MNZN FERRITE AND ITS PRODUCTION METHOD

Abstract

A method for producing MnZn ferrite comprising Fe, Mn and Zn as main components, and Ca, Si and Co, and at least one selected from the group consisting of Ta, Nb and Zr as sub-components, comprising a step of molding a raw material powder for the MnZn ferrite to obtain a green body, and a step of sintering the green body; the sintering step comprising a temperature-elevating step, a high-temperature-keeping step, and a cooling step; the cooling step including a slow cooling step of cooling in a temperature range of 1100 C. to 1250 C. at a cooling speed of 0 C./hour to 20 C./hour for 1 hours to 20 hours, and a cooling speed before and after the slow cooling step being higher than 20 C./hour; the MnZn ferrite having a volume resistivity of 8.5 .Math.m or more at room temperature, an average crystal grain size of 7 m to 15 m, and core loss of 420 kW/m.sup.3 or less between 23 C. and 140 C. at a frequency of 100 kHz and an exciting magnetic flux density of 200 mT.

Claims

1. A method for producing MnZn ferrite comprising Fe, Mn and Zn as main components, and Ca, Si and Co, and at least one selected from the group consisting of Ta, Nb and Zr as sub-components, comprising a step of molding a raw material powder for said MnZn ferrite to obtain a green body, and a step of sintering said green body; said sintering step comprising a temperature-elevating step, a high-temperature-keeping step, and a cooling step; said cooling step including a slow cooling step of cooling in a temperature range of 1100 C. to 1250 C. at a cooling speed of 0 C./hour to 20 C./hour for 1 hours to 20 hours, and a cooling speed before and after said slow cooling step being higher than 20 C./hour, said MnZn ferrite having a volume resistivity of 8.5 .Math.m or more at room temperature, an average crystal grain size of 7 m to 15 m, and core loss of 420 kW/m.sup.3 or less between 23 C. and 140 C. at a frequency of 100 kHz and an exciting magnetic flux density of 200 mT.

2. The method for producing MnZn ferrite according to claim 1, wherein said high-temperature-keeping step is conducted at a keeping temperature of higher than 1250 C. and 1350 C. or lower in an atmosphere having an oxygen concentration of more than 0.2% by volume and 10% by volume or less.

3. The method for producing MnZn ferrite according to claim 2, wherein the concentration of oxygen in said cooling step is controlled so that the relationship between the concentration of oxygen P [O.sub.2] (volume fraction) and temperature T ( C.) satisfies a formula:
log(P[O.sub.2])=ab/(T+273), wherein a is a constant of 6.4 to 11.5, and b is a constant of 10000 to 18000.

4. The method for producing MnZn ferrite according to claim 1, wherein the MnZn ferrite comprises Fe, Mn and Zn as main components, and Si, Ca and Co, and at least one selected from the group consisting of Ta, Nb and Zr as sub-components, said main components comprising 53-54% by mol of Fe (calculated as Fe.sub.2O.sub.3), and 8.2-10.2% by mol of Zn (calculated as ZnO), the balance being Mn calculated as MnO, and said sub-components comprising more than 0.001 parts by mass and 0.015 parts by mass or less of Si (calculated as SiO.sub.2), more than 0.1 parts by mass and 0.35 parts by mass or less of Ca (calculated as CaCO.sub.3), 0.4 parts by mass or less (not including 0) of Co (calculated as Co.sub.3O.sub.4), 0.1 parts by mass or less (including 0) of Ta (calculated as Ta.sub.2O.sub.5), 0.1 parts by mass or less (including 0) of Zr (calculated as ZrO.sub.2), and 0.05 parts by mass or less (including 0) of Nb (calculated as Nb.sub.2O.sub.5), the total amount of Ta.sub.2O.sub.5, ZrO.sub.2 and Nb.sub.2O.sub.5 being 0.1 parts by mass or less (not including 0), per 100 parts by mass in total of said main components (calculated as said oxides).

5. MnZn ferrite comprising 53-54% by mol of Fe (calculated as Fe.sub.2O.sub.3), and 8.2-10.2% by mol of Zn (calculated as ZnO), the balance being Mn (calculated as MnO), as main components, and more than 0.001 parts by mass and 0.015 parts by mass or less of Si (calculated as SiO.sub.2), more than 0.1 parts by mass and 0.35 parts by mass or less of Ca (calculated as CaCO.sub.3), 0.4 parts by mass or less (not including 0) of Co (calculated as Co.sub.3O.sub.4), 0.1 parts by mass or less (including 0) of Ta (calculated as Ta.sub.2O.sub.5), 0.1 parts by mass or less (including 0) of Zr (calculated as ZrO.sub.2), and 0.05 parts by mass or less (including 0) of Nb (calculated as Nb.sub.2O.sub.5), the total amount of Ta.sub.2O.sub.5, ZrO.sub.2 and Nb.sub.2O.sub.5 being 0.1 parts by mass or less (not including 0), as sub-components, per 100 parts by mass in total of said main components (calculated as said oxides), said MnZn ferrite having a volume resistivity of 8.5 .Math.m or more at room temperature, an average crystal grain size of 7 m to 15 m, core loss of 420 kW/m.sup.3 or less between 23 C. and 140 C. at a frequency of 100 kHz and an exciting magnetic flux density of 200 mT, and initial permeability i of 2800 or more at a frequency of 100 kHz and at 20 C.

6. The MnZn ferrite according to claim 5, wherein said sub-components comprises more than 0.003 parts by mass and 0.012 parts by mass or less of Si (calculated as SiO.sub.2), more than 0.1 parts by mass and 0.35 parts by mass or less of Ca (calculated as CaCO.sub.3), and 0.2 parts by mass or more and 0.4 parts by mass or less of Co (calculated as Co.sub.3O.sub.4), and said sub-components further comprises at least one selected from the group consisting of 0.015 parts by mass or more and 0.1 parts by mass or less of Ta (calculated as Ta.sub.2O.sub.5), 0.03 parts by mass or more and 0.1 parts by mass or less of Zr (calculated as ZrO.sub.2), and 0.02 parts by mass or more and 0.05 parts by mass or less of Nb (calculated as Nb.sub.2O.sub.5), the total amount of Ta.sub.2O.sub.5, ZrO.sub.2 and Nb.sub.2O.sub.5 being 0.1 parts by mass or less (not including 0), per 100 parts by mass in total of said main components (calculated as said oxides), said MnZn ferrite having core loss of 400 kW/m.sup.3 or less at 23 C. at a frequency of 100 kHz and an exciting magnetic flux density of 200 mT.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0032] FIG. 1 is a graph showing temperature conditions in a sintering step according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] A production method of MnZn ferrite and the MnZn ferrite obtained by the method according to an embodiment of the present invention will be specifically explained below. It should be noted, however, that the present invention is not restricted thereto, but modifications may be made properly within the scope of the technical idea. The numerical range expressed by - in this specification means a range including the numbers described before and after - as the upper and lower limits.

[0034] A method for producing MnZn ferrite of the present invention comprises a step of molding a raw material powder for the MnZn ferrite to obtain a green body, and a step of sintering the green body, the raw material powder containing Fe, Mn and Zn as main components, and Ca, Si and Co, and at least one selected from the group consisting of Ta, Nb and Zr as sub-components. Si, Ca, Ta, Zr and Nb are present in grain boundaries of MnZn ferrite and insulate crystal grains by increasing resistance of grain boundary layers, thereby reducing relative loss coefficient tan /i and thus reducing core loss. Each of Ta, Zr and Nb may be contained individually or in plural.

[0035] Although Si exclusively segregates in grain boundaries and triple points, Ca, Ta, Zr and Nb are dissolved in spinel phase in the course of the sintering step, and may be partly dissolved after sintering and remain in the crystal grains in some cases. When the content of Ca, Ta, Zr and Nb dissolved in the spinel phase increases, resistance in the crystal grain is increased and a volume resistivity can be increased. However, the content of Ca, Ta, Zr and Nb in the grain boundaries relatively decreases. To obtain MnZn ferrite having low core loss by achieving a high volume resistivity, it is effective to increase the resistance in crystal grains and to form high-resistance grain boundaries by appropriately adjusting the content of Ca, Ta, Zr and Nb dissolved in spinel phase and segregated in crystal grain boundaries. Such adjustment can be carried out by controlling sintering temperature and sintering atmosphere as described later.

[0036] By adding Co.sup.2+ in addition to Fe.sup.2+, temperature change of the core loss decreases, and low core loss over a wide temperature range can be obtained. In addition, since residual magnetic flux density Br can be reduced by adding Co.sup.2+, it is possible to reduce the hysteresis loss Ph to obtain MnZn ferrite having low core loss.

[0037] The sintering step comprises a temperature-elevating step, a high-temperature-keeping step, and a cooling step. The cooling step has a slow cooling step of cooling in a temperature range of 1100 C.-1250 C. at a cooling speed of 0-20 C./hour for 1-20 hours, and a cooling speed before and after the slow cooling step is made to be higher than 20 C./hour. In the present invention, it is preferable to reduce the core loss by segregating Ca, Ta, Zr and Nb at grain boundaries and properly controlling them dissolving in crystal grains.

[0038] To increase a resistance of crystal grain boundaries, a slow cooling step of cooling in a temperature range of 1100 C. to 1250 C. at a cooling speed of 0 C./hour to 20 C./hour for 1 hours to 20 hours, is provided in the cooling step. When the slow cooling step is provided in a temperature range exceeding 1250 C., the core loss between 23 C. and 140 C. increases due to an influence of internal distortion increase caused by volatilization of zinc from the surface layer. When the slow cooling step is provided in a temperature range of less than 1100 C., the core loss on a low temperature side increases due to excessive grain boundary segregation of Ca etc., thereby it is difficult to obtain a desired core loss.

[0039] When the cooling speed is more than 20 C./hour, segregation of Ca etc. to the grain boundaries is not sufficient and a high volume resistivity cannot be obtained, thereby the core loss on the high temperature side increases and thus it is difficult to obtain a desired core loss. It is noted that the cooling speed of 0 C./hour means to hold at a constant temperature. When the slow cooling step is conducted for less than 1 hour, the effect of reducing core loss cannot be sufficiently obtained. When it exceeds 20 hours, the crystal growth proceeds and the particle diameter increases, thereby volume resistivity decreases, and thus core loss may increase in some cases. Further, a cooling speed before and after the slow cooling step is set to be higher than the cooling speed in the slow cooling step, namely, more than 20 C./hour. When the cooling speed before the slow cooling step is 20 C./hour or less, the amount of volatilization of zinc from the surface layer increases, thereby an internal strain increases and thus core loss increases. When the cooling speed after the slow cooling step is 20 C./hour or less, Ca etc. are excessively segregated at grain boundaries and core loss increases. It is preferable to set a cooling speed to 50-150 C./hour from the high-temperature-keeping step to the slow cooling step and the cooling step after the slow cooling step, namely, before and after the slow cooling step. Through such processes, volume resistivity at room temperature can be 8.5 .Math.m or more. Further, the volume resistivity is preferably 10 .Math.m or more so as to reduce the eddy current loss Pe.

[0040] In the slow cooling step, when an oxygen concentration is high, a sintered body is oxidized, resulting in precipitation of hematite from spinel, whereas when an oxygen concentration is low, wustite precipitates and crystal strain is generated, resulting in undesirable increase of core loss. Therefore, it is preferable to control an oxygen concentration so that precipitation of hematite and precipitation of wustite do not occur. It is further preferable to control an oxygen concentration in the cooling step so that the relationship between an oxygen concentration P [O.sub.2] (volume fraction) and a temperature T ( C.) satisfies a formula:

log(P [O.sub.2])=ab/(T+273),

wherein a and b are constants, a is preferably 3.1-12.8 and b is preferably 6000-20000. a is defined from a temperature and an oxygen concentration in the high-temperature-keeping step. When b is smaller than the above range, even if a temperature is lowered, an oxygen concentration is high and oxidation proceeds, which may result in precipitation of hematite from spinel in some cases. When b is large, an oxygen concentration decreases and wustite precipitates and so on, thereby both crystal grains and grain boundary layers are not oxidized sufficiently, resulting in low resistance. More preferably, a is 6.4-11.5 and b is 10000-18000.

[0041] In the temperature-elevating step, it is carried out in the air between room temperature and a temperature of 750 C. or more and 950 C. or less (first temperature-elevating step) so as to remove a binder from the green body. It is preferable to reduce an oxygen concentration in the atmosphere to 0.1-2% by volume in a second temperature-elevating step, that is, from the first temperature-elevating step to the high-temperature-keeping step. The temperature-elevating speed in the temperature-elevating step may be appropriately selected depending on the state of residual carbon in binder removal and its composition. The average temperature-elevating speed is preferably in a range of 50-200 C./hour.

[0042] The temperature in the high-temperature-keeping step is higher than 1250 C. and 1350 C. or less, and an oxygen concentration in the atmosphere is higher than 0.2% by volume and 10% by volume or less, and is preferably set higher than the oxygen concentration adjusted in the second temperature-elevating step.

[0043] In the present invention, MnZn ferrite comprises Fe, Mn and Zn as main components, and Ca, Si and Co, and at least one selected from the group consisting of Ta, Nb and Zr as sub-components. The main components mainly are elements or compounds constituting spinel ferrite, whereas the sub-components are elements or compounds which are supplementarily used for its formation, and include the elements, a part of which dissolves in the spinel ferrite. Those constituting the spinel ferrite such as Co are also contained as sub-components because their content is smaller than those of the main components.

[0044] It is preferable that the main components comprises 53-54% by mol of Fe (calculated as Fe.sub.2O.sub.3), and 8.2-10.2% by mol of Zn (calculated as ZnO), the balance being Mn calculated as MnO, and the sub-components comprises more than 0.001 parts by mass and 0.015 parts by mass or less of Si (calculated as SiO.sub.2), more than 0.1 parts by mass and 0.35 parts by mass or less of Ca (calculated as CaCO.sub.3), 0.4 parts by mass or less (not including 0) of Co (calculated as Co.sub.3O.sub.4), 0.1 parts by mass or less (including 0) of Ta (calculated as Ta.sub.2O.sub.5), 0.1 parts by mass or less (including 0) of Zr (calculated as ZrO.sub.2), and 0.05 parts by mass or less (including 0) of Nb (calculated as Nb.sub.2O.sub.5), the total amount of Ta.sub.2O.sub.5, ZrO.sub.2 and Nb.sub.2O.sub.5 being 0.1 parts by mass or less (not including 0), per 100 parts by mass in total of the main components (calculated as the oxides).

[0045] In the MnZn ferrite of the present invention, when the contents of Si and Ca are in the above-mentioned range, Si and Ca are present in the grain boundaries to insulate crystal grains, thereby volume resistivity is increased, and thus relative loss coefficient tan /i can be reduced. The contents of Si and Ca are preferably more than 0.001 parts by mass and 0.015 parts by mass or less calculated as SiO.sub.2, and more than 0.1 parts by mass and 0.035 parts by mass or less calculated as CaCO.sub.3, respectively, per 100 parts by mass in total of the main components (calculated as the oxides). It is more preferably 0.003 parts by mass or more and 0.012 parts by mass or less calculated as SiO.sub.2 and more than 0.1 parts by mass and 0.25 parts by mass or less calculated as CaCO.sub.3.

[0046] By adding Co.sup.2+, temperature change of the core loss decreases, and low core loss over a wide temperature range can be obtained, also since residual magnetic flux density Br is reduced, it is possible to reduce hysteresis loss Ph. However, when the content of Co is too large, a magnetization curve tends to be a Perminber type, and a crystal magnetic anisotropy constant on the low temperature side is too large on the positive side, and may degrade on the contrary in some cases. The content of Co is preferably 0.4 parts by mass or less (not including 0) calculated as Co.sub.3O.sub.4, more preferably 0.2 parts by mass or more and 0.4 parts by mass or less calculated as Co.sub.3O.sub.4, and most preferably 0.25 parts by mass or more and 0.35 parts by mass or less calculated as Co.sub.3O.sub.4, per 100 parts by mass in total of the main components (calculated as the oxides).

[0047] Ta, Zr and Nb appear in the grain boundary layers together with Si and Ca, contribute to reducing core loss by increasing the resistance of the grain boundary layers. Ta, Zr and Nb may be contained individually or in combination of two or more. When contained alone, the contents of Ta, Zr and Nb are preferably 0.1 parts by mass or less (including 0) calculated as Ta.sub.2O.sub.5, 0.1 parts by mass or less (including 0) calculated as ZrO.sub.2, and 0.05 parts by mass or less (including 0) calculated as Nb.sub.2O.sub.5, respectively, per 100 parts by mass in total of the main components (calculated as the oxides). When two or more of Ta, Zr and Nb are contained, the total amount calculated as Ta.sub.2O.sub.5, ZrO.sub.2 and Nb.sub.2O.sub.5 is preferably 0.1 parts by mass or less (not including 0). When Ta, Zr and Nb are contained alone, the lower limit of the contents of Ta, Zr and Nb are preferably 0.03 parts by mass calculated as Ta.sub.2O.sub.5, ZrO.sub.2 and Nb.sub.2O.sub.5, respectively. When two or more of Ta, Zr and Nb are contained, the total amount calculated as Ta.sub.2O.sub.5, ZrO.sub.2 and Nb.sub.2O.sub.5 is preferably 0.03 parts by mass or more.

[0048] Sulfur S, chlorine Cl, phosphorus P, boron B, etc. may be contained as impurities in raw materials constituting MnZn ferrite. In the present invention, these impurities are not specifically defined, however, it is empirically known that reduction in core loss and improvement in magnetic permeability can be obtained by decreasing these impurities. Particularly, with respect to S, a compound with Ca may be generated and segregated as foreign matter at the grain boundaries, thereby decreasing the volume resistivity and increasing the eddy current loss in some cases. Therefore, for further reduction of the core loss, it is preferable to reduce impurities and to be 0.03 parts by mass or less of S, 0.01 parts by mass or less of Cl, 0.001 parts by mass or less of P, and 0.0001 parts by mass or less of B, per 100 parts by mass in total of the main components (calculated as the oxides).

[0049] Although a preferable average crystal grain size of the MnZn ferrite varies depending on a frequency to be used, when a frequency is less than 500 kHz, it is preferable to reduce coercive force He and reduce hysteresis loss by over 5 m. It is more preferably 7 m or more and 15 m or less.

[0050] The present invention will be explained in further detail by Examples below, without intention of restriction.

Example 1

[0051] As shown in Table 1, 53.4 mol % of Fe.sub.203, 9.2 mol % of ZnO, and 37.4 mol % of Mn.sub.3O.sub.4 calculated as MnO as a main component were wet-mixed and dried, and calcined at 900 C. for 3 hours. 100 parts by mass of the calcined powder was then mixed with SiO.sub.2, CaCO.sub.3, Co.sub.3O.sub.4, Ta.sub.2O.sub.5, ZrO.sub.2 and Nb.sub.2O.sub.5 to provide the MnZn ferrite compositions shown in Table 1, and pulverized to an average pulverized particle size of 1.2-1.4 m in a ball mill. With polyvinyl alcohol added as a binder, the resulting mixture was granulated by a mortar, and compression-molded to a ring-shaped green body, which was sintered to obtain a magnetic core (sintered ferrite body) having an outer diameter of 25 mm, an inner diameter of 15 mm and a thickness of 5 mm.

TABLE-US-00001 TABLE 1 Main components Sub-components Sample (mol %) (parts by mass .sup.(3)) No..sup.(1) Fe.sub.2O.sub.3 ZnO MnO.sup.(2) Co.sub.3O.sub.4 SiO.sub.2 CaCO.sub.3 Ta.sub.2O.sub.5 ZrO.sub.2 Nb.sub.2O.sub.5 1 53.4 9.2 37.4 0.3 0.003 0.18 0.05 0 0 2 53.4 9.2 37.4 0.3 0.006 0.18 0.05 0 0 3 53.4 9.2 37.4 0.3 0.015 0.18 0.05 0 0 *4 53.4 9.2 37.4 0.3 0.02 0.18 0.05 0 0 *5 53.4 9.2 37.4 0.3 0.006 0.07 0.05 0 0 6 53.4 9.2 37.4 0.3 0.006 0.1 0.05 0 0 7 53.4 9.2 37.4 0.3 0.006 0.35 0.05 0 0 *8 53.4 9.2 37.4 0.3 0.006 0.4 0.05 0 0 *9 53.4 9.2 37.4 0.3 0.006 0.18 0 0 0 10 53.4 9.2 37.4 0.3 0.006 0.18 0.015 0 0 11 53.4 9.2 37.4 0.3 0.006 0.18 0.03 0 0 12 53.4 9.2 37.4 0.3 0.006 0.18 0.1 0 0 *13 53.4 9.2 37.4 0.3 0.006 0.18 0.13 0 0 14 53.4 9.2 37.4 0.3 0.006 0.18 0 0.03 0 15 53.4 9.2 37.4 0.3 0.006 0.18 0 0.1 0 *16 53.4 9.2 37.4 0.3 0.006 0.18 0 0.15 0 17 53.4 9.2 37.4 0.3 0.006 0.18 0 0 0.02 18 53.4 9.2 37.4 0.3 0.006 0.18 0 0 0.05 *19 53.4 9.2 37.4 0.3 0.006 0.18 0 0 0.07 20 53.4 9.2 37.4 0.3 0.006 0.18 0.03 0.07 0 21 53.4 9.2 37.4 0.3 0.006 0.18 0.03 0 0.02 22 53.4 9.2 37.4 0.3 0.006 0.18 0.03 0.03 0.02 *23 53.4 9.2 37.4 0 0.003 0.18 0.05 0 0 24 53.4 9.2 37.4 0.16 0.003 0.18 0.05 0 0 25 53.4 9.2 37.4 0.4 0.003 0.18 0.05 0 0 *26 53.4 9.2 37.4 0.5 0.003 0.18 0.05 0 0 Note: .sup.(1)Sample No. with * indicates a Comparative Example. Note: .sup.(2)Mn.sub.3O.sub.4 was used as a raw material, and the composition was shown as MnO. Note: .sup.(3) The amount per 100 parts by mass of calcined powder consisting of the main component.

[0052] FIG. 1 shows the temperature conditions in the sintering step. Sintering comprises a temperature-elevating step of elevating the temperature from room temperature to 1310 C., a high-temperature-keeping step of keeping at 1310 C. for 4 hours, and a cooling step of cooling a temperature from 1310 C. to room temperature. The temperature-elevating step was carried out at a temperature-elevating speed of 150 C./hour in the air (in an atmosphere having an oxygen concentration of 21% by volume) in a temperature range from room temperature to 800 C., and in an atmosphere having an oxygen concentration of 1% by volume in a temperature range over 800 C. In the high-temperature-keeping step, the oxygen concentration was kept at 1% by volume. The cooling step was carried out at cooling speed of 100 C./hour in a temperature range from 1310 C. (high-temperature-keeping temperature) to 1250 C., at cooling speed of 10 C./hour in a temperature range from 1250 C. to 1200 C., at cooling speed of 100 C./hour in a temperature range from 1200 C. to 1000 C., and at cooling speed of 150 C./hour in a temperature range under 1000 C. The oxygen concentration (volume fraction) in the cooling step down to 1000 C. was shifted according to the formula: log (P [O.sub.2])=ab/(T+273), where a=6.9 and b=14000. Specifically, the oxygen concentration was controlled so as to be 0.5% by volume at 1250 C., 0.25% by volume at 1200 C., and 0.01% by volume (100 ppm) at 1000 C. Cooling was conducted in N.sub.2 flow lower than 1000 C., and the final oxygen concentration decreased to about 0.003% by volume (30 ppm).

[0053] With respect to the obtained magnetic core, core loss Pcv, initial permeability i, volume resistivity , and average crystal grain size were evaluated. The evaluation methods are as follows.

[0054] Core Loss Pcv

[0055] Using a B-H analyzer (SY-8232 available from Iwatsu Electric Co., Ltd.), the core loss Pcv of a magnetic core having a five-turn primary winding and a five-turn secondary winding was measured between 23 C. and 140 C. at a frequency of 100 kHz and an exciting magnetic flux density of 200 mT.

[0056] Initial Permeability i

[0057] The initial permeability i of a 10-turn magnetic core was measured at 23 C. and 100 kHz in a magnetic field of 0.4 A/m by HP-4284A available from Hewlett-Packard.

[0058] Volume Resistivity

[0059] A flat plate sample cut out from the magnetic core was coated with a gallium indium alloy as an electrode on the two opposing planes, and the electric resistance R () was measured using 3224 available from Hioki Electric Corporation. From the area A (m.sup.2) of the plane on which the electrode was formed and the thickness t (m), the volume resistivity (.Math.m) was calculated by the following formula:

Volume resistivity (.Math.m)=R(A/t).

[0060] Average Crystal Grain Size

[0061] The average crystal grain size was calculated by a quadrature method in a square region of 100 m100 m in a photograph (400 times) taken by an optical microscope on a mirror-polished sintered ferrite body thermally etched at 1100 C. for 1 hour in N.sub.2.

[0062] The results of the initial permeability i, the volume resistivity , the average crystal grain size and the core loss Pcv are shown in Table 2.

TABLE-US-00002 TABLE 2 Volume Average Initial Resistivity Crystal Perme- Sample Grain Size ability Core Loss Pcv (kW/m.sup.3) No..sup.(1) ( .Math. m) (m) (i) 23 C. 100 C. 140 C. 1 11.9 10.7 3500 379 279 367 2 18.3 9.5 3410 297 273 377 3 19.6 9.0 3090 415 309 385 *4 1.2 8.9 2820 663 448 566 *5 6.2 10.8 3730 397 294 483 6 12.0 10.3 3470 338 282 383 7 16.5 9.6 3200 370 303 377 *8 0.2 7.2 2240 949 717 700 *9 5.3 8.9 3660 400 308 503 10 10.0 10.3 3690 342 281 388 11 14.1 11.5 3660 315 262 371 12 17.0 10.2 3230 347 280 357 *13 5.6 11.4 3200 431 354 502 14 10.3 9.1 3720 351 292 383 15 11.4 10.4 3720 355 278 359 *16 6.1 10.1 3110 520 420 535 17 14.3 10.2 3630 343 293 408 18 13.1 9.3 3340 335 291 395 *19 0.1 11.3 3110 618 650 850 20 16.3 10.4 3100 379 277 373 21 13.7 9.1 3360 359 305 394 22 17.5 10.6 3020 385 280 379 *23 14.8 9.5 2340 532 308 335 24 11.9 10.5 2950 409 295 346 25 11.2 11.3 3150 352 279 367 *26 5.0 11.5 2860 446 279 490 Note: .sup.(1)Sample No. with * indicates a Comparative Example.

[0063] From Table 2, it is understood that the MnZn ferrites of Examples of the present invention have high volume resistivity of 8.5 .Math.m or more, and have core losses of 420 kW/m.sup.3 or less even under a high temperature environment at 140 C. On the other hand, it is understood that the MnZn ferrites of Comparative Examples excluding Sample No. *23 have volume resistivity of less than 8.5 .Math.m, and have high magnetic core losses. The MnZn ferrite of Sample No. *23 not containing Co had core loss of more than 420 kW/m.sup.3 at 23 C. Some Comparative Examples had core losses of 420 kW/m.sup.3 or less at 23 C. and 100 C., however, under a high temperature environment at 140 C. core losses of these Comparative Examples were more than 420 kW/m.sup.3. As described above, by having a composition comprising Ca, Si and Co, and at least one selected from the group consisting of Ta, Nb and Zr as sub-components, and providing a slow cooling step under predetermined conditions in a cooling step of sintering, MnZn ferrite having low core loss from low temperature (23 C.) to high temperature (140 C.) could be obtained.

Example 2

[0064] MnZn ferrites were produced in the same manner as in Sample 1, except that the compositions of the main components were changed as shown in Table 3. The results of the initial permeability i, the volume resistivity , the average crystal grain size, and the core loss Pcv were shown in Table 4. Every MnZn ferrites exhibited high volume resistivity of 10 .Math.m or more, however, the MnZn ferrites of Comparative Examples exhibited core losses of higher than 420 kW/m.sup.3 at high temperature or low temperature. On the other hand, the all MnZn ferrites of Examples exhibited core losses of 420 kW/m.sup.3 or less.

TABLE-US-00003 TABLE 3 Main Components Sub-Components Sample (mol %) (parts by mass .sup.(3)) No..sup.(1) Fe.sub.2O.sub.3 ZnO MnO.sup.(2) Co.sub.3O.sub.4 SiO.sub.2 CaCO.sub.3 Ta.sub.2O.sub.5 ZrO.sub.2 Nb.sub.2O.sub.5 27 53.0 10.2 36.8 0.3 0.003 0.18 0.05 0 0 28 53.4 9.2 37.4 0.3 0.003 0.18 0.05 0 0 29 54.0 8.2 37.8 0.3 0.003 0.18 0.05 0 0 *30 54.5 8.5 37.0 0.3 0.003 0.18 0.05 0 0 *31 53.0 7.5 39.5 0.3 0.003 0.18 0.05 0 0 *32 52.5 8.5 39.0 0.3 0.003 0.18 0.05 0 0 *33 54.0 11.0 35.0 0.3 0.003 0.18 0.05 0 0 Note: .sup.(1)Sample No. with * indicates a Comparative Example. Note: .sup.(2)Mn.sub.3O.sub.4 was used as a raw material, and the composition was shown as MnO. Note: .sup.(3) The amount per 100 parts by mass of calcined powder consisting of the main component.

TABLE-US-00004 TABLE 4 Volume Average Initial Resistivity Crystal Perme- Sample Grain Size ability Core Loss Pcv (kW/m.sup.3) No..sup.(1) ( .Math. m) (m) (i) 23 C. 100 C. 140 C. 27 13.0 11.0 3560 390 295 350 28 11.9 10.7 3500 379 279 367 29 10.9 10.0 3560 349 256 411 *30 10.1 10.2 3700 321 287 452 *31 12.8 10.2 2760 503 352 382 *32 10.3 9.7 2600 543 405 423 *33 13.5 11.5 3650 332 309 456 Note: .sup.(1)Sample No. with * indicates a Comparative Example.

Example 3

[0065] MnZn ferrites were produced in the same manner as in Sample 1, except that the compositions were changed as shown in Table 5 and the temperatures in the high-temperature-keeping step were changed as shown in Table 6. The results of the initial permeability i, the volume resistivity , the average crystal grain size, and the core loss Pcv were shown in Table 6. Every MnZn ferrites exhibited high volume resistivity of 8.5 .Math.m or more, however, the MnZn ferrites of Sample No. *40 (Comparative Example) having the average crystal grain size of less than 7 m exhibited core losses of higher than 420 kW/m.sup.3 at 140 C. On the other hand, the all MnZn ferrites of Examples exhibited core losses of 420 kW/m.sup.3 or less.

TABLE-US-00005 TABLE 5 Main Components Sub-Components (mol %) (parts by mass .sup.(2)) Fe.sub.2O.sub.3 ZnO MnO.sup.(1) Co.sub.3O.sub.4 SiO.sub.2 CaCO.sub.3 Ta.sub.2O.sub.5 ZrO.sub.2 Nb.sub.2O.sub.5 53.4 9.2 37.4 0.3 0.009 0.18 0.04 0 0 Note: .sup.(1)Mn.sub.3O.sub.4 was used as a raw material, and the composition was shown as MnO. Note: .sup.(2) The amount, per 100 parts by mass of calcined powder consisting of the main component.

TABLE-US-00006 TABLE 6 Temper- ature kept Volume Average at High- Resis- Crystal Initial Sam- Temper- tivity Grain Perme- Core Loss Pcv (kW/m.sup.3) ple ature Size ability 23 100 140 No..sup.(1) ( C.) ( .Math. m) (m) (i) C. C. C. 34 1335 12.2 13 3020 388 268 336 35 1310 14.9 10.2 2970 371 265 325 36 1295 16.8 9.3 2900 349 262 332 37 1280 18.1 8.4 2870 345 267 337 38 1265 18.7 7.4 2830 340 260 345 39 1250 18.9 7.0 2780 340 277 382 *40 1235 19.5 6.3 2690 334 291 423 Note: .sup.(1)Sample No. with * indicates a Comparative Example.

[0066] MnZn ferrites were produced in the same manner as in Sample 1, except that the compositions were changed as shown in Table 5 and the ranges of the slow cooling step were changed as shown in Table 7. The results of the initial permeability i, the volume resistivity , and the core loss Pcv were shown in Table 7. By setting the slow cooling temperature within the range specified by the method of the present invention, MnZn ferrite having core losses of 420 kW/m.sup.3 or less from low temperature (23 C.) to high temperature (140 C.) could be obtained.

TABLE-US-00007 TABLE 7 Temperature Volume Initial Sam- Range of Resistivity Perme- ple Slow Cooling ability Core Loss Pcv (kW/m.sup.3) No..sup.(1) ( C.) ( .Math. m) (i) 23 C. 100 C. 140 C. *41 8.2 3140 360 326 450 *42 1300-1250 12.6 2760 462 385 430 43 1250-1200 14.9 2970 371 265 325 44 1200-1150 18.5 2960 385 275 320 45 1150-1100 20.3 2850 395 298 345 *46 1100-1050 22.3 2610 430 320 350 *47 1050-1000 22.5 2320 460 340 370 *48 1300-1200 14.2 2720 442 378 425 *49 1150-1050 22.5 2530 445 338 370 Note: .sup.(1)Sample No. with * indicates a Comparative Example.

Example 5

[0067] MnZn ferrites were produced in the same manner as in Sample 1, except that the compositions of the main components were changed as shown in Table 5 and the cooling speed in the slow cooling step were changed as shown in Table 8. The results of the initial permeability i, the volume resistivity , and the core loss Pcv were shown in Table 8. By setting the cooling speed within the range specified by the method of the present invention, MnZn ferrite having core losses of 420 kW/m.sup.3 or less from low temperature (23 C.) to high temperature (140 C.) could be obtained.

TABLE-US-00008 TABLE 8 Cooling Volume Initial Sam- Speed in Slow Resistivity Perme- ple Cooling Step ability Core Loss Pcv (kW/m.sup.3) No..sup.(1) ( C./hours) ( .Math. m) (i) 23 C. 100 C. 140 C. *50 100 8.2 3140 360 326 450 *51 50 10.0 3110 366 317 431 52 20 13.5 3050 370 276 340 53 10 14.9 2970 371 265 325 54 5 16.3 2930 384 276 310 55 3 19.8 2850 395 275 315 Note: .sup.(1)Sample No. with * indicates a Comparative Example.

[0068] As described above, according to the method for producing MnZn ferrite of the present invention, low core loss can be achieved in a wide temperature range.