Compression molding process of zinc oxide varistor

Abstract

The invention relates to the technical field of piezoresistor manufacturing, and discloses a compression molding process of a zinc oxide piezoresistor, which is characterized by comprising the following steps: carrying out spray granulation on a zinc oxide piezoresistor raw material according to an existing production process of the zinc oxide piezoresistor, and carrying out water-containing treatment on powder after spray granulation to obtain the zinc oxide piezoresistor. The preparation method comprises the following steps: uniformly wetting the surface of powder by using an acid or an acid solution, slightly dissolving the surfaces of particles to generate a liquid phase in an interface, carrying out compression molding on the powder subjected to water-containing treatment according to an existing zinc oxide varistor production process, and carrying out sintering, heat treatment, aluminum spraying, glazing, detection and other processes to prepare the zinc oxide-based varistor disc. According to the compression molding process of the zinc oxide varistor, acid is used for controllably dissolving the surface of the zinc oxide varistor raw material, so that the surface energy and the chemical polarity of the zinc oxide varistor raw material are improved, and the compression molding process has the effects of avoiding cracks during calcination, improving the grain boundary performance and avoiding the generation of volatile organic compounds (VOC).

Description

Technical Field

[0001] The present invention relates to the technical field of varistor manufacturing, and specifically to a pressing and forming process of a zinc oxide varistor. In modern power systems and various electronic devices, varistors are important protective components, and the quality of their performance is directly related to the safety and stability of the entire system or equipment. Zinc oxide varistors have been widely used in many fields due to their unique performance advantages, and their pressing and forming process is one of the key links that determine their final performance. The technical field involved in the present invention revolves around this key link and is committed to innovating and optimizing the pressing and forming process of zinc oxide varistors. Background Art

[0002] Zinc oxide varistors are widely used in high-voltage lightning arresters and surge protection devices because of their nonlinear volt-ampere characteristics and excellent current-carrying capacity. Zinc oxide varistors are a multiphase polycrystalline ceramic material made of the main component ZnO and additives (such as Bi2O3, Sb2O3, etc.) that are ground, granulated, pressed and sintered at high temperatures. Their excellent nonlinear volt-ampere characteristics depend on the microstructure of grains and grain boundaries.

[0003] In the molding process of ZnO varistor, binder is the most important additive. It enables the powder to be pressed into a green embryo and gives the green embryo strength through the physical adsorption force and intermolecular force between binder and binder and between binder and powder particles. Polyvinyl alcohol (PVA) is the most commonly used binder in the molding preparation of zinc oxide varistor. In addition, acrylic emulsion is used as a binder, and polymers containing polar functional groups are used as binders. The green embryo must be subjected to high-temperature debinding treatment to thermally decompose the polymer. Most of its thermal decomposition products are volatilized in the form of volatile organic compounds (VOC), and some remain in the embryo in the form of residual carbon. However, the polymer will soften and melt during the high-temperature decomposition process, resulting in the agglomeration of zinc oxide and additive particles and the generation of pinhole defects, which destroys the uniformity of the grains after high-temperature sintering. The residual carbon after high-temperature decomposition will also affect the performance of the grain boundary in the form of impurities. Therefore, in order to improve the microstructure and macroscopic properties of ZnO varistor, the introduction of polymer binders in its molding process should be avoided. Summary of the invention

[0004] 1. Technical issues to be resolved

[0005] In view of the shortcomings of the prior art, the present invention provides a pressing molding process for zinc oxide varistor, which has the advantages of uniform grain distribution, improved grain boundary performance and reduced environmental pollution, and solves the problems that high-temperature decomposition of the adhesive will produce pinhole defects that destroy the uniformity of the grains, residual impurities will affect the performance of the grain boundaries, and organic compounds (VOC) volatile gases that will pollute the environment will be generated.

[0006] (II) Technical solution

[0007] In order to achieve the above-mentioned purpose of uniform grain distribution, improving grain boundary performance and reducing environmental pollution, the present invention provides the following technical solution: a compression molding process of a zinc oxide varistor, characterized in that it includes the following steps:

[0008] 1) spray granulating the zinc oxide varistor raw material according to the existing zinc oxide varistor production process;

[0009] 2) The powder after spray granulation is subjected to water treatment, that is, the surface of the powder is uniformly wetted with acid or acid solution, so that the surface of the particles is slightly soluble and a liquid phase is generated between the interfaces;

[0010] 3) The powder after the water treatment is pressed and formed according to the existing production process of zinc oxide varistor, and then sintered, heat treated, aluminum sprayed, glazed, tested and other processes are carried out to prepare zinc oxide-based varistor sheets.

[0011] Preferably, the acid or acid solution is specifically an inorganic acid and an organic acid, or an alcohol-water solution of an inorganic acid and an organic acid. From the perspective of process effect and performance optimization, an organic acid with a dissociation constant (pKa) greater than 1 and a molecular weight less than 300 is preferred, because such an organic acid can achieve effective dissolution of the raw material surface without introducing too many complex chemical substances, which is beneficial to the control of subsequent processes and the stability of product performance. Different organic acids have different chemical properties and dissociation abilities. A dissociation constant (pKa) greater than 1 means that the organic acid will not completely dissociate in the solution, and the rate and degree of the dissolution reaction can be controlled to a certain extent. A molecular weight less than 300 ensures that the organic acid has a moderate molecular size and can interact well with the particles on the surface of the zinc oxide varistor raw material to form a stable soluble salt coating structure. For example, common small molecules such as formic acid and acetic acid have Organic acids have suitable pKa values ​​and molecular weights and show good treatment effects in practical applications. The mass ratio of the acid to zinc oxide and other additives is between 0.1% and 10%, preferably between 0.5% and 5%. This ratio range has been verified by a large number of experiments and can ensure that the surface of the raw materials reacts fully while avoiding the degradation of the raw material performance caused by excessive dissolution. When the acid ratio is too low, it may not be possible to form a sufficient soluble salt coating layer on the surface of the raw materials, thereby failing to effectively improve the binding force between the raw materials. When the acid ratio is too high, it may cause excessive dissolution of the raw materials, destroy the crystal structure of the raw materials, and affect the final performance of the varistor. The alcohol in the alcohol-water solution is a monohydric alcohol or polyhydric alcohol with 2 to 11 carbon atoms and a solubility greater than 5 [g·(100g water)-1], and the mass fraction of the alcohol in the alcohol-water solution is 0 to 100%.

[0012] Preferably, the acid can be mixed with zinc oxide and other oxide powders and stirred evenly at any stage before spray granulation, preferably mixed in the ball milling stage. During the ball milling process, the raw material particles continuously collide and grind. At this time, the introduction of acid can contact the raw material more evenly, promote the surface dissolution reaction, and form a structure of soluble salt-coated zinc oxide. This special structure greatly improves the bonding force between the raw materials and lays a good foundation for the subsequent molding process. Parameters such as the speed of the ball mill and the ball milling time will also affect the acid treatment effect. Generally speaking, the ball mill speed is controlled at 150-250 rpm, and the ball milling time is more suitable for 8-12 hours. Within this parameter range, the raw materials can be fully mixed, and the reaction between the acid and the raw material surface can also be fully carried out. It is also possible to choose to introduce acid or acid solution in the aqueous process, that is, use acid or acid solution to uniformly wet the zinc oxide varistor raw material. When it is aqueous, a lubricant with a mass ratio of 0.2% to 2% should be added to the acid or acid solution.

[0013] Preferably, the slurry is spray granulated to obtain zinc oxide composite powder. The spray granulation process can make the powder have good fluidity and uniform particle size distribution, which is beneficial to the subsequent dry pressing molding. During the spray granulation process, parameters such as the inlet air temperature, the outlet air temperature, and the spray pressure have an important influence on the quality of the powder. The inlet air temperature is generally controlled at 180-220°C, and the outlet air temperature is controlled at 80-100°C. The spray pressure is appropriately adjusted according to the properties of the equipment and the slurry. Under such parameter conditions, zinc oxide composite powder with uniform particle size and good fluidity can be obtained.

[0014] Preferably, the water content is 0.3% to 3% of the total mass of the composite powder, preferably 0.5% to 1.5%. The water used is pure water or an aqueous solution containing 1% to 10% of a lubricant. The lubricant is a low molecular weight polymer containing one or more of an amino group, an amide group, a carboxyl group or a hydroxyl group. Preferably, a low molecular weight polymer containing an amino group and an amide group is used. The number of carbon atoms on the carbon chain of the low molecular weight polymer should be less than 10. The role of the lubricant is to further improve the fluidity and molding performance of the powder during dry pressing and to reduce stress and defects inside the green embryo. Different types of lubricants have different effects on the powder. The low molecular weight polymer containing an amino group and an amide group can form a good adsorption with the powder surface through intermolecular hydrogen bonds and electrostatic effects, thereby reducing the friction between the powder particles and improving the fluidity. At the same time, this type of lubricant can decompose and volatilize during high-temperature sintering, and no impurities will remain inside the varistor to affect its performance.

[0015] Preferably, during compression molding, pressing pressure and holding time are also key parameters. The pressing pressure is generally controlled at 25-40 MPa, and the holding time is 3-6 minutes. Within this pressure and time range, the green embryo can achieve sufficient density and strength, while avoiding defects such as cracks inside the green embryo due to excessive pressure or too long holding time.

[0016] Preferably, the sintering process generally includes three stages: heating, heat preservation and cooling. In the heating stage, the temperature is raised from room temperature to the sintering temperature at a certain rate. The heating rate is generally controlled at 3-5°C / minute. Too fast a heating rate may cause thermal stress inside the green embryo, causing defects such as cracks. Too slow a heating rate will extend the production cycle and reduce production efficiency. In the heat preservation stage, the sintering temperature is maintained for a certain time to fully sinter the green embryo and form a stable crystal structure. The heat preservation time is generally 2-4 hours. The specific time is adjusted according to factors such as the size and formula of the varistor. In the cooling stage, the temperature is reduced from the sintering temperature at an appropriate rate. To room temperature, the cooling rate is generally controlled at 4-6℃ / minute. Slow cooling can reduce the thermal stress inside the embryo and avoid problems such as cracks and deformation caused by sudden temperature drop. During the sintering process, the atmosphere in the furnace needs to be controlled. Generally, air atmosphere is used for sintering, but for some varistors with special formulas, inert gas atmosphere or reducing atmosphere may be required. The control of atmosphere has an important influence on the grain boundary structure and electrical properties of the varistor. For example, sintering in a reducing atmosphere can cause certain oxides at the grain boundaries to undergo reduction reactions, thereby changing the electrical properties of the grain boundaries and improving the nonlinear coefficient and energy density of the varistor.

[0017] Compared with the prior art, the present invention provides a compression molding process for zinc oxide varistor, which has the following beneficial effects:

[0018] 1. The present invention uses acid to controllably dissolve the surface of the zinc oxide varistor raw material, thereby increasing its surface energy and chemical polarity, thereby enhancing the interaction between powders. Even without adding a binder, the green embryo obtained by pressing can maintain good strength and will not crack during calcination.

[0019] 2. The present invention uses acid to controllably dissolve the surface of the zinc oxide varistor raw material, and generates a liquid phase (soluble composite salt solution) between the surfaces of the zinc oxide varistor raw material. On the one hand, it can enhance the binding force between particles to form a dense body through the dissolution, recrystallization and replacement reaction of salt under a certain pressure. On the other hand, it can improve the efficiency of doping ZnO with additives such as Bi2O3 and Sb2O3 and improve the grain boundary performance.

[0020] 3. The present invention no longer introduces polymer binders in its molding process, thereby reducing the debinding process, avoiding the emission of volatile organic gases (VOCs), improving green productivity and product performance, and reducing production costs. DETAILED DESCRIPTION

[0021] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] Embodiment 1:

[0023] The raw materials of this embodiment are prepared according to the following molar percentages, and the specific formula is as follows: Bi2O3: 3.12%; Sb2O3: 3.33%; Co2O3: 1.95%; Mn3O4: 0.55%; Cr2O3: 0.42%; NiO: 0.38%; SiO2: 0.71%; MgO: 0.06%; La2O3: 0.05%; Al(NO3)3·9H2O: 0.03%; silver glass powder: 0.15%; ZnO: 89.25%.

[0024] The present invention provides a process for preparing a zinc oxide varistor, comprising the following steps:

[0025] (1) Mixing in a ball mill according to the formula ratio, adding a 30% by mass ethylene glycol-water solution of citric acid, and adding acetic acid in an amount of 1.5% by mass ratio of zinc oxide to the total mass of the additive, and fully ball milling to obtain a uniformly mixed slurry; and spray granulating the slurry to obtain a zinc oxide composite powder.

[0026] (2) The zinc oxide powder is hydrated, and the water content is 1% of the weight of the zinc oxide composite powder.

[0027] (3) The hydrated zinc oxide powder is dry-pressed at a pressure of 9 MPa, a holding time of 10 s, 4 exhaust times, and a total exhaust time of 6 s to obtain a green embryo.

[0028] (4) The green body is produced according to the conventional production process of zinc oxide varistor such as sintering, heat treatment, aluminum spraying, glazing, and testing to prepare a zinc oxide varistor.

[0029] Embodiment 1:

[0030] The raw materials of this embodiment are prepared according to the following molar percentages, and the specific formula is as follows: Bi2O3: 3.12%; Sb2O3: 3.33%; Co2O3: 1.95%; Mn3O4: 0.55%; Cr2O3: 0.42%; NiO: 0.38%; SiO2: 0.71%; MgO: 0.06%; La2O3: 0.05%; Al(NO3)3·9H2O: 0.03%; silver glass powder: 0.15%; ZnO: 89.25%.

[0031] The present invention provides a process for preparing a zinc oxide varistor, comprising the following steps:

[0032] (1) Mixing in a ball mill according to the formula ratio, and obtaining a uniformly mixed slurry after sufficient ball milling; and spray granulating the slurry to obtain a zinc oxide composite powder.

[0033] (2) The composite powder is hydrated, specifically, an ethylene glycol alcohol-water solution containing 30% by mass of citric acid is used to uniformly wet the surface of the powder.

[0034] (3) The hydrated composite powder is dry-pressed at a pressure of 9 MPa, a holding time of 10 s, 4 exhaust times, and a total exhaust time of 6 s to obtain a green embryo.

[0035] (4) The green body is produced according to the conventional production process of zinc oxide varistor such as sintering, heat treatment, aluminum spraying, glazing, and testing to prepare a zinc oxide varistor.

[0036] Comparative Example:

[0037] The raw materials of the comparative example are prepared according to the following molar percentages, and the specific formula is as follows: Bi2O3: 3.12%; Sb2O3: 3.33%; Co2O3: 1.95%; Mn3O4: 0.55%; Cr2O3: 0.42%; NiO: 0.38%; SiO2: 0.71%; MgO: 0.06%; La2O3: 0.05%; Al(NO3)3·9H2O: 0.03%; silver glass powder: 0.15%; ZnO: 89.25%. The zinc oxide varistor is produced according to the conventional production process of the zinc oxide varistor.

[0038] The electrical properties of the zinc oxide-based varistors of the above-mentioned Example 1, Example 2 and Comparative Example obtained by testing are shown in Table 1.

[0039] Table 1 Comparison of electrical performance parameters of zinc oxide based varistors

[0040]

[0041] It can be seen from Table 1 that the zinc oxide varistor prepared by the process of the present invention has significantly improved potential gradient and current carrying capacity, while the pressure ratio is significantly reduced, compared with the zinc oxide resistor prepared by the traditional process (comparative example), which fully verifies the beneficial technical effects described in the present invention.

[0042] The beneficial effects of the present invention are as follows: the surface of the zinc oxide varistor raw material is controllably dissolved by using acid, so that its surface energy and chemical polarity can be increased, thereby enhancing the interaction force between powders. Even without adding a binder, the green embryo obtained by pressing can maintain good strength and will not have cracks during calcination. A liquid phase (soluble composite salt solution) is generated between the surfaces of the zinc oxide varistor raw material, and the binding force between particles can be enhanced by dissolution, recrystallization and substitution reaction of salt under a certain pressure to form a dense body. The efficiency of doping ZnO with additives such as Bi2O3 and Sb2O3 can be improved, and the grain boundary performance can be improved. Since a polymer binder is no longer introduced, the debinding process is reduced, the emission of volatile organic gases (VOCs) is avoided, the green productivity and product performance are improved, and the production cost is reduced.

[0043] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A compression molding process for a zinc oxide varistor, characterized in that: The following steps are involved: 1) Spray granulate the raw materials of zinc oxide varistor according to the existing production process of zinc oxide varistor; 2) Treat the powder after spray granulation with water, that is, use acid or acid solution to evenly wet the powder surface, so that the particle surface is slightly soluble and a liquid phase is generated between the interfaces; 3) The powder after the water treatment is pressed and formed according to the existing production process of zinc oxide varistor, and sintered, heat treated, aluminum sprayed, glazed, tested and other processes are carried out to prepare zinc oxide-based varistor sheets.

2. The compression molding process of the zinc oxide varistor according to claim 1, characterized in that: The acid or acid solution is specifically an inorganic acid and an organic acid, or an alcohol-water solution of an inorganic acid and an organic acid. From the perspective of process effect and performance optimization, an organic acid with a dissociation constant (pKa) greater than 1 and a molecular weight less than 300 is preferred, because such an organic acid can achieve effective dissolution of the surface of the raw material without introducing too many complex chemicals, which is beneficial to the control of subsequent processes and the stability of product performance. Different organic acids have different chemical properties and dissociation abilities. A dissociation constant (pKa) greater than 1 means that the organic acid will not completely dissociate in the solution, and can control the rate and degree of the dissolution reaction to a certain extent. A molecular weight less than 300 ensures that the organic acid has a moderate molecular size and can interact well with the particles on the surface of the zinc oxide varistor raw material to form a stable soluble salt coating structure. For example, common small molecule organic acids such as formic acid and acetic acid have suitable pKa. The value and molecular weight show good treatment effect in practical applications, wherein the mass ratio of acid to zinc oxide and other additives is between 0.1% and 10%, preferably between 0.5% and 5%. This ratio range has been verified by a large number of experiments, and can ensure sufficient reaction on the surface of the raw materials while avoiding degradation of raw material performance caused by excessive dissolution. When the acid ratio is too low, it may not be possible to form a sufficient soluble salt coating layer on the surface of the raw materials, thereby failing to effectively improve the binding force between the raw materials. When the acid ratio is too high, it may cause excessive dissolution of the raw materials, destroy the crystal structure of the raw materials, and affect the final performance of the varistor. The alcohol in the alcohol-water solution is a monohydric alcohol or polyhydric alcohol with a carbon atom number of 2 to 11 and a solubility greater than 5 [g·(100 g water)-1]. The mass fraction of the alcohol in the alcohol-water solution is 0 to 100%.

3. The compression molding process of the zinc oxide varistor according to claim 1, characterized in that: The acid can be mixed with zinc oxide and other oxide powders and stirred uniformly at any stage before spray granulation, preferably mixed in the ball milling stage. During the ball milling process, the raw material particles continuously collide and grind. At this time, the acid can be introduced to contact the raw material more evenly, promote the surface dissolution reaction, and form a structure of soluble salt-coated zinc oxide. This special structure greatly improves the bonding force between the raw materials and lays a good foundation for the subsequent molding process. Parameters such as the rotation speed and ball milling time of the ball mill will also affect the acid treatment effect. Generally speaking, the ball mill speed is controlled at 150-250 rpm, and the ball milling time is 8-12 hours. It is more appropriate. Within this parameter range, the raw materials can be fully mixed, and the reaction between the acid and the surface of the raw material can also be fully carried out. It is also possible to choose to introduce an acid or an acid solution in an aqueous process, that is, to use an acid or an acid solution to uniformly wet the zinc oxide varistor raw material. When it is aqueous, a lubricant with a mass ratio of 0.2% to 2% should be added to the acid or the acid solution.

4. The compression molding process of the zinc oxide varistor according to claim 1, characterized in that: The slurry is spray granulated to obtain zinc oxide composite powder. The spray granulation process can make the powder have good fluidity and uniform particle size distribution, which is beneficial to subsequent dry pressing molding. During the spray granulation process, parameters such as air inlet temperature, air outlet temperature, and spray pressure have an important influence on the quality of the powder. The air inlet temperature is generally controlled at 180-220°C, the air outlet temperature is controlled at 80-100°C, and the spray pressure is appropriately adjusted according to the properties of the equipment and the slurry. Under such parameter conditions, zinc oxide composite powder with uniform particle size and good fluidity can be obtained.

5. The compression molding process of the zinc oxide varistor according to claim 1, characterized in that: The water content is 0.3% to 3% of the total mass of the composite powder, preferably 0.5 to 1.5%. The water used is pure water or an aqueous solution containing 1% to 10% of a lubricant. The lubricant is a low molecular weight polymer containing one or more of an amino group, an amide group, a carboxyl group or a hydroxyl group. Preferably, a low molecular weight polymer containing an amino group and an amide group is used. The number of carbon atoms on the carbon chain of the low molecular weight polymer should be less than 10. The function of the lubricant is to further improve the fluidity and molding performance of the powder during dry pressing and to reduce stress and defects inside the green embryo. Different types of lubricants have different effects on the powder. The low molecular weight polymer containing an amino group and an amide group can form a good adsorption with the powder surface through intermolecular hydrogen bonds and electrostatic effects, thereby reducing the friction between the powder particles and improving the fluidity. At the same time, this type of lubricant can decompose and volatilize during high temperature sintering, and no impurities will remain inside the varistor to affect its performance.

6. The compression molding process of the zinc oxide varistor according to claim 1, characterized in that: During the compression molding, the pressing pressure and holding time are also key parameters. The pressing pressure is generally controlled at 25-40MPa, and the holding time is 3-6 minutes. Within this pressure and time range, the green embryo can achieve sufficient density and strength, while avoiding defects such as cracks inside the green embryo caused by excessive pressure or too long holding time.

7. The compression molding process of the zinc oxide varistor according to claim 1, characterized in that: The sintering process generally includes three stages: heating, heat preservation and cooling. In the heating stage, the temperature is raised from room temperature to the sintering temperature at a certain rate. The heating rate is generally controlled at 3-5°C / minute. Too fast a heating rate may cause thermal stress inside the green embryo, causing defects such as cracks. Too slow a heating rate will extend the production cycle and reduce production efficiency. In the heat preservation stage, the sintering temperature is maintained for a certain time to fully sinter the green embryo and form a stable crystal structure. The heat preservation time is generally 2-4 hours. The specific time is adjusted according to factors such as the size and formula of the varistor. In the cooling stage, the temperature is reduced from the sintering temperature to room temperature at an appropriate rate. The cooling rate is generally controlled at 4-6°C / minute. Minutes, slow cooling can reduce the thermal stress inside the embryo and avoid problems such as cracks and deformation caused by sudden temperature drop. During the sintering process, the atmosphere in the furnace needs to be controlled. Generally, air atmosphere is used for sintering, but for some varistors with special formulas, inert gas atmosphere or reducing atmosphere may be required. The control of atmosphere has an important influence on the grain boundary structure and electrical properties of the varistor. For example, sintering in a reducing atmosphere can cause certain oxides at the grain boundaries to undergo reduction reactions, thereby changing the electrical properties of the grain boundaries and improving the nonlinear coefficient and energy density of the varistor.

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