CA1093701A - Voltage-dependent resistor and method of making the same - Google Patents

Abstract

VOLTAGE-DEPENDENT RESISTOR AND METHOD
OF MAKING THE SAME

Abstract of the Disclosure A voltage-dependent resistor comprising a sintered body comprising ZnO as a major part and additives wherein at least 10 weight percent of the ZnO is composed of ZnO grains having a grain size in the range from 50 to 500 microns; and method of making the same wherein the starting mixture comprises ZnO grains having a grain size in the range from 20 to 200 microns. This voltage-dependent resistor has both a low C-value and a high surge energy withstanding capability. It also has a low leakage current at a high temperature due to the addition of an antimony component as a spinel type poly-crystalline Zn7/3Sb2/3O4.

Description

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This inyention relates -to a voltage-dependent resistor (varistorl having npn-ohmic properties (voltage-dependent pro~
perties) due to the bulk thereof and method of making the same, more particularly to a yoltage-dependent resistor, which is suitable for a surge absorber and a D,C. stabilizer used in low voltage circuits.
Various voltage~dependent resistors have bee~ wiclely used for stabillzation of voltage of electrical cirFults or suppression of abnormally high surge induced in electrical circuits. ~he electrical charactèristics of such voltage-. . .
dependent resistors are expressed by the relation-I = (V/C)n (1) where,V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numerical ::
value greater than l.~ The value of~n lS calculated by the following equation~
n~= ¦loglO~(I2J~ [loglo(v2/vl);] (2) where Vl and~V2 are the voltages at glVen currents Il and I2, ~respectlvely. The~desired value~of C'depends upon the kind of ap,plication to which the resistor is to be put. It is ordinarily deslrable that the value of n;be as large~ as possible since this exponent determines the extent to which~the resistors depart from ohmic~characte.ristics. Conveniently, the n-value defined by Il, I2,'Vl and V2;as shown in equation (2) is expressed by In2 to dlstlngulsh lt from~the n-value calculated by other cur~
rents~or voltages.
There have been known voltage-dependent resistors o~
the bulk type comprising a sintered~body~of zinc oxide with :
30~ additives, as seen in U.S. Patents 3,663,458, May 16, 1972, 3,632,529, January 4, 1972, 3,~634,~3~37, January 11, 1972, 3,598,763, August 10, I971, 3,&82J841, August 8, 1972, 3,642,664, :

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Februar~ 15~ 1972, 3~658~725( ~pril 25~ 1972, 3~687~871~ Au~ust 29, lq72, 3~723,175! March 27~ 1~73! 3,778,743, December 11, 1973,

3,806,765, April 23, 1974, 3,811,103, May 14, 1974, 3,936,396, February 3, 1976, 3,863,193, January 28, 1975, 3~872,582, March 25, 1975 and 3,953,373, April 27, 1976, all of which are assigned to Matsushita Electric Industrial Co., Ltd. These zinc oxide voltage-dependent resistors of the bulk type contain, as addi-tives, one or more combinations of oxides or fluorides of bismuth, cobalt, manganese, barium, boron, berylium, magnesium, calcium, strontium, titanium, antimony, germanium, chromium,;nic]cel, niobium, tantalum, tungsten, uranium, iron, cadmium, aluminum, gallium, indiumj silicon,-tin, lead, lanthanum, praseodymiu~, neodymium and samarium. The C-value thereof may be controlled, primarily by changing the compositions of said sintered body ~ -and the distance between electrodes.~ They have excellent vo1tage-dependent properties for the N-values in a region of current below lOA/cm2. For a current higher than~lOA/cm2, however; the n-value~falls to below 10.
~ Thls defect of these zinc oxide voltage-dependent resistors of bulk type is presumably due mainly to their low n-value for the lower C-value, espécially less than 80 volts.
~In~general, these zinc oxide vo~ltage-dependent resistors of the bulk type, mentioned above, have very low ~-value, i.e. less than 20, when the C-value is lcwer~than 80 volts. The power dissipation for surge energy, however,~ has a relatively low ~; value as compared with that of the conventional silicon carbide vo1tage-depende~t res1stor~ so that the~ohange rate of C-value exceeds e.g. 20 percent ~fter two standard surges of 8x?0 ~sec wave form in a peak~ourrent of 500A/cm2, applied to the zinc oxide voltage-dependent resistors~of the bulk type.

~ 3 _ 3~701 ~ nothex defect Qf these zinc Qxide volta~e-dependent resistors o~ the bulk type is a poor stabillty to D.C~ load, particularly their remarkable decrease of C~value measured even in a current region such as lOmA, a~ter applying a high D. C .
power to the voltage~dependent resistors especially when they have a C-value of less than 80 voltsO This deterioration in the C-value, especially less than 80 volts, is unfavorable e.g. for a voltage stabilizer which requires high accuracy and low loss for low voltage circuits. ~ ~
These defects of these zinc oxide voltage-dependent resistors of bulk type are presumably due mainly to~their low n-value for the lower C~values, especially of less th~an~80~vo1ts.
Th evelopment of the voltage-dependent resistors having a C-value e.g. less than 80 volts has~been strongly d~esired for the appllcatlon~;of~ the low voltage clrOU~Lts~; such~as in~the automoblle industry~and home appliances,;~but~the n-value~of conventional voltage-dependent resistors having lowe~C-values~is too~small to satls~fy~uses~such~as~voltage~s~ab~ zers~and surge~absorbers. ~-~ For~thes~e~reasons~ voltage~dependent reslstors of this type, 20 ~ ~having~a~C-value less than 80 volts,~have~ hardly been used in low~voltagè app1lcatlon.
In order to satisfy these desires, many~improvements were~tried and~, at~he~pr~sent,~s ~b~de~sl~es~are~satisfie~;by the~improvements~shown in U.~S.~Patents~3,962,144, June~8, 1976 and 4,028,277, June 7, 1977,~hoth.of;~which are assigned~to ; ;~
Matsushita Electric~Industria~ Co.~ Ltd, and~which inclùde ~he new,technology ln~compos~itions~and~`fabrlcatlon process o~
res~istor~bodles.~ However, the~;~des~ire~for~voltage-dependent resistors becomes stronger~ espec~ially~in~the~application 30~ of low voltage circuits such as~in~aut.omotive~use~. For this;

purpose, the voltage-dependent ~resistor must~satlsfy the newly~
desired electrical~properties.~ As;~the~ircuit voltage is D.C.

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12 tR 16 yolts in ~utomotiye use and the protection level ~or semiconductor elements is fairly low~ the C-value of voltage-dependent resistor should be smal].er than thak already satisfied by the previous techniques. ~ most important problem is to develop a new voltage~dependent resistor having low C-value below 40 volts, high n-value in the high current region, i.e.
above lOA~cm2 and additionally having a large surge energy withstanding capa~ility of 50 to 150 joules and a high operating temperature up to 150C, that is more speFifically a low : ~ . ., ~: :
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leakage current at a high temperature up to 150C. The latter two requirements are not yet satisfied by the improvements in the previous pa-tentsO
In order to suppress the surge observed in a battery circuit oE an automobile, the so called giant surge, a surge absorber is required to have a surge energy withstanding capability above 50~joules. The voltage-dependent resistor according to the previous patents has a surge energy withstanding capability of about 1 to 25 joules for the low C-values, which cannot satisfy the desired value mentioned above. The ambient temperature of voltage-dependent resistors set in the engine com-partment of an automobile is supposed to be 150C at the maxi ~ .
The voltage-dependent resistors according to the previous patents have a maximum operating temperature of 70C and such temperature lS too low to satisfy the new desire mentioned above. ~Conven-tionally, titanium oxide ~TlO2) or~beryllium oxide (BeO) is used as an additive for obtaining~a voltage-dependent resistor having a low C-value. However, by~the sole technique of using such an additive, the surge energy withstanding capability of the resistor is-poor. ;~
An object of this lnvention is to provide a voltage~
dependent resistor having a low C-value less than 40 volts, a high n-value éven in~a region of current above IOA/Cm2 and a high surge energy withstanding capability of above 50 joules.
Another object of thls~lnvention is to provide a voltage-dependent resistor which has a high operating tem-perature up to 150C~in addition to the above desired properties.
These and other objects of this invention will become apparent upon consideration of the follo~ing detailed des-cription taken together with the accomp~nying drawing in which the single FIGURE is a cross-sectional vlew of a voltage dep-endent resistor in accordance ~ith this invention.

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., Before proceeding with a detailed description of the manufacturing process of the voltage-dependent resistor contem-plated by this invention, its construction will be described ~, with reference to the single FIGURE wherein reference numeral 7 designates, as a whole, a voltage-dependent rësistor com-prising, as its active element, a sintered body having a pair of electrodes 2 and~3 in ohmic contact applied to opposite surfaces thereof The sintered body ]. is prepared in a manner hereinafter set forth and is in any form such as circular, square or rec-tangular plate form. Wire lead5 5 and 6 are attached conductively to the electrodes 2 and 3, respectively, by a connection means

4 such as solder or the like.
It has been discovered aacording to this .invention that a low C-value and a high surge energy withstanding capa-bil~ty, without deterioration of a high n-value due to an ad-ditive component for giving the~sintered body a voltage-dependent property, can be obtained by a voltage-dependent resistor comprlsing~a slnterad body of bulk type, whlch body comprises a zinc oxlde component as a main component and 0.1 20 ~ to 25 mole percent, in total, of an additive component for giving the sintered body a voltage-dependent property, charac~
terized in that the zinc oxide component comprises 10 to 100 wélght~percent of~zinc~oxide grains havlng a grain slae in the range rom 50 to 500 microns ~such zinc oxide gralns heing deined herein as zinc oxide~core grains) uniformly dispersed in the sintered body. It has been also dissovered ;according to thls lnventlon that~zuch a ~oltagé-dependent ~resistor can be made~by a method~comprising: homogeneously mixing zinc oxide grains havlng~:a graln size~o~ 20~to 200 ; 30 microns (such z~inc oxide gralna being deflned herein as zinc oxide seed grains, SG) with a zinc oxide powder and an ad-di~ive componsn for giving the sintered ~ody a voltage-:

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dependent property in an amount that the thus made mixture comprises 0.1 to 25 mole percent of the additive componen*, and that the zinc oxide component composed of the zinc oxide seed grains and the zinc oxide powder comprises 0.1 to 60 weight percent of the zinc oxide seed grains; compressing the thus made mixture into a compressed body; and sintering the thus made compressed body at a temperature of 1100 to 1400C, whereby the zinc oxide seed grains grow to an increased grain size in the range from 50 to 500 microns by taking the zinc oxide powder thereinto.
The thus made grains having an increased grain size are what are defined above as zinc oxide core grains~ The~
growth of the zinc oxide seed grains is caused by the phenomenon that the zinc oxide powder particles having a particle size usually in the range of 0.1 to 2 microns are adsorbed in neigh-boring z1nc oxide seed grains to form zinc oxlde grains having an increased grain size. The zinc oxide powder particles can have a larger particle size than 2 microns, but should be smaller than 20 microns. In order for the seed grain~ to grow~ the zinc oxide seed grains should have a grain size larger than the particle size of the zinc oxide powder particle. As the dlf~
ference between the grain size~of the seed grains and the par-ticle size of the zinc oxide becomes larger, the seed grains can grow more. Further, for the resultant sintered body to have a lowerporosity or a higher density, the zinc oxide powder should preferably have a smaller partlcle size. For this-` reason~ a preferred particle slze of the zinc oxiae powder isbetween 0.1 and 2 microns, more preferably between 0.1 and 1 micron. The grain size of the~seed~grains is measured by using a sieve or mesh. The grain size of the core grains is measured by: cutting the resultant sintered~body by a plane perpen-dicular to both the electrodes to be applied on opposite major ~ 7 -.. . :

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surfaces of the sintered body; and drawing, on the cut sur-face of the sintered body, two tangential lines which are par-allel to the opposite major surfaces of the sintered ~ody and i which respectively pass the points of each cut grain on the cut surface o the sintered body which points are nearest to the opposite major surfaces, respectively, of the sintered body. The grain size of each core grain is the distance between the thus drawn two tangential lines for the core grain.
It is known that a leakage current in a voltage-~
dependent resistor, which should be as small as possible, in creases as the temperature o~ the resistor increases. The operating temperature range of a voltage-dependent resistor~is~
the temperature range in which the leakàge current i 5 not too large to keep the resLstor operable. The maximum operating temperature of a voltage-dependent resistor depends on the composition of sintered body. Generally, the sintered body containing antimony oxide (Sb203) has a smaller leakage current at a high temperature. However, conventionally, the addition of antimony oxide to the sintered body causes a disadvantage in ~ that the C-value is greatly increased thereby. The sintered body of the resistor of this~invention, however, can conta~
antimony oxide to have a high operating temperature without a~great increase of the C-value. ~It is~the;~disoovery according to a further development of this invention that when the a~dition of the antimony component is carried Otlt in the form of a . .. :
compound of spinel type polycrystalline~Zn7/3Sb2/304, the leakage current at a high temperature c~an be more ef~fectively suppressed without an undesired increase of the C-value and without undesirably deterloratlng~the ~surge~energy~withs~anding capability.

The sintered body l oan be prepared by per se well known ceramic techniques. The starting materials oE ZnO, ~- .
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additives and ZnO seed grains with or without the spinel type polycrystalline Zn7/3Sb2/3O4 are mixed in a wet mill so as to produce homogeneous mixtures. The mixtures are dried and pressed in a mold into desired shapes at a pressure from 50 kg/cm2 to 500 kg/cm2. The pressed bodies arè sintered in air at 1100C
to 1400C for 0.5 to 20 hours, and then furnace-cooled to room temperature (about 15C to about 30C). The mixture to be pressed can be admixed with a suitable binder such as water, polyvinyl alcohol, etc. It is advantageous that the sinte~ed body be lapped at the opposite surfaces by abrasive powder such as silicon carbide in a particle size of about 10 to 50 ~ in mean diameter. The sintered bodies are;provided, at the op~
posite surfaces thereof, with electrod s in any available and suitable method such as silver painting, vacuum ~vaporation or .
flame spraying of metal such as Al, Zn~ Sn, etc.
The voltage-dependent propertles are not practically affected by the kind of electrodes l~sed, but are affected by the thickness of the sintered;~bodles. Particularly, the C-value varies in~proportion to the~thickness of the slntered bodies, while the n-value is~almost independent of the thicknes5.
This surely means that the voltage-dependent property is-dile~
to the bulk itself, not to the electrodes.
Lead wires can be attach~d to the electrodes in a per se conventional manner by using conventional solder. It :
is convenient to employ a conductive adhesive comprising silver powder and resin in an organic~ solvent in order to connect the lead wires to the electrodes.~ Voltage-dependent resistors according ~to this invention have~a high stability for the surge ~test~. The n-value does not change~remark~bly after the~
heating cycles, the load life~test,~humidity-test and surge life test. It is advantageous for achievement of high stability with respect to humidity that~the resultant voltage-dependent _ g ~
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resistors be embedded in a humidity proof resin such as epoxy resin and phenol resin in a per se well known manner.
Conventionally, a fine zinc oxide powder in the particle size usually between 0.1 and 2 microns is mixed with proper addi-tives for giving the resultant sintered body a voltage-dependent property, and the thus made mixture is compressed and sintered to make a voltage-dependent resistor. The feature of this inven-tion is that when zinc oxide grains (as seed grains), each of which is composed of or comprises a zinc oxide single crystal or a zinc oxide polycrystal in the grain size between 20 and 200 microns, are substituted for a portion of the fine zinc oxlde powder, the zinc oxide seed grains remarkably~grow to~
have an increased grain size (as zinc oxide~core gralns) by absorbing the fine zinc oxide powder. In the case that the thus made sintered body comprises æinc oxide core grains having a - grain size in the range between 50 and 500 microns in an amount between lO and 100 weight percent on the basis ~f the zinc oxide component composed of the zinc oxide core~grains~and~ the fine zinc oxlde powder, the~sintered body can~have~a desirably low C-value and a desirably~;high~surge~energy withstanding capability.
The zinc oxide seed grains are designated herein by SG, and`~
the other component of the starting mixture composed of the fine zlnc oxide powder and the~additives~(which~may contain a~spinel type polycrystalline powder, SP, ma1nly of Zn7/3Sb2/3O~
as will be described later) is designated herein by base powder, BP.
According to this invention, the preferred amount and grain size of the zinc oxide seed grains are from O.l~to 60 weight percent on the basis of~the~ total æinc oxlde component ~in the sintered body~and from 20 to~200 microns, respectively.
The preferred amount of the additives to be added to the ~: -sintered body and to give the~sintered body a voltage-dependent - 10 -: ~

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property is from 0.1 to 25 mole percent on the basis of the sintered body. Thereby, a sintered body comprising zinc oxide core ~rains having a grain size of 50 to 500 microns in an amount of 10 to 100 weight percent on the basis oF the total zinc oxide component can be obtained.
An example of the method of making a voltage-dependent resistor according to this in~ention will be described herein-below. In the first place, it is necessary to homogeneously mix a starting material containing zinc oxide seed grains.~ For this mixing step, a mixing method which does not pulverize the seed grains is necessarily used. For example, a wet ball mill method using resin balls (each having an iron core in~
which have a low pulverization power, can be used therefor. By .
compressing the thus made homogeneous mixture into a compressed body, and by sintering the compressed~body, and applying elec-trodes to the opposlte ma]or surfaces of the~thus made sintered body, a voltage-dependent resistor can be made. The grain growth rate o~ the~zinc oxide~seed grains ~lS determined malnly by the sintering temperature and the sintering time. When a higher sintering temperature is used,~ the sintering time can be shorter, or vice versa. A preferred sintering temperatu~re~
rom 1100 to 1400C,~and a preferred slntering time is from 0~5~to`20 hours.~ When the sintering temperature is too low, the seed grains cannot yrow to the desired core grains even : :
if the sintering time is very long.~ On the other hand, if the ~ ;
slntering tempeIature is too high,~the grain growth rate does ~not lncrease, and rather the additlve ~omponent may undesirably evaporate and the sintering furnace~may be~damagedO If the slntering time is too short, the grain growth rate of the seed grains is too low, and`the sintered~body may not be~sufficiently unlform. On the other hand, i~ the sintering time is too long, :
~ the grain growth rate of the seed grains does not increase with : :

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an increase in sintering time because the grain growth becomes saturated after a sufficient sintering time.
The grain size of the zlnc oxide seed gralns is pre-ferably between 20 and 200 microns. In a sintered body, the æinc oxide grains grown from zinc oxide particles usually havin~ a particle size of from 0.1 to 2 microns or of at least smaller than 20 mic~rons have a grain size usually between 10 an~
~0 microns. So, the effect of the addition of the zinc oxide seed grains appears with the grain size OI the seed grains of at least 20 microns. On the other~hand, if the grain size of the -seed grains is larger than 200 microns, the~distribution of the zinc oxide grains in the resultant sintered body loses its~
desired uniformity and density, although the C-value can be lowered by using seed grains having a larger grain size. By using seed grains having grain sizes d~istributed within~the , range of 20 to 200 microns, the C-value can be remarkably lowered without deteriorating other properties.~ The reason why the~preferred grain size of the zinc oxide core grains is between 50 and 500 microns is~slmilar to the reason why the preferred grain size of the zinc oxide seed grains is between 20 and 200 microns.
A preferred amount of the zinc oxide seed grains is ~rom 0.1 to 60 weight ~ercent on the~basis~of the total zlnc oxide component. If the amount of the seed grains is too small, -the distribution of the zinc oxide grains in the sintered body becomes undesirably~non~uniform, and the residual voltage ratio ~ ~ -VloA/VlmA, which will be described~later~,~becomes undeslrably high, and the surge~energy wlthstandlng capabillty of the sintered body becomes too low.~ On the cther hand, if the amount 30~ o the seed grains is too l~argel the porosity of the resultant sintered body becomes ~oo high, which~léads to a decrease of the contact areas between adjaaent zinc oxide grains in the sintered ~ - 12 -:

- ` ~033 7~31 body, resulting in an increase of the C-value and of the residual voltage ratio VlOA/Vl A and in the deterioration of the surge energy withstanding capability and of the stability to the ambient humidity. By using such zinc oxide seed grains, the seed grains grow to core grains in an amount of lO to lO0 weight percent on the basis of the total zinc oxide component in the sintered body. If the amount of the zinc oxide core grains is too small, similar disadvantages to those appearing in the case of seed grains having a too small grain size appear, such as too low surge energy withstanding capability.
The zinc oxide seed grains to be used in the method of making a voItage-dependent resistor according to thi~s in-vention can be made by pulverizing zinc oxlde single crystals having a very large crystal size. ~owever~ more preferably, the zinc oxide seed grains are made by the following method.
A zinc oxide powder having a particle size usually in the range ~of 0.1 to 2 microns~is prepared in~the flrst place. To the thus~prepared zinc oxide~powder as~a starting zinc oxlde powderl a grain growth promoting agent selected ~rom amongst barium oxide, ~;2~0 strontium oxide, calcium oxide, sodium oxide, potassium oxide, rubidium oxide, praseodymium oxide, samarium oxide, niob1um~
oxide, tantalum oxide, tungsten oxide, uranium oxide and bismuth ~oxide, is added in~an~amount that the s~tarting zinc oxide powder is 95 to 99.9 mole percent (which may contain cobaIt oxide, manganese oxlde or nickel oxide as wil1 be described later~
and the grain growth~promot~ng dgent is 0.1 to S mole percent.
If the amount of the~grain growth~promoting agent is too small,~the startiny zinc oxide powder particles do not suf-ficiently grow to seed grains, whereas the particle growth rate of the starting zlnc oxide powder to seed grains levels of at a certain amount of the grain growth promoting agent, - and thus an amount thereof exceeding the certaln amount ::
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(5 mole percent) is unnecessary or rather decreases the pro-duction yield rate of the seed grains.
The mixture of the starting zinc oxide powder and the grain growth promoting agent is heated or fired at a temperature preferably between 1100C and 1600C for a time period preferably between 0.5 and 50 hours. If the firing temperature is too low or the firing time ~is too short, the starting zinc oxide powder does not grow to grains having a sufficiently large grain size as seed grains. Qn the other hand, the particle growth leveIs off at a certain temperature (1600C) or at a certain firing time (50 hours), and thus a firing temperature higher than 1600C
and a firing time longer than 50 hours are unnecessary. ~
The desired zinc oxide seed grains can be made by pulverizing the thus made fired mixture and selecting grains in an appropriate grain size range with the ald of a sleve.~ In this case, the zinc oxide seed grains contain the slight a unt of the grain grow~h promoting agent remaining therein. However, more preferrably, a water soluble oxide i5 used, selected from amongst barlum oxide, strontlum oxlde, calcium oxide, sodium ., :
oxide, potassium oxide and rubidium~oxlde in the above described amount, or more preferably in an amount of 0.3 to 0.8 mole~
percent on~the basis of the ~sum~of the starting zinc oxide powder and the grain growth~promotlng agent. ~The most preferred one is barium oxide in view of the~grain growth of the starting zinc oxide powder and its water solubility. When the mixture of the starting zinc~oxide powder and;the water soluble grain growth promotoing agent is oompressed and flredj the graln growth promoting~agent gathers at the ~rain boundaries of the zinc oxlde seed grains in the fired~mixture. So, by immersing the flred mixture in water or further bolling the water, the grain growth promoting agent can be dissolved into the water. That is, the grain growth promoting agent is removed by washing.

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Thereby, the fired mixture is broken at the grain boundaries into separate seed grains.
Since the thus obtained seed grains have a grain size mostly in the range between 20 and 200 microns, the seed grains can be made by such method with a yield rate of nearly 100%.
In this case, if the amount of the water soluble promoting agent is too small, the s~tarting zinc oxide powder does not sufficiently grow, whereas if the amount is too large, it is difficult to completely remove the wa~er soluble grain growth promoting~
agent by washing. The seed grains produced by using and removing the water soluble grain growth promoting agent are better than the seed grains produced by using grain growth promoting agent~
and pulverizing the fired mixture, because the former seed grains are mainly composed of primary seed grains, whereas the latter seed grains often con~ain an agglomerates of plural seed grains and/or broken seed grains, so that the former seed grains cause:a more uniform and homogeneous sintered body having zinc ox~ide core grains of a higher grain sixe in the resultant voltage-dependent resistor.
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By using zinc oxide seed grains in the grain size range between 20 and 200 microns, voltage-dependent resist~ors~
having low C-values can be obtained. The C-value can be varied b~ selecting the grain size dlstribution~of the seed grain in accordance with the deslred use o the voltage-dependent resistors.
When the resistors are used for absorbing so-called giant surges which may appear in automobiles, the C-values are preferably in the range between 10 and 15 volts, and the residual voltaye ratio VlOA/Vlm~ lS preferably low~ For this usel the desired ~grain size~ of the zinc oxide s;eed~grains is in the range hetween - 30 44 and 150 microns, and the amount of the seed grains in this case on the basis of the tota~l zinc oxide component ln the 93~7~L
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resultant sintered body is more preferably from 2 to 15 weight percent.
~ According to the above method, the starting zinc oxide powder need not necessarily be pure. Generally, in voltage-dependent resistors using ~inc oxide sintered bodies, when cobalt oxide, manganese oxide and/or nickel oxide is used as an additive for giv~ing the sintered body a voltage dependent property, such additive is partially dissolved in zinc oxide grains. Such additive can be preliminarily dissolved in the zinc oxide seed grains to form a solid solution by Lncorporating such additive into the starting zlnc oxide p~owder before firing the mixture of the starting zinc oxide powder and~the~
grain growth promoting agent. In this case, the preferred amounts of cobalt oxide, manganese oxide and nickel oxide are 0.1 to 15 mole percent, 0.1 to 5.0 mole percent and 0.1 to 30 mole percent, respectively.

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Preferred;additives~;to be~added to the sintered body ~in conjunction with the~zinc oxide seed grains and the flne zinc oxide powder are known oxides (or known fluorides of some 20~ of~) magnesium, beryllium, calcium, strontium, barium, tltanium, niobium, tantalum,~ chromium, tungsten, uranlum, manganese;,;~ron,~
cobalt, nickel, cadmium, boron, aluminum, gallium, indium, si:icon, germanium, tin, lead, antimony, bismuth, lan~hanum, praseodymium, neodymlum and samarlum.~ However r when the additives among them other than strontium oxide, barium oxide, manganese oxide, ~obalt oxide~and bismu~h o~ide are used, they are desired to be used in con~junction with at least one of ~:
these five oxides, in order to obtàin practically suf~icient ~voltage-dependent properties o~ the resultant resistors.`

According~to a ~urther development of this invention, a voltage-~ependent resistor having a low leakage current even at a high temperature can be obtained. That~is, when the - 16 - ~

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voltage-dependent resistor is used for absorbing giant surges in an automobile, it is required to have not only a low C-value or a low varistor voltage and a low residual voltage ratio V10A/VlmA, but also a high operating temperature such as 150C, i.e. a low leakage current even at a high temperature such as 150C. It is the discovery according to the further development of this invention t~at such low leakage current can b~ attained by adding antimony oxide without undesirably increasing the C-value or the varistor voltage. It is known that the leak.age current at a high temperature can be reduced by the~addition of antimony oxide. (~hen antimony oxide is used, bismuth oxide is usually used at the same time.) However, in a conventio~al~
voltage-dependent resistor, the addition of antimony oxide causes the conventional resistor to increase in its C-value. However, in the voltage-dependent resistor~of this invention which~has ~inc oxide core grains in the grain size from 50 to 500 microns made from~zinc oxide seed grains in the grain si~e from ~0 to ; 200 microns, the addition~o~-antlmony oxide does not ~cause undesired increase of the C-value. This is presumably because the seed grains can yrow to the;~ore grains even in the presence of antimony oxide.
When antimony oxide is used and hence ~ismuth oxide s~used~at the same time, a preferred;~amount of each of antimony ~oxide and bismuth oxide is betw~en 0.1 to 10 mole percent on .
the basis of the resultant sintered body. If the àmount of each of these two oxides is too small, su~ficient effects of the -addltlons thereof do not appear. On the o her hand, if~the amount of antimony oxide is too large, the resultant C-value ~ becomes undesirably hlgh~ the~amount of bismuth~oxide is 30~ to~o large, when plural compressed bodles to be sintered are stacked on each other and sintered in a sintering furnace, the adjacent sintered bodies ar ~likely to be bonded to each other.

:
~ ~ 17 -' , , 3'7~

.. .
It is a further finding according to the further development of this in~ention that when antimony oxide is pre-liminarily mixed with a portion of the fine zinc oxide powder to be mixed with the zinc oxide seed grains and an additive or additives for giving the resultant sintered body volt~ge dependen-t properties, and is heated or fired to form a spinel type polycrystalline Zn7/3Sb2/3O4, and when the thus made spinel compound is pulverized to granules, and the thus made granules are added to the remaining fine zinc oxide powder and the zinc oxide seed grains and the additive or additives, then the more or less undesixed effect of the antimony oxide addition to increase the C-value of the ~resultant voltage-dependent~resi;~or - can more efectively be suppressed. Thereby, a lower C-value and a low leakage current at a high temperature can be attained.
The preferred heating temperature and time for making the spinel compound are between 13Q0C and 1~00Cj and between , ~ ~0. 5~and 10 hours, respectively.; If the heatlng temperature ; ~ and tim are too low an~d too short, respectively, the~desired ~splnel phase ls not su~iciently made, whereas~an excessively hlgher temperature and longer time than 140DC and 10 hours, respec~ively, are simply unnecessary. The~preferred granule~
size of the pulverized spinel compound is between 0.1 and 60 microns. If the granule slze~ls~ too~large,~the~residual voltage ratlo V10A/VlmA becomes undesirably~high and the surge energy withstanding capability becomes undesirably low. On the other handj if the granule size is ~oo small, the effect of the use of the spinel compound ~to suppress the lncrease of the C-value does not appear.

~ ~ In the case where 0~.l to 10 mole percent of antimony oxide~and 0.1 to 10 mole percent of bismuth oxide are used, when at least one member of cobalt oxlde, manganese oxide, ;chromium oxide and nickel oxide is also used in an amount that - 18 ~~
., :, '`` ~lO~37~L
, the amount of antimony oxide is between 99.2 and 7~7 mole percent on the basis of the sum of the antimony oxide and the above-mentioned at least one member, then the resultant voltage-dependent resistor can have better properties as a low C-value resistor for absorbing current surges. In this case, when the antimony oxide and the above-mentioned at least one member are mixed with a portion of the fine zinc oxide powder (to be mixed with the zinc oxide seed grains and an additive or additives for giving the resistor a voltage-dependent property) and heated or sintered to form a sintered po~der mainly of a spinel type polycrystalline compound, then the addition of such sintered powder to the remaining fine zinc oxide powder and the zinc~oxlde~
seed grains and the additive or~additives causes the resultant resistor to have a-lower C-value, a higher surge energy with-standing capability and a higher n-value than in the~case when the spinel type compound lS on1y of Zn7/35b2/3O4. In this case, the heating temperature and time for obtaining the sintered powder mainly of ~he spinel type polycrystalline material are :: :
pr~ferably between 1100 and 1400C and between 0.5 and 20 hours, respectively. If the heating temperature is too low, the sin-tered powder mainly of the splnel type polycrystalline material cannot be ma~e stably. On the other hand, an excessive ~empera-~-~ ture i.e. higher~than 1400C, is simply unnecessary. The granule size or the particle Si2~ of the slntered powder Ithis can be ob-tained by pulverizing the s1ntered mixture~ is preferably between 0.1 and 60 microns for similar reasons to those why the granule ;~size of the slngle use of the spinel~type polycrystalline 2n7/35b2/3O4 is preferably between 0.1 and 60 microns as set forth above. HereIn, the sintered~powder mainly of the spinel ~ type material or the single phase powder of spinel type Zn7/3Sb2/3O4 is designated by spinel powder, SP~

~ 1 9 ~ `

.
,.. . .. .

1~3~

This invention will more readily be understood with reference to the ~ollowing Examples 1 to 12, but these Examples are intended only to illustrate this invention, and are not to be construed to limit the scope of this invention.
(In the Examples, the C-value is designated by a voltage across each sintered body at 1 mA/cm2 of applied current per 1 mm thickness of the si~tered body.) Example 1 Zinc oxide with additives as shown in Table 1 were mixed in an agate mortar for 3 hours. Each of the thus made mixtures was pressed into a mold disc of 40 mm in diameter and 5 mm in thickness under a pressure of 250 kg/cm2 to a co pressed body. Each of the thus compressed bodies was~ sintered in air at 1400C for 10 hours, and then furnace-cooled to room temperature. The sintered body was~crushed into powder by an agate pestle, and then the thus made powders of 44 to 150 microns in diameter from each~body were selected by sieves.
The thus selected powders are designated as SG (seed grains).
On the other hand~ a zinc oxide powder having an 2~0 average particle size of 0.8 micron was mixed with additives as shown in Table 2 in an agate mortar for 3 hours. Each of~the thus made mixtures is designated as BP (basic powder). 10 weight parts of SG was mixed with 90 weight~parts of BP, and the mixture was mixed in~a we~: mlll with resin balls for 24 hours. The mixture was dried and~pressed into a mold~disc of 17~ mm in ~iiameter and 1 to 3 mm in thickness under a pressure of 250 1~g/cm2 into a compressed body. Each of the thus com-press,ed bodies was sintered in air at 1350C for 5 hours, and ~hen furnace-cooled to room temperature, and was then lapped to a suitable thickness. The opposite~major sur~aces of each of the thus sintered and lapped bodies were provided w1th a spray metallized film of alumlnum, as electrodes, by a per se :
:

)937al~

well known technique~
On the other hand, sintered bodies with electrodes similar to those prepared above were prepared, except that in ~s this case no SG was used for comparison.
The measured electrical characteristics of each of the thus made various sintered bodies are shown in Tables 3 and 4. It is apparent fro~ Tables 3 and 4 that the C-value decreases with the addition of SG without appreciably de-grading n-value and the residual voltage ratio, whlch are proper characteristics of BP, and that the energy withstanding capability increases with the addition of SG. ~erein, the residual voltage ratio V10A/V1 A means the ratio of the voltage across the sintered body supplied with the current of 10 A/cm2 to the voltage across the sintered body supplied with the current of 1 mA/cm2. Therefore, it is better Eor a surge absorber to have a smaller residual voltage. ~The surge energy withstanding capability~E mean~ the destruct~on energy of the :
sinte.red bodies with electrodes of 1 cm in diameter when the surge is applied to the sintered hody the varistor voltage ~20 (which means the voltage~across the sintered body when the current of l mA/cm2 is applied;) of which is~adjusted~to 20 volts. It will be readily recognized that the addition of SG~improYes~the C-value~and the energy wlthstanding aapabllity ~without degrading the inherent n-value and residual voltage r atio for each material composition.
Example 2 ~
Zinc oxide~and additives as shown by samples Nos. A
to F ln Table l were~fabricate;d into~the~SG by the same method as that o~ Example 1, except that in this Example 2, SG was made by washing and boiling in~pure~water for 10~hours the sintered bodies produced by using~water s~oluble grain ~rowth promoting agents. Various mixtures of zinc oxide with SG

' .

. , , . - , : :

37~

and additives as shown in Table 2 were fabricated into the sintered bodies with electrodes by the same method as that of Example 1, except that in this Example 2, the SG was made by washing and boiling the sintered bodies in pure water as des-cribed above.
The electrical characteristics of the thus made ` various sintered bo~ies are shown in Table 5. It is apparént from Table 5 that the C-values can be lowered from those in Example 1 by using the SG made by washing and boiling the 1~ sintered bodies in pure water employing water soluble grain growth promoting agents. The n-value and residual voltage ratio change only slightly. The energy withstanding capabllity increases in comparison to the result of Example 1. It can be easil~ understood that the addition of SG made by washing and boiling in pure water the slntered bodles employing water soluble grain growth promoting agents impro~es the C-value and the energy-withstanding capabllity.
: ` .
Example 3 `
.
Zinc oxide and additives as shown by sample Nos. A

and B in Table 1 were fabricated into the SG by the same :
method as that of Example 2. Zinc oxide with SG and additi~es as shown ln Table 2, sample No. 2, were fabricated into thè
sintered bodies~with electrodes~ by the~ same method as that of Example 1, except that the a~ount of SG in this Example 3 was varied from zero to 80 weight percent.

The electrical characteristics of the thus made various sintered bodies are shown in Table 6. It lS apparent rom Table 6 that C-value, residual voltage ratio and the energy withstanding capability~changes with the~amount of the added SG. It can be understood that the addition of SG of less than 0~1 weight percent and more than 60 weight percent-causes an undesired decrease of the energy withstanding ;:

capability and increase of the residual voltage ratio.
Example 4 zinc oxide and additives as shown in Table 1, sample No. A, were fabricated into the SG by the same method as that of Example 2, except that the grain size of SG in this Example 4 was varied from less than 20 microns to more than 200 microns.
Then, zinc oxide and additives as shown in Table 2, sample No. 2, were fabricated into the sintered bodies with electrodes by the same method as that oE Example 1, except that the grain size of the added SG in this Example 4 was ~aried rom less than 20 microns to more than 200 microns.
The electrlcal characteristics of the thus made~
various sintered bodies are shown in Table 7. It is apparent from Table 7 that C-value, the energy withstanding capabiIity and residual voltage ratio change wlth the graln size of the added SG. It can be understood~that the;addition of SG of less than 20 microns and more than 200 ml~crons~are not~pre-. . , ~ ~ferred for obtaining~excellent C-value, energy withstanding .
capability and residual voltage ratio~
Example 5 Zinc oxide and additives in Table 1, sample No.~A
were fabricat~d into the SG by the same methcd~as that of ~ ample 2, except that the amount of the additive in this Example 5 was~varied from zero to 10 mole percent. ~
.
The production yield rate of SG from 20 microns to 20~0 microns~are shown in Table~8.~ It is apparent from Table :: . ~ :
8~that the addition ~f additive of~less than O.l moIe % and the addition of the~additive~oE~more than 5 mole~% cause poor yield rate in production o~ the SG having a grain size useful ~for improving the electxical characteristics~ of ~the resUltant sintered bodies with electrodes~ ~
: .
- 23 - ~

937~.
... .
Example 6 zinc oxide and additiYes in Table 1, sample No. A, were abricated into SG by the sameimethod as that of Example 2, except that the sintering temperature and the sintering time in this Example 6 were varied from 1000C to 1600C and from 0.5 hour to 50 hours, respectively.
The produ~tion yield rate of SG having a grain size in the range from 20 microns to 200 microns ar~ shown in Table 9. It is apparent from Table 9 that the sintering a-t a tem, perature lower than 1100C and for a time period of shorter than 0.5 hour causes a poor production yield rate of SG because of poor growth of the ZnO grains~ The sintering at a temperature~

::
higher than 1600C causes saturation of the grain growth, so that temperatures higher than~1600C~cause little improvement of SG production yield rate in comparison with 1600C. The sintering for a time~perlod shorter than 0.5~hour causes little ~grain growth, resulting in poor~production~yleld rate. On the , other hand~, the slnt;ering for~ atime~period longer than 50 hours causes the saturation of the grain growtll, so that the sintering 20~ time longer~than 50 hours causes llttle improvement in the SG
productlon yield rate in comparison with 50 hours.
Example 7 Zlnc oxidè and additlves in Table 1,~ sampLe No. A, ~; were~fabricated lnto SG by the~same~ method as~that of Example 2.
-~ Zince oxide with SG and additlves as shown in Table 10~ were fabricated into the sintered~bodies with electrodes ~by the same method as~that of~Example l~
The electrical characterlstic~s of the~thus made various slntered bodies are shown in Table~ which shows the leakage ~;~current characterlstlcs~in addition~to the C-valuej n-value, ~residual voltage ratio and surge~energy withstanding capa-bility. Hereln, the leakage~current is a current flowing .. ~

~l~937~J~
.~ .

through the sin-tered body when 80 percent of its varistor voltage (VlmA~ is applied to the sintered body at 150C.
` For attaining high temperature operation, the leakage current is required to be smaller. It is deslred that the leakage current defined herein be smaller than 100 ~A. By comparing the leakage current characteristics of the materials made from BP using Sb2O3 with~those not using Sb2O3, it can be readily understood that the addition of Sb2O3 improves the leakage current characteristics.
Example 8 , Zinc oxide and additives in Table 1, sample No. A, were fabricated into SG by the same method as that of Example 2.
Meanwhile, antimony ox.ide as shown in Table 10, sample Nos. 27 to 31, was mixed with a portion of fine zinc oxide. The ratio of antimony oxide to zinc oxide was 7 to 1 . in molar ratio. The mixed:powders wexe sintered in air at 13:50C for 2 hours, and then f;urnace-cooled to room temperature.

: The sintered powders were crushed by an agate pestle, and then . .
2~ the powders of smaller than 60 microns and larger than 0.1 microns in diameter ~ere selected by sieves. The powders were ~ composed of splnel type polycrystalline ~SP) Z~,7/3Sb2/3O4.

: : ~ The rest of the fine zinc oxide powder and additives - : ` : : ' ' as shown in Table 10, sample Nos. 27 to 3], were mixed with the above prepared SG and SP .(employing the same amount of : ~ .
Sb2O3). Ths thue made mixtures were fabricated into the .
:sint~ered hodies with electrodes by the same method as that o~ Example 1.
`

The measured electrical characteristics of the thus ~: :
- 30 made various sintered bGdies a~re~shown in Table ]2, which ~shows better C-values and energy withsta:nding capabilitiQs than in the case when Sb2O3 is used without any preliminary - 25 - :

.. , . . ' . - - .

~093'7~

preparation of thP splnel type polycrystalline Zn7~3Sb2/304.
It can be understood that by adding the Sb2O3 in the form of Zn7/3Sb2/304, the C-value and energy withstanding capability are improved without deteriorating other electrical properties.
Example 9 Zinc oxide and additives in Table 1, sample No. A, were fabricated into SG by the same method as that of Example ~ .
Meanwhile, antimony oxide (and a portion of zinc ~xide powder for BP) as shown in Table 10, sample No. 28, was fabri~
cated into SP by the same method as that of Example 8, except that the granule slze of the~added SP ln the Example 9 was~
varied from 0.1 to 60 microns.
The rest of the zinc oxide powder and the additives as shown in Table lO, sample No. 28, were mixed with SG and SP, and the thus made mixtures were fabricated into the sin~tered bodies with electrodes by the same method as that o~ E~ample 8.
The elec~rical characteristics of the thus made sintéred bodies are shown in Table~13. It is apparent from :: :: : :
~o Table 13 that the residual voltage ratio becomes undeslrably high by adding the SP ha~ing a granule size larger than 60~
~microns.~ It can be understood~that by adding the SP having a granuIe slze in the~range~from~O.~ to 60 mlcrons,~ the residual voltage ratio is improved without degrading the leakage current characteristics.
Exam~le 10 Zinc oxide and addltives ln Table 1, sample~No~ A, were fabricated into SG by;the same~method as that of Example 2.
Meanwhile, antimony axide~(and~a portion of~zinc oxide powder for BP) as shown in Table 10~, sample N0. 28, wére fabri-cated into SP by the same method as that of Example 8, except that the sintering temperature and time in this Example 10 ~ 26 -, . ~ , .

37~

~, were from 1200C to 1400c and for from 0.5 hour to 10 hours, respectively.
The rest of the zinc oxide powder and the additives in Table 10, sample No. 28, were mixed wi.th SG and SP, and the thus made mixtures were fabricated into the sintered bodies with electrodes by the same method as ~hat of Example 8.
The electrical characteristics of the thus made sinter-ed bodies are shown in Table 14. It is apparent from Table 14 that the SP obtained by being sintered at a temperature lowbr than 1300C or for a time shorter than 0.5 hour causes undesir-ably high C-value and low energy withstanding capability, and that the SP obtained by being sintered at a temperature~higher~ :
than 1400C or for a time period longer than 10 hours does not cause much improvement~of C-value and the energy withstanding capability than by a temperature of 14Q0C or a time period of 10 hours.
Example 11 : :
Zinc oxide additi~es~ln Table 15, sample Nos. A
an~d N to Qj were fabricated into SG:by the same method as that .
~20 of Example 2, except that the additives in this Example 11 were those as shown in Table~15.
Zinc oxide and the additives as shown in Table 2, sample No. 2, and SG~were fabrlaated lnto the~sintered bodies ~;
with electrodes by the same method~as that oE E~mple.l,~except that the additives for SG in~this Example 11 were as ~shown in Table 15.
~: The electrical char~acteristics of the thus made :`sintered bodies are ~shown~in:Table 16, which:shows improvement of n-values in comparison with those of the sintered bodies :made by SG without further~additivés (except barium~oxlde) as shown in Table 15. It can:be understood that the addition of cobalt oxide, manganese oxide or nickel o~ide to SG causes .
- 27 ~

937~

improvement of the n-value without degrading other electrical properties.
Example 12 zinc oxide and additives in Table l, sample No. A, were fabricated into SG by the same method as that of Example 2.
Meanwhile; antimony oxide and a portion of zinc oxide for BP and additives as shown in Table 17 were mixed. The thus made mixtures were fabricated into SP by the same method as that of Example ~, except that the additives for SP in~this Example 12 were those as shown in Table 17.
The rest of the zin~ oxide powder and additlves~a~s shown in Table lO, sample No. 28, were mixed with SG and SP
~composed of the same amount of Sb2O3~and fabricated into the sintered body with electrodes by the~same method as that of Example lj except that the SP~in this~Example 12, was composed of~zinc~oxide, antimony~oxlde and one of cobalt oxide, manganese oxide, nicke1 oxide and chromium oxi~de.
The electrical characteristics of the thus made :
~20 ~sintered bodies are~shown in Table 18, which shows better n- ~
- :
values in comparison with those of the sintered bodies~with~
SP~without a further additive~ of cobalt oxide, manganese oxide nlckel~oxide or chromium ox~de~.~ It~can be understood that the ~further addition o~;cobalt oxide, manganese oxide, chromium oxide or nickel oxide for SP improves the n-value;without de-grading other electrical properties.
While part~icular embodiments~of this lnvention have been shown and described, it wi~lI be~obvIous to those sXilled ~ in thè art that changes and m~difications may be made without ~departing from this invention in its broader~aspects and, there fore, the aim in the appended claims is;to cover all such changes and modifications as fall with-in the true spirit and scope of thi5 invention.
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Table 3 _ .... ~
Sample Electrical Characteristics No of C-value (V) n-value / E (Joule) ____ . : -- --__,, , . _ i 1 ~.23 44 1.6 12 2 118 45 1.5 12 3 133 47 1.5 12 4 35 ` 28 2.0 25 1~ 2.0 22 6 125 25 1.6 12 7 90 35 : 1.6 18 8 151 45 ~ 1.6 6 9 120 45 ~1.6 12 ~ 105 ~ 4o ~ .6 12 ;~11 - 120 ~ 43 ~~1.6 12 12 ;123 43 ~ 1.7 ~ ~ ~12 13 118 ~ 45~ ~: 1.7 ~12 ~14` 123 40~ ~~ 1.6 ~; ~12 ;:15 ~ 115 ~ ~38~ ~: 1.7~ ~ ~ 12 16 ~ ~116 ~ ~39 ~ 1.7: : 12 ~: ~17 121 40 1.6 ~ ; 12 18 126 ~33 ; 1.6 12 19 118~ 30 : 1.5 12 20~ ~ ~120 ~ ~ 31 ~ .5~ ~12 21 124 33~: 1.5 ; 12 ;22 115`~ ~ 40~ ~ .7 ~12 : ~23: 119 ~ ~ 31 ~ ~ 1.7 ~ 12 2~4 ~ ~:~ 121 ~ ~ 35~ ~ :~1.6 ~ 12 ~25~ 121 ;~ :~ 37~ : ~1.6~ : : 12 : ~~ :' ~ ~ ~ : ~ : . - ~

.
2 - : : : :

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: ~:

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t7C~9L
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Table 4 .
Sample Sample - Electrical Characteristics SG No of C-Value (V) n-Value V1oA/V1mA E (Joule) , . . .... . . _ I
1 32 42 1.6 65 2 33 45 1.6 67 3 38 47 1 ~ 6 51 4 ` 21 27 ' 2.0 80 24 19 2.0 75 6 36 2~ 1.6 53 7 30 33 1.7 56 8 39 45 - 1.6 50 9 33 ~41 1.~ 58 38 1.6 63 11 33 43 1~ 6 59 12 32 ~40 ~ 1.7 58 13 29 40 1.7 67 14 29 38 1.7 67 1.7 69 16 27 39 1.8 66 17 33 40 1.6 58 18 33 30 1.7 58 19 3 ~ 30 1.6 63 31 ~ 28 ` 1.5 55 21 32 30 ~ .5 52 22 ~ 27 35 1.7 70 23 27 ~ 31 1.7 63 24 30 ~ ~ 35 1.6 - 61 ~ ~35 1.6 61 ~ . ~: ~ .-'~ : ' L0~93 7C~
~ . ~
Table 4 ~Continued) : - ' . .._ Sample Sample - E~ectrical Characteristics No. of No. of ____ SG BP C-yalue:(V). n-value VloA/~lmA E (Joule) ~'' ... _ .. . . _ .
` 1 36 44 1.6 53 B 2 36 46 1.6 53 18 2.0 70 6 38 24 1.7 54 __ . _ _ _ _ .
. 1 38 40 1.8 54 C 2 37 45. 1.7 53 26 15 2.2 65' 6 40 - ~ 20 1.8 51 ~ :
. : : . .~... ~ ..
1 38 30 1.8 - 55 ~ ~:

: D 2 37 : 35~ 1.7 53 :
27 ~ 16 ;2.1 : 63 :
~ 6 39 - ~ -~20 ~ 1~.8~ ~.~ . :
-: :: : 1 39 ~35 ~ 1.8 : 51 E:: ~2 : 37 ~: 35 ~ 1.8 ~ 53 ~ 20 1.9 63 6 38 . -22 : : 1.8 : 51 . . . ,. .:__. _ . ~
:~ ~ 1 37 :~ 30 I.9 51 : ~ : F 2 ; 33 : 1.~8 ~ 53

5~ ~ 25~ ~ ;21`~ ~ ~2.;2~ ~ 62 : : ~ 6-: 38 ~: 23 1.8 : 54 .
: '. : 1 : ~ 43~ 1.6 55 G 2 36 ~ ~ 4~5; : 1.5 54 24~ ; : ~18 ; - 2.0: 72 : ~ 6 : 37: ~: :21 1.6:~ : 53 :: : ~ ~: - : - : ~ ~ ~
~: ~ ~ ~ 1 39 ~ ~ 40:~:~ ~ 1.7 53 : ~ ~ ~ 40 ~~46~ -: 1.8 51 : : ~ ~ 5 26~ :~ 16 200 ~ 61 ~ _ 6 40 21 _ 1.8 51 -Table 4 (Continued) - -Sample Sample - Electrical Characteristics.
No. o No. of .
; SG BP C-~alue tV) n-value VloA/vlmA E (Joule) . . _ , 1 36 43f 1.7 53 I 2 35 45 1.6 55 23 19 2.1 71

6 39 23 1.7 53 . _ _ . _ 1 36 4~ 1.7 53 J 2 35 43 1.7 53 : : 5 23 19 2.0 67 6 39 : : 21 1.7 52 . _ . 1 35 40 1.6 : 55 ~ K 2 34 ~ 45 1.6 55 : 5 23 16 2.0 60 ~
: 6 40 ~ 23 ~ 1.6. 52 -_ _ _ 1 ~38 : 35 :1.7 51 L 2 ; 3~3 35 1.7 53 : 5 26 ~:~ 16 ~ 2.0 ~3 : 6 38 30 1.7 52 _.. , .
1 33 :46 1.6 57 M 2 40 ~ ~ 46 1.5: 51 ~ ~
:: 23 ~ 2~.0 62 :
6 39 ~ 26 ~ 1.:6 53 . _ _ _ : ' :

- : :~ ; .
~ ~ 35 ~ :
.

:' :

. ~

37 C3~L

~` Table 5 .. __ ._ . ~ . . _ . .... _ . .. ,.. _ Sample Sample Electrical Characteristics No. of No. of _. . . ._ _ _ SB BP C-value (~) n-value VlOA/V1mA E (Joule) i 1 13 41 1.6 101 ,.~ 2 15 46 1.5 96 3 16 46. 1.7 108 ~ 9 25 2.0 12~
18 ` 2.0 120 6 12 25 1.7 108

7 10 34 1.7 120

8 13 46 1.7 103

9 12 46 1.7 ~ 104 ~ ;~
12 3~ l o 7 105 11 12 40 1.6 105 : : A 12 13 40 : 1.6 :lQ5 :
13 12 : ~ 1.7~ ~ :107 14 12 4~1 ~1:.6 101 ~ : 15 12 41~ ~1.7 ~ 103 : : 16 12 4o 1 . 7 101 : ~ 17 13~ ~;;35 1.7 10~
; 18 13 35 1.7 100 :~ ~ ~ 19~ 13 ~ 28 ~ 1.7 105 :
:~ 2~0~ 13~ ~ :~29~ 1.6~; : ~121 . 21 ~ 13 31 1.6 115 . 22 12 ~ 3s 1:. 7 113 :; - ~ ~ 23 12 ~ 29 1.7 107 ~:: 24 13 : ~34 ~ 1.6 ~ 106 ;~ ~ 25 :13 ~36 1.6 ~l05 :

~ : , ~ ~ ~ ~ :
= ' . ~ . :. . : ~, , ,:
: : : :~

: - 36 -7~
~ Table 5 (Continued) _ . . ~
Sample Sample Electrica1 Characteristics No. of No. of _ _ SB BP C-value (y~ n-value VloA/VlmA E (Joule) _ ~ .;, -.-~ _. __ . ...
1 15 44 1.6 90 2 17 45 1.6 90 3 18 , 40 1.5 83 ~ `~ 11 30 2.1 114 12 19 1~9 123 6 13 26 1.7 114 7 12 ~35 1.7 ~ 120 8 13 40 1.6 110 9 ~ 15 ~-41 1.6 ~85 ~33 ~1.6 ~ 88 ~` 11 15 37 ~ 1.6 ~ 93 B 12 15 37 1.7 90 ~13 ~ 14; ~ ~ ~40 ~1.6 ~ ~90 14 ~14~ ; ~ 41 ; 1.7 ~ ~ 91 14~ ~ ~ 30 ~1.7~ 92 16 ~ 14 ~ ~0 1.7~ 93 17 15 ` 31 ~ 1.6~ 90 18 14 ` 35 1.6~ 91 ~; ~ ~~ 19 14 ~ ~30 ~ 1~.5~ ~9'1~
, ~ ~20~ ~ ~ ; ~ ~31~` ~ 6~ ~ ~ 93 21 15 :~30 loS 97 22 ~ 15 ~ ~ 39~ ~ ~1.7 ~9 r i ~ 23 ~14 ~ ~31 ;~ 1.7 ~ ~ ~ 95 ~~ ~ 24 ~ 15 ~~33 ~ 1-7 95 ; 25 ~ ~ 15 ~ ~ 36~ 1.7 90 ~ : ~ ~ . ~
~ ~ ~ : : ~ ~ ~ : : ~
- : ~ ~ ~ ~; _, ..... :::_., .
:
~ 37 ~

.
, . ~ -' ' ' -~: . : . . . .

Table 5 (Continued) ,, ~
Sample Sample - Electrical Characteristics No. of No. of SB BP C-value (y) n-value VlOA/VlmA E (Joule) .__ _ . . .___ _ 1 18 43 1.7 80 2 16 45 1.7 95 3 20 45 1.5 83 4 ~ 13 30 2.1 108 C 5 13 15 2.0 120 6 15 30 1.8 12 7 14 35 1.8 108 . 8 15 40 1.6 ~ 88 .
9 ~ 15 40 1.7 :92 ~ 41 1.7 91 . . .. . _ .
~, 1 17 ~ 39 :1.7 85 D 2 17 45~ :1.6 87 17 : ~:~ 18 ~ ~2~.0 ~ 89 6 : ~15 ~ 26~ ;~1.7:~` : ~90 :
; :1 . ~;17~ ; ~ 41~ ~1.6 ~92 : :~
2 18 43 ; :1.6 ~90 . ~12 ~ ~ 19 ~ 2.~0 ~ 110 6 14 26 1.7 103 ;
: ~ : .: _ ,~ . : ~
1 -~ 15 ~ ~:;~39~ : 1.7 ~ ~93 -~
F ~ ~ ~ 2 :~ 16 ~ ~ ~ 46~ ~ 1~.6 91 :~
~15 ~ 17 l.Y~ ;~ g5 ;;( ~~ ~ ~ ; ~ ~C 1.`7 ; ~ ~1 :

:
: : ~ :

~ - '. : ~ ~ ~ : :
~ - .... ~_ ~ ~ .. _ -:

-:38 - ~
.

.

3~10 gl Table 6 : Sample Sample Amount of : - Electrical Characteristics No. of No. of additiye C-Value n _ E
BP SG to SG ~(V) ~ value ~lOA/VlmA (Joule) . (mol-e %) . . -. . . . . . . .,, . _ . _ _ ~
0.05 63 45 1.9 24 0.1 28 44 1.7 72 1 16 45 1.6 75 2 15 45 1.5 94 A 5 15 46 1.5 95 46 1.5 96.
4S 1.5 95 . 20 16 46 ~ 1.5 78 . 40 17 45 1.5 73 :~ ~0 ~18 45 1.6 63 : .~ ~2 _ 80~_ 25 43 . 2.5 _ ~ 35 0.0569 45 l.g 25 : ~ ~ : 0~.135 ~ :44 1.7~ ~ 65 . 1 19 ~44 1.6 84 . .
~: ~ ~ 2 ;~ ~17~ ~ 45 1.5 86 17 ~ 45 1.5 89 . : I0 17 46 1.6 ~: 90 . : B 15 ~ 17 45 1.6 90 : : : ~: 20 ~ ~~ 18 45 1.6 77 :
~0 19 44 1.6 65 60~20~: 43 1.7 51 ~0 31 : 43 . _ 30 , -:

3 9 _ r ~ : :

" ~937a~
.
.

Table 7 . .
Sample Sample Grain size Electric-al Characteristics No. of No. of (microns) . C-Value n~ V /V E
: _ BP SG . . ........ : (V1 va1ue l~A 1mA (Joule) ,. less than ~5 45 1,5 36 20 to 44 25 46 1.5 78 2 A 44 to 10515 47 1.5 96 105 to 150 11 ~7 1.5 118 150 to 200 9 46 ~ 85 more than 20~ _ _ 7: _ 45 2.~3 _ 42 _ , Table 8~
:
Sample Amount ofYield rate ~ : -:: No. of additive ~o~ SG : ~ :
to SG : (weight percent) : (mole:~) . ~ : :
_ ~ _ : ~ .
0.05 ~ ~45 : : 0~1 ~3 ~: ~` 0. : ~8~

. 5.0 ~ ~ ~:1 : ~ 10.0 ~ 36 ~ :
' : ~ :
~: :

:
- 40 - .
:
..

.~

~ 1937~0gL

Table 9 ...... ... ~. .
.. _ . . _ _ . _ _ , Sample Sintering Sintering Yield rate No. of temperature time of SG
SB (C) ~ lhours) ~weight percent3 ,. , .. .. __ . ... ......
~: 1000C 0.5 23 . lO 39 S0 ~8 A . _ ..... .. .. __ . _ 1100C 0~5 75 ... _ . ~ ~: , _._ -1200C ~ 0.5 ~ 80 ~
; lO ~ 88 ` ;
: 50~ 96 A .. _. ... . . .
- ' 1 1400C I 0.5 ~ ~ 97 _ ~ ~ . .
1600C 0:.5 : : : 97 ; : : ~ ~ :~ : 10 ~ ~ ~: 98 ~: . :~ 50 : 99 . . . . ~. _ .. . .. _ ~ .
~ .
:~:
` ::: ; :

:

:

: .

: - 41~-~: :: ' ~:
- . ' ' :

1~93701 .

.
Table 10 Sample - = Additives (mole percent) _ _ _ No of 3nO Bl2O3 Co2O3 MnO2 Sb203 NiO Cr2O3 SnO2 TiO2 3eO

, 26 96.9 1.0 0.5 0.51.0 0.1 27 96.8 1.0 0.5 0.50.1 1.0 0.1 28 95.9 1.0 0.~ 0.51.0 1.0 0.1 29 93.9 1.0 ` 0.5 0.5 3.0 1.0 0.1 91.9 1.0 0.5 0.55.0; 1.0 0.1 31 86.9 1.0 0.5 0.5 10.0 1.0 Ool 32 99.0 0.5 0.5 33 98.5 0.5 0.5 0.5 34 98.0 0.5 0.5 0.5 0.5 97.0 0.5 0.5 0.5 1.0 0.5 36 97.0 0.5 0.5 0.5 1.0 0.5 37 96.9 0.5 0.5 0.5 0.1 1.0 0.5 38 97.5 0.5 0.5 0.5~ 0.5 0.5 39 97.4 0.5 0.5 0.5 0.1 3.5~ 0.5 98.0 0.5 0.5 0.5 ~ 0.5 41 97.9 0.5 0.5 0.5 0.1 0.5 97.8 0.1 ~ 0.5 0.5 1.0 0.1 43 97~.3 0.1 0.5 0.5 0.5 1.0 0.1 44 ~7.9 10.0 0.5 0.5 1.0 0.1 87.4 10.0 0.5 0.5 0~.5 1.0 0.1 46 99.0 0.5 0.5 ~
~7 98,5 0.5 0 5 0.5 _ _ ,, ~:)937~

,p .
Table 11 . . ... .. ..
_ . . . .. . _ . .
Sample Sample -- Electr;i-cal-Chaxacteristics No of SG C-value n-value ~loA/vlmA (Joule) Current . . . . . . (uA) . _ 26 15 45 1.5 96 385 27 15 45 1.5 96 78 28 I6 46 1.5 96 31 29 23 4~ 1.7 78 32 28 45 1.7 60 30 31 35 45 1.8 51 31 32 12 28 1.8 ~ 108 565 33 : 15 30 ~ 10 8 96 97 34 10 ~34 ~~,7 120 435 l2 34 ~ 1.7 108 ~~75 36 A 9 25~ ~ 2.0 ~ 120 634 37 10 25; ~ ~ 2.0 120 98 38~ 8 23 ~ ; ;2.0 138~ 535 9 9 24~ ~ 2~.0 138 89 ~ ~7 22~ ~ 2.1 ~ 150 516 d l : 8 24; ~ 2.1 138 93 42 13 39 1.7 108 321 ~43 13 ; 43 ~ ~1.7 102 83 44 ~ ~11 ~ ~46~ ~ 1.7 114 313 12 45 1~7 114 78 46 13~~28 ~; ~ 1.9 ~102 ~513 47 ~ 14 ~2~8 ~ ~ 1 9 102 89 _ , , ~~

- 43 - ~
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. . . C~ In ~ ~.
. a~
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~ . ~ ~ N ~ :
u~ ~_ ~ m .:
U ~1 ' ~U ~ o ~ ~

S-i _ - ---------- ~, '~ ~:) ~ ~ :
' ~ ~' ~? ~ , '. h ~1/lLt) u) ~D ~ : ~ ~ :
~` 1~ ~ . ~ .. fd' ~
~, o ~ . ~; ~ tn :~ o ; ~ ' ~: ~ : r-l ~: ~: . ~ ~ ~
, rd: ~. : ~ ~ ~ ~ ~ : ~ .' -: h ~` ~- ~ ~ U ~ ~ : :
`` ` :~ : ~ . : ~ ~ :: ~ h : ::
U ~ ~ ~ : ~ ~ R ~ ~: ~ :~ :
Ln Ln :: ~ ~ ~ I '~ U) L~l Ln ~ ~ ' ~ : ~ ~ , ~
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~: ~ ~; ~ r~ 9: _ _ : ~ ~. ,, :~ C) ~ ~ ~ ~0~ ~ ~: `0 ~
. I _ _ _ = _ . .. ~ .,~ ~ ~ ~ ~: . ` ' I' - ~ :: ~ ~ ~ : ~ ~ ~ o ~D ~ O ~ ~
a) ~ : ~ : : : :: ~ o ~ ~:
-`. : ~1 0 : : ~ : , ~ _ _ ---- :
~ : : ~ :~: ;:; ~ 4~ : :~ , :~z~A : ; ~ ` ~n~z~ ` : ~ ;.

~: ~ Q~ O ~ '~ o ~ ~ , ~ ~ .
~` ~ I~ CO ~ o ,~ ~ p,, ~: ~ P~ ~ ~ ~ : ~ O m: ~ co ::
U~ Zi : : : ~ U~ Z : :
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Id ~ ~ ~ ~1 ~ ~ ~ .`
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h ~ ~I r-l ~1 ,1 ~ ` ~J ~:1 - ~
C) ~' ______ `: ' , '..
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. ~, 'a)~ ~:1 :, . ~ ~
~1 ~ - u) u~ ' G~l ~ ' ~ ~ r : ~ ' 1:: : ~
; ~_ - - --: ~' :
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Q - t~D ~ :
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a) ~
:, ~ ~ ~ LO u~ In : U~ o o; o o o o : _ ~
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i-J ~d ~ o o o O O a O
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:- :
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'- ... _ ~ 3'7Q~
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` Table 15 - :- : : . : :
. .
Sample Addi:ti:ves (mole percent) No of ZnO. BaO. Co.2O3 MnO2. .Nio __ , A 99.5 0.5 N 98.5 0.5Q~5 Q~51.0 O 9~3. 0~50.1 5.00.1 P 84.3 Q~515.0 0.10.1 Q 69.3 0.50.1 0.130.0 .

Table 16 Sample Sample _ Électrical Characteristics : No. of No. of C-value n-value VloA/VlmA E Leakage ¦
SG BP (V) : (~oule) Curre~t .~ A . 16 ~46 ~ 1.5 ~ 96 33 : N 16 ;51 ~ 1.5 102 31 2 16 50~ 1~5 102 32 ~ ~: P 16 ~ 52 1.5 101 33 : . ~ Q 16 50 : l.S 100 33 .
:; ' ' ~:
: ~:: ` :

~ ' -~ 46 ::

_ .. . - - :

~L~)937C~L

` Table 17 ., Sample Additl~es (mole percent) No. o . . . . .
SP ZnO sb23 C23 ~nO2 Nio . Cr23 __~ . __ __ .. _. ... .
1 87.5 12.5 2 35 5 14.~14.428.2 3.0 3 87.412.2 0.10.1 0.1 0.1 4 35 5 60 .

7 35 5 60 .
8 74.351~.8 3.29~3.296.58 1.69 . g . 55.258.6 8.598.5917.18 1.79 ~ _. : .

;~ Table 18 .
. ~.. . . . . .. : .. _ : Sample Sample Sample Electrical Characteristics : No. of No. of No. of- . ~ .
: BP. SG SP C-value n-value V10A/VlmA E Leakag~
~(:V) ~ ; ~ (Joule Current . ~ ~ - _ ~
: 1 ~16 46;~ ~1.5 : 96 31 2 15 51 1.5 113:: 30 ~ :
: 3 ~ 17~ 50 :1,5. 100 32 . 4 ~16 51 105 105 28 : ` 28 A : 5 16 50: 1.5 : 10~ 28 :
6 16 50 ~ ~ 1.5 ~ 104 31 ~ :: 7 16 50~ 1.5 105 30 8 ~:~15~53 1.5 113 28 9 _~15~ 53 1.5 115 23 , - 47 ~

:: : :

. .

Claims (35)

The embodiments of the invention in which exclusive property or privilege is claimed are defined as follows:

1. A voltage-dependent resistor comprising a sin-tered body of the bulk type, which body comprises a zinc oxide component as a main component and 0.1 to 25 mole percent, in total, of an additive component for imparting to the sintered body a voltage-dependent property; 10 to 100 weight percent of said zinc oxide component being zinc oxide core grains having a grain size in the range from 50 to 500 microns uniformly dispersed in said sintered body.

2. A voltage-dependent resistor according to claim 1, wherein said zinc oxide component comprises more than 50 weight percent of said zinc oxide core grains.

3. A voltage-dependent resistor according to claim 1, wherein said zinc oxide core grains have a grain size in the range from 100 to 300 microns.

4. A voltage-dependent resistor according to claim 1, wherein said additive component includes 0.1 to 10 mole percent of antimony oxide and 0.1 to 10 mole percent of bis-muth oxide on the basis of said sintered body.

5. A voltage-dependent resistor according to claim 4, wherein said antimony oxide is present in said sintered body in the form of a spinel type polycrystalline Zn7/3Sb2/3O4.

6. A voltage-dependent resistor according to claim 4, wherein said additive component further includes a member selected from the group consisting of cobalt oxide, manganese oxide, nickel oxide and chromium oxide, wherein the amount of said member is in the range from 0.8 to 92.3 mole percent on the basis of the sum of said member and said antimony oxide, said antimony oxide being present in said sintered body in the form of a spinel type polycrystalline composed of said antimony oxide, said member and a portion of said zinc oxide component.

7. A voltage-dependent resistor according to claim 1, wherein each of said zinc oxide core grains is a solid solution of zinc oxide and a member selected from the group consisting of 0.1 to 15 mole percent of cobalt oxide, 0.1 to 5.0 mole percent of manganese oxide and 0.1 to 30 mole percent of nickel oxide.

8. A voltage-dependent resistor according to claim 1, wherein said zinc oxide core grains are grains grown from zinc oxide seed grains having a grain size in the range from 20 to 200 microns.

9; A voltage-dependent resistor according to claim 8, wherein said zinc oxide seed grains are grains made by firing a pressed mixture of a zinc oxide powder component and 0.1 to 5 mole percent of a grain growth promoting agent selected from the group consisting of barium oxide, strontium oxide , calcium oxide, sodium oxide, potassium oxide, rubidium oxide, praseodymium oxide, samarium oxide, niobium-oxide, tantalum oxide, tungsten oxide, uranium oxide and bismuth oxide.

10. A voltage-dependent resistor according to claim 9, wherein said grain growth promoting agent is a member selected from the group consisting of barium oxide, strontium oxide, cal-cium oxide, sodium oxide, potassium oxide and rubidium oxide, and said grain growth promoting agent being removed from the fired mixture of said grain growth promoting agent and said zinc oxide power component.

11. A voltage-dependent resistor according to claim 10, wherein said grain growth promoting agent is barium oxide.

12. A voltage -dependent resistor according to claim 1, wherein said additive component is a member selected from the group consisting of magnesium oxide, beryllium oxide, calcium oxide, strontium oxide, barium oxide, titanium oxide, niobium oxide, tantalum oxide, chromium oxide, tungsten oxide, uranium oxide, manganese oxide, iron oxide, cobalt oxide, nickel oxide, cadmium oxide, boron oxide, aluminum oxide, gallium oxide, indium oxide, silicon oxide, germanium oxide, tin oxide, lead oxide, antimony oxide, bismuth oxide, lanthanum oxide, praseodymium oxide, neodymium oxide and samarium oxide.

13. A method of making a voltage-dependent resistor comprising a sintered body of the bulk type, said method comprising:
homogeneously mixing zinc oxide seed grains having a grain size of 20 to 200 microns with a zinc oxide powder and an additive component for imparting to the sintered body a voltage-dependent property in an amount that the thus made mixture comprises 0.1 to 25 mole percent of said additive-component, said zinc oxide component comprising said zinc oxide seed grains and said zinc oxide powder comprising 0.1 to 60 weight percent of said zinc oxide seed grains; compressing the thus made mixture into a compressed body; and sintering the thus made compressed body at a temperature of 1100 to 1400°C, whereby said zinc oxide seed grains take said zinc oxide powder thereinto to grow and have an increased grain size in the range from 50 to 500 microns, a voltage-dependent sintered body being made thereby.

14. A method of making a voltage-dependent resistor according to claim 13, wherein said zinc oxide seed grains are made by firing a mixture of 95 to 99.9 mole percent of a starting zinc oxide powder and 0.1 to 5 mole percent of a grain growth promoting agent selected from the group consisting of barium oxide, strontium oxide, calcium oxide, sodium oxide, potassium oxide, rubidium oxide, praseodymium oxide, samarium oxide, niobium oxide, tantalum oxide, tungsten oxide, uranium oxide, and bismuth oxide.

15. A method of making a voltage-dependent resistor according to claim 14, wherein said grain growth promoting agent is one member selected from the group consisting of barium oxide, strontium oxide, calcium oxide, sodium oxide, potassium oxide and rubidium oxide, and said grain growth promoting agent being removed from said fired mixture by washing said fired mixture.

16. A method of making a voltage-dependent resistor according to claim 15, wherein said grain growth promoting agent is barium oxide.

17. A method of making a voltage-dependent resistor according to claim 15, wherein said starting zinc oxide powder to be mixed with said grain growth promoting agent comprises one member selected from the group consisting of 0.1 to 15 mole percent of cobalt oxide, 0.1 to 5.0 mole percent of manganese oxide and 0.:1 to 30 mole percent of nickel oxide, to form a solid solution in each of said zinc oxide seed grains.

18. A method of making a voltage-dependent resistor according to claim 14, wherein said mixture of said starting zinc oxide powder and said grain growth promoting agent is fired at a temperature of 1100 to 1600°C.

19. A method of making a voltage-dependent resistor according to claim 18, wherein said mixture of said starting zinc oxide powder and said grain growth promoting agent is fired for 0.5 to 50 hours.

20. A method of making a voltage-dependent resistor according to claim 13, wherein said sistering is carried out for 0.5 to 20 hours

21. A method of making a voltage-dependent resistor according to claim 13, wherein said zinc oxide seed grains have a grain size of 44 to 150 microns.

22. A method of making a voltage-dependent resistor according to claim 13, wherein the amount of said zinc oxide seed grains in said zinc oxide component is from 2 to 15 weight percent.

23. A method of making a voltage-dependent resistor according to claim 13, wherein said increased grain size of said zinc oxide seed grains is from 100 to 300 microns.

24. A method of making a voltage-dependent resistor according to claim 13, wherein said additive component includes 0.1 to 10 mole percent of antimony oxide and 0.1 to 10 mole percent of bismuth oxide on the basis of said sintered body.

25. A method of making a voltage-dependent resistor according to claim 24, wherein said antimony oxide and a portion of said zinc oxide powder to be added to said zinc oxide seed grains are mixed and heated to form a spinel type polycrystal-line Zn7/3Sb2/3O4, prior to the preparation of said mixture of said zinc oxide seed grains, said zinc oxide powder and said antimony oxide.

26. A method of making a voltage-dependent resistor according to claim 25, wherein the temperature for said heating of said mixture of said antimony oxide and said portion of zinc oxide powder to form Zn7/3Sb2/3O4 is from 1300 to 1400°C.

27. A method of making a voltage dependent resistor according to claim 26, wherein said heating to form Zn7/3Sb2/3O4 is carried out for 0.5 to; 10 hours.

28. A method of making a voltage-dependent resistor according to claim 25, wherein said polycrystalline Zn7/3Sb2/3O4 is crushed to granules having a granule size in the range of 0.1 to 60 microns, prior to the preparation of said mixture to said zinc oxide seed grains, said zinc oxide powder and said antimony.

29. A method of making a voltage-dependent resistor according to claim 24, wherein said additive component includes 0.1 to 10 mole percent, in total, of antimony oxide and one member selected from the group consisting of cobalt oxide, manganese oxide, nickel oxide and chromium oxide in an amount that the amount of said antimony oxide is in the range from 99.2 to 7.7 mole percent on the basis of the sum of said antimony oxide and said one member.

30. A method of making a voltage-dependent resistor according to claim 29, wherein said antimony oxide, said one member and a portion of said zinc oxide powder to be added to said zinc oxide seed grains are mixed and heated to a sintered powder mainly of a spinel type polycrystalline material, prior to the preparation of said mixture of said zinc oxide seed grains, said zinc oxide powder, said antimony and said one member.

31. A method of making a voltage-dependent resistor according to claim 30, wherein the temperature for said heating of said mixture of said antimony oxide, said one member and said portion of said zinc oxide powder to form said spinel type poly-crystalline is from 1100 to 1400°C.

32. A method of making voltage-dependent resistor according to claim 31, wherein said heating to form said spinel type polycrystalline material is carried out for 0.5 to 20 hours

33. A method of making a voltage-dependent resistor according to claim 30, wherein said spinel type polycrystalline material is crushed to granules having a granule size in the range of 0.1 to 60 microns, prior to the preparation of said mixture of said zinc oxide seed grains said zinc oxide powder, said antimony oxide and said one member.

34. A method of making a voltage-dependent resistor according to claim 13, wherein each of said zinc oxide seed grains is a solid solution of zinc oxide and a member selected from the group consisting of 0.1 to 15 mole percent of cobalt oxide, 0.1 to 5.0 mole percent of manganese oxide and 0.1 to 30 mole percent of nickel oxide.

35. A method of making a voltage-dependent resistor according to claim 13, wherein said additive component is a member selected from the group consisting of mangnesium oxide, beryllium oxide, calcium oxide, strontium oxide, barium oxide, titanium oxide, niobium oxide, tantalum oxide, chromium oxide, tungsten oxide, uranium oxide, manganese oxide, iron oxide, cobalt oxide, nickel oxide, cadmium oxide, boron oxide, aluminum oxide, gallium oxide, indium oxide, silicon oxide, germanium oxide, tin oxide, lead oxide, antimony oxide, bismuth oxide, lanthanum oxide, praseodymium oxide, neodymium oxide and samarium oxide.