JP2000327411A - Production of nickel - zinc based ferrite - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decrease core loss by forming a firing raw material to have a specific composition after firing, firing at a temp. of a specific range and controlling the concentration of oxygen in an atmosphere at least in the meantime to reach a specific temp. in a cooling process from the half way of the firing process to equal to or below a specific ratio. SOLUTION: The firing raw material is prepared by adding and blending molybdenum oxide in the quantity satisfying <=1 pt.wt. per 100 pts.wt. base raw material expressed in terms of MoO3 after firing into the base raw material having the component composition after firing, which satisfies 48-51 mol% iron oxide expressed in terms of Fe2O3, 1-30 mol% zinc oxide expressed in terms of ZnO, <=2 mol% cobalt oxide expressed in terms of CoO and the balance nickel oxide. The firing temp. is controlled to 1,000-1,130 deg.C and the concentration of oxygen in the atmosphere in the meantime from the half way of the firing process to reach 500 deg.C in the cooling process is controlled to <=5 vol.%. The core loss is reduced even in the excitation with heavy current in a high frequency band of 1-10 MHz and a desirable core material of a transformer is obtained by integrating a winding and a magnetic body.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a Ni-Zn ferrite having a small core loss in a frequency band of 1 to 10 MHz. Ni-Zn obtained by the present invention
The system ferrite is used for the main transformer for switching power supply used especially in the high frequency band of 1 to 10 MHz, and especially for the core material for low profile type transformer using thin-shaped core and the winding like chip transformer and thin film transformer. The magnetic material is suitable as a core material for a transformer having an integral structure.

[0002]

BACKGROUND OF THE INVENTION Switching power supplies have been
It is generally used at a frequency of 0 kHz band, and Mn-Zn based ferrite has been conventionally used as a transformer material of such a switching power supply. However, recently, with the reduction in size and weight of electronic devices, a magnetic material having a small core loss has been required even in a higher frequency band.

[0003] In order to meet such demands, even in the case of Mn-Zn ferrite, the basic composition, a small amount of additives, the pulverization method, the sintering method, and the like are devised to improve the frequency range up to about 2 to 3 MHz. For example, a material having small core loss and desired temperature characteristics can be obtained. However, conventional Mn-
In the case of Zn-based ferrite, it has been difficult to stably obtain a ferrite having a small core loss in a high frequency band exceeding 2 to 3 MHz. Here, the core loss is a hysteresis loss,
It means the sum of eddy current loss and residual loss.

[0004] Therefore, in a high-frequency band exceeding 2 to 3 MHz, Ni-Zn-based ferrite, which has a small core loss and a high resistance in a high-frequency band, has been attracting attention as a substitute for the Mn-Zn-based ferrite.

[0005] With the recent trend of miniaturization and automatic mounting of components, small-sized inductance elements and small-sized transformer components have been formed into chips. In such chip components, windings and electrode terminals and magnetic materials are used. Has a high specific resistance as the magnetic material used for
-Zn ferrite is already used. However, since the conventional Ni-Zn ferrite was developed for processing a weak signal of the exciting current, there was a problem that the hysteresis loss increased when excited with a large current like a transformer for a switching power supply. .

[0006] In this regard, the inventors have previously described Japanese Patent Application Laid-Open No. 10-2560.
No. 24 shows small core loss in high frequency band
A Ni-Zn ferrite was proposed. This technique aims to reduce the core loss in the high-frequency band of Ni-Zn ferrite by using CoO as one of the basic components. In fact, by using CoO, Ni-Zn containing no CoO is used. A core loss smaller than that of the system ferrite was obtained.

However, in this technique, as shown in FIG. 1, the temperature Tmin at which the core loss reaches a minimum value decreases (curve A: corresponding to the invention of claim 1 of Japanese Patent Application Laid-Open No. 10-256024), It deviates from the operating temperature of 80 to 100 ° C. In this regard, the temperature characteristics can be controlled by adjusting the mixing ratio of ZnO, but in this case, as shown by the curve B (corresponding to the invention of claim 3 of Japanese Patent Application Laid-Open No. 10-256024). The disadvantage is that the core loss in the operating temperature range of the transformer increases.

[0008]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problem, and has a small core loss even when excited with a large current in a high-frequency band of 1 to 10 MHz [for example,
Under the condition of 0 ° C. and Bm × f ≦ 40 kHz · T (where Bm is the maximum magnetic flux density during driving, f is the driving frequency), the core loss is
180 kW / m ^{3 or} less] It is an object of the present invention to propose an advantageous method for producing Ni-Zn ferrite. Here, the smaller the core loss, the less heat is generated, and the higher the specific resistance, the more excellent the insulating property.

[0009]

Means for Solving the Problems Now, the present inventors have proposed a Co-containing Ni-Zn which has become a problem in Japanese Patent Application Laid-Open No. Hei 10-256024.
Focusing on the core ferrite core loss temperature characteristics, as a result of intensive research, ascertained that a small core loss can be stably obtained in the operating temperature range of the transformer by devising the firing conditions, and complete the present invention. It was reached.

That is, according to the present invention, the component composition after calcination is 48 to 51 mol% of iron oxide in terms of Fe ₂ O ₃ , and 1 in terms of ZnO.
In a basic raw material adjusted to a composition satisfying 〜30 mol% of zinc oxide, less than 2 mol% of cobalt oxide in terms of CoO, and the balance being nickel oxide, after sintering, 1 part by weight in terms of MoO ₃ ( (Basic raw material: 100 parts by weight) Ni-Zn ferrite is manufactured by baking and cooling the Ni-Zn ferrite raw material to which the following amount of molybdenum oxide is added and blended. Ni, wherein the sintering temperature is in a temperature range of 1000 to 1130 ° C., and the oxygen concentration in the atmosphere is at least 5 vol% during at least the middle of the sintering step to 500 ° C. in the cooling step. This is a method for producing a Zn-based ferrite.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. First, the reason for limiting the component composition of the Ni—Zn ferrite raw material to the above range in the present invention will be described. Iron oxide: 48 to 51 mol% (calculated as Fe ₂ O ₃₎ content of iron oxide and less than 48 mol% calculated as Fe ₂ O _3, it is possible to reduce the hysteresis loss at 100 ° C. at the time of high frequency driving On the other hand, if it exceeds 51 mol%, not only does the electrical resistance drop sharply and eddy current loss increases, but also direct winding to the core, which is one of the advantages of Ni-Zn ferrite, is not possible. Occurs. A more preferred content of the iron oxide is from 48.5 to 50 mol% calculated as Fe ₂ O _3, the smallest core loss in this range can be stably obtained.

Zinc oxide: 1 to 30 mol% (in terms of ZnO) If the content of zinc oxide is less than 1 mol%, the hysteresis loss cannot be reduced, while if it exceeds 30 mol%, the core loss in a high frequency band becomes large. At 100 ° C., the residual loss increases significantly (particularly at 3 MHz or higher). A more preferred content of zinc oxide is 18 in terms of ZnO.
2828 mol%, and the smallest core loss can be stably obtained in this range.

Cobalt oxide: 2 mol% or less (in terms of CoO) If the content of cobalt oxide exceeds 2 mol%, it becomes difficult to adjust the temperature (Tmin) at which the core loss shows the minimum value to 80 to 100 ° C.

The basic component other than iron oxide, zinc oxide and cobalt oxide is nickel oxide (NiO), with a preferred content of 19 to 50 mol%. Nickel oxide (NiO equivalent)
If the content of is less than 19 mol%, the core loss at high frequency becomes large, so the residual loss at 100 ° C increases, while the 50m
If it exceeds ol%, hysteresis loss will increase. The more preferred content of nickel oxide is 22 to 32 mol% in terms of NiO, and the smallest core loss can be stably obtained in this range.

Although the component composition of the basic raw material has been described above, in the present invention, in order to obtain a Ni-Zn ferrite having a small core loss, molybdenum oxide is further contained as an auxiliary raw material in addition to the basic raw material described above. Let it. Molybdenum oxide: 1 part by weight or less (in terms of MoO ₃ ) Molybdenum oxide effectively contributes to a reduction in core loss, but when the content exceeds 1 part by weight with respect to 100 parts by weight of the above basic material, abnormal grain growth of crystal grains. And the effect of reducing core loss cannot be obtained. More preferably MoO ₃
0.15 to 0.6 parts by weight, more preferably 0.25 to 0.45
Parts by weight.

Although the composition range of ferrite has been described above, in the present invention, the average crystal grain size of the Ni—Zn ferrite obtained by firing is 0.5 to 4 μm.
Is preferably limited to the range of That is, if the average particle size is smaller than 0.5 μm,
Since it is necessary to finely pulverize the powder to m or less, it takes a long time for the pulverization and the production efficiency is deteriorated. In addition, since the amount of impurities mixed from the pulverization medium increases, the magnetic properties such as core loss are liable to fluctuate. In addition, there is a problem that it is very difficult to stably manufacture the fine powder, for example, a defect is easily generated and a molding yield is reduced. On the other hand, the average crystal grain size is 4 μm
If it is larger, the residual loss increases and the core loss in the high frequency band increases.

Further, the sintered density of the Ni-Zn ferrite is preferably set to 85% or more of the theoretical density. This is because if the sintered density is less than 85%, the effective magnetic material occupation ratio is low, so that the core loss cannot be sufficiently reduced.

By the above-mentioned addition of molybdenum oxide,
The mechanism for improving the core loss of the Ni—Zn ferrite can be explained as follows according to the study by the inventors. That is, when MoO ₃ having a boiling point in the firing temperature range (boiling point of a single substance: 1257 ° C.) is added, part of MoO ₃ evaporates from the grain boundaries during firing, and the compressive stress remaining at the grain boundaries is relaxed. It can be inferred that the hysteresis loss becomes smaller. MoO ₃ is a low-melting point oxide (melting point: 795 ° C.), which generates a liquid phase during the heating process during firing, promotes the densification of the sintered body, and promotes the soaking process during firing. Contributes to the uniformization of the crystal grain size. As a result, it is presumed that the ratio of coarse crystal grains (> 4 μm) that causes residual loss decreases, and core loss in a high frequency band decreases.

Furthermore, since Co ferrite has a negative crystalline magnetic anisotropy, if Ni-Zn ferrite contains cobalt oxide, CoO forms a solid solution and the crystalline magnetic anisotropy in the crystal grains is almost zero. Becomes As a result, it is assumed that the core loss in the high frequency band is reduced.

However, when firing in the air as in the method disclosed in Japanese Patent Application Laid-Open No. 10-256024,
The extent of decrease in core loss due to solid solution of
At around room temperature, a significant reduction effect is obtained, but 80 to 12
In the temperature range of 0 ° C., the effect of lowering around room temperature cannot be obtained. As a result, the temperature (Tmin) at which the core loss shows the minimum value shifts to around room temperature. Generally, a ferrite core used for a transformer has a Tmin
= 80 to 100 ° C., it is necessary to suppress the fluctuation of Tmin due to the introduction of CoO. In the above-mentioned Japanese Patent Application Laid-Open No. Hei 10-256024, in order to set the Tmin of the Ni—Zn ferrite containing CoO to 80 to 100 ° C., ZnO,
Although the mixing ratio of NiO was adjusted, such a method provided a desired temperature characteristic, but had a problem that hysteresis loss increased and core loss increased as a whole.

In order to clarify the reason why the effect of lowering the core loss due to the introduction of CoO differs depending on the temperature, the present inventors increased the core loss of Ni—Zn ferrites having different CoO contents by changing the frequency f: 200 Hz to 200 Hz. Core loss components were analyzed by measuring over various ranges of 5 MHz, applied magnetic flux density Bm: 20 mT, and measurement temperature T: 23 to 140 ° C. The core loss per cycle was calculated by dividing the core loss by the measurement frequency, and the frequency characteristics were examined.
That a core loss peak exists in a frequency band of less than Hz, that the peak intensity increases with the CoO content, that the frequency showing the peak changes with the measurement temperature, and that T =
Since the frequency showing the peak shifts to the high frequency side with the temperature from 60 to 140 ° C, 1 MH hits the base of the high frequency side.
It was found that the core loss near z also increased with temperature.

Further, from the Arrhenius plot of the relationship between the frequency showing the peak and the temperature, the relaxation time τ∞ = 2 × 16
^-16 (sec), the activation energy Ea was found to be a loss peak due to a relaxation phenomenon of Ea = 0.9 (eV). J.Phys.Sc
According to o.Jpn.18, p1441 (1963), similar τ∞ and Ea
The relaxation phenomenon with is also measured by the loss factor tanδ of Ni-Zn ferrite containing CoO.
It is concluded that this is due to the diffusion of electrons between the Co ²⁺ and Co ³⁺ ions.

From the above considerations, the present inventors have found that in order to improve the temperature characteristics of core loss of Ni—Zn ferrite containing cobalt oxide, electron diffusion between Co ²⁺ -Co ³⁺ ions is suppressed. It is necessary to achieve this, and the inventors have reached the idea that it is effective to keep the Co ³⁺ concentration in the Ni—Zn ferrite as low as possible.

In the case of a Ni—Zn ferrite having a stoichiometric composition (Fe ₂ O ₃ = 50 mol%), CoO is present in the spinel lattice as Co ²⁺ , but non-amount of Fe ₂ O ₃ <50 mol% In the theoretical composition, Co ³⁺
Co ²⁺ coexists. Therefore, as a method of keeping the Co ³⁺ concentration as low as possible, it is effective to set Fe ₂ O ₃ ≧ 50 mol%. However, when Fe ₂ O ₃ > 51 mol%, the concentration of Fe ²⁺ increases,
Since there are problems such as a decrease in specific resistance due to the transfer of electrons between Fe ^{3+ and} Fe ²⁺ and an increase in eddy current loss associated therewith, Fe ₂ O ₃
The concentration cannot be increased unnecessarily. So, F
As a method of keeping the Co ³⁺ concentration low while keeping e ₂ O ₃ = 48 to 51 mol%, a method of reducing the oxygen concentration in the firing atmosphere can be considered. With this method, Fe ₂ O ₃ , NiO, Zn
Co ³⁺ without changing the mixing ratio of other basic components such as O
The concentration can be suppressed.

As described above, the Ni-Zn ferrite of the present invention achieves an improvement in core loss utilizing the thermal properties of molybdenum oxide and an appropriate core loss temperature characteristic by suppressing the Co ³⁺ concentration in the ferrite. It was done. Therefore, in the method for producing a Ni—Zn ferrite according to the present invention, the soaking temperature (sintering temperature) in the sintering process is maintained in the range of 1000 to 1130 ° C., and at least 500 ° C. It is important to adjust the oxygen concentration in the atmosphere up to 5 vol% to a range of 5 vol% or less.

That is, if the firing temperature is lower than 1000 ° C.,
Hysteresis loss is increased due to the small sintering density and crystal grain size.On the other hand, when the temperature exceeds 1130 ° C, the crystal grain size is coarsened and the residual loss is increased. As a result, the evaporation of MoO ₃ is suppressed, so that the effect of reducing the hysteresis loss cannot be obtained. Further, if the oxygen concentration in the atmosphere during the period from the firing step to 500 ° C. during the cooling step exceeds 5 vol%, a sufficient effect of suppressing the Co ³⁺ concentration cannot be obtained. If the oxygen concentration is too low, a different phase may precipitate at the grain boundaries and the hysteresis loss may increase.
The lower limit of the oxygen concentration is preferably set to about 0.001 vol%.

Further, the temperature range in which the oxygen concentration is adjusted to a range of 5 vol% or less may be applied to the entire temperature range in the firing process. Since the core is likely to be cracked due to rapid evaporation, it is desirable to lower the oxygen concentration after the soaking process. However, if the time to lower the oxygen concentration becomes late, it becomes difficult to suppress the Co ³⁺ concentration uniformly even to the inside of the core, so at least 30 minutes in a temperature range of 1000 to 1130 ° C should be in a low oxygen atmosphere It is desirable. In order to prevent re-oxidation during the cooling process, it is necessary to cool in an atmosphere having a low oxygen concentration up to at least 500 ° C.

That is, as shown in FIG.
It is desirable that the oxygen concentration in the atmosphere is rather high up to about 600 ° C at which the decomposition of the binder is completed, but then the oxygen concentration in the first half of the temperature raising process and the soaking process up to the firing temperature of 1000 to 1130 ° C May be less than 5 vol% or more than 5 vol%. However, it is essential that the oxygen concentration in the atmosphere during the last half of the soaking process and during the cooling process up to 500 ° C. be 5 vol% or less. In addition, 500 ℃
In the following temperature range, the oxygen concentration in the atmosphere may be 5 vol% or less or more than 5 vol%, as in the first half of the temperature raising process and the soaking process.

The firing step according to the present invention has been described above.
It is good to follow the usual law. That is, after mixing the basic raw materials first, firing at a temperature of about 800 to 950 ° C for several hours (calcination)
Then, after adding molybdenum oxide, pulverizing, mixing and drying, and after adding a binder, granulation is performed. Also, after adding molybdenum oxide to the calcined product, pulverize,
Then, after adding the binder, mixing and drying may be performed. Note that the addition of cobalt oxide may be performed simultaneously with the addition of molybdenum oxide, and an oxide or carbonate of each metal can be used as a basic material. Then, it is molded into a predetermined shape, such a molding method,
As well as a normal compression molding method, a method may be used in which a binder is added to a raw material for firing to form a paste, which is then formed into a laminate by a doctor blade method, a paste printing method, or the like.

Thus, in the present invention, Co is contained.
In order to suppress the increase in core loss in the operating temperature range of the transformer, which was a problem with Ni-Zn ferrite, and to obtain appropriate temperature characteristics, the oxygen concentration in the atmosphere during firing was adjusted to The Co ³⁺ concentration is suppressed, and as a result, as shown by the curve C in FIG. 1, a reduction in core loss in the operating temperature range of the transformer is realized, and a desired core loss temperature characteristic is realized. In this regard, JP-A-10-256024
In the sintering in the atmosphere as disclosed in Japanese Patent Application Laid-Open Publication No. H11-129, it is not possible to control the Co ³⁺ concentration in the sintered body, so that it was necessary to increase the amount of ZnO at the expense of hysteresis loss in order to obtain the desired core loss temperature characteristics. However, according to the present invention, the temperature characteristics can be optimized without changing the basic composition.

Example 1 Fe was prepared such that the basic component composition shown in Table 1 was obtained after firing.
Weigh ₂ O ₃ , ZnO, CoO (or Co ₃ O ₄ ) and NiO,
After wet mixing, the mixture was calcined at 850 ° C for 3 hours to obtain a calcined Ni-Zn ferrite powder. Next, for this calcined powder, Table 1
After adding MoO ₃ by the amount shown in the following, wet pulverization and drying, after adding PVA as a binder and granulating,
Molding pressure: molded at 1 ton / cm ^2, an outer diameter: 24 mm, inner diameter: 12
A toroidal shaped body having a thickness of 5 mm and a height of 5 mm was obtained. Then, the temperature of this molded body was raised to 1120 ° C. in air, and after holding for 3 hours, air and nitrogen gas were mixed to obtain an oxygen concentration of 1 vol%.
After holding for 1.5 hours in the same atmosphere, it was cooled to about 100 ° C. in the same atmosphere to obtain a Ni—Zn ferrite. Table 1 also shows the results of measurement of the core loss (3 MHz, 10 mT, 100 ° C.) of the Ni—Zn ferrite thus obtained.

[0032]

[Table 1]

As is clear from the table, any material satisfying the proper component composition range of the present invention is 3 MHz, 10 m
Excellent characteristic values with core loss at T, 100 ° C of 37 kW / m ³ or less could be obtained.

Example 2 Fe was adjusted to have the same basic component composition as No. 2 in Table 1._TwoO_Three,
ZnO, CoO (or Co _ThreeO_Four) And NiO are weighed and wet mixed.
After combining, Ni-Zn ferrite
A calcined powder was obtained. Next, for this calcined powder: 100 parts by weight
And MoO_Three0.3 parts by weight, wet-pulverized and dried
After adding PVA as a binder and granulating,
Molding pressure: 1ton / cm^TwoOuter diameter: 24mm, inner diameter: 12
A toroidal shaped body having a thickness of 5 mm and a height of 5 mm was obtained. About
Then, the obtained molded body is fired in air at 1000 to 1130 ° C.
Temperature, then change to an atmosphere with an oxygen concentration of 2 vol%
After firing for 1 to 10 hours,
℃, Ni with different sintering density and average grain size
-A Zn ferrite was obtained. The Ni-Zn based
Average grain size, relative sintered density and 1MHz
The results of a study on core loss at 20 mT and 100 ° C are as follows.
It is shown in Table 2.

[0035]

[Table 2]

As is clear from the table, any one satisfying the preferred particle size and sintered density range of the present invention is as follows:
The core loss at 1 MHz, 20 mT, and 100 ° C shows excellent characteristic values of 140 kW / m ³ or less.

The average crystal grain size was determined as follows. A diagonal line was drawn on a SEM photograph of the fracture surface of the Ni—Zn ferrite or an optical microscope photograph after polishing, and the number of crystal grains orthogonal to the diagonal line was obtained, and calculated by the following equation. Average crystal grain size (μm) = length of diagonal line 結晶 number of crystal grains orthogonal to diagonal line × magnification of photograph Here, the magnification of the photograph is set so that the number of crystal particles perpendicular to the diagonal line is 20 to 40. Selected. The relative sintering density was determined from the following equation using the sintering density and the theoretical density measured by the Archimedes method. Relative sintered density (%) = Sintered density ÷ Theoretical density × 100 Theoretical density (g / cm ³ ) = (density of ZnFe ₂ O ₄ ) × [ZnO (mol
%) ZnO (mol%) + NiO (mol%) + CoO (mol%)
｝] + (Density of NiFe ₂ O ₄ ) × [NiO (mol%) ÷ ｛ZnO (mo
l%) + NiO (mol%) + CoO (mol%)｝] + (density of CoFe ₂ O ₄ ) × [CoO (mol%) ÷ ｛ZnO (mol%) + NiO (mol
%) + CoO (mol%)}] Here, the density of ZnFe ₂ O ₄ = 5.33 g / cm ³ The density of NiFe ₂ O ₄ = 5.38 g / cm ³ The density of CoFe ₂ O ₄ = 5.29 g / cm ³

Example 3 Fe was adjusted to have the same basic component composition as No. 2 in Table 1._TwoO_Three,
ZnO, CoO (or Co _ThreeO_Four) And NiO are weighed and wet mixed.
After combining, Ni-Zn ferrite
A calcined powder was obtained. Next, for this calcined powder: 100 parts by weight
And MoO_Three0.5 part by weight, wet-pulverized and dried
Then, after adding PVA as a binder and granulating,
Form pressure: 1ton / cm^TwoMolded with outer diameter: 24mm, inner diameter: 12m
m, a toroidal shaped body having a height of 5 mm was obtained. About
Then, the obtained molded body was heated to a firing temperature shown in Table 3 in air.
After adjusting the temperature to the oxygen concentration shown in Table 3,
It was baked for 3 hours. Then, about 100 in the same atmosphere as firing
It cooled to ° C and obtained Ni-Zn system ferrite. Thus gain
Core loss of Ni-Zn ferrite (3MHz, 20mT,
100 ° C.) are also shown in Table 3.

[0039]

[Table 3]

As is clear from the table, any of the present invention satisfying the proper firing temperature and oxygen concentration is 3%.
Excellent characteristic values with core loss of 500 kW / m ³ or less at MHz, 20 mT, and 100 ° C are obtained.

Example 4 Fe was adjusted to have the same basic component composition as No. 2 in Table 1._TwoO_Three,
ZnO, CoO (or Co _ThreeO_Four) And NiO are weighed and wet mixed.
After combining, Ni-Zn ferrite
A calcined powder was obtained. Next, for this calcined powder: 100 parts by weight
And MoO_ThreeAfter adding 0.4 parts by weight, wet pulverization and drying
After adding PVA as a binder and granulating,
Molding pressure: 1ton / cm^TwoOuter diameter: 24mm, inner diameter: 12
A toroidal shaped body having a thickness of 5 mm and a height of 5 mm was obtained. About
Then, the obtained molded body was fired under the conditions shown in Table 4,
Upon cooling, a Ni-Zn ferrite was obtained. Thus obtained
Core loss of Ni-Zn ferrite (3MHz, 20mT, 80
C) are also shown in Table 4.

[0042]

[Table 4]

As can be seen from the table, according to the present invention, any of the cases where the oxygen concentration was reduced at an appropriate time,
The core loss at 3 MHz, 20 mT, and 80 ° C was 500 kW / m ³ or less, and excellent characteristic values were obtained.

Example 5 As shown in Table 5, Ni—Zn ferrite (same as No. 2 in Table 1) manufactured according to the present invention and (a) CoO manufactured under similar conditions to No. 2 Ni-Zn ferrite not containing, (b) Ni-Zn containing CoO but fired in air
Fig. 3 shows the results of examining the temperature characteristics of core loss (3MHz, 10mT, 100 ° C) for the system ferrite and (c) the Ni-Zn system ferrite in which the amount of ZnO was reduced by the method disclosed in JP-A-10-256024. Show.

[0045]

[Table 5]

As shown in the figure, (a) Ni-Zn ferrite containing no CoO has a large core loss over the entire temperature range, but by containing CoO as shown in (b), the core loss is reduced near room temperature. We were able to. on the other hand,
As shown in (c), when the ZnO content is reduced to make Tmin = 80 ° C., the core loss is lower than that of (a), but the core loss increases in the temperature range of 23 to 100 ° C. as compared with (b). On the other hand, when manufactured by the method of the present invention, Tmin can be shifted to a high temperature side while maintaining the small loss of (b), and Ni-Ni having good temperature characteristics as a transformer material for a high-frequency power supply can be obtained. Zn ferrite was obtained.

[0047]

Thus, according to the present invention, 1 to 10 M
Even when excited with a large current in the high frequency band of Hz,
Ni-Zn with small core loss in the operating temperature range of the transformer
A system ferrite can be obtained stably. Therefore, the Ni-Zn ferrite of the present invention is effective as a core material of a transformer having a structure in which a winding and a magnetic material are integrated, such as a switching power supply chip transformer and a thin film transformer used at 1 to 10 MHz.

[Brief description of the drawings]

FIG. 1 is a graph comparing temperature characteristics of core loss.

FIG. 2 is a diagram showing a suitable atmosphere in a temperature raising, soaking, and cooling process.

FIG. 3 is a graph showing a comparison between core loss temperature characteristics of an invention example and a comparative example.

Claims (1)

[Claims] The composition of the composition after firing is 48 to 51 mol% iron oxide in terms of Fe ₂ O ₃ , 1 to 30 mol% zinc oxide in terms of ZnO, 2 mol% or less in terms of CoO, and The same amount of molybdenum oxide that satisfies not more than 1 part by weight (based on 100 parts by weight of the basic raw material) in terms of MoO ₃ is added to the basic raw material adjusted to have a composition that satisfies nickel oxide in the remainder. The raw material for firing the compounded Ni-Zn-based ferrite, after molding, firing, upon producing the Ni-Zn-based ferrite by cooling, the firing temperature in the temperature range of 1000 ~ 1130 ° C., at least in the firing step A method for producing a Ni-Zn-based ferrite, wherein the oxygen concentration in the atmosphere from the middle to 500 ° C during the cooling step is 5 vol% or less.