JP2023095907A - MnZn BASED FERRITE POWDER - Google Patents

Abstract

To obtain useful MnZn based ferrite powder in a high frequency region.SOLUTION: MnZn based ferrite powder includes: 53-56 mol% of Fe in terms of Fe2O3, 3-9 mol% of Zn in terms of ZnO and a remainder Mn in terms of MnO as a main component; and 0.05-0.4 pt.mass of Co in terms of Co3O4 to the total 100 pts.mass of the main component in terms of the oxide as a sub-component. The MnZn based ferrite powder having a surface on which two fracture modes of transgranular fracture and intergranular fracture mix with each other is obtained by coarsely and finely pulverizing a sintered body subjected to heating and passing through a sieve having an opening of 190 μm.SELECTED DRAWING: Figure 3

Description

The present invention relates to a method for producing MnZn-based ferrite powder used for electronic parts, which are functional elements such as transformers such as switching power supplies and choke coils.

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

From the viewpoint of energy efficiency, there is an increasing demand for electronic devices of recent years to have low power consumption as well as to be made smaller and lighter. Therefore, LSIs (Large-Scale Integration) such as DSPs (Digital Signal Processors) and MPUs (Micro-processing Units) and functional elements used in electronic devices are also required to be smaller and have higher performance as well as lower power consumption. there is On the other hand, in recent years, as the integration of transistors in LSIs has increased due to finer wiring, the withstand voltage of transistors has decreased and the current consumption has increased, resulting in a decrease in operating voltage and an increase in current.

A power supply circuit such as a DC-DC converter that supplies power to an LSI also needs to cope with a lower operating voltage and a larger current of the LSI. For example, as the operating voltage of an LSI is lowered, the voltage range in which it can operate normally becomes narrower. In order to increase the switching frequency of the power supply circuit, for example, a switching frequency of 500 kHz or higher has been adopted.

Such adaptation to high frequency and high current power supply circuits also has the advantage of reducing the size of magnetic cores that constitute electronic components such as transformers and choke coils used in circuits. For example, when driving a transformer with a sine wave, the voltage Ep (V) applied to the primary coil is determined by the number of turns Np of the primary coil, the cross-sectional area A (cm ² ) of the magnetic core, the frequency f (Hz) and Formula using the excitation magnetic flux density Bm (mT):
Ep=4.44×Np×A×f×Bm×10 ⁻⁷
is represented by

From this equation, it can be seen that if the frequency (switching frequency) f is increased with respect to a predetermined voltage Ep applied to the primary side coil, the cross-sectional area A of the magnetic core can be reduced and the size can be reduced. In addition, as the current increases, the maximum exciting magnetic flux density (hereinafter referred to as "exciting magnetic flux density") Bm increases, so the magnetic core is further miniaturized.

MnZn-based ferrite is mainly used as a magnetic material for a magnetic core that operates at a high excitation magnetic flux density in a high frequency region and is suitable for miniaturization. MnZn-based ferrite has a higher initial permeability and saturation magnetic flux density than Ni-based ferrite, etc. It has a characteristic that the magnetic core loss is smaller than that of a magnetic core using a metallic magnetic material such as Al. A small magnetic core loss is advantageous in terms of suppressing the power consumption of the power supply circuit.
Japanese Patent Application Laid-Open No. 2002-200301 describes a MnZn ferrite core for high frequency range.

WO2017/164351 JP-A-6-204023

Patent Document 1 describes a MnZn-based ferrite magnetic core that provides excellent magnetic properties in a high frequency range of 1 MHz or higher. 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, the shape that can be formed is limited to some extent, and there has been a problem in obtaining a free-form magnetic core.
Patent Document 2 describes a method for producing ferrite powder by pulverizing after sintering, in which the pulverized ferrite powder is annealed. However, Patent Document 2 does not describe a detailed pulverization method, and the described annealing treatment is a high-temperature, long-time heat treatment with a heat treatment temperature of 700 to 950° C. and a firing time of 4 hours. Furthermore, the material disclosed in Patent Document 2 is Ni--Cu--Zn based ferrite, which is a different material from MnZn based ferrite.

Therefore, there is a demand for MnZn ferrite powder that can be used in a high frequency range of 500 kHz or higher, particularly in a high frequency range of 1 to 5 MHz.
Accordingly, an object of the present invention is to provide a method for producing MnZn-based ferrite powder by which useful MnZn-based ferrite powder can be obtained in a high frequency range of 500 kHz or higher, especially 1 to 5 MHz.

Specific means for solving the above problems include the following aspects.
<1> 53 to 56 mol% of Fe in terms of Fe ₂ O ₃ , 3 to 9 mol% of Zn in terms of ZnO, and the balance Mn in terms of MnO as main components, and the sum of the main components in terms of oxides A method for producing MnZn-based ferrite powder containing Co as an auxiliary component in an amount of 0.05 to 0.4 parts by mass in terms of Co ₃ O ₄ per 100 parts by mass,
a forming step of forming a raw material powder of MnZn-based ferrite to obtain a formed 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 pulverizing step of pulverizing the obtained sintered body of MnZn-based ferrite to obtain MnZn-based ferrite powder,
Further, at least one of a heat treatment step of heat-treating the sintered body of the MnZn-based ferrite and a heat-treating step of heat-treating the MnZn-based ferrite powder obtained by pulverizing the sintered body of the MnZn-based ferrite, The heat treatment process
Condition 1: 200° C. or higher, and Condition 2: (Tc-90)° C. to (Tc+100)° C. [where Tc is calculated from the mole % of Fe ₂ O ₃ and ZnO contained in the main components of the MnZn-based ferrite. is the Curie temperature (°C) ]
A method for producing MnZn-based ferrite powder, characterized in that it is a heat treatment step in which the temperature is heated to a temperature satisfying the above, held for a certain period of time, and then the temperature is lowered from the holding temperature at a rate of 50 ° C./hour or less.

<2> The MnZn-based ferrite powder has a particle size in which, in the volume-based particle size distribution obtained by a laser diffraction/scattering particle size distribution measurement method, the cumulative amount (%) of passage from the small particle size side of the particle size of 1 μm is 15% or less. The method for producing the MnZn-based ferrite powder according to <1>, comprising a distribution.
<3> The method for producing MnZn-based ferrite powder according to <1> or <2>, wherein the MnZn-based ferrite powder has an average particle diameter D50 of 100 μm or less.

<4> The MnZn-based ferrite powder further contains 0.003 to 0.015 parts by mass of Si and CaCO as subcomponents in terms of SiO ₂ with respect to a total of 100 parts by mass of the main components in terms of oxides. 0.06 to 0.3 parts by weight of Ca in terms of ₃ , 0 to 0.1 parts by weight of V in terms of V ₂ O ₅ and a total of 0 to 0.3 parts by weight of Nb (as in terms of Nb ₂ O ₅ ) and/or Ta (in terms of Ta ₂ O ₅ ).

<5> The sintering step includes a temperature raising step, a high temperature holding step, and a temperature lowering step,
In the high temperature holding step, the holding temperature is higher than 1050 ° C. and lower than 1150 ° C., and the oxygen concentration in the atmosphere is 0.4 to 2% by volume,
During the temperature lowering step, the oxygen concentration when the temperature is lowered from 900 ° C. to 400 ° C. is in the range of 0.001 to 0.2% by volume, and the temperature lowering rate from (Tc + 70) ° C. to 100 ° C. is 50 ° C./hour or more. The method for producing MnZn-based ferrite powder according to any one of <1> to <4>.
<6> The method for producing MnZn-based ferrite powder according to <5>, wherein during the temperature lowering step, the temperature lowering rate from the holding temperature to 100° C. is 50° C./hour or more.

According to the present invention, useful MnZn ferrite powder can be obtained in a high frequency range of 500 kHz or higher.

It is a figure which shows the temperature conditions of the heat processing process of an Example. 1 is a particle size distribution of MnZn-based ferrite powder of an example. 1 is an electron micrograph of MnZn-based ferrite powder of an example.

In this specification, a numerical range represented by "to" means a range including the numerical values before and after "to" as lower and upper limits. In the numerical ranges described step by step 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 the numerical range described in other steps. . Moreover, in the numerical ranges described in this specification, the upper and lower limits of the numerical ranges may be replaced with the values 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. .
Embodiments of the present invention will be described below, but the present invention is not limited to the embodiments described below, and can be modified as appropriate within the scope of the technical idea.

One embodiment of the present invention contains 53 to 56 mol% of Fe in terms of Fe ₂ O ₃ , 3 to 9 mol% of Zn in terms of ZnO, and the balance Mn in terms of MnO as main components, and A method for producing MnZn-based ferrite powder containing 0.05 to 0.4 parts by mass of Co as an auxiliary component in terms of Co ₃ O ₄ with respect to a total of 100 parts by mass of the main components,
a forming step of forming a raw material powder of MnZn-based ferrite to obtain a formed 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 pulverizing step of pulverizing the obtained sintered body of MnZn-based ferrite to obtain MnZn-based ferrite powder,
Further, at least one of a heat treatment step of heat-treating the sintered body of the MnZn-based ferrite and a heat-treating step of heat-treating the MnZn-based ferrite powder obtained by pulverizing the sintered body of the MnZn-based ferrite, The heat treatment process
Condition 1: 200° C. or higher, and Condition 2: (Tc-90)° C. to (Tc+100)° C. [where Tc is calculated from the mole % of Fe ₂ O ₃ and ZnO contained in the main components of the MnZn-based ferrite. is the Curie temperature (°C) ]
is heated to a temperature that satisfies the above conditions, held for a certain period of time, and then cooled from the holding temperature at a rate of 50° C./hour or less.

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

The MnZn-based ferrite of the present embodiment contains Fe, Zn, and Mn as main components and at least Co as an auxiliary component, and the main component is 53 to 56 mol% of Fe and ZnO in terms of Fe ₂ O ₃ . 3 to 9 mol % _of Zn and the balance Mn in terms of MnO _. Contains 4 parts by mass of Co. The subcomponents further contain 0.003 to 0.015 parts by mass of Si in terms of _SiO2 and 0.06 to 0.3 parts in terms of _CaCO3 with respect to a total of 100 parts by mass of the main components in terms of oxides. parts by mass of Ca, 0 to 0.1 parts by mass of V calculated as V ₂ O ₅ , and a total of 0 to 0.3 parts by mass of Nb (as calculated as Nb ₂ O ₅ ) and/or Ta (as calculated as Ta ₂ O ₅ ) may be included.

Fe, together with Co, has the effect of controlling the temperature characteristics of core loss. It is difficult to keep the temperature between 20 and 100.degree. If the Fe content is between 53 and 56 mol % in terms of Fe ₂ O ₃ , low loss can be achieved in a high frequency range of 1 MHz or higher. The Fe content is more preferably 54 to 55 mol % in terms of Fe ₂ O ₃ .

Zn has the effect of controlling the frequency characteristics of magnetic permeability, and in the core loss, it particularly affects the control of residual loss related to loss such as domain wall resonance, and the smaller the amount, the lower the core loss in the high frequency region. Become. If the Zn content is 3 to 9 mol % in terms of ZnO, low loss can be achieved in a high frequency range of 1 MHz or higher, particularly in a frequency range up to 3 MHz. The Zn content is more preferably 5 to 8 mol % in terms of ZnO. Mn is the remainder in terms of MnO.

The Curie temperature (Tc) obtained by calculation from the mol % 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.

The MnZn-based ferrite of this embodiment contains at least Co as an accessory component. Co ²⁺ is a metal ^ion having a positive magnetocrystalline anisotropy constant K1 together with Fe ²⁺ , and has the effect of adjusting the temperature at which the core loss is minimized. Therefore, it is an effective element for improving the temperature dependence of the core loss. If the amount is too small, the effect of improving the temperature dependence is small, and if the amount is too large, the loss in the low temperature range increases remarkably, which is not preferable for practical use. Further, when 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 oxides, heat treatment is performed to obtain Co ²⁺ ions together with Fe ²⁺ ions. are rearranged to control the induced magnetic anisotropy, the core loss can be further reduced in the practical temperature range, and the temperature dependence can be improved. The Co content is more preferably 0.1 to 0.3 parts by mass in terms of Co3O4.

It preferably further contains Ca and Si as subcomponents. Si segregates at grain boundaries to increase grain boundary resistance, reduce eddy current loss, and has the effect of reducing core loss in the high frequency range. If there is too much, conversely, enlargement of crystals is induced and core loss is deteriorated. If the Si content is 0.003 to 0.015 parts by mass in terms of SiO ₂ with respect to a total of 100 parts by mass of the main components in terms of oxides, the grain boundary resistance is sufficient to reduce eddy current loss. can be ensured, and low loss can be achieved in a high frequency region of 1 MHz or higher. The Si content is more preferably 0.005 to 0.01 parts by mass in terms of SiO ₂ .

Ca, like Si, segregates at grain boundaries, increases grain boundary resistance, reduces eddy current loss, and has the effect of reducing core loss in a high frequency region. If the amount is too small, the effect of increasing the grain boundary resistance is small, and if the amount is too large, conversely, enlargement of crystals is induced and core loss is deteriorated. If the Ca content is 0.06 to 0.3 parts by mass in terms of CaCO ₃ with respect to a total of 100 parts by mass of the main components in terms of oxides, grain boundaries sufficient to reduce eddy current loss Resistance can be ensured, and low loss can be achieved in a high frequency region of 1 MHz or higher. The Ca content is more preferably 0.06 to 0.2 parts by mass in terms of CaCO ₃ .

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

V has a melting point lower than that of Nb and Ta, and also has the function of promoting the growth of crystal grains. Since V has a lower melting point than other group 5a elements, it is considered 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 and the like. If the amount of V is too large, it induces enlargement of the crystal and deteriorates the core loss. Grain boundary resistance sufficient to reduce eddy current loss when the V content is 0 to 0.1 parts by mass in terms of V ₂ O ₅ with respect to a total of 100 parts by mass of the main components in terms of oxides. can be ensured, and low loss can be achieved in a high frequency region of 1 MHz or higher. The V content is more preferably 0 to 0.05 parts by mass in terms of V ₂ O ₅ .

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 melting point higher than that of 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 will segregate in the grains and deteriorate the core loss. Eddy current loss if the total amount of Nb (in terms of Nb ₂ O ₅ ) and Ta (in terms of Ta ₂ O ₅ ) 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 oxides Grain boundary resistance sufficient to reduce is ensured, and low loss can be achieved in a high frequency region of 1 MHz or higher. Furthermore, Nb and Ta have the effect of reducing the hysteresis loss and residual loss, especially at high temperatures (100°C), among the core losses after heat treatment, and achieve low loss in a wide temperature range in the high frequency region. effective for The total amount of Nb (as Nb ₂ O ₅ ) and Ta (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, more preferably 0 to 0.05 parts by mass in terms of Ta ₂ O ₅ . The Nb content is preferably 0.05 parts by mass or less (not including 0), more preferably 0.01 to 0.04 parts by mass in terms of Nb ₂ O ₅ .

The MnZn-based 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, the residual loss is reduced due to the reduction of the domain wall, and the core loss in the high frequency region is reduced. However, if the average crystal grain size is less than 2 μm, the grain boundaries act as pinning points of the domain walls, and 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 core loss tends to increase in a high frequency range of 1 MHz or higher due to an increase in eddy current loss.
The average crystal grain size was determined by mirror-polishing the cross section of the MnZn ferrite sintered body, thermally etching it (950 to 1050° C. for 1 hour, N ₂ treatment), and examining the cross section with an optical microscope or scanning electron microscope. A photograph is taken at a magnification of 2000, and calculation can be performed by the quadrature method (corresponding to JIS H0501-1986) based on a rectangular area of 60 μm×40 μm on this 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 to be observed is 300 or more.

[2] Manufacturing method (1) Forming step As raw material powders of MnZn ferrite, powders of Fe ₂ O ₃ , Mn ₃ O ₄ and ZnO are used as raw materials of main components, and Co ₃ O ₄ is used as raw materials of auxiliary components. , SiO ₂ , CaCO ₃ and other powders are used. The molded body to be subjected to the sintering process is obtained 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 it until it reaches a predetermined average particle size, and adding it to the resulting mixture as a binder. For example, it is formed using granulated powder obtained by adding polyvinyl alcohol. Note that Co ₃ O ₄ may be added together with the raw material of the main component before calcination. The binder is an organic substance and decomposes during the heating process, but depending on the conditions, carbon may remain after sintering and degrade the magnetic properties. It is desirable to adjust as appropriate so that it decomposes into
The shape of the molded body can be, for example, a flat plate shape of 100 mm×100 mm×3 mm. Since it is pulverized after sintering, it may be formed into a shape that is convenient for the pulverization process. Although the shape of the flat plate is not particularly limited, it is preferably a rectangular shape with a side of about 10 mm to 100 mm and a thickness of about 1 mm to 5 mm.

(2) Sintering step By sintering the molded body of the raw material powder of the MnZn-based ferrite, a sintered body of the MnZn-based ferrite is obtained. The sintering has a temperature raising step, a high temperature holding step, and a temperature lowering step. In the high-temperature holding step, 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 step, the temperature-lowering rate from at least (Tc+70)°C to 100°C is preferably 50°C/hour or more, and the temperature-lowering rate from the holding temperature to 100°C is preferably 50°C/hour or more. preferably.

(a) Temperature Raising Step In the temperature raising step, the temperature is preferably at least 900° C. and the oxygen concentration in the atmosphere is preferably in the range of 0.4 to 2% by volume. By controlling the oxygen concentration at a temperature of 900° C. or higher at which 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 less, a sufficient sintered density cannot be obtained, and the structure tends to contain many fine crystals and pores. When the holding temperature is 1150° C. or higher, sintering is promoted, but the obtained crystal grains tend to have relatively large grain sizes, 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 regulation, the 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 enlargement of crystals, and to further suppress an increase in eddy current loss. In the present invention, the holding temperature in the high temperature holding step is preferably 1060-1140°C, more preferably 1070-1130°C.

If the oxygen concentration in the high-temperature holding step is less than 0.4% by volume, the atmosphere becomes reductive, and the resistance of the MnZn-based ferrite obtained by sintering becomes low, increasing the eddy current loss. On the other hand, when the oxygen concentration exceeds 2% by volume, the atmosphere becomes too oxidizing, so that low-resistance hematite is likely to be formed, and the grain size of the resulting crystal grains becomes relatively large, resulting in partial crystal formation. prone to obesity. As a result, eddy current loss increases, and core loss tends to increase over the entire temperature range (0 to 120° C.) from low to high temperatures at high frequencies and high excitation magnetic flux densities.

The oxygen concentration is preferably set according to the holding temperature, and the higher the holding temperature is, the higher the oxygen concentration is set. By setting the oxygen concentration in accordance with the holding temperature, Ca segregates at the grain boundaries, increasing the resistance of the grain boundaries and reducing the core loss.

As the oxygen concentration decreases, the amount of Fe ²⁺ having a positive magnetocrystalline anisotropy constant increases, and the temperature at which the core loss is minimized tends to decrease. Therefore, the oxygen concentration is set within the above range. is preferred.

(c) Temperature Lowering Step In the temperature lowering step following the high temperature holding step, first, the oxygen concentration is lowered from the atmosphere of the high temperature holding step, and the oxygen concentration is set to prevent excessive oxidation and excessive reduction. By setting the oxygen concentration in the atmosphere to 0.001 to 0.2% by volume within the temperature range of 900° C. to 400° C., the amount of Fe ²⁺ produced can be adjusted within a preferred range. Here, in the temperature lowering step following the high temperature holding step, the period from 900° C. to 400° C. in which the atmosphere is adjusted to have a predetermined oxygen concentration is called a first temperature lowering step.

Following the high-temperature holding step, the oxygen concentration is controlled and adjusted to the above range in the temperature-lowering step as well, thereby segregating Ca at the grain boundaries of the MnZn-based ferrite and appropriately controlling the amount of Ca dissolved in the crystal grains. Therefore, the resistance in the crystal grains and the grain boundaries can be increased to reduce the core loss associated with the eddy current loss.

The temperature-lowering rate in the first temperature-lowering step is not particularly limited as long as the temperature and oxygen concentration in the sintering furnace can be adjusted, but it is preferably 50 to 300° C./hour. If the temperature lowering rate in the first temperature lowering step is less than 50°C/hour, the sintering step will take a long time, and the residence time in the sintering furnace will increase, resulting in a decrease in productivity and an increase in cost. I don't like it. On the other hand, if the temperature drop rate exceeds 300° C./hour, it may be difficult to maintain 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 the oxygen concentration in the high-temperature holding step to predetermined ranges, and controlling the oxygen concentration when the temperature is lowered from 900° C. to 400° C. in the first temperature-lowering step within a specific range, variations in crystal grain size are suppressed. , Co ²⁺ ions and Fe ²⁺ ions can be controlled to appropriate amounts to reduce core loss.

In the temperature-lowering step, when the Curie temperature obtained by calculation from the mol % of iron oxide (Fe ₂ O ₃ ) and zinc oxide (ZnO) that constitute the main components of the MnZn-based ferrite is Tc (° C.), (Tc+70) C. to 100.degree. C. is preferably 50.degree. C./hour to 300.degree. C./hour. Typically, it is desirable to reduce the temperature from 400° C. to 100° C. at a rate of 50° C./hour to 300° C./hour. Here, in the temperature-lowering step, the temperature range from (Tc+70)° C. including Tc to 100° C. at a predetermined temperature-lowering rate is called a second temperature-lowering step.

If the temperature lowering rate in the second temperature lowering step is less than 50° C./hour, the influence of induced magnetic anisotropy caused by Co ²⁺ and Fe ²⁺ is likely to occur, and core loss on the high temperature side may deteriorate, which is undesirable. On the other hand, if the temperature drop rate exceeds 300° C./hour, it may be difficult to adjust the temperature inside the sintering furnace and the temperature drop rate, 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 in which the oxygen concentration is controlled in the first temperature-lowering step may be maintained, or an air atmosphere or an inert gas atmosphere may be used during the second temperature-lowering step.

(3) Pulverization step The MnZn ferrite sintered body is pulverized to obtain MnZn ferrite powder. In the pulverization process, it is preferable to divide into a coarse pulverization process and a fine pulverization process. An intermediate pulverization step may be provided between coarse pulverization and fine pulverization. The number of grinding steps can be selected as appropriate.

In the present embodiment, the coarsely pulverizing step is preferably performed so as to obtain coarsely pulverized powder having a size of about several millimeters square. For example, there is a continuous jaw crusher that has two plate-shaped crushing teeth of a fixed type and an oscillating type, and has a structure in which the inlet and the outlet are set at an angle so that the outlet side is narrowed and the coarsely crushed powder is discharged from the gap. can be used. Granularity adjustment can be performed by adjusting the set width of the outlet interval.

The coarsely pulverized powder obtained in this coarsely pulverizing step can be classified, for example, using a sieve with an opening of 1.5 mm, and the coarsely pulverized powder that has passed through the sieve can be sent to the finely pulverizing step. The coarsely pulverized powder remaining on the sieve may be returned to the coarsely pulverizing step and coarsely pulverized to a desired size. The sieve used here preferably has an opening of 3 mm or less. Furthermore, 2 mm or less is preferable. A vibrating sieve machine can be used for the classification using this sieve.

In the finely pulverizing step, the coarsely pulverized powder is finely pulverized to obtain finely pulverized powder of about 100 μm or less. In this pulverization step, for example, a vibration mill can be used. There are continuous and batch types of vibrating mills. Grinding media (spherical or rod-shaped) are placed in the grinding drum, and the material to be processed and the media are violently vibrated to pulverize the material. Granularity can be adjusted by the amount of media, its form, the amount of material to be processed, the processing time, the amount of amplitude, and the like.

The finely pulverized powder obtained in this finely pulverizing step is classified using, for example, a sieve with an opening of 198 μm, and the finely pulverized powder that has passed through the sieve can be used as the MnZn ferrite powder after the pulverizing step. The pulverized powder of MnZn-based ferrite remaining on the sieve may be returned to the pulverizing step again and pulverized to a desired size. The upper limit of the mesh size of the sieve used here may be adjusted according to the target particle size, for example, by appropriately selecting a sieve having a mesh size of about 30 μm to 200 μm for general use. A vibrating sieve machine can be used for the classification using this sieve. This sieve determines the upper limit of the particle size of the MnZn-based ferrite powder, but a sieve may be used to determine the lower limit of the particle size in order to remove pulverized powder that is too fine.

(4) Heat treatment step In the present embodiment, the heat treatment step of heat-treating the MnZn-based ferrite sintered body before pulverization and the heat-treatment step of heat-treating the MnZn-based ferrite powder obtained by pulverizing the MnZn-based ferrite sintered body. , at least one heat treatment step. Both heat treatment steps may be performed.
This heat treatment process
Condition 1: 200° C. or higher, and Condition 2: (Tc-90)° C. to (Tc+100)° C. [where Tc is calculated from the mole % of Fe ₂ O ₃ and ZnO contained in the main components of the MnZn-based ferrite. is the Curie temperature (°C) ]
It is a heat treatment step in which the temperature is heated to a temperature that satisfies the above conditions, held for a certain period of time, and then the temperature is lowered from the holding 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 core loss of the MnZn ferrite. When the temperature exceeds (Tc+100)° C., the effect of reducing core loss reaches the upper limit. If the rate of temperature drop from the holding temperature exceeds 50° C./hour, the effect of reducing core loss will not be sufficiently exhibited. Note that the temperature drop rate is calculated in the temperature range from the holding temperature to 150°C.

The heat treatment may be performed in the 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 so as to prevent deterioration of the magnetic properties due to oxidation of the MnZn ferrite, and the cooling rate is about 5° C./hour. If it is slow, it is preferably less than 350°C. In the reducing atmosphere, the upper limit of the heat treatment temperature is not limited by oxidation, but considering that the effect of reducing the core loss reaches the upper limit, it is preferable to set the temperature to 400° C. or less as in the heat treatment in the 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 apparatus and strain due to thermal stress, typically 100° C. to 300° C./hour. .

The holding time in the heat treatment is not particularly limited, but the time necessary for the sample placed in the apparatus to reach a predetermined temperature may be set, typically about 1 hour. The heat treatment of the present invention can be performed using a heat treatment furnace (electric furnace, constant temperature bath, etc.).

As for the MnZn-based ferrite powder of the present embodiment, the smaller the particle diameter of 1 μm or less, the better. Excessive pulverization destroys the crystals forming the sintered body, and the properties tend to deteriorate as the average particle size of the pulverized powder becomes smaller. The MnZn-based ferrite powder preferably has a particle size distribution in which the integrated amount (%) of particles having a particle size of 1 μm is 15% or less. It is more preferably 10% or less, and may be 0% by classification. Since the average crystal grain size of this MnZn ferrite is about 2 to 5 μm, those of 1 μm or less have a low contribution to the properties, and the smaller the content, the better.
Moreover, it is conceivable that this MnZn-based ferrite powder is mixed with a resin or the like and molded into a magnetic core or the like for use. At this time, if the particle size is large, uniform kneading and packing density cannot be improved. Therefore, the average particle diameter D50 is preferably 100 μm or less. The MnZn-based ferrite powder of the present embodiment preferably has an average particle diameter D50 of 1 μm or more, more preferably 5 μm or more, and still 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.

Raw material powders of MnZn ferrite having the compositions shown in Table 1 were prepared. Fe ₂ O ₃ , Mn ₃ O ₄ (in terms of MnO), and ZnO were used as raw materials for the main components, which were wet-mixed, dried, and calcined 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 are added as shown in Table 1 to 100 parts by mass of the calcined powder in a ball mill, and the average The powder was pulverized and mixed until the pulverized particle size (by air permeation method) reached 0.8 to 1.0 μm. Polyvinyl alcohol was added as a binder to the obtained mixture, and the mixture was granulated with a spray dryer, and then pressure-molded at 196 MPa to obtain a flat molded body (100 mm×100 mm×3 mm). The resulting molded body was sintered in an electric sintering furnace capable of adjusting the atmosphere to obtain a flat sintered body. The average grain size was 3 μm.

Sintering is carried out in the atmosphere during the temperature rising process from room temperature to 750 ° C. At 750 ° C., replacement with N ₂ gas is started to gradually decrease the oxygen concentration, and at 900 ° C., the oxygen concentration is reduced to 0. 0.65% by volume, and the temperature was raised at a heating rate of 130°C/hour to the temperature of the high temperature holding step, which was set at 1115°C. In the high temperature holding step, the oxygen concentration was set to 0.65% by volume and held for 4 hours. In the temperature lowering step, the oxygen concentration is gradually lowered from 1000°C to 850°C, adjusted to 0.65% by volume at 1000°C, 0.05% by volume at 900°C, and 0.005% by volume at 850°C or lower. bottom. In the temperature lowering step, the temperature was lowered to 100° C. at a temperature lowering 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 oxygen analyzer, and the temperature was measured with a thermocouple provided in the sintering furnace.

(Curie temperature)
Formula described in Ferrite (Maruzen Co., Ltd., November 30, 1986, 6th edition, page 79):
Tc = 12.8 x [y - (2/3) x z] - 358 (°C), where y and z are mole percent of Fe ₂ O ₃ and ZnO, respectively. ]
calculated by The Curie temperature of the example was 270°C.

The plate-like sintered body was heat-treated as follows. FIG. 1 shows the temperature conditions of the heat treatment process of the example. The heat treatment is performed by raising the temperature from room temperature for 1.5 hours, maintaining the temperature for 1 hour after reaching 250 ° C., stabilizing the temperature in the furnace, and then dropping the temperature to 150 ° C. at a rate of 5 ° C./hour. After reaching a temperature of less than 150° C., outside air was introduced into the furnace to cool the sample. Heat treatment was performed in air.

The plate-shaped sintered body after the heat treatment step was coarsely pulverized using a fine jaw crusher (registered trademark) SC-1007 manufactured by Mayekawa Kogyo Co., Ltd., which is a continuous jaw crusher, at an outlet interval of 20 mm. The coarsely ground powder was then classified using a vibrating sieve. Here, using a sieve with an opening of 1.4 mm, the coarsely pulverized powder passed through the sieve was pulverized.

A vibrating mill was used for fine grinding. In this embodiment, a finely pulverized powder was obtained by using a batch-type vibration mill with iron spherical media and performing pulverization for 15 minutes.
The finely ground powder was then classified using a vibrating screener. Here, a sieve with an opening of 198 μm was used, and the finely pulverized powder passed through the sieve was used as MnZn ferrite powder. The same heat treatment as that applied to the flat plate-shaped sintered body can also be applied to this MnZn-based ferrite powder.

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

According to this example, an MnZn-based ferrite powder having an average particle size D50 of 100 μm or less was obtained. The present MnZn ferrite is a useful material in a high frequency range of 1 to 5 MHz, and is useful for reducing the loss of parts using the present MnZn ferrite powder.

The MnZn-based ferrite powder of the present invention can be a useful material as a magnetic material used in electronic parts and the like used in the high frequency range of 1 to 5 MHz. This MnZn-based ferrite powder can be mixed with resin or the like, molded into a required shape, and functioned as a magnetic core or the like. This MnZn-based ferrite exhibits excellent magnetic properties in a high frequency range of 1 to 5 MHz, and is expected to contribute to the reduction of losses in components using this MnZn-based ferrite powder.

Claims (6)

53 to 56 mol% of Fe in terms of Fe ₂ O ₃ , 3 to 9 mol% of Zn in terms of ZnO, and the balance Mn in terms of MnO as main components, and the total of the main components in terms of oxides is 100 parts by mass. In contrast, MnZn ferrite powder containing 0.05 to 0.4 parts by mass of Co as an auxiliary component in terms of Co ₃ O ₄ . 2. The MnZn-based ferrite powder according to claim 1, having a surface in which both intragranular and intergranular fracture modes are mixed. 2. The MnZn-based ferrite powder according to claim 1, wherein the heat-treated sintered body is coarsely pulverized, finely pulverized, and passed through a sieve with an opening of 190 μm. The MnZn-based ferrite powder has a volume-based particle size distribution obtained by a laser diffraction/scattering particle size distribution measurement method, and has a particle size distribution in which the cumulative passing amount (%) from the small particle size side with a particle size of 1 μm is 15% or less. The MnZn-based ferrite powder according to any one of claims 1 to 3. The MnZn-based ferrite powder according to any one of claims 1 to 3, wherein the MnZn-based ferrite powder has an average particle diameter D50 of 100 µm or less. The MnZn-based ferrite powder contains 0.003 to 0.015 parts by mass of Si in terms of _SiO2 and _CaCO3 in terms of 0.06 to 0.3 parts by weight of Ca, 0 to 0.1 parts by weight of V calculated as V ₂ O ₅ and a total of 0 to 0.3 parts by weight of Nb (as calculated as Nb ₂ O ₅ ) and/or The MnZn-based ferrite powder according to any one of claims 1 to 3, containing Ta (in terms of Ta ₂ O ₅ ).

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