JPH02296302A - Manufacture of voltage nonlinear resistance element - Google Patents

Abstract

PURPOSE:To make it excellent in mechanical strength and surge quantity resistance, and to facilitate process management by applying laser beam on the face on which an electrode of a sintered body formed by mixing and baking powder of an zinc oxide and a very small amount of additive powder which generates voltage nonlinearity is provided so as to apply heat treatment to the surface and then forming an electrode. CONSTITUTION:Proper amounts of Pr, Co, B, etc., in the form of compounds such as oxides, etc., are added to ZnO powder and then those are mixed and granulated so as to make up granulated powder for ZnO varistor. This granulated powder is formed into a disc shape 1.25mm in thickness using a mold 17mm in diameter, and next this compact is baked for 4 hours at a temperature of 1350 deg.C in the oxidizing atmosphere. The size of the sintered body being obtained is 14mm in diameter and 1.0mm in thickness. Both opposed main faces of the sintered body being made this way is heat-treated in N2 gas by Ar laser, and then silver paste is applied on the sintered body so as to constitute a varistor. Hereby, it becomes excellent in mechanical strength and surge quantity resistance and process management in manufacturing becomes easy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a voltage nonlinear resistance element for low voltage circuits containing zinc oxide as a main component, and particularly to the formation of metal electrodes on a zinc oxide sintered body.

[Conventional technology]

Ceramics, which are made by mixing ZnD with a small amount of additives and sintering it as a main component, are known to exhibit excellent voltage nonlinearity, and are useful for suppressing abnormal voltages (surges) in electrical circuits. It is widely used as a control varistor. Such a varistor is manufactured according to the steps shown in FIG.

The voltage nonlinearity of the ZnO varistor is due to the double Schottky barrier formed at the grain boundaries of the 2nD grains. In a practical varistor, the varistor voltage per grain boundary layer formed by the combination of ZnO crystal grains is almost constant regardless of the crystal grain size, and its value is 2V.
That's about it. (Varistor voltage is the voltage between the terminals when a current of 1 m is passed through the varistor, usually V + mA
It is expressed as ) Therefore, the varistor voltage of the voltage nonlinear resistance element is determined by the number of grain boundary layers existing between the electrodes provided on the opposing surfaces of the ZnO sintered body.

[Problem to be solved by the invention]

By the way, silver and aluminum are used for the electrodes of ZnO varistors, and except for ZnO elements for lightning arresters, it is common to bake silver paste and use this as the electrode.
In this case, a Schottky barrier is formed at the interface between the silver electrode and the 2rrO sintered body, and when a current of 1 mA is passed, the electrode/Z
A voltage of about 3 V is generated due to the Schottky barrier at the interface of the n O sintered body. Therefore, this electrode/ZnD sintered body interface barrier becomes impossible to ignore as the varistor voltage becomes lower.

For example, when applying a ZnO varistor to a DC 12V circuit, a varistor voltage of 22V is generally used in consideration of circuit voltage fluctuations, but as mentioned above, the varistor voltage per grain boundary layer is approximately 2V. Therefore, there are 11 layers of grain boundaries between the opposing electrodes of this element.
However, due to the above-mentioned electrode/ZnO sintered body interface barrier, the varistor voltage of the grain boundaries of the ZnO sintered body and the electrode/
The total varistor voltage at the sintered body interface must be 22V. Therefore, in reality, as the varistor voltage decreases, the difference between the varistor voltage determined from the number of sintered bodies and the actually measured varistor voltage increases, so the number of grain boundaries in the sintered body must be reduced. If the thickness of the sintered body is necessarily reduced, (1) the mechanical strength of the sintered body will decrease [2] The ability to absorb surge energy (surge resistance) such as lightning surges and switching surges will depend on the volume of the sintered body. Since the ratio is proportional, there were problems such as a decrease in surge resistance characteristics.

In addition, the Schottky barrier at the electrode/sintered body interface is
The Schottky barrier existing at the grain boundaries of the sintered body itself has a low relative Vesage resistance, so the lower the varistor voltage is, the greater the change in varistor voltage due to surges becomes. The problem with the Schottky barrier is that it tends to fluctuate during the assembly process, such as the soldering process and the resin molding process, making process control of the varistor voltage difficult.

This invention was made in view of the above points, and its purpose is to eliminate the Schottky barrier between the sintered body and the metal electrode, thereby achieving a low voltage with excellent mechanical strength and surge resistance, and easy process control. An object of the present invention is to provide a voltage non-linear resistance element for use.

[Means to solve the problem]

According to the present invention, the above-mentioned object is achieved by irradiating the surface of the sintered body, which is made by mixing zinc oxide powder and a small amount of additive powder that causes voltage nonlinearity and firing it, with a laser beam on the surface on which the electrode is to be provided. Heat treatment is performed, and then electrodes are formed.

[Effect]

Zinc oxide is an η-type semiconductor, and donors are formed by oxygen vacancies. Therefore, by heating and rapidly cooling the surface on which the electrode is to be provided by irradiation with laser light in advance, oxygen vacancies are formed in the surface layer of several surfaces, thereby creating a low resistance region. Although oxygen vacancies are formed in the atmosphere, they are preferably formed in a neutral gas or in a vacuum.

Due to the low resistance layer, the Schottky barrier that would be formed at the sintered body/electrode interface disappears.

〔Example〕

Next, embodiments of the present invention will be described based on the drawings.

FIG. 2 shows the manufacturing process of a resistance element according to an embodiment of the present invention. First, appropriate amounts of Pr, Co, B, etc. in the form of compounds such as oxides are added to ZnO powder, and then mixed and granulated to prepare granulated powder for ZnO varistors.

This granulated powder is molded to a thickness of 1 mm using a mold with a diameter of 1.7 mm.
．． Shape into a 25mm disc. Next, this molded body is fired at a temperature of 1350° C. for 4 hours in an oxidizing atmosphere. The size of the obtained sintered body was 14n+m in diameter and 1.2m in thickness.
It was 0 mm.

Both opposing main surfaces of the sintered body produced as described in 1 above were heat-treated using an Ar laser in N2 gas. At this time, the surface temperature was approximately 1000°C.

The surface resistance of the surface heat-treated with a laser was less than 1Ω when a current of 1 mA was applied. The untreated one is 20
It was more than Ω.

A varistor was constructed by applying silver paste to the sintered body treated as described above and then baking it. A schematic cross-sectional view is shown in FIG. 3
is a sintered body, 2A and 2B are heat-treated layers heat-treated with a laser, and IA and 1.8 are metal electrodes. Subsequently, the parisphere characteristics were measured. Furthermore, the characteristics of the harisph after assembly were also evaluated.

The results are shown in Table 1.

Table 1 Table 2 shows the surge resistance (A) after assembly when a surge is applied. However, the surge resistance is 8/20
It was defined as a current at which the rate of change in V + mA after passing a μs standard current pulse through the device twice at 2-minute intervals was ±1O%.

Table 2 Table 1 shows VlffiA, coefficient of variation of V+□4, current 1
The voltage nonlinear coefficient α in the 00 μΔ to 1 mA region is shown in comparison with the conventional method after one assembly after electrode attachment.

Further, as reference data, characteristics when using a ZnO sintered body and an In-Ga electrode that does not form a Schottky barrier are also shown. From these results, it can be seen that the method of the present invention has no characteristic variation after electrode attachment and after assembly, and is superior to the conventional method.

As is clear from Table 2, the method of the present invention is superior. The reason for this is thought to be as follows. That is, the surge resistance of the sintered body itself is 1.2000Δ, but in the conventional element, the barrier formed at the electrode/sintered body interface is not easily changed by the surge, so in the case of an element where V l ff1A is 20V, This is because a 2V change in the electrode/sintered body interface barrier results in a 10% change. In this way, the lower the varistor voltage, the lower the Zn produced by the conventional method.
This eliminates the drawback that the surge resistance of the O varistor element is apparently low.

In addition, although the above-mentioned example described a method of heat treatment using a laser in N2 gas, similar effects were confirmed in a gaseous atmosphere, in the atmosphere, and in a vacuum.

Further, although the embodiments have been described with respect to a disk-shaped sintered body, the effects of the present invention are not dependent on the shape, and can be confirmed not only when the electrodes are opposed to each other but also when two electrodes are provided on the same plane.

〔Effect of the invention〕

According to this invention, by heat-treating the surface of a sintered body made by mixing zinc oxide powder and a small amount of additive powder that causes voltage nonlinearity and irradiating the surface on which the electrode is to be provided with a laser beam. , it is possible to introduce a low resistance layer between the sintered body and the electrode, which eliminates the Schottky barrier between the sintered body and the electrode, resulting in excellent mechanical strength and surge resistance, and easy manufacturing process control. A voltage nonlinear resistance element can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing a voltage non-linear resistance element according to an embodiment of the present invention, FIG. 2 is a flowchart showing the manufacturing process of a resistance element according to an embodiment of this invention, and FIG. 3 is a diagram of a conventional element. It is a flow chart showing a manufacturing process. 1Δ, IB metal electrode, 2A, 2B heat treatment layer, Fig.

Claims (1)

[Claims] 1) Heat-treat the surface by irradiating the surface of the sintered body, which is made by mixing zinc oxide powder and a small amount of additive powder that causes voltage nonlinearity and firing it, with a laser beam to provide the electrode, and then form the electrode. A method of manufacturing a voltage nonlinear resistance element, characterized by: