A method for preparing nano strontium hexaboride by salt-assisted microwave solid-phase combustion

Abstract

The present invention belongs to the technical field of nanopowder preparation, and discloses a method for preparing nano strontium hexaboride by salt-assisted microwave solid-phase combustion. According to the molar ratio of 1:9.6:(0.5-2):(0.5-1), strontium nitrate, amorphous boron powder, urea, and strontium chloride are respectively weighed and mixed evenly; the mixture is placed in a microwave reactor, and a solid-phase combustion reaction is carried out under microwave irradiation; the solid-phase combustion product is taken out, washed, separated, and dried to obtain nano strontium hexaboride. The present invention utilizes strontium chloride to strongly absorb microwaves during the solid-phase combustion process, promotes the complete reaction of amorphous boron powder and refines the product crystal grains, and rapidly obtains nano strontium hexaboride with controllable particle size and uniform dispersion in one step. This method has good industrial application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanopowder preparation, and particularly 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 crystal system, and the stoichiometry is represented by RB 6 (R = Ca, Sr, Ba). Due to the strong covalent bond between B atoms, a stable spatial grid structure is formed, which endows alkaline earth metal hexaborides with many advantages such as high melting point, high thermal stability, neutron absorption, and large negative Seebeck coefficient. At present, alkaline earth metal hexaborides have been widely used in many aspects such as neutron shielding materials, new semiconductor materials, and boron-containing additives. And SrB 6 has a continuous light absorption effect in the ultraviolet to mid-infrared band and is expected to be applied in the fields of sunlight modulation and application.

[0003] In recent years, many scholars have proposed various methods for the controlled synthesis of strontium hexaboride. The boron carbide reduction method requires a long reaction time under an inert atmosphere protection and the product particles are large; the sodium borohydride reduction method and the metallothermal reduction need to be carried out under an inert atmosphere protection. 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 the 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 for rapid, efficient, green, energy-saving, and large-scale preparation has always been a major obstacle restricting 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 Temperatures [D]. Shandong University, 2010) disclose a method for synthesizing metal boride nanopowders by iodine-assisted magnesium co-reduction solid-phase reaction. Mix SrO, boron powder, iodine particles and Mg powder in a high-pressure autoclave, and use magnesium powder as fuel to react at 350 °C for 12 h to prepare SrB with uneven morphology and particle size 6 , and at the same time, the use of magnesium powder and high-pressure autoclave is relatively dangerous and not suitable for large-scale production.

[0005] Alessia Aprea (Solid State Sci. 2012, 14(11-12): 1587-1590) used SrCl 2 and MgB 2 as raw materials to react at above 800 °C under vacuum for 12 h to synthesize SrB 6 . This method not only has a long reaction time, but also introduces magnesium diboride as an impurity, resulting in low product purity.

[0006] In US Patent US2012177556A1, solid-phase / solution combustion synthesis is adopted. Alkaline earth metal nitrate, boron powder and fuel are mixed evenly, and then ignited in a muffle furnace to obtain alkaline earth metal hexaboride powder. James T. Cahill (Cryst. Growth Des. 2017, 17: 3450-3461) adopted solution combustion synthesis. Alkaline earth metal nitrate and carbohydrazide were dissolved in deionized water together, and then boron powder was added. After evaporation and drying, it was placed in a muffle furnace and ignited. After washing the combustion product, alkaline earth metal hexaboride powder was obtained. However, the particles of the above methods are about 500 nm and the particle size distribution is wide. The reaction raw material carbohydrazide is toxic and harmful to human health. Moreover, the reaction process is violent and dangerous, and the product is difficult to collect. Summary of the Invention

[0007] Aiming at the problems in the preparation method of strontium hexaboride nanoparticles in the prior art, such as the need for atmosphere protection, high energy consumption, long time consumption, especially large and uneven particles, etc., the present invention proposes a method for preparing strontium hexaboride nanoparticles by salt-assisted microwave solid-phase combustion. Strontium nitrate is used as the main strontium source and oxidant, amorphous boron powder is used as the boron source and reductant, urea is used as the fuel, and strontium chloride is used as a reaction assistant to participate in the reaction process to achieve the purpose of regulating the reaction path. This 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 realized through the following technical solutions. A method for preparing strontium hexaboride nanoparticles by salt-assisted microwave solid-phase combustion, comprising the following steps:

[0009] S1. Weigh strontium nitrate, amorphous boron powder, urea, and strontium chloride respectively according to the molar ratio of 1:9.6:(0.5-2):(0.5-1), and mix them evenly to obtain a mixture;

[0010] S2. Place the mixture in a microwave reactor and carry out a solid-phase combustion reaction under microwave irradiation;

[0011] S3. Take out the solid-phase combustion product, wash it, separate and dry it to obtain strontium hexaboride nanoparticles.

[0012] More specifically, in step S1, the strontium nitrate is introduced by any one or more of strontium nitrate and its hydrates.

[0013] More specifically, in step S1, the strontium chloride is introduced by any one or more of strontium chloride and its hydrates.

[0014] More specifically, in step S2, the microwave frequency of the microwave reactor is 915 MHz - 2450 MHz.

[0015] More specifically, in step S2, the time for the solid-phase combustion reaction under microwave irradiation is 0.5 - 10 minutes.

[0016] More specifically, in step S3, the solid-phase combustion product is first washed with hydrochloric acid and then washed with water.

[0017] Compared with the existing preparation methods of nano strontium hexaboride, the beneficial effects of the present invention are as follows: 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 control agent directly participating in the reaction, nano strontium hexaboride powder with uniformly dispersed particles is prepared in a very short time. Strontium chloride introduced into the reactants in the present invention strongly absorbs microwaves during the solid-phase microwave combustion process, and at the same time wraps the generated strontium oxide during the microwave combustion process to prevent its agglomeration 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 participates in the boriding reaction process, promoting a more efficient boriding reduction reaction in the system to obtain nano strontium hexaboride with uniform particles (above 900 °C). During the whole reaction process, strontium chloride not only shortens the reaction time, but also achieves the effects of controlling the microstructure and improving the conversion rate by wrapping strontium oxide and converting it into strontium oxide. This 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 industrial application prospects. Description of the Drawings

[0018] Figure 1 X-ray diffraction pattern of the comparative example.

[0019] Figure 2 Scanning electron microscope image of the comparative example.

[0020] Figure 3 X-ray diffraction pattern of Example 1.

[0021] Figure 4 Scanning electron microscope image of Example 1.

[0022] Figure 5 X-ray diffraction pattern of Example 2.

[0023] Figure 6 Scanning electron microscope image of Example 2. Detailed Embodiments

[0024] Next, the technical solutions of the present invention will be further described and illustrated in combination with the drawings in the comparative examples and embodiments of the present invention. The described embodiments are only part of the embodiments of the present invention and do not represent all the embodiments.

[0025] Comparative Example

[0026] 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.

[0027] S2. The uniformly mixed mixture was put into a microwave reactor and subjected to a solid-phase combustion reaction for 3 min under microwave irradiation.

[0028] S3. The microwave solid-phase combustion product was taken out, washed with dilute hydrochloric acid and ultrapure water, and dried to obtain ultrafine SrB 6 powder.

[0029] Example 1

[0030] 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.

[0031] S2. The mixture was placed in a microwave reactor and subjected to a solid-phase combustion reaction for 3 min under microwave irradiation at a microwave frequency of 915 MHz.

[0032] S3. The solid-phase combustion product was taken out, washed with dilute hydrochloric acid and ultrapure water, and dried to obtain nano SrB 6 powder.

[0033] Example 2

[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 6.666 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 a solid-phase combustion reaction for 10 min under microwave irradiation at a microwave frequency of 915 MHz.

[0036] S3. The solid-phase combustion product was taken out, washed with dilute hydrochloric acid and ultrapure water, and dried to obtain nano SrB 6 powder.

[0037] Example 3

[0038] 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.

[0039] S2. The mixture was placed in a microwave reactor and subjected to a solid-phase combustion reaction for 5 min under microwave irradiation at a microwave frequency of 1500 MHz.

[0040] S3. Take out the solid combustion product, wash it with dilute hydrochloric acid and ultrapure water, and dry it to obtain nano SrB 6 powder.

[0041] Example 4

[0042] S1. Based on the preparation of 12.2 g of strontium hexaboride, weigh 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 respectively to obtain a mixture.

[0043] S2. Place the mixture in a microwave reactor and carry out a solid-phase combustion reaction for 1 min under microwave irradiation with a microwave frequency of 2450 MHz.

[0044] S3. Take out the solid combustion product, wash it with dilute hydrochloric acid and ultrapure water, and dry it to obtain nano SrB 6 powder.

[0045] Figure 1 is the X-ray diffraction pattern of the comparative sample. As shown, the characteristic diffraction peaks of the sample match the international standard card of calcium hexaboride (PDF #87-0286), indicating that strontium hexaboride can be successfully synthesized under this system.

[0046] Figure 2 is the scanning electron microscope photo of the comparative sample. As shown, the microscopic morphology of the comparative sample presents as cubes with clear edges and corners, but the particle size distribution is uneven, with cubes of significantly different sizes, and the average particle size is 187.7 nm.

[0047] Figure 3 and Figure 4 are the X-ray diffraction pattern and scanning electron microscope photo of the sample of Example 1. As Figure 3 shown, the characteristic diffraction peaks of the sample of Example 1 are consistent with the international standard card of strontium hexaboride (PDF #87-0286), indicating that strontium hexaboride can be successfully synthesized under this system and there are no other impurity phases. As Figure 4 shown, the microscopic morphology of the sample of Example 1 presents as cubes, with uniform particle size and an average particle size of 74.8 nm, indicating that strontium chloride hexahydrate can effectively control the product morphology and refine the product particle size.

[0048] Figure 5 and Figure 6 are the X-ray diffraction pattern and scanning electron microscope photo of the sample of Example 2. As Figure 5 shown, the characteristic diffraction peaks of the sample of Example 2 are consistent with the international standard card of strontium hexaboride (PDF #87-0286), without any other impurity peaks, indicating that strontium hexaboride can be successfully synthesized under this system and there are no other impurity phases. As Figure 6As shown, the microscopic morphology of the sample in Example 2 presents as cubes with an average particle size of 86.9 nm, indicating that strontium chloride hexahydrate can effectively control the product morphology and refine the product particle size.

[0049] Table 1

[0050]

[0051] Table 1 shows the comparison of the average particle sizes of the samples in the comparative example, Example 1, and Example 2. Among them, 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 187.7 nm of the sample in the comparative example, the grains in Example 1 and Example 2 are more refined, indicating that the introduction of strontium chloride can refine the product grains.

[0052] Finally, 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 them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various 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. 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 hexahydrate 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. When the system temperature rises to above 800°C, strontium chloride will be converted into strontium oxide. When the temperature reaches above 900°C, strontium oxide participates in a boronization reduction reaction to obtain nano strontium hexaboride. 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 S2, the microwave frequency of the microwave reactor is 915 MHz-2450 MHz.

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 solid phase combustion reaction is carried out under microwave irradiation for 0.5-10 minutes.

5. 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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