CA1186806A - Metal oxide varistor with controllable breakdown voltage and capacitance - Google Patents
Metal oxide varistor with controllable breakdown voltage and capacitanceInfo
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- CA1186806A CA1186806A CA000408962A CA408962A CA1186806A CA 1186806 A CA1186806 A CA 1186806A CA 000408962 A CA000408962 A CA 000408962A CA 408962 A CA408962 A CA 408962A CA 1186806 A CA1186806 A CA 1186806A
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- Prior art keywords
- varistor
- lithium
- heating
- varistor material
- capacitance
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Classifications
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01C—RESISTORS
- H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
- H01C7/10—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material voltage responsive, i.e. varistors
- H01C7/105—Varistor cores
- H01C7/108—Metal oxide
- H01C7/112—ZnO type
Abstract
METAL OXIDE VARISTOR WITH CONTROLLABLE BREAKDOWN
VOLTAGE AND CAPACITANCE
Abstract of the Disclosure A metal oxide varistor with controllable breakdown voltage and capacitance characteristics is fabricated by controlled diffusion of lithium into conventional metal oxide varistor material at elevated temperature. The varistor layer containing lithium exhibits an increased breakdown voltage, lowered capacitance, and low leakage current while maintaining a high co-efficient of nonlinearity.
VOLTAGE AND CAPACITANCE
Abstract of the Disclosure A metal oxide varistor with controllable breakdown voltage and capacitance characteristics is fabricated by controlled diffusion of lithium into conventional metal oxide varistor material at elevated temperature. The varistor layer containing lithium exhibits an increased breakdown voltage, lowered capacitance, and low leakage current while maintaining a high co-efficient of nonlinearity.
Description
~ 6~
METAL OXIDE VARISTOR WITH CONTROLLABLE BREAKDO~N
VOLTAG AND CAPACITANCE
Back~round of the Inventicn This invention relates to metal oxide varistors and, in part.icular, to lithium-doped zinc oxide based varistors with controllable breakdown voltage and capacitance.
In general, a metal oxide varistor comprises a zinc oxide (ZnO3 based ceramic semiconduct.or device with a highly nonlinear current~voltage relationshi? ~hich may be represented ~y ~.he equation I = (V/C)a, where V is the voltage between two points separated by the varistor material~ I is the current flowing between the points, C is a constant, and ~ is 2 measure of device nonlinearity. If ~ = 1, the device exhibits ohmic propertiesO For values of ~ greater thar 1 ~typically 20~-50 or more for ~nO based varistors~ the voltage-current character istics are similar to those exhibited by back-to-bark connected Zener diodes. Varistors, however, have much greater voltage, curren~., and energy-handling capabilities. If the voltage applied to t.he varistor is less than the varistor breakdown voltaye, only a small leakage current will flow between the electrodes and the device is essentially an insulaton having a resistance of many megohms. However, if the applied voltage is greater than the varist.or breakdown voltage, the varistof resistance drops to low values permitting large currents to flow through the varistor. Under varistor breakdown conditions, ~ RD 12743 the curren~. throu~h the varistor varies greatly for small chdnges in applied voltage so that the voltage across the varistor is effectively limited to a narrow range of values. The voltage limitiny or clamping action is enhanced dt higher values of ~.
Metal oxide varistors have been widely employed as surge arresters for protecting electrical equipment from transients on AC power lines created by lightning strikes or switching of electrical apparatus. Such applications require the use of varistors having breakdown voltages slightly greater than ~he maximum input voltage of the system to be protected. Thus, for exa~nple, d ty~ical system powered from l70 volts peak voltage (120 volts ~ns) AC power Inains would require the use of a varistor having a breakdo~n Yoltage somewhat greater than 170 volts.
Varistor device behaviorlnay be approximately modeled by a variable resistor in parallel with a capacitor. The parasitic capacitance modeled by the capacitor is an intrinsic property associated with the particular varistor composition9 and is generally undesirable as it may affect varistor performanc2 in surge-protective or swi tching applications, fo r example. In typical surge-arrest;r applications, the varistor is subjected ~o a continuously applied voltage. Although the applied voltage is lower than t.he varistor breakdown voltage, an undesirable current, due prednminantly to the parasitic capacitance~ flows through the varistor. In high ~requency circuits this current flow may be large enouyh to affect. normal operation of the cireuitO
Another capacitance-related problem (described in greater detail in U.S. Patent. 4,276,578, issued to L~M. Levinson, and assigned ~.o the same assignee as the present invention) arises in surge-arrest.er devices made up of stacked metal oxide varistors. In such devices, eaeh varistor in the stack has ~L~ R~ 743 in addition t.o the pdrasitic capacitance assooiated therewith, a coupling cap~citance to ground~ As a result of the combined effect of ~he parasitic an~ ground capacitance, particularly ground capacitance, a larger current flows through the top varistors (t.hose nearesf. the line3 in the stack since these varistors also pass the capacitive ground currents which flow through the lower varistors. The u~per varistors therefore are required to dissipate greater power, resulting in higher operating temperature, inferior stability, and ~oncomittantly shorter useful li~e due ~.o premature failure. In conventional systems, discrete, low dissipation capacitors are connected in parallel with the varistors to achieve a more uniform voltage and power distribution throughout the staoked varistors. Use of capacitors with graded intrinsic capacitances, as described in the aforementioned patent, is a more effective solution.
Varistor elements may also be used as switching elPments forInultiplexing, for example, liquid crystal displaysO In such applications, the parasitic capacitance is also a problem, since it appears in series with the capacitance of the liquid crystal material~ fonning a capacitive voltage divider. A
lower electric field t.han would otherwise be available is thus used to maintain the liquid crystal material in its active state. Additionally, if the varistor capacitance is too high nonselected ele~ents in the liquid crystal array may be inadvertently activated by pulses applied to the displayO
A more detailed description of multiplexing liquid crystal displays usi ng Ya ristors appears in U.S. Patent 4~?23~6o3 issued to D.E. Castleberry and in Canadia~ Serial No. 39.4,621 filed ~an 21/82 - by L.M. Levinson, both assigned to the same assignee as the present invention.
~ 7~3 Fro~ll the ~oregoiny the importance and desirability of reducing varistor capacitance is apparent. Aforementioned U.S. Patent 4,276,578 discloses the inclusion of antimony oxide (Sb203) in the varistor for the purpose of decreasing intrinsic capacitance. The present invention provides varistors with high breakdown volt.age and low capacitance by controlled diffusion of lithium into conventional zinr oxide varistor material.
.~L~
In accordance with the present invention, a zinc oxid~ based varistor exhibiting a high breakdown voltage and low capacitance is fabricated by diffusing lithium into conventional metal oxide varistor material at elevated temperatures. The diffusion of lithium mus~. be carefully controlled, otherwise the varistor becomes insulating for applied voltages even as high as ten or morP
times the normal breakdown voltageO Lithium may be diffused into the varistor materidl by placing a so1ution containing LiN03 or Li20 on the varistor surface. Solvents such as alconol or acet~"e ~ay be air dried while aqueous solutions should be heated in air to remove t.he water. Following the drying stept lithium surface concentration should not exceed approximately 2 mg/cm2O
The varistor material is then heated at9 for instance~ 800C for approximat.ely one hour. Temperatures between 500C and 1100C, however may ~e employedO The penetration of lithiwn into the varistor is determined by the time and temper~ture o~ the diffusion step. GiYen sufficient time, lit.hium may be diffused completely throuyh the varis~.or mat.eriat. For varistors in which lithium diffusion is limited t.o d thin layer on one side of the varistor, conventional surface electrodes may be employed.
~ RD-12743 It is an obJec~. of the invention to provide a metal oxide varistor exhibiting high breakdown voltage and low capacitance~
It is another object of the invention to provide a metal oxide varistor exhibiting controllable breakdown vol~age and capacitance characteristics.
It is still another object of the invention to provide a zinc oxide varistor containing diffused lithium and which has high breakdown vol~.a~e, low capacitance, and low leakage current.
Brie~ Descri~tion of the ~
The features of the invention believed to be novel are set fort.h with particularity in tne appended claimsO The invention itself~ however, both as to its organization and method of operation, together with fur~her obJects and advantages thereof, may best ~e understood by reference to the fallo~ing description taken in conjunction with the acc~npanying drawings in which /the single Figure depicts voltage-current characteristic curves of a metal oxide varistor produced in accordance with the present i nvention.
Det.ailed Deseription of the Invention In the past, high resistance surface layers containing lithium and potassium have been produced by diffusion of Li2C03 or Li~0 and K2C03 or K20 into zinc oxide varistor materials.
The lithium and potassium are diffused into the sides of the varistor disk or rod~ for example, while the eleotrodes are affixed to the flat end portions. In this manner, the non-linearity fo the varistor is unaffeeted in the undoped varistor ~L~3~8~j RD-12743 material portions, while the doped regions provide a high-resistance. Since the doped layer has a high resistance9 it does not appear to havP a nonohmic voltage characteristic, typical of varistor behavior. In Fact, by virtue of its high resistance, the doped layer could aid in avoiding voltage flashover between the electrodes from occurring along the sides of the varistor disk or rod.
In contrast, in accordance with ~he present invention,the quantity of lithium dif-fused into the varistor material is carefully controlled to preserve the nonohmic voltage character-istics associat~d with the varistor material, If rel aki vely large anlounts of lithium (described hereinaFter) are diffused, the varistor material becomes insulating for applied voltages even as high as ten or more times the nonnal breakdown voltage.
Such highly doped varistor materials do not exhibit varistor breakdown conduction. If the applied voltage is increased sufficiently, catastrophic conduction results. For smaller amounts of lithium dopant, however, a varistor having a high ~, increased breakdown voltage, and lower capacitance than that obtained with similar undoped varistor material is realized.
In order to practice the invention, lithium may be diffused into any conventional zinc oxide varistor materialO
Such varistor materials may conveniently comprise any of the standard cwnpositions employed in fabricating metal oxide varistors by conventional methods. Typically9 such varistors have zinc oxide (ZnO) as the primary constituent (typically, 90 mole ~ercent or more) and include smaller quantities of other metal oxide additives, such as bismuth oxide (Bi~03), cobalt oxide ~Co203), chromium oxide (Cr203) as well as other additives which may include additional metal oxides. Examples of such addikives include ~L3L~ 3(~6 RD-12743 mdnyanese oxide (MnO2), antilnony trioxide (Sb203), silicon dioxide (SiO2), nickel oxide (NiO), magnesium oxide ~MgO), aluminum nitrate (Al(N03)3 9(H20)), tin oxide (SnO2), titanium oxide (TiO2), nickel fluoride (NiF2), barium carbonate (BaC03), and boric acid (H3B03). The li5t of additives is not intended to be exhaustive~ nor, generally are all of the above-enumerated materials employed in a single varistor composition.
By way of example, and not limitation, a varistor material suitable for practicing the invention may comprise 0.5 mole percent each of Bi203, Co2039 MnO2, and SnO2, 0.1 mole percent each cf H3~03 and ~aC03, 1 mole percent Sb203, the remainder being ZnO~ The additive elernenrs may be added to ~he unfired varistor mixture as any convenient salt of the additive element since upon sintering these compounds decompose into oxides of the element.
Lithium may be diffused into varistor material by placing thereon a suitable paste or a solution of lithium nitrate (LiN03) or lithium oxide (Li20). Solutions using alcohol (sueh as, methanol) or acetone may be air dried. If an aqueous solution is used, the varistor is initially heated at a low temperature such as 100~C to evaporate the w~terO Resulting surface concentration of LiN03 or Li20 on the varistor should not exceed approximately Z mg/om2. The varistor material is then heated in air at temperatures as high as 1100Co The usual time versus temperature tradeoffs apply and the penetration of lithium into the varistor is determined by the time and temperature of the diffusion step. For a varistor heated for one hour at 600C, lithium penetration is in ~he order of a few mils9 while at 9Q0C it is on the order of. a few millimeters~ If sufficient time is allQwed, the lithium can be made to completely penetratQ the varistor.
~ RD-12743 6~
In applications where attaching electrodes to the opposite sides of the varistor material is inconvenient7 irnpractical, or where it is desired to control electrode separation, electrodes may be attached adjacent to one another on the doped side of the varistor material.
The Figure illustrates voltage-current characteristics of lithium doped and undoped varistor material having the afore-described ex~lnplary composition into which lithium has been diffused by heating in air at 800C for one hour, and on which surface electrodes were positiuned 1 mm apart. Varistor breakdown vol~age is indicated on the ver~ical axis9 while corresponding current values are shown on the horizortal axis. Curves A9 B, and C depict varistor characterilstics oF a lithium-doped varistor surf;ace correspondiny to depths of ~, 7.5, and 15 thousandths of an inch, respectively. In obtaining the voltage-current character-istics at various depths, to illustrate the dependence of breakdown voltage and varistor capacitance on lithium dopant concentration, successive varistor material layers were removed by lapping, electrodes attached, and the varistor characteristics measured.
Curves A, B, and C represent progressively lower lithium concentrations. Curve D depicts the charcteristics o~ an undoped varistor surface. It will be observed that for curves A, B, and C, capacitance values are 20 pf, 40 pf9 and 70 pf, respectively, while breakdown voltages are ~40, 410, and 155 voltsg respectively.
For undoped varistor material the capacitance and breakdown voltage are 1~0 pf and 11~ volts, resepctively. It is apparent, there~ore, tha~ near the varistor surface (Curve A7 highest lithium doping), the breakdown voltage is approximately eight times larger and t.he cdpacitance dpproximately fiYe times smaller than the undoped surface (Curve D)~
~ 36~tj ~D-12743 It is apparent from the foreyoing that the present invention provides a me~al oxide based varistor with a controllable breakdown voltage and capacitance. More specifically, the invention provides d zinc oxide varistor containing lithium and which has high breakdown voltage, low capacitance, and low leakage current.
While certain preferred features of the invention have been shown by way of illusl:ration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
METAL OXIDE VARISTOR WITH CONTROLLABLE BREAKDO~N
VOLTAG AND CAPACITANCE
Back~round of the Inventicn This invention relates to metal oxide varistors and, in part.icular, to lithium-doped zinc oxide based varistors with controllable breakdown voltage and capacitance.
In general, a metal oxide varistor comprises a zinc oxide (ZnO3 based ceramic semiconduct.or device with a highly nonlinear current~voltage relationshi? ~hich may be represented ~y ~.he equation I = (V/C)a, where V is the voltage between two points separated by the varistor material~ I is the current flowing between the points, C is a constant, and ~ is 2 measure of device nonlinearity. If ~ = 1, the device exhibits ohmic propertiesO For values of ~ greater thar 1 ~typically 20~-50 or more for ~nO based varistors~ the voltage-current character istics are similar to those exhibited by back-to-bark connected Zener diodes. Varistors, however, have much greater voltage, curren~., and energy-handling capabilities. If the voltage applied to t.he varistor is less than the varistor breakdown voltaye, only a small leakage current will flow between the electrodes and the device is essentially an insulaton having a resistance of many megohms. However, if the applied voltage is greater than the varist.or breakdown voltage, the varistof resistance drops to low values permitting large currents to flow through the varistor. Under varistor breakdown conditions, ~ RD 12743 the curren~. throu~h the varistor varies greatly for small chdnges in applied voltage so that the voltage across the varistor is effectively limited to a narrow range of values. The voltage limitiny or clamping action is enhanced dt higher values of ~.
Metal oxide varistors have been widely employed as surge arresters for protecting electrical equipment from transients on AC power lines created by lightning strikes or switching of electrical apparatus. Such applications require the use of varistors having breakdown voltages slightly greater than ~he maximum input voltage of the system to be protected. Thus, for exa~nple, d ty~ical system powered from l70 volts peak voltage (120 volts ~ns) AC power Inains would require the use of a varistor having a breakdo~n Yoltage somewhat greater than 170 volts.
Varistor device behaviorlnay be approximately modeled by a variable resistor in parallel with a capacitor. The parasitic capacitance modeled by the capacitor is an intrinsic property associated with the particular varistor composition9 and is generally undesirable as it may affect varistor performanc2 in surge-protective or swi tching applications, fo r example. In typical surge-arrest;r applications, the varistor is subjected ~o a continuously applied voltage. Although the applied voltage is lower than t.he varistor breakdown voltage, an undesirable current, due prednminantly to the parasitic capacitance~ flows through the varistor. In high ~requency circuits this current flow may be large enouyh to affect. normal operation of the cireuitO
Another capacitance-related problem (described in greater detail in U.S. Patent. 4,276,578, issued to L~M. Levinson, and assigned ~.o the same assignee as the present invention) arises in surge-arrest.er devices made up of stacked metal oxide varistors. In such devices, eaeh varistor in the stack has ~L~ R~ 743 in addition t.o the pdrasitic capacitance assooiated therewith, a coupling cap~citance to ground~ As a result of the combined effect of ~he parasitic an~ ground capacitance, particularly ground capacitance, a larger current flows through the top varistors (t.hose nearesf. the line3 in the stack since these varistors also pass the capacitive ground currents which flow through the lower varistors. The u~per varistors therefore are required to dissipate greater power, resulting in higher operating temperature, inferior stability, and ~oncomittantly shorter useful li~e due ~.o premature failure. In conventional systems, discrete, low dissipation capacitors are connected in parallel with the varistors to achieve a more uniform voltage and power distribution throughout the staoked varistors. Use of capacitors with graded intrinsic capacitances, as described in the aforementioned patent, is a more effective solution.
Varistor elements may also be used as switching elPments forInultiplexing, for example, liquid crystal displaysO In such applications, the parasitic capacitance is also a problem, since it appears in series with the capacitance of the liquid crystal material~ fonning a capacitive voltage divider. A
lower electric field t.han would otherwise be available is thus used to maintain the liquid crystal material in its active state. Additionally, if the varistor capacitance is too high nonselected ele~ents in the liquid crystal array may be inadvertently activated by pulses applied to the displayO
A more detailed description of multiplexing liquid crystal displays usi ng Ya ristors appears in U.S. Patent 4~?23~6o3 issued to D.E. Castleberry and in Canadia~ Serial No. 39.4,621 filed ~an 21/82 - by L.M. Levinson, both assigned to the same assignee as the present invention.
~ 7~3 Fro~ll the ~oregoiny the importance and desirability of reducing varistor capacitance is apparent. Aforementioned U.S. Patent 4,276,578 discloses the inclusion of antimony oxide (Sb203) in the varistor for the purpose of decreasing intrinsic capacitance. The present invention provides varistors with high breakdown volt.age and low capacitance by controlled diffusion of lithium into conventional zinr oxide varistor material.
.~L~
In accordance with the present invention, a zinc oxid~ based varistor exhibiting a high breakdown voltage and low capacitance is fabricated by diffusing lithium into conventional metal oxide varistor material at elevated temperatures. The diffusion of lithium mus~. be carefully controlled, otherwise the varistor becomes insulating for applied voltages even as high as ten or morP
times the normal breakdown voltageO Lithium may be diffused into the varistor materidl by placing a so1ution containing LiN03 or Li20 on the varistor surface. Solvents such as alconol or acet~"e ~ay be air dried while aqueous solutions should be heated in air to remove t.he water. Following the drying stept lithium surface concentration should not exceed approximately 2 mg/cm2O
The varistor material is then heated at9 for instance~ 800C for approximat.ely one hour. Temperatures between 500C and 1100C, however may ~e employedO The penetration of lithiwn into the varistor is determined by the time and temper~ture o~ the diffusion step. GiYen sufficient time, lit.hium may be diffused completely throuyh the varis~.or mat.eriat. For varistors in which lithium diffusion is limited t.o d thin layer on one side of the varistor, conventional surface electrodes may be employed.
~ RD-12743 It is an obJec~. of the invention to provide a metal oxide varistor exhibiting high breakdown voltage and low capacitance~
It is another object of the invention to provide a metal oxide varistor exhibiting controllable breakdown vol~age and capacitance characteristics.
It is still another object of the invention to provide a zinc oxide varistor containing diffused lithium and which has high breakdown vol~.a~e, low capacitance, and low leakage current.
Brie~ Descri~tion of the ~
The features of the invention believed to be novel are set fort.h with particularity in tne appended claimsO The invention itself~ however, both as to its organization and method of operation, together with fur~her obJects and advantages thereof, may best ~e understood by reference to the fallo~ing description taken in conjunction with the acc~npanying drawings in which /the single Figure depicts voltage-current characteristic curves of a metal oxide varistor produced in accordance with the present i nvention.
Det.ailed Deseription of the Invention In the past, high resistance surface layers containing lithium and potassium have been produced by diffusion of Li2C03 or Li~0 and K2C03 or K20 into zinc oxide varistor materials.
The lithium and potassium are diffused into the sides of the varistor disk or rod~ for example, while the eleotrodes are affixed to the flat end portions. In this manner, the non-linearity fo the varistor is unaffeeted in the undoped varistor ~L~3~8~j RD-12743 material portions, while the doped regions provide a high-resistance. Since the doped layer has a high resistance9 it does not appear to havP a nonohmic voltage characteristic, typical of varistor behavior. In Fact, by virtue of its high resistance, the doped layer could aid in avoiding voltage flashover between the electrodes from occurring along the sides of the varistor disk or rod.
In contrast, in accordance with ~he present invention,the quantity of lithium dif-fused into the varistor material is carefully controlled to preserve the nonohmic voltage character-istics associat~d with the varistor material, If rel aki vely large anlounts of lithium (described hereinaFter) are diffused, the varistor material becomes insulating for applied voltages even as high as ten or more times the nonnal breakdown voltage.
Such highly doped varistor materials do not exhibit varistor breakdown conduction. If the applied voltage is increased sufficiently, catastrophic conduction results. For smaller amounts of lithium dopant, however, a varistor having a high ~, increased breakdown voltage, and lower capacitance than that obtained with similar undoped varistor material is realized.
In order to practice the invention, lithium may be diffused into any conventional zinc oxide varistor materialO
Such varistor materials may conveniently comprise any of the standard cwnpositions employed in fabricating metal oxide varistors by conventional methods. Typically9 such varistors have zinc oxide (ZnO) as the primary constituent (typically, 90 mole ~ercent or more) and include smaller quantities of other metal oxide additives, such as bismuth oxide (Bi~03), cobalt oxide ~Co203), chromium oxide (Cr203) as well as other additives which may include additional metal oxides. Examples of such addikives include ~L3L~ 3(~6 RD-12743 mdnyanese oxide (MnO2), antilnony trioxide (Sb203), silicon dioxide (SiO2), nickel oxide (NiO), magnesium oxide ~MgO), aluminum nitrate (Al(N03)3 9(H20)), tin oxide (SnO2), titanium oxide (TiO2), nickel fluoride (NiF2), barium carbonate (BaC03), and boric acid (H3B03). The li5t of additives is not intended to be exhaustive~ nor, generally are all of the above-enumerated materials employed in a single varistor composition.
By way of example, and not limitation, a varistor material suitable for practicing the invention may comprise 0.5 mole percent each of Bi203, Co2039 MnO2, and SnO2, 0.1 mole percent each cf H3~03 and ~aC03, 1 mole percent Sb203, the remainder being ZnO~ The additive elernenrs may be added to ~he unfired varistor mixture as any convenient salt of the additive element since upon sintering these compounds decompose into oxides of the element.
Lithium may be diffused into varistor material by placing thereon a suitable paste or a solution of lithium nitrate (LiN03) or lithium oxide (Li20). Solutions using alcohol (sueh as, methanol) or acetone may be air dried. If an aqueous solution is used, the varistor is initially heated at a low temperature such as 100~C to evaporate the w~terO Resulting surface concentration of LiN03 or Li20 on the varistor should not exceed approximately Z mg/om2. The varistor material is then heated in air at temperatures as high as 1100Co The usual time versus temperature tradeoffs apply and the penetration of lithium into the varistor is determined by the time and temperature of the diffusion step. For a varistor heated for one hour at 600C, lithium penetration is in ~he order of a few mils9 while at 9Q0C it is on the order of. a few millimeters~ If sufficient time is allQwed, the lithium can be made to completely penetratQ the varistor.
~ RD-12743 6~
In applications where attaching electrodes to the opposite sides of the varistor material is inconvenient7 irnpractical, or where it is desired to control electrode separation, electrodes may be attached adjacent to one another on the doped side of the varistor material.
The Figure illustrates voltage-current characteristics of lithium doped and undoped varistor material having the afore-described ex~lnplary composition into which lithium has been diffused by heating in air at 800C for one hour, and on which surface electrodes were positiuned 1 mm apart. Varistor breakdown vol~age is indicated on the ver~ical axis9 while corresponding current values are shown on the horizortal axis. Curves A9 B, and C depict varistor characterilstics oF a lithium-doped varistor surf;ace correspondiny to depths of ~, 7.5, and 15 thousandths of an inch, respectively. In obtaining the voltage-current character-istics at various depths, to illustrate the dependence of breakdown voltage and varistor capacitance on lithium dopant concentration, successive varistor material layers were removed by lapping, electrodes attached, and the varistor characteristics measured.
Curves A, B, and C represent progressively lower lithium concentrations. Curve D depicts the charcteristics o~ an undoped varistor surface. It will be observed that for curves A, B, and C, capacitance values are 20 pf, 40 pf9 and 70 pf, respectively, while breakdown voltages are ~40, 410, and 155 voltsg respectively.
For undoped varistor material the capacitance and breakdown voltage are 1~0 pf and 11~ volts, resepctively. It is apparent, there~ore, tha~ near the varistor surface (Curve A7 highest lithium doping), the breakdown voltage is approximately eight times larger and t.he cdpacitance dpproximately fiYe times smaller than the undoped surface (Curve D)~
~ 36~tj ~D-12743 It is apparent from the foreyoing that the present invention provides a me~al oxide based varistor with a controllable breakdown voltage and capacitance. More specifically, the invention provides d zinc oxide varistor containing lithium and which has high breakdown voltage, low capacitance, and low leakage current.
While certain preferred features of the invention have been shown by way of illusl:ration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
Claims (19)
1. A method for controlling the intrinsic capacitance and breakdown voltage of a body of sintered zinc oxide based varistor material, said body possessing at least two substantially planar, parallel surfaces for electrode attachment, said body nonetheless retaining the nonohmic voltage-current properties of said varistor material, said method comprising:
diffusing lithium into the bulk of said varistor material by applying, to at least one of said parallel planar surfaces of said varistor body, a composition containing lithium such that the lithium concentration thereon is less than 2 mg/cm2, and then heating said varistor material at elevated temperatures for a time sufficient to cause diffusion of at least a portion of said lithium into said varistor body, whereby the intrinsic capacitance of said varistor material decreases and the breakdown voltage of said varistor body increases as the concentration of diffused lithium therein increases; and attaching at least one electrode to said planar surface having said lithium composition applied thereto.
diffusing lithium into the bulk of said varistor material by applying, to at least one of said parallel planar surfaces of said varistor body, a composition containing lithium such that the lithium concentration thereon is less than 2 mg/cm2, and then heating said varistor material at elevated temperatures for a time sufficient to cause diffusion of at least a portion of said lithium into said varistor body, whereby the intrinsic capacitance of said varistor material decreases and the breakdown voltage of said varistor body increases as the concentration of diffused lithium therein increases; and attaching at least one electrode to said planar surface having said lithium composition applied thereto.
2. The method of claim 1 wherein said composition comprises a solution of at least one compound selected from the group consisting of LiNO3 and Li2O.
3. The method of claim 2 further comprising the step of evaporating the solvent in said solution prior to said step of heating.
4. The method of claim 1 wherein said step of heating comprises heating said varistor material in air at a temperature of between 500°C and 1100°C.
5. The method of claim 4 wherein said varistor comprises a composition consisting essentially of 0.5 mole percent each of Bi2O3, Co2O3, MnO2, and SnO2, 0.1 mole percent each of H3BO3 and BaCO3, 1 mole percent of Sb2O3.
6. The method of claim 5 wherein said step of heating comprises heating said varistor material at 800°C
for one hour.
for one hour.
7. A method for controlling the intrinsic capacitance of a body of sintered zinc oxide based varistor material, said body possessing at least two substantially planar, parallel surfaces for electrode attachment, said body nonetheless retaining the nonohmic voltage-current properties of said varistor material, said method comprising:
diffusing lithium into the bulk of said varistor material by applying, to at least one of said parallel, planar surfaces of said varistor body, a composition consisting essentially of lithium as the active constituent, and then by heating said varistor material at elevated temperatures for a time sufficient to cause diffusion of at least a portion of said lithium into said varistor body, whereby the intrinsic capacitance of said varistor material decreases as the concentration of diffused lithium therein increases, and attaching at least one electrode to said planar surface having said lithium composition applied thereto.
diffusing lithium into the bulk of said varistor material by applying, to at least one of said parallel, planar surfaces of said varistor body, a composition consisting essentially of lithium as the active constituent, and then by heating said varistor material at elevated temperatures for a time sufficient to cause diffusion of at least a portion of said lithium into said varistor body, whereby the intrinsic capacitance of said varistor material decreases as the concentration of diffused lithium therein increases, and attaching at least one electrode to said planar surface having said lithium composition applied thereto.
8. The method of claim 7 wherein said composition comprises a solution of at least one compound selected from the group consisting of LiNO3 and Li2O.
9. The method of claim 7 further comprising the step of evaporating the solvent in said solution prior to said step of heating.
10. The method of claim 7 wherein the surface concentration of lithium applied to said varistor material is less than 2 mg/cm2.
11. The method of claim 10 wherein said step of heating comprises heating said varistor material at a temperature of between 500°C and 1100°C.
12. The method of claim 11 wherein said varistor comprises a composition consisting essentially of 0.5 mole percent each of Bi2O3, Co2O3, MnO2, and SnO2, 0.1 mole percent each of H3BO3 and BaCO3, 1 mole percent of Sb2O3, the remainder being ZnO.
13. The method of claim 12 wherein said step of heating comprises heating said varistor material at 800°C for one hour.
14. The varistor produced in accordance with claim 1.
15. The varistor produced in accordance with claim 4.
16. The varistor produced in accordance with claim 5.
17. The varistor produced in accordance with claim 7.
18. The varistor produced in accordance with claim 11.
19. The varistor produced in accordance with claim 12.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US29590181A | 1981-08-24 | 1981-08-24 | |
| US295,901 | 1981-08-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1186806A true CA1186806A (en) | 1985-05-07 |
Family
ID=23139706
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000408962A Expired CA1186806A (en) | 1981-08-24 | 1982-08-06 | Metal oxide varistor with controllable breakdown voltage and capacitance |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0074177A3 (en) |
| JP (1) | JPS5866304A (en) |
| BR (1) | BR8204971A (en) |
| CA (1) | CA1186806A (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| SE466826B (en) * | 1990-06-28 | 1992-04-06 | Asea Brown Boveri | MANUFACTURED TO MANUFACTURE A METAL OXID VARISTOR WITH FORBETTRAD ENERGY QUALITY |
| DE10350343B4 (en) * | 2002-10-29 | 2016-10-06 | Tdk Corp. | Chip-shaped varistor and method for its production |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3723175A (en) * | 1967-10-09 | 1973-03-27 | Matsushita Electric Ind Co Ltd | Nonlinear resistors of bulk type |
| CA970476A (en) * | 1971-08-27 | 1975-07-01 | Matsushita Electric Industrial Co., Ltd. | Process for making a voltage dependent resistor |
| DE2345753C3 (en) * | 1972-09-11 | 1978-03-09 | Tokyo Shibaura Electric Co., Ltd., Kawasaki, Kanagawa (Japan) | Metal oxide varistor |
| US4147670A (en) * | 1975-12-04 | 1979-04-03 | Nippon Electric Co., Ltd. | Nonohmic ZnO ceramics including Bi2 O3, CoO, MnO, Sb2 O.sub.3 |
| FR2373497A1 (en) * | 1976-12-10 | 1978-07-07 | Europ Composants Electron | CERAMIC BODY WITH RESISTANCE DEPENDING ON THE TENSION APPLIED |
-
1982
- 1982-08-04 EP EP82304111A patent/EP0074177A3/en not_active Withdrawn
- 1982-08-06 CA CA000408962A patent/CA1186806A/en not_active Expired
- 1982-08-18 JP JP57142243A patent/JPS5866304A/en active Pending
- 1982-08-23 BR BR8204971A patent/BR8204971A/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| BR8204971A (en) | 1983-08-02 |
| JPS5866304A (en) | 1983-04-20 |
| EP0074177A2 (en) | 1983-03-16 |
| EP0074177A3 (en) | 1983-08-31 |
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| MKEC | Expiry (correction) | ||
| MKEX | Expiry |