JP7498378B1 - Method for producing copper powder, copper paste containing the same, and conductive film - Google Patents

Abstract

A copper powder comprising the following copper particles A and copper particles B, in which the content of copper particles A relative to the total of copper particles A and copper particles B is 60% by mass or more and 99% by mass or less, and the content of copper particles B relative to the total of copper particles A and copper particles B is 1% by mass or more and 40% by mass or less. [Copper particles A] Copper particles comprising a core particle made of copper and a coating layer coating the surface of the core particle, the coating layer being formed of a copper salt of an aliphatic organic acid, and having a primary particle diameter of 0.1 μm or more and 0.6 μm or less. [Copper particles B] Copper particles in which a ratio (S1/B) of a first crystallite size S1 calculated from the half width of a peak derived from the (111) plane of copper in an X-ray diffraction measurement to a BET diameter B is 0.23 or less, a ratio (S1/S2) of S1 to a second crystallite size S2 calculated from the half width of a peak derived from the (220) plane is 1.35 or less, and a primary particle diameter is 0.1 μm or more and 2.0 μm or less.

Description

The present invention relates to copper powder and a copper paste containing the same. The present invention also relates to a method for producing a conductive film.

Copper is a highly conductive metal and a versatile material, and is therefore widely used industrially as a conductive material. For example, copper powder, an aggregate of copper particles, is widely used as a raw material for manufacturing various electronic components, such as the external and internal electrodes of multilayer ceramic capacitors (hereinafter also referred to as "MLCC"), and wiring for various substrates.

As one such copper powder, the applicant previously proposed a technology relating to spherical copper particles whose primary particle average particle size is 0.1 μm to 0.6 μm and whose particle surfaces are treated with a surface treatment agent (see Patent Document 1). This technology has the advantage of improving the low-temperature sintering properties of the copper particles.

JP 2015-168878 A

However, when a paste is prepared using spherical copper particles and various conductive films are produced from the paste, it may be difficult to ensure the continuity of the conductive film due to the particle shape.
On the other hand, flat copper particles are also known as copper particles. However, when a paste is prepared using the flat copper particles and various conductive films are produced from the paste, it may be difficult to improve the density of the conductive film due to the particle shape.
Therefore, in order to compensate for each other's shortcomings, spherical copper particles and flat copper particles are often used in combination. However, in recent years, in addition to the continuity and density of the conductive film, the low-temperature sintering property of the copper particles when manufacturing the conductive film is also required. Simply mixing spherical copper particles and flat copper particles does not improve the low-temperature sintering property of the mixed copper particles.

Therefore, the objective of the present invention is to provide copper powder that can produce a conductive film with high continuity and density and has a low sintering temperature.

The present invention includes the following copper particles A and copper particles B,
The copper powder has a content of copper particles A of 60% by mass or more and 99% by mass or less relative to the total content of copper particles A and copper particles B, and a content of copper particles B of 1% by mass or more and 40% by mass or less.
[Copper particles A]
The present invention comprises a core particle made of copper and a coating layer that coats the surface of the core particle,
the coating layer is formed from a copper salt of an aliphatic organic acid,
Copper particles having a primary particle diameter of 0.1 μm or more and 0.6 μm or less.
[Copper particles B]
a ratio (S1B) of a first crystallite size S1 calculated by the Scherrer formula from the half-width of a peak derived from the copper (111) plane in an X-ray diffraction measurement to a BET diameter B calculated from a BET specific surface area is 0.23 or less;
The ratio (S1/S2) of the first crystallite size S1 to the second crystallite size S2 calculated by the Scherrer formula from the half-width of the peak derived from the (220) plane of copper in X-ray diffraction measurement is 1.35 or less;
Copper particles having a primary particle diameter of 0.1 μm or more and 2.0 μm or less.

The present invention will be described below based on preferred embodiments thereof. The copper powder of the present invention includes copper particles A and copper particles B described below.
[Copper particles A]
The present invention comprises a core particle made of copper and a coating layer that coats the surface of the core particle,
the coating layer is formed from a copper salt of an aliphatic organic acid,
Copper particles having a primary particle diameter of 0.1 μm or more and 0.6 μm or less.
[Copper particles B]
a ratio (S1B) of a first crystallite size S1 calculated by the Scherrer formula from the half-width of a peak derived from the copper (111) plane in an X-ray diffraction measurement to a BET diameter B calculated from a BET specific surface area is 0.23 or less;
The ratio (S1/S2) of the first crystallite size S1 to the second crystallite size S2 calculated by the Scherrer formula from the half-width of the peak derived from the (220) plane of copper in X-ray diffraction measurement is 1.35 or less;
Copper particles having a primary particle diameter of 0.1 μm or more and 2.0 μm or less, except for those that fall under Copper Particles A.

After extensive research, the inventors have surprisingly found that the copper powder containing the copper particles A and B has a low sintering temperature, and that the conductive film produced from the copper paste containing the copper powder has high continuity and density.

In order to further enhance the above-mentioned effects, it is preferable that copper particles A have a spherical shape. On the other hand, it is preferable that copper particles B have a flat shape.

From the viewpoint of achieving both improved sinterability at low temperatures of the copper powder of the present invention and improved electrical conductivity of the conductive film obtained by sintering the particles, the average image analysis diameter of the primary particles of the copper particles A is preferably 0.1 μm or more and 0.6 μm or less, more preferably 0.12 μm or more and 0.4 μm or less, and even more preferably 0.15 μm or more and 0.3 μm or less. The primary particle refers to an object that is recognized as the smallest unit of a particle, judging from the geometric shape of its outer shape.
From the same viewpoint, the average image analysis diameter of the primary particles of copper particles B is preferably 0.1 μm or more and 2.0 μm or less, more preferably 0.15 μm or more and 1.0 μm or less, and even more preferably 0.2 μm or more and 0.6 μm or less.

The particle diameter of the copper particles A calculated from the BET specific surface area (hereinafter also referred to as BET diameter A) is preferably 0.1 μm or more and 0.6 μm or less, more preferably 0.12 μm or more and 0.4 μm or less, and even more preferably 0.15 μm or more and 0.3 μm or less. By having the BET diameter A in such a range, the thermal conductivity of the copper powder of the present invention can be increased and the sintering temperature can be effectively reduced.
From the same viewpoint, the particle diameter of copper particles B calculated from the BET specific surface area (hereinafter also referred to as BET diameter B) is preferably 0.1 μm or more and 2.0 μm or less, more preferably 0.15 μm or more and 1.0 μm or less, and even more preferably 0.2 μm or more and 0.6 μm or less. By having the BET diameter B in such a range, the thermal conductivity of the copper powder of the present invention can be increased, and the sintering temperature can be effectively reduced.
In this specification, the BET diameter B is also referred to as the primary particle diameter of the copper particles B.

The average image analysis diameter of the primary particles of copper particles A and B can be determined, for example, by observing the copper particles at a magnification of 10,000 times or 30,000 times using a scanning electron microscope (JSM-6330F manufactured by JEOL Ltd.), measuring the maximum Feret's diameter in the horizontal direction for 200 particles in the field of view, and calculating the volume average particle diameter converted into a sphere from these measured values.
In this specification, the average image analysis diameter of the primary particles of copper particles A calculated in this manner is also referred to as the primary particle diameter of copper particles A.

The BET diameters A and B calculated from the BET specific surface area can be measured under the following conditions based on the BET method. Specifically, the measurements can be performed by nitrogen adsorption using "Macsorb" manufactured by Mountech Co., Ltd. The amount of powder to be measured is 0.2 g, and the preliminary degassing conditions are under vacuum at 80°C for 30 minutes. The BET diameters A and B are calculated from the measured BET specific surface area by the following formula (I).
In formula (I), d is the BET diameter A or B [μm], A _BET is the specific surface area measured by the BET single point method [m ² /g], and ρ is the density of copper [g/cm ³ ].
d = 6 / (A _BET × ρ) ... (I)
Preferred embodiments of the copper particles A and B will be described in detail below.

<Preferable embodiment of copper particles A>
The copper particles A have a surface treatment agent containing a copper salt of an aliphatic organic acid applied to the surface of the particles. As a result, a coating layer made of the surface treatment agent is formed so as to cover the surface of the copper core particle continuously or discontinuously. The surface treatment agent is used to suppress both the oxidation of copper and the aggregation of the particles.
The core particles preferably consist of only copper with the remainder being unavoidable impurities.

As mentioned above, the surface treatment agent used in the present invention contains a copper salt of an aliphatic organic acid.

In this technical field, surface treatment agents such as fatty acids and fatty acid amines have been used to simultaneously suppress the oxidation of copper in copper particles and the aggregation of the particles. However, such treatment agents have a high decomposition temperature, and there are cases where they cannot be sufficiently removed during sintering of the copper particles. This can cause the sintering start temperature to rise and the resistance of the conductive film obtained after sintering of the copper particles to increase. In order to solve this problem, the present inventors have conducted extensive research and found that by using a copper salt of an aliphatic organic acid as a surface treatment agent, it is possible to suppress both the oxidation of copper and the aggregation of the particles while lowering the sintering start temperature, and as a result, it is possible to improve the low-temperature sintering property of the particles while lowering the resistance of the conductive film obtained after sintering.

From the viewpoint of lowering the sintering temperature of the copper powder of the present invention while suppressing oxidation of copper and agglomeration between particles, the number of carbon atoms of the aliphatic organic acid constituting the copper salt of an aliphatic organic acid is preferably 6 to 18, more preferably 8 to 18, even more preferably 10 to 18, and even more preferably 12 to 18. Examples of such aliphatic organic acids include linear or branched, saturated or unsaturated carboxylic acids, and sulfonic acids having linear or branched, saturated or unsaturated hydrocarbon groups, and preferably linear and saturated or unsaturated carboxylic acids. The valence of copper in the copper salt of an aliphatic organic acid is monovalent or divalent, and preferably divalent.

Specific examples of carboxylic acids include citric acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, lauric acid, palmitic acid, oleic acid, stearic acid, etc., of which lauric acid, oleic acid, and stearic acid are preferred, and lauric acid and stearic acid are more preferred.
Specific examples of sulfonic acids include hexanesulfonic acid, heptanesulfonic acid, octanesulfonic acid, nonanesulfonic acid, decanesulfonic acid, laurinsulfonic acid, palmitic acid, oleinsulfonic acid, stearinsulfonic acid, etc. These aliphatic organic acids can be used alone or in combination of two or more kinds.

The surface treatment agent can be applied to the particle surface, for example, in a process after the production of core particles made of copper, by contacting the obtained core particles with a copper salt of an aliphatic organic acid, which is a surface treatment agent. The amount of the surface treatment agent applied is expressed as the ratio (mass%) of the entire surface treatment agent to the copper particles A in the state in which the surface treatment agent is applied, and is preferably 0.2 mass% or more and 2.0 mass% or less in terms of carbon atoms, and more preferably 0.3 mass% or more and 1.0 mass% or less. By being in such a range, the melting temperature between the copper particles can be lowered by the effect of removing the oxide film on the copper particle surface by the surface treatment agent and co-melting, and as a result, the sintering temperature can be lowered.

The proportion (mass %) of the surface treatment agent applied to the surface of the copper particles A can be measured as follows: 0.5 g of copper powder, which is an aggregate of copper particles A to which a surface treatment agent has been applied, is heated in an oxygen stream using a carbon/sulfur analyzer (Horiba, Ltd., EMIA-320V), and the carbon content in the copper powder is decomposed into CO or _CO2 , and the amount of the decomposed carbon or CO2 is quantified to calculate the proportion of the surface treatment agent.

Qualitative and quantitative analysis of surface treatment agents can be performed using methods such as nuclear magnetic resonance (NMR) spectroscopy, Raman spectroscopy, infrared spectroscopy, liquid chromatography, and time-of-flight secondary ion mass spectrometry (TOF-SIMS), either alone or in combination.

Copper particles A have a coating layer formed on the surface of a core particle using a copper salt of an aliphatic organic acid as a surface treatment agent, and whether or not the coating layer was formed using a copper salt of an aliphatic organic acid can be determined, for example, by the following method. In detail, the copper particles A are diluted with KBr so that the mass of the copper particles A becomes 5 mass%, and the measurement sample is mixed in a mortar and measured by a diffuse reflectance method using an infrared spectrophotometer (model number: FT-IR4600) manufactured by JASCO Corporation under conditions of a resolution of 4 cm ^-1 and an accumulation number of 128 times, and a graph (spectrum) is obtained in which the vertical axis represents the value obtained by Kubelka-Munk conversion of absorbance and the horizontal axis represents wave numbers (500 to 4000 cm ^-1 ). In this case, if an infrared absorption peak is observed in the range of 1504 cm ^-1 or more and 1514 cm ^-1 or less and not in the range of 1584 cm ^-1 or more and 1596 cm ^-1 or less, it can be determined that the coating layer is formed using a copper salt of an aliphatic organic acid. That is, it is preferable that, in the measurement by infrared spectroscopy, the copper particles A have an infrared absorption peak in the range of 1504 cm ^-1 or more and 1514 cm ^-1 or less and no infrared absorption peak is observed in the range of 1584 cm ^-1 or more and 1596 cm ^-1 or less.
"Having an infrared absorption peak" is defined according to the following method. First, IR spectrum data normalized with the maximum value of the peak observed in the range of 2910 cm ^-1 to 2940 cm ^-1 is differentiated twice, and waveform separation is performed in the range of 1500 cm ^-1 to 1600 cm ^-1 based on the zero-up crossing method. Next, an arithmetic mean value is calculated from the absolute value of the amplitude from the reference line (zero) in each waveform obtained by waveform separation. Then, when the absolute value of the peak height is greater than half the value of the arithmetic mean value, it is deemed to "have an infrared absorption peak."
In addition, in the case of copper particles using a fatty acid or an aliphatic amine as a surface treatment agent, an infrared absorption peak is detected in the range of 1584 cm ^-1 or more and 1596 cm ^-1 or less, and in this respect, they can be distinguished from copper particles A.

The reason why the use of a copper salt of an aliphatic organic acid can produce copper particles that can lower the sintering temperature of the copper powder of the present invention while suppressing both copper oxidation and aggregation between particles is unclear, but the inventors speculate as follows.
As described above, there is a difference between the copper particles of the present invention and copper particles using a fatty acid or an aliphatic amine as a surface treatment agent in terms of the presence or absence of an infrared absorption peak at a specific wave number.
The principle of infrared spectroscopy is to measure the absorption of light energy equivalent to the kinetic energy of bonds in a molecule by irradiating infrared rays to a substance or molecule to be measured. In general, when infrared absorption is observed in infrared spectroscopy, it indicates that some bonds exist in the molecule. In particular, when infrared absorption is observed at a high wave number position, it can be said that bonds with high bond energy exist in the molecule because infrared rays at high wave numbers have high energy.
When copper particles A are compared with copper particles using a fatty acid or an aliphatic amine as a surface treatment agent, infrared absorption is observed in the low wave number region of 1504 cm ⁻¹ or more and 1514 cm ⁻¹ or less for both particles, and it is presumed that the absorption in this region means that a coating layer is bonded to the surface of the core particles. For this reason, it is considered that both the oxidation of the copper of the core particles and the aggregation of the particles can be suppressed.
On the other hand, when focusing on the high wavenumber region in the range of 1584 cm ^-1 or more and 1596 cm ^-1 or less, the copper particles A do not exhibit infrared absorption observed in the high wavenumber region, whereas the copper particles using fatty acids or aliphatic amines as surface treatment agents exhibit infrared absorption in the high wavenumber region. In other words, compared with the copper particles using fatty acids or aliphatic amines as surface treatment agents, the copper particles of the present invention have fewer bonds with large bond energy in the molecule. This is thought to be because the bond between the surface treatment agent and the core particles in the copper particles of the present invention is relatively weak, so that the surface treatment agent is easily detached at low temperatures, and sintering between the particles can be achieved at low temperatures.
For the above reasons, it is believed that by applying a surface treatment agent containing a copper salt of an aliphatic organic acid to the surface of copper particles A, it is possible to suppress both copper oxidation and aggregation between particles while achieving a reduction in the sintering temperature.

In addition, in order to identify which aliphatic organic acid constitutes the copper salt of an aliphatic organic acid in copper particles A, analysis can be performed, for example, by TOF-SIMS.

From the viewpoint of further lowering the sintering temperature of the copper powder of the present invention, in a thermogravimetric analysis when copper particles A are heated from 25°C to 1000°C, the temperature at which the ratio of the mass reduction value to the mass reduction value at 500°C is 10% is preferably 150°C or higher and 220°C or lower, more preferably 180°C or higher and 220°C or lower.

The above-mentioned thermogravimetric analysis can be carried out, for example, by the following method. That is, using a TG-DTA2000SA manufactured by Bruker AXS, a measurement sample of 50 mg is used, and the mass loss rate is measured when heated from 25°C to 1000°C. The atmosphere is nitrogen, and the heating rate is 10°C/min. The lower the temperature at which the mass loss rate reaches a predetermined rate, the lower the temperature at which the aliphatic organic acid that forms the coating layer can be removed, and thus this is a measure of the low-temperature sintering property of copper particles A.

As described above, the shape of the copper particles A is preferably spherical. In order to obtain spherical copper particles A, for example, the shape of the core particles may be spherical. In addition, the spherical shape of the particles means that the circularity coefficient measured by the following method is preferably 0.85 or more, more preferably 0.90 or more. The circularity coefficient is calculated by the following method. A scanning electron microscope image of the metal particles is taken, and 1000 particles that do not overlap with each other are randomly selected. When the area of the two-dimensional projection image of the particle is S and the perimeter is L, the circularity coefficient of the particle is calculated from the formula 4πS/ ^L2 . The arithmetic average value of the circularity coefficient of each particle is the above-mentioned circularity coefficient. When the two-dimensional projection image of the particle is a perfect circle, the circularity coefficient of the particle is 1.

<Preferable embodiment of copper particles B>
Copper particles B have a predetermined relationship in terms of crystallite size on a specific crystal plane calculated by X-ray diffraction measurement. Specifically, when the particle diameter calculated from the BET specific surface area is the BET diameter B, and the crystallite size calculated from the diffraction peak derived from the (111) plane of copper in the X-ray diffraction measurement by the Scherrer formula is the first crystallite size S1, the ratio (S1/B) of the first crystallite size S1 to the BET diameter 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.

The diffraction peak originating from the (111) plane of copper is the peak with the maximum height of the X-ray diffraction pattern obtained by X-ray diffraction measurement of copper particles B. From this, it is considered that the first crystallite size is larger than the crystallite size calculated from the diffraction peaks originating from other crystallite planes, and is also representative of crystallinity. Therefore, it is presumed that the first crystallite size S1 is small relative to the BET diameter B, so that there are many crystal grain boundaries in one particle. As a result, the thermal energy applied when the particles are heated makes the crystallite interface more unstable, which activates atomic diffusion, thereby increasing the fusion between particles at low temperatures and lowering the sintering temperature.
Such copper particles can be obtained, for example, by the manufacturing method described below.

The first crystallite size S1 of copper particles B is preferably 10 nm or more and 80 nm or less, more preferably 20 nm or more and 75 nm or less, and even more preferably 25 nm or more and 70 nm or less. By having the crystallite size S1 in such a range, it becomes easier to form more crystal grain boundaries in each particle, further increasing the fusion property of the particles during heating, and effectively lowering the sintering temperature.

In addition, when the second crystallite size S2 is the crystallite size calculated by the Scherrer formula from the half-width of the peak derived from the (220) plane of copper in X-ray diffraction measurement, it is also preferable that the ratio (S1/S2) of the first crystallite size S1 to the second crystallite size S2 is a predetermined value or less.
Specifically, the S1/S2 ratio is preferably 1.35 or less, more preferably 0.1 or more and 1.3 or less, and further preferably 0.1 or more and 1.2 or less.

Since metallic copper is prone to have a face-centered cubic crystal structure, copper particles B have a copper (111) plane on a specific surface of the particle surface, and a copper (220) plane on a surface intersecting the (111) plane. The smaller the S1/S2 ratio, the more the copper particles do not grow in the (111) plane direction, or grow in the (220) plane direction. Therefore, the fact that S1/S2 is in the above-mentioned predetermined range generally correlates with the copper particles B having anisotropy in particle shape, such as 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 copper particles B have a flat shape, it is presumed that the copper (111) plane is present on the main surface of copper particles B, and the copper (220) plane is present on the side surface of copper particles B.
Therefore, by having the S1/S2 ratio in the above-mentioned range, when the copper particles B are arranged during sintering, the main surfaces of the copper particles B or the side surfaces of the particles are likely to contact each other, and the contact parts of the copper particles B are likely to be the same crystal plane. When the particles to which thermal energy is applied are in contact with each other on the same crystal plane, the utilization efficiency of the thermal energy is higher than when the particles are in contact with each other on different crystal planes, and the atoms at the crystallite interface are more likely to diffuse. As a result, the fusion property between the particles at low temperatures can be improved, and the sintering temperature of the copper powder can be reduced. This is advantageous in that the sinterability can be further improved compared to spherical particles and mechanically produced flat copper particles.
In addition, since the contact between the copper particles B is likely to be face-to-face contact as described above, the contact area is larger than that of spherical copper particles, etc. Due to this, the conductive film produced from the copper powder of the present invention containing the copper particles B has high continuity.
Such copper particles can be obtained, for example, by the manufacturing method described below.

The second crystallite size S2 of copper particles B is preferably 10 nm or more and 80 nm or less, more preferably 20 nm or more and 75 nm or less, and even more preferably 30 nm or more and 70 nm or less. By having the crystallite size S2 in such a range, it is possible to form many conductive paths due to the shape of the copper particles while improving the low-temperature sintering property due to the relatively small crystallite size, and to form a low-resistance conductive film after sintering.

In copper particles B, when the third crystallite size S3 is determined by the Scherrer formula from the half-width of the peak derived from the (311) plane of copper in X-ray diffraction measurement, it is preferable that the ratio (S1/S3) of the first crystallite size S1 to the third crystallite size S3 is a predetermined value or less.
Specifically, the S1/S3 ratio is preferably 1.35 or less, more preferably 0.20 or more and 1.30 or less, and further preferably 0.50 or more and 1.25 or less.

Since metallic copper is prone to have a face-centered cubic crystal structure, copper particles B have a copper (111) plane on a specific surface of the particle surface, and a copper (311) plane on a surface intersecting the (111) plane. The smaller the S1/S3 ratio, the more copper particles B do not grow in the (111) plane direction, or grow in the (311) plane direction. Therefore, the fact that S1/S3 is in the above-mentioned predetermined range generally correlates with the copper particles B having anisotropy in particle shape, such as being flat. In this case, it is presumed that the copper (111) plane is present on the main surface of the copper particles B, and the copper (311) plane is present on the side surface of the copper particles.
Therefore, by having the S1/S3 ratio in the above-mentioned range, when the copper particles B are arranged during sintering, the main surfaces of the copper particles B or the side surfaces of the copper particles B are likely to contact each other, and the contact parts of the copper particles B are likely to have the same crystal plane. As a result, when the copper powder of the present invention is heated, atomic diffusion at the crystallite interface of the copper particles B is activated, the fusion property of the particles at low temperatures is improved, and the sintering temperature of the copper powder can be reduced. This is advantageous in that the sinterability can be further improved compared to spherical particles and mechanically produced flat copper particles.
Such copper particles can be obtained, for example, by the manufacturing method described below.

The third crystallite size S3 of the particles B is preferably 10 nm or more and 80 nm or less, more preferably 20 nm or more and 75 nm or less, and even more preferably 30 nm or more and 70 nm or less. By having the crystallite size S3 in such a range, it is possible to form many conductive paths due to the shape of the copper particles B while improving the low-temperature sintering property due to the relatively small crystallite size, and to form a low-resistance conductive film after sintering.

The first crystallite size S1, the second crystallite size S2, and the third crystallite size S3 can be calculated from the full width at half maximum of the diffraction peak derived from the (110), (220) or (311) plane of copper obtained by X-ray diffraction measurement, using the Scherrer formula shown below. The conditions of the X-ray diffraction measurement will be described in detail in the Examples described later. The PDF number used is 00-004-0836.
Scherrer's formula: D = Kλ/βcosθ
D: crystallite size K: Scherrer constant (0.94)
・λ: X-ray wavelength ・β: Half-width [rad]
θ: Diffraction angle

It is preferable that the copper particles B mainly contain copper element. "Mainly containing copper element" means that the copper element content in the copper particles is 97.0% by mass or more, preferably 97.5% by mass or more, more preferably 98.0% by mass or more, and even more preferably 98.5% by mass or more. The copper element content can be measured, for example, by ICP atomic emission spectrometry.

Copper particles B contain, in addition to copper element, other elements other than copper element, or consist of copper element and do not contain other elements other than copper element except inevitable impurities. Copper particles B are allowed to contain trace amounts of inevitable impurity elements such as oxygen element, as long as the effect of the present invention is not impaired. In either embodiment, the content of elements other than copper element in the copper particles is preferably 1.5 mass% or less. The content of these elements can be measured, for example, by ICP atomic emission spectrometry.

It is also preferable that the copper particles B have a low content of carbon element contained in the particles. In detail, the carbon element content in the copper particles B is preferably 5000 ppm or less, more preferably 4500 ppm or less, and even more preferably 4000 ppm or less. The lower the content, the better, but 100 ppm or more is realistic. By having the carbon element content in such a range, it is possible to relatively suppress sintering inhibition due to organic matter present on the copper particle surface. Such copper particles can be manufactured, for example, by the manufacturing method described below.

The carbon element content can be measured by, for example, gas analysis or combustion carbon analysis. In measuring the carbon element content, first, it is determined whether the copper particle B surface is coated. This confirmation method includes, for example, X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), Raman spectroscopy, infrared spectroscopy, liquid chromatography, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and other methods, either alone or in combination. If it is determined that the particle surface is coated by this method, the above-mentioned methods are used alone or in combination to qualitatively and quantitatively analyze the type and amount of elements contained in the coating layer formed by the coating process. In addition, the physical properties of the organic material can be evaluated by thermogravimetry (TG) by measuring the mass change occurring before and after the firing temperature and the amount of carbon after heating to that temperature.
When it is determined that no coating treatment has been applied to the particle surface, the copper particles B to be measured are directly subjected to the measurement, and the quantitative value obtained is regarded as the carbon element content contained in the copper particles B.

It is also preferable that the content of phosphorus contained in the copper particles B is within a predetermined range. In detail, the content of phosphorus in the copper particles is preferably 300 ppm or more, more preferably 300 ppm or more and 1500 ppm or less, and even more preferably 300 ppm or more and 1000 ppm or less. By setting the content of phosphorus in such a range, the melting point can be lowered while sufficiently maintaining the conductivity of copper, and the sintering temperature can be further lowered. Such copper particles can be manufactured, for example, by the manufacturing method described below. The presence or absence of phosphorus in the copper particles B and its content can be measured, for example, by ICP emission spectroscopy.

As mentioned above, the lower the carbon content of copper particles B, the less likely sintering inhibition occurs, and the lower the temperature at which copper powder can be sintered. However, as long as the carbon content is within the range described above, it is possible to relatively suppress sintering inhibition caused by organic matter present on the surface of copper particles B, so organic matter may be intentionally applied to the surface of copper particles B.

Examples of organic substances applied to the surface of copper particles B include copper salts of various fatty acids and aliphatic organic acids, and aliphatic amines. By applying such organic substances to the surface of copper particles B, aggregation between copper particles can be suppressed. In particular, it is preferable to use saturated or unsaturated fatty acids or aliphatic amines having 6 to 18 carbon atoms, particularly 10 to 18 carbon atoms, from the viewpoint of improving the low-temperature sintering property of copper powder. Specific examples of such fatty acids or aliphatic amines include benzoic acid, pentanoic acid, hexanoic acid, octanoic acid, nonanoic acid, decanoic acid, lauric acid, palmitic acid, oleic acid, stearic acid, pentylamine, hexylamine, octylamine, decylamine, laurylamine, oleylamine, and stearylamine. These fatty acids or aliphatic amines can be used alone or in combination of two or more.

The shape of the copper particles B is not particularly limited as long as the effects of the present invention are achieved, but when produced by the method described below, the particles are preferably flat. Such particles have a pair of nearly flat main surfaces facing each other and side surfaces intersecting both main surfaces, and are plate-like in that the maximum diameter of the main surfaces is greater than the thickness. In this case, when the main surfaces of the copper particles B are viewed in plan, it is also preferable that the shape has an outline defined by a combination of straight lines or a combination of straight lines and curves.

The conductive film produced from the copper paste containing the copper powder of the present invention has high density and continuity. From the viewpoint of further increasing the density and continuity of the conductive film, the content ratio of copper particles A to the total of copper particles A and copper particles B is preferably 60% by mass or more and 99% by mass or less, more preferably 65% by mass or more and 88% by mass or less, and even more preferably 70% by mass or more and 85% by mass or less.
From the same viewpoint, the content ratio of copper particles B to the total of copper particles A and copper particles B is preferably 1 mass% or more and 40 mass% or less, more preferably 12 mass% or more and 35 mass% or less, and even more preferably 15 mass% or more and 30 mass% or less.

Next, a preferred method for producing the copper powder of the present invention will be described. The copper powder of the present invention is preferably produced by mixing copper particles A and copper particles B in the preferred ratio described above. Below, a preferred method for producing copper particles A and copper particles B, and a method for mixing copper particles A and copper particles B will be described in detail in order.

<Method of producing copper particles A>
First, we will explain a suitable method for producing copper particles A. This production method involves contacting core particles made of copper with a solution containing a copper salt of an aliphatic organic acid to form a coating layer that coats the surfaces of the core particles.

First, prior to the surface treatment with a copper salt of an aliphatic organic acid, core particles made of copper are prepared. The method for producing copper core particles can be, for example, the wet method described in JP 2015-168878 A. That is, a reaction solution containing a monovalent or divalent copper source such as copper chloride, copper acetate, copper hydroxide, copper sulfate, copper oxide, or cuprous oxide is prepared in a liquid medium containing water and a monohydric alcohol preferably having 1 to 5 carbon atoms. This reaction solution is mixed with hydrazine in a ratio of preferably 0.5 to 50 moles per mole of copper, and the copper source is reduced to obtain core particles made of copper. The core particles obtained by this method do not have a surface treatment agent such as a copper salt of an aliphatic organic acid on their surfaces, and have a small particle size.

The core particles obtained in the above-mentioned process are preferably washed. Examples of washing methods include decantation and rotary filter methods. When washing the core particles by the rotary filter method, for example, an aqueous slurry is prepared by dispersing the core particles in a solvent such as water, and washing is performed until the conductivity of the slurry is preferably 2.0 mS or less. The washing conditions at this time can be, for example, when water is used as the washing solvent, a washing temperature of 15°C to 30°C and a washing time of 10 minutes to 60 minutes. By setting the conductivity of the slurry to the above range, the core particles to be washed can be uniformly dispersed without agglomeration, and the surface treatment described below can be efficiently performed. The content ratio of the core particles made of copper in this slurry is preferably 5% by mass to 50% by mass from the viewpoint of achieving both improved washing efficiency and improved particle dispersibility.

In addition, instead of the above-mentioned method, a direct current thermal plasma (DC plasma) method described in WO 2015/122251 may be used as another method for producing core particles made of copper. In detail, copper mother powder can be subjected to a direct current thermal plasma method, which is a type of PVD method, to generate core particles from the mother powder. The core particles obtained by this method also have no surface treatment agent such as a copper salt of an aliphatic organic acid on their surfaces, and have a small particle size. If necessary, the obtained core particles may be subjected to a crushing process or a classification process to separate or remove coarse particles and fine particles.

Next, the core particles obtained by the above-mentioned method are surface-treated with a surface treatment agent to form a coating layer that covers the surface of the core particles. As a method of surface treatment, for example, a method of contacting the core particles with a solution in which a copper salt of an aliphatic organic acid is dissolved in a solvent can be adopted. The form of the core particles to be contacted with the copper salt of an aliphatic organic acid in this step may be an aqueous slurry in which the core particles are dispersed in a solvent such as water, or may be in a dry state in which the core particles are not dispersed in a solvent or the like. In addition, as the order of contact in this step, one of the core particles and the copper salt solution of an aliphatic organic acid may be added to the other, or the core particles and the copper salt solution of an aliphatic organic acid may be contacted simultaneously.
From the viewpoint of uniformly treating the surface of the core particles with the copper salt of an aliphatic organic acid, it is preferable to employ a method in which a solution of the copper salt of an aliphatic organic acid is added to a slurry in which the core particles are dispersed.

The method of adding core particles to a copper salt solution of an aliphatic organic acid and performing surface treatment will be described below as an example. First, the solvent used in the copper salt solution of an aliphatic organic acid is heated to a temperature below the boiling point of the solvent used (for example, 25°C to 80°C), and under that condition, the copper salt of an aliphatic organic acid is added to the solvent to prepare a copper salt solution of an aliphatic organic acid. Next, while maintaining the temperature of the copper salt solution above the melting point of the copper salt of the aliphatic organic acid, the dry core particles or core particle-containing slurry is added to the copper salt solution of an aliphatic organic acid, and then stirred for 1 hour to perform surface treatment on the surface of the core particles. The copper particles A obtained by this method are core particles made of copper and have a coating layer made of a copper salt of an aliphatic organic acid formed on the surface of the core particles. When performing surface treatment using a core particle-containing slurry, it is preferable that the slurry is heated to a temperature above the melting point of the copper salt of the aliphatic organic acid in order to uniformly form a coating layer on the surface of the core particles.

In the surface treatment using a solution of a copper salt of an aliphatic organic acid, the content of the copper salt of an aliphatic organic acid in the reaction solution containing the core particles is preferably 0.1 parts by mass or more and 3.0 parts by mass or less, more preferably 0.2 parts by mass or more and 2.0 parts by mass or less, relative to 100 parts by mass of core particles that have not been surface-treated. By performing the surface treatment in such an amount, copper particles A that have been surface-treated with the above-mentioned carbon atom ratio can be obtained.

Examples of the solvent for dissolving the copper salt of an aliphatic organic acid include organic solvents such as monohydric alcohols, polyhydric alcohols, esters of polyhydric alcohols, ketones, and ethers having 1 to 5 carbon atoms. Of these, from the viewpoints of compatibility with water, economy, ease of handling, and ease of removal, it is preferable to use monohydric alcohols having 1 to 5 carbon atoms, and it is even more preferable to use an aqueous methanol solution, ethanol, 1-propanol, or isopropyl alcohol.

The copper particles A obtained through the above steps may be washed or separated into solid and liquid as necessary, and then mixed with copper particles B in the form of a slurry in which the copper particles A are dispersed in a solvent such as water or an organic solvent, or the copper particles A may be dried and mixed with copper particles B in the form of a dry powder that is an aggregate of copper particles. In either case, by including copper particles A in the copper powder of the present invention, oxidation of the constituent metal copper is suppressed, and particle aggregation is suppressed, while an excellent copper powder with a low sintering temperature is obtained.

<Method of producing copper particles B>
Next, a description will be given of a preferred method for producing copper particles B. This production method includes 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 higher polyphosphoric acid or a salt thereof (hereinafter, also referred to as polyphosphoric acids) to produce copper particles.
The polyphosphoric acids are present in the reaction system either when the second reduction step is performed or before the second reduction step is performed. In other words, the polyphosphoric acids may be present in the reaction system before or when the first reduction step is performed, and the second reduction step may be performed in that state. Alternatively, the polyphosphoric acids may not be present in the reaction system in the first reduction step, but may be present in the reaction system after the completion of the first reduction step, or when the second reduction step is performed or immediately before the second reduction step.

In this manufacturing method, from the viewpoint of uniformly controlling the reduction reaction, improving the productivity of the copper particles obtained as a result of the reduction reaction, and reducing manufacturing costs, it is preferable that all reduction steps are performed under wet conditions in an aqueous liquid, and it is also preferable that all reduction steps are performed in the same reaction system. Below, an example of a manufacturing method under wet conditions and in the same reaction system is explained.

First, a reaction solution containing a copper source and a reducing compound is prepared, and a first reduction step is carried out to reduce copper ions and generate cuprous oxide in the solution. The reaction solution may be prepared by adding each raw material to a solvent at the same time to form the reaction solution, or each raw material may be added to the solvent in any order.
From the viewpoint of making it easier to control the reduction reaction of copper ions and improving the ease of handling during production, it is preferable to premix the copper source with a solvent to prepare a copper-containing solution, and then add a solid reducing compound or a solution of the reducing compound predissolved in a solvent to the copper-containing solution. The reducing compound may be added all at once or gradually.

In the first reduction step, as described above, the reaction solution may or may not contain polyphosphates. When polyphosphates are present in the reaction solution, it is preferable to add the copper source, polyphosphates, and reducing compound in that order, since this allows the reduction of copper ions by the reducing compound and the control of crystal growth to be effectively carried out.

The solvent for the reaction solution may be water or a lower alcohol such as methanol, ethanol, or propanol. These may be used alone or in combination.

The copper source used in the first reduction step includes 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 organic copper salts, such as copper formate, copper acetate, and copper propionate, and inorganic copper salts, such as copper nitrate and copper sulfate. These copper compounds may be anhydrides or hydrates. These copper compounds may be used alone or in combination.

The content of the copper source in the reaction system in the first reduction step is preferably 0.01 mol/L or more and 2.0 mol/L or less, more preferably 0.1 mol/L or more and 1.5 mol/L or less, expressed as the molar concentration of copper element. By being in such a range, copper particles having a small particle size and a small crystallite size on a specific crystal plane can be produced with high productivity.

As the reducing compound, a water-soluble compound is preferably used. Specific examples of the reducing compound include hydrazine-based compounds such as hydrazine, hydrazine hydrochloride, hydrazine sulfate, and hydrazine hydrate, boron compounds and salts thereof such as sodium borohydride and dimethylamine borane, sulfur oxoacid salts such as sodium sulfite, sodium hydrogen sulfite, and sodium thiosulfate, nitrogen oxoacid salts such as sodium nitrite and sodium hyponitrite, phosphorus oxoacids and salts thereof such as phosphorous acid, sodium phosphite, hypophosphorous acid, and sodium hypophosphite. These reducing compounds may be anhydrides or hydrates. These reducing compounds may be used alone or in combination of two or more.

From the viewpoint of easily controlling the reduction product in the first reduction step to be cuprous oxide, thereby easily controlling the grain growth of copper in the subsequent reduction steps and easily obtaining particles having a predetermined crystallite size, and from the viewpoint of reducing the unintended mixing of impurities such as carbon element 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 hydrazine anhydride or hydrate.

The content of the reducing compound in the reaction solution in the first reduction step is preferably 0.1 to 2 moles, more preferably 0.1 to 1 mole, per mole of copper element. By controlling the concentration of the reducing compound within this range, the progress of the reduction reaction of copper ions and particle growth can be appropriately controlled, and copper particles having small particle diameters and small crystallite sizes on specific crystal faces can be obtained with high productivity.

It is preferable that the reaction solution in the first reduction step be acidic, with a pH of 3 to 5 at 25°C, in order to make it easier to impart anisotropy to the copper crystal growth that proceeds in the second reduction step while appropriately controlling the degree of reduction so that reduction to cuprous oxide proceeds but does not proceed to metallic copper, when a reducing compound, particularly a hydrazine-based compound, is used. In the first reduction step, it is preferable to add a reducing compound after adjusting the pH, in order to appropriately control the degree of reduction of copper ions.

The pH can be adjusted by using various acids or basic substances, or by adding polyphosphoric acids to the reaction solution, as long as the effects of the present invention are achieved. In particular, by using polyphosphoric acids to adjust the pH, the subsequent reaction can be carried out efficiently without adding other substances to the reaction system, which is advantageous in preventing the inclusion of unintended impurities and efficiently obtaining the desired copper particles.

The reduction reaction in the first reduction step may be carried out with the reaction liquid in an unheated state or in a heated state. In either case, the temperature of the reaction liquid is preferably 10°C or higher and 60°C or lower, more preferably 20°C or higher and 50°C or lower. The reaction time in the first reduction step is preferably 0.1 hours or higher and 2 hours or lower, more preferably 0.2 hours or higher and 1 hour or lower, provided that the temperature is within the above-mentioned range. In addition, from the viewpoint of uniformity of the reduction reaction, it is also preferable to continue stirring the reaction liquid from the start of the reaction to the end of the reaction.

Next, a second reduction step is carried out in which the cuprous oxide obtained in the first reduction step is reduced to produce metallic copper particles. The second reduction step is also preferably carried out under wet conditions, as in the first reduction step, and it is more preferable that both reduction steps are carried out in the same reaction system.

As described above, it is preferable to have polyphosphoric acids present in the reaction system when the second reduction step is performed or at any stage before the second reduction step is performed. The polyphosphoric acids used in the present production method include polyphosphoric acids and their salts, such as diphosphoric acid (H ₄ P ₂ O ₇ ), triphosphoric acid (tripolyphosphoric acid, H ₅ P ₃ O ₁₀ ), tetrapolyphosphoric acid (H ₆ P ₄ O ₁₃ ), etc., each of which has preferably 2 to 8, more preferably 2 to 5, phosphoric acid monomer units in its structure. The polyphosphate salts include alkali metal salts, alkaline earth metal salts, other metal salts, ammonium salts, etc. These can be used alone or in combination.

The content of polyphosphates in the second reduction step is preferably 0.1 millimole or more, more preferably 0.1 millimole or more and 1 mole or less, per mole of copper element. By setting the concentration of polyphosphates in such a range, the crystal growth of copper resulting from the reduction reaction of cuprous oxide can be made anisotropic, and copper particles having a small particle size and a small crystallite size on a specific crystal face can be obtained with high productivity. In addition, when polyphosphates are contained at the time of the first reduction step, the polyphosphates are not consumed in the reaction in the first reduction step, and the concentration of polyphosphates does not change substantially before and after the first reduction step, so that by adding polyphosphates in the above-mentioned concentration range to the reaction system in the first reduction step, the amount of polyphosphates suitable for reduction to metallic copper and grain growth in the second reduction step can be sufficiently achieved.

In the second reduction step, the above-mentioned reducing compound can be added to perform reduction to metallic copper. The content of the reducing compound in the reaction solution in the second reduction step is preferably 1 mole or more and 8 moles or less, more preferably 2 moles or more and 6 moles or less, relative to 1 mole of copper element. When the second reduction step is performed in the same reaction system as the first reduction step, it is preferable to further add the reducing compound to the solution so as to have the above-mentioned content, from the viewpoint of achieving both improvement of reduction 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 progressed, and copper particles having a small particle size and a small crystallite size on a specific crystal plane can be obtained with high productivity.

The reducing compound in the second reduction step may be added all at once or gradually. From the viewpoint of efficiently obtaining copper particles that satisfy the above-mentioned crystallite size ratio and particle diameter, it is preferable to adopt gradually adding the reducing compound.

The reaction solution in the second reduction step is preferably kept under non-acidic conditions (neutral or alkaline conditions) with a pH of 7.0 or more at 25° C., in that when a reducing compound, particularly a hydrazine-based compound, is used, the reduction of the copper ions and cuprous oxide remaining in the reaction solution to metallic copper can be efficiently promoted, and the crystal growth of copper can be easily made anisotropic. The pH is preferably adjusted before the addition of the reducing compound in the second reduction step, in that the degree of reduction of the copper ions can be appropriately controlled. Various acids and basic substances can be used for adjusting the pH.
When the second reduction step is carried out in the same reaction system as the first reduction step, since the reaction solution after the first reduction step is in an acidic condition, 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, since this allows copper ions and cuprous oxide to be efficiently reduced to metallic copper.

From the viewpoint of efficiently proceeding with the reduction of the copper ions and cuprous oxide in the reaction solution and obtaining 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 of the reaction solution are preferably such that the temperature is maintained at 10°C or higher and 60°C or lower, particularly 20°C or higher and 50°C or lower, from the start of the second reduction step, i.e., from the time of adding the reducing compound, to the end of the reaction. The reaction time is preferably 30 minutes or longer and 720 minutes or shorter under the above-mentioned temperature conditions. In addition, from the viewpoint of uniformly causing the reduction reaction to occur and obtaining copper particles with little variation in particle size, it is also preferable to continue stirring the reaction solution from the start of the reaction to the end of the reaction.

The inventors speculate as follows about the reason why copper particles with a low sintering temperature can be obtained in this manufacturing method by performing a two-step reduction process in which copper ions are reduced to cuprous oxide and then to metallic copper, and by having polyphosphoric acids present during the second reduction process.
First, in the first reduction step, copper ions are reduced by a reducing compound in the reaction solution, and very small particles of cuprous oxide are generated in the reaction solution. Then, in the second reduction step, monovalent copper ions eluted from the cuprous oxide particles are reduced to form metallic copper nuclei. Since these nuclei are very unstable, they repeatedly combine with each other or redissolve in the reaction solution, and eventually the particles grow. If polyphosphates are present during this particle growth, the polyphosphates are adsorbed to specific crystal faces of copper, and growth in the direction of the crystal faces is suppressed. On the other hand, growth is not suppressed on crystal faces to which polyphosphates are not adsorbed, and growth in the direction of the crystal faces proceeds.
Based on the fact that metallic copper is likely to have a face-centered cubic crystal structure and the results of X-ray diffraction measurement of the obtained copper particles, it is estimated that the crystal face to which polyphosphates are adsorbed is the (111) face of copper in the particles, and the crystal face to which polyphosphates are not adsorbed is the (220) face of copper located perpendicular to the (111) face of copper. For this reason, it is considered that anisotropic growth occurs in which the growth of the (111) face of copper is suppressed and the growth of the (220) face of copper progresses, resulting in flat copper particles with a low sintering temperature.

As a preferred method for producing copper particles B, the reduction reaction is carried out under acidic conditions, particularly in the first reduction step, so that the reducing power can be controlled to a level where copper ions can be reduced to cuprous oxide but not to metallic copper. In addition, it also becomes easier to control the subsequent metallic copper production reaction. Then, by changing the conditions to non-acidic conditions, the dissolution rate of cuprous oxide can be reduced and the supply of monovalent copper ions can be controlled. By carrying out the second reduction in this environment, the reduction reaction rate to metallic copper can be adjusted to gentle conditions, which is particularly advantageous in that the nucleus growth rate can be controlled.

The copper particles B obtained through the above steps, even if they do not contain organic components that control crystal growth, such as organic amines, amino alcohols, or reducing sugars, will satisfy the above-mentioned suitable crystallite sizes and ratios thereof, suitable particle diameter, and suitable contents of various elements such as carbon, and will have a flattened shape.
In addition, the copper particles B thus obtained have a crystal face of the crystal present on the main surface and grown in a direction perpendicular to the main surface, and a crystal face of the crystal present on the side surface and grown in a direction along the main surface, each of which has a specific orientation direction, and each crystal face is formed uniformly in one direction. Therefore, when copper powder containing copper particles B is used and sintered in a state where the main surfaces of copper particles B are in contact with each other or the side surfaces of copper particles B are in contact with each other, sintering at a low temperature is possible without excessive energy required for fusion due to the contact of the same evenly aligned crystal faces.

The copper particles B obtained through the above steps may be washed or separated into solid and liquid as necessary, and then mixed with the copper particles A in the form of a slurry in which the copper particles B are dispersed in a solvent such as water or an organic solvent, or the particles may be dried and mixed with the copper particles A in the form of a dry powder that is an aggregate of the copper particles B. In either case, the copper particles B have a low sintering temperature and are excellent. If necessary, the copper particles B may be further subjected to a surface coating treatment with an organic substance such as a fatty acid or a salt thereof, or an inorganic substance such as a silicon-based compound, in order to improve the dispersibility between the particles. As long as the effects of the present invention are achieved, it is acceptable that the obtained copper particles B contain elements other than the copper element, for example by unavoidably oxidizing the surface slightly.

<Method of mixing copper particles A and copper particles B>
The copper particles A and the copper particles B can be mixed in a dry or wet manner, but from the viewpoint of ease of mixing, it is preferable to mix them in a dry manner. Dry mixing can be performed using a known dry mixing device. Wet mixing can be performed in an organic solvent or an aqueous solvent.

The copper powder of the present invention can also be further dispersed in an organic solvent, a resin, or the like and used in the form of a conductive composition such as a conductive ink or a copper paste.
When the copper powder of the present invention is in the form of a conductive composition, the conductive composition is composed of at least copper powder and an organic solvent. As the organic solvent, those similar to those used in the technical field of conductive compositions containing metal powders can be used without any particular limitation. 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 alone or in combination of two or more. Among these, it is preferable to use polyethers such as polyethylene glycol and polypropylene glycol, which have a high reducing action and prevent unintended oxidation of copper particles during sintering. From the same viewpoint, when polyethylene glycol is used as the organic solvent, its number average molecular weight is preferably 120 to 400, and more preferably 180 to 400.

At least one of a dispersant, an organic vehicle, and a glass frit may be added to the conductive composition containing the copper powder of the present invention as necessary. Examples of the dispersant include dispersants such as nonionic surfactants that do not contain sodium, calcium, phosphorus, sulfur, or chlorine. Examples of the organic vehicle include mixtures containing resin components such as acrylic resins, epoxy resins, ethyl cellulose, and carboxyethyl cellulose, and solvents such as terpene-based solvents such as terpineol and dihydroterpineol, and ether-based solvents such as ethyl carbitol and butyl carbitol. Examples of the glass frit include borosilicate glass, barium borosilicate glass, and zinc borosilicate glass.

The conductive composition containing the copper powder of the present invention can be applied to a substrate to form a coating film, which can then be fired to form a conductive film containing copper. The conductive film is preferably used, for example, to form a circuit on a printed wiring board or to ensure electrical continuity of an external electrode of a ceramic capacitor. Depending on the type of electronic circuit in which the copper particles are used, examples of the substrate include a printed circuit board made of glass epoxy resin or a flexible printed circuit board made of polyimide.

The amounts of copper powder and organic solvent in the conductive composition containing copper powder of the present invention can be adjusted according to the specific use of the conductive composition and the coating method of the conductive composition, but the content of copper powder in the conductive composition is preferably 5% by mass or more and 95% by mass or less, more preferably 80% by mass or more and 90% by mass or less. Coating methods that can be used include, for example, the inkjet method, the dispenser method, the microdispenser method, the gravure printing method, the screen printing method, the dip coating method, the spin coating method, the spray coating method, the bar coating method, and the roll coating method.

The heating temperature when sintering the formed coating film may be equal to or higher than the sintering start temperature of the copper powder, and may be, for example, 150°C to 220°C. The heating atmosphere may be, for example, an oxidizing atmosphere or a non-oxidizing atmosphere. Examples of the oxidizing atmosphere include an oxygen-containing atmosphere. Examples of the non-oxidizing atmosphere include a reducing atmosphere such as hydrogen or carbon monoxide, a weakly reducing atmosphere such as a hydrogen-nitrogen mixed atmosphere, and an inert atmosphere such as argon, neon, helium, and nitrogen. Regardless of which atmosphere is used, the heating time is preferably 1 minute to 3 hours, more preferably 3 minutes to 2 hours, provided that heating is performed within the above-mentioned temperature range.

The conductive film thus obtained is obtained by sintering the copper powder of the present invention, so that sintering can proceed sufficiently even when sintering is performed under relatively low temperature conditions. Furthermore, during sintering, the copper particles that make up the copper powder melt even at low temperatures, so that the contact area between the copper particles or between the copper particles and the surface of the base material can be increased, resulting in a high degree of adhesion to the objects to be joined and an efficient formation of a dense sintered structure. Furthermore, the conductive film obtained has high continuity, density, and conductive reliability.

Although the present invention has been described above based on its preferred embodiments, the present invention is not limited to the above embodiments. For example, the copper powder of the present invention may contain copper particles other than copper particles A and copper particles B as long as the desired effect is achieved.

The above-described embodiment of the present invention encompasses the following technical ideas.
[1] Contains the following copper particles A and copper particles B,
A copper powder in which the content of copper particles A is 60% by mass or more and 99% by mass or less, and the content of copper particles B is 1% by mass or more and 40% by mass or less, relative to the total of copper particles A and copper particles B.
[Copper particles A]
The present invention comprises a core particle made of copper and a coating layer that coats the surface of the core particle,
the coating layer is formed from a copper salt of an aliphatic organic acid,
Copper particles having a primary particle diameter of 0.1 μm or more and 0.6 μm or less.
[Copper particles B]
a ratio (S1/B) of a first crystallite size S1 calculated by the Scherrer formula from the half-width of a peak derived from the copper (111) plane in an X-ray diffraction measurement to a BET diameter B calculated from a BET specific surface area is 0.23 or less;
The ratio (S1/S2) of the first crystallite size S1 to the second crystallite size S2 calculated by the Scherrer formula from the half-width of the peak derived from the (220) plane of copper in X-ray diffraction measurement is 1.35 or less;
Copper particles having a primary particle diameter of 0.1 μm or more and 2.0 μm or less.
[2] The copper particles B have a ratio (S1/S3) of the first crystallite size S1 to the third crystallite size S3 calculated by the Scherrer equation from the half-width of the peak derived from the (311) plane of copper in an X-ray diffraction measurement, of 1.35 or less. The copper powder according to [1].
[3] The copper powder according to [1] or [2], wherein the copper particles B contain carbon element and the content of the carbon element is 5000 ppm or less.
[4] The copper powder according to any one of [1] to [3], wherein the copper particles B contain phosphorus element and the content of the phosphorus element is 300 ppm or more.
[5] A copper paste containing the copper powder according to any one of [1] to [4].
[6] A method for producing a conductive film, comprising applying the copper paste according to [5] to a substrate to form a coating film, and firing the coating film.

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 such examples. Unless otherwise specified, "%" means "% by mass."

In the following examples and comparative examples, copper particles A-1, A-2, and B-1 to B-3 were used as copper particles. Of these, spherical copper particles described in JP 2015-168878 A were used as copper particles A-2.
Other copper particles were produced by the following method.

[Production of copper particles A-1]
According to the method described in Example 1 of JP 2015-168878 A, a slurry was produced in which spherical core particles (copper: 100% by mass) not treated with a surface treatment agent were dispersed in water. This slurry was washed with a rotary filter at 25°C for 30 minutes to obtain a slurry of washed core particles. The conductivity after washing was 1.0 mS and the core particles had a spherical shape. The content of the copper core particles in the slurry was 1000 g (10% by mass).

Next, the slurry of the washed core particles was heated to 50° C., and under this condition, a solution in which 17 g of copper (II) laurate was dissolved in 4 L of isopropyl alcohol was instantly added as a surface treatment agent, and the mixture was stirred at 50° C. for 1 hour. Then, solid-liquid separation was performed by filtration, and copper particles having a coating layer of a copper salt of an aliphatic organic acid formed on the surface of the core particles were obtained as a solid content. The content of the surface treatment agent in the obtained copper particles was 0.7% by mass in terms of carbon atoms.
Next, the copper particles A-1 and A-2 were subjected to the following evaluations.

[Measurement of the average image analysis diameter of primary particles]
The average image analysis diameter of the primary particles of the copper particles was measured by the method described above, and the results are shown in Table 1 below.

[Calculation of BET diameter A based on BET specific surface area]
First, the specific surface areas of the copper particles A-1 and A-2 were measured based on the BET single point method by the above-mentioned measurement method, and the BET diameter A was calculated based on the specific surface areas. The results are shown in Table 1.

[Evaluation of temperature at 10% mass loss]
In the thermogravimetric analysis when heated from 25° C. to 1000° C., the temperature at which the ratio of the mass loss to the mass loss at 500° C. was 10% was measured under the above-mentioned conditions. The results are shown in Table 1.

[Evaluation of infrared absorption peaks]
Copper particles A-1 and A-2 were subjected to infrared spectroscopy measurement by the above-mentioned method. For each range of 1504 cm ^-1 or more and 1514 cm ^-1 or less, and 1584 cm ^-1 or more and 1596 cm ^-1 or less, those having an infrared absorption peak were evaluated as "present", and those not having an infrared absorption peak were evaluated as "absent". The results are shown in Table 1.

[Production of copper particles B-1]
<First reduction step>
Into a stainless steel tank containing 5.0 L of warm pure water and 5.0 L of methanol, 2.5 kg of copper acetate monohydrate as a copper source and 8.0 g of sodium tripolyphosphate (molar ratio relative to 1 mole of copper element: 0.002) as a polyphosphate were placed, and the mixture was stirred at a liquid temperature of 25° C. for 30 minutes to dissolve both components.
Next, 235.0 g of hydrazine (molar ratio relative to 1 mole of copper: 1.55) was added to the liquid, and stirring was continued for 30 minutes under non-heated conditions with a liquid temperature of 25° C. to generate cuprous oxide fine particles in the liquid. After the cuprous oxide was generated, the reaction liquid was stirred for 30 minutes.

<Second reduction step>
Next, a 25% NaOH aqueous solution was added to the reaction solution in the first reduction step to adjust the pH of the solution to 7.0. The solution was then heated to 40° C., and 1900.0 g of hydrazine (molar ratio relative to 1 mole of copper: 3.0) was quantitatively and sequentially added to the solution over 10 minutes to perform the second reduction step. The solution was then cooled to 30° C. and stirred for 150 minutes to obtain copper particles in which the cuprous oxide particles were reduced to metallic copper.

The aqueous slurry of copper particles thus obtained was subjected to decantation washing until the electrical conductivity reached 1.0 mS (washed slurry).
The slurry of the washed core particles was heated to 50°C, and under this condition, a solution in which 4 g of copper (II) laurate was dissolved in 1 L of isopropyl alcohol was instantly added as a surface treatment agent, and the mixture was stirred at 50°C for 1 hour. Then, solid-liquid separation was performed by filtration, and copper particles in which a coating layer of a copper salt of an aliphatic organic acid was formed on the surface of the core particles were obtained as a solid content. Then, the mixture was dried to obtain copper powder consisting of an aggregate of copper particles. The obtained copper particles had a copper element content of more than 98% by mass and had a flat shape.

[Production of copper particles B-2]
Copper particles B-2 were obtained in the same manner as in the production of copper particles B-1, except that the amount of sodium tripolyphosphate added was 24 g (molar ratio relative to 1 mole of copper: 0.006). The obtained copper particles had a copper content of more than 98% by mass and a flat shape.

[Production of copper particles B-3]
4 kg of copper sulfate pentahydrate and 120 g of aminoacetic acid were dissolved in water to prepare 8 L of copper salt aqueous solution at a liquid temperature of 70 ° C. Then, while stirring this aqueous solution, 6.6 kg of 25% by mass sodium hydroxide solution was added at a constant rate over about 5 minutes. Next, the solution was stirred at a liquid temperature of 70 ° C. for 60 minutes, and the solution was aged until the liquid color became completely black to generate cupric oxide. After that, the solution was left for 30 minutes, 1.5 kg of glucose was added, and the solution was aged for 1 hour to reduce cupric oxide to cuprous oxide. Furthermore, 1 kg of hydrazine hydrate was quantitatively added over 5 minutes to reduce cuprous oxide to metallic copper, thereby generating a copper powder slurry. The obtained copper powder slurry was filtered, thoroughly washed with pure water, filtered again, and then dried. The obtained Cu powder was plastically deformed using the flattening method described in Patent No. 4227373 to convert the approximately spherical copper powder into flat copper powder. At this time, the bead diameter used was changed from 0.7 mm to 0.2 mm.

Next, the following evaluations were performed on copper particles B-1 to B-3.

[Measurement of the average image analysis diameter of primary particles]
For copper particles B-1 to B-3, the average image analysis diameter of the primary particles of each copper particle was measured by the above-mentioned method. The results are shown in Table 2.

[Calculation of BET diameter B based on BET specific surface area]
The BET diameter B of copper particles B-1 to B-3 was measured in the same manner as the BET diameter A of copper particles A. The results are shown in Table 2 below.

[Measurement of Carbon and Phosphorus Content]
The carbon element content in the copper particles B-1 to B-3 was measured using a carbon/sulfur analyzer (CS844 manufactured by LECO Japan LLC) by placing 0.50 g of any of the copper particles B-1 to B-3 in a magnetic crucible, using oxygen gas (purity: 99.5%) as the carrier gas, and setting the analysis time to 40 seconds. The measurement results are shown in Table 2 below.
The content of phosphorus element in the copper particles was measured by dissolving 1.00 g of any of copper particles B-1 to B-3 in 50 mL of 15% aqueous nitric acid solution, and introducing the solution into an ICP emission spectrometer (PS3520VDDII manufactured by Hitachi High-Tech Science Co., Ltd.). The measurement results are shown in Table 2 below.

[Measurement of crystallite size]
Measurements were performed on copper particles B-1 to B-3 by the following method. First, a 20% by mass aqueous slurry was prepared using the washed slurry of copper particles B-1 to B-3. Then, an 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 the mixture was stirred for 1 hour. Then, the solid content obtained by solid-liquid separation by filtration was vacuum dried, and the copper powder obtained by obtaining copper particles subjected to surface coating treatment was classified using a sieve with a mesh size of 75 μm, and the undersized portion was used as a sample. This sample was filled into a sample holder, and measurements were performed under the following conditions using an X-ray diffractometer (Ultima IV manufactured by Rigaku Co., Ltd.).
Then, among the diffraction peaks, the main peaks at the positions corresponding to the (220) plane, (111) plane or (311) plane of copper are used as the target, and based on the full width at half maximum of the peak, the above-mentioned Scherrer formula is used to calculate each crystallite size S1 to S3, as well as the S1/S2 and S1/S3 ratios.Furthermore, the S1/B ratio is calculated from each crystallite size obtained.The results are shown in the following Table 2.

<X-ray diffraction measurement conditions>
Tube: CuKα ray Tube voltage: 40 kV
Tube current: 50mA
Measurement diffraction angle: 2θ = 20 to 100°
Measurement step width: 0.01°
Acquisition time: 3 sec/step Receiving slit width: 0.3 mm
Divergence vertical limit slit width: 10 mm
・Detector: High-speed one-dimensional X-ray detector D/teX Ultra250

<Method of preparing samples for X-ray diffraction>
The copper powder to be measured was spread on a measurement holder, and smoothed using a glass plate so that the copper powder had a thickness of 0.5 mm and was smooth.

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

<Measurement data analysis conditions>
Analysis software: Rigaku PDXL2
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

The peaks of the X-ray diffraction pattern used in the analysis are as follows: The Miller indices shown below are synonymous with the above-mentioned copper crystal planes.
A peak indexed with Miller index (220) in the vicinity of 2θ=71° to 76°.
A peak indexed with Miller index (111) in the vicinity of 2θ=40° to 45°.
A peak indexed with Miller index (311) in the vicinity of 2θ=87.5° to 92.5°.

[Examples 1 to 7 and 9 and Comparative Examples 1 to 7]
Copper particles A-1, A-2, and B-1 to B-3 were mixed in the ratios shown in Table 3 below to obtain copper powders of each Example and Comparative Example. Specifically, each copper particle was added to a 100 mL container in the ratios shown in Table 3, and then mixed using a small ball mill (AV-1 manufactured by Asahi Rika) to obtain copper powders of each Example and Comparative Example. Mixing was performed at 100 rpm for 1 hour.
The copper powder obtained as described above and polyethylene glycol having a number average molecular weight of 200 were mixed using a three-roll kneader to obtain a copper paste containing 85% by mass of copper powder.
The content of each copper particle component relative to 100 parts by mass of the total of the copper particles is shown in Table 3. The "solid content concentration" indicates the ratio of the mass of the copper powder to the mass of the entire copper paste.

[Evaluation of Paste Printability]
The paste printability of each of the Examples and Comparative Examples was evaluated based on the following evaluation criteria.
<Evaluation Criteria for Paste Printability>
Possible: Can be applied to a masked glass plate measuring 2 cm in length and 1 cm in width.
Unacceptable: A continuous coating film cannot be formed under the above conditions.

[Production of Conductive Film]
The copper paste of each of the Examples and Comparative Examples was applied onto a glass substrate, and the substrate was baked at 190° C. for 10 minutes in a nitrogen atmosphere to form a conductive film on the glass substrate.
The obtained conductive film had a length of 2 cm, a width of 1 cm and a thickness of 30 μm, and was evaluated as follows.

[Evaluation of film strength]
The film strength of each conductive film was evaluated based on the following evaluation criteria.
<Evaluation criteria for film strength>
High: When the conductive film surface is scratched, no powder remains on the hands.
Low: The conductive film surface is scratched and powder comes off onto your hands.

[Measurement of resistivity of conductive film]
The resistivity of each conductive film was measured using a resistivity meter (Loresta-GP MCP-T610, manufactured by Mitsubishi Chemical Analytech Co., Ltd.). The conductive film to be measured was measured three times, and the arithmetic average value was taken as the resistivity (μΩ·cm). The lower the resistivity, the smaller the resistance of the conductive film, and it is preferable that the resistivity is 50 μΩ·cm or less. The results are shown in Table 3 below.

[Evaluation of Conductive Film Thickness]
The thickness of the fired film of the Cu paste was determined by measuring the thickness of the conductive film and glass substrate, and the thickness of the glass substrate alone using a digital length measuring machine (Nikon MFC-101), and calculating the difference between these thicknesses as the thickness of the conductive film.

[Measurement of Surface Roughness Ra of Conductive Film]
The surface roughness (average roughness Ra) of each conductive film was measured at three points using a surface roughness/contour shape measuring instrument (SURFCOM 130A manufactured by Tokyo Seimitsu Co., Ltd.) and the average value of the obtained values was calculated. The results are shown in Table 3.
From the viewpoint of electrical resistance, the surface roughness Ra is preferably 2.0 or less. Furthermore, a conductive film having a small surface roughness Ra means that the conductive film has high density.

[Measurement of Bonding Strength]
A copper paste was screen-printed on the polished side of a 5 mm square copper chip with one side polished with a #800 polishing sheet in the form of 2 mm x 2 mm x 30 μm, and a 3 mm square copper chip with one side polished with a #800 polishing sheet was placed so that the polished surface was the bonding surface. Thereafter, the chip was bonded by firing at 200 ° C. for 30 minutes under a nitrogen atmosphere while applying pressure of 5 MPa using a pressure firing machine (IMC-1AB6 manufactured by Imoto Seisakusho). The bond strength of the resulting bonded body was measured using a bond tester (Condor Sigma manufactured by XYZTEC Corporation).
The above-mentioned measurement was carried out three times, and the maximum and average values of the bonding strength were calculated. These are shown in Table 3. In Table 3, the bonding strength is a standardized value such that the bonding strength of Comparative Example 3 is 1.0. In Table 3, "-" indicates that the measurement was not performed.

As shown in Table 3, all of the conductive films of the examples had low resistivity even though they were produced by firing at a relatively low temperature of 190° C. This shows that the copper powder used in each example has a low sintering temperature. In addition, the conductive films of each example have excellent paste printability and low surface roughness Ra, which shows that they have good continuity and denseness.
Furthermore, as can be seen from a comparison between Example 2 and Comparative Example 1, even when the same flat copper particles are used, when mechanically flattened copper particles such as copper particles B-3 are used, the sintering temperature of the copper powder increases and the resistivity of the conductive film increases.

[Example 8, Comparative Example 8 and Comparative Example 9]
As shown in Table 4 below, when copper powder and polyethylene glycol having a number average molecular weight of 200 were mixed using a three-roll mixer, the mixing ratio of copper powder and polyethylene glycol was changed to prepare copper pastes containing 90 mass% copper powder. Except for this, the copper pastes and conductive films thereof of Example 8, Comparative Example 8, and Comparative Example 9 were prepared in the same manner as in Example 2, Comparative Example 3, and Comparative Example 5. These copper pastes and conductive films thereof were evaluated in the same manner as in Examples 1 to 7 and Comparative Examples 1 to 7. The results are shown in Table 4.

As shown in Table 4, the copper powders of Examples 2 and 8, which contain both copper particles A-1 and B-1, are less likely to be adversely affected in resistivity and surface roughness Ra of the conductive film even when the content of organic solvent in the copper paste is changed. On the other hand, as shown in Comparative Examples 8 and 9, when copper powder containing only copper particles A-1 or B-1 is used, if the content of organic solvent in the copper paste is 10 mass%, it can be seen that the resistivity of the conductive film in particular increases significantly.

According to the present invention, a copper powder is provided that can produce a conductive film having high continuity and density and can be sintered at a low temperature.

Claims (6)

The copper particles A and B include:
A copper powder in which the content of copper particles A is 60% by mass or more and 99% by mass or less, and the content of copper particles B is 1% by mass or more and 40% by mass or less, relative to the total of copper particles A and copper particles B.
[Copper particles A]
The present invention comprises a core particle made of copper and a coating layer that coats the surface of the core particle,
the coating layer is formed from a copper salt of an aliphatic organic acid,
Copper particles having a primary particle diameter of 0.1 μm or more and 0.6 μm or less.
[Copper particles B]
a ratio (S1/B) of a first crystallite size S1 calculated by the Scherrer formula from the half-width of a peak derived from the copper (111) plane in an X-ray diffraction measurement to a BET diameter B calculated from a BET specific surface area is 0.02 or more and 0.23 or less;
The ratio (S1/S2) of the first crystallite size S1 to the second crystallite size S2 calculated by the Scherrer formula from the half-width of the peak derived from the (220) plane of copper in X-ray diffraction measurement is 0.1 or more and 1.35 or less;
Copper particles having a primary particle diameter of 0.1 μm or more and 2.0 μm or less. The copper powder according to claim 1, wherein the ratio (S1/S3) of the first crystallite size S1 to the third crystallite size S3 calculated by the Scherrer formula from the half-width of the peak derived from the (311) plane of copper in X-ray diffraction measurement is 1.35 or less. The copper powder according to claim 1, wherein copper particles B contain carbon element and the carbon element content is 5000 ppm or less. The copper powder according to claim 1, wherein copper particles B contain phosphorus element and the content of the phosphorus element is 300 ppm or more. A copper paste containing the copper powder according to any one of claims 1 to 4. A method for producing a conductive film, comprising applying the copper paste according to claim 5 to a substrate to form a coating film, and then firing the coating film.