CN108129153B - Multicomponent rare earth boride (La)xSr1-x)B6Polycrystalline cathode material and preparation method thereof - Google Patents

Multicomponent rare earth boride (La)xSr1-x)B6Polycrystalline cathode material and preparation method thereof Download PDF

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CN108129153B
CN108129153B CN201711463453.8A CN201711463453A CN108129153B CN 108129153 B CN108129153 B CN 108129153B CN 201711463453 A CN201711463453 A CN 201711463453A CN 108129153 B CN108129153 B CN 108129153B
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周身林
周奕
樊后坤
刘世粮
叶子飘
胡强林
罗小兵
余晓光
温玉锋
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Jinggangshan University
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Abstract

Multicomponent rare earth boride (La)xSr1‑x)B6A preparation method of a polycrystalline cathode material belongs to the technical field of rare earth and alkaline earth hexaboride cathode materials. The composition of the multicomponent rare earth hexaboride provided by the invention is (La)xSr1‑x)B6Wherein x is more than or equal to 0.1 and less than or equal to 0.9. The method provided by the invention uses La2O3SrO and B powder are taken as raw materials, ball milling and vacuum hot pressing reaction sintering are adopted, the maximum sintering temperature is 1500-xSr1‑x)B6Solid solution. The method integrates the powder synthesis process and the sintering densification process, simplifies the preparation process, is beneficial to reducing the sintering temperature, improving the purity and the density, reducing the production cost, and is suitable for industrial production and application. (La) obtained by the present inventionxSr1‑x)B6The solid solution polycrystal has the characteristics of single phase, high compactness and high emission performance, and can be widely applied to the field of a plurality of cathodes.

Description

Multicomponent rare earth boride (La)xSr1-x)B6Polycrystalline cathode material and preparation method thereof
Technical Field
The invention belongs to the technical field of rare earth and alkaline earth hexaboride cathode materials, and particularly relates to (La)xSr1-x)B6Solid solution polycrystal and a method for producing the same.
Background
The cathode is a heart device of various modern vacuum electronic equipment and is widely applied to the fields of national defense industry and civil use. The hot cathode is used as an electron emission source of a vacuum electronic device, plays a crucial role in the performance of a high-power microwave device, is widely applied to various high-power microwave devices, and is more mature particularly in military equipment such as satellites and radars.
Lanthanum hexaboride (LaB)6) The material is a hot cathode material with excellent performance, has the advantages of high current emission density, long service life, poisoning resistance, high chemical stability, ion bombardment resistance and the like, and has important research significance and application value. LaB6The radar has wide application range in military affairs, is mainly and generally applied to radars, has too close range of the existing radars, is difficult to meet the requirements of modern war, and urgently needs to replace a high-power electronic tube. It is also used in plasma engine and propeller, and is designed for generating artificial plasma in near-earth space and controlling static electricity of spacecraft.
With the rapid development of technology and economy, the electron beam technology and equipment at present develop to micro electron beams on one hand, and develop to high power on the other hand, and put forward higher requirements for the research of cathode materials. The cathode needs to further increase the emission current density, reduce the operating temperature, prolong the operating life, and the like. Therefore, the research on improving the cathode performance focuses on how to lower the work function of the material.
Recently, scholars at home and abroad have conducted research on composite multi-element rare earth hexaboride cathode materials, and the main research idea is to perform research on LaB6La in the alloy is doped and substituted by other rare earth elements to reduce the work function. These studies have made some progress, but compared with low-temperature cathodes such as barium tungsten, various existing LaBs6The work function of the basic multi-element rare earth hexaboride cathode is still larger, and the working temperature is higher. Therefore, in order to expand the application of the organic electroluminescent material in wider fields (including medium and low temperature fields), the work function needs to be further reduced, and the emission performance needs to be improved.
It is found that whenMixing LaB6With alkaline earth metal borides such as SrB6Or BaB6When mixed together, the thermal electron emission performance ratio of the materials is LaB6And better, the research interest of the multi-element rare earth and alkaline earth metal hexaboride is aroused. Recently, the applicant has calculated Sr doped LaB by adopting the principle of first property6The work function of (La) of certain specific components was foundxSr1-x)B6Such as (La)0.6Sr0.4)B6And LaB6Compared with the lower work function and higher hot electron emission performance. This indicates that proper amount of Sr doping is expected to reduce LaB6The working temperature of the cathode is greatly improved, and the thermionic emission performance of the cathode is greatly improved. SrB6Inherently having a lower work function in LaB6The Sr-doped hexaboride cathode can replace part of La to improve the emission performance and the resistivity greatly, and is beneficial to the application of the hexaboride cathode in the field of direct heating cathodes. Therefore, LaB is regulated and controlled by doping Sr to replace La in a disordered way6The crystal structure of (A) and the preparation of the multicomponent rare earth boride (La) with excellent performancexSr1-x)B6The solid solution polycrystalline cathode can realize the controllability of the performance of the cathode material and has important significance in improving the thermal emission performance.
At present, the traditional preparation method of the multi-element metal hexaboride comprises two processes of powder synthesis and sintering densification. The powder synthesis method mainly includes an element synthesis method, a boron carbide method, a boron thermal reduction method, and the like. The purity of the powder obtained by the element synthesis method is highest, but because the price of boron powder is expensive, the metal Sr powder is easy to oxidize, and the high-temperature vapor pressures of Sr and B are different, the method has strict process, high equipment requirement and difficult process control, and is not suitable for large-scale industrial production. The purity of boride powder produced by the boron carbide process is relatively low, but B4The price of C is much lower than that of pure boron, and the shape of the synthesized powder is similar to that of the raw material B4C has a great relationship and has the phenomena of hard agglomeration and delamination. Therefore, the boron thermal reduction method is commonly adopted to prepare rare earth hexaboride powder at present.
The densification of the powder mainly adopts a hot-pressing sintering method or a Spark Plasma Sintering (SPS) method. Boride has high chemical activity and low plasticity at high temperature, and the densification process of boride has great process difficulty. The sintering temperature of hot-pressing sintering preparation is high (2000 plus 2200 ℃), so that the product has large grains, more pores and low density (the relative density is generally lower than 90%), the mechanical property and the emission property of the material are influenced, and the product is difficult to apply in industrial production. SPS is a rapid sintering technology, but the method has high requirements on equipment and low yield, and is not suitable for industrial production of products.
In the Chinese patent application with publication number 201210331645.4, a LaB is disclosed6The powder synthesis method comprises the following steps: adopts a full wet process and closed cycle, namely, magnesium powder, lanthanum oxide powder and boron oxide powder are taken as raw materials, proper component proportion is selected, the raw materials are pressed into a blank after being fully mixed by ball milling, and the blank is reacted in a combustion synthesis reaction kettle under protective atmosphere to generate the LaB-containing material6Mechanically pulverizing and grinding the block material into powder, and hydrometallurgy in which impurities are introduced into liquid phase by hydrochloric acid leaching reaction and LaB6The powder exists in a solid phase form, and a suction filtration device is utilized to carry out solid-liquid separation to obtain LaB6Drying the powder in a drying box at low temperature to obtain the final product.
In the Chinese patent application with publication number 201310492257.9, a LaB is disclosed6A process for producing a polycrystalline body. The method comprises the following steps: charging furnace with LaB6Placing the powder mold in a sintering furnace; raising the temperature, namely gradually raising the temperature from room temperature to a preset temperature in four stages; stopping pressure and slowly cooling to obtain LaB6A polycrystalline body. For LaB6For the preparation of polycrystalline blocks, the method does not involve the synthesis of powder, but only the second step of the conventional two-step process, namely, the step of preparing the raw material LaB6And (3) powder sintering densification. The sintering temperature is higher, the density of the product is not high, and the relative density is only 92-96%.
A multi-element rare earth boride (La) is disclosed in the Chinese patent application with publication number 200810225029xRE1-x)B6The cathode material and the preparation method thereof, wherein RE is the second rare earth element (namely RE is Ce, Pr, Nd, Sm, Eu) except La in the light rare earth elementsAnd Gd). The method comprises the following steps: 1) respectively preparing LaH by taking simple substance rare earth metal lanthanum block and RE block as raw materials and adopting a direct current arc evaporation condensation method in hydrogen and argon atmosphere2Nanopowder and REH2And (4) nano powder. 2) The LaH prepared in the step 1) is used2Nanopowder, REH2Grinding and uniformly mixing the nano powder and the raw material B nano powder in a low-oxygen argon environment, loading the mixture into a graphite mold, placing the graphite mold into an SPS sintering cavity, applying axial pressure of 50MPa, sintering in an argon atmosphere or under a vacuum condition with the vacuum degree superior to 8Pa, keeping the sintering temperature at 1300-1700 ℃, preserving the temperature for 10min, and then cooling the mixture to room temperature along with a furnace. The method prepares (La)xRE1-x)B6The polycrystallization requires a total of two steps: firstly preparing rare earth nano powder by adopting an arc evaporation condensation method, and then adopting SPS sintering densification to obtain a polycrystalline block. The preparation process is complex in process, high in technical difficulty, expensive in equipment, high in energy consumption, high in cost and low in yield, and the raw materials need high-purity elemental rare earth metal, so that the preparation method is not suitable for industrial production.
A highly dense (La) is disclosed in the Chinese patent application publication No. 201510213310.6xCa1-x)B6Polycrystalline cathode materials and methods of making the same. The invention uses LaB6And CaB6The powder is taken as a raw material, and ball milling and hot pressing sintering are adopted to prepare (La)xCa1-x)B6A solid solution polycrystal. The method can prepare (La)xCa1-x)B6The solid solution, however, because the raw materials are two kinds of metal hexaboride, the solid solution in sintering needs very high sintering temperature, the maximum sintering temperature is 1700-1900 ℃, the requirement on a sintering furnace is high, and the solid solution is not convenient to apply to production.
Disclosure of Invention
To increase LaB6The invention provides a multi-element rare earth boride (La) which has the emission performance of a polycrystalline cathode material and overcomes the defects of the existing preparation method of multi-element metal hexaboride polycrystalsxSr1-x)B6Polycrystalline cathode materials and methods of making the same. The composition of the multicomponent rare earth hexaboride provided by the invention is (La)xSr1-x)B6Wherein x is more than or equal to 0.1 and less than or equal to 0.9. The method provided by the invention uses La2O3SrO and B powder are taken as raw materials, ball milling and vacuum hot pressing reaction sintering are adopted, the maximum sintering temperature is 1500-xSr1-x)B6Solid solution. The method integrates the powder synthesis process and the sintering densification process, simplifies the preparation process, is beneficial to reducing the sintering temperature, improving the purity and the density, reducing the production cost, and is suitable for industrial production and application. (La) obtained by the preparation method provided by the inventionxSr1-x)B6The solid solution polycrystal has the characteristics of single phase, high compactness and high emission performance, and can be widely applied to the field of a plurality of cathodes.
One aspect of the present invention is to provide a multicomponent rare earth hexaboride (La)xSr1-x)B6A method for preparing a solid solution polycrystal. The preparation method comprises the following specific steps:
1) proportioning and mixing materials:
according to the reaction equation
xLa2O3+2(1-x)SrO+(14+x)B----2(LaxSr1-x)B6+(x+2)BO↑
In the glove box, La was weighed separately2O3And raw material powder of SrO and B is filled into a stainless steel grinding tank, and stainless steel balls or agate balls are selected as grinding media. And taking the ball milling tank out of the glove box, putting the ball milling tank into a high-energy ball mill, carrying out ball milling for 2-4h, and then sampling in the glove box.
2) Charging, namely, ball-milling and uniformly mixing the La obtained in the step 1)2O3Putting raw material powder of SrO and B into a graphite mold in a glove box, and placing the mold in a hot-pressing sintering furnace;
in the step 1) and the step 2), the glove box is filled with argon atmosphere;
3) heating and sintering, wherein in the first heating stage: room temperature to 400 ℃ and 500 ℃; a second temperature rising stage: raising the temperature from 400-500 ℃ to 700-800 ℃ in the first stage; a third temperature rise stage: the temperature is increased from 700-800 to 1100-1200 ℃ in the second stage; a fourth temperature rise stage: the temperature is raised from 1100-1200 ℃ to 1500-1800 ℃ in the third stage. The heating rate of the first heating stage to the third heating stage is 10-15 ℃/min, and the heating rate of the fourth heating stage is 5-10 ℃/min. Wherein, the first temperature-rising stage and the second temperature-rising stage apply 10-15Mpa axial pressure to the powder, and the third temperature-rising stage applies 15-20Mpa axial pressure to the powder; in the fourth temperature rising stage, 20-30Mpa axial pressure is applied to the powder;
4) preserving heat, preserving heat for 2-4h at the temperature of 1800 ℃ after the temperature is raised in the fourth stage of the step 3); and applying 30-50Mpa axial pressure to the powder in the heat preservation process;
5) cooling, and after the heat preservation in the step 4), removing pressure, and cooling along with the furnace to obtain the (La)x Sr1-x)B6A solid solution polycrystal.
Preferably, said La is used in step 1)2O3The purity of SrO raw material powder is more than or equal to 99.9 percent, the purity of B raw material powder is more than or equal to 99.0 percent, and the particle size range is 1-100 mu m. However, the present invention is not limited to the above raw materials, and La2O3 and SrO of different purities and particle sizes and other rare earth and alkaline earth metal oxide powders can be applied to the present invention.
Preferably, in the step 1) and the step 2), the oxygen content and the water vapor content in the argon atmosphere in the glove box are less than or equal to 50 ppm.
Preferably, the heating rate of each heating stage in the step 3) is 5-10 ℃/min.
Preferably, before sintering in step 3), the hot pressing furnace is pre-vacuumized to the air pressure less than or equal to 2 x 10-2Pa。
Another aspect of the present invention is to provide a single phase, highly dense (La)xSr1-x)B6The solid solution polycrystalline cathode material is characterized in that x is more than or equal to 0.1 and less than or equal to 0.9. The cathode material comprises (La) prepared by the methodxSr1-x)B6Polycrystalline body, (La) obtained by the method of the present inventionxSr1-x)B6The relative density of the solid solution polycrystal is 98.23-99.81%, the solid solution polycrystal has excellent emission performance, and can meet the requirements of various electron emission devices on the performance of a cathode (La)xSr1-x)B6Polycrystalline processing can obtain the cathode with the required shape and size.
Compared with the prior art, the invention has the following advantages:
1) the method adopts vacuum hot-pressing reaction sintering to sinter the La2O3Preparation of (La) from raw material powder of SrO and BxSr1-x)B6Polycrystal, which combines the powder synthesis and sintering densification into one. The preparation process is simplified, the sintering temperature is reduced, the process is simple, and the operation is convenient.
2) Using La2O3The SrO and B powder are used as raw materials, so that the production cost is reduced, the method is suitable for industrial production and application, and the method is favorable for expanding the application field of rare earth and alkaline earth hexaboride in the aspect of cathode materials.
3) Synthesized (La)xSr1-x)B6The solid solution polycrystalline cathode material has high purity, high density and excellent emission performance. Prepared (La)xSr1-x)B6The obtained product is detected to be a single hexaboride phase through X-ray diffraction, and the relative density can reach 99.81 percent at most. The emission performance test shows that (La)0.9Sr0.1)B6The saturated emission current density value of the cathode reaches 25.34A/cm at 1400 ℃, 1500 ℃ and 1600 ℃ respectively2、 37.61/cm2And 62.50A/cm2Has strong application prospect.
Drawings
FIG. 1 (La) prepared in example 10.3Sr0.7)B6XRD spectrum of the polymorph.
FIG. 2 (La) prepared in example 20.4Sr0.6)B6XRD spectrum of the polymorph.
FIG. 3 (La) prepared in example 30.7Sr0.3)B6XRD spectrum of the polymorph.
FIG. 4 (La) prepared in example 40.8Sr0.2)B6XRD spectrum of the polymorph.
FIG. 5 (La) prepared in example 50.9Sr0.1)B6XRD spectrum of the polymorph.
Detailed Description
The invention is further illustrated with reference to the following figures and detailed description, without however being restricted to the following examples. The invention will be described in detail hereinafter with reference to the drawings in conjunction with embodiments, in which the embodiments and features of the embodiments can be combined with each other without conflict. The rotating speed of the high-energy ball mill is 400-600r/min, in the example, 500r/min is taken.
La as raw material in examples of the present invention2O3The purity of the SrO raw material powder is more than or equal to 99.9 percent, the purity of the B raw material powder is more than or equal to 99.0 percent, and the particle size range of the raw material powder is 1-100 mu m.
Example 1
1) According to the reaction equation
0.3La2O3+1.4SrO+14.3B----2(La0.3Sr0.7)B6+2.3BO↑
In the glove box, La was weighed separately2O3And SrO and B raw material powder are filled into a stainless steel grinding tank, a stainless steel ball is used as a grinding medium, and the mass ratio of the ball to the powder is 10: 1. and taking the ball milling tank out of the glove box, putting the ball milling tank into a high-energy ball mill, carrying out ball milling for 2 hours, and then sampling in the glove box.
2) Charging, namely, ball-milling and uniformly mixing the La obtained in the step 1)2O3And charging the SrO and B raw material powder into a graphite mold in a glove box, and placing the mold into a hot-pressing sintering furnace.
In the step 1) and the step 2), the glove box is filled with argon atmosphere, and both the oxygen content and the water vapor content are less than or equal to 10 ppm; the La2O3And the purity of the SrO raw material powder is 99.99%, and the purity of the B raw material powder is 99.0%.
3) Heating and sintering, pre-vacuumizing in a hot pressing furnace to the air pressure of 1 × 10 before sintering-2Pa. A first temperature rise stage: room temperature to 500 ℃; a second temperature rising stage: heating from 500 ℃ to 800 ℃; a third temperature rise stage: heating to 1200 ℃ from 800 ℃; a fourth temperature rise stage: the temperature is increased to 1800 ℃ at 1200 ℃. The heating rate of the first to third heating stages is 10 ℃/min, and the heating rate of the fourth heating stage is 5 ℃/min. Wherein the first temperature-raising stage and the second temperature-raising stage apply 10MPa axial pressure to the powder, and the third temperature-raising stage applies 20MPa axial pressure to the powderForce, the fourth temperature rise stage applies 30MPa axial pressure to the powder.
4) Preserving heat, namely preserving heat for 2 hours at the temperature of 1800 ℃ after the temperature is raised in the fourth stage of the step 3); and 30MPa of axial pressure is applied to the powder in the heat preservation process.
5) Cooling, and after the heat preservation in the step 4), removing pressure, and cooling along with the furnace to obtain single-phase compact (La)0.3Sr0.7)B6A solid solution polycrystal.
(La0.3Sr0.7)B6The XRD result after polishing is shown in figure 1, and the figure shows that the sample is hexaboride single phase, the diffraction peak intensity is high, and the crystallization is good. Measured using an electronic gravity balance (La)0.3Sr0.7)B6The relative density of the polycrystalline body is 99.81%.
Example 2
1) According to the reaction equation
0.4La2O3+1.2SrO+14.4B----2(La0.4Sr0.6)B6+2.4BO↑
In the glove box, La was weighed separately2O3And SrO and B raw material powder are put into a stainless steel grinding tank, agate balls are used as grinding media, and the mass ratio of the balls to the powder is 15: 1. and taking the ball milling tank out of the glove box, putting the ball milling tank into a high-energy ball mill for ball milling for 4 hours, and then sampling in the glove box.
2) Charging, namely, ball-milling and uniformly mixing the La obtained in the step 1)2O3And charging the SrO and B raw material powder into a graphite mold in a glove box, and placing the mold into a hot-pressing sintering furnace.
In the step 1) and the step 2), the glove box is filled with argon atmosphere, and both the oxygen content and the water vapor content are less than or equal to 50 ppm; the La2O3And the purity of the SrO raw material powder is 99.99%, and the purity of the B raw material powder is 99.0%.
3) Heating and sintering, pre-vacuumizing in a hot pressing furnace before sintering until the air pressure is 8.5 multiplied by 10-3Pa. A first temperature rise stage: room temperature to 400 ℃; a second temperature rising stage: heating from 400 ℃ to 700 ℃; a third temperature rise stage: heating from 700 to 1100 ℃; a fourth temperature rise stage: heating to 1100 deg.C to 1500 deg.C. First to third litersThe temperature rise rate in the temperature stage is 10 ℃/min, and the temperature rise rate in the fourth temperature rise stage is 5 ℃/min. The first temperature rise stage and the second temperature rise stage apply 15MPa axial pressure to the powder, the third temperature rise stage applies 20MPa axial pressure to the powder, and the fourth temperature rise stage applies 20MPa axial pressure to the powder.
4) Preserving heat, namely preserving heat for 4 hours at the temperature of 1500 ℃ after the temperature is raised in the fourth stage of the step 3); and 50MPa of axial pressure is applied to the powder in the heat preservation process.
5) Cooling, and after the heat preservation in the step 4), removing pressure, and cooling along with the furnace to obtain single-phase compact (La)0.4Sr0.6)B6A solid solution polycrystal.
(La0.4Sr0.6)B6The XRD result after polishing is shown in figure 2, and the figure shows that the sample is hexaboride single phase, the diffraction peak intensity is high, and the crystallization is good. Measured using an electronic gravity balance (La)0.4Sr0.6)B6The relative density of the polycrystalline body was 99.46%. This example illustrates that high quality (La) can be produced at a sintering temperature of 1500 deg.CxSr1-x)B6The solid solution polycrystalline cathode material greatly reduces the sintering temperature.
Example 3
1) According to the reaction equation
0.7La2O3+0.6SrO+14.7B----2(La0.7Sr0.3)B6+2.7BO↑
In the glove box, La was weighed separately2O3And SrO and B raw material powder are filled into a stainless steel grinding tank, a stainless steel ball is used as a grinding medium, and the mass ratio of the ball to the powder is 20: 1. and taking the ball milling tank out of the glove box, putting the ball milling tank into a high-energy ball mill, carrying out ball milling for 3 hours, and then sampling in the glove box.
2) Charging, namely, ball-milling and uniformly mixing the La obtained in the step 1)2O3And charging the SrO and B raw material powder into a graphite mold in a glove box, and placing the mold into a hot-pressing sintering furnace.
In the step 1) and the step 2), the glove box is filled with argon atmosphere, and both the oxygen content and the water vapor content are less than or equal to 5 ppm; the La2O3And SrO atomThe purity of the material powder is 99.99%, and the purity of the material powder B is 99.0%.
3) Heating and sintering, pre-vacuumizing in a hot pressing furnace before sintering until the air pressure is 8.8 multiplied by 10-3Pa. A first temperature rise stage: room temperature to 500 ℃; a second temperature rising stage: heating from 500 ℃ to 800 ℃; a third temperature rise stage: heating to 1200 ℃ from 800 ℃; a fourth temperature rise stage: the temperature is increased to 1700 ℃ at 1200 ℃. The heating rate of the first to third heating stages is 10 ℃/min, and the heating rate of the fourth heating stage is 10 ℃/min. Wherein, the first temperature-rising stage and the second temperature-rising stage apply 10MPa axial pressure to the powder, and the third temperature-rising stage applies 20MPa axial pressure to the powder; the fourth temperature rise stage applies 20MPa axial pressure to the powder.
4) Preserving heat, namely preserving heat for 2 hours at 1700 ℃ after the temperature is raised in the fourth stage of the step 3); and applying 40MPa axial pressure to the powder in the heat preservation process;
5) cooling, and after the heat preservation in the step 4), removing pressure, and cooling along with the furnace to obtain single-phase compact (La)0.7Sr0.3)B6A solid solution polycrystal.
(La0.7Sr0.3)B6The XRD result after polishing is shown in figure 3, and the figure shows that the sample is hexaboride single phase, the diffraction peak intensity is high, and the crystallization is good. Measured using an electronic gravity balance (La)0.7Sr0.3)B6The relative density of the polycrystal is 98.23%.
Example 4
1) According to the reaction equation
0.8La2O3+0.4SrO+14.8B----2(La0.8Sr0.2)B6+2.8BO↑
In the glove box, La was weighed separately2O3And raw material powder of SrO and B is filled into a stainless steel grinding tank, and a stainless steel ball is used as a grinding medium. And taking the ball milling tank out of the glove box, putting the ball milling tank into a high-energy ball mill, carrying out ball milling for 2 hours, and then sampling in the glove box.
2) Charging, namely, ball-milling and uniformly mixing the La obtained in the step 1)2O3Putting raw material powder of SrO and B into a graphite mold in a glove box, and placing the mold in a hot-pressing sintering furnace;
in the step 1) and the step 2), the glove box is filled with argon atmosphere, and both the oxygen content and the water vapor content are less than or equal to 3 ppm; the La2O3And the purity of the SrO raw material powder is 99.99%, and the purity of the B raw material powder is 99.0%.
3) Heating and sintering, pre-vacuumizing in a hot pressing furnace before sintering until the air pressure is 9.1 multiplied by 10-3Pa. A first temperature rise stage: room temperature to 500 ℃; a second temperature rising stage: heating from 500 ℃ to 800 ℃; a third temperature rise stage: heating to 1200 ℃ from 800 ℃; a fourth temperature rise stage: the temperature is increased to 1800 ℃ at 1200 ℃. The heating rate of the first to third heating stages is 15 ℃/min, and the heating rate of the fourth heating stage is 5 ℃/min. The first temperature rise stage and the second temperature rise stage apply 10MPa axial pressure to the powder, the third temperature rise stage applies 20MPa axial pressure to the powder, and the fourth temperature rise stage applies 20MPa axial pressure to the powder;
4) preserving heat, namely preserving heat for 2 hours at the temperature of 1800 ℃ after the temperature is raised in the fourth stage of the step 3); and applying 30MPa axial pressure to the powder in the heat preservation process;
5) cooling, and after the heat preservation in the step 4), removing pressure, and cooling along with the furnace to obtain single-phase compact (La)0.8Sr0.2)B6A solid solution polycrystal.
(La0.8Sr0.2)B6The XRD result after polishing is shown in figure 4, and the figure shows that the sample is hexaboride single phase, the diffraction peak intensity is high, and the crystallization is good. Measured using an electronic gravity balance (La)0.8Sr0.2)B6The relative density of the polycrystalline body is 98.59%.
Example 5
1) According to the reaction equation
0.9La2O3+0.2SrO+14.9B----2(La0.9Sr0.1)B6+2.9BO↑
In the glove box, La was weighed separately2O3And raw material powder of SrO and B is filled into a stainless steel grinding tank, and a stainless steel ball is used as a grinding medium. And taking the ball milling tank out of the glove box, putting the ball milling tank into a high-energy ball mill, carrying out ball milling for 2 hours, and then sampling in the glove box.
2) Charging, namely, ball-milling and uniformly mixing the La obtained in the step 1)2O3Putting raw material powder of SrO and B into a graphite mold in a glove box, and placing the mold in a hot-pressing sintering furnace;
in the step 1) and the step 2), the glove box is filled with argon atmosphere, and both the oxygen content and the water vapor content are less than or equal to 5 ppm; the La2O3And the purity of the SrO raw material powder is 99.99%, and the purity of the B raw material powder is 99.0%.
3) Heating and sintering, pre-vacuumizing in a hot pressing furnace to the air pressure of 9.6 multiplied by 10 before sintering-3Pa. A first temperature rise stage: room temperature to 500 ℃; a second temperature rising stage: heating from 500 ℃ to 800 ℃; a third temperature rise stage: heating to 1200 ℃ from 800 ℃; a fourth temperature rise stage: the temperature is increased to 1800 ℃ at 1200 ℃. The heating rate of the first to third heating stages is 10 ℃/min, and the heating rate of the fourth heating stage is 5 ℃/min. The first temperature rise stage and the second temperature rise stage apply 10MPa axial pressure to the powder, the third temperature rise stage applies 20MPa axial pressure to the powder, and the fourth temperature rise stage applies 20MPa axial pressure to the powder;
4) preserving heat, namely preserving heat for 2 hours at the temperature of 1800 ℃ after the temperature is raised in the fourth stage of the step 3); and applying 30MPa axial pressure to the powder in the heat preservation process;
5) cooling, and after the heat preservation in the step 4), removing pressure, and cooling along with the furnace to obtain single-phase compact (La)0.9Sr0.1)B6A solid solution polycrystal.
(La0.9Sr0.1)B6The XRD result after polishing is shown in figure 5, and the figure shows that the sample is hexaboride single phase, the diffraction peak intensity is high, and the crystallization is good. Measured using an electronic gravity balance (La)0.9Sr0.1)B6The relative density of the polycrystalline body is 99.65%.
Each of (La) prepared in the above 5 examplesxSr1-x)B6The cathode emission performance test is as follows: the saturated emission current density values respectively reach 20.2-26.7A/cm at 1400 ℃, 1500 ℃ and 1600 DEG C2，29.6-37.5/cm2And 55.4-66.8A/cm2And has good emission performance and strong application prospect.

Claims (5)

1. Multicomponent rare earth boride (La)xSr1-x）B6The preparation method of the polycrystalline cathode material is characterized by comprising the following specific steps of:
1) proportioning and mixing materials:
according to the reaction equation
xLa2O3+2(1-x)SrO+(14+x)B----2(LaxSr1-x)B6+(x+2)BO
Figure DEST_PATH_IMAGE002
Wherein x = 0.9;
in the glove box, La was weighed separately2O3Raw material powder of SrO and B is filled into a stainless steel grinding tank, and stainless steel balls or agate balls are selected as grinding media; taking out the ball milling tank from the glove box, putting the ball milling tank into a high-energy ball mill, carrying out ball milling for 2-4h, and then sampling in the glove box;
2) charging, namely, ball-milling and uniformly mixing the La obtained in the step 1)2O3Putting raw material powder of SrO and B into a graphite mold in a glove box, and placing the mold in a hot-pressing sintering furnace;
in the step 1) and the step 2), the glove box is filled with argon atmosphere;
3) heating and sintering, wherein in the first heating stage: room temperature to 400 ℃ and 500 ℃; a second temperature rising stage: raising the temperature from 400-500 ℃ to 700-800 ℃ in the first stage; a third temperature rise stage: the temperature is increased from 700-800 to 1100-1200 ℃ in the second stage; a fourth temperature rise stage: heating from 1100-1200 deg.c to 1500-1800 deg.c in the third stage; the heating rate of the first heating stage to the third heating stage is 10-15 ℃/min, and the heating rate of the fourth heating stage is 5-10 ℃/min; wherein, the first temperature-rising stage and the second temperature-rising stage apply 10-15MPa axial pressure to the powder, the third temperature-rising stage applies 15-20MPa axial pressure to the powder, and the fourth temperature-rising stage applies 20-30MPa axial pressure to the powder;
4) preserving heat, preserving heat for 2-4h at the temperature of 1800 ℃ after the temperature is raised in the fourth stage of the step 3); and applying 30-50MPa axial pressure to the powder in the heat preservation process;
5) cooling, and after the heat preservation in the step 4), removing pressure, and cooling along with the furnace to obtain the (La)xSr1-x）B6A solid solution polycrystal.
2. The method according to claim 1, wherein the La is used in the step 1)2O3The purity of SrO raw material powder is more than or equal to 99.9 percent, the purity of B raw material powder is more than or equal to 99.0 percent, and the particle size range is 1-100 mu m;
in the step 1) and the step 2), the oxygen content and the water vapor content in the argon atmosphere in the glove box are both less than or equal to 50 ppm.
3. The method according to claim 1, wherein, before sintering in step 3), the autoclave is pre-evacuated to a pressure of 2 x 10 or less-2Pa。
4. (La) produced by the production process according to any one of claims 1 to 3xSr1-x）B6A polycrystalline body.
5. (La) produced by the production process according to any one of claims 1 to 3xSr1-x）B6The polycrystalline body is used as a cathode material.
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