A method for preparing nanometer lanthanum hexaboride by hydrated lanthanum chloride-assisted microwave solid-phase combustion

Abstract

The present invention discloses a method for preparing nano lanthanum hexaboride by hydrous lanthanum chloride-assisted microwave solid-phase combustion, which belongs to the technical field of nanopowder preparation. According to the molar ratio of 10:96:(50-75):1:6, hydrous lanthanum nitrate, amorphous boron powder, urea, ammonium nitrate and hydrous lanthanum chloride are weighed respectively and mixed. The mixture is placed in a microwave reactor and undergoes a solid-phase combustion reaction under microwave irradiation. After the solid-phase combustion reaction product is pickled, washed with water, separated and dried, nano lanthanum hexaboride with an average particle size of 50 nm is obtained. The present invention utilizes the strong absorption of microwave by hydrous lanthanum chloride during the solid-phase microwave combustion process to quickly generate lanthanum oxychloride to wrap the target product, promote the complete boride reaction and refine the product crystal grains, and quickly obtain highly dispersed and uniformly sized nano lanthanum hexaboride in one step. This 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 lanthanum hexaboride by microwave solid phase combustion assisted by hydrated lanthanum chloride. Background Art

[0002] LaB 6 It was found to be a stable compound with a cubic structure and a stoichiometric ratio of LaB 6 The sample appears purple, and the boron-rich LaB 9 Displayed in blue. Due to the strong bonding of B atoms, LaB 6 It has the characteristics of low work function (2.4eV~2.6eV), high melting point (2715℃), high thermal stability, low volatility and high mechanical strength. 6 It can be used in aerospace, electronic information, 5G medical, environmental protection and other fields. Due to its unique properties, LaB 6 The particles have a local surface plasmon resonance effect in the near-infrared region, and have extremely strong light absorption properties in the near-infrared region with a wavelength of 750 to 1000 nm. This makes the material have higher light absorption properties at a lower addition amount, and is expected to be used in building / automotive glass insulation.

[0003] In recent years, people have proposed a variety of controlled synthesis methods for ultrafine / nano lanthanum hexaboride. Since lanthanum hexaboride is a superhard material, the solid phase mechanochemical method requires long-term high-energy ball milling, which is carried out under high temperature and vacuum conditions, has a long reaction time, and the particles are micron-sized and have low purity. The metal thermal reduction and boron carbide reduction methods need to be carried out under inert gas protection, have a long reaction time, and have uneven particles. NaBH 4 RAPET with NaBH as a boron source 4 -Mg reduction method needs to be carried out under high pressure and the time is relatively short, but the reaction is difficult to control and prone to explosion; plasma method to prepare LaB 6 The conditions are simpler, but it requires 30KW of energy, long reaction time, high energy consumption, and uneven particles.

[0004] A Chinese patent (publication number CN111285380A) discloses a method for preparing and using a multi-rare earth doped boride and its heat insulation powder. The method of the invention is to dry a rare earth source and a boron source containing two rare earth elements and mix them with an additive, ball mill them, and then subject the mixture to microwave treatment. The temperature used in this method is lower than that of other methods, and the temperature is 500~1000℃. However, the additives used in this method, such as magnesium powder, aluminum powder, iron powder, metallic sodium, and metallic potassium, are all active metal elements, which are very likely to generate electric sparks during microwave heating, causing combustion and explosion, and are extremely dangerous.

[0005] Chinese Patent (Publication No. CN114873601B) discloses a preparation method for synthesizing ultrafine rare earth boride materials at low temperature using microwave heating. The method of this invention is to co-mill hydrated monovalent rare earth chloride, hydrated monovalent rare earth bromide, and magnesium diboride, then dry them, and then perform microwave treatment. This method requires a temperature of 100 - 180 °C in the microwave. It needs to be carried out under vacuum conditions, and the reaction energy is provided by microwaves. The microwave power is high and the time is long, resulting in extremely high energy consumption. At the same time, the boron source is magnesium diboride, which will introduce metal impurities into the system, and in addition, magnesium diboride is difficult to remove.

[0006] "Unique Preparation of Hexaboride Nanocubes: A First Example of Boride Formation by Combustion Synthesis" [J]. J. Am. Ceram. Soc., 93

[10] 3136–3141 (2010) proposed a method for synthesizing lanthanum hexaboride by solid-phase combustion synthesis. The method is to manually mix lanthanum nitrate, amorphous boron powder, and carbohydrazide, then heat and ignite them in a muffle furnace, and then perform washing and drying treatments to obtain lanthanum hexaboride. This method occurs in a short time and is relatively simple and convenient. However, the reaction during the synthesis process of this method is violent, the size of the obtained submicron-sized products is uneven and the agglomeration is serious, and there is residual amorphous boron powder in the products, which will greatly affect the optical properties of lanthanum hexaboride. In addition, the fuel used in this method is carbohydrazide, which is expensive and toxic, and is not conducive to industrial application.

[0007] The Chen Weifan team previously studied the preparation of nano lanthanum hexaboride by solution combustion method and submitted a Chinese patent application (Publication No. CN 113666382 A). The relevant research paper (Peng Rong, Combustion Synthesis and Performance Research of LaB 6 Nano Powder [D]. Nanchang University, 2023) proposed a method for synthesizing lanthanum hexaboride by solution combustion / solution microwave combustion. The method is to mix lanthanum nitrate (La(NO 3 ) 3 ·6H 2 O) and amorphous micron boron powder (B), the fuel carbohydrazide, and glycine evenly in an agate mortar, then put it into a muffle furnace or microwave oven to ignite. After combustion, first use hydrochloric acid and deionized water for centrifugal washing, and finally dry to obtain the powder. The synthesis process of this method is violent. In the most ideal state, the particle size is 100 - 200 nm. It can be clearly seen in the SEM image of nano lanthanum hexaboride that the products synthesized by this method contain unreacted amorphous boron powder and other by-products, which greatly affects the optical properties and the application of high-purity powder. Summary of the Invention

[0008] In view of the problems existing in the above-mentioned preparation method of ultrafine / nano lanthanum hexaboride, such as long time, high temperature and high pressure, the need for atmosphere protection, especially the difficulty in controlling the particle size at the nanoscale, incomplete reaction when amorphous boron powder is used as the boron source and the difficulty in removing the remaining amorphous boron powder, etc., the present invention proposes a method for preparing nano lanthanum hexaboride by microwave solid-phase combustion assisted by hydrated lanthanum chloride.

[0009] The present invention uses hydrated lanthanum nitrate as the lanthanum source and oxidant, amorphous boron powder as the boron source, urea as the fuel, ammonium nitrate as the additional oxidant, and the addition of hydrated lanthanum chloride provides an excessive amount of lanthanum source to promote complete reaction, and at the same time inhibits the agglomeration and sintering of the intermediate nano lanthanum oxide during the reaction process. The specific process is as follows: Due to the strong microwave absorption property of hydrated lanthanum chloride, it rapidly heats up under microwave irradiation, first loses part of its crystal water and then forms lanthanum oxychloride (LaCl 3 ·7H 2 O = LaCl 3 +7H 2 O, LaCl 3 +2H 2 O = La(OH) 2 Cl + 2HCl, La(OH) 2 Cl = LaOCl + H 2 O) at about 400 - 450 °C. It in-situ wraps the surface of the nano lanthanum oxide formed by low-temperature combustion, preventing its agglomeration and sintering. At about 800 °C, lanthanum oxychloride can be completely converted into lanthanum oxide (2LaOCl + 1 / 2O 2 = La 2 O 3 + Cl 2 ↑). Finally, uniform and fine intermediate lanthanum oxide particles are obtained, which further promotes the boron thermal and boride reactions initiated by combustion heat to tend to be complete (La 2 O 3 + 7B = LaB 6 + LaBO 3 ), solving the problems of large particle size in previous combustion synthesis, incomplete reaction of amorphous boron powder and difficulty in removing it. (According to the literature: Yao Chao, Preparation of Nano Lanthanum Oxide [J]. Journal of Chemical Engineering of Chinese Universities, (2003)06 - 0685 - 04, it is shown that the thermal decomposition temperature of lanthanum chloride is relatively low, and all crystal water will be lost at about 400 - 450 °C and it will be completely converted into lanthanum oxychloride). This method is carried out under air, without the need for a reducing atmosphere, and has the advantages of one-step phase formation, mild reaction, controllable particle size, short time consumption, high yield and simple equipment. The overall reaction equation is as follows:

[0010] 2La(NO 3 ) 3 ·7H 2 O + 5CH4 N 2 O + 7B = LaB 6 + LaBO 3 + 22H 2 O↑ + 5CO 2 ↑ + 8N 2 ↑;

[0011] 2LaCl 3 ·7H 2 O + 1 / 2O 2 + 7B = LaB 6 + LaBO 3 + Cl 2 ↑ + 4HCl + 12H 2 O。

[0012] The present invention is realized by the following technical solutions: A method for preparing nano lanthanum hexaboride by microwave solid-phase combustion assisted by hydrated lanthanum chloride, comprising the following steps:

[0013] S1. Weigh hydrated lanthanum nitrate, amorphous boron powder, urea, ammonium nitrate and hydrated lanthanum chloride respectively according to the molar ratio of 10:96:(50 - 75):1:6, and mix them evenly to obtain a mixture;

[0014] S2. Put the mixture into a microwave reactor and carry out a solid-phase combustion reaction under microwave conditions;

[0015] S3. Take out the solid-phase combustion reaction product, after pickling with acid and washing with water, separate and dry it to obtain nano lanthanum hexaboride.

[0016] Specifically, in step S2, the solid-phase combustion reaction time is 1 - 10 minutes.

[0017] Specifically, in step S2, the microwave frequency of the microwave reactor is 915 MHz - 2450 MHz.

[0018] Specifically, the acid used for pickling in step S3 is hydrochloric acid or sulfuric acid.

[0019] Specifically, the average particle size of the obtained nano lanthanum hexaboride is about 50 nm.

[0020] Compared with the existing methods for preparing lanthanum hexaboride, the beneficial effects of the present invention are as follows: Using hydrated lanthanum nitrate as the lanthanum source and oxidant, amorphous boron powder as the boron source, urea as the fuel, ammonium nitrate as the additional oxidant, and hydrated lanthanum chloride as the reaction promoter and morphology control agent and providing an additional lanthanum source, nano lanthanum hexaboride powder with uniformly dispersed particles is successfully prepared in an extremely short time. In the present invention, the total amount of lanthanum chloride introduced into the reactants strongly absorbs microwaves during the solid-phase microwave combustion process, promoting the complete reaction of amorphous boron powder and refining the product crystal grains. At the same time, during the microwave combustion process, it is quickly converted into nano lanthanum oxychloride to wrap lanthanum oxide and refine the crystal grains. The additional lanthanum oxychloride is converted into lanthanum oxide during the subsequent combustion, and then undergoes an efficient boride reaction with boron to obtain uniform nano lanthanum hexaboride, solving the problem of residual amorphous boron powder in the product. This method is synthesized in air without the need for a reducing atmosphere, and has the advantages of one-step phase formation, mild and controllable reaction, controllable particle size, short time consumption, high yield, and simple equipment, and has broad industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is the X-ray diffraction pattern of the sample of the comparative example.

[0022] Figure 2 It is the scanning electron microscope image of the sample of the comparative example.

[0023] Figure 3 It is the X-ray diffraction pattern of the sample of Example 1.

[0024] Figure 4 It is the scanning electron microscope image of the sample of Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0025] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the comparative examples and embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0026] A method for preparing nano lanthanum hexaboride by hydrated lanthanum chloride-assisted microwave solid-phase combustion includes the following steps:

[0027] S1. Weigh hydrated lanthanum nitrate, amorphous boron powder, urea, ammonium nitrate, and hydrated lanthanum chloride respectively according to a molar ratio of 10:96:(50 - 75):1:6, and mix them evenly to obtain a mixture;

[0028] S2. Put the mixture into a microwave reactor and carry out a solid-phase combustion reaction for 1 - 10 minutes under microwave conditions with a microwave frequency of 915 MHz - 2450 MHz;

[0029] S3. Take out the solid-phase combustion reaction product, wash it with acid and then with water, separate and dry it to obtain lanthanum hexaboride nanoparticles with an average particle size of about 50 nm. The acid used for acid washing is hydrochloric acid or sulfuric acid.

[0030] Comparative Example

[0031] S1. Based on the preparation of 22 g of lanthanum hexaboride, weigh out 43.3 g of lanthanum nitrate hexahydrate, 6.49 g of amorphous boron powder, 4.5 g of urea, and 0.8 g of ammonium nitrate respectively, and mix them evenly to obtain a mixture.

[0032] S2. Put the mixture into a microwave reactor and react for 5 minutes under microwave conditions with a microwave frequency of 915 MHz.

[0033] S3. Take out the solid-phase combustion reaction product, clean it with dilute hydrochloric acid and deionized water, centrifuge and dry it to obtain the comparative example sample.

[0034] Example 1

[0035] S1. Based on the preparation of 35 g of the product, accurately weigh out 43.3 g of lanthanum nitrate hexahydrate, 10.4 g of amorphous boron powder, 4.5 g of urea, 0.8 g of ammonium nitrate, and 22.3 g of lanthanum chloride heptahydrate, and mix them evenly to obtain a mixture.

[0036] S2. Put the mixture into a microwave reactor and carry out a solid-phase combustion reaction for 5 minutes under microwave conditions with a microwave frequency of 915 MHz.

[0037] S3. Take out the solid-phase combustion reaction product, clean it with dilute hydrochloric acid and deionized water, centrifuge and dry it to obtain the Example 1 sample.

[0038] Example 2

[0039] S1. Based on the preparation of 35 g of the product, accurately weigh out 43.3 g of lanthanum nitrate hexahydrate, 10.4 g of amorphous boron powder, 7.5 g of urea, 0.8 g of ammonium nitrate, and 22.3 g of lanthanum chloride heptahydrate, and mix them evenly to obtain a mixture.

[0040] S2. Put the mixture into a microwave reactor and carry out a solid-phase combustion reaction for 1 minute under microwave conditions with a microwave frequency of 2450 MHz.

[0041] S3. Take out the solid-phase combustion reaction product, clean it with dilute hydrochloric acid and deionized water, centrifuge and dry it to obtain the Example 2 sample.

[0042] Example 3

[0043] S1. Based on a production amount of 35 g of the prepared product, accurately weigh 43.3 g of lanthanum nitrate hexahydrate, 10.4 g of amorphous boron powder, 7.5 g of urea, 0.8 g of ammonium nitrate, and 22.3 g of lanthanum chloride heptahydrate, and mix them evenly to obtain a mixture.

[0044] S2. Put the mixture into a microwave reactor and carry out a solid-phase combustion reaction for 10 minutes under microwave conditions with a microwave frequency of 915 MHz.

[0045] S3. Take out the solid-phase combustion reaction product, clean it with dilute sulfuric acid and deionized water, perform centrifugal separation, and dry it to obtain the sample of Example 3.

[0046] Figure 1 The X-ray diffraction pattern of the sample for the comparative example. As shown in the figure, the characteristic diffraction peaks of the sample are the same as those of lanthanum hexaboride in PDF#73-1699.

[0047] Figure 2 The scanning electron microscope photograph of the sample for the comparative example. As shown in the figure, the sample particles show an obvious cubic structure, the local surface is smooth, the particle size is between 500 and 2000 nm, and there is flocculent amorphous boron powder nearby, indicating that the amorphous boron powder in this comparative example was not completely reacted and there is residue.

[0048] Figure 3 and Figure 4 are the X-ray diffraction pattern and scanning electron microscope image of the sample of Example 1 respectively. As Figure 3 shown, the characteristic diffraction peaks of the sample of Example 1 are the same as those of lanthanum hexaboride in PDF#73-1699. Compared with the sample of the comparative example, the diffraction peaks are significantly broadened, indicating that the crystal grains are significantly reduced; as Figure 4 shown, the particles of the sample of Example 1 are uniform, spherical-like, with an average particle size of 50 nm, and no flocculent amorphous boron powder is found.

[0049] Table 1

[0050]

[0051] As shown in Table 1, for the sample of the comparative example, based on the EDS energy spectrum surface analysis, the atomic ratio of La and B is 1:8.43, deviating from 1:6 of lanthanum hexaboride, indicating that the amorphous boron powder was not completely reacted and there is residue. The color of the sample is light brown, which can be verified from the Figure 2 presence of flocculent amorphous boron powder. The atomic ratio of La and B of the sample of Example 1 is 1:5.99, which is very close to 1:6 of lanthanum hexaboride, indicating that the amorphous boron powder has almost completely reacted, which can be verified from the Figure 4 absence of flocculent amorphous boron powder.

[0052] As described above, it is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes should be covered within the scope of the present invention.

Claims

1. A method for preparing nano-lanthanum hexaboride by microwave solid phase combustion assisted by hydrated lanthanum chloride, characterized in that: The steps include: S1. According to the molar ratio of 10:96:(50-75):1:6, respectively weigh hydrated lanthanum nitrate, amorphous boron powder, urea, ammonium nitrate and hydrated lanthanum chloride, mix them evenly to obtain a mixture; S2, placing the mixture into a microwave reactor, wherein the microwave frequency of the microwave reactor is 915 MHz -2450 MHz, and performing a solid phase combustion reaction under microwave conditions for 1 to 10 minutes; S3. Take out the solid phase combustion reaction product, wash it with acid and water, separate it and dry it to obtain nano lanthanum hexaboride.

2. The method for preparing nano-lanthanum hexaboride by microwave solid phase combustion assisted by hydrated lanthanum chloride according to claim 1, characterized in that: The acid used for pickling in step S3 is hydrochloric acid or sulfuric acid.

3. The method for preparing nano lanthanum hexaboride by microwave solid phase combustion assisted by hydrated lanthanum chloride according to claim 1, characterized in that: The average particle size of the obtained nano lanthanum hexaboride is 50 nm.

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