JPH0969419A - Method of manufacturing varistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To resultantly facilitate and stabilize the control of the electric characteristics by a method wherein respective particles comprising particle component and grain boundary component are previously sintered for mixing, molding and baking these calcined particles later to control the solid phase reaction. SOLUTION: The particle component is mixed (3) and calcinated (9) in the air for the main component to be crushed (5) later for the formation of the calcined particles of the particle component. Besides, the particle component is added and mixed (6) and after the calcination in the air to be crushed (8) for the formation of the calcined particles of the particle component. Next, after the adjustment to make the mean particle diameter of the calcinated particles of the particle component exceed that of the calcinated particles of grain boundary component, the calcined particles of the particle component and grain boundary component are added and mixed (9) to be granulated by adding an organic binder for molding (10) in a specific shape. Next, the molding is deashed (11) in the air to be baked in the reducing atmosphere (12). Finally, after finishing the baking step, the surface of the element is coated with an electrode paste (13) to be heat-treated in the air (14) and then the reoxidation and the electrode seizure are simultaneously performed.

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 In a conventional manufacturing method, varistor is manufactured by ignoring reactivity and function of raw material powder and mixing various raw material powders at once.

[0003]

In the conventional manufacturing method,
Since various raw material powders are mixed at once, it is difficult to control the solid-phase reaction in the firing process, and as a result, it is difficult to control the electrical characteristics.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for producing a varistor in which the solid-state reaction is controlled and, as a result, the electrical characteristics can be easily controlled and which is stable. is there.

[0005]

In order to achieve this object, the present invention is to pre-calculate each powder that becomes a particle component and a grain boundary component, and then mix, mold and fire these calcined powders. The varistor is manufactured.

[0006]

According to the above method, even if the chemical composition is the same, it is possible to control the solid-phase reaction by pre-calcining a part of the raw material powder used for blending. Can be controlled easily, and a varistor having stable electric characteristics can be manufactured.

[0007]

【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 is a varistor element, and electrodes 2 are provided on the front and back surfaces thereof. Varistor element 1
Contains SrTiO ₃ as a main component and Nb ₂ O as a secondary component.
It is formed by adding ₅ , MnO, SiO ₂ , CuO or the like. The electrode 2 is made of Ag or Ag-Pd.

In the above structure, the fine structure of the varistor element 1 is SrTi made into semiconductor by Nb ₂ O ₅ or the like.
Particles containing O ₃ as a main component and metal oxides such as Mn and Cu segregate so as to surround the particles and become a main component forming a grain boundary.

FIG. 1 shows the manufacturing process. First, as a particle component, 0.1 to 0.5 wt% of Nb ₂ O ₅ was added to 100 wt% of SrTiO ₃ and mixed (3), and 1
Calcination (4) was performed in the temperature range of 000 to 1300 ° C. After calcination, crush to an average particle size of 0.9-1.3 μm (5)
Then, a calcined powder of particle components was formed. Further, as grain boundary components, MnO 52 ± 5 wt% and TiO ₂ 14 ± 5 wt%
%, SiO ₂ 34 ± 5 wt%, three components were added, mixed (6), and calcined (7) in the temperature range of 1150 to 1200 ° C. in air. After calcination, average particle size is 0.7-1.1μ
The powder was pulverized (8) to about m to form a calcined powder of grain boundary components.

Next, after adjusting so that (the average particle size of the calcined powder of the particle component) is equal to or larger than the (average particle size of the calcined powder of the grain boundary component), the former calcined powder 100 wt 0.5 to 2.5 wt of the latter calcined powder is added and mixed (9), and then an organic binder is added and granulated to form a predetermined shape (diameter 13 mm, thickness 1.0 mm). (10) I did. After molding, the mold was degassed (11) in air and fired (12) in a temperature range of 1200 to 1400 ° C. in a reducing atmosphere. After firing, an Ag or Ag-Pd electrode paste was applied (13) to the surface of the device and heat-treated (14) in the temperature range of 700 to 850 ° C. in air to re-oxidize the device by electrode baking (electrode area of 0. 5 cm ² ) was carried out at the same time to fabricate a varistor element 1.

(Second Embodiment) Next, a second embodiment of the present invention will be described.

In addition to the particle component and grain boundary component described in Example 1, 0.1 to 2.0 wt% of CuO powder was added and mixed (15) as shown in FIG. 2, and then the same as in Example 1. The varistor element 1 was manufactured by the manufacturing method. The (average particle diameter of CuO) was adjusted to be equal to or smaller than (average particle diameter of calcined powder of particle component).

The electrical characteristics of the varistor element 1 obtained in Examples 1 and 2 are shown below (note that the firing temperature: 1250 ° C., Ag
(Baking temperature: 850 ° C.), in the case of Example 1, varistor voltage V _{1.0 mA / mm} : 15 V, apparent dielectric constant ε: 1.5 × 1
0 ⁴ , in the case of Example 2, V _{1.0mA / mm} : 33V, ε:
It was 5.5 × 10 ⁴ . Further, as a result of analyzing the fine structure with a scanning electron microscope or an X-ray microanalyzer, the average grain size is 4 to 5 μm, and Mn and C are present in the grain boundary portion.
It was confirmed that the u element was segregated and had a dense structure. In addition, the process capability index Cp of V _{1.0 mA / mm} in both examples (however, the standard range was ± 10% of the average value) was
A varistor element 1 which is stable at Cp = 1.1 and has very little variation is obtained.

Further, as a comparison, when a varistor element 1 having the same chemical composition was manufactured by a conventional manufacturing method, the process capability index Cp was 0.7 and the variation was large. Furthermore, as a result of observing the fine structure, it was confirmed that there are many variations in grain size and many pores, and the structure is clearly different from the former example.

As described above, it is considered that the reason why the finished varistor elements 1 have different electrical characteristics and structures even though they have the same chemical composition is that the solid-phase reaction in the firing process is different. That is, in the method of mixing various raw material powders in a large amount at the same time as in the conventional manufacturing method and firing the mixture, a more complicated solid phase reaction occurs, and as a result, it becomes difficult to control the electrical characteristics and the structure.

Also, regarding the average particle size of each component at the time of mixing, it becomes a factor that greatly affects the solid phase reaction, and the average particle size of the particle component is equal to or larger than the average particle size of the grain boundary component or CuO. It was confirmed that the effect of the invention was further improved. The reason is that finer grain boundary components and CuO
It is considered that this is because it becomes easy to cover the entire area of the particle component.

Further, it was better in that the calcination temperature of the grain boundary component was higher than the temperature at which it was in the liquid phase and the reaction was performed, in terms of the shrinkage ratio of the varistor element 1 and the variation in the electrical characteristics. .

The reason for this is that it is due to the reactivity of SiO ₂ , and it is because it is calcined at a high temperature to complete the reaction of the MnO--TiO ₂ --SiO ₂ system and to avoid the residual of SiO ₂ as a simple substance.

Although the disk type varistor element 1 has been described in the first and second embodiments, the laminated type varistor element 21 will be described next.

(Embodiment 3) A third embodiment of the present invention will be described below.

In FIG. 4, reference numeral 21 denotes a laminated varistor element, inside of which a plurality of internal electrodes 22 are provided, and external electrodes 23 are provided at both ends thereof. Internal electrode 22
Is formed by adding Ni as a main component and Li ₂ CO ₃ or the like as a subcomponent. Furthermore, the external electrode 23 is
The lower layer 23a contains Ni as a main component and Li ₂ C as a sub-component.
It is formed by adding O ₃ or the like, and the upper layer 23b is formed of Ag.

FIG. 5 shows the manufacturing process. As shown in (24), a ceramic sheet 21a was produced by blending, mixing, calcining, crushing, mixing calcined powder, making slurry, and sheet forming.

The ceramic sheet 21a and the internal electrodes 22
And were laminated (25), which were cut (26), de-baked and calcined (27), and chamfered (28).

Next, an Ni external electrode to be the lower layer 23a is applied (29) to the end face of the laminated varistor element 21, and 120
Reduction firing (30) at 0 to 1300 ° C., then upper layer 3
The Ag external electrode which becomes b is applied (31) and 700-85
Heat treatment (32) was carried out at 0 ° C., element reoxidation and Ag external electrode baking were carried out at the same time, and a laminated varistor element 21 was produced.

At this time, particle components, grain boundary components, Cu
The average particle diameters of O are 0.9, 0.7 and 0.7 μm, respectively.
Of calcined powder was used.

Multilayer varistor element 21 obtained in Example 3
Then, the stable varistor voltage and capacitance appeared as in Examples 1 and 2.

As a result of analyzing the fine structure, the average grain size was 2 to 3 μm, and segregation of Mn and Cu elements was confirmed at the grain boundary portion.

Here, the items considered to be important particularly in the average particle size of the calcined powder when the slurry is compounded in the manufacturing process of the laminated type are: (1) the average particle size of the calcined powder when the slurry is compounded. 0.7 to the diameter
1.0 μm is preferred. The reason is that the ceramic sheet 21a is excellent in air permeation, and the occurrence of delamination can be suppressed as much as possible.

(2) When the average particle size is smaller than 0.7 μm, air permeation becomes long, delamination frequently occurs, and slurry powder agglomerates to produce a ceramic sheet 21a having poor dispersibility. This is because it becomes easier.

(3) On the contrary, if the average particle size is larger than 1.0 μm, the packing density of the ceramic sheet 21a tends to decrease and the smoothness of the surface tends to deteriorate.

As described above, when the laminated varistor element 21 is manufactured, it is necessary to strictly control the average particle diameter of the mixed powder as compared with the case where the disk type varistor element 1 is manufactured.

Inferring the firing process of this example, (1) in the temperature range of 1100 to 1150 ° C., the MnO—TiO ₂ —SiO ₂ system of the grain boundary component is in the liquid phase.

(2) The liquid phase covers the particle components existing in the solid phase all over. (3) When the liquid phase penetrates deeply into the grain boundaries, capillary pressure acts to strongly attract and densify the particles. In addition, a part of them diffuses inside the particles and sintering proceeds.

(4) During the second heat treatment (reoxidation), the grain boundaries and the grain boundary components diffused inside the grains are rearranged and segregated at the grain boundary portions.

On the other hand, as in the conventional manufacturing method, in the method of mixing various raw material powders in a large amount at a time and firing,
The above reactions (1) to (4) are less likely to occur smoothly,
A more complicated solid-phase reaction occurs, resulting in a large number of variations in electrical characteristics.

Next, the effect of CuO will be described. C
uO has a function of diffusing into the grain boundary during the second heat treatment (reoxidation), increasing the resistance of the grain boundary, and strengthening the grain boundary. Further, the particle size of CuO added is equal to or smaller than that of the calcined powder of the particle component, and the effect is more exhibited. In addition, in Examples 2 and 3, although CuO was added alone, it was confirmed that the same effect can be obtained even in the composite type containing CuO as a main component.

Further, in the first, second, and third embodiments, a semiconductor is used.
Nb as an agent₂O_FiveWas used, but Ta ₂O_Five, La₂O_Three, Y
₂O_ThreeSimilar effects can be obtained by using other semiconductor agents such as
I was sure that.

[0039]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, each powder that becomes a particle component and a grain boundary component is preliminarily calcined, and then these calcined powders are mixed, molded and fired to produce a varistor. According to the above method, even if the chemical composition is the same, it is possible to control the solid-phase reaction by pre-calcining a part of the raw material powder used for compounding, and as a result, the electrical characteristics A varistor that is easy to control and has stable electrical characteristics can be manufactured.

[Brief description of drawings]

FIG. 1 is a manufacturing process diagram in a first embodiment of the present invention.

FIG. 2 is a manufacturing process diagram in the second embodiment of the present invention.

FIG. 3 is a sectional view showing a varistor element according to a first embodiment of the present invention.

FIG. 4 is a sectional view showing a laminated varistor element according to a third embodiment of the present invention.

FIG. 5 is a manufacturing process diagram in the third embodiment of the present invention.

[Explanation of symbols]

 1 Varistor Element 2 Electrode 21 Multilayer Varistor Element 22 Internal Electrode 23 External Electrode

Claims (8)

[Claims] 1. A method of manufacturing a varistor, which comprises pre-calcining each of powders to be a particle component and a grain boundary component, and then mixing, molding and firing these calcined powders. 2. The method for producing a varistor according to claim 1, wherein the particle size of the calcined powder of the particle component is equal to or larger than the particle size of the calcined powder of the grain boundary component. 3. The method for producing a varistor according to claim 1, wherein the grain boundary component is in a liquid state at a lower temperature than the grain component. 4. The method for producing a varistor according to claim 1, wherein the calcination temperature of the grain boundary component is higher than the temperature at which it becomes in a liquid phase. 5. The method of manufacturing a varistor according to claim 1, wherein the particle component contains SrTiO ₃ as a main component. 6. The grain boundary component is MnO—TiO ₂ —SiO ₂
The method of manufacturing a varistor according to claim 1, wherein the main component is a system. 7. A method for producing a varistor, which comprises mixing, molding and firing a calcined powder containing CuO as a main component in addition to the particle component and the grain boundary component. 8. The method of manufacturing a varistor according to claim 7, wherein the particle size of the calcined powder containing CuO as a main component is equal to or smaller than that of the calcined powder of the particle component.