EP3031551A1

EP3031551A1 - Composite copper particles and production method therefor

Info

Publication number: EP3031551A1
Application number: EP14834408.8A
Authority: EP
Inventors: Toshihiro Kohira; Nobuhiro Sasaki; Hikaru Minowa
Original assignee: Mitsui Mining and Smelting Co Ltd
Current assignee: Mitsui Kinzoku Co Ltd
Priority date: 2013-08-07
Filing date: 2014-08-01
Publication date: 2016-06-15
Also published as: KR20160040538A; WO2015019959A1; CN105451914A; JP2015034309A; TW201511035A; TWI556257B; EP3031551A4

Abstract

The composite copper particle of the invention is a composite copper particle including a flake-like copper particle and a plurality of inorganic oxide particles which are finer than the flake-like copper particle. The inorganic oxide particles are unevenly distributed on a surface of the flake-like copper particle. The composite copper particle preferably has a volume cumulative particle diameter D 50 at a cumulative volume of 50 vol% as measured by a laser diffraction scattering method of 0.1 µm to 10 µm. An aspect ratio of the maximum diameter d of a plane of the composite copper particle to the maximum thickness t of the composite copper particle, d/t, is preferably 5 to 30. The inorganic oxide particles preferably have a higher hardness than copper.

Description

Technical Field

This invention relates to a composite copper particle and a process for producing the same.

Background Art

Flake-like copper particles have a large specific surface area and a large contact area between themselves due to their flatness. Therefore, addition of flake-like copper particles to an electroconductive composition improves electric conductivity and allows for viscosity adjustment. For example, the assignee common to this patent application proposes a flake-like copper powder and a conductive paste containing the flake-like copper powder (see Patent Literature 1).
Patent Literature 1 discloses a flake-like copper powder having a particle diameter of 10 µm or less, an SD/D₅₀ of 0.5 or less (wherein SD is a standard deviation of particle size distribution, and D₅₀ is a particle diameter at 50% in the cumulative weight-based distribution), and a D₉₀/D₁₀ of 4.0 or less (wherein D₉₀ is a particle diameter at 90% in the cumulative weight-based distribution, and D₁₀ is a particle diameter at 10% in the cumulative weight-based distribution). Patent Literature 1 also discloses a flake-like copper powder having a particle diameter of 10 µm or less, an SD/D₅₀ of 0.15 to 0.35, and an aspect ratio ([thickness]/[D₅₀] of 0.3 to 0.7. The flake-like copper powder described in Patent Literature 1 allows for forming a fine electrical circuit.

Citation List

Patent Literature

Patent Literature 1: JP 2003-119501A

Summary of Invention

The ever decreasing size and improving performance of electronic components have boosted the demand for still finer materials for use therein. Accordingly, particle size reduction is demanded in a flake-like copper powder for use as a material of conductive compositions. However, when a conductive film is formed using a conductive paste containing a flake-like copper powder with a smaller particle size, there is a tendency that the gas generated during sintering hardly escapes. As a result, the resulting electrode film may have poor continuity.
An object of the invention is to provide composite copper particles free from the various disadvantages of the aforementioned conventional techniques and a process for making the same.
The present invention provides a composite copper particle including a flake-like copper particle and a plurality of inorganic oxide particles which are finer than the flake-like copper particle,
the inorganic oxide particles being unevenly distributed on a surface of the flake-like copper particle.
The present invention provides a process for producing composite copper particles comprising subjecting a mixed powder of a starting spherical copper powder and a particulate inorganic oxide to a dispersion process using beads thereby plastically flattening the copper particles of the starting copper powder and locating the particulate inorganic oxide on the surface of the flattened copper particles,
the particulate inorganic oxide powder having a ratio of a volume cumulative particle diameter D₅₀ (nm) measured by a dynamic light scattering method to a particle size D_BET calculated from its BET specific surface area, D₅₀/D_BET, of 60 or greater.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a copper mapping image of the composite copper particles obtained in Example 1.
[Fig. 2] Fig. 2 is a zirconium mapping image of the composite copper particles obtained in Example 1.
[Fig. 3] Fig. 3 is an aluminum mapping image of the composite copper particles obtained in Example 1.
[Fig. 4] Fig. 4 is a graph showing the results of thermomechanical analysis (TMA) of the copper particles obtained in Examples 1 to 3 and Comparative Examples 1 and 2.

Description of Embodiments

The invention will be described generally based on its preferred embodiments. The composite copper particle of the invention is composed of a copper particle as a matrix and a plurality of inorganic oxide particles combined with the matrix. The copper particle as a matrix is a flake-like copper particle having a flat shape, and the inorganic oxide particles are finer than the flake-like copper particles as a matrix.
The composite copper particle of the invention is characterized by the fashion of the inorganic oxide particles being combined with the flake-like matrix copper particle. Specifically, the inorganic oxide particles are unevenly distributed on the surface of the flake-like copper particle. When described as being unevenly distributed on the surface, the inorganic oxide particles are not uniformly distributed on the entire surface of the flake-like copper particle but exist in part of the surface. Thus, the surface of the flake-like copper particle has an area where the inorganic oxide particles are present (hereinafter "composite area") and an area where the inorganic oxide particles are substantially absent (hereinafter "composite-free area"). When the inorganic oxide particles are unevenly distributed on the surface of the flake-like copper particle, the following advantage is obtained. When a coating film of a conductive composition, such as a conductive paste, prepared using the composite copper particles of the invention is fired to form, e.g., an electronic circuit, the composite areas, where the inorganic oxide particles are present, of the composite copper particles are less likely to bind together than the composite-free areas. The sites at which the composite copper particles are less likely to bind together provide passageways for escape of the gas generated during firing. As a result, blistering of an electrode that might occur during firing is prevented effectively, whereby the electronic circuit and the like formed by using the composite copper particles of the invention will be prevented from increasing electric resistivity and have improved surface smoothness. In contrast, flake-like copper particles having no inorganic oxide particles on their surface are liable to bind together on their flat surfaces, failing to provide a passageway for escape of the gas, which can result in blistering of the electrode during firing.
When inorganic oxide particles are described as being unevenly distributed, it means that both a composite area and a composite-free area are observed on the outer peripheral surface of a composite copper particle of the invention when the composite copper particle is ultramicrotomed into sections and the sections are subjected to elemental mapping as shown in Figs. 1 through 3. When the entire peripheral surface of a composite copper particle is found to be a composite area or, conversely, a composite-free area, the inorganic oxide particles are not described as being unevenly distributed.
To effectively prevent the continuity of an electrode, it is preferred that, in the composite area, a plurality of inorganic oxide particles be agglomerated into agglomerates as shown in Fig. 2 as will be described later.
The inorganic oxide particles are located on the surface of a flake-like copper particle through an anchor effect in such a fashion that, for example, part of an inorganic oxide particle is embedded into the surface of the flake-like copper particle. Alternatively, the inorganic oxide particles are located on the surface of a flake-like copper particle through the cohesive force (surface energy) produced between the inorganic oxide particle and the flake-like copper particle. In some cases, a plurality of the inorganic oxide particles are agglomerated by the surface energy generated between the inorganic oxide particles.
The composite copper particles of the invention having the inorganic oxide particles distributed unevenly on the surface of a flake-like copper particle can be produced by, for example, the process described infra.
The effect of unevenly distributing the inorganic oxide particles on the surface of a flake-like copper particle can be enhanced when the proportion of the inorganic oxide particles in the composite copper particles of the invention ranges from 0.001 mass% to 5.0 mass%, preferably 0.01 mass% to 3.0 mass%, more preferably 0.01 mass% to 2.0 mass%. The proportion of the inorganic oxide particles is measured by, for example, inductively coupled plasma-atomic emission spectrometry (ICP-AES).
It is only necessary for the inorganic oxide particles to be localized only on the surface of a flake-like copper particle. The inorganic oxide particles do not need to be present inside the flake-like copper particle, which does not mean to exclude the presence of the inorganic oxide particle inside the flake-like copper particle. Nevertheless, the smaller the ratio of the inorganic oxide particles present inside the flake-like copper particle, the better for making the effect of the uneven distribution of the inorganic oxide particles more pronounced. From this standpoint, the ratio of the inorganic oxide particles present inside the flake-like copper particles to the total content of the inorganic oxide particles of the composite copper particles of the invention is preferably 1.0 mass% or less, more preferably 0.7 mass% or less. This ratio can be measured by, for example, elemental mapping of a section of a composite copper particle of the invention. As used herein, the phrase "an inorganic oxide particle present inside a flake-like copper particle" denotes an inorganic oxide particle that is not at all exposed on the surface of the composite copper particle of the invention.
The composite copper particles of the invention have a flaky shape reflecting the shape of the flake-like copper particles as a matrix. The flakiness of the composite copper particles of the invention can be represented by an "aspect ratio" of the maximum diameter d of a plane of the composite copper particle to the maximum thickness t of the composite copper particle, d/t. The aspect ratio d/t of the composite copper particles is preferably 5 to 30, more preferably 5 to 25, even more preferably 7 to 20. An electronic circuit and the like formed by using the composite copper particles with such flakiness exhibit increased denseness and are effectively prevented from increasing electrical resistance.
The aspect ratio of the composite copper particles is obtained by measuring the maximum diameter d of the plane of the particle and the thickness t of the particle. Specifically, particle are photographed using a scanning electron microscope (SEM), and the ratio of the maximum diameters d of the plane of the particle to the thicknesses t of the particle of the micrograph is calculated.
It is preferred for the composite copper particles of the invention to be not only flake-like but microfine. To use flaky and microfine composite copper particles of the invention provides an electronic circuit and the like with further increased denseness and further ensured prevention from an increase of electrical resistance. From that viewpoint, the composite copper particles of the invention preferably have a volume cumulative particle diameter D₅₀ at a cumulative volume of 50 vol% as measured by a laser diffraction scattering method of 0.1 µm to 10 µm, more preferably 0.2 µm to 9.0 µm, even more preferably 0.3 µm to 7.0 µm.
Copper particles tend to have a reduced sintering onset temperature with a decrease in particle size. This tendency also applies to the composite copper particles of the invention. Reduction in sintering onset temperature is undesirable for some applications of the composite copper particles of the invention. In this regard, the composite copper particles of the invention are protected from reduction of the sintering onset temperature by virtue of the presence of the inorganic oxide particles on the surface of the flake-like matrix copper particles. That is, although the composite copper particles of the invention are microfine particles, the composite copper particles of the invention has approximately the same sintering onset temperature as conventionally used copper particles.
It is preferred for the composite copper particles of the invention to be flake-like and have a broad particle size distribution. An electronic circuit and the like formed by using the composite copper particles that have a flaky shape with a broad size distribution exhibit increased denseness and are thereby effectively prevented from increasing in electrical resistance. In this regard, a ratio of the maximum diameter D_max to D₅₀, D_max/D₅₀, provides a parameter showing the breadth of the particle size distribution, wherein D_max is measured by a laser diffraction scattering method, and D₅₀ is measure by a laser diffraction scattering method. The composite copper particles of the invention preferably have a D_max/D₅₀ of 3 to 10, more preferably 3 to 9, even more preferably 3 to 8.
The composite copper particles of the invention which have the above-described particle size distribution can be obtained by properly selecting the conditions in the step of flattening starting copper particles in the hereinafter described preferred process for producing the composite copper particle of the invention.
The size of the flake-like copper particles as a matrix of the composite copper particles of the invention is equal to that of the composite copper particles of the invention. On the other hand, the size of the inorganic oxide particles as calculated from their BET specific surface area (hereinafter referred to as BET particle size) is preferably 1 nm to 500 nm, more preferably 1 nm to 400 nm, even more preferably 1 nm to 300 nm, provided that it is smaller than the size of the flake-like copper particles. By combining the flake-like copper particles with the inorganic oxide particles whose size is in that range, blistering of an electrode can effectively be prevented, and the reduction in sintering onset temperature can effectively be reduced.
The BET specific surface area of the inorganic oxide particles, from which the BET particle size is calculated, is measured by, for example, a gas adsorption method in which the specific surface area of particles is calculated from the amount of gas adsorbed on the surface of the particles. Specifically, MonoSorb from Yuasa Ionics Co., Ltd. may be used as a measuring instrument.
The inorganic oxide particles that can be used in the invention preferably have higher hardness than copper so that the inorganic oxide particles may easily be located on the surface of the flake-like copper particles in the hereinafter described preferred method for producing the composite copper particles of the invention. As used herein, the term "hardness" denotes a hardness measured with a Mohs hardness meter.
In that connection, preferred materials of the inorganic oxide particles include alumina, zirconia, silica, barium titanate, yttrium oxide, and zinc oxide. These materials may be used either individually or in combination of two or more thereof.
The flake-like copper particle as a matrix to be combined with the inorganic oxide particles may be made solely of copper or may comprise at least one other metal element or semimetal element (hereinafter inclusively referred to as a metallic element for the sake of simplicity) in addition to copper. Examples of the other metallic elements if used in combination with copper include those that behave differently from copper during sintering, such as aluminum, zirconium, yttrium, and silicon. These metallic elements may be used either individually or in combination of two or more thereof. The inclusion of such a metallic element in the flake-like copper particle makes it possible to control the behavior of copper during sintering.
The other metallic element included in the flake-like copper particle may be present in the form of an elemental metal, an alloy with copper, or a compound (e.g., an oxide). To ensure the effect described above, it is preferred that the other metallic element be present in the flake-like copper particle in the form of a compound, such as an oxide.
The other metallic element may be evenly distributed in the flake-like copper particle or localized at specific sites. As a result of the inventor's study, it has been revealed to be desirable that the other metallic element be localized in the surface portion of the flake-like copper particles. This is because, the inventor considers, the vicinities of the surface are influential on the sintering behavior.
When the other metallic element is localized on the surface of the flake-like copper particle, it is preferred for the metallic element to be uniformly distributed over the entire surface of the flake-like copper particle, so that the sintering behavior during firing is controlled easily. The ease of sintering behavior control combined with the broad particle size distribution of the flake-like copper particles makes electrode thickness design easier.
The content of the other metallic element is preferably 0.001 mass% to 5.0 mass%, more preferably 0.01 mass% to 3.0 mass%, even more preferably 0.05 mass% to 1.0 mass%, based on the mass of copper in the flake-like copper particles. When the other metallic element is present in that ratio, the other metallic element produces a more pronounced effect.
The preferred process for producing the aggregate of microfine particles according to the invention will then be described. In the process, a mixed powder of a starting, spherical copper powder and an inorganic oxide powder is subjected to a dispersion process using beads. By the dispersion process, the copper particles are plastically flattened, and the inorganic oxide particles are located on the surface of the flattened copper particles. What is important in this operation is to use inorganic oxide powder having a high degree of agglomeration. While being combined with the spherical copper powder, a highly agglomerated inorganic oxide powder can be localized unevenly. From that point of view, it is advantageous to use an inorganic oxide powder having a D₅₀/D_BET of 60 or greater, where D₅₀ is a volume cumulative particle diameter (nm) at a cumulative volume of 50 vol% as measured by a dynamic light scattering method, and D_BET is a BET particle size calculated from BET specific surface area of the particulate inorganic oxide powder. The D₅₀/D_BET value is an index representing the degree of agglomeration. A greater D₅₀/D_BET value means a higher degree of agglomeration. The agglomerated state of an inorganic oxide powder having a D₅₀/D_BET of 60 or greater will be reflected on the resulting composite copper particles such that the inorganic oxide particles will unevenly be distributed on the surface of the flake-like copper particle.
In order merely to locate the inorganic oxide particles unevenly on the surface of the flake-like copper particles, the D₅₀/D_BET of the inorganic oxide particles is desirably as high as possible. However, too high a D₅₀/D_BET value can cause difficulty in mixing the starting spherical copper powder and the inorganic oxide powder. Therefore, the D₅₀/D_BET is preferably 60 to 300, more preferably 60 to 100.
The beads used to subject the mixed powder of the spherical copper powder and the inorganic oxide powder to a dispersion process preferably have a diameter of 0.005 to 1.0 mm, more preferably 0.05 to 0.5 mm, even more preferably 0.05 to 0.3 mm. The beads may be of any material that is harder than copper and the inorganic oxide particles, such as alumina, zirconia, or silica.
The amount of the beads to be used is preferably 50 mass% to 90 mass%, more preferably 60 mass% to 85 mass%, even more preferably 65 mass% to 85 mass%, relative to the capacity of the device used to carry out the process.
The dispersion process using beads may be carried out, for example, by means of a bead mill. In using a bead mill, the time required for the dispersion process varies mainly according to the capacity of the device. Generally speaking, the time required for treating 1 kg of copper powder using a bead mill having a capacity of 0.1 liters to 300 liters is preferably 5 minutes to 90 minutes, more preferably 10 minutes to 70 minutes. Under that condition, the starting spherical copper powder is successfully flattened while retaining to some extent the agglomerated state of the inorganic oxide powder, and the agglomerated inorganic oxide powder can be fixedly located on the surface of the flattened, flake-like copper particles.
The inorganic oxide particles do not change in particle size (primary particle diameter) between before and after being combined with the flattened copper powder. Therefore, the particle size of the inorganic oxide powder used as a raw material is equal to that of the inorganic oxide particles contained in the composite copper particles of the invention. On the other hand, the starting copper particles used as a raw material are flattened by the dispersion process using beads, i.e., they change in shape and size between before and after being combined. The starting copper powder before the dispersion process is an aggregate of spherical copper particles. The starting copper powder has the volume cumulative particle diameter D₅₀ at a cumulative volume of 50 vol% as measured by a laser diffraction scattering method of preferably 0.03 µm to 8 µm, more preferably 0.05 µm to 7 µm, for ease of obtaining microfine composite copper particles. Although it is possible to use, as a starting copper powder, copper particles other than spherical copper particles, there may be a difficulty in obtaining composite copper particles having a desired flaky shape. Additionally, spherical copper particles are easier to produce than otherwise shaped copper particles.
The starting copper powder preferably has a high degree of agglomeration thereby to provide composite copper particles having a high degree of agglomeration. From that viewpoint, the starting copper powder has a ratio of the maximum diameter D_max to D₅₀, D_max/D₅₀, of preferably 2 to 15, more preferably 3 to 13, even more preferably 3 to 10, wherein D_max and D₅₀ are measured by a laser diffraction scattering method. A starting copper powder having such a particle size distribution can be prepared by properly selecting the conditions for the preparation by, for example, a dry process (e.g., atomizing) or a wet reduction process. Otherwise, copper powders prepared by these processes may be mixed or classified to obtain powder having a desired particle size distribution.
In the case when a metallic element other than copper is included into the desired composite copper particles, it is advantageous to previously incorporate the other metallic element into the starting copper powder. For example, aluminum may be incorporated as follows. In a dry process, aluminum is added to molten copper. In a wet process, an oxide of aluminum, such as alumina, is added in the course of copper reduction. In the thus prepared starting copper particles, aluminum element is chiefly present close to the surface of the particles.
As a result of the above described dispersion process, there are obtained desired composite copper particles. The resulting composite copper particles are used in the form of an electroconductive composition including the composite copper particles, such as conductive paste or conductive ink. A conductive paste may include the composite copper particles of the invention, an organic vehicle, and a glass frit. The organic vehicle comprises a resin component and a solvent. Examples of the resin component are acrylic resins, epoxy resins, ethyl cellulose, and carboxyethyl cellulose. Examples of the solvent include terpene solvents, such as terpineol and dihydroterpineol, and ether solvents, such as ethyl carbitol and butyl carbitol. Examples of the glass frit include borosilicate glass, barium borosilicate glass, and zinc borosilicate glass. The amount of the aggregate of microfine particles in the conductive paste are preferably 36 to 97.5 mass%. The amount of the glass frit in the conductive paste 1.5 to 14 mass%. The amount of the organic vehicle in the conductive paste 1 to 50 mass%. The conductive component in the conductive paste may consist solely of the composite copper particles of the invention or a mixture of the aggregate of microfine particles and other fine copper particle. In the latter case, the viscosity of the paste can be adjusted more accurately.

Examples

The invention will now be illustrated in greater detail with reference to Examples, but it should be understood that the invention is not limited thereto. Unless otherwise noted, all the percents and parts are by mass.

Example 1

(1) Preparation of starting copper powder

"CB-3000" available from Mitsui Mining & Smelting Co., Ltd. was used as a starting copper powder. CB-3000 had a D_max/D₅₀ of 3.5 and a D₅₀ of 3.2 µm. The D₅₀ and D_max were measured using Microtrac X-100 from Nikkiso Co., Ltd. The starting copper powder contained 0.25% aluminum. Aluminum existed in the form of a simple substance both at and in the vicinity of the surface of the particles.

(2) Preparation of inorganic oxide powder

Zirconia powder was used as an inorganic oxide powder. The powder had a D₅₀/D_BET of 70 and a BET particle size D_BET of 15 nm. The D₅₀ was measured using Zeta Sizer ZS available from Malvern. The D_BET was measured using MonoSorb available from Yuasa Ionics.

(3) Preparation of composite copper particles

A thousand grams of the starting copper powder and 100 g of the inorganic oxide powder were put in a 2 liter-bead mill and mixed. Then, Zirconia beads with a diameter of 0.2 mm were put into the mill to carry out a dispersion process for 20 minutes to obtain composite copper particles as desired. The amount of the beads was 70% of the capacity of the bead mill. The proportion of the inorganic oxide particles in the composite copper particles was found to be 0.5% as measured by the method described supra.
The resulting composite copper powder was ultramicrotomed into sections, and the sections were subjected to elemental mapping by scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) for copper, zirconium, and aluminum. The results are shown in Figs. 1 through 3. As is apparent from the images of Figs. 1 through 3, the composite copper particles are flake-like, and zirconia particles are unevenly distributed over their surface. In particular, it is seen that a plurality of the zirconia particles are agglomerated to form zirconia agglomerates. It is also seen that the aluminum is present not inside but in the surface portion of the flake-like copper particles.

Example 2

Composite copper particles were obtained in the same manner as in Example 1, except for replacing the zirconia powder used in Example 1 with alumina powder. The alumina powder had a D₅₀/D_BET of 60 and a BET particle size D_BET of 10 nm. The proportion of the inorganic oxide particles in the resulting composite copper particles was found to be 0.5% as determined by the method described above.

Example 3

Composite copper particles were obtained in the same manner as in Example 2, except for replacing the starting copper power described in Example 1 with a starting copper powder having a D_max/D₅₀ of 2.5 and a D₅₀ of 3.3 µm. The starting copper powder contained 0.13% aluminum. The elemental aluminum existed both at and in the vicinity of the surface of the particle. The proportion of the inorganic oxide particles in the composite copper particles was found to be 0.5% as determined by the method described above.

Comparative Example 1

Flake-like copper particles were obtained in the same manner as in Example 1, except that the treatment in a bead mill was performed without adding zirconia powder.

Comparative Example 2

Flake-like copper particles were obtained in the same manner as in Example 3, except that the treatment in a bead mill was performed without adding alumina powder.

Evaluation 1

The copper particles obtained in Examples and Comparative Examples were subjected to thermomechanical analysis (TMA) using TMA/SS6300 available from Seiko Instruments. TMA was carried out using a nitrogen atmosphere and a rate of temperature rise of 10°C/min. The results obtained are shown in Fig. 4. As is apparent from the figure, the composite copper particles obtained in Examples have a thermal shrinkage onset temperature, i.e., a sintering onset temperature, equal to or higher than that of the copper particles of Comparative Examples.

Evaluation 2

A conductive paste was prepared using the copper particles obtained in Examples and Comparative Examples. The conductive paste comprised 70% of the copper particles, 25% of terpineol, and 5% of ethyl cellulose. The conductive paste was applied to an alumina substrate using an applicator to form a 20 µm thick coating film, which was fired in a nitrogen atmosphere at 800°C for 1 hour. The surface condition of the thus formed conductive film (electrode) was observed with the naked eye to evaluate continuity of the electrode. When the electrode had continuity, the electrode was evaluated as "yes", and when the electrode did not have continuity, the electrode was evaluated as "no". The results were shown in Table 1. As is apparent from the results in Table 1, discontinuity, such as blisters, was not observed with the electrode formed by using the copper particles of Examples, whereas blistering was observed with the electrode formed by using the copper particles of Comparative Examples.

Table 1

	Starting Copper Powder		Inorganic Oxide Particles			Composite Copper Particles
	D_max/ D₅₀	D₅₀ (µm)	Kind	D_BET (nm)	D₅₀/D_BET	Aspect Ratio	BET particle size D₅₀ (µm)	D_max/D₅₀	Proportion of Inorganic Oxide Particles (%)	Proportion of Other Metallic Material (wrt Cu, %)	Continuity of Fired Film
Example 1	3.5	3.2	zirconia	15	70	12	4.1	3.8	0.5	0.25	yes
Example 2	3.5	3.2	alumina	10	60	12	4.1	3.8	0.5	0.25	yes
Example 3	2.5	3.3	alumina	10	60	11	4.2	3.6	0.5	0.13	yes
Comparative Example 1	3.5	3.2	nil			11	3.9	3.5	0	0.25	no
Comparative Example 2	2.5	3.3	nil			13	4.3	3.7	0	0.13	no

Industrial Applicability

The invention provides composite copper particles the sintering temperature of which is approximately the same as those of conventionally used copper powders and which are capable of making an electrode with reduced occurrence of blistering. The invention also provides a process for producing the composite copper particles.

Claims

A composite copper particle comprising a flake-like copper particle and a plurality of inorganic oxide particles which are finer than the flake-like copper particle,
the inorganic oxide particles being unevenly distributed on a surface of the flake-like copper particle.
The composite copper particle according to claim 1, having a volume cumulative particle diameter D₅₀ at a cumulative volume of 50 vol% as measured by a laser diffraction scattering method of 0.1 µm to 10 µm.
The composite copper particle according to claim 1 or 2, wherein an aspect ratio of the maximum diameter d of a plane of the particle to the maximum thickness t of the particle, d/t, is 5 to 30.
The composite copper particle according to any one of claims 1 to 3, wherein the inorganic oxide particles have a particle size of 1 nm to 500 nm as calculated from their BET specific surface area.
The composite copper particle according to any one of claims 1 to 4, having a ratio of the maximum diameter D_max to D₅₀, D_max/D₅₀, of 3 to 10, D_max and D₅₀ being measured by a laser diffraction scattering method.
The composite copper particle according to any one of claims 1 to 5, wherein the inorganic oxide particles have a higher hardness than copper.
An electroconductive composition comprising the composite copper particle according to any one of claims 1 to 6.
A process for producing composite copper particles comprising subjecting a mixed powder of a starting spherical copper powder and a particulate inorganic oxide to a dispersion process using beads thereby plastically flattening the copper particles of the starting copper powder and locating the particulate inorganic oxide on the surface of the flattened copper particles,
the particulate inorganic oxide powder having a ratio of a volume cumulative particle diameter D₅₀ (nm) at a cumulative volume of 50 vol% as measured by a dynamic light scattering method to a particle size D_BET calculated from its BET specific surface area, D₅₀/D_BET, of 60 or greater.
The process for producing composite copper particles according to claim 8, wherein the starting copper powder has a ratio of the maximum diameter D_max to D₅₀, D_max/D₅₀, of 2 to 15, D_max and D₅₀ being measured by a laser diffraction scattering method.
The process for producing composite copper particles according to claim 8 or 9, wherein the starting copper powder comprises copper and at least one other metallic element.