WO2003056578A1

WO2003056578A1 - Magnetoplumbite-type ferrite magnet having improved properties and preparation thereof

Info

Publication number: WO2003056578A1
Application number: PCT/KR2002/002451
Authority: WO
Inventors: Dongyoung Lee; Gilsoo Park; Jungwhan Lee; Gilwhan Jung; Jaegyun Yang
Original assignee: Ssangyong Materials Corporation
Priority date: 2001-12-27
Filing date: 2002-12-27
Publication date: 2003-07-10
Also published as: KR100359547B1

Abstract

A magnetoplumbite-type ferrite magnet powder having a composition of formula (I) of the present invention has a high residual magnetic flux density and a high coercive force with improved temperature stability, and it enables miniaturization, reduction in weight and increased performance of magnet products: A1-xRxFen-yMyO19± δ (I) wherein, A is at least one element selected from Sr and Ba; R is at least one element selected from La, Nd, Y and Pr; M is at least one element selected from Co, Cr and Al; 0.03≤x≤0.5; 0.03≤y≤0.5; 12<n≤ 14.

Description

MAGNETOPLUMBITE-TYPE FERRITE MAGNET

HAVING IMPROVED PROPERTIES AND

PREPARATION THEREOF

FTF.T T) OF THF, TNVFNTTON

The present invention relates to a magnetoplumbite-type(hexagonal) ferrite magnet powder having improved magnetic properties, i.e., high residual magnetic flux density and high coercive force with improved temperature stability.

BAC GROUND OF TFΓF. TNVF.NTION

Ferrite magnets, strong magnetic materials comprising chemically stable iron oxides as a primary component, are widely used in various applications, e.g., automobile rotors, electric apparatus motors and high performance speakers.

Recently, miniaturized light-weight rotors are required for automobiles, and high performance motors, for electric apparatuses.

Conventional Sr- or Ba-based ferrite magnets are generally prepared by the following process steps. First, an iron oxide is mixed with an oxide or carbonate of Sr or Ba, calcined, and pulverized to obtain a coarse powder. The coarse powder is mixed with a sintering aid such as Si0₂, SrC0₃, CaC0₃, AI2O3 and Q-₂O3, and then pulverized to obtain a fine powder having an average diameter ranging from 0.7 to l.CPn. A slurry of the fine powder is wet-molded while being subjected to a magnetic field, sintered and processed to prepare a magnet with a desired shape. In order to improve magnetic properties of the ferrite magnet having a general composition of AFe_nOi9±s (A is Sr or Ba and 12<n— 14) thus produced, the following four met ods have been attempted. CD make the density of a sintered body close to the theoretical density. make the crystal grain size as uniform as possible to approach the critical single magnetic domain diameter (0.9/^1 in Ba ferrite, 0.94 m in Sr ferrite).

CD make the crystal orientation close to 100%.

® improve the saturation magnetization(σ _s) of the ferrite component of the ferrite magnet which directly determines the magnetic flux density(Br). Among the above methods,®,© and Φ are widely used at present, but with limited effectiveness. Thus, there have recently been many studies on method ® by way of mixing a ferrite represented by AFe_nOi9± g (A is Sr or Ba and 12<n^≤ 14) with other metal compounds to partially replace A and Fe with other elements to improve the saturation magnetization( σ _s). The magnetism of a magnetoplumbite-type ferrite magnet is determined by the magnetic moment of Fe ions, but some of the Fe ions are arranged in an antiparallel manner. Thus, in order to improve the saturation magnetization of such magnetic structure, there have been attempts to replace the Fe ions having antiparallel magnetic moment with another element which is non-magnetic or has a smaller magnetic moment than Fe ion, and also to replace the parallel-oriented Fe ions with another element having a lager magnetic moment than Fe. However, when only Fe ions are replaced with other elements, charge unbalance results to generate undesirable phases. Accordingly, element A is replaced with other elements together with Fe ions for the purpose of charge compensation. Hitherto, such high-performance magnets having partially replaced A and

Fe have been commercialized. For example, such a magnet having a residual magnet flux density(Br) of 4250 G and a coercive force(iHc) of 3300 Oe is employed in motors for power windows of automobiles, and another having a Br of 3900 G and an iHc of 4800 Oe, in starting motors for automobiles. Efforts are continuing to develop magnets having even higher magnetic performance characteristics using such replacement method.

ST TMM AR Y OF THE INVENTION

Accordingly, it is a primary object of the present invention to provide a magnetoplumbite-type ferrite magnet powder having improved magnetic properties such as high residual magnetic flux density, high coercive force with improved temperature stability and a wide permeance coefficient range to meet the requirements arising from miniaturization, weight reduction and improved performance of magnets used in various applications.

In accordance with one aspect of the present invention, there is provided a magnetoplumbite-type ferrite magnet powder having a composition represented by formula (I):

Ai.xRxFen-ylViyOig± β (I) wherein, A is at least one element selected from Sr and Ba; R is at least one element selected from La, Nd, Y and Pr; M is at least one element selected from Co, Cr and Al; 0.03≤ x≤ 0.5; 0.03≤ y≤ 0.5; 12<n≤ 14.

BRTEF DESCRIPTION OF THE PR A WINGS

The above and other objects and features of the present invention will become apparent from the following description, when taken in conjunction with the accompanying drawings, which respectively show:

FIG. 1: magnetic properties of the respective ferrite magnets prepared in Examples 1 and 2, and Comparative Example; FIG. 2: variations in the coercive force as function of temperature of the respective ferrite magnets prepared in Example 1 and Comparative Example;

FIG. 3: the residual magnet flux density(Br) and coercive force(iHc) of the magnets having various n values prepared in Example 3;

FIGs. 4 and 5: the residual magnet flux density(Br) and coercive force(iHc), respectively, of the magents having various x=y values prepared in Example 4; and

FIGs. 6 and 7: magnetic properties of the ferrite magnets prepared in Examples 5 and 1, respectively.

DETAILED DESCRIPTION OF THE TNVENTTON

The ferrite magnet powder of the present invention is characterized by being prepared by replacing a part of A(Sr or Ba) of the conventional magnet having a basic composition of AFe_nOi₉± s with at least one element(R) selected from La, Nd, Y and Pr, and a part of Fe thereof with at least one element(M) selected from Co, Cr and Al, respectively, wherein the A : R : Fe : M atomic ratio is 1-x : x : n-y : y (0.03 ≤ x≤ 0.5, 0.03 ≤ y≤ 0.5, 12<n≤ 14).

Among the elements used as M in the present invention, preferred is Co which enhances both the residual magnetic flux density and coercive force. When n is less than 12 or more than 14, undesirable phases such as ^Q -Fe₂0₃ are generated, lowering the magnetic properties. Further, x and y have values ranging from 0.03 to 0.5, preferably from 0.05 to 0.4, and when less than 0.03, no desired effect is obtained, and when more than 0.5, the magnetic properties such as the residual magnetic flux density and coercive force become poor.

The inventive magnet powder may be prepared by mixing an iron oxide with an oxide or carbonate of element A selected from Sr and Ba, and then calcining and pulverizing the mixture, precursor compounds (oxides or hydroxides) of elements R and M being added at the calcination or pulverization step. At the pulverization step, a conventional sintering aid such as Si0₂, CaC0₃, A1₂0₃, Al(OH)₃ and Cr₂θ3 may be added for controlling the sintering behavior.

In accordance with the present invention, in order to obtain a sintered body having improved magnetic properties, the calcined product may be wet-pulverized to an average diameter ranging from 0.40 to 0.70^m. When pulverized to less than 0.40^πi₅ the ultra-fine powder is difficult to dehydrate after wet molding and undesirable particles grow during sintering, thereby decreasing the magnetic properties.

Furthermore, a sintered magnet of the present invention may be prepared by molding and sintering the ferrite magnet powder thus obtained by the conventional method. At the pulverization or molding step, a dispersing agent such as a surfactant may be added in an amount ranging 0.1 to 1.5% by weight, preferably 0.2 to 1.0% by weight based on the total solid amount of raw materials to inhibit coagulation of ferrite particles and improve orientation. Anionic organic polymers may be employed as the surfactant, representative examples thereof being sodium, ammonium and potassium salts of acrylate polymers. When the amount of the surfactant is less than 0.1% by weight, it is difficult to obtain the desired effect, and when more than 1.5% by weight, nonmagnetic compounds and holes generated after sintering lead to poor residual magnetic flux density. Alternatively, a bonded magnet of the present invention may be prepared by adding a binder to the ferrite magnet powder thus obtained and molding by the conventional method.

Therefore, the magnetoplumbite-type ferrite magnet powder of the present invention has a high residual magnetic flux density, a high coercive force with improved temperature stability and a wide range of permeance coefficient due to the enhanced saturation magnetization of the calcined powder, and it can be advantageously used to meet the requirements arising from m iaturization, weight reduction and high performance of magnet products such as rotors.

The present invention is further described and illustrated in Examples and Comparative Example, which is, however, not intended to limit the scope of the present invention.

Example 1

SrC0₃ and Fe₂0₃ were formulated to provide a basic composition of SrFenOi9± δ , wherein n=T2.2, wet-mixed, and then calcined at 1260 C for 2 hours in the air. The calcined material was dry-milled to obtain a coarse powder using a Roll Mill, which was mixed with appropriate amounts of La₂0₃ and CoO to provide a composition represented by

s , wherein x=y=0.2, R=La and M=Co. The resultant mixture was wet-milled with an Attritor to produce a slurry containing a fine powder having an average diameter of 0.65 ^m. At the wet-milling step, Si0₂ and CaO were further added as sintering aids in amounts corresponding to 0.55 and 0.40% by weight, respectively, based on the weight of total raw materials. Subsequently, the slurry was wet-molded under a magnetic field of 10 kOe, and the molding was sintered at 1200~1240^°C for 2 hours to prepare a magnetoplumbite-type sintered magnet having a composition represented by Sro.sLao^Fe^Coo^O^± δ .

Example 2

The procedure of Example 1 was repeated except that an acrylate polymer having sodium, ammonium and potassium ions were further added to the slurry as a dispersing agent in an amount corresponding to 0.3% by weight based on the total solid amount of raw materials to prepare a magnetoplumbite-type sintered magnet having a composition represented by Sro.8 ao.₂Fei2Cθo.2θi9± δ .

Comparative Example

The procedure of Example 1 was repeated except that La₂0₃ and CoO were not added (x=y=0) to prepare a magnetoplumbite-type sintered magnet having a composition represented by SrFei₂.₂θi₉± δ .

Residual magnetic flux density (Br) - coercive force (iHc) plots of the respective ferrite magnets prepared in Examples 1 and 2, and Comparative Example are shown in FIG.l, and variations of coercive force as function of temperature (temperature coefficient of coercive force), in FIG. 2. Br and iHc were measured by KS C 2501 permanent magnet test method. As shown in FIGs. 1 and 2, as compared with the conventional magnet prepared in Comparative Example, the inventive magnet having La and Co exhibits a higher residual magnetic flux density, a higher coercive force, and a lower temperature coefficient of coercive force of 0.1 to 0.15%/^°C, i.e., higher stability against temperature fluctuation (see the magnetic properties of Table 1).

Table 1

Example 3

The procedure of Example 1 was repeated except that the n value was changed among 12, 12.5, 13, 13.5 and 14 while keeping x=y=0.2, to prepare magnetoplumbite-type sintered magnets having various compositions.

Residual magnetic flux densities (Br) and coercive forces (iHc) of the magnets prepared in Example 3 are shown in FIG.3. This result suggests that a magnet having an n value in the range of 12 to 12.5 exhibits excellent magnetic properties.

Example 4

The procedure of Example 1 was repeated except that while keeping x=y, the x (y) value was changed among 0, 0.2, 0.4 and 0.5, and that while keeping n=12.2, La₂θ3 and CoO were added either at the prior- or post-calcination step, or at both steps, to prepare magnetoplumbite-type sintered magnets having various compositions.

Residual magnetic flux density (Br) - coercive force (iHc) plots of the magnets prepared in Example 4 are shown in FIGs. 4 and 5, respectively. The results suggest that magnets having x (=y) in the range of 0.07 to 0.3 exhibit excellent magnetic properties.

Example 5

The procedure of Example 1 was repeated except that La, Co, Cr and Al oxides were each added at the wet-milling step in amounts corresponding to n=T2.2, x=y=0.2, R=La and M^Co, Cr, Al, to prepare a magnetoplumbite-type sintered magnet having a composition represented by Sro.8Lao.2Fei₂Coo.i₇Cro.o₂Alo.oiOi₉+ s .

The magnetic properties of the magnets prepared in Examples 5 and 1 are shown in FIGs. 6 and 7, resepectively. It is confirmed in FIGs. 6 and 7, respectively, that the magnet prepared in Example 5 has a residual magnetic flux density (Br) of 4200 G or more and a coercive force (iHc) of 4800 Oe or more, and that the magnet prepared in Example 1 has Br of 4300 G or more and iHc of 4300 Oe or more, at room temperature.

Further, these magnets possess a recoil magnetic permeability (μ_rec: average slope of minor curve in B-H curve) of 1.04 to 1.07 when the permeance coefficient (P_c=magnetic flux density at operation point/magnetic field tenacity at operation point) is in the range of 0.7 to 2.5. As the permeance coefficient represents a stable operation point of a motor, an important parameter in motor design, a wide permeance coefficient range means that a stable motor which is little affected by outer environments can be conveniently designed.

As described above, the magnetoplumbite-type ferrite magnet powder of the present invention has a high residual magnetic flux density, a high coercive force with improved temperature stability and a wide range of permeance coefficient^" due to saturation magnetization of the calcined powder that enables miniaturization, reduction in weight and increased performance of magnet products such as rotors. While the invention has been described with respect to the specific embodiments, it should be recognized that various modifications and changes may be made by those skilled in the art to the invention which also fall within the scope of the invention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A magnetoplumbite-type ferrite magnet powder having a composition represented by formula (I): A_1-xR_xFe_n-_yMyOi9± δ (I) wherein,

A is at least one element selected from Sr and Ba; R is at least one element selected from La, Nd, Y and Pr; M is at least one element selected from Co, Cr and Al; 0.03≤ x≤ 0.5; 0.03≤ y≤ 0.5; 12<n≤ 14.

2. A sintered magnet prepared from a mixture of iron oxide and precursor compounds of elements A, R and M by calcining and pulverizing the mixture to foπn the ferrite magnet powder of claim 1, and then molding and sintering the magnet powder, wherein the precursor compounds of elements R and M are added at the calcination or pulverization step.

3. The sintered magnet of claim 2 which has a residual magnetic flux density

(Br) of 4200 G or more and a coercive force (iHc) of 4800 Oe or more at room temperature, and a temperature coefficient of coercive force of 0.1 to 0.15%/ C, wherein the precursor compounds of elements R and M are added in the foπn of oxide or hydroxide.

4. The sintered magnet of claim 2 which has a residual magnetic flux density (Br) of 4300 G or more, a coercive force (iHc) of 4300 Oe or more at room temperature, and a recoil magnetic permeability (U_rec) of 1.04 to 1.07 when the permeance coefficient is in the range of 0.7 to 2.5, wherein the precursor compounds of elements R and M are added in the foπn of oxide or hydroxide.

5. The sintered magnet of claim 2 which is prepared by further adding ^"a dispersing agent in an amount ranging from 0.1 to 1.5% by weight based on the total solid amount of raw materials at the pulverization or molding step.

6. A bonded magnet prepared using the ferrite magnet powder of claim 1.