US3245019A - Voltage dependent resistor - Google Patents

Voltage dependent resistor Download PDF

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US3245019A
US3245019A US406141A US40614164A US3245019A US 3245019 A US3245019 A US 3245019A US 406141 A US406141 A US 406141A US 40614164 A US40614164 A US 40614164A US 3245019 A US3245019 A US 3245019A
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Prior art keywords
resistor
voltage
temperature
resistance value
range
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US406141A
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English (en)
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Heywang Walter
Schofer Rudolf
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Siemens and Halske AG
Siemens AG
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Siemens AG
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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
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01C—RESISTORS
    • H01C1/00—Details
    • H01C1/14—Terminals or tapping points or electrodes specially adapted for resistors; Arrangements of terminals or tapping points or electrodes on resistors
    • H01C1/1406—Terminals or electrodes formed on resistive elements having positive temperature coefficient
    • 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
    • H01C7/02—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 having positive temperature coefficient
    • H01C7/022—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 having positive temperature coefficient mainly consisting of non-metallic substances
    • 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
    • H01C7/04—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 having negative temperature coefficient
    • H01C7/042—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 having negative temperature coefficient mainly consisting of inorganic non-metallic substances
    • H01C7/043—Oxides or oxidic compounds
    • H01C7/045—Perovskites, e.g. titanates
    • 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

Definitions

  • the invention relates to resistors with high positive temperature coetficient of the resistance value over a range of temperatures, having the following features, namely: (a) the resistor is made of a ceramic ferroelectrical material whose Curie temperature, at which the material loses its permanent polarization, lies at least below the upper limit of the operating temperature range, especially at or below the lower limit of the range at which the resistor should have a positive temperature coefiicient of the resistance value, and preferably below 20 C.; (b) the resistor material is made conductive by the inclusion of impurity centers by donor or acceptor (preferably n-conductive whereby the respective spacing E between the energy level of the donor atoms and the conduction band, or between the energy level of the acceptor atoms and the valence band, is smaller, and especially appreciably smaller, than half of the width E of the prohibited zone between valence band and conduction band; and (c) the intrinsic conductivity or another impurity center conductivity of the resistor material not imparted by said donor or accept
  • the present invention is directed to a voltage-dependent ceramic resistor of the type disclosed in said Patent No. 3,027,529, wherein the concentration of the donor or acceptor atoms (n in the interior of the particle and the particle size d are such that the magnitude of the product 61,71 is so much larger or smaller than that magnitude of the product dJ'I at which a maximum value (E for the critical field strength (E would be produced with any surface state density (11 that the actual value of the critical field strength is less than half of said maximum value (see FIG. 1a).
  • the present invention proposes to place the Curie temperature of the resistor material with respect to the operating temperature for which the resistor is designed, so that the value of the resistance is at the corresponding operating voltages to a high degree temperature independent while being strongly voltage dependent.
  • This invention 3,245,019 Patented Apr. 5, 1966 is based on the concept, explained in the above referred to patent that potential walls are formed at the particle borders (see FIG. 3 of such patent) due to surface impurity terms, which are in the presence of sufficiently high field strength, resulting in the resistor material upon connection of the operating voltage, overcome thermally by the electrons flowing in the conducitivity band of the resistor material.
  • FIG. 1 which corresponds to FIG. 3 in said Patent No. 3,027,529, shows the effect of the particle size on the field strength, producing a critical field strength 13,, at which resistance value begins to break down;
  • FIG. 1a shows the critical field strength E plotted against the factors of the particle size
  • FIG.2 shows an example of a resistor in which the invention may be incorporated
  • FIGS. 3 and 4 are performance curves.
  • FIG. 1 corresponding to FIG. 3 of said patent, shows in schematic manner an n-doped titanate and the formation of the previously mentioned potential wall.
  • the vertical line 1 indicates the particle border, for example, of two intersintered barium titanate bodies.
  • Numerals 2 and 2" represent donor terms of two particles I and II lying closely to the lower border 3' and 3" of the conductivity band of the barium titanate.
  • the upper limit of the valence band of those titanates is indicated at 4' and 4".
  • the height of this bend is indicated by (p thus designating the height of the potential wall which is now within the range of the Curie temperature due to the very strong temperature dependence of the dielectric constant (e), and is to a high degree temperature dependent. It rises, for example, in the case of barium titanate at an assumed acceptor density at the particle border, of l0 l0 /cm. and at a donor density of about 10 -10 /cm. from about 0.1 volt at 20 C. to about 1 volt at- 200 C.
  • E g the maximum field strength at which the resistor is operated, must not be appreciably greater, at the particle borders, even at the lowest temperatures, than the quotient formed by the height to of the potential wall divided by the particles size d.
  • E /d is the mean critical field strength 13,; in the volume of the resistance material at which the resistance value caused by the potential wall at the particles boundaries starts to collapse height of the potential wall, see FIG. 1, d length of particle measured in the direction of the working field strength in the volume of the resistance material hereinafter referred to as particle size.
  • the mean critical field strength 15 is given by the formula:
  • n is the surface state density (number of acceptor of donor atoms for unit area of the particle boundaries), m the impurity center concentration (total number of donor or acceptor atoms for unit volume of the particles), s the dielectric constant of a vacuum, and e is the dielectric constant of the resistance material.
  • the surface state density n is a constant and, if (see FIG. 1a of the present application) the magnitude of the mean critical field strength E of the potential wall is plotted against the product n -d for a particular ceramic the curve shown in FIG. 1a is obtained.
  • the particle size d must either be kept below the magnitude which corresponds to half the maximum field strength for a given density of the impurities (11 embedded in the resistance material, or else must be made so large that the product n -d is so much greater than that corresponding to the broken line in FIG. 1a that the critical field strength falls to less than half its maximum value.
  • the particle diameter of the major portion of this ceramic voltage-dependent resistor either less than microns and preferably less than 2 microns, or else greater than 15 microns and preferably between 20 and and 30 microns.
  • FIG. 3 hereof is a graph illustrating the dependence of the resistance of the resistor at very low voltages on the temperature. It will be seen that well below the Curie temperature which is indicated by an arrow 11 the resist ance is at a low value which is even below approximately 100 ohm-cm. At higher temperatures, the resistance rises to nearly ohm-cm. and has therefore, increased by several powers of ten. In order to obtain in such a resistor according to the invention a resistance value which varies with temperature as little as possible, the resistance value of the resistor body measured at a low voltage of the Working voltage range and at low temperatures of the working temperature range should be large compared with the resistance value of the same resistor measured at the same voltage but at the Curie temperature or at lower temperatures. In FIG.
  • 3 arrow b indicates the lower limit of the working temperature range at which a resistor according to the invention in which the ferroelectric substance is barium titanate operates as a voltage-dependent resistor and has only a small or no temperature dependence.
  • This temperature b is between 50 and 100 above the Curie temperature indicated by the arrow a. If, therefore, the lowest working temperature at which the resistor is required to serve as a varistor independent of temperature lies near room temperature (approximately 20 C.) it is advisable to use as starting material a material the Curie temperature of which lies below 20 C., and preferably below 0.
  • a starting material is advisable whose Curie temperature lies more than 100 below the lowest limit of the working temperature range; some temperature dependence of the resistance value must then be tolerated, see for instance FIG. 3 in which above approximately 300 C. the resistance begins to drop again in dependence on the temperature. If, however, as high a temperature in dependence as possible is of importance, it is advisable to operate the resistor on which FIG. 3 is based bebetween the temperatures b and 0 (approximately between 180 and 340 0.). It is, therefore, advisable to choose the material of the resistor for a given working temperature range to be such that the maximum resistance value measured at low voltages of the working voltage range lies within the working temperature range. In the case illustrated in FIG. 3, the maximum resistance value lies at point M, that is at a temperature at approximately 250 C. which is between the limits b and c of the working temperature range.
  • FIG. 4 is a graph illustrating the dependence of the resistance of the resistor, the R-T characteristic of which is illustrated in FIG. 3 against the field strength in the resistor for various temperatures.
  • the resistance is strongly voltage-dependent at field strengths above approximately 10 v./cm.
  • FIGS. 3 and 4 are based on a resistor of the type specified, the starting material of which is pure barium titanate. Consequently the Curie temperature lies at approximately C. and the working temperature at which this resistor is to be operated as a varistor, that is with a strong voltage dependence but with a small temperature dependence, lies above approximately C. It is possible to shift the Curie temperature and, therefore, the steeply rising branch of the resistance curve to corresponding low temperatures in those cases in which other working temperature ranges are desirable such as, for instance, a temperature range between 0 and 100 C. For this purpose a different ferro-electric material with a considerably lower Curie point may be used as a starting material for a resistor according to the invention.
  • a barium strontium titanate may be used in which the proportion of the strontium in the titanate amounts to 30 to 50 mol percent or more of the titanium in the titanate.
  • the Curie temperature of the starting material may lie at or below 0 C., for instance at approximately -20 C., for a working temperature range extending from approximately 0 C. or room temperature up to approximately 100 C. As mentioned above, it is, however, advisable to locate the Curie temperature for this working temperature range still lower, in particular at between 50 and -l00 C.
  • the resistance material should, therefore, be sintered from particles which are in general smaller than 5 microns and preferably smaller than 2 microns.
  • the critical field strength E is inversely proportional to the abscissa d-n in the right hand portion of FIG. 1a. The curve extends therefore hyperbolically to the right of the broken line in FIG. 1a.
  • the particle diameter d Since the magnitude n frequently cannot be made as small as desired the particle diameter d must often be made undesirably small in order to arrive at sufliciently low value B In these cases it is advantageous to make the particle diameter particularly large, that is to make it so large that the product n -d is greater than that corresponding to the broken line in FIG. 1a.
  • the critical field strength E; at which the resistance of the ceramic body starts to collapse and its resistance value becomes, therefore, strongly voltage-dependent is then achieved at relatively low values of the voltage applied to the resistor.
  • the particles preferably are to be used which are as uniform as possible so that the particle sizes of the major portion of the ceramic resistance material deviate relatively little, that is by only approximately :L-20 to 25% from their mean value in this body.
  • the invention may be applied in the construction of resistors of the type as shown, for example, in FIGS. 1 and 2 of said patent, one such embodiment being shown herein in FIG. 2.
  • the contacts are placed upon the resistor body substantially free of barrier or blocking layer, especially vaporized thereon, so as to reduce the voltage dependence of the total resistance value of the resistor or to make it negligibly low at room temperature.
  • the material for the contacts is for this purpose selected so that it does not form a barrier or blocking layer with the resistor material; more particularly, in the case of aresistor sintered of ferroelectric crystallite particles made n-conductive, the current supply contacts will consist of a base metal, preferably aluminum or zinc or of an alloy containing at least a high propor tion of one of these metals.
  • the surface parts of the ceramic material serving for the contacting are more- :over prior to or if desired incident to the contacting preferably particularly treated as compared with the interior particles of the resistor body, especially mechanically treated, for example, they are made to be 'well conductive by solder rubbing or by sanding or by electrical or chemical treatment applied prior to vaporizing metal thereon.
  • the purpose of this pretreatment is to provide a clean surface free of troublesome terms. This may be obtained, for example, by glow treatment or by chemical reduction.
  • the treatment may be such as to affect not only the surface but penetrating to some depth the marginal layer of the resistor material.
  • the resistor material be sintered at suitable high temperature for a sufiicient time so as to avoid these transition resistances as far as possible.
  • the volume effect may be subsequently reduced by forming operations.
  • a sufliciently voltage-independent resistance may be produced by first sintering the resistor body without regard to the voltage dependence of its resistance, thereupon contacting the resistor body in the manner explained above, and thereafter passing through the resistor a strong current surge which reduces the voltage dependence of the transition resistances between the crystallites and therewith the conductivity of the resistor body to a tolerable point, by effecting in a manner weldingtogether of the individual crystallites.
  • the surface treatment is in a particularly advantageous embodiment carried out by subjecting the surface to a glow effect.
  • the semi-conductor is suitably disposed in a vacuum vessel, at relatively low gas pressure, opposite an electrode and the surface parts of the semi-conductor body which are subsequently to be contacted are brought to a glowing condition by the connection of an alternating voltage or, preferably, a direct voltage, which becomes effective between the resistor body and the electrode.
  • a direct voltage it is advisable to place the semi-conductor on the positive pole of the voltage source.
  • the resistor material may be subjected to a strong glow effect, for example, at a current density from about to 30 milliamperes per square centimeter, for example, at roughly 3000 to 5000 volt, so as to liberate the surface to be metallized also from adhering residual gas or other contaminations such as deposited hydrogen.
  • the contact metal which does not form a blocking layer with the semi-conductor is thereupon applied, advantageously by vaporization in the same vacuum vessel at further reduced gas pressure
  • the con- 6 tact metals to be used in the case of n-doped ferroelec'tric resistor materials are preferably base metals with a normal potential below that of silver, especially below that of copper, for example Al or Zn, which are particularly suitable.
  • the use of noble metals as contact materials is, however, not inherently excluded.
  • the current connections may be mechanically secured on the semi-conductor resistor, for example, in the case of rod-shaped resistors (not shown herein), in the form of caps attached thereto by press-fit or in the form of clasps embracing the resistor body.
  • FIG. 2 shows a disc or wafer-shaped resistor comprising a resistor body 1 with a width L.
  • the resistor body 1' is made of sintered ferroelectric crystallites. Its opposite sides 11 and 12 are pro-treated as descnibed before, for example, chemically or electrically to remove surface terms.
  • base metal for example, aluminum 13, 14 is placed upon the resistor body 1, preferably by vaporizing. These vaporized layers or coatings 13, 14 form with the surfaces 11, 12 of the ceramic resistor body 1 very good and substantially barrier-free contact.
  • the base metal coatings are thereupon provided, chemically or electrochemically, With solderable material, for example, silver, forming thin reinforcing layers 15, 16, to which are soldered disc shaped portions 171, 181 carried by the respective current terminals 17 and 18, numerals 19 and 20 indicating the resulting solder pearls.
  • solderable material for example, silver
  • Ceramic resistor exhibiting strong voltage dependence of its resistance value, said resistor being made of ferroelectric-al material having a Curie temperature, above which the material loses its permanent polarization, lying below the lower limit of the range at which the resistor shall have said positive temperature coefl'icient of the resistance value, said resistor having conductive impurity content with a spacing of the donors from the conduction band and of the acceptors from the valence band which is smaller than one-half of the width of the prohibited zone between the valence band and the conduction band, the intrinsic conductivity of the resistor material being, at least within the range of the positive temperature coeificient, small as compared with the conductivity of said impurity content centers, the resistance value of said resistor, measured at a low voltage of the voltage range and at low temperatures of the operating temperature range, being high as compared with the resistance value measured at identical voltage but at a temperature not exceeding the Curie temperature of the basic resistor material, the concentration of the impurity atoms (n in the interior of the particles and the
  • a resistor according to claim 1 wherein the Curie temperature lies at about 20 C. to 0 C.
  • a resistor according to claim 1 wherein the diameter of the particles of the resistor material, measured in the direction of the operating field strength and the main part of the resistance material, is smaller than 2 microns.
  • a resistor according to claim 1 wherein the diameter of the particles of the resistor material, measured in the direction of the operating field strength and the main part of the resistance material exceeds 15 microns.
  • a resistor according to claim 1, wherein the diameter of the particles of the resistor material, measured in the direction of the operating field strength and the main part of the resistance material is from about 20 microns to about 30 microns.
  • a resistor according to claim 1 wherein theresistor body is disk shaped, contacts of base metal disposed barrier-free on said body, said contacts reinforced by layers of solderable metal disposed thereon.

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  • Engineering & Computer Science (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Ceramic Engineering (AREA)
  • Chemical & Material Sciences (AREA)
  • Materials Engineering (AREA)
  • Thermistors And Varistors (AREA)
US406141A 1958-06-04 1964-10-23 Voltage dependent resistor Expired - Lifetime US3245019A (en)

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CH (1) CH380807A (de)
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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3359521A (en) * 1965-10-26 1967-12-19 Cognitronics Corp Bistable resistance memory device
US4417227A (en) * 1980-05-24 1983-11-22 U.S. Philips Corporation Voltage-dependent resistor and method of producing such a resistor
US4635026A (en) * 1983-09-09 1987-01-06 Tdk Corporation PTC resistor device

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3544647A1 (de) * 1985-12-27 1987-06-19 Gen Electric Fehlerstromschalter

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2864713A (en) * 1955-09-09 1958-12-16 Gen Electric Co Ltd Ceramic dielectric compositions
US2911370A (en) * 1959-11-03 Time after polarization

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2911370A (en) * 1959-11-03 Time after polarization
US2864713A (en) * 1955-09-09 1958-12-16 Gen Electric Co Ltd Ceramic dielectric compositions

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3359521A (en) * 1965-10-26 1967-12-19 Cognitronics Corp Bistable resistance memory device
US4417227A (en) * 1980-05-24 1983-11-22 U.S. Philips Corporation Voltage-dependent resistor and method of producing such a resistor
US4635026A (en) * 1983-09-09 1987-01-06 Tdk Corporation PTC resistor device

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NL6605449A (de) 1966-07-25
FR1226309A (fr) 1960-07-11
CH380807A (de) 1964-08-15

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