Non-linear Resistors

Abstract

New non-linear resistors having a remarkably stable volt-ampere characteristic are provided. Such resistors comprise several sintered wafers superposed, said sintered wafer consisting of iron oxide as the main component and the respective prescribed quantities of calcium oxide, rare earth oxide and the like, and electrodes applied to opposite surfaces of said several sintered wafers superposed and are based on utilization of the property of the contact surfaces between said several sintered wafers superposed.

Description

BACKGROUND OF THE INVENTION

This invention relates to economical non-linear resistors having a remarkably stable volt-ampere charecteristic manufactured by superposing several sintered wafers, each sintered wafer comprising, as the main component, iron oxide which contains calcium oxide, rare earth oxide, etc. and has resistance of about or not more than 1K.OMEGA. and also small thermal negative resistance, and applying electrodes to opposite surfaces of the several sintered wafers superposed.

Conventional non-linear resistors (the term "non-linear resistor" is referred to as "varistor" hereinafter) comprise silicon carbide varistors, silicon varistors, barium titanate varistors and the like.

The volt-ampere characteristic of such a varistor is generally given by the following Equation (1):

I = I.sub.o (V/V.sub.o).sup..alpha. (1)

where I is the current flowing through the varistor, V is the voltage accross the varistor, V.sub.o is the voltage applied to the opposite surfaces of the varistor at I=I.sub.o mA, that is, the threshhold voltage, and exponent .alpha., an index representing non-linearity is a numerical value greater than 1, usually 3.about.5.

Silicon varistors and barium titanate varistors are limited in used because their V.sub.o values are both only 0.5.about.1.2V, while silicon carbide varistors have large .alpha. above 30V, but small .alpha. under 30V. Moreover, in such varistors based on utilization of the mutual contact between internal particles of sintered bodies as silicon carbide varistors, it is difficult to completely control the state of said mutual contact between the internal particles even under the same manufacturing conditions, consequently the products are inevitably variable in the characteristics.

This invention has been devised to overcome the defects of the conventional varistors as described hereinabove.

SUMMARY OF THE INVENTION

A primary object of this invention is to provide a varistor which has a remarkably stable volt-ampere characteristic.

Another object of this invention is to provide a varistor which is inexpensive in cost.

According to this invention, there is provided a non-linear resistor comprising several sintered wafers superposed, said sintered wafer consisting of iron oxide as the main component, 0.1 .about.34 weight percent of calcium oxide and 0.1.about.34 weight percent of rare earth oxide, and electrodes applied to opposite surfaces of said several sintered wafers superposed.

The sintered wafer may also comprise iron oxide as the main component, 0.1.about.34 weight percent of calcium oxide (CaO), 0.1.about.34 weight percent of rare earth oxide and 0.1.about.10 weight percent of at least one oxide selected from the group comprising aluminum oxide (Al.sub.2 O.sub.3), zirconium oxide (ZrO.sub.2) and cobalt oxide (CoO).

The invention is now explained in detail with reference to the attached drawings.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

FIG. 1 is a sectional view of an embodiment of this invention, a non-linear resistor manufactured by superposing two sintered wafers, each sintered wafer comprising iron oxide as the main component.

FIG. 2 shows the volt-ampere characteristic of the non-linear resistor of FIG. 1.

In FIG. 2, voltage E is plotted as abscissa and current I as ordinate.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides for, example, a non-linear resistor manufactured, as shown in FIG. 1, by superposing two sintered wafers 1, 1, each sintered wafer obtained by sintering a molded material comprising iron oxide as the main component and the respective prescribed quantities of calcium oxide (CaO) and rare earth oxide, or a molded material comprising iron oxide as the main component and the respective prescribed quantities of calcium oxide (CaO), rare earth oxide and at least one oxide selected from the group consisting of aluminum oxide (Al.sub.2 O.sub.3), zirconium oxide (ZrO.sub.2) and cobalt oxide (CaO), applying electrodes for example silver electrodes 2, 2 to the opposite surfaces of the two sintered wafers superposed respectively, attaching lead wires 5, 5 to the silver electrodes 2, 2 by using solder 4 and fixing the whole with suitable insulating coating material 6.

The wafer 1 is a sintered plate having any one of various shapes such as circular, square, rechtangular, etc.

In that case, because of usual ohmic junction at the surface of the sintered wafer 1 which the electrode 2 is applied to, there exists no surface barrier there. But a surface barrier exists at the other surface 3 of the sintered wafer 1 which no electrode is applied to. Therefore, when such sintered wafer 1 is superposed upon the other, there generates a high resistance R.

Furthermore, as the non-linear resistor of this invention shows both very stable V.sub.o and .alpha., especially V.sub.o of a little less than 20V, it proves to have an aptitude for a varistor. Such stable V.sub.o and .alpha. also makes the yield rate of the product to be improved greatly. In addition, as the non-linear resistor of this invention utilizes iron oxide as the main raw material, it is also advantageous in cost.

This invention also provides non-linear resistors manufactured by superposing more than three plates of said sintered wafer and applying electrodes to opposite surfaces of said sintered wafers superposed respectively. The thus manufactured resistors also have remarkably stable volt-ampere characteristics, proving to be suitable for a varistor.

This invention, as described hereinabove, provides varistors having remarkably more stable characteristics at much higher yield rate and at quite lower cost, as compared with the conventional varistors, consequently it is industrially of great value.

The invention will be understood more readily with reference to the following examples. However, the examples are intended to illustrate the invention and are not to be construed to limit the scope of the invention.

EXAMPLE 1

The sintered body is prepared by a conventional technique. The starting material in the composition defined in Table 1 is respectively mixed in a pot mill so as to produce a homogeneous mixture. The mixture is dried in a dryer, pressed in a mold at a pressure of about 1,000kg/cm.sup.2 into a disc of 15mm in diameter and 2mm in thickness. The molded disc is sintered in air at above 1,200.degree. C, thus the sintered disc 1 is obtained.

A non-linear resistor is manufactured, as shown in FIG. 1, by superposing two sintered discs 1, 1, applying silver electrodes 2, 2 to opposite surfaces of the superposed sintered discs 1, 1, attaching lead wires 5, 5 to silver electrodes 2, 2 by using solder 4 and fixing the whole with epoxy resin 6. Because of the ohmic junction at the surface of sintered disc 1 which silver electrode 2 is applied to, there exists no surface barrier there. But a surface barrier exists at the other surface 3 of sintered disc 1 which no silver electrode is applied to. Therefore, when sintered discs 1, 1 are superposed, there generates a high resistance R. The measured value of R, together with these of V.sub.o and .alpha. which are constants of Equation (1) at I.sub.o =1mA, are shown in Table 1. ##SPC1##

As Table 1 shows, there are obtained varistors having especially stable V.sub.o and in the range of 5.about.15wt percent of La.sub.2 O.sub.3 and 0.1.about.10wt percent of CaO. This means much higher yield rate of the product, as compared with the conventional silicon carbide varistors.

Moreover, superposing more than three plates of said sintered body 1 enables to increase V.sub.o value by about integral multiples of a little less than 20V. For example, the measured values of R, .alpha., V.sub.o of the varistors manufactured by superposing two.about.four plates of the sintered body consisting of 5 weight percent of La.sub.2 O.sub.3 and 10 weight percent of CaO as shown in Table 1 are shown in Table 2.

TABLE 2

Number of plates: 2 3 4 __________________________________________________________________________ R (K.OMEGA.) 40 80 119 .alpha. (-) 5 5 5 V.sub.o (V) 19.5 39.0 78.5

EXAMPLE 2

Example 1 shows that 5.about.15wt percent of La.sub.2 O.sub.3 is required to make .alpha. up to 5. In order to decrease the quantity of La.sub.2 O.sub.3 which is expensive, Example 2 is carried out. The starting materials in Example 2 are Fe.sub.2 O.sub.3 containing 15wt percent of CaO and 0.1wt percent of La.sub.2 O.sub.3 added with 0.1.about.10wt percent of at least one oxide selected from the group consisting of Al.sub.2 O.sub.3, ZrO.sub.2 and CoO as shown in Table 2.

The starting material is respectively processed and sintered in the same manner as Example 1, thus the sintered disc is obtained respectively. A non-linear resistor is manufactured by using said sintered disc in the same manner as Example 1. The measured values of R, .alpha., V.sub.o of the thus manufactured varistors are shown in Table 3. ##SPC2##

As table 3 shows, each of the sintered bodies gives a non-linear resistor of which .alpha. and V.sub.o are both remarkably stable because .alpha. is 5 and V.sub.o is 19.5V and which is more inexpensive in cost because of the decreased quantity of La.sub.2 O.sub.3.

Moreover, the measured values of R, .alpha., V.sub.o of the varistors manufactured by superposing two.about.four plates of the sintered body consisting of 10 weight percent of Al.sub.2 O.sub.3, 0.1 weight percent of ZrO.sub.2 and 0.5 weight percent of CoO as shown in Table 3 are shown in Table 4.

TABLE 4

Number of plates 2 3 4 __________________________________________________________________________ R (K.OMEGA.) 20 40 60 .alpha. (-) 5 5 5 V.sub.o (V) 19.5 39.0 18.5

Table 4 shows that varistors of good quality having different V.sub.o values can be manufactured by superposing more than three plates of such sintered bodies as shown in Table 3.

Claims

What is claimed is:

1. A non-linear resistor comprising several sintered wafers superposed, said sintered wafer consisting of iron oxide as the main component, 0.1.about.34 weight percent of calcium oxide (CaO) and 0.1.about.34 weight percent of rare earth oxide, and electrodes applied to opposite surfaces of said several sintered wafers superposed respectively,

2. A non-linear resistor as described in claim 1, wherein said sintered wafer comprises iron oxide as the main component, 0.1.about.34 weight percent of calcium oxide (CaO), 0.1.about.34 weight percent of rare earth oxide and 0.1.about.10 weight percent of at least one oxide selected from the group consisting of aluminum oxide (Al.sub.2 O.sub.3), zirconium oxide (ZrO.sub.2) and cobalt oxide (CoO).