# Patent

## Title
Mn—Zn ferrite core and method for producing the same

## Abstract

Disclosed is a MnZn ferrite core comprising basic components, subcomponents and unavoidable impurities. To the basic components comprising: iron oxide (as Fe2O3): 51.0-54.5 mol %, zinc oxide (as ZnO): 8.0-12.0 mol % and manganese oxide (as MnO): remainder, are added silicon oxide (as SiO2): 50-400 mass ppm and calcium oxide (as CaO): 50-4000 ppm as subcomponents and in the unavoidable impurities, phosphorous, boron, sulfur and chlorine are respectively kept to: less than 3 mass ppm, less than 3 mass ppm, less than 5 mass ppm, and less than 10 mass ppm. The ratio of the measure specific surface area to the ideal specific surface area of the MnZn ferrite core satisfies the formula: Measured specific surface area/ideal specific surface area<1500.

## Background
TECHNICAL FIELD
The present invention relates to a Mn—Zn ferrite core that is suitably used as, for example, a magnetic core for a pulse transformer in an Ethernet (registered trademark) device; and a method for producing such a Mn—Zn ferrite core.
BACKGROUND ART
Ethernet devices include pulse transformers for the purpose of achieving impedance match and electrical insulation in input/output terminals. Such transformers include magnetic cores generally composed of soft magnetic materials. Such pulse transformers are required to have a high incremental permeability μΔ under the application of a direct-current magnetic field in a temperature range of −40 to 85° C., for example, as defined in American standards ANSI X3.263-1995[R2000]. The incremental permeability μΔ is a value that indicates the degree of magnetization of a magnetic core (core) under the application of a magnetic field.
With a progress in the communication technology in recent years, in Ethernet devices, there has been a trend toward, in addition to a higher transmission speed, a direct supply of driving power of such devices together with transmission signals. In this case, pulse transformers are used under conditions in which large currents may be superposed, compared with conventional conditions. Furthermore, since components around magnetic cores (cores) of pulse transformers within devices generate heat due to such large currents, the temperature of the environment in which the cores are used probably increases.
Accordingly, Mn—Zn ferrite used in such an application is demanded to have a high inductance, that is, a high incremental permeability μΔ, at a higher temperature and under superposition of higher magnetic fields.
A soft magnetic material used in the application is generally Mn—Zn ferrite and various improvements have been proposed.
For example, Patent Literature 1 discloses a technique in which Mn—Zn ferrite is made to contain cobalt oxide to thereby improve magnetic characteristics at a high temperature. However, since Mn—Zn ferrite for magnetic cores of pulse transformers has been designed in terms of composition so as to have a high initial permeability μi, such Mn—Zn ferrite has a low saturation flux density and hence it is difficult to achieve a sufficiently high incremental permeability μΔ at a high temperature and in a high magnetic field.
Patent Literature 2 proposes that reduction in phosphorus and boron are effective to increase incremental permeability μΔ. However, Mn—Zn ferrite disclosed in Patent Literature 2 has a composition selected for the purpose of reduction in core loss and an increase in effective permeability at 100° C. Accordingly, the Mn—Zn ferrite has too low an initial permeability μi at room temperature or less, which is not described in Examples, and hence it is unlikely that the Mn—Zn ferrite has a sufficiently high incremental permeability μΔ in a low-temperature environment.
The above-described impurities are defined in techniques disclosed in Patent Literatures 3 to 6.
Patent Literature 3 proposes a technique in which the content of chlorine is defined to achieve improvements in terms of core loss and amplitude ratio permeability at 100° C. or more. However, it is impossible to make an incremental permeability μΔ at 23° C. be 200 or more by defining the content of chlorine only.
Patent Literature 4 proposes a technique in which the content of sulfur is defined to achieve improvements in terms of power loss. However, it is impossible to make an incremental permeability μΔ at 23° C. be 200 or more by defining the content of sulfur only.
Patent Literature 5 proposes a technique in which the contents of phosphorus, boron, sulfur, and chlorine are defined to suppress exaggerated grain growth in ferrite so that adverse influence on characteristics of ferrite is suppressed. This technique allows a Mn—Zn ferrite having a high resistivity and a small squareness ratio. However, the Mn—Zn ferrite does not have a sufficiently high incremental permeability μΔ in a high magnetic field.
Patent Literature 6 proposes a technique in which the content of phosphorus in a CoO-added ferrite is made very low to suppress exaggerated grain growth in the ferrite so that a high effective permeability is achieved under the application of a direct-current magnetic field. However, since the content of ZnO is high, this technique allows a high incremental permeability μΔ in a low magnetic field of 33 A/m but does not allow a sufficiently high incremental permeability μΔ in a high magnetic field of 80 A/m.
As described above, no existing technique allows a sufficiently high incremental permeability μΔ at a high temperature and in a high magnetic field. To overcome such a problem, the inventors of the present invention developed a Mn—Zn ferrite core having the following configuration and disclosed it in Patent Literature 7:
“A Mn—Zn ferrite core comprising a basic component, sub-components, and unavoidable impurities,
wherein, as the sub-components,
silicon oxide (in terms of SiO2): 50 to 400 mass ppm, and
calcium oxide (in terms of CaO): 50 to 4000 mass ppm; are added to the basic component consisting of
iron oxide (in terms of Fe2O3): 51.0 to 54.5 mol %,
zinc oxide (in terms of ZnO): 8.0 to 12.0 mol %, and
manganese oxide (in terms of MnO): balance and amounts of phosphorus, boron, sulfur, and chlorine in the unavoidable impurities are reduced as follows
phosphorus: less than 3 mass ppm,
boron: less than 3 mass ppm,
sulfur: less than 5 mass ppm, and
chlorine: less than 10 mass ppm.”
CITATION LIST


    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2004-196632
    Patent Literature 2: Japanese Unexamined Patent Application Publication No. 7-297020
    Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2006-213532
    Patent Literature 4: Japanese Unexamined Patent Application Publication No. 2001-64076
    Patent Literature 5: Japanese Unexamined Patent Application Publication No. 2005-179092
    Patent Literature 6: Japanese Unexamined Patent Application Publication No. 2007-197246
    Patent Literature 7: Japanese Patent Application No. 2008-13135


SUMMARY OF INVENTION
Technical Problem
As a result of the development of the Mn—Zn ferrite core disclosed in Patent Literature 7 above, a higher incremental permeability μΔ can be achieved at a high temperature and in a high magnetic field than before.
The present invention relates to an improvement of the Mn—Zn ferrite core and is intended to further increase the incremental permeability μΔ.
Solution to Problem
Features of the present invention are as follows.
1. A Mn—Zn ferrite core comprising a basic component, sub-components, and unavoidable impurities,
wherein, as the sub-components,
silicon oxide (in terms of SiO2): 50 to 400 mass ppm and
calcium oxide (in terms of CaO): 50 to 4000 mass ppm are added to the basic component consisting of
iron oxide (in terms of Fe2O2): 51.0 to 54.5 mol %,
zinc oxide (in terms of ZnO): 8.0 to 12.0 mol %, and
manganese oxide (in terms of MnO): balance; amounts of phosphorus, boron, sulfur, and chlorine in the unavoidable impurities are reduced as follows
phosphorus: less than 3 mass ppm,
boron: less than 3 mass ppm,
sulfur: less than 5 mass ppm, and
chlorine: less than 10 mass ppm; and
a ratio of a measured specific surface of the Mn—Zn ferrite core to an ideal specific surface of the Mn—Zn ferrite core satisfies formula (I) below

Measured specific surface/ideal specific surface<1500  (1)

where the measured specific surface represents a specific surface (m2/g) determined by a BET method (multipoint method) in JIS Z 8830 (2001); and the ideal specific surface represents a specific surface (m2/g) calculated from size and mass of the core on an assumption that the core is in an ideal state of having no cavities.
2. The Mn—Zn ferrite core according to 1 above, wherein the sub-components further include
Cobalt oxide (in terms of CoO): 50 to 3000 mass ppm.
3. The Mn—Zn ferrite core according to 1 or 2 above, wherein the sub-components further include one or more selected from zirconium oxide (in terms of ZrO2): 0.005 to 0.075 mass %;
tantalum oxide (in terms of Ta2O5): 0.005 to 0.075 mass %;
hafnium oxide (in terms of HfO2): 0.005 to 0.075 mass %; and
niobium oxide (in terms of Nb2O5): 0.005 to 0.075 mass %.
4. A method for producing the Mn—Zn ferrite core according to any one of 1 to 3 above, the method comprising calcining the basic component according to 1 above to provide a calcined powder, mixing the calcined powder with the sub-components according to any one of 1 to 3 above, subsequently granulating the mixture of the calcined powder with the sub-components into a granulated powder having a compressive rupture strength of 1.10 MPa or less, and then compacting and subsequently firing the granulated powder.
Advantageous Effects of Invention
According to the present invention, a Mn—Zn ferrite core having excellent characteristics can be obtained in which, under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ is continuously 400 or more in a wide temperature range of 0 to 85° C. and the incremental permeability μΔ at 65° C. is 700 or more.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates a section of a Mn—Zn ferrite core, the section being observed for cavities.
FIG. 2(a) illustrates a state of a core according to the present invention in which no cavities remain.
FIG. 2(b) illustrates a state of an existing core in which cavities remain due to insufficient disintegration of a granulated powder.
FIG. 3 illustrates the way of calculating an ideal specific surface.



DESCRIPTION OF EMBODIMENTS
A Mn—Zn ferrite core in the present application is mainly used in the form of a closed magnetic circuit with a small size represented by a toroidal core having an outer diameter of about 2 to 6 mm. In the case of such a small size, since there is a high probability of mold breakage in compaction, it is impossible to apply a high compaction pressure. Accordingly, when a section of a core that is a fired compact as illustrated in FIG. 1 is observed with a scanning electron microscope (SEM), there are cases where cavities due to insufficient disintegration of granulated powder remain as illustrated in FIG. 2(b).
When the core includes such cavities, the volume occupied by the magnetic material is small and hence magnetic fluxes concentrate in the magnetic material region and a magnetic flux density locally increases. Accordingly, the same phenomenon as an increase in a superposed magnetic field seemingly occurs in the magnetic material region and, as a result, the incremental permeability decreases.
To overcome such a problem, the inventors have performed thorough studies. As a result, the inventors have achieved the following findings.
The inventors have found that cavities in a core are represented by numerical variation of an increase in the specific surface of the core; and, when an ideal specific surface of the core is calculated from the size and form of the core on the assumption that the surface of the core is in an ideal state of complete flatness, and a ratio of the measured specific surface to the ideal specific surface is made less than a specific value, the core has a small number of cavities and a desirable incremental permeability.
The inventors have also found that, to make the measured specific surface/ideal specific surface be less than a specific value, it is necessary to make the compressive rupture strength of a granulated powder be 1.10 MPa or less in the production process of a Mn—Zn ferrite, the compressive rupture strength being measured by a procedure defined in JIS Z 8841 in which measurements of granules in terms of compressive rupture strength are defined.
The present invention is based on these findings.
Hereinafter, the present invention will be specifically described.
The reasons why the composition of the basic component of a Mn—Zn ferrite core according to the present invention is limited to the above-described ranges will be described.
Iron Oxide (in Terms of Fe2O3): 51.0 to 54.5 Mol %
When iron oxide in the basic component is less than 51.0 mol % or is more than 54.5 mol %, the incremental permeability μΔ under the application of a direct-current magnetic field decreases in a low-temperature range and a high-temperature range. Accordingly, the content of iron oxide in terms of Fe2O3 is made in the range of 51.0 to 54.5 mol %, preferably in the range of 52.0 to 54.0 mol %.
Zinc Oxide (in Terms of ZnO): 8.0 to 12.0 Mol %
When the content of zinc oxide is less than 8.0 mol %, a sufficiently high incremental permeability μΔ is not achieved under the application of a direct-current magnetic field. When the content of zinc oxide is more than 12.0 mol %, in a low-temperature range, the incremental permeability μΔ under the application of a direct-current magnetic field decreases; and, in a high-temperature range, the Curie temperature at which a ferromagnetic material loses its magnetism decreases and the incremental permeability μΔ under the application of a direct-current magnetic field also decreases. Accordingly, the content of zinc oxide in terms of ZnO is made in the range of 8.0 to 12.0 mol %, preferably in the range of 9.0 to 11.0 mol %.
Manganese Oxide (in Terms of MnO): Balance
The present invention relates to a Mn—Zn ferrite and the balance in the composition of the basic component needs to be manganese oxide. This is because, by making the ferrite contain manganese oxide, a high incremental permeability μΔ of 400 or more cannot be achieved under the application of a direct-current magnetic field of 80 A/m. The preferred range of manganese oxide in terms of MnO is 34.5 to 40.0 mol %.
The contents of iron oxide, zinc oxide, and manganese oxide that constitute the basic component are adjusted such that the total amount of iron oxide in terms of Fe2O3, zinc oxide in terms of ZnO, and manganese oxide in terms of MnO is 100 mol %.
The reasons why the composition of the sub-components of a Mn—Zn ferrite core according to the present invention is limited to the above-described ranges will be described.
Silicon Oxide (in Terms of SiO2): 50 to 400 Mass Ppm
Silicon oxide has an effect of reducing the number of vacancies remaining in crystal grains to thereby increase the incremental permeability μΔ under the application of a direct-current magnetic field. However, when the content of silicon oxide is less than 50 mass ppm, the effect of adding silicon oxide is not sufficiently exhibited; and when the content of silicon oxide is more than 400 mass ppm, exaggerated grains appear and the incremental permeability μΔ considerably decreases under the application of a direct-current magnetic field. Accordingly, the content of silicon oxide in terms of SiO2 is made in the range of 50 to 400 mass ppm, preferably in the range of 100 to 250 mass ppm.
Calcium Oxide (in Terms of CaO): 50 to 4000 Mass Ppm
Calcium oxide segregates in crystal grain boundaries of a Mn—Zn ferrite to provide an effect of suppressing growth of crystal grains. As a result, the initial permeability is appropriately decreased and the incremental permeability μΔ under the application of a direct-current magnetic field is effectively increased. However, when the content of calcium oxide is less than 50 mass ppm, the effect of suppressing growth of grains is not sufficiently provided; and when the content of calcium oxide is more than 4000 mass ppm, exaggerated grains appear and the incremental permeability μΔ considerably decreases under the application of a direct-current magnetic field. Accordingly, the content of calcium oxide in terms of CaO is made in the range of 50 to 4000 mass ppm, preferably in the range of 250 to 2500 mass ppm.
Note that the initial permeability μi at 23° C. is preferably made about 2500 to 4500.
In the present invention, to increase the incremental permeability μΔ under the application of a direct-current magnetic field, it is important to limit the contents of impurities in a ferrite, in particular, phosphorus, boron, sulfur, and chlorine simultaneously to ranges below.
Phosphorus: Less than 3 Mass Ppm, Boron: Less than 3 Mass Ppm
Phosphorus and boron are unavoidable impurities derived from raw material iron oxide. When the content of phosphorus or boron is 3 mass ppm or more, exaggerated grain growth is induced and the incremental permeability μΔ considerably decreases under the superposition of a high magnetic field of 80 A/m. Accordingly, the contents of phosphorus and boron are limited to less than 3 mass ppm.
The contents of phosphorus and boron can be limited to less than 3 mass ppm by, for example, a method of using raw material powders of iron oxide, zinc oxide, and manganese oxide that have a high purity and contain phosphorus and boron as less as possible. In addition, the contents of phosphorus and boron in a medium used in mixing and pulverization such as a ball mill or an attritor are preferably low to avoid entry of phosphorus and boron due to abrasion of the medium.
Note that all the values defined herein are quantified by an analytical procedure defined in “Molybdophosphate extraction-separation/molybdophosphoric blue spectrophotometric method” in JIS G 1214 (1998) in terms of the P component and by a procedure defined in “Curcumin spectrophotometric method” in JIS G 1227 (1999) in terms of the B component.
Sulfur: Less than 5 Mass Ppm
Sulfur is an unavoidable impurity derived from raw material iron oxide obtained from ferrous sulfide. When the content of sulfur is 5 mass ppm or more, exaggerated grain growth is induced and the incremental permeability considerably decreases under the superposition of a high magnetic field of 80 A/m. Accordingly, the content of sulfur is limited to less than 5 mass ppm. The content of sulfur is preferably limited to less than 4 mass ppm.
The content of sulfur can be limited to less than 5 mass ppm by, for example, a method in which time for calcination performed at 800° C. or more in the air atmosphere in the production of a Mn—Zn ferrite is increased to allow reaction between sulfur and oxygen to sufficiently proceed to thereby reduce the content of sulfur.
Note that the S values defined herein are quantified by an analytical procedure for the S component defined in “Methylene blue spectrophotometric method after separation of hydrosulfide” in JIS G 1215 (1994).
Chlorine: Less than 10 Mass Ppm
Chlorine is an unavoidable impurity derived from raw material iron oxide obtained from iron chloride. When the content of chlorine is 10 mass ppm or more, exaggerated grain growth is induced and the incremental permeability μΔ considerably decreases under the application of a direct-current magnetic field of 80 A/m. Accordingly, the content of chlorine is limited to less than 10 mass ppm. The content of chlorine is preferably limited to less than 8 mass ppm.
The content of chlorine can be limited to less than 10 mass ppm by, for example, a method in which raw material iron oxide is sufficiently washed with pure water in the production of a Mn—Zn ferrite to dissolve chlorine highly ionizable in pure water to thereby reduce the content of chlorine.
Note that the Cl values defined herein are quantified by an analytical procedure for the Cl component: “nitric acid decomposition-iron chloride nephelometry”.
The content of each unavoidable impurity other than the above-described phosphorus, boron, sulfur, and chlorine is not particularly limited, but is preferably reduced to 50 mass ppm or less.
The basic component, the sub-components, and the reduced components of a Mn—Zn ferrite core according to the present invention have been described so far. In the present invention, the limitation of the component composition of such a core to the above-described ranges only is insufficient and a formula below in terms of surface area needs to be satisfied.
It is important to satisfy the relationship represented by the following formula (I) in terms of the surface area of the core.

Measured specific surface/ideal specific surface<1500  (1)

The measured specific surface is a value determined by a BET method (multipoint method) in JIS Z 8830 (2001) and the unit thereof is m2/g. The ideal specific surface is a value obtained by, on the basis of the size and mass of the ferrite core, dividing the surface area of the core calculated on the assumption that the core is in an ideal state of having no cavities, by the mass of the core; and the unit thereof is also m2/g.
For reference, the way of calculating the ideal specific surface is illustrated in FIG. 3.
When the assumption is made that the core surface is in an ideal state of complete flatness, the specific surface can be calculated with the following formula.

Ideal specific surface=[2×{(outer diameter)2−(inner diameter)2}/4×π+{(outer diameter)+(inner diameter)}×π×height]/core mass

When a ferrite core includes a large number of cavities, which results in a problem of a decrease in the incremental permeability, a large number of cavities also remain in the surface of the core and hence the measured specific surface is large. Thus, the ratio of measured specific surface/ideal specific surface is high. Thorough studies have been performed on the ratio of measured specific surface/ideal specific surface. As a result, it has been revealed that, when the ratio can be reduced to less than 1500, it can be regarded that a core is obtained in which the incremental permeability does not decrease, that is, a dense core is obtained in which the number of cavities remaining in the surface of the core is small. The ratio of (measured specific surface/ideal specific surface) is preferably 1150 or less.
To reduce the ratio of measured specific surface/ideal specific surface to less than 1500, it is important to optimize granulation conditions in the production process of a Mn—Zn ferrite to provide a soft granulated powder. The production process of a Mn—Zn ferrite is a known technique in which, as the granulation method, a spray drying method is mainly employed, which is described in detail in documents, for example, p. 52 in “Ferrite” (Hiraga, Okutani, and Oshima; Maruzen: 1986):
The hardness of a granulated powder can be represented in a numerical value by the measurement of the compressive rupture strength of a granulated powder defined in JIS Z 8841 (1993). It has been revealed that, when the compressive rupture strength is 1.10 MPa or less, the ratio of measured specific surface/ideal specific surface can be reduced to less than 1500. The compressive rupture strength is preferably 1.00 MPa or less.
Since Mn—Zn ferrite has a low resistivity of less than 102 Ωm, it is often used after a surface thereof is subjected to an insulation coating treatment. However, the measured specific surface defined in the present invention is a measured value of a core that is not subjected to the coating treatment. This is because the coating treatment makes the surface be smooth and hence it becomes impossible to accurately measure the specific surface of a Mn—Zn ferrite.
A Mn—Zn ferrite according to the present invention may contain, in addition to the above-described components, other components described below as additional sub-components appropriately.
Cobalt Oxide (in Terms of CoO): 50 to 3000 Mass Ppm
By making an appropriate amount of cobalt oxide having positive magnetic anisotropy be contained, the incremental permeability μΔ under the application of a direct-current magnetic field can be increased in a wide temperature range of 0° C. to 85° C. However, when the content of cobalt oxide is less than 50 mass ppm, such an effect of adding cobalt oxide is not sufficiently provided. When the content of cobalt oxide is more than 3000 mass ppm, the incremental permeability μΔ under the application of a direct-current magnetic field decreases in the entire temperature range. Accordingly, the content of cobalt oxide in terms of CoO is made in the range of 50 to 3000 mass ppm.
One or more selected from zirconium oxide (in terms of ZrO2): 0.005 to 0.075 mass %; tantalum oxide (in terms of Ta2O5): 0.005 to 0.075 mass %; hafnium oxide (in terms of HfO2): 0.005 to 0.075 mass %; and niobium oxide (in terms of Nb2O5): 0.005 to 0.075 mass %
All these components are compounds having a high melting point and, in a Mn—Zn ferrite, decrease the size of crystal grains. Thus, the components suppress generation of coarse crystal grains and increase the incremental permeability μΔ under the application of a direct-current magnetic field. Such an effect is not sufficiently provided when the content of such a component is too low. When the content of such a component is too high, exaggerated grain growth is caused and the incremental permeability μΔ decreases under the application of a direct-current magnetic field. Accordingly, the components are made to be contained in the above-described ranges.
Note that addition of an additive in this group together with cobalt oxide can result in a considerable increase in the incremental permeability μΔ under the application of a magnetic field of 80 A/m. The reason for this is not clearly understood; however, a compound that increases the incremental permeability μΔ is probably generated upon the addition of cobalt oxide and zirconium oxide, tantalum oxide, hafnium oxide, or niobium oxide.
Next, a preferred method for producing a Mn—Zn ferrite according to the present invention will be described.
The powders of iron oxide, zinc oxide, and manganese oxide that constitute the basic component are weighed so as to achieve predetermined percentages. The powders are sufficiently mixed and then calcined. The calcined powder is then pulverized. The sub-components are added to the calcined powder so as to achieve predetermined proportions and are pulverized together with the calcined powder. In this process, the powder needs to be made sufficiently uniform such that concentrations of the added components are not localized and the particle size of the calcined powder needs to be decreased to a target average size. The resultant powder is mixed with an organic binder such as polyvinyl alcohol and granulated by a spray drying method or the like into a soft granulated powder having a compressive rupture strength of less than 1.10 MPa. The granulated powder is then compacted into a desired form and subsequently fired under appropriate firing conditions. The pressure applied in the compaction is preferably about 115 to 120 MPa. The firing conditions are preferably a temperature of 1200 to 1400° C. and a time for about 18 to 30 hours.
The compressive rupture strength of the granulated powder can be effectively reduced to less than 1.10 MPa by decreasing the temperature in granulation: specifically, to about 150 to 200° C., which is about 50 to 100° C. lower than the conventional temperature of 250 to 300° C.
When granulation is performed at the conventional temperature of 250 to 300° C., the granulated powder has a compressive rupture strength of about 1.2 to 1.4 MPa. As described above, a target condition of the present invention

Measured specific surface/ideal specific surface<1500  (1)

cannot be satisfied with such a compressive rupture strength.

The thus-obtained Mn—Zn ferrite can have, under the application of a high direct-current magnetic field of 80 A/m, a high incremental permeability μΔ of 400 or more in a temperature range of 0 to 85° C. and a high incremental permeability μΔ of 700 or more at 65° C., which cannot be achieved in existing Mn—Zn ferrites.
EXAMPLE 1
Raw material powders were weighed such that the compositions of iron oxide, zinc oxide, and manganese oxide that constituted the basic component satisfied percentages in Table 1 in terms of Fe2O3, ZnO, and MnO. The raw material powders were mixed with a ball mill for 16 hours and then calcined in the air at 925° C. for 3 hours. The calcined powders were then mixed with, as sub-components, silicon oxide and calcium oxide having been respectively weighed so as to achieve proportions in Table 1 in terms of SiO2 and CaO, and pulverized with the ball mill for 12 hours. The resultant powder mixtures were mixed with polyvinyl alcohol, granulated at 180° C., and compacted into toroidal cores under the application of a pressure of 118 MPa. The compacts were then put into a kiln and fired at a maximum temperature of 1350° C. Thus, sintered cores having an outer diameter of 6.0 mm, an inner diameter of 3.0 mm, and a height of 4.0 mm were obtained.
A wire was wound ten turns around each specimen obtained in this way. While the core was under the application of a direct-current magnetic field of 80 A/m with a direct-current application apparatus (42841A, manufactured by Agilent Technologies, Inc.), the specimen was measured in terms of incremental permeability μΔ at 0° C., 23° C., 65° C., and 85° C. at a measurement voltage of 100 mV and at a measurement frequency of 100 kHz with a LCR meter (4284A, manufactured by Agilent Technologies, Inc.). Note that the initial permeability μi was measured at 23° C. with the LCR meter (4284A).
In the preparation of the specimens, all the raw materials including iron oxide were highly pure; the ball mill serving as the mixing and pulverization medium had low contents of phosphorus and boron; the calcination was performed under sufficient air flow; and pure water used scarcely contained Cl. As a result, in all the specimens, the final contents of P, B, S, and Cl were respectively 2 mass ppm, 2 mass ppm, 3 mass ppm, and 6 mass ppm.
The compressive rupture strength of the granulated powders measured in accordance with JIS Z 8841 was 0.90±0.05 MPa and hence the number of cavities remaining in the surfaces of the ferrite cores was small. Accordingly, the measured specific surface was 0.453 to 0.493 m2/g and the ideal specific surface was 4.44×10−4 m2/g. All the ratios of (measured specific surface/ideal specific surface) were 1020 to 1110, which were less than 1500.
As for the crystal grain size of each specimen, the core was cut; the fracture section was polished; micrographs of three different fields of view of the polished surface at 500× magnification were taken with an optical microscope; size of grains in the micrographs was measured; and the average crystal grain size was calculated from the measured grain size.
The results are also described in Table 1.















TABLE 1













Compressive
Measured






Initial
Incremental permeability
Average
rupture
specific




Sub-
perme-
under application of magnetic
crystal
strength of
surface/


Speci-
Basic component
components
ability
field of 80 A/m
grain
granulated
ideal


men
(mol %)
(mass ppm)
μi
(μΔ)
size
powder
specific





















No.
Fe2O3
ZnO
MnO
SiO2
CaO
23° C.
0° C.
23° C.
65° C.
85° C.
(μm)
(MPa)
surface
Remarks
























1-1
55.0
10.0
Balance
100
500
3100
360
440
590
350
8
0.90 ± 0.05
1020-1110
Comparative
















example


1-2
50.5
10.0
Balance
100
500
3500
370
480
610
340
8


Comparative
















example


1-3
53.4
10.5
Balance
100
500
3700
420
540
770
660
8


Example of
















invention


1-4
52.0
10.0
Balance
100
500
4000
430
560
750
640
8


Example of
















invention


1-5
52.0
12.5
Balance
100
500
4600
450
570
560
360
8


Comparative
















example


1-6
52.0
 7.5
Balance
100
500
3000
320
410
590
380
8


Comparative
















example


1-7
51.8
10.0
Balance
100
500
4200
430
570
740
610
8


Example of
















invention


1-8
52.0
10.0
Balance
 20
500
5200
330
410
570
380
19


Comparative
















example


1-9
52.0
10.0
Balance
100
 35
5600
320
400
560
370
25


Comparative
















example


 1-10
52.0
10.0
Balance
250
2500 
2800
420
530
780
680
7


Example
















of invention


 1-11
52.0
10.0
Balance
450
500
2200
210
260
360
200
190


Comparative
















example


 1-12
52.0
10.0
Balance
100
4500 
1900
180
230
290
160
171


Comparative
















example


 1-13
52.0
10.0
Balance
450
4500 
1700
 90
130
150
 70
253


Comparative
















example









As described in Table 1, Specimen Nos. 1-3, 1-4, 1-7, and 1-10 serving as examples of the invention had excellent characteristics in which, under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ was continuously 400 or more in a wide temperature range of 0 to 85° C. and the incremental permeability μΔ at 65° C. was 700 or more.
In contrast, in a comparative example (Specimen No. 1-1) in which the content of Fe2O3 was more than 54.5 mol % and a comparative example (Specimen No. 1-2) in which the content of Fe2O3 was less than 51.0 mol %, under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ was less than 400 at 0° C. and 85° C. and the incremental permeability μΔ at 65° C. was less than 700.
In a comparative example (Specimen No. 1-5) in which the content of ZnO was high and more than the upper limit, under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ at 85° C. was less than 400 and the incremental permeability μΔ at 65° C. decreased to less than 700. In a comparative example (Specimen No. 1-6) in which the content of ZnO was less than the lower limit, under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ decreased in the entire temperature range, the incremental permeability μΔ at 0° C. and 85° C. was less than 400 and the incremental permeability μΔ at 65° C. was less than 700.
As for SiO2 and CaO, in comparative examples (Specimen Nos. 1-8 and 1-9) in which the content of SiO2 or CaO is less than the appropriate ranges, the initial permeability μΔ excessively increased and, as a result, the incremental permeability μΔ decreased in the entire temperature range, compared with the examples of the invention: under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ at 0° C. and 85° C. was less than 400 and the incremental permeability μΔ, at 65° C. was less than 700. In contrast, in comparative examples (Specimen Nos. 1-11, 1-12, and 1-13) in which the content of SiO2 and/or CaO was more than the appropriate ranges, exaggerated grains appeared and, as a result, the incremental permeability μΔ considerably decreased in the entire temperature range under the application of a direct-current magnetic field of 80 A/m.
EXAMPLE 2
Several iron oxide raw materials having different contents in terms of P, B, S, and Cl were used. The raw materials were weighed on the basis of calculation for achieving the contents of P, B, S, and Cl in Tables 2-1 and 2-2 such that the composition of iron oxide (in terms of Fe2O3), zinc oxide (in terms of ZnO), and manganese oxide (in terms of MnO) that constituted the basic component respectively satisfied Fe2O3: 52.0 mol %, ZnO: 10.0 mol %, and MnO: the balance. The raw materials were mixed with a ball mill for 16 hours and then calcined in the air at 925° C. for 3 hours. Some of the calcined powders were then mixed with, as sub-components, silicon oxide (100 mass ppm in terms of SiO2) and calcium oxide (500 mass ppm in terms of CaO). All the raw material powders were then pulverized with the ball mill for 12 hours. The resultant powder mixtures were mixed with polyvinyl alcohol, granulated, and compacted into toroidal cores under the application of a pressure of 118 MPa. At this time, the granulation temperature was varied such that the resultant granulated powders had various compressive rupture strengths. The compacts were then put into a kiln and fired at a maximum temperature of 1350° C. Thus, sintered cores having an outer diameter of 6.0 mm, an inner diameter of 3.0 mm, and a height of 4.0 mm were obtained.
A wire was wound ten turns around each specimen obtained in this way. The specimen was measured in terms of incremental permeability μΔ at 0° C., 23° C., 65° C., and 85° C. at a measurement voltage of 100 mV and at a measurement frequency of 100 kHz under the application of a direct-current magnetic field of 80 A/m with the same direct-current application apparatus and LCR meter as in EXAMPLE. The compressive rupture strength of the granulated powders was measured in accordance with requirements of JIS Z 8841. The measured specific surface was also measured by a BET method (multipoint method) in JIS Z 8830 (2001). The ideal specific surface was calculated from size and weight that were measured in accordance with JIS C 2560. The ratio of (measured specific surface/ideal specific surface) was calculated from the ideal specific surface of 4.44×10−4 m2/g). The initial permeability μ1 and the average crystal grain size were measured as in EXAMPLE 1.
The results are also described in Tables 2-1 and 2-2.











TABLE 2-1











Incremental permeability under



Sub-

Initial
application of magnetic field of



component
Contents of impurities
permeability
80 A/m


Specimen
(mass %)
(mass ppm)
μi
(μΔ)


















No.
ZrO2
CoO
P
B
S
Cl
23° C.
0° C.
23° C.
65° C.
85° C.





1-4 
0
0
2
2
3
6
4000
430
560
750
640


2-1 


2
2
4
8
3700
410
530
720
610


2-2 


5
2
3
6
3000
300
380
520
300


2-3 


2
5
3
6
2900
310
380
520
300


2-4 


2
2
8
6
3000
230
310
480
200


2-5 


2
2
3
15 
3000
210
300
450
210


2-6 


5
5
3
6
2700
170
260
310
160


2-7 


2
2
8
15 
2700
160
240
280
160


2-8 


5
2
8
15 
1600
160
240
300
150


2-9 


5
5
3
15 
1200
130
190
260
120


2-10


5
5
8
6
1200
130
180
250
110


2-11


5
5
8
15 
800
110
160
230
100


2-12


10 
10 
15 
30 
700
 60
100
130
 40


2-13
0.050
0
5
2
3
6
2200
280
350
470
280


2-14
0
0.100
5
2
3
6
2200
290
360
480
290


2-15
0.050
0.100
2
5
3
6
1900
320
380
490
290


2-16
0
0.100
2
5
3
6
2000
310
370
490
290


2-17
0.050
0
2
2
8
6
2100
220
280
450
200


2-18
0
0.100
2
2
8
6
2100
230
290
460
210


2-19
0.050
0.100
2
2
3
15 
2000
230
300
440
220


2-20
0
0.100
2
2
3
15 
2100
220
290
430
210



















Compressive

Measured





Average
rupture

specific




crystal
strength of
Measured
surface/




grain
granulated
specific
ideal



Specimen
size
powder
surface
specific



No.
(μm)
(MPa)
(m2/g)
surface
Remarks







1-4 
8
0.90 ± 0.05
0.448-0.493
1020-1110
Example of invention



2-1 
10



Example of invention



2-2 
19



Comparative example



2-3 
27



Comparative example



2-4 
22



Comparative example



2-5 
21



Comparative example



2-6 
36



Comparative example



2-7 
39



Comparative example



2-8 
48



Comparative example



2-9 
56



Comparative example



2-10
66



Comparative example



2-11
193



Comparative example



2-12
226



Comparative example



2-13
17
1.25 ± 0.05
0.730-0.808
1640-1800
Comparative example



2-14
17



Comparative example



2-15
24



Comparative example



2-16
25



Comparative example



2-17
20



Comparative example



2-18
20



Comparative example



2-19
18



Comparative example



2-20
19



Comparative example





















TABLE 2-2











Incremental permeability



Sub-

Initial
under application of magnetic



component
Contents of impurities
permeability
field of 80 A/m


Specimen
(mass %)
(mass ppm)
μi
(μΔ)


















No.
ZrO2
CoO
P
B
S
Cl
23° C.
0° C.
23° C.
65° C.
85° C.





2-21
0.050
0.100
5
2
3
6
2600
330
410
540
340


2-22
0
0.100
5
2
3
6
2800
320
400
530
330


2-23
0.050
0
2
5
3
6
2700
330
400
530
330


2-24
0
0.100
2
5
3
6
2600
340
410
540
320


2-25
0.050
0.100
2
2
8
6
2700
260
340
510
230


2-26
0
0.100
2
2
8
6
2800
260
330
500
220


2-27
0.050
0
2
2
3
15 
2800
230
320
470
230


2-28
0
0.100
2
2
3
15 
2800
240
330
480
240


2-29
0
0
5
2
3
6
2400
210
270
380
200


2-30


2
5
3
6
2200
200
260
360
190


2-31


2
2
8
6
2300
220
280
380
220


2-32


2
2
3
15 
2300
230
280
380
220


2-33


5
5
3
6
1900
170
230
350
150


2-34


2
2
8
15 
2000
160
220
330
160


2-35


5
2
8
15 
1200
130
190
230
120


2-36


5
5
3
15 
900
120
180
210
120


2-37


5
5
8
6
900
120
180
220
110


2-38


5
5
8
15 
700
100
140
190
 90


2-39


10 
10 
15 
30 
600
 40
 70
100
 30



















Compressive

Measured






rupture

specific




Average
strength of
Measured
surface/




crystal
granulated
specific
ideal



Specimen
grain size
powder
surface
specific



No.
(μm)
(MPa)
(m2/g)
surface
Remarks







2-21
17
0.90 ± 0.05
0.448-0.493
1020-1110
Comparative example



2-22
18



Comparative example



2-23
26



Comparative example



2-24
26



Comparative example



2-25
20



Comparative example



2-26
21



Comparative example



2-27
20



Comparative example



2-28
20



Comparative example



2-29
22
1.25 ± 0.05
0.730-0.808
1640-1800
Comparative example



2-30
25



Comparative example



2-31
21



Comparative example



2-32
23



Comparative example



2-33
39



Comparative example



2-34
42



Comparative example



2-35
52



Comparative example



2-36
59



Comparative example



2-37
64



Comparative example



2-38
178



Comparative example



2-39
213



Comparative example










As described in Tables 2-1 and 2-2, examples of the invention (Specimen Nos. 1-4 and 2-1) in which the content of S was less than 5 mass ppm, the content of Cl was less than 10 mass ppm, and the contents of P and B were less than 3 mass ppm, had excellent characteristics in which, under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ was 400 or more in a temperature range of 0 to 85° C. and the incremental permeability μΔ at 65° C. was 700 or more.
In contrast, in all the comparative examples (Specimen Nos. 2-2 to 2-39) in which the content of at least one of P, B, S, and Cl was more than the appropriate ranges, under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ was less than 400 in the range of 0 to 85° C. and the incremental permeability μΔ at 65° C. was less than 700.
EXAMPLE 3
A pulverized powder having the same composition as in Specimen No. 1-4 was used as a raw material. By changing the temperature condition in granulation by a spray drying method, granulated powders having different compressive rupture strengths in the range of 0.70 to 1.40 MPa were obtained. The granulated powders were then compacted and fired under the same conditions as in EXAMPLE 1 to obtain sintered cores having an outer diameter of 6.0 mm, an inner diameter of 3.0 mm, and a height of 4.0 mm (ideal specific surface: 4.44×10−4 m2/g).
A wire was wound ten turns around each specimen obtained in this way. While the core was under the application of a direct-current magnetic field of 80 A/m, the specimen was measured in terms of incremental permeability μΔ at 0° C., 23° C., 65° C., and 85° C. at a measurement voltage of 100 mV and at a measurement frequency of 100 kHz with the same direct-current application apparatus and LCR meter as in EXAMPLE.
The compressive rupture strength of the granulated powders was measured in accordance with requirements of JIS Z 8841. The measured specific surface was also measured by a BET method (multipoint method) in JIS Z 8830 (2001) and the ratio of (measured specific surface/ideal specific surface) was determined. The initial permeability μi and the average crystal grain size were measured as in EXAMPLE 1.
The results are described in Table 3.















TABLE 3









Compressive

Measured








rupture

specific

Incremental permeability
Average




strength of
Measured
surface/
Initial
under application of
crystal



Granulation
granulated
specific
ideal
permeability
magnetic field of 80 A/m
grain


Specimen
temperature
powder
surface
specific
μi
(μ )
size


















No.
(° C.)
(MPa)
(m2/g)
surface
23° C.
0° C.
23° C.
65° C.
85° C.
(μm)
Remarks





3-1
150
0.70
0.426
 960
4200
440
570
770
670
9
Example of invention


3-2
170
0.80
0.444
1000
4100
440
570
770
660
9
Example of invention


3-3
190
1.03
0.559
1260
3900
420
530
720
610
8
Example of invention


3-4
200
1.08
0.635
1430
3700
400
520
710
590
8
Example of invention


3-5
220
1.14
0.684
1540
3600
380
440
650
470
8
Comparative example


3-6
250
1.25
0.768
1730
3200
330
410
550
410
8
Comparative example


3-7
290
1.40
0.875
1970
2900
300
350
450
370
8
Comparative example









Examples of the invention (Specimen Nos. 3-1 to 3-4) in which the granulated powders had a compressive rupture strength of less than 1.10 MPa and, as a result, the specific surfaces of the cores decreased and the ratios of measured specific surface/ideal specific surface were less than 1500, had excellent characteristics in which, under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ was 400 or more in a temperature range of 0 to 85° C. and the incremental permeability μΔ at 65° C. was 700 or more.
However, comparative examples (Specimen Nos. 3-5 to 3-7) in which the granulated powders had a compressive rupture strength of 1.10 MPa or more and the ratios of measured specific surface/ideal specific surface were 1500 or more, that is, a large number of cavities were contained due to insufficient disintegration of the granulated powders, did not achieve characteristics in which the incremental permeability μΔ is 400 or more in the temperature range of 0 to 85° C. and the incremental permeability μΔ at 65° C. is 700 or more.
EXAMPLE 4
A calcined powder having the same composition as in Specimen No. 1-4 (note that adjustment was performed so as to achieve P: 2 mass ppm, B: 2 mass ppm, S: 3 mass ppm and Cl: 6 mass ppm) was mixed with cobalt oxide as a sub-component such that the final compositions satisfied proportions in Table 4, and pulverized with a ball mill for 12 hours. The pulverized powders were mixed with polyvinyl alcohol, granulated at 180° C., and compacted into toroidal cores under the application of a pressure of 118 MPa. The compacts were then put into a kiln and fired at a maximum temperature of 1350° C. Thus, sintered cores having an outer diameter of 6.0 mm, an inner diameter of 3.0 mm, and height of 4.0 mm (ideal specific surface: 4.44×10−4 m2/g) were obtained.
Each specimen obtained in this way was measured as in EXAMPLE 1 in terms of the incremental permeability μΔ at 0° C., 23° C., 65° C., and 85° C. under the application of a direct-current magnetic field of 80 A/m; the compressive rupture strength of the granulated powders; the ratio of (measured specific surface/ideal specific surface); the initial permeability μi; and the average crystal grain size. The compressive rupture strength of the granulated powders was 0.90±0.05 MPa. The measured specific surface measured by a BET method (multipoint method) in JIS Z 8830 (2001) was in the range of 0.453 to 0.493 m2/g. Thus, all the ratios of (measured specific surface/ideal specific surface) were in the range of 1020 to 1110, which were less than 1500.
The results are described in Table 4.














TABLE 4












Compressive
Measured






Incremental permeability
Average
rupture
specific




Initial
under application of
crystal
strength of
surface/



Amount of
permeability
magnetic field of 80 A/m
grain
granulated
ideal


Specimen
CoO added
μi
(μΔ)
size
powder
specific

















No.
(mass ppm)
23° C.
0° C.
23° C.
65° C.
85° C.
(μm)
(MPa)
surface
Remarks




















1-4
 0
4000
430
560
750
640
8
0.90 ± 0.05
1020-1110
Example of invention


4-1
 500
3800
530
670
870
710
7


Example of invention


4-2
1000
3500
550
690
890
680
7


Example of invention


4-3
2000
3000
570
700
890
640
7


Example of invention


4-4
2500
2700
590
720
870
590
7


Example of invention


4-5
3500
2300
250
370
440
370
14


Comparative example


4-6
5000
1800
210
310
390
320
17


Comparative example


4-7
10000 
1500
 80
100
170
140
26


Comparative example









As described in Table 4, all the examples of the invention (Specimen Nos. 4-1 to 4-4) in which appropriate amounts of CoO having positive magnetic anisotropy were added had excellent values: under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ was continuously 500 or more in a temperature range of 0 to 85° C. and the μΔ at 65° C. was 850 or more. Thus, these examples had characteristic values equivalent to or better than those of the example of the invention (Specimen No. 1-4) having a composition without CoO.
However, in all the comparative examples (Specimen Nos. 4-5 to 4-7) in which CoO was added in large amounts more than the upper limit, the incremental permeability μΔ under the application of a direct-current magnetic field of 80 A/m considerably decreased in the entire temperature range.
EXAMPLE 5
A calcined powder having the same composition as in Specimen No. 1-4 (note that adjustment was performed so as to achieve P: 2 mass ppm, B: 2 mass ppm, S: 3 mass ppm and Cl: 6 mass ppm) was mixed with zirconium oxide (in terms of ZrO2), tantalum oxide (in terms of Ta2O5), hafnium oxide (in terms of HfO2), and niobium oxide (in terms of Nb2O5) as sub-components such that the final compositions satisfied percentages in Table 5, and pulverized with a ball mill for 12 hours. The pulverized powders were mixed with polyvinyl alcohol, granulated at 180° C., and compacted into toroidal cores under the application of a pressure of 118 MPa. The compacts were then-put into a kiln and fired at a maximum temperature of 1350° C. Thus, sintered cores having an outer diameter of 6.0 mm, an inner diameter of 3.0 mm, and a height of 4.0 mm (ideal specific surface: 4.44×10−4 m2/g) were obtained.
Each specimen obtained in this way was measured as in EXAMPLE 1 in terms of the incremental permeability μΔ at 0° C., 23° C., 65° C., and 85° C. under the application of a direct-current magnetic field of 80 A/m; the compressive rupture strength of the granulated powders; the ratio of (measured specific surface/ideal specific surface); the initial permeability μi; and the average crystal grain size. The compressive rupture strength of the granulated powders was 0.90±0.05 MPa. The measured specific surface measured by a BET method (multipoint method) in JIS Z 8830 (2001) was in the range of 0.453 to 0.493 m2/g. Thus, all the ratios of (measured specific surface/ideal, specific surface) were in the range of 1020 to 1110, which were less than 1500.
The results are described in Table 5.














TABLE 5












Compressive
Measured






Incremental permeability

rupture
specific




Initial
under application of
Average
strength of
surface/




permeability
magnetic field of 80 A/m
crystal
granulated
ideal


Specimen
Sub-components (mass %)
μi
(μΔ)
grain size
powder
specific




















No.
ZrO2
Ta2O5
HfO2
Nb2O5
23° C.
0° C.
23° C.
65° C.
85° C.
(μm)
(MPa)
surface
Remarks























1-4
0
0
0
0
4000
430
560
750
640
8
0.90 ± 0.05
1020-1110
Example















of invention


5-1
0.050
0
0
0
3800
460
580
810
680
7


Example















of invention


5-2
0
0.050
0
0
3800
460
580
820
680
7


Example















of invention


5-3
0
0
0.050
0
3800
460
580
810
670
7


Example















of invention


5-4
0
0
0
0.050
3800
460
590
830
680
7


Example















of invention


5-5
0.030
0.030
0
0
3800
470
590
830
680
6


Example















of invention


5-6
0.030
0
0.030
0
3800
460
580
810
670
7


Example















of invention


5-7
0.030
0
0
0.030
3700
470
580
830
670
7


Example















of invention


5-8
0
0.030
0.030
0
3800
460
580
810
660
7


Example















of invention


5-9
0
0.030
0
0.030
3700
460
590
830
680
7


Example















of invention


 5-10
0
0
0.030
0.030
3800
460
580
820
660
7


Example















of invention


 5-11
0.020
0.020
0.020
0
3700
470
580
830
670
6


Example















of invention


 5-12
0.020
0.020
0
0.020
3600
460
580
830
670
6


Example















of invention


 5-13
0.020
0
0.020
0.020
3700
450
580
820
660
7


Example















of invention


 5-14
0
0.020
0.020
0.020
3700
460
590
830
670
7


Example















of invention


 5-15
0.020
0.020
0.020
0.020
3500
450
590
810
660
6


Example















of invention


 5-16
0.080
0
0
0
2400
240
360
480
270
116


Comparative















example


 5-17
0
0
0.080
0.080
2000
100
120
160
 90
163


Comparative















example


 5-18
0.080
0.080
0.080
0.080
1300
 50
 70
120
 70
205


Comparative















example









As is clear from Table 5, all the examples of the invention (Specimen Nos. 5-1 to 5-15) in which appropriate amounts of one or more selected from ZrO2, Ta2O5, HfO2, and Nb2O5 were added had excellent values: under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ was continuously 450 or more in a temperature range of 0 to 85° C. and the μΔ at 65° C. was 800 or more. Thus, these examples had characteristic values equivalent to or better than those of the example of the invention (Specimen No. 1-4) without such components.
However, in comparative examples (Specimen Nos. 5-16 to 5-18) in which at least one of the four components was added in large amounts more than the upper limit, exaggerated grain growth was caused and the incremental permeability μΔ under the application of a direct-current magnetic field of 80 A/m considerably decreased in the entire temperature range.
EXAMPLE 6
A calcined powder having the same composition as in Specimen No. 4-2 (note that adjustment was performed so as to achieve P: 2 mass ppm, B: 2 mass ppm, S: 3 mass ppm and Cl: 6 mass ppm) was mixed with zirconium oxide (in terms of ZrO2), tantalum oxide (in terms of Ta2O5), hafnium oxide (in terms of HfO2), and niobium oxide (in terms of Nb2O5) as sub-components such that the final compositions satisfied percentages in Table 6, and pulverized with a ball mill for 12 hours. The pulverized powders were mixed with polyvinyl alcohol, granulated at 180° C., and compacted into toroidal cores under the application of a pressure of 118 MPa. The compacts were then put into a kiln and fired at a maximum temperature of 1350° C. Thus, sintered cores having an outer diameter of 6.0 mm, an inner diameter of 3.0 mm, and a height of 4.0 mm (ideal specific surface: 4.44×10−4 m2/g) were obtained.
Each specimen obtained in this way was measured as in EXAMPLE 1 in terms of the incremental permeability μΔ at 0° C., 23° C., 65° C., and 85° C. under the application of a direct-current magnetic field of 80 A/m; the compressive rupture strength of the granulated powders; the ratio of (measured specific surface/ideal specific surface); the initial permeability μi; and the average crystal grain size. The compressive rupture strength of the granulated powders was 0.90±0.05 MPa. The measured specific surface measured by a BET method (multipoint method) in JIS Z 8830 (2001) was in the range of 0.453 to 0.493 m2/g. Thus, all the ratios of (measured specific surface/ideal specific surface) were in the range of 1020 to 1110, which were less than 1500.
The results are described in Table 6.














TABLE 6












Compressive
Measured






Incremental permeability under

rupture
specific




Initial
application of magnetic field of
Average
strength of
surface/




permeability
80 A/m
crystal
granulated
ideal


Specimen
Sub-components (mass %)
μi
(μΔ)
grain size
powder
specific




















No.
ZrO2
Ta2O5
HfO2
Nb2O5
23° C.
0° C.
23° C.
65° C.
85° C.
(μm)
(MPa)
surface
Remarks























4-2
0
0
0
0
3500
550
690
890
680
7
0.90 ± 0.05
1020-1110
Example















of invention


6-1
0.050
0
0
0
3300
590
750
950
740
7


Example















of invention


6-2
0
0.050
0
0
3300
590
730
950
750
7


Example















of invention


6-3
0
0
0.050
0
3300
580
720
940
740
7


Example















of invention


6-4
0
0
0
0.050
3300
610
750
960
750
7


Example















of invention


6-5
0.030
0.030
0
0
3300
590
720
950
740
6


Example















of invention


6-6
0.030
0
0.030
0
3300
590
720
950
750
7


Example















of invention


6-7
0.030
0
0
0.030
3200
600
730
960
760
7


Example















of invention


6-8
0
0.030
0.030
0
3300
600
730
970
750
7


Example















of invention


6-9
0
0.030
0
0.030
3200
610
750
970
760
7


Example















of invention


 6-10
0
0
0.030
0.030
3300
600
740
960
750
7


Example















of invention


 6-11
0.020
0.020
0.020
0
3200
590
740
950
740
6


Example















of invention


 6-12
0.020
0.020
0
0.020
3100
600
730
960
760
6


Example















of invention


 6-13
0.020
0
0.020
0.020
3200
600
740
950
750
7


Example















of invention


 6-14
0
0.020
0.020
0.020
3200
600
730
950
750
7


Example















of invention


 6-15
0.020
0.020
0.020
0.020
3100
610
750
960
750
6


Example















of invention


 6-16
0
0
0
0.080
2300
270
370
510
250
103


Comparative















example


 6-17
0.080
0.080
0
0
1700
100
150
180
 90
138


Comparative















example


 6-18
0.080
0.080
0.080
0.080
1100
 90
120
140
 80
197


Comparative















example









As is clear from Table 6, in all the examples of the invention (Specimen Nos. 6-1 to 6-15) in which appropriate amounts of one or more selected from ZrO2, Ta2O5, HfO2, and Nb2O5 were added, generation of coarse crystal grains is suppressed and the incremental permeability μΔ under the application of a direct-current magnetic field of 80 A/m increased. Due to a synergistic effect provided by the addition of CoO, these examples had particularly excellent characteristic values: the incremental permeability μΔ was continuously 550 or more in a temperature range of 0 to 85° C. and the μΔ at 65° C. was 900 or more. Thus, these examples had μΔ values equivalent to or higher than those of the example of the invention (Specimen No. 4-2) without such components.
However, in comparative examples (Specimen Nos. 6-16 to 6-18) in which at least one of the four components was added in large amounts more than the upper limit, exaggerated grain growth was caused and the incremental permeability μΔ under the application of a direct-current magnetic field of 80 A/m considerably decreased in the entire temperature range.
INDUSTRIAL APPLICABILITY
According to the present invention, a Mn—Zn ferrite core having excellent characteristics can be obtained in which, under the application of a direct-current magnetic field of 80 A/m, the incremental permeability μΔ is continuously 400 or more in a wide temperature range of 0 to 85° C. and the incremental permeability μΔ at 65° C. is 700 or more. Such a Mn—Zn ferrite core is advantageously applicable to, for example, a magnetic core of a pulse transformer in an Ethernet device.

## Claims

1. A Mn—Zn ferrite core comprising a basic component, subcomponents, and unavoidable impurities, wherein, as the sub-components,
silicon oxide (in terms of SiO2): 50 to 400 mass ppm and
calcium oxide (in terms of CaO): 50 to 4000 mass ppm are added to the basic component consisting of
iron oxide (in terms of Fe2O3): 51.0 to 54.5 mol %,
zinc oxide (in terms of ZnO): 8.0 to 12.0 mol %, and
manganese oxide (in terms of MnO): balance;

amounts of phosphorus, boron, sulfur, and chlorine in the unavoidable impurities are reduced as follows
phosphorus: less than 3 mass ppm,
boron: less than 3 mass ppm,
sulfur: less than 5 mass ppm, and
chlorine: less than 10 mass ppm; and

a ratio of a measured specific surface of the Mn—Zn ferrite core to an ideal specific surface of the Mn—Zn ferrite core satisfies formula (1) below


Measured specific surface/ideal specific surface<1500  (1)

where the measured specific surface represents a specific surface (m2/g) determined by a BET method (multipoint method) in JIS Z 8830 (2001); and the ideal specific surface represents a specific surface (m2/g) calculated from size and mass of the core on an assumption that the core is in an ideal state of having no cavities.

2. The Mn—Zn ferrite core according to claim 1, wherein the sub-components further include Cobalt oxide (in terms of CoO): 50 to 3000 mass ppm.

3. The Mn—Zn ferrite core according to claim 1, wherein the sub-components further include one or more selected from
zirconium oxide (in terms of ZrO2): 0.005 to 0.075 mass %;
tantalum oxide (in terms of Ta2O5): 0.005 to 0.075 mass %;
hafnium oxide (in terms of HfO2): 0.005 to 0.075 mass %; and
niobium oxide (in terms of Nb2O5): 0.005 to 0.075 mass %.

4. A method for producing the Mn—Zn ferrite core according to claim 1, the method comprising calcining the basic component to provide a calcined powder, mixing the calcined powder with the subcomponents, subsequently granulating the mixture of the calcined powder with the sub-components into a granulated powder having a compressive rupture strength of 1.10 MPa or less, and then compacting and subsequently firing the granulated powder.

5. The Mn—Zn ferrite core according to claim 2, wherein the sub-components further include one or more selected from
zirconium oxide (in terms of ZrO2): 0.005 to 0.075 mass %;
tantalum oxide (in terms of Ta2O5): 0.005 to 0.075 mass %;
hafnium oxide (in terms of HfO2): 0.005 to 0.075 mass %; and
niobium oxide (in terms of Nb2O5): 0.005 to 0.075 mass %.

6. A method for producing the Mn—Zn ferrite core according to claim 2, the method comprising calcining the basic component to provide a calcined powder, mixing the calcined powder with the subcomponents, subsequently granulating the mixture of the calcined powder with the sub-components into a granulated powder having a compressive rupture strength of 1.10 MPa or less, and then compacting and subsequently firing the granulated powder.

7. A method for producing the Mn—Zn ferrite core according to claim 3, the method comprising calcining the basic component to provide a calcined powder, mixing the calcined powder with the subcomponents, subsequently granulating the mixture of the calcined powder with the sub-components into a granulated powder having a compressive rupture strength of 1.10 MPa or less, and then compacting and subsequently firing the granulated powder.