Mn-Zn ferrite and production process thereof

Details Mn-Zn ferrite and production process thereof Mn-Zn ferrite and production process thereof

2001-03-01

2002-10-22

Wood, Elizabeth D. (Department: 1755)

Compositions

Magnetic

Iron-oxygen compound containing

C252S062590, C252S062620, C423S594120, C501S126000, C501S133000, C501S134000, C501S154000

Reexamination Certificate

active

06468441

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxide magnetic material having soft magnetism, and more particularly, to a Mn—Zn ferrite suitable for use as a switching power transformer, a rotary transformer and the like, and to a production process thereof.
2. Description of the Related Art
Typical oxide magnetic materials having soft magnetism include a Mn—Zn ferrite. This Mn—Zn ferrite of the prior art usually has a basic component composition containing 52 to 55 mol % of Fe
2
O
3
on the average exceeding 50 mol % which is the stoichiometric composition, 10 to 24 mol % of ZnO and the remainder of MnO. And the Mn—Zn ferrite is usually produced by mixing the respective material powders of Fe
2
O
3
, ZnO and MnO in a prescribed ratio, subjecting the mixed powders to the respective steps of calcination, milling, component adjustment, granulation and pressing to obtain a prescribed shape, and then sintering the resulting product at 1200 to 1400° C. for 2 to 4 hours in a reducing atmosphere in which a relative partial pressure of oxygen is controlled to a low level by supplying nitrogen. The Mn—Zn ferrite is sintered in the reducing atmosphere in order to produce a part of Fe
3+
thereby forming Fe
2+
. This Fe
2+
has positive crystal magnetic anisotropy and cancels negative crystal magnetic anisotropy of Fe
3
thereby enhancing soft magnetism.
Amount of the above-mentioned Fe
2+
formed depends on relative partial pressures of oxygen in sintering and cooling after the sintering. Therefore, when the relative partial pressure of oxygen is improperly set, it becomes difficult to ensure excellent soft magnetic properties. Thus, in the prior art, the following expression (1) has been experimentally established and the relative partial pressure of oxygen in sintering and in cooling after the sintering has been controlled strictly in accordance with this expression (1):
 
log Po
2
=−14540/(
T
+273)+
b
  (1)
where T is temperature (° C.), Po
2
is a relative partial pressure of oxygen, and b is a constant. Usually, the constant b is set to 7 to 8. The fact the constant b is set to 7 to 8 means that the relative partial pressure of oxygen in the sintering must be controlled in a narrow range, which makes the sintering treatment very troublesome thereby increasing the production costs.
In recent years, with miniaturization and performance improvement of electronic equipments, there is an increasing tendency that frequencies of processing signals become higher. Thus, a magnetic material having excellent magnetic properties even in a higher frequency region has been needed.
However, when the Mn—Zn ferrite is used as a magnetic core material, as a frequency region applied becomes higher, an eddy current flows to result in a larger loss. Therefore, to extend the upper limit of the frequency at which the Mn—Zn ferrite can be used as a magnetic core material, an electrical resistivity of the material must be made as high as possible. However, since the above-mentioned usual Mn—Zn ferrite contains Fe
2
O
3
in an amount larger than 50 mol % which is the stoichiometric composition, a large amount of Fe
2+
ion is present thereby making easy the transfer of electrons between the above-mentioned Fe
3+
and Fe
2+
ions. Thus, the electrical resistivity of the Mn—Zn ferrite is in the order of about 1 &OHgr;m (order of one digit) or less. Accordingly, an applicable frequency is limited to about several hundred kHz maximum, and in a frequency region exceeding this limit, permeability (initial permeability) is significantly lowered and the properties of the soft magnetic material are completely lost.
In order to increase an apparent resistance of the Mn—Zn ferrite, in some cases, CaO, SiO
2
and the like are added as additive to impart a higher resistance to grain boundaries and at the same time the Mn—Zn ferrite is sintered at as low as about 1200° C. to diminish grain sizes from their usual dimension, about 20 &mgr;m, to 5 &mgr;m, thereby taking measures to increase the ratio of the grain boundary. However, even if such measures are adopted, it is difficult to obtain an electrical resistivity exceeding 1 &OHgr;m order because resistance of the grain boundary itself is low, and the above-mentioned measures fall short of a thorough solution.
Further, a Mn—Zn ferrite in which, for example, CaO, Sio
2
, SnO
2
and TiO
2
are added to obtain a higher resistance has been developed and is disclosed in Japanese Patent Application No. Hei 9-180925. However the electrical resistivity of the Mn—Zn ferrite is as low as 0.3 to 2.0 &OHgr;m, which is insufficient for use in a high frequency region. Similarly, a Mn—Zn ferrite to which SnO
2
and the like are added is disclosed in EPC 1,304,237. The Mn—Zn ferrite described in this EPC patent contains as much as 3 to 7 mol % of Fe
2+
. An electrical resistivity depends on amount of Fe
2+
as described above, and the electrical resistivities of the Mn—Zn ferrite in this EPC patent cannot exceed the electrical resistivities of a usual Mn—Zn ferrite of the prior art.
On the other hand, Mn—Zn ferrites which exhibit a higher resistance by containing less than 50 mol % of Fe
2
O
3
, have been developed for use as a core material for a deflection yoke and are disclosed in Japanese Patent Application Laid-open Nos. Hei 7-230909, Hei 10-208926, Hei 11-199235 and the like.
However, judging from the fact that their usage is a core material for a deflection yoke and from the examples of the invention described in each publication, the Mn—Zn ferrites described in any of the publications are ferrite materials intended for applications in a frequency region of 64 to 100 kHz. The purpose of setting Fe
2
O
3
content to less than 50 mol % for a high electrical resistivity is to enable a copper wire to be wound directly around a core for a deflection yoke. In the ferrite materials, excellent magnetic properties are not obtained in such a high frequency region as exceeding 1 MHz. Thus, it does not enable the ferrites to be used as a magnetic core material in such a high frequency region as exceeding 1 MHz to only set the Fe
2
O
3
content to less than 50 mol % for a high electrical resistivity.
Further, a Mn—Zn ferrite containing 50 mol % or less of Fe
2
O
3
to which 1.3 to 1.5 mol % of CoO is added in order to decrease the temperature coefficient of initial permeability is disclosed in Japanese Examined Patent Publication No. Sho 52-4.753. This Mn—Zn ferrite is not intended for obtaining a property of low loss in such a high frequency region as exceeding 1 MHz, either, and relative partial pressure of oxygen in sintering and cooling after the sintering is not strictly controlled.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above-mentioned conventional problems. An object of the present invention is to provide a Mn—Zn ferrite that has, of course, excellent magnetic properties and also has both a higher electrical resistivity than 1 &OHgr;m order (a single digit order) and a low core loss in such a high frequency region as exceeding 1 MHz, and a production process as well, by which such a Mn—Zn ferrite can be obtained easily and inexpensively.
One of the Mn—Zn ferrites according to the present invention for attaining the above-mentioned object is characterized in that its basic component composition includes 44.0 to 49.8 mol % of Fe
2
O
3
, 6.0 to 15.0 mol % of ZnO (15.0 mol % is excluded), 0.1 to 3.0 mol % of CoO, 0.02 to 1.20 mol % of Mn
2
O
3
and remainder of MnO, and that the average grain size is less than 10 &mgr;m.
Another Mn—Zn ferrite according to the present invention is characterized in that its basic component composition includes 44.0 to 49.8 mol % of Fe
2
O
3
, 6.0to 15.0 mol % of ZnO (15. 0 mol % is excluded), 0.1 to 3.0 mol % of CoO, 0.1 to 6.0 mol % of CuO, 0.02 to 1.20 mol % of Mn
2
O
3
and remainder of MnO, and that the average grain size is less than 10 &mgr;m.
Still another Mn—Zn ferrite according

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