EP0081270A2 - Process for producing a thermionic cathode, and thermionic cathode manufactured by this process - Google Patents

Abstract

The cathode (4) - the material of which is essentially high-melting metal, such as W, Mo, Ta, Nb, Re and / or C - consists of a finely crystalline, mechanically stable support layer (5), one with an emissive substance, as a rule the scandium group, in particular from the group of rare earth metals, preferably with Th or its compounds, highly enriched layer sequence (6) and a thermally stable, preferred-oriented cover layer (7). All layers are over the gas phase, e.g. by CVD process, applied to a base (1) shaped according to the desired cathode geometry. The support (1) is removed after the deposition has ended.

Description

Process for producing a thermionic cathode and thermionic cathode produced by this process

The invention relates to a method for producing a thermionic cathode with a polycrystalline cover layer made of a high-melting metal, which is deposited on layers underneath.

The invention further relates to a thermionic cathode which has been produced by this method.

A refractory metal is a metal that melts at a high temperature, e.g. W, Mo, Ta, Nb, Re, Hf, Ir, Os; Pt, Rh, Ru, Th, Ti, V, Yb, Zr.

A method of the aforementioned type is known from DE-OS 14 39 890.

An overview of the most important types of thermionic monolayer cathodes and their mode of action is given in Vacuum 19 (1969) 353-359. The problems with high-performance cathodes for UHF tubes are discussed in DE-AS 24 15 384, especially with regard to the mesh cathodes previously used. The conclusion (not specified there) can be drawn from the last-mentioned design specification that cylindrical unipotential cathodes are the ideal cathodes for UHF tubes, provided the selected emitting system already meets the other boundary conditions when used in high-frequency tubes.

In order to avoid the problems regarding emission and stray impedance with the previously used thoriated mesh cathodes, have been specified in DE-OS 27 32 960 and later in DE-AS 28 38 020 directly heated unipotential cathodes for electron tubes with a coaxial structure of the electrodes, which consist of a hollow cylinder made of pyrolytic graphite and a thin metal layer as an emission layer. The thin metal layer should consist of tungsten carbide and thorium or thorium oxide. In one of the processes provided for the production, tungsten thorium is deposited from the gas phase on the hollow cylinder made of pyrolytic graphite. Layers of this type produced by chemical vapor deposition (CVD process) are also referred to below as “CVD layers”.

However, it has been found that thermionic cathodes with a support made of pyrolytic graphite and an electron-emitting body arranged thereon are problematic in three respects and are not very suitable for commercial use.

The main problem is the different coefficients of thermal expansion of the support and the emitting cathode part. For example, pyrolytic graphite has a linear thermal expansion coefficient of 10 ^-6 K ^-1 with respect to its layer structure in a direction referred to as the a direction. In contrast, in the c direction perpendicular to it, it is 20 to ₃₀ . 10 ^-6 k ^-1 , while for tungsten at 4.5. 10 ^-6 K ^-1 and for thorium at ₁₂ . 10 ^-6 K ^-1 lies. Given the strong temperature differences to which the cathodes are exposed during operation, this leads to a partial detachment of the emitting cathode part from the carrier. Also an adhesive layer between the support and the emitting cathode part, in which, for example, the coefficient of thermal expansion is an average of Coefficient of the substrate and the emitting cathode part, does not create a permanent connection at the usual operating temperatures of 2000 K.

The second disadvantage is the diffusion of carbon into the crystal structure of the emitting cathode part, against which there are no suitable diffusion barriers at an operating temperature of 2000 K. In the case of a cathode with a support made of pyrolytic graphite and an emitting cathode part made of thoriated tungsten, tungsten carbide (W ₂ C and WC) is formed, which in turn causes detachment via different expansion coefficients. Thirdly, thorium carbide (ThC) is formed, which is preferentially deposited along the grain boundaries of the tungsten crystals and clogs the diffusion paths of the thorium to the emitting surface. This interrupts the diffusion of the thorium to the surface, which is necessary for the continuous addition of the monoatomic thorium layer on the emitting surface, which greatly reduces the emission current density. The service life of the cathodes is therefore only short.

However, the low mechanical stability and columnar structure of the deposited CVD layers normally make it impossible to produce self-supporting cathodes without a support made of pyrolytic graphite.

Arbitrarily curved cathode surfaces, e.g. in the form of a cylindrical unipotential cathode can generally only be achieved with polycrystalline material. It is now known that in single-phase as well as monolayer cathodes, the electron work function depends on the surface before the type of the respective crystal plane. Different surface orientations result in very different electron emissions.

As a rule, the usual manufacturing processes, e.g. Powder metallurgy, cathodes made of polycrystalline surfaces with statistically oriented crystallites. Accordingly, a few crystallites or monolayer-covered crystallites with a correspondingly favorable orientation emit very strongly, but the far larger part of the crystallites hardly contributes to the emission.

The growth of crystallites with the orientation that e.g. has the lowest work function with a monolayer coverage, consequently leads to an immense increase in the emission current density.

From the already mentioned DE-OS 14 39 890 such cathodes with a preferred oriented polycrystalline surface and a method for their production are now known. "Preferred oriented" is intended to mean that almost all crystallite surfaces contribute to the emission and have such a crystal plane on the surface that the normal to this plane and the normal to the macroscopic cathode surface lie within a specified angle at this point. Some of the few ways to produce such a preferred polycrystalline surface is, according to the above publication, chemical deposition from the gas phase, with certain combinations of the deposition parameters, especially substrate temperature and flow rates of the gas mixture, being observed. A conventional cathode is generally used as the substrate, to which a polycrystalline layer is then additionally applied by means of the CVD method. This layer can either consist of a pure, high-melting metal such as W, Mo, Ta, Nb, Re, Hf, Ir, Os, Pt, Rh, Ru, Th, Ti, V, Yb, Zr or C. and should have a suitable preferred orientation or it can be a high emission substance, preferably a rare earth oxide, ZrC, ThC, UC ₂ , UN, LaB ₆ or N ^dB 6 _.

Particularly preferred in all of the exemplary embodiments is a polycrystalline tungsten cover layer on the cathode with the crystallographic <111> plane on the surface. The monoatomic emitter layer formed thereon by diffusion from the inside of the cathode or by adsorption from the vapor preferably consists of Th, Ba or Cs and, together with the preferred orientation, has a lower work function than that of the respective pure materials or monolayers on unoriented tungsten.

However, the cathodes produced in this way also have a number of disadvantages. An important disadvantage is, for example, that conventional cathodes must first be manufactured using the usual powder metallurgical methods, which are then additionally coated with the preferred oriented CVD layer, with a number of surface processing steps also having to be inserted to achieve the preferred orientation. The production of such cathodes is therefore very complex. Furthermore, the cathode shaping is very restricted due to the powder metallurgical production of the substrates. Thus, according to DE-OS 14 39 890, thoriated wires are coated with <111> -oriented tungsten, which in turn can be used to produce a mesh cathode, but the method does not allow the production of a cylindrical unipotential cathode made of thoriated tungsten, since the correspondingly shaped substrate cathode is not can be produced by powder metallurgy if it is also to be heated directly and effectively at the same time. A Another difficulty is that the recrystallization or crystal growth at longer operating times and normal operating temperatures (2000 K for Th- [W] cathodes) leads to an increasing destruction of the preferred orientation, whereby the emission naturally drops. Unfortunately, this happens much earlier than after the minimum cathode life of 10,000 hours required for UHF tubes (cf. I. Weissman, "Research on Thermionic Electron Emitting Systems" Varian Ass. Final Report (1966) Navy Department Bureau of Ships ( UNITED STATES)). In most of the cases, the preferred orientation is destroyed even in the activation phase of the cathode. In the case of CVD deposition of a surface layer from a rare earth metal oxide or from ZrC, ThC, UC or UN, it is a further disadvantage that the specific advantages of monolayer cathodes, in particular the higher emission, are not exploited. Instead, one obtains, for example, the significantly lower emission of oxide cathodes, the semiconducting oxide layer of which has the usual problems such as charge carrier depletion and lower resilience. When borides are applied, the problem arises that the contact layers (border areas) to the metal base generally pulverize. Realizations of the methods known from this laid-open specification therefore do not provide any more suitable cathodes for UHF tubes.

DE-AS 10 29 943 and 10 37 599 dispenser cathodes with porous sintered bodies are known, which are built up in layers such that layers with high-melting metal such as tungsten or molybdenum and layers with emission-promoting material such as thorium or thorium compounds or Alternate barium aluminate with the top layer of tungsten or molybdenum of the emitting surface is somewhat pronounced thicker and the pad is composed of the layer structure made of _W ungsten molybdenum or carbon. It is important for the function of these cathodes that they are porous and that the emitting substance can easily reach the surface. The layer-by-layer production only has the task of achieving a uniform distribution of the emitting substance in the storage area; the layers should even overlap or be interlocked by means of a coarse-grained structure. Such cathodes are produced by sintering powder layers onto the support or also by (physical) vapor deposition of the support.

However, such cathodes have several blatant disadvantages. First of all, the (clear) porosity leads to excessive evaporation of the emitting substance and thus to very poor vacuum properties, which makes its use in UHF electron tubes questionable. Secondly, the total material thickness required for production requires an excessive heating power. Thirdly, the production takes place essentially using powder metallurgical methods, with all the disadvantages that a powder metallurgical cathode production entails, whereas physical vapor deposition as an alternative only produces very thin layers. The disadvantages mentioned are, in particular, restrictions in the shaping (selection of the geometry), caused by production and (pressure) sintering, but also also a lack of mechanical stability of the porous structure, since pressing the layers in any chosen geometric shapes is out of the question and in addition, the sintering temperature must be kept so low that the emitting substance does not evaporate during manufacture. A lot of processing steps are necessary. Finally, the layer arrangement only the task of ensuring a uniform distribution of the emitting substance in the subsequent delivery area, which can also be achieved by other, less complex processes such as impregnation or powder mixing. In addition, the layer structure is not intended to be maintained over the life of the cathode. Above all, however, these cathodes are metal capillary cathodes (MK cathodes), that is to say not compact post-delivery cathodes which are the subject of the invention.

In contrast, the present invention has the task of creating a thermionic cathode which is suitable as a unipotential cathode for use in UHF and microwave tubes, and which has the advantages of a large-area cathode with largely freely selectable geometric shape, high emission current and stable high-frequency behavior over a long service life gets away.

• a) auf eine entsprechend der gewünschten Kathodengeometrie geformte Unterlage durch Transport über die Gasphase, gegebenenfalls verbunden mit Reduktionsreaktionen während oder nach Aufbringung der Schichten, folgende Schichtstruktur aufbringt:
  • α) Eine Trägerschicht aus hochschmelzendem Metall als Basismaterial und mindestens einem Dotierstoff zur mechanischen Strukturstabilisierung,
  • β) eine bei Betrieb der Kathode als Nachlieferungsbereich wirkende Schicht oder Schichtenfolge, bestehend aus einem hochschmelzenden Metall als Basismaterial und einem Vorrat an elektronenemittierendem Stoff, und
  • y) die polykristalline Deckschicht oder eine vorzugsorientierte polykristalline Deckschicht aus einem hochschmelzenden Metall als Basismaterial und mindestens einem Dotierstoff zur Textur- und Strukturstabilisierung, wobei die Vorzugsorientierung durch Wahl der Abscheide-Parameter derart festgelegt wird, daß die Austrittsarbeit aus der Emitter-Monoschicht, die sich bei Betrieb der Kathode auf dieser Deckschicht ausbreitet, minimal ist,
• b) die Unterlage entfernt und
• c) die Trägerschicht mit Anschlüssen zur Heizung versieht.

This object is achieved in that in a method of the type mentioned

• a) applying the following layer structure to a base shaped according to the desired cathode geometry by transport over the gas phase, possibly combined with reduction reactions during or after application of the layers:
  • α) a carrier layer made of high-melting metal as the base material and at least one dopant for mechanical structural stabilization,
  • β) a layer or layer sequence acting as a subsequent delivery area during operation of the cathode, consisting of a high-melting metal as the base material and a supply of electron-emitting material, and
  • y) the polycrystalline cover layer or a preferentially oriented polycrystalline cover layer made of a high-melting metal as the base material and at least one dopant for texture and structure stabilization, the preferred orientation being determined by the choice of the deposition parameters such that the work function from the emitter monolayer, which is spreads during operation of the cathode on this cover layer, is minimal,
• b) the pad is removed and
• c) provides the carrier layer with connections for heating.

The layers are preferably applied by reactive deposition, e.g. CVD processes, pyrolysis, cathode sputtering, vacuum condensation or plasma sputtering.

W, Mo, Ta, Nb, Re and / or C are preferably used as the base material, the composition of the base material being the same or different in the individual layers.

In a particularly advantageous embodiment of the method according to the invention, the gases involved in the deposition reaction are excited by generating a plasma for chemical conversion and the associated deposition of cathode material (so-called plasma-activated CVD method = PCVD).

The method according to the invention, which can be used in particular for the production of thermionic monolayer cathodes with a high electron emission density, accordingly a layer structure, at least consisting of a high-melting metal and a substance with high electron emission as a monolayer, successively deposited in a continuous process, for example by reactive deposition from the gas phase (CVD process) of at least two components on a base, the base being removed after the deposition so that a self-supporting whole CVD cathode is obtained. Such a cathode - designed as a cylindrical unipotential cathode - is particularly suitable for transmitter and amplifier tubes at high frequencies and / or high powers.

The thermionic cathode produced according to the invention - the material of which is essentially high-melting metal, such as W, Mo, Ta, Nb and / or Re and / or carbon - consists of a very finely crystalline, mechanically stable support or base layer, a layer sequence which is highly enriched with emitting substance and an optionally preferred oriented top layer, wherein all layers have been applied via the gas phase, preferably by CVD method, and the base has been removed after the deposition has ended.

In the method according to the invention, a finely crystalline carrier layer made of high-melting metal with good mechanical properties and grain growth suppressed by doping is applied to a suitable (and suitably shaped) base, for example by reactive deposition from the gas phase (CVD method). This is followed by a layer or a sequence of layers of alternating electron-emissive substance and base material, the layer composition being controlled by varying the gas flows, for example in CVD deposition. Finally, there is preferably a top layer preferred-oriented columnar layer made of a high-melting metal, which is protected by additives against grain growth and destruction of the preferred orientation. After the deposition has ended, the substrate or substrate preform is detached from the positive (ie the layer structure) and a self-supporting cathode with the desired properties is obtained, for example in the form of a cylindrical, self-supporting, directly heated unipotential cathode with high emission and long service life.

The base preferably consists of an easily and precisely mouldable material which has little adhesion to the cathode material deposited thereon. The substrate is removed according to the invention either by selective etching, mechanically or by evaporation when heated in vacuo, e.g. in a vacuum oven, or in a suitable gas atmosphere, e.g. Hydrogen, by burning off or by a combination of the methods mentioned, depending on the material used for the base.

According to the invention, the base is, for example, a body made of graphite, in particular pyrolytic graphite, or vitreous carbon, which is removed by mechanical processing, burning and / or mechanical-chemical micropolishing, or the base consists of copper, nickel, iron, molybdenum or one Alloy with a predominant proportion of these metals and is removed by selective etching or initially in its predominant mass mechanically and in the remaining residues by evaporation with heating in a vacuum, for example in a vacuum oven, or under a suitable gas atmosphere, for example under hydrogen. The base used for the method according to the invention must be made with the layer material, ie the material from which the supporting parts of the cathode are made are produced, as little compatible as possible, ie easily removable from it. Graphite advantageously fulfills this requirement. Graphite, for example polycrystalline electrographite, is easy to machine mechanically, so that even complicated moldings can be produced easily. However, since electrographite is porous, a thin layer of pyrolytic graphite is deposited on the preforms made from it, which is practically non-porous and is a good base for the deposition of the cathode material.

Depending on the shape of the underlay body, different methods are available for graphite to detach the finished cathode from the base. In many cases, the cathode can be pulled off the graphite shaped body very simply and with only a small force by pulling or pushing in the direction of the layering axis (a-axis) of the pyrolytic graphite. A reliable detachment is achieved by taking advantage of the different thermal expansion coefficients of the graphite base and the cathode formed, for example, from tungsten. As tungsten expands more than graphite in the heat, the finished cathode is split by heating to e.g. 300 ° C above the separation temperature. When coating the inner lateral surface of a cylindrical hollow body made of graphite, preferably at 500 ° C., the desired cleavage is obtained in an even simpler manner by cooling to room temperature. Another simple method for removing graphite, for example in inaccessible places, is by burning it off. Micropolishing results in particularly clean and uniform surfaces.

Underlay molded articles made of copper or nickel are also easy to machine and can be removed. Copper will initially mostly removed mechanically, e.g. by machining. Copper residues can be removed in a vacuum furnace by evaporation at 1800 to 1900 ° C or, like nickel, by selective etching or micropolishing. A mixture of HN03, H ₂ 0 and H ₂ O ₂ in a mixing ratio of 6: 3: 1 parts by volume or an aqueous solution of 220 g Ce (NH ₄ ) ₂ (NO ₃ ) ₆ and 110 is used as the etchant, in particular for nickel ml HN0 ₃ to 1 1 H ₂ 0 used. Copper supports can be removed in a solution of 200 g FeC1 ₃ each 1 1 H ₂ 0 at a working temperature of 50 ° C. Molybdenum substrates are preferably etched away by immersing them in a boiling solution from one part each of HN0 ₃ , HC1 and H ₂ 0.

A thermionic cathode produced by the method according to the invention is self-supporting and flat and has a thickness of 50 _/ um to 500 _/ um, preferably 100 to 150 _/ um, and even larger thicknesses can be realized without problems.

In order to be able to produce thin and stable self-supporting forms from high-melting, brittle metals by reactive deposition from the gas phase, a modification of the CVD process is advisable. With the usual deposition, columnar structures with low mechanical and thermal stability and a tendency to strong crystallite growth are frequently obtained under operating conditions.

• Eine einfache, aber etwas zeitraubende Möglichkeit besteht darin, daß man das CVD-Schichtwachstum durch wiederholte Substratabkühlung auf Zimmertemperatur immer wieder unterbricht und die Keimbildung durch Heizen neu startet oder eine periodische Variation der Substrattemperatur im Bereich zwischen 300 und 700°C durchführt. Man erhält eine Abfolge verschiedener Schichten - z.B. aus Wolfram -, deren Eigenschaften gegenüber dem kontinuierlich abgeschiedenen Material bereits deutlich verbessert sind. In einigen Fällen, z.B. bei direkter Widerstandsheizung des Substrats bzw. der Unterlage bei einer "Kaltwand"-Beschichtung, ist es auch möglich, die Zusammensetzung des Reaktionsgemisches periodisch zu variieren, insbesondere den Anteil desjenigen Reaktionspartners, der die stärkste Kühlung des Substrats bewirkt. Im Fall der Wolfram-CVD aus WF₆ + H₂ ist es z.B. der Wasserstoffanteil des eingeleiteten Gasgemischs, der variiert wird.

For this reason, modifications of the CVD method are preferably used to produce the carrier layer, ie the supporting cathode base, which produce ultra-crystalline structures with increased mechanical-thermal strength. This can be done in three ways:

• A simple, but somewhat time-consuming possibility is to interrupt the CVD layer growth again and again by repeatedly cooling the substrate to room temperature and to restart the nucleation by heating or to carry out a periodic variation of the substrate temperature in the range between 300 and 700 ° C. A sequence of different layers - for example made of tungsten - is obtained, the properties of which have already been significantly improved compared to the continuously deposited material. In some cases, for example in the case of direct resistance heating of the substrate or the base in the case of a “cold wall” coating, it is also possible to vary the composition of the reaction mixture periodically, in particular the proportion of the reaction partner which brings about the greatest cooling of the substrate. In the case of tungsten CVD from WF ₆ + H ₂ , it is, for example, the hydrogen content of the introduced gas mixture that is varied.

The second possibility of structural stabilization is the deposition of wafer-thin crystallite growth-inhibiting intermediate layers. As an example, tungsten is again to be used, the deposition of which from the gas phase is repeatedly interrupted by throttling the WF ₆ + H ₂ gas stream. Instead, carrier gas is alternately introduced with, for example, an organometallic thorium compound from a saturator, so that, for example, a ThO ₂ intermediate layer is deposited.

A similar effect can be achieved by carbon deposition in the intermediate layer at a very high saturator temperature. The thickness of the tungsten layers is of the order of 1 _/ um, that of the intermediate layers containing thorium or carbon is significantly less (about 0.2 _/ um).

The third method is based on the fact that the base material is deposited together with a doping material which has a negligibly low solubility in solid in the crystal lattice of the base material. For example, tungsten is deposited with 2% Th0 ₂ to produce the layers. Such a separation from the gas phase (multi-component CVD process) results in an extremely fine and uniform distribution of the admixture in the layer material. On the one hand, this significantly increases the flexural strength of the layer material, in the example of the tungsten doped with 2% Th0 _2, doubles, on the other hand, the addition mentioned inhibits the crystal growth in the layer material at operating temperatures and thereby stabilizes the crystal structure, in particular the grain size, which preferably Values of approximately 1 _/ µm and below, and the preferred orientation of the crystals over longer operating times. The cathodes according to the invention achieve a lifespan of 10 ⁴ hours due to the addition mentioned, in fact under normal operating conditions and with increased emission rates.

Because the backing layer, i.e. the load-bearing base layer, which is deposited from refractory metal by means of external doping in a highly crystalline and grain-stabilized manner, the mechanical strength is about three times greater than that of the pure CVD material. Because the dopants, which are practically insoluble in the base metal, are either finely dispersed simultaneously or alternately in a high-frequency layer sequence by CVD, excessive germ growth is interrupted again and again. In particular, this foreign doping strongly inhibits grain growth under normal operating temperatures, so that the mechanical stability is also guaranteed over a longer service life.

The stabilization of the base material is achieved by the addition of Th0 ₂ in tungsten, which is mentioned by way of example, also by other substances, provided that they have a small or negligible solid solubility in tungsten (e.g. scandium, yttrium) and their melting point is above 2000 K. These substances include in particular Zr, Zr0 ₂ , Ru, U0 ₂ , Sc ₂ 0 ₃ and Y203, which moreover can advantageously be separated from the gas phase simultaneously with the layer material.

In principle, the same also applies to other high-melting base materials, in which a material component which is insoluble in them must be deposited alternately or simultaneously in a fine admixture.

Structural stabilization of the carrier layer, that is to say the base, can only be brought about by correspondingly small additions, which as a rule do not have to be identical to the emitting substance. To extend the lifespan and increase the emission, additional layers with a significantly higher doping concentration of emitting substance are required.

A supply and subsequent delivery layer with a high doping concentration of emissive substance is therefore applied to the structure-stabilized base. This subsequent delivery area expediently consists of a high-frequency layer sequence, layers with emitting material alternating with layers of base material in such a way that these layers are still mechanically sufficiently stable and adhere well to the CVD carrier layer and at the same time have a high mean emitter concentration in the subsequent delivery area of preferably 10 to 20% by weight.

This layer sequence is produced according to the invention by reactive deposition from the gas phase with temporal variation of the parameters, in particular the flow rates of the gases involved in the reaction and / or the substrate temperature.

The variation in time of the CVD parameters is preferably periodic, in particular alternating between the optimal parameters for deposition of the emitting substance and those for CVD of the base material. A corresponding change in the respective gas flow rates is usually sufficient; in some cases, however, the substrate temperature must also be lowered and raised appropriately.

The electron-emitting substance is preferably selected from the scandium group (Sc, Y, La, Ac, lanthanides, actinides) and in metal, oxide, boride and / or carbide form with the base material, preferably W, Mo, Nb, Ta, Re , separated from the gas phase. According to the invention, the following combinations of substances serve in particular as emitting substance + base material: Th / Th0 ₂ + W, Th / Th0 ₂ + Nb, ThB ₄ ⁺ Re, Y / Y ₂ 0 ₃ + Ta, Y ₂ 0 ₃ + Nb, or Sc ₂ 0 ₃₁ Y ₂ 0 ₃ or La ₂ 0 ₃ in combination with molybdenum or tungsten as the base material are deposited as emitting substances. Favorable combinations are also Ce, Sm and Eu oxide with tungsten or molybdenum. ThB ₄ is preferably applied by pyrolysis of Th (BH ₄ ) ₄ , enriched in, for example, argon as the carrier gas, on a layer of rhenium with an underlying structurally stabilized tungsten carrier at substrate temperatures greater than or equal to 300 ° C.

When the emissive substance is deposited in oxide form, a further improvement in the cathode properties can be achieved by additionally using an activator component, preferably boron or carbon, to release the emitter in atomic form, and also a diffusion-enhancing component are also deposited using the CVD method. Pt, Os, Ru, Rh, Re, Ir or Pd in concentrations of 0.1 to 1% by weight are preferably used as diffusion-promoting or reinforcing constituents for the emitting substance.

• i. Metallhalogenide; vorzugsweise Fluoride, mit H₂ als Reduktionsmittel. Abscheidung der Metalle Mo, W, Re bei Temperaturen von 400 bis 1400°C, vorzugsweise 500 bis 800°C, insbesondere 500 bis 600°C.
• 2. Metallcarbonyle M(CO)_n; ein Teil der CO-Gruppen kann durch H, Halogene, NO, PF₃ ersetzt sein. Abscheidung von Mo, W, Re und Pt-Metallen bei Temperaturen von 300 bis 600°C.
• 3. Metalltrifluorphosphane M(PF₃)_n; Fluor kann ganz oder teilweise durch H, Cl, Br, J, Alkyle und Aryle ersetzt sein, die PF₃-Gruppen durch CO, H, Cl, Br, J, CO, NO. Im physikalischen und chemischen Verhalten ähnelt diese Gruppe den Metallcarbonylen. Die Abscheidung von Mo, W, Re und Pt-Metallen ist bei Temperaturen von 200 bis 600°C möglich.
• 4. Metallocene M(C₅H₅)_n; sie gehören zur Gruppe der metallorganischen Sandwich-Verbindungen. Die (C₅H₅)-Gruppen können teilweise durch H, Halogene, CO, NO, PF₃ und PR₃ ersetzt sein. Mo, W, Pt-Metalle können durch Pyrolyse abgeschieden werden. Mit H₂ als Reaktionskomponenten wird die Reaktionstemperatur wesentlich herabgesetzt.
• 5. Metall-β-Diketonate; Acetylacetonate M(aa)_n und die 1,1,1-Trifluoracetylacetonate M(tfa)_n und 1,1,1,5,5,5-Hexafluoracetylacetonate M(hfa)_n; aus diesen Verbindungen können Metalle der Platin-Gruppe und Oxide der Lanthaniden einschließlich Sc₂0₃ und Y₂0₃ und Oxide der Actiniden einschließlich Th0₂ abgeschieden werden. Die Abscheidungstemperaturen liegen bei 400 bis 600°C bei den Acetylacetonaten und bei 250°C bei den fluorierten Acetylacetonaten.
• 6. Metallalkoholate M(OR)_n; die Abscheidung der Oxide der Lanthaniden und Aktiniden einschließlich Sc203; Y₂0₃ und Th0₂ ist bei Temperaturen von 400 bis 600°C möglich. Es können in manchen Fällen auch Doppeloxide, z.B. MgAl₂O₄, abgeschieden werden.

Underlay temperatures of 200 ° C. to 600 ° C. (so-called low-temperature CVD process) are preferably used in the production of the cathodes according to the invention. The following volatile starting compounds are used to separate Mo, W, Re, Pt metals, rare earths, thorium and actinides:

• i. Metal halides; preferably fluorides, with H ₂ as a reducing agent. Deposition of the metals Mo, W, Re at temperatures from 400 to 1400 ° C, preferably 500 to 800 ° C, in particular 500 to 600 ° C.
• 2. metal carbonyls M (CO) _n ; some of the CO groups can be replaced by H, halogens, NO, PF ₃ . Deposition of Mo, W, Re and Pt metals at temperatures from 300 to 600 ° C.
• 3. metal trifluorophosphanes M (PF ₃ ) _n ; Fluorine can be replaced in whole or in part by H, Cl, Br, J, alkyls and aryls, the PF ₃ groups by CO, H, Cl, Br, J, CO, NO. This group is similar to metal carbonyls in physical and chemical behavior. The deposition of Mo, W, Re and Pt metals is possible at temperatures from 200 to 600 ° C.
• 4. Metallocenes M (C ₅ H ₅ ) _n ; they belong to the group of organometallic sandwich compounds. The (C ₅ H ₅ ) groups can be partially replaced by H, halogens, CO, NO, PF ₃ and PR ₃ . Mo, W, Pt metals can be deposited by pyrolysis. With H ₂ as the reaction components, the reaction temperature is significantly reduced.
• 5. metal β-diketonates; Acetylacetonates M (aa) _n and the 1,1,1-trifluoroacetylacetonates M (tfa) _n and 1,1,1,5,5,5,5-hexafluoroacetylacetonates M (hfa) _n ; metals of the platinum group and oxides of lanthanides including Sc ₂ 0 ₃ and Y ₂ 0 ₃ and oxides of actinides including Th0 _{2 can be} deposited from these compounds. The deposition temperatures are 400 to 600 ° C for the acetylacetonates and 250 ° C for the fluorinated acetylacetonates.
• 6. Metal alcoholates M (OR) _n ; the deposition of the oxides of lanthanides and actinides including Sc203; Y ₂ 0 ₃ and Th0 ₂ is possible at temperatures from 400 to 600 ° C. In some cases, double oxides, for example MgAl ₂ O ₄ , can also be deposited.

Tungsten and thorium or Th0 _{2 are} preferably allowed to alternate or simultaneously from WF ₆ + H ₂ and Th-diketonate, in particular Th-acetylacetonate, preferably Th-trifluoroacetylacetonate or Th-hexafluoroacetylacetonate, but also Th-heptafluorodimethyloctanedione or Th-dipivaloylmethane, at temperatures grow between 400 ° C and 650 ° C by reactive deposition from the gas phase, the organometallic Th starting compound being in powder form in a saturator which is heated to a temperature close to the respective melting point and by an inert gas, in particular argon, as Carrier gas is flowed through.

The layer structure of Nachlieferungsbereichs is typically formed such that the layer thicknesses of the base material layers is about 1 to 10 _/ um and that of the emissive material about 0.1 to 1 _/ um, respectively. In a preferred embodiment of the method according to the invention in particular 100 of Nachlieferungsbereich with emissive material in the form of a layer sequence by CVD method on a structure-stabilized doped CVD carrier layer of 30 to 300 _/ um thickness to thickness, is applied, in which one layer alternates from high-melting metal with small admixtures of electron-emissive material and optionally stabilizing doping with one with high concentrations of electron-emissive material, which is somewhat thinner, the layer spacings being of the order of the grain sizes. In particular, the individual layer thickness is 0.5 to 10 _/ μm at a concentration of the emitting substance of up to 5% by weight and 0.1 to 2 _/ μm at a concentration of the emitting substance of 5 to 50% by weight. The average concentration of emissive substance is preferably 15 to 20% by weight.

• Da die meisten gebräuchlichen emittierenden Stoffe nur geringe Löslichkeiten in den hochschmelzenden Materialien, aus denen das tragende Grundgerüst der Kathode (Basis) mit der Deckschicht besteht, haben, findet die Diffusion der vom Inneren an die Kathodenoberfläche nachgelieferten emittierenden Stoffe entlang der Korngrenzen statt. Um also eine hinreichende Nachlieferung an die Oberfläche zum Ausgleich der durch Abdampfen entstehenden Verluste an emittierenden Stoffen und eine hinreichende Oberflächenbedeckung durch dieselben zu gewährleisten, dürfen die Zahl der Korngrenzen nicht zu gering und die Diffusionswege entlang der Oberfläche nicht zu lang sein.

A preferential cover layer is then applied to the subsequent delivery area, which guarantees increased emissions. This cover layer can consist of the same material as the base, or of a further material which is selected such that the work function for the combination of emitter monolayer cover layer is even lower than that of the emitter-base combination. As a rule, the cover layer consists of a metal with a high work function, which reduces the work function accordingly by means of a high dipole moment between the emitter film and the cover layer. This dipole moment to the electropositive emitter film is not only dependent on the material, but also also of its crystallite surface orientation. A means of further strengthening this subtractive dipole field and thereby increasing the emission is now to apply a suitably oriented polycrystalline surface layer instead of an untextured surface. This preferred orientation can practically only be achieved by deposition from the gas phase on suitably pretreated surfaces. In the case of a thorium monolayer on tungsten, <111> is the suitable preferred orientation for tungsten. The applied surface layer must also meet other conditions. An important additional condition is that it must be very fine crystalline. This has the following cause:

• Since most of the common emissive substances have only low solubilities in the high-melting materials that make up the supporting framework of the cathode (base) with the cover layer, the diffusion of the emissive substances supplied from the inside to the cathode surface takes place along the grain boundaries. In order to ensure sufficient subsequent delivery to the surface to compensate for the losses of emitting substances caused by evaporation and adequate surface coverage by the same, the number of grain boundaries must not be too small and the diffusion paths along the surface must not be too long.

This condition is usually met by conventional cathodes at operating temperatures that are not too high. At elevated temperatures, which usually also mean higher emissions, the desorption of the emitting substance increases so much in comparison with surface diffusion that sufficient monolayer coverage is no longer guaranteed. The resulting drop in Emission depends critically on the mean grain diameter and only occurs at higher temperatures, the smaller the mean grain size. With Th - [W] cathodes, an average tungsten grain diameter of ≦ 1 _/ um means that the usable temperature range can be extended to> 2400 K. Such small, stable grain sizes can practically only be achieved using the CVD method, and only by a suitable choice of Parameters. This surface structure must of course also meet the further requirement that it remains stable against longer thermal loads. If, for example, the grain size increases too much due to recrystallization during operation of the cathode, this finally causes a drop in the emission current and thus a shorter service life due to the decrease in the monoatomic coverage. The same stability requirement also applies to the texture, ie the preferred orientation set on the surface must be maintained.

This recrystallization is prevented analogously to the mechanical stabilization of the support layer by adding a substance which is insoluble in the crystal lattice of the cover layer material and which is simultaneously separated from the gas phase. In the case of tungsten as a cover layer or base material, doping with Th, Th0 ₂ , Zr, Zr0 ₂ , U0 ₂ , Y, Sc, Y ₂ ⁰ ₃ , Sc ₂ 0 ₃ and Ru are suitable because of their low solubility in tungsten. If one assumes a working temperature of 2000 K (ie the melting point of the doping must be higher) and demands easy handling, Th0 ₂ , Zr0 ₂ , Y ₂ 0 ₃ , Sc ₂ O ₃ and Ru remain as preferred CVD doping. The doping can in particular also be identical to the emitting substance if Th, Y or Sc form the emitter monolayer.

The prevention of crystallite growth also means a stabilization of the texture, which in the majority of cases is destroyed without doping in the activation phase of the cathode. The destruction of the texture at higher operating temperatures is probably caused in pure materials by the fact that minority crystallites grow strongly at the expense of the preference-oriented majority, or that crystallite growth starts from the unoriented base.

This means that cathodes with a preferred cover layer - which also means a higher emission than conventional cathodes - can be produced, which also have a correspondingly long service life.

The different parts or layers of such a cathode, which is produced entirely according to the invention using the CVD method, therefore have to perform distinctly different tasks and must therefore be tailored to these tasks. In many cases it is advisable to apply an easy-to-remove, loose intermediate layer to the surface. The subsequent very fine crystalline and doped base layer serves to mechanically stabilize the cathode structure even under thermal stress and enables self-supporting substrate-free CVD structures to be produced. Finally, in the subsequent delivery area, a large stock of emissive material is important. The mechanical properties and the grain structure are less critical in this area, as long as only a high doping concentration of emissive substance - advantageously about 10 to 30% by weight - is achieved.

The preferred oriented cover layer, on the other hand, ensures a very low electron work function from the surface dipole layer and also thanks to its fine crystalline structure for good coverage with the monoatomic emitter film. It is also texture stabilized by low insoluble doping.

Instead of coating the outer surface of a substrate body, an inner coating of a suitable hollow body can also be carried out. However, the layers are applied in reverse order, i.e. one separates the preferred oriented cover layer first, then the subsequent delivery zone and finally the mechanically stable load-bearing base, i.e. the carrier layer. The finished cathode body is finally provided with power supplies for direct heating.

The advantages of the invention are that large-area thermionic cathodes with high emission currents, stable high-frequency behavior and freely selectable geometric shape and long service life are available, which are suitable for automated large-scale production with low manufacturing costs without the many time-consuming process steps as with mesh cathodes. By using the CVD process, the complex and difficult mechanical processing of the known high-melting and very hard cathode materials, for example tungsten, is avoided and at the same time an almost arbitrary layer structure can be produced.

The production of the entire cathode with all material layers by reactive deposition from the gas phase in a continuous process is particularly advantageous.

In a further embodiment of the invention, the layer structure is applied in such a way that the three layers α, β and y mentioned above are identical. This ensures that a single layer takes over the functions of the layers α, β and y. This single layer has a suitable texture and a high emitter and doping concentration; due to the finely divided doping, it is also texture stabilized, microstructure stabilized and mechanically stable under thermal stress.

The cathodes produced according to the invention are distinguished by the combination of a long service life, high emitter concentration and great mechanical stability.

• Fig. 1 einen Schnitt entlang der Längsachse durch eine Abscheidevorrichtung für eine Kathode,
• Fig. 2 einen Schnitt durch die Anordnung nach Fig. 1 mit einer nach Ausführungsbeispiel 1 hergestellten Kathode senkrecht zur Längsachse,
• Fig. 3a einen Querschnitt durch eine Th + W-CVD-Kathode nach Ausführungsbeispiel 2,
• Fig. 3b das dazugehörige (W₂C)Th0₂-Konzentrationsprofil,
• Fig. 4 die zeitliche Variation von WF₆- und Ar-Gasdurchflußraten zur Erzielung der Kathodenstruktur von Abb. 3a,
• Fig. 5 einen Schnitt durch die Vorrichtung nach Fig. 1 mit einer nach Ausführungsbeispiel 3 hergestellten Kathode senkrecht zur Längsachse,
• Fig. 6 eine fertige, mit einem'inneren Leiter und einem Ringkontakt zum direkten Beheizen versehene Kathode nach Ausführungsbeispiel 3,
• Fig. 7 einen Schnitt parallel zur Längsachse durch ein außen beschichtetes Kathodensubstrat nach Beispiel 4 und
• Fig. 8 einen vergrößert wiedergegebenen Bereich aus Fig. 7.

To illustrate the invention, some embodiments are shown in the drawing and are described below. Show it

• 1 shows a section along the longitudinal axis through a deposition device for a cathode,
• 2 shows a section through the arrangement according to FIG. 1 with a cathode produced according to embodiment 1 perpendicular to the longitudinal axis,
• 3a shows a cross section through a Th + W CVD cathode according to embodiment 2,
• 3b shows the associated (W ₂ C) Th0 ₂ concentration profile,
• 4 shows the temporal variation of WF ₆ and Ar gas flow rates to achieve the cathode structure of FIG. 3a.
• 5 shows a section through the device according to FIG. 1 with a cathode produced according to embodiment 3 perpendicular to the longitudinal axis,
• 6 shows a finished cathode according to embodiment 3 provided with an inner conductor and a ring contact for direct heating,
• 7 shows a section parallel to the longitudinal axis through an externally coated cathode substrate according to Example 4 and
• FIG. 8 shows an enlarged area from FIG. 7.

Embodiment 1

Inside a device, known in principle, for the reactive separation of substances from the gas phase (CVD reactor), which consists of a gas supply system with mass flow controllers, a reaction chamber and a gas disposal system; is the arrangement shown in Fig. 1. A hollow cylinder 1 serving as a base made of pyrolytic graphite with an inner diameter of 12 mm, a length of 95 mm and a wall thickness of approximately 200 μm is surrounded over its entire length by a heating coil 3 made of tungsten wire and at its ends also made of heat-resistant material Cover plates 2 held. The pyrolytic graphite of the base 1 is layered parallel to the inner surface, i.e. the crystallographic c-axis lies in the direction of the surface normal of the lateral surface. The graphite cylinder can also be heated by direct current passage through the cylinder.

The cathode 4 is formed in the CVD process by growing on the inner circumferential surface of the base 1 in the reverse order of the layers of the cathode, i.e. the later surface layer of the cathode is deposited first and finally the later inner support layer of the cathode.

In the present embodiment, the base 1 is heated to a temperature of 550 to 600 ° C, the reaction gases are introduced at a pressure of about 50 mbar.

2 shows the grown layers of the cathode in a section transverse to the longitudinal axis of the underlay hollow cylinder 1. First, a fine-crystalline (grain sizes 1 _/ um and...) Based on the underlay surface in the <1,1,1> direction is first placed on the substrate smaller) and deposited with 1% Th0 ₂ to stabilize the crystal structure W layer 7 in a thickness of 5 _/ um. For this purpose, the reaction gases WF ₆ with a flow rate of 30 to 50 cm ³ / min, H ₂ with a flow rate of 400 to 500 cm3 / min and Ar saturated with thorium acetylacetonate with a flow rate of 100 cm ³ / min as a mixture about 3 passed over the pad for up to 5 minutes. The hydrogen serves as a reducing gas for the metal compounds. The thorium acetylacetonate is in powder form in a saturation vessel, which is kept at a temperature of 160 ° C and is flushed out by the Ar serving as the carrier gas. The reaction gases are mixed in a mixing chamber, which is heated to a temperature of approximately 180 ° C., and passed through a nozzle onto the surface of the substrates.

The saturator temperature of 160 ° C must be strictly observed, since below + 150 ° C the Th (AcAc) ₄ vapor pressure for one Coating is too low and premature decomposition of this compound already occurs in the saturation vessel at + 170 ° C. After the preferred outer layer of the cathode has grown on, the subsequent delivery layer 6 enriched with electron-emissive material is deposited. For this purpose, an argon flow rate of about 85 cm ³ / min is set at flow rates of about 15 cm ³ / min for WF ₆ or 150 cm / min for H ₂ . A W layer is formed with an admixture of about 20% Th0 ₂ - possibly with the help of an additional oxidizing gas such as C0 ₂ . After a deposition time of approximately 100 minutes, the layer reaches a thickness of approximately 40 _μm . A carburization as with conventional thoriated tungsten cathodes is not necessary because enough carbon from Th C ₂₀ B ₂₈ O ₈ is already deposited. Another solution also practiced for the subsequent delivery area is to grow Th (Th0 ₂ ) and W layers alternately, in particular the WF ₆ flow rate between 10 and 60 cm ³ / min and the Ar rate between 85 and 30 cm ³ / min changes. The H ₂ rate is usually ten times the WF ₆ rate and the intervals are 1 min for W and about 5 min for Th layers, which are then about 4 or 1 _/ um thick. Subsequently, the cathode-supporting member 5 is formed with a layer thickness of about 50 to 100 _/ um. For this purpose, either the initial flow rates are set again, this time at a temperature of 500 ° C, or the parameters of the layer sequence of the subsequent delivery area are switched over in quick succession, the duration of the W intervals being 20 s each and that of the Th intervals being about 1 min is. As the uppermost layer nor a pure W layer can then addition of about 10 _/ um can be deposited.

With the fast switching processes between different A computer control of the gas volume flow controller is generally used for parameter sets.

A high-frequency modulation of all flow rates is recommended, in particular, in order to achieve uniformly thick layers within the graphite tube.

After the coating processes have been completed, the substrate and cathode are slowly cooled to room temperature. Due to the different thermal expansion coefficients of both materials, and by the poor adhesion of the tungsten to the pyrolytic graphite, the thoriated tungsten cathode 4 shrinks on cooling to about 500 ° C in diameter by approximately 10 _/ um stronger than the documents hollow cylinder 1 and separates from it . Because of the gap 10 formed, the tungsten-thorium cathode is pulled out of the underlay cylinder with ease. Since the inner cylinder surface of the base consists of pyrolytic graphite with a very smooth, uniform surface, the outer surface of the finished cathode has a high surface quality without polishing, which is also not influenced by irregularities in the deposited layers.

The finished tubular cathode body is cut into several short tube sections perpendicular to its longitudinal axis, e.g. with a laser beam. Each of the sections then forms the cathode of a tube.

Implementation example 2

3a shows a cross section through the layer structure of a planar (planar) cathode, but this can also be identical to a section of the jacket of a cylindrical cathode. The top layer 7 is one <111> -oriented, polycrystalline W layer with average grain sizes of about 1 to 2 _/ um. It has a thickness of about 10 _/ um and is doped with about 1% finely dispersed Th0 ₂ . Among them is of about 50 _/ um thick Nachlieferungsbereich 6, the micrometers of individual layers 9 2 1% thoriated W, with intermediate layers 8 of 0.2 _/ um with about 20 to 40 mole% Th0 ₂ and a carbon enrichment of the same order . The high-frequency layer sequence serves to stabilize the grain structure and to preserve grain sizes from 1 to 2 µm.

The subsequent delivery area 6, together with the load-bearing part 5, forms the base B. With the exception of the intermediate layers mentioned, it consists consistently of W with 1% ThO ₂ . Instead of 1% Th0 ₂ but also 1% Zr0 ₂ or 1% Sc ₂ O _{3 is used} for mechanical and structural stabilization against thermal stress. All layers 5 to 9 are produced by deposition from the gas phase on a base made of Mo or graphite. The underlay is removed again after the coating. Fig. 3b shows in addition to Fig. 3a again the Th0 ₂ - and C concentration profile over the cathode cross section. FIG. 4 shows the variation in time of the WF ₆ and Ar flow rates Q, 11 and 12, respectively, which is necessary to achieve the above cathode structure, as a function of the time after the start of the CVD deposition. Ar is the carrier gas for thorium acetylacetonate Th (C ₅ H ₇ 0 ₂ ) ₄ , with which it has accumulated after flowing through a saturator which is heated to a temperature of 160 ° C. The other gases that flow through the reactor are H ₂ , the flow rate of which is about 10 times higher than that of WF ₆ , and N ₂ , which serves as a purge gas for an observation window. The substrate temperature is measured using a radiation pyrometer through the observation window measured and kept constant at a value of around 500 ° C. The average pressure in the reactor is in the range from 10 to 100 mbar, preferably 40 mbar. The reactor itself is at a temperature of approximately 180 ° C. Fluorinated thorium acetylacetonate is even more suitable for ^Th - ^CVD than ^T h (C ₅ H ₇ 0 ₂ ) ₄ . Other special organometallic compounds with higher vapor pressure, such as Th-dipivaloylmethane or Th-heptafluorodimethyloctanedione, are also suitable. Th0 ₂ as an emitter material can be replaced without significant changes by rare earth metals, preferably by Ce0 ₂ , Sm ₂ 0 ₃ , ^E u ₂ ⁰ ₃ , Y ₂ O ₃ , while Th0 ₂ or Zr0 ₂ continues as doping of W for mechanical-thermal stabilization or Sc ₂ 0 ₃ can be used.

Embodiment 3

innerhalb von 3 Min eine 5 µm dicke Re-Schicht 7 abgeschieden, die im Falle ihres späteren Verbleibs in der Regel vorzugsorientiert ist. Die Re-Abscheidung wird durch langsame Verminderung des Gasstromes aus ReF₆ und H₂ beendet, bis nach 2 Min der Zustrom dieser Gase völlig unterbunden ist. Gleichzeitig mit dieser Verminderung des Gaszustromes wird die Unterlagentemperatur auf 400°C eingestellt und Th(BH₄)₄ wird mit Ar als Trägergas zum Substrat geleitet, wobei die Ar-Durchflußrate etwa 90 cm³/Min beträgt. Th(BH₄)₄ befindet sich in Pulverform in einem auf etwa 190°C geheizten Sättiger. Die Reaktortemperatur soll bei der Abscheidung 2C0 bis 210°C betragen. Durch pyrolytische Zersetzung scheidet sich auf der Re-Schicht 7 innerhalb von etwa 40 Min eine 30 _/um starke Schicht 6 aus ThB₄ ab. Auf diese läßt man bei einer kontinuierlichen Änderung der Unterlagentemperatur von 400°C auf 800°C und Durchflußraten 60 cm³/Min für ReF₆, 90 cm³/Min für das Th(BH₄y₄-Trägergas Ar und 90 bis 600 cm³/Min für H₂ in 5 bis 10 Min eine Ubergangsschicht 14 aus Re und ThB₄ zu einer Stärke von 5 _/um aufwachsen. Dann wird der Zufluß von Th(BH₄)₄-Trägergas beendet und mit den für die Schicht 7 genannten Verfahrensparametern in 6 Min eine 10 _/um dicke Schicht 13 aus Re abgeschieden. Den Abschluß bildet eine 100 _/um starke Schicht 5 aus mit 1 % Th0₂ dotiertem Wolfram, die unter Verwendung der im Beispiel 1 für die Schichten 5 aufgeführten Verfahrensparameter in einer Zeit von 25 Min bei einer Substrattemperatur von 600°C abgeschieden wird; diese Schicht 5 bildet die tragende Schicht der Kathode.In the apparatus described in Example 1, as shown in FIG. 5 as a result, at 500 ° C. and with a cold reactor (flow rate Q (Ar) = O), an approximately 2 _μm thick layer 15 of pure tungsten is used as the first layer within deposited on the substrate 1 for 1 min, all other process parameters correspond to those for the layer 5 from example 1, FIG. 2. Then the WF ₆ inflow is stopped and the substrate temperature is set to 800 ° C. A gas mixture of ReF ₆ with a flow rate of approximately 60 cm ³ / min and H ₂ with a flow rate of 600 cm ³ / min is passed over the substrate and then by means of the reaction

a 5 µm thick re-layer 7 is deposited within 3 minutes, which is generally preferred if it remains later. The re-deposition is through Slow reduction of the gas flow from ReF ₆ and H ₂ ended until after 2 minutes the inflow of these gases has been completely prevented. Simultaneously with this reduction in the gas inflow, the substrate temperature is set to 400 ° C. and Th (BH ₄ ) ₄ is fed to the substrate with Ar as the carrier gas, the Ar flow rate being approximately 90 cm ³ / min. Th (BH ₄ ) ₄ is in powder form in a saturator heated to about 190 ° C. The reactor temperature during the deposition should be 2C0 to 210 ° C. By pyrolytic decomposition layer Re-7 is deposited on the course of about 40 min a 30 _/ um thick layer 6 of ThB ₄ from. These are left with a continuous change in the substrate temperature from 400 ° C to 800 ° C and flow rates 60 cm ³ / min for ReF ₆ , 90 cm ³ / min for the Th (BH ₄ y ₄ carrier gas Ar and 90 to 600 cm ³ / min for H ₂ in 5 to 10 min grow a transition layer 14 made of Re and ThB ₄ to a thickness of 5 _/ um, then the inflow of Th (BH ₄ ) ₄ carrier gas is stopped and with those mentioned for layer 7 Process parameters in 10 min deposited a 10 _/ um thick layer 13 made of Re. The conclusion is formed by a 100 _/ um thick layer 5 made of tungsten doped with 1% Th0 ₂ , which uses the process parameters listed in Example 1 for layers 5 at one time of 25 min at a substrate temperature of 600 ° C; this layer 5 forms the supporting layer of the cathode.

After the coating processes have been completed, the base and cathode are slowly cooled to room temperature, the entire cathode shrinking away from the base 1 and a gap 16 forming, as described in Example 1.

Fig. 6 shows a finished cathode according to this embodiment. The cylindrical one manufactured in the CVD system Cathode body 4 is divided into several sections perpendicular to its longitudinal axis with a laser beam. A circular disk 18 of the same diameter made of tungsten or molybdenum is attached to the edge 17 of one of these sections 4 by spot welding. This circular disk carries in its center a pin 19, also made of tungsten or molybdenum, which is used for supplying the heating current and which is aligned in such a way that its longitudinal axis coincides with the cylinder axis. The heating current is dissipated again via the edge 20 of the cylinder jacket 4 facing away from the disk 18. Finally, the cathode is etched for about 30 s in a solution of 0.1 1 H ₂ 0 + 10 g potassium ferricyanide + 10 g potassium hydroxide and the outer layer 15 made of tungsten is thereby removed. The (preferred-oriented) re-layer 7 is also optionally removed. During operation of the cathode, an essentially monoatomic, electron-emitting layer of Th is formed on the surface of the exposed ThB ₄ layer (or on the Re layer) by diffusion of the Th.

Embodiment 4

Another example of the method according to the invention is explained with reference to FIGS. 7 and 8. The base is formed by a hollow cylinder made of nickel 21, which is closed off in the direction of flow and which is heated via a central power supply pin and a current discharge via the cylinder jacket, is heated directly or indirectly via a W coil 22 and on the outer surface of which the cylindrical cathode body 4 is deposited. On the base 5 of tungsten is deposited as a first layer, which is doped with 1% Th0 ₂ and 5 is prepared in Example 1 by the same method as the innermost layer, with in 20 minutes growth time at 600 ° C a 80 _/ um thick Layer forms. Then one begins to introduce ReF ₆ at the same time, the flow rate of which is increased to the same extent as that of the WF _{6 is} reduced, until after 2 minutes only ReF ₆ in the same amount as before the WF _{6 is} introduced, with the substrate temperature of 600 at the same time ° C is increased to 800 ° C and the supply of Th (C ₅ H ₇ O ₂ ) ₄ saturated Ar carrier gas is stopped.

In a period of 6 min setting parameters grows a 10 _/ um thick layer 13 of pure Re on this. Then the substrate temperature is reduced to 400 ° C. within 2 minutes, at the same time the inflow of ReF ₆ and H _{2 is} slowly reduced to zero and the inflow of Ar carrier gas saturated with Th (BH ₄ ) _{4 is} increased over the same period to a flow rate of 90 cm ³ / min, whereby the deposition of ThB _{4 is} started. The inflow of Ar saturated with Th (BH ₄ ) ₄ is continued for 40 minutes, and thus a 30 μm thick layer 6 of ThB _{4 is} grown. At the end of the layer sequence, one changes over to the deposition of pure Re in a sequence that is exactly the reverse of the time sequence to that described for the production of the transition between the Re layer 13 and the ThB ₄ layer 6, and strikes on the ThB ₄ layer 6 in 3 min a 5 _/ um thick layer 7 of Re down. The base 21 is then detached from the cathode 4 in the manner described by selective etching, the last deposited Re layer 7 protecting the ThB ₄ layer 6 from attack by the etching solution. A mixture of HN0 ₃ , H ₂ 0 and H ₂ 0 ₂ in a mixing ratio of 6: 3: 1 parts by volume or an aqueous solution of 220 g Ce (NH ₄ ) ₂ (NO ₃ ) ₆ and 110 ml HNO is used as the etchant for nickel ₃ on 1 1 H ₂ 0 used. Contacting the cathode body and optionally finally removing the re-layer 7 then take place, as in exemplary embodiment 2 shown. When the cathode substrate is heated directly by a central power supply 19 and a lead 20, only Ni is etched away under the emitting cathode jacket, which can be ensured, for example, by an Mo supply pin and an Mo cover plate, which are not attacked during the etching. With a suitable preferred orientation, the Re layer usually remains on the cathode surface.

Embodiment 5

The device described in Example 1 is used. In contrast to Example 1, a cathode is produced in which the layer 7 extends over the entire cathode body. The pad 1 is heated to 650 ° C; the total pressure in the reaction chamber is 67 mbar. A fine crystalline tungsten layer with a preferred orientation in the <1,1,1> direction with respect to the surface of the substrate, doped with 2 wt.% Th0 ₂ to stabilize the microstructure, is deposited on the inside of a cylinder by reactive deposition from the gas phase applied from pyrolytic graphite until the layer reaches a thickness of 150 _/ um. The corresponding flow rates of the supplied gases are 20 cm ³ / min for _W g ₆ , 150 cm ³ / min for H ₂ and 100 cm ³ / min for Ar saturated with Th-diketonate, the saturator being at a temperature just below the melting point of the organometallic Th compound is held. In this example, the dopant Th0 ₂ serves as an emissive substance and at the same time ensures the microstructural and mechanical stabilization of the cathode.

The invention thus provides a cathode which has hitherto only had singular advantages in known cathode types unites, whose layer sequence is produced entirely via the gas phase in a single operation with variation of the parameters, which is self-supporting, has a continuous and large surface (without intentionally created holes, such as mesh cathodes) and is therefore suitable as a unipotential cathode and at which, as a result of the substrate detachment after the deposition, avoids the mostly harmful interaction with the substrate. In particular, the cantilever design is made possible by simultaneously with deposited structure-stabilizing (insoluble) additives, which additives in a similar form also stabilize the texture of the preferred oriented top layer and make the advantage of high electron emission with a suitably adjusted preferred orientation available even for long lifetimes or operating times.

The high emission and long lifespan are also due in particular to the high doping concentration with emissive substance in the subsequent delivery and storage area, which has not previously been possible with powder metallurgy methods for any substrate shape, as well as the finely crystalline structure of the top layer with average grain diameters ⁹ 1 _/ um, which good subsequent delivery of the emissive substance by grain boundary diffusion to the surface, good monoatomic surface coverage even at higher temperatures and low desorption rates guaranteed.

Claims (30)

1. A method for producing a thermionic cathode with a polycrystalline cover layer made of a high-melting metal, which is deposited on the underlying layers, characterized in that a) applying the following layer structure to a base shaped according to the desired cathode geometry by transport over the gas phase, possibly combined with reduction reactions during or after application of the layers: α) a carrier layer made of high-melting metal as the base material and at least one dopant for mechanical structural stabilization, β) a layer or layer sequence acting as a subsequent delivery area during operation of the cathode, consisting of a high-melting metal as the base material and a supply of electron-emitting material, and γ) the polycrystalline cover layer or a preferentially oriented polycrystalline cover layer made of a high-melting metal as base material and at least one dopant for texture and structure stabilization, the preferential orientation being determined by the choice of the deposition parameters such that the work function from the emitter monolayer, which spreads during operation of the cathode on this cover layer, is minimal, b) the pad is removed and c) provides the carrier layer with connections for heating. 2. The method according to claim 1, characterized in that the application of the layers by reactive deposition such as CVD process, pyrolysis, cathode sputtering, vacuum condensation or plasma sputtering takes place. 3. The method according to claim 1 or 2, characterized in that W, Mo, Ta, Nb, Re and / or C is used as the base material, the composition of the base material in the individual layers being the same or different. 4. The method according to claim 1 to 3, characterized in that the gases involved in the deposition reaction are excited by generating a plasma for chemical conversion and the associated deposition of cathode material. 5. The method according to claim 1, characterized in that a body made of an easily and precisely mouldable material is used as a base, which has little adhesion to the material deposited thereon or which can be easily detached from the positive. 6. The method according to claim 1 and 5, characterized in that the base is removed by selective etching, mechanically, by evaporation when heated in a vacuum or in a suitable gas atmosphere, by burning off or a combination of the methods mentioned. 7. The method according to claim 5 or 6, characterized in that a body made of graphite, in particular pyrolytic graphite, or vitreous carbon is used as a base, which is removed by mechanical processing, burning and / or mechanical-chemical micropolishing. 8. The method according to claim 5 or 6, characterized in that a body made of copper, nickel, iron, molybdenum or an alloy is used as a base with a predominant proportion of these metals, the mechanical and by selective etching or initially in its predominant mass in the remaining residues is removed by evaporation when heated in vacuo or under a suitable gas atmosphere. 9. The method according to claim 5, characterized in that a body made of electrographite is used as the base, which is coated with a layer of pyrolytic graphite. 10. The method according to claim 1 to 4, characterized in that in the production of the carrier layer, the CVD layer growth is repeatedly interrupted by repeated substrate cooling to room temperature and the nucleation is restarted by heating or a periodic variation of the substrate temperature in the range between 300 and 700 ° C. 11. The method according to claim 1 to 4, characterized by the deposition of wafer-thin crystallite growth-inhibiting intermediate layers in the manufacture of the carrier layer. 12. The method according to claim 1 to 4, characterized in that in the production of the carrier layer, the base material is deposited together with a doping material which has a negligibly low solubility in solid in the crystal lattice of the base material. 13. The method according to claim 1 to 4, characterized in that as the base material tungsten and as structure-stabilizing doping ThO ₂ , Zr, ZrO ₂ , UO ₂ , Y ₂ 0 ₃ , Sc ₂ 0 ₃ , Ru, Y and / or Sc in one Concentration of about 0.5 to 2 wt.%, In particular about 1 wt.%, Can be deposited simultaneously or alternatively with tungsten using the CVD method. 14. The method according to claim 1 to 4, characterized in that a high concentration of electron-emitting substance from the scandium group (Sc, Y, La, Ac, lanthanides, actinides) is selected in the manufacture of the subsequent delivery area and in metallic oxide, boride and / or carbide form is alternately or simultaneously deposited with the high-melting metal. 15. The method according to claim 1 to 4, characterized in that the following material combinations of electron-emitting substance and high-melting metal are selected and deposited by CVD method: Th / Th0 ₂ + W, Th / ThO ₂ + Nb, ThB ₄ ⁺ Re, Y / Y ₂ O ₃ ⁺ Ta, Y ₂ O ₃ + Nb, Y ₂ 0 ₃ + W or Mo, Sc ₂ 0 ₃ + W or Mo, ^L a ₂ 0 ₃ + W or Mo. 16. The method according to claim 1 to 4, characterized in that lanthanide oxides, preferably Ce0 ₂₁ Sm ₂ 0 ₃ and Eu ₂ 0 ₃₁ in combination with W or Mo as the base and cover layer material are deposited as electron-emitting substances. 17. The method according to claim 15, characterized in that ThB ₄ by pyrolysis of Th (BH ₄ ) ₄ , enriched in argon as a carrier gas, applied to a layer of rhenium with an underlying structure-stabilized tungsten carrier at substrate temperatures greater than / equal to 300 ° C. becomes. 18. The method according to claim 1 to 4 and 14, characterized in that the electron-emitting substance in its oxidized form together with an activator component, preferably boron or carbon, and a diffusion-enhancing component - preferably Pt, Ir, Os, Ru, Rh or Pd, in a concentration of 0.1 to 1% by weight. 19. The method according to claim 1 to 4, characterized in that the reactive deposition or pyrolysis is carried out at temperatures of the substrate from 200 ° C to 600 ° C, preferably 400 to 550 ° C, with corresponding starting compounds for the electron-emitting substance , volatile organometallic compounds are used even at these temperatures and the desired layer structure is achieved by repeated changes in the gas composition and / or the other deposition parameters. 20. The method according to claim 15, characterized in that tungsten and thorium or Th0 ₂ alternately or simultaneously from WF ₆ + H ₂ and Th-diketonate, in particular Th-acetylacetonate, preferably Th-trifluoroacetylacetonate or Th-hexafluoroacetylacetonate, but also _T. h-heptafluorodimethyloctanedione or Th-dipivaloylmethane, can be grown at temperatures between 400 ° C and 650 ° C by reactive deposition from the gas phase, the organometallic Th starting compound being in powder form in a saturator which is at a temperature close to the respective melting point is heated and an inert gas, in particular argon, flows through it as the carrier gas. 21. The method according to claim 1 to 4, characterized in that on a structure-stabilized doped CVD carrier layer of 30 to 300 _/ um thickness, in particular 100 _/ um thickness, the subsequent delivery area with emissive substance is applied in the form of a layer sequence by CVD method, in which a layer of high-melting metal with low additions of electron-emissive material and possibly stabilizing doping alternates with one with high concentrations of electron-emissive material, which is somewhat thinner, the layer spacings being of the order of the grain sizes, the individual layer thickness in particular 0.5 to 10 _/ um at a concentration of the emitting substance up to 5 wt.% And in particular 0.1 to 2 _/ um at a concentration of the emitting substance from 5 to 50 wt.% And the mean concentration of emitting substance is preferably at 15 to 20% by weight. 22. The method according to claim 1 to 4, characterized in that a polycrystalline, preferentially oriented cover layer is applied, in which the crystalline preferential orientation is set in such a way by the parameters of the CVD deposition process, in particular the flow rates of the gases involved in the reaction and / or the substrate temperature is that the electron emission current density from the essentially monoatomic film of the electron-emitting substance on the cover layer at a given temperature becomes maximum or the work function is minimal and the cover layer is texture-stabilized by means of insoluble, simultaneously deposited dopants with respect to longer temperature loads. 23. The method according to claim 1 to 4 and 22, characterized in that essentially W, Re, Os or Nb is applied as a covering layer, with tungsten with thorium as monoatomic film on the surface the <111) orientation of tungsten as a preferred orientation is set and ThO ₂ , Zr0 ₂ , Y ₂ 0 ₃ , Sc ₂ 0 ₃ and / or ruthenium are simultaneously deposited with a concentration of 1 to 2% as texture-stabilizing components. 24. The method according to claim 1 to 4, 22 and 23, characterized in that the cover layer has a thickness of 2 to 20 _/ um and the substrate temperature is set such that the average grain diameter is ≦ 1 _/ um. 25. The method according to claim 15, characterized in that emitting substance and structure-stabilizing doping of the carrier or cover layer material are identical to each other. 26. Modification of the method according to claim 1 to 25, characterized in that the base is formed as a hollow body, preferably as a tube, in particular made of graphite, and the reactive deposition from the gas phase is carried out on the inside of the hollow body, with the temporal coating sequence in in the opposite direction and first the preferred oriented cover layer and lastly the carrier layer is deposited. 27. The method according to claim 26, characterized in that the hollow body consists of pyrolytic graphite and the cathode material has a linear coefficient of thermal expansion which is significantly greater than that of pyrolytic graphite (in the stratification direction), so that the cathode shrinks more strongly when cooled to room temperature as the base made of pyrolytic graphite, the top layer loosens and the cathode can be pulled out of the hollow body. 28. The method according to claim 1 to 27, characterized in that the entire cathode with all material layers is produced by reactive deposition from the gas phase in a continuous process. 29. The method according to claim 1 to 28, characterized in that the layer structure is applied in such a way that the three layers α, β and y are identical. 30. Thermionic cathode with a polycrystalline cover layer made of a high-melting metal, which is deposited on layers below, produced by the method according to one or more of claims 1 to 29, characterized in that the cathode has the following layers: a) a carrier layer (5) made of high-melting metal as the base material and at least one dopant for mechanical structural stabilization, b) a layer (6) or layer sequence (8, 9) which acts as a subsequent delivery area when the cathode is in operation, consisting of a high-melting metal as the base material and a supply of electron-emitting material, and c) the polycrystalline cover layer or a preferentially oriented polycrystalline cover layer (7) made of a high-melting metal as the base material and at least one dopant for texture and structure stabilization, the preferred orientation being determined in such a way that the work function from the emitter monolayer which occurs during operation of the Spreading cathode on this cover layer is minimal.

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