EP0559283B1 - Cathode with porous cathode element - Google Patents

Description

The invention relates to a cathode with a porous cathode element which contains at least one high-melting metal and nanostructured particles with an average diameter <1000 nm.

Due to increasing performance requirements, electron emitters of vacuum electron tubes, for example picture tubes, X-ray tubes and generally tubes in high-frequency and microwave technology, should have high emission current densities at the lowest possible operating temperature, a long service life, high resistance to poisoning from residual gases and stable behavior when bombarded with electrons.

HDTV cathodes must allow emission current densities of 20 A / cm ² , whereby the operating temperatures should be well below 1000 ° C and lifetimes of more than 20,000 h are required. Cathodes of modern high-performance X-ray tubes are said to deliver emission current densities of 10A / cm ² at relatively high residual gas pressures of up to 10 ^-3 mbar and with intensive ion bombardment.

The most important property criterion of a cathode is a constant high emission current density over the lifetime. In a known material with a given work function for electrons, the electron emission can be increased exponentially by increasing the temperature.

However, this can increase the power loss, reduce the mechanical stability of the emitting solid or its heater and, as a result of evaporation of the emitting material, rapid depletion of the supply and harmful contamination of system components of a tube (for example a grid), so that one with regard to the Electron emission advantageous temperature increase limits are set.

Only a few high-melting metals, in particular W, Re and Ta, are suitable as refractory metals for high-temperature operation, since at the same time the requirement for a low evaporation rate must be met.

An advantage of high-temperature operation of pure metals is the low contamination (poisoning) of the cathode surface and the low sensitivity to ion bombardment. However, even pure tungsten is unsuitable for high emission current densities> 5A / cm ² and lifetimes of> 10 ³ h. In contrast, such emission conditions can be met by building up suitable surface complexes (adsorbate / substrate dipole layers), whereby the electron work function is reduced. This achieves high emission current densities at relatively low operating temperatures and low energy losses. In practice, material combinations of W / ThO ₂ , W / Th, W / Ba, W / BaO, W / SC ₂ O ₃ / BaO / CaO / Al ₂ O ₃ are used.

It is desirable to have the most uniform possible coverage of the emitting surface with complexes that have a low work function and a low evaporation rate. This goal is tried in practice with the help of structure-modified cathodes, e.g. Impregnation, top-layer, mixed metal matrix, multilayer and controlled porosity dispenser cathodes. The reasons for the inadequate emission of the conventional structure-modified cathodes are that the degree of coverage of the emitting cathode surface is too low - e.g. with BaO for impregnated cathodes - due to insufficient subsequent delivery from the pores or grain boundaries after evaporation / sputtering or because of contamination of surface areas by residual gases.

The basic possibility of setting high emission current densities via temperature increases is paid for in the case of such cathode embodiments over a short service life. In the worst case, surface spaces become irreversible with elements or molecules that increase the work function even more than the value of the pure matrix metal. Although regenerable, the disadvantage is similarly disadvantageous in the loss of an adsorbate which lowers the work function (for example Sc from Sc ₂ 0 ₃ ) by evaporation or ion bombardment.

A dispenser cathode with a thermionically emitting cathode element of the type mentioned at the beginning is known from EP-A-0442 163 as an I-cathode. Oxide particles such as BaO are embedded in a material structure.

A cathode with a porous metal body is known from FR-A 1 410 641. The coarse porous body has a thin emitting layer on one surface, the porosity of which is so fine that there is a uniform emission.

From GB-A 21 16 356 a cathode with impregnated pores is known. This cathode contains both particles of high-melting metal and particles of scandium oxides.

The invention has for its object to design a cathode element of the type mentioned in such a way that higher emission current densities with a long service life are made possible at a given temperature.

The solution is achieved in that the cathode element is formed entirely from particles whose average diameter is less than 1000 nm, and which are connected to one another in a homogeneous distribution, in that 5 to 90% of the total volume of the cathode element consists of unfilled and open pores, the free distances between adjacent particles formed by the pores being less than 1000 nm.

Another cathode is characterized in that the porous cathode element contains both particles of high-melting metals and particles of metal oxides (such as Sc ₂ O ₃ , Y ₂ O ₃ , Eu ₂ O ₃ , La ₂ O ₃ , ThO ₂ ) a heatable Substrate are applied.

A cathode element according to the invention is referred to below as an “effusion cathode element”, since electrons “operate” from the near-surface, densely distributed pores into a vacuum during operation. An effusion cathode element in the sense of the present invention, if it does not itself contain alkaline earth oxides, can be used as a cover element e.g. can be used for an I-cathode of known type. On the other hand, an effusion cathode element according to the invention can also contain particles of alkaline earth oxides, so that it can then be used as a full-fledged cathode element.

)³ die Anzahlen der jeweiligen Partikel mit einem mittleren Durchmesser $\overline{\text{d}}$

um weniger als ± 20% voneinander abweichen, wobei d als Mittelwert der statistisch streuenden Durchmesser d der Partikel definiert ist.A homogeneous distribution of the particles means that in each volume element with a volume (20 $\overline{\text{d}}$

) ³ the numbers of the respective particles with an average diameter $\overline{\text{d}}$

deviate from each other by less than ± 20%, d being defined as the mean of the statistically scattering diameter d of the particles.

The diameter d of naturally not precisely spherical particles is to be understood as the mean value of the spatial extent of the particles measured in different angular positions.

Effusion cathode elements according to the invention can additionally be provided with cover layers made of Os or Ru, which are known per se. Such cover layers should also be designed with open pores.

In the case of an effusion cathode element according to the invention, the elements and compounds important for electron emission adjoin in a homogeneous fine distribution with a large total area of pores. Since the pores are open and thus accessible from the outer surfaces of the effusion cathode element in a manner free of solids, the active area of the effusion cathode body according to the invention is considerably enlarged. Subsequent delivery of alkaline earth atoms to near-surface areas is through the open pores facilitated. As a result, high emission current densities can be achieved even at relatively low operating temperatures.

An arrangement according to the invention is also insensitive to ion bombardment (sputtering) and to residual gas occupancy (poisoning). The sputtering effects only affect the outer surface, but not the areas near the surface in the porous structure. Undesired occupancy of residual gas in the interior of the structure is difficult under operating conditions because the free surfaces of the particles located there are occupied to a large extent with desired atoms / molecules.

These advantages are particularly significant compared to known cathodes if the effusion cathode element consists of particles, of which at least 90% have a diameter in the range from 5 to 1000 nm, preferably 30 to 500 nm. The smaller the particle diameter, the larger the ratio of the total available active surface to the outer surface of the effusion cathode element. With particle diameters that are too small (d <1 nm) and also with pore diameters that are too small, however, the effectiveness of the pores becomes too low, since structures made of particles with d <1 nm do not remain stable in the long run at operating temperatures of 750 °. If more than 10% of the particles of the solid structure had diameters of more than 500 nm, the intrinsically excellent resistance of the cathodes according to the invention to ion bombardment was noticeably deteriorated.

The high-melting metal components should have a proportion of> 30%, in particular about 50-90%, of the total volume of the solid particles. A value of about 50% has proven advantageous for effusion cathode bodies according to the invention containing alkaline earth oxide particles. With a metal content of less than 30%, a sufficiently good metallic conduction of the effusion cathode element can no longer be guaranteed.

According to a development of the invention,
that the oxidic particles are at least partially surrounded by a thin metal shell or that at least some of the metallic particles are enveloped by a thin oxidic cover layer. The cladding layers must of course be structured so thin or so permeable that the covered core can become active through the cladding.

Particularly high emission current densities were made possible in that the pore volume in the upper part of the effusion cathode element is larger than in the lower part.

Further embodiments of the invention result from the subclaims.

Fig. 1

zeigt einen Schnitt durch ein Volumenelement eines erfindungsgemäßen Effusionskathodenelements, welches W- und Sc₂O₃-Partikel, aber keine Erdalkalioxid-Partikel enthält und als Auflage für ein I-Kathodenelement (Fig. 3) geeignet ist.

Fig. 2

zeigt eine Ausführung mit Erdalkalioxid-Partikeln, welches als vollwertiges Dispenserkathodenelement verwendbar ist.

Fig. 3

zeigt einen Schnitt durch ein Kathodenelement, bei welchem ein Effusionskathodenelement nach Fig. 1 auf einem I-Kathodenelement aufgebracht ist.

Fig. 4

zeigt eine strombeheizte mäanderförmige Ausführungsform eines Effusionskathodenelements mit einer Struktur nach Fig. 2.

The invention is explained in more detail on the basis of the description of advantageous exemplary embodiments shown in the drawing.

Fig. 1

shows a section through a volume element of an effusion cathode element according to the invention, which contains W and Sc ₂ O ₃ particles but no alkaline earth oxide particles and is suitable as a support for an I-cathode element (FIG. 3).

Fig. 2

shows an embodiment with alkaline earth oxide particles, which can be used as a full dispenser cathode element.

Fig. 3

shows a section through a cathode element in which an effusion cathode element according to FIG. 1 is applied to an I-cathode element.

Fig. 4

shows a current-heated meandering embodiment of an effusion cathode element with a structure according to FIG. 2.

3, an effusion cathode element 1 is applied to an I-cathode element 2. The I cathode element 2 consists of a porous W matrix 3, which is impregnated with BaCa aluminate 4. The structure of a volume element of the effusion cathode element 1 is indicated in FIG. 1. Tungsten particles 5 with an average diameter of 30 nm form a supporting framework around pore spaces 6, in which an electron gas cloud is created during operation. Separate oxide particles 7 of Sc ₂ O ₃ with approximately the same diameter are embedded. As a result of the delivery of Ba in the direction of arrow 8 from the I-cathode element 2 (FIG. 3), when the cathode is operated at, for example, 900 ° C. on surfaces of the W-particles 5 directed towards the pores, a surface complex 9 made of Ba-Sc -O formed. At 900 ° C and a field strength of approx. 4 kV / mm, an emission current density of 110 A / cm ^{2 was} measured after 50 hours of operation (electron current direction according to arrows 10 and 11). These laboratory-made samples already showed significantly better values than were achieved with previously known cathode elements. At 965 ° C, lifetimes of more than 3000 hours were easily achieved.

2, Sc ₂ 0 ₃ cores 12 and BaO cores 13 (or also CaO cores) are coated with W layers 14a and 14b. Emitting layers of Ba-Sc-O are formed on the W surfaces of the two-layer particles facing the pore spaces 15. Current densities of 95 A / cm ² at 850 ° C. and field strengths of approximately 4 kV / mm were measured for the electron current flowing in the direction of arrows 16, 17. In known powder metallurgy cathode elements, which therefore have no open-pore structure, a maximum of 80 A / cm ² could be achieved under comparable conditions. It should be borne in mind that the favorable values achieved with the invention were measured on non-optimized laboratory-made samples. The in the invention required open porosity could be achieved with precipitation methods such as PCVD in particular. Sintering processes turned out to be unsuitable.

FIG. 4 shows a meandering foil-like effusion cathode element 20, with a structure according to FIG. 2, which is arranged on a support plate 21 which is preferably made of W, Ni or Ti. The carrier plate 21 can also be a substrate for the production of the layer of the effusion cathode element 20.

Because of the metallic conductivity of effusion cathode elements according to the invention and the nanostructured homogeneous structure, the meandering effusion cathode element 20 can also be heated directly without a support plate 21. If the heating current (arrow 18) is passed through, an electron current can be generated in the direction of arrow 19.

Indirect heating can of course also be provided. For example, structured effusion cathode elements can be arranged on a resistance heating conductor according to FIG. 2.

Claims (10)

1. A cathode including a porous cathode element which comprises a high melting point metal between two facing surfaces, characterized in that the cathode element (1) comprises particles (5,7) of a high melting point metal, in that less than 10% of the particles (5,7) have a diameter in excess of 500 nm, and in that the cathode element (1) has pores which are accessible from one or both surfaces.
2. A cathode including a porous cathode element comprising a high melting point metal between two facing surfaces, characterized in that the cathode element (1) comprises particles (5) of a high melting point metal as well as particles of metal oxides (7), in that less than 10% of the particles (5,7) have a diameter in excess of 500 nm, and in that the cathode element (1) has pores which are accessible from one or both surfaces.
3. A cathode as claimed in Claim 2, characterized in that the porous cathode element (1) is provided with heating means.
4. A cathode including a porous cathode body (3) which is provided with an impregnant (4) and with an emissive surface, characterized in that the cathode comprises a porous cathode element (1) on its emissive surface, which element comprises a high melting point metal between two facing surfaces, the cathode element (1) comprising particles (5) of a high melting point metal as well as particles of metal oxides (7), in that less than 10% of the particles (5,7) have a diameter in excess of 500 nm, and in that the cathode element (1) has pores which extend between the two surfaces.
5. A cathode including a porous cathode element comprising at least a high melting point metal as well as nano-structured particles having an average diameter of < 1000 nm, characterized in that the cathode element (1) completely consists of particles (5, 7) whose average diameter is smaller than 1000 nm and which in a homogeneous distribution are connected to each other, in that 5 to 90% of the overall volume of the cathode element (1) consists of unfilled pores (6, 15) which are open to their surroundings, the free distances, constituted by the pores (6, 15), between neighbouring particles (5, 7, 14a, 14b) being smaller than 1000 nm.
6. A cathode as claimed in Claim 5, characterized in that the porous cathode element comprises particles of high melting point metals (5), as well as particles of metal oxides (7), which are provided on a heatable substrate.
7. A cathode as claimed in Claim 5, characterized in that the porous cathode element exclusively comprises particles of a high melting point metal (5) or particles of metal oxides (7) and in that the cathode element (1) is provided on an I cathode element (2).
8. A cathode as claimed in Claim 5 or 6, characterized in that the porous cathode element comprises particles of a high melting point metal and particles of metal oxides and particles of alkaline earth metal oxides.
9. A cathode as claimed in any one of Claims 5 to 8, characterized in that the metal fraction (5, 14a, 14b) amounts to a volume fraction of > 30% of all the solid-state particles.
10. A cathode as claimed in any one of Claims 5 to 9, characterized in that the porous cathode element (1) is composed of particles (5, 7, 14a, 14b) of which at least 30% has a diameter in the range from 5 to 1,000 nm, preferably from 30 to 500 nm.

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