Copper particles and method for manufacturing the same

Abstract

The copper particles of the present invention contain copper element as a main body. The ratio (S1 / B) of the first crystallite size S1 calculated from the half value width of the peak from the (111) plane of copper by the Scherrer formula to the particle diameter B calculated from the BET specific surface area is 0.23 or less in X-ray diffraction measurement. The ratio (S1 / S2) of the aforementioned first crystallite size S1 of the copper particles to the second crystallite size S2 calculated from the half value width of the peak from the (220) plane of copper by the Scherrer formula in X-ray diffraction measurement is 1.35 or less. The present invention also provides a method for producing copper particles.

Description

TECHNICAL FIELD

[0001] The present application relates to a copper particle and a method for producing the same. BACKGROUND

[0002] The present applicant has previously proposed a technology relating to a flat copper particle having a substantially hexagonal profile in plan view (see Patent Document 1). This copper particle has the advantage of being able to improve packing density and reduce surface roughness of a resulting conductor.

[0003] PRIOR ART DOCUMENTS

[0004] PATENT DOCUMENTS

[0005] Patent Document 1: Japanese Patent Application Publication No. 2012-041592 SUMMARY

[0006] Problems to be solved by the invention

[0007] In the technology described in Patent Document 1, since the crystallinity of the particle is high, there is room for improvement from the viewpoint of achieving sintering at a lower temperature.

[0008] Therefore, an object of the present application is to provide a copper particle that can be sintered at a low temperature.

[0009] The present application provides a copper particle comprising a copper element as a main body,

[0010] The ratio (S1 / B) of the first crystallite size S1 calculated from the half-value width of the peak from the (111) plane of copper in X-ray diffraction measurement using the Scherrer formula with respect to the particle diameter B calculated from the BET specific surface area is 0.23 or less,

[0011] The ratio (S1 / S2) of the first crystallite size S1 described above with respect to the second crystallite size S2 calculated from the half-value width of the peak from the (220) plane of copper in X-ray diffraction measurement using the Scherrer formula is 1.35 or less.

[0012] The present application provides a method for producing a copper particle, the method comprising the steps of:

[0013] a first reduction step of generating cuprous oxide by reducing copper ions, and

[0014] a second reduction step of generating a copper particle by reducing the cuprous oxide described above,

[0015] In any stage of performing the second reduction step or before performing the second reduction step, a polyphosphoric acid of diphosphoric acid or more or a salt thereof is made to exist in the reaction system. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 Images (a) to (d) are scanning electron microscope images of copper particles before sintering in Examples 1 to 4, respectively.

[0017] Figure 2 Images (a) to (c) are scanning electron microscope images of copper particles before sintering in Comparative Examples 1 to 3, respectively.

[0018] Figure 3 (a) is a scanning electron microscope image of the copper particles in Example 2 before sintering. Figure 3 (b) is a scanning electron microscope image of the copper particles in Example 2 after sintering. Detailed Implementation

[0019] The present invention will now be described based on its preferred embodiments. The copper particles of the present invention comprise copper as the main component. Furthermore, for the copper particles, there is a prescribed relationship between the crystallite sizes of specific crystal planes calculated by X-ray diffraction.

[0020] Containing copper as the main component means that the copper content in the copper particles is 50% by mass or more, preferably 80% by mass or more, more preferably 98% by mass or more, and even more preferably 99% by mass or more. The copper content can be determined by, for example, ICP emission spectroscopy.

[0021] The copper particles can be particles containing elements other than copper, or particles composed entirely of copper and containing no elements other than copper except for unavoidable impurities. The latter type, i.e., composed entirely of copper, is preferred, but trace amounts of unavoidable impurity elements such as oxygen are permissible as long as they do not impair the effects of the invention. In either case, the content of elements other than copper in the copper particles is preferably 2% by mass or less. The content of these elements can be determined, for example, by ICP emission spectroscopy.

[0022] For the copper particles of the present invention, it is preferable that the particle size calculated from its BET specific surface area has a predetermined relationship with the crystallite size calculated from the X-ray diffraction peaks of the (111) plane derived from copper.

[0023] Specifically, when the particle size calculated from the BET specific surface area is set as particle size B, and the crystallite size calculated from the diffraction peaks originating from the (111) plane of copper in X-ray diffraction measurement is set as the first crystallite size S1, the ratio of the first crystallite size S1 to the particle size B (S1 / B) is preferably 0.23 or less, more preferably 0.02 or more and 0.23 or less, and even more preferably 0.05 or more and 0.23 or less.

[0024] The diffraction peak derived from the (111) plane of copper is the peak having the largest height in the X-ray diffraction pattern obtained when the copper particles of the present application are subjected to X-ray diffraction measurement. It is thus considered that the first crystallite size is larger than the crystallite sizes calculated from the diffraction peaks derived from other crystal planes, and it also represents the crystallinity. It is thus presumed that, since the structure in which the first crystallite size S1 is smaller than the particle diameter B is formed, the grain boundaries are more in one particle. As a result, the crystallite interfaces are easily destabilized due to the heat energy applied when the particles are heated, atomic diffusion becomes active, the fusion property of the particles to each other at low temperatures can be improved, and the low-temperature sinterability can be improved.

[0025] Such copper particles can be obtained by, for example, the production method described later.

[0026] The particle diameter B calculated from the BET specific surface area is preferably 100 nm or more and 500 nm or less, more preferably 100 nm or more and 400 nm or less, and further preferably 120 nm or more and 400 nm or less. By bringing the particle diameter B to such a range, the thermal conductivity can be improved, and the low-temperature sinterability can be effectively improved.

[0027] The particle diameter B can be measured based on the BET method under the following conditions. Specifically, "Macsorb" manufactured by MOUNTECH Corporation can be used, and the measurement can be performed by the nitrogen adsorption method. The amount of the powder to be measured is set to 0.2 g, and the pre-degassing conditions are set to 80°C under vacuum for 30 minutes. Then, the particle diameter B is calculated from the measured BET specific surface area by the following formula (I).

[0028] In formula (I), d is the particle diameter B [nm], A is the specific surface area [m 2 / g] measured by the BET single-point method, and p is the density of copper [g / cm 3 ].

[0029] d = 6000 / (A x p)...(I)

[0030] The first crystallite size S1 is preferably 10 nm or more and 60 nm or less, more preferably 20 nm or more and 60 nm or less, and further preferably 25 nm or more and 55 nm or less. By bringing the crystallite size S1 to such a range, it is easy to form more grain boundaries in one particle, and the fusion property of the particles when heated can be further improved, and the low-temperature sinterability can be effectively improved.

[0031] In addition, when the crystallite size calculated from the half-value width of the peak derived from the (220) plane of copper by the Scherrer formula in the X-ray diffraction measurement is taken as the second crystallite size S2 for the copper particles, it is also preferable that the ratio of the first crystallite size S1 to the second crystallite size S2 (S1 / S2) be a prescribed value or less.

[0032] Specifically, the S1 / S2 ratio is preferably 1.35 or less, more preferably 0.1 or more and 1.35 or less, further preferably 0.1 or more and 1.2 or less.

[0033] The copper particles of the present application have a (111) plane of copper on a specific plane of the particle surface and a (220) plane of copper on a plane intersecting the (111) plane. Further, the smaller the S1 / S2 ratio, the more it indicates that the copper particles do not grow in the (111) plane direction or grow in the (220) plane direction. Therefore, the S1 / S2 being within the above-mentioned range is roughly related to the copper particles of the present application having anisotropy in the particle shape such as a flat shape. The flat shape refers to a shape having a pair of main surfaces opposite to each other and a side surface intersecting the main surfaces. In the case where the copper particles have a flat shape, it is presumed that the (111) plane of copper is present on the main surface of the copper particles and the (220) plane of copper is present on the side surface of the copper particles.

[0034] Therefore, by making the S1 / S2 ratio within the above-mentioned range, when the particles are arranged at the time of sintering, the main surfaces of the particles easily come into contact with each other or the side surfaces of the particles easily come into contact with each other, and the contact portions of the particles with each other easily form the same crystal plane. In the case where the particles after application of heat energy come into contact with each other on the same crystal plane, the efficiency of use of the heat energy is high compared to the case where the particles come into contact with each other on different crystal planes, and the atoms of the grain boundaries easily diffuse. As a result, the fusion property of the particles with each other at low temperatures can be improved, and the low-temperature sinterability can be improved. This is advantageous in that the sinterability can be further improved compared to spherical particles or mechanically manufactured flat copper particles.

[0035] Such copper particles can be obtained by, for example, the manufacturing method described later.

[0036] The 2nd grain size S2 is preferably 10 nm or more and 60 nm or less, more preferably 20 nm or more and 50 nm or less, further preferably 30 nm or more and 50 nm or less. By making the grain size S2 within this range, the low-temperature sinterability due to the small grain size can be improved, and more conductive paths derived from the shape of the copper particles can be formed, and a conductor with low resistance can be formed after sintering.

[0037] For the copper particles of the present application, when the grain size calculated from the half-value width of the peak derived from the (311) plane of copper using the Scherrer formula in X-ray diffraction measurement is the 3rd grain size S3, the ratio of the 1st grain size S1 to the 3rd grain size S3 (S1 / S3) is preferably a prescribed value or less.

[0038] Specifically, the S1 / S3 ratio is preferably 1.35 or less, more preferably 0.2 or more and 1.30 or less, further preferably 0.5 or more and 1.25 or less.

[0039] The metal copper easily forms a crystal structure of a face-centered cubic structure, and therefore the copper particles of the present application have a (111) plane of copper on a specific plane of the particle surface, and a (311) plane of copper on a plane intersecting the (111) plane. Further, the smaller the S1 / S3 ratio, the more it indicates that the copper particles do not grow in the (111) plane direction or grow in the (311) plane direction. Therefore, the S1 / S3 being in the above-described range is roughly correlated with the particle shape of the copper particles being anisotropic, such as being flat. In this case, it is presumed that the (111) plane of copper is present on the major plane of the copper particles, and the (311) plane of copper is present on the side plane of the copper particles.

[0040] Therefore, by making the S1 / S3 ratio be in the above-described range, when the particles are arranged at the time of sintering, the major planes of the particles easily come into contact with each other or the side planes of the particles easily come into contact with each other, and the contact portions of the particles with each other easily form the same crystal plane. As a result, the atomic diffusion at the crystal grain boundaries is activated at the time of heating the particles, and the fusion of the particles at low temperatures can be improved, and the low-temperature sinterability can be improved. This is advantageous in that the sinterability can be further improved compared to spherical particles or mechanically manufactured flat copper particles.

[0041] Such copper particles can be obtained by, for example, the manufacturing method described later.

[0042] The third crystal grain size S3 is preferably 10 nm or more and 60 nm or less, more preferably 20 nm or more and 50 nm or less, and further preferably 30 nm or more and 50 nm or less. By making the crystal grain size S3 be in this range, the low-temperature sinterability due to the small crystal grain size can be improved, and more conductive paths derived from the shape of the copper particles can be formed, and a conductor with low resistance can be formed after sintering.

[0043] As for the first crystal grain size S1, the second crystal grain size S2, and the third crystal grain size S3, the full width at half maximum of the half-value width of the diffraction peak derived from the (111) plane, the (220) plane, or the (311) plane of copper, which is obtained by X-ray diffraction measurement, can be calculated using the Scherrer formula shown below. The conditions for the X-ray diffraction measurement will be described in detail in the Examples described later. The PDF number is 00-004-0836.

[0044] • Scherrer formula: D = Kλ / βcosθ

[0045] • D: Crystal grain size

[0046] • K: Scherrer constant (0.94)

[0047] • λ: Wavelength of X-rays

[0048] • β: Half-value width [rad]

[0049] • θ: Diffraction angle

[0050] The copper particle is also preferably low in the content of carbon element contained in the particle. Specifically, the content of carbon element in the copper particle is preferably 1000 ppm or less, more preferably 900 ppm or less, and further preferably 800 ppm or less, and is more preferably lower, but is 100 ppm or more in practice. By setting the content of carbon element to such a range, the sintering hindrance caused by the organic substance present on the surface of the copper particle can be more suppressed. Such a copper particle can be produced by, for example, the production method described later.

[0051] The content of carbon element can be measured by, for example, gas analysis, combustion-type carbon analysis, or the like. In measuring the content of carbon element, it is first determined whether or not the surface of the particle has been subjected to coating treatment. The method of such determination can be exemplified by a method performed by, for example, X-ray photoelectron spectroscopy (XPS) method, nuclear magnetic resonance (NMR) method, Raman spectroscopy, infrared spectroscopy, liquid chromatography, time-of-flight secondary ion mass spectrometry (TOF-SIMS), or the like, alone or in combination. If it is determined by the above method that the surface of the particle has been subjected to coating treatment, the kind and amount of element contained in the coating layer formed by the coating treatment are qualitatively and quantitatively analyzed by the above method alone or in combination. On this basis, the mass change before and after calcination and the amount of carbon after heating to the temperature are measured by thermogravimetric analysis (TG), and thus the physical properties of the organic substance can be evaluated.

[0052] In the case where it is determined that the surface of the particle has not been subjected to coating treatment, the copper particle to be measured is directly subjected to measurement, and the obtained quantitative value is taken as the content of carbon element contained in the copper particle.

[0053] The copper particle is also preferably low in the content of phosphorus element contained in the particle. Specifically, the content of phosphorus element in the copper particle is preferably 300 ppm or more, more preferably 300 ppm or more and 1500 ppm or less, and further preferably 300 ppm or more and 1000 ppm or less. By setting the content of phosphorus element to such a range, the conductivity possessed by copper can be sufficiently maintained, and the melting point can be lowered, and thus the sinterability at low temperature can be further improved. Such a copper particle can be produced by, for example, the production method described later. The presence or absence of phosphorus element in the copper particle and the content thereof can be measured by, for example, ICP emission spectroscopic analysis.

[0054] The copper particle of the present application is not particularly limited in shape as long as it exhibits the effects of the present application, and is preferably a flat shape when produced by the method described later. Such a particle is a plate-like shape having a pair of main faces that are opposed to each other and are substantially flat, and a side face that intersects the two main faces, and the maximum span length of the main faces is larger than the thickness. In this case, it is also preferable that the shape has a profile drawn by the combination of straight lines with each other or the combination of straight lines and curved lines when the main face of the copper particle is viewed from above.

[0055] Next, the preferred production method of the above copper particles will be described. The production method has two reduction steps, i.e., a first reduction step in which cuprous oxide is produced by reducing copper ions; and a second reduction step in which copper particles are produced by reducing the cuprous oxide in the presence of a polyphosphoric acid having two or more phosphoric acid groups or a salt thereof (hereinafter, also referred to as polyphosphoric acid-based).

[0056] In either case of performing the second reduction step or at any stage before performing the second reduction step, the polyphosphoric acid-based is allowed to exist in the reaction system. That is, the polyphosphoric acid-based can be allowed to exist in the reaction system before performing the first reduction step or at the time of performing the first reduction step, and the second reduction step is performed in this state. Alternatively, the polyphosphoric acid-based can not be allowed to exist in the reaction system in the first reduction step, but the polyphosphoric acid-based is allowed to exist in the reaction system at the time of performing the second reduction step or just before performing the second reduction step.

[0057] From the viewpoint of achieving both uniform control of the reduction reaction and the resulting improvement in the productivity of the copper particles and reduction in the production cost, it is preferred that both the reduction steps be performed under wet conditions in which reduction in an aqueous solution is performed, and it is further preferred that both the reduction steps be performed in the same reaction system. Hereinafter, the production method in which both the reduction steps are performed under wet conditions and in the same reaction system will be described.

[0058] First, a reaction solution containing a copper source and a reducing compound is prepared, and the first reduction step is performed to reduce the copper ions and produce cuprous oxide in the solution. The reaction solution can be prepared by simultaneously adding the respective raw materials in a solvent, or the respective raw materials can be added in any order in the solvent.

[0059] From the viewpoint of easily controlling the reduction reaction of the copper ions and thus improving the handling properties during production, it is preferred that a copper-containing solution be prepared by previously mixing the copper source and the solvent, and then a solid reducing compound or a solution of the reducing compound previously dissolved in the solvent be added to the copper-containing solution. The reducing compound can be added at one time or sequentially.

[0060] In the first reduction step, as described above, the reaction solution can or can not contain the polyphosphoric acid-based. In the case where the polyphosphoric acid-based is contained in the reaction solution, from the viewpoint of being able to effectively perform control of the reduction of the copper ions based on the reducing compound and the crystal growth, it is preferred that the copper source, the polyphosphoric acid-based, and the reducing compound be added sequentially.

[0061] The solvent in the reaction solution can be water; a lower alcohol such as methanol, ethanol, or propanol. These can be used alone or in a combination of two or more.

[0062] As the copper source used in the first reduction step, a compound that generates copper ions in the reaction solution can be mentioned, and a water-soluble copper compound is preferably selected. As specific examples of such a copper source, copper formate, copper acetate, copper propionate and the like copper organic acid salts; copper nitrate, copper sulfate and the like copper inorganic acid salts; and various copper compounds can be mentioned. These copper compounds can be anhydrous or a hydrate. These copper compounds can be used alone or in combination of a plurality of kinds.

[0063] The content of the copper source in the reaction solution of the first reduction step is preferably 0.5 mol / L or more and 5 mol / L or less, and more preferably 1 mol / L or more and 4 mol / L or less, in terms of the molar concentration of copper element. By being in such a range, copper particles having a small particle diameter and a small crystallite size of a specific crystal plane can be produced at a high productivity.

[0064] As the reducing compound, a water-soluble compound is preferably selected. As specific examples of the reducing compound, hydrazine, hydrazine hydrochloride, hydrazine sulfate, and hydrazine hydrate and the like hydrazine-based compounds; sodium borohydride, dimethylamine borane and the like boron compounds and salts thereof; sodium sulfite, sodium bisulfite, and sodium thiosulfate and the like sulfur oxoacid salts; sodium nitrite and sodium subnitrite and the like nitrogen oxoacid salts; phosphorous acid, sodium phosphite, hypophosphorous acid, and sodium hypophosphite and the like phosphorus oxoacids and salts thereof can be mentioned. These reducing compounds can be anhydrous or a hydrate. These reducing compounds can be used alone or in combination of two or more kinds.

[0065] From the viewpoint of easily controlling the reduction product of the first reduction step to be cuprous oxide, easily controlling the crystal grain growth of copper in the subsequent reduction step to obtain particles having a prescribed crystallite size, and from the viewpoint of reducing the unintended mixing of impurities such as carbon element after reduction, as the reducing compound in the reducing solution, a hydrazine-based compound is preferably used, and more preferably anhydrous or a hydrate of hydrazine is used.

[0066] The content of the reducing compound in the reaction solution of the first reduction step is preferably 0.5 mol or more and 3.0 mol or less, and more preferably 1.0 mol or more and 2.0 mol or less, per 1 mol of copper element. By controlling the concentration of the reducing compound in such a range, the reduction reaction of copper ions and the progress of the crystal grain growth can be moderately controlled, and thus copper particles having a small particle diameter and a small crystallite size of a specific crystal plane can be obtained at a high productivity.

[0067] From the viewpoint of appropriately controlling the degree of reduction to a degree that is reduced to cuprous oxide and is not reduced to metallic copper in the case of using a reducing compound, particularly a hydrazine-based compound, and easily imparting anisotropy to the crystal growth of copper performed in the second reduction step, the reaction solution of the first reduction step is preferably set to an acidic condition in which the pH is 3.5 or more and 5.5 or less at 25°C. In the first reduction step, the reducing compound is added after the adjustment of the pH value, which is preferable from the viewpoint of being able to appropriately control the degree of reduction of copper ions.

[0068] The adjustment of the pH can use various acidic or basic substances or cause a polyphosphoric acid-based compound to exist in the reaction solution, as long as the effect of the present application can be exerted. In particular, in adjusting the pH, by using a polyphosphoric acid-based compound, the subsequent reaction can be efficiently performed even without adding other substances to the reaction system, and thus, it is advantageous from the viewpoint of preventing the unintended mixing of impurities and efficiently obtaining the target copper particles.

[0069] The reduction reaction of the first reduction step can be performed in a non-heated state or in a heated state. In either case, the temperature of the reaction solution is preferably set to 5°C or more and 35°C or less, and more preferably set to 10°C or more and 30°C or less. The reaction time of the first reduction step is preferably set to 0.1 hours or more and 3 hours or less, and more preferably set to 0.2 hours or more and 2 hours or less, with the above temperature range as a condition. In addition, from the viewpoint of the uniformity of the reduction reaction, it is preferable to continuously stir the reaction solution from the start time to the end time of the reaction.

[0070] Next, the second reduction step of generating a particle of metallic copper by reducing the cuprous oxide obtained in the first reduction step is performed. Also with respect to the second reduction step, it is also preferable to perform it in a wet condition as with the first reduction step, and further preferably, the two reduction steps are performed in the same reaction system.

[0071] As described above, it is preferable to cause a polyphosphoric acid-based compound to exist in the reaction system at any stage when the second reduction step is performed or before the second reduction step is performed.

[0072] As the polyphosphoric acid-based compound used in the present production method, a polyphosphoric acid having two or more and eight or less, and more preferably two or more and five or less, of phosphoric acid monomer units in the structure of dipolyphosphoric acid (H4P2O7), triphosphoric acid (tripolyphosphoric acid, H5P3O 10 ), tetraphosphoric acid (tetrapolyphosphoric acid, H6P4O 13 ), and the like, and salts thereof can be mentioned. As the polyphosphoric acid salt, an alkali metal salt, an alkaline earth metal salt, another metal salt, an ammonium salt, and the like can be mentioned. These can be used alone or in a plurality of combinations.

[0073] The content of the polyphosphoric acid in the second reduction step is preferably 0.001 mol or more and 0.05 mol or less, more preferably 0.001 mol or more and 0.01 mol or less, per 1 mol of copper element. By setting the concentration of the polyphosphoric acid to such a range, the crystal growth of copper caused by the reduction reaction of cuprous oxide can be made anisotropic, and copper particles having a small particle diameter and a small crystallite size of a specific crystal plane can be obtained at a high productivity.

[0074] Note that, in the case where the polyphosphoric acid is contained at the time of the first reduction step, the polyphosphoric acid is not consumed in the reaction of the first reduction step, and the concentration of the polyphosphoric acid is substantially unchanged before and after the first reduction step. Therefore, by adding the polyphosphoric acid to the reaction system in the first reduction step at the above concentration range, the amount of the polyphosphoric acid present, which is suitable for the reduction to copper metal and the crystal growth in the second reduction step, can be sufficiently achieved.

[0075] In the second reduction step, the above reducing compound can be added to perform the reduction to copper metal. The content of the reducing compound in the reaction solution of the second reduction step is preferably 3 mol or more and 15 mol or less, more preferably 4 mol or more and 13 mol or less, per 1 mol of copper element. In the case where the second reduction step is performed in the same reaction system as the first reduction step, the reducing compound is preferably further added to the liquid to reach the above content from the viewpoint of balancing the improvement of the reducing property and the control of the reduction of impurities. In addition, the type of the reducing compound is also preferably the same in each reduction step.

[0076] By controlling the concentration of the reducing compound to such a range, the reduction reaction to copper metal can be sufficiently performed, and copper particles having a small particle diameter and a small crystallite size of a specific crystal plane can be obtained at a high productivity.

[0077] The reducing compound of the second reduction step can be added at once or sequentially. From the viewpoint of efficiently obtaining copper particles satisfying the above crystallite size ratio and particle diameter, sequential addition is preferable.

[0078] From the viewpoint that the residual copper ions and cuprous oxide in the reaction solution can be effectively reduced to copper metal and the crystal growth of copper can be easily made anisotropic in the case of using a reducing compound, particularly a hydrazine-based compound, the reaction solution of the second reduction step is preferably set to a non-acidic condition (neutral or basic condition) in which the pH is 7.0 or more at 25°C. From the viewpoint that the degree of reduction of copper ions can be appropriately controlled, the adjustment of the pH is preferably performed before the addition of the reducing compound in the second reduction step. The adjustment of the pH can be performed using various acids or basic substances.

[0079] In the case where the second reduction step is performed in the same reaction system as the first reduction step, since the reaction solution after the first reduction step is under acidic conditions, it is preferable to adjust the pH of the reaction solution by adding an alkaline substance such as sodium hydroxide, potassium hydroxide, or the like. From the viewpoint of being able to effectively reduce copper ions and cuprous oxide to metallic copper, in the second reduction step, it is preferable to add the reducing compound after the adjustment of the pH value.

[0080] From the viewpoint of efficiently performing the reduction of copper ions and cuprous oxide in the reaction solution and obtaining copper particles having a prescribed crystallite size with high productivity, in the second reduction step, it is preferable to heat the reaction solution. With respect to the heating conditions of the reaction solution, it is preferable to perform heating in such a manner that the reaction solution is maintained at 30°C or higher and 80°C or lower, particularly 30°C or higher and 50°C or lower, from the start time of the second reduction step, that is, from the time of addition of the reducing compound, to the end time of the reaction. The reaction time is preferably set to 60 minutes or longer and 180 minutes or shorter under the above temperature conditions. In addition, from the viewpoint of causing the reduction reaction to occur uniformly, thereby obtaining copper particles having little deviation in particle diameter, it is also preferable to continuously stir the reaction solution from the start time of the reaction to the end time of the reaction.

[0081] With respect to the reason why, in the present production method, by performing a two-stage reduction step in which copper ions are reduced to metallic copper via cuprous oxide, and a polyphosphoric acid is present at the time of performing the second reduction step, copper particles capable of achieving low-temperature sinterability are obtained, the present inventors made the following conjecture.

[0082] First, in the first reduction step, copper ions are reduced by the reducing compound in the reaction solution, and very small particles of cuprous oxide are generated in the reaction solution. Next, in the second reduction step, the monovalent copper ions that are dissolved out of the cuprous oxide particles are reduced, and a nucleus of metallic copper is formed. Since this nucleus is very unstable, the nuclei repeatedly coalesce with each other or are redissolved into the reaction solution, and the particles eventually gradually grow. If a polyphosphoric acid is present at the time of this particle growth, the polyphosphoric acid will be adsorbed to a specific crystal plane of copper, and the growth in this crystal plane direction is inhibited. On the other hand, the crystal plane that is not adsorbed to the polyphosphoric acid is not inhibited from growing, and the growth in this crystal plane direction proceeds.

[0083] Based on the viewpoint that metallic copper easily forms a crystal structure of face-centered cubic structure, and the results of X-ray diffraction measurement of the obtained copper particles, the crystal plane to which the polyphosphoric acid is adsorbed is conjectured to be the (111) plane of copper in the particles, and the crystal plane that is not adsorbed to the polyphosphoric acid is conjectured to be the (220) plane of copper that is in the perpendicular direction to the (111) plane of copper. From this, it is considered that anisotropic growth is formed, that is, the growth of the (111) plane of copper is inhibited and the growth of the (220) plane of copper proceeds, as a result of which flat copper particles capable of achieving low-temperature sinterability are formed.

[0084] Further, as a suitable production method of the present application, in particular, in the first reduction step, by performing the reduction reaction under acidic conditions, it is possible to control the reducing power to the extent that the copper ions are reduced to cuprous oxide and not to the extent that it is reduced to metallic copper. On this basis, the control of the subsequent metallic copper generation reaction also becomes easy. Thereafter, by setting to non-acidic conditions, it is possible to reduce the elution rate of the cuprous oxide and control the supply of the monovalent copper ions. By performing the second reduction under this environment, it is possible to adjust the reduction reaction rate to metallic copper to a slow condition, and thus it is particularly advantageous in terms of being able to control the nucleus growth rate.

[0085] The copper particles of the present application obtained through the above procedure satisfy the above suitable crystallite size and its ratio, suitable particle diameter, suitable content of various elements such as carbon element, and the like, even in the case where no organic components such as organic amines, amino alcohols, reducing sugars, and the like that control the crystal growth are contained, and further have a flat shape.

[0086] Further, the crystal planes of the crystal that exist on the main surface and grow in the direction perpendicular to the main surface, and the crystal planes of the crystal that exist on the side surface and grow in the direction along the main surface, of the copper particles thus obtained each have a specific orientation direction, and each of the crystal planes is uniformly formed in one direction. Therefore, when the copper particles are used and are fired in a state where the main surfaces of the copper particles contact each other or in a state where the side surfaces of the copper particles contact each other, since the same crystal planes that are uniformly arranged contact each other, there is no excessive need for energy required for melting, and sintering can be performed at a low temperature.

[0087] The copper particles obtained through the above procedure can be used in the form of a slurry in which the copper particles are dispersed in a solvent such as water or an organic solvent, after being cleaned and subjected to solid-liquid separation as needed, or the particles can be dried and used in the form of a dried powder that is an aggregate of the copper particles. In either case, the copper particles of the present application are excellent in low-temperature sinterability. The copper particles can also be further subjected to surface coating treatment using an organic substance such as a fatty acid or a salt thereof, an inorganic substance such as a silicon compound, and the like, as needed, for the purpose of improving the dispersibility of the particles with respect to each other.

[0088] Note that, as long as the effects of the present application can be exerted, it is permissible for the surface of the obtained copper particles to be inevitably slightly oxidized and the like, so as to contain other elements in addition to the copper element.

[0089] Further, the copper particles of the present application can also be further dispersed in an organic solvent, a resin, and the like, and used in the form of a conductive composition such as a conductive ink or a conductive paste.

[0090] In a case where the copper particles of the present application are made into a conductive composition, the conductive composition is composed of at least the copper particles and an organic solvent. As the organic solvent, the same substances as those used hitherto in the technical field of the conductive composition containing metal powder can be used without particular limitation. As such an organic solvent, for example, monohydric alcohol, polyhydric alcohol, polyhydric alcohol alkyl ether, polyhydric alcohol aryl ether, polyether, ester, nitrogen-containing heterocyclic compound, amide, amine, and saturated hydrocarbon, etc. can be exemplified. These organic solvents can be used alone or in combination of two or more.

[0091] The conductive composition can further add at least one of a dispersant, an organic excipient, and a glass powder as needed. As the dispersant, a dispersant such as a nonionic surfactant not containing sodium, calcium, phosphorus, sulfur, chlorine, etc. can be exemplified. As the organic excipient, for example, a mixture of solvents containing a resin component such as acrylic resin, epoxy resin, carboxyethyl cellulose, and a terpene-based solvent such as terpineol and dihydroterpineol, an ether-based solvent such as ethyl carbitol and butyl carbitol, etc. can be exemplified. As the glass frit, for example, borosilicate glass, barium borosilicate glass, zinc borosilicate glass, etc. can be exemplified.

[0092] For the conductive composition, a coating film is formed by applying it on a substrate, and the coating film is heated to sinter it, whereby a conductor film containing copper can be formed. The conductor film is suitable for, for example, circuit formation of a printed circuit board, ensuring electrical conduction of an external electrode of a ceramic capacitor. As the substrate, according to the kind of electronic circuit using the copper particles, for example, a printed circuit board formed of glass epoxy resin, etc., a flexible printed circuit board formed of polyimide, etc. can be exemplified.

[0093] The compounding amount of the copper particles and the organic solvent in the conductive composition can be adjusted according to the specific use of the conductive composition, the coating method of the conductive composition, and the content ratio of the copper particles in the conductive composition is preferably 5% by mass or more and 95% by mass or less, and more preferably 20% by mass or more and 90% by mass or less. As the coating method, for example, a method performed in the technical field of the present technology such as inkjet method, spray method, roll coating method, gravure printing method, etc. can be used.

[0094] The heating temperature (baking temperature) at the time of sintering the formed coating film can be, for example, 150°C or higher and 220°C or lower, provided that it is higher than the sintering start temperature of the copper particles. The atmosphere at the time of heating can be, for example, an oxidizing atmosphere or a non-oxidizing atmosphere. As the oxidizing atmosphere, for example, an oxygen-containing atmosphere can be mentioned. As the non-oxidizing atmosphere, for example, a reducing atmosphere such as hydrogen, carbon monoxide, and the like; a weakly reducing atmosphere such as a hydrogen-nitrogen mixed atmosphere; and a non-active atmosphere such as argon, neon, helium, and nitrogen can be mentioned. In the case of using any of these atmospheres, the heating time can be, for example, 1 minute or longer and 3 hours or shorter, further preferably 3 minutes or longer and 2 hours or shorter, provided that heating is performed in the above temperature range.

[0095] The conductor film thus obtained is obtained by sintering the copper particles according to the present application, and therefore, even in the case where sintering is performed under a relatively low temperature condition, the sintering can be performed sufficiently. In addition, at the time of sintering, the copper particles are fused at a low temperature, and therefore, the contact area of the copper particles with each other or the contact area of the copper particles with the surface of the substrate can be increased, as a result of which a sintered structure which is dense and has high adhesion to the object to be joined can be formed efficiently. Furthermore, the obtained conductor film has high electric conductivity reliability.

[0096] Example

[0097] Hereinafter, the present application will be described in more detail by way of examples. However, the scope of the present application is not limited to the examples.

[0098] [Example 1]

[0099] <1st Reduction Step>

[0100] In a stainless steel tank equipped with 5.0 liters of warm pure water and 5.0 liters of methanol, 2.5 kg of copper acetate monohydrate was added as a copper source, and 5.0 g of sodium diphosphate (molar ratio with respect to copper element: 0.002) was added as a polyphosphoric acid type, and the mixture was stirred at a liquid temperature of 25°C for 30 minutes to dissolve the components.

[0101] Next, 235.0 g of hydrazine (molar ratio with respect to copper element: 1.55) was added to the liquid, and the stirring was continued for 30 minutes under a non-heating condition at a liquid temperature of 25°C, and copper(I) oxide fine particles were generated in the liquid. After the generation of copper(I) oxide, the reaction liquid was stirred for 30 minutes.

[0102] <2nd Reduction Step>

[0103] Next, 25% NaOH aqueous solution was added to the reaction liquid in the 1st reduction step, and the pH of the liquid was adjusted to 7.0. Thereafter, the liquid was warmed to 40°C, and 1900.0 g of hydrazine (molar ratio of 1 mole with respect to copper element: 12.5) was added to the liquid in portions over 10 minutes to perform the 2nd reduction step. Then, the liquid was cooled to 30°C, and stirring was continued for 150 minutes to obtain copper particles in which copper oxide fine particles were reduced to metallic copper.

[0104] The aqueous slurry of the copper particles thus obtained was subjected to decantation washing, and the washing was performed until the electric conductivity reached 1.0 mS (washed slurry).

[0105] The obtained slurry was filtered using a nutsche. The solid component thus obtained was once put into 0.9 kg of methanol to perform solvent replacement. Thereafter, drying was performed to obtain copper powder formed of aggregates of copper particles. The copper particles thus obtained had a copper element content of more than 98 mass% and had a flat shape.

[0106] A scanning electron microscope image of the copper particles of Example 1 is shown in Figure 1 (a) of FIG. 1.

[0107] [Examples 2 to 4]

[0108] The kind of polyphosphoric acid used was changed as shown in Table 1 below, and only Example 4 was changed to 50°C in the liquid temperature at the time of addition of hydrazine in the 2nd reduction step. Except for these conditions, the same conditions as those of Example 1 were performed to obtain copper powder formed of aggregates of copper particles. The copper particles thus obtained had a copper element content of more than 98 mass% and had a flat shape.

[0109] A scanning electron microscope image of the copper particles of Example 2 to 4 is shown in Figure 1 (b) to (d) of FIG. 2, respectively.

[0110] [Comparative Example 1]

[0111] Copper particles having a flat shape were obtained by the method described in Example 1 of Japanese Patent Application Publication No. 2012-041592. This comparative example was manufactured by a manufacturing method in which polyphosphoric acid was not used.

[0112] Specifically, copper sulfate pentahydrate 4 kg, aminoacetic acid 120 g, and monosodium phosphate 50 g were added to 6 liters of pure water at 70°C, and stirring was performed. Further, pure water was added thereto, and the liquid amount was adjusted to 8 L, and stirring was performed for 30 minutes to obtain a copper-containing aqueous solution.

[0113] Next, 5.8 kg of a 25% NaOH solution was added to the aqueous solution while continuing to stir, and copper oxide particles were generated in the liquid. Stirring was continued for 30 minutes in this state.

[0114] Next, 1.5 kg of glucose was added to the aforementioned aqueous solution, and a first reduction process was performed to reduce the copper oxide to cuprous oxide. Stirring was continued for 30 minutes in this state.

[0115] Then, a second reduction process was performed by adding 1 kg of hydrazine monohydrate and 3 g of sodium borohydride all at once while stirring the liquid, to reduce the cuprous oxide to metallic copper. Stirring was continued for 1 hour and the reaction was completed.

[0116] After the reaction was completed, the aqueous slurry of the copper particles thus obtained was decanted and washed until the conductivity reached 1.0 mS (washed slurry).

[0117] The slurry obtained was filtered using a nutsche. The solid component thus obtained was added all at once to 0.9 kg of methanol to perform solvent replacement, and then dried to obtain copper powder formed of aggregates of copper particles.

[0118] A scanning electron microscope image of the copper particles of Comparative Example 1 is shown in Figure 2 (a).

[0119] [Comparative Example 2]

[0120] Copper particles having a flat shape were obtained by the method described in Comparative Example 1 of Japanese Patent Application Publication No. 2012-041592. This comparative example was produced by a production method that does not use polyphosphoric acid.

[0121] Specifically, copper sulfate pentahydrate 4 kg, aminoacetic acid 120 g, and trisodium phosphate 50 g were added to 6 L of pure water at 70°C and stirred. Further, pure water was added and the liquid volume was adjusted to 8 L, and stirring was continued for 30 minutes in this state to obtain a copper-containing aqueous solution.

[0122] Next, 5.8 kg of a 25% NaOH solution was added to the aqueous solution while continuing to stir, and copper oxide particles were generated in the liquid. Stirring was continued for 30 minutes, and then 1.5 kg of glucose was added to perform a first reduction reaction to reduce the copper oxide to cuprous oxide. Stirring was continued for 30 minutes, and then hydrazine monohydrate was added all at once while stirring the liquid, and stirring was continued for 1 hour and the reaction was completed.

[0123] After the reaction was completed, the aqueous slurry of the copper particles thus obtained was decanted and washed until the conductivity reached 1.0 mS (washed slurry).

[0124] The obtained slurry was filtered using a nutsche filter. The solid component thus obtained was put into methanol 0.9 kg at a time for solvent replacement, and then dried to obtain copper powder formed of aggregates of copper particles.

[0125] A scanning electron microscope image of the copper particles of Comparative Example 2 is shown in Figure 2 of (b).

[0126] 〔Comparative Example 3〕

[0127] The copper particles of the present comparative example were obtained in the following manner. The copper particles were spherical. The present comparative example was produced by a production method that did not use polyphosphoric acid.

[0128] Specifically, copper sulfate (pentahydrate) 4 kg and aminoacetic acid 120 g were dissolved in water to prepare an 8 L (liters) aqueous copper salt solution at a liquid temperature of 60°C. Then, while stirring the aqueous solution, 6.55 kg of a 25 wt% sodium hydroxide solution was added quantitatively over about 5 minutes, and stirring was performed for 60 minutes at a liquid temperature of 60°C to allow the copper oxide to mature until the liquid color completely turned black. After 30 minutes, glucose 1.5 kg was added and allowed to mature for 1 hour, whereby the copper oxide was reduced to cuprous oxide. Further, hydrazine hydrate 1 kg was added quantitatively over 1 minute to reduce the cuprous oxide, whereby metallic copper was formed, and a copper powder slurry was produced.

[0129] The aqueous slurry of the copper particles thus obtained was subjected to decantation washing until the conductivity reached 1.0 mS (washed slurry).

[0130] The obtained slurry was filtered using a nutsche filter. The solid component thus obtained was put into methanol 0.9 kg at a time for solvent replacement. Then, drying was performed to obtain copper powder formed of aggregates of copper particles.

[0131] A scanning electron microscope image of the copper particles of Comparative Example 3 is shown in Figure 2 of (c).

[0132] 〔Evaluation of sinterability〕

[0133] For the copper particles of the examples and comparative examples, evaluation of sinterability was performed by the following method.

[0134] First, a 20 mass% aqueous slurry was prepared using the washed slurry of the copper particles of the examples and comparative examples. Thereafter, isopropyl alcohol solution in which 12 g of copper laurate as a surface coating treatment agent was dissolved was added at a time to the slurry heated to 50°C, and stirred for 1 hour. Then, solid-liquid separation was performed by filtration to obtain a solid component, and the solid component was vacuum-dried to obtain copper particles on which surface coating treatment was performed.

[0135] Next, 8.5 g of the copper particles subjected to the surface coating treatment were mixed with polyethylene glycol having a number average molecular weight of 200 using a three-roll kneader to obtain a conductive paste containing 85 mass% of the copper particles. The obtained paste was applied to a glass substrate, and the substrate was sintered at 190°C for 10 minutes under a nitrogen atmosphere to form a conductor film on the glass substrate. The sintered copper particles in the conductor film were observed using an electron microscope to observe the degree of fusion of the copper particles to each other, and the sinterability was evaluated according to the following evaluation criteria. The results are shown in Table 1 below.

[0136] When sintering was performed using the copper particles of Example 2, a scanning electron microscope image of the state before sintering was taken and is shown in Figure 3 (a), and a scanning electron microscope image of the state after sintering was taken and is shown in Figure 3 (b).

[0137] < Evaluation Criteria for Sinterability >

[0138] A: A region in which the interface between the particles is unclear is present, and fusion between the particles is confirmed, and the sinterability at a low temperature is excellent.

[0139] D: The particles are not fused to each other, and the sinterability is poor.

[0140] [ Evaluation of Resistivity of Conductor Film ]

[0141] The resistivity of the conductor film formed in the above [Evaluation of Sinterability] was measured using a resistivity meter (Mitsubishi Chemical Analytech Co., Ltd., Loresta-GP MCP-T610). The measurement was performed three times on the conductor film as the measurement target, and the arithmetic mean value thereof was taken as the resistivity (μΩ·cm). The lower the resistivity, the smaller the resistance of the conductor film. The results are shown in Table 1 below.

[0142] [ Calculation of Particle Size Based on BET Specific Surface Area ]

[0143] The copper particles of Examples and Comparative Examples were measured by the following method.

[0144] First, a 20 mass% aqueous slurry was prepared using the cleaning slurry of the copper particles of Examples and Comparative Examples. Thereafter, 12 g of copper laurate as a surface coating treatment agent was dissolved in isopropyl alcohol, and the solution was added to the slurry heated to 50°C at once, and the mixture was stirred for 1 hour. Then, the solid component obtained by solid-liquid separation by filtration was vacuum-dried to obtain copper particles subjected to surface coating treatment. The specific surface area of the particles was measured based on the BET single-point method based on the measurement method based on the above BET method, and the particle size B was calculated based on the specific surface area. The results are shown in Table 1 below.

[0145] [ Measurement of Content of Carbon Element and Phosphorus Element ]

[0146] For the content of carbon element in the copper particles, 0.50 g of the copper particles of the example or the comparative example was put into a magnetic crucible using a carbon / sulfur analysis device (LECO Co., Ltd. CS844), the carrier gas was set to oxygen (purity: 99.5%), the analysis time was set to 40 seconds to perform the measurement. The measurement results are shown in Table 1 below.

[0147] For the content of phosphorus element in the copper particles, 1.00 g of the copper particles of the example or the comparative example was dissolved in 50 mL of 15% nitric acid aqueous solution to prepare a dissolved solution, the dissolved solution was introduced into an ICP emission spectroscopy analysis device (Hitachi High-Tech Corporation PS3520VDDII) and measured. The measurement results are shown in Table 1 below.

[0148] 〔Measurement of crystallite size〕

[0149] For the copper particles of the example and the comparative example, the measurement was performed by the following method.

[0150] First, 20 mass% aqueous slurry was prepared using the cleaning slurry of the copper particles of the example and the comparative example. Thereafter, isopropyl alcohol solution in which 12 g of copper laurate was dissolved as a surface coating treatment agent was added at once to the slurry heated to 50°C, and stirred for 1 hour. Then, the solid component obtained by solid-liquid separation by filtration was vacuum-dried, and the copper powder on which the surface coating treatment was performed was classified using a sieve having a mesh size of 75 μm, and the sieve underpart thereof was used as a sample. The sample was filled into a sample holder, and using an X-ray diffraction device (Ultima IV, Rigaku Corporation), the measurement was performed under the following conditions.

[0151] Thereafter, the main peak corresponding to the position of the (220) plane, the (111) plane, or the (311) plane of copper in the diffraction peak was taken as an object, and each crystallite size S1 and S2 and the S1 / S2 ratio were calculated using the above Scherrer formula based on the full width of the half value width of the peak. In addition, the S1 / B ratio was calculated from each crystallite size obtained. The results are shown in Table 1 below.

[0152] <Measurement conditions of X-ray diffraction>

[0153] • Tube: Cu Kα ray

[0154] • Tube voltage: 40 kV

[0155] • Tube current: 50 mA

[0156] • Measurement diffraction angle: 2θ = 20 to 100°

[0157] • Measurement step: 0.01°

[0158] • Collection time: 3 seconds / step

[0159] • Light receiving slit width: 0.3 mm

[0160] • Divergence longitudinal restriction slit width: 10 mm

[0161] • Detector: High-speed one-dimensional X-ray detector D / teX Ultra 250

[0162] Preparation method of sample for X-ray diffraction

[0163] The copper powder to be measured was laid on a measurement holder, and smoothed using a glass plate so that the thickness of the copper powder reached 0.5 mm and was smoothed.

[0164] The X-ray diffraction pattern obtained under the above measurement conditions was analyzed using analysis software under the following conditions. In the analysis, the peak width was corrected using a LaB6 value. The crystallite size was calculated using the full width at half maximum of the peak value and the Scherrer constant (0.94).

[0165] Analysis conditions of measurement data

[0166] • Analysis software: PDXL2 manufactured by Rigaku

[0167] • Smoothing process: Gaussian function, smoothing parameter = 10

[0168] • Background subtraction: fitting method

[0169] • Kα2 removal: intensity ratio = 0.497

[0170] • Peak search: second-order differential method

[0171] • Curve fitting: FP method

[0172] • Crystallite size distribution type: Lorenz model

[0173] • Scherrer constant: 0.9400

[0174] Note that the peaks of the X-ray diffraction pattern used in the analysis were as follows. The Miller indices shown below are synonymous with the crystal planes of the above copper.

[0175] • Peak indexed with Miller indices (220) in the vicinity of 2θ = 71° to 76°.

[0176] • Peak indexed with Miller indices (111) in the vicinity of 2θ = 40° to 45°.

[0177] • Peak indexed with Miller indices (311) in the vicinity of 2θ = 87.5° to 92.5°.

[0178] [Table 1]

[0179]

[0180] As shown in Table 1, the copper particles of the examples are excellent in sinterability at low temperature as compared with the copper particles of the comparative examples, and it is understood that the resistance of the conductor film obtained by sintering the copper particles is sufficiently small.

[0181] Industrial applicability

[0182] According to the present application, it is possible to provide a copper particle excellent in low-temperature sinterability.

Claims

1. A copper particle comprising copper element as a main body, a ratio (S1 / B) of a first crystallite size S1 calculated from a half width of a peak derived from a (111) plane of copper by a Scherrer formula in X-ray diffraction measurement to a particle diameter B calculated from a BET specific surface area is 0.23 or less, a ratio (S1 / S2) of the first crystallite size S1 to a second crystallite size S2 calculated from a half width of a peak derived from a (220) plane of copper by the Scherrer formula in the X-ray diffraction measurement is 1.35 or less, the copper particle contains a carbon element, and a content of the carbon element is 100 ppm or more and 1000 ppm or less, the first crystallite size S1 is 10 nm or more and 60 nm or less.

2. The copper particles of claim 1, wherein, the particle diameter is 100 nm or more and 500 nm or less.

3. The copper particles according to claim 1 or 2, wherein, a ratio (S1 / S3) of the first crystallite size S1 to a third crystallite size S3 calculated from a half width of a peak derived from a (311) plane of copper by the Scherrer formula in the X-ray diffraction measurement is 1.25 or less.

4. The copper particle according to claim 1 or 2, which contains a phosphorus element, and a content of the phosphorus element is 300 ppm or more.

5. A method for producing a copper particle, which is the method for producing the copper particle according to claim 1, the method comprising: a first reduction step of generating cuprous oxide by reducing copper ions, and a second reduction step of generating a copper particle by reducing the cuprous oxide, in any stage of performing the second reduction step or before performing the second reduction step, a polyphosphoric acid of di-phosphoric acid or more or a salt thereof is allowed to exist in a reaction system, a reaction solution of the first reduction step is set to an acidic condition in which pH is 3.5 or more and 5.5 or less at 25°C.

6. The manufacturing method according to claim 5, wherein, the first reduction step and the second reduction step are performed in the same reaction system.

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