Patent Application
20120176212
The present invention relates to a method and system for producing a slim-shaped sintered NdFeB magnet having excellent magnetic properties, and particularly a high level of coercivity and high degree of orientation. It also relates to a sintered NdFeB magnet produced by the aforementioned method.
A sintered NdFeB (neodymium-iron-boron) magnet, which was discovered by Sagawa (one of the present inventors) et al. in 1982, is characterized in that its properties are far superior to any previously used permanent magnets and yet it can be produced from relatively abundant, inexpensive materials, i.e. neodymium (a rare-earth element), iron and boron. Due to these merits, this magnet is currently used in various products, such as the voice coil motors for hard disk drives or similar devices, drive motors for hybrid cars or electric cars, motors for battery-assisted bicycles, industrial motors, high-quality speakers, head phones, and magnetic resonance imaging (MRI) apparatuses using permanent magnets.
Three methods have been previously known to be available for producing NdFeB sintered magnets: (1) a sintering method; (2) a method including the process steps of casting, hot working and aging; and (3) a method including the step of die upsetting of a rapidly cooled alloy. Among these methods, the sintering method is superior to the other two in terms of magnetic properties and productivity and has already been established on the industrial level. With the sintering method, a dense, uniform and fine structure necessary for permanent magnets can be obtained.
Patent Document 1 discloses a method for producing a sintered NdFeB magnet by a sintering method. A brief description of this method is as follows: Initially, an NdFeB alloy is created by melting and casting. This alloy is pulverized into fine powder and filled into a mold. A magnetic field is applied to this alloy powder, while pressure is applied to the powder with a pressing machine. In this manner, both the creation of a compact and the magnetic orientation of the compact are simultaneously performed. Subsequently, the compact is removed from the mold and heated for sintering to obtain a sintered NdFeB magnet.
Fine powder of an NdFeB alloy is easily oxidized and can ignite by reacting with oxygen in air. Therefore, the previously described process should preferably be performed entirely in an airtight container whose internal space is free from oxygen or filled with inert gas. However, this is impractical because creating the compact requires a large-sized pressing machine capable of applying a high pressure of tens or hundreds MPa to the alloy powder. Such a pressing machine is difficult to be set within an airtight container.
Patent Document 2 discloses a method for producing a sintered magnet without using a pressing machine (i.e. without creating a compact). This method includes the three processes of filling, orienting and sintering, which are performed in this order to create a sintered magnet. A brief description of this method is as follows: In the filling process, an alloy powder is supplied into a filling container (which is hereinafter called the “mold”), after which the density of the alloy powder in the mold is increased by a pushing, tapping or similar operation to a level of approximately 3M-4.2 g/cm3, which is higher than a natural filling density and lower than that of the press compact. In the orienting process, a magnetic field is applied to the alloy powder in the mold, without applying any pressure, to orient and align the crystal axes of the particles of the alloy powder in one direction. In the sintering process, the alloy powder which has been aligned in one direction in the orienting process is heated, together with the mold, to be sintered.
In the method of Patent Document 2, since no pressure is applied to the alloy powder in the magnetic orienting process and the density of the alloy powder is lower than that of the compact in the press-molding process, the friction among the particles of the alloy powder is reduced. Accordingly, in the orienting process, the powder particles can be aligned with high degree of orientation. As a result, an NdFeB magnet with even higher magnetic properties is obtained.
Patent Document 2 also discloses a system for producing a sintered magnet using an airtight container whose internal space can be maintained in an oxygen-free or inert-gas atmospheric condition, in which a filling unit, an orienting unit and a sintering unit are provided together with a conveyor for moving the filling container initially from the filling unit to the orienting unit and then from the orienting unit to the sintering unit. In this system, the alloy powder is handled under oxygen-free or inert-gas atmosphere throughout the entire process, so that the oxidization of the powder and the deterioration of magnetic properties due to the oxidization will not occur. Such a method in which no press compact is created, and in which the alloy powder is held in a mold until it is sintered into a magnet, will be hereinafter referred to as the “press-less process” or “PLP” method.
Patent Document 1: JP-A S59-046008
Patent Document 2: JP-A 2006-019521
In recent years, expectations for slim-shaped sintered NdFeB magnets (with a small thickness in its direction of magnetization) that can be used at an ambient temperature of 100° C. or higher have been increasing. A typical example is the magnets for automobiles, whose market has begun to rapidly expand due to the necessity for addressing environmental problems or other reasons. However, the magnetic properties of sintered NdFeB magnets significantly deteriorate with an increase in the temperature. An irreversible demagnetization is likely to occur at an ambient temperature of 100° C. or higher.
To avoid this problem, it is necessary to produce a sintered NdFeB magnet having a coercive force HcJ equal to or higher than a predetermined level, e.g. 15 kOe≈1.2 MA/m. (The coercive force HcJ is the value of magnetic field H at which magnetization J becomes zero as the magnetic field H is decreased on the magnetization curve.) This is because a magnet with a high level of coercive force is difficult to be demagnetized and less likely to undergo irreversible demagnetization. A commonly used method for improving the coercive force of a sintered NdFeB magnet is to replace a portion of Nd with Dy and/or Tb.
However, this technique cannot be successfully applied to the method of Patent Document 2 since this method allows a relatively high degree of freedom of motion among the powder particles. For example, replacing a portion of Nd with Dy and/or Tb to improve the coercive force of the sintered NdFeB magnet also increases the coercive force of the individual particles of the alloy powder, causing a stronger magnetic interaction among the particles. Due to this magnetic interaction, the orientation of the crystal axes of the aligned particles become disturbed before the alloy powder is completely sintered, which lowers the degree of orientation of the sintered NdFeB magnet after the sintering process and decreases the residual magnetic flux density from the expected level computed from the alloy composition.
The decrease in the residual magnetic flux density is more noticeable in the case of a slim-shaped sintered NdFeB magnet. This is due to the fact that the decreased amount of the alloy powder in the magnetizing direction strengthens the demagnetizing field acting on the alloy powder during the orienting process, and this demagnetizing field tends to disturb the alignment of the powder particles.
Given such a problem, a conventional method for producing a sintered magnet includes the steps of creating a block-shaped sintered NdFeB magnet having an adequate thickness in the direction of magnetization and cutting the block into thin plates to obtain a product that meets the aforementioned demand. One problem with this method is the waste dust produced by the cutting process, which cannot be reused for magnets and therefore not only deteriorates the use efficiency of the material but also increases the production cost. Another problem is that the cutting process mechanically damages the product, causing a decrease in the squareness (HK/HcJ) of the demagnetization curve or other magnetic properties.
The problem to be solved by the present invention is to provide a method and system for inexpensively producing a slim-shaped sintered NdFeB magnet having excellent magnetic properties, such as the residual magnetic flux density and coercive force.
A number of experiments and studies conducted by the present inventors have revealed that heating the NdFeB alloy powder in the orienting process to decrease the coercive force of the individual particles of the alloy powder is effective for suppressing the disturbance of the alignment of a magnetically oriented alloy powder. This technique makes it possible to maintain the degree of orientation of the alloy powder and thereby prevents a decrease in the residual magnetic flux density of the sintered NdFeB magnet even when the coercive force of the individual particles of the alloy powder increases due to the mixture of Dy into the alloy powder or when a strong demagnetizing field is formed due to a reduced amount of alloy powder in the magnetizing direction.
That is to say, a method for producing a sintered NdFeB magnet according to the present invention aimed at solving the aforementioned problem includes a filling process for filling an NdFeB alloy powder into a mold to a density within a range from 3.0 to 4.2 g/cm3, an orienting process for orienting the alloy powder in the mold by a magnetic field, and a sintering process for sintering the oriented alloy powder together with the mold, and the method further includes a heating process for heating the alloy powder in the mold before and/or after the application of the orienting magnetic field in the orienting process.
The heating temperature in the heating process should preferably be equal to or higher than 50° C. and equal to or lower than 300° C. At temperatures lower than 50° C., the coercive force of the individual particles of the alloy powder will barely decrease, so that the aforementioned effect of improving the degree of orientation by decreasing the coercive force cannot be obtained. At temperatures higher than 300° C., the individual particles of the alloy powder will be completely demagnetized by heat, so that the alloy powder will no longer be oriented even when the orienting magnetic field is applied.
The content of Dy in the alloy powder should preferably be equal to or higher than 1 wt % and lower than 6 wt %. When the content of Dy is lower than 1 wt %, the produced sintered NdFeB magnet cannot have an adequately high level of coercive force. When the content of Dy is equal to or higher than 6 wt %, the magnetic properties of the produced sintered NdFeB magnet other than the coercive force will deteriorate and, additionally, the production cost will be too high. More preferably, the content of Dy should be equal to or higher than 1 wt % and lower than 5 wt %, and even more preferably equal to or higher than 1 wt % and equal to or lower than 4 wt %.
Thus, the inclusion of the heating process in the orienting process promotes the demagnetization of the individual particles of the alloy powder and suppresses the disturbance of the alignment of the alloy powder after the magnetic orientation process. The process of magnetically orienting the alloy powder after heating the same powder is hereinafter referred to as the “heating orientation.”
The heating orientation process does not completely demagnetize the entire amount of the alloy powder; although the heating process reduces the amount of deterioration in the degree of orientation, the residual magnetization still has the potential of disturbing the alignment of the crystal axes of the powder particles. The disturbance of the crystal axes is particularly noticeable in the surface regions where the particles are less bound by their mutual friction. As a result, the surface shape of the produced sintered magnet will be unstable. This is unfavorable for the “near-net-shape” molding (the capability of producing sintered magnets in a shape near the final product), which is one of the characteristics of the PLP method.
Another problem is that the molds containing the alloy powder attract or repel each other due to the residual magnetization, impeding the handling of the molds after the orienting process.
To solve the previously described problems, it is preferable to perform, at the end of the orienting process, a heating demagnetization process for applying a demagnetizing magnetic field to the alloy powder maintained in the heated state created by the heating process.
The orienting magnetic field for orienting the alloy powder is applied at a high strength of several T (tesla) so that the force of moving the particles will be adequately stronger than the frictional force among the particles. The demagnetizing magnetic field to be applied for demagnetizing the oriented alloy powder must at least exceed the coercive force of the powder particles. However, using an excessively strong demagnetizing field will disturb the directions of the crystal axes that have been aligned by the orienting magnetic field.
The ease of motion of powder particles depends on the frictional force among the particles. At a filling density used in the PLP method (from 3.0 to 4.2 g/cm3), it is possible to completely demagnetize the individual particles of the powder by applying a demagnetizing magnetic field having a strength of 480 kA/m (≈6 kOe) or less, without causing the situation that the demagnetizing magnetic field exceeds the frictional force among the particles and disturbs the directions of their crystal axes. A more preferable upper limit of the magnetic field strength is 240 kA/m (≈3 kOe). A strength value of 480 kA/m approximately corresponds to 0.6 T, and 240 kA/m approximately corresponds to 0.3 T. These values demonstrate that the demagnetizing magnetic field is much weaker than the orienting magnetic field, which has a strength of several T, as already explained.
The temperature of the alloy powder in the process of applying the demagnetizing magnetic field should preferably be equal to or higher than a temperature at which the coercive force of the powder particles is 120 kA/m (≈1.5 kOe). This is because, if the coercive force of the powder particles is higher than this level, the application of the demagnetizing magnetic field causes rotation of the individual particles of the powder, and ultimately disturbing their alignment.
The demagnetizing magnetic field may be a damped AC (alternating-current) magnetic field which is gradually damped from the aforementioned magnetic field strength as an initial (maximum) peak strength (an AC magnetic field whose amplitude gradually decreases with time to an adequately small value, which is normally zero) or a DC (direct-current) magnetic field applied opposite to the direction of magnetization of the alloy powder that has undergone the heating orientation at the aforementioned magnetic field strength.
A system for producing a sintered NdFeB magnet according to the present invention aimed at solving the previously described problems is a system including a filling system for filling an NdFeB alloy powder into a mold to a density within a range from 3.0 to 4.2 g/cm3, an orienting device for orienting the alloy powder in the mold, and a sintering device for sintering the oriented alloy powder together with the mold, wherein the orienting device includes:
a magnetic-field applying device for applying a magnetic field to the alloy powder; and
a heating device for heating the alloy powder in the mold before and/or after the magnetic-field applying device applies an orienting magnetic field to the alloy powder.
In one mode of the present invention, the system further includes a controller for controlling the heating device and the magnetic-field applying device so that, after the alloy powder has been subjected to heating orientation by the heating device and the magnetic-field applying device, a demagnetizing magnetic field is applied to the alloy powder maintained in the heated state.
In the method and system for producing a sintered NdFeB magnet according to the present invention, an alloy powder held in a mold is heated either before or after, or both before and after the alloy powder is oriented by an orienting magnetic field. This heat treatment prevents the disturbance of the alignment of alloy powder which has been magnetically oriented. The high degree of orientation is maintained throughout the sintering process of the alloy powder even when a predetermined amount of Dy is added to the alloy powder to increase the coercive force or when the amount of the alloy powder in the magnetizing direction is decreased in order to produce a slim-shaped sintered magnet. As a result, a slim-shaped sintered NdFeB magnet having high coercive force and high residual magnetic flux density can be inexpensively produced.
In the case where the process of applying a demagnetizing magnetic field to the alloy powder maintained in the heated state is provided after the orienting process, the residual magnetization is decreased to zero without affecting the orientation of the crystal axes of the previously aligned powder particles. As a result, the surface shape of the sintered magnet to be eventually obtained will be stabilized. Furthermore, the handling of the molds after the orienting process will be easier since the molds containing the alloy powder will not attract or repel each other.
A general configuration of a sintered magnet production system used in the conventional PLP method is shown in the vertical sectional view of
An operation of the sintered magnet production system of
In the filling unit 1, after a mold 10 is placed directly below the supply port of a hopper 14, a predetermined amount of alloy powder 11 is supplied into the mold 10. At this point, the bulk density of the powder is low since the filling density of the powder is close to the natural filling density. Accordingly, a guide 15 is attached to the mold 10 so that the predetermined amount of alloy powder 11 can be supplied into the mold 10. Subsequently, this mold 10, with the guide 15 attached thereto, is moved to a position under a pusher 16, which applies pressure from above. The pressure from this pusher 16 only needs to be at a level of approximately 15 kgf/cm2 MPa) or may be even lower. Simultaneously with the application of the pressure from the pusher 16, a tapping device 17 supporting the mold 10 from below begins to slightly vibrate the mold 10. This operation causes the alloy powder 11 to be evenly filled into the mold 10 at a predetermined density, while pressing the alloy powder in the mold 10 down to the level of the upper end of the container. Subsequently, the guide 15 is removed from the mold 10.
The filling density of the powder in the mold should preferably be within a range from 3.0 to 4.2 g/cm3. If the filling density is lower than this range, the sintering of the powder may possibly be insufficient, failing to achieve the required density. Conversely, if the filling density exceeds 4.2 g/cm3, the friction among the powder particles will be too strong to achieve a high degree of orientation. A preferable range of the filling density is from 3.5 to 4.0 g/cm3, and more preferably from 3.6 to 4.0 g/cm3.
The mold 10 holding the alloy powder 11 is conveyed to the containing unit 2 by the belt conveyor. In the conveying unit 2, a plurality of molds 10 are stacked by the manipulator and loaded into the mold container 12. Each of the molds 10 contained in the mold container 12 has its upper side covered with either the bottom of another mold 10 located immediately above or the mold container 12. Thus, the alloy powder 11 is prevented from being scattered during the orienting process in the orienting unit 3. Furthermore, the simultaneous production of a plurality of sintered magnets improves the working efficiency.
After a plurality of molds 10 are loaded into the mold container 12, the mold container 12 is transferred onto a lift 18. Then, the lift 1.8 is moved upward, whereby the mold container 12 on the lift 18 is inserted into a magnetic-field applying coil 19. Subsequently, a DC or AC current is passed through the coil 19 to generate a DC or AC magnetic field, whereby the alloy powder 11 in each of the molds 10 contained in the mold container 12 is oriented in the axial direction of the coil 19. The thereby applied magnetic field should preferably be a pulsed magnetic field. This pulsed magnetic field should be as strong as possible; its strength should be at least 3T, and preferably 5T or higher. Using a pulsed magnetic field weaker than 3T prevents the degree of orientation from reaching a desired level. Combining an AC magnetic field and a DC magnetic field as the aforementioned magnetic field is particularly effective. There are many possible combinations of magnetic fields. Typical examples of the patterns for applying magnetic fields are: consecutive application of an AC magnetic field and a DC magnetic field, consecutive application of one AC magnetic field and another AC magnetic field, and consecutive application of an AC magnetic field, another AC magnetic field and a DC magnetic field in this order. After the crystal axes of the alloy powder 11 have been aligned by the magnetic field, the lift 18 is lowered.
At the last stage, the mold container 12 is transferred into the sintering furnace, where the alloy powder 11 with the crystal axes in the aligned state are heated, together with the mold 10 and the mold container 12, to a temperature of 950° to 1050° C. to sinter the alloy powder 11. Subsequently, the powder is subjected to an additional heat treatment at 900° C. or lower temperature. Thus, sintered magnets are completed.
In the PLP method, as compared to the method using a pressing machine, the friction among the particles of the alloy powder is reduced. Accordingly, the particles of the alloy powder can be aligned with a higher degree of orientation in the orienting process. Therefore, the produced sintered magnets have better magnetic properties than those produced by using a pressing machine.
However, when the coercive force of the individual particles of the alloy powder is increased, the magnetic interaction among the powder particles after the removal of the applied magnetic field increases. Therefore, even if a high degree of orientation is achieved in the orienting process, the degree of orientation decreases before the sintering process. The principle of this phenomenon is hereinafter explained by means of
As shown in
For the previously described problem, the present inventors have discovered that the disturbance of the alignment of magnetically oriented alloy powder can be suppressed by increasing the temperature of the alloy powder by heating this powder together with the mold containing it. This method is hereinafter explained by means of
The first embodiment of the system for producing a sintered NdFeB magnet according to the present invention is hereinafter described by means of
The heating method is not limited to induction heating; there are many other possible choices, such as electrical resistance heating or laser-irradiation heating. Any type of heating method may be used as long as the temperature of the molds holding the alloy powder can be evenly increased to a predetermined level within a predetermined period of time and the heater can be installed in an inert-gas atmosphere in which the PLP method is performed.
In the present embodiment, the molds 10 are not contained in the mold-container 12 so that the molds 10 and the alloy powder 11 contained therein can be easily heated by the induction heating coil 20. Accordingly, the system of the present embodiment has no containing unit 2; the stacking of the molds 10 is performed on the lift 18. The absence of the mold container 12 means that no cover is put on the top of the stack of the molds 10. To prevent the scattering of the alloy powder 11 from the top of the stack of the molds 10 during the heating and orienting processes, a fixing base 21 composed of an air cylinder, a lid and other components for pressing the stacked molds 10 from above is provided in the upper portion of the orienting unit 3. The fixing base 21 should preferably be made of a magnetic material whose permeability and saturation magnetization are comparable to those of the alloy powder. Examples of such materials include a magnetic steel sheet, a SmCo magnet, a powder magnetic core, and a laminate of Fe graphite sheets. Using any of these materials has the effect of aligning the directions of the lines of the magnetic field perpendicularly applied to the molds 10. Furthermore, as shown in
An operation of the system for producing a sintered NdFeB magnet according to the present embodiment is hereinafter described. The operation of the system for producing a sintered NdFeB magnet according to the present embodiment is the same as that of the sintered magnet production system using the conventional PLP method shown in
In the system for producing a sintered NdFeB magnet according to the present embodiment, the stacking of the molds 10 is performed on the lift 18. After a predetermined number of molds 10 have been stacked on the lift 18, the fixing base 21 is lowered to hold the molds 10 from above and below by the fixing base 21 and the lift 18. Therefore, for example, in the magnetic orientation process, the molds 10 are prevented from moving and the alloy powder 11 in the molds 10 is prevented from being scattered. After being held by the lift 18 and the fixing base 21, the molds 10 are moved into the magnetic-field applying coil 19, where an AC magnetic field is initially applied. After the application of the AC magnetic field is completed, the molds 10 are lowered to the level of the induction heating coil 20, where the molds 10 and the alloy powder 11 held therein are heated to a predetermined temperature. After this heating process, the molds 10 are once more moved into the magnetic-field applying coil 19, where, this time, a DC magnetic field is applied. After the application of the DC magnetic field is completed, the molds 10 are conveyed to the sintering furnace and sintered.
The combination of AC and DC magnetic fields applied by the system for producing a sintered NdFeB magnet according to the present embodiment may be different from the previous example. Each of the applied magnetic fields should preferably be a pulsed magnetic field with a strength of 3 T or higher, and more preferably 5 T or higher, as in the case of the conventional PLP method.
The average particle size of the alloy powder should be as small as possible, since a smaller particle size leads to a higher coercive force. However, if the particle size of the powder is too small, the coercive force rather deteriorates due to oxidation of the powder particles. In view of this problem, the average particle size of the alloy powder should preferably be equal to or larger than 1 μm and equal to or smaller than 5 μm, and more preferably equal to or larger than 1 μm and equal to or smaller than 3.5 μm.
The heating of the alloy powder 11 in the molds 10 cannot be simultaneously performed with the orienting process. Accordingly, to achieve a desired temperature of the mold 10 (or the alloy powder 11) in the magnetic orientation process, the temperature to be achieved by induction heating must be set at a slightly higher level, taking into account a decrease in the temperature during the movement of the molds 10 from the induction heating coil 20 to the magnetic-field applying coil 19.
The timing of heating the mold and applying the magnetic field can be arbitrarily set on a case-by-case basis according to the composition of the alloy powder. For example, when the applications of the magnetic fields in the orienting process are performed in the order of AC and DC magnetic fields, the heating process can be performed between the applications of AC and DC magnetic fields. There are many other possible methods, such as heating the mold before the application of the AC magnetic field, before the application of the AC magnetic field as well as immediately after the application of the DC magnetic field, or before the application of the AC magnetic field as well as before the application of the DC magnetic field. The heating temperature should also be appropriately set. For example, for the alloy powder having a Dy content of 4.1 wt % in
The magnetic properties of various samples of the sintered NdFeB magnets produced by the system according to the present embodiment are hereinafter presented.
In the first experiment, an alloy powder with a Dy content of 4.1 wt % (Alloy No. 4 in Table 1) was filled into a mold shown in
In Table 3, Br is the residual magnetic flux density (the magnitude of magnetization or magnetic flux density B at the point where magnetic field H is zero on the magnetization curve (J-H curve) or demagnetization curve (B-H curve)), Js is the saturation magnetization (the maximum value of magnetization J), HcB is the coercive force of the demagnetization curve, HcJ is the coercive force of the magnetization curve, (BH)Max is the maximum energy product (the maximum value of the product of magnetic flux density B and magnetic field H on the demagnetization curve), Br/Js is the degree of orientation, HK is the value of magnetic field H at the point where magnetization J equals 90% of residual magnetic flux density Br, and SQ is the squareness (HK/HcJ). The larger these values are, the better the magnet properties are. All the sintered NdFeB magnets in the “as-sintered” state had a shape of 38 mm in width, 60 mm in height and 2 mm in the thickness in the orientation direction, with a permeance coefficient of approximately 0.1. The “as-sintered” state means that “the magnet has not undergone any machine work, such as grinding or cutting, and therefore maintains the same state since its removal from the sintering furnace.”
Table 3 demonstrates that the sintered magnets produced by the methods of Examples 1 and 2 yielded the best results for almost all the magnetic properties. In Examples 1 and 2, the heating process was not performed before the application of the AC magnetic field but before the application of the DC magnetic field or before and after it. The improvements in the magnet properties due to the introduction of the heating process were evident when compared to the results obtained in Comparative Example 1, in which the temperature was constantly maintained at room temperature. It was also found that, under the conditions used in the present experiment, the degree of orientation would rather deteriorate when the heating process was performed before the application of the AC magnetic field (Comparative Examples 2 and 3).
In the second experiment, the heating process was performed before the application of the DC magnetic field under the conditions shown in Table 4. As a result, the magnet properties changed as shown in Table 5.
Table 5 demonstrates that the degree of orientation deteriorated when the heating temperature was 200° C., or when the heating temperature was 150° C. and the heating time was the shorter period of 60 seconds (Comparative Examples 4 and 5). A probable reason for the deterioration in the degree of orientation in the case of a high heating temperature is that increasing the temperature causes a reduction in the magnetic anisotropy of the alloy powder, which compromises the orienting effect of the applied magnetic fields. A probable reason for the deterioration in the degree of orientation in the case of a short heating time is that the temperature distribution of the alloy powder inside the mold becomes significantly varied, so that the demagnetizing effect due to the temperature increase occurs only in limited portions of the alloy powder, leaving the other portions at lower temperatures. By contrast, the results obtained in Example 3 were comparable to Examples 1 and 2. The degree of orientation exceeded 95%, and the other magnetic properties also improved.
In the third experiment, an alloy powder with a Dy content of 1.2 wt % (Alloy No. 2 in Table 1) was filled into a mold shown in
The sintered NdFeB magnets produced by using the mold shown in
In the fourth experiment, an alloy powder with a Dy content of 2.5 wt % (Alloy No. 3 in Table 1) was filled into a mold shown in
The sintered NdFeB magnets produced by using the mold shown in
In the previously described production method, the particles of the alloy powder are demagnetized by the heating orientation process, whereby a higher degree of orientation after the orienting process is achieved and the magnetic properties of the sintered magnet are improved. However, if the coercive force is decreased by only the heating process, a portion of the particles remain without any magnetic domain formed, as shown in
For this problem, the present inventors have discovered that the magnetization of the individual particles of alloy powder can be decreased to zero (i.e. completely demagnetized), without lowering the degree of orientation, by applying a predetermined magnetic field to those particles after decreasing their coercive force by heat. The process of demagnetizing the individual particles of alloy powder by applying a demagnetizing magnetic field after heating is hereinafter referred to as the “heating demagnetization.”
A method for producing a sintered NdFeB magnet using the heating demagnetization is hereinafter described by means of
Initially, similar to the previous embodiment, molds 10 are stacked on the lift 18 in the orienting unit 3. When a predetermined number of molds 10 have been stacked, the fixing base 21 is lowered to hold the molds 10 from above and below by the fixing base 21 and the lift 18.
After being held by the lift 18 and the fixing base 21 from above and below, the molds 10 are moved into the magnetic-field applying coil 19, where an AC magnetic field (as an orienting magnetic field) is initially applied to orient the alloy powder. After the orientation by the AC magnetic field is completed, the molds 10 are lowered to the level of the induction heating coil 20 and heated to a predetermined temperature. After this heating process, the molds 10 are once more moved into the magnetic-field applying coil 19, where, this time, a DC magnetic field (as an orienting magnetic field) is applied to orient the alloy powder. After the orientation by the DC magnetic field is completed, a damped AC magnetic field (as a demagnetizing magnetic field) with a predetermined peak strength is applied while the molds 10 and the alloy powder 11 are in the heated state, whereby the individual particles of the alloy powder are completely demagnetized. After the demagnetization is completed, the molds 10 are transferred to the sintering furnace and sintered.
A preliminary experiment has revealed that, in the process of demagnetizing the particles of the alloy powder, if the heating temperature of the alloy powder 11 is too low, the coercive force of the powder particles does not adequately decrease, which impedes the formation of the magnetic domains 114 shown in
The orienting magnetic field must have a strength of 3T or higher, and if possible, 5T or higher, as in the conventional PLP method. The peak intensity of the damped AC magnetic field applied in the demagnetizing process must exceed the coercive force of the particles of the alloy powder. However, it should not be too high, because an excessively high magnetic field makes the individual particles rotate against the frictional binding among the particles, causing a decrease in the degree of orientation. Table 9 below shows a change in the degree of orientation (Br/Js) of sintered magnets produced from alloy powder N50 with an average particle size of 3.3 μm by performing a demagnetizing process in which the peak strength of the damped AC magnetic field (“AC demagnetization”) applied to the alloy powder was changed from 0 T (no AC demagnetization) through 0.2 T, 0.4 T and 0.6 T. The orientation of the alloy powder was performed in such a manner that a pulsed AC magnetic field of 5.5 T (“AC orientation”) was initially applied two times, after which the powder was heated to 180° C., and 45 seconds later, a pulsed DC magnetic field of 5.5 T (“DC orientation”) was applied. The heating temperature of the alloy powder and the coercive force of the powder particles in the AC demagnetization process were 100° C. and 80 kA/m, respectively.
Table 10 shows that the degree of orientation decreases with an increase in the peak strength of the damped AC magnetic field used for AC demagnetization. To prevent this decrease in the degree of orientation, it is preferable to set the peak strength at 0.6 T or lower, and more preferably 0.3 T or lower. The peak strength of 0.6 T approximately corresponds to a coercive force of 480 kA/m, and 0.3 T approximately corresponds to 240 kA/m. In this manner, the peak strength of the damped AC magnetic field applied for demagnetization must be appropriately set according to the composition and particle size of the alloy powder, the heating temperature, the coercive force of the powder particles and the degree of orientation.
The sintered magnet production system shown in
Thus far, the system for producing a sintered NdFeB magnet according to the present invention has been described by means of the embodiment. It is evident that the previous embodiment is a mere example, and any change, modification or addition may be appropriately made within the spirit and scope of the present invention. For example, as an alternative to the damped AC magnetic field used as the demagnetizing magnetic field in the previous embodiment, a DC magnetic field having a strength equal to the peak strength of the aforementioned damped AC magnetic field may be applied in the direction opposite to the direction of magnetization of the alloy powder in the heating orientation process, to demagnetize the alloy powder.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-197932 | Aug 2009 | JP | national |
| 2010-100268 | Apr 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/064558 | 8/27/2010 | WO | 00 | 3/21/2012 |
