JPH02135705A - Manufacture of varistor - Google Patents

Abstract

PURPOSE:To prevent current concentration due to dispersion of grain size for improving surge strength by specifying a rate of temperature rise at the time of baking a varistor. CONSTITUTION:A green sheet made of a ceramic material is cut in a rectangular shape to form a plurality of varistor layers 2, on whose upper surface an internal electrode 3 made of Pt is formed, while the internal electrode 3 and the varistor layer 2 are alternately laminated to obtain a laminated body. Next, the laminated body is so baked that rate of temperature rise may become above 300 deg.C/hr in above 600 deg.C. Then, growth of particles at a temperature rise is suppressed so that the sintered body 4 having small dispersion of grain size can be obtained. Thereby, current concentration due to dispersion of grain size is eased up while so much improving surge strength.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a varistor that functions as a voltage non-linear resistor, and in particular improves surge resistance by preventing variation in particle size in the thickness direction of the varistor element. It relates to a manufacturing method that makes it possible to do so.

[Conventional technology]

Conventionally, a varistor has been known as a resistor element whose resistance value changes non-linearly depending on an applied voltage. Examples of such a varistor include a disk-shaped varistor, which is formed by forming electrodes on both sides of a varistor element made of semiconductor ceramics;
Alternatively, there is a laminated type varistor in which varistor layers and internal electrodes are alternately laminated and integrally sintered.

When manufacturing such a varistor, various additives that make the varistor function appear are mixed with ZnO, and then this is formed into a desired shape, and this is 100~b to obtain a sintered body. . The above-mentioned varistor is required to have a large nonlinear coefficient α and surge resistance.

[Problem that the invention seeks to solve]

However, although the above-mentioned conventional varistor can obtain relatively desired characteristics with respect to α, there is a problem in that the surge resistance is insufficient.

One of the causes of this is the variation in grain size in the thickness direction of the sintered body. In other words, if there is variation in the grain size in the thickness direction, the current tends to concentrate mainly on the larger grains,
In some cases, Joule heat is generated in this portion, causing a problem of damaging the element.

Furthermore, in the case of a multilayer varistor with a low varistor voltage, the varistor layer between the internal electrodes is very thin, so the number of particles in the thickness direction of this varistor layer may be as small as at least 2 to 3 particles. Therefore, if large particles are mixed in this varistor layer, it is very susceptible to the above-mentioned influence.

An object of the present invention is to provide a method for manufacturing a varistor that can improve surge resistance by preventing current concentration due to the above-mentioned variation in particle size.

[Means for solving problems]

The inventors of the present invention have focused on the fact that the temperature increase rate during firing is the cause of the above-mentioned variation in particle size. In other words, the conventional heating rate is 100 to 100 mm long.Although there are relatively few nuclei with a length of 100 to 100 mm, some of them grow to three times the average grain size.The present inventors believe that this low heating rate The present invention was developed based on the idea that the object of the present invention can be achieved by finding a temperature increase rate that can suppress the growth of particles.

Accordingly, the present invention is characterized in that, in a method for manufacturing a varistor, the temperature increase rate at at least 600° C. or higher during firing of the varistor is set to 300° C./hr or higher.

Here, the upper limit of the temperature increase rate is limited depending on the capacity of the heating device, for example, 1000°C/hr, 3000°C/hr, etc.
C./hr or more, and there is no need to specifically limit it. In the temperature raising step, the rate of temperature increase until reaching 600.degree. C. is not particularly specified, but it is preferably lower than 300.degree. C./hr.

[Effect]

According to the method for manufacturing a varistor according to the present invention, the temperature increase rate during firing is set at 300°C/hour or more, that is, faster than the conventional temperature increase rate of 100~b, so that particles are reduced during the temperature increase process. Numerous growth nuclei (occur, but they do not grow sufficiently; therefore, the average particle size is smaller (the particle size distribution is narrower, 3 times the average particle size) compared to the conventional case where the temperature is raised slowly. There are almost no particles that exceed 100 nm.As a result, the variation in particle size can be reduced, which alleviates the concentration of current and improves surge resistance accordingly.

〔Example〕

Hereinafter, the present invention will be described in detail.6 Embodiment 1 This embodiment will be explained by taking as an example a case where the present invention is applied to a disk type varistor.

■ First, Z n O (97,811o
i'%) powder, B is 0s (0
,5go1%) ,MnO(0,5so 1%)
, Cot 03 (0,5rgoR%) , S
bz 0s (0.7 moj!%), and an organic binder is mixed thereto to form a green sheet.

Punch out this green sheet into a disk shape, 8flφX1
．． A varistor element of Omt is formed.

(2) Next, the above varistor element is heated and baked at 1100° C. in air. In this case, the temperature is raised slowly from the start of the temperature rise until it reaches 600°C to prevent the occurrence of cranks and the like. When the temperature exceeds 600°C, increase the temperature increase rate to 300°C or more per hour, and
Hold when it reaches 0°C. As a result, the growth of particles during the temperature rise is suppressed, and a sintered body with small variations in particle size can be obtained.

■ Next, Ag paste is applied to both surfaces of the sintered body and baked to form electrodes, and then lead processing is performed.
The disk-shaped varistor of this embodiment is manufactured by performing the dip-exterior.

As described above, according to the manufacturing method of this example, the temperature increase rate of the varistor element was set to 300°C for an hour or more when the temperature exceeded 600°C from the start of temperature rise, so that the particle size could be reduced without causing any cranking or the like. Variations can be reduced, and as a result, current concentration can be alleviated, and surge resistance can be improved.

Next, let's look at the effects of this example! ! I will explain the experiment I conducted to confirm this.

In this experiment, we prepared a large number of varistor elements prepared in the above ①, and set the temperature increase rate in the above ① to 200℃/hr ~
VI of each varistor when changing to 3000℃8r
IIA+ surge resistance was measured.

The results are shown in Table 1. As is clear from the table,
When the heating rate is 200.250°C/hr, little improvement in surge resistance is observed, but when the heating rate is 300°C/hr,
It can be seen that the surge resistance is greatly improved to 1370 to 1400 A when the temperature rise rate is exceeded, regardless of the temperature increase rate set.

Embodiment 2 Next, a case in which the manufacturing method of the present invention is applied to a laminated varistor will be described as an example.

1 to 3 are diagrams showing a rectangular parallelepiped-shaped multilayer varistor. This laminated varistor 1 has left and right end surfaces 4a of a sintered body 4 formed by alternately laminating varistor layers 2 and internal electrodes 3 and sintering them together.

4b is covered with an external electrode 5.

Further, the end faces 3a of each of the internal parts 1j3 are the left 1 right end faces 4a, 4. b, and the other parts are buried in the sintered body 4.

When manufacturing the laminated varistor 1 having the above structure,
As explained in Example 1 above, a green sheet having the same composition as in (1) above is cut into rectangular shapes to form a plurality of varistor layers 2, and internal electrodes 3 made of PT are formed on the upper surfaces of the varistor layers 2. In addition, the internal electrode 3. Barista layer 2
are alternately laminated to obtain a laminate.

Next, the above laminate is fired in the same manner as in (1) above so that the temperature increase rate at 600 °C or higher is 300 °C/hr or higher,
A sintered body 4 is obtained. After that, the left and right end surfaces 4 of the sintered body 4 are
External electrodes 5 made of Ag--Pd are formed on a and 4b by baking.

In this way, also in the method for manufacturing the multilayer varistor 1 of this embodiment, since the temperature increase rate during firing is set to 300° C./hr or more, it is possible to eliminate variations in grain size and improve surge resistance.

Furthermore, in the case of the laminated varistor 1, since each varistor layer 2 is very thin, it is possible to solve the conventional problem that the varistor layer 2 is susceptible to current concentration due to variations in particle size.

Next, an experiment conducted to confirm the effects of the multilayer varistor 1 of this example will be described.

In this experiment, a large number of laminates prepared by the method of the above example were prepared, and the heating rate of each varistor was varied from 200 to 30 OO℃/hr. vlIIA+ surge resistance was measured. Note that the thickness of the varistor layer was adjusted so that the varistor voltages of each multilayer varistor were separated.

The results are shown in Table 2.As is clear from the table,
Also in the case of the multilayer varistor 1 of this embodiment, the temperature increase rate is 30
If the temperature exceeds 0°C/hr, the surge resistance will be 75 in any case.
It can be seen that there is a significant improvement of ~100 A.

〔Effect of the invention〕

As described above, according to the method for manufacturing a varistor according to the present invention, the temperature increase rate at at least 600°C or higher during firing is set to 300°C/hr or more, thereby preventing variation in particle size and improving surge resistance. There is an effect that can be done.

[Brief explanation of the drawing]

1 to 3 are diagrams for explaining a method of manufacturing a laminated varistor according to a second embodiment of the present invention, in which FIG. 1 is an exploded perspective view of the Mi-layer pear-shaped varistor, and FIG. 2 is a perspective view thereof. FIG. 3 is a sectional view thereof. In the figure, 1 is a laminated varistor (varistor) and 4 is a sintered body.

Claims (1)

[Claims] (1) A method for producing a varistor that functions as a voltage nonlinear resistor, characterized in that the heating rate at least 600°C or higher is 300°C/hour or higher when firing the ceramic raw material powder that exhibits the varistor function. A method of manufacturing a barista.