FIBROUS COPPER MICROPARTICLES AND PROCESS FOR PRODUCING SAME

Info

Publication number: 20160214169
Type: Application
Filed: Dec 13, 2013
Publication Date: Jul 28, 2016
Applicant: UNITIKA LTD. (Hyogo)
Inventors: Munenori Yamada (Kyoto), Kou Takeuchi (Kyoto), Mutsumi Matsushita (Kyoto), Airi Horikoshi (Kyoto), Akira Shigeta (Kyoto), Masahiro Hosoda (Kyoto), Yoshiaki Echigo (Kyoto), Kenta Shibata (Kyoto)
Application Number: 14/649,393

Abstract

The present invention provides fibrous copper microparticles suppressed in the occurrence of irregularities on the surface thereof and the aggregates of the fibrous copper microparticles having an average crystallite diameter controlled so as to fall within a specific range. In the fibrous copper microparticles of the present invention, the number of fibrous copper microparticles each including one or more irregularities each having a dimensional difference of 0.02 μm or more, in a range of 1 μm in the lengthwise direction of a fibrous body, between the maximum diameter portion of the fibrous body and the minimum diameter portion of the fibrous body falling in a diameter dimension range of 0.01 to 0.5 μm, and each having a length of 1 μm or more is 10 or less per 100 of the fibrous copper microparticles.

Description

Description

TECHNICAL FIELD

The present invention relates to fibrous copper microparticles suppressed in the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles and a process for producing the same, and the aggregates of fibrous copper microparticles having an average crystallite diameter controlled so as to fall within a specific range and a process for producing the same.

BACKGROUND ART

Copper microparticles are excellent in electrical conductivity, are an inexpensive material as compared with silver or the like, and hence are widely used as raw materials for electrically conductive coating agents and the like. Such electrically conductive coating agents are widely used in materials for forming circuits on printed wiring boards or the like by using various printing methods, and various electrical contact members.

Various investigations have been made on metal microparticle including copper microparticles, and processes for producing the metal microparticles. For example, fibrous copper microparticles and aggregates thereof, and processes for producing these (Patent Literature 1 and Non Patent Literature 1) have been proposed.

CITATION LIST Patent Literature

Patent Literature 1: International Publication No. WO 2011/071885

Non Patent Literature

Non Patent Literature 1: Nano Lett., 2012, 12, pp. 234-239 (ACS Publications, Dec. 15, 2011)

SUMMARY OF INVENTION Technical Problem

The fibrous copper microparticles and the aggregate thereof in Patent Literature 1 are excellent in electrical conductivity, and hence are promising as raw materials for various electrically conductive materials. However, a large number of irregularities occur on the surface of each of the fibrous copper microparticles of Patent Literature 1. When such fibrous copper microparticles are used as the raw materials for various electrically conductive materials, it is anticipated that various industrial troubles due to the irregularities will occur.

By controlling the average crystallite diameter of the aggregates of the fibrous copper microparticles, the number of the crystallites per unit length of the fibrous copper microparticles constituting the aggregates can be controlled, accordingly, the interfaces between the crystallites can be reduced, and thus, the electrical conductivity of the aggregates of the fibrous copper microparticles or the like can be improved. However, Patent Literature 1 has not made a study on the preparation of the aggregates of the fibrous copper microparticles controlled so as for average crystallite diameter to fall within a small diameter range or a large diameter range.

Non Patent Literature 1 describes a preparation of fibrous copper microparticles and the aggregates thereof by using a continuous reaction vessel. The shapes of the fibrous copper microparticles obtained by a continuous reaction are suggested to be somewhat different in fiber length or fiber diameter as compared with the shapes of the fibrous copper microparticles obtained by a non-continuous reaction.

However, in Non Patent Literature 1, no remarks have been made on the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles. In Non Patent Literature 1, no investigation has been made on the provision of fibrous copper microparticles suppressed in the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles.

Moreover, in Non Patent Literature 1, no investigation has been made on the control of the average crystallite diameter of the aggregates of the fibrous copper microparticles.

An object of the present invention is to provide fibrous copper microparticles suppressed in the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles and the aggregates of the fibrous copper microparticles, in order to solve such problems as described above in the related art. Another object of the present invention is to provide, on the basis of the suppression of the occurrence of the one or more irregularities on the surface of each of fibrous copper microparticles, fibrous copper microparticles and the aggregates thereof having an average crystallite diameter controlled so as to fall within a specific range.

Yet another object of the present invention is to provide a process for producing fibrous copper microparticles, the process being capable of producing by simple operations fibrous copper microparticles suppressed in the occurrence of one or more irregularities on the surface each of the fibrous copper microparticles, and a process for producing the aggregates of the fibrous copper microparticles; and to provide a process for producing the aggregates of the fibrous copper microparticles, the process being capable of producing by simple operations the aggregates of the fibrous copper microparticles having the average crystallite diameter controlled so as to fall within a specific range, on the basis of the suppression of the occurrence of the one or more irregularities on the surface of each of the fibrous copper microparticles.

Solution to Problem

The present inventors made a diligent study in order to solve the foregoing problems, and consequently have perfected the present invention by discovering for the first time the fibrous copper microparticles (namely, the fibrous copper microparticles suppressed in the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles) wherein the number of fibrous copper microparticles each including one or more irregularities having a dimensional difference of 0.02 μm or more, in the range of 1 μm in the lengthwise direction of a fibrous body, between the maximum diameter portion of the fibrous body and the minimum diameter portion of the fibrous body falling in a diameter dimension range of 0.01 to 0.5 μm, and each having a length of 1 μm or more is controlled so as to be a specific proportion, and by discovering for the first time the aggregates of the fibrous copper microparticles.

The present inventors have perfected the present invention by discovering for the first time the aggregates of the fibrous copper microparticles having the average crystallite diameter controlled so as to fall within a specific range, on the basis of the suppression of the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles.

The present inventors have perfected the process for producing the fibrous copper microparticles of the present invention, by discovering for the first time that fibrous copper microparticles each suppressed in the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles are capable of being produced by simple operations, without needing complicated operations.

The present inventors have perfected the process for producing the aggregates of the fibrous copper microparticles of the present invention, by discovering for the first time that it is possible to produce by simple operations, without needing complicated operations, the aggregates of the fibrous copper microparticles having the average crystallite diameter controlled so as to fall within a specific range on the basis of the suppression of the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles.

Specifically, the gist of the present invention is as follows.

(1) Fibrous copper microparticles, wherein the number of fibrous copper microparticles each including one or more irregularities each having a dimensional difference of 0.02 μm or more, in a range of 1 μm in the lengthwise direction of a fibrous body, between the maximum diameter portion of the fibrous body and the minimum diameter portion of the fibrous body falling in a diameter dimension range of 0.01 to 0.5 μm, and each having a length of 1 μm or more is 10 or less per 100 of the fibrous copper microparticles.

(2) The fibrous copper microparticles according to (1), wherein for each of the fibrous copper microparticles, the fiber diameter is 0.01 to 0.5 μm, and the aspect ratio is 10 or more.

(3) Aggregates of fibrous copper microparticles, formed by allowing the fibrous copper microparticles according to (1) or (2) to aggregate, wherein the average crystallite diameter is 0.045 to 0.1 μm and the average fiber diameter is 0.05 to 0.15 μm.

(4) Aggregates of fibrous copper microparticles, formed by allowing the fibrous copper microparticles according to (1) or (2) to aggregate, wherein the average fiber diameter is 0.05 to 0.15 μm, and the average crystallite diameter of the aggregates is 0.45 or more times the average fiber diameter.

(5) Aggregates of fibrous copper microparticles formed by allowing the fibrous copper microparticles according to (1) or (2) to aggregate, wherein the average crystallite diameter is 0.015 to 0.03 μm, and the average fiber diameter is 0.03 to 0.1 μm.

(6) A process for producing fibrous copper microparticles, wherein the production process is a process for producing the fibrous copper microparticles according to (1) or (2); and the production process includes the following step (I) and the following step (II) or (III), in this order:

the step (I) of heating, to 50 to 100° C., an aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound;

the step (II) of maintaining for 20 minutes or more the temperature of the aqueous solution after passing through the step (I), and continuously precipitating the fibrous copper microparticles;

the step (III) of cooling the temperature of the aqueous solution after passing through the step (I) to decrease the temperature thereof by 20° C. over a period of time of 15 minutes or more from immediately after the start of cooling, and continuously precipitating the fibrous copper microparticles.

(7) The process for producing fibrous copper microparticles according to (6), wherein in the step (II) or (III), the reducing compound is further added to the aqueous solution.

(8) A process for producing aggregates of fibrous copper microparticles, wherein the production process is a process for producing the aggregates of the fibrous copper microparticles according to (3) or (4); and the production process includes the following step (I) and the following step (IIa), in this order, and continuously precipitates the fibrous copper microparticles or the aggregates of the fibrous copper microparticles:

the step (I) of heating to 50 to 100° C. the aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound;

the step (IIa) of maintaining for 30 minutes or more the temperature of the aqueous solution after passing through the step (I).

(9) A process for producing aggregates of fibrous copper microparticles, wherein the production process is a process for producing the aggregates of the fibrous copper microparticles according to (5); and the production process includes the following step (Ia) and the following step (IIIa), in this order, and continuously precipitates the fibrous copper microparticles or the aggregates of the fibrous copper microparticles:

the step (Ia) of heating to 65 to 100° C. the aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound;

the step (IIIa) of decreasing the temperature of the aqueous solution after passing through the step (Ia) by 20° C. over a period of time of 15 minutes or more from immediately after the start of cooling.

(10) The process for producing aggregates of fibrous copper microparticles according to (8), wherein in the step (IIa), the reducing compound is further added to the aqueous solution.

(11) The process for producing aggregates of fibrous copper microparticles according to (9), wherein in the step (IIIa), the reducing compound is further added to the aqueous solution.

(12) The process for producing fibrous copper microparticles according to (6) or (7), wherein as the reducing compound, one or more selected from ascorbic acid, erythorbic acid and glucose are used.

(13) The process for producing aggregates of fibrous copper microparticles according to any one of (8) to (11), wherein as the reducing compound, one or more selected from ascorbic acid, erythorbic acid and glucose are used.

Advantageous Effects of Invention

The fibrous copper microparticles of the present invention are suppressed in the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles, and hence, when the fibrous copper microparticles are used as raw materials for electrically conductive materials such as electrically conductive coating agents, electrical conductivity coats or electrical conductivity films, it is possible to prevent the occurrence of various industrial troubles caused by the irregularities.

According to the present invention, it is possible to obtain the aggregates of the fibrous copper microparticles having the average crystallite diameter controlled so as to fall within a specific range on the basis of the suppression of the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles. In particular, according to the present invention, it is possible to obtain the aggregates of the fibrous copper microparticles having an average crystallite diameter controlled so as to fall within a larger range than conventional ranges relative to an average fiber diameter (“the aggregates of the fibrous copper microparticles having an average crystallite diameter of 0.045 to 0.1 μm and an average fiber diameter of 0.05 to 0.15 μm”; and “the aggregates of the fibrous copper microparticles having an average fiber diameter of 0.05 to 0.15 μm and an average crystallite diameter of 0.45 or more times the average fiber diameter”).

The aggregates of the fibrous copper microparticles of the present invention having the average crystallite diameter and the average fiber diameter respectively falling within the foregoing ranges are excellent in electrical conductivity.

According to the present invention, it is possible to obtain the aggregates of the fibrous copper microparticles having an average crystallite diameter controlled so as to fall within a smaller range (0.015 to 0.03 μm) than conventional ranges on the basis of the suppression of the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles.

The aggregates of the fibrous copper microparticles of the present invention having the average crystallite diameter controlled so as to fall within such a specific range as described above can expect a drastically wider range of applications as compared with the aggregates of the fibrous copper microparticles having an average crystallite diameter far from being controlled in the average crystallite diameter.

According to the process for production of the present invention, fibrous copper microparticles suppressed in the occurrence of one or more irregularities on the surface of each thereof and the aggregates of the fibrous copper microparticles can be easily produced by an extremely simple process of continuously precipitating the fibrous copper microparticles in a reducing compound-containing aqueous solution.

According to the process for production of the present invention, after the suppression of the occurrence of one or more irregularities on the surface each of the fibrous copper microparticles, the aggregates of the fibrous copper microparticles having the average crystallite diameter and the average fiber diameter controlled so as to respectively fall within the foregoing ranges can be easily produced without needing complicated operations.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating the irregularities occurring on the surface of a fibrous copper microparticle.

FIG. 2 is a view obtained by observing with a digital microscope a state in which the fiber diameters, the lengths and the irregularities of the fibrous copper microparticles cannot be properly measured or evaluated because the fibrous copper microparticles crowd.

FIG. 3 is a view obtained by observing with a digital microscope a state in which the fiber diameters, the lengths and the irregularities of the fibrous copper microparticles can be properly measured or evaluated because the fibrous copper microparticles do not crowd.

FIG. 4 is a graph showing the reactivity of each of the various reducing compounds with the dissolved oxygen in an alkaline aqueous solution.

FIG. 5 is a chart showing the base line of the peak for determining the average crystallite diameter obtained by subjecting to X-ray diffraction the aggregates of the fibrous copper microparticles.

FIG. 6 is an electron microscopic observation view of the fibrous copper microparticles of the present invention in Example 13.

FIG. 7 is an electron microscopic observation view of the conventional fibrous copper microparticles in Comparative Example 2.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the fibrous copper microparticles of the present invention are described in detail.

The fibrous copper microparticles of the present invention are the fibrous copper microparticles wherein the number of fibrous copper microparticles including irregularities having a dimensional difference of 0.02 μm or more, in a range of 1 μm in the lengthwise direction of a fibrous body, between the maximum diameter portion of the fibrous body and the minimum diameter portion of the fibrous body falling in a diameter dimension range of 0.01 to 0.5 μm, and each having a length of 1 μm or more is 10 or less per 100 of the fibrous copper microparticles. In other words, the fibrous copper microparticles of the present invention are the fibrous copper microparticles wherein the occurrence of one or more irregularities on the surface of each thereof is suppressed.

On the surface of each of the conventional fibrous copper microparticles, a large number of irregularities occur. The use of such fibrous copper microparticles as raw materials for electrically conductive materials causes various industrial problems to occur. For example, when the fibrous copper microparticles each having a large number of irregularities occurring on the surface thereof are used as a raw material in an electrically conductive material such as an electrically conductive coating agent, unfortunately there are obtained only electrically conductive materials poor in the adhesiveness to the substrate, the surface smoothness, the electrical conductivity, the transparency and the like.

The present inventors have discovered for the first time that the use of fibrous copper microparticles, for electrically conductive materials, each suppressed in the occurrence of one or more irregularities on the surface thereof prevents the occurrence of such problems as described above, and provides industrially advantageous effects in electrically conductive materials. In other words, the essential feature of the present invention is the suppression of the occurrence of one or more irregularities on the surface of each of fibrous copper microparticles (namely, the control of the proportion of the fibrous copper microparticles each undergoing the occurrence of one or more irregularities on the surface thereof so as to fall within a specific range).

In the present invention, the number of the fibrous copper microparticles each having one or more irregularities per 100 fibrous copper microparticles is required to be 10 or less, is preferably 8 or less and more preferably 5 or less, and it is most preferable that the fibrous copper microparticles each having one or more irregularities be not present at all. When the number of the fibrous copper microparticles each having one or more irregularities per 100 fibrous copper microparticles exceeds 10, the use of such fibrous copper microparticles as a raw material for an electrically conductive material such as an electrically conductive coating agent causes such industrial problems as described above.

The one or more irregularities defined in each of the fibrous copper microparticles of the present invention are described as follows with reference to FIG. 1.

As shown in FIG. 1, the irregularity 3 defined in the present invention is defined as an irregularity in the fibrous copper microparticle 1, having a dimensional difference 6 of 0.02 μm or more, in a range 2 of 1 μm in the lengthwise direction of a fibrous body, between the maximum diameter portion 4 of the fibrous body and the minimum diameter portion 5 of the fibrous body falling in a diameter dimension range of 0.01 to 0.5 μm. Here, the dimensional difference 6 means the difference between the radius of the maximum diameter portion 4 and the radius of the minimum diameter portion 5. The irregularity 3 occurs not only in the lateral portion of the fibrous copper microparticle 1 but also at the end of the fibrous copper microparticle 1.

In the present invention, in the case where the dimensional difference 6 is less than 0.02 μm, or even in the case where the dimensional difference 6 is 0.02 μm or more, when the dimensional difference 6 is present outside the range of 1 μm in the lengthwise direction of the fibrous body, the irregularity involved is not regarded as the irregularity 3. When the fibrous copper microparticles 1 each having a dimensional difference 6 not regarded as the irregularity 3 are used (for example, when used for various electrically conductive materials), various performances are not adversely affected and no industrial problems are caused.

The minimum diameter in the minimum diameter portion 5 is the diameter in the cross section perpendicular to the fiber lengthwise direction of the fibrous copper microparticle 1, and the diameter at the minimum diameter position in the range 2 of 1 μm in the lengthwise direction of the fibrous body. The diameter dimension of the minimum diameter portion 5 is required to be 0.01 to 0.5 μm, and is preferably 0.01 to 0.1 μm.

The maximum diameter in the maximum diameter portion 4 is the diameter in the cross section perpendicular to the fiber lengthwise direction of the fibrous copper microparticle 1, and means the diameter having a maximum dimension in the range 2 of 1 μm in the lengthwise direction of the fibrous body. The lower limit of the diameter dimension in the maximum diameter portion 4 is 0.03 μm as determined from the relation with the dimensional difference 6 between the maximum diameter portion 4 and the minimum diameter portion 5.

In the present invention, the fiber diameter of the fibrous copper microparticle 1 is the diameter of the minimum diameter portion 5 in the total length of the fibrous copper microparticle 1. The fiber diameter is preferably 0.01 to 0.5 μm and more preferably 0.01 to 0.1 μm. When the fiber diameter of the fibrous copper microparticles 1 exceeds 0.5 μm, the use of the fibrous copper microparticles in various electrically conductive materials causes problems in the transparency, the dispersibility or the like of the electrically conductive materials. When the fiber diameter is less than 0.01 μm, the electrical conductivity, the coatability or the like of an electrically conductive material using the fibrous copper microparticles as a raw material undergoes the occurrence of problems.

The length (fiber length) of each of the fibrous copper microparticles 1 is required to be 1 μm or more from the viewpoint of the assessment of one or more irregularities 3. In particular, the length of the fibrous copper microparticles 1 is preferably 5 μm or more and more preferably 10 μm or more. When the length of the fibrous copper microparticles 1 is less than 1 μm, the electrical conductivity, the transparency or the like of an electrically conductive material using the fibrous copper microparticles as a raw material undergoes the occurrence of problems. On the other hand, from the viewpoint of the handling of the electrically conductive coating agent in the formation of an electrically conductive coat or an electrically conductive film using as a raw material the fibrous copper microparticles 1, the length of the fibrous copper microparticles 1 preferably does not exceed 500 μm.

The aspect ratio (the length of the fibrous copper microparticles 1/the fiber diameter of the fibrous copper microparticles 1) of the fibrous copper microparticles 1 is preferably 10 or more, more preferably 100 or more and furthermore preferably 300 or more. When the aspect ratio of the fibrous copper microparticles 1 is less than 10, the transparency, the electrical conductivity or the like of various electrically conductive materials using as raw materials the fibrous copper microparticles undergoes the occurrence of problems.

In the present invention, the number (proportion) of the fibrous copper microparticles 1 each having one or more irregularities 3 per 100 of the fibrous copper microparticles 1 is derived by the following method.

The aggregates of the fibrous copper microparticles 1 are observed by using, for example, a transmission electron microscope (TEM), a scanning electron microscope (SEM) or a digital microscope. Next, from the aggregates of the fibrous copper microparticles 1, 100 of the fibrous copper microparticles 1 each having a length of 1 μm or more are selected, and the surfaces of 100 of these fibrous copper microparticles 1 are observed. As described above, when in a fibrous copper microparticle 1, in a range of 1 μm in the lengthwise direction of the fibrous body, there is a minimum diameter portion 5 falling in a diameter dimension range of 0.01 to 0.5 μm, and the dimensional difference 6 (the difference between the radius of the maximum diameter portion 4 and the radius of the minimum diameter portion 5) between the minimum diameter portion 5 and the maximum diameter portion 4 is 0.02 μm or more, the fibrous copper microparticle 1 is determined to be a fibrous copper microparticle 1 having a irregularity 3. By counting the number of the fibrous copper microparticles 1 each undergoing the occurrence of one or more irregularities 3, it is possible to determine the number of the fibrous copper microparticles 1 each undergoing the occurrence of one or more irregularities 3 on the surface thereof in relation to 100 of the fibrous copper microparticles 1.

The fiber diameter and the fiber length of the fibrous copper microparticles of the present invention can also be measured by using such a TEM, SEM or digital microscope as described above. Specifically, the diameters (the diameters of the minimum diameter portions in the total lengths of the individual fibrous copper microparticles) of the 100 fibrous copper microparticles each having a length of 1 μm or more, selected from the aggregates of the fibrous copper microparticles are measured, and the average value of the measured values can be taken as the fiber diameter. The lengths of the 100 fibrous copper microparticles each having a length of 1 μm or more, selected from the aggregates of the fibrous copper microparticles are measured, and the average value of the measured values can be taken as the fiber length. The aspect ratio of the fibrous copper microparticles of the present invention can be derived by dividing the fiber length determined as described above of the fibrous copper microparticles (the average value of the lengths of the 100 fibrous copper microparticles) by the fiber diameter determined as described above of the fibrous copper microparticles (the average value of the fiber diameters of the 100 fibrous copper microparticles).

When the fibrous copper microparticles of the present invention are observed, in the case where the adjacent fibrous copper microparticles overlap each other and crowd each other in the aggregates of the fibrous copper microparticles, it is impossible to accurately evaluate the shapes of the fibrous copper microparticles. In such a case, by using, for example, an ultrasonic disperser, the fibrous copper microparticles crowding each other to an excessive degree are disentangled to be subjected to observation.

FIGS. 2 and 3 show the observation views of the degree of the mutual crowding (possibility or impossibility of shape evaluation) of the fibrous copper microparticles as observed by using a digital microscope (“VHX-1000, VHX-D500/510,” manufactured by Keyence Corp.). FIG. 2 shows a state in which the fiber diameters, the fiber lengths and the irregularities defined in the present invention of the fibrous copper microparticles cannot be properly measured or evaluated because the fibrous copper microparticles crowd. FIG. 3 shows a state in which the fiber diameters, the fiber lengths and the irregularities defined in the present invention of the fibrous copper microparticles can be properly measured or evaluated because the adjacent fibrous copper microparticles do not crowd to an excessive extent. The magnification factor of each of the observation views of FIGS. 2 and 3 is approximately 10000.

A process for producing fibrous copper microparticles is described in detail.

The present inventors made a diligent study from various aspects in order to obtain fibrous copper microparticles suppressed in the occurrence of one or more irregularities on the surface of each thereof. Consequently, the present inventors have discovered for the first time that by a production process including the following process (I) and the following process (II) or (III) in this order, the number of the fibrous copper microparticles each having one or more irregularities on the surface thereof per 100 fibrous copper microparticles can be controlled to be 10 or less, namely “fibrous copper microparticles suppressed in the occurrence of one or more irregularities on the surface of each thereof can be easily produced”:

the step (I) of heating, to 50 to 100° C., an aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound

the step (II) of maintaining for 20 minutes or more the temperature of the aqueous solution after passing through the step (I), and continuously precipitating the fibrous copper microparticles

the step (III) of cooling the temperature of the aqueous solution after passing through the step (I) to decrease the temperature thereof by 20° C. over a period of time of 15 minutes or more from immediately after the start of cooling, and continuously precipitating the fibrous copper microparticles

The phrase, “continuously precipitating the fibrous copper microparticles” in the steps (II) and (III), means the consecutive precipitation of the fibrous copper microparticles by using a reaction vessel made of a glass tube or a stainless steel tube and by allowing the aqueous solution to flow continuously.

Hereinafter, the individual components contained in the aqueous solution of the step (I) are described.

The copper ion is a divalent cation capable of being produced by dissolving a water-soluble copper salt in water. Examples of the water-soluble copper salt include: copper sulfate, copper nitrate, copper chloride and copper acetate. Among these, copper sulfate or copper nitrate is preferable from the viewpoint of the easiness in forming fibrous copper microparticles.

As the alkaline compound, without being particularly limited, for example, sodium hydroxide or potassium hydroxide can be used.

The concentration of the alkaline compound in the aqueous solution is preferably 10 to 50% by mass, more preferably 20 to 45% by mass and furthermore preferably 20 to 40% by mass. When the concentration of the alkaline compound is less than 10% by mass, the fibrous copper microparticles are hardly formed. On the other hand, when the concentration of the alkaline compound exceeds 50% by mass, the handling of the aqueous solution is difficult.

The concentration of copper ion in the aqueous solution is specified by the molar ratio between the hydroxide ion of the alkaline compound and copper ion. Specifically, the concentration of copper ion in the aqueous solution is preferably set so as for the ratio (hydroxide ion of alkaline compound)/(copper ion) in terms of molar ratio to fall within a range preferably from 1500/1 to 6000/1 and more preferably from 1500/1 to 5000/1. When the molar ratio is less than 1500/1, the shape of the copper microparticles tends to be spherical without being fibrous. On the other hand, when the molar ratio exceeds 6000/1, the formation efficiency of the fibrous copper microparticles is degraded.

Examples of the nitrogen-containing compound include: ammonia, ethylenediamine and triethylenetetramine. Among these, ethylenediamine is preferable from the viewpoint of the easiness in forming the fibrous copper microparticles.

The nitrogen-containing compound is preferably used in a proportion of 1 mole or more in relation to 1 mole of copper ion from the viewpoint of the formation efficiency of the fibrous copper microparticles.

In the step (I), as the reducing compound beforehand contained in the aqueous solution heretofore known compounds can be used. Examples of such compounds include: hydrogen gas; hydrogen compounds such as hydrogen iodide, hydrogen sulfide, lithium aluminum hydride and sodium borohydride; lower oxides such as carbon monoxide, sulfur dioxide and sulfite or the salts of these; sulfur compounds such as sodium sulfide, sodium polysulfide and ammonium sulfide; metals such as alkali metals, magnesium, calcium and aluminum, or amalgams of these; and organic compounds such as aldehydes, saccharides, formic acid, oxalic acid, hydrazine, ascorbic acid, erythorbic acid, glucose, amines and thiols.

Among these, as the reducing compound beforehand contained in the aqueous solution in the step (I), for example, ascorbic acid, erythorbic acid or glucose can be preferably used, and it is particularly preferable to use ascorbic acid or erythorbic acid.

As the reducing compound beforehand contained in the aqueous solution in the step (I), it is preferable to use “a reducing compound not reacting with the dissolved oxygen in the alkaline aqueous solution” from the viewpoint of the capability of producing the fibrous copper microparticles sufficiently suppressed in the occurrence of one or more irregularities on the surface of each thereof. When “a reducing compound reacting with the dissolved oxygen in the alkaline aqueous solution” is used as the reducing compound, the proportion of the fibrous copper microparticles each having one or more irregularities on the surface of each thereof in the fibrous copper microparticles sometimes exceeds 10 per 100 of the fibrous copper microparticles. In other words, it is sometimes impossible to produce fibrous copper microparticles sufficiently suppressed in the occurrence of one or more irregularities on the surface of each thereof.

“The reducing compound not reacting with the dissolved oxygen in the alkaline aqueous solution” in the present invention is defined by the following index.

First, the dissolved oxygen concentration in the alkaline aqueous solution is measured. The resulting dissolved oxygen concentration is taken as the dissolved oxygen concentration 1. Next, the reducing compound is added to and dissolved in the alkaline aqueous solution. While the stirring is being continued successively even after the dissolution, the dissolved oxygen concentration at the elapsed time of 10 minutes after the addition of the reducing compound is measured. The dissolved oxygen concentration of this case is taken as the dissolved oxygen concentration 2.

Then, the dissolved oxygen concentration retention rate is determined by the following formula (1):

(Dissolved oxygen concentration retention rate)=(dissolved oxygen concentration 2)/(dissolved oxygen concentration 1) (1)

In the present invention, the reducing compound having the dissolved oxygen concentration retention rate of 0.5 or more obtained by the formula (1) is defined as “the reducing compound not reacting with the dissolved oxygen in the alkaline aqueous solution.” And, the reducing compound having the dissolved oxygen concentration retention rate of less than 0.5 obtained by the formula (1) is defined as “the reducing compound reacting with the dissolved oxygen in the alkaline aqueous solution.”

FIG. 4 shows the relations between the dissolved oxygen concentration (mg/L) in the alkaline aqueous solution having a pH of 10.4 and the time (after the elapsed times of 0.5 minute, 5 minutes, 10 minutes, 15 minutes, 30 minutes, 45 minutes and 60 minutes) for ascorbic acid, erythorbic acid, glucose and hydrazine as the reducing compounds.

As FIG. 4 shows, when ascorbic acid, erythorbic acid and glucose are used as the reducing compounds, the dissolved oxygen concentration retention rates after the elapsed time of 10 minutes are 0.5 or more. Accordingly, in the present invention, ascorbic acid, erythorbic acid and glucose are each defined as “the reducing compound not reacting with the dissolved oxygen in the alkaline aqueous solution.” As FIG. 4 shows, when ascorbic acid, erythorbic acid and glucose having a dissolved oxygen concentration retention rate of 0.5 or more are used, high dissolved oxygen concentrations are maintained even after the elapsed time of 30 minutes. The dissolved oxygen concentration retention rate obtained by the formula (1) is 0.90 for ascorbic acid, 0.96 for erythorbic acid and 0.97 for glucose.

On the other hand, as FIG. 4 shows, when hydrazine is used as the reducing compound, the dissolved oxygen concentration in the alkaline aqueous solution is rapidly and remarkably decreased, and the dissolved oxygen concentration retention rate after the elapsed time of 10 minutes is less than 0.5. Accordingly, in the present invention, hydrazine is defined as “a reducing compound reacting with the dissolved oxygen in the alkaline aqueous solution.” For hydrazine, the dissolved oxygen concentration retention rate obtained by the formula (1) is 0.03.

Such a reducing compound as described above is contained in a proportion such that the number of moles of the reducing compound is preferably 0.1 to 10 times and more preferably 0.2 to 5 times the number of moles of the copper ion in the aqueous solution. When the proportion of the contained reducing compound is less than 0.1 times the number of moles of the copper ion, the efficiency of the reduction reaction is degraded. On the other hand, even when the reducing compound is contained in a proportion exceeding 10 times the number of moles of the copper ion, the reduction reaction is saturated to be unfavorable from the viewpoint of, for example, the cost.

In the preparation of the aqueous solution containing such components as described above, the individual components may be combined simultaneously and mixed by stirring. Alternatively, the aqueous solution may also be prepared by adding the reducing compound after the components other than the reducing compound are stirred and mixed.

Next, in the step (I), such an aqueous solution (blue aqueous solution) as described above is heated until the aqueous solution turns almost colorless and transparent by using an appropriate heat source. In this case, the heating temperature is appropriately 50 to 100° C. The heating can use a method of continuous heating with a reaction vessel made of a glass tube or a stainless steel tube allowing the aqueous solution to continuously flow, or a method of heating the aqueous solution in an appropriate reaction vessel. In light of the simplicity in transition to the next step (II), the former method of continuous heating is preferable. This is because the continuous heating allows the aqueous solution before reaction to be continuously fed, and the aqueous solution after reaction to be continuously collected, and thus allows the continuous transition to the step (II).

The heating temperature of the aqueous solution in the step (I) set at 50 to 100° C. allows the reaction efficiency and the controllability to be excellent. In other words, when the heating temperature is lower than 50° C., the reaction efficiency is degraded; when the heating temperature exceeds 100° C., it is difficult to control the shape or the precipitation rate of the precipitated fibrous copper microparticles. The heating temperature is preferably 60 to 80° C.

In the step (I), when the aqueous solution is heated, the aqueous solution may be stirred by an appropriate technique and under appropriate conditions.

In the related art, after the step (I), the fibrous copper microparticles are precipitated in a short time; however, when the fibrous copper microparticles are precipitated without passing through a sufficient temperature maintenance time, it is impossible to produce fibrous copper microparticles sufficiently suppressed in the occurrence of one or more irregularities on the surface of each thereof.

Accordingly, in the present invention, the step (II) maintains the temperature of the aqueous solution after passing through the step (I) for 20 minutes or more, and thus controls the occurrence of one or more irregularities on the surface of each of the obtained fibrous copper microparticles so as to fall within the foregoing range. Here, “the maintenance of the temperature of the aqueous solution” means the operation allowing the temperature of the aqueous solution not to decrease by 20° C. or more.

Moreover, in the present invention, the step (III) decreases the temperature of the aqueous solution after passing through the step (I) by 20° C. over a time of 15 minutes or more, and thus, controls the occurrence of one or more irregularities on the surface of each of the obtained fibrous copper microparticles so as to fall within the foregoing range.

In the step (II), while the aqueous solution is being continuously heated by using a reaction vessel allowing the aqueous solution to flow continuously, the temperature of the aqueous solution is maintained. In the step (II), for the purpose of maintaining the temperature of the aqueous solution or controlling the heating time of the aqueous solution, for example, the size and the flow rate of the reaction vessel and/or the flow path may be appropriately selected.

Similarly, also in the step (III), the temperature of the aqueous solution can be continuously decreased by using a reaction vessel allowing the aqueous solution to continuously flow.

In the steps (II) and (III), a further addition of the reducing compound to the aqueous solution and the maintenance of the temperature of the aqueous solution allows the fiber diameters of the individual precipitated fibrous copper microparticles to be made uniform, and also allows the fibrous copper microparticles to be obtained in a higher formation yield.

As the reducing compound to be added in the steps (II) and (III), a heretofore known reducing compound such as the reducing compound contained in the aqueous solution in the step (I) can be used, and “the reducing compound not reacting with the dissolved oxygen in the alkaline aqueous solution” as described above is preferable. The reducing compound to be further added to the aqueous solution in the steps (II) and (III) may be of the same type as or of the type different from the type of the reducing compound beforehand contained in the aqueous solution in the step (I).

The addition amount of the reducing compound to be further added in the steps (II) and (III) is preferably in terms of the number of moles 0.5 to 100 times and more preferably 1 to 10 times the number of moles of the copper ion in the aqueous solution. When the addition number of moles of the reducing compound exceeds 100 times the number of moles of the copper ion, the formation effect of the fibrous copper microparticles is saturated to be unfavorable from the viewpoint of, for example, the cost.

After passing through the steps (II) and (III), the precipitated fibrous copper microparticles can be collected by performing solid-liquid separation based on a method such as filtration, centrifugal separation or pressure floatation. Moreover, if necessary, for example, the collected fibrous copper microparticles may also be washed or dried. When the fibrous copper microparticles are taken out, the surface of the fibrous copper microparticles tends to be oxidized, and hence the collecting operation is preferably performed in an inert gas atmosphere (for example, nitrogen gas atmosphere).

When the taken-out fibrous copper microparticles are stored, the fibrous copper microparticles are preferably stored in an atmosphere of an inert gas such as nitrogen gas, or preferably stored as redispersed, for example, in a solution in which a trace amount of a reducing compound is dissolved or in a solution in which an organic substance having a function to prevent the oxidation of copper is dissolved in a trace amount.

The aggregates of the fibrous copper microparticles of the present invention are described in detail.

The aggregates of the fibrous copper microparticles of the present invention are prepared by allowing the fibrous copper microparticles suppressed in the occurrence of one or more irregularities on the surface of each thereof to aggregate to each other as described above, and further by controlling the average crystallite diameter and the average fiber diameter so as to fall respectively within predetermined ranges as described below.

In the aggregates of the fibrous copper microparticles of the present invention, after the suppression of the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles as described above, the average crystallite diameter can be controlled to be larger than the average crystallite diameter of the aggregates of the conventional fibrous copper microparticles. Specifically, the average crystallite diameter is 0.045 to 0.1 μm, and the average fiber diameter is 0.05 to 0.15 μm. In particular, the average crystallite diameter is preferably 0.045 to 0.07 μm. The average fiber diameter is preferably 0.05 to 0.08 μm.

“The crystallite” means the largest aggregate regarded as a single crystal present in the aggregates of the fibrous copper microparticles.

“The average fiber diameter of the aggregates of the fibrous copper microparticles” means the average value of the fiber diameters of the individual fibrous copper microparticles constituting the aggregates. “The fiber diameter of the fibrous copper microparticle” means the diameter in the cross section perpendicular to the fiber lengthwise direction in each of the fibrous copper microparticles. The method for determining the average fiber diameter of the aggregates of the fibrous copper microparticles is described later.

In the aggregates of the fibrous copper microparticles having an average crystallite diameter of 0.045 to 0.1 μm and an average fiber diameter of 0.05 to 0.15 μm, the number of crystallites per unit length of the fibrous copper microparticles constituting the aggregates can be reduced. The interfaces between the crystallites can be thereby reduced. The interfaces are the factors disturbing the electrical conductivity, and hence the reduction of the interfaces improves the electrical conductivity of the aggregates of the fibrous copper microparticles. The aggregates of the fibrous copper microparticles having a large average crystallite diameter (0.045 to 0.1 μm) are excellent in stability and hardly undergo the effect of oxidation or the like.

Another aspect of the aggregates of the fibrous copper microparticles is the aggregates being suppressed in the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles as described above, having an average fiber diameter of 0.05 to 0.15 μm and having an average crystallite diameter of 0.45 or more times the average fiber diameter. In particular, the aggregates of the fibrous copper microparticles having an average fiber diameter of 0.05 to 0.08 μm are preferable. The average crystallite diameter is preferably 0.6 or more times and more preferably 0.7 or more times the average fiber diameter.

Similarly, in the aggregates of the fibrous copper microparticles having an average fiber diameter of 0.05 to 0.15 μm and an average crystallite diameter of 0.45 or more times the average fiber diameter, the number of crystallites per unit length of the fibrous copper microparticles constituting the aggregates can be reduced, and the electrical conductivity of the aggregates is improved.

On the other hand, in the aggregates of the fibrous copper microparticles, after the suppression of the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles as described above, the average crystallite diameter can be controlled to be smaller than the average crystallite diameter of the aggregates of the conventional fibrous copper microparticles. Specifically, the average crystallite diameter is 0.015 to 0.03 μm, and the average fiber diameter is 0.03 to 0.1 μm. In particular, the average crystallite diameter is preferably 0.015 to 0.025 μm. The average fiber diameter is preferably 0.05 to 0.07 μm.

In the aggregates of the fibrous copper microparticles having an average crystallite diameter of 0.015 to 0.03 μm, namely, the aggregates of the fibrous copper microparticles having a small average crystallite diameter, the average fiber diameter of the aggregates tends to be small. Examples of such aggregates include the aggregates of the fibrous copper microparticles having an average fiber diameter of 0.03 to 0.1 μm, and the aggregates of the fibrous copper microparticles having an average fiber diameter of 0.05 to 0.07 μm. The aggregates of the fibrous copper microparticles having an average crystallite diameter of as small as 0.015 to 0.03 μm are excellent in the reactivity with other substances, and can have drastically wide ranges of applications.

For example, when the average fiber diameter of the aggregates of the fibrous copper microparticles falls within a small range (for example, a range of 0.1 μm or less), the use of the aggregates of the fibrous copper microparticles as a raw material for an electrically conductive material results in, for example, remarkably satisfactory transparency of the electrically conductive material or remarkably satisfactory adhesiveness to the substrate of the electrically conductive material. Accordingly, in the applications more requiring, for example, the transparency and the adhesiveness to the substrate of electrically conductive materials, aggregates of fibrous copper microparticles having small average fiber diameters are preferably used.

As described above, in the individual fibrous copper microparticles constituting the aggregates of the fibrous copper microparticles, there are no fine copper granules formed in a state of attaching to the ends or the lateral portions of such fibrous copper microparticles so as to be integrated with the fibrous copper microparticles, or in a state of being brought into contact with but not integrated with the fibrous copper microparticles. When fine copper granules are formed, there are crystal interfaces between the main portions of the fibers constituting the aggregates and the copper granules, and hence the electrical conductivity of the aggregates of the fibrous copper microparticles is degraded.

Next, a method for deriving the average crystallite diameter of the aggregates of the fibrous copper microparticles is described below. In the present invention, the full width at half maximum (the width of the diffraction intensity curve at the intensity of half the peak intensity) of the peak (the peak for identification of copper) corresponding to the (111) plane of copper is determined by X-ray diffraction method, and by using the full width at half maximum, the average crystallite diameter of the aggregates of the fibrous copper microparticles can be determined. Specifically, the aggregates of the fibrous copper microparticles are subjected to X-ray diffraction using a wide angle X-ray diffractometer “RINT-TTR IV” (manufactured by Rigaku Corp.), thus the full width at half maximum β of the output peak corresponding to the (111) plane of copper is determined, and the average crystallite diameter is determined by substituting the full width at half maximum β into the following formula (2):

(Average crystallite diameter) (μm)=(K×λ)/(β×cos θ) (2)

In the formula (2), K is the Scherrer constant and has a value of 0.9, λ represents the wavelength of the X-ray used, and θ represents the diffraction angle (2θ/θ) (rad).

The average length of the aggregates of the fibrous copper microparticles is preferably 1 μm or more and more preferably 5 μm or more. When the average length is less than 1 μm, for example, the electrical conductivity or the transparency of the electrically conductive materials using the aggregates of the fibrous copper microparticles undergoes the occurrence of problems. On the other hand, from the viewpoint of the handling of the coating agent or the like in the formation of an electrically conductive coat or an electrically conductive film using the aggregates of the fibrous copper microparticles, the average length of the aggregates of the fibrous copper microparticles is preferably 500 μm or less. “The average length of the aggregates of the fibrous copper microparticles” in the present invention is the average value of the lengths of the individual fibrous copper microparticles constituting the aggregates. “The lengths of the individual fibrous copper microparticles” means the lengths in the fiber lengthwise direction. The method for determining the average length of the aggregates of the fibrous copper microparticles is described later.

The average aspect ratio (average length/average fiber diameter) of the aggregates of the fibrous copper microparticles preferably having such an average fiber diameter and an average length is preferably 10 or more, more preferably 100 or more and furthermore preferably 300 or more.

A method for determining the average fiber diameter and the average length of the aggregates of the fibrous copper microparticles is described below.

By using, for example, a TEM, SEM or digital microscope, the aggregates of the fibrous copper microparticles are observed. From the aggregates, 100 fibrous copper microparticles are selected. The fiber diameter and the length of each of these 100 fibrous copper microparticles are measured, and the average value of the fiber diameters and the average value of the lengths for these 100 fibrous copper microparticles are taken respectively as the average fiber diameter and the average length of the aggregates of the fibrous copper microparticles. The average aspect ratio of the aggregates of the fibrous copper microparticles is derived by dividing the average length determined as described above by the average fiber diameter determined as described above.

When the aggregates of the fibrous copper microparticles are observed, in the case where the fibrous copper microparticles constituting the aggregates overlap each other and crowd each other, it is impossible to accurately evaluate the average fiber diameter and the average length of the aggregates of the fibrous copper microparticles. In such a case, by using, for example, an ultrasonic disperser, the aggregates of the fibrous copper microparticles are disentangled to such an extent that the adjacent fibrous copper microparticles do not crowd each other, and then the fibrous copper microparticles are observed.

A process for producing the aggregates of the fibrous copper microparticles is described in detail. Description is omitted for the same constitution as in the process for producing the fibrous copper microparticles.

The present inventors made a diligent study from various aspects in order to obtain the aggregates of the fibrous copper microparticles capable of controlling the average crystallite diameter so as to fall within a predetermined range wherein the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles is suppressed as described above. Consequently, the present inventors have for the first time discovered that by a production process including the following step (I) and the following step (IIa) in this order, the aggregates of the fibrous copper microparticles having a large average crystallite diameter (0.045 to 0.1 μm) can be easily produced wherein the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles is suppressed as described above:

the step (I) of heating to 50 to 100° C. the aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound

the step (IIa) of maintaining for 30 minutes or more the temperature of the aqueous solution after passing through the step (I)

The step (I) is the same as the step (I) of the process for producing the fibrous copper microparticles.

The step (IIa) maintains for “30 minutes or more” the temperature of the aqueous solution after passing through the step (I). Otherwise, the step (IIa) is the same as the step (II) of the process for producing the fibrous copper microparticles.

When in the step (IIa), the time for maintaining the temperature of the aqueous solution in less than 30 minutes, the average crystallite diameter of the aggregates of the precipitated fibrous copper microparticles cannot be made sufficiently large. Specifically, the average crystallite diameter cannot be controlled so as to fall within a range from 0.045 to 0.1 μm.

On the contrary, when in the step (IIa), the time for maintaining the temperature of the aqueous solution exceeds 480 minutes, the effect of the suppression of the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles is saturated. Moreover, in this case, the average crystallite diameter in the aggregates of the fibrous copper microparticles cannot be made large so as to exceed 0.1 μm to be unfavorable from the viewpoint of, for example, the cost.

On the other hand, the present inventors have for the first time discovered that by a production process including the following step (Ia) and the following step (IIIa) in this order, the aggregates of the fibrous copper microparticles having a small average crystallite diameter (0.015 to 0.03 μm) can be easily produced wherein the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles is suppressed as described above:

the step (Ia) of heating to 65 to 100° C. the aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound

the step (IIIa) of decreasing the temperature of the aqueous solution after passing through the step (Ia) by 20° C. over a period of time of 15 minutes or more from immediately after the start of cooling.

In the step (Ia), the temperature for heating the aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound is set at “65 to 100° C.” Otherwise, the step (Ia) is the same as the step (I) of the process for producing the fibrous copper microparticles.

When in the step (Ia), the temperature for heating the aqueous solution is lower than 65° C., the average crystallite diameter of the aggregates of the precipitated fibrous copper microparticles cannot be made sufficiently small. Specifically, the average crystallite diameter cannot be controlled so as to fall within a range from 0.015 to 0.03 μm.

The step (IIIa) cools the aqueous solution after the step (Ia). Otherwise, the step (IIIa) is the same as the step (III) of the process for producing the fibrous copper microparticles.

In the step (IIIa), the temperature of the aqueous solution passing through the step (Ia) is cooled not rapidly but gradually (the cooling by 20° C., over a time of 15 minutes or more), and thus it is possible to obtain the aggregates of the fibrous copper microparticles sufficiently suppressed in the occurrence of one or more irregularities on the surface of each thereof and having an average crystallite diameter controlled so as to fall within a range from 0.015 to 0.03 μm (preferably so as to fall within a range from 0.015 to 0.025 μm).

When in the step (IIIa), the cooling time is less than 15 minutes, namely, the aqueous solution is cooled not gradually but rapidly, the aggregates of the fibrous copper microparticles are not precipitated.

The fibrous copper microparticles of the present invention and the aggregates thereof are mixed with and dispersed in a binder component, a solvent and the like, and thus, an electrically conductive coating agent can be prepared.

The binder component contained in the electrically conductive coating agent is not particularly limited, and examples of the usable binder component include: acrylic resins (such as acrylic silicone-modified resin, fluorine-modified acrylic resin, urethane-modified acrylic resin and epoxy-modified acrylic resin); polyester-based resin, polyurethane-based resin, olefin-based resin, amide resin, imide resin, epoxy resin, silicone resin and vinyl acetate-based resin; natural polymers such as starch, gelatin and agar; cellulose derivatives (semisynthetic polymers) such as carboxymethyl cellulose, hydroxy ethyl cellulose, methyl cellulose, hydroxy ethyl methyl cellulose, hydroxy propyl methyl cellulose; and water-soluble polymers (synthetic polymers) such as polyvinyl alcohol, polyacrylic acid-based polymer, polyacrylamide, polyethylene oxide and polyvinylpyrrolidone.

The solvent contained in the electrically conductive coating agent is not particularly limited, and examples of the contained solvent include: water, and organic solvent such as alcohols, glycols, cellosolves, ketones, esters, ethers, amides and hydrocarbons. These can be used each alone or in combinations of two or more thereof. Among these, it is preferable to use a solvent mainly composed of water or alcohols.

The mixing ratio between the fibrous copper microparticles or the aggregates thereof and the binder component in the electrically conductive coating agent is, in terms of the volume ratio (A/B) between the volume (A) of the fibrous copper microparticles or the aggregates thereof and the volume (B) of the binder component, preferably 1/100 to 5/1 and more preferably 1/20 to 1/1. When the amount of the fibrous copper microparticles or the aggregates thereof is small to such an extent that the volume ratio of the fibrous copper microparticles or the aggregates thereof to the binder component is (less than 1)/100, the electrical conductivity of, for example, the obtained electrically conductive coating agent or the electrically conductive coat obtained from the coating agent is degraded. On the other hand, when the amount of the binder component is small to such an extent that the volume ratio is (more than 5)/1, for example, the surface smoothness or the transparency of the obtained electrically conductive coat suffers from the occurrence of problems, or when the obtained electrically conductive coating agent is applied to a substrate, the adhesiveness to the substrate is degraded.

The solid content (the total content of the fibrous copper microparticles or the aggregates thereof of the present invention, the binder component, and if necessary, the solid content of the other additive(s)) concentration in the electrically conductive coating agent is preferably 1 to 99% by mass and more preferably 1 to 50% by mass, from the viewpoint of being excellent in the balance, for example, between the electrical conductivity and the handleability.

The viscosity of the electrically conductive coating agent at 20° C. is preferably 0.5 to 100 mPa·s and more preferably 1 to 50 mPa·s from the viewpoint of being excellent, for example, in the handleability and the easiness in application to substrates.

In the electrically conductive coating agent, a cross-linking agent for cross-linking the binder component, such as an aldehyde-based, epoxy-based, melamine-based or isocyanate-based cross-linking agent may also be used, if necessary, within a range not impairing the advantageous effects of the present invention.

By forming a coat with such an electrically conductive coating agent as described above, the electrically conductive coat can be obtained. Moreover, by forming the electrically conductive coat on a substrate, an electrically conductive film can be obtained. The electrically conductive coating agent, the electrically conductive coat and the electrically conductive film contain the fibrous copper microparticles of the present invention, suppressed in the occurrence of one or more irregularities on the surface of each thereof, and hence can prevent the various industrially disadvantageous problems caused by the irregularities.

Examples of the method for forming the electrically conductive coat include a method (liquid phase coat formation method) in which the electrically conductive coating agent is applied to the surface of a substrate such as a plastic film, subsequently dried, and then, if necessary, cured to form a coat. As the application method in the liquid phase coat formation method, for example, the following methods can be used: a roll coating method, a bar coating method, a dip coating method, a spin coating method, a casting method, a die coating method, a blade coating method, a gravure coating method, a curtain coating method, a spray coating method and a doctor coating method.

The thickness of the electrically conductive coat may be, for example, about 0.1 to 10 μm from the viewpoint of practicability.

In order to form an electrically conductive coat or an electrically conductive film containing the fibrous copper microparticles of the present invention, a method can also be used in which only the fibrous copper microparticles of the present invention are sprayed to the surface of a substrate such as a plastic film, and if necessary, a coating layer for protecting the sprayed fibrous copper microparticles is formed.

EXAMPLES

Hereinafter, Examples of the present invention are described. It is to be noted that the present invention is not limited by these Examples.

The evaluation methods and the measurement methods for the fibrous copper microparticles and the aggregates thereof obtained in Examples and Comparative Examples are as follows.

1. Evaluation of Reducing Compound not Reacting with Dissolved Oxygen

First, a few drops of a 10% sodium hydroxide aqueous solution were added to 500 g of pure water to prepare an alkaline aqueous solution (water temperature: 25° C.) the pH of which was adjusted to 10.4 (the dissolved oxygen concentration of the alkaline aqueous solution at this time is “the dissolved oxygen concentration 1.” Specifically, the dissolved oxygen concentration 1 is 8.3 mg/L). For the measurement of the dissolved oxygen concentration, a dissolved oxygen meter “DO-5509” (manufactured by Lutron Electronic Enterprise Co., Ltd.) was used.

Then, in an open cylindrical vessel of 7.0 cm in diameter, 100 mL of the alkaline aqueous solution was placed. Next, a reducing compound was added to the alkaline aqueous solution in an amount to give the concentration of the reducing compound of 0.50 mol/L, and the reducing compound was dissolved by stirring with a magnetic stirrer to such an extent that the aqueous solution did not swirl. While the aqueous solution was being stirred even after the completion of dissolution, the dissolved oxygen concentration after 10 minutes from the addition of the reducing compound was measured (the dissolved oxygen concentration of the alkaline aqueous solution at this time is “the dissolved oxygen concentration 2”).

On the basis of the evaluation standard for the reaction between the reducing compound and the dissolved oxygen, based on the foregoing formula (1) [(dissolved oxygen concentration retention rate)=(dissolved oxygen concentration 2)/(dissolved oxygen concentration 1)], the reactivity between the reducing compound used in each of Examples and Comparative Examples and the dissolved oxygen was evaluated. The reducing compounds having a dissolved oxygen concentration retention rate of 0.5 or more were evaluated as “the reducing compounds not reacting with the dissolved oxygen.”

2. Number of Fibrous Copper Microparticles Each Having One or More Irregularities Per 100 Fibrous Copper Microparticles

The aggregates of the fibrous copper microparticles were prepared, and were lightly disentangled by using an ultrasonic disperser in order that the fibrous copper microparticles might not crowd each other. Then, the aggregates were observed with the digital microscope and an electron microscope (Field Emission Scanning Electron Microscope, “S-800,” manufactured by Hitachi High-Technologies Corp.), and 100 of the fibrous copper microparticles each having a length of 1 μm or more were selected from the aggregates. The surface of each of the fibrous copper microparticles was observed, and in a range of 1 μm in the lengthwise direction of the fibrous body, and for each of the fibrous copper microparticles each having a minimum diameter portion falling in a diameter dimension range from 0.01 to 0.5 μm, the occurrence or non-occurrence of one or more irregularities each having a dimensional difference of 0.02 μm or more from the minimum diameter portion was verified. For the 100 fibrous copper microparticles, the number of the fibrous copper microparticles each undergoing the occurrence of one or more irregularities was counted, and thus the number of the fibrous copper microparticles each having one or more irregularities on the surface thereof per 100 of the fibrous copper microparticles was determined.

3. Average Fiber Diameter of Fibrous Copper Microparticles

In each of the 100 fibrous copper microparticles selected in the foregoing 2. the diameter of the minimum diameter portion in the total length was taken as the fiber diameter, and each of the fiber diameters was measured with the digital microscope and the electron microscope. The average value of the fiber diameters of the 100 fibrous copper microparticles was calculated, and was taken as the average fiber diameter of the fibrous copper microparticles.

4. Average Fiber Length and Average Aspect Ratio of Fibrous Copper Microparticles

The length of each of the 100 fibrous copper microparticles selected in the foregoing 2. was measured with the digital microscope and the electron microscope, and the average value of the lengths of the 100 fibrous copper microparticles was calculated, and was taken as the average fiber length of the fibrous copper microparticles. The average aspect ratio of the fibrous copper microparticles was derived by dividing the average fiber length of the fibrous copper microparticles by the average fiber diameter of the fibrous copper microparticles determined in the foregoing 3.

5. Full Width at Half Maximum

The aggregates of the fibrous copper microparticles obtained in each of the Examples and Comparative Examples were subjected to X-ray diffraction in the atmospheric air by using the wide angle X-ray diffractometer, the width of the diffraction intensity curve at half the intensity of the peak (at around 43 (deg)) corresponding to the copper (111) plane was determined by wide angle X-ray diffraction method and was taken as the full width at half maximum.

For X-ray diffraction, the powder reflection method was adopted. The measurement conditions involved are as follows.

Irradiation conditions: Cu-Kα ray (voltage: 50 kV, current: 300 mA), parallel beam method (CBO unit), 25° C.

Scanning conditions: 2 deg/min, 2θ/θ continuous scanning

Goniometer radius: 285 mm

Slit width conditions: Divergence slit: 1 mm, Divergence slit length: 10 mm, Scattering slit: 1 mm, Receiving slit: 0.2 mm

Filter: Nickel filter (thickness: 0.013 to 0.017 mm)

Scintillation counter: model SC-70C

Analysis software: JADE (version 7.5)

For each of the peaks, the baseline was set as shown in FIG. 5, and the peak intensity was measured from the baseline.

6. Average Crystallite Diameter

For the aggregates of the fibrous copper microparticles obtained in each of Examples and Comparative Examples, the average crystallite diameter was obtained on the basis of the foregoing formula (2) [(average crystallite diameter) (μm)=(K×λ)/(β×cos θ)]. For β in the foregoing formula (2), the full width at half maximum obtained in the foregoing 5. was substituted.

Production of Fibrous Copper Microparticles and Aggregates Thereof Example 1

In 186 g of pure water, 108.0 g of sodium hydroxide (manufactured by Nacalai Tesque, Inc.), 0.15 g of copper nitrate trihydrate (manufactured by Nacalai Tesque, Inc.) and 0.81 g of ethylenediamine (manufactured by Nacalai Tesque, Inc.) were mixed by stirring at 200 rpm at room temperature, and thus an aqueous solution dissolving these compounds was prepared. The obtained aqueous solution exhibited a clear, bright blue color. The molar ratio between the hydroxide ion and the copper ion in the aqueous solution was set at 4500/1.

Moreover, to the aqueous solution, 1.2 g of an ascorbic acid aqueous solution (manufactured by Nacalai Tesque, Inc.) (4.4% by mass) (in an amount of 0.5 times the number of moles of copper ion) was added as the reducing compound, and stirred at 200 rpm for a few minutes to prepare a uniform aqueous solution.

The aqueous solution prepared above was injected from the lower part of a glass column vessel (volume: 30 mL) equipped with a jacket circulating hot water at 70° C., in such a way that the flow rate of the aqueous solution was controlled so as for the column passage time (regarded as the heating time in the step (I)) to be 30 minutes. In this case, the aqueous solution was heated to 70° C. Consequently, with the heating time of 30 minutes, the aqueous solution turned almost colorless and transparent. The aqueous solution having turned colorless and transparent was successively discharged from the upper part of the column vessel. The description up to here is for the step (I) of the present invention.

The description from here on is for the step (II) (or the step (IIa)) of the present invention.

In the aqueous solution after passing through the step (I), 4.8 g (an amount of 2.0 times the number of moles of copper ion) of an ascorbic acid aqueous solution (4.4% by mass) was mixed as the reducing compound. Then, successively to the step (I), the aqueous solution was injected continuously from the lower part of a glass column vessel equipped with a jacket circulating hot water at 70° C. in such a way that the flow rate of the aqueous solution was controlled so as for the temperature of the aqueous solution to be maintained at 70° C. and so as for the column passage time (regarded as the temperature maintenance time in the step (II)) to be 30 minutes. As a result of allowing the aqueous solution to flow continuously, the successive precipitation of the fibrous copper microparticles and the aggregates thereof was visually verified.

The fibrous copper microparticles and the aggregates thereof precipitated in the step (II) were collected by filtration under pressure of compressed nitrogen [PTFE (polytetrafluoroethylene) membrane filter having a pore size of 1 μm, manufactured by Advantec Co., Ltd.], and were once washed with an ascorbic acid aqueous solution (10% by mass), and then washed three times with pure water. Subsequently, in a dryer set at 50° C., the liquid contained in the fibrous copper microparticles and the aggregates thereof was dried and removed, and thus the fibrous copper microparticles and the aggregates thereof of Example 1 were obtained. The evaluation results of the fibrous copper microparticles and the aggregates thereof are shown in Table 1.

TABLE 1 Production steps Reducing compound Step (I) Step (II) Addition amount (number Time for turning Temperature of moles relative to Temperature Heating colorless and Temperature maintenance one mole of copper ion) (° C.) time (min) transparent (min) (° C.) time (min) Type Step (I) Step (II) Example 1 70 30 30 70 30 Ascorbic acid 0.5 2.0 Example 2 70 30 30 70 90 Ascorbic acid 0.5 2.0 Example 3 70 30 30 70 180 Ascorbic acid 0.5 2.0 Example 4 70 30 30 70 90 Ascorbic acid 0.5 5.0 Example 5 70 30 30 70 90 Ascorbic acid 0.5 100.0 Example 6 80 20 20 80 90 Ascorbic acid 0.5 2.0 Example 7 60 80 80 60 260 Ascorbic acid 0.5 2.0 Example 8 70 10 10 70 60 Ascorbic acid 2.5 0 Example 9 70 10 10 70 180 Ascorbic acid 2.5 0 Example 10 70 30 30 70 20 Ascorbic acid 0.5 2.0 Example 11 70 30 30 70 30 Erythorbic acid 0.5 2.0 Example 12 70 30 30 70 30 Glucose 0.5 0.5 Comparative 70 1 1 70 10 Hydrazine 1.4 0 Example 1 Comparative 70 1 1 70 <10 Hydrazine 2.0 0 Example 2 Comparative 70 1 1 70 5 Hydrazine 2.5 0 Example 3 Comparative 70 5 5 70 <10 Hydrazine 0.5 2.0 Example 4 Comparative 70 30 30 — Ascorbic acid 0.5 2.0 Example 5 Shapes and properties of fibrous copper microparticles and aggregates thereof Number of fibrous Average Average Average Average Average copper microparticles crystallite crystallite fiber diameter fiber length aspect each having one or diameter diameter/average (μm) (μm) ratio more irregularities (μm) fiber diameter Example 1 0.07 50 714 4 0.0455 0.65 Example 2 0.08 55 688 8 0.063 0.79 Example 3 0.08 55 688 6 0.076 0.95 Example 4 0.09 47 522 4 0.0661 0.73 Example 5 0.10 43 430 6 0.074 0.74 Example 6 0.08 53 663 4 0.077 0.96 Example 7 0.08 60 750 8 0.052 0.65 Example 8 0.11 43 391 7 0.0731 0.66 Example 9 0.11 45 409 8 0.0713 0.65 Example 10 0.075 48 640 5 0.0326 0.43 Example 11 0.08 52 650 5 0.0524 0.66 Example 12 0.10 24 240 7 0.0471 0.47 Comparative 0.13 17 131 46 0.0304 0.23 Example 1 Comparative 0.12 26 217 55 0.0335 0.30 Example 2 Comparative 0.17 21 124 71 0.0406 0.24 Example 3 Comparative 0.45 16 36 90 0.0388 0.09 Example 4 Comparative No precipitation Example 5

Examples 2 to 7

In each of Examples 2 to 7, the heating temperature and the heating time (the column passage time) in the step (I), the temperature maintenance time (the column passage time) in the step (II) and the addition amount of the reducing compound in the step (II) were altered as described in Table 1. In each of Examples 2 to 7, fibrous copper microparticles and the aggregates thereof were obtained otherwise in the same manner as in Example 1. The evaluation results of these fibrous copper microparticles and the aggregates thereof are shown in Table 1.

Examples 8 and 9

In each of Examples 8 and 9, the heating time (the column passage time) in the step (I) and the temperature maintenance time (the column passage time) in the step (II) were altered as described in Table 1, the amount of the reducing compound beforehand added in the step (I) was altered as described in Table 1, and no reducing compound was added in the step (II). In each of Examples 8 and 9, fibrous copper microparticles and the aggregates thereof were obtained otherwise in the same manner as in Example 1. The evaluation results of these fibrous copper microparticles and the aggregates thereof are shown in Table 1.

Example 10

The temperature maintenance time (the column passage time) in the step (II) was set at 20 minutes. Fibrous copper microparticles and the aggregates thereof were obtained otherwise in the same manner as in Example 1. The evaluation results of the fibrous copper microparticles and the aggregates thereof are shown in Table 1.

Example 11

The reducing compound added in the step (I) and the step (II) was erythorbic acid. Fibrous copper microparticles and the aggregates thereof were obtained otherwise in the same manner as in Example 1. The evaluation results of the fibrous copper microparticles and the aggregates thereof are shown in Table 1.

Example 12

The reducing compound added in the step (I) and the step (II) was glucose, and the amount of the glucose added in the step (II) was 0.5 times the number of moles of copper ion. Fibrous copper microparticles and the aggregates thereof were obtained otherwise in the same manner as in Example 1. The evaluation results of the fibrous copper microparticles and the aggregates thereof are shown in Table 1.

Comparative Examples 1 to 3

In each of Comparative Examples 1 to 3, the heating time (the column passage time) in the step (I) and the temperature maintenance time (the column passage time) in the step (II) were altered as described in Table 1, hydrazine (added as hydrazine monohydrate, manufactured by Wako Pure Chemical Industries, Ltd.) was added in place of ascorbic acid as the reducing compound in an amount described in Table 1 in the step (I), and no reducing compound was added in the step (II). Fibrous copper microparticles and the aggregates thereof were obtained otherwise in the same manner as in Example 1. In the step (II), the prepared aqueous solution was injected from the lower part of a glass column vessel (volume: 30 mL) equipped with a jacket circulating hot water at 70° C. in such a way that the flow rate of the aqueous solution was controlled so as for the column passage time (regarded as the heating time) to be 20 minutes. The evaluation results of these fibrous copper microparticles and the aggregates thereof are shown in Table 1.

Comparative Example 4

The heating time (the column passage time) in the step (I) and the temperature maintenance time (the column passage time) in the step (II) were altered as described in Table 1, and hydrazine (added as hydrazine monohydrate, manufactured by Wako Pure Chemical Industries, Ltd.) was added in place of ascorbic acid as the reducing compound in the amounts described in Table 1 in the step (I) and in the step (II). Fibrous copper microparticles and the aggregates thereof were obtained otherwise in the same manner as in Example 1. The evaluation results of these fibrous copper microparticles and the aggregates thereof are shown in Table 1.

Comparative Example 5

The same operations as in Example 1 were performed up to the step (I), and then, 4.8 g (an amount of 2.0 times the number of moles of copper ion) of an ascorbic acid aqueous solution (4.4% by mass), a reducing compound, was mixed with the aqueous solution. Then, the mixed aqueous solution was rapidly cooled to 30° C. in 10 minutes by allowing the aqueous solution to pass through a glass column vessel equipped with a jacket circulating cool water (20° C.). It is to be noted that in Table 1, “-” means rapid cooling with cool water and the omission of the step (II).

In each of Examples 1 to 12, the aqueous solution was heated in the step (I), the temperature maintenance time in the step (II) was set at 20 minutes or more, and the fibrous copper microparticles were continuously produced by using a column vessel equipped with a hot water jacket. Consequently, as shown in Table 1, the number of the fibrous copper microparticles each having one or more irregularities on the surface thereof was 10 or less per 100 of the fibrous copper microparticles, showing that the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles was sufficiently suppressed.

In Examples 1 to 9, 11 and 12, the temperature maintenance time in the step (II) was set at “30 minutes or more,” and the fibrous copper microparticles were produced continuously by using the column vessel equipped with a hot water jacket. Consequently, as shown in Table 1, the fibrous copper microparticles having a fiber diameter of 0.01 to 0.5 μm and an average aspect ratio of 10 or more were obtained. Moreover, the aggregates of the fibrous copper microparticles having an average crystallite diameter of 0.045 to 0.1 μm and an average fiber diameter of 0.05 to 0.15 μm were obtained. Moreover, the aggregates of the fibrous copper microparticles having an average fiber diameter of 0.05 to 0.15 μm and an average crystallite diameter of 0.45 or more times the average fiber diameter were obtained. In other words, in the step (II), the temperature of the aqueous solution passing through the step (I) was maintained for “30 minutes or more,” and consequently the aggregates of the fibrous copper microparticles having a large average crystallite diameter (0.045 to 0.1 μm) were obtained.

In particular, in Examples 1 to 7, 11 and 12 (Examples in which the reducing compound was further added in the step (II)), as compared with Examples 8 and 9 (Examples in which the reducing compound was not added in the step (II), but the total amount of the reducing compound was added in the step (I)), the aggregates of the fibrous copper microparticles, having a small distribution of largish fiber diameter, and having a somewhat smaller average fiber diameter (specifically, 0.10 μm or less) were obtained.

In each of Comparative Examples 1 to 4, the color of the solution changed from blue in the step (I) in a short time, to yield a colorless and transparent solution; then, within 10 minutes, the fibrous copper microparticles and the aggregates thereof were completely precipitated. Accordingly, it was impossible to continuously precipitate the fibrous copper microparticles and the aggregates thereof by setting the temperature maintenance time at 20 minutes or more. In each of Comparative Examples 1 to 4, the number of the fibrous copper microparticles each having one or more irregularities on the surface thereof exceeded 10 per 100 of the fibrous copper microparticles, and thus it was impossible to suppress the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles.

In each of Comparative Examples 1 to 4, in the step (II), the temperature of the aqueous solution passing through the step (I) was not able to be maintained for “30 minutes or more,” and hence, only the aggregates of the fibrous copper microparticles having an average crystallite diameter of 0.0304 to 0.0406 μm falling outside the range specified in the present invention (0.045 to 0.1 μm) were obtained. Only the aggregates of the fibrous copper microparticles having an average crystallite diameter of less than 0.45 times the average fiber diameter were obtained.

Comparative Example 5 adopted a production process not including the step (II) and performing a rapid cooling, and hence was unable to obtain even the aggregates of the fibrous copper microparticles.

Example 13

In 186 g of pure water, 108.0 g of sodium hydroxide (manufactured by Nacalai Tesque, Inc.), 0.15 g of copper nitrate trihydrate (manufactured by Nacalai Tesque, Inc.) and 0.81 g of ethylenediamine (manufactured by Nacalai Tesque, Inc.) were mixed by stirring at 200 rpm at room temperature, and thus an aqueous solution dissolving these compounds was prepared. The obtained aqueous solution exhibited a clear, bright blue color. The molar ratio between the hydroxide ion and the copper ion in the aqueous solution was set at 4500/1.

Moreover, to the aqueous solution, 1.2 g of an ascorbic acid aqueous solution (manufactured by Nacalai Tesque, Inc.) (4.4% by mass) (in an amount of 0.5 times the number of moles of copper ion) was added as the reducing compound, and stirred at 200 rpm for a few minutes to prepare a uniform aqueous solution.

The aqueous solution prepared above was injected from the lower part of a glass column vessel (volume: 30 mL) equipped with a jacket circulating hot water at 80° C., in such a way that the flow rate of the aqueous solution was controlled so as for the column passage time (regarded as the heating time in the step (I)) to be 20 minutes. In this case, the aqueous solution was heated to 80° C. Consequently, with the heating time of 20 minutes, the aqueous solution turned almost colorless and transparent. The aqueous solution having turned colorless and transparent was successively discharged from the upper part of the column vessel. The description up to here is for the step (I) (or the step (Ia)) of the present invention.

The description from here on is for the step (III) (or the step (IIIa)) of the present invention.

In the aqueous solution after passing through the step (I), 4.8 g (an amount of 2.0 times the number of moles of copper ion) of an ascorbic acid aqueous solution (4.4% by mass) was mixed as the reducing compound. Then, successively to the step (I), the aqueous solution was injected from the lower part of a glass column vessel not equipped with a heating jacket and allowed to flow continuously through the column vessel, and thus the temperature of the aqueous solution was decreased to 70° C. after 10 minutes and to 60° C. after 30 minutes. In other words, in the step (III), the time required for decreasing the temperature of the aqueous solution by 20° C. was 30 minutes. In the mentioned slow cooling process, the successive precipitation of the fibrous copper microparticles and the aggregates thereof was visually verified.

The fibrous copper microparticles and the aggregates thereof precipitated in the step (III) were collected by filtration under pressure of compressed nitrogen [PTFE membrane filter having a pore size of 1 μm, manufactured by Advantec Co., Ltd.], and were once washed with an ascorbic acid aqueous solution (10% by mass), and then washed three times with pure water. Subsequently, in a dryer set at 50° C., the liquid contained in the fibrous copper microparticles and the aggregates thereof was dried and removed, and thus the fibrous copper microparticles and the aggregates thereof of Example 13 were obtained. The evaluation results of the fibrous copper microparticles and the aggregates thereof are shown in Table 2.

TABLE 2 Production steps Reducing compound Step (I) Step (III) Addition amount (number Time for turning Time required for decreasing of moles relative to Temperature Heating colorless and by 20° C. temperature of one mole of copper ion) (° C.) time (min) transparent (min) aqueous solution (min) Type Step (I) Step (III) Example 13 80 20 20 30 Ascorbic acid 0.5 2.0 Example 14 80 20 20 30 Ascorbic acid 0.5 1.0 Example 15 80 45 45 30 Ascorbic acid 0.25 0.5 Example 16 70 30 30 45 Ascorbic acid 0.5 2.0 Example 17 70 30 30 30 Glucose 0.5 0.5 Comparative 70 30 1 — Hydrazine 2.5 0 Example 6 Shapes and properties of fibrous copper microparticles and aggregates thereof Number of fibrous Average Average Average Average Average copper microparticles crystallite crystallite fiber diameter fiber length aspect each having one or diameter diameter/average (μm) (μm) ratio more irregularities (μm) fiber diameter Example 13 0.06 58 967 4 0.0166 0.28 Example 14 0.07 65 929 3 0.0293 0.42 Example 15 0.07 55 786 4 0.0194 0.28 Example 16 0.06 48 800 4 0.0275 0.46 Example 17 0.09 25 277 5 0.0180 0.20 Comparative 0.20 18 90 53 0.0420 0.21 Example 6

Examples 14 to 17

In each of Examples 14 to 17, the heating temperature and the heating time (the column passage time) in the step (I), the time required in the step (III), for decreasing by 20° C. the temperature of the aqueous solution after passing through the step (I), and the addition amounts and the type (as the reducing compound in Example 17, glucose (manufactured by Nacalai Tesque, Inc.) was used) of the reducing compound in the step (I) and the step (III) were altered as described in Table 2. Fibrous copper microparticles and the aggregates thereof were obtained otherwise in the same manner as in Example 13. The evaluation results of these fibrous copper microparticles and the aggregates thereof are shown in Table 2.

Comparative Example 6

The heating temperature and the heating time (the column passage time) in the step (I), the time required in the step (III), for decreasing by 20° C. the temperature of the aqueous solution after passing through the step (I), and the addition amounts and the type (hydrazine (added as hydrazine monohydrate, manufactured by Wako Pure Chemical Industries, Ltd.)) of the reducing compound in the step (I) and the step (III) were altered as described in Table 2. Fibrous copper microparticles and the aggregates thereof were obtained otherwise in the same manner as in Example 13. The evaluation results of the fibrous copper microparticles and the aggregates thereof are shown in Table 2.

It is to be noted that in Table 2, “-” means the start of the precipitation of the fibrous copper microparticles and the aggregates thereof, before the aqueous solution was subjected to the slow cooling.

In each of Examples 13 to 17, due to the fact that “in the step (I), the aqueous solution was heated” and the fact that “in the step (III), the temperature of the aqueous solution after passing through the step (I) was decreased by 20° C. by cooling over a time of “15 minutes or more” immediately after the start of the cooling”, as shown in Table 2, the number of the fibrous copper microparticles each having one or more irregularities on the surface thereof was 10 or less per 100 of the fibrous copper microparticles, and thus the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles was sufficiently suppressed.

Also, in each of Examples 13 to 17, due to the fact that in the step (III), the temperature of the aqueous solution after passing through the step (I) was decreased by 20° C. by cooling over a time of “15 minutes or more” immediately after the start of the cooling, as shown in Table 2, the aggregates of the fibrous copper microparticles having an average crystallite diameter of 0.015 to 0.03 μm, an average fiber diameter of 0.03 to 0.1 μm and an average aspect ratio of 10 or more was able to be obtained.

In Comparative Example 6, immediately after the step (I), the fibrous copper microparticles and the aggregates thereof precipitated, and completely precipitated before the aqueous solution was subjected to the slow cooling in the step (III). Accordingly, in Comparative Example 6, the number of the fibrous copper microparticles each having one or more irregularities on the surface thereof exceeded 10 (was 53) per 100 of the fibrous copper microparticles, and the occurrence of one or more irregularities on the surface of each of the fibrous copper microparticles fell outside the range (10 or less) specified in the present invention. Moreover, in the aggregates of the fibrous copper microparticles obtained in Comparative Example 6, the average crystallite diameter was 0.042 μm and the average fiber diameter was 0.2 μm, both deviating the ranges specified in the present invention (the average crystallite diameter range from 0.015 to 0.03 μm and the average fiber diameter range from 0.03 to 0.1 μm).

FIG. 6 shows an observation view (magnification factor: 40000) obtained by observing with a SEM the fibrous copper microparticles obtained in Example 13. As can be seen from FIG. 6, the fibrous copper microparticles obtained in Example 13 were sufficiently suppressed in the occurrence of one or more irregularities on the surface of each thereof.

FIG. 7 shows an observation view (magnification factor: 40000) obtained by observing with a SEM the fibrous copper microparticles obtained in Comparative Example 2. As can be seen from FIG. 7, in the fibrous copper microparticles obtained in Comparative Example 2, a large number of the fibrous copper microparticles having one or more irregularities on the surface of each thereof were formed.

REFERENCE SIGNS LIST

1 Fibrous copper microparticle
2 Range of 1 μm in lengthwise direction of fibrous body
3 Irregularity defined in the present invention
4 Maximum diameter portion of fibrous copper microparticle
5 Minimum diameter portion of fibrous copper microparticle
6 Dimensional difference between maximum diameter portion and minimum diameter portion

Claims

1. Fibrous copper microparticles, wherein a number of fibrous copper microparticles each including one or more irregularities each having a dimensional difference of 0.02 μm or more, in a range of 1 μm in a lengthwise direction of a fibrous body, between a maximum diameter portion of the fibrous body and a minimum diameter portion of the fibrous body falling in a diameter dimension range of 0.01 to 0.5 μm, and each having a length of 1 μm or more is 10 or less per 100 of the fibrous copper microparticles.

2. The fibrous copper microparticles according to claim 1, wherein for each of the fibrous copper microparticles, a fiber diameter is 0.01 to 0.5 μm, and an aspect ratio is 10 or more.

3. Aggregates of fibrous copper microparticles, formed by allowing the fibrous copper microparticles according to claim 1 to aggregate, wherein an average crystallite diameter is 0.045 to 0.1 μm and an average fiber diameter is 0.05 to 0.15 μm.

4. Aggregates of fibrous copper microparticles, formed by allowing the fibrous copper microparticles according to claim 1 to aggregate,

wherein the average fiber diameter is 0.05 to 0.15 μm, and an average crystallite diameter is 0.45 or more times the average fiber diameter.

5. Aggregates of fibrous copper microparticles, formed by allowing the fibrous copper microparticles according to claim 1 to aggregate,

wherein the average crystallite diameter is 0.015 to 0.03 μm, and the average fiber diameter is 0.03 to 0.1 μm.

6. A process for producing fibrous copper microparticles,

wherein the production process is a process for producing the fibrous copper microparticles according to claim 1; and

the production process comprises a following step (I) and a following step (II) or (III), in this order:

the step (I) of heating, to 50 to 100° C., an aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound;

the step (II) of maintaining for 20 minutes or more a temperature of the aqueous solution after passing through the step (I), and continuously precipitating the fibrous copper microparticles;

the step (III) of cooling the temperature of the aqueous solution after passing through the step (I) to decrease the temperature thereof by 20° C. over a period of time of 15 minutes or more from immediately after a start of cooling, and continuously precipitating the fibrous copper microparticles.

7. The process for producing fibrous copper microparticles according to claim 6, wherein in the step (II) or (III), the reducing compound is further added to the aqueous solution.

8. A process for producing aggregates of fibrous copper microparticles,

wherein the production process is a process for producing the aggregates of the fibrous copper microparticles according to claim 3; and the production process comprises the following step (I) and a following step (IIa), in this order, and continuously precipitates the fibrous copper microparticles or the aggregates of the fibrous copper microparticles:

the step (I) of heating to 50 to 100° C. the aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound;

the step (IIa) of maintaining for 30 minutes or more the temperature of the aqueous solution after passing through the step (I).

9. A process for producing aggregates of fibrous copper microparticles,

wherein the production process is a process for producing the aggregates of the fibrous copper microparticles according to claim 5; and the production process comprises a following step (Ia) and a following step (IIIa), in this order, and continuously precipitates the fibrous copper microparticles or the aggregates of the fibrous copper microparticles:

the step (Ia) of heating to 65 to 100° C. the aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound;

the step (IIIa) of decreasing the temperature of the aqueous solution after passing through the step (Ia) by 20° C. over a period of time of 15 minutes or more from immediately after a start of cooling.

10. The process for producing aggregates of fibrous copper microparticles according to claim 8, wherein in the step (IIa), the reducing compound is further added to the aqueous solution.

11. The process for producing aggregates of fibrous copper microparticles according to claim 9, wherein in the step (IIIa), the reducing compound is further added to the aqueous solution.

12. The process for producing fibrous copper microparticles according to claim 6, wherein as the reducing compound, one or more selected from ascorbic acid, erythorbic acid and glucose are used.

13. The process for producing aggregates of fibrous copper microparticles according to claim 8, wherein as the reducing compound, one or more selected from ascorbic acid, erythorbic acid and glucose are used.