A low-temperature conductive paste suitable for 3D printing, its preparation method and its application

Abstract

This invention relates to the field of conductive pastes, specifically a low-temperature conductive paste suitable for 3D printing, its preparation method, and its applications. The low-temperature conductive paste comprises silver nanoparticles, a diluent, and a binder; wherein the surface of the silver nanoparticles is coated with ligands; the diluent is an organic alcohol; and the binder is an alcohol-soluble resin. This invention aims to overcome the problems of easy agglomeration of silver nanoparticles and poor dispersibility during long-term storage in existing conductive pastes, leading to easy clogging of the 3D printing nozzle, poor conductivity after sintering, and poor adhesion. The low-temperature conductive paste prepared by this invention not only has good dispersibility and stability but also exhibits good density and conductivity after sintering, enabling high-precision direct-write 3D printing of 10 μm and below.

Description

Technical Field

[0001] This invention relates to the field of conductive pastes, and more particularly to a low-temperature conductive paste suitable for 3D printing, its preparation method, and its application. Background Technology

[0002] Precision 3D printing technology is a cutting-edge advanced manufacturing technology worldwide. Through printing control in different dimensions, it can realize the preparation of complex micro- and nano-scale three-dimensional structures. It has attracted widespread attention in fields such as precision optics, micro- and nano-chips, and printed electronics, and has great industrial potential.

[0003] This technology, applied in the microelectronics industry, places high demands on the performance of conductive pastes. The pastes used must not only ensure smooth material flow from fine nozzles but also possess excellent shape retention and conductivity. Currently, the market primarily uses conductive metal pastes prepared from nano-silver particles. These pastes generally exhibit high conductivity and have attracted attention in the field of 3D integrated electronics.

[0004] Existing methods for preparing silver nanoparticles can generally be categorized into two types: physical methods (plasma and atomization methods) and chemical methods (thermal decomposition of silver nitrate and liquid-phase reduction methods). Among these, the liquid-phase reduction method is currently the most prevalent method for preparing silver nanoparticles. In the liquid-phase reduction method, silver salts (such as silver nitrate) are dissolved in water, a chemical reducing agent is added, and silver nanoparticles of the appropriate size are prepared. These nanoparticles are then purified by centrifugation.

[0005] For example, patent CN110842191B discloses a method for preparing spherical silver powder for photovoltaic positive and negative electrodes. This method uses liquid-phase reduction to prepare spherical silver powder with advantages such as high dispersibility, good sphericity, high tap density, and narrow particle size distribution. However, the subsequent slurry preparation in this patent involves treating the surface with a surface treatment agent such as oleic acid and then dispersing it in a corresponding oil-soluble diluent. This process can lead to the silver nanoparticles easily agglomerating in the diluent due to incomplete surface treatment or lack of ligands.

[0006] Patent CN113593750A discloses a method for preparing a water-soluble nano-metal slurry. This method improves the adhesion of the slurry to different substrate surfaces by introducing a water-soluble resin. However, in actual use, we find that water, as a diluent for the slurry, is prone to agglomeration due to its high volatility. Summary of the Invention

[0007] The present invention aims to overcome the defects of existing conductive pastes as described above, and therefore provides a low-temperature conductive paste suitable for 3D printing, its preparation method and its application.

[0008] To achieve the above-mentioned objectives, the present invention is implemented through the following technical solution:

[0009] In a first aspect, the present invention provides a low-temperature conductive paste suitable for 3D printing.

[0010] Includes silver nanoparticles, diluents, and binders; among which,

[0011] The surface of the silver nanoparticles is coated with ligands;

[0012] The diluent is an organic alcohol;

[0013] The adhesive is an alcohol-soluble resin.

[0014] The conductive paste of this invention contains silver nanoparticles coated with ligands, which prevents agglomeration of the silver nanoparticles due to physical or electrostatic adsorption, thus ensuring good dispersibility and stability. Therefore, the conductive paste of this invention is suitable for high-precision direct-write 3D printing processes of 10 μm and below, and will not experience clogging issues during prolonged operation.

[0015] In addition, existing methods for preparing silver nanoparticles are mostly aqueous synthesis, and the surface ligands of the resulting silver particles are mostly aqueous ligands. Therefore, water is usually used as a diluent for the slurry after it is prepared. Due to the high volatility of water, it is prone to agglomeration during long-term use.

[0016] The diluent selected in this invention is an organic alcohol, which ensures good dispersibility of the silver nanoparticles synthesized in the aqueous phase while also exhibiting good dispersibility with the corresponding alcohol-soluble resin, thus achieving the preparation of a highly dispersible silver paste. The slurry obtained from aqueous phase synthesis primarily uses water as its solvent. Due to its volatility, the slurry is prone to agglomeration during long-term storage. Replacing water with a non-volatile alcohol-soluble solvent as the diluent allows for uniform dispersion of the silver nanoparticles and improves their storage stability.

[0017] Preferably, the silver nanoparticles are prepared as follows:

[0018] (1) Add the silver precursor to a solution containing the ligand to form a mixture;

[0019] (2) Add a reducing agent to the mixture to reduce the silver precursor to obtain silver nanoparticles;

[0020] (3) The obtained silver nanoparticles were washed with a mixed solvent;

[0021] The reducing agent mentioned in step (2) is an alcohol amine with the general formula HO-X-NH2, where X is an alkane chain with 2 ≤ carbon number ≤ 6.

[0022] In existing technologies, polyol amines such as diethanolamine are commonly used as reducing agents in the preparation of silver nanoparticles. However, the applicant discovered during the preparation process that using polyol amines as reducing agents leads to an increase in the degree of cross-linking between free organic ligands in the slurry, making it difficult to purify them completely in subsequent purification processes. As a result, existing silver nanoparticles can only achieve relatively good conductivity at higher sintering temperatures (such as 300 °C). At the same time, due to the vaporization and decomposition of free organic ligands during sintering, the porosity of the sintered silver wires increases significantly, leading to a decrease in electrical conductivity.

[0023] Therefore, the reducing agent used in this invention is an alcoholamine with the general formula HO-X-NH2, which ensures that no cross-linked ligand byproducts are generated during the synthesis of silver nanoparticles, thereby achieving the purpose of low-temperature sintering. Furthermore, the use of this reducing agent in this invention also ensures effective removal of excess free organic ligands during subsequent purification.

[0024] Preferably, in step (3):

[0025] The mixed solvent consists of a good solvent with strong solubility for the ligand and a less good solvent with weak solubility for the ligand.

[0026] The good solvent contains hydroxyl or carbonyl groups;

[0027] The suboptimal solvent contains ester groups.

[0028] In existing technologies, silver nanoparticles are typically used directly after centrifugation after preparation. However, this method still leaves behind a significant amount of reducing agent and unbound ligands. These impurities significantly affect the conductivity and adhesion after sintering. Therefore, some literature describes multiple purification processes using highly polar solvents (such as water and ethanol) after silver nanoparticle preparation. While this technique removes unbound free ligands, it also damages the ligands bound to the nanoparticle surface. This results in the loss of ligands on the nanoparticle surface during purification and subsequent processing, exposing some silver atoms or ions. Consequently, aggregation due to physical or electrostatic adsorption easily occurs between particles, affecting the uniformity of particle dispersion in the slurry and causing needle clogging during 3D printing, making it unsuitable for 3D printing processes.

[0029] Therefore, this invention optimizes the purification process of silver nanoparticles by first selecting a combination of good and slightly inferior solvents to appropriately reduce the polarity of the purification solvent. This allows for the effective removal of free ligands while retaining sufficient organic ligands on the surface of the silver nanoparticles, ensuring the stability of the slurry during storage and the surface density after sintering, thereby improving the overall stability of the slurry and the conductivity after sintering.

[0030] Preferably, the good solvent is any one or a combination of methanol, ethanol, and acetone.

[0031] The suboptimal solvent is any one or a combination of ethyl acetate, propyl acetate, and butyl acetate.

[0032] In this invention, organic alcohols are selected as diluents.

[0033] Preferably, the diluent is an organic alcohol with a carbon number / hydroxyl number ≤ 4 and a hydroxyl number ≥ 2.

[0034] It ensures good dispersibility of silver nanoparticles synthesized in the aqueous phase, while also exhibiting good dispersibility with corresponding alcohol-soluble resins, thus achieving the preparation of highly dispersible silver paste. Furthermore, it effectively reduces its volatility during use, further minimizing the possibility of agglomeration during long-term use.

[0035] Preferably, the diluent includes any one or more combinations of ethylene glycol, glycerol, 1,4-butanediol, isopentyl glycol, and trimethylolpropane.

[0036] Preferably, the alcohol-soluble resin includes any one or a combination of alcohol-soluble epoxy resin, alcohol-soluble acrylic resin, and alcohol-soluble polyurethane.

[0037] Preferably, the ligand includes any one or more combinations of PAA, PVP, Triton, and PEG.

[0038] Preferably, the solid content of the low-temperature conductive paste is 50-90%;

[0039] The low-temperature conductive paste, after sintering at any temperature between 150 and 200°C, exhibits a conductivity ≥ 6.3 * 10⁻⁶. 6 S / m;

[0040] The adhesion performance after sintering is 4B or higher, according to the test results of ASTM D3359-2017 test standard.

[0041] In a second aspect, the present invention also provides a method for preparing the conductive paste, comprising the following steps:

[0042] (S.1) Preparation of silver nanoparticles;

[0043] (S.2) The silver nanoparticles are mixed with a diluent and a binder to obtain the conductive paste.

[0044] A third aspect of the present invention also provides the application of the conductive paste described above in high-precision direct-write 3D printing of 10μm and below, precision optics, micro-nano chips, photovoltaics, or printed electronics.

[0045] Therefore, the present invention has the following beneficial effects:

[0046] (1) This invention uses low-boiling-point alkanolamine as a reducing agent to obtain low-temperature sintered nano-silver paste. At any temperature of 150~200℃, the conductivity after sintering is ≥ 6.3*10. 6 S / m, with good density after sintering;

[0047] (2) At the same time, after the silver nanoparticles are reduced, a purification method combining good solvent and sub-good solvent is used to fully wash away the free ligands, retain the ligands on the surface of the silver nanoparticles, improve the dispersibility and sintering performance of the silver nanoparticles, and facilitate the subsequent miscibility with alcohol-soluble resin, improve the adhesion of the slurry after sintering. The silver paste with added alcohol-soluble diluent and alcohol-soluble resin has significantly improved adhesion after sintering at 200℃. According to the cross-cut test of the ASTM D3359-2017 test standard, its adhesion is 4B and above.

[0048] (3) The stability of the slurry during long-term storage is significantly improved. After being stored at room temperature for more than 3 months, the slurry can still be stably discharged from a 10 μm diameter needle for more than 4 hours.

[0049] (4) After being stored at room temperature for more than 3 months, the conductivity of the low-temperature conductive slurry after sintering changes by ≤10%. Attached Figure Description

[0050] Figure 1 This is a SEM image of the silver nanoparticles (A1) prepared in Example 1.

[0051] Figure 2 This is a SEM image of the low-temperature conductive paste (B1) prepared in Example 1 after sintering.

[0052] Figure 3 This is a SEM image of the silver nanoparticles (A2) prepared in Example 2.

[0053] Figure 4 This is a SEM image of the low-temperature conductive paste (B2) prepared in Example 2 after sintering.

[0054] Figure 5 This is a SEM image of the silver nanoparticles (A3) prepared in Example 3.

[0055] Figure 6 This is a SEM image of the low-temperature conductive paste (B3) prepared in Example 3 after sintering.

[0056] Figure 7 This is a SEM image of the silver nanoparticles (A4) prepared in Example 4.

[0057] Figure 8 This is a SEM image of the low-temperature conductive slurry (B4) prepared in Example 4 after sintering.

[0058] Figure 9 This is a SEM image of the silver nanoparticles (A5) prepared in Example 5.

[0059] Figure 10 This is a SEM image of the low-temperature conductive paste (B5) prepared in Example 5 after sintering.

[0060] Figure 11 This is a SEM image of the silver nanoparticles (A6) prepared in Example 6.

[0061] Figure 12 This is a SEM image of the low-temperature conductive paste (B6) prepared in Example 6 after sintering.

[0062] Figure 13 This is a SEM image of the silver nanoparticles (A7) prepared in Example 7.

[0063] Figure 14 This is a SEM image of the low-temperature conductive paste (B7) prepared in Example 7 after sintering.

[0064] Figure 15 This is a SEM image of the silver nanoparticles (A8) prepared in Example 8.

[0065] Figure 16 This is a SEM image of the low-temperature conductive paste (B8) prepared in Example 8 after sintering.

[0066] Figure 17 This is a SEM image of the silver nanoparticles (A9) prepared in Comparative Example 1.

[0067] Figure 18 This is a SEM image of the low-temperature conductive paste (B9) prepared in Comparative Example 1 after sintering.

[0068] Figure 19This is a SEM image of the silver nanoparticles (A10) prepared in Comparative Example 2.

[0069] Figure 20 This is a SEM image of the low-temperature conductive paste (B10) prepared in Comparative Example 2 after sintering.

[0070] Figure 21 This is a SEM image of the silver nanoparticles (A11) prepared in Comparative Example 3.

[0071] Figure 22 This is a SEM image of the low-temperature conductive paste (B11) prepared in Comparative Example 3 after sintering. Detailed Implementation

[0072] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0073] Preparation of silver paste

[0074] Example 1

[0075] Preparation of silver paste

[0076] (1) Take 20g AgNO3 (118mmol), dissolve it in 20g deionized water to prepare silver nitrate solution, add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence, turn on the stir, add the silver nitrate solution dropwise to the conical flask, and finally pour 18g (295mmol) ethanolamine into the conical flask at a relatively fast rate, and stir at 300r / min for 2h;

[0077] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0078] (3) Cool to room temperature, add 100 ml of ethyl acetate, then add 300 ml of ethanol in portions, stir for 10 min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50 ml centrifuge tube, and centrifuge at 9000 rpm for 20 min.

[0079] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add 10 mL of ethyl acetate, then add ethanol to 50 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A1), the SEM image of which is shown below. Figure 1 As shown, the particle size is <400nm.

[0080] [Preparation of Low-Temperature Conductive Paste]

[0081] The silver paste obtained above was treated with 5% ethylene glycol and then mixed with epoxy-ethylene glycol (9:1) (epoxy resin content 3wt%) to obtain a low-temperature conductive paste (B1) with a solid content of 87%. The SEM image of the silver film obtained after sintering is shown below. Figure 2 As shown.

[0082] Example 2

[0083] (1) Take 20g AgNO3 (118mmol), dissolve it in 20g deionized water to prepare silver nitrate solution, add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence, turn on the stir, add the silver nitrate solution dropwise to the conical flask, and finally pour 22g (295mmol) propanolamine into the conical flask at a relatively fast rate, stir at 300r / min for 2h;

[0084] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0085] (3) Cool to room temperature, add 100 ml of ethyl acetate, then add 300 ml of ethanol in portions, stir for 10 min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50 ml centrifuge tube, and centrifuge at 9000 rpm for 20 min.

[0086] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add 10 mL of ethyl acetate, then add ethanol to 50 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A2), the SEM image of which is shown below. Figure 3 As shown, the particle size is <400nm.

[0087] [Preparation of Low-Temperature Conductive Paste]

[0088] The silver paste obtained above was treated with 5% ethylene glycol and then mixed with epoxy-ethylene glycol (9:1) (epoxy resin content 3wt%) to obtain a low-temperature conductive paste (B2) with a solid content of 84%. The SEM image of the silver film obtained after sintering is shown below. Figure 4 As shown.

[0089] Example 3

[0090] (1) Take 20g AgNO3 (118mmol), dissolve it in 20g deionized water to prepare silver nitrate solution, add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence, turn on the stir, add the silver nitrate solution dropwise to the conical flask, and finally pour 26g (295mmol) butanolamine into the conical flask at a relatively fast rate, stir at 300r / min for 2h;

[0091] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0092] (3) Cool to room temperature, add 100 ml of ethyl acetate, then add 300 ml of ethanol in portions, stir for 10 min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50 ml centrifuge tube, and centrifuge at 9000 rpm for 20 min.

[0093] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add 10 mL of ethyl acetate, then add ethanol to 50 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A3), the SEM image of which is shown below. Figure 5 As shown, the particle size is <400nm.

[0094] [Preparation of Low-Temperature Conductive Paste]

[0095] The silver paste obtained above was treated with 5% ethylene glycol and then mixed with epoxy-ethylene glycol (9:1) (epoxy resin content 3wt%) to obtain a low-temperature conductive paste (B3) with a solid content of 82%. The SEM image of the silver film obtained after sintering is shown below. Figure 6 As shown.

[0096] Example 4

[0097] Preparation of silver paste

[0098] (1) Take 20g AgNO3 (118mmol), dissolve it in 20g deionized water to prepare silver nitrate solution, add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence, turn on the stir, add the silver nitrate solution dropwise to the conical flask, and finally pour 18g (295mmol) ethanolamine into the conical flask at a relatively fast rate, and stir at 300r / min for 2h;

[0099] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0100] (3) Cool to room temperature, add 100 ml of ethyl acetate, then add 300 ml of ethanol in portions, stir for 10 min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50 ml centrifuge tube, and centrifuge at 9000 rpm for 20 min.

[0101] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add 10 mL of ethyl acetate, then add ethanol to 50 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A4), whose SEM image is shown below. Figure 7 As shown, the particle size is <400nm.

[0102] [Preparation of Low-Temperature Conductive Paste]

[0103] The silver paste obtained above was treated with 5% glycerol (solid content adjustable), then mixed with epoxy-glycerol (9:1) (epoxy resin content 3wt%) to obtain a low-temperature conductive paste (B4) with a solid content of 87%. The SEM image of the silver film obtained after sintering is shown below. Figure 8 As shown.

[0104] Example 5

[0105] Preparation of silver paste

[0106] (1) Take 20g AgNO3 (118mmol), dissolve it in 20g deionized water to prepare silver nitrate solution, add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence, turn on the stir, add the silver nitrate solution dropwise to the conical flask, and finally pour 18g (295mmol) ethanolamine into the conical flask at a relatively fast rate, and stir at 300r / min for 2h;

[0107] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0108] (3) Cool to room temperature, add 100 ml of ethyl acetate, then add 300 ml of ethanol in portions, stir for 10 min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50 ml centrifuge tube, and centrifuge at 9000 rpm for 20 min.

[0109] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add 10 mL of ethyl acetate, then add ethanol to 50 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A5), the SEM image of which is shown below. Figure 9 As shown, the particle size is <400nm.

[0110] [Preparation of Low-Temperature Conductive Paste]

[0111] The silver paste obtained above was treated with 5% trimethylolpropane (solid content adjustable), and then mixed with epoxy-trimethylolpropane (9:1) (epoxy resin content 3wt%) to obtain a low-temperature conductive paste (B5) with a solid content of 87%. The SEM image of the silver film obtained after sintering is shown below. Figure 10 As shown.

[0112] Example 6

[0113] Preparation of silver paste

[0114] (1) Take 20g AgNO3 (118mmol), dissolve it in 20g deionized water to prepare silver nitrate solution, add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence, turn on the stir, add the silver nitrate solution dropwise to the conical flask, and finally pour 18g (295mmol) ethanolamine into the conical flask at a relatively fast rate, and stir at 300r / min for 2h;

[0115] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0116] (3) Cool to room temperature, add 150 ml of butyl acetate, then add 250 ml of methanol in portions, stir for 10 min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50 ml centrifuge tube, and centrifuge at 9000 rpm for 20 min.

[0117] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add 13 mL of butyl acetate, then add methanol to 50 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A6), the SEM image of which is shown below. Figure 11 As shown, the particle size is <400nm.

[0118] [Preparation of Low-Temperature Conductive Paste]

[0119] The silver paste obtained above was treated with 5% ethylene glycol and then mixed with epoxy-ethylene glycol (9:1) (epoxy resin content 3wt%) to obtain a low-temperature conductive paste (B6) with a solid content of 87%. The SEM image of the silver film obtained after sintering is shown below. Figure 12 As shown.

[0120] Example 7

[0121] Preparation of silver paste

[0122] (1) Take 20g AgNO3 (118mmol), dissolve it in 20g deionized water to prepare silver nitrate solution, add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence, turn on the stir, add the silver nitrate solution dropwise to the conical flask, and finally pour 18g (295mmol) ethanolamine into the conical flask at a relatively fast rate, and stir at 300r / min for 2h;

[0123] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0124] (3) Cool to room temperature, add 200 ml of propyl acetate, then add 200 ml of acetone in portions, stir for 10 min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50 ml centrifuge tube, and centrifuge at 9000 rpm for 20 min.

[0125] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add 17.5 mL of propyl acetate, then add acetone to 50 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A7), the SEM image of which is shown below. Figure 13 As shown, the particle size is <400nm.

[0126] [Preparation of Low-Temperature Conductive Paste]

[0127] The silver paste obtained above was treated with 5% ethylene glycol and then mixed with epoxy-ethylene glycol (9:1) (epoxy resin content 3wt%) to obtain a low-temperature conductive paste (B7) with a solid content of 86%. The SEM image of the silver film obtained after sintering is shown below. Figure 14 As shown.

[0128] Example 8

[0129] Preparation of silver paste

[0130] (1) Take 20g AgNO3 (118mmol), dissolve it in 20g deionized water to prepare silver nitrate solution, add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence, turn on the stir, add the silver nitrate solution dropwise to the conical flask, and finally pour 18g (295mmol) ethanolamine into the conical flask at a relatively fast rate, and stir at 300r / min for 2h;

[0131] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0132] (3) Cool to room temperature, add 100 ml of ethyl acetate, then add 300 ml of ethanol in portions, stir for 10 min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50 ml centrifuge tube, and centrifuge at 9000 rpm for 20 min.

[0133] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add 10 mL of ethyl acetate, then add ethanol to 50 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A8), the SEM image of which is shown below. Figure 15 As shown, the particle size is <400nm.

[0134] [Preparation of Low-Temperature Conductive Paste]

[0135] The silver paste obtained above was treated with 40% ethylene glycol and then mixed with epoxy-ethylene glycol (9:1) (epoxy resin content 3wt%) to obtain a low-temperature conductive paste (B8) with a solid content of 54%. The SEM image of the silver film obtained after sintering is shown below. Figure 16 As shown.

[0136] Comparative Example 1

[0137] Preparation of silver paste

[0138] (1) Take 20g AgNO3 (118mmol), dissolve it in 20g deionized water to prepare silver nitrate solution, add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence, turn on the stir, add the silver nitrate solution dropwise to the conical flask, and finally pour 31g (295mmol) diethanolamine into the conical flask at a relatively fast rate, and stir at 300r / min for 2h;

[0139] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0140] (3) Cool to room temperature, add 100 ml of ethyl acetate, then add 300 ml of ethanol in portions, stir for 10 min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50 ml centrifuge tube, and centrifuge at 9000 rpm for 20 min.

[0141] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add 10 mL of ethyl acetate, then add ethanol to 50 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A9), the SEM image of which is shown below. Figure 17 As shown, the particle size is <400nm.

[0142] Preparation of conductive paste

[0143] The silver paste obtained above was treated with 5% ethylene glycol and then mixed with epoxy-ethylene glycol (9:1) (epoxy resin content 3wt%) to obtain a conductive paste (B9) with a solid content of 75%. The SEM image of the silver film obtained after sintering is shown below. Figure 18 As shown.

[0144] Comparative Example 2

[0145] Preparation of silver paste

[0146] (1) Take 20g AgNO3 (118mmol) and dissolve it in 20g deionized water to prepare silver nitrate solution. Add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence. Turn on the stirring and add the silver nitrate solution dropwise to the conical flask. Finally, pour 18g (295mmol) ethanolamine into the conical flask at a relatively fast rate and stir at 300r / min for 2h.

[0147] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0148] (3) Cool to room temperature, add 400ml of ethanol in portions, stir for 10min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50ml centrifuge tube, and centrifuge at 9000rpm for 20min.

[0149] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add ethanol to 60 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A10), the SEM image of which is shown below. Figure 19 As shown, the particle size is <400nm.

[0150] Preparation of conductive paste

[0151] The silver paste obtained above was treated with 5% ethylene glycol and then mixed with epoxy-ethylene glycol (9:1) (epoxy resin content 3wt%) to obtain a conductive paste (B10) with a solid content of 88%. The SEM image of the silver film obtained after sintering is shown below. Figure 20 As shown.

[0152] Comparative Example 3

[0153] Preparation of silver paste

[0154] (1) Take 20g AgNO3 (118mmol), dissolve it in 20g deionized water to prepare silver nitrate solution, add 50g deionized water to the conical flask, then add 3g PAA (MW 3000) and 1g PAA (MW 20,000) in sequence, turn on the stir, add the silver nitrate solution dropwise to the conical flask, and finally pour 18g (295mmol) ethanolamine into the conical flask at a relatively fast rate, and stir at 300r / min for 2h;

[0155] (2) Heat and stir on a hot plate at 120℃ for 2 hours;

[0156] (3) Cool to room temperature, add 100 ml of ethyl acetate, then add 300 ml of ethanol in portions, stir for 10 min, the product precipitates out as flocculent precipitate, remove the supernatant, transfer the precipitate to a 50 ml centrifuge tube, and centrifuge at 9000 rpm for 20 min.

[0157] (4) Remove the supernatant, add deionized water to 15 mL, shake well, filter once with a 10 μm filter, then filter again with a 1 μm filter, add 10 mL of ethyl acetate, then add ethanol to 50 mL, shake well, centrifuge at 9000 rpm for 20 min, remove the supernatant and mix well to obtain silver nanoparticles (A11), the SEM image of which is shown below. Figure 21As shown, the particle size is <400nm.

[0158] Preparation of conductive paste

[0159] The silver paste obtained above was treated with 5% ethanol and then mixed with epoxy-ethanol (9:1) (epoxy resin content 3wt%) to obtain a conductive paste (B11) with a solid content of 87%. The SEM image of the silver film obtained after sintering is shown below. Figure 22 As shown.

[0160] [Performance Testing]

[0161] (1) Particle size observation: The size and morphology of silver nanoparticles were characterized using a Phenom scanning electron microscope.

[0162] The specific method is as follows: ① Take a small amount of silver paste and dilute it in deionized water or organic solvent at a ratio of 1:1000; ② Disperse the diluted solution thoroughly, take a small amount with a dropper and add it to the surface of the carrier, and heat it to remove excess solvent; ③ After preparing the sample and spraying it with gold, send the sample for observation to obtain image information on the size and morphology of silver nanoparticles.

[0163] (2) Resistivity test: The resistivity of silver paste and low-temperature conductive paste with added resin was tested using the Ruikeweiye FT-340 four-probe sheet resistance tester.

[0164] The specific method is as follows: ① The prepared silver paste and low-temperature conductive paste are coated onto the glass substrate using a coating machine and sintered in air at 200℃ for 60 minutes to obtain a conductive silver film; ② The thickness of the sintered silver film sample is measured using a step meter; ③ The corresponding parameters are set in the sheet resistance meter, and the four probes are adjusted to be above the sample to be tested and then pressed down vertically to touch it; ④ After the readings stabilize, the sheet resistance, resistivity and other data are recorded.

[0165] (3) Adhesion test

[0166] ① The prepared low-temperature conductive paste is coated onto the glass substrate using a coating machine and sintered in air at 200°C for 60 minutes to obtain a conductive silver film;

[0167] ② Using ASTM D3359-2017 as the test standard, use a cross-cutting tool to cut the surface of the test piece at a cutting speed of 20-50 mm / s;

[0168] ③ Rotate the specimen 90° and repeat the above operation on the cut to form a grid pattern;

[0169] ④ Use a soft brush to brush the diagonals on both sides of the grid pattern, brushing lightly 5 times each way.

[0170] ⑤ Use 3M 600-1PK test tape to perform a peel test and evaluate the material's adhesion performance.

[0171] (4) Stability test

[0172] The prepared low-temperature conductive paste was sealed and stored at room temperature. The state of the paste was monitored after 90 days of storage. The time it took for the paste to stably appear at a 10 μm diameter needle was tested, and the monitoring time was 4 hours.

[0173] [Characteristics of Experimental Results]

[0174] The performance test results of the silver nanoparticles (A1) to (A11) prepared in Examples 1-8 and Comparative Examples 1-3 are shown in Table 1 below:

[0175] Table 1. Performance test results of silver nanoparticles (A1) to (A11) in Examples 1-8 and Comparative Examples 1-3.

[0176] .

[0177] The performance test results of the conductive pastes (B1) to (B11) prepared in Examples 1-8 and Comparative Examples 1-3 are shown in Table 2 below:

[0178] Table 2 Performance test results of conductive pastes (B1) to (B11) in Examples 1-8 and Comparative Examples 1-3

[0179] .

[0180] [Performance Analysis]

[0181] As can be seen from the data in Tables 1 and 2 above, the silver nanoparticles (A1) to (A8) and conductive pastes (B1) to (B8) prepared by the methods in Examples 1 to 8 of the present invention have significantly better performance than the silver nanoparticles (A9) to (A11) and conductive pastes (B9) to (B11) prepared by the methods in Comparative Examples 1 to 3.

[0182] Comparing Example 1 with Comparative Example 1, we found that the difference between Example 1 and Comparative Example 1 lies in the reducing agent used. Example 1 used ethanolamine as the reducing agent, while Comparative Example 1 used diethanolamine. Figure 1 as well as Figure 17 In the comparison, we can see that the surface of the silver nanoparticles in the silver paste synthesized in Example 1, which uses ethanolamine as a reducing agent, is smoother, while the surface of the silver nanoparticles in the silver paste synthesized in Comparative Example 1, which uses diethanolamine as a reducing agent, is rougher. This indicates that cross-linked ligands were produced as byproducts during the reduction process of diethanolamine.

[0183] SEM images of the silver films obtained by sintering conductive paste show that the silver nanoparticles obtained by reduction with ethanolamine exhibit good density after being sintered into a conductive paste (e.g., ...). Figure 2 As shown), this improves its conductivity; while the silver film obtained by sintering silver nanoparticles prepared from diethanolamine after reduction into a conductive paste has poor density (as shown). Figure 18 As shown in the figure, its resistivity is even higher.

[0184] Comparing Example 1 with Comparative Example 2, we found that the polarity of the solvent during the purification process has a significant impact on the stability and conductivity of the final conductive slurry. When a stronger solvent, ethanol, is used to clean the silver paste, the ligands attached to the surface of the silver nanoparticles are also removed to some extent, leading to a decrease in the dispersibility of the silver nanoparticles and their adhesion after sintering, and the sintered silver film is prone to brittleness. In Example 1, the combination of a polar solvent and a less suitable solvent can appropriately reduce the polarity of the purification solution. This allows for the effective removal of free ligands during the purification process while retaining sufficient organic ligands on the surface of the silver nanoparticles, ensuring the stability of the slurry during storage and its adhesion after sintering, thus improving the overall stability of the slurry and its adhesion after sintering.

[0185] Comparing Example 1 with Comparative Example 3, we found that in Comparative Example 3, volatile ethanol was used as a diluent during the preparation of the conductive paste. Due to its high volatility, the silver nanoparticles in the paste tended to agglomerate during long-term use, leading to a decrease in its storage stability. Furthermore, ethanol and silver paste separated, making film formation almost impossible and resulting in rapid drying. In contrast, Example 1 used ethylene glycol, a less volatile diluent. This ensured good dispersibility of the aqueous-synthesized silver nanoparticles while also providing good dispersibility for the corresponding alcohol-soluble resin, achieving a highly dispersible silver paste. This allowed for uniform dispersion of the silver nanoparticles and improved storage stability.

Claims

1. A low-temperature conductive paste suitable for 3D printing, characterized in that, Includes silver nanoparticles, diluents, and binders; among which, The surface of the silver nanoparticles is coated with ligands; The diluent is an organic alcohol with a carbon number / hydroxyl number ≤ 4 and a hydroxyl number ≥ 2; The adhesive is an alcohol-soluble resin; The silver nanoparticles are prepared as follows: (1) Add the silver precursor to a solution containing the ligand to form a mixture; (2) Add a reducing agent to the mixture to reduce the silver precursor to obtain silver nanoparticles; (3) The obtained silver nanoparticles were purified by using a mixed solvent; The reducing agent is an alcoholamine with the general formula HO-X-NH2, wherein X is an alkane chain with 2 ≤ carbon number ≤ 6; The mixed solvent consists of a good solvent containing hydroxyl or carbonyl groups and a less good solvent containing ester groups.

2. The low-temperature conductive paste suitable for 3D printing according to claim 1, characterized in that, The ligands include any one or more combinations of PAA, PVP, Triton, and PEG.

3. The low-temperature conductive paste suitable for 3D printing according to claim 1, characterized in that, The alcohol-soluble resin includes any one or a combination of alcohol-soluble epoxy resin, alcohol-soluble acrylic resin, and alcohol-soluble polyurethane.

4. The low-temperature conductive paste suitable for 3D printing according to claim 1, characterized in that, The solid content of the low-temperature conductive paste is 50-90%; The low-temperature conductive paste, after sintering at any temperature between 150 and 200°C, exhibits a conductivity ≥ 6.3 * 10⁻⁶. 6 S / m; The adhesion performance after sintering is 4B or higher, according to the test results of ASTM D3359-2017 test standard.

5. A low-temperature conductive paste suitable for 3D printing according to claim 1, characterized in that, After being stored at room temperature for more than 3 months, the low-temperature conductive slurry can be stably dispensed from a 10 μm diameter needle for more than 4 hours. After being stored at room temperature for more than 3 months, the conductivity of the low-temperature conductive slurry after sintering changes by ≤10%.

6. A method for preparing the conductive paste according to any one of claims 1 to 5, characterized in that, Includes the following steps: (S.1) Preparation of silver nanoparticles; (S.2) The silver nanoparticles are mixed with a diluent and a binder to obtain the conductive paste.

7. The application of the conductive paste as described in any one of claims 1 to 5 in the fields of high-precision direct-write 3D printing of 10 μm and below, precision optics, micro-nano chips, photovoltaics, or printed electronics.

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