variety has grains of 10 to 50 µm in diameter. 7. Heating element according to one of the resale cations 1 to 6, characterized in that the filament (3) is made of tungsten or tungsten rhenium covered by cataphoresis with an insulating layer of alumina or constituted by the mixture of alumina and oxide which constitutes the remainder of the heating element. 8. Method for manufacturing a heating element according to one of claims 1 to 7, characterized in that it comprises the following steps: a) - intimately mixing by stirring for at least 24 hours, the powder of an oxide of one of the elements of column III A of the periodic table of the elements and one or more alumina powders of different particle size, the oxide powder not exceeding 10% by weight of the mixture; b) - a binder is added to the mixture so as to obtain a paste c) - the surface of the emissive disc (1) located towards the inside of the cylinder (2) made of non-emissive material is coated with this paste, then the binder is slowly evaporated; d) - the filament (3) is introduced into the cylinder; e) - the cylinder (2) is filled several times with the paste, the consistency of which can be modified by adding the binder, by evaporating the binder each time; f) - sintered under hydrogen, between 1700 and 1800 ° C. 9. Cathode with indirect heating, characterized in that it comprises a heating element according to one of claims 1 to 7 and in that it consists of a cylinder (2) of molybdenum brazed to one of its ends on an emissive disc (1) made of porous tungsten hot impregnated with barium aluminate and calcium.

Images