JP3060900B2 - Varistor manufacturing method - Google Patents

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 a varistor.

[0002]

2. Description of the Related Art Conventionally, a varistor has been used to bake electrodes in air in order to re-oxidize grain boundaries that have been converted into semiconductors by firing.
The re-oxidation of the grain boundaries is performed at the same time by performing the process at 00 to 900 ° C.

In other words, electrical characteristics are developed by controlling grain boundaries.

[0004]

The electrical characteristics of the varistor can be easily exhibited by controlling the grain boundaries. On the other hand, the electrical characteristics of the varistor are changed by changing the method of controlling the grain boundaries, in this case, the electrode baking conditions. This means that the characteristics are greatly affected.

Accordingly, an object of the present invention is to provide electrode baking conditions under which the electrical characteristics of a varistor are maximized.

[0006]

In order to achieve the above object, a method for manufacturing a varistor according to the present invention comprises a first step of firing the varistor element to make it a semiconductor, and at least a pair of varistor elements on the surface of the varistor element. A second step of applying an electrode paste; and a third step of baking the electrode paste in an oxidizing atmosphere and reoxidizing the grain boundaries of the varistor element. In the third step, the temperature decreasing speed is equal to the temperature increasing speed. It is equivalent or faster.

[0007]

According to this method, the varistor element grain boundary portion
The precipitation of impurities is small, the thickness of the grain boundaries is thin and uniform,
In addition, the grain boundaries are not
Becomes a "strong barrier", surge tolerance and voltage non-linear index
It is possible to improve α and reduce variation in varistor voltage.
Diffusion of impurities such as glass contained in the electrode paste
Is further blocked by a strong barrier,
Can be improved.

[0008]

【Example】

(Embodiment 1) Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.

In FIG. 3, reference numeral 1 denotes a varistor element, in which a plurality of internal electrodes 2 are provided so as to face each other with a ceramic sheet 1a interposed therebetween, and external electrodes 3 are provided at both ends. . The varistor element 1 is made of SrT
iO ₃ as the main component and Nb ₂ O ₅ , Ta as the sub-components
It is formed by adding ₂ O ₅ , MnO ₂ , SiO _{2 and the} like. The internal electrode 2 is formed by using Ni as a main component and adding Li ₂ CO ₃ or the like as a sub-component. Further, the external electrode 3 is formed by forming the lower layer 3a with Ni as a main component and adding Li ₂ CO ₃ or the like as a sub component, and forming the upper layer 3b.
Is formed of at least one of Ag, Pd and Pt,
An Ni plating 4a and a solder plating 4b are formed thereon.

FIG. 4 shows a manufacturing process. As shown in (5), a ceramic sheet 1a was manufactured by mixing, pulverizing, slurrying, and sheet forming the raw materials.

Next, the ceramic sheet 1a and the internal electrodes 2
Were laminated (6), and cut (7), debindered / calcined (8), and chamfered (9).

Next, a lower layer 3a is formed on the end face of the varistor element 1.
Ni external electrode to be applied (10)
After reduction firing at 00 ° C. (11), an Ag paste made of Ag powder serving as an Ag external electrode serving as an upper layer 3b, lead borosilicate glass, ethyl cellulose, butyl carbitol, etc. is applied onto the lower layer 3a (12). ) And the temperature decreasing speed (800 ° C./h) shown in FIG.
Under the condition that the varistor element 1 was re-oxidized at the same time as Ag baking under the condition of being faster than (00 ° C./h), the varistor element 1 was heated at a constant temperature of 700 to 900 ° C. (13).

According to the results of Example 1, under the baking condition in which the temperature decreasing speed is higher than the temperature increasing speed, the varistor element 1
It was confirmed that the voltage non-linear index α and the surge resistance Weibull breakdown value improved.

In the first embodiment, as a furnace for baking Ag, a fan for supplying air is arranged outside the heat insulating wall, so that air is continuously introduced.

The varistor element 1 to which the Ag paste was applied was subjected to Ag baking and reoxidation (13) so as not to overlap to prevent sticking.

From the results of Example 1, when the temperature decreasing speed is faster than the temperature increasing speed, the voltage non-linear index α and the surge withstand capability are remarkably improved.

The reason for this is as follows: (I) In the case where the temperature lowering speed is lower than the temperature increasing speed In the temperature lowering where the grain boundary insulating layer is formed, if the temperature lowering speed is slow and long, only the re-oxidation of the grain boundary is performed. In addition, (1) precipitation of impurities at the grain boundary portion, (2) diffusion of glass contained in the Ag paste, (3) increase in impurity concentration at the grain boundary portion, and (4) increase in thickness of the grain boundary. And as a result of oxidation of the particles that have become semiconductors,
It is considered that the withstand voltage (withstand surge) and the voltage non-linear index α of the grain boundary part decrease.

(II) Conversely, when the cooling rate is equal to or higher than the heating rate: (1) the precipitation of impurities at the grain boundary is small; (2) the grain boundaries are thin and uniform, and the grains are semiconductors. It is considered that (3) the grain boundary is formed as a "stronger barrier" due to the fact that is not oxidized. Further, in this case, the glass contained in the Ag paste also has a “stronger barrier” and diffusion to particles and grain boundaries is suppressed by rapid cooling at the time of temperature decrease, and as a result, surge resistance,
It is considered that the voltage non-linear index α is improved.

Further, the above-mentioned effect is remarkably exhibited when air (oxygen) is continuously supplied into the furnace.
(1) uniform oxidation of the grain boundary part by the supplied air,
It is considered that (2) the temperature in the furnace is made uniform and (3) the binder contained in the Ag paste is removed (de-buy). However, in this case, it is more effective to install a fan or the like outside the heat insulating wall and supply the air indirectly, and the variation in characteristics is smaller than that of drawing the flow of air directly into the furnace. However, the air is directly varistor element 1
In this case, the temperature inside the furnace is cooled and the temperature of the baking (re-oxidation) between the varistor elements 1 is different, which causes variations in characteristics. Therefore, it is preferable to make indirect contact.

In order to quickly remove the binder contained in the Ag paste, the following method can be considered.

(1) Decomposition temperature of ethyl cellulose as a binder used for Ag paste at a temperature rise of 200 to 250
The temperature is maintained at 300 to 500 ° C. or higher for 10 to 30 minutes, and a debuy zone as shown in FIG. 2 is provided.

(2) The amount of oxygen introduced at the time of temperature rise is 1.5 to 2 times that at the time of reoxidizing the varistor element 1 and at the time of temperature decrease.
Do more in double the range.

(3) At the time of raising the temperature, steam is first introduced to hydrolyze the binder. The amount of water vapor at this time is related to the amount of the varistor element 1 to be processed.
Approximately 0.5 to 2.0 cc / min is required to process 10,000 pieces, and the inflow temperature is optimally in the range of 100 to 500 ° C.

(4) It is desirable to install an exhaust duct in the furnace in order to prevent the re-adhesion and re-circulation of the degassing exhaust gas.

(Embodiment 2) Next, a second embodiment will be described.

After the reduction firing (11), an Ag external electrode was applied (12) and heated (13) for reoxidation simultaneously with the baking of Ag under the conditions shown in FIG.

As a result, the area A of the triangle at the time of temperature rise is:
(1) In the case of A ≧ B, the surge withstand voltage and the voltage non-linear index α are the same as in the case of the above-mentioned temperature lowering speed ≧ temperature increasing speed.
Improvement was confirmed.

Conversely, (2) In the case of A <B, the same result as in the case of the above-mentioned temperature lowering speed <temperature raising speed was obtained.

In these cases, as described above, it is considered that the result is caused by the difference in the formation of the grain boundary portion due to the difference in the holding time.

(3) Further, even when the test was carried out under the condition shown in FIG. 2 in which a de-buying zone was provided in the course of raising the temperature, in the case of A ≧ B, it was confirmed that the surge resistance and the voltage nonlinearity index α were improved. .

In the first and second embodiments, a laminated varistor has been described as an example. However, the present invention can be applied to any type such as a disk type, a cylindrical type, and a laminated type.

Although only Ag was used as the external electrode 3, it was confirmed that the same effect could be obtained by using other Pd, Pt, or an alloy thereof.

The main component of the varistor element 1 is SrTi
Although O ₃ was used, the same effect can be obtained also in the case where a part of Sr is replaced with Ba, Mg, Ca or in the case where two or more kinds of them are used.

The heating speed is 400 to 600 ° C.
/ H, the subsequent holding temperature is 700 to 900 ° C, the time is 20 to 40 minutes, and the temperature decreasing speed is 600 to 800 ° C /
h is desirable. When a debuying zone is provided during the heating, the heating speed in the first half is 200 to 400.
° C / h, heating rate in the latter half is 400-600 ° C / h
It is desirable to divide into two stages.

When the temperature drops to about 500 ° C.,
The temperature inside the furnace is unlikely to drop, and the temperature drops in a gentle parabola.

However, it is effective to control the cooling rate in a temperature range from the holding temperature for baking the electrodes to the temperature at which the temperature in the furnace is hardly lowered.

[0037]

As described above, according to the present invention, the varistor element grain boundary
Low precipitation of impurities on the part, thin and uniform grain boundaries
In addition, since the particles that are semiconducting are not oxidized,
Grain boundaries become "strong barriers", and surge withstand and voltage
Improved line index α and reduced variation in varistor voltage
Become. In addition, impurities such as glass contained in the electrode paste
Diffusion is blocked by a strong barrier,
The air quality can be improved.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing electrode baking conditions in one embodiment of the present invention.

FIG. 2 is a characteristic diagram showing electrode baking conditions in one embodiment of the present invention.

FIG. 3 is a sectional view of a multilayer varistor according to an embodiment of the present invention.

FIG. 4 is a manufacturing process diagram of a laminated varistor according to one embodiment of the present invention.

[Description of Signs] 1 Multilayer varistor element 3 External electrode

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01C ^7/ 02-7/22

Claims (9)

(57) [Claims] And 1. A first step of firing in order to the varistor element is semiconductive, then a second step of applying at least one pair of electrode paste on the varistor element surface and then acid
Baking the electrode paste in a oxidizing atmosphere and reoxidizing the grain boundaries of the varistor element. In the third step, the temperature decreasing speed is equal to or faster than the temperature increasing speed. 2. The method for manufacturing a varistor according to claim 1, wherein the temperature rising speed is 400 to 600 ° C./h and the temperature decreasing speed is 600 to 800 ° C./h. 3. The method for manufacturing a varistor according to claim 1, wherein the electrode paste comprises at least one of Ag, Pd, and Pt powders, a glass powder, and an organic substance. 4. The method for manufacturing a varistor according to claim 1, wherein in the third step, oxygen is continuously introduced. 5. The method for manufacturing a varistor according to claim 1 , wherein in the third step, the varistor element is held at a certain temperature for a certain time when the temperature is raised. 6. The method for manufacturing a varistor according to claim 5, wherein a certain temperature is higher than a decomposition temperature of an organic substance contained in the electrode paste. 7. The method for manufacturing a varistor according to claim 1 , wherein in the third step, the oxygen partial pressure at the time of temperature rise is higher than that at the time of temperature fall. 8. The method for manufacturing a varistor according to claim 1 , wherein, in the third step, steam is caused to flow in at the time of raising the temperature to decompose organic substances contained in the varistor element, and thereafter oxygen is caused to flow. 9. The method according to claim 3, wherein the product of the holding temperature and the holding time at the highest temperature (the area of the square B) is larger than the area of the polygon A when the temperature is raised, and the relationship of A ≧ B is satisfied. Item 7. A method for manufacturing a varistor according to Item 1.