COPPER POWDER AND METHOD FOR PRODUCING COPPER POWDER

TECHNICAL FIELD

[0001]The present invention relates to copper powder obtained by heating copper oxide powder or copper powder in a polyol solvent, and a method for producing the copper powder.

BACKGROUND ART

[0002]Copper powder is used as a material for conductive paste for forming an internal electrode and an external electrode of a multilayer ceramic capacitor (MLCC), electrodes of a multilayer ceramic substrate, and the like, which are electronic components. In recent years, the use of thin film internal electrodes has been increased with a decrease in size and an increase in capacity of MLCC. Fine copper powder is required for this application as being used for the above-described conductive paste (internal electrode paste). In particular, metal fine particles having an average particle diameter of 250 nm or less have a low firing temperature unlike particles that are equal to or larger than general submicron particles, and are considered to be applied to low temperature-firing paste and the like.

[0003]Metal powder to be sintered at a low temperature has the following advantages: a heat load during sintering and a residual stress during cooling remain low, and the metal powder sintered once can be stable without melting until a bulk metal's melting point. Accordingly, low-temperature sintering metal powder has attracted attention in the fields of printed electronics, in which heat damage to a substrate should be avoided, and die bonding of a power module applicable to high-temperature operation.

[0004]Currently, nano-sized silver powder has been widely used as the low-temperature sintering metal powder, but has disadvantages such as expensiveness and ion migration properties. As a countermeasure against this, development for low-temperature sintering of inexpensive copper powder having excellent ion migration resistance has been actively performed in recent years.

[0005]Fined particles with an increased surface energy are effective in the low-temperature sintering of copper powder, like other types of metal powder. However, H fined copper powder tends to be oxidized, and under surface oxidation, sinterability is deteriorated. The surface of the copper powder therefore requires an oxidation resistance film (oxidation prevention film), and the oxidation resistance film itself needs to be decomposed at a low temperature in order not to prevent sintering.

[0006]That is, the low-temperature sintering of copper powder requires two conditions: the powder is fine particles, and is provided with an oxidation resistance film so as not to inhibit low-temperature sintering.

[0007]To address this, Patent Literature 1 discloses copper powder having a particle diameter and a particle size distribution that are adjusted within constant ranges by physical adsorption of an aliphatic carboxylic acid having an aliphatic group having 5 or more carbon atoms on the surface of the copper particles and achieving both oxidation resistance and low-temperature sintering.

[0008]According to Patent Literature 1, after copper powder having a SEM primary particle diameter of 100 nm or less is allowed to stand in the air for an extended period of time, oxidation is not confirmed by XRD, and the copper powder can be sintered at 300° C. in a nitrogen atmosphere.

[0009]Patent Literature 2 discloses copper powder that can be fired at a low temperature under a high-vacuum condition of 0.01 Pa or less due to the presence of a cuprous oxide film regulated at a constant proportion or so as to have an average coating thickness on the surface of the copper powder. According to Patent Literature 2, copper powder covered with a cuprous oxide layer that is further protected with caproic acid achieves sintering at 100° C. under the condition of a degree of vacuum of 3×10−6 Pa.

[0010]As a method other than the methods described above, Patent Literature 3 discloses a method (polyol method) for heating and reducing copper oxide powder (raw material) in a polyol solvent. Copper powder produced by this method has features such as excellent oxidation resistance due to an organic film formed during the reduction. According to Patent Literature 3, copper fine particles containing silver as a nuclear and having a particle diameter of 100 nm or less can be obtained by adding a silver salt for nucleation and adding polyvinylpyrrolidone as a dispersant.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

[0014]However, it is found that the copper powder described in Patent Literature 1 contains a large amount of organic components and is covered with an aliphatic carboxylic acid at a high density close to that of a liquid-condensed film. Such a large amount of organic components on the copper surface is volatilized by thermal decomposition to generate gas, and therefore may lead to failure caused by the generated gas, such as bulge and short-circuiting in formation of MLCC internal electrodes, for example.

[0015]In the copper powder described in Patent Literature 3, polyvinylpyrrolidone, which is a high molecular polymer, is added as a dispersant in an amount of 40% by mass relative to copper.

[0016]The oxide film formed on the surface of the copper powder may inhibit sintering even when it is a very thin layer of several nanometers. It is thus assumed that, for the copper powder described in Patent Literature 1, an XRD peak derived from an oxide is not detected immediately after synthesis and after storage in the air at 25° C. for 4 months. In consideration of detection sensitivity of XRD, however, it is difficult to judge whether the oxide film in an amount that may inhibit sintering is present by using XRD.

[0017]In a similar manner, for the copper powder described in Patent Literature 2, the formed oxide film may generate water vapor by reduction of the oxide film in a reducing atmosphere and cause inhibition of sintering in an inert atmosphere. Furthermore, low-temperature sintering as disclosed requires a high-vacuum or ultra-high vacuum condition, and is difficult to apply to a heating furnace for mass production, such as a conveying-type continuous heating furnace.

[0018]The present invention has been made in view of the circumstances of the related art, and an object of the present invention is to provide copper powder having an organic film for preventing the formation of an oxide film that may inhibit sintering, and having excellent low-temperature sinterability.

Solution to Problem

[0019]The inventors of the present invention have intensively investigated to solve the above-described problems, and as a result, found that a surface of copper powder produced using a polyol is covered with a mixture of an organic substance derived from a polyol solvent and a chain organic substance having an appropriate molecular weight and an appropriate functional group to obtain copper powder having both excellent oxidation resistance and low-temperature sinterability. Thus, the present invention has been completed. Specifically, the present invention provides the following.

[0031]An aspect of the present invention provides a method for producing copper powder including: heating copper oxide powder slurry at 230° C. or lower, the copper oxide powder slurry being obtained by mixing copper oxide powder in a polyol solvent having a boiling point of 230° C. or lower to which the chain organic substance is added.

[0032]The amount of the chain organic substance added is preferably 0.005% by mass or more and 10% by mass or less relative to the total amount of copper contained in the copper oxide powder.

[0033]The polyol solvent having a boiling point of 230° C. or lower more preferably includes one or more selected from the group consisting of ethylene glycol (boiling point: 196° C.), propylene glycol (boiling point: 188° C.), 1,3-propanediol (boiling point: 214° C.), 1,2-butanediol (boiling point: 194° C.), 1,3-butanediol (boiling point: 207° C.), 1,4-butanediol (boiling point: 228° C.), 1,2-pentanediol (boiling point: 210° C.), and 1,2-hexanediol (boiling point: 223° C.).

[0034]An aspect of the present invention provides a method for producing copper powder including: heat-treating copper powder slurry at 230° C. or higher in which copper powder having an average particle diameter of 250 nm or less and not satisfying at least any of the conditions (3) and (4) is dispersed in a polyol solvent having a boiling point of 250° C. or higher to which the chain organic substance is added.

[0035]The polyol solvent having a boiling point of 250° C. or higher more preferably includes one or more selected from the group consisting of triethylene glycol (boiling point: 287° C.), tetraethylene glycol (boiling point: 327° C.), and polyethylene glycol having an average molecular weight of 200 or more and 600 or less (boiling point: approximately 300° C.).

[0036]The temperature of the heat-treating is preferably 230° C. or higher, more preferably 250° C. or higher, and further preferably 270° C. or higher. The amount of the chain organic substance added is preferably 0.005% by mass or more and 10% by mass or less relative to the total amount of copper contained in the copper powder to be heat-treated.

Effects of Invention

[0037]Copper powder according to the present embodiment is copper powder having an organic film for preventing the formation of an oxide film that may inhibit sintering, and having excellent low-temperature sinterability.

[0038]With the method for producing copper powder according to the present embodiment, copper powder having an organic film for preventing the formation of an oxide film that may inhibit sintering, and having excellent low-temperature sinterability can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is a thermal mechanical analysis (TMA) profile of copper powder produced in Example 10 in a nitrogen atmosphere and in a 2% hydrogen, 98% nitrogen atmosphere.

[0040]FIG. 2 is a diagram illustrating results of gas chromatography-mass spectrometry in Example 17 and Conventional Example 3.

DESCRIPTION OF EMBODIMENTS

[0041]Hereinafter, specific embodiments according to the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiments, and various modifications can be made without modifying the gist of the present invention. The expression “X to Y” (X and Y are any numerical values) herein means “X or more and Y or less”.

[0042]In a polyol method, copper oxide powder is suspended in a polyol solvent and heated, and the polyol solvent acts as a reductant to promote reduction to copper powder. The use of copper oxide (CuO) as the copper oxide powder causes reduction of copper oxide (CuO) to copper (Cu) via cuprous oxide (Cu₂O). The use of cuprous oxide (Cu₂O) as the copper oxide causes reduction of cuprous oxide (Cu₂O) to copper (Cu). In all the cases, copper powder (hereinafter also referred to as “polyol copper powder”) is finally obtained. The obtained copper powder is washed with pure water and the like, filtered, then washed again if necessary, and dried. Specifically, as an example of washing, a method in which the copper powder obtained by the reduction (polyol copper powder) is precipitated, subjected to decantation, and then stirred and washed under supply of pure water and the like is used. As an example of filtration, a dehydration method by centrifugation, or the like is used.

[0043]The copper powder according to the present embodiment is copper powder having an average particle diameter of 250 nm or less and having a surface covered with an organic substance, and the surface of the copper powder is covered with both a specific organic substance that is detected by gas chromatography-mass spectrometry (GC-MS) and a specific organic substance that is detected by liquid chromatography-mass spectrometry (LC-MS). According to this configuration, in a green compact obtained by pressure-molding the copper powder, the temperature at which the thermal shrinkage ratio in an inert atmosphere is 1% on the basis of the thickness of the green compact at 25° C. during heating at a temperature increasing rate of 10° C./min is 230° C. or lower, which is excellent in low-temperature sinterability. Similarly, in a green compact obtained by pressure-molding the copper powder, the temperature difference between the temperature at which the thermal shrinkage ratio in an inert atmosphere is 3% and the temperature at which the thermal shrinkage ratio in a reducing atmosphere is 3% on the basis of the thickness of the green compact at 25° C. during heating at a temperature increasing rate of 10° C./min is smaller than 10° C., which indicates that an oxide film that may inhibit sintering is not formed on the surface of the copper powder or that the oxide film is formed, but is so thin that it does not inhibit sintering. The copper powder of the present embodiment is copper powder having an organic film for preventing the formation of an oxide film that may inhibit sintering, and having excellent low-temperature sinterability.

[0044]The average particle diameter of the copper powder is preferably 250 nm or less, more preferably 200 nm or less, and further preferably 150 nm or less. The average particle diameter of the copper powder may be, for example, 110 nm or less, 100 nm or less, 90 nm or less, 70 nm or less, or 60 nm or less, which are described in Examples. The copper powder that is fine can be suitably used for an electronic component, such as an electrode of a multilayer ceramic capacitor. The lower limit of the average particle diameter of the copper powder is not particularly limited, and is approximately 20 nm according to a production method described below. The average particle diameter of the copper powder according to the present embodiment is a value measured by the method described in Examples. The average particle diameter of the copper powder can be controlled under a production condition as described in a section of Production Method below.

[Gas Chromatography—Mass Spectrometry]

[0061]Since GC-MS is a measurement procedure for detecting a gasified organic substance, the specific organic substance detected by the GC-MS is an organic substance that has a relatively small molecular weight and is easy to gasify. A method for detecting the organic substance by GC-MS in the present embodiment is the method described in Examples. The structures of the organic substances (Chem. 1) to (Chem. 8) are as follows.

[Liquid Chromatography-Mass Spectrometry]

[0062]The specific organic substance detected by liquid chromatography-mass spectrometry (LC-MS) is preferably a chain organic substance having a molecular weight of 200 or more and 1,000 or less and having, at each terminal of the molecule, one selected from the group consisting of a carboxy group (—COOH), a hydroxy group (—OH), an amino group (—NH₂), an aldehyde group (—CHO), a nitro group (—NO₂), a thiol group (—SH), a sulfo group (—SO₃HHH), a phosphate group (—PO₄H₂), a cyan group (—CN), a chloro group (—Cl), a bromo group (—Br), and an iodo group (—I), which are each a functional group capable of being coordinated with a copper ion.

[0063]Among these, the chain organic substance having a molecular weight of 200 or more and 700 or less is more preferred. The chain organic substance more preferably has a carboxy group or a hydroxy group at each terminal of the molecule, and still more preferably has a carboxy group or a hydroxy group at both the terminals. Examples of such a chain organic substance include (Chem. 9) sebacic acid (molecular weight: 202), (Chem. 10) dodecanedioic acid (molecular weight: 230), (Chem. 11) tetradecanedioic acid (molecular weight: 258), (Chem. 12) hexadecanedioic acid (molecular weight: 286), (Chem. 13) octadecanedioic acid (molecular weight: 314), (Chem. 14) eicosanedioic acid (molecular weight: 342), (Chem. 15) poly(ethylene glycol)bis(carboxymethyl) ether (average molecular weight: 250), and (Chem. 16) poly(ethylene glycol)bis(carboxymethyl) ether (average molecular weight: 600), and these compounds are preferred in terms of easy availability. The structures of the organic substances (Chem. 9) to (Chem. 16) are as follows.

[0064]A mechanism in which the copper powder of the present embodiment has low-temperature sinterability as described above is not obvious in detail. However, the proportion of a functional group capable of being coordinated with a copper ion in a long chain molecule in the specific organic substance detected by the GC-MS and the specific organic substance detected by the LC-MS is low, and therefore the density of binding via the functional group to the surface of the copper powder, which is fully covered with the organic substances, may be decreased to facilitate detachment from the surface of the copper powder due to heat.

[0065]The organic substance detected by the GC-MS may be detected by the LC-MS, and the organic substance detected by the LC-MS may be detected by the GC-MS.

[0066]The copper powder of the present embodiment is pressure-molded at 100 MPa into a green compact, the thermal shrinkage ratio on the basis of the thickness of the green compact at 25° C. when the green compact is heated in an inert atmosphere from 25° C. at a temperature increasing rate of 10° C./min is measured, and as a result, the temperature at which the thermal shrinkage ratio is 18 is 230° C. or lower. The temperature at which the thermal shrinkage ratio of the green compact is 18 is a temperature at which the volume of the copper powder begins to shrink under a sintering phenomenon, and can be referred to as a sintering initiation temperature. For example, the temperature at which the thermal shrinkage ratio of the copper powder of the present embodiment is 1% may be 220° C. or lower, 215° C. or lower, 210° C. or lower, 205° C. or lower, 200° C. or lower, 195° C. or lower, or the like, as described in Examples.

[0067]The copper powder of the present embodiment is pressure-molded at 100 MPa into a green compact, the thermal shrinkage ratio on the basis of the thickness of the green compact at 25° C. when the green compact is heated in an inert atmosphere and a reducing atmosphere from 25° C. at a temperature increasing rate of 10° C./min is measured, and as a result, the temperature difference between the temperature at which the thermal shrinkage ratio in the inert atmosphere is 3% and the temperature at which the thermal shrinkage ratio in the reducing atmosphere is 3% is smaller than 10° C. At this time, since sintering behavior does not largely change between when firing (heating) is performed in the inert atmosphere and in the reducing atmosphere, restrictions associated with difficulty in sintering the copper powder are relaxed in selecting the firing atmosphere. The temperature at which the thermal shrinkage ratio of the green compact is 3% is a temperature when the volumetric shrinkage under the sintering phenomenon of the copper powder advances to some extent, and is an indicator of sinterability of the copper powder. Furthermore, from the comparison of the temperatures in the inert atmosphere and in the reducing atmosphere, the presence or absence of an oxide film that inhibits sintering can be presumed. That is, the temperature difference between the temperature at which the thermal shrinkage ratio of the green compact in the inert atmosphere is 3% and the temperature at which the thermal shrinkage ratio in the reducing atmosphere is 3% being smaller than 10° C. means that the oxide film that may inhibit sintering is not formed on the surface of the copper powder. For example, in the copper powder of the present embodiment, the temperature difference between the temperature at which the thermal shrinkage ratio in the inert atmosphere is 3% and the temperature at which the thermal shrinkage ratio in the reducing atmosphere is 3% may be 8° C. or lower, 6° C. or lower, 5° C. or lower, or the like, as described in Examples.

[0068]Specifically, when the temperature at which the thermal shrinkage ratio of the green compact in the reducing atmosphere is 3% is lower than the temperature at which the thermal shrinkage ratio of the green compact in the inert atmosphere is 3% by 10° C. or higher, the surface of the copper powder used in the measurement of the thermal shrinkage ratio is covered with a thin oxide film. Accordingly, it is considered that the oxide film is reduced by a reductive gas, resulting in easy sintering (in the inert atmosphere, sintering is inhibited by the oxide film). On the other hand, when the thermal shrinkage ratio is measured in the reducing atmosphere under a condition where the amount of an oxide on the surface of the copper powder used for thermal mechanical analysis is large, the amount of water vapor produced by reduction is increased, the expansion amount of the green compact due to the water vapor is larger than the shrinkage amount due to sintering of the copper powder, and a tendency of shift of a thermal shrinkage behavior to a high temperature side in a thermal mechanical analysis profile is recognized. This corresponds with a case where the temperature at which the thermal shrinkage ratio of the green compact in the reducing atmosphere is 3% is higher than the temperature at which the thermal shrinkage ratio of the green compact in the inert atmosphere is 3% by 10° C. or higher.

[0069]A mechanism in which the copper powder of the present embodiment has an effect of preventing the formation of an oxide film as described above is not obvious in detail. The specific organic substance detected by the GC-MS and the specific organic substance detected by the LC-MS are a combination of organic substances having different molecular lengths, and can efficiently and fully cover the surface of the copper powder, and therefore it is considered that the organic substances may have a high function as an oxidation prevention film.

[0070]All the organic substance on the surface of the copper powder of the present embodiment, the organic substance detected by the GC-MS, and the organic substance detected by the LC-MS may contain another substance, such as another organic substance or a compound, without departing from the spirit of the present invention.

[0082]A configuration other than the above-described configuration of the copper powder of the present embodiment is any configuration. The copper powder of the present embodiment is copper powder having an organic film for preventing the formation of an oxide film that may inhibit sintering, and having excellent low-temperature sinterability.

[0083]Next, a method for producing copper powder according to the present embodiment (Method 1 for Producing Copper Powder and Method 2 for Producing Copper Powder) will be described. In the following description, previously described items may be omitted or simplified. In the method for producing copper powder according to the present embodiment, the items described in the copper powder according to the present embodiment may be appropriately applied. The items described in the method for producing copper powder according to the present embodiment can be appropriately applied to the copper powder according to the present embodiment.

(Method 1 for Producing Copper Powder)

[0084]The method for producing copper powder of the present embodiment is a method for producing copper powder including reducing copper oxide powder (raw material) in a polyol solvent having a boiling point of 230° C. or lower (hereinafter, the polyol solvent having a boiling point of 230° C. or lower is also referred to as polyol solvent A) to obtain the copper powder. In the reduction process, to the polyol solvent A in which the copper oxide powder is suspended (hereinafter, the liquid in which the copper oxide powder is mixed and suspended in this polyol solvent A is also collectively referred to as a reaction liquid), the specific chain organic substance described above is added, followed by thermal reduction to obtain target copper powder. Through the reduction process, the organic substance containing the added chain organic substance can be applied to the surface of the copper powder to obtain the copper powder of the present embodiment described above.

[0085]The chain organic substance to be added may have a form of a salt or the like as long as it has a form of the chain organic substance in the reaction liquid.

[0086]As the copper oxide powder used as the raw material in the reduction process, both copper oxide (CuO) and cuprous oxide (Cu₂O) can be used. The copper oxide powder may be pulverized in advance and subjected to a reduction reaction.

[0087]The polyol solvent A used in the reduction process is a polyol solvent having a boiling point of 230° C. or lower, and is more preferably one or more selected from the group consisting of ethylene glycol (boiling point: 196° C.), propylene glycol (boiling point: 188° C.), 1,3-propanediol (boiling point: 214° C.), 1,2-butanediol (boiling point: 194° C.), 1,3-butanediol (boiling point: 207° C.), 1,4-butanediol (boiling point: 228° C.), 1,2-pentanediol (boiling point: 210° C.), and 1,2-hexanediol (boiling point: 223° C.). The amount of the chain organic substance added to the polyol solvent A is preferably 0.005% by mass or more and 10% by mass or less relative to the total amount of copper contained in the copper oxide powder.

[0088]The reaction temperature in the reduction process is preferably equal to or higher than a temperature that is lower than the boiling point of the polyol solvent A by 50° C., and more preferably equal to or higher than a temperature that is lower than the boiling point of the polyol solvent A by 40° C. The reaction temperature is preferably equal to or lower than a temperature that is lower than the boiling point of the polyol solvent A by #0° C., and more preferably equal to or lower than a temperature that is lower than the boiling point of the polyol solvent A by 5. The reaction temperature is preferably equal to or higher than a temperature that is lower than the boiling point of the polyol solvent A by 50° C. and equal to or lower than a temperature that is lower than the boiling point of the polyol solvent A by +0° C., and further preferably equal to or higher than a temperature that is lower than the boiling point of the polyol solvent A by 40° C. and equal to or lower than a temperature that is lower than the boiling point of the polyol solvent A by 5° C. When the reaction temperature is lower than a temperature that is lower than the boiling point of the polyol solvent A by 50° C., the reduction reaction does not sufficiently advance, the copper oxide powder (raw material) remains, the oxygen content in the resultant copper powder (polyol copper powder) may be high, and the reaction time is largely increased leading to low productivity. When the heating temperature is made higher than the boiling point of the polyol solvent A, the polyol solvent is significantly decreased (consumed) due to decomposition and volatilization, and reduction cannot be sufficiently achieved.

[0089]In the reduction process, the copper oxide powder is heated and reduced in the polyol solvent A to which the chain organic substance is added, to obtain the copper powder according to the present embodiment. In the reduction reaction, the chain organic substance is added to the reaction liquid, and heating is then initiated. Herein, the chain organic substance may be post-added to the reaction liquid during an increase in temperature.

[0090]When the chain organic substance that does not become neutral (pH: approximately 6.5 to 7.5) in an aqueous solution form is used, it is preferable that an alkali or an acid be added together as a neutralizer to neutralize the chain organic substance added to the reaction liquid. When an alkali is used as the neutralizer, a hydroxide of an alkali metal is preferred. Specifically, sodium hydroxide or potassium hydroxide can be used. When an acid is used as the neutralizer, an inorganic acid is preferred. Specifically, sulfuric acid or hydrochloric acid can be used. When the neutralizer is added to the reaction liquid, an aqueous solution of the neutralizer may be formed in advance and added.

[0091]With an increase in amount of the chain organic substance in the reaction liquid, the particle diameter (average particle diameter) of the obtained copper powder is decreased. Therefore, the average particle diameter of the copper powder can be controlled by the amount of the chain organic substance. Further, the chain organic substance is incorporated into the organic film formed on the surface of the copper powder, to improve sinterability. The amount of the chain organic substance in the reaction liquid is set according to conditions, such as the particle diameter of the copper powder obtained after the reduction reaction. The amount of the chain organic substance in the reaction liquid can be set by a pilot study. The amount of the chain organic substance in the reaction liquid is preferably 10% by mass or less relative to the total amount of copper contained in the copper oxide as the raw material. The amount of the chain organic substance in the reaction liquid (the amount of the chain organic substance added to the polyol solvent A) is preferably 0.005% by mass or more relative to the total amount of copper contained in the copper oxide as the raw material. The amount of the chain organic substance in the reaction liquid is preferably 0.005% by mass or more and 10% by mass or less relative to the total amount of copper contained in the copper oxide as the raw material. The addition of the chain organic substance in an amount of less than 0.005% by mass relative to the total amount of copper contained in the copper oxide as the raw material may be insufficient to exert an effect of controlling a particle diameter and an effect of improving sinterability. Since an effect of the addition of the chain organic substance is decreased by an increase in addition amount, the addition of the chain organic substance in an amount of more than 10% by mass relative to the total amount of copper contained in the copper oxide as the raw material increases a production cost and thus is not cost-effective. The amount of the chain organic substance in the reaction liquid (the amount of the chain organic substance added to the polyol solvent A) is preferably 0.001 mol % or more, and may be, for example, 0.01 mol % or more, or 0.1 mol % or more, relative to the total amount of copper contained in the copper oxide as the raw material. The amount of the chain organic substance in the reaction liquid is preferably 1 mol % or less relative to the total amount of copper contained in the copper oxide as the raw material. The amount of the chain organic substance in the reaction liquid is preferably 0.001 mol % or more and 1 mol or less, and may be 0.01 mol % or more and 1 mol % or less, or 0.1 mol % or more and 1 mol % or less, relative to the total amount of copper contained in the copper oxide as the raw material.

[0092]The average particle diameter of the copper powder obtained by the method 1 for producing copper powder according to the present embodiment is preferably 250 nm or less. The copper powder (polyol copper powder) obtained by the production method described above is controlled so as to have a particle diameter of equal to or less than the average particle diameter by using the chain organic substance in the reduction reaction. The copper powder can be used for an electronic component, such as an electrode of a multilayer ceramic capacitor, since the copper powder is fine. For example, by the method 1 for producing copper powder, copper powder having an average particle diameter of 200 nm or less, 150 nm or less, 110 nm or less, 100 nm or less, 90 nm or less, 70 nm or less, or 60 nm or less, as described in Examples, can be obtained.

[0093]As described above, the method 1 for producing copper powder of the present embodiment is a method for producing copper powder in which copper oxide powder is mixed with the polyol solvent having a boiling point of 230° C. or lower to which the chain organic substance is added, to obtain copper oxide powder slurry and the copper oxide powder slurry is heated at 230° C. or lower. The method 1 for producing copper powder of the present embodiment may be a method capable of producing the copper powder of the present embodiment described above. In the method 1 for producing copper powder of the present embodiment, a configuration other than the above-described configuration is any configuration. With the method 1 for producing copper powder of the present embodiment, copper powder having an organic film for preventing the formation of an oxide film that may inhibit sintering, and having excellent low-temperature sinterability can be obtained. In the method 1 of the present embodiment, a substance other than the above-described substances may be added to the reaction liquid without departing from the spirit of the present invention.

(Method 2 for Producing Copper Powder)

[0094]A method for producing copper powder according to another aspect of the present embodiment includes a heat-treating process in which copper powder having an average particle diameter of 250 nm or less and not satisfying at least any of the conditions (3) and (4) of the copper powder in the above-described present embodiment is dispersed in a polyol solvent having a boiling point of 250° C. or higher to which the chain organic substance is added (hereinafter, the polyol solvent having a boiling point of 250° C. or higher is also referred to as polyol solvent B) (hereinafter, a liquid in which the copper powder is mixed and suspended in this polyol solvent B is collectively referred to as “treatment liquid”), and heated to obtain target copper powder. The heat-treating process enables to modify the organic substance on the surface of the copper powder, which includes applying the chain organic compound added to the surface of the copper powder. Thus, the copper powder of the present embodiment can be obtained. For example, by the method 2 for producing copper powder, copper powder having an average particle diameter of 200 nm or less, 150 nm or less, 110 nm or less, 100 nm or less, 90 nm or less, 70 nm or less, or 60 nm or less, as described in Examples, can be obtained.

[0095]The copper powder having an average particle diameter of 250 nm or less and not satisfying at least any of the conditions (3) and (4) of the copper powder in the above-described present embodiment is, for example, copper powder produced by a method for producing copper powder described in Japanese Unexamined Patent Application Publication No. 2022-128445. The copper powder is not, of course, limited to this copper powder, and may be copper powder produced by another wet method or gas phase method as long as it is copper powder not satisfying at least any of the conditions (3) and (4) of the copper powder of the above-described present embodiment.

[0096]The polyol solvent B used in the heat-treating process is a polyol having a boiling point of 250° C. or higher, and is more preferably one or more selected from the group consisting of triethylene glycol (boiling point: 287° C.), tetraethylene glycol (boiling point: 327° C.), and polyethylene glycol having an average molecular weight of 200 or more and 600 or less (boiling point: approximately 300° C.). The amount of the chain organic substance added is preferably 10% by mass or less relative to the total amount of copper contained in the treated copper powder. The amount of the chain organic substance added is preferably 0.005% by mass or more relative to the total amount of copper contained in the treated copper powder. The amount of the chain organic substance added is preferably 0.005% by mass or more and 10% by mass or less relative to the total amount of copper contained in the treated copper powder.

[0097]The heat-treating temperature of the heat-treating process is preferably 230° C. or higher. The heat-treating temperature can be set, for example, similarly to conditions and the like described in Examples, and may be 240° C. or higher, 250° C. or higher, 260° C. or higher, or 270° C. or higher. The heat-treating temperature is preferably equal to or lower than a temperature that is lower than the boiling point of the polyol solvent B by #0° C., and more preferably equal to or lower than a temperature that is lower than the boiling point of the polyol solvent B by 5° C. The heat-treating temperature is preferably 230° C. or higher and equal to or lower than a temperature that is lower than the boiling point of the polyol solvent B by ±0° C., more preferably 250° C. or higher and equal to or lower than a temperature that is lower than the boiling point of the polyol solvent B by 5° C., and further preferably 270° C. or higher and equal to or lower than a temperature that is lower than the boiling point of the polyol solvent B by 5° C. When the heat-treating temperature is lower than 230° C., the substitution of the organic substance on the surface of the copper powder is not sufficiently promoted, and a target organic film is not obtained. Accordingly, excellent oxidation resistance and low-temperature sinterability cannot be imparted to the copper powder. When the heat-treating temperature is made higher than the boiling point of the polyol solvent B, the polyol solvent is significantly decreased (consumed) due to decomposition and volatilization, and the heat-treating cannot be sufficiently achieved. The time for the heat-treating is preferably 1 hour or less, more preferably 30 minutes or less, and further preferably 15 minutes or less.

[0098]In the heat-treating process, the copper powder not satisfying at least any of the conditions (3) and (4) of the copper powder in the above-described present embodiment is heated in the polyol solvent B to which the chain organic substance is added to obtain the copper powder according to the present embodiment. In the heat-treating process, the heating is initiated after the chain organic substance is added to the treatment liquid. Herein, the chain organic substance may be post-added to the reaction liquid during an increase in temperature.

[0099]When the chain organic substance that does not become neutral (pH: approximately 6.5 to 7.5) in an aqueous solution form is used, it is preferable that an alkali or an acid be added together as a neutralizer to neutralize the chain organic substance added to the treatment liquid. When an alkali is used as the neutralizer, a hydroxide of an alkali metal is preferred. Specifically, sodium hydroxide or potassium hydroxide can be used. When an acid is used as the neutralizer, an inorganic acid is preferred. Specifically, sulfuric acid or hydrochloric acid can be used. When the neutralizer is added to the reaction liquid, an aqueous solution of the neutralizer may be formed in advance and added.

[0100]The chain organic substance in the treatment liquid also functions as a coupling inhibitor for copper powder. The amount of the chain organic substance in the treatment liquid is set according to conditions, such as the particle diameter of the treated copper powder. The amount of the chain organic substance in the treatment liquid can be set by a pilot study. The amount of the chain organic substance in the treatment liquid is preferably 10% by mass or less relative to the total amount of copper contained in the treated copper powder. Further, the amount of the chain organic substance in the treatment liquid (the amount of the chain organic substance added to the polyol solvent B) is preferably 0.005% by mass or more relative to the total amount of copper contained in the treated copper powder. The amount of the chain organic substance in the treatment liquid is preferably 0.005% by mass or more and 10% by mass or less relative to the total amount of copper contained in the treated copper powder. The addition of the chain organic substance in an amount of less than 0.005% by mass relative to the total amount of copper contained in the treated copper powder may be insufficient to exert an effect of the coupling inhibitor for copper powder. Since an effect of the addition of the chain organic substance is decreased by an increase in addition amount, the addition of the chain organic substance in an amount of more than 10% by mass relative to the total amount of copper contained in the treated copper powder increases a production cost and thus is not cost-effective. The amount of the chain organic substance in the treatment liquid (the amount of the chain organic substance added to the polyol solvent B) is preferably 0.001 mol % or more, and may be, for example, 0.01 mol % or more, or 0.1 mol % or more, relative to the total amount of copper contained in the treated copper powder. The amount of the chain organic substance in the treatment liquid is preferably 1 mol % or less relative to the total amount of copper contained in the treated copper powder. The amount of the chain organic substance in the treatment liquid is preferably 0.001 mol % or more and 1 mol or less, and may be 0.01 mol % or more and 1 mol % or less, or 0.1 mol % or more and 1 mol % or less, relative to the total amount of copper contained in the treated copper powder.

[0101]As described above, the method 2 for producing copper powder of the present embodiment is a method for producing copper powder including: mixing copper powder having an average particle diameter of 250 nm or less and not satisfying at least any of the conditions (3) and (4) of the copper powder of the above-described embodiment with a polyol solvent having a boiling point of 250° C. or higher to which the chain organic substance is added, to obtain copper powder slurry, and heat-treating the copper powder slurry at 230° C. or higher. The method 2 for producing copper powder of the present embodiment may be a method capable of producing the copper powder of the present embodiment described above. In the method 2 for producing copper powder of the present embodiment, a configuration other than the above-described configuration is any configuration. With the method 2 for producing copper powder of the present embodiment, copper powder having an organic film for preventing the formation of an oxide film that may inhibit sintering, and having excellent low-temperature sinterability can be obtained. In the method 2 of the present embodiment, a substance other than the above-described substances may be added to the reaction liquid without departing from the spirit of the present invention.

EXAMPLES

[0102]Hereinafter, Examples of the present invention are shown with Comparative Examples for specific description, but the present invention is not limited to the following Examples. In the following description, a polyol solvent used in a reduction process is referred to as a first polyol solvent, and a polyol solvent used in a heat-treating process is referred to as a second polyol solvent. Method for measuring physical values are as follows.

(1) Average Particle Diameter

[0103]The average particle diameter of resultant copper powder (polyol copper powder) is a number average particle diameter determined from an image analysis in which a subject is 200 or more particles observed with a scanning electron microscope (SEM).

(2) Analysis of Organic Substance on Surface of Copper Powder

[0104]For an organic component on the surface of copper powder, 10 mg of copper powder was immersed in 70 μL of 1 wt % solution of tetramethylammonium hydroxide in methanol to extract an organic substance on the surface of the copper powder, the organic substance was dried under conditions of 50° C. and 10 minutes, and the organic component was then gasified under conditions of 300° C. and 30 seconds with a thermal decomposition furnace (PY-3030D manufactured by Frontier Laboratories Ltd.) and detected within the range of m/z of 33 to 550 by GC-MS (manufactured by Shimadzu Corporation, GC section: GC-2010Plus, MS section: QP-2010Ultra).

<Gas Chromatography Conditions>

[0110]Concurrently with this operation, 0.5 g of copper powder was immersed in 10 mL of 0.5 N sodium hydroxide aqueous solution to extract an organic substance on the surface of the copper powder, and a supernatant liquid after centrifugation was detected as an evaluation sample by LC-MS (manufactured by Agilent Technologies, Inc., LC section: Agilent 1290 infinity2, MS section: Agilent 6530). Using the data obtained above, the qualitative analysis of the organic component was performed.

<Liquid Chromatography Conditions>

<Mass Spectrometry Conditions (for LC)>

(3) Measurement of Thermal Shrinkage Behavior (TMA)

Example 1

[Reduction Process]

[0129]3.6 g of cuprous oxide (Cu₂O) powder (manufactured by American Chemet Corporation, product number: Ultrafine) as copper oxide powder was placed in a 50-mL tall beaker, and 13.5 g of propylene glycol (abbreviation: PG, boiling point: 188° C., molecular weight: 76) was added as the first polyol solvent, and 0.302 g (9.44% by mass (1 mol %) relative to the total amount of copper in cuprous oxide) of poly(ethylene glycol)bis(carboxymethyl) ether (average molecular weight: 600) (abbreviation: PEG600 dibasic acid, manufactured by Sigma-Aldrich) and 162 μL of 25 w/v % sodium hydroxide aqueous solution for neutralization were then added and mixed to obtain uniform slurry. This slurry was heated to 185° C., and then kept at this temperature with stirring for 45 minutes to perform a reduction reaction. The reaction liquid was cooled, and the produced polyol copper powder was then centrifuged, washed, and dried.

Example 2

[0130]Polyol copper powder was obtained by the same operation as in Example 1 except that the first polyol solvent was changed to 1,3-propanediol (abbreviation: 1,3-PDO, boiling point: 214° C., molecular weight: 76).

Example 3

[0131]Polyol copper powder was obtained by the same operation as in Example 1 except that the first polyol solvent was changed to 1,2-butanediol (abbreviation: 1,2-BDO, boiling point: 194° C., molecular weight: 90).

Example 4

[0132]Polyol copper powder was obtained by the same operation as in Example 1 except that the first polyol solvent was changed to 1,3-butanediol (abbreviation: 1,3-BDO, boiling point: 207° C., molecular weight: 90).

Example 5

[0133]Polyol copper powder was obtained by the same operation as in Example 3 except that the amount of PEG600 dibasic acid added was changed to 0.0302 g (0.94% by mass (0.1 mol %) relative to the total amount of copper in cuprous oxide) and 16 μL of a 25 w/v % sodium hydroxide aqueous solution for neutralization was added.

Example 6

[0134]Polyol copper powder was obtained by the same operation as in Example 3 except that the amount of PEG600 dibasic acid added was changed to 0.0030 g (0.094% by mass (0.01 mol %) relative to the total amount of copper in cuprous oxide) and 16 μL of a 25 w/v % sodium hydroxide aqueous solution for neutralization was added.

Example 7

[0135]Polyol copper powder was obtained by the same operation as in Example 3 except that the amount of PEG600 dibasic acid added was changed to 0.0003 g (0.0094% by mass (0.001 mol %) relative to the total amount of copper in cuprous oxide) and 16 μL of a 25 w/v % sodium hydroxide aqueous solution for neutralization was added.

Example 8

[Reduction Process]

[0136]Copper powder was produced by a method described in Example 5 of Japanese Unexamined Patent Application Publication No. 2022-128445. Specifically, 27 g of cuprous oxide powder as copper oxide powder was placed in a 200-mL separable flask, and 100 g of ethylene glycol (abbreviation: EG, boiling point: 197° C., molecular weight: 62) was added as the first polyol solvent, and 0.65 g (2.71% by mass (1 mol %) relative to the total amount of copper in cuprous oxide) of cis-1,2-cyclohexanedicarboxylic acid and 1.21 mL of a 25 w/v % sodium hydroxide aqueous solution for neutralization were then added and mixed to obtain uniform slurry. This slurry was heated to 190° C., and then kept at this temperature with stirring for 45 minutes to perform a reduction reaction. The reaction liquid was cooled, and the produced polyol copper powder was then centrifuged, washed, and dried.

[Heat-Treating Process]

[0137]From the product obtained by centrifuging the produced polyol copper powder, 3.2 g of copper powder was separated and then dispersed again in 13.5 g of tetraethylene glycol (abbreviation: TeEG, boiling point: 327° C., molecular weight: 194) as the second polyol solvent. After that, 0.302 g (9.44% by mass (1 mol %) relative to the total amount of copper in the treated copper) of poly(ethylene glycol)bis(carboxymethyl) ether (average molecular weight: 600) (PEG600 dibasic acid) and 162 μL of a 25 w/v % sodium hydroxide aqueous solution for neutralization were then added and mixed to obtain uniform slurry. This slurry was heated to 280° C., and then kept at this temperature with stirring for 10 minutes to perform heat-treating. The reaction liquid was cooled, and the produced polyol copper powder after the heat-treating was then centrifuged, washed, and dried.

Example 9

[0138]Polyol copper powder after heat-treating was obtained by the same operation as in Example 8 except that the temperature of the reduction reaction was changed to 180° C.

Example 10

[Reduction Process]

[0139]Copper powder was produced by a method described in Example 24 of Japanese Unexamined Patent Application Publication No. 2022-128445. Specifically, 27 g of cuprous oxide powder as copper oxide powder was placed in a 200-mL separable flask, and 100 g of propylene glycol (abbreviation: PG, boiling point: 188° C., molecular weight: 76) was added as the first polyol solvent, and 0.65 g (2.71% by mass (1 mol %) relative to the total amount of copper in cuprous oxide) of cis-1,2-cyclohexanedicarboxylic acid and 1.21 mL of a 25 w/v % sodium hydroxide aqueous solution for neutralization were then added and mixed to obtain uniform slurry. This slurry was heated to 185° C., and then kept at this temperature with stirring for 45 minutes to perform a reduction reaction. The reaction liquid was cooled, and the produced polyol copper powder was then centrifuged, washed, and dried.

[Heat-Treating Process]

[0140]From the product obtained by centrifuging the produced polyol copper powder, 3.2 g of copper powder was separated and then dispersed again in 13.5 g of tetraethylene glycol as the second polyol solvent. After that, 0.302 g (9.44% by mass (1 mols) relative to the total amount of copper in the treated copper) of poly(ethylene glycol)bis(carboxymethyl) ether (average molecular weight: 600) (PEG600 dibasic acid) and 162 μL of a 25 w/v % sodium hydroxide aqueous solution for neutralization were then added and mixed to obtain uniform slurry. This slurry was heated to 280° C., and then kept at this temperature with stirring for 10 minutes to perform heat-treating. The reaction liquid was cooled, and the produced polyol copper powder after the heat-treating was then centrifuged, washed, and dried.

Example 11

[0141]Polyol copper powder was obtained by the same operation as in Example 10 except that in the heat-treating process, the amount of PEG600 dibasic acid added was changed to 0.0302 g (0.94% by mass (0.1 mol %) relative to the total amount of copper in the treated copper) and 16 μL of a 25 w/v % sodium hydroxide aqueous solution for neutralization was added.

Example 12

[0142]Polyol copper powder was obtained by the same operation as in Example 10 except that in the heat-treating process, the amount of PEG600 dibasic acid added was changed to 0.0030 g (0.094% by mass (0.01 mol %) relative to the total amount of copper in the treated copper) and 16 μL of a 25 w/v % sodium hydroxide aqueous solution for neutralization was added.

Example 13

[0143]Polyol copper powder was obtained by the same operation as in Example 10 except that in the heat-treating process, the amount of PEG600 dibasic acid added was changed to 0.0003 g (0.0094% by mass (0.001 mol %) relative to the total amount of copper in the treated copper) and 16 μl of a 25 w/v % sodium hydroxide aqueous solution for neutralization was added.

Example 14

[0144]Polyol copper powder after heat-treating was obtained by the same operation as in Example 10 except that in the heat-treating process, the heating temperature of the slurry was changed to 260° C.

Example 15

[0145]Polyol copper powder after heat-treating was obtained by the same operation as in Example 10 except that in the heat-treating process, the heating temperature of the slurry was changed to 240° C.

Example 16

[0146]Polyol copper powder after heat-treating was obtained by the same operation as in Example 10 except that in the heat-treating process, the heating temperature of the slurry was changed to 230° C.

Example 17

[0147]Polyol copper powder after heat-treating was obtained by the same operation as in Example 10 except that, in the heat-treating process, 0.1724 g (5.39% by mass (1 mol %) relative to the total amount of copper in the treated copper) of eicosanedioic acid was added instead of poly(ethylene glycol)bis(carboxymethyl) ether (average molecular weight: 600) (PEG600 dibasic acid).

Example 18

[0148]Polyol copper powder after heat-treating was obtained by the same operation as in Example 10 except that, in the heat-treating process, 0.1018 g (3.18% by mass (1 mol %) relative to the total amount of copper in the treated copper) of sebacic acid was added instead of poly(ethylene glycol)bis(carboxymethyl) ether (average molecular weight: 600) (PEG600 dibasic acid).

Example 19

[0149]Polyol copper powder after heat-treating was obtained by the same operation as in Example 10 except that, in the heat-treating process, 0.1159 g (3.63% by mass (1 mol %) relative to the total amount of copper in the treated copper) of dodecanedioic acid was added instead of molecular weight: 600) (PEG600 dibasic acid).

Comparative Example 1

[Reduction Process]

[0150]30 g of copper oxide (CuO) powder (manufactured by Furukawa Chemicals Co., Ltd., product number: FCO-M6) as copper oxide powder was placed in a 200-mL separable flask, and 100 g of tetraethylene glycol was added as the first polyol solvent and then mixed to obtain uniform slurry. This slurry was heated to 300° C., and then kept at this temperature with stirring for 45 minutes to perform a reduction reaction. The reaction liquid was cooled, and the produced polyol copper powder was then centrifuged, washed, and dried.

Comparative Example 2

[0151]Polyol copper powder was obtained by the same operation as in Example 2 except that in the reduction process, PEG600 dibasic acid and a 25 w/v % sodium hydroxide aqueous solution for neutralization were not added.

Comparative Example 3

[0152]Polyol copper powder was obtained by the same operation as in Example 3 except that in the reduction process, PEG600 dibasic acid and a 25 w/v % sodium hydroxide aqueous solution for neutralization were not added.

Comparative Example 4

[0153]Polyol copper powder was obtained by the same operation as in Example 4 except that PEG600 dibasic acid and 25 w/v % sodium hydroxide solution for neutralization were not added in the reduction process.

Conventional Example 1

[Reduction Process]

[0154]Copper powder was produced by the method described in Example 5 of Japanese Unexamined Patent Application Publication No. 2022-128445. The reaction liquid was cooled, and the produced polyol copper powder was then centrifuged, washed, and dried. This polyol copper powder was not subjected to heat-treating, unlike Example 8.

Conventional Example 2

[0155]Polyol copper powder was obtained by the same operation as in Conventional Example 1 except that the temperature of the reduction reaction was changed to 180° C. This polyol copper powder was not subjected to heat-treating, unlike Example 9.

Conventional Example 3

[Reduction Process]

[0156]Copper powder was produced by the method described in Example 24 of Japanese Unexamined Patent Application Publication No. 2022-128445. The reaction liquid was cooled, and the produced polyol copper powder was then centrifuged, washed, and dried. This polyol copper powder was not subjected to heat-treating, unlike Examples 10 to 19.

Comparative Example 5

[0157]Polyol copper powder after heat-treating was obtained by the same operation as in Example 10 except that in the heat-treating process, the heating temperature of the slurry was changed to 200° C.

Comparative Example 6

[0158]Polyol copper powder was obtained by the same operation as in Example 10 except that in the heat-treating process, PEG600 dibasic acid and a 25 w/v % sodium hydroxide aqueous solution were not added.

[0159]Production conditions in Examples 1 to 19, Comparative Examples 1 to 6, and Conventional Examples 1 to 3 described above are listed in Table 1.

(Evaluation Results)

[0160]For the polyol copper powders obtained in Examples 1 to 19, Comparative Examples 1 to 6, and Conventional Examples 1 to 3, the average particle diameter, detection of an organic substance on the surface of the copper powder by GC-MS and LC-MS, the temperature at which the thermal shrinkage ratio of the green compact of each of the polyol copper powders obtained in an inert atmosphere by TMA measurement was 1% (hereinafter also abbreviated as TMA 1%), and the temperature difference (absolute value) between the temperature at which the thermal shrinkage ratio of the green compact in an inert atmosphere was 3% (hereinafter also abbreviated as TMA 3%) and the temperature at which the thermal shrinkage ratio of the green compact in a reducing atmosphere was 3% were determined by the above-described methods. The results are listed in Table 2. In Table 2, “not detected” means detection limit or less. The results of GC-MS in Example 17 and Conventional Example 3 are illustrated in FIG. 2.

[0161]In all Examples, an organic substance satisfying the conditions (1) and (2) described above was detected by GC-MS and LC-MS. In all Examples, the temperature at TMA 1% in an inert atmosphere was 230° C. or lower, and the temperature difference between the temperature at TMA 3% in the inert atmosphere and the temperature at TMA 3% in the reducing atmosphere was smaller than 10° C. For example, as indicated in a temperature-TMA profile (FIG. 1) in Example 10, a sintering (thermal shrinkage) behavior in the inert atmosphere and the reducing atmosphere is hardly changed, and this shows that an oxide film that inhibits sintering is not formed on the surface of the copper powder or the oxide film is so thin that sintering is not inhibited.

[0162]On the other hand, in all Comparative Examples and Conventional Examples, an organic substance satisfying both the conditions (1) and (2) described above was not detected by GC-MS and LC-MS. In Conventional Examples 1 to 3, the temperature at TMA 1% in the inert atmosphere was higher than 230° C., and in all Comparative Examples and Conventional Examples, the temperature difference between the temperature at TMA 3% in the inert atmosphere and the temperature at TMA 38 in the reducing atmosphere was 10° C. or higher.

[0163]In Comparative Example 1, the surface of the copper powder was covered with an organic substance satisfying the above-described condition (1) that can be detectable by GC-MS, but an organic substance satisfying the above-described condition (2) that can be detectable by LC-MS was not present on the surface of the copper powder. It is considered that the surface is insufficiently covered with the organic substance as compared with Examples 1 to 4, and the oxide film that inhibits sintering is easily formed on the surface of the copper powder. Accordingly, it is considered that sinterability in the inert atmosphere is deteriorated and the sintering behavior in the inert atmosphere differs from that in the reducing atmosphere. (The above-described condition (4) is not satisfied.)

[0164]In Comparative Examples 2 to 5 and Conventional Examples 1 to 3, the organic substance detected by GC-MS was not an organic substance satisfying the above-described condition (1). It is therefore considered that the organic substance on the surface of the copper powder inhibits sintering and does not satisfy the above-described condition (3). Further, the organic substance satisfying the above-described condition (2) that can be detectable by LC-MS was not present on the surface of the copper powder. Accordingly, it is considered that the oxide film that inhibits sintering is easily formed on the surface of the copper powder, sinterability in the inert atmosphere is deteriorated, and the sintering behavior in the inert atmosphere differs from that in the reducing atmosphere. (The above-described condition (4) is not satisfied.)

[0165]In Comparative Example 6, the copper powder to be treated during the heat-treating was aggregated to form a large clump, and hence in the subsequent analysis, measurement was impossible. This is considered to be because the chain organic substance functioning as a coupling inhibitor for copper powder is not added.

[0166]Because in Examples 8 to 19, heat-treating was added to Conventional Examples 1 to 3, it is considered that the organic substance satisfying the above-described conditions (1) and (2) by GC-MS and LC-MS is reformed on the surface of the copper powder by the heat-treating, and therefore the copper powder satisfying the above-described conditions (3) and (4) is obtained.

[0167]As confirmed from Examples, Comparative Examples, and Conventional Examples, the copper powder of the present embodiment is copper powder having an average particle diameter of 250 nm or less and having a surface covered with both the specific organic substance detected by GC-MS and the specific organic substance detected by LC-MS, in measurement of a thermal shrinkage ratio when a green compact obtained by pressure-molding the copper powder at 100 MPa is heated from 25° C. at a temperature increasing rate of 10° C./min in an inert atmosphere on the basis of the thickness of the green compact at 25° C., the temperature at which the thermal shrinkage ratio is 18 is 230° C. or lower, and in measurement of a thermal shrinkage ratio when a green compact obtained by pressure-molding the copper powder at 100 MPa is heated from 25° C. at a temperature increasing rate of 10° C./min in an inert atmosphere and in a reducing atmosphere on the basis of the thickness of the green compact at 25° C., the temperature difference between the temperature at which the thermal shrinkage ratio in the inert atmosphere is 3% and the temperature at which the thermal shrinkage ratio in the reducing atmosphere is 3% is smaller than 10° C.

[0168]The technical range of the present invention is not limited to the aspects described in the above-described embodiments and the like. One or more of the requirements described in the above-described embodiments and the like may be omitted. The requirements described in the above-described embodiments and the like can be appropriately combined. The contents of all the references cited in Japanese Patent Application No. 2022-030202 and the above-described embodiments and the like are incorporated herein as long as they are permitted under law.