CA1182278A - Varistor with tetragonal antimony zinc oxide additive - Google Patents
Varistor with tetragonal antimony zinc oxide additiveInfo
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- CA1182278A CA1182278A CA000389657A CA389657A CA1182278A CA 1182278 A CA1182278 A CA 1182278A CA 000389657 A CA000389657 A CA 000389657A CA 389657 A CA389657 A CA 389657A CA 1182278 A CA1182278 A CA 1182278A
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- varistor
- zinc oxide
- oxide
- voltage
- additive
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Abstract
VARISTOR WITH TETRAGONAL
ANTIMONY ZINC OXIDE ADDITIVE
ABSTRACT OF THE DISCLOSURE
A low varistor voltage, low leakage metal oxide varistor and especially a zinc oxide varistor is provided by adding a few percent of tetragonal antimony zinc oxide manufacturing the additive as well as a method for manufacturing a varistor including the additive are described.
ANTIMONY ZINC OXIDE ADDITIVE
ABSTRACT OF THE DISCLOSURE
A low varistor voltage, low leakage metal oxide varistor and especially a zinc oxide varistor is provided by adding a few percent of tetragonal antimony zinc oxide manufacturing the additive as well as a method for manufacturing a varistor including the additive are described.
Description
36-~P-1184 VARISTOR WITH TETRAGONAL
ANTIMONY ZINC OXIDE ADDITIVE
I'his invention relates in general to metal oxide varistors and more particularly to an additive for metal oxide varistors and a process employing the additive which decreases leakage and greatly increases the size of metal oxide grains in the varis-tor body so as to decrease the varistor voltage.
Voltage dependent resistors (varistors) have become increasingly widely employed for the suppression of electrical transients and the stabilization of voltage in electrical circui-ts. The electrical characteristics of such varistors may be expressed by the relation: I = (V/C)~;
where V is the vol-tage applied to the terminals of the device, I is the current flowing therethrough and C is a constant, the varistor voltage, corresponding to the voltage at a given current and alpha is a number greater than one. The value of alpha represents the degree of non-linearity of the varistor and is generally accepted to be a significant Eigure of merit :Eor the device. The value for C determines -the magnitude of impxessed voltage at which further increases in voltage result in relatively greater increases in curren-t than occur for similar changes in voltage at lower magnitudes.
The value for alpha may be calculated according to the following equation: e~= loglO(I2/Il)/log10( 2/ 1 Vl and V2 are the device voltages at currents Iland I2, respectively. The values of C and alpha are functions of the varistor formulation and the manufacturing process employed.
In certain varistor applications where relatively low circuit voltages are employed, that is to say on the order of a few tens of volts, such as applications in vehicles, low voltage power supplies and the like, it is desirable to provide a varistor having a low value for C. Heretofore the fabrication of such varistors has been complicated by the apparent interdependence between the value of C and the alpha of the varistor. In general, varistors having low C values have had correspondingly low ~ values.
Recently, U.S. Patent No~ 4,169,071, to Eda et al, issued September 25, 1~79, has described a varistor having the desired low varistor voltage characteristics, which includes in the body ~hereof, a zinc oxide component comprising 10 to 100 weigh-t percent of zinc oxide grains having a grain size in the range from 100 to 500 microns and preferably from 100 to 300 microns. These relatively large zinc oxide grains are formed by providing, during manu:Eacture of the zinc oxide varistor, .1 to 60 weight percent of zinc oxide seed grains having an initia] size of 20 to 200 microns which grow during sintering to the aforementioned 100 to 500 micron size. The seed grains grow by absorbing neighboring zinc oxide particles which particles have a size usually in the range of .1 to 2 microns. The seed grains themselves may be formed of either single crystal or polycrystalline zinc oxide. Single crystal zinc oxide seed grains can be made by pulverizing zinc oxide single crystals having a very large crystal size.
Alternatively, zinc oxide powder in combination with a soluble grain growth promoting agent is heated and fired for a time between about .5 and 50 hours. By crushing the thus fired mixtured followed by soaking and leaching to remove the soluble grain growth promoting agents from the grain boundaries, large size single crystal grains are produced from which grains the desired size range may be 7~
obtained by classification through a sieve. In order for the large grains to be isolated by leaching of grain growth promoting agents, the presence of nonsoluble materials such as bismuth oxide which segrega-te -to the grain boundries, must be avoided.
In addition to the above described seed grains, Eda describes the use of an antimony zinc oxide spinel in the ~form: Zn7/3SB2/3O4 in polycrystalline form for reducing leakage current.
While the foregoing technique may well produce varistors having the desired low vol-tage charac-teristics, it is expensive and time consuming and greatly increases the cost of the varistor inasmuch as many addi-tional steps are required in order to produce the zinc oxide seed grains ].5 as well as the antimony zinc oxide spinel. Large size single crystal zinc oxide material is both expensive to manufacture and not commonly available. Th~ production of seed grains through the use of a water soluble grain growth promoting agent is undesirable inasmuch as many additional steps are required over standard varistor processing. In addition, the seed grain sizes as well as the amount of seed grain material produced by the method described in -the foregoing patent have not been found to be particularly effec-tive.
Accordingly, it is an object of this invention to provide a voltage dependent varistor having low varistor voltage, high alpha, low cost, low leakage and ease of manufacture.
It is another object of this invention to provide a new and improved me-thod for manufacturing zinc oxide seed grains which may be utilized to produce a low varis-tor voltage metal oxide varistor having the foregoing character-istics without the need for a separate leakage reducing additive.
Briefly stated and in accordance with one aspect of this invention metal oxide varistors and preferably zinc oxide varistors having low varistor voltage, high alpha ~z~
and low leakage current as well as other desirable elec-trical characteris-tics are manufactured by mixing zinc oxide as a major constituent with one or more additives which are preferably metal oxides as minor constituents and a relatively small amount of tetragonal-an-timony-zinc-oxide (hereinafter TAZO). The mixture is pressed and sintered at an elevated temperature, electrodes and leads are attached and the device may then be utilized. While the percen-tage of TAZO which is added -to the varistor mixture may vary over a wide range in accordance wi-th -this invention, it is preferred to employ a few percen-t by weight.
The prepara-tion of TAZO proceeds by mixing xinc oxide and antimony oxide, pressing the mixture and sintering at an elevated temperature. The sintered body includes a white outer layer and a yellow-orange core which core contains the desired TAZO material. The white layer is removed, and the core is crushed and classified to produce the desired size TAZO particles~ Preferably particles on the order of 20 microns or less are employed.
While the weight percent of TAZO particles required to achieve the desired reduction in varistor voltage is not critical once a minimum amount is passed~ it is preferred to use a few percent by weight of T~ZO par-ticles Eormed as described above. The varis-tor mix-ture including the TAZO particles is pressed and sintered, for example, at 1300C for about one hour to form the varis-tor body.
The features of the invention which are believed to be novel are pointed out with particularity in the appended claims. The invention itself, however, both as -to its organi~ation and method of operation together with fur-ther objects and advantages thereof may best be unders-tood by reference to the following description taken in connection with the accompanying dxawings in which:
Figure l is a cross sectional view of a voltage dependent resistor in accordance with this invention.
227~
sefore proceeding with a detailed description of the manufacturing process of the vol-tage dependent resistor contemplated by this invention, its construction will be described with reference to the single figure wherein reference 10 designates, as a whole, a voltage dependent resistor comprising as its active element a sintered body having a pair of electrodes 12 and 13 in ohmic contac-t therewith applied to opposite surfaces thereof. The sintered body ].6 is prepared in a matter h~reinafter set forth and is in any form such as circular, square or rectangular plate :Eorm. Wire leads 18 and 20 are at-tached conductively to electrodes 12 and 14 respectively by connecting means 22 such as solder or the like.
It is known that a metal oxide varistor of the type shown exhibits low varistor voltage when the sintered body portion has a high percentage of relatively large grains of metal oxide and especially zinc oxide. For example, in a zinc oxide varistor, a grain size in the range o:E from 100 to 500 microns and preferably 100 to 200 microns is provided, uniformly dispersed in the sintered body. Normally, in the heretofore commonly employed methods for forming zinc oxide varistors, such large grain sizes are not produced.
It has been reported by Eda, et al, that such large grain sizes are formed when seed grains of zinc oxide having a grain size between about 20 and 200 microns are included in the zinc oxide formulation prior to sintering. In addition to the large seed grains required by the process described by Eda, in order to achieve satisfactory leakage characteristics especially at high -temperatures, Eda requires the addition of spinel type polycrystalline Zn7/3Sb2/3O4 to the mixture which is then mixed in a wet mill so as to produce homogeneous mixtures.
It has been discovered in accordance with this invention that the expensive single crystal zinc oxide seed grains heretofore produced either by crushing large zinc oxide single crystals or by dissolving the water soluble intergranular layers from sintered zinc oxide bodies especially formed using grain growth enhancing additives to produce large grains, are not required~and that substantially easier to manufacture nuclei may be employed to provide equally satisEactory and in many ways superior results. Further, the necessity for separately adding spinel type poly-crystalline Zn7/3Sb2/3O4 is elimina-ted by applicant's use oE tetragonal-antimony-zinc-oxide (TAZO) nuclei.
In accordance with a presently preferred embodiment of this invention, a low voltage varistor is formed by combining zinc oxide powder as a major constituent with one or more additives which produce the desired nonlinear characteris-tics and between .1 and 20% by weiyht of TAZO
nuclei for promoting the growth of large zinc oxide grains in the device and producing a low varistor voltage as well as low high temperature leakage current characteristics.
Preferably the TAZO nuclei have a particle size several times that of the zinc oxide powder so, for example, nuclei of 1 micron size or greater axe preferred.
The preparation of the TAZO nuclei relies upon the chemical reaction between antimony trioxide and zinc oxide according to the following chemical equation:
Sb203 + Zno --~ZnSb20~
the reaction product being in tetragonal form. The TAZO
nuclei are prepared by combing particulate zinc oxide wlth particulate antimony oxide and thoroughly mixing to insure even distribution. The mixture is then pressed into large discs or other convenient forms in the same manner tha-t metal oxide varistors are pressed prior to sintering. The discs are then sintered at a temperature between abou-t 500 and 1300C and preferably between 900 and 1200C.
The sintered discs exhibit a yellow-orange core surrounded by a light-colored peripheral layer. The peripheral layer consists primarily of zinc oxide and is thought -to result from the evaporation of antimony ~rom the body during sintering. The peripheral layer is mechanically removed and the core material which is rich in TAZO is pulverized and classified to provide the desired ranye and distribution of particle sizes up to about 50 microns.
Preferably classification through a 325 mesh sieve is performed to provide a nominal particle size of about 6 microns wherein greater than 90% of the particles are below 20 microns.
It has been determined that the form of an-timony zinc oxide produced by reacting zinc oxide and antimony oxide depends upon the relative amount of each material available as well as the amount of oxygen present. Where an excess of zinc oxide and oxygen is presen-t as is the case when the powders are combined and sintered in oxygen or in air, the reaction proceeds as described the following e~uation:
Sb23 + 7ZnO + 2 ~~~ Zn7O12 which equation product is often written in equivalent form as Zn7/3Sb2/3O~ and is in the spinel form.
By pressing the zinc oxide-antimony oxide mixture prior to sintering, the effect of oxygen on the reaction and the evaporation of antimony oxide are red~lced. The tetragonal form of antimony zinc oxide is primarily produced.
The pressing of the zinc oxide and antimony powders may be conveniently performed in the same manner as the varis-tor discs themselves are pressed prior to sinteriny.
Preferably the mixture oE powders is pressed to a density oE at least 3 grams/cc.
In accordance with an exemplary embodiment of this Eirst aspect of the present invention, a relatively low voltage varistor formulation was pressed into discs and sintered at about 1300C for 1 hour. The varistor formulation includes zinc oxide as a major constituent as well as bismuth oxide, cobalt oxide, manganese oxide, titanium oxide, chromium oxide, boron oxide and TAZO. The following characteristics were measured:
V
IL(~A) C~ 1 V/mm 0.6 21 23 18 ~2~
Wherein IL is the leakage current of the varistor,Cc is the alpha ag hereinabove described, V1is -the varistor voltage a-t 1 milliampere and ~/mm is the vol-ts per millimeter of thickness of the device.
In order to appreciate ~he significance of the resul-ts illustrated at Table 1, a varistor was prepared identically wi-th that hereinabove described except the TAZO nuclei were deleted and a like amoun-t of zinc oxide seed grains as described by Eda et al, were substituted therefore with the results presented in Table II.
TAsLE II
IL(~A) ~ Vl V/mm 1.2 17 20 14 It wlll be seen that the ~, Vl and volts per millimeter characteristics of the device including the TAZO nuclei and the device including the zinc oxide seed grains are similar but that the leakage current of the varistor Eormed in accordance with the teachings of this invention is substantially lower than the leakage current of the varistor formed without benefit of this invention. It will be recognized, that the varistor whose characteristics are presented in Table II
did not include the spinel type polycrystalline antimony zinc oxide in the form Zn7/3Sb2/3O4. Eda, et al claims that the addition oE the spinel would have a posi-tive efEect on leaka~e current. The advantage of this inven-tion resides in the elimina-tion of the necessity for preparing and adding the spinel to the varistor mixture in addition to the seed grains thus providing great economy of fabrication.
The amount of TAZO nuclei added to the varistor formulation may vary over a wide range wi-thout departing from the true spiri-t and scope of this invention. I-t has been determined, however, that beyond about 10~ by weight of TAZO, the effect on varistor voltage is reduced substantially. Preferably less than about 5~ by weight of TAZO nuclei are utilized with substantial decrease in ~2~
36-SP-11~4 in varistor voltage and ~he desirable reduction in leakage and varistor voltage may be obtained utilizing substantially less than 1% by weight of TAZO provided uniform distribution in the varistor mixture is achieved.
While this invention has been described in connection with certain presently preferred embodiments -thereof, those skilled in the art will recogni~e tha-t many modifications and changes may be made therein withou-t departing from the true spirit and scope of the invention which accordingly is intended to be defined solely by the appended claims.
ANTIMONY ZINC OXIDE ADDITIVE
I'his invention relates in general to metal oxide varistors and more particularly to an additive for metal oxide varistors and a process employing the additive which decreases leakage and greatly increases the size of metal oxide grains in the varis-tor body so as to decrease the varistor voltage.
Voltage dependent resistors (varistors) have become increasingly widely employed for the suppression of electrical transients and the stabilization of voltage in electrical circui-ts. The electrical characteristics of such varistors may be expressed by the relation: I = (V/C)~;
where V is the vol-tage applied to the terminals of the device, I is the current flowing therethrough and C is a constant, the varistor voltage, corresponding to the voltage at a given current and alpha is a number greater than one. The value of alpha represents the degree of non-linearity of the varistor and is generally accepted to be a significant Eigure of merit :Eor the device. The value for C determines -the magnitude of impxessed voltage at which further increases in voltage result in relatively greater increases in curren-t than occur for similar changes in voltage at lower magnitudes.
The value for alpha may be calculated according to the following equation: e~= loglO(I2/Il)/log10( 2/ 1 Vl and V2 are the device voltages at currents Iland I2, respectively. The values of C and alpha are functions of the varistor formulation and the manufacturing process employed.
In certain varistor applications where relatively low circuit voltages are employed, that is to say on the order of a few tens of volts, such as applications in vehicles, low voltage power supplies and the like, it is desirable to provide a varistor having a low value for C. Heretofore the fabrication of such varistors has been complicated by the apparent interdependence between the value of C and the alpha of the varistor. In general, varistors having low C values have had correspondingly low ~ values.
Recently, U.S. Patent No~ 4,169,071, to Eda et al, issued September 25, 1~79, has described a varistor having the desired low varistor voltage characteristics, which includes in the body ~hereof, a zinc oxide component comprising 10 to 100 weigh-t percent of zinc oxide grains having a grain size in the range from 100 to 500 microns and preferably from 100 to 300 microns. These relatively large zinc oxide grains are formed by providing, during manu:Eacture of the zinc oxide varistor, .1 to 60 weight percent of zinc oxide seed grains having an initia] size of 20 to 200 microns which grow during sintering to the aforementioned 100 to 500 micron size. The seed grains grow by absorbing neighboring zinc oxide particles which particles have a size usually in the range of .1 to 2 microns. The seed grains themselves may be formed of either single crystal or polycrystalline zinc oxide. Single crystal zinc oxide seed grains can be made by pulverizing zinc oxide single crystals having a very large crystal size.
Alternatively, zinc oxide powder in combination with a soluble grain growth promoting agent is heated and fired for a time between about .5 and 50 hours. By crushing the thus fired mixtured followed by soaking and leaching to remove the soluble grain growth promoting agents from the grain boundaries, large size single crystal grains are produced from which grains the desired size range may be 7~
obtained by classification through a sieve. In order for the large grains to be isolated by leaching of grain growth promoting agents, the presence of nonsoluble materials such as bismuth oxide which segrega-te -to the grain boundries, must be avoided.
In addition to the above described seed grains, Eda describes the use of an antimony zinc oxide spinel in the ~form: Zn7/3SB2/3O4 in polycrystalline form for reducing leakage current.
While the foregoing technique may well produce varistors having the desired low vol-tage charac-teristics, it is expensive and time consuming and greatly increases the cost of the varistor inasmuch as many addi-tional steps are required in order to produce the zinc oxide seed grains ].5 as well as the antimony zinc oxide spinel. Large size single crystal zinc oxide material is both expensive to manufacture and not commonly available. Th~ production of seed grains through the use of a water soluble grain growth promoting agent is undesirable inasmuch as many additional steps are required over standard varistor processing. In addition, the seed grain sizes as well as the amount of seed grain material produced by the method described in -the foregoing patent have not been found to be particularly effec-tive.
Accordingly, it is an object of this invention to provide a voltage dependent varistor having low varistor voltage, high alpha, low cost, low leakage and ease of manufacture.
It is another object of this invention to provide a new and improved me-thod for manufacturing zinc oxide seed grains which may be utilized to produce a low varis-tor voltage metal oxide varistor having the foregoing character-istics without the need for a separate leakage reducing additive.
Briefly stated and in accordance with one aspect of this invention metal oxide varistors and preferably zinc oxide varistors having low varistor voltage, high alpha ~z~
and low leakage current as well as other desirable elec-trical characteris-tics are manufactured by mixing zinc oxide as a major constituent with one or more additives which are preferably metal oxides as minor constituents and a relatively small amount of tetragonal-an-timony-zinc-oxide (hereinafter TAZO). The mixture is pressed and sintered at an elevated temperature, electrodes and leads are attached and the device may then be utilized. While the percen-tage of TAZO which is added -to the varistor mixture may vary over a wide range in accordance wi-th -this invention, it is preferred to employ a few percen-t by weight.
The prepara-tion of TAZO proceeds by mixing xinc oxide and antimony oxide, pressing the mixture and sintering at an elevated temperature. The sintered body includes a white outer layer and a yellow-orange core which core contains the desired TAZO material. The white layer is removed, and the core is crushed and classified to produce the desired size TAZO particles~ Preferably particles on the order of 20 microns or less are employed.
While the weight percent of TAZO particles required to achieve the desired reduction in varistor voltage is not critical once a minimum amount is passed~ it is preferred to use a few percent by weight of T~ZO par-ticles Eormed as described above. The varis-tor mix-ture including the TAZO particles is pressed and sintered, for example, at 1300C for about one hour to form the varis-tor body.
The features of the invention which are believed to be novel are pointed out with particularity in the appended claims. The invention itself, however, both as -to its organi~ation and method of operation together with fur-ther objects and advantages thereof may best be unders-tood by reference to the following description taken in connection with the accompanying dxawings in which:
Figure l is a cross sectional view of a voltage dependent resistor in accordance with this invention.
227~
sefore proceeding with a detailed description of the manufacturing process of the vol-tage dependent resistor contemplated by this invention, its construction will be described with reference to the single figure wherein reference 10 designates, as a whole, a voltage dependent resistor comprising as its active element a sintered body having a pair of electrodes 12 and 13 in ohmic contac-t therewith applied to opposite surfaces thereof. The sintered body ].6 is prepared in a matter h~reinafter set forth and is in any form such as circular, square or rectangular plate :Eorm. Wire leads 18 and 20 are at-tached conductively to electrodes 12 and 14 respectively by connecting means 22 such as solder or the like.
It is known that a metal oxide varistor of the type shown exhibits low varistor voltage when the sintered body portion has a high percentage of relatively large grains of metal oxide and especially zinc oxide. For example, in a zinc oxide varistor, a grain size in the range o:E from 100 to 500 microns and preferably 100 to 200 microns is provided, uniformly dispersed in the sintered body. Normally, in the heretofore commonly employed methods for forming zinc oxide varistors, such large grain sizes are not produced.
It has been reported by Eda, et al, that such large grain sizes are formed when seed grains of zinc oxide having a grain size between about 20 and 200 microns are included in the zinc oxide formulation prior to sintering. In addition to the large seed grains required by the process described by Eda, in order to achieve satisfactory leakage characteristics especially at high -temperatures, Eda requires the addition of spinel type polycrystalline Zn7/3Sb2/3O4 to the mixture which is then mixed in a wet mill so as to produce homogeneous mixtures.
It has been discovered in accordance with this invention that the expensive single crystal zinc oxide seed grains heretofore produced either by crushing large zinc oxide single crystals or by dissolving the water soluble intergranular layers from sintered zinc oxide bodies especially formed using grain growth enhancing additives to produce large grains, are not required~and that substantially easier to manufacture nuclei may be employed to provide equally satisEactory and in many ways superior results. Further, the necessity for separately adding spinel type poly-crystalline Zn7/3Sb2/3O4 is elimina-ted by applicant's use oE tetragonal-antimony-zinc-oxide (TAZO) nuclei.
In accordance with a presently preferred embodiment of this invention, a low voltage varistor is formed by combining zinc oxide powder as a major constituent with one or more additives which produce the desired nonlinear characteris-tics and between .1 and 20% by weiyht of TAZO
nuclei for promoting the growth of large zinc oxide grains in the device and producing a low varistor voltage as well as low high temperature leakage current characteristics.
Preferably the TAZO nuclei have a particle size several times that of the zinc oxide powder so, for example, nuclei of 1 micron size or greater axe preferred.
The preparation of the TAZO nuclei relies upon the chemical reaction between antimony trioxide and zinc oxide according to the following chemical equation:
Sb203 + Zno --~ZnSb20~
the reaction product being in tetragonal form. The TAZO
nuclei are prepared by combing particulate zinc oxide wlth particulate antimony oxide and thoroughly mixing to insure even distribution. The mixture is then pressed into large discs or other convenient forms in the same manner tha-t metal oxide varistors are pressed prior to sintering. The discs are then sintered at a temperature between abou-t 500 and 1300C and preferably between 900 and 1200C.
The sintered discs exhibit a yellow-orange core surrounded by a light-colored peripheral layer. The peripheral layer consists primarily of zinc oxide and is thought -to result from the evaporation of antimony ~rom the body during sintering. The peripheral layer is mechanically removed and the core material which is rich in TAZO is pulverized and classified to provide the desired ranye and distribution of particle sizes up to about 50 microns.
Preferably classification through a 325 mesh sieve is performed to provide a nominal particle size of about 6 microns wherein greater than 90% of the particles are below 20 microns.
It has been determined that the form of an-timony zinc oxide produced by reacting zinc oxide and antimony oxide depends upon the relative amount of each material available as well as the amount of oxygen present. Where an excess of zinc oxide and oxygen is presen-t as is the case when the powders are combined and sintered in oxygen or in air, the reaction proceeds as described the following e~uation:
Sb23 + 7ZnO + 2 ~~~ Zn7O12 which equation product is often written in equivalent form as Zn7/3Sb2/3O~ and is in the spinel form.
By pressing the zinc oxide-antimony oxide mixture prior to sintering, the effect of oxygen on the reaction and the evaporation of antimony oxide are red~lced. The tetragonal form of antimony zinc oxide is primarily produced.
The pressing of the zinc oxide and antimony powders may be conveniently performed in the same manner as the varis-tor discs themselves are pressed prior to sinteriny.
Preferably the mixture oE powders is pressed to a density oE at least 3 grams/cc.
In accordance with an exemplary embodiment of this Eirst aspect of the present invention, a relatively low voltage varistor formulation was pressed into discs and sintered at about 1300C for 1 hour. The varistor formulation includes zinc oxide as a major constituent as well as bismuth oxide, cobalt oxide, manganese oxide, titanium oxide, chromium oxide, boron oxide and TAZO. The following characteristics were measured:
V
IL(~A) C~ 1 V/mm 0.6 21 23 18 ~2~
Wherein IL is the leakage current of the varistor,Cc is the alpha ag hereinabove described, V1is -the varistor voltage a-t 1 milliampere and ~/mm is the vol-ts per millimeter of thickness of the device.
In order to appreciate ~he significance of the resul-ts illustrated at Table 1, a varistor was prepared identically wi-th that hereinabove described except the TAZO nuclei were deleted and a like amoun-t of zinc oxide seed grains as described by Eda et al, were substituted therefore with the results presented in Table II.
TAsLE II
IL(~A) ~ Vl V/mm 1.2 17 20 14 It wlll be seen that the ~, Vl and volts per millimeter characteristics of the device including the TAZO nuclei and the device including the zinc oxide seed grains are similar but that the leakage current of the varistor Eormed in accordance with the teachings of this invention is substantially lower than the leakage current of the varistor formed without benefit of this invention. It will be recognized, that the varistor whose characteristics are presented in Table II
did not include the spinel type polycrystalline antimony zinc oxide in the form Zn7/3Sb2/3O4. Eda, et al claims that the addition oE the spinel would have a posi-tive efEect on leaka~e current. The advantage of this inven-tion resides in the elimina-tion of the necessity for preparing and adding the spinel to the varistor mixture in addition to the seed grains thus providing great economy of fabrication.
The amount of TAZO nuclei added to the varistor formulation may vary over a wide range wi-thout departing from the true spiri-t and scope of this invention. I-t has been determined, however, that beyond about 10~ by weight of TAZO, the effect on varistor voltage is reduced substantially. Preferably less than about 5~ by weight of TAZO nuclei are utilized with substantial decrease in ~2~
36-SP-11~4 in varistor voltage and ~he desirable reduction in leakage and varistor voltage may be obtained utilizing substantially less than 1% by weight of TAZO provided uniform distribution in the varistor mixture is achieved.
While this invention has been described in connection with certain presently preferred embodiments -thereof, those skilled in the art will recogni~e tha-t many modifications and changes may be made therein withou-t departing from the true spirit and scope of the invention which accordingly is intended to be defined solely by the appended claims.
Claims (5)
1. An additive for reducing the varistor voltage and the leakage current of zinc oxide varistors consisting of particulate tetragonal-antimony zinc-oxide of the form ZnSb2O4.
2. The additive of claim 1 wherein said particulate antimony zinc oxide has a particle size less than about 50 microns.
3. The additive of claim 1 wherein said particulate antimony zinc oxide has a particle size between about 5 and about 20 microns.
4. A method for making a metal oxide varistor having low voltage and low leakage comprising combining zinc oxide as a major constituent, bismuth oxide and one or more other metal oxides as minor constituents, and a varistor voltage-reducing additive of less than 10 weight percent of particulate tetragonal ZnSb2O4 having a particle size of less than about 50 microns; pressing said combination to form a body; and sintering said body at a temperature of about 1300°C.
5. A metal oxide varistor having low varistor voltage and low leakage comprising zinc oxide as a major constituent, bismuth oxide and at least one other metal oxide as minor constituents, and a varistor voltage-reducing additive of from 0.1 to 10% by weight of particulate tetragonal ZnSb2O4.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000389657A CA1182278A (en) | 1981-11-06 | 1981-11-06 | Varistor with tetragonal antimony zinc oxide additive |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000389657A CA1182278A (en) | 1981-11-06 | 1981-11-06 | Varistor with tetragonal antimony zinc oxide additive |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1182278A true CA1182278A (en) | 1985-02-12 |
Family
ID=4121356
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000389657A Expired CA1182278A (en) | 1981-11-06 | 1981-11-06 | Varistor with tetragonal antimony zinc oxide additive |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1182278A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2857374A1 (en) | 2013-10-02 | 2015-04-08 | Razvojni Center eNem Novi Materiali d.o.o. | Method for manufacturing varistor ceramics and varistors having low leakage current |
-
1981
- 1981-11-06 CA CA000389657A patent/CA1182278A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2857374A1 (en) | 2013-10-02 | 2015-04-08 | Razvojni Center eNem Novi Materiali d.o.o. | Method for manufacturing varistor ceramics and varistors having low leakage current |
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