阅读说明：本技术 一种低熔点金属或合金辅助重稀土元素晶界扩散的方法 (Method for assisting grain boundary diffusion of heavy rare earth element by low-melting-point metal or alloy ) 是由 李家节 李飞飞 屈鹏鹏 何磊 仲洁 王磊 杨斌 于 2020-12-21 设计创作，主要内容包括：一种低熔点金属或合金辅助重稀土元素晶界扩散的方法,其特征在于：通过固相、液相、气相包覆的方法使重稀土金属粉末表面包覆一层低熔点金属/合金粉末,或者使低熔点金属/合金粉末表面包覆一层重稀土粉末,形成具有表面改性的异质核壳结构的复合粉作为扩散源。通过晶界扩散技术制备高性能钕铁硼永磁材料,能够在减少重稀土用量的同时,大幅度提高稀土永磁体的内禀矫顽力。(A method for assisting grain boundary diffusion of heavy rare earth elements by low-melting-point metal or alloy is characterized by comprising the following steps: the surface of the heavy rare earth metal powder is coated with a layer of low-melting-point metal/alloy powder by a solid-phase, liquid-phase and gas-phase coating method, or the surface of the low-melting-point metal/alloy powder is coated with a layer of heavy rare earth powder to form composite powder with a surface-modified heterogeneous core-shell structure as a diffusion source. The high-performance neodymium iron boron permanent magnet material is prepared by a grain boundary diffusion technology, so that the intrinsic coercive force of the rare earth permanent magnet can be greatly improved while the use amount of heavy rare earth is reduced.)

1. A method for assisting grain boundary diffusion of heavy rare earth elements by low-melting-point metal or alloy is characterized by comprising the following steps: the method comprises the following steps:

a. coating a layer of low-melting-point metal/alloy powder on the surface of the heavy rare earth powder by a solid-phase, liquid-phase or gas-phase coating method, or coating a layer of heavy rare earth powder on the surface of the low-melting-point metal/alloy powder to form composite powder with a heterogeneous core-shell structure as a diffusion source;

b. uniformly covering the prepared diffusion source on the upper and lower surfaces of the magnet through a grain boundary diffusion technology;

c. putting the processed magnet into a sintering furnace, wherein the vacuum degree is lower than 1x10^-3Pa, heating to 750-950 ℃, preserving heat for 2-16 h, cooling to room temperature, heating to 450-650 ℃, and preserving heat for 1-6 h.

2. The method of claim 1, wherein the low melting point metal or alloy assists in grain boundary diffusion of the heavy rare earth element, and the method comprises the following steps: in step a, the solid phase coating method comprises a mechanical ball milling method and a high energy method.

3. The method of claim 1, wherein the low melting point metal/alloy assists in grain boundary diffusion of the heavy rare earth element, and wherein: in the step a, the liquid phase coating method comprises a heterogeneous flocculation method, a sol-gel method and a polymer coating method.

4. The method of claim 1, wherein the low melting point metal or alloy assists in grain boundary diffusion of the heavy rare earth element, and the method comprises the following steps: in the step a, the gas phase coating method is to realize the surface coating of the micro/nano powder by directly utilizing gas or converting a shell layer substance into gas to ensure that the gas is subjected to biological change or chemical change in a gaseous state; the vapor phase coating method includes a physical vapor deposition coating method and a chemical vapor deposition coating method.

5. The method of claim 1, wherein the low melting point metal or alloy assists in grain boundary diffusion of the heavy rare earth element, and the method comprises the following steps:

the heavy rare earth element refers to one or more of Dy, Tb, Ho and Gd, and the heavy rare earth powder also comprises one or more of heavy rare earth fluoride, hydride and oxide powder besides the heavy rare earth metal powder or alloy powder.

6. The method of claim 1, wherein the low melting point metal or alloy assists in grain boundary diffusion of the heavy rare earth element, and the method comprises the following steps:

the low-melting-point metal refers to one or more of Al, Cu, Mg, Zn, Sn and Ga.

7. The method of claim 1, wherein the low melting point metal or alloy assists in grain boundary diffusion of the heavy rare earth element, and the method comprises the following steps:

the low-melting-point alloy consists of RE according to atomic percentage_xM_yWherein RE is one or more of Pr, Nd, La, Ce and Y, M is one or more of Cu, Al, Zn, Sn, Ga and Mg, x is more than or equal to 50 and less than or equal to 90, and Y is more than or equal to 10 and less than or equal to 50.

8. The method of claim 1, wherein the low melting point metal or alloy assists in grain boundary diffusion of the heavy rare earth element, and the method comprises the following steps: the particle size of the heavy rare earth powder is 0.3-10 mu m, the particle size of the low-melting-point metal is 0.3-10 mu m, and the particle size of the low-melting-point alloy is 0.5-300 mu m.

9. The method of claim 1, wherein the low melting point metal or alloy assists in grain boundary diffusion of the heavy rare earth element, and the method comprises the following steps:

the grain boundary diffusion technology comprises coating, spraying, vapor deposition, magnetron sputtering and metal printing.

Technical Field

The invention belongs to the field of rare earth magnetic materials, and relates to a method for assisting grain boundary diffusion of a heavy rare earth element by using low-melting-point metal or alloy.

Background

The permanent magnet material is an important basic functional material, has higher and higher application value in science and technology and daily life, and comprises the fields of wind power generation, aerospace, medical machinery, computers, household appliances, office automation and the like. However, with the increasing of high-end applications, the requirements on product performance are higher and higher, and the high-performance neodymium iron boron depends on expensive elements such as heavy rare earth Dy and Tb. How to obtain high-performance magnets at reduced cost has become a major and difficult point of recent research.

The coercive force is an important parameter for evaluating the performance of the neodymium-iron-boron magnet, and the compound Dy of heavy rare earth elements Dy and Tb₂Fe₁₄B、Tb₂Fe₁₄B has a large magnetocrystalline anisotropy field, and thus can increase the coercive force. But the price is expensive, the regeneration is not possible, and the application is greatly limited. Grain boundary diffusion is widely applied in enterprises as an effective method for improving the coercivity. However, the element diffusion depth is limited, and the ordinary grain boundary diffusion is only suitable for magnets with the thickness of less than 5mm, and has little effect on thicker magnets. Therefore, how to improve the diffusion depth under the condition of saving cost has profound significance for research and application.

Micro/nano powder is widely concerned by people with unique properties, but due to the phenomena of flammability, agglomeration and the like, the micro/nano powder needs to be subjected to surface modification in the preparation and application processes to fully exert the performance. Therefore, the method has important research significance for surface modification of micro/nano powder. The surface modification of the micro/nano powder is mainly to wrap a coating layer on the micro/nano surface to form a heterogeneous core-shell structure. The structure can lead the micro/nano powder to have better stability and weather resistance. Common coating methods include solid phase coating, liquid phase coating and gas phase coating.

The coating technology has wide application in the preparation and application fields of micro/nano powder, andthe technology is mature. The grain boundary diffusion technology has attracted wide attention since the proposal, and is one of the important methods for preparing the high-performance sintered neodymium iron boron. The diffusion source used for grain boundary diffusion is often micro/nano powder, such as TbH₂，DyF₃And the like, so that the application of the coating technology to the preparation of the diffusion source has important research significance. It is known that the melting point of heavy rare earth elements is rather high, and the heavy rare earth elements which are solid at high temperature are diffused to the liquid grain boundary phase and form (Nd, HRE)₂Fe₁₄And B, a core-shell structure. But its diffusion capacity is limited and the diffusion rate is low. Therefore, it is necessary to provide a method for increasing the diffusion rate while reducing the amount of heavy rare earth.

CN106710765A discloses a high-coercivity sintered neodymium-iron-boron magnet, and the preparation method thereof adopts a grain boundary addition method to mix, orient, form and sinter main alloy powder and auxiliary alloy powder. The Dy in the main alloy powder and the Dy in the auxiliary alloy powder need time for decomposition, the diffusion rate is slowed down, the melting point is increased along with the increase of the Dy content, the dosage of heavy rare earth cannot be adjusted at will, and the flexibility of component blending is poor.

CN110473684A discloses a preparation method of a high-coercivity sintered neodymium iron boron magnet, which comprises the steps of uniformly mixing a cobalt-based hard alloy ball and heavy rare earth compound powder, and enabling the cobalt-based hard alloy and the heavy rare earth powder to impact the surface of the sintered neodymium iron boron magnet through high-pressure airflow, so that the contact area between a diffuser and the magnet is increased, and the diffusion efficiency is improved. However, the method needs to impact the surface of the neodymium iron boron magnet for 10-30min before sintering the magnet, and also needs to accurately control the depth of an amorphous layer or a microcrack area to be 0.2-1.0 mm, has high requirement on the control precision of the process, and is not beneficial to large-scale production.

The invention creatively provides a method for assisting the grain boundary diffusion of heavy rare earth elements by using low-melting-point metal/alloy, which increases the diffusion rate and greatly improves the coercive force while reducing the use amount of heavy rare earth.

Disclosure of Invention

The invention provides a method for low-melting-point metal/alloy to assist grain boundary diffusion of heavy rare earth elements, which is characterized in that the heavy rare earth powder is coated with a layer of low-melting-point metal/alloy powder by a solid phase, liquid phase and gas phase coating method, or the surface of the low-melting-point metal/alloy powder is coated with a layer of heavy rare earth powder to form composite powder with a heterogeneous core-shell structure as a diffusion source. The coercive force of the rare earth permanent magnet can be greatly improved while the dosage of heavy rare earth is reduced.

In order to achieve the purpose, the invention provides the following technical scheme:

a. the heavy rare earth powder and the target material which are prepared in advance are coated with a layer of low-melting-point metal/alloy powder by a solid-phase, liquid-phase and gas-phase coating technology, or the low-melting-point metal/alloy powder is coated with a layer of heavy rare earth powder to form composite powder with a heterogeneous core-shell structure as a diffusion source.

b. And uniformly covering the prepared diffusion source on the upper surface and the lower surface of the magnet through a grain boundary diffusion technology.

c. Placing the processed magnet into a sintering furnace, wherein the vacuum degree is lower than 1 × 10^-3Pa, heating to 750-950 ℃, preserving heat for 2-16 h, cooling to room temperature, heating to 450-650 ℃, and preserving heat for 1-6 h.

Further, the solid phase coating method in step a comprises a mechanical ball milling method and a high energy method.

1. The mechanical ball milling method is to activate the powder surface purposefully by using the action of mechanical stress, so that the powder surface can adsorb other substances to achieve the purpose of surface coating.

2. The high energy method is a method of coating a micro/nano powder with ultraviolet rays, radioactive rays, or the like.

Further, the liquid phase coating method in step a comprises a heterogeneous flocculation method, a sol-gel method and a polymer coating method.

1. The heterogeneous flocculation method refers to the coagulation of powders with different charges by mutual attraction, and the heterogeneous flocculation method utilizes the function to carry out surface coating.

2. The sol-gel coating method is to disperse the core layer powder in medium with good compatibility and to coat the core layer powder through hydrolysis, condensation and other reactions of the shell layer source material.

3. The polymer coating method comprises two coating modes, wherein the coating is directly realized by the action of the micro/nano powder and the polymer, and the coating is realized by the polymerization of the monomer on the surface of the micro/nano powder. Common polymers with good film forming and medium compatibility are PVP, 4-dodecylbenzene sulfonic acid (DBSA).

Further, the gas phase method in step a is to directly use gas or convert the shell material into gas by various means, and make the gas undergo biological change or chemical change in the gas state to realize the coating of the surface of the micro/nano powder. The vapor phase method includes a physical vapor deposition coating method and a chemical vapor deposition coating method.

1. Physical vapor deposition utilizes the action of van der waals forces to achieve surface coating.

2. The chemical vapor deposition coating method is to utilize gaseous matter to generate solid deposition on the surface of powder to realize coating.

Further, the heavy rare earth element refers to one or more of Dy, Tb, Ho and Gd, and besides the heavy rare earth metal powder or the alloy powder, the heavy rare earth powder also comprises one or more of heavy rare earth fluoride, hydride and oxide powder.

Furthermore, the low-melting-point metal refers to one or more of Al, Cu, Mg, Zn, Sn and Ga.

Further, the low melting point alloy is composed of RE according to atomic percentage_xM_yWherein RE is one or more of Pr, Nd, La, Ce and Y, M is one or more of Cu, Al, Zn, Sn, Ga and Mg, x is more than or equal to 50 and less than or equal to 90, and Y is more than or equal to 10 and less than or equal to 50.

Furthermore, the particle size of the heavy rare earth powder is 0.3-10 mu m, the particle size of the low-melting-point metal is 0.3-10 mu m, and the particle size of the low-melting-point alloy is 0.5-300 mu m.

Further, the grain boundary diffusion technology refers to coating, spraying, vapor deposition, magnetron sputtering, metal printing and the like.

Compared with the prior art, the invention has the following advantages:

1. the heterogeneous core-shell structure composite powder diffusion source preferentially melts low-melting-point metal/alloy into a liquid phase during high-temperature diffusion, and the heavy rare earth powder is coated in the liquid phase and subjected to liquid-liquid diffusion with a rare earth-rich grain boundary phase, so that the diffusion efficiency is higher.

2. Relative to low melting point alloys containing heavy rare earths (e.g. Pr)₃₅Dy₃₅Al₃₀Etc.), the melting point of Pr-Dy-Al increases with the increase of Dy content, the melting point of the composite powder of the invention is fixed, the low melting point metal/alloy powder and the heavy rare earth powder are mixed in different proportions, the dosage of the heavy rare earth can be adjusted at will, the melting point of the composite powder is not changed, and the composite powder has flexible and adjustable freedom of components.

3. Low melting point alloys containing heavy rare earth elements (e.g. Pr)₃₅Dy₃₅Al₃₀) The decomposition of Dy requires time at high temperature, and the diffusion rate is slowed down, while the heterogeneous core-shell structure composite powder diffusion source prepared by the coating method reduces the melting point of a rare earth-rich grain boundary phase with the help of low-melting-point metal/alloy, so that heavy rare earth elements have larger diffusion coefficient, and the diffusion rate is further improved.

4. The composite powder with heterogeneous core-shell structure forms a hard shell layer with high magnetocrystalline anisotropy field on the main phase crystal grain epitaxial layer, namely (Nd, HRE)₂Fe₁₄The core-shell structure B effectively inhibits the nucleation and growth of the anti-magnetization domain, and obviously improves the intrinsic coercivity of the material.

5. The low-melting-point light rare earth eutectic alloy can improve the wettability between a neodymium-rich crystal boundary phase and a main phase crystal grain, optimize a microstructure, enable the distribution of the neodymium-rich phase to be more continuous and smooth, play a role in demagnetization coupling and improve the intrinsic coercivity of the material.

6. The coercive force of the rare earth permanent magnet can be greatly improved while the dosage of heavy rare earth is reduced by the heterogeneous core-shell structure composite powder diffusion source.

7. In addition, if the surface of the diffusion source is coated with a substance with stronger oxidation resistance, the oxidation resistance of the diffusion source can be enhanced.

Drawings

FIG. 1 is a schematic diagram of a composite powder diffusion source with a heterogeneous core-shell structure

FIG. 2 is a schematic view of a coating layer prepared by a solid phase coating method

FIG. 3 is a schematic view of the coating layer after high temperature melting

FIG. 4 is a schematic diagram of a polymer coating process for preparing a double coating

FIG. 5 is a diagram showing the relationship between the magnetic remanence and intrinsic coercivity of the initial magnet and the grain boundary diffusion magnet.

Detailed description of the preferred embodiments

As shown in fig. 1, in the embodiment of the present invention, a layer of low-melting-point metal/alloy powder is coated on a heavy rare earth powder by a solid-phase, liquid-phase, or gas-phase method, or a layer of heavy rare earth powder is coated on the surface of the low-melting-point metal/alloy powder to form a composite powder with a heterogeneous core-shell structure as a diffusion source, the low-melting-point metal or low-melting-point alloy powder of a shell layer is melted while the core remains in a solid state under the action of high temperature, and with reference to fig. 2 and 3, the heterogeneous core-shell structure composite powder is coated on the surface of a magnet, when the temperature is raised, the low-melting-point metal or alloy is preferentially melted, and. FIG. 4 shows a polymer coating process with double coating.

Example 1:

configuring atomic percent as Pr₈₀Al₂₀The raw materials are smelted into alloy ingots by an electric arc smelting mode, the smelted alloy ingots are crushed into small blocks, the small blocks are made into strips by a rapid quenching mode, and then the strips are crushed into powder with larger particle size. With DyF having a particle size of 10 μm₃Mixing the powders at a ratio of 1:1, ball milling for 3 hr under the protection of ethanol at a rotation speed of 300r/min (DyF after filtering)₃The powder can be recycled). Purposeful p-Pr by mechanical stress₈₀Al₂₀Activating the surface of the alloy powder to make the surface absorb DyF₃And (3) powder. Pr thus prepared having a particle size of 20 μm₈₀Al₂₀The surface of the alloy powder is coated with a layer of DyF with the granularity of 800nm₃And (3) powder. And mixing the prepared heterogeneous core-shell structure composite powder diffusion source with alcohol and citric acid monohydrate to prepare slurry, and uniformly covering the upper surface and the lower surface of the N50 magnet in a spraying manner.

Putting the processed magnet into a sintering furnace, wherein the vacuum degree is lower than 1x10^-3Pa, heating to 850 ℃ and preserving heat for 8h, and heating to 500 ℃ and preserving heat for 3h after cooling to room temperature.

TABLE 1

Example 2

Tb metal with the purity of 99.7% is put into a hydrogen breaking furnace, and is broken by hydrogen at 300 ℃ for 3h and dehydrogenated at 500 ℃ for 3 h. Preparing TbH with slightly larger granularity₂And (3) coarse powder. TbH₂Putting the coarse powder into a ball mill, and ball-milling for 3h at 200r/min to prepare powder with the particle size of 5 mu m. Due to TbH₂Fine powders have limited antioxidant capacity and are flammable, which must be done in a glove box.

Adopting Cu powder with the particle size of 500nm to assist TbH₂Grain boundary diffusion, according to Cu: TbH₂: mixing the alcohol in a ratio of 1:1:1 to obtain slurry, placing the slurry in the alcohol and shaking for 30min to enable the Cu nano powder to be adsorbed on the TbH₂The surface of the powder.

And (3) immersing the polished magnet into the slurry for 2-3 s, taking out, drying by using argon, and repeating for 3 times. Putting the processed magnet into a sintering furnace, wherein the vacuum degree is lower than 1x10^-3Pa, heating to 900 ℃ and preserving heat for 8h, and heating to 520 ℃ and preserving heat for 4h after cooling to room temperature.

TABLE 2

Example 3

Is provided with the atomic percentage of La₇₅Al₂₅The raw materials are smelted into alloy ingots by an arc smelting mode, the smelted alloy ingots are crushed into small blocks, the small blocks are made into strips by a rapid quenching mode, and the strips are ground into coarse powder (La) with the diameter of 200 mu m under the protection of argon₇₅Al₂₅It is ground into coarse powder because of its weak oxidation resistance).

Due to La₇₅Al₂₅Has weak oxidation resistance, is more suitable for adopting a polymer coating mode, and has DyF₃Inorganic substance and polyvinyl butyral (PVB) have good compatibility, and DyF with particle size of 2 μm₃The powder can be uniformly dispersed in the PVB alcohol solution. Thus, the polymer coating method is adopted to coat La₇₅Al₂₅DyF of alloy powder₃And a bilayer of PVBAnd (4) coating. The method is simple to operate, convenient to operate and strong in oxidation resistance, the slurry brushing mode can be carried out in the air, the production cost is reduced, the binding force between the sample and the slurry is strong, and waste caused by falling of a diffusion source is avoided. The method is especially suitable for low-melting-point alloys containing abundant rare earth (La, Ce and Y).

According to La₇₅Al₂₅：DyF₃: PVB: preparing slurry according to the proportion of 1:1:1:1, wherein the preparation process is carried out under the protection of argon. And first La is added₇₅Al₂₅Coarse powder and DyF₃The fine powder is put into alcohol and vibrated in ultrasonic wave for 20min to be mixed evenly, and then PVB is added and stirred evenly.

The slurry is uniformly coated on the upper surface and the lower surface of the N42 magnet by means of brushing, and the thickness of the coating can be controlled by the number of times of brushing. And (4) putting the treated magnet into a drying box for drying.

Putting the processed magnet into a sintering furnace, wherein the vacuum degree is lower than 1x10^-3Pa, heating to 850 ℃ and preserving heat for 10h, and heating to 490 ℃ and preserving heat for 4h after cooling to room temperature.

TABLE 3

As can be seen from Table 3, even La₇₅Al₂₅The coercive force of 6.51kOe can be improved by assisting the grain boundary diffusion of the heavy rare earth Dy element, and the remanence is only slightly reduced.

Example 4

Configuring atomic percent as Pr₈₀Ga₂₀The raw materials are smelted into alloy ingots by an electric arc smelting mode, the smelted alloy ingots are crushed into small blocks, the small blocks are made into strips by a rapid quenching mode, and then the strips are crushed into powder with larger particle size. Ball milling is carried out for 4 hours under the protection of ethanol, the rotating speed is 300r/min, and powder with the granularity of 50 mu m is obtained. Tb metal with the purity of 99.7% is put into a hydrogen breaking furnace, and is broken by hydrogen at 300 ℃ for 3h and dehydrogenated at 500 ℃ for 3 h. Preparing TbH with slightly larger granularity₂And (3) coarse powder. TbH₂Putting the coarse powder into a ball mill, and ball-milling for 5 hours at 200r/min to obtain the powder with the granularity of 1-2Powder of μm.

According to Pr₈₀Ga₂₀：TbH₂: PVP: preparing slurry according to the proportion of 1:1:1:1, wherein the preparation process is carried out under the protection of argon. And first Pr₈₀Ga₂₀And TbH₂Putting into alcohol, shaking in ultrasonic wave for 30min to mix well, then adding PVP and stirring well to prepare slurry.

The prepared slurry is placed on a metal mesh with holes and uniformly brushed on the upper surface and the lower surface of the N38 magnet in a screen printing mode.

Putting the processed magnet into a sintering furnace, wherein the vacuum degree is lower than 1x10^-3Pa, heating to 850 ℃ and preserving heat for 10h, and heating to 490 ℃ and preserving heat for 3h after cooling to room temperature.

TABLE 4

It can be seen from Table 4 that with Pr₈₀Ga₂₀The alloy powder assists Tb element in grain boundary diffusion, the coercive force is improved by 12.44kOe, the coercive force is greatly improved, and the remanence is only slightly reduced.

It should be understood that the above examples are only for clarity of illustration and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications therefrom are within the scope of the invention.