Patent Application
20160293305
The present invention relates to a method for producing a sintered magnet containing a rare-earth element R, such as an RFeB (R2Fe14B) or RCo (RCo5 or R2Co17) system sintered magnet.
For the production of sintered magnets, a method has been conventionally used which includes the steps of filling the cavity of a mold with a fine powder of a starting alloy (“filling process”; this powder is hereinafter called the “alloy powder”), 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”). Another method has also been used in which, after the filling process, the orienting process and the compression-molding process are simultaneously performed by applying a magnetic field to the alloy powder while applying pressure with a pressing machine. In any of these cases, compression molding is performed with a pressing machine. Therefore, in the present application, these methods are called the “pressing method.”
Due to their high magnetic properties, RFeB system sintered magnets are expected to be increasingly in demand in the future as permanent magnets for motors used in hybrid cars and electric cars as well as for other applications. Automobiles must be designed for use under extreme loading conditions, and accordingly, their motors also need to be guaranteed to operate under high-temperature environments (e.g. 180° C.). Therefore, RFeB system sintered magnets which have a high level of coercivity that can suppress the decrease in magnetization (magnetic force) due to an increase in the temperature have been in demand.
In general, reducing the size of the grains which form the main phase within a sintered magnet increases its coercivity. However, if the particle size of the alloy powder used for the production of the sintered magnet is reduced for that purpose, the alloy powder becomes more easily oxidized, which leads to a decrease in the coercivity.
In recent years, a method has been increasingly used in which a sintered magnet having a shape that is nearly the same as that of a cavity (“near-net shape”) is produced by performing the orienting and sintering process without applying pressure to the alloy powder in the cavity (Patent Literature 1). In the present application, such a method for producing a sintered magnet without performing the compression-molding process is called the “PLP (press-less process) method.” Compared to the pressing method, the PLP method facilitates the handling in an oxygen-free atmosphere (a vacuum or an inert-gas atmosphere) since it does not require the use of a pressing machine. Therefore, the PLP method is advantageous over the pressing method in that an alloy powder having a small particle size can be used in an oxygen-free atmosphere with almost no oxidization, so that a sintered magnet having a high level of coercivity can be obtained.
Patent Literature 1: WO 2006/004014 A
Thus, the PLP method allows the use of an alloy powder with a smaller particle size than the pressing method. However, an observation of the inner structure of the thereby produced sintered magnet with an optical microscope or similar device will reveal that the average grain size of the main phase grains is larger than the average particle size of the used alloy powder. This is most likely due to the fusion (growth) of the particles of the alloy powder into larger sizes during the sintering process. If such a growth of the particles can be suppressed, the coercivity and squareness of the sintered magnet will be further enhanced. It is also expected that the grains in the sintered body will be more compacted and consequently make the sintered body stronger.
The problem to be solved by the present invention is to provide a sintered magnet production method capable of suppressing the growth of the particles of the alloy powder during the sintering process and thereby enhancing the coercivity and squareness as well as increasing the density of the sintered body.
The sintered magnet production method according to the present invention developed for solving the previously described problem is a sintered magnet production method including a filling process in which the cavity of a container is filled with an alloy powder of a raw material for a sintered magnet, an orienting process in which the alloy powder in the cavity is oriented by applying an magnetic field to the alloy powder without applying a mechanical pressure, and a sintering process in which the alloy powder oriented by the orienting process is sintered by heating the alloy powder without applying a mechanical pressure, wherein:
the median D50 of a particle size distribution of the alloy powder measured by a laser diffraction method is 3 μm or less, and a powder of a high-melting-point material having a higher melting point than the heating temperature in the sintering process is mixed in the alloy powder before or in the filling process, where the median D50 of the powder of the high-melting-point material is 0.3 μm or less.
The heating temperature in the sintering process (which is hereinafter called the “sintering temperature”) is normally around 1000° C. For such a temperature, for example, the following compounds can be used as the high-melting-point material: Al2O3 (melting point, 2072° C.), MgO (2852° C.), CeO2 (1950° C.), αFe2O3 (1566° C.), SiO2 (1650° C.), ZrO2 (2715° C.), Mn2O3 (1080° C.), Mn3O4 (1564° C.), Ta2O5 (1468° C.), Nb2O5 (1520° C.) and other oxides, as well as TaC (3880° C.), NbC (3500° C.) and other carbides. The powder may consist of a single kind of high-melting-point material or a mixture of powders of two or more kinds of high-melting-point materials.
In the sintered magnet production method according to the present invention, a powder of a high-melting-point material having a melting point equal to or higher than the sintering temperature (which is hereinafter called the “high-melting material powder”) is mixed in the alloy powder as a pretreatment for the PLP method. Since the average particle size (D50) of the high-melting material powder is sufficiently smaller than that of the alloy powder, the powder can enter the spaces between the particles of the alloy powder. The high-melting material powder retains its solid form even when they are heated during the sintering process, preventing the fusion of the particles of the alloy powder. This most likely suppresses the growth of the particles of the alloy powder during the sintering process and thereby reduces the grain size of the main phase grains within the sintered magnet. Therefore, a sintered magnet with a higher coercivity, squareness and sintered-body density can be produced than in the case of the conventional PLP method.
Furthermore, in the sintered magnet production method according to the present invention, the entry of the particles of the high-melting material powder into the spaces between the particles of the alloy powder can almost certainly prevent the formation of pores or voids and thereby hinder an occurrence and development of cracks starting from such pores or voids.
The high-melting material powder should preferably be mixed so that there are on average 10-1000 particles of the high-melting material powder per one particle of the alloy powder. Using fewer particles makes it difficult to obtain the effect of preventing the fusion of the particles of the alloy powder, while using more particles hinders movement of the particles of the alloy powder and impedes the orientation of these particles during the orienting process, which consequently deteriorates various magnetic properties.
In the sintered magnet production method according to the present invention, the PLP method is performed after the high-melting material powder is mixed in the alloy powder. This most likely prevents the fusion of the particles of the alloy powder during the sintering process and thereby suppresses the growth of those particles. Therefore, a sintered magnet with a higher coercivity, squareness and sintered-body density can be produced than in the case of the conventional PLP method.
Examples of the sintered magnet production method according to the present invention are hereinafter described with reference to the drawings.
The sintered magnet production method of the present example has the following processes: a mixing process in which a powder of a high-melting material having a higher melting point than the heating temperature (“sintering temperature”) in the sintered process (which will be mentioned later) is mixed in a fine powder of a starting alloy for a sintered magnet (“alloy powder”); a filling process in which the cavity of a mold is filled with the mixed powder of the alloy powder and the high-melting material powder; an orienting process in which the mixed powder in the cavity is oriented by applying a magnetic field without applying a mechanical pressure; and a sintering process in which the mixed powder oriented in the cavity is sintered by heating it together with the mold without applying a mechanical pressure. A sintered magnet is produced by performing those processes in the previously mentioned order in an oxygen-free atmosphere.
The average particle size of the high-melting material powder is sufficiently smaller than that of the alloy powder: the average particle size of the alloy powder in terms of the median D50 of the particle size distribution measured by the laser diffraction method is 3 μm or less, while that of the high-melting material powder in terms of D50 is 0.3 μm or less (hereinafter, the “average particle size” always represents the median D50 of the particle size distribution measured by the laser diffraction method).
This high-melting material powder is heated to approximately 400° C. in vacuum to dehydrate it.
Subsequently, a predetermined amount of high-melting material powder is mixed in the alloy powder, and the mixture is kneaded after a lubricant is added to it. As a result, the particles of the high-melting material powder adhere to the surface of the particles of the alloy powder. The lubricant will help the alloy powder smoothly move in the filling and orienting processes.
The amount of high-melting material powder to be mixed is determined according to the number of particles of the high-melting material powder to be adhered per one particle of the alloy powder, on the assumption that all particles of the high-melting material powder will be adhered to those of the alloy powder. For example, consider the case where an Al2O3 powder (high-melting material powder, whose specific gravity is approximately 3.98) having an average particle size of 0.05 μm is mixed in a Nd2F14B powder (alloy powder, whose specific gravity is approximately 7.5) having an average particle size of 2 μm. The volume ratio per particle of these powders is 23:0.053=64000:1, and the weight ratio is 64000×7.5:1×3.98=1:0.000008291. Given the aforementioned average particle sizes, if on average 100 particles of the Al2O3 powder need to be adhered per one particle of the Nd2F14B powder, the amount of Al2O3 powder to be mixed is 0.08291 wt % of the Nd2F14B powder.
Considering another example, if an Al2O3 powder with an average particle size of 0.05 μm is to be mixed in a Nd2F14B powder with an average particle size of 3 μm, the volume ratio per particle of these powders is 33:0.053=216000:1, and the weight ratio is 216000×7.5:1×3.98=1:0.000002456. Given the aforementioned average particle sizes, if on average 100 particles of the Al2O3 powder need to be adhered per one particle of the Nd2F14B powder, the amount of Al2O3 powder to be mixed is 0.02456 wt % of the Nd2F14B powder.
The average number of particles of the high-melting material powder to be adhered per one particle of the alloy powder is hereinafter called the “MP value” (which is 100 in both of the previous examples).
An alloy powder with an average particle size of 2 μm (this value, as well as the particle sizes to be mentioned later were measured with a laser diffraction type particle size distribution measurement system, HELOS&RODOS, manufactured by Sympatec GmbH) was prepared from a starting alloy having a composition shown in Table 1 below (the unit of the values is wt %), and an Al2O3 powder (high-melting point material powder) with an average particle size of 0.05 μm was mixed in the alloy powder at an MP value of 200 or 400. After methyl laurate (lubricant) was added by 0.105 wt % of the mixed powder and the mixture was stirred in a beaker, the mixture was kneaded by pouring it into a rotary crusher “Wonder Blender” (Osaka Chemical Co., Ltd.) two times, in ten seconds each time (mixing process).
This mixed powder was placed in the cavity of a mold at a filling density of 3.2 g/cm3 (filling process) and oriented in this state by a magnetic field with a maximum strength of 5.4 T (orienting process). After the mixed powder oriented in this manner was placed in a sintering furnace together with the mold, the temperature within the furnace was increased to 950-963° C. (this temperature is called the “sintering temperature”, which differs depending on the sample) in 8 hours and further maintained at that temperature for 4 hours to sinter the alloy powder (sintering process). Subsequently, the powder was heated at 800° C. for 0.5 hours (the first-stage aging treatment) and then rapidly cooled, after which it was further heated at 490-540° C. for 1.5 hours (the second-stage aging treatment) and then rapidly cooled. In the sintering process, argon gas (inert gas) was passed through the sintering furnace at a flow rate of 2 L per minute (this operation of supplying argon gas during the sintering process is hereinafter called the “argon-gas supply”) until the temperature in the sintering furnace reached 425° C., after which the furnace was evacuated to 1×10−4 Pa or lower. The obtained sintered body was machined into a sintered magnet of 3 mm in thickness with 7-mm-square pole faces.
The magnetic properties of the obtained sintered magnets are shown in the table of
The graph of
As for the degree of orientation Br/Js and the residual magnetic flux density Br, as can be seen in
An experiment was performed using an alloy powder of a starting alloy having a composition shown in Table 2 below, under the following conditions: the average particle size of the alloy powder was 2.96 μm; the average particle size of the high-melting material powder (Al2O3 powder) was 0.05 μm; the MP value was one of the following values: 50, 100, 200, 400 and 800; the additive amount LL of methyl laurate was either 0.07 wt % or 0.14 wt %; the filling density Df of the mixed powder in the mold cavity was 3.3 g/cm3; the orienting magnetic field was 5.4 T; the sintering temperature was 995° C. (the temperature was increased to this level in 13 hours 25 minutes and subsequently maintained at this level for 4 hours, with the argon-gas supply performed at 2 L/min until 400° C.); the first-stage aging treatment was performed at 800° C. for 0.5 hours; and the second-stage aging treatment was performed at 530-560° C. for 1.5 hours. The result is shown in the table of
The graph of
The reason for the comparatively low magnetic properties of the samples with high MP values is that the degree of orientation was decreased since an excessive amount of particles of the high-melting material powder were adhered to the surface of the particles of the alloy powder and hindered the movement of these particles. This problem can be solved by increasing the additive amount of lubricant (methyl laurate) to reduce the friction caused by the particles of the high-melting material powder.
The sintered magnets for which the additive amount LL of methyl laurate was increased to 0.14 wt % had greater values of the squareness ratio SQ, degree of orientation Br/Js and residual magnetic flux density Br, although their coercivity HcJ was decreased. This decrease in the coercivity HcJ is due to the methyl laurate which became an impurity and remained in the sintered magnets. Thus, there are trade-offs between the coercivity HcJ and the three other magnetic properties (the squareness ratio SQ, degree of orientation Br/Js, and residual magnetic flux density Br). Therefore, the additive amount of lubricant should be appropriately controlled according to the use of the sintered magnet.
Using an alloy powder of a starting alloy having the aforementioned composition shown in Table 2, three sintered magnets with different high-melting material powders were produced under the following conditions: the average particle size of the alloy powder was 3 μm; the average particle size of the high-melting material powder (made of Al2O3, CeO2 or MgO) was 0.3 μm; the MP value was 200; the additive amount LL of methyl laurate was 0.07 wt %; the filling density Df of the mixed powder in the mold cavity was 3.3 g/cm3; the orienting magnetic field was an AC field (two times) and a DC field (one time), with a strength of 5.4 T each time; the sintering temperature was 995° C. (the temperature was increased to this level in 8 hours and subsequently maintained at this level for 4 hours, with the argon-gas supply performed at 2 L/min until 425° C.); the aging treatment was performed at 800° C. for 0.5 hours and 560° C. for 1.5 hours. As a comparative example, another sintered magnet was produced under the same conditions except that no high-melting material powder was mixed (i.e. with the MP value of zero). The densities of the obtained sintered magnets were 7.527 g/cm3 in the case of comparative example, 7.542 g/cm3 in the case where the Al2O3 powder was used, 7.543 g/cm3 in the case where the CeO2 powder was used, and 7.552 g/cm3 in the case where the MgO powder was used. Thus, the sintered magnets produced using the high-melting material powders other than the Al2O3 powder also had higher densities than the comparative example.
An experiment was performed using an alloy powder of a starting alloy having a composition shown in Table 3 below, under the following conditions: the average particle size of the alloy powder was 3 μm; the average particle size of the high-melting material powder (Al2O3 powder) was 0.05 μm; the MP value was 200; the additive amount LL of methyl laurate was 0.09 wt %; the filling density Df of the mixed powder in the mold cavity was 3.2 g/cm3 or 3.3 g/cm3; the orienting magnetic field was an AC field of 5.4 T; the sintering temperature was 995° C. (the temperature was increased to this level in 12 hours and subsequently maintained at this level for 4 hours, with the argon-gas supply performed at 2 L/min until 500° C.); the first-stage aging treatment was performed at 800° C. for 0.5 hours; and the second-stage aging treatment was performed at 520° C. for 1.5 hours. The result is shown in the table of
As can be seen in
An experiment was performed using an alloy powder of a starting alloy having the aforementioned composition shown in Table 2, under the following conditions: the average particle size of the alloy powder was 3 μm; the average particle size of the high-melting material powder (Al2O3 powder) was 0.05 μm; the MP value was 200; the additive amount LL of methyl laurate was 0.07 wt %; the filling density Df of the mixed powder in the mold cavity was 3.2 g/cm3 or 3.3 g/cm3; the orienting magnetic field was an AC field (two times) and a DC field (one time), with a strength of 5.4 T each time; the sintering temperature was 1005° C. (the temperature was increased to this level in 13 hours 25 minutes and subsequently maintained at this level for 4 hours); the first-stage aging treatment was performed at 800° C. for 0.5 hours; and the second-stage aging treatment was performed at 520° C. for 1.5 hours, or both the first and second stages of the aging treatment were omitted. The result is shown in the table of
As can be seen in
The results shown in
An experiment was performed using an alloy powder of a starting alloy having the aforementioned composition shown in Table 2, under the following conditions: the average particle size of the alloy powder was 3 μm; the average particle size of the high-melting material powder (Al2O3 powder) was 0.05 μm; the MP value was 200; the additive amount LL of methyl laurate was 0.07 wt %; the filling density Df of the mixed powder in the mold cavity was 3.2 g/cm3 or 3.3 g/cm3; the orienting magnetic field was an AC field (two times) and a DC field (one time), with a strength of 5.4 T each time; the sintering temperature was 1020° C. (the temperature was increased to this level in 12 hours and subsequently maintained at this level for 4 hours, with the inside of the sintering furnace constantly in vacuum); the first-stage aging treatment was performed at 800° C. for 0.5 hours; and the second-stage aging treatment was performed at 530° C. for 1.5 hours. The result is shown in the table of
As can be seen in the graph of
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-061344 | Mar 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/056180 | 3/10/2014 | WO | 00 |
