Metal composite powder and process for its production

Abstract

A method for producing a metal composite powder, comprising the steps of: providing a silver-coated copper powder, wherein the surface of the copper powder is coated with silver; spraying the silver-coated copper powder into an outlet flame region of a thermal plasma, wherein the outlet flame region of the thermal plasma has a temperature of 2000 to 5000 K to cause silver on the surface of the copper powder to diffuse into the entire grain boundaries of the copper on the interior of the copper powder; and subsequently coating the exposed surface of the copper powder with silver.

Description

BACKGROUND OF THE INVENTION Field of the Invention

[0001] The present invention relates generally to a metal composite powder and a method of making the same. More particularly, the invention relates to a metal composite powder for use in a conductive paste or the like and a method for producing the same. Description of the prior art

[0002] Conventionally, for forming electrodes and wires of electronic parts by printing methods and so on, a conductive paste prepared by mixing a solvent, a resin, a dispersant and so on in a conductive metal powder such as silver or copper powder is used.

[0003] However, silver powder is expensive because it is a powder of a noble metal, although it has a very low volume resistivity to be a good conductive material. On the other hand, copper powder is superior in storage stability (reliability) to that of silver powder because it is easily oxidized although it is low in volume resistivity to be a good conductive material.

[0004] In order to solve these problems, a silver-coated copper powder in which the surface of the copper powder is coated with silver has been proposed as a metal powder for use in a conductive paste (see, for example, Japanese Patent Publication Nos. 2010-174311 and 2010-077495).

[0005] However, in the silver-coated copper powders disclosed in Japanese Patent Publication Nos. 2010-174311 and 2010-077495, when there is a portion of the area of ​​copper not coated with silver, oxidation of that portion proceeds, so that the storage stability ( reliability) of which is insufficient. In particular, since oxygen easily diffuses into grain boundaries, oxidation of the grain boundaries of copper by the diffusion (grain boundary diffusion) of oxygen along the grain boundaries of copper proceeds. SUMMARY OF THE INVENTION

[0006] It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a metal composite powder containing copper and silver and capable of improving the storage stability (reliability) thereof by preventing the progress of oxidation from the surface thereof and the grain boundaries of copper is prevented, and to provide a method for producing the same.

[0007] In order to achieve the aforementioned and other objects, the inventors have made extensive studies and found that it is possible to produce a metal composite powder capable of improving the storage stability (reliability) thereof by preventing the progress of oxidation from the surface thereof and the grain boundaries of copper is prevented when a silver-coated copper powder, wherein the surface of a copper powder is coated with silver, is sprayed into a tail flame region of a thermal plasma to cause silver to the surface of the copper powder is diffused into a grain boundary of copper at the inside of the copper powder, and then the surface of the copper powder is coated with silver. Thus, the inventors have made the present invention.

[0008] According to the present invention there is provided a method of producing a metal composite powder, the method comprising the steps of: providing a silver-coated copper powder, the surface of a copper powder being coated with silver; the silver-coated copper powder is sprayed into a tail flame portion of a thermal plasma to cause silver on the surface of the copper powder to diffuse into a grain boundary of copper on the inside of the copper powder; and then the surface of the copper powder is coated with silver.

[0009] With this method for producing a metal composite material powder, the outlet flame region of the thermal plasma preferably has a temperature of 2000 to 5000 k. The copper powder is preferably produced by atomization. The copper powder preferably has an average particle diameter of 0.1 to 100 μm. The content of silver with respect to the silver-coated copper powder is preferably 5% by weight or more.

[0010] According to the present invention, there is provided a metal composite powder comprising: a copper powder; and silver diffusing into a grain boundary of copper at the inside of the copper powder and coating the surface of the copper powder. In this metal composite powder, the copper powder preferably has an average particle diameter of 0.1 to 100 μm. The content of silver with respect to the metal composite powder is preferably 5% by weight or more. The percentage of an area occupied by silver in a cross section of the metal composite powder is preferably 3 to 20%.

[0011] In the specification, the term “the average particle diameter of a copper powder” means the particle diameter (D50 diameter) corresponding to 50% of a cumulative distribution aggregate of the copper powder as measured by a laser diffraction particle size analyzer.

[0012] According to the present invention, it is possible to obtain a metal composite powder containing copper and silver and capable of improving the storage stability (reliability) thereof by preventing progress of oxidation of the surface thereof and the grain boundaries of copper, and a Provide methods for producing the same. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will be better understood from the following detailed description and accompanying drawings of the preferred embodiments of the invention. However, the drawings are not intended to limit the invention to a specific embodiment, but are for explanation and understanding only.

[0014] In the drawings are:

[0015] figure 1 is a BE (electron backscattered) mode composition image (COMPO image) obtained by observing a cross section of a silver-coated copper powder obtained in Comparative Example 1 by means of a field emission scanning electron microscope (FE-SEM);

[0016] figure 2 is a COMPO image obtained by observing a cross section of a metal composite powder obtained in Comparative Example 2 by the FE-SEM;

[0017] figure 3 is an association image obtained by observing the cross section of the metal composite powder obtained in Comparative Example 2 by means of an energy dispersive X-ray spectrometer (EDS) and a field emission Auger electron spectrometer (FE-AES);

[0018] figure 4 is a COMPO image obtained by observing a cross section of a metal composite powder obtained in Comparative Example 3 by the FE-SEM;

[0019] figure 5 is a COMPO image obtained by observing a cross section of a metal composite powder obtained in Example 1 by the FE-SEM;

[0020] figure 6 is a silver allocation image obtained by observing the cross section of the metal composite powder obtained in Example 1 by the FE-SEM;

[0021] figure 7 is a copper allocation image obtained by observing the cross section of the metal composite powder obtained in Example 1 by the FE-SEM;

[0022] figure 8 is a COMPO image obtained by observing a cross section of the metal composite powder obtained in Example 2 by the FE-SEM;

[0023] figure 9 is a silver allocation image obtained by observing the cross section of the metal composite powder obtained in Example 2 by the FE-SEM;

[0024] figure10 is a copper allocation image obtained by observing the cross section of the metal composite powder obtained in Example 2 by the FE-SEM;

[0025] figure 11 is a graph showing the measured results in TG-DTA of the silver-coated copper powder obtained in Comparative Example 1;

[0026] figure 12 is a graph showing the measured results in the TG-DTA of the metal composite powder obtained in Comparative Example 2;

[0027] figure 13 is a graph showing the measured results in the TG-DTA of the metal composite powder obtained in Comparative Example 3;

[0028] figure 14 is a graph showing the measured results in the TG-DTA of the metal composite powder obtained in Example 1;

[0029] figure 15 is a graph showing the measured results in the TG-DTA of the metal composite powder obtained in Example 2; and

[0030] figure 16 is a graph showing the measured results in TG-DTA of the silver-coated copper powder obtained in Comparative Example 4. FIG. DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] In a preferred embodiment of a method for producing a metal composite powder according to the present invention, a silver-coated copper powder, wherein the surface of a copper powder is coated with silver, is sprayed into an outlet flame region of a thermal plasma to cause silver on the surface of the copper powder into the Grain boundaries of the copper are diffused at the inside of the copper powder, and then the copper powder is coated with silver.

[0032] Although the copper powder used as a raw material can be produced by the wet reduction method, electrolysis method, vapor phase method or the like, it is preferably produced by a so-called atomization method (such as a gas atomization method or a water atomization method) to produce a fine powder by rapidly cooling and solidifying Copper melted at a temperature not less than its melting temperature is produced by colliding a high-pressure gas or water with the molten copper to cause it to drip from the lower portion of a ladle. In particular, when the copper powder is produced by a so-called water atomization method for spraying a high-pressure water, it is possible to obtain a copper powder with a small particle diameter, so that it is possible to increase the electrical conductivity of an electrically conductive paste due to the increase in the number of contact points between the particles of the copper powder when the copper powder is used for preparing the electrically conductive paste.

[0033] The average particle diameter of the copper powder is preferably in the range of 0.1 μm to 100 μm, more preferably in the range of 0.5 μm to 20 μm, and most preferably in the range of 1 μm to 10 μm. If the average particle diameter of the copper powder is less than 0.1 μm, it is not preferable because it has a bad influence on the electrical conductivity of the silver-coated copper powder. On the other hand, when the average particle diameter of the copper powder exceeds 100 μm, it is not preferable because it is difficult to form fine wires.

[0034] As a method of plating the copper powder with silver, a method of depositing silver on the surface of the copper powder by a substitution method using a substitution reaction for substituting silver for copper or by a reducing method using a reducing agent can be used. For example, a method of depositing silver on the surface of the copper powder can be used while stirring a solution containing the copper powder and silver ions in a solvent, or a method of depositing silver on the surface of the copper powder can be used while stirring a mixed solution of a solution containing the copper powder and an organic material in a solvent and a solution containing silver ions and an organic material in a solvent.

[0035] As the solvent, water, an organic solvent, or a mixed solvent thereof can be used. When a solvent prepared by mixing water with an organic solvent is used, it is necessary to use an organic solvent that is liquid at room temperature (20 to 30°C) and the mixing ratio of water to the organic Solvent can be suitably adjusted according to the organic solvent used. When water is used as the solvent, distilled water, ion-exchanged water, industrial water or the like can be used unless there is a possibility that impurities are mixed therein.

[0036] As raw materials of silver, silver nitrate having a high solubility with respect to water and many organic solvents is preferably used because it is necessary for silver ions to exist in a solution. In order to carry out a reaction for coating the copper powder with silver (silver plating reaction) as uniformly as possible, a silver nitrate solution prepared by dissolving silver nitrate in a solvent (water, an organic solvent or a mixed solvent thereof) is preferred, rather than solid silver nitrate preferably used. The amount of the silver nitrate solution used, the concentration of silver nitrate in the silver nitrate solution, and the amount of the organic solvent can be determined according to the amount of the intended silver-containing layer.

[0037] To make silver more uniform, a chelating agent can be added to the solution. As the chelating agent, a chelating agent having a high complex stabilization constant with respect to copper ions and so forth is preferably used so as to prevent redeposition of copper ions and so forth formed by substitution reaction for substituting silver ions for metallic copper as reverse formed products . In particular, since the copper powder serving as the core of the silver-coated copper powder contains copper as a main compositional element, the chelating agent is preferably selected in view of the complex stabilization constant with respect to copper. Specifically, as the chelating agent, a chelating agent selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), iminodiacetic acid, diethylenetriamine, triethylenediamine, and salts thereof can be used.

[0038] In order to carry out the silver plating reaction stably and safely, a pH buffer may be added to the solution. As the pH buffer, ammonium carbonate, ammonium hydrogen carbonate, ammonia water, sodium hydrogen carbonate or the like can be used.

[0039] When the silver plating reaction is carried out, a solution containing a silver salt is preferably added to a solution in which the copper powder is sufficiently dispersed by stirring the solution after the copper powder is added therein before the silver salt is added. The reaction temperature in the silver plating reaction may be a temperature at which solidification or evaporation of the reaction solution is not caused. The reaction temperature is adjusted to preferably 10 to 40°C, and more preferably 15 to 35°C. The reaction time can be set in the range of 1 minute to 5 hours, although it varies according to the amount of plating silver and the reaction temperature.

[0040] The content (coating amount) of silver with respect to the silver-coated copper powder is preferably 5% by weight or more, and is more preferably in the range of 7% by weight to 50% by weight, more preferably in the range of 8% by weight to 40% by weight and most preferably in the range of 9% to 20% by weight. If the content of silver is less than 5% by weight, it is not preferable because it has a bad influence on the electrical conductivity of the silver-coated copper powder. On the other hand, if the content of silver exceeds 50% by weight, it is not preferable since the cost thereof is high due to increase in the amount of silver to be used.

[0041] The silver-coated copper powder thus obtained is sprayed into the tail flame portion of a thermal plasma for heat treatment to cause silver on the surface of the copper powder to diffuse into the grain boundaries of copper on the inside of the copper powder. Since plasma flames use pure gases, there is no likelihood of impurities being attached to the silver-coated copper powder sprayed in the tail or tail portions of a thermal plasma flame. The period of time for applying heat to the silver-coated copper powder by the discharge flame portion of the thermal plasma is a short period of time, so it is possible to prevent the aggregation of the silver-coated copper powder.

[0042] In a typical method of using a thermal plasma to generate ultrafine particles (nanoparticles) by directly feeding a raw material into a plasma flame, the raw material is instantaneously heated to thousands of degrees Celsius in a high-temperature region of not less than 10,000°C of the plasma flame, so that it is decomposed into atoms and / or radicals to be rapidly cooled to about 1000°C at which homogeneous nucleation takes place in a downstream low-temperature region to synthesize ultrafine particles. However, in the preferred embodiment of a method for producing a metal composite powder according to the present invention, the silver-coated copper powder is fed into the plasma tail flame portion at a temperature of 2000 to 5000 K, so that silver, which has a lower melting point than copper, melts for diffusion while the silver-coated copper powder is passed through the plasma tail flame region in a very short period of time. Therefore, it is possible for the silver on the surface of the copper powder to diffuse into the grain boundaries of copper in the interior of the copper powder while maintaining the shape of the copper powder serving as the core of the silver-coated copper powder to some extent. Further preferably, silver on the surface of the copper powder is caused to diffuse into the grain boundaries of copper at the inside of the copper powder up to a third or more of the particle diameter of the copper powder from the surface of the copper powder, and is more preferably caused to diffuse into the entire grain boundaries of the copper diffuses at the inside of the copper powder.

[0043] The spraying of the silver-coated copper powder into the thermal plasma tail flame region can be performed by means of a thermal plasma device. In order to feed the silver-coated copper powder into the discharge flame region of the thermal plasma, which has a temperature of 2000 to 5000 K, by means of the thermal plasma device, the output of the plasma device is preferably 2 to 10 kW, more preferably 4 to 8 kW and most preferably 5 to 7kW. The flow rate of argon gas for plasma is preferably from 5 to 40 L / min, and more preferably from 15 to 25 L / min. The flow rate of carrier nitrogen gas for supplying the silver-coated copper powder is preferably 0 to 3 L / min, and more preferably 0 to 0.5 L / min. The pressure in the device is preferably 0 to 100 kPa, and more preferably 50 to 100 kPa. The supplied amount of the silver-coated copper powder is preferably 0.1 to 400 g / min, and more preferably 100 to 400 g / min.

[0044] After thus diffusing silver at the surface of the copper powder into the grain boundaries of copper at the interior of the copper powder, the surface (of the obtained metal composite powder) (at least the exposed surface of the copper powder) is coated with silver. As the method for plating the surface with silver, the same method as the method for plating the surface of the copper powder with silver described above can be used.

[0045] In the above-described preferred embodiment of a method for producing a metal composite powder according to the present invention, it is possible to produce a metal composite powder in which silver is diffused into grain boundaries of copper in the interior of a copper powder and the surface thereof is coated with silver. The content of silver with respect to that of the metal composite powder may be 5% by weight or more (preferably 7 to 50% by weight, more preferably 8 to 40% by weight, and most preferably 9 to 20% by weight). The percentage of an area occupied by silver on a cross section of the metal composite powder may be 3 to 20% (preferably 8 to 20%).

[0046] In the grain boundaries, the arrangement of crystals falls into disorder, and oxygen is easy to diffuse, so that oxidation of the grain boundaries of copper proceeds by the diffusion (grain boundary diffusion) of oxygen along the grain boundaries of copper. However, in the metal composite powder according to the present invention, silver is caused to diffuse into the grain boundaries of copper at the inside of the copper powder to fill the grain boundaries of copper at the inside of the copper powder, and then the surface of the copper powder is coated with silver. Therefore, it is possible to suppress oxidation of the surface thereof and the grain boundaries of copper, so that it is possible to provide metal composite powder having high oxidation resistance.

[0047] Furthermore, the metal composite powder (the metal composite powder whose surface is coated with silver) can be added by the above-described preferred embodiment of a method for producing a metal composite powder according to the present invention to a silver-supported solution, such as a silver-potassium cyanide solution, to cause that Silver is supported on the surface of the metal composite powder. Thus, when silver is supported on the surface of the metal composite powder, even if the copper powder is exposed on a part of the surface of the metal composite powder (the metal composite powder whose surface is coated with silver), the exposed part of the copper powder (which is not coated with silver) can be coated with silver, so that it is possible to provide a metal composite powder having higher oxidation resistance.

[0048] Examples of a metal composite powder and a method for producing the same according to the present invention are explained in more detail below. Comparative example 1

[0049] A commercially available copper powder produced by atomization (spherically atomized copper powder manufactured by Nippon Atomized Metal Powders Corporation, the copper powder having a purity of 99.9% by weight and an average particle diameter of 5 μm) was prepared.

[0050] Also prepared were a solution (solution 1) obtained by dissolving 2.6 kg of ammonium carbonate in 450 kg of pure water and a solution (solution 2) obtained by adding 92 kg of an aqueous silver nitrate solution containing 16.904 kg contained silver to a solution obtained by dissolving 319 kg of EDTA-4Na (43%) and 76 kg of ammonium carbonate in 284 kg of pure water.

[0051] Then, in the nitrogen atmosphere, 100 kg of the copper powder described above was added to the solution 1, and the temperature of the solution was raised to 35°C while the solution was stirred. Subsequently, the solution containing the copper dispersed therein was added to the solution 2 and was stirred for 30 minutes.

[0052] Subsequently, a solid content obtained by filtration was washed with ion-exchanged water until a transparent filtrate was obtained, and then the washed solid content was vacuum-dried at 70°C to obtain a copper powder coated with silver (a silver-coated copper powder ).

[0053] After a cross section of the silver-coated copper powder thus obtained was produced by a cross section polisher (CP), the cross section was observed by a field emission scanning electron microscope (FE-SEM). The BE mode composition image (COMPO image) of the cross section of the silver-coated copper powder in this observation is in figure 1 shown. In this COMPO image, since the brightness is higher as the atomic weight is larger, silver appears lighter than copper, so the relatively low portion of the brightness corresponds to silver and the dark portion thereof corresponds to copper. It can be seen from the COMPO image that the copper powder is coated with silver in the silver-coated copper powder obtained in this comparative example. Furthermore, the black lines that can be observed on the inside of the copper powder serving as the core of the silver-coated copper powder show the grain boundaries of copper.

[0054] Subsequently, a thermographic / differential thermal analyzer (TG-DTA device) (Thermo Plus EVO2 TG-8120 manufactured by Rigaku Co., Ltd.) was used to carry out the TG-DTA measurement of 40 mg of the silver-coated copper powder manufactured by the obtained silver-coated copper powder by raising its temperature from room temperature (25°C) to 400°C (diffused) at a rate of temperature increase of 10°C / min while flowing air therein at a flow rate of 200 ml / min. The measured results of this are in figure 11 shown. On the basis of a rate (%) of weight increase calculated from a difference (the weight increased by heating) between each of the weights of the silver-coated copper powder heated at temperatures of 200°C, 250°C, 300°C, 350 °C and 400°C was obtained in this measurement and the weight of the silver-coated copper powder before heating was obtained with respect to the weight of the silver-coated copper powder before heating, the storage stability (reliability) of the silver-coated copper powder was evaluated by evaluating the high-temperature stability (in regarding oxidation) of the silver-coated copper powder in air, assuming that all weights increased by heating were weights increased by oxidation of the silver-coated copper powder. As a result, the rates of weight gain at 200°C, 250°C, 300°C, 350°C, and 400°C were equal to 0.16%, 0.46%, 1.27%, 3.80%, and 6%, respectively. 54% In the TG-DTA measurement of the silver-coated copper powder obtained in this comparative example, an exothermic peak (having a weight increase due to oxidation) occurred.

[0055] The COMPO image of the cross section of the in figure1 and a particle analyzer software (Region Adviser manufactured by SYSTEM IN FRONTIER INC.) were used to carry out the image analysis of the cross section of the silver-coated copper powder in this comparative example. In this image analysis, after the data smoothing of the COMPO image was performed, its contrast was adjusted to 100 and its brightness was controlled between 60 and 100 in an automatic contrast-brightness control section (ACB), and binary-coded processing was performed in a histogram system (processing for forming a histogram of brightness values ​​on the image to binarize the image based on the tendency of the histogram) by area segmentation. As a result, the percentage of silver with respect to the entire cross-sectional area of ​​the silver-coated copper powder (the amount of silver at the cross-section) was 3.85%, which was smaller than the silver content (11.06%). Furthermore, the content of silver in the silver-coated copper powder in this comparative example was obtained as follows. First, 5.0 g of the silver-coated copper powder was added to 40 ml of an aqueous nitric acid solution prepared by diluting an aqueous nitric acid solution having a specific gravity of 1.38 with pure water to a volume ratio of 1:1, and the solution was boiled by a heater to completely dissolve the silver-coated copper powder therein. Then, an aqueous hydrochloric acid solution prepared by diluting an aqueous hydrochloric acid solution having a specific gravity of 1.18 with pure water at a volume ratio of 1:1 was added to the above-described aqueous solution in which the silver-coated copper powder was gradually and after being completely dissolved to deposit silver chloride, and the aqueous hydrochloric acid solution was added until precipitates of silver chloride were no longer generated. The silver content was calculated from the weight of the silver chloride obtained to obtain the silver content in the silver-coated copper powder. Comparative example 2

[0056] The silver-coated copper powder obtained in Comparative Example 1 was sprayed into the discharge flame portion of a thermal plasma by means of a thermal plasma apparatus (Nanoparticle Synthesis Experimental Apparatus manufactured by JEOL Ltd.) for heat treatment to obtain a metal composite powder. This plasma tail flame area was red, so it can be determined that its temperature was between 3000 and 5000K. In this process, the output of the thermal plasma device was 6 kW. The flow rate of argon gas for plasma was 20 L / min, and the flow rate of carrier nitrogen gas for supplying the silver-coated copper powder was 2 L / min. The pressure in the device was 50 kPa and the amount of silver-coated copper powder supplied was 2.5 g / min.

[0057] After a cross section of the metal composite powder thus obtained was obtained by the cross section polisher (CP), the cross section was observed by means of the field emission scanning electron microscope (FE-SEM). The COMPO image of the cross section of the metal composite powder in this observation is in figure 2 shown. From this COMPO image, it can be seen that although the surface of the copper powder in the metal composite powder obtained in this comparative example is not coated with silver, silver is caused to diffuse into the grain boundaries of copper.

[0058] Then, the cross section of the metal composite powder obtained in this comparative example was observed using an energy dispersive X-ray spectrometer (EDS) and a field emission Auger electron spectrometer (FE-AES). The mapping image of the cross section of the metal composite powder in this consideration is in figure 3 shown. It can also be seen from this map that the silver has been caused to diffuse into the grain boundaries of copper.

[0059] With respect to the obtained metal composite powder, TG-DTA measurement was carried out by the same method as in Comparative Example 1. The measured results thereof are shown in FIG figure 12 shown. On the basis of a rate (%) of weight increase calculated from a difference (the increased weight by heating) between each of the weights of the metal composite powder measured at temperatures of 200°C, 250°C, 300°C, 350°C and 400 °C were obtained in this measurement, and the weight of the metal composite powder before heating in relation to the weight of the metal composite powder before heating were obtained, the storage stability (reliability) of the metal composite powder was evaluated by evaluating the high-temperature stability (in terms of oxidation) of the metal composite powder in Air evaluated assuming that all weights increased by heating were weights increased by oxidation of the metal composite powder. As a result, the rates of weight gain at 200°C, 250°C, 300°C, 350°C, and 400°C were equal to 0.42%, 0.73%, 1.38%, 2.44%, and 3%, respectively. 99%. From these results, it can be seen that the high-temperature stability (in terms of oxidation) of the metal composite powder in air is improved, so that the storage stability (reliability) of the metal composite powder is improved because the rates of weight increase at high temperatures in the metal composite powder obtained from this Comparative Example is smaller than those in the silver-coated copper powder obtained in Comparative Example 1. Further, in the TG-DTA measurement of the metal composite powder obtained in this comparative example, no exothermic peak (having a weight increase due to oxidation) occurred.

[0060] The COMPO image of the cross section of the metal composite powder used in figure 2 and the particle analysis software (Region Adviser produced by SYSTEM IN FRONTIER INC.) were used to carry out the image analysis of the cross section of the metal composite powder in this comparative example. As a result, the percentage of silver with respect to the total cross-sectional area of ​​the metal composite powder (the amount of silver at the cross-section) was 12.00%, which was larger than the silver content (10.92%). Furthermore, the content of silver in the metal composite powder in this comparative example was obtained as follows. First, 0.5 g of the metal composite powder was added to 5 ml of an aqueous nitric acid solution prepared by diluting an aqueous nitric acid solution having a specific gravity of 1.38 with pure water at a volume ratio of 1:1, and the solution was a heater to completely dissolve the metal composite powder therein. Then, the filtrate obtained from the filtration was caused to have a constant volume by adding pure water, and the content of silver in the metal composite powder was determined by quantitative analysis by an inductively coupled plasma (ICP) emission spectrophotometric analyzer (iCAP 6300 manufactured by Thermo Scientific). Comparative example 3

[0061] A metal composite powder was obtained by the same method as that in Comparative Example 2, except that the output of the thermal plasma device was 2 kW (in this case, the end portion of the plasma flame was green, so it can be determined that the temperature of the end portion of the plasma flame was at lower temperature (2000 to 4000 K) than 3000 to 5000 K which was the temperature thereof when the output of the thermal plasma device was 6 kW). Then, a cross section of the metal composite powder thus obtained was obtained by the cross section polisher (CP), and the cross section was observed by the field emission scanning electron microscope (FE-SEM). The COMPO image of the cross section of the metal composite powder in this observation is in figure4 shown. It can be seen from this COMPO image that silver is caused to diffuse into a part of the grain boundaries of copper on the inside of the copper powder in the metal composite powder obtained in this comparative example.

[0062] With respect to the obtained metal composite powder, TG-DTA measurement was carried out by the same method as in Comparative Example 1. The measured results thereof are shown in FIG figure 13 shown. On the basis of a rate (%) of weight increase calculated from a difference (the increased weight by heating) between each of the weights of the metal composite powder measured at temperatures of 200°C, 250°C, 300°C, 350°C and 400 °C were obtained in this measurement, and the weight of the metal composite powder before heating in relation to the weight of the metal composite powder before heating were obtained, the storage stability (reliability) of the metal composite powder was evaluated by evaluating the high-temperature stability (in terms of oxidation) of the metal composite powder in Air evaluated assuming that all weights increased by heating were weights increased by oxidation of the metal composite powder. As a result, the rates of weight gain at 200°C, 250°C, 300°C, 350°C, and 400°C were equal to 0.19%, 0.42%, 1.24%, 3.86%, and 6%, respectively. 52% From these results, it can be seen that the storage stability (reliability) of the metal composite powder obtained in this comparative example does not vary much compared to that of the silver-coated copper powder obtained in comparative example 1. Further, in the TG-DTA measurement of the metal composite powder obtained in this comparative example, an exothermic peak (with a weight increase due to oxidation) occurred.

[0063] The COMPO image of the cross section of the metal composite powder used in figure 4 and the particle analysis software (Region Adviser produced by SYSTEM IN FRONTIER INC.) were used to carry out the image analysis of the cross section of the metal composite powder in this comparative example. As a result, the percentage of silver based on the total cross-sectional area of ​​the metal composite powder (the amount of silver at the cross-section) was 11.56%, which was larger than the content of silver (10.90%) (determined by the same method as in Comparative Example 2) was obtained. example 1

[0064] Also, a solution (Solution 1) obtained by dissolving 21.00 g of EDTA-4Na (43%) and 5.00 g of ammonium carbonate in 32.40 g of pure water was prepared, and a solution (Solution 2) was prepared , which was obtained by adding 3.45 g of an aqueous silver nitrate solution containing 1.11 g of silver to a solution prepared by dissolving 21.00 g of EDTA-4Na (43%) and 5.00 g of ammonium carbonate in contained 32.40 g of pure water.

[0065] Then, in the nitrogen atmosphere, 10.00 g of the metal composite powder obtained in Comparative Example 2 was added to the solution 1, and the temperature of the solution was raised to 35° C. while the solution was stirred. Subsequently, the solution containing the copper powder dispersed therein was added to the solution 2 and was stirred for 30 minutes.

[0066] Subsequently, a solid content obtained by filtration was washed with ion-exchanged water until a transparent filtrate was obtained, and then the washed solid content was vacuum-dried at 70°C to obtain a metal composite powder coated with silver.

[0067] After a cross section of the metal composite powder thus obtained was obtained by the cross section polisher (CP), the cross section was observed by means of the field emission scanning electron microscope (FE-SEM). The COMPO image of the cross section of the metal composite powder in this observation is in figure 5 shown. From this COMPO image, it can be seen that silver is caused to diffuse into the grain boundaries of copper on the inside of the copper powder while the surface of the copper powder is coated with silver in the metal composite powder obtained in this example.

[0068] Then, the cross section of the metal composite powder obtained in this example was observed using an energy dispersive X-ray spectrometer (EDS) and the field emission Auger electron spectrometer (FE-AES). The silver mapping image of the cross section of the metal composite powder in this observation is in figure 6 and the copper allocation image thereof is in FIG figure 7 shown. It can also be seen from these map images that silver is caused to diffuse into the grain boundaries of copper on the inside of the copper powder while the surface of the copper powder is coated with silver.

[0069] With respect to the obtained metal composite powder, TG-DTA measurement was carried out by the same method as in Comparative Example 1. The measured results thereof are shown in FIG figure 14 shown. On the basis of a rate (%) of weight increase calculated from a difference (the increased weight by heating) between each of the weights of the metal composite powder measured at temperatures of 200°C, 250°C, 300°C, 350°C and 400 °C were obtained in this measurement, and the weight of the metal composite powder before heating in relation to the weight of the metal composite powder before heating were obtained, the storage stability (reliability) of the metal composite powder was evaluated by evaluating the high-temperature stability (in terms of oxidation) of the metal composite powder in Air evaluated assuming that all weights increased by heating were weights increased by oxidation of the metal composite powder. As a result, the rates of weight gain at 200°C, 250°C, 300°C, 350°C, and 400°C were equal to 0.15%, 0.43%, 0.85%, 1.78%, and 3%, respectively. 51% From these results it can be seen that the high-temperature stability (in terms of oxidation) of the metal composite powder in air is improved, so that the storage stability (reliability) of the metal composite powder is improved because the rates of weight increase in the metal composite powder obtained in this example , are smaller than those in the silver-coated copper powder obtained in Comparative Example 1 and in the metal composite powders obtained in Comparative Examples 2 and 3. Further, in the TG-DTA measurement of the metal composite powder obtained in this example, no exothermic peak (having a weight increase due to oxidation) occurred.

[0070] The COMPO image of the cross section of the metal composite powder used in figure 5 and the particle analysis software (Region Adviser produced by SYSTEM IN FRONTIER INC.) were used to perform the image analysis of the cross section of the metal composite powder in this example. As a result, the percentage of silver based on the total cross-sectional area of ​​the metal composite powder (the amount of silver at the cross-section) was 15.05%, which was smaller than the content of silver (22.72%) (which was obtained by the same method as that obtained in Comparative Example 2). example 2

[0071] A metal composite powder coated with silver was obtained by the same procedure as in Example 1, except that the metal composite powder obtained in Comparative Example 3 was substituted for the metal composite powder obtained in Comparative Example 2 .

[0072] After a cross section of the metal composite powder thus obtained was made by the cross section polisher (CP), the cross section was observed by the field emission scanning electron microscope (FE-SEM). The COMPO image of the cross section of the metal composite powder in this observation is in figure 8 shown. From this COMPO image, it can be seen that silver is caused to diffuse into part of the grain boundaries of copper on the inside of the copper powder while the surface of the copper powder is coated with silver in the metal composite powder obtained in this example.

[0073] Then, the cross section of the metal composite powder obtained in this example was observed using an energy dispersive X-ray spectrometer (EDS) and a field emission Auger electron spectrometer (FE-HES). The silver mapping image of the cross section of the metal composite powder in this observation is in figure 9 and the copper allocation image thereof is in FIG figure 10 shown. It can also be seen from these map images that silver is caused to diffuse into part of the grain boundaries of copper on the inside of the copper powder while the surface of the copper powder is coated with silver.

[0074] With respect to the obtained metal composite powder, the TG-DTA measurement was carried out by the same method as in Comparative Example 1.

[0075] The measured results of this are in figure 15 shown. On the basis of a rate (%) of weight increase calculated from a difference (the increased weight by heating) between each of the weights of the metal composite powder measured at temperatures of 200°C, 250°C, 300°C, 350°C and 400 °C were obtained in this measurement, and the weight of the metal composite powder before heating in relation to the weight of the metal composite powder before heating were obtained, the storage stability (reliability) of the metal composite powder was evaluated by evaluating the high-temperature stability (in terms of oxidation) of the metal composite powder in Air evaluated assuming that all weights increased by heating were weights increased by oxidation of the metal composite powder. As a result, the rates of weight gain at 200°C, 250°C, 300°C, 350°C, and 400°C were equal to 0.07%, 0.32%, 1.09%, 3.12%, and 5, respectively. 53% From these results it can be seen that the high-temperature stability (in terms of oxidation) of the metal composite powder in air is improved, so that the storage stability (reliability) of the metal composite powder is improved because the rates of weight increase in the metal composite powder obtained in this example , are smaller than those in the silver-coated copper powder obtained in Comparative Example 1 and in the metal composite powders obtained in Comparative Example 3. Further, in the TG-DTA measurement of the metal composite powder obtained in this comparative example, an exothermic peak (with a weight increase due to oxidation) occurred.

[0076] The COMPO image of the cross section of the metal composite powder used in figure 8 and the particle analysis software (Region Adviser produced by SYSTEM IN FRONTIER INC.) were used to perform the image analysis of the cross section of the metal composite powder in this example. As a result, the percentage of silver based on the total cross-sectional area of ​​the metal composite powder (the amount of silver at the cross-section) was 12.05%, which was smaller than the content of silver (19.84%) (which was obtained by the same method as that obtained in Comparative Example 2). Comparative example 4

[0077] Also, a solution (Solution 1) obtained by dissolving 112.61 g of EDTA-4Na (43%) and 9.10 g of ammonium carbonate in 1440.89 g of pure water was prepared, and a solution (Solution 2) was prepared , which was obtained by adding 255.68 g of an aqueous silver nitrate solution containing 82.1 g of silver to a solution prepared by dissolving 1551.67 g of EDTA-4Na (43%) and 185.29 g of ammonium carbonate in contained 407.95 g of pure water.

[0078] Then, in the nitrogen atmosphere, 350 g of the same copper powder as in Comparative Example 1 was added to the solution 1, and the temperature of the solution was raised to 35°C while the solution was stirred. Subsequently, the solution containing the copper dispersed therein was added to the solution 2 and was stirred for 30 minutes.

[0079] Subsequently, a solid content obtained by filtration was washed with ion-exchanged water until a transparent filtrate was obtained, and then the washed solid content was vacuum-dried at 70°C to obtain a copper powder coated with silver (a silver-coated copper powder ).

[0080] The cross section of the silver-coated copper powder thus obtained was observed by the field emission scanning electron microscope (FE-SEM) by the same method as in Comparative Example 1. It was found from the COMPO image of the cross section of the silver-coated copper powder in this observation that the copper powder was coated with silver in the silver-coated copper powder obtained in this comparative example.

[0081] With respect to the obtained silver-coated copper powder, TG-DTA measurement was carried out by the same method as in Comparative Example 1. The measured results thereof are shown in FIG figure 16 shown. On the basis of a rate (%) of weight increase calculated from a difference (the weight increased by heating) between each of the weights of the silver-coated copper powder heated at temperatures of 200°C, 250°C, 300°C, 350 °C and 400°C was obtained in this measurement and the weight of the silver-coated copper powder before heating was obtained with respect to the weight of the silver-coated copper powder before heating, the storage stability (reliability) of the silver-coated copper powder was evaluated by evaluating the high-temperature stability (in regarding oxidation) of the silver-coated copper powder in air, assuming that all weights increased by heating were weights increased by oxidation of the silver-coated copper powder. As a result, the rates of weight gain at 200°C, 250°C, 300°C, 350°C, and 400°C were equal to 0.08%, 0.45%, 1.17%, 3.34%, and 5, respectively. 81% From these results, it can be seen that the high-temperature stability (in terms of oxidation) of the silver-coated copper powder in air is inferior to that of the metal composite powders obtained in Examples 1 and 2, so that the storage stability (reliability) of the silver-coated copper powder is superior to that of the is inferior to the metal composite powder obtained in Examples 1 and 2 because the rates of weight increase at high temperatures in the silver-coated copper powder obtained in this comparative example are larger than those in the metal composite powders obtained in Examples 1 and 2.

[0082] Then, according to the same method as in Example 1, the image analysis of the cross section of the silver-coated copper powder in this comparative example was carried out. As a result, the percentage of silver based on the total cross-sectional area of ​​the silver-coated copper powder (the amount of silver on the cross-section) was 7.73%, which was smaller than the content of silver (20.02%) (determined by the same method as obtained in Comparative Example 1).

[0083] While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate understanding thereof, it should be appreciated that the invention may be embodied in various ways without departing from the principles of the invention. Therefore, it is to be understood that there are all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principles of the invention as set forth in the appended claims. QUOTES INCLUDED IN DESCRIPTION

[0084] This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Patent Literature Cited

[0085] JP 2010-174311 [0004, 0005] JP 2010-077495 [0004, 0005]

Claims

[1] A method for producing a metal composite powder, the method comprising the steps of: a silver-coated copper powder is provided, wherein the surface of a copper powder is coated with silver; the silver-coated copper powder is sprayed into an outlet flame region of a thermal plasma to cause silver at the surface of the copper powder to diffuse into a grain boundary of the copper at the interior of the copper powder; and The surface of the copper powder is then coated with silver. [2] Method for producing a metal composite powder according to claim 1, wherein the outflow flame region of the thermal plasma has a temperature of 2000 to 5000 K. [3] Method for producing a metal composite powder according to claim 1, wherein the copper powder is produced by atomization. [4] Method for producing a metal composite powder according to claim 1, wherein the copper powder has an average particle diameter of 0.1 to 100 μm. [5] Method for producing a metal composite powder according to claim 1, wherein the silver content in relation to the silver-coated copper powder is not less than 5 wt%. [6] Metal composite powders, comprising: a copper powder; and Silver diffuses into a grain boundary of copper at the interior of the copper powder and coats the surface of the copper powder. [7] Metal composite powder according to claim 6, wherein the copper powder has an average particle diameter of 0.1 to 100 μm. [8] Metal composite powder according to claim 6, wherein the silver content in relation to the metal composite powder is not less than 5 wt%. [9] Metal composite powder according to claim 6, wherein the percentage of an area occupied by silver on a cross-section of the metal composite powder is between 3 and 20%.

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