Sintered magnet based on MnBi having improved heat stability and method of preparing the same

Abstract

Disclosed are an MnBi sintered magnet exhibiting excellent thermal stability as well as excellent magnetic characteristics at high temperature, an MnBi anisotropic complex sintered magnet, and a method of preparing the same.

Claims

1. A method of preparing a MnBi-based sintered magnet, the method comprising: (a) preparing a non-magnetic phase MnBi-based alloy; (b) subjecting the non-magnetic phase MnBi-based alloy to heat treatment to convert into a magnetic phase MnBi-based alloy; (c) pulverizing the magnetic phase alloy to prepare MnBi hard magnetic phase powders; (d) mixing the MnBi hard magnetic phase powders with a low-melting point metal powder into a mixture; (e) molding the mixture in a magnetic field applying an external magnetic field into a molded product; and (f) sintering the molded product to obtain the MnBi-based sintered magnet comprising the MnBi hard phase powder particles and the low-melting point metal in the interface between the MnBi hard magnetic phase powder particles, wherein the low-melting point metal is Sn, wherein the MnBi-based alloy prepared in (a) has a crystal grain size of 50 to 100 nm, wherein the low-melting point metal powder is added in an amount greater than 0 wt % and less than or equal to 2 wt %, wherein the pulverizing in (c) is performed by a ball milling, and wherein a ball milling time of the ball milling is 3 to 5 hours.

2. The method of claim 1, wherein the non-magnetic phase MnBi-based alloy is prepared in (a) by a rapidly solidification process (RSP).

3. The method of claim 2, wherein a wheel speed in the rapidly solidification process is 55 to 75 m/s.

4. The method of claim 1, wherein the heat treatment is performed in (b) at a temperature of 280 to 340 C.

5. The method of claim 1, wherein (c) and (d) are simultaneously performed.

6. The method of claim 1, wherein in (d), a rare earth hard magnetic phase powder is further added to and mixed with the MnBi hard magnetic phase powders and the low-melting point metal powder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a schematic view of the process of preparing an MnBi sintered magnet with improved thermal stability according to an exemplary embodiment of the present invention;

(2) FIG. 2 illustrates a schematic view of a process of complexing an MnBi hard magnetic phase powder/rare earth hard magnetic phase powder and preparing an anisotropic sintered magnet with improved thermal stability according to an exemplary embodiment;

(3) FIG. 3 illustrates a result of observing the micro structure of the MnBi sintered magnet to which Sn is added in an amount of 2 wt % through the measurement of energy dispersive X-ray spectrometry (EDS) selected area scanning. The yellow color indicates Sn; and

(4) FIG. 4 is a graph illustrating the relationship between intrinsic coercive force (HCi) and residual flux density (Br) of an MnBi sintered magnet to which an Sn powder is added in an amount of 2 wt % over the ball milling time according to an exemplary embodiment of the present invention.

MODES FOR CARRYING OUT THE PREFERRED EMBODIMENTS

(5) Hereinafter, the present invention will be described in more detail through the Examples. These Examples are provided only for more specifically describing the present invention, and it will be obvious to a person with ordinary skill in the art to which the present invention pertains that the scope of the present invention is not limited by these Examples.

EXAMPLE

(6) <Preparation and Magnetic Characteristics of MnBi Sintered Magent>

(7) 1. Preparation of MnBi Sintered Magnet Including Low-Melting Point Metal at Grain Boundary

(8) First, manganese (Mn) metal particles and bismuth (Bi) metal particles were mixed, and the mixed powder was charged into a furnace, and then melted through an induction heating method. In this case, the temperature of the furnace was instantaneously increased to 1,400 C. to prepare a mixed melt. And then, the mixed melt was injected into a cooling wheel in which the wheel speed was adjusted to about 65 m/s to prepare a non-magnetic phase MnBi-based ribbon in the solid state through a rapid cooling method.

(9) The non-magnetic phase MnBi-based ribbon prepared may comprise non-magnetic phase in an amount of 90% or more, preferably 99% or more. If non-magnetic phase MnBi-based ribbon comprises 90% or more of non-magnetic phase, it is possible to inhibit rapid grain growth in the heat treatment for forming an MnBi low temperature phase (LTP), and to have uniform MnBi LTP.

(10) In order to impart magnetic characteristics to the non-magnetic MnBi ribbon thus prepared, a low-temperature heat treatment was performed under the vacuum and inert gas atmosphere conditions to prepare an MnBi-based magnetic body.

(11) And then, a process of pulverizing the magnetic body using a ball milling was performed, and during the milling of the MnBi magnetic body, Sn was added thereto in an amount of 0 wt %, 1 wt %, and 2 wt %, respectively, and the milling process of pulverization and mixing was simultaneously performed.

(12) In particular, when the Sn powder was included in an amount of 2 wt %, the milling process was performed for the ball milling time of 3, 5, 6, and 7 hours, respectively to prepare a mixed powder in order to evaluate the effect of the ball milling time.

(13) Each of the mixed powder thus prepared was subjected to magnetic field molding under a magnetic field of about 1.6 T, and then sintered to an MnBi sintered magnet to which the low-melting point metal was added.

(14) In order to analyze the micro structure of the MnBi sintered magnet to which Sn was added in an amount of 2 wt % in the sintered magnet thus prepared, the distribution of Sn at the grain boundary surface was observed through the scanning measurement of the energy dispersive X-ray spectrometry selective region, and is illustrated in FIG. 3. In FIG. 3, the yellow color indicates Sn, and it can be confirmed that Sn is distributed at the boundary surface of crystal grains.

(15) 2. Measurement of Magnetic Characteristics of MnBi Sintered Magnet According to Amount of Low-Melting Point Metal Added

(16) The intrinsic coercive force (H.sub.Ci), residual flux density (B.sub.r), induced coercive force (H.sub.CB), density, and maximum magnetic energy product [(BH).sub.max] of the MnBi sintered magnet with improved thermal stability were measured, and the magnetic characteristics were measured at normal temperature (25 C.) using a vibrating sample magnetometer (VSM, Lake Shore #7300 USA, maximum 25 kOe), and the values are shown in the following Table 1.

(17) TABLE-US-00001 TABLE 1 H.sub.Ci B.sub.r H.sub.CB Density (BH).sub.max MnBi Sintered Manet (kOe) (kG) (kG) (g/cm.sup.3) (MGOe) Sn 2 wt % Addition 8.7 6.0 5.4 8.2 8.3 Sn 1 wt % Addition 7.5 6.1 5.2 8.2 8.4 Sn 0 wt % Addition 5.1 6.4 4.8 8.3 9.4

(18) Through Table 1, it can be confirmed that when the Sn powder was added in an amount of 2 wt %, the intrinsic coercive force was increased from 5.1 kOe to 8.7 kOe. The increase in intrinsic coercive force brings about a magnetic insulation effect, and thus improves the coercive force by maximally suppressing the generation of magnetization reversal due to the production and growth of a reverse magnetic domain produced from the surface of crystal grains because Sn is formed along the grain boundary.

(19) When defects are not present and only a domain and a domain wall are present inside the crystal grains in a general magnetic material, if external magnetic field is applied thereto, the domain is aligned in the same direction as the external magnetic field while the domain wall easily moves, so that saturation is achieved at low magnetic field. When the magnetic field is applied thereto in a state where saturation is achieved, domains are rotated at 180 at certain magnetic field, and in this case, the external magnetic field value will be the coercive force.

(20) As confirmed in FIG. 3, the diffusion of the low-melting point metal into the grain boundary brings about a result in which the coercive may be increased while reducing a decrease in the residual magnetization value. The decrease in the residual magnetization value is thought to be due to an effect resulting from the increase in content of the non-magnetic phase Sn.

(21) 3. Measurement of Magnetic Characteristics of MnBi Sintered Magnet According to Ball Milling Time

(22) As the case where the Sn powder is included in an amount of 2 wt %, the intrinsic coercive force (H.sub.Ci), residual flux density (B.sub.r), induced coercive force (H.sub.CB), density, and maximum magnetic energy product [(BH).sub.max] were measured at normal temperature (25 C.) using a vibrating sample magnetometer (VSM, Lake Shore #7300 USA, maximum 25 kOe) in order to measure the magnetic characteristics of the MnBi sintered magnet according to the ball milling time, and the values are shown in the following Table 2.

(23) TABLE-US-00002 TABLE 2 Ball milling H.sub.Ci B.sub.r H.sub.CB Density (BH).sub.max (hr.) (kOe) (kG) (kG) (g/cm.sup.3) (MGOe) 3 8.7 6.0 5.4 8.2 8.3 5 10.3 5.9 5.3 8.2 8.0 6 11.4 5.6 5.2 8.0 7.5 7 12.6 5.5 5.2 8.0 7.3

(24) From Table 2, the magnetic characteristics of the MnBi sintered magnet to which the Sn powder was added according to the ball milling time, showing a tendency that the intrinsic coercive force was increased and the residual flux density was decreased according to the increase in milling energy (ball milling time) as illustrated in FIG. 4. Due to the micronization of the powder according to the increase in milling time, the coercive force of the MnBi sintered magnet is increased.

(25) When the crystal grains are small, a single domain is enegetically stable rather than a multi-domain, and in a permanent magnet in the multi-domain state, the magnetization reversal into adjacent domains with low energy easily propagates like a domino phenomenon, thereby leading to a decrease in coercive force. However, in the single domain state, the magnetization reversal may be generated by the larger energy, thereby limiting the demagnetization and increasing the coercive force. Further, an increase in milling weakens the crystallinity of crystal grains, and is also a factor which decreases the residual flux density.

(26) 4. Measurement of Magnetic Characteristics According to Measurement Temperature of MnBi Sintered Magnet When Low-Melting Point Metal is Added and is not Added

(27) Magnetic characteristics of an MnBi sintered magnet to which the Sn powder was added in an amount of 2 wt % (ball milling time 3 hr) and an MnBi sintered magnet to which the Sn powder was not added (ball milling time 8 hr) were measured at a measurement temperature of 40 C., 25 C., and 150 C., respectively, and the results are shown in the following Table 3.

(28) TABLE-US-00003 TABLE 3 MnBi Measurement Den- Sintered Temperature H.sub.Ci B.sub.r H.sub.CB sity (BH).sub.max Magnet ( C.) (kOe) (kG) (kG) (g/cm.sup.3) (MGOe) Sn Addition 150 16.4 5.3 5.1 8.2 6.8 (2 wt %) 25 8.7 6.0 5.4 8.2 8.3 Ball milling 40 3.7 6.3 3.5 8.2 7.9 3 hr. Sn Addition 150 25.0 5.0 4.8 8.2 5.9 (0 wt %) 25 9.7 6.0 5.5 8.2 8.2 Ball milling 40 4.3 6.2 3.9 8.2 8.0 8 hr.

(29) As confirmed in Table 3, a long-term (7 hours or more) of ball milling time is required to show high-coercive force characteristics without adding the Sn powder, but when the Sn powder is added, high-coercive force characteristics may be obtained with the ball milling for a relatively short time.

(30) In particular, when the Sn powder was added thereto, it was confirmed that the change width in coercive force was so narrow over a wide temperature range that high thermal stability could be secured.

(31) Further, when the Sn powder was added thereto, a sintered magnet having high maximum magnetic energy product [(BH).sub.max] at particularly high temperature was prepared. In contrast, in the case of an MnBi sintered magnet prepared after a long-term ball milling was performed, it could be confirmed that due to the deterioration in crystallinity resulting from the high milling energy, the residual flux density (B.sub.r) was reduced at high temperature (150 C.), and thus, the performance of the magnet relatively deteriorated.

(32) <Preparation and Magnetic Characteristics of MnBi and Rare Earth Hard Magnetic Phase Sintered Magent>

(33) 1. Preparation of Anisotropic Complex Sintered Magnet Including Low-Melting Point Metal in Grain Boundary

(34) A mixed powder of manganese (Mn) metal particles and bismuth (Bi) metal particles was charged into a furnace, and then the temperature of the furnace was instantaneously increased to 1,400 C. to prepare a mixed melt through an induction heating method, and the mixed melt was injected into a cooling wheel in which the wheel speed was adjusted to about 65 m/s to prepare a non-magnetic phase MnBi-based ribbon in the solid state through a rapid cooling method.

(35) The non-magnetic phase MnBi-based ribbon prepared may comprise non-magnetic phase in an amount of 90% or more, preferably 99% or more. If non-magnetic phase MnBi-based ribbon comprises 90% or more of non-magnetic phase, it is possible to inhibit rapid grain growth in the heat treatment for forming an MnBi low temperature phase (LTP), and to have uniform MnBi LTP.

(36) In order to impart magnetic characteristics to the non-magnetic MnBi ribbon thus prepared, a low-temperature heat treatment was performed under the vacuum and inert gas atmosphere conditions to prepare an MnBi-based magnetic body.

(37) And then, a process of pulverizing the magnetic body using a ball milling was performed, and during the milling of the MnBi magnetic body, Sn was added thereto in an amount of 0 wt %, 1 wt %, and 2 wt %, respectively, and the milling process of pulverization and mixing was simultaneously performed by adding an SmFeN hard magnetic body powder in an amount of 35 wt % thereto. In this case, a complex process was performed for 3 hours, and the ratio of the magnetic phase powder, balls, a solvent, and a dispersing agent was about 1:20:6:0.12 (by mass), and the balls were set to 3 to 5. Subsequently, the magnetic powder prepared by the ball milling was molded under a magnetic field of about 1.6 T, and then sintering was performed to prepare an MnBi/SmFeN anisotropic complex sintered magnet including a low-melting point metal.

(38) 2. Magnetic Characteristics of MnBi/SmFeN Complex Sintered Magnet According to Addition of Sn

(39) In order to measure the effects according to the addition of Sn, magnetic characteristics were measured using a vibrating sample magnetometer (VSM, Lake Shore #7300 USA, maximum 25 kOe), and the results are shown in Table 4.

(40) TABLE-US-00004 TABLE 4 MnBi/SmFeN H.sub.Ci B.sub.r H.sub.CB Density (BH).sub.max Sintered Magnet (kOe) (kG) (kG) (g/cm.sup.3) (MGOe) Sn 2 wt % Addition 9.9 7.3 6.4 7.7 12.4 Sn 0 wt % Addition 8.7 7.7 6.6 7.9 13.8

(41) From Table 4, it could be confirmed that when the Sn powder was added in an amount of 2 wt % in the MnBi/SmFeN sintered magnet prepared in the same process, the intrinsic coercive force was increased from 8.7 kOe to 9.9 kOe. The increase in intrinsic coercive force brings about a magnetic insulation effect, and thus improves the coercive force by maximally suppressing the generation of magnetization reversal due to the production and growth of reverse magnetic domain produced from the surface of crystal grains because Sn is formed along the grain boundary. The decrease in the residual magnetization value is thought to be due to an effect resulting from the increase in content of the non-magnetic phase Sn.