JPS649724B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is to thermally diffuse a diffusing agent into the grain boundaries of conductive ceramic porcelain to form a high resistance layer with excellent varistor properties at the grain boundaries. The present invention relates to a method for manufacturing a bulk type varistor whose obtained sintered body itself has voltage nonlinearity.

(Structure of conventional example and its problems) Varistor is a general term for solid-state elements whose resistance value changes significantly depending on applied voltage and whose voltage-current characteristics exhibit nonlinearity. The operating principle differs depending on the type, but what they have in common is that they utilize the nonlinearity of the current that passes through potential barriers such as PN junctions on the surface or inside of semiconductors and ceramics, and shotgun barriers. .

Generally, the voltage-current characteristic of a varistor is I=
It is expressed as (V/C)〓. Here, α is called the voltage nonlinearity index of the varistor, and it represents the degree of non-ohmic property, that is, the performance of the varistor, and it varies greatly depending on the type, material, or manufacturing conditions of the varistor, and C is the voltage nonlinearity index of the varistor at the applied voltage V. The resistance value is the applied voltage V
It changes depending on the value of.

Varistors using conventional grain boundary layers, e.g.
In varistors, which are made by adding oxides such as Bi, Pb, Mn, Co, and Ba to ZnO and sintering them, the degree of segregation is determined by temperature, crystal grain size, raw material grain size, etc. This has disadvantages such as variations in the voltage nonlinearity index α due to fluctuations in the voltage, and a decrease in surge resistance due to the formation of thin grain boundary layers. Another method is to apply a diffusing agent in which additive components such as Bi and Pb are dispersed as a solid powder in a vehicle, and then thermally diffuse them to the grain boundaries.However, a diffusing agent layer with uniform components and a uniform film is applied to the ZnO surface. It is very difficult to do so, and the next reaction Bi ₂ O ₃ →2Bi ³⁺ +30 ^2- must be carried out on the ZnO surface in order to diffuse, but since the particle size is 0.5 to 1 μm, it is difficult to This has disadvantages such as a decrease in the apparent grain boundary diffusion coefficient and an increase in the diffusion temperature.

(Object of the invention) The present invention eliminates the above-mentioned conventional drawbacks, provides a method for manufacturing a varistor that has a stable voltage nonlinearity index α, and can form a uniform high-resistance layer in the grain boundary layer. This is what we provide.

(Structure of the Invention) The present invention provides a ceramic substrate for a varistor using an organic metal soap solution containing a metal component as a diffusing agent, a mixed solution of the organic metal soap solution and an organic binder, or an inorganic metal salt and a thickener. A varistor having a stable voltage non-linearity index α can be obtained by soaking the ceramic in a mixed solution and drying it to form a metal component layer on the ceramic surface, and then performing a diffusion treatment.

(Description of an Example) An example of the method for manufacturing a varistor of the present invention will be described below.

In the sample preparation process, industrial raw materials (purity 99.9
% or more), ZnO, Nb ₂ O ₅ , Bi ₂ O ₃ , and BiCl ₃ are prepared. After adding Nb ₂ O ₅ powder to ZnO powder and mixing thoroughly and uniformly, it is compression molded into a disk shape with a diameter of 13 mm and a thickness of 3 mm, and fired at a temperature in the range of 1400 to 1450°C. The sintered body thus obtained is an oxide semiconductor with low resistivity.

As a diffusing agent, a solution of bismuth naphthenate diluted with a mixed solvent of methanol and butanol,
A mixed solution of BiCl ₃ in methanol, IPA, and butanol, and a conventional example A in which Bi ₂ O ₃ was made into a paste with a vehicle were prepared. Using the above-mentioned diffusing agent, in this example, the semiconductor ceramic element was placed in a fluororesin net, immersed in a metal soap solution or an inorganic metal salt solution, and then dried in the air at 80 to 100°C. . In conventional example A, the paste-like diffusing agent was
It was applied evenly to the surface of the semiconductor ceramic element.

The semiconductor ceramic element coated with the diffusing agent as described above was subjected to a diffusion treatment in the atmosphere at a temperature of 700 to 1200°C. In this thermal diffusion treatment, care was taken to ensure that the applied diffusion component was not lost to the outside of the sample due to evaporation, melting, or diffusion to the outside of the sample. Ag--Ni ohmic electrodes were formed on both sides of the porcelain thus obtained to form a varistor element, and various characteristics shown in FIGS. 1 to 4 were investigated.

Figure 1 shows the variation of the voltage nonlinearity index α due to different methods of increasing the resistance of the grain boundary layer, Figure 2 shows the effective diffusion temperature range due to different diffusing agents, and Figure 3 shows the method of increasing the resistance of the grain boundary layer. FIG. 4 is a graph showing the relationship between the current withstand voltage and the surge voltage depending on the difference, and FIG. 4 is a graph showing the relationship between the usage amount of the diffusing agent and the difference in the diffusing agent.

In each figure, 1 means 2 when using the metal soap method.
3A and 3B show the case of conventional example A and conventional example B, respectively.

Conventional example B is an example in which no diffusing agent is used, and ZnO
After adding Nb ₂ O ₅ and Bi ₂ O ₃ and mixing them thoroughly, they were compression molded into a disc shape with a diameter of 13 mm and a thickness of 3 mm.
Fired at a temperature range of 1450℃. The sintered body thus obtained contains semiconducting crystal grains and Bi ₂ O ₃
It consists of grain boundary layers with high resistance due to segregation of particles. Ag--Ni ohmic electrodes were formed on this porcelain in the same manner as described above, and a varistor element was obtained.

1 to 4, examples of the present invention are a metal soap method and an inorganic metal salt method using a diffusion method,
The others are comparative examples. As is clear from the above graphs, the varistor obtained by the present invention is excellent in voltage nonlinearity index α, diffusion temperature, current withstand capacity, and amount of diffusing agent used. These good results are obtained by uniformly adhering the diffusing agent component in the form of fine particles to the surface of the varistor substrate.

In addition, in the relationship between the current withstand capacity against surge voltage due to the difference in the method of increasing the resistance of the grain boundary layer in Fig. 3,
● indicates normal use capacity, 〇 indicates destructive capacity, and these are:
This data is for a standard impulse current waveform of 8×20μS. In addition, in the relationship between the amount of diffusing agent used depending on the difference in the diffusing agent shown in FIG. 4, the amount of diffusing agent is the amount when converted to oxide powder.

In the above explanation, a metal soap solution was used as the solution for soaking the ceramic substrate for the varistor, but equivalent results can be obtained with a mixture of that solution and an organic binder solution such as PVB. In addition, although bismuth ofthenate was used as the metal soap, other metal soaps such as bismuth octylate may also be used, and as a diffusing agent.
Although Bi was used, other components may be used as long as they increase the resistance of the grain boundary layer. Also, as an inorganic metal salt
Although BiCl ₃ was used, there is no particular limitation as long as it becomes an oxide such as Bi ₂ O ₃ during the diffusion treatment. Also,
Although ZnO was used as the varistor substrate, TiO ₂ ,
Similar results can be obtained with other components such as BaTiO ₃ .
Further, as a method of applying metal soap or inorganic metal salt solution to the varistor substrate, equivalent results can be obtained by spraying or the like.

(Effects of the Invention) As explained above, the varistor manufacturing method of the present invention has a high value of the voltage non-linearity index α and is stable, it is also possible to lower the diffusion temperature, and the current withstand capacity against surge voltage is high. It has advantages such as high price and is extremely stable even in industrial mass production, and has extremely great industrial value.

[Brief explanation of drawings]

Fig. 1 is a graph showing the variation of the voltage nonlinearity index α due to different methods of increasing the resistance of the grain boundary layer, Fig. 2 is a graph showing the effective range of diffusion temperature due to different diffusing agents,
Figure 3 is a graph showing the relationship between current withstand capacity and surge voltage due to different methods of increasing the resistance of the grain boundary layer.
The figure is a graph showing the relationship between the amount of diffusing agent used and the amount of diffusing agent used.

Claims (1)

[Scope of Claims] 1. A ceramic substrate for a varistor is immersed in an organic metal soap solution containing a metal component of a diffusing agent or a mixed solution of the organic metal soap solution and an organic binder, and then subjected to a drying treatment, A method for manufacturing a varistor, which comprises forming a metal component layer on the surface of a ceramic substrate and subjecting it to a diffusion treatment. 2. A method for manufacturing a varistor, which comprises immersing a ceramic substrate for a varistor in a mixed solution of a thickener and an inorganic metal salt containing a metal component of a diffusing agent, and then drying and performing a diffusion treatment.