A 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 obtainable 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, as acknowledged in the opening clause of claim 1. The present invention also relates to a method for the preparation of above-mentioned composite ferrite materials as stated in the opening clause of claim 9. 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. Figure 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 700-1000°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 1000-1400°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 µm 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 (700-1000°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. Furthermore EP-A 0 105 375 shows an oxide-containing magnetic material which can be sintered at low temperatures and is prepared by adding 0.1 to 5 wt % of a glass containing 3 to 50 mol % Li₂0, 10 to 97 mol % B₂O₃ and 0 to 70 mol % SiO₂ to ferrite containing 0.5 mol % or more Li₂O in order to lower the sintering temperature to 1000°C or less. Patent Abstracts of Japan, volume 7, no. 255 (E-210) (1400) of November 12, 1983 and JP-A 58-141 511 propose to cover the ferrite particles with glass powder in order to supress the denseness of ferrite and to attain a low contraction coefficient. To improve compactibility and molding density Patent Abstracts of Japan vol. 7, no. 264 (E-212) (1409) of November 24, 1983 and JP-A 58 147 008 propose to add a lubricant when ferrite and glass are kneaded for the preparation of a ferrite containing glass powder. As a lubricant 0.1 to 6 wt % stearic acid calcium is added to a mixture of powder consisting of Fe₂O₃, MgO, ZnO, CuO at a predetermined mol %. Lastly a composite ferrite material as acknowledged in the opening clause of claim 1 and a method for the preparation of a composite ferrite material as acknowledged in the opening clause of claim 9 are known from Patent Abstracts of Japan vol. 7, no. 247 (E-208) (1392) of November 2, 1983 and JP-A 58-135 609. For preparing this ferrite magnetic material 0.3 to 15 wt % glass powder is added to a soft magnetic ferrite powder, and the mixture is kneaded at a temperature higher than the softening point of the glass by 200°C to coat the ferrite powder material with the glass. Then the mixture is heated at temperatures higher than those at which the shrinkage of the ferrite begins. Although a compact obtained by pressurizing the ferrite powder in a metal mold contains void holes, diffusion will occur among atoms if the ferrite is heated at temperatures higher than 800°C to 1000°C, causing the void holes to decrease and the compact to shrink. Therefore such ferrite magnets have a high dimensional acuracy and an increased performance. SUMMARY OF THE INVENTION It is the object of the invention to provide a composite ferrite material as acknowledged in the opening clause of claim 1 and a method for the preparation of a composite ferrite material as acknowledged in the opening clause of claim 9 leading to magnetic ferrite articles with low dielectric losses. This object is achieved by the features of claims 1 or 9 respectively. Suitable embodiments of this composite ferrite material are defined by the features of subclaims 2 to 8 and suitable embodiments of this method are defined by the features of subclaims 10 to 15. As will be evident from the examples, the addition of zinc oxide in a proportion between 1-30 % by weight to the glass leads to magnetic ferrite articles with low dielectric losses. 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 above-mentioned 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: Figures 1-3 are enlarged schematic illustrations showing the structure of the composite ferrite material of the present invention. Figure 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, zinc oxide and other metal oxides in the form of MxO (wherein M is a metal of valence n and x = 2/n) such as NiO etc., heating the mixture at a temperature of 1000°C or more, preferably in the range of 1000-1400°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 µm 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 µm 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°C. Moreover, since the firing temperature of the magnetic ferrite powder is 700°C or higher, and the heat-resistant temperature of metal molds is ordinarily in the order of 700°C, the said softening temperature should desirably be lower than 700°C. More specifically, glass with a softening temperature not exceeding 650°C is used. Furthermore, this glass contains 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 µm 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. Figure 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 Figure 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°C, then the binding effect of the glass is increased, and a composite ferrite material with excellent magnetic properties is obtained. Figure 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 20-30°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 50-100°C is employed. Figure 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 µm 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 Figure 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 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°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 µm. 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 µm 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/cm², 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°C for 60 minutes, thereby obtaining a glass-bonded annular ferrite core. The characteristics of the ferrite core are shown in Table 11. The values 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 these measurements are shown in Table 11. 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 Å/m, and the initial magnetic permeability at a frequencey of 1 MHz was calculated from the results of this measurement. Saturation magnetic flux density was measured in accordance with JIS C2561 in a 796 A/m (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. 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. 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. 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 max , where Q denotes the reciprocal of the dielectric loss tan δ. Comparative Example 11 The same procedure was repeated as in Example 81, except that glass powder was not added.

Claims: A composite ferrite material with dimensional stability obtainable by mixing a magnetic ferrite powder containing zinc oxide and having high cristallinity prepared by firing at a temperature of at least 1000°C and pulverizing, and a glass powder having a softening temperature lower than said firing temperature, and heat treating said mixture at a temperature which is higher than or equal to said softening temperature of said glass powder and lower than said firing temperature for binding said magnetic ferrite powder by said glass material characterized in that said glass powder contains 1 to 30 % by weight of zinc oxide. A composite ferrite material according to claim 1 characterized in that said magnetic ferrite powder is composed of granules with at least two different size distributions. A composite ferrite material according to one of claims 1 or 2 characterized in that said firing temperature is in the range of 1000°C to 1400°C. A composite ferrite material according to one of claims 1 to 3, characterized in that the temperature of said heat treatment is at least 800°C. A composite ferrite material according to one of claims 1 to 4, characterized in that said magnetic ferrite powder containing zinc oxide and having a high cristallinity is a soft type of magnetic powder. A composite ferrite material according to one of claims 1 to 4, characterized in that said magnetic ferrite powder containing zinc oxide and having high cristallinity is a hard type of magnetic powder. A composite ferrite material according to one of claims 1 to 6, characterized in that said magnetic 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 containing zinc oxide and having high cristallinity. A composite ferrite material according to one of claims 2 to 7, characterized in that the grain diameter of the powder granules with the smallest grain size distribution among said magnetic ferrite powders with at least two different size distributions is 5 µm or less. A method for the preparation of a composite ferrite material according to claim 1, comprising mixing a magnetic ferrite powder containing zinc oxide and having a high cristallinity prepared by firing at a temperature of at least 1000°C and pulverising and a glass powder containing 1 to 30 % by weight of zinc oxide and having a softening temperature lower than said temperature, subjecting said mixture to press-forming, and subjecting said press-formed mass to a heat treatment at a temperature which is higher than or equal to said softening temperature of said glass powder and lower than said firing temperature for binding said magnetic ferrite powder with said fused glass. A method according to claim 9 wherein said magnetic ferrite powder is composed of granules with at least two different size distributions. A method according to one of claims 9 or 10, wherein said firing temperature is in the range of 1000°C-1400°C. A method according to one of claims 9 to 11, wherein said softening temperature of said glass powder is 650°C or lower. A method according to one of claims 9 to 12, wherein the temperature of said heat treatment is at least 800°C. A method according to one of claims 9 to 13, 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 containing zinc oxide and having high cristallinity. A method according to one of claims 9 to 14, wherein the press-forming and the heat treatment are performed simultaneously, and wherein the obtained mass after that heat treatment is fired at a temperature which is lower than or equal to the firing temperature of said magnetic ferrite powder containing zinc oxide.

Inventors: HARADA SHINJI, KAWAMATA TADASHI, HARADA, SHINJI, KAWAMATA, TADASHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: H01F1
IPC codes: H01F1