Zinc oxide varistor and method for producing zinc oxide varistor

Abstract

The present invention relates to a zinc oxide varistor and a method for producing a zinc oxide varistor. This zinc oxide varistor comprises a varistor element and multiple electrodes. The varistor element contains zinc oxide as the main component and is formed of a mixture obtained by adding an oxide and lithium fluoride to zinc oxide.

Description

Zinc oxide varistor and method for manufacturing zinc oxide varistor

[0001] The present invention relates to a zinc oxide varistor and a method for manufacturing a zinc oxide varistor.

[0002] In recent years, the rapid increase in frequency and capacity of electronic devices has led to a rapid spread of various electronic devices, including mobile phones. Voltage limiting elements such as zinc oxide varistors are widely used in such devices to protect against various surges, pulse noise, and ESD, as well as to protect the device's circuits, ensure operational stability, and comply with noise regulations.

[0003] Zinc oxide (ZnO) and bismuth oxide (Bi ２ O ３ Zinc oxide varistors with additives such as palladium (Ag-Pd) are known to exhibit excellent nonlinearity in their current-voltage characteristics. This material is generally fired at high temperatures of around 1200°C. In particular, in multilayer chip varistors with internal electrodes in their layers, silver-palladium (Ag-Pd) alloys with palladium added are used as the electrode material.

[0004] Japanese Patent Application Laid-Open No. 2007-005499

[0005] However, if sintering at a low temperature becomes possible, there will be no need to use noble metals such as palladium (Pd), or the amount of noble metal used can be reduced.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a zinc oxide varistor that can be sintered at a low temperature and a method for producing a zinc oxide varistor.

[0007] In one embodiment, a zinc oxide varistor is provided, comprising a varistor element body formed using a mixed material containing zinc oxide as a main component and having an oxide and lithium fluoride added to the zinc oxide, and a plurality of electrodes formed on the varistor element body.

[0008] In one embodiment, the lithium fluoride is added at a ratio of 0.01 to 0.2 mol % relative to the zinc oxide.

[0009] In one embodiment, there is provided a method for manufacturing a zinc oxide varistor, which comprises preparing a mixed material containing zinc oxide as a main component and adding an oxide and lithium fluoride to the zinc oxide, forming a compact using the mixed material, and firing the compact to form a sintered body, thereby obtaining a varistor element.

[0010] In one embodiment, the lithium fluoride is added in a ratio of 0.01 to 0.2 mol % relative to the zinc oxide. In another embodiment, the molded body is fired at a temperature of 1000°C.

[0011] Zinc oxide varistors, which are manufactured by adding oxides and lithium fluoride to zinc oxide, can be sintered at low temperatures.

[0012] Fig. 1 is a diagram showing one embodiment of a zinc oxide varistor; Fig. 2 is a diagram showing another embodiment of a zinc oxide varistor; Fig. 3 is a diagram showing a comparative example of sintered pellets and samples 1 to 7; Fig. 4 is a graph showing the dependency of the nonlinearity coefficient α calculated from the current-voltage characteristics on the amount of lithium fluoride (LiF) added;

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings described below, identical or corresponding components are designated by the same reference numerals, and duplicated descriptions will be omitted. In the multiple embodiments described below, the configuration of an embodiment that is not particularly described is the same as that of other embodiments, and therefore duplicated descriptions will be omitted.

[0014] Fig. 1 is a diagram showing one embodiment of a zinc oxide varistor. Fig. 2 is a diagram showing another embodiment of a zinc oxide varistor. In the embodiment shown in Fig. 1, zinc oxide varistor 1A is a disk-type zinc oxide varistor. In the embodiment shown in Fig. 2, zinc oxide varistor 1B is a laminated chip-type zinc oxide varistor.

[0015] In the embodiment shown in Figure 1, the zinc oxide varistor 1A comprises an annular varistor element body 2A, a circular electrode 3A formed on the varistor element body 2A, and a lead wire 4 connected to the electrode 3A.

[0016] 2, the zinc oxide varistor 1B comprises a varistor element body 2B and internal electrodes 3Ba stacked on top of each other, and external electrodes 3Bb covering the varistor element body 2B and internal electrodes 3Ba. The varistor element body 2B and internal electrodes 3Ba have a plate shape and are arranged alternately along the vertical direction.

[0017] Hereinafter, in this specification, zinc oxide varistors 1A and 1B may be referred to without any particular distinction as zinc oxide varistor 1. Similarly, varistor elements 2A and 2B may be referred to without any particular distinction as varistor element 2.

[0018] The varistor element body 2 is a mixed material containing zinc oxide (ZnO) as the main component, with oxides and lithium fluoride (LiF) added to the zinc oxide. Such a mixed material is prepared, a compact is formed using the mixed material, and the compact is fired to form a sintered body, which becomes the varistor element body 2. Furthermore, by forming a plurality of electrodes on the sintered body, it is possible to manufacture the zinc oxide varistor 1. The zinc oxide varistor 1 can eliminate or reduce the amount of precious metals such as palladium (Pd) used.

[0019] Furthermore, a sheet material is prepared from the above-mentioned mixed material, and a paste containing silver (Ag) is printed on this sheet material. The sheet material that will become the varistor element body 2 and internal electrode 3Ba is constructed, and multiple sheets of this sheet material are prepared, stacked, and cut to a predetermined size to form a compact. This compact is fired to form a sintered body. External electrodes 3Bb are formed on the end faces of this sintered body. The zinc oxide varistor 1 formed in this way allows the internal electrodes and varistor element body to be fired simultaneously at a lower temperature than conventional methods.

[0020] In the manufacturing process of the zinc oxide varistor 1, Li is added to the interstitial sites of zinc oxide (ZnO). ＋ It is believed that the introduction of these elements has the effect of increasing the carrier density. Therefore, samples were prepared by adding oxides and lithium fluoride (LiF) to zinc oxide, and an evaluation was carried out as a varistor element 2 based on the sinterability of the zinc oxide varistor and the nonlinearity in the current-voltage characteristics. The evaluation method and results are explained below.

[0021] The evaluation method is as follows: As raw material powder, zinc oxide (ZnO), bismuth oxide (Bi ２ O ３ ), antimony oxide (Sb ２ O ３ ), and lithium fluoride (LiF) were used. Raw material powders were weighed out in a molar ratio of Zn:Bi:Sb:Li = 100:1:2:x (x = 0 to 0.4), and trace amounts of other additives were added to the raw material powders. These raw material powders were wet-mixed in 2-propanol for 2 hours using a mortar and pestle, and the mixed powder was dried.

[0022] The mixed powder was dried and then calcined in air at 600°C for 5 hours. After calcination, the mixture was pulverized for 1 hour. A pressure of 40 MPa was applied to the pulverized powder, and pellets with a diameter of 10 mm were produced by uniaxial pressing.

[0023] The pellets were subjected to main firing in air for 2 hours at temperatures of 900°C, 1000°C, and 1100°C. Note that these firing temperatures include not only the actual temperatures but also temperatures controlled or set to such temperatures. Of these, the pellets fired at 1000°C for 2 hours were selected as the comparative example and samples 1 to 7, respectively. The amounts of lithium fluoride (LiF) added to the comparative example and samples 1 to 7 are shown in Table 1.

[0024]

[0025] 3 shows a comparative example of sintered pellets and samples 1 to 7. As shown in FIG. 3, the microstructures of the comparative example and samples 1 to 7 obtained were observed using a scanning electron microscope (SEM), and the crystalline phases were identified using X-ray diffraction (XRD). In addition, an In—Ga alloy was applied to both sides of the sample as electrodes, and the current-voltage characteristics were measured.

[0026] The evaluation results are as follows. As is clear from Table 1 and Figure 3, grain growth is promoted as the amount of lithium fluoride (LiF) added increases. Furthermore, voids were observed between zinc oxide (ZnO) particles in the sample to which lithium fluoride (LiF) was added.

[0027] 4 is a graph showing the dependence of the nonlinearity coefficient α calculated from the current-voltage characteristics on the amount of lithium fluoride (LiF) added. As the amount of lithium fluoride (LiF) added increases, the nonlinearity coefficient α increases, reaching a maximum value at 0.03 mol%.

[0028] However, when 0.03 mol % or more of lithium fluoride (LiF) is added, the nonlinearity coefficient α decreases. It is thought that a small amount of Li ions enters the interstitial sites of zinc oxide (ZnO) and acts as donors to increase the carrier density, which increases the nonlinearity coefficient α.

[0029] On the other hand, when the amount of added Li ions exceeds a certain value, Li ions substitute for Zn sites, and the Li ions at the Zn sites act as acceptors, reducing the carrier density. This is thought to be why the nonlinearity coefficient α decreases.

[0030] Varistor characteristics were obtained for each of Samples 1 to 7. In particular, Samples 1 to 5 (containing lithium fluoride added at a ratio of 0.01 to 0.2 mol % relative to zinc oxide) are more suitable for use as varistor elements.

[0031] According to this embodiment, sintering at a lower temperature than in conventional methods is possible, and the efficiency of manufacturing zinc oxide varistors can be improved. Therefore, the manufacturing method of this embodiment is advantageous in that it reduces energy consumption and CO2 emissions in the production process. ２ This contributes to reducing the use of precious metals.

[0032] Therefore, the zinc oxide varistor 1 manufactured by the manufacturing method according to this embodiment can contribute to the achievement of Goal 3 "Good health and well-being", Goal 7 "Affordable and clean energy", and Goal 12 "Responsible consumption and production" in the Sustainable Development Goals (SDGs) led by the United Nations.

[0033] The above-described embodiments have been described for the purpose of enabling a person of ordinary skill in the art to practice the present invention. Various modifications of the above-described embodiments would be obvious to a person skilled in the art, and the technical concept of the present invention may be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is to be interpreted in the broadest scope in accordance with the technical concept defined by the claims.

[0034] The present invention can be used in zinc oxide varistors and methods for manufacturing zinc oxide varistors.

[0035] 1A, 1B Zinc oxide varistor 2A, 2B Varistor element 3A Electrode 3Ba Internal electrode 3Bb External electrode 4 Lead wire

Claims

1. A zinc oxide varistor comprising: a varistor element formed using a mixed material containing zinc oxide as a main component and having oxides and lithium fluoride added to the zinc oxide; and a plurality of electrodes formed on the varistor element.

2. A zinc oxide varistor according to claim 1, wherein the lithium fluoride is added at a ratio of 0.01 to 0.2 mol % relative to the zinc oxide.

3. A method for manufacturing a zinc oxide varistor, comprising the steps of preparing a mixed material containing zinc oxide as a main component and adding an oxide and lithium fluoride to the zinc oxide, forming a molded body using the mixed material, and firing the molded body to form a sintered body to form a varistor element.

4. A method for producing a zinc oxide varistor according to claim 3, wherein the lithium fluoride is added at a ratio of 0.01 to 0.2 mol % relative to the zinc oxide.

5. A method for producing a zinc oxide varistor according to claim 3 or 4, wherein the molded body is fired at a temperature of 1000°C.

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