High-gradient ZnO voltage-sensitive ceramic and simple low-temperature sintering method

Abstract

The invention discloses a ZnO voltage-sensitive ceramic and a simple low-temperature sintering method, the ZnO voltage-sensitive ceramic is composed of [100-(a + b + c + d + e)] mol% ZnO, amol% Bi2O3, bmol% Co3O4, cmol% Mn2O3, dmol% Cr2O3 and emol% SnO2, and a + b + c + d + e is more than or equal to 2.1 and less than or equal to 7.5. The preparation method comprises the following steps: sintering the ceramic at the sintering temperature of 800-1000 DEG C to obtain the ZnO voltage-sensitive ceramic with the voltage gradient Eb of 1529-2362V / mm, the nonlinear coefficient alpha of 63-86 and the leakage current density IL of 0.2-0.7 [mu] A / cm < 2 >, the performance of the ZnO voltage-sensitive ceramic is superior to that of commercial ZnO voltage-sensitive ceramic at the present stage, and the requirements of actual production links on the performance of the voltage-sensitive ceramic can be met.

Description

Technical Field

[0001] The invention belongs to the technical field of ceramic production, and particularly relates to a high-performance ZnO varistor ceramic and a simple low-temperature sintering method. Background Art

[0002] ZnO-based varistors are an important type of semiconductor ceramics, with ZnO powder as the main component and optimized by doping with metal oxides. ZnO varistors are widely used in multiple technical fields due to their excellent electrical properties, including communications, power systems, traffic control, industrial automation, automotive electronics, and household appliances. They have high nonlinear coefficients, low leakage currents, fast response times, and excellent resistance to large current shocks. With the rapid development of high-voltage AC and DC transmission lines and the trend of electronic equipment towards integration and miniaturization, the market demand for ZnO varistors with high breakdown voltage is increasing. The voltage gradient of this type of varistor is one of its key performance indicators, which directly affects its reliability in high-voltage applications. However, the ZnO varistors currently available on the market generally have a low voltage gradient (<400V / mm), which makes it difficult for them to meet the needs of miniaturization and high breakdown voltage.

[0003] The high-temperature solid-phase sintering method currently used in the production of commercial ZnO varistor ceramics is usually carried out at a high temperature of >1000°C. This process has exposed some significant defects in modern applications. First, during the high-temperature sintering process, Bi2O3 (melting point is about 825°C) will undergo severe volatilization, which will cause the nonlinear IV characteristics of ZnO varistor ceramics to drop significantly and reduce their electrical performance. Secondly, high-temperature sintering will also lead to excessive growth of ZnO grains, which not only affects the voltage gradient of the varistor ceramic, but also reduces its nonlinear coefficient, making it difficult for traditional methods to meet the needs of miniaturization.

[0004] In order to solve these problems, researchers began to explore more advanced sintering processes to reduce the sintering temperature of ZnO varistor ceramics, thereby controlling the growth of grains and improving the performance of the material. Some emerging sintering technologies, such as selective laser sintering (SLS), flash sintering (FS) and spark plasma sintering (SPS) have been proposed as improvement solutions. These advanced sintering processes can effectively improve the densification of materials at lower temperatures, thereby controlling the growth of grains and ultimately improving the electrical properties and voltage gradient of ZnO varistor ceramics. For example, selective laser sintering uses laser energy to locally heat the material, allowing the material to densify rapidly at a lower temperature. Flash sintering promotes the densification of the material through high-energy pulse heating in a short period of time, while spark plasma sintering uses the high temperature generated by arc discharge to accelerate the sintering process of the material. Although these advanced sintering technologies have significant advantages, they also face some challenges. First, the equipment required for these technologies is usually expensive and complex, resulting in a significant increase in production costs. Second, operating these advanced equipment requires a high level of technology and experience, which increases the difficulty of production. Therefore, although these sintering technologies have shown good potential in laboratory research, due to their high cost and complex operation, they are currently still mainly in the laboratory research stage and have not yet been widely used in actual production.

[0005] In conclusion, as a semiconductor ceramic device with important application prospects, the improvement of ZnO varistor ceramic production process is crucial to meet the needs of modern electronic devices for high breakdown voltage and miniaturization. Future research will continue to explore more efficient and economical sintering processes to promote the upgrading of commercial ZnO varistor ceramics and realize their wide application in various technical fields. Summary of the invention

[0006] In view of the above problems existing in the prior art, the purpose of the present invention is to develop a high-gradient ZnO varistor ceramic and a simple low-temperature sintering method.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A ZnO varistor ceramic, the composition expression of the ZnO varistor ceramic is:

[0009] [100-(a+b+c+d+e)]mol%ZnO+amol%Bi2O3+bmol%Co3O4+cmol%Mn2O3+dmol%Cr2O3+emol%SnO2, wherein 2.1≤a+b+c+d+e≤7.5. The ceramic is sintered at a sintering temperature of 800-1000°C to obtain a voltage gradient E of the ZnO varistor ceramic. b is 1529~2362V / mm, the nonlinear coefficient α is 63~86, and the leakage current density IL 0.2~0.7μA / cm 2 The performance is better than the currently commercially available ZnO varistor ceramics and can meet the requirements of the actual production process for the performance of varistor ceramics.

[0010] A simple low-temperature sintering method for ZnO varistor ceramics comprises the following steps:

[0011] S1: Mixing: Take the original powder according to the above ratio, put it into a ball mill, use anhydrous ethanol zirconium balls as the medium for ball milling and fully mixing, then take out the mixed powder, dry it, and sieve it with a 60-mesh screen for use. Specifically, weigh the original powder according to the ratio of ingredients and put it into a ball mill, use anhydrous ethanol or deionized water and other polar solutions as the medium, and mix it evenly by ball milling. After mixing, take out the mixed powder and dry it in an oven, and finally sieve it with a 60-mesh screen to obtain a mixed ceramic powder with uniform mixing and suitable particle size.

[0012] S2: Powder solid phase reaction: the mixed powder after drying in S1 is loaded into a high temperature crucible and placed in a muffle furnace, and reacted at a temperature of 800-1000°C for 1-3h to obtain a fully reacted ZnO-based varistor ceramic powder;

[0013] S3: Secondary mixing: Grind the ZnO-based varistor ceramic powder in S2 into fine powder, then put it into the ball mill again, and ball mill it with anhydrous ethanol as the medium for 8-24 hours. After the ball milling is completed, dry it in an oven and pass it through a 100-mesh sieve to obtain ZnO-based varistor ceramic powder with uniform particle size;

[0014] S4: Molding: Weigh an appropriate amount of the ZnO-based varistor ceramic powder in S3, use 15-30wt% of the mass of the ZnO-based varistor ceramic powder as a wetting agent to uniformly wet the ZnO-based varistor ceramic powder, and after fully grinding it evenly, transfer the ZnO-based varistor ceramic powder into a mold and apply a pressure of 200-500MPa. At the same time, heat it to 200-400℃ and keep it warm for 30min-2h to obtain a preliminary densified ZnO varistor ceramic green embryo.

[0015] S5: Sintering: Place the ZnO varistor ceramic green body into a muffle furnace and sinter it at a sintering temperature of 800-1000℃ for 2-5h to obtain a ZnO varistor ceramic product. The sintering temperature only needs to be 800-1000℃ to obtain a varistor ceramic with a potential gradient, nonlinear coefficient and leakage current density far superior to commercial methods.

[0016] Furthermore, the mass ratio of the powder, zirconium balls and ball milling liquid medium in S1 is 1:10:1.5, wherein the mass ratio of the zirconium balls with a diameter of 1 cm, a diameter of 0.5 cm and a diameter of 0.2 cm is 2:1:1, and the speed of the high-energy ball mill is 380 r / min.

[0017] Furthermore, the particle size of the original ZnO powder in the S1 is ~500nm, and the particle sizes of the original Bi2O3, Co3O4, Mn2O3, Cr2O3 and SnO2 powders are ~100nm, ~200nm, 100~300nm, ~200nm and <250nm respectively.

[0018] Furthermore, the density of the ZnO varistor ceramic green body in S4 is higher than 90%.

[0019] The heating rates in S4 and S5 are 5-20°C / min and 0.5-2°C / min respectively.

[0020] The present invention involves three key temperatures, the first is the S2 powder solid phase reaction temperature, the second is the S4ZnO piezoceramic green body forming temperature, and the third is the S5 ceramic sintering temperature. The functions of these three temperatures are:

[0021] 1. Powder solid phase reaction temperature: The temperature at which oxides such as ZnO, Bi2O3, Co3O4, Mn2O3, Cr2O3 and SnO2 react with each other mainly depends on the type of chemical reaction between them and the required activation energy. The reaction temperature range of different oxides is different. Therefore, when discussing their reaction temperatures, the following aspects can be considered:

[0022] 1) Formation temperature of Bi-rich phase: Reaction of ZnO and Bi2O3: ① The reaction temperature of ZnO and Bi2O3 is relatively low, usually between 800-900℃. They may form composite oxides such as Bi2ZnO4 (bismuth zinc oxide) at higher temperatures. ② The reaction of Cr2O3 with other oxides usually occurs above 900℃, especially when Cr2O3 interacts with oxides such as ZnO and Bi2O3, which often requires higher temperatures. This is because chromium oxide has strong chemical activity at higher temperatures and can form different chromium-based oxides or solid solutions with other metal oxides.

[0023] 2) The formation temperature of the spinel phase: The reaction temperature between Co3O4 and Mn2O3 is relatively high, usually at 900°C or above. Cobalt oxide and manganese oxide can undergo oxygen exchange reaction at high temperature to form complexes such as cobalt manganese oxide (such as CoMn2O4), which requires a higher temperature to drive the reaction.

[0024] 3) Reaction of SnO2 with other oxides: The reaction temperature with other metal oxides is usually high, usually between 800-1000°C. Tin oxide easily forms a solid solution or complex of metallic tin and other oxides with other oxides at higher temperatures. For example, SnO2 reacts with ZnO or Bi2O3 to form oxides with a perovskite structure.

[0025] In summary, among these oxides, higher temperatures are usually required to promote the reaction between them. Generally speaking, the reaction temperature range is about 800-1000°C, and the specific temperature depends on the specific ratio of the reaction and the types of oxides involved.

[0026] 2. ZnO varistor ceramic green body molding temperature: Traditionally, it is often necessary to add the adhesive polyvinyl alcohol (PVA) when molding ceramics, which will cause the PVA to be completely removed during the sintering process, otherwise it may affect the density and performance of the final ceramic; in addition, the addition of PVA may cause cracks or pores in the molding process, especially during high-temperature sintering. In addition, if PVA is used improperly as a binder, it may lead to insufficient uniformity of the molded body. The present invention overcomes the above-mentioned shortcomings, that is, by introducing acetic acid solution as a transient liquid phase and pressing and molding at 300°C, the steps of adding adhesives and granulation are omitted, which is simpler and more efficient, and can also omit the pre-burning and debinding steps in the subsequent sintering process.

[0027] 3. Ceramic sintering temperature: The sintering temperature of ZnO varistor ceramics is usually higher than 1100°C, but the present invention can significantly reduce the sintering temperature to below 1000°C by sintering the high-density (>90%) original embryo body, which effectively controls the abnormal growth of grains and optimizes electrical performance.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] 1. The stoichiometric ratio of the preparation process of the present invention is accurately controlled, the process is relatively simple, the preparation equipment cost is simple, and the production efficiency can be greatly improved;

[0030] 2. The molding method of the present invention eliminates the steps of adding binder and granulation in the traditional sintering method, consumes less raw materials, and reduces the cost in the production process;

[0031] 3. The relative density of the embryo during molding of the present invention is higher than 90%, which is much higher than the embryo prepared by the traditional molding method (relative density is usually 60%-80%). The extremely high initial density can make the final sintering temperature lower, which can greatly reduce the sintering energy consumption, which is beneficial to energy saving and emission reduction and reduce industrial production costs;

[0032] 4. The ZnO varistor ceramic prepared by the present invention has excellent electrical properties and good repeatability, and can meet various requirements for the preparation of commercial surge protection devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a microscopic morphology diagram of a specific embodiment of the low-temperature sintered ZnO varistor ceramic of the present invention.

[0034] Figure 2This is a nonlinear JE characteristic diagram of a specific embodiment of the low-temperature sintered ZnO varistor ceramic of the present invention. DETAILED DESCRIPTION

[0035] The present invention is further described in detail below by specific examples. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of protection of the present invention.

[0036] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can understand other advantages and effects of the present invention from the contents of the present invention. The present invention can also be applied through different embodiments, and the details in this specification can also be modified or adjusted according to different viewpoints and applications without changing the basic spirit of the present invention.

[0037] Please note that the process equipment or devices not specifically described in the following embodiments are all conventional equipment or devices in the art. All pressure values ​​and ranges refer to relative pressures, and the raw materials used are also materials conventionally used in the art.

[0038] Embodiment 1: A simple low-temperature sintering method for high-gradient ZnO varistor ceramics, comprising the following steps:

[0039] (1) Mixing: According to the composition expression [100-(a+b+c+d+e)] mol% ZnO+amol% Bi2O3+bmol% Co3O4+cmol% Mn2O3+dmol% Cr2O3+emol% SnO2, wherein 2.1≤a+b+c+d+e≤7.5, a total of 30 g of original powder was weighed using a precision electronic analytical balance and put into a ball mill, wherein ZnO accounted for 97.9 mol%, Bi2O3, Co3O4, Mn2O3, Cr2O3 and SnO2 accounted for 0.5 mol%, 0.5 mol%, 0.5 mol%, 0.1 mol% and 0.5 mol%, respectively. Then, 300 g of zirconium balls were added and 45 g of anhydrous ethanol was used as the ball milling medium. The mixture was evenly milled in a high-energy ball mill at a speed of 380 r / min. After the ball milling was completed, the mixed powder was taken out and dried in an oven at 80°C. Finally, it was sieved with a 60-mesh sieve to obtain a mixed ceramic powder with a uniform mixture and a suitable particle size.

[0040] (2) Powder solid phase reaction: the mixed ceramic powder dried in (1) is placed in a high temperature crucible and then placed in a muffle furnace, and reacted at a temperature of 800° C. for 1 hour to obtain a fully reacted ZnO-based varistor ceramic powder;

[0041] (3) Secondary mixing: Grind the ZnO-based varistor ceramic powder in (2) into fine powder in a mortar, and then put it into a ball mill again, and ball mill it for 8 hours using anhydrous ethanol as a medium. After the ball milling is completed, dry it in an oven and pass it through a 100-mesh sieve to obtain a ZnO-based varistor ceramic powder with uniform particle size;

[0042] (4) Molding: Weigh an appropriate amount of the powder in (3), use 15 wt% acetic acid solution as a wetting agent to uniformly wet the powder, grind the powder evenly, transfer the powder into a ceramic mold, apply a uniaxial pressure of 200 MPa through a manual tablet press, and apply a temperature of 400° C. to the mold through a heating sleeve, with a heating rate of 20° C. / min. After keeping the temperature for 30 min, a cylindrical ceramic green body with a diameter of 12.7 mm and a height of 2 mm is obtained;

[0043] (5) Sintering: Place the ceramic green body into a muffle furnace, heat to 1000°C at a heating rate of 2°C / min, and sinter for 2h to obtain a ZnO varistor ceramic product.

[0044] (6) Sample testing: Through relevant tests on the ZnO varistor ceramic products in (5), the final product can reach a relative density of 98.74% and has excellent electrical properties, voltage gradient E b is 1858.93 V / mm, the nonlinear coefficient α is 56.45, and the leakage current density I L 0.63μA / cm 2 .

[0045] Embodiment 2: A simple low-temperature sintering method for high-gradient ZnO varistor ceramics, comprising the following steps:

[0046] (1) Mixing: According to the composition expression [100-(a+b+c+d+e)] mol% ZnO+amol% Bi2O3+bmol% Co3O4+cmol% Mn2O3+dmol% Cr2O3+emol% SnO2, wherein 2.1≤a+b+c+d+e≤7.5, a total of 30 g of original powder was weighed using a precision electronic analytical balance and put into a ball mill, wherein ZnO accounted for 96.7 mol%, Bi2O3, Co3O4, Mn2O3, Cr2O3 and SnO2 accounted for 1.0 mol%, 0.5 mol%, 0.5 mol%, 0.3 mol% and 1.0 mol%, respectively. Then, 300 g of zirconium balls were added and 45 g of anhydrous ethanol was used as the ball milling medium. The mixture was evenly milled in a high-energy ball mill at a speed of 380 r / min. After the ball milling was completed, the mixed powder was taken out and dried in an oven at 80°C. Finally, it was sieved with a 60-mesh sieve to obtain a mixed ceramic powder with a uniform mixture and a suitable particle size.

[0047] (2) Powder solid phase reaction: the mixed ceramic powder dried in (1) is placed in a high temperature crucible and then placed in a muffle furnace, and reacted at a temperature of 850° C. for 2 h to obtain a fully reacted ZnO-based varistor ceramic powder;

[0048] (3) Secondary mixing: Grind the ZnO-based varistor ceramic powder in (2) into fine powder in a mortar, and then put it into a ball mill again, and ball mill it for 16 hours using anhydrous ethanol as a medium. After the ball milling is completed, dry it in an oven and pass it through a 100-mesh sieve to obtain a ZnO-based varistor ceramic powder with uniform particle size;

[0049] (4) Molding: Weigh an appropriate amount of the powder in (3), use 20 wt% acetic acid solution as a wetting agent to uniformly wet the powder, grind the powder evenly, transfer the powder into a ceramic mold, apply a uniaxial pressure of 300 MPa through a manual tablet press, and apply a temperature of 400° C. to the mold through a heating sleeve, with a heating rate of 15° C. / min. After keeping warm for 2 h, a cylindrical ceramic green body with a diameter of 12.7 mm and a height of 2 mm is obtained;

[0050] (5) Sintering: The ceramic green body is placed in a muffle furnace, heated to 850°C at a heating rate of 2°C / min, and sintered for 3 hours to obtain a ZnO varistor ceramic product.

[0051] (6) Sample testing: Through relevant tests on the ZnO varistor ceramic products in (5), the final product can reach a relative density of 98.65% and has excellent electrical properties, voltage gradient E b is 2232.48V / mm, the nonlinear coefficient α is 60.81, and the leakage current density I L 0.66μA / cm 2 .

[0052] Embodiment 3: A simple low-temperature sintering method for high-gradient ZnO varistor ceramics, comprising the following steps:

[0053] (1) Mixing: According to the composition expression [100-(a+b+c+d+e)] mol% ZnO+amol% Bi2O3+bmol% Co3O4+cmol% Mn2O3+dmol% Cr2O3+emol% SnO2, wherein 2.1≤a+b+c+d+e≤7.5, a total of 30 g of original powder was weighed using a precision electronic analytical balance and put into a ball mill, wherein ZnO accounted for 94.75 mol%, Bi2O3, Co3O4, Mn2O3, Cr2O3 and SnO2 accounted for 1.5 mol%, 1.0 mol%, 0.75 mol%, 0.5 mol% and 1.5 mol%, respectively. Then, 300 g of zirconium balls were added and 45 g of anhydrous ethanol was used as the ball milling medium. The mixture was evenly milled in a high-energy ball mill at a speed of 380 r / min. After the ball milling was completed, the mixed powder was taken out and dried in an oven at 80°C. Finally, it was sieved with a 60-mesh sieve to obtain a mixed ceramic powder with a uniform mixture and a suitable particle size.

[0054] (2) Powder solid phase reaction: the mixed ceramic powder dried in (1) is placed in a high temperature crucible and then placed in a muffle furnace, and reacted at a temperature of 900° C. for 2 h to obtain a fully reacted ZnO-based varistor ceramic powder;

[0055] (3) Secondary mixing: Grind the ZnO-based varistor ceramic powder in (2) into fine powder in a mortar, and then put it into a ball mill again, and ball mill it for 16 hours using anhydrous ethanol as a medium. After the ball milling is completed, dry it in an oven and pass it through a 100-mesh sieve to obtain a ZnO-based varistor ceramic powder with uniform particle size;

[0056] (4) Molding: Weigh an appropriate amount of the powder in (3), use 20 wt% acetic acid solution as a wetting agent to uniformly wet the powder, grind the powder evenly, transfer the powder into a ceramic mold, apply a uniaxial pressure of 300 MPa through a manual tablet press, and apply a temperature of 300° C. to the mold through a heating sleeve, with a heating rate of 10° C. / min. After keeping warm for 1 h, a cylindrical ceramic green body with a diameter of 12.7 mm and a height of 2 mm is obtained;

[0057] (5) Sintering: Place the ceramic green body into a muffle furnace, heat to 900°C at a heating rate of 1°C / min, and sinter for 3 hours to obtain a ZnO varistor ceramic product.

[0058] (6) Sample testing: Through relevant tests on the ZnO varistor ceramic products in (5), the final product can reach a relative density of 97.95% and has excellent electrical properties, voltage gradient E b is 1943.53 V / mm, the nonlinear coefficient α is 73.42, and the leakage current density I L 0.31μA / cm 2 .

[0059] Embodiment 4: A simple low-temperature sintering method for high-gradient ZnO varistor ceramics, comprising the following steps:

[0060] (1) Mixing: According to the composition expression [100-(a+b+c+d+e)] mol% ZnO+amol% Bi2O3+bmol% Co3O4+cmol% Mn2O3+dmol% Cr2O3+emol% SnO2, wherein 2.1≤a+b+c+d+e≤7.5, a total of 30 g of original powder was weighed using a precision electronic analytical balance and put into a ball mill, wherein ZnO accounted for 93.75 mol%, Bi2O3, Co3O4, Mn2O3, Cr2O3 and SnO2 accounted for 2.0 mol%, 1.5 mol%, 0.75 mol%, 0.5 mol% and 1.5 mol%, respectively. Then, 300 g of zirconium balls were added and 45 g of anhydrous ethanol was used as the ball milling medium. The mixture was evenly milled in a high-energy ball mill at a speed of 380 r / min. After the ball milling was completed, the mixed powder was taken out and dried in an oven at 80°C. Finally, it was sieved with a 60-mesh sieve to obtain a mixed ceramic powder with a uniform mixture and a suitable particle size.

[0061] (2) Powder solid phase reaction: the mixed ceramic powder dried in (1) is placed in a high temperature crucible and then placed in a muffle furnace, and reacted at a temperature of 950° C. for 3 h to obtain a fully reacted ZnO-based varistor ceramic powder;

[0062] (3) Secondary mixing: Grind the ZnO-based varistor ceramic powder in (2) into fine powder in a mortar, and then put it into a ball mill again, and ball mill it with anhydrous ethanol as a medium for 24 hours. After the ball milling is completed, dry it in an oven and pass it through a 100-mesh sieve to obtain a ZnO-based varistor ceramic powder with uniform particle size;

[0063] (4) Molding: Weigh an appropriate amount of the powder in (3), use 30 wt% acetic acid solution as a wetting agent to uniformly wet the powder, grind the powder evenly, transfer the powder into a ceramic mold, apply a uniaxial pressure of 400 MPa through a manual tablet press, and apply a temperature of 300° C. to the mold through a heating sleeve, with a heating rate of 10° C. / min. After keeping warm for 2 h, a cylindrical ceramic green body with a diameter of 12.7 mm and a height of 2 mm is obtained;

[0064] (5) Sintering: Place the ceramic green body into a muffle furnace, heat to 800°C at a heating rate of 1°C / min, and sinter for 5 hours to obtain a ZnO varistor ceramic product.

[0065] (6) Sample testing: Through relevant tests on the ZnO varistor ceramic products in (5), the final product can reach a relative density of 97.95% and has excellent electrical properties, voltage gradient Eb is 2035.82V / mm, the nonlinear coefficient α is 85.61, and the leakage current density I L 0.23μA / cm 2 .

[0066] Embodiment 5: A simple low-temperature sintering method for high-gradient ZnO varistor ceramics, comprising the following steps:

[0067] (1) Mixing: According to the composition expression [100-(a+b+c+d+e)] mol% ZnO+amol% Bi2O3+bmol% Co3O4+cmol% Mn2O3+dmol% Cr2O3+emol% SnO2, wherein 2.1≤a+b+c+d+e≤7.5, a total of 30 g of original powder was weighed using a precision electronic analytical balance and put into a ball mill, wherein ZnO accounted for 92.5 mol%, Bi2O3, Co3O4, Mn2O3, Cr2O3 and SnO2 accounted for 2.0 mol%, 1.5 mol%, 1.0 mol%, 1.0 mol% and 2.0 mol%, respectively. Then, 300 g of zirconium balls were added and 45 g of anhydrous ethanol was used as the ball milling medium. The mixture was evenly milled in a high-energy ball mill at a speed of 380 r / min. After the ball milling was completed, the mixed powder was taken out and dried in an oven at 80°C. Finally, it was sieved with a 60-mesh sieve to obtain a mixed ceramic powder with a uniform mixture and a suitable particle size.

[0068] (2) Powder solid phase reaction: the mixed ceramic powder dried in (1) is placed in a high temperature crucible and then placed in a muffle furnace, and reacted at a temperature of 1000° C. for 2 h to obtain a fully reacted ZnO-based varistor ceramic powder;

[0069] (3) Secondary mixing: Grind the ZnO-based varistor ceramic powder in (2) into fine powder in a mortar, and then put it into a ball mill again, and ball mill it with anhydrous ethanol as a medium for 24 hours. After the ball milling is completed, dry it in an oven and pass it through a 100-mesh sieve to obtain a ZnO-based varistor ceramic powder with uniform particle size;

[0070] (4) Molding: Weigh an appropriate amount of the powder in (3), use 30 wt% acetic acid solution as a wetting agent to uniformly wet the powder, grind the powder evenly, transfer the powder into a ceramic mold, apply a uniaxial pressure of 500 MPa through a manual tablet press, and apply a temperature of 200° C. to the mold through a heating sleeve, with a heating rate of 5° C. / min. After keeping the temperature for 30 min, a cylindrical ceramic green body with a diameter of 12.7 mm and a height of 2 mm is obtained;

[0071] (5) Sintering: Place the ceramic green body into a muffle furnace, heat to 900°C at a heating rate of 0.5°C / min, and sinter for 3 hours to obtain a ZnO varistor ceramic product.

[0072] (6) Sample testing: Through relevant tests on the ZnO varistor ceramic products in (5), the final product can reach a relative density of 97.95% and has excellent electrical properties, voltage gradient E b is 1600.53V / mm, the nonlinear coefficient α is 74.72, and the leakage current density I L 0.45μA / cm 2 .

[0073] Table 1 Performance parameters of ZnO varistor ceramics under different conditions and formulations

[0074]

[0075]

[0076] See also Figure 1 , Figure 2 And Table 1, through the above experimental results in the present invention, we can find the following experimental rules:

[0077] 1) The voltage gradient of ZnO varistor ceramics shows a decreasing trend with the increase of sintering temperature. As the sintering temperature increases, the grain growth rate accelerates, resulting in an increase in grain size. According to the conductive mechanism of varistor ceramics, a larger grain size will reduce the number of grain boundaries, thereby reducing the grain boundary barrier, making it easier for electrons to cross the grain boundaries, which in turn leads to a decrease in the voltage gradient.

[0078] 2) The nonlinear coefficient of ZnO varistor ceramics tends to decrease with the increase of sintering temperature. As the sintering temperature increases, the growth of ZnO grains and the volatilization of some dopants (the melting point of bismuth oxide is 825°C) lead to changes in the grain boundary structure. Grain boundaries are the key areas that determine the nonlinear characteristics of varistor ceramics. At high temperatures, the defect concentration and grain boundary phase composition at the grain boundaries change, which reduces the grain boundary barrier and thus reduces the nonlinear coefficient.

[0079] 3) By comparing the performance data of different formulations at the same or similar sintering temperatures, the contribution of each component to the performance of the varistor ceramic can be evaluated. For example, comparing Example 3 and Example 5 (both sintered at 900°C), as the content of components such as Bi2O3, Cr2O3 and SnO2 increases, the voltage gradient increases, the nonlinear coefficient changes slightly, and the leakage current density increases. This shows that the change in the content of these components has an important influence on the performance, and the formulation composition can be further optimized according to actual needs to obtain the best performance.

Claims

1. A ZnO varistor ceramic, characterized in that: The composition expression of the ZnO varistor ceramic is: [100-(a+b+c+d+e)] mol% ZnO+amol% Bi2O3+bmol% Co3O4+cmol% Mn2O3+dmol% Cr2O3+emol% SnO2, wherein 2.1≤a+b+c+d+e≤7.

5.

2. A simple low-temperature sintering method for high-gradient ZnO varistor ceramics, characterized in that: The steps include: S1: Mixing: Take the original powder according to the ratio in claim 1, put it into a ball mill, use anhydrous ethanol zirconium ball as the medium, and mix it thoroughly. Then take out the mixed powder, dry it, sieve it with a 60-mesh screen, and set it aside; S2: Powder solid phase reaction: the mixed powder after drying in S1 is loaded into a high temperature crucible and placed in a muffle furnace, and reacted at a temperature of 800-1000°C for 1-3h to obtain a fully reacted ZnO-based varistor ceramic powder; S3: Secondary mixing: Grind the ZnO-based varistor ceramic powder in S2 into fine powder, then put it into the ball mill again, and ball mill it with anhydrous ethanol as the medium for 8-24 hours. After the ball milling is completed, dry it in an oven and pass it through a 100-mesh sieve to obtain ZnO-based varistor ceramic powder with uniform particle size; S4: Molding: Weigh an appropriate amount of the ZnO-based varistor ceramic powder in S3, use an acetic acid solution with a weight percentage of 15-30wt% of the ZnO-based varistor ceramic powder as a wetting agent to uniformly wet the ZnO-based varistor ceramic powder, fully grind it evenly, transfer the ZnO-based varistor ceramic powder into a mold, apply a pressure of 200-500MPa, and heat it to 200-400℃ for 30min-2h to obtain a preliminarily densified ZnO varistor ceramic green body; S5: Sintering: Place the ZnO varistor ceramic green body into a muffle furnace and sinter it at a sintering temperature of 800-1000° C. for 2-5 hours to obtain a ZnO varistor ceramic product.

3. A simple low-temperature sintering method for a high-gradient ZnO varistor ceramic as claimed in claim 2, characterized in that: The mass ratio of the powder, zirconium balls and ball milling liquid medium in S1 is 1:10:1.5, wherein the mass ratio of the zirconium balls is 2:1:1 for diameters of 1 cm, 0.5 cm and 0.2 cm, and the rotation speed of the high-energy ball mill is 380 r / min.

4. A simple low-temperature sintering method for a high-gradient ZnO varistor ceramic as claimed in claim 2, characterized in that: The particle size of the original ZnO powder in the S1 is 500 nm, and the particle sizes of the original Bi2O3, Co3O4, Mn2O3, Cr2O3 and SnO2 powders are 100 nm, 200 nm, 100-300 nm, 200 nm and <250 nm, respectively.

5. A simple low-temperature sintering method for a high-gradient ZnO varistor ceramic as claimed in claim 2, characterized in that: The density of the ZnO varistor ceramic green body in the S4 is higher than 90%.

6. A simple low-temperature sintering method for a high-gradient ZnO varistor ceramic as claimed in claim 2, characterized in that: The heating rates in S4 and S5 are 5-20°C / min and 0.5-2°C / min respectively.

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