US3689863A

US3689863A - Voltage dependent resistors in a surface barrier type

Info

Publication number: US3689863A
Application number: US92380A
Authority: US
Inventors: Michio Matsuoka; Takeshi Masuyama; Yoshio Iida
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1969-12-08
Filing date: 1970-11-24
Publication date: 1972-09-05
Anticipated expiration: 1989-09-05
Also published as: DE2061670A1; SE361549B; DE2061670C3; GB1330010A; IE34799L; DE2061670B2; IE34799B1; NL7017694A; FR2072991A5

Abstract

A voltage dependent resistor of the surface barrier type. A sintered body consisting essentially of, as a major part, zinc oxide (ZnO) and 0.05 to 10.0 mole percent of, as an additive, beryllium oxide (BeO), has electrodes in contact therewith. At least one of the electrodes is in non-ohmic contact with the body. The body can have minor amounts of further additives such as nickel oxide, titanium oxide, barium oxide, stannic oxide, aluminum oxide, lead oxide, cadmium fluoride and thallium oxide.

Description

United States Patent Matsuoka et al.

1451 Sept. 5, 1972 VOLTAGE DEPENDENT RESISTORS IN A SURFACE BARRIER TYPE Inventors: Michio Matsuoka; Takeshi Masuyarna; Yoshio Iida, all of Osaka-fu, Japan Assignee: Matsushita Electric Industrial Co.,

, Ltd., Osaka, Japan Foreign Application Priority Data Dec. 8, 1969 Japan ..44/98789 Apr. 4, i970 Japan..................45/29908 Filed: Nov. 24, 1970 Appl. No.: 92,380

US. Cl. -3338/20, 317/235 AP, 317/238, 317/235 UA, 252/62.3 ZT Int. Cl. ..H0lc 7/10 Field of Search ..317/235 AP, 238, 234, 235; 338/20; 252/623 ZT [56] References Cited UNITED STATES PATENTS 3,611,073 10/1971 Hamamoto ..317/235 3,570,002 3/1971 Masuyama ..317/238 Primary Examiner-J. D. Miller Assistant Examiner-Harvey Fendelman Attamey-wenderoth, Lind & Ponack ABSTRACT A voltage dependent resistor of the surface barrier type. A sintered body consisting essentially of, as a major part, zinc oxide (ZnO) and 0.05 to 10.0 mole percent of, as an additive, beryllium oxide (BeO), has electrodes in contact therewith. At least one of the electrodes is in non-ohmic contact with the body. The body can have minor amounts of further additives such as nickel oxide, titanium oxide, barium oxide, stannic oxide, aluminum oxide, lead oxide, cadmium fluoride and thallium oxide.

10 Claims, 1 Drawing Figure PATENTEDSEP 51912 MI CHI O MATSUOKA, TAKESHI MASUYAMA and YOSHIO IIDA,

INVENTORS ATTORNEYS VOLTAGE DEPENDENT RESISTORS IN A SURFACE BARRIER TYPE This invention relates to voltage dependent resistors of a surface barrier type and more particularly to varistors comprising zinc oxide and beryllium oxide and having nonohmic electrodes applied thereto.

Various'voltage dependent resistors such as silicon carbide varistors, selenium or cuprous oxide rectifiers and germanium or silicon p-n junction diodes, are known. The electrical characteristics of such a voltage dependent resistor are expressed by the relation:

rent flowing through the resistor, C is a constant equivalent to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation:

n: lO 2/ 1) gic (V2/V1) where V and V are. the voltages at given currents I, and 1,, respectively. Conveniently, I and I are 10 mA and 100 mA, respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the degree to which the resistors depart from ohmic characteristics.

Silicon carbide varistors are most widely used as voltage dependent resistors and are manufactured by mixing fine particles of silicon carbide with water, ceramic binder and/or conductive material such as graphite, pressing the mixture in a mold to the desired shape, and then drying and firing the pressed body in air or nonoxidizing atmosphere. Silicon carbide varistors with conductive materials are characterized by a low electric resistance, i.e., a low value of C and low value of n whereas silicon carbide varistors without conductive materials have a high electric resistance, i.e., a high value of C and a high value of n. It has been difficult to manufacture silicon carbide varistors characterized by a high n and a low C. For example, silicon carbide varistors with graphite have been known to exhibit n values from 2.5 to 3.3 and C-values from 6 to 13 at a given current of 100 mA, and silicon carbide varistors without graphite show n-valves from 4 to 7 and C- values from 30 to 800 at a given current of 1 mA with respect to a given size of varistor, e.g., 30 mm in diameter and 1 mm in thickness.

Conventional rectifiers comprising selenium or cuprous oxide have an n-value less than 3 and a C-value of to 10 at a given current of 100 mA with respect to a specimen 20 mm in diameter. In this case, the thickness of the sample does not affect the C-value.

A germanium or silicon p-n junction resistor has an extremely high value of n but its C-value is constant, e.g., on the order of 0.3 to 0.7 at a given current of 100 mA because its diffusion voltage in the V-I characteristic is constant and cannot be changed very greatly. It is necessary for obtaining a desirable C-value to combine several diodes in series and/or in parallel. Another disadvantage of such diodes is the complicated steps involved in their manufacture, with resultant high cost. As a practical matter, the use of diode resistors is not "age dependent resistor, the C-valueof which can be controlled.

These objects are achieved by providing a voltage dependent resistor of the surface barriertype having a sintered body consisting essentially of, as a major part, zinc oxide (ZnO) and 0.05 to 10.0 mole percent of, as an additive, "beryllium oxide (BeO), and electrodes in contact therewith. At least one of the electrodes is in non-ohmic contact with the body. The body can have minor amounts of further additives such as nickel oxide, titanium oxide, barium oxide stannic oxide, aluminum oxide, lead oxide, cadmium fluoride and thallium oxide.

These and other objects of the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the single FIGURE is a partly cross-sectional view through a voltage dependent resistor in accordance with the invention.

Before proceeding with a detailed description of the voltage dependent resistors contemplated by the invention, their construction will be described with reference to the aforesaid figure of the drawing wherein reference character 10 designates, as a whole, a voltage dependent resistor having, as its active element, a sintered wafer l of electrically conductive ceramic material according to the present invention.

Sintered wafer 1 is prepared in a manner hereinafter set forth, and is provided with a pair of

electrodes

2 and 3 having specified compositionsand applied in a suitable manner hereinafter set forth, on two opposite surfaces of the wafer.

' The wafer 1 is a sintered plate having any one of various shapes such as circular, square, rectangular, etc. Wire leads 5 and 6 are attached conductively to the

electrodes

2 and 3, respectively, by a connection means 4 (solder or the like).

According to the present invention, a voltage dependent resistor with an n-value higher-than 5 can be obtained when the resistor comprises a sintered body consisting essentially of, as a major part, zinc oxide (ZnO) and 0.05 to 10.0 mole percent of, as an additive, beryllium oxide (BeO) and electrodes in contact with said body, at least one of which makes non-ohmic contact.

It has been discovered according to the invention that said sintered body 1 has a superior voltage dependent properties when it is provided with silver electrodes prepared by applying silver paint to opposite surfaces thereof and firing at to 850 C. in an oxidizing atmosphere such as air and oxygen. The n-value and C-value of thus produced voltage dependent'resistors vary with the compositions of the sintered body and electrodes, and their method of preparation. The stability of the resistor with silver paint electrodes is improved when said additive consists essentially of 1.0 to 8.0 mole percent of beryllium oxide (BeO).

Since the voltage dependent property of the novel resistoris attributable to the a non-ohmic property of a barrier formed between said sintered body 1 and electrodes 2 and/or 3, it is necessary for obtaining a desirable C-value and n-value to control the compositions of the sintered body 1 and the

electrodes

2 and 3.

It is necessary for achieving a low'value of'C for the resultant voltage dependent resistors that the sintered body have an electrical resistivity less than ohm-cm, said electrical resistivity being measured by a four point method in a per se conventional way.

Table 1 shows optimal compositions of sintered body 1 for producing a voltage dependent resistor having an n-value higher than 7 and a high stability with respect to temperature, humidity and electric load.

Table 2 shows operable and optimal compositions of silver electrodes 2 and/or 3 after heat treatment.

In the Table 2, the sum of the weight percents of all ingredients should be adjusted so as to be 100 weight percent by controlling the weight percent of individual ingredients within operable or optimal weight percents as indicated in the Table.

The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials in the compositions defined above are mixed in a wet mill so as to produce homogeneous mixtures. The mixtures are dried and pressed in a mold into desired shapes at a pressure from 100 kg/cm ml ,000 kg/cm The pressed bodies are sintered in air at 1,000 to l,450 C. for l to 3 hours, and then furnace-cooled to room temperature (about to about 30 C). The pressed bodies are preferably sintered in a non-oxidizing atmosphere such as nitrogen and argon when it is desired to reduce the electrical resistivity. The electrical resistivity also can be reduced by air-quenching from the sintering temperature to room temperature even when the pressed bodies are fired in air.

The mixture may be preliminarily calcined at 700 to 1,000 C. and pulverized for easy fabrication in the subsequent pressing step. The mixture to be pressed may be admixed with a suitable binder such as water, polyvinyl alcohol, etc.

It is advantageous that the sintered body have the opposite surfaces lapped by abrasive powder such as silicon carbide having a particle size of 300 mesh to 1,500 mesh.

The sintered bodies are coated at least on one of the opposite surfaces thereof by a silver electrode paint in a per se conventional manner such as by a spray method, screen printing method or brushing method. It is necessary that the silver electrode paint have a solid ingredient composition as defined in Table 2 after it is fired at 100 to 850 C. in air. Solid ingredients having compositions defined in Table 2 can be prepared in a per se conventional manner by mixing commercially available powders with organic resin such as epoxy, vinyl and phenyl resin in an organic solvent such as butyl acetate, toluene or the like so as to produce silver electrode paints.

The silver powder may be in the form of metallic silver, or in the form of silver carbonate or silver oxide, or in any other form which in firing at the temperatures employed will be converted to metallic silver. Therefore, the term silver" as used throughout this specification and the claims appended hereto in connection with the silver composition before it is fired, is meant to include silver in any form which during firing will be converted to metallic silver. The viscosity of the resultant silver electrode paints can be controlled by the amounts of resin and solvent. Particle sizes of solid ingredients also are required to be in the range of 0.1 p. to 5 11-.

Lead wires can be applied to the silver electrodes in a per se conventional manner by using conventional solder having a low. melting point. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent for connecting the lead wires to the silver electrodes.

Voltage dependent resistors according to this invention have a high stability with respect to temperature and in a load life test, which is carried out at C. at a rating power for 500 hours. The n-value and C-value do not change greatly after heating cycles and the load life test. It is preferable for achieving a high stability with respect to humidity that the resultant voltage dependent resistors be embedded in a humidity proof resin such as epoxy resin and phenol resin in a per se well known manner.

According to the invention, it has been discovered that the method of curing the applied silver electrode paint has'a great effect on the n-value of the resultant voltage dependent resistors. The n-value will not be optimal when the applied silver electrode paint is heated in a non-oxidizing atmosphere such as nitrogen and hydrogen for curing. It is necessary for obtaining a high n-value that the applied silver electrode paint be cured by heating in an oxidizing atmosphere such as air and oxygen.

Silver electrodes prepared by any other method than by silver painting result in a poor n-value. For example, the sintered body does not become a voltage dependent resistor when it is provided with silver electrodes on the opposite surfaces by electroless plating on electrolytic plating in a conventional manner. Silver electrodes prepared by vacuum evaporation or chemical deposition result in an n-value less than 3. i The following examples are given as illustrative of the presently preferred method of proceeding accord ing to the present invention; however, it is not intended that the scope of said invention be limited to the specific examples.

EXAMPLE 1 Respective starting materials according to Table 3 are mixed in a wet mill for 5 hours.

The mixture is dried and pressed in a mold into disc of 13 mm diameter and 2.5mm thickness at a pressure of 340 kg/cm.

Each pressed body is sintered in air at l,350 C. for 1 hour, and then quenched to room temperature (about 15 to about 30 C). Eachsintered disc is lapped at the opposite surfaces thereof lapped by silicon carbide having a particle size of 600 mesh. The resulting sintered disc has a size of 10 mm diameter and 1.5 mm thickness. Each sintered disc is coated on the opposite surfaces thereof with a silver electrode paint by a conventional brushing method. The silver electrode paint employed has a solid ingredient composition according to Table 4 and is prepared by mixing these ingredients with vinyl resin in amyl acetate. Each coated disc is fired at 800 C. for 30 minutes in air.

Claims

2. A voltage dependent resistor as claimed in claim 1, wherein said at least one of electrodes consists of a silver paint electrode which is in non-ohmic contact with said body.
3. A voltage dependent resistor as claimed in claim 2, wherein said additive consists essentially of 1.0 to 8.0 mole percent of beryllium oxide (BeO).
4. A voltage dependent resistor as claimed in claim 3, wherein said additive further includes at least one member selected from the group consisting of 0.1 to 3.0 mole percent of nickel oxide (NiO) and 0.1 to 3.0 mole percent of titanium oxide (TiO2).
5. A voltage dependent resistor according to claim 3, wherein said additive further includes 0.1 to 3.0 mole percent of nickel oxide (NiO), 0.1 to 3.0 mole percent of titanium oxide TiO2) and 0.02 to 1.0 mole percent of barium oxide (BaO).
6. A voltage dependent resistor according to claim 3, wherein said additive further includes 0.1 to 3.0 mole percent of nickel oxide (NiO), 0.1 to 3.0 mole percent of titanium oxide (TiO2), 0.02 to 1.0 mole percent of barium oxide (BaO) and 0.1 to 3.0 mole percent of stannic oxide (SnO2).
7. A voltage dependent resistor according to claim 3, wherein said additive further includes 0.1 to 3.0 mole percent of nickel oxide (NiO), 0.1 to 3.0 mole percent of aluminum oxide (Al2O3), 0.1 to 3.0 mole percent of lead oxide (PbO) and 0.1 to 3.0 mole percent of cadmium fluoride (CdF2).
8. A voltage dependent resistor according to claim 3, wherein said additive further includes 0.1 to 3.0 mole percent of thallium oxide (Tl2O3) and 0.1 to 3.0 mole percent of titanium oxide (TiO2).
9. A voltage dependent resistor according to claim 2, wherein said silver electrode has a composition comprising 70 to 99.5 percent by weight of silver, 0.3 to 27 percent by weight of bismuth oxide (Bi2O3), 0.1 to 15 percent by weight of silicon dioxide (SiO2) and 0.1 to 15 percent by weight of boron trioxide (B2O3).
10. A voltage dependent resistor according to claim 2, wherein said silver electrode has a composition comprising 70 to 99.45 percent by weight of silver, 0.3 to 27 percent by weight of bismuth oxide (Bi2O3), 0.1 to 15 percent by weight of silicon dioxide (SiO2), 0.1 to 15 percent by weight of boron trioxide (B2O3) and 0.05 to 6.0 percent by weight of cobalt oxide (CoO).