MnZn ferrite powder

Abstract

This produces MnZn ferrite powder that is useful in the high frequency range. [Solution] The composition contains, 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, The MnZn-based ferrite powder is characterized in that it 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.

Description

[Technical Field]

[0001] The present invention relates to a method for producing MnZn ferrite powder used in electronic components such as transformers for switching power supplies and functional elements such as choke coils. [Background technology]

[0002] Switching power supplies are used in the power supply 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.

[0003] Recent electronic devices are being 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) such as DSPs (Digital Signal Processors) and MPUs (Micro-processing Units) and other functional elements used in electronic devices are also being required to be smaller, perform better, 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, resulting in lower operating voltages and higher currents.

[0004] 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, and fluctuations in the supply voltage (ripple) from the power supply circuit cause the LSI to exceed or fall below the power supply voltage range, leading 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.

[0005] Responding to higher frequencies and currents in power supply circuits also has the advantage of miniaturizing the magnetic cores that make up the electronic components, such as transformers and choke coils, used in the 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 windings Np of the primary coil and the cross-sectional area A (cm 2 ), frequency f (Hz) and excitation magnetic flux density Bm (mT) are used to obtain the formula: Ep=4.44×Np×A×f×Bm×10 -7 It is expressed as:

[0006] From this formula, we can see that for a given voltage Ep applied 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. Also, 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.

[0007] MnZn-based ferrite is the main magnetic material used 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 than magnetic cores that use metallic magnetic materials such as Fe-based and Co-based amorphous, pure iron, Fe-Si, Fe-Ni, Fe-Si-Cr, and Fe-Si-Al. Lower core loss is advantageous in terms of reducing power consumption in power supply circuits. Patent Document 1 describes this MnZn-based ferrite magnetic core for high-frequency ranges. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2017 / 164351 Summary of the Invention [Problem to be solved by the invention]

[0009] Patent Document 1 describes an MnZn ferrite core that provides excellent magnetic properties in the high frequency range of 1 to 5 MHz. 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 are certain limitations on the shapes that can be formed, and there have been challenges in obtaining a magnetic core of any shape.

[0010] Therefore, there is a demand for MnZn-based ferrite powder that can be used at high frequencies of 500 kHz or more, particularly in the high frequency range of 1 to 5 MHz, but a method for obtaining such a powder has not been clarified.Therefore, an object of the present invention is to provide a method for producing MnZn-based ferrite powder that can obtain MnZn-based ferrite powder that is useful in the high frequency range of 500 kHz or more, particularly in the high frequency range of 1 to 5 MHz. [Means for solving the problem]

[0011] 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 % of Fe calculated as Fe2O3, 3 to 9 mol % of Zn calculated as ZnO, and the remainder Mn calculated as MnO, and containing, as an accessory component, 0.05 to 0.4 parts by mass of 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 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 granular MnZn-based ferrite powder; a heat treatment step of heat treating the granular MnZn ferrite powder, The heat treatment step Condition 1: 200°C or higher, and Condition 2: A method for producing MnZn-based ferrite powder, characterized by a heat treatment step of heating to a temperature that satisfies (Tc-90)°C to (Tc+100)°C (wherein Tc is the Curie temperature (°C) calculated from the mole percentages of Fe2O3 and ZnO contained as main components of the MnZn-based ferrite), holding the temperature for a certain period of time, and then lowering the temperature from the temperature held for the certain period at a rate of 50°C / hour or less.

[0012] <2> The MnZn ferrite powder has an average particle size of 20 μm to 200 μm as measured by a dry sieving test method. <1> 1. A method for producing the MnZn ferrite powder according to claim 1. <3> 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 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, <1> or <2> 1. A method for producing the MnZn ferrite powder according to claim 1.

[0013] <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; During the temperature-lowering step, the oxygen concentration is set to a range of 0.001 to 0.2 volume % when the temperature is lowered from 900°C to 400°C, and the temperature-lowering rate is set to 50°C / hour or more when the temperature is lowered from (Tc+70)°C to 100°C. <1> ~ <3> 3. A method for producing the MnZn ferrite powder according to claim 1, wherein the MnZn ferrite powder is a powder containing ferrite particles.

[0014] <5> During the temperature-lowering step, the temperature-lowering rate from the holding temperature to 100°C is 50°C / hour or more. <4> 1. A method for producing the MnZn ferrite powder according to claim 1. [Effects of the Invention]

[0015] According to the present invention, MnZn ferrite powder that is useful in the high frequency range of 500 kHz or more can be obtained. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 2 is a diagram showing the temperature history in an electric furnace during the heat treatment process of the example. [Figure 2] 2 shows the particle size distribution of the MnZn ferrite powder of the example. [Figure 3] 1 is a microscope photograph of an MnZn-based ferrite powder according to an example. DETAILED DESCRIPTION OF THE INVENTION

[0017] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the upper and lower limits. In numerical ranges described in stages in this specification, the upper or lower limit described in one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in numerical ranges described in this specification, the upper or lower limit 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.

[0018] One embodiment of the present invention is a method for producing an MnZn-based ferrite powder containing, as main components, 53 to 56 mol % of Fe calculated as Fe2O3, 3 to 9 mol % of Zn calculated as ZnO, and the remainder Mn calculated as MnO, and containing, as an accessory component, 0.05 to 0.4 parts by mass of 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 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 granular MnZn-based ferrite powder; a heat treatment step of heat treating the granular MnZn ferrite powder, The heat treatment step Condition 1: 200°C or higher, and Condition 2: A method for producing MnZn-based ferrite powder, characterized by a heat treatment step of heating to a temperature that satisfies (Tc-90)°C to (Tc+100)°C (wherein Tc is the Curie temperature (°C) calculated from the molar percentages of Fe2O3 and ZnO contained as main components of the MnZn-based ferrite), holding the temperature for a certain period of time, and then lowering the temperature from the temperature held for the certain period at a rate of 50°C / hour or less.

[0019] [1] Composition The composition of the MnZn ferrite of this embodiment is described below. MnZn ferrites have the 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 secondary component to adjust the magnetocrystalline anisotropy constant, the temperature characteristics of core loss can be improved.

[0020] The MnZn-based ferrite of this embodiment contains Fe, Zn, and Mn as the main components and at least Co as the auxiliary component, the main components being 53 to 56 mol% Fe as calculated as Fe2O3, 3 to 9 mol% Zn as calculated as ZnO, and the remainder Mn as calculated as MnO, and the auxiliary component containing 0.05 to 0.4 parts by mass of Co as calculated as Co3O4 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 of Si as calculated as SiO2, 0.06 to 0.3 parts by mass of Ca as calculated as CaCO3, 0 to 0.1 parts by mass of V as calculated as V2O5, and a total of 0 to 0.3 parts by mass of Nb (as calculated as Nb2O5) and / or Ta (as calculated as Ta2O5) relative to a total of 100 parts by mass of the main components as calculated as oxides.

[0021] 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 becomes minimum becomes too high, while if the amount is too large, the temperature at which core loss becomes minimum becomes too low, making it difficult to keep the temperature at which core loss becomes minimum between 20 and 100°C, and core loss deteriorates at 0 to 120°C. 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 higher. The Fe content is more preferably between 54 and 55 mol% calculated as Fe2O3.

[0022] Zn has the effect of controlling the frequency characteristics of magnetic permeability, and in particular has an effect on controlling residual loss related to losses such as domain wall resonance in magnetic core loss. The smaller the amount, 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 high frequency range up to 3 MHz. The Zn content is more preferably 5 to 8 mol % calculated as ZnO. Mn makes up the remainder calculated as MnO.

[0023] The Curie temperature (Tc) calculated from the molar percentages of Fe2O3 and ZnO falls within the range of 250 to 330°C, which is practically acceptable, provided that the Fe content and Zn content are within the above ranges.

[0024] The MnZn-based ferrite of this embodiment contains at least Co as a secondary component. 2+ Fe 2+ As a metal ion with a positive magnetocrystalline anisotropy constant K1, it has the effect of adjusting the temperature at which the core loss is minimized. 2+ Since Co has a large magnetocrystalline anisotropy constant K1 compared to Fe, it is an effective element for improving the temperature dependency of magnetic core loss. If the amount is too small, the effect of improving temperature dependency is small, and if the amount is too large, the loss increases significantly in the low temperature range, which is not preferable in practical use. Furthermore, if the Co content is 0.05 to 0.4 parts by mass in terms of Co3O4 per 100 parts by mass of the total of the main components in terms of the oxides, Fe can be obtained by heat treatment. 2+ Co with ions 2+By controlling the induced magnetic anisotropy through ion rearrangement, it is possible to further reduce core loss within the practical temperature range and improve temperature dependency.The Co content is more preferably 0.1 to 0.3 parts by mass in terms of Co3O4.

[0025] It is preferable that the alloy further contains 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, the crystals become thicker, deteriorating 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.

[0026] 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 actually 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 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 higher. The Ca content is more preferably 0.06 to 0.2 parts by mass, calculated as CaCO3.

[0027] The alloy may further contain a Group 5a metal such as 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, along with Si and Ca, segregate mainly as oxides at grain boundaries, increasing the resistivity of the grain boundary phase, thereby further reducing core loss.

[0028] V has a lower melting point than Nb and Ta, and also functions to promote the growth of crystal grains. 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 sintered bodies and suppressing the occurrence of chipping. Too much V induces crystal thickening and deteriorates core loss. When 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, and low loss can be achieved 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.

[0029] 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, 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 (as calculated in Nb2O5) and Ta (as calculated in Ta2O5) is 0 to 0.3 parts by mass per 100 parts by mass of the total of the main components calculated in terms of their 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 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 realizing low loss over a wide temperature range in a high frequency range. The total amount of Nb (as calculated in Nb2O5) and Ta (as calculated in Ta2O5) is more preferably 0 to 0.2 parts by mass.

[0030] The Ta content is preferably 0 to 0.1 parts by mass, more preferably 0 to 0.05 parts by mass, calculated as Ta2O5. 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 Nb2O5.

[0031] [2] Manufacturing method (1)Mixing process As the raw material powder for MnZn-based ferrite, Fe2O3, Mn3O4, and ZnO powders are used as the main component raw materials, and Co3O4, SiO2, CaCO3, 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 several 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 rate, or in a nozzle-type device, the nozzle diameter and discharge rate, etc. Considering handling, a particle size of about 50 to 200 μm is preferable.

[0032] (2) Calcination process 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 furnace setting temperature, and the mixed powder is heated to substantially the same temperature. In the calcination process, 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 process 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 process. 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 properties, 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 an N2 atmosphere.

[0033] (3) Crushing process The calcined powder obtained is first pulverized, as some particles may stick together. This pulverization preferably involves coarse pulverization and then pulverization. A vibration mill can be used for the 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 the coarse pulverization, pulverization is performed. This pulverization can be performed wet using an attritor. 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 process without adding powders such as Co3O4, SiO2, or CaCO3 as the raw materials for the auxiliary components in the mixing process. 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 SS100 manufactured by Shimadzu Corporation.

[0034] (4) Granulation process 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 process 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 will deteriorate and it will no longer be able to maintain its shape.

[0035] (5) Sintering process The granulated powder is sintered to obtain a spherical MnZn-based ferrite powder. The sintering process 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. The oxygen concentration in the atmosphere is preferably 0.4 to 2% by volume. In the temperature-lowering step, the temperature is preferably lowered at a rate of 50°C / hour or more from at least (Tc+70)°C to 100°C, and the temperature is preferably lowered at a rate of 50°C / hour or more from the holding temperature to 100°C. The spherical shape refers to a shape whose outer surface is not flat but is mostly curved, giving it a spherical appearance. An example of this is shown in FIG. 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. It is more preferably 30 μm to 180 μm.

[0036] (a) Temperature rising process In the temperature-raising step, it is preferable that the oxygen concentration in the atmosphere is set to a range of 0.4 to 2% by volume at least at 900° C. or higher, at which point ferrite formation begins.

[0037] (b) High temperature holding process 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.

[0038] If the oxygen concentration in the high-temperature holding step is less than 0.4% by volume, the atmosphere becomes reducing, and the MnZn 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.

[0039] The oxygen concentration is preferably set 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, calcium can segregate at the grain boundaries, making the grain boundaries highly resistive.

[0040] Fe with a positive magnetocrystalline anisotropy constant as the oxygen concentration decreases 2+ The amount of Fe increases. 2+ It is known that an increase in the oxygen concentration tends to lower the temperature at which the core loss of MnZn ferrite is minimized. 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 higher and in a wide temperature range from 0°C to 120°C.

[0041] (c) Temperature cooling process In the temperature-reducing step following the high-temperature holding step, the oxygen concentration in the atmosphere of the high-temperature holding 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% by volume in the temperature range of 900°C to 400°C, Fe 2+ The amount of generated oxygen can be adjusted within a preferred range. Here, in the temperature-reducing step following the high-temperature holding step, the period from 900°C to 400°C until the atmosphere is adjusted to a predetermined oxygen concentration is referred to as the first temperature-reducing step.

[0042] By controlling the oxygen concentration in the temperature-lowering step following the high-temperature holding step and adjusting it to the above-mentioned range, Ca can be segregated at the grain boundaries of the MnZn-based ferrite, and the amount of Ca dissolved in the crystal grains can be appropriately controlled, thereby increasing the resistance within the crystal grains and at the grain boundaries.

[0043] The temperature drop rate in the first temperature drop step is not particularly limited as long as it allows adjustment of the temperature and oxygen concentration in the sintering furnace, but is preferably 50 to 300°C / hour. A temperature drop rate of less than 50°C / hour in the first temperature drop 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 drop rate of more than 300°C / hour may make it difficult to maintain uniformity in the temperature and oxygen concentration in the sintering furnace, depending on the capacity of the sintering furnace. This temperature drop rate is calculated from the time it takes to reach 400°C from 900°C and the temperature difference (500°C) (500°C / time from 900°C to 400°C). The temperature drop rate from the holding temperature to 900°C can also be a similar value.

[0044] The holding temperature and oxygen concentration in the high-temperature holding step are set within a predetermined range, and the oxygen concentration when the temperature is lowered from 900°C to 400°C in the first temperature lowering step is controlled within a specific range. 2+ ions and Fe 2+ The amount of ions can be controlled appropriately to reduce core loss.

[0045] In the temperature-reducing step, when Tc (°C) is the Curie temperature calculated from the molar percentages of iron oxide (Fe2O3) 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 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).

[0046] When the cooling rate in the second cooling step is less than 50°C / hour, Co 2+ and Fe 2+This is undesirable because it is susceptible to the influence of induced magnetic anisotropy due to the above, and the magnetic core loss on the high temperature side may increase. On the other hand, if the temperature drop rate exceeds 300°C / hour, it may be difficult to adjust the temperature and temperature drop rate inside the sintering furnace, although this depends on the capacity of the sintering furnace.

[0047] The atmosphere in the second temperature-reducing step may be an inert gas atmosphere or air atmosphere. The oxygen concentration-controlled atmosphere in the first temperature-reducing step may remain the same, or may be changed to air atmosphere or an inert gas atmosphere midway through the second temperature-reducing step.

[0048] (6) Heat treatment process In this embodiment, the spherical MnZn 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 molar percentages of Fe2O3 and ZnO contained as 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 temperature held for the certain period (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 ferrite. Furthermore, if it 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).

[0049] The heat treatment may be carried out in air or in a reducing atmosphere. In an oxidizing atmosphere such as air, the upper limit of the holding temperature for the heat treatment is preferably 400°C or less to prevent deterioration of the magnetic properties due to oxidation of the MnZn ferrite, and is preferably less than 350°C when the temperature drop rate is slow, such as about 5°C / hour. Furthermore, in a reducing atmosphere, the upper limit of the holding temperature for the heat treatment is not limited by oxidation, but considering that the effect of reducing magnetic core loss will reach an upper limit, it is preferably 400°C or less, as in the case of heat treatment in an oxidizing atmosphere.

[0050] The rate of temperature rise 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 or distortion due to thermal stress, and is typically set to 100° C. to 300° C. / hour.

[0051] 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, which is 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.).

[0052] (7) Classification process 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 carried out 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. The sieve used here preferably has an opening of 300 μm or less, more preferably 250 μm or less. The granulated powder may also be classified. 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.

[0053] 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 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 as 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. The weight frequency was calculated 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 where the total weight frequency obtained was 50% was taken as the average particle size of the powder. This MnZn-based ferrite powder is thought to be mixed with resin or the like and molded into 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 flowability, and is easy to handle when producing magnetic cores or the like. [Example]

[0054] A raw powder of MnZn-based ferrite was prepared to have the composition shown in Table 1. The main raw materials were Fe2O3, Mn3O4 (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 was 80 to 150 μm as determined by a dry sieving test. The granulated powder was compacted into plates several mm thick using a roller compactor and crushed into pellets less than 10 mm in size. The pelletized mixed powder was then calcined at 900°C for 1.5 hours. The calcination was performed in a rotary kiln. The calcined powder was then coarsely crushed using a vibration mill to an average particle size of 1.6 ± 0.25 μm (measured by air permeability measurement). Next, Co3O4, SiO2, CaCO3, V2O5, Ta2O5, and Nb2O5 were added to 100 parts by mass of the calcined powder in an attritor as shown in Table 1, and the mixture was pulverized and mixed until the average pulverized particle size (air permeation method) reached 0.8-1.0 μm. Polyvinyl alcohol was added to the resulting mixture as a binder, and spherical granulated powder was produced using a spray dryer (hot air 170-210°C, exhaust air 115-125°C). The granulated powder was placed in a zirconia-coated mullite case and sintered in an atmosphere-controllable electric furnace (sintering furnace), yielding spherical MnZn ferrite powder.

[0055] [Table 1]

[0056] Sintering was performed in air during the temperature-raising process from room temperature to 750°C. At 750°C, the oxygen concentration was gradually reduced by replacing the atmosphere with N2 gas. The oxygen concentration reached 0.65 vol% at 900°C. The temperature was then raised at a rate of 130°C / hour to 1115°C, a high-temperature holding step. The oxygen concentration was maintained at 0.65 vol% during the high-temperature holding step, and held for 4 hours. During the temperature-lowering process, the oxygen concentration was maintained at the same level as in the high-temperature holding step up to 1000°C. The oxygen concentration was gradually reduced from 1000°C to 850°C, reaching 0.65 vol% at 1000°C, 0.05 vol% at 900°C, and 0.005 vol% below 850°C. The oxygen concentration was then adjusted to 0.005 vol% from 850°C to 400°C. In the temperature-reducing process, the temperature was lowered 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.

[0057] (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 × [y - (2 / 3) × 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.

[0058] The sintered MnZn ferrite powder was heat-treated as follows. The temperature conditions for the heat treatment process were as follows: the temperature was raised from room temperature over 1.5 hours, and after reaching 250°C, it was held at 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 performed in air using an electric furnace, with the sintered MnZn ferrite powder placed in a zirconia-coated mullite case. The temperature inside the furnace was measured using a thermocouple, and the results are shown in Figure 1.

[0059] The heat-treated MnZn ferrite powder was classified using a sieve with a mesh size of 198 μm, and the powder that passed through the sieve was designated as MnZn ferrite powder. Classification was performed using a vibrating sieve.

[0060] The particle size distribution of the obtained spherical MnZn ferrite powder is shown in Figure 2. Figure 2 plots 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 calculated from the weight ratio. For example, the particle size distribution for each particle was calculated using a mean value of 38.5 μm ((32 + 45) / 2 = 38.5), which is the midpoint between sieves with 32 μm and 45 μm mesh, and the point where the total weight frequency was 50% was taken as the mean 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 minimum opening of 32 μm 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.

[0061] The MnZn-based ferrite powder of the present invention is granular, and the 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 MnZn-based ferrite powder.

Claims

1. Fe ２ O ３ The composition contains, as main components, 53 to 56 mol % Fe calculated as Fe, 3 to 9 mol % Zn calculated as ZnO, and the remainder Mn calculated as MnO, Co as an auxiliary component relative to a total of 100 parts by mass of the main components in terms of oxides ３ O ４ 0.05 to 0.4 parts by mass of Co, SiO ２ Converted to 0.003 to 0.015 parts by mass of Si, CaCO ３ 0.06 to 0.3 parts by mass of Ca and V in terms of ２ O ５ 0 to 0.1 parts by mass of V converted into Nb (Nb ２ O ５ Conversion) and / or Ta (Ta ２ O ５ It is a spherical MnZn-based ferrite powder containing 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. 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. 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.

5. 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.

6. 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.

7. 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.

Images