MANGANESE-ZINC FERRITE

Abstract: The initial permeability of a manganese-zinc ferrite in the high-frequency zone of 10-500 Khz is improved by mixing a manganese-zinc ferrite comprising 50-56 mole % of Fe₂O₃, 22-39 mole % of Mno and 8-25 mole % of Zno with at most 800 ppm (in terms of Bi₂O₃) of bismuth oxide component and at most 1,200 ppm (in terms of MoO₃) of molybdenum oxide component and firing the mixture. The mean crystal grain diameter is adjusted to 5-50 µm or less.

Description: FIELD OF THE INVENTION This invention relates to manganese-zinc system ferrite. BACKGROUND ART Among manganese-zinc system ferrites, high magnetic permeability ferrites comprising 50 to 56 mol% of Fe 2 O 3 , 22 to 39 mol% of MnO, and 8 to 25 mol% of ZnO, for example, have been widely used as magnetic cores for high-frequency band transformers and magnetic cores for line filters and noise filters of various communication equipment or the like. These high magnetic permeability ferrites are now required to have high magnetic permeability particularly in the wide-frequency range of about 100 kHz to about 500 kHz. Manganese-zinc system polycrystalline ferrites have the general tendency that initial magnetic permeability µi increases with a crystal grain size. One exemplary substance known to promote grain growth is Bi 2 O 3 , which is practically used in Japanese Patent Publication (JP-B) No. 29439/1977 to establish a mean grain size of 50 µm or greater for improving µi. However, a greater grain size concurrently entails increased losses, especially at high frequencies, due to which only lower frequencies are available. Then JP-B No. 49079/1976 attempted to improve high-frequency response by adding Bi 2 O 3 and CaO in admixture. This improvement, however, is effective only at frequencies of up to about 100 kHz, and this material is not practically acceptable as the high magnetic permeability material for line filters or the like which is required to have high µi at higher frequencies, for example, over the entire range of from 10 to 500 kHz. DISCLOSURE OF THE INVENTION An object of the present invention is to provide a manganese-zinc system ferrite having high magnetic permeability in the high-frequency range, that is, high magnetic permeability over the entire range of from 10 to 500 kHz. This and other objects are achieved by the present invention which is defined below as (1) to (7). (1) A manganese-zinc system ferrite comprising 50 to 56 mol% calculated as Fe 2 O 3 of iron oxide, 22 to 39 mol% calculated as MnO of manganese oxide, and 8 to 25 mol% calculated as ZnO of zinc oxide, the manganese-zinc system ferrite having been sintered after adding thereto up to 800 ppm calculated as Bi 2 O 3 of a bismuth oxide component and up to 1,200 ppm calculated as MoO 3 of a molybdenum oxide component. (2) The manganese-zinc system ferrite of (1) having an average grain diameter of 5 to 50 µm. (3) A manganese-zinc system ferrite comprising 50 to 56 mol% calculated as Fe 2 O 3 of iron oxide, 22 to 39 mol% calculated as MnO of manganese oxide, and 8 to 25 mol% calculated as ZnO of zinc oxide, the manganese-zinc system ferrite having an average grain diameter of 5 to 45 µm. (4) The manganese-zinc system ferrite of any one of (1) to (3) which further includes 50 to 500 ppm calculated as CaO of calcium oxide. (5) The manganese-zinc system ferrite of any one of (1) to (4) having an initial magnetic permeability of at least 8,000, at least 8,000, and at least 2,000 at 10 kHz, 100 kHz, and 500 kHz, respectively. (6) The manganese-zinc system ferrite of any one of (1) to (4) having an initial magnetic permeability of at least 9,000, at least 9,000, and at least 3,000 at 10 kHz, 100 kHz, and 500 kHz, respectively. (7) The manganese-zinc system ferrite of any one of (1) to (6) wherein during the sintering step, an oxygen partial pressure of at least 25% is maintained at least upon heating from a temperature of at least 1,000°C to the holding temperature. EFFECT AND ADVANTAGES OF THE INVENTION At frequencies of up to 100 kHz, eddy-current losses can be reduced by increasing the electric resistance at grain boundaries. However, grain boundaries switch to act as a capacitance at high frequencies of from 100 to 500 kHz, which means that an increase in electric resistance at grain boundaries does not lead to an increase in the resistance of the entire core, failing to reduce the loss. This is the reason for the drop of µi at high frequencies in the prior art. Presuming that the grain size must be large and distributed uniformly before the loss at 500 kHz can be reduced and µi at 100 to 500 kHz can be increased, the inventors have found that use of MoO 3 in combination with Bi 2 O 3 is effective to this end and thereby succeeded in significantly improving µi at 10 to 500 kHz. It is believed that while MoO 3 partially sublimates during firing, it is effective for suppressing the growth of crystal grains probably due to this sublimation, thus yielding uniform crystal grains. In the prior art, JP-B No. 28078/1974 proposed in conjunction with magnetic head material to improve the frequency response of µi at about 1 MHz by adding MoO 3 thereto and controlling firing conditions (two stage sintering in air and nitrogen). However, no example of using MoO 3 in high magnetic permeability material is found in the prior art. This is because it was believed that high magnetic permeability materials can be produced by increasing grain size. In practice, however, if MoO 3 is singly added to high magnetic permeability material as in the above-referred publication, there occurs a substantial drop of µi at about 100 kHz or lower. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph under optical microscope of a manganese-zinc system ferrite according to the invention (Sample No. 22 in Example 3). FIG. 2 is a photograph under optical microscope of a manganese-zinc system ferrite according to the invention (Sample No. 23 in Example 3). FIG. 3 is a photograph under optical microscope of a prior art manganese-zinc system ferrite (sample No. 21 in Example 3). FIG. 4 is a graph showing the relationship of initial magnetic permeability µ 100 at frequency 100 kHz, µ 100 range R 100 , and loss factor tanδ/µ 100 to the oxygen partial pressure P O2 at the holding temperature during firing. BEST MODE FOR CARRYING THE INVENTION INTO EFFECT Below the illustrative construction of the present invention is described in detail. The manganese-zinc system ferrite as sintered according to the invention contains iron oxide, manganese oxide, and zinc oxide as major components. These major components are present in amounts of about 50 to 56 mol% calculated as Fe 2 O 3 , about 22 to 39 mol% calculated as MnO, and about 8 to 25 mol% calculated as ZnO. High magnetic permeability is not available outside this range. Also the manganese-zinc system ferrite of the invention may contain calcium oxide and silicon dioxide as subordinate components. These subordinate components are present in amounts of about 50 to 500 ppm, especially about 100 to 300 ppm calculated as CaO and about 50 to 150 ppm calculated as SiO 2 . It is to be noted that CaO and SiO 2 are generally present at grain boundaries. The ferrite of the invention defined above further contains bismuth oxide and molybdenum oxide, especially in the form of Bi 2 O 3 and MoO 3 . In this regard, since part of the bismuth and molybdenum oxide components added, especially molybdenum oxide component can evaporate or sublimate upon firing, the contents of bismuth oxide and molybdenum oxide in the ferrite do not coincide with the addition amounts in some cases. More specifically, the content of bismuth oxide is about 30 to 100% by weight of the addition amount calculated as Bi 2 O 3 and the content of molybdenum oxide is about 30 to 100% by weight, especially about 40 to 60% by weight of the addition amount calculated as MoO 3 . It is to be noted that the ferrite of the invention may further contain at least one of niobium oxide, indium oxide, vanadium oxide, etc. if necessary. These oxides are preferably present in a total amount of 0 to about 3,000 ppm, provided that they are calculated as Nb 2 O 5 , In 2 O 3 , and V 2 O 5 . The ferrite of the invention containing the above-mentioned components has an average grain diameter of 5 to 50 µm. The high-frequency response of µi would deteriorate whether the mean grain size is larger or smaller. Better results are obtained when the mean grain size is 5 to 45 µm, especially 10 to 45 µm, further preferably 15 to 35 µm, most preferably 20 to 30 µm. It is to be noted that the mean grain size is determined by etching a mirror finished surface with acid, observing polycrystalline grains under an optical microscope, and averaging the mean diameters of circles converted from the grains. When the mean grain size is large and distributed uniformly and the coefficient of variation of grain size CV (standard deviation of grain size divided by mean grain size) is small, the high-frequency response of µi is improved to accomplish an initial magnetic permeability µi of at least 8,000, especially at least 9,000, further preferably at least 9,500, for example, about 9,500 to 13,000 at a frequency of 100 kHz and 25°C and an initial magnetic permeability µi of at least 2,000, especially at least 3,000, further preferably at least 3,500, for example, about 3,500 to 6,000 at a frequency of 500 kHz and 25°C. The manganese-zinc system ferrite of the invention is prepared by first furnishing a mixture of a conventional iron oxide component, manganese oxide component and zinc oxide component as major components. These major components are mixed so as to provide the above-defined proportion in the final composition of ferrite, with the mixture being used as a source material. Source materials added thereto as subordinate components are calcium oxide or a compound which will convert to calcium oxide upon firing such as calcium carbonate, silicon oxide or a compound which will convert to silicon oxide upon firing, and the like. The source materials for subordinate components are added so as to provide the above-defined proportion in the final composition of magnetic material. Further, a bismuth oxide component and a molybdenum oxide component are added. The bismuth oxide component used may be Bi 2 O 3 or instead Bi 2 (SO 4 ) 3 , etc., with the Bi 2 O 3 being preferred. The amount of bismuth oxide component added is up to 800 ppm, especially up to 600 ppm, preferably 150 to 450 ppm calculated as Bi 2 O 3 . Addition amounts in excess of this range rather result in a lowering of µi. The molybdenum oxide component used may be MoO 3 or instead MoCl 3 , etc., with the MoO 3 or the like being preferred. The amount of molybdenum oxide component added is up to 1,200 ppm, especially up to 1,000 ppm, preferably 100 to 1,000 ppm calculated as MoO 3 . Addition amounts in excess of this range rather result in a lowering of µi. It is to be noted that one or more of niobium oxide, indium oxide, and vanadium oxide are added to the source mixture if necessary. After the major components and additive trace components are mixed in this way, an appropriate binder such as polyvinyl alcohol is added thereto in a small amount, for example, 0.1 to 1.0% by weight, and the mixture is granulated into granules having a diameter of about 80 to 200 µm through a spray dryer, etc. and molded. Next, the compact is fired, for example, by slowly heating it to a desired sintering temperature at a heating rate of about 50 to 300°C/hr. in an atmosphere having a controlled oxygen concentration, with sintering being completed at that temperature. In this regard, firing is generally carried out by holding at the desired temperature of at least 1,250°C, especially in the range of 1,300 to 1,400°C for about 4 to 5 hours. After the completion of sintering, the compact is preferably cooled at a cooling rate of about 50 to 300°C/hr. in an atmosphere having a controlled oxygen concentration. During such sintering, it is preferred to maintain an oxygen partial pressure of at least 25%, especially at least 30%, more preferably 30 to 100% at least upon heating from a temperature of at least 1,000°C to the holding temperature, more preferably in a temperature range of 1,000°C and higher. Crystal grains with a relatively large diameter can be created by providing a longer holding time at a higher firing temperature. However, when a combined system of bismuth oxide and molybdenum oxide is used, oxygen firing enables firing at a relatively lower temperature for a relatively short time so that µi at frequencies from 10 kHz to high 500 kHz can be increased without reducing the life of the firing furnace or lowering productivity. If it is attempted to increase µi by providing a longer holding time at a higher firing temperature, there generally is a variation of µi and an increase of loss factor. The present invention restrains such a variation of µi and an increase of loss factor by using a combined system of bismuth and molybdenum and effecting oxygen firing. INDUSTRIAL APPLICABILITY The manganese-zinc system ferrite of the present invention has high magnetic permeability in a high-frequency band of 100 kHz to 500 kHz, especially at 500 kHz and at the same time, high magnetic permeability on a low-frequency side ranging from frequency 10 kHz to 100 kHz. EXAMPLE Examples of the invention are given below by way of illustration. Example 1 To a mixture of 26 mol% of MnO, 21 mol% of ZnO and 53 mol% of Fe 2 O 3 as major components were added CaCO 3 (200 ppm calculated as CaO) and SiO 2 (100 ppm) as subordinate components. To this mixture, Bi 2 O 3 and MoO 3 were added as shown in Table 1, obtaining sample Nos. 1 to 5. Each of the mixtures, after mixing, was combined with a binder, granulated to a mean particle size of 150 µm through a spray dryer, and compacted. The compact was heated in a nitrogen atmosphere having an oxygen partial pressure P O2 of 0.5%, held at 1,270°C for 3 hours for sintering, and then cooled in an atmosphere having a controlled oxygen partial pressure, obtaining a toroidal core of 31 mm in outer diameter, 19 mm in inner diameter and 8 mm in height. The final composition was subject to fluorescent X-ray analysis to find that the contents of the major components, Ca and Si substantially corresponded to the source material composition and the contents of bismuth oxide and molybdenum oxide were 40 to 60% by weight of their addition amounts. The thus obtained toroidal cores were measured for initial magnetic permeability µi at frequencies 10 kHz, 100 kHz, and 500 kHz and 25°C. Note that an impedance analyzer was used for the measurement of permeability. The results are shown in Table 1. Sample Nos. 4 and 5 within the scope of the invention had a mean grain size of 20 to 35 µm. Sample No. Addition amount (ppm) µi Bi 2 O3 MoO 3 10 kHz 100 kHz 500 kHz 1 (comparison) 0 0 6300 5900 1800 2 (comparison) 0 200 5300 5000 2700 3 (comparison) 200 0 7900 7700 1700 4 (invention) 200 200 8600 8700 2200 5 (invention) 400 200 9600 9800 2300 The effectiveness of the invention is evident from the data shown in Table 1. More particularly, it is evident that the invention presents large and uniform crystal grains and significantly increased values of µi at 10 to 500 kHz. Example 2 Sample Nos. 11 to 14 were obtained by the procedure of Example 1 except that the holding step for sintering was changed to 1,380°C and 5 hours, the atmosphere used until the holding step was ambient air, and subsequent cooling was in an atmosphere having a controlled oxygen partial pressure. The results are shown in Table 2. Sample No. Addition amount (ppm) µi Bi 2 O3 MoO 3 10 kHz 100 kHz 500 kHz 11 (comparison) 0 0 7200 7400 3600 12 (comparison) 0 200 6900 7200 3800 13 (comparison) 200 0 7800 8100 3300 14 (invention) 200 200 8500 9100 4000 It is to be noted that sample No. 14 within the scope of the invention had a grain size of 20 to 35 µm and the contents of the respective components were substantially equal to those of sample No. 4 in Example 1. It is evident from Table 2 that the frequency response of µi is improved by increasing P O2 . Example 3 Sample Nos. 21 to 23 were obtained by the procedure of Example 2 except that P O2 = 100% was maintained until the holding step. The results are shown in Table 3. Sample No. Addition amount (ppm) Mean grain size (µm) µi Bi 2 O3 MoO 3 10 kHz 100 kHz 500 kHz 21 (comparison) 200 0 >50 9800 10200 2900 22 (invention) 200 500 27 9900 10600 4100 23 (invention) 200 1000 22 10500 11200 3900 It is evident from Table 3 that a better frequency response of µi is obtained by further increasing P O2 . FIGS. 1 and 2 are photographs under an optical microscope (300X) of sample Nos. 22 and 23 and FIG. 3 is a photograph under an optical microscope (same magnification) of sample No. 21. The mean grain sizes determined from these photographs are also shown in Table 3. Example 4 The compositions shown in Table 4 were sintered under the sintering conditions of Example 3. Sample No. Addition amount (ppm) µi Bi 2 O3 MoO 3 10 kHz 100 kHz 500 kHz 31 (comparison) 0 0 9000 9200 3400 32 (comparison) 0 400 8400 8800 3800 33 (invention) 200 400 10000 10500 4100 34 (invention) 200 800 10700 11200 3900 Example 5 There was obtained a sample containing 200 ppm of Bi 2 O 3 and 400 ppm of MoO 3 in Example 3 while changing P O2 during the process. The samples were measured for µi at 100 kHz (designated µ 100 ), with a range of µ 100 of total 27 samples designated R 100 and an average of loss factor tanδ/µ 100 being calculated therefrom. Their relationship to P O2 is shown in FIG. 4. In general, if µi is increased by elevating the holding temperature or extending the holding time, then the variation range R of µi increases and tanδ/µi increases, too. In contrast, FIG. 4 indicates that the variation of µi is rather reduced (R 100 is constant) and the loss factor does not increase.

Claims: A manganese-zinc system ferrite comprising 50 to 56 mol % calculated as Fe 2 O 3 of iron oxide, 22 to 39 mol % calculated as MnO of magnesium oxide, and 8 to 25 mol % calculated as ZnO of zinc oxide, said manganese-zinc system ferrite having been sintered after adding thereto 150 to 800 ppm calculated as Bi 2 O 3 of a bismuth oxide component and 100 to 1,200 ppm calculated as MoO 3 of a molybdenum oxide component. The manganese-zinc system ferrite of claim 1 having an average grain diameter of 5 to 50 µm. The manganese-zinc system ferrite of any one of claims 1 and 2 which further includes 50 to 500 ppm calculated as CaO of calcium oxide. The manganese-zinc system ferrite of any one of claims 1 and 2 having an initial magnetic permeability of at least 8,000, at least 8,000, and at least 2,000 at 10 kHz, 100 kHz, and 500 kHz, respectively. The manganese-zinc system ferrite of any one of claims 1 and 2 having an initial magnetic permeability of at least 9,000, at least 9,000, and at least 3,000 at 10 kHz, 100 kHz, and 500 kHz, respectively. The manganese-zinc system ferrite of any one of claims 1 and 2 wherein during said manganese-zinc ferrite system being sintered, an oxygen partial pressure of at least 25% is maintained at least upon heating from a temperature of at least 1,000°C to the sintering temperature. The manganese-zinc system ferrite of claim 1, wherein the amount of said bismuth oxide component added is 150 to 450 ppm, and the amount of said molybdenum oxide component added is 100 to 1,000 ppm.

Inventors: INOUE SHOJI, MATSUKAWA ATSUHITO, INOUE, SHOJI, MATSUKAWA, ATSUHITO
Applicants: TDK CORP, TDK CORPORATION
Application year: 1993
Main classification: C04B35
IPC codes: C01G49, H01F1, C04B35