EP0087826A2 - Thermionic cathode and manufacturing method - Google Patents

Abstract

The cathode has a layer structure, the individual layers, which essentially consist alternately of emitter material (3) and base material (1), being arranged obliquely to the macroscopic emitting cathode surface. In a preferred embodiment, the surface has a microscopic step structure, the outlet steps (2) forming the continuation of the emitter material layers (3). In a further embodiment, the surface is not stepped, but is formed by a polycrystalline or a preferentially oriented polycrystalline cover layer which is arranged on the sequence of inclined layers. The layer sequence is produced by alternating deposition from the gas phase, combined with subsequent bevel grinding of the layers. The polycrystalline cover layer is in turn applied by deposition from the gas phase. The step surface is e.g. generated by selective structure etching after bevel grinding.

Description

The invention relates to a thermionic cathode with a cathode body, which consists of a high-melting base material and a supply of emitter material, and with an electron-emitting monolayer on the surface of the cathode body, the monolayer being supplemented from the supply of emitter material during operation of the cathode. The invention further relates to a method for producing such a thermionic cathode. In the following, such cathodes are also referred to as delivery cathodes or monolayer cathodes.

Thermionic monolayer cathodes with thorium as the emitter material, i.e. as an electron-emitting substance, on tungsten as a high-melting base material, i.e. as a base matrix, have long been known (US Pat. No. 1,244,216) and have been investigated in detail early on, but because of their wide commercial distribution due to their good vacuum behavior, their fairly high emission and their favorable properties when used in UHF and microwave tubes a further improvement, especially in terms of emissions, is necessary due to increased requirements.

In general, such thermionic monolayer consist cathode of a base matrix of refractory metal, a compound is incorporated into the emitter material elemantar or in the form, the olumendiffusion at the operating temperature in the form of atoms, for example as a result grain boundary diffusion, _V or pores on the surface of the cathode diffuses and there is a surface monolayer forms or complements. The formation of a monolayer, ie an approximately monoatomic layer of emitter atoms on the surface, is supported by the increasing desorption with a greater degree of coverage. Especially with thoriated tungsten cathodes (Th- [W] cathodes) Th is released from Th0 ₂ thermally or preferably by reaction with W ₂ C and diffuses along the grain boundaries to the tungsten surface.

With a suitable choice of emitter and base material, the dipole field between the monolayer and the underlying atoms of the base material brings about an additional reduction in the emitter work function for thermionic electrons, so that monolayer cathodes have a higher electron emission than cathodes made of pure emitter material. For example, the work function for pure Th at about 3.5 eV, while for a Th monolayer on tungsten it is only 2.8 eV.

However, the cathode will only function properly if the entire emitting surface is actually covered by this monolayer, ie by a monoatomic film. This condition becomes critical at higher temperatures at which sufficient coverage and thus emission are no longer guaranteed due to strong desorption of the emitter atoms. With Th-Fwj cathodes, such a drop in emissions occurs at around 2200 K. The emission finally drops to that of pure tungsten. The temperature at which the drop in emissions occurs depends, in particular in the case of monolayer cathodes, with subsequent delivery of the emitter material via grain boundary diffusion on the grain size. Since the emitter atoms spread on the surface by surface diffusion, the sources for the emitter atoms being the Grain boundaries, smaller crystallites with the same diffusion length naturally lead to better coverage.

It is crucial for the desired improvement of the cathode emission, however, that there has been a problem that has not been solved for decades regarding emission and thorium diffusion length. From measurements of the thorium desorption rates Y _D of tungsten and measurements of the surface diffusion constant D _d for thorium on polycrystalline tungsten, the diffusion length can be given as √D _o .c _o / ν _D , where c _{o =} 1 is the relative thorium concentration at the edge of the Source is. This theoretically required diffusion length is, however, orders of magnitude larger than that which can be calculated from the mean grain sizes and the temperature of the drop in emissions.

I. Langmuir gave a possible plausible explanation of this phenomenon with the help of the so-called "edge effect" (Journal of The Franklin Institute 217 (1934) 543-569). Thereafter, at the edges of the individual tungsten crystallites, i.e. increased thorium desorption at the thorium exit sites, e.g. due to strongly inhomogeneous fields. This of course means an increased resistance to spreading and a shortening of the actual diffusion length. The goal of a cathode improvement must therefore be to eliminate the edge effect by means of a suitable structuring of the cathode.

In addition to the edge effect, there is another limitation of the cathode emission to be eliminated: The subtractive dipole field between the emitter monolayer and the base material is strongly dependent on the orientation of the base. With the usual polycrystalline non-textured cathodes, such as with all conventional powder metallurgy Monolayer cathodes, this leads to a locally strongly varying electron emission, the lowest work function being achieved only with a few randomly favorably oriented crystallites. So-called "spotty emitters" are obtained.

From DE-OS 14 39 890 a method is known, conventional monolayer cathodes with a polycrystalline preferential layer e.g. to be coated from the base material, producing the preferred orientation of the cover layer which causes the greatest reduction in the work function. In this way, a cathode which is homogeneously emitting with an increased emission current density is obtained to a good approximation, since all surfaces contribute to the emission to a similar extent. With Th- [W] cathodes e.g. <111> is the cheapest W orientation. However, the high electron emission of such suitably preferred cathodes does not remain stable over time; the texture is partially destroyed when activated.

In contrast, the invention has the object of specifying a suitable cathode structure and of creating a method for producing this structure, with which it is possible to avoid the edge effect in Th-ON1 and analog monolayer cathodes and also to emit the fine crystallinity of the base material and a suitable one Texture, as well as by ensuring the thermal stability of the texture, and to keep it stable over time.

This object is achieved in that, in the case of a cathode of the type mentioned at the outset, the cathode body consists of a sequence of layers of base material and intermediate layers with a high concentration of emitter material, and that the macroscopic Cathode surface extends obliquely to the main surfaces of at least that part of the layers which is in the vicinity of the macroscopic cathode surface.

According to the invention, the layer sequence is preferably produced by alternating deposition of the high-melting base material and the electron-emitting substance from the gas phase, and then the macroscopic emitting surface is produced by an angle grinding.

Advantageous refinements of the cathode according to the invention and advantageous developments of the method according to the invention are described in the subclaims.

A preferred cathode structure according to the invention looks as follows: The cathode consists of a layer sequence of layers arranged obliquely to the emitting cathode surface, which alternately consist primarily of high-melting base material and of emitter material. The thickness of these layers is in the range 'g a few _/ µm to 0.01 _/ µm, the emitter material layers being significantly thinner than the base material layers. The electron-emitting substance, which is preferably an element of the scandium group, in particular thorium, or one of its compounds, is characterized in that it essentially reaches the surface by grain boundary diffusion through the high-melting base material, in particular tungsten, and spreads there by surface diffusion. In addition to W, Mo, Ta, Nb, Re and / or C are also used as base materials, the composition of the base material being the same or different in the individual layers of the layer sequence.

The surface has a stepped structure, with the strongly emitting treads of the steps (hereinafter also referred to as outlet steps) forming the continuation of the layers of emitter material. The emitter atoms diffuse directly to the outlet stages without edge inhomogeneities and form a monolayer there. In a preferred embodiment of the invention, the base material layers have a suitable preferred orientation with respect to the layer normal, for Th [W] cathodes this is, for example, the <111> orientation for the W base material. The cathode material is finely crystalline with grain sizes -9 1 _/ um. It is also favorable if the grain diameters are somewhat larger than the step widths. The temporal stability of the texture is achieved by doping the base material with components which are only slightly or not soluble at all. Further doping in the edge zone of the emitter material layers serve to better release the emitter atoms if the emitter material is in the form of a compound.

In a further embodiment of the invention, the surface of the inclined layer structure is covered with a polycrystalline, optionally preferred-oriented layer made of base material or another material which, in combination with the emitter monolayer, brings about a sharp reduction in the electron work function. The boundary of the sloping layer towards the top layer is usually smooth, without pronounced steps. The top layer is finely crystalline.

The cathode according to the invention is preferably produced in three processing steps. In the first step, a layer sequence is first produced by alternating deposition of the high-melting base material and the electron-emitting substance from the gas phase.

A method for alternating deposition of base material and electron-emitting substance has been proposed in patent application P 31 48 441.7; this method and its configurations (also for simultaneous deposition) can also be used in the method according to the invention. The layers are applied by reactive deposition such as CVD processes, pyrolysis, cathode sputtering, vacuum condensation or plasma sputtering. In a particularly advantageous embodiment of the proposed method, the gases involved in the deposition reaction are caused by generating a plasma for chemical conversion and the associated deposition of cathode material (so-called plasma-activated CVD method = PCVD). Instead of high-frequency excitation, the chemical reaction can also be excited or induced by photons or by electron impact. Applied to the preferred material combination _T hW, this means that a layer sequence of pure or tungsten doped with a stabilizer is alternately deposited with ThO ₂ layers from the gas phase on a suitable substrate. If organometallic starting compounds are used, the Th-CVD also carburizes the base material deposited with it. In a preferred embodiment, tungsten is deposited by setting the CVD parameters <111> in a preferred manner.

The layer sequence is preferably produced by reactive deposition with temporal variation of the parameters, in particular the flow rates of the gases involved in the reaction and / or the substrate temperature. According to a special embodiment of the method according to the invention, the temporal variation of the parameters of the reactive deposition takes place essentially periodically (alternating CVD method).

In the second processing step, the layers are ground at an angle after the deposition, preferably at an angle of 20 to 70 °, in particular 45 °. The bevel cut according to the invention takes place e.g. by mechanical processing, such as grinding or milling, and / or mechanical-chemical micropolishing, or by cutting to size with the aid of a laser beam.

In the third processing step, in a preferred embodiment of the invention, a step-like structure of the surface is produced by etching. A suitable etchant for the combination Th-W is, for example, a 3% by weight solution of H ₂ 0 ₂ . The step-like microstructure of the surface can also be created using other methods. This includes, for example, the local evaporation of base material using an intense laser beam or electron beam, which is guided over the ground surface in accordance with the trailing edges of the emitter layers. In addition, there is also the possibility of roughening the surface by mechanical processing such as fine sanding and then carrying out a heat treatment for recrystallization, especially of surface crystallites. The inclined emitter material intermediate layers with their lower mechanical stability are the cause of the formation of the step structure or of the inhibition of the base material recrystallization on the intermediate emitter material layer in the latter process combination. The steps are designed so that their treads are in the extension of the layers with a high concentration of emitter material, while the step grooves are perpendicular to it. As a result, the emitter material can diffuse directly from the layers of high emitter material concentration to the surface of the outlet stages - and this without strong desorption at grain boundaries.

An appropriately set preferred orientation of the base layers also ensures that the lowest work function from the emitter-monolayer base combination is achieved everywhere on the outlet stages. The crystallites in the step throats are of course still randomly oriented. However, their share of the total area can be determined by a shallower angle of inclination of the layers than 45 ° - e.g. 25 ° - to be greatly reduced to the macro surface.

In order to stabilize the microstructure and the finest crystallinity of the cathode material of the monolayer cathodes according to the invention with subsequent grain boundary addition, the production method according to the invention is supplemented by a simultaneous deposition of additional dopants. This is again demonstrated using the typical example of the Th [W] cathodes. If the temperature of Th- [W] _C cathodes is increased above the normal working temperature of 2000 to 2100 K, then due to increasing Th-desorption from the monolayer, ie decreasing Th-coverage, a significant decrease in the emission occurs, in particular from 2200 K, so that an increase in emissions by increasing the temperature can no longer be realized. This drop in emissions 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 grain sizes can practically only be achieved using a CVD process, and because only through a suitable choice of parameters, produce. Of course, this very fine crystallinity must also remain stable with regard to longer thermal loads. If, for example, the grain size increases too much due to recrystallization during operation of the cathode, this finally causes a decrease in the emission current and thus a decrease in the monoatomic coverage lower lifespan. 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 a carrier 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, and at the same time also stabilizes the texture. 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 O ₃ , Sc ₂ 0 ₃ and Ru are suitable because of their low solid solubility in tungsten. At a working temperature of 2000 K, which means that the melting point of the doping must be higher, and if simple handling is required, Th0 ₂ , ZrO ₂ , Y ₂ O ₃ , Sc ₂ 0 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.

When producing a monolayer cathode according to the invention with any surface shape, a further processing step can be inserted after grinding, namely the assembly of individual cut facets to form a cathode body of the desired surface geometry, e.g. by means of an inlay technique. Another possibility, which is described in more detail in the exemplary embodiments, is the use of notched substrates (cf. FIG. 4).

In a further preferred embodiment of the method according to the invention, a polycrystalline cover layer or a layer is deposited on the surface produced by bevel grinding by means of a deposition from the gas phase preferred oriented polycrystalline top layer applied. One of the few possibilities for producing a preferentially oriented polycrystalline cover layer is again chemical deposition from the gas phase, it being expedient to adhere to certain combinations of the deposition parameters, especially the substrate temperature and flow rates of the gas mixture. The cover layer consists of a pure, high-melting metal, such as W, Mo, Ta, Nb, Re, Hf, Ir, Os, Pt, Rh, Ru, Zr, or of C and should have a preferred orientation. The material and its texture are chosen so that the work function from the combination of the emitter monolayer and the top layer is even lower than that from the combination of the emitter and the base. 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. A prerequisite for good surface coverage is either fine crystallinity of the cover layer of the emitter material or the presence of sufficient volume diffusion in the cover layer.

• Fig. 1 einen Querschnitt durch eine Kathode im Ausschnitt,
• Fig. 2 die Kathode nach Fig. 1 im Gesamt-Querschnitt,
• Fig. 3 einen Querschnitt durch eine zylinderförmige Kathode mit stufiger Außenmantelfläche,
• Fig. 4 einen Querschnitt durch eine Kathode mit einem ebenen Substrat mit sägezahnförmiger Kerbung und
• Fig. 5 eine graphische Darstellung der Abhängigkeit der Sättigungsemissionsstromdichte von der Kathodentemperatur.

Some embodiments of the invention are shown in the drawing and are explained in more detail below. Show it

• 1 shows a cross section through a cathode in detail,
• 2 shows the cathode of FIG. 1 in total cross section,
• 3 shows a cross section through a cylindrical cathode with a stepped outer surface,
• Fig. 4 shows a cross section through a cathode with a flat substrate with sawtooth notch and
• 5 shows a graphical representation of the dependence of the saturation emission current density on the cathode temperature.

In FIG. 1, 1 denotes base layers made of grain-stabilized, ie doped, tungsten. These layers are 1 to 2 _/ um thick. 2 denotes Th monolayers on W <111>. 3 denotes intermediate layers of Th0 ₂ from 0.1 to 0.5 _/ um thickness. There is a W ₂ C enrichment in the edge zone of the intermediate layer, which serves to release Th from Th0 ₂ . The intermediate layer 3 can also consist of Th0 ₂ and W ₂ C (as a mixture). 4 indicates the direction of deposition.

The entire cathode is usually a flat cathode, which is heated directly or indirectly. The layer sequence itself is achieved by a high-frequency alternating deposition of optionally doped W and Th0 ₂ . The high-frequency layer sequence is achieved by computer control of the process, in particular the flow of the individual gaseous components. The substrate temperature is about 500 ° C, the pressure in the reactor 10 to 100 mbar, preferably 40 mbar. In the W-CVD, the WF ₆ flow rate is approximately 30 cm ³ / min at an approximately 10 times H ₂ flow rate. The interval is up to a few minutes, especially 1 minute. In the intervals between, Ar is also used as the carrier gas for thorium acetylacetonate or fluorinated Th-acetylacetonate and WF ₆ Th0 ₂ or Th0 ₂ + W ₂ C for about 1 minute. Th (C ₅ H ₇ O ₂ ) ₄ is in powder form in a saturator flowed through by Ar at about 85 cm ³ / min, which is heated to a temperature of about 160 ° C. or close to the melting point of the Th compound. The reaction temperature is about 20 ° higher.

An additional W ₂ C enrichment at the edge of 3 is either by briefly (about 8 s) introducing a hydrocarbon-containing gas at the beginning of the new W-CVD interval or by stronger WF ₆ enrichment towards the end of the Th deposition, especially for Th -Trifluoroacetylacetonat achieved as a starting compound. As an alternative to carburization, boronization of the edge zone is also advantageous.

In the case of very high-frequency deposition of W and Th, doping of W can optionally be dispensed with, since grain stabilization is already caused by the intermediate layers. In layer sequences with more than 2 _/ um distance doping of the CVD-W with a hardly soluble in W or insoluble substance, such as 1 wt.% ThO _2, Zr0 _2, Y ₂ 0 _3, Sc ₂ 0 ₃ or Ru advantageous . The flow rate of WF ₆ is set so high that it just leads to a deposition of W in the <111> direction at the respective substrate temperature. After deposition of approximately 1000 to 2000 layer sequences, the CV _D sample is poured or clamped and ground flat at an angle of 45 ° to the growth direction or cut to size using a laser. Subsequently, the other sides of the sample to be abraded and by CVD deposition with an about 50 to 150 _/ um thick Re or W-coating 6 is provided (Fig. 2). Then the resulting sample is spotted on a hairpin 7 for heating. The device provided for emission uncovered sharpened cathode surface is etched again to a few tenths _/ um micro-polished and then with a suitable W for Strukturätzmittel carefully so that the desired step-like surface structure is obtained. A suitable structure etchant for W is, for example, a 3% by weight solution of H ₂ O ₂ .

If a partial conversion of the Th compound or of Th0 ₂ to metallic thorium is carried out after the CVD deposition, an electrochemical etching with a 14 CH ₃ COOH: 4HC10 ₄ : 'H ₂ ^0- Solution (temperature 10 ° C) for current periods (i ≦ 0.1 _{A /} cm) carried out 1 1 s, which acts directly on the intermediate layers. Even in the case of a tungsten carbide enrichment in the intermediate layer, pre-etching for step structuring can first take place using known etching agents which act on WC or W ₂ C (for example electrochemically with 2 g NaOH, 2 g Na tungsten and 100 ml water).

However, the cathode structure described in this exemplary embodiment and its method of manufacture apply not only to the Th-W emitter-base combination, but also to any combination of an emitter with a high-melting metal in a monolayer cathode in which the emitter is essentially supplied via grain boundary diffusion. Such emitter materials are e.g. can also be found in the scandium group: the above cathode structure is also a preferred structure for the combination Y-W and Sc-W. The corresponding acetylacetonates can also be used for the deposition of Y or Sc oxide.

In contrast to the production of the planar cathode of FIG. 2, the production of a cylindrical cathode with a stepped outer surface area becomes significantly more difficult. This problem can be solved either by assembling the cylinder jacket from a few (slightly curved) pieces, for example using spot welding, or by another mosaic or inlay technique that can also be used for cathodes of any surface shape. For cylindrical cathodes it is also advisable to use an elliptical substrate or a substrate 8 with a gear-shaped cross section (= longitudinally ribbed cylinder jacket) as in FIG. 3 coat and then grind and then carry out the step structuring. With a large number of ribs 9, a longitudinally ribbed cylinder substrate 8 provides a fairly uniform electron emission density distribution over the circumference of the jacket. By increasing the number of ribs in relation to the circumference, substrates of smaller thickness can be used because of the associated reduction in the rib depth, which is advantageous for the cathode heating. However, for special applications, for example for magnetron cathodes, the opposite effect can be exploited, such as with cylindrical substrates with an elliptical cross-section, and an inhomogeneous distribution of the emerging electrons due to the widely differing step widths, which, for example, results in four maxima in the electron exit density. Ribbed substrates are also advantageously used to produce cathodes with any surface geometry, for example flat substrates with ribbing or substrates with any curved surface with ribbing. In the case of flat cathodes, this saves in particular the facetted assembly of larger areas, for which one would normally use a mosaic or inlay technique. If, for example, a macroscopically "flat" substrate as in FIG. 4 with sawtooth-shaped notch is used, the boundary condition for parallel growth on the inclined notch surfaces is that the reactive deposition from the gas phase takes place in the area regulated by the surface reaction kinetics, ie the subsequent delivery The gaseous starting compounds to the surface must not yet be limited by the gas phase diffusion, the deposition temperatures must therefore be selected taking into account a turning point in the growth characteristics in the lower temperature range. The notch depths are in the range from 10 to 20 _/ um and about 10 to 20 layer sequences are applied. With a Th [W] cathode, the W layers again <111> -preferred and deposited with a structure-stabilizing component.

After the CVD coating, the surface is ground smooth in accordance with the selected substrate geometry, and the surface is provided with micro-steps according to one of the methods specified, the tread surfaces 2 of which in turn correspond to the outlet steps of the intermediate layers 3 of emitter material. The levels are e.g. generated by structure etching. The substrate 8 is e.g. made of molybdenum, in which the notches 9 are produced by mechanical processing. In FIG. 4, 1 again designates the base material layers, 3 the emitter material intermediate layers, 2 the outlet stages covered with the monoatomic emitter layer and 4 the deposition directions in the CVD deposition. The removed part of the CVD layers is indicated by dashed lines.

The decisive advantages of cathodes according to the invention with a stepped surface are as follows: The most important advantage is based on the suppression of the edge effect. Without strong desorption, the emitter atoms diffuse freely at the grain boundaries near the surface through the outlet stages and form a monolayer there. For Th- [W] cathodes according to the invention, because of the much lower edge desorption, the critical temperature shifts upwards by about 200 ° and the emission maximum also only occurs at a higher cathode temperature (about 2100 K). Step cathodes according to the invention thus open up the possibility of also achieving a higher emission current density by increasing the temperature than is customary in the conventional Th [W] cathodes. On the other hand, at the usual operating temperatures the consumption of emitter material is lower, the service life is consequently extended with the same supply of emitter material.

Another advantage is that the effective emitting surface is increased by the step structure; with a cut below 45 ° the magnification factor is approximately 1.4, which is desirable for Th [W] cathodes at temperatures ^S 2000 K.

Another important advantage of the invention is based on the deposition of the base material layers with that preferred orientation for which the work function of an emitter monolayer on this crystalline-oriented basis is minimal. For Th- [W] cathodes, this is the <111> -orientation of W. Thus, the outlet stages are <111> -oriented even in the normal direction for stratification; the side surfaces of the steps are statistically oriented and therefore make little contribution to the overall emission. It is therefore expedient to correspondingly increase the preferred area proportion of the run-off stages by means of a flatter ground angle, for example of 30 °, which in turn means an increase in the overall emission curve 11 (FIG. 5). FIG. 5 shows the approximate course of the emission current density i _s (T) of a stepped Th [W] cathode according to the invention as a function of the cathode temperature T. In comparison, curve 10 i _s (t) are presented for a conventional thoriated W wire cathode. The texture of the W layers is stabilized by additions of about 1% by weight, which are practically insoluble in W, for example Th0 ₂₁ ZrO ₂ , Y ₂ 0 ₃₁ Sc ₂ 0 ₃ and / or Ru. This doping also causes an inhibition of grain growth, which because of the intermediate layers anyway only plays a role laterally in the base material layers. The diffusion of the emitter material to the surface takes place along the intermediate layers 3 and is not hindered by lateral crystallite growth of the base layers.

This undisturbed subsequent delivery of the emitter material for Surface is used in the following further embodiment of the invention: the sequence of inclined layers, which in this case need not have a preferred orientation, is after grinding by reactive deposition from the gas phase with a polycrystalline, preferably preferentially oriented polycrystalline cover layer made of base material, for example <111 ) W for a Th [W] cathode, or with another high-melting material with a low work function from the emitter-monolayer-top layer combination. The thickness of this cover layer is in the range of about 2 to 20 _/ µm, preferably 5 to 10 _/ µm. The average grain sizes or grain diameters are set to values ≦ 1 _/ um by choosing the CVD parameters (low temperature ≦ 500 ° C and doping as above). When using an inlay technique for any surface shape, the CVD coating takes place according to the composition of the individual pieces to the desired surface shape. The range of favorable grinding angles in this embodiment of the invention is between 20 ° and 90 °.

The most important advantage of this embodiment lies in the subsequent delivery of the emitter material to the surface undisturbed by grain growth, combined with a high stock and less desorption than e.g. with MK cathodes (metal capillary cathodes), which means an overall increase in the service life in comparison to conventional Th [W] cathodes. At the same time, the emission is increased by the <111> -textured and texture-stabilized cover layer compared to known Th- [W] cathodes.

Claims (16)

1. Thermionic cathode with a cathode body, which consists of a high-melting base material and a supply of emitter material, and with an electron-emitting monolayer on the surface of the cathode body, the monolayer being supplemented from the supply of emitter material during operation of the cathode, characterized in that that the cathode body consists of a sequence of layers (1) of base material and intermediate layers (3) with a high concentration of emitter material, and that the macroscopic cathode surface extends obliquely to the main surfaces of at least that part of the layers (1, 3) that extends located near the macroscopic cathode surface. 2. Cathode according to claim 1, characterized in that the cathode surface has a step structure, the tread surfaces (2) of the steps forming a continuation of the intermediate layers (3) made of the emitter material. 3. Cathode according to claim 1, characterized in that on the surface of the sequence of inclined layers (1, 3) a polycrystalline cover layer (4), which is optionally preferred, is arranged. 4. Cathode according to claim 1, 2 or 3, characterized in that the emitter material is an element of the scandium group, in particular thorium, or one of its compounds, and the base material is tungsten. 5. Cathode according to one or more of claims 1 to 4, characterized in that the layer sequence (1, 3) consists of alternating deposits of high and low concentrations of the emitter material. 6. Cathode according to one of several of claims 1 to 5, characterized in that the macroscopic cathode surface is arranged at an angle between 10 ° and 70 °, in particular 45 °, to the main surface of the layers (1,3). 7. Cathode according to one or more of claims 1 to 6, characterized in that in an edge zone of the intermediate layers (3) or in the intermediate layers (3) itself there is additionally carbon and / or boron in a concentration in the order of magnitude of the emitter material concentration. 8. Cathode according to one or more of claims 1 to 7, characterized in that the layers (1) of the base material have a thickness of 0.5 to 20 _/ um, preferably 1 to 2 _/ um, and the intermediate layers (3) made of emitter material have a thickness of 0.1 to 0.5 _/ µm, in particular 0.2 _/ µm, the optionally present carbon and / or boron-containing edge zone having a thickness of 0.2 _/ µm. 9. A method for producing the thermionic cathode according to one or more of claims 1 to 8, characterized in that the layer sequence is produced by alternating deposition of the high-melting base material and the emitter material from the gas phase and then the macroscopic surface is produced by an angle grinding. 10. The method according to claim 9, characterized in that the layer sequence is produced by reactive deposition with temporal variation of the parameters, in particular the flow rates of the gases involved in the reaction and / or the substrate temperature. 11. The method according to claim 10, characterized in that the temporal variation of the parameters of the reactive deposition takes place essentially periodically. 12. The method according to claim 10 or 11, characterized in that tungsten layers, which are optionally doped for structural stabilization with up to 2 wt.% Th0 ₂ , Zr0 ₂ , Y ₂ O ₃ , SC203 and / or Ru, by adjusting the CVD parameters <111> are preferentially deposited. 13. The method according to claim 9, 10, 11 and / or 12 for producing a thermionic cathode according to claim 2, characterized in that the stepped microstructure of the surface a) etching or structure etching and / or b) local evaporation of material by means of an electron beam and / or c) local material evaporation by means of a laser beam and / or d) mechanical processing of the surface and / or e) a heat treatment
is produced. 14. The method according to claim 9, 10, 11 and / or 12 for the production of a thermionic cathode according to claim 3, characterized in that a polycrystalline cover layer, which is optionally preferred, is applied to the surface produced by bevel grinding via a deposition from the gas phase . 15. The method according to one or more of claims 9 to 13, characterized in that a ribbed and / or grooved or a sawtooth-shaped substrate in cross-section in the surface of any geometry, in particular flat or cylindrical geometry, or a substrate with a spatial somehow curved surface is used for the reactive deposition of the layers from the gas phase, the coating is continued until a thickness is reached which is at least equal to the notch depth, and then the coated surface is substantially smoothed down to the substrate notch edges such that the resulting layer sequence is arranged at an angle to the new surface, the layering direction within the notches being offset by 90 ° again and again, and then the step-like microstructure being produced. 16. The method according to one or more of claims 9 to 14, characterized in that not only the surface is prepared by angled grinding of the CVD layers, but also the sides of the CVD sample are ground in such a way that a pill-shaped cathode with a macroscopically flat surface and inclined layers is formed, the _/ um is applied to the non-emissive sides by CVD with a W, re or Mo-coating of about 20 to 200 provided, and then on a thin, temperature resistant hairpin-shaped bent wire is aufgepunktet, which serves for the direct cathode heating , or which is provided for indirect heating as a cap of a cylinder jacket with a tungsten filament inside, the step surface being produced by micropolishing and structural etching only after the heating leads have been attached.

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