Carbon-coated metal powder and paste

Abstract

To provide a carbon-coated metal powder and a paste, the metal powder having a relatively small particle size and being capable of effectively suppressing surface oxidation and aggregation in a liquid such as a powder or paste.SOLUTION: A carbon-coated metal powder has metal particles, and a coating layer that covers at least a part of the surface of each metal particle and contains carbon. The metal particles comprise: (1) tin, copper or silver, and unavoidable impurities; or (2) an alloy of tin, copper or silver in which the content of tin, copper or silver is 50 mass% or more, and unavoidable impurities. The average particle diameter calculated from the BET specific surface area is 150 nm or less.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] This specification describes carbon coated metal powders and pastes. [Background technology]

[0002] Metal powders made of a predetermined pure metal or alloy may be mixed with a liquid containing an organic substance, such as an organic binder or an organic solvent, to form a paste. Such pastes can be used, for example, as materials for electronic devices or energy devices, such as conductive pastes for forming wiring by printing or for filling holes or joining, or as solder pastes for soldering.

[0003] Although not related to metal powder for paste, for example, Patent Document 1 proposes, under the objective of "providing a carbon-coated metal powder or the like which has excellent fluidity and is capable of efficiently absorbing energy rays and is suitable as a powder material for additive manufacturing," "a carbon-coated metal powder having a carbon coating formed by chemically coating the surfaces of metal particles having an average particle diameter of 1 μm or more and 100 μm or less with carbon derived from an organic compound, wherein the amount of carbon coating in the carbon-coated metal powder is 0.001% by mass or more and 2.0% by mass or less." [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-199862 Summary of the Invention [Problem to be solved by the invention]

[0005] Depending on the application, metal powders may be required to have a particle size smaller than the "average particle size of 1 μm or more and 100 μm or less" described in Patent Document 1. Reducing the particle size of metal powders has advantages such as increasing the surface area and making them more active, resulting in a lower melting point, and making it possible to apply a thin film or a narrow area when made into a paste.

[0006] However, such extremely fine metal powders have a large surface area, making them susceptible to oxidation. Oxidation reduces the proportion of metal in the total metal particles. For example, when fine Cu or Ag metal powders are made into a paste and used as a wiring material, oxidation can cause a problem of reduced conductivity. In the case of fine Sn metal powder, surface oxidation reduces the melting of the Sn, reducing its functionality as a solder paste. Furthermore, extremely fine metal powders, whether in their original state or when mixed with a liquid such as a paste, are prone to agglomeration of metal particles. For example, when a paste containing agglomerated metal powders is applied thinly or over a narrow area, the presence of these agglomerations can cause uneven application, potentially resulting in defects.

[0007] In view of these problems, embodiments of the present invention provide carbon-coated metal powders and pastes that have relatively small particle sizes and can effectively suppress surface oxidation and aggregation in liquids such as powders or pastes. [Means for solving the problem]

[0008] The embodiments of the present invention that solve the above problems are defined below. 1. A carbon-containing coating layer comprising metal particles and covering at least a portion of the surface of the metal particles, The metal particles are (1) tin, copper, or silver and unavoidable impurities, or (2) an alloy of tin, copper, or silver, each of which contains 50% by mass or more of tin, copper, or silver, and unavoidable impurities; A carbon-coated metal powder having an average particle size of 150 nm or less as calculated from the BET specific surface area. 2. The carbon-coated metal powder according to 1 above, having a carbon content of 0.5 to 3.0 mass %. 3. The carbon-coated metal powder according to 1 or 2 above, wherein the hydrogen content is 0.01 to 0.25 mass %. 4. The carbon-coated metal powder according to any one of 1 to 3 above, wherein the carbon-containing coating layer is composed of a mixture of graphite carbon and hydrocarbon. 5. The carbon-coated metal powder according to any one of 1 to 4 above, which has an oxygen content of 3.0 mass % or less. 6. The carbon-coated metal powder according to any one of 1 to 5 above, which is used in a paste. 7. A paste comprising the carbon-coated metal powder according to any one of 1 to 6 above and a liquid containing an organic substance. [Effects of the Invention]

[0009] According to an embodiment of the present invention, it is possible to provide a carbon-coated metal powder and paste that have a relatively small particle size and can effectively suppress surface oxidation and aggregation in a liquid such as a powder or paste. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view schematically showing a surface-coated particle contained in a carbon-coated metal powder according to an embodiment of the present invention. [Figure 2] 1 is an XRD spectrum of the carbon-coated metal powder according to Example 1. [Figure 3] 3 is an XRD spectrum of the carbon-coated metal powder according to Example 2. [Figure 4] 1 is an XRD spectrum of the carbon-coated metal powder according to Example 3. [Figure 5] 1 shows SEM images of carbon-coated metal powders according to Examples 1 to 3. [Figure 6] 1 is an SEM image of the metal powder according to Comparative Example 1. [Figure 7] 2 is an SEM image of a coarse particle of the carbon-coated metal powder according to Example 1. [Figure 8]1 is an SEM image of coarse particles of the metal powder according to Comparative Example 1. [Figure 9] 1 is a Raman spectrum of the carbon-coated metal powder according to Example 3. [Figure 10] 1 is a Raman spectrum of a metal powder according to Comparative Example 1. [Figure 11] 3 is a photograph showing the appearance of the carbon-coated metal powder according to Example 1 when mixed with silicone oil. [Figure 12] 1 is a photograph showing the appearance of the carbon-coated metal powder according to Example 1 when mixed with PEG1000. [Figure 13] 1 is a photograph showing the appearance of the carbon-coated metal powder according to Example 1 when mixed with carbon paste. DETAILED DESCRIPTION OF THE INVENTION

[0011] Preferred embodiments of the present invention will be described in detail below, but the present invention should not be construed as being limited thereto. Various modifications and improvements can be made based on the knowledge of those skilled in the art without departing from the gist of the present invention. The multiple components disclosed in this embodiment can be appropriately combined to form various inventions. For example, some components may be omitted from all the components shown in this embodiment.

[0012] <Carbon-coated metal powder> A carbon-coated metal powder according to an embodiment of the present invention comprises metal particles and a coating layer containing carbon and covering at least a portion of the surface of the metal particles, The metal particles are (1) tin, copper, or silver and unavoidable impurities, or (2) an alloy of tin, copper, or silver, each of which contains 50% by mass or more of tin, copper, or silver, and unavoidable impurities; The average particle size calculated from the BET specific surface area is 150 nm or less.

[0013] More specifically, the carbon-coated metal powder according to an embodiment of the present invention is an aggregate of particles including surface-coated particles 3, each of which is formed by covering a metal particle 1 with a coating layer 2, as illustrated in Fig. 1, and most or almost all of the aggregate is composed of surface-coated particles 3. In Fig. 1, the entire surface of the surface-coated particle 3 is covered with the coating layer 2, but it is sufficient that the coating layer 2 is attached to the surface of the metal particle 1 so as to cover at least a portion of the surface of the metal particle 1. In other words, a portion of the surface of the metal particle 1 may be exposed and not covered with the coating layer 2.

[0014] As described above, the carbon-coated metal powder according to the embodiment of the present invention has a relatively small average particle size of 150 nm or less, but is characterized by excellent dispersibility in liquids such as pastes. Although the reason for this is not entirely clear, it is believed that compatibility with pastes is improved by covering at least a portion of the surface of the metal particles with a carbon-containing coating layer. The carbon-coated metal powder according to the embodiment of the present invention may have an average particle size of 120 nm or less, or may have an average particle size of 100 nm or less. The carbon-containing coating layer also has the function of inhibiting oxidation of the surface of the metal particles. The average particle size of the carbon-coated metal powder according to the embodiment of the present invention can be measured using a Macsorb / HM model-1208 manufactured by MOUNTEC Corporation.

[0015] (composition) The metal particles contained in the carbon-coated metal powder according to an embodiment of the present invention are composed of (1) tin, copper, or silver and unavoidable impurities, or (2) an alloy of tin, copper, or silver, each of which contains 50 mass% or more of tin, copper, or silver, and unavoidable impurities.

[0016] When the metal particles of the carbon-coated metal powder according to an embodiment of the present invention contain a metal, the content of tin, copper or silver in the metal portion of the carbon-coated metal powder may be, for example, 99.0 mass% or more, typically 99.9 mass% or more, and preferably 99.99 mass% or more.

[0017] When the metal particles of the carbon-coated metal powder according to the embodiment of the present invention contain an alloy, the tin, copper, or silver content in the carbon-coated metal powder is preferably 50% by mass or more. The tin, copper, or silver content in the carbon-coated metal powder is more preferably 60% by mass or more. For these tin, copper, or silver alloys, the tin, copper, or silver content in the carbon-coated metal powder is preferably controlled depending on the application. For example, in the case of carbon-coated metal powders used in solder pastes, specific examples of alloys constituting the metal particles include alloys having compositions, by mass, such as 63% Sn, 37% Pb, 60% Sn, 40% Pb, 96.5% Sn, 3% Ag, 0.5% Cu, 95% Sn, 4% Ag, 1% Cu, and 95% Sn, 5% Ag.

[0018] Examples of unavoidable impurities contained in the metal particles of the carbon-coated metal powder according to the embodiment of the present invention include Si, Fe, Na, Mg, etc. The content of unavoidable impurities contained in the metal particles of the carbon-coated metal powder is preferably 1.0 mass% or less, and more preferably 0.1 mass% or less. Note that when multiple types of unavoidable impurities are present, the content of the unavoidable impurities refers to the total content of the unavoidable impurities.

[0019] The content of tin, copper, or silver in carbon-coated metal powder, the content of added elements when the metal particles are alloys, and the content of impurities can be measured by ICP-MS if the sample to be measured is liquid, or by GD-MS if the sample is solid. For example, a solid sample of carbon-coated metal powder can be placed in a measurement container and solidified under pressure, and then measured by GD-MS. The content of impurities can be analyzed, for example, as a ratio (ppm) relative to Sn.

[0020] The carbon-coated metal powder according to an embodiment of the present invention includes a coating layer containing carbon, and therefore contains carbon. The carbon content of the carbon-coated metal powder is preferably 0.5 to 3.0 mass %, more preferably 0.5 to 2.0 mass %. If the carbon content is too low, there is a concern that aggregation and oxidation will not be sufficiently suppressed. On the other hand, if the carbon content is too high, the conductivity between particles will decrease, or the surface will become inactive, which may hinder fusion between particles during heating. In the carbon-coated metal powder according to an embodiment of the present invention, the carbon-containing coating layer may be composed of a mixture of graphite-based carbon and hydrocarbon. Here, graphite-based carbon is generated by heating hydrocarbon in an inert atmosphere (e.g., a gas atmosphere such as nitrogen or argon) or in a sealed air environment. The coating on the metal powder can be formed by spraying hydrocarbon such as methane onto the heated metal powder. Raman analysis of graphite-based carbon reveals a Raman shift of 1350 cm -1 Nearby is "D Band", 1600cm -1 Nearby is "G Band", 2680cm -1 A 2D band or G' band peak is detected near the peak. Thus, it is preferable for the coating layer of the carbon-coated metal powder to contain hydrocarbons, with the hydrogen content preferably being 0.01 to 0.25% by mass, more preferably 0.01 to 0.1% by mass. A higher hydrogen content improves compatibility with the paste and dispersibility. Because the hydrogen contained in this coating layer is hydrogen from hydrocarbons such as methane, too much hydrogen also increases the carbon content, potentially reducing the conductivity between particles and inactivating the surface, which may inhibit fusion between particles during heating. The carbon content or hydrogen content of the carbon-coated metal powder can be analyzed by non-dispersive infrared absorption spectroscopy and measured as a percentage (ppm) of the total mass of the carbon-coated metal powder.

[0021] In the carbon-coated metal powder according to an embodiment of the present invention, the metal particles are covered with a carbon-containing coating layer, which inhibits oxidation of the metal particles, resulting in almost no oxygen or a low oxygen content. The oxygen content of the carbon-coated metal powder may be, for example, 3.0 mass% or less, typically 2.5 mass% or less, or even 1.5 mass% or less. A low oxygen content is thought to make the metal more likely to melt when heated. The oxygen content can be analyzed by non-dispersive infrared absorption spectroscopy and measured as a percentage (ppm) of the total mass of the carbon-coated metal powder.

[0022] (particle size) The carbon-coated metal powder according to the embodiment of the present invention has a relatively small particle size. Specifically, the average particle size (so-called BET diameter) calculated from the BET specific surface area of ​​the carbon-coated metal powder is 150 nm or less, typically 100 nm or less, and more typically 50 to 80 nm. If the average particle size is too large, it becomes difficult to form a thin film when applied as a paste. Increasing the average particle size may improve handleability and make it less likely to aggregate.

[0023] The BET specific surface area of ​​the carbon-coated metal powder according to the embodiment of the present invention can be measured in accordance with JIS Z8830:2013, for example, using a Macsorb / HM model-1208 manufactured by MOUNTEC Corporation. The average particle size d (μm) calculated from the BET specific surface area can be calculated from this BET specific surface area using the formula: d = 6 / (ρ × s). In this formula, ρ is the density (g / cm) of tin, copper, or silver, which is determined depending on the composition of the metal particles. 3 ) or the density (g / cm 3 ) and s is the BET specific surface area (m 2 / g).

[0024] When a 2000x SEM image of the carbon-coated metal powder according to the embodiment of the present invention is obtained, the presence or absence of coarse particles is confirmed by observing as wide an area as possible. For example, 10 to 20 fields of view of a 2000x SEM image may be observed. In this case, it is preferable that there are no more than two coarse particles with a particle size of 5 μm or more, more preferably no more than one, and most preferably no particles. This configuration makes it easier to achieve thin film application of the paste or precise printing in a narrow area. The "particle size" can be measured using a scanning electron microscope (SEM).

[0025] (Manufacturing method) In a preferred method for producing the carbon-coated metal powder according to the embodiment of the present invention, a large metal powder having a particle size of about several μm to several tens of μm, which is produced by gas atomization or the like, is first prepared, and then the metal powder is refined by RF plasma, which evaporates the metal on the surface of the metal particles of the metal powder, thereby reducing the diameter of the metal particles.

[0026] Then, while the metal powder is in a high-temperature state after the RF plasma treatment, a hydrocarbon gas containing methane or the like can be sprayed onto it. The spraying of the hydrocarbon gas causes a decomposition and carbonization reaction on the surface of the metal particles, forming a coating layer containing carbon. The proportion of hydrocarbons in the carbon coating can be altered by changing the amount of gas sprayed. If necessary, air classification or other classification can be performed after the RF plasma treatment to remove coarse particles.

[0027] In this way, a carbon-coated metal powder is obtained, which has a predetermined small average particle size and includes surface-coated particles in which at least a portion of the surface of the metal particle is covered with a coating layer. In such a carbon-coated metal powder, since at least a portion of the surface of the metal particle is covered with a coating layer, the metal particle is less susceptible to oxidation and has a relatively low oxygen content.

[0028] <Paste> The carbon-coated metal powder according to the embodiment of the present invention described above is preferably used in a conductive paste, a solder paste, or other paste. Such a paste contains a liquid containing an organic substance in addition to the carbon-coated metal powder according to the embodiment of the present invention, and can be produced, for example, by mixing and stirring the carbon-coated metal powder according to the embodiment of the present invention with the liquid containing the organic substance.

[0029] The liquid containing the organic substance in the paste includes an organic binder, an organic solvent, etc. As the organic binder and the organic solvent, those that are commonly used may be used. [Example]

[0030] Next, the carbon-coated metal powder described above was produced as a prototype, and its effects were confirmed, which will be described below. However, this description is for illustrative purposes only and is not intended to be limiting.

[0031] In Examples 1 to 3, atomized powder (average particle size D50: 5.5 μm) manufactured by Nippon Atomize Processing Co., Ltd. was prepared as a raw material. The raw materials in Examples 1 to 3 were each atomized using different RF plasma outputs, and then coated with carbon by spraying methane after the RF plasma. The RF plasma and methane spraying conditions were the same in all Examples 1 to 3. In this manner, samples of carbon-coated metal powders according to Examples 1 to 3 were prepared. As Comparative Example 1, Sn powder (purity 99.7%) manufactured by Kanto Chemical Co., Inc. was prepared as a raw material, and this was used as a metal powder sample according to Comparative Example 1.

[0032] (BET diameter) For the carbon-coated metal powder samples according to Examples 1 to 3 and the metal powder sample according to Comparative Example 1, the BET diameter (average particle size calculated from the BET specific surface area) was measured as follows. The BET specific surface area was measured by the BET method in accordance with JIS Z8830:2013 using a Macsorb / HM model-1208 manufactured by MOUNTEC Corporation. The average particle size d calculated from the BET specific surface area was calculated from this BET specific surface area using the formula: d = 6 / (ρ × s). In this formula, ρ is the density (g / cm) of tin, copper, or silver, which is determined depending on the composition of the metal particles. 3 ) or the density (g / cm 3 ) and s is the BET specific surface area (m 2 / g). The obtained BET diameter (average particle size calculated from the BET specific surface area) is shown in Table 1.

[0033] [Table 1]

[0034] (XRD measurement) Measurements were carried out using the following XRD diffractometer for the carbon-coated metal powder samples according to Examples 1 to 3. The obtained XRD spectra for Examples 1 to 3 are shown in FIGS. As a result, it was found that the carbon-coated metal powder samples according to Examples 1 to 3 were made of pure tin.

[0035] (SEM observation) Samples of the carbon-coated metal powders according to Examples 1 to 3 and the metal powder according to Comparative Example 1 were observed using a scanning electron microscope (SEM) JXA-8500F (5CH) manufactured by JEOL Ltd. at an acceleration voltage of 15.0 kV and magnifications of 2000x and 10000x. The obtained SEM images are shown in Figures 5 and 6. As shown in Figure 5, it was confirmed that the carbon-coated metal powders according to Examples 1 to 3 were all composed of particles of several tens of nanometers. Furthermore, as shown in Figure 6, it was confirmed that the carbon-coated metal powder according to Comparative Example 1 was agglomerated.

[0036] Furthermore, the carbon-coated metal powder samples according to Examples 1 to 3 and the metal powder sample according to Comparative Example 1 were subjected to SEM observation (magnification: 2000 times) in 20 locations in the same manner as above to search for coarse particles. As a result, a relatively large number of coarse particles with particle sizes of 1 to 2 μm were found in Example 1, but no coarse particles with particle sizes of 5 μm or more were found. An SEM image of the coarse particles with particle sizes of 1 to 2 μm in Example 1 is shown in FIG. In addition, in Examples 2 and 3, no coarse particles with a particle size of 5 μm or more were found. In addition, in Comparative Example 1, coarse particles with a particle size of about 30 μm were found as shown in FIG.

[0037] (Structural analysis of carbon-containing coating layers) For samples of the carbon-coated metal powder according to Example 3 and the metal powder according to Comparative Example 1, structural analysis was performed using a Raman microscope manufactured by Renishaw Ltd., measuring at a laser wavelength of 532 nm, a Raman shift range of 100 to 3500 cm-1, and an accumulation count of 100. The Raman spectrum of the carbon-coated metal powder according to Example 3 is shown in Fig. 9, and the Raman spectrum of the metal powder according to Comparative Example 1 is shown in Fig. 10. For Example 3, Raman shift 1350 cm -1 Nearby is "D Band", 1600cm -1 Nearby is "G Band", 2680cm -1 The peak of the 2D band or G' band was detected around 2931 cm, confirming that the material contains graphite-based carbon. -1 and 3221cm -1 A peak due to a C-H bond was observed, confirming that the carbon-coated metal powder of Example 3 contained hydrocarbons. Thus, it was found that the carbon-coated metal powder of Example 3 was a mixture of graphite-based carbon and hydrocarbons. In Comparative Example 1, as shown in FIG. 10, no peaks derived from graphite-based carbon or hydrocarbons were detected.

[0038] The carbon (C), hydrogen (H), nitrogen (N), oxygen (O), and sulfur (S) contents of the carbon-coated metal powder samples according to Examples 1 to 3 and the metal powder according to Comparative Example 1 were analyzed using a non-dispersive infrared absorption method (LECO Japan LLC, model numbers: CS600(C,S), TC600(O,N), TCH600(H)), and were measured as a percentage (ppm) of the total mass of the carbon-coated metal powder. Similarly, the total content of impurities excluding CHNOS was analyzed and measured as a percentage (ppm) of the total mass of the carbon-coated metal powder. The measurement results are shown in Table 2. As shown in Table 2, for Examples 1 to 3, the carbon content was 0.5 to 3.0 mass% (5000 to 30000 ppm), the hydrogen content was 0.01 to 0.25 mass% (100 to 2500 ppm), and the oxygen content was 3.0 mass% or less (30000 ppm or less). On the other hand, for Comparative Example 1, the oxygen content was very high at 8 mass% (80000 ppm). The hydrogen content was also high, but based on the results of the Raman spectrum above, it is believed that this is not due to hydrocarbons but is related to the hydrogen of hydroxyl groups.

[0039] [Table 2]

[0040] (dispersibility) To confirm dispersibility, 0.05 g of a sample of the carbon-coated metal powder according to Example 1 was added to 5.00 g of various viscous liquids (silicone oil, polyethylene glycol (PEG1000), or carbon paste) and mixed using a pencil mixer. Regarding polyethylene glycol (PEG1000), the liquid was heated to 50°C, and then the carbon-coated metal powder was added and mixed. Photographs of these samples are shown in Figures 11 to 13.

[0041] 11 to 13, it was visually confirmed that the carbon-coated metal powder was uniformly mixed in all cases. This suggests that the aforementioned carbon-coated metal powder may be able to effectively suppress aggregation in liquid, even though it has a relatively small particle size.

[0042] (Melting point and endothermic amount) Differential scanning calorimetry (DSC) was performed on the carbon-coated metal powder samples according to Examples 1 to 3 and the metal powder sample according to Comparative Example 1 to confirm their melting points and endothermic heat amounts. The DSC device used was a Thermo plus EVOS series DSC8231 manufactured by Rigaku Corporation, and measurements were performed under the following measurement conditions. The evaluation results are shown in Table 3. Measurement atmosphere: N2, Heating rate: 5°C / min, Measurement temperature: Room temperature to 300°C

[0043] [Table 3]

[0044] From Table 3, it was confirmed that Examples 1 to 3, which are carbon-coated metal powders, have a larger endothermic amount because oxidation is suppressed compared to Comparative Example 1, which is a metal powder that is not carbon-coated. Also, while the melting point of Sn bulk is 232°C, the melting points (endothermic peak onset points) of the carbon-coated metal powders of Examples 1 to 3 are 216 to 221°C, which is a lower melting point than Sn bulk, and it was confirmed that they have higher activity.

[0045] (Potential contribution to SDGs) According to the above-described embodiment, agglomeration in a liquid can be effectively suppressed, which may improve the yield and performance of products manufactured using the carbon-coated metal powder. Therefore, this embodiment may contribute to Goal 9 "Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation" and Goal 12 "Ensure sustainable consumption and production patterns" of the United Nations-led Sustainable Development Goals (SDGs). [Explanation of symbols]

[0046] 1 metal particles 2 Covering layer 3 Surface coated particles

Claims

1. A carbon-containing coating layer comprising metal particles and covering at least a portion of the surface of the metal particles, The metal particles are (1) tin, copper, or silver and inevitable impurities, or (2) an alloy of tin, copper, or silver, each of which contains 50 mass% or more of tin, copper, or silver, and inevitable impurities; A carbon-coated metal powder having an average particle size calculated from a BET specific surface area of ​​150 nm or less.

2. 2. The carbon-coated metal powder according to claim 1, wherein the carbon content is 0.5 to 3.0 mass %.

3. 2. The carbon-coated metal powder according to claim 1, wherein the hydrogen content is 0.01 to 0.25 mass %.

4. 2. The carbon-coated metal powder according to claim 1, wherein the carbon-containing coating layer is composed of a mixture of graphitic carbon and a hydrocarbon.

5. 2. The carbon-coated metal powder according to claim 1, wherein the oxygen content is 3.0 mass % or less.

6. The carbon-coated metal powder according to claim 1 , which is used in a paste.

7. A paste comprising the carbon-coated metal powder according to any one of claims 1 to 6 and a liquid containing an organic substance.

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