Process For Making A Voltage Dependent Resistor

Abstract

A voltage dependent resistor comprising a sintered body of zinc oxide having: (1). voltage dependent properties by itself, (2). at least one member selected from the group consisting of L: ions and Na ions diffused in the side surface thereof, and (3). two electrodes applies to the opposite surfaces thereof. The invention also provides a process for making said resistor.

Description

This invention relates to a preparation of a voltage dependent resistor due to the bulk thereof and more particularly a varistor comprising zinc oxide sintered body having Li ions or Na ions diffused from the side surface of the sintered body.

Various voltage dependent resistor such as silicon carbide varistors, selenium rectifiers and germanium or silicon p-n junction diodes have been widely used for stabilization of voltage or current of electrical circuits. The electrical characteristics of such a voltage dependent resistor 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 corresponding 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 voltages at a given currents I.sub.1 and I.sub.2, 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.

There have been known voltage dependent resistors comprising sintered bodies of zinc oxide with or without additives and silver paint electrodes applied thereto, as seen in the U.S. Pat. No. 3,496,512. The non-linearity of such voltage dependent resistors is attributed to the interface between the sintered body of zinc oxide with or without additives and the silver paint electrode and is controlled mainly by changing the compositions of said sintered body and silver paint electrode. Therefore, it is not easy to control the C-value over a wide range after the sintered body is prepared. Similarly, the voltage dependent resistors comprising germanium or silicon p-n junction diodes are difficult to control the C-value over a wide range because the non-linearity of these voltage dependent resistors is not attributed to the bulk but to the p-n junction. On the other hand, the silicon carbide varistors have the non-linearity due to the contacts among individual grains of silicon carbide bonded together by a ceramic binding material i.e., to the bulk and are controlled in the C-value by changing a dimension in a direction to which the current flows through the varistors. The silicon carbide varistors, however, have a relatively low n-value ranging from 3 to 6 and are prepared by firing in non-oxidizing atmosphere, especially, in a purpose to obtain a lower C-value. In U.S. Pat. applications Ser. No. 763,285 filed on Sept. 27, 1968, No. 866,820 filed on Oct. 16, 1969, No. 866,819 filed on Oct. 16, 1969, No. 866,821 filed on Oct. 16, 1969, No. 869,470 filed on Oct. 27, 1969, No. 872,590 filed on Oct. 30, 1969, there have been disclosed voltage dependent resistors comprising sintered bodies of zinc oxide with additives such as bismuth oxide, uranium oxide, strontium oxide, lead oxide, barium oxide, cobalt oxide and manganese oxide. The non-linearity of such voltage dependent resistors is attributed to the bulk thereof and is independent of the interface between the sintered bodies and electrodes. Therefore, it is easy to control the C-value over a wide range by changing the thickness of sintered body itself. Such voltage dependent resistors in a bulk type have more excellent properties in n-value, transient power dissipation and AC power dissipation than SiC varistors. A disadvantage of the zinc oxide voltage-dependent resistors exists in their poor stability in an electric load life test in high humidity ambient. When D.C. power is applied to the zinc oxide sintered body in a high humidity ambient, the sintered body shows a decrease in the surface electrical resistance. The decrease causes particularly an increase in the leakage current in the zinc oxide voltage-dependent resistor of a bulk type and results in the poor non-linear property. The deterioration of the non-linear property of the voltage-dependent resistor occurs still in the load of low power such as that lower than 0.01 watt in high humidity ambient, for example 90 percent R.H at 70.degree. C ambient. Therefore, it is necessary that the sintered body are assured completely for outer moisture by protective coating.

An object of the present invention is to provide a method for making a voltage dependent resistor characterized by a high stability for d.c. load in high humidity ambient.

Another object of the present invention is to provide a method for making a voltage dependent resistor characterized by both a high n-value and a high stability for d.c. load in high humidity ambient.

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 of a voltage-dependent resistor in accordance with the invention.

Before proceeding with a detailed description of the manufacturing process of the voltage-dependent resistor contemplated by the invention, the construction of the resultant resistor will be described with reference to the aforesaid figure of drawing wherein reference character 10 designates, as a whole, a voltage-dependent resistor comprising, as its active element, a sintered body having surfaces consisting of a side surface 2 and opposite surfaces 3 and 4 to which a pair of electrodes 5 and 6 are applied. Said sintered body 1 is prepared in a manner hereinafter set forth and have a diffusion layer of Li ions or Na ions 11 at said side surface 2 and is in any form such as circular, square or rectangular plate form. Wire leads 8 and 9 are attached conductively to the electrodes 5 and 6, respectively, by a connection means 7 such as solder or the like.

A process for making a voltage dependent resistor characterized by a high humidity resistance according to the invention comprises:

1. providing zinc oxide sintered body having voltage dependent properties by itself,

2. diffusing at least one member selected from the group consisting of Li ions and Na ions into said zinc oxide sintered body from the side surface of said zinc oxide and

3. applying two electrodes to the opposite surfaces of said zinc oxide sintered body.

Said zinc oxide sintered body havng voltage dependent properties by itself can be prepared by using a composition described in U.S. Pat. applications Ser. No. 763,285, No. 866,820, No. 866,819, No. 866,821, No. 869,470, No. 872,590. Among various compositions, a better result can be obtained with a composition consisting essentially of, as a major part, 80.0 to 99.9 mole percent of zinc oxide, and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi.sub.2 O.sub.3) and 0.05 to 10.00 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO), antimony oxide (Sb.sub.2 O.sub.3), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).

The diffusion process referred to herein can be achieved by any suitable and available method such as firing said sintered body covered, at the side surface, with powder of lithium compound or sodium compound which is converted into lithium oxide or sodium oxide at the firing temperature. A prefereable method is to coat said sintered body with a paste including the lithium compound or the sodium compound at the side surface and to heat at a given temperature for a given time.

An assurance of humidity stability requires a diffusion length not less than 0.01mm in accordance with the present invention. The diffusion length can be easily controlled by diffusion temperature and diffusion time in a manner per se well known in the art. The higher diffusion temperature or the longer diffusion time results in the longer diffusion length.

Among lithium ions and sodium ions, lithium ions achieve a higher stability for humidity at the same diffusion length.

It has been discovered according to the invention that the D.C. stability of resultant resistor in high humidity is remarkably improved when said paste comprises, as a solid ingredient, 0.5 to 10.0 wt. parts of Li.sub.2 O and at least one member selected from the group consisting of 0.01 to 10.0 wt. parts of CoO. 0.01 to 10.0 wt. parts of MnO, 0.01 to 10.0 wt. parts of Ag.sub.2 O, 0.01 to 10.0 wt. parts of Cr.sub.2 O.sub.3 and 0.01 to 10.0 wt. parts of NiO.

The D.C. stability of resultant resistor is extremely improved when said paste comprises, as a solid ingredient, 0.5 to 10.0 wt. percent of lithium oxide (Li.sub.2 O), 3.0 to 35.0 wt. percent of boron trioxide (B.sub.2 O.sub.3), 5.0 to 43.0 wt. percent of silicon dioxide (SiO.sub.2) and 12.0 to 91.5 wt. percent of one member selected from the group consisting of bismuth oxide (Bi.sub.2 O.sub.3) and lead oxide (PbO).

According to the invention, the resultant resistor shows excellent D.C. stability in high humidity test when said paste comprises, as a solid ingredient, 0.8 to 10.0 wt. percent lithium oxide (Li.sub.2 O), 50.0 to 80.0 wt. percent of barium oxide (BaO) and 10.0 to 40.0 wt. percent of boron trioxide (B.sub.2 O.sub.3).

It has been discovered according to the invention that the optimal results can be obtained with the D.C. stability of the resultant resistor in humidity test when said paste comprises, as a solid ingredient, 1.0 to 2.5 wt. parts of Li.sub.2 O and 1.0 to 3.0 wt. parts of K.sub.2 O.

The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials comprising zinc oxide powder and additives such as bismuth oxide, cobalt oxide, manganese oxide, antimony oxide, barium oxide, strontium oxide, lead oxide, uranium oxide and tin oxide 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 100Kg/cm.sup.2 to 1,000Kg/cm.sup.2. When the rod-shaped resistor is desired, the mixed slurry can be fabricated into the desired shape by extruding method and then dried. The pressed or extruded bodies are sintered in air at a temperature of 1,000.degree. to 1,450.degree. C for 1 to 5 hours, and then furnace-cooled to room temperature. The sintering temperature is determined from the view of electrical resistivity, nonlinearity and stability. The electrical resistivity also can be reduced by air-quenching from the sintering temperature to room temperature. The mixtures may be preliminarily calcined at 700.degree. to 1,000.degree. C and pulverized for easy fabrication in the subsequent pressing step. The mixtures may be admixed with a suitable binder such as water, polyvinyl alcohol, etc. The said sintered body has non-ohmic resistance due to the bulk itself. Therefore, its C-value can be changed without impairing the n-value by changing the distance between said opposite surfaces. The shorter distance results in the lower C-value.

The sintered body is coated, at a side surface, with a paste including, Li oxide powder or Na oxide powder fired at a given temperature in oxidizing atmosphere so as to diffuse Li ions or Na ions into the bulk of said sintered body and then cooled to room temperature. Said paste comprises, as a solid ingredient, lithium oxide powder with or without further additives or sodium oxide powder and, as a binding material, an organic resin such as epoxy, vinyl and phenol resin in an organic solvent such as butyl acetate, toluene or the like. Said lithium oxide or sodium oxide can be replaced with any lithium compound or sodium compound such as oxalate, carbonate, nitrate, sulfate, iodide, bromide, fluoride, amid, hydroxyde, imide, or oxychloride which is converted into, lithium oxide or sodium oxide which diffuses easily into said sintered body as lithium ions or sodium ions at the firing temperature. The binding material is burned out during firing. The firing temperature and time depend on the weight of lithium or sodium component included in the applied paste and should be controlled so that the Li ions or Na ions diffuses into said sintered body to the depth not less than 0.01 mm. Therefore, the higher diffusion temperature requires the shorter diffusion time. The side surface layer of sintered body having Li ions or Na ions diffused therein shows very high electrical resistivity and assures a high humidity stability. The firing temperature higher than 1,000.degree. C results in the rapid diffusion of Li ions and Na ions and makes it too difficult to control the diffusion time to a given value of the diffusion depth. On the other hand, it takes too much time to diffuse said Li ions or Na ions into said sintered body at a firing temperature lower than 600.degree. C for Li ions and 650.degree. C for Na ions. Therefore, diffusion temperature is more desirable to be 600.degree..about.1,000.degree. C for Li ions and 650.degree..about.1,000.degree. C for Na ions.

After diffusing process, the sintered body is applied with electrodes at the opposite surfaces of the sintered body. Said electrodes can be made by any available method such as heating of noble metal paint, electroless or electrolytic plating of Ag, Cu, Ni, Sn etc., vacuum evapolating Al, Zn, Sn etc. and flame spraying of Cu, Sn, Al, Zn etc. in accordance with the prior well known technique. When said electrodes are formed by heating noble metal paint at a higher temperature than said diffusion temperature, said process of forming two electrodes is preferably carried out before said diffusing process.

Lead wires can be attached to the electrodes in a per se conventional manner by using conventional solder. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent in order to connect the lead wires to the silver electrodes. Voltage-dependent resistor according to this invention have a high stability to temperature and humidity and in the load life test, which is carried out at 70.degree. C, 90 percent RH at a rating power for 500 hours. The n-value and C-value do not change remarkably after load life test.

EXAMPLE 1

Starting materials listed in Table 1 are mixed in a wet mill for 5 hours.

The mixture is dried and pressed in a mold into a disc or a cylinder of 13 mm in diameter and the thickness listed in Table 1 at the pressure of 340 Kg/cm.sup.2. The pressed body is sintered in air at the temperature listed in Table 1 and then furnace-cooled to room temperature (about 15.degree. to about 30.degree. C). The sintered body is coated, at the side surface, with the paste containing 50 wt. percent of lithium carbonate or sodium carbonate and 50 wt. percent of an epoxy resin butyl alcohol solution. The amount of applied paste is controlled by an weight of lithium carbonate or sodium carbonate converted into lithium oxide or sodium oxide distributed through unit area of the side surface as shown in Table 1. The applied paste is fired at a temperature of 800.degree. C for 1 hr in air. A chemical analysis of fired body indicates that converted lithium oxide or sodium oxide diffuses into the sintered body of zinc oxide to the depth more than 0.01 mm. Then the opposite surfaces of sintered body are provided with electrodes of a spray-metalized film of aluminium in a per se well known technique. Lead wires are attached to the aluminum electrodes by means of conductive silver paint. The electrical characteristics of the resultant resistor are shown in TAble 1. It will be readily understood that the C value changes in proportion to the thickness of the sintered body. The resultant resistor is tested in accordance with a methods widely used in the electronic components parts. The load life test is carried out at 70.degree. C ambient temperature, 90 percent R. H atmosphere at 1 watt rating power for 500 hours. Table 1 shows the change rates of C-value and n-value of resistors after load life test. The sintered body having no Li ions or Na ions diffused therein shows the change rates of -20 percent for C value at a given current of 1mA and -50 percent for n value at the same load life test. It is readily understood that said diffusing process have great effect for improving the D.C. stability in high humidity. ##SPC1## ##SPC2## ##SPC3## ##SPC4## ##SPC5## ##SPC6## ##SPC7##

EXAMPLE 2

Starting materials according to Table 2 are completed to the voltage dependent resistor and tested in the same manner as that of Example 1 except the following processes;

Pressed size: 13mm.phi. and 10mm thickness

Sintering conditions: Table 2.

Firing temperature and Time: 800.degree. C for 1Hr.

Weight of applied Li.sub.2 O or Na.sub.2 O: 1 mg/cm.sup.2

The results of tests are shown in Table 2. ##SPC8## ##SPC9## ##SPC10## ##SPC11## ##SPC12##

EXAMPLE 3

The starting materials composing of 99.0 mole percent of zinc oxide, 0.5 mole percent of bismuth oxide and 0.5 mole percent of cobalt oxide are completed to the voltage dependent resistors and tested in the same manner as that in Example 2 except the following processes.

Sintering temperature and time: 1,350.degree. C and 1Hr. Li compound or Na compound included in the paste: Table 3,

Weight of applied Li compound or Na compound (with weight converted into Li.sub.2 O and Na.sub.2 O): 1mg/cm.sup.2 . Firing temperature and time: Table 3.

The results of test are shown in Table 3. ##SPC13## ##SPC14## ##SPC15##

EXAMPLE 4

The starting materials comprising 99.0 mole percent of zinc oxide, 0.5 mole percent of bismuth oxide and 0.5 mole percent of manganese oxide are completed to the voltage dependent resistors and tested in the same manner as that in Example 3 except the following processes:

Solid ingredients included in the paste: Table 4.

Weight of applied paste (with weight converted into Li.sub.2 O): 1mg/cm.sup.2

Firing temperature and time: Table 4.

The results of test are shown in Table 4. ##SPC16## ##SPC17## ##SPC18##

EXAMPLE 5

The starting materials comprising 99.0 mole percent of zinc oxide, 0.5 mole percent of bismuth oxide and 0.5 mole percent of manganese oxide are completed to the voltage dependent resistors and tested in the same manner as that in Example 3 except the following processes:

Solid ingredients included in the paste: Table 5.

Weight of applied paste (with weight converted into Li.sub.2 O): 1mg/cm.sup.2.

Firing temperature and time: Table 5.

The results of test are shown in Table 5. ##SPC19## ##SPC20##

EXAMPLE 6

The starting materials comprising 99.0 mole percent of zinc oxide, 0.5 mole percent of bismuth oxide and 0.5 mole percent of manganese oxide are completed to the voltage dependent resistors and tested in the same manner as that in Example 3 except the following processes:

Solid ingredients included in the paste: Table 6,

Weight of applied paste (with weight converted into Li.sub.2 O and K.sub.2 O): Table 6.

Firing temperature and time: Table 6

The results of test are shown in Table 6. ##SPC21##

EXAMPLE 7

The starting materials comprising 99.0 mole percent of zinc oxide, 0.5 mole percent of bismuth oxide and 0.5 mole percent of manganese oxide are completed to the voltage dependent resistors and tested in the same manner as that in Example 3 except the following processes:

Solid ingredients included in the paste: Table 7,

Weight of applied paste (with weight converted into Li.sub.2 O: 1mg/cm.sup.2

Firing temperature and time: Table 7.

The results of test are shown in Table 7. ##SPC22##

Claims

What we claim is:

1. A process for making a voltage dependent resistor comprising zinc oxide sintered body having voltage dependent properties by itself, said process comprising: (1.) providing zinc oxide sintered body having voltage dependent properties by itself, (2.) diffusing at least one member selected from the group consisting of Li ions and Na ions into said zinc oxide sintered body from the side surface of said zinc oxide sintered body, and (3.) applying two electrodes to the opposite surfaces of said zinc oxide sintered body.

2. A process for making a voltage dependent resistor comprising zinc oxide sintered body having voltage dependent properties by itself, said process comprising: (1.) providing zinc oxide sintered body having voltage dependent properties by itself, (2) diffusing Li ions into said zinc oxide sintered body from the side surface of said zinc oxide sintered body, and (3.) applying two electrodes to the opposite surfaces of said zinc oxide sintered body.

3. A process according to claim 1, in which said zinc oxide sintered body consists essentially of, as a major part, 99.9 to 80.0 mole percent of zinc oxide, and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi.sub.2 O.sub.3) and 0.05 to 10.0 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO), antimony oxide (Sb.sub.2 O.sub.3), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).

4. A process according to claim 1, in which said Li ions or Na ions are diffused into said sintered body to the depth not less than 0.01 mm from the side surface of said zinc oxide sintered body.

5. A process according to claim 2, in which said Li ions are diffused by applying a paste comprises, as a solid ingredient, 0.5 to 10.0 wt. parts of Li.sub.2 O and at least one member selected from the group consisting of 0.01 to 10.0 wt. parts of CoO, 0.01 to 10.0 wt. parts of MnO, 0.01 to 10.0 wt. parts of Ag.sub.2 O, 0.01 to 10.0 wt. parts of Cr.sub.2 O.sub.3 and 0.01 to 10.0 wt. parts of NiO.

6. A process according to claim 5, in which said paste comprises, as a solid ingredient, 0.5 to 10.0 wt. percent of lithium oxide (Li.sub.2 O), 3.0 to 35.0 wt. percent of boron trioxide (B.sub.2 O.sub.3), 5.0 to 43.0 wt. percent of silicon dioxide (SiO.sub.2) and 12.0 to 91.5 wt. percent of one member selected from the group consisting of bismuth oxide (Bi.sub.2 O.sub.3) and lead oxide (PbO).

7. A process according to claim 5, in which said paste comprises, as a solid ingredient, 0.8 to 10.0 wt. percent of lithium oxide (Li.sub.2 O), 50.0 to 80.0 wt. percent of barium oxide (BaO) and 10.0 to 40.0 wt. percent of boron trioxide (B.sub.2 O.sub.3).

8. A process according to claim 5, in which said paste comprises, as a solid ingredient, 1.0 to 2.5 wt. parts of Li.sub.2 O and 1.0 to 3.0 wt. parts of K.sub.2 O.

9. A voltage dependent resistor comprising zinc oxide sintered body having: (1.) voltage dependent properties by itself, (2.) at least one member selected from the group consisting of Li ions and Na ions diffused in the side surface thereof, and (3.) two electrodes applied to the opposite surfaces thereof.

10. A voltage dependent resistor comprising zinc oxide sintered body having: (1.) voltage dependent properties by itself, (2.) Li ions diffused in the side surface thereof, and (3.) two electrodes applied to the opposite surfaces thereof.

11. A voltage dependent resistor accroding to claim 9, in which said zinc oxide sintered body consists essentially of, as a major part, 99.9 to 80.0 mole percent of zinc oxide, and, as an additive, 0.05 to 10.0 mole percnet of bismuth oxide (Bi.sub.2 O.sub.3) and 0.05 to 10.0 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO), antimony oxide (Sb.sub.2 O.sub.3), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).

12. A voltage dependent resistor according to claim 9, in which the depth of said Li ions or Na ions diffused in said zinc oxide sintered body is not less than 0.01 mm from the side surface of said zinc oxide sintered body.