JP7484086B2 - Manufacturing method of MnZn ferrite core - Google Patents

Description

The present invention relates to a method for producing an MnZn-based ferrite core that constitutes electronic components such as transformers for switching power supplies and choke coils.

Switching power supplies are used in the power circuits of various electronic devices that require a power supply, such as EVs (electric vehicles), HEVs (hybrid electric vehicles), mobile communication devices (cell phones, smartphones, etc.), personal computers, and servers.

In recent years, electronic devices have been increasingly required to be small and lightweight, as well as to consume less power from the perspective of energy efficiency. For this reason, LSIs (Large-Scale Integration) and functional elements such as DSPs (Digital Signal Processors) and MPUs (Micro-processing Units) used in electronic devices are also required to be small and highly functional, as well as consume less power. On the other hand, in recent years, as LSIs have become more highly integrated with transistors due to finer wiring, the withstand voltage of transistors has decreased and current consumption has increased, leading to lower operating voltages and larger currents.

Power supply circuits such as DC-DC converters that supply power to LSIs also need to be able to handle the lower operating voltages and higher currents of LSIs. For example, as the operating voltage of an LSI is lowered, the voltage range in which it can operate normally becomes narrower. If the voltage supplied from the power supply circuit fluctuates (ripples) and goes above or below the power supply voltage range of the LSI, this can lead to unstable operation of the LSI. To prevent this, measures have been taken to increase the switching frequency of the power supply circuit, for example to 500 kHz or higher.

Such support for higher frequencies and larger currents in power supply circuits also has the advantage of allowing the magnetic cores constituting the electronic components such as transformers, choke coils, etc., used in the circuits to be made smaller. For example, when driving a transformer with a sine wave, the voltage Ep (V) applied to the primary coil is expressed by the following formula, using the number of turns Np of the primary coil, the cross-sectional area A ( ^cm2 ) of the magnetic core, the frequency f (Hz), and the excitation magnetic flux density Bm (mT):
Ep = 4.44 × Np × A × f × Bm × ^10-7
This is expressed as:

From this formula, we can see that for a given applied voltage Ep to the primary coil, if the frequency (switching frequency) f is increased, the cross-sectional area A of the magnetic core can be reduced, resulting in a smaller size. In addition, as the current increases, the maximum excitation magnetic flux density (hereafter referred to as excitation magnetic flux density) Bm increases, making the magnetic core even smaller.

MnZn-based ferrite is mainly used as the magnetic material for magnetic cores that operate at high excitation magnetic flux density in the high frequency range and are suitable for miniaturization. MnZn-based ferrite has a higher initial permeability and saturation magnetic flux density than Ni-based ferrite, and has a smaller core loss than magnetic cores that use Fe-based or Co-based amorphous, pure iron, Fe-Si, Fe-Ni, Fe-Si-Cr, Fe-Si-Al, or other metal-based magnetic materials. Small core loss is advantageous in terms of reducing power consumption in power supply circuits.
A description of this MnZn-based ferrite core for use in the high frequency range is given in Patent Document 1.

International Publication No. 2017/164351 Japanese Patent Application Laid-Open No. 6-204023

Patent Document 1 describes an MnZn-based ferrite core that provides excellent magnetic properties in the high frequency range of 1 MHz or more. However, Patent Document 1 only describes a magnetic core made of a sintered body. In the case of a magnetic core made of a sintered body, there is a certain degree of limitation on the shape that can be formed, and there is a problem in obtaining a magnetic core of a free shape.
Patent Document 2 describes a method for producing ferrite powder by sintering and then pulverizing the pulverized ferrite powder, and then annealing the pulverized ferrite powder. However, Patent Document 2 does not describe the pulverization method in detail, and the annealing treatment described is a high-temperature, long-term heat treatment at a heat treatment temperature of 700 to 950°C and a sintering time of 4 hours. Furthermore, the ferrite in Patent Document 2 is a Ni-Cu-Zn ferrite, which is a different material from MnZn ferrite.

For this reason, there is a demand for MnZn-based ferrite powder that can be used in the high frequency range of 500 kHz or more, particularly in the high frequency range of 1 to 5 MHz, but a method for obtaining such powder has not yet been clarified.
Therefore, an object of the present invention is to provide a method for producing an MnZn-based ferrite magnetic core using MnZn-based ferrite powder that is useful in the high frequency range of 500 kHz or more, particularly 1 to 5 MHz.

Specific means for solving the above problems include the following aspects.
<1> A method for producing an MnZn- based ferrite core, comprising Fe, Zn, and Mn as main components and at least Co as an auxiliary component, the main components being ₅₃ to 56 mol % Fe calculated as _Fe2O3 , 3 to ₉ mol % Zn calculated as ZnO, and the remainder Mn calculated as MnO, the auxiliary component comprising 0.05 to 0.4 parts by mass of Co calculated as Co3O4 _per 100 parts by mass of the main components calculated as oxide,
a molding step of molding the raw material powder to obtain a molded body;
a sintering step of sintering the molded body and cooling it to a temperature of less than 150° C. to obtain a MnZn-based ferrite sintered body;
A pulverization step of pulverizing the obtained MnZn-based ferrite sintered body to obtain MnZn-based ferrite powder;
and a step of molding the MnZn ferrite powder .
The method further includes at least one of a heat treatment step of heat treating the MnZn-based ferrite sintered body and a heat treatment step of heat treating the MnZn-based ferrite powder obtained by pulverizing the MnZn-based ferrite sintered body, and the heat treatment step is
Condition 1: 200°C or higher, and Condition 2: (Tc-90)°C to (Tc+100)°C [where Tc is the Curie temperature ( _° C) calculated from the mole percentages of _Fe2O3 and ZnO contained in the main components of the MnZn-based ferrite powder.]
a heat treatment process in which the temperature is increased to a temperature that satisfies the above-mentioned condition, the temperature is maintained for a certain period of time, and then the temperature is decreased from the maintained temperature at a rate of 50° C./hour or less ,
The pulverization step includes a coarse pulverization step and a fine pulverization step,
The MnZn-based ferrite powder has a particle size distribution in which the cumulative percentage of particles passing through from the small particle size side of 1 μm in a volumetric particle size distribution obtained by a laser diffraction scattering type particle size distribution measurement method is 15% or less .

<2> The method for producing MnZn-based ferrite powder according to <1> , wherein the MnZn-based ferrite powder has an average particle size D50 of 100 μm or less .

<4> The method for producing an MnZn-based ferrite magnetic core according to any one of <1> to <3>, wherein the MnZn-based ferrite powder further contains, as auxiliary components, 0.003 to 0.015 parts by mass of Si in terms of _SiO2 , _0.06 to 0.3 parts by mass of Ca in terms of _CaCO3 , 0 to 0.1 parts by mass of V in terms of _V2O5 , and a total of 0 to 0.3 parts by mass of Nb (in terms of _Nb2O5 ) and/or Ta (in terms of _Ta2O5 ), relative to a total of ₁₀₀ parts _by mass of the main components in terms of the oxides.

<5> The sintering step includes a temperature increasing step, a high-temperature holding step, and a temperature decreasing step,
The high-temperature holding step has a holding temperature of more than 1050° C. and less than 1150° C., and an oxygen concentration in the atmosphere is 0.4 to 2% by volume;
The method for producing an MnZn-based ferrite magnetic core according to any one of <1> to <3>, wherein, in the temperature-lowering step, the oxygen concentration when lowering the temperature from 900° C. to 400° C. is in the range of 0.001 to 0.2 volume %, and the temperature-lowering rate from (Tc+70)° C. to 100° C. is 50° C./hour or more.
<6> The method for producing an MnZn ferrite core according to <5>, wherein, during the temperature decreasing step, the temperature decreasing rate from the holding temperature to 100° C. is 50° C./hour or more.
<7> The method for producing an MnZn-based ferrite core according to any one of <1> to <6>, wherein the MnZn-based ferrite core is for a transformer or a choke coil.

According to the present invention, it is possible to obtain an MnZn-based ferrite core that is useful in the high frequency range of 500 kHz or more.

FIG. 4 is a diagram showing temperature conditions in a heat treatment process in an example. 3 shows the particle size distribution of the MnZn ferrite powder of the embodiment. 3 is an electron microscope photograph of the MnZn ferrite powder of the embodiment.

In this specification, a numerical range expressed using "to" means a range including the numerical values described before and after "to" as the lower and upper limits. In the numerical ranges described in stages in this specification, the upper limit or lower limit described in one numerical range may be replaced with the upper limit or lower limit of another numerical range described in stages. In addition, in the numerical ranges described in this specification, the upper limit or lower limit of the numerical range may be replaced with a value shown in the examples.
In this specification, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.
Hereinafter, an embodiment of the present invention will be described, however, the present invention is not limited to the embodiment described below and can be modified as appropriate within the scope of the technical concept.

One embodiment of the present invention is a method _for producing an MnZn-based ferrite powder containing, as main components, 53 to 56 mol % Fe calculated as _Fe2O3 , 3 to 9 mol % Zn calculated as ZnO, and the remainder Mn calculated as MnO, and containing, as an auxiliary component, 0.05 to _0.4 mass parts Co calculated as _Co3O4 per 100 mass parts of the total of the main components calculated as oxides,
a molding step of molding a raw material powder of MnZn-based ferrite to obtain a molded body;
a sintering step of sintering the molded body and cooling it to a temperature of less than 150° C. to obtain a sintered body of MnZn-based ferrite;
A pulverization step of pulverizing the obtained sintered body of MnZn-based ferrite to obtain MnZn-based ferrite powder,
The method further includes at least one of a heat treatment step of heat treating the sintered body of MnZn-based ferrite and a heat treatment step of heat treating MZn-based ferrite powder obtained by pulverizing the sintered body of MnZn-based ferrite, and the heat treatment step is
Condition 1: 200°C or higher, and Condition 2: (Tc-90)°C to (Tc+100)°C [where Tc is the Curie temperature ( _° C) calculated from the mole percentages of _Fe2O3 and ZnO contained in the main components of the MnZn-based ferrite.]
and then, after maintaining the temperature for a certain period of time, lowering the temperature from the maintained temperature at a rate of 50° C./hour or less.

[1] Composition The composition of the MnZn-based ferrite of this embodiment will be described below.
MnZn ferrite has the desired magnetic properties such as initial permeability, saturation magnetic flux density, etc., by setting Fe, Zn, and Mn within the specified ranges. Furthermore, by adding Co as an auxiliary component to adjust the magnetocrystalline anisotropy constant, the temperature characteristics of the magnetic core loss can be improved.

The MnZn-based ferrite of this embodiment contains Fe, Zn, and Mn as main components, and at least Co as an auxiliary component, the main components being 53 to 56 mol% Fe as calculated as Fe ₂ O ₃ , 3 to 9 mol% Zn as calculated as ZnO, and the remainder Mn as calculated as MnO, and the auxiliary components contain 0.05 to 0.4 parts by mass Co as calculated as Co ₃ O ₄ relative to a total of 100 parts by mass of the main components as calculated as oxides. The auxiliary components may further contain 0.003 to 0.015 parts by mass Si as calculated as SiO ₂ , 0.06 to 0.3 parts by mass Ca as calculated as CaCO ₃ , 0 to 0.1 parts by mass V as calculated as V ₂ O ₅ , and a total of 0 to 0.3 parts by mass Nb (as calculated as Nb ₂ O ₅ ) and/or Ta (as calculated as Ta ₂ O ₅ ) relative to a total of 100 parts by mass of the main components as calculated as oxides.

Fe, together with Co, has the effect of controlling the temperature characteristics of core loss; if the amount is too small, the temperature at which core loss is minimal becomes too high, while if the amount is too large, the temperature at which core loss is minimal becomes too low, making it difficult to set the temperature at which core loss is minimal between 20 and 100°C, and core loss at 0 to ₁₂₀ °C deteriorates. If the Fe content is between 53 and 56 mol% calculated as _Fe2O3 , low loss can be achieved in the high frequency range of 1 _MHz or more. The Fe content is more preferably between 54 and 55 mol% calculated as _Fe2O3 .

Zn has the effect of controlling the frequency characteristic of magnetic permeability, and has a particular effect on controlling residual loss related to losses such as domain wall resonance in magnetic core loss, and the smaller the content, the lower the magnetic core loss in the high frequency range. If the Zn content is 3 to 9 mol % calculated as ZnO, low loss can be achieved in the high frequency range of 1 MHz or more, particularly in the frequency range up to 3 MHz. The Zn content is more preferably 5 to 8 mol % calculated as ZnO.
Mn is the balance calculated as MnO.

The Curie temperature (Tc) calculated from the molar percentages of Fe ₂ O ₃ and ZnO is in the range of 250 to 330° C. if the Fe content and Zn content are within the above ranges, which is a practically acceptable temperature range.

The MnZn-based ferrite of this embodiment contains at least Co as an auxiliary component. Co ²⁺ , together with Fe ²⁺ , is a metal ion having a positive magnetocrystalline anisotropy constant K1, and has the effect of adjusting the temperature at which the core loss is minimized. Furthermore, since Co 2+ has a larger magnetocrystalline anisotropy constant K1 than Fe ²⁺ , it is an effective element for improving the temperature dependency of the core loss. If the amount is too small, the effect of improving the temperature dependency is small, and if the amount is too large, the loss in the low temperature range increases significantly, which is not preferable in practical use. Furthermore, if the Co content is 0.05 to 0.4 parts by mass in terms of Co ₃ O ₄ with respect to a total of 100 parts by mass of the main components in terms of the oxide, the Co ²⁺ ions can be rearranged together with the Fe ²⁺ ions by heat treatment to control the induced magnetic anisotropy, thereby further reducing the core loss in the practical temperature range and improving the temperature dependency. The Co content is more preferably 0.1 to 0.3 parts by mass in terms of Co ₃ O ₄ .

It is preferable to further include Ca and Si as minor components. Si segregates at grain boundaries to increase grain boundary resistance, reduce eddy current loss, and thus has the effect of reducing core loss in the high frequency range. If the amount is too small, the effect of increasing grain boundary resistance is small, and if the amount is too large, it induces crystal thickening and deteriorates core loss. If the Si content is 0.003 to 0.015 parts by mass in _SiO2 equivalent to a total of 100 parts by mass of the main components in terms of the oxides, sufficient grain boundary resistance can be secured to reduce eddy current loss, and low loss can be achieved in the high frequency range of 1 MHz or more. The Si content is more preferably 0.005 to 0.01 parts by mass in _SiO2 equivalent.

Ca, like Si, segregates at grain boundaries, increases grain boundary resistance, reduces eddy current loss, and thus has the effect of reducing core loss in the high frequency range. If the amount is too small, the effect of increasing grain boundary resistance is small, and if the amount is too large, it induces crystal thickening and deteriorates core loss. If the Ca content is 0.06 to 0.3 parts by mass in _CaCO3 equivalent per 100 parts by mass of the total of the main components in terms of the oxides, sufficient grain boundary resistance can be secured to reduce eddy current loss, and low loss can be achieved in the high frequency range of 1 MHz or more. The Ca content is more preferably 0.06 to 0.2 parts by mass in terms of _CaCO3 .

The alloy may further contain Group 5a metals V, Nb, or Ta as a secondary component (Group 5a metal is at least one selected from the group consisting of V, Nb, and Ta, hereinafter collectively referred to as Group 5a). Group 5a metals, together with Si and Ca, segregate mainly as oxides at grain boundaries, and by increasing the resistance of the grain boundary phase, have the effect of further reducing core loss.

V has a lower melting point than Nb and Ta, and also has the function of promoting the growth of crystal grains. V has a lower melting point than other 5a group elements, so it is thought to have good wettability with grain boundaries, and it also has the effect of improving the workability of the sintered body and suppressing the occurrence of chipping. If the amount of V is too large, it induces crystal thickening and deteriorates the magnetic core loss. If the V content is 0 to 0.1 parts by mass in V ₂ O ₅ conversion with respect to 100 parts by mass of the total of the main components in terms of the oxides, it is possible to ensure sufficient grain boundary resistance to reduce eddy current loss, and low loss can be achieved in the high frequency range of 1 MHz or more. The V content is more preferably 0 to 0.05 parts by mass in V ₂ O ₅ conversion.

Nb and/or Ta also have the effect of suppressing the growth of crystal grains, forming a uniform crystal structure, and reducing core loss. Nb and Ta have a higher melting point than V, and together with Ca and Si, have the effect of preventing the melting point from being lowered by oxides with Fe. If the amount of Nb and Ta is too large, they segregate within the grains and deteriorate the core loss. If the total amount of Nb (Nb ₂ O ₅ conversion) and Ta (Ta ₂ O ₅ conversion) is 0 to 0.3 parts by mass with respect to a total of 100 parts by mass of the main components in terms of the oxides, sufficient grain boundary resistance can be secured to reduce eddy current loss, and low loss can be achieved in a high frequency range of 1 MHz or more. Furthermore, Nb and Ta have the effect of reducing hysteresis loss and residual loss, especially at high temperatures (100°C), among the core losses after heat treatment, and are effective in realizing low loss over a wide temperature range in the high frequency range. The total amount of Nb (calculated as Nb ₂ O ₅ ) and Ta (calculated as Ta ₂ O ₅₎ is more preferably 0 to 0.2 parts by mass.

The Ta content is preferably 0 to 0.1 parts by mass, and more preferably 0 to 0.05 parts by mass, calculated as Ta ₂ O _5. The Nb content is preferably 0.05 parts by mass or less (excluding 0), and more preferably 0.01 to 0.04 parts by mass, calculated as Nb ₂ O ₅ .

The MnZn ferrite of this embodiment preferably has an average crystal grain size of 2 to 5 μm. If the average crystal grain size is 5 μm or less, the eddy current loss is reduced, and the residual loss is reduced due to the reduction in the domain walls, and the core loss in the high frequency range is reduced. However, if the average crystal grain size is less than 2 μm, the grain boundaries act as pinning points for the domain walls, and the influence of the demagnetizing field tends to induce a decrease in magnetic permeability and an increase in core loss. If the average crystal grain size exceeds 5 μm, the eddy current loss increases, and the core loss in the high frequency range of 1 MHz or more tends to increase.
The average crystal grain size can be calculated by mirror-polishing a cross section of a sintered MnZn ferrite body, thermally etching (treatment in _N2 at 950-1050°C for 1 hour), photographing the cross section with an optical microscope or scanning electron microscope at 2000x magnification, and calculating by quadrature method (equivalent to JIS H0501-1986) based on a rectangular area of 60 μm x 40 μm on the photograph. If a sufficient number of particles (300 or more) cannot be counted due to the size of the crystal grain size, the observation area can be appropriately adjusted so that the number of particles observed is 300 or more.

[2] Manufacturing method (1) Molding process As the raw material powder of MnZn-based ferrite, _Fe2O3 , _Mn3O4 and ZnO powders _are used as the raw material of the main component, and _Co3O4 , _{SiO2 , CaCO3} , etc. are used as the raw material of the subcomponent. The molded body to be subjected to the sintering process is formed by adding the raw material of the subcomponent to the calcined powder _obtained by calcining the raw material of the main component, pulverizing and mixing until a predetermined average particle size is obtained, and using a granulated powder obtained by adding, for example, _polyvinyl alcohol as a binder to the obtained mixture. Note that _Co3O4 may be added before the calcination together with the raw material of the main component. The binder is an organic matter and is almost decomposed in the heating process, but depending on the conditions, carbon may remain after sintering and deteriorate the magnetic properties, and it is desirable to appropriately adjust the timing of switching to a low oxygen concentration atmosphere so that the binder is sufficiently decomposed.
The shape of the molded body can be, for example, a flat plate of 100 mm x 100 mm x 3 mm. Since it is crushed after sintering, it should be in a shape that is convenient for the crushing process. The flat plate shape is not particularly limited, but is preferably a rectangular shape with one side of about 10 mm to 100 mm and a thickness of about 1 mm to 5 mm.

(2) Sintering process A sintered body of MnZn-based ferrite is obtained by sintering a compact of the raw material powder of MnZn-based ferrite. The sintering process includes a heating process, a high-temperature holding process, and a temperature lowering process. In the high-temperature holding process, the holding temperature is preferably higher than 1050°C and lower than 1150°C, and the oxygen concentration in the atmosphere is preferably 0.4 to 2% by volume. In the temperature lowering process, the temperature lowering rate from at least (Tc+70)°C to 100°C is preferably 50°C/hour or more, and further the temperature lowering rate from the holding temperature to 100°C is preferably 50°C/hour or more.

(a) Heating Step In the heating step, it is preferable to set the oxygen concentration in the atmosphere to a range of 0.4 to 2% by volume at least at 900° C. By controlling the oxygen concentration at a temperature of 900° C. or higher where ferrite formation starts, a denser and higher density sintered body can be obtained.

(b) High-temperature holding step If the holding temperature in the high-temperature holding step is 1050°C or lower, sufficient sintering density cannot be obtained, and the structure tends to contain many fine crystals and voids. If the holding temperature is 1150°C or higher, sintering is promoted, but the crystal grains obtained tend to have a relatively large grain size, and as a result, eddy current loss tends to increase. Therefore, if the holding temperature in the high-temperature holding step deviates from the above-mentioned regulation, the magnetic core loss tends to increase. By lowering the holding temperature in the high-temperature holding step to less than 1150°C, it is possible to suppress the enlargement of the crystals, and the increase in eddy current loss can be further suppressed. In the present invention, the holding temperature in the high-temperature holding step is preferably 1060 to 1140°C, and more preferably 1070 to 1130°C.

If the oxygen concentration in the high-temperature holding step is less than 0.4% by volume, the atmosphere becomes reducing, and the resistance of the MnZn ferrite obtained by sintering becomes low, resulting in increased eddy current loss. On the other hand, if the oxygen concentration exceeds 2% by volume, the atmosphere becomes too oxidizing, making it easier for low-resistance hematite to be produced, and the grain size of the resulting crystal grains becomes relatively large, making it easier for partial crystal enlargement to occur. As a result, eddy current loss increases, and there is a tendency for core loss to increase at high frequencies and high excitation magnetic flux densities over the entire temperature range from low to high temperatures (0 to 120°C).

It is preferable to set the oxygen concentration according to the holding temperature, and the higher the holding temperature, the higher the oxygen concentration should be set. By setting the oxygen concentration according to the holding temperature, Ca segregates at the grain boundaries, increasing the resistance of the grain boundaries and reducing core loss.

The lower the oxygen concentration, the greater the amount of Fe ²⁺ having a positive magnetocrystalline anisotropy constant, and the lower the temperature at which the core loss is minimized. Therefore, it is preferable to set the oxygen concentration so as not to deviate from the above range.

(c) Temperature-reducing step In the temperature-reducing step following the high-temperature-retaining step, the oxygen concentration is first reduced from the atmosphere in the high-temperature-retaining step, and set to an oxygen concentration that prevents excessive oxidation and excessive reduction. In the temperature range from 900°C to 400°C, the amount of Fe2 ⁺ generated can be adjusted within a preferred range by setting the oxygen concentration in the atmosphere to 0.001 to 0.2 volume %. Here, in the temperature-reducing step following the high-temperature-retaining step, the period from 900°C to 400°C in which the atmosphere is adjusted to a predetermined oxygen concentration is called the first temperature-reducing step.

By controlling the oxygen concentration in the cooling process following the high-temperature holding process and adjusting it to the above range, Ca is segregated at the grain boundaries of the MnZn ferrite, and the amount of Ca dissolved in the crystal grains is appropriately controlled, increasing the resistance within the crystal grains and at the grain boundaries, thereby reducing the magnetic core loss related to eddy current loss.

The temperature drop rate in the first temperature drop step is not particularly limited as long as it is within a range that allows adjustment of the temperature and oxygen concentration in the sintering furnace, but is preferably 50 to 300°C/hour. If the temperature drop rate in the first temperature drop step is less than 50°C/hour, the sintering step takes time, the residence time in the sintering furnace increases, productivity decreases, and costs increase, which is not preferable. On the other hand, if the temperature drop rate exceeds 300°C/hour, it may be difficult to maintain the uniformity of the temperature and oxygen concentration in the sintering furnace, depending on the capacity of the sintering furnace.

By setting the holding temperature and oxygen concentration in the high-temperature holding step within a predetermined range, and controlling the oxygen concentration within a specific range when lowering the temperature from 900°C to 400°C in the first cooling step, it is possible to suppress variation in crystal grain size, control the amounts of Co2 ⁺ ions and ^Fe2+ ions to appropriate levels, and reduce core loss.

In the temperature decreasing step, when the Curie temperature calculated from the mole percentages of iron oxide (Fe ₂ O ₃ ) and zinc oxide (ZnO) constituting the main components of the MnZn ferrite is Tc (° C.), the temperature decreasing rate from (Tc+70)° C. to 100° C. is preferably 50° C./hour to 300° C./hour. Typically, the temperature decreasing rate from 400° C. to 100° C. is desirably 50° C./hour to 300° C./hour. Here, the period during which the temperature is decreased at a predetermined temperature decreasing rate in the temperature range from (Tc+70)° C. to 100° C., including Tc, in the temperature decreasing step is referred to as the second temperature decreasing step.

If the cooling rate in the second cooling step is less than 50° C./hour, the magnetic core loss on the high temperature side may deteriorate due to the influence of induced magnetic anisotropy caused by Co ²⁺ and Fe ²⁺ , which is undesirable. On the other hand, if the cooling rate exceeds 300° C./hour, it may be difficult to adjust the temperature and cooling rate in the sintering furnace, depending on the capacity of the sintering furnace.

The atmosphere in the second cooling step may be an inert gas atmosphere or an air atmosphere. The atmosphere with the controlled oxygen concentration in the first cooling step may remain the same, or may be changed to an air atmosphere or an inert gas atmosphere midway through the second cooling step.

(3) Crushing Step The sintered body of MnZn-based ferrite is crushed to obtain MnZn-based ferrite powder.
The pulverization step is preferably divided into a coarse pulverization step and a fine pulverization step. A medium pulverization step may be provided between the coarse pulverization and the fine pulverization. The number of pulverization steps can be appropriately selected.

In this embodiment, the coarse crushing process is preferably carried out so that the coarsely crushed powder is a few mm square. For example, a continuous jaw crusher can be used that has two plate-shaped crushing teeth, one fixed and one swinging, and has an angle at the inlet and outlet, narrowing the outlet side and discharging the crushed coarsely crushed powder through the gap. Particle size adjustment can be performed by adjusting the set width of the outlet gap.

The coarsely pulverized powder obtained in this coarse pulverization process can be classified, for example, using a sieve with 1.5 mm openings, and the coarsely pulverized powder that passes through the sieve can be sent to the fine pulverization process. The coarsely pulverized powder that remains on the sieve can be returned to the coarse pulverization process again and coarsely pulverized to the desired size. The sieve used here preferably has an opening of 3 mm or less, more preferably 2 mm or less. A vibrating sieve can be used for classification using this sieve.

In the fine pulverization step, the coarsely pulverized powder is pulverized to obtain a fine pulverized powder having a size of about 100 μm or less.
In this fine grinding process, for example, a vibrating mill can be used. Vibrating mills are available in continuous and batch types, and are pulverized by placing pulverizing media (spherical or rod-shaped) in a grinding drum and vigorously vibrating the material to be processed and the media. The particle size can be adjusted by the amount and shape of the media, the input amount of the material to be processed, the processing time, the amount of vibration, etc.

The finely pulverized powder obtained in this pulverization process is classified, for example, using a sieve with a mesh size of 198 μm, and the finely pulverized powder that passes through the sieve can be used as the MnZn-based ferrite powder after the pulverization process. The pulverized powder of MnZn-based ferrite that remains on the sieve can be returned to the pulverization process again and pulverized to the desired size. The upper limit of the mesh size of the sieve used here can be appropriately selected and adjusted according to the target particle size, for example, a sieve with a mesh size of about 30 μm to 200 μm that is used for general purposes. In addition, a vibrating sieve can be used for classification using this sieve. This sieve determines the upper limit of the particle size of the MnZn-based ferrite powder, but in order to remove pulverized powder that is too fine, a sieve that determines the lower limit of the particle size may also be used.

(4) Heat Treatment Step In this embodiment, at least one of a heat treatment step of heat treating the MnZn-based ferrite sintered body before crushing it and a heat treatment step of heat treating the MnZn-based ferrite powder obtained by crushing the MnZn-based ferrite sintered body is provided. Note that both heat treatment steps may be performed.
This heat treatment process is
Condition 1: 200°C or higher, and Condition 2: (Tc-90)°C to (Tc+100)°C [where Tc is the Curie temperature (°C) calculated from the mole percentages of Fe2O3 and ZnO contained in the main components of the MnZn-based ferrite.]
This is a heat treatment process in which the material is heated to a temperature that satisfies the above condition, held for a certain period of time, and then cooled from the held temperature at a rate of 50° C./hour or less.
If the holding temperature is less than 200°C or less than (Tc-90)°C, it becomes difficult to obtain the effect of reducing the magnetic core loss of the MnZn ferrite. If the temperature exceeds (Tc+100)°C, the effect of reducing the magnetic core loss reaches an upper limit. If the cooling rate from the holding temperature exceeds 50°C/hour, the effect of reducing the magnetic core loss is not sufficiently exhibited. The cooling rate is calculated in the temperature range from the holding temperature to 150°C.

The heat treatment may be performed in air or in a reducing atmosphere. In the case of an oxidizing atmosphere such as air, the upper limit of the heat treatment temperature is preferably 400°C or less to prevent the magnetic properties from deteriorating due to oxidation of the MnZn ferrite, and is preferably less than 350°C when the temperature drop rate is slow, about 5°C/hour. In the case of a reducing atmosphere, the upper limit of the heat treatment temperature is not limited by oxidation, but considering that the effect of reducing magnetic core loss will reach an upper limit, it is preferable to set it to 400°C or less, as in the case of heat treatment in an oxidizing atmosphere.

The heating rate in the heat treatment is not particularly limited, but may be appropriately selected so as not to be affected by the performance of the device or distortion due to thermal stress, and is typically set to 100°C to 300°C/hour.

The holding time in the heat treatment is not particularly limited, but it is sufficient to provide a time required for the sample placed in the apparatus to reach a predetermined temperature, typically about one hour.
The heat treatment of the present invention can be carried out using a heat treatment furnace (electric furnace, constant temperature bath, etc.).

In the present embodiment, the MnZn ferrite powder should preferably have a small particle size of 1 μm or less. If the powder is over-pulverized, the crystals constituting the sintered body are destroyed, and the properties tend to deteriorate as the average particle size of the pulverized powder becomes smaller. The MnZn ferrite powder preferably has a particle size distribution in which the cumulative percentage of particles passing through a particle size of 1 μm is 15% or less. More preferably, it is 10% or less, and it may be 0% by classification. Since the average crystal particle size of this MnZn ferrite is about 2 to 5 μm, particles of 1 μm or less have a low contribution to the properties, and therefore a small content is preferable.
In addition, this MnZn ferrite powder is mixed with resin and molded into a magnetic core. If the particle size is large, uniform mixing and filling density are not achieved. Therefore, it is preferable that the average particle size D50 is 100 μm or less.
The MnZn ferrite powder of this embodiment has an average particle size D50 of preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more. Also, it is preferably 100 μm or less, more preferably 90 μm or less, and even more preferably 80 μm or less.

The raw powder of MnZn-based ferrite was prepared so as to have the composition shown in Table 1. Fe ₂ O ₃ , Mn ₃ O ₄ (MnO equivalent) and ZnO were used as the raw materials of the main components, which were wet mixed, dried and pre-fired at 900°C for 1.5 hours. Next, Co ₃ O ₄ , SiO ₂ , CaCO ₃ , V ₂ O ₅ , Ta ₂ O ₅ and Nb ₂ O ₅ were added to a ball mill with respect to 100 parts by mass of the pre-fired powder as shown in Table 1, and the mixture was ground and mixed until the average ground particle size (air permeation method) was 0.8 to 1.0 μm. Polyvinyl alcohol was added as a binder to the resulting mixture, which was granulated with a spray dryer and then pressure-molded at 196 MPa to obtain a plate-shaped molded body (100 mm x 100 mm x 3 mm). The resulting molded body was sintered in an electric sintering furnace capable of adjusting the atmosphere to obtain a plate-shaped sintered body. The average crystal grain size was 3 μm.

Sintering was performed in the air during the temperature increase process from room temperature to 750 ° C., and at 750 ° C., replacement with _N2 gas was started to gradually reduce the oxygen concentration, and the oxygen concentration was set to 0.65 vol.% at 900 ° C., and the temperature was increased at a temperature increase rate of 130 ° C. / hour to the temperature of the high-temperature holding process set at 1115 ° C. In the high-temperature holding process, the oxygen concentration was set to 0.65 vol.% and held for 4 hours. In the temperature reduction process, the oxygen concentration was gradually reduced from 1000 ° C. to 850 ° C., and adjusted to 0.65 vol.% at 1000 ° C., 0.05 vol.% at 900 ° C., and 0.005 vol.% at 850 ° C. or less. In the temperature reduction process, the temperature was reduced to 100 ° C. at a temperature reduction rate of 150 ° C. / hour, and then the flat sintered body was taken out from the electric sintering furnace. The oxygen concentration was measured with a zirconia type oxygen analyzer, and the temperature was measured with a thermocouple installed in the sintering furnace.

(Curie temperature)
Ferrite (Maruzen Co., Ltd., published on November 30, 1986, 6th printing, page 79):
Tc = 12.8 x [y - (2/3) x z] - 358 (°C), where y and z are the mole percentages of _Fe2O3 and ZnO _, respectively.
The Curie temperature in this example was 270° C.

The flat sintered body was subjected to heat treatment as follows. Figure 1 shows the temperature conditions of the heat treatment process in the example. The heat treatment was carried out by raising the temperature from room temperature over 1.5 hours, and after reaching 250°C, maintaining that temperature for 1 hour to stabilize the temperature inside the furnace, lowering the temperature to 150°C at a rate of 5°C/hour. After the temperature fell below 150°C, outside air was introduced into the furnace to cool the sample. The heat treatment was carried out in the atmosphere.

The flat sintered body after the heat treatment process was coarsely crushed using a continuous jaw crusher, Mayekawa Industries Fine Jaw Crusher (registered trademark) SC-1007, with an outlet gap of 20 mm. The coarsely crushed powder was then classified using a vibrating sieve. Here, a sieve with 1.4 mm openings was used, and the coarsely crushed powder that passed through the sieve was finely crushed.

The fine pulverization was carried out using a vibration mill. In this embodiment, the fine pulverization was carried out for 15 minutes using a batch type vibration mill with iron spherical media to obtain a finely pulverized powder.
The finely pulverized powder was then classified using a vibrating sieve with a mesh size of 198 μm, and the finely pulverized powder that passed through the sieve was designated as MnZn-based ferrite powder.
This MnZn ferrite powder can be subjected to the same heat treatment as that given to the flat sintered body.

The particle size distribution of the obtained MnZn ferrite powder is shown in Figure 2. Figure 2 shows the particle size distribution with the particle size (particle diameter) (μm) on the horizontal axis and the frequency (%) and cumulative passing amount (%) on the vertical axis. The vertical axis is volume %. This particle size distribution was measured by a laser diffraction scattering type particle size distribution measuring method using a HORIBA LA-920. The measurement conditions are as follows:
<Transmittance>
- "Optimal upper limit" 95%, "optimal lower limit" 70%
<Sample preparation>
・"Circulation speed" 15 (device range)
・"Ultrasonic operation time" 3 minutes ・"Ultrasonic intensity" 7 (device range)
<Measurement condition settings>
・"Data reading count" 10 times ・"Ultrasonic operation during measurement" Yes <Display condition settings>
・"Distribution shape" Standard ・"Number of repetitions" 30 times ・"Relative refractive index" 2.50-4.00i
"Particle-based" Volume The MnZn ferrite powder had an average particle size D50 (median diameter) of 5.1 μm. The cumulative percentage of particles with a particle size of 1 μm passing through was about 10%.
An electron microscope photograph of this MnZn ferrite powder is shown in Figure 3. The obtained MnZn ferrite powder has a surface on which both intragranular and intergranular fracture modes are mixed.

In this example, MnZn-based ferrite powder with an average particle size D50 of 100 μm or less was obtained. This MnZn-based ferrite is a useful material in the high frequency range of 1 to 5 MHz, and is useful for reducing loss in components that use this MZn-based ferrite powder.

The MnZn-based ferrite powder of the present invention can be a useful material as a magnetic body used in electronic components and the like used in the high frequency range of 1 to 5 MHz. This MnZn-based ferrite powder can be mixed with resin, etc., molded into the required shape, and can function as a magnetic core, etc. This MnZn-based ferrite exhibits excellent magnetic properties in the high frequency range of 1 to 5 MHz, and is expected to contribute to reducing loss in components and the like that use this MZn-based ferrite powder.

Claims (7)

A method for producing an MnZn-based ferrite core, comprising Fe, Zn and Mn as main components and at least Co as an auxiliary component, the main components being ₅₃ to 56 mol % Fe calculated as _Fe2O3 , 3 to 9 mol % Zn calculated as ZnO and the remainder Mn calculated as MnO, the auxiliary component comprising 0.05 to 0.4 parts by mass of Co calculated as Co3O4 per 100 parts by mass of the main components _{calculated as} oxide ,
a molding step of molding the raw material powder to obtain a molded body;
a sintering step of sintering the molded body and cooling it to a temperature of less than 150° C. to obtain a MnZn-based ferrite sintered body;
A pulverization step of pulverizing the obtained MnZn-based ferrite sintered body to obtain MnZn-based ferrite powder;
and a step of molding the MnZn ferrite powder .
The method further includes at least one of a heat treatment step of heat treating the MnZn-based ferrite sintered body and a heat treatment step of heat treating the MnZn-based ferrite powder obtained by pulverizing the MnZn-based ferrite sintered body, and the heat treatment step is
Condition 1: 200°C or higher, and Condition 2: (Tc-90)°C to (Tc+100)°C [where Tc is the Curie temperature ( _° C) calculated from the mole percentages of _Fe2O3 and ZnO contained in the main components of the MnZn-based ferrite powder.]
a heat treatment process in which the temperature is increased to a temperature that satisfies the above-mentioned condition, the temperature is maintained for a certain period of time, and then the temperature is decreased from the maintained temperature at a rate of 50° C./hour or less,
The pulverization step includes a coarse pulverization step and a fine pulverization step,
The MnZn-based ferrite powder has a particle size distribution in which the cumulative percentage of particles passing through from the small particle size side of 1 μm in a volumetric particle size distribution obtained by a laser diffraction scattering type particle size distribution measurement method is 15% or less . 2. The method for producing a MnZn based ferrite magnetic core according to claim 1, wherein the MnZn based ferrite powder has an average particle size D50 of 100 μm or less . 2. The method for producing an MnZn ferrite core according to claim 1, wherein the pulverizing step further includes a medium pulverizing step between the coarse pulverizing step and the fine pulverizing step. The method for producing an MnZn-based ferrite magnetic core according to any one of claims 1 to 3, wherein the MnZn-based ferrite powder further contains, as auxiliary components, 0.003 to 0.015 parts by mass of Si in terms of _SiO2 , _0.06 to 0.3 parts by mass of Ca in terms of _CaCO3 , 0 to 0.1 parts by mass of V in terms of _V2O5 , and a total of 0 to _{0.3 parts by mass of Nb (in terms of Nb2O5) and/or Ta (in terms of Ta2O5 ) ,} relative to a total of 100 parts _by mass of the main components in terms of the oxides. The sintering step includes a temperature increasing step, a high temperature holding step, and a temperature decreasing step,
The high-temperature holding step has a holding temperature of more than 1050° C. and less than 1150° C., and an oxygen concentration in the atmosphere is 0.4 to 2% by volume;
The method for producing a MnZn-based ferrite magnetic core according to any one of claims 1 to 3, wherein, during the temperature-lowering step, the oxygen concentration when lowering the temperature from 900°C to 400°C is in the range of 0.001 to 0.2 volume %, and the temperature-lowering rate from (Tc+70)°C to 100°C is 50° C /hour or more. 6. The method for producing an MnZn ferrite magnetic core according to claim 5, wherein, during the temperature decreasing step, the temperature is decreased at a rate of 50[deg.] C./hour or more from the holding temperature to 100[deg.] C. 7. The method for producing a MnZn ferrite core according to claim 1, wherein the MnZn ferrite core is for a transformer or a choke coil.

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