Copper particles and methods for producing the same

Abstract

This copper particle mainly contains a copper element. In this copper particle, the ratio (S1 / B) of a first crystallite size S1, which is obtained during X-ray diffraction measurement by using the Scherrer equation from the half-value width of a peak derived from the (111) plane of copper, to a particle size B, which is calculated from a BET specific surface area, is 0.23 or less. In this copper particle, the ratio (S1 / S2) of the first crystallite size S1 to a second crystallite size S2, which is obtained during the X-ray diffraction measurement by using the Scherrer equation from the half-value width of a peak derived from the (220) plane of copper, is 1.35 or less. The present invention also provides a method for manufacturing the copper particle.

Description

Technical Field

[0001] The present invention relates to copper particles and a method for producing the same.

Background Art

[0002] The applicant has previously proposed a technique related to flat copper particles having a substantially hexagonal outline in plan view (see Patent Document 1). These copper particles have the advantages that the packing density can be increased and the surface roughness of the obtained conductor is reduced.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

[0004] In the technique described in Patent Document 1, since the crystallinity of the particles is high, there is room for improvement in order to achieve sintering at a lower temperature.

[0005] Therefore, an object of the present invention is to provide copper particles capable of sintering at a low temperature.

[0006] The present invention mainly contains copper element, The ratio (S1 / B) of the first crystallite size S1 obtained by the Scherrer formula from the half-value width of the peak derived from the (111) plane of copper in X-ray diffraction measurement to the particle size B calculated from the BET specific surface area is 0.23 or less, The present invention provides copper particles in which the ratio (S1 / S2) of the first crystallite size S1 to the second crystallite size S2 obtained by the Scherrer formula from the half-value width of the peak derived from the (220) plane of copper in X-ray diffraction measurement is 1.35 or less.

[0007] The present invention includes a first reduction step of reducing copper ions to produce cuprous oxide, The process includes a second reduction step in which the cuprous oxide is reduced to produce copper particles, The present invention provides a method for producing copper particles, wherein a polyphosphate of diphosphate or more, or a salt thereof, is present in the reaction system during the second reduction step, or at any stage before the second reduction step. [Brief explanation of the drawing]

[0008] [Figure 1] Figures 1(a) to 1(d) are scanning electron microscope images of copper particles before sintering in Examples 1 to 4, respectively. [Figure 2] Figures 2(a) to 2(c) are scanning electron microscope images of copper particles before sintering in Comparative Examples 1 to 3, respectively. [Figure 3] Figure 3(a) is a scanning electron microscope image of the copper particles of Example 2 before sintering, and Figure 3(b) is a scanning electron microscope image of the copper particles of Example 2 after sintering. [Modes for carrying out the invention]

[0009] The present invention will be described below based on its preferred embodiments. The copper particles of the present invention mainly contain the element copper. Furthermore, the crystallite size of the copper particles at a specific crystal plane, calculated by X-ray diffraction measurement, has a predetermined relationship.

[0010] "Mainly containing copper" 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 measured, for example, by ICP emission spectrometry.

[0011] Copper particles may contain elements other than copper in addition to copper, or they may consist of copper and, excluding unavoidable impurities, not other elements. The latter embodiment, i.e., consisting of copper, is preferred, but the inclusion of trace amounts of unavoidable impurity elements such as oxygen is permissible as long as it does not impair the effects of the present invention. In either embodiment, 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 measured, for example, by ICP emission spectrometry.

[0012] In the present invention, it is preferable that the copper particles have a predetermined relationship between the particle size calculated from their BET specific surface area and the crystallite size calculated from the X-ray diffraction peaks originating from the (111) plane of copper. Specifically, when the particle diameter calculated from the BET specific surface area is defined as particle diameter B, and the crystallite size calculated from the diffraction peak originating from the (111) plane of copper in X-ray diffraction measurement is defined as the first crystallite size S1, the ratio of the first crystallite size S1 to the particle diameter B (S1 / B) is preferably 0.23 or less, more preferably 0.02 to 0.23, and even more preferably 0.05 to 0.23.

[0013] The diffraction peak originating from the (111) plane of copper is the peak with the greatest height in the X-ray diffraction pattern obtained when the copper particles of the present invention are subjected to X-ray diffraction. From this, it can be inferred that the first crystallite size is larger than the crystallite size calculated from the diffraction peaks originating from other crystal planes and is representative of the crystallinity. Therefore, it is inferred that there are many grain boundaries within a single particle because the first crystallite size S1 is small relative to the particle diameter B. As a result, the thermal energy applied when the particles are heated makes the crystallite interface more unstable, leading to active atomic diffusion, which in turn enhances the fusion properties between particles at low temperatures and improves low-temperature sinterability. Such copper particles can be obtained, for example, by the manufacturing method described later.

[0014] The particle size B calculated from the BET specific surface area is preferably between 100 nm and 500 nm, more preferably between 100 nm and 400 nm, and even more preferably between 120 nm and 400 nm. By having the particle size B within this range, thermal conductivity can be increased, and low-temperature sinterability can be effectively improved.

[0015] Particle size B can be measured based on the BET method under the following conditions. Specifically, it can be measured using the nitrogen adsorption method with "Macsorb" manufactured by Mountec Co., Ltd. The amount of powder to be measured is 0.2 g, and the pre-degassing conditions are under vacuum at 80°C for 30 minutes. Then, particle size B is calculated from the measured BET specific surface area using the following formula (I). In equation (I), d is the particle size B [nm], and A is the specific surface area [m²] measured by the BET single-point method. 2 [g / g], where ρ is the density of copper [g / cm³]. 3 ] d = 6000 / (A × ρ) ... (I)

[0016] The first crystallite size S1 is preferably 10 nm to 60 nm, more preferably 20 nm to 60 nm, and even more preferably 25 nm to 55 nm. Having the crystallite size S1 within this range makes it easier to form more grain boundaries within a single particle, further enhancing the fusion properties of the particles during heating and effectively improving low-temperature sinterability.

[0017] Furthermore, it is preferable that, when the copper particles are measured in X-ray diffraction and the crystallite size is determined by Scherrer's formula from the full width at half maximum of the peak originating from the (220) plane of copper, the ratio of the first crystallite size S1 to the second crystallite size S2 (S1 / S2) is less than or equal to a predetermined value. Specifically, the S1 / S2 ratio is preferably 1.35 or less, more preferably 0.1 to 1.35, and even more preferably 0.1 to 1.2.

[0018] Since metallic copper tends to have a face-centered cubic crystal structure, the copper particles of the present invention have the (111) plane of copper on a specific plane of the particle surface and the (220) plane of copper on a plane intersecting the (111) plane. And the smaller the S1 / S2 ratio, the more it indicates that the copper particles are not growing in the (111) plane direction or are growing in the (220) plane direction. Therefore, the fact that S1 / S2 is within the above-described predetermined range generally correlates with the anisotropy in the particle shape such as the copper particles of the present invention being in a flat shape. The flat shape means a shape having a pair of main surfaces facing each other and side surfaces intersecting these main surfaces. When the copper particles are in a flat shape, it is presumed that the (111) plane of copper exists on the main surface of the copper particles and the (220) plane of copper exists on the side surface of the copper particles. Therefore, when the S1 / S2 ratio is within the above range, when the particles are arranged during sintering, the main surfaces of the particles or the side surfaces of the particles are likely to contact each other, and the contact portions between the particles are likely to become the same crystal plane. The particles to which thermal energy is applied have higher utilization efficiency of thermal energy and are more likely to diffuse the atoms at the crystallite interface when contacting with the same crystal plane than when contacting with different crystal planes. As a result, the fusion property between the particles at a low temperature can be enhanced, and the low-temperature sinterability can be improved. This is advantageous in that the sinterability can be further improved as compared with spherical particles or mechanically produced flat copper particles. Such copper particles can be obtained, for example, by the production method described below.

[0019] The second crystallite size S2 is preferably 10 nm or more and 60 nm or less, more preferably 20 nm or more and 50 nm or less, still more preferably 30 nm or more and 50 nm or less. By the crystallite size S2 being within such a range, while enhancing the low-temperature sinterability due to the relatively small crystallite size, many conductive paths derived from the shape of the copper particles can be formed, and a low-resistance conductor can be formed after sintering.

[0020] When the crystallite size of the copper particles of the present invention determined by Scherrer's formula from the half-width of the peak derived from the (311) plane of copper in X-ray diffraction measurement is defined as the third crystallite size S3, the ratio (S1 / S3) of the first crystallite size S1 to the third crystallite size S3 is preferably not more than a predetermined value. Specifically, the S1 / S3 ratio is preferably 1.35 or less, more preferably 0.2 or more and 1.30 or less, and still more preferably 0.5 or more and 1.25 or less.

[0021] Since metallic copper tends to have a face-centered cubic crystal structure, in the copper particles of the present invention, the (111) plane of copper exists on a specific plane of the particle surface, and the (311) plane of copper exists on a plane intersecting the (111) plane. And the smaller the S1 / S3 ratio, the more it indicates that the copper particles have not grown in the (111) plane direction or have grown in the (311) plane direction. Therefore, the fact that S1 / S3 is within the above-mentioned predetermined range generally correlates with the anisotropy in the particle shape such as the copper particles being flat. In this case, it is presumed that the (111) plane of copper exists on the main surface of the copper particles and the (311) plane of copper exists on the side surface of the copper particles. Therefore, when the S1 / S3 ratio is within the above-mentioned range, when the particles are arranged during sintering, the main surfaces of the particles or the side surfaces of the particles are likely to come into contact with each other, and the contact portions between the particles are likely to become the same crystal plane. As a result, when the particles are heated, the atomic diffusion at the crystallite interface can be activated, the fusion property of the particles at low temperature can be enhanced, and the low-temperature sintering property can be improved. This is advantageous in that the sintering property can be further improved as compared with spherical particles and mechanically produced flat copper particles. Such copper particles can be obtained, for example, by the production method described later.

[0022] The third crystallite size S3 is preferably 10 nm to 60 nm, more preferably 20 nm to 50 nm, and even more preferably 30 nm to 50 nm. By having the crystallite size S3 within this range, it is possible to enhance the low-temperature sinterability caused by the relatively small crystallite size, while forming many conductive paths derived from the shape of the copper particles, thereby forming a low-resistance conductor after sintering.

[0023] The first crystallite size S1, the second crystallite size S2, and the third crystallite size S3 can each be calculated from the total width at half maximum of the diffraction peaks originating from the (111), (220), or (311) planes of copper obtained by X-ray diffraction measurement, using Scherrer's formula shown below. The conditions for the X-ray diffraction measurement will be described in detail in the examples below. Use PDF number 00-004-0836. Scherrer's formula: D = Kλ / βcosθ • D: Crystallite size • K: Scherrer constant (0.94) • λ: Wavelength of X-ray β: Half-width [rad] ·θ: Diffraction angle

[0024] It is also preferable that the copper particles contain a low amount of carbon. More specifically, the carbon content in the copper particles is preferably 1000 ppm or less, more preferably 900 ppm or less, and even more preferably 800 ppm or less. The lower the amount, the better, but 100 ppm or more is practical. By keeping the carbon content within this range, it is possible to relatively suppress sintering inhibition by organic matter present on the surface of the copper particles. Such copper particles can be manufactured, for example, by the manufacturing method described later.

[0025] The carbon content can be measured by methods such as gas analysis or combustion carbon analysis. To measure the carbon content, it is first necessary to determine whether or not a coating treatment has been applied to the particle surface. This can be done using methods such as X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), Raman spectroscopy, infrared spectroscopy, liquid chromatography, or time-of-flight secondary ion mass spectrometry (TOF-SIMS), either individually or in combination. If it is determined that a coating treatment has been applied to the particle surface using these methods, the types and amounts of elements contained in the coating layer formed by the coating treatment are analyzed qualitatively and quantitatively using either the aforementioned methods individually or in combination. In addition, thermogravimetric analysis (TG) can be used to evaluate the properties of organic materials by measuring the mass change before and after the firing temperature and the amount of carbon after heating to that temperature. If it is determined that the particle surface has not been coated, the copper particles to be measured are subjected to measurement as is, and the obtained quantitative value is taken as the carbon element content of the copper particles.

[0026] It is also preferable that the copper particles contain phosphorus within a predetermined range. More specifically, the phosphorus content in the copper particles is preferably 300 ppm or more, more preferably 300 ppm to 1500 ppm, and even more preferably 300 ppm to 1000 ppm. By setting the phosphorus content within this range, it is possible to maintain sufficient conductivity of copper while causing a melting point depression, thereby further improving sinterability at low temperatures. Such copper particles can be manufactured, for example, by the manufacturing method described later. The presence or absence of phosphorus in the copper particles and their content can be measured, for example, by ICP emission spectrometry.

[0027] The copper particles of the present invention are not particularly limited in shape as long as the effects of the present invention are achieved, but when manufactured by the method described later, they are preferably flattened. Such particles are plate-like in shape, having a pair of substantially flat main surfaces facing each other and side surfaces intersecting both main surfaces, with the maximum span of the main surfaces being greater than the thickness. In this case, when the main surfaces of the copper particles are viewed from above, it is also preferable that their shape has a contour defined by a combination of straight lines, or a combination of straight lines and curves.

[0028] Next, a preferred method for producing the copper particles described above will be explained. This production method comprises two reduction steps: a first reduction step in which copper ions are reduced to produce cuprous oxide, and a second reduction step in which cuprous oxide is reduced in the presence of diphosphate or more polyphosphates or salts thereof (hereinafter also referred to as polyphosphates) to produce copper particles. Polyphosphates are present in the reaction system either when the second reduction step is performed or at any stage before the second reduction step. In other words, polyphosphates may be present in the reaction system before or during the first reduction step, and the second reduction step may be performed in that state. Alternatively, polyphosphates may not be present in the reaction system during the first reduction step, and may be present in the reaction system after the completion of the first reduction step, when the second reduction step is performed, or immediately before it.

[0029] This manufacturing method, from the viewpoint of uniformly controlling the reduction reaction, improving the productivity of the copper particles obtained therefrom, and reducing manufacturing costs, preferably carries out all reduction steps under wet conditions in an aqueous solution, and preferably carries out all reduction steps in the same reaction system. The following describes a manufacturing method using wet conditions and the same reaction system as an example.

[0030] First, a reaction solution containing a copper source and a reducing compound is prepared, and the first reduction step is performed to reduce copper ions and generate cuprous oxide in the solution. The reaction solution may be prepared by adding each raw material to the solvent simultaneously, or by adding each raw material to the solvent in any order. From the viewpoint of facilitating control of the reduction reaction of copper ions and improving handling during manufacturing, it is preferable to first mix the copper source and the solvent to form a copper-containing solution, and then add a solid reducing compound or a reducing compound solution pre-dissolved in the solvent to the copper-containing solution. The reducing compound may be added all at once or sequentially.

[0031] In the first reduction step, as described above, polyphosphates may or may not be present in the reaction solution. When polyphosphates are present in the reaction solution, it is preferable to add them in the order of copper source, polyphosphates, and reducing compound, as this allows for effective control of copper ion reduction and crystal growth by the reducing compound.

[0032] The solvent in the reaction solution can be water or lower alcohols such as methanol, ethanol, or propanol. These can be used individually or in combination.

[0033] Examples of copper sources used in the first reduction step include compounds that generate copper ions in the reaction solution, and water-soluble copper compounds are preferred. Specific examples of such copper sources include various copper compounds such as copper organic salts like copper formate, copper acetate, and copper propionate, and copper inorganic salts like copper nitrate and copper sulfate. These copper compounds may be anhydrous or hydrated. These copper compounds can be used individually or in combination.

[0034] The copper source content in the reaction solution during the first reduction step is preferably 0.5 mol / L to 5 mol / L, more preferably 1 mol / L to 4 mol / L, expressed as the molar concentration of copper element. This range allows for the productive manufacture of copper particles with small particle size and small crystallite size on specific crystal planes.

[0035] As reducing compounds, water-soluble compounds are preferred. Specific examples of reducing compounds include hydrazine compounds such as hydrazine, hydrazine hydrochloride, hydrazine sulfate, and hydrazine hydrate; boron compounds such as sodium borohydride and dimethylamine borane and their salts; sulfur oxoates such as sodium sulfite, sodium bisulfite, and sodium thiosulfate; nitrogen oxoates such as sodium nitrite and sodium hyponitrite; phosphorus oxoacids such as phosphorous acid, sodium phosphite, hypophosphorous acid, and sodium hypophosphite and their salts. These reducing compounds may be anhydrous or hydrated. These reducing compounds can be used individually or in combination of two or more.

[0036] From the viewpoint of making it easier to control the reduction product in the first reduction step to be cuprous oxide, thereby making it easier to control the growth of copper particles in subsequent reduction steps and making it easier to obtain particles with a predetermined crystallite size, and from the viewpoint of reducing the unintended inclusion of impurities such as carbon elements after reduction, it is preferable to use a hydrazine-based compound as the reducing compound in the reducing solution, and it is even more preferable to use anhydrous or hydrated hydrazine.

[0037] In the first reduction step, the content of the reducing compound in the reaction solution is preferably 0.5 moles to 3.0 moles, more preferably 1.0 mole to 2.0 moles, per mole of copper element. By controlling the concentration of the reducing compound within this range, the reduction reaction of copper ions and the progress of grain growth can be appropriately controlled, making it possible to obtain copper particles with small particle size and small crystallite size on specific crystal planes with high productivity.

[0038] In the first reduction step, it is preferable to maintain an acidic pH of 3.5 to 5.5 at 25°C. This allows for appropriate control of the degree of reduction, particularly when using reducing compounds, especially hydrazine compounds, so that reduction to cuprous oxide proceeds without reduction to metallic copper, while also facilitating anisotropy in the copper crystal growth that occurs in the second reduction step. In the first reduction step, it is preferable to add the reducing compound after adjusting the pH, as this allows for appropriate control of the degree of reduction of copper ions.

[0039] pH adjustment can be performed using various acids and basic substances, or by adding polyphosphates to the reaction solution, as long as the effects of the present invention are achieved. In particular, using polyphosphates to adjust the pH is advantageous because it allows the subsequent reaction to proceed efficiently without adding other substances to the reaction system, thus preventing the unintended contamination of impurities and efficiently obtaining the desired copper particles.

[0040] The reduction reaction in the first reduction step may be carried out under unheated conditions or under heated conditions. In either case, the temperature of the reaction solution is preferably 5°C to 35°C, more preferably 10°C to 30°C. The reaction time in the first reduction step is preferably 0.1 hours to 3 hours, more preferably 0.2 hours to 2 hours, provided that the temperature is within the above-mentioned temperature range. Furthermore, from the viewpoint of uniformity of the reduction reaction, it is also preferable to continue stirring the reaction solution from the start of the reaction to the end of the reaction.

[0041] Next, a second reduction step is performed to reduce the cuprous oxide obtained in the first reduction step to produce metallic copper particles. The second reduction step is also preferably carried out under wet conditions, similar to the first reduction step, and it is even more preferable that both reduction steps be carried out in the same reaction system.

[0042] As described above, it is preferable to have polyphosphates present in the reaction system when performing the second reduction step, or at any stage before performing the second reduction step. The polyphosphates used in this manufacturing method include diphosphate (H4P2O7) and triphosphate (tripolyphosphate, H5P3O7). 10 ), tetrapolyphosphate (H6P4O 13 Examples include polyphosphates and salts thereof, which preferably have 2 to 8, more preferably 2 to 5, phosphate monomer units in their structure. Examples of polyphosphates include alkali metal salts, alkaline earth metal salts, other metal salts, and ammonium salts. These can be used individually or in combination.

[0043] In the second reduction step, the polyphosphate content is preferably 0.001 moles to 0.05 moles, more preferably 0.001 moles to 0.01 moles, per mole of copper element. By setting the concentration of polyphosphates within this range, it is possible to induce anisotropy in the copper crystal growth resulting from the reduction reaction of cuprous oxide, thereby enabling the productive acquisition of copper particles with small particle size and small crystallite size on specific crystal planes. Furthermore, if polyphosphates are included at the first reduction step, they are not consumed in the reaction of the first reduction step, and the concentration of polyphosphates does not substantially change before and after the first reduction step. Therefore, by adding polyphosphates to the reaction system within the above-mentioned concentration range during the first reduction step, a sufficient amount of polyphosphates suitable for reduction to metallic copper and grain growth in the second reduction step can be achieved.

[0044] In the second reduction step, the above-mentioned reducing compound can be added to reduce the material to metallic copper. The content of the reducing compound in the reaction solution in the second reduction step is preferably 3 moles to 15 moles, more preferably 4 moles to 13 moles, per mole of copper element. When the second reduction step is carried out using the same reaction system as the first reduction step, it is preferable to further add the reducing compound to the solution in the above-mentioned content in order to achieve both improved reducing power and control of impurity reduction. It is also preferable to use the same type of reducing compound in each reduction step. By controlling the concentration of the reducing compound within this range, the reduction reaction to metallic copper can be sufficiently advanced, allowing for the highly productive production of copper particles with small particle size and small crystallite size on specific crystal planes.

[0045] The reducing compound in the second reduction step may be added all at once or sequentially. From the viewpoint of efficiently obtaining copper particles that satisfy the crystallite size ratio and particle size described above, sequential addition is preferable.

[0046] In the second reduction step, it is preferable to maintain the reaction solution under non-acidic conditions (neutral or alkaline conditions) with a pH of 7.0 or higher at 25°C. This is because, when using reducing compounds, particularly hydrazine compounds, it efficiently promotes the reduction of residual copper ions and cuprous oxide in the reaction solution to metallic copper, and facilitates anisotropy in the growth of copper crystals. It is preferable to adjust the pH before adding the reducing compound in the second reduction step, as this allows for appropriate control of the degree of reduction of copper ions. Various acids and basic substances can be used to adjust the pH. When the second reduction step is carried out using the same reaction system as the first reduction step, the reaction solution after the first reduction step is acidic, so it is preferable to adjust the pH of the reaction solution by adding a basic substance such as sodium hydroxide or potassium hydroxide. In the second reduction step, it is preferable to add a reducing compound after adjusting the pH, as this allows for efficient reduction of copper ions and cuprous oxide to metallic copper.

[0047] In order to efficiently reduce copper ions and cuprous oxide in the reaction solution and to obtain copper particles having a predetermined crystallite size with high productivity, it is preferable to heat the reaction solution in the second reduction step. The heating conditions for the reaction solution are preferably maintained at 30°C to 80°C, particularly 30°C to 50°C, from the start of the second reduction step, i.e., from the time of addition of the reducing compound, until the end of the reaction. The reaction time is preferably 60 minutes to 180 minutes under the above temperature conditions. Furthermore, in order to ensure a uniform reduction reaction and to obtain copper particles with little variation in particle size, it is also preferable to continue stirring the reaction solution from the start of the reaction until the end of the reaction.

[0048] The inventors speculate that the reason why copper particles capable of low-temperature sintering can be obtained by performing a two-step reduction process in which copper ions are reduced to metallic copper via cuprous oxide, and by including polyphosphates during the second reduction process, is as follows. First, in the first reduction step, copper ions are reduced by reducing compounds in the reaction solution, generating very fine cuprous oxide particles in the reaction solution. Subsequently, in the second reduction step, monovalent copper ions eluted from the cuprous oxide particles are reduced to form metallic copper nuclei. Because these nuclei are very unstable, they repeatedly combine with each other or redissolve in the reaction solution, ultimately causing the particles to grow. If polyphosphates are present during this particle growth, they adsorb to specific crystal planes of copper, suppressing growth in that direction. On the other hand, growth is not suppressed on crystal planes where polyphosphates do not adsorb, and growth in that direction proceeds. Based on the fact that metallic copper readily adopts a face-centered cubic crystal structure and the results of X-ray diffraction measurements of the obtained copper particles, it is estimated that the crystal plane on which polyphosphates are adsorbed is the (111) plane of copper in the particle, and the crystal plane on which polyphosphates are not adsorbed is estimated to be the (220) plane of copper, which is located perpendicular to the (111) plane. From this, it is thought that anisotropic growth occurs in which the growth of the (111) plane of copper is suppressed and the growth of the (220) plane of copper progresses, resulting in flattened copper particles that can be sintered at low temperatures.

[0049] Furthermore, as a preferred manufacturing method of the present invention, the first reduction step, in particular, is performed under acidic conditions to control the reducing power to such an extent that copper ions can be reduced to cuprous oxide but not to metallic copper. In addition, this also facilitates the control of subsequent metallic copper formation reactions. Subsequently, by using non-acidic conditions, the dissolution rate of cuprous oxide can be reduced, and the supply of monovalent copper ions can be controlled. Performing the second reduction under these conditions allows the rate of reduction to metallic copper to be adjusted to a gradual state, which is particularly advantageous in that it allows control of the nucleation rate.

[0050] The copper particles of the present invention obtained through the above process satisfy the above-mentioned preferred crystallite size and its ratio, preferred particle diameter, preferred content of various elements such as carbon, even when organic components that control crystal growth, such as organic amines, amino alcohols, and reducing sugars, and also have a flattened shape. Furthermore, the copper particles obtained in this way have crystal planes on the main surface that have grown perpendicular to the main surface, and crystal planes on the side surfaces that have grown along the main surface, each having a specific orientation direction, and each crystal plane is uniformly formed in one direction. Therefore, when these copper particles are fired with the main surfaces of the copper particles in contact with each other, or with the side surfaces of the copper particles in contact with each other, the contact between identical, uniformly aligned crystal planes does not require excessive energy for fusion, and sintering at low temperatures becomes possible.

[0051] The copper particles obtained through the above process may be washed and separated into solid and liquid components as needed, and then used in the form of a slurry in which the copper particles are dispersed in a solvent such as water or an organic solvent. Alternatively, the particles may be dried and used in the form of a dried powder, which is an aggregate of copper particles. In any case, the copper particles of the present invention exhibit excellent low-temperature sinterability. The copper particles may be further coated with organic substances such as fatty acids or their salts, or inorganic substances such as silicon-based compounds, as needed, to improve the dispersibility of the particles. Insofar as the effects of the present invention are achieved, it is permissible for the obtained copper particles to contain elements other than copper, such as due to the inevitable trace oxidation of their surface.

[0052] Furthermore, the copper particles of the present invention can be further dispersed in an organic solvent or resin and used in the form of a conductive composition such as a conductive ink or conductive paste. When the copper particles of the present invention are in the form of a conductive composition, the conductive composition comprises at least copper particles and an organic solvent. The organic solvent can be any organic solvent that has been used in the art of conductive compositions containing metal powders, without any particular limitations. Examples of such organic solvents include monohydric alcohols, polyhydric alcohols, polyhydric alcohol alkyl ethers, polyhydric alcohol aryl ethers, polyethers, esters, nitrogen-containing heterocyclic compounds, amides, amines, and saturated hydrocarbons. These organic solvents can be used individually or in combination of two or more.

[0053] The conductive composition may optionally contain at least one of a dispersant, an organic vehicle, and a glass frit. Examples of dispersants include nonionic surfactants that do not contain sodium, calcium, phosphorus, sulfur, and chlorine. Examples of organic vehicles include mixtures containing resin components such as acrylic resin, epoxy resin, ethylcellulose, and carboxyethylcellulose, and solvents such as terpene solvents such as terpineol and dihydroterpineol, and ether solvents such as ethyl carbitol and butyl carbitol. Examples of glass frit include borosilicate glass, barium borosilicate glass, and zinc borosilicate glass.

[0054] A conductive composition can be applied to a substrate to form a coating, and this coating can be heated and sintered to form a conductive film containing copper. The conductive film is suitably used, for example, for circuit formation on printed circuit boards or for ensuring electrical conductivity of the external electrodes of ceramic capacitors. Depending on the type of electronic circuit in which copper particles are used, suitable substrates include printed circuit boards made of glass epoxy resin or the like, and flexible printed circuit boards made of polyimide or the like.

[0055] The amounts of copper particles and organic solvent in the conductive composition can be adjusted according to the specific application of the conductive composition and the method of application, but the copper particle content in the conductive composition is preferably 5% by mass or more and 95% by mass or less, more preferably 20% by mass or more and 90% by mass or less. As for the application method, methods used in the art, such as inkjet printing, spray printing, roll coating, and gravure printing, can be employed.

[0056] The heating temperature (sintering temperature) when sintering the formed coating film should be above the sintering start temperature of the copper particles, for example, 150°C to 220°C. The atmosphere during heating can be, for example, an oxidizing atmosphere or a non-oxidizing atmosphere. An oxidizing atmosphere is, for example, an oxygen-containing atmosphere. A non-oxidizing atmosphere is, for example, a reducing atmosphere such as hydrogen or carbon monoxide, a weakly reducing atmosphere such as a hydrogen-nitrogen mixed atmosphere, or an inert atmosphere such as argon, neon, helium, and nitrogen. In any case, the heating time is preferably 1 minute to 3 hours, and more preferably 3 minutes to 2 hours, provided that the heating is within the above temperature range.

[0057] Since the conductive film obtained in this manner is obtained by sintering the copper particles of the present invention, sintering can proceed sufficiently even when sintering is performed under relatively low temperature conditions. Furthermore, since the copper particles fuse together even at low temperatures during sintering, the contact area between the copper particles themselves, or between the copper particles and the surface of the substrate, can be increased. As a result, a highly adhesive and dense sintered structure can be efficiently formed with respect to the object being joined. Moreover, the obtained conductive film has high conductivity reliability. [Examples]

[0058] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to these examples.

[0059] [Example 1] <First Reduction Process> In a stainless steel tank containing 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 to 1 mole of copper: 0.002) was added as a polyphosphate. The mixture was stirred at a liquid temperature of 25°C for 30 minutes to dissolve both substances. Next, 235.0 g of hydrazine (molar ratio to 1 mole of copper: 1.55) was added to the solution, and stirring was continued for 30 minutes under non-heating conditions at a liquid temperature of 25°C to generate cuprous oxide particles in the solution. After generating the cuprous oxide, the reaction solution was stirred for 30 minutes.

[0060] <Second Reduction Process> Next, a 25% NaOH aqueous solution was added to the reaction solution from the first reduction step to adjust the pH of the solution to 7.0. Then, the solution was heated to 40°C, and 1900.0 g of hydrazine (molar ratio to 1 mole of copper: 12.5) was quantitatively and sequentially added to the solution over 10 minutes to carry out the second reduction step. After that, the solution was cooled to 30°C, and stirring was continued for 150 minutes to obtain copper particles in which cuprous oxide fine particles were reduced to metallic copper.

[0061] The aqueous slurry of copper particles obtained in this manner was subjected to decantation washing until the conductivity reached 1.0 mS (washed slurry). The obtained slurry was filtered using a Nutsche filter. The resulting solid was added all at once to 0.9 kg of methanol and the solvent was replaced. After drying, copper powder consisting of aggregates of copper particles was obtained. The obtained copper particles had a copper element content of over 98% by mass and had a flattened shape. Figure 1(a) shows a scanning electron microscope image of copper particles in Example 1.

[0062] [Examples 2-4] The type of polyphosphate used was changed as shown in Table 1 below, and only in Example 4 was the liquid temperature during hydrazine addition in the second reduction step changed to 50°C. Except for these conditions, the procedure was carried out in the same manner as in Example 1 to obtain copper powder consisting of aggregates of copper particles. All of the obtained copper particles had a copper element content of more than 98% by mass and had a flattened shape. Scanning electron microscope images of copper particles in Examples 2 to 4 are shown in Figures 1(b) to 1(d), respectively.

[0063] [Comparative Example 1] Flattened copper particles were obtained by the method described in Example 1 of Japanese Patent Publication No. 2012-041592. This comparative example was produced by a manufacturing method that does not use polyphosphate. In detail, 4 kg of copper sulfate pentahydrate, 120 g of aminoacetic acid, and 50 g of trisodium monophosphate were added to 6 liters of pure water at 70°C and stirred. Further pure water was added to adjust the volume to 8 L, and the mixture was stirred for 30 minutes to obtain a copper-containing aqueous solution. Next, while continuing to stir, 5.8 kg of 25% NaOH solution was added to the aqueous solution to generate copper oxide particles in the solution. The mixture was then stirred for 30 minutes.

[0064] Next, 1.5 kg of glucose was added to the aqueous solution to perform the first reduction step, reducing copper oxide to cuprous oxide. The mixture was then stirred for 30 minutes. Subsequently, while the liquid was being stirred, 1 kg of hydrazine monohydrate and 3 g of sodium borohydride were added all at once to carry out the second reduction step, reducing cuprous oxide to metallic copper. The reaction was then completed by continuing to stir for 1 hour. After the reaction was complete, the aqueous slurry of copper particles obtained in this way was decanted and washed until the conductivity reached 1.0 mS (washed slurry). The obtained slurry was filtered using a Nutsche filter. The resulting solid was added all at once to 0.9 kg of methanol for solvent replacement, and then dried to obtain copper powder consisting of aggregates of copper particles. Figure 2(a) shows a scanning electron microscope image of copper particles in Comparative Example 1.

[0065] [Comparative Example 2] Flattened copper particles were obtained by the method described in Comparative Example 1 of Japanese Patent Publication No. 2012-041592. These comparative examples were produced by a manufacturing method that does not use polyphosphate. In detail, 4 kg of copper sulfate pentahydrate, 120 g of aminoacetic acid, and 50 g of trisodium phosphate were added to 6 L of pure water at 70°C and stirred. Further pure water was added to adjust the volume to 8 L, and stirring was continued for 30 minutes to obtain a copper-containing aqueous solution. Next, while stirring the aqueous solution, 5.8 kg of 25% sodium hydroxide solution was added to generate cupric oxide in the solution. After stirring for another 30 minutes, 1.5 kg of glucose was added to carry out the first reduction reaction, reducing cupric oxide to cuprous oxide. After stirring for another 30 minutes, hydrazine monohydrate was added all at once while the solution was stirring, and stirring was continued for 1 hour to complete the reaction. After the reaction was complete, the aqueous slurry of copper particles obtained in this way was decanted and washed until the conductivity reached 1.0 mS (washed slurry). The obtained slurry was filtered using a Nutsche filter. The resulting solid was added all at once to 0.9 kg of methanol for solvent replacement, and then dried to obtain copper powder consisting of aggregates of copper particles. Figure 2(b) shows a scanning electron microscope image of copper particles in Comparative Example 2.

[0066] [Comparative Example 3] The copper particles of this comparative example were obtained by the following method. These copper particles were spherical. This comparative example was manufactured using a method that does not use polyphosphate. In detail, 4 kg of copper sulfate (pentahydrate) and 120 g of aminoacetic acid were dissolved in water to prepare an 8 L (liter) copper salt aqueous solution at a liquid temperature of 60°C. Then, while stirring this aqueous solution, 6.55 kg of 25 wt% sodium hydroxide solution was quantitatively added over approximately 5 minutes, and the mixture was stirred at 60°C for 60 minutes until the liquid color became completely black to produce cupric oxide. After that, it was left to stand for 30 minutes, and 1.5 kg of glucose was added, and the mixture was aged for 1 hour to reduce the cupric oxide to cuprous oxide. Furthermore, 1 kg of hydrated hydrazine was quantitatively added over 1 minute to reduce the cuprous oxide to metallic copper, and a copper powder slurry was produced. The aqueous slurry of copper particles obtained in this manner was subjected to decantation washing until the conductivity reached 1.0 mS (washed slurry). The obtained slurry was filtered using a Nutsche filter. The resulting solid was added all at once to 0.9 kg of methanol for solvent replacement, and then dried to obtain copper powder consisting of aggregates of copper particles. Figure 2(c) shows a scanning electron microscope image of copper particles in Comparative Example 3.

[0067] [Evaluation of sinterability] The sinterability of the copper particles in the examples and comparative examples was evaluated using the following method. First, a 20% by mass aqueous slurry was prepared using the washing slurry of copper particles from the examples and comparative examples. Then, an isopropyl alcohol solution containing 12 g of copper laurate was added all at once to the slurry, which had been heated to 50°C, and the mixture was stirred for 1 hour. After that, the solids obtained by solid-liquid separation by filtration were vacuum-dried to obtain copper particles with surface coating treatment. Next, 8.5 g of surface-coated copper particles and polyethylene glycol with a number-average molecular weight of 200 were mixed using a three-roll kneader to obtain a conductive paste containing 85% by mass of 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 conductive film on the glass substrate. The degree of fusion between copper particles in the sintered conductive film was observed using an electron microscope, and the sinterability was evaluated according to the following evaluation criteria. The results are shown in Table 1 below. Figure 3(a) shows a scanning electron microscope image of the copper particles before sintering, and Figure 3(b) shows a scanning electron microscope image of the copper particles after sintering.

[0068] <Evaluation Criteria for Sinterability> A: There are many regions where the interfaces between particles are unclear, indicating fusion between particles, and demonstrating excellent sinterability at low temperatures. D: The particles are not fused together, resulting in poor sinterability.

[0069] [Evaluation of the resistivity of conductive films] The resistivity of the conductive film formed in the above-described [Evaluation of Sinterability] was measured using a resistivity meter (Loresta-GP MCP-T610, manufactured by Mitsubishi Chemical Analytec Co., Ltd.). Three measurements were taken for each conductive film, and the arithmetic mean was taken as the resistivity (μΩ·cm). A lower resistivity indicates a lower resistance of the conductive film. The results are shown in Table 1 below.

[0070] [Calculation of particle size based on BET specific surface area] The copper particles in the examples and comparative examples were measured using the following method. First, a 20% by mass aqueous slurry was prepared using the washing slurry of copper particles from the examples and comparative examples. Then, an isopropyl alcohol solution containing 12 g of copper laurate was added all at once to the slurry, which was heated to 50°C, and the mixture was stirred for 1 hour. After that, the solids obtained by solid-liquid separation by filtration were vacuum-dried to obtain copper particles with surface coating treatment. The specific surface area of ​​these particles was measured using the BET method described above, based on the BET single-point method, and the particle size B was calculated based on this specific surface area. The results are shown in Table 1 below.

[0071] [Measurement of carbon and phosphorus content] The carbon content in copper particles was measured using a carbon-sulfur analyzer (CS844, manufactured by LECO Japan LLC). 0.50 g of copper particles from the examples or comparative examples were placed in a magnetic crucible, oxygen gas (purity: 99.5%) was used as the carrier gas, and the analysis time was 40 seconds. The measurement results are shown in Table 1 below. The phosphorus content in the copper particles was measured by dissolving 1.00 g of copper particles from the examples or comparative examples in 50 mL of a 15% nitric acid aqueous solution and introducing the solution into an ICP emission spectrometer (PS3520VDDII, Hitachi High-Tech Science Corporation). The measurement results are shown in Table 1 below.

[0072] [Measurement of crystallite size] The copper particles in the examples and comparative examples were measured using the following method. First, a 20% by mass aqueous slurry was prepared using the washing slurry of copper particles from the examples and comparative examples. Then, an isopropyl alcohol solution containing 12 g of copper laurate was added all at once to the slurry, which had been heated to 50°C, and the mixture was stirred for 1 hour. After that, the solids obtained by solid-liquid separation by filtration were vacuum-dried to obtain copper particles with surface coating treatment. The copper powder was classified using a sieve with a mesh size of 75 μm, and the portion below the sieve was used as the sample. This sample was packed into a sample holder, and measurements were performed using an X-ray diffractometer (Ultima IV, manufactured by Rigaku Corporation) under the following conditions. Subsequently, among the diffraction peaks, the main peaks at positions corresponding to the (220), (111), or (311) planes of copper were targeted, and based on the full width at half maximum of these peaks, the crystallite sizes S1 and S2, as well as the S1 / S2 ratio, were calculated using the Scherrer's equation described above. Also, the S1 / B ratio was calculated from the obtained crystallite sizes. The results are shown in Table 1 below.

[0073] <X-ray Diffraction Measurement Conditions> · Tube target: CuKα ray · Tube voltage: 40 kV · Tube current: 50 mA · Measurement diffraction angle: 2θ = 20 - 100° · Measurement step width: 0.01° · Collection time: 3 sec / step · Receiving slit width: 0.3 mm · Divergent vertical limiting slit width: 10 mm · Detector: High-speed 1D X-ray detector D / teX Ultra250

[0074] <Method for Preparing X-ray Diffraction Sample> The copper powder to be measured was spread evenly in the measurement holder and smoothed using a glass plate so that the thickness of the copper powder was 0.5 mm and it became smooth.

[0075] Using the X-ray diffraction pattern obtained under the above measurement conditions, it was analyzed with analysis software under the following conditions. For the analysis, the LaB6 value was used for correcting the peak width. The crystallite size was calculated using the full width at half maximum of the peak and the Scherrer constant (0.94).

[0076] <Measurement Data Analysis Conditions> · Analysis software: PDXL2 manufactured by Rigaku · Smoothing process: Gaussian function, smoothing parameter = 10 · Background removal: Fitting method · Kα2 removal: Intensity ratio 0.497 · Peak search: Second derivative method · Profile fitting: FP method • Crystallite size distribution type: Lorentz model Scherrer constant: 0.9400

[0077] The peaks of the X-ray diffraction pattern used in the analysis are as follows. The Miller indices shown below are equivalent to the copper crystal planes mentioned above. • A peak indexed by the Miller index (220) located around 2θ = 71° to 76°. • A peak indexed by the Miller index (111) located around 2θ = 40° to 45°. • A peak indexed by the Miller index (311) located around 2θ = 87.5° to 92.5°.

[0078] [Table 1]

[0079] As shown in Table 1, the copper particles of the example exhibit superior sinterability at low temperatures compared to the copper particles of the comparative example, and the resistance of the conductive film obtained by sintering these copper particles is sufficiently low. [Industrial applicability]

[0080] According to the present invention, copper particles with excellent low-temperature sinterability are provided.

Claims

1. It mainly contains copper, The ratio (S1 / B) of the first crystallite size S1, determined by Scherrer's formula from the full width at half maximum of the peak originating from the (111) plane of copper in X-ray diffraction measurements, to the particle size B calculated from the BET specific surface area, is 0.23 or less. Copper particles in which the ratio (S1 / S2) of the first crystallite size S1 to the second crystallite size S2, determined by Scherrer's formula from the full width at half maximum of the peak originating from the (220) plane of copper in X-ray diffraction measurements, is 1.35 or less.

2. The copper particles according to 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 the ratio (S1 / S3) of the first crystallite size to the third crystallite size S3, determined by Scherrer's formula from the full width at half maximum of the peak originating from the (311) plane of copper in X-ray diffraction measurement, is 1.25 or less.

4. Copper particles according to any one of claims 1 to 3, wherein the carbon element is contained and the carbon element content is 1000 ppm or less.

5. Copper particles according to any one of claims 1 to 4, wherein the phosphorus element is contained in a quantity of 300 ppm or more.

6. A first reduction step involves reducing copper ions to produce cuprous oxide, The process includes a second reduction step in which the cuprous oxide is reduced to produce copper particles, When performing the first reduction step, a polyphosphate of diphosphate or more, or a salt thereof, is present in the reaction system. In the first reduction step, the pH of the reaction solution at 25°C is set to an acidic condition of 3.5 to 5.

5. A method for producing copper particles, wherein in the second reduction step, the pH of the reaction solution at 25°C is set to a neutral or alkaline condition of 7.0 or higher.

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

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