Voltage-dependent nonlinear resistor

FIELD OF THE INVENTION
The present invention relates to a voltage-dependent nonlinear resistor, and more particularly, to a voltage-dependent nonlinear resistor that contains zinc oxide (ZnO) as a primary component and which is used as an overvoltage protective device.
BACKGROUND OF THE INVENTION
Varistors that contain silicon carbide (SiC), selenium (Se), silicon (Si) or zinc oxide (ZnO) as a primary component are used to protect electronic and electrical apparatuses against overvoltage. Those which conain zinc oxide as a primary component have low voltage limits and large voltage-dependent nonlinearity indices. Thus, they are used more often in protecting apparatuses that are composed of a semiconductor and other devices that have small resistance to overcurrent than those made of silicon carbide.
Voltage-dependent nonlinear resistors fabricated by sintering a mix that contains zinc oxide as a primary component and five auxiliary components in elemental or compound form, i.e., a rare earth element; cobalt (Co); at least one of magnesium (Mg) and calcium (Ca); at least one of potassium (K), rubidium (Rb) and cesium (Cs); and chromium (Cr), are known to have good voltage-dependent linearity. However, these types of resistors are not suitable for incorporation in small devices because they have a relatively small resistance to both long and short wave-tail surges and a short service life.
Resistors having good voltage-dependent nonlinear characteristics can also be fabricated by sintering a mix that contains zinc oxide as a primary component and auxiliary components in elemental or compound form, i.e., a rare earth element; cobalt (Co); at least one of potassium (K), rubidium (Rb) and cesium (Cs); and chromium (Cr). However, these resistors are also not suitable for incorporation in compact devices because they also have a relatively small resistance to long wave-tail surges and a short service life.
SUMMARY OF THE INVENTION
Therefore, one object of the present invention is to provide a voltage-dependent nonlinear resistor that is small in size and which has improved resistance to both long and short wave-tail surges as well as an extended service life.
In the course of research, it has been found that when a large amount of long or short wave-tail surge current is applied to a conventional voltage-dependent nonlinear resistor that contains ZnO as a main component and five auxiliary components, i.e., a rare earth element; Co; at least one of Mg and Ca; at least one of K, Cs and Rb; and Cr, current concentrations due to the concentrated electric field around the electrodes on the surface of the device causes a breakdown. It has also been found that when a D.C. current is impressed on the device a localized heterogeneity within the resistor becomes the center of current concentration and impairs the characteristics of the device.
The same phenomena were found to occur when a large amount of long wave-tail surge current was applied to another conventional type of voltage-dependent nonlinear resistor that contained ZnO as a main component and four auxiliary components, i.e., a rare earth element; Co; at least one of K, Ca and Rb; and Cr. The mechanism behind this phenomena was also the same as above.
As a result of various studies made to solve these problems, it was discovered that by using as an additional auxiliary component, boron (B) which may be combined with at least one substance selected from among aluminum (Al), gallium (Ga), and indium (In), the peripheral area of the voltage-dependent nonlinear resistor became slightly more resistant than the central part and that this was effective in preventing current concentration from occurring in the area around the electrodes, thereby increasing the resistance to both long and short wave-tail surges. At the same time, the undesired heterogeneity disappeared from the inside of the resistor and its service life was appreciably extended.
Therefore, the present invention provides a voltage-dependent nonlinear resistor that contains ZnO as a main component and six auxiliary components, i.e., (1) a rare earth element, (2) Co, (3) at least one of Mg and Ca, (4) at least one of K, Rb and Cs, (5) Cr and (6) B which may be combined with at least one of Al, Ga and In.
The present invention also provides a voltage-dependent nonlinear resistor that contains ZnO as a main component and six auxiliary components, i.e., (1) a rare earth element, (2) Co, (3) at least one of K, Rb and Cs, (4) Cr, (5) B, and (6) at least one of Al, Ga and In.
DETAILED DESCRIPTION OF THE INVENTION
Either type of the voltage-dependent nonlinear resistors of the present invention can be produced by sintering a mix of ZnO and the necessary additives in metallic or compound form in an oxygen-containing atmosphere. The additives are usually employed in the form of metal oxides. However, those compounds which may become oxides during the subsequent sintering step, such as carbonate salts, hydroxides, fluorides and solutions thereof, may be used. The additives may also be used in elemental form if they are converted into oxides during the sintering step. In a particularly preferred embodiment, the voltage-dependent nonlinear resistor of the present invention is produced by intimately mixing ZnO powder with the necessary additives in either metallic or compound form, firing the mix at a temperature between 500.degree. and 1000.degree. C. for several hours, grinding the fired product into adequately small particles compacting the particles into the desired shape, and sintering the compacted particles in air at a temperature between 1100.degree. and 1400.degree. C. for several hours. If the sintering temperature is less than 1100.degree. C., sufficient sintering to produce stable characteristics is not achieved. If the sintering temperature is more than 1400.degree. C., a homogeneous product suitable for practical use is difficult to obtain and the only product that can be produced has a low degree of voltage-dependent nonlinearity. Further, with this product the properly controlled characteristics cannot always be obtained.
The present invention is described by the following examples to which the scope of the invention is by no means limited.

EXAMPLE 1
Samples of ZnO powder were mixed thoroughly with Pr.sub.6 O.sub.11, Co.sub.3 O.sub.4, MgO, K.sub.2 CO.sub.3, Cr.sub.2 O.sub.3, B.sub.2 O.sub.3 and Al.sub.2 O.sub.3 powders in the atomic percents prescribed in Table 1 below. Each mix was fired for several hours at between 500.degree. and 1000.degree. C. and ground into adequately small particles. After adding a binder, the particles were shaped into a disk having a diameter of 17 mm and sintered in air for 1 hour at between 1100.degree. and 1400.degree. C. Forty sintered disks were prepared in this manner. All 40 sintered disks were polished to a thickness of 2 mm and provided with an electrode on each side. Four electrical characteristic parameters were measured; i.e., (1) the interelectrode voltage V.sub.1 mA that developed when a current of 1 mA was applied to the device at 25.degree. C.; (2) the nonlinearity index .alpha. at 1-10 mA; (3) the resistance to long wave-tail surge current as measured in terms of the change in V.sub.1 mA following 20 applications of a square-wave current of 100 A for 2 milliseconds; and (4) the service life as measured in terms of the change in interelectrode voltage V.sub.1 .mu.A at 1 .mu.A following the application of 20 mA D.C. for 5 minutes.
The nonlinearity index .alpha. was calculated by the following approximation:
I=(V/C).sup..alpha.
wherein I is the current through the device at voltage V; and C is the voltage across the device per unit thickness at a current density of 1 mA/cm.sup.2. The results are shown in Table 1. The atomic percents indicated in Table 1 were calculated from the ratio of the number of atoms of a specific additive element to the sum of the number of atoms of the metallic elements present in each mix.
TABLE 1__________________________________________________________________________ Resistance to long wave- Service Non- tail surge lifeSample Additives (atm %) V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. Pr Co Mg K Cr B Al (V) Index .alpha. (%) (%)__________________________________________________________________________1 0.10 5.0 0.1 0.1 0.1 0 0 384 41 -75.4 -20.12 0.01 5.0 0.1 0.1 0.1 0.01 0.005 213 37 -86.2 -35.13 0.08 5.0 0.1 0.1 0.1 0.01 0.005 225 35 -8.3 -3.84 0.1 5.0 0.1 0.1 0.1 0.01 0.005 245 38 -2.4 -4.25 1.0 5.0 0.1 0.1 0.1 0.01 0.005 258 43 -1.9 -9.26 5.0 5.0 0.1 0.1 0.1 0.01 0.005 294 41 -28.8 -19.67 7.0 5.0 0.1 0.1 0.1 0.01 0.005 303 40 -63.1 -25.28 0.1 0.05 0.1 0.1 0.1 0.01 0.005 183 34 -69.7 -37.19 0.1 0.1 0.1 0.1 0.1 0.01 0.005 191 35 -36.3 -18.510 0.1 0.5 0.1 0.1 0.1 0.01 0.005 205 33 -12.4 -7.211 0.1 1.0 0.1 0.1 0.1 0.01 0.005 227 31 -3.1 -2.112 0.1 10.0 0.1 0.1 0.1 0.01 0.005 283 38 -13.5 -10.213 0.1 15.0 0.1 0.1 0.1 0.01 0.005 312 37 -80.3 -19.314 0.1 5.0 0.005 0.1 0.1 0.01 0.005 214 31 -86.2 -27.815 0.1 5.0 0.01 0.1 0.1 0.01 0.005 221 29 -10.1 -13.516 0.1 5.0 1.0 0.1 0.1 0.01 0.005 273 38 -7.4 -9.217 0.1 5.0 5.0 0.1 0.1 0.01 0.005 281 41 -8.9 -15.118 0.1 5.0 7.0 0.1 0.1 0.01 0.005 292 42 -45.1 -25.319 0.1 5.0 0.1 0.005 0.1 0.01 0.005 224 33 -79.6 -30.320 0.1 5.0 0.1 0.01 0.1 0.01 0.005 231 35 -2.3 -8.421 0.1 5.0 0.1 0.5 0.1 0.01 0.005 258 35 -1.5 -9.322 0.1 5.0 0.1 1.0 0.1 0.01 0.005 292 37 -19.4 -18.323 0.1 5.0 0.1 2.0 0.1 0.01 0.005 331 40 -37.2 -28.224 0.1 5.0 0.1 0.1 0.005 0.01 0.005 225 37 -78.1 -19.625 0.1 5.0 0.1 0.1 0.01 0.01 0.005 232 35 -5.3 -2.526 0.1 5.0 0.1 0.1 0.5 0.01 0.005 259 35 -13.7 -3.427 0.1 5.0 0.1 0.1 1.0 0.01 0.005 273 38 -28.5 -10.228 0.1 5.0 0.1 0.1 2.0 0.01 0.005 307 37 -80.4 -15.129 0.1 5.0 0.1 0.1 0.1 0.0001 0.005 371 37 -79.2 -27.130 0.1 5.0 0.1 0.1 0.1 0.0005 0.005 352 38 -20.3 -10.431 0.1 5.0 0.1 0.1 0.1 0.005 0.005 257 30 -2.3 -6.232 0.1 5.0 0.1 0.1 0.1 0.05 0.005 187 28 -1.5 -5.333 0.1 5.0 0.1 0.1 0.1 0.1 0.005 147 24 -7.6 -8.134 0.1 5.0 0.1 0.1 0.1 0.5 0.005 112 7 -8.3 -12.335 0.1 5.0 0.1 0.1 0.1 0.01 0.00001 272 37 -86.1 -18.436 0.1 5.0 0.1 0.1 0.1 0.01 0.0001 275 42 -67.2 -17.137 0.1 5.0 0.1 0.1 0.1 0.01 0.001 257 45 -5.1 -10.238 0.1 5.0 0.1 0.1 0.1 0.01 0.01 231 41 -1.7 -5.439 0.1 5.0 0.1 0.1 0.1 0.01 0.05 -198 29 -10.5 -4.240 0.1 5.0 0.1 0.1 0.1 0.01 0.1 114 9 -16.2 -19.4__________________________________________________________________________
As Table 1 shows, Sample No. 1 which corresponds to a conventional sintered product containing only ZnO, Pr, Co, Mg, K and Cr had a resistance to long wave-tail surge current of -75.4%, a service life of -20.1% and a nonlinearity index .alpha. of 41. Those products having a greater resistance to long wave-tail surge current and a longer service life were Sample Nos. 3 to 6, 9 to 12, 15 to 17, 20 to 22, 25 to 27, 30 to 34 and 36 to 40. Sample Nos. 34 and 40 had low nonlinearity indices and were not suitable for practical applications.
It is therefore concluded that to achieve the intended object of the present invention, Pr, Co, Mg, K, Cr, B and Al must be added in 0.08 to 5.0 atm %, 0.1 to 10 atm %, 0.01 to 5.0 atm %, 0.01 to 1.0 atm, %, 0.01 to 1.0 atm %, 5.times.-10.sup.4 to 1.times.10.sup.-1 atm %, and 1.times.10.sup.-4 to 5.times.10.sup.-2 atm %, respectively.
As is clear from Table 1, the resistance to long wave-tail surge current and service life of the systems containing Pr, Co, Mg and K as auxiliary components were greatly improved by incorporating B and Al as additional auxiliary components. This effect could be achieved only when ZnO was combined with Pr, Co, Mg, K, B and Al. Products containing these auxiliary components individually have a very low voltage-dependent nonlinearity (i.e., substantially ohmic) and are not well suited for practical purposes. In the experiment summarized in Table 1, only Pr was used as a rare earth element, but it was found that even when other rare earth elements were used or if two or more rare earth elements were employed, great improvements in the resistance to long wave-tail surge current and service life could be accomplished by the addition of B and Al without sacrificing good nonlinearity. The results of these experiments are shown in Table 2.
The experiment described immediately above was repeated, except that Mg was replaced with Ca or Mg and Ca. The results are shown in Table 3. The results demonstrate that the addition of B and Al was equally effective in improving the resistance to long wave-tail surge current and service life without reducing nonlinearity.
Table 4 shows the results of an experiment wherein K was replaced with Rb and Cs individually, as well as by K and Rb, K and Cs or Rb and Cs. The effect of the addition of B and Al was the same as that when K was used alone.
The results seen from using Ga or In in place of Al are shown in Table 5. Table 5 clearly demonstrates that the effect of adding B and Ga or In was the same as that of adding B and Al.
TABLE 2__________________________________________________________________________ Resistance toAdditives (atm %) long wave- Service Rare earth Non- tail surge lifeSample element V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. (atm %) Co Mg K Cr B Al (V) Index .alpha. (%) (%)__________________________________________________________________________41 Tb 1.0 1.0 0.1 0.1 0.1 0.01 0.005 227 31 -8.4 -13.142 Tb 1.0 1.0 0.1 0.1 0.1 0.01 0.01 236 33 -2.2 -9.543 Tb 1.0 1.0 0.1 0.1 0.1 0.01 0.05 203 28 -1.7 -6.144 La 1.0 2.0 0.1 0.1 0.1 0.01 0.005 205 34 -7.4 -8.245 La 1.0 2.0 0.1 0.1 0.1 0.01 0.01 211 35 -4.2 -9.446 La 1.0 2.0 0.1 0.1 0.1 0.01 0.05 185 25 -2.1 -7.247 Nd 1.0 5.0 0.1 0.1 0.1 0.01 0.005 212 29 -8.5 -5.448 Nd 1.0 5.0 0.1 0.1 0.1 0.01 0.01 209 30 -2.1 -3.649 Nd 1.0 5.0 0.1 0.1 0.1 0.01 0.05 174 25 -1.7 -2.850 Sm 1.0 5.0 0.1 0.1 0.1 0.01 0.005 185 28 -6.2 -8.551 Sm 1.0 5.0 0.1 0.1 0.1 0.01 0.01 191 30 -2.1 -2.152 Sm 1.0 5.0 0.1 0.1 0.1 0.01 0.05 153 24 -1.7 -3.553 Dy 1.0 1.0 0.1 0.1 0.1 0.01 0.005 197 31 -9.7 -7.254 Dy 1.0 1.0 0.1 0.1 0.1 0.01 0.01 203 34 -5.2 -6.855 Dy 1.0 1.0 0.1 0.1 0.1 0.01 0.05 198 30 -5.5 -8.256 Pr + La 0.5 + 0.5 1.0 0.1 0.1 0.1 0.01 0.005 217 32 -2.8 -10.157 Pr + La 0.5 + 0.5 1.0 0.1 0.1 0.1 0.01 0.01 210 28 -1.7 -4.558 Pr + La 0.5 + 0.5 1.0 0.1 0.1 0.1 0.01 0.05 187 26 -4.3 -6.2__________________________________________________________________________
TABLE 3__________________________________________________________________________ Resistance to long wave- Service Non- tail surge lifeSample Additives (atm %) V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. Pr Co Mg Ca K Cr B Al (V) Index .alpha. (%) (%)__________________________________________________________________________59 0.1 5.0 0 0.1 0.1 0.1 0 0 341 33 -84.1 -23.160 0.1 5.0 0 0.005 0.1 0.1 0.01 0.05 221 35 -86.3 -25.261 0.1 5.0 0 0.01 0.1 0.1 0.01 0.05 219 34 -38.1 -10.162 0.1 5.0 0 0.5 0.1 0.1 0.01 0.05 225 31 -3.7 -5.463 0.1 5.0 0 1.0 0.1 0.1 0.01 0.05 232 30 -8.9 -7.264 0.1 5.0 0 2.0 0.1 0.1 0.01 0.05 245 37 -42.1 -27.265 0.1 5.0 0.1 0.1 0.1 0.1 0.01 0.05 235 37 -5.2 -9.2__________________________________________________________________________
TABLE 4__________________________________________________________________________ Resistance to long wave- Service Non- tail surge lifeSample Additives (atm %) V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. Pr Co Mg K Rb Cs Cr B Al (V) Index .alpha. (%) (%)__________________________________________________________________________66 0.1 5.0 0.1 0 0.01 0 0.1 0.01 0.005 231 34 -5.7 -10.367 0.1 5.0 0.1 0 0.1 0 0.1 0.01 0.005 249 37 -3.2 -5.268 0.1 5.0 0.1 0 1.0 0 0.1 0.01 0.005 312 38 -20.5 -9.369 0.1 5.0 0.1 0 0 0.01 0.1 0.01 0.005 247 31 -10.2 -6.270 0.1 5.0 0.1 0 0 0.1 0.1 0.01 0.005 262 35 -2.7 -5.471 0.1 5.0 0.1 0 0 1.0 0.1 0.01 0.005 316 38 -10.9 -3.872 0.1 5.0 0.1 0.1 0.1 0 0.1 0.005 0.005 265 33 -4.2 -9.773 0.1 5.0 0.1 0.1 0.1 0 0.1 0.05 0.005 196 29 -3.8 -7.174 0.1 5.0 0.1 0.1 0.1 0 0.1 0.1 0.005 97 16 -8.3 -8.275 0.1 5.0 0.1 0.1 0.1 0 0.1 0.01 0.001 281 37 -6.3 -7.976 0.1 5.0 0.1 0.1 0.1 0 0.1 0.01 0.01 272 40 -2.3 -3.877 0.1 5.0 0.1 0.1 0.1 0 0.1 0.01 0.05 203 38 -5.7 -6.178 0.1 5.0 0.1 0.1 0 0.1 0.1 0.01 0.005 251 34 -7.2 -3.879 0.1 5.0 0.1 0 0.1 0.1 0.1 0.01 0.005 268 37 -9.6 -4.280 0.1 5.0 0.1 0.05 0.05 0.05 0.1 0.01 0.005 243 33 -13.1 -9.3__________________________________________________________________________
TABLE 5__________________________________________________________________________ Resistance to long wave- Service Non- tail surge lifeSample Additives (atm %) V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. Element Atm % Pr Co Mg K Cr B (V) Index .alpha. (%) (%)__________________________________________________________________________81 Ga 0.001 0.1 5.0 0.1 0.1 0.1 0.001 236 31 -5.3 -9.882 " 0.005 0.1 5.0 0.1 0.1 0.1 0.001 217 35 -3.2 -4.183 " 0.01 0.1 5.0 0.1 0.1 0.1 0.001 196 37 -2.9 -3.884 " 0.05 0.1 5.0 0.1 0.1 0.1 0.001 151 35 -6.1 -7.985 In 0.001 0.1 5.0 0.1 0.1 0.1 0.001 208 32 -7.2 -10.686 " 0.005 0.1 5.0 0.1 0.1 0.1 0.001 184 38 -6.1 -9.287 " 0.01 0.1 5.0 0.1 0.1 0.1 0.001 143 27 -5.9 -10.388 " 0.05 0.1 5.0 0.1 0.1 0.1 0.001 97 18 -13.4 -15.2__________________________________________________________________________
As Tables 1 to 5 show, examples of voltage-dependent nonlinear resistors according to the present invention have greatly improved resistance to long wave-tail surge current and appreciably extended service lives while retaining good nonlinearity. Therefore, they are expected to make a very efficient varistor.
EXAMPLE 2
Samples of ZnO powder were mixed thoroughly with Pr.sub.6 O.sub.11, Co.sub.3 O.sub.4, MgO, K.sub.2 CO.sub.3, Cr.sub.2 O.sub.3 and B.sub.2 O.sub.3 powders in the atomic percents noted in Table 6. Each mix was fired for several hours at between 500.degree. and 1000.degree. C. and ground into adequately small particles. After adding a binder, the particles were shaped into a disk having a diameter of 42 mm and sintered in air for 1 hour at between 1100.degree. and 1400.degree. C. Thirty-seven sintered disks were prepared in this manner. All 37 sintered disks were polished to a thickness of 2 mm and provided with an electrode on each side. As in Example 1, four electrical characteristic parameters were measured; i.e., (1) the interelectrode voltage V.sub.1 mA that developed when a current of 1 mA was applied to the device at 25.degree. C.; (2) the nonlinearity index .alpha. at 1-10 mA; (3) the resistance to short wave-tail surge current as measured in terms of the change in V.sub.1 mA following 2 applications of an impact current of 65 kA for a period of 4.times.10.sup.4 microseconds; and (4) the service life as measured in terms of the change in interelectrode voltage V.sub.1 .mu.A at 1 .mu.A following the application of 100 mA D.C. for 5 minutes. The results are shown in Table 6. The atomic percents indicated in Table 6 were calculated from the ratio of the number of atoms of a specific additive element to the sum of the number of atoms of the metallic elements present in each mix.
Sample No. 1 in Table 6, which corresponds to a conventional sintered product containing only ZnO, Pr, Co, Mg, K and Cr, had a resistance to short wave-tail surge current of -58.5%, a service life of -32.7% and a nonlinearity index .alpha. of 41. Those products having a greater resistance to short wave-tail surge current and a longer service life were Sample Nos. 3 to 6, 9 to 12, 15 to 18, 21 to 23, 26 to 29 and 32 to 37, Sample No. 37 had a low nonlinearity index and was not suitable for practical purposes.
Thus, in order to achieve the intended object of the present invention, Pr, Co, Mg, K, Cr and B must be added in 0.08 to 5.0 atm %, 0.1 to 10 atm %, 0.01 to 5.0 atm %, 0.01 to 1.0 atm %, 0.01 to 1.0 atm % and 5.times.10.sup.-4 to 1.times.10.sup.-1 atm %, respectively.
As is clear from Table 6, the resistance to short wave-tail surve current and service life of the systems containing Pr, Co, Mg, K and Cr, as auxiliary components were greatly improved by incorporating B as an additional auxiliary component. This effect could be achieved only when ZnO was combined with Pr, Co, Mg, K, Cr and B. Products containing these auxiliary components individually have a very low voltage-dependent nonlinearity (i.e., substantially ohmic) and are not very well suited for practical applications.
In the experiment reported in Table 6, only Pr was used as a rare earth element, but it was found that even when other rare earth elements were used or if two or more rare earth elements were employed, great improvements in the resistance to short wave-tail surge current and service life could be accomplished by the addition of B without sacrificing good nonlinearity. The results of these experiments are shown in Table 7.
The same experiment described immediately above was repeated, except that Mg was replaced by Ca. The results are shown in Table 8. The results of an experiment using Mg and/or Ca and at least one of K, Rb and C.sub.s are shown in Table 9.
TABLE 6__________________________________________________________________________ Resistance to short wave- Service Non- tail surge lifeSample Additives (atm %) V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. Pr Co K Cr Mg B (V) Index .alpha. (%) (%)__________________________________________________________________________1 0.10 5.0 0.1 0.1 0.1 0 371 41 -58.5 -32.72 0.05 5.0 0.1 0.1 0.1 0.010 261 28 -43.1 -35.33 0.08 5.0 0.1 0.1 0.1 0.010 285 32 -13.4 -10.24 0.10 5.0 0.1 0.1 0.1 0.010 309 34 -3.2 -5.35 0.50 5.0 0.1 0.1 0.1 0.010 334 45 -7.2 -7.26 5.0 5.0 0.1 0.1 0.1 0.010 385 40 -19.6 -18.47 7.0 5.0 0.1 0.1 0.1 0.010 413 42 -29.6 -38.58 0.10 0.05 0.1 0.1 0.1 0.010 251 28 -53.1 -34.39 0.10 0.10 0.1 0.1 0.1 0.010 272 34 -13.1 -19.710 0.10 0.50 0.1 0.1 0.1 0.010 295 37 -4.3 -2.711 0.10 1.0 0.1 0.1 0.1 0.010 302 35 -10.6 -9.712 0.10 10.0 0.1 0.1 0.1 0.010 288 38 -37.2 -21.513 0.10 15.0 0.1 0.1 0.1 0.010 408 18 -62.1 -33.014 0.10 5.0 0.005 0.1 0.1 0.010 264 17 -60.1 -34.215 0.10 5.0 0.01 0.1 0.1 0.010 280 28 -26.1 -21.316 0.10 5.0 0.05 0.1 0.1 0.010 292 31 -13.5 -10.617 0.10 5.0 0.5 0.1 0.1 0.010 359 37 -8.2 -8.518 0.10 5.0 1.0 0.1 0.1 0.010 273 38 -25.2 -18.719 0.10 5.0 2.0 0.1 0.1 0.010 424 35 -30.2 -35.120 0.10 5.0 0.1 0.005 0.1 0.010 442 19 -43.2 -36.221 0.10 5.0 0.1 0.01 0.1 0.010 372 27 -27.5 -16.422 0.10 5.0 0.1 0.2 0.1 0.010 305 36 -2.7 -8.323 0.10 5.0 0.1 1.0 0.1 0.010 284 38 -18.5 -21.324 0.10 5.0 0.1 2.0 0.1 0.010 262 33 -36.3 - 43.225 0.10 5.0 0.1 0.1 0.005 0.010 295 38 -42.1 -33.126 0.10 5.0 0.1 0.1 0.01 0.010 302 35 -18.6 -24.227 0.10 5.0 0.1 0.1 0.1 0.010 209 34 -3.4 -2.728 0.10 5.0 0.1 0.1 1.0 0.010 343 27 -9.8 -6.329 0.10 5.0 0.1 0.1 3.0 0.010 369 20 -37.5 -20.330 0.10 5.0 0.1 0.1 7.0 0.010 388 15 -64.1 -37.231 0.10 5.0 0.1 0.1 0.1 0.0001 357 38 -57.8 -34.232 0.10 5.0 0.1 0.1 0.1 0.0005 348 40 -10.1 -24.233 0.10 5.0 0.1 0.1 0.1 0.0010 345 43 -3.7 -10.334 0.10 5.0 0.1 0.1 0.1 0.0050 324 38 -4.2 -7.235 0.10 5.0 0.1 0.1 0.1 0.0050 286 29 -7.6 -8.536 0.10 5.0 0.1 0.1 0.1 0.10 247 19 -13.4 -28.537 0.10 5.0 0.1 0.1 0.1 0.20 198 9 -26.5 -27.2__________________________________________________________________________
TABLE 7__________________________________________________________________________ Resistance to long wave- Service Non- tail surge lifeSample Additives (atm %) V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. Element Atm % Co K Cr Mg B (V) Index .alpha. (%) (%)__________________________________________________________________________31 Tb 1.0 1.0 0.1 0.1 0.1 0.005 371 39 -7.2 -10.732 Tb 1.0 1.0 0.1 0.1 0.1 0.01 347 34 -3.1 -5.733 Tb 1.0 1.0 0.1 0.1 0.1 0.05 165 22 -4.3 -8.334 La 1.0 2.0 0.1 0.1 0.1 0.005 357 28 -8.2 -9.635 La 1.0 2.0 0.1 0.1 0.1 0.01 302 25 -4.2 -3.736 La 1.0 2.0 0.1 0.1 0.1 0.05 169 18 -3.8 -8.437 Nd 1.0 5.0 0.1 0.1 0.1 0.005 355 38 -5.8 -7.138 Nd 1.0 5.0 0.1 0.1 0.1 0.01 308 31 -4.2 -2.739 Nd 1.0 5.0 0.1 0.1 0.1 0.05 147 29 -8.1 -3.340 Sm 1.0 5.0 0.1 0.1 0.1 0.005 361 37 -9.6 -6.541 Sm 1.0 5.0 0.1 0.1 0.1 0.01 317 28 -3.2 -2.542 Sm 1.0 5.0 0.1 0.1 0.1 0.05 208 22 -2.7 -3.143 Dy 1.0 1.0 0.1 0.1 0.1 0.005 348 35 -6.1 -8.544 Dy 1.0 1.0 0.1 0.1 0.1 0.01 284 28 -3.8 -4.245 Dy 1.0 1.0 0.1 0.1 0.1 0.05 241 23 -4.2 -3.146 Pr + La 1.0 1.0 0.1 0.1 0.1 0.005 349 33 -8.3 -8.947 Pr + La 1.0 1.0 0.1 0.1 0.1 0.01 301 29 -2.7 -2.448 Pr + La 1.0 1.0 0.1 0.1 0.1 0.05 247 20 -9.2 -5.7__________________________________________________________________________
TABLE 8__________________________________________________________________________ Resistance to short wave- Service Non- tail surge lifeSample Additives (atm %) V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. Pr Co K Cr Ca B (V) Index .alpha. (%) (%)__________________________________________________________________________49 0.1 5.0 0.1 0.1 0.1 0 364 38 -57.1 -34.950 0.1 5.0 0.1 0.1 0.005 0.010 295 37 -52.5 -36.551 0.1 5.0 0.1 0.1 0.01 0.010 301 40 -27.6 -18.752 0.1 5.0 0.1 0.1 0.1 0.010 307 35 -4.2 -5.653 0.1 5.0 0.1 0.1 1.0 0.010 285 23 -8.4 -10.754 0.1 5.0 0.1 0.1 5.0 0.010 261 18 -26.3 -22.155 0.1 5.0 0.1 0.1 7.0 0.010 247 10 -63.1 -29.656 0.1 5.0 0.1 0.1 0.1 0.0001 347 39 -60.3 -30.257 0.1 5.0 0.1 0.1 0.1 0.0005 338 36 -37.6 -21.358 0.1 5.0 0.1 0.1 0.1 0.0010 340 40 -5.4 -9.659 0.1 5.0 0.1 0.1 0.1 0.005 328 38 -4.2 -4.760 0.1 5.0 0.1 0.1 0.1 0.05 285 30 -6.1 -8.361 0.1 5.0 0.1 0.1 0.1 0.1 236 18 -10.7 -7.662 0.1 5.0 0.1 0.1 0.1 0.2 205 8 -31.5 -26.4__________________________________________________________________________
TABLE 9__________________________________________________________________________ Resistance to long wave- Service Non- tail surge lifeSample Additives (atm %) V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. Pr Co K Rb Cs Cr Mg Ca B (V) Index .alpha. (%) (%)__________________________________________________________________________63 0.1 5.0 0.1 0 0 0.1 0.1 0.1 0.001 357 37 -27.6 -31.264 0.1 5.0 0.1 0 0 0.1 0.1 0.1 0.05 232 29 -13.4 -19.665 0.1 5.0 0.1 0 0 0.1 0.1 0.1 0.7 168 21 -26.3 -30.366 0.1 5.0 0 0.1 0.1 0 0.1 0 0.01 329 38 -4.3 -8.267 0.1 5.0 0 0 0 0.1 0.1 0 0.01 318 40 -3.2 -10.168 0.1 5.0 0.1 0.1 0.1 0.1 0.1 0 0.01 343 41 -5.7 -9.7__________________________________________________________________________
In either case, the addition of B was effective in remarkably improving the resistance to short wave-tail surge current and service life without losing high nonlinearity as in the case where Mg or K alone was used.
Therefore, in order to achieve the desired object of the present invention, a rare earth element, Co, Cr and B must be added in 0.08 to 5.0 atm %, 0.1 to 10.0 atm %, 0.01 to 1.0 atm % and 5.times.10.sup.-4 to 1.times.10.sup.-1 atm %, respectively. Furthermore, at least one of Mg and Ca must be present in 0.01 to 5.0 atm % and at least one of K, Cs and Rb should be present in a total amount of 0.01 to 1.0 atm %. The desired advantage is achieved only when ZnO is combined with a rare earth element, Co, at least one of Mg and Ca, at least one of K, Cs and Rb, as well as Cr and B. Products containing these auxiliary components individually have a very low voltage-dependent nonlinearity (i.e., substantially ohmic) and are not suitable for practical purposes.
As is clear from the foregoing data, the voltage-dependent nonlinear resistor of the present invention which contains ZnO as the primary component and six auxiliary components, i.e., a rare earth element, Co, at least one of Mg and Ca, at least one of K, Cs and Rb, as well as Cr and B has greatly improved resistance to short wave-tail surge current and extended service life without reducing nonlinearity. Therefore, the resistor is expected to make a very efficient varistor.
EXAMPLE 3
Samples of ZnO powder were mixed thoroughly with Pr.sub.6 O.sub.11, Co.sub.3 O.sub.4, K.sub.2 CO.sub.3, Cr.sub.2 O.sub.3, B.sub.2 O.sub.3 and Al.sub.2 O.sub.3 powders in the atomic percents listed in Table 10 below. Each mix was fired for several hours at between 500.degree. and 1000.degree. C., and subsequently treated as in Example 1. Thirty-six sintered disks were prepared in this manner and subjected to the measurement of the four electrical characteristic parameters described. The results are shown in Table 10, wherein the atomic percents were calculated from the ratio of the number of atoms of a specific additive element to the sum of the number of atoms of the metallic elements present in each mix.
Sample No. 1 in Table 10 corresponds to a conventional sintered product containing only ZnO, Pr, Co, K and Cr. This product had a resistance to long wave-tail surge current of -79.3%, a service life of -23.5% and a nonlinearity index .alpha. of 34. Those products having a greater resistance to long wave-tail surge current and a longer service life were Sample Nos. 3 to 7, 10 to 13, 16 to 19, 22 to 24, 27 to 31 and 33 to 35. Sample No. 31 had a low nonlinearity index and was not suitable for practical purposes.
Thus, in order to achieve the intended object of the present invention, Pr, Co, K, Cr, B and Al has to be added in 0.08 to 5.0 atm %, 0.1 to 10 atm %, 0.01 to 1.0 atm %, 0.01 to 1.0 atm %, 5.times.10.sup.-4 to 1.times.10.sup.-1 atm % and 1.times.10.sup.-4 to 5.times.10.sup.-2 atm %.
TABLE 10__________________________________________________________________________ Resistance to short wave- Service Non- tail surge lifeSample Additives (atm %) V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. Pr Co K Cr B Al (V) Index .alpha. (%) (%)__________________________________________________________________________1 0.10 5.0 0.1 0.1 0.0 0.0 381 34 -79.3 -23.52 0.01 5.0 0.1 0.1 0.010 0.005 221 25 -80.5 -34.13 0.08 5.0 0.1 0.1 0.010 0.005 242 28 -1.4 -5.74 0.10 5.0 0.1 0.1 0.010 0.005 274 42 -1.3 -1.55 0.50 5.0 0.1 0.1 0.010 0.005 305 45 -1.5 -8.96 1.0 5.0 0.1 0.1 0.010 0.005 331 38 -2.3 -14.17 5.0 5.0 0.1 0.1 0.010 0.005 374 39 -23.2 -20.68 7.0 5.0 0.1 0.1 0.010 0.005 410 33 -77.3 -37.19 0.10 0.05 0.1 0.1 0.010 0.005 184 19 -87.3 -22.510 0.10 0.10 0.1 0.1 0.010 0.005 205 28 -34.1 -13.111 0.10 0.50 0.1 0.1 0.010 0.005 221 30 -18.3 -3.512 0.10 1.0 0.1 0.1 0.010 0.005 242 33 -5.7 -2.113 0.10 10.0 0.1 0.1 0.010 0.005 305 37 -38.5 -10.114 0.10 15.0 0.1 0.1 0.010 0.005 347 34 -43.5 -25.115 0.10 5.0 0.005 0.1 0.010 0.005 253 35 -83.1 -17.416 0.10 5.0 0.01 0.1 0.010 0.005 261 38 -24.2 -8.317 0.10 5.0 0.05 0.1 0.010 0.005 268 42 -15.3 -2.118 0.10 5.0 0.2 0.1 0.010 0.005 285 40 -2.5 -1.519 0.10 5.0 1.0 0.1 0.010 0.005 307 38 -8.5 -8.320 0.10 5.0 2.0 0.1 0.010 0.005 341 34 -76.2 -27.921 0.10 5.0 0.1 0.005 0.010 0.005 352 38 -75.1 -30.422 0.10 5.0 0.1 0.01 0.010 0.005 334 41 -5.2 -17.123 0.10 5.0 0.1 0.5 0.010 0.005 262 37 -6.3 -9.424 0.10 5.0 0.1 1.0 0.010 0.005 253 34 -8.5 - 23.825 0.10 5.0 0.1 2.0 0.010 0.005 241 35 -81.8 -37.526 0.10 5.0 0.10 0.10 0.0001 0.005 344 38 -83.1 -27.627 0.10 5.0 0.10 0.10 0.0005 0.005 340 35 -25.3 -12.128 0.10 5.0 0.10 0.10 0.0010 0.005 275 38 -3.2 -5.329 0.10 5.0 0.10 0.10 0.050 0.005 189 27 -8.5 -2.830 0.10 5.0 0.10 0.10 0.10 0.005 152 25 -13.1 -16.531 0.10 5.0 0.10 0.10 0.50 0.005 113 9 -24.2 -18.332 0.10 5.0 0.10 0.10 0.01 0.00001 311 38 -87.4 -20.133 0.10 5.0 0.10 0.10 0.01 0.0001 302 41 -27.5 -16.334 0.10 5.0 0.10 0.10 0.01 0.01 242 31 -8.4 -8.435 0.10 5.0 0.10 0.10 0.01 0.05 227 27 -23.2 -5.136 0.10 5.0 0.10 0.10 0.01 0.1 138 12 -79.6 -16.3__________________________________________________________________________
As is clear from Table 10, the resistance to long wave-tail surge current and service life of the systems containing Pr, Co, K and Cr as auxiliary components were greatly improved by incorporating B and Al as additional auxiliary components. This effect could be achieved only when ZnO was combined with Pr, Co, K, Cr, B and Al. Products containing these auxiliary components individually have a very low voltage-dependent nonlinearity (i.e., substantially ohmic) and are not highly suitable for practical applications.
In the experiment summarized in Table 10, only Pr was used as a rare earth element, but it was found that even when other rare earth elements were used or if two or more rare earth elements were employed, great improvements in the resistance to long wave-tail surge current and service life could be accomplished by the addition of B and Al without sacrificing good nonlinearity. The results of these are shown in Table 11.
The same experiment described immediately above was repeated, except that K was replaced by Rb or Cs. The results are shown in Table 12. The results using K and Rb, or K, Rb and Cs are shown in Table 13.
TABLE 11__________________________________________________________________________ Resistance to long wave- ServiceAdditives (atm %) Non- tail surge lifeSample Rare earth V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. element Atm % Co K Cr B Al (V) Index .alpha. (%) (%)__________________________________________________________________________37 Tb 1.0 1.0 0.1 0.1 0.010 0.005 321 35 -7.6 -13.438 Tb 1.0 1.0 0.1 0.1 0.010 0.01 308 43 -4.2 -5.739 Tb 1.0 1.0 0.1 0.1 0.010 0.05 242 37 -5.1 -8.340 La 1.0 2.0 0.1 0.1 0.010 0.005 284 38 -3.8 -9.641 La 1.0 2.0 0.1 0.1 0.010 0.01 271 39 -1.1 -2.542 La 1.0 2.0 0.1 0.1 0.010 0.05 237 37 -8.4 -3.743 Nd 1.0 5.0 0.1 0.1 0.010 0.005 248 33 -7.2 -8.444 Nd 1.0 5.0 0.1 0.1 0.010 0.01 242 30 -3.1 -5.145 Nd 1.0 5.0 0.1 0.1 0.010 0.05 213 25 -3.3 -6.346 Sm 1.0 5.0 0.1 0.1 0.010 0.005 307 38 -6.9 -9.247 Sm 1.0 5.0 0.1 0.1 0.010 0.01 275 34 -4.1 -5.248 Sm 1.0 5.0 0.1 0.1 0.010 0.05 243 27 -2.3 -7.249 Dy 1.0 1.0 0.1 0.1 0.010 0.005 353 36 -5.8 -5.350 Dy 1.0 1.0 0.1 0.1 0.010 0.01 329 38 -4.1 -8.451 Dy 1.0 1.0 0.1 0.1 0.010 0.05 282 31 -8.5 -3.252 Pr + La 0.5 + 0.5 1.0 0.1 0.1 0.010 0.005 352 41 -7.2 -6.153 Pr + La 0.5 + 0.5 1.0 0.1 0.1 0.010 0.01 372 43 -3.1 -1.854 Pr + La 0.5 + 0.5 1.0 0.1 0.1 0.010 0.05 328 38 -2.9 -3.2__________________________________________________________________________
TABLE 12__________________________________________________________________________ Resistance to long wave- ServiceAdditives (atm %) Non- tail surge lifeSample Alkali V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 .mu.ANo. element Atm % Pr Co Cr B Al (V) Index .alpha. (%) (%)__________________________________________________________________________55 Cs 0.01 0.1 5.0 0.1 0.01 0.005 264 33 -35.1 -10.656 Cs 0.1 0.1 5.0 0.1 0.01 0.005 285 38 -3.5 -7.257 Cs 1.0 0.1 5.0 0.1 0.01 0.005 317 41 -12.3 -6.358 Rb 0.01 0.1 5.0 0.1 0.01 0.005 259 34 -27.5 -12.159 Rb 0.1 0.1 5.0 0.1 0.01 0.005 272 31 -5.4 -5.160 Rb 1.0 0.1 5.0 0.1 0.01 0.005 337 41 -8.3 -9.2__________________________________________________________________________
TABLE 13__________________________________________________________________________ Resistance to long wave- Service Non- tail surge lifeSample Additives (atm %) V.sub.1 mA linearity .DELTA.V.sub.1 mA .DELTA.V.sub.1 mANo. Pr Co K Rb Cs Cr B Al (V) Index .alpha. (%) (%)__________________________________________________________________________61 0.1 5.0 0.1 0.1 0 0.1 0.001 0.005 334 38 -15.3 -12.362 0.1 5.0 0.1 0.1 0 0.1 0.01 0.005 312 39 -1.7 -8.263 0.1 5.0 0.1 0.1 0 0.1 0.1 0.005 241 31 -3.2 -4.364 0.1 5.0 0.1 0.1 0 0.1 0.01 0.001 349 43 -26.3 -19.265 0.1 5.0 0.1 0.1 0 0.1 0.01 0.01 307 39 -5.7 -3.166 0.1 5.0 0.1 0.1 0 0.1 0.01 0.05 304 34 -32.5 -8.367 0.1 5.0 0.1 0.1 0.1 0.1 0.001 0.05 342 40 -26.4 -9.268 0.1 5.0 0.1 0.1 0.1 0.1 0.01 0.05 308 33 -3.4 -4.269 0.1 5.0 0.1 0.1 0.1 0.1 0.1 0.05 253 29 -2.1 -13.570 0.1 5.0 0.1 0.1 0.1 0.1 0.01 0.001 332 40 -16.2 -16.371 0.1 5.0 0.1 0.1 0.1 0.1 0.01 0.01 301 37 -8.3 -9.272 0.1 5.0 0.1 0.1 0.1 0.1 0.01 0.05 284 30 -6.1 -18.373 0.1 5.0 0.1 0.1 0.1 0.1 0.001 0.05 342 36 -13.1 -10.474 0.1 5.0 0.1 0.1 0.1 0.1 0.01 0.05 331 37 -2.6 -3.275 0.1 5.0 0.1 0.1 0.1 0.1 0.1 0.05 274 35 -1.7 -10.976 0.1 5.0 0.1 0.1 0.1 0.1 0.01 0.001 351 37 -9.7 -10.177 0.1 5.0 0.1 0.1 0.1 0.1 0.01 0.01 303 31 -6.2 -4.278 0.1 5.0 0.1 0.1 0.1 0.1 0.01 0.05 285 28 -10.1 -15.3__________________________________________________________________________
In either case, the addition of B and Al was effective in remarkably improving the resistance to long wave-tail surge current and service life without losing high nonlinearity as in the case where K alone was used.
Thus, in order to achieve the intended object of the present invention, a rare earth element, Co, Cr, B and Al must be added in 0.08 to 5.0 atm %, 0.1 to 10.0 atm %, 0.01 to 1.0 atm %, 5.times.10.sup.-4 to 1.times.10.sup.-1 atm %, and 0.0001 to 0.05 atm %, respectively. Furthermore, at least one of K, Cs and Rb should be present in a total amount of 0.01 to 1.0 atm %. The desired advantage is achieved only when ZnO is combined with a rare earth element, Co, at least one of K, Cs and Rb, as well as Cr, B and Al. Products containing these auxiliary components individually have a very low voltage-dependent nonlinearity (i.e., substantially ohmic) and are not suitable for practical purposes. When Al was replaced by Ga or In, the results were the same as those summarized in Table 10-13.
As is clear from the foregoing data, the voltage-dependent nonlinear resistor of the present invention which contains ZnO as the primary component and six auxiliary components, i.e., a rare earth element, Co, at least one of K, Cs and Rb, Cr, B, and at least one of Al, Ga and In has greatly improved resistance to long wave-tail surge current and an extended service life without reducing nonlinearity. Therefore, the resistor is expected to make a very efficient varistor.
While the invention has been described in detail and with respect to various embodiments thereof, it is apparent that various changes and modifications may be made therein without departing from the spirit and scope thereof.

Number	Date	Country
57-193725	Nov 1982	JPX
57-193726	Nov 1982	JPX
57-193727	Nov 1982	JPX

Number	Name	Date
4038217	Namba et al.	Jul 1977
4045374	Nagasawa et al.	Jul 1977
4069061	Nagasawa et al.	Jan 1978
4091144	Dresner et al.	May 1978
4160748	Yodogawa et al.	Jul 1979
4169071	Eda et al.	Sep 1979
4285839	Wong	Aug 1981
4326187	Miyoshi et al.	Apr 1982
4383237	Eda et al.	May 1983
4386022	Nagasawa et al.	May 1983
4397775	Levinson	Jul 1983
4436650	Bowen	Mar 1984

Voltage-dependent nonlinear resistor

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