EP0441698B1 - Impregnated cathode manufacturing procedure and cathode obtained therewith - Google Patents

Description

The present invention relates to a method of manufacturing an impregnated cathode and a cathode obtained by this method. It finds an application in the production of cathodes for electronic tubes and more particularly but not exclusively for cathode ray display tubes.

Impregnated cathodes are commonly used to provide electronic current densities of up to 1 to 2 A / cm² continuously and more in pulses.

The impregnated cathodes known from the prior art consist of a porous body of refractory metal, such as pure tungsten, or else of a mixture of tungsten, either with a metal originating from the platinum mine (mixed matrix), such as known from document FR A 2 356 263, either with scandium oxide or other rare earths in low concentration (3 to 5% by weight).

This porous body is generally obtained by compressing a finely divided powder of the metal (or mixture of metals) using an isostatic press or a uniaxial press.

The compacts thus obtained are then heated under hydrogen at high temperature, in order to sinter the particles together and to increase the density of the porous body.

To facilitate the machining of the porous body, it is infiltrated with copper or plastic, then machined to the desired shape. Subsequently, the copper or plastic is removed by dissolving in an acid or by heating.

The porous body of the desired shape is then brazed on a molybdenum skirt which serves to hold, on one side, the emissive patch and, on the other, a filament potted in alumina which allows the heating of the cathode . Once the filament is in place the pores of the porous body can be filled with barium and calcium aluminates. In other words, the body is impregnated with these aluminates, which constitute the emissive material of the finished cathode.

For this operation, the porous body is kept in close contact with an aluminate composition brought, under a reducing atmosphere, to a temperature above its melting point. Contact is ensured, either by immersing the porous body in the aluminate, or by placing the aluminate on the porous body. At the time of fusion, the aluminate diffuses by capillarity or by flow inside the open pores and fills them. The cathode is then cleaned mechanically and chemically, in order to remove the aluminate residues which have remained stuck on the surfaces.

Finally, the cathode is activated, under vacuum, at a temperature at which tungsten reduces barium and calcium aluminate to release barium oxide. Metallic barium is produced in areas where the aluminate is in contact with the refractory metal (pores). The metallic barium reaches the end of the pores and diffuses over the entire emissive surface where it forms with oxygen a surface monolayer which promotes electronic emissivity by lowering the work of electron output.

Furthermore, the deposition, on the emissive surface of these impregnated cathodes, of a film of osmium, iridium, ruthenium, or an alloy of these bodies, this film having a thickness of a few thousand Angstroms , can improve emissivity by a factor of around 3.

The mixed matrix cathode, covered with a refractory metal film, is known from document FR 4 2 469 792 in the name of the applicant.

The performances obtained from the cathodes produced by the methods known from the prior art are satisfactory for most professional applications, because high current densities can be obtained during a lifetime which does not limit the lifetime of the equipment. in which the cathode, or the electronic tube comprising the cathode, will be installed.

However, the methods known from the prior art and briefly summarized above are long, complicated, and expensive because they include many stages, of different types and of execution critical for the quality of the finished product. These drawbacks make their cost prohibitive for consumer applications where the price must fall with the increase in the number of cathodes produced.

It is also known from patent WO89 / 09480 to manufacture impregnated cathodes by a single pressing step of a mixture of tungsten powders and aluminates.

The method according to the present invention aims precisely to remedy these drawbacks. To this end, the invention recommends an original method which provides the advantages of impregnated cathodes, which makes it possible to obtain cathodes of equal quality but with a procedure which is substantially simplified compared to those known from the prior art.

The process according to the invention has for characteristic those of claim 1.

According to the invention, the powder of tungsten or of a mixture of tungsten and of a metal of the platinum mine or of a scandium oxide or of the three materials is mixed with a powder of aluminate, barium and calcium in the desired stoichiometric proportions, then this mixture is pressed in the form of pellets and sintered, under a hydrogen atmosphere, at a temperature higher than the melting temperature of aluminates. This gives a gangue of consistency equal to the porous body, which can be handled, which is placed in a molybdenum or tantalum support by light pressing.

According to one characteristic of the invention, the mixture comprises tungsten or tungsten powder and other materials as above, with barium and calcium and alumina carbonates in the desired stoichiometric proportions. This mixture is then compressed and sintered at the same temperature than before. In this way, the aluminate is formed during "in situ" sintering.

According to another characteristic of the invention, the emissive surface of the pellet obtained according to the method of the invention is covered with a film of osmium, iridium or rhenium to increase its emissive properties.

Then, the filament is brought and potted in a conventional manner, and the cathode is activated in the same manner as above.

Thus, the method according to the invention makes it possible to obtain all the advantages known from the prior art of impregnated cathodes, whether they are of simple matrix (pure tungsten) or mixed, covered or not, but with simplified procedures, shorter and less expensive, with a significantly reduced number of steps compared to the prior art, which makes it possible to obtain an equal quality of finished product with less critical handling and therefore with fewer controls.

The method according to the invention is therefore particularly suitable for industrial production at high speed and at low cost of cathodes with high current density and with a relatively long service life, which makes it possible to envisage their use in equipment intended for wide distribution.

Specifically, the invention therefore relates to a method of manufacturing an impregnated cathode, characterized in that an emissive pellet is produced by co-pressing and sintering a mixture of at least one powder of refractory metal with a powder of barium and calcium aluminates, or with barium and calcium carbonates added with alumina.

The document EP-A-0 409 275 filed on July 20, 1990, claiming a Japanese priority of July 21, 1989 and published on January 23, 1991 designating France, Germany and the Netherlands describes a process for manufacturing a impregnated cathode in which an emissive pellet is produced by co-pressing and sintering under an argon atmosphere of a mixture of a refractory metal powder with barium and calcium carbonates added with alumina.

The subject of the invention is also an impregnated cathode as obtained by implementing the method which has just been defined.

In ancillary manner, the invention also relates to variants of impregnated cathodes which can be produced using the process which has just been defined; for example cathodes produced according to the method of the invention and then covered with a metal film of platinum mine or the like in order to increase its electronic emissivity or to lower the operating temperature while keeping the emissivity constant. The subject of the invention is also variants of impregnated cathodes which can be produced starting from the very principle of the process of the invention, for example of cathodes produced according to the process of the invention, but with, in addition to the mixture of the powder of a refractory metal and the aluminates or carbonates of barium and calcium, addition of the oxide of scandium or rare earths, Other variants of the process according to the invention could easily be imagined and implemented by a person skilled in the art, in order to reap the advantages obtained by the invention with particular advantages known by elsewhere, for specific applications.

• la figure 1 représente, de façon schématique, les étapes principales d'un procédé simplifié selon l'invention de fabrication d'une cathode imprégnée ;
• la figure 2 représente une application possible de ces cathodes en tant qu'émetteur pour tube à rayon cathodique.

In any case, the characteristics and advantages of the invention will appear better after the description which follows with its examples given by way of illustration and in no way limiting, and its attached drawings in which:

• Figure 1 shows, schematically, the main steps of a simplified method according to the invention for manufacturing an impregnated cathode;
• FIG. 2 represents a possible application of these cathodes as a transmitter for cathode ray tube.

In FIG. 1, we see an example of an impregnated cathode manufactured according to the method of the invention, illustrated in these main steps in this FIG. 1.

The emitting tablet (1) is formed by pressing (c) and sintering (d), in a conventional manner, of a mixture (b), of a powder (w) of at least one refractory metal with a powder (y) barium and calcium aluminate or barium and calcium carbonates with alumina.

At least one of the starting powders (w) is a powder of known elements such as tungsten, molybdenum, tantalum, rhenium or the alloys containing them, or a powder of an element capable of improving the electronic emission, such as osmium, ruthenium, iridium or alloys containing at least one of these elements or, finally, a powder of scandium oxide or particles of oxides containing scandium.

In Figure 1 (e) we see the emissive patch encapsulated in a cup, which will then be crimped (f) in a skirt (4) made of molybdenum or tantalum. All that remains is to add a tungsten-rhenium filament (5) covered with an insulating film (not shown) and to hold it in the skirt (4) by an "alumina potting" body (6), as seen in Figure 1 (g).

• les poudres à mélanger seront tamisées et de granulométrie de l'ordre de 5 à 10 microns. Elles seront ensuite mélangées dans des proportions stoechiométriques désirées pour obtenir les qualités requises de la cathode. Ces proportions appropriées seront déterminées par expérimentation pour une application donnée, mais pourrait être, par exemple : W = 80 %, Sc₂O₃ = 2 %, BaO = 12 %, CaO = 3 %, Al₂O₃ = 3 % ; ou bien la poudre de tungstène pourrait être remplacé par un mélange de poudres de tungstène et un autre métal, par exemple : W = 45 %, Os = 35 %.
• Les poudres mélangées sont pressées ensemble (c) dans une presse isostatique ou uniaxiale sous pression de l'ordre de 10 tonnes au cm², par exemple, pour former une pastille.
• la pastille est frittée (d) à haute température (de l'ordre de 2 000° C, par exemple) sous atmosphère d'hydrogène. La température choisie sera suffisante pour atteindre la température de fusion des aluminates contenus dans la pastille.
• la pastille émissive ainsi obtenue est ensuite montée mécaniquement sur une jupe (4) en Mo ou Ta, éventuellement à l'aide d'une coupelle (3) dans laquelle la pastille sera insérée par un léger pressage.

As an explanation, we can do it with the following parameters:

• the powders to be mixed will be sieved and with a particle size of the order of 5 to 10 microns. They will then be mixed in desired stoichiometric proportions to obtain the required qualities of the cathode. These appropriate proportions will be determined by experimentation for a given application, but could be, for example: W = 80%, Sc₂O₃ = 2%, BaO = 12%, CaO = 3%, Al₂O₃ = 3%; or the tungsten powder could be replaced by a mixture of tungsten powders and another metal, for example: W = 45%, Os = 35%.
• The mixed powders are pressed together (c) in an isostatic or uniaxial press under pressure of the order of 10 tonnes per cm², for example, to form a pellet.
• the pellet is sintered (d) at high temperature (of the order of 2000 ° C., for example) under a hydrogen atmosphere. The temperature chosen will be sufficient to reach the melting temperature of the aluminates contained in the tablet.
• the emissive patch thus obtained is then mechanically mounted on a skirt (4) in Mo or Ta, possibly using a cup (3) into which the patch will be inserted by light pressing.

The skirt (4) can be made integral with the assembly by crimping (f) on the cup (3).

Then the heating filament (5), previously covered with an alumina film (not shown), can be mounted in the skirt and held in place by an alumina body (6) commonly known by the English word "potting ". This "potting" operation can be done, for example, by sintering at 1800 ° C. under hydrogen from an alumina powder deposited using a suspension around the filament inside the skirt.

Optionally, the emissive patch could be covered with a thin metallic film with a thickness of between 10 and 30,000 Angstroms, for example, the metallic material being able to be selected from the group comprising osmium, ruthenium, iridium, and the alloys containing any of these. This film can be deposited by conventional means of sputtering, vacuum deposition, or any other suitable means.

In Figure 2, we can see schematically and in section a possible mounting of a cathode manufactured according to the method of the invention, for application as an electron emitter for cathode ray tube.

To the assembly of the impregnated cathode of FIG. 1 (g), for this application, it is only necessary to add a support (7) to hold the assembly at the desired location in the equipment. The cathode generally operating at high voltage in an electron gun this support (7) will probably be electrically insulating, in alumina or ceramic, for example.

The method according to the invention has the advantage, compared to the prior art, of being feasible with a significantly reduced number of steps, and with less critical handling for the quality of the product. This results in the possibility of a better production yield, simultaneously with an accelerated rate and at lower cost per part.

These cumulative advantages make it possible to envisage the use of these cathodes, of performances comparable to those previously intended only for professional applications because of their high price, for applications with wider diffusion and possibly for applications for the general public.

Claims (5)

1. Process for the manufacture of an impregnated cathode, characterized in that an emissive pellet (1) is produced by copressing and sintering a mixture of at least one powdered refractory metal (w) with a powder (y) of barium and calcium aluminates or of barium and calcium carbonates to which alumina has been added, under hydrogen atmosphere at a temperature which is sufficient to reach the melting temperature of the aluminates.
2. Process according to Claim 1, characterized in that the said mixture of at least one powdered refractory metal contains tungsten powder mixed with the powder of a metal of the platinum group.
3. Process according to either of Claims 1 and 2, with addition of powdered scandium oxide or rare-earth oxide in a low concentration of the order of 5 %.
4. Process according to any one of Claims 1 to 3, characterized in that the emissive pellet (1) is covered, after copressing and sintering, with a film of metal of the platinum group.
5. Impregnated cathode manufactured according to any one of Claims 1 to 4.

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