Composite functional device and method for producing the same

Abstract: A composite functional device with good varistor characteristics and good capacitor characteristics is provided. The device comprises a magnetic ceramic including an Ni-Zn ferrite; and a semiconductive including zinc oxide and an oxide of at least one of nickel and iron in an amount of from 0.001 to 0.1 mol% in terms of NiO or Fe 2 O 3 .

Description: FIELD OF THE INVENTION The present invention relates to a small-sized, composite functional device with varistor characteristics, capacitor characteristics and magnetic characteristics, and to a method for producing it. Such multi-functional devices are, for instance, know from US-A-5 476 728 and EP-A-0 581 206. BACKGROUND OF THE INVENTION In current electronic instruments, it is important to prevent integrated circuits and other semiconductor devices from malfunctioning and breaking down due to noise, and also to prevent propagation of noise to the exterior of the electronic instruments. To accomplish this, varistor chips, capacitor chips and ferrite chips are mounted on electronic circuit boards. However, the mounting of a large number of such devices on circuit boards results in an increase in the total area occupied by circuit boards in the electronic instruments and even in an increase in the cost of the electronic instruments. Therefore, composite devices for preventing noise are desired. One method of meeting the above-mentioned requirement is to provide a composite functional device by integrally sintering a magnetic ceramic composition and a semiconductive ceramic composition with varistor characteristics. In order to produce such a ccmposite functional device, however, the interaction between compositions of different materials that occurs during integral sintering should be restricted to such a degree that a negative influence on the characteristics of the device is avoided. For example, while integrally sintering a semiconductive ceramic composition with varistor characteristics and a magnetic ceramic composition can result in the compositions mutually diffusing into each other. Where the semiconductive ceramic composition comprises ZnO and the magnetic ceramic composition comprises a Ni-Zn ferrite, Ni and Fe compounds diffuse into the varistor material. The diffusion of Ni and Fe compounds results in deterioration of both the varistor characteristics and the capacitor characteristics of the semiconductive ceramic composition. As a result of the diffusion, the linearity for the varistor characteristics is lowered and the dielectric loss for the capacitor characteristics is increased. SUMMARY OF THE INVENTION Accordingly, the object of the present invention is to provide a noise-preventing, composite functional device with good varistor characteristics and good capacitor characteristics which comprises an integrally sintered body composed of a magnetic ceramic and a semiconductive ceramic and in which diffusion of the components constituting the magnetic ceramic into the semiconductive ceramic is retarded or prevented, and also to provide a method for producing the device. To attain the above-mentioned object, the present invention provides a composite functional device prepared by integrally sintering a magnetic ceramic consisting essentially of an Ni-Zn ferrite, and a semiconductive ceramic with varistor characteristics consisting essentially of zinc oxide and containing an oxide of at least one of nickel and iron in an amount of from 0.001 to 0.1 mol% in terms of NiO or Fe 2 O 3 , and preferably 0.005 to 0.5 mol%, based on the mols zinc oxide. The present invention also provides a method for producing such a composite functional device comprising combining a shaped body of a magnetic material that consists essentially of an Ni-Zn ferrite and a shaped body of a semiconductive material with varistor characteristics that consists essentially of zinc oxide and an oxide of at least one of nickel and iron in an amount of from 0.001 to 0.1 mol% in terms of NiO or Fe 2 O 3 , followed by integrally sintering the combination. Thus, it is possible, according to the present invention, to prevent the components constituting the magnetic ceramic from diffusing into the semiconductive ceramic during sintering, and is therefore possible to obtain a composite functional device with good varistor characteristics and good capacitor characteristics. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective, exploded view illustrating the lamination of green sheets prior to baking, which constitute one embodiment of the composite functional device of the present invention. Fig. 2 is a perspective view illustrating the laminate of green sheets of Fig. 1 prior to baking, which is for the composite functional device of the present invention. Fig. 3 is a perspective view illustrating a composite functional device that comprises the sintered laminate illustrated in Fig. 1 and Fig. 2. Fig. 4 shows the equivalent circuit of the composite functional device of Fig. 3. PREFERRED MODES OF CARRYING OUT THE INVENTION Embodiments of the composite functional device of the present invention are described below with reference to the drawings. With reference to Fig. 1 and Fig. 2, the method for producing the composite functional device 10 of Fig. 3 is described below. In Fig. 3, 11 and 12 are semiconductive ceramic sheets; 13 is a magnetic ceramic sheet; 14 and 15 are terminal electrodes for the source of potential; and 16 and 17 are terminal electrodes for the ground. First prepared were green sheets Ia, Ib, Ic, Id, Ie and If comprising a varistor material, that is, a semiconductive material. More precisely, prepared were raw material powders of ZnO, Bi 2 0 3 , CoCO 3 , MnO 2 and Sb 2 0 3 all having a purity of not lower than 99 %. These were weighed in proportions of 98 mol% of ZnO, 0.5 mol % of Bi 2 O 3 , 0.5 mol% of CoCO 3 , 0.5 mol% of MnO 2 and 0.5 mol% of Sb 2 O 3 , and mixed. To the mixture was added 3 % by weight of zinc borosilicate glass (Zn: 25 mol%, B: 40 mol%, Si: 35 mol%). Next, NiO and/or Fe 2 O 3 were/was added thereto in amounts of from 0.001 to 1.0 mol%, as shown in Table 1. The thus-prepared raw material mixture was next milled with pure water in a ball mill, then dewatered, and thereafter calcined at 800°C for 2 hours. The resulting calcinate was further milled with pure water in a ball mill, filtered and dried to obtain a raw material powder for a varistor. The raw material powder was dispersed in a solvent along with an organic binder to prepare a slurry, which was then shaped into 50 µm -thick green sheets through doctor-blading. From these green sheets, rectangular sheets were stamped out having a predetermined size. Thus were obtained a plurality of green sheets each comprising the varistor material. Next, on these green sheets Ib, Ic, le and If, formed were electrodes 3, 4, 4 and 3, respectively, through screen printing which used an electroconductive paste consisting essentially of silver and palladium (Ag/Pd = 70/30, by weight). The electrode 3 is essentially cross-shaped in which the facing ends of one pair of legs individually reach the facing ends of the green sheet. The electrode 4 is essentially I-shaped with one end reaching one end of the green sheet. Next prepared were green sheets 2a and 2b each comprising a ferrite material, that is, a magnetic material. More precisely, prepared were raw material powders of Fe 2 O 3 , NiO and ZnO all having a purity of not lower than 99 %. These were weighed in proportions of 47 mol% of Fe 2 O 3 , 30 mol% of NiO and 23 mol% of ZnO, and mixed. To the resultant mixture was added 3 % by weight of zinc borosilicate glass (Zn: 25 mol%, B: 40 mol%, Si: 35 mol%). The thus-prepared raw material mixture was milled with pure water in a ball mill, filtered and dried. The resulting mixture was calcined at 800°C for 2 hours. After this, the thus-prepared calcinate was roughly ground, combined with 0.1 % by weight of Bi 2 O 3 and 3 % by weight of zinc borosilicate glass (this is the same as that used hereinabove for the preparation of the raw material powder for the varistor). The resulting mixture was further milled with pure water in a ball mill for 24 hours, and then filtered and dried to obtain a raw material powder for a ferrite. The raw material powder was next dispersed in a solvent along with an organic binder to prepare a slurry, which was then shaped into 50 µm-thick green sheets through doctor-blading. From these green sheets, rectangular sheets of predetermined size were stamped out. Thus were obtained a plurality of green sheets each comprising the ferrite material. On the green sheet 2b was formed an electrode 5 through screen printing an electroconductive paste consisting essentially of silver and palladium (Ag/Pd = 70/30, by weight). The electrode 5 is essentially I-shaped with both ends reaching the facing ends of the green sheet. Next, as shown in Fig. 1, green sheet Ia, green sheet Ib with electrode 3, green sheet Ic with electrode 4, green sheet 2a, green sheet 2b with electrode 5, green sheet Id, green sheet Ie with electrode 4, green sheet If with the electrode 3 were laminated together in that order starting from the top. After this, the thus-laminated green sheets Ia, Ib, Ic, 2a, 2b, Id, Ie and If were set in a pressing machine and pressed under a pressure of 2 tons/cm 2 to obtain the integrated laminate of Fig. 2. The laminate was then baked at 950°C for 2 hours. After this, using an electroconductive paste consisting essentially of silver, formed were terminal electrodes 14 and 15 for connection to a sourse of potential. These electrodes 14 and 15 were electrically connected with the exposed ends of the electrodes 4, 5 and 4. In the same manner, also formed were terminal electrodes 16 and 17 for the ground and were electrically connected with the ground electrodes 3 and 3. Fig. 4 shows the equivalent circuit of the composite functional device produced herein. The characteristics of the composite functional device 10 thus obtained herein were measured. In particular, for the varistor part, measured were the varistor voltage (V 1 mA), the non-linear coefficient (α), and also the capacitance and the dielectric loss under the conditions of 25°C, 1 kHz and 1 Vrms. For the ferrite part, measured were the inductance at 1 MHz and the impedance at 100 MHz. The data obtained are shown in Table 2, in which the samples marked with (*) are outside the scope of the present invention and the other samples are within the scope of the present invention. As shown in Table 2, the characteristics of the samples of the present invention in which the varistor comprises NiO and/or Fe 2 0 3 are better than those of the sample No. 22 containing neither NiO nor Fe 2 O 3 in that the non-linear coefficient (α) of the former is large and that the dielectric loss of the former is small. However, as seen in the samples Nos. 6, 7, 13, 14, 18 and 21, if the content of either one of NiO and Fe 2 O 3 is larger than 0.1 mol%, such is unfavorable since the varistor voltage (V 1 mA) and the non-linear coefficient (α) are small and since the dielectric loss is large. Regarding the characteristics of the ferrite, the addition of NiO and/or Fe 2 O 3 to the varistor layer makes the components constituting the ferrite hard to diffuse into the varistor composition, resulting in an increase in the inductance and the impedance of the ferrite. However, as seen in the samples Nos. 6, 7, 13, 14, 18 and 21, if the amount of either one of NiO and Fe 2 0 3 added is more than 0.1 mol%, such is unfavorable since both the inductance and the impedance are lowered. In the above-mentioned samples, the addition of NiO and/or Fe 2 0 3 to the varistor composition resulted in no change in the degree of shrinkage of the sintered samples and neither delamination nor peeling which may be caused by the difference in the degree of shrinkage between the constitutive layers. The composite functional device of the present invention is not limited to the examples mentioned hereinabove, but can be varied and modified within the scope and the spirit of the invention. For example, the number and the order of the green sheets to be laminated, the number and the shape of the varistor electrodes, the number and the shape of the coils, and thespecific compositions of the ferrite and semiconductor can be suitably varied and modified. As will be obvious from the above-mentioned description, it is possible to obtain according to the present invention, a noise-preventing, composite functional device with good varistor characteristics and good capacitor characteristics, since nickel oxide and/or iron oxide are/is previously added to the shaped body of a semiconductor material with varistor characteristics. The addition of the oxide prevents the nickel and iron compounds that exist in the magnetic material from diffusing into the semiconductive material during the step of baking a laminate of green sheets into an integrated monolithic device. Sample Number Amount Added (mol%) NiO Fe 2 O 3 1 0.001 0 2 0.005 0 3 0.01 0 4 0.05 0 5 0.1 0 6* 0.5 0 7* 1.0 0 8 0 0.001 9 0 0.005 10 0 0.01 11 0 0.05 12 0 0.1 13* 0 0.5 14* 0 1.0 15 0.05 0.01 16 0.05 0.05 17 0.05 0.1 18* 0.05 0.5 19 0.01 0.05 20 0.1 0.05 21* 0.5 0.05 22* Not added. Sample Number Varistor Voltage, V 1mA (V) Non-linear Coefficient (8,1,a) Capacitance (pF) Dielectric Loss (%) Inductance, 1 Mhz (µH) Impedance, 100 Mhz (Ω) 1 32.0 31.2 322 1.8 130 37 2 31.6 32.6 333 1.7 131 35 3 30.4 35.0 342 1.5 132 42 4 30.7 35.4 336 1.6 127 46 5 30.6 34.5 327 1.8 122 43 6* 21.3 22.6 354 5.1 109 34 7* 18.0 16.8 391 7.6 104 35 8 32.7 31.4 330 1.2 136 37 9 33.9 35.2 322 1.4 127 39 10 34.1 38.6 315 1.3 120 45 11 36.8 37.6 300 1.6 128 49 12 39.1 32.2 285 1.2 126 40 13* 38.4 21.1 346 5.7 107 35 14* 38.5 11.6 390 8.8 101 37 15 32.4 34.3 346 1.6 131 46 16 33.7 35.8 321 1.4 122 47 17 34.3 33.4 337 2.1 121 42 18* 37.3 15.7 319 9.1 98 34 19 32.7 36.6 327 1.8 124 45 20 30.0 32.5 350 1.6 122 51 21* 19.8 14.2 383 7.0 105 34 22* 38.5 13.3 265 10.2 91 21

Claims: A composite functional device exhibiting varistor, capacitor and magnetic characteristics comprising: a shaped body of a magnetic ceramic consisting essentially of a Ni-Zn ferrite; and a shaped body of a semiconductive material consisting essentially of zinc oxide and containing at least one of nickel oxide and iron oxide in an amount of from 0.001 to 0.1 mol% in terms of NiO or Fe 2 O 3 . A composite functional device according to Claim 1, characterized in that the amount is from 0.005 to 0.05 mol% in terms of NiO or Fe 2 O 3 . A method for producing a composite functional device exhibiting varistor, capacitor and magnetic characteristics, comprising the steps of: 1) providing a green sheet of magnetic material consisting essentially of a Ni-Zn ferrite; 2) providing a green sheet of semiconductive material consisting essentially of zinc oxide and containing at least one of nickel oxide and iron oxide in an amount of from 0.001 to 0.1 mol% in terms of NiO or Fe 2 O 3 ; 3) laminating said green sheet of a magnetic material and said green sheet of a semiconductive material; and 4) firing said laminate. A method according to Claim 3 for producing a composite functional device, characterized in that the amount is from 0.005 to 0.05 mol% in terms of NiO or Fe 2 O 3 .

Inventors: KANEKO KAZUHIRO, NAKAMURA KAZUTAKA, KANEKO, KAZUHIRO, NAKAMURA, KAZUTAKA
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H05K1
IPC codes: H01F17, H03H7, H01C7, H01F1, H05K1, H01F41