Oxide-containing copper particles and method for producing same, sintering material containing said oxide-containing copper particles, joined body in which said sintering material is used, and method for producing fired product using said sintering material

Abstract

Oxide-containing copper particles according to the present invention are coated with a carboxylic acid, said oxide-containing copper particles including: copper particles; an oxide coating that is composed of one or more substances selected from the group consisting of Cu2O, Cu8O, and Cu64O and is disposed on the surface of the copper particles; and Cu64O particles disposed on at least a part of the surface of the oxide coating. The ratio of the total contained amount of Cu2O, Cu8O, and Cu64O relative to the total contained amount of Cu, Cu2O, Cu8O, and Cu64O is 0.1-10 mass%, and the grain diameter of the Cu64O particles is 1-20 nm.

Description

Oxide-containing copper particles and method for producing the same, sintering material containing the oxide-containing copper particles, bonded body using the sintering material, and method for producing fired product using the sintering material

[0001] The present disclosure relates to oxide-containing copper particles and a method for producing the same, a sintering material containing the oxide-containing copper particles, a joined body using the sintering material, and a method for producing a fired product using the sintering material.

[0002] In recent years, printable electronics, a technology that directly forms fine wiring using inkjet or printing methods, has attracted attention in the manufacture of printed circuit boards. This technology eliminates the need for conventional exposure and etching processes and does not emit harmful chemicals. Solder has traditionally been used to bond components, such as between substrates and components in semiconductor devices and between circuits. High-melting-point lead solder has been used as a bonding material for power semiconductors and LSIs that operate at temperatures above 150°C. In contrast, with the recent emergence of silicon carbide (SiC)-based power semiconductors, the operating temperature of semiconductors has risen to 250-300°C, close to the melting point of high-melting-point lead solder, making ensuring connection reliability and heat dissipation design significant challenges. Furthermore, with the tightening of RoHS regulations, lead-free bonding materials are in demand. For example, techniques that involve low-temperature sintering of silver nanoparticles to form a sintered silver layer have been proposed as lead-free bonding materials.

[0003] As a bonding material using silver, Patent Document 1 proposes a silver microparticle composition that can obtain high adhesive strength at low bonding temperatures. However, silver microparticles are expensive and there are concerns about the influence of wiring short circuits due to ion migration. As an alternative to silver microparticles, copper microparticles, which are inexpensive and highly resistant to ion migration, have attracted attention. However, copper microparticles have the disadvantage of being difficult to sinter at low temperatures.

[0004] To address the above drawbacks, the present inventors have attempted to obtain particles suitable for low-temperature sintering using copper oxides. ２ O, Cu ４ O ３ , Cu ８ O and Cu６４ Five types of Cu are known. ６４ O and Cu ８ O (especially Cu ６４ O) is a stable phase of CuO and Cu ２ Compared to O, the proportion of oxygen atoms is smaller, the stability is low, and it can be reduced to metallic copper with low energy. ６４ O and / or Cu ８ O (especially Cu ６４ By using O), it is thought that low-temperature sintering at 250°C or less can be achieved.

[0005] For example, Patent Document 2 discloses Cu ６４ O and Cu as needed ２ O and coated with a carboxylic acid, ６４ O and Cu ２ Cu relative to the total mass of O ６４ Oxide-containing copper fine particles having an O mass ratio of 0.5 to 2.0 mass % are disclosed.

[0006] In recent years, electrical components have been required to have better electrical conductivity. In addition to the submicron oxide-containing copper particles described in Patent Document 2, which have improved surface necking characteristics due to a carboxylic acid coating, the following Patent Documents 3 and 4 are cited as conductive material technologies that utilize a nano-sized melting point depression. Patent Document 3 describes Cu ６４ O particles and Cu ８ O particles, and the Cu ６４ O particles and the Cu ８ Patent Document 4 discloses a copper oxide particle composition containing copper clusters having an average particle size of 0.1 nm or more and 1 nm or less, and copper oxide particles having an average particle size of more than 1 nm and 20 nm or less, and the copper oxide particles are Cu ６４ O particles and the Cu ８or copper clusters having an average particle size of 0.1 nm or more and 1 nm or less, copper oxide particles having an average particle size of more than 1 nm and 20 nm or less, and metallic copper particles having an average particle size of more than 20 nm and 1 μm or less, wherein the copper oxide particles are Cu ６４ O particles and the Cu ８ Mixed particles are shown, at least one of which is O particles.

[0007] International Publication No. 2016 / 166948 International Publication No. 2022 / 045252 Japanese Patent Application Laid-Open No. 2020-29392 Japanese Patent Application Laid-Open No. 2020-100893

[0008] In Patent Documents 3 and 4, copper salts such as copper acetate are used as starting materials to produce the copper oxide particles contained in the mixed particles. However, copper salts are bulky and heavy, and the reaction process, including dissolving the copper salt in a solvent, requires a long time. Furthermore, although the concentration of copper ions is high at the beginning of the reduction, the concentration changes during the reaction, making it difficult to control the reaction and the particle size obtained, especially in a concentrated system. Therefore, further investigation is considered necessary for industrial mass production. Furthermore, the obtained mixed particles are composed of metallic copper particles and fine copper oxide particles (Cu ６４ O particles and Cu ８ The present disclosure has been made in view of the above circumstances, and one of its objects is to provide oxide-containing copper particles that can be produced more efficiently than conventional particles and can be sufficiently sintered even at low temperatures, a method for producing the same, a sintering material containing the oxide-containing copper particles, a bonded body using the sintering material, and a method for producing a sintered product using the sintering material.

[0009] Aspect 1 of the present invention is an oxide-containing copper particle coated with a carboxylic acid, comprising copper particles and Cu ２ O, Cu ８ O and Cu ６４ and an oxide film comprising at least one selected from the group consisting of Cu and O, and Cu disposed on at least a portion of the surface of the oxide film. ６４ O particles, Cu, Cu ２O, Cu ８ O and Cu ６４ Cu relative to the total content of O ２ O, Cu ８ O and Cu ６４ The ratio of the total content of O is 0.1 mass % or more and 10 mass % or less, and the Cu ６４ The copper particles are oxide-containing particles, and the equivalent circle diameter of the O particles is 1 nm or more and 20 nm or less.

[0010] Aspect 2 of the present invention is Cu, Cu ２ O, Cu ８ O and Cu ６４ Cu relative to the total content of O ６４ The oxide-containing copper particles according to aspect 1, wherein the ratio of the O content is 0.1% by mass or more and 5% by mass or less.

[0011] A third aspect of the present invention is the oxide-containing copper particle according to the first or second aspect, wherein the average thickness of the oxide film is 0.1 nm or more and 10 nm or less.

[0012] A fourth aspect of the present invention is a material for sintering, comprising the oxide-containing copper particles according to any one of the first to third aspects.

[0013] A fifth aspect of the present invention is a method for producing oxide-containing copper particles according to any one of the first to third aspects, comprising: a step of mixing CuO particles with a solution containing a carboxylic acid and a complexing agent to obtain a mixture; and a step of adding a reducing agent to the mixture and reacting the mixture in an oxygen-containing atmosphere.

[0014] A sixth aspect of the present invention is the method according to the fifth aspect, wherein the complexing agent is an amine compound.

[0015] A seventh aspect of the present invention is the production method according to the fifth or sixth aspect, further comprising a step of heating the mixture to a temperature of 40° C. or higher and 60° C. or lower after the mixing step and before adding the reducing agent.

[0016] Aspect 8 of the present invention is the production method according to any one of Aspects 5 to 7, wherein the reaction is carried out by heating to a temperature higher than 60°C and not higher than 80°C.

[0017] A ninth aspect of the present invention is the material for sintering according to the fourth aspect, which is a paste.

[0018] A tenth aspect of the present invention is a joined body formed by joining a plurality of materials with the sintering material according to the fourth or ninth aspect.

[0019] An eleventh aspect of the present invention is a method for producing a sintered product, comprising the steps of: preparing the material for sintering according to the fourth or ninth aspect; and sintering the material for sintering at a temperature of 250°C or less.

[0020] According to the present disclosure, it is possible to provide oxide-containing copper particles that have improved particle productivity compared to conventional methods and can be sufficiently sintered even at low temperatures, a method for producing the same, a sintering material containing the oxide-containing copper particles, a bonded body using the sintering material, and a method for producing a fired product using the sintering material.

[0021] FIG. 1 is an XRD pattern of oxide-containing copper microparticles of an example. FIG. 2 is an STEM image of oxide-containing copper microparticles of an example. FIG. 3 is a highly detailed STEM image of oxide-containing copper microparticles of an example. FIG. 4 is a TG-DTA measurement result of an example. FIG. 5 is an STEM image of oxide-containing copper microparticles of a comparative example. FIG. 6 is a highly detailed STEM image of oxide-containing copper microparticles of a comparative example. FIG. 7 is a TG-DTA measurement result of a comparative example. FIG. 8 is a diagram showing the procedure for producing a sample for bonding and sintering evaluation. FIG. 9A is a schematic diagram of a copper test piece used to produce a sample for bonding and sintering evaluation. FIG. 9B is a schematic diagram of a metal mask used to produce a sample for bonding and sintering evaluation. FIG. 9C is a schematic diagram illustrating hot pressing in a bonding and sintering evaluation test. FIG. 9D is a schematic cross-sectional view illustrating a method for the bonding and sintering evaluation test. FIG. 10A shows the temperature rise and fall profile (fired at a maximum of 200°C) when preparing a sample for bonding and firing evaluation of an example. FIG. 10B shows the temperature rise and fall profile (fired at a maximum of 200°C) when preparing a sample for bonding and firing evaluation of a comparative example. FIG. 10C shows the temperature rise and fall profile (fired at a maximum of 150°C) when preparing a sample for bonding and firing evaluation of an example. FIG. 11 shows the relationship between paste concentration and adhesive strength of an example. FIG. 12 is an SEM image of a fracture surface after a bonding and firing evaluation test of an example (fired at a maximum of 150°C). FIG. 13 is an SEM image of a fracture surface after a bonding and firing evaluation test of an example (fired at a maximum of 200°C). FIG. 14A is a schematic diagram of a sintering material containing a relatively large amount of solvent. FIG. 14B is a schematic diagram of a sintering material with the solvent reduced from FIG. 14A. FIG. 14C is a schematic diagram of a sintering material with the solvent reduced from FIG. 14B.

[0022] The present inventors conducted extensive research to realize oxide-containing copper particles that offer improved particle productivity and can be sufficiently sintered even at low temperatures. As a result, they discovered a method in which CuO particles are mixed with a solution containing a carboxylic acid and a complexing agent, followed by the addition of a reducing agent and reaction in an oxygen-containing atmosphere. This method uses CuO particles that are not copper salts as a starting material, which does not require a long reaction time and does not make it difficult to control the reaction and the resulting particle size in a concentrated system, resulting in higher particle productivity than conventional methods. Furthermore, this method has made it possible to realize oxide-containing copper particles that can be sufficiently sintered even at low temperatures. The oxide-containing copper particles are oxide-containing copper particles coated with a carboxylic acid, and further comprise copper particles and CuO particles arranged on the surfaces of the copper particles. ２ O, Cu ８ O and Cu ６４ and an oxide film comprising at least one selected from the group consisting of Cu and O, and Cu disposed on at least a portion of the surface of the oxide film. ６４ O particles, and the oxide content and Cu ６４ The oxide-containing copper particles have fine Cu particles on the surface of the copper particles, which are different from conventional copper particles. ６４ Because O particles are arranged, Cu ６４ The aggregation of O particles is suppressed, and the Cu ６４ Sufficient sintering can be achieved even at low temperatures due to O particles, etc. First, the oxide-containing copper particles according to this embodiment will be described in detail below.

[0023] [Oxide-containing copper particles] The oxide-containing copper particles according to this embodiment are oxide-containing copper particles coated with a carboxylic acid, and the oxide-containing copper particles comprise copper particles and Cu particles disposed on the surfaces of the copper particles. ２ O, Cu ８ O and Cu ６４ and an oxide film comprising at least one selected from the group consisting of Cu and O, and Cu disposed on at least a portion of the surface of the oxide film. ６４ O particles, Cu, Cu ２ O, Cu ８ O and Cu６４ Cu relative to the total content of O ２ O, Cu ８ O and Cu ６４ The ratio of the total content of O is 0.1 mass % or more and 10 mass % or less, and the Cu ６４ The equivalent circle diameter of the O particles is 1 nm or more and 20 nm or less, thereby obtaining oxide-containing copper particles that can be sufficiently sintered even at low temperatures.

[0024] The oxide-containing copper particles according to this embodiment are coated with a carboxylic acid. In this embodiment, the flux effect of the carboxylic acid is utilized during heating (sintering) to form copper oxide (Cu ２ O, Cu ８ O and Cu ６４ O) can be removed. The carboxylic acid may cover a part or the whole of the surface of the oxide-containing copper particle. In the present embodiment, the surface of the oxide-containing copper particle is covered with an oxide film and Cu. ６４ The carboxylic acid may be a compound that dissolves the oxide film and Cu particles on the surface of the oxide-containing copper particles. ６４ The O particles may be partially or entirely coated.

[0025] Examples of the carboxylic acids that coat the oxide-containing copper microparticles according to this embodiment include saturated fatty acids, unsaturated fatty acids, hydroxy acids, aromatic carboxylic acids, and terpene carboxylic acids. One type may be used alone, or two or more types may be used in combination. As the carboxylic acid, it is preferable to use at least one selected from aliphatic monocarboxylic acids, aliphatic dicarboxylic acids, aromatic carboxylic acids, and terpene carboxylic acids. In addition, it is preferable that the carboxylic acid is hydrophobic. By being hydrophobic, the equilibrium reaction represented by the following formula (1) is suppressed during storage (when not heated), and H ３ O ＋ The corrosion reaction caused by RCOOH+H can be suppressed. ２ O ⇔ RCOO － +H ３ O ＋In the above formula (1), R is, for example, a monovalent hydrocarbon group.

[0026] The aliphatic monocarboxylic acid preferably has 5 or more carbon atoms to exhibit hydrophobicity. The aliphatic monocarboxylic acid may be either linear or branched, and may be either saturated or unsaturated. Among these, linear saturated aliphatic monocarboxylic acids are preferred. The aliphatic monocarboxylic acid preferably has 5 to 18 carbon atoms. The aliphatic monocarboxylic acid may be used alone or in combination of two or more. Preferred aliphatic monocarboxylic acids have even carbon numbers, such as caproic acid (6 carbon atoms), caprylic acid (8 carbon atoms), capric acid (10 carbon atoms), lauric acid (12 carbon atoms), myristic acid (14 carbon atoms), palmitic acid (16 carbon atoms), and stearic acid (18 carbon atoms), which are inexpensive and readily available.

[0027] Aliphatic dicarboxylic acids having 6 or more carbon atoms are preferred because they exhibit hydrophobicity. The aliphatic dicarboxylic acids may be either linear or branched, and may be either saturated or unsaturated aliphatic dicarboxylic acids. Among these, linear saturated aliphatic dicarboxylic acids are preferred. The aliphatic dicarboxylic acids preferably have 6 to 18 carbon atoms. The aliphatic dicarboxylic acids may be used alone or in combination of two or more. Examples of aliphatic dicarboxylic acids include adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, 1,0-nonanedicarboxylic acid, 1,10-decanedicarboxylic acid, brassylic acid, 1,12-dodecanedicarboxylic acid, 1,13-tridecanedicarboxylic acid, thapsic acid, 1,15-pentadecanedicarboxylic acid, and 1,16-hexadecanedicarboxylic acid.

[0028] Examples of aromatic carboxylic acids include benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, hemimellitic acid, trimellitic acid, and trimesic acid.

[0029] As the terpene-based carboxylic acid, those contained in rosin etc. can be used, for example, abietic acid, neoabietic acid, palustric acid, pimaric acid, isopimaric acid, desidroabietic acid etc.

[0030] In this embodiment, whether the oxide-containing copper microparticles are coated with carboxylic acids can be confirmed by known mass spectrometry, for example, by ionizing the carboxylic acids on the surface using laser desorption ionization (LDI) or the like, and determining the mass-to-charge ratio using time-of-flight (TOF) or the like.

[0031] The component composition of the copper particles contained in the oxide-containing copper particles according to this embodiment may be such that the main component is copper, and the copper content may be, for example, 80% by mass or more, 90% by mass or more, or 95% by mass or more. The component composition preferably consists of copper and unavoidable impurity elements. This can improve the electrical conductivity of the sintered body. The total amount of unavoidable impurity elements may be, for example, 1.0% by mass or less.

[0032] The size and shape of the copper particles contained in the oxide-containing copper particles according to the embodiment of the present invention are not limited, and the average particle size may be, for example, 20 nm to 2 μm. The average particle size may exceed 2 μm, but when used in printable electronics, for example, coarse particles may cause a decrease in print quality, so the average particle size is preferably 2 μm or less. In this specification, "particle size" refers to the primary particle size and the circle-equivalent diameter, and "average particle size" refers to the median diameter of 150 or more particles randomly selected from an SEM or TEM image.

[0033] The surface of the copper particles contains Cu ２ O, Cu ８ O and Cu ６４ O. The presence of this film allows the oxide film to be formed on the surface of the substrate. ６４Not only does it enable low-temperature sintering even on particle surfaces where no O particles are present, but it also improves the chemical stability of the oxide-containing copper particles, making it easier to maintain their properties over a long period of time.

[0034] At least a portion of the surface of the oxide film contains Cu. ６４ O particles are arranged. The presence of these particles allows for a certain degree of fluidity during sintering, making it easier to fill voids and facilitating sintering at low temperatures. ６４ The O particles may be disposed on a part of the surface of the oxide film, or may be disposed on the entire surface so as to cover the surface.

[0035] Cu, Cu ２ O, Cu ８ O and Cu ６４ Cu relative to the total content of O ２ O, Cu ８ O and Cu ６４ The total content of O (i.e., Cu contained in the oxide film) ２ O, Cu ８ O and Cu ６４ O, and Cu ６４ Cu contained in O particles ６４ O) is 0.1 mass % or more and 10 mass % or less, and Cu ６４ The particle diameter of the O particles is 1 nm or more and 20 nm or less. ６４ While the above-mentioned effects of the O particles are exhibited, the fluxing action of the carboxylic acids and the reducing ability of the dispersion medium contained in the paste or ink are utilized to form the oxide film and Cu. ６４ When the content ratio is less than 0.1 mass %, the oxide film and / or Cu particles can be sufficiently reduced, and sufficient sintering properties can be obtained at low temperatures. ６４ If the content ratio exceeds 10 mass %, the oxide film and / or Cu particles may not be formed. ６４ There is a risk that the O particles may not be sufficiently reduced by the flux action or the reducing ability of the dispersion medium contained in the paste or ink. ６４ When the particle size of the O particles is less than 1 nm or more than 20 nm, Cu ６４There is a risk that the above-mentioned effects of the O particles may not be obtained.

[0036] Cu, Cu ２ O, Cu ８ O and Cu ６４ Cu relative to the total content of O ２ O, Cu ８ O and Cu ６４ The ratio of the total content of O is preferably 7 mass % or less, which makes it easier to reduce the oxide film and the like.

[0037] Cu ６４ The amount of O is Cu ２ O content and Cu content ８ It is preferable that the amount of O is greater than either one of the two, and Cu ２ O content and Cu content ８ It is more preferable that the amount of Cu is larger than the amount of O. This makes it easier to reduce the oxide film and the like. ２ O, Cu ８ O and Cu ６４ Cu relative to the total content of O ６４ The ratio of the content of O is preferably 0.1 mass % or more and 5 mass % or less. ２ O, Cu ８ O and Cu ６４ Cu relative to the total content of O ２ O and Cu ８ The ratio of the content of O to the content of Cu is preferably 0 mass % or more and 1 mass % or less. ６４ The above-mentioned effect of the O particles is more easily obtained, and the oxide film and Cu ６４ O particles become more easily reduced.

[0038] In the oxide-containing copper microparticles according to an embodiment of the present invention, the mass ratio can be calculated by acquiring an XRD pattern and using the RIR (Reference Intensity Ratio) method. The RIR method is a method for calculating a quantitative value using the RIR value stored in a database from the integrated intensity obtained by subtracting the background intensity determined by subtracting the baseline from the strongest line of the test component. In an embodiment of the present invention, the strongest line of each component (Cu(111), Cu(111)) is used. ２ O(111), Cu ８ O(022) and Cu ６４ The integrated intensity of CuO(044) and the RIR values ​​(Cu: 8.86, Cu ２ O: 8.28, Cu ８ O: 4.97 and Cu ６４ The mass ratio was calculated using the 2θ=43.317° for Cu(111) and 4.89 for Cu(111). ２ O(111) is 2θ=36.521°, Cu ８ O(022) is 35.452°, Cu ６４ O(044) has a diffraction peak at 2θ=40.710°.

[0039] In this embodiment, the average thickness of the oxide film is preferably 0.1 nm or more and 10 nm or less, and more preferably 0.1 nm or more and 6 nm or less. This improves the chemical stability of the oxide-containing copper particles while making it easier to reduce the oxides in the oxide film. In this specification, the "average thickness" refers to the arithmetic mean value of the thicknesses of 10 or more points randomly selected from a TEM image.

[0040] By using the oxide-containing copper particles according to this embodiment, sufficient sintering can be achieved at low temperatures of 250°C or less, 200°C or less, or even 150°C or less under normal pressure (preferably higher pressure).

[0041] [Sintering Material Comprising Oxide-Containing Copper Particles] The present disclosure also includes a sintering material containing the oxide-containing copper particles according to this embodiment. Examples of the sintering material include a paste or ink containing the oxide-containing copper particles according to this embodiment, intended for forming a conductive film or the like. Known materials can be used as the dispersion medium contained in the paste or ink. Examples of the dispersion medium include amines such as 2-(dimethylamino)ethanol, N-butyldiethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, diethanolamine, triethanolamine, 1-[bis(2-hydroxyethyl)amino]-2-propanol, and ethylenediamine-N,N,N',N'-tetraethanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, hexaethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, polypropylene glycol (diol type, triol type), and glycerol. A bonded body with high adhesive strength can be obtained by, for example, applying a sintering material containing the oxide-containing copper particles according to this embodiment to the bonding surfaces of multiple materials, such as metals (pure metals, alloys), ceramics, etc., and bonding them together. The sintered body according to this embodiment is obtained by sintering (sometimes also referred to as "firing") the sintering material containing the oxide-containing copper particles according to this embodiment. The bonded body according to this embodiment is obtained by bonding (sometimes also referred to as "bonding and firing" or simply "firing") multiple materials together with the sintering material containing the oxide-containing copper particles according to this embodiment.

[0042] The sintering material according to this embodiment is preferably in a "capillary state," "fungular state," or "pendular state," more preferably in a "fungular state" or "pendular state," and even more preferably in a "fungular state." This makes it easier to sinter at low temperatures. These classifications are described in "Particle Size Enlargement" by Capes CE (Elsevier Scientific Publishing Company, 1980). Those skilled in the art can achieve these states without excessive trial and error by appropriately adjusting the amounts of oxide-containing copper particles and dispersion medium contained in the sintering material depending on the type of dispersion medium used, etc.

[0043] In one embodiment of the present invention, the sintering material preferably contains 75.0 mass% or more and 99.0 mass% or less of the oxide-containing copper particles according to this embodiment. This makes it easier to obtain a "capillary state," "fungular state," or "pendular state." In addition, in one embodiment of the present invention, the sintering material preferably contains more than 86.0 mass% and 99.0 mass% or less of the oxide-containing copper particles according to this embodiment. This makes it easier to obtain a "fungular state" or "pendular state." In addition, in one embodiment of the present invention, the sintering material preferably contains more than 86.0 mass% and less than 90.5 mass% of the oxide-containing copper particles according to this embodiment. This makes it easier to obtain a "fungular state."

[0044] [Method for producing oxide-containing copper particles] The method for producing oxide-containing copper particles according to this embodiment includes the steps of: mixing CuO particles with a solution containing a carboxylic acid and a complexing agent to obtain a mixture; and adding a reducing agent to the mixture and reacting the mixture in an oxygen-containing atmosphere. The requirements for the production method according to this embodiment are described in detail below.

[0045] [Mixing step] First, CuO particles are mixed with a solution containing a carboxylic acid and a complexing agent to obtain a mixture. During this mixing, the complexing agent acts to form the cores of the copper particles in the finally obtained oxide-containing copper particles and the fine Cu６４ It is believed that the nuclei of O particles are quickly formed, and as the crystal growth of copper progresses in the reaction step described below, they are coated with carboxylic acids. The materials necessary for the mixing step will be described below.

[0046] (CuO particles) First, CuO particles are prepared as raw material particles. The size of the CuO particles is not limited. From the viewpoint of cost, coarse particles may be used, and for example, the average particle size of the CuO particles may be 2.4 μm or more. The raw material oxide particles are not limited to CuO, but CuO is desirable from the viewpoint of storage stability, etc.

[0047] (Carboxylic Acids) Carboxylic acids are mainly added as protective agents. These carboxylic acids coat the surface of the oxide-containing copper particles that are finally obtained. Carboxylic acids can also contribute to the formation of copper complex ions. Examples of carboxylic acids include those described above in the section on [Oxide-containing copper particles].

[0048] The amount of the carboxylic acid to be added is not particularly limited, but the molar ratio of the carboxylic acid to the copper contained in the raw material particles (CuO particles) is preferably 0.02 or more. The molar ratio may be, for example, 20 or less. If the molar ratio is less than 0.02, the coating with the carboxylic acid becomes insufficient, which is not preferable.

[0049] (Complexing Agent) In this embodiment, copper complex ions are formed using raw material particles (CuO particles) and a complexing agent. By ionizing the copper in the raw material particles, the reduction rate can be significantly increased in the reduction using a reducing agent, as described below, compared to the case where solid CuO is reduced without forming copper complex ions. As the complexing agent, a compound having an amino group (hereinafter referred to as an "amine compound") is added. The amine compound is not particularly limited, and examples thereof include alkanolamines and diamines. In terms of improving the function as a complexing agent, linear compounds are preferred, such as 2-aminoethanol.

[0050] The amount of the complexing agent to be added is not particularly limited, but from the viewpoint of promoting the formation of copper complex ions, it is preferable that the amount of the complexing agent be 0.05 to 15 times the molar ratio of the copper contained in the raw material particles (CuO particles).

[0051] (Solvent) The solvent is not particularly limited, and examples thereof include polyhydric alcohols such as ethylene glycol, lower alcohols such as methanol, ethanol, and 2-propanol, ketones such as acetone, and water.

[0052] (Heating after Mixing) After the mixing step, and before adding a reducing agent described later, it is preferable to further include a step of heating to 40° C. or more and 60° C. or less. This allows complex formation to proceed sufficiently, even when coarse raw material particles (for example, an average particle size of 2.4 μm or more) are used, and ultimately allows the desired particles to be prepared.

[0053] [Reaction step] Next, a reducing agent is added to the mixture and reacted in an oxygen-containing atmosphere. As a result, copper particles (and an oxide film) are formed from the copper particle nuclei obtained in the mixing step, and the formed fine copper particles are oxidized to Cu. ６４ O particles are formed, and Cu is formed on at least a part of the surface of the copper particles (and the oxide film). ６４ It is thought that O particles are arranged. The materials required for the reaction process will be explained below.

[0054] (Reducing Agent) The reducing agent is not particularly limited, and examples of the reducing agent that can be used include hydrazine-based reducing agents such as hydrazine, hydrazine hydrochloride, hydrazine sulfate, and hydrazine hydrate, citric acid, ascorbic acids, and borohydrides such as sodium borohydride. After adding the reducing agent, the mixture may be stirred to allow the reaction to proceed sufficiently until the raw material oxide or the reducing agent is consumed.

[0055] (Reaction Conditions) When a reducing agent is added and the reaction is carried out in an oxygen-containing atmosphere, it is preferable to heat the mixture to a temperature higher than 60°C and not higher than 80°C. It is also preferable to stir the mixture at 2000 rpm or higher during the reaction. This allows the particle surface to be appropriately oxidized via the carboxylic acids that coat the surface, and also allows Cu to be removed. ６４ It is believed that the generation of O particles progresses and ultimately makes it easier to adjust to the desired particles.

[0056] In the production method according to this embodiment, the atmosphere during the reaction is limited to an oxygen-containing atmosphere such as air, and the reaction must be constantly exposed to the oxygen-containing atmosphere. If an inert gas atmosphere such as nitrogen gas or argon gas is used, or if the reaction vessel is sealed in air, the oxide-forming reaction may not proceed, resulting in the formation of metallic copper particles.

[0057] [Other Steps] The production method of this embodiment may further include steps other than those described above. For example, it may include a step of recovering oxide-containing copper particles by, for example, centrifuging and / or filtering a slurry containing the particles obtained by the reduction, followed by a purification step, a drying step, etc. In the purification step, purification is preferably performed using a washing solvent. The washing solvent is not particularly limited, and for example, an organic solvent such as N,N-dimethylacetamide, toluene, hexane, acetone, or ethanol can be used.

[0058] The method for producing oxide-containing copper particles according to this embodiment has been described above. However, a person skilled in the art who understands the desired properties of the oxide-containing copper particles according to this embodiment may, through trial and error, discover a method for producing oxide-containing copper particles according to this embodiment other than the above-described method.

[0059] The method for producing a sintered product according to this embodiment includes the steps of preparing a sintering material containing the oxide-containing copper particles according to this embodiment and firing the sintering material at a temperature of 250°C or lower (preferably 200°C or lower, more preferably 150°C or lower). Examples of the fired product include a sintered body obtained by sintering (firing) the sintering material according to this embodiment, and a bonded body obtained by joining (bonding and firing) multiple materials using the sintering material according to this embodiment. The step of preparing the sintering material can be performed by appropriately mixing the oxide-containing copper particles according to this embodiment with a dispersion medium. As the dispersion medium, known dispersion mediums such as those described above can be used. In the step of firing the sintering material, the material can be fired, for example, under atmospheric pressure (preferably a higher pressure) at a low temperature of 250°C or lower, preferably 200°C or lower, and more preferably 150°C or lower. The lower limit of the firing temperature is not particularly limited.

[0060] When the sintered product is a bonded body, the method for producing the sintered product (i.e., the bonded body) according to this embodiment further includes, after the step of preparing a sintering material containing the oxide-containing copper particles according to this embodiment, a step of applying the sintering material between multiple materials before the step of firing the sintering material at a temperature of 250° C. or less. The method for applying the sintering material between multiple materials is not particularly limited, and can be carried out, for example, by coating the sintering material on the bonding surfaces of multiple materials, such as metals (pure metals, alloys), ceramics, etc.

[0061] The present embodiment will be described in more detail below with reference to examples. The present embodiment is not limited to the following examples, and can be implemented with appropriate modifications within the scope of the above-mentioned and below-mentioned aims, and all such modifications are included in the technical scope of the present embodiment.

[0062] (Preparation of oxide-containing copper particles) 1 mol of copper oxide II (N-300 manufactured by Nisshin Chemco) as a raw material, 1 L of 1-propanol (manufactured by Junsei Chemical Co., Ltd.) as a solvent, 60 mmol of hexanoic acid (manufactured by Junsei Chemical Co., Ltd., carbon number 6) as a protective agent, and 500 mmol of 2-aminoethanol (manufactured by Junsei Chemical Co., Ltd.) as a complexing agent were mixed and heated to 50°C. Subsequently, 2 mol of hydrazine monohydrate (manufactured by Kanto Chemical Co., Ltd.) as a reducing agent was added and heated to 70°C, and then the mixture was reacted for 2 hours while stirring at a stirring speed of 4000 rpm in the atmosphere. After the reaction, the mixture was purified twice each with acetone and ethanol, and recovered as a slurry containing oxide-containing copper particles.

[0063] The water contact angle of the resulting oxide-containing copper particles was measured, and the results showed high hydrophobicity, suggesting that the particle surfaces were coated with hexanoic acid.

[0064] Fig. 1 shows the X-ray diffraction pattern of the obtained oxide-containing copper particles. The measurement was performed using a powder X-ray diffractometer (Miniflex II, D / teX Ultra, manufactured by Rigaku) ​​with Cu-Kα radiation at a scan rate of 20° (2θ) / min. As shown in Fig. 1, the obtained oxide-containing copper particles contain Cu in addition to Cu. ２ O and / or Cu ８ O and Cu ６４ It was confirmed that the sample was composed of copper oxide of CuO. ２ O and Cu ８ Although it is difficult to clearly distinguish the O peak, if the peak is Cu, ２ When O is used, Cu: 96.73 mass%, Cu ６４ O: 2.93 mass%, and Cu ２ O: 0.34 mass%, and the peak is Cu ８ When O is used, Cu: 96.52 mass%, Cu ６４ O: 2.92 mass%, and Cu ８ O: 0.56 mass%. Furthermore, the peak was Cu. ２ O and Cu ８ When both O and Cu are present, Cu: 96.52 to 96.73 mass%, Cu ６４ O: 2.92 to 2.93% by mass, Cu２ O: 0 to 0.34% by mass, Cu ８ O: It is considered to be in the range of 0 to 0.56 mass %, and in any case, one of the requirements of this embodiment (Cu, Cu ２ O, Cu ８ O and Cu ６４ Cu relative to the total content of O ２ O, Cu ８ O and Cu ６４ The ratio of the total content of O satisfied the condition of 0.1 mass % or more and 10 mass % or less.

[0065] FIG. 2 shows an example of an STEM image of the obtained oxide-containing copper particles. The apparatus used was a scanning transmission electron microscope (JEOL Ltd., JEM-ARM200F, accelerating voltage 200 kV). From the observation results shown in FIG. 2, it was found that the obtained oxide-containing copper particles have large particles with a diameter of 50 to 250 nm, and fine particles with a diameter of 10 nm or less (1 nm or more) are arranged on at least a part of the surface. From the lattice spacing analysis of the highly detailed STEM image shown in FIG. 3, it was found that the large particles with a diameter of 50 to 250 nm (black parts in FIG. 3) are copper fine particles, and the fine particles with a diameter of 10 nm or less are Cu. ６４ On the surface of the large particles, a phase with a different crystal structure from Cu (the phase outside the dotted line in Figure 3) was confirmed, and lattice spacing analysis revealed that this phase was Cu. ６４ O and Cu ２ O and / or Cu ８ It was found that the oxide film was 1.5 nm or less (0.1 nm or more) and composed of O. In addition, the original drawings of Figure 3 (and Figure 6 described below) have been submitted together with this application as a document for submission in order to enable a more detailed understanding of the differences in the lattice fringes. Please refer to these original drawings as necessary.

[0066] (Production of a paste (sintering material) containing oxide-containing copper particles) The slurry of oxide-containing copper particles and ethanol obtained as described above was centrifuged to remove the supernatant, thereby obtaining a wet cake of oxide-containing copper particles. 7.5 parts by mass of triethanolamine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added as a paste solvent to 92.5 parts by mass of particles in the wet cake calculated by the following formula (2). After stirring for 5 minutes with a vortex mixer (manufactured by Heathrow Scientific), a thin film swirling high-speed mixer (Filmix 56-L manufactured by Primix) was used to perform a 5-minute intermittent dispersion treatment at a peripheral speed of 30 m / s. The obtained paste after the dispersion treatment was divided into small portions, and different amounts of triethanolamine were added to each portion to adjust the concentration. Each portion was then stirred for 16 minutes using a planetary centrifugal mixer. The mixture was then held in a vacuum at room temperature until no weight change was observed, and the ethanol was removed, obtaining six levels of pastes with different particle contents. The resulting pastes were subjected to thermogravimetry and differential thermal analysis (TG-DTA) using a TG / DTA simultaneous measurement device (Shimadzu Corporation) in a 3% hydrogen / nitrogen mixed gas at a heating rate of 5°C / min. The results are shown in Figure 4. As shown in Figure 4, from the weight loss indicated by TGA, it was confirmed that each paste contained 92.8 mass%, 90.5 mass%, 88.2 mass%, 86.0 mass%, 82.3 mass%, and 78.7 mass% of particles, respectively.

[0067]

[0068] In formula (1), Y: weight of particles in the slurry (g) M ｓ V: Slurry weight (g) ｓ : Slurry volume (cm ３ ) ρ ｍ : Solvent density (g / cm ３ ) ρ Ｃｕ : Copper density (g / cm ３ )

[0069] When the X-ray diffraction pattern of the oxide-containing copper particles in the obtained sintering material (paste) was obtained, it was found that Cu was ６４ Since the peak of O was observed, it is clear that Cu ６４It was suggested that the O particles were unchanged.

[0070] (Cu ６４ Synthesis of oxide-containing copper particles not containing O particles) As a comparative example, Cu ６４ Oxide-containing copper particles not containing O particles were synthesized as follows. 1 mol of copper oxide II (Nissin Chemco NB-2) was added as a raw material, 1 L of ethyl carbitol (Sankyo Chemical) as a solvent, and 60 mmol of hexanoic acid (Junsei Chemical) as a protective agent, and the mixture was heated to 70°C. 2 mol of hydrazine monohydrate (Kanto Chemical) was then added as a reducing agent, and the mixture was reacted for 2 hours while stirring at a stirring speed of 4000 rpm. After the reaction, the mixture was purified twice each with acetone and 2-propanol, and recovered as a slurry containing oxide-containing copper particles of the comparative example.

[0071] The oxide-containing copper particles of the comparative example were observed with a transmission electron microscope (JEOL Ltd., JEM-2000FX, acceleration voltage 200 kV). An example of an STEM image is shown in FIG. 5. As shown in FIG. 5, Cu was extracted from the oxide-containing copper particles of the comparative example. ６４ An example of a more detailed STEM image is shown in FIG. 6. The lattice spacing analysis of the highly detailed STEM image shown in FIG. 6 revealed that Cu particles were present on the surface of the copper particles, as in the example. ６４ O and Cu ２ O and / or Cu ８ It was found that a copper oxide film composed of O was formed.

[0072] (Cu ６４(Production of a Paste Containing Oxide-Containing Copper Particles Containing No O Particles) A ​​slurry of oxide-containing copper particles and 2-propanol of the comparative example was centrifuged to remove the supernatant, yielding a wet cake of oxide-containing copper particles of the comparative example. 10 parts by mass of triethanolamine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added as a paste solvent to 90 parts by mass of the particles in the wet cake calculated by equation (2). The mixture was premixed using a rotation-revolution hybrid mixer (AR-100 manufactured by Thinky Co., Ltd.), and then dispersed using an ultra-high-pressure disperser (Starburst mini manufactured by Sugino Machine Co., Ltd.). The mixture was then held at room temperature in a vacuum until no weight change occurred, and the 2-propanol was removed, yielding a paste of the comparative example. The resulting paste was subjected to thermogravimetry and differential thermal analysis (TG-DTA) at a heating rate of 5°C / min in a 3% hydrogen / nitrogen mixed gas using a simultaneous TG / DTA analyzer (manufactured by Shimadzu Corporation). The results are shown in FIG. 7 . As shown in FIG. 7, the weight loss indicated by TGA confirmed that the resulting paste contained 88.7% by mass of particles.

[0073] (Bonding and Firing Evaluation) Two copper test pieces were bonded together using the pastes of the examples and comparative examples, and bonded samples for bonding and sintering evaluation were prepared by sintering the paste, and the adhesive strength was measured.

[0074] 1. Preparation of Samples for Bonding and Sintering Evaluation The procedure for preparing samples for bonding and sintering evaluation is as shown in Figure 8. First, circular copper test pieces, Copper Test Piece 1 with a diameter of 12 mm and Copper Test Piece 2 with a diameter of 5 mm, were cut from a 5 mm thick oxygen-free copper plate (C1020P) as shown in Figure 9A. Copper test pieces 1 and 2 were prepared. The surface of each copper test piece was polished with waterproof abrasive paper (SiC paper) P4000, then mirror-finished with a polishing cloth soaked in a suspension of 0.5 μm alumina powder, and degreased. Before applying the paste, each copper test piece was immersed in 2.5 M hydrochloric acid to remove the copper oxide film on the surface, thoroughly washed with pure water and methanol, and dried.

[0075] Next, metal mask printing was performed. Specifically, a metal mask having an opening size of 5 mm diameter x 0.15 mm thickness, as shown in FIG. 9B, was used. As shown in FIG. 9C, the paste (paste containing oxide-containing copper particles) 3 was applied to the center of a 12 mm diameter copper test piece 1, and then bonded to a 5 mm diameter copper test piece 2. Then, a load of 15 MPa was applied to the two bonded copper test pieces in the direction of the arrow in FIG. 9C using a hot press, and the temperature was rapidly increased to 200 ° C. at 90 ° C. / min. After reaching 200 ° C. and holding for 15 minutes, the test pieces were removed from the hot press and rapidly cooled by water cooling to obtain a sample for bond firing evaluation. The temperature rise and fall profiles are shown in FIG. 10A (Example) and FIG. 10B (Comparative Example). 10A and 10B, the temperature rise and fall profiles of the Example and the Comparative Example are different, and the time exposed to a heating temperature of 100°C or higher, which is thought to contribute to sintering, was 18 minutes in total for the Example, while it was 50 minutes in total for the Comparative Example, so the firing conditions of the Example were more unfavorable. The series of operations were carried out in a nitrogen atmosphere.

[0076] 2. Bonding and Sintering Evaluation Test (Measurement of Adhesion Strength) The adhesive strength of the bonding and sintering evaluation sample was evaluated using a 5 kN materials testing machine (manufactured by Shimadzu Corporation). Specifically, as shown in the schematic cross-sectional view of FIG. 9D , a load was applied parallel to the coated surface of the bonding and sintering evaluation sample fixed to the fixture 4 of the testing machine at a rate of 1 mm / min, and the breaking load of the copper test piece was measured as adhesive strength. Table 1 shows the results of a comparison of adhesive strengths between samples with equivalent paste concentrations (88.2% by mass for the Example and 88.7% by mass for the Comparative Example). The term "paste concentration" refers to the content of oxide-containing copper particles in the sintering material (paste).

[0077]

[0078] As described above, the Examples were subjected to more unfavorable firing conditions than the Comparative Examples, but as shown in Table 1, they exhibited higher adhesive strength than the Comparative Examples. This is because the Examples were subjected to firing conditions that were more unfavorable than the Comparative Examples. ６４ This is thought to be because all the requirements of this embodiment, such as the arrangement of O particles, are satisfied.

[0079] Preferred sintering materials are described below. Figure 11 shows the relationship between paste concentration and adhesive strength (the error bars on the vertical axis for each measurement indicate the variation in the results obtained from duplicate measurements). When preparing samples for bond firing evaluation, a temperature rise / fall profile with a maximum temperature of 150°C (shown in Figure 10C) was also performed in addition to the one shown in Figure 10A. Figure 11 shows the measurement results for samples fired at a maximum temperature of 150°C (open circles) and 200°C (filled circles). As shown in Figure 11, sufficient adhesive strength was observed at both paste concentrations (average of 10 MPa or more at 150°C (open circles) and 20 MPa or more at 200°C (filled circles)). Sufficient sintering was also possible at low temperatures below 200°C, preferably below 150°C. The adhesive strength reached its maximum value at a paste concentration of 88.2% by mass, and it was found that adhesive strength decreased with increasing or decreasing paste concentrations. The 88.2 mass% paste, which showed the best average adhesive strength, showed high values ​​of 42 MPa at 150°C and 71 MPa at 200°C. The fracture surfaces after each adhesive strength measurement were observed with an SEM, and the results are shown in Figure 12 (when fired at a maximum of 150°C) and Figure 13 (when fired at a maximum of 200°C). As shown in Figures 12 and 13, it was confirmed that the particles were densely sintered in both cases.

[0080] The change in adhesive strength with respect to paste concentration shown in Figure 11 can be explained by the rheological considerations shown in Figures 14A to 14C. As shown in the schematic diagram of the paste in the upper left of Figure 14A, when a large amount of solvent 12 is present, the voids between the copper particles 11 are filled with solvent 12, forming a viscoelastic paste. In this example, a paste concentration of 86.0 mass% or less (and at least 75.0 mass% or more) corresponds to this, which is considered to correspond to the so-called capillary region. In the state shown in the schematic diagram in the upper left of Figure 14A, the copper particles 11 are covered with solvent 12, which acts as a reducing agent, and the gas phase is discontinuous, which is thought to facilitate a reduction reaction rather than an oxidation reaction. On the other hand, as shown in the schematic diagram of the preparation of a sample for evaluation of bonding and firing in the lower right of Figure 14A, application of a load P causes the paste 14 to flow easily, thinning the adhesive layer and potentially causing contact with the copper test piece 13. It is also thought that the evaporation and / or flow of the solvent causes voids and / or cracks, which reduces adhesive strength. As shown in the schematic diagram of the paste in the upper left of Figure 14B, when the solvent 12 is reduced below the capillary region, the funicular region, or plastic region, exhibits clay-like behavior. In this example, the sample with a concentration of more than 86.0 mass% and less than 90.5 mass%, corresponds to this. As shown in the schematic diagram of the paste in the upper left of Figure 14B, the gas phase is continuous, but the surfaces of the copper particles 11 are evenly covered with the solvent 12, which acts as a reducing agent, making oxidation difficult and facilitating the reduction reaction. Furthermore, as shown in the schematic diagram of the sample preparation for bonding and firing evaluation in the lower right of Figure 14B, when a load P is applied to the copper test piece 13, plastic flow occurs. However, because the amount of solvent in the paste 14 is low, the flow rate is low and voids and / or cracks are unlikely to occur, which is thought to improve the adhesive strength. As shown in the schematic diagram of the paste in the upper left of Figure 14C, further reduction of the solvent 12 leads to the pendular region. Because the solvent 12 does not completely cover the surfaces of the copper particles 11, the copper test piece 13 enters the elastic region and exhibits more powder-like behavior. In this example, the sample having a concentration of 90.5% by mass or more (and may be, for example, 99.0% by mass or less) corresponds to this. In this sample, by kneading the paste, the solvent 12 covers the surfaces of the copper particles 11 to some extent, and the sample exhibits hard clay-like behavior, making it possible to apply the paste using a metal mask.As shown in the schematic diagram of the preparation of the sample for evaluation of bonding and firing in the lower right of Fig. 14C, because the copper test piece 13 is an elastic body, it hardly flows even when a load P is applied to it, and it is thought that there is almost no effect of voids and / or cracks because there is very little solvent in the paste 14. On the other hand, as shown in the schematic diagram of the paste in the upper left of Fig. 14C, because only a portion of the surface of the copper particle 11 is covered with the solvent 12 and a gas phase is continuously present, an oxidation reaction occurs on the surface of the copper particle 11, which is likely to inhibit sintering and reduce the adhesive strength.

[0081] The oxide-containing copper particles according to this embodiment can be sufficiently sintered at low temperatures of 250°C or less, 200°C or less, or even 150°C or less under normal pressure (preferably higher pressure). Therefore, the oxide-containing copper particles according to this embodiment can be used, for example, as circuit-forming materials for printed circuit boards (particularly flexible boards) and other microwiring materials, as well as for heat conduction applications such as die bonding materials for power semiconductors, as shown in this example. They can also be used as antistatic materials, electromagnetic wave blocking materials, infrared blocking materials, etc.

[0082] This application claims priority from Japanese Patent Application No. 2024-042277, filed March 18, 2024. Japanese Patent Application No. 2024-042277 is incorporated herein by reference.

[0083] REFERENCE SIGNS LIST 1, 2 Copper test piece 3 Paste containing oxide-containing copper particles 4 Fixing jig for tester 11 Copper particles 12 Solvent 13 Copper test piece 14 Paste

Claims

1. Oxide-containing copper particles coated with a carboxylic acid, comprising copper particles and Cu ２ O, Cu ８ O and Cu ６４ and an oxide film comprising at least one selected from the group consisting of Cu and O, and Cu disposed on at least a portion of the surface of the oxide film. ６４ O particles, Cu, Cu ２ O, Cu ８ O and Cu ６４ Cu relative to the total content of O ２ O, Cu ８ O and Cu ６４ The ratio of the total content of O is 0.1 mass % or more and 10 mass % or less, and the Cu ６４ The oxide-containing copper particles have an equivalent circle diameter of 1 nm or more and 20 nm or less.

2. Cu, Cu ２ O, Cu ８ O and Cu ６４ Cu relative to the total content of O ６４ 2. The oxide-containing copper particles according to claim 1, wherein the ratio of the O content is 0.1% by mass or more and 5% by mass or less.

3. Oxide-containing copper particles according to claim 1 or 2, wherein the average thickness of the oxide film is 0.1 nm or more and 10 nm or less.

4. A sintering material comprising the oxide-containing copper particles according to any one of claims 1 to 3.

5. A method for producing oxide-containing copper particles according to any one of claims 1 to 3, comprising the steps of: mixing CuO particles with a solution containing a carboxylic acid and a complexing agent to obtain a mixture; and adding a reducing agent to the mixture and reacting the mixture in an oxygen-containing atmosphere.

6. The method of claim 5, wherein the complexing agent is an amine compound.

7. The method of claim 5 or 6, further comprising a step of heating to 40°C or higher and 60°C or lower after the mixing step and before adding the reducing agent.

8. The method of any one of claims 5 to 7, wherein the reaction is carried out by heating at a temperature higher than 60°C and not higher than 80°C.

9. The sintering material according to claim 4, which is a paste.

10. A joined body formed by joining a plurality of materials with the sintering material according to claim 4 or 9.

11. A method for producing a sintered product, comprising the steps of: preparing a sintering material according to claim 4 or 9; and sintering the sintering material at a temperature of 250°C or less.

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