JPH04280603A - Laminated varistor - Google Patents

Abstract

PURPOSE:To enhance surge resistivity by covering the edge of the tip part of an inner electrode extended into a calcined body. CONSTITUTION:A laminated body is obtained by laminating a ceramic green sheet, a suitable number of ceramic green sheets are laminated and press-bonded on the upper side and the lower side. Inner electrodes 23 and 24, consisting of electrode paste, are formed in the above-mentioned sintered body 31. Insulating films 25 and 26, on which insulating paste is baked, are formed on the tip part of the inner electrodes 23 and 24. To be more precise, the edge of the inner electrodes 23 and 24, positioned in the sintered body 31, is covered by the insulating films 25 and 26. As a result, concentration of current is prevented, and surge withstand voltage can be enhanced.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laminated varistor in which a plurality of internal electrodes are laminated via semiconductor ceramic layers, and more particularly to a laminated varistor suitably used as a chip-type noise filter.

0002

2. Description of the Related Art Conventionally, varistors have been widely used as noise absorbing elements for absorbing abnormal voltages. In recent years, as electronic components have become more and more chip-based, varistors are also required to be ultra-miniaturized in order to achieve high-density packaging, and as circuits become more integrated, demands for lower voltages have also become stronger. It's coming.

[0003] In response to the above requirements,
A laminated varistor has been proposed (Special Publication No. 58-2392)
Publication No. 1, etc.). An example of a conventional multilayer varistor will be described with reference to FIGS. 2 and 3. First, as shown in FIG. 2, electrode pastes 3 and 4 are printed on the upper surfaces of rectangular ceramic green sheets 1 and 2 mainly made of a material exhibiting varistor characteristics. Next, a plurality of ceramic green sheets 1 and 2 printed with electrode pastes 3 and 4 are laminated alternately, and further, ceramic green sheets 5 and 6 and ceramic green sheets 7 and 8 are laminated above and below. Get a body. The obtained laminate is compressed in the thickness direction and then sintered to obtain the sintered body 10 shown in FIG. 3. Inside the sintered body 10, an internal electrode 3 based on the electrode paste is formed.
, 4 are arranged so as to overlap each other with a semiconductor ceramic layer interposed therebetween.

[0004] In this specification, the electrode paste for internal electrodes and the internal electrodes formed after firing will be described with the same reference numerals. By providing external electrodes 11 and 12 on both end faces of the sintered body 10, a laminated varistor 13 is obtained. In the laminated varistor 13,
A plurality of internal electrodes 3 and 4 are stacked so as to overlap with each other via a semiconductor ceramic layer, and in this structure, 50A to
To obtain surge resistance of about 100A, internal electrodes 3 and 4
The total number must be about 10 to 20.

On the other hand, as the number of laminated internal electrodes 3 and 4 increases, the capacitance increases, making it impossible to use it as a signal line noise filter. Therefore, in the conventional multilayer varistor, in order to reduce the capacitance, the number of laminated ceramic green sheets 1 and 2 coated with internal electrodes, that is, the number of laminated internal electrodes, was reduced. For example, several 10p
In order to achieve a capacitance of F, the number of semiconductor ceramic layers sandwiched between the internal electrodes was one layer, that is, the internal electrodes 3 and 4 had two layers in total.

[0006]

[Problem to be Solved by the Invention] However, as described above, the smaller the number of stacked internal electrodes 3 and 4, the more
Current concentration at the edges of the internal electrodes 3 and 4 located within the sintered body 10 became strong, and the surge resistance was forced to decrease significantly. As a result, it has become difficult to use the multilayer varistor 13 as a noise filter.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a multilayer varistor having a structure capable of increasing surge resistance without increasing the number of semiconductor ceramic layers sandwiched between internal electrodes, that is, the number of laminated layers.

[0008]

[Means for Solving the Problems] The present invention provides a laminated varistor in which a plurality of internal electrodes are laminated via semiconductor ceramic layers having varistor properties within a sintered body, and is characterized by having the following configuration. That is, in the present invention,
Among the edge portions of the internal electrode located within the sintered body, at least the edge of the tip portion of the internal electrode extended into the sintered body is covered with an insulating coating.

[0009]

[Function] In view of the fact that current concentration, which is one of the factors that reduces surge resistance, occurs at the edge portion of the internal electrode, in the present invention, the edge of the internal electrode is covered with an insulating coating. . That is, by covering at least the edge of the tip portion of the internal electrode with an insulating coating, current concentration is prevented, thereby significantly increasing surge resistance.

0010

DESCRIPTION OF THE EMBODIMENTS A laminated varistor according to an embodiment of the present invention will be described with reference to FIGS. 1, 4 and 5. To obtain the laminated varistor of this example, first, electrode pastes 23 and 24 for forming internal electrodes are printed on the upper surfaces of rectangular ceramic green sheets 21 and 22 shown in FIG. 4. The ceramic green sheets 21 and 22 are made mainly of semiconductor ceramic material powder having varistor properties, to which an organic binder, vehicle, etc. are added. As a semiconductor ceramic having such varistor characteristics, a material mainly composed of ZnO to which an appropriate semiconducting agent is added is used. Further, as the electrode pastes 23 and 24, a material made by mixing an organic vehicle with a powder of a metal such as Ag or Pd or an alloy such as Ag-Pd can be used, and a known method such as a screen printing method can be used. printed by a method.

In producing the laminated varistor of this embodiment, after printing the electrode pastes 23 and 24, the edge portions of the electrode pastes 23 and 24 located within the ceramic green sheets 21 and 22 are coated. Insulating pastes 25 and 26 are printed on. Insulating paste 25,2
As No. 6, glass powder coated with an organic vehicle and a binder and kneaded is used, but other insulating materials may be used instead of glass powder as long as they can withstand the firing process described below. can.

Next, ceramic green sheets 21, 2
A suitable number of ceramic green sheets 27, 28 and 29, 30 are laminated above and below, and the ceramic green sheets 27, 28 and 29, 30 are laminated and pressed together in the thickness direction to obtain a laminate. By firing the obtained laminate at a predetermined temperature, a sintered body 31 shown in FIG. 1 is obtained. Inside this sintered body 31, the electrode paste 23 is
, 24 are formed. Note that the insulating pastes 25, 26 are baked onto the tip portions of the internal electrodes 23, 24 located within the sintered body 31 to form insulating coatings 25, 26. That is,
The edges of the internal electrodes 23 and 24 located within the sintered body 31 are covered with insulating coatings 25 and 26.

[0013] In this specification, insulating paste 2
The insulating coating formed by baking 5 and 26 will be described with the same reference numerals as those of the insulating paste. Finally, both end surfaces 31a of the sintered body 31
, 31b are provided with external electrodes 32, 33, thereby obtaining the laminated varistor 34 of this embodiment. external electrode 32,
33 can be formed by a known and commonly used method conventionally used to obtain a laminated varistor. FIG. 5 shows the appearance of the obtained laminated varistor 34.

In the multilayer varistor 34 of this embodiment, the internal electrodes 23 and 2 in the sintered body 31 are separated by the insulating coatings 25 and 26.
Since the edges of 4 are covered, the surge resistance can be effectively increased as is clear from the experimental examples described later. In the above embodiment, the laminated varistor 34 in which two layers of internal electrodes 23 and 24 were formed was explained, but as shown in FIG. Three or more internal electrodes may be formed within the sintered body 31. In this case as well, since the edges of the internal electrodes 23 and 24 disposed within the sintered body 31 are covered with the insulating coatings 25 and 26, the surge resistance is greatly increased as in the above embodiment.

Furthermore, as shown in FIG. 4, the insulating pastes 25 and 26 were applied to all the edges of the internal electrodes 23 and 24 located within the ceramic green sheets 21 and 22. The edge portion to which the insulating paste is applied, that is, the edge portion of the internal electrode covered with the insulating film, is not limited to the pattern of the embodiment shown in FIG. That is, as shown in FIG. 7, as long as the insulating coating 25 is formed to at least cover the edge 23a of the tip portion of the internal electrode 23 extending inside the sintered body 31, the surge resistance can be greatly increased. be able to.

Next, a specific experimental example will be explained. Z
97.8 mol% nO powder, 0.5 mol% CoO powder
, 0.5 mol% of MnO powder, 0 of Sb2O3 powder
．． Each powder was weighed to contain 7 mol % and Bi2 O3 powder at a ratio of 0.5 mol %, and was used as a raw material. Note that each oxide powder used had a purity of 99.
9% or more was used. The prepared raw materials were mixed with pure water in a ball mill for 24 hours. After mixing, the mixture was filtered, dried and calcined at a temperature of 800° C. for 2 hours.

Next, the calcined raw material was pulverized and dispersed in alcohol together with polyvinyl butyral resin to obtain a slurry. Using the obtained slurry, a ceramic green sheet was formed by a doctor blade method. Next, the obtained ceramic green sheet was punched into a rectangular shape of a predetermined size to prepare ceramic green sheets 21 and 22 shown in FIG. 4. Electrode pastes 23 and 24 made of an Ag-Pd alloy and an organic vehicle were printed on the upper surfaces of the ceramic green sheets 21 and 22, and dried. Next, after drying the electrode pastes 23 and 24, insulating pastes 25 and 26 are printed as shown in FIG.
were laminated as shown in FIG. 4 and pressed together in the thickness direction to obtain a laminate.

Next, the obtained laminate was fired at a temperature of 950° C. for 2 hours. After that, an external electrode paste mainly composed of Ag was applied to both end faces of the obtained sintered body, and the paste was heated at 700°C.
The external electrodes 32 and 33 shown in FIG. 1 were formed by baking at a temperature of . An impact current of 8/20 μsec was applied twice at an interval of 5 minutes to the laminated varistor of the example obtained as described above, and the varistor voltage (V1 mA) was 10
The limiting current value (surge withstand capacity) that does not change by more than % was investigated.

For comparison, a laminated varistor was prepared as Comparative Example 1, which was manufactured in exactly the same manner as in the above embodiment, except that the insulating coating using the insulating paste was not formed. In the same way, we investigated the surge resistance. Furthermore, a laminated varistor having a structure using the same materials as in Comparative Example 1 but with a total number of internal electrodes of 10 was prepared as Comparative Example 2, and its surge resistance was measured in the same manner as the laminated varistor of the example.

Table 1 shows the characteristics of each laminated varistor of Example and Comparative Examples 1 and 2.

[0021]

[Table 1]

As is clear from Table 1, the surge resistance of the laminated varistor of the example is about 1.5 times higher than that of the laminated varistor of Comparative Example 1 having the same number of internal electrodes. Furthermore, it can be seen that the capacitance is significantly lower and the surge resistance is also significantly increased compared to the laminated varistor of Comparative Example 2 in which the total number of internal electrodes is 10.

[0023]

Effects of the Invention According to the present invention, at least the edges of the tip portions of the internal electrodes extended into the sintered body are covered with an insulating film, so that the lamination of the semiconductor ceramic layers sandwiched between the internal electrodes can be avoided. It becomes possible to significantly increase surge resistance without increasing the number of internal electrodes. Therefore, it is possible to provide a multilayer varistor that is small but has a large surge resistance. The multilayer varistor of the present invention is suitably used as a chip-type noise filter that is required to be ultra-miniaturized.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a laminated varistor according to an embodiment of the present invention.

FIG. 2 is an exploded perspective view illustrating a ceramic green sheet used to obtain a conventional laminated varistor and the printed shape of an electrode paste printed thereon.

FIG. 3 is a cross-sectional view showing an example of a conventional laminated varistor.

FIG. 4 is an exploded perspective view showing the shapes of a ceramic green sheet and an electrode paste and an insulating paste printed thereon used to obtain a laminated varistor according to an embodiment of the present invention.

FIG. 5 is a perspective view showing the appearance of the laminated varistor of the example.

FIG. 6 is a sectional view showing another embodiment of the present invention.

FIG. 7 is a plan cross-sectional view for explaining another example of an edge portion on which an insulating coating is formed.

[Explanation of symbols]

23, 24...Internal electrode 25, 26...Insulating coating 31...Sintered body 34...Laminated varistor

Claims (1)

[Claims] 1. A laminated varistor in which a plurality of internal electrodes are laminated within a sintered body through semiconductor ceramic layers having varistor characteristics, at least of the edge portions of the internal electrodes located within the sintered body. A laminated varistor characterized in that an edge of a tip portion of an internal electrode extending into a sintered body is covered with an insulating coating.