Abstract:
A particle ( 10 ) of ferromagnetic powder for use in preparation of soft magnetic core components has a core-shell structure. The particle includes a central core ( 12 ) and a shell ( 14 ) coated on the central core. The central core is made of magnetic material and is used for providing the necessary magnetic property for the magnetic core components made from the ferromagnetic powder. The shell has a higher electrical resistance than the central core so as to reduce an eddy current loss of the magnetic core component. The shell also functions to provide an excellent bonding strength between particles of the powder.

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
   Relevant subject matter is disclosed in two copending U.S. patent application filed on the same date and each having a title “motor stator”, which are assigned to the same assignee with the present application. 
   FIELD OF THE INVENTION 
   The present invention relates generally to soft magnetic materials, and more particularly to ferromagnetic powders used for producing soft magnetic core components for use as a dust core for a motor, inductor, transformer, generator or the like. 
   DESCRIPTION OF RELATED ART 
   Magnetic material includes hard magnetic material (Hc&gt;200 Oe) and soft magnetic material (Hc&lt;20 Oe), wherein the former can be permanently magnetized while the latter can be easily magnetized and demagnetized at an applied, relatively low magnetic field. Particularly, soft magnetic material has a high magnetic permeability and the magnetization thereof can be reversed easily at an applied field. The permeability of a magnetic material is an indication of its ability to become magnetized or its ability to carry a magnetic flux. Currently, soft magnetic material is widely used as material for producing the dust core for an electric/magnetic conversion device such as motors, generators, transformers, inductors and the like. 
   Some soft magnetic cores, such as rotors and stators in electric machines, are made of stacked steel laminations. For example, in a fan motor, silicon steel laminations have been used for decades as constituting the stator core of the fan motor. The silicon steel laminations, which are usually made from soft magnetic Fe—Si alloy via hot rolling, have an eddy current loss that is proportional to the square of the thickness of the laminations. The eddy current loss is brought about by the production of electric currents in the magnetic core component due to the changing flux caused by an alternating magnetic field. Thus, the laminations are expected to have a thickness as small as possible in order to reduce the eddy current loss problem. However, since the hot rolling technique requires each of the laminations to have a minimum thickness, and laminations with an excessively thin structure are prone to deformation during assembly, the laminations often are selected to have a thickness which is typically restricted at 0.20 mm, 0.35 mm or 0.50 mm. Furthermore, the shape of the stator core made from laminated steel sheets is also unduly limited. Certain three-dimensional configurations are very difficult and expensive to achieve with the silicon steel laminations. 
   Therefore, it is desirable to provide a soft magnetic material suited for the production of a dust core wherein one or more of the foregoing disadvantages may be overcome or at least alleviated. 
   SUMMARY OF INVENTION 
   The present invention relates to ferromagnetic powder for use in manufacturing of soft magnetic core components. A particle of the ferromagnetic powder has a core-shell structure, which includes a central core and a shell coated on the central core. The central core is made of magnetic material and is used for providing the necessary magnetic property for the magnetic core component made from the ferromagnetic powder. The shell has a higher electrical resistance than the central core and is used for providing a bonding strength between particles of the powder and for reducing an eddy current loss of the magnetic core component. 
   Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic representation of a particle of the ferromagnetic powder in accordance with an embodiment of the present invention; 
       FIG. 2  is a schematic representation of a particle of the ferromagnetic powder in accordance with an alternative embodiment of the present invention; and 
       FIG. 3  is a schematic representation of a particle of the ferromagnetic powder in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  schematically illustrates a particle  10  of the ferromagnetic powder in accordance with an embodiment of the present invention. The particle  10  has a core-shell structure, which includes an inner core  12  made of magnetic material and an outer shell  14  covering the core  12 . The shell  14  is a thin insulating layer coated on an outer peripheral surface of the core  12 . The shape of the particle  10  is subject to no limitations, which may be spherical, flat or other suitable shapes. When the particle  10  is spherical, an average diameter of the particle  10  is from 5 to 150 μm. 
   The magnetic material used for the core  12  is typically selected from a soft magnetic material of high magnetic permeability and low magnetic loss, such as soft magnetic metals, amorphous iron-based magnetic powder, pure iron powder, iron-based powder compositions, soft magnetic non-metals and the like. For example, magnetic powder such as iron, sendust, ferrosilicon, permalloy, supermalloy, iron nitride, iron-aluminum alloys or iron-cobalt alloys is suitable for the core  12 . Among these magnetic materials mentioned above, iron or iron-based powder compositions having high saturation magnetization is preferred when the powder is used to prepare dust cores as a substitute for the dust core prepared from silicon steel laminations currently widely employed in fan motors. 
   The shell  14  of the particle  10  is made from such materials as to enable the shell  14  to have an electrical resistance that is higher than that of the core  12  for the purpose of reducing the eddy current loss associated with the ferromagnetic powder. In these embodiments, the shell  14  is made of metal composites or piezoelectric materials. 
   As an example, the particle  10  with the core-shell structure is prepared by employing a diffusion/precipitation mechanism, based on powder sintering. Specifically, the soft magnetic material for the core  12  such as iron is melted firstly and coating material used to form the shell  14  is then added to the melted magnetic material to form a mixture. By using an atomizing or pulverization method, powder is prepared from the mixture. Then the powder is sintered at high temperature (e.g., in the range of about 300 to 900° C.) to cause the coating material contained in the powder to become supersaturated and accordingly precipitate out from the magnetic material. The magnetic material forms as the core  12  of the particle  10  and the precipitated coating material forms as the shell  14  coated on the core  12 . 
   In another example, the core  12  is previously obtained by, for example, an atomizing method from a soft magnetic material such as iron. A thin layer of film having high electrical resistance is then deposited on an outer surface of the core  12 , wherein the film is provided as the shell  14 . Such deposition method may be physical vapor deposition (PVD) or chemical vapor deposition (CVD). The material used for depositing of the film may be ferrites, piezoelectric materials, ferroelectric materials or ceramic materials. 
     FIG. 2  schematically illustrates another embodiment of the present invention, in which a particle  10   a  of the ferromagnetic powder has a multi-layer structure. As shown in this embodiment, the particle  10   a  includes a central core  12  and multiple layers of shells  14  concentrically surrounding the central core  12 . Every two adjacent shells  14  are spaced apart by a magnetic layer  16  made of magnetic material. The outmost part of the particle  10   a  is a shell layer  14 . The material for the magnetic layers  16  includes soft magnetic metals, amorphous iron-based magnetic powder, pure iron powder and composites thereof, soft magnetic non-metals and the like. In this preferred embodiment, the core  12  and the magnetic layers  16  are made of the same magnetic material. 
     FIG. 3  schematically illustrates a further embodiment of the present invention, in which a particle  10   b  includes multiple particles  10  of  FIG. 1  which are combined together by a binder  18  to form the particle  10   b . Each of the elementary particles  10  includes a magnetic central core  12  and an insulation shell  14  enclosing the central core  12 . In this embodiment, the binder  18  and the shell  14  are made of the same material. 
   The ferromagnetic powder as described above can be used to produce soft magnetic core components such as dust cores for transformers, inductors, motors, generators, and other electric/magnetic conversion devices through powder metallurgy. Powder metallurgy is a process of making parts by pressing powdered particles in die presses. A dust core can be made by pressure molding the ferromagnetic powder at a high temperature, for example, in the range of 300 to 800 centigrade degrees. After molding, the dust core can be desirably annealed to release the strain induced in the powder during the molding process in order to increase the magnetic performance thereof. The magnetic core  12  of each particle  10  of ferromagnetic powder provides the necessary magnetic property for the dust core. The shell  14  of the particle  10  operates to improve the bonding strength between the particles  10  as the ferromagnetic powder is pressure molded into the dust core. The shell  14  permits adjacent ferromagnetic particles  10  to strongly bond together, thereby increasing the mechanical performance of the dust core. Also, due to the presence of the shell  14 , the insulation between the ferromagnetic particles  10  is enhanced, thereby decreasing the eddy current loss of the dust core. Therefore, the dust core made of the ferromagnetic powder as illustrated above exhibits a high magnetic flux density, low eddy current loss as well as high mechanical strength. 
   The dust core made from the ferromagnetic powder is suitably used as a substitute for the conventional stator core of a fan motor made from laminated steel sheets. By using the powder metallurgy process, it is possible to produce dust cores with relatively complex shapes. The use of the coated ferromagnetic particles  10  avoids the manufacturing limits in laminated steel sheets and provides a higher freedom with respect to the shape of the dust core to be formed. By using the ferromagnetic particles  10  having the core-shell structure as described above, many advantages such as improved mechanical bonding strength, reduced eddy current loss and the ability to make magnetic core components having complex shapes are achieved. 
   It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.