A metal magnetic powder with a multi - cooperative protective film on its surface and a preparation method thereof

Abstract

The present invention discloses a metal magnetic powder with a multiple synergistic protective film on its surface and a preparation method thereof. Specifically, under an inert atmosphere protection, the metal magnetic powder is placed in an organic solution containing concentrated phosphoric acid and stirred to obtain a metal magnetic powder with a phosphated surface; under an inert atmosphere protection, the metal magnetic powder with a phosphated surface is placed in an alkaline solution containing an oxidant, fully stirred at 25-80 °C, left to stand for reaction, then washed and dried to obtain a dried metal magnetic powder treated with an oxidant; finally, it is placed in an oxygen partial pressure furnace for heat treatment, so that a uniform and dense multiple protective film that is well combined with the substrate is formed on the surface of the metal magnetic powder. Compared with the magnetic powder treated by single phosphating or oxidation, the metal magnetic powder with a multiple synergistic protective film on its surface in the present invention has better oxidation resistance under high-temperature service.

Description

Technical Field

[0001] The present invention belongs to the field of preparation and surface modification treatment of metal magnetic powder, and in particular relates to a metal magnetic powder with a multiple synergistic protective film on its surface and a preparation method thereof. Background Art

[0002] Sm2Fe 17 N x is a new type of rare earth permanent magnet material developed in recent years. Compared with NdFeB permanent magnets, such magnets have similar magnetic properties, and the theoretical maximum magnetic energy product is equivalent to that of NdFeB, and its theoretical value can reach 480 KJ / m 3 or more. At the same time, such magnets have better temperature and chemical stability. Its Curie temperature is higher than 750 K (477 °C), which is nearly 160 °C higher than that of NdFeB (590 K or 317 °C), and it has excellent intrinsic magnetic properties and a large anisotropy field.

[0003] However, such fine magnetic powder has high surface activity and is extremely easy to oxidize when directly placed in air, and will even burn at slightly higher temperatures. Therefore, it is necessary to coat the magnetic powder to improve its antioxidant ability. The coating of Sm-Co permanent magnet materials is mainly used to improve the high-temperature oxidation resistance of the magnet, and the basic principle is to block or reduce the diffusion of oxygen in the magnet. For example, a layer of metal (such as Al, Cu, Ni, Cr, Mo, and W, etc.) is plated on the surface of the magnet. Experiments have found that a uniform and thick enough coating (not less than 10 μm) can provide effective protection for the magnet. Professor Kenichi Machida of Osaka University in Japan (Advanced Science and Technology Joint Research Center) developed a technique for surface treatment of anisotropic SmFeN magnetic powder by galvanizing and organosilicon compounds. After treatment, not only the oxidation resistance is improved, but also the powder particles are tightly combined with the resin, and the particles are easy to rotate during orientation. Zhang Dongtao et al. used the silane coupling agent KH550 to perform surface coating treatment on Sm2Fe 17 N x permanent magnet powder, and studied the oxidation behavior and the change law of magnetic properties of the magnetic powder in different coating states below 300 °C. The results show that the coupling agent is adsorbed on the surface of the magnetic powder in a chemical bond manner, forming a network-like thin film, which effectively blocks the contact between the magnetic powder and oxygen in the environment, thereby improving the antioxidant ability of the magnetic powder at high temperatures.

[0004] The oxidation and corrosion protection of rare earth permanent magnet materials are very important. With the broadening of their application fields, the increasingly harsh service environments and the improvement of user requirements, higher requirements are put forward for oxidation and corrosion protection technologies. For specific application requirements, it is necessary to develop new protective coatings and their preparation technologies with good effects, environmental friendliness, and low costs. Summary of the Invention

[0005] The object of the present invention is to propose a metal magnetic powder with a multiple synergistic protective film on its surface and a preparation method thereof in view of the deficiencies of the prior art. The surface of the metal magnetic powder has multiple synergistic protective films such as a phosphating film and a metal oxide film. The preparation method of the multiple synergistic protective film is that the metal magnetic powder is first subjected to phosphating treatment, then chemically oxidized using an oxidant, and finally heat-treated in a low-oxygen environment in an oxygen partial pressure furnace, and finally a uniform, dense and stable-quality multiple synergistic protective film containing a phosphating film and an oxide film is formed on the powder surface. Further improve the oxidation resistance of the metal powder under high-temperature service. In summary, there is an urgent need to propose a method for forming a uniform and dense protective film on the surface of metal powder through the synergistic treatment of phosphating and chemical oxidation to improve the oxidation resistance of the powder under high-temperature service.

[0006] The method of the present invention adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a preparation method of a metal magnetic powder with a multiple synergistic protective film on its surface, including:

[0008] Under the protection of an inert atmosphere, the metal magnetic powder is placed in an organic solution containing concentrated phosphoric acid and stirred to obtain a metal magnetic powder with a phosphated surface;

[0009] Under the protection of an inert atmosphere, the metal magnetic powder with a phosphated surface is placed in an alkaline solution containing an oxidant, and after sufficient stirring at 25-80 °C, it is left to stand for reaction to obtain a metal magnetic powder after chemical oxidation treatment;

[0010] Under the protection of an inert atmosphere, the metal magnetic powder after chemical oxidation treatment is washed and dried to obtain a dried metal magnetic powder after oxidation treatment;

[0011] The dried metal magnetic powder after oxidation treatment is placed in an oxygen partial pressure furnace for heat treatment, so that a uniform and dense multiple protective film that is well combined with the substrate is formed on the surface of the metal magnetic powder.

[0012] Preferably, the metal magnetic powder includes one or a composite of samarium-iron-nitrogen magnetic powder, neodymium-iron-boron magnetic powder, neodymium-iron-nitrogen magnetic powder, samarium-cobalt magnetic powder, samarium-iron-cobalt magnetic powder, and iron-nitrogen compound magnetic powder.

[0013] Preferably, the oxidant includes one of sodium nitrate, sodium nitrite, hydrogen peroxide, potassium dichromate, and potassium permanganate. More preferably, the oxidant is sodium nitrite, and its content is 0.5%.

[0014] Preferably, the solute in the organic solution containing concentrated phosphoric acid is concentrated phosphoric acid with a mass content greater than 80%.

[0015] Preferably, the inert atmosphere includes one or both of argon and helium.

[0016] Preferably, the phosphoric acid content in the organic solution containing concentrated phosphoric acid is 0.5% - 10% of the mass of the magnetic powder.

[0017] Preferably, the pH of the alkaline solution containing an oxidant is greater than 8, and the oxidant content is 0.1% - 1.0% of the mass of the magnetic powder, more preferably 0.1% - 0.5%.

[0018] Preferably, in the oxygen partial pressure furnace, a mixed gas of nitrogen and air or a mixed gas of nitrogen and pure oxygen is introduced, and the flow rate of air or pure oxygen is controlled so that the oxygen content in the furnace chamber is 1 - 15 vol.%. The oxygen partial pressure furnace is controlled at a temperature of 150 - 250 °C, with a heating and cooling rate of 2 - 4 °C / min and a holding time of 30 min - 2 h.

[0019] The solvent of the acid solution is an organic solvent.

[0020] In a second aspect, the present invention provides a metal magnetic powder with a multi - synergistic protective film on its surface, prepared by the above method.

[0021] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0022] The metal magnetic powder of the present invention is protected by the synergistic effect of the phosphating film and the metal oxide film, avoiding the problem of the monolayer film being non - dense, effectively isolating the powder from contact with air, ensuring high - temperature oxidation resistance, and thus improving the coercivity of the metal magnetic powder at high temperatures.

[0023] After heat treatment in a high - temperature furnace under controlled oxygen partial pressure, the heat treatment can not only dehydrate and dry the chemical phosphating film molecules to ensure the bonding force between the synergistic film and the metal powder, but also micro - oxidize the surface of the powder at high temperature under a small amount of oxygen to form an oxidation protective film again to make up for the non - dense parts of the synergistic film. At the same time, the powder pre - treated at high temperature can avoid the decrease in permanent magnetic properties of the metal magnetic powder when serving in an environment below 300 °C, realizing the possibility of the metal magnetic powder serving at higher temperatures and broadening the applicable market of the metal magnetic powder. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of an oxygen partial pressure tube furnace device for heat treatment of metal magnetic powder.

[0025] Figure 2The TEM images and EDS spectra of samarium iron nitrogen magnetic powder particles prepared by multiple synergistic surface treatments in Example 1 and Control Group 1, wherein the elements are Fe, Sm, O, and P, respectively. Among them, (A-1) is the morphology of the magnetic powder particles in Example 1, (A-2) is the iron element distribution diagram of the magnetic powder particles in Example 1, (A-3) is the samarium element distribution diagram of the magnetic powder particles in Example 1, (A-4) is the oxygen element distribution diagram of the magnetic powder particles in Example 1, (B-1) is the morphology of the magnetic powder particles in Control Group 1, (B-2) is the iron element distribution diagram of the magnetic powder particles in Control Group 1, (B-3) is the samarium element distribution diagram of the magnetic powder particles in Control Group 1, (B-4) is the oxygen element distribution diagram of the magnetic powder particles in Control Group 1, (C) is the morphology of the magnetic powder in Example 1, and (D) is the phosphorus element distribution diagram of the magnetic powder in Example 1.

[0026] Figure 3 It is a schematic diagram of the effect of the entire experimental process. The product of the multiple synergistic surface treatment is a multiple synergistic protective film on the surface of the SmFeN magnetic powder particles.

[0027] Figure 4 The flowchart of the whole experiment. DETAILED DESCRIPTION

[0028] As mentioned above, in view of the shortcomings of the prior art, the inventor of this case proposed the technical solution of the present invention after long-term research and extensive practice, the main basis of which at least includes:

[0029] The present invention firstly phosphates the metal magnetic powder (taking samarium iron nitrogen metal magnetic powder as an example), then uses an oxidant to perform chemical oxidation treatment, and finally performs heat treatment in an oxygen partial pressure furnace in a low oxygen environment, so that the surface of the samarium iron nitrogen magnetic powder finally obtains a multiple synergistic protective film containing both a phosphating film and an oxide film. Due to the formation of the protective film on the surface of the metal magnetic powder, the permanent magnetic properties of the metal magnetic powder at room temperature are greatly improved, the effect of the metal magnetic powder in resisting the decline of the permanent magnetic properties below 300°C and the oxidation resistance at high temperature are improved, the possibility of the metal magnetic powder serving at higher temperatures is realized, and the applicable market of the metal magnetic powder is broadened.

[0030] The present invention provides a method for preparing a metal magnetic powder having multiple synergistic protective films on the surface, such as Figure 4 include:

[0031] Step 1: Under the protection of an inert atmosphere, place the metal magnetic powder in an organic solution containing concentrated phosphoric acid and stir to obtain a surface-phosphorized metal magnetic powder. Figure 3 .

[0032] In one embodiment, the phosphoric acid content in the organic solution containing concentrated phosphoric acid is 0.5%-10% of the mass content of the magnetic powder, and the stirring treatment time is 10 min-30 min.

[0033] Step 2: Under the protection of an inert atmosphere, place the surface phosphated metal magnetic powder in an alkaline solution containing an oxidant, stir it thoroughly at 25-80°C, and let it stand for reaction to obtain the metal magnetic powder after chemical oxidation treatment. Figure 3 .

[0034] In one embodiment, the oxidant content in the alkaline solution containing the oxidant is 0.1%-1.0% of the mass content of the magnetic powder, the stirring treatment time is 20min-40min, and the standing time is 40min-60min.

[0035] Step 3: Under the protection of an inert atmosphere, the metal magnetic powder after chemical oxidation treatment is cleaned and dried to obtain dry metal magnetic powder after oxidation treatment;

[0036] For example, the solution for cleaning the metal magnetic powder may be washed with deionized water until it is colorless and transparent, and then the metal magnetic powder is washed twice with an ethanol solution and a normal hexane solution, respectively. The cleaned metal magnetic powder is placed in a glove box with inert atmosphere protection, and dried naturally at room temperature, and finally the dry metal magnetic powder treated with the alkaline pro-oxidant is obtained.

[0037] Step 4: Place the obtained metal magnetic powder Figure 1 Heat treatment is carried out in an oxygen partial pressure furnace, so that a uniform and dense multi-layer protective film that is well bonded to the matrix is formed on the surface of the metal magnetic powder.

[0038] Heat treatment can not only dehydrate and dry the chemical phosphating film molecules, ensuring the bonding force between the synergistic film and the metal powder, but also micro-oxidize the powder surface under a trace of oxygen and high temperature to compensate for the formation of an oxide protective film again at the loose part of the synergistic film. At the same time, the high-temperature treatment of the powder in advance can prevent the permanent magnetic properties of the metal magnetic powder from decreasing when it is used in an environment below 300°C.

[0039] In one embodiment, the metal magnetic powder mentioned in step 1 includes one of samarium iron nitrogen magnetic powder, neodymium iron boron magnetic powder, neodymium iron nitrogen magnetic powder, samarium cobalt magnetic powder, samarium iron cobalt magnetic powder and iron nitrogen compound magnetic powder, or a composite thereof.

[0040] In one embodiment, the solute of the organic solution containing concentrated phosphoric acid mentioned in step 1 is concentrated phosphoric acid with a mass content greater than 80%.

[0041] The acid solution in this embodiment can be a mixture of concentrated phosphoric acid and an organic solvent. Because the metal magnetic powder particles are small and highly active, they are prone to chemical corrosion in aqueous solvents to generate loose products that affect the antioxidant effect. Therefore, the concentrated phosphoric acid used in the phosphoric acid solution has a concentration of more than 80%, and an organic solvent such as anhydrous ethanol, isopropanol or n-hexane is used as the solvent, and water cannot be added.

[0042] In one embodiment, the pH of the alkaline solution containing the oxidant mentioned in step 2 is greater than 8, the alkaline solution can reduce the corrosion problem of the magnetic powder in the aqueous solution, and the oxidant can cause the magnetic powder to undergo an oxidation reaction, wherein the oxidant includes one of sodium nitrate, sodium nitrite, hydrogen peroxide, potassium dichromate and potassium permanganate.

[0043] The oxidant in this embodiment is preferably sodium nitrite. When preparing the solution of the oxidant sodium nitrite, deionized water is finally selected as the solvent because sodium nitrite is slightly soluble in organic solvents such as ethanol. Since metal magnetic powder has a high surface activity, it is very easy to oxidize in the air and even burn in the air, and since sodium nitrite has a strong pro-oxidation property, it may further lead to oxidation and combustion of the metal powder. Therefore, the content of sodium nitrite in the solution should be controlled in a relatively low range, and finally a sodium nitrite content of 0.1%-1.0% of the mass content of the magnetic powder is selected, preferably 0.5%.

[0044] The chemical oxidation treatment of the metal magnetic powder surface is carried out in a pure inert atmosphere vacuum glove box to prevent the highly surface active metal magnetic powder from being oxidized or even burned in the air. The metal magnetic powder is placed in a reagent in which sodium nitrite is completely dissolved, stirred thoroughly and left to stand for a sufficient time to ensure that the surface of the magnetic powder can be more evenly and fully contacted with the reagent, thereby ensuring the uniformity and consistency of the chemical reaction.

[0045] In one embodiment, the carrier gas in steps one, two and three is a pure inert gas, wherein the inert gas includes one or both of argon and helium; so that the oxygen content in the vacuum glove box is less than 5 PPm and the water content is less than 0.1 PPm.

[0046] In one embodiment, the cleaning treatment of the metal powder mentioned in step three is to use three solvents (deionized water, ethanol, and n-hexane) to clean the metal magnetic powder after chemical oxidation treatment. After the surface of the metal magnetic powder is evenly and fully treated, the sodium nitrite remaining on the surface of the metal magnetic powder should be fully cleaned with deionized water, and then the small amount of sodium nitrite and water remaining on the surface of the metal magnetic powder should be further cleaned with ethanol, so that the metal powder is fully cleaned to avoid the residue of oxidant and water solvent, which affects the next step of heat treatment experiment. Finally, the metal magnetic powder is cleaned with n-hexane solution because the n-hexane solution contains almost no oxygen, which can effectively avoid the influence of oxygen element on the vacuum environment and the oxidation of the metal magnetic powder surface in the next step of vacuum drying.

[0047] In one embodiment, the surface heat treatment of the dried metal magnetic powder mentioned in step four is to form a uniform and dense oxide film on the surface of the metal magnetic powder. In order to avoid drying in an oven at a certain temperature, which may cause the metal magnetic powder to oxidize under the influence of high temperature, humidity and air environment, it is finally chosen to air-dry naturally in a vacuum drying environment. Place the dried metal powder in a crucible and put it into an oxygen partial pressure tube furnace. Theoretically, if the wet powder is placed in the tube furnace for heat treatment, it will further promote the oxidation of the metal magnetic powder and even cause the oxidation of the metal magnetic powder matrix. Therefore, the metal magnetic powder should be fully dried before the experiment. Heat-treat the metal magnetic powder in an oxygen partial pressure tube furnace, using air or a mixture of pure oxygen and nitrogen as the reaction atmosphere, controlling the oxygen content in the furnace to be 1-15 vol.%, the temperature of the oxygen partial pressure furnace to be 150-250 °C, and the holding time to be 30 min - 2 h. A uniform and dense oxide film is obtained on the surface of the metal magnetic powder. The oxidation effect is mainly related to the oxygen content, oxidation temperature and oxidation time. If the oxidation time is too long and the oxygen content and oxidation temperature are too high, theoretically, it will cause serious oxidation of the metal magnetic powder, and even the metal powder matrix will be oxidized. High temperature may even cause the decomposition of small particle size powder, and ultimately lead to the complete loss of various properties of the metal magnetic powder; if the oxidation time is too short and the oxygen content and oxidation temperature are too low, theoretically, the oxide film on the surface of the metal magnetic powder will not be dense enough, porous, and the antioxidant property will be severely reduced. Even during the manufacturing and processing process, the oxide film is more likely to fall off and cannot achieve a good antioxidant effect. During the heat treatment of the metal magnetic powder, the oxygen content in the furnace should be controlled at a certain value, and the temperature should be slowly increased and decreased at a rate of 2-4 °C / min. Theoretically, the slow rise and fall of the temperature can make the reaction on the surface of the metal magnetic powder slow and uniform, which is helpful for the formation of a uniform and dense oxide film on the surface of the metal magnetic powder.

[0048] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to 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.

[0049] The present invention will be further described below 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

[0050] Step 1: Under the protection of an inert atmosphere (which can be one or both of argon and helium), dissolve 1% phosphoric acid (concentration 85%) by mass of the magnetic powder in absolute ethanol and stir well to dissolve it to form a 1 wt.% phosphoric acid phosphating solution; place the metal magnetic powder in the 1 wt.% phosphoric acid phosphating solution and stir to obtain the surface-phosphated metal magnetic powder;

[0051] Step 2: Under the protection of an inert atmosphere, place solid NaNO2 with a mass content of 0.5% of magnetic powder in a small amount of deionized water, and adjust the pH of the solution to 9 with NaOH. Stir well to form a 0.5% NaNO2 oxidizing alkaline solution. Place the surface-phosphated metal magnetic powder in the 0.5% NaNO2 oxidizing alkaline solution and stir well at 25 °C. After standing for reaction, the chemically oxidized metal magnetic powder is obtained. Then, still under the protection of an inert atmosphere, first wash the solution for cleaning the metal magnetic powder with deionized water until it is colorless and transparent, and then wash the metal magnetic powder twice with an ethanol solution and a n-hexane solution respectively. Place the washed metal magnetic powder in a glove box with an inert atmosphere protection and air-dry it naturally at room temperature. Finally, the dried chemically oxidized metal magnetic powder is obtained.

[0052] Step 3: Heat-treat the chemically oxidized samarium-iron-nitrogen magnetic powder in an oxygen partial pressure furnace: Place the samarium-iron-nitrogen magnetic powder treated by phosphating and chemical oxidation in an oxygen partial pressure tube furnace respectively, and introduce a mixed gas of nitrogen and air or pure oxygen. Control the flow rate of air or pure oxygen so that the oxygen content in the furnace is 1-15 vol.%. The oxygen partial pressure furnace controls the temperature at 150-250 °C, the heating and cooling rates are 2 °C / min, and the holding time is 30 min-2 h. Take out and collect the samarium-iron-nitrogen magnetic powder with a multiple synergistic protective film from the oxygen partial pressure tube furnace, and observe the microscopic morphology of the powder particles.

[0053] Set up control groups 1, 2, 3, and 4, where:

[0054] Control group 1 only has steps 1 and 2 in Example 1 and no step 3;

[0055] Control group 2 only has steps 1 and 3 in Example 1 and no step 2;

[0056] Control group 3 only has steps 2 and 3 in Example 1 and no step 1;

[0057] Control group 4 is the original samarium-iron-nitrogen powder without any treatment.

[0058] Figure 2 Show the transmission electron microscope (TEM) and EDS energy spectrum analysis results of Fe, Sm, O, and P elements of single samarium-iron-nitrogen powder in Example 1 and Control Group 1. It can be seen from the experimental results that a multiple synergistic protective layer of a uniform and dense phosphating film and metal oxide film is formed on the surface of the powder particles after multiple synergistic surface treatment. For Control Group 1 lacking step 3, due to the absence of the heat treatment process, the content of O element outside the magnetic powder is significantly less than that in Example 1, indicating that the oxide film outside the magnetic powder is not dense enough, which will reduce the high-temperature antioxidant ability of the magnetic powder.

[0059] Weigh 3.5 g - 4.5 g of the samarium iron nitride powders with different synergistic surface treatments collected from Example 1 and Control Groups 1 - 3, and the untreated samarium iron nitride raw powder of Control Group 4. Place the samarium iron nitride powders in a crucible, and then place the crucible in a high-temperature heat loss test chamber for heat treatment at 200 °C, 250 °C, and 300 °C for 30 min respectively. Weigh the heat-treated powders on a high-precision electronic balance, calculate the oxidation weight gain rate of the samarium iron nitride powders, complete the oxidation weight gain experiment of the samarium iron nitride powders, analyze the antioxidant properties of the samarium iron nitride powders at each temperature, and the corresponding data of the oxidation weight gain experiment are shown in Table 1.

[0060] The results of the oxidation weight gain experiments at different temperatures in Example 1 show that at 200 °C, the samarium iron nitride powders with multiple synergistic surface treatments in Example 1 hardly show oxidation behavior, and the antioxidant property is significantly enhanced. As the temperature rises to 250 °C and 300 °C, the weight gain rate increases with the increase of temperature, but the increase amplitude is not large.

[0061] However, for Control Groups 1, 2, and 3, due to the lack of one of the steps, the weight gain rate is increased compared with Example 1 at high temperatures. At 200 °C, the antioxidant property of the samarium iron nitride powders is not much different from that of Example 1, and the increase is not very obvious. But as the temperature rises to 250 °C and 300 °C, the weight gain rate is significantly higher than that of Example 1, indicating that the antioxidant property of the powders is weakened compared with Example 1. Among them, the antioxidant property of the powders in Control Group 3 without Step 1 is weakened more significantly.

[0062] The results of the oxidation weight gain experiments at different temperatures in Control Group 4 show that at 200 °C, 250 °C, and 300 °C, the samarium iron nitride powders all show severe oxidation. Compared with Example 1, after multiple synergistic surface treatments in Example 1, a uniform and dense protective film is formed on the surface of the samarium iron nitride powders, which significantly enhances the high-temperature antioxidant property of the samarium iron nitride powders at 200 °C - 300 °C.

[0063] Place the samarium iron nitride powders with multiple synergistic surface treatments collected from Example 1 and Control Groups 1 - 3, and the untreated samarium iron nitride raw powder of Control Group 4 in a crucible, and then place the crucible in a high-temperature heat loss test chamber for heat treatment at 200 °C, 250 °C, and 300 °C for 30 min respectively. Weigh 3 g of samarium iron nitride powders, weigh 0.18 g of paraffin according to the mass ratio of powder to paraffin of 3:0.18, then place the weighed powder and paraffin in a mortar and grind them evenly. Then use a manual hydraulic molding machine to cold-press the powder placed in the mold into a cylindrical block. The cold-pressing pressure is 40 KN, and the cold-pressing time is 3 min. Finally, use a permanent magnetic property automatic measuring instrument (AMT - 4) to test the permanent magnetic properties of the demolded blocks, and the corresponding data of the magnetic properties are shown in Table 2.

[0064] The permanent magnetic performance test results of Example 1 and Control Groups 1-4 by the automatic magnetic tester (AMT-4) show that the coercivity of the samarium iron nitride powder is basically corresponding to the antioxidant performance. The stronger the high-temperature antioxidant performance is, the smaller the loss of coercivity at high temperature is.

[0065] Table 1 Comparison of antioxidant properties of powders obtained in different examples

[0066] Sample Name ΔW Oxidation Weight Gain Rate at 200°C ΔW Oxidation Weight Gain Rate at 250°C ΔW Oxidation Weight Gain Rate at 300°C Example 1 0.0002 0.0057 0.0281 Control Group 1 0.0009 0.0121 0.0596 Control Group 2 0.0004 0.0096 0.0371 Control Group 3 0.0011 0.0154 0.0632 Control Group 4 0.2294 0.5303 0.9370

[0067] Table 2 Comparison of permanent magnetic properties of powders obtained in different examples

[0068] Sample Name Hcj / KOe at Room Temperature Hcj / KOe at 200°C Hcj / KOe at 250°C Hcj / KOe at 300°C Example 1 9.598 9.687 7.587 3.361 Control Group 1 9.687 9.34 7.096 3.005 Control Group 2 9.587 9.596 7.476 3.295 Control Group 3 9.456 9.496 7.106 3.132 Control Group 4 9.246 0.39487 0.15774 0.09774

[0069] As mentioned above, the above embodiments are only used to illustrate the technical solutions of the present invention, rather than 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 recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing metal magnetic powder with a multi - cooperative protective film on its surface, characterized in that, The method comprises: Step S1, under the protection of an inert atmosphere, placing a metal magnetic powder in an organic solution containing concentrated phosphoric acid and stirring it to obtain a surface-phosphorized metal magnetic powder; the solute in the organic solution containing concentrated phosphoric acid is concentrated phosphoric acid with a mass content greater than 80%, and the solvent is an alkyl alcohol; the phosphoric acid content in the organic solution containing concentrated phosphoric acid is 0.5%-10% of the mass content of the magnetic powder; Step S2, under the protection of an inert atmosphere, placing the surface phosphated metal magnetic powder in an alkaline solution containing an oxidant, stirring the solution sufficiently at 25-80° C., and allowing the solution to stand for reaction to obtain the metal magnetic powder after chemical oxidation treatment; the oxidant comprises one of sodium nitrate, sodium nitrite, hydrogen peroxide, potassium dichromate and potassium permanganate; the oxidant content in the alkaline solution containing the oxidant is 0.1%-1.0% of the mass content of the magnetic powder; Step S3, under the protection of an inert atmosphere, the metal magnetic powder after the chemical oxidation treatment is sequentially cleaned and dried to obtain dry metal magnetic powder after oxidation treatment; the cleaning and drying specifically include first washing the solution for cleaning the metal magnetic powder with deionized water until it is colorless and transparent, and then washing the metal magnetic powder twice with an ethanol solution and a n-hexane solution respectively; placing the cleaned metal magnetic powder in a glove box with an inert atmosphere protection and drying it naturally at room temperature; Step S4, placing the dried oxidized metal magnetic powder in an oxygen partial pressure furnace for heat treatment, so that a layer of uniform and dense multi-layer protective film that is well bonded to the substrate is formed on the surface of the metal magnetic powder.

2. The method according to claim 1, wherein The metal magnetic powder includes one of samarium iron nitrogen magnetic powder, neodymium iron boron magnetic powder, neodymium iron nitrogen magnetic powder, samarium cobalt magnetic powder, samarium iron cobalt magnetic powder and iron nitrogen compound magnetic powder or a composite thereof.

3. The method according to claim 1, wherein The inert atmosphere includes one or both of argon and helium.

4. The method according to claim 1, wherein A mixed gas of nitrogen and air or a mixed gas of nitrogen and pure oxygen is introduced into the oxygen partial pressure furnace, and the flow rate of air or pure oxygen is controlled so that the oxygen content in the furnace is 1-15 vol.%.

5. The method according to claim 1, wherein The oxygen partial pressure furnace controls the temperature at 150-250° C., the heating and cooling rates at 2-4° C. / min, and the heat preservation time at 30 min-2 h.

6. A metal magnetic powder having multiple synergistic protective films on its surface, prepared by the method according to any one of claims 1 to 5.

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