Impregnated cathode preparation method and impregnated cathode

Abstract

The invention discloses a preparation method of an impregnated cathode and the impregnated cathode. The preparation method comprises the following steps: obtaining a heater assembly with a porous tungsten sponge body; reducing powder and an active substance are mixed according to the set mass ratio to form mixed powder, and the active substance can be reduced by the porous tungsten sponge body and the reducing powder respectively; inverting the heater assembly to enable the porous tungsten sponge body to be placed in the molybdenum boat, fixing the heater assembly, and adding a proper amount of mixed powder into the molybdenum boat to bury the porous tungsten sponge body; placing the molybdenum boat in which the heater assembly is placed in a hydrogen furnace, preserving heat for a set time according to a set temperature to melt the active substance, and impregnating the active substance and the reducing powder in holes of the porous tungsten sponge body to form an impregnated cathode; the heater assembly is taken out and the surface of the impregnated cathode is polished. According to the method, enough free barium can be generated at a relatively low temperature and transported to the emission surface, so that the production rate of the free barium is increased, the work function of the emission surface is reduced, and the emission capability is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of vacuum electronic devices, and in particular relates to a preparation method of an impregnated cathode and the impregnated cathode. Background Art

[0002] The cathode is widely used as an electron source in vacuum devices, including X-ray tubes, klystrons, traveling wave tubes, magnetrons and gyrotrons. The emission performance of the cathode directly determines whether the vacuum electronic device can work stably and reliably. According to the different electron emission mechanisms, the cathode is mainly divided into thermal cathodes, field emission cathodes, photocathodes and secondary electron cathodes. As a representative thermal cathode, the impregnated barium tungsten cathode has many advantages such as high current density, simple structure and process, and no need to trigger the laser, and is widely used.

[0003] The emission principle of the barium tungsten cathode is that the tungsten matrix reduces the active substances in its pores to produce free barium at high temperature, and the free barium migrates to the emission surface to reduce the work function of the emission surface so that electrons are emitted from the surface. Within the operating temperature range, the higher the temperature, the stronger the emission ability, but the evaporation rate of barium is also greater. Too high an operating temperature will accelerate the consumption of barium and shorten the life of the cathode. In addition, a higher operating temperature will also lead to a series of problems such as increased cathode gas outgassing, cation back bombardment, increased stray electrons, and cathode short circuit.

[0004] Therefore, it is of great significance to increase the current emission density at a relatively low temperature. There are many ways to improve the cathode emission capacity, such as optimizing the structure and composition of active materials, optimizing the pore structure of tungsten matrix, and eliminating cathode poisoning substances as much as possible, but these technologies are relatively mature and it is difficult to achieve significant results in the short term. Summary of the invention

[0005] The purpose of the embodiment of the present invention is to provide a method for preparing an impregnated cathode, so as to solve the problem in the prior art that it is difficult to increase the emission current density at a relatively low operating temperature.

[0006] The embodiments of the present invention provide a method for preparing an impregnated cathode and an impregnated cathode.

[0007] The method for preparing the impregnated cathode of the first embodiment of the present invention comprises the following steps:

[0008] Obtain a thermal subassembly with a porous tungsten sponge;

[0009] Mixing a reducing powder and an active material in a set mass ratio to form a mixed powder, wherein the active material can be reduced by the porous tungsten sponge and the reducing powder respectively;

[0010] Invert the hot sub-assembly so that the porous tungsten sponge is placed in the molybdenum boat and fix the hot sub-assembly, add an appropriate amount of the mixed powder into the molybdenum boat to bury the porous tungsten sponge;

[0011] Place the molybdenum boat with the hot sub-assembly in a heating furnace, keep it at a set temperature for a set time to melt the active substance, and impregnate the pores of the porous tungsten sponge with the active substance and the reducing powder to form an impregnated cathode;

[0012] Take out the hot sub-assembly and polish the surface of the impregnated cathode.

[0013] Further, the diameter of the reducing powder is 10 nm to 1 μm.

[0014] Further, the mass ratio of the reducing powder to the active substance is 1:50 to 1:5.

[0015] Further, the set temperature is 1600 °C to 1750 °C.

[0016] Further, the set time is 1 min to 5 min.

[0017] Further, the reducing powder is at least one of tungsten powder and carbon powder; and / or,

[0018] The active substance is at least one of aluminate and scandate.

[0019] The impregnated cathode according to the embodiment of the second aspect of the present invention includes:

[0020] A hot sub-assembly, the hot sub-assembly includes a porous tungsten sponge, and a plurality of pores are defined in the porous tungsten sponge;

[0021] Reducing powder and active substance, the reducing powder and the active substance are mixed and jointly filled in a plurality of the pores.

[0022] Further, the diameter of the reducing powder is 10 nm to 1 μm; and / or,

[0023] The mass ratio of the reducing powder to the active substance is 1:50 to 1:5.

[0024] Further, the reducing powder is at least one of tungsten powder and carbon powder; and / or,

[0025] The active substance is at least one of aluminate and scandate.

[0026] The vacuum electronic device according to the embodiment of the third aspect of the present invention includes the impregnated cathode described in the above embodiment.

[0027] The preparation method of the impregnated cathode according to the embodiments of the present invention can generate sufficient free barium at a relatively low temperature and transport it to the emission surface by increasing the contact area between tungsten and the active substance, increasing the production rate of free barium, reducing the work function of the emission surface, and improving the emission ability. Description of the Drawings

[0028] Figure 1 It is a flowchart of the preparation method of the impregnated cathode according to the embodiments of the present invention;

[0029] Figure 2 It is a schematic diagram of impregnating the active substance into the porous tungsten sponge according to the embodiments of the present invention;

[0030] Figure 3 It is a schematic diagram of the principle of the preparation method of the impregnated cathode according to the embodiments of the present invention;

[0031] Figure 4 It is a graph showing the relationship between the rising distance and time of tungsten powder and active substance with different diameters in the porous tungsten sponge according to the embodiments of the present invention;

[0032] Figure 5 It is a schematic diagram of the surface of the impregnated cathode in the prior art;

[0033] Figure 6 It is a schematic diagram of the surface of the impregnated cathode in Embodiment 1 of the present invention;

[0034] Figure 7 It is a schematic diagram of the surface of the impregnated cathode in Embodiment 2 of the present invention;

[0035] Figure 8 It is a schematic diagram of the surface of the impregnated cathode in Embodiment 3 of the present invention.

[0036] Reference Signs

[0037] Impregnated cathode 100; Porous tungsten sponge 10; Pores 11; Heater assembly 20; Reducing powder 40; Active substance 30; Molybdenum boat 200; Pressing block 300. Detailed Embodiments

[0038] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0039] The terms "first", "second", etc. in the description and claims of the present invention are used to distinguish similar objects, rather than to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged under appropriate circumstances, so that the embodiments of the present invention can be implemented in an order other than those illustrated or described herein. In addition, "and / or" in the description and claims means at least one of the connected objects, and the character " / " generally indicates an "or" relationship between the associated objects before and after.

[0040] The following Figures 1 to 8 will, through specific embodiments and their application scenarios, describe in detail the preparation method of the impregnated cathode provided by the embodiments of the present invention.

[0041] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0042] As Figure 1 shown, the preparation method of the impregnated cathode according to the embodiments of the present invention includes the following steps:

[0043] S10. Obtain a heater sub-assembly 20 with a porous tungsten sponge 10;

[0044] S20. Mix a reducing powder 40 and an active substance 30 in a set mass ratio to form a mixed powder, wherein the active substance 30 can be reduced by the porous tungsten sponge 10 and the reducing powder 40 respectively;

[0045] S30. As Figure 2 shown, invert the heater sub-assembly 20 so that the porous tungsten sponge 10 is placed in a molybdenum boat 200 and fix the heater sub-assembly 20, and add an appropriate amount of the mixed powder into the molybdenum boat 200 to bury the porous tungsten sponge 10;

[0046] S40. Place the molybdenum boat 200 with the heater sub-assembly 20 in a hydrogen furnace, keep it at a set temperature for a set time to melt the active substance 30, and impregnate the pores of the porous tungsten sponge 10 with the active substance 30 and the reducing powder 40 to form an impregnated cathode 100, as Figure 3 shown.

[0047] S50. Take out the heater sub-assembly 20 and polish the surface of the impregnated cathode 100.

[0048] Specifically, the preparation method of the impregnated cathode 100 according to the embodiments of the present invention increases the contact area between the active substance 30 and the reducing substance, enabling the cathode to generate sufficient free barium even at a relatively low temperature and transporting it to the emission surface. Specifically, the active substance 30 (e.g., aluminate) is uniformly mixed with the reducing powder (e.g., nano tungsten powder). When dipping in salt, the nano tungsten powder is inhaled into the pores 11 of the porous tungsten sponge 10 together with the active substance 30. As Figure 5 shown, in the original process, tungsten powder was not mixed, and the active substance 30 could only react with the inner wall of the pores 11 of the porous tungsten sponge 10 to generate free barium. As Figures 6 to 8 shown, after adding tungsten powder, the surface of the tungsten powder contacts the active substance 30, significantly increasing the reaction contact area. At the same working temperature, the generation rate of free barium is increased, so the barium content of the emission surface can be increased, thereby improving the electron emission ability. Details are as follows:

[0049] The emission ability of the barium-tungsten cathode can be measured by the zero-field emission current density, and its magnitude is determined by the following formula:

[0050]

[0051] where J is the zero-field emission current density; A is the emission constant related to factors such as the material properties and surface microstructure of the emission surface; T is the absolute temperature; φ is the work function of the emission surface; k is the Boltzmann constant.

[0052] To ensure that the cathode has sufficient emission ability, it must work at a certain high temperature. On the one hand, it can be seen from formula (1) that as the temperature T increases, the zero-field emission current density J increases rapidly. On the other hand, sufficient free barium can only be generated and transported to the surface at a relatively high temperature, and the work function φ is reduced to further increase the zero-field emission current density J. However, the increase in temperature will affect the evaporation of barium, and the evaporation rate satisfies the following relationship:

[0053]

[0054] where v is the evaporation rate of barium, C is the pre-exponential factor, and φ′ is the activation energy of barium evaporation. Therefore, the increase in temperature will cause a sharp rise in the barium evaporation rate, accelerating the consumption of barium and shortening the cathode life. In addition, a higher working temperature will also cause a series of problems such as increased cathode outgassing, cation back-bombardment, increased stray electrons, and cathode short-circuit.

[0055] The present invention uniformly mixes tungsten powder (particle size 10 nm to 1 μm) in the active substance 30. When dipping in salt, at high temperature, the active substance 30 melts and is inhaled into the pores of the tungsten matrix by capillary action. At the same time, the melted active substance 30 will bring the tungsten powder into the pores during the flowing process.

[0056] As the core content of the present invention, it is necessary to explore whether tungsten powder with a relatively high density can be smoothly inhaled into the pores, and its kinetic process is analyzed as follows:

[0057] To simplify the model, it is assumed that the pores are cylindrical in the vertical direction, the horizontal movement of the fluid and tungsten powder is ignored, the tungsten powder is regarded as spherical, and the forces acting on the tungsten powder mainly consider gravity and the drag force of the fluid on it, as Figure 3 shown.

[0058] After the active substance 30 melts, it is inhaled into the pores under capillary action, and the distance into the pores can be described by the following differential equation:

[0059]

[0060] where t is the flow time; h is the height of the fluid rise at time t; γ is the gas-liquid interfacial tension; R is the pore radius; θ is the contact angle between the fluid and the pore wall; μ is the fluid viscosity; g is the acceleration due to gravity.

[0061] The driving force of the active substance 30 on the tungsten powder during the flow process mainly considers the drag force, and its value is given by the following formula:

[0062]

[0063] where F is the magnitude of the drag force; ρ g is the fluid density; v g is the fluid flow rate; v is the movement speed of the tungsten powder; C D is the drag coefficient. The kinetic equation of the tungsten powder is:

[0064]

[0065] where r is the radius of the tungsten powder, ρ W is the density of tungsten. Through Equation (5), it can be solved that

[0066]

[0067] where is a characteristic velocity, and v g can be obtained by numerically solving the nonlinear differential equation (3). The rising distance x of the tungsten powder can be expressed as

[0068]

[0069] From Equations (3) and (7), the relationship between the meniscus of the active substance 30 inhaled into the capillary and the rising distance of the tungsten powder with time under different tungsten powder particle sizes can be obtained, as Figure 4 shown. The abscissa in the figure is time, and the ordinate is the rising distance of the tungsten powder with time. It can be seen from the figure that the smaller the tungsten powder particles, the faster they rise.

[0070] It can be seen that the tungsten powder can be fully inhaled into the pores, and it can be estimated that the contact area can be increased by more than double. In addition, it is also necessary to control the salt impregnation process to prevent a large amount of tungsten powder from agglomerating and sintering with the porous tungsten sponge body 10. The sintering of some tungsten powder with the inner wall of the pores or the agglomeration of a small amount of tungsten powder can also increase the contact area between the active substance 30 and tungsten, achieving the effect of improving the free barium yield.

[0071] Thus, according to the preparation method of the impregnated cathode 100 of the embodiment of the present invention, by increasing the contact area between tungsten and the active substance 30, sufficient free barium can be generated at a relatively low temperature and transported to the emission surface, increasing the production rate of free barium, reducing the work function of the emission surface, and improving the emission ability.

[0072] According to an embodiment of the present invention, in step S20, an appropriate amount of tungsten powder with a diameter of 10 nm to 1000 nm is fully mixed with the active substance 30 evenly.

[0073] Furthermore, in step S30, the hot subassembly 20 is placed upside down in the center of the molybdenum boat 200, and an appropriate amount of the above-mentioned mixed powder is added to bury part of the tungsten substrate, and a pressing block 300 is pressed, as Figure 4 shown.

[0074] Optionally, the mass ratio of the reducing powder 40 to the active substance 30 is 1:50 to 1:5.

[0075] Preferably, the set temperature is 1600 °C to 1750 °C.

[0076] In an implementation of the present invention, the set time is 1 min to 5 min.

[0077] According to an embodiment of the present invention, the reducing powder 40 is one or more, not limited to tungsten powder and carbon powder; optionally, the active substance 30 is one or more, such as aluminate or scandate with different ratios, and is not limited thereto.

[0078] For comparison, Figure 5 is the surface of the impregnated cathode 1 in the prior art. As can be seen from Figure 5 , there is only the active substance 30 in the pores 11 on the surface of the impregnated cathode in the prior art.

[0079] Example 1: Mix 1:50 of the reducing powder 40 (tungsten powder) with the active substance 30 (aluminate) evenly; place the hot subassembly 20 upside down in the center of the molybdenum boat 200, add an appropriate amount of the above-mentioned mixed powder to bury part of the tungsten substrate, and press a pressing block 300; after assembly, put it into a hydrogen furnace, carry out salt impregnation at 1750 °C for 1 min of heat preservation; after taking out the cathode, polish its outer surface. The surface of the obtained impregnated cathode 100 is shown in the schematic diagram as Figure 6 shown.

[0080] Example 2: Thoroughly mix 40 parts of the original powder (carbon powder) and 30 parts of the active substance (scandate) at a ratio of 1:25; invert the hot sub-assembly 20 and place it at the center of the molybdenum boat 200, add an appropriate amount of the above mixed powder to partially bury the tungsten substrate, and press on the pressing block 300; after assembly, place it in a hydrogen furnace and perform salt impregnation at 1600 °C for 5 minutes; after taking out the cathode, polish its outer surface. The surface of the impregnated cathode 100 obtained is shown schematically in Figure 7 the figure shown below.

[0081] Example 3: Thoroughly mix 40 parts of the original powder (tungsten powder) and 30 parts of the active substance (aluminate) at a ratio of 1:5; invert the hot sub-assembly 20 and place it at the center of the molybdenum boat 200, add an appropriate amount of the above mixed powder to partially bury the tungsten substrate, and press on the pressing block 300; after assembly, place it in a hydrogen furnace and perform salt impregnation at 1700 °C for 3 minutes; after taking out the cathode, polish its outer surface. The surface of the impregnated cathode 100 obtained is shown schematically in Figure 8 the figure shown below.

[0082] As Figures 5 to 8 shown in the figure, as the amount of the reducing powder 40 in the pores 11 increases, the contact area between the active substance 30 and the reducing powder 40 gradually increases. Thus, the active substance 30 can be reduced not only by the porous tungsten sponge 10 but also by the reducing powder 40. With the increase of the reducing powder 40, the generation rate of free barium at the same working temperature increases, so the barium content on the emission surface can be increased, thereby improving the electron emission ability.

[0083] The impregnated cathode 100 according to the second aspect embodiment of the present invention includes a hot sub-assembly 20, a reducing powder 40, and an active substance 30.

[0084] Specifically, the hot sub-assembly 20 includes a porous tungsten sponge 10, a reducing powder 40, and an active substance 30. A plurality of pores 11 are defined in the porous tungsten sponge 10; the reducing powder 40 and the active substance 30 are mixed and jointly filled in the plurality of pores 11. After filling, the surface of the porous tungsten sponge 10 is as Figure 5 shown in the figure. The reducing powder 40 (such as tungsten powder) is evenly distributed inside the pores 11 of the porous tungsten sponge 10, which can effectively increase the contact area between the tungsten powder and the active substance. At the same temperature, more free barium can be generated per unit time, thereby increasing the barium coverage on the emission surface, further reducing the work function of the emission surface, and playing a role in improving the emission ability.

[0085] Furthermore, the diameter of the reducing powder 40 is 10 nm to 1 μm.

[0086] According to an embodiment of the present invention, the reducing powder 40 is tungsten powder, carbon powder or other reducing powder 40; the active substance 30 can be one of aluminates and scandates with different ratios, and is not limited thereto.

[0087] Preferably, the mass ratio of the reducing powder 40 to the active substance 30 is 1:50 to 1:5.

[0088] The vacuum electronic device according to the third aspect embodiment of the present invention includes the impregnated cathode 100 of the above embodiment.

[0089] The other structures and technologies of the vacuum electronic device according to the embodiment of the present invention belong to the prior art and will not be elaborated here.

[0090] The embodiments of the present invention have been described above in conjunction with the accompanying drawings. However, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the spirit and scope protected by the claims of the present invention, and all of them belong to the protection scope of the present invention.

Claims

1. A method for preparing an impregnated cathode, characterized in that, It includes the following steps: Obtain a hot sub-assembly with a porous tungsten sponge body; Mix reducing powder and active substance with a set mass ratio to form a mixed powder, wherein the active substance can be reduced by the porous tungsten sponge body and the reducing powder respectively; Invert the hot sub-assembly so that the porous tungsten sponge body is placed in a molybdenum boat and fix the hot sub-assembly, add an appropriate amount of the mixed powder into the molybdenum boat to bury the porous tungsten sponge body; Place the molybdenum boat with the hot sub-assembly in a heating furnace, keep it at a set temperature for a set time to melt the active substance, and impregnate the pores of the porous tungsten sponge body with the active substance and the reducing powder to form an impregnated cathode; Take out the hot sub-assembly and polish the surface of the impregnated cathode.

2. The preparation method according to claim 1, wherein The diameter of the reducing powder is 10 nm to 1 μm.

3. The preparation method according to claim 1, characterized in that The mass ratio of the reducing powder to the active substance is 1:50 to 1:

5.

4. The preparation method according to claim 1, characterized in that, The set temperature is 1600 °C to 1750 °C.

5. The preparation method according to claim 1, wherein, The set time is 1 min to 5 min.

6. The preparation method according to claim 1, characterized in that, The reducing powder is at least one of tungsten powder and carbon powder; and / or, The active substance is at least one of aluminate and scandate.

7. An impregnated cathode, characterized in that, It includes: A hot sub-assembly, the hot sub-assembly includes a porous tungsten sponge body, and a plurality of pores are defined in the porous tungsten sponge body; Reducing powder and active substance, the reducing powder and the active substance are mixed and jointly filled in the plurality of pores.

8. The impregnated cathode according to claim 7, characterized in that, The diameter of the reducing powder is 10 nm to 1 μm; and / or, The mass ratio of the reducing powder to the active substance is 1:50 to 1:

5.

9. The impregnated cathode according to claim 7, wherein, The reducing powder is at least one of tungsten powder and carbon powder; and / or, The active substance is at least one of aluminate and scandate.

10. A vacuum electronic device, characterized in that, It includes the impregnated cathode according to any one of claims 7-9.

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