A method for preparing high-current ZnO varistor pressed sheet

Abstract

The present invention discloses a method for preparing a high-current ZnO varistor tablet, which belongs to the field of varistors. The preparation method of the present invention includes the preparation of raw materials for the varistor tablet, taking each raw material according to the ratio, mixing by ball milling, spray drying and pressing into a quadrangular prism embryo, and then sintering and grinding to obtain a finished tablet. Compared with the traditional cylindrical embryo, the pressing molding process of the present invention uses the existing grinding process to grind the side, reducing the porosity of the varistor and increasing the uniformity. At the same time, it solves the problem that the upper part of the same column is thinner than the lower part when coating the high-resistance layer and the insulating layer, and greatly improves the uniformity of the high-resistance layer; the coating repair layer replaces the traditional high-resistance layer process, and oxides such as silicon oxide and antimony oxide are removed from the repair layer system, reducing the generation of spinel substances such as willemite and zinc antimonate during the sintering process, so that the repair medium can fully repair the side grains and grain boundaries, and suppress the side flashover phenomenon.

Description

Technical Field

[0001] The present invention relates to the technical field of varistors, and in particular to a method for preparing a high-current ZnO varistor pressed sheet. Background Art

[0002] With the development of electric power, power grid transformation, and the expansion of the electronics, information, and home appliance industries, the demand for varistors is increasing, and performance requirements are also becoming increasingly stringent. The modernization and informatization of military equipment, in particular, have placed even higher demands on the performance of varistors. Key requirements for varistors include low residual voltage, good nonlinearity, high current capacity, low leakage current, high voltage gradient, and good aging performance. While the low-current characteristics of domestically produced ZnO varistors are comparable to those of foreign products, there is still a significant gap in energy withstand capability. When subjected to a 2-ms high-current surge, varistors typically fail due to lateral flashover or body cracking. Coating the lateral surfaces of ZnO varistors with a high-performance high-resistance layer can prevent damage under high-current surges and improve their 2-ms surge withstand capability. The high-resistance layer on the lateral surfaces of varistors is a crucial component in varistor manufacturing and a key to the development and production of high-performance ZnO varistors.

[0003] In terms of improving the large current flow capacity per unit area of ​​varistors, domestic ZnO varistor manufacturers have, after years of exploration and practice, achieved or exceeded the international leading indicators of similar products in many indicators. However, there is still a gap with the world's top products. The flow capacity shows how much current shock the ZnO varistor can withstand and the stability of its performance after the large current shock. Therefore, improving the flow capacity is very important for improving the performance of ZnO varistors, which is also the focus of research by varistor manufacturers around the world.

[0004] At present, varistors are generally pressed into cylindrical resistor sheets using cylindrical molds, and a high-resistance layer or glass glaze material is coated on the side of the varistor. This manufacturing method has the following defects: (1) When the inorganic high-resistance layer is coated using the common process, due to the effect of gravity, the upper part of the same column is thinner than the lower part, resulting in poor uniformity of the high-resistance layer; (2) The traditional cylindrical sheeting method only grinds the upper and lower surfaces of the sintered embryo, which cannot solve the problem of side pores and side unevenness, affecting the firmness of the bonding between the insulating layer and the porcelain surface, and then causing side flashover or breakdown damage in the adjacent side area, reducing the current capacity; (3) The existing high-resistance layer and the resistor body have poor matching properties such as bonding, thermal expansion coefficient, elasticity and tensile strength. Summary of the Invention

[0005] The purpose of the present invention is to provide a high-current ZnO varistor pressed sheet and a preparation method to solve the above technical problems.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for preparing a high-throughput ZnO varistor sheet comprises the following steps:

[0008] Raw material preparation of S100 varistor sheet: The varistor sheet is composed of the following raw materials in molar percentage: zinc oxide 92.25-95.18%, bismuth oxide 0.8-1.0%, cobalt oxide 0.6-1.2%, manganese dioxide 0.5-1.0%, chromium trioxide 0.3-0.7%, antimony trioxide 0.6-1.2%, silicon dioxide 0.8-1.5 mol%, yttrium nitrate hexahydrate 0.5-1.0%, gallium nitrate nonahydrate 0.3-0.7%, indium nitrate nonahydrate 0.02-0.04%;

[0009] S200 compression molding: Take the raw materials according to the ratio, mix them through ball milling, spray dry the powder, place it in a square tableting mold, cover the mold tightly, and mold the powder into a quadrangular prism embryo with a square cross-section;

[0010] S300 sintering: Use a high-temperature electric furnace to sinter the green body in a closed condition, heating from room temperature to 400℃ for 10-15h, maintaining low-temperature debinding at 280℃ for 5h, and then cooling to room temperature at a cooling rate of 15℃ / h;

[0011] S400 tableting: from room temperature to 900℃, heating time is 9h, average heating rate is 100℃ / h; from 900℃ to 1250℃, heating time is 5h, average heating rate is 100℃ / h; keep at 1250℃ for 2-4h; cool naturally, average cooling rate is 100℃ / h, and obtain varistor tablets;

[0012] S500 grinding sheet: The four sides of the varistor pressed sheet are ground by a vertical grinding machine with a grinding depth of 1 to 2 mm. A large number of pores in the side area are ground away to obtain a finished varistor pressed sheet.

[0013] Preferably, the varistor pressing sheet is composed of the following raw materials in molar percentage: 93.12% zinc oxide, 1.05% bismuth oxide, 1.0% cobalt oxide, 0.75% manganese dioxide, 0.5% chromium trioxide, 1.0% antimony trioxide, 1.25% silicon dioxide, 0.9% yttrium nitrate hexahydrate, 0.4% gallium nitrate nonahydrate, and 0.03% indium nitrate nonahydrate.

[0014] As a preferred embodiment of the present invention, the step of coating a repair layer is further included: coating the repair layer on the four side surfaces of the finished varistor pressed sheet obtained in step S500, sintering the repair layer, and then polishing the upper and lower end surfaces of the finished varistor pressed sheet;

[0015] The repair layer is composed of the following raw materials in molar percentage: 91.7-95.2% zinc nitrate, 3.5-6.5% bismuth nitrate, 0.5-1.5% cobalt oxide, and 0.1-0.3% lithium vinyl acetate.

[0016] Furthermore, the repair layer is composed of the following raw materials in molar percentage: 93.8% zinc nitrate, 5% bismuth nitrate, 1.0% cobalt oxide, and 0.2% lithium vinyl acetate.

[0017] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0018] (1) The present invention uses a tableting mold to form a ZnO varistor, pressing the varistor powder into a quadrangular prism-shaped embryonic body and sintering it. Compared with the traditional circular cross-section, the problem of the upper part of the same column being thinner than the lower part and poor uniformity when coating the inorganic high-resistance layer and the insulating layer is solved, and the uniformity of the high-resistance layer is greatly improved. At the same time, during the grinding process, the problem of the uneven side of the cylindrical shape and the difficulty of grinding it flat is solved. The existing grinding process can be used to grind the side surface, reducing the porosity of the varistor and increasing the uniformity. It also improves the firmness of the bonding between the insulating layer and the porcelain surface, and greatly improves the energy tolerance of the varistor under large current impact.

[0019] (2) Based on the process of pressing into a quadrangular prism-shaped blank, the traditional high-resistance layer is replaced by the repair layer formula system of the present invention. The oxides such as silicon oxide and antimony oxide are removed from the repair layer formula system, which reduces the generation of spinel substances such as willemite and zinc antimonate during the sintering process, repairs the side grains and grain boundaries, and suppresses the side flashover phenomenon; in addition, the lower spinel content increases the matching of the expansion coefficient of the side edge area and the interior of the resistor sheet, and when the ZnO resistor sheet withstands high energy impact, the probability of cracking or peeling on the side of the resistor sheet is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The figure is a flow chart of the preparation process of the high-throughput ZnO varistor pressed sheet of the present invention;

[0021] Figure 2 This is a three-dimensional schematic diagram of a high-throughput ZnO varistor tablet pressing die of the present invention;

[0022] Figure 3 This is a schematic cross-sectional view of a high-throughput ZnO varistor tablet pressing die according to the present invention;

[0023] Figure 4 Schematic diagram of X-ray Micro-CT testing of varistor sample B as a control example;

[0024] Figure 5 This is a three-dimensional rendering of the pore distribution on the side of the varistor sample B as a control example;

[0025] The numbers in the figure are: 1-lid handle; 2, lid; 3, outer wall of tablet pressing mold; 4, inner wall of tablet pressing mold. DETAILED DESCRIPTION

[0026] The following is a further description of specific embodiments of the present invention in conjunction with the accompanying drawings. It should be noted that the description of these embodiments is intended to facilitate understanding of the present invention and does not constitute a limitation of the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0027] like Figure 1 Flowchart, a method for preparing a high-throughput ZnO varistor sheet of the present invention comprises the following steps:

[0028] The raw material preparation of the S100 varistor sheet is as follows: the varistor sheet is composed of the following raw materials in molar percentage: zinc oxide (ZnO) 92.25-95.18%, bismuth oxide (Bi2O3) 0.8-1.0%, cobalt oxide (Co2O3) 0.6-1.2%, manganese dioxide (MnO2) 0.5-1.0%, chromium trioxide (Cr2O3) 0.3-0.7%, antimony trioxide (Sb2O3) 0.6-1.2%, silicon dioxide (SiO2) 0.8-1.5 mol%, yttrium nitrate hexahydrate (Y(NO)3·6H2O) 0.5-1.0%, gallium nitrate nonahydrate (Ga(NO)3·9H2O) 0.3-0.7%, indium nitrate nonahydrate (In(NO)3·9H2O) 0.02-0.04%;

[0029] S200 pressing molding: Take the raw materials according to the ratio, mix them by ball milling, and spray dry the powder, such as Figure 2 and 3 As shown, the powder is placed in a square tablet pressing mold, the mold lid is tightly closed, and the powder is formed into a quadrangular prism embryo with a square cross-section;

[0030] S300 sintering: Use a high-temperature electric furnace to sinter the green body in a closed condition, heating from room temperature to 400℃ for 10-15h, maintaining low-temperature debinding at 280℃ for 5h, and then cooling to room temperature at a cooling rate of 15℃ / h;

[0031] S400 tableting: from room temperature to 900℃, heating time is 9h, average heating rate is 100℃ / h; from 900℃ to 1250℃, heating time is 5h, average heating rate is 100℃ / h; keep at 1250℃ for 2-4h; cool naturally, average cooling rate is 100℃ / h, and obtain varistor tablets;

[0032] S500 grinding sheet: The four sides of the varistor pressed sheet are ground by a vertical grinding machine with a grinding depth of 1 to 2 mm. A large number of pores in the side area are ground away to obtain a finished varistor pressed sheet.

[0033] Preferably, the varistor pressing sheet is composed of the following raw materials in molar percentage: zinc oxide (ZnO) 93.12%, bismuth oxide (Bi2O3) 1.05%, cobalt oxide (Co2O3) 1.0%, manganese dioxide (MnO2) 0.75%, chromium trioxide (Cr2O3) 0.5%, antimony trioxide (Sb2O3) 1.0%, silicon dioxide (SiO2) 1.25%, yttrium nitrate hexahydrate (Y(NO)3·6H2O) 0.9%, gallium nitrate nonahydrate (Ga(NO)3·9H2O) 0.4%, and indium nitrate nonahydrate (In(NO)3·9H2O) 0.03%.

[0034] As a preferred embodiment of the present invention, the process further includes applying a repair layer: applying a repair layer to the four side surfaces of the varistor pressed sheet obtained in step S500, sintering the repair layer, and then polishing the upper and lower end surfaces of the varistor pressed sheet. Generally, the requirements for sintering the repair layer are substantially the same as those for sintering step S300, generally sintering at 450° C. for 75 minutes.

[0035] The repair layer is composed of the following raw materials in molar percentages: 91.7-95.2% zinc nitrate (Zn(NO3)2), 3.5-6.5% bismuth nitrate (Bi(NO3)3), 0.5-1.5% cobalt oxide (Co2O3), and 0.1-0.3% vinyl lithium acetate (C2H3LiO2).

[0036] Furthermore, the repair layer is composed of the following raw materials in molar percentages: 93.8% zinc nitrate (Zn(NO3)2), 5% bismuth nitrate (Bi(NO3)3), 1.0% cobalt oxide (Co2O3), and 0.2% lithium vinyl acetate (C2H3LiO2).

[0037] The following comparison between the present invention and the traditional ZnO varistor sheet verifies that the varistor sheet obtained by the formulation system and process of the present application has excellent performance:

[0038] 1. The pressing and grinding processes are different

[0039] Example 1: Pressing a quadrangular prism embryo with a square cross section, and grinding the upper and lower surfaces and sides of the quadrangular prism embryo

[0040] The varistor sheet of Example 1 was composed of the following raw materials in molar percentages: 93.12% zinc oxide (ZnO), 1.05% bismuth oxide (Bi2O3), 1.0% cobalt oxide (Co2O3), 0.75% manganese dioxide (MnO2), 0.5% chromium trioxide (Cr2O3), 1.0% antimony trioxide (Sb2O3), 1.25% silicon dioxide (SiO2), 0.9% yttrium nitrate hexahydrate (Y(NO)3·6H2O), 0.4% gallium nitrate nonahydrate (Ga(NO)3·9H2O), and 0.03% indium nitrate nonahydrate (In(NO)3·9H2O). A varistor sheet sample A was prepared using the raw materials and process described in Example 1.

[0041] The control example used a conventional pressing process to press a circular cross-section embryo, and the upper and lower surfaces were polished using a conventional polishing process. The raw material formula of the varistor tablet of this Example 2 was the same as that of Example 1. The varistor tablet sample B was prepared using the raw materials and process of this Example 2.

[0042] Various performance tests were performed on the ZnO varistor samples obtained in Example 1 and the comparative example, as shown in Table 1, where A represents a quadrangular prism-shaped ZnO varistor sample and B represents a cylindrical ZnO varistor sample.

[0043] Table 1: Comparison of various properties of ZnO varistor samples obtained in Example 1 and the control example

[0044] sample Voltage gradient (V / mm) Nonlinear coefficient <![CDATA[Leakage current (μA / cm 2 )]]> Residual pressure ratio 2ms withstand current (A) A (Example 1) 453 65.5 1.55 1.72 900 B (control example) 409 59.2 1.29 1.86 500

[0045] The results in Table 1 show that compared to ZnO varistors with circular cross-sections, ZnO varistors with quadrangular prisms exhibit significantly higher voltage gradients, better nonlinear coefficients, lower residual voltage ratios, and significantly higher 2ms withstand currents. This process solves the problem of the upper portion of the same prism being thinner than the lower portion and exhibiting poor uniformity when coating with conventional inorganic high-resistance and insulating layers, significantly improving the uniformity of the high-resistance layer. Furthermore, during the grinding process, quadrangular prism-shaped resistors can overcome the difficulty of grinding the uneven sides of cylindrical varistors, enhancing the bond between the insulating layer and the body surface. This significantly improves the varistor's energy tolerance under high current surges, eliminates pores, and reduces edge porosity, further enhancing energy tolerance.

[0046] like Figure 4 and Figure 5 As shown in the figure, the dense pores on the side need to be solved. The varistor sheet B sample obtained in Example 2 (control example) was tested by X-ray Micro-CT. Figure 4(a)(b) is the edge area test diagram, (c)(d) is the internal area test diagram, (a)(b)(c)(d) indicates that the internal tissue of the tablet is gradually approached from the outside to the inside, further confirming the excellent pressing process of the quadrangular prism embryo. Figure 5 Sample B has densely distributed pores on its sides. The quadrangular prism pressing process, combined with a polishing process, polishes the four sides of the varistor's pressed sheet by 1-2 mm, eliminating the pores at the edges. This reduces porosity and improves overall uniformity. Even if the polished varistor is coated with only an insulating layer without a high-resistance layer, arcing or flashover will not occur on its edges or sides.

[0047] 2. The repair layer formula system replaces the traditional high-resistance layer

[0048] Common high-resistance layer formulations, such as Chinese patent CN201811435152.9, disclose a process for preparing a novel inorganic high-resistance layer for ZnO varistors. This novel inorganic high-resistance layer includes zinc oxide (ZnO), bismuth oxide (Bi2O3), antimony oxide (Sb2O3), manganese oxide (MnO2), tin dioxide (SnO2), chromium oxide (Cr2O3), polyvinyl alcohol (C2H4O), and deionized water. Other high-resistance layer formulations currently include ZnO-SiO2-Bi2O3-Sb2O3-Li2CO3, ZnO-SiO2-Bi2O3-Sb2O3-ZrO2, and ZnO-SiO2-Bi2O3-Fe2O3.

[0049] Example 2, based on Example 1, added a coating step: the outer surface of the formed square embryo was coated to form a repair layer. The repair layer formulation was as follows: 93.8% zinc nitrate (Zn(NO3)2), 5% bismuth nitrate (Bi(NO3)3), 1.0% cobalt oxide (Co2O3), and 0.2% lithium vinyl acetate (C2H3LiO2). Varistor sheet sample C was produced using the same raw materials and process as in Example 1.

[0050] Example 3, based on the control example, added a coating step: a repair layer was applied to the outer surface of the formed square embryo. The repair layer formulation was as follows: 93.8% zinc nitrate (Zn(NO3)2), 5% bismuth nitrate (Bi(NO3)3), 1.0% cobalt oxide (Co2O3), and 0.2% lithium vinyl acetate (C2H3LiO2). Varistor sheet D was produced using the same raw materials and process as in Example 2.

[0051] Various performance tests were performed on the ZnO varistor samples obtained in Example 2 and Example 3. See Table 2 for details, where C represents a quadrangular prism-shaped ZnO varistor sample and D represents a cylindrical ZnO varistor sample.

[0052] Table 2: Comparison of various properties of ZnO varistor samples in various embodiments and comparative examples

[0053] sample Voltage gradient (V / mm) Nonlinear coefficient Leakage current (μA / cm2) Residual pressure ratio 2ms withstand current (A) A (Example 1) 453 65.5 1.55 1.72 900 B (control example) 409 59.2 1.29 1.86 500 C (Example 3) 452 78.8 4.03 1.75 1100 D (Example 4) 388 49.5 1.56 1.77 750

[0054] Table 3: Performance comparison of 2ms withstand current of various samples

[0055]

[0056] Note: √: passed; ●: burst; ■: crack; ▲: perforation; ◎: flashover.

[0057] The performance comparisons in Tables 2 and 3 show that sample C (with a repair layer added, a quadrangular prism body) has a significant improvement in the 2ms square wave current, representing high-current performance, from 500A to 1100A, compared to samples D (with a repair layer added, a cylindrical body), A (without a repair layer, a quadrangular prism body), and B (without a repair layer, a cylindrical body). This also shows that the change in the pressing process and the addition of a repair layer instead of a traditional high-resistance layer significantly improve the nonlinear coefficient. Similarly, the residual voltage ratio and voltage gradient performance have also been improved to varying degrees. Although the leakage current is higher than that of the other samples, it is still below 5μA / cm 2 5 microamperes is a relatively small leakage current and also meets general needs.

[0058] The above experiments show that the repair layer formula system of the present invention removes oxides such as silicon oxide and antimony oxide, reduces the generation of spinel substances such as willemite and zinc antimonate during the sintering process, enables the repair medium to repair the side grains and grain boundaries, and suppresses the side flashover phenomenon; in addition, the lower spinel content increases the matching of the expansion coefficient of the side edge area and the interior of the resistor sheet, and when the ZnO resistor sheet withstands higher energy impact, it reduces the probability of cracking or peeling on the side of the resistor sheet.

[0059] The main functions of each component in the repair layer are as follows:

[0060] During the sintering process, Bi acts as a flux to promote the growth of ZnO grains.

[0061] ZnO makes the spinel evenly distributed in the repair layer during sintering, improving the bonding properties.

[0062] During the sintering process, Co loses oxygen to form CoO. Co2+ enters the ZnO grains to form substitutional impurity defects, generating supplementary energy levels in the ZnO band gap. This reduces the ZnO grain resistance, increases the grain conductivity, flattens the nonlinear section of the volt-ampere characteristic curve, and correspondingly increases the nonlinear coefficient. At the same time, it dissolves in the spinel particles during the sintering process, affecting the formation of spinel.

[0063] Li ions are highly active and have a strong diffusion and penetration effect into the body during the sintering process of the resistor coated with the repair layer.

[0064] In summary, compared with the traditional circular cross-section, the pressing molding process of the present invention solves the problem that the upper part of the same column is thinner than the lower part and the uniformity is poor when coating the inorganic high-resistance layer and the insulating layer, and greatly improves the uniformity of the high-resistance layer; the high-resistance layer of the traditional varistor pressing sheet is replaced by the repair layer of the present invention, and the traditional high-resistance layer process is replaced with the main body for sintering. The formula system of the repair layer removes oxides such as silicon oxide and antimony oxide, reduces the generation of spinel substances such as willemite and zinc antimonate during the sintering process, and enables the repair medium to fully repair the side grains and grain boundaries, thereby suppressing the side flashover phenomenon.

[0065] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. It is apparent to those skilled in the art that various changes, modifications, substitutions, and variations to these embodiments may be made without departing from the principles and spirit of the present invention, and these changes and modifications still fall within the scope of protection of the present invention.

Claims

1. A method for preparing a high-current ZnO varistor sheet, characterized in that: The following steps are involved: Raw material preparation of S100 varistor sheet: The varistor sheet is composed of the following raw materials in molar percentage: zinc oxide 92.25-95.18%, bismuth oxide 0.8-1.0%, cobalt oxide 0.6-1.2%, manganese dioxide 0.5-1.0%, chromium trioxide 0.3-0.7%, antimony trioxide 0.6-1.2%, silicon dioxide 0.8-1.5 mol%, yttrium nitrate hexahydrate 0.5-1.0%, gallium nitrate nonahydrate 0.3-0.7%, indium nitrate nonahydrate 0.02-0.04%; S200 compression molding: Take the raw materials according to the ratio, mix them through ball milling, spray dry the powder, place it in a square tableting mold, cover the mold tightly, and mold the powder into a quadrangular prism embryo with a square cross-section; S300 sintering: Use a high-temperature electric furnace to sinter the green body in a closed condition, heating from room temperature to 400℃ for 10-15h, maintaining low-temperature debinding at 280℃ for 5h, and then cooling to room temperature at a cooling rate of 15℃ / h; S400 tableting: from room temperature to 900℃, heating time is 9h, average heating rate is 100℃ / h; from 900℃ to 1250℃, heating time is 5h, average heating rate is 100℃ / h; keep at 1250℃ for 2-4h; cool naturally, average cooling rate is 100℃ / h, and obtain varistor tablets; S500 grinding: The four sides of the varistor pressed sheet are ground by a vertical grinding machine to a depth of 1 to 2 mm to obtain a finished varistor pressed sheet; S600: coating a repair layer: coating the four side surfaces of the varistor pressed sheet obtained in step S500 with a repair layer, sintering the repair layer, and then polishing the upper and lower end surfaces of the varistor pressed sheet; The repair layer is composed of the following raw materials in molar percentage: 91.7-95.2% zinc nitrate, 3.5-6.5% bismuth nitrate, 0.5-1.5% cobalt oxide, and 0.1-0.3% lithium vinyl acetate.

2. The method for preparing a high-throughput ZnO varistor sheet according to claim 1, characterized in that: The varistor pressing sheet is composed of the following raw materials in molar percentage: 93.12% zinc oxide, 1.05% bismuth oxide, 1.0% cobalt oxide, 0.75% manganese dioxide, 0.5% chromium trioxide, 1.0% antimony trioxide, 1.25% silicon dioxide, 0.9% yttrium nitrate hexahydrate, 0.4% gallium nitrate nonahydrate, and 0.03% indium nitrate nonahydrate.

3. The method for preparing a high-throughput ZnO varistor sheet according to claim 1, wherein: The repair layer is composed of the following raw materials in molar percentage: 93.8% zinc nitrate, 5% bismuth nitrate, 1.0% cobalt oxide, and 0.2% lithium vinyl acetate.

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