Polycrystalline lanthanum hexaboride, and preparation method and application thereof

Abstract

The application discloses polycrystalline lanthanum hexaboride and a preparation method and application thereof, and belongs to the technical field of lanthanum hexaboride preparation, and the preparation method comprises the following steps: ball milling mixed powders of lanthanum hexaboride powders and spherical nickel powders in a protective gas to prepare ball-milled powders; screening the ball-milled powders through an ultrasonic vibration sieve classifier, and collecting mixed powders with a particle size of 1-2 microns; placing the screened mixed powders in a mold, and then performing gradient hot-pressing sintering under vacuum conditions; and cooling after sintering to obtain the polycrystalline lanthanum hexaboride. The relative density of the material is as high as 99.42%, the material has the advantages of high density and strong thermal electron emission performance, the production cost of the method is low, the process difficulty is low, and the method can effectively solve the problems existing in the preparation of polycrystalline lanthanum hexaboride by using the existing sintering method.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lanthanum hexaboride material preparation, and particularly relates to polycrystalline lanthanum hexaboride and a preparation method and application thereof. BACKGROUND

[0002] Lanthanum hexaboride (LaB6) is a kind of excellent thermal electron emission cathode material, which has low electron work function, high conductivity, high brightness and high current density compared with traditional ordinary cathode materials such as tungsten and tantalum. Lanthanum hexaboride has a very high melting point (2715℃), is suitable for working in high-power and high-temperature environments, has good resistance to cathode pollution, is not easy to be corroded by chemical substances, and has a high cathode service life. It is widely used in key components of national defense industry and civil industry, such as high-power emission cathode, electron tube, electron microscope and electron beam device.

[0003] Lanthanum hexaboride cathode material can be divided into lanthanum hexaboride single crystal material and polycrystalline lanthanum hexaboride. The lanthanum hexaboride single crystal material has fewer internal defects and no extra grain boundaries, and the emission performance is better than that of polycrystalline lanthanum hexaboride. However, due to the complex production process, high production cost and great technical difficulty of the lanthanum hexaboride single crystal material, it is difficult to meet the urgent demand for high-performance cathode material. It is also pointed out in the prior art that the emission performance of sintered dense polycrystalline lanthanum hexaboride can reach the level of lanthanum hexaboride single crystal material. Compared with the lanthanum hexaboride single crystal material, the production process of polycrystalline lanthanum hexaboride is relatively simple, and the requirement for equipment is relatively low. Therefore, polycrystalline lanthanum hexaboride has become the most widely used cathode material.

[0004] Hot-pressing sintering method and SPS discharge plasma sintering method are relatively widely used for preparing polycrystalline lanthanum hexaboride. However, the sintering temperature of polycrystalline lanthanum hexaboride prepared by the hot-pressing sintering method is high, and it is difficult to sinter densely, the intergranular gap is large, and the relative density of the material is low, which greatly affects the thermal electron emission performance. Although the relative density of polycrystalline lanthanum hexaboride prepared by the discharge plasma sintering method is high, and the thermal electron emission performance is good, the powder performance required by this method is very high, the technical difficulty is relatively large, and the production cost is also very high. SUMMARY

[0005] In view of the above problems in the prior art, the application provides a polycrystalline lanthanum hexaboride, a preparation method and application thereof. The relative density of the material is as high as 99.42%, the material has the advantages of high density and strong thermal electron emission performance, the production cost of the method is low, the technical difficulty is low, and the problems existing in the preparation of polycrystalline lanthanum hexaboride by the existing sintering method can be effectively solved.

[0006] To achieve the above-mentioned purposes, the technical solution adopted by the application to solve its technical problems is as follows:

[0007] A preparation method of polycrystalline lanthanum hexaboride, comprising the following steps:

[0008] (1) Ball milling the mixed powder of lanthanum hexaboride powder and spherical nickel powder in a protective gas to obtain a ball-milled powder;

[0009] (2) Screening the ball-milled powder through an ultrasonic vibration sieve to collect mixed powder with a particle size of 1-2 μm;

[0010] (3) Placing the screened mixed powder in a mold, and then performing gradient hot-pressing sintering under vacuum, and cooling after sintering to obtain.

[0011] Further, in step (1), the mass percentage of lanthanum hexaboride powder and spherical nickel powder is 98.5-99.9%:0.1-1.5%, the particle size of the lanthanum hexaboride powder is 4-5 μm, and the particle size of the spherical nickel powder is 2-4 μm.

[0012] Further, in step (1), the protective gas is a mixed gas of nitrogen and argon, wherein the proportion of nitrogen is 80%, and the rest is argon.

[0013] Further, in step (1), when ball milling, hard zirconia grinding balls with diameters of 10 mm, 8 mm and 5 mm are used, the ball milling time is 10-14 h, and the powder after ball milling is subjected to drying treatment.

[0014] Further, in step (2), the vacuum degree during gradient pressure sintering is 1-9 × 10 -2 Pa.

[0015] Further, in step (2), the specific process of gradient pressure sintering is as follows:

[0016] a. uniformly heating to 600-900 ℃ while applying an axial pressure of 60-90 MPa, and pressure sintering for 60-90 min;

[0017] b. uniformly heating to 1400-1700 ℃ while applying an axial pressure of 91-125 MPa;

[0018] c. uniformly heating to 1700-1990 ℃ while pressure holding, and holding for 90-240 min;

[0019] d. uniformly cooling to 800-1000 ℃ and uniformly releasing pressure to 20-60 MPa;

[0020] e. after releasing pressure to 0 MPa, naturally cooling in the furnace to obtain.

[0021] Further, in a, the temperature is raised at a rate of 5-10 DEG C / min, in b, the temperature is raised at a rate of 10-15 DEG C / min, in c, the temperature is raised at a rate of 5-10 DEG C / min, and in d, the temperature is lowered at a rate of 10-15 DEG C / min, and the pressure is released at a rate of 0.3-0.7 MPa / min.

[0022] In the above scheme, the uniform temperature rising and pressure operation in step a can promote particle rearrangement and initial necking, and preliminarily reduce the porosity in the material; the temperature rising speed is increased in step b to trigger the liquid phase sintering effect of nickel, and the high pressure is used to inhibit the migration of grain boundaries and control the size of the grains; the temperature rising speed is reduced in step c in combination with the pressure operation to continue discharging the gas in the material and improve the density of the material; the axial pressure can reduce the sintering activation energy, inhibit the excessive migration of grain boundaries, inhibit the grain coarsening, and make the internal structure of the material more dense; and the uniform temperature lowering in step d can reduce the residual stress in the material, avoid the micro-crack initiation, and maintain the stability of the microstructure of the polycrystalline lanthanum hexaboride.

[0023] A polycrystalline lanthanum hexaboride is prepared by the above method.

[0024] The polycrystalline lanthanum hexaboride is applied to the preparation of a cathode emission material.

[0025] The beneficial effects of the present application are as follows:

[0026] 1. The present application innovatively adopts the doping method of high-purity spherical nickel powder, and the spherical nickel powder improves the mechanical properties and thermionic emission properties of the polycrystalline lanthanum hexaboride through dispersion strengthening and solid solution strengthening. The doping of nickel makes the grain boundary combination more compact, and the ductility of nickel compensates for the brittleness of the lanthanum hexaboride, reduces the surface crack propagation under high temperature or high stress, reduces the sintering temperature of the lanthanum hexaboride, reduces the pore defects, improves the material density, reduces the work function of the polycrystalline lanthanum hexaboride, improves the electron escape efficiency, reduces the sputtering loss of the cathode material under a strong electric field, and prolongs the service life of the cathode material.

[0027] 2. The material is ball milled under a protective atmosphere, which can realize the activation of the material surface, inhibit the oxidation of the lanthanum hexaboride powder, control the lattice defect density, and improve the sintering performance of the subsequent polycrystalline lanthanum hexaboride.

[0028] 3. The present application adopts gradient pressure sintering operation, slowly raises the temperature at a lower temperature rising rate, and simultaneously applies pressure, which can control the particle rearrangement and necking of the material, promote the discharge of the gas in the material, make the lanthanum hexaboride green body reach a more compact particle accumulation before sintering, reduce the porosity in the green body, improve the density of the material, and further improve the performance of the material.

[0029] 4、The present application optimizes and improves the traditional low-efficiency sintering process, effectively greatly improves the material performance while keeping a low technical difficulty, reduces the performance requirements for technical equipment while controlling the production cost. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 An electron micrograph of polycrystalline lanthanum hexaboride in Example 4;

[0031] Figure 2 An electron micrograph of polycrystalline lanthanum hexaboride in Comparative Example 1;

[0032] Figure 3 A physical photograph of polycrystalline lanthanum hexaboride in Example 4. DETAILED DESCRIPTION

[0033] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application, i.e., the described embodiments are only a part of the embodiments of the present application, but not all the embodiments.

[0034] Therefore, the detailed description of the embodiments of the present application provided below is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application.

[0035] It should be noted that the relational terms such as "first" and "second" and the like are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. Moreover, the terms "comprise", "include" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a list of elements not only includes those elements, but also includes other elements not explicitly listed, or further includes elements inherent in such a process, method, article or apparatus. Without more limitations, the element defined by the statement "comprises a" does not exclude the presence of additional identical elements in the process, method, article or apparatus that includes the element.

[0036] The features and performances of the present application will be further described in detail below with reference to the embodiments and the accompanying drawings.

[0037] Example 1

[0038] A polycrystalline lanthanum hexaboride, the preparation method comprising the following steps:

[0039] (1) Weighing 99.9% by mass of lanthanum hexaboride powder with a particle size of 5 μm and 0.1% by mass of high-purity spherical nickel powder with a particle size of 4 μm, the mixed powder of lanthanum hexaboride powder and spherical nickel powder is placed in a polyurethane ball mill, and zirconia grinding balls with diameters of 10 mm, 8 mm and 5 mm are used as grinding media. The mixed powder is subjected to all-round planetary ball milling for 14 h in a protective gas to obtain ball-milled powder. The protective gas is a mixture of nitrogen and argon, wherein nitrogen accounts for 80% and the rest is argon. After the ball-milled powder is taken out, it is placed in an electric vacuum oven at 95°C for 24 h, and then the powder is taken out and sieved on an ultrasonic vibration sieving instrument with a vibration sieving frequency of 18 kHz to collect powder below 2 μm.

[0040] (2) The powder obtained after screening was placed in a graphite mold padded with graphite paper, and then the mold was placed in a hot pressing sintering furnace and vacuumed to 1×10 -2 Pa, and then gradient hot pressing sintering is carried out. The specific sintering conditions are:

[0041] a. Raise the temperature to 600℃ at a rate of 5℃ / min, apply an axial pressure of 60Mpa, and sinter for 90min;

[0042] b. Raise the temperature uniformly at a rate of 10°C / min to 1400°C while applying an axial pressure of 90 MPa;

[0043] c. Under pressure holding conditions, heat the sample to 1700°C at a rate of 5°C / min and keep the temperature for 90 minutes;

[0044] d. Cool down to 800℃ at a rate of 10℃ / min and release the pressure to 20Mpa at a rate of 0.3Mpa / min;

[0045] e. After the pressure is released to 0Mpa, the mixture is naturally cooled in the furnace to obtain the product.

[0046] Example 2

[0047] A polycrystalline lanthanum hexaboride, the preparation method of which comprises the following steps:

[0048] (1) Take 99.7% by mass of lanthanum hexaboride powder with a particle size of 4 μm and 0.3% by mass of high-purity spherical nickel powder with a particle size of 2 μm, place the mixed powder of the lanthanum hexaboride powder and the spherical nickel powder in a polyurethane ball mill jar, use zirconia grinding balls with diameters of 10 mm, 8 mm and 5 mm as grinding media, and perform omnidirectional planetary ball milling in a protective gas for 14 h, wherein the protective gas is a mixture of nitrogen and argon, and the proportion of nitrogen is 80%; after the ball milling powder is taken out, it is dried in an electric vacuum oven at 95℃ for 24 h, and then the powder is taken out and sieved on an ultrasonic vibration sieve instrument, the vibration sieving frequency is 18 KHz, and the powder below 2 μm is collected;

[0049] (2) Place the sieved powder in a graphite mold lined with graphite paper, then place the mold in a hot-pressing sintering furnace, and vacuumize to 5×10 -2 Pa, and then perform gradient hot-pressing sintering, and the specific sintering conditions are as follows:

[0050] a. uniformly increase the temperature to 900℃ at a rate of 10℃ / min, while applying an axial pressure of 90Mpa, and pressure-sinter for 60 min;

[0051] b. uniformly increase the temperature to 1700℃ at a rate of 15℃ / min, while applying an axial pressure of 125Mpa;

[0052] c. uniformly increase the temperature to 1900℃ at a rate of 10℃ / min under pressure holding conditions and keep it for 90 min;

[0053] d. uniformly decrease the temperature to 1000℃ at a rate of 15℃ / min and uniformly release the pressure to 60Mpa at a rate of 0.7Mpa / min;

[0054] e. after releasing the pressure to 0Mpa, naturally cool down in the furnace, and obtain.

[0055] Example 3

[0056] A polycrystalline lanthanum hexaboride, and a preparation method thereof, the preparation method comprising the following steps:

[0057] (1) Take 99.5% by mass of lanthanum hexaboride powder with a particle size of 4 μm and 0.5% by mass of high-purity spherical nickel powder with a particle size of 3 μm, place the mixed powder of the lanthanum hexaboride powder and the spherical nickel powder in a polyurethane ball mill jar, use zirconia grinding balls with diameters of 10 mm, 8 mm and 5 mm as grinding media, and perform omnidirectional planetary ball milling in a protective gas for 14 h, wherein the protective gas is a mixture of nitrogen and argon, and the proportion of nitrogen is 80%; after the ball milling powder is taken out, it is dried in an electric vacuum oven at 95℃ for 24 h, and then the powder is taken out and sieved on an ultrasonic vibration sieve instrument, the vibration sieving frequency is 18 KHz, and the powder below 2 μm is collected;

[0058] (2) Place the sieved powder in a graphite mold lined with graphite paper, then place the mold in a hot-pressing sintering furnace, and vacuumize to 1×10 -2 Pa, and then perform gradient hot-pressing sintering, and the specific sintering conditions are as follows:

[0059] a. uniformly increase the temperature to 800℃ at a rate of 8℃ / min, while applying an axial pressure of 80Mpa, and pressure-sinter for 70 min;

[0060] b. uniformly increase the temperature to 1500℃ at a rate of 15℃ / min, while applying an axial pressure of 100Mpa;

[0061] c. uniformly increase the temperature to 1900℃ at a rate of 9℃ / min under pressure, and keep the temperature for 150 min;

[0062] d. uniformly decrease the temperature to 900℃ at a rate of 12℃ / min, and uniformly release the pressure to 30Mpa at a rate of 0.6Mpa / min;

[0063] e. after releasing the pressure to 0Mpa, naturally cool down in the furnace, and obtain.

[0064] Example 4

[0065] A polycrystalline lanthanum hexaboride, and a preparation method thereof, the preparation method comprising the following steps:

[0066] (1) Take 99.2% by mass of lanthanum hexaboride powder with a particle size of 5 μm and 0.8% by mass of high-purity spherical nickel powder with a particle size of 4 μm, place the mixed powder of the lanthanum hexaboride powder and the spherical nickel powder in a polyurethane ball mill jar, use zirconia grinding balls with diameters of 10 mm, 8 mm and 5 mm as grinding media, and perform omnidirectional planetary ball milling in a protective gas for 14 h, wherein the protective gas is a mixture of nitrogen and argon, and the proportion of nitrogen is 80%; after the ball milling powder is taken out, it is dried in an electric vacuum oven at 95℃ for 24 h, and then the powder is taken out and sieved on an ultrasonic vibration sieve instrument, the vibration sieving frequency is 18 KHz, and the powder below 2 μm is collected;

[0067] (2) Place the sieved powder in a graphite mold lined with graphite paper, then place the mold in a hot-pressing sintering furnace, and vacuumize to 1×10 -2 Pa, and then perform gradient hot-pressing sintering, and the specific sintering conditions are as follows:

[0068] a. uniformly increase the temperature to 800℃ at a rate of 10℃ / min, while applying an axial pressure of 80Mpa, and pressure-sinter for 80 min;

[0069] b. uniformly increase the temperature to 1600℃ at a rate of 15℃ / min, while applying an axial pressure of 120Mpa;

[0070] c. uniformly increase the temperature to 1990℃ at a rate of 8℃ / min under pressure holding conditions and keep it for 200 min;

[0071] d. uniformly decrease the temperature to 900℃ at a rate of 10℃ / min and uniformly release the pressure to 50Mpa at a rate of 0.5Mpa / min;

[0072] e. after releasing the pressure to 0Mpa, naturally cool down in the furnace, and obtain.

[0073] Example 5

[0074] A polycrystalline lanthanum hexaboride, and a preparation method thereof, the preparation method comprising the following steps:

[0075] (1) Weighing 98.5% by mass of lanthanum hexaboride powder with a particle size of 4 μm and 1.5% by mass of high-purity spherical nickel powder with a particle size of 2 μm, placing the mixed powder of lanthanum hexaboride powder and spherical nickel powder in a polyurethane ball mill, using zirconia grinding balls with diameters of 10 mm, 8 mm and 5 mm as grinding media, and performing all-round planetary ball milling for 14 h in a protective gas to obtain ball-milled powder. The protective gas is a mixture of nitrogen and argon, wherein nitrogen accounts for 80% and the rest is argon; taking out the ball-milled powder and placing it in an electric vacuum oven at 95°C for 24 h, then taking out the powder and placing it on an ultrasonic vibration sieving instrument for sieving, the vibration sieving frequency being 18 kHz, and collecting powder below 2 μm;

[0076] (2) The powder obtained after screening was placed in a graphite mold padded with graphite paper, and then the mold was placed in a hot pressing sintering furnace and vacuumed to 1×10 -2 Pa, and then gradient hot pressing sintering is carried out. The specific sintering conditions are:

[0077] a. Raise the temperature to 700℃ at a rate of 8℃ / min, apply an axial pressure of 70MPa, and sinter for 80min;

[0078] b. Raise the temperature uniformly at a rate of 13°C / min to 1600°C while applying an axial pressure of 110 MPa;

[0079] c. Under pressure holding conditions, heat up to 1850°C at a rate of 9°C / min and hold for 210 minutes;

[0080] d. Cool down to 800℃ at a rate of 14℃ / min and depressurize to 50Mpa at a rate of 0.5Mpa / min;

[0081] e. After the pressure is released to 0Mpa, the mixture is naturally cooled in the furnace to obtain the product.

[0082] Comparative Example 1

[0083] A polycrystalline lanthanum hexaboride, the preparation method of which comprises the following steps:

[0084] (1) Take 98% by mass of lanthanum hexaboride powder with a particle size of 5 μm and 2% by mass of high-purity spherical nickel powder with a particle size of 4 μm, place the mixed powder of the lanthanum hexaboride powder and the spherical nickel powder in a polyurethane ball mill jar, use zirconia grinding balls with diameters of 10 mm, 8 mm and 5 mm as grinding media, and perform omnidirectional planetary ball milling in a protective gas for 14 h, the protective gas being a mixture of nitrogen and argon, wherein the proportion of nitrogen is 80%, and the rest is argon; after the ball milling powder is taken out, it is dried in an electric vacuum oven at 95℃ for 24 h, and then the powder is taken out and sieved on an ultrasonic vibration sieve instrument, the vibration sieving frequency being 18 KHz, and the powder below 2 μm is collected;

[0085] (2) Place the sieved powder in a graphite mold lined with graphite paper, and then place the mold in a hot-pressing sintering furnace, vacuumize to 1×10 -2 Pa, and then perform gradient hot-pressing sintering, the specific sintering conditions being:

[0086] a. uniformly increase the temperature to 800℃ at a rate of 10℃ / min, while applying an axial pressure of 80Mpa, and keep the pressure for 80 min;

[0087] b. uniformly increase the temperature to 1600℃ at a rate of 15℃ / min, while applying an axial pressure of 120Mpa;

[0088] c. under the pressure keeping condition, uniformly increase the temperature to 1990℃ at a rate of 8℃ / min and keep the temperature for 200 min;

[0089] d. uniformly decrease the temperature to 900℃ at a rate of 10℃ / min and uniformly release the pressure to 50Mpa at a rate of 0.5Mpa / min;

[0090] e. after releasing the pressure to 0Mpa, naturally cool down in the furnace, and obtain.

[0091] Comparative Example 2

[0092] A polycrystalline lanthanum hexaboride, the preparation method thereof comprising the following steps:

[0093] (1) Take 99.2% by mass, 5 μm particle size lanthanum hexaboride powder and 0.8% by mass, 4 μm particle size high purity spherical nickel powder, place the mixed powder of lanthanum hexaboride and spherical nickel powder in a polyurethane ball mill jar, use zirconium oxide grinding balls with diameters of 10 mm, 8 mm and 5 mm as grinding media, and mill in all directions in a planetary ball mill for 14 h in a protective atmosphere, wherein the protective atmosphere is a mixture of nitrogen and argon, and the nitrogen accounts for 80%; after the ball milling powder is taken out, it is dried in an electric vacuum oven at 95°C for 24 h, and then the powder is taken out and sieved on an ultrasonic vibration sieve instrument, the vibration sieving frequency is 18 KHz, and the powder below 2 μm is collected;

[0094] (2) Place the sieved powder in a graphite mold lined with graphite paper, then place the mold in a hot-pressing sintering furnace, and vacuumize to 1×10 -2 Pa, and then gradient hot-pressing sintering is performed, and the specific sintering conditions are as follows:

[0095] a. uniformly increase the temperature to 800°C at a rate of 15°C / min, while applying an axial pressure of 80 MPa, and pressure-sintering for 80 min;

[0096] b. uniformly increase the temperature to 1600°C at a rate of 15°C / min, while applying an axial pressure of 120 MPa;

[0097] c. uniformly increase the temperature to 1800°C at a rate of 15°C / min under pressure, and keep the temperature for 200 min;

[0098] d. uniformly decrease the temperature to 900°C at a rate of 15°C / min, and uniformly release the pressure to 50 MPa at a rate of 1 MPa / min;

[0099] e. after releasing the pressure to 0 MPa, naturally cool down in the furnace, and obtain.

[0100] Test Example

[0101] The relative densities of the polycrystalline lanthanum hexaboride prepared in Examples 1-5 and Comparative Examples 1-2 are measured, and the results are shown in Table 1. The pulsed electron emission performance of the polycrystalline lanthanum hexaboride in Examples 1-5 and Comparative Examples 1-2 is tested, and the results of the thermal electron emission performance are shown in Table 2.

[0102] Table 1: Results

[0103] Relative density (%) Example 1 96.42 Example 2 97.64 Example 3 97.72 Example 4 98.98 Example 5 99.42 Comparative Example 1 99.51 Comparative Example 2 98.83

[0104] Table 2: Electron emission performance

[0105]

[0106]

[0107] From the data in the above table, it can be seen that the incorporation of high-purity spherical nickel powder can effectively improve the thermionic emission performance of the polycrystalline lanthanum hexaboride material, and improve the density of the material, increase the relative density, and the thermionic emission density increases first and then decreases with the incorporation of high-purity spherical nickel powder. Excessive incorporation will also cause intergranular impurity defects, resulting in a significant decrease in performance. When the incorporation amount of high-purity spherical nickel powder is constant, the final sintering temperature and the heating rate are the key factors affecting the final thermionic emission performance.

[0108] Taking the polycrystalline materials prepared in Example 4 and Example 5 as examples, scanning electron microscope detection was performed, and the specific results are shown in Figures 1-3 .

[0109] Figure 1 The SEM image of the polycrystalline material in Example 4 can be seen that when the doping amount of high-purity spherical nickel powder is 1%, the lanthanum hexaboride grains in the material are uniformly distributed, and no impurities are generated;

[0110] Figure 2 The SEM image of the polycrystalline material in Example 5 can be seen that when the doping amount of high-purity spherical nickel powder is increased to 2%, impurity phases are generated in the material, causing intergranular pore defects.

[0111] Figure 3 The physical image of the polycrystalline material in Example 4.

Claims

1. A method for producing polycrystalline lanthanum hexaboride, characterized by, Comprising the following steps: (1) Ball milling mixed powders of lanthanum hexaboride powder and spherical nickel powder in a protective gas to obtain a ball-milled powder, the mass percentage of lanthanum hexaboride powder and spherical nickel powder being 98.5-99.9%:0.1-1.5%, the particle size of the lanthanum hexaboride powder being 4-5 μm, and the particle size of the spherical nickel powder being 2-4 μm; the protective gas being a mixture of nitrogen and argon, wherein the nitrogen accounts for 80%, and the rest is argon; (2) Screening the ball-milled powder through an ultrasonic vibration sieve to collect mixed powder with a particle size of 1-2 μm; (3) The sieved mixed powder is placed in a mold, and then gradient hot-press sintering is performed under vacuum, the vacuum degree during gradient press sintering is 1-9x10 -2 Pa, after sintering, cooling, and thus a product is obtained; The gradient pressure sintering process is as follows: a. uniformly increasing the temperature to 600-900 ℃ at a rate of 5-10 ℃ / min while applying an axial pressure of 60-90 MPa, and pressure sintering for 60-90 min; b. uniformly increasing the temperature to 1400-1700 ℃ at a rate of 10-15 ℃ / min while applying an axial pressure of 91-125 MPa; c. while pressure holding, uniformly increasing the temperature to 1700-1990 ℃ at a rate of 5-10 ℃ / min, and holding for 90-240 min; d. uniformly decreasing the temperature to 800-1000 ℃ at a rate of 10-15 ℃ / min and uniformly releasing the pressure to 20-60 MPa at a rate of 0.3-0.7 MPa / min; e. after releasing the pressure to 0 MPa, naturally cooling in the furnace to obtain the product.

2. The method of claim 1, wherein the polycrystalline lanthanum hexaboride is prepared by the steps of: In step (1), the hard zirconia grinding balls with diameters of 10 mm, 8 mm and 5 mm are used during ball milling, and the ball milling time is 10-14 h, and the powder after ball milling is subjected to drying treatment. ​ 3. Polycrystalline lanthanum hexaboride, characterized in that The method is prepared by using any one of claims 1-2.

4. Use of the polycrystalline lanthanum hexaboride in claim 3 in the preparation of cathode emission materials.

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