Preparation method of strontium-doped lanthanum hexaboride powder

Abstract

The invention provides a preparation method of strontium-doped lanthanum hexaboride powder, and relates to the technical field of alkaline earth / rare earth hexaboride powder preparation, and the preparation method comprises the following steps: according to the requirements of a chemical equation [(1-x) La2O3 + 2xSrO + (15-x) B--2La1-xSrxB6 + (3-x) BO superscript], calculating the weight of La2O3 powder, SrO powder and B powder according to a molar ratio, weighing and canning, then putting into a powder mixing machine for powder mixing, and finally obtaining the strontium-doped lanthanum hexaboride powder. And uniform powder is obtained. According to the method, the La1-xSrxB6 powder is prepared through spark plasma reaction pressureless sintering, the preparation period is short, the process is simple, the subsequent acid / alkali / distilled water washing process is not needed, Na impurities cannot be introduced, the production cost is greatly reduced, and the method is suitable for industrial large-scale production and application.

Description

Technical Field

[0001] The invention relates to the technical field of preparation of alkaline earth / rare earth hexaboride powder, and in particular to a preparation method of strontium-doped lanthanum hexaboride powder. Background Art

[0002] Lanthanum hexaboride (LaB 6 ) has the advantages of high melting point, low evaporation rate at high temperature, good chemical stability, strong resistance to poisoning and ion bombardment. At the same time, due to its excellent thermal electron emission performance, lanthanum hexaboride is widely used as a hot cathode material in vacuum electronic devices such as Hall electric thrusters, high-resolution electronic analysis instruments and electron beam welders. With the rapid development of deep space exploration and high-power and high-frequency vacuum electronic devices in my country, higher requirements are placed on the performance and life of hot cathode materials. This requires reducing the work function of lanthanum hexaboride, thereby increasing its thermal electron emission current density.

[0003] The study found that in LaB 6 When alkaline earth metal elements such as Sr or Ba are doped in the molten metal, its thermal electron emission performance is better than that of LaB 6 The researchers used first-principles calculations to find out whether Sr-doped LaB 6 The work function of some specific components of La 1-x Sr x B 6 Such as La 0.75 Sr 0.25 B 6 Has more than LaB 6 Lower work function and higher thermal electron emission performance. Appropriate amount of Sr doping is expected to reduce LaB 6 The cathode operating temperature and its thermal electron emission performance are greatly improved. At the same time, due to the LaB 6 The characteristics of high melting point make LaB 6 The raw materials are mostly powders, and most of the molding methods are powder sintering. The quality of the raw material powder will seriously affect the final performance of the material. 1-x Sr x B 6 ) Powder preparation is the primary part of alkaline earth metal doped hexaboride research. 6 The main methods for preparing powder-like products include pure element chemical synthesis, carbon / boron carbide thermal reduction reaction, etc.

[0004] The in-situ synthesis method is to directly prepare LaB by reacting metal element lanthanum and element boron. 6 The specific process in the literature is to mix lanthanum powder and amorphous boron powder evenly, and then heat to 1300℃~2000℃ in a vacuum argon or hydrogen environment to obtain LaB 6Powder. However, this method has the disadvantages of high production and storage costs and strict requirements on production processes and equipment due to the use of high-purity metal elements. Therefore, this method is not used in large-scale industrial production.

[0005] The carbon / boron carbide thermal reduction method has many intermediate reactions, the reaction process is difficult to control, and the numerous by-products require complex treatment of the reaction products. 2 O 3 、 CeO 2 , B 4 After C and Al were kept at 1500℃ for 4h, the sintered products were ground to remove by-products, and then soaked in 20 wt.% NaOH solution at 90℃ for 3h, and the remaining solid was washed twice with NaOH solution. 2 SO 4 The residual alkali solution was neutralized with 9.8 wt. % solution, and the powder was washed with deionized water. 1-x Ce x B 6 Powder. It can be seen that the obtained product needs to undergo complex washing, which increases the process flow.

[0006] Currently, there is no pure element chemical synthesis method or carbon / boron carbide thermal reduction reaction method to prepare La. 1-x Sr x B 6 Related reports on powder. 2 O 3 , SrO and NaBH 4 As raw materials, La is obtained by mixed grinding, pressing, vacuum sintering, etc. 1- x Sr x B 6 The powder was obtained by heating for 2 hours and washing with hydrochloric acid, distilled water and anhydrous ethanol was required for 3 times. 4 , it is difficult to avoid Na contamination in the product.

[0007] Therefore, a method with short cycle, simple process and high quality single-phase La 1-x Sr x B 6 The method for preparing the powder needs to be discovered urgently. Summary of the invention

[0008] The purpose of the present invention is to solve the shortcomings of the prior art and to propose a method for preparing strontium-doped lanthanum hexaboride powder.

[0009] In order to achieve the above object, the present invention adopts the following technical scheme: a method for preparing strontium-doped lanthanum hexaboride powder, comprising the following steps: S1, mixed powder, according to the chemical reaction equation [(1-x)La 2 O 3 +2xSrO+(15-x)B----2La 1-x Sr x B 6 +(3-x)BO↑] requires that La 2 O 3 The weight of the powder, SrO powder and B powder is calculated according to the molar ratio, weighed and filled into a can, and then put into a powder mixer for mixing to obtain a uniform powder; S2, ball milling, putting the powder into a stainless steel ball milling jar, adding stainless steel balls as a ball milling medium according to a ball-to-material ratio of 10:1, evacuating the ball milling jar, filling it with argon gas, and then putting it into a high-energy ball mill for ball milling to obtain a ball-milled powder; S3, screening, screening the ball-milled powder through a sieve, leaving fine powder and filling it into a can, and then putting it into a powder mixer again for mixing to obtain fine powder; S4, pressing, the fine powder is loaded into a graphite mold with an inner diameter of 13.4 mm and a height of 50 mm, the powder is wrapped with a thin carbon paper to isolate the graphite mold and the pressing head, and then the powder is pre-pressed on a hydraulic press with a pre-pressure of 1 to 2 MPa; S5, reaction sintering, after pre-pressing, a layer of carbon felt is wrapped on the surface of the graphite mold, and it is placed in a spark plasma sintering furnace. The position of the graphite mold is adjusted by a graphite pad and the upper and lower electrodes of the sintering furnace are connected to form a sintering current. Then the vacuum is evacuated to below 5Pa, and the reaction sintering is started. After the sintering is completed, the current is turned off and the furnace is cooled. After cooling to room temperature, the sintered sample is taken out to obtain La 1-x Sr x B 6 Prefabricated body; S6, grinding, using an agate mortar, 1-x Sr x B 6 The preform was ground for 30 min to obtain La 1-x Sr x B 6 powder.

[0010] Preferably, in step S2, the process of evacuating the ball mill and then filling it with argon gas needs to be repeated three times.

[0011] Preferably, in step S4, the fine powder needs to be placed in the middle of the graphite mold.

[0012] Preferably, in step S1, La 2 O3 The purity of SrO and B powders is ≥99%, La 2 O 3 The particle size of SrO powder is 10-20 μm, and the particle size of B powder is 30-40 μm.

[0013] Preferably, in step S5, the process parameters of the reaction sintering are: axial pressure 0 MPa; sintering temperature 1600° C.; and holding time 30 to 60 min.

[0014] Preferably, in step S5, the heating method of the reaction sintering is as follows: First, the temperature was raised to 600°C at a rate of 75-85°C / min. The sample was dried at 600°C for 15 min to remove the oxide La. 2 O 3 and the moisture absorption effect of SrO powder; then heat to 1500℃ at a heating rate of 65-75℃ / min; finally heat to 1600℃ at a heating rate of 25-35℃ / min, and enter the insulation stage for 30-60min.

[0015] Preferably, in step S2, the ball milling time is 6 hours and the ball milling frequency is 45 to 50 Hz.

[0016] Preferably, in step S3, the sieve is 100 mesh.

[0017] Compared with the prior art, the advantages and positive effects of the present invention are: The method of the present invention adopts spark plasma reaction pressureless sintering to prepare La 1-x Sr x B 6 Powder, short preparation cycle, simple process, no need for subsequent acid / alkali / distilled water washing process, and no introduction of Na impurities. 1-x Sr x B 6 The average particle size of the powder is 10 μm, and the production cost is greatly reduced, making it suitable for industrial large-scale production and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The La obtained in Example 1 0.75 Sr 0.25 B 6 XRD pattern of powder; Figure 2 The La obtained in Example 1 0.75 Sr 0.25 B 6 SEM image and mapping image of powder; Figure 3The La obtained in Example 1 0.75 Sr 0.25 B 6 Particle size distribution of powders; Figure 4 The La obtained in Example 2 0.5 Sr 0.5 B 6 XRD pattern of powder; Figure 5 The La obtained in Example 2 0.5 Sr 0.5 B 6 Particle size distribution of powders; Figure 6 The La obtained in Example 3 0.25 Sr 0.75 B 6 XRD pattern of powder; Figure 7 The La obtained in Example 3 0.25 Sr 0.75 B 6 Particle size distribution of powders. DETAILED DESCRIPTION

[0019] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0020] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways than those described herein. Therefore, the present invention is not limited to the specific embodiments of the following disclosure.

[0021] Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the present technical field.

[0022] Example 1: See Figure 1-3 The present invention provides a technical solution: a method for preparing strontium-doped lanthanum hexaboride powder, comprising the following steps: S1, mixed powder, according to the chemical reaction equation 0.75La 2 O 3 +0.5SrO+14.75B----2La 0.75 Sr 0.25 B 6 +2.75BO↑ La 2 O 3The weight of the powder, SrO powder and B powder was calculated according to the molar ratio of 0.75:0.5:14.75, weighed and canned, and then loaded into a powder mixer for mixing for 12 hours to obtain a uniform powder; S2, ball milling, put the powder into a stainless steel ball milling jar, add stainless steel balls as ball milling medium according to the ball-to-material ratio of 10:1, evacuate the ball milling jar, fill it with argon, and then put it into a high-energy ball mill for ball milling. The ball milling time is 6 hours and the ball milling frequency is 50 Hz to obtain the ball-milled powder; the evacuation of the ball milling jar and the filling of argon need to be repeated three times; S3, sieving, the ball-milled powder is sieved through a 100-mesh sieve, leaving the fine powder and canning it, and then it is put into the powder mixer again for mixing for 12 hours to obtain fine powder; S4, pressing, the fine powder is loaded into a graphite mold with an inner diameter of 13.4 mm and a height of 50 mm. The fine powder needs to be placed in the middle of the graphite mold, and then the powder is wrapped with a thin carbon paper to isolate the graphite mold and the pressing head, and then the powder is pre-pressed on a hydraulic press with a pre-pressure of 2 MPa; S5, reaction sintering, after pre-pressing, a layer of carbon felt is wrapped on the surface of the graphite mold, and it is placed in a spark plasma sintering furnace. The position of the graphite mold is adjusted by a graphite pad and the upper and lower electrodes of the sintering furnace are connected to form a sintering current. Then, the vacuum is evacuated to below 5Pa and reaction sintering is started. The process parameters are as follows: Axial pressure 0MPa; sintering temperature 1600℃; holding time 30min; The specific heating method is as follows: first, heat to 600℃ at a heating rate of 75℃ / min; keep dry at 600℃ for 15min to remove the oxide La 2 O 3 and the moisture absorption effect of SrO powder; then heat to 1500℃ at a heating rate of 75℃ / min; finally heat to 1600℃ at a heating rate of 35℃ / min, and enter the insulation stage for 30min.

[0023] After sintering, turn off the current and cool down with the furnace. After the temperature drops to room temperature, take out the sintered sample to obtain La 1-x Sr x B 6 Prefabricated body; S6, grinding, using an agate mortar to 1-x Sr x B 6 The preform was ground for 30 min to obtain La 1-x Sr x B 6 powder.

[0024] Figure 1 The La prepared in this example0.75 Sr 0.25 B 6 The XRD spectrum of the powder shows that the product is a single phase without any impurity phase, which is high-purity La 0.75 Sr 0.25 B 6 pink.

[0025] Figure 2 The La prepared in this example 0.75 Sr 0.25 B 6 The SEM image of the powder shows that the powder shape is basically consistent, the powder particles are relatively dispersed, the particle size is small, and the La, Sr and B elements are evenly distributed.

[0026] Figure 3 The La prepared in this example 0.75 Sr 0.25 B 6 Particle size distribution diagram of the powder. From the figure, it can be seen that the powder particle size distribution is uniform and the average particle size is 9.67μm.

[0027] Example 2: See Figure 4-5 The present invention provides a technical solution: a method for preparing strontium-doped lanthanum hexaboride powder, comprising the following steps: S1, mixed powder, according to the chemical reaction equation 0.5 La 2 O 3 + SrO + 14.5 B----2 La 0.5 Sr 0.5 B 6 + 2.5 BO↑ La 2 O 3 The weight of the powder, SrO powder and B powder was calculated according to the molar ratio of 0.5:1:14.5, weighed and canned, and then put into a powder mixer for mixing for 12 hours to obtain a uniform powder; S2, ball milling, put the powder into a stainless steel ball milling jar, add stainless steel balls as ball milling medium according to the ball-to-material ratio of 10:1, evacuate the ball milling jar, fill it with argon, and then put it into a high-energy ball mill for ball milling. The ball milling time is 6 hours and the ball milling frequency is 45Hz to obtain the ball-milled powder; the evacuation of the ball milling jar and the filling of argon need to be repeated three times; S3, sieving, the ball-milled powder is sieved through a 100-mesh sieve, leaving the fine powder and canning it, and then it is put into the powder mixer again for mixing for 12 hours to obtain fine powder; S4, pressing, the fine powder is loaded into a graphite mold with an inner diameter of 13.4 mm and a height of 50 mm. The fine powder needs to be placed in the middle of the graphite mold, and then the powder is wrapped with a thin carbon paper to isolate the graphite mold and the pressing head, and then the powder is pre-pressed on a hydraulic press with a pre-pressure of 1 MPa; S5, reaction sintering, after pre-pressing, a layer of carbon felt is wrapped on the surface of the graphite mold, and it is placed in a spark plasma sintering furnace. The position of the graphite mold is adjusted by a graphite pad and the upper and lower electrodes of the sintering furnace are connected to form a sintering current. Then, the vacuum is evacuated to below 5Pa and reaction sintering is started. The process parameters are as follows: Axial pressure 0MPa; sintering temperature 1600℃; holding time 60min; The specific heating method is as follows: first, heating to 600℃ at a heating rate of 80℃ / min; keeping and drying at 600℃ for 15min to remove the oxide La 2 O 3 and the moisture absorption effect of SrO powder; then heat to 1500℃ at a heating rate of 70℃ / min; finally heat to 1600℃ at a heating rate of 30℃ / min, and enter the insulation stage for 60min.

[0028] After sintering, turn off the current and cool down with the furnace. After the temperature drops to room temperature, take out the sintered sample to obtain La 1-x Sr x B 6 Prefabricated body; S6, grinding, using an agate mortar to 1-x Sr x B 6 The preform was ground for 30 min to obtain La 1-x Sr x B 6 powder.

[0029] Figure 4 The La prepared in this example 0.5 Sr 0.5 B 6 The XRD spectrum of the powder shows that the product is a single phase without any impurity phase, which is high-purity La 0.5 Sr 0.5 B 6 pink.

[0030] Figure 5 The La prepared in this example 0.5 Sr 0.5 B 6 Particle size distribution diagram of the powder. From the figure, it can be seen that the powder particle size distribution is uniform, and the average particle size is about 10.61μm.

[0031] Example 3: See Figure 6-7 The present invention provides a technical solution: a method for preparing strontium-doped lanthanum hexaboride powder, comprising the following steps: S1, mixed powder, according to the chemical reaction equation 0.25 La 2 O 3 + 1.5 SrO + 14.25 B----2 La 0.25 Sr 0.75 B 6 + 2.25 BO↑ La 2 O 3 The weight of the powder, SrO powder and B powder was calculated according to the molar ratio of 0.25:1.5:14.25, weighed and canned, and then loaded into a powder mixer for mixing for 12 hours to obtain a uniform powder; S2, ball milling, put the powder into a stainless steel ball milling jar, add stainless steel balls as ball milling medium according to the ball-to-material ratio of 10:1, evacuate the ball milling jar, fill it with argon, and then put it into a high-energy ball mill for ball milling. The ball milling time is 6 hours and the ball milling frequency is 50 Hz to obtain the ball-milled powder; the evacuation of the ball milling jar and the filling of argon need to be repeated three times; S3, sieving, the ball-milled powder is sieved through a 100-mesh sieve, leaving the fine powder and canning it, and then it is put into the powder mixer again for mixing for 12 hours to obtain fine powder; S4, pressing, the fine powder is loaded into a graphite mold with an inner diameter of 13.4 mm and a height of 50 mm. The fine powder needs to be placed in the middle of the graphite mold, and then the powder is wrapped with a thin carbon paper to isolate the graphite mold and the pressing head, and then the powder is pre-pressed on a hydraulic press with a pre-pressure of 2 MPa; S5, reaction sintering, after pre-pressing, a layer of carbon felt is wrapped on the surface of the graphite mold, and it is placed in a spark plasma sintering furnace. The position of the graphite mold is adjusted by a graphite pad and the upper and lower electrodes of the sintering furnace are connected to form a sintering current. Then, the vacuum is evacuated to below 5Pa and reaction sintering is started. The process parameters are as follows: Axial pressure 0MPa; sintering temperature 1600℃; holding time 60min; The specific heating method is as follows: first, heat to 600℃ at a heating rate of 85℃ / min; keep dry at 600℃ for 15min to remove the oxide La 2 O 3 and the moisture absorption effect of SrO powder; then heat to 1500℃ at a heating rate of 65℃ / min; finally heat to 1600℃ at a heating rate of 25℃ / min, and enter the insulation stage for 60min.

[0032] After sintering, turn off the current and cool down with the furnace. After the temperature drops to room temperature, take out the sintered sample to obtain La 1-x Sr x B 6 Prefabricated body; S6, grinding, using an agate mortar to 1-x Sr x B 6 The preform was ground for 30 min to obtain La 1-x Sr x B 6 powder.

[0033] Figure 6 The La prepared in this example 0.25 Sr 0.75 B 6 The XRD spectrum of the powder shows that the product is a single phase without any impurity phase, which is high-purity La 0.25 Sr 0.75 B 6 pink.

[0034] Figure 7 The La prepared in this example 0.25 Sr 0.75 B 6 Particle size distribution diagram of the powder. From the figure, it can be seen that the powder particle size distribution is uniform, and the average particle size is about 9.93μm.

[0035] The above are only preferred embodiments of the present invention and are not intended to limit the present invention in other forms. Any technician familiar with the profession may use the technical contents disclosed above to change or modify them into equivalent embodiments with equivalent changes and apply them to other fields. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing strontium-doped lanthanum hexaboride powder, characterized in that: The following steps are involved: S1, mixed powder, according to the chemical reaction equation [(1-x)La2O3+2xSrO+(15-x)B----2La 1-x Sr x B6+(3-x)BO↑] requires that La2O3 powder, SrO powder and B powder be weighed and canned according to the molar ratio, and then put into a powder mixer for mixing to obtain uniform powder; S2, ball milling, putting the powder into a stainless steel ball milling jar, adding stainless steel balls as a ball milling medium according to a ball-to-material ratio of 10:1, evacuating the ball milling jar, filling it with argon gas, and then putting it into a high-energy ball mill for ball milling to obtain a ball-milled powder; S3, screening, screening the ball-milled powder through a sieve, leaving fine powder and filling it into a can, and then putting it into a powder mixer again for mixing to obtain fine powder; S4, pressing, the fine powder is loaded into a graphite mold with an inner diameter of 13.4 mm and a height of 50 mm, the powder is wrapped with a thin carbon paper to isolate the graphite mold and the pressing head, and then the powder is pre-pressed on a hydraulic press with a pre-pressure of 1 to 2 MPa; S5, reaction sintering, after pre-pressing, a layer of carbon felt is wrapped on the surface of the graphite mold, and it is placed in a spark plasma sintering furnace. The position of the graphite mold is adjusted by a graphite pad and the upper and lower electrodes of the sintering furnace are connected to form a sintering current. Then the vacuum is evacuated to below 5Pa, and the reaction sintering is started. After the sintering is completed, the current is turned off and the furnace is cooled. After cooling to room temperature, the sintered sample is taken out to obtain La 1-x Sr x B6 preform; S6, grinding, using an agate mortar, 1-x Sr x The B6 preform was ground for 30 min to obtain La 1-x Sr x B6 powder.

2. The method for preparing strontium-doped lanthanum hexaboride powder according to claim 1, characterized in that: In step S2, the ball mill jar is evacuated and then filled with argon gas, which needs to be repeated three times.

3. The method for preparing strontium-doped lanthanum hexaboride powder according to claim 1, characterized in that: In step S4, the fine powder needs to be placed in the middle of the graphite mold.

4. The method for preparing strontium-doped lanthanum hexaboride powder according to claim 1, characterized in that: In step S1, the purity of La2O3, SrO and B powders are all ≥99%, the particle size of La2O3 and SrO powders is 10-20 μm, and the particle size of B powder is 30-40 μm.

5. The method for preparing strontium-doped lanthanum hexaboride powder according to claim 1, characterized in that: In step S5, the process parameters of the reaction sintering are: axial pressure 0 MPa; sintering temperature 1600° C.; and holding time 30 to 60 min.

6. The method for preparing strontium-doped lanthanum hexaboride powder according to claim 1, characterized in that: In step S5, the temperature raising method of the reaction sintering is as follows: First, heat to 600°C at a heating rate of 75-85°C / min; keep dry at 600°C for 15 minutes to remove the moisture absorption effect of the oxide La2O3 and SrO powders; then heat to 1500°C at a heating rate of 65-75°C / min; finally heat to 1600°C at a heating rate of 25-35°C / min, and then enter the insulation stage for 30-60 minutes.

7. The method for preparing strontium-doped lanthanum hexaboride powder according to claim 1, characterized in that: In step S2, the ball milling time is 6 hours, and the ball milling frequency is 45-50 Hz.

8. The method for preparing strontium-doped lanthanum hexaboride powder according to claim 1, characterized in that: In step S3, the sieve is 100 mesh.

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