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

2002-10-08

Wood, Elizabeth D. (Department: 1755)

Compositions

Magnetic

Iron-oxygen compound containing

C252S062560, C423S594120, C501S126000, C501S133000, C501S134000, C501S154000

Reexamination Certificate

active

06461531

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxide magnetic material having soft magnetism, and more specifically 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. Conventionally, this Mn—Zn ferrite usually has a basic component composition containing 52 to 55 mol % Fe
2
O
3
on the average, exceeding 50 mol % which is the stoichiometric composition, 10 to 24 mol % ZnO and remainder MnO. The Mn—Zn ferrite is usually produced by mixing respective material powders of Fe
2
O
3
, ZnO and Mno in a prescribed ratio, subjecting the mixed powders to respective steps of calcination, milling, component adjustment, granulation and pressing to obtain a desired shape, then performing sintering treatment 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 reduce 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
to thereby enhance 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, conventionally, 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 strictly controlled 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 which is usually 7 to 8. The fact that 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, whereby such a problem arises that the sintering treatment becomes significantly troublesome and therefore production costs are increased.
In recent years, with miniaturization and performance improvement of electronic equipments there is such an increasing tendency that signals are processed at a higher frequency. Thus, a magnetic material having excellent magnetic properties even in a higher frequency region as well has been needed
However, when the Mn—Zn ferrite is used as a magnetic core material, an eddy current flows in a higher frequency region applied resulting in a larger loss. Therefore, in order to extend an upper limit of the frequency at which the Mn—Zn ferrite can be applied as a magnetic core material, an electrical resistivity of the material must be made as high as possible. However, since the above-mentioned general 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 1 &OHgr;m or less. Accordingly, an applicable frequency is limited to about several hundred kHz maximum, and in a frequency region exceeding the limit, permeability (initial permeability) is significantly lowered to completely take away properties of the soft magnetic material.
In order to increase an apparent resistance of the Mn—Zn ferrite, in some cases, CaO, SiO
2
or the like is added as additive to impart a higher resistance to grain boundary and at the same time the Mn—Zn ferrite is sintered at as low as about 1200° C. to diminish the grain size from its 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 as the grain boundary itself has a low resistance, and the above-mentioned measures fall short of a thorough solution.
Further, a Mn—Zn ferrite to which, for example, CaO, SiO
2
, SnO
2
and TiO
2
are added to obtain a higher resistivity 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 does not sufficiently satisfy application in a high frequency region. Further, 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 % Fe
2+
. An electrical resistivity depends on amount of Fe
2+
as described above. Therefore, 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, a Mn—Zn ferrite which contains less than 50 mol % Fe
2
O
3
for a higher resistance has been developed for use as a core material for a deflecting yoke and is disclosed in Japanese Patent Laid-open Nos. Hei 7-230909, Hei 10-208926, Hei 1-99235 and the like.
However, judging from the fact that the application thereof is a core material for a deflecting yoke and from examples of the invention described in each publication, the Mn—Zn ferrites described in any of the above publications are ferrite materials intended to be used in a frequency region of 64 to 100 kHz. It is described that the purpose in setting Fe
2
O
3
content to 50 mol % or less for a high electrical resistivity is to enable a copper wire to be wound directly around a core for a deflecting yoke. Excellent magnetic properties are not obtained in such a high frequency region as exceeding 1 MHz. Thus, only setting the Fe
2
O
3
content to less than 50 mol % for a high electrical resistivity is not good enough to enable the ferrites to be used as a magnetic core material in such a high frequency region as exceeding 1 MHz.
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 by which such a Mn—Zn ferrite can be obtained easily and inexpensively.
A Mn—Zn ferrite according to the present invention to attain the above-mentioned object is characterized in that its basic component composition includes 44.0 to 49.8 mol % Fe
2
O
3
, 6.0 to 15.0 mol % ZnO (15.0 mol % is excluded), 0.1 to 4.0 mol % at least one of TiO
2
and SnO
2
, and remainder 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 % Fe
2
O
3
, 6.0 to 15.0 mol % ZnO (15.0 mol % is excluded), 0.1 to 4.0 mol % at least one of TiO
2
and SnO
2
, 0.1 to 6.0 mol % CuO, and remainder MnO, and that the average grain size is less than 10 &mgr;m.
Still another Mn—Zn ferrite according to the present invention may contain as additive, in addition to the basic component compositions of the above-described two inventions, at least one component selected from the group consisting of 0.005 to 0.200 mass % CaO, 0.005 to 0.050 mass % SiO
2
, 0.010 to 0.200 mass % ZrO
2
, 0.010 to 0.200 mass % Ta
2
O
5
, 0.010 to 0.200 mass % HfO
2
and 0.010 to 0.200 mass % Nb
2
O
5
.
On the other hand, a production process according to the present invention to attain the above-mentioned object is characterized in that a mixed powder whose components are adjusted so as to compose

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