JPH05326216A - Manufacture of varistor - Google Patents

Abstract

PURPOSE:To suppress fluctuation of electrical characteristics such as varistor voltage or nonlinear coefficient by making uniform the temperature or the atmosphere when oxide is thermally diffused to a semiconductor ceramics while furthermore prevent adhesion of oxides during cooling process of heat treatment and enhance life characteristics and surge resistance. CONSTITUTION:When an oxide or a compound of Li, K is thermally diffused to a semiconductor ceramic subjected to reduction baking thus producing a varistor, the semiconductor ceramic and the oxide or compound of Li, K are placed in a ceramic container which is then subjected to heat treatment while rotating. Air or oxygen is introduced during cooling process of heat treatment where the temperature is in the range of 900-700 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a varistor in which a voltage non-linear characteristic is obtained by oxidizing a ceramic grain boundary, and in particular, when a Li or K oxide or its compound is thermally diffused in a semiconductor ceramic. Manufacturing in which temperature and atmosphere are made uniform to avoid variations in varistor voltage, non-linear coefficient, etc., and also to prevent oxides from adhering in the cooling process during heat treatment and improve life characteristics and surge withstand capability. Regarding the method.

[0002]

2. Description of the Related Art Generally, varistors containing SrTiO ₃ based semiconductor ceramics as a main component are widely used as surge absorbing elements and noise absorbing elements in electronic devices and the like. In this varistor, a perovskite-based polycrystalline sintered body in which SrTiO ₃ and Sr are partially replaced with Ca or Ba as main components is converted into a semiconductor by electron valence control or reduction firing, and L
By diffusing the oxides of i and K, or their compounds, an electrical barrier is formed at the crystal grain boundary, and thereby a large dielectric constant and voltage non-linear characteristic are obtained. In the case of manufacturing such a varistor, conventionally, a molded body made of a SrTiO ₃ based semiconductor ceramic is fired in a reducing atmosphere to form a semiconductor porcelain, and the semiconductor porcelain is formed from an oxide such as Li or a compound thereof. A general method is to apply the above paste, then arrange the semiconductor porcelain in a dish-shaped box, and heat-treat in this state to thermally diffuse the oxide into the semiconductor porcelain.

[0003]

By the way, a high dielectric constant,
In order to obtain a varistor with high surge resistance and excellent life characteristics, it is necessary to delicately control the diameter of semiconductor particles and the state of grain boundaries that make up the varistor. It is necessary to control it uniformly so that there is no variation in temperature. However, the above-mentioned conventional manufacturing method has a limit in controlling the temperature and the atmosphere uniformly, and as a result, there is a problem that variations in electrical characteristics such as varistor voltage, nonlinear coefficient, and capacitance are likely to occur. is there. Further, in the above conventional manufacturing method, during the cooling process during the heat treatment, the oxides of Li and K tend to adhere to the surface of the semiconductor porcelain and thus remain in the state of being easily ionized. There is a problem that it is a factor that lowers the

The present invention has been made in view of the above-mentioned conventional circumstances. The temperature and atmosphere during heat treatment can be made uniform to avoid variations in electrical characteristics and to prevent oxides from adhering during cooling. It is an object of the present invention to provide a varistor manufacturing method capable of improving life characteristics and surge resistance.

[0005]

Means for Solving the Problems The inventors of the present invention examined the process of heat treatment from the viewpoint that the heat treatment process of thermally diffusing an oxide or the like into a semiconductor porcelain is an important process that determines the electrical characteristics. .. That is, in the conventional method, since the semiconductor porcelain is placed on the box and the box is fixed in the heat treatment furnace, the oxide applied to the semiconductor porcelain scatters from the temperature rise during the heat treatment to the top temperature. However, the oxide forms the main atmosphere. As a result, we paid attention to the fact that the diffusion of oxides to the grain boundaries was suppressed. Therefore, the semiconductor porcelain is housed in a container to prevent the oxide from scattering, and the semiconductor porcelain is moved by rotating the container together, whereby the temperature and the atmosphere can be made uniform and variations can be eliminated. Found. Further, in order to prevent the adhesion of oxides during cooling, by actively supplying air or oxygen during this cooling,
The present invention is based on the idea that the above oxide can be removed and oxygen can be applied to the oxide diffused in the grain boundary to stabilize the oxide.

Therefore, the present invention provides a method for producing a varistor in which a voltage non-linear characteristic is obtained by thermally diffusing Li, K oxides or their compounds into a semiconductor ceramic, and in the method, the semiconductor ceramic and the Li, K are combined. The oxide or its compound is heat-treated in a magnetic container while rotating the container, and the
It is characterized by introducing air or oxygen between 0 ℃ and 700 ℃.

The reason why the air or oxygen is introduced is between 900 ° C. and 700 ° C. because the anti-adhesion effect cannot be obtained outside this range. That is, when the temperature exceeds 900 ° C., the oxide diffused in the crystal grain boundaries is released again to the outside, and as a result, the electrical characteristics become unstable. On the other hand, if the temperature is lower than 700 ° C., the oxide will adhere to the surface of the semiconductor porcelain, and the effect introduced will not be obtained.

[0008]

According to the varistor manufacturing method of the present invention, the semiconductor porcelain, the oxides of Li and K, and the like are housed in the magnetic container, and the heat treatment is performed while rotating the container. Oxygen diffusion to the grain boundaries can be prevented and the semiconductor porcelain moves with the rotation of the magnetic container, so that the temperature and atmosphere in the container can be made uniform. As a result, variations in electrical characteristics such as varistor voltage, non-linear coefficient, and electrostatic capacitance can be suppressed, and a large amount of heat treatments can be performed at one time, cost can be reduced, and productivity can be improved.

Further, according to the present invention, since air or oxygen is introduced between 900 ° C. and 700 ° C. in the cooling process during the heat treatment, oxides adhering to the semiconductor porcelain can be removed and the grain boundaries can be removed. Oxygen can be supplied to the oxides diffused into the substrate, and as a result, life characteristics and surge withstand capability can be improved.

[0010]

EXAMPLES Examples of the present invention will be described below. In this embodiment, a varistor is manufactured by the manufacturing method according to the present invention,
The test conducted for confirming the effect of the varistor thus obtained will be described. First, SrTi of this example
One manufacturing method for obtaining an O ₃ -based varistor will be described. S
Raw material powders of rCO ₃ , CaCO ₃ , TiO ₂ , and Er ₂ O ₃ were respectively added to Sr _0.9 Ca _0.1 Er _0.003 TiO 2.
Mix so as to be ₃ , add pure water to this, and mix with 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, the above calcined body is dry pulverized to form a calcined powder, and SiO ₂ is added to the powder in a proportion of 0.5 wt%, and polyvinyl alcohol is added as a binder to the powder.
%, At the same time, pure water is further added and mixed by a ball mill to form a slurry. Next, this slurry is dried with a spray dryer and then granulated. Next, this granulated powder was applied with a pressure of ² t / cm ² to obtain a diameter of 10 mmφ and a thickness of
Form a 1.5 mm disc-shaped body.

Next, the above-mentioned molded body is heated in air from 400 ° C. to 600 ° C. to burn the binder, and subsequently, it is replaced with a reducing atmosphere and heated to 1400 ° C. It is fired for 2 hours to obtain a semiconductor ceramic.

Further, Li ₂ CO ₃ and TiO ₂ are mixed in a ratio of 2: 1.
Varnish is added to the oxide powder mixed at a mol ratio to form an oxidation paste, and this paste is applied to the above semiconductor porcelain.
Apply wt%.

Then, a cylindrical alumina porcelain pot having a diameter of 150 mm, in which an introduction hole having a diameter of 10 mm is formed in the lid and the bottom, is prepared, and the alumina porcelain pot is filled with the semiconductor porcelain coated with the above-mentioned oxidation paste. To do. In this case, the shape of the porcelain pot may be a prismatic shape or a polygonal shape.

Then, the alumina porcelain pot is placed at 0.5 rpm.
While rotating at a rotation speed of 1, the temperature is raised to 1200 ° C. and heat treatment is performed for 5 hours. Then, air or oxygen is introduced at 100 l / hour while the temperature drops from 900 ° C to 700 ° C during the cooling. By this heat treatment, the semiconductor porcelain is agitated in the porcelain pot, and the oxide is thermally diffused in the crystal grain boundaries of the semiconductor porcelain to form an electrical barrier. Further, during the cooling, the oxide attached to the semiconductor porcelain is blown off, and oxygen is supplied to the oxide diffused in the crystal grain boundaries.

Finally, a diameter of 7 is formed on both main surfaces of the semiconductor porcelain.
After applying mmφ silver paste, bake at 800 ℃ for 10 minutes to form the electrode. As a result, the varistor of this embodiment is manufactured.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Table 3]

Tables 1 to 3 show the results of tests conducted to confirm the effect of the varistor manufactured by the method of the above embodiment. In this test, heat treatment was performed while rotating the alumina porcelain pot, and at the time of cooling this, air,
And oxygen are introduced, and the introduction timing of these is from 1100 ° C to 100
A large number of samples were prepared by changing the temperature to 500 ° C in steps of ° C. Then, the varistor voltage V _1mA , the non-linear coefficient α, and the capacitance nF of each sample were measured and their variations were examined (see Table 1). This variation is 3
Shown as σ / average × 100 (%). In addition, 2000 of each sample above
The capacitance, non-linear coefficient, and varistor voltage change rate after A surge were measured (see Table 2). This 2000A surge was applied twice with a triangular current wave of 8 × 20 μsec at 5 minute intervals, and the characteristics before and 30 minutes after the application were compared. In addition, 15
A DC voltage of 85% of the varistor voltage was applied for 100 hours at a temperature of 0 ° C, the rate of change of each characteristic before and after this load was measured, and the life characteristics of each sample were investigated (see Table 3). Note that a monitor was prepared for comparison, and the same measurement was performed for this. In this monitor, the above-mentioned semiconductor porcelain is lined up on a co-fabric setter (box), and this is set to 1200 by a box-type batch furnace.
It was fired at 5 ° C. for 5 hours and prepared without introducing air and oxygen during cooling.

As is clear from each table, in the case of the conventional sample in which air and oxygen were not introduced, the variation of the varistor voltage was as large as 26.8%, and the rate of change after the surge application was -12.5 to
Large at 18.1%, and the life characteristics are 7.1 to −2.
It is as low as 4.3%. Also, the introduction time of air and oxygen is 1100,1000.
When the temperature is as high as ℃, the varistor voltage is 44.1,38.2%, and the capacitance is 13.8,23.9%. Furthermore, the rate of change after application of surge was large at -28.4 to -9.5%, and the life characteristics were low at 4.6 to -17.3%, respectively, because oxides diffused to the grain boundaries were released. On the other hand, if the above introduction time is as low as 600 ℃ and 500 ℃,
The variation in each characteristic, the rate of change after the application of surge, and the life characteristic tend to increase, and it can be said that a part of the oxide is attached.

On the other hand, in the case of the sample of this embodiment in which the introduction time was between 900 ° C. and 700 ° C., the variations in both varistor voltage and electrostatic capacitance were small at 8.6% to 7.6%. In addition, the rate of change after applying surge is −3 for ΔV _1mA .
1 to −1.1%, Δα is −4.4 to 0.3%, ΔCap is −5.3
It is significantly reduced to ~ 1.8%. Furthermore, in the life characteristics, ΔV _1mA is -3.2 to 2.3%, Δα is -6.2 to 0.1%.
%, ΔCap is significantly improved from -4.6 to -1.9%,
Above all, the temperature around 800 ℃ is the most stable, and this ± 100 ℃
The change in each characteristic is less than 5%. This allows
By performing heat treatment while rotating in a porcelain pot, and introducing air and oxygen between 900 and 700 ° C during cooling, it is possible to greatly reduce variations in electrical characteristics and to significantly improve surge withstand and life characteristics. You can see that it can be improved.

[0023]

As described above, according to the method of manufacturing a varistor of the present invention, the semiconductor porcelain and the oxide of Li or K, etc. are housed in a magnetic container, and the container is heat-treated while rotating the container. , 900 ℃ -700 in the cooling process
Since air or oxygen was introduced between ℃, the temperature and atmosphere during heat treatment can be made uniform, and there is the effect that variations in electrical characteristics such as varistor voltage, nonlinear coefficient, and capacitance can be prevented. Oxides adhering to the semiconductor porcelain can be removed, and the life characteristics and surge withstand capability can be improved.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yasunobu Yoneda 2-26-10 Tenjin Tenjin, Nagaokakyo-shi, Kyoto Murata Manufacturing Co., Ltd.

Claims (1)

[Claims] 1. A method for producing a varistor in which a voltage nonlinear characteristic is obtained by thermally diffusing Li or K oxide or a compound thereof into a semiconductor ceramic containing SrTiO ₃ as a main component. The semiconductor porcelain and the oxide of Li or K, or a compound thereof,
While heat-treating the magnetic container in a magnetic container while rotating the container, the cooling process during this heat treatment should be performed at 900 ℃ to 700 ℃.
A method of manufacturing a varistor, characterized in that air or oxygen is introduced between the two.