WO2014064227A1

WO2014064227A1 - Quick-starting thermionic cathode and method for producing same

Info

Publication number: WO2014064227A1
Application number: PCT/EP2013/072338
Authority: WO
Inventors: Pierre Nugues; Olivier HERBELIN; Nicolas LE SECH
Original assignee: Thales
Priority date: 2012-10-26
Filing date: 2013-10-24
Publication date: 2014-05-01
Also published as: FR2997548A1

Abstract

The invention proposes a thermionic cathode (1 ) comprising: - a first emitting block (10) comprising at least one electron emission surface (12) configured to emit electrons in at least one predefined direction and at least one non-emission surface (13), - a second heating block (11) comprising a first thermally insulating and refractory material (14), the first material (14) being covered with a layer of a second resistive material (15) intended to be connected to a power source, the layer of the second resistive material (15) providing the heating of the second block (11), the second heating block (11) being brazed to at least one non-emission surface (12) of the first block (10), allowing conductive heat transfer.

Description

FAST-START THERMOELECTRONIC EMISSION CATHODE AND PROCESS FOR PREPARING THE SAME

The invention relates to thermoelectronic emission cathodes. More particularly, the invention relates to a novel cathode structure with thermoelectronic emission and the method of producing this type of cathode. The invention more specifically relates to a thermoelectronic emission cathode particularly suitable for applications requiring a very fast start, less than about 2 seconds. The cathodes according to the invention can find applications in small traveling wave tubes used in missiles.

The term "thermoelectronic emission cathode" means a conductive surface from which electrons are extracted by applying an electric field when this surface is heated to high temperature. Different types of thermoelectronic cathodes exist in the prior art, FIGS. 1a to 1e illustrate these embodiments.

FIG. 1a shows a cathode 1 comprising an emitting block 2 and a filament 3. The emitting block 2, possibly comprising tungsten, comprises an emitting surface 4 configured to emit electrons in a previously established direction. The filament 3 may comprise tungsten, is often wound to form a spiral to increase the heat exchange surface near the emitting block 2. The filament 3 is intended to be connected to a power source, and the circulation of the current through the filament 3 allows to heat it. The filament 3 is disposed near the emitting block 2 allowing a transfer of the thermal energy of the filament 3 to the emitting block 2 by radiation. According to this prior art, the filament 3 is said to be "bare", which means that the filament 3 is not coated with a material. The disadvantage of using a bare filament 3 is that it is very sensitive to vibrations which limits its life. In order to overcome this drawback, the prior art illustrated in FIG. 1b proposes placing alumina beads along the entire length of the filament 3 in the manner of a "pearl necklace", which allows, on the one hand, , to improve the resistance of the filament 3 to the vibrations, and, on the other hand, to electrically isolate the filament 3 of the emitting block 2. This type of cathode requires heating longer to reach the temperature necessary for the emission of electrons.

Figure 1c illustrates another type of cathode 1 commonly presented in the literature. The cathode 1 comprises an emitting block 2 and a heating pad 6, the heating block 6 being brazed on the emitting block 2. The heating pad 6 comprises a filament 3 embedded in a block of alumina called "potting", the filament is said to "Pooped" 7a. The potting is maintained by a cylinder 7b called "skirt" comprising a refractory metal. The transfer of heat energy from the heating pad 6 to the emitting block 2 is achieved by conduction. A cathode 1 made according to this prior art requires a relatively long heating time, typically of the order of 3 minutes since it is necessary to heat a relatively large mass of alumina. In addition, the solder between the heating pad 6 and the emitting block 2 is relatively fragile given the large mass of the heating pad 6 and the electron emission temperature.

Furthermore, the method for producing the cathode 1 comprises a first soldering step of the skirt 7b on the emitting block 2, the skirt 7b serving to maintain the potting, a second step of producing the pleated filament 7a consisting in inserting the filament 3 inside the skirt 7b and cover the filament 3 with alumina powder, and a third heat treatment step. This method of elaboration is long and it does not make it possible to control the integrity of the filament 3 during the process of elaboration of the cathode 1. When the cathode 1 is made, the quality control of the cathode 1 is not possible since there is no method of analysis to control the quality of the potting or the quality of the solder between the keypad 2 and the heating pad 6. However, the mere presence of a bubble in the potting near the filament 3 weakens the filament 3 and reduces its duration of use. An alternative to the previous prior art is illustrated in Figure 1d. The cathode 1 according to this prior art comprises an emitting block 2 and a heating pad 6. The heating pad 6 comprising alumina is brazed on the emitting block 2, a bare filament 3 being brazed on the heating pad 6. The transfer Thermal energy of the heating pad 6 to the emitting block 2 is achieved by conduction but a portion of the thermal energy is dissipated by radiation. Furthermore, the brazing of the heating pad 6 on the emitting block 2 is relatively fragile because of the large mass of the heating pad 6 and the temperature, about 1000 ° C, necessary to facilitate the emission of electrons.

According to the patent application EP 2498372, there is provided a cathode 1 illustrated in Figure 1 e. The cathode 1 comprises a graphite support 8 serving as heating pad 6 on which an emitting coating 9 comprising a mixture of tungsten and rare earth oxides is deposited by cathodic sputtering. The graphite serving as heating pad 6 is heated by Joule effect.

This solution makes it possible to produce a cathode 1 devoid of filament 3. However, the graphite support used requires a relatively long time before reaching a sufficiently high temperature to allow electron emission. Moreover, the circulation of the current inside the graphite support 8 is in the least resistive way, the heating is not homogeneous. For example, if the electrical contact is made in the middle of the graphite support 8, the current will preferentially flow in the central part of the material, the heating will be preferably in the central part of the material.

Furthermore, the method of producing a cathode according to this document is relatively long since it requires a vacuum of the graphite support to achieve the deposition of the emissive coating, the brazing of an emitting block on the graphite support 8 being particularly difficult to achieve, the solder material migrating into the pores of the graphite. In this context, it is proposed a cathode 1 of relatively low mass and low cost for a quick start. It is also proposed a method of rapid elaboration of this cathode 1.

According to one aspect of the invention, there is provided a thermoelectronic emission cathode comprising: a first emissive block and a second heating block, the second heating block being brazed on at least one non-emissive surface of the first block allowing a thermal transfer by conduction. The first emissive block includes at least one electron emission surface configured to emit electrons in at least one predetermined direction and at least one non emissive surface. The second heating block comprising a first thermally insulating and refractory material, the first material being covered with a layer of a second resistive material intended to be connected to a current source, the layer of resistive material heating the second block .

The cathode thus produced does not include a filament which renders it insensitive to vibrations and facilitates the control of the quality of the cathode during its preparation or during its use. Advantageously, the first emissive block comprises at least one housing adapted to receive at least a portion of the second heating block. Preferably, the housing and the portion of the second heating block have a complementary shape which facilitates the method of developing the cathode.

In one embodiment, the second heating block is in the form of a ball which facilitates machining. In addition, once the ball disposed in the housing, it does not require maintenance during the brazing step.

The layer of the second resistive material comprises graphite. Alternatively, the layer of the second resistive material comprises an alloy comprising molybdenum and manganese. Advantageously, the layer of the second resistive material has a thickness of less than 20 microns, this range of values allowing rapid heating of the second heating block. Advantageously, the first thermally insulating and refractory material comprises alumina or zirconia.

Alternatively, the first material and the second material are identical. According to another aspect of the invention, there is provided a method for producing a cathode comprising: a first step of preparing a first emissive block, the first emissive block comprising at least one electron emission surface and at least one non-emissive surface, a second step of preparing a second heating block, the second heating block comprising a first thermally insulating and refractory material and being covered with a layer of a second resistive material. The method further comprises a single brazing step comprising a brazing sub-step of the second heat block on the first emissive block and two brazing sub-steps of the electrical contacts. A single soldering step saves hours of labor.

In a first embodiment, the brazing step is performed by applying a strong electric current, greater than about ten amperes, between a first contact disposed on the second heat block and a second contact disposed on the first block. emissive.

In a second embodiment, the soldering step is performed by induction. These brazing methods make it possible to carry out the three solders almost instantaneously.

Advantageously, the deposition of the layer of the second resistive material on the first thermally insulating material is carried out with a brush or by a dipping method.

These deposit methods can once again save labor time. Alternatively, the deposition of the layer of the second resistive material on the first insulating material is performed by sputtering which allows better control of the thickness of the deposited layer.

The invention will be better understood from the study of some embodiments described by way of non-limiting examples, and illustrated by the appended drawings in which: FIGS. 1 to 1 d previously described represent different cathodes produced according to the art FIG. 2 shows an embodiment of a cathode, according to one aspect of the invention, FIG. 3 represents a preferred embodiment of the cathode, according to one aspect of the invention.

FIG. 2 represents a thermoelectronic cathode, according to one aspect of the invention. The cathode 1 comprises a first emissive block 10 and a second heating block 11, the second heating block 11 being brazed on the first emissive block 10.

The first emissive block 10 comprises at least one emitting surface 12, in this case the first emissive block 10 comprises two emitting surfaces 12, an emitting surface 12 being intended to emit electrons in a previously defined direction. The emissive block 10 also comprises at least one non-emissive surface 13.

The second heating block 1 1 comprises a first thermally insulating and refractory material 14, the first material 14 being covered with a layer of a second resistive material 15 intended to be connected to a power supply source. The arrangement of the second resistive material 15 over the entire surface of the first material 14 allows a homogeneous heating, and consequently, a homogeneous electron emission over the entire emission surface 12. In addition, the coating the entire surface of the first material makes it possible to obtain layers of the second very thin resistive material. It is also possible to partially cover the surface of the second material with a strip or a wire thicker than 30 μιτι. Advantageously, the first material 14 comprises a ceramic known in particular for their ability to withstand high temperatures. Advantageously, the second material 15 is sufficiently conductive to allow the passage of the current while being sufficiently resistive, typically the resistance is of the order of one ohm, to allow the heating of the second heating block. Advantageously, the thickness of the layer of the second resistive material 15 is less than twenty microns, beyond this limit the layer of the second material 15 is not sufficiently resistive to allow rapid heating of the second heating block January 1.

Advantageously, the second heating block 1 1 is brazed on one of the non-emissive faces 13 of the first emissive block 10 which allows a heat transfer of the heating block 1 1 to the emissive block 10 by conduction.

Figure 3 shows a preferred embodiment of the invention.

In this case, the first emissive block 10 comprises porous tungsten, the pores being filled with a mixture of barium oxide, calcium oxide and aluminum oxide. The first emissive block 10 is cylindrical in shape of 3 mm in diameter and 1 mm in thickness. It comprises a concave-shaped emission surface 12 making it possible to focus the electrons expelled under the effect of the rise in temperature and a housing 16 of concave shape and of radius of curvature adapted to receive a portion of the heating block 1 .

The second heating block 1 1 is spherical in shape of 3 mm in diameter. The ball comprises a first thermally insulating and refractory material 14 which may be alumina or zirconia, the first material 14 being covered with a layer of a second resistive material 15 which may be graphite or an alloy comprising molybdenum and manganese. Alternatively, the first material 14 and the second material 15 are identical. Advantageously, the ball may comprise a ceramic weakly electrically conductive. Alternatively, the ceramic may comprise metal particles or metal oxides homogeneously distributed or defining a resistive path inside the ceramic. In the examples presented, the ceramic associates the functions of the thermal insulation corresponding to the first material 14 and of the electrical conductor corresponding to the second material 15.

Advantageously, the cathode comprises a skirt 7b limiting the radiation losses of the first emissive block 10 and the second heating block 1 1 consisting of the ball, the skirt 19 comprising an electrically conductive and refractory material such as molybdenum or rhenium. The skirt 7b is fixed on the first emissive block 10 so as not to hinder the extraction of electrons, advantageously the skirt 7b is cylindrical.

The cathode 1 is intended to be connected to a current supply source 19 via electrical contacts 17a and 17b comprising tungsten and preferably molybdenum, the electrical contacts 17a and 17b being soldered to the second heating block 11 and on the first emissive block 10, respectively.

Brazing between the first emissive block 10 and the second heating block 1 1 is made using a suitable brazing material 18.

The cathode 1 thus produced has a mass of approximately 200 mg, which is 2 to 3 times smaller than the mass of a so-called "potted" cathode and a mass less than the cathode proposed according to the prior art EP 2498372, in effect for obtaining a resistivity equivalent to that obtained according to the invention, the graphite layer 8 must be relatively thick. The low mass cathode is well suited for use in projectiles.

The cathode 1 according to the invention allows a rapid heating of the first emissive block 10 which can then emit very rapidly electrons. It is well suited to rapid-start traveling wave tubes, less than 2 seconds, used in particular missiles.

Moreover, the quality control of the cathode 1 is easily achievable by a simple visual inspection of the integrity of the second layer. resistive material which is not the case of a cathode 1 comprising a spun filament 7 for example.

The method for producing the thermoelectronic emission cathode according to the invention comprises a first step of preparing the first emissive block 10, a second step of preparing the second heating block 11 and a third step comprising a substep of brazing the block. heating 1 1 on one of the non-emissive faces 13 of the first emissive block 10 and two brazing sub-steps of the electrical contacts 17a and 17b on the second heating block 1 1 and the first emitting block 10, the three substeps being performed simultaneously.

The first step of the process consists essentially of producing emission surfaces 12 in a material comprising porous tungsten and metal oxides within the pores.

The second step of the process consists in covering the first material 14 comprising a ceramic such as alumina or zirconia with a second resistive material. This step can be carried out by cathodic sputtering, which allows better control of the thickness of the layer of the second deposited material. Preferably, the deposition of the layer of the second resistive material is carried out by dipping the first material 14 in a solution comprising the second material 15 or by brushing, these deposition techniques making it possible to increase the rate of production of the cathodes because they do not require evacuation and have a relatively low cost.

The third brazing step of the method further comprises a sub-step of contacting the different elements to be brazed. Advantageously, the first emissive block 10 comprises a housing 16 of shape adapted to receive at least a portion of the second heating block January 1. Thus, it is possible to deposit the second heating block 1 1 in the housing 16 of the first emissive block 10, it is then no longer necessary to maintain the second heating block 1 1 when soldering it on the first emissive block 10. Furthermore, the first electrical contact 17a is brought into contact with the second heating block 1 1 and the second electrical contact 18b is brought into contact with the first emissive block 10.

According to a first embodiment of the method, the electrical contacts 17a; 17b are connected to a power supply source for delivering a high current of about 10A under a voltage of about 1V. The circulation of the strong electric current allows a sufficient heating to realize the various solders.

According to a second embodiment of the method, the brazing step is performed by induction. A turn energized at high frequency generates a magnetic field that produces a current. The passage of the current through a resistive brazing material allows sufficient heating to allow soldering the solder material used being preferably graphite, a good electrical conductor material is not suitable for this type of brazing.

Preferably, the soldering step is carried out under a reducing atmosphere so as to avoid the oxidation of the solder which would weaken it.

The method proposed according to one aspect of the invention avoids multiple successive heat treatments which reduces the hours of labor required to achieve a cathode 1 by a factor of about 5 compared to a so-called cathode. potted "for example.

Claims

1. A thermoelectronic emission cathode (1) comprising: a first emissive block (10) comprising at least one electron emission surface (12) configured to emit electrons in at least one defined direction and at least one non-surface (13) emissive

a second heating block (1 1) comprising a first thermally insulating and refractory material (14), the first material (14) being covered with a layer of a second resistive material (15) intended to be connected to a source of current, the layer of the second resistive material (15) heating the second block (1 1), the second block (1 1) of heating being soldered on at least one non-emissive surface (12) of the first block (10) allowing a thermal transfer by conduction.

2. Cathode according to claim 1 wherein the first emissive block (10) comprises at least one housing (16) adapted to receive at least a portion of the second block (1 1) heating.

3. The cathode of claim 2 wherein the housing (16) and the portion of the second block (1 1) heating have a complementary shape.

4. Cathode according to one of the preceding claims wherein the second block (1 1) heating is in the form of a ball.

5. Cathode according to one of the preceding claims wherein the layer of resistive material (15) comprises graphite.

6. Cathode according to one of claims 1 to 4 wherein the layer of the second resistive material (15) comprises an alloy comprising molybdenum and manganese.

7. Cathode according to one of the preceding claims wherein the layer of the second material (15) resistive has a thickness less than 20 microns.

8. Cathode according to one of the preceding claims wherein the first material (14) thermally insulating and refractory comprises alumina or zirconia.

9. Cathode according to claims 1 to 4 wherein the first material (14) and the second material (15) are identical.

10. Process for producing a cathode (1) comprising a first step of preparing a first emissive block (10), the first emissive block (10) comprising at least one electron emission surface (12) and at least one non-emissive surface (13), a second step of preparing a second heating block (1 1), the second heating block (1 1) comprising a first material (14) thermally insulating and refractory and being covered with a layer of a second resistive material (15), the method being characterized in that it comprises a single brazing step.

1 1. Process according to Claim 10, in which the soldering step is carried out by applying a strong electric current, greater than about ten amperes, between a first electrical contact (17a) arranged on the second heating block (1 1) and a second electrical contact (17b) disposed on the first emissive block (10).

12. The method of claim 10 wherein the brazing step is performed by induction.

13. Method according to one of claims 10 to 12 wherein the deposition of the layer of the second material (15) resistive on the first material (14) thermally insulating is carried out with a brush or by a dipping method.

14. Method according to one of claims 10 to 12 wherein the deposition of the layer of the second material (15) resistive on the first material (14) thermally insulating is carried out by sputtering.