JPH05129108A - Manufacture of varistor - Google Patents

Abstract

PURPOSE:To provide the method for manufacturing a varistor by which life characteristics and a surge resistance can be increased, the manufacturing process can be shortened and the productivity can be increased. CONSTITUTION:When manufacturing a varistor by heating and baking a ceramic compact to form a sintered body and then diffusing a univalent metal oxide or a chemical compound into the sintered body, the temperature of the compact is increased to 1000 deg.C-1450 deg.C in the air and then the air is replaced with a reducing atmosphere and the compact is kept in the reducing atmosphere for 30 minutes or over.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an SrTiO ₃ varistor, which is used as a noise absorbing element, and in particular, it can improve the life characteristics and surge resistance and shorten the manufacturing process to improve the productivity. The present invention relates to a manufacturing method made possible.

[0002]

2. Description of the Related Art A SrTiO ₃ varistor designed to absorb noise by utilizing a voltage non-linear characteristic is a perovskite polycrystal calcination obtained by partially replacing SrTiO ₃ and Sr as main components with Ca and Ba. The compound is converted into a semiconductor by controlling the electron valence and reducing firing, and Na, K, or Li is added to this.
By diffusing a monovalent metal oxide or a compound thereof as described above, an electric barrier is formed at the crystal grain boundaries of the ceramic, and thereby high dielectric constant and varistor characteristics are obtained. In the case of manufacturing such a varistor, conventionally, a ceramic molded body is fired in air to burn a binder, then kept at a temperature rise, then cooled, and then fired again in a reducing atmosphere for firing. Form a union. After that, a metal oxidant is applied to the sintered body and heat-treated to diffuse the oxidant.

[0003]

However, the varistor obtained by the above-mentioned conventional manufacturing method has problems that the electrical characteristics are easily deteriorated, the life characteristics are short, and the surge resistance is small. This is because the monovalent metal oxides or compounds diffused in the crystal grain boundaries have poor ion stability and are therefore inferior in stability, and even after they are diffused, they are likely to be ionized due to environmental changes or biases. The electrical barrier is changed, resulting in deterioration of electrical characteristics, and improvement in this respect is required. In addition, in the above-described conventional manufacturing method, since the two steps of the step of firing the molded body in air to burn the binder and the step of firing it again in a reducing atmosphere are required, the productivity is low, There is also the problem of increased costs.

The present invention has been made in view of the above conventional circumstances, and an object of the present invention is to provide a method of manufacturing a varistor, which can improve the life characteristics and surge withstand capability and shorten the manufacturing process to improve the productivity. I am trying.

[0005]

Means for Solving the Problems The inventors of the present invention have investigated the cause of the deterioration of the life characteristics and surge withstand capability, and have found that it is related to the firing process of the ceramic molded body. That is, in the conventional process in which the molded body is once fired in the air, cooled, and then fired again in a reducing atmosphere to obtain a sintered body, particles are abnormally grown and the particle size is likely to vary. As a result, the surge resistance is lowered, the metal oxide is not smoothly diffused to the grain boundaries of the sintered body, and the oxygen supply is insufficient. As a result of further studies to solve such a problem, it was conceived that the above problems can be improved by substituting a reducing atmosphere in the middle of temperature-elevated sintering in air, and the present invention is realized. It was made. Therefore, the present invention is directed to the production of a varistor in which a ceramic molded body is heated and fired to form a sintered body, and a voltage non-linear characteristic is obtained by diffusing a monovalent metal oxide or compound into the sintered body. In the method, the molded body is heated to 1000 ° C in air.
The temperature is raised to ~ 1450 ° C, and then replaced with a reducing atmosphere for 30
It is characterized by holding for more than a minute. Where the above 30
The reason for setting the time longer than this is that if the time is shorter than this time, the reduction cannot be sufficiently performed and the effect of improving the surge resistance cannot be obtained. If the temperature is raised in the above air, once 200-
It is desirable to keep at 600 ℃ for 30 minutes or more. Furthermore,
The manufacturing method of the present invention can be applied to a single-plate or square-plate varistor, a laminated varistor in which ceramic layers and internal electrodes are alternately laminated, or the like, and is not particularly limited.

[0006]

According to the method of manufacturing a varistor according to the present invention, after the temperature was raised to 1000 ° C to 1450 ° C in the air, the atmosphere was replaced with a reducing atmosphere. Can be made uniform, and appropriate pores are generated at the crystal grain boundaries, so that the metal oxide can be smoothly diffused and oxygen can be sufficiently supplied. As a result, the resistance in the crystal grains is reduced, so that the surge resistance can be improved, and since the reactivity of the metal oxide is increased and stabilized, the deterioration of electrical characteristics can be avoided and the life characteristics can be improved. .. Further, in the manufacturing method of the present invention, since the reducing atmosphere is substituted during the temperature-elevated sintering in the air, only one step is required. Therefore, the productivity is higher than that in the case where two conventional steps are required. While improving
The cost can be reduced.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram for explaining a varistor manufacturing method according to an embodiment of the present invention. In this example, a test performed to confirm the effect of a varistor produced by the manufacturing method of the present invention will be described. First, a method for manufacturing the varistor of this embodiment will be described. SrCO ₃ , C
Raw material powders of aCO ₃ , TiO ₂ , and Er ₂ O ₃ are mixed so as to be Sr _0.9 Ca _0.1 Er _0.003 TiO _3, and pure water is added thereto and mixed by a ball mill. This is dehydrated and dried with a filter, granulated with a mesh, and then calcined at 1200 ° C for 2 hours.

Next, after dry pulverizing the calcinated body, SiO ₂ is added to the calcinated powder at a rate of 0.5 wt%, and polyvinyl alcohol is added as a binder in an amount of 5%.
Further, pure water is added and mixed by a ball mill to form a slurry. Then, this slurry is dried and granulated by a spray dryer, and a pressure of ² ton / cm ² is applied from the granulated powder by a press to form a ceramic compact having a diameter of 10 mmφ and a thickness of 1.5 mmφ.

Then, the molded body is heated and fired according to the firing pattern shown in FIG. First, in the air 200 ~
Heat to a Ta temperature of 600 ° C at a heating rate of 4 ° C / min, and
Hold the time and burn the binder. Subsequently, it is also heated in air at a temperature rising rate of 4 ° C./min to a Tc temperature of 1000 to 1450 ° C., and when this Tc temperature is reached, the atmosphere is replaced with a reducing atmosphere. Tb at 1350 to 1450 ℃ in this reducing atmosphere
Heat to temperature over a tA time of 0-100 min. Subsequently, when the temperature reaches the above Tb temperature, it is held for 30 hours to 120 minutes for tB, and then cooled to obtain a sintered body. When the above Tc temperature = Tb temperature, the tA time is the time for holding in the air after reaching the Tc temperature.

Next, Na ₂ CO ₃ and TiO ₂ were added in a ratio of 2: 1.
Add varnish to the powder mixed in mol ratio to form a paste-like metal oxidizer, and add 1 wt% of this oxidizer to the above sintered body.
%, And heat treated at 1200 ° C. for 3 hours. As a result, the oxidizer is diffused into the crystal grain boundaries of the sintered body to form an electrical barrier.

On both principal surfaces of the thus obtained sintered body, Ag paste having a diameter of 7 mm is applied, and the paste is applied at 800 ° C. for 10 minutes.
Bake for minutes to form electrodes. As a result, the disc type varistor of this embodiment is manufactured.

[0012]

[Table 1]

[0013]

[Table 2]

Tables 1 and 2 show the results of tests conducted to confirm the effect of the varistor obtained by the above manufacturing method. This test was conducted at a Ta temperature of 500 as shown in Table 1.
℃, Tc temperature and Tb temperature are 1000 to 1450 respectively
℃, and 1350 〜 1450 ℃ range, tA time, and tB time 0 to 100 minutes, and 30 to 120, respectively
Samples No. 1 to No. 9 of this example were prepared by changing the range of minutes. Then, the capacitance of each of the sample Nos. 1 to 9, t
An δ, varistor voltage, and nonlinear coefficient were measured. Also,
The change rate of varistor voltage after 2000A surge was measured. For this 2000A surge, a triangular current wave of 8 × 20 μsec was applied twice at 5-minute intervals, and the characteristics before and after application were compared. Furthermore, at a temperature of 150 ° C, a DC voltage of 85% of the varistor voltage is
After applying for 0 hours, the varistor voltage before and after this load and the rate of change of capacitance were measured. Further, for comparison, Comparative Samples No. 10 and No. 11 in which the holding time in the reducing atmosphere, that is, Tb time was set to 10 and 20 minutes, were prepared. Furthermore, a conventional sample M was prepared by a conventional manufacturing method, and the same measurement was performed for these. The conventional sample M was heated in air at 500 ° C. for 2 hours to burn the binder,
Further, it was baked at 1200 ° C. for 2 hours and cooled, and then again baked at 1400 ° C. for 2 hours in a reducing atmosphere.

As is clear from Table 2, in the case of the conventional sample M, the change rate of the varistor voltage after the 2000 A surge was -7.6.
%, The rate of change in varistor voltage and capacitance after 150 ° C / 100 hours is large at -2.3% and -13.4%, respectively, and values that satisfy surge resistance and life characteristics have not been obtained. On the other hand, in the case of Comparative Samples No. 10 and No. 11, the reduction was insufficient due to the short holding time in the reducing atmosphere, and the change rate of the varistor voltage was large at -9.6% and 13.6%. On the other hand, sample Nos. 1 to 9 of this example
In both cases, the change rate of varistor voltage after 2000A surge was small at -2.3 to + 1.0%, and the change rate of varistor voltage and capacitance after 150 ° C / 100 hours were + 2.1% respectively.
It is as small as ~ + 0.7%, -5.6 ~ -1.6%. From this, it can be seen that the surge withstand capability and life characteristics can be improved by substituting a reducing atmosphere during the heating process in air. ..

[0016]

As described above, according to the varistor manufacturing method of the present invention, the ceramic molded body is heated to 1000 ° C. in air.
After heating up to ~ 1450 ° C, it was replaced with a reducing atmosphere and held for 30 minutes or more, so that the grain size can be made uniform and appropriate pores can be formed at the grain boundaries, resulting in a surge withstand capability. In addition to the effects of improving electrical characteristics, life characteristics can be improved by avoiding deterioration of electrical characteristics. Further, since only one step is required, productivity can be improved and cost can be reduced.

[Brief description of drawings]

FIG. 1 is a characteristic diagram for explaining a firing pattern in a varistor manufacturing method according to an embodiment of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenjira Mihara 2-26-10 Tenjin Tenjin, Nagaokakyo, Kyoto Prefecture Murata Manufacturing Co., Ltd.

Claims (1)

[Claims] 1. A method of manufacturing a varistor in which a voltage non-linear characteristic is obtained by forming a sintered body by heating and firing a ceramic molded body and diffusing a monovalent metal oxide or compound into the sintered body. In the above,
A method for producing a varistor, which comprises heating to 00 ° C to 1450 ° C, replacing the atmosphere with a reducing atmosphere, and holding the atmosphere for 30 minutes or more.