CA1065496A - Voltage dependent resistor and the manufacturing process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A non-linear voltage dependent resistor element and a method for manufacturing the same wherein a poly-crystalline, high resistance layer which contains at least one or both of zinc silicate (Zn2SiO4) and zinc antimonate (Zn7Sb2012) formed on a side of a sintered body mainly consisting of zinc oxide which itself has non-linear voltage dependent properties and a non-crystalline, high resistance layer is formed on the polycrystalline layer, whereby the reduction in non-linear expoment a in an atmosphere of moisture or carbon dioxide gas, sulfur dioxide gas, chlorine gas or the like is prevented and the stable non-linear voltage dependent characteristics are maintained.

Description

10t;5~9~;

1 The present invention relates to a non-linear voltage dependent resistor element having dual layers of high resistances each conslsting of a polycrystalline material and a non-crystalllne material, respectively, formed on a side of a sintered body mainly consisting of zinc oxide which has non-linear voltage dependent properties by itself, and a method for manufacturing the same.
A non-linear voltage dependent resistor element is generally called a varistor and it has been widely used for the purpose of voltage stabilization and surge absorption. A typical example is a silicon carbide (SiC) `~ varistor which makes use of the high sensitivity of contact resistance of silicon carbide particles, and it has been used as a spark killer for a contact or in other applica-tions. The voltage (V) - current (I) characteristic of a varistor can be generally approximated by the relation, I = (C)~
where C is a constant corresponding to the voltage at a given current, and an exponent ~ has a numeric value greater than 1~ and is called the non-linear exponent. The characteristic of the varistor is conveniently represented, ~-in general~ by a voltage at a given current and the d- ~ -value.
The SiC varistor is inexpensive in cost and has been widely used, but because the d-value is small such as 3 - 7~ the effect of the use of the SiC varistor for voltage stabilization or surge absorption has not been sufficient and hence the realization of - 1 - ~

10~5~9~
l an element having a high ~-value has been strongly desired.
As one approach to the above need, a zinc oxide varistor has recently been developed. It i8 manu-factured by adding small amounts of bismuth oxide, lead oxide, barium oxide or the like to zinc oxide, mlxing them together, molding the mixture and sintering it ln the air at 800C - 1~500C to produce a sintered body, on which electrodes are provided. The non-linear voltage dependent properties thereof depends on a sedimentatlon layer which surrounds zinc oxide fine particles and mainly consists of the additives, and the ~-value reaches as high as 50 or higher. Therefore, the zinc oxide varistor has been finding broad usage in various applications.
While the zinc oxide varistor has many advantages as described above, it also includes problems to be resolved owing to its high performance. Cne of the problems lies in that the resistance value of a side of the varistor element reduces in a moist atmosphere, resulting in a substantial decrease in the d-value. This tendency is particularly remarkable when a D.C. voltage is applied in a moist atomsphere. Such deterioration in the characteristic ~-value is due to the increase of leakage current by the decrease in the resistance value of the side of the varistor element.
In order to prevent the ab~ve deterioration in the characteristic ~-value, the side of the varistor has been coated with epoxy resin or the like. However, the effect of this moisture-proofing approach has not been satis~actory because the adhesion between the sintered body mainly 10~5496 1 col~sisting of ~inc oxide und the epoxy re~in wa~ not sufl`icicnt. To overcome the above drawback, it has recent-ly been tricd ~o upply blsmuth oxide (Bi203), ~ilicon dioxide (SiO2), antimony oxide (Sb203) or the like, either sin61y or in combination, on a side o~ a molded body mainly consisting of zinc oxide, and sinter the body to form a high resistance layer on the ~ide.
This approach has overcomed the above difficulty to a certuin extent. It has been effective in improving the moisture-proofing property and preventing the occurrence of a creeping di~charge path for a high voltage applied across the element. However, the element manufactured in the above method also has a drawback. Namely, the thickness of the high resiqtance layer formed on the `~ 15 element i9 very thin, i.e. not more than in the order of several tens microns. Consequently, if the side is con-taminated, not only the moisture-proofing property but also the stability of the side against high voltage tend to be lost and hence it has been required to con-tinuously keep the side clean. Accordingly, the treatment after the firing step was critical, which resulted in inconvenience in the manufacture.
- The contamination of the side of the element occurs not only during the manufacturing proces~ but also by the environment surrounding the element. For example, when the element i~ left in an atmosphclc of carbon dioxide ~as, sulfur dioxide gas, chlorine gas or the like, the gas deposits on the side ~f the elements, which gas react~ with water molecule~ in the air to producc curbonic acid ions (C03 ), hydrogcl) slllri~

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10~5496 ions (HSO3 ) or chlorine ions (C~ ), which in turn significantly deteriorate the stability of the side of the element. In a loaded durability test in such an atmosphere, the varistor voltage and the -value tend to significantly decrease in 100 to 200 hours. Furthermore, it has been proved that when the element manufactured by the above method was boiled in pure water no substantial change in the characteristic varistor voltage or -value were observed, but when it was boiled in salt water considerable changes in the varistor voltage and the a-value before and after the boiling were observed.
As described above, the element manufactured by the above method includes the problems concerning the characteristic varistor voltage and the -value as well as drawbacks in appear-ance and commercial value.
It is an object of the present invention to provide a non-linear voltage dependent resistor element which overcomes -~
the above drawbacks. -., It is another object of the present invention to -provide a method for manufacturing such a non-linear voltage dependent element.
Embodiments of the invention will now be described with reference to the accompanying drawings in which:
Fig. 1 is a sectional view of one embodiment of a non-linear voltage dependent element of the present invention.
Fig. 2 shows a comparative chart of test results for the element produced in accordance with the method of the present invention and elements produced by other methods, in atmosphere of chlorine gas, carbon dioxide gas and sulfur dioxide gas.

~ ~ - 4 -10~5496 1 Fig. 3 shows the change in characteristic voltage of the element by heat treatment.
Figs. 4 A and B show the comparison of the adherence of a glass to the sintered body when the glass is baked by the process of the present invention and when the glass is directly baked onto the sintered body mainly consisting of zinc oxide.
In accordance with the present invention there is provided a non-linear voltage dependent resistor element comprising a polycrystalline~ hi8h resistance layer including at least one or both of zinc silicate (Zn2SiO4) and zinc antimonate (Zn7Sb2012) formed on a side of a sintered body mainly consisting of zinc oxide which itself has non-linear voltage dependent properties and a non-crystalline, high resistance layer formed on the polycrystalline layer.
The non-linear voltage dependent resistor element of the present invention is provided with dual ' polycrystalline and non-crystalline layers of high resistance by forming the polycrystalline, high resistance layer on the side of the sintered body mainly consisting of zinc oxide simultaneously with sintering and then applying the non-crystalline, high resistance material layer on the polycrystalline layer. The element thus produced exhibits a very high resistance to the contamina-tion of the side and is stable against humidity and an impact current as well as various gases. Further-more, the element of the present invention is of a high commercial value because it shows surface gloss of the side.
.

10~;5496 1 The present invention wlll now be explained in detail with reference to an example.

Example:
Added to p~wders of zinc oxide (ZnO) were powders of bismuth oxide (Bi2O3), cobalt oxide (CoO), manganese oxide (NnO) and antimony oxide (Sb2O3) in the ran8e of 0.01 - 10 MX, respectively, and they were fully blended and compressed and molded into a body of 40 mm in diameter and 30 mm in thickness.
On the other hand~ a mixture of silicon dioxide (SiO2), antimony oxide (Sb2O3) and bismuth oxide (Bi2O3) , was mixed with a binder consisting of one part by weight ; of ethyl cellulose and three parts by weight of butyl carbitol, at the mixing ratio of 1 to 3 by weight, and they were blended to produce a homogeneous blend. The paste thus produced was applied on the side of the molded body, dried and then fired at the temperature of 800 -1,500C. The sintered body thus produced had its upper and lower planes polished. Glass frits having melting points in the range of 360 to 650C were blended with a binder consisting of 5 parts by weight of ethyl cellulose and 95 parts by weight of butyl carbitol at the mixing ratio of 5 to 2 by weight, and they were mixed homogeneously The resulting paste was applied on the side of the sintered body and baked at a proper temperature. Thereafter, aluminum electrodes were formed on the upper and lower planes of the sintered body.
Fig. 1 is a sectional view of the non-linear ~-voltage dependent resistor element produced in the manner 10f~549~;
1 described above, wherein reference numeral 1 designates the sintered body mainly consisting of zinc oxide,

2 designates the polycrystalline, hlgh resistance layer (first layer) which may be fonmed by the diffusion of the silicon, antimony or bismuth component or by the reaction with zinc oxide. The polycrystalline, hlgh resistance layer 2 is a high resistance layer including at least one or both of zinc silicate (Zn2SiO4) and zinc antimonate n7Sb2012). The reference numeral 3 designates the non-crystalline, high resistance layer (second layer) formed by glass, and 4 and 5 designate the electrodes. It has been proved by X-ray diffraction that the polycrystalline, high resistance layer consisted of zinc silicate and zinc antimonate. Table 1 shows the effects of changing the glass frit co~position to various electric charac-teristics. In the Table, when a mixture consisting of silicon dioxide (SiO2), antimony oxide (Sb203) and bismuth oxide (Bi203) at the ratio of 70 to 20 to 10 in M ratio was used as the first layer forming material, -VlpA/mm and VImA/mm represent voltage5 per unit thick_ ness when currents of 1 ~A and 1 mA were flown, respectively Such voltages per unit thickness were used because the sintered body mainly consisting of zinc oxide showed a non-linear bulk characteristic. The symbol a represents the non-linear exponent which can be determined by the pre-viously shown relationship between the current and the voltage using the voltage value across the electrodes when currents of 0.1 mA and 1 mA are flown. The voltage variation rate avluA shows the change of the voltage Vl~A before and after the test. The test 10~5496 condition represented by the symbol I is atmosphere temperature of 70C, relative humidity of 95%, applied D.C. voltage of 1000 volts, and voltage application period of 1000 hours. The test conditions represented by the symbols II, III and IV include, in addition to the same condition as I, atmosphere of chlorine (CQ2) gas, carbon dioxide (CO2) gas and sulfur dioxide (SO2) gas, respectively, at partial pressures of 50 mmHg, 250 mmHg, and 50 mmHg, respectively. The test conditions represented by the symbols V and VI include boiling in pure water for 100 hours and in salt water for 100 hours, respectively. IF
represents a maximum current which does not cause creeping discharge when an impulse current (wave form: 4 x 10 ~s) is applied.
The ingredients for the various compositions shown in Table 1, the amounts of the ingredients, the baking temperature (fusing temperature) and the coefficients of thermal expansion are shown in Table 2. In Table 2, the figures shown in the columns of the composition numbers represent the amounts of the ingredients shown in the leftmost column by M%.
As seen from Tables 1 and 2, for the prior art example ~ -having only the first layer, considerable deterioration in voltage value occurs in the atmosphere of the conditions II, III, IV, and VI, but for the compositions 1 to 4 in which the second layers of glass having melting points in the range of at least 350C to 650C are formed, are little influenced by the atmosphere of the conditions II, III, IV or VI, and their other characteristics are not inferior to those of the prior art.
Particularly, the composition 4 shows the most excellent effect.

~ ~_ 0~ j ~O,~ ~ ~ ~ ~ O ~
.` ~ 0 U~ ~ ,~ ~ ~ ~
l ~ ~
-- N l ~ ~-~ I ~ ~
¦H ~ O O O r~ ~ ~ O
. ~i _ _ _ ~ rl rl ~ ~ ~
~ l ~ I ~ ~ ~_ ,' H O O O O l l O O ,~-~
_ ___ _ __ ~ _._ .- , ~ ~D i i~ 1~ ~1-~ d- ~1 ~ ~, ~ ~ ~ ~ ~0_ _p;~\~ o~ o~ o~ o a~ o L,~
1~ 1 I rrr l 1~

B 9_ 10f~5496 Table 2 ( ~5 %) ~tiOII 1 2 3 4 5 I 6 Ingredi \ .

SiO2 3 6 66 77 . . .
~23 22 ~ 48 32.5 25 lS
ZnO ¦ 9 ~ 20 ¦ 44 7 4.5 ` l ~r2 1. 2 S l ~ I ~ ~2~

: PbF 59 20 30 15.5 ¦ _ ~ . 1 5 ~ 2 : ~aking Temp.(C) ~5 480 600560 I700 900 (Fusing Temp.) Coeff of Ther-mal Expansion 125 72 83 64 75 48 : (X10-7/C) .

1 Fig~ 2 shows relations between test time and the voltage variation rate ~Vl~A for an element on whicn the ~econd layer has been fo~ned by baking the gla:3~
of the com~)osition 4 at 560C, tested under the conditjol,-- 5 II, llI arld IV. For compari~on, the relations for the - prior art example are shown by dotted lines.

- 10 ..

10~5496 1 As is apparent from Fig. 2, the element in accordance with the present invention shows very small voltage varia-tion rate ~Vl~A and is stable in comparison w$th the prlor art example. Howe~er, when the glasses of the compositions 5 and 6 are used as the second layer, the magnitudes of the voltage V and V and the a-value are lmA/mm l~A/mm slightly smaller than those of other examples.
Fig. 3 plots the ~elation between the voltage variation rate ~Vl~A and the heat treatment temperature, with ~ belng a parameter~ when a sintered body having only the f$rst layer was heat treated at various tempera-tures. As an apparent from Fig. 3, the magnitude of ~V~ A slightly increases in the negative direction in the range of 650C to 1000C and tbe a-value tends to decrease.
It is thus readily understood that when a glass having a melting point at a high temperature such as the glass of the composition 5 or 6 in the Table 1 is baked the characteristics are slightly changed by the high tempera-ture. It should be understood, however, that the change by the heat treatment at high temperature is not large enough to deteriorate the characteristics of the element as a varistor.
It would not be unnatural fro~ the above des-cription that one might consider that a method of baking the glass directly on the side of the sintered body malnly consisting of zinc oxide~ to form a high resistance layer is also effective. However~ this is not so as shown by composition X in Table 1. The composition X represents the one in which the glass of composition 4 was dlrectly baked onto the side _ 11 -10t~5496 1 of the body. In thls case, the creeping discharge is more likely to occur than in the other cases and large voltage variation rate ~Vl~A is exhibited in the loaded durability test in the environment of chlorine gas, carbon dioxide gas or sulfur oxide gas. Thls is due to the adherency of the glass to the body. When dual high resistance layers are provided, the creeping discharge always occurs at the interface between the second layer and the atmosphere. On the other hand, where the glass only is provided~ the creeping discharge tends to occur at the interface between the body and the glass layer.
It is apparent from Fig. 4 that the affinity between the glass and the body during fusing is remarkably improved by provid$ng the first layer. Fi8. 4 A and B
show the geometries of fused planes formed by placing 0.1 gram of glass paste of composition 4 in Table 2 on a sintered body mainly consisting of zinc oxide and on a similar sintered body having a plane of the first layer, respectively, in such a manner that the contact area with the plane is a circular plane of 0.02 cm2 area, and firing it at 560C for five minutes. In Fig. 4, the reference numeral 1 designate the sintered body mainly consisting of zinc oxide~ 2 the polycrystalline, high resistance layer formed by the diffusion of silicon, antimony or bismuth component or by the reaction with zinc oxide. 6 designates the paste including glass component, and 3 the non-crystalline, high resistance layer after having been baked. As seen from the drawing, when the first layer is formed on the surface of the body, the fusing is considerably facilitated in comparison , 10~;5496 1 wlth that without the first layer. Such fusing condition of the glass is also obserbed for the glasses of other compositions~
As described hereinabove, when the dual high resistance layers sre provided in accordance with the present invention, the characteristics of the element are very stable not only for the humidity and impulse current but also for the external atmosphere contamina-tion. Such stability could not been attained heretofore when the polycrystalline, high resistance layer or the non-crystalline, high resistance layer was singly used.
Consequently, the effect of the present invention is not a mere combination of both layers. me element of the present invention, which is stable to the various atmosphere contaminants can be advantageously used in an out-door application such as an arrester for a distribution line and in a microwave oven application.
While bismuth oxide (Bi2O3), cobalt oxide (CoO), manganese oxide (MnO) and antimony oxide (Sb2O3) were added to zinc oxide in the above example, lead oxide (PbO), calcium oxide (CaO), strontium oxide (SrO), barium oxide (BaO) and uranium oxide (UO2)~ or other element which for~s a layer having non-linear voltage dependent properties at the particle boundary in the zinc oxide sintered body, can be used. For the purpose of controlling the resistance of the sintered body or for other purpose, nic~el oxide (NiO), magnesium oxide (MgO), chromium oxide (Cr2O3)~ tin oxide (SnO2)~ silicon oxide (SiO2) or titanium oxide (TiO2) may be added without affecting the present invention.

,., 1~5496 1 Furthermore~ although sllicon dloxlde and antlmony oxide were lncluded in the past which formed - the first layer in the above example, it should be under-stood that those, such as carbonate or hydroxide, whlch result in oxides during firing and react with zinc oxide to produce zinc silicate or zinc antimonate, can attain a similar effect. Bismuth oxide (Bi203) need not be -~
added but other component which takes place thereof may be added. Furthermore~ it should be understood that depending on the particular purpose of the varistor, a material which does not appreciably damage the above effect may be added.
Glass of any composition can be used so long as it reacts with the body or the first layer, without adverse effect, to form a non-crystalline, high resistance `
layer.

_ 14 -

Claims (5)

WHAT IS CLAIMED IS:

1. A non-linear voltage dependent resistor element comprising a polycrystalline, high resistance layer including at least one or both of zinc silicate (Zn2SiO4) and zinc antimonate (Zn7Sb2O12) formed on a side of a sintered body mainly consisting of zinc oxide and itself having non-linear voltage dependent properties, and a non-crystalline, high resistance layer formed on the polycrystalline layer.

2. A non-linear voltage dependent resistor element according to Claim 1 wherein said non-crystalline, high resistance layer is a glass layer.

3. A method for manufacturing a non-linear voltage dependent resistor element with a sintered body which shows non-linear voltage dependent properties itself comprising the steps of applying material including at least one or both of silicon and antimony on a side of a molded body mainly consisting of zinc oxide, firing them to form a polycrystalline, high resistance layer on the side of the sintered body, applying material including glass component thereon, and heat treating it to form a non-crystalline, high resistance layer.

4. A method for manufacturing a non-linear resistor element according to Claim 3 wherein a mixture consisting of silicon dioxide (SiO2), antimony oxide (Sb203) and bismuth oxide (Bi203) in the M ratio of 70 to 20 to 10 is applied on the side of the molded body mainly consisting of zinc oxide and then fired to form the polycrystalline, high resistance layer on the side of the molded body.

5. A non-linear voltage dependent resistor element according to Claim 1 wherein said sintered body comprises zinc oxide (ZnO), to which bismuth oxide (Bi203), cobalt oxide (CoO), manganese oxide (MnO) and antimony oxide (Sb203) are essentially added by the amount of 0.01 to 10 M%, respectively.