Abstract:
A method of making a permanent magnet is described. In one embodiment, the method includes providing a first alloy powder having a desired composition, the alloy powder containing neodymium, iron, and boron; coating the first alloy powder with dysprosium, dysprosium alloy. terbium, or terbium alloy so that the first alloy powder has a surface concentration of dysprosium, terbium, or both in excess of a bulk concentration of dysprosium, terbium, or both; and forming the permanent magnet from the coated alloy powder using a powder metallurgy process, the permanent magnet having a non-uniform distribution of dysprosium, terbium, or both therein. Permanent magnets are also described.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Permanent magnets find applications in a variety of devices, including electric motors for hybrid and electric vehicles. Sintered Nd—Fe—B permanent magnets have very good magnetic properties at low temperatures. After magnetization, permanent magnets are in a thermodynamically non-equilibrium state. Any changes in the external conditions, in particular the temperature, result in a transition to another, more stable state. These transitions are typically accompanied by changes in the magnetic properties. Due to the low Curie temperature of the Nd 2 Fe 14 B phase, the magnetic remanence and intrinsic coercivity decrease rapidly with increased temperature. 
         [0002]    It is important to improve the thermal stability of this material and to increase the magnetic properties further in order to obtain compact, lightweight, and powerful motors for hybrid and electrical vehicles. There are two common approaches to improving thermal stability and magnetic properties. One is to raise the Curie temperature by adding Co, which is completely soluble in the Nd 2 Fe 14 B phase. However, the coercivity of the Nd—Fe—B magnets with Co decreases, possibly because of the nucleation sites for reverse domains. The second approach is to add heavy rare-earth (RE) elements. It is known that the substitution of dysprosium for neodymium or iron 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. 
         [0003]    However, Dy and Tb are very rare and expensive RE elements. Heavy REs contain only about 2-7% Dy. The price of Dy has increased sharply in recent times (from about $50/kg for DyO in 2005 to about $140/kg in 2010). Tb is needed if higher magnetic properties are required than Dy can provide, and it is much more expensive than Dy (about $400/kg for TbO). 
         [0004]    Typical magnets for 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 uniformly distributed within the magnet. 
         [0005]    Assuming the weight of permanent magnet pieces is about 1-1.5 kg per electric motor, and a yield of the machined permanent magnet (PM) pieces of typically about 55-60%, 2-3 kg of PM per motor would be required, or 4-6 kg per vehicle (some hybrid vehicles may use one induction motor and one PM motor). Moreover, Dy is also widely used by other industries. The only RE mine in the United States does not have any significant amounts of Dy. Therefore, reducing the Dy or Tb usage in permanent magnets would have a very significant cost impact. 
         [0006]    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), screening, and mixing alloy powders for the desired chemical composition. A typical powder metallurgy process follows: weighing and pressing (vacuum bagging), isostatic pressing, sintering and aging (e.g., about 30 hrs, at about 1100 C, in vacuum), and machining to magnet pieces. Finally, the magnets are surface treated by phosphating, electroless Ni plating, epoxy coating, etc. 
         [0007]    The ideal microstructure for sintered Nd—Fe—B based magnets is Fe 14 Nd 2 B grains perfectly isolated by the nonferromagnetic Nd-rich phase (a eutectic matrix of mainly Nd plus some Fe 4 Nd 1.1 B 4  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 Fe 14 Nd 2 B grains. The addition of elements to improve the magnetic properties should desirably fulfill the following conditions: 1) the intermetallic phase should be nonferromagnetic to separate the ferromagnetic grains; 2) the intermetallic phase should have a lower melting point than the Fe 14 Nd 2 B phase to produce a dense material via liquid phase sintering; and 3) the elements should have a low solubility in Nd 2 Fe 14 B to keep good magnetic properties. 
         [0008]    The microstructures of Nd—Fe—B sintered magnets have been extensively investigated in order to improve the magnetic properties. In general, sintered magnets are mainly composed of the hard-magnetic Nd 2 Fe 14 B phase and a nonmagnetic Nd-rich phase. The coercivity is known to be greatly influenced by the morphology of the boundary phases between Nd 2 Fe 14 B grains. The magnetic properties of the Nd—Fe—B sintered magnets are degraded when the magnet size is decreased because the machined surface causes nucleation of magnetic reversed domains. Machida et al. (Machida, K., Suzuki, S., Ishigaki, N., et al., Improved magnetic properties of small-sized magnets and their application for DC brush-less micro-motors. Coll. Abstr. Magn. Soc. Jpn. 142 (2005), 25-30), found that the degraded coercivity of small-sized Nd—Fe—B sintered magnets can be improved by surface treating of the formed magnet with Dy and Tb-metal vapor sorption so that there is a uniformly distributed coating of Dy or Tb on the outside of the formed magnet and no Dy or Tb inside. 
       SUMMARY OF THE INVENTION 
       [0009]    One aspect of the invention is a method of making a permanent magnet. In one embodiment, the method includes providing a first alloy powder having a desired composition, the first alloy powder containing neodymium, iron, and boron; coating the first alloy powder with dysprosium, dysprosium alloy, terbium, or terbium alloy so that the first alloy powder has a surface concentration of dysprosium, terbium, or both in excess of a bulk concentration of dysprosium, terbium, or both; and forming the permanent magnet from the coated alloy powder using a powder metallurgy process, the permanent magnet having a non-uniform distribution of dysprosium, terbium, or both therein. 
         [0010]    Another aspect of the invention is a permanent magnet. In one embodiment, the permanent magnet includes a neodymium, iron, and boron based magnet having a bulk concentration of dysprosium, or terbium, or both in a range of about 0.3 to about 5 wt % and a non-uniform distribution of dysprosium, terbium, or both therein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic of a mechanical mill. 
           [0012]      FIG. 2  is a schematic of a particle gun based on spark erosion. 
           [0013]      FIG. 3  is a schematic of a particle gun based on high pressure sputtering. 
           [0014]      FIG. 4  is a schematic of a swirl coater. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Magnets made using the present process use much less Dy or Tb than those made using the conventional methods while obtaining similar magnetic properties. In the present process, the Dy or Tb 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, which can be seen and measured using a scanning electron microscope with a microprobe. This enables the present process to use much less Dy or Tb for the similar magnetic properties. 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. By non-uniform distribution, we mean that Dy and/or Tb_are distributed or concentrated at the interface of the powder particles, with little or none inside the particles. 
         [0016]    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 using a variety of coating methods. Suitable processes include, but are not limited to the following. One process involves mechanical milling with Dy or Tb metals or alloys. For example, Dy or Tb containing iron based alloy powder(s) (e.g., containing about 15-80 wt % Dy or Tb) is milled or mechanically alloyed with Nd—Fe—B based powder to create new powders with Dy or Tb enriched surfaces. Another process involves coating the Nd—Fe—B based powders with Dy or Tb metal vapor via physical vapor deposition (PVD). A third method involves coating the Nd—Fe—B based powders with very fine metal powder of Dy or Tb metal, or Fe—Dy or Fe—Tb alloy mixed with a solvent. 
         [0017]    Using these methods, the coating thickness can be about one micrometer to about 100 micrometers, for example, about 2 to about 100 micrometers, or about 5 to about 90 micrometers, or about 5 to about 80 micrometers, or about 5 to about 70 micrometers, or about 5 to about 60 micrometers, or about 10 to about 50 micrometers. 
         [0018]    The powder coating process allows the average Dy or Tb concentration to be reduced and changes the distribution of the Dy or Tb in the magnet. The average Dy or Tb concentration of the magnet can be in a range of about 0.3 to about 6 wt %, or about 0.3 to about 5 wt %, or about 0.3 to about 4 wt %, or about 0.3 to about 3 wt %, compared with about 6-9 wt % for traditional magnets having similar high magnetic properties. The coating process creates powder particles with a Dy or Tb surface concentration as high as about 5 to about 80 wt. % or more, and a low Dy or Tb bulk concentration (i.e., inside the particles). Dy and/or Tb could be intentionally added or partially diffused into the powder particle from the particle surface, if desired. However, the bulk concentration of Dy and/or Tb inside the particles is less than the surface concentration of Dy and/or Tb. The coating process is introduced into the current preparation for the powder metallurgy process as an extra step. 
         [0019]    Dy or Tb or both can be used, as desired. If Tb is included, not as much Dy is needed. For example, the combination of Dy and Tb could be less than about 6 wt %. Tb can be much more effective than Dy in improving magnetic properties. However, this should be balanced against the significantly higher cost of Tb. A ratio of Dy:Tb of up to about 1:5 can be used if desired, but a ratio of up to about 1:3 would be more typical due to cost considerations. 
         [0020]    The Dy or Tb concentration distribution feature can be manipulated by various heat treatments of the magnets, especially annealing schedules. A longer time or higher temperature can make the distribution wider and less concentrated at the particle surface. 
         [0021]    The magnet manufacturing process includes: 1) melting and strip casting, 2) hydrogen decrepitation (hydride and de-hydride), 3) pulverizing (with nitrogen), 4) mixing alloy powder to adjust the chemical composition and optional screening, 5) coating the powder with Dy and/or Tb rich powder, and 6) optional screening. This is followed by a typical powder metallurgy process, such as: weighing and pressing (vacuum bagging), isostatic pressing, sintering and aging (e.g., about 30 hrs, at about 1100° C., in vacuum) and machining to magnet pieces. Finally, the magnets are surface treated (e.g., phosphate, electroless Ni plating, epoxy coating, etc.). 
         [0022]    The three coating methods described above will be discussed in more detail. 
         [0023]    Mechanical alloying is a solid-state powder processing technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. It can be used to synthesize a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or pre-alloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. 
         [0024]    Mechanical alloying uses a high energy mill to favor plastic deformation required for cold welding and to reduce the process times. It allows a mixture of elemental and master alloy powders to be used. The use of master alloy powders reduces the activity of the element because it is known that the activity in an alloy or a compound could be orders of magnitude less than in a pure metal. Mechanical alloying eliminates the use of surface-active agents which would produce fine pyrophoric powder as well as contaminate the powder. It relies on a constant interplay between welding and fracturing to yield a powder with a refined internal structure, typical of very fine powders normally produced, but which has an overall particle size which is relatively coarse, and therefore stable. 
         [0025]    The mechanical alloying process starts with mixing the powders in the desired proportion. The powder mix is loaded into the ball mill along with the grinding medium (e.g., steel balls). The powder mixture is then milled for the desired length of time. The important components of the mechanical alloying process are the raw materials, the mill, and the process variables. Parameters include the type of mill, the milling container, milling speed (generally about 50 to about 400 rpm, typically about 250 rpm), milling time (generally about 0.5 to about 12 hours), the type, size, and size distribution of the grinding medium (e.g., hardened steel, stainless steel etc.), ball-to-powder weight ratio (generally about 1:1 to as high as about 220:1, with about 10:1 being typical), the extent of filling the vial, the milling atmosphere (e.g., vacuum, nitrogen, or argon), and temperature of milling (generally room temperature to about 250° C.). 
         [0026]    The raw materials used for mechanical alloying can have particle sizes in the range of 1-200 μm. The powder particle size is not critical, except that it should be smaller than the grinding ball size, because the powder particle size decreases exponentially with time and reaches a few microns after only a few minutes of milling. The raw powders can be pure metals, master alloys, or pre-alloyed powders. 
         [0027]    Different types of high-energy milling equipment can be used to produce mechanically alloyed powders. They differ in their capacity, efficiency of milling, and additional arrangements for cooling, heating, etc. A conventional ball mill  10  includes a rotating horizontal drum  15  partially filled with small steel balls  20 , as shown in  FIG. 1 . As the drum  15  rotates, the balls  20  drop on the metal powder that is being ground. The grinding tanks or containers are available in stainless steel or stainless steel coated inside with alumina, silicon carbide, silicon nitride, etc., for example. The ball mill  10  includes a rotating impeller  25 . Coolant flows through the jacket of the drum  15  from an inlet  30  to an outlet  35  to control the temperature of the powder during milling. 
         [0028]    Another method involves coating the Nd—Fe—B based powders with Dy or Tb metal using physical vapor deposition (PVD). PVD methods using a particle gun based on spark erosion and sputtering are illustrated in  FIGS. 2-3 , although other PVD processes could be used if desired. The “substrate” can be located on the bottom. The substrate is basically a container containing the Nd—Fe—B powders to be coated. If desired, there can be a mixer (not shown) in the container to stir the powders to ensure a uniform coating on the powders. 
         [0029]      FIG. 2  illustrates a spark erosion PVD process. There is a fixed electrode holder  100  and an moveable electrode holder  105 . The fixed electrode holder  100  is connected to an electrical power supply (not shown). The moveable electrode holder  105  is connected to an electrical power supply and a mechanical oscillator (not shown). The fixed electrode holder  100  and moveable electrode holder  105  have electrodes  110 . A carrier gas inlet  115  introduces a carrier gas. A treatment gas inlet  120  introduces a treatment gas into the carrier gas. The coating material is directed to the substrate  135 . 
         [0030]    A sputtering PVD coating process is shown in  FIG. 3 . There are two magnetron sputter sources  150  on the top directed toward the rotating substrate table  155  on the bottom. In sputtering, atoms are ejected from the surface of a target material (Dy and/or Tb or the alloys) due to the impact of high energy particles (such as nitrogen ions) in plasma. The ejected atoms condense on the surface of the substrate creating a thin film. 
         [0031]    The third coating method involves coating the Nd—Fe—B based powders with a very fine metal powder of Dy or Tb metal and/or alloys mixed with a solvent. A high velocity jet (about 30 to about 60 ft/sec) is established by accelerating a stream of air or inert gas with a swirl accelerator. By adjusting the flow rate and pressure of the air/gas stream, a laminar flow pattern can be established at Reynolds numbers where turbulent flow would normally occur. The gas is directed to a “coating tube”. The swirl accelerator is available from GEA Process Engineering Inc., of Columbia, Md. 21045, for example. 
         [0032]    As shown in  FIG. 4 , a reservoir of the powder to be coated in the area of “down flow bed”  200  surrounds the coating tube  205  and is kept lightly aerated by a low velocity gas stream  210  that enters the powder bed from the bottom. There is a low volume flow in this area. A gap  215  between the inlet fluidizing plate  220  and the bottom of the coating tube  205  under the wetting and contacting zone  225  allows powder to be exposed to the high velocity gas stream. Particles of powder are picked up at this interface and accelerated by the gas stream. 
         [0033]    A fine spray  230  of the coating that contains Dy or Tb metal or alloy is introduced into the bottom of the high velocity gas stream through the spray nozzle  235 . The coating spray  230  is moving faster than the solid particles so contact occurs and a coating is deposited. 
         [0034]    The boundary layer effect causes a velocity gradient from high gas velocity at the center of the tube to zero at the wall. This gradient causes the powder to be tumbled by the gas stream so that all particle surfaces are exposed to the coating spray. Once the coating is applied, the coated particle travels on up the coating tube. The particle velocity is always lower than the gas velocity so there is always a movement of gas across the particle surface. This gas movement evaporates the solvent and dries the coating in the drying zone  240 . The particle is substantially dry by the time it reaches the end of the coating tube  205 . 
         [0035]    At the end of the tube, the particles disengage from the high velocity stream and fall back to the holding area (not shown). 
         [0036]    The “solvent” or carrier can be alcohols, chlorinated solvents, or almost any other solvent used industrially. The actual coating time is extremely short so that only a thin layer of coating is applied with each coating. The Nd—Fe—B powders are repeatedly coated to obtain a desired coating thickness. The final coating thickness is generally a few micrometers or less, e.g., less than about 10 micrometers, or about 1 to about 10 micrometers, or about 2 to about 5 micrometers, depending on the process parameters. The typical conveying velocity is about 20 to about 40 meter/sec. At any given time, most of the powder in the system is dry because the actual wetting and drying process is very short, making the process relatively easy to control. The rapid coating and drying cycle also means that the powder is only wetted by the solvent for a very short time. The solvent has little opportunity to penetrate the interior of the particles. This means that solvent/powder reactions are generally not a problem, and it is often possible to use solvents that would normally be considered incompatible with the powder. 
         [0037]    It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. 
         [0038]    For the purposes of describing and defining the present invention it is noted that the term “device” is utilized herein to represent a combination of components and individual components, regardless of whether the components are combined with other components. For example, a “device” according to the present invention may comprise an electrochemical conversion assembly or fuel cell, a vehicle incorporating an electrochemical conversion assembly according to the present invention, etc. 
         [0039]    For the purposes of describing and defining the present invention it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0040]    Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.