TW202423570A - Copper powder - Google Patents

Abstract

This copper powder has a BET specific surface area of 1.0 m2/g to 10.0 m2/g and includes an organic matter having a molecular weight of 500 or less and a polyether and/or a polyol.

Description

Copper powder

This specification discloses a technology related to copper powder.

For example, submicron copper powder with a particle size of less than 1 μm is sometimes used in the form of conductive paste to make internal and external electrode materials for electronic parts such as multilayer ceramic capacitors or inductors or for the manufacture of inkjet wiring.

Conductive paste containing copper powder is printed on a substrate and heated to sinter the copper powder for the purpose of forming a circuit or bonding a semiconductor element to a substrate. Copper powder used in conductive paste may be required to be sintered at a low temperature, i.e., low-temperature sinterability. The reason is that copper powder sintered at a low temperature is more cost-effective than copper powder sintered at a high temperature, and can also be applied to substrates with low heat resistance.

As a technology related to this, there is one described in Patent Document 1, for example. In patent document 1, under the topic of "providing a copper powder with excellent low-temperature sintering properties", it is proposed that "a copper powder has a packed bulk density of 1.30 g/cm ³ to 2.96 g/cm ³ , and D50 and a crystallite diameter D satisfy D/D50≧0.060, wherein the above D50 is the 50% particle size when the cumulative frequency reaches 50% in the particle size histogram based on the volume of the copper particles, and the above crystallite diameter D is obtained by using the Scherrer equation from the diffraction peak of the Cu (111) plane in the X-ray diffraction curve obtained by powder X-ray diffraction of the copper powder." [Prior art literature] [Patent literature]

[Patent Document 1] Japanese Patent No. 7122436

[The problem that the invention wants to solve]

Although the copper powder described in Patent Document 1 can be sintered at a relatively low temperature, sometimes different techniques or viewpoints are required to effectively improve the low-temperature sinterability of the copper powder.

In this specification, a copper powder with excellent low-temperature sintering properties is provided. [Technical means to solve the problem]

The copper powder disclosed in this specification has a BET specific surface area of 1.0 m ² /g to 10.0 m ² /g and contains an organic substance with a molecular weight of 500 or less, and polyether and/or polyol. [Effects of the Invention]

The copper powder has excellent low temperature sintering properties.

The following is a detailed description of the above copper powder embodiments. In one embodiment, the copper powder has a BET specific surface area of 1.0 m ² /g to 10.0 m ² /g. The specific surface area of the copper powder is relatively large to a certain extent, and the smaller the particle size, the easier it is to sinter in the low temperature region.

Furthermore, the copper powder contains an organic substance with a molecular weight of 500 or less (hereinafter, also referred to as "low molecular organic substance"), and polyether and/or polyol. When the polyether or polyol in the copper powder is heated and thermally decomposed, the copper oxide generated on the surface of the copper powder will be reduced, and it is believed that the sintering of the copper powder at a low temperature is promoted by removing the surface oxide in this way. Furthermore, in fact, after analyzing the sintered body of the copper powder by X-ray diffraction method, it was confirmed that the copper oxide present in the copper powder was reduced. In addition, by thermal analysis, etc., it was also confirmed that the decomposition temperature of the polyether or polyol is roughly the same as the sintering temperature of the copper powder. Moreover, the low molecular organic substance has the effect of inhibiting the aggregation of copper powder. If such low molecular weight organic matter is added during the manufacture of copper powder, it can be inferred that the above-mentioned polyether or polyol is fully attached to most of the copper particles dispersed by the low molecular weight organic matter. In this way, the low temperature sintering effect brought by the polyether or polyol can be significantly exerted. Therefore, from the perspective of improving low temperature sintering properties, the key is that the copper powder contains both low molecular weight organic matter and polyether and/or polyol. In addition, the molecular weight of the low molecular weight organic matter is less than 500, and the amount of carbon residue that can hinder the sintering of copper powder during thermal decomposition is small.

As a result, it is considered that the low-temperature sintering property of the copper powder of this embodiment becomes excellent. However, it is not limited to the above theory.

(Specific surface area) The BET specific surface area of copper powder is 1.0 m ² /g to 10.0 m ² /g. Copper powder with such a large BET specific surface area has a small particle size and a relatively low sintering starting temperature in an inert gas. Furthermore, when the BET specific surface area is too large, it is difficult to ensure oxidation resistance, and there is a concern that the paste properties of conductive pastes may be affected due to moisture absorption or agglomeration. From this point of view, the BET specific surface area of copper powder is more preferably 2.0 m ² /g to 7.0 m ² /g.

The BET specific surface area of copper powder can be measured according to JIS Z8830:2013, for example, using BELSORP-mini II of MicrotracBEL. More specifically, 3 g of copper powder sample is degassed at 70°C in a vacuum for 5 hours, the nitrogen adsorption isotherm is measured, and the results obtained are analyzed by the BET method to calculate the BET specific surface area.

(Composition) Copper powder is mostly copper, and further contains low molecular weight organic matter, polyether and/or polyol. Typically, copper powder is a copper powder in which at least a portion of the surface of the copper particles is coated with low molecular weight organic matter, polyether and/or polyol.

As mentioned above, the copper powder is suppressed from agglomeration and has high dispersibility by containing low molecular weight organic matter. When the copper powder is subjected to surface treatment of polyether and/or polyol during the manufacture of the copper powder, if low molecular weight organic matter is contained, the copper powder to which the polyether and/or polyol is effectively attached can be obtained. Polyether or polyol is used to improve the low temperature sintering property of the copper powder, so the copper powder containing not only polyether and/or polyol but also low molecular weight organic matter effectively achieves the sintering temperature reduction by using the polyether or polyol.

In addition, low molecular weight organic matter does not contain that much carbon when the molecular weight is less than 500 and the oxygen content is more than 50 mass%. Therefore, when the copper powder is heated for sintering, the amount of carbon remaining when the low molecular weight organic matter undergoes thermal decomposition is relatively small, and it is not easy to hinder sintering.

For reference, specific examples of oxygen contents of low molecular weight organic substances are shown below. Glucose (C ₆ H ₁₂ O ₆ ): 53.29 mass% Galactose (C ₆ H ₁₂ O ₆ ): 53.29 mass% Mannose (C ₆ H ₁₂ O ₆ ): 53.29 mass% Maltose (C ₁₂ H ₂₂ O ₁₁ ): 51.42 mass% Sucrose (C ₁₂ H ₂₂ O ₁₁ ): 51.42 mass% Lactose (C 12 H ₂₂ O ₁₁ ): 51.42 mass% Citric acid (C ₆ H ₈ O ₇ ): 58.29 mass% Acetic acid (C ₂ H ₄ O ₂ ): _53.29 mass% Apple acid (C ₄ H ₆ O ₅ ): 59.66 mass% Malonic acid (C ₃ H ₄ O ₄ ): 61.50 mass % Succinic acid (C ₄ H ₆ O ₄ ): 54.19 mass % Fumaric acid (C ₄ H ₄ O ₄ ): 55.14 mass % Tartaric acid (C ₄ H ₆ O ₆ ): 63.96 mass % Gluconic acid (C ₆ H ₁₂ O ₇ ): 57.10 mass % Formic acid (CH ₂ O ₂ ): 69.52 mass % Oxalic acid (C ₂ H ₂ O ₄ ): 71.08 mass % Aconitic acid (C ₆ H ₆ O ₆ ): 55.14 mass % Pyruvic acid (C ₃ H ₄ O ₃ ): 54.50 mass % Oxalacetic acid (C ₄ H ₄ O ₅ ): 60.57 mass % Lactic acid (C ₃ H ₆ O ₃ ): 53.29 mass % Ascorbic acid (C ₆ H ₈ O ₆ ): 54.50 mass%

The copper powder preferably contains at least one selected from the group consisting of carboxylic acid, carboxylic acid salt, glucose, maltose, sucrose and lactose among low molecular weight organic substances. Thus, the copper powder can be produced which suppresses aggregation and does not hinder sintering.

Among the above-mentioned low molecular weight organic substances, if carboxylic acid is added during the production of copper powder, it will coordinate with copper, reducing the reaction rate or the growth rate of copper particles. As a result, the copper powder particles not only become closer to spherical in shape, but also have a narrower particle size distribution and less agglomeration. Therefore, copper powder preferably contains carboxylic acid among the low molecular weight organic substances.

As carboxylic acid, specifically, there can be mentioned: citric acid, acetic acid, malic acid, methyl dihydroxyvaleric acid, malonic acid, succinic acid, fumaric acid, tartaric acid, gluconic acid, formic acid, oxalic acid, aconic acid, pyruvic acid, oxalacetic acid, lactic acid, and salts thereof. In particular, the copper powder preferably contains citric acid and/or a citrate as a low molecular weight organic substance. Citric acid or a citrate has a high effect of inhibiting aggregation and not hindering sintering.

Furthermore, copper powder should preferably not contain high molecular weight organic substances (organic substances with a molecular weight of more than 500) such as gum arabic. The reason is that high molecular weight organic substances will cause a large amount of carbon residue during thermal decomposition when heated, which may hinder the sintering of copper powder at relatively low temperatures.

In addition to the above-mentioned low molecular weight organic substances, copper powder also contains polyethers and/or polyols. The polyethers or polyols in the copper powder function when heated to reduce copper oxide that may be naturally generated on the surface of the copper powder to copper. This can promote the sintering of the copper powder at low temperatures. It is believed that the reducing power of polyethers or polyols on the surface of copper powder is stronger than that of low molecular weight organic substances.

When the copper powder contains a polyether, the polyether preferably comprises a compound represented by the general formula (1): RO(C ₂ H ₄ O) _n H (R is H, or a saturated or unsaturated hydrocarbon of C4 to 18, and n is an integer of 2 to 30) and/or a compound represented by the general formula (2): RO(C ₃ H ₆ O) _n H (R is H, or a saturated or unsaturated hydrocarbon of C4 to 18, and n is an integer of 2 to 30). When the copper powder contains the compound of the general formula (1) or (2), copper oxide can be reduced at a relatively low temperature. Examples of the compound of the general formula (1) include polyethylene glycol of HO(C ₂ H ₄ O) _n H and polyoxyethylene alkyl ether of H _{2m ＋ 1} C _m O(C ₂ H ₄ O) _n H (m: an integer of 4 to 18). Examples of the compound of the general formula (2) include polypropylene glycol of HO(C ₃ H ₆ O) _n H and polyoxypropylene alkyl ether of H _{2m ＋ 1} C _m O(C ₃ H ₆ O) _n H (m: an integer of 4 to 18). Examples of the compound of the general formula (2) include polyoxyethylene polyoxypropylene alkyl ether of H _{2m ＋ 1} C _m O(C ₂ H ₄ O) _n (C ₃ H ₆ O) _l H in which a polyoxyethylene moiety and a polyoxypropylene moiety are combined.

The polyether that can be contained in the copper powder preferably has an alkyl chain (i.e., a linear or branched alkyl group). If it has an alkyl chain, the dispersibility of the copper powder in the paste is improved. The reason is that since the alkyl chain is a hydrophobic functional group, it has a good affinity with the hydrophobic organic solvent of the paste. Therefore, it is believed that when the copper powder with an alkyl chain is mixed with the organic solvent, the dispersion state of the copper powder can be maintained for a long time.

The copper powder may also contain a polyol. Examples of the polyol include glycerol, polyglycerol, butanediol, pentanediol, hexanediol, heptanediol, octanediol, nonanediol, decanediol, pentaerythritol, and dipentaerythritol.

Whether copper powder contains low molecular weight organic substances, polyethers, or polyols can be confirmed by infrared spectroscopy or mass spectrometry.

In the identification using infrared spectroscopy, the diffuse reflectance method can be used, and the device can be FT/IR-6700 manufactured by JASCO Corporation. Regarding the sample holder of the diffuse reflectance unit, a plurality of slots for placing the samples are used, and the sample to be measured can be changed by rotating the holder. The copper powder as the sample is directly placed in the slot of the sample holder of the diffuse reflectance unit without dilution with potassium bromide, etc., and the surface is flattened by scraping with the flat part of the medicine spoon. The background measurement is performed in the following form: For the part of the slot of the sample holder where no substance is placed, the wavelength of 400 to 4000 cm ^-1 is scanned at a resolution of 4 cm ^-1 . Thereafter, the holder is rotated to align with the slot where the copper powder has been placed, and the measurement is performed under the same conditions as above. Furthermore, the measurement was performed at room temperature, and the number of accumulations was set to 256 or more in order to obtain a sufficient spectrum.

When the copper powder contains the polyether represented by the general formula (1) or (2), the infrared spectroscopy method described above detects a peak derived from a saturated hydrocarbon (alkyl) at 3000 to 2840 cm ^-1 and a peak derived from an ether bond at 1260 to 1000 cm ^-1 . Therefore, it can be determined that the copper powder contains the polyether. In addition, a peak derived from a carboxylic acid or a carboxylic acid salt in a low molecular weight organic substance is detected at 1500 to 1750 cm ^-1 , so it can also be confirmed whether the copper powder contains a carboxylic acid or a carboxylic acid salt.

In addition, in the identification by mass spectrometry, a liquid chromatography-orbitrap mass spectrometer (manufactured by Thermo Fisher Scientific, LC-Orbitrap MS, LC: Vanquish analytical purification LC system, Orbitrap MS: Orbitrap Exploris240 mass spectrometer) can be used. In this way, it is possible to confirm whether the copper powder contains polyols or low molecular weight organic substances. The measurement conditions are as follows. Copper powder (4 mL of mixed solvent relative to 1 g of copper powder) was added to a solution of 10 mmol/L ammonium acetate aqueous solution and acetonitrile (volume ratio of 1:1), stirred by a shaker and an ultrasonic cleaner, separated into copper powder and extract by centrifugal separation, and the extract was collected by syringe filter filtration (Millex (registered trademark)-LCR manufactured by Merck Millipore, material: hydrophilic PTEF, pore size: 0.45 μm). The extract was measured by the above-mentioned liquid chromatography-orbitrap mass spectrometer. The column used is Hypersil GOLD (C18), and the mobile phase is introduced by changing from 10 mmol/L ammonium acetate aqueous solution to acetonitrile, and the column temperature can be set to 40°C. In the negative ion detection mode, when a peak with an m/z value of 500 or less is detected, the copper powder can be said to contain an organic substance with a molecular weight of 500 or less. In addition, as an example, in the positive ion detection mode, when a peak of ion addition to C ₃ H ₈ O ₃ is detected, the copper powder can be said to contain glycerol, which is a type of polyol.

The copper powder preferably has a carbon content of 0.15% to 1.00% by mass. The carbon in the copper powder includes carbon from low molecular weight organic substances and carbon from polyols and/or polyethers. When the carbon content of the copper powder is too low, the low molecular weight organic substances and/or polyols or polyethers required for low temperature sintering may be insufficient. On the other hand, if the carbon content is too high, the amount of carbon residue after the copper powder is sintered increases, and there is a concern that the sinterability will be reduced or it will be disadvantageous for low resistance. From this point of view, the carbon content of the copper powder is further preferably 0.15% to 0.70% by mass.

The carbon content of copper powder is measured by high-frequency induction furnace combustion-infrared absorption method. Specifically, a carbon-sulfur analysis device such as CS844 manufactured by LECO can be used, the sample collection amount is set to 0.2 g, adjusted to the intensity range of the calibration curve, the combustion aid is set to LECOCEL II and Fe chips manufactured by LECO, and a probe of a standard substance is used as the calibration curve to measure the carbon content of copper powder. Furthermore, after the sample is placed in an aluminum crucible for measurement, the aluminum crucible is used to measure the carbon content of copper powder after the following pretreatment, which is to heat the sample from room temperature to 1000°C at a certain heating rate in air for 2 hours, and then maintain it at 1000°C for 2 hours.

(Sintering start temperature) When the copper powder is heated in an inert gas as described above, it is sometimes sintered at a relatively low temperature, for example, the sintering start temperature is below 300°C. The sintering start temperature of the copper powder in an inert gas is preferably below 250°C.

The above sintering starting temperature is measured using thermomechanical analysis (TMA). Specifically, it is carried out as follows. That is, copper powder (about 0.3 g) is placed in a pelletizing mold with a hole of 5 mm in diameter, and compressed with a force of 1 kN to produce cylindrical (height: about 3 mm, diameter: about 5 mm) copper powder particles. The height is measured using a micrometer (manufactured by Mitutoyo, coolant proof micrometer (Coolant Proof Micrometer) MDC-25MX, maximum allowable error ±1 μm), and it is used as the initial particle height. The particles were placed in a thermomechanical analysis device (TMA4000SE manufactured by NETZSCH), evacuated to below -0.1 MPa, and then N ₂ was introduced to form an inactive environment. N ₂ was flowed in at a flow rate of 500 mL/min, while a load of 10 g was applied. The temperature was raised from room temperature (25°C) to 700°C at a rate of 10°C/min. During the heating period from room temperature to 700°C, the height of the particles was measured every 1 second, and the temperature at which the height of the particles shrunk by 2% from the initial height was taken as the sintering starting temperature.

(Manufacturing method) To manufacture copper powder, various methods such as chemical reduction method or disproportionation method and liquid phase method can be used. However, in the case of liquid phase method, it is important to add a predetermined organic substance and preferably make the copper particles contact with the surface treatment agent after the copper particles are generated by reaction. The following is a detailed description of a specific example of the case of using chemical reduction method.

In the chemical reduction method, the following steps are performed: copper salts such as copper sulfate, low molecular weight organic substances (organic substances with a molecular weight of 500 or less), reducing agents, and bases are mixed in a liquid and reacted to generate copper particles, thereby obtaining copper slurry containing copper particles. The low molecular weight organic substances include, as described above, citrate salts such as citric acid, sodium citrate, or potassium citrate. As the reducing agent, hydrazine or sodium borohydride can be used, and as the base, sodium hydroxide or ammonia can be used.

As a more detailed example of this step, after heating the copper sulfate aqueous solution to an appropriate reaction temperature, the pH value is adjusted with a sodium hydroxide aqueous solution or an ammonia aqueous solution, and then a hydrazine aqueous solution is added at once to react, thereby reducing the copper sulfate to cuprous oxide particles with a particle size of about 100 nm. After heating the cuprous oxide slurry containing the cuprous oxide particles to the reaction temperature, an aqueous solution containing sodium hydroxide and hydrazine is added dropwise, and then a hydrazine aqueous solution is added dropwise, thereby reducing the cuprous oxide particles to copper particles.

Next, the copper slurry is washed to obtain washed copper slurry. The washing method is not particularly limited, and a filter press or decanting may be used.

Next, a surface treatment agent such as polyoxyethylene alkyl ether is added to the above-mentioned copper slurry after washing, and the copper particles in the copper slurry after washing are subjected to surface treatment. At this time, since the aggregation of copper particles is inhibited by the low molecular weight organic matter contained in the copper slurry after washing, the majority of copper particles are effectively treated by polyoxyethylene alkyl ether and the like. When the copper particles are aggregated, the copper particles are in contact with each other at the aggregated part, so the surface treatment cannot be fully performed.

As mentioned above, the surface treatment is preferably performed after the reaction of generating copper particles. If polyoxyethylene alkyl ether is added during the reaction, there is a risk that the liquid will overflow from the reaction container due to foaming. When hydrazine is used as a reducing agent, foaming is undesirable from the perspective of safety. When a defoaming agent is used, in addition to the increase in cost, the components of the defoaming agent may remain in the copper powder, which may affect the sintering characteristics or paste dispersibility.

Regarding the surface treatment agent, in order to uniformly contact the copper particles in the slurry, it is preferred to use a water-soluble polyoxyethylene alkyl ether. If it is a non-water-soluble agent, there is a concern that it will separate from the slurry and cannot be mixed uniformly, or the amount of adhesion cannot be controlled.

Next, the surface treated copper slurry is dried to obtain a dry powder. The drying method is not particularly limited, but from the perspective of controlling the amount of the surface treatment agent attached, a method that can perform full drying is preferred, such as spray drying, FM mixer, vacuum drying, vacuum heating drying, or heating drying in an inert gas environment.

Next, the dried powder is crushed using a jet mill, a planetary ball mill, or a mortar. Thereafter, the crushed powder is dried by vacuum drying, vacuum heating drying, or heating drying in an inert gas environment. In this way, copper powder can be produced.

As described above, although the copper particles in the washed copper slurry are in contact with the surface treatment agent before drying, the contact period between the copper particles and the surface treatment agent may also be after drying. For example, after the drying step of crushing the powder, the copper particles may be in contact with polyoxyethylene alkyl ether or polyoxyethylene alkyl ether aqueous solution as a surface treatment agent. In this case, drying or crushing may be performed as needed. [Example]

Next, since the copper powder as described above was produced and its characteristics were confirmed, it will be described below. However, the description here is for illustrative purposes only and is not intended to be limiting.

(Example 1) A solution prepared by mixing 1 kg of 1 mol/L copper sulfate aqueous solution with 2.7 g of citric acid was prepared as solution A. A solution prepared by mixing 369 g of 30 mass% sodium hydroxide aqueous solution and 37.5 g of 80 mass% hydrazine aqueous solution in 1 L of pure water was prepared as solution B. A solution prepared by mixing 374 g of 30 mass% sodium hydroxide aqueous solution and 31.4 g of 80 mass% hydrazine aqueous solution in 1 L of pure water was prepared as solution C. A solution prepared by mixing 489 g of 30 mass% sodium hydroxide aqueous solution in 1 L of pure water was prepared as solution D. A solution prepared by mixing 200 g of citric acid in 1 L of pure water was prepared as solution E. Prepare a solution of 76.7 g of 80 mass% hydrazine aqueous solution in 1 L of pure water as solution F.

The following addition ratios are described as addition ratios relative to 1 L of solution A unless otherwise specified. Solution A was placed in a reaction vessel, heated to 50°C, and solution B was added at a ratio of 0.66 L. Next, the temperature was raised to 70°C, and solution C was added at a ratio of 0.24 kg. Next, solution D was added so that the pH value became 10.5, and solution E was added as a citric acid component at a ratio of 1.5 g. Next, solution F was added at a ratio of 0.18 kg to obtain copper slurry. The washed copper slurry was obtained by washing it with water.

The mass % of copper in the washed copper slurry is calculated by measuring the amount of water evaporated using an infrared moisture meter. A polyoxyethylene alkyl ether (AE, EMULMIN NL-70 manufactured by Sanyo Chemical Industries, Ltd.) is diluted to 10 mass % to obtain an aqueous solution as a surface treatment agent. The polyoxyethylene alkyl ether is added to the washed copper slurry in an amount of 1 mass % relative to the mass of copper in the washed copper slurry, and then stirred to obtain a surface treated copper slurry. The surface treated copper slurry is dried, air flow crushed, and vacuum dried to obtain copper powder.

(Example 2) Solution B was added in a ratio of 0.65 L, solution D was replaced with a 30 mass % sodium hydroxide aqueous solution, and the pH value was added in a way that the pH value became 10.3, and the above-mentioned polyoxyethylene alkyl ether was added in an amount of 0.65 mass % relative to the copper mass in the copper slurry after washing. Except for this, the same operation as in Example 1 was performed to obtain copper powder.

(Example 3) Except for adding the above-mentioned polyoxyethylene alkyl ether in an amount of 0.14 mass % relative to the copper mass in the copper slurry after washing, the same operation as in Example 2 was performed to obtain copper powder.

(Example 4) Solution B was added in a ratio of 0.67 L, solution C was added in a ratio of 0.25 kg, and the polyoxyethylene alkyl ether was added in an amount of 0.03 mass % relative to the copper mass in the copper slurry after washing. The same operation as in Example 2 was performed to obtain copper powder.

(Example 5) Except for adding the above-mentioned polyoxyethylene alkyl ether in an amount of 0.55 mass % relative to the copper mass in the copper slurry after washing, the same operation as in Example 4 was performed to obtain copper powder.

(Example 6) Before adding solution D, a 30% by mass sodium hydroxide aqueous solution was added to a pH value of 8.0, and solution D was added to a pH value of 10.4. The same operation as in Example 2 was performed except that polyoxyethylene alkyl ether was not added. To the copper powder thus obtained, an aqueous solution of polyethylene glycol 200 (PEG 200, manufactured by FUJIFILM Wako Pure Chemical Co., Ltd.) was added as a surface treatment agent in an amount of 1% by mass relative to the mass of copper, and the mixture was dried and crushed to obtain copper powder.

(Example 7) Except that the surface treatment agent was changed to polyethylene glycol 400 (PEG 400, manufactured by FUJIFILM Wako Pure Chemical Co., Ltd.), the same operation as in Example 6 was performed to obtain copper powder.

(Example 8) Except that the surface treatment agent was changed to polyethylene glycol 1000 (PEG 1000, manufactured by FUJIFILM Wako Pure Chemical Co., Ltd.), the same operation as in Example 6 was performed to obtain copper powder.

(Example 9) Except that the surface treatment agent was changed to polypropylene glycol 400 diol type (PPG 400 diol type, manufactured by FUJIFILM Wako Pure Chemical Co., Ltd.), the same operation as in Example 6 was performed to obtain copper powder.

(Example 10) Except that the surface treatment agent was changed to polypropylene glycol 700 diol type (PPG 700 diol type, manufactured by FUJIFILM Wako Pure Chemical Co., Ltd.), the same operation as in Example 6 was performed to obtain copper powder.

(Example 11) Except that the surface treatment agent was changed to polypropylene glycol 1000 diol type (PPG 1000 diol type, manufactured by FUJIFILM Wako Pure Chemical Co., Ltd.), the same operation as in Example 6 was performed to obtain copper powder.

(Example 12) Except that the surface treatment agent was changed to glycerin (manufactured by FUJIFILM Wako Pure Chemical Co., Ltd.), the same operation as in Example 6 was performed to obtain copper powder.

(Comparative Example 1) Except for not adding polyoxyethylene alkyl ether as a surface treatment agent, the same operation as Example 1 was performed to obtain copper powder.

(Comparative Example 2) The addition ratio in Comparative Example 2 is the addition ratio relative to 1 kg of cuprous oxide. Cuprous oxide is mixed with pure water to prepare 15.8 mass% cuprous oxide slurry, and 0.016 mass% of an aqueous gum arabic solution is added in a ratio of 6 g of gum arabic. 32 mass% sulfuric acid is added thereto in a ratio of 1.8 kg. Thereafter, 0.016 mass% of an aqueous gum arabic solution is added in a ratio of 4 g of gum arabic to obtain copper slurry. Washed copper slurry is obtained by washing it with water. The washed copper slurry is dried, air flow crushed, and vacuum dried to obtain copper powder.

(Comparative Example 3) Prepare the copper powder synthesized under the manufacturing conditions of Comparative Example 2, use an aqueous solution of polyoxyethylene alkyl ether (AE, EMULMIN NL-70 manufactured by Sanyo Chemical Industries, Ltd.) as a surface treatment agent, add the polyoxyethylene alkyl ether to the copper powder in an amount of 1 mass % relative to the mass of copper, and dry and crush to obtain copper powder.

(Comparative Example 4) The ratio of the raw materials in Comparative Example 1 was changed to obtain a copper powder having a larger BET specific surface area than the copper powder in Comparative Example 1.

(Identification of organic matter) As an example, the copper powder of Example 5 was analyzed by the above-mentioned infrared spectroscopy method, and peaks were observed near 2900 cm ^-1 (from saturated hydrocarbons), near 1600 cm ^-1 (from carboxylic acids and carboxylic acid salts), and near 1100 cm ^-1 (from ether bonds). According to the results, the copper powder of Example 5 contains the polyether described by the general formula (1) or the general formula (2) (from its hydrocarbons and ether bonds), and carboxylic acids or carboxylic acid salts.

As an example, the copper powder of Example 5 was analyzed by the mass spectrometry method. As a result, in the positive ion detection mode, the m/z value ranged from 275.2579 to 1040.7299, and the m/z value was detected at a scale of 44. The intensity was the highest when the m/z value was 556.4418, which was roughly consistent with the monoisotopic mass (monoisotopic mass) of [C ₂₈ H ₅₈ O ₉ +NH ₄ ] ⁺ to which ammonium ions were added to the chemical formula corresponding to n=8 of the general formula (1), which was 556.4425. In addition, in the negative ion detection mode, the m/z value was detected to be 191.0196. This is roughly consistent with the single isotopic mass of [C ₆ H ₈ O ₇ -H] ^-, which is 191.0192, when a proton is released from the chemical formula equivalent to citric acid. From this result, it can be seen that the copper powder of Example 5 contains the compound represented by the general formula (1) and citric acid.

(Specific surface area) The BET specific surface area of each copper powder obtained in Examples 1 to 12 and Comparative Examples 1 to 4 was measured by the above method. The results are shown in Tables 1 and 2.

(SEM image) The scanning electron microscope images (SEM images) of each copper powder obtained in Examples 1, 2 and 4 and Comparative Examples 1 and 2 are shown in Figures 1 to 5, respectively.

If the BET specific surface areas of the copper powders of Examples 1, 2 and 4 and Comparative Examples 1 and 2 shown in Table 1 are compared with Figures 1 to 5, it can be seen that the BET specific surface area depends on the particle size. Although the BET specific surface areas of the copper powders of Examples 6 to 12 and Comparative Example 3, which have BET specific surface areas of "1 to 10" in Table 1, have not been measured, since the particle size is determined by the step of generating copper particles, the copper powders of Examples 6 to 12, which generated copper particles in the same manner as Example 2, and the copper powder of Comparative Example 3, which generated copper particles in the same manner as Comparative Example 2, may have BET specific surface areas in the range of 1 m ² /g to 10 m ² /g.

Furthermore, if Figures 1 and 4 are compared with Figure 5, it can be seen that the copper powders coated with citric acid in Example 1 and Comparative Example 1 (Figures 1 and 4) have less necking between particles than the copper powders not coated with citric acid in Comparative Example 2 (Figure 5), and each particle is closer to a true sphere. It is believed that the dispersibility of the copper powders coated with citric acid in the paste is improved.

(Carbon content) The carbon content of each copper powder obtained in Examples 1 to 12 and Comparative Examples 1 to 4 was measured by the above method. The results are shown in Tables 1 and 2.

(Low-temperature sintering properties) For each copper powder obtained in Examples 1 to 12 and Comparative Examples 1 to 4, the sintering starting temperature in an inert gas was confirmed by the above method, and the results are shown in Table 1.

As can be seen from Table 1, the copper powders of Examples 1 to 12 contain both low molecular weight organic matter and polyether and/or polyol, and therefore have lower sintering starting temperatures than the copper powders of Comparative Examples 1 to 4.

(Preparation of copper powder paste) For the copper powder of each of Example 3 and Comparative Example 4, a copper powder paste was prepared. Specifically, α-terpineol (80.5 g), oleic acid (6.5 g), and ethyl cellulose (49% ethoxy) 10 (13.0 g) were mixed with a rotary mixer and used as a medium. Thereafter, copper powder (8.0 g) and the medium (2.0 g) were mixed with a rotary mixer to obtain a copper powder paste.

(Viscosity of the paste) The viscosity of the prepared copper powder paste was measured by the following method. The viscosity of the copper powder paste was measured using a rotational viscometer MCR102 manufactured by Anton Paar. The copper powder paste was placed on a constant temperature plate set at 25°C, and a cone plate with a cone angle of 2° (model: CP25-2) was used as a measuring fixture. The gap at the measuring position was set to 1 mm, and the cone plate was pressed against the copper powder paste. Thereafter, the copper powder paste overflowing from the cone plate was removed. The measurement procedure lasted 392 seconds, and the shear rate was slowly increased from 0 to 1000 s ^-1 . In this way, the viscosity of the paste at each shear rate was measured. The viscosity of the copper powder paste at a shear rate of 1 s ^-1 is shown in Table 2.

As can be seen from Table 2, the difference between the copper powder of Example 3 and the copper powder of Comparative Example 4 is whether polyoxyethylene alkyl ether is added, and the BET specific surface areas of the two are equal. The viscosity of the copper powder paste obtained by using the copper powder of Example 3 added with polyoxyethylene alkyl ether is lower than the viscosity of the copper powder paste obtained by using the copper powder of Comparative Example 4 without adding polyoxyethylene alkyl ether.

The viscosity of the copper powder paste is lower as the copper powder is more dispersed in the paste. Therefore, it can be said that the copper powder of Example 3 has a higher dispersibility in the paste than the copper powder of Comparative Example 4. As the reason for this, the inventors believe that by adding polyoxyethylene alkyl ether containing a hydrophobic alkyl chain to the copper powder, the compatibility of the hydrophobic medium and the copper powder is increased, and the dispersibility of the copper powder in the paste is improved.

[Table 1] Copper particle preparation method Low molecular weight organic matter Polyether polyol BET specific surface area [m ² /g] C [wt%] Sintering starting temperature [℃] Embodiment 1 Chemical reduction method Citric Acid AE (polyether, general formula (1)) 2.1 0.46 277 Embodiment 2 Chemical reduction method Citric Acid AE (polyether, general formula (1)) 4.3 0.47 236 Embodiment 3 Chemical reduction method Citric Acid AE (polyether, general formula (1)) 4.8 0.22 257 Embodiment 4 Chemical reduction method Citric Acid AE (polyether, general formula (1)) 4.8 0.19 289 Embodiment 5 Chemical reduction method Citric Acid AE (polyether, general formula (1)) 4 0.47 235 Embodiment 6 Chemical reduction method Citric Acid PEG200 (polyether, general formula (1)) 1～10 0.41 241 Embodiment 7 Chemical reduction method Citric Acid PEG 400 (polyether, general formula (1)) 1～10 0.64 247 Embodiment 8 Chemical reduction method Citric Acid PEG 1000 (polyether, general formula (1)) 1～10 0.66 239 Embodiment 9 Chemical reduction method Citric Acid PPG 400 diol type (polyether, general formula (2)) 1～10 0.66 274 Embodiment 10 Chemical reduction method Citric Acid PPG 700 diol type (polyether, general formula (2)) 1～10 0.67 270 Embodiment 11 Chemical reduction method Citric Acid PPG 1000 diol type (polyether, general formula (2)) 1～10 0.66 269 Embodiment 12 Chemical reduction method Citric Acid Glycerin (polyol) 1～10 0.43 245 Comparison Example 1 Chemical reduction method Citric Acid - 1.8 0.05 398 Comparison Example 2 Disproportionation Gum Arabic - 2.9 0.26 376 Comparison Example 3 Disproportionation Gum Arabic AE (polyether, general formula (1)) 1～10 Not determined 327 Comparison Example 4 Chemical reduction method Citric Acid - 4.8 0.12 344

[Table 2] Copper particle preparation method Low molecular weight organic matter Polyether polyol BET specific surface area [m ² /g] C [wt%] Sintering starting temperature [℃] Paste viscosity [Pa･s] Embodiment 3 Chemical reduction method Citric Acid AE (polyether, general formula (1)) 4.8 0.22 257 1025 Comparison Example 4 Chemical reduction method Citric Acid - 4.8 0.12 344 1272

Based on the above content, it is suggested that the copper powder has the possibility of excellent low-temperature sintering properties.

without

[Figure 1] is a SEM image of the copper powder obtained in Example 1. [Figure 2] is a SEM image of the copper powder obtained in Example 2. [Figure 3] is a SEM image of the copper powder obtained in Example 4. [Figure 4] is a SEM image of the copper powder obtained in Comparative Example 1. [Figure 5] is a SEM image of the copper powder obtained in Comparative Example 2.

Claims (8)

A copper powder has a BET specific surface area of 1.0 m ² /g to 10.0 m ² /g and contains an organic substance with a molecular weight of 500 or less, and a polyether and/or a polyol. The copper powder of claim 1 contains a compound represented by the following general formula (1) and/or a compound represented by the following general formula (2) as the polyether: RO(C ₂ H ₄ O) _n H (1) (in the general formula (1), R is H or a saturated or unsaturated hydrocarbon of C4 to 18, and n is an integer of 2 to 30) RO(C ₃ H ₆ O) _n H (2) (in the general formula (2), R is H or a saturated or unsaturated hydrocarbon of C4 to 18, and n is an integer of 2 to 30). The copper powder of claim 1 or 2, wherein the organic substance with a molecular weight of 500 or less comprises at least one selected from the group consisting of carboxylic acid, carboxylic acid salt, glucose, maltose, sucrose and lactose. The copper powder of claim 1 or 2, wherein the organic substance with a molecular weight of 500 or less contains citric acid and/or citrate. The copper powder of claim 1 or 2, which contains the polyether having an alkyl chain. The copper powder of claim 1 or 2, comprising glycerol as the polyol. The copper powder of claim 1 or 2 has a carbon content of 0.15 mass % to 1.00 mass %. The copper powder of claim 1 or 2 has a sintering starting temperature of 300°C or less in an inert gas.