Zinc oxide based varistor sheet and manufacturing method thereof

Abstract

The present invention relates to a zinc oxide-based varistor sheet and a method for manufacturing the same, comprising zinc oxide as a primary raw material, other oxides, and auxiliary raw materials. The present invention employs specific raw material ratios and components, and utilizes a double-layer structure comprising a zinc oxide-based varistor core portion and a zinc oxide-based varistor shell portion surrounding the core portion. The well-insulated outer layer limits grain growth within the varistor sheet during the sintering process, thereby improving the sintering density of the ZnO primary material. Simultaneously, surface-treated ZnO nanomaterials are combined, utilizing the covalent bonding of carbon and nitrogen at sites on the ZnO particles and atomic surfaces to further improve the electrical properties of the ZnO-based varistor sheet. Thus, the ZnO varistor sheet formulation, raw material processing, and selection, combined with the sintering process, improve the nonlinear characteristics of the varistor sheet, achieving a low residual voltage ratio and a high energy density.

Description

Technical Field

[0001] The present invention relates to an electric component and a manufacturing method thereof, and more particularly to a zinc oxide-based varistor sheet and a manufacturing method thereof. Background Art

[0002] Currently, power systems require effective overvoltage limiting to protect the insulation of high-voltage equipment. Metal oxide arresters (MOAs) are key devices for overvoltage limiting in power systems, and their overvoltage protection largely determines the insulation level of various power system equipment. ZnO varistor valve discs are the core components of arresters, with excellent nonlinear characteristics and high energy absorption capacity, which can effectively limit overvoltage.

[0003] Compared with foreign high-performance valve discs, the ZnO varistor valve discs currently produced by domestic manufacturers have problems such as low varistor voltage gradient, large voltage limiting ratio, small flow capacity or mechanical aging.

[0004] Many domestic scientific research and engineering workers have conducted experimental and production research on zinc oxide-based resistors. For example, Chinese patent publication CN103021608 A relates to a high-gradient, large-capacity zinc oxide varistor and its preparation method. The resistor is composed of components such as zinc oxide, bismuth trioxide, cobalt tetroxide, cobalt trioxide, and antimony trioxide. Its preparation method uses ball milling, granulation, debinding, pre-burning coating, firing, grinding, heat treatment, electrode spraying, and glazing. The patented technology records that the potential gradient of the prepared zinc oxide varistor is greatly improved, and the 2ms square wave current capacity remains unchanged compared to ordinary resistors of the same specifications; this preparation method not only reduces the number of processes and saves energy, but also avoids secondary pollution from auxiliary materials and additives. However, this type of technology does not refine the formula and raw material selection of ZnO series nonlinear resistors, nor does it optimize nonlinear resistance characteristics (high potential gradient) and residual voltage ratio. Summary of the Invention

[0005] The present invention improves one or more of the above needs and deficiencies. In a first aspect of the present invention, a zinc oxide-based varistor sheet is provided, wherein the zinc oxide-based varistor sheet comprises the following raw material components calculated in molar percentage:

[0006] ZnO: 80-85%, Sb2O3: 3-7%, Bi2O3: 3-5%, Co2O3: 0.5-2%, Co3O4: 0.5-1.0%, SiO2: 1.5-3%, MnO2: 1-3%, Cr2O3: 0.5-1.5%, NiO: 0.2-0.8%, B2O3: 0.5-1.0%, Al(NO3)3: 0.1-0.5%, Er2O3: 0.1-0.5%; and optionally additional auxiliary components, the auxiliary components comprising one or more of a binder, a dispersant, and a defoaming agent;

[0007] The particle size of the ZnO raw material of the zinc oxide-based varistor sheet is between about 50-100 nm, and

[0008] The zinc oxide-based varistor sheet includes a zinc oxide-based varistor sheet core portion and a zinc oxide-based varistor sheet shell portion surrounding the zinc oxide-based varistor sheet core portion.

[0009] Preferably, the molar percentage content of the Zn component in the shell part is lower than the molar percentage content of the Zn component in the core part.

[0010] Further, according to an optional technical solution, the average particle size of the core part of the zinc oxide-based varistor sheet is about 150 nm to 200 nm, and the average particle size of the shell part of the zinc oxide-based varistor sheet is about 500 nm to 5 μm.

[0011] According to an optional technical solution, the shell portion of the zinc oxide-based varistor sheet is composed of ZnO, SiO2 and Al2O3.

[0012] According to an optional technical solution, in the shell portion of the zinc oxide-based varistor sheet, the molar percentage content of the ZnO component accounts for 10% to 40% of the total content of the shell portion.

[0013] According to an optional technical solution, in the shell portion of the zinc oxide-based varistor sheet, the shell portion is composed of ZnO, SiO2 and Al2O3 in a molar ratio of 1:1:1.

[0014] A second aspect of the present invention provides a method for manufacturing the above-mentioned zinc oxide-based varistor sheet, the method comprising the following steps:

[0015] Step 1): preparing raw materials for the zinc oxide-based varistor sheet, wherein, calculated by mole percentage, ZnO: 80-85%, Sb2O3: 3-7%, Bi2O3: 3-5%, Co2O3: 0.5-2%, Co3O4: 0.5-1.0%, SiO2: 1.5-3%, MnO2: 1-3%, Cr2O3: 0.5-1.5%, NiO: 0.2-0.8%, B2O3: 0.5-1.0%, Al(NO3)3: 0.1-0.5%, Er2O3: 0.1-0.5%; and auxiliary components, wherein the particle size of the ZnO raw material is between about 50-100 nm;

[0016] Step 2): The above raw materials except the above ZnO are mixed in a ball mill and ball milled, calcined at a temperature below 600° C. for 0.5-2 hours, and then crushed again in a ball mill;

[0017] Step 3): The ZnO raw material is subjected to a surface treatment process;

[0018] Step 4): Adding ZnO after surface treatment process to a mixing device, adding other raw material components crushed in a ball mill, and then adding auxiliary components accounting for no more than 5% of the total mass of the raw materials, and mixing in the mixing device;

[0019] Step 5): Pressing the mixed material into a resistor green sheet, and performing preliminary low-temperature sintering on the green sheet, in the preliminary low-temperature sintering, heating from room temperature to 200° C. at a heating rate of 50-60° C. / h; heating to 400° C. at a heating rate of 65-75° C. / h; heating to 600° C. at a heating rate of 75-85° C. / h; and holding at 600° C. for 20 minutes;

[0020] Step 6): preparing a shell raw material composed of raw material nano ZnO, raw material SiO2, and raw material Al2O3, ball milling them in a ball mill, and then adding 0.5-1.5% of the total mass of the shell raw material binder, 0.5-2% of the dispersant and deionized water to mix to form a thermal insulation coating slurry;

[0021] Step 7): spraying or brushing the thermal insulation coating slurry on the side surface, top surface and bottom surface of the resistor green sheet that has been preliminarily sintered at a low temperature in step 5), to form a shell structure and a core structure of the resistor green sheet covered with the thermal insulation coating slurry;

[0022] Step 8): The resistor sheet having the shell structure and the core structure is subjected to secondary sintering, wherein the temperature is raised to 400°C at a heating rate of 70-90°C / h, and then raised to a sintering temperature of 850°C at a heating rate of 40-50°C / h, and maintained at 850°C for 3 hours, and then slowly raised to 950°C at a heating rate of 40-50°C / h and maintained for 1 hour; then, the sintered sheet is cooled in the furnace to obtain a zinc oxide-based varistor sheet including a shell portion and a core portion.

[0023] According to an optional technical solution, the surface treatment process of the ZnO raw material in step 3) is implemented according to the following method:

[0024] Ammonium citrate and purified water are added to a closed polytetrafluoroethylene reaction tank at a molar ratio of ammonium citrate to purified water of 1:20 for a hydrothermal reaction. The reaction temperature is controlled at 100-150° C. for 2-5 hours. After the hydrothermal reaction is completed, the mixture is filtered using a microporous filter, and the filtered material is dried and ground. The obtained product is mixed with the nano ZnO raw material in step 1) to obtain a surface-treated ZnO raw material.

[0025] Preferably, according to an optional technical solution, in step 6), the molar ratio of the slurry composed of nano ZnO, SiO2, and Al2O3 is 1:1:1; and

[0026] The thickness of the thermal insulation coating formed in step 7) is about 300 μm to 500 μm.

[0027] Preferably, in step 4), the auxiliary components added in an amount not exceeding 5% by weight of the total weight of the raw material components are:

[0028] Adding 3% of the total weight of the raw material components as a binder polyvinyl ketone (PVP), 0.5% of the total weight of the raw material components as a dispersant sodium hexametaphosphate, and 0.2% of the defoaming agent tributyl phosphate, and thoroughly mixing the components; and

[0029] The step 8) further includes polishing the shallow surface of the zinc oxide-based varistor sheet and attaching electrodes to the upper and lower surfaces of the zinc oxide-based varistor sheet.

[0030] The oxide-based varistor device obtained according to the raw material ratio, processing technology and manufacturing steps of the present invention adopts the ratio and composition between specific raw materials, and uses a double-layer structure including a core portion of a zinc oxide-based varistor sheet and a shell portion of a zinc oxide-based varistor sheet surrounding the core portion of the zinc oxide-based varistor sheet. The good thermal insulation outer layer (shell portion) is used to limit the growth of grains inside the resistor sheet and improve the sintering density of the ZnO main material. At the same time, combined with the surface-treated ZnO nanomaterial, the covalent bonding of carbon and nitrogen at the sites of the ZnO particles and the atomic surface is utilized to further improve the electrical properties of the ZnO-based varistor sheet. Therefore, the ZnO resistor sheet formula, raw material processing and selection combined with the sintering process of the present invention improve the nonlinear characteristics of the resistor sheet, achieve a low residual voltage ratio and a high energy density. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings are used to provide a further understanding of the technical solution of the present disclosure and constitute a part of the specification. Together with the specific embodiments of the present application, they are used to explain the technical solution of the present disclosure and do not constitute a limitation on the technical solution of the present disclosure.

[0032] Figure 1 Schematic diagram of the oxide-based varistor device prepared by the present invention and scanning electron microscope (SEM) images of the central region (core portion), transition region, and edge region (shell portion). DETAILED DESCRIPTION

[0033] The present invention is described in more detail below to facilitate understanding of the invention.

[0034] Before describing the specific embodiments, it should be noted that those skilled in the art can select appropriate raw materials based on the inspiration and teachings of this disclosure, and use relevant testing equipment to perform relevant tests and obtain corresponding results. For raw materials for which the specific manufacturer or route is not specified, those skilled in the art can select raw materials that meet the corresponding requirements as reaction starting materials based on the disclosure and requirements of this specification. The raw materials for the reactions between the compounds or components in the process are derived from the initial products synthesized in the preceding steps of the present invention, which is also understandable based on this disclosure.

[0035] The main oxide raw materials of the present invention are purchased from commercial sources, among which the main raw material zinc oxide is purchased from Jiangsu Xianfeng Nanomaterial Technology Co., Ltd., which is a nano zinc oxide powder with a purity of more than 99.8wt% and an average particle size of 60-80nm, or a zinc oxide fine powder of 100-150 nanometers (used as a comparative example experiment).

[0036] Other submicron oxide powders and other raw materials were also purchased commercially, with a purity of 99.5 wt% or greater. Those skilled in the art can also further refine the raw material powders to the desired particle size range using methods known in the art (e.g., high-energy ball milling), and can also purify or remove impurities based on the raw material purity requirements.

[0037] In the electron microscope test of the present invention, a scanning electron microscope JSM-IT200 of JEOL Corporation of Japan was used for testing;

[0038] The Xi'an Hongduo KG-5kA / 500A zinc oxide resistor impulse current tester was used to conduct an 8 / 20μs lightning impulse current simulation test on the resistor. The residual voltage value U5kA after the resistor passed the 5kA impulse current was measured, and the residual voltage ratio was calculated.

[0039] The U-I test of the resistor was performed using a KEITHLEY 2410 digital source meter to obtain the potential gradient (E1mA).

[0040] Example 1 (E1)

[0041] This embodiment provides an oxide-based varistor device, which is a zinc oxide-based varistor sheet. The preparation method involves the following steps.

[0042] Step 1): Prepare the raw materials for the oxide-based varistor device. The raw materials used include the following components calculated in molar percentage:

[0043] ZnO: 81%, Sb2O3: 5.5%, Bi2O3: 5%, Co2O3: 0.5%, Co3O4: 1.0%, SiO2: 3%, MnO2: 1%, Cr2O3: 0.5%, NiO: 0.5%, B2O3: 1.0%, Al(NO3)3: 0.5%, Er2O3: 0.5%. The zinc oxide-based varistor sheet may also contain other auxiliary components.

[0044] In this embodiment, the zinc oxide raw material is zinc oxide fine powder with an average particle diameter of 60-80 nm. The zinc oxide fine powder is subjected to a surface treatment process described in detail below before use.

[0045] In this embodiment, the particle size of the above raw material components except ZnO is selected to be 0.2 μm or less, for example, 0.1 to 0.2 μm, so as to match the particle size of the ZnO raw material.

[0046] Step 2): The above raw materials except the above ZnO are mixed and ball-milled in a ball mill at 100 rpm for 5 hours, calcined at 500° C. for 1 hour, and pulverized again in the ball mill.

[0047] Step 3): The ZnO raw material is subjected to a surface treatment process, which will be described in detail below.

[0048] Step 4): Add the surface-treated ZnO and the other raw material components crushed in the ball mill to a mixing device, then add 3% of the total weight of the above raw material components as a binder polyvinyl ketone (PVP), 0.5% of the total weight of the above raw material components as a dispersant sodium hexametaphosphate, and 0.2% of the defoaming agent tributyl phosphate, and thoroughly mix the components.

[0049] Step 5): The mixed material is pressed into resistor chip green sheets, and the green sheets are subjected to preliminary low-temperature sintering. In the preliminary low-temperature sintering step, the temperature is raised from room temperature to 200°C at a heating rate of 50-60°C / h, then to 400°C at a heating rate of 65-75°C / h, and finally to 600°C at a heating rate of 75-85°C / h. The temperature is then maintained at 600°C for 20 minutes.

[0050] The resistor sheet green sheet obtained in the above steps can be used as the core part of the final zinc oxide-based varistor sheet.

[0051] Step 6): Next, a slurry consisting of the nano-ZnO raw materials, SiO2, and Al2O3 raw materials is prepared. The powdered raw materials are added in a molar ratio of 1:1:1 and ball-milled. The mixture is then mixed with 1% binder (1% by weight of the total weight of the mixed powders), 1% dispersant, and excess deionized water to form a thermal insulation coating slurry.

[0052] Step 7): Spray or brush the coating slurry onto the side, top, and bottom surfaces of the resistor chip blank to form a resistor chip blank shell and core structure covered with the thermal insulation coating slurry. The thickness of the thermal insulation coating slurry is about 500 μm.

[0053] Step 8): The resistor sheet with the shell and core structure is subjected to a secondary sintering process. The temperature is raised to 400°C at a heating rate of 70-90°C / h, then slowly increased to a sintering temperature of 850°C at a heating rate of 40-50°C / h. The temperature is maintained at 850°C for 3 hours, and then slowly increased to 950°C at a heating rate of 40-50°C / h and maintained for 1 hour. The sintered sheet is then cooled in the furnace to obtain a zinc oxide-based varistor sheet comprising a shell portion and a core portion.

[0054] The shallow surface of the zinc oxide-based varistor sheet is polished. Then, optionally, an electrode is attached (electrode layer is applied) or a glazing treatment is performed according to the subsequent use requirements.

[0055] The resulting zinc oxide-based varistor sheet was subjected to relevant electrical performance tests, as well as SEM scanning electron microscopy testing of the cross-section of the varistor sheet. This will be described in detail in the performance testing and analysis section below.

[0056] The inventors have discovered that surface modification of untreated nano zinc oxide powder can further improve the gradient performance, residual voltage ratio and energy density of oxide-based varistor devices.

[0057] Therefore, the inventors performed the following surface treatment process on the raw material nano-zinc oxide fine powder in Example 1: ammonium citrate and purified water were added to a sealed polytetrafluoroethylene reaction vessel at a molar ratio of 1:20, and the reaction temperature was controlled at 120°C for 3 hours. After the hydrothermal reaction, the mixture was filtered through a microporous filter, dried, and ground. The resulting product was then mixed with the nano-ZnO raw material from step 1) to obtain a surface-treated ZnO raw material.

[0058] Here, more specifically, zinc oxide fine powder with an average particle diameter of 60-80 nm is added to an excess ethanol solvent, and the above-mentioned hydrothermal treated product is added according to a molar ratio of zinc oxide fine powder to ammonium citrate of 10:1, and the mixture is continuously stirred for 15 minutes. After filtration and drying, the surface-treated ZnO raw material is formed.

[0059] Example 2 (E2)

[0060] The inventors implemented Example 2 for preparing a zinc oxide-based varistor sheet. In Example 2, the same experimental steps as in Example 1 were performed, except that the raw materials used involved the following components calculated in molar percentage:

[0061] ZnO: 85%, Sb2O3: 3%, Bi2O3: 3%, Co2O3: 2%, Co3O4: 0.5%, SiO2: 3%, MnO2: 1.5%, Cr2O3: 0.5%, NiO: 0.5%, B2O3: 0.5%, Al(NO3)3: 0.3%, Er2O3: 0.3%.

[0062] The zinc oxide-based varistor sheet of Example 2 was also subjected to relevant electrical performance tests.

[0063] Comparative Example 1 (C1)

[0064] In this experiment, the same preparation steps as in Example 1 were used, except that the raw materials used involved the following components calculated in molar percentages, as shown in Table 2:

[0065] Table 2: Molar ratio of zinc oxide-based varistor raw materials used in Comparative Example 1

[0066]

[0067]

[0068] In Table 2 of Comparative Example 1, the raw material components marked with horizontal lines were not selected and proportioned according to the components of the examples of the present invention.

[0069] Comparative Example 2 (C2)

[0070] This experiment involves the preparation and comparative experiment of the shell part of zinc oxide-based varistor sheets.

[0071] In the experiment of Comparative Example 2-1 (C2-1), compared to the experimental steps in Example 1, the experimental steps of preparing the coating slurry in Example 1 and spraying or brushing it on the side, top, and bottom surfaces of the resistor sheet to form a resistor sheet covered with the thermal insulation coating slurry were not included. In other words, the zinc oxide-based varistor sheet did not include a shell structure with thermal insulation properties.

[0072] Comparative Example 2-2 (C2-2) involved spraying or brushing the coating slurry onto the side, top, and bottom surfaces of the resistor chip green sheet, forming a thermally insulating coating slurry-coated resistor chip green sheet shell and core structure. The differences were that Sb2O3 and Bi2O3 were added to a powdered raw material at a molar ratio of 1:1 and ball-milled in a ball mill. The coating slurry was then mixed with 1% binder (compared to the total weight of the mixed powder), 1% dispersant, and an excess of deionized water to form the coating slurry. The coating slurry was then sprayed or brushed onto the side, top, and bottom surfaces of the resistor chip green sheet, forming a resistor chip green sheet shell and core structure. The coating slurry was then applied in the same spraying or brushing process as in Example 1 to form a resistor chip coated with the coating slurry.

[0073] Comparative Example 2-3 (C2-3) followed the same experimental steps as in Example 1, including spraying or brushing the coating slurry onto the side, top, and bottom surfaces of the resistor chip green sheet to form a resistor chip green sheet shell and core structure covered with the thermal insulation coating slurry. However, the coating slurry was prepared using a slurry containing ZnO, Sb2O3, and Bi2O3 at a molar ratio of 2:1:1. Subsequently, the same spraying or brushing process as in Example 1 was performed to form a resistor chip covered with the coating slurry.

[0074] Comparative Example 3 (C3)

[0075] This experiment involves the preparation and comparative experiment of the shell part of zinc oxide-based varistor sheets.

[0076] In the experiment of Comparative Example 3-1 (C3-1), a preparation process similar to that of Example 1 was carried out. The difference was that the zinc oxide raw material was a zinc oxide fine powder with an average particle diameter of 60-80 nm. However, this zinc oxide fine powder was not subjected to the surface treatment process of Example 1. Instead, it was directly mixed with the above raw materials except ZnO and the remaining processes of Example 1 were carried out.

[0077] In Comparative Example 3-2 (C3-2), a similar preparation process as in Example 1 was followed. The difference was that the zinc oxide raw material was a fine zinc oxide powder with an average particle diameter of 150-200 nm. This zinc oxide fine powder was not subjected to the surface treatment process of Example 1, but was directly mixed with the above-mentioned raw materials, excluding ZnO, and the remaining processes of Example 1 were followed.

[0078] In Comparative Example 3-2 (C3-3), a similar preparation process was used as in Example 1. The difference was that the zinc oxide raw material was fine zinc oxide powder with an average particle diameter of 150-200 nm. The remaining processes, including the surface treatment of the zinc oxide powder, were the same as in Example 1.

[0079] Test Experiment

[0080] In the test of this application, the potential gradient (E1mA, V / mm), residual voltage ratio (K) and energy density (J / cm 3 The test process complies with the test procedures and test standards of "AC Metal Oxide Surge Arrester" of GB / T 11032-2020.

[0081] During the test, the voltage across the two ends of each zinc oxide-based varistor sheet when a direct current of 1 mA is passed through the sheet (U1mA) is first measured, and the voltage across the two ends of the sheet when a lightning strike of 8 / 20s is passed through the sheet (U5kA) is measured. The ratio of the two is the residual voltage ratio (K), which reflects the nonlinear characteristics of the prepared sheet. In addition, the ratio of the voltage corresponding to the 1mA current to the thickness of the sheet is used to obtain the above-mentioned potential gradient of the sheet (E1mA, V / mm). The sheet of this embodiment can achieve a potential gradient of 600V / mm or even higher. The residual voltage ratio (K), potential gradient (E1mA) and energy density (J / cm 3 ) are shown in Table 3.

[0082] Table 3: Performance tests of various embodiments and comparative examples

[0083]

[0084]

[0085] It can be seen from the test data of the embodiments and comparative examples that the samples of Examples 1 and 2 of the present invention achieve the highest nonlinear characteristics of the potential gradient, better energy density and lower residual voltage ratio.

[0086] The inventors do not wish to be bound by any theory. However, based on the inventors' test and experimental results, combined with corresponding test methods, some corresponding explanations and possible theoretical analysis can be obtained. The inventors conducted SEM scanning electron microscopy tests on the longitudinal cross-sections (cross-sections) of the zinc oxide-based varistor sheet prepared in Example 1 at different positions. Figure 1 As shown. Figure 1 In the embodiment of the present invention, the zinc oxide-based varistor sheet 1 includes a core portion (i.e., the portion from region a to region b), and a shell portion 3 sprayed or brushed onto the side, top, and bottom surfaces of the resistor sheet green sheet to form a resistor sheet green sheet shell coated with a thermal insulation coating slurry. The material sprayed or brushed onto the shell portion 3 (forming region a) of the embodiment of the present invention is composed of a slurry composed of ZnO, SiO2, and Al2O3. The coating composed of oxides of Zn, Al, and Si has excellent thermal insulation properties and effectively inhibits the growth of ceramic particles in the central region a of the zinc oxide-based varistor sheet. As can be seen from the SEM image, the average particle size of the sintered particles in the central region a of the zinc oxide-based varistor sheet obtained after sintering is approximately 150-200 nm. Combined with the short sintering process of the present invention, the growth of ceramic particles in the central region is effectively restricted. Here, region 2 shows the area where the central region and the shell portion at the edge intersect and transition with each other. From the longitudinal cross-sectional SEM photograph corresponding to the region b closer to the edge (shell part), it can be seen that the ceramic particles have grown significantly during the high-temperature sintering step, reaching the level of hundreds of nanometers to microns. At the edge c (the region including the shell part), the particles including the shell part grow rapidly under high-temperature sintering, and the outermost particles sintered from the slurry composed of ZnO, SiO2, and Al2O3 have a particle size of more than 2μm. Therefore, in the inventor's design concept, the slurry composed of ZnO, SiO2, and Al2O3 acts as an insulating protective layer to protect the growth of the ceramic particles in the core part, so that the particle density of the zinc oxide-based varistor sheet of Examples (1 and 2) is compact, the nonlinear characteristics are better, and the optimal energy density (J / cm 3) and potential gradient (E1mA). Therefore, in contrast, in the series of experiments of comparative example C2, the resistor sheet that does not contain a shell structure cannot achieve optimized potential gradient and energy density. It is worth mentioning that the inventors surprisingly found that the use of Zn and Zn oxides in the shell structure plays a role in further optimizing the sintering effect and resistance characteristics. However, when other raw material components that do not contain the Zn element are used as the coating of the shell part (C2-2), the potential gradient and energy density are deteriorated compared with the embodiment. The inventors speculate that the ceramic grains at the boundary of the joint part of the shell part that does not contain Zn and the core part that contains a large amount of Zn are unevenly staggered with each other, which deteriorates the overall sintering density and compactness of the ceramic sheet, affecting the further improvement of the nonlinear characteristics. In addition, although the other components in the comparative example (such as Sb and Bi oxides) have good insulation properties, their thermal insulation properties are poor, so other types of oxides cannot serve as a good thermal insulation layer. However, when the Zn content in the shell portion is high (equal to or greater than 50% by mole), or when the shell element does not contain Si oxide elements (such as C2-3), the thermal insulation performance of the shell portion cannot be optimally guaranteed, and therefore the nonlinear resistance characteristics and potential gradient cannot be optimally improved. Therefore, in the embodiment of the present invention, preferably, the molar percentage content of the Zn component in the shell portion of the resistor sheet accounts for 10% to 40% of the total components of the shell portion. More preferably, the shell portion of the zinc oxide-based varistor sheet is composed of ZnO, SiO2 and Al2O3 raw materials in a molar ratio of 1:1:1.

[0087] In addition, the inventors surprisingly discovered that surface modification of commercially available ZnO nanopowders or nanoparticles, for example, can further improve the energy density and nonlinear characteristics of the potential gradient of the resistor. When subjected to a hydrothermal reaction using ammonium citrate, quantum dots containing carbon are formed. Furthermore, the surfaces of the carbon quantum dots may have oxygen-containing coordination groups, such as carbonyl (-C=O-) and amino (-NH2). When further mixed with nano-zinc oxide particles, these can fully coordinate with the dangling bonds of metal cations on the surface of the nano-zinc oxide before and after sintering, thereby reducing the amount of adsorbed oxygen molecules and the easily polymerizable hydroxyl groups bound to the surface of the nano-zinc oxide. This increases the sintered density of the ZnO material and, consequently, improves the energy density and nonlinear characteristics.

[0088] Therefore, compared to Comparative Example C3 of the present invention, when the ZnO nanoparticles were prepared without surface treatment, the energy density was not optimal. It is worth noting that if the initial ZnO raw material particle size is too large (such as C3-3), while achieving good energy density, the nonlinear characteristics are slightly degraded. This may be related to the excessive growth of the main component particles during sintering, which affects the optimization of the nonlinear characteristics of the ZnO resistor.

[0089] Finally, regarding the selection of raw materials for nonlinear resistors, the inventors discovered through experiments that the types of Si, multivalent Co, rare earth elements, and the selection and proportion of the main ingredient, ZnO, influence the electrical properties of the nonlinear resistor. Specifically, when the SiO2 ratio is too high, the sintering density decreases, potentially affecting the nonlinear conduction between Zn components. Therefore, in the preferred embodiment, the molar ratio of SiO2 does not exceed 3%. Furthermore, a proper mixture of Co2O3 and Co3O4 is the optimal solution for optimizing nonlinear resistors. When the Co3O4 ratio is too low (such as in C1-2), it is not conducive to optimizing nonlinear characteristics. In the preferred embodiment of the present invention, the molar ratio of Co3O4 in the raw materials should be greater than or equal to 0.5%. This may be related to the sintering atomic structure of Co oxide itself. Finally, the inventors discovered that Er2O3 is a preferred rare earth trace element, with a synergistic effect superior to Y2O3 reported in related art, which may be related to Er's superior electron transport properties. When Al(NO 3 ) 3 is used as one of the raw materials, nitrate can also serve as a site for bonding with the ZnO surface, so the sintering density and particle bonding degree in the embodiment using Al nitrate are better.

[0090] Although the present disclosure includes specific embodiments, it is obvious to those skilled in the art that various formal and detailed substitutions or changes can be made to these embodiments without departing from the gist and scope of the invention of the present claims and their equivalent technical solutions. The embodiments described herein should be considered in an illustrative sense only and not for the purpose of limitation. The description of the features and aspects in each embodiment is considered to be applicable to similar features and aspects in other embodiments. Therefore, the scope of the present disclosure should not be limited by the specific description, but by the technical solutions of the claims, and all changes within the scope of the claims and their equivalents are interpreted as being included within the technical solutions of the present disclosure.

Claims

1. A zinc oxide-based varistor sheet, characterized in that: The core part of the zinc oxide-based varistor sheet comprises the following raw material components calculated in molar percentage: ZnO: 80-85%, Sb2O3: 3-7%, Bi2O3: 3-5%, Co2O3: 0.5-2%, Co3O4: 0.5-1.0%, SiO2: 1.5-3%, MnO2: 1-3%, Cr2O3: 0.5-1.5%, NiO: 0.2-0.8%, B2O3: 0.5-1.0%, Al(NO3)3: 0.1-0.5%, Er2O3: 0.1-0.5%; and optionally additional auxiliary components, the auxiliary components comprising one or more of a binder, a dispersant, and a defoaming agent; The particle size of the ZnO raw material of the zinc oxide-based varistor sheet is between 50-100 nm, and The zinc oxide-based varistor sheet includes a core portion of the zinc oxide-based varistor sheet and a shell portion of the zinc oxide-based varistor sheet surrounding the core portion of the zinc oxide-based varistor sheet; wherein the shell portion of the zinc oxide-based varistor sheet is composed of ZnO, SiO2 and Al2O3, and in the shell portion of the zinc oxide-based varistor sheet, the shell portion is composed of ZnO, SiO2 and Al2O3 in a molar ratio of 1:1:

1. 2 . The zinc oxide-based varistor sheet according to claim 1 , wherein a molar percentage content of the Zn component in the shell portion is lower than a molar percentage content of the Zn component in the core portion.

3. The zinc oxide-based varistor sheet according to claim 1, characterized in that: The average particle size of the core part of the zinc oxide-based varistor sheet is 150 nm to 200 nm, and the average particle size of the shell part of the zinc oxide-based varistor sheet is 500 nm to 5 μm.

4. A method for manufacturing a zinc oxide-based varistor sheet according to any one of claims 1 to 3, the method comprising the following steps: Step 1): preparing raw materials for the zinc oxide-based varistor sheet, wherein, calculated by mole percentage, ZnO: 80-85%, Sb2O3: 3-7%, Bi2O3: 3-5%, Co2O3: 0.5-2%, Co3O4: 0.5-1.0%, SiO2: 1.5-3%, MnO2: 1-3%, Cr2O3: 0.5-1.5%, NiO: 0.2-0.8%, B2O3: 0.5-1.0%, Al(NO3)3: 0.1-0.5%, Er2O3: 0.1-0.5%; and auxiliary components, wherein the particle size of the ZnO raw material is between 50-100 nm; Step 2): The above raw materials except the above ZnO are mixed in a ball mill and ball milled, calcined at a temperature below 600° C. for 0.5-2 hours, and then crushed again in a ball mill; Step 3): The ZnO raw material is subjected to a surface treatment process; Step 4): Adding ZnO after surface treatment process to a mixing device, adding other raw material components crushed in a ball mill, and then adding auxiliary components accounting for no more than 5% of the total mass of the raw materials, and mixing in the mixing device; Step 5): Pressing the mixed material into a resistor green sheet, and performing preliminary low-temperature sintering on the green sheet, in the preliminary low-temperature sintering, heating from room temperature to 200° C. at a heating rate of 50-60° C. / h; heating to 400° C. at a heating rate of 65-75° C. / h; heating to 600° C. at a heating rate of 75-85° C. / h; and holding at 600° C. for 20 minutes; Step 6): preparing a shell raw material composed of raw material nano ZnO, raw material SiO2, and raw material Al2O3, ball milling them in a ball mill, and then adding 0.5-1.5% of the total mass of the shell raw material binder, 0.5-2% of the dispersant and deionized water to mix to form a thermal insulation coating slurry, wherein the molar ratio of the slurry composed of nano ZnO, SiO2, and Al2O3 is 1:1:1; Step 7): spraying or brushing the thermal insulation coating slurry on the side surface, top surface and bottom surface of the resistor green sheet that has been preliminarily sintered at a low temperature in step 5), to form a shell structure and a core structure of the resistor green sheet covered with the thermal insulation coating slurry; Step 8): The resistor sheet having the shell structure and the core structure is subjected to secondary sintering, wherein the temperature is raised to 400°C at a heating rate of 70-90°C / h, and then raised to a sintering temperature of 850°C at a heating rate of 40-50°C / h, and maintained at 850°C for 3 hours, and then slowly raised to 950°C at a heating rate of 40-50°C / h and maintained for 1 hour; then, the sintered sheet is cooled in the furnace to obtain a zinc oxide-based varistor sheet including a shell portion and a core portion.

5. The method according to claim 4, wherein The surface treatment process of the ZnO raw material in step 3) is carried out according to the following method: Ammonium citrate and purified water are added to a closed polytetrafluoroethylene reaction tank at a molar ratio of 1:20 for a hydrothermal reaction. The reaction temperature is controlled at 100-150° C. for 2-5 hours. After the hydrothermal reaction is completed, the mixture is filtered using a microporous filter, and the filtered material is dried and ground to obtain a surface-treated ZnO raw material.

6. The method according to claim 4, wherein: The thickness of the thermal insulation coating formed in step 7) is 300 μm to 500 μm.

7. The method according to claim 4, wherein in step 4), the auxiliary component that accounts for no more than 5% by weight of the total weight of the raw material components is added: Adding 3% of the total weight of the raw material components as a binder polyvinyl ketone (PVP), 0.5% of the total weight of the raw material components as a dispersant sodium hexametaphosphate, and 0.2% of the defoaming agent tributyl phosphate, and thoroughly mixing the components; and The step 8) further includes polishing the shallow surface of the zinc oxide-based varistor sheet and attaching electrodes to the upper and lower surfaces of the zinc oxide-based varistor sheet.

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