JP2012041592A - Flat copper particle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide flat copper particles having a high filling density of particles, and enabling easy control of viscosity when being changed into paste.SOLUTION: This flat copper particle has a contour defined by a plurality of approximately linear sides in a plan view, comprises a flat body in which each angle formed by adjacent sides is ≥60° and <180°, and has an average particle size of 0.05-0.5 μm. The flat copper particles are produced preferably by a method including the first reduction process and the second reduction process performed thereafter. In the first reduction process, reducing sugar or hydrazine is used as a reducing agent, and in the second reduction process, two or more kinds of reducing agents whose hydrogen reduction standard potential Eis -1.11 to -1.24 V are used as the reducing agent.

Description

  The present invention relates to a copper particle having a flat shape. The flat copper particles of the present invention are particularly preferably used as a raw material for a copper paste used, for example, for circuit formation of a printed wiring board and ensuring electrical continuity of external electrodes of a ceramic capacitor.

  Conventionally, as a method of forming electrodes and circuits for electronic components, etc., after a conductive paste in which copper powder, which is a conductive material, is dispersed in a paste is printed on a substrate, the paste is baked or cured and cured. A method of forming is known.

  As the copper particles contained in the conductive paste, those having a substantially spherical shape have been used so far. In recent years, flaky particles have also been used from the viewpoint of improving the fillability of via holes in printed wiring boards and improving the accuracy of the shape of the conductor to be formed. Since the flake copper particles have a large specific surface area and a large contact area between the particles, they are effective in reducing electrical resistance and improving the accuracy of the conductor shape.

  Conventionally, flaky copper particles have been manufactured by mechanically deforming, for example, spherical copper particles with a ball mill or the like (see, for example, Patent Document 1). Moreover, it replaces with such a mechanical method and the method of manufacturing a flaky copper particle chemically is also proposed (for example, refer patent document 2).

JP 2003-119501 A JP 2005-314755 A

  When flaky copper particles are produced by the mechanical method described in Patent Document 1, it is not easy to make the obtained particles uniform in size, and there is a limit to producing particles having a small particle size. was there. Moreover, since it takes a long time to manufacture and the yield decreases, it is not easy to suppress the manufacturing cost. On the other hand, according to the chemical method described in Patent Document 2, flaky particles having a small particle diameter can be produced with a higher yield than the technique described in Patent Document 1. However, there is a limit to precisely controlling the shape and size of the particles produced by the reaction.

  Accordingly, an object of the present invention is to provide flat copper particles having various characteristics further improved as compared with the copper particles of the prior art described above.

The present invention comprises a flat body having a contour defined by a plurality of substantially straight sides in a plan view, and the angles formed by adjacent sides are all 60 degrees or more and less than 180;
The object is solved by providing flat copper particles having an average particle diameter Dia of 0.05 to 0.5 μm.

In addition, the present invention provides a suitable method for producing the flat copper particles,
In the method of producing flat copper particles having a reduction step of reducing copper by adding a reducing agent to an aqueous solution containing a water-soluble copper compound,
The reduction step includes a first reduction step and a second reduction step performed thereafter;
In the first reduction step, reducing sugar or hydrazine is used as a reducing agent,
Provided is a method for producing flat copper particles, wherein two or more reducing agents having a hydrogen reduction standard potential E ⁰ of −1.11 to −1.24 V are used as reducing agents in the second reduction step. It is.

  The flat copper particles of the present invention are excellent in particle dispersibility and aspect ratio uniformity, and have a high packing density. Further, the viscosity can be easily controlled when the paste is formed. Moreover, the surface roughness of the conductor formed using the paste is lower than that of the conventional one.

1 is a scanning electron microscope image of the flat copper particles obtained in Example 1. FIG. FIG. 2 is a scanning electron microscope image of the flat copper particles obtained in Example 2. FIG. 3 is a scanning electron microscope image of the flat copper particles obtained in Example 3. 4 is a scanning electron microscope image of the flat copper particles obtained in Example 4. FIG. FIG. 5 is a scanning electron microscope image of the copper particles obtained in Comparative Example 1. FIG. 6 is a scanning electron microscope image of the copper particles obtained in Comparative Example 2. FIG. 7 is a scanning electron microscope image of the copper particles obtained in Comparative Example 3.

  Hereinafter, the present invention will be described based on preferred embodiments thereof. The copper particles of the present invention are characterized by their flat shape. Specifically, it has a plate-like shape having two substantially flat surfaces facing each other, and the size (transverse length) of the surfaces is larger than the thickness. When the copper particles are viewed in plan, this plate-like shape is a shape having an outline defined by a plurality of substantially straight sides. For example, it has a polygonal outline.

  When the plate surface of the flat copper particles is viewed in plan, the angle formed by two adjacent sides among the sides defining the plate surface is 60 degrees or more and less than 180 degrees, preferably 80 degrees or more and 160 degrees or less, More preferably, it is 100 degrees or more and 120 degrees or less. The flat copper particles of the present invention satisfy this angle at all corners formed by two adjacent sides in the side defining the plate surface. However, it is not necessary that all corners have the same angle. Since the flat copper particles have such a shape, the copper powder made of the particles has a uniform shape, and the packing density of the particles is increased. Further, the viscosity can be easily controlled when the paste is formed. In addition, "plan view" is a state when the flat copper particles of the present invention are viewed from a direction orthogonal to the plate surface.

  From the viewpoint of further increasing the packing density of the particles, the flat copper particles of the present invention preferably have a polygonal shape with a plate surface of a triangle or more, particularly a hexagon or more. In this case, the polygon does not need to be a regular polygon, but it is preferable that the shape is closer to the regular polygon.

  The copper particles of the present invention are also characterized by their small size, that is, they are fine particles despite being flat. Specifically, the flat copper particles are observed with a scanning electron microscope (SEM), and the average particle diameter Dia of the plate surface calculated by image analysis is a fine particle having a diameter of 0.05 to 0.5 μm, preferably Is 0.1 to 0.45 μm, more preferably 0.15 to 0.4 μm. When the copper particles of the present invention are such fine particles, the packing density of the particles is further increased by a synergistic effect with the flatness of the particles.

  The average particle diameter Dia obtained by the above image analysis is based on the SEM image obtained by direct observation using a SEM with a magnification of 5000 to 20000 times, and the maximum crossing of individual copper particles (the number of measurement samples is 10 or more). It is obtained by actually measuring the length and averaging with the number of measurement samples.

  While the average particle size of the plate surface in the flat copper particles of the present invention is in the above range, the thickness of the particle, that is, the distance between two opposing plate surfaces is smaller than the average particle size of the plate surface. It has become. Specifically, it is preferably 0.01 to 0.1 μm, more preferably 0.02 to 0.08 μm. Therefore, the aspect ratio defined by the particle size / thickness is preferably 2 to 25, more preferably 5 to 10. The thickness of the flat copper particles is obtained by averaging measured values measured by direct observation with an SEM (number of measurement samples: 10 or more).

The flat copper particles of the present invention are preferably produced by the method described later, and the flat copper particles produced by the method have a sharp particle size distribution. As a measure of the particle size distribution, for example, the value of SD / D ₅₀ can be used. Here, SD is the standard deviation (μm) of the particle size distribution obtained by the laser diffraction / scattering particle size distribution measurement method, and D ₅₀ is the volume cumulative particle at a cumulative volume of 50% by volume by the laser diffraction / scattering particle size distribution measurement method. Diameter (μm). The flat copper particles of the present invention have an SD / D ₅₀ value of preferably 0.4 or less, more preferably 0.38 or less, and even more preferably 0.35 or less. By having such a sharp particle size distribution, the flat copper particles of the present invention have a higher particle packing density.

The value itself of the denominator of the D ₅₀ in SD / D ₅₀ is an expression of the particle size distribution of said, 0.1 to 0.4 [mu] m, it is preferable that particularly 0.25～0.35Myuemu. D ₅₀ is a representative value of the agglomerated particle diameter of the flat copper particles of the present invention, and the closer this value is to the average particle diameter Dia described above, the lower the degree of aggregation of the particles. Since the average particle diameter Dia of the flat copper particles of the present invention is 0.05 to 0.3 μm as described above, as is clear from a comparison between this value and 0.1 to 0.4 which is the value of D ₅₀ , It can be said that the flat copper particles of the present invention have a small degree of aggregation. This also characterizes the flat copper particles of the present invention.

  The sharp particle size distribution of the flat copper particles of the present invention means that the distribution of the aspect ratio of the particles is also sharp. As described above, the aspect ratio is defined by the particle size / thickness. Since the width of the thickness variation is small compared to the variation of the particle size, if the particle size distribution is sharp, the distribution of the aspect ratio results. Also become sharper. Specifically, the distribution of the aspect ratio is preferably 4 or less, more preferably 2 or less, expressed by the standard deviation σ. The sharp distribution of the aspect ratio is advantageous in that when the paste is produced using the flat copper particles of the present invention, the viscosity of the paste can be easily controlled. It is also advantageous in that the surface roughness of the conductor formed using the paste is further reduced.

  When the flat copper particles of the present invention are produced by the method described later, the particles have a crystallite size larger than that of the mechanically produced flat copper particles. A large crystallite diameter is advantageous from the viewpoint of improving the sintering resistance. Specifically, when producing a paste using the flat copper particles of the present invention and forming a conductor using the paste, a step of sintering the conductor is performed. When copper particles with low sintering resistance are used, the dimensional stability of the conductor is likely to be impaired due to thermal contraction of the particles, but by using the flat copper particles of the present invention with high sintering resistance, sintering is performed. It is possible to suppress the resulting change in dimensional stability. From this viewpoint, the crystallite diameter in the flat copper particles of the present invention is preferably 20 nm or more, and more preferably 25 nm or more. The crystallite diameter of the flat copper particles can be determined from, for example, the half width of the diffraction angle peak measured by X-ray diffraction of the particles.

  The flat copper particles of the present invention may be composed only of copper, or may contain a small amount of copper and other elements. By containing other elements, various properties of the flat copper particles can be improved. For example, the advantageous effect that oxidation resistance improves by containing boron preferably 1-50 ppm, more preferably 10-40 ppm is produced. Furthermore, the particle size can be made uniform and atomized. The ability to atomize is advantageous in terms of ease of viscosity control of a paste prepared using the flat copper particles of the present invention. Ppm means parts per million by weight.

  In addition to or in place of boron, the flat copper particles of the present invention may contain phosphorus. By containing phosphorus, the oxidation resistance of the flat copper particles can be improved. However, since inclusion of a large amount of phosphorus contributes to a decrease in electrical conductivity of the copper particles, the phosphorus content in the flat copper particles of the present invention is 10 to 200 ppm, particularly 50 to 180 ppm. Is preferred.

  In order to contain boron and phosphorus in the flat copper particles of the present invention, for example, in the production method described later, a phosphorus-containing compound may be added, or a boron-containing reducing agent may be used as a reducing agent for copper ions. . Further, the content of boron or phosphorus in the flat copper particles of the present invention can be measured by, for example, an ICP emission analyzer.

  From the viewpoint of further improving the oxidation resistance, the surface of the flat copper particles of the present invention may be treated with an organic compound. For example, by treating the surface of the flat copper particles with a saturated fatty acid, an unsaturated fatty acid, a nitrogen-containing organic compound, a sulfur-containing organic compound, or a silane coupling agent, the oxidation resistance of the particles is further improved. Details of these various organic compounds are described in, for example, Japanese Patent Application Laid-Open No. 2005-314755 related to the earlier application of the present applicant.

  Next, the suitable manufacturing method of the flat copper particle of this invention is demonstrated. In this production method, flat copper particles are obtained by a chemical method in which a reducing agent is added to an aqueous solution containing a water-soluble copper compound to reduce copper. The mechanical method described in Patent Document 1 described above is not adopted in this manufacturing method. In this production method, the reduction process is performed in two stages. That is, the reduction process includes a first reduction process and a second reduction process performed thereafter.

  In this production method, first, an aqueous solution containing a water-soluble copper compound (hereinafter also referred to as “copper-containing aqueous solution”) is prepared. As the water-soluble copper compound, for example, copper sulfate, copper nitrate, copper acetate, or a hydrate thereof can be used. Among these copper compounds, copper sulfate pentahydrate and copper nitrate are highly water-soluble, can increase the copper concentration in the aqueous solution, and can easily obtain flat copper particles with high particle size uniformity. Therefore, it is preferably used.

  The copper-containing aqueous solution preferably contains 10 to 50 parts by weight, more preferably 20 to 40 parts by weight of the copper compound with respect to 100 parts by weight of water. By containing the copper compound in a proportion within this range, it becomes easy to obtain flat copper particles having a highly uniform particle size.

  The copper-containing aqueous solution preferably contains a copper (II) ion complexing agent. By coexisting the complexing agent in the aqueous solution, copper (II) ions can be successfully reduced. As the complexing agent, for example, amino acids, tartaric acid and the like can be used. These can be used alone or in combination of two or more. Examples of amino acids that can be used include aminoacetic acid, alanine, and glutamic acid. Of these amino acids, use of aminoacetic acid is preferred because flat copper particles having a high uniformity in particle size can be easily obtained.

  The copper-containing aqueous solution preferably contains 0.005 mol to 10 mol, more preferably 0.01 mol to 5 mol of a complexing agent with respect to 1 mol of copper contained therein. It is preferable that the complexing agent is contained in a proportion within this range because flat copper particles having a fine particle size and a large crystallite diameter can be obtained. Moreover, since the aspect ratio of flat copper particle becomes large, it is preferable.

  The copper-containing aqueous solution is prepared by dissolving a copper compound and, if necessary, a complexing agent in water. There are no particular limitations on the method and order of dissolution of the copper compound and complexing agent in water. Examples of the method for dissolving the copper compound and the complexing agent include a method in which water is stirred and the copper compound and the complexing agent are added and stirred. The liquid temperature during the preparation of the copper-containing aqueous solution is preferably 50 ° C. to 90 ° C., more preferably 60 ° C. to 80 ° C., from the viewpoint of obtaining flat copper particles having a uniform particle size.

  A basic compound is added to the copper-containing aqueous solution thus obtained to produce cupric oxide (CuO). Examples of the basic compound used for this purpose include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, and ammonia. The produced cupric oxide is suspended in the liquid in the form of fine solid particles.

  Examples of the method for adding the basic compound to the copper-containing aqueous solution include a method in which the copper-containing aqueous solution is kept in a stirred state, and an aqueous solution of the basic compound is added thereto and stirred. The liquid temperature at this time is preferably 50 ° C to 90 ° C, more preferably 60 ° C to 80 ° C. When the liquid temperature is within this range, it is preferable because flat copper particles having a high particle size with little primary particle aggregation are easily obtained.

  The amount of the basic compound added to the copper-containing aqueous solution is such that the amount of the basic compound relative to 1 equivalent of the copper compound is preferably 1.05 equivalents to 1.5 equivalents, more preferably 1.1 equivalents to 1.3 equivalents. The amount is such that By making the addition amount of the basic compound within this range, it is preferable because flat copper particles having a high uniformity in particle diameter can be easily obtained. The equivalent of a copper compound and a basic compound means the equivalent as an acid and the equivalent as a base, respectively.

  After cupric oxide is formed by adding a basic compound to the copper-containing aqueous solution, it is preferable to continue aging by continuing the stirring of the solution. The aging is preferably performed for 10 minutes to 60 minutes, particularly 20 minutes to 40 minutes. It is preferable because cupric oxide is sufficiently formed by aging, and thereby flat copper particles having high particle size uniformity are easily obtained.

When cupric oxide is produced in this way, the first reduction step is performed next. In this reduction step, cupric oxide contained in the liquid is reduced to cuprous oxide (Cu ₂ O) by adding a reducing agent while stirring the liquid. Therefore, the reducing agent used in this reduction step has an action of reducing cupric oxide to cuprous oxide. As this reducing agent, for example, reducing sugar and hydrazine can be used. A reducing agent can be used individually or in combination of 2 or more types. As the reducing sugar, for example, glucose, fructose, lactose and the like can be used. Among these reducing sugars, glucose is preferably used because it easily controls the reduction reaction.

  In this reduction step, the reducing agent is preferably added in an amount of 0.1 mol to 3 mol, more preferably 0.3 mol to 1.5 mol, per 1 mol of copper contained in the liquid. When the addition amount of the reducing agent is within this range, the reduction reaction of cupric oxide to cuprous oxide is sufficiently performed. As a result, the target flat copper particles are unlikely to aggregate the primary particles. This is preferable.

  Even after cupric oxide is reduced to cuprous oxide by this reduction step, it is preferable to continue aging by continuing the stirring of the liquid. The aging is preferably performed for 10 minutes to 60 minutes, particularly 20 minutes to 40 minutes. The cuprous oxide is sufficiently produced by aging, and the intended flat copper particles are preferable because the primary particles are less likely to aggregate.

When the first reduction process is completed, the second reduction process is continued. In this reduction process, cuprous oxide contained in the liquid is reduced to copper by adding a reducing agent while stirring the liquid. Therefore, the reducing agent used in this reduction step has an action of reducing cuprous oxide to copper. As the reducing agent, in the present invention, two or more reducing agents are used in combination. As this reducing agent, one having a hydrogen reduction standard potential E ⁰ of −1.11 to −1.24V is used. By using a combination of two or more such reducing agents, cuprous oxide can be reduced at an appropriate rate, and target flat copper particles, particularly hexagonal flat copper particles can be successfully obtained. As a result of the study by the present inventors, it has been found that this can be done. Examples of the reducing agent that can be used include hydrazine (−1.11 V); sodium borohydride (−1.24 V), sodium cyanoborohydride (−1.24 V), and lithium triethylborohydride (−1. 24V), metal hydrides such as lithium borohydride (-1.24V), zinc borohydride (-1.24V), sodium triacetoxyborohydride (-1.24V); dimethylamine borane, diethylamine borane, etc. Of amine borane; The numerical value in parentheses is the hydrogen reduction standard potential E ⁰ . Of these reducing agents, a combination of hydrazine and another reducing agent is preferably used, and a combination of hydrazine and a borohydride compound or a combination of hydrazine and an amine borane is more preferable.

  When hydrazine is used as one of the two or more reducing agents used in the second reduction step, the hydrazine is preferably 1 mol to 10 mol, more preferably 3 mol per 1 mol of copper contained in the liquid. Mole to 7 moles are added. Regarding the reducing agent other than hydrazine, the total amount of the reducing agent is preferably 0.01 mol to 1.1 mol, more preferably 0.03 mol to 1 mol of copper contained in the liquid. Add 1.0 mole. The ratio of addition of hydrazine and one or more reducing agents other than hydrazine is expressed in moles, and the total amount of reducing agents other than hydrazine / hydrazine = preferably 1-1000, more preferably 100-700. And By setting the amount of these reducing agents used within this range, the intended flat copper particles, particularly hexagonal flat copper particles, can be obtained more successfully.

  In this reduction step, it is preferable to add hydrazine and a reducing agent other than hydrazine to the solution at the same time. Further, hydrazine and a reducing agent other than hydrazine may be added all at once in the solution, or may be added sequentially. From the viewpoint of obtaining the desired flat copper particles more successfully, it is preferable to perform batch addition. In any addition method, the liquid temperature is preferably set to 50 ° C to 90 ° C, particularly 60 ° C to 80 ° C. When the liquid temperature is within this range, it is preferable because flat copper particles having a high particle size with little primary particle aggregation are easily obtained.

  Even after cuprous oxide is reduced to copper by this reduction step, it is preferable to continue aging by agitating the liquid. Aging is preferably performed for 20 minutes to 120 minutes, particularly 40 minutes to 90 minutes. Reduction is sufficiently progressed by aging, and the intended flat copper particles are preferable because the primary particles are less likely to aggregate.

  In the second reduction step, when a borohydride compound is used as the reducing agent, a small amount of boron is contained in the flat copper particles produced by the reduction due to the borohydride compound. Due to the action of boron, the flat copper particles of the present invention have the advantageous effect of improving heat resistance, as described above. Furthermore, there is an advantageous effect that the particle size is made uniform and atomized.

  The produced flat copper particles are separated from the liquid by filtration using Nutsche or the like. Next, cleaning with pure water is performed once or a plurality of times. Then, if necessary, the surface treatment is performed by washing with a methanol solution containing an organic compound such as oleic acid.

  In the above manufacturing method, a phosphorus compound may be added in any step so that the target flat copper particles contain phosphorus. When the flat copper particles contain phosphorus, the oxidation resistance of the particles is improved, and the crystallite diameter of the copper is easily increased. The timing for adding the phosphorus compound is, for example, (i) before adding the basic compound to the copper-containing aqueous solution, (ii) simultaneously with or after adding the basic compound to the copper-containing aqueous solution, and (Iii) at the same time or after the addition of the reducing agent in the first reduction step and before the second reduction step, and (iv) the reducing agent in the second reduction step. Any one time or two or more time may be mentioned at the same time as or after the addition. In particular, it is preferable to add a phosphorus compound at the time of (i) because the target flat copper particles, particularly hexagonal flat copper particles can be obtained more successfully.

  As the phosphorus compound, it is preferable to use a compound capable of generating phosphate ions such as orthophosphate ions, pyrophosphate ions, and metaphosphate ions in the presence of water. Examples of such phosphorus compounds include polyphosphoric acid such as phosphoric acid and pyrophosphoric acid, metaphosphoric acid such as trimetaphosphoric acid, phosphate such as sodium phosphate and potassium phosphate, and polyphosphate such as sodium pyrophosphate and potassium pyrophosphate. And metaphosphates such as sodium trimetaphosphate and potassium trimetaphosphate. Among these phosphorus compounds, when trisodium phosphate is used, flat copper particles having sharp corners, particularly hexagonal flat copper particles having sharp corners, can be successfully obtained.

  The total amount of the phosphorus compound added in this production method is expressed in terms of P (phosphorus), and is preferably 0.001 to 3 mol, more preferably 0.01 to 1 mol per 1 mol of copper. Is a mole. It is preferable that the addition amount of the phosphorus compound be within this range because the oxidation resistance is easily increased and the crystallite diameter of the copper is easily increased without impairing the electrical conductivity of the target flat copper particles.

  The target flat copper particles are obtained by the above method. The flat copper particles thus obtained are suitably used as a raw material for conductive paste, for example. This conductive paste contains the flat copper particles of the present invention and a resin. Examples of this resin include acrylic resin, epoxy resin, ethyl cellulose, carboxyethyl cellulose, and the like. As the copper particles in the conductive paste, only the flat copper particles of the present invention may be used, or a combination of the flat copper particles and copper particles having other shapes such as a spherical shape may be used. By using the flat copper particles of the present invention in combination with spheres or other shapes of copper particles, it becomes easy to precisely adjust the viscosity of the paste.

  The conductive paste thus obtained is preferably used for, for example, circuit formation of a printed wiring board, ensuring electrical continuity of an external electrode of a ceramic capacitor, and measures against EMI.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, “%” means “% by weight”.

[Example 1]
To 6 liters of pure water at 70 ° C., 4 kg of copper sulfate pentahydrate, 120 g of aminoacetic acid and 50 g of trisodium phosphate were added and stirred. Pure water was further poured into this to adjust the liquid volume to 8 L, and stirring was continued for 30 minutes to obtain a copper-containing aqueous solution. Next, with this aqueous solution stirred, 5.8 kg of 25% sodium hydroxide solution was added to the aqueous solution to form cupric oxide in the liquid. Subsequently, after stirring for 30 minutes, 1.5 kg of glucose was added to perform a first reduction reaction, and cupric oxide was reduced to cuprous oxide. Subsequently, after stirring for 30 minutes, 1 kg of hydrazine monohydrate and 3 g of sodium borohydride were added all at once while the liquid was stirred, and a second reduction reaction was performed to reduce cuprous oxide to copper. Subsequently, stirring was performed for 1 hour to complete the reaction. After completion of the reaction, the resulting slurry was filtered using a Nutsche, then washed with pure water and methanol, and further dried to obtain the desired flat copper particles. The SEM image of this copper particle is shown in FIG. As is clear from the figure, it was confirmed that the obtained copper particles had a hexagonal plate shape with each side being substantially straight.

[Examples 2 to 4]
Copper particles were obtained in the same manner as in Example 1 except that the production was performed under the conditions shown in Table 1 below. SEM images of the obtained copper particles are shown in FIG. 2 (Example 2), FIG. 3 (Example 3), and FIG. 4 (Example 4).

[Comparative Example 1]
To 6 L of pure water at 70 ° C., 4 kg of copper sulfate pentahydrate, 120 g of aminoacetic acid, and 50 g of trisodium phosphate were added and stirred. Pure water was further poured into this to adjust the liquid volume to 8 L, and stirring was continued for 30 minutes to obtain a copper-containing aqueous solution. Next, with this aqueous solution stirred, 5.8 kg of 25% sodium hydroxide solution was added to the aqueous solution to form cupric oxide in the liquid. Subsequently, after stirring for 30 minutes, 1.5 kg of glucose was added to perform a first reduction reaction, and cupric oxide was reduced to cuprous oxide. Subsequently, after stirring for 30 minutes, hydrazine monohydrate was added all at once while the liquid was stirred, and stirring was continued for 1 hour to complete the reaction. After completion of the reaction, the obtained slurry was filtered using a Nutsche, then washed with pure water and methanol, and further dried to obtain target copper particles. An SEM image of the obtained copper particles is shown in FIG.

[Comparative Example 2]
In this comparative example, flat copper particles were produced by mechanical treatment. That is, spherical copper particles having an average particle diameter of 5 μm (made by Mitsui Metal Mining Co., Ltd.) were used and pulverized with a planetary ball mill for 72 hours to obtain flat copper particles. As media, stainless steel beads having a diameter of 1 mm were used. Grinding was performed for 180 minutes using ethanol as a solvent. The SEM image of the obtained copper particle is shown in FIG.

[Comparative Example 3]
In this comparative example, spherical copper particles were produced according to a general production method. That is, 4 kg of copper sulfate pentahydrate and 120 g of aminoacetic acid were dissolved in water to obtain 8 liters of an aqueous solution of copper salt. To this was added 5 liters of 25% sodium hydroxide over 5 minutes to obtain a cupric oxide slurry. To this solution, 1.5 kg of glucose was added to obtain cuprous oxide. Further, 1 kg of hydrated hydrazine was added over 5 minutes to obtain the copper particles. The SEM image of the obtained copper particle is shown in FIG.

[Evaluation]
For copper particles obtained in Examples and Comparative Examples, the average particle diameter of Dia, the particle size D ₅₀ by laser diffraction scattering particle size distribution measuring method, to measure the aspect ratio in the manner described above. Further, the SD / D ₅₀ and aspect ratio distributions were measured by the methods described above. Moreover, the crystallite diameter of the copper particles was measured by the method described above. Furthermore, the amount of boron and phosphorus contained in the copper particles was measured by the method described above. Furthermore, the thixo ratio and the oxidation start temperature were measured by the following methods. These results are shown in Table 2 below.

[Thixo ratio]
Using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd.), the ratio of the viscosity at 1/10 rotations was determined, and this was defined as the thixo ratio. The thixo ratio indicates that the larger the value, the higher the viscosity.

[Oxidation start temperature]
Thermogravimetry (TG) was performed using a thermal analyzer (manufactured by Seiko Instruments Inc.), and the change temperature of the weight was defined as the oxidation start temperature.

  As is apparent from the results shown in Table 2 and FIGS. 1 to 7, it can be seen that the copper particles obtained in each example are hexagonal plate-shaped fine particles. It can also be seen that the crystallite size is large. The copper particles of Comparative Example 1 that did not use a specific reducing agent combination in the second reduction step had a rounded outline. The copper particles of Comparative Example 3 obtained by a general production method are spherical, and as shown in FIG. 7, all the angles formed by adjacent sides are not more than 60 degrees and less than 180.

Claims (8)

In plan view, it has a contour defined by a plurality of substantially linear sides, and is composed of a flat body in which the angles formed by adjacent sides are all 60 degrees or more and less than 180,
Flat copper particles having an average particle diameter Dia of 0.05 to 0.5 μm.   The flat copper particle of Claim 1 which has a hexagonal outline in planar view.   The flat copper particle of Claim 1 or 2 containing 1-50 ppm of boron.   The flat copper particle in any one of Claim 1 thru | or 3 containing 10-200 ppm of phosphorus.   The flat copper particles according to any one of claims 1 to 4, having an aspect ratio (particle diameter / thickness) of 2 to 25. In the method of producing flat copper particles having a reduction step of reducing copper by adding a reducing agent to an aqueous solution containing a water-soluble copper compound,
The reduction step includes a first reduction step and a second reduction step performed thereafter;
In the first reduction step, reducing sugar or hydrazine is used as a reducing agent,
A method for producing flat copper particles, wherein two or more reducing agents having a hydrogen reduction standard potential E ⁰ of −1.11 to −1.24 V are used as reducing agents in the second reduction step. Adding a basic compound to an aqueous solution containing a water-soluble copper compound to produce cupric oxide, and then reducing the produced cupric oxide to cuprous oxide in the first reduction step;
The method for producing flat copper particles according to claim 6, wherein cuprous oxide is reduced to copper in the second reduction step.   The method for producing flat copper particles according to claim 6 or 7, wherein the reducing agent used in the second reduction step is one or more of hydrazine and a reducing agent other than hydrazine, and all of these reducing agents are added together. .