Method for preparing nano strontium hexaboride through salt-assisted microwave solid-phase combustion

Abstract

The invention belongs to the technical field of nano powder preparation, and discloses a method for preparing nano strontium hexaboride through salt-assisted microwave solid-phase combustion. The preparation method comprises the following steps: respectively weighing strontium nitrate, amorphous boron powder, urea and strontium chloride according to a molar ratio of 1: 9.6: (0.5-2): (0.5-1), and uniformly mixing; putting the mixture into a microwave reactor, and carrying out solid-phase combustion reaction under microwave irradiation; and taking out a solid-phase combustion product, washing, separating and drying to obtain the nano strontium hexaboride. Strontium chloride strongly absorbs microwaves in the solid-phase combustion process, complete reaction of amorphous boron powder is promoted, product grain refinement is promoted, nano strontium hexaboride with controllable grain size and uniform dispersion is rapidly obtained in one step, and the method has good industrial application prospects.

Description

Technical Field

[0001] The invention belongs to the technical field of nano powder preparation, and in particular relates to a method for preparing nano strontium hexaboride by salt-assisted microwave solid phase combustion. Background Art

[0002] Alkaline earth metal hexaborides have a CsCl-type face-centered cubic crystal structure, belonging to the simple cubic system, and the stoichiometry is represented by RB6 (R=Ca, Sr, Ba). Due to the strong covalent bonds between B atoms, they form a stable spatial grid structure, which makes alkaline earth metal hexaborides have many advantages such as high melting point, high thermal stability, neutron absorption, and a large negative Seebeck coefficient. At present, alkaline earth metal hexaborides are widely used in many aspects such as neutron shielding materials, new semiconductor materials, and boron-containing additives. SrB6 has continuous light absorption in the ultraviolet to mid-infrared band, and is expected to be used in the field of solar light modulation and application.

[0003] In recent years, many scholars have proposed a variety of controlled synthesis methods for strontium hexaboride. The boron carbide reduction method requires a long reaction time under the protection of an inert atmosphere and the product particles are large; the sodium borohydride reduction method and metal thermal reduction need to be carried out under the protection of an inert atmosphere. Although the reaction time is short, the reaction process is violent and difficult to control, and the raw materials are dangerous; the plasma method and molten salt electrochemical method have simple processes, but there are problems such as long reaction time, high energy consumption, and uneven particles. Therefore, the key technology of rapid, efficient, green and energy-saving, and large-scale preparation has always been a major obstacle to its application and development.

[0004] Chinese patent CN201010178836.2 and related research papers (Wang Liancheng, Preparation of Boron Nitride, Boron Carbide and Rare Earth (or Alkaline Earth) Metal Boride Nanomaterials at Mild Temperature [D]. Shandong University, 2010) disclose a method for synthesizing metal boride nanopowders by iodine-assisted magnesium co-reduction solid phase reaction. SrO, boron powder, iodine particles and Mg powder are mixed in an autoclave, and magnesium powder is used as fuel to react at 350°C for 12 hours to prepare SrB6 with uneven morphology and particle size. At the same time, the use of magnesium powder and autoclave is dangerous and not suitable for large-scale production.

[0005] Alessia Aprea (Solid State Sci. 2012, 14(11-12): 1587-1590) synthesized SrB6 using SrCl2 and MgB2 as raw materials at 800℃ or above in vacuum for 12h. This method not only takes a long time to react, but also introduces magnesium diboride as an impurity, resulting in low product purity.

[0006] US Patent US2012177556A1 adopts solid phase / solution combustion synthesis, using alkaline earth metal nitrate, boron powder and fuel to mix evenly, and then put it into a muffle furnace to ignite to obtain alkaline earth metal hexaboride powder. James T. Cahill (Cryst. Growth Des. 2017, 17: 3450-3461) adopts solution combustion synthesis, using alkaline earth metal nitrate and carbohydrazide to dissolve in deionized water, then adding boron powder, after evaporation and drying, put it into a muffle furnace to ignite, and wash the combustion product to obtain alkaline earth metal hexaboride powder. However, the above method has particles of about 500nm and a wide particle size distribution. The reaction raw material carbohydrazide is toxic and harmful to human health; and the reaction process is violent and dangerous, and the product is difficult to collect. Summary of the invention

[0007] In view of the problems existing in the preparation method of nano strontium hexaboride in the prior art, such as the need for atmosphere protection, high energy consumption, long time consumption, and especially large and uneven particles, the present invention proposes a method for preparing nano strontium hexaboride by salt-assisted microwave solid phase combustion, using strontium nitrate as the main strontium source and oxidant, amorphous boron powder as the boron source and reducing agent, urea as the fuel, and strontium chloride as the reaction aid to participate in the reaction process, so as to achieve the purpose of regulating the reaction path. The method is carried out in an air environment and has the advantages of low energy consumption, one-step phase formation, fast reaction, and controllable particle size.

[0008] The present invention is achieved by the following technical solutions: A method for preparing nano strontium hexaboride by salt-assisted microwave solid phase combustion, comprising the following steps: S1. According to the molar ratio of 1:9.6:(0.5-2):(0.5-1), weigh strontium nitrate, amorphous boron powder, urea, and strontium chloride respectively, mix them evenly, and obtain a mixture; S2, placing the mixture in a microwave reactor, and performing a solid phase combustion reaction under microwave irradiation; S3. Take out the solid phase combustion product, wash it, separate it and dry it to obtain nano strontium hexaboride.

[0009] More specifically, in step S1, the strontium nitrate is introduced through any one or more of strontium nitrate and its hydrate.

[0010] More specifically, in step S1, the strontium chloride is introduced through any one or more of strontium chloride and its hydrate.

[0011] More specifically, in step S2, the microwave frequency of the microwave reactor is 915 MHz-2450 MHz.

[0012] More specifically, in step S2, the solid phase combustion reaction is carried out under microwave irradiation for 0.5-10 minutes.

[0013] More specifically, in step S3, the solid phase combustion product is first washed with hydrochloric acid and then washed with water.

[0014] Compared with the existing preparation method of nano strontium hexaboride, the present invention has the beneficial effects of: using strontium nitrate as the main strontium source, amorphous boron powder as the boron source, urea as the fuel, and strontium chloride as the reaction promoter and morphology controller to directly participate in the reaction, and preparing nano strontium hexaboride powder with uniform particle dispersion in a very short time. The strontium chloride introduced into the reactant of the present invention strongly absorbs microwaves during the solid-phase microwave combustion process, and at the same time encapsulates the generated strontium oxide during the microwave combustion process to prevent it from agglomerating and sintering. After the system temperature rises (above 800°C), strontium chloride will be converted into strontium oxide, and then all the generated strontium oxide will participate in the boration reaction process, prompting the system to undergo a more efficient boration reduction reaction to obtain nano strontium hexaboride with uniform particles (above 900°C). In the entire reaction process, strontium chloride not only shortens the reaction time, but also achieves the effect of controlling the microstructure and improving the conversion rate by encapsulating strontium oxide and converting it into strontium oxide. The method is carried out in an air environment and has the advantages of low energy consumption, one-step phase formation, fast reaction, controllable particle size, simple equipment, etc., and has broad prospects for industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is the X-ray diffraction pattern of comparative example.

[0016] Figure 2 The scanning electron microscope image is a comparative example.

[0017] Figure 3 This is the X-ray diffraction pattern of Example 1.

[0018] Figure 4 This is a scanning electron microscope image of Example 1.

[0019] Figure 5 This is the X-ray diffraction pattern of Example 2.

[0020] Figure 6 This is a scanning electron microscope image of Example 2. DETAILED DESCRIPTION

[0021] The technical solution of the present invention will be further illustrated and described below in conjunction with the accompanying drawings in the comparative examples and embodiments of the present invention. The described embodiments are only some embodiments of the present invention and do not represent all embodiments. Comparative Example

[0022] S1. Based on the preparation of 12.2 g of strontium hexaboride, 16.93 g of strontium nitrate, 3.459 g of amorphous boron powder and 4.805 g of urea were weighed respectively to obtain a mixture.

[0023] S2. The uniformly mixed mixture is placed in a microwave reactor and subjected to solid phase combustion reaction for 3 minutes under microwave irradiation.

[0024] S3. Take out the microwave solid phase combustion product, wash it with dilute hydrochloric acid and ultrapure water, and then dry it to obtain ultrafine SrB6 powder. Example 1

[0025] S1. Based on the preparation of 12.2 g of strontium hexaboride, 10.581 g of strontium nitrate, 5.189 g of amorphous boron powder, 4.805 g of urea and 10.665 g of strontium chloride hexahydrate were weighed respectively to obtain a mixture.

[0026] S2. The mixture was placed in a microwave reactor and subjected to solid phase combustion reaction for 3 min under microwave irradiation at a microwave frequency of 915 MHz.

[0027] S3. Take out the solid phase combustion product, wash it with dilute hydrochloric acid and ultrapure water, and then dry it to obtain nano SrB6 powder. Example 2

[0028] S1. Based on the preparation of 12.2 g of strontium hexaboride, 10.581 g of strontium nitrate, 5.189 g of amorphous boron powder, 5.405 g of urea and 6.666 g of strontium chloride hexahydrate were weighed respectively to obtain a mixture.

[0029] S2. The mixture was placed in a microwave reactor and subjected to solid phase combustion reaction for 10 min under microwave irradiation at a microwave frequency of 915 MHz.

[0030] S3. Take out the solid phase combustion product, wash it with dilute hydrochloric acid and ultrapure water, and then dry it to obtain nano SrB6 powder. Example 3

[0031] S1. Based on the preparation of 12.2 g of strontium hexaboride, 10.581 g of strontium nitrate, 5.189 g of amorphous boron powder, 4.805 g of urea and 6.666 g of strontium chloride hexahydrate were weighed respectively to obtain a mixture.

[0032] S2. The mixture was placed in a microwave reactor and subjected to solid phase combustion reaction for 5 minutes under microwave irradiation at a microwave frequency of 1500 MHz.

[0033] S3. Take out the solid phase combustion product, wash it with dilute hydrochloric acid and ultrapure water, and then dry it to obtain nano SrB6 powder. Example 4

[0034] S1. Based on the preparation of 12.2 g of strontium hexaboride, 10.581 g of strontium nitrate, 5.189 g of amorphous boron powder, 5.405 g of urea and 13.331 g of strontium chloride hexahydrate were weighed respectively to obtain a mixture.

[0035] S2. The mixture was placed in a microwave reactor and subjected to solid phase combustion reaction for 1 min under microwave irradiation at a microwave frequency of 2450 MHz.

[0036] S3. Take out the solid phase combustion product, wash it with dilute hydrochloric acid and ultrapure water, and then dry it to obtain nano SrB6 powder.

[0037] Figure 1 The X-ray diffraction pattern of the comparative sample is shown in the figure. As shown in the figure, the characteristic diffraction peaks of the sample are consistent with the international standard card of calcium hexaboride (PDF #87-0286), indicating that strontium hexaboride can be successfully synthesized under this system.

[0038] Figure 2 The scanning electron microscope photo of the comparative sample is shown in the figure. As shown in the figure, the microscopic morphology of the comparative sample is a cube with clear edges and corners, but the particle size distribution is uneven, and there are cubes with large size differences. The average particle size is 187.7nm.

[0039] Figure 3 and Figure 4 The X-ray diffraction pattern and scanning electron microscope photo of the sample in Example 1 are shown in FIG. Figure 3 As shown, the characteristic diffraction peaks of the sample in Example 1 are consistent with the international standard card of strontium hexaboride (PDF #87-0286), indicating that the system can successfully synthesize strontium hexaboride without the presence of other impurities. Figure 4 As shown, the microscopic morphology of the sample in Example 1 is cubic, the particle size is uniform and the average particle size is 74.8 nm, indicating that strontium chloride hexahydrate can effectively regulate the product morphology and refine the product particle size.

[0040] Figure 5 and Figure 6 The X-ray diffraction pattern and scanning electron microscope photo of the sample in Example 2 are shown in FIG. Figure 5 As shown, the characteristic diffraction peaks of the sample in Example 2 are consistent with the international standard card of strontium hexaboride (PDF #87-0286), and there are no other impurity peaks, indicating that the system can successfully synthesize strontium hexaboride without the presence of other impurities. Figure 6 As shown, the microscopic morphology of the sample in Example 2 is cubic and the average particle size is 86.9 nm, indicating that strontium chloride hexahydrate can effectively regulate the product morphology and refine the product particle size.

[0041] Table 1

[0042] Table 1 is a comparison of the average particle sizes of the samples in the comparative example, example 1, and example 2. The average particle size of the sample in example 1 is 74.8 nm, and the average particle size of the sample in example 2 is 86.9 nm. Compared with the average particle size of the comparative example sample of 187.7 nm, the grains in examples 1 and 2 are more refined, indicating that the introduction of strontium chloride can refine the product grains.

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing nano-strontium hexaboride by salt-assisted microwave solid phase combustion, characterized in that: The following steps are involved: S1. Weigh strontium nitrate, amorphous boron powder, urea, and strontium chloride respectively according to a molar ratio of 1:9.6:(0.5-2):(0.5-1), and mix them evenly to obtain a mixture; S2, placing the mixture in a microwave reactor, and performing a solid phase combustion reaction under microwave irradiation; S3. Take out the solid phase combustion product, wash it, separate it and dry it to obtain nano strontium hexaboride.

2. The method for preparing nano-strontium hexaboride by salt-assisted microwave solid phase combustion according to claim 1, characterized in that: In step S1, the strontium nitrate is introduced through any one or more of strontium nitrate and its hydrate.

3. The method for preparing nano-strontium hexaboride by salt-assisted microwave solid phase combustion according to claim 1, characterized in that: In step S1, the strontium chloride is introduced through any one or more of strontium chloride and its hydrate.

4. The method for preparing nano-strontium hexaboride by salt-assisted microwave solid phase combustion according to claim 1, characterized in that: In step S2, the microwave frequency of the microwave reactor is 915 MHz-2450 MHz.

5. The method for preparing nano-strontium hexaboride by salt-assisted microwave solid phase combustion according to claim 1, characterized in that: In step S2, the solid phase combustion reaction is carried out under microwave irradiation for 0.5-10 minutes.

6. The method for preparing nano-strontium hexaboride by salt-assisted microwave solid phase combustion according to claim 1, characterized in that: In step S3, the solid phase combustion product is first washed with hydrochloric acid and then washed with water.

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