A micro-nano rare earth alloy coated with an in-situ antioxidant layer and a preparation method thereof

Abstract

The present invention provides a micro-nano rare earth alloy coated with an in-situ antioxidant layer and a preparation method thereof. The method is to mix a water-soluble samarium salt, a water-soluble transition metal salt, an SiO2 donor, a substance that is soluble and has reducibility at high temperature or generates a reducing substance through high-temperature decomposition, and dissolve them in water to form a spray precursor solution. Then, spherical particles in which Sm2O3, transition metal, and SiO2 are mutually embedded are prepared by ultrasonic spray pyrolysis method. Subsequently, through calcium reduction diffusion and in-situ chemical reaction, a samarium-containing metal alloy coated with calcium silicate with a size of 0.1 - 6 μm is obtained. The present invention directly obtains a samarium-containing metal alloy powder with a micro-nano size and in-situ CaSiO3 coating without ball milling and without secondary surface treatment. The morphology and size of the samarium-containing metal alloy are controllable, and the antioxidant performance is improved. The preparation process of this method is simpler, more economical, and convenient for industrialization, which is beneficial to the preparation of high-performance samarium-containing rare earth alloy magnetic powder.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of rare earth alloy magnetic materials, and relates to a micro-nano rare earth alloy coated with an in-situ antioxidant layer and a preparation method thereof. Background Art

[0002] Rare earth alloys composed of rare earth elements and transition elements (such as iron, nickel, cobalt) include materials such as neodymium iron boron, samarium iron nitrogen, and samarium cobalt. They all have excellent intrinsic magnetic properties and are thus widely studied and applied. The properties of rare earth alloy powders determine the properties of rare earth alloy magnets. Usually, the particle size of the powder determines the magnitude of its coercivity. Research shows that the closer the magnetic powder is to the single-domain size (~0.1 - 0.4 μm), the stronger its coercivity. Therefore, when preparing rare earth alloy powders, the magnetic powder is refined as much as possible to make it close to the single-domain size. However, since rare earth alloys are all very reactive, the activity of the powder will increase exponentially after refinement. Therefore, how to avoid its oxidation is the key to improving the performance of the refined magnetic powder. In addition, the shape of the magnetic powder plays a key role in the preparation of high-orientation and high-remnant magnetization magnets. Near-spherical magnetic powder is beneficial to reducing friction, improving the orientation degree and density during the orientation pressing process.

[0003] Currently, the commonly used methods for preparing rare earth alloy powders include rapid quenching method, reduction diffusion method, spray pyrolysis method, etc. However, rare earth alloy powders have high surface activity and are extremely easy to oxidize during the washing and drying processes. None of the above methods mention the antioxidant treatment process; and in some reports, the antioxidant treatment is to perform a separate surface modification or antioxidant treatment process on the premise that the magnetic powder has been prepared to improve the antioxidant ability. For example, it is reported that the surface of Sm2Fe 17 N x permanent magnet powder was surface-coated with the silane coupling agent KH550. The coupling agent was adsorbed on the magnetic powder in a chemical bond manner to form a network film, effectively preventing the magnetic powder from contacting oxygen and water in the environment, thereby improving the high-temperature antioxidant ability of the magnetic powder. Patent CN114156033A discloses a method of mixing Sm powder, Fe powder, an anionic surfactant, a phosphating solution, and an ammonia-containing liquid, and preparing samarium iron nitride magnetic powder with a dense phosphating film on the surface by high-energy ball milling. In addition, patent CN1745440A proposes organic pigments such as indanthrene-based or phthalocyanine-based pigments, and inorganic pigments such as carbon black. The surface of the magnetic powder is a pigment attachment layer, which can not only improve the high-temperature antioxidant property of the magnetic powder but also enhance the fluidity of the magnetic powder.

[0004] The above methods all perform separate surface treatment on the magnetic powder after its synthesis. This additional treatment process not only increases the complexity of the production process, but also the transfer between processes involves an increase in the total process time and the replacement of different equipment, which will undoubtedly lead to the possibility of further oxidation of the highly active magnetic powder. Moreover, the commonly used magnetic powder surface treatment processes are carried out in solutions, such as phosphating or silanization, which may cause corrosion to the highly active rare earth alloy powder. After the ball milling and refinement process of rare earth alloy magnetic powder, the particle size distribution is wide and the shape is irregular; during reduction diffusion, the magnetic powder sinters and grows excessively, making it difficult to control the size; the prepared magnetic powder has low antioxidant performance and requires subsequent separate surface treatment processes to improve its antioxidant property, etc.;

[0005] In addition, in 2018, the inventor proposed CN108274016A, which uses a mixed aqueous solution of samarium salt and iron salt as the spray precursor solution, and prepares spherical samarium-iron composite oxide by ultrasonic spray pyrolysis method, and then obtains samarium-iron alloy by pre-reducing with hydrogen and then calcium-reducing or directly calcium-reducing. In order to further solve the problem of sintering and growth of powder during high-temperature calcium reduction, CN111014714A was then proposed in 2019, using samarium salt, iron salt, high-melting-point salt and substances that are soluble and have reducibility at high temperature or produce reducing substances through high-temperature decomposition as the spray precursor solution, and preparing spherical particles with an intercalated structure of Sm2O3 and α-Fe with a size of 0.1 - 2 μm by ultrasonic spray pyrolysis method, and then obtaining samarium-iron alloy with a size of 0.3 - 3 microns and near-spherical particles by calcium reduction diffusion. However, due to the fine and highly active powder prepared by the above two technologies, it is extremely easy to oxidize and reduce the magnetic properties of the powder. Similarly, subsequent surface treatment technologies such as phosphating are needed to solve the antioxidant problem.

[0006] In summary, there is an urgent need to propose a samarium-containing rare earth alloy powder with high antioxidant property, a near-spherical structure, and a micro-nano size, as well as a preparation method thereof. Summary of the Invention

[0007] The first object of the present invention is to propose a method for micro-nano rare earth alloy coated with an in-situ antioxidant layer in view of the deficiencies of the prior art. The method synthesizes spherical particles with an intercalated structure of samarium oxide Sm2O3, metal M and silicon dioxide SiO2 by ultrasonic spray pyrolysis, and then generates a near-spherical micro-nano size core-shell structure rare earth alloy powder with high antioxidant property with a core of Sm-M (M is iron or cobalt) alloy and a shell of Ca-Si-O through reduction diffusion and in-situ reaction coating. Without subsequent crushing, the particle size of the alloy powder can be controlled to be 0.1 - 5 microns, and during the preparation of the samarium-containing rare earth alloy powder, a calcium silicate nano-layer can be in-situ coated on the powder surface synchronously, and good antioxidant performance can be achieved without subsequent secondary surface treatment.

[0008] The method of the present invention adopts the following technical solutions:

[0009] Step 1: Weigh a certain amount of water-soluble samarium salt, water-soluble transition metal salt, SiO2 donor, and a substance that is water-soluble and has reducibility at high temperature or produces a reducing substance through high-temperature decomposition, and mix and dissolve them in water to form an atomized precursor solution; wherein the SiO2 donor is hydrophilic nano-silica SiO2 or a water-soluble silicate that can produce silica through high-temperature hydrolysis oxidation.

[0010] Step 2: Place the atomized precursor solution obtained in Step 1 in an ultrasonic atomizer. Using the principle of ultrasonic spraying, turn the precursor solution into an atomized state, and then under the action of a carrier gas, transport it to a tubular furnace with a temperature gradient at a certain flow rate.

[0011] Step 3: The atomized precursor solution undergoes evaporation and dehydration, salt thermal decomposition, and oxidation reactions in the tubular furnace, decomposes to produce reducing gases and undergoes reduction reactions. Finally, spherical particles in which samarium oxide, transition metal M, and silica are embedded in each other are formed and are transported by the carrier gas to a collector with a magnetic field attached to the end of the tubular furnace.

[0012] Step 4: Mix the collected spherical particles evenly with an excessive amount of calcium grains and place them in a sealed pure iron crucible. Then place the pure iron crucible in a high-temperature atmosphere reaction furnace. The high-temperature atmosphere reaction furnace is first evacuated and then filled with Ar at room temperature, repeated several times, and then heated to 850 - 1100 °C for reduction diffusion and in-situ coating treatment for 1 - 5 h. At this time, the reduction diffusion side reaction product CaO reacts with the SiO2 migrated to the surface to form calcium silicate (CaSiO3), and at the same time, calcium silicate uniformly coats the surface of the samarium-containing metal alloy; the mass ratio of calcium grains to samarium oxide in the spherical particles is 1 - 3:1.

[0013] Step 5: Wash the product of Step 4 repeatedly with water and dilute acetic acid - magnetic separation until the pH of the upper aqueous solution reaches 7, then perform magnetic separation several times with organic alcohol to extract and wash the water, and then dry it in vacuum at room temperature for 12 h. Finally, collect the micro-nano-sized powder of the samarium-containing metal alloy coated with calcium silicate.

[0014] Preferably, other aqueous solution metal salts can also be added to the atomized precursor solution in Step 1. More preferably, the other aqueous solution metal salts include one or more of titanium salts, zirconium salts, and copper salts.

[0015] Preferably, the water-soluble samarium salt in Step 1 is a trivalent water-soluble samarium salt, specifically one or more of samarium chloride, samarium nitrate, and samarium sulfate.

[0016] Preferably, the water-soluble transition metal salts in step one include water-soluble iron salts and water-soluble cobalt salts; the water-soluble iron salts are one or more of ferric chloride, ferric nitrate, ferric sulfate, ferrous chloride, ferrous nitrate, and ferrous sulfate, and the water-soluble cobalt salts are one or more of cobalt chloride, cobalt nitrate, and cobalt sulfate.

[0017] Preferably, the hydrophilic nano-silica SiO2 or water-soluble silicate that can produce silica by high-temperature hydrolysis oxidation in step one, the provided or produced SiO2 has a diameter ≤ 50 nm, and the mass of the provided or produced SiO2 accounts for 5% - 20% of the total mass of the atomized product.

[0018] Preferably, the substances that are water-soluble and have reducibility at high temperature or produce reducing substances by high-temperature decomposition in step one are specifically one or more of urea, ammonium carbonate, ammonium bicarbonate, and ethanol, and the mass ratio to the water-soluble transition metal salt is 1.1 - 1.7:1.

[0019] Preferably, the molar ratio of the water-soluble samarium salt to the water-soluble transition metal salt in the atomized precursor solution in step one is 10% - 40% higher than the molar ratio of samarium to the transition metal in the final target product.

[0020] Preferably, the frequency of the ultrasonic atomizer in step two is 0.5 - 10 MHz.

[0021] Preferably, the carrier gas in steps two and three is pure inert gas or a mixed gas containing reducing gas and inert gas, where the inert gas includes one or two of argon or helium. In order to prevent metallic iron from being nitrided at high temperature, nitrogen cannot be used as the carrier gas; the reducing gas is hydrogen; the flow rate of the carrier gas is 1 - 3 L / min, and the size of the obtained target product is 0.1 - 6 μm.

[0022] The evaporation dehydration, salt thermal decomposition, oxidation reaction, decomposition to produce reducing gas, and reduction reaction mentioned in the above step three refer to that the small droplets containing nano-SiO2 formed by atomizing the spray precursor solution by an ultrasonic atomizer are transported to a tubular furnace by the carrier gas. In the tubular furnace with a temperature gradient, the small droplets first evaporate and dehydrate to form spherical small particles with a certain proportion of samarium salt, transition metal salt, and SiO2 evenly embedded. Then the metal salt undergoes thermal decomposition to form spherical composite oxides (Sm-M-O + M-O + SiO2). At the same time, the substances that are reducible at high temperature or produce reducing substances by high-temperature decomposition reduce the M-O therein to pure metal M. The evaporation, dehydration, oxidation, decomposition, and reduction reactions all occur in the tubular furnace. The temperature of the tubular furnace is in a stepwise manner, with the central temperature being 1000 - 1200 °C, and the temperature gradually decreases from the center to both sides along the length direction. Finally, spherical particles with samarium oxide, metal M, and silica particles mutually embedded are collected in a collector with an attached magnetic field.

[0023] The reduction diffusion treatment mentioned in the above step 4 is to perform calcium reduction diffusion on spherical particles in which particles such as samarium oxide, metal M, and silicon dioxide are mutually embedded, and finally reduce them to a pure samarium-containing metal alloy. The reaction vessel for calcium reduction diffusion is carried out in a pure iron crucible. To reduce the loss of samarium escaping in the form of vapor during the reduction diffusion process, the pure iron crucible can be sealed by covering it with a lid. The purpose of calcium reduction is to directly obtain a samarium-containing metal alloy coated with CaSiO3 by reducing spherical particles in which particles such as samarium oxide, metal M, and silicon dioxide are mutually embedded with metallic calcium. Under high-temperature conditions, metallic calcium first reacts with Sm2O3 to generate metallic samarium. The generated metallic samarium adsorbs on the surface of metal M and diffuses with each other to form a samarium-containing metal alloy. The samarium-containing metal alloy generated by the reduction diffusion of a single particle undergoes densification sintering shrinkage and aggregates in the center of the particle. On the other hand, SiO2 does not undergo sintering, and the particle size of the composite particles does not change. At the same time, SiO2 on the particle surface reacts with the by-product CaO of the reduction diffusion reaction to generate CaSiO3. When the generated samarium-containing metal alloy is further sintered, the samarium-containing metal alloy inside a single particle sinters and grows and aggregates in the center of the particle. At the same time, SiO2 located inside the particle migrates to the particle surface due to incompatibility with the interface of the samarium-iron alloy and continues to react with the by-product to generate CaSiO3 because of the densification sintering of the samarium-iron alloy, forming core-shell structured particles with CaSiO3 as the shell and the samarium-containing metal alloy as the core. Under the coating effect of calcium silicate, sintering between adjacent particles is avoided, and the size is maintained stable. From the perspective of a single particle, in the state with SiO2 as the matrix, the dispersed samarium-containing metal alloy has a very large surface area and is unstable in terms of energy. By sintering with each other inside the particle, the surface free energy is reduced to the lowest, forcing the internal SiO2 to migrate to the particle surface to react with CaO, and then a core-shell structure is formed.

[0024] The calcium reduction diffusion process is as follows: Uniformly mix spherical particles in which particles such as samarium oxide, metal M, and silicon dioxide are mutually embedded with metallic calcium particles with a particle size of ≤5 mm, put them into a sealed iron crucible, and place the iron crucible in a high-temperature atmosphere furnace. First, evacuate and then introduce argon at room temperature, and repeat this gas washing process 3 - 5 times. Then heat up to 850 - 1200 °C and reduce for 1 - 5 h in a flowing Ar atmosphere to obtain a samarium-containing metal alloy coated with calcium silicate. The melting point of metallic calcium is about 850 °C and it is in a liquid state at the set reduction temperature. The saturated vapor pressure will cause calcium volatilization and partial oxidation of the surface of the metallic calcium particles. Therefore, the iron crucible used for calcium reduction needs to be sealed and the amount of calcium used is required to be excessive. Generally, the amount of calcium used is 1 to 3 times the theoretical value. Theoretically, 1 g of samarium oxide requires 0.344 g of calcium to be reduced.

[0025] The pure samarium obtained by calcium reduction and the pure metal M will form a samarium-containing metal alloy compound Sm-M through an adsorption-diffusion process at the reduction temperature. That is, the Sm obtained by reduction is adsorbed on the surface of the surrounding pure metal M particles in gaseous form. Under the coating effect of CaSiO3, most of the gaseous samarium diffuses into the iron particles, and finally a uniform and stable samarium-containing metal alloy is formed. Compared with the existing atomization reduction diffusion, due to the coating effect of CaSiO3, the obtained powder has better monodispersity and antioxidant properties. At the same time, it can also play a role in reducing the volatilization of Sm during the reduction diffusion process, reducing the total amount of samarium used by reducing the loss of samarium, thereby achieving the effect of cost savings.

[0026] The chemical reactions involved in calcium reduction diffusion and in-situ coating are as follows.

[0027] Calcium reduction: 3Ca + Sm2O3 = 2Sm + 3CaO

[0028] Diffusion: Sm + M → Sm-M

[0029] In-situ coating: CaO + SiO2 = CaSiO3

[0030] The calcium reduction diffusion temperature has a great influence on the purity of the target product of the insulated-coated samarium-containing metal alloy. If the calcium reduction temperature is low, the alloying reaction is insufficient, the sphericity of the particles is poor and not round enough; if the calcium reduction temperature is too high, the volatilization of samarium increases, and α-Fe that is not conducive to the permanent magnetic properties is easily generated. The samarium-iron alloy with a calcium silicate coating layer can, to a certain extent, reduce the volatilization of samarium, and at the same time play a role in particle coating, preventing particle sintering caused by liquid samarium between particles, inhibiting the agglomeration between alloy powder particles, and thus controlling the product size, morphology and monodispersity. The influence of the reduction time is the same as that of the reduction temperature. Generally, the calcium reduction temperature is 850~1200 °C, and the reduction time is 1~5 h. The optimal condition is to reduce at 1050 °C for 1.5 h.

[0031] After calcium reduction diffusion, since the addition amount of metallic calcium is 1~3 times the theoretical value, in addition to the consumption for generating calcium silicate, there are still a large number of by-products of calcium remaining, which need to be removed by washing. The magnetic separation cleaning process is adopted, that is, a permanent magnet is placed at the bottom of the mortar during the water washing process. Using the property of magnetic separation, the insulated-coated samarium-containing metal alloy powder is separated from the waste liquid. At the same time, grinding is additionally carried out during water washing until there is no obvious particle feeling, so that calcium oxide is completely separated from the insulated-coated samarium-containing metal alloy powder. In this step, one or two drops of acetic acid solution are often dropped into the water to increase the dissolution amount of calcium oxide and dissolve part of the samarium-rich phase. After water washing, the product surface is washed repeatedly with absolute ethanol to remove the water; then in the glove box small chamber, the chamber is evacuated to ≤ 0.05 MPa, and vacuum dried at room temperature for 12 h.

[0032] The second objective of the present invention is to provide a rare earth alloy prepared according to the above method. The rare earth alloy has a core-shell structure, where the inner core is a metal alloy containing samarium with a size of 0.1 - 6 microns, and the outer shell is a composite oxide containing silicon and calcium with a thickness of 5 - 50 nanometers.

[0033] Preferably, the metal alloy containing samarium includes an alloy containing samarium and iron, an alloy containing samarium and cobalt, specifically Sm2Fe 17 , SmFe9, Sm2Co 17 , Sm(FeCoTi) 12 and other alloys.

[0034] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0035] (1) Using ultrasonic spray pyrolysis to prepare rare earth alloy composite oxide powder, the prepared powder has a spherical shape, good control of alloy ratio, and no waste water generation;

[0036] (2) Combining powder preparation with subsequent surface modification (surface antioxidant treatment), directly in-situ coating calcium silicate during the calcium reduction and diffusion preparation of the powder, avoiding the subsequent surface treatment process of magnetic powder, with a simple process and improved efficiency;

[0037] (3) Without ball milling, micro-nano sized near-spherical samarium-iron alloy can be directly prepared;

[0038] (4) Through in-situ generation of insulating CaSiO3 coating, effectively preventing the sintering and growth between magnetic powder during the calcium reduction and diffusion stage, effectively controlling the particle size of magnetic powder, and making the normal distribution more concentrated;

[0039] (5) The in-situ insulating coating of the core-shell structure solves the problem that the magnetic properties of magnetic powder are severely deteriorated due to easy oxidation. Compared with magnetic powder without in-situ insulating coating, the magnetic powder prepared by insulating coating significantly improves the antioxidant performance of magnetic powder, especially the antioxidant performance at high temperatures. Description of the Drawings

[0040] Figure 1 Schematic diagram of the spray pyrolysis device for preparing spherical particles with mutually embedded particles of samarium oxide, metal M, and silicon dioxide, etc.; in the figure, 1 is the carrier gas, 2 is the ultrasonic atomizer, 3 is the precursor solution, 4 is the tubular furnace, 5 is the collection device, 6 is the spherical particles with mutually embedded structures, 7 is the NdFeB permanent magnet, and different transparencies below the tubular furnace represent different temperature gradients.

[0041] Figure 2 SEM image of the spherical particles with mutually embedded structures (Sm2O3 + α-Fe + SiO2) prepared by spray pyrolysis method in Example 1.

[0042] Figure 3 To obtain the position distribution (Map) diagrams of four elements, namely samarium, iron, silicon, and oxygen, for the structurally interlocked spherical particles in Figure 2 , where (a) represents Sm, (b) represents Fe, (c) represents Si, and (d) represents O.

[0043] Figure 4 SEM diagram and corresponding XRD diagram of the CaSiO3-coated Sm2Fe 17 alloy prepared in Example 1.

[0044] Figure 5 To obtain the position distribution (Map) diagrams of five elements, namely samarium, iron, calcium, silicon, and oxygen, for the CaSiO3-coated Sm2Fe Figure 4 alloy in 17 . Among them, (1) represents Sm, (2) represents Fe, (3) represents Ca, (4) represents Si, and (5) represents O.

[0045] Figure 6 Schematic diagram of the overall experimental process effect. The atomized product is structurally interlocked spherical particles, and after calcium reduction diffusion, calcium silicate is in-situ coated on the samarium-iron alloy.

[0046] Figure 7 Schematic diagram of the in-situ coating of calcium silicate on the samarium-iron alloy during the reduction diffusion process. As the Sm2Fe 17 alloy inside the particles is gradually sintered, it forces SiO2 to gradually migrate to the particle surface and react with CaO to form a CaSiO3 coating shell. Detailed implementation method

[0047] As mentioned above, in view of the deficiencies of the prior art, the inventors of this case have proposed the technical solution of the present invention through long-term research and a large number of practices. The main basis includes at least:

[0048] Based on the ultrasonic spray pyrolysis method, the present invention realizes the direct preparation of micro-nano-sized samarium-containing metal alloy powder with in-situ CaSiO3 coating without ball milling and secondary surface treatment. By synchronizing reduction diffusion and in-situ coating, the morphology and size of the samarium-containing metal alloy can be controlled, and the antioxidant performance of the powder is improved. The preparation process of this method is simpler, facilitating industrialization, and the raw material cost is low, which is conducive to the preparation of high-performance samarium-containing rare earth alloy magnetic powder.

[0049] In order to make the purpose, technical solution, and advantages of the present invention clearer, the following further elaborates on the present invention in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0050] The specific implementation plan of the present invention is as follows:

[0051] Step 1: Weigh 25 mL of 0.2 mol / L water-soluble samarium salt, 170 mL of 0.2 mol / L water-soluble transition metal salt, 0.7 g of hydrophilic nano-silica or water-soluble silicate, and a substance that is water-soluble and has reducibility at high temperature or produces reducing substances upon high-temperature decomposition. Mix and dissolve them in water to form an atomized precursor solution.

[0052] Other aqueous solution metal salts, such as titanium salts, zirconium salts, and copper salts, can also be added to the atomized precursor solution described in Step 1.

[0053] Step 2: Heat the tube furnace to 1100 °C at a heating rate of 10 °C / min. Place the precursor solution in an ultrasonic atomizer, adjust its frequency to 0.5 - 10 MHz, and using the principle of ultrasonic spraying, turn the spray precursor solution into an atomized state. Relying on the carrier gas, transport the atomized small liquid droplets to the tube furnace at a certain flow rate.

[0054] The atomized precursor solution undergoes successive evaporation of water, salt thermal decomposition, and oxidation reactions under the high temperature of the tube furnace. Reduction gases are generated by decomposition and reduction reactions occur. Finally, spherical particles with mutually embedded samarium oxide, metal, and silica particles are formed and transported by the carrier gas to a collector with a magnetic field attached to the end of the tube furnace.

[0055] Step 4: Mix the collected spherical particles with several mutually embedded particles evenly with an appropriate amount of calcium grains and place them in a sealed pure iron crucible. Then place the pure iron crucible in a high-temperature atmosphere reaction furnace. The high-temperature atmosphere reaction furnace is first evacuated and then filled with Ar at room temperature, repeated several times, and then heated to 850 - 1100 °C for reduction diffusion and in-situ coating treatment for 1 - 5 h. At this time, the reduction diffusion side reaction product CaO reacts with the SiO2 migrated to the surface to form CaSiO3, which uniformly coats the surface of the samarium-containing metal alloy. The dosage of calcium grains is 1 - 3 times the mass of samarium oxide.

[0056] Step 5: Wash the reduction diffusion product repeatedly with water and dilute acetic acid - magnetic separation until the pH of the upper aqueous solution finally reaches 7. Then perform magnetic separation several times with organic alcohol to extract and wash the water, and then dry it under vacuum at room temperature for 12 h. Finally, collect the micro-nano-sized powder of the samarium-containing metal alloy coated with calcium silicate.

[0057] The present invention will be further described below in conjunction with specific embodiments, but the methods and technical parameters involved in the solutions should not be construed as limitations to the present invention. Example 1

[0058] Prepare the precursor solution: Take 25 mL of 0.2 mol / L SmCl3 solution, 170 mL of 0.2 mol / L FeCl3 solution, 0.7 g of 25 nm hydrophilic SiO2, and 2.8 g of urea, mix them well and stir thoroughly.

[0059] Set control group 1, and its precursor solution is: Take 25 mL of 0.2 mol / L SmCl3 solution, 170 mL of 0.2 mol / L FeCl3 solution and 2.8 g of urea, mix them and stir well.

[0060] Set control group 2, and its precursor solution is: Take 25 mL of 0.2 mol / L SmCl3 solution, 170 mL of 0.2 mol / L FeCl3 solution, 2.2 g of CaCl2 and 2.8 g of urea, mix them and stir well.

[0061] Pour the three precursor solutions into Figure 1 the ultrasonic atomizer shown in the figure. Heat the tube furnace to 1100 °C at a heating rate of 10 °C / min, then turn on the atomizer, and the frequency of the ultrasonic atomizer is 10 MHz; Loosen the main valve of the nitrogen gas cylinder and tighten the pressure reducing valve, and control the gas flow rate to 2.5 L / min through the gas flow meter. The carrier gas argon transports the atomized small liquid droplets into the tube furnace; After atomizing for a period of time, turn off the ultrasonic atomizer and collect the powder in the collection device at the tube tail. Observe the micro-morphology of the powder. The powders obtained from the example and the two control groups are all spherical, and there is no obvious difference in size. The micro-structure of the powder obtained from the example is as shown in Figure 2 the figure. Perform Map analysis on the spherical particles obtained from the example for four elements of samarium, iron, oxygen, and silicon, and the results are as shown in Figure 3 the figure. The results show that the four elements are evenly distributed in the spherical particles, proving that the thermal decomposition products Sm2O3 and α-Fe and nano-SiO2 exist in an intercalated structure, and the energy spectrum analysis shows that the samarium-iron ratio in each particle is consistent with the samarium-iron ratio in the precursor solution.

[0062] Weigh 2 g of the spray pyrolysis powder collected above (about containing 0.5 g of samarium oxide), then add 0.35 g of metallic calcium granules. After mixing evenly, put them into an iron crucible with a lid, and then push it into the tube furnace. React at 1050 °C for 1.5 h under a flowing Ar atmosphere. After the reaction is completed, pour the product in the iron crucible into a mortar, add an appropriate amount of deionized water, and grind and wash for a few minutes until there is no obvious particle feeling. Then place a NdFeB magnet under the mortar, and use the principle of magnetic separation to pour off the washing liquid; Then wash with dilute acetic acid 2 - 3 times, and then wash with deionized water 3 - 4 times to ensure that the pH of the washing liquid is about 7. Then wash with absolute ethanol 3 times to extract and remove the water on the surface of the samarium-iron alloy powder. Then in the glove box small chamber, evacuate the chamber to ≤0.05 MPa and dry at room temperature in vacuum for 12 h. Finally, collect the powder, observe the micro-morphology of the powder and perform phase analysis using an X-ray diffractometer (XRD).

[0063] Example 1 used an X-ray diffractometer (XRD) to analyze the composition of the powder and determined that it was Sm2Fe 17 alloy and CaSiO3 crystal diffraction peaks, and no crystal diffraction peaks of α-Fe and other alloys were found. The scanning electron microscope (SEM) was used to observe the microscopic morphology of the alloy powder, and the results were as Figure 4 shown. It was found that all were near-spherical particles with a size distribution of about 0.5 - 3 microns, and there was no sign of sintering between the particles. The Sm2Fe 17 alloy coated with CaSiO3 was used to make the position distribution (Map) diagrams of the five elements of samarium, iron, calcium, silicon, and oxygen. The results were as Figure 5 shown. The surface near-spherical particles were samarium-iron alloy coated with silica, and SiO2 was distributed in the shell layer, and the samarium and iron elements were evenly distributed. Figure 6 Figure 10 is a schematic diagram of the effect of the entire experimental process. The atomized product is spherical particles with interlocked structures. After calcium reduction and diffusion, calcium silicate in-situ coated samarium-iron alloy is generated. Figure 7 Figure 11 is a schematic diagram of calcium silicate in-situ coating samarium-iron alloy during the reduction and diffusion process. As the Sm2Fe 17 alloy inside the particles is gradually sintered, it forces SiO2 to gradually migrate to the particle surface and react with CaO to form a CaSiO3 coating shell.

[0064] Control group 1 used an X-ray diffractometer (XRD) to analyze the composition of the samarium-iron alloy powder and determined that it was Sm2Fe 17 alloy crystal diffraction peaks, and no crystal diffraction peaks of α-Fe and other alloys were found. The scanning electron microscope (SEM) was used to observe the microscopic morphology of the alloy powder, and it was found that the particle size distribution was wide, the sizes were obvious, and the near-spherical particles had a size distribution of about 1 - 10 microns, and there was serious sintering and aggregation between the particles.

[0065] Control group 2 used an X-ray diffractometer (XRD) to analyze the composition of the samarium-iron alloy powder and determined that it was Sm2Fe 17 alloy crystal diffraction peaks, and no crystal diffraction peaks of α-Fe and other alloys were found. The scanning electron microscope (SEM) was used to observe the microscopic morphology of the alloy powder, and it was found that all were near-spherical particles with a size distribution of about 2 - 5 microns, and there was a slight sign of sintering between the particles.

[0066] The Sm2Fe 17 alloy powders prepared in Example 1, Control group 1, and Control group 2 were respectively treated in a conventional oven at 150 °C for 30 min, and the changes in the oven were observed with the naked eye. It was found that there was no obvious change in Example 1, while the powders in the two control groups became red-hot due to heating. This indicates that the powders obtained in the control groups became red-hot due to oxidation and heating at high temperatures, while the powders obtained in Example 1 had better high-temperature oxidation resistance. The weight changes of the powders before and after drying treatment were tested, as shown in Table 1.

[0067] The Sm2Fe alloy powders prepared in Example 1, Control Group 1 and Control Group 2 were nitrided in a tube furnace at 450 °C for 2 hours. The atmosphere in the tube furnace was a mixed gas of ammonia and hydrogen. After the nitrided powders were heat-treated at room temperature and 100 °C respectively, their magnetic properties were measured using a vibrating sample magnetometer (VSM). The corresponding data are shown in Table 1. It can be seen from Table 1 that the powders obtained in the example have a more concentrated size distribution, better high-temperature oxidation resistance, and less loss of comprehensive magnetic properties after high-temperature treatment compared with the powders obtained in the two control groups. 17

[0068] Table 1 Comparison of the properties of powders obtained in different cases

[0069]

[0070] Example 2

[0071] Prepare the precursor solution: Mix 40 mL of 0.2 mol / L Sm(NO3)3 solution, 170 mL of 0.2 mol / L Co(NO3)3 solution, 3 g of NaSiO3 and 80 mL of ethanol and stir well.

[0072] Set up Control Group 3, and its precursor solution is: Mix 40 mL of 0.2 mol / L Sm(NO3)3 solution, 170 mL of 0.2 mol / L Co(NO3)3 solution and 80 mL of ethanol and stir well.

[0073] Set up Control Group 4, and its precursor solution is: Mix 40 mL of 0.2 mol / L Sm(NO3)3 solution, 170 mL of 0.2 mol / L Co(NO3)3 solution, 2.5 g of Ca(NO3)2 and 80 mL of ethanol and stir well.

[0074] Pour the three precursor solutions into Figure 1 the ultrasonic atomizer as shown. Heat the tube furnace to 1100 °C at a heating rate of 10 °C / min, then turn on the atomizer. The frequency of the ultrasonic atomizer is 8 MHz. Loosen the main valve of the nitrogen gas cylinder and tighten the pressure reducing valve. Control the gas flow rate to 2.5 L / min through the gas flow meter. The carrier gas argon transports the atomized small liquid droplets into the tube furnace. After atomizing for a period of time, turn off the ultrasonic atomizer and collect the powder in the collection device at the tail of the tube. Observe the microstructure of the powder. The powders obtained in the example and the two control groups are all spherical, and there is no obvious difference in size.

[0075] ​Weigh 2 g of the spray pyrolysis powder collected above (about 0.45 g of samarium oxide), then add 0.32 g of metallic Ca granules. After mixing evenly, place them in an iron crucible with a lid, and then push it into a tube furnace. React for 2 h under a flowing Ar atmosphere at 1050 °C. After the reaction, pour the product in the iron crucible into a mortar, add an appropriate amount of deionized water, grind and wash for several minutes until there is no obvious granular feeling. Then place an NdFeB magnet under the mortar, and pour off the washing solution using the principle of magnetic separation. Then wash it with dilute acetic acid 2 - 3 times, and then wash it with deionized water 3 - 4 times to ensure that the pH of the washing solution is about 7. Then wash it with absolute ethanol 3 times to extract and remove the water on the surface of the samarium-iron alloy powder. Then, in the small chamber of the glove box, evacuate the chamber to ≤0.05 MPa and dry it in vacuum at room temperature for 12 h. Finally, collect the powder, observe the micro-morphology of the powder, and perform phase analysis using an X-ray diffractometer (XRD).

[0076] In the example, an X-ray diffractometer (XRD) was used to analyze the composition of the powder, and it was determined that the diffraction peaks of SmCo5 alloy and CaSiO3 crystals were present, and no diffraction peaks of other alloys were found. The scanning electron microscope (SEM) was used to observe the micro-morphology of the alloy powder, and it was found that all were near-spherical particles with a size distribution of about 0.4 - 4 microns, and there was no sign of sintering between the particles.

[0077] In control group 3, an X-ray diffractometer (XRD) was used to analyze the composition of the powder, and it was determined that the diffraction peak of SmCo5 alloy crystal was present, and no diffraction peaks of other alloys were found. The scanning electron microscope (SEM) was used to observe the micro-morphology of the alloy powder, and it was found that all were near-spherical particles with a size distribution of about 1 - 8 microns, and there was serious sintering and aggregation between the particles.

[0078] In control group 4, an X-ray diffractometer (XRD) was used to analyze the composition of the powder, and it was determined that the diffraction peak of SmCo5 alloy crystal was present, and no diffraction peaks of other alloys were found. The scanning electron microscope (SEM) was used to observe the micro-morphology of the alloy powder, and it was found that all were near-spherical particles with a size distribution of about 0.8 - 5 microns, and there was slight sintering and aggregation between the particles.

[0079] The SmCo5 alloy powders prepared in Example 2, control group 3, and control group 4 were respectively treated in a conventional oven at 150 °C for 30 min, and the changes in the oven were observed with the naked eye. It was found that there was no obvious change in the example, while the powders in the two control groups became hot and red. This indicates that the powders obtained in the control groups became red due to oxidation and heat generation at high temperatures, while the powders obtained in the example have better high-temperature oxidation resistance. The weight changes of the powders before and after drying treatment were tested, and the corresponding data are shown in Table 2.

[0080] The SmCo5 alloy powders prepared in Example 2, Control Group 3, and Control Group 4 were tested for magnetic properties using a vibrating sample magnetometer (VSM) at room temperature and after heat treatment at 100 °C. The corresponding data are shown in Table 2. It can be seen from Table 2 that compared with the powders obtained in the two control groups, the powder obtained in the example has a narrow particle size distribution, better high-temperature oxidation resistance, and less loss of comprehensive magnetic properties after heat treatment.

[0081] Table 2: Performance comparison of powders obtained from different cases

[0082]

Claims

1. A preparation method of a micro-nano rare earth alloy coated with an in-situ antioxidant layer, characterized in that, It includes the following steps: Step 1: Mix a water-soluble samarium salt, a water-soluble transition metal salt, an SiO2 donor, and a substance that is water-soluble and has reducibility at high temperature or produces a reducing substance upon high-temperature decomposition in water to form an atomized precursor solution; wherein the SiO2 donor is hydrophilic nano-silica SiO2 or a water-soluble silicate that can produce silica upon high-temperature hydrolysis and oxidation; Step 2: Place the atomized precursor solution in an ultrasonic atomizer. Using the principle of ultrasonic spraying, turn the atomized precursor solution into an atomized state, and then under the action of a carrier gas, transport it to a tubular furnace with a temperature gradient at a certain flow rate; Step 3: The atomized precursor solution undergoes evaporation and dehydration, salt thermal decomposition, and oxidation reactions in the tubular furnace, decomposes to produce reducing gases and undergoes reduction reactions. Finally, spherical particles in which samarium oxide Sm2O3, transition metal M, and silica SiO2 are mutually embedded are formed and are transported by the carrier gas to a collector with a magnetic field provided at the end of the tubular furnace; Step 4: Mix the above spherical particles evenly with an excessive amount of calcium grains and place them in a sealed crucible. Place the crucible in a high-temperature atmosphere reaction furnace. The high-temperature atmosphere reaction furnace first evacuates and then fills with Ar at room temperature, repeats several times, and then raises the temperature to 850 - 1100 °C for reduction diffusion and in-situ coating treatment for 1 - 5 h. At this time, the reduction diffusion by-product CaO reacts with the SiO2 migrated to the surface to form calcium silicate CaSiO3, and at the same time, calcium silicate uniformly coats the surface of the samarium-containing metal alloy; the mass ratio of calcium grains to samarium oxide in the spherical particles is 1 - 3:1; Step 5: Wash the product of Step 4 repeatedly with water and dilute acetic acid - magnetic separation until the pH of the upper aqueous solution reaches 7, then perform magnetic separation with organic alcohol several times to extract and wash the water, and then dry it under vacuum at room temperature. Finally, collect the micro-nano-sized powder of the samarium-containing metal alloy coated with calcium silicate.

2. The method according to claim 1, characterized in that The water-soluble samarium salt in Step 1 is one or more of samarium chloride, samarium nitrate, and samarium sulfate.

3. The method according to claim 1, characterized in that The transition metal is iron or cobalt.

4. The method according to claim 1, wherein The diameter of SiO2 provided or produced by the SiO2 donor in Step 1 is ≤50 nm, and the mass of SiO2 provided or produced accounts for 5% - 20% of the total mass of the atomized precursor solution.

5. The method according to claim 1, characterized in that, The substance that is water-soluble and has reducibility at high temperature or produces a reducing substance upon high-temperature decomposition in Step 1 is one or more of urea, ammonium carbonate, ammonium bicarbonate, and ethanol. The mass ratio of the substance that is water-soluble and has reducibility at high temperature or produces a reducing substance upon high-temperature decomposition to the water-soluble transition metal salt is 1.1 - 1.7:

1.

6. The method according to claim 1, wherein The molar ratio of the water-soluble samarium salt to the water-soluble transition metal salt in the atomized precursor solution in Step 1 is 10% - 40% higher than the molar ratio of samarium to the transition metal in the final target product.

7. The method according to claim 1, characterized in that, The frequency of the ultrasonic atomizer in Step 2 is 0.5 - 10 MHz.

8. The method according to claim 1, characterized in that, The carrier gas in Steps 2 and 3 is a pure inert gas or a mixed gas containing a reducing gas and an inert gas. The inert gas includes one or both of argon and helium, and the reducing gas is hydrogen. The flow rate of the carrier gas is 1 - 3 L / min.

9. The method according to claim 1, wherein Other aqueous metal salts are also added to the atomized precursor solution described in Step 1.

10. A micro-nano rare earth alloy coated with an in-situ antioxidant layer, which is prepared by the method described in any one of claims 1-8, and is characterized in that, The rare earth alloy powder has a core-shell structure. The inner core is a metal alloy containing samarium with a size of 0.1 - 6 microns, and the outer shell is a composite oxide containing silicon and calcium with a thickness of 5 - 50 nanometers.

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