Highly densified voltage non-linear resistor and method of manufacturing the same

Abstract: When manufacturing a voltage non-linear resistor, the sintering process is divided into two completely separated steps, i.e. primary and secondary sinterings. The primary sintering is carried out under a reduced pressure and the secondary sintering is conducted under atmospheric pressure with a sufficient amount of oxygen. The primary sintering is effected such that the relative density and open porosity of the primarily sintered body are limited to 85% or more and 1% or less, respectively. Then in the second sintering the voids in the body are removed to a large extent and also the oxidation is sufficiently effected. Therefore, the finally sintered body has the high density, large surge withstanding capability, and high non-linearity index.

Description: The present invention relates to a voltage non-linear ceramic resistor composed mainly of zinc oxide. More particularly, the invention relates to a method of manufacturing a voltage non-linear resistor to be used in overvoltage-protecting devices such as lightning arrestors, and also relates to a highly densified voltage non-linear resistor. Since the voltage non-linear resistors composed mainly of zinc oxide have excellent non-linear voltage-current characteristics, they are widely used in lightning arrestors and surge absorbers to stabilize the voltage and to absorb surges. In case of manufacturing the voltage non-linear resistor, a small amount of an oxide or oxides of bismuth, antimony, cobalt and/or manganese, which serve as a substance for introducing the voltage non-linearity in the sintered body, is mixed with zinc oxide which serves as the main component, and then the mixture is granulated and shaped into a desired configuration. Next the shaped body is subjected to a sintering process. In a preferable case, an inorganic material is applied on a side surface of the sintered body and, thereafter the assembly is subjected to a secondary sintering process, to form a high resistance layer. After that electrodes made of aluminum, for example, are applied on opposite surfaces of the finally sintered body. In order to use the thus obtained voltage non-linear resistor in the lightning arrestor in which very large surges have to be absorbed, it is desirable to make the surge withstanding capability of the voltage non-linear resistor as large as possible. The surge withstanding capability of the voltage non-linear resistor may be represented by the maximum electric current value at which the resistor is not broken down or a flashover does not occur under the application of the impulse electric current having a waveshape of 4/10 microseconds two times for each five minutes and stepping up the electric current value. It is considered that the value of surge withstanding capability of the voltage non-linear resistor depends on the amount and diameter of voids existing in the sintered body. That is to say, it is considered that when applying the 4/10 µs impulse electric current to the voltage non-linear resistor, the destruction of the resistor is caused by the thermal stress. Therefore, if the mechanical strength of the sintered body is made high by decreasing the voids in the sintered body, it is expected that the surge withstanding capability thereof would be improved. And, at the tip of the void, the electric current is likely to be concentrated. If such local concentration of electric current is occurred at the tip of the void, the temperature at the tip of the void is locally increased, because the heat conduction of the sintered body surrounding the void is small under applying the electric current for only short time such as 4/10 µs. If the thermal stress is generated by this temperature increase becomes to be more than the mechanical strength of the sintered body, the resistor would be broken. Therefore, it is necessary to make the mechanical strength of the sintered body high and to remove the voids for the purpose that the local concentration of electric current would not be likely to occur. Further, in order to effectively prevent an electric discharge caused by the flashover of the voltage non-linear resistor, it is necessary to improve the coherency of the high resistor layer onto the side surface of the sintered body. Japanese Patent Laid-open Publication, Kokai Sho No. 58-28,802 discloses the method of reducing the voids in the voltage non-linear resistors, in which the shaped body is heated up to 1,300°C and during this heating cycle, the sintering is carried out under a reduced pressure lower than the atmospheric pressure within a temperature range from 800°C to 1,150°C. In this publication, there is only indicated that the surge withstanding capability under the application of 2 ms rectangular electric current is improved, but there is not indicated the characteristic with respect to the surge withstanding capability under the application of 4/10 µs impulse electric current. In case that the 2 ms rectangular electric current is applied to the voltage non-linear resistor and the value thereof is stepped up until the resistor is broken, the feedthrough breakdown would be occurred in the resistor. On the other hand, in case of applying the 4/10 µs impulse electric current, the feedthrough breakdown is not generated, but the parting breakdown would be occurred. Therefore, it is considered that the voids existing in the sintered body act in different manners on the surge withstanding capability for the 2 ms rectangular electric current and 4/10 µs impulse electric current. The feedthrough breakdown is a breakdown such that a hole having a diameter of about 1 mm is formed through the voltage non-linear resistor and thus the resistance thereof becomes 1 kΩ or less so that the non-linear voltage current characteristic is substantially completely disappeared. And, the parting breakdown is a breakdown by which the voltage non-linear resistor is cracked or crushed and is broken into many pieces. As explained above, it is considered that the parting breakdown is attributable to the thermal stress generated in the sintered body when the impulse electric current is applied thereto. Also, in the method disclosed in the Japanese Patent Laid-open Publication Kokai Sho No. 58-28,802, the shaped body is sintered under the reduced pressure until the sintering temperature becomes to 1,150°C, so that the added component or components as an additive are vapored, so that the uniformly sintered body can not be obtained. And, the oxidation of the sintered body is started when the sintering temperature becomes over 1,150°C. Therefore, if the shaped body has a large dimension such as 47 mm in diameter, 25 mm thickness, the oxidation is not effected sufficiently up to the center of the body, so that the non-linear voltage current characteristics same as that of resistor sintered under the normal pressure can not be attained. If sintering time is extended in order that the sufficient oxidation is effected inside the sintered body, zinc oxide grains are grown too much in the sintered body, so that the threshold voltage (V 1mA /mm) at which the resistor begins to show the non-linear voltage-current characteristic becomes unfavorably low. This threshold voltage (V 1mA /mm) is a voltage such that the non-linear voltage current characteristic is appeared, and may be defined as a voltage appearing across unit thickness viewed in the direction of the electric current when the electric current of 1 mA is supplied to the resistor under the application. As a measure for restraining the evaporation of the added component during the sintering under the reduced pressure, it is suggested that the shaped body is buried in powders including the relevant component and then is sintered. In this case, if the sintering temperature is increased until the sintered body is densified, the powders are adhered or applied to the sintered body so strongly that the side surface of the body is not smooth. Furthermore, it is necessary to form high resistance layers on the side surfaces of voltage non-linear resistors to be used in the over-voltage protecting devices such as normal lightning arrestors in order to effectively prevent flashover along the side surface. The resistance layer is usually formed by applying an inorganic material layer on the side surface of the body to be sintered, and reacting the inorganic material with the material constituting said surface by sintering the body. Therefore, it is very important that the inorganic material applied on the surface is not separated therefrom during the sintering. In the known method disclosed in Japanese Patent Publication Kokai Sho No. 58-28,802, the coherency between the body to be sintered and the inorganic material is small because the body to which the inorganic material should be applied is a shaped body or a degreased body. Also, since the body to be sintered is suddenly shrinked at the sintering temperature of about 850°C, there is so large difference in the shrinkage between the inorganic material and the shaped body to be sintered that the inorganic material would be peeled from the body. Thus, there is a drawback in the conventional art that the high resistance layer can not be formed firmly and uniformly on the side surface of the voltage non-linear resistor. The object of the present invention is, obviating the above-mentioned inconvenience, to provide a method of manufacturing a voltage non-linear resistor having an excellent voltage non-linear characteristic and a high density. It is another object of the invention to provide a method of manufacturing a voltage non-linear resistor in which the high resistance layer can be formed easily and positively on the side surfaces of the resistor. It is still another object of the invention to provide a highly densified voltage non-linear resistor. According to the invention, there is provided a method of manufacturing a voltage non-linear resistor as set out in claim 1. According to a further aspect of the invention, there is provided a voltage non-linear resistor as set out in claim 6 the relative density is preferably at least 98%. In the method according to the invention, the sintering is carried out in completely separated two steps. That is to say, the primary sintering (provisional sintering) is carried out under the reduced pressure, and thereafter the secondary sintering (regular sintering) is performed under the partial pressure of oxygen which is higher than that of the primary sintering. Since during the primary sintering under reduced pressure, voids are removed to a large extent, and a small amount of remaining voids are almost all removed from the body during the secondary sintering. Further, the oxidation is sufficiently carried out during the secondary sintering. Thus, a sintered body having high density and excellent non-linear voltage-current characteristics can be obtained and the surge withstanding capability of the thus obtained body is improved. By the invention, a voltage non-linear resistor having a relative density equal to or higher than 98% may be obtained by sintering the body under the normal pressure without using a complicated and expensive densification technique such as HIP (Hot Isostatic Press), etc. That is to say, in order to remove the voids sufficiently from the finally sintered body during the secondary sintering process under the normal pressure, the sintered body after the primary sintering should satisfy the condition that the density and open porosity thereof are 85% or more and 1% or less, respectively. It has been experimentally confirmed that the above mentioned condition could be satisfied when the primary sintering under the reduced pressure is carried out for 1∼10 hours at a temperature of 900∼1,000°C. The density of shaped body and the dispersion of additives (Bi₂O₃, etc.) also effect to the quality of the preliminarily sintered body. That is to say, when the density of shaped body is higher, or when the dispersion of additives is higher, the shaped body is densified even at a lower temperature. Therefore, it is possible to make the primary sintering temperature low, so that the evaporation of additives is restricted to a large extent, and thus uniformly sintered body can be obtained. It is possible to obtain the primarily sintered body having the density of 85% or more and the porosity of 1% or less by sintering the shaped body under the atmospheric pressure. However, in this case, the pressure in the voids in the sintered body becomes high, and a viscosity of liquid phase formed by the additives becomes high so that the distribution of the liquid phase becomes ununiform. Therefore, if the thus sintered body is subjected to the secondary sintering under the same condition as that according to the present invention, the relative density of 98% or more could not be achieved. Namely, the very high relative density of 98% or more can never be achieved unless the primary sintering is carried out under the reduced pressure as defined in the present invention. In the present invention, since the primary sintering is carried out under the reduced pressure, in case that an additive having a high vapor pressure such as Bi₂O₃ is used, Bi₂O₃ is likely to be evaporated. In order to prevent the evaporation of Bi₂O₃, it is desirable to effect the primary sintering, while the shaped body is buried in powders which are consisting of zinc oxide as the main component and at least Bi₂O₃. Further, it is more desirable that the powders have the same chemical composition as that of the body to be sintered. The effect of such buried sintering under the reduced pressure will be explained in the following. That is to say, in the vicinity of the boundary between the powders and the sintering atmosphere, the high vapor pressure component in the powders, such as Bi₂O₃, is actively evaporated, but in the vicinity of the surface of the body to be sintered, the evaporation of Bi₂O₃ from the body is restrained because the Bi₂O₃ vapor pressure is almost saturated therein. On the other hand, since the partial pressures of oxygen and nitrogen are reduced in a furnace, the air which goes out of the body is exhausted into the atmosphere in the furnace. Even if the buried sintering is carried out under the atmospheric pressure, the air would be also restrained to go out into the atmosphere, so that the voids are not removed sufficiently. In case the shaped body is not buried in the powders during preliminary sintering, the powders should not cohere with the body so strong otherwise they would not be separated from each other thereafter, and there should not be produced any ununiformity of the chemical composition in the sintered body. Such effects of the buried sintering is achieved only when the primary and secondary sinterings are independently conducted. If the secondary sintering is conducted also in the buried manner, the powders for burying the body would be adhered strongly onto the surface of the body, so that the sintered body having a smooth outer surface can not be obtained. It has been found experimentally that the desired secondary sintering temperature is 1,050∼1,300°C, otherwise the body would not be densified, oxidation would not be carried out sufficiently up to the inside of the body and therefore an excellent non-linear voltage current characteristic would not be attained. It is necessary to increase the partial pressure of oxygen during the secondary sintering such that the main component of the sintered body and additives are sufficiently oxidized. According to the invention, it is necessary to effect the secondary sintering in the oxidizing atmosphere having the partial pressure of oxygen higher than that of the primary sintering condition. The normal atmospheric pressure is more desirable because the atmosphere in the furnace can be controlled easily. In this case, it is possible to pressurize the air or oxygen in the furnace during the secondary sintering in order to promote the oxidation of the sintered body. As explained in the above, the primary sintering density guarantees the high densification, and the secondary sintering promotes the oxidation and densification as well as the grain growth of zinc oxide in the sintered body. Thus, the diameter of the grain of zinc oxide in the sintered body can be easily controlled, and thus the voltage non-linear resistor having the desired threshold voltage (V 1mA) can be manufactured. In a preferred embodiment of the method according to the invention, after the primary sintering, an inorganic material layer is applied on the side surface of the body and thereafter the assembly is subjected to the secondary sintering. In this case, since the adhesive force between the first sintered body and the inorganic material layer is strong and the primarily sintered body is no more shrinked so much during the secondary sintering, and thus the difference in shrinkage between the body and the inorganic material layer applied thereon is small. Therefore, the high resistance layer is firmly adhered onto the side wall of sintered body, so that the flashover can be effectively prevented. Example 1 To ZnO powders were mixed additive powders Bi₂O₃, Sb₂O₃, Cr₂O₃, Co₂O₃, MnO₂, NiO, SiO₂ and Aℓ₂O₃ at rates listed in a Table 1, column 1. After the mixture was mixed with a binding agent to form a slurry, the slurry was granulated to obtain grains. Then, the paste was shaped into a cylindrical body. In this manner, forty cylindrical bodies were made. The thus formed cylindrical bodies were embedded in powders consisting of the same chemical composition as that of the mixture and were placed in a furnace. The shaped bodies were embedded in the powders in a depth of 10 mm from the surface thereof. Then, the furnace was heated from the room temperature to 900°C at a heating rate of about 50°C/H. It should be noted that this heating step is continued for about eighteen hours. Before initiating the heating, the pressure inside the furnace was reduced to 1 Torr or when the temperature of the furnace was increased near 900°C, the pressure inside the furnace is reduced to 1 Torr. Then, the shaped body was heated at 900°C for two hours under the reduced pressure of 1 Torr. Then, the furnace was cooled at the usual cooling rate of about 60°C/H to the room temperature. In this manner, the primary sintering process was carried out for about thirty six hours. Then, the relative density and open porosity of primarily sintered bodies were measured by means of the usual methods. The results of these measurements are also listed in the Table 1. Next, an inorganic material paste consisting of Bi₂O₃, Sb₂O₃ and SiO₂ was applied on the side wall of the body. After the inorganic material layer was dried to evaporate a binder solvent, the bodies were placed in a furnace and the furnace was heated from the room temperature to 1,300°C at the rate of 50°C/H. Then, the furnace was kept at 1,300°C for five hours under the atmospheric pressure of 760 Torr. After that, the furnace was cooled at the rate of about 60°C/H to the room temperature. In this manner, the secondary sintering was carried out under the atmospheric pressure for more than fifty hours. Then the relative density of ten sintered bodies was measured. At the same time, these ten sintered bodies were used to measure the mechanical strength. This measurement was effected under the testing method defined by JIS (Japanese Industrial Standards) R1601, i.e. the flexural strength was measured by applying a load at four points. An average value and its standard deviation were derived in a unit of mega Pascal (MPa). The measured values are also listed in the Table 1. Opposite surfaces of the remaining twenty cylindrical sintered bodies were polished and aluminum electrodes were applied on the polished surfaces by aluminum flange spraying. In this manner, there were obtained twenty voltage non-linear resistors having a diameter of 47 mm and a thickness of 22.5 mm with the electrodes having a diameter of 46 mm. Then the threshold voltage V 1mA /mm under the application of the electric current of 1 mA, the non-linear index α, and the surge withstanding capability were measured. It should be noted that the non-linear index α is represented by an equation, I=(V/C) α , wherein I represents the current, V the voltage and C denotes a constant. Further, the surge withstanding capability was measured by supplying 4/10 µs impulse current to the resistors twice with interposing a pause of five minutes and by increasing the amplitude of the current from 60 KA in a stepwise manner at a step of 10 KA until the resistor was broken. An average current at which the twenty resistors were broken and its standard deviation are indicated in the Table 1 together with V 1mA /mm and α. Similar experiments were conducted by varying various factors such as the composition of the mixture, and the maximum temperature and time period of the primary and secondary sintering processes. The similar data as that explained above with reference to the Example 1 are also shown in the Table 1. As is seen in Table 1, in Examples 1∼6 according to the present invention the relative density and open porosity of the primarily sintered bodies are larger than 88% and smaller than 0.5%, respectively, and the relative density of the secondarily sintered body is larger than 98%. It has been experimentally confirmed that the threshold voltage at which the non-linearity begins to appear can be adjusted by controlling the secondary sintering temperature. In this manner, according to the invention, it is possible to manufacture the voltage non-linear resistor having the high density and high surge withstanding capability. And, it was proved from the Examples 7∼10 that even if the composition constituting the body to be sintered are different, the same results described above can be obtained. In the Table 1, there are also shown eleven Comparative Examples. In the Comparative Examples 1∼3, the primary sintering temperature was 850°C, so that the relative density and open porosity of the primarily sintered bodies are less than 84% and more than 16%, respectively. In the Comparative Example 4, during the primary sintering process the bodies were heated at 850°C for ten hours, so that the relative density is higher than 88%, but the open porosity is larger than 9%. In the Comparative Example 5, the bodies were heated up to 1,000°C at the rate of 200°C/H. In this case, although the open porosity is smaller than 0.5%, the relative density is smaller than 85%. The Comparative Examples 6∼8 are similar to the known method disclosed in the above mentioned Japanese Laid-open Publication, Kokai Sho 58-28,802. In these examples, the relative density of the sintered bodies is smaller than 97%. It was further found that the inorganic material layer was not firmly adhered to the side wall of the cylindrical body, so that the flashover could not be prevented efficiently. From the Comparative Example 6, it was proved that the oxidation was not carried out sufficiently, so that the non-linearity index α is very small. From the Comparative Example 8, it was also confirmed that when the heating rate is made higher, the densitification of the sintered body could not be achieved even if the sintering is partially effected under the reduced pressure. In the Comparative Examples 9 and 10, the primary sintering was carried out under the atmospheric pressure instead of the reduced pressure. In this case, although the primarily sintered bodies had the relative density higher than 84% and the open porosity smaller than 0.6%, the finally sintered bodies could not have the relative density higher than 96%. In the Comparative Example 11, the second sintering was conducted under the reduced pressure. In this case, the relative density of the finally sintered bodies was higher than 99%, but the non-linear index α was too small to carry out the withstanding capability test. From the above experiments, it has been found that the primary sintering has to be preferably conducted such that the primarily sintered body has the relative density equal to or higher than 85% and the open porosity equal to or lower than 1%. In order to satisfy the above mentioned preferable property, it has been confirmed that the primary sintering temperature should be set to a value within a range of 900∼1,000°C. Then, it is possible to obtain the finally sintered body having the relative density equal to or higher than 98%. The inventors of the present application further conducted experiments, and the experimental data is shown in a Table 2. In these experiments the finally sintered cylindrical body had the diameter of 28 mm and the thickness of 18 mm, and the aluminum electrode had the diameter of 25 mm. In the Table 2, the void evaluation ○ represents the condition that there is no void having the diameter of 10 µm or more, and the mark X expresses the condition that voids having the diameter larger than 10 µm are produced in the sintered body. It should be noted that the composition of the starting material and the sintering conditions of the Example 2 in the Table 2 are identical with those of the Comparative Example 1 in the Table 1, but the finally sintered body of the Example 2 in the Table 2 has the desired property. This is due to the fact that the size of the sintered body of the Example 1 in the Table 2 is smaller than that of the Comparative Example 1 in the Table 1. In the Comparative Example 1 in the Table 2, the primary sintering was carried out under the atmospheric pressure of 760 Torr, in the Comparative Example 2, the secondary sintering was conducted under the reduced pressure of 1 Torr, and in the Comparative Example 3, the inorganic material layer was applied on the side surface of the shaped body before the primary sintering was effected. As is seen in the Table 2, in the voltage non-linear resistor manufactured by the method according to the invention any void having the diameter larger than 10 µm could not be found, and the bulk density and four points flexure strength are sufficiently high. And also, it is seen from the Table 2 that in the voltage non-linear resistors according to the invention, the voltage non-linearity index α is very large and the surge withstanding capability is also high. The reason why the bulk density and surge withstanding capability are improved in the present invention, compared with the Comparative Example 1 in which the primary sintering is effected under the atmospheric pressure, is as follows. That is to say, Bi₂O₃, one of the compositions of the shaped body is molten at the temperature about 850°C and forms the liquid phase, so the body is shrinked suddenly about at this temperature of 850°C. The sudden shrinkage of the body is due to the capillary pressure of the liquid phase, however, under the reduced pressure, the liquid phase is likely immersed into the spaces between the particles, and bubbles in the liquid phase are liable to escape from the liquid phase, and thus the body is shrinked largely. In other words, the voids are decreased and the bulk density becomes high. As a result, the local electric current concentration at the tip of the void is hardly occurred. And as the voids are decreased, the mechanical strength of the sintered body becomes high. Thus, the breakdown of the resistor due to the thermal stress is so prevented that the surge withstanding capability of the resistor is increased. In the Comparative Example 2, the bulk density is much better than that of the Comparative Example 1, but the threshold voltage V 1mA /mm and the voltage non-linearity index α are smaller than those of examples according to the present invention because the oxidation during the secondary sintering could not be carried out sufficiently. In the Comparative Example 3, there is recognized the improvement in the bulk density, but the inorganic material layer applied on the side surface of the body was peeled off due to the sudden shrinkage of the body during the primary sintering. Thus, when 4/10 µs impulse electric current was supplied to the resistor, the flashover occurred and the surge withstanding capability was low. It is considered that the non-linear voltage current characteristic is caused by the intergranular layers of the additives existing among zinc oxide grains. The non-linear voltage current characteristic of the sintered body is disappeared by the reduction heat treatment, and is appeared again by the oxidation heat treatment (see Journal of Applied Physics, 1983 vol 54, No. 6, pp. 3566∼3572). Therefore, it is considered that the supply of oxygen to the intergranular layer is necessary to attain the non-linear voltage current characteristic in the sintered body. The reason why the threshold voltage V 1mA /mm and the non-linearity index α are small in the Comparative Example 2 is that oxygen was not supplied to the intergranular layer sufficiently. As can be seen from the examples according to the present invention in the Tables 1 and 2, the sintered bodies were densified regardless of the composition of the additives, and therefore the present invention should not be limited to the compositions of additives listed in the Tables 1 and 2. As is evident from the foregoing explanation, in the method according to the present invention, sintering is carried out in two completely separated steps, and the primary sintering is carried out under the reduced pressure at a relatively low temperature and the secondary sintering is conducted under the partial pressure of oxygen higher than that of the primary sintering at a higher temperature. The relative density and open porosity of the primarily sintered body are made 85% or more and 1% or less, respectively. Then, the sufficient oxidation is effected in the sintered body during the secondary sintering. As a result, the finally sintered body having a relative density of 98% or more and an excellent non-linear voltage current characteristic can be obtained, and further the surge withstanding capability is also improved.

Claims: A method of manufacturing a voltage non-linear resistor comprising the following steps: forming a mixture of zinc oxide powder and at least one additive powder, which mixture exhibits voltage non-linearity in a sintered body; granulating the mixture to form mixture grains; shaping the mixture grains into a shaped body having desired shape and size; effecting a primary sintering by heating the shaped body under a reduced pressure lower than atmospheric pressure, so that there is produced a primarily sintered body having a relative density of 85% or more and an open porosity of 1% or less; and effecting a secondary sintering by heating the primarily sintered body under an oxidizing atmosphere having a partial pressure of oxygen higher than that of the primary sintering. A method according to claim 1, further comprising applying an inorganic material layer at least on a side surface of the primarily sintered body after the primary sintering. A method according to claim 1 or claim 2 wherein said primary sintering is performed at a temperature within the range 850∼1000°C. A method according to claim 3 wherein said primary sintering is performed at a temperature within the range 900∼1000°C. A method according to any one of claims 1 to 4 wherein said secondary sintering is carried out at a temperature within a range of 1050∼1300°C. A voltage non-linear resistor comprising a sintered body comprising zinc oxide as a main composition and at least one kind of additive, which body exhibits voltage non-linearity and having a relative density of at least 97%. A voltage non-linear resistor according to claim 6 having a relative density of at least 98%.

Inventors: KAWASAKI SHINJI, NEMOTO HIROSHI, UMEMOTO KOICHI, KAWASAKI, SHINJI, NEMOTO, HIROSHI, UMEMOTO, KOICHI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1988
Main classification: H01C17
IPC codes: H01C7, H01C17