Mn-Zn ferrite and production process thereof

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

2001-01-26

2002-08-20

Koslow, C. Melissa (Department: 1755)

Compositions

Magnetic

Iron-oxygen compound containing

Reexamination Certificate

active

06436308

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 relates to a Mn—Zn ferrite suitable for use as various inductance elements, impedance elements for EMI countermeasures 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 the 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 mixed powders to respective steps of calcination, milling, component adjustment, granulation and pressing to obtain a desired shape, then conducting 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 reason why the Mn—Zn ferrite is sintered in the reducing atmosphere is that Fe
2+
is formed as the result of reducing a part of Fe
3 +
. This Fe2+ has positive crystal magnetic anisotropy and cancels out 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 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, which is usually set at 7 to 8. The fact that the constant b is 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 production costs.
Additionally, in recent years, with miniaturization and performance improvement of electronic equipments there is 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 used as a magnetic core material, an electrical resistivity thereof 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+ ions are present, thereby making the transfer of electrons between the above-mentioned Fe3+ and Fe
2+
ions easy. 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 hundreds kHz at highest, and in a frequency region exceeding the limit, permeability (initial permeability) is significantly lowered to 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 boundaries 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, which constitutes 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 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 resistance has been developed and is disclosed in Japanese Patent Application No. Hei 9-18092. However, the electrical resistivity of the Mn—Zn ferrite is as low as 0.3 to 2.0 &OHgr;m, which is insufficient for application in a high frequency region. Further, a Mn—Zn ferrite containing 50 mol % or less Fe
2
03 to which SnO
2
or the like is added is disclosed in EPC 1,304,237. Although it is supposedly very difficult for Fe
2+
to be formed when Fe
2
O
3
content is 50 mol % or less, the Mn—Zn ferrite described in this EPC patent contains as much as 3 to 7 mol % Fe
2+
. Therefore, the electrical resistivity of the Mn—Zn ferrite in the EPC patent cannot exceed the electrical resistivity of a conventional general Mn—Zn ferrite.
On the other hand, a Mn—Zn based 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 11-199235 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 based ferrites described in any of the above publications are ferrite materials intended to be applied in a frequency region of 64 to 100 kHz. It is described that reason for setting the Fe
2
O
3
content to 50 mol % or less for obtaining a high resistance is to make it possible to wind a copper wire directly around a core for a deflecting yoke. Thus, those publications do not suggest the application of the Mn—Zn based ferrite in such a high frequency region as exceeding 1 MHz. All the Mn—Zn based ferrites have an initial permeability of about 1100 at 100 kHz, and excellent soft magnetic properties cannot be obtained by merely setting the Fe
2
O
3
content to less than 50 mol % so as to obtain a high electrical resistivity.
Further, Japanese Patent Examined Application No. Sho 52-4753 discloses a Mn—Zn ferrite containing 50 mol % or less Fe
2
O
3
, to which 1.3 to 1.5 mol % CoO was added in order to decrease the temperature coefficient of initial permeability. This Mn—Zn ferrite also contains as low as 11 mol % ZnO and its relative partial pressure of oxygen at sintering and cooling is not strictly controlled. Thus, the initial permeability at 100 kHz is about 2000.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above-mentioned conventional problems, and an object of the present invention is therefore to provide a Mn—Zn ferrite which has a higher electrical resistivity than 1 &OHgr;m order and at the same time high initial permeabilities of 4000 or more at 100 kHz and of 100 or more at 10 MHz, and a production process by which such a Mn—Zn ferrite can be obtained easily and inexpensively.
A Mn—Zn ferrite according to an aspect of the present invention to attain the above-mentioned object is characterized in that the main components include 44.0 to 49.8 mol % Fe
2
O
3
, 15.0 to 26.5 mol % ZnO, 0.1 to 3.0 mol % CoO, 0.02 to 1.00 mol % Mn
2
O
3
and the remainder MnO.
The present Mn—Zn ferrite may contain, in addition to the above-mentioned main components, at least one of 0.010 to 0.200 mass % V
2
O
5
, 0.005 to 0.100 mass % Bi
2
O
3
, 0.005 to 0.100 mass % In
2
O
3
, 0.005 to 0.100 mass % PbO, 0.001 to 0.100 mass % MoO
3
and 0.001 to 0.100 mass % WO
3
as additive.
Further, the present Mn—Zn ferrite has an initial permeability of 4000 or more at 100 kHz and 100 or more at 10 MHz at room temperature (25° C.).
Still further, a production process according to the pres

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