Nonlinear voltage resistor

Abstract

A nonlinear voltage resistor is formed by sintering a resistor body made from a powder predominantly comprising ZnO. The powder also includes a first element selected from the group of Li and Na and a second element selected from the group of Al, In, and Ga. The powder preferably includes 50 to 1,000 atomic ppm of the first element and a concentration of the second element sufficient to set the carrier concentration of the powder in the range of 5 atomic ppm to 120 atomic ppm.

Description

FIELD OF THE INVENTION

This invention relates to a nonlinear voltage resistor, and more particularly to a nonlinear voltage resistor formed of a sintered article having zinc oxide (ZnO) as a main component and intended to be used as a protective element against overvoltage.

BACKGROUND OF THE INVENTION

Heretofore for the purpose of protecting electronic devices and electric devices against damage due to overvoltage, there have been utilized nonlinear voltage resistors using silicon carbide (SiC), selenium (Se), and silicon (Si). In recent years, a nonlinear voltage resistor (hereinafter referred to as a "ZnO varistor") has made of a sintered article produced by admixing ZnO as a main component with various additives, molding the resultant mixture into a prescribed shape, and sintering the molded mixture. The ZnO varistor has characteristics such as a low discharge voltage and a large nonlinear voltage coefficient. It is, therefore, useful for protecting against overvoltage devices made up of semiconductor elements that have small capacities for resistance to overvoltage. The popularity of the ZnO varistor has been growing as a replacement for a SiC varistor and other overvoltage protection devices.

Among the types of ZnO varistors so far adopted for actual use is counted the ZnO-Pr.sub.6 O.sub.11 type. It is known that the ZnO-Pr.sub.6 O.sub.11 type varistor is produced by admixing ZnO as a main component with cobalt (Co), magnesium (Mg), calcium (Ca), potassium (K), and chromium (Cr) beside Pr as auxiliary components used in the form of elements or compounds and firing the resultant mixture. This is described in Japanese Patent Publication No. SHO 57(1982)-42,962.

The ZnO varistor is well known as a semiconductor ceramic and has electrical properties that are notably altered by the presence of a minute amount of impurities. When a monovalent metal such as sodium (Na) or lithium (Li) is added, the ZnO varistor increases its magnitude of resistance because the added metal functions as an acceptor. When a trivalent metal such as aluminum (Al) or iron (Fe) is added, the ZnO varistor loses its magnitude of resistance because the added metal functions as a donor. As demonstrated in Japanese Patent Publication No. SHO 55(1980)-37,846, Al is known to be an element capable of improving the discharge voltage property at a very low addition rate. Among the trivalent metals, Fe is a problematic element because this element, even at a very low addition rate, increases leakage current and deteriorates the discharge voltage property.

Since the ZnO varistor has its properties altered by impurities as described above, the compound ZnO used as the raw material therefore must be highly pure. Ordinary grade ZnO powder used as a white pigment is not acceptable.

The ZnO powder that is used for the ZnO varistor is mostly produced by the "France Method" that comprised fusing metallic Zn in a kettle made of carbon, for example, and oxidizing the Zn vapor issuing from the molten Zn with air. By this method, ZnO powder of extremely high purity can be produced. The kettle for the fusion of the metallic Zn is used for a long time and, therefore it produces a sediment of impurities. This sediment contains Fe in a large proportion. As a result, the proportion of Fe sediment that passes into the Zn vapor increases with the increasing number of days of use of the kettle. The Fe content of the produced ZnO powder, therefore, gradually increased with the increasing number of days of use of the kettle from an initial level of 2 atomic ppm to a level about 15 atomic ppm.

The varying Fe content of the ZnO powder from one lot to another of the raw material increased the possibility that the V-I property of the ZnO varistor will vary from one lot to another of the ZnO raw material. The conventional method has not always been capable of providing elements with stable properties.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is a ZnO varistor that may be produced with consistent V-I properties.

Another object of the present invention is a ZnO varistor that is compensated for the present of Fe impurities.

A further object of the present invention is an improved method for forming ZnO varistors.

These and other objects are obtained by a nonlinear voltage resistor comprising a sintered article having a nonlinear voltyage drop over a range of currents passing therethrough, the article having formed of a sintered powder predominantly comprising zinc oxide and including a first element selected from the group of Li and Na and a second element selected from the group of Al, In, and Ga.

BRIEF DESCRIPTION OF THE DRAWINGS

The manner by which the above objects and other objects, features, and advantages of the present invention are attained will become fully apparent when the following detailed description is considered in view of the drawings, wherein:

FIG. 1 is a graph showing the change of carrier concentration of the raw ZnO material due to different amounts of Al added for a varying Li concentration.

FIG. 2 is a graph showing the relation between the carrier concentration of the raw ZnO material and a nonlinear voltage coefficient, .alpha., obtained at the combinations of Li and Al concentrations of Table 1 wherein the Li concentration is in the range of 5 to 1,000 atomic ppm;

FIG. 3 is a graph showing the carrier concentration and the ratio, V(40 A)/V(1 mA), obtained at the combinations of Li and Al concentrations of Table 1 wherein the Li concentration is in the range of 5 to 1,000 ppm;

FIG. 4 is a graph showing the relation between the nonlinear voltage coefficient, .alpha., and the ratio, V(40 A)/V(1 mA), obtained for varistors formed from raw ZnO material obtained by adding 100 atomic ppm of Li and 230 atomic ppm of Al where the raw ZnO material varies in the number of days after production by the France Method;

FIG. 5 is a graph showing the relation between the nonlinear voltage coefficient, .alpha., and the ratio V(40 A)/V(1 mA), obtained for varistors formed from raw ZnO material obtained by adding 100 atomic ppm of Na and 230 atomic ppm of Al where the raw ZnO material varies in the number of days after production by the France Method;

FIG. 6 is a graph showing the relation between the nonlinear voltage coefficient, .alpha., and the ratio, V(40 A)/V(1 mA), obtained for varistors formed from raw ZnO material obtained by adding 100 atomic ppm of Li and 230 atomic ppm of In where the raw ZnO material varies in the number of days after production by the France Method; and

FIG. 7 is a graph showing the relation between the nonlinear voltage coefficient, .alpha., and the ratio, V(40 A)/V(1 mA), obtained for varistors formed from raw ZnO material obtained by adding 100 atomic ppm of Li and 230 atomic ppm of Ga where the raw ZnO material varies in the number of days after production by the France Method.

DETAILED DESCRIPTION

The present invention is directed to alleviating the effect of Fe entrained in ZnO thereby permitting production of a ZnO varistor exhibiting a satisfactory V-I property from a low current region through a high current region. This is accomplished by adding to the ZnO powder at least one element selected from the group of Na and Li in an amount necessary for counteracting the adverse effects of any Fe that may be mixed with the ZnO powder and, at the same time, adding at least one element selected from the group of Al, In, and Ga in an amount corresponding to the amount of Na or Li to be added.

When the ZnO powder as the raw material for the varistor has incorporated in advance at least one element selected from the group of Na and Li in an amount several times the amount of the Fe that is suspected to be mingled with the raw material, the Na or Li functions as an acceptor in contrast to the Fe that functions as a donor. The effects of the Fe are in this way counteracted. After the effect of the Fe has been eliminated as described above, the addition of at least one element selected from the group of Al, In, and Ga in an amount appropriate for the improvement of discharge voltage property and level of surge resistance permits production of a ZnO varistor having stable properties not affected by Fe that may be entrained in the ZnO powder as the raw material.

The present invention will be described more specifically with reference to the following examples.

(Example 1):

In a ball mill, a powder prepared by adding to ZnO powder 0.5 atomic % of Pr, 2.0 atomic % of Co, 0.2 atomic % of K, 0.15 atomic % of Cr, 0.1 atomic % of Mg, and 0.1 atomic % of Ca invariably in the form of oxides was mixed with aqueous solutions of Li and Al. The resultant wet mixture was dried and then calcined at 500.degree. to 1,000.degree. C. for several hours. The calcination product was thoroughly comminuted, combined with a binder, compression molded in the shape of a circular plate 17 mm in diameter, and fired in air at 1,100.degree. to 1,400.degree. C. for several hours to produce a sintered article. The sintered article so produced was ground to produce a test piece 1 mm in thickness. An element was prepared by baking electrodes to the opposite sides of the test piece and the element was tested for electrical properties. The ZnO powder used as the raw material had the purity of a guaranteed reagent and had Fe, Li, and Na contents of not more than 1 atomic ppm.

The electrical properties determined by the test were a voltage, V(1 mA), produced between the electrodes of a sample element when an electric current of 1 mA was passed therethrough at room temperature, a nonlinear voltage coefficient, .alpha., determined at a current in the range of 10 .mu.A to 1 mA, the ratio, V(40 A)/V(1 mA), of the voltages between the electrodes which were measured when electric currents of 40 A and 1 mA, respectively were passed with a standard waveform of 8.times.20 .mu.s through the element.

The nonlinear voltage coefficient, .alpha., was found by approximating the change of the current I of the element relative to the voltage according to the following formula:

I=KV.sup..alpha. (1) wherein K denotes a constant to be determined by the element. The results are shown in Table 1.

The samples of the compositions shown in Table 1 were tested for carrier concentration by the C-V method. The results are shown in FIG. 1. The C-V method is intended to find carrier concentration and other factors of a given ZnO varistor based on the relation between the capacitance (C) and the voltage (V). It is well known as a method for the evaluation of physical properties of Si and other semiconductors. By a careful study of the relation between the carrier concentration and the V-I property, it was found that the nonlinear voltage coefficient, .alpha., was large and the discharge voltage property was satisfactory so long as the carrier concentration was in the range of 5 atomic ppm to 120 atomic ppm, without reference to the amounts of Li and Al added. It is noted from Table 1 that the compensation by Al was not effected satisfactorily and the V-I property was inferior when the amount of Li added exceeded 2,000 atomic ppm.

FIG. 2 shows the relation between the carrier concentration and the nonlinear voltage coefficient, .alpha.. FIG. 3 shows the relation between the carrier concentration and the ratio, V(40 A)/V(1 mA). It has been demonstrated by the inventors that the behavior of carrier concentration shown in FIG. 1 represents proper data in the light of the theoretical calculation performed with the aid of model semiconductors containing both a donor and an acceptor.

(Example 2):

The procedure of Example 1 was faithfully repeated, except that Na was added in the place of Li. The results are shown in Table 2. It is noted from the results that the effect of Na was virtually the same as that of Li.

(Example 3):

A model test was carried out by varying the amount of Fe added in the range of 0 to 20 atomic ppm to find the effect of the addition of Fe.

The test will be descrbed in detail below.

From the results of Example 1, a total of 8 combinations of Li and Al which had carrier concentrations of about 50 atomic ppm were selected from these combinations which produced elements with satisfactory properties. To samples of each of the 8 combinations, Fe was added in amounts in the range of 0 to 20 atomic ppm as stepped by the unit of 5 atomic ppm. The samples so prepared were tested for the relations between the amount of Fe added and the quantities V(1 mA), .alpha., and V(40 A)/V(1 mA). The results are shown in Table 3.

From Table 3, it is noted that so long as the Li content was not less than 50 atomic ppm, the effect of Fe was substantially nil even when the amount of Fe added was 20 atomic ppm.

(Example 4):

The procedure of Example 1 was faithfully repeated, except that Na was added in the place of Li. The results are shown in Table 4. From the result, it is noted that the effect of Na was virtually the same as that of Li.

(Example 5):

For the purpose of examining the results of the model test of Example 3 on a mass-production scale, ZnO powder lots of varying age (number of days) after their production in the kettle and incorporating therein 100 atomic ppm of Li and 230 atomic ppm of Al were subjected to the same test. The results are shown in FIG. 4. FIG. 4 also shows the results obtained by the conventional method for comparision. It is clearly noted from FIG. 4 that the present invention virtually eliminates the effect of a presumed increase in Fe contamination that occurs with increased usage of the kettle.

(Example 6):

For the purpose of examining the results of the model test of Example 4 on a mass-production scale, ZnO powder lots of varying age (number of days) after their production in the kettle and incorporating therein 100 atomic ppm of Na and 230 atomic ppm of Al were subjected to the same test. The results are shown in FIG. 5. FIG. 5 also shows the results obtained by the conventional method for comparison. It is clearly noted from FIG. 5 that the present invention brings about the same neutralizing effect of Fe contamination as was shown in FIG. 4.

In none of the test runs of the present examples were Li and Na added simultaneously. In a separate experiment, it has been demonstrated that the V-I property is virtually constant when Li and Na are simultaneously added in a combined amount equal to the amount of Li or Na to be added independently.

(Example 7):

The procedure of Example 1 was faithfully repeated, except In was added in the place of Al. The results are shown in Table 5. From the results, it is noted that the effect of In was virtually the same as that of Al.

(Example 8):

The procedure of Example 1 was faithfully repeated, except that Ga was added in the place of Al. The results are shown in Table 6. It is noted from the results that the effect of Ga was substantially the same as that of Al.

(Example 9):

For the purpose of examining the results of the model test of Example 3 on a mass-production scale, ZnO powder lots of varying ages (number of days) after their production in the kettle and incorporating therein 100 atomic ppm of Li and 230 atomic ppm of In were subjected to the same test. The results are shown in FIG. 6. FIG. 6 also shows the results obtained by the conventional method for comparison. The same effects as exhibited in FIGS. 4 and 5 were achieved.

(Example 10):

For the purpose of examinating the results of the model test of Example 3 on a mass-production scale, ZnO powder lots of varying ages (number of days) after their poduction in the kettle and incorporating therein 100 atomic ppm of Li and 230 atomic ppm of Ga were subjected to the same test. The results as shown in FIG. 7 also show the results obtained by the conventional method for comparison. The same effects as exhibited in FIGS. 4, 5, and 6 were achieved.

In none of the test runs of the present examples were In and Ga used in comination with Na. In a separate experiment, it has been demonstrated that the same neutralizing effect is obtained when In and Ga are used in combination with Na.

Al, In, and Ga fulfill substantially the same function. From this fact, it can be easily inferred that the V-I property is virtually constant when these three elements are simultaneously added in a total amount equal to the amount in which they are independently added.

In this example, only the compositions of the raw material based on the invention of Japanese Patent Publication No. SHO 57(1982)-42,962 were used. The effect of this invention is not limited to these compositions. The same effect of the present invention is recognized with the ZnO-Bi.sub.2 O.sub.3 type ZnO varistor and the ZnO varistor incorporating a rare earth element other than Pr.

In accordance with the present invention, after the effect of the Fe contamination of the ZnO raw material is counteracted by Li or Na, the addition of Al in an amount selected for the carrier concentration to fall in the range of 5 to 120 atomic ppm permits production of a ZnO varistor having properties that are not substantially affected by the amount of Fe intermingled with the ZnO powder.

TABLE 1______________________________________ Amount of Li Amount of AlSample added added V(1 mA) V(40 A)No. (atomic ppm) (atomic ppm) (V) .alpha. V(1 mA)______________________________________1 5 5 499 6 4.232 " 6 394 21 2.213 " 10 250 78 1.814 " 16 211 87 1.645 " 30 189 91 1.556 " 90 187 84 1.427 " 220 192 62 1.378 " 430 182 32 1.659 " 700 158 11 1.8310 10 10 504 7 4.2311 " 11 398 23 2.2212 " 16 252 79 1.8613 " 22 244 86 1.6114 " 35 189 91 1.5715 " 90 189 85 1.4316 " 220 194 64 1.3717 " 430 184 36 1.6318 " 700 160 16 1.8519 20 20 510 6 4.2320 " 21 402 21 2.2421 " 28 255 78 1.8222 " 36 216 87 1.6223 " 52 191 93 1.5524 " 110 191 81 1.4225 " 250 196 62 1.3926 " 450 186 32 1.6527 " 700 162 12 1.8528 50 50 515 6 4.2329 " 51 406 20 2.2330 " 60 257 77 1.8431 " 68 213 88 1.6632 " 84 995 93 1.5733 " 150 193 81 1.4134 " 320 198 64 1.3835 " 560 183 34 1.6336 " 900 163 11 1.8837 100 100 520 5 4.2038 " 102 410 20 2.2039 " 110 260 77 1.8240 " 120 220 88 1.6241 " 140 195 90 1.5842 " 230 195 82 1.4043 " 420 200 63 1.3844 " 700 190 30 1.6245 " 1100 165 10 1.8346 200 200 530 5 4.2447 " 202 418 20 2.2648 " 220 265 76 1.8849 " 230 224 85 1.6650 " 260 199 88 1.6351 " 380 198 78 1.4352 " 640 204 59 1.4153 " 960 194 31 1.6654 " 1500 168 12 1.8855 500 500 541 6 4.3256 " 502 424 18 2.2857 " 520 279 72 1.8858 " 580 228 81 1.6759 " 620 202 83 1.6460 " 800 204 78 1.4461 " 1300 208 62 1.4262 " 1800 199 29 1.6763 " 2600 171 9 1.9164 1000 1000 551 6 4.4965 " 1010 435 18 2.4266 " 1080 277 63 1.8967 " 1100 233 78 1.7168 " 1200 207 80 1.6969 " 1600 206 72 1.5170 " 2500 215 61 1.4571 " 3400 201 20 1.7372 " 4500 175 8 1.9873 2000 2000 886 4 5.6274 " 2010 743 11 3.3975 " 2100 532 11 2.1876 " 2200 543 12 1.9377 " 2400 412 10 1.9378 " 3200 439 12 1.9279 " 5000 468 13 2.0480 " 7000 410 8 2.1881 " 9800 345 5 2.83______________________________________ TABLE 2______________________________________ Amount of Na Amount of AlSample added added V(1 mA) V(40 A)No. (atomic ppm) (atomic ppm) (V) .alpha. V(1 mA)______________________________________82 5 5 499 6 4.2383 " 6 394 21 2.2184 " 10 250 78 1.8185 " 16 211 87 1.6486 " 30 189 91 1.5587 " 90 187 84 1.4288 " 220 192 62 1.3789 " 430 182 32 1.6590 " 700 158 11 1.8391 10 10 504 7 4.2392 " 11 398 23 2.2293 " 16 252 79 1.8694 " 22 244 86 1.6195 " 35 189 91 1.5796 " 90 189 85 1.4397 " 220 194 64 1.3798 " 430 184 36 1.6399 " 700 160 16 1.85100 20 20 510 6 4.23101 " 21 402 21 2.24102 " 28 255 78 1.82103 " 36 216 87 1.62104 " 52 191 93 1.55105 " 110 191 81 1.42106 " 250 196 62 1.39107 " 450 186 32 1.65108 " 700 162 12 1.85109 50 50 515 6 4.23110 " 51 406 20 2.23111 " 60 257 77 1.84112 " 68 213 88 1.66113 " 84 995 93 1.57114 " 150 193 81 1.41115 " 320 198 64 1.38116 " 560 188 34 1.63117 " 900 163 11 1.88118 100 100 520 5 4.20119 " 102 410 20 2.20120 " 110 260 77 1.82121 " 120 220 88 1.62122 " 140 195 90 1.58123 " 230 195 82 1.40124 " 420 200 63 1.38125 " 700 190 30 1.62126 " 1100 165 10 1.83127 200 200 530 5 4.24128 " 202 418 20 2.26129 " 220 265 76 1.88130 " 230 224 85 1.66131 " 260 199 88 1.63132 " 380 198 78 1.43133 " 640 204 59 1.41134 " 960 194 31 1.66135 " 1500 168 12 1.88136 500 500 541 6 4.32137 " 502 424 18 2.28138 " 520 279 72 1.88139 " 580 228 81 1.67140 " 620 202 83 1.64141 " 800 204 78 1.44142 " 1300 208 62 1.42143 " 1800 199 29 1.67144 " 2600 171 9 1.91145 1000 1000 551 6 4.49146 " 1010 435 18 2.42147 " 1080 277 63 1.89148 " 1100 233 78 1.71149 " 1200 207 80 1.69150 " 1600 206 72 1.51151 " 2500 215 61 1.45152 " 3400 201 20 1.73153 " 4500 175 8 1.98154 2000 2000 886 4 5.62155 " 2010 743 11 3.39156 " 2100 532 11 2.18157 " 2200 543 12 1.93158 " 2400 412 10 1.93159 " 3200 439 12 1.92160 " 5000 468 13 2.04161 " 7000 410 8 2.18162 " 9800 345 5 2.83______________________________________ TABLE 3______________________________________Sample Li Al Fe V(1 mA) V(40 A)No. (ppm) (ppm) (ppm) (V) .alpha. V(1 mA)______________________________________6 5 90 0 187 84 1.42163 " " 5 184 73 1.44164 " " 10 178 58 1.49165 " " 15 173 51 1.53166 " " 20 170 41 1.6215 10 90 0 189 85 1.43167 " " 5 185 77 1.43168 " " 10 179 69 1.48169 " " 15 174 63 1.52170 " " 20 172 45 1.6024 20 110 0 191 81 1.42171 " " 5 186 78 1.43172 " " 10 183 72 1.48173 " " 15 181 68 1.53174 " " 20 177 51 1.6233 50 150 0 193 81 1.41175 " " 5 195 83 1.42176 " " 10 194 82 1.41177 " " 15 194 82 1.42178 " " 20 193 80 1.4342 100 230 0 195 82 1.40179 " " 5 196 82 1.41180 " " 10 195 82 1.41181 " " 15 195 83 1.42182 " " 20 194 81 1.4251 200 380 0 198 78 1.43183 " " 5 198 78 1.43184 " " 10 198 78 1.43185 " " 15 199 79 1.44186 " " 20 198 77 1.4360 500 800 0 204 78 1.44187 " " 5 204 78 1.44188 " " 10 203 77 1.44189 " " 15 203 78 1.43190 " " 20 204 79 1.4469 1000 1600 0 206 72 1.51191 " " 5 206 73 1.52192 " " 10 207 72 1.51193 " " 15 207 71 1.51194 " " 20 207 72 1.51______________________________________ TABLE 4______________________________________Sample Na Al Fe V(1 mA) V(40 A)No. (ppm) (ppm) (ppm) (V) .alpha. V(1 mA)______________________________________ 87 5 90 0 187 84 1.42195 " " 5 184 73 1.44196 " " 10 178 58 1.49197 " " 15 173 51 1.53198 " " 20 170 41 1.62 96 10 90 0 189 85 1.43199 " " 5 185 77 1.43200 " " 10 179 69 1.48201 " " 15 174 63 1.52202 " " 20 172 45 1.60105 20 110 0 191 81 1.42203 " " 5 186 78 1.43204 " " 10 183 72 1.48205 " " 15 181 68 1.53206 " " 20 177 51 1.62114 50 150 0 193 81 1.41207 " " 5 195 83 1.42208 " " 10 194 82 1.41209 " " 15 194 82 1.42210 " " 20 193 80 1.43123 100 230 0 195 82 1.40211 " " 5 196 82 1.41212 " " 10 195 82 1.41213 " " 15 195 83 1.42214 " " 20 194 81 1.42132 200 380 0 198 78 1.43215 " " 5 198 78 1.43216 " " 10 198 78 1.43217 " " 15 199 79 1.44218 " " 20 198 77 1.43141 500 800 0 204 78 1.44219 " " 5 204 78 1.44220 " " 10 203 77 1.44221 " " 15 203 78 1.43222 " " 20 204 79 1.44150 1000 1600 0 206 72 1.51223 " " 5 206 73 1.52224 " " 10 207 72 1.51225 " " 15 207 71 1.51226 " " 20 207 72 1.51______________________________________ TABLE 5______________________________________ Amount of Li Amount of InSample added added V(1 mA) V(40 A)No. (atomic ppm) (atomic ppm) (V) .alpha. V(1 mA)______________________________________227 5 5 501 5 4.24228 " 6 396 20 2.22229 " 10 252 77 1.82230 " 16 213 86 1.65231 " 30 191 90 1.56232 " 90 189 83 1.43233 " 220 194 61 1.38234 " 430 184 31 1.66235 " 700 160 10 1.84236 10 10 506 6 4.24237 " 11 400 22 2.23238 " 16 254 78 1.87239 " 22 246 85 1.62240 " 35 191 90 1.58241 " 90 191 84 1.44242 " 220 196 63 1.38243 " 430 186 35 1.64244 " 700 162 15 1.86245 20 20 511 5 4.24246 " 21 404 20 2.25247 " 28 256 77 1.83248 " 36 217 86 1.63249 " 52 192 92 1.56250 " 110 193 80 1.43251 " 250 197 61 1.40252 " 450 188 31 1.66253 " 700 163 11 1.86254 50 50 516 5 4.24255 " 51 407 19 2.24256 " 60 258 76 1.85257 " 68 215 87 1.67258 " 84 996 92 1.58259 " 150 195 80 1.42260 " 320 199 63 1.39261 " 560 190 33 1.64262 " 900 165 10 1.89263 100 100 521 5 4.21264 " 102 412 19 2.21265 " 110 261 76 1.83266 " 120 222 87 1.63267 " 140 197 89 1.59268 " 230 196 81 1.41269 " 420 201 62 1.39270 " 700 192 29 1.63271 " 1100 167 9 1.84272 200 200 530 5 4.25273 " 202 419 19 2.27274 " 220 267 75 1.89275 " 230 226 84 1.67276 " 260 201 87 1.84277 " 380 200 77 1.44278 " 640 206 58 1.42279 " 960 195 30 1.67280 " 1500 169 11 1.89281 500 500 543 6 4.32282 " 502 426 17 2.28283 " 520 281 71 1.88284 " 580 230 80 1.67285 " 620 204 82 1.64286 " 800 206 77 1.45287 " 1300 209 61 1.43288 " 1800 201 28 1.68289 " 2600 173 9 1.92290 1000 1000 553 6 4.50291 " 1010 437 17 2.43292 " 1080 279 62 1.90293 " 1100 235 77 1.72294 " 1200 209 79 1.70295 " 1600 208 71 1.52296 " 2500 217 60 1.46297 " 3400 203 19 1.74298 " 4500 177 8 1.99299 2000 2000 888 4 5.63300 " 2010 745 11 3.40301 " 2100 534 11 2.19302 " 2300 545 12 1.94303 " 2400 414 10 1.94034 " 3200 441 12 1.93305 " 5000 470 13 2.05306 " 7000 413 8 2.10307 " 9800 347 5 2.84______________________________________ TABLE 6______________________________________ Amount of Li Amount of GaSample added added V(1 mA) V(40 A)No. (atomic ppm) (atomic ppm) (V) .alpha. V(1 mA)______________________________________308 5 5 502 6 4.24309 " 6 397 20 2.23310 " 10 253 76 1.82311 " 16 214 86 1.66312 " 30 192 89 1.56313 " 90 190 83 1.43314 " 220 195 60 1.39315 " 430 185 31 1.68316 " 700 161 10 1.85317 10 10 507 7 4.24318 " 11 401 22 2.24319 " 16 255 77 1.87321 " 22 247 85 1.63321 " 35 192 89 1.58322 " 90 192 84 1.45323 " 220 197 62 1.38324 " 430 187 35 1.64325 " 700 163 14 1.87326 20 20 513 6 4.25327 " 21 405 20 2.25328 " 28 258 76 1.84329 " 36 219 86 1.63330 " 52 194 91 1.57331 " 110 194 79 1.43332 " 250 199 61 1.41333 " 450 189 30 1.66334 " 700 165 11 1.87335 50 50 518 6 4.25336 " 51 409 19 2.24337 " 60 260 75 1.85338 " 68 217 87 1.68339 " 84 998 91 1.58340 " 150 196 80 1.43341 " 320 201 62 1.39342 " 560 191 33 1.65343 " 900 166 10 1.90344 100 100 523 5 4.24345 " 102 413 19 2.22346 " 110 263 75 1.83347 " 120 223 87 1.61348 " 140 198 88 1.59349 " 230 198 81 1.42350 " 420 203 61 1.39351 " 700 193 29 1.64352 " 1100 168 10 1.85353 200 200 533 5 4.25354 " 202 421 19 2.28355 " 220 268 74 1.90356 " 230 227 84 1.67357 " 260 202 86 1.64358 " 380 201 77 1.44359 " 640 207 57 1.43360 " 960 197 30 1.68361 " 1500 171 10 1.90362 500 500 544 6 4.34363 " 502 427 17 2.29364 " 520 282 70 1.90365 " 580 231 80 1.69366 " 620 205 81 1.65367 " 800 207 77 1.45368 " 1300 211 60 1.43369 " 1800 202 28 1.69370 " 2600 174 9 1.93371 1000 1000 554 6 4.51372 " 1010 438 17 2.44373 " 1080 280 61 1.91374 " 1100 236 76 1.72375 " 1200 210 79 1.71376 " 1600 209 70 1.53377 " 2500 218 60 1.46378 " 3400 241 18 1.75379 " 4500 178 8 2.01380 2000 2000 889 4 5.64381 " 2010 746 10 3.41382 " 2100 535 11 2.20383 " 2200 548 12 1.95384 " 2400 415 10 1.94385 " 3200 442 11 1.93386 " 5000 471 12 2.06387 " 7000 413 8 2.19388 " 9800 348 5 2.85______________________________________

Claims

1. A nonlinear voltage resistor comprising a sintered article exhibiting a nonlinear voltage drop over a range of currents passing therethrough, said article being formed of a sintered powder predominantly comprising zinc oxide and including a first element selected from the group of Li and Na and a second element selected from the group of Al, In, and Ga.

2. The nonlinear voltage resistor according to claim 1, wherein said powder contains 50 atomic ppm to 1,000 atomic ppm of said first element.

3. The nonlinear voltage resistor according to claim 2, wherein said powder contains said second element in a proportion sufficient to set the carrier concentration thereof in the range of 5 atomic ppm to 120 atomic ppm.

4. The nonlinear voltage resistor according to claim 1, wherein said powder contains said second element in a proportion sufficient to set the carrier concentration thereof in the range of 5 atomic ppm to 120 atomic ppm.

5. The nonlinear voltage resistor according to claim 1, wherein said first element is Na.

6. The nonlinear voltage resistor according to claim 1, wherein said second element is selected from the group of Al and Ga.

7. The nonlinear voltage resistor according to claim 1, wherein said resistor is adapted to include electrodes baked to the opposite sides of the sintered article.

8. A method for producing a voltage resistor having a nonlinear voltage drop over a range of currents passing therethrough comprising the steps of:

providing a powder primarily comprising ZnO and including a first element selected from the group of Li and Na and a second element selected from the group of Al, In, and Ga;

forming a resistor body from said powder; and

sintering said resistor body.

9. The method according to claim 8, further comprising the step of baking electrodes to the opposite sides of the sintered body.

10. A nonlinear voltage resistance ceramic product comprising a sintered article exhibiting a nonlinear voltage drop over a range of currents passing therethrough, said article being formed of a sintered powder predominantly comprising zinc oxide and including a first element selected from the group of Li and Na and a second element selected from the group of Al and Ga.

11. A nonlinear voltage resistance ceramic product comprising a sintered article exhibiting a nonlinear voltage drop over a range of currents passing therethrough, said article being formed of a sintered powder predominantly comprising zinc oxide and including Na and In.