EP0074177A2 - Varistance en oxyde métallique à tension de claquage et capacité contrôlables - Google Patents

Varistance en oxyde métallique à tension de claquage et capacité contrôlables Download PDF

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Publication number
EP0074177A2
EP0074177A2 EP82304111A EP82304111A EP0074177A2 EP 0074177 A2 EP0074177 A2 EP 0074177A2 EP 82304111 A EP82304111 A EP 82304111A EP 82304111 A EP82304111 A EP 82304111A EP 0074177 A2 EP0074177 A2 EP 0074177A2
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EP
European Patent Office
Prior art keywords
varistor
lithium
varistor material
breakdown voltage
capacitance
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP82304111A
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German (de)
English (en)
Other versions
EP0074177A3 (fr
Inventor
Herbert Reynold Philipp
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
General Electric Co
Original Assignee
General Electric Co
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Filing date
Publication date
Application filed by General Electric Co filed Critical General Electric Co
Publication of EP0074177A2 publication Critical patent/EP0074177A2/fr
Publication of EP0074177A3 publication Critical patent/EP0074177A3/fr
Withdrawn legal-status Critical Current

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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

Definitions

  • This invention relates to metal oxide varistors and, in particular, to lithium-doped zinc oxide based varistors with control able breakdown voltage and capacitance.
  • the voltage applied to the varistor is less than the varistor breakdown voltage, only a small leakage current will flow between the electrodes and the device is essentially an insulator having a resistance of many megohms.
  • the varistor resistance drops to low values permitting large currents to flow through the varistor.
  • the current through the varistor varies greatly for small changes in applied voltage so that the voltage across the varistor is effectively limited to a narrow range of values. The voltage limiting or clamping action is enhanced at higher values of a.
  • 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 the maximum input voltage of the system to be protected. Thus, for example, a typical system powered from 170 volts peak voltage (120 volts nns) AC power mains would require the use of a varistor having a breakdown voltage somewhat greater than 170 volts.
  • Varistor device behavior may 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 composition, and is generally undesirable as it may affect varistor performance in surge-protective or switching applications, for example.
  • the variste- is subjected to a continuously applied voltage. Although the applied voltage is lower than the varistor breakdown voltage, an undesirable current, due predominantly to the parasitic capacitance, flows through the varistor. In high frequency circuits this current flow may be large enough to affect normal operation of the circuit.
  • the upper varistors therefore are required to dissipate greater power, resulting in higher operating temperature, inferior stability, and concomittantly shorter useful life due to premature failure.
  • discrete, low dissipation capacitors are connected in parallel with the varistors to achieve a more uniform voltage and power distribution throughout the stacked 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 elements for multiplexing, for example, liquid crystal displays.
  • the parasitic capacitance is also a problem, since it appears in series with the capacitance of the liquid crystal material forming a capacitive voltage divider. A lower electric field than would otherwise be available is thus used to maintain the liquid crystal material in its active state.
  • the varistor capacitance is too high, nonselected elements in the liquid crystal array may be inadvertently activated by pulses applied to the display.
  • Aforementioned U.S. Patent 4,276,578 discloses the inclusion of antimony oxide (Sb 2 0 3 ) in the varistor for the purpose of decreasing intrinsic capacitance.
  • the present invention provides varistors with high breakdown voltage and low capacitance by controlled diffusion of lithium into conventional zinc oxide varistor material.
  • a zinc oxide 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 must be carefully controlled, otherwise the varistor becomes insulating for applied voltages even as high as ten or more times the normal breakdown voltage.
  • Lithium may be diffused into the varistor material by placing a solution containing LiN0 3 or Liz0 on the varistor surface. Solvents such as alcohol or acetone may be air dried while aqueous solutions should be heated in air to remove the water. Following the drying step, lithium surface concentration should not exceed approximately 2 mg/cm 2 .
  • the varistor material is then heated at, for instance, 800°C for approximately one hour.
  • the present invention attempts to provide a metal oxide varistor exhibiting high breakdown voltage and low capacitance characteristics which are both controllable.
  • the invention also attempts to provide a zinc oxide varistor containing diffused lithium and which has high breakdown voltage, low capacitance, and low leakage current.
  • high-resistance surface layers containing lithium and potassium have been produced by diffusion of Li 2 C0 3 or Li 2 O and K 2 CO 3 or K 2 O into zinc oxide varistor materials.
  • the lithium and potassium are diffused into the sides of the varistor disk or rod, for example, while the electrodes are affixed to the flat end portions.
  • the non-linearity of the varistor is unaffected in the undoped varistor material portions, while the doped regions provide a high-resistance.
  • the doped layer has a high resistance, it does not appear to have 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.
  • the quantity of lithium diffused into the varistor material is carefully controlled to preserve the nonohmic voltage characteristics associated with the varistor material. If relatively large amounts of lithium (described hereinafter) are diffused, the varistor material becomes insulating for applied voltages even as high as ten or more times the normal 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 a, increased breakdown voltage, and lower capacitance than that obtained with similar undoped varistor material is realized.
  • lithium may be diffused into any conventional zinc oxide varistor material.
  • varistor materials may conveniently comprise any of the standard compositions employed in fabricating metal oxide varistors by conventional methods.
  • such varistors have zinc oxide (Zn0) as the primary constituent (typically, 90 mole percent or more) and include smaller quantities of other metal oxide additives, such as bismuth oxide (Bi 2 0 3 ), cobalt oxide (Co 2 O 3 ), chromium oxide (Cr 2 O 3 ) as well as other additives which may include additional metal oxides.
  • additives examples include manganese oxide (MnO 2 ), antimony trioxide (Sb 2 O 3 ), silicon dioxide (Si0 2 ), nickel oxide (Ni0), magnesium oxide (M g 0), aluminum nitrate (Al(NO 3 ) 3 9(H 2 O)), tin oxide (Sn0 2 ), titanium oxide (Ti0 2 ), nickel fluoride (NiF 2 ), barium carbonate (BaC03), and boric acid (H 3 BO 3 ).
  • MnO 2 manganese oxide
  • Sb 2 O 3 silicon dioxide
  • Ni0 nickel oxide
  • magnesium oxide M g 0
  • Al(NO 3 ) 3 9(H 2 O) aluminum nitrate
  • titanium oxide Ti0 2
  • NiF 2 nickel fluoride
  • barium carbonate BaC03
  • boric acid H 3 BO 3 BO 3
  • a varistor material suitable for practicing the invention may comprise 0.5 mole percent each of Bi 2 O 3 , Co 2 O 3 , Mn0 2 , and Sn0 2 , 0.1 mole percent each of H 3 BO 3 and BaCO 3 , 1 mole percent Sb 2 O 3 , the remainder being ZnO.
  • the additive elements may be added to the 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 (LiN0 3 ) or lithium oxide (Li 2 0). Solutions using alcohol (such 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 water. Resulting surface concentration of LiN0 3 or Li 2 0 on the varistor should not exceed approximately 2 mg/an 2 . The varistor material is then heated in air at temperatures as high as 1100°C. 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.
  • a suitable paste or a solution of lithium nitrate (LiN0 3 ) or lithium oxide (Li 2 0). Solutions using alcohol (such as, methanol) or acetone may be air dried. If an aqueous solution is used, the varistor is initially heated at a low temperature such
  • lithium penetration is in the order of a few mils, while at 900°C it is on the order of a few millimeters. If sufficient time is allowed, the lithium can be made to completely penetrate the varistor.
  • 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 aforedescribed exemplary composition into which lithium has been diffused by heating in air at 800°C for one hour, and on which surface electrodes were positioned 1 mm apart.
  • Varistor breakdown voltage is indicated on the vertical axis, while corresponding current values.are shown on the horizontal axis.
  • Curves A, B, and C depict varistor characteristics of a lithium-doped varistor surface corresponding to depths of 2, 7.5, and 15 thousandths of an inch, respectively.
  • Curves A, B, and C represent progressively lower lithium concentrations.
  • Curve D depicts the charcteristics of an undoped varistor surface. It will be observed that for curves A, B, and C, capacitance values are 20 pf, 40 pf, and 70 pf, respectively, while breakdown voltages are 840, 410, and 155 volts, respectively. For undoped varistor material the capacitance and breakdown voltage are 100 pf-and 115 volts, resepctively. It is apparent, therefore, that near the varistor surface (Curve A, highest lithium doping), the breakdown voltage is approximately eight times larger and the capacitance approximately five times smaller than the undoped surface (Curve D).
  • the present invention provides a metal oxide based varistor with a controllable breakdown voltage and capacitance. More specifically, the invention provides a zinc oxide varistor containing lithium and which has high breakdown voltage, low capacitance, and low leakage current.

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  • Engineering & Computer Science (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Thermistors And Varistors (AREA)
EP82304111A 1981-08-24 1982-08-04 Varistance en oxyde métallique à tension de claquage et capacité contrôlables Withdrawn EP0074177A3 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US29590181A 1981-08-24 1981-08-24
US295901 1981-08-24

Publications (2)

Publication Number Publication Date
EP0074177A2 true EP0074177A2 (fr) 1983-03-16
EP0074177A3 EP0074177A3 (fr) 1983-08-31

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ID=23139706

Family Applications (1)

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EP82304111A Withdrawn EP0074177A3 (fr) 1981-08-24 1982-08-04 Varistance en oxyde métallique à tension de claquage et capacité contrôlables

Country Status (4)

Country Link
EP (1) EP0074177A3 (fr)
JP (1) JPS5866304A (fr)
BR (1) BR8204971A (fr)
CA (1) CA1186806A (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1992000593A1 (fr) * 1990-06-28 1992-01-09 Asea Brown Boveri Ab Procede de fabrication d'un varistor d'oxyde metallique a capacite energetique amelioree
DE10350343B4 (de) * 2002-10-29 2016-10-06 Tdk Corp. Chipförmiger Varistor und Verfahren zu dessen Herstellung

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3723175A (en) * 1967-10-09 1973-03-27 Matsushita Electric Industrial 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 (de) * 1972-09-11 1978-03-09 Tokyo Shibaura Electric Co., Ltd., Kawasaki, Kanagawa (Japan) Metalloxid-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 (fr) * 1976-12-10 1978-07-07 Europ Composants Electron Corps ceramique a resistance dependant de la tension appliquee

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1992000593A1 (fr) * 1990-06-28 1992-01-09 Asea Brown Boveri Ab Procede de fabrication d'un varistor d'oxyde metallique a capacite energetique amelioree
DE10350343B4 (de) * 2002-10-29 2016-10-06 Tdk Corp. Chipförmiger Varistor und Verfahren zu dessen Herstellung

Also Published As

Publication number Publication date
JPS5866304A (ja) 1983-04-20
BR8204971A (pt) 1983-08-02
CA1186806A (fr) 1985-05-07
EP0074177A3 (fr) 1983-08-31

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Inventor name: PHILIPP, HERBERT REYNOLD