the method of fabricating a zinc oxide varistor for high voltage according to the present invention will be explained in detail with reference to the fabrication process diagram of fig1 as follows . a mixture of oxides is prepared comprising zinc oxide as the major ingredient , and very small amounts of antimony oxide , cobalt oxide , manganese oxide and chromium oxide , which are additives , in the absence of bismuth oxide . the oxides are added in a predetermined ratio , and are mixed , milled and then dried . the dried mixture is then formed with pressure into a disc of a predetermined thickness , and sintered at a temperature of 1200 ° c .- 1350 ° c . to form a primary sintered body . a series of process steps such as mixing , milling , drying and sieving are then carried out with a metal oxide comprising lead oxide , bismuth oxide and borax . a solvent is then mixed into the mixed powder , to thereby produce a metal oxide paste . this metal oxide paste is then printed on the whole of both surfaces of said primary sintered body , and a secondary heating process is conducted at a temperature of 1000 ° c .- 1200 ° c ., so that the metal oxide printed on the surface of the primary sintered body is diffused to the grain boundaries in the interior of the sintered body . finally , electrodes are attached to the front and rear disc surfaces of a sintered body which is then subjected to a secondary heat treatment . the zinc oxide varistor for high voltage fabricated according to the method of the present invention exhibits good voltage - current characteristics . this is attained by either preventing any abnormal grain growth caused by liquid phase sintering during the fabrication process or by decreasing the volatilization of bismuth oxide , and by diffusing the bismuth oxide to the zinc oxide grain boundaries . moreover , the thus obtained varistor exhibits a high breakdown voltage , 500 - 900 v / mm , which therefore makes it suitable as a high voltage varistor element . the present invention will be explained more clearly through the following examples . all metal oxides , except for bismuth oxide , present in a composition of a zinc oxide varistor are weighed in a composition ratio ( mole ratio ) of 97 . 5 zno + 1 . 0 sb 2 o 3 + 0 . 5 co 3 o 4 + 0 . 5 mno 2 + 0 . 5 cr 2 o 3 . the oxides are then mixed and ground in a ball mill , and formed with pressure into a disc of 10 mm diameter and 1 . 2 mm thickness by using polyvinylalcohol as a binder and sintered at a temperature of 1200 - 1350 ° c . for one hour to thereby produce a primary sintered body . lead oxide , bismuth oxide , and borax are separately mixed in a weight ratio of 45 : 45 : 10 and formed into a metal oxide paste . the mixed powder is then mixed with a butylcarbitol solution containing 5 . 0 wt . % cellulose so as to have a viscosity of about 400 poise . the thus manufactured metal oxide paste is printed on the two entire surfaces of the primary sintered body by silk screen printing . this is carried out by a secondary heat treatment at a temperature of 1000 ° c . for two hours . the metal oxide material is thus diffused to the grain boundaries in the interior of the sintered body . silver electrodes ( element area ratio 80 %) are then bonded with fire to both opposite peripheral surfaces of the element to thereby fabricate the element . the electrical characteristics of the element fabricated by this method are shown in table 1 below . table 1__________________________________________________________________________ secondaryprimary heatingsintering temperature leakage . sup . α 1 . sup . α 2temperature and time v lma / mm current ( 10 . sup .- 6 - ( 10 . sup .- 4 - cap . (° c .) (° c . × hr .) ( v ) ( μa ) 10 . sup .- 4 a ) 10 . sup .- 2 a ) ( pf ) __________________________________________________________________________1200 1000 × 2 697 0 . 50 13 . 3 41 . 1 1601250 1000 × 2 641 0 . 65 11 . 3 44 . 8 1751300 1000 × 2 668 0 . 42 11 . 1 47 . 2 1801350 1000 × 2 510 1 . 4 9 . 5 34 . 9 220__________________________________________________________________________ all steps of the process for fabricating the element are the same as in example 1 except that the metal oxide paste composition comprised bismuth oxide and zinc oxide and the compositing was carried out at near ( weight ratio : 98 . 5 : 1 . 5 ) the eutectic point ( 750 ° c .) in order to lower the melting temperature of the paste as far as possible . the electrical characteristics of the element fabricated by this method are shown in table 2 below . table 2__________________________________________________________________________ secondaryprimary heatingsintering temperature leakage . sup . α 1 . sup . α 2temperature and time v lma / mm current ( 10 . sup .- 6 - ( 10 . sup .- 4 - cap . (° c .) (° c . × hr .) ( v ) ( μa ) 10 . sup .- 4 a ) 10 . sup .- 2 a ) ( pf ) __________________________________________________________________________1200 1000 × 2 755 0 . 28 18 . 9 41 . 6 1501250 1000 × 2 757 0 . 32 15 . 4 45 . 2 1601300 1000 × 2 687 0 . 25 17 . 8 47 . 7 1751350 1000 × 2 600 0 . 60 12 . 5 45 . 0 190__________________________________________________________________________ all steps in the process for fabricating the element are the same as in example 1 except that the metal oxide paste comprises bismuth oxide and b 2 o 3 and that the paste was composed at a 96 : 4 weight ratio near the eutectic point ( 622 ° c .) in order to lower the melting temperature of the paste as far as possible . the electrical characteristics of the element fabricated by this method are shown in table 3 below . table 3__________________________________________________________________________ secondaryprimary heatingsintering temperature leakage . sup . α 1 . sup . α 2temperature and time v lma / mm current ( 10 . sup .- 6 - ( 10 . sup .- 4 - cap . (° c .) (° c . × hr .) ( v ) ( μa ) 10 . sup .- 4 a ) 10 . sup .- 2 a ) ( pf ) __________________________________________________________________________1200 1000 × 2 884 0 . 15 22 . 6 27 . 5 1601250 1000 × 2 734 0 . 32 13 . 8 41 . 5 1651300 1000 × 2 704 0 . 34 15 . 5 57 . 9 1801350 1000 × 2 582 0 . 38 13 . 8 35 . 9 195__________________________________________________________________________ the primary sintering temperature is 1350 ° c ., and the composition of the paste used for the secondary heating process is the same as in example 1 , except that the secondary heat treatment was carried out at 1000 ° c ., 1100 ° c . and 1200 ° c . for one hour . the electrical characteristics of the element fabricated by this method are as shown in table 4 below . table 4__________________________________________________________________________ secondaryprimary heatingsintering temperature leakage . sup . α 1 . sup . α 2temperature and time v lma / mm current ( 10 . sup .- 6 - ( 10 . sup .- 4 - cap . (° c .) (° c . × hr .) ( v ) ( μa ) 10 . sup .- 4 a ) 10 . sup .- 2 a ) ( pf ) __________________________________________________________________________1350 1000 × 1 590 0 . 40 13 . 3 48 . 8 1951350 1100 × 1 450 0 . 95 8 . 1 31 . 7 2551350 1200 × 1 330 1 . 32 6 . 5 24 . 5 320__________________________________________________________________________ the following is a brief review of the above examples . in examples 1 to 3 , the process was conducted at different primary sintering temperatures ranging from 1200 ° c . to 1350 ° c . it can be seen that the higher the primary sintering temperature the lower the breakdown voltage regardless of the kind of paste used . this is because the higher the primary sintering temperature , the faster is the speed of grain growth , and the larger is the diameter of the grain . in addition , it can be seen that when the matter is sintered primarily at 1300 ° c ., it not only exhibits a lower leakage current , but also the highest non - linear voltage - current characteristic . in example 4 the primary sintering temperature is 1350 ° c . and the secondary heating temperature is changed respectively to 1000 ° c ., 1100 ° c ., and 1200 ° c . the best results are exhibited when the secondary heating step is carried out at 1000 ° c . the characteristics of the varistor are largely changed in response to the conditions of the secondary heating step . this is particularly the case where comparing the voltage - current characteristics exhibited by the primarily sintered body and the test piece that was subjected to the secondary heating step in fig2 . in conclusion , a primary sintered body lacking bismuth oxide , will exhibit a voltage - current characteristic that is almost linear . on the contrary , in the case where the paste comprises bi 2 o 3 as a major ingredient , and printing and a secondary heating step are carried out to diffuse bismuth oxide to the zinc oxide grain boundaries , a remarkably non - linear voltage - current characteristic curve is exhibited by the varistor . thus , it can be appreciated that a potential barrier is only formed when bismuth oxide is present in the vicinity of the zinc oxide grain boundaries , and that this potential barrier controls the voltage - current characteristics of the zinc oxide varistor .