JP7759907B2 - MnZn ferrite powder - Google Patents

Description

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

Switching power supplies are used in the power supply circuits of a variety of 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.

Recently, electronic devices have been increasingly required to be smaller and lighter, as well as to consume less power from the perspective of energy efficiency. Consequently, LSIs (Large-Scale Integration) and functional elements such as DSPs (Digital Signal Processors) and MPUs (Micro-processing Units) used in electronic devices are also being required to be smaller, more powerful, and consume less power. Meanwhile, as LSIs have become more highly integrated with transistors due to finer wiring in recent years, the transistor's withstand voltage has decreased and its current consumption has increased, leading to lower operating voltages and higher currents.

Power supply circuits, such as DC-DC converters that supply power to LSIs, also need to adapt to the lower operating voltages and higher currents of LSIs. For example, as the operating voltage of LSIs decreases, the voltage range in which they can operate normally narrows. If fluctuations (ripples) in the supply voltage from the power supply circuit cause the LSI's power supply voltage to exceed or fall below the range, 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.

Responding to higher frequencies and currents in such power supply circuits also has the advantage of miniaturizing the magnetic cores that make up electronic components such as transformers, choke coils, etc. For example, when driving a transformer with a sine wave, the voltage Ep (V) applied to the primary coil is calculated using the number of windings Np of the primary coil, the cross-sectional area A (cm ² ), the frequency f (Hz), and the excitation magnetic flux density Bm (mT) as follows:
Ep=4.44×Np×A×f×Bm×10 ^-7
It is expressed as:

From this equation, we can see that for a given applied voltage Ep to the primary coil, increasing the frequency (switching frequency) f reduces the cross-sectional area A of the magnetic core, resulting in a smaller size. Furthermore, as the current increases, the maximum excitation magnetic flux density (hereinafter referred to as excitation magnetic flux density) Bm increases, further reducing the size of the magnetic core.

MnZn-based ferrite is primarily used as the magnetic material for magnetic cores that operate at high excitation magnetic flux densities in the high-frequency range and are suitable for miniaturization. MnZn-based ferrite has higher initial permeability and saturation magnetic flux density than Ni-based ferrite, and is characterized by lower core loss compared to magnetic cores that use metallic magnetic materials such as Fe-based and Co-based amorphous materials, pure iron, Fe-Si, Fe-Ni, Fe-Si-Cr, and Fe-Si-Al. Low core loss is advantageous in terms of reducing power consumption in power supply circuits. Patent Document 1 describes MnZn-based ferrite magnetic cores for use in the high-frequency range.

International Publication No. 2017/164351

Patent Document 1 describes an MnZn-based ferrite core that offers excellent magnetic properties in the high-frequency range of 1 to 5 MHz. However, Patent Document 1 only describes magnetic cores made from sintered bodies. With magnetic cores made from sintered bodies, there are certain limitations on the shapes that can be formed, making it difficult to obtain magnetic cores with freely defined shapes.

For this reason, there is a demand for MnZn-based ferrite powder that can be used at high frequencies of 500 kHz or higher, particularly in the high frequency range of 1 to 5 MHz, but a method for obtaining such powder has not been identified. Therefore, the object of the present invention is to provide a method for producing MnZn-based ferrite powder that can produce MnZn-based ferrite powder that is useful in the high frequency range of 500 kHz or higher, particularly in the high frequency range of 1 to 5 MHz.

Specific means for solving the above problems include the following aspects.
<1> 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 a subcomponent, 0.05 to 0.4 parts by mass Co calculated as _Co3O4 relative to a total of 100 parts by mass of the main components calculated as oxides,
a calcination step of mixing raw material powders of MnZn ferrite and then calcining the mixed powder at 800°C to 1000°C to obtain a calcined powder;
a pulverization step of pulverizing the calcined powder to obtain a pulverized powder;
A granulation step of granulating the pulverized powder to obtain granulated powder;
a sintering step of sintering the granulated powder at a temperature higher than 1050°C and lower than 1150°C, and then cooling it to a temperature lower than 150°C to obtain a granular MnZn-based ferrite powder;
a heat treatment step of heat treating the granular MnZn-based ferrite powder,
The heat treatment step
A method for producing MnZn-based ferrite powder, characterized by a heat treatment process in which a material is heated to a temperature that satisfies 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 as main components of the MnZn-based ferrite], held for a certain period of time, and then cooled from the temperature held for the certain period of time at a rate of 50°C/hour 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 of 20 μm to 200 μm as measured by a dry sieving test method.
<3> The method for producing MnZn-based ferrite powder according to <1> or <2>, 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 oxides.

<4> The sintering step includes a temperature increasing step, a high-temperature holding step, and a temperature decreasing step, and 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,
<1> to <3>, wherein, 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.

<5> The method for producing MnZn-based ferrite powder described in <4>, wherein during the temperature-lowering step, the temperature-lowering rate from the holding temperature to 100°C is 50°C/hour or more.

The present invention makes it possible to obtain MnZn-based ferrite powder that is useful in the high frequency range of 500 kHz or higher.

FIG. 2 is a diagram showing the temperature history in an electric furnace during the heat treatment process of the example. 1 shows the particle size distribution of the MnZn ferrite powder of the example. 1 is a microscope photograph of an MnZn-based ferrite powder according to an example.

In this specification, a numerical range expressed using "to" means a range that includes the numerical values written before and after "to" as the upper and lower limits. In numerical ranges described in stages in this specification, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in numerical ranges described in this specification, the upper or lower limit value of that numerical range may be replaced with a value shown in an example.
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, but 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 Fe ₂ O ₃ , 3 to 9 mol % Zn calculated as ZnO, and the remainder Mn calculated as MnO, and containing, as a subcomponent, 0.05 to 0.4 parts by mass Co calculated as Co ₃ O ₄ relative to a total of 100 parts by mass of the main components calculated as oxides,
a calcination step of mixing raw material powders of MnZn ferrite and then calcining the mixed powder at 800°C to 1000°C to obtain a calcined powder;
a pulverization step of pulverizing the calcined powder to obtain a pulverized powder;
A granulation step of granulating the pulverized powder to obtain granulated powder;
a sintering step of sintering the granulated powder at a temperature higher than 1050°C and lower than 1150°C, and then cooling it to a temperature lower than 150°C to obtain a granular MnZn-based ferrite powder;
a heat treatment step of heat treating the granular MnZn-based ferrite powder,
The heat treatment step
and Condition 2: (Tc-90)°C to (Tc+100)°C [where Tc is the Curie temperature (°C) calculated from the mole percentages of Fe ₂ O ₃ and ZnO contained in the main components of the MnZn-based ferrite], and then maintaining the temperature for a certain period of time, followed by lowering the temperature from the temperature maintained for the certain period at a rate of 50°C/hour or less.

[1] Composition The composition of the MnZn-based ferrite of this embodiment is described below.
MnZn-based ferrites have desired magnetic properties such as initial permeability and saturation magnetic flux density when the Fe, Zn, and Mn contents are within predetermined ranges. Furthermore, by adding Co as a minor component to adjust the magnetocrystalline anisotropy constant, the temperature characteristics of 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 accessory component, the main components being 53 to 56 mol % of Fe _calculated as _Fe2O3 , 3 to 9 mol % of Zn calculated as ZnO, and the remainder being Mn calculated as MnO, and the accessory component containing 0.05 to 0.4 parts by mass of _Co calculated as _Co3O4 per 100 parts by mass of the total of the main components calculated as oxides. The auxiliary components may further include, relative to a total of 100 parts by mass of the main components calculated as oxides, 0.003 to 0.015 parts by mass of Si calculated as _SiO2 , 0.06 to 0.3 parts by mass of Ca calculated as _CaCO3 , 0 _to 0.1 parts by mass of V calculated as _V2O5 , and a total of 0 to 0.3 parts by mass of Nb (calculated _{as Nb2O5} ) and/or Ta (calculated _{as Ta2O5 )} .

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

Zn has the effect of controlling the frequency characteristics of magnetic permeability, and in particular has an impact on controlling residual loss related to losses such as domain wall resonance in magnetic core loss. The lower the amount, the lower the magnetic core loss in the high frequency range. If the Zn content is 3 to 9 mol% converted to ZnO, low loss can be achieved in the high frequency range of 1 MHz or higher, particularly in the high frequency range up to 3 MHz. The Zn content is more preferably 5 to 8 mol% converted to ZnO. Mn makes up the remainder converted to 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 a secondary component. Co ²⁺ , together with Fe ²⁺ , is a metal ion with a positive magnetocrystalline anisotropy constant K1, and thus has the effect of adjusting the temperature at which core loss is minimized. Furthermore, because Co 2+ has a larger magnetocrystalline anisotropy constant K1 than Fe ²⁺ , it is an effective element for improving the temperature dependence of core loss. Too little Co 2+ reduces the effect of improving temperature dependence, while too much Co 2+ significantly increases loss at low temperatures, making it undesirable for practical use. Furthermore, if the Co content is 0.05 to 0.4 parts by mass, calculated as Co ₃ O ₄ , per 100 parts by mass of the total of the main components calculated as the oxides, heat treatment can rearrange the Co ²⁺ ions together with the Fe ²⁺ ions to control induced magnetic anisotropy, further reducing core loss within the practical temperature range and improving temperature dependence. The Co content is more preferably 0.1 to 0.3 parts by mass, calculated as Co ₃ O ₄ .

It is preferable to further include Ca and Si as minor components. Si segregates at grain boundaries, increasing grain boundary resistance and reducing eddy current loss, thereby reducing core loss in the high-frequency range. If the amount is too small, the effect of increasing grain boundary resistance is small, while 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 , calculated as SiO2, per 100 parts by mass of the total of the main components calculated as oxides, sufficient grain boundary resistance can be ensured to reduce eddy current loss, and low loss can be achieved in the high-frequency range of 1 MHz or higher. The Si content is more preferably 0.005 to 0.01 parts by mass, calculated as _SiO2 .

Like Si, Ca segregates at grain boundaries, increasing grain boundary resistance and reducing eddy current loss, thereby reducing core loss in the high-frequency range. If the amount is too small, the effect of increasing grain boundary resistance is small, while 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, calculated as _CaCO3, per 100 parts by mass of the total of the main components calculated as oxides, sufficient grain boundary resistance can be ensured to reduce eddy current loss, and low loss can be achieved in the high-frequency range of 1 MHz or higher. The Ca content is more preferably 0.06 to 0.2 parts by mass, calculated as _CaCO3 .

The alloy may further contain Group 5a metals V, Nb, or Ta as a secondary component (Group 5a metals are at least one selected from the group consisting of V, Nb, and Ta, hereinafter collectively referred to as Group 5a). Group 5a metals, along with Si and Ca, segregate at grain boundaries primarily as oxides, increasing the resistivity of the grain boundary phase and thereby further reducing core loss.

V has a lower melting point than Nb and Ta and also functions to promote crystal grain growth. Because V has a lower melting point than other 5a group elements, it is thought to have good wettability with grain boundaries, improving the workability of the sintered body and suppressing chipping. Too much V induces crystal thickening and deteriorates core loss. If the V content is 0 to 0.1 parts by mass, calculated as _V2O5 , per 100 parts by mass of the total of the main components calculated as the _oxides , sufficient grain boundary resistance can be secured to reduce eddy current loss, resulting in low loss in the high-frequency range of 1 MHz or higher. The V content is more preferably 0 to 0.05 parts by _mass , calculated as _V2O5 .

Nb and/or Ta also have the effect of suppressing crystal grain growth, creating a uniform crystal structure, and reducing core loss. Nb and Ta have a higher melting point than V, and together with Ca and Si, they also have the effect of preventing a lower melting point due to the oxides with Fe. If the amount of Nb and Ta is too large, they segregate within the grains and deteriorate core 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 per 100 parts by mass of the total of the main components in terms of their oxides, sufficient grain boundary resistance can be ensured to reduce eddy current loss, resulting in low loss in the high-frequency range of 1 MHz or higher. Furthermore, Nb and Ta have the effect of reducing hysteresis loss and residual loss, particularly at high temperatures (100°C), among core losses after heat treatment, and are effective in achieving 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, 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), more preferably 0.01 to 0.04 parts by mass, calculated as Nb ₂ O ₅ .

[2] Manufacturing Method (1) Mixing Step As the raw material powder for MnZn-based ferrite, Fe ₂ O ₃ , Mn ₃ O _{4 ,} and ZnO powders are used as the main component raw materials, and Co ₃ O ₄ , SiO ₂ , CaCO ₃ , etc. are used as the subcomponent raw materials. These powders are mixed to obtain a mixed powder with a predetermined composition. Mixing can be performed wet using a ball mill. After wet mixing, the mixture is in a slurry state, so it is dehydrated and dried. After dehydration using a centrifuge or filter press, the mixture may be dried using a tray dryer, vibration dryer, belt dryer, etc., or it may be dehydrated, dried, and granulated into spherical particles using a spray dryer. Granulation can also be performed using a roller compactor. The hot air temperature of the spray dryer used here is preferably 200°C to 300°C. A spray dryer produces roughly spherical granulated powder. There are many ways to adjust the particle size of this spherical granulated powder depending on the type, but for example, in a disk-type device, it can be adjusted to any particle size within the range that can be adjusted for each device by the atomizer rotation speed and discharge amount, or in a nozzle-type device, the nozzle diameter and discharge amount, etc. Considering handling, etc., a particle size of about 50 to 200 μm is preferable.

(2) Calcination Step: The mixed powder is calcined at 800°C to 1000°C to obtain calcined powder. For ease of handling, the calcined mixed powder is preferably dehydrated and granulated using a spray dryer, then pelletized using a roller compactor. Calcination can be performed using an electric furnace, preferably a continuous rotary kiln or pusher furnace. The calcination temperature is the set temperature of the furnace, and the mixed powder is heated to a substantially equivalent temperature. In the calcination step, impurities are removed from the mixed powder, and at least 20% to 30% of the mixed powder is ferritized (converted into spinel). If the calcination temperature is below 800°C, ferritization is insufficient, resulting in significant shrinkage of the granulated powder in the sintering step described below, making it difficult to obtain powder with a uniform shape. If the calcination temperature is above 1000°C, ferritization progresses, increasing necking between particles, which may affect the ease of pulverization of the calcined powder in the subsequent pulverization step. Furthermore, in an atmosphere containing oxygen, the ferritized particles are oxidized during the cooling process, and components other than ferrite are generated, which tends to cause deterioration of characteristics, so the calcination temperature is preferably 800° C. to 1000° C., and more preferably 850° C. to 950° C. Although the calcination can be performed in air, it is preferable to perform the calcination in an inert atmosphere such as a _N2 atmosphere.

(3) Pulverization Step The obtained calcined powder is first pulverized, as some particles may stick together. This pulverization preferably involves coarse pulverization and pulverization. A vibration mill can be used for coarse pulverization. While the particle size range of this coarse pulverization is not particularly limited, it is performed to shorten the pulverization time in the subsequent attritor process, adequately separate the powder that has aggregated during calcination, and homogenize the resulting particle size. It is preferable to pulverize the coarsely pulverized powder so that the average particle size of the coarsely pulverized powder measured by the air permeability method is approximately 1.3 to 2.2 μm. After coarse pulverization, pulverization is performed. Pulverization can be performed wet using an attritor. At this time, the composition can be adjusted by adding trace amounts of the main component raw materials. It is preferable to add the auxiliary components after calcination. It is also possible to mix the auxiliary components in the pulverization step without adding powders such as Co ₃ O ₄ , SiO ₂ , and CaCO ₃ as raw materials for the auxiliary components in the mixing step. In other words, mixing and pulverization can be performed simultaneously using an attritor. It is preferable that the pulverization be carried out so that the average particle size of the pulverized powder measured by the air permeability method is about 0.8 to 1.2 μm. The evaluation by the air permeability method was carried out using an SS100 manufactured by Shimadzu Corporation.

(4) Granulation Step: This pulverized powder (mixed pulverized powder) is used to obtain granulated powder. To obtain this granulated powder, a spray dryer can be used to granulate the powder. A binder or the like is added to the obtained pulverized powder (mixed pulverized powder), and the mixture is dried using a spray dryer to obtain spherical granulated powder. The spray dryer used here preferably has a hot air temperature of 150°C to 250°C. The reason for setting the upper limit of the hot air temperature of the spray dryer in the granulation step to 250°C is to suppress thermal decomposition of the binder added during spray drying. The decomposition temperature of the binder depends on the type of binder, so the upper limit temperature can be adjusted according to the binder. For example, if the hot air temperature is too high and the binder begins to decompose, the strength of the formed granulated powder deteriorates and it becomes unable to maintain its shape.

(5) Sintering Step: The granulated powder is sintered to obtain spherical MnZn-based ferrite powder. 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 greater than 1050°C and less than 1150°C. 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. Note that the term "spherical" refers to a powder whose outer surface is not flat but is mostly curved, giving it a spherical appearance. An example of such a powder is shown in Figure 3, which will be described later. The MnZn-based ferrite powder preferably has an average particle size of 20 μm to 200 μm, as measured by a dry sieving test method. It is more preferably 30 μm to 180 μm.

(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. or higher, at which point ferrite formation begins.

(b) High-Temperature Holding Step If the holding temperature in the high-temperature holding step is 1050°C or lower, sufficient ferritization may not be achieved. If the holding temperature is 1150°C or higher, sintering is promoted, but the resulting MnZn-based ferrite powder tends to be in a state where the granulated powder particles stick together. Therefore, the holding temperature in the high-temperature holding step is set to be higher than 1050°C and lower than 1150°C. It is preferably 1060 to 1140°C, and more preferably 1070 to 1130°C.

If the oxygen concentration during the high-temperature holding process is less than 0.4% by volume, the atmosphere becomes reducing, and the MnZn-based ferrite obtained by sintering is likely to have low resistance. On the other hand, if the oxygen concentration exceeds 2% by volume, the atmosphere becomes too oxidizing, and low-resistivity hematite is likely to be produced.

The oxygen concentration is preferably set according to the holding temperature, with the higher the holding temperature, the higher the oxygen concentration should be set. Setting the oxygen concentration according to the holding temperature allows Ca to segregate at the grain boundaries, making the grain boundaries more resistive.

The lower the oxygen concentration, the greater the amount of Fe2 ⁺ , which has a positive magnetocrystalline anisotropy constant. It is known that the temperature at which the magnetic core loss of MnZn ferrite is minimized tends to decrease as the amount of Fe2 ⁺ increases. Therefore, it is preferable to set the oxygen concentration within the above range so as to achieve low loss in the high frequency region of 500 kHz or more and in a wide temperature range from 0°C to 120°C.

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

Following the high-temperature holding process, by controlling the oxygen concentration in the cooling process to keep it within the above range, Ca is segregated at the grain boundaries of the MnZn ferrite, and the amount of Ca dissolved within the crystal grains is appropriately controlled, thereby increasing the resistance within the crystal grains and at the grain boundaries.

The temperature reduction rate in the first temperature reduction step is not particularly limited as long as it allows for adjustment of the temperature and oxygen concentration inside the sintering furnace, but is preferably 50 to 300°C/hour. A temperature reduction rate of less than 50°C/hour in the first temperature reduction step is undesirable because it requires a long time for the sintering process, increases the residence time in the sintering furnace, reduces productivity, and increases costs. On the other hand, a temperature reduction rate of more than 300°C/hour may make it difficult to maintain uniformity in the temperature and oxygen concentration inside the sintering furnace, depending on the capacity of the sintering furnace. This temperature reduction rate is calculated from the time it takes to heat from 900°C to 400°C and the temperature difference (500°C) (500°C/time from 900°C to 400°C). The temperature reduction rate from the holding temperature to 900°C can also be a similar value.

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 temperature-lowering step, it is possible to control the amounts of Co2 ⁺ ions and Fe2 ⁺ ions to appropriate levels and reduce core loss.

In the temperature-reducing step, when Tc (°C) is the Curie temperature calculated from the mole percentages of iron oxide (Fe ₂ O ₃ ) and zinc oxide (ZnO) that constitute the main components of the MnZn-based ferrite, the temperature-reducing rate from (Tc + 70)°C to 100°C is preferably 50°C/hour to 300°C/hour. Typically, the temperature-reducing rate from 400°C to 100°C is desirably 50°C/hour to 300°C/hour. Here, the period in the temperature-reducing step in which the temperature is reduced at a predetermined temperature-reducing rate through the temperature range from (Tc + 70)°C to 100°C, including Tc, is referred to as the second temperature-reducing step. Here, the temperature-reducing rate is calculated from the time from the start temperature to the end temperature of that section and the temperature difference (temperature difference/time from start temperature to end temperature).

If the temperature-reducing rate in the second temperature-reducing step is less than 50°C/hour, the sintered body is susceptible to the influence of induced magnetic anisotropy due to Co2 ⁺ and Fe2 ⁺ , and the magnetic core loss on the high-temperature side may deteriorate, which is undesirable. On the other hand, if the temperature-reducing rate exceeds 300°C/hour, it may be difficult to adjust the temperature and temperature-reducing rate in the sintering furnace, although this depends on the capacity of the sintering furnace.

The atmosphere in the second temperature-reducing step may be an inert gas atmosphere or air. The oxygen concentration-controlled atmosphere of the first temperature-reducing step may remain, or the atmosphere may be changed to air or an inert gas atmosphere midway through the second temperature-reducing step.

(6) Heat Treatment Step In this embodiment, the spherical MnZn-based ferrite powder obtained in the sintering step is heat treated.
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).
and then lowering the temperature from the temperature (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 core loss of MnZn-based ferrite. Furthermore, if the temperature exceeds (Tc + 100)°C, the effect of reducing core loss reaches an upper limit. If the cooling rate from the holding temperature exceeds 50°C/hour, the effect of reducing core loss is not fully exhibited. This cooling rate is calculated from the temperature difference between the holding temperature and 150°C and the time (temperature difference/time from holding temperature to 150°C).

The heat treatment may be carried out in air or in a reducing atmosphere. When an oxidizing atmosphere such as air is used, the upper limit of the heat treatment holding temperature is preferably set to 400°C or less to prevent deterioration of the magnetic properties due to oxidation of the MnZn ferrite. If the temperature drop rate is slow, such as about 5°C/hour, it is preferably set to less than 350°C. Furthermore, in a reducing atmosphere, the upper limit of the heat treatment holding 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 with heat treatment in an oxidizing atmosphere.

There are no particular restrictions on the heating rate during heat treatment, but it should be selected appropriately to avoid being affected by the performance of the equipment or distortion due to thermal stress, and is typically set to 100°C to 300°C per hour.

The holding time in the heat treatment (the time for which the sample is held at the holding temperature) is not particularly limited, but it is sufficient to set the time required for the sample placed in the apparatus to reach the specified temperature, and typically it is 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.).

(7) Classification Step The MnZn-based ferrite powder after the heat treatment step may be subjected to a classification step, if necessary. By classification, MnZn-based ferrite powder of a desired particle size can be obtained. The classification step can be performed using a sieve. For example, a sieve with an opening of 198 μm (80 mesh) can be used and a vibrating sieve machine can be used. Note that the sieve used here preferably has an opening of 300 μm or less, more preferably 250 μm or less. In addition, classification can also be performed on the granulated powder.
This classification allows the particle size to be adjusted by removing powders that are too large or too small. In order to remove powders that are too fine, classification may be performed to determine the lower limit of particle size. In this case, the sieve preferably has an opening of 20 μm or more, more preferably 30 μm or more.

The MnZn-based ferrite powder of this embodiment preferably has an average particle size of 20 μm to 200 μm as measured by the dry sieving test method (JIS 2510). It is more preferably 30 μm to 180 μm. This dry sieving is performed, for example, using sieves with a mesh size classification specified by the JIS test sieves, ranging from a lower limit of 32 μm to 45 μm, 53 μm, 63 μm, 75 μm, 90 μm, 106 μm, 125 μm, 150 μm, 180 μm, 250 μm, and 355 μm, and an upper limit of approximately 420 μm, in order from largest to smallest. The particle size was determined by taking the median value between the mesh size of the sieve through which the powder passed and the mesh size of the sieve through which the powder did not pass as the particle size, and by using the weight frequency as the ratio of the weight of the powder that did not pass through the sieve to the total weight of the powder obtained by sieving. The point at which the total weight frequency reached 50% was taken as the average particle size of the powder.
It is conceivable that this MnZn-based ferrite powder is mixed with resin or the like and molded into the form of a magnetic core or the like for use. In this case, the appropriate particle size varies depending on the application, but by performing classification, it is possible to obtain MnZn-based ferrite powder with a particle size distribution that matches the application. Furthermore, in order to obtain good magnetic properties and to increase uniform mixing and packing density, the average particle size is preferably 20 μm to 200 μm. Furthermore, the granular MnZn-based ferrite powder of this embodiment is spherical, has excellent fluidity, and is easy to handle when producing magnetic cores or the like.

A raw powder of MnZn-based ferrite was prepared to have the composition shown in Table 1. The main raw materials were Fe ₂ O ₃ , Mn ₃ O ₄ (MnO equivalent), and ZnO. These were wet-mixed in a ball mill for 4 hours. The mixture was then dehydrated and dried in a spray dryer (hot air 270°C, exhaust air 120°C) and granulated. The resulting granulated powder was spherical, and the spray dryer conditions were adjusted so that the average particle size of the granulated powder measured by a dry sieving test was 80 to 150 μm. The granulated powder was compacted into plates several mm thick using a roller compactor and crushed into pellets less than 10 mm in diameter. The pelletized mixed powder was then calcined at 900°C for 1.5 hours. A rotary kiln was used for calcination. The calcined powder was then coarsely crushed using a vibration mill. The material was coarsely pulverized to an average particle size (air permeation method) of 1.6±0.25 μm after coarse pulverization. Then, Co ₃ O ₄ , SiO ₂ , CaCO ₃ , V ₂ O ₅ , Ta ₂ O _{5 ,} and Nb ₂ O ₅ were added to an attritor per 100 parts by mass of the calcined powder as shown in Table 1, and the mixture was pulverized and mixed until the average pulverized particle size (air permeation method) was 0.8 to 1.0 μm. Polyvinyl alcohol was added as a binder to the resulting mixture, and spherical granulated powder was produced using a spray dryer (hot air 170°C to 210°C, exhaust air 115°C to 125°C). The granulated powder was placed in a zirconia-coated mullite case and sintered in an atmosphere-controllable electric furnace (sintering furnace) to obtain spherical MnZn-based ferrite powder.

Sintering was performed in air during the temperature increase process from room temperature to 750 ° C. At 750 ° C, _N2 gas replacement was started to gradually decrease the oxygen concentration, and the oxygen concentration was 0.65 vol% at 900 ° C. The temperature was then increased at a rate of 130 ° C/hour to the high-temperature holding step temperature set at 1115 ° C. In the high-temperature holding step, the oxygen concentration was set to 0.65 vol% and held for 4 hours. In the temperature decrease step, the oxygen concentration was the same as in the high-temperature holding step up to 1000 ° C., and the oxygen concentration was gradually decreased from 1000 ° C to 850 ° C., adjusted to 0.65 vol% at 1000 ° C, 0.05 vol% at 900 ° C, and 0.005 vol% below 850 ° C. Then, it was adjusted to 0.005 vol% from 850 ° C to 400 ° C. In the temperature-reducing step, the temperature was reduced from the holding temperature to 100°C at a rate of 150°C/hour, and then the MnZn ferrite powder was removed from the electric furnace. The oxygen concentration was measured using a zirconia-type oxygen analyzer, and the temperature was measured using a thermocouple installed in the electric furnace.

(Curie temperature)
The Curie temperature is calculated by the formula given in Ferrite (Maruzen Co., Ltd., published 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 Fe ₂ O ₃ and ZnO, respectively.
The Curie temperature in this example was 270°C.

The sintered MnZn ferrite powder was heat-treated as follows. The temperature conditions for the heat treatment process were to raise the temperature from room temperature over 1.5 hours, and after reaching 250°C, hold that temperature for 1 hour. After stabilizing the temperature inside the furnace, the temperature was lowered to 150°C at a rate of 10°C/hour. Once the temperature dropped below 150°C, outside air was introduced into the furnace to cool the sample. The heat treatment was carried out in the atmosphere using an electric furnace, with the sintered MnZn ferrite powder placed in a mullite case with a zirconia coating on the surface. The temperature inside the furnace was measured using a thermocouple installed in the electric furnace, and the results are shown in Figure 1.

After heat treatment, the MnZn ferrite powder was classified using a sieve with 198 μm openings, and the material that passed through the sieve was designated as MnZn ferrite powder. Classification was performed using a vibrating sieve.

The particle size distribution of the obtained spherical MnZn-based ferrite powder is shown in Figure 2. Figure 2 shows the particle size distribution, with particle size (μm) on the horizontal axis and weight frequency (%) on the vertical axis. This particle size distribution was measured using a dry sieving test method in which multiple sieves specified by JIS mesh size were combined, at least 10 g of powder was passed through the coarse mesh, the weight of the powder remaining on the sieve was measured, and the mass frequency distribution was determined from the weight ratio. Note that, for example, for sieves with 32 μm and 45 μm mesh sizes, the distribution was determined using the mean value of 38.5 μm ((32 + 45)/2 = 38.5), which is the intermediate value between the two sieves, and the point where the total weight frequency was 50% was taken as the average particle size.
The MnZn ferrite powder had an average particle size of 100 μm as measured by a dry sieving test. The weight frequency (%) of powder (particle size 16 μm) that passed through a sieve with a lower limit of 32 μm opening was approximately 2%.
A photograph of this MnZn ferrite powder taken at 50x magnification using a microscope (Keyence Digital Microscope VHX) is shown in Figure 3. The MnZn ferrite powder of the example was spherical.

The MnZn-based ferrite powder of the present invention is granular, and its particle size can be easily adjusted by classification, and it can be mixed with resins and the like with good dispersibility, and it can be easily molded into the required shape. This MnZn-based ferrite exhibits excellent magnetic properties in the high frequency range of 500 kHz or more, typically 1 to 5 MHz, and is expected to contribute to reducing loss in parts and the like that use this MZn-based ferrite powder.

Claims (7)

The main components are 53 to 56 mol % Fe calculated as Fe ₂ O ₃ , 3 to 9 mol % Zn calculated as ZnO, and the remainder Mn calculated as MnO,
The spherical MnZn-based ferrite powder contains, as auxiliary components, 0.05 to 0.4 parts by mass of Co in terms _{of Co3O4} , 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 0 to 0.3 parts by mass in total 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 _oxides ,
A MnZn-based ferrite powder characterized in that a mixed powder obtained by mixing raw material powders is calcined, then pulverized to form a pulverized powder, and the spherical granulated powder composed of the pulverized powder is sintered at a temperature higher than 1050°C and lower than 1150°C, and no pulverization is performed after sintering . 2. The MnZn-based ferrite powder according to claim 1, wherein the average particle size of the pulverized powder is 0.8 to 1.2 μm. 3. The MnZn ferrite powder according to claim 1 , wherein the MnZn ferrite powder has an average particle size of 20 μm to 200 μm as measured by a dry sieving test method. 4. The MnZn-based ferrite powder according to claim 3 , wherein the MnZn-based ferrite powder does not contain powder that passes through a sieve with 32 μm openings in a dry sieving test. 4. The MnZn-based ferrite powder according to claim 3 , wherein the MnZn-based ferrite powder does not contain powder that does not pass through a sieve with 250 μm openings in a dry sieving test. 4. The MnZn-based ferrite powder according to claim 3 , wherein the MnZn-based ferrite powder does not contain powder that does not pass through a sieve with 198 μm openings in a dry sieving test. The MnZn-based ferrite powder according to claim 3 , wherein the MnZn-based ferrite powder does not include powder that passes through a sieve with a mesh size of 32 μm in a dry sieving test method, and does not include powder that does not pass through a sieve with a mesh size of 198 μm in a dry sieving test method.