JP2000077209A - Varistor and producing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a varistor which has stable characteristics and high reliability. SOLUTION: A varistor 10 has a varistor element assembly 12, inner electrodes 14, 16, which are formed inside the varistor element assembly 12 and outer electrodes 18, 20 to be connected to the inner electrodes 14, 16. Moisture content contained in the varistor element assembly 12 is adjusted less than 10 ppm equivalent in molecule H2O. To obtain the varistor 10, first a mold is manufactured by molding varistor materials, and a binder contained in the mold is combusted, and then the varistor element 12 is formed by sintering the mold. At this time, concentration of steam in a furnace where the binder is combusted is set below -5 deg.C when converted into the dew point temperature of atmospheric pressure, and the concentration of steam in the furnace where the mold is sintered is set lower than -10 deg.C when converted into the dew point temperature of the atmospheric pressure.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a varistor and a method of manufacturing the same, and more particularly to, for example, a varistor used to protect an IC or the like by applying a current when a predetermined voltage or more is applied.

[0002]

2. Description of the Related Art As shown in FIG. 1, a varistor 10 includes a varistor element body 12, for example. A plurality of internal electrodes 14 and 16 are formed in the varistor element 12. These internal electrodes 14 and 16 are formed so as to be exposed on alternately opposed side surfaces. External electrodes 18 and 20 are respectively provided on the side surfaces of the varistor element 12 where the internal electrodes 14 and 16 are exposed.
Is formed. The varistor element 12 has a characteristic that the resistance value sharply decreases when a voltage higher than a certain voltage is applied.
Therefore, when a high voltage is applied between the external electrodes 18 and 20, a current flows through the varistor element 12.

Utilizing such characteristics, it is used for protection of a circuit including an IC or the like. When a high voltage is applied, a current flows through the varistor 10 to prevent the high voltage from being applied to the IC. Can be. That is, the varistor 10 is characterized by exhibiting a high voltage suppression capability by utilizing the voltage-current nonlinearity of the ceramic grain boundary.
This voltage-current non-linearity is exerted, for example, by adding several kinds of additives to zinc oxide and presenting these additives at the grain boundaries. To this end, it is of course important to uniformly mix these additives from the raw material preparation process to the processing process, but since there are many factors that affect the grain boundary formation during firing, There are many points to keep in mind, including controlling the material of the firing box and the atmosphere in the furnace. The shape of the varistor 10 is not limited to the stacked type, and there is a varistor 10 in which an electrode is formed on an opposing surface of a plate-like varistor element.

In order to manufacture the varistor 10, for example, oxides such as Bi, Co, Mn, Sb, and Al are mixed with ZnO at a predetermined ratio, wet-pulverized, calcined, and wet-pulverized. A material is formed. After granulating the varistor material, a binder is mixed to form a green sheet, and the electrode material is printed on the green sheet. A plurality of green sheets on which the electrode material is printed are laminated, and a green sheet as an exterior is further laminated and pressed, and cut into a predetermined size to form a molded body. The obtained compact is put into a furnace, and the binder contained in the compact is burned. Further, the molded body obtained by burning the binder is fired in a firing furnace, and the laminated green sheets become varistor element bodies, and the electrode materials printed on the green sheets become internal electrodes 14 and 16. And the internal electrode 1
The varistor 10 is formed by forming the external electrodes 18 and 20 on the exposed portions 4 and 16. Before the molded body is placed in a firing furnace, a material for an external electrode is printed on the side surface of the molded body, and then fired to form the varistor element and the internal electrode, and simultaneously form the external electrodes 18 and 20. May be.

[0005]

However, since zinc oxide, which is a raw material, has semiconductor properties, it tends to adsorb water molecules and nitrogen molecules to the powder. In particular, when the water content of the powder increases, the properties are adversely affected. H ₂ O molecule is Z
It has been confirmed by Okuno et al. in, for example, No. 4 of the Journal of the Ceramic Society of Japan, Volume 89, that the interface state changes when it is ionized by adhering to the nO surface. Even when moisture is adsorbed on the zinc oxide varistor, a change in the grain boundary level or a change in the grain boundary resistance due to ionized water molecules occurs, and as a result, acts in a direction that prevents the voltage-current nonlinearity characteristic of the varistor. .

If zinc oxide and water molecules are bonded not by adsorption but by chemical bonding or the like, the binding energy of both becomes such that the thermal energy of firing dissociates them for the first time. Alternatively, water molecules or hydroxyl groups remain bonded to the surface of the zinc oxide particles without being dissociated even after firing, which causes a change in grain boundary characteristics. In order to avoid such a problem, there is a method of using a dry method instead of a wet method in manufacturing a varistor material. However, a uniform dispersion method of additives has not been established in the dry method. Therefore, if the water used in the wet method remains in the varistor material and cannot be finally dissociated by firing, the characteristics cannot be stabilized.

Therefore, a main object of the present invention is to provide a varistor having stable characteristics and high reliability. Another object of the present invention is to provide a method for manufacturing a highly reliable varistor having stable characteristics.

[0008]

SUMMARY OF THE INVENTION According to the present invention, there is provided a varistor including a varistor element and an electrode formed with the varistor element interposed therebetween, wherein the moisture contained in the varistor element is H ₂ O.
A varistor characterized by having a molecular weight of 10 ppm or less. Further, the present invention provides a step of mixing a varistor material with a binder to form a molded article, a step of burning a binder contained in the molded article, and a step of firing the molded article obtained by burning the binder to form a varistor element. Obtaining a varistor element, wherein the steam concentration of the atmosphere in which the molded product is fired in the step of obtaining the varistor element is -10 ° C. or less in terms of an atmospheric dew point temperature. It is a manufacturing method. Further, the present invention provides a step of mixing a varistor material with a binder to form a molded article, a step of burning a binder contained in the molded article, and a step of firing the molded article obtained by burning the binder to form a varistor body. And a method for producing a varistor, comprising: supplying air containing water vapor of -10 ° C. or less in terms of an atmospheric dew point to a furnace for firing a molded product in the step of obtaining a varistor body. A varistor manufacturing method. Further, the present invention provides a step of mixing a varistor material with a binder to form a molded article, a step of burning a binder contained in the molded article, and a step of firing the molded article obtained by burning the binder to form a varistor element. Obtaining a varistor in the step of obtaining the varistor body, wherein the vapor concentration of the atmosphere in the step of burning the binder is −5 ° C. or less in terms of the atmospheric dew point, and the step of obtaining the varistor body is performed. A method for manufacturing a varistor, characterized in that a water vapor concentration in an atmosphere is -10 ° C or less in terms of an atmospheric pressure dew point temperature. In addition, the present invention
A varistor comprising: a step of mixing a varistor material with a binder to form a molded article; a step of burning a binder contained in the molded article; and a step of firing the molded article produced by burning the binder to obtain a varistor element body. In the method for producing a varistor, an air containing water vapor of -5 ° C. or less in terms of an atmospheric dew point is supplied into the furnace in the step of burning the binder, and the molded product in the step of obtaining the varistor element is fired in the furnace. And supplying air containing water vapor of not more than −10 ° C. in terms of an atmospheric dew point. The atmospheric pressure dew point temperature is determined by setting the water vapor concentration to 1
It is shown in terms of the temperature of saturated steam at atmospheric pressure.

By controlling the water contained in the varistor element to be 10 ppm or less in terms of H ₂ O molecules, it is possible to prevent a change in grain boundary characteristics due to water molecules, and to obtain a good voltage-current A varistor having linearity and stable characteristics can be obtained. And
In the step of obtaining the varistor element body, a varistor having good characteristics can be manufactured by converting the water vapor concentration of the atmosphere to −10 ° C. or less in terms of the atmospheric dew point. Further, the water vapor concentration in the atmosphere in the step of burning the binder of the molded product is converted into an atmospheric pressure dew point temperature by -5.
The varistor having good characteristics can be manufactured by setting the water vapor concentration of the atmosphere in the step of obtaining the varistor element to -10 ° C. or less in terms of the atmospheric pressure dew point temperature in the step of obtaining the varistor element body.

The above objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As an example of a varistor according to the present invention, there is a laminated varistor 10 as shown in FIG. That is, the plurality of internal electrodes 14 and 16 are formed in the varistor element body 12 formed of ceramic or the like. And
The adjacent internal electrodes 14 and 16 are drawn out to the opposing side surfaces of the varistor element 12. External electrodes 18 and 20 are formed on the side surfaces from which the internal electrodes 14 and 16 are drawn.
In the varistor 10, the moisture contained in the varistor element body 10 is adjusted to be 10 ppm or less in terms of H ₂ O molecules. In such a varistor 10, since the varistor element body 12 has a voltage-current nonlinearity,
When a voltage higher than a certain voltage is applied between the external electrodes 18 and 20, the resistance value of the varistor element 12 sharply decreases.
Therefore, the varistor element body 10 can be used in a circuit using the IC to protect the IC from an excessive voltage.

In order to manufacture such a varistor 10, a raw material such as zinc oxide is prepared. Then, an oxide such as Bi, Co, Mn, Sb, or Al is mixed with zinc oxide at a predetermined ratio, and wet pulverization is performed. Further, after calcining, wet pulverization is performed to form a varistor material. After the varistor material is granulated, a binder is mixed to form a green sheet, and the electrode material is printed on the green sheet. A plurality of green sheets on which the electrode material is printed are laminated, and a green sheet as an exterior is further laminated and pressed, and cut into a predetermined size to form a molded body. The obtained compact is put into a furnace, and the binder contained in the compact is burned. Further, the molded body obtained by burning the binder is fired in a firing furnace, and the laminated green sheets become varistor element bodies, and the electrode materials printed on the green sheets become internal electrodes 14 and 16. Then, the varistor 10 is formed by forming the external electrodes 18 and 20 on the exposed portions of the internal electrodes 14 and 16.

In the manufacturing process of such a varistor 10, the steam concentration of the atmosphere in the furnace when the binder is burned is adjusted so as to be -5 ° C. or less in terms of the atmospheric dew point, and the binder is burned. The water vapor concentration in the atmosphere when the molded body is fired is adjusted so as to be -10 ° C or less in terms of the atmospheric dew point. By doing so, the water content of the varistor element body 12 of the varistor 10 can be reduced to 10 ppm or less in terms of H ₂ O molecules.

In the varistor 10, the varistor element 12
, The change in grain boundary level due to moisture and the change in grain boundary resistance due to ionized water molecules are small, and good voltage-current nonlinearity can be obtained. Also, since the influence of moisture is small, the varistor 10
Characteristics can be stably maintained.

[0015]

(Example 1) A 99% or more pure oxide of Bi, Co, Mn, Sb and Al is mixed at a predetermined ratio with a 99% or more pure zinc oxide raw material and wet pulverized. After baking, wet grinding was performed to obtain a varistor material. This varistor material was stored in a dried container after granulation, and was controlled so that the water content was 0.1% or less. A green sheet was formed by mixing a binder with the varistor material, an electrode material for forming an internal electrode was applied by screen printing, and a plurality of green sheets were laminated. Further, an exterior green sheet was laminated on the upper and lower sides of the green sheet laminate on which the electrode material was printed, and this was pressed and cut into a predetermined size to form a molded body.

The obtained compact is put into a furnace for binder combustion, and the vapor density in the atmosphere in the furnace is converted into the atmospheric dew point temperature, and the firing temperature is maintained at -50 ° C. while being sent while being held. At 500 ° C., the binder of the molded body was burned. The molded body after burning the binder is put into a firing furnace, and the water vapor concentration is converted into atmospheric dew point temperature using an air filter, and is + 5 ° C, 0 ° C, -3 ° C, -5 ° C,
The sintering was performed at 950 ° C. while flowing sintering air set at −10 ° C. and −50 ° C., and an external electrode was formed to obtain a varistor. At this time, the water vapor concentration was measured in the cooling zone from the top temperature to 100 ° C with a dew point thermometer,
Confirmed that the set dew point temperature was reached.

With respect to the varistor thus obtained, a varistor voltage: V1 mA, a nonlinear coefficient: α, a capacitance: Cap. , Dielectric loss: tan δ was measured. At the same time, the amount of water contained in the varistor was measured in terms of H ₂ O molecules. After that, a DC power supply was connected to the external electrodes, and the battery was charged in a thermostat controlled at a temperature of 125 ° C. and a humidity of 25% or less, and a high-temperature load life test was performed. At this time, the DC voltage was continuously charged at 85% of the varistor voltage of the varistor. After a lapse of 1000 hours in this state, the varistor is taken out of the thermostat, and the varistor voltage, the nonlinear coefficient,
The capacitance and the dielectric loss were measured, and the water content inside the varistor was converted into H ₂ O molecules. Table 1 shows the amounts of water contained in the varistor samples fired under different conditions and the characteristic values thereof.

[0018]

[Table 1]

As can be seen from Table 1, when the amount of water contained in the varistor is 30 ppm or less in terms of H ₂ O molecules, a non-linear coefficient of 20 or more is obtained, and the varistor is excellent in voltage suppression ability. Can be obtained. And H
_When the water content in terms of ₂ O molecules was 10 ppm or less, the change in characteristics by the constant temperature life test was 10% or less. From this result, it is desirable that the moisture content inside the varistor be 10 ppm or less in consideration of reliability. Further, in order to satisfy such a condition of the amount of water inside the varistor, it is desirable that the water vapor concentration in the firing furnace is -10 ° C. or less in terms of the atmospheric dew point.

(Example 2) Bi, Co, Mn, Sb and Al oxides having a purity of 99% or more are mixed at a predetermined ratio with a zinc oxide raw material having a purity of 99% or more, and wet pulverization is performed. After that, wet pulverization was performed to obtain a varistor material. This varistor material was stored in a dried container after granulation, and was controlled so that the water content was 0.1% or less. A green sheet is formed by mixing a binder with this varistor material,
An electrode material for forming an internal electrode was applied by screen printing, and a plurality of green sheets were laminated. Further, an exterior green sheet was laminated on the upper and lower sides of the green sheet laminate on which the electrode material was printed, and this was pressed and cut into a predetermined size to form a molded body.

The obtained compact was put into a furnace for binder combustion. Then, the water vapor concentration in the furnace between 300 ° C. and 500 ° C. is converted into the atmospheric dew point temperature of the sintering air using an air filter, and is + 5 ° C., 0 ° C., −3 ° C., −5 ° C.
The binder of the molded body was burned at 500 ° C. while feeding while maintaining the temperature at -10 ° C., −10 ° C., and −50 ° C. It has been confirmed by thermal differential analysis that the binder used in this example has an exothermic peak at 320 ° C. to 360 ° C. when heated at 1 ° C. per minute. It is easy to imagine what is going on. At this time, gases composed of hydrocarbon compounds including carbon monoxide gas and carbon dioxide gas generated by binder combustion are generated, and it is expected that the type and amount of these gases will change depending on the water vapor concentration in the furnace. Is done.
The molded body after burning the binder is put into a firing furnace, and the steam concentration is converted to the atmospheric dew point by using an air filter. The resultant was fired at 950 ° C. to form an external electrode to obtain a varistor.

With respect to the varistor thus obtained, a varistor voltage: V1 mA, a nonlinear coefficient: α, a capacitance: Cap. , Dielectric loss: tan δ was measured. At the same time, the amount of water contained in the varistor was measured in terms of H ₂ O molecules. After that, a DC power supply was connected to the external electrodes, and the battery was charged in a thermostat controlled at a temperature of 125 ° C. and a humidity of 25% or less, and a high-temperature load life test was performed. At this time, the DC voltage was continuously charged at 85% of the varistor voltage of the varistor. After a lapse of 1000 hours in this state, the varistor is taken out of the thermostat, and the varistor voltage, the nonlinear coefficient,
The capacitance and the dielectric loss were measured, and the water content inside the varistor was converted into H ₂ O molecules. Tables 2, 3 and 4 show the amounts of water contained in the varistor samples and the characteristic values thereof with respect to the respective conditions of the steam concentration of the baking air poured into the baking furnace.

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

[Table 4]

As can be seen from these tables, if the water vapor concentration in the furnace at the time of burning the binder and the water vapor concentration in the furnace at the time of firing the compact are -10 ° C. or less in terms of the atmospheric pressure dew point, the water vapor concentration is 20 or more. It is clear that the non-linear coefficient and reliability can be secured. Further, when the steam concentration in the firing furnace at the time of firing the molded body is −50 ° C. or less at the atmospheric pressure dew point temperature, the steam concentration at the time of burning the binder is −5 ° C. or less in terms of the atmospheric pressure dew point temperature. Thus, good characteristics and reliability can be secured.

[0027]

According to the present invention, a varistor having stable characteristics and high reliability can be obtained by reducing the water contained in the varistor element to 10 ppm or less in terms of H ₂ O molecules. Can be. Moreover, by converting the water vapor concentration in the furnace for firing the molded body to obtain a varistor to -10 ° C. or less in terms of the atmospheric pressure dew point temperature, it has the above-described stable characteristics and reliability. High varistors can be manufactured. In addition, the steam concentration in the furnace for burning the binder is converted to an atmospheric pressure dew point temperature of -5 ° C or less, and the steam concentration in the furnace for firing the molded body is converted to the atmospheric pressure dew point temperature of -10 ° C or less. By doing so, a highly reliable varistor having the above-described stable characteristics can be manufactured.

[Brief description of the drawings]

FIG. 1 is an illustrative view showing a laminated varistor as an example of a varistor;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Varistor 12 Varistor element 14 Internal electrode 16 Internal electrode 18 External electrode 20 External electrode

Claims (5)

[Claims] 1. A varistor comprising a varistor element and an electrode formed with the varistor element interposed therebetween, wherein the varistor element has a water content of 10 ppm or less in terms of H ₂ O molecules. Characteristic varistor. 2. a step of mixing a varistor material with a binder and molding to obtain a molded article; and burning the binder contained in the molded article.
And a varistor manufacturing method including a step of firing the molded article obtained by burning the binder to obtain a varistor element, wherein the water vapor concentration of the atmosphere in which the molded article is fired in the step of obtaining the varistor element is an atmospheric pressure dew point. Converted to temperature-
A method for producing a varistor, wherein the temperature is 10 ° C. or lower. 3. A step of mixing a varistor material with a binder and molding to obtain a molded article; a step of burning the binder contained in the molded article;
And a method for manufacturing a varistor including a step of obtaining a varistor element by firing the molded article obtained by burning the binder, wherein the step of obtaining the varistor element sets an atmosphere dew point temperature in a furnace for firing the molded article. A method for producing a varistor, characterized in that air containing water vapor of not higher than -10 ° C is supplied. 4. A step of mixing a varistor material with a binder and molding to obtain a molded article; a step of burning the binder contained in the molded article;
And a method for manufacturing a varistor including a step of firing the molded product obtained by burning the binder to obtain a varistor element, wherein the water vapor concentration of the atmosphere in the step of burning the binder is converted into an atmospheric pressure dew point temperature of -5. C. or less, and the steam concentration of the atmosphere in which the molded product is fired in the step of obtaining the varistor element is -10.degree. C. or less in terms of the atmospheric dew point temperature. 5. A step of mixing a varistor material with a binder and molding to obtain a molded product, a step of burning the binder contained in the molded product,
And a method for manufacturing a varistor including a step of firing the molded product obtained by burning the binder to obtain a varistor element body. In the furnace in the step of burning the binder, the temperature is converted into an atmospheric dew point temperature of -5 ° C or less. In a furnace for baking the molded product in the step of supplying air containing water vapor and obtaining the varistor element at an atmospheric pressure dew point temperature of −10 ° C.
It is characterized by supplying air containing the following steam,
Varistor manufacturing method.