EP1459347B1 - Vacuum tube and method of manufacturing thereof - Google Patents

Abstract

The invention concerns vacuum tubes, and in particular electronic tubes. The invention concerns a method for making a tube, in particular an electron collector with several electrodes (30, 40, 50, 60), which consists in producing electrodes in the form of ceramic blocks with high thermal conductivity. The blocks have (at least) electrically conductive surfaces. They are preferably made of insulating ceramic such as aluminium nitride, and are made conductive over part of their surface. The conductive surface portion preferably consists of a conductive ceramic, preferably based on titanium nitride, or similar conductive ceramic materials. Thus robustness, heat discharge capacity and weight are improved.

Description

The invention relates to a method for manufacturing electrodes, designed mainly for producing vacuum electronic tubes, and in particular tubes using an electron collector; the collector collects the electrons after using a fraction of the energy that has been communicated to them by an acceleration electric field created in the vacuum tube. This manufacturing method is usable for producing other electrodes than electronic tube collectors. More generally it is applicable to the production of electrodes of all kinds of total or partial vacuum devices involving in this vacuum a physical transport of charged elementary particles (electrons but also ions) which can be accelerated, slowed down or collected by electrodes of the device. The method will however be described in connection with the most interesting application which is the embodiment of the collector of a linear beam vacuum tube.

Examples of linear beam tubes include mono or multibeam klystrons, traveling wave tubes, carcinotrons, inductive output tubes (IOTs).

These tubes generally work by interacting, in an area called interaction zone, an electromagnetic wave and a linear electron beam, the beam communicating a portion of its kinetic energy to the electromagnetic wave to amplify it. Generally, the beam still retains a portion of its kinetic energy after passing through the interaction zone and the residual electrons must be collected in a collector placed at the exit of the interaction zone. Sometimes, the kinetic energy remaining downstream of the interaction zone can reach 50% to 80% of the energy initially transmitted to the beam upstream of this zone. This results in strong constraints for the construction of the collector, in terms of heat dissipation; this results in strong constraints for the construction of the collector, in terms of heat dissipation; there are also other constraints, such as voltage withstand, etc.

In the past, the collector consisted of a simple metal electrode, usually copper, brought to a suitable potential (most often that of the anode which was used to accelerate the electrons). However, to increase the efficiency of the tubes, more sophisticated collectors, called mono or multi-stage depressed collectors, consisting of several successive electrodes carried at different potentials and thus electrically insulated from each other, have been made.

The realization of these collector electrodes poses different problems, all the more difficult to solve that the tubes must be both more and more powerful and more and more compact. Among these problems, there is the problem of the evacuation of the heat produced by the impact of the electrons on an electrode as well as the problem of secondary electronic re-emission of the electrodes; there is the problem of the electrical insulation of the electrodes with each other and with respect to the outside; there is the problem of vacuum tightness of the tube, with the joint problem of the crossing of electrical connections from the inside to the outside of the tube to bring a current or voltage of an inner electrode to the outside from the tube or from the outside to an inner electrode of the tube, and this for each of the electrodes of the collector, as well as for the other electrodes of the tube (anode, cathode).

The solutions adopted for producing these depressed collectors most often use brazed or shrink-wrapped copper electrodes on insulating ceramic parts; the ceramic provides electrical insulation between electrodes carried at different potentials and in the case of a collector with internal electrical insulation, it must also ensure the transfer of heat flow. Brazing provides mechanical strength and vacuum tightness. These assemblies are complex and expensive. Their heterogeneous structure made of ceramics and metal makes them particularly sensitive to thermomechanical stresses and vibrations. Their limited performance in particular as regards the efficiency of the heat dissipation, the admissible operating voltages, the compactness, sometimes also the weight (for applications of space tubes for example).

The problems are particularly delicate for the realization of a multi-stage collector, but it will be understood that they may also exist for insulated electrodes for which it is also necessary to provide on the one hand an electrical insulation relative to the rest of the tube, on the other hand a supply voltage or current, and finally an evacuation of the heat produced.

Note that it is suggested in the patent US 4,277,721 , to produce electrodes whose surfaces have characteristics that minimize secondary emission through coatings of materials such as pyrolytic graphite, titanium carbide, tungsten carbide, titanium diboride. The thermal conductivity of these materials is not mentioned, and they are in principle deposited on electrodes that are conventionally made of copper.

JP-61214327 describes a vacuum tube according to claim 1.

The present invention is intended to achieve an improved construction tube in terms of the ratio between the performance obtained and the manufacturing cost.

For this, the invention proposes firstly a vacuum tube according to claim 1 and secondly a method of manufacturing such a tube according to claim 8.

The particularity of these electrodes of the vacuum tube according to the invention is that they are made in the form of ceramic blocks and not metal blocks.

Ceramics are refractory mineral compounds such as metal oxides, metal nitrides, metal carbides, treated by sintering, that is to say by agglomeration under high temperature (and possibly under pressure) of a powder of the compound or of a paste of the compound (the paste being a powder mixed with an organic binder, the latter disappearing during the agglomeration operation). Some ceramics are electrically insulating, others are conductive, depending on the nature of the mineral compounds that compose it. In a mixture of insulating compounds and electrically conductive compounds, ceramics can also have intermediate conductivities.

The electrode is thus made in this case in the form of a composite ceramic block (with two different compositions, one conductive the other insulating but both ceramic) and not in the form of a brazing of an electrode metal (copper) on an insulating ceramic (alumina) as could be done in the prior art.

The assembly is all the more advantageous as the ceramic (and in particular that which composes the insulating block) has a high thermal conductivity. Ceramic with a high thermal conductivity means a ceramic with a thermal conductivity of at least 100 watts / m ° K at 20 ° C, which is about a quarter of the conductivity of copper, but about three times at least the conductivity of the alumina.

The electrode thus made of ceramic may participate directly in the vacuum seal of the tube if it directly constitutes a part of the wall of the tube casing. But it can also be cofrittée with another insulating ceramic constituting (partially or completely) the sealed envelope of the tube. For example, the electrodes are ceramic blocks (at least superficially electrically conductive) inserted in an insulating ceramic sheath and co-sealed with this sheath. The sleeve then constitutes the vacuum-tight envelope of the tube.

To make the connection of the electrode towards the outside of the tube, a conductive ceramic pin will also preferably be used; this pin is in contact on one side with a conductive ceramic portion of the electrode, inside the tube; and it passes through an insulating ceramic forming part of the electrode and / or the jacket of the tube, and co-sealed with this insulating ceramic.

The vacuum tightness of these conductive vias is excellent because on the one hand the bond obtained by high temperature heat treatment is strong and on the other hand the alloy materials exhibit similar thermomechanical behavior. This is particularly true when the crossing is made by coiling ceramics. The bushings can, however, also be made of refractory metal co-sealed with the ceramic during sintering thereof.

Thanks to this electrode embodiment in the form of a ceramic block which is highly heat-conductive, it is possible to adopt tube arrangements which are particularly effective from the point of view of the heat dissipation, the insulation between the electrodes, the compactness of the tube, its weight, provisions that could not be adopted with conventional metal electrodes or with conventionally soldered ceramic / metal assemblies.

The conductive ceramic made in a thin layer may in particular be silicon carbide, or titanium carbide, or tungsten carbide, or titanium nitride, or a mixture of two or more of these materials. It may also include additions of compounds facilitating sintering, such as for example yttrium oxide, the presence of additions being conventional in the sintering of ceramics.

The ceramic used to make an insulating ceramic block forming part of the electrode or the jacket of the tube is preferably essentially based on aluminum nitride, which has very good thermal conduction properties (about 180 watts / mK at 20 ° C) and very good dielectric strength (withstand at electric fields of at least 20 kV / mm). It can be made of almost pure aluminum nitride or composite ceramic comprising aluminum nitride co-cured with carbide silicon or titanium nitride in low proportion in the aluminum nitride. Sinter additions can also be present.

The invention is applicable to vacuum tubes (total or partial vacuum). The main application is the application to electron tubes, that is to say tubes in which the charged particles that are transported are electrons (and in this case the vacuum is usually very high). Another possible application is a device (also referred to as the "tube" for simplification) in which the particles transported are not electrons but ions. For example, the invention can be applied to the production of acceleration electrodes of an ionic propellant; an ion thruster is a motor intended to act to move an object in a vacuum (for a satellite or a spaceship); it produces continuously, when operating, a plasma of charged ions which are accelerated under partial vacuum by an electric field (thanks to electrodes) and ejected through a nozzle. The ejection acts as a conventional jet propellant, with the difference that the ejected material is ionic (charged) and is ejected by an acceleration by an electric field acting directly on the ions because of their charge. In this patent application, and in particular in the claims, it will be considered that the term "tube" includes all electrode devices using the transport of charged particles in a total vacuum (that is to say very high ) or partial (less advanced), whether the tube is closed or partially open (as in the case of a thruster).

• les figures 1 et 2 représentent des constructions de collecteur de tubes à vide de l'art antérieur ;
• la figure 3 représente en coupe un collecteur de tube selon l'invention ;
• les figures 4 à 7 représentent les différentes électrodes de céramique séparées ;
• la figure 8 représente le fourreau de céramique destiné à enfermer les différentes électrodes ;
• la figure 9 représente en perspective partiellement coupée le collecteur monté.

Other features and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which:

• the Figures 1 and 2 represent vacuum tube collector constructions of the prior art;
• the figure 3 represents in section a tube collector according to the invention;
• the Figures 4 to 7 represent the different ceramic electrodes separated;
• the figure 8 represents the ceramic sheath for enclosing the different electrodes;
• the figure 9 represents in perspective partially cut the mounted collector.

The invention will be described with regard to the embodiment of the collector of a multi-stage depressed collector traveling wave tube, but it is applicable in many other cases: other vacuum electronic tubes than a TOP, non-depressed collector a single electrode, other electrodes than collector electrodes. But it is particularly interesting in the case of a multi-stage depressed collector and this is why this example was chosen to be described in detail. Similarly, with regard to the manufacturing method according to the invention, which will be described about the same traveling wave tube, it will be understood that it is applicable to the realization of a TOP collector as to the realization of other tube electrodes, with the overall meaning given above for the word tube.

It is recalled that a traveling wave tube (TOP in French abbreviation, TWT in English abbreviation) is a vacuum tube comprising a cathode emitting a linear electron beam (focused by permanent magnets), and, successively from upstream downstream in the direction of travel of the electrons: an anode of acceleration of these electrons; a radiofrequency signal input receiving a radio frequency signal to be amplified, this input being connected to the input of a slowing structure which is for example a helix surrounding the electron beam; an output of the deceleration structure, constituting the output of the TOP, providing a radiofrequency signal; and a collector for collecting the electrons from the beam downstream of the deceleration structure. These electrons have lost some of their energy by communicating it to the radiofrequency wave, in the interaction zone situated between the upstream part and the downstream part of the helix. The collector that receives the electrons is subjected to intense heating due to the impact energy of the electrons and this heating is one of the main causes of difficulties for the realization of the tube.

The collector is typically a multi-stage depressed collector, i.e., several electrodes with different potentials and insulated from each other by electrically insulating parts. The potentials are chosen so that the electrons have a certain energy if possible reach the electrode which is at a potential corresponding substantially to this energy. In this way we obtain a good performance of the tube, but this requires providing a connection of several electrodes to the outside of the tube.

All the elements that have just been described are enclosed inside a sealed envelope in which is made a high vacuum. The envelope comprises insulating parts and possibly also conductive parts. Some of the elements described above, electrodes or insulators between electrodes can themselves be part of the sealed envelope and thus ensure themselves a vacuum seal. Welds or solders between elements, for example between a metal electrode and an insulating ceramic, also participate in this seal. Finally, when an electrode is located completely inside the envelope (that is to say, it does not constitute a part of the outer envelope and is therefore not accessible directly from the outside ), it is generally necessary, to connect it to the outside, to provide a conductive crossing, through an insulating portion of the casing, to connect the casing to an outer pin.

The figure 1 represents an embodiment of a multi-stage depressed sink with internal isolation of the prior art, which will better understand the differences brought by the invention in the overall construction of the collector.

The collector, of generally cylindrical shape, comprises in this example three massive copper electrodes E1, E2, E3 having conical shapes whose apex, open for the first two electrodes and closed for the last, is turned towards the arrival side. electrons (left on the figure 1 ). Electrodes E1 and E2 also comprise a cylindrical portion sandwiched by insulating ceramic bars (or plates) 10, themselves enclosed in an outer metal shell ENV constituting both an electromagnetic protective cover and a vacuum-tight envelope. The insulating ceramic is generally alumina for low powers to dissipate and beryllium oxide BeO at higher powers. The collector is closed on the right by an assembly of insulating parts and conductive parts brazed together, also performing vacuum sealing. Conductive vias are provided to connect the electrodes E1, E2, E3 to the outside. These bushings comprise a conductor 12, 13 or 14 surrounded by insulating ceramic 16, 17 or 18. In the example of the figure 1 , the ceramic bars 10 surrounding the electrodes E1 and E2 also serve to pass a conductor of the electrode E1 to the bottom of the collector, to the conductive passage 12, isolating the conductor of the electrode E2 and the outer envelope ENV.

The figure 2 represents another example of a TOP collector embodiment in which the electrodes are less massive than on the figure 1 : they are thin copper revolution cores brazed over their entire cylindrical periphery inside a ceramic sleeve 20; the holding of this structure to thermal stresses is possible only if the thinness of the electrodes makes it possible to accommodate the differential expansions without excessive stress. The ceramic sleeve is again surrounded by another metal sleeve 22 serving as an electromagnetic protection cover. Vacuum tightness is achieved by both metal parts and insulating ceramic parts. On the figure 2 it can be seen that conductive vias 24 may be provided radially through the insulating sleeves for the connection of the electrode E1 with the outside of the vacuum tube. A metal conductor such as brazed nickel is used on the internal electrode E1. Vacuum tightness is provided by brazing on the ceramic sleeve. For the connection with the electrode E2, the electrode itself has been taken out of the bottom of the tube to the outside, and this electrode E2 therefore itself participates directly in the vacuum seal. For the E3 electrode, a complex assembly of metal, insulating ceramic and conducting bushing must be provided to ensure the connection with the outside by the bottom of the tube.

In all cases, we see in these figures the complexity of the assembly that allows to hold the mechanical stresses, electrical stresses and thermal stresses.

The figure 3 represents the general principle of tube construction according to the invention with a collector whose particularity is that at least some of the electrodes (but preferably all) are made mainly of ceramic: they each consist of a ceramic block (similar to copper blocks from the figure 1 ); this ceramic is at least superficially conductive (to perform the function of electrode collecting electrons); this ceramic has very good thermal conduction properties to evacuate the heat generated by the impact of electrons.

Preferably, each electrode consists of a thin layer of conductive ceramic sintered on the surface of an insulating ceramic. In this case, it is the electrically insulating ceramic which must have very good thermal conduction properties.

The preferred construction of the collector is as follows: the ceramic blocks constituting the different electrodes are placed in contact with the inner periphery of an insulating ceramic sheath.

Conductive vias are preferably provided in this sleeve to provide the electrical connection between the outside of the tube and the conductive portion of at least some of the ceramic electrodes.

The electrodes, the insulating sheath and the conductive vias are preferably made integral in a single heat treatment operation (cofritting) or else several successive heat treatments which ensure a strong bond and therefore a seal of the inside of the vacuum tube. .

On the figure 3 , there is shown a collector with four electrodes which are respectively, following the direction of displacement of the electrons, a first electrode 30, a second electrode 40, a third electrode 50, and a final electrode 60. The first three electrodes are pierced axially at their center to let the electron beam through, with apertures (respectively 31, 41, 51) becoming larger to take account of the increasing divergence of the beam downstream. The final electrode 60 is not pierced.

The electrodes are made of electrically conductive ceramic in certain areas, in area, and electrically insulating in the mass. The ceramic may be conductive over its entire surface or only in areas drawn in a pattern that depends of course on the general design of the tube, the rest of the electrode being constituted by an insulating ceramic block.

The four electrodes are preferably mounted in a cylindrical sleeve 70 of electrically insulating ceramic and strongly conductor of heat. This cylindrical sheath 70 constitutes the outer casing of the tube and it is preferably provided with radial fins 80 facilitating the evacuation of heat generated in operation from inside the tube. This sleeve 70 may, like the electrodes 30, 40, 50, 60, have a locally conductive surface, both inside and outside the tube. In practice, it will be seen that the sheath may constitute an electrode with the same potential (its inner surface only) as the electrode 50.

The bottom of the tube, on the right of the figure 3 , can be completely constituted by the mass of the final electrode 60, especially if it is conductive only in its inner surface portion to the tube.

On the figure 3 the conductive areas of each electrode have not been detailed. However, to illustrate the principle of the invention, there is shown by a dashed line 90, along the inner wall of the sleeve 70 and along a portion of the electrode 50, a surface area which is conductive.

The electrical connection of the different electrodes with the outside, to ensure the passage of currents or bias voltages, is carried out in the following manner: for the electrode 30, a radial conductive passage is provided through the cylindrical insulating sleeve 70. The bushing comprises a conductive rod 32 which passes through a bore in the electrode 30 and a corresponding bore in the sleeve 70. The conductive rod 32 is preferably made of conducting ceramic, but it could also be made of refractory conductive metal such as tungsten. It comes into contact, inside the tube with a conductive zone of the first electrode 30.

For the second electrode 40, the assembly is quite similar, with a radial conductive feedthrough comprising a conductive rod 42.

As regards the third electrode 50, it would also have been possible to provide a conductive passage, but it has been provided in this example that the inner surface of the sleeve 70 is rendered conductive in the same manner as the conductive surface of the electrodes, that is, that is, preferably by cofiring a conductive ceramic on an insulating ceramic. The conductive zone is represented by the dashed line 90 already mentioned. Conductive electrical continuity can thus be established from the electrode 50 to the outside of the tube, as shown in the line dashed 90 which starts from the electrode 50 and which goes beyond the electrode 60. The conductive portion outside the tube can then constitute an external connection of the third electrode 50. For this reason, it can be considered that the sheath itself constitutes an electrode, at the same potential as the electrode 50.

The connection of the final electrode 60 with the outside can also be done by the bottom of the tube, or by direct contact with the ceramic if its outer face is conductive and in electrical conduction contact with its inner face to the tube or if it is entirely made of conductive ceramic, or by a conductive passage, with a rod 62, from the inner face of the electrode to the outside of the tube if only the ceramic surface inside the tube is conductive. The passage passes in this case through the insulating ceramic block constituting the electrode 60 and not through the sleeve 70. It extends axially and not radially.

The entire collector is then formed of ceramics, some parts being electrically insulating ceramic but very good thermal conductivity, and other parts being electrically conductive ceramic and connected to conductive rods passing through the insulating ceramic. A collector block is thus obtained whose parts have homogeneous thermomechanical properties.

It is advantageous to achieve the entire collector by cofritting ceramics, that is to say by mounting the electrodes and the sleeve in place relative to each other while these parts are still in the state of raw ceramics and sintering for all ceramics at a time. However, it is also possible to strongly associate the electrodes to the body by successive heat treatments or also to partially subcompact some subassemblies and then to associate the subassemblies together with or without further sintering operation.

The figure 4 represents in isolation the first electrode 30. In this embodiment, the electrode is superficially conductive over most of its surface, but not at its periphery. It will also be in contact at its periphery with the sheath 70. The electrode is made by machining a raw paste of insulating ceramic.

The machined electrode is coated with a thin layer of green conductive ceramics 35 represented by a dashed line. The delimitation of the conductive zone can be done either by masking the areas that must remain insulating, or by selective removal after uniform deposition on all surfaces. The electrode 30 may be sintered before insertion into the sleeve 70, or inserted first into the sleeve 70 and then co-sealed with the sleeve. If it is sintered at the same time as the sleeve, the mechanical connection between the electrode and the sleeve will be all the more resistant and the thermal conductivity improved. The conductive bushing for connecting the electrode to the sheath is made by providing a radial bore 36 into which the conductive rod 32 visible to the figure 3 ; this rod will preferably be placed in the bore before common sintering of the electrode and the sheath. It is in contact on one side with the conductive ceramic layer 35. The sintering ensures the adhesion of the surface conductive ceramic 35 with the insulating ceramic which forms the body of the electrode 30.

The figure 5 represents the second electrode 40 taken alone. It is constituted in principle in the same way as the first one, namely by sintering a green electrically insulating ceramic body having the shape of the desired electrode, partially coated with a thin layer of raw conducting ceramic 45. A drilling 46 serves to pass a rod 42 for the establishment of the conductive bushing.

The figure 6 represents the third electrode 50 taken alone, constituting like the others with a local surface layer 55 of conductive ceramic, but no drilling in the case where there is no conductive bushing for its connection to the outside.

The figure 7 represents the fourth electrode 60 with its local surface conductive ceramic layer 65, and its bore 66 for a conductive bushing.

The figure 8 represents the cylindrical sleeve 70 taken alone, with its radial fins 80. There are bores 72 and 73 in the sleeve, which are opposite holes 36 and 46 of the first and the second electrode 30 and 40 when they are mounted in the sheath, to allow passage to the conductive rods 32 and 42. Indeed, the conductive rods pass through in this case not only the thickness of the blocks of insulating ceramic which constitute the electrodes 30 and 40, but also the thickness of the sleeve 70. There are no fins at the holes 72 and 73 so that the conductive rods that will be placed in the holes are accessible. The holes 72, 73, 36 and 46 serve at the same time to ensure the correct positioning of the ceramic electrodes in the sheath 70.

The various constituent elements of the collector (electrodes, sheath) can be made with the aid of conventional ceramic techniques. Preferably, the sleeve 70 with its fins 80 is preferably made, because it is cylindrical, by extrusion of a ceramic paste raw. The fins may have a grooved surface (grooves also made during extrusion) to improve heat dissipation. The shape of the sheath can be completed by other machining operations and drilling of the raw ceramic paste.

The electrodes are preferably made by extrusion and then machining these blocks to give them the desired shape (conical with an opening at the top and recesses facilitating their introduction into the sleeve). The blocks of electrically insulating raw ceramic are coated with a slurry of electrically conductive ceramic raw. Alternatively, they could be coated with a conductive ink based on refractory metal (especially tungsten).

In the figures, there are shown electrodes whose conductive parts are symmetrical of revolution. However, any conductive zone pattern can be provided without particular difficulty, while machining metal blocks into unsymmetrical shapes was much more problematic in the prior art.

This arrangement makes it possible to limit the reflected electrons by creating an asymmetry of the electric field applied by the electrode thus formed while maintaining an axisymmetric electrode easy to manufacture.

The raw composite ceramic blocks coated with an electrically conductive layer are inserted into the sleeve, the rods of the conducting bushings are put in place, a conductive paste (ceramic or conductive tungsten ink) may be deposited, for example with a brush, on the ends of these rods to facilitate the electrical connection between these rods and the conductive surfaces of the electrodes.

Similarly, a conductive tungsten ink or a conductive ceramic paste may be deposited inside the sheath, with a brush and / or by dipping and / or by spraying or spraying, to produce the conductive surface represented by line 90 of the figure 3 . A conductive film may also be deposited outside the sleeve (without establishing an electrical connection with the inner surfaces of the tube), to ensure the electromagnetic shielding of the collector.

The last electrode 60, which forms the bottom of the tube, is put in place, with its conducting rod 62, after these operations.

The set of electrodes, the sheath, and the conductive rods is cofired to result in the desired collector structure.

The figure 9 represents, in partially open view, the collector block thus produced. In its preferred version, all the electrodes and the sleeve are at least superficially conductive ceramic.

The preferred ceramic for all insulating parts is preferably based on aluminum nitride AlN (up to 100%). The thermal conductivity of aluminum nitride is about 180 watts / mK. Aluminum nitride can be mixed in a small proportion of silicon carbide SiC or titanium nitride TiN. Sintering additions in a small proportion (less than 10%) can be included in the raw ceramic paste to facilitate sintering or co-sintering with other ceramics.

For the electrically conductive parts of the electrode, the ceramic is preferably of titanium nitride TiN, but may also be titanium carbide TiC, tungsten carbide WC, silicon carbide SiC. These materials can be mixed with aluminum nitride. In the case where the conductive surface portions are metallic, the metal is preferably tungsten or molybdenum. Again sintering additions are advantageously provided, especially to facilitate the cofritting with aluminum nitride.

The particle size of the powders used to make the ceramics makes it possible to modify the texture of the conductive surface of the electrode, a controlled particle size of the order of one micrometer (0.5 to 2 micrometers) resulting in the formation of superficial microcavities tending to limit the secondary emission of electrons when the electrode is bombarded by electrons.

The conductive rods constituting the bushings in the insulating ceramics may be titanium nitride, titanium carbide, or silicon carbide, or a mixture of these materials. Again, sintering additions can be provided. The stems may also be tungsten or molybdenum.

The sintering additions may typically be yttrium oxide Y2O3, calcium oxide CaO, yttrium fluoride YF3, calcium fluoride CaF2.

Aluminum nitride (insulator) and titanium nitride (conductor) have similar characteristics, particularly in terms of densification kinetics during cofiring, resulting in a strong inorganic bond, of the ionocovalent type.

It is known that the raw ceramic undergoes significant shrinkage during sintering (of the order of 15 to 30%). This shrinkage is obviously taken into account in order to determine the machining of the raw ceramic pieces. This shrinkage will be used to ensure shrinking (radial inward tightening) of the sleeve on the electrodes during the sintering operation.

The electrode assemblies thus produced can withstand very high operating temperatures without causing degassing phenomena as on metal electrodes of the prior art.

With regard to the sheath, it may be noted that the invention can facilitate the cooling of the tube by fluid (and in particular a liquid such as an electrical insulating oil or deionized water) if the channels formed in the barrel body are mixed in the barrel body. during the extrusion of the sheath.

Indeed, in the prior art, the fluid had to have sufficient dielectric strength to accommodate the external faces of the electrodes and the outer envelope subjected to different voltages between them.

In the invention, the structure of the outer ceramic shell may advantageously be traversed in the longitudinal direction by capillaries in which a cooling fluid can be circulated. In addition to the proximity of the cooling fluid with respect to the internal surfaces to be cooled, this arrangement makes it possible to use a standard fluid such as water since the fluid is no longer directly in contact with the electrodes. The fluid is in direct contact with the envelope over the entire length thereof.

This new arrangement also makes it possible to avoid the occurrence of galvanic torque, of chemical corrosion. Aluminum nitride being particularly chemically inert.

Claims (11)

1. A vacuum tube, characterised in that it comprises at least one electrode (30, 40, 50, 60) produced in the form of an electrically insulating ceramic block with high thermal conductivity, characterised in that said block is cofired with a thin surface layer of electrically conductive ceramic on at least one portion of its surface.
2. The tube according to claim 1, characterised in that the ceramic block has a thermal conductivity of at least 100 watts/mK at 20°C.
3. The tube according to claim 2, characterised in that the insulating ceramic is mainly based on aluminium nitride.
4. The tube according to claim 3, characterised in that the surface conductive ceramic is based on titanium nitride or titanium carbide or silicon carbide or tungsten carbide or a mixture of two or more of these materials.
5. The tube according to any one of claims 1 to 4, characterised in that it comprises a rod (32) made from conductive ceramic so as to connect the electrode with the outside of the tube, one side of said rod being in contact with a conducting portion of the electrode inside the tube and passing through an insulating ceramic forming part of the electrode and/or an insulating envelope of the tube, and which is cofired with said insulating ceramic.
6. The tube according to any one of claims 1 to 5, characterised in that it comprises an external envelope made from electrically insulating ceramic and cooling means by circulation of liquid in direct contact with said envelope over the entire length of the envelope.
7. The tube according to any one of claims 1 to 6, characterised in that the electrode is an electron collector.
8. A process for producing a vacuum tube, characterised in that it comprises the production of at least one electrode for the tube from a composite ceramic, by cofiring an electrically conducting ceramic with an electrically insulating ceramic with high thermal conductivity.
9. The process according to claim 8, characterised in that the conducting ceramic is deposited as a thin layer on a portion of the surface of the insulating ceramic.
10. The process according to any one of claims 8 to 9, characterised in that the electrode is produced by depositing a thin layer of raw conductive ceramic on a raw compound of insulating ceramic.
11. The process according to any one of claims 8 to 10, characterised in that the electrode is an electron collector.

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