Preparation method of internal electrode silver paste for chip ZnO varistor

Abstract

The present invention provides a preparation method for internal electrode silver paste used in chip ZnO varistors. The method involves mixing spherical silver powder with surface modification, flaky silver powder, polymer resin, dispersant and organic solvent to obtain the internal electrode silver paste. The spherical silver powder with surface modification is obtained by coating the surface of the original silver powder with nano-metal oxides, having a specific surface area of 0.5 - 2.5 m² / g and a tapped density of 4 - 6 g / ml. The flaky silver powder has a specific surface area of 0.5 - 2 m² / g and a tapped density of 4 - 5 g / ml. The beneficial effects of the present invention are as follows: The silver powder with special surface modification is used to prepare the silver electrode paste for ZnO chip varistors, which improves the burn resistance of silver and reduces the resistivity after sintering. Palladium is no longer added, effectively solving the problem that palladium in the silver-palladium paste is prone to react with bismuth. At the same time, it can further reduce the resistivity after sintering and improve the product performance. Meanwhile, it also reduces the high cost brought by using the silver-palladium paste.

Description

Technical Field

[0001] The present invention belongs to the technical field of electronic component preparation, and in particular relates to a method for preparing an inner electrode silver paste for a chip ZnO varistor. Background Art

[0002] A chip varistor is a type of varistor. It is an electric shock protection device made with a zinc oxide nonlinear resistance element as the core. The zinc oxide nonlinear resistance element is a compound semiconductor element made of zinc oxide (ZnO) as the main material, added with various other trace elements, and processed by ceramic technology. With the improvement of the scale and speed of modern integrated circuits, the requirements for electronic components are becoming more and more stringent. Such as miniaturization, high performance, thinness, lightness, large capacitance, high surge resistance of resistors, etc. A multilayer chip varistor is composed of multiple discrete varistors connected in parallel, and has the advantages of small size, large current-carrying capacity, good surface mountability, and easy implementation of low voltage. As a miniaturized and high-performance surge suppression component, it has been widely used in various fields such as IC protection, CMOS, MOSFET device protection, and automotive electronic circuit protection.

[0003] The ZnO system is an ideal chip component material, mostly used in chip varistors. The conventional formula for multilayer chip ceramics uses the ZnO-Bi2O3-Sb2O3 system formula. Generally, silver-palladium paste is used as the inner electrode. Most high-end products use foreign manufacturers such as DuPont, Ferro, and Showa. Its main performance indicators are good co-firing compatibility, better surge resistance, and higher conductivity. The ZnO-based chip varistor green sheet generally contains bismuth, which is likely to react with palladium in the silver-palladium paste, causing product defects; the existing pure silver paste will cause silver migration, reducing the product performance; the pure silver paste has poor firing resistance, and silver flying phenomenon will occur at high temperature, which is not suitable for the green ceramic sheet with a relatively high sintering temperature. And the silver-palladium paste has a relatively large resistivity, which restricts the development of high-performance products, and the silver-palladium paste is expensive, which is not conducive to enterprise cost control. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a method for preparing an inner electrode silver paste for a chip ZnO varistor.

[0005] The technical solution adopted by the present invention is: a method for preparing an inner electrode silver paste for a chip ZnO varistor, which mixes surface-modified spherical silver powder, flaky silver powder, polymer resin, dispersant and organic solvent to prepare the inner electrode silver paste;

[0006] Among them, the surface-modified spherical silver powder is 70%-90%, the flaky silver powder is 5%-20%, the polymer resin is 1%-5%, the dispersant is 0.1%-3%, and the balance is organic solvent.

[0007] Preferably, the surface-modified spherical silver powder is formed by coating the surface of the original silver powder with nano-metal oxides, and the addition amount of the nano-metal oxides is 0.2%-2% of the silver powder content.

[0008] Preferably, the nano-metal oxides are one or a combination of more than one of nano-silica, nano-aluminum oxide, nano-zinc oxide, nano-titanium dioxide, and nano-zirconium oxide.

[0009] Preferably, the original silver powder is a single-crystalline spherical silver powder with a particle size of 1.5-5 μm, a specific surface area of 0.2-1 m 2 / g, and a tapped density of 4-6 g / ml.

[0010] Preferably, the nano-metal oxides are added to ethanol, and then a dispersant with a nano-metal oxide content of 1-5% is added, and stirred; the original silver powder is added, and the surface-modified spherical silver powder is obtained after stirring.

[0011] Preferably, the specific surface area of the flaky silver powder is 0.5-2 m 2 / g, and the tapped density is 4-5 g / ml.

[0012] Preferably, the polymer resin is one or a combination of more than one of acrylic resin, PVB resin, and ethyl cellulose;

[0013] The organic solvent is one or a combination of more than one of butyl carbitol, butyl carbitol acetate, terpineol, alcohol ester 12, diethylene glycol diethyl ether, and methyl ester of nylon acid;

[0014] The dispersant is one or a combination of more than one of polyethylene wax, acrylate, and phosphate ester.

[0015] Preferably, it further includes an additive, and the additive is an anti-settling thixotropic agent and / or an anti-foaming agent;

[0016] The anti-settling thixotropic agent is one or a combination of more than one of hydrogenated castor oil, polyurea, polyamide wax, and polyurethane.

[0017] The internal electrode silver paste prepared by the preparation method of the internal electrode silver paste for the chip ZnO varistor.

[0018] The chip ZnO varistor containing the internal electrode silver paste.

[0019] The advantages and positive effects of the present invention are: The silver powder with special surface modification is used to make the silver electrode paste for the ZnO chip varistor, which improves the burn resistance of silver and reduces the resistivity after sintering; palladium is no longer added, effectively solving the phenomenon that palladium in the silver-palladium paste is easy to react with bismuth, and at the same time, the resistivity after sintering can be further reduced and the product performance can be improved; at the same time, the high cost brought by using the silver-palladium paste is also reduced. Description of the Drawings

[0020] Figure 1 It is a comparison of the thermal analysis curves before and after the modification of silver powder. Specific embodiments

[0021] The embodiments of the present invention will be described below with reference to the accompanying drawings.

[0022] The present invention discloses a preparation method of an internal electrode silver paste for a chip ZnO varistor. To solve the defects of high resistivity, easy reaction with bismuth elements in green sheets, and high price of silver-palladium pastes, as well as poor firing resistance and easy silver migration of pure silver pastes, the present invention uses silver powder with special surface modification to prepare silver electrode paste for ZnO chip varistors, improving the firing resistance of silver, reducing the resistivity after sintering, and improving product performance. Mix the surface-modified spherical silver powder, flaky silver powder, polymer resin, dispersant and organic solvent evenly to prepare the internal electrode silver paste; wherein, the surface-modified spherical silver powder is 70%-90%, the flaky silver powder is 5%-20%, the polymer resin is 1%-5%, the dispersant is 0.1%-3%, and the balance is organic solvent.

[0023] The surface-modified spherical silver powder is coated with nano-metal oxides on the surface of the silver powder raw material. The addition amount of nano-metal oxides is 0.2%-2% of the silver powder content, preferably 0.5%-1%. The nano-metal oxides are one or a combination of nano-silica, nano-aluminum oxide, nano-zinc oxide, nano-titanium dioxide and nano-zirconium oxide; the silver powder raw material is a single-crystalline spherical silver powder with a particle size of 1.5-5um, a specific surface area of 0.2-1m 2 / g, and a tapped density of 4-6g / ml. The specific preparation process is as follows: Take a certain amount of the above-mentioned nano-oxide, add it to sufficient ethanol, add a dispersant with an oxide content of 1%-5%, and stir well; then add the silver powder raw material and stir under a stirrer to obtain the surface-modified spherical silver powder; by controlling the stirring time, modified silver powders with different coating degrees can be obtained; the specific surface area of the surface-modified spherical silver powder is 0.5-2.5m 2 / g, and a tapped density of 4-6g / ml.

[0024] The specific surface area of the flaky silver powder used in the silver paste is 0.5-2m 2 / g, the tap density is 4 - 5 g / ml; the polymer resin is one or a combination of acrylic resin, PVB resin and ethyl cellulose; the organic solvent is one or a combination of butyl carbitol, butyl carbitol acetate, terpineol, alcohol ester 12, diethylene glycol diethyl ether and methyl esters of nylon acid; the dispersant is one or a combination of polyethylene wax, acrylate and phosphate. Two steps of operations are involved in the present invention. First, an organic carrier is prepared by mixing the polymer resin and additives into a solvent; then an inner paste is prepared by mixing the surface-modified spherical silver powder, flaky silver powder, organic carrier and additives into a solvent. In the two steps of operations, the same or different types of organic solvents can be used, and the same or different types of additives can be used. For example, the additives may include a dispersant. The dispersant and the solvent are divided into two parts. One part is added when preparing the carrier, and the other part is added when preparing the inner paste. The dispersant and the solvent in the two steps can be of the same type or different types; in some embodiments of the present invention, the types of dispersants used may be different. For different purposes, the former is to make the organic carrier more uniform, and the latter is for the dispersion of silver powder.

[0025] In some embodiments of the present invention, an anti-settling thixotropic agent may also be added to the silver paste. The anti-settling thixotropic agent is one or a combination of hydrogenated castor oil, polyurea, polyamide wax and polyurethane. The silver paste includes, by mass percentage: 70% - 90% of surface-modified spherical silver powder, 5% - 20% of flaky silver powder, 1% - 5% of polymer resin, 0% - 1% of anti-settling thixotropic agent, 0.1% - 3% of dispersant, and the balance is organic solvent. In addition, in some embodiments of the present invention, other additives may also be added, such as adding an antifoaming agent.

[0026] The silver layer prepared from the prepared silver electrode paste has a high brightness, good co-firing match with the green sheet, and a low resistivity. Using the silver powder with special surface modification to make the inner paste of the chip varistor effectively solves the problem that palladium in the silver-palladium paste is easy to react with bismuth, and at the same time can further reduce the resistivity after sintering and improve the product performance; at the same time, it also reduces the high cost brought by using the silver-palladium paste. In addition, the present invention further solves the problems of poor firing resistance and silver migration of the pure silver paste.

[0027] The following describes the solution of the present invention in conjunction with the drawings. Among them, for the experimental methods where the operation steps are not specifically described, they are all carried out according to the corresponding product specifications. The instruments, reagents, and consumables used in the examples can be purchased from commercial companies without special instructions.

[0028] Example 1:

[0029] Surface modification of silver powder: Select nano-SiO2 for modification; Add 1 g of nano-SiO2 (30 nm) to 200 g of alcohol, dropwise add 0.05 g of dispersant (KD9 Croda) and stir for 10 min. Take 100 g of silver powder (specific surface area about 0.8 m 2 / g, D50 1.5 um) and add it to the above solution, continue stirring. After stirring for 1 h, put it into an oven to dry. Add sufficient alcohol to the dried modified silver powder, let it stand for 30 min, remove the upper turbid liquid, and dry. After repeating 2 - 3 times, pure surface-modified silver powder (specific surface area 0.5 - 2.5 m 2 / g) can be obtained.

[0030] Analyze the spherical silver powder before and after modification, and the thermal analysis curve is as Figure 1 shown; It can be seen that the burn resistance of the silver powder is significantly improved after surface modification.

[0031] Preparation of organic carrier: Take 20 g of ethyl cellulose, 2 - 4 g of hydrogenated castor oil, 0.1 - 0.3 g of dispersant and add them to 200 g of a mixed solvent of butyl carbitol and terpineol. Stir and dissolve at 60 °C. After stirring for 2 hours, take out the carrier, filter while it is hot, and obtain the required carrier after cooling

[0032] Preparation of internal slurry: Take 78 g of surface-modified spherical silver powder, 10 g of flake silver powder, 14 g of carrier, 0.3 g of dispersant, 0.3 g of defoamer and 0.4 g of solvent, mix them in a plastic tank, stir evenly (centrifugal stirring or stir with a mixer), and then grind them on a three-roll mill. After 5 - 7 passes of rolling, test the fineness. If it is less than 7 um, the dispersion is completed. After filtration, a silver electrode paste for ZnO chip varistors with good performance is obtained.

[0033] Example 2:

[0034] Prepare a silver electrode paste for ZnO chip varistors with good performance according to the method described in Example 1.

[0035] Among them, the weight percentage composition: 80% of surface-modified spherical silver powder, 8% of flake silver powder, 0.5 - 1% of polyurea, 1 - 3% of ethyl cellulose, 0.3 - 0.6% of dispersant, 4 - 6% of terpineol, and the balance is butyl carbitol.

[0036] Among them, the surface-modified spherical silver powder is modified with nano-silica, and the specific surface area is 0.5 - 2.5 m 2 / g, and the flake silver powder is 0.5 - 2 m 2 / g.

[0037] Example 3:

[0038] The silver electrode paste for ZnO chip varistors with good performance was prepared according to the method described in Example 1.

[0039] Among them, the weight percentage composition is: 80% of spherical silver powder with surface modification, 8% of flaky silver powder, 0.5 - 1% of polyurea, 1 - 3% of ethyl cellulose, 0.3 - 0.6% of dispersant, 4 - 6% of terpineol, and the balance is butyl carbitol.

[0040] Among them, the spherical silver powder with surface modification is modified by nano - zirconia, and the specific surface area is 0.5 - 2.5 m 2 / g, and the specific surface area of the flaky silver powder is 0.5 - 2 m 2 / g.

[0041] Example 4:

[0042] The silver electrode paste for ZnO chip varistors with good performance was prepared according to the method described in Example 1.

[0043] Among them, the weight percentage composition is: 80% of spherical silver powder with surface modification, 8% of flaky silver powder, 0.5 - 1% of hydrogenated castor oil, 1 - 3% of ethyl cellulose, 0.3 - 0.6% of dispersant, 4 - 6% of terpineol, and the balance is butyl carbitol.

[0044] Among them, the spherical silver powder with surface modification is modified by nano - alumina, and the specific surface area is 0.5 - 2.5 m 2 / g, and the specific surface area of the flaky silver powder is 0.5 - 2 m 2 / g.

[0045] Example 5:

[0046] The silver electrode paste for ZnO chip varistors with good performance was prepared according to the method described in Example 1.

[0047] Among them, the weight percentage composition is: 70% of spherical silver powder with surface modification, 15% of flaky silver powder, 0.5 - 1% of polyurea, 1 - 3% of ethyl cellulose, 0.3 - 0.6% of dispersant, 4 - 6% of terpineol, and the balance is butyl carbitol.

[0048] Among them, the spherical silver powder with surface modification is modified by nano - silica, and the specific surface area is 0.5 - 2.5 m 2 / g, and the specific surface area of the flaky silver powder is 0.5 - 2 m 2 / g.

[0049] Example 6:

[0050] The silver electrode paste for ZnO chip varistors with good performance was prepared according to the method described in Example 1.

[0051] Among them, the weight percentage composition is: 70% of surface-modified spherical silver powder, 15% of flaky silver powder, 0.5 - 1% of polyurea, 1 - 3% of polyurethane, 0.3 - 0.6% of dispersant, 4 - 6% of terpineol, and the balance is butyl carbitol.

[0052] Among them, the surface-modified spherical silver powder is modified by nano-silica, and the specific surface area is 0.5 - 2.5 m 2 / g, and the specific surface area of the flaky silver powder is 0.5 - 2 m 2 / g.

[0053] Take the silver electrode paste prepared in Examples 1 - 6, and test the resistance and co-firing matching after the paste is sintered (at 930°C). The results are shown in Table 1.

[0054] Table 1

[0055]

[0056] It can be found from the data in Table 1 that the sheet resistance of the comparative sample (silver-palladium paste) is relatively high, and the comparative sample (ordinary pure silver paste) has defects such as silver flying during high-temperature sintering. However, Examples 1 - 6 not only achieve better results in terms of sheet resistance, but also can prevent silver flying during the high-temperature sintering process. The prepared silver electrode has low resistivity and excellent firing resistance.

[0057] The above has described the embodiments of the present invention in detail, but the content described is only the preferred embodiments of the present invention and cannot be considered as limiting the scope of implementation of the present invention. All equivalent changes and improvements made according to the scope of the application of the present invention shall still fall within the scope covered by the patent of the present invention.

Claims

1. A preparation method of internal electrode silver paste for chip ZnO varistors, characterized in that: Mix the surface-modified spherical silver powder, flaky silver powder, polymer resin, dispersant and organic solvent to prepare the inner electrode silver paste; first prepare the organic carrier by mixing the polymer resin and additives into the solvent; then prepare the inner paste by mixing the surface-modified spherical silver powder, flaky silver powder, organic carrier and additives into the solvent. Among them, the surface-modified spherical silver powder is 70%-90%, the flaky silver powder is 5%-20%, the polymer resin is 1%-5%, the dispersant is 0.1%-3%, and the balance is organic solvent. The surface-modified spherical silver powder is obtained by coating the surface of the original silver powder with nano-metal oxide or nano-silica, and the addition amount of nano-metal oxide or nano-silica is 0.2%-2% of the silver powder content; add nano-metal oxide or nano-silica to ethanol, then add a dispersant with a nano-metal oxide or nano-silica content of 1%-5%, and stir; add the original silver powder and stir to obtain the surface-modified spherical silver powder. The dispersant is a combination of one or more of polyethylene wax, acrylate and phosphate ester.

2. The preparation method of the internal electrode silver paste for the chip ZnO varistor according to claim 1, characterized in that: The nano-metal oxide is a combination of one or more of nano-aluminum oxide, nano-zinc oxide, nano-titanium dioxide and nano-zirconium oxide.

3. The preparation method of the internal electrode silver paste for the chip ZnO varistor according to claim 1, characterized in that: The original silver powder is single-crystalline spherical silver powder with a particle size of 1.5 - 5 μm, a specific surface area of 0.2 - 1 m 2 / g, and a tapped density of 4 - 6 g / ml.

4. The preparation method of the internal electrode silver paste for the chip ZnO varistor according to claim 1, characterized in that: The specific surface area of the flaky silver powder is 0.5 - 2 m 2 / g, and the tapped density is 4 - 5 g / ml.

5. The preparation method of the internal electrode silver paste for the chip ZnO varistor according to claim 1, characterized in that: The polymer resin is a combination of one or more of acrylic resin, PVB resin and ethyl cellulose. The organic solvent is a combination of one or more of butyl carbitol, butyl carbitol acetate, terpineol, alcohol ester 12, diethylene glycol diethyl ether and methyl ester of nylon acid.

6. The preparation method of the internal electrode silver paste for the chip ZnO varistor according to claim 1, characterized in that: It also includes additives, and the additives are anti-settling thixotropic agent and / or defoaming agent. The anti-settling thixotropic agent is a combination of one or more of hydrogenated castor oil, polyurea, polyamide wax and polyurethane.

7. The inner electrode silver paste prepared by the preparation method of the inner electrode silver paste for a chip ZnO varistor according to any one of claims 1-6.

8. A chip ZnO varistor comprising the inner electrode silver paste according to claim 7.

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