Voltage Dependent Resistors In A Surface Barrier Type

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

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: 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: 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 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 non-oxidizing 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 5 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 widespread at the present in view of their high cost even though they may have a high value of n.

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

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

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

These objects are achieved by providing a voltage dependent resistor of the surface barrier type 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 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 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 100.degree. to 850.degree. 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 resistor is 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 10 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.sup.2 to 1,000 kg/cm.sup.2. The pressed bodies are sintered in air at 1,000.degree. to 1,450.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 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.degree. to 1,000.degree. 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.degree. to 850.degree. 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 .mu. to 5 .mu..

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 70.degree. 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.

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

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 2.5 mm thickness at a pressure of 340 kg/cm.sup.2.

Each pressed body is sintered in air at 1,350.degree. C. for 1 hour, and then quenched to room temperature (about 15.degree. to about 30.degree. C). Each sintered 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.degree. C. for 30 minutes in air.

Lead wires are attached to the silver electrodes by means of silver paint. The electric characteristics of the resultant resistors are shown in Table 3.

EXAMPLE 2

Sintered discs having composite according to Table 5 are prepared and then lapped in the same manner as in Example 1. Each lapped disc is coated on one of the opposite surfaces thereof with a silver electrode paint by a conventional brushing method and has applied to the other side of said opposite surfaces an ohmic electrode by metallizing Al or Sn metal thereon. The silver electrode paint employed has a solid ingredient composition according to Table 4 and is prepared in the same manner as Example 1.

Lead wires are attached to the silver electrodes by means of silver paint. The electric characteristics of the resultant resistors are measured by applying positive electric potential to the ohmic electrode of the sintered disc. The results are shown in Table 5.

EXAMPLE 3

A series of sintered discs each having a composition of 96.0 mole. percent of zinc oxide and 4.0 mol. percent of beryllium oxide is prepared in the same manner as in Example 1. Each sintered disc has a size of 10 mm diameter and 1.5 mm thickness after lapping. Various silver electrode paints are applied to the opposite surfaces of the sintered discs and they are and fired at 800.degree. C. for 30 minutes in air. The silver electrode paints have solid ingredient compositions shown in Table 6 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 voltage dependent resistors have desirable C-value and n-values as indicated in Table 6. It will be readily understood that the electrode compositions have a great effect on the electrical characteristics of the resultant non-linear resistors.

EXAMPLE 4

The resistors of Example 1 are tested according to the methods used in testing 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 a cycle in which said resistors are kept at 85.degree. C. ambient temperature for 30 minutes, cooled rapidly to -20.degree. C. and then kept at such temperature for 30 minutes. After heating cycles and load life test, the change rates of the C-value and n-value are shown in Table 7. ##SPC1##

TABLE 2

Preferable Composition of Silver Electrode (wt.%) Ag Bi.sub.2 O.sub.3 SiO.sub.2 B.sub.2 O.sub.3 70 to 99.5 0.3 to 27 0.1 to 15 0.1 to 15

Optimal Composition of Silver Electrode (wt.%) Ag Bi.sub.2 O.sub.3 SiO.sub.2 B.sub.2 O.sub.3 CoO 70 to 99.45 0.3 to 2 7 0.1 to 15 0.1 to 15 0.05 to 6.0

TABLE 3 Composition of Sintered Body (mol.%) Electric characteristics _________________________________________________________________________ _ ZnO BeO NiO TiO.sub.2 BaO Further C(at n additives 100mA) _________________________________________________________________________ _ 99.95 0.05 -- -- -- -- 6.2 7.0 99.90 0.1 -- -- -- -- 5.5 7.5 99.0 1 -- -- -- -- 4.9 8 96.0 4 -- -- -- -- 4.5 9.5 92.0 8 -- -- -- -- 5.1 8 90.0 10 -- -- -- -- 6.0 7.0 98.9 1 0.1 -- -- -- 6.0 10 96.0 1 3 -- -- -- 5.9 10 91.9 8 0.1 -- -- -- 5.8 10 89.0 8 3 -- -- -- 6.1 11 95.5 4 0.5 -- -- -- 4.4 14 98.9 1 -- 0.1 -- -- 5.5 10 96.0 1 -- 3 -- -- 5.6 10 91.9 8 -- 0.1 -- -- 5.5 11 89.0 8 -- 3 -- -- 5.7 11 95.5 4 -- 0.5 -- -- 4.0 14 98.8 1 0.1 0.1 -- -- 5.5 11 95.9 1 0.1 3 -- -- 5.6 11 95.9 1 3 0.1 -- -- 5.7 12 93.0 1 3 3 -- -- 5.4 11 91.8 8 0.1 0.1 -- -- 5.3 12 88.9 8 0.1 3 -- -- 5.4 12 88.9 8 3 0.1 -- -- 5.2 12 86.0 8 3 3 -- -- 5.4 11 95.0 4 0.5 0.5 -- -- 3.7 15 97.78 1 0.1 0.1 0.02 -- 5.1 13 97.8 1 0.5 0.5 0.2 -- 4.6 16 92.0 1 3 3 1 -- 5.3 14 95.78 4 0.1 0.1 0.02 -- 5.4 16 94.8 4 0.5 0.5 0.2 -- 3.0 25 89.0 4 3 3 1 -- 5.2 18 91.78 8 0.1 0.1 0.02 -- 5.2 15 90.8 8 0.5 0.5 0.2 -- 4.7 16 85.0 8 3 3 1 -- 5.3 13 98.68 1 0.1 0.1 0.02 snO.sub.2 0.1 5.3 14 97.3 1 0.5 0.5 0.2 SnO.sub.2 0.5 4.1 17 89.0 1 3 3 1 SnO.sub.2 3 4.5 15 95.68 4 0.1 0.1 0.02 SnO.sub.2 0.1 3.9 17 94.3 4 0.5 0.5 0.2 SnO.sub.2 0.5 3.0 25 86.0 4 3 3 1 SnO.sub.2 3 4.2 19 91.68 8 0.1 0.1 0.02 SnO.sub.2 0.1 4.8 15 90.3 8 0.5 0.5 0.2 SNO.sub.2 0.5 4.7 16 82.0 8 3 3 1 SnO.sub.2 3 5.1 15 {al.sub.2 O.sub.3 0.1 98.6 1 0.1 -- -- {PbO 0.1 5.8 14 {CdF.sub.2 0.1 {Al.sub.2 O.sub.3 0.5 97.0 1 0.5 -- -- {PbO 0.5 4.7 16 {CdF.sub.2 0.5 {Al.sub.2 O.sub.3 3 87.0 1 3 -- -- {PbO 3 5.5 14 {CdF.sub.2 3 {Al.sub.2 O.sub.3 0.1 95.6 4 0.1 -- -- {PbO 0.1 4.7 17 {CdF.sub.2 0.1 {Al.sub.2 O.sub.3 0.5 94.0 4 0.5 -- -- {PbO 0.5 3.0 20 {CdF.sub.2 0.5 {al.sub.2 O.sub.3 3 84.0 4 3 -- -- {PbO 3 4.1 17 {CdF.sub.2 3 {Al.sub.2 O.sub.3 0.1 91.6 8 0.1 -- -- {PbO 0.1 5.4 14 {CdF.sub.2 0.1 {Al.sub.2 O.sub.3 0.5 90.0 8 0.5 -- -- {PbO 0.5 4.6 16 {CdF.sub.2 0.5 {Al.sub.2 O.sub.3 3 80.0 8 3 -- -- {PbO 3 5.9 13 {CdF.sub.2 3 98.8 1 -- 0.1 -- tl.sub.2 O.sub.3 0.1 6.3 12 95.9 1 -- 0.1 -- Tl.sub.2 O.sub.3 3 5.6 13 95.9 1 -- 3 -- Tl.sub.2 O.sub.3 0.1 5.7 13 93.0 1 -- 3 -- Tl.sub.2 O.sub.3 3 4.7 14 91.8 8 -- 0.1 -- Tl.sub.2 O.sub.3 0.1 5.1 12 88.9 8 -- 0.1 -- Tl.sub.2 O.sub.3 3 5.3 15 88.9 8 -- 3 -- Tl.sub.2 O.sub.3 0.1 5.0 15 86.0 8 -- 3 -- Tl.sub.2 O.sub.3 3 6.1 12 95.0 4 -- 0.5 -- Tl.sub.2 O.sub.3 0.5 3.0 25 _________________________________________________________________________ _

table 4 composition of Silver Electrode (wt.%) _________________________________________________________________________ _ Ag Bi.sub.2 O.sub.3 SiO.sub.2 B.sub.2 O.sub.3 CoO 78 14 3.0 3.0 2.0 _________________________________________________________________________ _

table 5 electric Composition of Sintered Body (mol.%) characteristics _________________________________________________________________________ _ Further C(at ZnO BeO NiO TiO.sub.2 BaO additives 100mA) n _________________________________________________________________________ _ 99.95 0.05 -- -- -- -- 5.2 7.0 99.90 0.1 -- -- -- -- 4.6 8.0 99.0 1 -- -- -- -- 3.9 8.8 96.0 4 -- -- -- -- 3.5 9.5 92.0 8 -- -- -- -- 4.1 8.2 90.0 10 -- -- -- -- 5.0 7.0 98.9 1 0.1 -- -- -- 5.0 11 96.0 1 3 -- -- -- 4.8 12 91.9 8 0.1 -- -- -- 4.7 11 89.0 8 3 -- -- -- 5.1 12 95.5 4 0.5 -- -- -- 3.5 16 98.9 1 -- 0.1 -- -- 4.5 12 96.0 1 -- 3 -- -- 4.6 12 91.9 8 -- 0.1 -- -- 4.4 13 89.0 8 -- 3 -- -- 4.7 12 95.5 4 -- 0.5 -- -- 3.0 16 98.8 1 0.1 0.1 -- -- 4.5 13 95.9 1 0.1 3.0 -- -- 4.6 13 95.9 1 3 0.1 -- -- 4.7 13 93.0 1 3 3 -- -- 4.3 14 91.8 8 0.1 0.1 -- -- 4.4 14 88.9 8 0.1 3 -- -- 4.4 12 88.9 8 3 0.1 -- -- 4.2 13 86.0 8 3 3 -- -- 4.3 12 95.0 4 0.5 0.5 -- -- 3.0 17 98.78 1 0.1 0.1 0.02 -- 4.1 15 97.8 1 0.5 0.5 0.2 -- 3.6 17 92.0 1 3 3 1 -- 4.3 16 95.78 4 0.1 0.1 0.02 -- 4.4 18 94.8 4 0.5 0.5 0.2 -- 2.2 27 89.0 4 3 3 1 -- 4.2 20 91.78 8 0.1 0.1 0.02 -- 4.3 16 90.8 8 0.5 0.5 0.2 -- 3.9 18 85.0 8 3 3 1 -- 4.3 15 98.68 1 0.1 0.1 0.02 SnO.sub.2 0.1 4.3 15 97.3 1 0.5 0.5 0.2 SnO.sub.2 0.5 3.2 18 89.0 1 3 3 1 SnO.sub.2 3 3.6 16 95.68 4 0.1 0.1 0.02 SnO.sub.2 0.1 2.9 18 94.3 4 0.5 0.5 0.2 SnO.sub.2 0.5 2.2 27 86.0 4 3 3 1 SnO.sub.2 3 3.2 21 91.68 8 0.1 0.1 0.02 SnO.sub.2 3.9 17 90.3 8 0.5 0.5 0.2 SnO.sub.2 3.7 18 82.0 8 3 3 1 SnO.sub.2 3 4.1 17 {al.sub.2 O.sub.3 0.1 98.6 1 0.1 -- -- {PbO 0.1 4.8 16 {CdF.sub.2 0.1 {Al.sub.2 O.sub.3 0.5 97.0 1 0.5 -- -- {PbO 0.5 3.7 18 {CdF.sub.2 0.5 {Al.sub.2 O.sub.3 3 87.0 1 3 -- -- {PbO 3 4.5 16 {CdF.sub.2 3 {Al.sub.2 O.sub.3 0.1 95.6 4 0.1 -- -- {PbO 0.1 3.9 20 {CdF.sub.2 0.1 {Al.sub.2 O.sub.3 0.5 94.0 4 0.5 -- -- {PbO 0.5 2.1 23 {CdF 0.5 {al.sub.2 O.sub.3 3 84.0 4 3 -- -- {PbO 3 3.1 20 {CdF.sub.2 3 {Al.sub.2 O.sub.3 0.1 91.6 8 0.1 -- -- {PbO 0.1 4.3 17 {CdF.sub.2 0.1 {Al.sub.2 O.sub.3 0.5 90.0 8 0.5 -- -- {PbO 0.5 3.8 19 {CdF.sub.2 0.5 {Al.sub.2 O.sub.3 3 80.0 8 3 -- -- {PbO 3 4.8 15 {CdF.sub.2 3 98.8 1 -- 0.1 -- tl.sub.2 O.sub.3 0.1 5.3 14 95.9 1 -- 0.1 -- Tl.sub.2 O.sub.3 3 4.6 14 95.9 1 -- 3 -- Tl.sub.2 O.sub.3 0.1 4.8 15 93.0 1 -- 3 -- Tl.sub.2 O.sub.3 3 4.0 17 91.8 8 -- 0.1 -- Tl.sub.2 O.sub.3 0.1 4.2 16 88.9 8 -- 0.1 -- Tl.sub.2 O.sub.3 3 4.3 18 88.9 8 -- 3 -- Tl.sub.2 O.sub.3 0.1 4.0 17 86.0 8 -- 3 -- Tl.sub.2 O.sub.3 3 5.2 17 95.0 4 -- 0.5 -- Tl.sub.2 O.sub.3 0.5 2.2 28 _________________________________________________________________________ _

table 6 electric Composition of Silver Electrode (wt.%) Characteristics _________________________________________________________________________ _ Ag Bi.sub.2 O.sub.3 SiO.sub.2 CoO sub.2 O.sub.3 C(at 100mA) n _________________________________________________________________________ _ 99.5 0.3 0.1 0.1 -- 6.4 7.0 84.6 0.3 0.1 15 -- 5.9 7.2 84.6 0.3 15 0.1 -- 6.3 8.5 72.8 27 0.1 0.1 -- 7.0 7.0 70.0 21 4.5 4.5 -- 5.3 8.8 80.0 14 3.0 3.0 -- 4.8 9.5 90.0 7 1.5 1.5 -- 5.5 8.5 79.95 14 3.0 3.0 0.05 5.4 8.5 79.9 14 3.0 3.0 0.1 4.9 9 79.0 14 3.0 3.0 1 4.7 9 78.0 14 3.0 3.0 2 4.5 9.5 74.0 14 3.0 3.0 6 4.9 9.0 78.0 14 5.0 1.0 2 6.2 8.0 78.0 14 1.0 5.0 2 5.2 9.0 78.0 16 1.0 3.0 2 5.3 9.5 78.0 12 5.0 3.0 2 5.8 8.5 78.0 16 3.0 1.0 2 5.5 9.0 78.0 12 3.0 5.0 2 5.3 8.7 _________________________________________________________________________ _

TABLE 7 Composition of Sintered Body Change Rate (%) (mol.%) Load Heating Further life test cycle test ZnO BeO NiO BaO additives ub.2 .DELTA. C .DELTA.n .DELTA.C .DELTA. n _________________________________________________________________________ _ 99.95 0.05 -- -- -- -- -9.3 -9.5 -9.3 -9.0 99.90 0.1 --- -- -- -8.4 -8.1 -8.1 -8.0 99.0 1 --- -- -- -5.0 -6.8 -5.0 -6.7 96.0 4 --- -- -- -3.6 -5.2 -3.8 -5.0 92.0 8 --- -- -- -6.8 -7.1 -6.5 -7.1 90.0 10 --- -- -- -8.9 -9.3 -8.6 -9.0 98.9 ` 0.1 -- -- -- -6.4 -7.0 -6.3 -6.7 96.0 1 -- -- -- -7.0 -7.5 -6.5 -7.1 91.9 8 0.1 -- -- -- -6.1 -5.9 -6.1 -5.9 89.0 8 3 -- -- -- -6.2 -5.4 -6.2 -5.3 95.5 4 0.5 -- -- -- -2.5 -2.1 -2.4 -2.0 98.9 1 -- 0.1 -- -- -6.5 -6.5 -6.2 -6.2 96.0 1 -- 3 -- -- -6.8 -6.6 -6.7 -6.7 91.9 8 -- 0.1 -- -- -5.4 -4.9 -5.0 -4.9 89.0 8 -- 3 -- -- -4.9 -5.3 -4.9 -5.5 95.5 4 -- -- -- -2.0 -3.7 -2.1 -3.5 98.8 1 0.1 -- -- -5.8 -6.3 -5.5 -6.5 95.9 1 3.0 -- -- -6.0 111 -7.0 -6.9 -6.8 95.9 1 0.1 -- -- -5.9 -6.5 -6.0 -6.3 93.0 1 3 -- -- -4.8 -5.5 -4.7 -5.5 91.8 8 0.1 -- -- -3.8 -6.0 -4.0 -6.1 88.9 8 3 .1 -- -- -5.1 -5.8 -5.2 -5.9 88.9 8 0.1 -- -- -4.4 -6.1 -4.4 -6.0 86.0 8 3 -- -- -3.5 -4.5 -3.5 -4.7 95.0 4 0.5 -- -- -1.5 -2.0 -1.7 -2.1 98.78 1 0.1 0.02 -- -6.0 -5.3 -6.0 -5.5 97.8 1 0.5 0.2 -- -3.0 -3.8 -3.1 -3.7 92.0 1 3 1.0 -- -4.1 -4.4 -4.0 -4.5 95.78 4 0.1 0.02 -- -3.4 -3.4 -3.5 -3.5 94.8 4 0.5 0.2 -- -1.0 -1.0 -1.2 -1.2 89.0 4 3 1.0 -- -2.9 -2.9 -2.9 -3.1 91.78 8 0.1 0.02 -- -3.8 -4.1 -3.8 -4.2 90.8 8 0.5 0.2 -- -2.1 -3.0 -1.8 -3.1 85.0 8 3 1.0 -- -5.1 -4.9 -5.0 -5.0 98.68 1 0.1 0.02 SnO.sub.2 0.1 -5.5 -3.8 -5.6 -3.9 97.3 1 0.5 0.2 SnO.sub.2 0.5 -2.5 -2.1 -2.6 -2.2 89.0 1 3 1.0 SnO.sub.2 3 -3.4 -4.1 -3.5 -4.2 95.68 4 0.1 0.02 SnO.sub.2 0.1 -3.2 -2.4 -3.3 -2.2 94.3 4 0.5 0.2 SnO.sub.2 0.5 -0.5 -0.5 -0.6 -0.7 86.0 4 3 1.0 SnO.sub.2 3 -2.3 -3.4 -2.2 -2.1 91.68 8 0.1 0.02 SnO.sub.2 0.1 -3.8 -3.4 -3.7 -3.1 90.3 8 0.5 0.2 SnO.sub.2 0.5 -2.0 -1.5 -1.9 -1.7 82.0 8 3 1.0 SnO.sub.2 3 -4.3 -4.5 -4.2 -4.5 {al.sub.2 O.sub.3 0.1 98.6 1 --.1 -- {PbO 0.1 -4.8 -5.0 -5.0 -5.0 {CdF.sub.2 0.1 {Al.sub.2 O.sub.3 0.5 97.0 1 --.5 -- {PbO 0.5 -3.0 -3.8 -3.2 -3.9 {CdF.sub.2 0.5 {Al.sub.2 O.sub.3 3 87.0 1 -- -- {PbO 3 -4.1 -4.2 -4.1 -4.2 {CdF.sub.2 3 {Al.sub.2 O.sub.3 0.1 95.6 4 --.1 -- {PbO 0.1 -4.3 -3.9 -4.3 -3.8 {CdF.sub.2 0.1 {Al.sub.2 O.sub.3 0.5 94.0 4 --.5 -- {PbO 0.5 -2.0 -2.0 -2.0 -2.1 {CdF.sub.2 0.5 {al.sub.2 O.sub.3 3 84.0 4 -- -- {PbO 3 -3.9 -3.9 -3.7 -4.0 {CdF.sub.2 3 {Al.sub.2 O.sub.3 0.1 91.6 8 --.1 -- {PbO 0.1 -4.1 -3.7 -4.0 -3.5 {CdF.sub.2 0.1 {Al.sub.2 O.sub.3 0.5 90.0 8 --.5 -- {PbO 0.5 -2.5 -2.5 -2.6 -2.4 {CdF.sub.2 0.5 {Al.sub.2 O.sub.3 3 80.0 8 -- -- {PbO 3 -4.8 -3.9 -4.9 -4.2 {CdF.sub.2 3 98.8 1 0.1 -- tl.sub.2 O.sub.3 0.1 -5.4 -5.5 -5.1 -5.1 95.9 1 0.1 -- Tl.sub.2 O.sub.3 3 -6.0 -5.3 -6.0 -5.4 95.9 1 3 - -- Tl.sub.2 O.sub.3 0.1 -4.1 -3.9 -4.0 -4.0 9.30 1 3 - -- Tl.sub.2 O.sub.3 3 -4.8 -4.1 -4.7 -4.2 91.8 8 -- 0.1 -- Tl.sub.2 O.sub.3 0.1 -5.0 -3.7 -5.3 -4.0 88.9 8 0.1 -- Tl.sub.2 O.sub.3 3 -4.5 -3.9 -4.6 -3.9 88.9 8 3 - -- Tl.sub.2 O.sub.3 0.1 -4.2 -3.8 -4.2 -3.9 86.0 8 3 - -- Tl.sub.2 O.sub.3 3 -3.4 -4.1 -3.5 -4.1 95.0 4 0.5 -- Tl.sub.2 O.sub.3 0.5 -2.0 -1.5 -2.0 -1.6 _________________________________________________________________________ _

Claims

1. A voltage dependent resistor of the surface barrier type comprising 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 said electrodes

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

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

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

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 TiO.sub.2) and 0.02 to 1.0 mole percent of

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 (TiO.sub.2), 0.02 to 1.0 mole percent of barium oxide (BaO) and 0.1 to 3.0 mole percent of stannic oxide

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 (Al.sub.2 O.sub.3), 0.1 to 3.0 mole percent of lead oxide (PbO) and 0.1 to 3.0 mole percent of cadmium fluoride

8. A voltage dependent resistor according to claim 3, wherein said additive further includes 0.1 to 3.0 mole percent of thallium oxide (Tl.sub.2

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 (Bi.sub.2 O.sub.3), 0.1 to 15 percent by weight of silicon dioxide (SiO.sub.2) and 0.1 to 15

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 (Bi.sub.2 O.sub.3), 0.1 to 15 percent by weight of silicon dioxide (SiO.sub.2), 0.1 to 15 percent by weight of boron trioxide (B.sub.2 O.sub.3) and 0.05 to 6.0 percent by weight of cobalt oxide (CoO).