Process for producing Mn-Zn ferrite

Details Process for producing Mn-Zn ferrite Process for producing Mn-Zn ferrite

1999-11-18

2002-06-11

Derrington, James (Department: 1731)

Plastic and nonmetallic article shaping or treating: processes

Outside of mold sintering or vitrifying of shaped inorganic...

Of magnetic article or component

C264S611000, C264S612000, C252S062560, C252S062590, C252S062620

Reexamination Certificate

active

06403017

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing an oxide magnetic material having soft magnetism, particularly Mn—Zn ferrite. More particularly, the invention relates to a process for producing Mn—Zn ferrite which is suitable for use as low loss materials used in switching power supply transformer, flyback transformer or deflection yoke, various inductance elements, impedance elements for EMI countermeasure, electromagnetic wave absorbers, and the like.
2. Background of Related Art
There is Mn—Zn ferrite as the representative oxide magnetic material having soft magnetism. This Mn—Zn ferrite generally has a composition comprising basic components of more than 50 mol % of Fe
2
O
3
, 52 to 55 mol % of Fe
2
O
3
on the average, 10 to 24 mol % of ZnO and the remainder being MnO. The Mn—Zn ferrite has conventionally been produced by mixing each raw material powder of Fe
2
O
3
, ZnO and MnO in predetermined proportion, forming the resulting mixture into a predetermined shape through each step of calcination, milling, component adjustment, granulation, pressing and the like, and then subjecting the green compact to sintering treatment such that the green compact is maintained at 1,200 to 1,400° C. for 3 to 4 hours in a reduced atmosphere having greatly decreased oxygen concentration by flowing of nitrogen gas. The reason for sintering in a reduced atmosphere is that since the green compact contains Fe
2
O
3
in a large amount of 50 mol % or more, if it is sintered in the air, densification does not proceed sufficiently, and as a result, good soft magnetism is not obtained. Further, Fe
2+
to be formed by reduction of Fe
3+
has a positive crystal magnetic anisotropy, and therefore has the effect that it offsets a negative crystal magnetic anisotropy of Fe
3+
, thereby increasing soft magnetism. However, if sintered in the air, formation of Fe
2+
by such a reduction reaction cannot be expected.
However, it is known that the densification depends on oxygen concentration at the time of temperature rising in sintering, and formation of Fe
2+
depends on oxygen concentration at the time of temperature dropping after sintering. Accordingly, if setting of oxygen concentration in sintering is mistaken, it is difficult to secure good soft magnetism. For this reason, conventionally the following equation (1) has been established experimentally, and oxygen concentration in sintering has been administered according to this equation (1).
log
Po
2
=−14,540/(
T
+273)+
b
  (1)
wherein T is temperature (° C.), Po
2
is oxygen concentration, and b is a constant. The term “oxygen concentration” in the present specification means the proportion of oxygen gas when the volume of all gases is set to 1, and has the same meaning as partial pressure of oxygen. The numerical value of about 7-8 has conventionally been employed as the constant b. The constant b being 7-8 means that oxygen concentration during sintering must be controlled to a narrow range, and due to this fact, there have conventionally been the problems that sintering treatment is very complicated and production cost is very high.
On the other hand, where Mn—Zn ferrite is used as a magnetic core material, eddy current flows with becoming a frequency used high, and loss by such an eddy current increases. Therefore, in order to raise the upper limit of the frequency that can be used as a magnetic core material, it is necessary to make its electrical resistance large as much as possible. However, there have been the problems that the electrical resistance in the above-described conventional Mn—Zn ferrite is a value smaller than 1 &OHgr;m due to enjoyment of electron between Fe
3+
and Fe
2+
(interionic) as mentioned above, frequency that can be used is up to about several hundred kHz, and initial permeability remarkably decreases in a high frequency region exceeding 1 MHz, resulting in losing properties as soft magnetic material.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-described conventional problems. An object of the present invention is to provide a production process which can easily and inexpensively obtain Mn—Zn ferrite which has large electrical resistance and is sufficiently durable to the use in a high frequency region exceeding 1 MHz.
The above object can be achieved by the following aspects.
According to a first aspect of the present invention, there is provided a process for producing Mn—Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol % of Fe
2
O
3
, 4.0 to 26.5 mol % of ZnO, 0.1 to 8.0 mol % of at least one member selected from the group consisting of TiO
2
and SnO
2
, and the remainder being MnO, sintering the resulting green compact in the air, and then cooling the green compact in the air.
According to a second aspect of the present invention, there is provided a process for producing Mn—Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol % of Fe
2
O
3
, 4.0 to 26.5 mol % of ZnO, 0.1 to 8.0 mol % of at least one member selected from the group consisting of TiO
2
and SnO
2
, 0.1 to 16.0 mol % of CuO and the remainder being MnO, sintering the resulting green compact in the air, and then cooling the green compact in the air like the first aspect of the present invention.
According to a third aspect of the present invention, there is provided a process for producing Mn—Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol % of Fe
2
O
3
, 4.0 to 26.5 mol % of ZnO, 0.1 to 8.0 mol % of at least one member selected from the group consisting of TiO
2
and SnO
2
, and the remainder being MnO, sintering the resulting green compact in an atmosphere having an oxygen concentration as defined by the following equation, and then cooling the green compact after sintering the same at a temperature up to at least 300° C.:
log
Po
2
=−14,540/(
T
+273)+
b
wherein T is temperature (° C.), Po
2
is oxygen concentration, and b is a constant selected from the range of 6 to 21. In this case, if the temperature is lower than 300° C., since the reaction of oxidation and reduction can be disregarded without depending on the oxygen concentration, the adjustment of the atmosphere is sufficient such that the cooling after sintering advances to the point of 300° C.
According to a fourth aspect of the present invention, there is provided a process for producing Mn—Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol % of Fe
2
O
3
, 4.0 to 26.5 mol % of ZnO, 0.1 to 8.0 mol % of at least one member selected from the group consisting of TiO
2
and SnO
2
, 0.1 to 16.0 mol % of CuO and the remainder being MnO, sintering the resulting green compact in an atmosphere having an oxygen concentration as defined by the following equation, and then cooling the green compact after sintering the same at a temperature up to at least 300° C.:
log
Po
2
=−14,540/(
T
+273)+
b
wherein T is temperature (° C.), Po
2
is oxygen concentration, and b is a constant selected from the range of 6 to 21. In this case, if the temperature is lower than 300° C., since the reaction of oxidation and reduction can be disregarded without depending on the oxygen concentration, the adjustment of the atmosphere is sufficient such that the cooling after sintering advances to the point of 300° C.
It is known that iron component in Mn—Zn ferrite is present in the form of Fe
3+
and Fe
2+
, but Ti and Sn form Fe
2+
by receiving electron from Fe
3+
. Therefore, Fe
2+
can be formed even by sintering in the air or an atmosphere containing an appropriate amount of oxygen by containing Ti and Sn.
The first to fourth aspects of the present invention make it

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