JPS586287B2 - Manufacturing method of thick film varistor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention mainly relates to a method of manufacturing a thick film varistor suitable for eliminating spark discharge generated between a commutator and a brush of a small electric motor.

Generally, when an electric contact breaks a circuit, a sudden change in current occurs, and the accompanying self-induction induces a surge voltage, causing a spark discharge between the contacts.

This spark discharge not only accelerates the wear of contacts or commutators and brushes, thereby shortening the life of the motor;
As a noise source, it causes various disturbances to various devices.

For this purpose, a resistor or a capacitor is connected between commutator segments, or a varistor made by sintering a SiC varistor or an oxide semiconductor into a ring shape has come to be used.

However, resistors do not have a significant effect on eliminating sparks or suppressing noise, and SiC varistors, etc. are more effective than resistors, but their shape leaves problems in the installation method and installation space. .

One of the performances required of varistors these days is a large nonlinear coefficient.

In particular, when applied to small electric motors, it is necessary to suppress spark voltage, absorb electrical noise, and reduce wear on commutators and brushes, and on the other hand, it is necessary to have a cleaning effect using commutator sparks.

In other words, a varistor having an appropriate nonlinear coefficient α is optimal, and for mass production, it is necessary that this nonlinear coefficient α does not vary.

An object of the present invention is to suppress spark discharge to an appropriate level, reduce variations in the nonlinear coefficient α, and improve load withstand capacity.

In other words, in conventional thick-film varistors made of varistor powder and glass frit that binds it together, zinc oxide, bismuth oxide, cobalt oxide, and manganese oxide are used as varistor powder to obtain an appropriate nonlinear coefficient α. etc. are added, mixed, molded, and fired.

After firing, it was coarsely ground in a mortar and then finely ground in a pot mill to obtain barista powder with a particle size suitable for printing.

However, the particles of barista powder obtained by such mechanical pulverization have different shapes and large variations in particle size.

In contrast, according to the present invention, cobalt oxide, manganese oxide, nickel oxide, and barium carbonate are added to zinc oxide as varistor powder, mixed, molded, and fired, and after firing, the sintered body is placed in water and hydrolyzed. By this process, varistor powder with a particle size suitable for printing can be obtained.

Can it be obtained by chemical methods like this? Compared to varistor powder, which has conventionally been mechanically pulverized, Lista powder has more uniform particle shapes and smaller variations in particle size.

Therefore, in a thick film varistor using this varistor powder, the variation in the nonlinear coefficient α is small, and the load withstand capacity is also improved.

Hereinafter, an explanation will be given based on an example with reference to FIG. 1.

The composition of the varistor powder is ZnO, CO03+Mn0
2, NiO, and BaC03 were weighed, transferred to a pot mill, added pure water, and wet-mixed for 20 hours. After mixing, the slurry was taken out and dried.

The dried powder is passed through a 32 mesh, and the passed powder is molded into a disc shape with a pressure of 300 kg/cm', a diameter of 20 mm, and a thickness of about 1 mm.

This molded product is fired at a temperature of 1050°C to 1350°C.

The sintered body is placed in water and left for 2 to 3 days.

After 2 to 3 days, BaCO3 is dissolved in water and the sintered body becomes powder.

The amount of the varistor powder and glass frit obtained in this way is 2 to 2 to the total amount of the varistor powder and glass frit.
A vehicle is added to the solid content of 50 wt% to form a paste.

The vehicle is necessary for making the paste, and there is no particular restriction on the type as long as it is dispersed during firing of the varistor film, but a vehicle prepared by dissolving ethyl cellulose in carbitol acetate was used.

The varistor paste thus obtained was applied to the electrically insulating substrate 1.
The varistor film 3 is formed by printing and baking the lower electrode 2 printed thereon.

Further, by printing and firing the upper electrode 4 on the varistor film 3, a thick film varistor can be obtained.

Now, for a varistor with a varistor film thickness of 30μ and a varistor area of 5-, the voltage between the terminals (varistor voltage) when a current of 10mA is applied, V1o1, the voltage nonlinear coefficient 1α1o when currents of 1mA and 10mA are applied, and Figure 2. The rate of change in VIO when pulses having an energy of 100 mJ were applied 10 times at 5 second intervals using the pulse test circuit shown was investigated and is shown in the table below.

As is clear from the table, the thick film varistor of the present invention has a current of 1
Voltage nonlinear coefficient 1α1o at mA and 10 mA
The variation in is smaller than that of the conventional method, and it is also effective in terms of pulse load withstand capacity.

In the barista composition, barium carbonate is 0.1 mol%
The reason for setting it to 5.0 mol% is because hydrolysis becomes difficult if it is less than 0.1 mol%, and 5.0 mol% or less is difficult.
This is because if the amount exceeds mol %, grains of ZnO will grow excessively and it will be difficult to remove barium carbonate after hydrolysis.

The present invention is intended to reduce variations in the varistor voltage and nonlinear coefficient α and to improve pulse withstand capacity by aligning the shape of the particles of varistor powder and reducing variations in particle size.

Therefore, the metal oxide added to ZnO as a varistor composition is not limited to the metal oxides shown in the above table, but may be any metal oxide that improves the nonlinear coefficient α.

Furthermore, the amount of glass frit relative to the total amount of varistor powder and glass frit is not limited as shown in the table, but may be in the range of 2 to 50 wt%.

As described above, the thick film varistor manufacturing method according to the present invention has a small variation in the voltage nonlinear coefficient α and a large pulse load capacity. It protects the children and brushes.

[Brief explanation of drawings]

FIG. 1 is a front sectional view of a thick film varistor obtained by the method of the present invention, and FIG. 2 is a pulse test circuit for the varistor. DESCRIPTION OF SYMBOLS 1... Electric insulating substrate, 2... Lower electrode, 3... Varistor film, 4... Upper electrode.

Claims (1)

[Claims] 1. In a thick film varistor consisting of varistor powder and glass frit bonding it together, several metal oxides and BaCO3 are added to ZnO at 0.00%. 1 to 5. After mixing, molding and firing, the above varistor powder is obtained from the sintered body by hydrolysis, and a paste made by adding a solvent containing a thickener to this varistor powder and glass frit is applied to an electrically insulating substrate. A method for manufacturing a thick film varistor, characterized in that a varistor film is formed by applying the film directly or via an electrode on the thick film varistor.