WO2020189037A1 - MnZn-BASED FERRITE AND METHOD FOR MANUFACTURING SAME - Google Patents

Abstract

Provided is a MnZn-based ferrite that has excellent magnetic and mechanical properties, and can be suitably used for vehicle-mounted electronic components. This MnZn-based ferrite has a basic component and a sub component adjusted to within appropriate ranges, has the amounts of P, B, and Ti, which are inevitable impurities, limited to less than 10 mass ppm, less than 10 mass ppm, and less than 50 mass ppm, respectively, and has a surface residual stress of less than 40 MPa.

Description

MnZn-based ferrite and its manufacturing method

The present invention relates to MnZn-based ferrite and a method for producing the same, which are particularly suitable for magnetic cores of automobile-mounted parts.

MnZn ferrite is a material widely used as a magnetic core for noise filters such as switching power supplies, transformers, and antennas. Among the soft magnetic materials, MnZn ferrite has high magnetic permeability and low loss in the kHz region, and is cheaper than amorphous metals and the like.

Here, the magnetic core of electronic devices for automobile mounting applications, whose needs are expanding due to the recent hybridization and electrical equipment of automobiles, is that they are not damaged during their use, that is, their fracture toughness value (Kic) is particularly high. Desired. This is because oxide magnetic materials such as MnZn ferrite are ceramics and are easily damaged because they are brittle materials. In addition, they are constantly vibrated in automobile-mounted applications compared to conventional home appliance applications. This is because it will continue to be used in a fragile environment.
However, in automobile applications, weight reduction and space saving are also required at the same time. Therefore, it is important that MnZn ferrite has not only a high fracture toughness value but also suitable magnetic properties as in the conventional application.

Various developments have been made in the past as MnZn ferrites for automobile mounting applications.
Patent Documents 1 and 2 and the like are reported as references to good magnetic properties, and Patent Documents 3 and 4 and the like are reported as MnZn ferrite having an increased fracture toughness value.

JP-A-2007-51052 Japanese Unexamined Patent Publication No. 2012-76983 Japanese Unexamined Patent Publication No. 4-318904 Japanese Unexamined Patent Publication No. 4-177808

Generally, in order to increase the initial magnetic permeability of MnZn-based ferrite, it is effective to reduce the magnetic anisotropy and magnetostriction. In order to realize these, it is necessary to set the blending amounts of Fe ₂ O ₃ , ZnO and MnO, which are the main components of MnZn-based ferrite, in a suitable range.
Further, there are the following methods as a method for increasing the initial magnetic permeability of the MnZn-based ferrite in the high frequency region. That is, by applying sufficient heat in the firing step to appropriately grow the crystal grains in the ferrite, the movement of the domain wall in the crystal grains in the magnetization step is facilitated, and a component that segregates at the grain boundaries is further added. Produces grain boundaries of appropriate and uniform thickness. By this method, the specific resistance of the MnZn-based ferrite is maintained to suppress the attenuation due to the frequency increase of the initial magnetic permeability, and a high initial magnetic permeability is realized even in the 100 kHz region.

In addition to the above magnetic characteristics, the magnetic core of electronic components for automobiles is required to have a high fracture toughness value so that it will not be damaged even in an environment subject to constant vibration. If the MnZn-based ferrite, which is the magnetic core, is damaged, the inductance is greatly reduced, so that the electronic component cannot perform the desired function, and as a result, the entire automobile becomes inoperable.
From the above, both the magnetic property of high initial permeability and the high fracture toughness value are required. As a specific example, it has good magnetic properties with an initial magnetic permeability of 4000 or more at 23 ° C and 100 kHz, and excellent mechanical properties with a fracture toughness value of 1.0 MPa ・ m ^1/2 or more based on JIS R1607. Characteristics are required.

However, in Patent Document 1 and Patent Document 2, although the composition for realizing the desired magnetic properties is mentioned, the fracture toughness value is not described at all, and it is not suitable as a magnetic core of an in-vehicle electronic component. I think that the.
Further, although Patent Document 3 and Patent Document 4 mention the improvement of the fracture toughness value, the magnetic characteristics are insufficient as the magnetic core of an in-vehicle electronic component, and it can be said that it is also unsuitable for this application.

Therefore, the present inventors first examined the appropriate amounts of the basic components of MnZn-based ferrite that can increase the initial magnetic permeability at 23 ° C. and 100 kHz, that is, Fe ₂ O ₃ , Zn O and Mn O.
As a result, the present inventors have created a secondary peak in the vicinity of 23 ° C., which has small magnetic anisotropy and magnetostriction, maintains specific resistance, and shows a maximum temperature characteristic of initial magnetic permeability. We have found an appropriate range of basic components that can achieve high initial magnetic permeability at ° C and 100 kHz.

Next, the present inventors add appropriate amounts of SiO ₂ , CaO, and Nb ₂ O ₅ , which are non-magnetic components segregating at the grain boundaries, to generate grain boundaries having a uniform thickness in MnZn-based ferrite, and to generate specific resistance. Was raised. Then, they have found that by using this component, it is possible to further suppress the attenuation of the initial magnetic permeability in the MnZn-based ferrite even if the frequency rises.

Furthermore, when the present inventors investigated factors effective in improving the fracture toughness value, the following two findings could be obtained.
First, the present inventors have found that suppression of abnormal grain growth is essential. Abnormal grain growth referred to in the present invention means coarse particles having a size of about 100 normal particles (abnormal particles in the present invention) due to the imbalance of grain growth during firing due to the presence of impurities or the like. (Also known as) appears. When this abnormal grain growth occurs, the strength of the portion is extremely low, so that the ferrite core is likely to break from this portion. Therefore, suppressing the growth of abnormal grains in ferrite is indispensable for improving the fracture toughness value of ferrite.

Next, the present inventors measured and considered the residual stress of the ferrite material from the X-ray diffraction of the ferrite surface. As a result, the present inventors have found that there is a correlation between the residual stress value and the fracture toughness value. That is, the brittle material breaks due to tensile stress, but if the residual stress on the surface is compressive stress or tensile stress below a certain value, crack propagation during fracture can be suppressed, so MnZn. The fracture toughness value of the ferrite material is improved.

From this point of view, the present inventors further investigated and found a means for reducing the tensile stress remaining on the surface.
It is a method of immersing a fired product after firing in the process of producing a ferrite core in an oxidizing liquid having a concentration of 10 N or more, for example, nitric acid, sulfuric acid, hydrochloric acid, or the like for more than 0.50 hours. The surface of conventional MnZn ferrite is slightly oxygen-deficient due to the reduction reaction during firing, and tensile stress is generated due to this. However, when the above-mentioned chemical oxidation with the oxidizing liquid is performed, oxygen is applied to the ferrite surface portion, and the tensile stress of the ferrite surface portion can be reduced.
Then, in the production method of the present invention, it is possible to effectively increase the fracture toughness value of the material by using this means.

Note that Patent Document 5 and Patent Document 6 disclose a process of immersing a ferrite fired product in an acid. However, in Patent Document 5, the acid concentration is 1 to 5% (corresponding to about 0.2 to 1.1 N for sulfuric acid, about 0.2 to 0.8 N for nitric acid, and about 0.3 to 1.5 N for hydrochloric acid). In Patent Document 6, the immersion time is as short as 6 to 30 minutes, so that the surface residual stress cannot be sufficiently reduced in any case. Further, in these documents, the purpose of immersing the ferrite is to elute Cu and adjust the inductance L value, respectively, and neither improvement of surface residual stress is described.

Japanese Unexamined Patent Publication No. 2003-286072 Japanese Unexamined Patent Publication No. 9-20554

In Patent Document 1 and Patent Document 2 described above, the fracture toughness value is not mentioned, and it can be said that improvement of such a value is impossible.
Further, in Patent Documents 3 and 4, although the toughness is improved, the desired magnetic properties cannot be realized because an appropriate composition range cannot be selected.

Here, Patent Document 7 and Patent Document 8 describe that the residual stress affects the bending strength. However, the bending strength in Patent Documents 7 and 8 is a means for evaluating those in which the strength on the outermost surface is particularly important, and is a case where no pre-crack is formed in order to evaluate the strength of the portion on the outermost surface. This is to evaluate the bending strength.
On the other hand, the improvement of fracture toughness, which is a subject of the present invention, utilizes treatment with a predetermined acid. Therefore, it is necessary to evaluate the strength at a certain depth from the surface. Therefore, the fracture toughness value in the present specification is evaluated by a bending test after making a pre-crack on the surface of the test piece.
As described above, in the MnZn-based ferrite in the present specification, the strength of the portion different from that of Patent Document 7 and Patent Document 8 is important, and in order to evaluate such a different portion, Patent Document 7 and Patent Document 8 are different from each other. Evaluating in different ways. That is, it can be seen that there is a large technical difference between the above-mentioned Patent Documents 7 and 8 and the MnZn-based ferrite in the present specification from the strength evaluation method.

Japanese Unexamined Patent Publication No. 2015-178442 Japanese Unexamined Patent Publication No. 2015-178443

As described above, it is not possible to produce an MnZn-based ferrite suitable for the magnetic core of an automobile-mounted component, particularly an in-vehicle electronic component, which is practically useful only by these known techniques.
The present invention has been made to solve such a problem, and is based on the above-mentioned new findings.

That is, the gist structure of the present invention is as follows.
1. 1. MnZn-based ferrite consisting of basic components, sub-components and unavoidable impurities.
As the above basic ingredients
Iron: 51.5 to 55.5 mol% in terms of Fe ₂ O ₃
Zinc: More than 15.5 mol% in terms of ZnO and 26.0 mol% or less and Manganese: 22.0 to 32.0 mol% in terms of MnO
Including
As opposed to the basic component, as the sub-component
SiO ₂ : 50-250 massppm,
CaO: 100 massppm or more, less than 1000 massppm and Nb ₂ O ₅ : 100 to 300 massppm
Including
The amounts of P, B and Ti in the above unavoidable impurities, respectively.
P: less than 10 mass ppm,
B: suppressed to less than 10 mass ppm and Ti: less than 50 mass ppm,
The MnZn-based ferrite having a surface residual stress value of less than 40 MPa.

2. 2. The MnZn-based ferrite according to 1 above, wherein the MnZn-based ferrite further contains CoO: 3500 mass ppm or less as a sub-component.

3. 3. The fracture toughness value of the MnZn-based ferrite in the fracture toughness measurement based on JIS R1607 is 1.00 MPa · m ^1/2 or more, and the value of the initial magnetic permeability at 23 ° C. and 100 kHz is 4000 or more. MnZn-based ferrite according to.

4. A calcination step in which a mixture of the basic components is calcined and cooled to obtain a calcination powder, and a subcomponent is added to the calcination powder obtained in the calcination step, mixed and crushed to produce crushed powder. After adding and mixing a binder to the crushed powder obtained in the mixing-crushing step and the mixing-crushing step, the granulation step of granulating and the granulating powder obtained in the granulation step are molded. A method for producing MnZn-based ferrite, which comprises a firing step of firing and a dipping step of immersing in an acid to obtain the MnZn-based ferrite according to any one of 1 to 3 above.
The dipping step is a method for producing MnZn-based ferrite in which the fired product obtained in the firing step is immersed in an oxidizing liquid having a concentration of 10 N or more for more than 0.50 hours.

5. The method for producing MnZn-based ferrite according to 4 above, wherein the oxidizing liquid is nitric acid, sulfuric acid or hydrochloric acid.

The MnZn-based ferrite of the present invention can achieve both good magnetic properties and excellent mechanical properties at a level that was not possible with conventional MnZn-based ferrites, and is particularly suitable for use in the magnetic core of electronic components mounted on automobiles. .. As good magnetic properties, for example, the initial magnetic permeability value at 23 ° C. and 100 kHz is 4000 or more, and as excellent mechanical properties, for example, the fracture toughness value based on JIS R1607 is 1.00 MPa · m ^{1. It is / 2} or more.

Hereinafter, the present invention will be specifically described.
First, in the present invention, the reason why the composition of MnZn-based ferrite is limited to the above-mentioned range will be described. In addition, iron, zinc, and manganese contained in the present invention as basic components are all shown as values converted into Fe ₂ O ₃ , ZnO, and MnO, respectively. The contents of Fe ₂ O ₃ , ZnO, and MnO are expressed in mol%, while the contents of sub-components and impurity components are expressed in mass ppm with respect to the basic component.

Fe ₂ O ₃ : 51.5 to 55.5 mol%
Of the basic components, whether Fe ₂ O ₃ is less than the appropriate amount range or more, the magnetic anisotropy becomes large and the magnetostriction also becomes large, which causes a decrease in the initial magnetic permeability. Therefore, the amount of Fe ₂ O ₃ is at least 51.5 mol%, and the upper limit is 55.5 mol%.

ZnO: More than 15.5 mol% and 26.0 mol% or less When the amount of ZnO is small, the Curie temperature becomes excessively high and the initial magnetic permeability at 23 ° C. decreases. Therefore, the content should be at least 15.5 mol% or more. I will do it. On the other hand, even when the content exceeds an appropriate amount, the secondary peak temperature at which the initial magnetic permeability shows a maximum value decreases, so that the initial magnetic permeability at 23 ° C. decreases. Therefore, the upper limit is set to 26.0 mol%. It is preferably in the range of 16.0 to 25.5 mol%, preferably 16.0 mol% or more, more preferably 18.0 mol% or more, and preferably 25.5 mol% or less.

MnO: 22.0 to 32.0 mol%
The present invention is an MnZn-based ferrite, and the balance of the principal component composition is MnO. The reason is that it is difficult to obtain good magnetic characteristics such as an initial magnetic permeability of 4000 or more at 23 ° C. and 100 kHz unless the MnO is used. The preferred range of MnO is 22.5 to 31.0 mol%, preferably 22.5 mol% or more, preferably 31.0 mol% or less, and more preferably 29.0 mol% or less.
Needless to say, the total amount of the basic components Fe ₂ O ₃ , ZnO, and MnO is 100 mol%.

The basic components have been described above, but the sub-components are as follows.
SiO ₂ : 50-250 massppm
SiO ₂ is known to contribute to the homogenization of the crystal structure of ferrite, and by adding an appropriate amount, abnormal grain growth can be suppressed and specific resistance can be increased. Therefore, by adding an appropriate amount of SiO ₂ , the initial magnetic permeability at 23 ° C. and 100 kHz can be increased, and the fracture toughness value can be increased. Therefore, at least 50 massppm of SiO _{2 is} to be contained. On the other hand, when the amount of SiO ₂ added is excessive, on the contrary, abnormal grain growth with low strength locally occurs, the fracture toughness value is remarkably lowered, and at the same time, the initial magnetic permeability is remarkably deteriorated. Therefore, the content of SiO ₂ needs to be limited to 250 mass ppm or less. The amount of SiO ₂ is preferably in the range of 60 to 230 mass ppm, preferably 60 mass ppm or more, and preferably 230 mass ppm or less.

CaO: 100 mass ppm or more and less than 1000 mass ppm CaO segregates at the grain boundaries of MnZn-based ferrite and has a function of suppressing the growth of crystal grains. Therefore, by adding an appropriate amount of CaO, the specific resistance can be increased and the initial magnetic permeability at 23 ° C. and 100 kHz can be increased. In addition, since the function of suppressing crystal grain growth suppresses abnormal grain growth, the fracture toughness value can be increased. Therefore, it is assumed that CaO is contained at least 100 mass ppm. On the other hand, when the amount of CaO added is excessive, abnormal particles appear, the fracture toughness value decreases, and the initial magnetic permeability also deteriorates. Therefore, the CaO content needs to be limited to less than 1000 mass ppm. The preferable CaO content is in the range of 130 to 850 mass ppm, preferably 130 mass ppm or more, and preferably 850 mass ppm or less.

Nb ₂ O ₅ : 100-300 massppm
Nb ₂ O ₅ segregates at the grain boundaries of MnZn-based ferrite, and has the effect of gently suppressing the grain growth and relaxing the stress. Therefore, by adding an appropriate amount of Nb ₂ O ₅ , the initial magnetic permeability can be increased, and the fracture toughness value can also be increased by suppressing the growth of abnormal grains that locally become low in strength. Therefore, it is assumed that at least Nb ₂ O ₅ is contained in 100 mass ppm. On the other hand, when the amount added is excessive, abnormal particles appear, which induces a significant decrease in fracture toughness value and deterioration of initial magnetic permeability. Therefore, it is necessary to suppress the amount of Nb ₂ O ₅ to 300 mass ppm or less. The content of Nb ₂ O ₅ is preferably in the range of 120 to 280 mass ppm, preferably 120 mass ppm or more, and preferably 280 mass ppm or less.

Next, the unavoidable impurity components to be suppressed will be described.
P: less than 10 mass ppm, B: less than 10 mass ppm, Ti: less than 50 mass ppm These are components inevitably contained mainly in the raw material iron oxide. There is no problem if the content of P and B is very small. However, when P and B are contained in a certain amount or more, abnormal grain growth of ferrite is induced, and since this site becomes the starting point of fracture, the fracture toughness value is lowered and the initial magnetic permeability is lowered. It has a serious adverse effect. Therefore, the contents of P and B were both suppressed to less than 10 mass ppm. Preferably, both the amounts of P and B are 8 mass ppm or less. The content of P is preferably 8 mass ppm or less, and the content of B is preferably 8 mass ppm or less.
Further, when the Ti content is high, not only the fracture toughness but also the initial magnetic permeability value deteriorates. Therefore, the Ti content is controlled to less than 50 mass ppm. The Ti content is preferably less than 40 mass ppm, more preferably less than 30 mass ppm.

Further, various characteristics of MnZn-based ferrite are greatly affected by various parameters regardless of the composition. Therefore, in the present invention, the following provisions can be further provided in order to have more preferable magnetic characteristics and strength characteristics.
Fracture toughness value of fine ceramics based on JIS R 1607: 1.00 MPa · m ^1/2 or more MnZn-based ferrite is a ceramic and is a brittle material, so it hardly undergoes plastic deformation. Therefore, the fracture toughness uses the SEBP method specified in JIS R 1607. In this SEBP method, the fracture toughness value (Kic) is measured by imprinting a Vickers indentation in the center of the object to be measured and performing a bending test with a pre-crack added. The MnZn-based ferrite of the present invention is intended for use in automobiles where high toughness is required, and it is desirable that the fracture toughness value obtained by the SEBP method is 1.00 MPa · m ^1/2 or more.

In order to satisfy the condition of the fracture toughness value, the value of the surface residual stress of the obtained MnZn-based ferrite needs to be less than 40 MPa. Here, the value of the surface residual stress is minute from the shift of the (551) plane peak appearing at 148.40 ° by X-ray diffraction, assuming that the surface of the MnZn-based ferrite (ferrite core) is MnFe ₂ O _4. This is the result of calculating the stress.
Since MnZn-based ferrite is a brittle material, it breaks due to tensile stress. Similarly, for glass, which is a brittle material, tempered glass in which compressive stress is applied to the surface in advance is known in order to cancel the tensile stress that causes this fracture. Inspired by this, the present inventors thought that the fracture toughness value of the ferrite core could be improved by controlling the surface stress even in MnZn-based ferrite, and repeated diligent research. As a result, tensile stress generated due to a slight oxygen deficiency due to the reduction reaction during firing remains on the surface of normal MnZn-based ferrite, and by reducing this tensile stress, MnZn-based ferrite as a material can be used. It was found that the fracture toughness value can be increased. There is a correlation between the fracture toughness value and the surface residual stress, and in order to obtain a desirable fracture toughness value of 1.00 MPa · m ^1/2 or more, the surface residual stress must be less than 40 MPa, and 38 MPa. The value is preferably 36 MPa or less, and more preferably 36 MPa or less.

In order to keep the surface residual stress value of the MnZn-based ferrite less than 40 MPa, it is necessary to immerse the fired product after firing in the ferrite core manufacturing process in an oxidizing liquid having a concentration of 10 N or more for more than 0.50 hours. The immersion temperature is preferably in the range of 20 to 60 ° C. The surface of the conventional MnZn-based ferrite is slightly oxygen-deficient due to the reducing action during firing, so that tensile stress is generated, and the surface residual stress is 40 MPa or more. Therefore, in the production method of the present invention, ferrite as a fired product is chemically oxidized by immersing it in an oxidizing liquid having a predetermined concentration. As a result of oxygen being applied to the ferrite surface portion by this method, the tensile stress on the surface is reduced and the residual stress becomes less than 40 MPa.
Here, the oxidizing liquid is preferably nitric acid, sulfuric acid or hydrochloric acid from the viewpoint of easy availability, ease of handling and the like.

The MnZn-based ferrite of the present invention may contain the following additives.
CoO: 3500 massppm or less CoO is a component containing Co ²⁺ ions having positive magnetic anisotropy, and the addition of this component can widen the temperature range of the secondary peak showing the maximum temperature of the initial magnetic permeability. On the other hand, when the amount of CoO added is excessive, the negative magnetic anisotropy of other components cannot be offset, resulting in a significant decrease in the initial magnetic permeability. Therefore, when adding CoO, it is necessary to limit it to 3500 mass ppm or less. The amount of CoO added is preferably 3000 mass ppm or less, more preferably 2500 mass ppm or less.
On the other hand, when CoO is added, the lower limit of the CoO content is not particularly limited, but is 100 mass ppm, more preferably 250 mass ppm, and further preferably 500 mass ppm from the viewpoint of the appearance of the effect of widening the temperature range of the secondary peak.

Next, the method for producing MnZn ferrite of the present invention will be described in detail.
Regarding the production of MnZn-based ferrite, Fe ₂ O ₃ , ZnO and MnO are first weighed so as to have a predetermined ratio, and after sufficiently mixing these, they are calcined and cooled to obtain a calcined powder (temporary firing step). .. Fe ₂ O ₃ , ZnO and MnO are usually powders. When crushing this calcined flour, an additive as an auxiliary component specified in the present invention is added at a predetermined ratio and mixed to obtain a pulverized powder (mixing-crushing step). In this step, the powder is sufficiently homogenized so that the concentration of the added component is not biased, and the calcined powder is refined to the target average particle size. An organic binder such as polyvinyl alcohol is added to the powdered pulverized powder having the target composition thus obtained, and the granulated powder is obtained through a granulation step by a spray-drying method or the like (granulation step). After performing steps such as sieving to adjust the particle size, pressure is applied with a molding machine to mold. After such molding, firing is performed under suitable firing conditions (calcination step), and the mixture is immersed in an oxidizing liquid having a concentration of 10 N or more, for example, nitric acid, sulfuric acid, hydrochloric acid, etc. for more than 0.50 hours, that is, more than 30 minutes. (Immersion step). Then, if necessary, it is washed with water and dried to obtain a ferrite sintered body according to the present invention, that is, a MnZn-based ferrite.
The obtained ferrite sintered body may be subjected to surface polishing or other processing.

The MnZn-based ferrite thus obtained exhibits extremely excellent fracture toughness and magnetic properties, which was not possible with conventional MnZn-based ferrites. These extremely excellent characteristics include, for example, a fracture toughness value of 1.00 MPa · m ^1/2 or more (preferably 1.05 MPa · m ^1/2 or more) as measured by a fracture toughness measurement of a flat plate sample based on JIS R1607. Moreover, the toughness-shaped core produced under the same conditions has an extremely excellent characteristic that the initial magnetic permeability value at 23 ° C. and 100 kHz is 4000 or more (preferably 4500 or more).

(Example 1)
Each raw material powder weighed so that the amounts of Fe ₂ O ₃ , ZnO and MnO have the ratio shown in Table 1 is mixed for 16 hours using a ball mill, and then calcined in air at 900 ° C. for 3 hours. , It was cooled to room temperature in the air for 1.5 hours to obtain a calcined powder. Next, SiO ₂ , CaO and Nb ₂ O ₅ were weighed equivalent to 130, 450 and 200 mass ppm, respectively, and then added to the calcined powder, and the mixture was pulverized in a ball mill for 12 hours. Then, polyvinyl alcohol was added to the pulverized powder obtained by such pulverization, spray-dried granulation was applied, and a pressure of 118 MPa was applied to form a toroidal core shape and a flat core shape to obtain a molded product. Then, these compacts were charged into a firing furnace and fired at a maximum temperature of 1320 ° C. for 2 hours in a gas stream in which nitrogen gas and air were appropriately mixed, and the fired products after firing were placed at room temperature of 23 ° C. After immersing in 13.0N (specified) nitric acid for 1.00 hours, it is taken out, washed with pure water and dried to obtain MnZn-based ferrite having an outer diameter of 25 mm, an inner diameter of 15 mm, and a height of 5 mm. A sintered toroidal core (hereinafter, also simply referred to as a toroidal core) and a sintered flat plate-shaped core (hereinafter, simply referred to as a rectangular core) having a length of 4 mm, a width of 35 mm, and a thickness of 3 mm were obtained.
Since a high-purity raw material was used as the raw material and the medium such as a ball mill was thoroughly washed before use to suppress the mixing of components from other materials, the impurities P contained in the toroidal core and the rectangular parallelepiped core after immersion were suppressed. The B and Ti contents were 4, 3 and 15 mass ppm, respectively. The contents of P, B and Ti were quantified according to JIS K 0102 (IPC mass spectrometry).

The initial magnetic permeability of the toroidal core was calculated based on the inductance measured using an LCR meter (4980A manufactured by Keysight Co., Ltd.) after winding the toroidal core obtained in the above procedure for 10 turns.
The surface residual stress was calculated by using a micro stress measuring device (AutoMATE manufactured by Rigaku), using Cr-Kα rays, and using the parallel tilt method. At this time, assuming that the ferrite surface is MnFe ₂ O ₄ , the shift of the (551) plane peak appearing at 148.40 ° was measured, and calculated using the values of Poisson's ratio 0.28 and elastic constant 147 GPa. .. For details of the above-mentioned parallel tilting method, refer to "Materials" (J. Soc. Mat. Sci., Japan), Vol. 47, No. 11, pp. 1189-1194, Nov. It is described in 1998.
The fracture toughness value of the rectangular parallelepiped core is based on JIS R 1607, which is based on the fracture load and the dimensions of the test piece after pre-cracking the sample dented in the center by Vickers and then breaking it in a three-point bending test. Calculated in.
The results obtained are also shown in Table 1.

As shown in the table, in Examples 1-1 to 1-5, which are examples of the invention, the initial magnetic permeability value at 23 ° C. and 100 kHz is 4000 or more, and the fracture toughness value is 1.00 MPa · m ^1/2. The above-mentioned good magnetic properties and high toughness are obtained together.
On the other hand, in the comparative example containing less than 51.5 mol% of Fe ₂ O ₃ (Comparative Example 1-1) and the comparative example containing more than 55.5 mol% of Fe ₂ O ₃ (Comparative Example 1-2), the value was high. Although toughness has been achieved, the initial magnetic permeability has decreased due to the increase in magnetic anisotropy and magnetostriction, and the value of the initial magnetic permeability at 23 ° C. and 100 kHz is less than 4000.
In Comparative Example (Comparative Example 1-3) in which ZnO was insufficient, the Curie temperature rose excessively, and in Comparative Example (Comparative Example 1-4) containing a large amount of ZnO from the range of the present invention, the initial magnetic permeability reached a maximum value. Since the indicated secondary peak has decreased, the value of the initial magnetic permeability at 23 ° C. and 100 kHz is less than 4000 in each case.

(Example 2)
The raw materials were weighed so that Fe ₂ O ₃ was 53.0 mol%, Zn O was 20.0 mol%, and MnO was 27.0 mol%, mixed for 16 hours using a ball mill, and then in air at 900 ° C. for 3 hours. It was calcined and cooled to room temperature in the air for 1.5 hours to obtain calcined powder. Next, SiO ₂ , CaO and Nb ₂ O ₅ , which are sub-ingredients in the amounts shown in Table 2, and CoO were added to some of the samples, and the mixture was pulverized with a ball mill for 12 hours. Then, polyvinyl alcohol was added to the pulverized powder obtained by such pulverization, spray-dried granulation was applied, and a pressure of 118 MPa was applied to form a toroidal core shape and a flat core shape to obtain a molded product. Then, these compacts are charged into a firing furnace and fired at a maximum temperature of 1320 ° C. for 2 hours in a gas stream in which nitrogen gas and air are appropriately mixed, and the fired products after firing are placed at room temperature of 23 ° C. After immersing in 13.0 N (specified) nitric acid for 1.00 hours, it is taken out, washed with pure water and dried to obtain MnZn-based ferrite having an outer diameter of 25 mm, an inner diameter of 15 mm, and a height of 5 mm. A sintered toroidal core and a sintered rectangular core having a length of 4 mm, a width of 35 mm, and a thickness of 3 mm were obtained. The contents of impurities P, B and Ti contained in the toroidal core and the rectangular parallelepiped core obtained after the immersion were all 4, 3 and 15 mass ppm, respectively.
The characteristics of each of these samples were evaluated using the same method and apparatus as in Example 1. The results of the obtained evaluation are also shown in Table 2.

As shown in the table, in Examples 2-1 to 2-9 in which the amounts of SiO ₂ , CaO, and Nb ₂ O ₅ are within the specified range, the value of the initial magnetic permeability at 23 ° C. and 100 kHz is 4000 or more. Moreover, a high toughness with a fracture toughness value of 1.00 MPa · m ^1/2 or more is also obtained.
On the other hand, in Comparative Examples 2-1, 2-3 and 2-5 in which even one of the three components of SiO ₂ , CaO and Nb ₂ O ₅ is contained in less than the specified amount, the grain boundary formation is insufficient. As a result, the resistivity decreases, resulting in deterioration of the initial magnetic permeability. Furthermore, due to insufficient suppression of grain growth, some low-strength coarse grains appear, resulting in a fracture toughness value. It's getting low. On the contrary, in Comparative Examples 2-2, 2-4 and 2-6 in which even one of the three components is excessive, the initial magnetic permeability is deteriorated due to the appearance of abnormal grains, and the site of the abnormal grains is local. Because of its low strength, the fracture toughness value is also greatly reduced.
In Example 2-9, in which the amount of CoO is more than 3500 mass ppm, the value of the initial magnetic permeability is slightly lowered because the magnetic anisotropy and the magnetostriction are excessively large.

(Example 3)
By the method shown in Example 1, the basic component and the sub-component have the same composition as in Example 1-2, while the amount of unavoidable impurities contained is different as shown in Table 3. The granulated powder obtained in use was molded into a toroidal core shape and a flat core shape by applying a pressure of 118 MPa to obtain a molded product. Then, these compacts are charged into a firing furnace and fired at a maximum temperature of 1320 ° C. for 2 hours in a gas stream in which nitrogen gas and air are appropriately mixed, and the fired products after firing are placed at room temperature of 23 ° C. After immersing in 13.0 N (specified) nitric acid for 1.00 hours, it is taken out, washed with pure water and dried to obtain MnZn-based ferrite having an outer diameter of 25 mm, an inner diameter of 15 mm, and a height of 5 mm. A sintered toroidal core and a sintered rectangular core having a length of 4 mm, a width of 35 mm, and a thickness of 3 mm were obtained.
The characteristics of each of these samples were evaluated using the same method and apparatus as in Example 1. The results of the obtained evaluation are also shown in Table 3.

As shown in the table, in Example 3-1 in which the unavoidable impurities P, B and Ti components are within the specified range, not only the initial magnetic permeability value at 23 ° C. and 100 kHz is 4000 or more, but also 1 An excellent fracture toughness value of 0.00 MPa · m ^1/2 or more is obtained.
On the other hand, in the comparative example in which any one or more of the above-mentioned impurity components exceeds the specified value, the initial magnetic permeability deteriorates due to the appearance of abnormal particles, and at the same time, the fracture toughness value decreases, both of which are desired. The value of is not obtained.

(Example 4)
The granulated powder obtained so as to have the same composition as that of Example 1-2 produced by the method shown in Example 1 was molded into a toroidal core shape and a flat core shape by applying a pressure of 118 MPa. And said. After that, these molded bodies were charged into a firing furnace and fired in a gas stream in which nitrogen gas and air were appropriately mixed for 2 hours at a maximum temperature of 1320 ° C., and the sintered products obtained are shown in Table 4. Under the conditions, it is immersed in nitric acid, sulfuric acid or hydrochloric acid, which is an oxidizing liquid, then taken out, washed with pure water and dried to obtain MnZn-based ferrite with an outer diameter of 25 mm, an inner diameter of 15 mm, and a height. A sintered toroidal core having a length of 5 mm and a sintered rectangular core having a length of 4 mm, a width of 35 mm and a thickness of 3 mm were obtained. The amounts of the P, B, and Ti components contained in the toroidal core and the rectangular parallelepiped core after the immersion were 4, 3, and 15 mass ppm, respectively.
The characteristics of each of these samples were evaluated using the same method and apparatus as in Example 1. The results obtained are also shown in Table 4.

In the dipping process 1) When the concentration of the oxidizing liquid to be immersed is 10 specified (N) or more,
2) In Examples 4-1 to 4-8 produced under the dipping process conditions satisfying both 1) and 2) with a dipping time of more than 0.50 hours (hr), the surface of the core as MnZn-based ferrite was formed. Due to chemical oxidation, the surface residual stress of the core was less than 40 MPa. As a result, the tensile stress is reduced, and a good fracture toughness value of 1.00 MPa · m ^1/2 or more is obtained for the core.
On the other hand, in Comparative Examples 4-1 to 4-8 produced through a dipping step that does not satisfy the above conditions, the tensile stress remaining on the surface is not sufficiently eliminated because the chemical oxidation is insufficient. .. As a result, the desired fracture toughness value has not been obtained.

Claims (5)

1. MnZn-based ferrite consisting of basic components, sub-components and unavoidable impurities.
   As the above basic ingredients
   Iron: 51.5 to 55.5 mol% in terms of Fe ₂ O ₃
   Zinc: More than 15.5 mol% in terms of ZnO and 26.0 mol% or less and Manganese: 22.0 to 32.0 mol% in terms of MnO
   Including
   As opposed to the basic component, as the sub-component
   SiO ₂ : 50-250 massppm,
   CaO: 100 massppm or more, less than 1000 massppm and Nb ₂ O ₅ : 100 to 300 massppm
   Including
   The amounts of P, B and Ti in the above unavoidable impurities, respectively.
   P: less than 10 mass ppm,
   B: suppressed to less than 10 mass ppm and Ti: less than 50 mass ppm,
   The MnZn-based ferrite having a surface residual stress value of less than 40 MPa.
2. The MnZn-based ferrite according to claim 1, wherein the MnZn-based ferrite further contains CoO: 3500 mass ppm or less as an auxiliary component.
3. Of the MnZn-based ferrite
   The MnZn system according to claim 1 or 2, wherein the fracture toughness value of the fracture toughness measurement based on JIS R1607 is 1.00 MPa · m ^1/2 or more, and the initial magnetic permeability value at 23 ° C. and 100 kHz is 4000 or more. Ferrite.
4. A calcining step of calcining a mixture of the basic components and cooling the mixture to obtain a calcined powder.
   A mixing-crushing step of adding sub-ingredients to the temporary baking powder obtained in the above temporary baking step, mixing and crushing to obtain crushed powder, and
   A granulation step of adding a binder to the crushed powder obtained in the above mixing-crushing step, mixing, and then granulating.
   A firing step in which the granulated powder obtained in the above granulation step is molded and then fired,
   The dipping process of dipping in acid and
   The method for producing MnZn-based ferrite according to any one of claims 1 to 3, wherein the MnZn-based ferrite is obtained.
   The dipping step is a method for producing MnZn-based ferrite in which the fired product obtained in the firing step is immersed in an oxidizing liquid having a concentration of 10 N or more for more than 0.50 hours.
5. The method for producing MnZn-based ferrite according to claim 4, wherein the oxidizing liquid is nitric acid, sulfuric acid or hydrochloric acid.