A silver-plated copper powder with high electrical conductivity, preparation method and conductive adhesive

Abstract

The present invention relates to a silver-plated copper powder with high electrical conductivity, a preparation method thereof, and a conductive adhesive, belonging to the technical field of silver-coated copper powder. The high-electrical-conductivity dendritic silver-plated copper powder of the present invention is composed of dendritic copper powder and a silver layer coated on the surface of the dendritic copper powder; based on the mass of the dendritic silver-plated copper powder being 100%, the silver layer accounts for 3-7%; the silver layer contains S element, and the content of S element in the silver layer is 0.5-3.0 wt.%. The dendritic silver-plated copper powder of the present invention has the characteristics of low cost and high electrical conductivity. The conductive adhesive containing this silver-plated copper powder can greatly reduce the cost of silver conductive adhesive and has excellent stability at the same time.

Description

Technical Field

[0001] The invention relates to high-conductivity silver-plated copper powder, a preparation method and conductive adhesive, and belongs to the technical field of silver-coated copper powder. Background Art

[0002] As electronic components gradually develop towards miniaturization and portability, traditional solder pollutes the environment and harms human health, and can no longer meet the demand. Conductive adhesives have attracted much attention due to their environmental protection, low curing temperature, simple process, and good aging resistance. They have been widely used as adhesive materials in microelectronic components and packaging manufacturing processes. For example, the main applications of conductive adhesives in microelectronic assembly are printing and preparing fine wire circuits and packaging and bonding of electronic components such as light-emitting diodes, liquid crystal displays, and integrated circuit chips.

[0003] Conductive adhesives are mainly composed of organic resins and conductive materials. Conductive materials determine the electrical and thermal conductivity of composite materials, while organic resins determine the mechanical properties of composite materials. Currently, conductive adhesives include metal series, carbon series, and several composite types. Metal series include silver powder, copper powder, nickel powder, etc. Carbon series include graphene, multi-walled carbon nanotubes, carbon fibers, etc. Among them, they have higher electrical and thermal conductivity and are better used as fillers in conductive adhesives.

[0004] Conductive adhesive synthesized with silver powder as filler has excellent adhesion, conductivity and chemical stability, but silver material is expensive and prone to electromigration under the action of electric field, which reduces performance; copper conductive adhesive has the advantages of environmental friendliness, low cost and small connection. When the conductivity is guaranteed, it is close to the performance of silver conductive adhesive, and the price is much lower than that of silver conductive adhesive. However, copper powder has active chemical properties and is easily oxidized in the air, so the application of copper conductive adhesive has certain limitations; silver-plated copper powder, as a new conductive functional material, has higher stability than single metal copper conductive adhesive, and highlights greater cost advantages and anti-migration performance compared with pure silver conductive adhesive.

[0005] At present, silver-plated copper powder is often prepared by chemical plating, which is mainly divided into displacement method and chemical reduction method. The displacement method is to place copper powder in a silver solution, and the silver ions or silver ammonium ions in the plating solution react with copper to form a silver coating on the surface of the copper powder. This method has the problem of partial dissolution of copper powder, resulting in silver particles dotted on the surface of copper powder, and the coating is not completely dense. The chemical reduction method is to use a reducing agent to reduce the silver ions in the silver solution to silver element, and continuously deposit it on the surface of the copper powder to form a coating. This method is simple and easy to implement.

[0006] There are many preparation processes for silver-plated copper powder. For example: a preparation method of silver-coated copper powder composite powder with a zinc intermediate layer added, that is, a Cu / Zn / Ag three-layer composite powder is prepared by combining mechanical plating and chemical plating. The preparation process using this method is lengthy and will introduce the pollution of third metal ions; a preparation method of silver-coated copper powder, that is, the copper powder is subjected to N times of silver coating treatment, where N is an integer greater than or equal to 2. In each silver coating treatment, at least one of silver ammonia solution and a solution containing ethylenediaminetetraacetic acid and silver salt is used as the complex solution, and a reduction reaction is carried out under the condition of a reducing agent. There is a problem of a large amount of reaction wastewater in the case of multiple coatings; a method of silver plating on dendritic copper powder, that is, the dendritic copper powder is pickled with dilute acid, and a main salt solution and a reducing agent solution are prepared; when the amount of copper powder is less than 100 g, the copper powder is placed in the reducing agent solution, and the main salt is added according to the segmented addition process of "first fast and then slow"; when the amount of copper powder is not less than 100 g, the reducing agent solution is added to the copper powder in segments, and the main salt is added dropwise according to the segmented addition process of "first faster and then slower". The remaining reducing agent solution is added step by step and evenly every 8 - 18 min, and the main salt solution is added dropwise according to the segmented addition process of "first faster and then slower" to complete the silver plating on the surface of the copper powder, and the silver content is between 17% and 30%. In order to obtain better conductivity, the silver content of this method is relatively high; silver-coated copper powder, the silver-coated copper powder particles are formed by coating the surface of copper powder particles with silver. It contains silver-coated copper powder particles in a dendritic shape. When observing these copper powder particles with a scanning electron microscope (SEM), it has a main axis, and more than 2 branches are obliquely separated from the main axis, thus showing a dendritic shape growing in two or three dimensions. The thickness a of the main axis is 0.3 μm - 5.0 μm, and the length b of the longest branch extending from the main axis is 0.6 μm - 10.0 μm. The dendritic silver-plated copper powder has underdeveloped branches, and when applied to conductive adhesives, the proportion of conductive phases in the unit space is relatively low.

[0007] As can be seen from the above, in the preparation methods of silver-plated copper powder, the oxidation effect during the copper powder pretreatment process has not been concerned, especially for dendritic copper powder with a high specific surface area. Summary of the Invention

[0008] With the development of electronic products towards "light, thin, short and small", in the fields of conductive pastes and conductive adhesives, finer patterns and smaller thicknesses are sought, and conductive materials used in these fields are required to have smaller particle sizes. However, due to silver-plated copper powder, especially when the particle size of dendritic silver-plated copper powder is small, the specific surface area increases significantly, and the copper powder is easily oxidized during the pretreatment process, and the phenomenon of missing plating is likely to occur. Therefore, it is necessary to increase the silver content per unit specific surface area, otherwise there will be a problem that it is difficult to improve the conductivity of silver-plated copper powder. This application proposes a silver-plated copper powder with high conductivity, a preparation method and a conductive adhesive, that is, using ammonium sulfate-ethylenediamine solution as a pretreatment solution to convert solid copper oxide and cuprous oxide on the surface of dendritic copper powder into soluble complexes of copper (II) diethylenediamine monosulfate and copper (I) diethylenediamine monosulfate, and using a sulfur-containing compound to prevent the oxidation of dendritic copper powder and improve the dispersion of dendritic copper powder, so as to increase the number of contact points between the prepared silver-plated copper particles, thereby establishing good electrical connection and improving the conductivity.

[0009] A dendritic silver-plated copper powder with high conductivity, which consists of dendritic copper powder and a silver layer coated on the surface of the dendritic copper powder; based on the mass of the dendritic silver-plated copper powder being 100%, the silver layer accounts for 3-7%; the silver layer contains S element, and the content of S element in the silver layer is 0.5-3.0 wt.%.

[0010] The average particle size D50 of the dendritic silver-plated copper powder is 4.0-10.0 μm, and the BET specific surface area is 15000-25000 cm 2 / g, and there are more than 20 branches on the main axis crystal.

[0011] The BET specific surface area of the silver-plated copper powder is S1, and the BET specific surface area of the dendritic copper powder is S2, and S1 / S2 = 1.02-1.05.

[0012] Furthermore, the surface of the silver-plated copper powder is coated with 0.1-0.3 wt% of alkyl mercaptan, and the number of carbon atoms in the alkyl mercaptan is greater than 8.

[0013] The preparation method of the dendritic silver-plated copper powder with high conductivity includes the following specific steps:

[0014] (1) Add dendritic copper powder into ammonium sulfate-ethylenediamine solution, stir for 30-60 min, add sodium mercaptobenzothiazole and continue to stir and react for 10-30 min, separate the solid and liquid, wash with deionized water until the pH is neutral, and vacuum dry to obtain pretreated dendritic copper powder;

[0015] (2) Disperse the dendritic copper powder into deionized water to obtain a copper powder dispersion. Adjust the pH value to 10 - 11 with sodium hydroxide. Dropwise add the silver plating solution containing a reducing agent into the copper powder dispersion. At a temperature of 25 - 50 °C, stir and react for 30 - 60 min. Perform solid-liquid separation, wash with ethanol to obtain dendritic wet silver-plated copper powder, and then vacuum dry to obtain high-conductivity dendritic silver-plated copper powder.

[0016] Furthermore, the preparation method also includes transferring the dendritic wet silver-plated copper powder to a high-speed mixer. Under the condition of stirring at room temperature, spray alkyl mercaptan onto the wet silver-plated copper powder, stir for 15 - 45 min, and then vacuum dry to obtain high-conductivity dendritic silver-plated copper powder.

[0017] The dendritic copper powder can be prepared by an electrolysis method, a mechanical pulverization method, or a chemical reduction method.

[0018] In the ammonium sulfate-ethylenediamine solution in step (1), the concentration of ammonium sulfate is 1.0 - 5.0 mol / L, and the concentration of ethylenediamine is 0.2 - 0.5 mol / L.

[0019] In step (2), the addition amount of sodium mercaptobenzothiazole is 0.5 - 1.0% of the mass of the copper powder.

[0020] In step (2), the reducing agent is glucose and / or trisodium citrate. The molar ratio of the reducing agent to silver nitrate is 0.8 - 1.2:1, and the concentration of the reducing agent is 0.2 - 0.5 mol / L.

[0021] A conductive adhesive contains the high-conductivity dendritic silver-plated copper powder.

[0022] Adsorb a surface treatment agent with a mercapto group onto the surface of the copper particles, and form a silver layer containing S on the surface of the copper particles by electroless plating. The presence of a certain amount of S element in the silver layer of the dendritic silver-plated copper powder can improve its conductivity, while adding a certain amount of S element to the spherical silver-plated copper powder cannot improve its conductivity. Since the dispersibility of the dendritic copper powder is improved after adsorbing a compound containing S element on its surface, the number of contact points between the prepared silver-plated copper particles increases, thus establishing a good electrical connection and enhancing the conductivity. However, the spherical silver-plated copper particles have good self-dispersibility, and the increase in contact points is not obvious using the same treatment process, so its conductivity cannot be improved.

[0023] The beneficial effects of the present invention are:

[0024] (1) The present invention uses ammonium sulfate - ethylenediamine solution as a pretreatment solution to convert the solid copper oxide and cuprous oxide on the surface of dendritic copper powder into soluble complexes of copper(II) diethylenediamine monosulfate and copper(I) diethylenediamine monosulfate. A sulfur - containing compound is used to prevent the oxidation of dendritic copper powder and improve its dispersibility, increasing the number of contact points between the prepared silver - plated copper particles, thereby establishing good electrical connection and enhancing the conductivity.

[0025] (2) The present invention sprays alkyl mercaptan. The mercapto group in alkyl mercaptan easily bonds with silver and copper to modify the silver - plated copper powder. The surface alkyl carbon chains will entangle to form an organic coating layer to inhibit the migration of oxygen into the interior of the particles and suppress the oxidation of silver - plated copper. However, when the number of carbon atoms in the alkyl mercaptan is less than 8, the carbon chain is short and the formed organic coating layer has poor antioxidant performance.

[0026] (3) The dendritic silver - plated copper powder of the present invention has the characteristics of low cost and high conductivity. The conductive adhesive containing this silver - plated copper powder can greatly reduce the cost of silver conductive adhesive and has excellent stability at the same time.

[0027] (4) The process of the dendritic silver - plated copper powder of the present invention is simple and easy to operate. The prepared conductive adhesive can be widely used in the production of special electronic information materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is the SEM image of the dendritic silver - plated copper powder in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0029] The following further elaborates on the present invention in detail in conjunction with the specific embodiments, but the protection scope of the present invention is not limited to the content described.

[0030] The high - conductivity dendritic silver - plated copper powder is composed of dendritic copper powder and a silver layer coated on the surface of the dendritic copper powder. Calculated based on the mass of the dendritic silver - plated copper powder being 100%, the silver layer accounts for 3 - 7%. The silver layer contains S element, and the content of S element in the silver layer is 0.5 - 3.0 wt.%. The fact that the silver layer of the dendritic silver - plated copper powder contains a certain amount of S element can improve its conductivity, while adding a certain amount of S element to spherical silver - plated copper powder cannot improve its conductivity. This is mainly because after the dendritic copper powder surface adsorbs the sulfur - containing compound, its dispersibility is improved, and the number of contact points between the prepared silver - plated copper particles increases, thereby establishing good electrical connection and enhancing the conductivity. While the spherical silver - plated copper particles have good self - dispersibility, and the increase in contact points is not obvious using the same treatment process, so its conductivity cannot be improved.

[0031] The S element in the silver layer of the silver - plated copper powder is preferably dispersed in the silver layer. By adsorbing a surface treatment agent with a mercapto group on the surface of copper particles and forming a sulfur - containing silver alloy layer on the surface of copper particles through electroless plating.

[0032] The content of S element in the silver alloy of silver-coated copper powder is preferably 0.5-3.0%. All of the total S element exists in the silver layer. When the above S element content exists in the silver layer, the conductivity of dendritic silver-coated copper powder will increase without increasing the silver content.

[0033] The average particle size (D50) of dendritic copper powder obtained by laser diffraction scattering particle size distribution is 4.0-10.0 μm, preferably 6.0-7.0 μm. When the average particle size (D50) is less than 4.0 μm, the refinement of copper powder particles is likely to cause surface oxidation, which is not preferred. When the average particle size (D50) exceeds 10.0 μm, the silver-coated copper powder obtained cannot meet the requirements of finer patterns and smaller thickness when applied to conductive adhesives.

[0034] The BET specific surface area of dendritic copper powder is 15000-25000 cm 2 / g, preferably 18000-20000 cm 2 / g. When the BET specific surface area of copper powder is greater than 25000 cm 2 / g, the specific surface area of copper powder particles is too large, which is likely to cause surface oxidation.

[0035] Dendritic copper powder with more than 20 branches on the main axis crystal of dendritic copper powder. This is to obtain silver-coated copper powder with a developed branch system, a large number of contact points between particles, and a relatively high proportion of conductive phase in the unit space when applied to conductive adhesives, resulting in high conductivity of the conductive adhesive.

[0036] The BET specific surface area of silver-coated copper powder is S1, and the BET specific surface area of dendritic copper powder is S2. S1 / S2 = 1.02-1.05, preferably S1 / S2 = 1.02-1.03. When S1 / S2 > 1.05, the deposition compactness of the silver layer of silver-coated copper powder is poor, the porosity is relatively high, and the particle oxidation resistance is poor, which cannot meet the stability requirements when applied to conductive adhesives.

[0037] For dendritic silver-coated copper powder, the surface coating content of alkyl mercaptan is 0.1-0.3%. The number of carbon atoms in the alkyl mercaptan is greater than 8, preferably the number of carbon atoms in the alkyl mercaptan is greater than 12. The mercapto group in the alkyl mercaptan is easy to bond with silver and copper. As a surface modifier, the alkyl carbon chains on the surface of silver-coated copper will entangle to form an organic coating layer to inhibit the migration of oxygen into the particles and inhibit the oxidation of silver-coated copper. When the number of carbon atoms in the alkyl mercaptan is less than 8, the formed organic coating layer has poor oxidation resistance.

[0038] The preparation method of dendritic silver-coated copper powder is to perform a pretreatment process of removing oxides and antioxidant treatment on copper powder, then place the pretreated copper powder in an alkaline dispersion liquid, and slowly add a silver plating solution prepared by mixing a reducing agent and silver nitrate dropwise to the copper powder dispersion liquid for electroless silver plating to prepare silver-coated copper powder.

[0039] Due to its large surface area, dendritic copper powder is easily oxidized in air. A complex solution of ammonium sulfate and ethylenediamine is used to convert the solid copper oxide and cuprous oxide on the surface of dendritic copper powder into soluble complexes of copper(II) diethylenediamine monoammonium sulfate and copper(I) diethylenediamine monoammonium sulfate. Then, sodium mercaptobenzothiazole containing a mercapto group in water-soluble form is used to treat the surface of copper powder to inhibit its oxidation. The concentration of ammonium sulfate is preferably 2.0 - 3.0 mol / L, and the concentration of ethylenediamine is preferably 0.3 - 0.5 mol / L. When the concentration of the above complex solution is too low, the effect of removing oxides is poor, which affects the subsequent silver plating effect. The amount of sodium mercaptobenzothiazole is preferably 0.5 - 0.8% of the mass of copper powder. When the amount of sodium mercaptobenzothiazole is less than 0.5%, the antioxidant treatment effect of copper powder is not good, while when the amount is greater than 1.0%, too much sodium mercaptobenzothiazole is adsorbed on the surface of copper powder, inhibiting the subsequent silver deposition effect. At the same time, in order to inhibit the oxidation of copper powder during drying, the treated copper powder needs to be washed to neutral and dried in a vacuum drying oven for standby;

[0040] Before electroless silver plating, copper powder and deionized water are mechanically stirred to form a copper powder dispersion. To ensure good suspension of copper powder in the solution, through zeta potential analysis, the pH value of the dispersion is preferably 10 - 11. When the pH is not within this range, copper powder is easily affected by charges to form aggregates, which affects silver plating. The reducing agent and silver nitrate are mixed in a certain proportion and then simultaneously dropped into the copper powder dispersion. By this method, the reduction rate of silver can be effectively controlled to obtain a dense silver layer. At the same time, the reducing agent and silver nitrate are controlled by pH and will not react immediately after being mixed in a certain proportion. The reaction temperature is preferably 30 - 40°C. When the temperature is lower than 25°C, the silver deposition rate is slow and the production efficiency is low. When the temperature is higher than 50°C, the silver reduction rate is fast, which is likely to cause the generation of free silver particles and affect the silver coating. An alkylthiol methanol solution is used to perform self-assembly surface modification on the silver-plated copper powder to improve the thermal stability of the silver-plated copper powder. At the same time, methanol forms an azeotrope with the water on the surface of the wet silver-plated copper powder, accelerating the drying of the silver-plated copper powder. At the same time, in order to inhibit the oxidation of copper powder during drying, the silver-plated copper powder needs to be placed in a vacuum drying oven for drying;

[0041] The silver plating solution prepared by mixing the reducing agent and silver nitrate: The reducing agent is one or more of glucose or trisodium citrate. The molar ratio of the reducing agent to silver nitrate is 0.8 - 1.2:1, and the concentration of the reducing agent is 0.2 - 0.5 mol / L. Controlling the molar ratio of the reducing agent to silver nitrate and the concentration of the reducing agent can effectively control the reduction rate and the stability of the mixed solution;

[0042] Using dendritic silver-plated copper powder to prepare conductive adhesive: The conductive adhesive includes the dendritic silver-plated copper powder of the present invention and a solvent, and can be formulated with components such as binder resin, curing agent, dispersant, defoaming agent, leveling agent, etc. according to needs. In addition to the silver-plated copper powder of the present invention, metal powders such as alloys, silver, copper, platinum, or any conductive filler such as carbon can also be used;

[0043] As the adhesive resin, substances well-known in the art can be used. For example, various modified polyester resins such as polyester resin, polyurethane-modified polyester resin, epoxy-modified polyester resin, acrylic-modified polyester, polyurethane resin, vinyl chloride-vinyl acetate copolymer, acrylic resin, epoxy resin, phenolic resin, melamine resin, alkyd resin, butyral resin, polyvinyl alcohol, polyimide resin, polyamide-imide resin, amino resin, styrene resin, resol, and inorganic adhesives such as glass frit, etc. can be listed; these adhesive resins can be used alone or in combination of two or more;

[0044] As the solvent, substances well-known in the art can be used. For example, hydrocarbon solvents such as tetradecane, toluene, xylene, ethylbenzene, diethylbenzene, isopropylbenzene, amylbenzene, p-cymene, tetrahydronaphthalene, and petroleum aromatic hydrocarbon mixtures, etc. can be listed; aldehyde or glycol aldehyde solvents such as ethylene glycol monoacetaldehyde, ethylene glycol monobutaldehyde, propylene glycol monomethyl ether, propylene glycol monoacetaldehyde, propylene glycol monon-butyl ether, propylene glycol monoter-butyl ether, diethylene glycol monoacetaldehyde, diethylene glycol monobutaldehyde, dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether, etc.; glycol ester solvents such as ethylene glycol monomethyl ether acetate, ethylene glycol monoacetaldehyde acetate, ethanol monobutaldehyde acetate, propylene glycol monomethyl ether acetate, propylene glycol monoacetaldehyde acetate, etc.; ester solvents such as ethyl acetate, butyl acetate, etc.; ketone solvents such as methyl isobutyl ketone, methyl ethyl ketone, cyclohexanone, etc.; terpene alcohols such as terpineol, linalool, geraniol, citronellol, etc.; alcohol solvents such as methanol, ethanol, propanol, n-butanol, sec-butanol, tert-butanol, etc.; glycol solvents such as ethylene glycol, diethylene glycol, etc.; γ-butyrolactone, dioxane, dimethylacetamide, dimethylformamide, N-methylpyrrolidone, and water, etc. The solvents can be used alone or in combination of two or more;

[0045] The silver-plated copper powder content in the electrically conductive adhesive of the present invention varies according to the use.

[0046] Example 1

[0047] <Pretreatment of copper powder>

[0048] 10 kg of dendritic copper powder was put into a 20 L solution of 1.0 mol / L ammonium sulfate and 0.2 mol / L ethylenediamine at a temperature of 25 °C to form a copper slurry; for the above copper powder, self-made electrolytic copper powder EC-1 was used (the average particle size (D50) obtained by laser diffraction scattering particle size distribution was 6.0 μm, and the BET specific surface area was 21340 cm 2 / g, there are 24 branches on the main axis crystal). After stirring the slurry for 30 min, 0.5% of sodium mercaptobenzothiazole based on the mass of dendritic copper powder is added and dissolved, and then the slurry is stirred for another 10 min. After solid-liquid separation, the copper powder is washed with deionized water until the pH is neutral, and then washed once with anhydrous ethanol. The centrifuged dendritic copper powder is dried in a vacuum drying oven at a temperature of 60 °C and a vacuum degree less than 100 Pa for 6 h for standby;

[0049] <Silver-plated copper powder>

[0050] Add the above 10 kg of pretreated dendritic copper powder to 50 L of deionized water to prepare a copper powder dispersion. Add sodium hydroxide to adjust the pH value and stabilize it at 10, and keep the copper powder dispersion at a constant temperature of 30 °C; In addition, add 889 g of trisodium citrate and 488 g of silver nitrate to 14.35 L of deionized water at 25 °C and stir to dissolve to form a mixed silver plating solution; The mixed silver plating solution is added to the copper powder dispersion at a dropping rate of 0.5 ml / min, and the reaction temperature is controlled at 30 °C. After the addition of the mixed silver plating solution is completed, stir the reaction for 30 min, perform solid-liquid separation, and wash once with ethanol; Finally, transfer the wet silver-plated copper powder to a high-speed mixer, and under the condition of stirring at room temperature, spray 0.1% of dodecyl mercaptan (with 12 carbon atoms) based on the mass of the silver-plated copper powder into the wet silver-plated copper powder, stir for 15 min, stop stirring, and dry the wet powder in a vacuum drying oven at 50 °C;

[0051] <Silver-plated copper conductive adhesive>

[0052] Relative to 100 parts by weight of the above silver-plated copper powder, add 31.92 parts by weight of a xylene solution of epoxy resin (solid content 87%) and 1.42 parts by weight of a curing agent to make the content of silver-plated copper powder in the conductive adhesive 75 wt%. After premixing, perform uniform dispersion and defoaming treatment to prepare a silver-plated copper conductive adhesive.

[0053] Example 2

[0054] <Pretreated copper powder>

[0055] Put 10 kg of dendritic copper powder into 20 L of 1.0 mol / L ammonium sulfate and 0.2 mol / L ethylenediamine solution at 25 °C to form a copper slurry; For the above copper powder, use self-made electrolytic copper powder EC-0 (the average particle size (D50) obtained by laser diffraction scattering particle size distribution is 4.0 μm, and the BET specific surface area is 24968 cm 2 / g, with 22 branches on the main axis crystal). After stirring the slurry for 30 min, 0.5% of sodium mercaptobenzothiazole based on the mass of the dendritic copper powder was added and dissolved, and then the slurry was stirred for another 10 min. After solid-liquid separation, the copper powder was washed with deionized water until the pH was neutral, washed once with anhydrous ethanol, and the centrifuged dendritic copper powder was placed in a vacuum drying oven at 60 °C and a vacuum degree less than 100 Pa for drying for 6 h for standby;

[0056] <Silver-plated copper powder>

[0057] 10 kg of the pretreated dendritic copper powder described above was added to 50 L of deionized water to prepare a copper powder dispersion. Sodium hydroxide was added to adjust the pH value and stabilize it at 10, and the copper powder dispersion was kept at a constant temperature of 30 °C. Additionally, 889 g of trisodium citrate and 488 g of silver nitrate were added to 14.35 L of deionized water at 25 °C and stirred to dissolve to form a mixed silver-plating solution. The mixed silver-plating solution was added to the copper powder dispersion at a dropping rate of 0.5 ml / min, and the reaction temperature was controlled at 30 °C. After the dropping of the mixed silver-plating solution was completed, the reaction was stirred for 30 min, followed by solid-liquid separation, and it was washed once with ethanol. Finally, the wet silver-plated copper powder was transferred to a high-speed mixer, and 0.1% of nonyl mercaptan (with 9 carbon atoms) based on the mass of the silver-plated copper powder was sprayed into the wet silver-plated copper powder under stirring conditions at room temperature, stirred for 15 min, and then the stirring was stopped. The wet powder was dried in a vacuum drying oven at 50 °C;

[0058] <Silver-plated copper conductive adhesive>

[0059] Relative to 100 parts by weight of the above silver-plated copper powder, 31.92 parts by weight of a xylene solution of epoxy resin (solid content 87%) and 1.42 parts by weight of a curing agent were added to make the content of silver-plated copper powder in the conductive adhesive 75 wt%. After premixing, it was uniformly dispersed and degassed to prepare a silver-plated copper conductive adhesive.

[0060] Example 3

[0061] <Pretreated copper powder>

[0062] 10 kg of dendritic copper powder was added to 20 L of a 1.0 mol / L ammonium sulfate and 0.2 mol / L ethylenediamine solution at 25 °C to form a copper slurry; for the above dendritic copper powder, self-made electrolytic copper powder EC-2 was used (the average particle size (D50) obtained by laser diffraction scattering particle size distribution was 10.0 μm, and the BET specific surface area was 16780 cm 2 / g, with 30 branches on the main axis crystal). After stirring this slurry for 30 min, 0.5% of sodium mercaptobenzothiazole based on the mass of dendritic copper powder is added and dissolved, and then the slurry is stirred for another 10 min. After solid-liquid separation, the copper powder is washed with deionized water until the pH is neutral, washed once with absolute ethanol, and the centrifuged dendritic copper powder is dried in a vacuum drying oven at a temperature of 60 °C and a vacuum degree less than 100 Pa for 6 h for standby;

[0063] <Silver-plated copper powder>

[0064] Add the above 10 kg of pretreated dendritic copper powder to 50 L of deionized water to prepare a copper powder dispersion. Add sodium hydroxide to adjust the pH value and stabilize it at 10, and keep the copper powder dispersion at a constant temperature of 30 °C. Additionally, put 889 g of trisodium citrate and 488 g of silver nitrate into 14.35 L of deionized water at 25 °C and stir to dissolve to form a mixed silver-plating solution. Add the mixed silver-plating solution to the copper powder dispersion at a dropping rate of 0.5 ml / min, control the reaction temperature at 30 °C. After the dropping of the mixed silver-plating solution is completed, stir and react for 30 min, perform solid-liquid separation, and wash once with ethanol. Finally, transfer the wet silver-plated copper powder to a high-speed mixer, and under the condition of stirring at room temperature, spray 0.1% of eicosanethiol (with 20 carbon atoms) based on the mass of the silver-plated copper powder into the wet silver-plated copper powder, stir for 15 min, stop stirring, and dry the wet powder in a vacuum drying oven at 50 °C;

[0065] <Silver-plated copper conductive adhesive>

[0066] Relative to 100 parts by weight of the above silver-plated copper powder, add 31.92 parts by weight of a xylene solution of epoxy resin (solid content 87%) and 1.42 parts by weight of a curing agent to make the content of silver-plated copper powder in the conductive adhesive 75 wt%. After premixing, perform uniform dispersion and defoaming treatment to prepare the silver-plated copper conductive adhesive.

[0067] Example 4

[0068] <Pretreated copper powder>

[0069] Put 10 kg of dendritic copper powder into 20 L of a solution of 5.0 mol / L ammonium sulfate and 0.5 mol / L ethylenediamine at 25 °C to form a copper slurry; for the above copper powder, use self-made electrolytic copper powder EC-1 (the average particle size (D50) obtained by laser diffraction scattering particle size distribution is 6.0 μm, and the BET specific surface area is 21340 cm 2 / g, there are 24 branches on the main axis crystal). After stirring the slurry for 60 min, 0.8% of sodium mercaptobenzothiazole based on the mass of the dendritic copper powder was added and dissolved, and then the slurry was stirred for another 30 min. After solid-liquid separation, the copper powder was washed with deionized water until the pH reached neutral, and then washed once with anhydrous ethanol. The centrifuged dendritic copper powder was dried in a vacuum drying oven at 60 °C and a vacuum degree less than 100 Pa for 6 h for standby;

[0070] <Silver-plated copper powder>

[0071] Add 10 kg of the above-mentioned pretreated dendritic copper powder to 50 L of deionized water to prepare a copper powder dispersion. Add sodium hydroxide to adjust the pH value and stabilize it at 11, and keep the copper powder dispersion at a constant temperature of 40 °C; In addition, add 1525.24 g of trisodium citrate and 1004.73 g of silver nitrate to 19.7 L of deionized water at 25 °C and stir to dissolve to form a mixed silver-plating solution; The mixed silver-plating solution was added to the copper powder dispersion at a dropping rate of 0.5 ml / min, and the reaction temperature was controlled at 30 °C. After the addition of the mixed silver-plating solution was completed, the reaction was stirred for 30 min, followed by solid-liquid separation and washing once with ethanol; Finally, transfer the wet silver-plated copper powder to a high-speed mixer, and under the condition of stirring at room temperature, spray 0.3% of dodecyl mercaptan (with 12 carbon atoms) based on the mass of the silver-plated copper powder into the wet silver-plated copper powder, stir for 15 min, stop stirring, and dry the wet powder in a vacuum drying oven at 50 °C;

[0072] <Silver-plated copper conductive adhesive>

[0073] Relative to 100 parts by weight of the above silver-plated copper powder, add 31.92 parts by weight of a xylene solution of epoxy resin (solid content 87%) and 1.42 parts by weight of a curing agent to make the content of silver-plated copper powder in the conductive adhesive 75 wt%. After premixing, perform uniform dispersion and defoaming treatment to prepare a silver-plated copper conductive adhesive.

[0074] Example 5

[0075] <Pretreated copper powder>

[0076] Put 10 kg of dendritic copper powder into 20 L of 5.0 mol / L ammonium sulfate and 0.5 mol / L ethylenediamine solution at 25 °C to form a copper slurry; For the above copper powder, use self-made electrolytic copper powder EC-1 (the average particle size (D50) obtained by laser diffraction scattering particle size distribution is 6.0 μm, and the BET specific surface area is 21340 cm 2 / g, there are 24 branches on the main axis crystal). After stirring the slurry for 60 min, 1.0% of sodium mercaptobenzothiazole based on the mass of dendritic copper powder is added and dissolved, and then the slurry is stirred for another 30 min. After solid-liquid separation, the dendritic copper powder is washed with deionized water until the pH is neutral, washed once with absolute ethanol, and the centrifuged dendritic copper powder is dried in a vacuum drying oven at a temperature of 60 °C and a vacuum degree less than 100 Pa for 6 h for standby;

[0077] <Silver-plated copper powder>

[0078] Add the above 10 kg of pretreated dendritic copper powder into 50 L of deionized water to prepare a copper powder dispersion liquid. Add sodium hydroxide to adjust the pH value and stabilize it at 11, and keep the copper powder dispersion liquid at a constant temperature of 40 °C; In addition, add 1525.24 g of trisodium citrate and 1004.73 g of silver nitrate into 19.7 L of deionized water at 25 °C and stir to dissolve to form a mixed silver-plating solution; The mixed silver-plating solution is added to the copper powder dispersion liquid at a dropping rate of 0.5 ml / min, and the reaction temperature is controlled at 30 °C. After the dropping of the mixed silver-plating solution is completed, stir and react for 30 min, carry out solid-liquid separation, and wash once with ethanol; Finally, transfer the wet silver-plated copper powder to a high-speed mixer, and under the condition of stirring at room temperature, spray 0.3% of octadecanethiol (with 18 carbon atoms) based on the mass of the silver-plated copper powder into the wet silver-plated copper powder, stir for 15 min, stop stirring, and dry the wet powder in a vacuum drying oven at 50 °C;

[0079] <Silver-plated copper conductive adhesive>

[0080] Relative to 100 parts by weight of the above silver-plated copper powder, add 31.92 parts by weight of a xylene solution of epoxy resin (solid content 87%) and 1.42 parts by weight of a curing agent to make the content of silver-plated copper powder in the conductive adhesive 75 wt%. After premixing, perform uniform dispersion and defoaming treatment to prepare a silver-plated copper conductive adhesive.

[0081] Example 6

[0082] <Pretreated copper powder>

[0083] Put 10 kg of dendritic copper powder into a 20 L solution of 5.0 mol / L ammonium sulfate and 0.5 mol / L ethylenediamine at 25 °C to form a copper slurry; The above copper powder uses self-made electrolytic copper powder EC-1 (the average particle size (D50) obtained by laser diffraction scattering particle size distribution is 6.0 μm, and the BET specific surface area is 21340 cm 2 / g, with 24 branches on the main axis crystal). After stirring the slurry for 60 min, 1.0% of sodium mercaptobenzothiazole based on the mass of dendritic copper powder was added and dissolved, and then the slurry was stirred for another 30 min. After solid-liquid separation, the copper powder was washed with deionized water until the pH was neutral, and then washed once with absolute ethanol. The centrifuged dendritic copper powder was dried in a vacuum drying oven at 60 °C with a vacuum degree less than 100 Pa for 6 h for standby;

[0084] <Silver-plated copper powder>

[0085] Add the above 10 kg of pretreated dendritic copper powder to 50 L of deionized water to prepare a copper powder dispersion. Add sodium hydroxide to adjust the pH value and stabilize it at 11, and keep the copper powder dispersion at a constant temperature of 40 °C; In addition, add 1220.19 g of trisodium citrate and 1004.73 g of silver nitrate to 11.82 L of deionized water at 25 °C and stir to dissolve to form a mixed silver-plating solution; The mixed silver-plating solution was added to the copper powder dispersion at a dropping rate of 0.5 ml / min, and the reaction temperature was controlled at 30 °C. After the addition of the mixed silver-plating solution was completed, the reaction was stirred for 30 min, followed by solid-liquid separation and washing once with ethanol; Finally, transfer the wet silver-plated copper powder to a high-speed mixer. Under the condition of stirring at room temperature, spray 0.1% of octadecanethiol (with 18 carbon atoms) based on the mass of the silver-plated copper powder into the wet silver-plated copper powder, stir for 15 min, stop stirring, and dry the wet powder in a vacuum drying oven at 50 °C;

[0086] <Silver-plated copper conductive adhesive>

[0087] Relative to 100 parts by weight of the above silver-plated copper powder, add 31.92 parts by weight of a xylene solution of epoxy resin (solid content 87%) and 1.42 parts by weight of a curing agent to obtain a mixed slurry with a silver-plated copper powder content of 75 wt% in the conductive adhesive. After premixing, perform uniform dispersion and degassing treatment to prepare the silver-plated copper conductive adhesive.

[0088] Example 7

[0089] Prepare dendritic silver-plated copper powder using the same preparation method as in Example 6;

[0090] <Silver-plated copper conductive adhesive>

[0091] Relative to 100 parts by weight of the above silver-plated copper powder, add 23.8 parts by weight of a toluene solution of epoxy resin (solid content 87%) and 1.20 parts by weight of a curing agent to obtain a mixed slurry with a silver-plated copper powder content of 80 wt% in the conductive adhesive. After premixing, perform uniform dispersion and degassing treatment to prepare the silver-plated copper conductive adhesive.

[0092] Example 8

[0093] Prepare dendritic silver-plated copper powder using the same preparation method as in Example 6;

[0094] <Silver-plated copper conductive adhesive>

[0095] Relative to 100 parts by weight of the above silver-plated copper powder, 31.92 parts by weight of a solution of polyester resin in ethylene glycol monoacetaldehyde acetate (solid content 87%) and 1.42 parts by weight of a curing agent are added to prepare a mixed slurry with a silver-plated copper powder content of 75 wt% in the conductive adhesive. After pre-mixing, it is uniformly dispersed and degassed to prepare the silver-plated copper conductive adhesive.

[0096] Comparative Example 1

[0097] Dendritic silver-plated copper powder and conductive adhesive were prepared using the same preparation method as in Example 1, except that the copper powder was not surface-treated with sodium mercaptobenzothiazole.

[0098] Comparative Example 2

[0099] Dendritic silver-plated copper powder and conductive adhesive were prepared using the same preparation method as in Example 1, except that the silver-plated copper powder was not surface-treated with alkyl mercaptan.

[0100] Comparative Example 3

[0101] Spherical-like silver-plated copper powder and conductive adhesive were prepared using the same preparation method as in Example 1, where the spherical-like copper powder was self-made chemically reduced copper powder C-2 (average particle size (D50) obtained by laser diffraction scattering particle size distribution was 6.0 μm, BET specific surface area 3890 cm 2 / g).

[0102] Comparative Example 4

[0103] Spherical-like silver-plated copper powder and conductive adhesive were prepared using the same preparation method as in Comparative Example 1, where the spherical-like copper powder was self-made chemically reduced copper powder C-2 (average particle size (D50) obtained by laser diffraction scattering particle size distribution was 6.0 μm, BET specific surface area 3890 cm 2 / g).

[0104] Evaluation

[0105] For the silver-plated copper powder and conductive adhesive obtained according to the examples and comparative examples, their content and physical properties were measured by the following methods. The specific methods are as follows:

[0106] <Measurement of D50 of copper powder by laser diffraction scattering particle size distribution method>

[0107] 0.2 g of the sample was mixed with a 0.2 wt% aqueous solution of PVP K30 and dispersed for 5 min using an ultrasonic disperser (model KQ-600KDE from Kunshan Ultrasonic Instruments Co., Ltd.), and then the particle size distribution was measured using a laser diffraction scattering particle size distribution measuring device model LS900 (from Zhuhai Omicron Technology Co., Ltd.).

[0108] <BET Specific Surface Area of Copper Powder and Silver-Coated Copper Powder>

[0109] After degassing 2.0 g of copper powder or silver-coated copper powder at 75 °C for 10 min, it was measured using a specific surface area analyzer BSD-PS2 (Beishide Instrument).

[0110] <Determination of Ag and S Contents in Silver-Coated Copper Powder>

[0111] Dissolve 1.0 g of silver-coated copper powder with concentrated nitric acid, dilute the dissolved solution with a volumetric flask, and determine the content using an ICP-AES inductively coupled plasma atomic emission spectrometer (ICP2060T type of Tianrui Instrument).

[0112] <Determination of Powder Compaction Conductivity>

[0113] Take 3.0 g of silver-coated copper powder and use a powder resistivity tester (FT-300 of Ningbo Ruike Weiye Instrument Co., Ltd.) at a pressure of 10 MPa and a cross-sectional area of 78.5 mm 2 , and calculate the conductivity of the silver-coated copper powder according to Equation (1).

[0114]

[0115] Among them, δ ---- resistivity (S / m); R ---- resistance (Ω); S ---- area (78.5 mm 2 ); L ---- length (mm).

[0116] Regarding the conductivity of the silver-coated copper powder after accelerated aging, the newly prepared silver-coated copper powder was placed in an oven at 160 °C and left standing for 2 hours before measurement. Define the ratio of the powder compaction conductivity after accelerated aging to the powder compaction conductivity of the newly prepared powder, and calculate the powder compaction conductivity retention rate.

[0117] <Determination of Volume Resistivity of Conductive Adhesive>

[0118] Coat the conductive adhesive described in the examples on a flat glass plate, pre-dry it at 100 °C for 5 min, and then heat it at 160 °C for 30 min to obtain a conductive film. Use a four-probe resistance measuring instrument (TC-SZT-2 type of Tongde Chuangye) and a film thickness gauge (SR-C type of Yiguang Technology) to measure the volume resistivity of the obtained conductive film.

[0119] Table 1 Evaluation Results of Examples and Comparative Examples

[0120]

[0121] As shown in the results in Table 1, the dendritic silver-plated copper powders obtained in Examples 1 to 8 were analyzed for the elemental content by ICP. The silver or silver alloy content was 3 to 7%, the silver alloy contained S element, and the content of S in silver was 0.5 to 3.0%. It can be seen from Example 1 and Comparative Example 1 that the pretreatment of copper powder with sodium mercaptobenzothiazole can make the silver layer contain S element. After the dendritic copper powder surface adsorbs the S element-containing compound, the dispersibility is improved, the number of contact points between the prepared silver-plated copper particles increases, so as to establish good electrical connection and improve the conductivity. It can be seen from Comparative Example 2 that after the surface of the silver-plated copper powder is modified with alkyl mercaptan, its oxidation can be inhibited and the conductivity retention rate of the powder can be improved. It can be seen from Comparative Examples 3 and 4 that the spherical silver-plated copper particles have good self-dispersibility. When the same treatment process is adopted, the increase in contact points is not obvious and the conductivity cannot be improved.

[0122] The dendritic silver-plated copper powder obtained in the present invention has the characteristics of low cost and high conductivity. The conductive adhesive containing the silver-plated copper powder can greatly reduce the cost of silver conductive adhesive and has excellent stability at the same time.

[0123] The specific embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the knowledge scope of those of ordinary skill in the art, various changes can be made without departing from the purpose of the present invention.

Claims

1. A dendritic silver-plated copper powder with high electrical conductivity, characterized in that: It consists of dendritic copper powder and a silver layer coated on the surface of the dendritic copper powder; based on the mass of the dendritic silver-plated copper powder being 100%, the silver layer accounts for 3 - 7%; the silver layer contains S element, and the content of S element in the silver layer is 0.5 - 3.0 wt.%. The average particle size D50 of the dendritic silver-plated copper powder is 4.0 to 10.0 μm, the BET specific surface area is 15,000 to 25,000 cm 2 / g, and there are more than 20 branches on the main axis crystal; The preparation method of the high-conductivity dendritic silver-plated copper powder includes the following specific steps: (1) Add dendritic copper powder into an ammonium sulfate-ethylenediamine solution, stir for 30 - 60 min, add sodium mercaptobenzothiazole and continue stirring and reacting for 10 - 30 min. After solid-liquid separation, wash with deionized water until the pH is neutral, and vacuum dry to obtain pretreated dendritic copper powder; (2) Disperse dendritic copper powder into deionized water to obtain a dendritic copper powder dispersion. Adjust the pH value to 10 - 11 with sodium hydroxide. Dropwise add a silver plating solution containing a reducing agent to the dendritic copper powder dispersion. At a temperature of 25 - 50 °C, stir and react for 30 - 60 min. After solid-liquid separation, wash with ethanol to obtain dendritic wet silver-plated copper powder, and vacuum dry to obtain high-conductivity dendritic silver-plated copper powder.

2. The silver-plated copper powder according to claim 1, wherein: The BET specific surface area of the silver-plated copper powder is S1, and the BET specific surface area of the dendritic copper powder is S2, and S1 / S2 = 1.02 - 1.

05.

3. The silver-plated copper powder according to claim 1, wherein: The surface of the silver-plated copper powder is coated with 0.1 - 0.3 wt% of alkyl mercaptan, and the number of carbon atoms in the alkyl mercaptan is greater than 8.

4. The silver-plated copper powder according to claim 1, wherein: The preparation method of the high-conductivity dendritic silver-plated copper powder also includes transferring the dendritic wet silver-plated copper powder to a high-speed mixer, spraying alkyl mercaptan onto the wet silver-plated copper powder under stirring conditions at room temperature, stirring for 15 - 45 min, and vacuum drying to obtain high-conductivity dendritic silver-plated copper powder.

5. The silver-plated copper powder according to claim 1, wherein: In step (1), the concentration of ammonium sulfate in the ammonium sulfate-ethylenediamine solution is 1.0 - 5.0 mol / L, and the concentration of ethylenediamine is 0.2 - 0.5 mol / L.

6. The silver-plated copper powder according to claim 1, characterized in that: The addition amount of sodium mercaptobenzothiazole in step (1) is 0.5 - 1.0% of the mass of the dendritic copper powder.

7. The silver-coated copper powder according to claim 1, wherein: The silver plating solution in step (2) is prepared by mixing a reducing agent and silver nitrate; the reducing agent is glucose and / or trisodium citrate, and the molar ratio of the reducing agent to silver nitrate is 0.8 - 1.2:1, and the concentration of the reducing agent is 0.2 - 0.5 mol / L.

8. A conductive adhesive, characterized in that: It contains the high-conductivity dendritic silver-plated copper powder described in claim 1.

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