WO2015051756A1 - Surface treatment method and preparation method of a sintered ndfeb magnet - Google Patents

Abstract

A surface treatment method and a preparation method of a sintered NdFeB magnet. The surface treatment method comprising: dipping, coating or spraying a solution containing at least one of Dy and Tb onto the surface of the sintered NdFeB magnet, diffusion processing the sintered NdFeB magnet in a non-oxidizing environment. The intrinsic coercive force can be improved while the maximum energy product of the sintered NdFeB magnet is held on the basis of the surface treatment method and the preparation method.

Description

Surface treatment method and manufacturing method of sintered NdFeB magnet Technical field

The present invention relates to a surface treatment method and a manufacturing method for a sintered NdFeB magnet.

Background technique

Rare earth permanent magnet materials are permanent magnet materials with intermetallic compounds composed of different rare earth elements and transition metals (Fe, Co, Ni, etc.), which have been widely used in many fields and become important basic functions of contemporary new technologies. Materials, especially in the field of permanent magnet motors, play an irreplaceable role. Today, permanent magnet motors using rare earth permanent magnet materials have covered major types such as stepper motors, brushless motors, servo motors and linear motors, and are widely used in computers, printers, household appliances, air conditioner compressors, and automotive power steering motors. , hybrid or pure electric vehicle drive motor / generator, car starter motor, ground military motor, aviation motor and other important areas.

High-performance sintered NdFeB magnets with iron as the basic component and high coercivity and high magnetic energy product have been the rare earth permanent magnet materials. Applying such a rare earth permanent magnet material to various motors can significantly improve the performance of the motor, reduce the weight of the motor, reduce the size of the motor, and obtain an efficient energy saving effect. Moreover, NdFeB has a high performance-price ratio.

At present, the laboratory level of the maximum magnetic energy product of sintered NdFeB magnets is very close to the theoretical limit (about 93%), but its intrinsic coercivity is far below the ideal limit. Specifically, even with a very extreme means, it is only about 25% of the magnetocrystalline anisotropy field H _A of the main phase of the NdFeB. However, high internal coercivity is the most basic requirement for various new applications of sintered NdFeB magnets. Therefore, how to fully exert the internal neodymium magnetic characteristics of the main phase of NdFeB to improve the intrinsic coercive force of sintered NdFeB magnets is a hot issue in current research.

A method of adding a heavy rare earth element such as Dy(镝)/Tb(铽) to a magnet alloy smelting process to partially replace Nd in the magnet to increase the coercive force of the sintered NdFeB magnet is known. Since Dy ₂ Fe ₁₄ B or Tb ₂ Fe ₁₄ B has a higher magnetocrystalline anisotropy field than Nd ₂ Fe ₁₄ B, that is, it has a larger ideal value of intrinsic coercivity, and therefore, by Dy/Tb The solid solution phase (Nd, Dy) ₂ Fe ₁₄ B or (Nd, Tb) ₂ Fe ₁₄ B formed after partially replacing the Nd in the main phase Nd ₂ Fe ₁₄ B can effectively increase the magnetic crystal of Nd ₂ Fe ₁₄ B The anisotropy field can significantly increase the intrinsic coercivity of the sintered NdFeB magnet. However, the adverse effect of this elemental substitution is a significant reduction in the saturation magnetization of the Nd ₂ Fe ₁₄ B phase, thereby significantly reducing the remanence and maximum magnetic energy product of the magnet. This is because the magnetic moments of Nd and Fe are aligned in parallel in the Nd ₂ Fe ₁₄ B main phase, that is, the two are ferromagnetic coupling, and the magnetic moments of the two are enhanced superposition; and the magnetic moments of Dy/Tb and Fe For ferrimagnetic coupling, the magnetic moment of Dy/Tb is arranged in anti-parallel with the Fe magnetic moment, thus partially canceling the total magnetic moment of the main phase. In addition, Dy/Tb reserves are sparse and unevenly distributed relative to Nd, and the price of Dy/Tb is much higher than Nd, and this substitution causes an increase in cost.

In addition, some new processes have been used to increase the intrinsic coercivity of sintered NdFeB magnets. For example, CN101845637A, CN101517670A, CN101521068A, CN101615459A, CN101620904A, CN101563738A, and the like. These new processes coat or deposit a metal powder or compound containing Dy or Tb onto the surface of NdFeB, and then Dy or Tb into NdFeB by diffusion. In addition, CN20110024823 dissolves a mixture of rare earth fluoride, rare earth nitrate and phosphate in an alcohol or water, and then thermally diffuses the magnet to improve the intrinsic coercive force.

However, the type of metal powder and the coating uniformity of the metal powder and the rare earth fluoride are very sensitive to the effect of improving the intrinsic coercive force, and Dy or Tb is difficult to uniformly disperse, resulting in product consistency problems, or being A large amount of volatilization does not deposit on the surface of NdFeB, which results in the inability to save Dy or Tb.

Summary of the invention

The inventors of the present invention found that the theoretical maximum value of the maximum magnetic energy product of pure Nd ₂ Fe ₁₄ B intermetallic compound is 64 MGOe at normal temperature, and the magnetocrystalline anisotropy field is 76 kOe, which is the ideal limit of the intrinsic coercive force Hcj. value. In order to obtain high intrinsic coercivity, the presence of a rare earth-rich phase must exist in the grain boundary of the main phase having a crystal structure of Nd ₂ Fe ₁₄ B. Porosity, impurities, grain orientation, etc., also cause the maximum magnetic energy product and the intrinsic coercive force of the actual magnet to differ from their theoretical limit values or ideal limit values, especially the difference in intrinsic coercivity. It is known from the definition of the maximum energy product that it is proportional to the square of the remanence Br of the magnet, and the relationship between the remanence Br of the magnet and various influencing factors can be expressed by the following formula:

Br=(Is·β)·(ρ/ρO)·(1-α)·f

Where Is=4πMs is the saturation magnetic polarization of the main phase, β is the temperature influence factor of Is, ρ/ρO is the relative density, α is the volume percentage of the non-magnetic phase, and f is the orientation factor of the main phase grains.

In addition, the intrinsic coercive force Hcj of the NdFeB sintered magnet is not only closely related to the magnetocrystalline anisotropy field H _{A of the} main phase, but also to the saturation magnetic polarization Is of the main phase, and the relationship between them can be expressed as follows:

Hcj=C·H _A -N·Is

Where C depends on the interaction between the main phase grains and their interfacial grains, and N is the effective demagnetization factor. C and N are sensitively dependent on the grain size and distribution of the sintered magnet, as well as the orientation and boundary features between adjacent grains.

In the past, after adjusting the formulation and routing of the magnet, its Br and Hcj were basically determined. According to the above relationship of Hcj of the sintered NdFeB magnet, the intrinsic coercive force Hcj depends on the magnetocrystalline anisotropy field H _{A of the} main phase and on the mutual phase grains of the main phase and the interfacial grains thereof. Role, and boundary features between adjacent grains.

In view of the above, the object of the present invention is to increase the magnetocrystalline anisotropy field of the surface layer of the main phase by grain boundary diffusion method, and at the same time improve the boundary characteristics of the grain boundary and its interaction with the main phase grains. Increasing the intrinsic coercive force of the sintered NdFeB magnet, while having a minimal negative impact on the remanence and magnetic energy product of the magnet.

In order to achieve the above object, the present invention provides a surface treatment method for sintering NdFeB, comprising: impregnating, coating or spraying a solution containing at least one of Dy and Tb on a surface of sintered NdFeB in a non-oxidizing environment. The sintered NdFeB is subjected to diffusion treatment.

By using the surface treatment method of the present invention, Dy or Tb is diffused to the grain boundary of the sintered NdFeB, thereby improving Hcj. Use a volatile solvent such as alcohol, gasoline or petroleum ether The nitrate containing Dy or Tb can conveniently prepare a solution in which Dy or Tb atoms or ions are uniformly distributed, thereby avoiding the disadvantage that the powder is difficult to suspend and coat, and it is easier to achieve uniform coating such as dipping, coating or spraying.

detailed description

Hereinafter, a method for producing a sintered NdFeB magnet of the present invention will be specifically described.

First, a sintered NdFeB billet magnet is prepared according to a conventional process, for example, a batch-magnet magnet is prepared by a process of batch-alloy preparation-grinding-forming-forming-sintering-tempering. In the present invention, the sintered NdFeB magnet magnet is made to have a minimum size (thickness) of 10 mm or less, for example, a thickness of 0.5 to 10 mm, preferably 0.5 to 5 mm.

The blank magnet is then subjected to a conventional surface cleaning treatment.

Next, the solution was prepared. Wherein the solute is a hydrated nitrate containing at least one of Dy and Tb, and the solvent is, for example, alcohol, gasoline, water or the like. The mass concentration of the solution (solute mass: (solute mass + solvent mass)) is not less than 25%.

Next, the disposed solution is attached to the surface of the sintered NdFeB magnet by a process such as dipping, coating or spraying.

Next, the magnet is placed in a vacuum sintering furnace, and subjected to diffusion treatment under vacuum or under an inert gas (particularly argon) after evacuation. The diffusion treatment temperature is 800 to 1000 ° C and the time is 2 to 10 hours.

Finally, in the vacuum sintering furnace of the above, tempering treatment is carried out in a vacuum state or in a state where an inert gas (particularly argon gas) is charged after evacuation. The temperature of the tempering treatment is 450 to 620 ° C, and the time is 2 to 10 hours.

The magnetic properties of the magnet after the tempering treatment were measured.

Example 1 and Comparative Example 1

The sintered NdFeB was processed into 6 pieces of a disk having a size of D10×2. A hydrated nitrate Tb(NO ₃ ) ₃ ·5H ₂ O was used as a solute, and anhydrous alcohol was used as a solvent to prepare a solution having a mass concentration of 25%. Three wafers (Example 1) were immersed in the above solution for 3 minutes, taken out, placed in a boat and temporarily stored in a nitrogen atmosphere, and then placed simultaneously with three other untreated wafers (Comparative Example 1). Into the vacuum sintering furnace, the immersion and non-immersion products are separated by more than 10 cm. The diffusion treatment was carried out at 800 ° C for 10 hours in a vacuum atmosphere. Then, it was cooled with an inert gas to 100 ° C or lower, and then evacuated and tempered, and the tempering temperature was 450 ° C for 6 hours. Then, it is cooled by an inert gas and cooled to 70 ° C or lower. The BH curve was measured to obtain Br, Hcj, and (BH)max, as shown in Table 1.

Table 1

sample	B r (kGs)	H cj (kOe)	(BH) max (MGOe)
				Example 1 (surface soaked)	12.90	17.22	40.3
Comparative Example 1 (surface not soaked)	12.98	14.12	40.8

As can be seen from Table 1, the Hcj of Example 1 was increased by 3.10 kOe as compared with Comparative Example 1, and (BH) _max hardly decreased.

Example 2 and Comparative Example 2

The sintered NdFeB was processed into 6 pieces of a disk having a size of D10×3. A hydrated nitrate Tb(NO ₃ ) ₃ ·5H ₂ O was used as a solute, and anhydrous alcohol was used as a solvent to prepare a solution having a mass concentration of 35%. The entire surface of the three wafers (Example 2) was sprayed by spraying, then placed in a boat and temporarily stored in a nitrogen atmosphere, and then placed simultaneously with three other untreated wafers (Comparative Example 2). Into the vacuum sintering furnace, the sprayed and non-sprayed products were separated by more than 10 cm, and subjected to diffusion treatment at 850 ° C for 6 hours in a vacuum environment. Then, it was filled with an inert gas and cooled to 100 ° C or lower, and then evacuated and tempered, and the tempering temperature was 480 ° C for 4 hours. Then, it is cooled by an inert gas and cooled to 70 ° C or lower. The BH curve was measured to obtain Br, Hcj, and (BH)max, as shown in Table 2.

Table 2

sample	B r (kGs)	H cj (kOe)	(BH) max (MGOe)
				Example 2 (surface sprayed)	12.95	18.2	40.70
Comparative Example 2 (surface not sprayed)	13.12	16.5	41.74

As can be seen from Table 2, the Hcj of Example 2 was increased by 1.7 kOe compared to Comparative Example 2, and (BH) _max hardly decreased.

Example 3 and Comparative Example 3

The sintered NdFeB was processed into a total of 6 wafers having a size of D10×10. A hydrated nitrate Dy(NO ₃ ) ₃ ·6H ₂ O was used as a solute, and anhydrous alcohol was used as a solvent to prepare a solution having a mass concentration of 35%. The entire surface of the three wafers (Example 3) was coated by coating, then placed in a boat and temporarily stored in a nitrogen atmosphere, and then with three other untreated wafers (Comparative Example 3). At the same time, it was placed in a vacuum sintering furnace, and the coated article and the non-coated article were separated by more than 10 cm, and diffusion treatment was carried out at 850 ° C for 6 hours in a vacuum atmosphere. Then, it was filled with an inert gas and cooled to 100 ° C or lower, and then evacuated and tempered, and the tempering temperature was 520 ° C for 6 hours. Then, it is cooled by an inert gas and cooled to 70 ° C or lower. The BH curve was measured to obtain Br, Hcj, and (BH)max, as shown in Table 3.

table 3

sample	B r (kGs)	H cj (kOe)	(BH) max (MGOe)
				Example 3 (surface sprayed)	12.11	19.5	35.56
Comparative example 3 (surface not sprayed)	12.23	18.5	36.27

As can be seen from Table 3, the Hcj of Example 3 was increased by 1.0 kOe as compared with Comparative Example 3, and (BH) _max hardly decreased.

Examples 4 to 15 and Comparative Examples 4 to 15

Tables 4 and 5 show Examples 4 to 15, respectively. Among them, Comparative Examples 4 to 15 were not subjected to immersion, coating or spray treatment as compared with Examples 4 to 15, and the others were the same.

Table 4

table 5

As can be seen from the above examples and comparative examples, the sintered NdFeB magnet manufactured by the manufacturing method of the present invention can significantly increase Hcj while making the decrease in the maximum magnetic energy product small.

Although the invention has been described above by way of specific embodiments, the invention is not limited to the embodiments described above. It will be apparent to those skilled in the art that the present invention may be variously modified without departing from the spirit of the invention. The scope of protection of the invention is determined according to the claims.

Claims (14)

1. A surface treatment method for sintered NdFeB magnets, comprising:

   Soaking, coating or spraying a solution containing at least one of Dy and Tb on the surface of the sintered NdFeB magnet,

   The sintered NdFeB magnet is subjected to diffusion treatment in a non-oxidizing environment.
2. The method according to claim 1, wherein the solute of the solution is a hydrated nitrate containing at least one of Dy and Tb.
3. The surface treatment method according to claim 2, wherein

   The molecular formula containing Dy hydrated nitrate is Dy(NO ₃ ) ₃ ·XH ₂ O, where X=6, 5, 3.5, 3,

   The molecular formula containing Tb hydrated nitrate is Tb(NO ₃ ) ₃ ·yH2O, where Y=6, 5, 3.5.
4. The surface treatment method according to claim 1, wherein the solvent of the solution is a volatile solvent.
5. The surface treatment method according to any one of claims 1 to 4, wherein the solution has a mass concentration of not less than 25%.
6. The surface treatment method according to claim 1, wherein the temperature of the diffusion treatment is 800 to 1000 °C.
7. The surface treatment method according to claim 1, wherein the diffusion treatment time is 2 to 10 hours.
8. The surface treatment method according to claim 1, wherein the tempering treatment is performed after the diffusion treatment.
9. The surface treatment method according to claim 8, wherein the temperature of the tempering treatment is 450 to 620 °C.
10. The surface treatment method according to claim 8, wherein the tempering treatment time is 2 to 10 hours.
11. The surface treatment method according to claim 1, wherein the intrinsic coercive force is increased by at least 1000 Oe after the surface treatment method is performed as compared with the method of not performing the surface treatment.
12. The surface treatment method according to claim 1, wherein the non-oxidizing environment is a vacuum environment or is evacuated to be filled with an inert gas atmosphere.
13. The surface treatment method according to claim 1, wherein the sintered NdFeB magnet has a thickness of 10 mm or less.
14. A method of producing a sintered NdFeB magnet, characterized in that the sintered NdFeB magnet is subjected to the indicating treatment according to any one of claims 1 to 13.