JPH065198A - Cathode including cathode element - Google Patents

Abstract

PURPOSE: To obtain a high emission electric current density at a prescribed temperature and a long service life by allowing a negative electrode element to contain metal particles of a prescribed average diameter. CONSTITUTION: An emission negative electrode element 1 is formed in an I negative electrode element negative electrode 2. The element 2 consists of a porous matrix 3 impregnated with BaCa alminic acid 4. In the element 1, tungsten particles 5 having the average diameter of 30nm constitutes a support frame surrounding a porous space 6 where an ionized gaseous cloud is generated during its operation, and separated oxides of Sc2 O3 having the same diameter as the tungsten particles are mixed. Respondent to a supply of Ba to an arrow 8 direction, for an instance when the negative electrode is operated at 900 deg.C, a surface complex 9 of Ba-Sc-O is formed on the surface of tungsten elements 5 pointing towards holes. As the result, emission electric current density 110A/cm<2> is obtained at 900 deg.C and an intensity area 4kV/mm, and its service life of not less than 3000 hours at a temperature of 965 deg.C is obtained.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a cathode containing a porous cathode element composed of a refractory metal (especially tungsten, tantalum, rhenium, iridium, osmium) between two facing surfaces.

[0002]

2. Description of the Related Art Due to the increasingly stringent performance requirements, the electron emitters of vacuum electron tubes, such as picture tubes, x-ray tubes and common tubes of high frequency and microwave technology, have emission current densities at the lowest possible operating temperatures. It must be high, long-lived, highly resistant to residual gas toxicity and stable in behavior during electron impact.

HDTV cathodes have emission current densities of 20 A / cm ² , operating temperatures well below 1000 ° C., while long lifetimes above 20000 hours are required. The cathode of today's high-performance X-ray tubes has a relatively high residual gas pressure of up to 10 ^-3 mbar and under strong ion bombardment 10 A /
It has an emission current density of cm ² .

The most important property criterion of the cathode is its high emission current density which is stable throughout its life. In known materials with a certain operating function for electrons, the electron emission can increase exponentially with increasing temperature. However, this may result in increased power dissipation, reduced mechanical stability of the radiant solid structure or its heating elements, rapid exhaustion of the dispenser material due to evaporation of the radiant material and system components (eg, lattice) of the tube. It can cause harmful pollution, which must itself limit the temperature increase that is preferred for electron emission.

For high temperature operation, only a few refractory metals, especially the heat resistant metals W, Re and Ta, are suitable.
This is because the requirements of low evaporation rate must be met at the same time.

The advantages of high temperature operation of pure metals are low contamination of the cathode surface (poisoning) and low sensitivity to ion bombardment. Even pure tungsten is 5
Not suitable for high emission current densities exceeding A / cm ² and lifetimes exceeding 10 ³ h. On the other hand, such release conditions must be met by forming a suitable surface complex (adsorbate / substrate dipole layer) to reduce the electronic working function. Then, a high emission current density can be obtained at a relatively low operating temperature and with a small energy loss. Actually, W / ThO ₂ , W / Th, W / Ba, W / BaO, W /
A material combination of Sc ₂ O ₃ / Ba ₂ O / CaO / Al ₂ O ₃ is used.

One should consider as uniform a coating as possible of the complexed emission surface which simultaneously has a low operating function and a low evaporation rate. In practice, for this purpose one attempts to use structurally modified cathodes such as impregnation, top layers, mixed metal substrates, multilayers and controlled porosity dispenser cathodes. The causes for the still inadequate emission of conventional structurally deformed cathodes are, for example, impregnated after evaporation / sputtering due to inadequate distribution from the pores or grain boundaries, or due to contamination of the surface sites with residual gases. This is because the degree of coating on the surface of the emission anode with BaO at the cathode is insufficient.

In such a cathode embodiment, the high emission current densities achieved at higher temperatures are a life burden. In the worst case, the surface part is irreversibly coated with elements or molecules that increase the working function beyond the value of the pure base metal. Loss of operating function due to evaporation or ion bombardment −
Reducing adsorbates (eg Sc ₂ O ₃ to Sc) can be regenerated, but is equally detrimental in their effectiveness.

A dispenser cathode comprising a cathode element of the type described above is known from EP 0442163 as I cathode. Oxide particles, such as BaO, are embedded in the material structure.

[0010]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a cathode device of the type described above which is capable of achieving higher emission current densities and longer lifetimes at a given temperature. For this purpose, the cathode element has an average diameter of 1000 nm.
The solution is to have smaller refractory metal particles and have pores available on one or both sides.

[0011]

A cathode comprising a refractory metal and nano-structured particles having an average diameter of less than 1000 nm is characterized as follows. That is, the cathode element is made up of particles having an average diameter of less than 1000 nm and connected to the porous cathode element in a uniform distribution, and 5 to 90% of the total volume of the cathode element is open to the surroundings. , With no distance, composed of pores, consisting of unfilled pores between adjacent particles smaller than 1000 nm.

Further, in the cathode, the porous cathode element has high melting point metal particles and metal oxide particles (for example, Sc ₂ O ₃ , Y).
₂ O ₃ , Eu ₂ O ₃ , La ₂ O ₃ , TnO ₂ ) and is formed on a heatable substrate.

The cathode element of the present invention will be described below as an "effusion cathode element". This is because during operation electrons "emit" into the vacuum from the tightly packed holes closest to the surface. Even without alkaline earth metal oxides, the emission cathode electrons of the invention can be used as a top layer or coating element, for example for I-cathodes of known type. On the other hand, the emission cathode device of the present invention may also contain particles of an alkaline earth metal oxide so that it can be used as a complete cathode.

The uniform distribution of particles is determined by the volume (20)

[Outer 1] It can be seen that, in each volume element having, the number of particles having the diameter d means that the average diameter [outer 1] is different from less than ± 20%. Here, [outer 1] is defined as the average value of various statistically different diameters d of the particles.
Spatial extensio measured at different angular positions
It can be seen that the average value of n) is the diameter d of the imperfect spherical particles. The emission cathode device of the present invention is particularly suitable for Os or R
It may also have a known top coat of u.

In the emission cathode device of the present invention, the devices and compounds important for electron emission combine with each other in a uniform fine distribution and with a high total surface area of the pores. The holes are perforated and thus available on both outer sides of the emitting cathode element through the passages free of solid state material, which significantly increases the active surface of the emitting cathode body of the present invention. It is easier to partition the atoms of the alkaline earth metal through the holes to the region closest to the surface. As a result, high emission current densities can be achieved at relatively low operating temperatures.

The device of the present invention is also insensitive to ion bombardment (sputtering) and residual substrate (poisoning).
In fact, the sputtering effect only affects the outer surface, not the area of the pore structure closest to the surface. The undesired residual gas volume of the inner part of the structure is disturbed under operating conditions. This is because the free surface of the particles in this region has a high degree of desired atoms / molecules.

Compared with the known cathodes, these advantages are:
It is of particular significance if the emitting cathode element comprises particles, at least 90% of which have a diameter between 5 and 100 nm, preferably between 30 and 50 nm. The smaller the particle diameter, the greater the ratio of the total available active surface to the outer surface of the emissive cathode device. Particle diameter (d <1n
If m) is too small and the diameter of the pores is too small, the pores will be less effective. Because the diameter d is 1
This is because structures containing particles smaller than nm do not maintain constant stability at an operating temperature of 750 °. When more than 10% of the particles of the solid-state structure have a diameter of more than 50 nm, the remarkable resistance of the cathode of the invention to ion bombardment has a great influence.

The refractory metal compound comprises an amount of more than 30% of the total volume of the solid-state particles, in particular about 50-90%. A value of about 50% has been found to be advantageous for emission cathode bodies according to the invention which contain particles of alkaline earth metal oxides. The amount of metal is 3
Below 0%, a sufficiently satisfactory metallic conductivity of the emitting cathode element can no longer be guaranteed.

In another embodiment of the invention, the oxide particles are at least partially covered with a thin metal skin, or at least a portion of the metal particles are covered with a thin oxide coating layer. Of course, the coating layer is so thin and has a transparent structure that the coated core becomes active through the coating layer. Very high emission current densities are possible when the hole volume at the top of the emitting cathode device is larger than at the bottom.

[0020]

EXAMPLE In FIG. 3, an emission cathode element 1 is formed on an I cathode element 2. The I cathode element 2 comprises a porous W matrix 3 impregnated with BaCa 4 aluminate. The structure of the volume element of the emission cathode device 1 is shown in FIG. Tungsten particles 5 with a mean value diameter of 30 nm constitute the support frame around the porous space 6 which produces an electron gas cloud during operation. Sc ₂ O ₃ separated oxide particles 7 having almost the same diameter are mixed. B from the I cathode element 2 (FIG. 3) in the direction of arrow 8
In response to the replenishment of a, for example, when operating the cathode at 900 ° C., the Ba-Sc is deposited on the surface of the W particles 5 which are oriented towards the pores.
The surface complex 9 of —O is formed. 900 ° C and about 4kV /
In the intensity region of mm, an emission current density of 110 A / cm ² (electron flow direction indicated by arrows 10 and 11) was measured after 50 hours of operation. These laboratory tests have already obtained considerably better values than known cathode devices. 965
A life of more than 3000 hours at 0 ° C was achieved without any problems.

In the embodiment of FIG. 2, the Sc ₂ O ₃ core 12 and the BaO core 13 (or also the CaO core) are wrapped with W coatings 14a and 14b, respectively. A Ba-Sc-O release coating is formed on the W surface of the double-coated particles which is directed towards the pore space 15. A flow density of 95 A / cm ² was measured in the intensity region of 850 ° C. and about 4 kV / mm for the electron flow flowing in the directions of arrows 16 and 17. In the known powder-metallurgically produced cathode elements, which have no open pore structure, at most 80 A / cm ² could be achieved under comparable conditions. It should be considered that the preferred values achieved according to the invention have been determined in the laboratory on non-optimized test samples. The required apertures in the device of the invention could be achieved especially by deposition methods such as PCVD. It has been found that the sintering method is not very suitable.

FIG. 4 shows a bent foil type emission cathode device 20 having a structure as shown in FIG. It is arranged on a support 21, preferably made of W, Ni or Ti. The support 21 is at the same time a substrate for producing the coating of the emission electrode element 20.

Due to the metal conductivity and nano-structured uniform structure of the inventive emission cathode device, the folded emission cathode device 20 can also be heated directly without the support 21. As the heat flow (arrow 18) passes through, electron flow in the direction of arrow 19 may occur. Of course, it is possible to heat indirectly. For example, as shown in FIG. 2, the configured emission cathode element may be arranged on the resistance heating element.

[Brief description of drawings]

1 is a cross-sectional view of a volume element of an emission cathode device of the present invention, comprising W and Sc ₂ O ₃ particles as an upper layer of an I cathode device (FIG. 3) instead of alkaline earth metal oxide particles. Appropriate.

FIG. 2 shows an embodiment with alkaline earth metal oxide particles that can be used as a complete dispenser cathode.

FIG. 3 is a cross-sectional view of a cathode device in which the emission cathode device shown in FIG. 1 is formed on an I cathode device.

FIG. 4 shows a flow-heated bent embodiment of an emission cathode device having a structure as shown in FIG.

[Explanation of symbols]

1 Emission Cathode Element 2 I Cathode Element 3 Porous W Matrix 4 BaCa 5 Aluminate 5 W Particle 6 Porous Space 7 Oxide Particle 9 Surface Complex 12 Sc ₂ O ₃ 13 BaO 14 a W Coating 14 b W Coating 15 Pore Space 20 Emission Cathode Element 21 support

Claims (9)

[Claims] 1. A cathode comprising a porous cathode element comprising a refractory metal between two facing faces, wherein the cathode element (1) comprises particles of refractory metal (5) having an average diameter of less than 1000 nm. ) And having a hole that can be used from one side or both sides. 2. A cathode comprising a porous cathode element comprising a refractory metal between two facing surfaces, the cathode element (1) comprising particles of refractory metal (5) having an average diameter of less than 1000 nm. ) And metal oxide particles (7) having an average diameter of less than 1000 nm and having pores accessible from one or both sides. 3. The cathode according to claim 2, wherein the porous cathode element is formed in a heating device. 4. A cathode comprising an impregnating agent (4) and a porous cathode body (3) provided with an emission surface, the cathode comprising a porous cathode element (1) on the emission surface, said elements being two opposed. A cathode element (1) containing a refractory metal between the surfaces
Comprising a refractory metal particle (5) having an average diameter of less than 1000 nm and a metal oxide particle (7) having an average diameter of less than 1000 nm, and having a hole extending between two surfaces. . 5. At least a refractory metal and an average diameter of 1
In a cathode comprising a porous cathode element comprising nano-structured particles smaller than 000 nm, the cathode element (1) has an average diameter of less than 1000 nm and particles (5, 5 connected to the porous cathode element in a uniform dispersion). Completed from 7),
5 to 90% of the total volume of the cathode element (1) is open to the surroundings, there is no distance, it is constituted by the holes (6, 15) and the adjacent particles (5, 7, 1) smaller than 1000 nm
Cathode characterized by comprising unfilled holes (6, 15) between 4a, 14b). 6. The cathode according to claim 5, wherein the porous cathode element is composed of refractory metal particles (5) and metal oxide (7) particles formed on a heatable substrate. . 7. The porous cathode element contains only refractory metal particles (5) or refractory metal particles and metal oxide particles (7), and the cathode element (1) is an I cathode element ( The cathode according to claim 5, which is formed on 2). 8. A porous cathode device comprising a refractory metal (5) particle and an alkaline earth metal oxide particle (13), or a refractory metal particle and a metal oxide particle and an alkaline earth metal oxide. 7. The cathode according to claim 5, wherein the cathode contains particles of the product. 9. Cathode according to any one of claims 5 to 8, characterized in that the metal parts (5, 14a, 14b) make up more than 30% by volume of the total cathode element.