Ferrite magnet, and powder for ferrite magnet and production process thereof

It is an object of the present invention, in the W-type ferrite which is formulated as SrO.2(FeO).n(Fe.sub.2 O.sub.3), to provide the ferrite magnet and the manufacturing process thereof by which said W-type magnet maintains cost-performance characteristics recognized with the conventional M-type magnet and furthermore exhibits the maximum energy products more than 5 MGOe. In order to achieve the aforementioned object, carbon elements are admixed to raw powder which is a previously prepared mixture of SrCO.sub.3 and Fe.sub.2 O.sub.3 under a given condition such that n-value in the above formula is in a range between 7.2 and 7.7. After the calcining said mixture, CaO, SiO.sub.2 and C powders are furthermore mixed and pulverized to have an average particle size of less than 0.06 .mu.m, followed by forming into a green compact body under a magnetic field and sintering the formed product under a non-oxidizing atmosphere. Hence the W-type ferrite, which has not been realized before, can be produced easily and with low cost.

TECHNICAL FIELD 
The present invention relates to the W-type ferrite magnet which is 
basically formulated as SrO.2(FeO).n(Fe.sub.2 O.sub.3); more specifically 
the invention relates to ferrite magnet, and powders for fabricating said 
ferrite magnet and production process thereof which is characterized by 
the maximum energy products more than 5 MGOe, which has not ever achieved 
by the conventional M-type ferrite magnet, through mixing raw powders in 
such a way that the n-value in the aforementioned chemical formula is 
within the most optimum range, adding a certain type of additive 
element(s) after the calcining, controlling the particle size in a given 
range, forming the green compact using the calcined powders, and sintering 
the formed green compact. 
BACKGROUND ART 
The oxide magnet material typified by the SrO-6Fe2O3, which is a magnet 
plumbite type hexagonal ferrite and is so-called M-type ferrite, was 
proposed by J. J. Went et al., (Philips, 1952). Since then, it has been 
mass-produced and utilized in versatile fields due to its excellent 
magnetic characteristics and high cost-performance. 
Currently, an environmental demands requires the low fuel-cost ratio for 
automobiles, accordingly the light-weight structure for the main body of 
the automobiles is promoted. As a result, in order to produce electronic 
parts with smaller and lighter structure, magnets which serve as major 
components for these electric parts are urgently needed to be fabricated 
with much smaller size with higher efficiency. 
However, since the degree of magnetization of the above mentioned M-type 
magnet is small, it has been difficult to obtain the better magnetic 
properties; for example the maximum energy products--(BH)max--more than 5 
MGOe. 
In order to provide a ferrite material with larger degree of magnetization 
than the conventional M-type magnet, it has been proposed that 
SrO--Fe.sub.2 O.sub.3 which is a principle constituent of the M-type 
magnet is expanded to the ternary system such as SrO--MeO--Fe.sub.2 
O.sub.3 (where Me represents divalent metallic ions such as Co, Zn, or 
Fe), so that four types (W-type, X-type, Y-type, and Z-type) of more 
complicated hexagonal ferrite magnets having much stronger ferromagnetism 
can be fabricated. 
Among these, it was found that the W-type magnet has a similar crystalline 
structure as the conventional M-type magnet and exhibits superior 
properties such as saturation magnetization of about 10% higher than the 
M-type magnet and approximately same anisotropic magnetic field. However, 
the W-type magnet has not been realized. 
For example, F. K. Lotgerin et al. proposed the W-type magnets which were 
consisted of BaO.2(FeO).8(Fe.sub.2 O.sub.3) and SrO.2(FeO).8(Fe.sub.2 
O.sub.3) in Journal of Applied Physics (vol.51, p.5913, 1980). However, 
several drawbacks associated with the proposed magnet were reported; they 
included (1) a complicated control was required for the sintering 
atmosphere, and (2) the maximum energy products, (BH)max, for Ba-system 
magnet was 4.3 MGOe while the (BH)max value for the Sr-system magnet was 
3.8 MGOe. Hence these maximum energy product values indicated that these 
proposed magnets did not possess superior magnetic properties to the 
conventional M-type magnets. 
Moreover, S. Ram et al. reported that the maximum energy products, (BH)max 
of the Sr.sub.0.9 Ca.sub.0.1 O.2(ZnO).8(Fe.sub.2 O.sub.3) was 2.7 MGOe in 
IEEE Trans. Magn., vol.1, p.15; 1992. However, this type of magnet was not 
realized yet. 
As a consequence, in order to overcome the problems found in the above 
articles, it is an object of the present invention to provide a ferrite 
magnet, and powder for the ferrite magnet as well as production process 
thereof, by which the W-type magnet can be formulated as 
SrO.2(FeO).n(Fe.sub.2 O.sub.3), maintaining a similar cost-performance 
recognized with the conventional M-type magnet and exhibiting an excellent 
magnetic property such as the maximum energy product value exceeding 5 
MGOe. 
DISCLOSURE OF INVENTION 
The present inventors have recognized that the W-type magnet has a larger 
magnetization than the conventional M-type magnet. We have found that 
there was an optimum range for the n-value in the formula 
SrO.2(FeO).n(Fe.sub.2 O.sub.3). Furthermore, after the continuous and 
diligent efforts for finding appropriate types of additives in order to 
improve the magnetic properties, we came to a conclusion that the magnetic 
properties can be remarkably improved by adding certain types of plurality 
of additive elements after the calcining process. 
Moreover, we have completed the presently applied invention by finding that 
the ferrite magnet can be fabricated having the maximum energy products, 
(BH)max, exceeding 5 MGOe (which any one of the conventional M-type 
magnets can not be achieved) through the following sequential processes; 
i.e., (1) preparing raw powders being admixed with a certain type of 
additives, (2) pulverizing the raw powders in order to have an average 
particle size less than 0.06 .mu.m (by the BET measurement), (3) forming 
the green compact body under the magnetic field, and (4) sintering the 
formed compact in a non-oxidizing atmosphere. 
Namely, for more details, the present invention is characterized by the 
following processes; (1) preparing raw powders which are mixture of 
SrCO.sub.3 and Fe.sub.2 O.sub.3 with a given mol ratio ranging from 1:8.2 
to 1:8.7, (2) adding carbon with 0.3 to 5.0 weight %, (3) calcining the 
mixture, (4) further adding CaO with 0.3 to 1.5 weight %, SiO.sub.2 with 
0.1 to 0.6 weight % and C with 0.1 to 0.5 weight %, (5) pulverizing the 
mixture into fine particles having an average particle size of less than 
0.06 .mu.m, (6) forming the green compact body under the applying the 
magnetic field, and (7) sintering the formed compact body in a 
non-oxidizing atmosphere. As a result, the ferrite magnet can be obtained 
which has a chemical formula as SrO.2(FeO).n(Fe.sub.2 O.sub.3) having an 
optimum range of n-value in the above formula between 7.2 and 7.7, has an 
average grain size of the sintered body of less than 2 .mu.m, and exhibits 
the maximum energy products, (BH)max, of more than 5 MGOe. 
The present invention proposes also the production process for the 
aforementioned ferrite magnets, being characterized by the processes such 
as (1) a process in which, after the calcination process, in addition to 
the above mentioned additives, furthermore at least either one of Cr.sub.2 
O.sub.3 (0.2 to 0.8 weight %) or CoO (0.2 to 0.8 weight %) is added, (2) a 
process by which the formed green compact body is dried under a 
temperature range between 100.degree. C. and 200.degree. C., and (3) a 
process for which an oxidizing agent or a reducing agent is added during 
the sintering process. 
Moreover, the present invention is characterized by producing powders used 
for the ferrite magnets, which can be effectively utilized as powders for 
bonded magnets or sintered magnets, by adding carbon of 0.3 to 5.0 weight 
% to raw powders which are previously mixed with SrCO.sub.3 and Fe.sub.2 
O.sub.3 under a given mol ratio ranging from 1:8.2 to 1:8.7, calcining the 
thus prepared mixture, and pulverizing the calcined powders into fine 
particles with an average particle size less than 3 .mu.m. 
The above and many other objectives, features and advantages of the present 
invention will be fully understood from the ensuing detailed description 
of the examples of the invention, which description should be read in 
conjunction with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
The followings are sequential processes to produce the ferrite magnet 
according to the present invention; 
1) mixing SrCO.sub.3 and Fe.sub.2 O.sub.3 with a certain mol ratio to 
prepare raw powders, 
2) admixing carbon to raw powders, 
3) calcining the admixture, 
4) adding CaO, SiO.sub.2, and C to the calcined powders after the 
calcination, 
5) pulverizing the mixture of powders to have an average particle size of 
less than 0.06 .mu.m, 
6) forming the thus pulverized powders under the magnetic field, and 
7) sintering the formed green compact in a non-oxidizing atmosphere. 
First of all, the present inventors have chosen the appropriate n-value 
range in W-type magnet being formulated as SrO.2(FeO).n(Fe.sub.2 O.sub.3). 
When raw powders are prepared, a mixture of SrCO.sub.3 and Fe.sub.2 
O.sub.3 are admixed under various mol ratios. The mixture was then 
subjected to the calcination at a temperature of 1,340.degree. C. in 
nitrogen gas atmosphere. Furthermore, the calcined mixture was pulverized 
into fine particles having an average particle size of 0.06 .mu.m. The 
pulverized particles were then formed into a green compact body under 
applying the magnetic field. The formed green bodies were finally sintered 
at three different sintering temperatures of 1,150.degree. C., 
1,175.degree. C., and 1,200.degree. C. FIG. 1 shows the changes in the 
intrinsic coercive force, iHc, and residual flux density, Br, of variously 
prepared magnets when n-value is altered in a range from 7.0 to 8.75 (on 
x-axis). 
As clearly seen in FIG. 1, it was found that, when n-value in the 
aforementioned formula is either less than 7.2 or more than 7.7, the 
coersive force (iHc) reduced and the target value of the maximum energy 
products, (BH)max, of 5 MGOe was not achieved. Therefore, it is necessary 
to define the n-value within a range from 7.2 to 7.7. In order to achieve 
this limitation for n-values in the formula SrO.2(FeO).n(Fe.sub.2 
O.sub.3), it was found that SrCO.sub.3 and Fe.sub.2 O.sub.3 powders should 
be mixed under a range of mol ratio between 1:8.2 to 1:8.7. 
FIG. 2 shows x-ray diffractograms for identifying crystalline structures of 
calcined powder being heated at 1,150.degree. C., 1,200.degree. C., 
1,250.degree. C., and 1,300.degree. C. in nitrogen atmosphere, after said 
raw powders were mixed with carbon powders of 0.5 weight %. Moreover, the 
x-ray diffractograms of crystalline structures of the calcined powder 
being heated under the same conditions as above except that carbon powders 
were not admixed to raw powders were shown in FIG. 3. Different marks used 
in FIGS. 2 and 3 (for example, circles, triangles, and squares) represent 
the diffraction intensities for each respective calcining temperature. The 
closed marked represent data for W-type and the open marks indicate data 
obtained form M-type magnets, respectively. 
In a case when carbon powders are not mixed to raw powders, the sintering 
temperature for W-type magnets is limited to a relatively high 
temperature; while by adding carbon powders to raw powders the W-type 
magnets can be fabricated in a wider range of calcining temperature, so 
that production cost can be reduced, the productivity can be enhanced, and 
the improved intrinsic coercive force can be achieved by using refined 
particles. 
As a consequence, as described previously, 0.3 to 5.0 weight % of carbon 
powders are mixed to raw powders which are previously prepared by mixing 
SrCO.sub.3 and Fe.sub.2 O.sub.3 with a certain predetermined mol ratio. 
Carbon powders--which serves as a reducing agent--are added in order to 
prevent the oxidation of raw powders upon the subsequent calcination 
process. If the amount of carbon to be added prior to the calcination 
process is less than 0.3 weight %, the calcining temperature cannot be 
defined in a wider range of temperature; on the other hand, if the carbon 
addition amount exceeds 5.0 weight %, it will become more difficult to 
produce the W-type ferrite and also deteriorate the magnetic properties 
even if the W-type magnet would be fabricated. As a result, the amount of 
carbon to be added to raw powders prior to the calcination process should 
be limited within an appropriate range between 0.3 weight % and 5.0 weight 
%. 
In the next step, the raw powders admixed with carbon with a certain amount 
will be calcined. Since the W-type magnet contains divalent iron ion 
(Fe.sup.++), the atmosphere for calcination is needed to be controlled. 
Although nitrogen gas would be suitable as a non-oxidizing atmosphere, the 
calcination in air can be possible, depending upon the amount of added 
carbon powders. Furthermore, the calcining temperature depends upon the 
amount of added carbon, too. It was found that the calcining temperature 
ranging from 1,150.degree. C. to 1,400.degree. C. would be the most 
suitable to the present invention. 
As indicated in the x-ray diffractograms for structural phases formed in 
the calcined powders (see FIG. 2), W-type ferrite was obtained. As a 
consequence, the calcined powders can further be pulverized into fine 
particles to provide raw powders which can be utilized to produce bonded 
magnets or sintered magnets. If they are utilized to produce bonded 
magnets, it is preferable that the calcined powders are pulverized into 
fine particles in a range from 0.07 .mu.m to 3 .mu.m. If they are utilized 
to produce sintered magnet, it is desirable that the calcined powders are 
pulverized to a particle size less than 0.06 .mu.m. 
On the other hand, according to the present invention, CaO (0.3 to 1.5 
weight %), SiO.sub.2 (0.1 to 0.6 weight %) and C (0.1 to 0.5weight %) are 
added to calcined powders. These three additives contribute to improve the 
residual flux density (Br) and intrinsic coercive force (iHc) as well. 
FIG. 4 show the changes in the intrinsic coercive force (iHc) and residual 
flux density (Br) when CaO addition amount is altered from 0.15 weight % 
to 0.9 weight % (on x-axis) and SiO.sub.2 addition amount is varied from 
0.15 wright % (circle marks), 0.30 weight % (triangle marks) to 0.45 
weight % (square marks) with respect to a composition of 
SrO.2(FeO).7.5(Fe.sub.2 O.sub.3). Samples presented in FIG. 4 were 
calcined at 1,340.degree. C. in nitrogen atmosphere, pulverized into fine 
particles with an average particle size of 0.06 .mu.m, formed into green 
compact under the applied magnetic field, and sintered at 1,175.degree. C. 
Although the remarkable improvement in intrinsic coercive force can be 
found in relatively wide range of CaO addition amount, if it is less than 
0.3 weight %, the addition effect is not achieved. On the other hand, if 
the CaO addition amount exceeds 1.5 weight %, Ca ferrite could be 
produced, perhaps causing a deterioration of magnetic characteristics. 
Accordingly, the addition amount of CaO would be preferable if it is 
limited in a range from 0.3 to 1.5 weight %. It is further more preferable 
if it is limited in a range from 0.5 to 0.8 weight %. 
With regard to addition amount of SiO.sub.2, it was found that if it is 
less than 0.1 weight %, no improvement in intrinsic coercive force was 
recognized; while if it exceeds 0.6 weight %, both intrinsic coercive 
force and residual flux density were deteriorated. As a consequence, the 
range between 0.1 weight % and 0.6 weight % is preferable; more 
specifically it would more preferable if it is limited in a range between 
0.3 weight % and 0.4 weight %. 
FIG. 5 shows changes in maximum energy products, (BH)max, as a function of 
drying temperature ranging from 50.degree. C. to 175.degree. C. of the 
formed green compact when carbon powders are not added after the 
calcination process. As seen clearly from the figure, for the case when 
carbon was not added, excellent magnetic properties can be obtained in 
only a very narrow range of the drying temperature. Accordingly, adding 
carbon prior to the pulverizing the calcined powders makes the optimum 
temperature range for drying temperature to shift toward higher 
temperature side, resulting in stabilizing the excellent maximum energy 
products, (BH)max. 
Moreover, FIG. 6 shows the changes in the maximum energy products, (BH)max, 
when carbon addition amount is altered with respect to the composition 
SrO.2(FeO).7.5(Fe.sub.2 O.sub.3). The samples were calcined at 
1,340.degree. C. in nitrogen atmosphere, pulverized into fine particles 
having average particle size of 0.06 .mu.m, formed in green compact under 
the magnetic field, and sintered at 1,150.degree. C. 
It was found that carbon addition improves both intrinsic coercive force 
and residual flux density. Furthermore, carbon addition helps to widen the 
optimum drying temperature range and to stabilize the excellent maximum 
energy product, (BH)max, when the formed body is subjected to drying 
process; said formed body was formed under the magnetic field by using 
powders which is pulverized through the wet pulverizing process, as will 
be described later. 
The amount of carbon addition is different from the amount required to be 
added prior to the calcination process. Namely, if it is less than 0.1 
weight %, the maximum energy product, (BH)max, is not improved; on the 
other hand, if it exceeds 0.5 weight %, the maximum energy products tend 
to decrease. As a consequence, addition amount ranging from 0.1 weight % 
to 0.5 weight % is preferable; more specifically it would be more 
preferable if it is limited in a range from 0.1 to 0.3 weight %. 
In addition to the above mentioned additive, by adding at least one of 
Cr.sub.2 O.sub.3 (ranging from 0.2 to 0.8 weight %) or CoO (ranging from 
0.2 to 0.8 weight %), further improvements in both intrinsic coercive 
force and residual flux density can be achieved. 
Namely, with respect to the composition SrO.2(FeO).7.5(Fe.sub.2 O.sub.3), 
the addition amount of Cr.sub.2 O.sub.3 was altered from 0 to 1 weight % 
with constant CaO of 0.45 weight % and SiO.sub.2 of 0.45 weight %. FIG. 7 
demonstrates the changes in intrinsic coercive force, iHc, and residual 
flux density, Br, as a function of Cr.sub.2 O.sub.3 addition amount. As 
seen from FIG. 7 clearly, it was found that the intrinsic coercive force 
can be improved by adding Cr.sub.2 O.sub.3 powders. However, there appears 
to be a limitation; if it is less than 0.2 weight % or more than 0.8 
weight %, the intrinsic coercive force decreases. As a result, it is 
preferable to control the addition amount of Cr.sub.2 O.sub.3 within a 
range from 0.2 to 0.8 weight %. 
Moreover, FIG. 8 shows changes in intrinsic coercive force, iHc, and 
residual flux density, Br, when CaO and SiO.sub.2 are kept constant (i.e., 
0.45 weight %, respectively) and addition amount of CoO is altered from 0 
to 1 weight %. As seen in FIG. 8, it was found that the residual flux 
density was improved by adding CoO powders; however the beneficial effect 
of CoO addition was not recognized if it is less than 0.2 weight %. On the 
other hand, if it is more than 0.8 weight %, the intrinsic coercive force 
decreases. Accordingly, it is preferable to control the addition amount of 
CoO within a range from 0.2 and 0.8 weight %. 
Furthermore, in addition to the above mentioned effective additives, it was 
found that addition of SrCO.sub.3 ranging from 0.3 to 1.0 weight % is also 
effective in terms of improvements of magnetic properties; said addition 
amount being dependent on other conditions including calcining temperature 
and particle size of pulverized powders. 
The powders being added with various additives are now pulverized into fine 
particles having an average particle size less than 0.06 .mu.m. Although 
the means for pulverizing is not limited to specific technologies, it is 
preferable to employ the wet pulverization process such as ball mill or 
attritor mill. The pulverized powders are then subjected to forming into 
green compact under the applied magnetic field, as known as a prior art. 
FIG. 9 shows changes in residual flux density, Br, when the particles size 
is varied from 0.027 .mu.m, 0.047 .mu.m, 0.081 .mu.m, to 0.143 .mu.m. The 
samples--having a composition SrO.2(FeO).7.5(Fe.sub.2 O.sub.3)--were mixed 
with 0.5 weight % of carbon prior to the calcination process, calcined in 
nitrogen atmosphere at 1,250.degree. C., further added with 0.47 weight % 
of CaO, 0.3 weight % of SiO.sub.2, and 0.17 weight % of C, pulverized into 
certain particle size powders, followed by forming into the green compact 
under the applied magnetic field and sintering at 1,175.degree. C. 
If the average particle size exceeds 0.06 .mu.m, although the residual flux 
density, Br, is still improved, an adverse effect was found with the 
intrinsic coercive force, iHc. Moreover, if the average particle size is 
too small, although there is an improvement in intrinsic coercive force, 
iHc, the residual flux density, Br, was deteriorated. As a consequence, 
the average particle size to be added prior to the calcination process 
should be controlled to be less than 0.06 .mu.m. The most preferable range 
for the average particle size would be between 0.04 and 0.06 .mu.m. All 
data of particle size was obtained by the BET measurement. 
As described previously, in a case when the wet pulverizing method such as 
ball mill is employed, it is preferable to dry the formed green compact at 
the optimum temperature. Depending upon the addition amount of the carbon 
or other additives which are added prior to the pulverizing, the 
preferable temperature range would be in a range between 100.degree. C. 
and 200.degree. C. 
The final stage of the process is the sintering of the formed green 
compact. Although there is no specific requirements for the sintering 
process, the preferable sintering atmosphere is a non-oxidizing atmosphere 
such as an inert gas or vacuum, and the sintering temperature ranging from 
1,150.degree. C. to 1,250.degree. C. is preferable. Besides, it is 
preferable to add an oxidizing agent or a reducing agent if necessary; 
depending upon the composition of the compact, type of additives, 
calcining conditions, pulverized particle size, and drying conditions. 
Carbon or PVA can be used as a reducing agent; while iron sesquioxide 
(Fe.sub.2 O.sub.3) powder can be employed as an oxidizing agent. 
According to the present invention, the average grain size of produced 
ferrite magnet is limited less than 2 .mu.m. FIG. 10 shows various 
magnetic properties (intrinsic coercive force, iHc, residual flux density, 
Br, and maximum energy products, (BH)max as a function of average grain 
size in .mu.m. In the composition SrO.2(FeO).n(Fe.sub.2 O.sub.3) (where 
n=7.2.about.7.7) of the present invention, it was found that if the 
average grain size exceeds 2 .mu.m, there is a tendency of decreasing of 
magnetic properties (especially, intrinsic coercive force). Hence, it is 
necessary to control the average grain size to be less than 2 .mu.m in 
order to obtain excellent magnetic properties, particularly the maximum 
energy products, (BH)max, be 5 MGOe, which has not be achieved with the 
conventional M-type magnets. The more preferably, the average grain size 
is controlled within a range from 1.2 to 1.7 .mu.m. 
EMBODIMENT 
Raw powders were prepared by mixing SrCO.sub.3 and Fe.sub.2 O.sub.3 with a 
mol ratio of 1:8.5. Furthermore 1.5 weight % of carbon was added to raw 
powders. The mixture was calcined in nitrogen atmosphere at 1,350.degree. 
C. for 1 hour. To the calcined powders, 0.6 weight % of CaO, 0.3 weight % 
of SiO.sub.2, and 0.2 weight % of C were added, followed by pulverizing by 
the ball mill to produce fine powders having an average particle size of 
0.05 .mu.m. 
The pulverized powders were then subjected to the forming into the green 
compact body under the applied magnetic field. The formed * compact was 
dried at 200.degree. C. for 2 hours, followed by sintering at 
1,175.degree. C. for 1 hour in nitrogen atmosphere. 
The magnetic properties of the thus produced W-type magnet as follows; 4 
.pi.Is=5.0 kG, Br=4.8 kG, iHc=2.5 kOe, (BH)max=5.3 MGOe. FIG. 11 shows the 
magnetization curve of the W-type magnet which was produced according to 
the above procedures. 
INDUSTRIAL APPLICABILITY 
According to the present invention, it is easily and less-expensively to 
produce the W-type magnet which has not been realized. The present * 
invention can also provide the W-type magnet, maintaining a similar 
cost-performance as the M-type magnet, which said W-type magnet has the 
maximum energy product, (BH)max, more than 5.0 MGOe, being higher than 
those found with the conventional M-type magnet. 
While this invention has been described with respect to preferred 
embodiments and examples, it should be understood that the invention is 
not limited to that precise examples; rather many modifications and 
variations would present themselves to those of skill in the art without 
departing from the scope and spirit of this invention, as defined in the 
appended claims.