Method for producing copper powder

Abstract

To provide a copper powder production method, in a polyol method of reducing copper compound powder in a polyol solvent to obtain copper powder, capable of easily and efficiently obtaining fine and monodispersed copper powder having excellent oxidation resistance and low temperature sinterability.SOLUTION: A copper powder production method has a step of suspending copper compound powder into a polyol solvent and reducing the same to a boiling point of the polyol solvent or lower to obtain copper powder, where the average particle diameter of the copper compound powder is 1.0 μm or less, the polyol has OH groups of 2 or more and 6 or less, and the average molecular weight is 160 or more.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for producing copper powder obtained by reducing copper compound powder in a polyol.

Background Art

[0002] Copper powder is also used as a material for conductive paste for forming internal electrodes and external electrodes of multilayer ceramic capacitors (MLCC), which are electronic components, and electrodes of multilayer ceramic substrates. In recent years, with the miniaturization and increased capacitance of multilayer ceramic capacitors, the internal electrodes have been thinned. Therefore, in this application, the copper powder used in the conductive paste (internal electrode paste) is also required to be fine. Furthermore, it is desired that the copper powder has a small number of connected particles and is monodispersed (monodispersed particles), and has excellent oxidation resistance.

[0003] Furthermore, in recent years, with the spread of high-power motor power control inverters and the like, an increase in the use of power semiconductor devices made of silicon carbide or gallium nitride and operating in a high-temperature environment of 200°C or higher is expected. As a heat dissipation material for releasing the heat generated by the power semiconductor device to the substrate, a die attach paste using lead-free solder powder as a metal filler has been used. However, since the melting point of lead-free solder powder is about 200°C and its heat resistance is insufficient, it is difficult to cope with high-temperature operation.

[0004] As an alternative material, a die attach paste using nano silver powder as a metal filler has been used. Nano silver powder can be sintered at a low temperature of 250°C or lower and has the characteristic of high operational reliability at high temperatures. However, due to its high cost and the problem of ion migration, in response to these problems, in addition to being fine, monodispersed, and having excellent oxidation resistance, the development of copper powder capable of low-temperature sintering is desired.

[0005] As a method for producing copper powder, the so-called electrolysis method is the most common. However, the copper powder obtained by this method tends to form coarse aggregates. As a method for obtaining fine copper powder, for example, Patent Document 1 discloses a method of wet-reducing copper oxide in the presence of a coupling agent, and Patent Document 2 discloses a method of vapor-phase reducing copper chloride, and a method using a disproportionation reaction has been proposed. However, the copper powder obtained by these methods all have high surface activity, and when used as a paste, they may be oxidized by heating for resin curing or oxidation may progress due to oxygen slightly present in the atmosphere during firing for the purpose of volatilizing organic substances, and it was not possible to satisfy all of fineness, monodispersibility, and oxidation resistance.

[0006] Therefore, as a solution to the above problems, Patent Documents 3 and 4 disclose a method of heating and reducing copper compound powder in a polyol solvent (polyol method). The copper powder (polyol copper powder) obtained by this method is excellent in monodispersibility and oxidation resistance, and is suitable for the above-described conductor paste.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, the copper powder (polyol copper powder) obtained by the methods described in Patent Documents 3 and 4 above has an organic film formed on the surface of the copper powder that does not easily decompose at 250°C or lower, and inhibits the progress of sintering between the copper powders, so it does not have low-temperature sinterability suitable for die attach applications.

[0009] The present invention has been completed based on the above findings, and an object thereof is to provide a method for producing copper powder that is fine, has excellent monodispersity and oxidation resistance, and also has low-temperature sinterability.

Means for Solving the Problems

[0010] In view of such conventional problems, the present inventors have conducted intensive studies. As a result, in producing polyol copper powder, by heating a copper compound powder having a specific average particle size in a solvent (polyol) having a specific average molecular weight, it has been found that copper powder that is fine, has excellent monodispersity and oxidation resistance, and also has low-temperature sinterability can be obtained.

[0011] According to an aspect of the present invention, there is provided a method for producing copper powder in which a copper compound powder is suspended in a polyol solvent and reduced to a temperature below the boiling point of the polyol solvent to obtain copper powder, wherein the average particle size of the copper compound powder is 1.0 μm or less, the polyol has 2 or more and 6 or less OH groups, and the average molecular weight is 160 or more.

[0012] Further, in the above method for producing copper powder, the copper compound powder is preferably at least one selected from the group consisting of copper oxide and cuprous oxide. Also, the water content contained in the copper compound powder is preferably 10% by mass or less. Further, the copper powder preferably has an average particle size of 1.0 μm or less.

Effects of the Invention

[0013] According to the method for producing copper powder according to the present embodiment, in the polyol method in which a copper compound powder is reduced in a polyol solvent to obtain copper powder, copper powder having excellent monodispersity and oxidation resistance and also having low-temperature sinterability can be obtained simply and efficiently.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0015] Hereinafter, specific embodiments of the method for producing copper powder according to the present invention (hereinafter referred to as "the present embodiment") will be described in detail. It should be noted that the present invention is not limited to the following embodiments, and various modifications are possible without changing the gist of the present invention. Also, in this specification, the notation "X to Y" (X and Y are arbitrary numerical values) means "X or more and Y or less".

[0016] In the polyol method, when copper compound powder is suspended in a polyol solvent and heated, the polyol solvent acts as a reducing agent and the reduction to copper proceeds. When copper oxide (CuO) is used as the copper compound, reduction from copper oxide (CuO) to copper (Cu) occurs via cuprous oxide (Cu2O). When cuprous oxide (Cu2O) is used as the copper compound, cuprous oxide (Cu2O) is reduced to copper (Cu). In either case, copper powder (hereinafter sometimes referred to as "polyol copper powder") is finally obtained. The obtained copper powder is washed with pure water or the like, filtered, and then, if necessary, washed again and dried. Specifically, as an example of washing, after sedimenting the polyol copper powder obtained by reduction and performing decantation, a method such as supplying pure water or the like and stirring and washing is used. As an example of filtration, a method such as dehydrating by centrifugation is used.

[0017] Commercially available inexpensive copper compound powders used as raw materials for copper powder have various particle sizes depending on the production method. For example, the particle size of copper oxide powder produced from copper chloride is about 3 μm to 50 μm. When the polyol method is used for such a copper compound powder with such a particle size, the reaction rate is extremely slow and it is substantially impossible to reduce, or even if reduction is possible, the particle size of the obtained polyol copper powder becomes agglomerated powder, which is unsuitable for use in conductive paste.

[0018] The method for producing copper powder according to this embodiment is a method for producing copper powder in which a copper compound powder is suspended in a polyol solvent and reduced to a temperature below the boiling point of the polyol solvent to obtain copper powder. The average particle size of the copper compound powder is 1.0 μm or less, the polyol has 2 or more and 6 or less OH groups, and the average molecular weight is 160 or more. In the method for producing copper powder according to this embodiment, before the copper compound powder is subjected to a reduction reaction, the average particle size thereof is pulverized to 1.0 μm or less, so that the copper powder (polyol copper powder) obtained by the polyol method is fine and monodispersed. In the method for producing copper powder according to this embodiment, in particular, a copper compound powder having an average particle size of 3.0 μm or more, which was conventionally unsuitable as a raw material as described above, is pulverized to a predetermined particle size or less, so that it can be used as a raw material for the polyol method.

[0019] The method for producing copper powder according to this embodiment will be described in detail. FIG. 1 is a flowchart showing the method for producing copper powder of this embodiment. The method for producing copper powder of this embodiment includes a pulverization step S1 and a reduction step S2.

[0020] (Pulverization Step S1) The pulverization step S1 is a step of pulverizing a raw material to obtain a copper compound powder pulverized to an average particle size of 1.0 μm or less. In this specification, the average particle size of the copper compound powder is the particle size at which the relative particles are 50% on a volume basis using a laser diffraction / scattering particle size measuring device. Also, in this specification, the average particle size of the obtained copper powder (polyol copper powder) is the number average value of the particle sizes of the primary particles that can be confirmed for the entire sample in an observation image such as a scanning electron microscope (SEM).

[0021] The method for manufacturing copper powder according to this embodiment is particularly effective when using copper compound powder with an average particle size of 3.0 μm or more as a raw material. The copper powder used as a conductive filler material for conductive paste preferably has an average particle size of 1.0 μm or less and is monodisperse. Also, for copper powder used in low-temperature sintering applications, the smaller the particle size, the better the low-temperature sinterability. Therefore, it is preferably an average particle size of 0.3 μm or less and is monodisperse. If a copper compound powder with an average particle size of 1.0 μm or more is used as a raw material without pulverizing the copper compound, the average particle size of the copper powder (polyol copper powder) may exceed this suitable range or may aggregate and not be monodisperse. Of course, in the method for manufacturing copper powder of this embodiment, in order to obtain finer or more monodisperse copper powder (polyol copper powder), the copper compound powder with an average particle size of less than 3.0 μm may be pulverized.

[0022] The copper compound powder used as a raw material is preferably either copper oxide (CuO) or cuprous oxide (Cu2O), or a mixture thereof. That is, the copper compound powder used as a raw material is preferably one or more selected from copper oxide and cuprous oxide.

[0023] As the pulverization method, a method of mechanically pulverizing (mechanical pulverization) can be used. The pulverization method is not particularly limited, and known methods can be used. As the dry pulverization method, mortar pulverization, spiral jet mill, counter jet mill, and as the wet pulverization method using pulverization media, for example, ball mill, bead mill, and as the wet pulverization method not using pulverization media, for example, one or more selected from high-pressure collision method and high-pressure emulsification method are preferable. When using these pulverization methods, it is possible to easily obtain a copper compound powder with an average particle size of 1.0 μm or less. Also, when using the wet pulverization method, in order to prevent contamination of impurities by using a plurality of solvents, it is preferable to suspend the copper compound powder in a polyol solvent and then perform the treatment. Note that the above one or more means, for example, including pulverizing by combining a plurality of methods such as coarse pulverization and fine pulverization.

[0024] When reducing copper compound powder with an average particle size of 1.0 μm or less, regarding the mechanism of the copper powder (polyol copper powder) obtained after reduction being refined, in the reduction reaction process in the polyol solvent, the contact area between the copper compound and the polyol solution increases with the refinement of the copper compound, and the elution rate of copper ions increases, promoting nucleation. Furthermore, it is considered that for the refinement of the copper compound, the copper ion source is uniformly dispersed in the solvent, avoiding the localization of nucleation and maintaining the dispersibility of the polyol copper powder.

[0025] In the method for producing copper powder according to this embodiment, the average particle size of the copper compound powder to be subjected to the reduction step S2 is preferably 1.0 μm or less, and more preferably 0.8 μm or less. When the average particle size of the copper compound powder to be subjected to the reduction step S2 is 1.0 μm or less, the average particle size of the obtained copper powder (polyol copper powder) is refined to 1.0 μm or less, which is suitable for use as a conductive filler for conductive paste. In particular, when the average particle size of the pulverized copper compound powder is 0.8 μm or less, the average particle size of the copper powder (polyol copper powder) obtained by the polyol method is refined to 0.3 μm or less, which is more suitable for low-temperature sintering applications. The lower limit of the average particle size of the copper compound powder to be subjected to the reduction step S2 is not particularly limited, but when using the pulverization method described above, it is usually about 0.05 μm as the lower limit.

[0026] (Reduction step S2) The reduction step S2 is a step of obtaining copper powder (polyol copper powder) by heating copper compound powder with an average particle size of 1.0 μm or less by the polyol method to a temperature of the boiling point of the polyol solvent - 50 °C or higher and the boiling point of the polyol solvent ± 0 °C or lower. The temperature of the boiling point of the polyol solvent - 50 °C or higher and the boiling point of the polyol solvent ± 0 °C or lower means, for example, when the boiling point of the polyol solvent is 300 °C, 250 °C (boiling point - 50 °C) or higher and 300 °C (boiling point ± 0 °C) or lower.

[0027] The copper compound powder to be subjected to the reduction step S2 is, for example, the pulverized copper compound powder obtained in the above pulverization step S1. In addition, as the copper compound powder to be subjected to the reduction step S2, a commercially available product or the like may be used as long as it is a copper compound powder pulverized to an average particle size of 1.0 μm or less. In the case of using a commercially available product or the like as the copper compound powder to be subjected to the reduction step S2, the production method of the present embodiment may not include the pulverization step S1. Note that the copper compound powder to be subjected to the reduction step S2 only needs to have an average particle size of 1.0 μm or less, that is, it is not limited to being pulverized. As the copper compound powder to be subjected to the reduction step S2, for example, particles previously produced with an average particle size of 1.0 μm or less may be used. In this case, the method for producing copper powder according to the present embodiment may not include the above-described pulverization step S1.

[0028] The water content contained in the raw material or the pulverized copper compound powder is preferably 10% by mass or less, and more preferably the water content contained in the pulverized copper compound powder is 10% by mass or less. When water is present in the system in the polyol method, the oxidation of the polyol solvent proceeds, and the production of polyol copper powder starts before the total amount of copper oxide (CuO) becomes cuprous oxide (Cu2O) due to the generated aldehyde compound. When the water content in the copper compound powder exceeds 10% by mass, this action becomes remarkable, and it may not be possible to obtain homogeneous copper powder (polyol copper powder).

[0029] In addition, the above pulverization may be performed using a solvent other than polyol such as water. For example, when pulverization is performed using an aqueous solvent, it is preferable to perform water removal or drying by heating as shown in Example 1 so that the water content contained in the pulverized copper compound powder becomes 10% by mass or less.

[0030] In the method for producing copper powder according to this embodiment, the polyol (polyol solvent) used as the solvent is a polyhydric alcohol having a reducing action on the copper compound powder, preferably having 2 to 6 OH groups and an average molecular weight of 160 or more. Specifically, one or more selected from tetraethylene glycol, pentaethylene glycol, hexaethylene glycol, tripropylene glycol, polyethylene glycol (having an average molecular weight of 160 or more), and phenyldiglycol are preferred. Among them, it is particularly preferred to use one or more selected from tetraethylene glycol and polyethylene glycol (having an average molecular weight of 160 or more). Note that polyethylene glycol having an average molecular weight of 160 or more can be used, but it suffices that it can be stirred when the copper compound powder is suspended. For example, those having a molecular weight of about 10,000 can also be used. Further, the solvent used in the polyol method may contain other components as long as the gist of the present invention is not deviated from.

[0031] The temperature at which the polyol solvent in which the copper compound powder is suspended is heated (heating temperature) is preferably such that the liquid temperature is not lower than the boiling point of the polyol - 50°C and not higher than the boiling point of the polyol ±0°C, and more preferably not lower than the boiling point of the polyol - 40°C and not higher than the boiling point of the polyol - 5°C. When the heating temperature is lower than - 50°C with respect to the boiling point of the polyol solvent, the reduction reaction may not proceed sufficiently and cuprous oxide (Cu2O) may remain, resulting in an increase in the oxygen content in the obtained copper powder (polyol copper powder), and the reaction time may be significantly extended, deteriorating productivity. Further, when the heating temperature is higher than the boiling point of the polyol, the decrease (consumption) due to the volatilization of the polyol becomes significant, and there is a risk that sufficient reduction cannot be achieved.

[0032] Through the above reduction step S2, polyol copper powder can be obtained. In the method for manufacturing copper powder of the present embodiment, the average particle diameter of the polyol copper powder obtained by the reduction step S2 can be set to 1.0 μm or less, 0.5 μm or less, and 0.3 μm or less, and finely divided particles can be manufactured. Note that the average particle diameter of the polyol copper powder obtained by the reduction step S2 can be controlled by changing the average particle diameter of the copper compound powder to be subjected to the reduction step S2 as described above.

[0033] Moreover, in the method for manufacturing copper powder of the present embodiment, the polyol copper powder obtained by the reduction step S2 can be made into monodisperse particles. For example, monodisperse particles satisfying the conditions described in the following examples can be manufactured.

[0034] In addition, in the method for manufacturing copper powder of the present embodiment, for the polyol copper powder obtained by the reduction step S2, the temperature at which the shrinkage amount is 1% when TMA measurement (thermomechanical analysis) is performed under the conditions described in the following examples can be set to 250 °C or less, 240 °C or less, and 230 °C or less, and copper powder having low-temperature sinterability can be manufactured.

[0035] Furthermore, in the method for manufacturing copper powder of the present embodiment, for the polyol copper powder obtained by the reduction step S2, the oxygen concentration increase value described in the following examples can be set to 0.5 mass% or less and 0.4 mass% or less, and copper powder having excellent oxidation resistance can be manufactured.

[0036] As described above, the method for producing copper powder according to the present embodiment is a method for producing copper powder in which copper compound powder is suspended in a polyol solvent and reduced to a temperature below the boiling point of the polyol solvent to obtain copper powder, wherein the average particle size of the copper compound powder is 1.0 μm or less, the polyol has 2 or more and 6 or less OH groups, and the average molecular weight is 160 or more. In addition, in the method for producing copper powder according to the present embodiment, the configurations other than the above are arbitrary configurations. According to the method for producing copper powder according to the present embodiment, in the polyol method of reducing copper compound powder in a polyol solvent to obtain copper powder, by using predetermined ones for the particle size of the copper compound powder and the polyol, it is possible to implement, and copper powder that is monodisperse, refined, and has excellent oxidation resistance and low-temperature sinterability can be obtained simply and efficiently.

Examples

[0037] Hereinafter, examples of the present invention will be shown together with comparative examples for more specific explanation, but the present invention is not limited to the following examples. The method for measuring the average particle size of the copper compound powder before and after pulverization was the particle size at which the relative particles were 50% on a volume basis using a laser diffraction scattering particle size measuring device (LA950V2, manufactured by Horiba, Ltd.).

[0038] (Example 1) 10 kg of copper oxide (CuO) powder (manufactured by Furukawa Chemicals Co., Ltd., product number: FCO-M6, average particle size 32.3 μm) as copper compound powder was pulverized in one pass at a pulverization pressure of 0.6 MPa using a spiral jet device (PJM-100SP, manufactured by Nippon Pneumatic Mfg. Co., Ltd.) to obtain pulverized copper oxide (copper compound) powder. The average particle size of the obtained pulverized copper oxide powder was 0.68 μm.

[0039] 30 g of the pulverized copper oxide powder was uniformly mixed with 100 g of tetraethylene glycol (manufactured by Tokyo Chemical Industry Co., Ltd., abbreviation: TeEG, average molecular weight: 194) as a polyol solvent. After that, the mixture was stirred at a liquid temperature of 180 °C for 20 minutes to remove the moisture remaining in the solvent. After removing the moisture, a reduction reaction was carried out while stirring the polyol solvent at a liquid temperature of 290 °C for 60 minutes. After the reduction reaction was completed, the reaction solution was cooled, subjected to filtration washing and solid-liquid separation treatment, and the cake-like polyol copper powder was recovered. The recovered cake-like polyol copper powder was vacuum dried to obtain polyol copper powder. The obtained polyol copper powder was monodisperse particles with an average particle size of 0.20 μm. The production conditions of the polyol copper powder are summarized in Table 1.

[0040] (Example 2) 30 g of the copper oxide powder pulverized in Example 1 as a copper compound was uniformly mixed with polyethylene glycol 200 (manufactured by Fujifilm Wako Pure Chemical Corporation, abbreviation: PEG200, average molecular weight: 180 - 220) as a polyol solvent. After that, the mixture was stirred at a liquid temperature of 180 °C for 20 minutes to remove the moisture remaining in the solvent. After removing the moisture, a reduction reaction was carried out while stirring the polyol solvent at a liquid temperature of 270 °C for 60 minutes. After the reduction reaction was completed, the reaction solution was cooled, subjected to filtration washing and solid-liquid separation treatment, and the cake-like polyol copper powder was recovered. The recovered cake-like polyol copper powder was vacuum dried to obtain polyol copper powder. The obtained polyol copper powder was monodisperse particles with an average particle size of 0.24 μm. The production conditions of the polyol copper powder are shown in Table 1.

[0041] (Example 3) 30 g of the copper oxide powder pulverized in Example 1 as the copper compound and polyethylene glycol 300 (manufactured by Fujifilm Wako Pure Chemical Corporation, abbreviation: PEG300, average molecular weight: 260 - 340) as the polyol solvent were uniformly mixed, and then the water remaining in the solvent was removed while stirring at a liquid temperature of 180°C for 20 minutes. After removing the water, a reduction reaction was carried out while stirring the polyol solvent at a liquid temperature of 300°C for 60 minutes. After the reduction reaction was completed, the reaction solution was cooled, subjected to filtration washing and solid-liquid separation treatment, and the cake-like polyol copper powder was recovered. The recovered cake-like polyol copper powder was vacuum dried to obtain polyol copper powder. The obtained polyol copper powder was monodisperse particles with an average particle size of 0.22 μm. The production conditions of the polyol copper powder are shown in Table 1.

[0042] (Example 4) 30 g of the copper oxide powder pulverized in Example 1 as the copper compound and polyethylene glycol 400 (manufactured by Fujifilm Wako Pure Chemical Corporation, abbreviation: PEG400, average molecular weight: 360 - 440) as the polyol solvent were uniformly mixed, and then the water remaining in the solvent was removed while stirring at a liquid temperature of 180°C for 20 minutes. After removing the water, a reduction reaction was carried out while stirring the polyol solvent at a liquid temperature of 300°C for 60 minutes. After the reduction reaction was completed, the reaction solution was cooled, subjected to filtration washing and solid-liquid separation treatment, and the cake-like polyol copper powder was recovered. The recovered cake-like polyol copper powder was vacuum dried to obtain polyol copper powder. The obtained polyol copper powder was monodisperse particles with an average particle size of 0.20 μm. The production conditions of the polyol copper powder are shown in Table 1.

[0043] (Example 5) 30 g of the copper oxide powder pulverized in Example 1 as the copper compound and polyethylene glycol 600 (manufactured by Fujifilm Wako Pure Chemical Corporation, abbreviation: PEG600, average molecular weight: 560 - 640) as the polyol solvent were uniformly mixed. After that, while stirring at a liquid temperature of 180 °C for 20 minutes, the moisture remaining in the solvent was removed. After removing the moisture, a reduction reaction was carried out while stirring the polyol solvent at a liquid temperature of 300 °C for 60 minutes. After the reduction reaction was completed, the reaction solution was cooled, subjected to filtration washing and solid-liquid separation treatment, and the cake-like polyol copper powder was recovered. The recovered cake-like polyol copper powder was vacuum dried to obtain polyol copper powder. The obtained polyol copper powder was monodisperse particles with an average particle size of 0.22 μm. The production conditions of the polyol copper powder are shown in Table 1.

[0044] (Comparative Example 1) A reduction reaction was carried out in the same manner as in Example 1, except that the copper oxide powder (average particle size 32.3 μm) before pulverization in the spiral jet device was used as the copper compound to produce polyol copper powder. When the obtained polyol copper powder was observed by SEM, it was found that the particles were connected to each other and were not monodisperse particles. The production conditions of the polyol copper powder are shown in Table 1.

[0045] (Comparative Example 2) Polyol copper powder was obtained in the same manner as in Example 1, except that triethylene glycol (manufactured by Fujifilm Wako Pure Chemical Corporation, abbreviation: TEG, average molecular weight: 150) was used as the polyol solvent and the heating temperature after moisture removal was set to 260 °C. When the obtained polyol copper powder was observed by SEM, it was monodisperse particles with an average particle size of 0.42 μm. The production conditions of the polyol copper powder are shown in Table 1.

[0046] (Comparative Example 3) Polyol copper powder was obtained in the same manner as in Example 1, except that the copper oxide powder (average particle size 32.3 μm) before pulverization in the spiral jet device was used as the copper compound, triethylene glycol was used as the polyol solvent, and the heating after moisture removal was set to 260 °C. When the obtained polyol copper powder was observed by SEM, it was found that the particles were connected to each other and were not monodisperse particles.

[0047] Manufacturing Conditions of Polyol Copper Powder

Table 1

[0048] (Evaluation of Polyol Copper Powder) For the polyol copper powders obtained in Examples 1 to 5 and Comparative Examples 1 to 3, the evaluation of various powder characteristics was carried out as follows.

[0049] (1) Average Particle Size From the image observed using a scanning electron microscope (JSM-7100F, manufactured by JEOL Ltd.), the particle sizes of 300 or more primary particles that could be uniformly observed were measured, and the number average value was obtained and taken as the average particle size (SEM diameter).

[0050] (2) Monodispersity From the image observed using a scanning electron microscope (JSM-7100F, manufactured by JEOL Ltd.), for 300 or more primary particles that could be uniformly observed, the area S and the perimeter L of the particles were measured using image analysis software (Mac-View Version.5, manufactured by Mountech Co., Ltd.), and the circularity represented by 4πS / L 2 Particles with a circularity exceeding 0.85 were counted as monodisperse particles, and particles with 0.85 or less were counted as aggregated particles. Among the counted particles, when the proportion of aggregated particles was 10% or less, it was regarded as monodisperse (indicated by "〇" in Table 2), and when it exceeded 10%, it was regarded as aggregated (indicated by "×" in Table 2).

[0051] (3) Oxidation Resistance The oxygen concentration (mass %) of the polyol copper powder was measured using an oxygen-nitrogen analyzer (ON836, manufactured by LECO Corporation) immediately after production and after being left in an air atmosphere at 25°C for 1000 hours, respectively. The oxygen concentration increase value is the difference between the oxygen concentration of the latter and the former. Those with an oxygen concentration increase value of 0.5 mass % or less have good oxidation resistance (indicated by "〇" in Table 2), and those with an oxygen concentration increase value exceeding 0.5 mass % have poor oxidation resistance (indicated by "×" in Table 2). Also, for the aggregated polyol copper powder, oxidation resistance evaluation was not performed.

[0052] (4) Sinterability Approximately 0.3 g of polyol copper powder was compressed at 100 MPa to form a cylindrical shape with a diameter of 5 mm to obtain a copper powder pellet. The obtained copper powder pellet was measured for the shrinkage amount (%) at a heating rate of 10°C / min in a reducing gas (a mixed gas of 2% hydrogen and 98% nitrogen) using a TMA measuring device (TMA4000SA, manufactured by BRUKER AXS). The shrinkage amount at a certain temperature is obtained as a percentage (%) represented by the pellet shrinkage length (μm) at a certain temperature / the pellet height (μm) at the time of pellet production × 100. In the measurement results, those with a shrinkage amount of 1% or more at 250°C have low-temperature sinterability (indicated by "〇" in Table 2), and those with less than 1% have no low-temperature sinterability (indicated by "×" in Table 2).

[0053] (Evaluation Results) The evaluation results obtained for Examples 1 to 5 and Comparative Examples 1 to 3 are summarized in Table 2. Examples 1 to 5 and Comparative Example 1 are all examples in which polyol copper powder was produced using a polyol solvent with an average molecular weight of 160 or more. The polyol copper powder obtained in Examples 1 to 5 using copper oxide powder with an average particle size of 1 μm or less after pulverization treatment became monodispersed and had a fine average particle size of 1 μm or less. Also, in all examples, the oxidation resistance was good and they had low-temperature sinterability. On the other hand, in Comparative Example 1 using the copper oxide powder before pulverization, the obtained polyol copper powder was aggregated, and due to the presence of aggregated powder, the average particle size became 0.36 μm, which is larger than that of Example 1. Also, in Comparative Example 1, due to aggregation, the surface free energy of the polyol copper powder decreased and it did not have low-temperature sinterability.

[0054] In Comparative Example 2, pulverized copper oxide powder was used. However, the average molecular weight of the polyol solvent (triethylene glycol) used was 150, which is smaller than 160. Therefore, due to the reaction temperature being restricted by the boiling point of the solvent and becoming lower, the average particle size of the obtained polyol copper powder was 0.42 μm, which was larger than that of Examples 1 to 5. The oxidation resistance in Comparative Example 2 was good as in Examples 1 to 5 because the specific surface area decreased due to the increase in particle size. On the other hand, the sinterability of Comparative Example 2 was inferior to that of Examples 1 to 5. This is considered to be because although the polyol copper powder in Comparative Example 2 was monodisperse, the average particle size was large, so the surface free energy of the polyol copper powder was smaller than that of Examples 1 to 5.

[0055] In Comparative Example 3, the average molecular weight of the polyol solvent (triethylene glycol) used was 150, and copper oxide powder before pulverization was used. The obtained polyol copper powder was aggregated, and because aggregated powder was mixed therein, the average particle size became 0.47 μm, which was larger than that of Example 1. Also, due to the aggregation, the surface free energy of the polyol copper powder decreased, and it did not have low-temperature sinterability.

[0056] From the examples and comparative examples, it is confirmed that the method for producing copper powder according to this embodiment can simply and efficiently obtain copper powder that is fine, monodisperse, excellent in oxidation resistance, and has low-temperature sinterability in the polyol method of reducing copper compound powder in a polyol solvent to obtain copper powder.

[0057] Properties of Polyol Copper Powder

Table 2

[0058] Note that the technical scope of the present invention is not limited to the aspects described in the above embodiments and the like. One or more of the requirements described in the above embodiments and the like may be omitted. Also, the requirements described in the above embodiments and the like can be combined as appropriate. Further, to the extent permitted by law, the disclosures of all the documents cited in the above embodiments and the like are incorporated by reference and made part of the description herein.

Claims

**Claim 1** A method for producing copper powder, comprising suspending copper compound powder in a polyol solvent and reducing it to a temperature below the boiling point of the polyol solvent to obtain copper powder, wherein the average particle size of the copper compound powder is 1.0 μm or less, and the polyol has 2 to 6 OH groups and an average molecular weight of 160 or more. **Claim 2** The method for producing copper powder according to claim 1, wherein the copper compound powder is at least one selected from the group consisting of copper oxide and copper suboxide. **Claim 3** The method for producing copper powder according to claim 1 or 2, wherein the water content contained in the copper compound powder is 10% by mass or less. **Claim 4** The method for producing copper powder according to any one of claims 1 to 3, wherein the copper powder has an average particle size of 1.0 μm or less.

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