Oxide varistor

Abstract

An oxide varistor is prepared from a basic composition formed of 70 to 14 mol% of at least one Me.sup.I O.sub.2, 29 to 85 mol% of ZnO and 1 to 20 mol% of Sb.sub.2 O.sub.3, 1 to 20% by weight of Bi.sub.2 O.sub.3, and 0.5 to 10% by weight of at least one Me.sup.II.sub.2 O.sub.3 based on said basic composition, Me.sup.I being selected from Ti, Sn and Zr, and Me.sup.II being selected from Fe, Cr, Mn and Co.

Description

This invention relates to a varistor prepared from an oxide semiconducttor, and more particularly to an oxide varistor having greater voltage-current nonlinear coefficient.

As one of circuit elements based on a semiconductor, a voltage nonlinear resistance element is known. SiC varistors are known as a typical example. The element of this kind has nonlinear voltage-current characteristics, namely, is sharply reduced in resistance with increasing voltage to permit electric current to be markedly increased accordingly and has consequently been widely used for absorbing abnormally high voltage or for stabilization of voltage.

With the recent marked advance in electronic art there is a growing demand for devices (IC, transistors, etc.) in which semiconductor are used. However, these electronic parts have disadvantage in common that they are weak against abnormal voltage and a settlement to this problem has been sought. Arresters, surge absorbers, etc., using the conventional SiC varistor are slow in charge response speed to impulses and incomplete for protection against surges involved in devices in which semiconductors are used.

With rotational devices, the art of breakers has been markedly developed. With the advent of a vacuum breaker, an on-off surge due to current suppression poses a problem. The protection against the on-off surge has heretofore been made by using a gap type arrester or a capacitor for electric power. In the gap type arrester, it is possible to absorb abnormal voltage due to a normal current suppression. However, there is still a problem in respsonding to impluses of the order of MHz in current unstable areas and impulses involved during re-ignition. When the capacitor is used, problems arise from the price consideration as well as from the setting of capacitance. Therefore, there is a demand for an inexpensive varistor element having excellent varistor characteristics.

Generally, the voltage-current characteristics of the varistor may be expressed approximately in the following equation:

I - (V/C).sup..alpha.

where

I = current flowing through the varistor

V = voltage across the varistor

C = constant

.alpha. = nonlinear voltage coefficient

Therefore, the characteristics of the varistor may be indicated by C and .alpha. or the two other constants which can replace them. Since accurate determination of C presents extreme difficulties, C is generally substituted by voltage Vc at a certain current C (mA). With the varistor voltage thus designated as Vc the voltage-current characteristics of the varistor may be indicated by Vc and the nonlinear coefficient .alpha.. An SiC varistor heretofore well known as a varistor element has as small a voltage nonlinear coefficient .alpha. as 3-7. A zener diode is known as having a larger voltage nonlinear coefficient. The zener diode is expensive and in addition has a limited usage voltage of 200 V at most and is consequently inconvenient for use with electronic devices requiring for high voltage. The zener diode involves a greater variation with temperature in the rising voltage and a smaller surge resistance, thus presenting practical problems.

U.S. Pat. No. 3,632,529 discloses a voltage variable resistor ceramic composition consisting essentially of zinc oxide and 0.05 to 10.0 mol % of strontium oxide and, as an additive, 0.05 to 8.0 mol % of bismuth oxide, lead oxide, calcium oxide or cobalt oxide. This ceramic composition is different from an oxide semiconductor composition of an oxide varistor according to this invention. The voltage nonlinear coefficient of the ceramic composition is of the order of 10 and is lower than that (more than 30) of the oxide varistor according to this invention.

U.S. Pat. No. 3,663,458 discloses a nonlinear bulk-type resistor consisting essentially of a sintered body of 80.0 to 99.9 mol % of Zn0, 0.05 to 10 mol % Bi.sub.2 O.sub.3 and 0.05 to 10 mol % in total of at least one member selected from the group consisting of CoO, MnO.sub.2, In.sub.2 O.sub.3, Sb.sub.2 O.sub.3, TiO.sub.2, B.sub.2 O.sub.3, Al.sub.2 O.sub.3, SnO.sub.2, BaO, NiO, MoO.sub.3, Ta.sub.2 O.sub.5, Fe.sub.2 O.sub.3 and Cr.sub.2 O.sub.3. It will be evident from the following description that the composition of the sintered body is different from that of an oxide varistor according to this invention. According to this patent the content of SnO.sub.2 or TiO.sub.2 is as low as 0.05 to 10 mol % and it is impossible to attain a predetermined performance in the practice of this invention. Furthermore, the resistor represents a relatively great change when electric current is supplied for a load life test.

An object of this invention is to provide an oxide varistor having a greater voltage nonlinear coefficient (.alpha.>30).

Another object of this invention is to provide a high performance oxide varistor capable of representing a lower rising voltage, a small change with temperature, a high surge resistance and a smaller change with time.

The other object will be apparent from the following description.

In accordance with this invention there is provided an oxide varistor prepared from a basic composition formed of 70 to 14 mol % of at least one Me.sup.I O.sub.2, 29 to 85 mol % of ZnO and 1 to 20 mol % of Sb.sub.2 O.sub.3, 1 to 20 % by weight of Bi.sub.2 O.sub.3, and 0.5 to 10 % by weight of at least one Me.sup.II.sub.2 O.sub.3 based on said basic composition, wherein said Me.sup.I is a metal selected from the group consisting of Ti, Sn and Zr, and said Me.sup.II is a metal selected from the group consisting of Fe, Cr, Mn and Co.

This invention can be more fully understood from the following detailed description when taken in connection with reference to the accompanying drawings, in which:

FIG. 1 shows the content of Sb.sub.2 O.sub.3 and the resistivity of an Me.sup.I O.sub.2 -Zno-Sb.sub.2 O.sub.3 system in which the molar ratio of Me.sup.I O to ZnO is rendered constant;

FIG. 2 shows the molar ratio of Me.sup.I O.sub.2 to ZnO and the resistivity of the Me.sup.I O.sub.2 -Zno-Sb.sub.2 O.sub.3 system in which the content of Sb.sub.2 O.sub.3 is rendered constant; and

FIGS. 3A to 6C show a relation between the content of Bi.sub.2 O.sub.3 and the voltage nonlinear coefficient .alpha. of an Me.sup.I O.sub.2 -Zno-Sb.sub.2 O.sub.3 -Bi.sub.2 O.sub.3 -Me.sup.II.sub.2 O.sub.3 system in which Me.sup.II.sub.2 O.sub.3 is used as a parameter.

An oxide varistor according to this invention can be prepared in the following manners. Accurately weighed out raw material metal oxides having a predetermined composition ratio are mixed together at a ball mill, etc., and after preliminarily sintered at a relatively low temperature, for example, at 600.degree. to 900.degree.C, are powdered by the ball mill. As raw material used in this case, use may be made of metal compound, such as hydroxide, carbonate, oxalate, etc., which is convertible into oxide upon heating. The powder so obtained is mixed with a binder such as polyvinyl alcohol, etc. The mixture is shaped, under pressure of about 100 to 1,000 Kg/cm.sup.2, into a disc having a diameter of about 20 mm and a thickness of about 1 mm. The disc is sintered generally in an air atmosphere at a temperature of about 1,000.degree. to 1,300.degree.C. During the sintering it is kept at a maximum temperature for about 1 to 6 hours. Then, electrodes are baked to the resultant disc to obtain a varistor.

As mentioned above, oxide varistor according to this invention includes a basic composition formed of 70 to 14 mol % of metal dioxide Me.sup.I O.sub.2, 29 to 85 mol % of ZnO and 1 to 20 mol % of Sb.sub.2 O.sub.3. The metal dioxide is selected from TiO.sub.2, SnO.sub.2, Zro.sub.2 or a mixture thereof.

The content of Sb.sub.2 O.sub.3 is 1 to 20 mol %. When the content (mol %) of Sb.sub.2 O.sub.3 of the basic composition i.e. Me.sup.I O.sub.2 -ZnO-Sb.sub.2 O.sub.3 system is varied with the molar ratio of Me.sup.I O.sub.2 to ZnO fixed at 1:2, resistivity values as shown in FIG. 1 are obtained. As will be understood from this figure, when Sb.sub.2 O.sub.3 exceeds 1 mol % the resistivity value is sufficiently lowered and is suitable for use as a varistor. However, when Sb.sub.2 O.sub.3 exceeds 20 mol %, then the resistivity value is increased and is unsuitable for use as a varistor. Even if the resistivity presents no problem, sintered bodies are so porous that no desirable varistor is obtained from the practical viewpoint. It is preferred that the content of Sb.sub.2 O.sub.3 be 5 to 15 mol %.

The contents of Me.sup.I O.sub.2 and ZnO are 70 to 14 mol % and 29 to 85 mol %, respectively. When the molar ratio of Me.sup.I O.sub.2 to ZnO of the base Me.sup.I O.sub.2 -Zno-Sb.sub.2 O.sub.3 is varied with the content of Sb.sub.2 O.sub.3 fixed at 6 mol %, then the resistivity of the basic composition is as shown in FIG. 2. As will be appreciated from this figure, when the content of Me.sup.I O.sub.2 is outside the scope of 70 to 14 mol % and the content of ZnO is outside the scope of 29 to 85 mol %, a resistivity is increased and the composition is unsuitable for the practice of this invention. Even if Me.sup.I is replaced by Ti, Sn or Zr, the same trend will also be observed. Where it is desired to obtain a varistor having a relatively great voltage nonlinear coefficient, it is preferred that the content of Me.sup.I O.sub.2 be in the range of 60 to 30 mol % and the content of ZnO be in the range of 35 to 57 mol %. When it is desired to obtain a varistor whose rising voltage is relatively low, it is preferred that the content of Me.sup.I O.sub.2 be 14 to 20 mol % or around 70 mol % and the content of ZnO be 66 to 85 mol % or around 30 mol %.

An oxide varistor according to this invention contains, as an additive to the base Me.sup.I O.sub.2 -ZnO-Sb.sub.2 O.sub.3 composition, 1 to 20 weight % of Bi.sub.2 O.sub.3 and 0.5 to 10 weight % of Me.sup.II.sub.2 O.sub.3. The Me.sup.II.sub.2 O.sub.3 is selected from Fe.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Mn.sub.2 O.sub.3, Co.sub.2 O.sub.3 or a mixture thereof. Determination was made of the voltage nonlinear coefficient .alpha. of resistors obtained by adding varying amounts of Bi.sub.2 O.sub.3 to a basic composition formed of 30 mol % of Me.sup.I O.sub.2, 60 mol % of ZnO and 10 mol % of Sb.sub.2 O.sub.3 in which Me.sup.II.sub.2 O.sub.3 is used as a parameter. The results are shown in FIGS. 3A to 6C. Figures with the suffix a added show the case where Me.sup.I is Ti; figures with the suffix b added show the case where Me.sup.I is Sn; and figures with the suffix c added show the case where Me.sup.I is Zr. FIGS. 3A to 3C show the case where Me.sup.II is Fe; FIGS. 4A to 4C where Me.sup.II is Cr; FIGS. 5A to 5C where Me.sup.II is Mn; and FIGS. 6A to 6C where Me.sup.II is Co.

As will be understood from FIGS. 3A to 6C, when the contents of Bi.sub.2 O.sub.3 and Me.sup.II.sub.2 O.sub.3 are ouside the scope indicated, a variator whose voltage nonlinear coefficient is great (.alpha.>30) is not obtained. Furthermore, those outside the scope have the drawback that a high rise voltage is involved. Even when a composition is outside the scope, there may be obtained a varistor whose voltage nonlinear coefficient exceeds 30. However, the varistor is higher (about 1.5 to 2 times higher) in its rise voltage than a variator according to this invention which shows a voltage nonlinear coefficient of much the same magnitude. Therefore, it is difficult to handle as well as to be put to practical use.

The above-mentioned voltage-current characteristics are not varied in any composition of this invention even when Ag or an In-Ga alloy is used as electrode material.

An oxide varistor according to this invention has as high a voltage nonlinear coefficient .alpha. as more than 30 and presents extremely small changes with temperature in the rising voltage and a high surge resistance, thus attaining a high performance. Therefore, the varistor is suitably applicable to an arrester, a surge suppressor for vacuum breaker, etc., as well as to the protection of communication instruments against surge and the suppression of abnormal voltage involved in a microwave oven.

Furthermore, an oxide varistor according to this invention can be manufactured at lower cost, since the raw material is obtained at low cost.

This invention will be more fully understood upon reading the following examples.

EXAMPLES

Mixtures were prepared as shown in tables by adding to a basic composition formed of 75 to 9 mol % of Me.sup.I O2, 24 to 90 mol % of ZnO and 1 to 22 mol % of Sb.sub.2 O.sub.3 (totalling 100 mol %), 0.5 to 25 wt % of Bi.sub.2 O.sub.3 and 0.3 to 12 wt % of Me.sup.II.sub.2 O.sub.3 based on the basic composition, and intimately mixed at a ball mill. The mixtures were preliminarily sintered at 800.degree.C for 1 hour and powdered at a ball mill. Then, the powders were mixed with polyvinyl alcohol acting as a binder and shaped under a pressure of 1,000 Kg/cm.sup.2. The shaped powders were heated to a temperature ranging 1,100.degree. to 1,300.degree.C and kept at that temperature for 2 hours for sintering, thereby obtaining 147 discs having a diameter of 20 mm and a thickness of 1 mm. Then, silver electrodes were baked to the disc, resulting in a varistor. An elemental Ag or Ag.sub.2 O may likewise be used as a starting material for silver electrodes. Since the sintered mass is stable to temperature, the electrode could be baked to the sintered disc over a wider range of about 400.degree. to 800.degree.C. Then, determination was made of the voltage-current characteristics of the samples, i.e., a varistor voltage Vc at room temperature and a voltage nonlinear coefficient .alpha. using a standard method. The results are shown in Tables I to III. Table I shows the case where Me.sup.I is Ti; Table II where Me.sup.I is Sn; and Table III where Me.sup.I is Zr.

Table I __________________________________________________________________________ Main component Supplemental Sample No (mol %) component (wt %) Vc .alpha. TiO.sub.2 ZnO Sb.sub.2 O.sub.3 Bi.sub.2 O.sub.3 Me.sup.II.sub.2 O.sub.3 (V) __________________________________________________________________________ Control 1 70 29 1 1 306 11.0 2 " " " " Me=Fe 0.6 143 34.1 3 " " " " Me=Cr " 131 32.8 4 " " " " Me=Mn " 138 33.6 5 " " " " Me=Co " 148 34.7 Control 6 60 35 5 5 557 25.4 7 " " " " Me=Fe 2.0 286 73.2 8 " " " " Me=Cr " 274 72.1 9 " " " " Me= Mn " 280 72.5 10 " " " " Me=Co " 293 74.0 11 " " " " Me=Fe 1.0 295 74.6 Me=Cr " 12 " " " " Me=Mn " 297 74.8 Me=Co " Control 13 50 42 8 7 726 41.4 14 " " " " Me=Fe 4.0 425 97.5 15 " " " " Me=Cr " 414 96.3 16 " " " " Me=Mn " 420 96.9 17 " " " " Me=Co " 433 98.6 18 " " " " Me=Fe 1.0 438 99.0 Me=Cr " Me=Mn " Me=Co " Control 19 40 50 10 9.5 776 45.8 20 " " " " Me=Fe 4.8 461 111.3 21 " " " " Me=Cr " 452 110.0 22 " " " " Me=Mn " 458 111.1 23 " " " " Me=Co " 474 115.4 24 " " " " Me=Fe 1.6 479 116.2 Me=Cr " Me=Mn " 25 " " " " Me=Cr 1.6 470 114.6 Me=Mn " Me=Co " Control 26 30 57 13 12 661 36.2 27 " " " " Me=Fe 6.0 362 79.8 28 " " " " Me=Cr " 353 78.1 29 " " " " Me=Mn " 360 78.5 30 " " " " Me= Co " 384 80.4 Control 31 20 64 16 14 542 24.4 32 " " " " Me=Fe 8.0 273 60.1 33 " " " " Me=Cr " 260 58.8 34 " " " " Me=Mn " 267 58.2 35 " " " " Me=Co " 282 61.3 36 " " " " Me=Fe 2.0 284 61.6 Me=Cr " Me=Mn " Me=Co " Control 37 14 66 20 20 315 11.5 38 " " " " Me=Fe 10.0 163 36.0 39 " " " " Me=Cr " 151 35.2 40 " " " " Me=Mn " 148 34.6 41 " " " " Me=Co " 170 37.3 Control 42 14 85 1 20 291 10.8 43 " " " " Me=Fe 0.5 125 32.7 44 " " " " Me=Cr " 117 31.9 45 " " " " Me=Mn " 120 32.2 46 " " " " Me=Co " 132 33.6 Control 47 75 24 1 25 Me=Fe 0.3 293 16.9 Control 48 9 90 1 0.5 Me=Cr 12.0 258 14.0 Control 49 48 30 22 6 Me=Mn 4.0 327 21.1 __________________________________________________________________________

Table II __________________________________________________________________________ Main Component Supplemental Sample No. (mol %) component (wt %) Vc .alpha. SnO.sub.2 ZnO Sb.sub.2 O.sub.3 Bi.sub.2 O.sub.3 Me.sup.II.sub.2 O.sub.3 (V) __________________________________________________________________________ Control 50 70 29 1 1 309 11.2 51 " " " " Me=Fe 0.6 130 31.5 52 " " " " Me=Cr " 136 32.4 53 " " " " Me=Mn " 142 33.1 54 " " " " Me=Co " 147 33.8 Control 55 60 35 5 5 568 25.9 56 " " " " Me=Fe 2.0 281 72.5 57 " " " " Me=Cr " 286 73.0 58 " " " " Me=Mn " 295 74.2 59 " " " " Me=Co " 299 75.3 60 " " " " Me=Fe 1.0 302 75.6 Me=Cr " 61 " " " " Me=Mn " 304 75.8 Me=Co " Control 62 50 42 8 7 735 42.5 63 " " " " Me=Fe 4.0 407 95.2 64 " " " " Me=Cr " 415 96.0 65 " " " " Me=Mn " 420 96.3 66 " " " " Me=Co " 426 97.7 67 " " " " Me=Fe 1.0 431 98.4 Me=Cr " Me=Mn " Me=Co " Control 68 40 50 10 9.5 783 47.2 69 " " " " Me=Fe 4.8 446 107.5 70 " " " " Me=Cr " 451 109.0 71 " " " " Me=Mn " 457 110.8 72 " " " " Me=Co " 464 112.3 73 " " " " Me=Fe 1.6 466 113.1 Me=Cr " Me=Mn " 74 " " " " Me=Cr " 472 113.9 Me=Mn " Me=Co " Control 75 30 57 13 12 663 36.6 76 " " " " Me=Fe 6.0 355 78.8 77 " " " " Me=Cr " 360 79.5 78 " " " " Me=Mn " 367 81.1 79 " " " " Me=Co " 371 82.4 Control 80 20 64 16 14 525 24.0 81 " " " " Me=Fe 8.0 250 55.9 82 " " " " Me=Cr " 256 57.2 83 " " " " Me=Mn " 263 58.3 84 " " " " Me=Co " 272 59.4 85 " " " " Me=Fe 2.0 277 60.1 Me=Cr " Me=Mn " Me=Co " Control 86 14 66 20 20 318 11.8 87 " " " " Me=Fe 10.0 153 34.6 88 " " " " Me=Cr " 165 35.0 89 " " " " Me=Mn " 174 35.9 90 " " " " Me=Co " 181 36.7 Control 91 14 85 1 20 288 10.6 92 " " " " Me=Fe 0.5 116 31.1 93 " " " " Me=Cr " 120 32.4 94 " " " " Me=Mn " 127 32.9 95 " " " " Me=Co " 133 33.5 Control 96 75 24 1 25 Me=Fe 0.3 297 17.8 Control 97 9 90 1 0.5 Me=Cr 12.0 255 13.7 Control 98 48 30 22 6 Me=Mn 4.0 320 20.4 __________________________________________________________________________

Table III __________________________________________________________________________ Main component Supplemental Sample No. (mol %) component (wt %) Vc .alpha. ZrO.sub.2 ZnO Sb.sub.2 O.sub.3 Bi.sub.2 O.sub.3 Me.sup.II.sub.2 O.sub.3 (V) __________________________________________________________________________ Control 99 70 29 1 1 303 10.9 100 " " " " Me=Fe 0.6 154 35.1 101 " " " " Me=Cr " 138 33.0 102 " " " " Me=Mn " 142 34.3 103 " " " " Me=Co " 160 36.2 Control 104 60 35 5 5 549 26.5 105 " " " " Me=Fe 2.0 290 74.4 106 " " " " Me=Cr " 273 73.0 107 " " " " Me=Mn " 284 73.6 108 " " " " Me=Co " 295 75.2 109 " " " " Me=Fe 1.0 297 75.8 Me=Cr " 110 " " " " Me=Mn " 301 76.1 Me=Co " Control 111 50 42 8 7 724 40.9 112 " " " " Me=Fe 4.0 436 98.7 113 " " " " Me=Cr " 425 98.0 114 " " " " Me=Mn " 418 97.1 115 " " " " Me=Co " 445 99.6 116 " " " " Me=Fe 1.0 451 100.3 Me=Cr " Me=Mn " Me=Co " Control 117 40 50 10 9.5 757 46.2 118 " " " " Me=Fe 4.8 473 114.6 119 " " " " Me=Cr " 465 112.3 120 " " " " Me=Mn " 461 111.5 121 " " " " Me=Co " 480 115.8 122 " " " " Me=Fe 1.6 484 116.1 Me=Cr " Me=Mn " 123 40 50 10 9.5 Me=Cr 1.6 486 116.3 Me=Mn " Me=Co " Control 124 30 57 13 12 657 35.8 125 " " " " Me=Fe 6.0 361 79.6 126 " " " " Me=Cr " 350 77.7 127 " " " " Me=Mn " 344 76.7 128 " " " " Me=Co " 365 80.2 Control 129 20 64 16 14 545 25.0 130 " " " " Me=Fe 8.0 282 61.5 131 " " " " Me=Cr " 271 60.3 132 " " " " Me=Mn " 264 59.8 133 " " " " Me=Co " 287 62.4 134 " " " " Me=Fe 2.0 290 62.7 Me=Cr " Me=Mn " Me=Co " Control 135 14 66 20 20 309 11.1 136 " " " " Me=Fe 10.0 173 38.0 137 " " " " Me=Cr " 161 36.9 138 " " " " Me=Mn " 155 36.2 139 " "" " Me=Co " 182 39.3 Control 140 14 85 1 20 290 10.5 141 " " " " Me=Fe 0.5 130 33.0 142 " " " " Me=Cr " 122 32.4 143 " " " " Me=Mn " 118 31.6 144 " " " " Me=Co " 137 34.5 Control 145 75 24 1 25 Me=Fe 0.3 291 16.6 Control 146 9 90 1 0.5 Me=Cr 12.0 256 13.7 Control 147 48 30 22 6 Me=Mn 4.0 320 20.3 __________________________________________________________________________

As will be appreciated from Tables I to III, the varistor having a basic composition formed of 60 to 30 mol % of Me.sup.I O.sub.2, 35 to 57 mol % of ZnO and 5 to 15 mol % of Sb.sub.2 O.sub.3, exhibits an extremely high voltage nonlinear coefficient. Furthermore, the varistor having a basic composition formed of 14 to 20 mol % or about 70 mol % of Me.sup.I O.sub.2, 64 to 85 mol % or about 30 mol % of ZnO and 1 to 20 mol % of Sb.sub.2 O.sub.3 has a particularly low rise voltage.

With respect to some of the above-mentioned examples determination was made of the change with temperature in the rising voltage (Vc) and surge resistance involved in a pulse current of 8 .times. 20 .mu.s. The results are shown in Table IV in which the voltage nonlinear coefficient .alpha. as shown in Tables I to III is restated.

Table IV ______________________________________ Change with Surge Voltage Sample No. temperature resistance nonlinear in the rising coefficient voltage (%/.degree.C) (A/cm.sup.2) .alpha. ______________________________________ Control 1 -0.004 2570 11.0 " 13 -0.005 2760 41.4 " 26 -0.005 2830 36.2 " 48 -0.008 2180 14.0 " 50 -0.005 2550 11.2 " 62 -0.004 2840 42.5 " 75 -0.004 2730 36.6 " 97 -0.009 2220 13.7 " 99 -0.005 2540 10.9 " 111 -0.005 2710 40.9 " 129 -0.004 2820 25.0 " 146 -0.008 2130 13.7 3 -0.003 3390 32.8 5 -0.002 3620 34.7 9 -0.002 3970 72.5 16 -0.003 4220 96.9 21 -0.001 4630 110.0 24 -0.002 4950 116.2 28 -0.001 4110 78.1 34 -0.003 3870 58.2 38 -0.002 3640 36.0 43 -0.001 3380 32.7 53 -0.002 3360 33.1 56 -0.003 3580 72.5 59 -0.001 3930 75.3 66 - 0.002 4240 97.8 71 -0.002 4610 110.8 74 -0.003 4950 113.9 78 -0.002 4100 81.1 84 -0.001 3890 59.4 88 -0.002 3630 35.0 93 -0.001 3420 32.4 100 -0.003 3240 35.1 102 -0.002 3580 34.3 105 -0.003 3880 74.4 113 -0.002 4070 98.0 118 -0.002 4560 114.6 121 -0.001 4930 115.8 125 -0.002 4440 79.6 131 -0.001 3950 60.3 134 -0.003 3720 62.7 139 -0.001 3490 39.3 ______________________________________

As will be understood from Table IV, the change with temperature in the rising voltage of the oxide varistor according to this invention is below -0.003 %/.degree.C and the varistor is very favorably compared with the conventional SiC varistor (-0.1 %/.degree.C) and zener diode (-0.1 %/.degree.C). Furthermore, the surge resistance of the varistor according to this invention is more than 3,000 A/cm.sup.2 and very favorably compared with the conventional zinc oxide varistor element (2,000 A/cm.sup.2) and zener diode (20 A/cm.sup.2).

With the varistors outside the scope of this invention, for example, the sample Nos. 13, 26, 62, 75, 111, etc., the voltage nonlinear coefficient .alpha. exceeds 30 and they are favorably compared with some of the varistors according to this invention. However, they are inferior, in the change with temperature in the rising voltage as well as in the surge resistance, to the varistors according to this invention.

The varistor according to this invention was supplied will an electric power of 1 watt for a load life test and kept at 70.degree.C for 500 hours. Therefore, the variation of the voltage nonlinear coefficient .alpha. was determined. The results are shown below in comparison with the resistors of U.S. Pat. No. 3,663,458 which are described in Table 12 of this patent.

Table V ______________________________________ Variation of voltage nonlinear Sample No. coefficient .alpha. after load life test (%) ______________________________________ 18 -0.8 35 -0.9 72 -0.3 87 -0.7 110 -0.6 133 -0.8 139 -0.2 Control ZnO 99.5 -4 Sb.sub.2 O.sub.3 0.5 ZnO 99.5 -2 SnO.sub.2 0.5 ______________________________________

From this table it will be understood that the varistor according to this invention undergoes a very small change with time. Though Me.sup.I is used in a single compound, the same result is obtained even when it is used in a mixed form.

Claims

What we claim is:

1. An oxide varistor prepared from a basic composition consisting of a total of 70 to 14 mol % of at least one compound of the formula Me.sup.I O.sub.2, 29 to 85 mol % of ZnO and 1 to 20 mol % of Sb.sub.2 O.sub.3, and containing 1 to 20 % by weight of Bi.sub.2 O.sub.3, and a total of 0.5 to 10 % by weight of at least one Me.sup.II.sub.2 O.sub.3 based on the weight of said basic composition, wherein said Me.sup.I is selected from the group consisting of Ti, Sn and Zr, said Me.sup.II is selected from the group consisting of Fe, Cr, Mn and Co.

2. The oxide varistor according to claim 1 wherein said basic composition consists of 60 to 30 mol % of Me.sup.I O.sub.2, 35 to 57 mol % of ZnO and 5 to 15 mol % of Sb.sub.2 O.sub.3.

3. The oxide varistor according to claim 1, wherein the amount of said Me.sup.I O.sub.2 of the basic composition is 20 to 14 mol % and the amount of said ZnO of the basic composition is 66 to 85 mol %.

4. The oxide varistor according to claim 1, wherein the amount of said Me.sup.I O.sub.2 of the basic composition is about 70 mol % and the amount of said ZnO of the basic composition is about 30 mol %.

5. An oxide varistor having a nonlinear voltage coefficient (.alpha.) greater than 30, a change with temperature in rising voltage of less than -0.003%/.degree.C and a surge resistance of more than 3,000 A/cm.sup.2, said oxide varistor being prepared from a basic composition consisting of a total of 70 to 14 mol % of at least one compound of the formula Me.sup.I O.sub.2, 29 to 85 mol % of ZnO and 1 to 20 mol % of Sb.sub.2 O.sub.3, and containing 1 to 20 % by weight of Bi.sub.2 O.sub.3, and a total of 0.5 to 10 % by weight of at least one Me.sup.II.sub.2 O.sub.3 based on the weight of said basic composition, wherein said Me.sup.I is selected from the group consisting of Ti, Sn and Zr, and said Me.sup.II is selected from the group consisting of Fe, Cr, Mn and Co.