Abstract:
A sintered magnet production system using a press-less method capable of minimizing the distortion of a sintered magnet includes: a filling device for filling a container cavity with alloy powder for a sintered magnet; an orienting section for orienting the powder in the cavity by applying a magnetic field without applying a mechanical pressure to the powder; and a sintering section for sintering the powder oriented by the orienting section, by heating the powder without applying a mechanical pressure to the powder. The orienting section includes: an air-core coil; and two ferromagnetic members made of a ferromagnetic material arranged within the coil at respective open ends of the coil with a space for containing the container in between. The ferromagnetic members adjust the magnetic field within the coil toward a direction parallel to the axis of the coil, whereby distortion of a sintered magnet is minimized.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a system and method for producing a sintered magnet containing a rare-earth element R, such as an RFeB system (R 2 Fe 14 B) or RCo system (RCo 5 , R 2 Co 17 ). 
       BACKGROUND ART 
       [0002]    For the production of sintered magnets, a method has been conventionally used which includes the steps of filling a cavity of a container with fine powder of a starting alloy (which is hereinafter called the “alloy powder”) (filling process), applying a magnetic field to the alloy powder in the cavity to orient the particles of the alloy powder (orienting process), subsequently applying pressure to the alloy powder to produce a compression-molded compact (compression-molding process), and heating the compression-molded compact to sinter it (sintering process). A variation of this method has also been used in which, after the cavity has been filled with the alloy powder, the orienting process and the compression-molding process are simultaneously performed by applying a magnetic field to the alloy powder while applying pressure by a pressing machine. In any cases, these methods uses a pressing machine for compression molding. Therefore, in the present application, these methods are called the “pressing method.” 
         [0003]    Meanwhile, in recent years, it has been found that a sintered magnet having a desired shape using corresponding shape of cavity can be obtained, without the compression-molding process, by a method in which the alloy powder that has been placed in the cavity is oriented in a magnetic field and subsequently, directly subjected to the sintering process (Patent Literature 1). In the present application, such a method of producing a sintered magnet without the compression-molding process is called the “press-less method.” 
         [0004]    In the pressing method, the orienting process also requires an application of a mechanical pressure; otherwise, when the compression-molding process is performed after the orienting process, the alloy-powder particles which have been methodically oriented in the orienting process would be disordered by the compression-molding. Therefore, a strong magnetic field must be used for the orientation, and it is difficult to contain the entire system (including the pressing machine) within the magnetic field. By contrast, the press-less method is advantageous in that it does not include application of such pressure and therefore does not require using an expensive magnetic field pressing machine; the application of the magnetic field can be performed within an air-core coil formed by a conductive wire wound around the cavity filled with the alloy powder. 
       CITATION LIST 
     Patent Literature 
       [0005]    Patent Literature 1: JP 2006-019521 A 
         [0006]    Patent Literature 2: JP 11-049101 A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    Whichever of the pressing method and the press-less method is used, the product obtained by sintering the powder is slightly smaller in size than the powder before the sintering (the compression-molded compact or the lump of powder in the cavity). Accordingly, the press die or the cavity shape is designed to be slightly larger than the intended shape of the sintered product. 
         [0008]    However, the present inventors have discovered that a sintered magnet obtained by the press-less method may not take the intended shape (the shape corresponding to the cavity) due to a variation in the degree of shrinkage or deformation depending on the position within the air-core coil (this phenomenon is hereinafter called the “distortion”). 
         [0009]    The problem to be solved by the present invention is to provide a sintered magnet production system and method using a press-less method capable of minimizing the distortion of the sintered magnet. 
       Solution to Problem 
       [0010]    The present inventors have studied the manufacturing conditions for the cases where the distortion of the sintered magnets occurred, and have found that significant amounts of distortion occurred in the sintered magnets obtained from the cavities placed near the open ends of the air-core coil in the magnetic orientation process of the alloy powder. For example, in the case where a cavity having a shape corresponding to a plate-shaped rectangular sintered magnet is placed within an air-core coil with the plate face perpendicular to the axis of the air-core coil (coil axis), the eventually obtained sintered magnet will not have a rectangular plate face; the side closest to the intersection point of the plate face and the coil axis will be inwardly warped, forming an arc shape. A similar distortion was observed in a sintered magnet obtained from a cavity placed near the longitudinal center of the coil axis, although it was less noticeable than in the case of the cavities near the ends (open ends). 
         [0011]    From these facts, the present inventors have inferred the cause of the distortion as follows: The magnetic field lines within the air-core coil form a “trumpet-like” shape which spreads in the radial direction from the coil axis as the position departs from the center of the air-core coil toward the open ends. If a cavity filled with alloy powder is placed in a magnetic field with such magnetic field lines in the orienting process, the direction of the magnetic field lines, or the direction of the magnetic field, deviates depending on the position within the cavity (relative to the coil axis). Consequently, the direction of the magnetic orientation of the alloy-powder particles varies depending on the position within the cavity. In general, the shrinkage factor is large in the direction of the magnetic orientation. Therefore, if the alloy powder under such a condition is sintered by a heating process, the obtained sintered magnet will have a different shrinkage factor in each portion. Such a variation in the direction of the magnetic field depending on the position within the cavity, and the thereby caused variation in the direction of the magnetic orientation as well as the difference in the shrinkage factor are not noticeable in the central region of the air-core coil, but noticeable at cavities placed near the open ends. 
         [0012]    Accordingly, the present inventors have studied the configuration of the air-core coil for making the magnetic field lines within the core of the air-core coil fully aligned to the direction parallel to the coil axis, and have developed the present invention. 
         [0013]    That is to say, the sintered magnet production system according to the present invention developed for solving the previously described problem is a system including: a filling device for filling a cavity of a container with alloy powder of a material for a sintered magnet; an orienting device for orienting the alloy powder held in the cavity by applying a magnetic field without applying a mechanical pressure to the alloy powder; and a sintering device for sintering the alloy powder oriented by the orienting device, by heating the alloy powder without applying a mechanical pressure to the alloy powder, wherein: 
         [0014]    the orienting device includes: 
         [0015]    a) an air-core coil; and 
         [0016]    b) two ferromagnetic members made of a ferromagnetic material to be arranged within the air-core coil at respective open ends of the air-core coil with a space for containing the container in between. 
         [0017]    In the sintered magnet production system according to the present invention, the two ferromagnetic members are arranged at the open ends of the air-core coil, with the container filled with the alloy powder by the filling device placed within the air-core coil. As a result, a magnetization parallel to the coil axis of the air-core coil is formed in the ferromagnetic members. By this magnetization, the magnetic field lines are adjusted to become aligned to the direction parallel to the coil axis, whereby a magnetic field closer to fully parallel to the coil axis is created. This reduces the variation in the direction of orientation of the particles depending on the position within the cavity. As a result, the difference in the shrinkage factor among the portions of the sintered magnet obtained through the sintering process will be smaller and the distortion will be minimized. 
         [0018]    Each of the ferromagnetic members should preferably be composed of laminated plates made of a ferromagnetic material, with their plate faces intersecting with a plane perpendicular to the axis of the air-core coil. Typically, the plate faces should orthogonally intersect with the aforementioned plane, but may also obliquely intersect with it. As the plate-shaped members, for example, magnetic steel sheets can be used. 
         [0019]    The reason why the plate faces of the plate-shaped members should preferably intersect with a plane perpendicular to, the coil axis is as follows: When the strength of the magnetic field created by the air-core coil changes, particularly in the initial phase of the formation of the magnetic field or in the case of using a pulsed magnetic field, an eddy current perpendicular to the coil axis occurs in the ferromagnetic member. When such an eddy current occurs, the magnetic field in the ferromagnetic member is weakened, and consequently, the effect of aligning the magnetic field lines closer to the direction parallel to the coil axis will also be weakened. The previously described structure composed of laminated plates made of a ferromagnetic material with their plate faces intersecting with a plane perpendicular to the coil axis blocks the current at the interfaces of respective plates and prevents occurrence of the eddy current, so that a magnetic field closer to the direction parallel to the coil axis can be created. 
         [0020]    The sintered magnet production method according to the present invention is a method including: a filling process in which a container is filled with alloy powder of a material for a sintered magnet; an orienting process in which the alloy powder held in the container is oriented by applying a magnetic field without applying a mechanical pressure to the alloy powder; and a sintering process in which the alloy powder oriented in the orienting process is sintered by heating the alloy powder without applying a mechanical pressure to the alloy powder, wherein: 
         [0021]    the orienting process includes the steps of: 
         [0022]    placing the container within an air-core coil and generating a magnetic field within the coil, with two ferromagnetic members to be arranged at respective open ends of the air-core coil with the container in between, to orient the alloy powder in the container. 
       Advantageous Effects of the Invention 
       [0023]    According to the present invention, it is possible to align the magnetic field lines within the coil closer to fully parallel to the coil axis and thereby minimize the distortion of a sintered magnet produced by a press-less method. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]      FIG. 1  is a schematic diagram showing the overall configuration of one embodiment of the sintered magnet production system according to the present invention. 
           [0025]      FIGS. 2A and 2B  are a vertical sectional view and a top view, respectively, of an alloy-powder container used in the sintered magnet production system of the present embodiment. 
           [0026]      FIG. 3  is a vertical sectional view showing the configuration of the orienting section in the sintered magnet production system of the present embodiment. 
           [0027]      FIG. 4A-1  is a vertical sectional view showing the magnetic field lines of a magnetic field created within a coil in the orienting section with only the containers placed within the coil,  FIG. 4A-2  is a sectional view at line A-A in  FIG. 4A-1 , and  FIG. 4A-3  is a partially enlarged view of  FIG. 4A-3 , while  FIG. 4B-1  is a vertical sectional view showing the magnetic field lines of a magnetic field created within the coil with the containers sandwiched between ferromagnetic members in the coil-axis direction, and  FIG. 4B-2  is a sectional view at line B-B in  FIG. 4B-1 . 
           [0028]      FIG. 5  is a top view showing a plate-shaped sintered magnet in a distorted form and the definition of the amount of distortion. 
           [0029]      FIG. 6  is a graph showing the amount of distortion of sintered magnets obtained with a sintered magnet production system of the present embodiment and that of a comparative example. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    One embodiment of the sintered magnet production system and method according to the present invention is described using  FIGS. 1-6 . The following descriptions deal with the case of producing a NeFeB system sintered magnet. The descriptions similarly apply in the case of producing a sintered magnet other than the NeFeB system, such as a SmCo system. 
       Embodiment 
     (1) Configuration of Sintered Magnet Production System  10  of Present Embodiment 
       [0031]    In this section, the overall configuration of the sintered magnet production system  10  of the present embodiment will be initially described, after which a detailed description of the configuration of the orienting section (orienting device)  13  will be given, and lastly, a description will be made about the configuration of a piling section  12 , elevator  139  and ferromagnetic member removing section  14  which serve to assist the operation of the orienting section  13 . 
       (1-1) Overall Configuration 
       [0032]    As shown in  FIG. 1 , the sintered magnet production system  10  of the present embodiment has a filling section  11 , a piling section  12 , an orienting section  13 , a ferromagnetic member removing section  14  and a sintering section  15 , as well as a conveyer (belt conveyer)  16  for sequentially conveying containers  21  among those sections in a roughly horizontal direction. No pressing machine is provided for the sintered magnet production system  10 . 
         [0033]    Furthermore, the sintered magnet production system  10  is provided with a closed chamber  17  containing the aforementioned sections exclusive of the sintering section  15 . This closed chamber  17  can be filled with an inert gas, such as argon or nitrogen gas. The sintering section  15  is located outside the closed chamber  17 , but as will be described later, it can be filled with an inert gas independently of the closed chamber  17 . The inert gas used in the closed chamber  17  and the sintering section  15  serves to prevent oxidization of alloy powder which is the material for the NeFeB system sintered magnet. 
         [0034]    The filling section  11  is a system for filling cavities  211  of the container  21  with alloy powder. It has a hopper  111  for storing an amount of alloy powder, a sieve member  112  provided at the opening at the lower end of the hopper  111 , and a gas supplier  113  for repeatedly supplying compressed gas (inert gas) in a pulsed form to the hopper  111 . Such a system for filling a container with powder using compressed gas repeatedly supplied in a pulsed form is called an “air-tapping system.” For example, a system described in Patent Literature 2 can be used. 
         [0035]    The configuration of the piling section  12 , orienting section  13  and ferromagnetic member removing section  14  will be described later along with the description of the orienting device in the present invention. 
         [0036]    The sintering section  15  has a sintering chamber  151  having a carry-in entrance and a carry-out exit for containing a number of containers  21 , a heat-insulating door  152  provided at the carry-in entrance, and a heater (not shown) for heating the inside of the sintering chamber  151 . The closed chamber  17  and the sintering chamber  151  communicate with each other at the carry-in entrance but can be thermally separated by closing the heat-insulating door  152 . The sintering chamber  151  can be filled with inert gas (independently of the closed chamber  17 ). 
         [0037]    As shown in  FIGS. 2A and 2B , the container  21  used in the present embodiment has two cavities  211  each of which has a roughly rectangular-parallelepiped shape measuring 95.2 mm in length, 17.9 mm in width and 7.7 mm in depth and which are arranged side-by-side in their width direction. The entire container  21  measures 126 mm in length, 76 mm in width and 10 mm in thickness. 
       (1-2) Configuration of Orienting Section (Orienting Device)  13   
       [0038]    The configuration of the orienting section (orienting device)  13  in the sintered magnet production system  10  is hereinafter described. As shown in  FIG. 3 , the orienting section  13  consists of an air-core coil  131  and ferromagnetic members  132 . The air-core coil  131  is positioned with its axis directed in a substantially vertical direction. The air-core coil  131  measures 150 mm in inner diameter and 200 mm in the distance between its open ends (coil length). The air-core coil  131  is located above the conveyer (belt conveyer)  16 . By means of the elevator  139  (which will be described later), the target of the orienting process is carried into and removed from the air-core coil  131  through the opening on the lower side. 
         [0039]    The ferromagnetic member  132  used in the present embodiment was composed of approximately 200 plate members made of magnetic steel (magnetic steel sheets) arranged next to each other with their easy axes of magnetization aligned in the direction of the coil axis (or with their plate faces intersecting with a plane perpendicular to the coil axis). In the present embodiment, non-oriented magnetic steel sheets were used, although oriented magnetic steel sheets may also be used. Each single magnetic steel sheet was 0.35 mm thick. The ferromagnetic member  132  was enclosed in a rectangular-parallelepiped containing case  133  made of a non-magnetic material. As another example of the ferromagnetic member  132 , a containing case  133  filled with magnetic particles of iron was also prepared. 
         [0040]    In the orienting section  13 , n containers  21  (where n is two or greater) piled in their, thickness direction are placed within the air-core coil  131 , with one ferromagnetic member  132  enclosed in the containing case  133  arranged above and below, i.e. on each of the two sides of the containers  21  facing the open ends of the air-core coil  131 . That is to say, a structure including one ferromagnetic member  132 , n containers  21  and another ferromagnetic member  132  sequentially piled from below is carried into the air-core coil  131  (this structure is hereinafter called the “pile  22 ”). 
       (1-3) Configuration of Piling Section  12 , Elevator  139  and Ferromagnetic Member Removing Section  14   
       [0041]    The piling section  12  is a system for building the pile  22 . It has a first elevating stage  121  and a first top lift  122 . The first elevating stage  121  can be vertically moved between a position which is level with the roughly horizontal conveyer line of the conveyer  16  and a position lower than the former position by an amount which is not less than the sum of the thickness of one containing case  133  and that of the (n−1) containers  21  in the pile  22 . 
         [0042]    The elevator  139  is located below the air-core coil  131 . The pile  22  conveyed from the piling section  12  by the conveyer  16  is placed onto this elevator, which pushes up the pile  22  into the air-core coil  131 . After the orienting process is completed, it lowers the pile  22  to the level of the conveyer line of the conveyer  16 . 
         [0043]    The ferromagnetic member removing section  14  is a system for removing the ferromagnetic members  132  from the pile  22  which has gone through the orienting process in the orienting section  13 . It has a second elevating stage  141  and a second top lift  142 . The second elevating stage  141  can be vertically moved between the level of the conveyer line of the conveyer  16  and a level lower than that by an amount equal to the thickness of one containing case  133 . 
       (2) Operation of Sintered Magnet Production System  10  of Present Embodiment 
     (2-1) Filling Process 
       [0044]    Initially, a container  21  is conveyed to a position below the hopper  111  of the filling section  11  by the conveyer  16 , and the hopper  111  is attached to the upper side of the container  21 . Next, compressed gas in a pulsed form is repeatedly supplied from the gas supplier  113  to the hopper  111  to make the alloy powder in the hopper  111  pass through the sieve member  112  and fill the cavity  211  of the container  21 . Subsequently, the hopper  111  is removed from the container  21 , and the container  21  is conveyed to the piling section  12  by the conveyer  16 . These operations are cyclically performed for each container  21  until n containers  21  are completed. 
       (2-2) Operation in Piling Section  12   
       [0045]    In the piling section  12 , initially, one ferromagnetic member  132  is placed onto the first elevating stage  121  by the first top lift  122 . Next, the first elevating stage  121  is lowered so that the top face of the containing case  133  of the ferromagnetic member  132  will be level with the conveyer line of the conveyer  16 . Then, one container  21  conveyed from the filling section  11  by the conveyer  16  is placed onto the top face of the containing case  133 . Subsequently, the first elevating stage  121  is lowered so that the top face of this container  21  will be level with the conveyer line of the conveyer  16 . This operation is repeatedly performed every time one container  21  is conveyed to the piling section  12  in the previously described manner until n containers are completed. After that, the second ferromagnetic member  132  is placed onto the top face of the uppermost container  21  by the first top lift  122 . Thus, the pile  22  is completed. Subsequently, the first elevating stage  121  is moved upward so that the lowest portion of the pile  22  (the bottom face of the lower containing case  133 ) will be level with the conveyer line of the conveyer  16 , and the pile  22  is conveyed to the elevator  139  by the conveyer  16 . 
       (2-3) Orienting Process 
       [0046]    The pile  22  conveyed to the elevator  139  is pushed up by the elevator  139  into the air-core coil  132  of the orienting section  13 . Then, a magnetic field is generated within the air-core coil  131 , whereby the alloy powder in the containers  21  of the pile  22  is oriented along the axial direction of the air-core coil  131 . The effect of the orienting section  13 , and particularly, that of the ferromagnetic members  132  in this operation will be described later. Although the magnetic field to be eventually applied is a pulsed DC magnetic field of 4 tesla, it is preferable to previously apply a pulsed AC magnetic field, which facilitates the subsequent magnetic orientation by the pulsed DC magnetic field. After the application of the magnetic field is completed, the elevator  139  is lowered to remove the pile  22  from the air-core coil  131 . Subsequently, the pile  22  is conveyed to the ferromagnetic member removing section  14  by the conveyer  16 . 
       (2-4) Operation in Ferromagnetic Member Removing Section  14   
       [0047]    In the ferromagnetic member removing section  14 , the pile  22  is placed onto the second elevating stage  141 , and the upper ferromagnetic member  132  is removed from the pile  22  by the second top lift  142 . Subsequently, the second elevating stage  141  is lowered by an amount equal to the thickness of the containing case  133  so that the bottom face of the lowermost container  21  in the n containers  21  will be level with the conveyer line of the conveyer  16 . Then, only the n containers  21  in the pile are conveyed to the sintering section  15  by the conveyer  16 , leaving the lower ferromagnetic member  132  behind. The upper and lower ferromagnetic members  132  left behind are returned to the piling section  12  by the conveyer  16 . 
       (2-5) Sintering Process 
       [0048]    In the sintering process  15 , the door  152  of the carry-in entrance is opened, and a predetermined number of piles of n containers  21  are carried into the sintering chamber  151 . After that, the door  152  is closed, and the inside of the sintering chamber  151  is heated by the heater to a predetermined sintering temperature (normally, 900 to 1100° C.). By this process, the alloy powder in the cavities  211  is sintered, and sintered magnets are obtained. 
         [0049]    Any of the previously described processes is performed in an inert gas atmosphere formed within the closed chamber  17  or the sintering chamber  151 . Furthermore, none of those processes include the step of press-molding the alloy powder. 
       (3) Effect of Orienting Section  13  and Reason for Minimized Distortion of Sintered Magnet 
       [0050]    The effect of the orienting section  13  in the sintered magnet production system  10  of the present embodiment, and the reason why the distortion of the sintered magnet is minimized are described, using  FIGS. 4A-1  through  4 B- 2 .  FIGS. 4A-1  through  4 A- 3  illustrate the direction of the magnetic field H within the air-core coil  131  in a conventional sintered magnet production system which does not use the ferromagnetic members  132 . Among them,  FIG. 4A-1  shows a vertical section (including the axis of the air-core coil  131 ),  FIG. 4A-2  shows a horizontal section (perpendicular to the axis of the air-core coil  131 ) through the container  21  closest to the open ends of the air-core coil  131 , and  FIG. 4A-3  shows a partially enlarged view of  FIG. 4A-2 .  FIGS. 4B-1  and  4 B- 2  respectively show the direction of the magnetic field H on the vertical and horizontal aforementioned sections in the sintered magnet production system  10  of the present embodiment. In  FIGS. 4A-2  and  4 A- 3 , a coordinate system is defined in which the x and y axes respectively extend along the long and short sides of the rectangular top face of the rectangular-parallelepiped cavity  211 , with the origin O lying at the coil axis. The arrows show the direction and magnitude of the projection H ⊥  of the magnetic field H onto the horizontal section (xy plane) as well as the magnitude of x component Hx. 
         [0051]    In the comparative example, i.e. in the sintered magnet production system which does not use the ferromagnetic members  132 , a magnetic field which spreads in the radial direction as the distance from the center of the coil-axis direction increases toward the open ends is formed within the air-core coil  131  ( FIG. 4A-1 ). Therefore, within the cavity  211  of the container  21 , a magnetic field whose direction varies depending on the position is formed ( FIG. 4A-3 ). Consequently, the alloy powder in the cavity  211  becomes magnetically oriented in different directions depending on the position, causing the sintered magnet being created to shrink in different directions at different points. Specifically, as shown in  FIG. 4A-3 , when two arbitrarily chosen points A and B at the same x coordinate (x0) are compared, point A whose y coordinate is closer to zero has a greater magnitude of H⊥, and furthermore, its direction is closer to the x direction, so that Hx at point A has a greater magnitude than at point B. Therefore, at any x coordinate, as the y coordinate becomes closer to zero or to the coil axis, Hx becomes stronger and more easily causes the shrinkage in the x direction during the sintering process. As a result, the sintered magnet as viewed on the aforementioned horizontal plane (xy plane) becomes distorted and bulges outward from the coil axis (see  FIG. 5 ). 
         [0052]    By contrast, in the sintered magnet production system  10  of the present embodiment, when a magnetic field is created within the air-core coil  131 , the direction of the magnetic field is adjusted by the magnetization which is formed in the ferromagnetic members  132 . As a result, the direction of the magnetic field becomes closer to the direction of the coil axis of the air-core coil  131  ( FIG. 4B-1 ), so that the variation in the direction and strength of the magnetic field within the cavity  211  of the container  21  will be reduced ( FIG. 4B-2 ). 
         [0053]    Furthermore, as in the present embodiment, in the case where laminated plate-shaped ferromagnetic members (magnetic steel sheets) with their plate faces inclined from the plane perpendicular to the coil axis are used as the ferromagnetic member  132 , when the pulsed DC magnetic field is created by the air-core coil  131 , the ferromagnetic member can prevent an occurrence of eddy current which cancels the magnetic field. Therefore, the aforementioned effect of adjusting the direction of the magnetic field will not be weakened. 
       (4) Result of Experiment  
       [0054]    To confirm the influence and effect by the sintered magnet production system  10  of the present embodiment, the following experiment has been conducted. As the ferromagnetic member  132 , the previously described one consisting of the non-oriented magnetic steel sheets and the one made of magnetic particles were used. Furthermore, as a comparative example, a similar experiment was conducted without using the ferromagnetic member  132 , as shown in  FIG. 4A-1 . The number of containers  21  placed within the air-core coil  131  in the present experiment was four. In the following description, those four containers  21  are labeled as Nos.  1  to  4  from bottom to top. As shown in  FIG. 5 , the sintered magnets S produced in the present embodiment and the comparative example had a distorted form with the long side shaped like an arc. The amount of distortion was defined as w max −w avg , where w max  is the width of the short side of a rectangle R including the entire plate face of the sintered magnet S (see  FIG. 5 ) and w avg  is the average width in the short-side direction of the sintered magnet S. The amount of distortion was determined for each of the containers Nos.  1 - 4  by measuring the sizes of the created sintered magnets S. 
         [0055]    The result of the experiment demonstrated that, in any of the containers  21  Nos.  1 - 4 , the amount of distortion was the largest in the comparative example, the second largest when the ferromagnetic member  132  made of magnetic particles was used, and the smallest when the ferromagnetic member  132  consisting of the non-oriented magnetic steel sheets was used. This result confirms that the sintered magnet production system  10  of the present embodiment can reduce the amount of distortion as compared to the comparative example. 
         [0056]    As for the variation in the amount of distortion among the containers  21  or with respect to the position within the air-core coil  131 , the following points should be mentioned: In the comparative example, the sintered magnets S produced in the containers  21  Nos.  1  and  4  placed near the open ends of the air-core coil  131  underwent a greater amount of distortion than those produced in the containers  21  Nos.  2  and  3  placed near the center of the air-core coil  131 . The most likely reason is that the magnetic field in the comparative example is curved by a greater amount in the regions near the open ends of the air-core coil  131 . When the ferromagnetic member  132  made of magnetic particles was used, there was no noticeable difference from the comparative example in terms of the variation depending on the position of the container. By contrast, when the ferromagnetic member  132  consisting of the non-oriented magnetic plate sheets was used, the variation in the amount of distortion depending on the position of the container was barely noticeable. This is most likely because the non-oriented magnetic plate sheets with the easy axes of magnetization orderly aligned in the direction of the coil axis improve the effect of correcting the distortion of the magnetic field, so that the magnetic field can be sufficiently aligned in the axial direction of the air-core coil  131  even in the region near the open ends. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           11  . . . Filling Section 
           111  . . . Hopper 
           112  . . . Sieve Member 
           113  . . . Gas Supplier 
           12  . . . Piling Section 
           121  . . . First Elevating Stage 
           122  . . . First Top Lift 
           13  . . . Orienting Section 
           131  . . . Air-Core Coil 
           132  . . . Ferromagnetic Member 
           133  . . . Containing Case 
           139  . . . Elevator 
           14  . . . Ferromagnetic Member Removing Section 
           141  . . . Second Elevating Stage 
           142  . . . Second Top Lift 
           15  . . . Sintering Section 
           151  . . . Sintering Chamber 
           152  . . . Door of Carry-in Entrance 
           16  . . . Conveyer 
           17  . . . Closed Chamber 
           21  . . . Container 
           211  . . . Cavity 
           22  . . . Pile 
           301  . . . Cavity