JPS6050322B2 - thick film varistor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thick film varistor in which a pair of electrodes is provided on a varistor film made of zinc oxide powder and glass frit for bonding the zinc oxide powder.

The purpose is to provide a thick film varistor with excellent electrical characteristics and no structural defects such as bubbles inside the varistor film. Thick film varistors made of zinc oxide powder and glass frit have been developed and put into practical use as microscopic and thin film components.

However, these thick film varistors have bubbles and the like inside the varistor film, which reduces the effective area of the varistor film and deteriorates the electrical characteristics, especially the pulse characteristics. This is because in the conventional method, the average particle size of the zinc oxide powder is 4 to 10 μm, and the average particle size of the glass frit is 4 to 10 μm.
Since the diameter is ~7 μm, even if the zinc oxide particles are sufficiently mixed, voids will be created in the contact areas between the zinc oxide particles, and the glass frit will not be able to enter these voids, and the voids will be surrounded by molten glass during baking. This is thought to be because it expands and becomes a board when it is wet. Therefore, the present invention aims to solve the conventional problems and provide a thick film varistor which does not have structural defects such as bubbles inside the varistor film and has excellent electrical characteristics, particularly pulse characteristics.

In order to achieve this objective, in the present invention, in addition to the conventional zinc oxide powder with an average particle size of 4 to 10 μm,
Prepare particles with an average particle size of 2 μm or less, and combine this with an average particle size of 4
A method was used in which a varistor paste was prepared by thoroughly mixing glass frit of ~7 μm, and this was applied onto an electrically insulating substrate directly or via an electrode to obtain a varistor film baked at a temperature higher than the melting point of the glass.

With this method, very fine zinc oxide powder with an average particle size of 2 μm or less enters the voids that were conventionally created in the contact area of zinc oxide powder, resulting in dense filling, which eliminates air bubbles, etc. This is an attempt to obtain a thick film varistor with extremely few structural defects. The following will explain based on the drawings.

In FIG. 1, 1 is an electrically insulating and heat-resistant substrate, 2 and 4 are electrodes, and 3 is a varistor film, which is made of zinc oxide powder and glass.

Further, FIG. 2 shows the voltage-current characteristics of the thick film varistor according to the present invention. As is well known, the voltage-current characteristics of a varistor are expressed by the following empirical formula. However, I: current;
α: nonlinear coefficient, ■: voltage, C: constant. Then, the voltage when the current flows is ■.

If so, ■. is defined as the varistor voltage at the current. The zinc oxide powder used in the present invention was produced as follows.

That is, zinc oxide (ZnO) powder is
By firing in an air atmosphere at a temperature of 0°C for 0.5 to 5 hours and finely pulverizing the obtained sintered body, one with an average particle size of 4 to 10 pm and one with an average particle size of 2 μm or less were obtained. . When the glass frit used as a binder used in the present invention is baked in air, it melts sufficiently at the baking temperature, binds the zinc oxide particles well to each other, and at the same time fixes well on the substrate or electrode. Bye.

A glass frit that exhibits favorable properties for this purpose is barium zinc borate glass. The glass composition exhibiting particularly favorable properties was 30-45% by weight boron oxide, 10-4% by weight barium oxide, and 15-45% by weight zinc oxide. Further, preferable properties were also shown when 2 to 15% by weight of bismuth oxide or cobalt oxide was added individually or both to the mixture of boron oxide, barium oxide, and zinc oxide. and,
The glass frit was prepared by a well-known method. That is, glass with an average particle size of 4 to 7 μm is produced by blending a predetermined glass composition, melting the well-mixed mixture at high temperature, quenching it in water, and then pulverizing it to the required particle size. Got frites. A barista paste is made from these glass frits, zinc oxide powder and a solvent containing a thickener.

The method involves blending these compositions, mixing them well with a kneader such as a three-stage roller or Hubermala, and uniformly dispersing them.
Obtain one with a predetermined viscosity. The solvent and thickener are necessary for making the paste, and there are no particular restrictions on their type as long as they are scattered during baking, but a solution of ethyl cellulose in carbitol acetate was used. The viscosity of the above barista paste is 200 to 2000 poise. The electrode material used in the present invention is a silver paste containing silver powder, bismuth oxide, and a thickener.

Note that in addition to silver, pastes containing powders of gold, platinum, and palladium can also be used for the electrodes. The silver paste consists of silver powder with an average particle size of 0.1 to 10 μm, bismuth oxide, and a thickener containing a solvent, and is obtained by mixing these together. Next, a method for manufacturing a thick film varistor will be described with reference to FIG.

First, a silver paste is applied to the surface of a substrate 1 having good heat resistance and electrical insulation properties, such as alumina, forsterite, crystallized glass, etc., and after drying, a tunnel furnace having a maximum temperature of 850 to 950°C is applied. A silver electrode 2 is made by baking in an air atmosphere.Next, varistor paste is applied on this electrode 2 and after drying, a varistor film 3 is made in the same way.Next, using the same silver paste, An electrode 4 is formed on the varistor film 3 by the same method. After baking this electrode 4, if necessary, it is protected with an overcoat glass or resin that can be coated at a temperature lower than the baking temperature to prevent characteristic deterioration. Hereinafter, the content of the invention will be described by giving more specific examples.

Zinc oxide (ZnO) powder was heated at a temperature of 1350°C in an air atmosphere for 1 hour.

The obtained sintered body was coarsely pulverized with a stamp mill and then finely pulverized with a ball mill to produce zinc oxide particles with an average particle size of 4 to 10 μm and zinc oxide powder with an average particle size of 2 μm or less. On the other hand, as a glass frit composition, boron oxide (B2O3)
, barium oxide (BaO), and zinc oxide (ZnO) were set to 35% by weight, 3% by weight, and 35% by weight, respectively, and 5% by weight of cobalt oxide (CO2O3) and bismuth oxide (Bl2O3) were added to this mixture, respectively. Added.

These mixtures were melted at 1250°C, poured into water, rapidly cooled, coarsely ground, and then milled using a ball mill to reduce the average particle size to 4.
A glass frit of ~7 μm was made.

These zinc oxide powders and glass frit were mixed in various ratios, and 8 parts by weight of solid content and 2 parts by weight of a solvent containing a thickener were added and thoroughly kneaded with a Hubermara to obtain a barista paste. The thickener consists of 15% by weight ethyl cellulose and 85% by weight carbitol acetate. The electrode material was prepared by adding ethyl cellulose dissolved in a carbitol acetate solvent to a powder having a weight ratio of 95 silver powder to 5 bismuth oxide (Bi2O3).

Silver paste was first applied onto an alumina substrate by screen printing, and after drying, it was heated to a maximum temperature of 920'C.
The powder was passed through a tunnel furnace and baked in an air atmosphere.

Next, the barista paste was applied in the same manner and baked. The thickness of the obtained varistor film was 40 μm. Subsequently, the same silver paste was applied to an electrode area of 5Tf0fL square and fired in the same manner. The varis of the thick film varistor obtained in this way, 8 voltage.

, pulse voltage resistance values are shown in the table below. Varistor voltage■.
is the voltage at a current of 10rT1A.■10. Expressed in nA. Further, the withstand pulse voltage was measured using a test circuit shown in FIG. In the test, first, a 35 μF capacitor is charged from a DC power supply, then the SW is placed on the sample (thick film varistor) side, and a charging voltage is applied to the sample. This is repeated 10 times, and the varistor voltage VlO of the thick film varistor. The DC power supply voltage value at which nA changed by 10% or more compared to the initial value was defined as the pulse withstand voltage value at that time. As is clear from the details detailed above and the table, by mixing the zinc oxide powder composition of the present invention with an average particle size of 4 to 10 pm and an average particle size of 2 μm or less, the pulse resistance characteristics can be improved. I found it to be significantly improved.

Furthermore, if the amount of glass is less than (to)% by weight based on the total amount of zinc oxide powder and glass, the pulse withstand voltage value will be low at 40V or less, whereas if the amount of glass exceeds 5% by weight, There were problems such as the glass seeping into the silver electrode, making it impossible to solder.

In the examples, only zinc oxide powder was described, but it can also be mixed with glass frit to form a varistor film, for example, zinc oxide powder mixed with bismuth oxide, cobalt oxide, manganese oxide, etc. A similar effect can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a thick film varistor showing an embodiment of the present invention, FIG. 2 is a diagram showing voltage-current characteristics of the same varistor, and FIG.
The figure is a circuit diagram showing a test circuit for pulse characteristics.

1... Electric insulating substrate, 2, 4... Electrode, 3... ．． ．． Ballista membrane.

Claims (1)

[Scope of Claims] 1. A thick film varistor comprising zinc oxide powder and glass bonding the same, wherein the zinc oxide powder has an average particle size of 4 to 10 μm and a mean particle size of 2 μm or less. Electrically insulating paste is made by adding a solvent containing a thickener to the solid content of zinc oxide powder and glass in an amount of 30 to 50% by weight based on the total amount of zinc oxide powder and glass. A thick film varistor characterized by forming a varistor film by coating directly or via an electrode on a substrate. 2. The thick film varistor according to claim 1, wherein the amount of zinc oxide powder having an average particle diameter of 2 μm or less is 10 to 65% by weight based on the zinc oxide particles.