A low-temperature preparation method of an alkaline earth metal-doped lanthanum hexaboride nanopowder

Abstract

The application discloses a low-temperature preparation method of alkaline earth metal doped lanthanum hexaboride nano powder, relates to the technical field of inorganic functional material preparation, and comprises the following steps: S1, dehydrating lanthanum chloride heptahydrate and alkaline earth metal chloride hydrate by heating at 200-300 DEG C to obtain anhydrous mixed powder; S2, uniformly grinding the mixed powder after adding sodium borohydride to the mixed powder to obtain a boron-containing mixture; S3, mixing KCl / LiCl mixed molten salt with the boron-containing mixture for 25-40 hours to obtain a reaction precursor mixture; S4, placing the reaction precursor mixture in an inert atmosphere, heating to 900-1000 DEG C at a heating rate of 10-20 DEG C / min, and keeping the temperature for 0.5-1.5 hours to obtain a reaction product; and S5, dissolving, filtering, acid washing, water washing and drying the reaction product to obtain the alkaline earth metal doped lanthanum hexaboride nano powder. The alkaline earth metal doped lanthanum hexaboride nano powder prepared at a low temperature is single-phase, has a cubic morphology, and has uniform particle size distribution.

Description

Technical Field

[0001] This invention relates to the field of inorganic functional materials preparation technology, and in particular to a low-temperature preparation method for alkaline earth metal-doped lanthanum hexaboride nanopowder. Background Technology

[0002] Lanthanum hexaboride (LaB6) is a rare-earth boride with unique physicochemical properties. It has a simple cubic CsCl-type crystal structure composed of lanthanum atoms and boron octahedrons. Due to its low work function, high melting point, low evaporation rate, excellent chemical stability, and good thermionic emission performance, this material is widely used as a thermionic cathode in high-tech fields such as satellites, deep space exploration, electron beam processing, 3D printing, nuclear energy, plasma sources, and high-resolution electron microscopy, both in defense and civilian applications.

[0003] With the rapid development of high-power vacuum electronic devices, the demand for hot cathode materials with lower cathode operating temperatures and longer service lives is increasing, which necessitates improving the thermionic emission performance of LaB6. According to first-principles calculations, appropriate doping with rare earth or alkaline earth metals (M) can effectively reduce the work function of LaB6, thereby improving its thermionic emission performance, especially the multi-component LaB6 doped with alkaline earth metals strontium (Sr) / barium (Ba). x M 1-x B6 material. Currently, the main methods for preparing alkaline earth metal-doped lanthanum hexaboride powder are the high-temperature solid-state method and the borocarbonothermic reduction method. The high-temperature solid-state method uses lanthanum oxide, alkaline earth metal oxides, and sodium borohydride as raw materials, which are mixed, ground, and pressed into shape in a specific ratio, and then subjected to high temperature (>1200)... o C) After vacuum sintering and multiple impurity removal processes, La was obtained. x M 1-x B6 nanoparticles. The boron-carbothermic reduction method uses alkaline earth metal carbonates, lanthanum oxide, boron carbide, and carbon powder as raw materials, involving three rounds of ball milling and two prolonged high-temperature processes (the first >1400°C). o C, Second time > 1900 o C) After the reaction, La was synthesized and prepared. x M 1-x B6 micron powder. However, both the high-temperature solid-state method and the boron-carbothermic reduction method have the problem of demanding preparation conditions, especially the high synthesis temperature, which leads to complex processes and increased preparation costs. In addition, the content and purity of the target product obtained by the above two methods are also relatively low.

[0004] In summary, a low-temperature preparation method for alkaline earth metal-doped lanthanum hexaboride nanopowder is urgently needed. Summary of the Invention

[0005] In view of the aforementioned existing problems, the present invention is proposed.

[0006] Therefore, this invention provides a low-temperature preparation method for alkaline earth metal-doped lanthanum hexaboride nanopowder, thereby solving the problem of harsh preparation conditions in existing high-temperature solid-state methods and boron-carbon thermal reduction methods.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution: 1. This invention provides a low-temperature preparation method for alkaline earth metal-doped lanthanum hexaboride nanopowder, comprising: S1, mixing lanthanum chloride heptahydrate with alkaline earth metal chloride hydrate, heating at 200-300°C for 4-8 hours to remove water of crystallization, and obtaining a mixed powder of anhydrous lanthanum chloride and anhydrous alkaline earth metal chloride; S2. Add sodium borohydride to the mixed powder and grind it at a speed of 40-90 r / min for 0.3-1 hour to make the material uniformly mixed and obtain a boron-containing mixture; S3. Add a mixed molten salt composed of lithium chloride and potassium chloride to the boron-containing mixture, and continue mixing for 25 to 40 hours to obtain a reaction precursor mixture; S4. The reaction precursor mixture is placed in an inert atmosphere and heated to 900-1000°C at a heating rate of 10-20°C / min. The mixture is kept at this temperature for 0.5-1.5 hours. After the heating is completed, the temperature is reduced to 600°C at a cooling rate of 5°C / min and then cooled to room temperature in the furnace to obtain the reaction product. S5. Dissolve the reaction product in water, filter it, acid wash it, wash it with water until neutral, and then dry it at 80-90℃ to obtain alkaline earth metal-doped lanthanum hexaboride nanopowder.

[0008] As a preferred embodiment of the low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder of the present invention, wherein: the heating temperature in S1 is 240°C, the holding time is 6 hours, and the alkaline earth metal chloride hydrate is one of BaCl2·2H2O, SrCl2·6H2O or CaCl2·2H2O.

[0009] As a preferred embodiment of the low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder of the present invention, wherein: the grinding speed in S2 is 60 r / min, the grinding time is 0.5 hours, the sodium borohydride is dried before being added, and the mixing process is carried out in a dry and inert environment.

[0010] In a preferred embodiment of the low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder of the present invention, the mass ratio of lithium chloride to potassium chloride in the mixed molten salt in S3 is 45:55, and the total mass ratio of the boron-containing mixture to the mixed molten salt is 1:10.

[0011] In a preferred embodiment of the low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder according to the present invention, the mixing time in step S3 is 28 hours, and uniform mixing is achieved by low-speed ball milling or mechanical stirring. The mixing process is carried out in a sealed container to prevent moisture absorption or oxidation. The rotation speed of the low-speed ball mill does not exceed 100 r / min to avoid initiating the decomposition of sodium borohydride, and the material temperature is controlled below 30°C during the mixing process to prevent side reactions.

[0012] As a preferred embodiment of the low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder of the present invention, wherein: the heating rate in S4 is 15℃ / min, the reaction temperature of alkaline earth metal Sr-doped LaB6 nanopowder is 900℃, the reaction temperature of alkaline earth metal Ba-doped or Sr / Ba co-doped LaB6 nanopowder is 1000℃, the holding time is 1 hour, the inert atmosphere is high-purity argon, and the gas flow rate is maintained at 40-60 mL / min.

[0013] In a preferred embodiment of the low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder of the present invention, the acid washing in step S5 uses a hydrochloric acid solution with a mass fraction of 36%, and the water washing continues until the pH value of the filtrate is 6.8-7.2 to ensure complete removal of soluble salts.

[0014] As a preferred embodiment of the low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder of the present invention, the mixed molten salt is dried at 200°C for 4 hours before use to remove adsorbed water, and the lithium chloride and potassium chloride are analytical grade or higher purity reagents.

[0015] The beneficial effects of this invention are as follows: By employing a chloride precursor and sodium borohydride in a low-temperature in-situ reaction in a eutectic LiCl-KCl molten salt system, the S2 step achieves molecular-level homogeneous mixing of the boron source and metal chloride through gentle grinding, effectively avoiding sodium borohydride decomposition or local overheating caused by high-energy ball milling, thus providing a highly homogeneous precursor environment for subsequent reactions; the S3 step introduces a mixed molten salt with a specific composition and mixes it for a long time, which not only serves as a reaction medium to lower the synthesis temperature but also acts as a nano-confined template to regulate the La 1-x M x B6 nucleation and growth kinetics. By combining complete dehydration in S1, controlled heating and inert protection in S4, and precise post-processing in S5, the entire process synergistically achieves the direct preparation of single-phase, cubical alkaline earth metal-doped lanthanum hexaboride nanopowder with uniform particle size distribution at temperatures far lower than those of traditional high-temperature solid-state methods and boron-carbothermic reduction methods. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a photograph of alkaline earth metal-doped lanthanum hexaboride nanopowder. Figure 2 XRD pattern of alkaline earth metal-doped lanthanum hexaboride nanopowder; Figure 3 La prepared in Example 1 0.5 Ba 0.5 SEM characterization results of B6 nanopowder; Figure 4 La prepared in Example 3 0.5 Sr 0.25 Ba 0.25 SEM characterization results of B6 nanopowder. Detailed Implementation

[0018] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0019] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0020] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0021] Figures 1-3 La is given 0.5 Ba 0.5 The physical image, XRD pattern, and microstructure of B6 nanopowder represent the first embodiment of this invention. This embodiment provides a low-temperature preparation method for alkaline earth metal barium-doped lanthanum hexaboride nanopowder, comprising the following steps: Example 1: Barium-doped lanthanum hexaboride nanopowder (La 0.5 Ba 0.5 B6) 1. Raw material preparation: Select 99.5% pure LaCl3·7H2O powder and 99.5% pure BaCl2·2H2O powder, and accurately weigh them according to the elemental molar ratio of La to Ba 1:1; Analytical grade LiCl and KCl (mass ratio 45:55) are dried at 200℃ for 4 hours before use to remove adsorbed water; 99% pure NaBH4 is dried before use.

[0022] 2. Step S1 (Decrystallization): Mix the two chloride powders above evenly, place them in a vacuum atmosphere furnace, keep them at 240°C for 6 hours to remove the crystal water, and obtain anhydrous LaCl3 and BaCl2 mixed powder (I powder).

[0023] 3. Step S2 (Grinding and Mixing): In a closed environment protected by dry argon gas, add NaBH4 to powder I, control the molar ratio of the sum of La and Ba elements to B element to be 1:6, grind at a speed of 60 r / min for 0.5 hours to obtain a boron-containing mixture (powder II), and control the material temperature below 30℃ during the mixing process.

[0024] 4. Step S3 (Preparation of precursor): Place the II powder and the mixed molten salt (LiCl+KCl) into a sealed container at a total mass ratio of 1:10, and mix them by low-speed ball milling (80 r / min) for 28 hours to obtain the reaction precursor mixture (III powder).

[0025] 5. Step S4 (molten salt reaction): Place III powder into a corundum crucible, put it in a vacuum atmosphere furnace, introduce high-purity argon gas (flow rate 50 mL / min), heat to 1000℃ at a heating rate of 15℃ / min, hold for 1 hour, after holding, reduce the temperature to 600℃ at a cooling rate of 5℃ / min, and then cool to room temperature with the furnace to obtain the reaction product (IV powder).

[0026] 6. Step S5 (Post-processing): Dissolve IV powder in deionized water, filter, wash with 36% hydrochloric acid, then wash with deionized water until the pH of the filtrate is 6.8-7.2, and finally dry at 85℃ to obtain La. 0.5 Ba 0.5 B6 nanopowder.

[0027] The product obtained from the above steps is grayish-black, a single-phase solid solution without impurities; it has a cubic morphology, an average particle size of 69.09 nm, high surface activity, large specific surface area, and good sintering activity.

[0028] Figures 1-2 La is given 0.75 Sr 0.25The physical image and XRD pattern of B6 nanopowder represent the second embodiment of the present invention. This embodiment provides a low-temperature preparation method for alkaline earth metal strontium-doped lanthanum hexaboride nanopowder, comprising the following steps: Example 2: Strontium-doped lanthanum hexaboride nanopowder (La 0.75 Sr 0.25 B6) 1. Raw material preparation: Select 99.5% pure LaCl3·7H2O powder and 99.5% pure SrCl2·6H2O powder, and accurately weigh them according to the La:Sr element molar ratio of 3:1; the remaining raw materials and pretreatment are the same as in Example 1. 2. Steps S1~S5: Except for S1 where the raw materials are LaCl3·7H2O and SrCl2·6H2O, S2 where the sum of La and Sr elements is in a 1:6 molar ratio to B, and S4 where the heating temperature is 900℃, the remaining process parameters are completely consistent with Example 1. The obtained product is blue-black, a single-phase solid solution, free of impurities; cubic morphology, with an average particle size of 83.12 nm, exhibiting good sintering activity.

[0029] Figure 1 , Figure 2 and Figure 4 La is given respectively 0.5 Sr 0.25 Ba 0.25 The physical image, XRD pattern, and microstructure of B6 nanopowder represent the third embodiment of this invention. This embodiment provides a low-temperature preparation method for alkaline earth metal strontium / barium co-doped lanthanum hexaboride nanopowder, comprising the following steps: Example 3: Strontium / Barium co-doped lanthanum hexaboride nanopowder (La 0.5 Sr 0.25 Ba 0.25 B6) 1. Raw material preparation: Select 99.5% pure LaCl3·7H2O powder, 99.5% pure SrCl2·6H2O powder, and 99.5% pure BaCl2·2H2O powder, and accurately weigh them according to the molar ratio of La, Sr, and Ba elements of 2:1:1; the remaining raw materials and pretreatment are the same as in Example 1. 2. Steps S1~S5: Except for S1, where the raw material is a mixture of three chlorides, and S2, where the molar ratio of the sum of La, Sr, and Ba elements to B element is 1:6, the remaining process parameters are completely consistent with Example 1. The obtained product is grayish-black, a single-phase solid solution, free of impurities; La, Sr, Ba, and B elements are uniformly distributed, with a cubic morphology and an average particle size of 94.58 nm, possessing both high surface energy and a large specific surface area, exhibiting good sintering activity.

[0030] Comparative Example 1: Uncontrolled Mixed Molten Salt Ratio 1. Raw materials and process: basically the same as in Example 1, except that the mass ratio of LiCl to KCl in the mixed molten salt in S3 is 20:80 (deviating from the optimized ratio of 45:55), and the mass ratio of boron-containing mixture to total mixed molten salt is 1:3 (deviating from the optimized ratio of 1:10).

[0031] 2. A small amount of impurity phase (not completely forming a single-phase solid solution) appeared in the powder. The powder morphology was irregular, with partial agglomeration. The average particle size was 220 nm, the specific surface area was reduced by 30% compared to Example 1, and the sintering activity was reduced.

[0032] Comparative Example 2: Deviation of high-temperature reaction parameters 1. Raw materials and process: basically the same as in Example 2, except that the heating rate in S4 is 25℃ / min (higher than 10~20℃ / min), the reaction temperature is 850℃ (lower than 900~1000℃), and the holding time is 0.3 hours (shorter than 0.5~1.5 hours).

[0033] 2. The reaction was incomplete, and unreacted raw material impurities remained in the powder. The XRD pattern showed multiple impurity peaks, and the powder morphology was a mixture of irregular blocks and granules with an average particle size of 210 nm.

[0034] Comparative Example 3: Simplified Post-processing 1. Raw materials and process: basically the same as in Example 3, except that the acid washing step was not performed in S5, the water washing was only performed once (not washed to pH 6.8~7.2), and the drying temperature was 70℃ (below the range of 80~90℃).

[0035] 2. The powder surface has residual soluble salt impurities (such as NaCl, LiCl, etc.), and the impurity element peaks were detected by EDS energy dispersive spectroscopy. The powder morphology is also affected by the adhesion of impurities, which cannot meet the application requirements of multifunctional ceramic materials.

[0036] In summary, this invention utilizes a chloride precursor and sodium borohydride in a low-temperature in-situ reaction within a eutectic LiCl-KCl molten salt system. In step S2, gentle grinding achieves molecular-level homogeneous mixing of the boron source and metal chloride, effectively avoiding sodium borohydride decomposition or localized overheating caused by high-energy ball milling, thus providing a highly homogeneous precursor environment for subsequent reactions. Step S3 introduces a mixed molten salt with a specific composition and undergoes prolonged mixing, serving not only as a reaction medium to lower the synthesis temperature but also as a nano-confined template to regulate the La content. 1-x M xB6 nucleation and growth kinetics. By combining complete dehydration in S1, controlled heating and inert protection in S4, and precise post-processing in S5, the entire process synergistically achieves the direct preparation of single-phase, cubical alkaline earth metal-doped lanthanum hexaboride nanopowder with uniform particle size distribution at temperatures far lower than those of traditional high-temperature solid-state methods and boron-carbothermic reduction methods.

[0037] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A low-temperature preparation method for alkaline earth metal-doped lanthanum hexaboride nanopowder, characterized in that: include, S1. Lanthanum chloride heptahydrate is mixed with alkaline earth metal chloride hydrate and heated at 200-300°C for 4-8 hours to remove water of crystallization, thereby obtaining a mixed powder of anhydrous lanthanum chloride and anhydrous alkaline earth metal chloride. S2. Add sodium borohydride to the mixed powder and grind it at a speed of 40-90 r / min for 0.3-1 hour to make the material uniformly mixed and obtain a boron-containing mixture; S3. Add a mixed molten salt composed of lithium chloride and potassium chloride to the boron-containing mixture, and continue mixing for 25 to 40 hours to obtain a reaction precursor mixture; S4. The reaction precursor mixture is placed in an inert atmosphere and heated to 900-1000°C at a heating rate of 10-20°C / min. The mixture is kept at this temperature for 0.5-1.5 hours. After the heating is completed, the temperature is reduced to 600°C at a cooling rate of 5°C / min and then cooled to room temperature in the furnace to obtain the reaction product. S5. Dissolve the reaction product in water, filter it, acid wash it, wash it with water until neutral, and then dry it at 80-90℃ to obtain alkaline earth metal-doped lanthanum hexaboride nanopowder.

2. The low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder as described in claim 1, characterized in that: The heating temperature in S1 is 240℃, the holding time is 6 hours, and the alkaline earth metal chloride hydrate is one or more of BaCl2·2H2O, SrCl2·6H2O, or CaCl2·2H2O.

3. The low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder as described in claim 1, characterized in that: The grinding speed in S2 is 60 r / min, the grinding time is 0.5 hours, the sodium borohydride is dried before being added, and the mixing process is carried out in a dry and inert environment.

4. The low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder as described in claim 1, characterized in that: In S3, the mass ratio of lithium chloride to potassium chloride in the mixed molten salt is 45:55, and the total mass ratio of the boron-containing mixture to the mixed molten salt is 1:

10.

5. The low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder as described in claim 1, characterized in that: The mixing time described in S3 is 28 hours, and uniform mixing is achieved using low-speed ball milling or mechanical stirring. The mixing process is carried out in a sealed container to prevent moisture absorption or oxidation. The rotation speed of the low-speed ball mill does not exceed 100 r / min to avoid decomposing sodium borohydride, and the material temperature is controlled below 30℃ during the mixing process to prevent side reactions.

6. The low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder as described in claim 1, characterized in that: The heating rate in S4 is 15℃ / min, the reaction temperature of alkaline earth metal Sr-doped LaB6 nanopowder is 900℃, the reaction temperature of alkaline earth metal Ba-doped or Sr / Ba co-doped LaB6 nanopowder is 1000℃, the holding time is 1 hour, the inert atmosphere is high-purity argon, and the gas flow rate is maintained at 40-60 mL / min.

7. The low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder as described in claim 1, characterized in that: The acid washing described in S5 uses a 36% hydrochloric acid solution, and the water washing continues until the pH of the filtrate is 6.8 to 7.2 to ensure complete removal of soluble salts.

8. The low-temperature preparation method of alkaline earth metal-doped lanthanum hexaboride nanopowder as described in claim 1, characterized in that: The mixed molten salt is dried at 200°C for 4 hours before use to remove adsorbed water, and both lithium chloride and potassium chloride are analytical grade or higher purity reagents.

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