A high-voltage large-current zinc oxide varistor disc and a preparation method thereof

Abstract

The application discloses a high-voltage large-current zinc oxide varistor disc and a preparation method and application thereof, and belongs to the technical field of overvoltage protection devices. 3 The zinc oxide varistor disc is composed of the following raw materials in percentage by weight: 95.0-97.5% of ZnO, 0.5-1.2% of Bi2O 3 0.8-1.5% of Sb2O 3 0.2-0.5% of Co2O 2 0.3-0.6% of MnO 3 0.1-0.3% of Cr2O 3 0.01-0.05% of Al2O 3 0.1-0.4% of SiO2 and 0.05-0.2% of a rare earth dopant RE2O The non-working surface of the zinc oxide varistor disc is sprayed with modified epoxy resin insulating paint, and the modified epoxy resin insulating paint is prepared by mixing tetrahydroxy octaphenyl double-layer cage-type silsesquioxane solution, epoxy resin, fillers and a curing agent. The zinc oxide varistor disc has a residual voltage ratio of less than or equal to 1.8, a voltage nonlinearity coefficient alpha of more than or equal to 50, and a performance attenuation rate of less than or equal to 5% after 1000 hours of aging at 150 DEG C under a 250kA 8 / 20 mu s impulse current, is excellent in electrical performance and weather resistance, and can meet the overvoltage protection requirements of extra-high-voltage and large-capacity power systems.

Description

Technical Field

[0001] This invention relates to the field of overvoltage protection device technology, specifically to a high-voltage, high-current zinc oxide varistor and its preparation method. Background Technology

[0002] Zinc oxide varistors are overvoltage protection devices with excellent nonlinear volt-ampere characteristics, widely used in power systems, rail transportation, and communication equipment. With the development of ultra-high voltage (UHV) transmission technology, the capacity and voltage levels of power systems are constantly increasing, placing higher demands on the current-carrying capacity and size of zinc oxide varistors. Large-size, high-current-carrying varistors in the 220 / 250kA class are core components of UHV surge arresters, and their performance directly determines the arrester's protective effect and service life.

[0003] Currently, there are many technical challenges in the preparation of large-size zinc oxide varistors: First, during the sintering process, uneven heat conduction in large-size blanks can easily lead to low internal density and uneven grain boundary structure, resulting in a decrease in the current carrying capacity and nonlinear characteristics of the resistor. Second, under traditional formulation systems, large-size resistors are prone to grain boundary breakdown and severe degradation of electrical performance after being subjected to high-energy impacts. Third, the stability of the forming and sintering processes is difficult to control, resulting in a low product qualification rate during industrial production. To address the aforementioned issues, the industry primarily improves the process by adjusting doping formulations and optimizing sintering procedures. For example, CN116143511A discloses a method for preparing a low-voltage-ratio, high-current-capacity zinc oxide resistor sheet. This method improves grain boundary properties and the uniformity of the resistor sheet structure by adding rare earth elements, thereby enhancing the current-capacity. However, it has not been proven that this method can meet the high current-capacity requirements of 220-250kA. CN116072365A discloses a sintering method for high-current-capacity resistor sheets. This method employs a multi-step sintering process, adjusting the heating rate and holding time to regulate the crystal structure of bismuth oxide, thereby improving the uniformity of oxide grain size and grain distribution in the resistor sheet.

[0004] However, during the preparation of zinc oxide varistors, the molding effect significantly affects the sintering effect. This is because the high strength of the powder particles results in high frictional resistance, and poor molding leads to low density after sintering, thus resulting in a low product yield. Furthermore, improving the weather resistance of zinc oxide varistors is of positive significance for their better application in lightning protection devices. Summary of the Invention

[0005] The purpose of this invention is to provide a high-voltage, high-current zinc oxide varistor and its preparation method to solve the technical problems mentioned in the background art.

[0006] The present invention achieves the above objectives through the following technical solutions: In a first aspect, the present invention provides a high-voltage, high-current zinc oxide varistor, comprising the following raw materials in weight percentages: ZnO 95.0-97.5%, Bi₂O₃ 0.5-1.2%, Sb₂O₃ 0.8-1.5%, Co₂O₃ 0.2-0.5%, MnO₂ 0.3-0.6%, Cr₂O₃ 0.1-0.3%, Al₂O₃ 0.01-0.05%, SiO₂ 0.1-0.4%, and rare earth dopant RE₂O₃ 0.05-0.2%. The non-working surface of the zinc oxide varistor is coated with a modified epoxy resin insulating coating, which is prepared by mixing a tetrahydroxyoctaphenyl bilayer cage-type silsesquioxane solution with epoxy resin, filler and curing agent.

[0007] As a further optimization of the present invention, the tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane solution is obtained by dissolving tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane in 20-30 times its mass of ethyl acetate.

[0008] As a further optimization of the present invention, the amount of the tetrahydroxyoctaphenyl bilayer cage-type silsesquioxane is 3-6% of the mass of the epoxy resin.

[0009] As a further optimization of the present invention, the rare earth dopant is at least one of Y2O3 and La2O3.

[0010] As a further optimization of the present invention, the diameter of the varistor is 100-150mm, the thickness is 15-25mm, the residual voltage ratio under a 220-250kA 8 / 20μs impact current is ≤1.8, the voltage nonlinearity coefficient α is ≥50, and the performance degradation rate after aging at 150℃ for 1000h is ≤5%.

[0011] A second aspect of the present invention also provides a method for preparing a high-voltage, high-current zinc oxide varistor as described in any of the above descriptions, comprising the following steps: (1) Dry and crush each raw material separately, mix them according to the formula, and use zirconia balls as grinding media and anhydrous ethanol as dispersant to ball mill to obtain a slurry. After vacuum drying, the slurry is sieved to obtain mixed powder. (2) Add molding aid solution to the mixed powder, stir evenly and then granulate. After granulation, the green blank is obtained by isostatic pressing. Then the green blank is dried at room temperature and degummed. (3) After debinding, the green blank is heated to 400-500℃ at 2-3℃ / min and held for 1-2h. Then it is heated to 900-1000℃ at 1-2℃ / min and held for 2-3h. Finally, it is heated to 1150-1200℃ at 0.5-1℃ / min and held for 4-6h. After sintering, it is cooled to 600℃ at 2-3℃ / min and cooled to room temperature in the furnace to obtain the sintered green blank. (4) After grinding and polishing the working surface of the sintered blank, apply electrode slurry, dry at 120-150℃, heat to 600-700℃ at a rate of 5-10℃ / min, keep warm for 1-2 hours to complete electrode firing, then spray modified epoxy resin insulating coating on the non-working surface of the sintered blank, after curing, perform edge chamfering to remove burrs.

[0012] As a further optimization of the present invention, in step (1), the drying temperature is 120-150℃ and the heat preservation time is 2-4h; Controlling the particle size D of the pulverized raw material 50 The thickness is 1.0-2.0 μm; During ball milling, the ball-to-material ratio is 4-6:1, the amount of dispersant is 30-50% of the total mass of raw materials, the ball milling speed is 200-300 r / min, and the ball milling time is 8-12 h. The slurry is vacuum dried at 60-80℃ for 6-10 hours and then passed through a 100-150 mesh sieve.

[0013] As a further optimization of the present invention, in step (2), the molding aid solution includes, by mass percentage, 3.0-4.0% polyvinyl alcohol, 0.1-0.3% pentaerythritol tetraoleate, 0.05-0.1% emulsifier, and the balance being deionized water.

[0014] As a further optimization of the present invention, in step (2), the particle size of the powder after granulation is controlled at 80-150 mesh; The isostatic pressing pressure is 150-200MPa, and the holding time is 3-5min; The drying time at room temperature is 24-48 hours; The debinding process conditions are: heat up to 500-600℃ at a rate of 5-10℃ / h, and keep warm for 2-4 hours.

[0015] As a further optimization of the present invention, in step (4), the electrode paste is silver paste or aluminum paste, and the coating thickness is 20-50 μm; The modified epoxy resin insulating coating shall be sprayed at least twice, with a spraying pressure of 0.1-0.3 MPa, a curing temperature of 80-160℃, and a curing time of 2-3 hours.

[0016] As a third aspect of the present invention, an application of a high-voltage, high-current zinc oxide varistor as described in any of the preceding claims in an ultra-high voltage surge arrester is also provided.

[0017] The beneficial effects of this invention are as follows: (1) In this invention, ZnO is used as the main raw material to form the crystalline matrix of the resistor sheet, and Bi2O3 is used as the main donor dopant. During the sintering process, Bi2O3 segregates at the grain boundaries to form a high resistivity Bi-rich grain boundary layer, which is the key component to ensure the nonlinear characteristics of the resistor sheet. Sb2O3 and ZnO form a spinel phase Zn7Sb2O 12 These components play a role in grain refinement, suppressing abnormal growth of ZnO grains during high-temperature sintering, and improving the mechanical strength and current-carrying capacity of the resistor sheet. Co2O3, MnO2, Cr2O3, and RE2O3, as modifying dopants, can improve the electrical properties of grain boundaries, increase the nonlinear coefficient and impact resistance of the resistor sheet. SiO2 and Al2O3, as auxiliary dopants, can promote the homogenization of the grain boundary layer and refine the grains, further improving the density of the resistor sheet.

[0018] (2) The molding aid provided by the present invention has excellent lubrication and wetting properties of pentaerythritol tetraoleate, which works synergistically with polyvinyl alcohol to significantly improve the strength and structural integrity of the green body, effectively suppress the risk of handling cracking after molding and before sintering, and improve the interface lubrication effect between ceramic powder and the inner wall of metal mold during molding, effectively reducing frictional resistance, thereby suppressing the cracking of the resistor body after sintering and the generation of annular cracks in subsequent grinding processes from the source.

[0019] (3) The resistor prepared by this invention has a residual voltage ratio ≤1.8 and a voltage nonlinearity coefficient α≥50 under a 250kA 8 / 20μs impact current. The performance decay rate is ≤5% after aging at 150℃ for 1000h. By adjusting the doping formula and optimizing the molding and sintering process, the ZnO grains are effectively refined and the grain boundary structure is improved, so that the resistor exhibits excellent low residual voltage ratio and excellent thermal stability. The nonlinear characteristics and large current carrying capacity of the resistor are significantly improved. The performance is stable under 220-250kA impact, and the energy loss under normal working conditions is reduced. It can meet the overvoltage protection requirements of ultra-high voltage and large capacity power systems.

[0020] (4) The present invention uses modified epoxy resin insulating coating to treat the coating. With the good adhesion, salt spray resistance and water resistance of the modified epoxy resin insulating coating, it is beneficial to improve the weather resistance of the varistor and meet the long-term use requirements of the varistor in harsh environments. Detailed Implementation

[0021] The present application will now be described in further detail. It should be noted that the following specific embodiments are only used to further illustrate the present application and should not be construed as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0022] The epoxy resin is preferably an epoxy resin with an epoxy value of 0.25-0.54. In the following examples, epoxy resin E-51 is specifically selected. In the following examples, tetrahydroxyoctaphenyl bilayer cage-type silsesquioxane (DDSQ-4OH) was prepared by the following method: 31.7 g of phenyltrimethoxysilane, 3.33 g of deionized water, and 4.3 g of NaOH were added sequentially to 160 mL of isopropanol. After thorough magnetic stirring, the mixture was transferred to an oil bath at 70°C and refluxed under nitrogen for 6 h. The reaction was then carried out at room temperature with stirring for 18 h. The resulting mixed solution was subjected to rotary evaporation to remove isopropanol, and then vacuum dried at 60°C for 12 h to obtain a white powder, DDSQ-4Na. 6 g of DDSQ-4Na was weighed, dissolved in anhydrous THF, and added to a flask. The mixture was vigorously stirred under nitrogen in a water bath at 25°C. 4.8 g of glacial acetic acid was slowly added dropwise, and the reaction was continued for 3.5 h. Then, 40 g of deionized water was added to the system, and stirring was continued for 40 min. After the reaction, the solvent was removed by vacuum rotary evaporation of the product, followed by extraction with CHCl3. The mixture was washed five times with saturated NaHCO3 solution, then ten times with deionized water. After separation, the layers were dried with anhydrous magnesium sulfate. The extractant was removed by rotary evaporation after filtration, and the product was dried under vacuum for 24 hours to obtain a white, blocky solid. The crude product was then subjected to silica gel column chromatography (V... 乙酸乙酯 ∶V 正己烷 After separation at a ratio of 5:1, rotary evaporation yields a white powder, DDSQ-4OH.

[0023] Unless otherwise specified, all methods used below are conventional methods known to those skilled in the art, and all reagents and materials used are commercially available products.

[0024] 1. Raw material composition The high-voltage, high-current zinc oxide varistor chip provided in this embodiment is composed of the following raw materials by weight percentage: ZnO 96.5%, Bi2O3 0.8%, Sb2O3 1.0%, Co2O3 0.4%, MnO2 0.5%, Cr2O3 0.2%, Al2O3 0.03%, SiO2 0.4%, and rare earth dopant Y2O3 0.17%.

[0025] 2. Preparation method 2.1 Raw material pretreatment ZnO, Bi₂O₃, Sb₂O₃, Co₂O₃, MnO₂, Cr₂O₃, Al₂O₃, SiO₂, and rare earth dopant Y₂O₃ were dried separately at 120℃ for 4 hours to remove adsorbed water. The dried raw materials were then ball-milled to control the particle size D. 50 The particle size is 1.0-2.0 μm, and the particle size D90 ≤ 3.0 μm.

[0026] 2.2 Ingredients and Mixing The pretreated raw materials were placed in a planetary ball mill, using zirconia balls as the grinding medium and anhydrous ethanol as the dispersant. The ball-to-material ratio was 5:1, and the amount of dispersant was 50% of the total mass of the raw materials. The ball milling speed was 300 r / min, and the milling time was 8 h to obtain a uniform slurry. The slurry was placed in a vacuum drying oven and vacuum dried at 80℃ for 6 h to obtain a dried mixed powder. The powder was then passed through a 150-mesh sieve to remove agglomerated particles.

[0027] 2.3 Granulation and Molding (1) Preparation of molding aid solution The molding additive solution is prepared according to the following mass percentages: 3.0% polyvinyl alcohol, 0.3% pentaerythritol tetraoleate, 0.05% ODEA emulsifier, and the balance being deionized water.

[0028] (2) Granulation and molding Add molding aid solution to the mixed powder to make the moisture content of the mixed powder about 1.5%. After stirring evenly, granulate the powder. After granulation, the particle size of the powder is controlled between 80-150 mesh. Mold using an isostatic press at a molding pressure of 150 MPa and a holding time of 5 min, to obtain a green blank with a diameter of φ100 mm and a thickness of 25 mm. Place the green blank in a ventilated and dry place and dry at room temperature for 48 h. Then place it in an oven and heat it to 600 °C at a heating rate of 10 °C / h and hold it for 2 h to complete the glue removal process.

[0029] 2.4 Sintering treatment The debinding green body was placed in a high-temperature sintering furnace and subjected to a segmented heating and sintering process: an oxygen atmosphere was introduced at a flow rate of 1.0 L / min, and the temperature was increased to 400℃ at a rate of 2℃ / min and held for 2 hours; then the temperature was increased to 900℃ at a rate of 1℃ / min and held for 3 hours; finally, the temperature was increased to 1150℃ at a rate of 0.5℃ / min and held for 6 hours. After sintering, the temperature was decreased to below 600℃ at a rate of 2℃ / min and then cooled to room temperature with the furnace to obtain the sintered green body.

[0030] 2.5 Post-processing (1) Preparation of modified epoxy resin insulating coating Weigh out tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane (DDSQ-4OH) and dissolve it in ethyl acetate to obtain a tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane solution, wherein the amount of ethyl acetate used is 20 times that of tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane.

[0031] A tetrahydroxyoctaphenyl bilayer cage-type silsesquioxane solution was added to epoxy resin (E-51) and dispersed until completely dissolved. The amount of tetrahydroxyoctaphenyl bilayer cage-type silsesquioxane used was 3% of the mass of epoxy resin. Add filler (silica, 10% of epoxy resin) to the above solution, disperse at 300 r / min for 3 h, add curing agent (H-256, 30% of epoxy resin) and stir evenly at 80℃, then adjust the viscosity of the mixture to 1200 mPa·s to obtain modified epoxy resin insulating coating.

[0032] (2) Electrode firing and coating treatment The working surfaces of the sintered blank are ground and polished to ensure that the parallelism error of the end faces is ≤0.05mm. Silver paste is then applied to both polished end faces to a thickness of 30μm. The blanks are dried in a drying oven at 150℃ for 1 hour, then placed in a muffle furnace and heated to 600℃ at a rate of 5℃ / min, held for 2 hours to complete electrode firing. A modified epoxy resin insulating coating with a thickness of 20μm is then sprayed onto the non-working surfaces of the sintered blank at a spraying pressure of 0.3MPa and dried at 80℃. This spraying-drying process is repeated five times. The coated resistor sheet is then cured at 80℃, 120℃, and 160℃ for 2 hours each. Finally, the resistor sheet is chamfered to remove burrs, yielding the finished zinc oxide varistor sheet.

[0033] Example 2 1. Raw material composition The high-voltage, high-current zinc oxide varistor chip provided in this embodiment is composed of the following raw materials by weight percentage: ZnO 96.5%, Bi2O3 1.1%, Sb2O3 0.84%, Co2O3 0.5%, MnO2 0.3%, Cr2O3 0.3%, Al2O3 0.01%, SiO2 0.4%, and rare earth dopant La2O3 0.05%.

[0034] 2. Preparation method ZnO, Bi₂O₃, Sb₂O₃, Co₂O₃, MnO₂, Cr₂O₃, Al₂O₃, SiO₂, and the rare earth dopant La₂O₃ were dried separately at 150℃ for 2 hours to remove adsorbed water. The dried raw materials were then ball-milled to control the particle size D. 50 The particle size is 1.0-2.0 μm, and the particle size D90 ≤ 3.0 μm.

[0035] 2.2 Ingredients and Mixing The pretreated raw materials were placed in a planetary ball mill, using zirconia balls as the grinding medium and anhydrous ethanol as the dispersant. The ball-to-material ratio was 6:1, and the amount of dispersant was 30% of the total mass of the raw materials. The ball milling speed was 300 r / min, and the milling time was 8 h to obtain a uniform slurry. The slurry was placed in a vacuum drying oven and vacuum dried at 60°C for 8 h to obtain a dried mixed powder. The powder was then passed through a 100-mesh sieve to remove agglomerated particles.

[0036] 2.3 Granulation and Molding (1) Preparation of molding aid solution The molding additive solution is prepared according to the following mass percentages: 4.0% polyvinyl alcohol, 0.1% pentaerythritol tetraoleate, 0.1% ODEA emulsifier, and the balance is deionized water.

[0037] (2) Granulation and molding Add molding aid solution to the mixed powder to make the moisture content of the mixed powder about 1.5%. After stirring evenly, granulate the powder. After granulation, the particle size of the powder is controlled between 80-150 mesh. Mold using an isostatic press at a molding pressure of 200MPa and a holding time of 2min, to obtain a green blank with a diameter of φ150mm and a thickness of 15mm. Place the green blank in a ventilated and dry place and dry at room temperature for 28h. Then place it in an oven and heat it to 500℃ at a heating rate of 5℃ / h and hold it for 4h to complete the glue removal process.

[0038] 2.4 Sintering treatment The debinding green body was placed in a high-temperature sintering furnace and subjected to a segmented heating and sintering process: an oxygen atmosphere was introduced at a flow rate of 0.5 L / min, and the temperature was increased to 500°C at a rate of 3°C / min and held for 1 hour; then the temperature was increased to 1000°C at a rate of 2°C / min and held for 2 hours; finally, the temperature was increased to 1200°C at a rate of 1°C / min and held for 4 hours. After sintering, the temperature was decreased to below 600°C at a rate of 3°C / min, and then cooled to room temperature with the furnace to obtain the sintered green body.

[0039] 2.5 Post-processing (1) Preparation of modified epoxy resin insulating coating Weigh out tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane (DDSQ-4OH) and dissolve it in ethyl acetate to obtain a tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane solution, wherein the amount of ethyl acetate used is 30 times that of tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane.

[0040] A tetrahydroxyoctaphenyl bilayer cage-type silsesquioxane solution was added to epoxy resin (E-51) and dispersed until completely dissolved. The amount of tetrahydroxyoctaphenyl bilayer cage-type silsesquioxane used was 5% of the mass of epoxy resin. Add filler (silica, 10% of epoxy resin) to the above solution, disperse at 300 r / min for 3 h, add curing agent (H-256, 30% of epoxy resin) and stir evenly at 80℃, then adjust the viscosity of the mixture to 1200 mPa·s to obtain modified epoxy resin insulating coating.

[0041] (2) Electrode firing and coating treatment The working surfaces of the sintered blank are ground and polished to ensure that the parallelism error of the end faces is ≤0.05mm. Aluminum paste is then applied to both polished end faces to a thickness of 50μm. The blanks are dried in a drying oven at 120℃ for 2 hours, then placed in a muffle furnace and heated to 700℃ at a rate of 10℃ / min, held for 1 hour to complete electrode firing. A modified epoxy resin insulating coating with a thickness of 30μm is then sprayed onto the non-working surfaces of the sintered blank at a spraying pressure of 0.1MPa and dried at 80℃. This spraying-drying process is repeated five times. The coated resistor sheet is then cured at 80℃, 120℃, and 160℃ for 2 hours each. Finally, the resistor sheet is chamfered to remove burrs, yielding the finished zinc oxide varistor sheet.

[0042] Example 3 The high-voltage, high-current zinc oxide varistor chip provided in this embodiment is composed of the following raw materials by weight percentage: ZnO 96.5%, Bi2O3 0.65%, Sb2O3 1.5%, Co2O3 0.2%, MnO2 0.6%, Cr2O3 0.1%, Al2O3 0.05%, SiO2 0.2%, and rare earth dopant La2O3 0.2%.

[0043] The preparation method of the high-voltage, high-current zinc oxide varistor in this embodiment is the same as in Embodiment 2.

[0044] To investigate the effect of the use of molding aid solution on the preparation of high-voltage, high-current zinc oxide varistor sheets, the following comparative examples were set up: Comparative Example 1 The raw material formulation of the high-voltage, high-current zinc oxide varistor sheet provided in this comparative example is the same as that in Example 1, but the preparation method differs from that in Example 1 in that the molding aid solution used in granulation and molding in 2.3 is prepared according to the following mass percentage: polyvinyl alcohol 3.0%, and the balance is deionized water.

[0045] Comparative Example 2 The raw material formulation of the high-voltage, high-current zinc oxide varistor sheet provided in this comparative example is the same as that in Example 1, but the preparation method differs from that in Example 1 in that the molding aid solution used in granulation and molding in sections 2.3 is prepared according to the following mass percentages: polyvinyl alcohol 3.0%, ODEA emulsifier 0.05%, and the balance being deionized water.

[0046] Comparative Example 3 The raw material formulation of the high-voltage, high-current zinc oxide varistor sheet provided in this comparative example is the same as that in Example 1, but the preparation method differs from that in Example 1 in that the molding aid solution used in granulation and molding in sections 2.3 is prepared according to the following mass percentages: polyvinyl alcohol 3.0%, pentaerythritol tetraoleate 0.3%, and the remainder is deionized water.

[0047] Comparative Example 4 The raw material formulation of the high-voltage, high-current zinc oxide varistor sheet provided in this comparative example is the same as that in Example 1, but the preparation method differs from that in Example 1 in that the molding aid solution used in granulation and molding in sections 2.3 is prepared according to the following mass percentages: polyvinyl alcohol 3.0%, trimethylolpropane oleate 0.3%, and the remainder is deionized water.

[0048] Comparative Example 5 The raw material composition of the high-voltage, high-current zinc oxide varistor provided in this comparative example is the same as that in Example 1, but the preparation method differs from that in Example 1 in that only electrode firing is performed in the post-processing stage 2.5, and no modified epoxy resin insulating coating is applied.

[0049] First, 100 varistors were prepared using the methods of Examples 1-3 and Comparative Examples 1-4 (without post-processing). The scrap rate after sintering and the scrap rate after grinding were statistically analyzed. The scrap rate after sintering refers to the proportion of resistors with delamination observed after sintering out of the total number of resistors. The scrap rate after grinding refers to the resistors with delamination defects on the rear end face after grinding. The results are shown in Table 1.

[0050] Table 1. Scrap Rate Statistics As can be seen from Table 1, the varistors in Examples 1-3, which contain a molding aid solution prepared from polyvinyl alcohol, pentaerythritol tetraoleate, ODEA emulsifier, and deionized water, had a scrap rate of 0 after sintering and a scrap rate of 0 after grinding. In contrast, the varistors in Comparative Example 1, which used a molding aid solution prepared from polyvinyl alcohol and deionized water, had the highest scrap rates after sintering and grinding. In Comparative Example 3, which used a molding aid solution prepared from polyvinyl alcohol, pentaerythritol tetraoleate, and deionized water, although the scrap rate was 0 after sintering, there was still a relatively high scrap rate after grinding. As can be seen, the molding aid provided by the present invention, with its excellent lubrication and wetting properties of pentaerythritol tetraoleate, synergistically enhances the strength and structural integrity of the green body with polyvinyl alcohol, effectively suppresses the risk of handling cracking after molding and before sintering, and improves the interface lubrication effect between ceramic powder and the inner wall of the metal mold during molding, effectively reducing frictional resistance, thereby suppressing the cracking of the resistor sheet body after sintering and the generation of annular cracks in the subsequent grinding process from the source.

[0051] Subsequently, the varistor sheets prepared in Examples 1-3 and Comparative Examples 1-5 were subjected to electrical performance tests under an 8 / 20μs impulse current (People's Republic of China Energy Industry Standard NB / T42152-2018 "General Technical Requirements for Nonlinear Metal Oxide Resistors"). The test results are shown in Table 2.

[0052] Table 2. Electrical Performance Test Results As shown in Table 2, the varistor sheets prepared by the methods in Examples 1-3 have a residual voltage ratio ≤1.8 and a voltage nonlinearity coefficient α≥50 under a 250kA 8 / 20μs impact current. Furthermore, the performance degradation rate / % is ≤5% after aging at 150℃ for 1000h. By adjusting the doping formula and optimizing the molding and sintering process, the ZnO grains were effectively refined, and the grain boundary structure was improved, resulting in excellent low residual voltage ratio and excellent thermal stability of the varistor sheet. This significantly improved the nonlinear characteristics and high current carrying capacity of the varistor sheet. The performance was stable under 220-250kA impact, reducing energy loss under normal operating conditions and meeting the overvoltage protection requirements of ultra-high voltage and large-capacity power systems. The electrical properties of the varistor sheets prepared by the methods in Comparative Examples 1-4 are inferior to those prepared by the methods in Examples 1-3, indicating that the use of molding aids during the molding process can effectively avoid structural defects in the ZnO resistor sheet after molding and sintering, and solve the problem of uneven density during the sintering of large-size green blanks affecting its electrical properties. Comparative Example 5 shows that the modified epoxy resin insulating coating treatment has a positive effect on improving the thermal stability of the resistor sheet.

[0053] To further investigate the effect of modified epoxy resin insulating coating on varistors, the following comparative examples were set up: Comparative Example 6 The raw material formulation of the high-voltage, high-current zinc oxide varistor provided in this comparative example is the same as that in Example 1, but the preparation method differs from that in Example 1 in that epoxy resin insulating coating is used instead of modified epoxy resin insulating coating in the post-treatment in section 2.5. The preparation method of epoxy resin insulating coating is as follows: add filler (silica, amount of 10% of epoxy resin) to epoxy resin, disperse at 300 r / min for 3 h, add curing agent (H-256, amount of 30% of epoxy resin), stir evenly at 80°C, and adjust the viscosity of the mixture to 1200 mPa·s to obtain epoxy resin insulating coating.

[0054] Comparative Example 7 The raw material formulation of the high-voltage, high-current zinc oxide varistor provided in this comparative example is the same as that in Example 1, but the preparation method differs from that in Example 1 in that the preparation method of the epoxy resin insulating coating in the post-treatment in section 2.5 is as follows: 3,3,3-trifluoropropyltrimethoxysilane is weighed and dissolved in ethyl acetate to obtain a 3,3,3-trifluoropropyltrimethoxysilane solution, wherein the amount of ethyl acetate is 30 times that of 3,3,3-trifluoropropyltrimethoxysilane.

[0055] A solution of 3,3,3-trifluoropropyltrimethoxysilane was added to epoxy resin (E-51) and dispersed until completely dissolved. The amount of 3,3,3-trifluoropropyltrimethoxysilane was 5% of the mass of epoxy resin. Filler (silica, 10% of epoxy resin) was added to the above solution and dispersed at 300 r / min for 3 h. Then, curing agent (H-256, 30% of epoxy resin) was added and stirred evenly at 80 °C. The viscosity of the mixture was adjusted to 1200 mPa·s to obtain a modified epoxy resin insulating coating.

[0056] The modified epoxy resin insulating coating prepared in Example 1 and the epoxy resin insulating coating prepared in Comparative Examples 6-7 were cured at 80℃, 120℃, and 160℃ for 2 hours each. The adhesion of the coatings was tested according to GB / T5210-2006 (pull-out method); the salt spray resistance of the coating film was tested according to GB / T1771-2007 standard, with tinplate as the substrate. The flexibility of the coating film was tested according to GB / T1731-2020 standard. Water resistance was tested according to GB / T1733-1993 method.

[0057] The results are shown in Table 3.

[0058] Table 3. Performance Test Results The results showed that, compared with Comparative Example 5, which did not contain tetrahydroxyoctaphenyl bilayer cage-type silsesquioxane, and Comparative Example 6, which contained conventional fluorinated silane, the modified epoxy resin insulating coating used in Example 1 to prepare the varistor sheet had better adhesion, salt spray resistance, and water resistance. Applying it to the surface coating treatment of the varistor sheet is beneficial to improving the weather resistance of the varistor sheet and meeting the long-term use requirements of the varistor sheet in harsh environments.

[0059] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A high-voltage, high-current zinc oxide varistor, characterized in that, It is composed of the following raw materials by weight percentage: ZnO 95.0-97.5%, Bi2O3 0.5-1.2%, Sb2O3 0.8-1.5%, Co2O3 0.2-0.5%, MnO2 0.3-0.6%, Cr2O3 0.1-0.3%, Al2O3 0.01-0.05%, SiO2 0.1-0.4%, and rare earth dopant RE2O3 0.05-0.2%; The non-working surface of the zinc oxide varistor is coated with a modified epoxy resin insulating coating, which is prepared by mixing a tetrahydroxyoctaphenyl bilayer cage-type silsesquioxane solution with epoxy resin, filler and curing agent.

2. The high-voltage, high-current zinc oxide varistor according to claim 1, characterized in that, The tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane solution is obtained by dissolving tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane in 20-30 times its mass of ethyl acetate. The amount of the tetrahydroxyoctaphenyl bilayer cage-like silsesquioxane used is 3-6% of the mass of the epoxy resin.

3. The high-voltage, high-current zinc oxide varistor according to claim 1, characterized in that, The rare earth dopant is at least one of Y2O3 and La2O3.

4. The high-voltage, high-current zinc oxide varistor according to claim 1, characterized in that, The diameter of the varistor is 100-150mm, the thickness is 15-25mm, the residual voltage ratio under a 220-250kA 8 / 20μs impact current is ≤1.8, the voltage nonlinearity coefficient α is ≥50, and the performance degradation rate after aging at 150℃ for 1000h is ≤5%.

5. A method for preparing a high-voltage, high-current zinc oxide varistor as described in any one of claims 1-4, characterized in that, Includes the following steps: (1) Dry and crush each raw material separately, mix them according to the formula, and use zirconia balls as grinding media and anhydrous ethanol as dispersant to ball mill to obtain a slurry. After vacuum drying, the slurry is sieved to obtain mixed powder. (2) Add molding aid solution to the mixed powder, stir evenly and then granulate. After granulation, the green blank is obtained by isostatic pressing. Then the green blank is dried at room temperature and degummed. (3) After debinding, the green blank is heated to 400-500℃ at 2-3℃ / min and held for 1-2h. Then it is heated to 900-1000℃ at 1-2℃ / min and held for 2-3h. Finally, it is heated to 1150-1200℃ at 0.5-1℃ / min and held for 4-6h. After sintering, it is cooled to 600℃ at 2-3℃ / min and cooled to room temperature in the furnace to obtain the sintered green blank. (4) After grinding and polishing the working surface of the sintered blank, apply electrode slurry, dry at 120-150℃, heat to 600-700℃ at a rate of 5-10℃ / min, keep warm for 1-2 hours to complete electrode firing, then spray modified epoxy resin insulating coating on the non-working surface of the sintered blank, after curing, perform edge chamfering to remove burrs.

6. The method for preparing a high-voltage, high-current zinc oxide varistor according to claim 5, characterized in that, In step (1), the drying temperature is 120-150℃, and the temperature is maintained for 2-4 hours; Controlling the particle size D of the pulverized raw material 50 The thickness is 1.0-2.0 μm; During ball milling, the ball-to-material ratio is 4-6:1, the amount of dispersant is 30-50% of the total mass of raw materials, the ball milling speed is 200-300 r / min, and the ball milling time is 8-12 h. The slurry is vacuum dried at 60-80℃ for 6-10 hours and then passed through a 100-150 mesh sieve.

7. The method for preparing a high-voltage, high-current zinc oxide varistor according to claim 5, characterized in that, In step (2), the molding aid solution, by mass percentage, includes 3.0-4.0% polyvinyl alcohol, 0.1-0.3% pentaerythritol tetraoleate, 0.05-0.1% emulsifier, and the balance being deionized water.

8. The method for preparing a high-voltage, high-current zinc oxide varistor according to claim 5, characterized in that, In step (2), the particle size of the granulated powder is controlled at 80-150 mesh. The isostatic pressing pressure is 150-200MPa, and the holding time is 3-5min; The drying time at room temperature is 24-48 hours; The debinding process conditions are: heat up to 500-600℃ at a rate of 5-10℃ / h, and keep warm for 2-4 hours.

9. The method for preparing a high-voltage, high-current zinc oxide varistor according to claim 5, characterized in that, In step (4), the electrode paste is silver paste or aluminum paste, and the coating thickness is 20-50 μm; The modified epoxy resin insulating coating shall be sprayed at least twice, with a spraying pressure of 0.1-0.3 MPa, a curing temperature of 80-160℃, and a curing time of 2-3 hours.

10. The application of a high-voltage, high-current zinc oxide varistor as described in any one of claims 1-4 in an ultra-high voltage surge arrester.

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