Microwave magnetic material and method of fabricating the same

Abstract: A microwave magnetic material used for a microwave non-reciprocal circuit element, which is constructed by sintering a laminated body 3 obtained by laminating a plurality of magnetic sheets 1.

Description: The present invention relates to a microwave magnetic material used for an non-reciprocal circuit element used in a microwave band and a method of fabricating the same. In a mobile communication equipment such as a portable telephone or a car telephone, the miniaturization and the diversity thereof have progressed in recent years. Correspondingly, miniaturization and diversity have been also required in an non-reciprocal circuit element used in the above described mobile communication equipment. Examples of the above described non-reciprocal circuit element include an element having a plurality of center electrodes disposed so as to cross each other in an electrically insulated state and plate-shaped microwave magnetic materials disposed on and beneath the plurality of center electrodes and further so constructed that a DC magnetic field is applied to respective portions of the plurality of center electrodes, that is, a so-called lumped-constant type non-reciprocal circuit element. Examples include a lumped-constant type circulator or isolator. One example of a method of fabricating the above described non-reciprocal microwave circuit element will be described with reference to Fig. 5. A center electrode 42a is disposed on a disc-shaped microwave magnetic material 41a. The center electrode 42a is in such a shape as to radially extend through the center of the upper surface of the microwave magnetic material 41a and further lead to the side surface of the microwave magnetic material 41a. An insulating film 43a made of an insulating material is then disposed on the above described center electrode 42a, and a center electrode 42b is disposed thereon so as to cross the center electrode 42a. Furthermore, an insulating film 43b, a center electrode 42c and an insulating film 43c are disposed in this order on the center electrode 42b, and a microwave magnetic material 41b is superimposed thereon and fixed. Thereafter, permanent magnets are disposed on and beneath a structure interposed between the above described microwave magnetic materials 41a and 41b so that a DC magnetic field is applied to the structure. Meanwhile, the above described microwave magnetic materials 41a and 41b have been conventionally fabricated in the following method. Specifically, magnetic powders are put into a metal mold and are press-formed, to obtain a formed body. The formed body obtained is sintered, thereby to obtain a microwave magnetic material 44 shown in Fig. 6. The microwave magnetic material 44 is so mechanically polished as to have a predetermined thickness, thereby to fabricate a microwave magnetic material 45 shown in Fig. 7. As described above, the microwave magnetic material 45 used for an non-reciprocal circuit element has been conventionally fabricated by obtaining a formed body in the powder press forming process and mechanically polishing a sintered body obtained by sintering the formed body. The reason for this is that a thin formed body cannot be fabricated in the powder press forming process, so that a thick formed body is forced to be previously fabricated to fabricate a thin microwave magnetic material 45 by polishing after sintering as described above. Furthermore, in the conventional method, the powder press forming process has been used so as to obtain a microwave magnetic material. Accordingly, metal molds corresponding to the sizes of objective microwave magnetic materials are respectively prepared, thereby to cope with the diversity of components. As the miniaturization and the diversity of the components have progressed, however, the type of metal mold or the like is increased, and the polishing process and the powder forming process are liable to be complicated. As a result, mass productivity is lowered, resulting in very high fabricating costs. US-A-4,388,131 concerns a method of fabricating magnets, wherein the magnets are formed of alternating layers of hard and soft ferrite powders which are subsequently fired together to form a magnetic structure. The structure is manufactured in such a manner that the magnet has a thickness of about 0.125 inch. IEEE Transactions On Components, Hybrids And Manufacturing Technology, Vol. 8, No. 1, March 1985, pages 221-227, C.K. Maiti et al. "Thick Film Ferrimagnetic Paste Using Lithium Ferrite" concerns the preparation of a thick-film ferrimagnetic paste and a resinous technique for the measurement of the dielectric constant, saturation magnetization, loss tangents, and resonance line width of the ferrite films at microwave frequencies. For microwave applications, relatively thick ferrimagnetic films are required, since for the application in a non-reciprocal microwave device a thickness of about 8 to 10 mils is necessary. IEEE Transactions On Microwave Theory And Techniques, Vol. 15, No. 12, December 1967, pages 748-750, B. Hershenov, "Microwave Strip Junction Circulator For Microwave Integrated Circuits" describes a microstrip junction circulator fabricated from a single garnet substrate and outlines the possibility to fabricate these garnets by lamination. The article in Onde Electricque, Vol. 71, No. 1, February 1991, pages 35-47, G. Forterre "Les Materiaux Ferrites et Leurs Applications en Hyperfréquence" concerns microwave applications using ferrite devices which are formed by sintering a ferrite garnet. GB-A-1023873 concerns insulating ferrite articles with embedded conductors and a method of making same. Magnetic elements are formed by thin sheets of green ferrite, printing conductive patterns on the sheets, and laminating a plurality of such sheets with conductive patterns in a desired registry and firing the lamination to provide a unitary homogenous sintered ferrite sheet having a desired two-state magnetic characteristic. GB-A-1175510 concerns mechanically oriented hard ferrite bodies. A raw ferrite body is formed by stacking a plurality of ferrite sheets to obtain a thickness of about 1/2 inch. Starting from this prior art, it is the object of the present invention to provide a microwave magnetic material so constructed as to easily cope with the miniturization and the diversity of a microwave circuit element, and a method of fabricating microwave magnetic material which can cope with the miniturization and the diversity of components and allows the microwave magnetic material to be supplied at low costs. This object is achieved by a microwave magnetic material body according to claim 1, and by a method according to claim 4. A first embodiment of the present invention provides a microwave magnetic material used for a microwave non-reciprocal circuit element, which is constructed by sintering a laminated body obtained by laminating a plurality of magnetic sheets. Furthermore, a second embodiment of the present invention provides a method of fabricating a microwave magnetic material, which comprises the steps of sheet-forming a magnetic paste obtained by thoroughly mixing magnetic powders with a binder resin and a solvent, laminating a plurality of magnetic sheets obtained to obtain a laminated body, and sintering the laminated body. In the first and second embodiments of the present invention, the plurality of magnetic sheets are laminated and the laminated body obtained is sintered, thereby to finally obtain a microwave magnetic material. In this case, the magnetic sheet can be formed by an arbitrary sheet forming process such as the Doctor blade process. However, a much thinner magnetic sheet can be easily obtained by the sheet forming process, as compared with the powder press forming process conventionally used. Consequently, the respective thicknesses of the plurality of magnetic sheets are adjusted and the number of magnetic sheets is further adjusted, thereby to make it possible to easily fabricate a microwave magnetic material having a desired thickness. In the conventional method of fabricating a microwave magnetic material, complicated polishing work has been required so as to finally adjust the thickness of the microwave magnetic material. On the other hand, according to the present invention, such polishing work can be omitted. Moreover, in the conventional fabricating method, the powder press forming process has been used, so that various high-cost metal molds must be prepared depending on the shape of the microwave magnetic material. On the other hand, in the present invention, such high-cost metal molds are not required, so that the microwave magnetic material having a desired shape and a thickness can be provided at low cost, thereby to make it possible to smoothly cope with the miniaturization and the diversity of the microwave non-reciprocal circuit element. Accordingly, the present invention can greatly contribute to the miniaturization and the diversity of a mobile communication equipment such as a car telephone. The microwave magnetic material according to the present invention can be utilized for a microwave non-reciprocal circuit element such as a circulator or an isolator conventionally known. In accordance with a particular aspect of the present invention, there is provided a microwave non-reciprocal circuit element comprising a pair of microwave magnetic materials and a plurality of center electrodes disposed in a state where they are electrically insulated from each other between the microwave magnetic materials and so as to cross each other in their central portions, and wherein the above described microwave magnetic material is constructed by sintering a laminated body obtained by laminating a plurality of magnetic sheets, and a DC magnetic field is applied to the portions where the center electrodes cross each other by a permanent magnet. The above described microwave non-reciprocal circuit element is constructed using the microwave magnetic material according to the present invention, thereby to make it possible to easily prepare a microwave magnetic material having a desired thickness by adjusting the respective thicknesses of the magnetic sheets and the number of magnetic sheets. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. Figs. 1A to 1C are respectively perspective views for explaining the fabricating processes of a microwave magnetic material according to the present embodiment, where Fig. 1A illustrates a plurality of magnetic sheets to be laminated, Fig. 1B illustrates a laminated body, and Fig. 1C illustrates a laminated body cut in a desired shape; Fig. 2 is a perspective view showing a microwave magnetic material according to one embodiment of the present invention; Fig. 3 is a perspective view for explaining the processes for assembling the microwave non-reciprocal circuit element using microwave magnetic materials in the embodiment of the present invention; Fig. 4 is a cross sectional view showing main portions of the microwave non-reciprocal circuit element shown in Fig. 3; Fig. 5 is a perspective view for explaining the processes for assembling a conventional microwave non-reciprocal circuit element; Fig. 6 is a perspective view showing a magnetic material prepared in fabricating a conventional microwave magnetic material; and Fig. 7 is a perspective view showing a microwave magnetic material obtained by a conventional fabricating method. A non-restrictive embodiment of a microwave magnetic material and a method of fabricating the same according to the present invention will be described to clarify the present invention. First, magnetic powders are thoroughly mixed with a binder resin, a solvent and the like, to obtain a magnetic paste. Examples of the magnetic powders includes magnetic powders mainly composed of yttrium oxide (Y 2 O 3 ) and iron oxide (Fe 2 O 3 ) and magnetic powders mainly composed of nickel oxide (NiO) and iron oxide (Fe 2 O 3 ). The above described binder resin is used so as to combine the above described magnetic powders with each other. Examples of the binder resin include polyvinyl alcohol. The solvent is used so as to obtain the above described magnetic paste using the magnetic powders and the binder resin. Examples of the solvent include toluene and ethanol. The above described magnetic paste is then formed by a sheet forming process, to obtain a thin magnetic sheet having a thickness of several microns to several ten microns. As the sheet forming process, known sheet forming processes such as the Doctor blade process can be employed. A plurality of magnetic sheets obtained are laminated as shown in Fig. 1A depending on the thickness of an objective microwave magnetic material. In Fig. 1A, reference numeral 1 denotes each magnetic sheet. A laminated body obtained by laminating the plurality of magnetic sheets 1 as described above is pressed in the direction of thickness, to obtain a laminated body 2 shown in Fig. 1B. Thereafter, the laminated body 2 is cut using a punch or the like, to obtain a disc-shaped laminated body 3 shown in Fig. 1C. The above described disc-shaped laminated body 3 is then sintered at temperatures of, for example, 1300°C to 1600°C, thereby to obtain a microwave magnetic material 4 shown in Fig. 2. As described in the foregoing, in the present invention, the thickness of the microwave magnetic material 4 finally obtained is determined depending on the thickness of the laminated body 2 obtained by laminating the plurality of magnetic sheets 1. Consequently, the number of magnetic sheets 1 used is adjusted considering the contraction of the magnetic sheets 1 by pressing and sintering in obtaining the above described laminated body 2, thereby to make it possible to easily obtain the microwave magnetic material 4 having a desired thickness. In the conventional fabricating method, the thick microwave magnetic material 44 must be mechanically polished in obtaining the microwave magnetic material 45. On the other hand, in the present embodiment, the adjustment of the thickness by the above described polishing work can be omitted, thereby to make it possible to easily provide a thin microwave magnetic material having a thickness of approximately several ten microns to several hundred microns. Furthermore, the microwave magnetic material 4 obtained in the above described embodiment can be directly used as the microwave magnetic materials 41a and 41b used in the conventional method of fabricating the microwave non-reciprocal circuit element described with reference to, for example, Fig. 5. One example of a microwave non-reciprocal circuit element constructed using the microwave magnetic material 4 obtained in the above described embodiment will be described with reference to Figs. 3 and 4. Fig. 3 is a perspective view for explaining the assembly processes of the microwave non-reciprocal circuit element, and Fig. 4 is a cross sectional view showing main portions of the assembled microwave non-reciprocal circuit element. In Figs. 3 and 4, microwave magnetic materials 4a and 4b are used. The microwave magnetic materials 4a and 4b are obtained in the same manner as the microwave magnetic material 4 in the above described embodiment. A through hole 31a containing the above described microwave non-reciprocal circuit element is first formed in the center of a rectangular substrate 31 made of an insulating material such as alumina. Electrodes for taking out capacitance 32 are formed on the upper surface of the substrate 31 by printing a conductive film. On the other hand, a ground electrode is formed on the lower surface of the substrate 31 so as to be opposed to the above described electrodes for taking out capacitance 32a while being separated by the substrate 31. In addition, a ground plate 33 as illustrated below is joined to the ground electrode by soldering, so that the substrate 31 and the ground plate 33 are integrated. The ground plate 33 is a metal plate, has a through hole 33a in its center, and has raised portions 33b in its portions facing the through hole 33a. The raised portions 33b are projected upward through the through hole 31a of the substrate 31 in a state where the substrate 31 and the ground plate 33 are joined to each other as described above. Furthermore, the above described microwave magnetic materials 4a and 4b are laminated while being separated by a plurality of center electrodes 42a to 42c as illustrated. The center electrodes 42a to 42c are constructed in the same manner as the center electrodes 42a to 42c in the prior art shown in Fig. 5. It would be noted that the illustration of members for electrically insulating the plurality of center electrodes 42a to 42c from each other is omitted in Figs. 3 and 4. As apparent from Fig. 4 showing main portions after the assembly, the above described raised portions 33b are connected to respective one ends of the center electrodes 42a to 42c in the above described microwave non-reciprocal circuit element by soldering or the like. In addition, reference numeral 37 shown in Fig. 4 denotes a ground electrode formed on the lower surface of the substrate 31. The above described electrodes for taking out capacitance 32, the substrate 31, and the ground electrode 37 formed on the reverse surface of the substrate 31 constitute a capacitance for impedance matching. On the other hand, the respective other ends of the center electrodes 42a to 42c in the microwave non-reciprocal circuit element are electrically connected to the electrodes for taking out capacitance 32 formed on the upper surface of the substrate 31, although only the center electrode 42 is illustrated in, for example, Fig. 4. Similarly, the other ends of the respective other center electrodes 42a and 42b are also electrically connected to the other electrodes for taking out capacitance 32. Returning to Fig. 3, the substrate 31 and the ground plate 33 are laminated, and the microwave non-reciprocal circuit element is incorporated into the through holes 31a and 33b and is interposed between yokes 34 and 35, thereby to construct a microwave non-reciprocal circuit device. A permanent magnet 36 is fixed to the lower surface of the yoke 34. The yokes 34 and 35 are made of a metal material, and is so constructed that a pair of opposed edges of one of the yokes is bent toward a pair of opposed edges of the other yoke and both the yokes are fixed to each other by solder or the like or mechanical engagement utilizing the bent portions. Furthermore, although in the above described embodiment, the microwave magnetic materials 4, 4a and 4b are so constructed as to finally have a disc shape, the plane shape of the microwave magnetic materials is not limited to the disc shape as illustrated. For example, the plane shape can be changed into an arbitrary shape such as a rectangular shape. Moreover, in the present embodiment, the above described laminated body 2 is cut by a punch or the like to obtain the laminated body 3 having a desired plane shape. Therefore, it is possible to provide a microwave magnetic material having a desired shape without requiring more complicated and higher-cost work such as a change in a metal mold, as compared with the conventional method of fabricating the microwave magnetic material using the powder press forming process.

Claims: A microwave magnetic material body (4) for use in a microwave non-reciprocal circuit element, which is a sintered body (2,3) obtained by sintering a plurality of laminated magnetic sheets (1), characterized in that each of the laminated magnetic sheets (1) has a thickness of several microns to several ten microns; and that the thickness of the microwave magnetic material body (4) is in the range of several tens microns to several hundred microns. The microwave magnetic material body (4) according to claim 1, wherein the microwave magnetic material body has a disc shape. A microwave non-reciprocal circuit element comprising: a pair of microwave magnetic material bodies (4a,4b) of claim 1; a plurality of center electrodes (42a,42b,42c) disposed so as to be electrically insulated from each other between the pair of microwave magnetic material bodies (4a,4b) and to cross each other in central portions thereof; and a permanent magnet (36) disposed so as to apply a DC magnetic field to locations where the center electrodes cross each other. A method for fabricating a microwave magnetic material body (4), comprising the steps of: forming a magnetic paste by mixing magnetic powders with a binder resin and a solvent; forming a plurality of magnetic sheets (1) from the magnetic paste, each of the magnetic sheets (1) having a thickness of several microns to several ten microns; determining a number of the magnetic sheets (1) to be laminated so that the microwave magnetic material body has a desired thickness in view of the contraction of the magnetic sheets due to pressing and sintering; laminating the number of magnetic sheets (1) to obtain a laminated body (2,3); and sintering the laminated body (2,3) to form a microwave magnetic material body (4) having a thickness of several tens microns to several hundred microns. The method for fabricating a microwave magnetic material body (4) according to claim 4, wherein the step of forming the sheets (1) is performed by the Doctor blade process. The method for fabricating a microwave magnetic material body (4) according to claim 4, wherein the magnetic powders comprise yttrium oxide and iron oxide. The method for fabricating a microwave magnetic material body (4) according to claim 4, wherein the magnetic powders comprise nickel oxide and iron oxide. The method for fabricating a microwave magnetic material body (4) according to claim 4, wherein the binder resin comprises polyvinyl alcohol.

Inventors: KOUNOIKE TAKEHIRO, MARUSAWA HIROSHI, TOMONO KUNISABURO, KOUNOIKE, TAKEHIRO, MARUSAWA, HIROSHI, TOMONO, KUNISABURO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1993
Main classification: H01P1
IPC codes: H01P1, H01P11, H01F1, H01F41