A low-evaporation coated diffusion cathode, preparation and application

Abstract

The application discloses a low-evaporation coated diffusion cathode, preparation and application. The coated diffusion cathode comprises a cathode base, an emitting material and a noble metal film layer; wherein the emitting material is a barium-strontium-calcium-aluminum oxide quaternary cathode emitting substance; the molar ratio of the barium-strontium-calcium-aluminum elements is 10-35:1-20:1-20:2-50. A new emitting material formula combination is designed through a large number of experimental researches. The introduction of the strontium element is beneficial to reduce the evaporation rate of the emitting material. Meanwhile, under the suitable ratio of the barium-strontium-calcium-aluminum elements, the evaporation rate of the emitting material is further reduced, and the current density is kept at a high level. Therefore, the coated diffusion cathode prepared from the above emitting material has a low evaporation rate and high reliable performance, and the cathode emitting performance can meet the use conditions of vacuum devices, and is especially suitable for being used in vacuum devices such as traveling wave tubes and klystrons with low evaporation and long service life requirements.

Description

Technical Field

[0001] This invention relates to the field of vacuum electronic components technology. More specifically, it relates to a low-evaporation coated diffusion cathode, its preparation, and its application. Background Technology

[0002] Vacuum devices are indispensable in fields such as aerospace and information interaction. The cathode is a core component of vacuum devices, and the coated diffusion cathode is one of the most widely used electron sources. Its characteristic feature is the deposition of an active emitting material (barium aluminate) into the pores of a porous tungsten sponge (cathode substrate) through melting and impregnation, followed by the deposition of a thin film of noble metal or alloy on the cathode surface to reduce electron work function. Coated diffusion cathodes possess characteristics such as precision shaping, high emission current density, and excellent resistance to poisoning, making them the preferred cathode type for the manufacture of microwave vacuum electronic devices. These characteristics also make this type of cathode an important reference model for the development of new cathodes.

[0003] In current applications of coated diffusion cathodes, problems caused by high evaporation rates include: 1) adsorption of evaporated material by other components in the device's operating environment, affecting normal device operation; 2) a significant reduction in device lifespan. Space-based traveling wave tubes (TWTs) and airborne or missile-borne klystrons require cathodes that meet emission requirements while possessing low evaporation performance. This is beneficial for extending device lifespan and preventing material evaporation from affecting normal device operation.

[0004] Therefore, in order to solve the above problems, it is urgent to study a coated diffusion cathode that can significantly reduce the evaporation rate while meeting the emission current density required by device design to ensure the normal operation of the device, so as to meet the needs of device design and development. Summary of the Invention

[0005] To address the aforementioned problems, the first objective of this invention is to provide a low-evaporation coated diffusion cathode. With a suitable ratio of barium, strontium, calcium, and aluminum elements in the emission material of this coated diffusion cathode, the evaporation rate of the coated diffusion cathode is significantly reduced, which is of great significance for solving the problem of high evaporation of cathode emission materials in existing vacuum devices.

[0006] A second objective of this invention is to provide a method for preparing a low-evaporation coated diffusion cathode as described above.

[0007] The third objective of this invention is to provide an application of the low-evaporation coated diffusion cathode described above in the fabrication of vacuum electronic components.

[0008] To achieve the first objective mentioned above, the present invention adopts the following technical solution:

[0009] This invention discloses a low-evaporation coated diffusion cathode, which includes a cathode substrate, an emitting material, and a noble metal film layer;

[0010] The emitting material is a barium-strontium-calcium-aluminum-oxygen quaternary cathode emitting material; the molar ratio of each element in the barium-strontium-calcium-aluminum quaternary cathode is 10-35:1-20:1-20:2-50.

[0011] To address the problems of high evaporation, short lifetime, and low current density in existing cathode emission materials, this invention, through extensive experimental research, designs a new emission material formulation. The introduction of strontium helps reduce the evaporation rate of the emission material, and the appropriate ratio of barium, strontium, calcium, and aluminum further reduces the evaporation rate while maintaining a high current density. Therefore, the coated diffusion cathode prepared with the above emission material exhibits a low evaporation rate, high reliability, and emission performance that meets the requirements of vacuum devices, making it particularly suitable for use in vacuum devices such as traveling wave tubes and klystrons that require low evaporation and long lifetime.

[0012] In this invention, the introduction of Sr is beneficial for reducing the evaporation rate. However, the amount of Sr should not exceed the limits set forth in this invention. This is because when the proportion of Sr is lower than the limits set forth in this invention, the reduction in evaporation rate is not significant. However, the amount of Sr introduced should also not be too large. Exceeding the limits set forth in this invention will cause a significant reduction in emission current density, and may even fail to meet the requirements. The advantage of this invention is that it can significantly reduce the evaporation rate while ensuring the required emission current density. Preferably, experimental studies have shown that when the molar ratio of the barium, strontium, calcium, and aluminum elements is 15-30:5-20:10-20:15-45, the prepared low-evaporation coated diffusion cathode exhibits better overall performance.

[0013] Furthermore, the cathode substrate is made of porous tungsten metal. Since it is a coated cathode, the porosity of the porous tungsten has an impact on the final performance of the cathode. When the porosity is 10-40%, the performance of the coated cathode is significantly improved.

[0014] Furthermore, the material of the noble metal film is selected from one or any two alloys formed from Os, Re, Ir or Ru. For example, the material of the noble metal film can be Os, Re, Ir, Ru, Os-Ru alloy, Os-Ir alloy or Os-Re alloy, etc.

[0015] To achieve the second objective mentioned above, the present invention adopts the following technical solution:

[0016] This invention discloses a method for preparing the coated diffusion cathode as described above, comprising the following steps:

[0017] First, the emission material is prepared by co-precipitation combined with high-temperature sintering. Then, the emission material is impregnated into the cathode substrate by heat treatment. Finally, a noble metal film is coated on the surface of the impregnated cathode substrate by DC sputtering to obtain a coated diffusion cathode.

[0018] Furthermore, the emission material is prepared according to the following steps:

[0019] 1) Weigh out the barium salt, strontium salt, calcium salt, and aluminum salt according to the proportion, dissolve them in deionized water to prepare a mixed salt solution, and use ammonium carbonate as a precipitant to add deionized water to prepare an ammonium carbonate solution.

[0020] 2) Add ammonium carbonate solution to mixed salt solution, stir and react at 15-35℃ to obtain suspension, filter, and dry to obtain precursor powder;

[0021] 3) The precursor powder is placed in a muffle furnace and sintered at high temperature under a hydrogen atmosphere to obtain the product.

[0022] Furthermore, the barium salt is selected from barium nitrate or barium chloride, the strontium salt is selected from strontium nitrate or strontium chloride, the calcium salt is selected from calcium nitrate or calcium chloride, and the aluminum salt is selected from aluminum nitrate or aluminum chloride; preferably, the concentration of the mixed salt solution is 0.1-0.6 mol / L; preferably, the concentration of the ammonium carbonate solution is 0.2-0.7 mol / L.

[0023] Furthermore, the drying temperature is 80-150℃, and the drying time is ≥7h; the high-temperature sintering temperature is 1200-1700℃, and the sintering time is 2-5h.

[0024] Furthermore, the heat treatment method involves melting the emission material at high temperature in a hydrogen atmosphere, and then allowing it to enter the pores of the cathode substrate through capillary action.

[0025] Furthermore, the high-temperature melting temperature is 1400-2000℃.

[0026] To achieve the third objective mentioned above, this invention discloses an application of the coated diffusion cathode described above in the fabrication of vacuum electronic components.

[0027] The beneficial effects of this invention are as follows:

[0028] This invention discloses a low-evaporation coated diffusion cathode, its preparation, and its application. Through extensive experimental research, this invention designs a novel emission material formulation. The introduction of strontium helps reduce the evaporation rate of the emission material. Furthermore, the appropriate ratio of barium, strontium, calcium, and aluminum further reduces the evaporation rate while maintaining a high current density. With the guaranteed low evaporation rate, the cathode exhibits a long lifespan. Therefore, the coated diffusion cathode prepared by this invention has a low evaporation rate, high reliability, and emission performance that meets the requirements of vacuum devices, making it particularly suitable for use in vacuum devices such as traveling wave tubes and klystrons that require low evaporation and long lifespan. Attached Figure Description

[0029] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0030] Figure 1 The XRD pattern of the strontium aluminate-containing emission material in Example 1 of the present invention is shown.

[0031] Figure 2 This image shows a SEM image of the strontium aluminate-coated diffusion cathode surface in Embodiment 1 of the present invention.

[0032] Figure 3 The image shows a SEM image of the fracture surface of the strontium aluminate-coated diffusion cathode of Embodiment 1 of the present invention.

[0033] Figure 4 The graph shows the diffusion cathode evaporation test (Beck method) curve of strontium aluminate-coated film in Example 1 of the present invention.

[0034] Figure 5 The diagram shows the pulse emission test curve of the diffusion cathode containing strontium aluminate coating in Example 1 of the present invention.

[0035] Figure 6 The graph shows a comparison of the lifetime test curves of the strontium aluminate-coated diffusion cathode of Embodiment 1 of the present invention and the conventional coated diffusion cathode. Detailed Implementation

[0036] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments and accompanying drawings, further clarifies the invention. It should be understood that the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0037] Unless otherwise specified, all methods used in this invention are conventional, and the raw materials used can be obtained from publicly available commercial sources unless otherwise specified.

[0038] According to a specific embodiment of the present invention, a low-evaporation strontium aluminate coated diffusion cathode is provided, the coated diffusion cathode comprising a cathode substrate, an emitting material, and a noble metal film layer;

[0039] The emitting material is a barium-strontium-calcium-aluminum-oxygen quaternary cathode emitting material; the molar ratio of each element in the barium-strontium-calcium-aluminum quaternary cathode is 10-35:1-20:1-20:2-50.

[0040] Furthermore, the coated diffusion cathode is prepared by the following steps:

[0041] First, the emission material is prepared by co-precipitation combined with high-temperature sintering. Then, the emission material is impregnated into the cathode substrate by heat treatment. Finally, a noble metal film is coated on the surface of the impregnated cathode substrate by DC sputtering to obtain a coated diffusion cathode.

[0042] Furthermore, the cathode substrate is made of tungsten metal with a porous structure and a porosity of 10-40%.

[0043] Furthermore, the emission material obtained by co-precipitation combined with high-temperature sintering is a strontium aluminate containing BaO, SrO, CaO, and Al2O3. Compared with traditional aluminates, the addition of SrO in this invention effectively reduces the evaporation rate of the emission material in traditional coated diffusion cathodes. Simultaneously, the reasonable combination of the components provides the required emission current density for the device. During cathode operation, the emission material reacts with the substrate to generate elemental Ba and Sr. At high temperatures, the generated Ba and Sr diffuse through the substrate voids to the cathode surface, and then evaporate into the vacuum on the cathode surface. In the above technical solution, the introduction of SrO reduces the generation and migration rate of Ba in the emission material, thereby reducing the evaporation rate of Ba, a key substance in the emission material, during cathode operation. On the other hand, Sr has a weakening effect on the emission current in the coated diffusion cathode system; therefore, the proportions of the four elements need to be reasonably and effectively controlled to achieve a reduction in cathode evaporation performance while simultaneously meeting the emission current density requirements for device use.

[0044] The technical solution of the present invention will be described below with reference to specific embodiments. It should be noted that the only difference between the conventional coated diffusion cathode described below and the embodiments and comparative examples is that it does not contain the element Sr:

[0045] Example 1

[0046] Weigh and mix Ba(NO3)2, Sr(NO3)2, Ca(NO3)2·4H2O and Al(NO3)3·9H2O in a molar ratio of 19:6:15:19. Dissolve the mixed nitrates in deionized water to prepare a mixed nitrate solution with a concentration of 0.24 mol / L.

[0047] Dissolve ammonium carbonate in deionized water and stir thoroughly to prepare an ammonium carbonate solution with a concentration of 0.35 mol / L;

[0048] The mixed nitrate solution and ammonium carbonate solution were mixed and reacted at room temperature. After the reaction was completed, a suspension was obtained, which was filtered. The reaction product was dried at 125°C for ≥7 hours to obtain the emission material precursor powder.

[0049] The precursor powder was loaded into a molybdenum boat and sintered at high temperature in a hydrogen atmosphere, with a sintering temperature range of 1200℃ and a time of 2 hours, to obtain an emission material containing strontium aluminate.

[0050] Figure 1 The image shows the X-ray diffraction (XRD) pattern of the strontium aluminate-containing emission material. The figure reveals that the strontium aluminate-containing emission material includes barium aluminate, calcium aluminate, strontium aluminate, and hydrated alumina. The preparation method of this emission material effectively achieves efficient Sr doping.

[0051] Strontium aluminate-containing emission material was impregnated in a porous tungsten cathode substrate with a porosity of 23%. The emission material was impregnated by a high-temperature melting method under a hydrogen atmosphere at a temperature of 1500°C for 1.5 minutes to obtain a cathode substrate impregnated with emission material.

[0052] A metallic Os film with a thickness of 0.5 μm was deposited on the surface of a cathode substrate impregnated with emission material using DC sputtering. This yielded a coated diffusion cathode.

[0053] Figure 2 The image shown is a SEM image of the prepared coated diffusion cathode surface. It can be seen that the coated diffusion cathode surface is smooth and the pores are evenly distributed.

[0054] Figure 3 The image shows a SEM image of the fracture surface of a strontium aluminate-coated diffusion cathode. As can be seen from the image, the cathode substrate and the emission material are tightly bonded, and the emission material is fully impregnated.

[0055] The strontium aluminate-coated diffusion cathode prepared in this embodiment was subjected to cathode emission material evaporation testing using the Beck method. Figure 4 The figure shows the evaporation test curve. The results show that at 1050℃, the evaporation rate of the emitting material is v = 4.21 × 10⁻⁶. -11 g / cm 2 Under the same conditions, the evaporation rate of the emission material in a conventional coated cathode prepared at 1050 °C is approximately 1.0 × 10⁻⁶ s. -10 g / cm 2 Under the same conditions, the evaporation rate of the emission material in a strontium aluminate cathode can be reduced by about 58%. Figure 5 The image shows the pulse emission test curve of the strontium aluminate-coated diffused cathode. The test pulse conditions were 200 Hz and 10 μs. The test results show that the strontium aluminate-coated diffused cathode can achieve an offset emission current density of 25.68 A / cm² at 1050 °C. 2 Under the same testing conditions, the emission current density of a conventional coated cathode is 15-25 A / cm². 2 This indicates that the emission performance of the strontium-containing aluminate-coated cathode prepared in this embodiment is comparable to that of a conventional coated cathode.

[0056] The low-evaporation coated cathode prepared in this embodiment was subjected to lifetime testing, such as... Figure 6 As shown, the cathode is subjected to a DC current of 4A / cm. 2 Under current density and temperature acceleration (1100℃) conditions, the measured lifetime is >8500 hours, and the current does not decrease. Under the same test conditions, the emission current density of a conventional coated cathode drops by 2.5% after 8000 hours. The results indicate that the cathode prepared in this embodiment has better lifetime characteristics.

[0057] Example 2

[0058] This invention discloses a strontium aluminate-coated diffusion cathode:

[0059] Weigh and mix Ba(NO3)2, Sr(NO3)2, Ca(NO3)2·4H2O and Al(NO3)3·9H2O in a molar ratio of 18:13:16:20. Dissolve the mixed nitrates in deionized water to prepare a mixed nitrate solution with a concentration of 0.3 mol / L.

[0060] Dissolve ammonium carbonate in deionized water and stir thoroughly to prepare an ammonium carbonate solution with a concentration of 0.28 mol / L;

[0061] The mixed nitrate solution and ammonium carbonate solution were mixed and reacted at room temperature. After the reaction was completed, a suspension was obtained, which was filtered. The reaction product was dried at 100°C for ≥7 hours to obtain the emission material precursor powder.

[0062] The precursor powder was loaded into a molybdenum boat and sintered at high temperature in a hydrogen atmosphere at 1300°C for 2 hours to obtain an emission material containing strontium aluminate.

[0063] Strontium aluminate emission material was impregnated in a porous tungsten cathode substrate with a porosity of 24%. The emission material was impregnated by a high-temperature melting method under a hydrogen atmosphere at a temperature of 1500°C for 1.5 minutes to obtain a cathode substrate impregnated with emission material.

[0064] The evaporation and emission performance of the strontium aluminate-coated diffusion cathode prepared in this embodiment were tested. The test results showed that the evaporation rate of the cathode emission material at 1050℃ was 3.66 × 10⁻⁶. -11 g / cm 2 At 1050℃, under pulsed (200Hz, 10μs) conditions, the emission current density can reach 23.80 A / cm². 2 Under the same preparation conditions, the evaporation rate of the emission material in a conventional coated cathode is 3.5 × 10⁻⁶. -10 g / cm 2 The result indicates that the strontium-aluminate coated cathode prepared in this embodiment can reduce the evaporation rate of the emission material to 1 / 9 of that of a conventional coated cathode at the same temperature. Meanwhile, its emission performance is comparable to that of a conventional coated cathode, meeting the requirements for device use.

[0065] Comparative Example 1

[0066] Ba(NO3)2, Sr(NO3)2, Ca(NO3)2·4H2O, and Al(NO3)3·9H2O were weighed and mixed in a molar ratio of 19:0.8:15:19 to prepare a mixed nitrate solution with a concentration of 0.24 mol / L.

[0067] Dissolve ammonium carbonate in deionized water and stir thoroughly to prepare an ammonium carbonate solution with a concentration of 0.35 mol / L;

[0068] The mixed nitrate solution and ammonium carbonate solution were mixed and reacted at room temperature. After the reaction was completed, a suspension was obtained, which was filtered. The reaction product was dried at 100°C for ≥7 hours to obtain the emission material precursor powder.

[0069] The precursor powder was loaded into a molybdenum boat and sintered at high temperature in a hydrogen atmosphere, with a sintering temperature range of 1200℃ and a time of 2 hours. This yielded an aluminate emission material.

[0070] Aluminate emission material was impregnated in a porous tungsten cathode substrate by high-temperature melting under a hydrogen atmosphere at 1500°C for 1.5 minutes, thus obtaining a cathode substrate impregnated with emission material.

[0071] A metallic Os film with a thickness of 0.5 μm was deposited on the surface of a cathode substrate impregnated with emission material using DC sputtering. This yielded a coated diffusion cathode.

[0072] The coated diffusion cathode prepared in this comparative example has a low Sr content in its cathode emitter material. Evaporation tests of the cathode emitter material were conducted using the Beck method. The results show that at 1060℃, the evaporation rate of the emitter material in the cathode of Comparative Example 1 is approximately 2.31 × 10⁻⁶.-10 g / cm 2 Under the same preparation conditions, the evaporation rate of the emission material in a conventional coated cathode is 3 × 10⁻⁶ s. -10 g / cm 2 The evaporation rate decreased by only 27% with Sr, which is a relatively small decrease. The results of Comparative Example 1 show that when the Sr doping level is too low, the reduction in emission material is small and cannot meet the low evaporation requirements of the device.

[0073] Comparative Example 2

[0074] Weigh and mix Ba(NO3)2, Sr(NO3)2, Ca(NO3)2·4H2O and Al(NO3)3·9H2O in a molar ratio of 18:25:16:20. Dissolve the mixed nitrates in deionized water to prepare a mixed nitrate solution with a concentration of 0.3 mol / L.

[0075] Dissolve ammonium carbonate in deionized water and stir thoroughly to prepare an ammonium carbonate solution with a concentration of 0.28 mol / L;

[0076] The mixed nitrate solution and ammonium carbonate solution were mixed and reacted at room temperature. After the reaction was completed, a suspension was obtained, which was filtered. The reaction product was dried at 150°C for ≥7 hours to obtain the emission material precursor powder.

[0077] The precursor powder was loaded into a molybdenum boat and sintered at high temperature in a hydrogen atmosphere, with a sintering temperature range of 1300℃ and a time of 2 hours, to obtain an aluminate emission material.

[0078] Aluminate emission material was impregnated in a porous tungsten cathode substrate by high-temperature melting under a hydrogen atmosphere at 1500°C for 1.5 minutes to obtain a cathode substrate impregnated with emission material.

[0079] The evaporation and emission performance of the strontium aluminate-coated diffusion cathode prepared in this comparative example were tested. The test results showed that the evaporation rate of the cathode emission material at 1060℃ was 3.23 × 10⁻⁶. -11 g / cm 2 At 1050℃, the pulse emission current density is only 10.5 A / cm². 2 Compared with traditional coated cathodes, the evaporation of the strontium aluminate coated diffusion cathode prepared in Comparative Example 2 was reduced by more than 70%, but the emission current density was 30-60% lower than that of traditional coated cathodes, and the emission could not meet the device requirements.

[0080] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A low-evaporation coated diffusion cathode, characterized in that, The coated diffusion cathode includes a cathode substrate, an emitting material, and a noble metal film layer; The emitting material is a quaternary cathode emitting material composed of barium, strontium, calcium, and aluminum oxides; the molar ratio of each element in the barium, strontium, calcium, and aluminum oxides is 15-30:5-20:10-20:15-45. The cathode substrate is made of porous tungsten metal with a porosity of 10-40%. The coated diffusion cathode is prepared according to the following steps: First, the emission material is prepared by co-precipitation combined with high-temperature sintering. Then, the emission material is impregnated into the cathode substrate by heat treatment. Finally, a noble metal film is coated on the surface of the impregnated cathode substrate by DC sputtering to obtain a coated diffusion cathode. The emission material is specifically prepared according to the following steps: 1) Weigh out barium salt, strontium salt, calcium salt, and aluminum salt according to the proportions, dissolve them in deionized water to prepare a mixed salt solution, and use ammonium carbonate as a precipitant to add deionized water to prepare an ammonium carbonate solution. 2) Add ammonium carbonate solution to mixed salt solution, stir and react at 15-35℃ to obtain suspension, filter, and dry to obtain precursor powder; 3) The precursor powder is placed in a muffle furnace and sintered at high temperature under a hydrogen atmosphere to obtain the product. The high-temperature sintering temperature is 1200-1700℃, and the sintering time is 2-5 hours. The heat treatment method involves melting the emission material at high temperature in a hydrogen atmosphere, and then allowing it to enter the pores of the cathode substrate through capillary action.

2. The coated diffusion cathode according to claim 1, characterized in that, The material of the noble metal film is selected from one or an alloy formed from any two of Os, Re, Ir or Ru.

3. A method for preparing a coated diffusion cathode as described in any one of claims 1-2, characterized in that, Includes the following steps: First, the emission material is prepared by co-precipitation combined with high-temperature sintering. Then, the emission material is impregnated into the cathode substrate by heat treatment. Finally, a noble metal film is coated on the surface of the impregnated cathode substrate by DC sputtering to obtain a coated diffusion cathode. The emission material is prepared according to the following steps: 1) Weigh out barium salt, strontium salt, calcium salt, and aluminum salt according to the proportions, dissolve them in deionized water to prepare a mixed salt solution, and use ammonium carbonate as a precipitant to add deionized water to prepare an ammonium carbonate solution. 2) Add ammonium carbonate solution to mixed salt solution, stir and react at 15-35℃ to obtain suspension, filter, and dry to obtain precursor powder; 3) The precursor powder is placed in a muffle furnace and sintered at high temperature under a hydrogen atmosphere to obtain the product. The high-temperature sintering temperature is 1200-1700℃, and the sintering time is 2-5 hours. The heat treatment method involves melting the emission material at high temperature in a hydrogen atmosphere, and then allowing it to enter the pores of the cathode substrate through capillary action.

4. The preparation method according to claim 3, characterized in that, The barium salt is selected from barium nitrate or barium chloride, the strontium salt is selected from strontium nitrate or strontium chloride, the calcium salt is selected from calcium nitrate or calcium chloride, and the aluminum salt is selected from aluminum nitrate or aluminum chloride.

5. The preparation method according to claim 3, characterized in that, The concentration of the mixed salt solution is 0.1-0.6 mol / L.

6. The preparation method according to claim 3, characterized in that, The concentration of the ammonium carbonate solution is 0.2-0.7 mol / L.

7. The preparation method according to claim 3, characterized in that, The drying temperature is 80-150℃, and the drying time is ≥7h.

8. The preparation method according to claim 3, characterized in that, The high-temperature melting temperature is 1400-2000℃.

9. The application of a coated diffusion cathode as described in any one of claims 1-2 in the fabrication of vacuum electronic components.

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