EP0594120A2 - A method for producing a ZnO Varistor - Google Patents

A method for producing a ZnO Varistor Download PDF

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Publication number
EP0594120A2
EP0594120A2 EP93116867A EP93116867A EP0594120A2 EP 0594120 A2 EP0594120 A2 EP 0594120A2 EP 93116867 A EP93116867 A EP 93116867A EP 93116867 A EP93116867 A EP 93116867A EP 0594120 A2 EP0594120 A2 EP 0594120A2
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Prior art keywords
zno
powders
varistor
producing
based varistor
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German (de)
French (fr)
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EP0594120A3 (en
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Atsushi Iga
Masahiro Ito
Hideyuki Okinaka
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Panasonic Holdings Corp
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Matsushita Electric Industrial Co Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01C—RESISTORS
    • H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01C—RESISTORS
    • H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
    • H01C7/10—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material voltage responsive, i.e. varistors
    • H01C7/105—Varistor cores
    • H01C7/108—Metal oxide
    • H01C7/112—ZnO type

Definitions

  • the present invention relates to a method for producing a ZnO-based varistor used for absorbing a surge caused in an electrical circuit, and more particularly to a method for producing a low-voltage ZnO-based varistor widely used for protecting a semi-conductor device in an electronic circuit from various surges.
  • ZnO-based varistors are formed of a sintered body containing ZnO varistor materials (i.e., zinc oxide (ZnO), basic additives such as bismuth oxide (Bi2O3), manganese oxide (MnO2) and cobalt oxide (CoO), and various oxides to be added for the purpose of improving the performance of the varistors).
  • ZnO varistor materials i.e., zinc oxide (ZnO), basic additives such as bismuth oxide (Bi2O3), manganese oxide (MnO2) and cobalt oxide (CoO), and various oxides to be added for the purpose of improving the performance of the varistors.
  • the clamping voltage of each ZnO-based varistor is known to increase nearly in proportion to the number of grain boundaries present between electrodes. That is to say, the clamping voltage of the ZnO-based varistor increases by 3 to 4 V per one grain boundary.
  • a grain-growth-enhancing additive such as titanium oxide (TiO2) has been added to enhance the growth of ZnO grains.
  • the method for producing a ZnO-based varistor of the present invention includes the steps of: heat-treating a mixture of TiO2 powders and Bi2O3 powders to prepare composite powders; and adding the composite powders to ZnO varistor materials.
  • the above-mentioned method further includes the step of adding an aluminum component to the ZnO varistor materials.
  • the aluminum component is added to the ZnO varistor materials so that the weight ratio of Al2O3 powders to the ZnO powders is in the range of 0.00062 : 100.0 to 0.372 : 100.0.
  • the aluminum component is a solution of an aluminum salt.
  • the aluminum salt is aluminum nitrate.
  • the aluminum salt is aluminum acetate.
  • the above-mentioned method further includes the step of adding an antimony component to the ZnO varistor materials.
  • the antimony component is added to the ZnO varistor materials so that the weight ratio of Sb2O3 powders to the ZnO powders is in the range of 0.018 : 100.0 to 0.72 : 100.0.
  • the above-mentioned method further includes the step of adding a tin component to the ZnO varistor materials.
  • the tin component is added to the ZnO varistor materials so that the weight ratio of SnO2 powders to the ZnO powders is in the range of 0.005 : 100.0 to 0.37 : 100.0.
  • the above-mentioned method further includes the step of adding a chrome component to the ZnO varistor materials.
  • the chrome component is added to the ZnO varistor materials so that the weight ratio of Cr2O3 powders to the ZnO powders is in the range of 0.005 : 100.0 to 0.18 : 100.0.
  • the present invention has been achieved as a result of the earnest study of a mechanism of grain-growth-enhancing by using TiO2. More specifically, the following was found: When TiO2 is reacted with ZnO, the growth of ZnO grains cannot be enhanced. On the other hand, when TiO2 is reacted with Bi2O3, the reaction product can enhance the growth of ZnO grains. Therefore, in the case where TiO2 is merely added as in the conventional method, in some parts of the sintered body, TiO2 is reacted with ZnO and in other parts thereof, TiO2 is reacted with Bi2O3.
  • the growth of ZnO grains are not enhanced, and in other parts of the sintered body, the growth of ZnO grains are enhanced.
  • a mixture of TiO2 and Bi2O3 is previously heat-treated to prepare composite powders.
  • the composite powders can uniformly enhance the growth of ZnO grains.
  • an antimony component, a tin component, or a chrome component together with an antimony component suppresses the abnormal growth of ZnO grains.
  • the addition of either of these components makes the ZnO grains twin crystal, thereby suppressing the abnormal growth of ZnO grains.
  • the ZnO grains having a large average grain size and a small distribution of grain sizes can be obtained.
  • These phenomenons were found by the inventors of the present invention as a result of their earnest study of a method for adding a bismuth component and a titanium component to ZnO varistor materials.
  • an aluminum component dissolves in the ZnO grains in a solid state to function as a donor, thereby decreasing electrical resistance.
  • the invention described herein makes possible the advantage of providing a method for producing a low-voltage ZnO-based varistor with high yield, which is excellent in electrical characteristics and reliability at a low voltage.
  • Figure 1 is a schematic view showing a ZnO-based varistor produced in the examples according to the present invention.
  • TiO2 fine powders and Bi2O3 powders were mixed in a weight ratio of 11.4 : 88.6.
  • the mixture thus obtained was heat-treated at 600°C for 5 hours. Then, the mixture was ground to obtain composite powders.
  • the composite powders thus obtained will be referred to as TiO2/Bi2O3 composite powders.
  • ZnO powders, TiO2/Bi2O3 composite powders, CoO powders and MnO2 powders were mixed in a weight ratio of 100.0 : 2.93 : 0.804 : 0.555 by a wet method and ground.
  • the resulting powders were dried and formed by pressure molding.
  • the molding thus obtained was provisionally sintered at 600°C and ground.
  • the ground molding was formed into a disk shape.
  • the disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1250°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body.
  • the sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • the ZnO-based varistor was measured for V 1mA/mm and non-linear resistance index 0.1 ⁇ 1mA .
  • V 1mA/mm refers to a voltage with respect to 1 mm thickness of the ZnO-based varistor between electrodes, in a case where an electric current of 1 mA flows through the lead wire 13 ;
  • the non-linear resistance index 0.1 ⁇ 1mA refers to a value obtained by using V 1mA and V 0.1mA .
  • the ZnO-based varistor was evaluated for reliability with respect to a DC load.
  • the ZnO-based varistor was added with a DC load of 0.2 watts at 80°C for 500 hours.
  • the change rate ⁇ V 1mA /V 1mA (change rate due to a DC load) of a clamping voltage V 1mA of the ZnO-based varistor was measured.
  • the ZnO-based varistor was evaluated for reliability with respect to a surge.
  • the ZnO-based varistor was twice applied with a pulse of 0.5 kA for 8 x 20 ⁇ sec to obtain the change rate ⁇ V 1mA /V 1mA (change rate due to a surge).
  • Table 1 The composition of the specimen of the present example is shown in Table 1, and the evaluation results of the electrical characteristics thereof are shown in Table 2. The numerals representing the evaluation results are the maximum and the minimum values of in each lot.
  • the ZnO-based varistor produced by the method of the present example has excellent reliability. That is to say, in the ZnO-based varistor of the present invention, the clamping voltage V 1mA was low and the absolute value of the change rate ⁇ V 1mA /V 1mA of the clamping voltage V 1mA with respect to the application of a DC load or a surge for a long period of time was not more than 5%. In addition, as shown in Table 2, the variation of the electrical characteristics between the ZnO-based varistors obtained from one lot was small.
  • the yield of the ZnO-based varistors were remarkably improved.
  • a ZnO-based varistor was produced by a conventional method, using a sintered body having the same composition as that of Example 1.
  • ZnO powders, Bi2O3 powders, TiO2 fine powders, CoO powders, and MnO2 powders were mixed in a weight ratio of 100.0 : 2.60 : 0.33 : 0.804 : 0.555 by a wet method and ground.
  • the powders thus obtained were dried and formed by pressure molding.
  • the molding thus obtained was provisionally sintered at 600°C and ground.
  • the ground molding was formed into a disk shape.
  • the disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1250°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body.
  • the sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • Example 1 a ZnO-based varistor was produced. Also, in the same way as in Example 1, the ZnO-based varistor thus obtained was evaluated for electrical characteristics. Table 3 shows the composition of the specimen of this comparative example, and Table 4 shows the evaluation results of the electrical characteristics thereof.
  • the clamping voltage V 1mA after the application of a DC load of 0.2 watts was remarkably decreased, and the absolute value of the change rate ⁇ V 1mA /V 1mA of the clamping voltage V 1mA with respect to the application of a DC load was not less than 25%.
  • the absolute value of the change rate ⁇ V 1mA /V 1mA of the application of a surge was more than 40%.
  • the reliability of the ZnO-based varistor produced by the conventional method was remarkably low.
  • the variation of the electrical characteristics between the ZnO-based varistors obtained from one lot was large.
  • the variation of the electrical characteristics between the ZnO-based varistors obtained from different lots was larger than that from one lot (not shown in Table 4).
  • the ZnO-based varistors had V 1mA/mm and a non-linear resistance index 0.1 ⁇ 1mA lower than those shown in Table 4.
  • the ZnO-based varistor produced by the method of the present invention is more excellent than the ZnO-based varistor produced by the conventional method in the initial electrical characteristics, the reliability, and the variation of the electrical characteristics between the ZnO-based varistors obtained from one lot and between the ZnO-based varistors obtained from different lots.
  • TiO2 fine powders and Bi2O3 powders were mixed in a weight ratio of 20.5 : 79.5.
  • the mixture thus obtained was heat-treated at 800°C for 5 hours.
  • the mixture was ground to obtain TiO2/Bi2O3 composite powders.
  • the composite powders contained Bi4Ti3O12 as a main component thereof.
  • ZnO powders, TiO2/Bi2O3 composite powders, CoO powders and MnO2 powders were mixed in a weight ratio of 100.0 : 3.15 : 0.922 : 0.534 by a wet method and ground.
  • the resulting powders were dried and formed by pressure molding.
  • the molding thus obtained was provisionally sintered at 600°C and ground.
  • the ground molding was formed into a disk shape.
  • the disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1250°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body.
  • the sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • Example 1 a ZnO-based varistor was produced in the same way as in Example 1. Moreover, in the same way as in Example 1, the ZnO-based varistor thus obtained was evaluated for electrical characteristics.
  • the composition of the specimen of the present example is shown in Table 5, and the evaluation results of the electrical characteristics thereof are shown in Table 6.
  • the ZnO-based varistor produced by the method of the present example has excellent reliability. That is to say, in the ZnO-based varistor of the present invention, the clamping voltage V 1mA was low and the absolute value of the change rate V 1mA /V 1mA of the clamping voltage V 1mA with respect to the application of a DC load or a surge for a long period of time was not more than 5%. In addition, as shown in Table 6, the variation of the electrical characteristics between the ZnO-based varistors obtained from one lot was small.
  • the yield of the ZnO-based varistors were remarkably improved.
  • TiO2 fine powders and Bi2O3 powders were mixed in a weight ratio of 11.4 : 88.6.
  • the mixture thus obtained was heat-treated at 600°C for 5 hours. Then, the mixture was ground to obtain TiO2/Bi2O3 composite powders.
  • the resulting powders were dried and formed by pressure molding.
  • the molding thus obtained was provisionally sintered at 600°C.
  • An aqueous solution of aluminum nitrate was added to the molding so that the weight ratio of the ZnO powders to Al2O3 was 100.0 : 0.007 and ground. Then, the ground molding was formed into a disk shape.
  • the disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1200°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body.
  • the sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • a ZnO-based varistor was produced in the same way as in Example 1. Also, in the same way as in Example 1, the ZnO-based varistor was evaluated for electrical characteristics.
  • the composition of the specimen of the present example is shown in Table 7, and the evaluation results of the electrical characteristics thereof are shown in Table 8.
  • the ZnO-based varistor produced by the method of the present example has excellent reliability. That is to say, in the ZnO-based varistor of the present example, the non-linear resistance index 0.1 ⁇ 1mA was large in the case where the added amount of the TiO2/Bi2O3 composite powders was not less than 0.2 in a weight ratio (Specimen Nos. 012 - 018), and the absolute value of the change rate ⁇ V 1mA /V 1mA of the clamping voltage V 1mA with respect to the application of a DC load or a surge for a long period of time was not more than 5%.
  • the added amount of the TiO2/Bi2O3 composite powders was not less than 20 in a weight ratio (Specimen No. 019), if a plurality of moldings were sintered under the condition that the moldings were layered, the sintered bodies stuck to each other, decreasing mass production. Thus, this added amount was not suitable for mass production.
  • TiO2 fine powders and Bi2O3 powders were mixed so that the weight ratio of the Bi2O3 powders to the TiO2 fine powders was in the range of 1.5 to 59.
  • the mixture thus obtained was heat-treated at 500°C for 5 hours. Then, the mixture was ground to obtain TiO2/Bi2O3 composite powders.
  • ZnO powders, CoO powders, MnO2 powders, Sb2O3 powders, and TiO2/Bi2O3 composite powders were mixed in a weight ratio of 100.0 : 0.555 : 0.291 : 3.0 by a wet method and ground.
  • the resulting powders were dried and formed by pressure molding.
  • the molding thus obtained was provisionally sintered at 600°C.
  • An aqueous solution of aluminum nitrate was added to the molding so that the weight ratio of ZnO to Al2O3 was 100.0 : 0.001 and ground.
  • the ground molding was formed into a disk shape.
  • the disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1200°C for 2 hours.
  • the molding was cooled at 100°C/hour to obtain a sintered body.
  • the sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • the sintered body thus obtained was observed by a scanning electron microscope (SEM), revealing that the sintered body had a characteristic fine structure having twin crystal grains of ZnO.
  • a ZnO-based varistor was produced in the same way as in Example 1. Also, in the same way as in Example 1, the ZnO-based varistor was evaluated for electrical characteristics. In particular, in order to inspect the reliability of the ZnO-based varistor with respect to a surge in detail, the change rate ⁇ V 0.01mA /V 0.01mA of the clamping voltage V 0.01mA of the ZnO-based varistor was measured, since the change of the reliability at 0.01 mA with respect to a surge was remarkably exhibited.
  • the ZnO-based varistor was applied with a pulse of 1.0 kA for 8 x 20 ⁇ sec to obtain the change rate ⁇ V 0.01mA /V 0.01mA of the clamping voltage V 0.01mA thereof.
  • the composition of the specimen of the present example is shown in Table 9, and the evaluation results of the electrical characteristics thereof are shown in Table 10.
  • TiO2 fine powders and Bi2O3 powders were mixed in a weight ratio of 25 : 75.
  • the mixture thus obtained was heat-treated at 650°C for 5 hours.
  • the mixture was ground to obtain TiO2/Bi2O3 composite powders.
  • the resulting powders were dried, and an aqueous solution of aluminum acetate was added to the dried powders so that the weight ratio of the ZnO powders to Al2O3 was 100.0 : 0.001.
  • the resulting powders were provisionally sintered at 600°C.
  • the molding was ground and mixed.
  • the resulting mixture was formed into a disk shape.
  • the disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1200°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body.
  • the sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • the sintered body thus obtained was observed by a scanning electron microscope (SEM), revealing that the sintered body had a characteristic fine structure having twin crystal grains of ZnO.
  • a ZnO-based varistor was produced in the same way as in Example 1. Also, in the same way as in Example 1, the ZnO-based varistor was evaluated for electrical characteristics. In particular, in order to inspect the reliability of the ZnO-based varistor with respect to a surge in detail, the change rate ⁇ V 0.01mA /V 0.01mA of the clamping voltage V 0.01mA of the ZnO-based varistor was measured, since the change of the reliability at 0.01 mA with respect to a surge was remarkably exhibited.
  • the ZnO-based varistor was applied with a pulse of 1.0 kA for 8 x 20 ⁇ sec to obtain the change rate ⁇ V 0.01mA /V 0.01mA of the clamping voltage V 0.01mA thereof.
  • the composition of the specimen of the present example is shown in Table 11, and the evaluation results of the electrical characteristics thereof are shown in Table 12.
  • the resulting powders were dried, and an aqueous solution of aluminum acetate was added to the dried powders so that the weight ratio of the ZnO powders to Al2O3 was 100.0 : 0.001.
  • the resulting powders were provisionally sintered at 600°C. Then, the molding was ground and mixed. The resulting mixture was formed into a disk shape. The disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1200°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body.
  • the sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • the sintered body thus obtained was observed by a scanning electron microscope (SEM), revealing that the sintered body had a characteristic fine structure having twin crystal grains of ZnO.
  • a ZnO-based varistor was produced in the same way as in Example 1. Also, in the same way as in Example 1, the ZnO-based varistor was evaluated for electrical characteristics. In particular, in order to inspect the reliability of the ZnO-based varistor with respect to a surge in detail, the change rate ⁇ V 0.01mA /V 0.01mA of the clamping voltage V 0.01mA of the ZnO-based varistor was measured, since the change of the reliability at 0.01 mA with respect to a surge was remarkably exhibited.
  • the ZnO-based varistor was applied with a pulse of 1.0 kA for 8 x 20 ⁇ sec to obtain the change rated ⁇ V 0.01mA /V 0.01mA of the clamping voltage V 0.01mA thereof.
  • the composition of the specimen of the present example is shown in Table 13, and the evaluation results of the electrical characteristics thereof are shown in Table 14.
  • composite powders obtained by previously heat-treating the mixture of TiO2 powders and Bi2O3 powders can uniformly enhance the growth of ZnO grains.
  • an antimony component, a tin component, or a chrome component together with an antimony component suppresses the abnormal growth of ZnO grains.
  • a ZnO grain having a large average grain size and a small distribution in grain size can be obtained. Namely, a ZnO-based varistor excellent in electrical characteristics and reliability can be produced with a high yield.

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Abstract

A method for producing a ZnO-based varistor includes the steps of: heat-treating a mixture of TiO₂ powders and Bi₂O₃ powders so as to prepare composite powders; and adding the composite powders to ZnO varistor materials.

Description

    BACKGROUND OF THE INVENTION 1. Field of the Invention:
  • The present invention relates to a method for producing a ZnO-based varistor used for absorbing a surge caused in an electrical circuit, and more particularly to a method for producing a low-voltage ZnO-based varistor widely used for protecting a semi-conductor device in an electronic circuit from various surges.
    • 2. Description of the Related Art:
  • ZnO-based varistors are formed of a sintered body containing ZnO varistor materials (i.e., zinc oxide (ZnO), basic additives such as bismuth oxide (Bi₂O₃), manganese oxide (MnO₂) and cobalt oxide (CoO), and various oxides to be added for the purpose of improving the performance of the varistors). The clamping voltage of each ZnO-based varistor is known to increase nearly in proportion to the number of grain boundaries present between electrodes. That is to say, the clamping voltage of the ZnO-based varistor increases by 3 to 4 V per one grain boundary. Therefore, in order to produce a low-voltage ZnO-based varistor, it is required that a sintered body containing ZnO grains having a large size is produced. Conventionally, a grain-growth-enhancing additive such as titanium oxide (TiO₂) has been added to enhance the growth of ZnO grains.
  • However, the addition of TiO₂ sometimes causes an abnormal growth of ZnO grains, resulting in a large distribution in size of ZnO grains. Thus, it has been difficult to uniformly enhance the growth of the grains.
  • An electric current is likely to flow through a path in the ZnO grains between electrodes, where the least grain boundaries are present. Therefore, in the case where the distribution in size of ZnO grains in the sintered body is large, an electric current intensively flows through that part of the sintered body. Accordingly, there has been problems in that the electrical characteristics of the low-voltage ZnO-based varistor are decreased within a short period of time and so the reliability thereof is low. In addition, since it has been difficult to sufficiently control the abnormal growth of ZnO grains, there has been a large variation of electrical characteristics and the reliability between Zno-based varistors obtained from one lot (in-lot variation) and between ZnO-based varistors obtained from different lots (inter-lot variation).
  • As described above, according to the conventional production method, low-voltage ZnO-based varistors with excellent electrical characteristics and reliability cannot be stably produced.
    • SUMMARY OF THE INVENTION
  • The method for producing a ZnO-based varistor of the present invention, includes the steps of:
       heat-treating a mixture of TiO₂ powders and Bi₂O₃ powders to prepare composite powders; and
       adding the composite powders to ZnO varistor materials.
  • In one embodiment of the present invention, the above-mentioned method further includes the step of adding an aluminum component to the ZnO varistor materials.
  • In another embodiment of the present invention, the aluminum component is added to the ZnO varistor materials so that the weight ratio of Al₂O₃ powders to the ZnO powders is in the range of 0.00062 : 100.0 to 0.372 : 100.0.
  • In still another embodiment of the present invention, the aluminum component is a solution of an aluminum salt.
  • In still another embodiment of the present invention, the aluminum salt is aluminum nitrate.
  • In still another embodiment of the present invention, the aluminum salt is aluminum acetate.
  • In still another embodiment of the present invention, the above-mentioned method further includes the step of adding an antimony component to the ZnO varistor materials.
  • In still another embodiment of the present invention, the antimony component is added to the ZnO varistor materials so that the weight ratio of Sb₂O₃ powders to the ZnO powders is in the range of 0.018 : 100.0 to 0.72 : 100.0.
  • In still another embodiment of the present invention, the above-mentioned method further includes the step of adding a tin component to the ZnO varistor materials.
  • In still another embodiment of the present invention, the tin component is added to the ZnO varistor materials so that the weight ratio of SnO₂ powders to the ZnO powders is in the range of 0.005 : 100.0 to 0.37 : 100.0.
  • In still another embodiment of the present invention, the above-mentioned method further includes the step of adding a chrome component to the ZnO varistor materials.
  • In still another embodiment of the present invention, the chrome component is added to the ZnO varistor materials so that the weight ratio of Cr₂O₃ powders to the ZnO powders is in the range of 0.005 : 100.0 to 0.18 : 100.0.
  • The present invention has been achieved as a result of the earnest study of a mechanism of grain-growth-enhancing by using TiO₂. More specifically, the following was found: When TiO₂ is reacted with ZnO, the growth of ZnO grains cannot be enhanced. On the other hand, when TiO₂ is reacted with Bi₂O₃, the reaction product can enhance the growth of ZnO grains. Therefore, in the case where TiO₂ is merely added as in the conventional method, in some parts of the sintered body, TiO₂ is reacted with ZnO and in other parts thereof, TiO₂ is reacted with Bi₂O₃. Namely, in some parts of the sintered body, the growth of ZnO grains are not enhanced, and in other parts of the sintered body, the growth of ZnO grains are enhanced. Thus, according to the conventional production method, it has been impossible to produce a sintered body with a small distribution in size using ZnO grains. According to the present invention, a mixture of TiO₂ and Bi₂O₃ is previously heat-treated to prepare composite powders. The composite powders can uniformly enhance the growth of ZnO grains.
  • Moreover, it was found that an antimony component, a tin component, or a chrome component together with an antimony component suppresses the abnormal growth of ZnO grains. The addition of either of these components makes the ZnO grains twin crystal, thereby suppressing the abnormal growth of ZnO grains.
  • Because of the above-mentioned two phenomenons (i.e., the addition of the composite powders obtained by previously heat-treating the mixture of TiO₂ and Bi₂O₃ and the addition of either of an antimony component, a tin component, or a chrome component together with an antimony component), the ZnO grains having a large average grain size and a small distribution of grain sizes can be obtained. These phenomenons were found by the inventors of the present invention as a result of their earnest study of a method for adding a bismuth component and a titanium component to ZnO varistor materials.
  • In addition, an aluminum component dissolves in the ZnO grains in a solid state to function as a donor, thereby decreasing electrical resistance.
  • Thus, the invention described herein makes possible the advantage of providing a method for producing a low-voltage ZnO-based varistor with high yield, which is excellent in electrical characteristics and reliability at a low voltage.
  • This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figure.
    • BRIEF DESCRIPTION OF THE DRAWING
  • Figure 1 is a schematic view showing a ZnO-based varistor produced in the examples according to the present invention.
    • DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • Hereinafter, the present invention will be described by way of illustrative examples with reference to the drawing and tables.
    • Example 1
  • TiO₂ fine powders and Bi₂O₃ powders were mixed in a weight ratio of 11.4 : 88.6. The mixture thus obtained was heat-treated at 600°C for 5 hours. Then, the mixture was ground to obtain composite powders. Hereinafter, the composite powders thus obtained will be referred to as TiO₂/Bi₂O₃ composite powders.
  • ZnO powders, TiO₂/Bi₂O₃ composite powders, CoO powders and MnO₂ powders were mixed in a weight ratio of 100.0 : 2.93 : 0.804 : 0.555 by a wet method and ground. The resulting powders were dried and formed by pressure molding. The molding thus obtained was provisionally sintered at 600°C and ground. Then, the ground molding was formed into a disk shape. The disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1250°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body. The sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • Next, referrering to Figure 1, a method for producing a ZnO-based varistor will be described. Aluminum and then copper were flame-coated onto both sides of a sintered body 11 obtained as described above, whereby electrodes 12 were formed thereon. The electrodes 12 were provided with a lead wire 13 by soldering, and then the sintered body 11 excluding the lead wire 13 was painted in order to produce a ZnO-based varistor.
  • The electrical characteristics of the ZnO-based varistor thus obtained were evaluated. As initial electrical characteristics, the ZnO-based varistor was measured for V1mA/mm and non-linear resistance index 0.1α1mA. Here, V1mA/mm refers to a voltage with respect to 1 mm thickness of the ZnO-based varistor between electrodes, in a case where an electric current of 1 mA flows through the lead wire 13; and the non-linear resistance index 0.1α1mA refers to a value obtained by using V1mA and V0.1mA.
  • In addition, the ZnO-based varistor was evaluated for reliability with respect to a DC load. The ZnO-based varistor was added with a DC load of 0.2 watts at 80°C for 500 hours. In this way, the change rate Δ V1mA/V1mA (change rate due to a DC load) of a clamping voltage V1mA of the ZnO-based varistor was measured. Moreover, the ZnO-based varistor was evaluated for reliability with respect to a surge. The ZnO-based varistor was twice applied with a pulse of 0.5 kA for 8 x 20 µsec to obtain the change rate Δ V1mA/V1mA (change rate due to a surge). The composition of the specimen of the present example is shown in Table 1, and the evaluation results of the electrical characteristics thereof are shown in Table 2. The numerals representing the evaluation results are the maximum and the minimum values of in each lot. Table 1
    specimen ZnO (wt ratio) Bi₂O₃+TiO₂ (heat-treated) (wt ratio) CoO (wt ratio) MnO₂ (wt ratio)
    #001 100.0 2.93 0.804 0.555
    Table 2
    specimen V₁mA/mm (V) 0.1α₁mA Change rate due to a DC load Δ V₁mA/V₁mA (%) Change rate due to a surge Δ V₁mA/V₁mA (%)
    #001 16-18 30-34 -1--2 -4-1
  • As is understood from Tables 1 and 2, the ZnO-based varistor produced by the method of the present example has excellent reliability. That is to say, in the ZnO-based varistor of the present invention, the clamping voltage V1mA was low and the absolute value of the change rate Δ V1mA/V1mA of the clamping voltage V1mA with respect to the application of a DC load or a surge for a long period of time was not more than 5%. In addition, as shown in Table 2, the variation of the electrical characteristics between the ZnO-based varistors obtained from one lot was small.
  • Moreover, in the present example, the variation of the electrical characteristics between the ZnO-based varistors obtained from different lots was also small (not shown in Table 2).
  • As described above, according to the method of the present example, the yield of the ZnO-based varistors were remarkably improved.
    • Comparative Example
  • A ZnO-based varistor was produced by a conventional method, using a sintered body having the same composition as that of Example 1.
  • ZnO powders, Bi₂O₃ powders, TiO₂ fine powders, CoO powders, and MnO₂ powders were mixed in a weight ratio of 100.0 : 2.60 : 0.33 : 0.804 : 0.555 by a wet method and ground. The powders thus obtained were dried and formed by pressure molding. The molding thus obtained was provisionally sintered at 600°C and ground. Then, the ground molding was formed into a disk shape. The disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1250°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body. The sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • Hereinafter, in the same way as in Example 1, a ZnO-based varistor was produced. Also, in the same way as in Example 1, the ZnO-based varistor thus obtained was evaluated for electrical characteristics. Table 3 shows the composition of the specimen of this comparative example, and Table 4 shows the evaluation results of the electrical characteristics thereof. Table 3
    specimen ZnO (wt ratio) Bi₂O₃ (wt ratio) TiO₂ (wt ratio) CoO (wt ratio) MnO₂ (wt ratio)
    #002 100.0 2.60 0.33 0.804 0.555
    Table 4
    specimen V₁mA/mm (v) 0.1α₁mA Change rate due to a DC load Δ V₁mA/V₁mA (%) Change rate due to a surge Δ V₁mA/V₁mA (%)
    #002 24-33 10-23 -25--40 -45--60
  • As is understood from Tables 3 and 4, in the ZnO-based varistor produced by the conventional method, the clamping voltage V1mA after the application of a DC load of 0.2 watts was remarkably decreased, and the absolute value of the change rate Δ V1mA/V1mA of the clamping voltage V1mA with respect to the application of a DC load was not less than 25%. Moreover, the absolute value of the change rate Δ V1mA/V1mA of the application of a surge was more than 40%. Thus, the reliability of the ZnO-based varistor produced by the conventional method was remarkably low. As shown in Table 4, the variation of the electrical characteristics between the ZnO-based varistors obtained from one lot was large.
  • Moreover, the variation of the electrical characteristics between the ZnO-based varistors obtained from different lots was larger than that from one lot (not shown in Table 4). In some cases, the ZnO-based varistors had V1mA/mm and a non-linear resistance index 0.1α1mA lower than those shown in Table 4.
  • As is apparent from the comparison between Example 1 and Comparative Example, the ZnO-based varistor produced by the method of the present invention is more excellent than the ZnO-based varistor produced by the conventional method in the initial electrical characteristics, the reliability, and the variation of the electrical characteristics between the ZnO-based varistors obtained from one lot and between the ZnO-based varistors obtained from different lots.
    • Example 2
  • TiO₂ fine powders and Bi₂O₃ powders were mixed in a weight ratio of 20.5 : 79.5. The mixture thus obtained was heat-treated at 800°C for 5 hours. Then, the mixture was ground to obtain TiO₂/Bi₂O₃ composite powders. The composite powders contained Bi₄Ti₃O₁₂ as a main component thereof.
  • ZnO powders, TiO₂/Bi₂O₃ composite powders, CoO powders and MnO₂ powders were mixed in a weight ratio of 100.0 : 3.15 : 0.922 : 0.534 by a wet method and ground. The resulting powders were dried and formed by pressure molding. The molding thus obtained was provisionally sintered at 600°C and ground. Then, the ground molding was formed into a disk shape. The disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1250°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body. The sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • Hereinafter, a ZnO-based varistor was produced in the same way as in Example 1. Moreover, in the same way as in Example 1, the ZnO-based varistor thus obtained was evaluated for electrical characteristics. The composition of the specimen of the present example is shown in Table 5, and the evaluation results of the electrical characteristics thereof are shown in Table 6. Table 5
    specimen ZnO (wt ratio) Bi₄Ti₃O₁₂ Bi₂O₃+TiO₂ (heat-treated) (wt ratio) CoO (wt ratio) MnO₂ (wt ratio)
    #003 100.0 3.15 0.92 0.53
    Table 6
    specimen V₁mA/mm (V) 0.1α₁mA Change rate due to a DC load Δ V₁mA/V₁mA (%) Change rate due to a surge Δ V₁mA/V₁mA (%)
    #003 15-16 28-32 -1--2 -1--2
  • As is understood from Tables 5 and 6, the ZnO-based varistor produced by the method of the present example has excellent reliability. That is to say, in the ZnO-based varistor of the present invention, the clamping voltage V1mA was low and the absolute value of the change rate V1mA/V1mA of the clamping voltage V1mA with respect to the application of a DC load or a surge for a long period of time was not more than 5%. In addition, as shown in Table 6, the variation of the electrical characteristics between the ZnO-based varistors obtained from one lot was small.
  • Moreover, in the present example, the variation of the electrical characteristics between the ZnO-based varistors obtained from different lots was also small (not shown in Table 6).
  • As described above, according to the method of the present example, the yield of the ZnO-based varistors were remarkably improved.
    • Example 3
  • TiO₂ fine powders and Bi₂O₃ powders were mixed in a weight ratio of 11.4 : 88.6. The mixture thus obtained was heat-treated at 600°C for 5 hours. Then, the mixture was ground to obtain TiO₂/Bi₂O₃ composite powders.
  • ZnO powders, CoO powders, MnO₂ powders, SnO₂ powders, and TiO₂/Bi₂O₃ composite powders were mixed in a weight ratio of 100.0 : 0.804 : 0.555 : 0.518 : X (X = 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 15.0, 20.0) by a wet method and ground. The resulting powders were dried and formed by pressure molding. The molding thus obtained was provisionally sintered at 600°C. An aqueous solution of aluminum nitrate was added to the molding so that the weight ratio of the ZnO powders to Al₂O₃ was 100.0 : 0.007 and ground. Then, the ground molding was formed into a disk shape. The disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1200°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body. The sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • Hereinafter, a ZnO-based varistor was produced in the same way as in Example 1. Also, in the same way as in Example 1, the ZnO-based varistor was evaluated for electrical characteristics. The composition of the specimen of the present example is shown in Table 7, and the evaluation results of the electrical characteristics thereof are shown in Table 8. Table 7
    specimen ZnO (wt ratio) Bi₂O₃+TiO₂ (heat-treated) (wt ratio) CoO (wt ratio) MnO₂ (wt ratio) SnO₂ (wt ratio) Al₂O₃ (wt ratio)
    #011 100.0 0.1 0.804 0.555 0.518 0.007
    #012 100.0 0.2 0.804 0.555 0.518 0.007
    #013 100.0 0.5 0.804 0.555 0.518 0.007
    #014 100.0 1.0 0.804 0.555 0.518 0.007
    #015 100.0 2.0 0.804 0.555 0.518 0.007
    #016 100.0 5.0 0.804 0.555 0.518 0.007
    #017 100.0 10.0 0.804 0.555 0.518 0.007
    #018 100.0 15.0 0.804 0.555 0.518 0.007
    #019 100.0 20.0 0.804 0.555 0.518 0.007
    Table 8
    specimen V₁mA/mm (V) 0.1α₁mA Change rate due to a DC load Δ V₁mA/V₁mA (%) Change rate due to a surge Δ V₁mA/V₁mA (%)
    #011 6-18 6-10 -10 --20 -25--35
    #012 19-22 12-18 -2 --4 -4-+1
    #013 18-20 28-32 -1 --2 -1-+1
    #014 15-16 26-30 -1 --2 -1-+1
    #015 16-19 25-31 -1 --2 -1-+1
    #016 16-19 24-30 -1 --2 -1-+1
    #017 15-17 24-30 -1 --2 -1-+1
    #018 16-18 25-29 -1 --2 -1-+1
    #019 16-19 25-30 -1 --2 -1-+1
  • As is understood from Tables 7 and 8, the ZnO-based varistor produced by the method of the present example has excellent reliability. That is to say, in the ZnO-based varistor of the present example, the non-linear resistance index 0.1α1mA was large in the case where the added amount of the TiO₂/Bi₂O₃ composite powders was not less than 0.2 in a weight ratio (Specimen Nos. 012 - 018), and the absolute value of the change rate Δ V1mA/V1mA of the clamping voltage V1mA with respect to the application of a DC load or a surge for a long period of time was not more than 5%. In the case where the added amount of the TiO₂/Bi₂O₃ composite powders was not less than 20 in a weight ratio (Specimen No. 019), if a plurality of moldings were sintered under the condition that the moldings were layered, the sintered bodies stuck to each other, decreasing mass production. Thus, this added amount was not suitable for mass production.
    • Example 4
  • TiO₂ fine powders and Bi₂O₃ powders were mixed so that the weight ratio of the Bi₂O₃ powders to the TiO₂ fine powders was in the range of 1.5 to 59. The mixture thus obtained was heat-treated at 500°C for 5 hours. Then, the mixture was ground to obtain TiO₂/Bi₂O₃ composite powders.
  • ZnO powders, CoO powders, MnO₂ powders, Sb₂O₃ powders, and TiO₂/Bi₂O₃ composite powders were mixed in a weight ratio of 100.0 : 0.555 : 0.291 : 3.0 by a wet method and ground. The resulting powders were dried and formed by pressure molding. The molding thus obtained was provisionally sintered at 600°C. An aqueous solution of aluminum nitrate was added to the molding so that the weight ratio of ZnO to Al₂O₃ was 100.0 : 0.001 and ground. Then, the ground molding was formed into a disk shape. The disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1200°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body. The sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • The sintered body thus obtained was observed by a scanning electron microscope (SEM), revealing that the sintered body had a characteristic fine structure having twin crystal grains of ZnO.
  • Hereinafter, a ZnO-based varistor was produced in the same way as in Example 1. Also, in the same way as in Example 1, the ZnO-based varistor was evaluated for electrical characteristics. In particular, in order to inspect the reliability of the ZnO-based varistor with respect to a surge in detail, the change rate Δ V0.01mA/V0.01mA of the clamping voltage V0.01mA of the ZnO-based varistor was measured, since the change of the reliability at 0.01 mA with respect to a surge was remarkably exhibited. Furthermore, the ZnO-based varistor was applied with a pulse of 1.0 kA for 8 x 20 µsec to obtain the change rate Δ V0.01mA/V0.01mA of the clamping voltage V0.01mA thereof. The composition of the specimen of the present example is shown in Table 9, and the evaluation results of the electrical characteristics thereof are shown in Table 10. Table 9
    specimen ZnO (wt ratio) Bi₂O₃+TiO₂ Heat-treated CoO (wt ratio) MnO₂ (wt ratio) Sb₂O₃ (wt ratio) Al₂O₃ (wt ratio)
    Bi₂O₃ (wt ratio) TiO₂ (wt ratio)
    #021 100.0 1.8 1.2 0.804 0.555 0.291 0.001
    #022 100.0 2.0 1.0 0.804 0.555 0.291 0.001
    #023 100.0 2.1 0.9 0.804 0.555 0.291 0.001
    #024 100.0 2.2 0.8 0.804 0.555 0.291 0.001
    #025 100.0 2.5 0.5 0.804 0.555 0.291 0.001
    #026 100.0 2.6 0.4 0.804 0.555 0.291 0.001
    #027 100.0 2.8 0.2 0.804 0.555 0.291 0.001
    #028 100.0 2.9 0.1 0.804 0.555 0.291 0.001
    #029 100.0 2.95 0.05 0.804 0.555 0.291 0.001
    Table 10
    specimen V₁mA/mm (V) 0.1α₁mA Change rate due to a DC load Δ V₁mA/V₁mA (%) Change rate due to a surge Δ V0.01mA/V0.01mA (%)
    #021 25-32 4-6 -50--35 -22--18
    #022 21-27 12-18 -4--2 -8-+1
    #023 18-23 29-31 -4--1 -5-+2
    #024 18-21 27-29 -2--2 -3-+1
    #025 19-23 27-32 -2--1 -3-+1
    #026 19-22 25-30 -2--1 -2-+2
    #027 18-23 24-32 -3--2 -2-+1
    #028 20-26 10-13 -4--2 -8-+2
    #029 30-36 3-6 -33--40 -20--15
  • As is understood from Tables 9 and 10, in the case where the weight ratio of the Bi₂O₃ powders to the TiO₂ powders was in the range of 2.0 to 29.0 (Specimen Nos. 022 to 028), ZnO-based varistors with satisfactory electrical characteristics and reliability were obtained. In the case where the weight ratio was not within this range (Specimen Nos. 021 and 029), the absolute value of the change rate Δ V0.01mA/V0.01mA of the clamping voltage V0.01mA with respect to the application of a pulse was not less than 10%, resulting in decreased reliability.
    • Example 5
  • TiO₂ fine powders and Bi₂O₃ powders were mixed in a weight ratio of 25 : 75. The mixture thus obtained was heat-treated at 650°C for 5 hours. Then, the mixture was ground to obtain TiO₂/Bi₂O₃ composite powders.
  • ZnO powders, TiO₂/Bi₂O₃ composite powders, CoO powders, MnO₂ powders, Sb₂O₃ powders, and Cr₂O₃ powders were mixed in a weight ratio of 100.0 : 2.5 : 0.804 : 0.555 : X : X/4 (X = 0.010, 0.018, 0.030, 0.060, 0.10, 0.20, 0.36, 0.720, 0.80) by a wet method and ground. The resulting powders were dried, and an aqueous solution of aluminum acetate was added to the dried powders so that the weight ratio of the ZnO powders to Al₂O₃ was 100.0 : 0.001. The resulting powders were provisionally sintered at 600°C. Then, the molding was ground and mixed. The resulting mixture was formed into a disk shape. The disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1200°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body. The sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • The sintered body thus obtained was observed by a scanning electron microscope (SEM), revealing that the sintered body had a characteristic fine structure having twin crystal grains of ZnO.
  • Hereinafter, a ZnO-based varistor was produced in the same way as in Example 1. Also, in the same way as in Example 1, the ZnO-based varistor was evaluated for electrical characteristics. In particular, in order to inspect the reliability of the ZnO-based varistor with respect to a surge in detail, the change rate Δ V0.01mA/V0.01mA of the clamping voltage V0.01mA of the ZnO-based varistor was measured, since the change of the reliability at 0.01 mA with respect to a surge was remarkably exhibited. Furthermore, the ZnO-based varistor was applied with a pulse of 1.0 kA for 8 x 20 µsec to obtain the change rate Δ V0.01mA/V0.01mA of the clamping voltage V0.01mA thereof. The composition of the specimen of the present example is shown in Table 11, and the evaluation results of the electrical characteristics thereof are shown in Table 12. Table 11
    specimen ZnO (wt ratio) Bi₂O₃ (75 wt ratio) + TiO₂ (25 wt) (heat-treated) (wt ratio) CoO (wt ratio) MnO₂ (wt ratio) Sb₂O₃ (wt ratio) Cr₂O₃ (wt ratio) Al₂O₃ (wt ratio)
    #031 100.0 2.5 0.804 0.555 0.010 0.0025 0.001
    #032 100.0 2.5 0.804 0.555 0.018 0.0045 0.001
    #033 100.0 2.5 0.804 0.555 0.030 0.0075 0.001
    #034 100.0 2.5 0.804 0.555 0.060 0.015 0.001
    #035 100.0 2.5 0.804 0.555 0.100 0.025 0.001
    #036 100.0 2.5 0.804 0.555 0.200 0.050 0.001
    #037 100.0 2.5 0.804 0.555 0.360 0.090 0.001
    #038 100.0 2.5 0.804 0.555 0.720 0.180 0.001
    #039 100.0 2.5 0.804 0.555 0.800 0.200 0.001
    Table 12
    specimen V₁mA/mm (V) 0.1α₁mA Change rate due to a surge Δ V0.01mA/V0.01mA (%)
    #031 19-23 12-18 -25--20
    #032 20-28 18-25 -8--5
    #033 20-28 22-30 -7--2
    #034 25-35 25-45 -5-+1
    #035 30-40 30-40 -3-+1
    #036 35-45 35-40 -2-+1
    #037 40-43 35-45 +0-+2
    #038 40-45 40-45 +2-+4
    #039 60-70 50-55 +3-+5
  • As is understood from Tables 11 and 12, the addition of a small amount of Sb₂O₃ powders to the ZnO varistor materials improved the reliability of the ZnO-based varistor with respect to a surge. That is to say, when the added amount of Sb₂O₃ powders was X = 0.018 or more, and the added amount of Cr₂O₃ was X/4 = 0.0045 or more (Specimen Nos. 032 to 039), the absolute value of the change rate Δ V0.01mA/V0.01mA with respect to the application of a surge was not more than 10%. However, when the added amount of Sb₂O₃ powders was X = 0.8 or more, and the added amount of Cr₂O₃ was X/4 = 0.2 or more (Specimen No. 039), V1mA/mm exceeded 50 V. In this case, a low-voltage varistor cannot be obtained.
    • Example 6
  • ZnO powders, Bi₂O₃ powders, TiO₂/Bi₂O₃ composite powders (which were obtained in the same method as in Example 5), CoO powders, MnO₂ powders, and SnO₂ powders were mixed in a weight ratio of 100.0 : 1.0 : 2.00 : 0.804 0.555 : X (X = 0.003, 0.005, 0.010, 0.030, 0.050, 0.10, 0.20, 0.370, 0.50) by a wet method and ground. The resulting powders were dried, and an aqueous solution of aluminum acetate was added to the dried powders so that the weight ratio of the ZnO powders to Al₂O₃ was 100.0 : 0.001. The resulting powders were provisionally sintered at 600°C. Then, the molding was ground and mixed. The resulting mixture was formed into a disk shape. The disk-shaped molding was heated in the atmosphere at 100°C/hour and kept at 1200°C for 2 hours. Then, the molding was cooled at 100°C/hour to obtain a sintered body. The sintered body had a thickness of 1.2 mm and a diameter of 14 mm.
  • The sintered body thus obtained was observed by a scanning electron microscope (SEM), revealing that the sintered body had a characteristic fine structure having twin crystal grains of ZnO.
  • Hereinafter, a ZnO-based varistor was produced in the same way as in Example 1. Also, in the same way as in Example 1, the ZnO-based varistor was evaluated for electrical characteristics. In particular, in order to inspect the reliability of the ZnO-based varistor with respect to a surge in detail, the change rate Δ V0.01mA/V0.01mA of the clamping voltage V0.01mA of the ZnO-based varistor was measured, since the change of the reliability at 0.01 mA with respect to a surge was remarkably exhibited. Furthermore, the ZnO-based varistor was applied with a pulse of 1.0 kA for 8 x 20 µsec to obtain the change rated Δ V0.01mA/V0.01mA of the clamping voltage V0.01mA thereof. The composition of the specimen of the present example is shown in Table 13, and the evaluation results of the electrical characteristics thereof are shown in Table 14. Table 13
    specimen ZnO (wt ratio) Bi₂O₃ (wt ratio) Bi₂O₃ + TiO₂ (heat-treated) (wt ratio) CoO (wt ratio) MnO₂ (wt ratio) SnO₂ (wt ratio) Al₂O₃ (wt ratio)
    #041 100.0 1.0 2.0 0.804 0.555 0.003 0.001
    #042 100.0 1.0 2.0 0.804 0.555 0.005 0.001
    #043 100.0 1.0 2.0 0.804 0.555 0.010 0.001
    #044 100.0 1.0 2.0 0.804 0.555 0.030 0.001
    #045 100.0 1.0 2.0 0.804 0.555 0.050 0.001
    #046 100.0 1.0 2.0 0.804 0.555 0.100 0.001
    #047 100.0 1.0 2.0 0.804 0.555 0.200 0.001
    #048 100.0 1.0 2.0 0.804 0.555 0.370 0.001
    #049 100.0 1.0 2.0 0.804 0.555 0.500 0.001
    Table 14
    specimen V₁mA/mm (V) 0.1α₁mA Charge rate due to a surge Δ V0.01mA/V0.01mA (%)
    #041 18-22 12-17 -12--10
    #042 21-24 16-21 -8-+1
    #043 25-29 20-25 -5-+2
    #044 28-34 20-28 -3-+1
    #045 32-39 22-35 -3-+1
    #046 36-43 30-38 -2-+2
    #047 40-45 35-45 +2-+2
    #048 44-48 40-50 +4-+3
    #049 65-75 42-55 +4-+5
  • As is understood from Tables 13 and 14, when the added amount of SnO₂ powders was X = 0.005 or more (Specimen Nos. 042 - 049), the absolute value of the change rate Δ V0.01mA/V0.01mA with respect to the application of a surge was not more than 10%. However, when the added amount of SnO₂ powders was X = 0.5 or more (Specimen No. 049), V1mA/mm exceeded 50 V. In this case, a low-voltage varistor cannot be obtained.
  • As described above, composite powders obtained by previously heat-treating the mixture of TiO₂ powders and Bi₂O₃ powders can uniformly enhance the growth of ZnO grains. In addition, an antimony component, a tin component, or a chrome component together with an antimony component suppresses the abnormal growth of ZnO grains. Thus, according to the present invention, a ZnO grain having a large average grain size and a small distribution in grain size can be obtained. Namely, a ZnO-based varistor excellent in electrical characteristics and reliability can be produced with a high yield.
  • Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.

Claims (12)

  1. A method for producing a ZnO-based varistor comprising the steps of:
       heat-treating a mixture of TiO₂ powders and Bi₂O₃ powders to prepare composite powders; and
       adding the composite powders to ZnO varistor materials.
  2. A method for producing a ZnO-based varistor according to claim 1, further comprising the step of adding an aluminum component to the ZnO varistor materials.
  3. A method for producing a ZnO-based varistor according to claim 2, wherein the aluminum component is added to the ZnO varistor materials so that the weight ratio of Al₂O₃ powders to the ZnO powders is in the range of 0.00062 : 100.0 to 0.372 : 100.0.
  4. A method for producing a ZnO-based varistor according to claim 3, wherein the aluminum component is a solution of an aluminum salt.
  5. A method for producing a ZnO-based varistor according to claim 4, wherein the aluminum salt is aluminum nitrate.
  6. A method for producing a ZnO-based varistor according to claim 4, wherein the aluminum salt is aluminum acetate.
  7. A method for producing a ZnO-based varistor according to claim 3, further comprising the step of adding an antimony component to the ZnO varistor materials.
  8. A method for producing a ZnO-based varistor according to claim 7, wherein the antimony component is added to the ZnO varistor materials so that the weight ratio of Sb₂O₃ powders to the ZnO powders is in the range of 0.018 : 100.0 to 0.72 : 100.0.
  9. A method for producing a ZnO-based varistor according to claim 3, further comprising the step of adding a tin component to the ZnO varistor materials.
  10. A method for producing a ZnO-based varistor according to claim 9, wherein the tin component is added to the ZnO varistor materials so that the weight ratio of SnO₂ powders to the ZnO powders is in the range of 0.005 : 100.0 to 0.37 : 100.0.
  11. A method for producing a ZnO-based varistor according to claim 8, further comprising the step of adding a chrome component to the ZnO varistor materials.
  12. A method for producing a ZnO-based varistor according to claim 11, wherein the chrome component is added to the ZnO varistor materials so that the weight ratio of Cr₂O₃ powders to the ZnO powders is in the range of 0.005 : 100.0 to 0.18 : 100.0.
EP93116867A 1992-10-20 1993-10-19 A method for producing a ZnO Varistor. Withdrawn EP0594120A3 (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0699641A1 (en) * 1994-08-29 1996-03-06 Matsushita Electric Industrial Co., Ltd. Method of manufacturing zinc oxide sintered compact body
EP0762438A2 (en) * 1995-09-07 1997-03-12 Mitsubishi Denki Kabushiki Kaisha Electric resistance element exhibiting voltage nonlinearity characteristic and method of manufacturing the same

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Publication number Priority date Publication date Assignee Title
WO1988002921A2 (en) * 1986-10-16 1988-04-21 Raychem Corporation Metal oxide varistors, precursor powder compositions and methods for preparing same
JPH01289218A (en) * 1988-05-17 1989-11-21 Matsushita Electric Ind Co Ltd Manufacture of varistor
EP0357113A2 (en) * 1988-08-03 1990-03-07 Philips Patentverwaltung GmbH Production process of a non-linear voltage-dependent resistor
GB2242065A (en) * 1990-03-16 1991-09-18 Ecco Ltd Varistor composition

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1988002921A2 (en) * 1986-10-16 1988-04-21 Raychem Corporation Metal oxide varistors, precursor powder compositions and methods for preparing same
JPH01289218A (en) * 1988-05-17 1989-11-21 Matsushita Electric Ind Co Ltd Manufacture of varistor
EP0357113A2 (en) * 1988-08-03 1990-03-07 Philips Patentverwaltung GmbH Production process of a non-linear voltage-dependent resistor
GB2242065A (en) * 1990-03-16 1991-09-18 Ecco Ltd Varistor composition

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* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 014 no. 070 (E-0886) ,8 February 1990 & JP-A-01 289218 (MATSUSHITA ELECTRIC IND CO LTD) 21 November 1989, *

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0699641A1 (en) * 1994-08-29 1996-03-06 Matsushita Electric Industrial Co., Ltd. Method of manufacturing zinc oxide sintered compact body
US5569414A (en) * 1994-08-29 1996-10-29 Matsushita Electric Industrial Co., Ltd. Method of manufacturing zinc oxide sintered compact body
US5811033A (en) * 1994-08-29 1998-09-22 Matsushita Electric Industrial Co., Ltd. Method of manufacturing zinc oxide sintered compact body
EP0762438A2 (en) * 1995-09-07 1997-03-12 Mitsubishi Denki Kabushiki Kaisha Electric resistance element exhibiting voltage nonlinearity characteristic and method of manufacturing the same
EP0762438A3 (en) * 1995-09-07 1997-12-10 Mitsubishi Denki Kabushiki Kaisha Electric resistance element exhibiting voltage nonlinearity characteristic and method of manufacturing the same
US5807510A (en) * 1995-09-07 1998-09-15 Mitsubishi Denki Kabushiki Kaisha Electric resistance element exhibiting voltage nonlinearity characteristic and method of manufacturing the same

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Publication number Publication date
KR940010133A (en) 1994-05-24
EP0594120A3 (en) 1995-07-26

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