Silver-coated copper powder for narrow line width printing with steel mesh and preparation method thereof

Abstract

The application discloses a silver-coated copper powder for narrow line width printing of steel mesh and a preparation method thereof, and relates to the technical field of silver-coated copper powder. The silver-coated copper powder is of a core-shell structure, comprising a copper core and a silver shell layer coated on the surface of the copper core, and a self-assembled monolayer is formed on the surface of the silver shell layer through chemical adsorption; the self-assembled monolayer is composed of organic molecules with a general formula of X-(CH2)n-Y. In the application, the self-assembled monolayer serves as a microwave absorption medium, interface polarization is generated in a microwave field, and heat is preferentially concentrated on the particle contact neck; meanwhile, the molecular film is decomposed at low temperature, in-situ reduction of trace oxides on the copper surface is promoted, metallurgical bonding is promoted, and low-temperature rapid densification is realized.

Description

Technical Field

[0001] This invention relates to the field of silver-coated copper powder technology, and in particular to a silver-coated copper powder for use in narrow linewidth stencil printing and its preparation method. Background Technology

[0002] Heterojunction solar cells combine the advantages of crystalline silicon solar cells and thin-film technology, possessing a series of advantages such as high conversion efficiency, simple manufacturing process, thin silicon wafer application, low temperature coefficient, and bifacial power generation. They are hailed as the next-generation high-efficiency battery technology with the greatest industrial potential. Currently, the mass production efficiency of heterojunction solar cells is around 24%, indicating significant room for improvement and excellent development prospects.

[0003] For example, Chinese patent CN119920522A discloses a low-temperature silver-coated copper paste for heterojunction batteries, its preparation method, and its application. The paste includes: 10%-70% spherical silver-coated copper powder, 15-80% submicron silver powder, 0.5-5% nano-silver powder, 1-6% resin, 0.2-4% additives, and 0.5-3% solvent. The resin comprises an epoxy resin with an epoxy equivalent of 200-260 and a rubber-toughened epoxy resin with a weight-average molecular weight of 10,000-20,000. The epoxy resin with an epoxy equivalent of 200-260 provides ink permeability, ensuring narrow linewidth printing on the stencil; the high molecular weight resin with a weight-average molecular weight of 10,000-20,000 acts as a thixotropic agent, ensuring high linewidth and aspect ratio in the printing.

[0004] However, with the development of photovoltaic heterojunction cells and flexible electronic devices, higher requirements have been placed on the width (≤30μm) and conductivity of the electrode grid lines. Silver-coated copper powder has become an ideal material due to its low cost and high conductivity, but existing technologies have the following problems.

[0005] 1. Under low-temperature sintering conditions (≤200℃), it is difficult to achieve densification of silver-coated copper powder, and the particles mainly have point contact, resulting in high contact resistance; 2. Traditional hot sintering takes a long time, and the copper core is easily oxidized, leading to a decrease in conductivity; 3. Narrow linewidth printing has stringent requirements for powder dispersion and the integrity of the line shape after sintering. Existing silver-coated copper powder paste is prone to grid breakage or burrs when printed on a steel mesh. Summary of the Invention

[0006] To address the aforementioned technical problems and overcome the shortcomings of existing technologies, this invention provides a silver-coated copper powder for use in narrow-line-width stencil printing and its preparation method.

[0007] The further defined technical solution of this invention is: a silver-coated copper powder for use in conjunction with narrow linewidth stencil printing, wherein the silver-coated copper powder has a core-shell structure, comprising a copper core and a silver shell layer coating the surface of the copper core, and a self-assembled monomolecular film is formed on the surface of the silver shell layer through chemical adsorption; the self-assembled monomolecular film is composed of organic molecules with the general formula X-(CH2)nY, wherein X is an anchoring group selected from at least one of mercapto, thiol, or dopamine groups, n is an integer from 8 to 18, and Y is a functional end group selected from at least one of methyl, amino, or carboxyl groups.

[0008] Furthermore, the copper core has a particle size D50 of 1.5-2.0 μm, the silver shell has a thickness of 20-100 nm, and the silver mass fraction is 15-25%.

[0009] Furthermore, the carbon chain length n of the self-assembled monolayer is 10-12, the functional end group Y is methyl or carboxyl, and the coating amount of the self-assembled monolayer is 0.1%-2.0% of the mass of the silver-coated copper powder.

[0010] Furthermore, the coating amount of the self-assembled monolayer is 0.5%-1.0% of the mass of the silver-coated copper powder.

[0011] This invention provides a method for preparing silver-coated copper powder for use in narrow-linewidth stencil printing, comprising the following steps:

[0012] Step 1: Provide silver-coated copper powder. The silver-coated copper powder has a core-shell structure, including a copper core and a silver shell layer covering the surface of the copper core.

[0013] Step 2: Disperse the silver-coated copper powder in an organic solvent, add an organic molecule with the general formula X-(CH2)nY, where X is at least one of mercapto, thiol or dopamine group, n is an integer from 8 to 18, and Y is at least one of methyl, amino or carboxyl group. Stir and react for 0.5 to 4 hours at room temperature to 60°C to allow the organic molecule to self-assemble on the surface of the silver shell through anchoring groups.

[0014] Step 3: Filter, wash and dry the product after the reaction to obtain surface-modified silver-coated copper powder.

[0015] Furthermore, the amount of organic molecules added is 0.1%-2.0% of the mass of the silver-coated copper powder, and the organic solvent is selected from at least one of ethanol, isopropanol or acetone.

[0016] This invention also provides a microwave sintering method for narrow linewidth printing, using any of the above-mentioned silver-coated copper powders for use with narrow linewidth stencil printing, comprising the following steps:

[0017] S1, take silver-coated copper powder, mix it with organic carrier and additives to prepare a conductive paste;

[0018] S2 uses a stencil to print conductive paste onto a substrate, forming fine lines with a linewidth of 8-14μm;

[0019] S3. The printed substrate is placed in a microwave sintering equipment for sintering. The sintering atmosphere is an inert gas, the microwave frequency is 2.45GHz or 5.8GHz, the output power is 500W-3000W, the sintering time is 10-120 seconds, and the substrate temperature is controlled at 150℃-220℃ during the sintering process.

[0020] Furthermore, during microwave sintering, the self-assembled monolayer generates interfacial polarization in the microwave field, and the heat is preferentially concentrated at the particle contact neck of the silver-coated copper powder. At the same time, the self-assembled monolayer decomposes and reduces the trace oxides on the surface of the silver shell and copper core in situ, forming a metallurgical bond.

[0021] Furthermore, the inert gas is nitrogen or argon, and the substrate is polyethylene terephthalate or polyimide flexible substrate, or silicon wafer substrate.

[0022] Furthermore, the microwave sintering output power is 1500W-2500W, the sintering time is 30-90 seconds, the substrate temperature is controlled at 180-210℃ during the sintering process, and the inert gas flow rate is 5-15L / min to maintain the oxygen content below 50ppm during the sintering process.

[0023] The beneficial effects of this invention are:

[0024] (1) Compared to the prior art where silver-coated copper powder requires hot sintering at above 200℃ for more than 30 minutes to achieve initial conductivity, this invention utilizes the interfacial polarization absorption effect of a surface self-assembled monolayer, combined with microwave sintering, to achieve densification in just 60 seconds at 1500W power, with the actual measured temperature of the substrate controlled at 195℃. In the unmodified + hot sintering scheme, the resistivity reaches as high as 2.5×10⁻⁶ under 200℃ / 30min conditions. -5 The resistivity of the present invention is reduced to 6.8 × 10⁻⁶ Ω·cm under microwave conditions of 60 seconds. -6 Ω·cm, a decrease of 73%;

[0025] (2) In this invention, selective heating of the self-assembled monolayer in a microwave field concentrates heat at the particle contact neck, promoting metallurgical bonding to form a sintered neck and significantly reducing contact resistance. The resistivity of this invention reaches 5.2 × 10⁻⁶. -6 The resistivity is reduced by approximately 80% compared to the unmodified + thermal sintering scheme, and by approximately 57% compared to Comparative Example 3 (unmodified + microwave). This resistivity is close to the conductivity of pure silver paste under the same process conditions, while significantly reducing the amount of precious metals used.

[0026] (3) This invention employs microwave rapid sintering, which has a short heat treatment time (on the order of seconds), thus avoiding prolonged high-temperature oxidation of the copper core from the source. At the same time, the self-assembled monolayer can generate a reducing atmosphere in situ during decomposition, which can reduce trace amounts of copper oxide. In the prior art, an increase in resistance caused by copper oxidation can be observed, while no obvious oxide layer is seen in the microscopic cross-section of this solution, and the sintered layer maintains the luster of pure copper;

[0027] (4) In this invention, the self-assembled monolayer modified on the surface of the silver-coated copper powder also acts as a dispersant, reducing the surface energy of the powder and improving its dispersion stability in the organic carrier. The printing test stencil of this invention on an 8μm linewidth stencil has neat edges, no broken grids, no burrs, and the linewidth accuracy is controlled within ±1.0μm, meeting the high precision requirements of fine grids in heterojunction batteries and flexible electronic devices;

[0028] (5) In this invention, by adjusting the carbon chain length (n=8-18) and functional end groups (methyl, amino, carboxyl) of the self-assembled monolayer, different microwave sintering parameters (power 500W-3000W, time 10-120 seconds) can be flexibly matched, providing a clear process window and industrial operability. Compared with the complex thermal sintering process in the prior art that requires precise temperature control, the microwave sintering equipment of this invention has lower cost and lower energy consumption, and is compatible with roll-to-roll continuous production, which is conducive to large-scale industrial application. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, a detailed description is provided below in conjunction with specific embodiments. Many specific details are set forth in the following description to provide a thorough understanding of the invention. However, the invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0031] This invention comprises two core components: powder surface modification and microwave sintering process.

[0032] The silver-coated copper powder has a core-shell structure, with a copper core particle size (D50) of 1.5-2.0 μm, a silver layer thickness of 20-100 nm, and a silver mass fraction of 15-25%. A self-assembled monolayer is formed on the outermost layer of the silver-coated copper powder through chemical adsorption. This self-assembled monolayer is composed of organic molecules with the general formula X-(CH2)nY, wherein:

[0033] X is the anchoring group, selected from mercapto (-SH), thiol, or dopamine groups, used to form strong chemical bonds with the silver surface; n is the carbon chain length, selected from 8-18; Y is the functional end group, selected from methyl (-CH3), amino (-NH2), or carboxyl (-COOH).

[0034] The modified silver-coated copper powder was formulated into a conductive paste with an organic carrier and additives. After being printed into fine lines with a linewidth of 8-14 μm by stencil printing, the paste was placed in a microwave sintering apparatus for sintering. The microwave sintering parameters were as follows:

[0035] Microwave frequency: 2.45 GHz or 5.8 GHz;

[0036] Output power: 500W-3000W;

[0037] Sintering time: 10-120 seconds;

[0038] Sintering atmosphere: nitrogen or inert gas protection;

[0039] Sintering temperature: 150-220℃.

[0040] The mechanism of action of this invention is as follows: the self-assembled monolayer acts as a microwave absorption medium, generating interfacial polarization in the microwave field, with heat preferentially concentrated at the particle contact neck; at the same time, the monolayer decomposes at low temperature, reducing trace oxides on the copper surface in situ, promoting metallurgical bonding, and achieving rapid densification at low temperature.

[0041] This invention provides several embodiments and comparative examples to verify the effects.

[0042] Example 1

[0043] 1. Powder preparation: Particulate copper powder with D50 = 1.2 μm was selected and silver-coated copper powder was prepared by chemical displacement plating. The silver content was 25%, and the silver layer thickness was approximately 100 nm. The above silver-coated copper powder was dispersed in ethanol, and 0.5% (by weight of powder) of dodecyl mercaptan (C) was added. 12 H 25 SH, n=12, X=-SH, Y=-CH3), stirred at room temperature for 2 hours, filtered, and vacuum dried to obtain surface-modified silver-coated copper powder.

[0044] 2. Slurry preparation: Take 85 parts of the above modified powder and mix it with ethyl cellulose resin, terpineol, dispersant, etc. to prepare a conductive slurry with a fineness ≤5μm and a viscosity of 30Pa·s (Brookfield, 10rpm).

[0045] 3. Printing and sintering: Using an 8μm line width steel mesh, fine lines with a line width of 20μm are printed on the PET flexible substrate. After leveling, the substrate is placed in a microwave sintering furnace under a nitrogen atmosphere, with a frequency of 2.45GHz, a power of 1500W, and a sintering time of 60 seconds. The measured peak temperature of the substrate is 195℃.

[0046] 4. Performance Testing:

[0047] Line width: 13.2μm, with neat edges and no broken grid lines;

[0048] Resistivity: 6.8 × 10⁻⁶ -6 Ω·cm;

[0049] Adhesion: 3M tape test showed no peeling;

[0050] Microscopic cross-section: SEM shows that the sintered layer is dense, with obvious sintering necks formed between particles.

[0051] Example 2

[0052] The difference from Example 1 is that the self-assembled molecule used is 11-mercaptoundecanoic acid (HS-(CH2)). 10 -COOH (n=11, X=-SH, Y=-COOH), added at a rate of 1.0%. Microwave sintering power 2000W, time 40 seconds.

[0053] Performance results:

[0054] Line width: 12.5μm;

[0055] Resistivity: 5.2 × 10⁻⁶ -6 Ω·cm;

[0056] Adhesion: Excellent;

[0057] Microscopically: It has better density, the functional end group -COOH has a stronger interaction with the silver surface, and the film layer decomposes more completely.

[0058] Example 3

[0059] The difference from Example 1 is that the self-assembled molecule used is octadecyl mercaptan (n=18, X=-SH, Y=-CH3), and the addition amount is 0.3%. The microwave sintering power is 1000W and the time is 90 seconds.

[0060] Performance results:

[0061] Line width: 12.0μm;

[0062] Resistivity: 7.5 × 10⁻⁶ -6 Ω·cm;

[0063] Adhesion: Excellent;

[0064] This indicates that longer carbon chains provide better dispersibility, but the microwave time needs to be extended appropriately to fully remove the film.

[0065] Comparative Example 1 (no surface modification, conventional hot sintering)

[0066] Using the same unmodified silver-coated copper powder as in Example 1, the same paste was prepared, and after printing, it was sintered in a hot air oven under a nitrogen atmosphere at 200°C for 30 minutes.

[0067] Performance results:

[0068] Line width: 11.8μm (slightly rough);

[0069] Resistivity: 2.5 × 10⁻⁶ -5 Ω·cm (approximately 3.7 times that of Example 1);

[0070] Adhesion: The tape test showed slight powder shedding;

[0071] Microscopically: the particles are mainly in point contact, with no obvious sintering necks observed, and some copper powder shows signs of oxidation.

[0072] Comparative Example 2 (with surface modification, conventional hot sintering)

[0073] The silver-coated copper powder modified with dodecyl mercaptan in Example 1 was sintered in a hot air oven at 200°C for 30 minutes.

[0074] Performance results:

[0075] Line width: 11.5μm;

[0076] Resistivity: 1.8 × 10⁻⁶ -5 Ω·cm;

[0077] Adhesion: Good;

[0078] Microscopically: The density is better than that of Comparative Example 1, but still lower than that of the microwave sintering example, indicating that although thermal sintering can remove the film layer, it cannot achieve instantaneous neck melting.

[0079] Comparative Example 3 (no surface modification, microwave sintering)

[0080] Unmodified silver-coated copper powder was used under the same microwave conditions (1500W, 60 seconds).

[0081] Performance results:

[0082] Line width: 11.0μm;

[0083] Resistivity: 1.2 × 10⁻⁶ -5 Ω·cm;

[0084] Adhesion: Average;

[0085] Microscopically: Some areas are densified, but the silver-coated copper powder has low microwave absorption efficiency, and there are unsintered areas between particles.

[0086] Comparative Example 4 (Surface modification but improper sintering parameters)

[0087] The modified powder was prepared using Example 1, but the microwave power was too high (3000W, 60 seconds).

[0088] Performance results:

[0089] Line width: 10.8μm (local burn marks were observed);

[0090] Resistivity: 9.5 × 10⁻⁶ -6 Ω·cm (slightly higher than in Example 1);

[0091] Adhesion: Excellent, but film brittleness increases;

[0092] Note: Excessive power can lead to localized overheating, causing the organic film to decompose too quickly and produce residual gas, which affects its density.

[0093] The above results show that the present invention has the following advantages over the prior art:

[0094] 1. Reduce sintering temperature and shorten time: Through the interfacial polarization absorption effect of the self-assembled monolayer, the traditional hot sintering time of 200℃ / 30min is shortened to microwave 1500W / 60s, and the substrate temperature is controlled within 200℃, which is suitable for flexible substrates such as PET.

[0095] 2. Improved conductivity: The resistivity of Example 2 reaches 5.2 × 10⁻ 6 Ω·cm, which is about 80% lower than Comparative Example 1 (unmodified + hot sintering) and about 57% lower than Comparative Example 3 (unmodified + microwave).

[0096] 3. Improved printability for narrow linewidths: The self-assembled film on the surface also acts as a dispersant, improving the permeability and leveling of the paste on the stencil. Printed edges with 8μm linewidths are neat, without broken grids or satellite dots.

[0097] 4. Suppressing copper oxidation: The synergistic effect of microwave rapid sintering and in-situ reduction of the film layer avoids copper core oxidation caused by prolonged heating, resulting in a dense microstructure.

[0098] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A copper-plated powder for use in narrow-line-width stencil printing, characterized in that: The silver-coated copper powder has a core-shell structure, including a copper core and a silver shell layer covering the surface of the copper core. A self-assembled monomolecular film is formed on the surface of the silver shell layer through chemical adsorption. The self-assembled monomolecular film is composed of organic molecules with the general formula X-(CH2)nY, where X is an anchoring group selected from at least one of mercapto, thiol, or dopamine groups, n is an integer from 8 to 18, and Y is a functional end group selected from at least one of methyl, amino, or carboxyl groups.

2. The copper-plated powder for use in narrow-line-width stencil printing according to claim 1, characterized in that: The copper core has a particle size D50 of 1.5-2.0 μm, the silver shell has a thickness of 20-100 nm, and the silver mass fraction is 15-25%.

3. The copper-plated powder for use in narrow-line-width stencil printing according to claim 1, characterized in that: The carbon chain length n of the self-assembled monolayer is 10-12, the functional end group Y is methyl or carboxyl, and the coating amount of the self-assembled monolayer is 0.1%-2.0% of the mass of the silver-coated copper powder.

4. The copper-plated powder for use in narrow-line-width stencil printing according to claim 1, characterized in that: The coating amount of the self-assembled monolayer is 0.5%-1.0% of the mass of the silver-coated copper powder.

5. A method for preparing silver-coated copper powder for use in narrow-linewidth stencil printing, used to prepare any one of the silver-coated copper powders for use in narrow-linewidth stencil printing as described in claims 1-4, characterized in that, Includes the following steps: Step 1: Provide silver-coated copper powder, wherein the silver-coated copper powder has a core-shell structure, including a copper core and a silver shell layer covering the surface of the copper core; Step 2: Disperse the silver-coated copper powder in an organic solvent, add an organic molecule with the general formula X-(CH2)nY, wherein X is at least one of mercapto, thiol or dopamine group, n is an integer from 8 to 18, and Y is at least one of methyl, amino or carboxyl group, and stir the reaction at room temperature to 60°C for 0.5 to 4 hours to allow the organic molecule to self-assemble on the surface of the silver shell through anchoring groups; Step 3: Filter, wash and dry the product after the reaction to obtain surface-modified silver-coated copper powder.

6. The method for preparing silver-coated copper powder for use in narrow-linewidth stencil printing according to claim 5, characterized in that: The amount of organic molecule added is 0.1%-2.0% of the mass of the silver-coated copper powder, and the organic solvent is selected from at least one of ethanol, isopropanol or acetone.

7. A microwave sintering method for narrow linewidth printing, using any one of the silver-coated copper powders as described in claims 1-4 for use with narrow linewidth stencil printing, characterized in that, Includes the following steps: S1, take silver-coated copper powder, mix it with organic carrier and additives to prepare a conductive paste; S2, The conductive paste is printed onto the substrate using a stencil to form fine lines with a line width of 8-14μm; S3. The printed substrate is placed in a microwave sintering equipment for sintering. The sintering atmosphere is an inert gas, the microwave frequency is 2.45GHz or 5.8GHz, the output power is 500W-3000W, the sintering time is 10-120 seconds, and the substrate temperature is controlled at 150℃-220℃ during the sintering process.

8. The microwave sintering method for narrow linewidth printing according to claim 7, characterized in that: During the microwave sintering process, the self-assembled monolayer generates interfacial polarization in the microwave field, and the heat is preferentially concentrated at the particle contact neck of the silver-coated copper powder. At the same time, the self-assembled monolayer decomposes and reduces trace oxides on the surface of the silver shell and copper core in situ, forming a metallurgical bond.

9. The microwave sintering method for narrow linewidth printing according to claim 7, characterized in that: The inert gas is nitrogen or argon, and the substrate is polyethylene terephthalate or polyimide flexible substrate, or silicon wafer substrate.

10. The microwave sintering method for narrow linewidth printing according to claim 7, characterized in that: The microwave sintering output power is 1500W-2500W, the sintering time is 30-90 seconds, and the substrate temperature is controlled at 180-210℃ during the sintering process; and the flow rate of the inert gas is 5-15L / min to maintain the oxygen content below 50ppm during the sintering process.