WO2010135958A1

WO2010135958A1 - Nd-fe-b permanent magnetic material and preparation method thereof

Info

Publication number: WO2010135958A1
Application number: PCT/CN2010/072854
Authority: WO
Inventors: Qing Gong; Zhiqiang Zhang; Surong Zhang; Xin Du; Xiaofeng Cheng
Original assignee: Byd Company Limited
Priority date: 2009-05-27
Filing date: 2010-05-17
Publication date: 2010-12-02
Also published as: CN101901657A; EP2436016B1; CN101901657B; EP2436016A4; EP2436016A1; US20120058003A1

Abstract

A permanent magnetic material comprising a Nd-Fe-B alloy and an additive including at least a cobalt ferrite, and a method for preparing the permanent magnetic material are provided. The method comprises the following steps: mixing the Nd-Fe-B alloy and the additive including at least a cobalt ferrite to obtain a mixture; magnetically orienting and pressing the mixture in a magnetic field; and sintering and tempering the mixture in vacuum or under the protection of inert gas.

Description

Nd-Fe-B PERMANENT MAGNETIC MATERIAL AND PREPARATION METHOD

THEREOF

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority and benefits of Chinese Patent Application No. 200910107649.2 filed with the State Intellectual Property Office of P.R. China on May 27, 2009, the entirety of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an Nd-Fe-B permanent magnetic material and a preparation method thereof.

BACKGROUND

Because of their magnetic properties, low cost and ample reserves, Nd-Fe-B permanent magnetic materials are widely used in vehicles, computers, electronics, mechanical and medical devices, etc. In addition, because of their high performance/price ratio, Nd-Fe-B materials have been the ideal materials to produce magnetic devices with high efficiency, small volume and light mass. However, as the continuous expansion of application fields and the development of technology, requirements for performance, operating temperature and corrosion resistance of permanent magnetic materials become higher and higher.

SUMMARY OF THE INVENTION

In view thereof, the present invention is directed to provide an Nd-Fe-B permanent magnetic material with good high temperature and corrosion resistance properties, and further to provide a preparation method of the Nd-Fe-B permanent magnetic material.

An embodiment of a first aspect of this invention provides a permanent magnetic material with good high temperature and corrosion resistance properties, comprising an Nd-Fe-B alloy and an additive comprising a cobalt ferrite.

An embodiment of a second aspect of this invention provides a method of preparing the permanent magnetic material described above, comprising steps of mixing an Nd-Fe-B alloy and an additive including at least a cobalt ferrite to obtain a mixture; magnetically orienting and pressing the mixture in a magnetic filed; and sintering and tempering the mixture under protection of vacuum or an inert gas. Additional aspects and advantages of the embodiments of present invention will be given in part in the following descriptions, become apparent in part from the following descriptions, or be learned from the practice of the embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

These and other aspects, solutions and advantages of the invention will become apparent and more readily appreciated from the following descriptions taken in conjunction with the drawings.

In a first aspect of the present invention, an embodiment of the present invention provides a permanent magnetic material comprising an Nd-Fe-B (neodymium-iron-boron) alloy and an additive including at least a cobalt ferrite. The inventors of the present invention have been found: by adding particles of a cobalt ferrite and distributing them uniformly along the grain boundary of the Nd-Fe-B alloy, the over-growth of the grain and magnetic domain size of the Nd-Fe-B alloy may be inhibited (i.e. pinning effect), thus improving the operating temperature effectively, and the cobalt element itself and neodymium can produce stable intergranular additional structure, thus improving the corrosion resistance property. The content of the heavy metal cobalt may be reduced because of adding a nano-cobalt ferrite, thus lowering the cost. An appropriate amount of oxygen in the cobalt ferrite may improve the high temperature resistance properties of the permanent magnetic material. Meanwhile, due to the presence of the cobalt ferrite, the corrosion resistance property of the permanent magnetic material may be improved greatly.

In some embodiments, the content of the cobalt ferrite may be about 0.1 wt % to about 20 wt %, particularly about 0.5 wt % to about 10 wt % of the Nd-Fe-B alloy. In some embodiments, the average particle diameter of the cobalt ferrite is about 20 nanometers to about 60 nanometers. In some embodiments, the cobalt ferrite is represented by a general formula of Co_nFe3__nθ4 , in which n is in a range of about 0.1<n<2.0. The particles of the cobalt ferrite are distributed uniformly along the grain boundary of the main phase of the Nd-Fe-B alloy, thus forming the pinning effect. However, the content of cobalt may not exceed about 20 wt % of the total weight of the Nd-Fe-B permanent magnetic material, otherwise the coercive force may be seriously reduced.

In some embodiments, the Nd-Fe-B alloy is represented by a general formula of Nd_aRebFe(ioo-a- b-_c-d)BcMd, where Re is at least one element selected from a group consisting of Pr, Dy, Tb, Ho, Gd, La, Ce and Y; M is at least one element selected from a group consisting of Co, Al, Cu, Zr, Ga, Nb and Mo; and a, b, c, and d are atomic weight ratios, in which a is in a range of about 1 < a < 10, b is in a range of about 5 < b < 12, c is in a range of about 5 < c < 8, and d is in a range of about 0 < d < 15. In a second aspect of the present invention, an embodiment of the present invention provides a method of preparing a permanent magnetic material, comprising steps of: mixing an Nd-Fe-B alloy and an additive including at least a cobalt ferrite to obtain a mixture; magnetically orienting and pressing the mixture in a magnetic filed; and sintering and tempering the mixture under protection of vacuum or an inert gas. In some embodiments, the sintering and tempering can be performed under the protection of vacuum. In some embodiments, the sintering and tempering step can be performed under the protection of an inert gas. In some embodiments, the method of preparing a permanent magnetic material employing the sintering process may include without limitation one or more of the following steps: formulating, melting, crushing, milling, magnetically orienting and pressing in a magnetic field, sintering in vacuum, mechanical processing and electroplating.

Some of the steps of the method are described as follows:

(1) The Nd-Fe-B alloy may be crushed and milled to form a powder. The crushing process may be a hydrogen decrepitation process or a mechanical crushing process using a crusher. In some embodiments, jet milling and ball milling under an inert gas may be utilized to produce a powder with an average particle diameter of about 2 microns to about 10 microns. In some embodiments, the Nd-Fe-B alloy may be an Nd-Fe-B alloy ingot or a strip casting flake, both of which are commercially available. The Nd-Fe-B alloy ingot can be prepared by a casting process, and the strip casting flake can be prepared by a strip casting flaking process. In some embodiments, the Nd-Fe-B alloy may be represented by the following general formula: Nd_aRebFe(ioo-a-b-c d)B_cMd , where Re is at least one element selected from the group consisting of Pr, Dy, Tb, Ho, Gd, La, Ce and Y; M is at least one element selected from the group consisting of Co, Al, Cu, Zr, Ga, Nb and Mo; and a, b, c, and d are atomic weight ratios, in which a is in a range of about 1 < a < 10, b is in a range of about 5 < b < 12, c is in a range of about 5 < c < 8, and d is in a range of about 0 < d < 15.

The casting process may be those well known in the art, and may comprise steps of casting a melted alloy melt in a water-cooled copper mould. The Nd-Fe-B alloy ingot may comprise columnar crystals, where the columnar crystals are separated by Nd-rich phase thin layers. Particularly, the distance between two adjacent Nd-rich phase layers may be about 100 microns to about 1500 microns.

In some embodiments, the strip casting flaking process may be those well known in the art, and may comprise steps of pouring a melted alloy onto a rotating copper roller surface, with a rotating linear velocity of the copper roller surface ranging from about 1 m/s to about 2 m/s, and then rapidly cooling the melted alloy to form flakes with different widths and with a thickness ranging from about 0.2 millimeter to about 0.5 millimeter. In some embodiments, the columnar crystals in the flakes may have a width ranging from about 5 microns to about 25 microns. In some embodiments, the hydrogen decrepitation process using a hydrogen decrepitation furnace may be those well known in the art, and may comprise, for example, steps of placing an Nd- Fe-B alloy with fresh surfaces into a stainless steel vessel, filling the vessel with high purity hydrogen until about one atmospheric pressure after vacuumizing, and then maintaining at the pressure for about 20 minutes to about 30 minutes until the alloy decrepitates and the temperature the vessel increases, this is resulted from the decrepitation of the alloy due to the formation of a hydride after the alloy absorbs hydrogen, and finally vacuumizing and dehydrogenating the hydride for about 2 hours to about 10 hours under the temperature about 400 ⁰C to about 600 ⁰C.

In some embodiments, the mechanical crushing process may be those well known in the art, and may comprise, for example, steps of rough crushing in a jaw crusher, followed by mechanical crushing in a fine crusher.

In some embodiments, the jet milling may be those well known in the art, and may comprise steps of accelerating powder particles to a supersonic speed using an air flow, and then causing the particles to clash with each other to break up.

(2) The Nd-Fe-B alloy powder and the additive are mixed uniformly using a mixer to obtain a mixed powder.

In some embodiments, the additive comprising a cobalt ferrite is subject to decentralized processing in advance. The amount of the cobalt ferrite may be about 0.5 wt % to about 10 wt % of the total weight of the Nd-Fe-B matrix powder. The cobalt ferrite may have an average particle diameter of about 10 nanometers to about 150 nanometers, particularly about 20 nanometers to about 60 nanometers.

In some embodiments, the alloy and the additive may be mixed in the presence of an antioxidant, or in the presence of an antioxidant and a lubricant. In some embodiments, based on the weight of the Nd-Fe-B alloy, the amount of the antioxidant may be about 0.1 wt % to about 5 wt %, and the amount of the lubricant may be about 0 wt % to about 5 wt %. There is not limitation to the antioxidant, for example, the antioxidant may be one or more selected from polyethylene oxide alkyl ether, polyethylene oxide monofatty ester and polyethylene oxide alkenyl ether. Particularly, the antioxidant may be an antioxidant commercially available from the Shenzhen Deepocean Chemical Industry Co. Ltd, P.R.C. In some embodiments, the lubricant may be one or more selected from gasoline, oleic acid, stearic acid, polyhydric alcohol, polyethylene glycol, sorbitan, and stearin.

The mixing process may be those well known in the art. For example, the mixing process may be carried out in a mixer. (3) The mixed powder obtained may be oriented and pressed in a magnetic field to form a parison.

Pressing the mixed powder in a magnetic filed to form a parison may be achieved by using a well known process and a magnetically orienting-forming-pressing machine. In some embodiments, the orienting magnetic field has an intensity of about 1.2 T to about 3.0 T, and the pressing may be carried out under a pressure of about 10 MPa to about 200 MPa for about 10 seconds to about 60 seconds. The orientation degree of the magnetic powder may be improved by further increasing the magnetic field intensity. In some embodiments, The forming of the parison is performed in a completely closed glove box with isolating the magnetic powder from the air, thus avoiding the fire risk due to the oxidation and heat generation of the magnet and reducing the content of oxygen in the final magnet.

(4) The parison is sintered and tempered under protection of vacuum or an inert gas to obtain the Nd-Fe-B permanent magnetic material.

In some embodiments, the sintering and tempering process may be a well known process, and may be carried out under protection of vacuum or an inert gas. The inert gas may be any gas which may not participate in the reaction and may be one or more selected from nitrogen, helium, argon, neon, krypton and xenon. In some embodiments, the parison may be sintered at a temperature of about 1030 ⁰C to about 1120 ⁰C for a period of about 2 hours to about 8 hours, then tempered in a first tempering step at a temperature of about 800 ⁰C to about 920 ⁰C for a period of about 1 hour to about 3 hours, and finally tempered in a second tempering step at a temperature of about 500 ⁰C to about 650 ⁰C for a period of about 2 hours to about 4 hours. The second tempering step may further improve the coercive force. Because the cobalt ferrite has a melting point above 1120⁰C, during being sintered at the above temperature, the cobalt ferrite may not be decomposed and melted. The present invention will be described in detail with reference to the following examples.

EXAMPLE 1

(1) An Nd-Fe-B alloy represented by the formula (PrNd)₁C₆₁Dy_3-STb₁JFe_77-Ss B_5.87C0_1.68Alo_.5Cuo_.i₆Gao_.13 (a%) was prepared by a strip casting flaking process with a rotating linear velocity of a copper roller surface of about 1.5 meters per second. The flake had a thickness of about 0.3 millimeter.

(2) The alloy was crushed by a hydrogen decrepitation process in a hydrogen decrepitation furnace. After absorbing hydrogen to saturation at room temperature and being dehydrogenated at about 550 ⁰C for about 6 hours to prepare a crushed powder, the crushed powder was milled via jet milling under a nitrogen atmosphere to produce a powder with an average particle diameter of about 3.5 microns.

(3) CoFe₂θ₄ with an average particle diameter of about 50 nanometers and an antioxidant (commercially available from the Shenzhen Deepocean Chemical Industry Co. Ltd, P.R.C.) were added to the Nd-Fe-B alloy powder. Based on the weight of the Nd-Fe-B alloy powder, the amount of the CoFe₂O₄ is about 1 wt % and the amount of the antioxidant is about 0.5 wt %.

(4) The mixed powder was pressed by using a magnetically orienting-forming-pressing machine in a closed glove box filled with a nitrogen gas to form a parison. The intensity of the orienting magnetic field was about 1.6 T, the pressure was about 100 MPa, and the pressing time was about 30 seconds.

(5) The compacted parison was sintered in a vacuum sintering furnace under a degree of vacuum of 2 X 10^"2 Pa at a temperature of about 1080 ⁰C for about 3 hours, then tempered at about 850 ⁰C for about 2 hours, and finally tempered at about 550 ⁰C for about 3 hours to prepare an Nd- Fe-B permanent magnetic material labeled as Tl.

COMPARATIVE EXAMPLE 1

In the process of the COMPARATIVE EXAMPLE 1, no nano-sized cobalt ferrite CoFe₂O₄ is added, and the other steps are substantially similar to those of EXAMPLE 1.

The Nd-Fe-B permanent magnetic material obtained was labeled as CTl.

EXAMPLE 2

The process of this example was substantially similar to that of EXAMPLE 1, except that Co₂Fe]O₄ was used as the additive in stead Of CoFe₂O₄, and the amount of Co₂Fe]O₄ was about 5 wt % of the Nd-Fe-B alloy powder.

The Nd-Fe-B permanent magnetic material obtained was labeled as T2.

EXAMPLE 3

The process of this example was substantially similar to that of EXAMPLE 1 , except that the average particle diameter of the CoFe₂O₄ is about 100 nanometers.

The Nd-Fe-B permanent magnetic material obtained was labeled as T3.

EXAMPLE 4

The process of this example was substantially similar to that of EXAMPLE 1 , except that the amount of the CoFe₂O₄ was about 10 wt % of the Nd-Fe-B alloy.

The Nd-Fe-B permanent magnetic material obtained was labeled as T4.

COMPARATIVE EXAMPLE 2 The process of this example was substantially similar to that of EXAMPLE 1 , except that Co was used as the additive instead of CoFe₂C^, and an average particle diameter of the Co was about 50 nanometers.

The Nd-Fe-B permanent magnetic material obtained was labeled as CT2.

TEST

1. Corrosion resistance property

Cylindrical samples with a diameter of 10 millimeters and a length of 7 millimeters were prepared from the permanent magnetic materials T1-T4, CTl and CT2, and then tested on a HAS- 70CP type Highly Accelerated Stress Tester commercially available from Terchy Environmental Technology Ltd, with a temperature of 130 ^°C, a humidity of 95%, a steam pressure of 2.1 bar, and a period of 10 days. The mass loss (W_l0Ss) of the permanent magnetic materials T1-T4, CTl and CT2 were recorded in Table 1.

2. Maximum operating temperature

Cylindrical samples with a diameter of 10 millimeters and a length of 7 millimeters were prepared from the permanent magnetic materials T1-T4, CTl and CT2 , and then heated using a curve measurement system NIM200C (National Institute of Metrology, P.R.C.) from a temperature of 60 ^°C with a 2 ^°C increment each time. When the line started to bend at a certain temperature, the permanent magnetic materials reached the maximum operating temperature.

Test results were shown in Table 1.

Table 1

It can be seen from the results of Table 1 that Tl has a Wi_oss of 2.1 mg/cm and a inflection temperature 190 ^°C and CT2 has a Wi_oss of 2.7 mg/cm² and a inflection temperature 170^°C, so that the permanent magnetic material according to the embodiments of the present invention exhibited a better corrosion resistance and higher temperature resistance properties.

Although explanatory embodiments have been shown and described, it would be appreciated by those skilled in the art that changes, alternatives, and modifications all falling into the scope of the claims and their equivalents can be made in the embodiments without departing from spirit and principles of the invention.

Claims

What is claimed is:

1. A permanent magnetic material, comprising: an Nd-Fe-B alloy; and an additive including at least a cobalt ferrite.

2. The permanent magnetic material of claim 1, wherein the cobalt ferrite is about 0.5 wt % to about 10 wt % of the Nd-Fe-B alloy.

3. The permanent magnetic material of claim 1 or 2, wherein an average particle diameter of the cobalt ferrite is in a range of about 10 nanometers to about 150 nanometers.

4. The permanent magnetic material of claim 1 or 2, wherein the cobalt ferrite is represented by a general formula of Co_nFe3__nθ4, where n is in a range of about 0.1<n<2.0.

5. The permanent magnetic material of claim 1 or 2, wherein the Nd-Fe-B alloy is represented by a general formula of Nd_aRebFe(ioo-a-b-c-d)B_cM_d, where:

Re is at least one element selected from a group consisting of Pr, Dy, Tb, Ho, Gd, La, Ce and Y; M is at least one element selected from a group consisting of Co, Al, Cu, Zr, Ga, Nb and Mo; and a, b, c, and d are atomic weight ratios, in which a is in a range of about 1 < a < 10, b is in a range of about 5 < b < 12, c is in a range of about 5 < c < 8, and d is in a range of about 0 < d < 15.

6. A method for preparing a permanent magnetic material, comprising steps of: mixing an Nd-Fe-B alloy and an additive including at least a cobalt ferrite to obtain a mixture; magnetically orienting and pressing the mixture in a magnetic filed; and sintering and tempering the mixture under the protection of vacuum or an inert gas.

7. The method of claim 6, wherein the cobalt ferrite is about 0.5 wt % to about 10 wt % of the Nd-Fe- B alloy.

8. The method of claim 6, wherein the cobalt ferrite is represented by a general formula of Co_nFe3_ _nθ4 , where n is in a range of about 0.1<n<2.0.

9. The method of claim 6, wherein the mixing step comprises mixing the Nd-Fe-B alloy and the additive in the presence of an antioxidant or in the presence of an antioxidant and a lubricant, in which based on the weight of the Nd-Fe-B alloy, the amount of the antioxidant is about 0.1 wt % to about 5 wt %, and the amount of the lubricant is about 0 wt % to about 5 wt %.

10. The method of claim 6, wherein the Nd-Fe-B alloy is represented by a general formula of NdaRebFe αoo-a-b-c-d) B_cM_d, where:

11. The method of claim 6, wherein an average particle diameter of the cobalt ferrite is in a range of about 10 nanometers to about 150 nanometers, and an average particle diameter of the Nd-Fe-B alloy is in a range of about 2 microns to about 5 microns.

12. The method of claim 6, wherein the magnetically orienting and pressing is performed under a magnetic filed intensity of about 1.2 T to about 3.0 T and a pressure of about 10 MPa to about 200 MPa for a period of about 10 seconds to about 60 seconds; the sintering is performed at a temperature of about 1030 ⁰C to about 1120 ⁰C for a period of about 2 hours to about 8 hours; and wherein the tempering steps comprising a first tempering step and a second tempering step, in which the first tempering step is performed at a temperature of about 800 ⁰C to about 920 ⁰C for a period of about 1 hour to about 3 hours, and the second tempering step is performed at a temperature of about 500 ⁰C to about 650 ⁰C for a period of about 2 hours to about 4 hours.