Copper particles and methods for manufacturing copper particles

Abstract

To provide a copper particle having a sufficiently small particle size and excellent in high temperature oxidation resistance, and a method for producing the copper particle.SOLUTION: In a copper particle 10 covered with an organic protective film 12 in which a surface of the particle is composed of an organic molecule derived from copper carboxylate, an average particle diameter of primary particles is within a range of more than 50 nm and 400 nm or less, and a variation in a ratio of maximum peak intensities of Cu2O / Cu in X-ray diffraction (XRD) is less than 10% before and after processing for 1 hour at 150°C in an air atmosphere.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to copper particles and a method for producing the same, which are used, for example, as raw materials for conductive or bonding pastes.

Background Art

[0002] Conventionally, as a method for forming electrodes of electronic components or wiring of electronic circuits, a method of printing a conductive paste or conductive ink (hereinafter referred to as a copper particle-containing paste or the like) containing copper particles of a bonding material as a conductive filler on a substrate has been widely known. For example, the copper particles are prepared in a slurry form and then mixed into an organic substance, and used as a bonding material such as a copper particle-containing paste. In recent years, a method of easily forming wiring or the like on a substrate or the like has been developed and put into practical use by directly applying such a copper particle-containing paste or the like to the substrate using, for example, an inkjet printer, a screen printer, or an offset printer.

[0003] <00​​​​​​​​If copper particles are oxidized and their surfaces become covered with copper oxide, this may affect their sinterability. Furthermore, the electrical resistance of electrodes and other components formed by sintering may increase. Therefore, copper particles that do not easily oxidize are required, even during the aforementioned binder removal process.

[0006] Therefore, for example, Patent Documents 1 and 2 propose copper particles that have improved high-temperature oxidation resistance. In Patent Document 1, oxidation resistance is ensured by applying an SiO2-based gel coating film with a thickness of 100 nm or less to the surface of copper particles with an average particle size of 100 μm or less. Furthermore, in Patent Document 2, oxidation resistance is ensured by applying a surface treatment layer of 100 to 2000 ppm of Si to copper powder with an average particle size of 0.5 to 5 μm. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 3646259 [Patent Document 2] Patent No. 6159505 [Overview of the project] [Problems that the invention aims to solve]

[0008] Recently, there has been a demand to further miniaturize electronic circuits formed using copper particle-containing pastes, or to further miniaturize and increase the density of electronic devices. As a result, finer wiring patterns are required when forming electronic circuits. For this reason, copper particles, which are a common conductive metal material in conductive slurries that are raw materials for conductive inks, etc., are increasingly desired to have extremely small particle sizes of nano-size or submicron size. Furthermore, for copper particles, the smaller the particle size, the larger the specific surface area and the more susceptible they are to oxidation, making high-temperature oxidation resistance of copper particles even more important. However, as mentioned above, the particle sizes in Patent Documents 1 and 2 were relatively large and could not be used to create fine wiring patterns.

[0009] This invention has been made in view of the circumstances described above, and aims to provide copper particles that have a sufficiently small particle size and excellent high-temperature oxidation resistance, and a method for producing these copper particles. [Means for solving the problem]

[0010] To solve this problem, the inventors conducted diligent research and found that by forming an organic protective film with excellent heat resistance on the surface of copper particles, it is possible to provide copper particles that are sufficiently small in size and have excellent high-temperature oxidation resistance.

[0011] The present invention is based on the above-mentioned findings, and the copper particles of embodiment 1 of the present invention have a particle surface Number of carbon atoms 8 or more The copper particles coated with an organic protective film composed of organic molecules derived from copper carboxylate are characterized by having an average primary particle size in the range of over 50 nm and up to 400 nm, and by a change of less than 10% in the ratio of the highest peak intensity of Cu2O / Cu in X-ray diffraction (XRD) before and after treatment at 150°C in an atmospheric environment for 1 hour.

[0012] According to the copper particles of Embodiment 1 of the present invention, the average particle size of the primary particles is within the range of more than 50 nm and 400 nm or less, so the particle size is sufficiently small, making it possible to create fine wiring patterns. In addition, the copper particles have a large reaction area and high reactivity upon heating, which allows the copper particles to be sintered at a relatively low temperature. Furthermore, the particle surface is coated with an organic protective film composed of organic molecules derived from copper carboxylate, and the change in the ratio of the highest peak intensity of Cu2O / Cu in X-ray diffraction (XRD) is less than 10% before and after treatment at 150°C in an air atmosphere for 1 hour, indicating excellent high-temperature oxidation resistance.

[0013] Aspect 2 of the present invention is characterized in that, in the copper particles of Aspect 1, the total concentration of metal impurities with an oxidation-reduction potential lower than copper contained in the copper particles is less than 10 ppm by mass. According to the copper particles of embodiment 2 of the present invention, the total concentration of metal impurities with an oxidation-reduction potential lower than copper contained in the copper particles is less than 10 ppm by mass. Therefore, when the copper particles are used as a wiring material or bonding material, there is no risk of impairing the properties of other components due to the diffusion of metal impurities.

[0014] Aspect 3 of the present invention is a method for producing copper particles of Aspect 1 or Aspect 2 without using a dispersant and a surface protectant to suppress aggregation and / or oxidation of particles, Number of carbon atoms 8 or more The method is characterized by comprising the steps of: adding a pH adjusting agent to an aqueous dispersion of copper carboxylate to adjust the pH of the aqueous dispersion of copper carboxylate to 3 or more and 10 or less; adding and mixing an aqueous solution of a hydrazine compound having an oxidation-reduction potential in the range of -1.0V to -0.5V to the pH-adjusted aqueous dispersion of copper carboxylate to obtain a mixed solution; heating the mixed solution to a temperature of 60°C to 80°C under an inert gas atmosphere and holding it for 1.5 hours to 3.0 hours to reduce the copper carboxylate and obtain a copper particle dispersion in which copper particles are dispersed; and washing the copper particle dispersion using a washing medium in which the proportion of water is 25% by mass or less.

[0015] According to the method for producing copper particles of embodiment 3 of the present invention, copper carboxylate is used as a source of copper ions, the pH of an aqueous dispersion of copper carboxylate is adjusted to 3 or more and 10 or less, and the dispersion is held at a temperature of 60°C to 80°C for 1.5 to 3.0 hours in a hydrazine compound with an oxidation-reduction potential of -1.0V to -0.5V, thereby producing copper particles with an average primary particle size in the range of over 50 nm and under 400 nm. Furthermore, the organic molecules, which are the non-metallic components of copper carboxylate, coat the surface of the copper particles as a protective film, resulting in excellent high-temperature oxidation resistance.

[0016] Aspect 4 of the present invention is, in the method for producing copper particles of Aspect 3, wherein the copper carboxylate is sparingly soluble in water, Number of carbon atoms: 8 or more and is one or more copper salts selected from the group consisting of copper carboxylates that are According to the method for producing copper particles of Aspect 4 of the present invention, the copper carboxylate is sparingly soluble in water, Number of carbon atoms: 8 or more and is one or more copper salts selected from the group consisting of copper carboxylates that are

[0017] Aspect 5 of the present invention is, in the method for producing copper particles of Aspect 3 or Aspect 4, wherein the pH adjuster is ammonium carboxylate. According to the method for producing copper particles of Aspect 5 of the present invention, since ammonium carboxylate is used as the pH adjuster without using sodium hydroxide containing, for example, sodium which is a cause of residual metal impurities, the metal impurities of the obtained copper particles can be reduced.

Advantages of the Invention

[0018] According to the present invention, it is possible to provide copper particles having a sufficiently small particle size and excellent high-temperature oxidation resistance, and a method for producing the copper particles.

Brief Description of the Drawings

[0019] [Figure 1] It is a partial cross-sectional explanatory view of copper particles which is one embodiment of the present invention. [Figure 2] It is a flowchart of a method for producing copper particles which is one embodiment of the present invention. [Figure 3] It is a flowchart of a method for producing copper particles which is one embodiment of the present invention. [Figure 4] It is a photographic view of an aggregate of copper particles of Example 10 taken with a scanning electron microscope. [Figure 5] It is a photographic view of an aggregate of copper particles of Example 24 taken with a scanning electron microscope. [Modes for carrying out the invention]

[0020] Below, a copper particle, which is one embodiment of the present invention, and a method for manufacturing the copper particle will be described with reference to the attached drawings.

[0021] [Copper particles] As shown in Figure 1, in the copper particles 10 of this embodiment, the surface of the core particles 11 made of metallic copper is covered with an organic protective film 12 composed of organic molecules derived from copper carboxylate. Furthermore, the copper particles 10 are in the primary particle state, and their average particle size is in the range of over 50 nm and up to 400 nm.

[0022] If the average particle size of the primary particles of the copper particles 10 is 50 nm or less, there is a problem in that the paste becomes thicker when using the copper particles 10 to prepare a paste with a predetermined composition. Also, if the average particle size of the primary particles of the copper particles 10 exceeds 400 nm, the reaction area of ​​the copper particles is not large, resulting in low reactivity upon heating, which may prevent sintering at relatively low temperatures. Therefore, in the copper particles 10 of this embodiment, the average particle size of the primary particles is set to be within the range of more than 50 nm and 400 nm or less. The average particle size of the primary particles of the copper particles 10 is preferably 70 nm or more, and more preferably 80 nm or more. On the other hand, the average particle size of the primary particles of the copper particles 10 is preferably 200 nm or less, and more preferably 150 nm or less.

[0023] Here, the average particle size of the primary particles described above is determined by the following method. First, a scanning electron microscope (SEM) is used to take an SEM image of the copper particles, with the magnification determined according to the size of the copper particles. It is preferable to take the image in the range of 10,000x to 50,000x. Next, the SEM image is analyzed using image analysis software to determine the Heywood diameter for 300 or more particles per sample, and the arithmetic mean of the Heywood diameters is taken as the average particle size of the primary particles.

[0024] Furthermore, in the copper particles 10 of this embodiment, the change in the ratio of the highest peak intensity of Cu2O / Cu in X-ray diffraction (XRD) is less than 10% before and after treatment at 150°C in an atmospheric environment for 1 hour. As described above, in the copper particles 10 of this embodiment, the change in the ratio of the highest peak intensity of Cu2O / Cu in X-ray diffraction (XRD) before and after heat treatment at 150°C in an air atmosphere is less than 10%. This indicates that oxide formation is sufficiently suppressed even when heat treatment is performed in an air atmosphere, and the high-temperature oxidation resistance is sufficiently excellent. Furthermore, it is preferable that the change in the ratio of the highest peak intensity of Cu2O / Cu in X-ray diffraction (XRD) is less than 5% before and after treatment at 150°C in an atmospheric environment for 1 hour, and more preferably less than 1%.

[0025] In this embodiment of copper particles 10, it is preferable that the total concentration of metal impurities with an oxidation-reduction potential lower than copper contained in the copper particles 10 is less than 10 ppm by mass. When the total concentration of the aforementioned impurities is kept below 10 ppm by mass, it is possible to suppress the degradation of the properties of other components due to the diffusion of metal impurities when copper particles are used as a wiring material. For example, it is possible to suppress contamination of substrates on which wiring is applied by impurities, thereby reducing the insulation properties of the substrate. Furthermore, it is more preferable that the total concentration of metal impurities with a lower oxidation-reduction potential than copper contained in the copper particles 10 is less than 1 ppm by mass.

[0026] Furthermore, metals with a lower oxidation-reduction potential than copper that are contained in the copper particles 10 include alkali metals such as potassium and sodium, alkaline earth metals, and transition metals. The concentrations of these metals are measured by ICP-MS (inductively coupled plasma mass spectrometry). The reason why metals with a lower oxidation-reduction potential than copper are considered impurity metals is that metals nobler than copper, such as gold and silver, have a higher oxidation-reduction potential than copper and therefore precipitate regardless of the type of reducing agent used.

[0027] The organic protective film 12 is composed of organic molecules derived from copper carboxylate. This organic protective film 12 coats the surface of the core particles 11 made of metallic copper and plays a role in preventing oxidation of the core particles 11 during storage from manufacturing until they become a paste.

[0028] Furthermore, as the copper carboxylate mentioned above, one or more copper salts selected from the group consisting of copper carboxylates that are sparingly soluble in water and have four or more carbon atoms are preferably used. Examples include copper tartrate (four carbon atoms), copper citrate (six carbon atoms), copper phthalate (eight carbon atoms), and copper benzoate (four carbon atoms). It is more preferable that the carboxylic acid has six or more carbon atoms. In the organic protective film 12, which is composed of organic molecules derived from copper carboxylate with a high number of carbon atoms, high-temperature oxidation resistance is further improved.

[0029] [Method for manufacturing copper particles] In the method for producing copper particles 10 of this embodiment, a pH adjusting agent is added to an aqueous dispersion of copper carboxylate to adjust the pH of the aqueous dispersion to 3 or more and 10 or less. An aqueous solution of a hydrazine compound having an oxidation-reduction potential in the range of -1.0V to -0.5V is added to the pH-adjusted aqueous dispersion of copper carboxylate and mixed to obtain a mixture. The mixture is then heated to a temperature of 60°C to 80°C under an inert gas atmosphere and held for 1.5 to 3.0 hours to reduce the copper carboxylate and obtain a copper particle dispersion in which copper particles are dispersed. This copper particle dispersion is then washed with a washing medium in which the proportion of water is 25% by mass or less. After washing, the copper particle dispersion is subjected to solid-liquid separation, and the solid phase is dried to obtain copper particles 10.

[0030] The starting material, copper carboxylate, can be commercially available copper carboxylate hydrate or a product synthesized by reacting industrial copper sulfate with sodium carboxylate or ammonium carboxylate. Alternatively, as shown in Figure 2, this copper carboxylate may be obtained by reacting an aqueous carboxylate salt solution and a copper electrolyte in a reaction vessel under an atmospheric atmosphere, stirring at a temperature of 60°C to 80°C to obtain a copper carboxylate suspension, then washing, separating the solid and liquid components, and drying the solid components to obtain high-purity powdered copper carboxylate. The aqueous carboxylate salt solution here is prepared by dissolving sodium or ammonium salts of carboxylic acids such as citric acid, phthalic acid, benzoic acid, and tartaric acid in pure water such as deionized water or distilled water. Then, as shown in Figure 2, powdered copper carboxylate is placed in pure water such as deionized water or distilled water at room temperature and stirred to uniformly disperse it, thereby obtaining an aqueous dispersion of copper carboxylate with a concentration of 25% to 40% by mass.

[0031] Next, as shown in Figure 3, a pH adjusting agent is added to the aqueous dispersion of copper carboxylate to adjust the pH of the dispersion to between 3 and 10. As the pH adjusting agent, ammonium carboxylate that does not contain metal components is preferred. If the pH of the aqueous dispersion of copper carboxylate adjusted with a pH adjuster is less than 3, the elution of copper ions from copper carboxylate is slow, the reaction does not proceed rapidly, and it is difficult to obtain the target particles. Furthermore, if the pH of the aqueous dispersion adjusted with a pH adjuster exceeds 10, the average particle size of the primary particles may increase. Furthermore, it is preferable that the pH of the aqueous dispersion of copper carboxylate, adjusted with a pH adjusting agent, be in the acidic range of 3 to less than 6. By setting the pH of the aqueous dispersion of copper carboxylate to less than 6, it is possible to suppress the precipitation of eluted copper ions as copper(II) hydroxide when copper carboxylate is reduced with a hydrazine compound, making it possible to produce copper particles 10 in high yield. Furthermore, it is even more preferable that the pH of the aqueous dispersion of copper carboxylate, adjusted with a pH adjusting agent, be between 4 and 5.

[0032] As shown in Figure 3, an aqueous solution of a hydrazine compound with an oxidation-reduction potential in the range of -1.0V to -0.5V is added to this pH-adjusted aqueous dispersion of copper carboxylate under atmospheric conditions and mixed to obtain a mixture. Hydrazine-based reducing agents, including hydrazine monohydrate, are known to react differently in acidic and alkaline environments. Here, the redox potential refers to the potential difference relative to the standard hydrogen electrode (NHE). If the redox potential is less than -1.0V, the redox potential difference with copper becomes large, which may result in the presence of many metal impurities. If it exceeds -0.5V, the redox potential difference with copper becomes small, which may result in incomplete reduction of copper carboxylate. The preferred range for the redox potential is -0.7V to -0.5V, and the even more preferred range is -0.6V to -0.5V.

[0033] The oxidation-reduction potential E(V) is expressed by the following formula (1) based on the pH value. (Acidic range) N2H5 + = N2 + 5H + + 4e - (Alkaline range) N2H4 + 4OH - = N2 + 4H2O + 4e - Oxidation-reduction potential E(V): -0.23 -0.075 × pH (1) For example, when the pH is 3, the above equation (1) becomes [-0.23 -0.075 × 3], and the oxidation-reduction potential is -0.455V. Note that in the examples and comparative examples described later, the oxidation-reduction potential is rounded to the second decimal place, for example, -0.455V is shown as -0.5V.

[0034] Next, as shown in Figure 3, the mixture is heated to a temperature of 60°C to 80°C under an inert gas atmosphere and held for 1.5 hours to 3.0 hours to reduce the copper carboxylate and generate core particles 11. An organic protective film 12 derived from copper carboxylate is formed on the surface of these core particles 11, thereby creating a dispersion of copper particles 10 of the desired particle size (copper particle dispersion). The reason for heating and holding the mixture under an inert gas atmosphere is to prevent oxidation of the core particles 11. If the heating temperature of the mixture is below 60°C, the reducing power of copper carboxylate is too low, and the reduction reaction will not be completed. If the temperature exceeds 80°C or the holding time exceeds 3.0 hours, the amount of copper ions eluted from copper carboxylate increases, increasing the reaction rate, which may result in the inclusion of many metal impurities and a decrease in the amount of organic protective film 12 covering the mixture. Furthermore, it may become difficult to control the particle size of the primary particles. Also, if the holding time is less than 1.5 hours, the copper carboxylate may not be completely reduced, and the desired particles may not be obtained. If the holding time exceeds 3.0 hours, particle growth occurs due to the disappearance of fine particles and the growth of coarse particles to alleviate the free energy caused by the particle size difference, which may result in the inability to obtain primary particles with an average particle size in the range of 400 nm or less. The preferred heating temperature is 65°C to 75°C, and a more preferred heating temperature is 65°C to 70°C. The preferred holding time is 1.5 hours to 2.5 hours, and a more preferred holding time is 2.0 hours to 2.5 hours.

[0035] The reduction of copper carboxylate is carried out under an inert gas atmosphere to prevent oxidation of copper that dissolves into the liquid. Examples of inert gases include nitrogen gas and argon gas. Hydrazine compounds have advantages when reducing copper carboxylate under acidic conditions, such as not producing residue after the reduction reaction, being relatively safe, and being easy to handle. Examples of hydrazine compounds include hydrazine monohydrate, anhydrous hydrazine, hydrazine hydrochloride, and hydrazine sulfate. Among these, hydrazine monohydrate is preferred because it is desirable that it does not contain components that can become impurities, such as sulfur and chlorine.

[0036] Generally, copper generated in acidic solutions with a pH of less than 6 dissolves. In this embodiment, even when the pH of the aqueous dispersion of copper carboxylate adjusted with a pH adjusting agent is in the acidic range of less than 6, when a hydrazine compound, which is a reducing agent, is added and mixed, core particles 11 are generated in the solution. Components derived from carboxylate ions generated from copper carboxylate quickly coat the surface of the core particles 11, suppressing the dissolution of the core particles 11. It is preferable to keep the aqueous dispersion of copper carboxylate with a pH of less than 6 at a temperature of 50°C to 70°C, as this facilitates the reduction reaction.

[0037] In this embodiment of the method for producing copper particles 10, the copper particle dispersion is washed as shown in Figure 3. A washing medium with a water content of 25% by mass or less is added to the copper particle dispersion and stirred. After standing and settling, the supernatant liquid is removed. This process is repeated to wash the copper particle dispersion. Here, by using a cleaning medium with a water content of 25% by mass or less, the organic protective film 12 formed on the surface of the core particles 11 can be sufficiently preserved. Preferably, the water content in the cleaning medium is 20% by mass or less, and more preferably 10% by mass or less. In addition, various organic solvents such as ethanol and acetone can be used as components other than water in the cleaning medium.

[0038] Next, the washed copper particle dispersion is subjected to solid-liquid separation, for example, using a centrifuge, and the solid phase is dried with hot air in an atmospheric or nitrogen atmosphere, thereby obtaining the copper particles 10 of this embodiment, in which an organic protective film 12 is formed on the surface of the core particles 11 described above.

[0039] In this embodiment of copper particles 10 with the configuration described above, the average particle size of the primary particles is within the range of over 50 nm and 400 nm or less, so the particle size is sufficiently small, making it possible to create fine wiring patterns. In addition, the copper particles 10 have a large reaction area and high reactivity when heated, which allows the copper particles 10 to be sintered at a relatively low temperature. Furthermore, the surface of the core particles 11 is coated with an organic protective film 12 composed of organic molecules derived from copper carboxylate, and the change in the ratio of the highest peak intensity of Cu2O / Cu in X-ray diffraction (XRD) is less than 10% before and after treatment at 150°C in an atmospheric environment for 1 hour, indicating excellent high-temperature oxidation resistance.

[0040] In this embodiment of copper particles 10, if the total concentration of metal impurities with an oxidation-reduction potential lower than copper is less than 10 ppm by mass, there is no risk of impairing the properties of other components due to the diffusion of metal impurities when the copper particles 10 are used as a wiring material or bonding material.

[0041] Furthermore, according to the method for producing copper particles 10 in this embodiment, copper carboxylate is used as a source of copper ions, the pH of the aqueous dispersion of copper carboxylate is adjusted to 3 or more and 10 or less, and by holding it in a hydrazine compound with an oxidation-reduction potential of -1.0V to -0.5V at a temperature of 60°C to 80°C for 1.5 to 3.0 hours, copper particles 10 with an average primary particle size in the range of over 50 nm and up to 400 nm can be obtained. Furthermore, the organic molecules, which are the non-metallic components of copper carboxylate, coat the surface of the core particles 11 as a surface protective film, resulting in excellent high-temperature oxidation resistance.

[0042] In the method for producing copper particles 10 according to this embodiment, if the copper carboxylate is one or more copper salts selected from the group consisting of copper carboxylates that are sparingly soluble in water and have 4 or more carbon atoms, the oxidation resistance of the organic protective film 12 formed on the surface of the core particles 11 will be further improved, and copper particles 10 with even greater high-temperature oxidation resistance can be produced.

[0043] In the method for producing copper particles 10 according to this embodiment, if the pH adjusting agent is ammonium carboxylate, the metal impurities in the resulting copper particles 10 can be further reduced.

[0044] Although an embodiment of the present invention, specifically copper particles and a method for producing copper particles, has been described above, the present invention is not limited thereto and can be modified as appropriate without departing from the technical spirit of the invention. For example, in this embodiment, the solid phase after solid-liquid separation is described as being dried with hot air to obtain copper particles, but there are no limitations on the drying method, and freeze-drying or vacuum drying may also be used. [Examples]

[0045] The results of the verification experiments conducted to confirm the effects of the present invention are described below.

[0046] First, Table 1 below shows the types of copper carboxylates and their carbon number used in the examples and comparative examples, the total concentration of metal impurities with oxidation-reduction potentials lower than copper, and the impurity concentration of each metal. The manufacturing methods for each copper carboxylate are also shown at the bottom of Table 1. The total concentration of metal impurities with oxidation-reduction potentials lower than copper shown in Table 1 is an approximate value.

[0047] [Table 1]

[0048] <Example 1> First, copper phthalate, as shown in Table 1, was prepared as the starting material, copper carboxylate. This copper phthalate was placed in deionized water at room temperature and stirred with a stirring blade to prepare an aqueous dispersion of copper phthalate with a concentration of 30% by mass. Next, an aqueous solution of ammonium phthalate was added to this aqueous dispersion of copper phthalate as a pH adjuster to adjust the pH of the dispersion to 3. Then, the pH-adjusted solution was heated to 50°C and, under a nitrogen gas atmosphere, an aqueous solution of hydrazine monohydrate (2-fold dilution) with an oxidation-reduction potential of -0.5V, equivalent to 1.2 times the amount that can reduce copper ions, was added all at once and mixed uniformly with a stirring blade. Furthermore, in order to synthesize the target copper particles, the mixture of the aqueous dispersion and the reducing agent was heated to the holding temperature of 70°C under a nitrogen gas atmosphere and held at 70°C for 2 hours. Then, a washing medium of 100% by mass ethanol was added to the obtained copper particle dispersion and stirred. After standing and settling, the supernatant was removed. This process was repeated three times to wash the copper particle dispersion. The washed copper particle dispersion was separated into solid and liquid components using a centrifuge, and the copper particles in the dispersion were converted into solid components. The recovered solid components were dried by hot air drying at 150°C to produce the copper particles of Example 1.

[0049] <Examples 2-44, Comparative Examples 1-22> Using the same or a different copper carboxylate (see Table 1) as the starting material for Example 1, the adjusted pH value was the same as or different from that of Example 1, the type of reducing agent and the oxidation-reduction potential were the same as or different from those of Example 1, and the holding temperature and holding time during the synthesis of copper particles were the same as or different from those of Example 1. The resulting copper particle dispersion was then washed using the same or a different washing medium as in Example 1. Otherwise, copper particles for Examples 2-44 and Comparative Examples 1-22 were produced in the same manner as in Example 1. In Comparative Examples 6-9, the additives shown in Table 1 were added. Figures 4 and 5 show photographs of the aggregates of copper particles obtained in Examples 10 and 24, respectively, taken with a scanning electron microscope. As shown in Figures 4 and 5, copper particles with uniform particle size were observed in Examples 10 and 24.

[0050] The manufacturing conditions for Examples 1 to 44 are shown in Tables 2 and 3 below, and the manufacturing conditions for the copper particles in Comparative Examples 1 to 22 are shown in Table 4 below.

[0051] [Table 2]

[0052] [Table 3]

[0053] [Table 4]

[0054] <Comparative Evaluation Test and Results> For the copper particles of Examples 1-44 and Comparative Examples 1-22, the average particle size of the primary copper particles, the total concentration of metal impurities with an oxidation-reduction potential lower than copper, and the rate of change in the ratio of the highest peak intensity of Cu2O / Cu in X-ray diffraction (XRD) before and after heat treatment held at 150°C in an air atmosphere for 1 hour were determined using the methods described in the embodiments. These results are shown in Tables 5-7 below. Here, the total concentration of metal impurities with an oxidation-reduction potential lower than copper is the sum of the concentrations of each metal impurity.

[0055] [Table 5]

[0056] [Table 6]

[0057] [Table 7]

[0058] In Comparative Examples 1 to 22, when washing the copper particle dispersant, a washing medium with a water content of 30% by mass or more was used. The change in the ratio of the highest peak intensity of Cu2O / Cu in X-ray diffraction (XRD) before and after heat treatment held at 150°C in an air atmosphere for 1 hour was very large. It is presumed that washing removed a portion of the organic protective film formed on the surface of the copper particles, reducing their high-temperature oxidation resistance. Furthermore, a similar trend was observed regardless of which copper carboxylate was used as the copper raw material.

[0059] In contrast, in Examples 1 to 44, when washing the copper particle dispersant, a washing medium with a water content of 25% by mass or less was used, and the change rate of the ratio of the highest peak intensity of Cu2O / Cu in X-ray diffraction (XRD) before and after heat treatment held at 150°C in an air atmosphere for 1 hour was small, less than 10%. Even after washing, the organic protective film formed on the surface of the copper particles remained sufficiently intact, suggesting excellent high-temperature oxidation resistance. Furthermore, a similar trend was observed regardless of which copper carboxylate was used as the copper raw material. Furthermore, by optimizing the manufacturing conditions, it was possible to reduce the average particle size of the primary particles and decrease the total concentration of metals less noble than copper.

[0060] From the above verification experiments, it was confirmed that the present invention can provide copper particles with a sufficiently small particle size and excellent high-temperature oxidation resistance, as well as a method for producing these copper particles. [Industrial applicability]

[0061] The copper particles of the present invention can be used as lead-free wiring or bonding particles for fine pitch applications. Wiring paste or bonding paste obtained using these wiring or bonding particles as raw materials can be suitably used for mounting fine electronic components. [Explanation of Symbols]

[0062] 10 copper particles 11 core particles 12 Organic protective film

Claims

1. In copper particles whose surface is coated with an organic protective film composed of organic molecules derived from copper carboxylate with 8 or more carbon atoms, The average particle size of the primary particles is in the range of over 50 nm and 400 nm or less. Cu in X-ray diffraction (XRD) ２ Copper particles characterized in that the change in the ratio of the highest peak intensity of O / Cu is less than 10% before and after treatment at 150°C in an atmospheric environment for 1 hour.

2. The copper particles according to claim 1, characterized in that the total concentration of impurities of metals with an oxidation-reduction potential lower than copper contained in the copper particles is less than 10 ppm by mass.

3. A method for producing copper particles according to claim 1 or claim 2, without using dispersants and surface protectants to suppress aggregation between particles and / or oxidation of particles, A step of adding a pH adjusting agent to an aqueous dispersion of copper carboxylate having 8 or more carbon atoms to adjust the pH of the aqueous dispersion of copper carboxylate to 3 or more and 10 or less, A step of adding and mixing an aqueous solution of a hydrazine compound having an oxidation-reduction potential in the range of -1.0 V to -0.5 V to the pH-adjusted aqueous dispersion of copper carboxylate to obtain a mixed solution, The process involves heating the mixture to a temperature of 60°C to 80°C under an inert gas atmosphere and holding it for 1.5 to 3.0 hours to reduce the copper carboxylate and obtain a copper particle dispersion in which copper particles are dispersed, A cleaning step in which the copper particle dispersion is cleaned using a cleaning medium in which the proportion of water is 25% by mass or less, A method for producing copper particles, characterized by containing [the specified ingredient].

4. The method for producing copper particles according to claim 3, wherein the copper carboxylate is one or more copper salts selected from the group consisting of copper carboxylates that are sparingly soluble in water and have eight or more carbon atoms.

5. The method for producing copper particles according to claim 3, wherein the pH adjusting agent is ammonium carboxylate.

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