Patent Description:
The present disclosure relates to magnetic powder and a method of preparing the same.

SmFe<NUM>-based magnets having a ThMn<NUM> structure have superior magnetic properties at room temperature as compared to the existing Nd<NUM>Fe<NUM>B structure as follows.

Sm(Fe<NUM>Co<NUM>)<NUM>: µ<NUM>Ms=<NUM>. 78T, µ<NUM>Ha=12T Nd<NUM>Fe<NUM>B: µ<NUM>Ms=<NUM>. 61T, µ<NUM>Ha=<NUM>.

(µ<NUM>: permeability of vacuum, Ms: intensity of spontaneous magnetization, Ha: strength of magnetic anisotropy).

In addition, its Curie temperature, which is the temperature at which the magnetic material loses its magnetism, is higher than <NUM>, which means higher thermal stability than Nd<NUM>Fe<NUM>B.

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 SmFe<NUM>-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.

<CIT> discloses a method for producing rare earth-transition metal-nitrogen based magnetic powder comprises: a first stage where a rare earth-transition metal based master alloy in which the amount of a rare earth element(s) present in the master alloy is a surplus by a specified amount or above than the stoichiometric composition of a rare earth element(s) present in the main phase thereof is produced from a raw material mixture comprising a surplus rare earth oxide powder, transition metal powder and a reducing agent by utilizing a reduction diffusion process, and the master alloy is thereafter nitrided in a nitrogen-containing atmosphere under heating, so as to obtain a rare earth-transition metal-nitrogen based magnetic powder; and a second stage where the obtained magnetic powder is cleaned with an acidic aqueous solution in such a manner that the rare earth element(s) present in the magnetic powder is removed till the surplus amount of the same reaches a specified amount or below to the stoichiometric composition of the rare earth element(s) present in the main phase, and thereafter, drying is performed.

<CIT> a rare earth-transition metal-nitrogen based magnet powder produced by a reduction diffusion method. The magnet powder contains at least one element selected from Ti, Zr and Al in an outer peripheral part of a rare earth-iron-alloy and the ratio of a particle size having an average particle diameter (D50) of more than <NUM> and <NUM> or less is less than <NUM>%.

<CIT> discloses a magnetic compound is expressed by an equation (R(<NUM>-x)Zrx)-α(Fe(<NUM>-y)Coy)bTcMdAe (in the same equation, R is a rare earth element of one type or more, and T is an element of one type or more selected in a group comprising Ti, V, Mo and W, and M is an inevitable impurity element and is an element of one type or more selected in a group comprising Al, Cr, Cu, Ga, Ag and Au; and A is an element of one type or more selected in a group comprising N, C, H and P, and <NUM> <= x <= <NUM>, <NUM> <= y <= <NUM>, <NUM> <= a <= <NUM>, b = <NUM>-a-c-d, <NUM> < c < <NUM>, <NUM> <= d <= <NUM>, <NUM> <= e <= <NUM>). A main phase of the magnetic compound has a ThMn<NUM> type crystalline structure, and a volume fraction of α-(Fe,Co) phase is <NUM>% or less.

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.

The above problems are solved in accordance with the subject-matter of the independent claims. Further embodiments result from the subclaims.

Magnetic powder according to an embodiment of the present disclosure for solving the above problems is a magnetic powder, which is powder particles obtainable by using a mixture of a rare earth oxide, a raw material, one of a metal and a metal oxide, and a reducing agent,.

The reducing agent may include at least one of Ca, Mg, CaH<NUM>, Na and Na-K alloy.

An average particle size of the particles constituting the magnetic powder may be <NUM> micrometers or less.

A method of preparing magnetic powder according to an embodiment of the present disclosure includes the steps of:.

The heat-treating may be performed for <NUM> minutes to <NUM> hours.

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 <NUM> 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 obtainable by using a mixture of a rare earth oxide, a raw material, one of a metal and a metal oxide, and a reducing agent,.

The reducing agent may include at least one of Ca, Mg, CaH<NUM>, Na and Na-K alloy. Particularly, CaH<NUM> is preferable.

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 is a method of preparing magnetic powder, comprising the steps of:.

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 <NUM> to <NUM> with a reduction-diffusion method.

The heat-treating may be performed in a tube furnace at a temperature of <NUM> to <NUM> under an inert atmosphere for <NUM> minutes to <NUM> hours. Reduction and diffusion between the mixtures at a temperature of <NUM> to <NUM> 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 <NUM> minutes or less, the metal powder may not be sufficiently synthesized. When the heat-treatment is performed for <NUM> 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.

After the step of reacting the mixture, a washing step for removing by-products of the reduction may further proceed. NH<NUM>NO<NUM> 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, NH<NUM>NO<NUM> is dissolved in ethanol or methanol, and then washed and pulverized together with the synthesized powder and ZrO<NUM> 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 N<NUM> atmosphere to minimize contact with air.

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 <NUM> of Sm<NUM>O<NUM>, <NUM> of Fe, <NUM> of Co, <NUM> of ZrO<NUM>, <NUM> of TiO<NUM>, <NUM> of Cu and <NUM> of CaH<NUM> (reducing agent). The mixture is tapped in SUS of any shape and then reacted in a tube furnace for <NUM> to <NUM> hours under an inert gas (Ar, He) atmosphere at a temperature of <NUM> to <NUM>. 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 N<NUM> atmosphere to minimize contact with air. After uniformly mixing <NUM> of NH<NUM>NO<NUM> with the synthesized magnetic powder, it is soaked in <NUM> of methanol and homogenized using a homogenizer once or twice for effective washing. Thereafter, the magnetic powder and <NUM> ZrO<NUM> ball are put together in ethanol or methanol in which <NUM> of NH<NUM>NO<NUM> 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.

<NUM> of Sm<NUM>O<NUM>, <NUM> of Fe, <NUM> of Co, <NUM> of TiO<NUM>, and reducing agents (<NUM> of Ca and <NUM> of Na-K alloy) are mixed uniformly, and then magnetic powder is synthesized by the method described in Example <NUM>. After the synthesized magnetic powder is pulverized using a mortar, washing is performed by the method described in Example <NUM>.

<NUM> of Sm<NUM>O<NUM>, <NUM> of Fe, <NUM> of Co, <NUM> of ZrO<NUM>, <NUM> of TiO<NUM>, <NUM> of CuF<NUM> and <NUM> of CaH<NUM> (reducing agent) are mixed uniformly, and then magnetic powder is synthesized by the method described in Example <NUM>. After the synthesized magnetic powder is pulverized using a mortar, washing is performed by the method described in Example <NUM>.

<NUM> of Sm<NUM>O<NUM>, <NUM> of Fe, <NUM> of Co, <NUM> of ZrO<NUM>, <NUM> of TiO<NUM>, <NUM> of Cu and <NUM> of CaH<NUM> (reducing agent) are mixed uniformly, and then magnetic powder is synthesized by the method described in Example <NUM>. After the synthesized magnetic powder is pulverized using a mortar, washing is performed by the method described in Example <NUM>.

<NUM> of Nd<NUM>O<NUM>, <NUM> of Fe, <NUM> of TiO<NUM>, <NUM> of CaF<NUM> and <NUM> of Ca (reducing agent) are mixed uniformly, and then magnetic powder is synthesized by the method described in Example <NUM>. After the synthesized magnetic powder is pulverized using a mortar, washing is performed by the method described in Example <NUM>.

An alloy raw material prepared by mixing <NUM> of Nd, <NUM> of Fe, <NUM> of Co, and <NUM> of Ti is dissolved by arc melting, and then rapidly quenched at a rate of <NUM>/sec to prepare flakes. The flakes are heat-treated at a temperature of <NUM> for <NUM> hours under an Ar atmosphere, and then pulverized using a cutter mill under an Ar atmosphere to prepare magnetic powder.

<NUM> of Nd, <NUM> of Fe, <NUM> of Co, and <NUM> 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 <NUM><NUM> 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 <NUM>. The flakes are heat-treated at a temperature of <NUM> for <NUM> 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 <NUM> to <NUM> (wherein Examples <NUM> to <NUM> are comparative) are shown in <FIG>, an XRD pattern of the magnetic powder prepared in Example <NUM> (comparative) is shown in <FIG>, and XRD patterns of the magnetic powders prepared in Comparative Examples <NUM> to <NUM> are shown in <FIG>. Si in <FIG> is a material added to set a reference point of each point. Referring to <FIG>, the magnetic powders according to Examples <NUM> to <NUM> (wherein Examples <NUM> to <NUM> are comparative) were confirmed to have weak peak intensity of Alpha Fe or FeTi. Referring to <FIG>, it was confirmed that the magnetic powder according to Example <NUM> (comparative) did not show a peak of secondary phase such as Alpha Fe. On the other hand, referring to <FIG>, the magnetic powders according to Comparative Examples <NUM> to <NUM> were confirmed to have apparent peak intensity of Alpha (Fe, Co) phase.

The volume fractions of secondary phase and unreacted materials of Examples <NUM>, <NUM>, Comparative Examples <NUM>, <NUM>, and <NUM> were measured according to Rietveld refinement method and EDS analysis, and the results are shown in Table <NUM> below.

All the magnetic powders prepared in Examples <NUM> to <NUM> have the volume fraction of secondary phase of <NUM>% 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 <NUM> to <NUM>.

Scanning electron microscope images of the Sm<NUM>Zr<NUM>(Fe<NUM>Co<NUM>)<NUM>Ti<NUM>Cu<NUM> magnet powder prepared in Example <NUM> are shown in <FIG>, and scanning electron microscope images of the Sm(Fe<NUM>Co<NUM>)<NUM>Ti<NUM> magnet powder prepared in Example <NUM> are shown in <FIG> and <FIG>. Referring to <FIG>, it can be confirmed that an average particle size of the particles constituting the magnetic powder according to Examples of the present disclosure is <NUM> micrometers or less.

Claim 1:
A magnetic powder, which is powder particles obtainable by using a mixture of a rare earth oxide, a raw material, one of a metal and a metal oxide, and a reducing agent,
wherein
the raw material comprises Fe and Co,
the metal optionally comprises one of Ti or Zr, and
the metal oxide comprises TiO<NUM>, and optionally ZrO<NUM>;
the rare earth oxide comprises samarium oxide;
wherein the mixture further optionally comprises Cu; characterised in that the magnetic powder has a composition of Sm<NUM>Zr<NUM>(Fe<NUM>Co<NUM>)<NUM>Ti<NUM>Cu<NUM> or Sm(Fe<NUM>Co<NUM>)<NUM>Ti<NUM> and the powder particles are single-phase meaning that a volume fraction of secondary phase is <NUM>% or less.