Diode Comprising Zinc Oxide Doped With Gallium Oxide Used As A Voltage Variable Resistor

Abstract

Voltage variable resistors having nonohmic resistance comprising a sintered wafer consisting essentially of zinc oxide with 0.05 to 10.0 mole of gallium oxide and two electrodes applied to opposite surfaces of said sintered wafer, at least one of said two electrodes being a silver paint electrode.

Description

This invention relates to voltage variable resistors having nonohmic resistance and more particularly relates to varistors comprising zinc oxide and having silver electrodes applied thereto.

Various voltage variable resistors such as silicon carbide varistors, selenium or cuprous oxide rectifiers and germanium or silicon PN junction diodes, are known. The electrical characteristics of such voltage variable resistors are expressed by the relation:

I=(V/C).sup.n

Where V is the voltage across the resistor, I is the current 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=(log.sub.10 (I.sub.2 /I.sub.1)/log.sub.10 (V.sub.2 /V.sub.1)

where V.sub.1 and V.sub.2 are the voltages at given currents I.sub.1 and I.sub.2, respectively. Conveniently, I.sub.1 and I.sub.2 are 2 are 10 ma. and 100 ma; respectively. The desired value of C depends upon the use to which the resistor is to be put. It is ordinarily desired 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 variable resistors and are manufactured by mixing fine particles of silicon carbide with water, a ceramic binder and/or conductive material such a graphite or metal powder, pressing the mixture in a mold to the desired shape, and then drying and firing the pressed body in air or a nonoxidizing atmosphere. Silicon carbide varistors with conductive materials are characterized by a low electric resistance, i.e. a low valve of C and a 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-valves 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 shown n-values from 4 to 7 and C-values from 30 to 800 at a given current of 1 ma. with respect for 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 of 5 to 10 and a C-valve less than 2 at a given current of 100 ma. with respect for a rectifier size of 20 mm. in diameter. In this case, the thickness of the rectifier does not effect the C-value.

A germanium or silicon PN junction resistor has an extremely high value of n but its C-value is constant, e.g. on the order of 0.3 or 0.7 at a given current of 100 ma. because its diffusion voltage in the V-I characteristic is constant and cannot be changed 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 widespread at the present because of their high cost even though they may have a high value of n.

An object of this invention is to provide a voltage variable resistor having a high value of n and a low value of C.

A further object of this invention is to provide a voltage variable resistor capable of being made by a simple manufacturing method at a low cost.

A further object of this invention is to provide a voltage variable resistor characterized by a high stability with respect to temperature, humidity and electric load.

Another object of this invention is to provide a voltage variable resistor, the C-value of which can be controlled.

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 variable resistor in accordance with the invention.

Before proceeding with a detailed description of the voltage variable resistors contemplated by the invention, their construction will be described with reference to the aforesaid figure of drawing wherein reference character 10 designates, as a whole, a voltage variable resistor having, as its active element, a sintered wafer 1 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 compositions and 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 in conductive relationship to the electrodes 2 and 3, respectively, by a connection means 4 (solder or the like).

According to the present invention, sintered wafer 1 consists essentially of zinc oxide (ZnO) and a minor portion of an additive of gallium oxide (Ga.sub.2 O.sub.3) It is preferable that said sintered wafer have further a minor proportion of an additive of bismuth oxide (Bi.sub.2 O.sub.3).

It has been discovered according to the invention that said sintered body 1 has electrical characteristics of superior nonlinearity when it is provided with silver electrodes prepared by applying silver paint to at least one of the opposite surfaces thereof and firing at 300.degree. C. to 900.degree. C. in an oxidizing atmosphere such as air or oxygen. The n-valve and C-value of voltage variable resistors produced in this manner vary with the compositions of the sintered body and the electrodes, and the method of their preparation.

Since the nonlinearity of the novel resistors is attributed to the nonohmic contact between said sintered body 1 and electrodes 2 and 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. The control of the C-value and the n-value can also be obtained by applying an ohmic electrode as the electrode 3 in place of the silver electrode.

It is necessary for achieving a low value of C for the resultant voltage variable resistors that the sintered body have an electrical resistivity less than 10 ohm-cm., said electrical resistivity being measured by a four point method in a per se conventional way.

According to the present invention, a voltage variable resistor with low C and high n can be obtained when said resistor comprises a sintered wafer consisting essentially of 90 to 99.95 mole percent of zinc oxide (ZnO) and 0.05 to 10.0 mole percent of gallium oxide (Ga.sub.2 O.sub.3) and two electrodes are applied to opposite surfaces of said sintered wafer, at least one of said two electrodes being a silver paint electrode.

It has been discovered according to the invention that the C-value is further lowered when one of said electrodes applied to said sintered wafer is a silver paint electrode and the other is an ohmic electrode

The n-value is further elevated when said sintered wafer is 99.9 to 97.0 mole percent of zinc oxide (ZnO) and 0.1 to 3.0 mole percent of gallium oxide.

According to the invention the combination of a lower C and a higher n can be obtained when said sintered wafer is 99.9 to 82.0 mole percent of zinc oxide (ZnO), 0.05 to 10.0 mole percent of gallium oxide (Ga.sub.2 O.sub.3) and gallium to 8.0 mole percent of bismuth oxide (Bi.sub.2 O.sub.3).

The stability during an electric load life test and at ambient temperature is improved when said silver electrode or electrodes has a composition of 70 to 99.5 wt. percent of silver 0.3 to 27 wt. percent of lead oxide (PbO), 0.1 to 15 wt. percent of silicon dioxide (SiO.sub.2) and 0.05 to 15 wt. percent of boron trioxide (B.sub.2 O.sub.3).

The stability during electric load life test and at ambient temperature is also improved when said silver electrode or electrodes have a composition of 70 to 99.5 wt. percent of silver, 0.3 to 27 wt. percent of bismuth oxide (Bi.sub.2 O.sub.3), 0.3 to 15 wt. percent of silicon dioxide (SiO.sub.2) and 0.05 to 15 wt. percent of boron trioxide (B.sub.2 O.sub.3). It has been discovered according to the invention that the n-value and the stability during an electric load life test and at ambient temperature of the resistors are greatly improved when said silver electrodes have a composition of 70 to 99.5 wt. percent silver, 0.3 to 27 wt. percent of bismuth oxide (Bi.sub.2 O.sub.3), 0.03 to 15 wt. percent of silicon dioxide (SiO.sub.2), 0.05 to 15 wt. percent of boron trioxide (B.sub.2 O.sub.3) and 0.05 to 6 wt. percent of at lest one member selected from the group of consisting of cobalt oxide (CoO) and of manganese oxide (MnO).

The n-value and the stability during an electric load life test and at ambient temperature are further improved when said silver electrodes have a composition of 70 to 99.5 wt. percent of silver, 0.3 to 27 wt. percent of lead oxide (PbO), 0.1 to 15 wt. percent of silicon dioxide (SiO.sub.2), 0.05 to 15 wt. percent of boron trioxide (B.sub.2 O.sub.3) and 0.05 to 6 wt. percent of at least one member selected from the group of consisting of cobalt oxide (CoO) and manganese oxide (MnO).

The sintered body 1 can be prepared by a per se well-known ceramic technique. The starting materials in the compositions described in the foregoing description are mixed in a wet mill so as to produce a homogeneous mixtures. The mixtures are dried and pressed in a mold into desired shapes at a pressure from 100 kg./cm..sup.2 to 1000 kg./cm.sup.2. The pressed bodies are sintered in air at 1100.degree.to 1450.degree. C. for 1 to 3 hours, and then furnace-cooled to room temperature (about 15.degree.to about 30.degree. C.). The pressed bodies are preferably sintered in a nonoxidizing 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 mixtures may be preliminarily calcined at 600.degree.to 1000.degree. C. and pulverized for easy fabrication in the subsequent pressing step. The mixture to be pressed can be admixed with a suitable binder such as water, polyvinyl alcohol, etc.

It is advantageous that the sintered body, have the opposite surfaces lapped with an abrasive powder such as silicon carbide having a particle size of 300 mesh to 1500 mesh.

The sintered bodies are coated on one or both 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. Solid ingredients having compositions described in the foregoing description can be prepared in a per se conventional manner by mixing commercially available powders with organic resin such as epoxy, vinyl and phenol resin in an organic solvent such as butyl acetate, toluene or the like so as to produce silver electrode paints.

The silver powder can 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 on 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 size of solid ingredients also are required to be controlled so as to be in the range of 0.1.mu. to 5.mu..

The sintered bodies with a silver electrode on only one surface thereof have applied to another surfaces an ohmic electrode in a manner such as by a spray method using Al, Zn and Sn, a vacuum evaporation method using Al. In on Zn and an electrolytic or electroless method using Ni, Cu and Sn.

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 variable resistors according to this invention have a high stability with respect to temperature and in a load life test, which is carried out at 70.degree. C. at a rating powder 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 voltage variable resistors be embedded in a humidity proof resin such as epoxy resin or 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 nonlinear resistors. The n-value will not be optimal when the applied silver electrode paint is heated in a nonoxidizing 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 or oxygen.

Silver electrodes prepared by any other method than by silver painting result in poor n-value. For example, the voltage variable resistor is not produced when the sintered body is provided with silver electrodes on the opposite surfaces by electroless plating or electrolytic plating in a conventional manner. Silver electrodes prepared by vacuum evaporation or chemical deposition result in an n-value less than 3.

The following examples are given as illustrative of the presently preferred method of proceeding according to the present invention. However it is not intended that the scope of said invention be limited to the specific examples.

Example 1

Starting material according to table 1 is mixed in a wet mill for 5 hours.

The mixture is dried and pressed in a mold into a disc of 13 mm. in diameter and 2.5 mm. in thickness at a pressure of 340 kg./cm..sup.2.

The pressed body is sintered in air at 1350.degree. C. for 1 hour, and then quenched to room temperature (about 15.degree. to about 30.degree. C.). The sintered disc has the opposite surfaces thereof lapping using silicon carbide having a particle size of 600 mesh. The thus produced sintered disc is 10 mm. in diameter and 1.5 mm. in thickness. The sintered disc is coated on the opposite surfaces thereof with a silver electrode paint by a conventional brushing method. The silver electrode paint employed is composed of solid ingredient according to table 2 and is prepared by mixing with vinyl resin in amyl acetate. The coated disc is fired at the temperature listed in table 2 for 30 minutes in air.

Lead wires are attached to the silver electrodes by means of silver paint. The electric characteristics of the resultant resistor and of other similarly prepared resistors are shown in table 1.

It will be readily understood that the zinc oxide sintered body having gallium oxide therein in has a low C value and a high N-value and combined additions of gallium oxide and bismuth oxide will produce a higher n-value. ##SPC1## ##SPC2##

Example 2

Starting material according to table 3 is mixed in a wet mill for 5 hours. The mixture is dried, pressed, sintered and lapped in the same manner as in the example 1. The lapped disc is coated on one surface thereof with the same silver electrode paints as those used in example 1 by a conventional brushing method. The coated disc is fired at the temperature listed on table 2 for 30 minutes in air . The fired disc is provided on another surface thereof with an ohmic electrode by spraying with aluminum or evaporating aluminum.

Lead wires are attached to the silver electrodes by means of silver paint. The electric characteristics of the thus produced resistor and of other similarly prepared resistors are shown in table 3, wherein electrical characteristics are measured with the polarity applying high voltage terminal to the silver electrode.

It will be easily realized that the combination of a silver electrode and an ohmic electrode produces a low C and a high n: ##SPC3##

Example 3

A sintered disc having a comparison listed in table 4 is prepared in the same manner as in example 1. The sintered disc is 10 mm. in diameter and 1.5 mm. in thickness after lapping. Various silver electrode paints are applied to the opposite surfaces of the sintered disc and fired at the temperature listed in table 4 for 30 minutes in air. The silver electrode paints are composed at ingredient as shown in table 4 and are prepared by mixing 100 weight parts of said solid ingredient compositions with 1 to 20 weight parts of epoxy resin in 20 to 40 weight parts of butyl alcohol. The resultant resistors exhibit desirable C-values and n-values as indicated in table 4. It will be readily understood that the electrode compositions have a great effect on the electrical characteristics of the resultant voltage variable resistors and particularly electrode compositions containing cobalt oxide and manganese oxide are especially useful obtaining a higher value of n. ##SPC4##

Example 4

A sintered disc having a composition listed in table 5 is made into resistor in the same manner as in example 1. The resistors are tested according to the methods used for electronic component parts. The load life test is carried out at 70.degree. C. ambient temperature at 1 watt rating power for 500 hours. The heating cycle test is carried out by repeating 5 times the cycle in which said resistors are cooled to -20.degree. C. and then kept at such temperature for 30 minutes. After the heating cycle and load life test, the rates of change of the C-value and n-value are shown in table 5. It will easily realized that the change rates are less than 10 percent and the composition of silver electrodes containing cobalt oxide and manganese oxide produce excellent stability. ##SPC5##

Claims

What we claimed is:

1. A voltage variable resistor comprising a sintered wafer consisting essentially of zinc oxide and a member taken from the group consisting of (a) 0.05 to 10.0 mole percent of gallium oxide (Ga.sub.2 O.sub.3) and (b) mixtures of 0.05 to 10.0 mole percent of gallium oxide (Ga.sub.2 O.sub.3) and 0.05 to 8.0 mole percent of bismuth oxide (Bi.sub.2 O.sub.3,) and two electrodes applied to opposite surfaces of said sintered wafer, at least one of said two electrodes consisting of silver paint electrodes.

2. A voltage variable resistor according to claim 1, wherein one of said electrodes is a silver paint electrode and the other is an ohmic electrode.

3. A voltage variable resistor according to claim 1, wherein said sintered wafer consists essentially of 99.9 to 97.0 mole percent of zinc oxide (ZnO) and 0.1 to 3.0 mole percent of gallium oxide (Ga.sub.2 O.sub.3).

4. A voltage variable resistor according to claim 1, wherein said sintered wafer consists essentially of 99.8 to 94.0 mole percent of zinc oxide (ZnO), 0.1 TO 3.0 mole percent of gallium oxide (Ga.sub.2 O.sub.3) and 0.1 TO 3.0 mole percent of bismuth oxide (Bi.sub.2 O.sub.3).

5. A voltage variable resistor according to claim 1, wherein said silver electrode has a composition comprising 70 to 99.5 wt. percent of silver, 0.3 to 27 wt. percent of load oxide (PbO), 0.1 to 15 wt. percent of silicon dioxide (SiO.sub.2) and 0.05 to 15 wt. percent of boron trioxide (B.sub.2 O.sub.3).

6. A voltage variable resistor according to claim 1, wherein said silver electrode has a composition comprising 70 to 99.5 wt. percent of silver 0.3 to 27 wt. percent of bismuth oxide (Bi.sub.2 O.sub.3), 0.03 to 15 wt. percent of silicon dioxide (Si.sub.2) and 0.05 TO 15 wt. percent of boron trioxide (B.sub.2 O.sub.3).

7. A voltage variable resistor according to claim 1, wherein said silver electrode has a composition comprising 70 to 99.5 wt. percent of silver, 0.3 to 27 wt. percent of lead oxide (PbO), 0.1 to 15 wt. percent of silicon dioxide (SiO.sub.2), 0.05 to 15 wt. percent of boron trioxide (B.sub.2 O.sub.3) and 0.05 to 6.0 wt. percent of at least one member selected from the group consisting of cobalt oxide (CoO) and manganese oxide (MnO).

8. A voltage variable resistor according to claim 1, wherein said silver electrode has a composition comprising 70 to 99.5 wt. percent of silver, 0.3 to 27 wt. percent of bismuth oxide (Bi.sub.2 O.sub.3), 0.03 to 15 wt. percent of silicon dioxide (SiO.sub.2), 0.05 to 15 wt. percent of boron trioxide (B.sub.2 O.sub.3) and 0.05 to 6.0 wt. percent of at least one member selected from the group consisting of cobalt oxide (CoO) and manganese oxide (MnO).