Patent Grant
11865623
The present application is a national phase entry under 35 U.S.C. § 371 of International Application No. PCT/KR2019/009813 filed Aug. 6, 2019 which claims priority from Korean Patent Applications No. 10-2018-0093981 filed on Aug. 10, 2018 and No. 10-2019-0092709 filed on Jul. 30, 2019 with the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference in their entirety.
The present disclosure relates to magnetic powder and a method of preparing the same. More specifically, the present disclosure relates to magnetic powder including a rare earth element having a ThMn12 structure and a method of preparing the magnetic powder.
SmFe12-based magnets having a ThMn12 structure have superior magnetic properties at room temperature as compared to the existing Nd2Fe14B structure as follows.
Sm(Fe0.8Co0.2)12: μ0Ms=1.78T, μ0Ha=12T Nd2Fe14B: μ0Ms=1.61T, μ0Ha=7.6T
In addition, its Curie temperature, which is the temperature at which the magnetic material loses its magnetism, is higher than 800K, which means higher thermal stability than Nd2Fe14B.
It is known that magnetic powder is generally prepared by a strip/mold casting or melt spinning method based on metal powder metallurgy. First of all, the strip/mold casting method refers to a process of melting metals such as rare earth metals, iron, etc. through heat-treatment to prepare an ingot; coarsely pulverizing crystal grain particles; and preparing microparticles through a refining process. This process is repeated to obtain powder, which then undergoes a pressing and sintering process under a magnetic field to produce an anisotropic sintered magnet.
Also, the melt spinning method is performed in such a way that metal elements are melt; then poured into a wheel rotating at a high speed to be quenched; then pulverized with a jet mill; then blended with a polymer to form a bonded magnet or pressed to prepare a magnet.
However, when the SmFe12-based magnet is prepared by a strip casting, it is difficult not only to obtain single-phase, but also to obtain powder whose particle size is controlled to several micrometers. In addition, phase separation occurs when hydrogen is absorbed to make particles small using a jet mill, and thus it is difficult to maintain single-phase.
A task to be solved by embodiments of the present disclosure is to solve the problems as above, and the embodiments of the present disclosure are to provide single-phase magnetic powder in which a particle size of particles of the magnetic powder is controlled to a certain size or less, and a method of preparing the same.
Magnetic powder according to an embodiment of the present disclosure for solving the above problems is powder particles synthesized using a mixture of a rare earth oxide, a raw material, a metal, a metal oxide and a reducing agent, wherein the powder particles are single-phase, the raw material includes at least one of Fe and Co, the metal includes at least one of Ti, Zr, Mn, Mo, V and Si, and the metal oxide includes at least one of MnO2, MoO3, V2O5, SiO2, ZrO2 and TiO2.
The reducing agent may include at least one of Ca, Mg, CaH2, Na and Na—K alloy.
The magnetic powder may have a ThMn12 structure.
The rare earth oxide may include neodymium oxide or samarium oxide.
The mixture further may include at least one of Cu, Al, Ga, CuF2, CaF2 and GaF3.
The magnetic powder may have a ThMn12 structure, and a composition of R1-xZrx(Fe1-yCoy)12-zTzM {(0≤x≤0.2), (0≤y≤0.2), (0≤z≤1)}, wherein the R is Nd or Sm, the M is Cu, Al or Ga, and the T is Mn, Mo, V, Si or Ti.
The magnetic powder may have a composition of Sm1-xZrx(Fe1-yCoy)12-zTzM {(0≤x≤0.2), (0≤y≤0.2), (0≤z≤1)}, wherein the M is Cu, Al or Ga, and the T is Mn, Mo, V, Si or Ti.
An average particle size of the particles constituting the magnetic powder may be 10 micrometers or less.
A method of preparing magnetic powder according to an embodiment of the present disclosure includes the steps of: preparing a mixture by mixing a rare earth oxide, a raw material, a metal, a metal oxide and a reducing agent; and synthesizing magnetic powder by heat-treating the mixture at a temperature of 800° C. to 1100° C. with a reduction-diffusion method, wherein the raw material comprises at least one of Fe and Co, the metal comprises at least one of Ti, Zr, Mn, Mo, V and Si, the metal oxide comprises at least one of MnO2, MoO3, V2O5, SiO2, ZrO2 and TiO2, and the magnetic powder has single-phase powder particles.
The reducing agent may include at least one of Ca, Mg, CaH2, Na and Na—K alloy.
The heat-treating may be performed for 10 minutes to 6 hours.
The synthesized magnetic powder may have a ThMn12 structure.
The rare earth oxide may include neodymium oxide or samarium oxide.
The mixture may further include at least one of Cu, Al, Ga, CuF2, CaF2 and GaF3.
The magnetic powder may have a ThMn12 structure, and a composition of R1-xZrx(Fe1-yCoy)12-zTzM {(0≤x≤0.2), (0≤y≤0.2), (0≤z≤1)}, wherein the R is Nd or Sm, the M is Cu, Al or Ga, and the T is Mn, Mo, V, Si or Ti.
The magnetic powder may have a composition of Sm1-xZrx(Fe1-yCoy)12-zTzM {(0≤x≤0.2), (0≤y≤0.2), (0≤z≤1)}, wherein the M is Cu, Al or Ga, and the T is Mn, Mo, V, Si or Ti.
An average particle size of the particles constituting the magnetic powder may be 10 micrometers or less.
According to embodiments of the present disclosure, it is possible to provide single-phase magnetic powder with reduced secondary phase by a reduction-diffusion method, and to control an average particle size of particles constituting the magnetic powder to 10 micrometers or less, thereby preventing a decrease in saturation magnetization of main phase and a decrease in coercive force of permanent magnet.
Hereinafter, with reference to the accompanying drawings, various embodiments of the present disclosure will be described in more detail such that those skilled in the art, to which the present disclosure pertains, may easily practice the present disclosure. The present disclosure may be implemented in various different forms, and is not limited to the embodiments described herein.
Also, throughout the present specification, when any part is said to “include” or “comprise” a certain component, this means that the part may further include other components rather than excluding the other components, unless otherwise particularly specified.
Hereinafter, the magnetic powder according to an embodiment of the present disclosure will be described in detail.
The magnetic powder according to an embodiment of the present disclosure are powder particles synthesized using a mixture of a rare earth oxide, a raw material, a metal, a metal oxide and a reducing agent, wherein the powder particles are single-phase, the raw material includes at least one of Fe and Co, the metal includes at least one of Ti, Zr, Mn, Mo, V and Si, and the metal oxide includes at least one of MnO2, MoO3, V2O5, SiO2, ZrO2 and TiO2.
The reducing agent may include at least one of Ca, Mg, CaH2, Na and Na—K alloy. Particularly, CaH2 is preferable. The rare earth oxide may include neodymium oxide or samarium oxide.
The magnetic powder may have a ThMn12 structure. The ThMn12 structure magnet has superior magnetic properties at room temperature than Nd2Fe14B structure magnet, and its Curie temperature is higher than 800K, which means higher thermal stability than Nd2Fe14B.
The mixture may further include at least one of Cu, Al, Ga, CuF2, CaF2 and GaF3. In this case, the magnetic powder with a ThMn12 structure may have a composition of R1-xZrx(Fe1-yCoy)12-zTzM {(0≤x≤0.2), (0≤y≤0.2), (0≤z≤1)}, wherein the R is Nd or Sm, the M is Cu, Al or Ga, and the T is Mn, Mo, V, Si or Ti. More specifically, the composition may be Sm1-xZrx(Fe1-yCoy)12-zTzM {(0≤x≤0.2), (0≤y≤0.2), (0≤z≤1)}, wherein the M is Cu, Al or Ga, and the T is Mn, Mo, V, Si or Ti. The composition can be single-phase magnetic powder even in the absence of Co, and Co is added to increase saturation magnetization of the magnetic powder.
The metal including at least one of Ti, Zr, Mn, Mo, V and Si and the metal oxide including at least one of MnO2, MoO3, V2O5, SiO2, ZrO2 and TiO2 are added to ensure phase stability.
The ThMn12 structure has four crystal sites consisting of 2a, 8i, 8j and 8f. Rare earth metal atoms are located at site 2a and Fe elements are located at sites 8i, 8j and 8f. A distance between the Fe atoms at sites 8i, 8j and 8f is equal to or greater than a radius of the Fe atom. When the Ti, Mn, Mo, V, and Si elements substitute the Fe atoms and are located at sites 8i, 8j and 8f, the phase is stabilized because the Ti, Mn, Mo, V, and Si atoms are larger than the distance between the Fe atoms and cohesive energy of the ThMn12 structure is reduced due to the substitution. This principle applies equally to the addition of TiO2, MnO2, MoO3, V2O5 and SiO2, which are oxides of the above metals.
On the other hand, Zr may be located at site 2a of the ThMn12 structure by substituting the rare earth metal atom. Since the Zr atom is relatively smaller than the rare earth metal atom such as Nd and Sm, it causes shrinkage of the crystal lattice, and the substitution makes a substructure of the site 8i where the Fe is located even smaller, thereby stabilizing the phase. This principle applies equally to the addition of ZrO2, which is an oxide of the Zr.
ThMn12-type crystal phase has a tetragonal crystal structure. Since the ThMn12 structure magnetic powder is poor in phase stability and contains a large amount of Fe as a by-product, a concentration of the Fe element is high and Alpha Fe phase or the like is easily precipitated. Therefore, it is difficult to obtain single-phase magnetic powder. However, as the magnetic powder according to an embodiment of the present disclosure is single-phase ThMn12 structure magnetic powder having a reduced content of secondary phase such as Alpha Fe, FeTi, or Fe2Ti, it is possible to prevent a decrease in the Fe concentration in the main phase caused by the precipitation of Alpha Fe, etc. Therefore, a decrease in saturation magnetization of the main phase and a decrease in coercive force of permanent magnet can be prevented.
Since the ThMn12 structure magnetic powder is poor in phase stability, it is difficult to control the particle size of the particles constituting the magnetic powder to 10 micrometers or less when hydrogen is absorbed for the pulverizing process using a jet mill. On the other hand, the magnetic powder according to an embodiment of the present disclosure may be ThMn12 structure magnetic powder in which the average particle size of the particles constituting the magnetic powder is controlled to 10 micrometers or less with a reduction-diffusion method. In the process of obtaining a sintered magnet by sintering the magnetic powder, the sintering process in a temperature range of 1000 to 1250° C. is necessarily accompanied by a growth of crystal grains, which acts as a factor for decreasing coercive force. Herein, a size of the crystal grain of the sintered magnet is directly related to a size of the initial magnetic powder. Therefore, as long as the average particle size of the magnetic powder is controlled to 10 micrometers or less as in the magnetic powder according to an embodiment of the present disclosure, a sintered magnet with improved coercive force may be obtained.
Subsequently, a method of preparing magnetic powder according to another embodiment of the present disclosure will be described in detail. The method of preparing magnetic powder according to an embodiment of the present disclosure may be a method of preparing rare earth magnetic powder. More specifically, the method may be a method of preparing ThMn12 structure magnetic powder.
The method of preparing magnetic powder according to an embodiment of the present disclosure includes the steps of: preparing a mixture by mixing a rare earth oxide, a raw material, a metal, a metal oxide and a reducing agent; and synthesizing magnetic powder by heat-treating the mixture at a temperature of 800° C. to 1100° C. with a reduction-diffusion method, wherein the raw material includes at least one of Fe and Co, the metal includes at least one of Ti, Zr, Mn, Mo, V and Si, the metal oxide includes at least one of MnO2, MoO3, V2O5, SiO2, ZrO2 and TiO2, and the magnetic powder has single-phase powder particles.
The reducing agent may include at least one of Ca, Mg, CaH2, Na and Na—K alloy. Particularly, CaH2 is preferable. The rare earth oxide may include neodymium oxide or samarium oxide.
The heat-treating may be performed in a tube furnace at a temperature of 800° C. to 1100° C. under an inert atmosphere for 10 minutes to 6 hours. Reduction and diffusion between the mixtures at a temperature of 800° C. to 1100° C. may synthesize the rare earth magnet powder without a separate pulverizing process such as coarse pulverization, hydrogen crushing, and jet milling or a surface treatment process. When the heat-treatment is performed for 10 minutes or less, the metal powder may not be sufficiently synthesized. When the heat-treatment is performed for 6 hours or more, there may be a problem in that the size of the metal powder becomes coarse and primary particles are formed together into lumps.
The metal and the metal oxide are added to ensure phase stability. The mixture may further include at least one of Cu, Al, Ga, CuF2, CaF2 and GaF3.
After the step of reacting the mixture, a washing step for removing by-products of the reduction may further proceed. NH4NO3 is evenly mixed with the powder synthesized by the heat-treating, then immersed in methanol, and then homogenized once or twice using a homogenizer. Thereafter, NH4NO3 is dissolved in ethanol or methanol, and then washed and pulverized together with the synthesized powder and ZrO2 ball in a Turbula mixer. Lastly, the powder is rinsed with acetone, and then vacuum dried to finish the washing step. The washing step may be performed under an N2 atmosphere to minimize contact with air.
The rare earth magnetic powder thus prepared may be ThMn12 structure magnetic powder. The magnetic powder may have a composition of R1-xZrx(Fe1-yCoy)2-zTzM {(0≤x≤0.2), (0≤y≤0.2), (0≤z≤1)}, wherein the R is Nd or Sm, the M is Cu, Al or Ga, and the T is Mn, Mo, V, Si or Ti. More specifically, the composition may be Sm1-xZrx(Fe1-yCoy)12-zTzM {(0≤x≤0.2), (0≤y≤0.2), (0≤z≤1)}, wherein the M is Cu, Al or Ga, and the T is Mn, Mo, V, Si or Ti.
ThMn12-type crystal phase has a tetragonal crystal structure. Since the ThMn12 structure magnetic powder is poor in phase stability and contains a large amount of Fe as a by-product, a concentration of the Fe element is high and secondary phase such as Alpha Fe, FeTi, or Fe2Ti is easily precipitated. Therefore, it is difficult to obtain single-phase magnetic powder. The precipitation of Alpha Fe or the like decreases the Fe concentration in the main phase, causing a decrease in saturation magnetization of the main phase and a decrease in coercive force of permanent magnet.
When the ThMn12 structure magnetic powder is prepared by the conventional strip casting method, it is difficult to obtain magnetic powder in which the particle size of the particles constituting the magnetic powder is controlled to 10 micrometers or less. In addition, since the ThMn12 structure magnetic powder is poor in phase stability, phase separation occurs when hydrogen is absorbed for the pulverizing process using a jet mill, and thus it is difficult to maintain single-phase.
According to an embodiment of the present disclosure, it is possible to provide single-phase ThMn12 structure magnetic powder having an average particle size of 10 micrometers or less of the particles constituting the magnetic powder with reduced secondary phase such as Alpha Fe, FeTi or Fe2Ti through a reduction-diffusion method by adding a metal oxide, a metal, or a metal fluoride without a separate pulverizing process such as coarse pulverization, hydrogen crushing, and jet milling or a surface treatment process.
Then, the method of preparing magnetic powder according to the present disclosure will be described through specific Examples hereinafter.
A mixture is prepared by uniformly mixing 8.500 g of Sm2O3, 23.957 g of Fe, 6.320 g of Co, 1.201 g of ZrO2, 3.893 g of TiO2, 0.309 g of Cu and 12.004 g of CaH2 (reducing agent). The mixture is tapped in SUS of any shape and then reacted in a tube furnace for 1 to 3 hours under an inert gas (Ar, He) atmosphere at a temperature of 900° C. to 1050° C. After the reaction is completed, it is pulverized using a mortar to make magnetic powder, and then a washing process is performed to remove Ca and CaO, which are by-products of the reduction. The washing process is performed under a N2 atmosphere to minimize contact with air. After uniformly mixing 50 g of NH4NO3 with the synthesized magnetic powder, it is soaked in 400 ml of methanol and homogenized using a homogenizer once or twice for effective washing. Thereafter, the magnetic powder and 200 g ZrO2 ball are put together in ethanol or methanol in which 0.5 g of NH4NO3 is dissolved to proceed the washing process accompanied by pulverization using a Turbula mixer. Then, it is rinsed with acetone and then dried in vacuum.
8.925 g of Sm2O3, 23.957 g of Fe, 6.320 g of Co, 3.893 g of TiO2, and reducing agents (10.477 g of Ca and 0.918 g of Na—K alloy) are mixed uniformly, and then magnetic powder is synthesized by the method described in Example 1. After the synthesized magnetic powder is pulverized using a mortar, washing is performed by the method described in Example 1.
2.086 g of Sm2O3, 6.148 g of Fe, 1.622 g of Co, 0.295 g of ZrO2, 0.478 g of TiO2, 0.122 g of CuF2 and 2.738 g of CaH2 (reducing agent) are mixed uniformly, and then magnetic powder is synthesized by the method described in Example 1. After the synthesized magnetic powder is pulverized using a mortar, washing is performed by the method described in Example 1.
2.086 g of Sm2O3, 6.148 g of Fe, 1.622 g of Co, 0.295 g of ZrO2, 0.478 g of TiO2, 0.076 g of Cu and 2.738 g of CaH2 (reducing agent) are mixed uniformly, and then magnetic powder is synthesized by the method described in Example 1. After the synthesized magnetic powder is pulverized using a mortar, washing is performed by the method described in Example 1.
2.215 g of Sm2O3, 5.989 g of Fe, 1.580 g of Co, 0.150 g of ZrO2, 0.973 g of TiO2, 0.077 g of Cu and 2.847 g of CaH2 (reducing agent) are mixed uniformly, and then magnetic powder is synthesized by the method described in Example 1. After the synthesized magnetic powder is pulverized using a mortar, washing is performed by the method described in Example 1.
2.215 g of Sm2O3, 6.098 g of Fe, 1.608 g of Co, 0.300 g of ZrO2, 0.778 g of TiO2, 0.077 g of Cu and 2.693 g of CaH2 (reducing agent) are mixed uniformly, and then magnetic powder is synthesized by the method described in Example 1. After the synthesized magnetic powder is pulverized using a mortar, washing is performed by the method described in Example 1.
2.086 g of Nd2O3, 7.652 g of Fe, 0.9409 g of TiO2, 0.2904 g of CaF2 and 2.6092 g of Ca (reducing agent) are mixed uniformly, and then magnetic powder is synthesized by the method described in Example 1. After the synthesized magnetic powder is pulverized using a mortar, washing is performed by the method described in Example 1.
An alloy raw material prepared by mixing 1.54 g of Nd, 13.275 g of Fe, 4.425 g of Co, and 0.76 g of Ti is dissolved by arc melting, and then rapidly quenched at a rate of 50 K/sec to prepare flakes. The flakes are heat-treated at a temperature of 1100° C. for 4 hours under an Ar atmosphere, and then pulverized using a cutter mill under an Ar atmosphere to prepare magnetic powder.
1.54 g of Nd, 13.275 g of Fe, 4.425 g of Co, and 0.76 g of Ti are mixed and dissolved in a melting furnace to prepare a molten metal. The molten metal is fed to a cooling roll and rapidly quenched at a rate of 104 K/sec to prepare flakes. Magnetic powder is prepared by pulverizing the flakes using a cutter mill under an Ar atmosphere.
Flakes are prepared in the same manner as in Comparative Example 2. The flakes are heat-treated at a temperature of 1200° C. for 4 hours under an Ar atmosphere, and then pulverized using a cutter mill under an Ar atmosphere to prepare magnetic powder.
XRD patterns of the magnetic powders prepared in Examples 1 to 6 are shown in
The volume fractions of secondary phase and unreacted materials of Examples 1, 2, Comparative Examples 1, 2, and 3 were measured according to Rietveld refinement method and EDS analysis, and the results are shown in Table 1 below.
All the magnetic powders prepared in Examples 1 to 2 have the volume fraction of secondary phase of 2% or less, and it can be confirmed that they are single-phase magnetic powders with high purity having a reduced content of the secondary phase compared to Comparative Examples 1 to 3.
Scanning electron microscope images of the Sm0.8Zr0.2(Fe0.8Co0.2)11Ti1Cu0.1 magnet powder prepared in Example 1 are shown in
Preferred Examples of the present disclosure have been described in detail as above, but the scope of the present disclosure is not limited thereto, and their various modifications and improved forms made by those skilled in the art using a basic concept of the present disclosure defined in the following claims also belong to the scope of the present disclosure.
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| WO2020/032547 | 2/13/2020 | WO | A |
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