Low-gradient zinc oxide varistor and preparation method thereof

Abstract

The invention relates to the field of electronic materials, and particularly discloses a low-gradient zinc oxide varistor and a preparation method thereof. The invention relates to a low-gradient zinc oxide piezoresistor disc, which is prepared from the following raw materials in parts by weight: 88 to 96 parts of zinc oxide, 1.5 to 3 parts of antimony trioxide, 2 to 5 parts of bismuth trioxide, 0.2 to 1.8 parts of nickel oxide, 0.1 to 0.9 part of cobalt sesquioxide, 0.1 to 0.4 part of manganese oxide, 0.02 to 0.07 part of chromium sesquioxide, 0.15 to 0.18 part of lithium acetate, 1.2 to 1.8 parts of dispersing agents, 1.8 to 2.5 parts of binding agents, 0.12 to 0.15 part of aluminum nitrate and 0.001 to 0.003 part of silver nitrate. Under the synergistic effect of all the components, by adjusting the use ratio of all the components, the potential gradient of the resistor disc can be reduced, the large-current endurance capacity of the resistor disc is improved, and the change of electrical performance after impact is reduced.

Description

Technical Field

[0001] The present application relates to the field of electronic materials, and in particular to a low-gradient zinc oxide varistor sheet and a preparation method thereof. Background Art

[0002] Resistors for lightning arrester monitors are a key component used in these devices. They are categorized by gradient: high-gradient, medium-gradient, and low-gradient. Low-gradient resistors generate a relatively low voltage per unit thickness, effectively limiting overvoltage at relatively low voltages. They also exhibit excellent nonlinear characteristics and high stability, making them widely used in power systems, industrial sectors, and rail transit.

[0003] However, in the existing technology, a large amount of bismuth trioxide is added to make the potential gradient of the low-gradient resistor greater than 120. The grain size is too small, resulting in an increased risk of local breakdown under large current shocks and a high rate of change of electrical parameters after multiple square wave tolerances. Summary of the Invention

[0004] In order to reduce the potential gradient of existing resistor sheets, the present application provides a low-gradient zinc oxide varistor sheet and a preparation method thereof.

[0005] In a first aspect, the present application provides a low-gradient zinc oxide varistor sheet, which adopts the following technical solution: A low-gradient zinc oxide varistor sheet comprises the following raw materials in parts by weight: 88-96 parts of zinc oxide, 1.5-3 parts of antimony trioxide, 2-5 parts of bismuth trioxide, 0.2-1.8 parts of nickel oxide, 0.1-0.9 parts of cobalt trioxide, 0.1-0.4 parts of manganese oxide, 0.02-0.07 parts of chromium trioxide, 0.15-0.18 parts of lithium acetate, 1.2-1.8 parts of a dispersant, 1.8-2.5 parts of a binder, 0.12-0.15 parts of aluminum nitrate, and 0.001-0.003 parts of silver nitrate. In the low-gradient zinc oxide varistor sheet raw materials, bismuth trioxide and antimony trioxide are pre-coated with stearic acid.

[0006] By adopting the above technical solution, this application uses zinc oxide as the main crystalline phase raw material, combined with other components, to improve the nonlinear coefficient of the varistor and reduce the potential gradient. Bismuth trioxide can promote the growth of zinc oxide grains. During sintering, bismuth trioxide melts to form a liquid phase, which encapsulates the zinc oxide grains, optimizes the grain boundary barrier, and thus reduces the potential gradient.

[0007] Cobalt trioxide, manganese oxide, and nickel oxide work synergistically to adjust the resistor's conductive properties, enhance its stability, improve its ability to withstand high currents, and reduce changes in electrical performance after impact. Antimony trioxide inhibits excessive zinc oxide grain growth, dissolving within the zinc oxide lattice to prevent grain boundary migration, stabilize the grain structure, and improve the nonlinear coefficient of the varistor, thereby reducing the potential gradient. This improves the aging resistance of the resistor and avoids localized hot spots. Reducing the bismuth trioxide content while increasing the cobalt trioxide, manganese oxide, and nickel oxide content in the raw material composition can reduce the grain boundary barrier and, in turn, the potential gradient of the resistor.

[0008] Bismuth trioxide and antimony trioxide are high-surface-energy powders. Stearic acid coating of the bismuth and antimony trioxide surfaces reduces their surface energy. The long-chain stearic acid molecules form a physical barrier between the powders, reducing contact and agglomeration and improving the uniformity of resistor sintering. The coated bismuth and antimony trioxide particles are more evenly dispersed, spreading uniformly across the zinc oxide grain boundaries during sintering. This strengthens the interfacial bonding between the bismuth and antimony trioxide particles and zinc oxide, alleviates sintering stress, and reduces grain boundary cracking.

[0009] Preferably, the bismuth trioxide and antimony trioxide in the raw material of the low-gradient zinc oxide varistor sheet are pre-coated with stearic acid, comprising the following specific steps: drying the bismuth trioxide and antimony trioxide, grinding and sieving to obtain dispersed powders, then mixing stearic acid with a solvent to obtain a stearic acid solution, adding the dispersed powder to the stearic acid solution, ultrasonically dispersing, heating and stirring, drying and sieving to obtain bismuth trioxide-antimony trioxide coated with stearic acid.

[0010] Preferably, the mass ratio of the total mass of the bismuth trioxide and antimony trioxide to the stearic acid and the solvent is 10:(0.5-0.8):(1-3).

[0011] Preferably, the heating temperature is 80-90°C.

[0012] Preferably, the binder is ethyl cellulose.

[0013] By adopting this technical solution, ethyl cellulose can provide sufficient green strength after powder mixing, ensuring that the resistor chip does not break during compression molding and reducing the risk of cracking during the drying and sintering processes. Furthermore, in the subsequent preparation process, it can act as a binder to help the various components form a uniform spherical granulated powder.

[0014] Preferably, the dispersant is one of polyvinyl alcohol and ethanol.

[0015] By adopting the above technical solution, polyvinyl alcohol and ethanol are used as dispersants to reduce the phenomenon of powder agglomeration and promote a more uniform distribution of grain size after sintering.

[0016] In a second aspect, the present application provides a method for preparing a low-gradient zinc oxide varistor sheet, which adopts the following technical solution: A method for preparing a low-gradient zinc oxide varistor sheet comprises the following specific steps: The coated bismuth trioxide and antimony trioxide are mixed with zinc oxide, nickel oxide, cobalt trioxide, manganese oxide, chromium trioxide, lithium acetate, a binder, aluminum nitrate, and silver nitrate, and then water and a dispersant are added to wet-ball mill, and the granulated powder is obtained after drying. The granulated powder is dry pressed into shape, and then subjected to binder removal pre-sintering, main firing, side insulation treatment in sequence, and finally ground into a low-gradient zinc oxide varistor.

[0017] By adopting the above technical solution, debinding pre-sintering is performed before the main firing, which can prevent excessive grain growth. Through the combination of various process steps, the potential gradient of the prepared resistor is reduced, the high current withstand capability is greatly improved, and the change rate of the electrical parameters of the resistor is reduced.

[0018] Preferably, the pressure during the dry pressing is 80-150 MPa, and the holding time is 10-15 s.

[0019] By adopting the above technical solution, the pressure during the forming process is controlled within a lower range, which can reduce excessive density increase, reduce the occurrence of cracks, ensure uniform density of the green body, and improve the stability of the electrical performance of the resistor.

[0020] Preferably, the binder removal pre-firing temperature is 400-600°C, the heating rate is 1-3°C / min, and the holding time is 2-4h; the main firing temperature is 950-1100°C, the holding time is 2-4h, and the cooling rate is 5-10°C / min.

[0021] By adopting this technical solution, a binder removal pre-sintering process can initially form the grain boundary structure in preparation for the main firing, while also removing binders and dispersants, improving the resistor's nonlinear coefficient and electrical performance. Controlling the binder removal pre-sintering time within an appropriate range can prevent raw material oxidation or reduction. Main firing temperatures are controlled between 950°C and 1100°C, reducing grain growth rates. Controlling the cooling rate can also prevent thermal stress cracking and improve resistor stability.

[0022] Preferably, the side insulation treatment is spraying a glass glaze layer on the side.

[0023] In summary, this application has the following beneficial effects: 1. Since this application uses zinc oxide as the primary crystalline phase, cobalt trioxide, manganese oxide, nickel oxide, and antimony trioxide are added to the raw materials to stabilize the grain structure, improve the nonlinear coefficient of the varistor, and reduce the potential gradient. Simultaneously, in the raw material ratio, reducing the bismuth trioxide content while increasing the cobalt trioxide, manganese oxide, and nickel oxide content can reduce the grain boundary barrier and, in turn, the potential gradient of the resistor.

[0024] 2. In this application, bismuth trioxide and antimony trioxide are pre-coated with stearic acid, which can utilize the long-chain molecules of stearic acid to form a physical barrier between the powder particles, reduce the agglomeration of the powder particles, and promote the more uniform dispersion of the coated bismuth trioxide and antimony trioxide particles. During sintering, they can be evenly spread on the zinc oxide grain boundaries, relieve sintering stress, and reduce grain boundary cracks. DETAILED DESCRIPTION

[0025] The present application is further described in detail below with reference to the embodiments.

[0026] All raw materials in the examples are commercially available.

[0027] Example

[0028] Example 1

[0029] This embodiment provides a low-gradient zinc oxide varistor sheet, comprising the following raw materials in parts by weight: 92g zinc oxide, 2.2g antimony trioxide, 3.5g bismuth trioxide, 1g nickel oxide, 0.5g cobalt trioxide, 0.3g manganese oxide, 0.045g chromium trioxide, 0.17g lithium acetate, 1.5g dispersant, 2.1g binder, 0.14g aluminum nitrate, and 0.002g silver nitrate. The dispersant is polyvinyl alcohol (PVA-205), and the binder is ethyl cellulose.

[0030] The preparation method of the low-gradient zinc oxide varistor sheet comprises the following specific steps: S1: Bismuth trioxide and antimony trioxide are dried, ground and sieved to obtain dispersed powders, and then stearic acid is mixed with a solvent, wherein the solvent is ethanol, and the mass ratio of the total mass of bismuth trioxide and antimony trioxide to stearic acid and solvent is 10:0.8:3 to obtain a stearic acid solution. The dispersed powders are added to the stearic acid solution, ultrasonically dispersed for 3 hours, and then heated with stirring to 85°C. After drying, the powders are sieved to obtain stearic acid-coated bismuth trioxide-antimony trioxide.

[0031] S2: Mix zinc oxide, fatty acid-coated bismuth trioxide-antimony trioxide, nickel oxide, cobalt trioxide, manganese oxide, chromium trioxide, lithium acetate, a binder, aluminum nitrate, and silver nitrate, then add water and a dispersant and wet-ball mill for 40 hours, wherein the mass ratio of water to total powder is 1:1, and obtain granulated powder after drying.

[0032] S3: The granulated powder is dry-pressed at 80 MPa for 15 seconds, then heated to 600°C at a heating rate of 3°C / min, kept warm for 2 hours, and then continued to heat to 1100°C for main firing, with a holding time of 2 hours. Finally, the temperature is cooled to room temperature at a cooling rate of 10°C / min to obtain a semi-finished resistor chip. A glass glaze layer is then sprayed on the side of the semi-finished resistor chip at 530°C and kept warm for 10 hours. Finally, the chip is ground and dried to obtain a low-gradient zinc oxide varistor chip.

[0033] Example 2

[0034] The difference between Example 2 and Example 1 is that the amount of zinc oxide used in the low-gradient zinc oxide varistor raw material is 88g, the amount of antimony trioxide used is 3g, the amount of bismuth trioxide used is 2g, the amount of nickel oxide used is 1.8g, the amount of cobalt trioxide used is 0.1g, the amount of manganese oxide used is 0.4g, the amount of chromium trioxide used is 0.02g, the amount of lithium acetate used is 0.18g, the amount of dispersant used is 1.2g, the amount of binder used is 1.8g, the amount of aluminum nitrate used is 0.15g, and the amount of silver nitrate used is 0.001g.

[0035] Example 3

[0036] The difference between Example 3 and Example 1 is that the amount of zinc oxide used in the low-gradient zinc oxide varistor sheet raw material is 96g, the amount of antimony trioxide used is 1.5g, the amount of bismuth trioxide used is 5g, the amount of nickel oxide used is 0.2g, the amount of cobalt trioxide used is 0.9g, the amount of manganese oxide used is 0.1g, the amount of chromium trioxide used is 0.07g, the amount of lithium acetate used is 0.15g, the amount of dispersant used is 1.8g, the amount of binder used is 2.5g, the amount of aluminum nitrate used is 0.12g, and the amount of silver nitrate used is 0.003g.

[0037] Example 4

[0038] The difference between Example 4 and Example 1 is that the method for preparing the low-gradient zinc oxide varistor sheet includes the following specific steps: S1: Bismuth trioxide and antimony trioxide are dried, ground and sieved to obtain dispersed powders, and then stearic acid is mixed with a solvent, wherein the solvent is ethanol, and the mass ratio of the total mass of bismuth trioxide and antimony trioxide to stearic acid and solvent is 10:0.8:3 to obtain a stearic acid solution. The dispersed powders are added to the stearic acid solution, ultrasonically dispersed for 3 hours, and then heated with stirring to 85°C. After drying, the powders are sieved to obtain stearic acid-coated bismuth trioxide-antimony trioxide.

[0039] S2: Mix zinc oxide, fatty acid-coated bismuth trioxide-antimony trioxide, nickel oxide, cobalt trioxide, manganese oxide, chromium trioxide, lithium acetate, a binder, aluminum nitrate, and silver nitrate, then add water and a dispersant and wet-ball mill for 40 hours, wherein the mass ratio of water to total powder is 1:1, and obtain granulated powder after drying.

[0040] S3: The granulated powder is dry-pressed at 150 MPa for 10 seconds, then heated to 600°C at a heating rate of 3°C / min, kept warm for 2 hours, and then continued to heat to 1100°C for main firing, and kept warm for 2 hours. Finally, the temperature is cooled to room temperature at a cooling rate of 10°C / min to obtain a semi-finished resistor chip. A glass glaze layer is then sprayed on the side of the semi-finished resistor chip at 530°C and kept warm for 10 hours. Finally, the chip is ground and dried to obtain a low-gradient zinc oxide varistor chip.

[0041] Example 5

[0042] The difference between Example 5 and Example 1 is that the method for preparing the low-gradient zinc oxide varistor sheet includes the following specific steps: S1: Bismuth trioxide and antimony trioxide are dried, ground and sieved to obtain dispersed powders, and then stearic acid is mixed with a solvent, wherein the solvent is ethanol, and the mass ratio of the total mass of bismuth trioxide and antimony trioxide to stearic acid and solvent is 10:0.8:3 to obtain a stearic acid solution. The dispersed powders are added to the stearic acid solution, ultrasonically dispersed for 3 hours, and then heated with stirring to 85°C. After drying, the powders are sieved to obtain stearic acid-coated bismuth trioxide-antimony trioxide.

[0043] S2: Mix zinc oxide, fatty acid-coated bismuth trioxide-antimony trioxide, nickel oxide, cobalt trioxide, manganese oxide, chromium trioxide, lithium acetate, a binder, aluminum nitrate, and silver nitrate, then add water and a dispersant and wet-ball mill for 40 hours, wherein the mass ratio of water to total powder is 1:1, and obtain granulated powder after drying.

[0044] S3: The granulated powder is dry-pressed at 80 MPa for 15 seconds, then heated to 400°C at a heating rate of 1°C / min, kept warm for 4 hours, and then continued to heat to 950°C for main firing, with a holding time of 4 hours. Finally, the temperature is cooled to room temperature at a cooling rate of 5°C / min to obtain a semi-finished resistor chip. A glass glaze layer is then sprayed on the side of the semi-finished resistor chip at 530°C and kept warm for 10 hours. Finally, the chip is ground and dried to obtain a low-gradient zinc oxide varistor chip.

[0045] Example 6

[0046] The difference between Example 6 and Example 1 is that the mass ratio of the total mass of bismuth trioxide and antimony trioxide in the low-gradient zinc oxide varistor sheet raw material to stearic acid and solvent is 10:0.5:1.

[0047] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that cobalt trioxide is not used in the raw material of the low-gradient zinc oxide varistor sheet.

[0048] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that cobalt trioxide and manganese oxide are not used in the raw materials of the low-gradient zinc oxide varistor sheet.

[0049] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that cobalt trioxide, nickel oxide and manganese oxide are not used in the raw materials of the low-gradient zinc oxide varistor sheet.

[0050] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the amount of bismuth trioxide used in the low-gradient zinc oxide varistor sheet raw material is 10 g.

[0051] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that bismuth trioxide and antimony trioxide are not pre-coated with stearic acid in the raw materials of the low-gradient zinc oxide varistor sheet.

[0052] The preparation method of the low-gradient zinc oxide varistor sheet comprises the following specific steps: S1: Mix zinc oxide, fatty acid-coated bismuth trioxide-antimony trioxide, nickel oxide, cobalt trioxide, manganese oxide, chromium trioxide, lithium acetate, a binder, aluminum nitrate, and silver nitrate, then add water and a dispersant and wet-ball mill for 40 hours, wherein the mass ratio of water to total powder is 1:1, and then dry to obtain a granulated powder.

[0053] S2: The granulated powder is dry pressed at 80 MPa for 15 seconds, then heated to 600°C at a heating rate of 3°C / min, kept warm for 2 hours, and then continued to heat to 1100°C for main firing, and kept warm for 2 hours. Finally, the temperature is cooled to room temperature at a cooling rate of 10°C / min to obtain a semi-finished resistor chip. Then, a glass glaze layer is sprayed on the side of the semi-finished resistor chip at 530°C and kept warm for 10 hours. Finally, the chip is ground and dried to obtain a low-gradient zinc oxide varistor chip.

[0054] According to the low-gradient zinc oxide varistors provided in Examples 1-6 and Comparative Examples 1-5 of the present application, varistors with a diameter of 99 mm and a height of 12 mm were fired and subjected to the following performance tests. The specific test results are shown in Table 1.

[0055] 1. Electrical performance The voltage gradient and varistor voltage (V1mA) of the varistor sheet sample fired in the present application were detected at a current density of 1mA / cm², and the leakage current when a voltage of 75% of V1mA was applied was detected. The voltage of the varistor sheet sample fired in the present application at 1mA / cm² and 10mA / cm² was detected, and the nonlinear coefficient β was measured, β= lg(U10 / U1) / lg(10).

[0056] 2. Appearance performance testing Appearance: After electrodes were fabricated from the varistor samples prepared in this application, the surface was examined under a microscope for cracks and electrode detachment. Vernier calipers were used to measure the thickness and diameter of the sample at the center and around its perimeter to test for dimensional deviation. The flexural strength of the varistor samples prepared in this application was tested using the three-point bending method.

[0057] Table 1: Performance test results data table

[0058] Table 1: Performance test results data table

[0059] The performance test results show that the varistor prepared in this application can effectively reduce the potential gradient, greatly improve the large current tolerance capability, and reduce the rate of change of electrical parameters after multiple square wave tolerance.

[0060] Comparison of Comparative Examples 1-4 with Example 1 shows that Comparative Example 1 does not use cobalt trioxide, Comparative Example 2 does not use cobalt trioxide and manganese oxide, Comparative Example 3 does not use cobalt trioxide, nickel oxide, and manganese oxide, and Comparative Example 4 increases the amount of bismuth trioxide. Performance test results show that adjusting the ratio of bismuth trioxide, cobalt trioxide, manganese oxide, and nickel oxide in the varistor raw materials significantly reduces the potential gradient of the varistor. Comparative Example 1, which does not use cobalt trioxide, can lead to uneven current distribution, making local breakdown prone under surge impacts, causing the varistor to fail and reducing its current withstand capability. Comparative Example 2, which removes cobalt trioxide and manganese oxide, weakens their grain-refining effect, leading to an increase in the breakdown voltage per unit thickness, failing to meet the low-gradient requirement. Comparative Example 3, which removes cobalt trioxide, nickel oxide, and manganese oxide, significantly increases leakage current, causing the varistor to heat severely at normal operating voltages. Its performance rapidly degrades under long-term operation or in high-temperature environments, significantly shortening its lifespan.

[0061] By comparing Examples 1-4 with Example 1, it can be seen that eliminating the use of these three elements one by one significantly reduces the stability and nonlinear coefficient of the resistor, and the current tolerance capability is also reduced. However, excessively increasing the bismuth trioxide content can cause a large change in the electrical performance of the resistor after impact, reducing the stability of the resistor.

[0062] By comparing Comparative Example 5 with Example 1, it can be seen that pre-coating bismuth trioxide and antimony trioxide with stearic acid can effectively improve the stability of the resistor, improve the dispersibility of bismuth trioxide and antimony trioxide particles in the resistor system, relieve sintering stress, reduce grain boundary cracks, and improve the dimensional stability of the resistor.

[0063] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed, but as long as they are within the scope of the claims of the present application, they are protected by the patent law.

Claims

1. A low-gradient zinc oxide varistor, characterized in that: The low-gradient zinc oxide varistor comprises the following raw materials in parts by weight: 88-96 parts of zinc oxide, 1.5-3 parts of antimony trioxide, 2-5 parts of bismuth trioxide, 0.2-1.8 parts of nickel oxide, 0.1-0.9 parts of cobalt trioxide, 0.1-0.4 parts of manganese oxide, 0.02-0.07 parts of chromium trioxide, 0.15-0.18 parts of lithium acetate, 1.2-1.8 parts of dispersant, 1.8-2.5 parts of binder, 0.12-0.15 parts of aluminum nitrate, and 0.001-0.003 parts of silver nitrate. In the low-gradient zinc oxide varistor raw material, bismuth trioxide and antimony trioxide are pre-coated with stearic acid.

2. The low-gradient zinc oxide varistor according to claim 1, characterized in that: The method for pre-coating bismuth trioxide and antimony trioxide in the raw materials of a low-gradient zinc oxide varistor sheet comprises the following specific steps: drying the bismuth trioxide and antimony trioxide, grinding and sieving to obtain dispersed powders, then mixing stearic acid with a solvent to obtain a stearic acid solution, adding the dispersed powders to the stearic acid solution, ultrasonically dispersing, heating and stirring, drying, and sieving to obtain bismuth trioxide-antimony trioxide coated with stearic acid.

3. The low-gradient zinc oxide varistor according to claim 2, characterized in that: The mass ratio of the total mass of the bismuth trioxide and antimony trioxide to the stearic acid and the solvent is 10:(0.5-0.8):(1-3).

4. The low-gradient zinc oxide varistor according to claim 2, characterized in that: The heating temperature is 80-90°C.

5. The low-gradient zinc oxide varistor according to claim 1, characterized in that: The binder is ethyl cellulose.

6. The low-gradient zinc oxide varistor according to claim 1, characterized in that: The dispersant is one of polyvinyl alcohol and ethanol.

7. A method for preparing a low-gradient zinc oxide varistor sheet according to any one of claims 1 to 6, characterized in that: The specific steps include: The coated bismuth trioxide and antimony trioxide are mixed with zinc oxide, nickel oxide, cobalt trioxide, manganese oxide, chromium trioxide, lithium acetate, a binder, aluminum nitrate, and silver nitrate, and then water and a dispersant are added to wet-ball mill, and the granulated powder is obtained after drying. The granulated powder is dry pressed into shape, and then subjected to binder removal pre-sintering, main firing, side insulation treatment in sequence, and finally ground into a low-gradient zinc oxide varistor.

8. The method for preparing a low-gradient zinc oxide varistor sheet according to claim 7, characterized in that: The pressure during the dry pressing process is 80-150 MPa, and the holding time is 10-15 seconds.

9. The method for preparing a low-gradient zinc oxide varistor sheet according to claim 7, characterized in that: The binder removal pre-firing temperature is 400-600°C, the heating rate is 1-3°C / min, and the holding time is 2-4h; the main firing temperature is 950-1100°C, the holding time is 2-4h, and the cooling rate is 5-10°C / min.

10. The method for preparing a low-gradient zinc oxide varistor according to claim 7, characterized in that: The side insulation treatment is to spray a glass glaze layer on the side.

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