# Patent

## Title
Method for producing sintered ferrite magnet, and sintered ferrite magnet

## Abstract

A sintered ferrite magnet comprising (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca1-x-yRxAyFe2n-zCoz, wherein the atomic ratios (1-x-y), x, y and z of these elements and the molar ratio n meet the relations of 0.3≦(1-x-y)≦0.65, 0.2≦x≦0.65, 0≦y≦0.2, 0.03≦z≦0.65, and 4≦n≦7, and (b) a grain boundary phase indispensably containing Si, the amount of Si being more than 1% by mass and 1.8% or less by mass (calculated as SiO2) based on the entire sintered ferrite magnet, and its production method.

## Background
This is a continuation of U.S. application Ser. No. 13/381,526 Dec. 29, 2011, which is a national stage of PCT/JP2010/060280 filed on Jun. 17, 2010, claiming priority based on Japanese Patent Application No. 2009-156297, filed Jun. 30, 2009, the contents of all of which are incorporated herein by reference in their entirety.


FIELD OF THE INVENTION
The present invention relates to a method for producing a sintered ferrite magnet, and a sintered ferrite magnet.
BACKGROUND OF THE INVENTION
Sintered ferrite magnets are used in various applications such as motors, power generators, speakers, etc. Known as typical sintered ferrite magnets are Sr ferrite (SrFe12O19) and Ba ferrite (BaFe12O19) each having a hexagonal M-type magnetoplumbite structure. These sintered ferrite magnets are produced from starting materials comprising iron oxide, carbonates of strontium (Sr) or barium (Ba), etc. by a powder metallurgy method at a relatively low cost.
Recently, to provide smaller, lighter-weight, higher-performance electronic parts for automobiles, parts for electric appliances, etc. for environmental advantages, sintered ferrite magnets are required to have higher performance. Particularly desired for motors for automobiles are sintered ferrite magnets having, in addition to high residual magnetic flux density Br, such high coercivity HcJ as to avoid demagnetization by a demagnetizing field generated when thinned.
To provide sintered ferrite magnets with improved magnetic properties, proposals have been made to improve coercivity HcJ and a residual magnetic flux density Br by substituting part of Sr with a rare earth element such as La, etc., and substituting part of Fe with Co in the above Sr ferrite (for example, JP 10-149910 A, and JP 11-154604 A).
The Sr ferrite with part of Sr substituted by a rare earth element such as La, etc. and part of Fe substituted by Co, etc. (hereinafter referred to as “Sr—La—Co ferrite”), which is described in JP 10-149910 A and JP 11-154604 A, has excellent magnetic properties, finding various applications in place of conventional Sr or Ba ferrite. However, further improvement in magnetic properties is desired.
Also known for the sintered ferrite magnets together with the above Sr or Ba ferrite is Ca ferrite. It is known that the Ca ferrite has a stable structure represented by the formula of CaO—Fe2O3 or CaO—2Fe2O3, forming hexagonal ferrite by the addition of La. However, it has magnetic properties on substantially the same levels as those of conventional Ba ferrite, not sufficiently high.
To improve a residual magnetic flux density Br and coercivity HcJ as well as the temperature characteristics of coercivity HcJ, Japanese Patent 3181559 discloses Ca ferrite with part of Ca substituted by a rare earth element such as La, etc. and part of Fe substituted by Co, etc. (hereinafter referred to as “Ca—La—Co ferrite”), which has an anisotropic magnetic field HA of 20 kOe or more, higher by 10% or more than that of Sr ferrite.
However, the Ca—La—Co ferrite described in Japanese Patent 3181559 has substantially the same Br and HcJ as those of Sr—La—Co ferrite, despite a higher anisotropic magnetic field HA than that of the Sr—La—Co ferrite. In addition, having an extremely poor squareness ratio, it does not meet high coercivity and a high squareness ratio simultaneously, failing to be used for various applications such as motors, etc.
With respect to the Sr—La—Co ferrite proposed by JP 10-149910 A and JP 11-154604 A, JP 2007-123511 A proposes a method for improving HcJ while maintaining Br. In general, a sintered ferrite magnet is produced by the steps of (1) mixing iron oxide with carbonate of Sr or Ba, etc. as starting materials, (2) calcining a starting material mixture for ferritization to obtain a calcined body, (3) pulverizing the calcined body to powder, (4) molding the powder to a green body, and (5) sintering the green body to obtain a sintered body. JP 2007-123511 A discloses a pulverization step comprising a first fine pulverization step, a powder heat-treating step and a second fine pulverization step, by which the percentage of crystals as large as 1.1 μm or less is made 95% or more, improving HcJ.
Though HcJ is improved by the method of JP 2007-123511 A, cost increase is unavoidable due to increase in the number of production steps. Particularly, Sr—La—Co ferrite containing expensive elements such as Co, La, etc. as indispensable components suffers a higher material cost than conventional Sr ferrite. Higher production cost deprives sintered ferrite magnets of the most important economic advantages, failing to meet cost requirements in the market.
Various measures for improving performance have been conducted on the Ca—La—Co ferrite proposed by Japanese Patent 3181559. For example, JP 2006-104050 A proposes Ca—La—Co ferrite containing La and Co at predetermined percentages with the molar ratios of elements and the value of n optimized. WO 2007/060757 proposes Ca—La—Co ferrite with part of Ca substituted by La and Ba. WO 2007/077811 proposes Ca—La—Co ferrite with part of Ca substituted by La and Sr. However, the Ca—La—Co ferrites of JP 2006-104050 A, WO 2007/060757 and WO 2007/077811 fail to provide sintered ferrite magnets having high HcJ satisfying demands in the market.
JP 2008-137879 A and WO 2008/105449 disclose a technology for improving HcJ by using the pulverization step described in JP 2007-123511 A in the production of Ca—La—Co ferrite. Though the Ca—La—Co ferrites described in JP 2008-137879 A and WO 2008/105449 have improved HcJ, they doubly suffer increase in a material cost and a production cost as in JP 2007-123511 A, failing to meet cost requirements in the market.
Thus, sintered ferrite magnets meeting recent requirements of magnetic properties and costs have not been proposed yet.
It is known to add SiO2, CaCO3, etc. as sintering aids to sintered ferrite magnets, to change balance between Br and HcJ, which are in a trade-off relation in the sintered ferrite magnets. To obtain high Br, it is effective to reduce the amounts of sintering aids acting as non-magnetic components in a range having liquid phase components necessary for sintering, or to increase the amount of CaCO3 relative to that of SiO2. However, such measures make it difficult to maintain fine sintered structures, resulting in low HcJ. On the other hand, to obtain high HcJ, it is effective to increase the amounts of sintering aids, or to increase the amount of SiO2 relative to that of CaCO3. However, such measures increase the amounts of non-magnetic components or lower sinterability, inevitably resulting in decrease in Br and a squareness ratio Hk/HcJ, wherein Hk was the value of H at J of 0.95Br on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant.
In conventional sintered ferrite magnets, particularly in Sr—La—Co ferrite and Ca—La—Co ferrite proposed recently, the amounts of sintering aids such as SiO2, CaCO3, etc. are generally as small as possible to maintain high Br. For example, JP 2006-104050 A describes that 0.3-1.5% by mass (calculated as CaO) of CaCO3 and 0.2-1.0% by mass of SiO2 are added preferably at the time of pulverizing a calcined body. WO 2007/060757 describes that 0.2-1.5% by mass (0.112-0.84% by mass when calculated as CaO) of CaCO3, and 0.1-1.5% by mass of SiO2 are added preferably at the time of pulverizing a calcined body. JP 2008-137879 A describes that 1.35% or less by mass of SiO2 is preferably added to a sintering material.
However, JP 2006-104050 A, WO 2007/060757 and JP 2008-137879 A describe in Examples only Ca—La—Co ferrite magnets, in which the amounts of both SiO2 and CaCO3 (calculated as CaO) are 0.9% or less by mass. Because emphasis is placed on the improvement of the Br of these ferrite magnets, the addition of SiO2 and CaCO3 in amounts of more than 0.9% by mass is not contemplated, providing no information about the magnetic properties (Br, HcJ and Hk/HcJ) of such Ca—La—Co ferrite magnets.
OBJECT OF THE INVENTION
Accordingly, an object of the present invention is to provide a sintered Ca—La—Co ferrite magnet having extremely improved HcJ while maintaining high Br and squareness ratio Hk/HcJ without suffering increase in a material cost and a production cost, which can therefore be made thinner, and a method for producing such sintered ferrite magnet at a low cost.
DISCLOSURE OF THE INVENTION
As a result of intensive research in view of the above object, the inventors have paid attention to sintering aids added at a pulverization step, finding that in Ca—La—Co ferrite, (a) the addition of SiO2 in an amount exceeding 1% by mass as a sintering aid specifically increases HcJ; (b) by adding 1% or more by mass of CaCO3 depending on the amount of SiO2 added, decrease in Br and Hk/HcJ can be avoided as much as possible; therefore, (c) a sintered ferrite magnet having high HcJ with high Br and Hk/HcJ maintained, which has not been obtained so far, is produced. The present invention has been completed based on such finding.
The specific increase in HcJ by the addition of SiO2 in an amount exceeding 1% by mass is a phenomenon peculiar to the Ca—La—Co ferrite. In the Sr—La—Co ferrites described in JP 10-149910 A and JP 11-154604 A, the addition of SiO2 and CaCO3 in amounts exceeding 1% by mass provides only small increase in HcJ, accompanied by extreme decrease in Br and Hk/HcJ (see Example 3 below).
Measurement by the inventors using a singular point detection (SPD) method has revealed that Ca—La—Co ferrite in which the atomic ratio of Co is 0.3 has an anisotropic magnetic field HA of 2.1 MA/m (about 26.4 kOe), higher than 1.8 MA/m (about 22.6 kOe) of general Sr—La—Co ferrite in which the atomic ratio of Co is 0.2. Namely, it is expected for Ca—La—Co ferrite to have higher HcJ than that of Sr—La—Co ferrite. The details of the SPD method is described in Asti and S. Rinaldi, J. Appl. Phys., 45 (1974), pp. 3600-3610.
However, the Ca—La—Co ferrite magnets having a Co atomic ratio of 0.3, which are described in Examples of JP 2006-104050 A, WO 2007/060757 and JP 2008-137879 A, have HcJ of about 400 kA/m (about 5 kOe) at highest. HcJ of about 400 kA/m is obtained even in the Sr—La—Co ferrite having a Co atomic ratio of 0.2 (JP 2007-123511 A). Namely, though the Ca—La—Co ferrite magnets described in Examples of JP 2006-104050 A, WO 2007/060757 and JP 2008-137879 A have a higher anisotropic magnetic field HA than that of the Sr—La—Co ferrite, they have substantially the same HcJ as that of the Sr—La—Co ferrite, not improved to an expected level, and thus failing to exhibit potential inherent in the materials. The fact that HcJ is specifically improved by the addition of SiO2 in an amount exceeding 1% by mass in the present invention is a phenomenon peculiar to the Ca—La—Co ferrite, providing a remarkable effect of achieving properties close to potential inherent in the materials.
Thus, the method of the present invention for producing a sintered ferrite magnet comprising (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure, and (b) a grain boundary phase indispensably containing Si, comprises
a step of preparing a calcined body of ferrite comprising a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca1-x-yRxAyFe2n-zCoz, wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:

0.3≦(1-x-y)≦0.65,

0.2≦x≦0.65,

0≦y≦0.2,

0.03≦z≦0.65, and

4≦n≦7;

a step of pulverizing the calcined body to powder;
a step of molding the powder to a green body; and
a step of sintering the green body to obtain a sintered body;
before the pulverization step, more than 1% by mass and 1.8% or less by mass of SiO2 being added to 100% by mass of the calcined body.
1-2% by mass (calculated as CaO) of CaCO3 is preferably added to 100% by mass of the calcined body, before the pulverization step.
The amount of the SiO2 added is preferably 1.1-1.6% by mass.
The amount of the CaO added is preferably 1.2-2% by mass.
The above (1-x-y), x, y and z and n are preferably

0.35≦(1-x-y)≦0.55,

0.4≦x≦0.6,

0≦y≦0.15,

0.1≦z≦0.4, and

4.5≦n≦6.

The above (1-x-y), x, y and z and n are more preferably

0.42≦(1-x-y)≦0.5,

0.45≦x≦0.55,

0≦y≦0.08,

0.2≦z≦0.3, and

4.8≦n≦5.2.

The above pulverization step preferably comprises a first fine pulverization step, a step of heat-treating powder obtained by the first fine pulverization step, and a second fine p. ulverization step of pulverizing the heat-treated powder again.
The sintered ferrite magnet of the present invention comprises
(a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca1-x-yRxAyFe2n-zCoz, wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:

0.3≦(1-x-y)≦0.65,

0.2≦x≦0.65,

0≦y≦0.2,

0.03≦z≦0.65, and

4≦n≦7, and

(b) a grain boundary phase indispensably containing Si, the amount of Si being more than 1% by mass and 1.8% or less by mass (calculated as SiO2), based on the entire sintered ferrite magnet.
The amount of Si is preferably 1.1-1.6% by mass (calculated as SiO2), based on the entire sintered ferrite magnet.
The above (1-x-y), x, y and z and n are preferably

0.35≦(1-x-y)≦0.55,

0.4≦x≦0.6,

0≦y≦0.15,

0.1≦z≦0.4, and

4.5≦n≦6.

The above (1-x-y), x, y and z and n are more preferably

0.42≦(1-x-y)≦0.5,

0.45≦x≦0.55,

0≦y≦0.08,

0.2≦z≦0.3, and

4.8≦n≦5.2.




BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the relation between the amount of SiO2 added and a residual magnetic flux density Br in the sintered ferrite magnet of Example 1.
FIG. 2 is a graph showing the relation between the amount of SiO2 added and coercivity HcJ in the sintered ferrite magnet of Example 1.
FIG. 3 is a graph showing the relation between the amount of SiO2 added and a squareness ratio Hk/HcJ in the sintered ferrite magnet of Example 1.
FIG. 4 is a graph showing the relation between the amount of SiO2 added and coercivity HcJ in the sintered ferrite magnet of Example 1.
FIG. 5 is a graph showing the relation between the amount of CaO added and coercivity HcJ in the sintered ferrite magnet of Example 1.
FIG. 6 is a graph showing the relation between the amount of SiO2 added and a residual magnetic flux density Br in the sintered ferrite magnet of Example 2.
FIG. 7 is a graph showing the relation between the amount of SiO2 added and coercivity HcJ in the sintered ferrite magnet of Example 2.
FIG. 8 is a graph showing the relation between the amount of SiO2 added and a squareness ratio Hk/HcJ in the sintered ferrite magnet of Example 2.
FIG. 9 is a graph showing the relation between the amount of SiO2 added and coercivity HcJ in the sintered ferrite magnet of Example 2.
FIG. 10 is a graph showing the relation between the amount of SiO2 added and a residual magnetic flux density Br in the sintered ferrite magnets of Examples 1-3.
FIG. 11 is a graph showing the relation between the amount of SiO2 added and coercivity HcJ in the sintered ferrite magnets of Examples 1-3.
FIG. 12 is a graph showing the relation between the amount of SiO2 and a squareness ratio Hk/HcJ in the sintered ferrite magnets of Examples 1-3.
FIG. 13 is a graph showing the relation between CaO/SiO2 and coercivity HcJ in the sintered ferrite magnets of Examples 1-3.
FIG. 14 is a graph showing the relation between the amount of SiO2 added and a residual magnetic flux density Br in the sintered ferrite magnet of Example 4.
FIG. 15 is a graph showing the relation between the amount of SiO2 added and coercivity HcJ in the sintered ferrite magnet of Example 4.
FIG. 16 is a graph showing the relation between the amount of SiO2 added and a squareness ratio Hk/HcJ in the sintered ferrite magnet of Example 4.
FIG. 17 is a graph showing the relation between CaO/SiO2 and coercivity HcJ in the sintered ferrite magnet of Example 4.
FIG. 18 is a graph showing the relation between the amount of SiO2 added and a residual magnetic flux density Br in the sintered ferrite magnet of Example 5.
FIG. 19 is a graph showing the relation between the amount of SiO2 added and coercivity HcJ in the sintered ferrite magnet of Example 5.
FIG. 20 is a graph showing the relation between the amount of SiO2 added and a squareness ratio Hk/HcJ in the sintered ferrite magnet of Example 5.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
[1] Sintered Ferrite Magnet
The sintered ferrite magnet of the present invention comprises
(a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca1-x-yRxAyFe2n-zCoz, wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:

0.3≦(1-x-y)≦0.65,

0.2≦x≦0.65,

0≦y≦0.2,

0.03≦z≦0.65, and

4≦n≦7; and

(b) a grain boundary phase indispensably containing Si, the amount of the above Si being more than 1% by mass and 1.8% or less by mass (calculated as SiO2), based on the entire sintered ferrite magnet.
The Ca content (1-x-y) is 0.3≦(1-x-y)≦0.65. When Ca is less than 0.3, the amounts of the elements R and A are relatively large, undesirably resulting in low Br and Hk/HcJ. When Ca is more than 0.65, the amounts of the elements R and A are relatively small, undesirably resulting in low Br and Hk/HcJ. The range of (1-x-y) is preferably 0.35≦(1-x-y)≦0.55, more preferably 0.42≦(1-x-y)≦0.5.
The element R is at least one of rare earth elements, indispensably including La. To obtain high magnetic properties, the percentage of La in the element R is preferably 50 atomic % or more, more preferably 70 atomic % or more, most preferably La alone except for inevitable impurities. because La is most dissolvable in the M phase among the elements R, the larger percentage of La provides larger effects of improving magnetic properties. The amount (x) of the element R is 0.2≦x≦0.65. When x is less than 0.2 or more than 0.65, low Br and Hk/HcJ are provided. The range of x is preferably 0.4≦x≦0.6, more preferably 0.45≦x≦0.55.
The element A is Ba and/or Sr. The amount (y) of the element A is 0≦y≦0.2. Although the effects of the present invention would not be deteriorated even if the element A were not added, the addition of the element A makes crystals finer in the calcined body with smaller aspect ratios, resulting in improved HcJ. The range of y is 0≦y≦0.15, more preferably 0≦y≦0.08.
The amount (z) of Co is 0.03≦z≦0.65. When z is less than 0.03, the addition of Co does not provide effects of improving magnetic properties. Also, because unreacted α-Fe2O3 remains in the calcined body, a slurry tends to leak from a mold cavity during wet molding. When z is more than 0.65, undesired phases containing a large amount of Co are formed, resulting in largely decreased magnetic properties. The range of z is preferably 0.1≦z≦0.4, more preferably 0.2≦z≦0.3.
Part of Co may be substituted by at least one of Zn, Ni and Mn. Particularly, the partial substitution of Co with Ni and Mn reduces production cost without lowering the magnetic properties. Also, the partial substitution of Co with Zn improves Br, despite slightly reduced HcJ. The total amount of substituting Zn, Ni and Mn is preferably 50 mol % or less of Co.
In the Ca—La—Co ferrite, increase in the amounts of Co and La essentially results in improvement in HcJ. However, because Co and La are rare metals, which are rare and expensive, their contents are desirably as small as possible for the purposes of reducing resource consumption and lowering the prices of sintered ferrite magnets. Because HcJ can be extremely improved while maintaining high Br and squareness ratio Hk/HcJ by the present invention, the amounts of Co and La can be reduced to provide magnets having the same HcJ as that of the conventional sintered Ca—La—Co ferrite magnets. It makes it possible to obtain HcJ on a practical level even in compositions containing Co at an atomic ratio of 0.2, whose practical use has been difficult because high HcJ cannot be expected, thereby providing inexpensive, high-performance sintered ferrite magnets containing reduced amounts of Co and La.
The value of n represents a molar ratio of (Fe+Co) to (Ca+R+A), 2n=(Fe+Co)/(Ca+R+A). The molar ratio n is preferably 4≦n≦7. When n is less than 4, the magnet contains a large percentage of non-magnetic portions with excessively flat calcined particles, resulting in drastically reduced HcJ. When n is more than 7, the unreacted α-Fe2O3 remains in the calcined body, resulting in the leakage of a slurry from a mold cavity during wet molding. The range of n is preferably 4.5≦n≦6, more preferably 4.8≦z≦5.2.
Si is an indispensable element, and contained in an amount of more than 1% by mass and 1.8% or less by mass (calculated as SiO2) based on the entire magnet. Si is preferably added in the form of SiO2 to the calcined ferrite, and the amount of SiO2 added is preferably more than 1% by mass and 1.8% or less by mass based on 100% by mass of the calcined body. SiO2 added to the calcined body essentially remains in the sintered magnet, too, without change. It should be noted, however, that some of SiO2 may flow out in the pulverization step and the molding step, resulting in a smaller amount in the sintered magnet than in the calcined body. Si essentially forms the grain boundary phase, without being contained in the ferrite phase having a hexagonal M-type magnetoplumbite structure. The preferred range of Si is 1.1-1.6% by mass (calculated as SiO2).
A molar ratio range x/z of the element R to Co is preferably 0.73≦x/z≦15.62, more preferably 1≦x/z≦3, most preferably 1.2≦x/z≦2. By selecting the composition meeting these values, the magnetic properties can be improved.
In the present invention, large effects of improving magnetic properties are obtained when the amount of the element R>the amount of Co>the amount of the element A, namely, x>z>y. Also, when the amount of Ca>the amount of the element A, namely, (1-x-y)>y, large effects of improving magnetic properties are obtained.
The sintered ferrite magnet of the present invention, and the calcined body used for producing it have a ferrite phase having a hexagonal M-type magnetoplumbite structure. The term “having a hexagonal M-type magnetoplumbite structure” used herein means that the X-ray diffraction pattern of the hexagonal M-type magnetoplumbite structure is mainly observed when the X-ray diffraction of the sintered ferrite magnet or the calcined body is measured under general conditions. There may be undesired phases (orthoferrite phase, spinel phase, etc.) and impurity phases in extremely small amounts (about 5% or less by mass) observed by X-ray diffraction, etc. The quantitative measurement of undesired phases by X-ray diffraction can be conducted by a Rietveld analysis method.
The sintered ferrite magnet of the present invention indispensably comprises the ferrite phase having a hexagonal M-type magnetoplumbite structure, and the grain boundary phase indispensably containing Si. Because the grain boundary phase cannot easily be observed by an X-ray diffraction pattern, it is preferably confirmed by a transmission electron microscope, etc.
In the present invention, both the calcined ferrite and the sintered ferrite magnet have the same ferrite phase, which has a hexagonal M-type magnetoplumbite structure. The calcined ferrite preferably contains the ferrite phase as a main component for the orientation of crystals in the molding and structure control in the sintering, the existence of the grain boundary phase being not critical. On the other hand, the sintered ferrite magnet contains the ferrite phase as a main component, and needs the grain boundary phase indispensably containing Si for structure control and densification in the sintering process.
With the above preferred ranges, the sintered ferrite magnet of the present invention has magnetic properties such as coercivity HcJ of 450 kA/m or more, a residual magnetic flux density Br of 0.4 T or more, and a squareness ratio Hk/HcJ of 80% or more. With more preferred ranges, it has magnetic properties such as coercivity HcJ of 460 kA/m or more, a residual magnetic flux density Br of 0.44 T or more, and a squareness ratio Hk/HcJ of 80% or more.
[2] Production Method of Sintered Ferrite Magnet
The sintered ferrite magnet is produced by a step of preparing calcined ferrite, a step of pulverizing the calcined ferrite to powder, a step of molding the powder to green body, and a step of sintering the green body to a sintered body. Before the pulverization step, more than 1% by mass and 1.8% or less by mass of SiO2 can be added to 100% by mass of the calcined body to provide the sintered magnet with remarkably improved coercivity HcJ.
The calcined ferrite comprises a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, the element R which is at least one of rare earth elements and indispensably includes La, the element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca1-x-yRxAyFe2n-zCoz, wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations: 0.3≦(1-x-y)≦0.65, 0.2≦x≦0.65, 0≦y≦0.2, 0.03≦z≦0.65, and 4≦n≦7.
The composition of the calcined ferrite including 0 (oxygen) is represented by the general formula of Ca1-x-yRxAyFe2n-zCozOα, wherein (1-x-y), x, y, z, α represents the atomic ratios of Ca, the element R, the element A, Co and O, and n represents a molar ratio, meeting 0.3≦(1-x-y)≦0.65, 0.2≦x≦0.65, 0≦y≦0.2, 0.03≦z≦0.65, and 4≦n≦7, and α=19 at a stoichiometric composition ratio in which the element R and Fe are trivalent, Co is bivalent, x=z, and n=6.
In the composition of the above calcined ferrite including O (oxygen), the mole ratio of oxygen may differ depending on the valences of Fe and Co, the value of n, and the kind of the element R. Also, in the sintered ferrite magnet, a ratio of oxygen to the metal element may change depending on the vacancy of oxygen when sintered in a reducing atmosphere, the change of the valences of Fe and Co in the ferrite phase, etc. Accordingly, the actual molar ratio α of oxygen may be deviated from 19. Therefore, the composition of metal elements, which are most easily identified, are used in the present invention.
(1) Step of Preparing Calcined Ferrite
The calcined ferrite is produced by mixing oxide powders and powders of compounds such as Ca compounds, compounds of the elements R, Ba and/or Sr compounds, if necessary, iron compounds, and Co compounds, which are converted to oxides by calcining, to the above composition; and calcining (ferritizing) the resultant mixture. The composition ranges of Ca, the element R, the element A, Fe and Co are restricted for the same reasons as in the case of the sintered ferrite magnet.
Usable as the starting material powders are powders of metals in the form of oxides, carbonates, hydroxides, nitrates, chlorides, etc., regardless of their valences. Solutions of starting material powders may also be used. Used as the Ca compounds are carbonates, oxides, chlorides, etc. of Ca. Used as the compounds of the elements R are oxides such as La2O3, hydroxides such as La(OH)3, carbonates such as La2(CO3)3.8H2O, etc. Particularly, the oxides, hydroxides, carbonates, etc. of mixed rare earth elements (La, Nd, Pr, Ce, etc.) are preferable because they are inexpensive, enabling cost reduction. Used as the compounds of the elements A are carbonates, oxides, chlorides, etc. of Ba and/or Sr. Used as the iron compounds are iron oxides, iron hydroxides, iron chlorides, mill scales, etc. Used as the Co compounds are oxides such as CoO, Co3O4, etc., hydroxides such as CoOOH, Co(OH)2, Co3O4.m1H2O (m1 is a positive number), etc., carbonates such as CoCO3, and basic carbonates such as m2CoCO3.m3Co(OH)2.m4H2O, etc. (m2, m3 and m4 are positive numbers).
Other starting material powders than CaCO3, Fe2O3 and La2O3 may be added at the time of mixing starting materials or after the calcination. For example, the sintered ferrite magnet can be produced, (1) by mixing CaCO3, Fe2O3, La2O3 and Co3O4, calcining the resultant mixture, and pulverizing, molding and sintering the calcined body, or (2) by mixing CaCO3, Fe2O3 and La2O3, calcining the resultant mixture, adding Co3O4 to the calcined body, and pulverizing, molding and sintering the calcined body.
To accelerate a reaction during calcining, about 1% or less by mass of a B-containing compound such as B2O3, H3BO3, etc. may be added, if necessary. Particularly, the addition of H3BO3 is effective for improving HcJ and Br. The amount of H3BO3 added is more preferably 0.3% or less by mass, most preferably about 0.2% by mass. Less than 0.1% by mass of H3BO3 provides only a small effect of improving Br, and more than 0.3% by mass of H3BO3 reduces Br. Because H3BO3 has effects of controlling the shapes and sizes of crystal grains during sintering, it may be added after calcining (before fine pulverization or before sintering), or both before and after calcining.
The mixing of the starting material powders may be conducted in a wet or dry state. When stirred with media such as steel balls, etc., the starting material powders can be mixed more uniformly. In the case of wet mixing, the solvent is preferably water. To increase the dispersibility of the starting material powders, known dispersants such as ammonium polycarboxylate, calcium gluconate, etc. may be used. A slurry of the starting materials is dewatered to obtain mixed starting material powders.
The mixed starting material powders are heated in an electric furnace, a gas furnace, etc. to cause a solid-phase reaction, thereby forming a ferrite compound having a hexagonal M-type magnetoplumbite structure. This process is called “calcining,” and the resultant compound is called “calcined body.”
The calcining step is conducted preferably in an atmosphere having an oxygen concentration of 5% or more. The oxygen concentration of less than 5% results in abnormal grain growth, the formation of undesired phases, etc. More preferably, the oxygen concentration is 20% or more.
In the calcining step, the solid-phase reaction forming the ferrite phase proceeds as the temperature is elevated, and is completed at about 1100° C. When the calcining temperature is lower than 1100° C., the unreacted hematite (iron oxide) remains, resulting in low magnetic properties. On the other hand, when the calcining temperature is higher than 1450° C., crystal grains grow excessively, likely needing a long period of time for the pulverization step. Accordingly, the calcining temperature is preferably 1100-1450° C., more preferably 1200-1350° C. The calcining time is preferably 0.5-5 hours.
When H3BO3 is added before calcining, the ferritization reaction is accelerated, enabling calcining at 1100-1300° C.
(2) Addition of SiO2 
The production method of the present invention is characterized by adding more than 1% by mass and 1.8% or less by mass of SiO2 to 100% by mass of the calcined body before the pulverization step. This specifically improves HcJ. The addition of 1% or less by mass of SiO2 fails to obtain an effect of improving HcJ, and the addition of more than 1.8% by mass of SiO2 undesirably reduces HcJ, Br and Hk/HcJ. The more preferred amount of SiO2 added is 1.1-1.6% by mass.
Though SiO2 is most preferably added to the calcined body, part of SiO2 may be added before calcining (when mixing the starting material powders). With SiO2 added before calcining, the size control of crystal grains can be conducted in calcining.
(3) Addition of CaCO3 
Depending on the amount of SiO2 added, 1-2% by mass (calculated as CaO) of CaCO3 is preferably added to 100% by mass of the calcined body before the pulverization step. The addition of CaCO3 prevents the decrease of Br and Hk/HcJ as much as possible, resulting in sintered ferrite magnets having high HcJ with high Br and Hk/HcJ maintained, which have not been achieved conventionally. When the amount of CaCO3 (calculated as CaO) is less than 1% by mass or more than 2% by mass, Br and Hk/HcJ are undesirably low. The amount of CaCO3 added is more preferably 1.2-2% by mass.
The preferred amount of SiO2 added may slightly change depending on the amount of CaCO3 (calculated as CaO). As shown in Examples below, it slightly changes also depending on the amount (z) of Co. For example, from the aspect of improving HcJ, increase in the amount of CaCO3 added tends to shift the preferred amount of SiO2 to a higher side, regardless of the amount (z) of Co. Also, decrease in the amount of Co tends to shift the preferred amount of SiO2 to a higher side. However, too much SiO2 reduces Br and Hk/HcJ. To achieve high HcJ while maintaining high Br and Hk/HcJ, it is preferable to add SiO2 in an amount of 1.1-1.5% by mass and CaCO3 in an amount of 1.2-2% by mass (calculated as CaO) when the amount (z) of Co is z≧0.3, and to add SiO2 in an amount of 1.4-1.6% by mass and CaCO3 in an amount of 1.5-2% by mass (calculated as CaO) when the amount (z) of Co is z<0.3. Considering both cases, as described above, the amount of SiO2 added is preferably 1.1-1.6% by mass. In this case, the amount (calculated as CaO) of CaCO3 added is preferably 1.2-2% by mass.
In the present invention, when both SiO2 and CaCO3 are added, the amounts of SiO2 and CaCO3 may be properly determined within the above ranges. As described above, it is preferable to add SiO2 in a range of 1.1-1.6% by mass and CaCO3 in a range of 1.2-2% by mass (calculated as CaO), providing sintered ferrite magnets with high HcJ, with high Br and Hk/HcJ maintained.
When both SiO2 and CaCO3 are added, the magnetic properties can be improved by adjusting a ratio of the amount (calculated as CaO) of CaCO3/the amount of SiO2 to 0.8-2. In this case, the preferred range of [CaCO3 (as CaO)/SiO2] slightly changes depending on the amount (z) of Co as shown in Examples below. When Z≧0.3, the ratio is preferably 1-1.7, more preferably 1.1-1.4. When z<0.3, the ratio is preferably 0.8-1.4, more preferably 0.9-1.1. Considering both cases of z≧0.3 and z<0.3, the ratio is preferably 0.8-1.7, more preferably 0.9-1.4. With [CaCO3 (as CaO)/SiO2] set in the above range, the sintered ferrite magnets has high HcJ, with high Br and Hk/HcJ maintained.
(4) Pulverization Step
The calcined body is pulverized by a vibration mill, a ball mill, an attritor, etc. to powder. The pulverized powder preferably has an average particle size of about 0.4-0.8 μm (measured by an air permeation method). The pulverization step may be either dry pulverization or wet pulverization, though their combination is preferable.
The wet pulverization uses water and/or non-aqueous solvents (organic solvents such as acetone, ethanol, xylene, etc.). The wet pulverization produces a slurry containing the calcined body in water (solvent). The slurry preferably contains known dispersant and/or surfactant in an amount of 0.2-2% by mass on a solid basis. After the wet pulverization, the slurry is preferably concentrated and kneaded.
In the pulverization step, Cr2O3, Al2O3, etc. may be added together with SiO2 and CaCO3 described above to improve the magnetic properties. Each of them is preferably 5% or less by mass.
Because the pulverized powder contains ultra-fine powder of less than 0.1 μm, which cause poor dewatering and molding defects, the pulverized powder is preferably heat-treated to remove the ultra-fine powder. The heat-treated powder is preferably pulverized again. Thus, using a pulverization step comprising a first fine pulverization step, a step of heat-treating powder obtained by the first fine pulverization step, and a second fine pulverization step of pulverizing the heat-treated powder again, which is called “heat-treating, repulverizing step,” HcJ can be further improved in addition to the effect of improving HcJ by the addition of SiO2 and CaCO3, thereby providing sintered ferrite magnets with extremely high HcJ, with high Br and Hk/HcJ maintained, which have not been obtained so far.
Ultra-fine powder of less than 0.1 μm is likely to be inevitably formed in a usual pulverization step, and the existence of ultra-fine powder lowers HcJ, results in poor dewatering in the molding step, provides green bodies with defects, and deteriorates pressing cycles because too much time is needed for dewatering. When the powder containing ultra-fine powder, which is obtained by the first fine pulverization step, is heat-treated, a reaction occurs between powder having relatively large particle sizes and the ultra-fine powder, resulting in decrease in the amount of the ultra-fine powder. In the second fine pulverization step, the particle sizes are controlled with necking removed, to produce powder with predetermined particle sizes. Thus, powder having an excellent particle size distribution with a small percentage of ultra-fine powder can be obtained, thereby improving HcJ, and thus solving the above problems in the molding step.
Utilizing the effect of improving HcJ by the heat-treating, repulverizing step, the same HcJ as when using powder having an average particle size of about 0.4-0.8 μm, which is produced by a usual pulverization step, is obtained, even if powder obtained by the second fine pulverization step has relatively large particle sizes (for example, an average particle size of about 0.8-1.0 μm). Accordingly, time saving can be achieved by the second fine pulverization step, with improved dewatering and pressing cycle.
As described above, though the heat-treating, repulverizing step provides various advantages, cost increase due to increased production steps cannot be avoided. However, magnetic properties are much more improved by the heat-treating, repulverizing step in the production of the sintered ferrite magnet of the present invention than in the production of conventional sintered ferrite magnets, canceling the above cost increase. Accordingly, the heat-treating, repulverizing step is practically significant in the present invention.
The first fine pulverization may be the same as the above-described usual pulverization, using a vibration mill, a jet mill, a ball mill, an attritor, etc. The pulverized powder preferably has an average particle size of about 0.4-0.8 μm (measured by an air permeation method). The pulverization step may be either dry pulverization or wet pulverization, though their combination is preferable.
After the first fine pulverization step, a heat treatment is conducted preferably at 600-1200° C., more preferably 800-1100° C. Though not particularly restricted, the heat treatment time is preferably 1 second to 100 hours, more preferably about 1-10 hours.
The second fine pulverization after the heat treatment step uses a vibration mill, a jet mill, a ball mill, an attritor, etc., like the first fine pulverization. Because the desired particle sizes are already substantially obtained by the first fine pulverization step, the control of particle sizes and the removal of necking are mainly conducted in the second fine pulverization step. Accordingly, milder pulverization conditions due to the shortened pulverization time, etc. are preferably used in the second fine pulverization step than in the first fine pulverization step. Pulverization under the same conditions as in the first fine pulverization step undesirably forms ultra-fine powder again.
The average particle size of the powder after the second fine pulverization is preferably about 0.4-0.8 μm (measured by an air permeation method) as in a usual pulverization step when seeking higher HcJ than that of sintered ferrite magnets obtained by the usual pulverization step, and preferably 0.8-1.2 μm, more preferably about 0.8-1.0 μm (measured by an air permeation method) when seeking the saving of time in the pulverization step, improving dewatering and a pressing cycle, etc.
(5) Molding Step
A slurry after the pulverization is press-molded with or without a magnetic field while removing water (solvent). Press-molding in a magnetic field aligns the crystal orientation of powder particles, thereby drastically improving the magnetic properties. To improve the orientation further, a dispersant and a lubricant may be added in an amount of 0.01-1% by mass. Also, the slurry may be concentrated before molding, if necessary. The concentration is preferably conducted by centrifugal separation, filter pressing, etc.
(6) Sintering Step
The press-molded green body is degreased if necessary, and then sintered. The sintering is conducted in an electric furnace, a gas furnace, etc.
The sintering is conducted preferably in an atmosphere having an oxygen concentration of 10% or more. When the oxygen concentration is less than 10%, abnormal grain growth, the formation of undesired phases, etc. occur, resulting in deteriorated magnetic properties. The oxygen concentration is more preferably 20% or more, most preferably 100%.
The sintering temperature is preferably 1150-1250° C. The sintering time is preferably 0.5-2 hours. The sintered magnet has an average crystal particle size of about 0.5-2 μm.
After the sintering step, the sintered ferrite magnet is obtained through known production steps such as a machining step, a washing step, an inspection step, etc.
The present invention will be explained in more detail referring to Examples without intention of restricting it thereto.
EXAMPLE 1
CaCO3 powder, La(OH)3 powder, Fe2O3 powder and Co3O4 powder were mixed to have the formula of Ca1-x-yLaxAyFe2n-zCO3O19-δ, wherein x=0.5, y=0, z=0.3, n=5.2, and δ≧0, and 0.1% by mass of H3BO3 powder was added to the total amount (100% by mass) of the mixed powders to produce a starting material powder. This starting material powder was mixed by a wet ball mill for 4 hours, and granulated by drying. It was calcined at 1300° C. for 3 hours in the air, and the calcined body was coarsely pulverized by a hammer mill to obtain coarse powder.
SiO2 powder and CaCO3 powder (calculated as CaO) were added in the amounts shown in Table 1 to the above coarse powder, and fine pulverization was conducted by a wet ball mill using water as a solvent until its average particle size (measured by an air permeation method) became 0.55 μm. Each of the resultant fine powder slurries was molded in a magnetic field of about 1.3 T under pressure of about 50 MPa with a compression direction parallel to the direction of the magnetic field, while removing the solvent. Each of the resultant green bodies was sintered at 1200° C. for 1 hour in the air to obtain a sintered magnet.
Each sintered magnet was measured with respect to a residual magnetic flux density Br, coercivity HcJ, and a squareness ratio Hk/HcJ, wherein Hk was the value of H at J of 0.95B, on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant. The measurement results are shown in FIGS. 1-3. In FIGS. 1-3, the axes of abscissas represent the amount (% by mass) of SiO2 added, and the axes of ordinates represent a residual magnetic flux density Br (T) (FIG. 1), coercivity HcJ (FIG. 2) and a squareness ratio Hk/HcJ (FIG. 3). In the figures, data obtained with the same amount of CaO were connected by straight lines. A dotted line at Hk/HcJ=80% in FIG. 3 is a line as a measure for practical use. Even if a sintered magnet had high Br and HcJ, it would not be able to be made thin when its Hk/HcJ is less than 80%, substantially failing to be useful for various applications such as electronic parts for automobiles, parts for electric appliances, etc. The same is true of Examples below.












TABLE 1







Sample
CaO
SiO2




No.
(% by mass)
(% by mass)
CaO/SiO2









101
0.7
0.0
—



102
0.7
0.3
2.33



103
0.7
0.6
1.17



104
1.0
0.0
—



105
1.0
0.3
3.33



106
1.0
0.6
1.67



107
1.0
0.9
1.11



108*
1.0
1.2
0.83



109*
1.3
1.1
1.18



110
1.5
0.3
5.00



111
1.5
0.6
2.50



112
1.5
0.9
1.67



113*
1.5
1.2
1.25



114*
1.5
1.5
1.00



115
1.5
1.9
0.80



116
2.0
0.6
3.33



117
2.0
0.9
2.22



118*
2.0
1.2
1.67



119*
2.0
1.5
1.33



120*
2.0
1.8
1.11







Note:



*Within the present invention.






As is clear from FIGS. 1-3, high magnetic properties are obtained when more than 1% by mass of SiO2 and 1% or more by mass of CaO are added, and higher magnetic properties are obtained when 1.1-1.5% by mass of SiO2 and when 1.3-2% by mass of CaO are added. Particularly, Sample 113 to which 1.2% by mass of SiO2 and 1.5% by mass of CaO were added, and Sample 119 to which 1.5% by mass of SiO2 and 2.0% by mass of CaO were added had specifically improved HcJ with maximally suppressed decrease in Br and KAU.
FIG. 4 compares Samples 113 and 119, and samples containing 1% or less by mass of SiO2, which are shown in FIG. 2. It is clear that as compared with samples to which 1% or less by mass of SiO2 was added, which have conventionally been considered optimum, Sample 113 to which 1.2% by mass of SiO2 and 1.5% by mass of CaO were added, and Sample 119 to which 1.5% by mass of SiO2 and 2.0% by mass of CaO were added had HcJ improved by about 20% or more.
As described above, because Ca—La—Co ferrite in which the atomic ratio of Co was 0.3 had an anisotropic magnetic field HA of 2.1 MA/m, the HcJ (400 kA/m or less) of the samples containing 1% or less by mass of SiO2 was about 19% or less of the above anisotropic magnetic field HA, but the HcJ of Samples 113 and 119 was about 23% of the above anisotropic magnetic field HA, closer to the inherent potential of the material. Further, taking into consideration Samples 113 and 119 having CaO/SiO2 of 1.25 and 1.33, and Sample 109 having better magnetic properties, it has been found that excellent magnetic properties can be obtained at a ratio of CaO/SiO2 in a range of about 1.1-1.4.
FIG. 5 compares Sample 103 (CaO=0.7% by mass, and SiO2=0.6% by mass), Sample 107 (CaO=1.0% by mass, and SiO2=0.9% by mass), Sample 109 (CaO=1.3% by mass, and SiO2=1.1% by mass), Sample 113 (CaO=1.5% by mass, and SiO2=1.2% by mass), and Sample 119 (CaO=2.0% by mass, and SiO2=1.5% by mass), which exhibit the highest HcJ at the CaO content of 0.7% by mass, 1.0% by mass, 1.3% by mass, 1.5% by mass, and 2.0% by mass, respectively, in FIG. 2. In FIG. 5, the axis of abscissas represents the amount (% by mass) of CaO added, and the axis of ordinates represents coercivity HcJ. As is clear from FIG. 5, as the amount of CaO added increases, the coercivity HcJ is improved. Particularly when the amount of CaO added was 1.5% by mass and 2% by mass, the highest coercivity HcJ was obtained. Also, when the amount of CaO added was in a range of 1.2-2% by mass, the coercivity HcJ was about 20% or more of the anisotropic magnetic field HA of Ca—La—Co ferrite in which the atomic ratio of Co was 0.3, and when the amount of CaO added was in a range of 1.5-2% by mass, the coercivity HcJ was about 23% of the above anisotropic magnetic field HA.
With respect to the sintered magnets of Samples 113 and 114, the amounts of SiO2 and CaO added and those measured by inductively coupled plasma (ICP) atomic emission spectroscopy are shown in Table 2. The amounts of SiO2 and CaO added are expressed by “% by mass” determined based on the entire composition.











TABLE 2








Amount of SiO2

Amount of CaO



Sample
(% by mass)

(% by mass)











No.
Measured
Added
Measured
Added





113
1.19
1.17
3.98
4.37


114
1.66
1.46
3.96
4.36









As shown in Table 2, SiO2 added to the calcined body remained in the sintered magnet. The amount of SiO2 measured was larger than that added, presumably because the amounts of other elements than SiO2 became smaller than when added.
EXAMPLE 2
Sintered magnets were produced in the same manner as in Example 1, except that SiO2 powder and CaCO3 powder (calculated as CaO) were added in the amounts shown in Table 3 to a composition having the formula of Ca1-x-yLaxAyFe2n-zCozO19-δ, wherein x=0.5, y=0, z=0.2, n=4.8, and δ≧0.
The sintered magnets were measured with respect to a residual magnetic flux density Br, coercivity HcJ, and a squareness ratio Hk/HcJ. The measurement results are shown in FIGS. 6-8. As in Example 1, in FIGS. 6-8, the axes of abscissas represent the amount (% by mass) of SiO2 added, and the axes of ordinates represent a residual magnetic flux density Br (T) (FIG. 6), coercivity HcJ (FIG. 7), and a squareness ratio Hk/HcJ (FIG. 8), data at the same CaO content being connected by a straight line.












TABLE 3







Sample
CaO
SiO2




No.
(% by mass)
(% by mass)
CaO/SiO2









201
0.6
0.2
3.00



202
0.6
0.4
1.50



203
0.6
0.6
1.00



204
0.6
0.8
0.75



205
0.6
1.0
0.60



206
1.0
0.2
5.00



207
1.0
0.4
2.50



208
1.0
0.6
1.67



209
1.0
0.8
1.25



210
1.0
1.0
1.00



211
1.5
1.0
1.50



212*
1.5
1.2
1.25



213*
1.5
1.4
1.07



214*
1.5
1.5
1.00



215*
1.5
1.6
0.94



216*
1.5
1.8
0.83



217
1.5
2.0
0.75



218
2.0
1.0
2.00



219*
2.0
1.5
1.33



220
2.0
2.0
1.00







Note:



*Within the present invention.






As is clear from FIGS. 6-8, even in the compositions in which the amount of Co was changed, as in Example 1, high magnetic properties were obtained when more than 1% by mass of SiO2 and 1% or more by mass of CaO were added, and better magnetic properties were obtained when 1.4-1.6% by mass of SiO2 and 1.5-2% by mass of CaO were added. Particularly, Sample 215 to which 1.6% by mass of SiO2 and 1.5% by mass of CaO were added had specifically improved HcJ, while suppressing decrease in Br and Hk/HcJ as much as possible.
FIG. 9 compares Sample 215 to which 1.6% by mass of SiO2 and 1.5% by mass of CaO were added, and samples to which 1% or less by mass of SiO2 was added, which are shown in FIG. 7. As is clear from FIG. 9, Sample 215 to which 1.6% by mass of SiO2 and 1.5% by mass of CaO were added had HcJ improved by about 27%, as compared with samples containing 1% or less by mass of SiO2, which have conventionally been considered optimum.
SPD measurement revealed that Ca—La—Co ferrite in which the atomic ratio of Co was 0.2 had an anisotropic magnetic field HA of 1.9 MA/m (about 23.9 kOe). Accordingly, samples to which 1% or less by mass of SiO2 was added had HcJ (300 kA/m or less), which was about 16% or less of the above anisotropic magnetic field HA, Sample 215 to which 1.6% by mass of SiO2 and 1.5% by mass of CaO were added had HcJ, which was about 20% of the above anisotropic magnetic field HA, close to the inherent potential of the material. Further, CaO/SiO2 was 0.94 in Sample 215. It was found that excellent magnetic properties were obtained at a CaO/SiO2 ratio in a range of about 0.9-1.1.
EXAMPLE 3
Sintered magnets were produced in the same manner as in Example 1, except that SiO2 powder and CaCO3 powder (calculated as CaO) were added in the amounts shown in Table 4 to a composition having the formula of Ca1-x-yLaxAyFe2n-zCOzO19-δ, wherein x=0.5, y=0, z=0.25, n=5.0, and δ≧0.
The sintered magnets were measured with respect to a residual magnetic flux density Br, coercivity HcJ, and a squareness ratio Hk/HcJ. The measurement results are shown in FIGS. 10-12. As in Example 1, in FIGS. 10-12, the axes of abscissas represent the amount (% by mass) of SiO2 added, and the axes of ordinates represent a residual magnetic flux density Br (T) (FIG. 10), coercivity HcJ (FIG. 11), and a squareness ratio Hk/HcJ (FIG. 12). To investigate influence by the Co content (z) in FIGS. 10-12, Samples 111-115 in Example 1 and Samples 211-217 in Example 2 were plotted, and data at the same Co content (z) were connected by a straight line.












TABLE 4







Sample
CaO
SiO2




No.
(% by mass)
(% by mass)
CaO/SiO2









301
1.5
0.9
2.00



302*
1.5
1.2
1.33



303*
1.5
1.5
1.00



304
1.5
1.9
0.78







Note:



*Within the present invention.






As is clear from FIGS. 10-12, even at the Co content (z) of 0.25, as in Examples 1 and 2, high magnetic properties were obtained when more than 1% by mass of SiO2 and 1% or more by mass of CaO were added. Particularly when SiO2 was 1.5% by mass, HcJ was specifically improved while suppressing decrease in Br and Hk/HcJ as much as possible.
It is also clear from FIGS. 10-12 that the preferred amount of SiO2 differs at z of 0.3, 0.25 and 0.2, and that as the Co content (z) decreases, the preferred amount of SiO2 tends to shift toward a higher side. Namely, the amount of SiO2 providing high HcJ while maintaining high Br and Hk/HcJ was in a range of 1.1-1.2% by mass when the Co content (z) was 0.3, and in a range of 1.4-1.6% by mass when the Co content (z) was 0.25 and 0.2, respectively.
FIG. 13 shows these data, the axis of abscissas representing CaO/SiO2, and the axis of ordinates representing coercivity HcJ, and data at the same Co content (z) being connected by a straight line. As is clear from FIG. 13, when both SiO2 and CaO are added, magnetic properties can be improved by setting CaO/SiO2 in a range of 0.8-2.0. In this case, it is clear that the preferred range of CaO/SiO2 slightly differs between when z=0.3 and when z<0.3 (z=0.25, z=0.2). Namely, the range of CaO/SiO2 for achieving high HcJ while maintaining high Br and Hk/HcJ is preferably 1-1.7, more preferably 1.2-1.4, when z=0.3, and preferably 0.8-1.4, more preferably 0.9-1.1, when z<0.3 (z=0.25, z=0.2). Considering both cases of z≧0.3 and z<0.3, it is preferably 0.8-1.7, more preferably 0.9-1.4.
It is clear from FIG. 11 that a lower Co content (z) tends to provide a lower maximum value of HcJ. However, as considered in Examples 1 and 2, HcJ was improved by 20% or more in the same Co content (z), as compared with samples to which 1% or less by mass of SiO2 (conventionally considered optimum) was added, the improvement of HcJ being 4% or more of the anisotropic magnetic field HA. It was thus confirmed that the sintered magnets had HcJ close to potential inherent in the materials. Accordingly, when magnets having the HcJ same as that of conventional sintered Ca—La—Co ferrite magnets are provided, the amounts of rare and expensive Co and La can be reduced. As in Example 2, even for compositions having a Co content z of 0.2, which are not expected to have high HcJ so that it has been difficult to put them into practical use, HcJ on a practical level can be obtained, thereby providing high-performance sintered ferrite magnets with reduced amounts of Co and La at a low cost.
EXAMPLE 4
Sintered magnets were produced in the same manner as in Example 1, except that CaCO3 powder, La(OH)3 powder, SrCO3 powder, BaCO3 powder, Fe2O3 powder and Co3O4 powder were mixed to provide the formula of Ca1-x-y1-y2LaxSry1Bay2Fe2n-zCozO19-δ wherein x=0.5, y1+y2=0.05, z=0.3, n=5.3, and δ≧0, and that the amounts of SiO2 powder and CaCO3 powder (calculated as CaO) relative to the total amount of these mixed powders were changed as shown in Table 5. The atomic ratios of Sr (y1) and Ba (y2) are also shown in Table 5. Samples 111-115 are sintered magnets evaluated in Example 1.
The sintered magnets were measured with respect to a residual magnetic flux density Br, coercivity HcJ and a squareness ratio Hk/HcJ, wherein Hk is the value of H, at which J is 0.95Br, on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant. The measurement results are shown in FIGS. 14-16. In FIGS. 14-16, the axes of abscissas represent the amount (% by mass) of SiO2 added, and the axes of ordinates represent a residual magnetic flux density Br (T) (FIG. 14), coercivity HcJ (FIG. 15) and a squareness ratio Hk/HcJ (FIG. 16), respectively. In the figures, data connected by a straight line are denoted by “Unsubstituted” representing Samples 111-115, “Ba” representing Samples 401-404 to which only Ba was added, “Sr” representing Samples 405-408 to which only Sr was added, and “Sr+Ba” representing Samples 409-412 to which both Sr and Ba were added.












TABLE 5






Atomic
Atomic





Sample
Ratio
Ratio
CaO
SiO2



No.
of Sr (y1)
of Ba (y2)
(% by mass)
(% by mass)
CaO/SiO2




















401
0
0.05
1.5
0.6
2.50


402
0
0.05
1.5
0.9
1.67


403*
0
0.05
1.5
1.2
1.25


404*
0
0.05
1.5
1.5
1.00


405
0.05
0
1.5
0.9
1.67


406*
0.05
0
1.5
1.2
1.25


407*
0.05
0
1.5
1.5
1.00


408
0.05
0
1.5
1.9
0.80


409
0.025
0.025
1.5
0.9
1.67


410*
0.025
0.025
1.5
1.2
1.25


411*
0.025
0.025
1.5
1.5
1.00


412
0.025
0.025
1.5
1.9
0.80


111
0
0
1.5
0.6
2.50


112
0
0
1.5
0.9
1.67


113*
0
0
1.5
1.2
1.25


114*
0
0
1.5
1.5
1.00


115
0
0
1.5
1.9
0.80





Note:


*Within the present invention.






As is clear from FIGS. 14-16, the compositions to which Sr and/or Ba were added also had high magnetic properties when more than 1% by mass of SiO2 and 1% or more by mass of CaO were added, as in Examples 1-3 not containing Sr and Ba.
FIG. 17 is a graph showing these data plotted with the axis of abscissas being CaO/SiO2, and the axis of ordinates being coercivity HcJ. As is clear from FIG. 17, the compositions to which Sr and/or Ba were added also had high magnetic properties at a CaO/SiO2 ratio of 0.8-2.0 when both SiO2 and CaO were added, as in Examples 1-3 not containing Sr and Ba.
EXAMPLE 5
Sintered magnets were produced in the same manner as in Example 1, except that SrCO3 powder, La(OH)3 powder, Fe2O3 powder and Co3O4 powder were mixed to have the formula of Sr1-xLaxFe2n-zCOzO19-δ, wherein x=0.2, z=0.2, n=5.8, and δ≧0, and that the amounts of SiO2 powder and CaCO3 powder (calculated as CaO) were changed as shown in Table 6. These sintered magnets are so-called Sr—La—Co ferrite magnets having basic compositions outside the present invention.
The sintered magnets were measured with respect to a residual magnetic flux density Br, coercivity HcJ, and a squareness ratio Hk/HcJ. The measurement results are shown in FIGS. 18-20.












TABLE 6







Sample
CaO
SiO2




No.
(% by mass)
(% by mass)
CaO/SiO2









501
0.7
0.0
—



502
0.7
0.3
2.33



503
0.7
0.6
1.17



504
1.0
0.0
—



505
1.0
0.3
3.33



506
1.0
0.6
1.67



507
1.0
0.9
1.11



508
1.0
1.2
0.83



509
1.5
0.3
5.00



510
1.5
0.6
2.50



511
1.5
0.9
1.67



512
1.5
1.2
1.25



513
1.5
1.5
1.00










It is clear from FIGS. 18-20 that when more than 1% by mass of SiO2 was added, Sr—La—Co ferrite magnets do not have good magnetic properties, with HcJ not extremely improved, and the squareness ratio lowered to less than 80%.
EXAMPLE 6
CaCO3 powder, La(OH)3 powder, Fe2O3 powder and Co3O4 powder were mixed to have the formula of Ca1-x-yLaxAyFe2n-zCOzO19-δ, wherein x=0.5, y=0, z=0.3, n=5.2, and δ≧0, and 0.1% by mass of H3BO3 powder was added to 100% by mass of the mixed powders to obtain a starting material powder. This starting material powder was mixed for 4 hours by a wet ball mill, and granulated by drying. It was calcined at 1300° C. for 3 hours in the air, and the calcined body was pulverized to coarse powder by a hammer mill.
1.2% by mass of SiO2 powder and 1.5% by mass (calculated as CaO) of CaCO3 powder were added to the coarse powder, and the first fine pulverization was conducted to an average particle size of 0.5 μm (measured by an air permeation method) by a wet ball mill with water as a solvent. The resultant powder was dried, and heat-treated at 1000° C. for 5 hours in the air. The heat-treated powder had an average particle size of 1.4 μm (measured by an air permeation method). The heat-treated powder was subject to the second fine pulverization to an average particle size of 0.8 μm by a wet ball mill. While removing the solvent, a fine powder slurry obtained by the above heat-treating, repulverizing step was molded in a magnetic field of about 1.3 T under pressure of about 50 MPa, with a compression direction parallel to the direction of a magnetic field. The resultant green body was sintered at 1200° C. for 1 hour in the air to obtain a sintered magnet.
In addition, a sintered magnet was produced in the same manner as above, except for conducting only the first fine pulverization to an average particle size of 0.8 μm (measured by an air permeation method) without the heat treatment and the second fine pulverization.
The sintered magnets were measured with respect to a residual magnetic flux density Br, coercivity HcJ, and a squareness ratio Hk/HcJ, wherein Hk was the value of H at J of 0.95Br on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant. The measurement results are shown in Table 7. Table 7 shows Sample 601 pulverized in the heat-treating, repulverizing step, and Sample 602 pulverized only in the first fine pulverization.












TABLE 7







Sample
Br
HcJ
Hk/HcJ



No.
(T)
(kA/m)
(%)









601
0.438
494.0
80.1



602
0.436
387.9
80.4










As is clear from Table 7, the use of the heat-treating, repulverizing step drastically improved HcJ with Br and Hk/HcJ unchanged, than the use of only one usual pulverization.
Sample 113 in Example 1 shown in Table 1 was obtained in the same manner as in this Example except for using only one usual pulverization to an average particle size of 0.55 μm, having magnetic properties of HcJ=476 kA/m, Br=0.443 T, and Hk/HcJ=81.6% as shown in FIGS. 1-3. Comparison of this sample with Sample 601 in this Example revealed that Sample in this Example had slightly improved HcJ with Br and Hk/HcJ substantially unchanged than the sample (average particle size 0.55 μm) in Example 1, despite the fact that the pulverized powder had a large average particle size (0.8 μm). Namely, even though powder after the second fine pulverization step has relatively large particle sizes, the use of the heat-treating, repulverizing step provides as small HcJ as when using powder having relatively small particle sizes, which is obtained by a usual pulverization step. Accordingly, the second fine pulverization step can be conducted in a shortened period of time, thereby improving dewatering and pressing cycles.
EFFECT OF THE INVENTION
Because the production method of the present invention can extremely improve HcJ while maintaining high Br and squareness ratio Hk/HcJ, even if the amounts of rare and expensive Co and La indispensable for Ca—La—Co ferrite are reduced than before, economically advantageous Ca—La—Co ferrite magnets can be obtained. Ca—La—Co ferrite magnets obtained by the production method of the present invention do not suffer demagnetization by a demagnetizing field generated by thinning the magnet. Accordingly, the sintered ferrite magnet of the present invention can be used for electronic parts for automobiles and parts for electric appliances, such as small, light-weight, high-performance motors, power generators, speakers, etc.

## Claims

1. A method for producing a sintered ferrite magnet comprising (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure, and (b) a grain boundary phase indispensably containing Si, said method comprising
a step of preparing calcined ferrite containing a ferrite phase having a hexagonal M-type magnetoplumbite structure, which comprises Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca1-x-yRxAyFe2n-zCoz, wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:


0.3≦(1-x-y)≦0.65,



0.2≦x≦0.65,



0≦y≦0.2,



0.25≦z≦0.65, and



4≦n≦7;


a step of pulverizing said calcined body to powder;
a step of molding said powder to a green body; and
a step of sintering said green body to obtain a sintered body;
more than 1% by mass and 1.6% or less by mass of SiO2 and 1.2-2% by mass (calculated as CaO) of CaCO3 being added to 100% by mass of said calcined body, before the pulverization step,
a ratio of the amount (calculated as CaO) of CaCO3 to the amount of SiO2 being 0.9-1.1 when 0.25≦z≦0.3 and 1.1-1.4 when 0.3≦z≦0.65.

2. The method for producing a sintered ferrite magnet according to claim 1, wherein the amount of SiO2 added is 1.1-1.6% by mass.

3. The method for producing a sintered ferrite magnet according to claim 1, wherein (1-x-y), x, y and z and n meet the relations of


0.35≦(1-x-y)≦0.55,



0.4≦x≦0.6,



0≦y≦0.15,



0.25≦z≦0.4, and



4.5≦n≦6.

4. The method for producing a sintered ferrite magnet according to claim 1, wherein said pulverization step comprises a first fine pulverization step, a step of heat-treating powder obtained by said first fine pulverization step, and a second fine pulverization step of pulverizing the heat-treated powder again.

5. A sintered ferrite magnet comprising
(a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca1-x-yRxAyFe2n-zCoz, wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:


0.3≦(1-x-y)≦0.65,



0.2≦x≦0.65,



0≦y≦0.2,



0.25≦z≦0.65, and



4≦n≦7, and


(b) a grain boundary phase indispensably containing Si, and Ca in amounts determined by more than 1% by mass and 1.6% or less by mass of SiO2 powder and 1.2-2% by mass (calculated as CaO) of CaCO3 powder added to 100% by mass of said ferrite phase,
a ratio of the amount (calculated as CaO) of CaCO3 to the amount of SiO2 being 0.9-1.1 when 0.25≦z≦0.3, and 1.1-1.4 when 0.3≦z≦0.65.

6. The sintered ferrite magnet according to claim 5, wherein the amount of SiO2 powder added is 1.1-1.6% by mass based on 100% by mass of said ferrite phase.

7. The sintered ferrite magnet according to claim 5, wherein (1-x-y), x, y and z and n meet the relations of


0.35≦(1-x-y)≦0.55,



0.4≦x≦0.6,



0≦y≦0.15,



0.25≦z≦0.4, and



4.5≦n≦6.