US3570002A

US3570002A - Non-linear resistor of sintered zinc oxide

Info

Publication number: US3570002A
Application number: US723746A
Authority: US
Inventors: Takeshi Masuyama; Michio Matsuoka
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1967-04-26
Filing date: 1968-04-24
Publication date: 1971-03-09
Anticipated expiration: 1988-03-09
Also published as: NL6805740A; DE1765097A1; DE1765097B2; GB1156436A; DE1765097C3; FR1561741A; NL139035B

Abstract

A NON-LINEAR RESISTOR. THE RESITOR HAS A SINTERED DISC OF ZINC OXIDE AND TWO ELECTRODES, ONE APPLIED TO EACH OF THE OPPOSITE SURFACES OF SAID SINTERED DISC. ONE OF SAID TWO ELCTRODES IN A SILVER ELECTRODE IN NON-OHMIC CONTACT WITH ONE OF SAID OPPOSITE SURFACES, AD THE OTHER OF SAID TWO ELECTRODES IN OHMIC CONTACT WITH ANOTHER OF SAID OPPOSITE SURFACES.

Description

March 9, 1971 TAKESHI MASUYAMA' ET AL ,5

NON-LINEAR RESISTOR OF SINTERED ZINC OXIDE Filed'April 24, 1968 INVENTORS TAKESHI MASUYAMA MIC HIO MATSUOKA ATTORNEYS United States Patent 3,570,002 NON-LINEAR RESISTOR 0F SINTERED ZINC OXIDE Takeshi Masuyama and Michio Matsuoka, Osaka, Japan, asslgnors to Matsushita Electric Industrial (10., Ltd., Osaka, Japan Filed Apr. 24, 1968, Ser. No. 723,746

Claims priority, application Japan, Apr. 26, 1967,

4207,34: Int. Cl. H01l 3/16 US. Cl. 317238 9 Claims ABSTRACT OF THE DISCLOSURE A non-linear resistor. The resistor has a sintered disc of zinc oxide and two electrodes, one applied to each of the opposite surfaces of said sintered disc. One of said two electrodes is a silver electrode in non-ohmic contact with one of said opposite surfaces, and the other of said two electrodes in ohmic contact with another of said opposite surfaces.

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

mi 2W1) 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 use 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 a 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 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 have n-values from 4 to 7 and C-values from 30 to 800 at a given current 3,570,002 Patented Mar. 9, 1971 of 1 ma. 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 less than 3 and a C-value of 5 to 10 at a given current of ma. for a specimen size of 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 it 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 for the VI characteristics is constant and can not 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 in view of their high cost even though they 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 and hence at a low cost.

A further object of this invention is to provide a nonlinear resistor characterized by a high stability with respect 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 objects are accomplished by providing a nonlinear resistor comprising a sintered disc of zinc oxide and two electrodes, one applied to each of the opposite surfaces of said sintered disc, one of said two electrodes being a silver electrode in non-ohmic contact with one of said opposite surfaces, and the other of said two electrodes being in ohmic contact with another of said opposite surfaces.

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 the 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 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.

One of said pair of electrodes, for example, the electrode 2 is a silver electrode in non-ohmic contact with the water 1, and the other electrode 3 is in ohmic contact with the wafer 1 in accordance with the invention.

The wafer 1 is a sintered plate having any one of a number 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, such as solder or the like.

According to the present invention, sintered water 1 consists essentially of, as an active ingredient, zinc oxide (ZnO). It is preferable that said zinc oxide have incorporated therein a proportion of 0.05 to 10.0 mol percent of at least one oxide selected from the group consisting of aluminum oxide (A1 0 iron oxide (Pesos), bismuth oxide (Bi O magnesium oxide (MgO), calcium 4 oxide (CaO), nickel oxide (NiO), cobalt oxide (C), Composition of silver electrode (wt. percent): niobium oxide (-Nb O tantalum oxide (Ta O zir- PbO1.2-l7 conium oxide (ZrO tungsten oxide (W0 cadmium SiO 0.1-6.0 oxide (CdO), and chromium oxide (Cr O 2 a It has been discovered according to the invention that r z :i sintered wafer 1 will be in non-ohmic contact with said 0 CdO-0*2.0 silver electrode 2 when said silver electrode is prepared (3110-0-40 applying Silver Paint to one Surface of said Wafel: and Since the non-linearity of the novel resistors can be i g F thus coated Wafer at 1,000 to m an attributed to the non-ohmic contact between said sintered oxldlzlng atmosphere such as alt f oxygen- 10 wafer 1 and said silver electrode 2, it is necessary for Table 1 Shows operable f optlmal FomPosltlons of obtaining a desirable C-value and n-value to control the sllver electrode 2 after beam? for Curmg m compositions of the sintered wafer 1 and the silver elecproduce the novel non-linear resistors in accordance with node 2 the invention. It is necessary for achieving alow value of C for the Important for eehlevmg a non-01111110 Contact that resultant non-linear resistors that the sintered wafer have Sald sllvel' electrode be lf p hl f at a p an electrical resistivity less than 10 ohm-cm, said electure of 100 to 111 all OXldlZlng atmosphere Such trical resistivity being measured by a four point method as air and oxygen, a silver paint which is applied to the in a per Se conventional surface of said sintered wafer and which has a composi- Th i d b d 1 can b prepared b a per Se w ll tion similar With respect to the SOlid ingredients to that known ceramic technique, The starting materials having 0f the fired Silver electrode the compositions according to the invention are mixed The other electrode, in Ohmic Contact With the Wafer, in a Wet mill so as to produce homogeneous mixtures. is formed y Vacuum-evaporating a metal Selected from The mixtures are dried and pressed in a mold into desired the g p consisting of aluminum, zine. tin and i i shapes at a pressure of from 100 kg./cm. to 1000 kg./ in accordance With t e inventioncm. The pressed bodies are sintered in air at 1250 C. A e method fo making Said electrode Which s to 1450 C. for l to 3 hours, and then furnace cooled in ohmic contact according to the invention is to plate to room temperature (from about 15 to about C.). electro-chemically a metal selected from the group con- The pressed bodies are preferably sintered in a nonsisting of silver, copper, nickel, zinc, and tin. oxidizing atmosphere such as nitrogen or argon when it is A further operable electrode which is in ohmic con- 30 desired to reduce the electrical resistivity. The electrical tact can be formed by a spray metalized film of a metal resistivity also can be reduced by air quenching from the selected from the group consisting of copper, tin, zinc sintering temperature to room temperature even when and aluminum. the pressed bodies are fired in air.

TABLE I Operable composi- Optimal composition of electrode (wt. percent) tion of electrode wt. percent (Ag; Ag PbO SiO2 B203 Biz03 CdO CuO It has been discovered according to the invention that The mixtures can, if desired, be preliminarily calcined a sintered wafer of zinc oxide of the aforesaid composiat 700 to 1000 C. and pulverized for easy fabrication tion has an excellent non-linearity as a non-linear resistor in the subsequent pressing step. The mixture to be pressed and a high stability with respect to humidity, temperacan be admixed with a suitable binder such as water, ture, and electric load when it is provided, at one surface polyvinyl alcohol, etc.

thereof, with a silver electrode which is in non-ohmic con- The sintered bodies in wafer form are coated on one tact therewith and has the aforesaid composition, and at surface thereof by a silver electrode paint in a per se the other surface thereof, with another electrode in ohmic conventional manner such as by a p y method, Screen contact therewith, as described hereinbefore. Such a nonprinting method or brushing method. It is necessary that linear resistor is a non-symmetrical type and can be conthe silver electrode paint have a solid ingredient comnected at said silver electrode to a positive terminal of P ition as defined in Tables 1 and 2 after it is fired at a DC source and, at said electrode in ohmic contact, to 100 C. to 850 C. in air- Solid ing having coma negative terminal of DC source. A great feature of PositionS defined in Tables 1 and 2 can be P p in the non-linear resistor of the non-symmetrical type aca P Se Conventional manner y mixing commercially cording to the invention is the combination of a high available powders with organic resin such as p y vinyl n-value and a low C-value, lower than 1.0 at a given and Phenol resin in a Organic solvent h as hutyl current of 100 tate, toluene or the like so as to produce silver electrode Table 2 shows optimal compositions for the combinap tion of the sintered wafer 1 and the silver electrode in T sllver Powder y be in the form of metallic Silver, accordance with the invention. A non-linear resistor with P 111 the form of Silver carbonate Silver OXide, or such optimal compositions has a C-value lower than 0.8 y otherform which during firinget h temperatures at a given current of 100 ma., an n-value higher than employed i be Converted to rrletalhe Y e q s 0 and a high stability with respect temperature, the term sllver as used throughout this specification humidity and electric load, when it is provided with the e the ms pp he in c i n w h h other electrode 3 consisting of any one of the aforesaid silver florrlpesl'flefl befere 1t fired; 1S meant to include operable electrodes in ohmic contact therewith in accordsllvel' 111 e 3 f which d1lr1I 1g firlrlg W111 be converted ance with the invention. to metallic silver. The viscosity of the resultant silver electrode paints can be controlled by the amounts of resin TABLE Z-QOP'DIMAL COMPOSITION OF SINTERED and solvent. The particle sizes of the solid ingredients AND SILVER ELECTRODE also are required to be controlled so as to be in the range Composition of sintered wafer (mol percent): of 0.1 to 5 ZnO-100-98 A sintered wafer, after being provided on one surface 'ZFe O 02.0 thereof with said silver electrode, is provided, on the other surface thereof, with an electrode in ohmic contact by vacuum evaporation, electrochemical plating or spray metallizing in a per se well known manner.

Lead Wires can be applied to the silver electrode and the electrode in ohmic contact 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 electrode and the ohmic contact electrode.

Non-linear 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 C. at rated power for 500 hours. The n-value and C-value do not change significantly after being subjected to heating cycles and a load life test. It is preferable for achieving a high stability with respect to humidity that the resultant non-linear 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 curing method for the applied silver electrode paint has a great effect on the n-value of the resultant nonlinear resistors. The n-value will not be optimal when the applied silver electrode paint is heated in a non-oxidizing atmosphere such as nitrogen or 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.

A silver electrode prepared by any method other than by silver painting results in a resistor having a poor n-value. For example, the sintered body does not act as a non-linear resistor when it is provided with a silver electrode by electroless plating or electrolytic plating in a conventional manner. A silver electrode prepared by vacuum evaporation or chemical deposition results in a resistor having 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.

Starting materials according to Table 3 were mixed in a wet mill for 5 hours.

The mixtures were dried and pressed in molds into discs of 13 mm. diameter and 2.0 mm. thick at a pressure of 340 kg./cm.

The pressed bodies were sintered in air at 1350 C. for 1 hour, and then quenched to room temperature (about 15 to about 30 C.). The resulting sintered discs were 10 mm. in diameter and 1.5 mm. thick. The sintered discs were coated on one surface thereof with a silver electrode paint by a conventional brushing method. The silver electrode paint employed had the solid ingredient composition according to Table 4 and was prepared by mixing with vinyl resin in amyl acetate. The coated discs were fired at 500 C. for 30 minutes in air. The other surfaces were provided with a spray metalized film of aluminum by a per se well known technique.

Lead wires were attached to the silver electrodes and the aluminum electrodes by means of conductive silver paint. The electric characteristics of the resultant resistors measured in that direction of easy current flow are shown in Table 3.

TABLE 4 Composition of silver electrode wt. percent:

Ag 9O PbO 7.0 SiO 2.0 B203 CuO 0.3

EXAMPLE 2 Sintered discs each having a composition of 99.5 mol. percent of zinc oxide and 0.5 mol. percent of iron oxide were prepared in the same manner as in Example 1. The sintered discs were 10 mm. in diameter and 1.5 mm. thick. Various silver electrode paints were applied to one surface of the respective sintered discs and the coated discs were fired at 500 C. for 30 minutes in air. The silver electrode paints had solid ingredient compositions as shown in Table 5 and are prepared by mixing 100 parts by weight of said solid ingredient compositions with 1 to 20 parts by Weight of epoxy resin in 20 to 40 parts by weight of butyl alcohol. The other surfaces were provided with a nickel electrode by an electroless plating method. The resultant non-linear resistors had desirable C-values and n-values as indicated in Table 5. It will thus be readily understood that the electrode compositions have a great effect on the electrical characteristics of the resultant nonlinear resistors.

EXAMPLE 3 The resistors of Example 1 were tested according to the methods used for testing electronic component parts. The load life test was carried out at 70 C. ambient temperature at a 1 watt rating power for 500 hours. The heating cycle test was carried out by repeating 5 times a cycle in which said resistors were kept at C. ambient temperature for 30 minutes, cooled rapidly to -20 C. and then kept at such temperature for 30 minutes. After the heating cycle and load life tests, the C-value and n-values did not change more than 1.5% and 3%, respectively.

1 At a given current of 100 ma.

What is claimed is:

1. A non-linear resistor comprising a sintered disc of zinc oxide and two electrodes, one applied to each of the opposite surfaces of said sintered disc, one of said two electrodes being a silver electrode in non-ohmic contact with one of said opposite surfaces, and the other of said two electrodes being in ohmic contact with another of said opposite surfaces.

2. A non-linear resistor according to claim 1, wherein said sintered disc consists essentially of zinc oxide.

3. A non-linear resistor according to claim 1, wherein said sintered disc consists essentially of 99.95 to mol percent of zinc oxide and 0.05 to 10.0 mol percent of at least one oxide selected from the group consisting of F6203, A1203, Bi203, CaO, COO, Nb205, T3205, ZrO W03, and Cr O 4. A non-linear resistor according to claim 1, wherein said silver electrode is percent by weight Ag.

5. A non-linear resistor according to claim 1, wherein said silver electrode consists essentially of 0.25 to 27 percent by weight PbO, 0.02 to 15 percent by weight SiO 0.01 to 15 percent by weight B 0 0 to 6.0 percent by weight Bi O 0 to 6.0 percent by weight CdO, 0 to 6.0 percent by weight CuO and the balance silver.

6. A non-linear resistor according to claim 1, wherein said sintered disc consists essentially of 100 to 98.0 mol percent ZnO and 0 to 2.0 mol percent R 0 and said silver electrode consists essentially of 1.2 to 17.0 percent by Weight PbO, 0.1 to 6.0 percent by weight SiO 0.06 to 6.0 percent by weight B 0 '0 to 2.0 percent by weight Bi O 0 to 2.0 percent by weight CdO, 0 to 2.0 percent by weight CuO and the balance silver.

7. A non-linear resistor according to claim 1, wherein said other electrode in ohmic contact consists essentially of an evaporated film of a metal selected from the group consisting of aluminum, zinc, tin and indium.

8. A non-linear resistor according to claim 1, wherein said other electrode in ohmic contact consists essentially of an electrochemically plated film of a metal selected 8 from the group consisting of silver, copper, nickel, zinc and tin.

9. A non-linear resistor according to claim 1, wherein said other electrode in an ohmic contact consists essentially of a spray metalized film of a metal selected from the group consisting of copper, tin, zinc and aluminum.

References Cited JAMES D. KALLAM, Primary Examiner 0 US. Cl. X.R.