# Patent

## Title
Ferrite sintered body and manufacturing method therefor

## Abstract

The ferrite sintered body of the present invention contains main components consisting of 52 to 54 mol % Fe2O3, 35 to 42 mol % MnO and 6 to 11 mol % ZnO as oxide equivalents and additives including Co, Ti, Si and Ca in specified amounts, and has a temperature at which the power loss is a minimal value (bottom temperature) of higher than 120° C. in a magnetic field with an excitation magnetic flux density of 200 mT and a frequency of 100 kHz, and a power loss of 350 kW/m3 or less at the bottom temperature.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ferrite sintered body containing main components including Fe, Mn and Zn together with additives including Co, Ti, Si and Ca, and to a manufacturing method therefor.
2. Related Background Art
Ferrite sintered bodies are used as magnetic core materials in power transformers and the like. The ferrite sintered body forming a magnetic core is called a ferrite core, and Mn-Zn ferrite containing Mn and Zn is widely used. In order to reduce the amount of heat generated when the device is used, the power loss value of the ferrite core should be small across a broad temperature range (see Japanese Patent Application Laid-open No. 2005-119892). The power loss of the core is called the “core loss”.
Conventional power transformers and other devices are designed to perform at operating temperatures around 50 to 70° C., and the temperature at which power loss is minimized (hereunder called the “bottom temperature”) is 80 to 100° C. for the materials of commonly used ferrite cores. If the operating temperature of the device is lower than the bottom temperature, thermal runaway is prevented from occurring because the amount of heat generated declines gradually even if the temperature of the ferrite core rises gradually during use.
However, electronic devices and power sources have gotten smaller in recent years, and there has been strong demand for thinner and smaller transformer cores because these occupy a large amount of parts volume. Higher parts densities have also been achieved in the field of electronics. Under these circumstances, the temperature tends to rise more due to heat generation, and the temperature of the ferrite core also tends to be higher. To answer demands for higher operating temperature of electronic devices, various investigations for ferrite materials were performed (see for example Japanese Patent Application Laid-open No. 2004-35372, Japanese Patent Application Laid-open No. 2006-213532, Japanese Patent Application Laid-open No. 2006-44971).
SUMMARY OF THE INVENTION
However, conventional ferrite sintered bodies are not always adaptable to higher operating temperatures in transformer power sources and the like, and there is room for improvement from the standpoint of reliably preventing thermal runaway of the device.
Under these circumstances, it is an object of the present invention to provide a ferrite sintered body suited to use under high-temperature conditions whereby thermal runaway can be adequately prevented, as well as a manufacturing method therefor.
The ferrite sintered body of the present invention contains main components consisting of 52 to 54 mol % Fe2O3, 35 to 42 mol % MnO and 6 to 11 mol % ZnO as oxide equivalents, and additives including Co, Ti, Si and Ca in the amounts shown in (1) to (4) below per each 1 part by mass of the total mass of the aforementioned oxides of the main components, and has a bottom temperature greater than 120° C. and a power loss of 350 kW/m3 or less at the bottom temperature in a magnetic field with an excitation magnetic flux density of 200 mT and a frequency of 100 kHz.
(1) Co in an amount corresponding to 1000×10−6 to 3500×10−6 parts by mass as CoO equivalent,
(2) Ti in an amount corresponding to 2000×10−6 to 5000×10−6 parts by mass as TiO2 equivalent,
(3) Si in an amount corresponding to 50×10−6 to 150×10−6 parts by mass as SiO2 equivalent,
(4) Ca in an amount corresponding to 300×10−6 to 1500×10−6 parts by mass as CaCO3 equivalent.
The temperature (bottom temperature) at which the power loss of this ferrite sintered body is minimized is higher than 120° C., and the power loss at the bottom temperature is 350 kW/m3 or less. Thus, with a magnetic core consisting of this ferrite sintered body the amount of heat generated can be adequately reduced even under high-temperature conditions of about 100° C. or even higher, and thermal runaway can be adequately prevented.
In the ferrite sintered body of the present invention, the additives preferably further include Nb and/or Ta in the amounts shown in (5) and (6) below per each 1 part by mass of the total mass of the aforementioned oxides of the main components.
(5) Nb in an amount corresponding to 50×10−6 to 600×10−6 parts by mass as Nb2O5 equivalent,
(6) Ta in an amount corresponding to 80×10−6 to 1000×10−6 parts by mass as Ta2O5 equivalent.
When the ferrite sintered body of the present invention is one that includes Nb and/or Ta in the amounts given in (5) and (6) above as additives, the uniformity of the crystal structure of the ferrite sintered body is improved, and power loss is faker reduced.
In the ferrite sintered body of the present invention, the additives preferably further include Zr and/or Hf in the amounts shown in (7) and (8) below per each 1 part by mass of the total mass of the aforementioned oxides of the main components.
(7) Zr in an amount corresponding to 200×10−6 parts by mass or less as ZrO2 equivalent,
(8) Hf in an amount corresponding to 400×10−6 parts by mass or less as HfO2 equivalent.
The Zr and Hf in the ferrite sintered body are components that help to increase the resistance of the grain boundaries. Thus, when the ferrite sintered body of the present invention includes Zr and/or Hf in the amounts given in (7) and (8) above as additives, power loss under high temperature conditions is further reduced.
The method for manufacturing the ferrite sintered body of the present invention has a step of mixing main components consisting of 52 to 54 mol % Fe2O3, 35 to 42 mol % MnO and 6 to 11 mol % ZnO as oxide equivalents with additives including Co, Ti, Si and Ca in the amounts shown in (1) to (4) above per 1 part by mass of the total mass of the oxides of the main components, and a main firing step wherein a ferrite sintered body is obtained by firing a powder containing the aforementioned main components and aforementioned additives in a furnace, with this main firing step having a temperature-holding step in which the firing temperature is maintained at 1250 to 1345° C. and an oxygen concentration adjustment step in which the oxygen concentration inside the furnace is lowered either continuously or in stages as the temperature drops from the holding temperature, so that by means of the oxygen concentration adjustment step, the oxygen concentration at 1250° C. is made to be 0.24 to 2.0 vol % while the oxygen concentration at 1100° C. is made to be 0.020 to 0.20 vol %.
With this ferrite sintered body manufacturing method, it is possible to effectively manufacture a ferrite sintered body with a bottom temperature higher than 120° C. and a power loss of 350 kW/m3 or less at the bottom temperature. Moreover, with a magnetic core made of a ferrite sintered body obtained by this manufacturing method it is possible to satisfactorily reduce the amount of heat generated even at about 100° C. or under even higher temperature conditions, and to satisfactorily prevent thermal runaway.
In the present invention, the “temperature at which power loss is mnimized (bottom temperature)” is the temperature at which the power loss reaches a minimal value within a temperature range of 25 to 150° C. in a magnetic field with an excitation magnetic flux density of 200 mT and a frequency of 100 kHz, and the power loss at this temperature range is the value measured with a SY-8232 B-H analyzer (brand name, Iwatsu Electric Co., Ltd.).
The present invention provides a ferrite sintered body whereby the occurrence of thermal runaway can be prevented and which is suited to use under high-temperature conditions, along with a manufacturing method therefor.



BRIEF EXPLANATION OF THE DRAWINGS
FIG. 1 is a perspective view of a ferrite core consisting of the ferrite sintered body of the present invention;
FIG. 2 is a graph showing the relationship between power loss and temperature;
FIG. 3 is a graph showing one example of temperature settings in the main firing step; and
FIG. 4 is a graph showing one example of oxygen concentration settings during cool-down in the main firing step.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are explained in detail below.
FIG. 1 is a perspective view of a ferrite core (magnetic core) consisting of the ferrite sintered body of the present invention. As shown in FIG. 1, E-shaped ferrite core 10 is called the E-shaped core or the like, and is used for transformers and the like. A transformer using an E-shaped core such as ferrite core 10 might be a transformer containing two internal E-shaped cores arranged facing one another.
Ferrite Sintered Body
Ferrite core 10 is composed of a ferrite sintered body, and this ferrite sintered body contains main components including Fe, Mn and Zn, along with additives including Co, Ti, Si and Ca. The main components of the ferrite sintered body consist of 52 to 54 mol % Fe2O3, 35 to 42 mol % MnO and 6 to 11 mol % ZnO, as oxide equivalents. The additives of the ferrite sintered body include Co, Ti, Si and Ca in the amounts shown in (1) to (4) below per each 1 part by mass of the total mass of the aforementioned oxides of the main components.

    (1) Co in an amount corresponding to 1000×10−6 to 3500×10−6 parts by mass as CoO equivalent,
    (2) Ti in an amount corresponding to 2000×10−6 to 5000×10−6 parts by mass as TiO2 equivalent,
    (3) Si in an amount corresponding to 50×10−6 to 150×10−6 parts by mass as SiO2 equivalent,
    (4) Ca in an amount corresponding to 300×10−6 to 1500×10−6 parts by mass as CaCO3 equivalent.


The ferrite sintered body making up ferrite core 10 also has a bottom temperature higher than 120° C. and a power loss of 350 kW/m3 or less at the bottom temperature in a magnetic field with an excitation magnetic flux density of 200 mT and a frequency of 100 kHz. FIG. 2 is a graph showing the properties of the ferrite sintered body of the present invention, and illustrates one example of the relationship between power loss (core loss) and temperature (see Example 1 below). Point A in FIG. 2 is the point of minimal power loss for the ferrite sintered body, and the temperature at this point is called the bottom temperature.
Because ferrite core 10 has a bottom temperature higher than 120° C. and a power loss of 350 kW/m3 or less at the bottom temperature, heat generation can be adequately reduced even at about 100° C. or under even higher temperature conditions (such as about 110° C.), and thermal runaway can be adequately prevented. Because ferrite core 10 is highly reliable under high-temperature conditions, it can be used favorably in compact devices or in devices in which parts are packed at high densities, which are likely to have higher operating temperatures.
The reasons for making this ferrite core with such a composition are as follows.
Main Components
If the Fe2O3 component of the ferrite sintered body is less than 52 mol %, the bottom temperature of the ferrite sintered body will be excessively high, and power loss will not be sufficiently reduced at room temperature. On the other hand, if the Fe2O3 content exceeds 54 mol %, it will be difficult to achieve a bottom temperature higher than 120° C., and performance will deteriorate markedly over time during use under high temperature conditions. A Fe2O3 content of 52.8 to 53.8 mol % is especially desirable.
If the ZnO content of the ferrite sintered body is less than 6 mol % or more than 11 mol %, power loss will not be sufficient reduced at temperatures above 120° C. A ZnO content of 7.5 to 9.5 mol % is especially desirable.
Once the contents of the other main components (Fe2O3 and ZnO) have been determined, the MnO content of the fertite sintered body is determined as the remainder of the main components.
Additives
In order to more reliably prevent thermal runaway of the device, it is desirable that the power loss value at the bottom temperature be maintained as much as possible without any dramatic increase in power loss even if the bottom temperature is exceeded. Because the magnetic anisotropic constant K1 of Co is a relatively large positive value, adding a suitable amount of Co has the effect of adequately controlling the temperature dependence of power loss in the temperature range at and above the bottom temperature.
If the Co content (CoO equivalent) of the ferrite sintered body is less than 1000×10−6 parts by mass per 1 part by mass of the total mass of the oxides of the main components, there will be a marked increase in power loss in the temperature range above the bottom temperature. However, if the Co content (CoO equivalent) exceeds 3500×10−6 parts by mass, although the temperature variance rate of the power loss will be controlled in the temperature range above the bottom temperature, the power loss will not be sufficiently reduced. The Co content (CoO equivalent) is preferably greater than 1000×10−6 parts by mass and less than 3500×10−6 parts by mass, or more preferably 1500×10−6 to 3000×10−6 parts by mass.
Including a suitable amount of Ti in a ferrite sintered body containing Co has the effect of allowing the deterioration over time of the properties due to use in a high temperature environment to be controlled without causing an increase in power loss. If the Ti content (TiO2 equivalent) of the ferrite sintered body is less than 2000×10−6 parts by mass per 1 part by mass of the total mass of the oxides of the main components, there will be marked deterioration over time in performance if the sintered body is used in a high temperature environment. However, if the Ti content (TiO2 equivalent) exceeds 5000×10−6 parts by mass, power loss will not be sufficiently reduced. The Ti content (TiO2 equivalent) is preferably more than 2000×10−6 parts by mass and less than 5000×10−6 parts by mass, or more preferably 2500×10−6 to 4000×10−6 parts by mass.
Because Si has the effect of improving the sinterability of the ferrite while contributing to greater resistance of the grain boundaries, power loss can be reduced by including a suitable amount of Si. If the Si content (SiO2 equivalent) of the ferrite sintered body is less than 50×10−6 parts by mass per 1 part by mass of the total mass of the oxides of the main components, the high resistance grain boundary of the ferrite sintered body will not form properly, and power loss will not be sufficiently reduced. However, if the Si content (SiO2 equivalent) exceeds 150×10−6 parts by mass, abnormal grain growth will result, and power loss will not be sufficiently reduced. The Si content (SiO2 equivalent) is preferably 70×10−6 to 130×10−6 parts by mass.
Like Si, Ca has the effect of increasing the degree of sintering of tie ferrite sintered body while contributing to greater resistance of the gram boundaries, and therefore power loss can be reduced by including a suitable amount of Ca. If the Ca content (CaCO3 equivalent) of the ferrite sintered body is less than 300×10−6 parts by mass per 1 part by mass of the total mass of the oxides of the main components, the high resistance layer of the ferrite sintered body will not form properly, and power loss will not be sufficiently reduced. However, if the Ca content (CaCO3 equivalent) exceeds 1500×10−6 parts by mass, abnormal grain growth will result, and power loss will not be sufficiently reduced. The Ca content (CaCO3 equivalent) is preferably 350×10−6 to 1250×10−6 parts by mass.
The additives of the ferrite sintered body of this embodiment preferably further include Nb and/or Ta in the amounts shown in (5) and (6) below per 1 part by mass of the total mass of the oxides of the main components.

    (5) Nb in an amount corresponding to 50×10−6 to 600×10−6 parts by mass as Nb2O5 equivalent.
    (6) Ta in an amount corresponding to 80×10−6 to 1000×10−6 parts by mass as Ta2O5 equivalent.


Because Nb contributes to the uniformity of the crystal structure of the ferrite sintered body, power loss can be reduced by including a suitable amount of Nb. If the Nb content (Nb2O5 equivalent) of the ferrite sintered body is less than 50×10−6 parts by mass per 1 part by mass of the total mass of the oxides of the main components, it is likely that the crystal structure will not be made sufficiently uniform, and power loss will not be sufficiently reduced. However, if the Nb content (Nb2O5 equivalent) exceeds 600×10−6 parts by mass, it will actually tend to make the crystal structure less uniform. The Nb content (Nb2O5 equivalent) is preferably 200×10−6 to 500×10−6 parts by mass.
Like Nb, Ta contributes to the uniformity of the crystal structure of the ferrite sintered body, and therefore power loss can be reduced by including a suitable amount of Ta. If the Ta content (Ta2O5 equivalent) of the ferrite sintered body is less than 80×10−6 parts by mass per 1 part by mass of the total mass of the oxides of the main components, it is likely that the crystal structure will not be made sufficiently uniform, and power loss will not be sufficiently reduced. However, if the Ta content (Ta2O5 equivalent) exceeds 1000×10−6 the crystal structure will actually tend to be less uniform. The Ta content (Ta2O5 equivalent) is preferably 300×10−6 to 900×10−6 parts by mass.
When both Nb and Ta are included in the ferrite sintered body, the total content of Nb and Ta can be adjusted appropriately based on the molecular weights of Nb2O5 and Ta2O5.
In the ferrite sintered body of this embodiment, the additives preferably further include Zr and/or Hf in the amounts shown in (7) and (8) below per 1 part by mass of the total mass of the oxides of the main components.

    (7) Zr in an amount corresponding to 200×10−6 parts by mass or less as ZrO2 equivalent,
    (8) Hf in an amount corresponding to 400×10−6 parts by mass or less as HfO2 equivalent.


Because Zr contributes to higher resistance of the grain boundaries, including an appropriate amount of Zr is one way of reducing power loss. However, if the Zr content (ZrO2 equivalent) of the ferrite sintered body exceeds 200×10−6 parts by mass per 1 part by mass of the total mass of the oxides of the main components, an excess of high-resistance layer is likely to form in the ferrite sintered body, and power loss may not be sufficiently reduced. The Zr content (ZrO2 equivalent) is preferably 50×10−6 to 200×10−6 parts by mass, or more preferably 80×10−6 to 150×10−6 parts by mass.
Like the Zr discussed above, Hf contributes to higher resistance of the grain boundaries, so power loss can be reduced by including a suitable amount of Hf. However, if the Hf content (HfO2 equivalent) of the ferrite sintered body exceeds 400×10−6 parts by mass per 1 part by mass of the total mass of the oxides of the main component, an excess of high-resistance layer is likely to form in the ferrite sintered body, and power loss may not be sufficiently reduced. The Hf content (HfO2 equivalent) is preferably 80×10−6 to 350×10−6 parts by mass, and more preferably 130×10−6 to 260×10−6 parts by mass.
When both Zr and Hf are included in the ferrite sintered body, the combined content of Zr and Hf can be adjusted appropriately based on the molecular weights of ZrO2 and HfO2.
The ferrite sintered body of this embodiment may also have components other than the aforementioned. For example, V (V2O5) and Mo (MoO3) contribute to uniformity of the crystal structure of the ferrite sintered body in the same way as Nb and Ta, and therefore including appropriate amounts of V and/or Mo is one way of reducing power loss.
Ferrite Core Manufacturing Method
Next, the method for manufacturing ferrite core 10 is explained.
First, iron oxide α-Fe2O3, manganese oxide Mn3O4 and zinc oxide ZnO are prepared as the source materials, and these oxides are mixed to obtain a mixture. The raw materials are mixed at this time so that the respective contents of Fe2O3 and ZnO are 52 to 54 mol % and 6 to 11 mol %, with the remainder consisting mainly of Mn3O4. Other compounds may also be mixed together with these oxides at this time so that the constituent ratio of each oxide component in the final mixture is within the aforementioned range when converted to the oxide equivalent.
Next, the mixture of the main components is pre-sintered to obtain a pre-sintered powder (pre-sintering step). pre-sintering can normally be performed in air. The pre-sintering temperature is dependent on the components making up the mixture, but is preferably 800 to 1100° C. The pre-sintering time is also dependent on the components of the mixture, but is preferably 1 to 3 hours. Next, the resulting pre-sintered product is pulverized in a ball mill or the like to obtain a powder.
Meanwhile, cobalt oxide CoO, titanium oxide TiO2, silicon oxide SiO2 and calcium carbonate CaCO3 are prepared as the additives, and specific amounts of these additives are mixed together to obtain a mixture. This mixture is added as the source materials for additives when pulverizing the pre-sintered product of the main component raw materials above, and the two are mixed together. In this way, a ferrite power is obtained for the main firing (mixing step). Additives other than those given above (Nb2O5, Ta2O5, ZrO2, HfO2, V2O5, MoO3 and the like) can also be added at this stage. Other compounds can be used in place of the aforementioned compounds so that the contents of the additives in the final mixture are within the aforementioned ranges. Also, CaO could be used in place of CaCO3 for example.
Next, a suitable binder such as polyvinyl alcohol is mixed with the ferrite power obtained as described above, which is then pressed in the same shape as ferrite core 10, or in other words in an E shape to obtain a compact body.
Next, the molded body is fired under atmospheric conditions (1 atmosphere) in a furnace (main firing step). FIG. 4 is a graph showing one example of temperature settings in the main firing step. As shown in FIG. 4, the main firing step has at least heating step S1 in which the molded body is gradually heated in the furnace, temperature holding step S2 in which the temperature is maintained at 1250 to 1345° C., slow cooling step S3 in which the temperature is gradually lowered from the holding temperature, and rapid cooling step S4 in which the temperature is rapidly lowered after the end of slow cooling step S3.
Heating step S1 is a step of raising the temperature inside the furnace to the holding temperature described below. The heating rate is preferably 10 to 300° C. per hour.
Once the specified temperature (1250 to 1345° C.) has been reached by means of heating step S1, this temperature is maintained in temperature holding step S2. If the holding temperature in temperature holding step S2 is less than 1250° C., power loss will not be sufficiently reduced due to insufficient grain growth of the ferrite sintered body and increased hysteresis loss. However, if the holding temperature exceeds 1345° C., there will be excess grain growth of the ferrite sintered body, and power loss will not be sufficiently reduced due to increased eddy-current loss. By keeping the holding temperature at 1250 to 1345° C., it is possible to balance hysteresis loss with eddy-current loss and adequately reduce power loss in the high-temperature range.
The time for firing at this holding temperature (holding) is preferably 2 hours 30 minutes or more. If the holding time is less than 2 hours 30 minutes, grain growth will be insufficient if firing is performed at 1250 to 1345° C., and power loss will probably not be sufficiently reduced. The holding time is dependent on the components of the powder, but is preferably 3 to 10 hours.
Slow cooling step S3 is performed after completion of temperature holding step S2. The slow cooling rate in slow cooling step S3 is preferably 150° C. per hour or less. If the slow cooling rate exceeds 150° C. per hour, there is likely to be more residual stress in the grains of the ferrite sintered body, and power loss probably will not be sufficiently reduced for this reason. This slow cooling rate signifies the average value within the slow cooling step, and there may be times when the temperature falls at a rate exceeding this.
When lowering the temperature from the holding temperature in slow cooling step S3, an operation is performed to control the oxygen concentration within the furnace and lower it either continuously or in stages (oxygen concentration adjustment step). By means of such an operation, the oxygen concentration at 1250° C. is made to be 0.24 to 2.0 vol % while the oxygen concentration at 1100° C. is made to be 0.020 to 0.20 vol %.
FIG. 4 is a graph showing one example of oxygen concentration settings in the furnace in slow cooling step S3. As the temperature is lowered from 1250° C. to 1100° C., the oxygen concentration in the furnace is preferably adjusted so that it moves in the region between line L1 and line L2 in FIG. 4. FIG. 4 shows the oxygen concentration set to decrease in stages. Such a stepwise decrease in oxygen concentration can be achieved by means of a stepwise decrease in the oxygen concentration of the oxygen-containing gas supplied to the furnace. The oxygen concentration can be adjusted continuously rather than in stages, or by a combination of the two. The oxygen concentration in the furnace may rise temporarily as long as it moves within the region between line L1 and line L2.
The maximum line L1 and minimum line L2 of the oxygen concentration at temperatures of 1250 to 1100° C. were calculated based on the following formula (1):

Log(PO2)=a−b/T   (1)

wherein PO2 is the molar ratio of oxygen (%), T is the absolute temperature (K), and a and b are each constants. The constants a and b corresponding to maximum line L1 and minimum line L2, respectively, in FIG. 4 are shown in Table 1.











TABLE 1







a
b




















Maximum line L1
9.45
13941



Minimum line L2
9.26
15044










The temperature at which slow cooling step S3 is complete and rapid, cooling step S4 begins (slow cooling end temperature) is preferably 950 to 1150° C. If the slow cooling end temperature exceeds 1150° C., there is likely to be more residual stress in the grains of the ferrite sintered body, and power loss may not be sufficiently reduced. However, if the slow cooling end temperature is lower than 950° C., hetero-phase components are likely to occur at the grain boundaries of the ferrite sintered body, and power loss may not be sufficiently reduced.
Rapid cooling step S4 is performed after completion of slow cooling step S3. It is desirable that the cooling rate be 200° C. per hour or more at least in the temperature range between the temperature at the end of the slow cooling step and 800° C. If the cooling rate in this temperature range is less than 200° C. per hour, hetero-phase components are likely to occur at the grain boundaries of the ferrite sintered body, and power loss may not be sufficiently reduced. After completion of slow cooling step S3, it is desirable from the standpoint of preventing oxidation of the ferrite that the atmosphere within the furnace be a nitrogen atmosphere (oxygen concentration 0.02 vol % or less).
The present invention is not limited by the embodiment described above. For example, in the method of manufacturing ferrite core 10 above the powder is mixed with a binder and molded before the main firing step so that ferrite core 10 can be made in the specified shape (E shape), but a ferrite core of the specified shape can also be manufactured by first main firing the powder, and then working it.
Moreover, the aforementioned embodiment is an example in which the mixed powder for main firing is prepared by adding the source materials for additives when pulverizing a pre-sintered product obtained by pre-sintering the main component materials, but this mixed powder can also be prepared as follows. For example, a mixture obtained by mixing the main component materials before pre-sintering with the source materials for additives can be pre-sintered, and the pre-sintered product can then be pulverized to obtain a mixed powder for main firing. Alternatively, a mixture obtained by mixing the main component materials before pre-sintering with the source materials for additives can be pre-sintered, and further source materials for additives and the like can then be added while pulverizing the pre-sintered product to thereby obtain a mixed power for main firing.
In this embodiment, the firing profile shown in FIG. 3 is adopted for the temperature settings in the main firing step, but this is not a limitation, and appropriate changes can be made.
Moreover, while an example of an E-shaped ferrite core 10 is used in this embodiment, the shape of the ferrite core is not restricted to this. The shape of the ferrite core can be determined according to the shape and usage of the device that will contain the ferrite core.
EXAMPLES
Examples 1 to 16 and Comparative Examples 1 to 9
The various component materials were weighed so as to finally obtain the compositions shown in Table 1, and wet mixed with a ball mill. The raw material mixture was dried, and pre-sintered at a temperature of about 900° C. in air. The resulting pre-sintered powder was placed in a ball mill and wet pulverized for 3 hours to obtain the desired particle diameter.
The resulting powder was dried and then granulated after addition of 0.8 parts by mass of polyvinyl alcohol per 100 parts by mass of powder, and the obtained granule were pressed at a pressure around 100 MPa to obtain a toroidal shape compact body. The compact body was fired under the conditions shown in Table 3 to obtain the ferrite cores of Examples 1 to 16 and Comparative Examples 1 to 9, which were toroidal molded bodies with dimensions of external diameter 20 mm, internal diameter 10 mm and height 5 mm.
The power loss of each of the ferrite cores manufactured in Examples 1 to 16 and Comparative Examples 1 to 9 was measured as follows. That is, power loss was measured within a temperature range of 25 to 150° C. under conditions of excitation magnetic flux density 200 mT, frequency 100 kHz. The temperature at which the measured value of power loss was minimal within the temperature range of 25 to 150° C. (bottom temperature) was determined. The value (minimal power loss value) of power loss at the bottom temperature was also determined. FIG. 2 is a graph showing measurement results for power loss for the ferrite core of Example 1.
After power loss measurement, each ferrite core was stored for 96 hours in a thermostatic chamber set at 200° C. The power loss of each ferrite core was then measured again by the same methods. The power loss measurements before and after storage in the thermostatic chamber were then substituted in Formula (2) below to calculate the rate of change in power loss.




  
    
      
        
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          ⁢
          
            
          
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          loss
          ⁢
          
            
          
          ⁢
          
            (
            %
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        =
        
          
            
              
                
                  
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                      ⁢
                      
                        
                      
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                      power
                      ⁢
                      
                        
                      
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                      ⁢
                      
                        
                      
                      ⁢
                      value
                      ⁢
                      
                        
                      
                      ⁢
                      after
                      ⁢
                      
                        
                      
                      ⁢
                      storage
                    
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                  -
                
              
            
            
              
                
                  (
                  
                    minimal
                    ⁢
                    
                      
                    
                    ⁢
                    power
                    ⁢
                    
                      
                    
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                    loss
                    ⁢
                    
                      
                    
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                    value
                    ⁢
                    
                      
                    
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                    before
                    ⁢
                    
                      
                    
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                    storage
                  
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            minimal
            ⁢
            
              
            
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            value
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Table 2 shows measurement results for bottom temperature, minimal power loss value, power loss at 150° C. and rate of change in power loss.














TABLE 2












Mini-








mal
Power






power
loss at
Rate



Main components
Additives
Bottom
loss
150° C.
of



(mol %)
(ppm mass)
temp.
(kW/
(kW/
change

























Fe2O3
MnO
ZnO
CoO
TiO2
SiO2
CaCO3
Nb2O5
Ta2O5
ZrO2
HfO2
V2O5
MoO3
(° C.)
m3)
m3)
(%)



























Ex 1
53.32
37.87
8.80
2414
3333
90
475
462
—
119
—
200
77
121
286
344
4.0


Ex 2
52.53
36.57
10.90
3400
4980
54
302
464
—
194
—
303
60
148
348
350
1.0


Ex 3
53.48
38.26
8.26
2369
2860
105
473
469
—
110
—
184
78
121
285
341
4.8


Ex 4
53.42
38.29
8.29
2390
3330
103
1270
209
—
119
—
199
80
127
300
351
4.4


Ex 5
53.98
40.00
6.02
1011
2100
103
1446
211
—
111
—
120
140
122
301
360
8.0


Ex 6
53.30
38.20
8.50
2400
3000
90
475
—
652
120
—
201
71
131
295
340
3.8


Ex 7
53.20
38.00
8.80
2414
3333
148
470
—
652
130
—
201
72
133
300
352
3.5


Ex 8
52.80
36.70
10.50
3300
4000
140
400
250
—
—
170
250
110
138
320
345
2.1


Ex 9
52.80
36.70
10.50
3288
3987
141
411
249
—
—
389
250
110
138
318
343
2.0


Ex 10
53.68
38.32
8.00
1316
2001
103
1499
450
—
121
—
121
50
124
292
355
6.0


Ex 11
53.20
37.35
9.45
2414
2810
85
475
455
—
123
—
350
70
129
292
343
3.5


Ex 12
53.50
38.97
7.53
2391
3101
75
809
457
—
120
—
301
73
127
284
340
4.9


Ex 13
53.33
37.97
8.70
2414
3333
90
475
462
—
 11
173
198
75
121
284
343
4.1


Ex 14
53.33
37.97
8.70
2412
3321
87
465
462
—
 13
 20
195
70
121
289
348
4.2


Ex 15
53.54
40.43
6.03
2501
4965
89
465
460
—
110
—
200
76
122
294
349
5.1


Ex 16
53.45
38.28
8.27
2399
2852
105
448
 50
665
 17
171
186
63
121
282
340
4.3


CE1
54.51
39.51
5.98
110
390
75
1500
200
—
131
—
251
60
91
295
601
>10


CE2
53.80
39.18
7.02
500
1011
103
450
211
—
111
—
120
140
129
312
480
>10


CE3
51.10
36.70
12.20
3500
4500
80
311
—
651
105
—
252
50
>150
—
511
<1


CE4
53.40
39.10
7.50
4532
2500
90
411
457
—
134
—
199
79
131
400
421
4.4


CE5
53.30
39.26
7.44
2200
5503
90
475
467
—
—
—
203
80
122
370
440
4.0


CE6
53.32
37.88
8.80
3420
3330
200
1800
460
—
120
—
201
77
134
500
590
3.9


CE7
53.33
37.87
8.80
2420
3330
90
475
462
—
344
—
200
77
120
360
414
4.0


CE8
53.33
37.87
8.80
2401
3300
99
466
—
—
121
—
—
—
120
375
432
4.3


CE9
53.40
39.10
7.50
2600
2830
90
411
457
—
—
561
199
79
135
383
432
4.6



















TABLE 3









Holding temperature
1300° C.



Holding time
5 hr



Slow cooling rate
30° C./hr











Oxygen concentrations
1250° C.
1.24 vol %



in slow cooling step
1100° C.
0.11 vol %










Temperature at end of slow cooling
1000° C.



Cooling rate during rapid cooling
300° C./hr



(to 800° C.)










Examples 17 to 25 and Comparative Examples 10 to 18
The ferrite cores of Examples 17 to 25 and Comparative Examples 10 to 18 were manufactured as in Example 1 except that main firing was performed under the conditions shown in Table 4 rather than the conditions shown in Table 3, and the evaluation tests described above were performed. Table 4 shows measurement results for bottom temperature, minimal power loss value, power loss at 150° C. and rate of change in power loss.


















TABLE 4











Slow
Oxygen
Temp. at
Rapid

Minimal
Power




Holding
Holding
cooling
concentration
end of
cooling
Bottom
power
loss at
Rate of



temp.
time
rate
(vol %)
holding
rate
temp.
loss
150° C.
change



















(° C.)
(hr)
(° C./hr)
1250° C.
1100° C.
time (° C.)
(° C./hr)
(° C.)
(kW/m3)
(kW/m3)
(%)






















Ex 1
1300
5
30
1.24
0.11
1000
300
121
286
344
4.0


Ex 17
1340
5
30
1.24
0.11
1000
300
121
336
396
3.6


Ex 18
1255
5
30
1.24
0.11
1000
300
121
320
369
4.5


Ex 19
1300
3
30
1.24
0.11
1000
300
121
340
398
4.0


Ex 20
1300
5
100
1.24
0.11
1000
300
121
345
401
4.1


Ex 21
1300
5
30
1.24
0.11
1100
300
121
293
350
4.1


Ex 22
1300
5
30
1.24
0.11
950
300
121
345
402
4.2


Ex 23
1300
5
30
2.00
0.20
1000
300
121
283
340
7.0


Ex 24
1300
5
30
0.30
0.03
1000
300
125
343
400
2.3


Ex 25
1300
5
30
1.24
0.11
1000
200
121
330
390
4.0


CE 10
1360
5
30
1.24
0.11
1000
300
121
390
455
3.4


CE 11
1200
5
30
1.24
0.11
1000
300
121
380
430
4.7


CE 12
1300
2
30
1.24
0.11
1000
300
121
356
413
4.0


CE 13
1300
5
150
1.24
0.11
1000
300
121
400
460
4.1


CE 14
1300
5
30
1.24
0.11
1200
300
121
360
450
4.1


CE 15
1300
5
30
1.24
0.11
850
300
121
450
520
4.0


CE 16
1300
5
30
2.40
0.30
1000
300
115
280
340
>10


CE 17
1300
5
30
0.10
0.01
1000
300
125
375
435
2.0


CE 18
1300
5
30
1.24
0.11
1000
100
121
370
430
4.1









It is clear from the results shown in Tables 2 and 4 that it is possible to manufacture a ferrite sintered body with a bottom temperature higher than 120° C. and sufficiently low power loss at the bottom temperature by giving the ferrite sintered body a specific composition and controlling the main firing conditions. Moreover, it is clear that a ferrite sintered body manufactured in this way can maintain a sufficiently low power loss value even when used long-term under high-temperature conditions, with little deterioration of performance over time.
It has thus been confirmed that because it has such properties, the ferrite sintered body of the present invention is sufficiently resistant to thermal runaway, and can be used favorably as a magnetic core.

## Claims

1. A ferrite sintered body containing:
main components consisting of 52 to 54 mol % Fe2O3, 35 to 42 mol % MnO and 6 to 11 mol % ZnO as oxide equivalents; and
additives including 1000×10−6 to 3500×10−6 parts by mass Co as CoO equivalent, 2100×10−6 to 5000×10−6 parts by mass Ti as TiO2 equivalent, 50×10−6 to 150×10−6 parts by mass Si as SiO2 equivalent and 300×10−6 to 1500×10−6 parts by mass Ca as CaCO3 equivalent per 1 part by mass of the total mass of said oxides of said main components,
wherein the temperature at which power loss is a minimal value is higher than 120° C. in a magnetic field with an excitation magnetic flux density of 200 mT and a frequency of 100 kHz, and the power loss at said temperature at which power loss is a minimal value is 350 kW/m3 or less.

2. The ferrite sintered body according to claim 1, wherein said additives further include 50×10−6 to 600×10−6 parts by mass of Nb as Nb2O5 equivalent and/or 80×10−6 to 1000×10−6 parts by mass of Ta as Ta2O5 equivalent per 1 part by mass of the total mass of said oxides of said main components.

3. The ferrite sintered body according to claim 1, wherein said additives further include 200×10−6 parts by mass or less of Zr as ZrO2 equivalent and/or 400×10−6 parts by mass or less of Hf as HfO2 equivalent per 1 part by mass of the total mass of said oxides of said main components.

4. The ferrite sintered body according to claim 2, wherein said additives further include 200×10−6 parts by mass or less of Zr as ZrO2 equivalent and/or 400×10−6 parts by mass or less of Hf as HfO2 equivalent per 1 part by mass of the total mass of said oxides of said main components.

5. A method for manufacturing a ferrite sintered body, comprising:
a mixing step of mixing main components consisting of 52 to 54 mol % Fe2O3, 35 to 42 mol % MnO and 6 to 11 mol % ZnO as oxide equivalents with additives including 1000×10−6 to 3500×10−6 parts by mass Co as CoO equivalent, 2000×10−6 to 5000×10−6 parts by mass Ti as TiO2 equivalent, 50×10−6 to 150×10−6 parts by mass Si as SiO2 equivalent and 300×10−6 to 1500×10−6 parts by mass Ca as CaCO3 equivalent per 1 part by mass of the total mass of said oxides of said main components; and
a main firing step of sintering a mixed powder containing said main components and said additives in a furnace to obtain a ferrite sintered body,
wherein said main firing step has a temperature holding step of maintaining the firing temperature at 1250 to 1345° C., and an oxygen concentration adjustment step of reducing the oxygen concentration inside said furnace either continuously or in stages when lowering the temperature from the holding temperature, so that by means of said oxygen concentration adjustment step, the oxygen concentration at 1250° is made to be 0.24 to 2.0 vol % while the oxygen concentration at 1100° is made to be 0.020 to 0.20 vol %.