Polycrystalline lanthanum hexaboride ceramic material as well as preparation method and application thereof

Abstract

The invention discloses a polycrystalline lanthanum hexaboride ceramic material and a preparation method and application thereof.The preparation method comprises the steps that lanthanum hexaboride and metal oxide are mixed, ball-milled to be uniform and dried to obtain first powder, the first powder is put into a mold, the first powder is located in the center of the mold and has the same apparent density, and the first powder is put into the mold; in an inert gas atmosphere, increasing the pressure from 6-8 MPa of P1 to 20-60 MPa of P2, keeping the temperature of 1400-2000 DEG C at the pressure P2 for 0.5-3 h to perform hot pressed sintering, cooling, and keeping the pressure at the pressure P2 for 0-3 h at the same time to obtain the polycrystalline lanthanum hexaboride ceramic material, with the P1 being 6-8 MPa and the P2 being 20-60 MPa. Lanthanum hexaboride is doped and modified through the metal oxide, and the work function of the polycrystalline lanthanum hexaboride ceramic material is reduced by utilizing the influence of different metal elements of the metal oxide on the surface potential energy of the lanthanum hexaboride surface.

Description

TECHNICAL FIELD

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

[0002] As a core device of vacuum electron equipment, the vacuum emission cathode has important applications in national defense, medical detection, electron beam welding, field emission display, etc. Since the equipment usually works in high temperature, airflow and other environments, the material is required to have good electron emission characteristics and good mechanical properties, and can serve for a long time at high temperature [1] . The traditional cathode emission material is mainly barium tungsten cathode material. Although the barium tungsten cathode material has low work function, strong anti-bombing property, and is currently mature in technology, it has been widely used as a cathode emission material by various countries. However, the barium tungsten cathode material has many defects. The emission mechanism of the barium tungsten cathode material determines that the electron emission density is low, and the surface will accumulate sediment during emission, which will cause pore blockage after long-term use, resulting in a decrease in emission efficiency [2] . Therefore, the lanthanum hexaboride cathode material with better emission performance and longer service life is gradually replacing the barium tungsten cathode material and becoming the main cathode emission material.

[0003] Studies have shown that the current emission density of lanthanum hexaboride material is much higher than that of other cathode materials [3] . By comparing lanthanum hexaboride material with tungsten, it can be found that the work function of lanthanum hexaboride material is very low, and the evaporation rate is very low. Compared with pure tungsten material, the evaporation rate of lanthanum hexaboride material is about one percent of that of pure tungsten material. Compared with barium tungsten cathode material, the evaporation rate of lanthanum hexaboride material is about one fifth of that of barium tungsten cathode material. The extremely low evaporation rate makes the service life of lanthanum hexaboride material much longer than that of barium tungsten cathode material. In summary, lanthanum hexaboride has become a cathode emission material with great application prospect, and has been widely used in military, aerospace and medical equipment industries.

[0004] At present, due to technical limitations, lanthanum hexaboride material cannot be produced and used on a large scale. Therefore, many researches are still being conducted to make lanthanum hexaboride practical. Studies on the cathode emission performance of single crystal lanthanum hexaboride and polycrystalline lanthanum hexaboride show that the single crystal lanthanum hexaboride has a higher emission current density than the polycrystalline lanthanum hexaboride [4]In normal circumstances, the work function of single crystal lanthanum hexaboride is 2.4eV, while the work function of polycrystalline lanthanum hexaboride is difficult to reach this value, generally above 3eV. Therefore, the emission performance of single crystal lanthanum hexaboride is better than that of polycrystalline lanthanum hexaboride, but the preparation process of single crystal lanthanum hexaboride is complex, such as chemical vapor deposition method, molten salt electrolysis method and the like, which has high requirements on the preparation conditions and high cost, and it is difficult to prepare large-size products, so it is difficult to be widely used. Therefore, preparing large-size polycrystalline lanthanum hexaboride (LaB6) by a simple preparation method and improving the emission performance of polycrystalline LaB6 are the keys to further application of LaB6 [5] .

[0005] Doping modification is an effective method to improve the emission performance of lanthanum hexaboride ceramics. In the 1960s, foreign researchers found that the current emission capacity of the mixed polycrystalline prepared by mixing BaB6 and SrB6 with lanthanum hexaboride is stronger than that of pure lanthanum hexaboride. Schmidt et al. [6] Pr and Nd are doped into the lanthanum hexaboride crystal by using zone melting method, and through testing, the work function of the lanthanum hexaboride crystal doped with the two elements is less than that of pure lanthanum hexaboride. There are two methods for solid solution doping of lanthanum hexaboride at present, the first method is to mix other hexaboride powders with lanthanum hexaboride powders and sinter under certain conditions to prepare multi-element hexaboride crystals. Ning et al. [7] The lanthanum hexaboride and cerium hexaboride are mixed and sintered by using a spark plasma sintering method to obtain a hexaboride block, and then a single crystal of the mixed hexaboride is obtained by zone melting. Another method is to perform solid solution doping during the preparation of the powder to prepare a mixed hexaboride powder. Yu Yiping uses a molten salt method [8] La and Ce mixed hexaboride powders are prepared by uniformly mixing lanthanum chloride and cerium chloride during the preparation process, so as to realize the solid solution doping of LaB6. However, the current doping modification is mainly reflected in the aspect of non-oxide[9], and the work function of LaB6 ceramic is not significantly improved. SUMMARY

[0006] In view of the deficiencies of the prior art, the purpose of the present application is to provide a preparation method of polycrystalline lanthanum hexaboride ceramic material.

[0007] The purpose of the present application is achieved by the following technical scheme.

[0008] A preparation method of polycrystalline lanthanum hexaboride ceramic material comprises the following steps:

[0009] Step 1, mixing lanthanum hexaboride (LaB6) and metal oxide, ball milling to be uniform, drying to obtain a first powder, wherein the ratio of lanthanum hexaboride (LaB6) and metal oxide is (92-98):(2-8) by mass fraction, and the metal oxide is an oxide of R, and R is one or more of Zn, Fe, Al, Ti and Si;

[0010] In step 1, the ball milling ratio is 1:(2-6).

[0011] In step 1, the ball milling speed is 250-350 r / min, and the ball milling time is 2-8 h.

[0012] In step 1, the oxide of R is a mixture of one or more of ZnO, Fe2O3, Al2O3, TiO2 and SiO2.

[0013] Step 2, the first powder is loaded into a mold, the first powder is located at the center of the mold and has the same loose bulk density, and the pressure P1 is increased to the pressure P2 under an inert gas atmosphere, and the pressure P2 is kept at 1400-2000℃ for 0.5-3 h to perform hot-pressing sintering, and then cooled while keeping the pressure P2 for 0-3 h to obtain a polycrystalline lanthanum hexaboride ceramic material, wherein P1 is 6-8 MPa, and P2 is 20-60 MPa.

[0014] In step 2, the rate of increasing the pressure P1 to the pressure P2 is 6-20 MPa / min.

[0015] In step 2, the 1400-2000℃ for 0.5-3 h is preferably 1600-1700℃ for 1-2 h.

[0016] In step 2, the heating rate is 6-30℃ / min, and when the temperature is above 600℃, the heating rate is preferably 15-20℃ / min.

[0017] In step 2, the first powder is located at the center of the mold and has the same loose bulk density by vibration.

[0018] In step 2, P2 is preferably 30-50 MPa.

[0019] In the above technical solution, the work function of the polycrystalline lanthanum hexaboride ceramic material is 0.9-3.2 eV, and preferably 1-2 eV.

[0020] In the above technical solution, the electrical conductivity of the polycrystalline lanthanum hexaboride ceramic material is 3-11 MS / m.

[0021] In the above technical solution, the secondary electron emission coefficient of the polycrystalline lanthanum hexaboride ceramic material is 1.5-1.9.

[0022] In the above technical solution, the relative density of the polycrystalline lanthanum hexaboride ceramic material is 90-98%.

[0023] Application of the polycrystalline lanthanum hexaboride ceramic material in an electron emission cathode device.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] The present application dopes and modifies lanthanum hexaboride (LaB6) by metal oxides, utilizes the influence of different metal elements of the metal oxides on the surface potential energy on the surface of LaB6, reduces the work function of the polycrystalline lanthanum hexaboride ceramic material (the optimal work function of the polycrystalline lanthanum hexaboride ceramic material of the present application can reach 1.32 eV (not shown in the specific embodiment), which is less than that of non-oxide-doped lanthanum hexaboride ceramic and single-crystal lanthanum hexaboride), improves the secondary electron emission coefficient of the polycrystalline lanthanum hexaboride ceramic material, realizes the optimization of the emission performance of LaB6 by metal oxides, fills the blank of improving the work function of borides by metal oxides, and obtains the polycrystalline lanthanum hexaboride ceramic material with a density higher than 98%. The polycrystalline lanthanum hexaboride ceramic material of the present application can realize cold cathode field emission in an electron emission cathode device. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 A photo of the polycrystalline lanthanum hexaboride ceramic material prepared in Example 1;

[0027] Figure 2 A microstructure picture of the polycrystalline lanthanum hexaboride ceramic material prepared in Example 1;

[0028] Figure 3 XRD of the polycrystalline lanthanum hexaboride ceramic material prepared in Example 1;

[0029] Figure 4 EDS of the polycrystalline lanthanum hexaboride ceramic material prepared in Example 1. DETAILED DESCRIPTION

[0030] The technical solution of the present application will be further described below in combination with specific embodiments.

[0031] The raw material information involved in the following examples is as follows:

[0032] Raw materials Molecular formula Purity Origin Lanthanum hexaboride LaB6 ≥99.9％ China Aluminium trioxide Al2O3 ≥99.9％ China Silicon dioxide SiO2 ≥99.9％ China Titanium dioxide TiO2 ≥99.9％ China

[0033] The instrument information involved in the following examples is as follows:

[0034]

[0035] Example 1

[0036] A method for preparing a polycrystalline lanthanum hexaboride ceramic material, comprising the following steps:

[0037] Step 1, mix lanthanum hexaboride (LaB6) and metal oxide, ball mill in a high-energy ball mill at a speed of 350 r / min for 3 h to be uniform (with ethanol as the grinding medium), place in a drying box, and dry at 120℃ for 12 h to obtain a first powder, wherein the ratio of lanthanum hexaboride (LaB6) to metal oxide is 98:2 by mass fraction, and the metal oxide is Al2O3, TiO2, and SiO2, and the ratio of Al2O3, TiO2, and SiO2 is 35:35:30 by mass fraction;

[0038] The lanthanum hexaboride (LaB6) is in powder form, the average particle size of the lanthanum hexaboride (LaB6) is 5.6 μm, the metal oxide is in powder form, the average particle size of the metal oxide is 3 μm, and the metal oxide needs to be ball milled in a high-energy ball mill at a speed of 250 r / min for 4 h to be uniform (with deionized water as the grinding medium) before being mixed with the lanthanum hexaboride (LaB6), and the ball milling in step 1 uses agate balls as the grinding balls, and the ball milling has a material-to-ball ratio of 1:3.

[0039] Step 2, sequentially load the first powder into a graphite mold (the graphite mold has an inner diameter of 30 mm and an outer diameter of 70 mm), vibrate the graphite mold to make the first powder occupy the center position of the graphite mold and have the same loose bulk density, vacuumize to below 10 Pa, introduce argon gas into the graphite mold, increase the pressure from P1 (P1 = 7 MPa) to P2 (P2 = 40 MPa) at a rate of 10 MPa / min, and simultaneously "first increase the temperature to 600℃ at a rate of 6℃ / min, and then increase the temperature to 1700℃ at a rate of 20℃ / min" during the pressure increase from P1 to P2, perform hot-press sintering at 40 MPa (P2) and 1700℃ for 1 h in a vacuum hot-press sintering furnace, cool after the hot-press sintering is completed, and simultaneously maintain the pressure at 40 MPa for 140 min to obtain a polycrystalline lanthanum hexaboride ceramic material.

[0040] The relative density of the polycrystalline lanthanum hexaboride ceramic material prepared in Example 1 is 95%, the electrical conductivity is 7 MS / m, the work function is 1.81 eV, and the secondary electron emission coefficient is 1.6.

[0041] Example 2 (for comparison)

[0042] A method for preparing a LaB6 ceramic, comprising the following steps:

[0043] Step 1: LaB6 raw material powder with a particle size of 20 μm (purchased from Shanghai Xuetai New Material Technology Co., Ltd.) was ball milled at a speed of 350 r / min for 6 h using a planetary ball mill with deionized water as the milling medium and a material-to-ball ratio of 1:4 to obtain LaB6 powder with a D50 of 4.45 μm;

[0044] Step 2: The LaB6 powder obtained in Step 1 was loaded into a graphite mold (with an inner diameter of 30 mm and an outer diameter of 70 mm) of a bulk material machine, the graphite mold was vibrated to allow the LaB6 powder to be located at the center of the graphite mold and have the same loose density, the graphite mold was vacuumed to below 10 Pa, argon was introduced into the graphite mold, the pressure was increased from P1 (P1 = 7 MPa) to P2 (P2 = 40 MPa) at a rate of 12 MPa / min, and the temperature was increased to 600°C at a rate of 6°C / min and then to 1975°C at a rate of 20°C / min while the pressure was being increased from P1 to P2, the hot-pressing sintering was performed at 40 MPa and 1975°C for 1 h in a vacuum hot-pressing sintering furnace, and the LaB6 ceramic was obtained after cooling and pressure holding at 40 MPa for 140 min.

[0045] The relative density of the LaB6 ceramic prepared in Example 2 was 96.26%, the electrical conductivity was 10.18 MS / m, and the work function was 3.7 eV.

[0046] Example 3 (for comparison)

[0047] A preparation method of a non-oxide doped lanthanum hexaboride ceramic, comprising the following steps:

[0048] Step 1: LaB6 and B4C powders in Example 1 were mixed and ball milled using deionized water as the medium and agate balls as the milling balls to obtain LaB6-B4C mixed powder, the material-to-ball ratio of the ball milling was 1:3, the rotation speed of the ball milling was 250 r / min, the ball milling time was 6 h, and the ratio of the LaB6 powder to the B4C powder was 96:4 by mass fraction;

[0049] Step 2: Load the LaB6-B4C mixed powder obtained in Step 1 into the graphite mold of the bulk material machine (the inner diameter of the graphite mold is 30 mm, and the outer diameter is 70 mm). Vibrate the graphite mold to ensure that the LaB6-B4C mixed powder is centered in the graphite mold and has the same loose density. Evacuate to below 10 Pa, and introduce argon gas into the graphite mold, increasing the pressure from P1 (P1 = 7 MPa) to P2 (P2 = 40 MPa). a) The pressure is increased from P1 to P2 at a rate of 7 MPa / min. While increasing the pressure from P1 to P2, the temperature is increased to 600℃ at 6℃ / min and then to 2000℃ at 20℃ / min. The temperature is then maintained at 40 MPa and 2000℃ for 1 hour in a vacuum hot pressing sintering furnace for hot pressing sintering. After hot pressing sintering, the temperature is cooled and simultaneously maintained at 40 MPa for 140 minutes to obtain non-oxide-doped lanthanum hexaboride ceramic.

[0050] The non-oxide-doped lanthanum hexaboride ceramic prepared in Example 3 has a relative density of 95%, but its resistivity is too high, making it impossible to measure the work function and secondary electron emission coefficient.

[0051] Example 4

[0052] A polycrystalline lanthanum hexaboride ceramic material is basically the same as that in Example 1, except that "the metal oxide is Al2O3, TiO2 and SiO2" is replaced with "the metal oxide is Al2O3".

[0053] The polycrystalline lanthanum hexaboride ceramic material prepared in Example 4 has a relative density of 96%, an electrical conductivity of 6.1 MS / m, and a work function of 2.1 eV.

[0054] Example 5

[0055] A polycrystalline lanthanum hexaboride ceramic material is basically the same as that in Example 1, except that "the metal oxides are Al2O3, TiO2 and SiO2" is replaced with "the metal oxides are ZnO, SiO2 and TiO2".

[0056] The polycrystalline lanthanum hexaboride ceramic material prepared in Example 5 had a relative density of 97%, an electrical conductivity of 7.6 MS / m, and a measured work function of 1.41 eV.

[0057] Depend on Figure 1 It can be seen that the polycrystalline lanthanum hexaboride ceramic material (LaB6 ceramic) prepared by this invention is a pure purple color, has a smooth surface, and can be processed into any shape.

[0058] Figure 2 The microstructure of the cross-section of the polycrystalline lanthanum hexaboride ceramic material prepared in Example 1 is shown by... Figure 2 It can be seen that its interior is very dense.

[0059] Figure 3 XRD of the polycrystalline lanthanum hexaboride ceramic material prepared in Example 1 was performed to determine the phase composition of the material. Figure 3 "LaB6+2% ATS" in the present disclosure refers to the polycrystalline lanthanum hexaboride ceramic material prepared in Example 1), which was performed by Figure 3 It can be seen that the phase composition of LaB6 is not changed after doping metal oxides. Figure 4 Elemental distribution EDS of the polycrystalline lanthanum hexaboride ceramic material prepared in Example 1 was performed to determine the elemental distribution of the material. Figure 4 The microstructure of the cross-section of the polycrystalline lanthanum hexaboride ceramic material is shown in the present disclosure), which was performed by Figure 4 It can be seen that the doped Al and Si elements are distributed at the grain boundary position.

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[0069] The above has made an exemplary description of the present application, it should be explained that, without departing from the core of the present application, any simple modification, change or other equivalent replacement which can not cost the creative labor of those skilled in the art falls into the protection scope of the present application.

Claims

1. A method for producing a polycrystalline lanthanum hexaboride ceramic material, characterized by, The method comprises the following steps: Step 1: mixing lanthanum hexaboride and metal oxide, ball-milling to be uniform, drying to obtain a first powder, wherein the ratio of lanthanum hexaboride to metal oxide is (92-98):(2-8) by mass fraction, and the metal oxide is oxide of R, and R is one or more of Zn, Fe, Al, Ti and Si; Step 2: loading the first powder into a mold, the first powder is in the center of the mold and has the same loose bulk density, under an inert gas atmosphere, increasing the pressure from P1 to P2, and performing hot-pressing sintering at P2 and 1400-2000℃ for 0.5-3h, cooling and simultaneously keeping the pressure at P2 for 0-3h to obtain a polycrystalline lanthanum hexaboride ceramic material, wherein P1 is 6-8MPa and P2 is 20-60MPa.

2. The production method according to claim 1, characterized by, The oxide of R is a mixture of one or more of ZnO, Fe2O3, Al2O3, TiO2 and SiO2.

3. The preparation method according to claim 1, characterized in that, The work function of the polycrystalline lanthanum hexaboride ceramic material is 0.9-3.2eV.

4. The production method according to claim 1, characterized by, The electrical conductivity of the polycrystalline lanthanum hexaboride ceramic material is 3-11MS / m.

5. The preparation method according to claim 1, characterized in that, The secondary electron emission coefficient of the polycrystalline lanthanum hexaboride ceramic material is 1.5-1.

9.

6. The method of claim 1, wherein, The relative density of the polycrystalline lanthanum hexaboride ceramic material is 90-98%.

7. The preparation method according to claim 2, characterized in that, In step 2, the rate of increasing the pressure from P1 to P2 is 6-20MPa / min.

8. The preparation method according to claim 2, characterized in that, In step 2, the first powder is in the center of the mold and has the same loose bulk density by vibration.

9. The polycrystalline lanthanum hexaboride ceramic material obtained by the method of any one of claims 1-8.

10. The polycrystalline lanthanum hexaboride ceramic material of claim 9 in an electron emission cathode device.

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