A method for manufacturing a high-temperature thermionic cathode

Abstract

A preparation method of high-temperature hot cathode relates to the technical field of vacuum electron device manufacturing. Ammonium metatungstate, ammonium perrhenate, lanthanum nitrate and calcium nitrate are dissolved in water respectively, and then mixed into a clear solution. The solute is precipitated by water bath heating, dried in an oven, and then reduced in a hydrogen atmosphere to obtain a cathode precursor powder. The cathode precursor powder is sintered in a sintering furnace. During the working process of the cathode, the rare earth oxide can rapidly diffuse to the surface of the cathode, thereby supplementing the evaporation of the surface composition caused by high temperature and ensuring the stable release of electrons during the working process of the cathode. The noble metal rhenium has excellent chemical stability, less surface adsorbed gas, good ion bombardment resistance, good high-temperature mechanical properties, low evaporation rate, high resistivity, and can form a tungsten-rhenium alloy with the cathode matrix after being added, or rhenium-coated tungsten, thereby improving the overall emission performance of the cathode. After the addition of the alkaline earth metal, the evaporation of the active substances on the surface of the cathode can be reduced, and the surface ablation can be reduced, thereby improving the service life of the cathode.

Description

Technical Field

[0001] This invention relates to the field of vacuum electronic device manufacturing technology, and is a novel method for preparing a high-temperature hot cathode. Background Technology

[0002] Vacuum-electronic devices have a wide range of applications in civilian and military fields, such as microwave communication, medical diagnostics, radar, electronic warfare, and aerospace. The cathode, as the electron source of a vacuum-electronic device, is considered its core and plays a crucial role in its application. In recent years, with the continuous development of vacuum-electronic devices, the requirements for the electron emission capability of the cathode have become increasingly stringent. For example, in high-power terahertz radiation sources, the required electron beam current density is typically as high as several hundred A / cm². 2 .

[0003] Pure metal cathodes, as the earliest invented cathodes, possess excellent thermal stability. They are typically fabricated from refractory metals (tungsten, molybdenum, rhenium, tantalum, niobium) through processing. Among these, tungsten cathodes are the most representative and widely used pure metal cathodes. The operating temperature of pure tungsten cathodes is usually above 2500K, while the emission current density is approximately 1 A / cm². 2 Its emission efficiency is relatively low, approximately 1 mA / W.

[0004] To improve the emission performance of cathodes, reinforcing phases of varying compositions are added to tungsten, such as 1.5–2% ThO2 or La2O3. After appropriate heat treatment, the thermionic emission current of a doped tungsten cathode at the same temperature is much higher than that of a pure tungsten cathode. For example, when the mass fraction of ThO2 in pure tungsten reaches 2%, the work function of the ThO2-W cathode decreases to 2.63 eV, the operating temperature is 1950–2000 K, and the current density is 2 A / cm². 2 The emission efficiency is 10 mA / W, which is 10 times that of a pure tungsten cathode. However, thorium-tungsten cathodes cause radioactive contamination during processing, use, and disposal. Currently, thorium-tungsten cathodes are gradually being replaced by other cathodes.

[0005] To date, even with the addition of rare earth oxides to replace thorium oxide, the resulting cathodes can only be used in environments below 1600℃. The Ta, W, Ta-W, and porous Ta-W cathodes already in use experience the most severe ablation at operating temperatures of 1900℃, due to the highest temperature they endure. This leads to widespread deformation, uneven end faces, and localized melting and overflow, directly impacting the cathode's lifespan. Rhenium is a chemically stable metal with a melting point as high as 3180℃. It has low surface gas adsorption, good resistance to ion bombardment, good high-temperature mechanical properties, low evaporation rate, and high resistivity. Its disadvantages include high cost, and it is rarely used alone. Generally, a small amount of rhenium is added to tungsten to create tungsten-rhenium alloys, or a rhenium layer is applied to improve the cathode's electron emission performance. In particular, Re3W, which appears when the rhenium content is increased to 75%, exhibits a single phase and the best emission performance.

[0006] In summary, tungsten is combined with noble metals as the main matrix component in the cathode, and rare earth lanthanum oxide is added using different preparation methods to refine the microstructure and improve cathode emission. Adding rhenium and rare earth oxides to the tungsten matrix has a positive effect on improving cathode emission performance. This invention achieves excellent electron emission capabilities through high-temperature activation by controlling the composition and dosage of raw materials, which is of great significance to the performance of the entire vacuum electronic device. Summary of the Invention

[0007] This invention provides a novel method for preparing a high-temperature hot cathode. The raw materials are dissolved in deionized water using a liquid-liquid mixing method. The solutes are precipitated by heating in a water bath, dried in an oven, and then reduced in a two-stage process under a hydrogen atmosphere to obtain cathode precursor powder. Finally, the powder is sintered in a sintering furnace.

[0008] The specific preparation process includes the following steps:

[0009] Step 1: Dissolve ammonium metatungstate, ammonium perrhenate, lanthanum nitrate, and calcium nitrate separately in water and then mix them to form a clear solution;

[0010] Step 2: The solution prepared in Step 1 is stirred and heated in a water bath to obtain a precipitate, which is then placed in an oven for drying.

[0011] Step 3: Grind and sieve the precipitate prepared in Step 2 to obtain powder;

[0012] Step 4: The powder from Step 3 is subjected to two-stage hydrogen reduction in a hydrogen environment to obtain cathode precursor powder;

[0013] Step 5: Press the cathode precursor powder from Step 4 into shape, and sinter it in a high-temperature tungsten mesh hydrogen furnace in a pure hydrogen atmosphere to prepare the cathode.

[0014] Step Six: The cathode prepared in Step Five is processed into the required size through physical processing methods such as machining to obtain the new type of cathode.

[0015] In step one, ammonium perrhenate (H4NO4Re) comprises 30–75 wt%; lanthanum nitrate (La(NO3)3·6H2O) + calcium nitrate (Ca(NO3)2) comprises 10–20 wt%; the remainder is ammonium metatungstate. The mass ratio of rhenium to tungsten is 1–4:1 before further steps.

[0016] This invention is actually a method for preparing a ternary mixed-base diffusion cathode. To avoid the volatilization of rhenium, a two-stage hydrogen reduction process is used to obtain the cathode precursor powder.

[0017] The prepared cathode precursor powder has a uniform and fine particle size distribution, is free of impurities, and has a uniformly dispersed phase. Rhenium metal, rare earth oxides, and alkaline earth metals are uniformly distributed in the precursor powder.

[0018] The prepared precursor powder was stored in a dry vacuum environment. The powder was then pressed into blocks by a molding process and sintered in a high-temperature hydrogen sintering furnace to finally obtain the cathode product.

[0019] The operating temperature of the high-temperature hot cathode obtained by this invention is not lower than 1600℃.

[0020] The actual effect of the cathode obtained by the novel high-temperature hot cathode preparation method of the present invention is as follows:

[0021] 1. This invention improves the emission current density of the mixed-base diffused cathode by controlling the composition and dosage of raw materials, thereby adjusting the composition of the final product. The test temperature was 1600℃. b (Brightness temperature), the effective current density that the sample can extract reaches 2.46 A / cm². 2 Furthermore, the work function is 2.842 eV, which is a significant improvement compared to a pure tungsten cathode.

[0022] 2. After sintering, the cathode obtained by this method can rapidly diffuse rare earth oxides, as emission active materials, to the cathode surface in the working state, thereby replenishing the surface components evaporated due to high temperature, ensuring stable electron release during cathode operation, and the multi-phase components work together to strengthen the cathode, thereby improving the cathode emission performance and service life.

[0023] 3. This method introduces nanoscale rare earth oxides, noble metals, and alkaline earth metals into a traditional tungsten cathode. During operation, the rare earth oxides rapidly diffuse to the cathode surface, replenishing the surface components lost due to high temperatures and ensuring stable electron release. Rhenium, a noble metal, exhibits excellent chemical stability, low surface gas adsorption, good resistance to ion bombardment, good high-temperature mechanical properties, low evaporation rate, and high resistivity. Its addition can form tungsten-rhenium alloys with the cathode substrate, or rhenium can coat tungsten, thereby improving the overall emission performance of the cathode. Furthermore, alkaline earth metals possess excellent thermal and electrical conductivity; their addition reduces the evaporation of active materials on the cathode surface and decreases surface ablation, thus extending the cathode's lifespan. Through the synergistic effect of these multiple components, a cathode with excellent overall performance can be prepared. Attached Figure Description

[0024] Figure 1 Cathode fabrication process route diagram;

[0025] Figure 2 Example 1: Laser particle size distribution diagram of precursor powder;

[0026] Figure 3 SEM images of cathode precursor powder and cathode;

[0027] Table 1. DC emission current density and work function of the novel hot cathode. Detailed Implementation

[0028] The specific implementation process of the preparation method of the novel high-temperature hot cathode of the present invention is as follows. The technical solution of the present invention will be described in detail below, but the present invention is not limited to the following embodiments.

[0029] A water bath heating process was used to precipitate an aqueous solution of ammonium metatungstate, ammonium perrhenate, lanthanum nitrate, and calcium nitrate. After drying, the precipitate was ground and sieved to obtain raw material powder. This powder was then subjected to two-stage hydrogen reduction at 600°C and 900°C in a tube furnace to obtain precursor powder. The particle size of the powder is as follows: Figure 2 As shown.

[0030] Example 1:

[0031] 2.8g of ammonium metatungstate, 10.8g of ammonium perrhenate, 1.6g of lanthanum nitrate, and 1.2g of calcium nitrate were weighed and dissolved in water. The mixture was heated and stirred in a water bath until a precipitate formed. After filtration and washing, the precipitate was dried in an 80℃ oven for 24 hours. After drying, the mixture was ground and passed through an 80-mesh sieve to obtain a powder. The powder was then subjected to a two-stage hydrogen reduction in a tube furnace. The reduction process was as follows: the first stage was held at 600℃ for 2 hours, and the second stage was held at 900℃ for 2 hours. The precursor powder was obtained, and its microstructure is as follows. Figure 3 As shown in (a).

[0032] Pressing and sintering of cathode precursor powder: The weighed precursor powder was loaded into a mold with an inner diameter of 3 mm and formed by bidirectional molding using a powder press. A sintered green body was obtained by holding the pressure at 0.8 MPa for 45 seconds. The green body was then held at 1800℃ for 120 minutes in a high-temperature hydrogen furnace to obtain a novel high-temperature cathode substrate. The microstructure is as follows: Figure 3 As shown in (d).

[0033] The prepared novel cathode was machined to obtain a finished cathode with a diameter of 2.5 mm and a thickness of 1 mm.

[0034] The test results obtained after the invention cathode underwent DC emission testing are shown in Table 1.

[0035] Example 2:

[0036] 3.5g of ammonium metatungstate, 10.8g of ammonium perrhenate, 1.6g of lanthanum nitrate, and 1.2g of calcium nitrate were weighed and dissolved in water. The mixture was heated and stirred in a water bath until a precipitate formed. After filtration and washing, the precipitate was dried in an 80℃ oven for 24 hours. After drying, the mixture was ground and passed through an 80-mesh sieve to obtain a powder. The powder was then subjected to a two-stage hydrogen reduction in a tube furnace. The reduction process was as follows: the first stage was held at 600℃ for 2 hours, and the second stage was held at 900℃ for 2 hours. The precursor powder was obtained, and its microstructure is as follows. Figure 3 As shown in (b).

[0037] As in Example 1, the cathode was machined to obtain a finished cathode with a diameter of 2.5 mm and a thickness of 1 mm. The test results obtained after DC emission testing are shown in Table 1.

[0038] Example 3:

[0039] 4.2g of ammonium metatungstate, 10.8g of ammonium perrhenate, 1.6g of lanthanum nitrate, and 1.2g of calcium nitrate were weighed and dissolved in water. The mixture was heated and stirred in a water bath until a precipitate formed. After filtration and washing, the precipitate was dried in an 80℃ oven for 24 hours. After drying, the mixture was ground and passed through an 80-mesh sieve to obtain a powder. The powder was then subjected to a two-stage hydrogen reduction in a tube furnace. The reduction process was as follows: the first stage was held at 600℃ for 2 hours, and the second stage was held at 900℃ for 2 hours. The precursor powder was obtained, and its microstructure is as follows. Figure 3 As shown in (c).

[0040] As in Example 1, the cathode is machined to obtain a finished cathode with a diameter of 2.5 mm and a thickness of 1 mm.

[0041] Example 4:

[0042] 12.7g of ammonium metatungstate, 3.6g of ammonium perrhenate, 1.6g of lanthanum nitrate, and 1.2g of calcium nitrate were weighed and dissolved in water. The mixture was heated and stirred in a water bath until a precipitate formed. After filtration and washing, the precipitate was dried in an oven at 80℃ for 24 hours. After drying, the mixture was ground and passed through an 80-mesh sieve to obtain a powder. The powder was then subjected to a two-stage hydrogen reduction in a tube furnace. The reduction process was as follows: the first stage was held at 600℃ for 2 hours, and the second stage was held at 900℃ for 2 hours. The precursor powder was obtained, and its microstructure is as follows. Figure 3 As shown in (c).

[0043] As in Example 1, the cathode is machined to obtain a finished cathode with a diameter of 2.5 mm and a thickness of 1 mm.

[0044] Example 5:

[0045] 12.7g of ammonium metatungstate, 5.4g of ammonium perrhenate, 1.6g of lanthanum nitrate, and 1.2g of calcium nitrate were weighed and dissolved in water. The mixture was heated in a water bath with stirring until a precipitate formed. After filtration and washing, the precipitate was dried in an 80℃ oven for 24 hours. After drying, the precipitate was ground and passed through an 80-mesh sieve to obtain a powder. The powder was then subjected to a two-stage hydrogen reduction in a tube furnace. The reduction process consisted of a first stage at 600℃ for 2 hours and a second stage at 900℃ for 2 hours. This yielded a precursor powder with the following microstructure: Figure 3 As shown in (c).

[0046] As in Example 1, the cathode is machined to obtain a finished cathode with a diameter of 2.5 mm and a thickness of 1 mm.

[0047] Example 6:

[0048] 12.7g of ammonium metatungstate, 7.2g of ammonium perrhenate, 1.6g of lanthanum nitrate, and 1.2g of calcium nitrate were weighed and dissolved in water. The mixture was heated and stirred in a water bath until a precipitate formed. After filtration and washing, the precipitate was dried in an 80℃ oven for 24 hours. After drying, the mixture was ground and passed through an 80-mesh sieve to obtain a powder. The powder was then subjected to a two-stage hydrogen reduction in a tube furnace. The reduction process was as follows: the first stage was held at 600℃ for 2 hours, and the second stage was held at 900℃ for 2 hours. The precursor powder was obtained, and its microstructure is as follows. Figure 3 As shown in (c).

[0049] As in Example 1, the cathode is machined to obtain a finished cathode with a diameter of 2.5 mm and a thickness of 1 mm.

[0050] Table 1

[0051]

[0052]

Claims

1. A method for preparing a high-temperature hot cathode, characterized in that, Includes the following steps: Step 1: Dissolve ammonium metatungstate, ammonium perrhenate, lanthanum nitrate, and calcium nitrate separately in water and then mix them to form a clear solution; Step 2: The solution prepared in Step 1 is stirred and heated in a water bath to obtain a precipitate, which is then placed in an oven for drying. Step 3: Grind and sieve the precipitate prepared in Step 2 to obtain powder; Step 4: The powder from Step 3 is subjected to two-stage hydrogen reduction in a hydrogen environment to obtain cathode precursor powder; Step 5: Press the cathode precursor powder from Step 4 into shape, and sinter it in a high-temperature tungsten mesh hydrogen furnace in a pure hydrogen atmosphere to prepare the cathode. Step Six: The cathode prepared in Step Five is processed into the required size through physical processing methods such as machining to obtain the new type of cathode. In step one, ammonium perrhenate (H4NO4Re) is 30-75 wt%; lanthanum nitrate (La(NO3)3·6H2O) + calcium nitrate (Ca(NO3)2) is 10-20 wt%; the remainder is ammonium metatungstate.

2. The method according to claim 1, characterized in that, The mass ratio of rhenium to tungsten in the cathode precursor powder is 1-4:

1.

3. A high-temperature hot cathode prepared according to the method of claim 1 or 2.

4. The application of a high-temperature hot cathode prepared according to the method of claim 1 or 2, wherein the working temperature is not lower than 1600℃.

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