Ferrite core and its production method

Details Ferrite core and its production method Ferrite core and its production method

2002-08-19

2004-06-22

Koslow, C. Melissa (Department: 1755)

Compositions

Magnetic

Iron-oxygen compound containing

Reexamination Certificate

active

06752932

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a ferrite core which is adapted for use in a transformer or a choke coil used at a high temperature, and its production method. To be more specific, this invention relates to a ferrite core which exhibits a high saturation flux density at a high temperature of 100° C. or higher, and in particular, at a temperature around 150° C., and which has high magnetic stability with reduced deterioration in high temperature storage, as well as its production method.
A soft ferrite which is used in producing a magnetic core should have a high saturation flux density and a low power loss. Such ferrite can be used as a ferrite core in a transformer or a choke coil of a DC—DC converter in an EV (electric vehicle) or HEV (hybrid electric vehicle), or as a ferrite core to be placed near the engine of an automobile which will be exposed to a high temperature.
Various properties are required for such soft ferrite core which is used at a high temperature. Exemplary such properties include excellent durability with reduced magnetic deterioration during use at a high temperature, the saturation flux density which experience no significant decrease at a high temperature, and low power loss.
Various proposals have been made to fulfill such requirements. For example, JP-A 10-64715 proposes a magnetic core material of low loss ferrite comprising a MnZnNi ferrite in order to provide a ferrite magnetic core material which exhibits a low loss and a high saturation flux density for a relatively broad frequency band of about 100 kHz to 500 kHz.
The magnetic core material of MnZnNi ferrite disclosed in JP-A 10-64715, however, was still insufficient in the saturation flux density Bs and loss at a high temperature of 100° C. or higher, and in particular, at around 150° C. as well as in the magnetic stability although it had sufficiently high saturation flux density Bs and sufficiently low loss at 80° C.
JP-A 2-83218 also proposes an oxide magnetic material of MnZnNi ferrite. This material has been developed to provide a material which has highly stable magnetic properties, high saturation flux density, and low power loss when used at a high temperature range of 100° C. or higher, and in particular, at 100 to 200° C. at magnetic field strength (flux density) of 1000 G (100 mT) or higher, and in particular, at 2000 to 5000 G (200 to 500 mT) or higher. In JP-A 2-83218, additives incorporated as auxiliary components are particularly defined. The material disclosed in JP-A 2-83218 exhibits dramatically improved saturation flux density in view of the state of the art at that time. However, there is an increasing demand for improving the properties of the material, and further improvements are required. In addition, despite the effectiveness of the Fe
2
O
3
rich composition of this material in attaining the high saturation flux density, it is quite unlikely that deterioration of the magnetic properties at a high temperature can be effectively avoided as in the case of JP-A 2-83218 in the highly Fe
2
O
3
rich region not tested in JP-A 2-83218 by merely limiting the content of the auxiliary components (additives) to predetermined ranges.
SUMMARY OF THE INVENTION
An object of the present invention is to obviate the situation as described above, and to provide a ferrite core which has high saturation flux density Bs at a high temperature of 100° C. or higher, and in particular, at around 150° C., and which has excellent magnetic stability at a high temperature, experiencing reduced deterioration of magnetic properties, and in particular, reduced core loss at such high temperature (even by trading off some improvement in the level of the loss).
Such an object is achieved by the present invention as defined below.
(1) A ferrite core containing 55 to 59 mol % of iron oxide calculated in terms of Fe
2
O
3
, more than 0 to 15 mol % of zinc oxide calculated in terms of ZnO, 2 to 10 mol % of nickel oxide calculated in terms of NiO, and the balance of manganese oxide (MnO) as its main components, wherein
when the main components has a composition represented by the formula:
(Zn
2+
a
, Ni
2+
b
, Mn
2+
c
, Mn
3+
d
, Fe
2+
e
,Fe
3+
f
)O
4+&dgr;
  (1)
wherein a, b, c, d, e and f meet the relations:
a+b+c+d+e+f
=3, and
&dgr;=
a+b+c
+(3/2)
d+e
+(3/2)
f
−4
the value of &dgr; in formula (1) is such that:
0≦&dgr;≦2.5×10
−3
.
(2) A ferrite core according to the above (1) wherein
0≦&dgr;≦2.0×10
−3
.
(3) A ferrite core according to the above (1) wherein
0≦&dgr;≦1.0×10
−3
.
(4) A ferrite core according to the above (1) wherein
0≦&dgr;≦0.5×10
−3
.
(5) A ferrite core according to the above (1) wherein
0<&dgr;.
(6) A ferrite core according to the above (1) containing 56 to 57 mol % of iron oxide calculated in terms of Fe
2
O
3
, 5 to 10 mol % of zinc oxide calculated in terms of ZnO, 3 to 6 mol % of nickel oxide calculated in terms of NiO, and the balance of manganese oxide (MnO) as its main components.
(7) A ferrite core according to the above (1) which has
a saturation flux density at 100° C. of at least 430 mT, and a saturation flux density at 150° of at least 350 mT when measured by applying a magnetic field of 1000 A/m, and
a core loss at 100° C. of up to 1200 kW/m
3
when measured by applying a sine-wave AC magnetic field of 100 kHz and 200 mT.
(8) A ferrite core according to the above (1) which has
a saturation flux density at 100° C. of at least 450 mT, and a saturation flux density at 150° of at least 380 mT when measured by applying a magnetic field of 1000 A/m, and
a core loss at 100° C. of up to 900 kW/m
3
when measured by applying a sine-wave AC magnetic field of 100 kHz and 200 mT.
(9) A ferrite core according to the above (1) wherein increase in the core loss is up to 4% when stored at 150° C. for 2000 hours.
(10) A ferrite core according to the above (1) wherein increase in the core loss is up to 3% when stored at 150° C. for 2000 hours.
(11) A ferrite core according to the above (1) wherein increase in the core loss is up to 10% when stored at 175° C. for 2000 hours.
(12) A ferrite core according to the above (1) wherein increase in the core loss is up to 50% when stored at 200° C. for 2000 hours.
(13) A method for producing the ferrite core of the above (1) comprising the step of firing a molded article, wherein
the firing step comprises heating stage, steady temperature stage, and cooling stage in this order, and
the article is kept in the steady temperature stage at a temperature (steady temperature) of at least 1250° C. with the oxygen concentration of the atmosphere kept at 0.05 to 2.0%.
(14) A method for producing the ferrite core according to the above (13) wherein the oxygen concentration of the atmosphere in the steady temperature stage is kept at 0.05 to 0.8%.
(15) A method for producing the ferrite core according to the above (13) wherein the temperature (steady temperature) in the steady temperature stage is up to 1400° C.
(16) A method for producing a ferrite core according to the above (13) wherein the cooling stage is accomplished such that,
when a specific temperature in 900 to 1200° C. is designated Tn, and when the temperature is reduced from the steady temperature to the temperature Tn,
the oxygen concentration P
O
2
(unit: %) of the atmosphere at temperature T (unit: K) is either gradually or incrementally reduced to satisfy the relation:
Log(
P
O
2
)=
a−b/T
wherein a is 3 to 14, and b is 5000 to 23000, provided that a and b may or may not alter with the decrease in the temperature T;
when the temperature reaches Tn, the oxygen concentration of the atmosphere is reduced to the level of 0 to 0.01%; and
the temperature is reduced from Tn to the room temperature at a cooling rate which is 2 to 10 times faster than the cooling rate used in the cooling from the steady temperature to the temperature Tn.
(17) A method for producing a ferrite core according t

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