Composite ferrite material

Abstract

A composite ferrite material is provided. The composite ferrite material obtained from a mixture of a magnetic ferrite powder with high crystallinity, prepared by firing at a prescribed temperature, and a glass powder, having a softening temperature lower than said firing temperature, by heat treatment of said mixture at a temperature which is higher than, or equal to said softening temperature of said glass powder and lower than, or equal to said firing temperature, to effect the binding of said magnetic ferrite powder by said glass material. The composite ferrite material has excellent magnetic characteristics and can be obtained in a form of the desired dimensions with high accuracy.

Description

BACKGROUND OF THE INVENTION

1. Field of the invention:

The present invention relates to a composite ferrite material obtained by consolidating a high-crystallinity magnetic ferrite powder with glass, more particularly to a composite ferrite material which can readily be produced in desired dimensions. The present invention also relates to a method for the preparation of above-mentioned composite ferrite materials.

2. Description of the prior art:

Magnetic ferrite articles are manufactured mainly by the powder metallurgical method. In this method, magnetic ferrite powder is sintered by firing at high temperatures in the following manner.

First, ferric oxide powder, and other metal oxide powders such as nickel oxide, zinc oxide, etc., are mixed in specified proportions in accordance with the characteristics of the desired magnetic article, and subjected to pre-sintering. This pre-sintering results in a certain degree of solid phase reaction at the grain boundaries, and the generation of gas. The material so obtained is then pulverized, and granules of an appropriate size are formed by adding water-soluble resin to consolidate the said powder (this process will hereinafter be referred to as granulation). This granular material is then press-formed and the resulting powder mass is subjected to the final firing in a suitable gaseous atmosphere at a temperature higher than the aforementioned pre-sintering temperature. In this manner, a polycrystalline magnetic ferrite article possessing the desired magnetic properties and mechanical strength is obtained.

FIG. 4 shows the microstructure of such a polycrystalline magnetic ferrite mass obtained by sintering. This sintered magnetic ferrite mass is composed of an aggregate of porous sintered magnetic powder 6 possessing numerous pores 9. Other pores 8 are also present to some extent at the grain boundaries between the grains of said magnetic powder 6.

The temperature at which the pre-sintering is carried out in the aforementioned method is set in the range of 700.degree.-1000.degree. C., wherein a solid phase reaction is initiated at the interfaces of the original raw materials, i.e., ferric oxide, nickel oxide, zinc oxide, etc. The temperature of the final firing, performed in order to attain an adequate degree of sintering, is ordinarily set in the higher range of 1000.degree.-1400.degree. C. The temperature of the final firing that is employed varies according to the composition of raw materials, the condition of pre-sintering, the shapes and grain size of the powder after pre-sintering. The gaseous atmosphere used when firing varies according to the type of magnetic powder product desired, both oxidizing and non-oxidizing atmospheres being employed.

In the aforementioned methods, the powder obtained by pre-sintering is of grain diameter 2-5 .mu.m or less. In the mass formed by compression of this powder, the granules of the said powder are in mutual contact, but considerable gaps still remain between the granules. When the powder mass formed from this pre-sintered powder is heated at a temperature exceeding the pre-sintering temperature (700.degree.-1000.degree. C.), mutual diffusion of the atoms that constitute the granules occurs at the areas of contact between pre-sintered powder granules, and thus sintering proceeds. As sintering progresses, the gaps between the pre-sintered powder granules decrease. As a result, the final firing causes a further densification of the pre-sintered powder mass, ordinarily by a ratio ranging from 10 to 20% and in some cases even higher, which may cause deterioration in the dimensional precision and yield of the final sintered product. In order to obtain final sintered compacts of the desired dimensions, machine finishing processes such as cutting or grinding are necessary.

In general, in order to form sintered articles of uniform composition that does not crack when subjected to abrupt rises or falls of temperature, comparatively gradual elevation and reduction of temperature during the final firing is essential. Consequently, the final firing process ordinarily requires at least half a day, and in some cases may even last for two days.

Considerable research has already been conducted into efforts to improve these defects in ferrite sintering methods. For example, Japanese Laid-Open Patent Publication Nos. 58-135133 and 58-135606, discloses that when a mixture of pre-sintered ferrite powder and glass powder is press-formed, and the resulting mass is fired at an appropriate temperature sufficiently high as to allow sintering of the said magnetic powder, the said glass powder fuses, the magnetic ferrite powder granules are bound by the glass, and as a result the degree of contraction of the ferrite mass becomes relatively small. However, in the above-mentioned process, because the mass made of the powder mixture is fired at a temperature exceeding the temperature of the pre-sintering that is carried out to obtain the pre-sintered ferrite powder, a contraction of several percent occurs. This is due to the fact that, although most of the ferrite powder grains are separated from each other by the fused glass, a solid phase reaction may occur at the interfaces between the ferrite powder grains during the final firing operation.

In general, if sintering is performed in order to obtain the desired characteristics in the manufacture of sintered ferrite articles, then the further the sintering process progresses, the greater the proportion of shrinkage of the said article. In the aforementioned method, if the content of glass powder, is increased in order to suppress shrinkage, then the essential characteristics of the ferrite cannot be adequately manifested in the final product. Sintered ferrite articles are widely used as materials for electronic parts and devices, and therefore ferrite articles which combine high-level functional characteristics with dimensional precision are important desiderata.

SUMMARY OF THE INVENTION

A composite material of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, is obtained from a mixture of a magnetic ferrite powder with high crystallinity, prepared by firing at a prescribed temperature, and a glass powder, having a softening temperature lower than said firing temperature, by heat treatment of said mixture at a temperature which is higher than, or equal to said softening temperature of said glass powder and lower than, or equal to said firing temperature, to effect the binding of said magnetic ferrite powder by said glass material.

A method for the preparation of composite ferrite material of this invention comprises mixing a magnetic ferrite powder with high crystallinity, prepared by firing at a prescribed temperature, and a glass powder with a softening temperature lower than said firing temperature, subjecting said mixture to press-forming, and subjecting said press-formed mass to heat treatment at a temperature which is higher than or equal to the softening temperature of said glass powder and lower than or equal to said firing temperature to fuse said glass powder contained in said mass thereby binding said magnetic ferrite powder with said fused glass.

A method for the preparation of composite ferrite material of this invention comprises mixing a magnetic ferrite powder with high crystallinity, prepared by firing at a prescribed temperature, and a glass powder with a softening temperature lower than said firing temperature, subjecting said mixture to press-forming and simultaneous heat treatment at a temperature which is higher than or equal to the softening temperature of said glass powder, and lower than or equal to, said firing temperature and thereby fusing said glass powder, thus effecting the binding of said magnetic ferrite powder by said fused glass, and firing the obtained mass after said heat treatment at a temperature which is lower than or equal to the firing temperature of said magnetic ferrite powder.

In a preferred embodiment, the magnetic ferrite powder is composed of granules with at least two different size distributions.

In a preferred embodiment, the glass contains zinc oxide.

In a preferred embodiment, the firing temperature is in the range of 1000.degree.-1400.degree. C.

In a preferred embodiment, the temperature of said heat treatment is 800.degree. C. or higher.

In a preferred embodiment, the glass powder is used in an amount of 0.3 to 30% by weight based on the total weight of said glass powder and said magnetic ferrite powder with high crystallinity.

Thus, the invention described herein makes possible the objectives of:

(1) providing a composite ferrite material with excellent magnetic characteristics that can be obtained in a form of the desired dimensions with high accuracy;

(2) providing a composite ferrite material with high electrical resistance, which achieves excellent high frequency characteristics even when magnesium-zinc type ferrite materials with low electrical resistance are used; and

(3) providing a method for producing abovementioned excellent composite ferrite material economically in a short period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows:

FIGS. 1-3 are enlarged schematic illustrations showing the structure of the composite ferrite material of the present invention.

FIG. 4 is an enlarged schematic illustration showing the structure of a conventional sintered ferrite mass.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The magnetic ferrite powder with high crystallinity used in the present invention is prepared by mixing ferric oxide and other metal oxides in the form of MxO (wherein M is a metal of valence n and .times.=2/n) such as NiO, ZnO, etc., heating the mixture at a temperature of 1000.degree. C. or more, preferably in the range of 1000.degree.-1400.degree. C., in order to effect adequate ferritization, and then pulverizing this material. This magnetic ferrite powder with high crystallinity is a ferrimagnetic substance possessing the characteristic spinel crystal structure of ferrite materials. If a soft magnetic ferrite material is desired, then, since a low magnetic coercive force Hc is desirable in the aforementioned magnetic ferrite powder, the grain size of the powder should be large. However, if the grain size is unduly large, then the packing density of the powder mass will be low, therefore magnetic ferrite powder with high crystallinity of grain diameter 100-200 .mu.m is ordinarily used. When hard ferrite materials are prepared, high coercive force Hc and large energy products are desirable: in order to achieve this, granules of diameter allowing the formation of particles of single magnetic domain are desirable. When magnetic ferrite powders with high crystallinity having two or more different grain size distributions, the smaller grains can fill the voids in the magnetic mass. Magnetic powders with grain diameters of 5 .mu.m or less are effective for the smaller grains.

The glass composing the glass powder used in the present invention has a softening temperature that is lower than the firing temperature for preparing the aforementioned magnetic ferrite powder with high crystallinity. In order that the composite ferrite material so obtained can be used at comparatively high temperatures, the softening temperature of the glass should desirably be at least 300.degree. C. Moreover, since the firing temperature of the magnetic ferrite powder is 700.degree. C. or higher, and the heat-resistant temperature of metal molds is ordinarily in the order of 700.degree. C., the said softening temperature should desirably be lower than 700.degree. C. More specifically, glass with a softening temperature not exceeding 650.degree. C. is used. Furthermore, this glass should desirably contain zinc oxide in a proportion not exceeding 30% by weight, preferably 1-30% by weight. If zinc oxide in a proportion between 1-30% by weight is contained in the glass, then magnetic ferrite articles with low dielectric losses are obtained.

The grain diameter of this glass powder should desirably be 10 .mu.m or less. The amount of this glass powder should desirably be from 0.3 to 30% by weight, based on the total weight of the aforementioned magnetic ferrite powder and the said glass powder. If the amount of glass powder is less than 0.3% by weight, then the effect of the glass in binding the magnetic powder granules is insufficient, and the magnetic ferrite powder article so obtained will be of low mechanical strength. Conversely, if the amount of glass powder exceeds 30% by weight, then the magnetic properties of the ferrite powder will not be adequately manifested in the product.

FIG. 1 shows an enlarged schematic illustration showing the structure of the composite ferrite material of the first embodiment of the present invention. This composite ferrite material is obtained by binding the magnetic ferrite powder with high crystallinity 1 with the glass material 2, which softens and fuses at a temperature lower than the firing temperature for the ferrite powder. For example, first, the aforementioned magnetic ferrite powder with high crystallinity 1 and the aforementioned glass powder are thoroughly mixed and granulated. This is then subjected to press-forming, and heated at a temperature not exceeding the aforementioned firing temperature but at least as high as the melting temperature of the said glass powder. This heat treatment is performed in order to melt the glass powder and allow the molten glass to permeate the gaps between the magnetic powder granules. The time required for heat treatment that includes the time required for elevation of the temperature to the prescribed value, the period of maintenance of the said temperature and the time required for subsequent temperature reduction, can sufficiently be 3 hours or less.

The softened glass permeates the gaps between the magnetic powder granules and binds the said granules together. As shown in FIG. 1, even after heat treatment, voids 4 still exist within the solidified glass. The void ratio is almost the same as before heating, and consequently the degree of shrinkage is extremely low. If the temperature of heat treatment is at least 800.degree. C., then the binding effect of the glass is increased, and a composite ferrite material with excellent magnetic properties is obtained.

FIG. 2 shows an enlarged schematic illustration showing the structure of the composite ferrite material of the second embodiment of the present invention. This composite ferrite material is obtained by applying pressure to bind the magnetic ferrite powder with high crystallinity 1 with the glass material 2, which softens and fuses at a temperature lower than the firing temperature for obtaining the ferrite powder. More specifically, first, the magnetic ferrite powder with high crystallinity 1 and the aforementioned glass material are thoroughly mixed and granulated. Then, during press forming, this material is heated at a temperature which is higher than or equal to the softening temperature of the glass powder, and lower than or equal to the aforementioned firing temperature, thereby softening and fusing the glass powder. The temperature used for this heat treatment is relatively low but sufficient to ensure the fusion of the glass powder and the ready permeation of the fused glass into the gaps between the magnetic powder granules. For example, a temperature that is higher than the softening temperature of the glass powder by 20.degree.-30.degree. C. is employed. Since the molten glass permeates the gaps between the magnetic powder granules and pressure is applied simultaneously, the voids between the magnetic powder granules are almost completely eliminated, and a high density compact with sporadic pores 3 is formed. The high density mass formed in this manner by binding the magnetic powder granules with glass are then heat-treated at a temperature lower than the firing temperature used when preparing the aforementioned magnetic powder with high crystallinity. The temperature used for this heat treatment is comparatively high, for example, a temperature that is lower than the firing temperature for preparing the magnetic ferrite powder with high crystallinity by 50.degree.-100.degree. C. is employed.

FIG. 3 shows an enlarged schematic illustration showing the structure of the composite ferrite material of the third embodiment of the present invention. This embodiment is almost identical with the first embodiment, however, in the present case, at least two varieties of magnetic ferrite powder with high crystallinity having different grain size distributions are used. The grain size of the magnetic powder with the smaller granules should desirably be 5 .mu.m or less, this magnetic powder being used to increase the packing density of the mass. This composite ferrite material can be obtained, for example, by the following procedure. First, the aforementioned two or more varieties of magnetic ferrite powder with high crystallinity, in the present case 1 and 5, are thoroughly mixed and granulated. This is then subjected to press-forming, and heated at a temperature that is higher than or equal to the softening temperature of the glass powder and lower than or equal to the firing temperature for preparing the magnetic ferrite powder with high crystallinity, thereby softening and fusing the aforementioned glass powder. The heating temperature and time in the present case are the same as in the aforementioned first embodiment. The softened glass permeates the gaps between the magnetic powder granules and binds the said granules together. In the first embodiment, as shown in FIG. 1, voids 4 are present within the solidified glass. However, in the present embodiment, the larger voids between the magnetic powder granules are filled with the granules of the smaller grain-sized magnetic powder, thereby obtaining a mass of higher density than the type produced in the first embodiment.

Magnetic ferrite powders with high crystallinity which are sufficiently ferritized by firing are used in the above-mentioned methods of first to third embodiments of the present invention. Therefore, when a powder mass made of the said magnetic ferrite powder and glass powder is subjected to heat treatment at a temperature which is higher than or equal to the softening temperature of the glass powder and lower than or equal to the firing temperature, no further solid phase reaction occurs between the magnetic ferrite powder granules, and consequently the volume of the final mass is almost undiminished. Moreover, since the magnetic powder granules are bound together by the fused glass, masses of high strength are obtained. The aforementioned heating temperature is lower than the firing temperature used for conventional types of ferrite articles, and moreover, this heating is completed in a short time, hence, the production cost is low. Thus, ferrite articles of high dimensional precision can be easily and economically produced. Furthermore, since the ferrite articles contain glass, high electrical resistance can be obtained even when magnesium-zinc type ferrite materials with low resistivity are used. Therefore, excellent high frequency characteristics are obtained even for the soft type of ferrite articles which are necessary to reduce eddy current losses. The composite ferrite materials of the present invention are therefore suitable for wide applications in various electronic parts and other industrial uses.

The present invention will be described in greater detail with reference to the following examples.

EXAMPLES 1-7

A mixed powder composed of ferric oxide powder, nickel oxide powder and zinc oxide powder mixed in the molar ratio of 50:18:32 was fired at 1320.degree. C. for 6 hours, and this mixture was then pulverized, obtaining a nickel-zinc soft-type magnetic ferrite powder with high crystallinity, the ferrite powder particles having a mean grain diameter of 70 .mu.m. An X-ray analysis of this powder revealed the sharp spinel diffraction lines characteristic of soft ferrite, and demonstrated that this was a magnetic powder with extremely high crystallinity.

To this magnetic ferrite powder, alkali-free lead borosilicate glass powder with mean grain diameter of 1 .mu.m and softening point (Td) of 370.degree. C. was then added in an amount shown in Table 1 (the value in Table 1 shows % by weight of the glass powder based on the total weight of the magnetic ferrite powder and the glass powder), and the powder was mixed and granulated. The mixed powder was then formed under a pressure of 3 ton/cm.sup.2, thereby preparing an annular mass with inner diameter 7 mm, outer diameter 12 mm and thickness 3 mm.

The mass was then placed in an electric furnace and heat treated in air at 1200.degree. C. for 60 minutes, thereby obtaining glass-bonded annular ferrite core.

The value of the initial magnetic permeability, saturation magnetic flux density, percentage of shrinkage and tensile strength of the core were measured by the following methods. The results of this measurements are shown in Table 1.

Initial permeability was measured in accordance with JIS C2561 by the following procedure. First, a layer of insulating tape was formed by winding the tape onto the ferrite core, after which a layer of insulated copper wire 0.26 mm in diameter was formed by winding the wire around the entire circumference of the core. Next, the self-inductance of this specimen was measured with a Maxwell bridge at a magnetic field strength not exceeding 0.3 .ANG./m, and the initial magnetic permeability at a frequency of 1 MHz was calculated from the results of this measurement.

Saturation magnetic flux density was measured in accordance with JIS C2561 in a 10 Oe magnetic field, using a self-recording flux meter.

The percentage of shrinkage was calculated from measurements of the outer diameter of the annular mass prior to heat treatment and the ferrite core obtained after the heat treatment.

Tensile strength was measured in accordance with JIS C2564 as follows. First, two fine wires were passed through the annular ferrite core, the two ends of one of these wires were fixed at a single point, the two ends of the other wire were placed together and subjected to traction at a velocity not exceeding 5 mm/min, and the strength was determined from the tensile load at the instant when the specimen broke.

COMPARATIVE EXAMPLE 1

The same procedure was repeated as in Example 1, except that glass powder was not used. The physical properties of the annular ferrite core so obtained are shown in Table 1, along with the corresponding results for the Comparative Examples 2 and 3 to be described below.

COMPARATIVE EXAMPLE 2

A mixed granulated powder with the same composition as that used in Comparative Example 1 was fired at 1000.degree. C. for 2 hours followed by pulverization to a grain diameter of 2-5 .mu.m. This powder was granulated, and using this material, an annular powder mass was prepared in the same manner as in Example 1.

This mass was placed in an electric furnace, fired in air at 1300.degree. C. for 3 hours and then slowly cooled, thus obtaining an annular nickel-zinc sintered ferrite core.

COMPARATIVE EXAMPLE 3

The same type of glass powder as used in Example 1 was added in a proportion of 5% by weight to the same type of pre-sintered powder as used in Comparative Example 2. After mixing and granulation, an annular mass was prepared from this material by the same procedure as used in Example 1. The mass obtained was then placed in an electric furnace and heat-treated in air at 1200.degree. C. for 60 minutes, thus obtaining an annular ferrite core. Table 1.

TABLE 1__________________________________________________________________________ Firing temperature and time for Temperature and Initial Saturation preparing magnetic time for magnetic magnetic Tensile Amount of glass ferrite powder heat treatment Density permeability flux density Shrinkage strength (wt %) (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 1 0.5 1320/6 1200/1 3.8 200 3100 0 4Example 2 1 1320/6 1200/1 3.8 220 3140 0 5Example 3 3 1320/6 1200/1 3.9 280 3200 0 7Example 4 5 1320/6 1200/1 3.9 270 3180 0.1 9Example 5 10 1320/6 1200/1 4.0 260 3160 0.7 15Example 6 30 1320/6 1200/1 4.1 180 3080 7.1 17Example 7 40 1320/6 1200/1 4.2 150 3040 12.5 18Comparative 0 1320/6 1200/1 3.8 120 3040 0 2Example 1Comparative 0 1000/2 1300/3 4.9 830 3900 18.7 18Example 2Comparative 5 1000/2 1200/1 4.3 640 3800 19.0 20 orExample 3 more__________________________________________________________________________

EXAMPLES 8-12

The same procedure was repeated as in Example 4, except that the temperature for heat treatment of the mass was varied as shown in Table 2. The physical properties of the annular ferrite core so obtained are shown in Table 2, along with the corresponding results for the Example 13 described below.

EXAMPLE 13

The same procedure was repeated as in Example 4, except that alkali-free lead borosilicate glass powder with softening temperature (Td) of 700.degree. C. was used in place of the previously mentioned alkali-free lead borosilicate glass powder with softening point of 370.degree. C.

TABLE 2__________________________________________________________________________ Firing temperature and time for Temperature and Initial Saturation preparing magnetic time for magnetic magnetic Tensile Amount of glass ferrite powder heat treatment Density permeability flux density Shrinkage strength (wt %) (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 8 5 1320/6 1300/1 3.9 460 3200 3.0 10Example 9 5 1320/6 1000/1 3.9 120 3020 0 8Example 10 5 1320/6 800/1 3.9 70 2980 0 7Example 11 5 1320/6 600/1 3.9 30 2850 0 5Example 12 5 1320/6 450/1 3.9 25 2800 0 4Example 13 5 1320/6 1200/1 3.9 200 3100 0.5 8__________________________________________________________________________

EXAMPLE 14

A mixed powder composed of barium oxide powder and ferric oxide powder mixed in a molar ratio of 1:6 was fired at 1300.degree. C. for 2 hours, after which the mixture was pulverized, thus obtaining a hard-type magnetic barium ferrite powder with high crystallinity, the ferrite powder particles having a mean grain diameter of 1 .mu.m.

To this magnetic barium ferrite powder, 5% by weight of alkali-free lead borosilicate glass powder with mean grain diameter of 1 .mu.m and softening point of 370.degree. C. was added. After mixing and granulation, this material was press-formed under a pressure of 3 ton/cm.sup.2, thereby preparing an annular mass with an inner diameter 7 mm, an outer diameter 12 mm and a thickness 3 mm.

The mass was then placed in an electric furnace and heat-treated in air at 1200.degree. C. for 30 minutes, thus obtaining an annular glass-bonded ferrite core. The dimensions of this core was almost identical with those of the original powder mass. The physical properties of the barium ferrite core obtained in this manner are shown in Table 3, along with the corresponding results for the Comparative Example 4 described below.

COMPARATIVE EXAMPLE 4

A mixed powder with the same composition as used in Example 14 was pre-sintered at 1000.degree. C. for 1 hour. After pulverization to a grain diameter of 0.5 .mu.m and granulation, an annular powder mass was prepared from this material in the same manner as in Example 14.

The mass so obtained was placed in an electric furnace, fired in air at 1250.degree. C. for 3 hours and then slowly cooled, thus obtaining an annular sintered barium ferrite core.

TABLE 3______________________________________ Maximum energy Shrink- Tensile Density product age strength (g/cm.sup.3) (BH).sub.max MGOe (%) (kg/m.sup.2)______________________________________Example 4.2 2.0 1.5 1014Comparative 4.8 2.4 10.5 20Example 4______________________________________

EXAMPLES 15-21

A mixed powder composed of ferric oxide powder, nickel oxide powder and zinc oxide powder mixed in a molar ratio of 50:18:32 was fired at 1320.degree. C. for 6 hours, after which the mixture was pulverized, thus obtaining a soft-type nickel-zinc magnetic ferrite powder with high crystallinity, the ferrite powder particles having a mean grain diameter of 50-100 .mu.m.

To this magnetic ferrite powder, alkali-free lead borosilicate glass powder with mean grain diameter of 1 .mu.m and softening point (Td) of 370.degree. C. was added in an amount as shown in Table 4. After mixing and granulation, a specified amount of this mixed powder was packed into a stellite mold and hot pressed for 2 minutes at 420.degree. C. in air under a pressure of 3 ton/cm.sup.2, thereby preparing an annular mass with an inner diameter of 7 mm, an outer diameter 12 mm and a thickness 3 mm.

The mass so obtained was then placed in an electric furnace and heat-treated in air at 1200.degree. C. for 60 minutes, thus obtaining an annular glass-bonded ferrite core.

The properties of the ferrite core are shown in Table 4, along with the corresponding results for Example 22 and the Comparative Example 5 described below.

COMPARATIVE EXAMPLE 5

The same procedure was repeated as in Example 15, except that glass powder was not used and heat was not applied when the annular mass was formed.

EXAMPLE 22

The same procedure was repeated as in Example 18, except that alkali-free lead borosilicate glass powder with softening temperature (Td) of 700.degree. C. was used in place of the previously mentioned alkali-free lead borosilicate glass powder with softening temperature of 370.degree. C., and that the heating temperature used when the annular mass was formed was 700.degree. C. in the present case. When the heating temperature was set at 800.degree. C., the core so formed could not be removed from the stellite mold due to the deformation of the mold.

TABLE 4__________________________________________________________________________ Firing temperature and time for Temperature and Initial SaturationAmount Heating preparing magnetic time for magnetic magnetic Shrink- Tensileof glass condi- ferrite powder heat treatment Density permeability flux density age strength(wt %) tions (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 15 0.5 A 1320/6 1200/1 4.0 260 3100 0.1 9Example 16 11 A 1320/6 1200/1 4.0 360 3150 0.2 12Example 17 3 A 1320/6 1200/1 4.1 390 3200 0.2 16Example 18 5 A 1320/6 1200/1 4.2 380 3190 0.2 20Example 19 10 A 1320/6 1200/1 4.2 330 3170 0.7 20 or moreExample 20 30 A 1320/6 1200/1 4.2 220 3070 5.0 20 or moreExample 21 40 A 1320/6 1200/1 4.2 180 3040 11.5 20 or moreExample 22 5 A 1320/6 1200/1 4.1 370 3180 0.2 20Comparative 0 B 1320/6 1200/1 3.8 120 3040 0 2Example 5__________________________________________________________________________ A: Hotpressed in air at 420.degree. C. for 2 minutes under a pressure of ton/cm.sup.2, and fired in air. B: Pressed at room temperature under a pressure of 3 ton/cm.sup.2, and fired in air.

EXAMPLES 23-27

The same procedure was repeated as in Example 18, except that the temperature used in the heat treatment of the mass was varied as shown in Table 5. The physical properties of the annular ferrite core obtained in this manner are shown in Table 5.

TABLE 5__________________________________________________________________________ Firing temperature and time for Temperature and Initial SaturationAmount Heating preparing magnetic time for magnetic magnetic Shrink- Tensileof glass condi- ferrite powder heat treatment Density permeability flux density age strength(wt %) tions (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 23 5 A 1320/6 1300/1 4.2 490 3210 4.5 20 or moreExample 24 5 A 1320/6 1000/1 4.2 150 3040 0 13Example 25 5 A 1320/6 800/1 4.1 100 2980 0 9Example 26 5 A 1320/6 600/1 4.1 50 2850 0 6Example 27 5 A 1320/6 450/1 4.1 30 2800 0 5__________________________________________________________________________ A: Hotpressed in air at 420.degree. C. for 2 minutes under a pressure of ton/cm.sup.2, and filled in air.

EXAMPLES 28-36

Using a mixed powder composed of ferric oxide powder, nickel oxide powder, zinc oxide powder and cupric oxide powder mixed in a molar ratio of 48:13:34:5, a soft-type magnetic nickel-zinc-copper ferrite powder with high crystallinity, the ferrite powder particles having a mean grain diameter of 70 .mu.m was prepared by the same procedure as in Example 15. The same type of glass powder as was used in Example 15 was then added to this magnetic powder in an amount shown in Table 6, and an annular glass-bonded ferrite core was obtained in the same manner as in Example 15. The properties of the ferrite core are shown in Table 6, along with the corresponding results for the Comparative Examples 6 and 7 described below.

COMPARATIVE EXAMPLE 6

The same procedure was repeated as in Example 28, except that glass powder was not added.

COMPARATIVE EXAMPLE 7

The same procedure was repeated as in Example 33, except that heat was not applied when the annular mass was prepared.

TABLE 6__________________________________________________________________________ Firing temperature and time for Temperature and Initial SaturationAmount Heating preparing magnetic time for magnetic magnetic Shrink- Tensileof glass condi- ferrite powder heat treatment Density permeability flux density age strength(wt %) tions (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 28 0.1 A 1320/6 1200/1 3.9 250 2560 0.1 6Example 29 0.3 A 1320/6 1200/1 4.0 360 2580 0.1 11Example 30 0.5 A 1320/6 1200/1 4.1 270 2590 0.2 15Example 31 1 A 1320/6 1200/1 4.1 350 2630 0.2 18Example 32 3 A 1320/6 1200/1 4.2 280 2600 0.2 19Example 33 5 A 1320/6 1200/1 4.2 270 2590 0.2 20Example 34 10 A 1320/6 1200/1 4.2 230 2540 0.4 20 or moreExample 35 30 A 1320/6 1200/1 4.2 220 2520 1.5 20 or moreExample 36 40 A 1320/6 1200/1 4.2 160 2490 4.5 20 or moreComparative 0 B 1320/6 1200/1 3.8 200 2500 0 3Example 6Comparative 5 B 1320/6 1200/1 4.0 260 2530 0.1 12Example 7__________________________________________________________________________ A: Hotpressed in air at 420.degree. C. for 2 minutes under a pressure of ton/cm.sup.2, and fired in air. B: Pressed at room temperature under a pressure of 3 ton/cm.sup.2, and fired in air.

EXAMPLE 7

A mixed powder composed of barium oxide powder and ferric oxide powder mixed in a molar ratio of 1:6 was fired at 1300.degree. C. for 2 hours, after which the mixture was pulverized, thus obtaining a hard-type magnetic barium ferrite powder with high crystallinity, the ferrite powder particles having a mean grain diameter of 1 .mu.m.

Then, using this magnetic barium ferrite powder, annular powder mass was prepared in the same manner as in Example 15. Next, the mass was placed in an electric furnace and heat-treated in air at 1200.degree. C. for 30 minutes, thus obtaining annular glass-bonded ferrite core. The dimensions of the core was almost identical to those of the original powder mass. The physical properties of the ferrite core obtained in this manner are shown in Table 7.

TABLE 7__________________________________________________________________________Density (g/cm.sup.3) Maximum energy product (BH).sub.max MGOe Shrinkage (%) Tensile Strength__________________________________________________________________________ (kg/m.sup.2)Example 4.3 2.1 1.0 1537__________________________________________________________________________

EXAMPLES 38-44

A mixed powder composed of ferric oxide powder, nickel oxide powder and zinc oxide powder mixed in a molar ratio of 50:18:32 was fired at 1320.degree. C. for 6 hours, after which the mixture was pulverized, thereby obtaining two varieties of soft-type magnetic nickel-zinc ferrite powder with high crystallinity, i. e., (1) a coarse powder with grain diameters ranging from 50 to 100 .mu.m, and (2) a fine powder with a grain diameter of 5 .mu.m or less.

Then, 100 parts by weight of the coarse powder 1 and 30 parts by weight of the fine powder 2 are mixed. Next, alkali-free lead borosilicate glass powder with a mean grain diameter of 1 .mu.m and softening point (Td) of 370.degree. C. was added to the mixture in the proportion shown in Table B based on the total weight of the two varieties of magnetic ferrite powders and glass powder. After mixing and granulation, this material was press-formed under a pressure of 3 ton/cm.sup.2, thereby obtaining an annular mass with an inner diameter 7 mm, an outer diameter 12 mm and a thickness 3 mm.

The mass was then placed in an electric furnace and heat-treated in air at 1200.degree. C. for 60 minutes, thus obtaining an annular glass-bonded ferrite core.

The properties of the ferrite core are shown in Table 8, along with the corresponding results for the Comparative Example 8 and Examples 45 and 46 described below.

COMPARATIVE EXAMPLE 8

The same procedure was repeated as in Example 38, except that glass powder was not added.

EXAMPLE 45

The same procedure was repeated as in Example 41, except that the fine magnetic ferrite powder used in the present case had a grain diameter distribution of 5-20 .mu.m.

EXAMPLE 46

The same procedure was repeated as in Example 41, except that the fine magnetic ferrite powder used in the present case had a grain diameter distribution of 20-50 .mu.m.

TABLE 8__________________________________________________________________________ Firing temper- Temperature InitialAmount Grain sizes and mixing ature and time for and time magnetic Saturationof ratio of magnetic preparing mag- for heat permea- magnetic Shrink- Tensileglass ferrite powders netic ferrite pow- treatment Density bility flux density age strength(wt %) (parts by weight) der (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 38 0.5 Coarse powder.sup.a 100 1320/6 1200/1 4.0 250 3090 0.2 5 Fine powder I.sup.b 30Example 39 1 Coarse powder.sup.a 100 1320/6 1200/1 4.0 250 3140 0.2 10 Fine powder I.sup.b 30Example 40 3 Coarse powder.sup.a 100 1320/6 1200/1 4.1 380 3190 0.2 14 Fine powder I.sup.b 30Example 41 5 Coarse powder.sup.a 100 1320/6 1200/1 4.1 370 3180 0.3 18 Fine powder I.sup.b 30Example 42 10 Coarse powder.sup.a 100 1320/6 1200/1 4.2 320 3160 1.0 20 Fine powder I.sup.b 30 or moreExample 43 30 Coarse powder.sup.a 100 1320/6 1200/1 4.2 220 3070 7.5 20 Fine powder I.sup.b 30 or moreExample 44 40 Coarse powder.sup.a 100 1320/6 1200/1 4.2 180 3040 14.5 20 Fine powder I.sup.b 30 or moreCompar- 0 Coarse powder.sup.a 100 1320/6 1200/1 4.0 150 3040 0.2 4ative Fine powder I.sup.b 30Example 8Example 45 5 Coarse powder.sup.a 100 1320/6 1200/1 3.9 290 3170 0.1 18 Fine powder II.sup.c 30Example 46 5 Coarse powder.sup.a 100 1320/6 1200/1 3.9 280 3160 0.1 18 Fine powder III.sup.d 30__________________________________________________________________________ .sup.a Grain diameter: 50-100 .mu.m .sup.b Grain diameter: 5 .mu.m or less .sup.c Grain diameter: 5-20 .mu.m .sup.d Grain diameter: 20-50 .mu.m

EXAMPLES 47-51

The same procedure was repeated as in Example 41, except that the temperatures used for heat treatment of the powder mass was varied as shown in Table 9. The physical properties of the annular ferrite core so obtained are shown in Table 9, along with the corresponding results for the Example 52 as described below.

EXAMPLE 52

The same procedure was repeated as in Example 41, except that alkali-free lead borosilicate glass powder with a softening point (Td) of 700.degree. C. was used in place of the previously mentioned alkali-free lead borosilicate glass powder with a softening point of 370.degree. C. and that the heating temperature used in the formation of the annular mass was 700.degree. C. When the heating temperature was raised to 800.degree. C., the stellite mold was deformed and the core could not be removed from the mold.

TABLE 9__________________________________________________________________________ Firing temper- Temperature InitialAmount Grain sizes and mixing ature and time for and time magnetic Saturationof ratio of magnetic preparing mag- for heat permea- magnetic Shrink- Tensileglass ferrite powders netic ferrite pow- treatment Density bility flux density age strength(wt %) (parts by weight) der (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 47 5 Coarse powder.sup.a 100 1320/6 1300/1 4.2 480 3200 5.0 20 Fine powder I.sup.b 30 or moreExample 48 5 Coarse powder.sup.a 100 1320/6 1000/1 4.1 140 3030 0 12 Fine powder I.sup.b 30Example 49 5 Coarse powder.sup.a 100 1320/6 800/1 4.1 90 2980 0 8 Fine powder I.sup.b 30Example 50 5 Coarse powder.sup.a 100 1320/6 600/1 4.1 40 2840 0 6 Fine powder I.sup.b 30Example 51 5 Coarse powder.sup.a 100 1320/6 450/1 4.1 30 2800 0 5 Fine powder I.sup.b 30Example 52 5 Coarse powder.sup.a 100 1320/6 1200/1 4.1 330 3090 1.0 10 Fine powder I.sup.b 30__________________________________________________________________________ .sup.a Grain diameter: 50-100 .mu.m .sup.b Grain diameter: 5 .mu.m or less

EXAMPLES 53-61

The same procedure was repeated as in Example 38, except that a mixed powder composed of ferric oxide powder, nickel oxide powder, zinc oxide powder and cupric oxide powder mixed in a molar ratio of 48:13:34:5 was used, and the glass powder was added in the proportion shown in Table 10. The properties of the ferrite core so obtained are shown in Table 10, along with the corresponding results for the Comparative Example 9 described below.

COMPARATIVE EXAMPLE 9

The same procedure was repeated as in Example 53, except that glass powder was not added.

TABLE 10__________________________________________________________________________ Firing temper- Temperature InitialAmount Grain sizes and mixing ature and time for and time magnetic Saturationof ratio of magnetic preparing mag- for heat permea- magnetic Shrink- Tensileglass ferrite powders netic ferrite pow- treatment Density bility flux density age strength(wt %) (parts by weight) der (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 53 0.1 Coarse powder.sup.a 100 1320/6 1200/1 3.9 250 2560 0.2 5 Fine powder I.sup.b 30Example 54 0.3 Coarse powder.sup.a 100 1320/6 1200/1 4.0 260 2580 0.2 10 Fine powder I.sup.b 30Example 55 0.5 Coarse powder.sup.a 100 1320/6 1200/1 4.1 270 2590 0.2 14 Fine powder I.sup.b 30Example 56 1 Coarse powder.sup.a 100 1320/6 1200/1 4.1 350 2630 0.2 18 Fine powder I.sup.b 30Example 57 3 Coarse powder.sup.a 100 1320/6 1200/1 4.2 280 2600 0.3 19 Fine powder I.sup.b 30Example 58 5 Coarse powder.sup.a 100 1320/6 1200/1 4.2 270 2590 0.5 20 Fine powder I.sup.b 30 or moreExample 59 10 Coarse powder.sup.a 100 1320/6 1200/1 4.2 230 2540 1.5 20 Fine powder I.sup.b 30 or moreExample 60 30 Coarse powder.sup.a 100 1320/6 1200/1 4.2 220 2520 3.0 20 Fine powder I.sup.b 30 or moreExample 61 40 Coarse powder.sup.a 100 1320/6 1200/1 4.2 160 2490 5.5 20 Fine powder I.sup.b 30 or moreCompar- 0 Coarse powder.sup.a 100 1320/6 1200/1 3.9 210 2520 0.1 4ative Fine powder I.sup.b 30Example 9__________________________________________________________________________ .sup.a Grain diameter: 50-100 .mu.m .sup.b Grain diameter: 5 .mu.m or less

EXAMPLES 62-68

A mixed powder composed of ferric oxide powder, nickel oxide powder and zinc oxide powder mixed in the molar ratio of 50:18:32 was fired at 1320.degree. C. for 6 hours, and this mixture was then pulverized, obtaining a nickel-zinc soft-type magnetic ferrite powder with high crystallinity, the ferrite powder particles having mean grain diameter of 70 .mu.m. An X-ray analysis of this powder revealed the sharp spinel diffraction lines characteristic of soft ferrite, and it was demonstrated that this was a magnetic powder with extremely high crystallinity.

To this magnetic ferrite powder, 5% by weight of lead borosilicate glass powder with a mean grain diameter of 1 .mu.m and containing zinc oxide in the proportion indicated in Table 11 was added, mixed and granulated. The mixed powder was then formed under a pressure of 3 ton/cm.sup.2, thereby preparing an annular mass with an inner diameter 7 mm, an outer diameter 12 mm and a thickness 3 mm.

The mass was then placed in an electric furnace and heat treated in air at 1200.degree. C. for 60 minutes, thereby obtaining a glass-bonded annular ferrite core.

The characteristics of the ferrite core are shown in Table 11.

TABLE 11__________________________________________________________________________ Firing temperature Temperature InitialAmount Amount of and time for and time for magnetic Dielec- Saturationof ZnO preparing magnetic heat permea- tric magnetic Shrink- Tensileglass in glass ferrite powder treatment Density bility loss flux density age strength(wt %) (wt %) (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz (O.sub.max) at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 62 5 0 1320/6 1200/1 3.9 260 40 3160 0.1 10Example 63 5 0.5 1320/6 1200/1 3.9 260 40 3160 0.1 10Example 64 5 1 1320/6 1200/1 3.9 260 70 3170 0.1 10Example 65 5 5 1320/6 1200/1 3.9 270 80 3180 0.1 11Example 66 5 10 1320/6 1200/1 3.9 280 100 3190 0.2 12Example 67 5 30 1320/6 1200/1 3.9 260 70 3170 0.1 10Example 68 5 40 1320/6 1200/1 3.9 220 50 3140 0.1 10__________________________________________________________________________

EXAMPLES 69-75

The same procedure was repeated as in Example 66, except that glass powder was added in an amount shown in Table 12.

The properties of the ferrite core so obtained are shown in Table 12, along with the corresponding results for the Comparative Example 10 described below.

COMPARATIVE EXAMPLE 10

The same procedure was repeated as in Example 66, except that glass powder was not added.

TABLE 12__________________________________________________________________________ Firing temperature Temperature InitialAmount Amount of and time for and time for magnetic Dielec- Saturationof ZnO preparing magnetic heat permea- tric magnetic Shrink- Tensileglass in glass ferrite powder treatment Density bility loss flux density age strength(wt %) (wt %) (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz (O.sub.max) at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 69 0.5 10 1320/6 1200/1 3.8 200 40 3100 0 3Example 70 1 10 1320/6 1200/1 3.8 210 70 3120 0 7Example 71 3 10 1320/6 1200/1 3.9 270 80 3180 0 8Example 72 5 10 1320/6 1200/1 3.9 260 100 3160 0.1 10Example 73 10 10 1320/6 1200/1 4.0 250 100 3150 0.6 15Example 74 30 10 1320/6 1200/1 4.1 170 70 3070 7.8 17Example 75 40 10 1320/6 1200/1 4.2 140 50 3030 13.0 18Compar- 0 0 1320/6 1200/1 3.8 120 40 3040 0 2ativeExample 10__________________________________________________________________________

EXAMPLES 76-80

The same procedure was repeated as in Example 66, except that the temperature used for heat treatment of the mass was varied as shown in Table 13. The physical properties of the ferrite core so obtained are also shown in Table 13.

TABLE 13__________________________________________________________________________ Firing temperature Temperature InitialAmount Amount of and time for and time for magnetic Dielec- Saturationof ZnO preparing magnetic heat permea- tric magnetic Shrink- Tensileglass in glass ferrite powder treatment Density bility loss flux density age strength(wt %) (wt %) (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz (O.sub.max) at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 76 5 10 1320/6 1300/1 3.9 450 20 3190 3.0 10Example 77 5 10 1320/6 1000/1 3.9 110 60 3010 0 8Example 78 5 10 1320/6 800/1 3.9 60 40 2970 0 7Example 79 5 10 1320/6 600/1 3.9 30 40 2850 0 5Example 80 5 10 1320/6 450/1 3.9 25 40 2800 0 4__________________________________________________________________________

EXAMPLES 81-89

The same procedure was repeated as in Example 66, except that a mixed powder composed of ferric oxide powder, nickel oxide powder, zinc oxide powder and cupric oxide powder mixed in a molar ratio of 48:13:34:5 was used, and the glass powder was added in an amount shown in Table 14. The properties of the ferrite core so obtained are shown in Table 14, along with the corresponding results for the Comparative Example 11 described below. The dielectric loss was expressed in terms of the maximum value Q.sub.max, where Q denotes the reciprocal of the dielectric loss tan .delta..

COMPARATIVE EXAMPLE 11

The same procedure was repeated as in Example 81, except that glass powder was not added.

TABLE 14__________________________________________________________________________ Firing temperature Temperature InitialAmount Amount of and time for and time for magnetic Dielec- Saturationof ZnO preparing magnetic heat permea- tric magnetic Shrink- Tensileglass in glass ferrite powder treatment Density bility loss flux density age strength(wt %) (wt %) (.degree.C./hours) (.degree.C./hours) (g/cm.sup.3) at 1 MHz (O.sub.max) at 10 Oe (%) (kg/m.sup.2)__________________________________________________________________________Example 81 0.1 10 1320/6 1200/1 3.9 230 40 2540 0 4Example 82 0.3 10 1320/6 1200/1 3.9 240 70 2560 0 9Example 83 0.5 10 1320/6 1200/1 3.9 250 110 2570 0 11Example 84 1 10 1320/6 1200/1 4.0 320 100 2600 0 11Example 85 3 10 1320/6 1200/1 4.0 260 100 2580 0 12Example 86 5 10 1320/6 1200/1 4.0 250 90 2520 0.1 12Example 87 10 10 1320/6 1200/1 4.1 210 80 2510 0.7 15Example 88 30 10 1320/6 1200/1 4.2 200 70 2500 2.0 17Example 89 40 10 1320/6 1200/1 4.2 150 40 2480 5.5 18Compar- 0 0 1320/6 1200/1 3.8 190 40 2500 0 3ativeExample 11__________________________________________________________________________

It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.

Claims

1. A method for the preparation of a composite ferrite material, comprising:

mixing a sintered magnetic ferrite powder with high crystallinity, and a glass powder with a softening temperature lower than the sintering temperature of said ferrite powder,

subjecting said mixture to press forming, and

subjecting said press-formed mass to heat treatment at a temperature which is higher than or equal to the softening temperature of said glass powder and lower than said sintering temperature to fuse said glass powder contained in said mass thereby binding said magnetic ferrite powder with said fused glass.

2. A method according to claim 1, wherein said magnetic ferrite powder is composed of granules with at least two different size distributions.

3. A method according to claim 1, wherein said sintering temperature is in the range of 1000.degree.-1400.degree. C.

4. A method according to claim 1, wherein said softening temperature of said glass powder is 650.degree. C. or lower.

5. A method according to claim 1, wherein the temperature of said heat treatment is 800.degree. C. or higher.

6. A method according to claim 1, wherein said glass contains zinc oxide.

7. A method according to claim 1, wherein said glass powder is used in an amount of 0.3 to 30% by weight based on the total weight of said glass powder and said magnetic ferrite powder with high crystallinity.

8. A method for the preparation of a composite ferrite material, comprising:

mixing a sintered magnetic ferrite powder with high crystallinity, and a glass powder with a softening temperature lower than the sintering temperature of said ferrite powder,

subjecting said mixture to press forming, and simultaneous heat treatment at a temperature which is higher than or equal to the softening temperature of said glass powder and lower than said sintering temperature and thereby fusing said glass powder thus effecting the binding of said magnetic ferrite powder by said fused glass, and

firing the obtained mass after said heat treatment at a temperature which is lower than or equal to the sintering temperature of said magnetic ferrite powder.

9. A method according to claim 8, wherein said magnetic ferrite powder is composed of granules with at least two different size distributions.

10. A method according to claim 8, wherein said firing temperature is in the range of 1000.degree.-1400.degree. C.

11. A method according to claim 8, wherein said softening temperature of said glass powder is 650.degree. C. or lower.

12. A method according to claim 8, wherein the temperature of said heat treatment is 800.degree. C. or higher.

13. A method according to claim 8, wherein said glass contains zinc oxide.

14. A method according to claim 8, wherein said glass powder is used in an amount of 0.3 to 30% by weight based on the total weight of said glass powder and said magnetic ferrite powder with high crystallinity.