Non-linear resistors

Abstract

Claims

Feb. 17, 1 970 MICHIO MATSUOKA ETAL 3,496,512 NON -LINEAR RESISTORS Filed May 10, 1967 ATTORNEYS United States Patent 3,496,512 NON-LINEAR RESISTORS Michio Matsuoka, Nishinomiya-shi, Takeshi Masuyama, Takatsuki-shi, and Yoshio Iida, Hirakata-shi, Japan, assignors to Matsushita Electric Industrial Co., Ltd., Osaka, Japan Filed May 10, 1967, Ser. No. 637,492 Claims priority, application Japan, May 16, 1966, 41/ 31,594 Int. Cl. Htllc 7/10, 7/04, 1/14 US. Cl. 33820 7 Claims ABSTRACT OF THE DISCLOSURE New non-linear resistors having non-ohmic resistance are provided. Such resistors comprise a sintered wafer comprising, as an active ingredient, zinc oxide, and silver electrodes applied to opposite surfaces of said sintered wafer. A method is provided for making such resistors comprising applying a silver electrode paint to the opposite surfaces of the sintered wafer, heating said silver electrode paint in an oxidizing atmosphere at 100 to 850 C. so as to produce silver electrodes adhered to said surfaces and connecting lead wires to said silver electrode by a conductive connection means. NON-LINEAR RESIST ORS 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 and V are the voltages at given currents I and I 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 nonlinear resistors and are manufactured by mixing fine particles of silicon carbide with water, ceramic binder and/ or conductive material such as 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 non-oxidizing atmosphere. Silicon carbide varistors with conductive materials are characterized by a low electric resistance, i.e. a low value of C and a low value of 11 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 diflicult 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 agiven current of 100 ma., and "ice silicon carbide varistors without graphite show n-values 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 ma. with respect to a specimen size of 20 mm. in diameter. In this case, the thickness of sample does not affect the C-value. A germanium or silicon p-n junction resistor has an extremely high value of 11 but its C-value is constant, e.g. of the order of 0.3 or 0.7 at a given current of 100 ma. because its diffusion voltage in the V-I characteristics is constant and cannot be changed remarkably. 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 their 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 non-linear resistor having a high value of n and a low value of C. A further object of this invention is to provide a nonlinear resistor capable of being made by a simple manufacturing method which results in a low cost. A further object of this invention is to provide a nonlinear resistor characterized by a high stability to temperature, humidity and electric load. Another object of this invention is to provide a nonlinear 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 non-linear resistor in accordance with the invention. Before proceeding with a detailed description of the non-linear 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 non-linear resistor having, as its active element, a sintered wafer 1 of electrically conductive ceramic material according to the present invention. Sintered water 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, sintered wafer 1 consists essentially of, as an active ingredient, zinc oxide (ZnO). It is preferable that said zinc oxide have incorporated therein a minor proportion of an additive selected from the group consisting of aluminum oxide (A1 0 iron oxide (Fe O bismuth oxide (Bi O magnesium oxide (MgO), calcium oxide (CaO), nickel oxide (NiO), cobalt oxide (C00), niobium oxide (Nb O tantalum oxide (Ta O zirconium oxide (ZrO tungsten oxide (W0 cadmium oxide (CdO), and chromium oxide (Cr O It has been discovered according to the invention that said sintered body 1 produces a superior non-linearity of the electrical characteristics when it is provided with silver electrodes prepared by applying silver paint to opposite surfaces thereof and firing at 100 C. to 850 C. in an oxidizing atmosphere such as air and oxygen. The - n-value and C-value of so-produced non-linear resistors 3 4 vary with the compositions of the sintered body and elecnitrogen and argon when it is desired to reduce the electrodes, and their preparation method. trical resistivity. The electrical resistivity also can be Since the non-linearity of the novel resistors is atreduced by air-quenching from the sintering temperature tributed to a. non-ohmic contact between said sintered to room temperature even when the pressed bodies are body 1 and electrodes 2 and 3, it is necessary for obtain- 5 fired in air. ing a desirable C-value and n-value to control the corn- The mixtures may be preliminarily calcined at 700 positions of the sintered body 1 and the electrodes 2 and to 1000 C. and pulverizedfor easy fabrication in the positions of the sintered body 1 and the elecsubsequent pressing step. The mixture to be pressed may trodes2 and 3. be admixed with a suitable binder such as water, poly- It is necessary for achieving a low value of C of revinyl alcohol, etc. sultant non-linear resistors that the sintered body have an It is advantageous that the sintered body he lapped electrical resistivity less than 10 ohm-cm. said electrical at the opposed surfaces by abrasive powder such as siliresistivity being measured by a four point method in a per con carbide in a particle size of 300 meshes to 1500 se conventional way. meshes. Table 1 shows operable and optimal compositions of The sintered bodies are coated at the opposed surfaces sintered body 1 for producing a non-linear resistor having thereof by a silver electrode paint in a per se convenan n-value higher than 3 and a high stability with temtional manner such as by a spray method, screen printing perature, humidity and electric load. method or brushing method. It is necessary that the silver Table 2 shows operable and optimal compositions of electrode paint have a solid ingredient composition as silver electrodes 2 and 3 after heating for curing in order defined in Tables 2 and 3 after it is fired at 100 C. to to produce the novel non-linear resistors in accordance 850 C. in air. Solid ingredients having compositions with the invention. defined in Tables 2 and 3 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 Table 3 shows optimal combinations of sintered body 1 and silver electrodes 2, 3 for producing non-linear resistors having a C-value lower than 6 at a given current of 100 ma., an n-value higher than 4 and a high stabili Or the like so as to produce silver electrode paints. with temperature, humidity and electric load. The silver powder may be in the form of metallic In the Tables 2 and 3, indicated in the specification, silver, or in the form of silver carbonate or silver oxide, a sum of the weight percents of all ingredients should be or in any other form which in firing at the temperatures 100 weight percent by controlling a weight percent of employed will be converted to metallic silver. Therefore, individual ingredient with operable or optimal weight perthe term silver as used throughout this specification cent indicated in the tables. and the claims appended hereto in connection with the silver composition before it is fired, is meant to include silver in any form which in firing will be converted to TABLE 1 5 0 b1 0 t, w t d O 10 f 3 metallic silver. The viscosity of the resultant silver elecpera e omposi 10H 0 1n ere p 11113. OIHDOSI 10D 0 Body (moLpemnt) sintered Body (moLpement) trode paints can be controlled by the amounts of resin and solvent. Particle slze of solid mgredients also are Zno Addmve Zno Addltwe required to be controlled in the range of 0.1/l. to 51.1.. 00 0 n 99.95 to 90.0..... 0.05 to 10, A1103... 99.9 to 98.0..-. 0.1 to 2.0, A1903 40 Lead f be apphed to l sllver ele.ctrodes m a 99.95 r 0 9 .8--- 8.82%018, 6183.-- 93.3 0 3. 8% 0 9183 per se conventional manner by using conventional solder 99.95o9.... o ,l23- 9.0. .o.,iz 99.95 to 90.0 0.05 to 10, MgO 99.9 to 98. 0.1 to 2.0, Mg() havlng low lti g polnt. lt 1s convenient to employer t0 gag 9 to 2.0, gag conductive adhesive comprising silver powder and resin 0 i to 2.0 i 9995 to (105m m, 000" Y in an organic solvent for connecting the lead wires to the 99.95 to 90.0--.-- 0.05 to 10, Nb205--- silver electrodes. 99.95 to 90.0 0.05 to 10, T8905- 99.95 to 90.0-.- 0.05 to 10, ZrO2 99.95 to 90.0 0.05 to 10, W03" 99.95 to 90.0 0.05 to 10, CdO 99.95 to 90.0. 0.05 to 10, CrzO Non-linear resistors according to this invention have a high stability to temperature and in the load life test, which is carried out at C. at a rating power for 500 hours. The n-valve and C-value do not change remark- TAB LE 2 [Operable Composition of Electrode, Wt. Percent] TABLE 3.OPTIMAL COMBINATIONS OF SINTERED BODY AND ELECTRODE Composition of Sintered Body (M01. Percent) Composition of Electrode (Wt. Percent) ZnO Additive Ag PbO SiO, B103 B110 CdO 0110 99.9 to 98 0.1 to 2.0, F6103.-- to 98. 1.2 to 17... 0.1 to 6.0-... 0.06 to 6.0... 0 to 2.0.... 0 to 2.0.... 0 to 2.0. The sintered body 1 can be prepared by a per se well ably after heating cycles and load life test. It is preferable known ceramic technique. The starting materials in the for achieving a high stability to humidity that the recompositions defined in Table 1 are mixed in a wet mill sultant non-linear resistors are embedded in a humidity so as to produce homogeneous mixtures. The mixtures are proof resin such as epoxy resin and phenol resin in a per dried and pressed in a mold into desired shapes at a 70 se well known manner. 1 pressure from 100 kg./cm. to 1000 kg./cm. The pressed According to the invention, it has been discovered tha bodies are sintered in air at 1250" C. to 1450 C. for 1 the curring method of the appliedsilver electrode paint to 3 hours, and then furnace-cooled to room temperahas a great effect on the n-value of the resultant nonture (about 15 to about 30 C,). The pressed bodies are linear resistors. The n-valve will not be optimal when preferably sintered in IlOn-QXidiZing atmosphere such as 75 the applied silver electrode paint is heated in a n0n-oxidiging 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 produce a non-linear resistor when it is provided with silver electrodes at the opposite surfaces by electroless plating or electolytic plating in 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 4 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 the pressure of 340 kg./cm. The pressed body is sinteredin air at 1350 C. for 1 hour, and then quenched to room temperature (about 15 to about 30 C.). The sintered disc is lapped at the opposite surfaces thereof by silicon carbide in a particle size of 600 meshes. Resulting sintered disc has a size of 10 mm. in diameter and 1.5 mm. in thickness. The sintered disc is coated at the Opposite surfaces thereof with a silver electrode paint by a conventional brushing method. The silver electrode paint employed has the solid ingredient composition according to Table 5 and is prepared by mixing with vinyl resin in amyl acetate. The coated disc is fired at 500 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 resistor and of other similarly prepared resistors are shown in Table 4. TABLE 5.COMPOSITION 0F SILVER ELECTRODE (WT. PERCENT) Ag PbO S103 B: CdO TABLE 4 Electric Characteristics of Resultant Starting Materials (mol. percent) Resistors C (at a given current of 100 III a n. EXAMPLE 2 A sintered disc in a composition of 99.5 mol. percent of zinc oxide and 0.5 mol. percent of iron oxide is prepared in the same manner as that in Example 1. The sintered disc has a size of 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 500 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 non-linear resistors exhibit desirable C-values 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 3 TAB LE 6 The Character istics of the Resultant Composition of Electrode (wt. percent) Resistors current of S102 B203 100 ma.) CdO C110 What is claimed is: 1. A non-linear resistor comprising a sintered wafer consisting essentially of zinc oxide, and silver paint electrodes applied to opposite surfaces of said sintered wafer, said electrodes consisting essentially of fired silver paint. 2. A non-linear resistor according to claim 1, wherein said sintered wafer comprises 99.95 to mol. percent of zinc oxide (ZnO) and 0.05 to 10.0 mol. percent of at least one oxide selected from the group consisting of iron oxide (Fe O aluminum oxide (A1 0 bismuth oxide (Bi O magnesium oxide (MgO), calcium oxide (CaO), nickel oxide (NiO), cobalt oxide (COO), niobium oxide (Nb O5), tantalum oxide (Ta O zirconium oxide (ZrO tungsten oxide (W0 cadmium oxide (CdO), and chromium oxide (Cr O 3. A non-linear resistor according to claim 1, wherein said sintered wafer comprises 99.9 to 98.0 mol. percent of zinc oxide (ZnO) and 0.1 to 2.0 mol. percent of at least one oxide selected from the group consisting of aluminum oxide (A1 0 iron oxide (Fe O bismuth oxide (Bi O magnesium oxide (MgO), calcium oxide (C210) and nickel oxide (NiO). 4. A non-linear resistor according to claim 2, wherein said silver electrodes have a composition of wt. percent of silver. 5. A non-linear resistor according to claim 1, wherein said silver electrodes have the composition comprising 70 to 99.5 wt. per-cent of silver, 0.25 to 27 wt. percent of lead oxide (PbO), 0.02 to 15 wt. percent of silicon dioxide (SiO 0.01to 15 wt. percent of boron trioxide (E 0 1, '0 to 60 Wt. percent of bismuth oxide (Bi O '0 to 6.0 wt. percent of cadmium oxide (CdO) and Oto 6.0 Wt. percent of cupric oxide (CuO). 6. A non-linear resistor according to claim 1, wherein said silver electrodes have the composition comprising 80 to 98 wt. percent of silver, 1.2 to-17 wt. percent of lead oxide (PbO), 0.1 to 6.0 Wt. percent of silicon dioxide (SiO 0.06 to 6.0 Wt. percent of boron trioxide (B 0 0 to 2.0 Wt. percent of bismuth oxide (Bi O 0 to 2.0 wt. percent of cadmium oxide (CdO) and 0 to 2.0 wt. percent cupric oxide (CuO) 7. A non-linear resistor according to'claim 1, wherein said sintered wafer comprises 99.9 to 98.0 mol. percent of zinc oxide (ZnO) and 0.1 to 2.0 mol. percent of iron oxide (Fe O and said silver electrodeshave the composition comprising 80 'to 98 wt. percent of silver, 1,2 to 17 Wt. percent of lead oxide (PbO), 0.1 to 6.0 wt. percent of silicon dioxide (SiO 0.06 to 6.0 wt. percent of boron trioxide (B 0 0 to 2.0 wt. percent of bismuth oxide (Bi O 0 to 2.0 wt. percent of cadmium oxide (CdO) and 0 to 2.0 wt.'percent of cupric oxide (CuO). 8 References Cited 2,786,819 3/1957 Smith et a1. 33822 2,977,558 3/1961 Hampton 338-22 3,037,942 6/ 1962 In'gold et a1. ' 3,075,122 1/1963 Lehmann 252518 X 3,219,480 11/1965 Girard 33822 X 3,264,229 8/1966 Klein 252518 1,822,742 9/1931 McEachron 338-21 2,027,277 1/1936 Habann 338-2O FOREIGN PATENTS 479,580- 12/1951 Canada. 3/1949 Great Britain. ".OTHER- REFERENCES REUBEN EPSTEIN, Primary Examiner US. Cl. X.R.

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