Method of Producing Sintered Magnets with Controlled Structures and Composition Distribution

Abstract

A method of making a permanent magnet includes a step of providing an alloy powder comprising at least one rare earth element. The alloy powder is shaped and then exposed to microwave radiation or a pulsed electric current to form a sintered magnet.

Description

FIELD OF THE INVENTION

In at least one aspect, the present invention relates to methods of making permanent magnets.

BACKGROUND

Permanent magnets (PMs) are used in a variety of devices, including traction electric motors for hybrid and electric vehicles. Sintered neodymium-iron-boron (Nd—Fe—B) permanent magnets have very good magnetic properties at low temperatures. However, due to the low Curie temperature of the Nd₂Fe₁₄B phase in Nd—Fe—B permanent magnets, the magnetic remanence and intrinsic coercivity decrease rapidly with increased temperature. It is known that the substitution of Dy for Nd or Fe in Nd—Fe—B magnets results in increases of the anisotropic field and the intrinsic coercivity, and a decrease of the saturation magnetization (C. S. Herget, Metal, Poed. Rep. V. 42, P. 438 (1987); W. Rodewald, J. Less-Common Met., V111, P77 (1985); and D. Plusa, J. J. Wystocki, Less-Common Met. V. 133, P. 231 (1987)). It is a common practice to add the heavy RE metals such as dysprosium (Dy) or terbium (Tb) into the mixed metals before melting and alloying. However, Dy and Tb are very rare and expensive. Heavy REs contain only about 2-7% Dy, and only a small fraction of the RE mines in the world contain heavy REs. The price of Dy has increased sharply in recent times. Tb, which is needed if higher magnetic properties are required than Dy can provide, is much more expensive than even Dy.

Typical magnets for traction electric motors in hybrid vehicles contain about 6-10 wt % Dy to meet the required magnetic properties. Conventional methods of making magnets with Dy or Tb result in the Dy or Tb being distributed in the grains and in the phases along grain boundaries within the magnet through solid diffusion. Nd—Fe—B permanent magnets can be produced using a powder metallurgy process, which involves melting and strip casting, hydrogen decrepitation (hydride and de-hydride), pulverizing (with nitrogen jet milling), screening, and mixing alloy powders for the desired chemical composition. A typical powder metallurgy process is as follows: weighing and pressing under a magnetic field for powder alignment (vacuum bagging), isostatic pressing, sintering and aging (e.g., about 5-30 hrs, at about 500-1100 C, in vacuum), and machining to magnet pieces. Finally, the magnets are surface treated by phosphating, electroless nickel (Ni) plating, epoxy coating, or the like (if needed).

The ideal microstructure for sintered Nd—Fe—B based magnets is Fe₁₄Nd₂B grains perfectly isolated by the nonferromagnetic Nd-rich phases (a eutectic matrix of mainly Nd plus some Fe₄Nd_1.1B₄ and Fe—Nd phases stabilized by impurities). The addition of Dy or Tb leads to the formation of quite different ternary intergranular phases based on Fe, Nd and Dy or Tb. These phases are located in the grain boundary region and at the surface of the Fe₁₄Nd₂B grains.

Dy or Tb (or their alloys) coated Nd—Fe—B powders are used to make the magnet, which results in a non-uniform distribution of Dy or Tb in the magnet microscopically. For example, the amount of Dy and/or Tb can be reduced by about 20% or more compared to conventional processes, or about 30% or more, or about 40% or more, or about 50% or more, or about 60% or more, or about 70% or more, or about 80% or more, or about 90% or more, depending on relative amount of surface powder to core powder and the Dy or Ty concentration in the surface powder, sintering schedule (which affects diffusion of Dy or Ty into the bulk from grain surface). The process involves coating the Nd—Fe—B based powder used to make sintered Nd—Fe—B permanent magnets with Dy or Tb metals or alloys. The Nd—Fe—B based powder can be coated via mechanical milling, physical vapor deposition (PVD), or chemical vapor deposition (CVD).

Accordingly, there is a need for improved methods of making permanent magnets, and in particular, Nd—Fe—B based magnets.

SUMMARY OF THE INVENTION

The present invention solves one or more problems of the prior art by providing in at least one embodiment, a method of making a permanent magnet. The method of making a rare-earth magnet includes a step of providing an alloy powder comprising at least one rare earth element. The alloy powder is shaped and then exposed to microwave radiation to form a sintered magnet.

In another embodiment, a method of making a permanent magnet is provided. The method of making a rare-earth magnet includes a step of providing an alloy powder comprising neodymium, iron, and boron. The alloy powder is shaped and then exposed to microwave radiation to form a sintered magnet.

In another embodiment, a method of making a permanent magnet is provided. The method of making a rare-earth magnet includes a step of providing an alloy powder comprising at least one rare earth element. The alloy powder is shaped and then exposed to a pulsed electric current to form a sintered magnet.

A BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is a flow chart illustrating a method for making a permanent magnet using microwave radiation;

FIG. 2 is a schematic of a microwave sintering apparatus;

FIG. 3 is a flow chart illustrating a method for making a permanent magnet using microwave radiation; and

FIG. 4 is a schematic of a pulsed electric current sintering apparatus that executes the method of claim 3.

DESCRIPTION OF THE INVENTION

Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.

Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.

It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.

Various embodiments of the present invention provide methods for sintering permanent magnets with controlled macrostructures (such as porosity and powder particle size and distribution) and microstructures (various phases and elemental compositions). Such embodiments include microwave sintering methods and electric current sintering method. The processed magnets which include Nd—Fe—B based magnets and Sm—Fe—N (samarium-iron-nitrogen) based magnets.

With reference to FIG. 1, a flow chart illustrating a method of making a permanent magnet is provided. The method comprises providing alloy powder 10 comprising at least one rare earth element. Alloy powder 10 is shaped in mold 12 and then exposed to microwave radiation 14 to form sintered magnet 16. In a variation, alloy powder 10 includes neodymium, iron, and boron. In a refinement, alloy powder 10 further includes a component selected from the group consisting of dysprosium, terbium, and combinations thereof. In still a further refinement, the dysprosium and/or terbium have a non-uniform distribution. U.S. application Ser. No. 13/007,203, filed Jan. 14, 2011, entitled Method Of Making Nd—Fe—B Sintered Magnets With Dy or Tb (same inventor as the current one) describes magnets and methods of making magnets that uses much less Dy or Tb than those made using the conventional methods while obtaining similar magnetic properties by coating the core magnetic powder particles with a Dy or Tb rich coating via physical vapor deposition. This patent is hereby incorporated by reference in its entirety. In a refinement, the alloy powders set forth in that patent are used in the present embodiment. In another variation, alloy powder 10 includes samarium and iron which are contacted with nitrogen gas during sintering. In this latter variation, sintered magnet 16 comprises samarium-iron-nitrogen magnetic domains.

With reference to FIG. 2, a schematic illustration of a microwave sintering apparatus for forming a rare earth permanent magnet is provided. Microwave sintering apparatus 20 includes microwave generator 22 which provides microwave radiation to sintering chamber 24. Microwave generator 22 provides microwave radiation to recirculator 26 which may provide radiation to R-H tuner 28. Microwave sintering apparatus 20 typically operates at frequency 300 MHz to about 300 GHz with power output in the range of 1-6 kW. In a refinement, the microwave radiation has a frequency from about 2 to about 3 GHz (e.g., 2.45 GHz). The microwave radiation is provided to then sintering chamber 24. Microwave sintering apparatus 20 also includes water load 30 to which radiation is directed when microwave generator is powered but radiation is not to be provided to sintering chamber 24. Alloy powder 10 is held in mold 32 which is surrounded by ceramic insulation housing 34 (batch system). In another variation, the ceramic insulation housing may be replaced with an alumina tube insulated with ceramic insulation. The primary function of the insulation is to preserve the heat generated in the magnet parts. The compressed green powder piece(s) 10 can also be placed inside ceramic housing (container(s)), or shelves so that they are heated through heat radiation from the heated ceramic housing. Pyrometer 36 is used to monitor the temperature of alloy powder 10 during sintering. IR sensors and/or sheathed thermocouples placed close to the surface of the sample may also be used to monitor the temperatures. In a refinement, alloy powder 10 achieves temperatures from about 500 to about 1600° C. In a refinement, alloy powder 10 achieves temperatures from about 500 to about 1200° C. In another refinement, the hold lengths are from about 1 minute to about 10 hours and the heating and cooling rates are from about 1 to 1000 degrees Centigrade per minute. Gases (e.g., argon, helium, nitrogen, hydrogen and the like) are introduced via gas system 40. In a refinement, alloy powder 10 is contacted with a gas prior to and/or during exposure to the microwave radiation. Vacuum system 42 is used to pump out the gases and/or to maintain sintering chamber 24 in a vacuum of about 10⁻⁴ Pa or greater.

The microwave process set forth above provides a method to meet the demands of producing fine microstructures, higher density and better properties, at a potentially lower cost. It yields better mechanical properties than conventional processing and produces a finer grain size. The shape of the porosity, if present, is quite different than that achieved by conventional sintering methods. Microwave-processed powder metal components are expected to produce round-edged porosities producing higher ductility and toughness. Microwave-metal interactions are more complex than those working actively in the field had expected. There are many factors that contribute significantly to the total microwave heating of powdered metals. The magnet part size and shape, the distribution of the microwave energy inside the cavity, and the magnetic field of the electromagnetic radiation are all important in the heating and sintering of powder metals.

With reference to FIG. 3, a flow chart illustrating a method of making a permanent magnet is provided by Pulsed Electric Current Sintering (PECS). Pulsed Electric Current Sintering is also known as Spark Plasma Sintering (SPS) or Field Assisted Sintering Technology (FAST). PECS employs a pulsed DC current to heat up electrically conductive powder compact parts by Joule heating. This direct way of heating allows the application of very high heating and cooling rates, enabling low sintering temperature than conventional sintering process, enhancing densification over grain growth, promoting solid diffusion, allowing maintaining the intrinsic properties of the magnetic powders in their fully dense products. The method comprises providing alloy powder 10 comprising at least one rare earth element. Alloy powder 10 is shaped and positioned between punches 52, 54 and dies 58, 60. Alloy powder 10 is then exposed to a pulsed electric current from source 62 to form sintered magnet 64.

With reference to FIG. 4, a schematic illustration of a pulsed electric current sintering system is provided. Sintering system 70 includes vacuum chamber 72 in which alloy powder 10 is sintered. Sintering system 70 includes upper punch 74 and lower punch 76 which are both typically formed from graphite or a metal. Sintering system 70 also includes die 78 for containing alloy powder 10 which is also typically made from a metal. Force is applied to alloy powder 10 as indicted by arrows 80, 82. Pulsed D.C. power supply 84 is used to apply a pulsed electric current to alloy sample 10. In a refinement, the pulsed electric current is from about 100 to about 10,000 amps and has a pulsed duration from about 1 ms to about 300 ms and a pause time of 1 to about 50 ms. In a refinement, the vacuum is about 10⁻⁴ Pa or greater.

Still referring to FIG. 4, sintering system 70's metal parts electrically resistive so that when an electric current is applied direct heat is very quickly generated. Therefore, holding time can be only one minute. Since PCES is a direct-heating method the crystal structure is prevented from changing by a rapid rise in temperature. In addition, by using pulse current method it becomes possible to promote bonding of the powder particle boundary surfaces without significantly raising the temperature of the powder. As a result, the magnet powder can be sintered without degrading its magnetic properties. Moreover, the density can be enhanced by using servo pressing with programmable loading control. In order to promote a homogeneous sintering behavior, the temperature gradients inside the specimen are minimized. Parameters that influence the temperature distribution inside the magnetic parts include the material's electrical conductivity, the die wall thickness and the presence of graphite papers used to prevent direct contact between the magnetic parts (if needed and used to guarantee electrical contacts between all parts). A finite element model can be used to evaluate the evolution of thermal gradients during PECS depending on the physical material properties, the geometrical parameters of the different parts and the pulsed current input. Depending on the electrical properties of the parts, the current flow as well as temperature distribution inside the working part differs drastically. In the case of electrically conductive parts, the pulsed DC current mainly flows through the parts and only a small part flows through the die.

In a variation of the present embodiment, the permanent magnet of the present embodiment has a non-uniform distribution of dysprosium and/or terbium. For example, in one refinement, the permanent magnet includes regions in which neodymium, iron, and boron magnetic domains are coated with a layer including dysprosium and/or terbium. In a refinement, the coating has a thickness from about 100 nm to about 100 microns. In another refinement, the coating has a thickness from about 5 microns to about 70 microns. In still another refinement, the coating has a thickness from about 10 microns to about 50 microns. In a refinement, the coated powder is shaped by placing the allow powder 10 into mold 22. The allow powder 10 is usually pressed during or after shaping. Typically, permanent magnet includes from about 0.01 to about 8 weight percent dysprosium and/or terbium of the total weight of the permanent magnet. However, the surface concentration of dysprosium and/or terbium may be from about 5 to about 50 weight percent of the total weight of the coating layer.

In variations of the embodiments set forth above, the alloy powder used in the microwave and PECS methods is formed as follows. An alloy containing neodymium, iron, and boron is melted and cast via spinning to make strips. The alloy strips are then hydrogen decrepitated by hydrogenating the alloy. Typically, this step is accomplished in a hydrogen furnace at a pressure of approximately 1 to 5 atm until the alloy is deprecated. The alloy is then typically dehydrogenated in a vacuum at elevated temperature (e.g., 300 to 600° C.) for 1 to 10 hours. The result of the hydrogenation and dehydrogenation is that the alloy is pulverized into a coarse powder typically with an average particle size from 0.1 mm to 4 mm. The coarse powder is then pulverized (by nitrogen jet milling) to make a starting powder. In a refinement, the alloy powder may be mixed with a second alloy powder in order to adjust the chemical composition and optional screening. In a refinement, the alloy powder is then coated by a mechanical milling, a physical vapor deposition process, or a chemical vapor deposition with a Dy and/or Tb containing layer. The resulting coated powder may then be optionally screened. Finally, a permanent magnet is formed by the processes set forth above.

While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.

Claims

1. A method of making a rare-earth magnet, the method comprising:

providing an alloy powder comprising at least one rare earth element;

hydrogen deprecating the alloy powder;

shaping the alloy powder; and

exposing the powder to microwave radiation to form a sintered magnet.

2. The method of claim 1 wherein the alloy powder is shaped by placing the alloy powder into a mold under a magnetic field for powder magnetic alignment.

3. The method of claim 1 wherein the alloy powder is pressed during or after shaping.

4. The method of claim 1 further comprising contacting the alloy powder with a gas prior to or during exposure to the microwave radiation.

5. The method of claim 4 wherein the gas comprises a component selected from the group consisting of helium, argon, hydrogen, nitrogen, and combinations thereof.

6. (canceled)

7. The method of claim 1 wherein the microwave radiation has a power output in from about 1 to about 6 kw and a frequency from about 300 MHz to about 300 GHz.

8. The method of claim 1 wherein the alloy powder includes neodymium, iron, and boron.

9. The method of claim 8 wherein the alloy powder further includes a component selected from the group consisting of dysprosium, terbium, and combinations thereof.

10. The method of claim 9 wherein the alloy powder includes dysprosium and/or terbium having a non-uniform distribution.

11. The method of claim 10 wherein the dysprosium and/or terbium coat the alloy powder.

12. The method of claim 1 wherein the alloy powder includes samarium and iron.

13. The method of claim 12 wherein the sintered magnet comprises samarium-iron-nitrogen magnetic domains.

14. A method of making a neodymium-iron-boron magnet, the method comprising:

providing an alloy powder comprising neodymium, iron, and boron;

hydrogen deprecating the alloy powder;

shaping and pressing the alloy powder; and

exposing the powder to microwave radiation to form a sintered neodymium-iron-boron magnet.

15. The method of claim 14 further comprising contacting the alloy powder with a gas prior to or during exposure to the microwave radiation.

16. The method of claim 15 wherein the gas comprises a component selected from the group consisting of helium, argon, hydrogen, nitrogen, and combinations thereof.

17. The method of claim 15 wherein the microwave radiation has a frequency from about 300 MHz to about 300 GHz a power output from about 1 to about 6 kw.

18. The method of claim 14 wherein the alloy powder further includes a component selected from the group consisting of dysprosium, terbium, and combinations thereof.

19. A method of making a rare-earth magnet, the method comprising:

providing an alloy powder comprising at least one rare earth element;

shaping the alloy powder; and

exposing the powder to pulsed electric current.

20. The method of claim 19 wherein the pulsed electric current is from about 100 to about 10,000 amps.

21. The method of claim 19 wherein the pulsed electric current has a pulsed duration from about 1 ms to about 300 ms and a pause time of 1 to about 50 ms.