Mn-Zn ferrite and production process thereof

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

2000-10-18

2002-08-27

Koslow, C. Melissa (Department: 1755)

Compositions

Magnetic

Iron-oxygen compound containing

C252S062600, C252S062590

Reexamination Certificate

active

06440323

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxide magnetic material having soft magnetism, particularly to a Mn—Zn ferrite and more particularly to a Mn—Zn ferrite suitable for use as a high permeability material used for various inductance elements, impedance elements for EMI countermeasure or the like, a low loss material used for switching power transformers, an electromagnetic wave absorbing material and the like, and a production process thereof.
2. Description of the Related Art
A Mn—Zn ferrite is counted among the typical oxide magnetic materials having soft magnetism. The Mn—Zn ferrite of the prior art usually has a basic component composition containing more than 50 mol % (52 to 55 mol % on the average) Fe
2
O
3
, 10 to 24 mol % ZnO and the remainder consisting 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 to the respective steps of calcination, milling, component adjustment, granulation, pressing and the like to obtain a desired shape, then conducting sintering treatment in which the resulting product is kept at 1200 to 1400° C. for 3 to 4 hours in a reducing atmosphere in which a partial pressure of oxygen is limited to a low level by supplying nitrogen. Incidentally, the reason why the Mn—Zn ferrite is sintered in the reducing atmosphere is that when it contains Fe
2
O
3
exceeding 50 mol % and is sintered in the air, densification is not attained sufficiently thereby failing to obtain excellent soft magnetism, and that although Fe
2+
formed by the reduction of Fe
3+
has positive crystal magnetic anisotropy and cancels negative crystal magnetic anisotropy of Fe
3+
thereby enhancing soft magnetism, such a reducing reaction cannot be expected if sintering is conducted in the air.
Incidentally, it has been known that the above-mentioned densification depends on the partial pressure of oxygen in the temperature rise at the time of sintering and the above mentioned formation of Fe
2+
depends on the oxygen in the temperature fall after sintering, respectively. Therefore, when the setting of the partial pressure of oxygen at the time of sintering is wrong, it becomes difficult to ensure an excellent soft magnetism. Thus, in the prior art, the following expression (1) was experimentally established and the partial pressure of oxygen at the time of sintering has been conventionally controlled strictly in accordance with this expression (1).
log
P
O
2
=−14540/(
T+
273)+
b
  (1)
where T is temperature (°C.), PO
2
is a relative partial pressure of oxygen, wherein PO
2
=P′O
2
/P
total
, P′O
2
is the absolute partial pressure of oxygen (Pa), and P
total
is the absolute total pressure (Pa), and b is a constant. The constant b has been set at about 7 to 8. The fact that the constant b is set at 7 to 8 means that the partial pressure of oxygen during sintering must be controlled at a narrow range, which makes the sintering treatment very troublesome thereby increasing the production costs.
On the other hand, when the Mn—Zn ferrite is used as a magnetic core material, eddy current flows at a higher frequency region, resulting in a larger loss. Therefore, to extend an upper limit of the frequency at which the Mn—Zn ferrite can be used as a magnetic core material, its electrical resistance must be set as high as possible. However, the electrical resistance in the above-mentioned usual Mn—Zn ferrite has values smaller than 1 &OHgr;m due to the transfer of electrons between the above-mentioned Fe
3+
ions and Fe
2+
ions and a frequency which is available for application is limited to about several hundred kHz maximum. Thus, in a frequency region exceeding this limit, permeability (initial permeability) is remarkably lowered and the properties of the soft magnetic material are lost.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above-mentioned conventional problems. The present invention has objects to provide a Mn—Zn ferrite which has a high electrical resistance and can sufficiently satisfy applications in a high frequency region exceeding 1 MHz, and to provide a production process thereof in which such Mn—Zn ferrite can be obtained easily and at low costs.
The present inventors recognized in a series of researches related to the Mn—Zn ferrite that even if Fe
2
O
3
content is limited to 50.0 mol % or less, the Mn—Zn ferrite has a high electrical resistance by allowing suitable amounts of TiO
2
and/or SnO
2
to be contained and further a suitable amount of CuO to be contained as desired and can sufficiently satisfy applications in a high frequency region exceeding 1 MHz, and have already disclosed the above in Japanese Patent Application No. Hei 11-29993 and Japanese Patent Application No. Hei 11-29994 (both applications are unpublished).
The inventions in the above-mentioned applications filed are made under the conviction that Fe
2+
can be formed by allowing the Mn—Zn ferrite to contain Ti and/or Sn even when the Mn—Zn ferrite is sintered in the air or in an atmosphere containing some amount of oxygen, which is derived from the findings that iron components in the Mn—Zn ferrite exist as Fe
3+
and Fe
2+
and that Ti and Sn receive electrons from this Fe
3+
to form Fe
2+
. Further, in the inventions of the above-mentioned filed applications, the content of TiO
2
and/or SnO
2
in the basic component composition is limited to 0.1 to 8.0 mol % for controlling the amount of Fe
2+
formed so that the coexistence ratio of Fe
3+
to Fe
2+
is optimized to offset positive and negative crystal magnetic anisotropy, whereby an excellent soft magnetism can be obtained. Further, since a number of Ti
4+
and Sn
4+
ions which have stable valences exist under the conditions, even if Fe
2
O
3
content is limited to a low level, an exchange of electrons between Fe
3+
and Fe
2+
is substantially blocked. Thus, an electrical resistance remarkably higher (about 10
3
times) than conventionally can be obtained.
The present inventors have made the invention by finding out in a series of researches related to the Mn—Zn ferrite that the initial permeability, particularly the initial permeability in a high frequency region is further enhanced by allowing one or more from CoO, NiO and MgO to be contained in a suitable amount as additive to a basic component composition in which Fe
2
O
3
content is limited to 50.0 mol % or less and in which TiO
2
and/or SnO
2
is contained in a suitable amount as described above.
That is, a Mn—Zn ferrite according to one aspect of the present invention to attain the above-mentioned objects is characterized in that the basic component composition contains 44.0 to 50.0 mol % Fe
2
O
3
, 4.0 to 26.5 mol % ZnO, 0.1 to 8.0 mol % one or two from TiO
2
and SnO
2
and the remainder consisting of MnO, and further contains 0.01 to 2.00 mass % one or more from CuO, NiO and MgO as additive.
Further, a Mn—Zn ferrite according to another aspect of the present invention is characterized in that the basic component composition contains 44.0 to 50.0 mol % Fe
2
O
3
, 4.0 to 26.5 mol % ZnO, 0.1 to 8.0 mol % one or two from TiO
2
and SnO
2
, 0.1 to 16.0 mol % CuO and the remainder consisting of MnO, and further contains 0.01 to 2.00 mass % one or more from CoO, NiO and MgO as additive.
The Mn—Zn ferrite according to the present invention is characterized in that Fe
2
O
3
content is limited to 50.0 mol % or less as described above. However, since too little Fe
2
O
3
content leads to reduction in the saturation magnetization or initial permeability, at least 44.0 mol % Fe
2
O
3
is adapted to be contained.
ZnO affects the Curie temperature and saturation magnetization. If ZnO is contained in a significant amount, the Curie temperature is lowered, resulting in practical problems. On the other hand, if ZnO is contained in too small an amount, the saturati

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