Production process of Mn-Zn ferrite

Details Production process of Mn-Zn ferrite Production process of Mn-Zn ferrite

2001-11-21

2003-06-10

Fiorilla, Christopher A. (Department: 1731)

Plastic and nonmetallic article shaping or treating: processes

Recycling of reclaimed or purified process material

C264S611000, C252S062200

Reexamination Certificate

active

06576169

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a production process of a Mn—Zn ferrite, and more particularly to a production process of a Mn—Zn ferrite that enables wastes of sintered products to be recycled.
2. Description of the Related Art
Typical oxide magnetic materials having soft magnetism include a Mn—Zn ferrite that has been used as a low loss material used for switching power transformers, flyback transformers and the like, various inductance elements, an impedanace element for EMI countermeasures, an electromagnetic wave absorber and the like. Conventionally, this Mn—Zn ferrite usually has a basic component composition containing over 50 mol % (52 to 55 mol % on the average) Fe
2
O
3
, 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 the 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 3 to 4 hours in a reducing atmosphere in which a relative partial pressure of oxygen is considerably lowered by supplying nitrogen. The reason why the Mn—Zn ferrite is sintered in the reducing atmosphere is that when the Mn—Zn ferrite containing over 50 mol % Fe
2
O
3
is sintered in the air, densification is not attained sufficiently thereby failing to obtain excellent soft magnetism, and that Fe
2+
which has positive crystal magnetic anisotropy is formed by reducing a part of Fe
2
O
3
exceeding 50 mol % thereby canceling negative crystal magnetic anisotropy of Fe
3+
and 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, 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 7 to 8.
In addition, the above-mentioned milling step is conducted so that an average grain size of a fine milled powder ranges 1.0 to 1.4 &mgr;m. If the average grain size is more than 1.4 &mgr;m, a desired density can not be obtained in sintering, and if the average grain size is less than 1.0 &mgr;m, it becomes difficult to handle the resultant powder.
A large amount of wastes are generated for several reasons, such as a surplus, defects and the like in each step of the production process of Mn—Zn ferrite described above. While wastes generated prior to the step of pressing can be recycled without particular problems, wastes generated in the step of sintering due to defects, such as dimensional error, cracking, chipping or the like, are difficult to recycle for the reason described below and are just scrapped as they are.
The step of sintering a Mu—Zn ferrite is largely affected by vacancy concentration of oxygen ions that have the lowest diffusing rate among its constituent ions. As the vacancy concentration of oxygen ions increases, the diffusion of oxygen ions, iron ions, manganese ions and zinc ions is accelerated and the sintered density increases. Fe
2
O
3
content and a relative partial pressure of oxygen in an atmosphere are factors governing the vacancy concentration of oxygen ions. The smaller the Fe
2
O
3
content is and the lower the relative partial pressure of oxygen is, the easier the vacancies of oxygen ions can be formed. Because a conventional Mn—Zn ferrite contains over 50 mol % Fe
2
O
3
, the vacancies of oxygen ions decrease, whereas the respective vacancies of iron ions, manganese ions and zinc ions increase. That is, in case a conventional sintered Mn—Zn ferrite is milled and pressed for recycling, it must be sintered with the relative partial pressure of oxygen in an atmosphere considerably lowered. However, the lowest relative partial pressure of oxygen available in actual mass production process is about 0.0001 in which a desired vacancy concentration of oxygen can not be obtained. As a result of this, the sintering can not be conducted smoothly making it difficult to obtain a desired density. Consequently, the resultant sintered cores do not have magnetic properties good enough to serve for practical use and therefore are simply scrapped.
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 production process of a Mn—Zn ferrite, which enables wastes of sintered cores to be recycled without serious difficulties in sintering.
In order to attain the above-mentioned object, a production process of a Mn—Zn ferrite according to the present invention comprises the steps of: milling a sintered core of Mn—Zn ferrite for recycling; subjecting a recycled powder to a component adjustment so as to have a composition of 44.0 to 49.8 mol % Fe
2
O
3
, 4.0 to 26.5 mol % ZnO, 0.02 to 1.00 mol % Mn
2
O
3
and a remainder being MnO; pressing a mixed powder subjected to the component adjustment; and sintering a green compact obtained by pressing the mixed powder.
Amount of powder to be recycled, that is a recycled powder, can be arbitrarily selected. When the recycled powder has a target component composition, all mixed powder for pressing may be recycled And, when the recycled powder does not have a target component composition, the components must be adjusted by appropriately adding respective raw material powders of Fe
2
O
3
, ZnO, MnO or the like.
As Fe
2
O
3
content is restricted to less than 50 mol % in the present invention as mentioned above, vacancies of oxygen ions are easily formed in the sintering step and the density of a sintered core is easily increased. Therefore, when the sintering (heating—maintaining temperature—cooling) is conducted in an atmosphere containing an appropriate amount of oxygen, the resultant sintered core has sufficiently high density even if a recycled powder is used. However, as too small Fe
2
O
3
content results in lowering the initial permeability, at least 44.0 mol % Fe
2
O
3
must be contained in the ferrite.
ZnO influences the Curie temperature and saturation magnetization. Too large amount of ZnO lowers the Curie temperature to result in practical problems, but on the contrary, too small amount of ZnO reduces the saturation magnetization, so it is desirable for ZnO content to be set to the above-mentioned range of 4.0 to 26.5 mol %.
A manganese component in the above-mentioned ferrite exists as Mn
2+
and Mn
3+
. Since Mn
3+
distorts a crystal lattice: thereby significantly lowering the initial permeability, Mn
2
O
3
content is set to 1.00 mol % or less. However, since too small Mn
2
O
3
content lowers the electrical resistivity significantly, at least 0.02 mol % Mn
2
O
3
must be contained in the ferrite.
It is desirable for the lower limit of the average gram size of the recycled powders to be set to about 1.0 &mgr;m similarly to the prior art, However, even when the average grain size exceeds 1.4 &mgr;m and measures, for example about 2.0 &mgr;m, sufficiently high density is obtained in the sintering.
Since the present invention relates to recycling of wastes of Wintered cores, a recycled ferrite naturally contains additives contained in the wastes of sintered cores. Generally, CaO, SiO
2
, ZrO
2
, Ta
2
O
5
, HfO
2
, Nb2O
5
, V
2
O
5
, Bi
2
O
3
, In
2
O
3
, CuO, MoO
3
, WO
3
, Al
2
O
3
, TiO
2
and SnO
2
are used as additive. Therefore, the recycled ferrite in the present invention may contain a slight amount of one or more of these additives.
In the presen

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